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

Abstract

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.

Molecules on Si: Electronics with Chemistry

“…[34,161,162] Experimentally, it is fairly simple to identify this transition bias (V T ) and indeed, in the study cited, [34,161,162] V T correlated well with the expected barrier height, the difference of the molecular level (highest occupied molecular orbital, HOMO) and the electrode Fermi level as deduced independently by UPS (see, however, Section 5.2). [52,161,162] This approach assumes a roughly linear J-V relation for tunneling transport [161] (i.e., positive ln(J/V 2 ) vs. 1/V slope), and in that case V T indeed indicates a change in transport mechanism to field-emission. However, as the barrier width/height ratio (which we called ''shape factor'' [149,159] ) increases, the tunneling J-V becomes more non-linear.…”

“…[149,159,161,162] One approach postulates that if the applied bias is larger than the barrier height, the transport mechanism changes from direct tunneling to field-emission. [34,161,162] Experimentally, it is fairly simple to identify this transition bias (V T ) and indeed, in the study cited, [34,161,162] V T correlated well with the expected barrier height, the difference of the molecular level (highest occupied molecular orbital, HOMO) and the electrode Fermi level as deduced independently by UPS (see, however, Section 5.2). [52,161,162] This approach assumes a roughly linear J-V relation for tunneling transport [161] (i.e., positive ln(J/V 2 ) vs. 1/V slope), and in that case V T indeed indicates a change in transport mechanism to field-emission.…”

A Molecular Half-Wave Rectifier

“…Their studies revealed that the dominant mechanism of charge transport across these junctions was tunneling for molecular wires shorter than ~ 4 nm, but the mechanism changed to hopping for molecular wires exceeding ~ 4 nm in length. Beebe et al 38,39 showed that A generally accepted mechanism to explain the difference in values of β in "insulating" media is super-exchange tunneling. 44 Interactions of the electrons with the orbitals of the organic molecule increase the probability of tunneling, and make "through-bond" tunneling more efficient than "through-space" tunneling.…”

“…Beebe et al 38,39 described a method to estimate this barrier height. Near zero bias, the barrier shape is rectangular and eq.…”

“…38,39 Beebe et al 38,39 argued that a transition of the mechanism of charge transport from tunneling (logarithmic) to field emission (linear)…”