Electron Tunneling through Fluid Solvents

Abstract

Despite the biological, chemical, and physical importance of electron tunneling across noncovalent matrices, relatively little is known about the ability of the various nonbonding interactions (hydrogen-bonding and van der Waals forces) to mediate charge transfer. Herein, we report the steady-state current-voltage (I-V) profiles of nanometer junctions filled with water and a variety of organic solvents. The maximum currents for the solvents studied span 6 orders of magnitude. The I-V data can be reasonably fit to a simple electron tunneling model with a rectangular energy barrier representing the solvent. Protic solvents provide the smallest barrier heights (greatest tunneling currents), and nonpolar solvents exhibit the largest energy barriers (lowest currents). Trends in the barrier heights with the strength of the solvent-solvent interactions (hydrogen-bonding < dipoledipole < dispersion interactions) indicate that the solvent's cohesive energy largely determines/limits the barrier heights of the fluid systems rather than the electronic structure of the solvent molecule (e.g., electron affinity or ionization potential). These results demonstrate that facile electron tunneling through nonbonding media must be accompanied by relatively strong intermolecular interactions.

Free Standing Nanocrossbar Arrays with Molecular Throughput

“…25 The top wires of the crossbar may relax with a small structural change resulting in somewhat smaller gap sizes than predicted by the chromium layer. 23 The currents of the vacant junctions are exponentially dependent on potential which is consistent with a tunneling behavior. 23 25 By contrast, the chromium-filled devices exhibit a linear I-V behavior characteristic of a conventional resistor.…”

“…25 If the geometric differences are assumed to occur entirely in the vertical separation and the crosssectional areas (wire widths) are assumed to be constant, the variability in the nanojunctions' vertical separations can be estimated. A cross-sectional area of 1600 nm 2 (40 nm wide wires) and the reported barrier height of toluene (3.38 eV) 23 yields a more narrow distribution of the gap sizes, Figure 5(B). With these currents, the calculations yield vertical separations that span less than 0.5 nm for the 36 nodes in the 6 × 6 array.…”

“…For example, electromigration techniques yield arrays of particle-free nanogaps with nearidentical electronic properties. 14 [18][19][20][21][22][23] Some of these approaches may even be applicable to creating arrays of vacant nanogaps in the future. In some instances the partial removal of an insulating layer leads to an unexpected decrease in the junction's resistance.…”

Probing Water Structures in Nanopores Using Tunneling Currents

“…Below 4.5Å water is completely excluded from the region between the electrodes and above 9.25Å we obtain sub-pA average currents which cannot be easily detected with present techniques. As a medium water acts to reduce the effective barrier height to about 1 eV [10,23], much lower than the work function of gold (4.3 eV [24]). To emphasize this point we have calculated the current of a rectangular tunneling barrier in which the barrier height is the work function of gold and the barrier width is the diameter minus twice the distance between the edge of a jellium electron model with the gold density (r s = 3) and the center of the closest plane of gold atoms [25].…”

Tunneling Junctions as Molecular Sensors

“…Recently, we reported the relative tunneling barriers of fluid solvents by measuring the steady-state current-voltage curves of nanojunctions immersed in a variety of organic liquids (7). The tunneling currents spanned over six orders of magnitude for a range of 25 solvents.…”