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On the fabrication of crystalline C60 nanorod transistors from solution
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On the fabrication of crystalline C60nanorod transistors from solutionChristian Larsen1, Hamid Reza Barzegar1, Florian Nitze,Thomas Wagberg and Ludvig Edman
Department of Physics, Umea University, SE-901 87 Umea, Sweden
Received 12 March 2012, in final form 10 May 2012Published 10 August 2012Online at stacks.iop.org/Nano/23/344015
AbstractFlexible and high-aspect-ratio C60 nanorods are synthesized using a liquid–liquid interfacialprecipitation process. As-grown nanorods are shown to exhibit a hexagonal close-packedsingle-crystal structure, with m-dichlorobenzene solvent molecules incorporated into thecrystalline structure in a C60:m-dichlorobenzene ratio of 3:2. An annealing step at 200 ◦Ctransforms the nanorods into a solvent-free face-centred-cubic polycrystalline structure. Thenanorods are deposited onto field-effect transistor structures using two solvent-basedtechniques: drop-casting and dip-coating. We find that dip-coating deposition results in apreferred alignment of non-bundled nanorods and a satisfying transistor performance. Thelatter is quantified by the attainment of an electron mobility of 0.08 cm2 V−1 s−1 and anon/off ratio of >104 for a single-crystal nanorod transistor, fabricated with a solution-basedand low-temperature process that is compatible with flexible substrates.
S Online supplementary data available from stacks.iop.org/Nano/23/344015/mmedia
(Some figures may appear in colour only in the online journal)
1. Introduction
Organic electronic materials are heralded since they promisean attractive combination of high-performance, low-costand scalable solution-based processing, and novel flexibledevice configurations. One particularly interesting organiccompound is the fullerene C60 and its derivatives [1–4], whichare currently employed in a manifold of applications includingsolar cells [5–11], and integrated electronic circuits [2,12–15]. Fullerenes are normally utilized as a homogeneousor blend thin film in such applications, but recent reportsshow that it is possible to fabricate C60 nanorods, witha diameter of a few tenths of a nanometre and a lengthexceeding a hundred micrometres [16, 17]. Such nanorodscan conveniently be synthesized from solution in a processtermed liquid–liquid interface precipitation (LLIP), where theC60 nanorods are formed at the interface between a goodand poor C60 solvent [18–20]. Recent studies have indicatedthat such carbon nanostructures could pave the way for
1 These authors contributed equally to this work.
high-performance, solution-processed and flexible electronicapplications, notably field-effect transistors (FETs) [21–27].However, the solution-based synthesis process also frequentlyresults in the presence of solvent molecules incorporatedinto the C60 structure, which could influence the propertiesof electronic applications. The specific effect of solventmolecules on the FET characteristics has only been sparselystudied previously [21].
Here, we demonstrate that flexible C60 nanorods witha high aspect ratio of 1000 can be fabricated from ablend solution comprising ethanol (the poor solvent) andm-dichlorobenzene (m-DCB; the good solvent). The as-grownnanorods exhibit a single-crystal hexagonal close-packed(hcp) structure with incorporated m-DCB solvent moleculesin a C60:m-DCB ratio of 3:2. By vacuum annealing thesesingle-crystal structures at 200 ◦C, a phase transformationwas attained resulting in the formation of a polycrystallineface-centred-cubic (fcc) nanorod structure free from solventmolecules. We further report that the nanorods can be alignedand applied in FET devices using a solution-based dip-coatingtechnique, and that we are able to attain a highly promising
Figure 1. (a) Optical micrograph showing bundles of flexible nanorods with typical lengths of 0.3–0.8 mm. (b) TEM image indicating thewidth distribution of individual and bundled nanorods.
transistor performance in the form of an electron mobilityof 0.08 cm2 V−1 s−1 and an on/off ratio of >104 for asingle-crystal nanorod transistor.
