Defect-Related Luminescence in Nanocrystals: Spectroscopy and Computation

Abstract

Dopants and defects play an important role in the luminescence of semiconductors, from classic bulk phosphors to more recently developed colloidal nanocrystals. This thesis describes several studies on the photophysics of defect-related luminescence in semiconductor nanocrystals, with a particular focus on copper and silver charge-transfer luminescence. The combination of density functional theory (DFT) and spectroscopy can provide deeper insight into the photophysics of these materials and help resolve unanswered questions about their luminescence. In chapter 2, DFT is used to investigate various aspects of the electronic structure and photoluminescence mechanism of copper-doped CdSe nanocrystals, including the copper oxidation state, the origin of the broad luminescence line width, and the excited-state singlet−triplet splitting. These calculations support and expand upon previous experimental results. Chapter 3 extends these experimental and computational studies to silver-doped CdSe nanocrystals. These materials have very similar photoluminescence properties to their copper-doped analogues, but they have significant electronic-structure differences due to inverted bonding between the silver dopant and its neighboring anions. Chapter 4 addresses the similarities between the photoluminescence of copper-doped and copper indium sulfide nanocrystals. DFT calculations demonstrate a significant tendency for the hole to localize in the valence band of copper indium sulfide, which has significant copper character and resembles the copper impurity level in doped nanocrystals. Other dopants and defects can also influence nanocrystal luminescence in different ways. Chapter 5 addresses the effects of slow electron trapping and detrapping on the intensity and dynamics of delayed luminescence in nanocrystals by varying the excitation pulse width. Chapter 6 is an investigation of photoinduced magnetization in manganese-doped CdSe nanocrystals; the formation of excitonic magnetic polarons is studied by continuous-wave and time-resolved magneto-optical spectroscopies. Together, these studies demonstrate a variety of ways that dopants and defects can affect the photoluminescence of semiconductor nanocrystals.