We compute the potential of mean force for two gold nanocrystals capped by alkylthiols from atomistic simulations and show how variables such as temperature, capping molecule length, and the presence of solvent affect these interactions. Our main findings are (1) the equilibrium distance in vacuum always equals approximately 1.25 times the core diameter, (2) incomplete capping layers promote sintering, and (3) the presence of a good solvent results in purely repulsive interactions.

Self-assembly of capped nanocrystals ͑NC͒ attracted a lot of attention over the past decade. Despite progress in manufacturing of NC superstructures, the current understanding of their mechanical and thermodynamic stability is still limited. For further applications, it is crucial to find the origin and the magnitude of the interactions that keep self-assembled NCs together, and it is desirable to find a way to rationally manipulate these interactions. We report on molecular simulations of interacting gold NCs protected by capping molecules. We computed the potential of mean force for pairs and triplets of NCs of different size ͑1.8-3.7 nm͒ with varying ligand length ͑ethanethiol-dodecanethiol͒ in vacuum. Pair interactions are strongly attractive due to attractive van der Waals interactions between ligand molecules. Three-body interaction results in an energy penalty when the capping layers overlap pairwise. This effect contributes up to 20% to the total energy for short ligands. For longer ligands, the three-body effects are so large that formation of NC chains becomes energetically more favorable than close packing of capped NCs at low concentrations, in line with experimental observations. To explain the equilibrium distance for two or more NCs, the overlap cone model is introduced. This model is based on relatively simple ligand packing arguments. In particular, it can correctly explain why the equilibrium distance for a pair of capped NCs is always Ϸ1.25 times the core diameter independently on the ligand length, as found in our previous work ͓Schapotschnikow, R. Pool, and T. J. H. Vlugt, Nano Lett. 8, 2930 ͑2008͔͒. We make predictions for which ligands capped NCs self-assemble into highly stable three-dimensional structures, and for which they form high-quality monolayers.

CdSe nanocrystals (NCs) capped by organic ligands are studied at the atomistic level using classical molecular simulations. We show for the first time that the NC−ligand bond strength can be explained using a simple model based on electrostatic interactions. The computed binding energies in vacuum for amine, thiol, thiolate, and phosphine oxide ligands are 86.8, 34.7, 1283, and 313.6 kJ/mol, respectively. These values are in good agreement with available quantum chemical calculations and experiments. It is crucial that one corrects for the dielectric constant of the solvent used in the experiment. We also show that the amine capping layer is formed in two stages: first, amine molecules binds to a single surface cation each, and then additional amines bind to less favorable sites forming hydrogen bonds with already adsorbed ligands. The crossover between these mechanisms can occur at ambient conditions. We speculate that this crossover may be responsible for transitions in optical properties reported earlier. The calculated adsorption isotherms show that amine ligands desorb from the nanocrystal surface under ultra-high vacuum at ambient temperatures.

The exchange kinetics of native ligands that passivate CdSe quantum dots (hexadecylamine (HDA), trioctylphosphine oxide (TOPO), and trioctylphosphine (TOP)) by thiols is followed in situ. This is realized by measuring, in real-time, the decrease in emission intensity of the QDs upon addition of hexanethiol (HT) which quenches the emission. The effect of adding an excess of native ligands prior to thiol addition on the capping exchange is studied to provide insight in the bond strength and exchange kinetics of the individual surfactants. Temperature-dependent measurements reveal faster kinetics with increasing temperature. A kinetic model to describe the time-dependent measurements is introduced, taking into account the equilibrium between native ligands before thiol addition and describing the evolution of surface coverage by all ligands over time. The model allows us to extract the quenching rate for a single thiol ligand (0.004 ns−1) as well as exchange rates, equilibrium constants, activation energies, and changes in Gibbs free energy for replacement of the different native surfactants by HT. The analysis reveals that the substitution half-time of HDA by HT (72 s) is much shorter than for TOP (5 h) or TOPO (2.5 h) under the same conditions. The temperature dependence of the kinetics shows that the activation energy for exchange of HDA/TOPO by hexanethiol (1.6 kJ/mol) is much smaller than for TOP (20.9 kJ/mol).

Molecular dynamics simulations are performed on capped and uncapped PbSe nanocrystals, employing newly developed classical interaction potentials. Here, we show that two uncapped nanocrystals fuse efficiently via direct surface attachment, even if they are initially misaligned. In sharp contrast to the general belief, interparticle dipole interactions do not play a significant role in this "oriented attachment" process. Furthermore, it is shown that presumably polar, capped PbSe{111} facets are never fully Pb-or Se-terminated.

We carried out Monte Carlo simulations of gold nanocrystals (NCs) and (111) slabs covered with alkyl thiols, with and without explicit solvent (n-hexane), at T ) 300 K. Adsorption isotherms for propane-and octanethiol showed a phase behavior measured previously in experiments. Comparison of the adsorption isotherm of octanethiol in hexane on a (111) slab with experimental data suggests that, in this system, no thiolate bond was formed. The geometry of a gold surface strongly influences the formation and structure of the capping monolayer. On a (111) surface, attractive interactions between carbon chains are more pronounced than on a NC. This leads to a stronger penetration of the capping layer by the solvent. Adsorption selectivity for binary alkyl thiol mixtures is stronger in vacuum than in solution. The convex shape of the NCs also reduces the adsorption selectivity of binary thiol mixtures. This result shows that the solvent cannot be ignored in simulations.

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The adsorption of mixtures of alkyl thiol surfactants on the Au(111) surface as well as on icosahedral gold nanocrystals (NCs) was investigated by molecular simulation. We compared the molfraction of each surfactant type on the gold structure with the molfraction of each surfactant type in the surrounding bulk solvent. For alkyl thiol surfactants with 1 to 4 carbon atoms difference, we found that, in contrast to the (111) surface, the adsorption selectivity on a NC is almost identical to that in the solvent.