Laboratory of Molecular Electronics, Chemistry DepartmentUniversity of Alabama, Tuscaloosa, AL 35487-0336, USA
THE UNIMOLECULAR RECTIFIER AND
BEYOND
Robert M. MetzgerRobert M. Metzger
Laboratory for Molecular Electronics Laboratory for Molecular Electronics Department of Chemistry Department of Chemistry The University of AlabamaThe University of Alabama
Tuscaloosa, AL 35487, USATuscaloosa, AL 35487, USATel = 1-205-348-5952, fax = 1-205-348-9104Tel = 1-205-348-5952, fax = 1-205-348-9104
Email = [email protected] = [email protected]
NSF DMR-00-95215 (R.M.Metzger) NSF-DMR-00-99674 (D.L.Mattern)NSF-DMR-01-20967 (L. R. Dalton)
International Workshop on Advances in Molecular Electronics: from Molecular Materials to Single-Molecule Devices
Max-Planck Institute for the Physics of Complex Systems, Dresden, Germany25 February 2004
Laboratory of Molecular Electronics, Chemistry DepartmentUniversity of Alabama, Tuscaloosa, AL 35487-0336, USA
GEOGRAPHY
ALABAMA
TUSCALOOSAIn 1539, Chief Tuscaloosa (“Black Warrior” in Choctaw) fought and died in the only full battle between the Indians and the Spaniards (led by Hernando de Soto) at the village of Mauvilia (now lost) on one of the rivers of Alabama. [The city of Mobile, AL is named after Mauvilia]. Hernando de Soto, in his exploration from Florida seeking gold, went on to the banks of the Mississippi river, where he died of syphilis in 1541.
Laboratory of Molecular Electronics, Chemistry DepartmentUniversity of Alabama, Tuscaloosa, AL 35487-0336, USA
UNIVERSITY OF ALABAMA QUAD, CAMPANILE, & CHERRY BLOSSOMS
Laboratory of Molecular Electronics, Chemistry DepartmentUniversity of Alabama, Tuscaloosa, AL 35487-0336, USA
ABSTRACTUNIMOLECULAR RECTIFICATIONr (asymmetric DC conductivity) for 1- molecule thick Langmuir-Blodgett
(LB) (vertical transfer) or Langmuir-Schaefer (LS) (horizontal pickup) monolayers was:
(a) confirmed in cells “Al | LB of 1 | Al” [J. Am. Chem. Soc. 119: 10455 (1997)] and found in cells “Au |
LB of 1 | Au” [Angew. Chem. Intl. Ed. 40: 1749 (2001); J. Phys. Chem. B105: 7280 (2001)]
(b) found in cells “Au | LB of 2 | Au” [J. Phys. Chem. B106: 12158 (2002)] (ion-pair rectifier?)
(c) found in cells “Au | LB of 3 | Au”: weak rectification; extremely high forward currents in some cells
are due to Au stalagmites [J. Phys. Chem. B107: 1021 (2003)]
================== Acc. Chem. Res. 32: 950 (1999); Chem. Reviews 103: 3803 (2003)
(d) found in cells “Au | LS of 4 | Au”:vert sturdy film; the asymmetry persists [unpublished]
Code : VIOLET: ONE-ELECTRON DONOR; BLUE: ONE-ELECTRON ACCEPTOR
NC CN
CNN1
O
O
N
NEt
O
O
N
NEt
4
N
N NI-
2
NH3C CH3
N3
Laboratory of Molecular Electronics, Chemistry DepartmentUniversity of Alabama, Tuscaloosa, AL 35487-0336, USA
WHY ?INORGANIC ELECTRONICS: Gordon E. Moore’s “Law”:
At present, speed of computation doubles every 18 months Design rule of components (their distance) halves every 18 months. Now at 100
nm. What is the limit? Cost of fabrication laboratory increases exponentially with time Field-effect transistors can be scaled down, until semiconductor or oxide fail (? 15 nm ?)
Junction transistors can be scaled down, but not so far (? 50 nm?)
…..
R. M. Metzger, Chem. Reviews 103: 3803 (2003).
(from David L. Allara)
Laboratory of Molecular Electronics, Chemistry DepartmentUniversity of Alabama, Tuscaloosa, AL 35487-0336, USA
RESULTS IN MOLECULAR ELECTRONICS
in large sense, (sensu lato) or in narrow sense (sensu stricto)
MOLECULAR ELECTRONICS (sensu lato) or MOLECULE-BASED ELECTRONICSo Organic metals [TTF TCNQ, 1973]
o Organic superconductors [TMTSF2PF6, 1979: Tc= 1 K, BEDT-TTF)2Cu(NCS)2, Tc = 13 K]
o Charge-transfer light-emitting diodes [Tang, 1987]o Charge-transfer polymers for electrostatic copiers [1967]
o Alkali fullerides [Cs2HC60, 1993, Tc ≈ 40 K]
o Conducting polymers [doped polyacetylene, 1977; polypyrrole, poly-p-phenylenevinylene, polythiophene]]o Organic polymeric light-emitting diodes [Friend, 1991]
(UNI)MOLECULAR ELECTRONICS (sensu stricto), or MOLECULAR-SCALE ELECTRONICSo Molecular lines, spacers, alligator clips, tinkertoys, meccano components, resistorso Molecular wires, antennas, conductors [conducting polymers, carotenes]o Unimolecular rectifiers (Aviram-Ratner), switches, .negative differential resistance devices; diode logico Single-electron transistors & single-atom transistor (Coulomb blockade): no gain .o Must reach out and touch molecules… STM, break junctions, macroscopic padso Molecules with gain? Unimolecular “transistor” (molecular amplifier) with gain?o When and if all components exist, we can start to plan organic interconnects, instead of metal wires…
