posted on 2021-01-20, 19:05authored byInco J. Planje, Ross J. Davidson, Andrea Vezzoli, Abdalghani Daaoub, Sara Sangtarash, Hatef Sadeghi, Santiago Martín, Pilar Cea, Colin J. Lambert, Andrew Beeby, Simon J. Higgins, Richard J. Nichols
Indium
tin oxide (ITO) is an attractive substrate for single-molecule
electronics since it is transparent while maintaining electrical conductivity.
Although it has been used before as a contacting electrode in single-molecule
electrical studies, these studies have been limited to the use of
carboxylic acid terminal groups for binding molecular wires to the
ITO substrates. There is thus the need to investigate other anchoring
groups with potential for binding effectively to ITO. With this aim,
we have investigated the single-molecule conductance of a series of
eight tolane or “tolane-like” molecular wires with a
variety of surface binding groups. We first used gold–molecule–gold
junctions to identify promising targets for ITO selectivity. We then
assessed the propensity and selectivity of carboxylic acid, cyanoacrylic
acid, and pyridinium-squarate to bind to ITO and promote the formation
of molecular heterojunctions. We found that pyridinium squarate zwitterions
display excellent selectivity for binding to ITO over gold surfaces,
with contact resistivity comparable to that of carboxylic acids. These
single-molecule experiments are complemented by surface chemical characterization
with X-ray photoelectron spectroscopy, quartz crystal microbalance,
contact angle determination, and nanolithography using an atomic force
miscroscope. Finally, we report the first density-functional theory
calculations involving ITO electrodes to model charge transport through
ITO–molecule–gold heterojunctions.