The effect of stretching thiyl- and ethynyl-Au molecular junctions

Hoft, R. C. and Ford, M. J. and Garcia-Suarez, V. M. and Lambert, Colin (2008) The effect of stretching thiyl- and ethynyl-Au molecular junctions. Journal of Physics: Condensed Matter, 20 (2). -. ISSN 0953-8984

Abstract

We perform density functional theory (DFT) calculations of the stretching of Au(111)-X-Au(111) molecular junctions where X is either a thiyl or ethynyl biradical. The equilibrium geometries for the radicals adsorbing on the surface are first calculated and the radicals then placed in the junction geometry. The unit cell is stepwise increased in length and the geometry relaxed at each step. When stretching the ethynyl junction, a single gold atom is detached from the rest of the surface and the gold-carbon bond does not break. In contrast, the gold-sulfur bond in the thiyl junction breaks without detaching any gold atoms. This behaviour can be attributed to the enhanced strength of the Au-C interaction over the Au-S interaction. In both junctions the conductance calculated using the non-equilibrium Green's function formalism (NEGF) decreases as the junction is stretched. After breakage of the Au-S bond, the thiyl radical contains an unpaired electron on the sulfur atom and the system is in a spin doublet state. Transmission spectra were calculated for the spin-unpolarized case only; evaluation of the spin- polarized density of states suggests that an enhanced conductance for electrons of one spin type may be observed after the Au-S bond is broken.