Colloidal CdSe/CdS heteronanorods: Synthesis, Self-Assembly and Optical properties

Abstract

This thesis addresses the study of CdSe/CdS dot-in-a-rod. The first part of the thesis offers a general introduction to colloidal semiconductor NCs and their self-assembly. The state of art of CdSe/CdS dot-in-a-rod systems is discussed, which includes both a review of the methods to synthesize the heteronanocrystals, and the effect of the spatial confinement on the NCs’ properties. Additionally, some key aspects of long-range assembly are discussed. The second part of this thesis focuses on the investigation of the electronic fine structures of CdSe/CdS NRs. We investigated the temperature-dependent exciton decay dynamics in the frame work of the exciton fine structure. We performed photoluminescence (PL) spectra and decay measurements as a function of temperature (1.5 -300 K). We proposed a model within which the low temperature exciton dynamics is governed by exciton excited states that arise from the dark state by coupling to an acoustic lattice phonon. In the third part of this thesis we studied the self-assembly of CdSe/CdS NRs. In particular we aimed to achieve high control and to understand the mechanism regulating the self-assembly at the liquid/air interface. We present a study of the self-assembly of the NRs at the liquid/air interface, combining time-resolved in-situ grazing-incidence small angle X-ray scattering(GISAXS) and ex-situ transmission electron microscopy (TEM). Our study showed that NR superstructure formation occurs at the liquid/air interface. We proved that a systematic variation of the NR length and initial concentration of the NR dispersion allowed us to tune the orientation of the NRs in the final superstructure. Furthermore, we propose a model of hierarchical self-organization for which in the first step the NRs form ordered bundles in the bulk dispersion, which are then adsorbed at the liquid/air interface, where organization in superlattice occurs. Additionally, we demonstrate the effects of the NR-NR interactions energy on the mechanism of NRs’ interfacial adsorption and self-organization. To further support our model, we study of bundle formation in the bulk solution by using Linear Dichrosim (LD) and Small Angle X-ray Scattering (SAXS). Our SAXS results provide strong evidence that well-ordered NR bundles are present in solution, which, under the influence of an external magnetic field, align parallel to . We show that the size and the concentration of the bundles in solution strongly depends on the length of the NRs, the monomer concentration, and the temperature of the suspension. The alignment of the bundles in the magnetic field as a function of their size and temperature is described with a simple statistical model. The last part of the thesis focuses on the incorporation of the NRs in silica by using the reverse micelle method, in order to render them processable in polar solvents while increasing their photochemical stability, which would be beneficial for their application in LED’s and as biolabels.