NICE IST-1999-29111 Nanoscale Integrated Circuits using Endohedral Fullerenes Final Project Report Covering period 1.8.2000-31.3.2004 Report Version: Version 2 Report Preparation Date: June 14, 2004 Classification: EU Contract Start Date: August 1, 2000 Duration: 44 months Project Co-ordinator: NMRC Partners: University of Cambridge, University of Gothenborg, IBM Zurich Project funded by the European Community under the “Information Society Technologies” Programme (1998 -2002)
Transcript
NICE
IST-1999-29111
Nanoscale Integrated Circuits using Endohedral Fullerenes
Final Project Report Covering period 1.8.2000-31.3.2004
Report Version: Version 2
Report Preparation Date: June 14, 2004
Classification: EU
Contract Start Date: August 1, 2000 Duration: 44 months
Project Co-ordinator: NMRC
Partners: University of Cambridge, University of Gothenborg, IBM Zurich
Project funded by the European Community under the “Information Society Technologies” Programme (1998 -2002)
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EXECUTIVE SUMMARY 3
PROJECT OBJECTIVES 4
METHODOLOGIES 5
PROJECT RESULTS AND ACHIEVEMENTS 7 Workpackage 1- Endohedral Materials and Substrate/Contacts 7
Endohedral Materials Production and Purification 7 Ion Source for Metallo -fullerenes 18 Substrates and Contact Fabrication 19
Workpackage 3- Fabrication: STM Experiments and Nanostencil Patterning 40 STM Experiments 40 Development of the Nanostencil 59 Nanostencilled Structures 62
Workpackage 4- Characterisation 72 Electrical Characterisation of Large Area Structures 72 X-ray Studies of Endohedral Films 86 Characterization of Au/C60 Interface 89
DELIVERABLES AND REFERENCES 96
FUTURE OUTLOOK 101
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Executive Summary
The NICE project addresses fundamental limits to circuit integration using endohedrally doped buckminsterfullerene and the nanostencil technique.
The primary objective of the NICE project was to develop a fullerene based nano-scale device technology. To achieve this objective, work on endohedral materials production, modelling, characterisation, and direct patterning using shadow mask techniques with both dynamic and static nanostencils was undertaken. Primarily, the project relied upon joining and development of two new technologies: endohedral buckminsterfullerene production and the nanostencil technique.
Research in four categories was pursued: endohedral buckminsterfullerene production, simulation, nanofabrication, and electrical characterisation.
The production and isolation of macroscopic amounts of endohedrally doped C60 molecules was furthered developed within the project. Production and purification of Li@C60 was refined within the project, with several milligram samples distributed to the partners. Smaller amounts of larger alkalis (Na,K) trapped within the fullerenes were produced and a new ion source for multi-valent metal atoms was developed within the project. Additionally, La@C82 was obtained from external sources for experiments requiring larger amounts of doped endohedrals.
Deposition studies of pristine and doped fullerenes (Li@C60, Na@C60, K@C60 and La@C82) on Si(100), Si(111), Ag, Au and Cu surfaces were performed. The doped fullerenes require special handling and careful attention to the temperatures in the deposition process. Large area layered structures were deposited and the interfaces of the doped fullerenes with various metal surfaces (Au, Ti, W, Ag) were characterised both electrically and by SIMS analysis.
Simulations were principally concerned with the properties of the endohedral atoms interaction with the fullerene cages and external electric fields, and the interaction of the fullerenes with surfaces. Fundamental physics was explored in terms of dielectric shielding and single electron orbitals for use in interpretation of STM and related experiments. Simple modelling of diode structures and device modelling of the crossed gate structure was performed.
The combination of shadow masking with scanning probe methods allows structures to be deposited locally through pinhole like apertures within the proximity of a cantilever tip. Within the project, doped fullerene structures were patterned and electrically characterised. Fullerene (both doped and undoped) wires were patterned and crossed with metal wires. The resulting structures were then electrically characterised.
Although difficult to handle, the doped fullerenes can be deposited with sub-monolayer coverage, as monolayers or thin films. They are prone to contamination so all processing and characterisation needed to be performed in UHV. Device structures can be fabricated with the materials, but the electrical properties of the materials do not lend themselves to conventional Si –like structures. The manipulation of the single atom within the fullerene cage for switching is a challenging problem, as interactions of the dopant with the surface is not negligible and the electric field shielding of the dopant atom by the fullerene cage makes STM methods difficult to apply to the problem.
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Project Objectives The goal of the NICE project was to use the nanostencil approach to the prototyping of nanoscale electronic devices. To achieve this goal, developments in the following areas were pursued within the project: Endohedral Buckminsterfullerene Production
Test Structure Fabrication Deliverable 3- Metallized Substrates and Contact Fabrication Simulation
Deliverable 5- Fullerene Modelling STM and Nanostencil Patterning
Deliverable 2- Single Atom Switch Effect Deliverable 10-MESFET Structure
Electrical and Physical Characterisation Deliverable 6- Demonstration of Rectification Deliverable 7- Nanowire Characterisation Deliverable 9- Interface Characterisation As the project developed, it became clear that detailed materials and deposition studies were required to achieve the project objectives. At a more fundamental level, the research was to explore how semiconductor metal structures could be patterned with the nanostencil technology for device fabrication. As the material must be evaporated for deposition through an aperture, conventional materials such as silicon are not suitable due to high temperatures required for deposition and the fact they will not deposit as a crystalline film. The fullerenes offer an attractive alternative in that they may be deposited at reasonable temperatures while remaining intact. Hence structures with well-defined material parameters are achievable. A further objective of the project was to explore if endohedrally doping was of a sufficient character to modify the structures to produce semi-metal or highly doped semiconductor structures. The project results would indicate that doping of the films in this manner is possible, but the control of the electrical properties is not sufficient for conventional device technologies. Finally, within the project, the manipulation of a single atom within a doped fullerene was to be explored for switching applications. Given the size of the fullerene, this could enable memory capability at the 1 nm dimension.
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Methodologies Materials - Li- and Na- endohedral complexes of C60 were produced by using the low energy ion implantation method (Nature, 382, 407(1996)): fullerenes under vacuum are deposited onto a substrate, and simultaneously irradiated by a metal ion beam with a selected energy. The method was extended to K@C60 within the project and a new ion source for implantation of Al, La and other metals was developed. The produced films are examined by LDI TOF mass spectrometry. The intensity of the endohedral mass peaks is critically dependent on ion energy, ion current and speed of deposition. The fullerene films, after low energy ion bombardment, are washed from the substrate by CS2 in an ultrasonic bath. The CS2 soluble part is then separated from the insoluble rest by filtration and examined by HPLC (High Performance Liquid Chromatography). After chromatographic separation the solutions of endohedrally doped fullerenes are concentrated in vacuum, transferred into glass vials and dried in a flow of nitrogen gas. The purified material is then sent to project partners for deposition. Simulations - The doped fullerenes and their interactions with surfaces were studied using density functional calculations with the Becke 3- Lee-Yang-Parr (B3-LYP) exchange correlation functional. A polarised double zeta (dzp) basis set has been used for all fullerene carbon atoms, while a polarised triple zeta (tzp) basis is been applied to the dopant atom. Metal surfaces have been simulated with cluster models with as many as 55 atoms, and were chosen for stability and their ability to mimic surface properties. For the electric field screening calculations, various basis sets were used to determine the sensitivity of the calculations and estimate the error in shielding. For visualisation of the single electron orbitals to compare with the energy resolved LDOS measurements, the OpenDX software package was used to generate a constant current surface and to project a single molecular orbital’s charge density onto the surface. The NANOTCAD software developed within the IST FET programme was used to investigate the electric fie ld gating of the crossed fullerene/metal wire junctions. Nanostencil- The nanostencils at IBM and Uni Cambridge were adapted to the NICE project work, and continued to be developed within the NICE project after conclusion of the ATOMS project. The technique uses a combination of a shadow mask and SPM technique, whereby materials are locally deposited through a pinhole (size < 100 nm) close to an SPM tip. Controlled movement of the mask leads to direct fabrication of arbitrary structure without the need for conventional lithography. This technique represents the only method capable of producing complex fullerene heterostructures. A comprehensive nanostencil system has been completed at Cambridge involving three components: UHV chamber, electronics and software for controlled patterning. The new, specialized UHV (ultra high vacuum) system built includes an e-beam evaporation source, a high precision X-Y stage for sample control and an AFM scanning and monitoring system. At IBM, advanced static and dynamic na nostencils were designed and constructed in the ATOMS project, with further instrumental developments important for the NICE project. The fabrication of small structures of “exotic” materials, which exist only in minute quantities calls for nanostencil requirements different from those found in other stencil applications. A key difference between static and dynamic
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stencils is the amount of material wasted during deposition because as it blocked by the mask: a static stencil allows deposition of simple structures in a simultaneous manner, while the dynamic stencil fabricates the structures sequentially. For simple structures, therefore, a static tool is best suited, while a dynamic stencil becomes import when more complicated structures are needed which cannot be produced in a mask (typically rings etc.). The “all-in-one” nanostencil is a system designed in the end phase of the ATOMS project at IBM and fully implemented within the NICE project. It has the following features:
1. The entire nanostencil is mounte d on a mechanical damping system using standard scanning probe microscopy technology.
2. The masks, evaporators and substrates can easily be changed and aligned within 10-15 µm precision.
3. The distance between mask and substrate is piezo-controlled. 4. A combined beam-deflection AFM and STM is included in the system to analyze
the produced patterns. Characterisation- The normal incidence X-ray standing wavefield (NIXSW) technique utilises the standing wave formed by interference of the incoming X-ray and diffracted beams from a single crystal. As X-ray photon energy is scanned through the Bragg reflection condition the standing wavefield shifts. This can be used to locate specific atoms on the surface by monitoring the photoemission response of these atoms to the standing wave. Cambridge have studied the distribution of the endohedral La atoms in La@C82 molecules adsorbed on Cu(111) and Ag(111). The XSW results show that upon adsorption on the metal surfaces the molecules adopt a preferential orientation with respect to the substrate surface, with the La atoms possessing a significant degree of ordering. These experimental results are supported by the theoretical calculations on La@C60/Cu(111) carried out at NMRC, which show that there is a significant difference in energies for La located in the upper and lower half of the cage. Thin films of Au and C60 were deposited at Cambridge for interface characterisation. The films were thermally deposited onto ultra-flat Si oxide substrates provided by NMRC. Cambridge also used Focused Ion Beam (FIB) technique to prepare cross sections of the films for examination at the interfaces. To prepare the cross section, a layer of Pt was first deposited to protect the film surface and the FIB was then used to mill down into the sample, which left behind a nearly prefect polished sidewall which can then be imaged by high-resolution field-emission SEM. SIMS, ULE-SIMS and RBS analyses of C60/Au structures provided have been performed at MATS (UK) Ltd acting as subcontractors for NMRC. Rutherford Backscattering was performed to determine the density value of the fullerene layer. The depth resolution of SIMS and ULE-SIMS analyses are limited in this case by the initial surface roughness of the samples. AFM analysis of the roughness of the C60 film shows that the roughness of the C60 film is in the range of several nanometers. Thus the depth resolution of the ULE-SIMS analysis is in the range of several nanometers as well.
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Project Results and Achievements
Workpackage 1- Endohedral Materials and Substrate/Contacts Endohedral Materials Production and Purification
Li@C60 Preparation Li- and Na- endohedral complexes of C60 have been produced by using the low energy ion implantation method (RT, NK, SL, IVH, EEBC, Nature, 382, 407(1996)) developed at UGOT: fullerenes under moderate vacuum are deposited onto a substrate, which is simultaneously irradiated by a metal ion beam of a certain energy (30eV for Li, and up to 80eV for Na). In order to facilitate bulk production of material, a substrate an Al foil on a rotating cylinder, the rotation speed being adjusted to deposit one C60 monolayer per rotation is used. The end thickness of the films produced, can reach 800nm (the duration of the process is highly dependant on the ion source and some other factors). The produced LiC 60 films are examined by LDI TOF mass spectrometry and all of them exhibited molecular ion signals at 720 amu (C 60) and 727 amu, corresponding to Li@C60. Na@C60 films showed in LDI TOF mass spectra a weak molecular ion signal at 743 amu. During the test MS experiments with C60 deposited on the Al-substrate without low energy ion bombardment the additional signals were not found. The intensity of the endohedral mass peaks is critically dependent on ion energy, ion current and speed of deposition. The fullerene film after low energy ion bombardment is washed from the substrate by CS2 in an ultrasonic bath. The CS2 soluble part is then separated from the insoluble rest by filtration and examined by HPLC (High Performance Liquid Chromatography). In addition to the C60 fraction two other components are found (further for simplicity, denoted as E1, of shorter retention time, and E2). A typical height ratio of E1 to E2 peaks in chromatogram (optical absorption at 340nm was measured) is equal to 2-2,5; however, after isolation and drying the weight of E2 species is typically higher (see table). In LDI TOF mass spectra both new components E1 and E2 showed Li@C60 signal. After chromatographic separation the solutions of endohedrally doped Li@C60 species are concentrated in vacuum, transferred into glass vials and dried in a flow of nitrogen gas. The production efficiency of Li endohedral species is rather high, ca. 20%. Macroscopic amounts of purified (>90% of chromatographic purity) were produced regularly to partner groups. The CS2-insoluble part (typically up to 20 weight % with respect to the soluble portion) of ion-treated fullerene films, according to MS data, also contains Li@C60. E1 and E2 fractions were studied by UV-Vis and Raman spectroscopy. E2 is the monomer Li@C60 species, whereas E1 shows characteristics of the dimer molecule. Gas phase experiments confirm the endohedral nature of E2. For comparison, experiments on exo-doping of C60 with Li were carried out by grinding the C60 powder with the excess of metallic Li. Among the products there was found (C 60)2, with an HPLC retention time close to the E1 species. However, no products of this reaction exhibited the signal at 727amu, corresponding to L i@C60, in LDI TOF MS.
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Problems with deposition of certain samples of Li@C60 have been identified. One issue is that there was no routine mass spectrometry done at the end of last year when the last samples were sent to IBM Zurich, as the mass spectrome ter was out of action for a number of months due to technical problems. It transpires that it is very important to check the mass spec at every stage of the purification process. The endohedral production apparatus has been dismantled, moved and rebuilt. There are strong Li@C60 peaks in the mass spectra of the as-produced films. The material can be sublimed also showing clear endohedral signals (with some shrink-wrapping of the fullerene cage). The anisole-enrichment procedure was applied to this new material. This method gave us strongly enriched endohedral fullerene material without having to use HPLC. As previously, this can be easily sublimed, using conditions similar to sublimation of C60 (400-600 C). However, other material is also sublimed with the fullerenes making this method unsuitable for the UHV STM experiments. The HPLC fractions were similarly studied by subliming and taking laser desorption mass spec of the evaporated material and/or deposited films within the same apparatus. The material without washing (to remove impurities) is sublimable and shows endohedral content in the sublimed material although the amount of endohedral content seems to vary (also from LDMS of the purified samples themselves, without evaporation). It is not clear why this is so. It may be due to variations during the production or handling of the material after production. Further investigation is required. The HPLC fractions of the endohedral materials arrive together with empty fullerene dimers and trimers and these can also be present in the "purified" fractions. The IR/Raman spectra of the HPLC purified material showed the presence of impurities especially C-H. A procedure was developed to remove these impurities. This "clean" material was the material sent to Zurich at the end of last year. Unfortunately, the cleaning procedure seems to make the material much more irreproducible in terms of the mass spectra and also the sublimation behaviour. In some cases endohedral species can be detected, in some cases they cannot. The material also degrades much more rapidly with time. It is believed that the impurities present in the HPLC purified samples (in particular the H) serve to stabilise the fullerene cage, making it possible to sublime the material. Since the H is rather weakly bound to the cage it tends to fragment during the laser desorption/ionisation process and is generally not detectable in the mass spectrum (although some evidence can be seen under weak desorption/ionisation conditions). It therefore does not seem pos sible to reproducibly sublime pure Li@C60 without destroying the material. Some chemical stabilisation is needed for the molecule to survive the elevated temperatures. The Na@C60 purification procedure has not been able to be optimised to the extent of Li@C60. It is found that CS2 extraction of Na@C60 films is not so efficient as for LiC60 films: about a half (or more) of the film material was CS2-insoluble stays on the filter after extraction. The chromatographic analysis of the CS2-soluble part showed the presence of E1 and E2-like fractions (with retention time close to that of Li@C60 E1 and E2 respectively), however height’s ratio E1/E2 for Na@C60 species was equal to 15-20. LDI TOF MS analysis of Na E1 fraction showed, so far, no presence of molecular ion peak at 743amu (Na@C60). The investigation of appropriate isolation procedure is in progress. Table 1- Summary of 1st Year Materials Production Film # (metal:C 60 ratio)
Total 16.8 mg * - ‘new’ Li-source ** - the weight of CS2 insoluble portion varied from run to run, dependent on the ion current and deposition rate used during production *# - ‘in stock’ Table 2- 2nd Year Summary of Production of Li@C60
Li@C60 LiC 60; Probe number(date)
Total weight of CS2 soluble material (mg)
Weight of CS2-insoluble part
Weight of LiC60-E1 fraction
Weight of LiC60-E2 fraction (mg)
Comments
Li043; 2002-01-30
3.1 0,2 1
Li044&Li045 2002-02-05
7.3 0,69 2.2
Sent to Zurich
Li046; 2002-02-08
5.9 0,3 The material Low ratio Li/C60
10
Li047; 2002-02-12
4.2 0,5 was used for sublimation experiments without weighing
6Li051& 6Li052& 6Li053 2002-02-27 – 2002-03-15
10.2 <1mg ca. 2mg Ratio 6Li/C 60 was 0,7 the material was completely consumed for making samples for IR -Raman experiments
Li054; 2002-03-23
6.1 1.2
Li055 & Li056 (sept. 2001?) separated 2002-04-01
12.9 ca. 2mg
The material was used for 1H NMR measurements, in order to clarify the nature of C-H stretching in IR; then – after recycling – for IR; After IR measurements had been performed, the samples of LiC 60-E1, LiC 60-E2 and C120 on KBr were sent to Linköping
Li060-Li063 2002-04-15 – 2002-04-29
5.4 (bad yield; very low ration Li:C 60)
0,5 Sublimation experiments after separation
Li064 2002-05-04
0,3 0,1 ratio Li/C60 - 3:1
Li065&Li066 2002-05-22
9.8 Ca. 0,8 1.6 Solutions in o-DCB were studied by ESI-MS in Aarhus(DK) Are supposed to be sent for XRD experiments to A.Balch
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Li067 – Li070 2002-05-30
83 (?) (a big fraction of substituted C60 (peak before C60) was observed
ca. 7 not weighted
1.3 Sent to Cambridge
Li071 – Li072 2002-06-26
9.5 0.8 1.3 IR/Raman study
Li073-74 7.7 Li075-78 16.7 2.5 Li79-82 16.9 2.1
~4mg Sent to Austin/Texas/US for 13C NMR measurements, after careful washing with methylene chloride.
