2020 Theses Doctoral

Formation Mechanism of Monodisperse Colloidal Semiconductor Quantum Dots: A Study of Nanoscale Nucleation and Growth

Greenberg, Matthew William

Since the fortuitous discovery of the existence of quantum size effects on the band structure of colloidal semiconductor nanocrystals, the development of synthetic methods that can form nanoscale crystalline materials of controllable size, shape, and composition has blossomed as an empirical scientific achievement. The fact that the term “recipe” is commonly used within the context of describing these synthetic methods is indicative of the experimentally driven nature of the field. In this respect, the highly attractive photophysical properties of semiconductor nanocrystals—as cheap wavelength tunable and high quantum yield absorbers and emitters of light for various applications in lighting, biological imaging, solar cells, and photocatalysis—has driven much of the interest in these materials. Nevertheless, a more rigorously predictive first-principles-grounded understanding of how the basic processes of nanocrystal formation (nucleation and growth) lead to the formation of semiconductor nanocrystals of desired size and size dispersity remains an elusive practical and fundamental goal in materials chemistry. In this thesis, we describe efforts to directly study these dynamic nucleation and growth processes for lead chalcogenide nanoparticles, in many cases in-situ, using a mixture of X-ray scattering and UV-Vis/NIR spectroscopy.

The lack of a rigorously predictive and verified mechanism for nanocrystal formation in solution for many material systems of practical interest is due both to the inherent kinetic complexity of these reactions, as well as the spectroscopic challenge of finding in-situ probes that can reliably monitor nanoscale crystal growth. In particular, required are direct time-resolved structural probes of metastable inorganic amorphous and crystalline intermediates formed under the high temperature inert conditions of nanocrystal synthesis. It is, at the very least, highly challenging to apply many of the standard spectroscopic tools of mechanistic inorganic and organic chemistry such as ¹H NMR spectroscopy, IR vibrational spectroscopy, and mass spectrometry to this task. A notable counterexample is, of course, UV-vis/NIR absorbance and emission spectroscopies, which are of great value to the studies described herein. Nevertheless, to address this relative dearth of conventional spectroscopic probes, here we explore the use of X-ray Total Scattering real space Pair Distribution Function (PDF) analysis and Small Angle X-ray Scattering (SAXS) techniques to directly probe the crystallization process in-situ. Time-resolved measurements of the small angle reciprocal space scattering data allow mapping of the time evolution of the colloidal size and concentration of the crystals during synthesis, while the Fourier transform of scattering data over a wide range of reciprocal space provides direct insight into the local structure. Through this approach, we compare direct observations of these nucleation and growth processes to the widely cited theoretical models of these processes (Classical Nucleation Theory and LaMer “Burst Nucleation”) and find a number of stark differences between these widely cited theories and our experiments.

The first two chapters cover the results of these 𝘪𝘯-𝘴𝘪𝘵𝘶 diffraction studies. Chapter 1 focuses on small angle X-ray scattering data collection and modeling. Chapter 2 focuses upon lead sulfide and lead selenide real space PDF analysis of local structural evolution during synthesis. Finally, Chapter 3 discusses a project in which we examine the origins of emergent semiconducting electronic structure in an increasing size series of atomically precise oligomers of [Ru₆C(CO)₁₆]²⁻ bridged by Hg²⁺ and Cd²⁺ atoms. Using an atomically well-defined series of molecules that bridge the small molecule and nanoscale size regimes, we discuss the factors that give rise to controllable semiconductor electronic structure upon assembly into extended periodic structures in solution. In all these projects, we seek to highlight the value of applying concepts of molecular inorganic chemistry—ligand binding models, relative bond strengths, in addition to kinetics and thermodynamics—to explain our observations regarding nanocrystal nucleation and growth. Consideration of the chemistry of nanocrystal formation processes provides a valuable compliment to the physics-based classical models of nucleation and growth that do not explicitly consider the system specific molecular structure and bonding.