15 research outputs found
Carbon-Based Molecular Junctions for Practical Molecular Electronics
ConspectusThe field of molecular electronics
has grown rapidly since its
experimental realization in the late 1990s, with thousands of publications
on how molecules can act as circuit components and the possibility
of extending microelectronic miniaturization. Our research group developed
molecular junctions (MJs) using conducting carbon electrodes and covalent
bonding, which provide excellent temperature tolerance and operational
lifetimes. A carbon-based MJ based on quantum mechanical tunneling
for electronic music represents the worldâs first commercial
application of molecular electronics, with >3000 units currently
in
consumer hands. The all-carbon MJ consisting of aromatic molecules
and oligomers between vapor-deposited carbon electrodes exploits covalent,
CâC bonding which avoids the electromigration problem of metal
contacts. The high bias and temperature stability as well as partial
transparency of the all-carbon MJ permit a wide range of experiments
to determine charge transport mechanisms and observe photoeffects
to both characterize and stimulate operating MJs. As shown in the
Conspectus figure, our group has reported a variety of electronic
functions, many of which do not have analogs in conventional semiconductors.
Much of the described research is oriented toward the rational design
of electronic functions, in which electronic characteristics are determined
by molecular structure.In addition to the fabrication of molecular
electronic devices
with sufficient stability and operating life for practical applications,
our approach was directed at two principal questions: how do electrons
move through molecules that are components of an electronic circuit,
and what can we do with molecules that we cannot do with existing
semiconductor technology? The central component is the molecular junction
consisting of a 1â20+ nm layer of covalently bonded oligomers
between two electrodes of conducting, mainly sp2-hybridized
carbon. In addition to describing the unique junction structure and
fabrication methods, this Account summarizes the valuable insights
available from photons used both as probes of device structure and
dynamics and as prods to stimulate resonant transport through molecular
orbitals.Short-range (<5 nm) transport by tunneling and
its properties
are discussed separately from the longer-range transport (5â60
nm) which bridges the gap between tunneling and transport in widely
studied organic semiconductors. Most molecular electronic studies
deal with the <5 nm thickness range, where coherent tunneling is
generally accepted as the dominant transport mechanism. However, the
rational design of devices in this range by changing molecular structure
is frustrated by electronic interactions with the conducting contacts,
resulting in weak structural effects on electronic behavior. When
the molecular layer thickness exceeds 5 nm, transport characteristics
change completely since molecular orbitals become the conduits for
transport. Incident photons can stimulate transport, with the observed
photocurrent tracking the absorption spectrum of the molecular layer.
Low-temperature, activationless transport of photogenerated carriers
is possible for up to at least 60 nm, with characteristics completely
distinct from coherent tunneling and from the hopping mechanisms proposed
for organic semiconductors. The Account closes with examples of phenomena
and applications enabled by molecular electronics which may augment
conventional microelectronics with chemical functions such as redox
charge storage, orbital transport, and energy-selective photodetection
Structure Controlled Long-Range Sequential Tunneling in Carbon-Based Molecular Junctions
Carbon-based
molecular junctions consisting of aromatic oligomers
between conducting sp<sup>2</sup> hybridized carbon electrodes exhibit
structure-dependent current densities (<i>J</i>) when the
molecular layer thickness (<i>d</i>) exceeds âŒ5 nm.
All four of the molecular structures examined exhibit an unusual,
nonlinear ln <i>J vs</i> bias voltage (<i>V</i>) dependence which is not expected for conventional coherent tunneling
or activated hopping mechanisms. All molecules exhibit a weak temperature
dependence, with <i>J</i> increasing typically by a factor
of 2 over the range of 200â440 K. Fluorene and anthraquinone
show linear plots of ln <i>J vs d</i> with nearly identical <i>J</i> values for the range <i>d</i> = 3â10
nm, despite significant differences in their free-molecule orbital
energy levels. The observed current densities for anthraquinone, fluorene,
nitroazobenzene, and bis-thienyl benzene for <i>d</i> =
7â10 nm show no correlation with occupied (HOMO) or unoccupied
(LUMO) molecular orbital energies, contrary to expectations for transport
mechanisms based on the offset between orbital energies and the electrode
Fermi level. UVâvis absorption spectroscopy of molecular layers
bonded to carbon electrodes revealed internal energy levels of the
chemisorbed films and also indicated limited delocalization in the
film interior. The observed current densities correlate well with
the observed UVâvis absorption maxima for the molecular layers,
implying a transport mechanism determined by the HOMOâLUMO
energy gap. We conclude that transport in carbon-based aromatic molecular
junctions is consistent with multistep tunneling through a barrier
defined by the HOMOâLUMO gap, and not by charge transport at
the electrode interfaces. In effect, interfacial âinjectionâ
at the molecule/electrode interfaces is not rate limiting due to relatively
strong electronic coupling, and transport is controlled by the âbulkâ
properties of the molecular layer interior
Solid State Spectroelectrochemistry of Redox Reactions in Polypyrrole/Oxide Molecular Heterojunctions
To understand the mechanism of bias-induced resistance
switching
observed in polypyrrole (PPy) based solid state junctions, in situ
UVâvis absorption spectroscopy was employed to monitor oxidation
states within PPy layers in solution and in PPy/metal oxide junctions.
