The Genesis of Molecular Electronics

Conventional Electronics: Transistor Conventional Electronics: Transistor developmentdevelopment

In 1915 AT&T opened their transcontinental In 1915 AT&T opened their transcontinental telephone system; required signal amplification.telephone system; required signal amplification.

1945: AT&T and Bell Labs set up the Solid State 1945: AT&T and Bell Labs set up the Solid State Physics group.Physics group.

First transistor invented in 1947 at Bell Labs.First transistor invented in 1947 at Bell Labs. Junction transistors used to develop first integrated Junction transistors used to develop first integrated

circuit in 1958; Jack Kilby at Texas Instruments circuit in 1958; Jack Kilby at Texas Instruments (2000 Nobel Prize in Physics).(2000 Nobel Prize in Physics).

FET’s in 1961.FET’s in 1961. 1965 Moore’s Law.1965 Moore’s Law.

Moore’s LawMoore’s Law

                                                                                                                                                                

        

Boundaries of conventional techniquesBoundaries of conventional techniques Miniaturization achieved by “top down” approach using Miniaturization achieved by “top down” approach using

improvements in lithography technique.improvements in lithography technique.

Even with the development of ever-improving lithographic Even with the development of ever-improving lithographic tools, silicon is approaching fundamental physical limitations of tools, silicon is approaching fundamental physical limitations of operation. As gate widths decrease below 100 nm, bulk operation. As gate widths decrease below 100 nm, bulk properties yield to quantum phenomena and leakage currents properties yield to quantum phenomena and leakage currents from electron tunneling prevent proper device operation.from electron tunneling prevent proper device operation.

Chemistry operates at the nanometer scale by controlling the Chemistry operates at the nanometer scale by controlling the placement of individual atoms and functional groups on placement of individual atoms and functional groups on molecules through synthetic chemistry, allowing macroscopic molecules through synthetic chemistry, allowing macroscopic properties from rigidity to optical and electronic behavior to be properties from rigidity to optical and electronic behavior to be engineered. engineered.

““Bottom up” approach is promising instead of carving Bottom up” approach is promising instead of carving lithographically bigger blocks into smaller and smaller chunks.lithographically bigger blocks into smaller and smaller chunks.

Molecular ElectronicsMolecular Electronics First coined by Mark Ratner, in 1974.First coined by Mark Ratner, in 1974.

Molecular electronics involves the replacement of a Molecular electronics involves the replacement of a wire, transistor or other basic solid-state (usually wire, transistor or other basic solid-state (usually silicon) electronic element with one or a few silicon) electronic element with one or a few molecules.molecules.

Molecular electronic device must exchange Molecular electronic device must exchange information, or transfer states or must be able to information, or transfer states or must be able to interface with components at the macroscopic level.interface with components at the macroscopic level.

Simple molecular electronic devices usually consist of Simple molecular electronic devices usually consist of organic molecules sandwiched between conducting organic molecules sandwiched between conducting electrodes.electrodes.

Molecules displaying functional behavior

Molecular Rectifiers

Early WorkEarly Work Computational results from Computational results from

study of one such D-study of one such D-σσ-A-A molecule composed of a molecule composed of a donor moiety donor moiety tetrathiafulvalenetetrathiafulvalene connected connected by a by a methylenemethylene bridge to an bridge to an acceptor moiety, acceptor moiety, tetracyanoquinodimethanetetracyanoquinodimethane, , showed a rectification of showed a rectification of current should be possible.current should be possible.

A. Aviram, M. A. Ratner, Chem. Phys. Lett. 1974, 29, 277.

In the 1970’s Aviram In the 1970’s Aviram and Ratner surmised that an and Ratner surmised that an organic analogue of a p-n organic analogue of a p-n junction would act as a junction would act as a molecular rectifier.molecular rectifier.

D-σ-A Rectifier

Difference in threshold voltage for the mechanisms under positive and negative bias gives rise to rectifying behavior.

Carroll, R. L.; Gorman, C. B. Angew. Chem. Int. Ed. 2002, 41, 4378-4440.

