Electron transport and redox reactions in carbon-based molecular electronic junctions

Abstract

A unique molecular junction design is described, consisting of a molecular mono- or multilayer oriented between a conducting carbon substrate and a metallic top contact. The sp2 hybridized graphitic carbon substrate (pyrolyzed photoresist film, PPF) is flat on the scale of the molecular dimensions, and the molecular layer is bonded to the substrate via diazonium ion reduction to yield a strong, conjugated C-C bond. Molecular junctions were completed by electron-beam deposition of copper, titanium oxide, or aluminium oxide followed by a final conducting layer of gold. Vibrational spectroscopy and XPS of completed junctions showed minimal damage to the molecular layer by metal deposition, although some electron transfer to the molecular layer resulted in partial reduction in some cases. Device yield was high (>80%), and the standard deviations of junction electronic properties such as low voltage resistance were typically in the range of 10-20%. The resistance of PPF/molecule/Cu/Au junctions exhibited a strong dependence on the structure and thickness of the molecular layer, ranging from 0.13 ohms cm2 for a nitrobiphenyl monolayer, to 4.46 ohms cm2 for a biphenyl monolayer, and 160 ohms cm2 for a 4.3 nm thick nitrobiphenyl multilayer. Junctions containing titanium or aluminium oxide had dramatically lower conductance than their PPF/molecule/Cu counterparts, with aluminium oxide junctions exhibiting essentially insulating behavior. However, in situ Raman spectroscopy of PPF/nitroazobenzene/AlO(x)/Au junctions with partially transparent metal contacts revealed that redox reactions occurred under bias, with nitroazobenzene (NAB) reduction occurring when the PPF was biased negative relative to the Au. Similar redox reactions were observed in PPF/NAB/TiO(x)/Au molecular junctions, but they were accompanied by major effects on electronic behavior, such as rectification and persistent conductance switching. Such switching was evident following polarization of PPF/molecule/TiO2/Au junctions by positive or negative potential pulses, and the resulting conductance changes persisted for several minutes at room temperature. The "memory" effect implied by these observations is attributed to a combination of the molecular layer and the TiO2 properties, namely metastable "trapping" of electrons in the TiO2 when the Au is negatively biased.

Molecular Self‐Assembled Monolayers and Multilayers for Organic and Unconventional Inorganic Thin‐Film Transistor Applications

“…Whatever the origin of the hopping, it is observed much less frequently in SAMs than is tunneling because the typical lengths of the molecules investigated are seldom greater than $2 nm. [162,189,190] Extensive theoretical work has been reported concerning the tunneling/hopping transition, [154,[191][192][193][194][195][196][197] and earlier data on some organics [198][199][200][201] and DNA [202,203] have clearly revealed a length dependent transition between tunneling and hopping conduction. Recently, the transition was demonstrated in Au-molecule-Au junctions, where the electrical resistance of oligophenyleneimine (OPI) molecules of various lengths were measured with a conductive AFM tip (Fig.…”

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

“…[10,65,66,81,84] Examples from three different laboratories where vapor deposition was carried out successfully onto covalently bonded molecular layers are shown in Figure 16 and 17. Figure 16 shows results for molecular junctions made using diazonium-derived aromatic molecular layers on carbon with e-beam-deposited Cu top contacts.…”

“…Changes in the oxidation state of molecules within junctions can accompany electron conduction and also result in the bistability important to memory devices. [10,14,15] As described in later sections, the concepts from these various disciplines are important for understanding the behavior and functions of molecular junctions, all of which are needed to meet key objectives in the field.…”

Electron Transport and Redox Reactions in Molecular Electronic Junctions

“…The original papers should be consulted for fabrication details and properties of PPF/molecule/metal molecular junctions. [19,[22][23][24][25][26] All current/voltage curves are presented as J (A cm 4), all between PPF and Cu contacts. As reported elsewhere, these PPF/molecule/Cu devices all show an activated region above~250 K, with Arrhenius slopes of 0.02-0.17 eV [22] .…”