For PPy layers in acetonitrile, oxidation led primarily to cationic
polaron formation, while oxidation in 0.1 M NaOH in H<sub>2</sub>O
resulted in imine formation, caused by deprotonation of polarons.
On the basis of these results in solution, spectroelectrochemistry
was used to monitor bias-induced formation of polarons and imines
in PPy layers incorporated into solid state carbon/PPy/Al<sub>2</sub>O<sub>3</sub>/Pt junctions. A positive bias on the carbon electrode
caused PPy oxidation, with the formation of polaron and imine species
strongly dependent on the surrounding environment. The spectral changes
associated with polarons or imines were stable for at least several
hours after the applied bias, while a negative bias reversed the absorbance
changes back to the initial PPy spectrum. These results indicate that
PPy can be oxidized in nominally solid state devices, and the formation
of stable polarons is dependent on the tendency for deprotonation
of the polaron to the imine. Since PPy conductivity depends strongly
on the polaron concentration, monitoring its concentration is critical
to determining resistance switching mechanisms. Furthermore, the importance
of ion mobility and OH<sup>â</sup> generation through H<sub>2</sub>O reduction at the Pt contact are discussed
Assembling Molecular Electronic Junctions One Molecule at a Time
Diffusion of metal atoms onto a molecular monolayer attached to a conducting surface permits electronic contact to the molecules with minimal heat transfer or structural disturbance. Surface-mediated metal deposition (SDMD) involves contact between âcoldâ diffusing metal atoms and molecules, due to shielding of the molecules from direct exposure to metal vapor. Measurement of the current through the molecular layer during metal diffusion permits observation of molecular conductance for junctions containing as few as one molecule. Discrete conductance steps were observed for 1â10 molecules within a monolayer during a single deposition run, corresponding to ârecruitmentâ of additional molecules as the contact area between the diffusing Au layer and molecules increases. For alkane monolayers, the molecular conductance measured with SDMD exhibited an exponential dependence on molecular length with a decay constant (ÎČ) of 0.90 per CH<sub>2</sub> group, comparable to that observed by other techniques. Molecular conductance values were determined for three azobenzene molecules, and correlated with the offset between the molecular HOMO and the contact Fermi level, as expected for hole-mediated tunneling. Currentâvoltage curves were obtained during metal deposition showed no change in shape for junctions containing 1, 2, and 10 molecules, implying minimal intermolecular interactions as single molecule devices transitioned into several molecules devices. SDMD represents a âsoftâ metal deposition method capable of providing single molecule conductance values, then providing quantitative comparisons to molecular junctions containing 10<sup>6</sup> to 10<sup>10</sup> molecules
Characterization of Growth Patterns of Nanoscale Organic Films on Carbon Electrodes by Surface Enhanced Raman Spectroscopy
Electrochemical
deposition of aromatic organic molecules by reduction
of diazonium reagents enables formation of molecular layers with sufficient
integrity for use in molecular electronic junctions of interest to
microelectronics. Characterization of organic films with thicknesses
in the 1â10 nm range is difficult with Raman spectroscopy,
since most molecular structures of electronic interest have Raman
cross sections which are too small to observe as either thin films
on solid electrodes or within intact molecular junctions. Layer formation
on a 10 nm thick Ag island film on a flat carbon surface (eC/Ag) permitted
acquisition of structural information using surface enhanced Raman
spectroscopy (SERS), in many cases for molecules with weak Raman scattering.
Raman spectra obtained on eC/Ag surfaces were indistinguishable from
those on carbon without Ag present, and the spectra of oligomeric
molecular layers were completely consistent with those of the monomers.
Layer growth was predominantly linear for cases where such growth
was sterically allowed, and linear growth correlated strongly with
the line width and splitting of the Cî»C phenyl ring stretches.
Molecular bilayers made by successive reduction of different diazonium
reagents were also observable and will be valuable for applications
of 1â20 nm organic films in molecular electronics
Orbital Control of Photocurrents in Large Area All-Carbon Molecular Junctions
Photocurrents
generated by illumination of carbon-based molecular
junctions were investigated as diagnostics of how molecular structure
and orbital energies control electronic behavior. Oligomers of eight
aromatic molecules covalently bonded to an electron-beam deposited
carbon surface were formed by electrochemical reduction of diazonium
reagents, with layer thicknesses in the range of 5â12 nm. Illumination
through either the top or bottom partially transparent electrodes
produced both an open circuit potential (OCP) and a photocurrent (PC),
and the polarity and spectrum of the photocurrent depended directly
on the relative positions of the frontier orbitals and the electrode
Fermi level (<i>E</i><sub>F</sub>). Electron donors with
relatively high HOMO energies yielded positive OCP and PC, and electron
acceptors with LUMO energies closer to <i>E</i><sub>F</sub> than the HOMO energy produced negative OCP and PC. In all cases,
the PC spectrum and the absorption spectrum of the oligomer in the
molecular junction had very similar shapes and wavelength maxima.