2. Materials and methods
2.1. Nanorod synthesis
The C60 nanorods were synthesized by the LLIP method.The C60 powder (99.9%, Mer Corporation) was degassed at150 ◦C for 12 h, before being dissolved in m-DCB (>99.0%,Sigma-Aldrich) in a concentration of 1 mg ml−1. 10 ml ofethanol (99.5%, Kemetyl) was gently added to 1 ml of the C60solution so that a clearly defined interface between the twosolutions could be observed. The blend solution was thereaftergently ultrasonicated for 10 s (using the lowest power setting1 on an ultrasonic bath, USC300D from VWR) to form ahomogeneous interface. The blend solution was stored atroom temperature in a tightly closed glass bottle, and C60nanorods were observed to deposit at the bottom of the bottleafter 2 days. The nanorod synthesis was considered completeafter 1 week of storage under such conditions.
2.2. Nanorod characterization
The nanorods were characterized with x-ray diffrac-tion (XRD, Siemens D5000 diffractometer, wavelength(Cu Kα) = 1.5418 A, accelerating voltage = 40 kV),Fourier-transform infrared spectroscopy (FT-IR, Tensor 27FT-IR microscope), Raman spectroscopy (inVia RamanMicroscope Renishaw, excitation wavelength = 785 nm),transmission electron microscopy (TEM, JEOL 2100F andJOEL 1230, accelerating voltage = 80 kV and 200 kV,respectively), thermal gravimetric analysis (TGA, MettlerToledo TGA/DSC 1 LF/948, heating rate = 5 ◦C min−1,measurement under Ar), and optical microscopy (OlympusBX51 equipped with an Infinity 2-1C CCD camera). Thenanorods were deposited by drop-casting a (∼25 µl) dropof the nanorod dispersion on the substrate of choice (siliconwafer for XRD, FT-IR, Raman spectroscopy and glass
substrate for optical microscopy), followed by drying atroom temperature. For the TEM measurements, the nanorodswere loaded on a holey carbon grid by dipping the gridinto the nanorod dispersion. The annealing of the nanorodstook place on the substrate of choice by introducing thenanorod–substrate assembly into a vacuum oven (Jeio Tech,model OV-11) for 24 h at T = 200 ◦C and p < 1 kPa.
2.3. FET fabrication and characterization
The FETs were fabricated in a bottom-gate, bottom-electrodeconfiguration. Heavily doped p-type Si substrates cappedwith 200 nm of thermally grown SiO2 (Siegert Consulting)were utilized as the gate-electrode/gate-oxide structures.Cr (thickness: 15 nm) and Au (thickness: 15 nm) weresequentially thermally evaporated on top of the SiO2 underhigh vacuum (p = 1× 10−6 mBar). The resulting Cr/Au filmwas patterned using lift-off photo-lithography, so that a setof inter-digitated source and drain electrodes, with a channellength of 20 µm and a channel width of either 160 or 830 mm,were realized on each substrate. The FET structure wasfinalized by applying the C60 nanorod dispersion on top of thesource/drain electrodes under an inert N2 atmosphere, usingeither a drop-casting or dip-coating deposition technique.The drop-casting was performed by applying a single drop(∼25 µl) of the nanorod dispersion onto the substrate usinga glass micropipette. The dip-coating was carried out using acustom-built set-up, with a variable withdrawal velocity andvariable dip angle (satisfying conditions, as described below,were achieved by a withdrawal velocity of 3 cm min−1 anda dip angle of 70◦). The nanorod FETs were annealed for24 h under vacuum (p < 1 kPa) at T = 200 ◦C. The FETcharacterization was performed under an inert N2 atmospherein a glove box ([O2], [H2O] < 1 ppm) using a KeithleySCS-4200 instrument, with the source electrode grounded.
3. Results and discussion
3.1. Characterization of as-grown and annealed nanorods
Figure 1(a) presents an optical micrograph of as-grownnanorods deposited by drop-casting on a glass substrate. The
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Figure 2. (a) TGA trace recorded on as-grown nanorods at a heating rate of 5 ◦C min−1. The data indicate a significant weight loss between85 and 171 ◦C, which is assigned to the evaporation of m-DCB solvent molecules incorporated into the nanorod structure. (b) Ramanspectra recorded on as-grown C60 nanorods (upper red trace (i)) and pristine C60 (lower blue trace (ii)). The asterisks indicate the peaksstemming from the presence of m-DCB molecules in the nanorod structure. The insets indicate the downshift in the Ag(1) and Hg(1) modesdue to the presence of m-DCB molecules in the as-grown nanorod structure.
micrograph highlights the flexible nature of the nanorodsand their tendency to form bundles. In optical imagesrecorded with lower magnification, it is established thatthe typical nanorod length is between 0.3 and 0.8 mmwith some nanorods exhibiting a length exceeding 1 mm.An analysis of TEM images (see figure 1(b)) reveals thatthe width distribution of individual non-bundled nanorodsis in the 100–400 nm range, with the dominant widthbeing 200–250 nm. This yields an aspect ratio for thesolution-processed and flexible nanorods of the order of1000.
