Selective Anchoring Groups for Molecular Electronic Junctions with ITO Electrodes

Abstract

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.

A review of oligo(arylene ethynylene) derivatives in molecular junctions

“…Historically, molecular junctions have relied on metallic electrodes, with single-molecule studies particularly reliant on gold electrodes. The above discussion has already made clear the growing interest in alternative junction materials and designs, including nickel electrodes in STM-BJ studies, 247 ITO-molecule-Au junctions 165 and bridging nanogaps between CNT 76,77 or graphene [73][74][75] electrodes with molecules. Graphene-based junctions are of particular interest owing to the notable properties of graphene and its compatibility with organic materials.…”

“…161 The dimethylated analogue of DHBT, namely DMBT, is also an effective anchor for gold surfaces. 165,166 Thiophene has been used as a sulfur-based anchor, usually as a 3-thienyl (-C 4 H 3 S) substituent. 156,167 Selenium-based anchoring groups, for example methyl selenoether (methylseleno, -SeMe), have been shown to have similar properties to their sulfur analogues.…”

“…ITO-molecule-Au junctions containing asymmetric OAE2 species were recently investigated using the I(t ) 51 modification of the STM-BJ method. 165 Carboxylic acid, 2-cyanoacrylic acid, and pyridinium squarate (PS) were all shown to be suitable anchoring groups for ITO. PS showed no affinity for Au surfaces, meaning selective junction orientation could be achieved using a species bearing both SAc and PS anchor groups.…”

“…PS showed no affinity for Au surfaces, meaning selective junction orientation could be achieved using a species bearing both SAc and PS anchor groups. 165 Selectivity studies of this type are of relevance to controlled device manufacturing, and comparable studies with other electrode materials would be insightful.…”

Uncapped Gold Nanoparticles for the Metallization of Organic Monolayers

“…More recently attention has turned to properties of molecular junctions that extend beyond the mimicry of electrical components at the single‐molecule level. Various studies have now shown that molecular junctions may have applications in areas as diverse as solar‐energy harvesting, [ 13 ] thermoelectric conversion, [ 14,15 ] and molecular sensing, [ 16,17 ] whilst single‐molecule light‐emitting diodes have also been demonstrated. [ 18 ] Mechanical manipulation of molecules within the junction has led to interests in mechanically switched molecular electronic components, [ 19,20 ] whilst single‐molecule reactions within the junction have emerged as a hot topic area in recent years.…”