Molecular Electronic Junctions Research Articles

Electron Transport and Redox Reactions in Molecular Electronic Junctions

Electron transport through single molecules or collections of molecules oriented in parallel can occur by several mechanisms, including coherent tunneling, activated transfer between potential wells, various "hopping" modes, etc. Given suitable energy levels and sufficiently long charge transport times, reduction or oxidation with accompanying nuclear reorganization can occur to generate "polarons", that is, localized redox centers in the molecule or monolayer. Redox events in molecular junctions are amenable to spectroscopic monitoring in working devices, and can have major effects on the electronic behavior of the junction. Several examples are presented, along with a possible application to molecular memory.

Anomalous Tunneling in Carbon/Alkane/TiO2/Gold Molecular Electronic Junctions: Energy Level Alignment at the Metal/Semiconductor Interface

Carbon/TiO(2)/gold electronic junctions show slightly asymmetric electronic behavior, with higher current observed in current density (J)/voltage (V) curves when carbon is biased negative with respect to the gold top contact. When a approximately 1-nm-thick alkane film is deposited between the carbon and TiO(2), resulting in a carbon/alkane/TiO(2)/gold junction, the current increases significantly for negative bias and decreases for positive bias, thus creating a much less symmetric J/V response. Similar results were obtained when SiO(2) was substituted for the alkane layer, but Al(2)O(3) did not produce the effect. The observation that, by the addition of an insulating material between carbon and TiO(2), the junction becomes more conductive is unexpected and counterintuitive. Kelvin probe measurements revealed that while the apparent work function of the pyrolyzed photoresist film electrode is modulated by surface dipoles of different surface-bound molecular layers, the anomalous effect is independent of the direction of the surface dipole. We propose that by using a nanometer-thick film with a low dielectric constant as an insertion layer, most of the applied potential is dropped across this thin film, thus permitting alignment between the carbon Fermi level and the TiO(2) conduction band. Provided that the alkane layer is sufficiently thin, electrons can directly tunnel from carbon to the TiO(2) conduction band. Therefore, the electron injection barrier at the carbon/TiO(2) interface is effectively reduced by this energy-level alignment, resulting in an increased current when carbon is biased negative. The modulation of injection barriers by a low-kappa molecular layer should be generally applicable to a variety of materials used in micro- and nanoelectronic fabrication.

Nanoencapsulation and Stabilization of Single-Molecule/Particle Electronic Nanoassemblies Using Low-Temperature Atomic Layer Deposition

This work addresses a significant challenge in engineered molecular systems regarding both understanding and controlling the stability of molecule/nanoparticle nanostructures under ambient exposure. Results deal specifically with molecular electronic junctions, where electronic contacts and transport are known to be sensitive to sample history and ambient exposure. We demonstrate that low-temperature atomic layer deposition can gently encapsulate and stabilize molecular electronic junctions, making it feasible to handle and transport junctions in air for many days with minimal change in electronic conduction. These findings indicate the potential for long-term stability of advanced synthetic nanoparticle/molecule nanoconstructs. For this study, conductivity through nanoparticle/molecule/nanoparticle junctions is analyzed and found to be consistent with nonresonant charge tunneling through a single or a small number of oligomeric phenylene ethynylene molecules in the electrical junction. The conductivity was stable in vacuum and inert gas, but under ambient exposure, the current initially decreased, then increased rapidly, followed by a slower rise, reaching a value exceeding 10 times larger than initially measured. After encapsulating functional devices using atomic layer deposition of aluminum oxide thin films at 30−50 °C, the junction showed conductance similar to the precoated values, and the current remained unchanged after more than 15 days under ambient exposure. The presence of the molecule junction after encapsulation was confirmed by the observed transition to Fowler−Nordheim tunneling and analysis of junction breakdown at high fields. The high conformality, precise thickness control, and low-temperature compatibility of the atomic layer deposition method make it uniquely qualified to stabilize and protect molecular junctions and systems.