Figure 2(a) shows a TGA measurement performed onas-grown C60 nanorods. The absence of a weight loss attemperatures below 85 ◦C verifies the ambient-temperaturestability of the nanorod structures. At higher temperatures,between 85 and 171 ◦C, a distinct and accumulated weightloss of 10.7% is assigned to the evaporation of m-DCBmolecules from the nanorod structure, in agreement withprevious studies reporting that m-DCB molecules can beincorporated into as-grown C60 nanorod structures duringsynthesis [28, 29]. This assignment is further supported bycomplementary experimental data, as presented below, andfrom here on we denote a 24 h vacuum oven treatmentat T = 200 ◦C and p < 1 kPa as the ‘annealing step’.With this information at hand, we are able to calculatethe C60:m-DCB molar ratio in the as-grown nanorods tobe slightly lower than 3:2. The weight loss in the highertemperature regime (400–600 ◦C) coincides well with thesublimation temperature of C60 (390 ◦C).
Raman spectra recorded on as-grown nanorods (upperred trace (i)) and on pristine C60 powder (lower blue trace(ii)) are presented in figure 2(b). The pristine C60 displays tenRaman peaks (8Hg and 2Ag), which are also observed in thenanorod spectrum. However, in addition to these characteristicC60 peaks, the as-grown nanorod spectrum displays threeadditional Raman peaks positioned at 341, 353 and 535 cm−1
(marked with asterisks), which disappear after the annealingstep at 200 ◦C (data not shown). In consideration of the
TGA results, and in agreement with earlier reports [30], wechoose to assign the existence of these three additional Ramanpeaks in the as-grown nanorod spectrum to the presence ofm-DCB molecules within the nanorod structure. The presenceof m-DCB molecules in the as-grown, but not in the annealed,C60 nanorods is further manifested by a small downshift ofthe Hg(1) and Ag(1) modes from 272 to 269 cm−1 and496 to 494 cm−1, respectively (see inset in figure 2(b)). Wenote that these observations are in agreement with previousRaman studies on C60-solvate crystalline structures, wherethe downshift has been assigned to interactions betweenC60 and solvent molecules [28–30]. Further support for thecomplete removal of the m-DCB solvent molecules followingthe annealing step was provided by FT-IR spectroscopy (seesupporting information available at stacks.iop.org/Nano/23/344015/mmedia), and from here on we denote the solvent-freeannealed nanorods as ‘annealed nanorods’. We also note thatthe Ag(2) Raman mode of C60 is a sensitive indicator ofintermolecular chemical bonding, and that the fact that thismode is invariably positioned at 1468 cm−1 in all nanorodRaman spectra provides evidence that both the as-grown andthe annealed nanorods are in a monomeric state [31–34].
Figure 3(a) displays an XRD pattern for the as-grownnanorods, and an analysis of the positions of the narrow XRDpeaks yields a hcp crystal structure, with a unit cell sizeof a = b = 23.71 A and c = 10.18 A. This assignment isalso supported by SEM images (not shown) demonstratingthat the as-grown nanorods exhibit hexagonal cross sections.Figure 3(b) shows a typical high-resolution TEM (HRTEM)image of an as-grown nanorod, and the inset shows thecorresponding selective area electron diffraction (SAED)pattern. No signs of crystalline faults or dislocations could beobserved with TEM while scanning over the entire nanorodsurface, and in combination with the distinct and consistentdot pattern detected in the SAED measurements, we findit established that the as-grown nanorods indeed are singlecrystals.
The XRD pattern of annealed nanorods is presented inthe upper part of figure 4(a) (upper red trace (i)), and an
Figure 3. (a) XRD pattern of as-grown nanorods, revealing a hcp single-crystal structure. The indices indicate the hcp crystal planes. (b)Typical HRTEM image of an as-grown nanorod, with the inset showing the SAED pattern.