Li83-86 17.2 2.7 >2mg Li 87-90 2002-10-17
23.3 >2mg After additional HPLC purification and washing, was sent to Zurich (2002-12-10)
Total: 21mg Table 3- Summary of 2nd Year Summary of Production of Li@C70 LiC70; Probe number(date)
Total weight of CS2 soluble material (mg)
Weight of CS2-insoluble part
Weight of LiC60-E2 fraction (mg)
comments
Li048 2002-02-12
1.6 1.2
Li049 2002-02-14
4.5 0,7
<0,4 (after washing with CH2Cl2)
The material was used for IR study
Li04 2002-11-27
11.1 1.5 >1mg (after washing with CH2Cl2)
The material was used for Raman study
Table 4- Summary of 2nd Year Summary of Production of Na- and K@C60
K@C60; Probe number (date)
Total weight of CS2 soluble material (mg)
Weight of CS2-insoluble part
Comments
K01 – K03, 80eV 1.3 3 HPLC showed 2 peaks at
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2002-03-15 – 2002-03-20 K04 80eV 2002-12-11
1
2,6
positions of LiC 60, however with E2-fraction of much lower intensity; MS characterisation in progress
Na@C60
A series of low energy ion bombardment experiments have been carried out with the different ion energies: 30 – 80 eV. Chromatograms of CS2 soluble parts of the films prepared at energies of 50 – 80 EV are looking similar, with E1/E2 peak heights ratio equal to 15 –20. So far, have we failed to carry out LDI MS characterisation of the extracted film material and fractions after HPLC separation. The vibrational structure of chromatographically isolated Li@C60 species and Li@C70 were studied by middle IR spectroscopy at UGOT. Li@C60-E1 and Li@C60-E2 show similar spectra with respect to peaks positions, especially in the higher frequency range. However, for Li@C60-E2 these modes exhibit additional splitting, indicating a further reduction of its molecular symmetry with respect to the first endohedral fraction. We ascribe to this species the structure of a single-bonded dimer (Li@C60)2 with molecular symmetry C2h, which corrected the initial assumption concerning the monomeric nature of Li@C60-E2 species. Comparison of IR spectra of Li@C60-E2 species produced with 6Li and 7Li isotopes allowed the vibration-rotational bands originating from motion of the Li inside the carbon cage to be identified. The IR spectrum of Li@C70 is reminiscent to the IR spectrum of C140, but with some extra peaks due to modes splitting. Taking into account the HPLC behaviour Li@C70 and its IR features we propose for this species the structure of a single -bonded dimer (Li@C70)2. HPLC behavior of K@C60 is highly reminiscent to that of Li@C60 films: in chromatogram there were observed 2 fractions, with retention times the same as for E1 and E2 fractions of Li@C60 (Fig.1). The intensity of E2 fraction for K@C60 is smaller than was observed for Li@C60 (1:2.6). However, if to compare the intensity of this fraction to (C60)3, which is produced during alkali metal catalyzed polymerisation of C60, it is much bigger (about of an order of magnitude). This may be indication that we have got K@C60. According to our IR/Raman study of Li@C60 species E1 fraction should have dimeric structure, while E2 may be also a dimer of lower symmetry or trimer (in favor of the latter assumption may serve the HPHL similarity between E2 and (C60) 3). MS characterisation of K@C60 species is still in progress at UGOT- the obtained spectra are rather noisy, and the intensity of K@C60 peak is small.
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Fig. 1- Chromatographic comparison of Li@C60 and K@C60, produced by low energy ion bombardment with thermal ionisation ion sources.
Some specific technical materials issues arising from the characterization work is: - IR/Raman study of Li@C60 species showed, that neither E1 nor E2 are monomeric
compunds (with respect to the number of C60 units) – this has to be taken into account when carrying out deposition experiments.
- sublimed Li@C60 –species look to be monomers as determined by IR spectra, however the appearance of strong C-H-like stretching cannot be explained so far
- it was shown by LDMS, that La@C82 films are oxidised quickly (within 1-2 hours) by air exposure. Li@C60-E2 doesn’t show changes in HPLC after 2 weeks in air – the exact reason is unknown and may be attributable to insufficient chromatographical resolution of the column used.
- I-V characteristics of sublimed LaC 82 were studied on air and these measurements will be repeated in situ under anaerobic conditions.
Table 5- Summary of 3rd Year Production and Isolation of Li@C60 Li@C60 Li@C60; Probe number(date)
Total weight of CS2 soluble material (mg)
Weight of CS2-insoluble part
Weight of LiC60-E1 fraction
Weight of LiC60-E2 fraction (mg)
Utilization/comments
Li073-74 2002-08-03
7.7
Li075-78 2002-08-31
16.7 2.5
Li79-82 2002-09-10
16.9 2.1
~4mg Sent to Austin/Texas/US for solid -state 13C /7Li NMR measurements, after careful washing with methylene chloride.
720 780 840 900 960 1020 1080 1140 1200 1260
0.0
0.1
0.2
0.3
abs.3
40nm
time, sec.
KC60 LiC60 KC60_021211
14
Li83-86 2002-10-07
17.2 2.7 >2mg
Li 87-90 2002-10-17
23.3 >2mg
After additional HPLC purification and washing, was sent to Zurich (2002-12-10)
Li 91-92 2002-12-06
11.1 ~0.7mg 1.2mg Both E1& E2 fractions used for IR measurements of the sublimed material
Li 93-97 2003-01-20
7.8 1.5 0,6 mg MS experiments with sublimed and thermally desorbed material
Li 98-101 2003-02-25
17.5 1.4
1.2 mg
>1mg After washing with methylene chloride and drying, the material was re-dissolved in o-DCB-d4, and sent to Austin/USA for 7Li NMR
Li102-103 2003-03-24
11.4
Li 104-105 2003-03-27
7.5 0,6
>1 mg (after washing)
Used for in situ conductivity measurements of the sublimed material
Li 106-108 2002-04-07
22 1.6 ~2mg The E2 material, after washing and analysis, had been sent to Zurich as suspension in degassed CS2 for
Li 109-111 2003-04-15
8.7 ~0,6mg Used for preparation of the sample for UV-Vis-NIR on quartz substrate
Li 112-117 2003-05-06
25.5 > 1.5mg
Li 118-120 2003-05-09
Was used without separation
2.9 mg (after washing with methylene chloride)
Used for gas-phase (thermal desorption) MS TOF experiments with ionisation with femto-second laser
15
Total: >5mg
Total: >16 mg
Table 6- Summary of 3rd Year Production of Li@C70 Li@C70; Probe number(date)
Total weight of CS2 soluble material (mg)
Weight of CS2-insoluble part
Weight of LiC60-E2 fraction (mg)
comments
Li04 2002-11-27
11.1 1.5 >1mg (after washing with CH2Cl2)
The material was used for Raman study
Table 7- Summary of 3rd Year Production of K@C60 films K@C60; Probe number(date)
Bomardment energy, eV / K-ion : fullerene ratio
Total weight of CS2 soluble material (mg)
Weight of CS2-insoluble part (mg)
comments
K04 2002-12-11
80eV 1 mg 2,6 mg
K05 2003-03-03
80 eV/ 4:1 1.7 mg 1.3 mg
K06 2003-03-05
80 eV / 1:1 1,9 mg 1,4 mg
K07 2003-03-10
100 eV / 2:1 1.4 mg 2 mg
HPLC showed 2 peaks at positions of LiC60, however with E2-fraction of much lower intensity; MS characterisation in progress
Research was continued on the optimisation of the yield of K(Na)@C60 and working out the isolation procedure for these species. The production charts of K@C60 are shown in Table 3. The low-energy ion bombardment procedure with the use of K-ions, has been carried out both with a commercial ion source and new home-made surface-ionization ion source. The testing of the new ion source showed problems with the electric power supply, which had to provide the heating of the crucible / metal vaporization. At the moment we try to fix this problem. For isolation of K(Na)@C60 species we applied at first the procedure used for Li@C60: (i) extraction of soluble species with CS2, and (ii) HPLC separation on Cosmosil 5PBB column (Nicalai Tesque, Japan) and elution with o-dichlorobenzene (oDCB). However, for K(Na)@C60, with ion bombardment energies of 70-100 eV, the amount of insoluble part increased to =40%. For comparison for Li@C60 films after ion bombardment at energies of 30 eV, the amount of insoluble portion does not typically exceed 10%. Fig. 2 shows the comparison of HPLC traces of CS2 soluble parts of Li/Na/K/C 60 films, produced by low -energy ion bombardment, compared to HPLC trace of Li/C60 mixture after ball milling (the synthetic method for production of C120 – a C60 dimer; as by-product a trimer C180 is formed) in the region of appearance of dimeric and trimeric species. It is clearly seen that E1(dimer)/E2(trimer) ratio increases while moving from Li (ion bombardment energy 30 eV) to Na and K (ion energy of 70 and 80eV respectively), and is the biggest for ball milling product. As it has been illustrated in fig. 2, the E1 peak from the Li@C60 film consists to at least about
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50% of endohedral dimers and in this case the intensity of accompanying trimeric Li@C60-E2 is fairly high (E1/E2 peak height at 340 nm ratio is equal to 2,5÷4). We suppose, that in case of structurally similar Na/K@C60 species the E1/E2 ratio should be close to this value. However, in practice for Na/K@C60 the E1/E2 ratio is equal to 12-15. We think that in case of higher ion bombardment energies the additional formation of C120 happens, the peak of which overlaps with the peak of dimeric Na/K@C60 species during HPLC separation. The ratio C120/C180 in the C60 material after ball-milling in the presence of Li has been found to be ~50. LDI TOF MS analysis of E2 fraction of K@C60 showed small mass peak at 759 u.
Fig. 2- HPLC traces of CS2 soluble Li/Na/K/C60 films, produced by low-energy ion bombardment procedure, compared with Li/C 60 material after ball milling. The extended region of E1 (dimers) and E2 (trimers) parts of chromatogram is shown. The traces are normalised to the height of E1/C120 peak.
The dependence of ionisation energy and metal/fullerene ratio on the yield of the soluble part of the film was studied. These data are presented in Table 3. Fig. 3 shows HPLC traces of CS2 soluble part of K/C60 films, prepared under different conditions. The lowest ratio of E1/E2 was obtained for metal fullerene ratio 1:1 for each monolayer of the film. The using the metal-to-fullerene ratio to 4:1 led to the increased formation of by-product C120. The use of 2 monolayers of C60 per rotation of cylinder in the production chamber with keeping the total metal-to-fullerene ratio 1:1 also yielded the higher E1/E2 ratio.
720 840 960 1080 1200
0,0
0,2
abs.
340n
m
time, sec.
LiC60, 30eV KC
60, 80eV
NaC60
, 70eV C
60+ Li, ball-milling
17
Fig. 3- HPLC traces of CS2 soluble part of K/C60 films, prepared under different conditions (see text for details).
The MS analysis of CS2 insoluble part of K@C60 films showed the presence of K@C60. The subsequent extraction of CS2 insoluble part with pyridine (Py) and aniline allowed transferring additional amounts of K@C60 into solution phase. The HPLC analysis of Py and aniline extracts, however, is hampered because of chemical incompatibility of these solvents with the material of stationary phase of HPLC column. We plan to try purification of K@C60 by sublimation of insoluble stuff after preliminary removal of not reacted fullerene species by subsequent extractions with CS2 and oDCB. Confirmation of optimum bombardment energy of ≤ 85 eV for production of endohedral containing K@C60 material (HPLC and mass spec analysis) was confirmed. Difficulties in finding efficient way to extract pure K@C60 were encountered. HPLC purified material appears to have a derivatised cage which can easily lead to opening on further treatment/sublimation. Insoluble pr oduction material shows K@C60 and appears to be polymerised fullerene-like material (both empty and endohedral fullerenes). It is not possible to sublime monomer endohedrals from the insoluble production material. The soluble production material shows monomer and oligomerised C60 (dimer, trimer). Mass spec shows predominantly C60 – not endohedral species (within available sensitivity of equipment). Extracting the empty fullerenes using o-DCB followed by methanol was used to further treat the insoluble material. The remaining material does not consist of empty fullerenes/oligomers according to spectroscopic investigations but does show fullerene species in the mass spec. Unfortunately, mass spec problems mean we cannot be more specific about the species present. With time, this material becomes partially soluble in ethanol and, to a lesser extent, in toluene. We believe it contains the endohedral species with some additional cage derivatisation. Spectroscopic characterisations are underway as well as more extensive mass spectrometry. Samples of these materials were sent to IBM.
720 780 840 900 960 1020 1080 1140 1200
0,0
0,5
1,0
1,5
K/C60
, CS2-extract
Li98 is shown for reference
abs.
340n
m
time, sec.
ratio 4_1, 80 eV ratio 1_1, 80 eV li98 80ev_2ML
18
Ion Source for Metallo -fullerenes
The work on the design of the set-up for low energy ion implantation production of M@C60 was completed during the NICE project duration, the construction of low energy La-ion sources for ion bombardment is however still in progress.
Fig. 4- Surface ionisation source for La. The ionisation takes place on the surface of a Rhenium sheet heated ohmically to ca 2000 °C. Similar designs have been shown to give currents of La+ up to a few mA. The crucible is heated by electron impact.
As alternative source of endohedral fullerenes, we studied the isolation of the La@Cn from commercially available La-enriched soot (purchased from BuckyUSA). LDI MS analysis (positive mode) of the material before chromatographic separation showed in the original soot significant amounts of La@C60 and La@C70 together with endohedral fullerenes with higher cages (La@C82 being the most abundant species). However, extraction of the soot with CS2 and further chromatographic separation of the extract allowed us to isolate mostly empty fullerenes and La@C82. From total amount of 100mg of commercial soot, we isolated about 3 – 5 mg of La@C82 with purity ca. 85% (according to LDI MS, the impurities are empty C84 and minor amounts of C82). La@C82 was partly used for sublimation behaviour experiments (see below) and partly delivered to the project partners for nanofabrication experiments. The newly installed ion source was tested using alkali metals. An overheating problem led to melting and required reconstruction of major components. The source has now been reconstructed and new test experiments are in progress for production of metallo -fullerenes. For higher conductivity films, for greate r stability of during deposition, and for greater stability in atmosphere, it was the intention of the project to focus primarily upon La@C60 materials and not the alkali endohedrals. To offset delays due to production of La@C60, outsourcing of La@C82 from the Shinohara group was undertaken for various deposition, STM and nanostencil experiments. The new source with nA current with Al ions has been verified. Mass spectra of Al implanted fullerenes show a small signal that can be attributed to endohedral species.
19
There is not yet sufficient material to attempt a separation. A µA current has been produced with alkali ions (K) and is reasonably constant over a period of hours and can be used for implantation.
Substrates and Contact Fabrication
A literature survey for metallisation schemes to form ohmic contacts with the fullerenes was performed. This was used as for the initial selection for sputtering of metals (Ti, Au, Al, Mb, Pt, Cu, …) and has resulted in the production of ceramic, glass and silicon wafers with these various metallization layers at NMRC. These metallized substrates were sent to UCAM for deposition and patterning of fullerene layers, and returned to NMRC for contact metallization and electrical characterisation of ohmic and diode contact test structures. The result of the electrical characterization is reported in the Workpackage 4 description. A set of <111> wafers with resistivities of 0.01 to 0.1 ohm -cm were purchased for use in the project; these wafers served as a reference substrate throughout the project duration for fullerene deposition and characterisation studies performed at UCAM. For wafers with an insulating oxide layer and for fabricated test structures on an insulating layer, standard oxidized wafers were found to be sufficiently flat. Various oxides layers (varying thickness and growth conditions) were processed on the <111> wafers at NMRC and surface roughness studies for the oxide layers were performed at UCAM. Nanostencilling on these oxide substrates for metal and fullerene structures was achieved. Contact structures were specified by UCAM and their lay-out and fabrication by e -beam was performed at NMRC. The metallized structures have four contact electrodes that match the four-finger probe in the UCAM nanostencil system. A mask set to include test structures for mobility and other transport properties were defined to be compatible with UCAM package and bonding specifications. There are two different structures with the spacing between electrodes of 8µm and 16 µm. Masks for the metal patterns are shown in figs. 5 and 6. Connected to the four electrodes are four converging metal lines. The distance between these lines varies from 50 nm to 1000 nm. The lines are 100 nm wide and 10 nm thick (3 nm Cr plus 7 nm Au) (the substrates used are the oxidized silicon wafers processed described above). The contacts have been produced at NMRC using the e-beam lithography and the lift -off process, and initial structures have been sent to UCAM for subsequent patterning with the nanostencil.
