Point Defects in 2D and 3D Nanomaterials: A Density Functional Theory Exploration

Abstract

In this thesis, a large number of point defects was studied in both 2D and 3D nanomaterials that are of utmost importance to nanoscience by means of first principles density functional theory calculations. First, we focused on the lead chalcogenide family: PbS, PbSe, and PbTe that are frequently used in quantum dots. Defects including monovacancies, interstitials, Schottky and Frenkel type defects were considered. We found that monovacancies and Schottky defects are more favorable as indicated by the lower defect formation energy. The lead monovacancy (VPb) was predicted to be a shallow acceptor and may participate the radiative recombination in photoluminescence. Whereas sulfur monovacancy (VS) is a deep donor and may reduce the photoluminescent yield by trapping the conducting holes. Considering their significantly lower formation energy, Schottky defects may play a much more important role than currently presumed in the cation exchange process of synthesizing the PbSe-CdSe heteronanoscrystals (HNCs). Our second target is native point defects in tungsten disulfide (WS2) monolayers (MLs), a member of the 2D stackable van der Waals transition metal dichalcogenide (TMDs) family. It was found that some of the defect states exhibit strong spin-orbit splitting, which can be as large as 296 meV for the Ws antisite, and is of the same scale of the SO splitting in native WS2 (433 meV). Furthermore, we confirmed that both the WS and WS2 antisite defects possess a local magnetic moment of 2 μB around the antisite W atom. Both of the findings of the SO splitting and the magnetic moment of point defects in ML WS2 hold great promise for spintronics applications. The research continued with the atomically thin 2D monolayers of the early transition metal oxides (TMOs, with TM=Sc, Ti, V, Cr, Mn) which exhibit hitherto unknown magnetism and conductivity. According to the various valence states of the TMs, four geometries together with all the possible collinear magnetic configurations were calculated. It was found that the hexagonal M2O3 phase is the most stable phase among all the four 2D TMO phases, while the rock salt TiO, CrO and MnO are more stable than their bulk counterpart, thereby strongly suggesting the possibility of the experimental realization of these 2D TMO phases. Further analysis revealed that these 2D TMO phases possess a spectacular variety of electronic and magnetic properties including semimetallic, half-metallic, semiconducting, with different magnetic arrangements such as non-magnetic, ferromagnetic, and antiferromagnetic. The magnetism of transition metal monovacancies was also investigated. All these results present the great potential of the 2D TMOs for future applications in electronic- and spin-related fields. The results in this thesis demonstrate the strong influence of point defects on materials properties, giving rise to e.g. additional electronic states in the band gap of semiconductors, defect-induced magnetism and strong spin-orbit coupling effects. All these findings show the importance of the role of point defects in the development of nanoscience.