Figure 4. (a) XRD pattern of annealed C60 nanorods (i) and a pristine C60 powder (ii), revealing an fcc crystalline structure for bothmaterials. The indices indicate the fcc crystal planes, and the asterisk in the upper spectrum marks a peak corresponding to the Si substrateon which the nanorods were deposited. (b) An STEM image of an annealed nanorod with a clearly visible pore structure. (c) An HRTEMimage and (d) a SAED pattern displaying the polycrystalline nature of the annealed nanorods.
analysis of the data yields an fcc structure, with a unit cellsize of a = b = c = 14.13 A. This is the same fcc structure asdisplayed by ‘normal’ three-dimensional C60 crystals (lowerblue trace (ii)) [18, 35]. Figure 4(b) presents a scanningtransmission electron microscopy (STEM) image that reveals
that an element of porosity has been introduced into thenanorod structure during the annealing step, in agreementwith results reported by Sathish et al [36]. Figure 4(c) showsa typical HRTEM image for an annealed nanorod, whichdisplays a polycrystalline structure with rather distinct grain
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Figure 5. (a–b) Schematic showing the deposition of C60 nanorods from an ethanol/m-DCB dispersion onto an Au-electrode patternedsubstrate. The process starts with evaporation of the low-boiling-point ethanol (a) and is completed by the evaporation of thehigh-boiling-point m-DCB (b). C60 crystalline needles are preferentially deposited on the (yellow) Au electrodes, and as a result only thosenanorods bridging the (dark grey) gap between the two electrodes will function as a transistor-active material. (c) Optical micrograph of(large) nanorods and (small) needles deposited on a gold surface. Note the lack of needles in the proximity of the nanorods. (d) Opticalmicrograph showing a section of a (dark grey) SiO2 transistor channel surrounded by two (light yellow) Au electrodes, which highlights thepreference for Au for the C60 needle structures. (e) Mapped Raman image displaying a (red) C60 nanorod bridging a (dark grey) transistorchannel.
boundaries; a finding that is confirmed by the characteristicpolycrystalline streak patterns in the corresponding SAEDpattern (see figure 4(d)). Thus, the annealing of the nanorodsdoes not only effectively remove the m-DCB molecules fromthe nanorod structure, but also transforms the structure frombeing single-crystal hcp to polycrystalline fcc.
3.2. Fabrication of nanorod transistors
For the transistor fabrication, the nanorods were depositedfrom an ethanol/m-DCB solvent mixture (in which theywere synthesized) onto a substrate surface comprising Ausource–drain electrodes separated by a SiO2 transistorchannel. It is to be anticipated that the ethanol will evaporatebefore the m-DCB on the merit of its lower boiling point(78.3 versus 173 ◦C) and higher vapour pressure (7.87 versus0.252 kPa at 25 ◦C) [37], and the effects of the consecutive
solvent-evaporation processes, based on observations ofdrop-cast samples, are schematically depicted in figures 5(a)and (b). An optical micrograph of the resulting nanorodconfiguration on top of an Au electrode is presented infigure 5(c). A group of high-aspect-ratio nanorods is clearlyvisible in the centre of the micrograph, but we call particularattention to the small needle-like structures (stemming fromthe nucleation and crystalline growth of small remnants ofdissolved C60 during the final stages of m-DCB evaporation)with a length of ∼10 µm that can be observed throughout theAu surface, with the exception of the regions surrounding thenanorods.
The shape and size of these C60 needles are found todepend strongly on the evaporation rate, in agreement withan observation reported in [29]. The fact that the needlestructures are absent close to the nanorods implies that thenanorods themselves function as a preferred nucleation site
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Figure 6. (a) Transfer characteristics for a typical drop-cast nanorod FET comprising as-grown (blue open circles) and annealed (red solidsquares) nanorods as the active material; dVGS/dt = 2 V s−1 and VDS = 60 V. (b) Output characteristics for an annealed drop-cast nanorodFET showing good IDS saturation and a superlinear IDS increase at low VDS; where the latter is a strong indicator of a significant contactresistance.
over the Au surface, and that the nanorods consequentlyshould grow in size during the last instants of m-DCBevaporation (see figure 5(b)). We speculate that such an in situgrowth of the nanorods on top of the Au electrode couldimprove the electrical contact between the electrode and thenanorods via an increased contact area, and mention thatthis hypothesis is in line with our observation that a slowevaporation of the solvent mixture (which supports crystallinegrowth over amorphous film formation) is concomitantwith a distinctly improved transistor performance. A videoshowing the consecutive solvent-evaporation processes andthe nucleation and growth of the needle structures is providedin the supporting information (available at stacks.iop.org/Nano/23/344015/mmedia).