20
Fig. 5- Metallised structure mask (distance
between contact electrodes is 8 µm) Fig. 6 - Metallised structure mask
(distance between contact electrodes is 16 µm)
Fig. 7- Chip layout for contact structures for the nanostencil work
Fig. 8- Structure of contact pads for the four-point electrical probes
Fig. 9- A metallic structure of the pre-patterned substrate for nanostencil fabrication. The gap between the metal lines is varied in the range between 1000 and 100 nm. Fullerene lines across the gap will be deposited using the nanostencil, allowing in-situ measurements for electrical conduction of fullerene nanowires and properties of fullerene/metal contact junctions.
22
Workpackage 2- Modelling and Simulation Fullerene Modelling
Modelling of Li@C60 Density functional calculations on Li@C60 have been performed using the Becke 3- Lee-Yang-Parr (B3-LYP) exchange correlation functional. A polarised double zeta (dzp) basis set has been used for all fullerene carbon atoms, while a polarised triple zeta (tzp) basis has been applied to the encaged lithium atom. The atom was placed on one of four symmetry axe of the fullerene cage. In each case, the full icosahedral Ih symmetry of the fullerene is broken. Below we depict the symmetry axe used in the calculations.
Fig. 10 - Symmetry labelling for the Li@C 60 calculations
The axis located at the centre of a pentagon face reduces the symmetry Ih → C5v and likewise the axis centred within a hexagonal reduces the symmetry Ih → C3v. We also consider the bond-centred sites between two six -membered rings (Ih → C2v) and the bond shared between a six- and five -membered ring Ih → Cs. A full geometry optimisation was performed for these symmetry constrained cases, and the total energies for the optimised structures were found to be within ten to twenty milli-electron volts of one other. Within the error of the calculations we conclude that the resulting geometries at the respective energy minima are essentially isoenergetic. The distance of the lithium atom from the cage center was found to be approximately 1.4 Ångström for all symmetries (in contrast to previous studies, a full geometry relaxation was performed for the calculations reported here). We next proceeded to calculate single point energies along three higher symmetry axe (all except Cs), and we determine the following potential energies as a function of displacement of the lithium atom with respect to the centre of the fullerene cage.
nu
n
ª ª C3v
n C5v
u C2v
n Cs
23
Fig. 11 - Potential energy of the endohedral lithium as a function of displacement from the fullerene cage center along three symmetry axe (blue stars C3v, red crosses C5v, brown circles C2v)
These calculations reveal important information concerning the motion of the lithium atom within the cage. Charge analysis reveals that the lithium is effectively ionized transferring essentially one full electron to the fullerene cage. The lithium cation in the cage is trapped on a spherically symmetric potential with a minimum at 1.4 Ångström from the center of the cage. The minimum energy site is opposite a six-membered ring, followed by the five membered ring. Small transition barriers occur when the lithium atom moves across from bonds on the cage (C2v and Cs sites), but these barriers are low enough that at room temperature, the lithium atom is essentially free to move on a “sphere” of radius 1.4 Ångström.
Fig. 12 - The effect of an external electric field to Li@C60 in C5v symmetry. Field values of 0.0, 0.001, 0.002, 0.004, 0.008 atomic units corresponding approximately to the range of 0 to 8 V/nm, which cover the range of field values typically applied within the STM experiments.
-0.2
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0.4
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0.8
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0.0 0.5 1.0 1.5 2.0 2.5
R Li / Angstrom
En
erg
y / e
lect
ron
vo
lt
Potent ia l energy wi th external f ie lds
-3
-2
-1
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3
-4 -2 0 2 4R L i / A n g s t r o m
Ene
rgy
/ ele
ctro
n vo
lts
24
We next apply an external electric field along the C5v symmetry axis and investigate the change in the potential energy as a function of the lithium atom position. The charge transfer to the lithium cage produces an effective Faraday cage, with shielding of the electric field at low field values (i.e. little change to the potential energies with increasing field strength). For larger field values the electron distribution on the cage is polarised and the lithium cation “sees” the electric field. The nearly spherical symmetry of the lithium atom’s potential is broken, and an energy minimum is created at a point furthest away from the field source, and the lithium atom becomes localised. This is the behaved desired for switching studies, in that if the Li (or other endo atoms) can be localised at different positions in the cage relative to the substrate, differences in the LDOS could be detected as the “state” of the switch. However, one aspect of the calculations above required further exploration- the fact that relatively high electric fields are needed to position the atom within the cage. To understand the screening of the electric field acting on the atom, further study of the screening by the fullerene cages was undertaken. Electric Field Screening by Buckminsterfullerene The motion of a lithium atom trapped inside C60 is confined to stay at radius of 1.4 Angstrom from the centre of the cage, and faces energy barriers in moving around this surface which are small, being of the order of kT at room temperature. We investigated the possibility of changing the motion of the atom inside the cage by applying an external electric field applied from an STM tip or a patterned electrode. To this end, we applied an external electric field along a five-fold axis (the results are approximately independent of the specific symmetry axis) and calculated the total energy of the atom plus C60 system as a function of the atomic position. It is found that the applied field breaks the degeneracy between the “up- and down- minima”, but it is noteworthy that the energy difference between these two local minima is quite small- the C60 screens the external field quite strongly. The energy difference between the two minima can be approximately thought of coming from the potential energy difference this electric field establishes between the two positions. For an applied electric field strength of 0.20 Volts/Angstrom, if the field completely penetrated the cage an energy difference of 0.20 eV x 2.8 = 0.56 eV between the two different energy minima would be found, whereas we find only a 0.10 eV energy difference. The shielding effect is maintained for higher applied fields. To investigate this further, we considered C60 without endohedral doping. Even at zero applied field, there is a cage field present inside the hollow, to investigate how much of the applied field gets though we subtract this from the total fie ld. This gives us the change in field when the external field is applied, and we find this to be, to a very good approximation, constant in magnitude and direction near the centre of the C60.
To quantify this effect, an isolated (no endohedral doping) C60 molecule was immersed in an external electric field of magnitude 8.23 V/nm (0.1 atomic unit) with the field vector directed along a five-fold symmetry axis. At zero applied field, there is a residual cage field E0 present inside the cage hollow, we subtract this from the total field to obtain the change in field as the external field is applied. As mentioned, we find this to be reduced to 25% of the magnitude of the applied field. Furthermore, the field inside is approximately constant in magnitude and direction over a large spherical region inside the cage; this is true for a region about the fullerene center of radius 2 Angstrom. To check convergence and estimate an error we performed three separate shielding calculations, each with a different carbon Gaussian basis set. Our value of 25% is an
25
average of the three shielding values, and we estimate an error in our prediction in the shielding effect of +/-5%. The average fraction <p> of the applied field that penetrates to the interior of the cage, is averaged over spherical surface, and plotted as a function of the radius of the sphere over which we average. This is shown for three calculations, using TZVPP (dashed), TZP (solid) and cc-pVTZ (dotted) basis sets (also shown is the standard deviation- the cur ves which start near the origin). We can understand the uniformity of the internal field by modelling the buckyball as a hollow spherical dielectric shell. When a uniform external field is applied to such a shell, the field inside the hollow is constant, and is reduced from the field outside by a fraction p that depends on the relative dielectric constant of the shell material and the ratio a/b of the inner and outer radii of the shell. Explicitly, the fraction of field p that penetrates to the interior is given by the formula
p = 9 er / [(2 er +1) (e r+2)] - 2 (er-1)2 a3/b3. To indicate how the model is applied, we note that in the high er limit the polarizability goes as b3. Two experimental values for the polarizability α are 76.5 +/- 8.0 Angstrom3 and 1000 Angstrom3. One can immediately calculate from our model using these experimental values an effective outer radius values of either b = 4.24 Angstrom or 10 Angstrom, respectively. Given that the radius of the C60 cage is 3.5 Angstrom, and the 2pz electrons might be expected to extend approximately 1 Angstrom about the nuclear radius, one can use our model to quickly show that the figure of 1000 Angstrom3 is too high, and that of 76.5 Angstrom3 is plausible. By fitting to experiment and our shielding calculation one can extract parameters for the model. Assuming a=3.5-∆, b = 3.5 + ∆. Solving, we obtain er = 18 and ∆ = 1.04 Angstrom. This is a simple model, but we can use it to perform quick estimates of the shielding and polarisability of fullerenes of all sizes. One fact that the model enables us to deduce immediately is that the percentage of field that penetrates a fullerene should increase linearly and slowly with the fullerene radius r, growing only at a rate of 2% per Angstrom. This linear for mula holds for radii r < 10 Angstrom; for larger r some downward curvature in the graph of p(r) is evident. Thus larger fullerenes enable the endohedral atom to be manipulated with greater ease, allowing endohedral atoms to be somewhat more easily manipulated in larger cages.
26
Fig. 13 – Electric field penetration <p> within C60
Rectification at C 60/Au(110) Interface In order to investigate the properties of C60 bonded to a 110 gold surface, we have used a 26 gold atom cluster with a C60 molecule bonded to the top atoms, as shown in Fig. 14. The geometry of this model has been determined by geometry optimisation (energy minimization with respect to atomic positions) constrained to have C2v symmetry, with the 8 outer most gold atoms of the gold cluster constrained to lie within a plane. The calculation has been performed using the density functional (DFT) method, with the BP generalized gradient approximation representation of the exchange-correlation potential. All calculations are with a polarized valence double zeta (VDZP) basis set and a 60-core electron, relativistic effective core potential (ECP) on gold. The C60 molecule is bonded fairly strongly to the gold cluster, in agreement with experimental knowledge of C60 bonded to Au (110). This is monitored indirectly by the bond elongations of the formal single and double bonds between the carbon atoms bonding to gold, which is between 0.01 and 0.02 Å for the single bonds and around 0.03 Å for the double bonds, indicating that C60 is less strongly bonded to Au (110) than to Cu (111). We are interested in the increasing asymmetry with film thickness observed in Workpackage 3. To model the molecular interactions of C60 films on a gold surface, a second C60 molecule has been placed on top of the one bonded to gold at a distance of 2 Å, keeping the C2v symmetry but twisting the structure of the second C60 so that the top double bond in C60(1) is perpendicular to the lowest double bond in C60(2) (See Fig. 14). While clearly not an accurate representation of the films, we are interested in the changes of the localized molecular states of the fullerenes molecules both near and away from the surface, as well as the interaction of the states between the two fullerene molecules.
27
Fig. 14 – C60 dimer bonded to a 110 Au gold cluster
To study the effect of the applied electric field arising from a bias from an STM-tip, we have performed calculations with different electric fields across the cluster. This is shown for the two C60 complex in Fig. 15. As can be seen, the levels of the gold cluster shift in energy quite significantly while the energy levels dominated by C60 stay more or less constant. The static electric fields that have been applied are +/-0.27 V/nm, with positive field defined as current going from surface to tip. The two highest occupied energy levels that are dominated by C60 contributions are delocalized over the two cages. On the other hand, the lower lying unoccupied states belonging to C60 are localized on either C60(1) or C60(2). The LUMO states correspond to the buckminsterfullerene t1u LUMO, split by the interaction with gold and the presence of the other fullerene. They are also shifted up in energy with positive applied field. For positive fields, the orbital energies localized on C60(2) goes up in energy and the C60(1) LUMO levels are lowered, resulting in an increased gap between them. For negative field they come closer together in energy.
28
Fig. 15- Energy level shifts for fullerene dimer on gold with applied bias. Dashed lines correspond to gold energy levels, solid lines to fullerene molecular levels. Note that the DFT in the LDA underestimates the band gap, otherwise the relative level spacings are representative.
Modelling of La@C60 The lowest unoccupied molecular orbital (LUMO) of C60 is t1u symmetry, which is a three-fold degenerate irrep (six-fold when spin is included). Placing an atom in the cage at a lower symmetry site reduces the overall molecular symmetry, the LUMO splits, either into a two-fold degenerate state (e) and non-degenerate state (a1), or three non-degenerate states (a1, b1, b2, for example). In the lower symmetries, these states are no longer constrained to be energetically equivalent and energy splitting occurs. We have calculated these energy level splittings for Li@C60 and La@C60 in lower symmetries, by occupying the C60 LUMO states with electrons transferred by the lithium and lanthanum dopant atoms. For La, we have found a b1↑↓b2↑ state to be the most stable, although the energy differences between various occupancies can be small, ranging from tens of meV to a few hundred meVs. We have been able to verify that La transfers nearly three full electron charges to the fullerene cage. The ground state of La is [Xe]5d16s2 and the occupancies of the La atomic orbitals when in the C60 cage is only 0.08 e- for the d state and 0.04 e- for the s state, indicating a very effective charge transfer from the dopant atom. The binding energy for La in the cage has been determined to be 2.29 electron volt, consistent with other estimates.
29
Fig. 16 – Examples of energy state splitting due to dopants in buckminsterfullerene Lanthanum in C60 on a copper (111) surface Experimental and computational work has shown that La is free to move around along the walls of freestanding fullerenes (C82) at room temperature. Conversely, it has been observed by UCAM that the La atoms in La@C82 on a Cu (111) surface are confined to regions of space, either near the surface or up in the cage (see Workpackage 3 description). It is unknown whether the La prefers to be up or down in the cage as the x-ray measurements described in Workpackage 3 do not give absolute positions, but ratios of the atomic distance to the Cu atom layer separation. NMRC have performed theoretical calc ulations on a La@C60 molecule bonded to a Cu31 cluster (see Fig. 17), for which the surface character has been mimicked by restricting the outer-most atoms in the top layer of the cupper cluster to be constrained to lie in a plane during geometry optimisation. Calculations have been performed using the density functional (DFT) method with the generalized gradient approximation of Becke and Perdew (BP) for the exchange and correlation functional. The polarized valence double zeta (VDZP) basis set of TURBOMOLE has been used for all atoms, for Cu and La in conjunction with 10 and 46 electron relativistic effective core potentials. In agreement with experiment, we find that C60 bonds strongly to Cu (111) as quantified by the changes in the bond lengths in the C60 molecule. In free C60, there is a formal single bond, with a bond length of 1.459 Å, and a partial double bond, 1.408 Å. When bonded to the Cu31 cluster these distances, between the carbon atoms directly bonded to Cu, are 1.509 Å and 1.471 Å, respectively, indicative of strong bonding to copper. When C60 is bonded to a Cu (111) surface, electrons are transferred from the surface to the fullerene. The charge transfer is estimated from photoemission and near-edge adsorption spectrum to be 1.5 - 2 electrons. In our model system, with a six membered ring of C60 bonded to the Cu31 cluster, a charge of –1.08 is shared between the carbon atoms bonded to Cu (-0.18 per carbon). A closer inspection of the partial charges of the complex shows that all this negative charge is not transferred from the surface. Some charge is withdrawn from the carbon atoms in the fullerene not directly bonded to the metal surface. The total charge transfer from the copper cluster to the C60 molecule is 0.67e.
t1u e a1 a1 b1 b2
???
30
Fig. 17- Endohedrally doped C60 on copper cluster used to study the preferential atomic position of the La atom
A large negative charge concentrated to the carbon atoms closest to the metal surface suggests that the positively charged lanthanum atom would prefer to reside close to bottom of the cage. But this assumption will be shown to be invalid based upon the computations presented here, since there are other charge transfer factors to be considered. La formally donates three electrons to the fullerene cage (both C82 and C60) upon endohedral encapsulation. These electrons do not completely leave the La atom, but are involved in bonding the La atom to the inside of the cage wall. In free La@C60 calculated partial charges show that the positive charge on La is 1.46 and the negative charge is shared between the carbon atoms closest to the La. While the six nearest bonding carbon atoms have charges between –0.08 and –0.12, and an additional six neighbouring carbon atoms have charges between –0.06 and –0.11. In total these twelve carbons carries a charge of –1.10. Since this negative charge on the carbons will follow the La atom’s movement on the inside cage wall, the positive La will not be attracted by the negative charge at the bottom of the cage. The negative charge that follows the La atom is repelled by the negative charge of the carbon atoms bonded to Cu. La positions within the cage were investigated at NMRC and find that the lowest energy minima are located within the top half of the fullerene. The most stable position is close to a double bond slightly above the midpoint of C60 (see Fig. 18). The energy for this configuration is 450 meV lower in energy than if the La atom is bonded to the bottom six membered ring. It should be pointed out for reference, there is little difference between the energies for La bonded to a double bond and to a six-membered ring in free La@C60.
31
a)
b)
Fig. 18 -a) Stable bonding site of La within C 60 bonded to copper cluster b) Depiction of the larger 55 atom metal cluster used to validate earlier calculations
To investigate the effects of using a finite cluster for the Cu model, calculations with a larger metal cluster (55 metal atoms) were undertaken as depicted in fig. 18b). For these calculations, no constraints for the geometry optimization were imposed. The calculations serve to validate our previously reported calculations for the system, but with the larger 55-atom metal cluster more representative of a surface, reflected by the fact that the surface topology is maintained without geometry constraints during the
32
energy minimization. This larger model confirms our findings using the smaller cluster model (as previously reported in previous NICE annual reports) and suggests that the effect of the local bonding of the fullerene to copper surface is the primary influence on the endohedral atom. These computations performed at NMRC have been used in the analysis of the experimental x-ray studies of endohedral films on metal surfaces performed by UCAM. Molecular orbitals of C60 bonded to copper Experiments at IBM have enabled the imaging of the energy resolved LDOS for C60 bonded to copper and this has led to calculations at NMRC for bonding of buckminsterfullerene to the Cu(111) surface. As seen for the experimental images (see description in Workpackage 3), the computed molecular oribtals display a 3-fold symmetry due to the C3v symmetry resulting from a fullerene hexagon bonded to a Cu(111) hollow site. In fig. 19, we display representative isocontours of orbitals above the Fermi level obtained from the calculations. For direct comparison to the experimental images, determination of a constant current surface must be obtained by integration of the LDOS, and the molecular orbital with energy corresponding to the tip voltage is to be projected onto the constant current surface.