Conducting Polymer Memory Devices Based on Dynamic Doping

Molecular electronic junctions consisting of a 20 nm thick layer of polypyrrole (PPy) and 10 nm of TiO2 between conducting layers of carbon and gold were investigated as potential nonvolatile memory devices. By making the polymer layer much thinner than conventional polymer electronic devices, it is possible to dynamically oxidize and reduce the polypyrrole layer by an applied bias. When the electrode in contact with the PPy is biased positive, oxidation of the PPy occurs to yield a conducting polaron state. The junctions exhibit a large increase in conductance in response to the positive bias, which is reversed by a subsequent negatively biased pulse. Switching between the conducting and nonconducting state can occur for pulses at least as short as 10 micros, and the conducting state persists after a positive bias pulse for at least 1 week. The read/write/read/erase cycle may be repeated for at least 1700 cycles, although with an error rate of approximately 3% due mainly to an incomplete "erase" step. The speed and retention of the PPy/TiO2 junctions are far superior to those of the analogous fluorene/TiO2 devices lacking the polymer, and the conductance changes are absent if SiO2 is substituted for TiO2. The observations are consistent with "dynamic doping" of the solid-state polymer layer, with the possible involvement of adventitious mobile ions. Although the speed of the current polymer/TiO2 junctions is slower than commercial dynamic random access memory, their retention is approximately 5 orders of magnitude longer.

In-Situ Optical Absorbance Spectroscopy of Molecular Layers in Carbon Based Molecular Electronic Devices

In-situ optical absorbance spectroscopy was used to monitor transparent carbon based molecular electronic junctions with various molecular and metal oxide layers. Junctions with molecular layers consisting of N-decylamine (C10N) and fluorene (FL) did not show absorbance changes upon the application of voltage pulses. Junctions with molecular layers consisting of 4-nitroazobenzene (NAB) and 9,10anthraquinone (AQ) showed absorbance changes upon the application of voltage pulses which were reversible for at least tens of cycles. For NAB junctions, a negative voltage pulse caused an increase in absorbance at 410 nm and a decrease in absorbance at 360 nm. For AQ junctions, a negative voltage pulse caused an absorbance increase at 395 nm and a decrease in absorbance at 320-350 nm. These absorbance changes are consistent with the reduction of the NAB and AQ layers when the carbon substrate is biased negative. Positive voltage pulses reversed the absorbance changes observed during a negative pulse which is consistent with the reoxidation of the molecular layer. The persistence of the absorbance changes depended strongly on the molecule, with absorbance changes persisting for tens of minutes for NAB junctions but only several seconds for AQ junctions. The in-situ optical absorption results are supported with solution based electrochemistry of both free molecules and chemisorbed molecular layers and time-dependent density functional theory. We have shown that in-situ optical absorbance spectroscopy can be used to probe changes in energy levels through absorption changes in biased molecular junctions, which should be useful for deducing structural and electronic changes that strongly effect electron transfer in molecular electronic devices.

Measuring Relative Barrier Heights in Molecular Electronic Junctions with Transition Voltage Spectroscopy

Though molecular devices exhibiting potentially useful electrical behavior have been demonstrated, a deep understanding of the factors that influence charge transport in molecular electronic junctions has yet to be fully realized. Recent work has shown that a mechanistic transition occurs from direct tunneling to field emission in molecular electronic devices. The magnitude of the voltage required to enact this transition is molecule-specific, and thus measurement of the transition voltage constitutes a form of spectroscopy. Here we determine that the transition voltage for a series of alkanethiol molecules is invariant with molecular length, while the transition voltage of a conjugated molecule depends directly on the manner in which the conjugation pathway has been extended. Finally, by examining the transition voltage as a function of contact metal, we show that this technique can be used to determine the dominant charge carrier for a given molecular junction.

Stochastic Modulation in Molecular Electronic Transport Junctions: Molecular Dynamics Coupled with Charge Transport Calculations

The experimental variation in conductance that can be expected through dynamically evolving Au-molecule-Au junctions is approximated using molecular dynamics to model thermal fluctuations and a nonequilibrium Green's function code (Hückel-IV 2.0) to calculate the charge transport. This generates a statistical set of conductance data that can be used to compare directly with experimental results. Experimental measurements on Au-single molecule junctions show a large variation in conductance values between different identically prepared junctions. Our computational results indicate that the Au-Au and the Au-molecule fluctuations provide extensive geometric freedom and an associated broad distribution in calculated conductance values. Our results show agreement with experimental measurements of the low bias voltage conductance and conductance distribution for both thiol-Au and amine-Au linker structures. -

Electron spectra of a self-assembled monolayer on gold: Inverse photoemission and two-photon photoemission spectroscopy

Ultraviolet photoemission spectroscopy, inverse photoemission spectroscopy, and two-photon photoemission spectroscopy are techniques commonly used to examine the electronic structure of surfaces and interfaces. We have used all three to study self-assembled monolayers of 4,4′-bis(phenylethynyl)benzenethiol on gold, a system often studied for potential application in molecular electronic junctions. We compare the results of these measurements and discuss them in relation to electronic structure calculations. The result is a better understanding of the valence electronic structure for this system, of the spectroscopic methods involved, and of previous measurements.

