Single nanocrystal microscopy and spectroscopy unveils hidden mechanistic information in cation exchange

Nanocrystals provide a whole new lens to visualize chemistry. These nanoscopic crystallites can be remarkably different in their reactivities from our expectations based on the macroscale. Metastable crystal morphologies, crystal domains smaller than the diffusion lengths of ions or electrons, and optical/electronic properties that are highly sensitive to small perturbations are some of the unique features of nanocrystals. However, heterogeneity often shrouds these unique features. Each individual nanocrystal is different from every other nanocrystal: defects, dopants, ligand coverage, size, and geometry can all vary vastly within a single sample of several nanocrystallites. Ensemble-averaged measurements, therefore, severely limit the development of new insights into nanoscale reactivity. Investigating these novel nanoscale phenomena requires new tools that can resolve the reactivity of each individual nanocrystal. In my PhD studies, I have used cation exchange as a model reaction to investigate nanoscale reactivity using single-nanoparticle-resolved optical microscopy techniques. Cation exchange is possibly the most revolutionary chemical method for nanocrystal synthesis. This class of reaction offers the ability to transform simple binary nanotemplates into nanocrystalline materials with arbitrarily new compositions. Control of the final phase/composition requires a precise atomistic understanding of the reaction, which is why the mechanistic insight provided by single-nanocrystal-level optical microscopy is important. Chapter 1 gives a brief summary of optical microscopy of nanostructures, outlining the huge impact it has had on the understanding of steady-state optical properties, catalytic properties, and chemical reactivity. The chapter then transitions to a description of cation exchange, its unique features, and the mechanistic understanding that has been gathered so far. Chapter 2 describes my work probing individual cadmium selenide quantum dot nanocrystals undergoing cation exchange. The fluorescence-based individual nanocrystal reaction trajectories led to the finding that cation exchange is a cooperative reaction, which had yet to be observed in nanoscience. Chapter 3 describes the intermittent fluorescent emission of individual CdSe nanocrystals in the course of their conversion to Ag2Se. It is found that close to the conversion point, a CdSe nanocrystal experiences a drastic increase in its fluorescence intermittency. This increase in intermittency is caused by defective/doped intermediates formed transiently in the course of the exchange process. Chapter 4 lays the ground work for the mechanistic understanding of a whole new reaction, the cation exchange of CdSe with mercury, which does not appear to be cooperative. This exchange seems to have very complex intermediate structures that a combination of in-situ optical probing and ex-situ high resolution electron microscopy and structural studies should resolve. Chapter 5 summarizes doped quantum dots. These doped quantum dots display highly tunable plasmon resonances due to free charge carriers created by doping. Cation exchange presents a powerful tool to synthesize these doped quantum dots with specificity of the composition. The control of composition allows tuning of the plasmon resonance properties for applications such as communications technology and biomedical imaging.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Chemistry

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2016

Author dc:creator

• Routzahn, Aaron Lynn

Contributors dc:contributor

• Jain, Prashant K.
• Dlott, Dana D.
• Rodríquez-López, Joaquín
• Shim, Moonsub

• Quantum dot
• nanocrystal
• cation exchange
• single nanocrystal micrsocopy

Statement dc:rights

• Copyright 2016 Aaron Routzahn

Language dc:language

en

Handle dc:identifier

http://hdl.handle.net/2142/92885

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/92885