Oxidation of thiol anchor groups in molecular junction devices: A density functional theory study

Thiols (RSH) or thiolates (RS) are one of the most popular anchor groups used for attaching molecules to gold electrodes in molecular junction devices. Operation of these devices under oxidizing conditions may lead to oxidation of the thiolate anchor groups. Herein we investigate the plausibility of this process as a potential source of current fluctuation. Density functional theory calculations on various oxide derivatives of ethanethiolate on gold clusters (EtSO_n/Au₁₃; n ) 0-3) suggest that oxidation of thiolate anchor groups (Au-S) into sulfoxides (Au-SO) should be unlikely under ambient conditions. Under oxidizing conditions, sulfoxides can form and oxidize further into sulfinates (Au-SO₂) and sulfonates (Au-SO₃) favorably via oxygen transfer from surface oxides or other active oxygen species. Nonequilibrium Green's function calculations on model devices with thiolate, sulfoxide, and sulfonate anchor groups suggest that thiolates show essentially the same insulating current-voltage characteristics before and after oxidation. However, oxidation of thiolates into sulfonates can increase the length of the electron transfer pathway (that is, the molecule), and this type of oxidation-induced change (molecular lengthening, SAM thickening, and possibly SAM disordering) can affect the robustness of the molecular junction devices.