Transcript Slide 1

The Genesis of Molecular Electronics
Conventional Electronics: Transistor
In 1915 AT&T opened their transcontinental
telephone system; required signal amplification.
1945: AT&T and Bell Labs set up the Solid State
Physics group.
First transistor invented in 1947 at Bell Labs.
Junction transistors used to develop first integrated
circuit in 1958; Jack Kilby at Texas Instruments
(2000 Nobel Prize in Physics).
FET’s in 1961.
1965 Moore’s Law.
Moore’s Law
Boundaries of conventional techniques
Miniaturization achieved by “top down” approach using
improvements in lithography technique.
Even with the development of ever-improving lithographic
tools, silicon is approaching fundamental physical limitations of
operation. As gate widths decrease below 100 nm, bulk
properties yield to quantum phenomena and leakage currents
from electron tunneling prevent proper device operation.
Chemistry operates at the nanometer scale by controlling the
placement of individual atoms and functional groups on
molecules through synthetic chemistry, allowing macroscopic
properties from rigidity to optical and electronic behavior to be
“Bottom up” approach is promising instead of carving
lithographically bigger blocks into smaller and smaller chunks.
Molecular Electronics
First coined by Mark Ratner, in 1974.
Molecular electronics involves the replacement of a
wire, transistor or other basic solid-state (usually
silicon) electronic element with one or a few
Molecular electronic device must exchange
information, or transfer states or must be able to
interface with components at the macroscopic level.
Simple molecular electronic devices usually consist of
organic molecules sandwiched between conducting
Molecules displaying functional behavior
Molecular Rectifiers
Early Work
In the 1970’s Aviram
and Ratner surmised that an
organic analogue of a p-n
junction would act as a
molecular rectifier.
D-σ-A Rectifier
Computational results from
study of one such D-σ-A
molecule composed of a
donor moiety
tetrathiafulvalene connected
by a methylene bridge to an
acceptor moiety,
showed a rectification of
current should be possible.
A. Aviram, M. A. Ratner, Chem. Phys. Lett. 1974, 29, 277.
Difference in
voltage for the
under positive
and negative
bias gives rise
to rectifying
Carroll, R. L.; Gorman, C. B. Angew.
Chem. Int. Ed. 2002, 41, 4378-4440.
Molecular Rectifiers
Metzger and co-workers have
studied Langmuir Blodgett
(LB) films of
between metal electrodes and
observed strong rectification
The donor is the quinolinium
moiety, connected to the
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
Metzger, R. M. Chem. Rev. 2003, 103, 3803-3834.
Comparison of Mechanisms
Aviram-Ratner model for neutral D-σ-A
Model for zwitterionic D+-π-A- species
Carroll, R. L.; Gorman, C. B. Angew. Chem. Int. Ed. 2002, 41, 4378-4440.
Langmuir-Blodgett Monolayer Photodiode
photodiode: D-S-A
Under positive bias, emoves from to D to S
excitation promotes eto first E.S. of S, to A
and finally to Au.
No current when light is
Sakomura, M.; Lin, S.; Moore, T. A.; Moore, A. L.; Gust, D.; Fujihira, M.
J. Phys. Chem. A 2002, 106, 2218.
Molecular Wires
Carroll, R. L.; Gorman, C. B. Angew. Chem. Int. Ed. 2002, 41, 4378-4440.
Tour Wires
James M.Tour’s group over 15 years have been synthesizing molecules with
aromatic, alkene, and alkyne bridges, terminating in thiols at one or both
ends. These are known as Tour Wires.
A wire is defined as a two-terminal entity that possesses a reasonably linear
I(V) curve prior to the breakdown limit.
Precise molecular wires bearing protected alligator clips (SAc) at one and two ends.
Tour Wire: Molecular Devices
Molecular devices could be systems having two or more termini with currentvoltage responses that would be expected to be nonlinear due to intermediate
barriers or heterofunctionalities in the molecular framework.
Two terminal wire with tunnel barrier; wire with a quantum well: RTD; three
terminal system: switch; four terminal system: logic gate
P(m,n) refers to the molecular electrostatic potential impedance of a system with m
1,4-phenylene moieties and n ethynylene moieties.
Tour, J. M.; Kozaki, M.; Seminario, J. M. J. Am. Chem. Soc. 1998, 120, 8486-8493.
