Mapping the Transmission Functions of Single-Molecule Junctions

Abstract

Charge transport phenomena in single-molecule junctions are often dominated by tunneling, with a transmission function dictating the probability that electrons or holes tunnel through the junction. Here, we present a new and simple technique for measuring the transmission functions of molecular junctions in the coherent tunneling limit, over an energy range of 1.5 eV around the Fermi energy. We create molecular junctions in an ionic environment with electrodes having different exposed areas, which results in the formation of electric double layers of dissimilar density on the two electrodes. This allows us to electrostatically shift the molecular resonance relative to the junction Fermi levels in a manner that depends on the sign of the applied bias, enabling us to map out the junction's transmission function and determine the dominant orbital for charge transport in the molecular junction. We demonstrate this technique using two groups of molecules: one group having molecular resonance energies relatively far from EF and one group having molecular resonance energies within the accessible bias window. Our results compare well with previous electrochemical gating data and with transmission functions computed from first principles. Furthermore, with the second group of molecules, we are able to examine the behavior of a molecular junction as a resonance shifts into the bias window. This work provides a new, experimentally simple route for exploring the fundamentals of charge transport at the nanoscale.

Gate controlling of quantum interference and direct observation of anti-resonances in single molecule charge transport

“…15. Because of the high efficiency of the electrochemical gating, we assume that the shift in transmission curves is roughly the same amplitude as the applied gate voltage 15,19,41 . In the absence of the gate voltage, the electrode Fermi energy was estimated from the DFT calculations using a slab model with bulk corrections 42 , which was found to be −5.32 eV for a gold(111) surface.…”

Electric-field induced bistability in single-molecule conductance measurements for boron coordinated curcuminoid compounds

“…It is important to stress that the ratio between the conductance values of the two peaks varies from sample to sample and is not exactly two as expected if two molecules were to be trapped at the same time 35–37. In addition, previously reported bistability features due to different anchoring groups or rectification38–42 can be disregarded; as for the former one, one would expect plateaus of different lengths (which is not observed) and with respect to the latter one that the molecule should present a clear asymmetric structure (which is not the case).…”

Tuning the polarity of charge carriers using electron deficient thiophenes

“…In this method, the molecular orbital alignment is pinned relative to the substrate potential; an increase in conductance with increasing (decreasing) tip bias thus implies HOMO (LUMO) dominated transport, as detailed previously. 26 The conductance of each of the seven thiophene pentamers was measured in propylene carbonate (PC) at more than ten different biases between –0.54 and +0.90 V. At each bias (applied to the tip), thousands of conductance versus displacement traces are collected, where an STM tip (coated with Apiezon) is repeatedly driven into and retracted from a gold substrate in a dilute solution of the molecules. 41,42 The conductance versus displacement traces for a compound at each bias are compiled into logarithmically binned histograms which are fit with a Gaussian; the peak value is the most probable conductance of the molecule.…”

“…For all our molecules, even though the HOMO and LUMO are close to the gold E F , we do not observe any crossing of molecular resonances at the voltages applied, since this would have led to charging effects, which alter the slope of the conductance versus voltage plots. 26 …”

“…We also recently developed a new strategy to experimentally determine the polarity of charge carriers in single molecule junctions by performing conductance measurements in a polar environment – a technique wherein the bias window is opened asymmetrically across the junction. 21,26 The method, used in this study, elucidates the dominant conducting orbital by mapping the molecular transmission functions in the region near to E F .…”