Molecular RectifiersMolecular Rectifiers

Metzger and co-workers have Metzger and co-workers have studied Langmuir Blodgett studied Langmuir Blodgett (LB) films of (LB) films of (nhexadecyl)quinolinium (nhexadecyl)quinolinium tricyanoquinodimethanidetricyanoquinodimethanide between metal electrodes and between metal electrodes and observed strong rectification observed strong rectification behavior. behavior. The donor is the The donor is the quinoliniumquinolinium moiety, connected to the moiety, connected to the acceptor, acceptor, tricyanoquinodimethanidetricyanoquinodimethanide by a bridge. by a bridge.

D-π-A Rectifier

Metzger, R. M. Chem. Rev. 2003, 103, 3803-3834.

I/V curves from two different LB-film configurations. a) 1 LB monolayer b) 4 LB monolayers.

Metzger, R. M. Chem. Rev. 2003, 103, 3803-3834.

Comparison of MechanismsComparison of Mechanisms

Carroll, R. L.; Gorman, C. B. Angew. Chem. Int. Ed. 2002, 41, 4378-4440.

Aviram-Ratner model for neutral D-σ-A species

Model for zwitterionic D+-π-A- species

LangmuirLangmuir--Blodgett Monolayer PhotodiodeBlodgett Monolayer Photodiode

Electrochemical Electrochemical photodiode: D-S-Aphotodiode: D-S-A

Under positive bias, eUnder positive bias, e-- moves from to D to S moves from to D to S (G.S).(G.S).

Photochemical Photochemical excitation promotes eexcitation promotes e-- to first E.S. of S, to A to first E.S. of S, to A and finally to Au.and finally to Au.

No current when light is No current when light is off.off.

Sakomura, M.; Lin, S.; Moore, T. A.; Moore, A. L.; Gust, D.; Fujihira, M.

J. Phys. Chem. A 2002, 106, 2218.

Carroll, R. L.; Gorman, C. B. Angew. Chem. Int. Ed. 2002, 41, 4378-4440.

Molecular WiresMolecular Wires

Tour WiresTour WiresJames M.Tour’s group over 15 years have been synthesizing molecules James M.Tour’s group over 15 years have been synthesizing molecules with aromatic, alkene, and alkyne bridges, terminating in thiols at one or with aromatic, alkene, and alkyne bridges, terminating in thiols at one or both ends. These are known as Tour Wires.both ends. These are known as Tour Wires.

A wire is defined as a two-terminal entity that possesses a reasonably linear A wire is defined as a two-terminal entity that possesses a reasonably linear I(V) curve prior to the breakdown limit.I(V) curve prior to the breakdown limit.

Precise molecular wires bearing protected alligator clips (SAc) at one and two ends.

Tour Wire: Molecular DevicesTour Wire: Molecular DevicesMolecular devices could be systems having two or more termini with currentMolecular devices could be systems having two or more termini with current--voltage responses that would be expected to be nonlinear due to intermediate voltage responses that would be expected to be nonlinear due to intermediate barriers or heterofunctionalities in the molecular framework.barriers or heterofunctionalities in the molecular framework.

Two terminal wire with tunnel barrier; wire with a quantum well: RTD; three Two terminal wire with tunnel barrier; wire with a quantum well: RTD; three terminal system: switch; four terminal system: logic gateterminal system: switch; four terminal system: logic gate

Tour, J. M.; Kozaki, M.; Seminario, J. M. J. Am. Chem. Soc. 1998, 120, 8486-8493.

P(m,n) refers to the molecular electrostatic potential impedance of a system with m 1,4-phenylene moieties and n ethynylene moieties.

Resonant Tunnelling DiodeResonant Tunnelling Diode

RTD allows voltage bias to RTD allows voltage bias to switch “on” and “off” the switch “on” and “off” the current.current.

Current passes equally well Current passes equally well in both directions.in both directions.

Aliphatic groups with high Aliphatic groups with high P.E. establish aromatic ring P.E. establish aromatic ring between them as narrow between them as narrow “island” of lower P.E. “island” of lower P.E. through which electrons through which electrons must pass to traverse the must pass to traverse the entire length of the wire.entire length of the wire.

Resonant Tunnelling Diode; OperationResonant Tunnelling Diode; Operation Smaller the region in which the Smaller the region in which the

electrons are confined, farther apart electrons are confined, farther apart are the allowed quantized energy are the allowed quantized energy levels, eg. “island” and regions to levels, eg. “island” and regions to left and right of barrier.left and right of barrier.

