Vacancy defects are known to play an important role in the structural and chemical properties of metallic and semiconductor nanoparticles. Here, we investigate the likelihood of vacancy formation at the surface, in the subsurfaces, and in the interior of a model system of Al nanocrystals. The depth dependence of the vacancy formation energy (VFE) in 14–17 layered low-indexed surfaces of aluminium is studied using LDA, PBE, and PBEsol exchange-correlation functionals. Within a depth of two subsurface layers, the functionals make a transition from a similar description of surfaces to the differences in VFEs observed in bulk Al. The VFE converges to the bulk value within 0.01 eV beyond a maximum depth of 3–6 atomic layers, depending on the crystallographic surface plane. We find that the different convergence behaviors are related to the relaxations of atomic planes, normal to the surface, which in turn depend on the packing density of these surfaces. For the (111) subsurfaces, surprisingly, the defect formation energies are found to be higher than that of bulk Al, which is related to the hindered relaxations in its close-packed atomic planes. Although our results predict considerably lower VFE for the topmost layers of all the surfaces, the likelihood of forming a vacancy in the immediate subsurfaces of multifaceted Al nanoparticles is predicted to be lower than in bulk Al, which is in contrast to expectation.@en

Cation exchange is a powerful tool for the synthesis of nanostructures such as core–shell nanocrystals, however, the underlying mechanism is poorly understood. Interactions of cations with ligands and solvent molecules are systematically ignored in simulations. Here, we introduce the concept of pseudoligands to incorporate cation-ligand-solvent interactions in molecular dynamics. This leads to excellent agreement with experimental data on cation
exchange of PbS nanocrystals, whereby Pb ions are partially replaced by Cd ions from solution. The temperature and the ligand-type control the exchange rate and equilibrium composition of cations in the nanocrystal. Our simulations reveal that Pb ions are kicked out by exchanged Cd interstitials and migrate through interstitial sites, aided by local relaxations at core–shell interfaces and point defects. We also predict that high-pressure conditions facilitate strongly enhanced cation exchange reactions at elevated temperatures. Our approach is easily extendable to other semiconductor compounds and to other families of
nanocrystals.@en

A transferable force field for the PbSe-CdSe solid system using the partially charged rigid ion model has been successfully developed and was used to study the cation exchange in PbSe-CdSe heteronanocrystals [A. O. Yalcin et al., "Atomic resolution monitoring of cation exchange in CdSe-PbSe heteronanocrystals during epitaxial solid-solid-vapor growth," Nano Lett. 14, 3661-3667 (2014)]. In this work, we extend this force field by including another two important binary semiconductors, PbS and CdS, and provide detailed information on the validation of this force field. The parameterization combines Bader charge analysis, empirical fitting, and ab initio energy surface fitting. When compared with experimental data and density functional theory calculations, it is shown that a wide range of physical properties of bulk PbS, PbSe, CdS, CdSe, and their mixed phases can be accurately reproduced using this force field. The choice of functional forms and parameterization strategy is demonstrated to be rational and effective. This transferable force field can be used in various studies on II-VI and IV-VI semiconductor materials consisting of CdS, CdSe, PbS, and PbSe. Here, we demonstrate the applicability of the force field model by molecular dynamics simulations whereby transformations are initiated by cation exchange@en