Nanoscopic tunnel junctions were formed by contacting Au-, Pt-, or Ag-coated atomic force microscopy (AFM) tips to self-assembled monolayers (SAMs) of alkanethiol or alkanedithiol molecules on polycrystalline Au, Pt, or Ag substrates. Current-voltage traces exhibited sigmoidal behavior and an exponential attenuation with molecular length, characteristic of nonresonant tunneling. The length-dependent decay parameter, beta, was found to be approximately 1.1 per carbon atom (C(-1)) or 0.88 A(-)(1) and was independent of applied bias (over a voltage range of +/-1.5 V) and electrode work function. In contrast, the contact resistance, R(0), extrapolated from resistance versus molecular length plots showed a notable decrease with both applied bias and increasing electrode work function. The doubly bound alkanedithiol junctions were observed to have a contact resistance approximately 1 to 2 orders of magnitude lower than the singly bound alkanethiol junctions. However, both alkanethiol and dithiol junctions exhibited the same length dependence (beta value). The resistance versus length data were also used to calculate transmission values for each type of contact (e.g., Au-S-C, Au/CH(3), etc.) and the transmission per C-C bond (T(C)(-)()(C)).

Current-voltage measurements of metal-molecule-metal junctions formed from pi-conjugated thiols exhibit an inflection point on a plot of ln(I/V(2)) vs 1/V, consistent with a change in transport mechanism from direct tunneling to field emission. The transition voltage was found to scale linearly with the offset in energy between the Au Fermi level and the highest occupied molecular orbital as determined by ultraviolet photoelectron spectroscopy. Asymmetric voltage drops at the two metal-molecule interfaces cause the transition voltage to be dependent on bias polarity.

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.

Using conducting probe atomic force microscopy (CP-AFM), we have formed molecular tunnel junctions consisting of alkanethiols and alkane isonitrile self-assembled monolayers sandwiched between gold, platinum, silver, and palladium contacts. We have measured the resistance of these junctions at low bias (dV/dI |V=0) as a function of alkane chain length. Extrapolation to zero chain length gives the contact resistance, R0 . R0 is strongly dependent on the type of metal used for the contacts and decreases with increasing metal work function; that is, R0,Ag > R0,Au > R0,Pd > R0,Pt. R0 is approximately 10% smaller for Au junctions with isonitrile versus thiol surface linkers. We conclude that the Fermi level of the junction lies much closer to the HOMO than to the LUMO.

Understanding electron transport in metal-molecule-metal (MMM) junctions is of great importance for the advancement of molecular electronics. Critical factors that determine conductivity in a MMM junction include the nature of metal-molecule contacts and the electronic structure of the molecular backbone. We have studied the electronic transport property and the valence electronic structure on rigid, conjugated oligoacenes of increasing length with either thiol (-S) or isocyanide (-CN) linkers using conducting probe atomic force microscopy (CP-AFM) and ultraviolet photoelectron spectroscopy (UPS). We find that for these conjugated systems the Au-CN contact is more resistive than Au-S. The difference in contact resistance correlates with UPS measurements that show the highest-occupied molecular orbital (HOMO) of the isocyanide series is lower in energy (relative to the Fermi level of Au) than the HOMO of the thiol series, indicating the presence of a higher tunneling barrier at the contact for the isocyanide-linked molecules. By contrast, the difference in the HOMO positions for the two series of molecules does not appear to affect the length dependence of the junction resistance (i.e., the beta value = 0.5 A-1).

We report the electrical transport behavior of a series of redox-active conjugated molecular wires as a function of temperature and molecular length. The wires consist of covalently coupled ruthenium(II) bis(σ-arylacetylide) complexes (Ru1-Ru3) and range in length from 2.4 to 4.9 nm. The molecules are unique in that they contain multiple metal-redox centers that are well-coupled by conjugated ligands. The molecules were self-assembled and their films were extensively characterized using ellipsometry, X-ray photoelectron spectroscopy, reflectionabsorption infrared spectroscopy, and cyclic voltammetry. We probed their electrical properties using conducting probe atomic force microscopy and crossed-wire junctions. At room temperature, we found a very weak dependence of the wire resistance with molecular length, consistent with a high degree of electronic communication along the molecular backbone. In low-temperature (5 K) experiments, Coulomb blockadelike behavior was observed in junctions incorporating Ru3; direct tunneling appears to be the dominant transport mechanism in Ru1 and Ru2 junctions.