Asymmetry of electronic coupling at the top and bottom electrodes
due to differences in bonding and contact area cause an internal potential
gradient which controls PC and OCP polarities. The results provide
a direct indication of which orbital energies are closest to <i>E</i><sub>F</sub> and also indicate that transport in molecular
junctions thicker than 5 nm is controlled by the difference in energy
of the HOMO and LUMO orbitals
Light Emission as a Probe of Energy Losses in Molecular Junctions
Visible light emission was observed
for molecular junctions containing
5â19 nm thick layers of aromatic molecules between carbon contacts
and correlated with their currentâvoltage behaviors. Their
emission was compared to that from Al/AlOx/Au tunnel junctions, which
has been previously attributed to transport of carriers across the
AlOx layer to yield âhot carriersâ which emit light
as they relax within the Au contact. The maximum emitted photon energy
is equal to the applied bias for the case of coherent tunneling, and
such behavior was observed for light emission from AlOx and thin (<5
nm) molecular junctions. For thicker films, the highest energy observed
for emitted photons is less than <i>eV</i><sub>app</sub> and exhibits an energy loss that is strongly dependent on molecular
layer structure and thickness. For the case of nitroazobenzene junctions,
the energy loss is linear with the molecular layer thickness, with
a slope of 0.31 eV/nm. Energy loss rules out coherent tunneling as
a transport mechanism in the thicker films and provides a direct measure
of the electron energy after it traverses the molecular layer. The
transition from elastic transport in thin films to âlossyâ
transport in thick films confirms that electron hopping is involved
in transport and may provide a means to distinguish between various
hopping mechanisms, such as activated electron transport, variable
range hopping, and Poole Frankel transport
Direct Optical Determination of Interfacial Transport Barriers in Molecular Tunnel Junctions
Molecular
electronics seeks to build circuitry using organic components
with at least one dimension in the nanoscale domain. Progress in the
field has been inhibited by the difficulty in determining the energy
levels of molecules after being perturbed by interactions with the
conducting contacts. We measured the photocurrent spectra for large-area
aliphatic and aromatic molecular tunnel junctions with partially transparent
copper top contacts. Where no molecular absorption takes place, the
photocurrent is dominated by internal photoemission, which exhibits
energy thresholds corresponding to interfacial transport barriers,
enabling their direct measurement in a functioning junction
Internal Electric Field Modulation in Molecular Electronic Devices by Atmosphere and Mobile Ions
The
internal potential profile and electric field are major factors
controlling the electronic behavior of molecular electronic junctions
consisting of âŒ1â10 nm thick layers of molecules oriented
in parallel between conducting contacts. The potential profile is
assumed linear in the simplest cases, but can be affected by internal
dipoles, charge polarization, and electronic coupling between the
contacts and the molecular layer. Electrochemical processes in solutions
or the solid state are entirely dependent on modification of the electric
field by electrolyte ions, which screen the electrodes and form the
ionic double layers that are fundamental to electrode kinetics and
widespread applications. The current report investigates the effects
of mobile ions on nominally solid-state molecular junctions containing
aromatic molecules covalently bonded between flat, conducting carbon
surfaces, focusing on changes in device conductance when ions are
introduced into an otherwise conventional junction design. Small changes
in conductance were observed when a polar molecule, acetonitrile,
was present in the junction, and a large decrease of conductance was
observed when both acetonitrile (ACN) and lithium ions (Li<sup>+</sup>) were present. Transient experiments revealed that conductance changes
occur on a microsecondâmillisecond time scale, and are accompanied
by significant alteration of device impedance and temperature dependence.
A single molecular junction containing lithium benzoate could be reversibly
transformed from symmetric currentâvoltage behavior to a rectifier
by repetitive bias scans. The results are consistent with field-induced
reorientation of acetonitrile molecules and Li<sup>+</sup> ion motion,
which screen the electrodes and modify the internal potential profile
and provide a potentially useful means to dynamically alter junction
electronic behavior
Control of Electronic Symmetry and Rectification through Energy Level Variations in Bilayer Molecular Junctions
Two layers of molecular oligomers
were deposited on flat carbon
electrode surfaces by electrochemical reduction of diazonium reagents,
then a top contact applied to complete a solid-state molecular junction
containing a molecular bilayer. The structures and energy levels of
the molecular layers included donor molecules with relatively high
energy occupied orbitals and acceptors with low energy unoccupied
orbitals. When the energy levels of the two molecular layers were
similar, the device had electronic characteristics similar to a thick
layer of a single molecule, but if the energy levels differed, the
current voltage behavior exhibited pronounced rectification. Higher
current was observed when the acceptor molecule was biased negatively
in eight different bilayer combinations, and the direction of rectification
was reversed if the molecular layers were also reversed. Rectification
persisted at very low temperature (7 K), and was activationless between
7 and 100 K. The results are a clear example of a âmolecular
signatureâ in which electronic behavior is directly affected
by molecular structure and orbital energies. The rectification mechanism
is discussed, and may provide a basis for rational design of electronic
properties by variation of molecular structure