The electronic properties of alkanethiolate [CH3(CH2)nS-, n = 9 and 11] and alkaneselenolate [CH3(CH2)nSe-, n = 9 and 11] self-assembled monolayers on Au{111} have been quantitatively compared. Simultaneously acquired apparent tunneling barrier height (ATBH) and scanning tunneling microscopy (STM) images reveal that alkanethiolate molecules have a lower barrier to tunneling, and therefore a higher conductance than alkaneselenolates of the same alkyl chain length. Molecular and contact conductance differences were elucidated by using observed STM topographic tunneling height differences between the analogous species. This apparent topographic difference combined with comparative ATBH data indicate that the observed decrease in conductance for alkaneselenolates compared to alkanethiolates originates exclusively from the Au-chalcogenide physical, chemical, and electronic contact.

Coexisting adsorbate phases in high-coverage decaneselenolate and dodecaneselenolate [CH 3 (CH 2 ) n Se, n ) 9 and 11] self-assembled monolayers on Au{111} have been characterized by scanning tunneling microscopy and consist of two types: a densely packed distorted hexagonal lattice incommensurate to the underlying gold substrate, as revealed by the observation of a moire ´pattern, and a commensurate linear missing-row structure. Examination of the nearest neighbor distances in the tightly packed lattice reveal two distinct repeat distances of 4.90 and 5.20 Å, which complements previous surface X-ray data. The linear missing row structure manifests in several variants of the ( 3 × 3 3)R30°unit cell differentiated by whether the molecules bind at 2-or 3-fold substrate sites. While the number of molecules within this unit cell is typically two, in some cases an additional alkaneselenolate molecule is located at a site one Au atom lower than the rest. The structural conclusions are supported by excellent agreement of experimental lattice parameters and those derived from molecular packing models. Comparison of the alkaneselenolate data with analogous structural phases reported for alkanethiolate monolayers on Au{111} shows that differences between the two systems can be understood on the basis that self-assembly is guided both by headgroup-headgroup as well as headgroup-substrate interactions.