The optical micrograph in figure 5(d) shows a typicalconfiguration of needle structures next to the interfacesbetween two (light yellow) Au electrodes and a (dark grey)SiO2 transistor channel, and it is clear that the needle shapedcrystalline structures have formed almost exclusively on topof the Au surface. It is further notable that the needles are notable to bridge the transistor channel due to their insufficientlength. Figure 5(e) displays a (red) transistor-active nanorodbridging the transistor channel between two (light grey) Auelectrodes, as visualized by a selective Raman spectroscopymapping of the characteristic C60 vibration at 1468 cm−1.This image is also important since the complete lack ofadditional (red) C60 in the (dark grey) transistor channelsupports that the measured transistor characteristics indeedare due solely to the nanorods.
3.3. Nanorod transistor fabricated by drop-casting
A straightforward method for realizing nanorod FETs isby drop-casting a single (∼25 µl) drop of the nanorod-in-ethanol/m-DCB dispersion onto a pre-patterned transistorsubstrate under an inert N2 atmosphere, and then letting thedeposited nanorods dry at room temperature for 24 h, beforeexecuting a transistor characterization of such as-grownnanorods. By further annealing the nanorods directly on thetransistor substrates, it is possible to compare the transistor
performance of (as-grown) single-crystal hcp nanorodscomprising incorporated m-DCB molecules with that of(annealed) polycrystalline and solvent-free fcc nanorods.
The transfer characteristics for a drop-cast nanorodFET in the as-grown state (blue open circles) and in theannealed state (red solid squares) are presented in figure 6(a).Both devices exhibit n-type operation, with a monotonouslyincreasing source–drain current (IDS) with increasing gatevoltage (VGS). The as-grown FET exhibits a large hysteresis atthe slow scan rate of dVGS/dt = 2 V s−1, which we attribute toa slow escape of trapped electrons on the incorporated m-DCBguest (solvent) molecules within the single-crystal nanorodstructure. This hysteresis is decreased, but not eliminated,following the annealing step when the m-DCB molecules areremoved from the nanorod structure, and we attribute thissmaller hysteresis to the existence of electron traps at thegrain boundaries within the now polycrystalline (annealed)nanorods. Figure 6(b) presents the output characteristics foran annealed nanorod FET. The device exhibits a well-behavedsaturation of the source–drain current at high drain voltage(VDS), but also a non-linear increase of IDS in the linearregime (at low VDS); where the latter is an indication of anon-ideal injection process [38]. In this context, we pointout that we employed a slow evaporation protocol for theattainment of good injection (see discussion in section 3.2),and although this procedure indeed resulted in better injectionand overall performance, it does obviously not yield Ohmicelectrode/active material contacts. One consequence of anon-Ohmic injection process is that standard equations forthe calculation of the mobility of the charge carriers willunderestimate its true value [22, 39]. Hysteresis effects arealso manifested in the output characteristics, and we mentionthat a complete de-trapping of gate-induced electrons couldtake up to one day to be accomplished.
The electron mobility (µn) and the threshold voltage (VT)of the active material in a FET (here, the nanorods) canbe calculated in the saturation regime using the followingequation:
Table 1. Transistor and structural metrics for drop-cast nanorod FETs.
