laboratory of molecular electronics, chemistry department university of alabama, tuscaloosa, al...

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


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.


UNIVERSITY OF ALABAMA QUAD, CAMPANILE, & CHERRY BLOSSOMS


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(d) found in cells “Au | LS of 4 | Au”:vert sturdy film; the asymmetry persists [unpublished]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


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)


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…



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)]



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


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.


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


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)


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


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


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


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).


-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)


Hiromi Sakurai

CNNC

N

CN

C16H33Q-3CNQ

D+

A-

π


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


CNNC

N

CN

C16H33Q-3CNQ

D+

A-

π


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


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).


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


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


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


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


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


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


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


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


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


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


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


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


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



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)


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


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


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


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


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)]


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…..


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


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


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).


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)


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 !!!


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.


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


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?


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


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

laboratory of molecular electronics, chemistry department university of alabama, tuscaloosa, al...

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