Effects of Inorganic Composition and Organic Ligands on Spin, Charge, and Thermal Transport Properties of Semiconductor Nanocrystals

Harvey, Samantha

doi:https://doi.org/10.21985/n2-4krq-8t93

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Effects of Inorganic Composition and Organic Ligands on Spin, Charge, and Thermal Transport Properties of Semiconductor Nanocrystals

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Semiconductor nanocrystals possess unique photophysical properties that make them desirable for many optoelectronic applications such as photovoltaics, LEDs, and quantum computing. When the size of a semiconductor is reduced to below the excitonic Bohr radius of the material, its carriers becomes quantum confined resulting in drastic changes to optical, electronic, phononic, and spintronic characteristics. Most notably these nanoscale systems exhibit absorption and emission tunable through size and shape in addition to composition. Their large surface-to-volume ratios and organic capping ligands also serve to set them apart from bulk semiconductors with ligand composition playing a key role in synthesis, crystal structure, and carrier dynamics/transfer. The tunability of these systems, from the inorganic core to the organic shell, coupled with solution processability, unique spin physics, and device incorporation make them a vibrant and critical area of research. This dissertation studies fundamental physical properties of semiconductor nanocrystals, namely thermal, electronic, and spintronic, using ultrafast spectroscopy and analytical characterization. The effects of ligands on both intraparticle and interparticle dynamics are also examined in detail. After introducing the basics of semiconductor nanocrystals and the techniques used to study them in chapters one and two, six stories are discussed. The first two focus on thermal properties, covering the effects of heat generated by photoexcitation. Femtosecond stimulated Raman spectroscopy allows monitoring of optical phonon dynamics on femtosecond and picosecond timescales. It is used in chapter three to study multiexciton recombination effects on longitudinal optical phonon dissipation in CdSe nanocrystals. As laser fluence is increased, and more excitons are generated, a phonon bottleneck limits particle cooling. A brief look into the interplay of optical phonon modes (transverse vs. longitudinal) in InP nanocrystals is also covered. Chapter four utilizes a different technique to monitor thermal properties, time-resolved X-ray diffraction. Instead of phonons, perturbations to the crystalline lattice such as expansion and disordering are examined. Cooling lifetimes and melting thresholds in CuInSe2 nanocrystals as a function of size and ligand composition (oleylamine vs. sulfide) are studied. It was found that exchange for a short anionic ligand such as sulfide does not affect the onset of melting but does allow more rapid cooling. Temperature dependent X-ray diffraction is also utilized to determine melting temperatures that are reduced from the bulk composition as well as calculate interfacial thermal conductivity. CuInSe2 nanocrystals are also the central focus of chapter five. Three different samples that were synthesized in the presence of different ligands (oleylamine, diphenylphosphine, and tributylphosphine) were examined and found to have lifetimes and quantum yields that vary by an order of magnitude. Investigations into the crystalline structure and surface compositions yield insight into a variety of defect states that limit photovoltaic device efficiency. CuInSe2 nanocrystals capped with diphenylphosphine show substantial brightening after heating to 600K and TA, Raman, and XRD are utilized to examine subsequent differences in structure. In chapter 6, intense photoexcitation and NMR is used to study the desorption of ligands (oleic acid) from the surface of CdSe nanocrystals. Both laser fluence and dosing time show strong effects on the percentage of ligand removed. For the sample with the highest absorbed photon dose, the nanocrystals begin to sinter together due to excessive ligand loss and photoluminescence quantum yield drops substantially. Most samples retain their size and emission characteristics although two show brightening after exposure. All photoexcited samples exhibit oleic acid fragmentation into aldehydes, terminal alkenes, hydrogen and water. The last two chapters (seven and eight) study spin-polarized electron transfer from CdSe/CdS core/shell nanocrystals to molecular acceptors. In the first, proof-of-concept is shown for a naphthalene diimide (NDI) acceptor coupled to three samples of varying shell thickness. Transient absorption confirms charge separation and recombination, while electron paramagnetic spectroscopy shows a spin-polarized NDI spectrum consistent with radical pair formation via the triplet mechanism. The second expands the work to include four other molecular acceptors as well as pulsed-EPR spectroscopy to determine T2 lifetimes. Using varying ligand equivalents, the ratio of triplet/singlet character and the coherence lifetime is determined to be controlled in part by the rate of initial electron transfer.

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