R. M. Metzger, Chem. Reviews 103: 3803 (2003).
Laboratory of Molecular Electronics, Chemistry DepartmentUniversity of Alabama, Tuscaloosa, AL 35487-0336, USA
EIGHT SIGNIFICANT MILESTONES IN UNIMOLECULAR ELECTRONICS1) STS: currents across alkanethiols on Au (111) << STS currents through aromatic thiols
on Au(111) [L. A. Bumm, et al., Science 271: 1705 (1996)].2) Break junction: resistance between two Au shards with a single 1,4-benzenedithiol
bonded to them is several M. [M. A. Reed, et al., Science 278: 252 (1997)]3) Molecules of 2’-amino-4-ethynylphenyl-4’-ethynylphenyl-2’-nitro-benzene-1-thiolate,
attached to Au on one side and topped by a Ti electrode on the other, exhibit negative differential resistance [J. Chen, et al., Science 286: 1550 (1999)].
4) The Landauer quantum of resistance, 12.9 k was measured at 300 K between a single-walled carbon nanotube, glued to a conducting atomic force microscope (AFM) tip, and a pool of liquid Hg [S. Frank, et al., Science 280: 1744 (1999)].
5) FET behavior was observed for a single-walled carbon nanotube curled over parallel Au lines, with the STM acting as a gate electrode; the power gain was 0.33. [S. J. Tans, et al., Nature 386: 474 (1997)].
6) For an LB monolayer of a bistable [2]catenane closed-loop molecule, with a naphthalene group as one ”station”, and TTF as the second “station”, and a tetracationic catenane hexafluorophospate salt traveling on the catenane, like a “train” on a closed track, deposited on poly-silicon, and topped by a 5 nm Ti layer, then Al, the current-voltage plot is asymmetric as a function of bias (which moves the train on the track),as the train stops in different stations [C. P. Collier et al., Science 289: 1172 (2000)]. (filaments??????)
7) The organometallic equivalent of a single-electron transistor has been realized at 0.1 K with a Co(II) tris-bipyridyl: this structure has no power gain, but can be ascribed to the redox behavior (Co(II) <--> Co(III)) [J. Park, et al., Nature 417: 722 (2002)].
8) Unimolecular rectification [R. M. Metzger et al., J. Am. Chem. Soc. 119: 10455 (1997)]
R. M. Metzger, Chem. Reviews 103: 3803 (2003).
Laboratory of Molecular Electronics, Chemistry DepartmentUniversity of Alabama, Tuscaloosa, AL 35487-0336, USA
AVIRAM & RATNER PROPOSAL OF UNIMOLECULAR RECTIFICATION (1973)
• Down-hill inelastic electron tunneling from A– to D+ strongly favored.• Molecule never synthesized.
Aviram & Ratner, Chem. Phys. Lett. 29: 277 (1974).
ET
+- +
+
ELECTRON FLOWIVT
++
-
-+D+--A-
-
-
++
+
LUMO(A)
-
ET
-
HOMO(D)
--
++
Step 2
Fermi level (metal 2)
D--A molecule
D0--A0
-D0--A0
-
Step 1 -
Forward bias: preferred direction of electron flow
S
S S
S
CNNC
CNNC
+
ETIVT
Fermi level (metal 1)
ET
D ––A
Laboratory of Molecular Electronics, Chemistry DepartmentUniversity of Alabama, Tuscaloosa, AL 35487-0336, USA
MOLECULAR ENERGY LEVELS & METAL WORK FUNCTIONS
• Aviram-Ratner rectifier requires strong donors D and strong acceptors A for conventional metal electrodes.
• Graphite or nanotube electrodes tolerate weaker D and weaker A , but D/A asymmetry is needed.
