Atomistic Simulations of the Surface Coverage of Large Gold Nanocrystals Takieddine Djebaili, † Johannes Richardi,* ,† Ste ́ phane Abel, ‡ and Massimo Marchi ‡ † Laboratoire des Maté riaux Me ́ soscopiques et Nanome ́ triques (LM2N), UMR CNRS 7070, Universite ́ Pierre et Marie Curie, bâ t F, BP 52, 4 Place Jussieu, 75252 Paris Cedex 05, France ‡ Commissariat a ̀ l’Energie Atomique et aux Energies Alternatives, DSV/iBiTEC-S/SB2SM/LBMS & CNRS UMR 8221, Saclay, France * S Supporting Information ABSTRACT: 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. 1. INTRODUCTION Because of their unique size-tunable properties, gold nano- crystals (AuNC) are currently studied for various applications including catalysis, electronic and photonic devices, and biomedical sensors. 1-4 Thus, their good biocompatibility lets them also be good candidates for therapeutic drug delivery in cancer diagnostics and therapy. 3 To prevent the aggregation of the AuNC, organic ligands are used to stabilize them. 1 Among these ligands, we can cite alkanethiols, amines, and phosphines. These ligands are able to form compact monolayers at the nanoparticle surface due to the high affinity between the NC gold atoms and the ligand head groups. The formation of these ligand monolayers on solid gold surfaces has been widely studied in the literature. 5-10 It was found that these monolayers are usually highly ordered molecular films called self-assembled monolayers (SAM) (see Figure 1). Figure 1A shows that the thiol head groups form a hexagonal overlayer structure denoted by (√3 × √3)R30°. The alkane chains are tilted by an angle of about 30° with respect to the surface normal. 5 DFT calculations and STM experiments 11,12 have shown that the adsorbed thiolates are between the three gold atoms, but the geometry of adsorption is better described by a shifted bridge position. Classical simulations often yield a hollow position as preferred positions for the thiol atoms (see Figure 1), which is a drawback of the interaction model we use. Several theoretical and experimental studies 7,13-20 have shown the importance of surface reconstruction for the formation of SAMs with the presence of adatoms and vacancies. A very recent simulation study allowing surface reconstruction 9 indicates that the vacancies and adatoms may form islands and we might obtain a slightly perturbed (√3 × √3)R30° lattice. However, one has to be very careful with these first results, which have to be confirmed in the future. Experiments have shown significant differences between the monolayers of alkanethiols formed on gold nanocrystals (NCs) and flat surfaces. 21-27 Elemental analysis using transmission electron microscopy for alkanethiol-derivated gold clusters with diameters of 2 nm gives a lower average surface per adsorbed Received: April 8, 2013 Revised: July 12, 2013 Published: July 24, 2013 Figure 1. Self-assembled monolayer on a flat Au(111) surface (A) and on the edge of NC facets (B) as obtained by classical simulations: Positions of the SH groups of the alkanethiols are shown by yellow spots. Article pubs.acs.org/JPCC © 2013 American Chemical Society 17791 dx.doi.org/10.1021/jp403442s | J. Phys. Chem. C 2013, 117, 17791-17800