Date of Award
Doctor of Philosophy (PhD)
Materials Science & Engineering
Christopher B. Murray
Monodisperse colloidal nanocrystals (NCs) provide an opportunity to access physical properties that cannot be realized in bulk materials, simply by tuning the particle size or shape. These NCs form the basis of an artificial periodic table that can be used as building blocks to engineer a new class of solid-state materials with emergent properties. The monodispersity offers a structural advantage for assembling NCs into an ordered superlattice, in addition to a narrow distribution of band energies which in principle promote more efficient transport when the NCs are electronically coupled in a thin film solid after undergoing surface chemistry treatments. However, previous methods for NC assembly have been limiting in their scalability, and while there has been much work in general on the effects of different ligand surface chemistries on semiconductor NC solids, little work has been done to controllably tune the Fermi level and quantify its position in order to promote better device engineering. Herein, we investigate dip-coating as a method by which to scale up NC superlattice assembly. We demonstrate large-area ordering on wafer-scale for both single component and binary nanocrystal superlattices with a diverse set of NC materials and binary crystal geometries. We confirm the extent over which these films are ordered via GISAXS, TEM, and SEM characterization. In the remainder of this work, we study the electronic effects of different ligand chemistry treatments of the NCs. We show that a sequential two step surface treatment can offer increased control over the tuning of the Fermi level and we quantify its positioning and band edge energies relative to vacuum level via a pairing of temperature dependent Seebeck measurements, cyclic voltammetry, and absorption spectroscopy. This provides a reference by which NC devices can be more precisely engineered. Furthermore, we apply that the AC magnetic field Hall effect measurement to a series of common ligand treatments used for making NC devices such as solar cells and field effect transistors to better understand their relative electronic transport properties. We demonstrate this method can be used to determine the hall mobility in these generally high resistivity, low mobility films.
Gaulding, Elizabeth Ashley, "Electronic and Structural Investigation of Nanocrystal Thin Films Tuned via Surface Chemistry" (2015). Publicly Accessible Penn Dissertations. 1731.