Nanorodtreatment
Crystalstructure Crystallinity
Existence ofguest molecules
µn (% ofmax) VT (V) Ion/Ioff
As-grown hcp Single crystal Yes 100 24.5 >104
Annealed fcc Polycrystalline No 85 28.1 >104
Figure 7. Histogram showing the angular distribution of thedrop-cast nanorods observed to bridge transistor channels, with 90◦
corresponding to a desired nanorod alignment perpendicular to theelectrode edges. A Gaussian fit of the measured data is indicated bythe (blue) solid line.
where CG is the gate-oxide capacitance (CG = 17.3 nF cm−2
for 200 nm SiO2) and W and L are the total width andthe average length, respectively, of the nanorods bridgingthe transistor channel. The Ion/Ioff ratio was calculated bydividing the source–drain current at VGS = VDS = 60 V withthat at VGS = 0 and VDS = 60 V.
Figure 7 presents a histogram, and a correspondingGaussian fit, of the observed alignment of drop-cast nanorodswithin the transistor channel. 90◦ corresponds to a nanorodpositioned with its long axis along the shortest transistor-channel (or source–drain) path, i.e. perpendicular to theelectrode edges; see figure 5(b) for an illustrative example of ananorod oriented at a 90◦ angle. The probability for a nanorodof a certain length to cross a transistor channel (or any othergap) obviously increases with increasing angle up to 90◦, andthe number of channel-bridging nanorods should accordinglyincrease with increasing angle to reach a maximum at 90◦,and then exhibit a mirror-symmetric decrease with furtherincreasing angle. On the basis of the observed broad nanoroddistribution in figure 7, it is clear that drop-casting results in ahighly random alignment of the drop-cast nanorods within thetransistor channel.
The number density and propensity for bundling ofdrop-cast deposited nanorods are found to be very high; seefigure 1. This results in it being very difficult to distinguishindividual, and count assemblies of, nanorods in a transistorchannel (using, e.g., an optical microscope), which in turnmakes the estimation of the values for W and L to be used
in equation (1) highly uncertain. We have also carefullymonitored the nanorod distribution within a number ofdifferent transistor channels during the annealing process,and find that the nanorod distribution remains unaffected bythe high-temperature treatment. With these observations inmind, we opted for a qualitative comparison of the electronmobility of drop-cast nanorods between the as-grown and theannealed state, and our measured transistor and structural dataare presented in table 1.
The drop-cast nanorod FET exhibits a 15% higherelectron mobility in the as-grown state than in the annealedstate, while the threshold voltage is similar for both types ofdevices. In the latter context, we point out that the hysteresiseffects are much more severe for the as-grown nanorods(see figure 6(a)), which indicates the existence of a deeperelectron-trap level on the m-DCB guest molecules than atthe polycrystalline boundaries. If we naively assume thatthe detrimental influence on the mobility of guest molecules(specific to the as-grown nanorods) cancels that of thecrystalline faults (specific to the annealed nanorods), it wouldbe possible to state that the hcp structure yields higherelectron mobility than the fcc structure. However, this is a veryspeculative statement at this stage, and further measurementsare clearly needed before it can be verified. We finally notethat the on/off ratio is very high at >10 000 for both theas-grown and the annealed nanorod FETs.
3.4. Nanorod transistor fabricated by dip-coating
We have utilized a dip-coating technique for the fabrication ofnanorod FETs in order to alleviate the bundling and countingissues related to drop-cast deposition, and a schematicof the dip-coating process is depicted in the left panelof figure 8. By tuning the dip angle and the substratewithdrawal speed, we were able to deposit a preferred lownumber density of non-bundled nanorods on the transistorstructures, so that the estimation of W and L in equation (1)could be done with high accuracy; thus allowing for aquantitative calculation of the electron mobility. Importantly,by orienting the source–drain electrode edges perpendicularto the substrate withdrawal direction, we were able to attain apreferential orientation of the long axis of the nanorods alongthe shortest transistor-channel (source–drain electrode) path.We found that a dip angle of 70◦ and a substrate withdrawalspeed of 3 cm min−1 results in very good alignment ofeasily distinguishable and non-bundled nanorods, and anoptical micrograph of such an assembly of aligned dip-coatednanorods on top of (dark grey) transistor channels is presentedin the right panel of figure 8.
The improved alignment attained by the dip-coatingprocess is also clearly visualized in the angular-distribution
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Figure 8. (Left) schematic drawing of the dip-coating of nanorods from an ethanol/m-DCB dispersion onto an Au-electrode-coatedsubstrate. (Right) optical micrograph showing the alignment of nanorods along the shortest (dark grey) transistor-channel path between(light yellow) Au electrodes.
