Effects of the Chemical Environment on the Optical Properties of Semiconductor Nanostructures

Abstract

Semiconductor nanostructures have an enormous potential to revolutionize the technology for electronic and optoelectronic devices. In recent decades, the chemical synthesis of semiconductor nanostructures, which is essential for their use on a large scale, has developed rapidly. As a result, the implementation of nanostructures in applications, such as optoelectronic devices, is now increasingly becoming the focus of research. Part of this research is to understand and control the properties of nanostructures in their entirety to unlock their full potential. Critical to the successful use of nanostructures in devices is a comprehensive understanding of how their properties are affected by their environment.

This thesis examines the influence of the chemical environment on the optical properties, particularly the photoluminescence, of semiconductor nanostructures. This question is not only interesting for applications in sensor technology, but also represents a part of the necessary basic knowledge that is of central importance for the use of nanostructures in general. Two types of nanostructures, nanowires and quantum dots, were analyzed using confocal spectroscopy.

Colloidal CdSe nanowires (or quantum wires) were prepared wetchemically. To explore the influence of the chemical environment on the optical properties, measurements were performed in flow channels and gas-flow setups. In particular, the influence of oxygen on the photoluminescence and the Raman scattering was investigated. Both irreversible and reversible effects were observed and analyzed. Based on the findings, models were constructed to identify and adequately describe the effects. This led to the elucidation of photoinduced processes, such as the activation of photoluminescence and other changes in the emission and Raman spectra. The irreversible photoinduced activation of photoluminescence could be attributed to the formation of oxides and the concomitant elimination of surface defects in the form of unpassivated selenium. The reversible changes in the spectra, observed both in the form of additional blue-shifted emission and shifts in the Raman bands, could be attributed to the action of oxygen as a reversible electron acceptor.

In addition, the influence of oxygen on the photoluminescence properties of quantum dots was investigated. Here, the focus was on the reversible processes and in particular the blinking of the photoluminescence was investigated. A detailed analysis of the experimental data and the comparison with simulated data revealed multiple different effects of oxygen on the photoluminescence of quantum dots. The studies showed that oxygen acts as an electron acceptor, neutralizing particles and thus stabilizing the photoluminescence of quantum dots excited at high excitation powers. On the other hand, the effect of oxygen on the photoluminescence blinking of quantum dots with thin shells suggests that oxygen can also act as a centre for non-radiative recombination.