Comparison of the Energy-Level Alignment of Thiolate- and Carbodithiolate-Bound Self-Assembled Monolayers on Gold

This paper presents an experimental study of a carbodithiolate-bound monolayer on gold and finds the electronic structure of this surface to be different from those of thiolate-bound monolayers in important ways. Specifically, self-assembled monolayers formed from benzyl mercaptan and dithiobenzoic acid (molecules identical to each other except for the linker group) chemisorbed on gold are compared. The electronic structure of these modified surfaces was investigated using photoemission spectroscopy and electronic structure calculations. Surfaces covered with monolayers formed from benzyl mercaptan were found to be comparable to other thiolate-bound adsorbates. In contrast, surfaces covered with monolayers formed from dithiobenzoic acid have a markedly different electronic structure; the work function of the such modified surface is lowered by 1.6 eV, as compared to that of a bare gold surface, and the highest occupied pi-band lies 1.3-1.6 eV below the Fermi level of the gold surface. Dithiobenzoic acid was also used to modify the contacts of organic transistors and found to be effective in engineering the metal-semiconductor interface to achieve ambipolar transport.