Progress with Molecular Electronic Junctions: Meeting Experimental Challenges in Design and Fabrication

Abstract

Molecular electronics seeks to incorporate molecular components as functional elements in electronic devices. There are numerous strategies reported to date for the fabrication, design, and characterization of such devices, but a broadly accepted example showing structure-dependent conductance behavior has not yet emerged. This progress report focuses on experimental methods for making both single-molecule and ensemble molecular junctions, and highlights key results from these efforts. Based on some general objectives of the field, particular experiments are presented to show progress in several important areas, and also to define those areas that still need attention. Some of the variable behavior of ostensibly similar junctions reported in the literature is attributable to differences in the way the junctions are fabricated. These differences are due, in part, to the multitude of methods for supporting the molecular layer on the substrate, including methods that utilize physical adsorption and covalent bonds, and to the numerous strategies for making top contacts. After discussing recent experimental progress in molecular electronics, an assessment of the current state of the field is presented, along with a proposed road map that can be used to assess progress in the future.

Molecular Series-Tunneling Junctions

“…The difference in β for aliphatic and aromatic carboxylates is in agreement with predictions based on molecular orbital (MO) theory. 2,4,8 Aromatic molecules are characterized by smaller energy gaps (∼3−5 eV) between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) than those of aliphatic molecules (∼7 eV) of similar length. 2,48 Furthermore, the HOMO of aromatic molecules aligns more favorably with the Fermi level of electrodes than does that of aliphatic molecules; this alignment, in a SAM-based tunneling junction, facilitates charge transport by lowering the effective height of the tunneling barrier.…”

Anomalously Rapid Tunneling: Charge Transport across Self-Assembled Monolayers of Oligo(ethylene glycol)

“…1- 14 Our studies have used a junction with the structure M/A-R-T//Ga 2 O 3 /EGaIn (where "A" is the group anchoring the molecules of the SAM to the metal "M", "R" is the backbone of the molecule making up the SAM, "T"…”

Nonequilibrium transport response from equilibrium transport theory

“…1,2 However, the consistent and thorough work of a range of experimental groups has allowed researchers to reach some consensus on the conductance values and variability of a few specific junctions. [3][4][5] In addition, the development of simulation codes based on a combination of density functional theory (DFT) and the nonequilibrium Green's function formalism (NEGF) [5][6][7][8][9][10][11][12] has enabled theoreticians to assist the above experiments with theoretical insights and predictions. However, the size and complexity of the experimentally active part of the junction is typically too large, and the above codes need to assume a number of simplifications regarding the size, geometry, number of feasible atomic arrangements, and the electronic correlations.…”