R. M. Metzger, Chem. Reviews 103: 3803 (2003).
C60
O
OOO2N
NO2
NO2
TTF DCNNaQI DDQTCNQ TCNQF4
Pt
CNO
OCNCl
Cl
NC CN
CNNC
N
NNC
CN
F
CNNC
CN
F
NC
F
F
TCNQF4DDQMg
TMPD TTFIDAA
S
S
S
SS
S
S
S
N(CH3)2
N(CH3)2SS
SS
TCNQ, DCNNaQI
trinitrofluorenonep-benzoquinone, C60
φ
Au(111)
Al(111)
TMPD &BEDT-TTF
benzene10 eV
5 eV
vacuum level
graphite graphitegraphite
trinitrofluorenone
BEDT-TTF
p-benzo-quinone1-ELECTRON DONORS
1-ELECTRON ACCEPTORS
METALS
Laboratory of Molecular Electronics, Chemistry DepartmentUniversity of Alabama, Tuscaloosa, AL 35487-0336, USA
ASSEMBLY STRATEGIES
A. PHYSISORBED MOLECULES:
1. Langmuir-Blodgett films [Langmuir, Blodgett, Gaines, Kuhn]:
Thermodynamically often metastableKinetically very ordered (for 10-100 layers)
2. Polymerizable Langmuir-Blodgett films [Wegner, Tripathy]
Thermodynamically stablepolydiacetylenes, other polymers
kinetically ordered monomers
Disordered polymers (except for topotactic polymers)
B. CHEMISORBED MOLECULES:Thermodynamically stable (single layer)Often not well ordered
1. Thiolates on Au, Ag, Cu [Dewar, Allara, Nuzzo, Sagiv, Whitesides]
Partially polar RS- Au+ bond 2. Organosilicon on oxides of Si, Al
[Sagiv, Allara, Ulman, Rondelez]
can order into a perfect monolayer at the right temperaturecovalent bond (not polar)
3. Alcohols on Pt [Nuzzo, Allara, Whitesides]
4. Amines on Pt [Nuzzo, Allara, Whitesides]
5. Carboxylic acids on oxide of Al and Ag [Nuzzo, Allara]
Polar carboxylate RCOO-Ag+
6. Sequential bonding of bifunctional monomers on surfaces
[Marks, Ratner]
To bond bifunctional molecules X-A-Y, Z-B-W, etc., onto a surface S, forming S-A-B-etc; used to make light-emitting diode structures
R. M. Metzger, Chem. Reviews 103: 3803 (2003)
Laboratory of Molecular Electronics, Chemistry DepartmentUniversity of Alabama, Tuscaloosa, AL 35487-0336, USA
LANGMUIR-BLODGETT (LB) VERSUS LANGMUIR-SCHAEFER (LS) TRANSFER
hydrophilic head group
hydrophobic tail
substrate
barrierbarrier
water
barrier
monolayer film
water
water
water
barrier
barrier
monolayer film
monolayer film
substrate
substrate
substrate
barrier
monolayer film
water
1
2
3
4
LB TRANSFER ONTO HYDROPHILIC SUBSTRATE (e.g. Al, “fresh” Au)
X MULTILAYER Y MULTILAYER Z MULTILAYER
LS TRANSFER ONTO HYDROPHOBICSUBSTRATE
(from dissertation of Xiang-Li Wu)
LB MULTLAYER TYPES
Laboratory of Molecular Electronics, Chemistry DepartmentUniversity of Alabama, Tuscaloosa, AL 35487-0336, USA
METZGER & PANETTA SYNTHESES AND MEASUREMENTS (UNIV. OF MISSISSIPPI, 1981-91)
• LB films chosen for monolayer assembly; Carbamate link between TCNQ and donors successful.• Rectification measurements were primitive, and failed.
Metzger & Panetta, New J. Chem. 15: 209 (1991).Metzger, Matrl. Sci. and Engrg. C3: 277 (1995).
NR
NH
R
NC CN
CNNC
CO
O
S
S
S
S NH
NC CN
CNNC
Br
CO O
O
NC CN
CNNC
Br
NH
CO O
O
R
NH
NC CN
CNNC
X
CO Y O
O
7a: X=Br: Py-C-BHTCNQ b: X=H : Py-C-HETCNQ
5, TTF-C-BHTCNQ 6a: R=O-(n-C12H25), X=Br, Y=CH2: DDOP-C-BHTCNQ b: R=N(n-C12H25)2, X=Br, Y=CH2: BDDAP-C-BHTCNQ c: R=N(n-C12H25)2, X=H, Y=CH2: BDDAP-C-HETCNQ d: R=N(n-C12H25)2, X=H, Y=C2H4: BDDAP-C-HPTCNQ e: R=N(n-C12H25)2, X=H, Y=C3H6: BDDAP-C-HBTCNQ
8a: R=n-C6H13: BDDAP-C-HMTCAQ b: R=n-C12H25: BDDAP-C-HMTCAQ
Laboratory of Molecular Electronics, Chemistry DepartmentUniversity of Alabama, Tuscaloosa, AL 35487-0336, USA
I - V PLOTS FOR Ag | Mg | C16H33Q-3CNQ LB | Pt
monolayer 3 layers 4 layers
Ashwell, Sambles, Martin, Parker, Szablewski, Chem. Comm., 1374 (1990)
To see whether Schottky barrier was responsible for rectification, LB layers of fatty acids were later put between C16H33Q-3CNQ and metal electrodes: rectification persisted [A. S. Martin, J. R. Sambles, and G. J. Ashwell, Phys. Rev. Lett 70, 218 (1993)].
Molecule is a zwitterion; it forms Z-type Langmuir-Blodgett (LB) multilayers (dipoles all oriented in the same direction) and has a high (resonance-enhanced) second-order non-linear optical susceptibility
CNNC
N
CN
C16H33Q-3CNQ
D+
A-
π
Laboratory of Molecular Electronics, Chemistry DepartmentUniversity of Alabama, Tuscaloosa, AL 35487-0336, USA
IS C16H33Q-3CNQ ZWITTERIONIC?If the molecule were flat, then the two states (zwitterionic and undissociated) would be in resonance, as shown at right .
N
HC
R
CN
CNNC
N
HC
R
CN
CNNC
No crystal structure is available for C16H33Q-3CNQ (reflection from micro-crystals with overlapping unit cells could be indexed), but there is a steric hindrance due to peri hydrogen (see arrow), so the torsion angle is probably non-zero.
