Understanding changes in materials properties as a function of size is crucial for both fundamental science development and technological applications. Size restriction results in quantum confinement effects that modify both energy level structures and electron dynamics of solid materials. This study investigates individual quantum states in a single nanocrystal. Single electron charging effects in gold and semiconductor nanocrystals are observed. Charging effects are found to be dominant in samples, where the nanocrystals are weakly coupled to the substrate. For nanocrystals strongly coupled to the substrate, nanocrystal-substrate tunneling rate is larger than tip-nanocrystal tunneling rate. Therefore, the resulting peaks in the dI/dV spectrum are attributed to tunneling through the energy levels of the nanocrystal. A newly developed nanocrystals BEES technique is used successfully to further explore quantized energy levels and electron dynamics in single gold nanocrystals. BEES samples were grown successfully by depositing $unit[10]{nm}$ thick gold on silicon substrates. Nanocrystals are chemically attached to the gold substrate using a self assembled monolayer (SAM) of xyelendithiol molecules. Immobile and single isolated nanocrystals were imaged at low temperature. A BEES turn-on voltage of $unit[0.84]{V}$ was found on nanocrystal-free region of the substrate. The BEES spectrum acquired on a single gold nanocrystal is found to be attenuated by a factor of 10 when compared with BEES acquired on the substrate. The attenuation is attributed to electron relaxation to lower energy states before tunneling out of the nanocrystal. The measured hot electron lifetimes from experimental data were found to be on the order of $unit[16]{picoseconds}$, which is a long time compared to lifetimes in bulk metals or large nanocrystals. The long measured lifetimes result from the molecular-like energy level structures of these small nanocrystals.

Non-volatile memory can keep the data without supplying power, and it is suitable for portable electronic products due to the advantage of low power consumption. In current industrial production, high-temperature and long-time process are necessary for the fabrication of non-volatile memory, which are heavy loadings on production capacity and lots cost. Therefore, decreasing the temperature of the process is a trend. Recently using the oxidation treatment of supercritical carbon dioxide fluid can efficiently decrease the temperature of the process. In this thesis, the mixture layer of Zn, Sn, and SiO2 is applied to reduce the temperature of process, and to employ the defects of ZnO and SnO2 as floating gate for electron storage to fabricate the nonvolatile memory device. Zn and Sn are applied due to the low temperature melting points. To ensure the layer of cosputtering with Zn and Sn to be able to successfully fabricate as nano material device, the process of traditional rapid temperature annealing treatment was applied for first step. The co-sputtered Zn-Sn-SiO2 thin film was deposited on the tunneling oxide layer, and then the thin film was treated with varied annealing temperature to precipitate ZnO and SnO2 nanocrystals. After that, the C-V measurement is applied to analyze the change of the electrical and material properties. Using a positive bias, the electrons are injected into the oxide layer, by the threshold voltage the offset is occurred, which is defined as the memory window of the memory effect, and the property of nonvolatile memory will be applied. In addition, no matter the charge is injected from the gate oxide or tunnel oxide, the defects position of DLTS¡¦s peak is with the same property. The supercritical carbon dioxide fluid technology has been performed to study the memory effect. The capability of electron injection, storages and the defect, in the storage layer were studied by the C-V measurement and DLTS. The experiment confirmed that the Zn-Sn alloy has the memory property after it been treated by the supercritical carbon dioxide fluid technology. It has shown that Zn can promote to the storage capability ability due to the formation of deep level defects of SnO2 from the DLTS spectra. A new species is found at 0.93 eV with low activation energy and high capability of electron storage. The defect formation mechanism of Zn, ZnO, Zn-O-Si, Sn, and SnO are analyzed by found by the XPS and DLTS. The device fabrication using Zn-Si alloy and supercritical carbon dioxide fluid technology has the potential to reduce the process temperature and to improve the memory property of nonvolatile memory device.

