Electronic Doping and Trap Reduction of Quantum Dots

Abstract

Both undoped and doped semiconductor quantum dots (QDs) offer unique opportunities for studying the fundamental physics of quantum confinement. Obtaining a thorough understanding of their physical properties is necessary for development of efficient and robust materials for use in a wide range of applications such as optoelectronics (optical switches, light emitting diodes (LEDs), photovoltaics, and lasers), biosensing, and nanoelectronics. This thesis involves studies that look specifically at the effects of electronic doping and trap reduction in undoped and Mn<super>2+</super> -doped QDs. Investigation of the effect of electron-Mn<super>2+</super> exchange interactions on Mn<super>2+</super> luminescence in Mn<super>2+</super>:CdS nanocrystal films through an electrochemical method reveals effective Auger de-excitation of photoexcited Mn<super>2+</super>. The doped QDs demonstrate increased sensitivity to Auger de-excitation <italic>versus</italic> undoped QDs due to the long lifetime of the Mn<super>2+</super> excited state. Photochemical electronic doping of colloidal CdSe nanocrystals is achieved for the first time through the use of a borohydride hole quencher, Li[Et₃BH], and the high spectroscopic quality of the resulting <italic>n</italic>-type nanocrystals allows for advanced characterization by absorption and photoluminescence. Additionally, chemical titrations of the n-type nanocrystals confirm electron accumulation and suggest significant electron trapping for some of the nanocrystals. Spectroelectrochemical measurements on undoped and Mn<super>2+</super>-doped ZnSe QDs target charge injection into traps within the semiconductor bandgap. In both the undoped and doped QDs, transfer of electrons into the nanocrystal film is directly correlated with enhanced photoluminescence quantum yield and dubbed "electrobrightening." This method of brightening through trap passivation is extended to colloidal systems through the use of outer-sphere reductants and ultimately improves the ensemble photoluminescence quantum yield of Mn<super>2+</super> -doped ZnSe QDs from 14% to 80%.