Fig. 19- Isocontours of various molecular orbitals for C60 on Cu(111)
33
Further analysis of the molecular orbitals of C60 on Cu(111) as described were necessary to compare with the low temperature STM measurement made at IBM. In fig. 20, we directly compare the images obtained from the calculations at NMRC with the IBM experiments. As can be seen, the agreement for the unoccupied states (positive voltages) is very good; as well, the primary resonances for the occupied states have been identified. The purpose of this work is to aid in the analysis of detection of the endohedral atoms within the fullerenes when adsorbed onto surfaces. Details of the low temperature STM for the endohedral systems are reported under Workpackage 3. In addition to the orbital maps, bonding and charge transfer for the molecules at the surface have been calculated.
Fig. 20- Comparison of the energy resolved LDOS for C60 of Cu(111). Positive voltages correspond to the experimentally resolved LDOS for unoccupied states, images below these are the corresponding theoretical results. Likewise, negative voltages correspond to the experimentally resolved LDOS for occupied states, images above these are the corresponding theoretical results.
Device Modelling
C60-Metal Diode Characterisation The purpose of this section is to examine the I-V characteristics of the fullerene diode structures using the experimental data obtained from measurements of large area metal/ C60 structures at IBM and NMRC [1]. All structures have an area of 97×97 µm2. The structures were fabricated at UCAM and NMRC are composed of several layers of metals and C60 on silicon substrate as follows:
34
Type 1: 50nm Au / 25nm In / 24nm C60 / 120nm In Type 2: 50nm Au / 26nm C60 / 140nm In
The measurements on the Type 1 structures displayed ohmic behaviour with conductance G=10-3 A/V. This large conductance value contradicts the results of the bulk C60 resistance measurements reported in [2], which report that the electrical resistivity is on the order of tens of MΩ-cm. After exposure to air at ambient pressure and temperature the resistivity of the C60 film increases by almost a factor of ten [3]. Thus it is believed that either in theses samples In has diffused through the C60 layer, or possibly a short-circuit has been formed between the top In layer and a bottom metal layer. The Type 2 structures, as reported previously, show rectifying behaviour. These experimental results are analysed in this report using the thermionic-emission theory. The fcc C60 crystal is a direct-gap semiconductor, with both the valence band top and the conduction band bottom located at the X point of the Brillouin Zone [4,5]. Molecular orientation may affect the band structure of fcc C60 and it may become an indirect-gap semiconductor [6,7]. For the purpose of this report, the semiconductor is assumed to be a direct-gap fcc C60 crystal. In this case the effective mass of the conduction band electron is estimated to be approximately 1.3 me [4]. We summarise estimates of the electrical properties of C60 relevant to the thermionic -emission theory in table 1. Table 8- Electrical properties of C60 Name Value References Electron effective mass, m* 1.3 me [4,8] Hole effective mass 1.5 me and 3.4 me [4] Band gap From 1.04 eV to 2.6 eV [4,8]
C60 work function 3.7 eV C60 /Au Schottky barrier height 0.8 eV (assumed) [9] The Type 1 structure is modelled as a circuit composed of a Schottky diode and a series resistor. The value of C60 resistivity is taken to be ρ=3.5×106 Ω-m, based on the results from ref. [3]. This gives the series resistance of the fullerene film RS=9×106 Ω . The Richardson constant corresponding to the electr on effective mass of C60 is
A* = 1.2×106 (m*/ me) A m-2 K-2 = 1.56×106 A m-2 K-2
The following equation describes the diode current [10]:
where φ eff is the effective Schottky barrier height, VD is the voltage on the C60/Au contact, SJ is the contact area, and n is the ‘ideality factor’. Fixing the current prefactor and fitting to the experimental data (fig. 21) allows for values of the Schottky barrier height and the ideality factor to be extracted: φeff = 0.98 eV, n = 6.
Fig. 21- Diode data fit The fit is be improved by taking much larger value of the series resistance. Fig. 22 shows the fit obtained by using RS= 2 GΩ. The value of the effective barrier height remains the same: φeff = 0.98 eV.
Fig. 22- Diode data fit
In summary, the fullerene/Au diode structure shows highly non-ideal behaviour which can be characterised by a large value of the ideality factor n. Theoretical fitting to the
36
experimental I-V curve suggests that the C60/Au Schottky barrier height is approximately 1 eV. I-V measurements performed by IBM several weeks later on the same diode structure show larger voltage gap than the measurements obtained at NMRC. Fitting to the experimental data obtained by IBM suggests that the C60/Au Schottky barrier height is approximately 1.1 eV. The fit and the corresponding values of parameters are shown in Fig. 23.
Fig. 23- Fit to IBM measurements From the above analysis, it can be concluded that the C60/Au barrier potential is approximately 1 eV. The samples as noted in previous reports are sensitive to air exposure and/or interface diffusion. The large value of the non-ideality factor indicates that the gold-fullerene junction in these samples has a high density of interface states, or that the interfaces are “rough”. We will discuss this in light of the SIMS analysis presented under workpackage 4. References 1 NICE Tech. Report, NMRC, May 2002. 2 NICE Periodic Progress Report No. 3, 2002, pages 34-35. 3 NICE Periodic Progress Report, UGOT. 2003, page 4. 4 S. Saito, A. Oshiyama. Phys. Rev. Lett. 66, 2637 (1991). 5 E. L. Shirley, S. G. Louie. Phys. Rev. Lett. 71 , 133 (1993). 6 S. Satpathy et. al. Phys. Rev. B. 46, 1773 (1992). 7 B.-L. Gu et. al. Phys. Rev. B. 49, 16 202 (1994). 8 P.J. Benning et.al. Phys. Rev. B. 45, 6899 (1992). 9 I. Hiromitsu et. al. Solid State Comm. 113, 165 (2000). 10 E. H. Rhoderick, R. H. Williams. Monographs in Electrical and Electronic Eng. 19 , Oxford (1988)
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Electrical Parameters for Transport Studies of C60 We further summarize the electrical properties of C60 to include those parameters needed for simulation of fullerene structures with use of the NANOTCAD software. Table 9- Electrical properties of C60 Name Value and References
Acceptor levels 60±30 meV and 170±30 meV [1]
Donor level 60±30 meV [1]
Dielectric constant 18±4 [1]
Electron effective mass, m* 1.3 me [1,2]
Hole effective masses 1.5 me and 3.4 me [1]
Band gap From 1.04 eV to 2.6 Ev [1,4]
C60 work function 3.7 eV [3]
Electron affinity 2.6 eV [4]
References 1 B.-L. Gu et. al. Phys. Rev. B. 49, 16 202 (1994). 2 P.J. Benning et. al. Phys. Rev. B. 45 , 6899 (1992). 3 C. Ton-That and M. E. Welland. NICE Review (2001). 4 R.C. Haddon, Acc. Chem. Res. 25, 127 (1992). These parameters have been compiled for use with the NANOTCAD software developed within the IST-FET program. These software tools have been obtained from the Pisa group, and will be used to simula te the MESFET (fullerene/metal crossed wire) structures. Installation of these codes has been slowed due to the discovery of a virus (the programs run under Linux and hence this a relatively rare occurrence). Re-compilation of the codes was completed and following re-installation, the crossed wire structures were simulated using the above best estimates for electrical parameters.
38
NANOTCAD Simulation Results Fig. 24 shows the schematic of the simulated structure. The black bar represents La@C82 wire, yellow strips represent metal contacts. The length (y) of the simulated structure is 20 microns (the distance between edges of source and drain terminals is 14 microns). The thickness (x, depth) of the fullerene wire is 5 nm. The width is 2 microns. The donor concentration is ND=4.2x1022 m-3. This value has been estimated from the results of La@C82 wire resistivity measurements. Drain and source electrodes have ohmic contacts with the fullerene wire. The work function of the gate electrode had been set to 4.7 eV , which corresponds to the workfunction of Cu.
Fig. 24- Schematic of the simulated structure. S, G, and D represent Source, Gate, and Drain areas, respectively. Y represents the distance in S-D direction, X is the depth (X=0 at the fullerene/metal contact).
At VGS=0V the drain-to-source channel is fully depleted. The potential and electron concentration distributions for different drain and gate voltages are shown in fig. 25 through fig. 28.
Fig. 28- Electron concentration. VDS=50 V, VGS=2V.
Workpackage 3- Fabrication: STM Experiments and Nanostencil Patterning STM Experiments
Deposition and STM Studies of Fullerenes/Endohedrals on Silicon Substrates Early within the project, UCAM has characterised two C60 and Li@C60 fullerene samples received from UGOT. The samples were characterised by adsorption onto
41
Si(111) surface and analysed by UHV scanning tunnelling microscopy (STM). The Si(111)-7×7 surfaces were prepared prior to fullerene adsorption. A piece of Si(111) cut from a boron doped wafer was loaded into a UHV chamber (pressure ~ 3 × 10-10 mbar) and outgassed for one hour. The sample temperature was then raised to ~ 1200oC for one minute to create a 7×7 reconstruction. STM images were taken at room temperature using electrochemically etched tungsten tips, which were cleaned by argon ion sputtering in UHV. Fig. 29 shows STM images of Si(111)-7×7 before adsorption of fullerenes. Fullerene samples were deposited from a well out-gassed Knudsen cell heated at 380oC and 500oC for C60 and Li@C60 samples, respectively.
Figure 29 - STM images of reconstructed Si(111) -7×7 surface
42
Figure 30 - STM images of C60 sample adsorbed on Si(111) -7×7 surface. Image size is 35 × 35nm (top) and 50 × 50nm (bottom). Arrows show the molecules at which tunnelling spectroscopy was taken.
In addition to ordinary constant current topographs, STM has been used to probe the electronic structure of the molecules shown in Fig. 30. In scanning tunnelling spectroscopy (STS), the tip is held at a fixed height above a chosen position on the sample, and the tunnelling current (I) from the sample is measured as a function of sample voltage (V). The tunnelling current depends on the electronic density of state (DOS) of the tip and the sample. However, the tip DOS is assumed not to influence the shape of IV curves because of the metallic nature of the tip. It has been shown that the sample DOS is proportional to the differential conductivity, d I/dV. However, this term contains a tunnelling matrix element, which is voltage -dependent. The logarithmic ratio or normalised conductivity d logI/dlogV = (dI/dV).(V/I) gives an accurate representation of the sample DOS.
43
-1
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2
3
4
5
6
-2 -1 0 1 2
(dI/d
V).
(V/I)
Sample voltage (V)
Fig. 31 - Tunneling spectrum of molecules shown above.
The normalised conductivity of the two molecules marked in Fig. 30 is presented in Fig. 31. From the tunnelling spectrum we conclude that the molecules on Si(111)-7×7 surface are semiconductor-like with an estimated band gap of about 2.2 eV. The highest occupied molecular orbital (HOMO) is centred at 1.5eV below the Fermi level and the lowest unoccupied molecular orbital (LUMO) is centred at 1.7eV above. Previous STM measurements show C60 molecules have a band gap of about 2eV on Au(110) and Al(111) surfaces.
44
Fig. 32 - STM images of Li@C 60 sample adsorbed on Si(111)-7×7 surface. Image size is 120 × 120 nm (top) and 45 × 45 nm (bottom).
STM images of the Li@C60 sample are shown in Fig. 32. The clusters on the surface have sizes ranging from about 4nm up to about 10nm, and are not single molecules. It is likely that the material already polymerises during evaporation before adsorbing onto the substrate. Previous studies show endohedral fullerenes and their derivatives undergo a range of degradation reactions at high temperatures. UCAM subsequently characterised two new endohedral fullerene samples deposited onto Si substrates: La@C82 and Li@C60. Samples were prepared by depositing the endofullerenes from a quartz or tantalum crucible heated at 500 – 600oC. The fullerenes were deposited onto pre-prepared Si(111) -7×7 or Cu(111) substrates, and characterised by STM, XPS and UPS. Fig. 33 shows STM images of an La@C82 multilayer film on Si(111)-7×7 surface. The round spots in the image are individual La@C82 molecules. The molecules are hexagonally packed with few defects, the inter-molecular distance is measured at 1.22 ± 0.08 nm. All the molecules have the same brightness and size, suggesting a high degree of ordering in the film. This indicates that the layers are formed predominantly by the van der Waals interactions and the dangling bonds at the Si(111) -7×7 surface are already passivated by the first layer. STM images of the Li@C60 adsorbed on the Si(111)-7×7 are shown in fig. 34. Unlike La@C82 which form ordered films on the surface, only some clusters were observed for Li@C60. The clusters on the surface have sizes ranged from about 5 nm up to tens of nm, and are not single molecules. Again it is conclude that for the deposition conditions used, it is likely that the material has already polymerised during heating and does not sublime as individual molecules.
45
A)
B)
Fig. 33- A) 50 × 50 nm2 STM image of a multilayer film of La@C 82 on Si(111)-7×7 B) Image (20 × 20 nm2) of the top monolayer showing hexagonally close-packed structure of La@C82 molecules. Tunneling current is 0.15 nA and bias voltage is 2.0 V.
[1 2 1]
46
Fig. 34- Typical STM images of endohedral Li@C60 adsorbed on Si(111) -7×7 surface. Image size is 60 × 60 nm 2 (top) and 35 × 35 nm2 of a Li@C 60 cluster (bottom).
Photoemission experiments were then conducted by UCAM on the deposited La@C82 and Li@C60 films on Cu(111). XPS spectra of the deposited La@C82 films displayed signals of Cu, La and C as expected. The results confirm the molecules in Fig. 33, at least a significant proportion, are La@C82. For Li@C60, we did not observe Li signals in any of the deposited films. Typical spectra of a deposited Li@C60 are shown in Fig. 35. Film survey by XPS with Mg Kα (hν = 1253 eV) X-rays shows signals of Cu and C (Note that Li, if existent, would not be expected to be detected at this photon energy). From the intensity ratios of the substrate signal (Cu) and film signal (C), the film thickness could be estimated ~ 5 nm, equivalent to about six C60 monolayers in a close-pack FCC structure. Using low photon energies (e.g. 100 eV) above the Li K-edge, it is expected to “see” Li in the film; however, Li 1s peak was not observed in the spectrum
47
(Fig. 35b). The results suggest that the clusters, seen in fig. 34, do not contain Li in their composition. In summary, we have been able to deposit and image La@C82 molecules in the deposited films on clean surfaces. Li@C60, however, appears to polymerise during the heating process before being sublimed.
(a)
200 300 400 500 600 700 800 900 1000
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Fig. 35- Typical photoelectron spectra of deposited Li@C60 films on the Cu(111) substrate. (a) XPS survey spectrum collected with Mg Kα (hν = 1253 eV) shows strong C 1s peak. (b) UPS spectrum collected with a synchrotron photon energy hν = 100 eV. No Li 1s peak at 54.7 eV was observed.
To investigate the possibility of manipulating the position of a La atom in the fullerene cage, STM experiments have been used to examine deposited La@C82 films and to apply a voltage bias on top of selected molecules in the film by UCAM. Fig. 36 shows the STM image of a 2-monolayer La@C82 film on Si(111)-7×7 surface, which shows the molecules form hexagonally close packed layers with the inter-molecular distance of ~ 1.15 nm. Several tunneling IV curves have been collected on a number of La@C82 molecules in the bias range from –5V to +5V, but there no indication of the atom switching sites within the cage to influence tunnelling characteristics.
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Fig. 36- STM images of a 2-monolayer La@C82 film on Si(111)-7×7 Deposition and STM studies of Fullerenes/Endohedrals on Metal Surfaces The approach taken at IBM is to grow and electrically characterise materials in-situ in ultrahigh vacuum. The existing expertise in growing monolayer (1 ML = 0.7 nm) films of C60 on top of Au(110) has been used to deposit and spectroscopically investigate films of C60 with a thickness of 1 ML and 2 ML, and these techniques have been refined and extended to grow Li@C60 on Au(110), and finally the combination of Li@C60 and C60. The results from the initial studies are summarised below. C60 on Au(110) C60 has been deposited on Au(110) following the standard procedures developed earlier at IBM. An STM image of C60 islands of 1 ML height is shown in Fig. 37A. The individual C60 molecules are identified within the islands. On the Au(11) substrate, the rows of Au atoms running along the [110] direction are clearly seen. Within the C60 island, a hexagonal arrangement of the molecules and the typical zig-zag reconstructions which are aligned with the rows of Au atoms are observed. Tunnelling spectroscopy shows the I/V curve on the Au substrate (Figure 37B) and on top of the C60 island (Figure 37C). Metallic behaviour is observed on Au, more semiconducting behaviour on C60. For 2 ML, the zig-zag reconstructions of the C60 molecules vanish, and a new ordered phase appears whose orientation is no longer in registry with the Au rows. A typical STM image for a 2 ML C60 film is shown in Fig. 38A. A detailed analysis shows that at the straight terrace edges the first ML is visible proving that the film consists of 2 ML. Tunneling spectroscopy shows a transition from metallic to semiconducting from the bare Au(110) to the 1 ML C60 on Au. 2 ML C60 on Au exhibit a gap of more than 2 eV. A typical I/V curve for the 2 ML C60 on Au is shown in Fig. 38 B.
49
A
B C
Fig. 37 - A) STM image of C60 islands of 1 ML height grown on top of Au(110) at 720 K. Image size 42 nm by 42 nm; B) I/V-curve from STM spectroscopy taken on the bare Au substrate, and C) on the 1 ML C60 island.