Knowledge of energy dissipation and relaxation in electron, spin, and lattice degrees of freedom is of fundamental importance from both a technological and scientific point of view. In this dissertation, the electronic and magnetization dynamics of photoexcited colloidal cobalt substituted iron oxide nanocrystals, CoxFe3-xO4, were investigated through transient absorption and pump-probe Faraday rotation measurements. In this dissertation, linearly polarized femtosecond optical pulses at 780 nm were used to excite the weak absorption originating from the intervalence charge transfer transition (IVCT) between Fe2+ and Fe3+ ions of Fe3O4 nanocrystals. The timescale and corresponding relaxation processes of electronic relaxation dynamics of the excited IVCT state were first discussed. Size effect on electronic relaxation dynamics in Fe3O4 nanocrystals is not distinct on the basis of result from this study. One interesting feature of electronic dynamics data of photoexcited Fe3O4 nanocrystals is the creation of coherent acoustic phonons. Information on lattice temperature was obtained by measuring the period of coherent acoustic phonon as a function of excitation fluence and fit into a simple model based on Lamb’s theory. Since optical control of the magnetization can be either through optical or heating mechanisms, quantitative estimation of degree of demagnetization caused by lattice temperature is made by using Langevin function. The result from such estimation indicates the effect of lattice temperature rise on magnetization is too small to significantly affect the magnetization of Fe3O4 nanocrystals. Magnetization dynamics were studied via pump-probe Faraday rotation measurements. Optical excitation with near-infrared pulse resulted in an ultrafast demagnetization in 100fs. The energy of the excited state then relaxed through spin-lattice relaxation (SLR). Effects of surface spin and chemical tuning on the SLR were investigated by comparing the magnetization recovery timescales of nanocrystal with different particle sizes and cobalt concentration respectively. The experimental result is explained by a simple model where interior and surface spins contributed to the spin-lattice relaxation process differently. The observations suggest that spin-orbit coupling of the surface is stronger and less sensitive to stoichiometric variation than the interior spins of the nanocrystals.

In this study, silicon (Si) rich silicon dioxide (SiO2) films and terbium (Tb) embedded in three different Si containing films has been produced by e-beam evaporation and magnetron sputtering techniques. Post deposition annealing was done for different temperatures and durations to study its effect on both Si nanocrystal formation and Tb luminescence. It was verified by X-ray diffraction technique (XRD) that Si nanocrystals were formed in Si rich matrices. Energy dispersive X-ray (EDS) spectroscopy analysis was carried out to determine the relative concentrations of the atoms inside the produced films. X-ray photoelectron spectroscopy (XPS) gave the evidence of different bonding structures inside the Tb-Si-O containing films. Depth profile measurements were carried out to analyze changes in the relative concentration during sputtering of the layers after annealing of the Tb containing film. Luminescence characteristics of Si nanocrystals and Tb3+ ions were studied by photoluminescence (PL) spectroscopy. It was observed that Tb3+ luminescence enhanced by an energy transfer from Si nanocrystals and trap levels in a matrix. This result supplies valuable information about the excitation paths of Tb3+ ion the way of intense luminescence.

This thesis examines the luminescence properties of nanoscale silicon (Si) by using spectroscopic techniques. Since the development of new optical devices requires understanding light emission mechanism optical spectroscopy has become more important tool in the analysis of these structures. In this thesis, Si nanocrystals embedded in SiO2 matrix will be studied. Photoluminescence (PL) and Time-resolved photoluminescence spectroscopy (TRPL) have been used to detect the light emission in UV-Vis-NIR range. Experiments have been performed in the temperature range 10-300 K. PL is sensitive to impurities and defects that affect materials quality and device performance. In this context, the role of defects in limiting the luminescence of Si nanocrystals and the removal of these defects by hydrogen passivation has been investigated. v TRPL was employed to determine the time evolution of photoluminescence as function of temperature. The decay time of the PL spectra was determined by a stretched exponential function and perfectly fitted to an expression based on three excitonic levels. Carrier lifetimes associated with these three levels were determined and compared with literature. Additionally, temporal variation of PL from free-standing Si nanoparticles is studied under a strong laser illumination. The observed bleaching behavior (time dependent emission intensity), which is reversible, have discussed in terms of exciton trapping at the interface between nanocrystal and the surrounding oxide layer. The results of this thesis will provide new insight on the understanding of light emission mechanism of Si nanocrytals.