N
R
CN CN
C
H
N
≠ 0But: a crystal structure was solved for the related compound, or -picolinium tricyano-quinodimethanide 2: the torsion angle = 30.13˚ N
CN CN
CN
H3C
= 30.13˚
P-3CNQ, 2
Thus: in C16H33Q-3CNQ the torsion angle must benon-zero, and the zwitterionic and undissociated states are not in resonance. Which is lower? The zwitterionic one, as we’ll see…
R. M. Metzger et al., J. Am. Chem. Soc. 119, 10455 (1997).
Laboratory of Molecular Electronics, Chemistry DepartmentUniversity of Alabama, Tuscaloosa, AL 35487-0336, USA
-2-1.5-1-0.500.511.5
V (Volts vs. SCE)
-1.24 V
-0.58
-0.49
0.48 E1/2 = -0.54 V
CYCLIC VOLTAMMOGRAM OF C16H33Q-3CNQ: MOLECULE IS REVERSIBLE WEAK ELECTRON ACCEPTOR (E1/2 IS SIMILAR
TO THAT OF p-BENZOQUINONE)
R. M. Metzger et al., J. Am. Chem. Soc. 119, 10455 (1997).
Hiromi Sakurai
CNNC
N
CN
C16H33Q-3CNQ
D+
A-
π
Laboratory of Molecular Electronics, Chemistry DepartmentUniversity of Alabama, Tuscaloosa, AL 35487-0336, USA
10
20
30
40
50
0 0.02 0.04 0.06 0.08
Concentration / mg·mL -1
DIPOLE MOMENT OF C16H33Q-3CNQ: MOLECULE IS A ZWITTERION!
Measured in dichloromethane solution; Kirkwood-Froehlich equation was used for calculation. Calculated from the temperature dependence of the dielectric constant. (-10°C < T < 30°C) For +1 charge on N and -1 charge were on bridgehead C, 50 Debyes would be expected….
Dominique Vuillaume
= 43 ± 8 Debyes
R. M. Metzger et al., J. Am. Chem. Soc. 119, 10455 (1997).
CNNC
N
CN
C16H33Q-3CNQ
D+
A-
π
Laboratory of Molecular Electronics, Chemistry DepartmentUniversity of Alabama, Tuscaloosa, AL 35487-0336, USA
0
0.2
0.4
0.6
0.8
1
600 800 1000 1200 1400 1600 1800
Intensity (arbitrary units)
wavelength / nm
CH3
CN-ACHCl
3-ACH
2Cl
2-A
CDCl3
-F
CH3
CN-F
CH2
Cl2
-F
VIS ABSORBANCE AND NIR-FLUORESCENCE OF C16H33Q-3CNQ
IVT Absorbance in visible region (A) is strongly hypsochromic (gnd > exc)
Near IR fluorescence (F) emission is solvatochromic; exc= 3 to 5 Debyes if gnd= 43 Debyes;
Kirkwood-Westheimer calculation (1938 paper) yields exc = 8.7 Debyes
J. Baldwin et al., J. Phys. Chem. B103: 4269 (1999)
Ground state ≈ D+-π-A- gnd=43 Debyes
Excited state ≈ D0-π-A0 exc = 3 to 9 Debyes
Intervalence transfer band (565 nm in LB films)
Jeffrey Baldwin, Camino Simpson, & RMM
Laboratory of Molecular Electronics, Chemistry DepartmentUniversity of Alabama, Tuscaloosa, AL 35487-0336, USA
C CN
CNNC16H33
N
PM3 / RHF / singlet as GS
LUMO -2.4 eV
HOMO -8.0 eV
= 30 °
MOLECULAR ORBITAL CALCULATION FOR C16H33Q-3CNQ (A TWIST ANGLE 30˚ IS
ASSUMED)
R. M. Metzger et al., J. Am. Chem. Soc. 119, 10455 (1997)
Hiromi Sakurai
HOMO-LUMO gap (5.6 eV) is probably too large. Dipole moment is maximum if twist angle becomes 90˚
In LUMO, charge density is localized on 3CNQ moiety
Other calculations:O. Kwon, M. L. McKee, and R. M. Metzger, Chem. Phys. Letters 313, 321 (1999).
C. Krzeminski, C. Delerue, G. Allan, D. Vuillaume, and R. M. Metzger, Phys. Rev. B64, 085405 (2001).
Laboratory of Molecular Electronics, Chemistry DepartmentUniversity of Alabama, Tuscaloosa, AL 35487-0336, USA
0
0.02
0.04
0.06
0.08
0.1
0.12
200 300 400 500 600 700 800 900
Wavelength / nm
570 nm = 2.17 eV
VISIBLE SPECTRUM OF 11-LAYER Z-TYPE LB FILM OF C16H33Q-3CNQ
R. M. Metzger et al., J. Am. Chem. Soc. 119, 10455 (1997)
Hiroaki TachibanaThe high (2) = 180 pm / V measured by Ashwell for Z-type multilayers of C16H33Q-3CNQ with a Nd-YAG laser at 1064 nm is due to resonance at 532 nm….
CNNC
N
CN
C16H33Q-3CNQ
D+
A-
π
Laboratory of Molecular Electronics, Chemistry DepartmentUniversity of Alabama, Tuscaloosa, AL 35487-0336, USA
GRAZING-ANGLE REFLECTION ABSORPTION IR SPECTRUM OF
C16H33Q+-3CNQ- LB MONOLAYER ON Au
==> The molecule cannot be lying flat on the Au surface
T. Xu, T. Morris, G. Szulczewski, R. R. Amaresh, Y. Gao, S. Street, L. D. Kispert, R. M. Metzger, and F. Terenziani, J. Phys. Chem. B106: 10374 (2002)
There is also a 2217 cm-1 CN stretch in C16H33Q-3CNQ that is Raman-active but is almost IR-silent.
There are three CN stretches….