A
B C
Fig. 38 - A) STM image of 2 ML C60 grown on top of Au(110) at 720 K. Image size 50 nm by 50 nm; B) I/V-curve from STM spectroscopy taken on the 2 ML C60 on Au showing that 2 ML C60 are semiconducting. C) The forward and backward rectifying characteristics compared to a fit to the diode equation with barrier height ϕ=0.241 volt
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Li@C60 on Au(110) Li@C60 produced by UGOT has been evaporated at IBM onto Au(110) from a thin glass tube. Care has to be taken to prevent decomposition of Li@C60 by evaporating at moderate temperatures (800 K). Films of 0.4 ML Li@C60 have been produced with a growth rate of 0.1 nm/min. Evaporation was done onto a warm Au(110) substrate (T=550 K) with the aim to enhance diffusion of the molecules to form larger islands. STM images clearly show extended islands of 1 ML height with individual molecules resolved; see Fig. 39A. The images differ from those of C60 on Au(110) by a higher corrugation. The Au rows are still visible, even though the image is noisier, possibly because solvents from the Li@C60 material used for evaporation are also present on the surface. The I/V characteristics show a clear metallic be haviour, in striking contrast to C60 on Au, see Fig. 39B.
A B
Fig. 39 - A) STM image of 1-ML high islands of Li@C60 grown on top of Au(110) at 550 K. Image size 27 nm by 27 nm; B) I/V-curve from STM spectroscopy taken on Li@C60 on Au showing that Li@C 60 is metallic.
Li@C60 on C60 on Au(110) The growth of this layer structure has been done sequentially with the recipes used for C60 deposition and Li@C60 deposition. Images have been acquired which show regions of Li@C60 on extended C60 layers, see Fig. 40A. I/V spectroscopy reveals as before the semiconducting extended C60 layers, while Li@C60 is rectifying, see Fig. 40B. The barrier rectification can be characterized by a built-in voltage of 0.3 V to 0.5 V. A fit to the log I(V) vs. V curve gives an ideality factor varying between 1.2 and 1.8. Apart from the demonstration of rectification, the successful realization of in-situ STM spectroscopy on Li@C60 is an essential step towards achieving deliverable Deliverable 2, Single Atom Switch Effect.
51
A B
Fig. 40 - A) STM image of a 1-ML high islands of Li@C60 grown on top of 2 ML C60 on Au(110). The individual Li@C60 molecules are clearly resolved (right), while the 2 ML C60 to the left appears noisy. Image size 36 nm by 22 nm; B) STM spectroscopy taken on the Li@C60 island demonstrating rectification.
Fullerene epitaxy on gold surfaces and I/V spectroscopy Following the initial studies of epitaxial C60 films on Au(110) of 1 ML and 2 ML thickness, work at IBM considered the details for producing defect-free, epitaxial C60 films of larger thickness up to 13 ML. This is an important intermediate step for an investigation relating defects to electrical characteristics for the fullerene semiconductor. The growth mode of C60 films at 300 K is 3D, with large corrugation and very mobile molecules which render imaging by STM difficult; see Fig. 41A. Subsequent annealing at 600 K during at least 1 hour makes the molecules to rearrange in perfect epitaxy; see Fig. 41B. Due to the large thickness and low conductance of these films, STM can be reasonably done only at very low current and comparably high voltage. With such parameters STM images of remarkably good molecular resolution could be obtained.
Fig. 41- A) STM image of a C60 film of a nominal thickness of 13 ML grown on top of Au(110) at 300 K; image size 100 nm by 100 nm; tunneling voltage Vt = +4.1V (at sample), tunneling current It = 23 pA; B) same film after annealing at 600 K for 1 hour. Note the dislocation formation; image size 50 nm by 50 nm; Vt = 3.32 V, It = 32 pA.
I/V-spectroscopy was taken for these thick films at various distances between tip and sample; see Fig. 42. For the largest distances, a large rectification is observed with an
A B
52
apparent gap of 5 eV, while for the closest approach of tip to sample, the gap width is reduced to less than 1.5 eV. The asymmetry in the IV curves is reduced but does not disappear.
-3 -2 -1 0 1 2
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Fig. 42- I/V-curves from STM spectroscopy taken on 10 ML C60 and varying distance between tip and sample. STM images taken at these various distances indicate that for the three smallest distances the tip was in contact with the C 60 film.
The I/V-characteristics can be understood by taking into account the appreciable voltage drop that is present within the C60 film. If the tip touches the sample, the vacuum gap disappears and the work function difference between the tungsten tip and the Au substrate is the remaining small asymmetry in the structure. Heterogeneous fullerene systems: Li@C60 / C60 The finding that C60 is highly mobile at room temperature without additional annealing has consequences for the layering of Li@C60 with C60. Without annealing, Li@C60 films on top of C60 are very difficult to image because Li@C60 molecules are also mobile in STM imaging. Fig. 43A shows an example of a well annealed, epitaxial 3 MLC60 film covered by 0.3 ML Li@C60 deposited at room temperature. While annealing again resolves the problem of poor epitaxy (see Fig. 43B), I/V-spectroscopy at different locations is not as reproducible as on pure C60 films. Therefore it cannot be excluded that the fullerenes and endohedral fullerenes intermix when Li@C60 films are annealed on C60.
53
A B
Fig. 43- A) STM image of 0.3 ML Li@C60 on an epitaxial 3 ML C60 film, grown at 300 K; image size 60 nm by 60 nm; Vt = 2.24 V, It = 21 pA; B) same film after annealing at 620 K during 30 minutes; image size 35 nm by 35 nm; Vt = -0.73 V, I t = 18 pA.
Low temperature STM microscopy of individual C60 molecules Our results from the fullerene epitaxy show that the high-mobility of these molecules is complicating the foreseen program. In order to resolve the very fundamental open issues with respect to the I/V-curves, it was decided that IBM put additional resources into the construction of a new variable temperature STM capable of operating to temperatures as small as 5 K. First experiments were done on C60 on Cu(331) and C60 on Cu(111). By cooling the sample during evaporation the formation of C60 islands could be hampered on both substrates (see Fig. 44), demonstrating the possibility to investigate the electronic properties of individual C 60 molecules.
Fig. 44- STM image of individual C60 molecules on Cu(331) (Vt = 0.43 V, I t = 0.38 nA) Islands of C60 on Cu(111) were grown at a deposition temperature of 360 K and are shown in Fig. 45. A series of STM images of the same area was recorded, each image taken at a different bias voltage ranging from –4.5 V to +3.3 V. At certain voltages the images show intramolecular features reported earlier by Hashizume et al. (Phys. Rev. Lett. 71, 2959 (1993)).
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A B Fig. 45- STM images of a C60 island on Cu(111). The same area was scanned several times with different bias voltages applied. The two images shown were taken at Vt = +1.7 V (A) and Vt = –2.3 V (B) with a tunneling current of It = 2.4 nA.
C60 molecules on Cu(111) C60 molecules where deposited by thermal evaporation onto the Cu(111) surface at substrate temperatures of 5 K and 77 K. These temperatures allowed us to study the internal structure of individual C60 molecules (5 K), but also the formation of C60 islands (77 K) or the adsorption of C60 at step edges. Individual C60 molecules on Cu(111) show a threefold symmetry in STM images, see Fig. 46. This suggests that C60 is bound with a hexagon parallel to the substrate. Moreover the C 60 molecules can be found in two different orientations. This reveals that the center of a C60 molecule is located above a hollow site, which can be either fcc or hcp hollow sites.
Fig. 46- Individual C60 molecules on Cu(111) displaying threefold symmetry
Scanning tunneling spectroscopy shows characteristic peaks, which clearly depend on the exact position of the tip above the molecule. Spectra taken over three different positions above the molecule are shown in Fig. 47.
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Fig. 47- Local I(V) (blue) and dI/dV(green) spectroscopy on individual C60 molecules
Even more detailed information can be gathered from STM dI/dV images. Fig. 48 shows a series of ten dI/dV images taken at different bias voltages. As with all STM images of molecules it is necessary to perform STM image calculations to understand the richness of the STM data in detail [Crommie et al, Phys. Rev. Lett. 90, 096802 (2003)]. These calculations have been performed by NMRC and are compared to the experimental spectra in fig. 48 and yield details on the binding of C60 molecules to the Cu(111) surface.
Fig. 48- dI/dV images (acquired in constant-current mode) of C60 molecules for a series of voltages given at each image
Li@C60 molecules on Cu(111) For these studies, IBM has focused on the study of individual Li@C60 molecules. Since only small amounts of Li@C60 are available, a special evaporation technique was applied. A small drop of the solution was put on a silicon wafer, which has been degassed before in UHV without removing the SiO2 layer. The CS2 solvent evaporated immediately and within a few minutes the wafer was transferred into vacuum. The whole material was then deposited onto the Cu(111) substrate by evaporation in a single flash. The resulting coverage was very low and showed only minute amounts of contamination. A low-temperature STM image is presented in Fig. 50. The image shows molecules (red arrows) with the same appearance as usual C60 molecules: Voltage -
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dependent imaging as well as local spectroscopy were identical to the results on previously studied C60 molecules. Apart from these molecules, however, some additional adsorbates (blue arrows) with a similar density/coverage as the other species were observed. These additional adsorbates appear as small protrusions, but can otherwise not be chemically identified.
Fig. 49- Sketch of the special evaporation technique developed for depositing minute am ounts of endohedral fullerenes, used for Li@C60 and K@C60.
Since one would expect Li@C60 molecules to appear significantly different in STM images as compared to C60 molecules, we are led to conclude that the Li@C60 molecules decompose into Li atoms and C60 molecules when being adsorbed on the Cu(111) surface. This came as a surprise since the evaporation procedure followed by IBM is similar to the one used in earlier experiments where it was shown that Li@C60 did not decompose during evaporation on Au( 110) or C60. Hence, the effect is attributed to a difference in adsorption on the Cu surface. Test of two different approaches to avoid decomposition were proposed by IBM. First, is to evaporate Li@C60 not on metal but on a less reactive surface, such as for example on a C60 film. A second approach could be to evaporate Na@C60 instead of Li@C60. It is expected this molecule to be more stable due to the larger ionic radius and larger mass of Na, but it might is difficult to produce large amounts of this material using the ion implantation procedure.
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Fig. 50- STM image after evaporation of Li@C60 molecules; image size 140 nm by 140 nm; red arrows point to single molecules which are identified as C60 molecules based on local spectroscopy; blue arrows point to small protrusions identified as possible contamination or Li atoms
La@C82 molecules on Cu(111) Using the same evaporation technique as for Li@C60, deposition of La@C82 was undertaken at IBM. Individual molecules were observed, but with varying, non-identical shape. This can be easily explained by the non-spherical geometry of the molecules and the existence of several different isomers. An STM image of a single La@C82 molecule and the corresponding local spectroscopy are displayed in Fig. 51. Compared to C60 molecules the spectra show a more metallic-like behaviour at low voltages. These experiments clearly showed that La@C82 can be evaporated on a metal surface without decomposition and that it is a suitable material from a technical point of view. On the other hand, it is less suited for a comparison of experimental arrangements with calculations because of the large number of different configurations for this molecule.
Li@C60
58
Fig. 51- STM image of an individual La@C82 molecule and the corresponding I(V) (blue) and dI/dV(V) (green) spectroscopy data.
K@C60 molecules on Cu(111) K@C60 has been delivered in a soluble and an insoluble fraction. Both were evaporated in two different runs onto a clean Cu(111) single crystal using the same flash evaporation technique as used for Li@C60. The resulting coverage was similar to that of the experiment with Li@C60. The STM images show many different adsorbates on the Cu surface. Almost none of the adsorbates had an apparent height which was similar or slightly larger than the one of pure C60, or which had a shape of threefold symmetry as expected for K@C60/Cu(111). The very few adsorbates which could not been ruled out of being K@C60 already at first sight were studied in more detail by means of low-temperature scanning tunneling spectroscopy. All these remaining candidates could be identified as pure C60. We ensured that these results are specific to the provided material and not to the material handling and evaporation procedure. For this crosscheck pure C60 molecules were evaporated onto the Cu(111) crystal using the very same technique. The C60 material was dispersed in ethanol and put onto the evaporator under ambient conditions, the evaporator was transferred into UHV and C60 was evaporated in a single flash. The resulting STM image, shown in fig. 52, shows very little contamination but individual C60 molecules at a coverage suitable for the study of individual molecules.
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Fig. 52- STM images after evaporation of pure C60. Image size 50 nm by 50 nm.
Development of the Nanostencil
The aim of this work is to use the nanostencil developed at UCAM and IBM for the ATOMS project to produce fullerene nanostructures with the ultimate aim of fabricating fullerene devices. The technique uses a combination of a shadow mask and SPM technique, whereby materials are locally deposited through a pinhole (size < 100 nm) close to an SPM tip. Controlled movement of the mask leads to direct fabrication of arbitrary structure without the need for conventional lithography. This technique represents the only method capable of producing complex fullerene heterostructures. A comprehensive nanostencil system has been completed at UCAM involving three components: UHV chamber, electronics and software for controlled patterning.
Fig. 53- (a) Overall view of the nanostencil system. Upper part contains the 4-source evaporator and the lower part is the AFM chamber. (b) The view looking into the chamber showing the AFM/nanostencil.
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The new, specialized UHV (ultra high vacuum) system built at UCAM includes an e-beam evaporation source, a high precision X-Y stage for sample control and an AFM scanning and monitoring system; see fig. 53. The mobilizing and aligning ability of this system has been demonstrated by placing patterns at desired positions on a pre-patterned substrate and by aligning patterns consisting of different materials on the same substrate. Various kinds of materials, including Au, Ag, Cu, Ni and most significantly C60 have been successively evaporated to form patterned structures in this system, with specific results presented in the following section. At IBM, an advanced static and dynamic nanostencil has been designed and constructed in the ATOMS project, and further developed since then within the NICE project (ATOMS has ended in May 2003). Here we describe the instrumental developments at IBM, which are important for the NICE project. The fabrication of small structures of “exotic” materials, which exist only in minute quantities calls for nanostencil requirements different from those found in other stencil applications. A key difference between static and dynamic stencils is the amount of material wasted during deposition because as it blocked by the mask: a static stencil allows us to deposit simple structures in a simultaneous manner, while the dynamic stencil fabricates the structures sequentially. For simple structures, therefore, a static tool is best suited, while a dynamic stencil becomes import when more complicated structures are needed which cannot be produced in a mask (typically rings etc.). All-in-one nanostencil and test patterns The “all-in-one” nanostencil is a system designed in the end phase of the ATOMS project at IBM and fully implemented within the NICE project during the last few months. It has the following features:
5. The entire nanostencil is mounted on a mechanical damping system using standard scanning probe microscopy technology.
6. The masks, evaporators and substrates can easily be changed and aligned within 10-15 µm precision.
7. The distance between mask and substrate is piezo-controlled. 8. A combined beam-deflection AFM and STM is included in the system to analyze
the produced patterns.
In fig. 54, the all-in-one nanostencil developed at IBM is shown, A) as a schematic and B) as the technical realization. A 40-µm-range scanning piezo with a sample holder is mounted on a standard XYZ-course-stepper motor from Omicron. This motor performs with a precision of 10% with respect to an absolute position. In order to achieve a mechanical precision of 10-15 µm (less than half the scan range of the piezo), the all-in-one nanostencil had to be equipped with a turning plate, which is embedded in a ball bearing (fig. 54B). Mask and STM/AFM tip are mounted on the plate, while the sample is attached to the piezo tube, which is moved by the XYZ stepper motor. Using a ball bearing snap-in the required mechanical precision can be achieved. The AFM is of beam-deflection type. The photodiode plus the amplification electronics is sit uated in the vacuum on a sapphire circuit print. With this method noise coupling due to long cables from the instrument to the electronics outside of the vacuum can be reduced. The bandwidth of the AFM electronics is 3 MHz. Sapphire was used as circuit print material to maintain a good heat flow from the in -situ amplifier to the large instrument mass. The
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AFM is also equipped with a cantilever excitation piezo, which allows measurements to be performed in non-contact (dynamic) mode. To circumvent mode hopping of lasers and to be flexible with the light power, a fiber-coupled superluminescent diode was used which can be continuously powered up to 10 mW light power. The fiber is directed into the vacuum system. An optical lens focuses the beam on the cantileve r. Standard stepper motor mirrors accomplish the beam adjustment to the cantilever and to the photodiode. The STM preamplifier is located on the turning plate of the instrument close to the probing tip (fig. 54) and is well shielded to protect the electronics from capacitive incoupling of the high-frequency excitation piezo. This system is finished and fabricates the structures required for the NICE project. It is remarkably stable and has shown atomic resolution on test samples even though the scan range with 40 µm is very large. The stencils used both for the static and dynamic modes are made from silicon nitride membranes from a focused ion beam technique. Structure sizes down to 50 nm have been made and smaller mask aperture sizes can be produced by massive material deposition at high rate to clog the mask holes partially. At “normal” deposition rates (typically 5 nm/min) the masks do not appreciably deteriorate under repeated operation.
B A Fig. 54- A) Schematic of the all-in-one nanostencil, and B) its technical realization. The entire instrument is mounted on a vibrational damping system. An XYZ-stepper motor accomplishes the coarse movement of the sample, whereas the attached tube-piezo performs the fine movement by scanning. A close up of the turning plate, the sample holder with the STM current preamplifier, and the AFM detection system is shown at the top.
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Nanostencilled Structures
For the initial nanostencil work, it was decided at IBM to perform test experiments concerning the electrical characterization of fullerene films within the plane and to pattern first fullerene lines at submicron dimensions. To test the feasibility of connecting molecular structures to large contact pads with the static nanostencil technique, a wire of C60 molecules crossing an Au gap was stenciled. For comparison and electrical characterization, a macroscopic C60 film was evaporated on top of the Au electrodes to cross the gap. Shadowing an insulating mica substrate with a 25 micron standard bonding wire and evaporating 20 nm Au onto the sample produced the Au gap; see Fig. 55.
