Pronounced odd-even changes in the molecular arrangement and packing density of biphenyl-based thiol SAMs: A combined STM and LEED study

Abstract

Self-assembled monolayers (SAMs) of ω-(4′-methylbiphenyl-4-yl) alkanethiols CH3(C6H4)2 (CH2)nSH(BPn, n = 3 and 4) on Au(111) substrates were studied using scanning tunnelling microscopy (STM) and low-energy electron diffraction (LEED). Preparation at elevated temperature results in highly ordered layers with large domains. Whereas the (2√3 × √3) structure of the BP3 SAMs is similar to that reported previously for other aromatic thiols, SAMs made from BP4 exhibit a very different structure. A (5√3 × 3)rect unit cell containing 8 molecules is found which corresponds to a packing density reduced by 25% compared to that of BP3. The odd-even effect observed in the molecularly resolved STM images of BP3 and BP4, therefore, confirms the pronounced influence of the spacer chain on the structure of these biphenyl-based SAMs.