Here, the adsorption of alkanethiols (from ethane to dodecanethiol) on icosahedral gold nanocrystals with diameters up to 10 nm is studied by molecular dynamics simulations in a vacuum. The surface coverage of the nanocrystals obtained in the simulations is in good agreement with experimental data. We show that the average surface per adsorbed thiol does not markedly depend on the nanocrystal size and ligand and is only about 10% lower than the value observed on a flat Au(111) surface. We observe two different molecular organizations of the thiolates on the edges and in the centers of the nanocrystal facets. The incompatibility between both organizations explains the fact that the formation of self-assembled monolayers usually observed on flat Au(111) surfaces is hindered for nanocrystals smaller than 6 nm. We also show that the organization of thiolates on the edges is at the origin of the lower average surface per adsorbed thiol found for the nanocrystal.

International audienceThis paper reports on a molecular dynamics investigation of the molecular organization of alkanethiolates (from ethane to dodecanethiolate) on octahedral and cubic gold nanocrystals with diameters up to 10 nm. We show that the average surface per adsorbed thiolate only slightly depends on the nanocrystal shape and the alkane chain length. Two different organizations of thiolates are observed on the facet centers and edges of octahedral nanocrystals, while on cubic nanocrystals only one appears. This explains the closer distance between thiolates on the edges of octahedral nanocrystals, which is not observed for nanocubes. The enhanced surface coverage of thiolates on nanocrystals is explained by the new organization for octahedral nanocrystals and can be attributed to the occupation of adsorption sites on the edges for cubic nanocrystals. Small differences observed in the molecular organizations on icosahedral and octahedral nanocrystals can be mainly explained by the larger facets of the latter ones

The influence of the interaction model on the adsorption of butanethiolate on gold surfaces and nanocrystals has been studied with molecular dynamics simulations. The results obtained for three different head group sizes are compared to experiments. The use of the largest head group induces new organizations of the ligands in the case of nanocrystals and Au(100) surfaces, while no such difference is observed for Au(111). As a consequence, this model does not reproduce the higher surface coverage experimentally observed for nanocrystals. Our results show that the evaluation of the quality of force fields cannot be restricted to the study of specific surfaces. Some properties such as the occupation frequencies of adsorption sites markedly depend on the nanocrystal size.