Resonant Tunnelling Diode
RTD allows voltage bias to
switch “on” and “off” the
Current passes equally well
in both directions.
Aliphatic groups with high
P.E. establish aromatic ring
between them as narrow
“island” of lower P.E.
through which electrons
must pass to traverse the
entire length of the wire.
Resonant Tunnelling Diode; Operation
Smaller the region in which the
electrons are confined, farther apart
are the allowed quantized energy
levels, eg. “island” and regions to
left and right of barrier.
Electrons injected under bias into
If the K.E. is’nt enough, no
tunneling occurs; switched “off”.
If bias is high enough, incoming
electron’s energy resonate with
energy levels inside well, tunneling
ocuurs, etc.; switched “on”.
“Peak” to “valley” ratio ~1.3:1
Negative Differential Resistance
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.
At 60 K, assembly was found to display a very
strong NDR with a peak-to-valley
ratio (PVR) of 1030:1.
Control molecules (having no
nitro 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.
Three terminal devices
Molecular three-terminal junction that could be used as a molecular interconnect.
Tour, J. M.; Kozaki, M.; Seminario, J. M. J. Am. Chem. Soc. 1998, 120, 8486-8493.
Low input potential:
impedance = 2P (in series)
High input potential:
impedance = 3P/2
Switch like properties.
Can behave like a NOT
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 Switch
Docking stations:
Benzidine and
Bulky stopper groups.
Bead: tetracationic
bead shifts to
Molecular shuttle
switched electrostatically
Carroll, R. L.; Gorman, C. B. Angew. Chem. Int. Ed. 2002, 41, 4378-4440.
Rotaxane: Logic Device
“Ring” and “Thread”
fluoresce separately.
Upon threading (CT
complex), fluorescence
Addition of protons or base
recovers the fluorescence.
Neutralization removes
fluorescence again.
If the fluorescence is taken
as an indicator of truth, and
B and H+are taken as inputs,
then the system has the
same behavior as an XOR
Carroll, R. L.; Gorman, C. B. Angew. Chem. Int. Ed. 2002, 41, 4378-4440.
Rotaxane: Logic Device
Stoddart et al. Science 2000, 289, 1172-1175.
Tetracationic cyclophane
with two bipyridinium
units interlocked with a
crown ether containing a
TTF and a NP unit on
opposite sides.
TTF inside : A0
On oxidation, TTF
outside: B+
At 0 V , goes to B0
Bistability is the basis of
the device.
Probing and Interconnecting Molecules:
Self Assembly and Directed Self Assembly
How to attach probe electrodes to either side of molecule? Self
assembly to adsorb molecules on an electrode.
Alligator clips: R-NC, R-S-S-R, R-COOH etc.
Alkanethiolates used as insulating host matrix; electronic
properties of embedded molecules can be explored.
Directed self assembly: grow rigid substrate molecules normal
to the adsorbate by selective insertion into host at defect sites,
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 Conductivity
Electron Transfer:
Coherent nonresonant tunneling :
Electronic states of the molecule are far from the energy of the
tunneling electrons; rate of electron transport exponentially
dependent on the length of the molecule.
Coherent resonant tunneling
Energy of tunneling electrons resonant with the energy of the
molecular orbitals’ rate of electron transport is essentially
independent of length .
Measuring Molecular Electronic
Mercury Drop Junction:
Mercury can form thiol-based
The junction is created by
forming a mechanical contact
of a SAM supported on a solid
substrate 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
Break Junctions
Some of the first single-molecule
conductivity measurements executed using
mechanically controllable break (MCB)
Benzene-1,4-dithiol, one of the simplest
molecules used in characterizing
molecular conductance.
The nanopore consists of a SAM of conjugated
molecules sandwiched between two electrodes.
Fabrication of the heterostructure.
(a) Cross section of a silicon wafer
showing 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 sandwich
structure in the nanopore.
(c) 4-thioacetylbiphenyl and detail diagram of
the sandwich heterostructure.
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.
Fabrication of the nanopore
The devices are fabricated using a combination of electronbeam 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 methods
Contact Conductive Probe AFM
Nanoparticle Coupled CP-AFM
Molecular electronics will mature into a
powerful technology only if its development is
based on sound scientific conclusions that
have been tried and tested at every step.
Detailed understanding of the
molecule/electrode interface, as well as
developing methods for manufacturing reliable
devices needed.