Date of Award

2015

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Graduate Group

Chemistry

First Advisor

Cherie R. Kagan

Second Advisor

Christopher B. Murray

Abstract

Colloidal semiconductor nanocrystals have emerged as fascinating new materials and gained interest in the last 30 years because of their size, shape, and compositionally tunable electronic and optical properties as well as their potential to serve as artificial atoms. Challenges and opportunities have arisen when assembling nanocrystals into nanocrystal solids for electronic and optoelectronic applications, largely because of the significant influence of nanocrystal surface chemistry on the electronic, optical, and structural properties of nanocrystal solids. In order to assemble nanocrystal solids for high performance devices, we must understand and be able to control the effects of nanocrystal surface organic capping ligands, non-stoichiometry, and impurity doping on the electronic and optoelectronic properties of the nanocrystal solids.

Here, we show that exposing CdSe nanocrystals to methanol removes oleic acid capping ligands and/or Cd ions from the nanocrystal surface, introducing mid-band-gap trap states that are recombination centers for photogenerated electron/hole pairs. Using photoluminescence spectroscopy and photoconductivity measurements we illustrate that these trap states have the adverse effects of decreased photoluminescence quantum yield and lifetime and decreased photoconductivity. Treatment with CdCl2 fills these traps states, realizing a significant increase in photocurrent magnitude and lifetime. We present examples of intentional nanocrystal surface chemical modifications: non-stoichiometry doping of PbSe nanocrystals by surface enrichment with excess Pb or Se, and surface impurity doping and passivation of CdSe nanocrystals by thermal diffusion of indium. We demonstrate how these modifications shift the Fermi level of the PbSe or CdSe nanocrystal solid through the density of electronic states of the valence band (PbSe nanocrystal solid only), mid-gap, and the conduction band. Using field effect transistor and flash-photolysis time-resolved microwave conductivity measurements we show that control of the Fermi level can be used to effectively tailor the charge carrier polarity (PbSe nanocrystal solid only), mobility, and lifetime. We find that Se-rich PbSe and non-indium treated CdSe nanocrystals have a greater tendency to fuse with nearest neighbor nanocrystals than do Pb-rich PbSe and indium-treated CdSe nanocrystals. Finally, we present a flexible, robust method for assembling nanocrystals into high quality, ordered (in some cases superlattices), uniform nanocrystal solids for application in electronic and optoelectronic devices.

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