Molecular Simulations of Nanoscale Transformations in Ionic Semiconductor Nanocrystals

Abstract

The aim of the study described in this thesis is to obtain a profound understanding of transformations in NCs at the atomic level, by performing molecular simulations for such transformations, and by comparing the simulation results with available experimental high resolution transmission electron microscopy (HRTEM) data to validate the simulations and to reveal underlying physical mechanisms. These transformations include structural and morphological transitions and cation exchange processes in ionic nanocrystals (II-VI and IV-VI semiconductors). The main simulation method used is classical Molecular Dynamics (MD) simulation. First principles density functional theory (DFT) calculations were used to develop empirical force fields that are able to accurately reproduce phase transitions. Using these newly developed force fields, large scaled classical MD simulations were carried out and linked to HRTEM experiments. The partially charged rigid ion model (PCRIM) was chosen for the force fields. This PCRIM approach has a simple functional form with a few number of parameters and has a clear physical meaning for ionic crystals. To simulate cation exchange in colloidal NC systems at the NC/solution interface, we used a combination of all-atom force fields and a coarse-grained model. In Chapter 2, an ab-initio based force field for ZnO is developed within the framework of the PCRIM approach. The values of the partial charges were determined by Bader charge analysis of DFT calculations on various ZnO phases. Beside Coulombic interactions, only short-ranged pairwise interatomic interactions were included. An initial guess of the parameters of the short-ranged pair potentials were first obtained by the lattice inversion method. The parameters were further adjusted by an ab-initio potential surface fitting procedure. The new ZnO force field has a very simple functional form is able to accurately reproduce several important physical properties of ZnO materials. These physical properties include the lattice parameters and phase stability of several ZnO polymorphs, as well as the elastic constants, bulk moduli, phonon dispersion, and melting points of wurtzite ZnO. The transition pressure of the wurtzite-to-rocksalt transition calculated with the force field equals 12.3 GPa, in agreement with experimental measurements and DFT calculations. A wurtzite-to-honeycomb phase transition is predicted at an uniaxial pressure of 8.8 GPa. We found a rational and effective way to derive force fields with simple functional forms for accurate simulations of phase transitions in ionic crystals. In Chapter 3, we developed a transferable force field for CdS-CdSe-PbSPbSe solid systems. The selection of the force field and the fitting procedure are similar to that of the ZnO force field in Chapter 2. The challenges when developing this force field were to maintain the transferability of this force field for four materials (CdS, CdSe, PbS, and PbSe) and to describe their mixed phases. This was solved by assuming that different cations/anions have the same values of the partial charges, and that shortranged interatomic interactions between two cations/anions are the same in different materials. For the mixed phases, DFT calculations of the mixed phases were included in both the training and validation sets. This work is the first step for further simulation studies of these II-VI and IV-VI semiconductor NCs and heteronanocrystals (HNCs). In Chapter 4, a thermally induced morphological and structural transition of CdSe NCs was investigated using MD simulations. In MD simulations, a CdSe nanosphere with the ZB structure transforms into a tetrapodlike morphology at 800 K. In a CdSe tetrapod, four WZ legs attach to the {111} facets of a tetrahedral ZB core. This transformation is achieved by a layer-by-layer slip of the ZB-{111} bilayer. Simulations show that the slips are mediated by the formation of Cd vacancies on the surface of the NCs to overcome the potentially large energy barriers associated with slip. The morphology of the annealed NCs is found to be temperature and size-dependent. An octapod-like morphology is found in NCs with a relatively large NC size and in a certain range of the heating temperature. Surprisingly, nanoscale transformations of CdSe NCs have been directly observed in HRTEM in situ heating experiments. Our findings provide a simple method to modify the morphology of ionic NCs and can potentially be used in the synthesis of branched NCs. The cation exchange process of PbSe/CdSe HNCs has been investigated by HRTEM in situ heating experiments in combination with MD simulations and DFT calculations in Chapter 5. In the HRTEM experiments, we bserved that Cd atoms in PbSe/CdSe nanodumbbells (CdSe rods with one or two PbSe tip(s)) are replaced by Pb atoms. The exchange rate depends on the heating temperature and the amount of Pb atoms present in the system. Sometimes, fully converted PbSe nanodumbells can be observed. MD simulations were performed to investigate the mechanism of this cation exchange process. It was found that the the CdSe domains near the PbSe/CdSe interfaces have significant structural disorder. These findings are in line with the experimental observation that the exchange process proceeds in a layer-by-layer fashion along the WZ- direction. We concluded that cation exchange in PbSe/CdSe HNCs is mediated by the local structural disorder which enables the formation of vacancies and accelerated the motion of cations. In Chapter 6, a coarse-grained psuedoligand model was introduced to simulate cation exchange in PbS colloidal NCs taking into account the cation-solvent interactions. Modelling colloidal NC systems including interactions with the solvent has long been a challenge due to the large system size and long time scales. Here, we incorporated the effects of ligands and solvents into negatively charged large spherical coarse-grained psuedoligands. MD simulations combining coarse-grained and all-atom models can successfully reproduce the cation exchange process in PbS colloidal NCs. Simulations show that the exchange rate and system equilibrium can be controlled by the temperature and by changing ligands. The exchange process is directly related to vacancy formation and the high mobility of Cd ions at the PbS/CdS interface. Our simulations also predict that high-pressure conditions will be beneficial for achieving fast exchange at elevated temperatures. Our coarse-grained model can be easily extended to other systems for the computational investigation of transformations in nanostructures.