Electrons injected under bias into Electrons injected under bias into LUMO on LHS.LUMO on LHS.

If the K.E. is’nt enough, no If the K.E. is’nt enough, no tunneling occurs; switched “off”.tunneling occurs; switched “off”.

If bias is high enough, incoming If bias is high enough, incoming electron’s energy resonate with electron’s energy resonate with energy levels inside well, tunneling energy levels inside well, tunneling ocuurs, etc.; switched “on”.ocuurs, etc.; switched “on”.

“Peak” to “valley” ratio ~1.3:1

A negative differential resistance (NDR) is characterized by a discontinuity in the monotonic increase of current as the voltage is increased.

Several of these devices can be combined to give I/V curves with multiple peaks–this behavior has been proposed to lead to multi-state memory and logic devices.

Reed and Tour et al. reported the clearest example of molecule-based NDR to date.

Negative Differential Resistance

At 60 K, assembly was found to display a very strong NDR with a peak-to-valleyratio (PVR) of 1030:1.Control molecules (having nonitro or amine moieties) showed no NDR.

In the singly reduced state, the LUMO becomes fully delocalized, allowing enhanced conduction, thus creating the onset of the NDR peak. As the bias voltage is increased the molecule becomes doubly reduced, the LUMO becomes localized across the molecule and decreases the conductivity of the molecule, reducing the current passed through the molecule.

Tour, J. M.; Kozaki, M.; Seminario, J. M. J. Am. Chem. Soc. 1998, 120, 8486-8493.

Three terminal devicesMolecular three-terminal junction that could be used as a molecular interconnect.

Low input potential: Low input potential: impedance = 2P (in series)impedance = 2P (in series)

High input potential: High input potential: impedance = 3P/2 impedance = 3P/2

Switch like properties.Switch like properties. Can behave like a NOT Can behave like a NOT

logic gate.logic gate.

Molecular-sized switch with corr. equivalent of source, drain, and gate terminals of a bulk solid-state FET.

Tour, J. M.; Kozaki, M.; Seminario, J. M. J. Am. Chem. Soc. 1998, 120, 8486-8493.

Rotaxane: Molecular SwitchRotaxane: Molecular Switch

Docking stations: Docking stations: Benzidine and Benzidine and Benzophenol.Benzophenol.

Bulky stopper groups.Bulky stopper groups. Bead: tetracationic Bead: tetracationic

cyclophane.cyclophane. Protonation/Oxidation: Protonation/Oxidation:

bead shifts to benzophenolbead shifts to benzophenol Molecular shuttle switched Molecular shuttle switched

electrostaticallyelectrostatically

Carroll, R. L.; Gorman, C. B. Angew. Chem. Int. Ed. 2002, 41, 4378-4440.

Rotaxane: Logic DeviceRotaxane: Logic Device ““Ring” and “Thread” Ring” and “Thread”

fluoresce separately.fluoresce separately. Upon threading (CT Upon threading (CT

complex), fluorescence complex), fluorescence extinguished.extinguished.

Addition of protons or base Addition of protons or base recovers the fluorescence.recovers the fluorescence.

Neutralization removes Neutralization removes fluorescence again.fluorescence again.

If the fluorescence is taken If the fluorescence is taken as an indicator of truth, and as an indicator of truth, and B and HB and H++are taken as inputs, are taken as inputs, then the system has the same then the system has the same behavior as an XOR gate.behavior as an XOR gate.

Carroll, R. L.; Gorman, C. B. Angew. Chem. Int. Ed. 2002, 41, 4378-4440.

Rotaxane: Logic DeviceRotaxane: Logic Device

Tetracationic cyclophane Tetracationic cyclophane with two bipyridinium units with two bipyridinium units interlocked with a crown interlocked with a crown ether containing a TTF and ether containing a TTF and a NP unit on opposite sides.a NP unit on opposite sides.

TTF inside : ATTF inside : A00

On oxidation, TTF outside: On oxidation, TTF outside: BB++

At 0 V , goes to BAt 0 V , goes to B00

Bistability is the basis of the Bistability is the basis of the device.device.

Stoddart et al. Science 2000, 289, 1172-1175.