Fig. 55- Optical micrograph showing the two Au electrodes spaced apart by a gap of 25 µm on top of mica.
The Au evaporation was done at room temperature at a pressure of 2×10–6 mbar. Upon annealing the Au film at 720 K for 30 min, the resistance of the gap dropped from several hundred GΩ to MΩ, indicating that Au atoms diffuse into the gap. Therefore, in order to make electrical device testing reliable, an unannealed Au structure was used for stencilling. In Fig. 56, a representative current/voltage (I/V) characteristic taken at IBM of a C60 layer covering a Au gap of 22 µm is shown, with a resistivity of 67.1 GΩ. Knowing the dimensions of the C60 bridge above mica (width 2.34 mm, length 22 µm (=gap distance), thickness 20 nm) a specific resistance of ρ = 14.2×106 Ωcm was calculated. The same experiment with a 10 nm thick C60 bridge but otherwise identical geometry gives a resistivity of 131.4 GΩ and hence a specific resistance of 14.0×106 Ωcm. These results are in rough agreement with bulk C 60 experiments performed by Wen et al., Appl. Phys. Lett. 61, 2162 (1992).
63
Fig. 56- I/V characteristics of a 20 nm thick C60 film covering the gap between Au electrodes on mica. The resistivity of the layer is 67.1 GΩ .
The experiment on bulk C60 indicates a poor conductivity of the fullerene film, which renders reliable conductivity measurements on long (mm), thin (µm) wires impossible. In Fig. 57, an AFM image taken at IBM of a C60 wire bridging a 25-µm Au gap is shown. The wire is 600 nm wide and 10 nm high, corresponding to 11 molecular layers of C60. The wire features a small gap of about 2 µm (Fig. 57A). The experiments demonstrate the successful production of a sub-µm scale fullerene structure connected to the macroscopic world.
Fig. 57- A) AFM image of a C60 wire bridging a gold gap of 25 µm; B) zoom-in on a step edge of the Au gap; and C) zoom-in on the C60 wire on the Au pad.
An example of Cu and C60 nanostructures fabricated at UCAM using the nanostencil system. The shadow mask consists of a thin (400 nm thick) silicon nitride membrane, supported by a silicon frame. Patterns on the mask can be fabricated using focused ion beam (FIB). A mask pattern and metal wires are shown in fig. 58.
64
Fig. 58- (a) Schematic of a mask pattern consisting of 150 nm holes. The arrow indicates the movement direction of the mask during the evaporation. (b) Image (9 µm × 8 µm) of silver wires fabricated. The mask was moved over a distance of 2 µm with a constant speed during the course of the fabrication. We can produce continuous lines of arbitrary lengths without using long slits, which might cause significant stress in the membrane.
The ability to fabricate complex 3D heterostructures and to align the mask at precise locations on a pre-patterned surface has been demonstrated at UCAM. An example of Cu and C60 nanostructures fabricated using the nanostencil system is shown in Fig. 59.
Cu
65
Fig. 59- AFM images of Cu and C60 nanowires fabricated by the nanostencil. Width of the lines is approximately 100 nm. Cross section of a fabricated structure demonstrates the pattern definition of better than 10 nm.
Further work at UCAM includes a micro four -point probe to be incorporated into the nanostencil system, which will allow in-situ electrical measurement of nanostenciled structures and eliminate contamination problems that we have thus far encountered with measurements in ambient conditions. Two probes with contact pad separation of 8 µm and 16 µm are available. Pre-patterned substrates with contact pads and metallic connections have been designed and fabricated at NMRC for the nanostencil work at UCAM (see Workpackage 1 description). Subsequent nanostencil masks have been produced for the fabrication of fullerene structures at UCAM. In fig. 60, structures fabricated by scanning the nanostencil mask over a sample surface are presented. The arrows indicate the moving direction of the mask during evaporation. A mask consisting of 5 µm long, 100 nm wide slits produced the Ag wedge structures by moving the mask with increasing speed. The wedge thickness decreases from 20 to 5 nm.
Fig. 60- Metal structures fabricated by scanning the nanostencil mask
66
An example of the performance of the nanostencil at IBM is shown in fig. 61. A long C60 line of 160 µm length and 500 nm width has been fabricated at IBM on top of an insulating SiO 2 substrate in the static mode. The stencil mask is shown in fig. 61A and the fabricated structure imaged in-situ with a sequence of overlapping AFM images is given in fig. 61B. This structure exemplifies the importance of the large scan range and the high-resolution imaging capability to locate fabricated patterns.
A B
Fig. 61- A) Si3N4 mask with structures produced by FIB. A long slit of 160 µm length and distance markers separated from each other by 10 µm are visible. B) A sequence of overlapping AFM images shows the C60 lines on SiO 2 fabricated with the all-in-one nanostencil in static mode.
The goal to build unconventional devices on the submicron scale requires accurate positioning of sequentially deposited patterns of different shapes. A typical example is the positioning of a nanowire bridging a gap between two electrodes. Fig. 62 shows an example in which this task has been achieved with two evaporation steps, with the additional challenge to position the wire exactly at the edge of the electrodes within less than 100 nm. A stencil mask has been used with two different structures in it, large area rectangles and a single line. In a first run, the large electrodes at the left of the image and the narrow line at the right have been deposited. With the accurate piezo-controlled positioning the distance between the edge of the electrode and the line is determined, the mask moved by exactly this amount, and in the second step an additional wire is stenciled exactly at the edge of the electrodes.
67
Fig. 62- A non -contact AFM image of a Cu line stenciled exactly at the edge of two large-area Cu electrodes. The Cu line to the right served as a distance calibrator to exactly position the stencil mask such that a Cu line can be fabricated at the electrode edges. The linewidth of the Cu lines is 200 nm, the distance 6.8 µm. The substrate is SiO 2.
This proof of concept of the new IBM nanostencil can be extended to materials relevant to the NICE project. Fig. 63 shows a multi-material patterned structure produced in two steps with a stencil mask exchange in between. The pattern consists of a narrow Cu wire and subsequently deposited C60 wires at right angles to the metallic Cu connector on an insulating substrate, SiO2. To distinguish between different materials the damping signal of the AFM is displayed in Fig. 63. Interestingly, the damping is strongly modified at the positions where the two materials make contact. The reason for this change in damping is not yet known. This last example shows that with this all-in-one nanostencil, all relevant structures for NICE can be deposited, and in particular the MESFET structures.
Fig. 63- A simple Cu/C60 crosspoint “device” on SiO 2 demonstrating the MESFET structure. The image shown is the damping signal taken in AFM mode to enhance the material contrast between Cu and C60. The Cu line appears white and the C60 lines grey, except for the positions where the two materials intersect. The regular array of faint points superimposed consists of small Cu dots evaporated simultaneously for other reasons. Image size: 10 µm by 15 µm.
68
For electrical characterization, more complicated fullerene/metal nanostructures than those shown in Fig. 63 must be fabricated. This has been done by sequential deposition of metal and fullerene material through two different stencil masks, see Fig. 64. Mask 1 contains large holes to deposit contact pads for Gate, Source, and Drain (labeled G, S, and D, respectively) and a thin horizontal wire electrode connected to G. This mask was used for Cu deposition in a first step. Mask 2 contains a lines structures to deposit the nanowire from fullerene material (C 60, La@C82), which is evaporated in the second processing step. Both masks have marks for precise optical alignment to place the masks such that the fullerene wire connects the S and D contact pads as shown in Fig. 64. The gap distance between S and D in this particular example is 14 µm.
Fig. 64: Masks 1 and 2 were used for sequential evaporation to fabricate the crossed wire structure. The fullerene wire connects the electrodes S and D. A Cu wire is connected to electrode G and crosses the fullerene wire in the center of the gap.
The structures were deposited on a prepatterned sample which consists of a Si substrate covered by SiO2 and two large Au electrodes spaced 100 µm apart, as shown in Figs. 64 and 65. The Cu electrodes S and D overlap with the large Au electrodes, thus providing electrical contacts to S and D. The third electrode G remains unconnected to the outside world, but can be connected using a conductive AFM cantilever.
69
Fig. 65: Sc hematics (left) and optical microscopy image (right) of a fabricated structure.
In a first experiment we have investigated a Cu/C60 crossed wire structure, similar to Fig. 63 but now connected to the large Au electrodes. This structure was imaged and geometrically characterized using the noncontact AFM and is shown in Fig. 66. The width of the Cu nanowire is about 400 nm and the length is 40µm. The horizontal wire is made of C60 and overlaps with the bigger Cu contact pad. The vertical Cu wire is connected to the third pad (not shown). A closer look at the junction area (Fig. 66b) shows a smooth Cu wire of 6 nm height crossed by a C 60 wire of about the same height. The C60 material appears smooth on a large scale, but is composed of small grains of about 10 nm size. In the junction area the grainy structure is less pronounced.
Fig. 66: Noncontact in-situ AFM images of the Cu/ C60 lines attached to contact pads. (a) Large scale image of the crossed wire structure, (b) zoom of the intersection. The edges of the big contact pads are visible at the left and right. The vertical Cu wire exceeds the image by about 20 µm and ends on the G contact pad, as shown in Fig. 65. Note that these images are rotated by 90o compared with Fig. 65.
70
Electrical characterization of crossed wires “MESFET” structure Deliverable D10 requires electrical characterization of MESFET structures similar to the one shown in Fig. 66. However, since C60 is not conducting, C60 has been replaced by the more conducting La@C82 as the wire material connecting S and D. The G electrode has been contacted by a conductive AFM cantilever (n+-doped, 0.01 Ohm cm). The La@C82
wire had a length of 14 µm, a width of 2 µm, and a height of 5 nm. The width of the structure is rather inaccurate (+/- 1 µm), possibly due to the high mobility of La@C82 at room temperature. In this three point geometry the crossed wire structure was electrically analyzed in two steps. First, the conductances between the three contacts were measured, and second, the metal/fullerene gate junction resistance was determined. In the first step the I/V characteristics of all the sections was measured: S to D, S to G, D to G. In Fig. 67 two representative I/V data sets are shown in a double logarithmic plot (absolute values are plotted). Only the conductance is shown, because in good agreement with the symmetry of the device, the two shorter sections between S–G and D–G have the same conductance. At voltages above >30 V, a cubic dependence of the current on voltage is observed. It is important to recognize that the conductance through the shorter S–G wire is smaller than through the longer S–D wire. Therefore we expect a significant contact resistance at the metal/fullerene gate junction. To determine this contact resistance we performed additional three point measurements.
Fig. 67: I/V characterization of the La@C82 wire. Absolute values of current versus applied voltage are shown in a double logarithmic plot. Blue: current measured at D, voltage applied at S–D fullerene wire section; red: current measured at G, voltage applied at D–G section, S not connected. The black line was fitted to the blue data set starting at 30 V.
71
Fig. 68 shows I/V curves with drain current I_D as function of gate-source voltage U_GS, with drain -source voltage U_DS as a parameter. Important are the marked points were I_D becomes zero for given U_DS. These points allow us to determine the voltage drop across the metal-fullerene interface at G and thus the contact resistance. For example, for U_DS = -20V, U_GS = -80V (red curve). This means the actual voltage across the fullerene wire is only 1/4 of the applied G–S voltage (for positive voltages this is 1/5, see green curve). Taking into account this voltage, we conclude that the actual conductance along the G–S wire is significantly higher (about a factor of 8) than across the S–D contact. We note that the G-fullerene contact resistance is probably nonlinear and cannot be represented by a simple ohmic resistance.
Fig. 68: I/V data sets of I_D as function of U_GS. U_DS was varied as a parameter. Depending on the bias applied to U_DS the zero crossing of I_D is shifted.
The electrical characterization of this crossed wire structure is still in a exploratory stage. For a detailed device characterization, for instance, temperature-dependent measurements will be needed, and the electronic structure of the endohedral fullerene material needs to be known better. Nevertheless, a few conclusions can be drawn from our results. First of all the conductance of the wire is very low, so the wire is described as an insulator or bad conductor, similar to organic films. Therefore the transport properties of the wire at higher voltages will be determined by the properties of the injected charge carriers from the metal electrodes. In such a case the current can be space charge limited due to the fact that the charge of the carriers is not compensated by a suitable opposite background charge in the host material. In the case of a defect free material and assuming constant carrier mobility, the current density can be described by the modified Child’s law or Mott-Gurney law. According to Child's law, the Space Charge Limited Current (SCLC) density jSCLC is
72
jSCLC = 9/8 ε r ε 0 L-3 µ V2, with a carrier mobility µ independent of electric field, and a wire length L. A fit to our data gives a cubic power law dependence, j ~ V3, rather than a quadratic dependence. If we assume that the carrier mobility is not constant but increases linearly with the local electric field, such a dependence is reconciled: jSCLC’ ~ L-4 V3. Several reasons exist why the mobility can be field dependent, such as the existence of deep impurity states in the bandgap which can trap the charge carriers. The important consequence is that the length of the wire strongly affects conductance. As discussed above we also see that the conductance of the S–D wire is about an order of magnitude smaller than the S–G wire which has only half the length. This also supports our space charge limited current (SCLC) model. These first results show that deliverable D10 has been fulfilled: Controlled fabrication and in -situ electrical characterization of a three-terminal device with fullerene and metal materials has been achieved. We also have to note, however, that the device does not work as a FET, i.e. the S–D conductance cannot be controlled with the applied gate voltage.
Workpackage 4- Characterisation Electrical Characterisation of Large Area Structures
Electrical characterisation measurements for metal/C60 film contacts Several samples of C60 films were deposited onto several metal-coated silicon wafers. For the example depicted in fig. 69, the contacts were directly evaporated onto the fullerene films with the structure Ti-Pt-Au and approximate layer thicknesses as depicted.
Fig. 69 – Typical contact layer structure
73
The contact pattern is circular with a diameter of approximately 150 micron. Due to the position of the mask relative to the film, the entire circular region was not deposited. Rather two patterns as depicted in Fig. 70 were deposited with approximate maximum dimensions of 20x100 sq. micron (P1) and 75x75 sq. micron (P2).
Fig. 70 - Contact patterns for the measurements presented Three sets of I-V characteristics were made: between P1 and the Ti substrate, between P2 and the Ti substrate, and between P1 and P2. In each case, the resulting I-V curves are symmetrical. However, the curves indicate a non-linear behaviour at low voltages suggesting a tunnel barrier at the interfaces (oxide), or other interfacial properties. However, the symmetrical nature of the contacts is such that any rectify behaviour observed using these contacts can be attributed to the deposited films and not the electrical contacts. Several variations of the experiments described (film and contact structures) were attempted. Due to the difficulties with interface oxides and degradation of the endohedral films in an oxygen ambient, we conclude it is required to investigate the contacts in UHV conditions; see workpackage 3. Also, for these initial test structures, the metal substrates were fabricated at NMRC, sent to UCAM for fullerene deposition, and then returned to NMRC for top contact metallization. To avoid the exposure of the fullerene layer to ambient prior to metallization, a new deposition chamber was developed at UCAM allowing metal deposition directly on the fullerene film, without exposing the uncapped fullerene to atmosphere.
Fig. 71- IV characteristics for pattern 1
-0.008
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-0.004
-0.002
0
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-1.5 -1 -0.5 0 0.5 1 1.5
Voltage / Volts
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rren
t / A
mp
eres
74
AFM/STM Characterisation of Fullerene/Metal Contacts on Deposited Films Using the chamber developed at UCAM, several films of fullerene and endohedral fullerenes have been evaporated onto the metal substrates and their electrical properties were measured by conducting AFM and STM. UCAM used La@C82 as a representative endohedral fullerene in the measurements. Pure C60 films have thickness in the range between 30 and 70nm; the La@C82 films are thinner, in the range 8 - 20nm due to the limited amount of the material available. Prior to deposition substrates were cleaned by argon sputtering for 20 minutes, C60 and La@C82 were evaporated in tandem from quartz crucibles. The fullerene contact areas were deposited over half of each metal-coated substrate using a mask. The metallic Ohmic contacts with substrates were prepared on the two corners of each sample. For electrical measurements the probe tips were made contact with the film, as shown in Fig. 72, the current between tip and sample is measured a function of applied bias voltage.
Fig. 72 - Contact layer structure and probe tip in contact with the film for electrical measurements
35nm
Si substrate
Au or Ti coating
C60
La@C82
Probe tip
75
-20
0
20
40
60
80
100
-10 -5 0 5 10
F=320nNF=640nNF=960nNF=1280nN
I (pA
)
V (V)
Fig. 73 - IV characteristics of C60/Ti contact measured by conducting AFM.
Fig. 73 shows IV curves obtained by conducting AFM with a diamond-coated tip for C60/Ti film at different tip-sample forces. The IV curves show a diode-like behaviour with no current flow for negative sample bias. At the same voltage bias the current is scaled up with the applied force due to increase in tip-sample contact area. The logI versus V, shown in Fig. 74, is linear in the forward bias range of 5-10V, which indicates the diode-like behaviour of diamond/C60/Ti contact. Fowler -Nordheim or ln (I/V2) versus (1/V) plot of the data (Fig. 75) shows deviation from a straight line, confirming that the measured current is not dominated by the current of field emission mechanism. Since Ti readily forms an oxide in air, interpretation of the results for C60/Ti contact is difficult. UCAM therefore switched to use Au-coated silicon as substrates to grow other fullerene films.
76
-3
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1
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3
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5
5 6 7 8 9 10
F=320nNF=640nNF=960nNF=1280nN
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Sample voltage (V)
Fig. 74- Semi-logarithmic plot of current versus bias voltage for C60/Ti sample.