Normal and Surface-Enhanced Raman Spectroscopy of Nitroazobenzene Submonolayers and Multilayers on Carbon and Silver Surfaces

Raman and ultraviolet-visible (UV-Vis) absorption spectra were obtained for nitroazobenzene (NAB) chemisorbed on smooth and rough silver, and they were compared to published spectra for NAB on sp(2) hybridized pyrolyzed photoresist film (PPF) surfaces. High signal-to-noise ratio Raman spectra were obtained for 4.5 nm thick NAB films on PPF and smooth Ag due to significant enhancement of the NAB scattering relative to that observed in solution. The UV-Vis spectra of chemisorbed NAB exhibited a significant shift toward longer wavelength, thus bringing the NAB absorption closer to the 514.5 nm laser wavelength. The red shift was larger for PPF than for smooth Ag, consistent with the approximately 5x stronger Raman signal obtained on PPF. Deposition of Ag onto quartz without a chromium adhesion layer produced a rough Ag surface that enhanced the Raman spectrum of chemisorbed NAB by a factor of approximately 1000, as expected for roughened Ag due to electromagnetic field enhancement. The strong Raman signal permitted observation of NAB at low coverage and revealed changes in the NAB spectrum as the film progressed from submonolayer to multilayer thicknesses. Finally, deposition of Ag onto PPF/NAB samples through a metal grid produced Ag squares on top of the NAB, which enhanced the Raman scattering of the NAB layer by a factor of approximately 100. Deposition of a final conducting film on the Ag squares should permit in situ observation of a wide range of molecules in operating molecular electronic junctions.

Vibronic Coupling in Semifluorinated Alkanethiol Junctions: Implications for Selection Rules in Inelastic Electron Tunneling Spectroscopy

Determining the selection rules for the interaction of tunneling charge carriers with molecular vibrational modes is important for a complete understanding of charge transport in molecular electronic junctions. Here, we report the low-temperature charge transport characteristics for junctions formed from hexadecanethiol molecules having varying degrees of fluorination. Our results demonstrate that C-F vibrations are not observed in inelastic electron tunneling spectroscopy (IETS). Because C-F vibrations are almost purely dipole transitions, the insensitivity to fluorine substitution implies that Raman modes are preferred over infrared modes. Further, the lack of attenuation of the C-H vibrational modes with fluorine substitution suggests that either the scattering cross section is not an additive quantity or the physical position of a vibrational mode within the junction influences whether the transition is observed in IETS.

Internal Structure of a Molecular Junction Device: Chemical Reduction of PtO2 by Ti Evaporation onto an Interceding Organic Monolayer

We present a physical and chemical characterization study of molecular electronic junctions previously shown1,2 to display tunable hysteric electrical switching behavior useful for nanoscale memory and logic circuitry applications.3-6 X-ray photoelectron spectroscopy (XPS) data were acquired from the internal buried interfaces in a series of related device structures. This was accomplished using a new technique based on stripping (delaminating) the molecular device stacks at the molecular layer for XPS examination in ultrahigh vacuum (UHV). These data reveal that evaporative deposition of titanium onto the organic monolayers in these junctions results in the chemical reduction of the 2.5 nm platinum oxide film underlying the monolayers. This understanding allows us to construct an improved model for the internal chemical structure of these electrically well-studied molecular junctions, and to suggest a new model for how the evaporation of titanium as the top contact in molecular electronic devices prevent...

Redox-Driven Conductance Switching via Filament Formation and Dissolution in Carbon/Molecule/TiO2/Ag Molecular Electronic Junctions

Carbon/molecule/TiO2/Au molecular electronic junctions show robust conductance switching, in which a metastable high conductance state may be induced by a voltage pulse which results in redox reactions in the molecular and TiO2 layers. When Ag is substituted for Au as the "top contact", dramatically different current/voltage curves and switching behavior result. When the carbon substrate is biased negative, an apparent breakdown occurs, leading to a high conductance state which is stable for at least several hours. Upon scanning to positive bias, the conductance returns to a low state, and the cycle may be repeated hundreds of times. Similar effects are observed when Cu is substituted for Au and for three different molecular layers as well as "control" junctions of the type carbon/TiO2/Ag/Au. The polarity of the "switching" is reversed when the Ag layer is between the carbon and molecular layers, and the conductance change is suppressed at low temperature. Pulse experiments show very erratic transitions between high and low conductivity states, particularly near the switching threshold. The results are consistent with a switching mechanism based on Ag or Cu oxidation, transport of their ions through the TiO2, and reduction at the carbon to form a metal filament.