Probing and Interconnecting Molecules:Probing and Interconnecting Molecules:Self Assembly and Directed Self AssemblySelf Assembly and Directed Self Assembly

How to attach probe electrodes to either side of molecule? Self How to attach probe electrodes to either side of molecule? Self assembly to adsorb molecules on an electrode.assembly to adsorb molecules on an electrode.

Alligator clips: R-NC, R-S-S-R, R-COOH etc.Alligator clips: R-NC, R-S-S-R, R-COOH etc. Alkanethiolates used as insulating host matrix; electronic Alkanethiolates used as insulating host matrix; electronic

properties of embedded molecules can be explored.properties of embedded molecules can be explored. Directed self assembly: grow rigid substrate molecules normal Directed self assembly: grow rigid substrate molecules normal

to the adsorbate by selective insertion into host at defect sites, to the adsorbate by selective insertion into host at defect sites, or at step edges.or at step edges.

B. A. Mantooth, P. S.Weiss, “Fabrication, Assembly And Characterization Of Molecular Electronic Components,” Proc. IEEE, vol. 91, pp. 1785-1802, Nov. 2003.

Molecular ConductivityMolecular Conductivity

Electron Transfer:Electron Transfer: Coherent nonresonant tunneling :Coherent nonresonant tunneling :

Electronic states of the molecule are far from the energy of the Electronic states of the molecule are far from the energy of the tunneling electrons; rate of electron transport exponentially tunneling electrons; rate of electron transport exponentially dependent on the length of the molecule.dependent on the length of the molecule.

Coherent resonant tunnelingCoherent resonant tunnelingEnergy of tunneling electrons resonant with the energy of the Energy of tunneling electrons resonant with the energy of the molecular orbitals’ rate of electron transport is essentially molecular orbitals’ rate of electron transport is essentially independent of length . independent of length .

Measuring Molecular Electronic Measuring Molecular Electronic ComponentsComponents

Mercury Drop Junction:Mercury Drop Junction:Mercury can form thiol-based SAMs. The junction is created by forming a mechanical contact of a SAM supported on a solidsubstrate and a SAM supported on a suspended mercury drop. The resulting metal–SAM–SAM–metal junction allows for the ensemble measurement of pure and mixed monolayers.

Break JunctionsBreak JunctionsSome of the first single-molecule conductivity measurements executed using mechanically controllable break (MCB) junctions.Benzene-1,4-dithiol, one of the simplest molecules used in characterizing molecular conductance.

NanoporeNanopore

Reed, et al. “The electrical measurement of molecular unctions,” in Molecular Electronics: Science and Technology, 1998, vol. 852, Annals of the New York Academy of Sciences, pp. 133–144.

The nanopore consists of a SAM of conjugated molecules sandwiched between two electrodes.

Fabrication of the heterostructure.(a) Cross section of a silicon wafershowing the bowl-shaped pore etched in suspended SiN membrane with a diameter about 300 A. (b) Au-Ti top electrode/self-assembled monolayer/Au bottom electrode sandwichstructure in the nanopore. (c) 4-thioacetylbiphenyl and detail diagram of the sandwich heterostructure.

Fabrication of the nanoporeFabrication of the nanopore The devices are fabricated using a combination of electron-

beam lithography, plasma etching, and use of an anisotropic etchant to create a suspended silicon nitride membrane with a 30–50 nm aperture .

An Au contact is evaporated on the top of the aperture and the device is immersed in a solution of the molecule of interest to form a SAM.

After deposition, the bottom electrode is formed by evaporating 200 nm of Au onto the sample, which is held at 77 K to minimize damage to the SAM.

Using the nanopore device, Reed and coworkers have measured the properties of biphenyl-4-thiol.

This molecule exhibited strong rectifying behavior arising from the asymmetry of the molecule.

Other methodsOther methodsSTM

Contact Conductive Probe AFMContact Conductive Probe AFMNanoparticle Coupled CP-AFMNanoparticle Coupled CP-AFM

ConclusionConclusion

Molecular electronics will mature into a Molecular electronics will mature into a powerful technology only if its development is powerful technology only if its development is based on sound scientific conclusions that based on sound scientific conclusions that have been tried and tested at every step.have been tried and tested at every step.

Detailed understanding of the Detailed understanding of the molecule/electrode interface, as well as molecule/electrode interface, as well as developing methods for manufacturing reliable developing methods for manufacturing reliable devices needed.devices needed.