77
-2.5
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0.1 0.12 0.14 0.16 0.18 0.2
F=320nNF=640nNF=960nNF=1280nN
ln (
I / V
^2)
1 / V
Fig. 75- Fowler -Nordheim plot for C60/Ti sample.
Current-voltage (IV) spectra of the films have been obtained by approaching the film towards a STM probe tip until a predetermined value of tunnelling current was obtained. With a current of between 0.2nA to 1nA and a bias voltage of between 1.5 to 2V the IV characteristics were dominated by the tunnelling current across the gap between the tip and the film surface. Increasing the tunnelling current about 1.5–4nA the current indicates an electrical contact between the tip and the film, and IV curves do not contain a tunnelling gap. This is interpreted as the position at which the tip is in contact with the film being imaged. Care was taken in approaching the sample in small steps in order to ensure that probe tips do not indent through the films during measurements. The shape of IV curves remained unchanged as the current was further increased up to 10nA. Fig. 76 shows typical IV characteristics measured on a gold-coated substrate and a La@C82 (9 nm) film using a Pt-Ir tip. The reason for using Pt-Ir tips is that the y do not form an oxide in the ambient atmosphere, and that they are mechanically harder, more wear-resistant than gold. The IV data of Pt-Ir tip and Au substrate show a linear line, which confirms the Ohmic tip-substrate contact. The contact resistance of the metal-metal tip-sample contact is on the order of a few hundred kilo-Ohms, the current density in the region of the tip can therefore can be very high. For the La@C82 (9 nm)/Au sample the IV curves are almost symmetrical indicating that the behaviour is not Schottky-like. However, the curves show a non-linear behaviour in the low voltage range, suggesting a tunnelling barrier. This barrier is attributed to cleanliness of Pt-Ir tips, which might be covered in a thin insulating contaminant layer.
78
-100
-50
0
50
100
-4 -3 -2 -1 0 1 2 3 4
AuC60/AuLa@C82/Au
I (nA
)
Sample voltage (V)
Fig. 76- IV curves obtained on gold-coated substrate, C60 and La@C82 films on Au.
The IV characteristics of C60 (51nm) layer on Au show strong rectification behaviour. The Au/C60 junction is conducting when a positive bias is applied to the substrate; when a negative bias is applied, the current becomes much smaller. The rectification ratio is about 4 at ± 3.5V. The semi-logarithmic plot of the C60/Au data, shown in Fig. 77, displays almost a linear line for forward biases above 1V, which indicates Schottky-like behaviour. The results are consistent with literature reports that C60 forms rectifying junctions with Au and Si. The rectification behaviour is attributed to a potential barrier at the C60 interface with the substrates.
79
-1
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1
2
3
4
5
0 0.5 1 1.5 2 2.5 3 3.5 4
ln I
Sample voltage (V)
Fig. 77- Semi-logarithmic plot of current versus bias voltage for C60/Au junction and the linear least squares regression line.
The IV curve of La@C82/C60 (12nm/35nm) multilayer film on Au substrate is shown in Fig. 78, which shows symmetrical behaviour around zero bias. Although the tip-sample contact area and current density are unknown, the higher conductivity of C60/La@C82 multilayer film than C60 film with similar thickness could be sensed by STM. This is due to the lower electrical resistivity of the endohedral layer. The IV shape of the multilayer film appears distinctively different from that of a C60 film, but similar to IV of a La@C82 film. Since C60/Au contact is rectifying, the current of La@C82/C60/Au in the reversed bias is expected to be controlled by the C60/Au junction. The symmetrical IV relationship of the multilayer film suggests the La@C82/C60 contact is rectifying with its direction opposite to that of the C60/Au junction in the multilayer film. Thus the film behaves like an equivalent circuit of two series diodes with opposite directions. The current measured on the La@C82/C60/Au film is of the same magnitude as the reversed bias current of the C60/Au film. This supports the argument that the current of the film is controlled by rectification current in the reverse direction for the two rectifying junctions.
80
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Sample voltage (V)
Fig. 78- IV curve obtained on C60/La@C82 multilayer film on gold-coated substrate
Electrical characterisation of fullerene/metal and fullerene/endofullerene contacts Several large area contact structures composed of variations of fullerene, endohedral fullerene and metal films have been deposited at UCAM and sent to NMRC for electrical characterisation. The films were evaporated through meshes to produce square structures of various sizes ranging from 5 µm to 200 µm. Initially, three types of contact have been produced:
(a) In/C60/In junction (b) In/C60/Au junction
It is predicted that metals with work functions comparable with the work function of C60 (In φ = 4.0 eV and C60 φ = 3.7 eV) will behave as an ohmic contact with C60, while metals with higher work functions (Au φ = 5.1 eV) tend to form rectifying junctions with C60. The former junction thus should show linear IV or ohmic behaviour, while the latter should behave like a diode. Indium is easy to evaporate (low melting point) and can be used to making connecting wires to fullerene devices, and due to its low work function, is anticipated to be an ohmic contact to C 60.
81
Fig. 79- Typical structures evaporated through a 200 mesh
In the following measurements, current – voltage characteristics of metal/C60 were measured using the HP 4156A precision semiconductor parameter analyzer at NMRC. The bias voltage has been applied on the Au surface, and the In metal is connected to the ground. During the experiments, since the metal In and C60 are relatively soft, extra care needs to be put to avoid possible punching through the layer when approaching the probe. This is due to the fact that the device layers sit atop each other, therefore, we must probe the metal top contact directly, which may result in punch through. Sample 1: May 02 This sample was of the 97 x 97 um2 square structures of the multi-layer films deposited on gold coated silicon substrates: 50nm Au / 25nm In / 24nm C 60 / 120nm In
Gold substrate
In/C60 layers
Si
Au C60
In
82
Following is the typical characteristics shown on sample 1 in the experiment. The voltage applied here is from -0.5V to +0.5V. As can be seen, the behaviour of the layers is ohmic. Sample 2: May 02 The second sample has the same large area mesh structure as that of sample 1 on which the 97 x 97 um2 square structures of the multi-layer films deposited on the gold coated silicon substrates: 50nm Au / 26nm C60 / 140nm In The voltage applied in the experiment was from -1.0V to +1.0V.
During measurement, the voltage applied is limited since we noticed that the devices on sample 1 begin to be damaged beyond a voltage of 0.8V. Another effect occurred in measurements at the same voltage: the film resistance increase. We suspected that it is it due to diffusion of In under the applied electrical field. We also observed that the devices on sample 2 were degraded if the applied voltage was greater than 1.5V. The diode characteristics seemed to be degraded with time as the IV curves became flatter with the repetition of measurements (voltage maintained under 1.5V).
Ohmic1
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I
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diode3
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I
83
These two samples were later re-investigated after two months at IBM using an AFM cantilever for a probe tip, and either due to sample degradation or other effects, the IV characteristics were not repeatable. However, at the time of the original measurements, the IV characteristics were repeatable from device to device, and at each device. Additional test structures fabricated suffered from layer shorting. Subsequent test structures and fabrication work indicate that the large area structures are prone to contamination and are unstable with time. Hence the project consortium finally adopted an emphasis on in situ characterisation of the nanofabricated structures within UHV. However, to understand interface properties and possible contamination issues, interface characterisation studies were undertaken and are reported below. Electrical Characterisation of Doped/undoped C60 Films At UGOT, four-probe measurements on thin films of endohedral fullerenes deposited under high vacuum conditions and after exposure to atmosphere were carried out. Two different endohedral fullerenes were electrically characterised at UGOT: La@C82 and Li@C60. Thin films of C60, Li@C60 and La@C82 were deposited on silicon chips specially prepared with gold electrodes in a vacuum of (5-8)×10-8 mbar. The fullerenes were evaporated at temperatures of 770-830 K (depending on the fullerene type). The vapour pressure of the endohedral fullerenes is somewhat lower than that for C60. The C 60 measurements gave a resistivity of 50 MΩcm, in good agreement with literature values for single-crystal C60. After five minutes exposure to air at ambient pressure and temperature the resistivity of the C60 film increased by almost a factor of 10. This is in agreement with studies that show the conductivity depends very much on the extent of oxygen diffusion within the films or crystals. The large range in conductivity values reported in the literature for pure C60 has been attributed to the effects of oxygen diffusion. The measurements made immediately after film deposition yield a resistivity of 1.5 kΩcm for the endohedral fullerene. This is over four orders of magnitude lower than for C60. On exposure to air, the Li@C60 film is affected much more than the C60 film. After two minutes exposure, the resistivity of the endohedral film increases to 4 MΩcm. After about 1 hour of exposure the resistivity of the Li@C60 film is comparable to that of C60. This dramatic increase could be attributed to the much higher reactivity of the endohedral fullerene and the much stronger effect of charge transfer reactions with oxygen. It is also possible that the monomer molecules begin to oligomerise thus also affecting the conductivity of the film. Further work is needed to elucidate the mechanisms involved. The more conventional endohedral fullerenes such as La@C82 are less reactive than the metallo -buckminsterfullerenes and are regarded as being stable molecules. Similar experiments were performed at UGOT on thin films of evaporated La@C82. The results are shown in Fig. 80.
84
Fig. 80- I/V measurement for a thin film of La@C82. The effect on exposure to air is much less pronounced than for Li@C60 or La@C82. Full line: measurement immediately after deposition under high vacuum. Dotted line: measurement after 5 minutes exposure to air.
The resistivity, as measured under vacuum conditions immediately after deposition, is found to be 300 Ωcm. After five minutes exposure this has increased, but only slightly. Interestingly, the increase in resistivity is considerably less than that observed for the C60 films. However, after a number of days exposure to air at room temperature and pressure, the resistivity of the La@C82 films does approach that of the pure C60. The lower resistivity measured for La@C82 compared to Li@C60 may be a consequence of the rapid reaction of Li@C60 with the background gas in the vacuum chamber even under the high vacuum conditions used for these studies ((5-8)×10-8 mbar). IBM reported the first successful realization of molecular nanowires composed of C60 on an insulating substrate. These wires were stenciled on mica connecting two macroscopic Au electrodes. Since the resistivity of undoped C60 is very high, a direct determination of current/voltage characteristics was not possible. Electrical characterization was done in -situ on extended films grown on top of the Au electrodes bridging the insulator gap. Thicknesses of the C60 films were either 10 nm or 20 nm. Given the dimensions of the C60 bridge above mica (width 2.34 mm, length 22 µm (=gap distance), thickness 10 nm or 20 nm), the specific resistance ρ has been calculated to be 14.0×106 Ωcm and 14.2×106 Ωcm, respectively. These results are in rough agreement with bulk C60 experiments performed by Wen [C. Wen et al, Appl. Phys. Lett. 61, 2162 (1992)]. The same experiment for Li@C60 films crossing the insulator gap have been performed at IBM. A typical current vs. voltage curve for a 6 ML thick film is shown in Fig. 81. The calculated resistivity is 170 Ωcm, i.e. about 5 orders of magnitude lower than for the undoped fullerene films. A drastic reduction is expected if Li@C60 indeed behaves metallic. However, typical metallic resistivities are smaller by another 3 to 4 orders of magnitude. For Li@C60, no literature value for ρ has been reported. With the aim to shrink devices to the smallest dimensions, it becomes important to determine the film thickness dependence of the resistance down to the thinnest films.
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100
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300La@C
8 2
as deposited after 5 min in air
Cur
rent
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]
Voltage [V]
85
Therefore, the resistance has been determined for Li@C60 films ranging in thickness from below 1 ML to 6 ML. The results of such a series are shown in Fig. 82. Films below 2 ML have infinite resistance (i.e. zero conductance). Above 4 ML, the conductance scales linearly with the film thickness. We therefore conclude that percolation of the unannealed Li@C60 film takes place between 2 and 3 ML.
-2 -1 0 1 2
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0200400600800
Li@C60: 6ML CoverageI [
nA]
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Fig. 81- Current vs. voltage characteristics of a Li@C60 film covering the gap between Au electrodes on mica. The thickness of the film is 6 ML (= 6 nm). The resistivity of the layer is 170 Ωcm, calculated from the measured dimensions of the Li@C60 film.
0 2 4 60
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/V]
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Fig. 82- Thickness dependence of the conductance for Li@C60 covering the gap between Au electrodes on mica. The onset at ~2 ML is interpreted by the coalescence and subsequent percolation of the film. Above 4 ML, the conductance varies linearly with film thickness. All measurements have been done in-situ in UHV.
86
X-ray Studies of Endohedral Films
La@C82 adsorbed on Cu(111) and Ag(111) The normal incidence X-ray standing wavefield (NIXSW) technique utilises the standing wave formed by interference of the incoming X-ray and diffracted beams from a single crystal. As X-ray photon energy is scanned through the Bragg reflection condition the standing wavefield shifts. This can be used to locate specific atoms on the surface by monitoring the photoemission response of these atoms to the standing wave. UCAM have studied the distribution of the endohedral La atoms in La@C82 molecules adsorbe d on Cu(111) and Ag(111). The XSW results show that upon adsorption on the metal surfaces the molecules adopt a preferential orientation with respect to the substrate surface, with the La atoms possessing a significant degree of ordering. The La atom resides on one side of the cage, either in the upper or lower half of the cage. The degree of La ordering in the monolayer increases as the film is cooled, which is due to reduced thermal intramolecular motions of the La within the cage. These experimental results are supported by the theoretical calculations on La@C60/Cu(111) carried out at NMRC, which show that there is a significant difference in energies for La located in the upper and lower half of the cage.
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Fig. 83- NIXSW La profiles from <111> reflection recorded at 295 K from La@C82 multilayer and one monolayer adsorbed on Ag(111). Inset shows the XPS spectrum of La 3d from one ML La@C82 acquired at hν = 2600 eV, peak La 3d5/2 was used to collect the NIXSW data. The difference between the profiles of the multilayer and one ML is discernible – the multilayer profile consists of a Gaussian-like peak, while the ML profile contains an asymmetrical peak and a dip in intensity, indicating significant La ordering in the monolayer.
The aim of this investigation is to study the feasibility of fullerene -based memory devices. In most cases the endohedral atom is not situated in the center of the fullerene cage but preferentially adsorbed on one or more sites on the inner surface of the
87
fullerene cage. We have used synchrotron X-ray to study the distribution of endohedral atomic position when endohedral fullerenes are adsorbed on metal surfaces. The X-ray standing wavefield (XSW) technique utilises the standing wave formed by interference of the incoming X-ray and back-scattered beams from a single crystal (see Fig. 84). As X-ray photon energy is changed the nodal and anti-nodal planes of the standing wave shifts in a systematic fashion and atoms at a specific position with respect to the scattering planes experience calculable X-ray intensity changes. This can be used to locate specific atoms on the surface by monitoring the photoemission response of these atoms to the standing wave. We have studied the distribution of endohedral La atoms in La@C82 monolayers adsorbed on Cu(111) surface using XSW. When adsorbed on metal surfaces the strong fullerene-surface interaction modifies the properties of fullerene molecules and the behaviour of encapsulated atoms. The XSW results show that La@C82 adsorbed anisotropically on Cu(111) and the La atoms reside on one side of the cage, either as close to or as far from the Cu(111) surface as possible. The atom switching effects within the cage was investigated.
Fig. 84- X-ray standing wave from the normal (111) reflection
Fig. 85 - Diagram showing two confinement regions of endohedral La atoms in La@C 82 monolayers on Cu(111). The dotted circle (diameter of 3.92 Å) represents an isotropic distribution of the La atoms inside the cage for multilayer films.
1.20 Å2.08 Å
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Cu atom
C82 cage La range
88
Structural studies of pristine and K-doped Y@C82 UCAM has conducted studies of the electronic structure and charge transfer in the metallofullerene Y@C82 using photoemission and X-ray absorption spectroscopy. The fullerene material, supplied by Nagoya Univer sity, was produced by the arc-discharge method and purified by HPLC. The molecules were deposited onto clean Ag(111) or Cu(111) surface using an evaporator mounted on a linear transport arm, that allows the source to move close to the sample surface and a multilayer film deposited. Monolayer films were prepared by deposition onto the substrate kept at ~ 300 oC.
The valence bands for undoped C82 and Y@C82 are essentially identical, except that Y@C82 shows a small peak at 0.8 eV below the Fermi level. This peak appears to be the singly occupied molecular orbital (SOMO), as previously found in La@C82. Upon doping the Y@C82 film with potassium (KxY@C82, x = 0 – 4), the valence band features evolve with the degree of doping. Surprisingly, we have observed no significant change in the peak position or broadening in the Y 2p spectra, indicating that the Y endoatom is not affected by the charge state of the molecule and that the atom is well screened from the external environment.
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Fig. 86- XPS spectrum of Y 2p fr om one Y@C82 monolayer on Ag(111)
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Fig. 87- Normal incidence Y L-edge EXAFS from Y@C82
89
UCAM have also conducted Normal Incidence X-ray Standing Wavefield (NIXSW) on Y@C82 adsorbed on Ag(111). Preliminary analysis indicates that, unlike La@C82, there is no significant Y ordering in the mono-layers. This is remarkable, since it shows that the intra-molecular motion of the endo-atom within the cage could have an important role in the ordering of the atom. Note that the Y atom in Y@C82, in contrast to La@C82, does not possess a dynamic motion on the inner wall of the fullerene cage. Characterization of Au/C 60 Interface
Thin films of Au and C60 were deposited at UCAM for interface characterisation. The films were thermally deposited onto ultra-flat Si oxide substrates provided by NMRC.
Fig. 88- (Top) 25 nm Au / 5 nm Cr deposited on ultra-flat SiO2 substrate. Image size 10 µm × 10 µm, z-range = 27 nm. (Bottom) 70nm C60 unannealed film deposited on the surface shown above. Image size 5 µm × 5 µm, z-range = 27 nm. The metal film is flat and featureless, while surface roughness is increased markedly as the C60 film is deposited on top.
