Abstract
We have investigated electrical transport through the molecular model systems benzenedithiol, benzenediamine, hexanedithiol and hexanediamine. Conductance histograms under different experimental conditions indicate that measurements using mechanically controllable break junctions in vacuum are limited by the surface density of molecules at the contact. Hexanedithiol histograms typically exhibit a broad peak around 7×10−4 G0. In contrast to recent results on scanning tunnelling microscope (STM) based break junctions in solution we find that the spread in single-molecule conductance is not reduced by amino anchoring groups. Histograms of hexanediamine exhibit a wide peak around 4×10−4 G0. For both benzenedithiol and benzenediamine we observe a large variability in low-bias conductance. We attribute these features to the slow breaking of the lithographic mechanically controllable break junctions and the absence of a solvent that may enable molecular readsorption after bond breaking. Nevertheless, we have been able to acquire reproducible current–voltage (I–V) characteristics of benzenediamine and benzenedithiol using a statistical measurement approach. Benzenedithiol measurements yield a conductance gap of about 0.9 V at room temperature and 0.6 V at 77 K. In contrast, the I–V characteristics of benzenediamine-junctions typically display conductance gaps of about 0.9 V at both temperatures.
Highlights
We have investigated electrical transport through the molecular model systems benzenedithiol, benzenediamine, hexanedithiol and hexanediamine
The absence of a solvent and a fast rupture of the metal–molecule bond must have reduced the probability of forming stable molecular junctions, which are a prerequisite for pronounced peaks in the histograms
We have studied the electronic properties of single prototypical molecules using lithographic MCBJs in vacuum
Summary
Phosphorous bronze wafers (50 mm × 50 mm × 0.3 mm) were polished and cleaned by ultrasonication in acetone and isopropanol (IPA). After the application of an adhesion promoter (VM651, HD Microsystems) the wafers were spin-coated with a commercial polyimide precursor solution (PI2610, HD Microsystems). For the subsequent electron-beam lithography step the wafers were spin-coated with a double layer of resist (320 nm of methylmethacrylate (8.5) methacrylic acid copolymer, followed by 110 nm of polymethylmethacrylate (PMMA) 950 k). The resists were spin-coated from ethyl-L-lactate and anisole, respectively (Microchem). Both layers were baked out for 15 min at 175 ◦C. We defined ten devices on a wafer with four break junctions in each device using a Leica electron-beam pattern generator. After lift-off in hot acetone and a rinsing step in IPA we protected the fully processed wafers with a 500 nm thick layer of PMMA 350 k. The individual break junction devices were obtained later by laser-cutting and removing the protection layer (see below)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
