Date of Award
Doctor of Philosophy (PhD)
Arthur J. Nozik
Matthew C. Beard
Colloidal semiconductor nanocrystals are a promising class of functional materials that have been the subject of intense research and development for over thirty years due to their highly tunable optical and electronic properties. Potential applications that can take advantage of these tunable parameters include nanoscale photonics, light-emitting diodes, biological labeling/imaging, and next-generation solar energy capture, conversion, and storage strategies. Nanoscale physicochemical structure drastically influences nanocrystal optical and electronic behavior; therefore, a more thorough understanding of how to manipulate, characterize, and optimize nanoscale composition, size, and shape is necessary for their wide-spread technological implementation. This thesis explores how postsynthetic modification of colloidal semiconductor nanocrystal physicochemical structure affects emergent optical and electronic properties.
To this extent, I studied the size-dependent electronic band edge energies of semiconductor nanocrystal thin films fabricated using a sulfur-containing ligand. We found that the Fermi level of lead sulfide quantum dot films prepared with 1,2-ethanedithiol shifted towards the valence band with increasing quantum dot core diameter due to ligand-induced stoichiometric effects, emphasizing the importance of surface chemistry on electronic band energetics. Additionally, through careful analysis of the valence band region spectra of bulk lead sulfide and the lead sulfide quantum dot thin films, we found that photoelectron spectroscopy is not sensitive to the lead sulfide valence band maximum due to low density of states; therefore, we developed a simple effective mass density of states model to more accurately characterize and extract the quantum dot film valence band edge energy.
I also developed a postsynthetic solution-phase X-type ligand exchange method to systematically and cleanly modify the surface chemistry composition of lead sulfide quantum dots. I established quantitative relationships between changes to the physicochemical nature of nanocrystal surfaces and their substantial impacts on optical and electronic properties - specifically broadband optical absorbance enhancement and electronic band edge energies. I studied the nanocrystal surface coordination thermodynamics of one such exchange using quantitative nuclear magnetic resonance and spectrophotometric absorbance titration spectroscopies and found a high degree of ligand binding cooperativity at the nanocrystal surface.
Finally, I helped develop cation exchange techniques to modify the composition of inorganic nanocrystal cores. In the first method, I used postsynthetic partial cation exchange to incorporate Ag+ ions into PbSe quantum dots and observed the spectroscopic signatures of p-type electronic impurity doping. In the second method, I studied multiple exciton generation (MEG) in lead/cadmium sulfide Janus-like, segmented heterostructure semiconductor nanocrystals also prepared using postsynthetic partial cation exchange. I found that the MEG threshold turns on right at the thermodynamic limit of twice the nanocrystal band gap, and I measured the most rapid increase in photon-to-exciton quantum yield with increasing excitation energy of any nanocrystal material system reported to date.
Overall, the culmination of this work highlights the incredible flexibility of nanocrystal based material systems, as well as the importance of establishing clean postsynthetic chemical modification strategies to clearly and quantitatively link nanocrystal physicochemical structure with emergent optical and electronic properties. Building on previous reports, we firmly establish that nanocrystal inorganic core composition, size, and shape, as well as surface chemistry, are important physicochemical parameters to consider for the functionality of nanocrystal systems. I expect that the relationships and principles presented her
Kroupa, Daniel McCray, "Manipulation of Colloidal Semiconductor Nanocrystal Optical and Electronic Properties via Postynthetic Chemical Modification" (2017). Chemistry & Biochemistry Graduate Theses & Dissertations. 278.