Effects of surface chemistry and electronic environment on cadmium selenide nanocrystal photoluminescence

CdSe nanocrystals are promising chromophores for use in a variety of technological applications such as biological markers and as the emissive material in light-emitting diodes. For both of these applications, it is important to maximize the photoluminescence efficiency of the nanocrystals and it is also important to control charge injection and the rate of energy transfer between the nanocrystals and the surrounding environment. The photoluminescence efficiency of the nanocrystals and their electronic and energetic accessibility are related, and are mediated by the nanocrystal surface chemistry. In this thesis, I will study the effects of surface chemistry and the local electronic environment on the photoluminescence of CdSe nanocrystals.;First, I will investigate the effects of organic ligands (specifically alkylamines and alkanethiols) on CdSe nanocrystal ensemble photoluminescence in solution. I will show that alkylamines both enhance and quench the photoluminescence of CdSe nanocrystals in solution, as well as blue-shift the nanocrystal emission energy at high amine concentration. In addition, I will show the alkanethiols quench the photoluminescence of CdSe and CdSe/CdS nanocrystals in solution. In Chapter 4, I will investigate CdSe photoluminescence quenching by alkanethiols in more detail, by single-nanocrystal spectroscopy and find that binding a single thiol molecule to a CdSe nanocrystal strongly quenches the nanocrystal photoluminescence.;Second, I will discuss the use of nanocrystals as the emissive material in light-emitting diodes and report the effects of ligand exchange on the device performance nanocrystal light-emitting diodes. Lastly, I will study the effects of electric field and current density on the photoluminescence of CdSe nanocrystals in nanocrystal light-emitting diodes. I will show that the photoluminescence intensity of the nanocrystals decreases with increasing current density. This work indicates that high photoluminescence efficiency of nanocrystals can be achieved by controlling nanocrystal surface chemistry and that improved charge balance in nanocrystal light-emitting diodes should increase their efficiency.