UCAM also used Focused Ion Beam (FIB) technique to prepare cross sections of the films for examination at the interfaces. To prepare the cross section, a layer of Pt was first deposited to protect the film surface and the FIB was then used to mill down into the sample, which left behind a nearly prefect polished sidewall which can then be imaged by high-resolution field-emission SEM. Beams currently used are 70 and 145
90
pA, and 30 kV. Experiments are currently being carried out to optimise the FIB milling conditions and minimise film damages in order to reveal the detailed structures of the interfaces.
Fig. 89- SEM image of FIB-fabricated cross-section Au/C60 multilayer film, showing several layers of Au and C60 layers with different thicknesses. The top surface has been roughened due to the material being sputtered during the deposition of a Pt protective layer.
SIMS, ULE-SIMS and RBS analyses of C60/Au structures provided by UCAM have been performed at MATS (UK) Ltd acting as subcontractors for NMRC. The sample compositions sent out for analysis were as follows:
Sample M02: 5nm Cr / 30 nm Au / 120 nm C60 / 60 nm Au Sample M03: 5nm Cr / 30 nm Au / 40 nm C60 / 40 nm Au
As stated above, all samples were deposited on flat SiO 2 substrates. Rutherford Backscattering was performed to determine the density value of the fullerene layer. RBS gave a ratio of fullerene layer atomic content of 2.7. Initial analysis of the samples was made using ULE-SIMS to give the best depth resolution. ULE-SIMS analysis results are shown in fig. 90 and 91. The analysis of Sample M02 was terminated before lower gold layer was reached. The results for both samples show that the depths of all layers determined from sputter times of the ULE-SIMS analysis are from 1.7 to 2.5 times larger than the anticipated depths. The depth resolution of SIMS and ULE-SIMS analyses are limited in this case by the initial surface roughness of the samples. AFM analysis of the roughness of the C60 film performed at UCAM shows that the roughness of the C60 film is in the range of several nanometers. Thus the depth resolution of the ULE-SIMS analysis is in the range of several nanometers as well. However, this does not resolve the difference between specified layer depths and the depths determined by the ULE-SIMS analysis. Rutherford Backscattering was performed to determine the density value of the fullerene layer. RBS gave a ratio of fullerene layer atomic content of 2.7.
20 nm Au 60 nm C60
80 nm Au
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91
Fig. 90- ULE-SIMS analysis result for Sample M02 Fig. 91- ULE-SIMS analysis result for Sample M03
Within figs. 90 and 91, the primary beam energy is 500 eV, primary ions are Cs+, and detected secondary ions are Au-, C-. A conventional SIMS analysis was undertaken on both samples choosing secondary ions, which are less sensitive to the surface chemistry.
High Depth Resolution SIMS Depth Profile
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Fig. 92- SIMS analysis result for Sample M02 Fig. 93- SIMS analysis result for Sample M03
Within figs. 92 and 93, the SIMS analysis: primary beam energy is 10 keV, primary ions are Cs+, and detected secondary ions are CsAu+, CsC+. The results of the conventional SIMS analysis are used to estimate the amount of intermixing between Au and C60 layers. Taking the layer thicknesses determined by the thickness monitor during the deposition to calculate the abruptness of the top gold/fullerene interface gives values of 1.7 nm/decade for the Sample M03 and 1.4 nm/decade for the Sample M02. These values are less than the value of the SIMS analysis depth resolution (several nm). Thus it can be concluded that the amount of intermixing between Au and C 60 layers (due to the diffusion during layer deposition and the roughness of the C60 layer) is small and the metal does not show significant diffusion into the fullerene layer.
SIMS Depth ProfileM02
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Fig. 94- RBS analysis result for Sample M02 Fig. 95- RBS analysis result for Sample M03 For figs. 94 and 95, the He beam energy was 2MeV. For the second part of this study, flat C60 structures were required in order to achieve better SIMS depth resolution. Therefore, a multilayer sample consisting of a mica substrate covered by an epitaxial 100 nm Au(111) layer, followed by a C60 layer of 10 ML thickness annealed at 620 K to make it epitaxial, and capped by a 10 nm Au layer deposited at 115 K was grown at IBM. The important point with this type of sample is that the Au buffer layer is flat and pinhole -free and that after deposition of C60 this flatness is maintained. Fig. 98 reveals these two conditions for the bare Au buffer layer and the initial stage of the C 60 film are met. ULE-SIMS analyses were performed on the original sample and on the same sample annealed at 250 degrees C for 30 minutes in order to obtain the information on C60/Au interface and the diffusion rate of gold in C 60. The results are shown in figs. 96 and 97.
94
High Depth Resolution SIMS Depth ProfileUnannealed C-60 layer in Au on Mica
Analysis 2 - small crater
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Figure 96- ULE-SIMS analysis result for the unannealed sample
Figure 97- ULE-SIMS analysis result for the annealed sample
The depth scale is shown as ‘frame’ number and represents sputtering time. The sputtering time to reach the C60 layer is higher in the case of the annealed sample by approximately 30%. This unexpected result may be due to a change in the surface topography (reduction of the surface roughness) caused by the anneal. The Au signal does not show a dip in the C60 layer as the presence of C enhances the Au ion yield. Because of this, it is impossible to determine the amount of intermixing between Au and C60 layers and whether the anneal has influenced the distribution of Au in the C60 layer.
A B
Fig. 98- STM images of A) a 100 nm thick epitaxial Au(111) layer on mica, and B) of 2 ML C60 deposited on top after subsequent annealing to 620 K. Image sizes are 310 nm by 310 nm and 200 nm by 200 nm, for A) and B), respectively. These images have been taken with the nanostencil in STM mode.
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Progress Overview Chart
Effort in person months for reporting period 1/8/2003 –31/3/2004
Status of deliverables At the filing of this report, only deliverable D10 remains outstanding. The work for deliverable D10 has been completed and is detailed on pp. 67-69 of this report. With submission of this final report, all deliverables have been completed and submitted.
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Publications and Conference Presentations Electron spectroscopy study of LiC 60: charge transfer and dimer formation, J. Schnadt, P. A. Brühwiler, N. Mårtensson, A. Lassesson, F. Rohmund, E. E. B. Campbell, Phys. Rev. B, 62 (2000) 4253-4256 Time-of-flight mass spectrometry studies of endohedral Li@C60 A. Lassesson, F. Rohmund, E.E.B. Campbell, Molecular Materials, 13 (2000) 225-230 Photoionisation and photofragmentation of Li@C60 F. Rohmund, A. Bulgakov, M. Heden, A. Lassesson, E.E.B. Campbell, Chem. Phys. Lett., 323 (2000) 173-179 Laser desorption studies of the fragmentation of Li@C60 A. Lassesson, A.V. Bulgakov, M. Heden, F. Rohmund, E.E.B. Campbell, in "The Physics and Chemistry of Clusters", Proceedings of Nobel Symposium 117, edited by E.E.B. Campbell, M Larsson, (World Scientific,2001) 298 Collisional Production and Characterization of Alkali Endohedral Fullerenes E.E.B. Campbell, in ”Endohedral Fullerenes” ed. T. Akasaka, S. Nagase (Kluwer Academic Publishers b.v. 2002), Chap. 3 Spectroscopical Investigations of Endohedral Li@C60 A. Gromov, N. Krawez, A. Lassesson, D. I. Ostrovskii, E.E.B. Campbell, Current Appl. Phys., 2 (2002) 51-55 Electrical Properties of Thin Films of the Doped Fullerenes Li@C60 and La@C82 A. Lassesson, V. Popok, A. Gromov, M. Jonsson, H. Shinohara, E.E.B. Campbell in Nanometer-scale Science and Technology: Proc. 7th International NANO Conference, Malmo, June 24-28 2002 Li@C60: STM Analysis of isolated molecules Cuong Ton-That, M.E. Welland, A.Gromov, A.Lassesson, M.Jönsson, J.Greer, E.E.B.Campbell, Presentation at 7-th International Conference on Nanometer-scale Science and Technology & 21-st European Conference on Surface Science, NANO-7 & ECOSS-21, Malmö, Sweden, June 2002. Faraday Screening of Electric Fields in Buckminsterfullerene Endohedrally Doped with Lithium P. Delaney, L. Tong, and J.C. Greer, E-MRS Spring Meeting, June 18-21, 2002, Strasbourg, France IR Spectroscopy investigation of purified endohedral Li@C60 and Li@C70 A. Gromov, A. Lassesson, M. Jönsson, D. Ostrovskii, E.E.B. Campbell, Electrochemical Society, Fullerene Symposium Proceedings (Indiana), ed. Kadish, (Electrochemical Soc., 2002)
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Investigations into the fragmentation and ionization of highly excited La@C82 A. Lassesson, K. Mehlig, A. Gromov, A. Taninaka, H. Shinohara, E.E.B. Campbell, J. Chem. Phys., 117 (2002) 9811-9817 Oxygen reactivity of La@C82 investigated with laser desorption mass spectrometry A. Lassesson, M. Jönsson, A. Gromov, A. Taninaka, H. Shinohara, E.E.B. Campbell, Int. J. Mass Spec., 228 (2003) 913-920 FTIR and Raman spectroscopical study of chromatographically isolated Li@C60 and Li@C70 A. Gromov, D. Ostrovskii, A. Lassesson, M. Jönsson, E.E.B. Campbell, J. Phys. Chem. B, 107 (2003) 11290-11301 Formation of small lanthanum-carbide ions from laser induced fragmentation of La@C82 A. Lassesson, A. Gromov, K. Mehlig, A. Taninaka, H. Shinohara, E.E.B. Campbell, J. Chem. Phys. 119 (2003) 5591-5600 Endohedral fullerenes: conductivity and air sensitivity of Li@C60 and La@C82 V. Popok, A. Lassesson, M. Jönsson, A. Gromov, A. Taninaka, H. Shinohara, E.E.B. Campbell, in ”Fullerenes and Nanotubes: The Building Blocks of Next Generation nanodevices” ed. D.M. Guldi, P.V. Kamat, F. D’Souza (Electrochemical Soc., 2003) p. 560-563 Fullerenes with Confined Atoms A. Lassesson, PhD Thesis (Göteborgs Universitet, 2003) Structure and electronic properties of endohedral La@C82 films C. Ton-That, A. G. Shard, S. Egger, A. Taninaka, H. Shinohara, and M. E. Welland, Poster, Trends in Nanotechnology, Santiago (Spain), September 2002. Orientation and Constraints of Endohedral Lanthanum in La@C82 Molecules Adsorbed on Cu(111) C. Ton-That, A. G. Shard, S. Egger, V. R. Dhanak, A. Taninaka, H. Shinohara, and M. E. Welland Phys. Rev. B 68, 045424 (2003) Modulations of Valence Band Photoemission Spectrum from C60 Monolayers on Ag(111) C. Ton-That, A. G. Shard, S. Egger, V. R. Dhanak, and M. E. Welland Phys. Rev. B 67, 155415 (2003) Structural and Electronic Properties of Ordered La@C82 films on Si(111) C. Ton-That, A. G. Shard, S. Egger, A. Taninaka, H. Shinohara, and M. E. Welland, Surf. Sci. 522, L15 (2003) Fullerenes with Confined Atom A. Lassesson, PhD Thesis (Göteborgs Universitet, 2003)
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C60 as a Faraday Cage P. Delaney and J. C. Greer, Applied Physics Letters, 84 pp. 431-433 (2004) Interaction of La@C60 with Copper and Silver Surfaces J.A. Larsson and J. C. Greer, in preparation, Chemic al Physics Letters, (2004) Orbital Density Maps and Surface Bonding of C60 on Cu(111), J. Repp, G. Meyer, R. Allenspach, R. Schlittler, J.A. Larsson, S. Elliott, J.C. Greer, in preparation (2004). Femtosecond laser ionisation of La@C82 A. Lassesson, M. Jönsson, A. Gromov, K. Hansen, E. E. B. Campbell, M. Boyle, D. Pop, C. P. Schulz, I. V. Hertel, A. Taninaka, H. Shinohara, in preparation. A novel all-in-one static and dynamic nanostencil AFM/STM system P. Zahl, M. Bammerlin, G. Meyer, R.R. Schlittler, in preparation. Invited Talks Electronic and optical properties of alkali endohedral fullerenes, E.E.B. Campbell, Keynote lecture, Endohedral Fullerene Symposium, ECS Spring Meeting, Washington DC, March 2001 Ion Implantation of Fullerenes, E.E.B. Campbe ll, Euresco Conference on Particle-Solid Interactions, Donostia Spain, September 2001 Endohedral Fullerenes and Carbon Nanotubes. E.E.B. Campbell, The high potential of low dimensions, Workshop on Emerging nanosciences: nanomaterials and nanobiology, Sigtuna October 2001 Production and Properties of Endohedral Fullerenes and Carbon Nanotubes, E.E.B. Campbell, Seminar, Dept of Solid State Physics, Lund University, June 2001 Production, properties and potential of fullerene-based nanostructures, E.E.B. Campbell, Chemistry Dept., Glasgow University Scotland, November 2001 Endohedral Fullerenes and Carbon Nanotubes. E.E.B. Campbell, The High Potential of Low Dimensions, 1st Korean-Swedish Bilateral Symposium on Quantum phenomena in Nanostructures of Molecula r Conductors and Biomolecules, Seoul Korea, November 2001 Production, properties and potential of fullerene-based nanostructures, E.E.B. Campbell, Physics Dept., Trinity College Dublin. Jan. 2002 Design for Emerging Nanoelectronic Technologies, J.C. Gree r, IDA & NMRC ICT Technology Forum “Future Perspectives in Photonics & Nanotechnology”, Cork, Ireland, May 2 (2002) Atoms Trapped in Buckminsterfullerene, J.C. Greer, Colloquium, Department of Chemistry, Maynooth University, Ireland, Jan. 18 (2002) Endohedral Fullerenes: Conductivity and Air Sensitivity of Li@C60 and La@C82, E.E.B. Campbell, Electrochemical Society Spring Meeting, Paris 27 April - 2 May 2003
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Doped Fullerenes, J.C. Greer, Colloquium, Department of Physics, University of Nottingham, UK, July 29 (2003) Manipulating Fullerenes, E.E.B. Campbell, LEIF European Workshop Meeting, Stockholm, December 2003 Electronic Structure of Doped Fullerenes, J. Greer, Royal Society of Chemistry Symposium, Maynooth (2004) The IBM Nanostencil: Design and first experiments P. Zahl, University of Bremen, May 2004. Dynamic Nanostencil AFM/DFM/STM P. Zahl, University of Basel, June 2004.
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Future Outlook The materials explored within the NICE project were primarily La@C82 and Li@C60. However, for doping and other properties, these have proven not to be ideal for our studies. However, other endohedral materials can be developed and explored in the context of their electrical properties, as well, other doping mechanisms for fullerenes deposited via nanostencil should be explored. Also, deposition and subsequent electrical characterisation of the films has proven difficult, and the purity and reproducibility of the starting material needs to be controlled to a very high level for high quality measurements. For the manipulation of individual atoms within the fullerenes, other dopant atoms and on different substrates can be examined. For example, the behaviour of the ordering of the La atoms within the fullerenes adsorbed onto metal surfaces is very different than for Y within the fullerenes. However, due to the large electric fields needed to manipulate the ions within the cage, new means for manipulating the dopant atom should be devised. Demonstration of metal/fullerene structures with the nanostencil has been achived, but further study of device structures fabricated from nanostencilling can be explored. For example, the gate covers only 1/28th of the La@C82 wire length in the structure fabricated near the project’s end, thus the current magnitude is greatly influenced by the resistance of the sections of the wire not covered by the gate and it could be expected that the effect of the gate would be stronger if the gate covered larger section of the La@C82 wire. Conductivity measurements from NICE project show that the conductivity of La@C82 is relatively low. This can be due to low carrier mobility or low carrier concentration. Measurements of La@C82 EPR and UV photoelectron spectra show that La donates three electrons to C82 to form La3+C82
3-. The concentration of La in the La@C82 crystal is approximately 3x1021 /cm3. If we assume that the donor concentration in La@C82 equals this high level doping, and taking into account that resistivity of La@C82 film is 300 Ohm-cm, then the mobility would be expected to be approximately 7x10-6 cm2/(Vs). For comparison, a mobility of 0.5 cm 2/(Vs) is considered large for an organic semiconductor. A study of a Dy@C82 FET (Chem. Phys. Lett. 379 (2003), p. 223) estimates the field-effect mobility to be 9x10-5 cm2/(Vs). Dy is a lanthanoid and transfers three electrons to C82, similarly to La. The low value of mobility is attributed in the paper to low crystallinity of the Dy@C82 film, however another conclusion is the charge transferred to the fullerene cages are not actively part icipating in conduction. It is difficult to tell if the mobility can be estimated using the equation for the Space Charge Limited Current, because the transport in fullerene films is not fully understood. However, a study of SCLC in thin semiconductor films (IEEE Tran. El. Dev. 36, no. 6, (1989)) shows that the SCL current in thin films is independent of film thickness (if film thickness d<<L, length of the film) and depends on contact geometry. Thus if the measurements of structures with different film thicknesses would show similar current, this may suggest SCLC transport. Also measuring current at different temperatures can point to possible transport mechanism. Further study of the transport mechanisms in these films is clearly required.
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Longer term, new ways of using the nanostencil to deposit fullerenes or to use fullerenes as precursors to nanotube formation could be considered. For example, one can envision experiments where a catalyst is deposited and on top of this layer deposition fullerenes is made with the nanostencil or similar technique. By heating or via other mechanisms, nanotubes could be formed. If the chirality of the nanotubes formed in the manner can be controlled, then patterning of nanotubes could be explored for interconnect and other applications.