Electron Transport and Redox Reactions in Carbon-based Molecular Electronic Junctions

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Molecular Electronic Junctions with Nanotubes

Molecular Electronic Junctions with Nanotubes

Carbon/molecule/metal molecular electronic junctions: the importance of “contacts”

Molecular electronic junctions fabricated by covalent bonding onto a graphitic carbon substrate were examined with Raman spectroscopy and characterized electronically. The molecular layer was a 4.5 nm thick multilayer of nitroazobenzene (NAB), and the top contact material was varied to investigate its effect on junction behavior. A 3.0 nm thick layer of copper, TiO2, or Al(III) oxide (AlO(x)) was deposited on top of the NAB layer, followed by a 7.0 nm thick layer of gold. Copper "contacts" yielded molecular junctions with low resistance and showed a strong dependence on molecular structure. Carbon/ NAB/AlO(x)/Au junctions exhibited high resistance, with current densities three orders of magnitude less than those for analogous Cu junctions. However, Raman spectroscopy revealed that the NAB layer was reduced when the carbon substrate was biased negative, to a product resembling that resulting from electrochemical reduction of NAB. Carbon/ NAB/TiO2/Au junctions showed rectifying J/V behavior, with high conductivity to electrons able to enter the TiO2 conduction band. Substitution of azobenzene for nitroazobenzene yielded junctions with similar spectroscopic and electronic behavior to NAB, indicating that the nitro group is not essential for rectification. The results are interpreted in terms of the energy levels of the molecule relative to those of TiO2. The combination of a covalently bonded molecular layer and a semiconducting oxide yields unusual electronic properties in a carbon/molecule/semiconductor/Au molecular junction.

Electron transport and redox reactions in carbon-based molecular electronic junctions

A unique molecular junction design is described, consisting of a molecular mono- or multilayer oriented between a conducting carbon substrate and a metallic top contact. The sp2 hybridized graphitic carbon substrate (pyrolyzed photoresist film, PPF) is flat on the scale of the molecular dimensions, and the molecular layer is bonded to the substrate via diazonium ion reduction to yield a strong, conjugated C-C bond. Molecular junctions were completed by electron-beam deposition of copper, titanium oxide, or aluminium oxide followed by a final conducting layer of gold. Vibrational spectroscopy and XPS of completed junctions showed minimal damage to the molecular layer by metal deposition, although some electron transfer to the molecular layer resulted in partial reduction in some cases. Device yield was high (>80%), and the standard deviations of junction electronic properties such as low voltage resistance were typically in the range of 10-20%. The resistance of PPF/molecule/Cu/Au junctions exhibited a strong dependence on the structure and thickness of the molecular layer, ranging from 0.13 ohms cm2 for a nitrobiphenyl monolayer, to 4.46 ohms cm2 for a biphenyl monolayer, and 160 ohms cm2 for a 4.3 nm thick nitrobiphenyl multilayer. Junctions containing titanium or aluminium oxide had dramatically lower conductance than their PPF/molecule/Cu counterparts, with aluminium oxide junctions exhibiting essentially insulating behavior. However, in situ Raman spectroscopy of PPF/nitroazobenzene/AlO(x)/Au junctions with partially transparent metal contacts revealed that redox reactions occurred under bias, with nitroazobenzene (NAB) reduction occurring when the PPF was biased negative relative to the Au. Similar redox reactions were observed in PPF/NAB/TiO(x)/Au molecular junctions, but they were accompanied by major effects on electronic behavior, such as rectification and persistent conductance switching. Such switching was evident following polarization of PPF/molecule/TiO2/Au junctions by positive or negative potential pulses, and the resulting conductance changes persisted for several minutes at room temperature. The "memory" effect implied by these observations is attributed to a combination of the molecular layer and the TiO2 properties, namely metastable "trapping" of electrons in the TiO2 when the Au is negatively biased.

Coupled Surface-Enhanced Raman Spectroscopy and Electrical Conductivity Measurements of 1,4-Phenylene Diisocyanide in Molecular Electronic Junctions

Probing the structure of molecules in a metal-molecule-metal junction under an applied voltage is critical for understanding molecular electron transport properties. We present an approach that allows recording surface-enhanced Raman spectra simultaneously with electrical measurements of a monolayer of molecules in molecular electronic junctions. 1,4-Phenylene diisocyanide in two different types of junctions was used to illustrate the approach. The results show that the molecular integrity was intact in the molecular junctions and under the applied bias. The monolayer sensitivity of the approach provides a new powerful tool for characterizing molecular structure in a molecular electronic junction.