CNNC
N
CN
C16H33Q-3CNQ
D+
A-
π
Laboratory of Molecular Electronics, Chemistry DepartmentUniversity of Alabama, Tuscaloosa, AL 35487-0336, USA
N(1s) CORE-LEVEL XPS OF THE D+--A- MOLECULE HEXADECYLQUINOLINIUM TRICYANOQUINODIMETHANIDE,
C16H33Q+-3CNQ: TWO PEAKS
T. Xu, T. Morris, G. Szulczewski, R. R. Amaresh, Y. Gao, S. Street, L. D. Kispert, R. M. Metzger, and F. Terenziani, J. Phys. Chem. B106: 10374 (2002)
Tao Xu
CNNC
N
CN
C16H33Q-3CNQ
D+
A-
π
≈ 10 Å
≈ 20 Å
Laboratory of Molecular Electronics, Chemistry DepartmentUniversity of Alabama, Tuscaloosa, AL 35487-0336, USA
Au wireAu wire
Ga/In
Ga/In
Glass, quartz, or Si
Al
Au wire
Ga/In
Glass, quartz, or Si
Al
Au wire
Ga/In Al
+ electrode for V > 0
+ electrode for V > 0LB Monolayer LB Multilayer
Al
ORIENTATION OF THE LB FILMS OF C16H33Q+-3CNQ- ON Al
R. M. Metzger et al., J. Am. Chem. Soc. 119, 10455 (1997)
Bo Chen & Dominique Vuillaume
Symmetrical electrodes.Two-probe measurements: all resistances add. Oxides exist on Ga/In, and defect oxides on Al electrodeslimit the current to where the oxide is not….
Monolayer thickness is 2.3 nm; Molecular length is 3.0 nm if extended;Hence angle of tilt on substrate is 45˚Z-type LB multilayers
Laboratory of Molecular Electronics, Chemistry DepartmentUniversity of Alabama, Tuscaloosa, AL 35487-0336, USA
Metzger et al., J. Am. Chem. Soc. 119, 10455 (1997)
Bo Chen & Dominique Vuillaume
RECTIFICATION IN “Al | 1LB C16H33Q-3CNQ | Al”
The best result. Rectification ratio at 1.5 Volts:RR = [ I(V=1.5 Volts) ] / [ - I(V=-1.5 Volts] = 26 RR decreases upon cycling……Current= 0.33 electrons per molecule per second
0
0.0001
0.0002
0.0003
0.0004
-2 -1.5 -1 -0.5 0 0.5 1 1.5 2
Voltage / V
Electron flow for V > 0
NC CN
CNN1
Laboratory of Molecular Electronics, Chemistry DepartmentUniversity of Alabama, Tuscaloosa, AL 35487-0336, USA
RECTIFICATION IN “Al | 1 LB OF C16H33Q-3CNQ | Al” AT 105 K
-2 10-8
0 100
2 10-8
4 10-8
6 10-8
8 10-8
1 10-7
1.2 10-7
-1.5 -1 -0.5 0 0.5 1 1.5
Current I / Amperes
Bias V/ Volts
B. Chen & R. M. Metzger, J. Phys. Chem. B103: 4447 (1999)
Rectification was observed for a monolayer of C16H33Q-3CNQ between Al electrodes, between 370 K and 105 K, with no temperature dependence for rectification ratio. Current increases as T increases.
NC CN
CNN1
Laboratory of Molecular Electronics, Chemistry DepartmentUniversity of Alabama, Tuscaloosa, AL 35487-0336, USA
SCANNING TUNNELING SPECTROSCOPY OF 15 LB MONOLAYERS OF C16H33Q-3CNQ ON HOPG
Ulf Höpfner
R. M. Metzger, H. Tachibana, X. Wu, U. Höpfner, B. Chen, M. V. Lakshmikantham, and M. P. Cava, Synth. Metals 85, 1359 (1997)
PUSH
IVT IVTIMT
IVT PUSH
- ++ - +-
ELECTRON FLOWHOPG
+-----
Pt/Ir nanotip
+
++
++LB layer 1 LB layer 2 ...etc.... LB layer 15
First monolayer adheres 50% to HOPG and is X-type; the other 14 layers transfer 100% as X-type.
Electron flow is much higher for direction of intervalence transfer (IVT) within layers 2 to 15.
Electron flow for V < 0
NC CN
CNN1
Laboratory of Molecular Electronics, Chemistry DepartmentUniversity of Alabama, Tuscaloosa, AL 35487-0336, USA
STM IMAGE OF C16H33Q-3CNQ MONOLAYER ON HOPG
Metzger et al., J. Am. Chem. Soc. 119, 10455 (1997)
Jeffrey Baldwin
Pattern agrees with image of dicyanomethylene tails standing closest to Pt/Ir tip
Pt/Ir nanotip
CC
N
CN
N N
HOPG
CC
N
CN
N N
HOPGPt
NC CN
CNN1
Laboratory of Molecular Electronics, Chemistry DepartmentUniversity of Alabama, Tuscaloosa, AL 35487-0336, USA
HOMO
Vacuum level
1.0
2.0
3.0
4.0
5.0
6.0
7.0
Mg 3.7
Al 4.2
graphite 4.4
Work functions
Pt 5.7
Au 5.1
TCNQ 3.3
Electron affinity
LUMO
Exc.Singlet
Theory on C16H33Q-3CNQ
ELUMO
hνCT = 2.17 eV
EHOMOGround Singlet
3.3 eV
Experiments on C16H33Q-3CNQ
8.0
eV
Energy Diagram
onset of asymmerty through bond tunneling
R. M. Metzger et al., J. Am. Chem. Soc. 119, 10455 (1997)
ENERGY LEVELS FOR METALS AND FOR C16H33Q-3CNQ
The LUMO energy is estimated from the appearance of enhanced rectification current from Al or HOPG
The HOMO-LUMO gap is taken from the energy of the IVT band(570 nm = 2.17 eV).
