A critical perspective on molecular electronic junctions: there is plenty of room in the middle

Abstract

The promise of molecular electronic devices stems from the possibilities offered by the rich electronic structure of organic molecules. The use of molecules as functional components in microelectronic devices has long been envisioned to augment or even replace silicon. However, the understanding of what controls charge transport in these devices involves complexities stemming from numerous variables that are often interactive and exert a controlling influence on transport, confounding the role of the molecular component. This perspective discusses various aspects of molecular electronics, from the initial "vision quests" of single molecule, functional electronic elements, to the molecular tunnel junctions that have been studied and characterized in-depth. Aspects of energy level alignment are discussed in the context of charge transport mechanisms, as are important electronic interactions when molecules are bonded to conducting "contacts". In addition, integration of molecular components with microelectronic processing is considered, as are the prospects for functional, real-world devices.

Characterizing the Metal–SAM Interface in Tunneling Junctions

“…[6][7][8][9][10][11][12][13][14][15][16][17][18] Recent studies of singlemolecule break junctions have been interpreted to indicate that the presence of covalent Au−C σ-bonds-formed using trimethyltin (-SnMe 3 )-terminated n-alkyl groups, 19,20 and SnMe 3 -terminated aromatics [19][20][21] or trimethylsilyl (TMS)-terminated conjugated systems 22 -increases rates of charge transport across these junctions by approximately a factor of 10-100, relative to amine or thiolate anchoring groups. One possible inference from the increase is that the Au−C σ-bond, and the absence of resistive anchoring heteroatoms, increases "conductivity" (although the meaning of this word is not entirely clear for tunneling junctions).…”

Defining the Value of Injection Current and Effective Electrical Contact Area for EGaIn-Based Molecular Tunneling Junctions

“…We point out that many of the results for molecular tunneling described in the earlier literature appear to rely on single, selected data points, or small numbers of points, and are literally uninterpretable because they do not distinguish between statistically defined values (means based on large numbers of data) and outliers or artifacts (which may be displaced many orders of magnitude from the mean). 18,45 Electrical Structure of the Ag TS -SAM//Ga 2 O 3 /EGaIn Junction. For a fixed voltage, we can consider junctions having structure Ag TS -SAM//Ga 2 O 3 /EGaIn as being two resistors in series ( Figure 2): the Ga 2 O 3 layer, with specific resistance R Ga 2 O 3 , and the tunnel gap established by the SAM, which has specific resistance R SAM .…”

Internal Photoemission in Molecular Junctions: Parameters for Interfacial Barrier Determinations

“…Moreover, it has become clear that interactions between the molecules and conductors in a molecular device lead to the requirement to consider a molecular junction as a system, where the properties of the individual, isolated components are insufficient or misleading for predicting the behavior of a completed device. 23,24 Since the specific design of electronic function has been a primary goal in molecular electronics, 12,25 information regarding system energy levels in complete, functioning devices, including possible interactions between the molecules and the contacts, is critical to further progress. Several recent reports on in situ characterization of molecular junctions have been published, including UV−vis, 26,27 infrared 28−31 and Raman spectroscopy, 32,33 thermopower measurements, 34,35 and inelastic tunnel-ing spectroscopy.…”