Two-dimensional self-assembly of melem and melemium cations at pH-controlled aqueous solution–Au(111) interfaces under electrochemical control

Abstract

Two-dimensional self-assembly of melem at pH-controlled aqueous solution-Au(111) interfaces has been investigated by electrochemical scanning tunneling microscopy. In the solutions with pH>pK b1 of melem, two ordered self-assembled structures (honeycomb and close-packed structures) and one disordered fibrillar structure were observed as a function of the surface coverage of melem controlled by the electrode potential. In contrast, in the acidic solution with pH<pK b1 of melem, only the self-assembled honeycomb network was observed in a relatively wide potential range probably due to the presence of monoprotonated melem cations. Dots attributed to counteranions were frequently observed in the pores of the honeycomb network. The lack of close-packed and fibrillar structures at low pH (<pK b1) is attributed to ionic repulsion of melemium cations.