Carbon/molecule/metal molecular electronic junctions: the importance of “contacts”

Abstract

Molecular electronic junctions fabricated by covalent bonding onto a graphitic carbon substrate were examined with Raman spectroscopy and characterized electronically. The molecular layer was a 4.5 nm thick multilayer of nitroazobenzene (NAB), and the top contact material was varied to investigate its effect on junction behavior. A 3.0 nm thick layer of copper, TiO2, or Al(III) oxide (AlO(x)) was deposited on top of the NAB layer, followed by a 7.0 nm thick layer of gold. Copper "contacts" yielded molecular junctions with low resistance and showed a strong dependence on molecular structure. Carbon/ NAB/AlO(x)/Au junctions exhibited high resistance, with current densities three orders of magnitude less than those for analogous Cu junctions. However, Raman spectroscopy revealed that the NAB layer was reduced when the carbon substrate was biased negative, to a product resembling that resulting from electrochemical reduction of NAB. Carbon/ NAB/TiO2/Au junctions showed rectifying J/V behavior, with high conductivity to electrons able to enter the TiO2 conduction band. Substitution of azobenzene for nitroazobenzene yielded junctions with similar spectroscopic and electronic behavior to NAB, indicating that the nitro group is not essential for rectification. The results are interpreted in terms of the energy levels of the molecule relative to those of TiO2. The combination of a covalently bonded molecular layer and a semiconducting oxide yields unusual electronic properties in a carbon/molecule/semiconductor/Au molecular junction.

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

“…However, a requirement for ordering is a relatively weak metal-thiolate or electrostatic bond which allows surface diffusion to occur, with bond strengths of $1.9 eV for Au-thiolate and <0.5 eV for the various interactions that anchor L-B films (i.e., electrostatic, van der Waals, etc.). The less commonly studied silicon-carbon [44,66,67,83,84] and carboncarbon [10,85] binding modes have the attraction of much stronger bonds (3-4 eV), but as a consequence they are generally less ordered. Once the surface bond is formed, usually by a radical or photochemical route, the molecules cannot move, and therefore cannot assemble into a low energy, ordered arrangement.…”

Synthesis, characterization, and application of silver nanoparticle‐thiophenol nanocomposite film on the glassy carbon surface

“…Modified surfaces can be characterized by electrochemical methods such as cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS); by spectroscopic methods such as XPS, electron spin resonance, Raman spectroscopy, and reflectance‐absorption infrared spectroscopy; by microscopic methods such as atomic force microscopy (AFM), scanning tunneling microscopy, scanning electrochemical microscopy, and scanning electron microscopy; and by optical methods such as ellipsometry and surface plasmon resonance techniques …”

“…[11] Another most versatile modification method is the organic self assembly film formation on the metal substrates for various purposes. [12] Modified surfaces can be characterized by electrochemical methods such as cyclic voltammetry (CV) [13] and electrochemical impedance spectroscopy (EIS); [9] by spectroscopic methods such as XPS, [14] electron spin resonance, [15] Raman spectroscopy, [9] and reflectance-absorption infrared spectroscopy; [16][17][18][19] by microscopic methods such as atomic force microscopy (AFM), [20] scanning tunneling microscopy, [21] scanning electrochemical microscopy, [22] and scanning electron microscopy; [23] and by optical methods such as ellipsometry [2] and surface plasmon resonance techniques. [24] Modification of electrodes by silver nanoparticle (SNP) is useful in the design and fabrication of electrochemical biosensors [25] and in the nanowire construction.…”

Gold/Molecule/p$^+$ Si Devices: Variable Temperature Electronic Transport

“…Much of the work that has been done to date has utilized metal electrodes, however, this has posed significant problems for device stability and interpretation of results. Replacing one or both of the metal electrodes with alternate materials such as semiconductors [4] or carbon [5] is a promising approach for addressing such issues. A variety of molecular layers can be covalently bound to semiconductor surfaces, leading to chemically stable junctions [6], [7].…”