Theoretical HOMO-LUMO gap is too big; theory and experiment must be brought into better agreement
Laboratory of Molecular Electronics, Chemistry DepartmentUniversity of Alabama, Tuscaloosa, AL 35487-0336, USA
Reverse bias: charge compensation
Forward bias: field-induced excitation to neutral excited state; preferred direction of electron flow
D+--A-
++++
D0-π-A0
----
++++
IVT
ET ET
D+-π-A-
----
++++
ELECTRON FLOW
Step 6 Step 7
D+-π-A-
----
++++
D0-π-A0
++++
----
D+-π-A-
++++
----
ELECTRON FLOW
Step 8 Step 9IVT
ET ET
+
---
--
-+
- +
AVIRAM-RATNER MODEL MODIFIED FOR DONOR(+)-π-ACCEPTOR(-) ZWITTERION
R. M. Metzger et al., J. Am. Chem. Soc. 119, 10455 (1997)
Laboratory of Molecular Electronics, Chemistry DepartmentUniversity of Alabama, Tuscaloosa, AL 35487-0336, USA
DEPOSITING THE TOP AU ELECTRODE, TO FORM THE “Au | LB MONOLAYER | Au” SANDWICH
cryocooling copper plate
Au source
Filled with liquid nitrogen
thickness sensor
Glass substrate
bottom gold layer(50nm)
LB film
Silicon coated mask
Cr layer (20nm)
Thermally conductive gel
Thermoinsulator
Aluminium foil as radiation reflector Chamber filled with
ca. 110-3 mbar Ar to scatter Au atoms
• Mean free path of gold atoms in 110-3 mbar Ar at 300 K is 7 cm;
• Therefore: about 7 Au-Ar collisions should occur before Au atom deposits onto the LB monolayer
50 cm
T. Xu, I. Peterson, M. Lakshmikantham and R. M. Metzger, Angew. Chem. Int. Ed. Engl. 40, 1749 (2001)
Laboratory of Molecular Electronics, Chemistry DepartmentUniversity of Alabama, Tuscaloosa, AL 35487-0336, USA
SETUP FOR RECTIFICATION MEASUREMENTS BETWEEN Au ELECTRODES
glass substrate
V > 0
I > 0
Ga/In
Ga/In
Au layer (50 nm thick)
Au pad A Au pad B
enhancedelectronflowunderforwardbias
Sn-coatedwire
0.6 mm- 5 mm 0.6 mm
17 nm
Sn-coatedwire
D+ part
C C
CN
N
N
N
C16H33-Q3CNQ, 1
°0
2.4 nm
A- part s part Direction ofenhanced
electron flow
/ 20 Cr nm
I
Laboratory of Molecular Electronics, Chemistry DepartmentUniversity of Alabama, Tuscaloosa, AL 35487-0336, USA
Resistance R = 2.47 k,
Current I = 0.891 mA/ pad = 9,830 electrons molecule-1 s-1,
Rectification ratio (RR) = 27.53 at 2.2 Volts.
ELECTRICAL RECTIFICATION IN “Au | LB MONOLAYER OF C16H33Q+-3CNQ- | Au”
SANDWICH
R. M. Metzger, T. Xu, and I. R. Peterson, J. Phys. Chem. B, 105, 7280 (2001);T. Xu, I. Peterson, M. Lakshmikantham and R. M. Metzger, Angew. Chem. Int. Ed. Engl. 40, 1749 (2001)
CNNC
N
CN
C16H33Q-3CNQ
D+
A-
π
Laboratory of Molecular Electronics, Chemistry DepartmentUniversity of Alabama, Tuscaloosa, AL 35487-0336, USA
RECTIFICATION RATIOS DECREASING IN LATER CYCLES : MOLECULES PROBABLY RE-ORIENT (OR ARE DESTROYED)
IN THE EXTERNAL ELECTRIC FIELD (UP TO 1 GV / m)
The subsequent scans show systematicdecreases in the forward current;RR (@2.2 V) decreases from 27.2 (first scan) to10.1, 4.76, and 2.44 in cycles 2-4, respectively.
Metzger, Xu, and Peterson, J. Phys. Chem. B 105: 7280 (2001)
CNNC
N
CN
C16H33Q-3CNQ
D+
A-
π
Laboratory of Molecular Electronics, Chemistry DepartmentUniversity of Alabama, Tuscaloosa, AL 35487-0336, USA
SATURATION OF THE FORWARD CURRENT IN“Au | LB MONOLAYER OF C16H33Q+-3CNQ- | Au”
SANDWICH
This cell shows a saturation current at Imax = 20.0 mA at 3.2 V; other pads show similar behavior (but not all of them)
Metzger, Xu, and Peterson, J. Phys. Chem. B 105: 7280 (2001)
Saturation of current seen, fits Aviram-Ratner prediction
CNNC
N
CN
C16H33Q-3CNQ
D+
A-
π
Laboratory of Molecular Electronics, Chemistry DepartmentUniversity of Alabama, Tuscaloosa, AL 35487-0336, USA
MECHANISMS FOR RECTIFICATION IN “Metal | MONOLAYER | Metal” SANDWICHES
1. Schottky barriers at metal | molecule interface(s) (“S” for Schottky)
[ W. Schottky, Z. f. Phys. 118: 539 (1942) ]
2. Asymmetrical placement of chromophore between metal electrodes (“A” for asymmetric)
[ C. Krzeminski, C. Delerue, G. Allan, D. Vuillaume, and R. M. Metzger, Phys. Rev. B64: # 085405 (2001);
M. L. Chabinyc et al., J. Am. Chem. Soc. 124: 11730 (2002)]
3. Asymmetry within the molecular chromophore (“U” for unimolecular)
[A. Aviram and M. A. Ratner, Chem. Phys. Lett. 29: 277 (1974)]
Laboratory of Molecular Electronics, Chemistry DepartmentUniversity of Alabama, Tuscaloosa, AL 35487-0336, USA
POSSIBLE MECHANISMS FOR UNIMOLECULAR (“U”)RECTIFICATION
Does it involve ONLY ONE molecular orbital or energy level in the gap?