Figure 9. (a) Histogram showing the measured angular distribution of nanorods bridging the transistor channel for a typical dip-coatedsubstrate. A Gaussian fit to the dip-coated data is included as the red solid line. A normalized drop-cast fit (blue dotted line) is also includedfor comparison. (b) Transfer characteristics of a typical dip-coated nanorod FET in the as-grown state (open blue circles) and in theannealed state (solid red squares). Note that IDS has been normalized so that it depicts the IDS current per nanorod.
Table 2. Transistor and structural metrics for dip-coated nanorod FETs.
Nanorodtreatment
Crystalstructure Crystallinity
Existence ofguest molecules µn (cm2 V−1 s−1)a VT (V)a Ion/Ioff
Annealed fcc Polycrystalline No 8.8(±2.6)× 10−3 18.9(±1.7) >102
a Mean values with standard deviation in parenthesis.
histogram in figure 9(a), where the Gaussian fit to thedip-coating data is indicated by the (red) solid line and the fitto the drop-cast data is indicated by the (blue) dashed line (thelatter fit has been imported from figure 7 and renormalized).The alignment can be quantified by a calculation of thestandard deviation (σ ) of the measured angular-distributiondata, and we find that σ = 18◦ for the dip-coated nanorodsand σ = 33◦ for the drop-cast nanorods. It should be notedthat the presented data for both the drop-casting and thedip-coating experiments originated from the same nanorod
batch, thus eliminating influences stemming from differencesin the nanorod size distribution.
Nanorod FETs fabricated with the dip-coating techniquedisplay well-behaved transistor performance, and figure 9(b)presents representative transfer characteristics in the as-grownstate (blue circles) and in the annealed state (red squares).Table 2 presents structural data, and the average and standarddeviation for the transistor metrics. The lower mobilityin annealed nanorods is related to their polycrystallinenature introduced by annealing. The highest recorded
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electron mobility is 0.081 cm2 V−1 s−1 for a dip-coatedas-grown nanorod FET, which is slightly higher than the0.05 cm2 V−1 s−1 reported by Ochiai et al for a nanorod FETsynthesized in a chlorobenzene solvent [21, 23]. The thresholdvoltage for the dip-coated FETs is found to be consistentlyslightly lower than for the drop-cast FETs (compare data intable 2 with table 1).
4. Conclusions
We report on the solution synthesis and comprehensivecharacterization of flexible C60 nanorods with a largeaspect ratio of ∼1000. The as-grown nanorods exist ina single-crystal hcp structure with incorporated m-DCBguest molecules in a C60:m-DCB ratio of 3:2, but anannealing step executed at 200 ◦C transforms the nanorodsinto a polycrystalline fcc structure void of guest molecules.Nanorod transistors were fabricated by drop-casting a nanoroddispersion onto transistor substrates, but the analysis of therecorded data was obscured by the bundling and randomalignment of the drop-cast nanorods. This problem waseliminated by the employment of a dip-coating technique thatresulted in a good alignment of non-bundled nanorods. Wemeasured a highest electron mobility of 0.081 cm2 V−1 s−1
for such a dip-coated as-grown nanorod FET. We also reportthat deep electron-trap levels on the incorporated m-DCBmolecules result in significant hysteresis effects, and thatthe employment of a slow solvent-evaporation procedurealleviates injection problems related to a poor physical contactbetween a nanorod and the source/drain electrodes. Finally,we call attention to the fact that the C60 nanorods werehandled in solution form at low temperatures, both during thesynthesis and the transistor fabrication, and that the presentedmethod therefore could allow for a scaled-up fabrication ofelectronic circuits using flexible substrates.
Acknowledgments
This project was supported by Solar Fuels Umea (UmeaUniversity) and the Artificial Leaf Project Umea (K&AWallenberg foundation). The authors acknowledge thevibrational spectroscopy platform (ViSP) at Umea University.HRB and TW thank the JC Kempe Foundation andAngpanneforeningen, respectively, for support. TW and LEacknowledge the Swedish research council for researchgrants. LE is a ‘Royal Swedish Academy of SciencesResearch Fellow’ supported by a grant from the Knut andAlice Wallenberg Foundation.
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