Carbon/Molecule/Metal and Carbon/Molecule/Metal Oxide Molecular Electronic Junctions

Molecular junctions were fabricated on the basis of a 1.7−4.5 nm thick layer of fluorene (FL) or nitroazobenzene (NAB) covalently bonded to a graphitic pyrolyzed photoresist film (PPF) substrate. The junction was completed with a top contact consisting of metallic Cu, TiO2, or aluminum(III) oxide (AlOx) and a final layer of Au. The current/voltage behavior of the junctions depended strongly both on the nature of the metal or metal oxide top layers and on the structure of the molecular layer. PPF/NAB/Cu/Au and PPF/FL/Cu/Au junctions were highly conducting, with resistances of 0.3−1.7 Ω cm2, depending on the identity and thickness of the molecular layer. Substitution of Cu with either AlOx or TiO2 caused a large increase in junction resistance by 2−4 orders of magnitude, but also yielded rectifying junctions in the case of PPF/NAB(4.5)/TiO2(3.1)/Au. For a positive bias (PPF relative to Au) above +2 V, the NAB(4.5)/TiO2(3.1) junction became highly conductive, apparently due to injection of electrons into the...

Strong Effects of Molecular Structure on Electron Transport in Carbon/Molecule/Copper Electronic Junctions

Carbon/molecule/copper molecular electronic junctions were fabricated by metal deposition of copper onto films of various thicknesses of fluorene (FL), biphenyl (BP), and nitrobiphenyl (NBP) covalently bonded to flat, graphitic carbon. A "crossed-wire" junction configuration provided high device yield and good junction reproducibility. Current/voltage characteristics were investigated for 69 junctions with various molecular structures and thicknesses and at several temperatures. The current/voltage curves for all cases studied were nearly symmetric, scan rate independent, repeatable at least thousands of cycles and exhibited negligible hysteresis. Junction conductance was strongly dependent on the dihedral angle between phenyl rings and on the nature of the molecule/copper "contact". Junctions made with NBP showed a decrease in conductivity of a factor of 1300 when the molecular layer thickness increased from 1.6 to 4.5 nm. The slope of ln(i) vs layer thickness for both BP and NBP was weakly dependent on applied voltage and ranged from 0.16 to 0.24 A(-1). These attenuation factors are similar to those observed for similar molecular layers on modified electrodes used to study electrochemical kinetics. All junctions studied showed weak temperature dependence in the range of approximately 325 to 214 K, implying activation barriers in the range of 0.06 to 0.15 eV. The carbon/molecule/copper junction structure provides a robust, reproducible platform for investigations of the dependence of electron transport in molecular junctions on both molecular structure and temperature. Furthermore, the results indicate that junction conductance is a strong function of molecular structure, rather than some artifact resulting from junction fabrication.

In Situ Raman Spectroscopy of Bias-Induced Structural Changes in Nitroazobenzene Molecular Electronic Junctions

Carbon/nitroazobenzene (NAB)/titanium/gold molecular electronic junctions with active thicknesses of 7-8 nm were constructed having partially transparent Ti/Au top contacts, which permitted in situ monitoring of molecular structure with Raman spectroscopy for applied biases between +3 and -3 V. Deposition of the Ti/Au top contacts resulted in spectral changes similar to those accompanying NAB reduction in a conventional spectroelectrochemical experiment. Upon application of +3 V (carbon relative to Ti), the spectrum indicated reoxidation of the NAB reduction product, and this redox cycle could be repeated at least three times. When a voltage excursion to -2 or -3 V occurred, the spectra indicated irreversible loss of the nitro group, and a dramatic but reversible decrease in Raman intensity over the entire shift range examined. Negative applied voltage causes formation of reduced NAB and a high oxidation state of titanium, while positive voltage forms oxidized NAB and injects electrons into the titanium oxide layer. The spectral changes were correlated with current/voltage curves in order to probe the mechanism of rectification and conductance switching reported previously. Overall, the combination of spectroscopic and voltammetric results implies a conduction mechanism involving both NAB and titanium oxide, possibly mediated by the injection of carriers into the semiconducting titanium oxide, or by reduction of an insulating titanium oxide to a more conductive form.