A logical progression from the maturing field of colloidal semiconductor quantum dots to the emerging subclass of impurity-doped colloidal semiconductor nanoparticles is underway. To this end, the present work describes the formation and analysis of a new form of Tin-doped Indium Oxide (ITO). The form is that of a colloidal dispersion comprised of pure-phase, 4-6 nanometer ITO particles possessing an essentially single crystalline character. This system forms a non-agglomerated, optically clear solution in a variety of non-polar solvents and can remain in this state, at room temperature, for months and potentially, years. ITO is the most widely used member of the exotic materials family known as Transparent Conductive Oxides (TCOs) and is the primary enabling material behind a wide variety of opto-electronic device technologies. Material synthesis was achieved by initiating a series of interrelated nucleophilic substitution reactions that provided sufficient intensity to promote doping efficiencies greater than 90% for a wide range of tin concentrations. The optical clarity of this colloidal system allowed the intrinsic properties of single crystalline ITO particles to be evaluated prior to their use in thin-films or composite structures. Monitoring the temporal progression of n-type degeneracy by its effects on the optical properties of colloidal dispersions shed light on the fundamental issues of particle formation, band filling (Burstein-Moss) dynamics, and the very origin of n-type degeneracy in ITO. Central to these studies was the issue of excess electron character. The two limiting cases of entirely free and entirely confined electron motion were evaluated by application of bulk-like band dispersion analysis and the effective mass approximation, respectively. This provided a means to estimate the number of excess conduction band electrons present within an individual particle boundary. The ability to control and optimize the level of n-type degeneracy within the colloidal ITO nanoparticle form by compositional variation was also demonstrated. A key to the widespread adoption of a new material by industry is an ability to produce multi-gram and perhaps, kilogram quantities with no significant sacrifice in quality. Accordingly, a modified synthesis process was developed to allow for the mass production of high-quality colloidal ITO nanocrystals.

Nanocrystalline materials have been under extensive study in the past two decades. The reduction in grain size induces many abnormal behaviors in the properties of nanocrystalline materials, that have been investigated systematically and quantitatively. As one of the most fundamental relations in materials science, the structure-property relation should still apply on materials of nano-scale grain sizes. The characterization of grain boundaries (GBs) and related entities remains a big obstacle to understanding the structure-property relation in nanocrystalline materials. It is challenging experimentally to determine the topological properties of polycrystalline materials due to the complex and disordered grain boundary network presented in the nanocrystalline materials. The constantly improving computing power enables us to study the structure-property relation in nanocrystalline materials via Monte Carlo and molecular dynamic simulations. In this study, we will first propose a geometrical construction method based on inverse Monte Carlo simulation to generate digital microstructures with desired topological properties such as grain size, interface area, triple junction length as well as their statistical distributions. The influences on the grain shapes by different topological properties are studied. Two empirical geometrical laws are examined including the Lewis rule and Aboav-Weaire law. Secondly, defect free nanocrystalline Copper (nc-Cu) samples are generated by filling atoms into the Voronoi structure and then relaxed by molecular dynamics simulations. Atoms in the relaxed nc-Cu samples are then characterized into grain atoms, GB interface atoms, GB triple junction atoms and vertex atoms using a newly proposed method. Atoms in each GB entity can also be identified. Next, the topological properties of nc-Cu samples before and after relaxation are calculated and compared, indicating that there exists a physical limit in the number of atoms to form a stable grain boundary interface and triple junction in nanocrystalline materials. In addition, we are able to obtain the statistical averages of geometrical and thermal properties of atoms across each GB interfaces, the so-called GB profiles, and study the grain size, misorientation and temperature effects on the microstructures in nanocrystalline materials. Finally, nc-Cu samples with different topological properties are deformed under simple shear using MD simulation in an attempt to study the structure-property relation in nanocrystalline materials.

