Synthesis and optical spectroscopy of (hetero)-nanocrystals: An exciting interplay between Chemistry and Physics

Abstract

This thesis describes the synthesis and study of the optical properties of various colloidal semiconductor (hetero)nanocrystals ((H)NCs). Before the experimental results are discussed in detail, the essential theoretical background on the chemical and physical aspects of this work is provided in chapters 2 and 3, respectively. In chapter 2 the concept of a nanocrystal as a dynamic hybrid particle consisting of an inorganic crystal core and organic surfactant shell is introduced. Furthermore, the synthesis of colloidal (H)NCs is discussed as well as the most important mechanisms on nucleation and growth of (H)NCs. Chapter 3 describes the effect of spatial confinement on the electronic structure of (H)NCs taking the properties of bulk semiconductors as a starting point. The formation of ZnTe magic sized nanocrystals is discussed in chapter 4. The experimental results indicate that the reaction temperature, concentration, precursors, and type of ligands play a crucial role in the formation of magic size NCs. Chapter 5 addresses the synthesis and optical properties of highly luminescent (Zn,Cd)Te-CdSe heteronanowires. The use of ZnTe MSC as seeds is found to be essential for the formation of the heteronanowires. The nanowires were formed via a growth mechanism in which the magic size clusters first undergo a fast cation exchange resulting in (Zn,Cd)Te magic size clusters. The resulting (Zn,Cd)Te magic size clusters form nanowires, via oriented attachment, in which CdSe is incorporated. The optical properties of the heteronanowires imply the formation of a spatially indirect exciton after photo-exciation. The degree of electron-hole overlap can be tuned by the amount of CdSe incorporated in the heteronanowires. Chapter 6 discusses the coupling of excitons with longitudinal optical phonons in Type-II heteronanocrystals at 4.2 K. The results clearly show that this coupling is larger for Type-II HNCs than for other types of (H)NCs. This increase is explained by the decreased electron-hole wavefunction overlap which leads to larger exciton polarizabilities that enhance the exciton-phonon coupling via Fröhlich interactions. A distinct demonstration of the size dependence of the exciton levels in CdTe QDs is given in chapter 7. Analysis of the size dependent CdTe QD absorption spectra allowed a comparison with calculated absorption spectra. Best agreement was found for tight binding calculations in which electron-hole configuration interactions were included. Chapter 8 deals with the formation and optical properties of concentric core/shell ZnSe/CdSe heteronanocrystals synthesized via a cation exchange reaction. The optical properties of the core/shell HNCs suggest that the HNCs are type-I1/2. Furthermore, the influence of the reaction temperature on the cation exchange reaction and the final HNC elemental distribution profile is investigated. In chapter 9 a study of the photoluminescence (PL) enhancement of suspensions of colloidal CdTe QDs as a function of the equilibration time after dilution is presented. Based on our results we propose a ligand mediated surface reconstruction and surface relaxation model. This model emphasizes the importance of the ligand-ligand and ligand-NC surface interactions which determine the evolution of the PL-enhancement of the colloidal CdTe QDs.