Effect of anchoring groups on the formation of self-assembled monolayers on Au(111) from cyclohexanethiol and cyclohexyl thiocyanate

Surface structures, binding conditions, and electrochemical behaviors of self-assembled monolayers (SAMs) formed by adsorption of cyclohexanethiol (CHT, C₆H₁₀-SH) and cyclohexyl thiocyanate (CHTC, C₆H₁₀-SCN) on Au(111) were characterized to understand the effect of anchoring groups on the formation of SAMs using scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and cyclic voltammetry (CV). STM observations showed that CHT SAMs had tri-directional small-ordered domains, whereas CHTC SAMs were composed of unidirectional long-range-ordered domains and disordered domains. CHT and CHTC SAMs had identical molecular arrangements with zig-zag-ordered molecular rows, unlike the SAMs of alkanethiols and alkyl thiocyanates, which have different packing structures. XPS measurements showed that CHTC SAMs on Au(111) can be formed via chemical reactions of sulfur atoms after cleavage of S-CN bonds, and the surface coverages of CHT and CHTC SAMs were similar. Moreover, the N 1s peak intensity of CHTC SAMs is very weak, which means that the CN species generated during the adsorption of CHTC molecules on Au(111) are easily removed from the surface. CV measurements revealed the reductive desorption peaks of CHT and CHTC SAMs at similar potentials of −0.980 and −0.988 V, respectively, suggesting that the electrochemical stabilities of CHT and CHTC SAMs were almost identical to each other. Overall, we clearly demonstrated that, unlike alkyl thiocyanate SAMs, alicyclic thiocyanates could be used as an excellent alternative to thiol analogues with the same alicyclic backbone for SAM preparation.