Thin film copper indium gallium selenide (CIGS) solar cells have exhibited single junction power conversion efficiencies above 20% and have been commercialized. The large scale production of CIGS solar cells, however, is hampered by the relatively high cost and poor stoichiometric control of coevaporating tertiary and quaternary semiconductors in high vacuum. To reduce the overall cost of production, CIGS nanocrystals with predetermined stoichiometry and crystal phase were synthesized in solution. Colloidal nanocrystals of CIGS provide a novel route for production of electronic devices. Colloidal nanocrystals combine the well understood device physics of inorganic crystalline semiconductors with the solution processability of amorphous organic semiconductors. This approach reduces the overall cost of CIGS manufacturing and can be used to fabricate solar cells on flexible and light-weight plastic substrates. As deposited CIGS nanocrystal solar cells were fabricated by ambient spray-deposition. Devices with efficiencies of 3.1% under AM1.5 illumination were fabricated. Examining the external and internal quantum efficiency spectrums of the devices reveal that in nanocrystal devices only the space charge region is actively contributing to the extracted photocurrent. The device efficiency of the as-deposited nanocrystal films is presently limited by the small crystalline grains (≈ 15 nm) in the absorber layer and the relatively large interparticle spacing due to the organic capping ligands on the nanocrystal surfaces. Small grains and large interparticle spacing limits high density extraction of electrons and holes from the nanocrystal film. A Mott-Schottky estimation of the space charge region reveals that only 50 nm depth of the nanocrystalline absorber is effectively contributing to the photogenerated current. One strategy to improve charge collection involves increased space charge region for extraction by vertical stacking of diodes. A much longer absorption path for the photons exists in the space charge region with the stacked devices, increasing the probability that the incident radiation is absorbed and then extracted. This method enables an increase in the collected short circuit current. The overall device efficiency, however, suffers with the increased series resistance and shunt conductance of the device. Growth of nanocrystal grains was deemed necessary to achieve power conversion efficiencies comparable to vapor deposited CIGS films. Simple thermal treatment of the nanocrystal layers did not contribute to the growth of the crystalline grain size. At the same time, because of the loss of selenium and increased trap density in the absorber layer, there was a measurable decrease in device efficiency with thermal processing. For increased grain size, the thermal treatment of the absorber layer took place in presence of compensating amounts of selenium vapor. The process of selenization, as it is called, took place at 500°C in a graphite box and led to an increase of the grain size from 15 nm to several microns in diameter. Devices with the increased grain size yielded efficiencies up to 5.1% under AM1.5 radiation. Mott-Schottky analysis of the selenized films revealed a reduction in doping density and a comparable increase in the space-charge region depth with the increased grain size. The increased collection combined with the much higher carrier mobility in the larger grains led to achieved Jsc values greater than 20 mA/cm2. Light beam induced current microscopy (LBIC) maps of the devices with selenized absorber layers revealed significant heterogeneity in photogenerated current. Distribution of current hotspots in the film corresponded with highly selenized regions of the absorber films. In an effort to improve the overall device efficiency, improvements in the selenization process are necessary. It was determined that the selenization procedure is dependent on the selenization temperature and processing environment. Meanwhile, the reactor geometry and nanocrystal inks composition played important roles in determining selenized film morphology and the resulting device efficiency. Further work is necessary to optimize all the parameters to improve device efficiency even further. / text

Colloidal nanocrystals with optical properties in the near-infrared (NIR) are of interest for many applications such as photovoltaic (PV) energy conversion, bioimaging, and therapeutics. For PVs and other electronic devices, challenges in using colloidal nanomaterials often deal with the surfaces. Because of the high surface-to-volume ratio of small nanocrystals, surfaces and interfaces play an enhanced role in the properties of nanocrystal films and devices. Organic ligand-capped CuInSe2 (CIS) and Cu(InXGa1-X)Se2 (CIGS) nanocrystals were synthesized and used as the absorber layer in prototype solar cells. By fabricating devices from spray-coated CuInSe nanocrystals under ambient conditions, solar-to-electric power conversion efficiencies as high as 3.1% were achieved. Many treatments of the nanocrystal films were explored. Although some treatments increased the conductivity of the nanocrystal films, the best devices were from untreated CIS films. By modifying the reaction chemistry, quantum-confined CuInSeXS2-X (CISS) nanocrystals were produced. The potential of the CISS nanocrystals for targeted bioimaging was demonstrated via oral delivery to mice and imaging of nanocrystal fluorescence. The size-dependent photoluminescence of Si nanocrystals was measured. Si nanocrystals supported on graphene were characterized by conventional transmission electron microscopy and spherical aberration (Cs)-corrected scanning transmission electron microscopy (STEM). Enhanced imaging contrast and resolution was achieved by using Cs-corrected STEM with a graphene support. In addition, clear imaging of defects and the organic-inorganic interface was enabled by utilizing this technique. / text

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