L. E. Hall, J. R. Reimers, N. S. Hush, and K. Silverbrook, J. Chem. Phys. 112: 1510 (2000)
I. R. Peterson, D. Vuillaume, and R. M. Metzger, J. Phys. Chem. A105: 4702 (2001).
Does it involve TWO molecular orbitals or energy levels in the gap?
A. Aviram and M. A. Ratner, Chem. Phys. Lett. 29: 277 (1974)
C. Krzeminski, C. Delerue, G. Allan, D. Vuillaume, and R. M. Metzger,
Phys. Rev. B64: # 085405 (2001)
Data are not conclusive either way,
but stay tuned…..
Laboratory of Molecular Electronics, Chemistry DepartmentUniversity of Alabama, Tuscaloosa, AL 35487-0336, USA
ELECTRICAL INTER-IONIC RECTIFICATION IN “Au | LB MONOLAYER OF (Bu2NφV)2BuPy+I- | Au” SANDWICH
J. W. Baldwin, R. R. Amaresh, I. R. Peterson, W. J. Shumate, M. P. Cava, M. A. Amiri, R. Hamilton, G. J. Ashwell, and R. M. Metzger, J. Phys. Chem., B106: 12158 (2002)
High forward current is in the direction of electronflow from iodide counterion (or amines) to pyridinium ring, and decreases with every successive cycle. Rectification ratio is as high as 60. Probably back- charge transfer from iodide to pyridinium
Au -0.005
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
-2 -1.5 -1 -0.5 0 0.5 1 1.5 2
Current / milliAmperes
Bias / Volts
C ycle 1
C ycle 2
Cycle 3
C ycle 4
Cycle 5
Cycle 6
1
6
(a)
0
5
10
15
20
25
30
35
40
0 50 100 150 200 250 300
Pressure ∏ / (mN m
-1
)
Area A / (Å
2
molecule
-1
)
A
o
= 142 Å
2
∏
c
= 35 mN m
-1
A
c
= 25 Å
2
Au
Electron flow for V > 0N
N NI-
2
Laboratory of Molecular Electronics, Chemistry DepartmentUniversity of Alabama, Tuscaloosa, AL 35487-0336, USA
ELECTRICAL RECTIFICATION IN “Au | LB MONOLAYER OF DMAn-NC60 | Au” SANDWICH
Dimethylanilino-azaC60
DMAn-N C60
R. M. Metzger, J. W. Baldwin, W. J. Shumate, I. R. Peterson, P. Mani, G. J. Mankey, T. Morris, G. Szulczewski, S. Bosi, M. Prato, A. Comito and Y. Rubin, J. Phys.Chem. B107: 102 (2003)
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
-2 -1.5 -1 -0.5 0 0.5 1 1.5 2
Current I / Amperes
Bias V / Volts
1
23
4
5
0
5
10
15
20
25
30
35
0 50 100 150 200 250 300 350
Pressure ∏ (mN/m)
Area A (Å
2
/ molecule)
Au
Au
Electron flow under forward bias
-0.04
-0.02
0
0.02
0.04
0.06
-1.5 -1 -0.5 0 0.5 1 1.5
Bias V / Volts
The molecule rectifies (RR=2)
Large ohmic current due to gold stalagmites!Strong film; small area; C60 staggered
NH3C CH3
N
Laboratory of Molecular Electronics, Chemistry DepartmentUniversity of Alabama, Tuscaloosa, AL 35487-0336, USA
0
10
20
30
40
50
0 10 20 30 40 50 60 70
Surface pressure (mN/m)
Area per molecule (Å
2
)
SC
CH3O
N
CNNC
NC
SYNTHESIS AND PRESSURE-AREA ISOTHERMOF CH3C(O)S-C11H22Q+-3CNQ:
COMBINE LB AND SELF-ASSEMBLY
A. Jaiswal, R. R. Amaresh, M. V.Lakshmikantham, A. Honciuc, M. P. Cava, and R. M. Metzger, Langmuir 19: 9043 (2003).
Laboratory of Molecular Electronics, Chemistry DepartmentUniversity of Alabama, Tuscaloosa, AL 35487-0336, USA
N
SC
H 3 C
O
NC
CN
C N
N
SC
H 3C
O
NC
CN
CN
N
SC
H 3 C
O
NC
CN
CN
N
SC
H 3C
O
NC
CN
CN
N
SC
H 3 C
O
NC
CN
C N
N
SC
H 3 C
O
NC
CN
CN
9 Å5.2 Å
STM IMAGE OF LB MONOLAYER OF -S-C11H22Q+-3CNQ- ON Au (111)
A. Jaiswal, R. R. Amaresh, M. V.Lakshmikantham, A. Honciuc, M. P. Cava, and R. M. Metzger, Langmuir 19: 9043 (2003).
S
N
CNNC
NC
Au (111)
Laboratory of Molecular Electronics, Chemistry DepartmentUniversity of Alabama, Tuscaloosa, AL 35487-0336, USA
N
CNNC
NC
(CH2)10
S
N
CNNC
NC
(CH2)10
S
C
H3C
O
Gold substrate
e-e-
Pt/Irnanotip
Case A: Above Left and
Right (Top and Middle)
Case B: Above Right and
Right (Bottom)
A. Jaiswal, R. R. Amaresh, M. V.Lakshmikantham, A. Honciuc, M. P. Cava, and R. M. Metzger, Langmuir 19: 9043 (2003)..
BONDING OF S-C11H22Q+-3CNQ- ON Au: THIOLATE AND C(CN)2 GROUPS COMPETE !!!
