Charge transport and rectification in molecular junctions formed with carbon-based electrodes

Abstract

Molecular junctions formed using the scanning-tunneling-microscope-based break-junction technique (STM-BJ) have provided unique insight into charge transport at the nanoscale. In most prior work, the same metal, typically Au, Pt, or Ag, is used for both tip and substrate. For such noble metal electrodes, the density of electronic states is approximately constant within a narrow energy window relevant to charge transport. Here, we form molecular junctions using the STM-BJ technique, with an Au metal tip and a microfabricated graphite substrate, and measure the conductance of a series of graphite/amine-terminated oligophenyl/Au molecular junctions. The remarkable mechanical strength of graphite and the single-crystal properties of our substrates allow measurements over few thousand junctions without any change in the surface properties. We show that conductance decays exponentially with molecular backbone length with a decay constant that is essentially the same as that for measurements with two Au electrodes. More importantly, despite the inherent symmetry of the oligophenylamines, we observe rectification in these junctions. State-of-art ab initio conductance calculations are in good agreement with experiment, and explain the rectification. We show that the highly energy-dependent graphite density of states contributes variations in transmission that, when coupled with an asymmetric voltage drop across the junction, leads to the observed rectification. Together, our measurements and calculations show how functionality may emerge from hybrid molecular-scale devices purposefully designed with different electrodes beyond the so-called "wide band limit," opening up the possibility of assembling molecular junctions with dissimilar electrodes using layered 2D materials.

Single-molecule conductance with nitrile and amino contacts with Ag or Cu electrodes

“…For comparison, Venkataraman et al found that the tunneling decay is 1.71 per phenyl group with Au-amine contact [5], while Wold et al reported 1.76 per phenyl group with Au-thiol contact by using conducting probe atomic force microscopy [31]. More recently, tunneling decay constant of 1.74 per phenyl group for Ag junctions was observed for oligophenyls with amine group [32]. Our results are thus in line with the literature.…”

New Insights into Single-Molecule Junctions Using a Robust, Unsupervised Approach to Data Collection and Analysis

“…Furthermore, given that the tip and substrate materials do not make physical contact, the method is ideal for studies of surface-orientated asymmetrical molecules (probing single-molecule rectification 36−38 ), on-surface (in situ) synthesized/modified components, 39,40 or experiments involving heterogeneous electrodes. 41, 42 In a first step toward pursuits along these lines, herein we report an automated I(s) data collection methodology (capable of recording 10 000 I(s) measurements in ∼8 h) and a novel, mathematically rigorous data sorting algorithm which makes no assumptions about plateau shape (processing 10 000 I(s) measurements in ∼2 h). With our approach, the number of measurements per experiment can be increased by 2 orders of magnitude compared to previous studies 5,13,15,[19][20][21]24 using this method.…”

Single-molecule diodes with high rectification ratios through environmental control

“…A single-molecule diode, a circuit element that directs current flow 4 , was first proposed more than 40 years ago 5 and consisted of an asymmetric molecule comprising a donor-bridge-acceptor architecture to mimic a semiconductor p-n junction. Several singlemolecule diodes have since been realized in junctions featuring asymmetric molecular backbones [6][7][8] , molecule-electrode linkers 9 or electrode materials 10 . Despite these advances, molecular diodes have had limited potential for applications due to their low conductance, low rectification ratios, extreme sensitivity to the junction structure and high operating voltages [7][8][9]11,12 .…”