Alkanethiol tunnel junctions were studied using conducting-probe atomic force microscopy to determine causes of variability in measured resistance behavior. Measurements were made on Au/decanethiol/Au monolayer junctions, and effects of substrate roughness, tip chemistry, presence of solvent, extensive tip usage, applied load, and tip radius were examined. Resistance measurements yielded log-normal distributions under a variety of conditions, indicating that the origin of the variance is likely to be either changes in tunneling length or electronic overlap. Spreads in resistance values for a given tip were much less when flat, template-stripped Au substrates were used rather than rough, evaporated Au substrates. Chemical modification of tips with ethanethiol (C2) or butanethiol (C4) and performing measurements under cyclohexane were also found to reduce variance by a factor of about 2-4. Experiments performed with unmodified tips showed an increase in junction resistance over the course of hundreds of consecutive measurements, whereas junctions made with modified tips or under cyclohexane did not. Attempts to ascribe variance between tips to varying tip radii failed; however, decreases in resistance with increasing applied load on the tip contact were observed and could be interpreted in terms of conventional contact mechanics models.

Using inelastic electron tunneling spectroscopy (IETS) to measure the vibronic structure of nonequilibrium molecular transport, aided by a quantitative interpretation scheme based on Green's functiondensity functional theory methods, we are able to characterize the actual pathways that the electrons traverse when moving through a molecule in a molecular transport junction. We show that the IETS observations directly index electron tunneling pathways along the given normal coordinates of the molecule. One can then interpret the maxima in the IETS spectrum in terms of the specific paths that the electrons follow as they traverse the molecular junction. Therefore, IETS measurements not only prove (by the appearance of molecular vibrational frequencies in the spectrum) that the tunneling charges, in fact, pass through the molecule, but also can be used to determine the transport pathways and how they change with the geometry and placement of molecules in junctions. molecular electronics ͉ molecular junctions ͉ molecular transport T he electron-transfer process is crucial in chemistry, materials science, condensed matter physics, and electrical engineering. Although it is always modeled either explicitly or implicitly by pathways (how electrons actually move within the molecule), there is as yet no direct measurement or observation of such pathways. The pathways idea has been present in physical organic chemistry for years in connection with reaction mechanisms and has been widely used in the interpretation of electron tunneling pathways in proteins (1), but no distinct observations have been made. The absence of direct measurement of pathways is because the measurements are usually made starting with an equilibrium structure, exciting quickly (optical spectroscopy), and then observing the new perturbed structure. Although it is instructive to observe these initial and final states, they are static snapshots and cannot capture the dynamics of the electrontransport process. In molecular transport junctions, where current is moving continuously through the molecule, the nonequilibrium inelastic electron tunneling spectroscopy (IETS) probe permits direct observation of how different modes modulate the transport and, therefore, can be used to deduce actual pathways.It is well established that tunneling electrons can lose energy through excitation of a molecular vibrational level contained within the tunnel junction (2-5). The threshold for such excitation is eV ϭ -h where V is the bias voltage and -h is the energy of the molecular vibration. Peaks in d 2 I/dV 2 versus V, or more commonly the normalized quantity (d 2 I/dV 2 )/(dI/dV) versus V, correspond to molecular vibrations. IETS has become quite popular in the field of molecular electronics over the last 3 years (6-9) and has distinguished itself as a unique spectroscopic probe of molecular junctions. Because an IET spectrum is acquired directly from the measured transport characteristics (Fig. 1), the only added experimental requirement is the ability to cool the ju...