Laboratory of Molecular Electronics, Chemistry DepartmentUniversity of Alabama, Tuscaloosa, AL 35487-0336, USA
O
O
N
NEt
O
O
N
NEt
4
NEW RECTIFIER: “Au | LS MONOLAYER OF 4 | Au”
Andrei Honciuc, Archana Jaiswal, Charles W. Spangler, RMM, et al. (unpublished)
Pressure-Area isotherm: very rigid solid monolayer (LS transfer is good)
Zoomed STM: LS monolayer on Au has good hexagonal local order (balls are 10 Å apart)
Au
Au
Electronflow under“forward bias”(V < -2 Volts)
Rectification onset at -2 Volt; it does NOT decay with cycles. Some hysteresis. No ohmic regions. LS monolayer is very robust.
Laboratory of Molecular Electronics, Chemistry DepartmentUniversity of Alabama, Tuscaloosa, AL 35487-0336, USA
NEW RECTIFIERS IN “Au | LB MONOLAYER | Au” SANDWICHES (UNDER STUDY)
Synthesis: D. L. Mattern, Univ. of Mississippi
Synthesis: M. Bryce & D. Perepichka, U. Durham
CC
N
N
NO2NO2
NO2
S
S
S
S
O
O
SO O
NN
O
O
O
O
Laboratory of Molecular Electronics, Chemistry DepartmentUniversity of Alabama, Tuscaloosa, AL 35487-0336, USA
WHAT NEXT? Unimolecular amplifier?
1. Rectifiers, bonded covalently to gold - in progress -
2. Other rectifiers - two more found
3. Can a single molecule exhibit power gain? (unimolecular junction transistor, not an FET or a single-electron transistor). To test this one needs:
A. Make at least 3 metal electrodes meet to within 1 to 2 nm of each other (emitter, base,collector).
Better break junctions (M. A. Reed)?
Burn ultra-thin wires (P. McEuen)?
B. How will power gain be possible? (different electron mobilities within a molecule? Back-to-back rectifiers plus a middle region?)
I0
A B
C
I0
Large DC forward bias
DC back bias
Small AC signal in
Large AC signal out
0.01 Io
0.99 Io
MOLECULAR AMPLIFIER?
Laboratory of Molecular Electronics, Chemistry DepartmentUniversity of Alabama, Tuscaloosa, AL 35487-0336, USA
HOW DO WE BUILD CIRCUITS?
Need “orthogonal synthesis”: attach molecule A covalently to surface, then molecule B covalently to A, then molecule C covalently to B, etc. (Li, Ratner, & Marks).
At first, “surface” will be thin inorganic wires.
Ultimately, “surface” will be a conducting organic wire.
NEEDED:
GOOD WORKING TEAMS OF SYNTHETIC CHEMISTS, PHYSICAL CHEMISTS, AND DEVICE PHYSICISTS
WHAT AFTER THAT ?
M
L
A
N
M†
B
O
N†
C
P
O†
D
A, B, C = electron donor, acceptor, wire, bridgeL, M, N, O, P, Q = functional groups for attachment;M† will bond only to M;N† will bond only to N; etc.
Q
NEW ZOO OF MOLECULES
ML ANM† B
ON† CPO† D
Q
(UNI)MOLECULAR DEVICE ATTACHED TO ELECTRODE
Laboratory of Molecular Electronics, Chemistry DepartmentUniversity of Alabama, Tuscaloosa, AL 35487-0336, USA
Colleagues: (Prof. Geoffrey J. Ashwell) Cranfield University, UKDr. M. V. Lakshmikantham University of AlabamaProf. Michael P. Cava University of AlabamaDr. Dominique Vuillaume Institut de Microéléctronique du Nord, Lille, France(Prof. Maurizio Prato) University of Trieste, ItalyProf. Gary J. Mankey University of Alabama
Post-docs: Prof. Ian R. Peterson Coventry University, UKDr. Hiroaki Tachibana AIST, Tsukuba, JapanDr. Tsuyoshi Kawai University of Kyushu, JapanDr. Hiromi Sakurai Tokyo, JapanDr. Ramiya R. Amaresh University of Virginia Dr. Rajugopal University of AlabamaDr. Archana Jaiswal University of AlabamaDr. Akihiko Otsuka Kyoto University
Grad. students:Dr. Xiangli Wu Etek Dynamics, San Jose, CADr. Jeffrey W. Baldwin Naval Research Laboratory, Washington, DCDr. Terry V. Hughes Howard Hughes Res. Ctr., La Jolla, CADr. Bo Chen Xeotron Corp., Houston, TXDr. Tao Xu Texas A&M UniversityMr. Petie Shumate University of AlabamaMr. Andrei Honciuc University of Alabama
Undergrads: Miss Christina Hosch Hatcher (Clarke College) University of Wisconsin at MadisonMiss Camino Simpson (Talladega College) United States Navy
University of Mississippi colleagues: Profs. Charles A. Panetta, Norman E. Heimer, Daniell L. Mattern
Yale University colleague: Prof. Mark A. Reed
Funding: NSF Past funding: ONR, DOE-EPSCoR, JSPS
ACKNOWLEDGEMENTS