Organic Electrodes Based on Grafted Oligothiophene Units in Ultrathin, Large-Area Molecular Junctions

Abstract

Molecular junctions were fabricated with the combined use of electrochemistry and conventional CMOS tools. They consist of a 5-10 nm thick layer of oligo(1-(2-bisthienyl)benzene) between two gold electrodes. The layer was grafted onto the bottom electrode using diazonium electroreduction, which yields a stable and robust gold-oligomer interface. The top contact was obtained by direct electron-beam evaporation on the molecular layers through masks defined by electron-beam lithography. Transport mechanisms across such easily p-dopable layers were investigated by analysis of current density-voltage (J-V) curves. Application of a tunneling model led to a transport parameter (thickness of ~2.4 nm) that was not consistent with the molecular thickness measured using AFM (~7 nm). Furthermore, for these layers with thicknesses of 5-10 nm, asymmetric J-V curves were observed, with current flowing more easily when the grafted electrode was positively polarized. In addition, J-V experiments at two temperatures (4 and 300 K) showed that thermal activation occurs for such polarization but is not observed when the bias is reversed. These results indicate that simple tunneling cannot describe the charge transport in these junctions. Finally, analysis of the experimental results in term of "organic electrode" and redox chemistry in the material is discussed.

Activationless charge transport across 4.5 to 22 nm in molecular electronic junctions

“…Fig. 2B shows an overlay of attenuation plots for BTB with a bias of +1 V obtained at five temperatures from 6 to 300 K. Importantly, for d < 8 nm both the value of β and the current magnitude are weakly dependent on temperature, consistent with transport controlled by quantum-mechanical tunneling, as previously reported for similar junctions (9,30,31). However, for the intermediate thickness range (8-12 nm), the current density decreases slowly with lower temperature, whereas β itself does not change significantly.…”

“…Such grafted oligo(BTB) layers can be easily p-doped and switched from insulating to conductive states in electrochemical conditions (25)(26)(27)(28). Previous transport measurements in planar Au junctions containing a 7-nm-thick BTB molecular layer were explained with a mixed-transport mechanism combining a conducting molecular region with a tunnel barrier (31). In the present work, temperature and electrical field dependence of transport were analyzed to determine the likely transport mechanisms in three thickness regions: less than 8 nm, 8-16 nm, and greater than 16 nm.…”

Inelastic electron tunneling spectroscopy in molecular junctions showing quantum interference

“…Molecular junctions are fabricated by conventional cleanroom microfabrication techniques in a cross-bar geometry [11,31]. Starting from a Si/SiO 2 substrate, the bottom electrode is first fabricated by optical lithography and e-beam evaporation (10 −8 mbar).…”

“…It consists in a 15 mm long, 20 μm wide metallic stripe made of Ti/Au (2 nm/50 nm). The stripe geometry is optimized for the successive electrochemical grafting procedure of the molecular layer by electroreduction of diazonium salts [31]. The latter is performed by cyclic voltammetry (scan rate of 100 mV/s; potential range −0.3 V < V < 0.4 V) inducing the electroreduction of 9,10-dioxo-1-anthracenediazonium salt in C 2 H 3 N-NBu 4 BF 4 solution.…”

“…Details on the electrografting procedure and molecular layer characterization are given in Refs. [11] and [31]. Redox grafting parameters, such as scan rate, switching potentials, and number of cycles, are chosen accordingly to Ref.…”