Plasmonic nanocrystals (NCs) have been a focus of intense research over the past decade due to their unique optical properties and wide applications. Indium (III) oxide (In2O3) is an ideal host lattice for plasmonic NCs, owing to its high charge carrier concentration and mobility. In this project, one pot colloidal synthesis has been utilized to prepare antimony-doped In2O3 (AIO) NCs and titanium-doped In2O3 (TIO) NCs. It is shown that both of these doped NC samples exhibit the tunability of the plasmon resonance in the mid-infrared (MIR). For AIO NCs, it is revealed that the plasmon resonance can be well-tuned from 0.25 eV to 0.37 eV, with the maximum electron concentration of ca. 1.24 x 10^20 cm^-3 determined for 10.6 % of Sb. Compared to the broad plasmon of AIO NCs, relatively narrow plasmon of TIO NCs can be tuned from 0.13 eV to 0.28 eV by varying the doping concentration of Ti from 1.12 % to 7.8 %. Furthermore, the highest electron concentration determined for TIO NCs (7.8 % of Ti) is ca. 6.85 x 10^19 cm^-3. Both XRD patterns and high-resolution TEM images indicate that all synthesized AIO and TIO NCs retain the body-centered cubic (bcc)-In2O3 structure. UV-visible absorption spectra confirm the blue shift of the band gap for both AIO NCs and TIO NCs, because of the Burstein-Moss effect. Post treatment of as-synthesized NCs by rapid annealing under H2 or Ar illustrates that the intensity of the plasmon band can be improved appreciably. Finally, electronic and optical properties of AIO and TIO NCs were further investigated by the Density Functional Theory (DFT) calculations. It is expected that AIO and TIO NCs broadly tunable in the MIR can be employed in a variety of potential applications, including sensing, enhanced spectroscopy, and thermal imaging.

Plasmonic nanostructure materials have been widely investigated recently because of their considerable potential for applications in biological and chemical sensors, nano-optical devices and photothermal therapy. Compared to metal nanocrystals (NCs), doped semiconductor NCs with tunable localized surface plasmon resonance (LSPR) from near-infrared (NIR) mid-infrared (MIR) region bring more opportunities to the applications of plasmonics. Magnetoplasmonic nanostructures which could be utilized in multifunctional devices also have attracted attention due to the combination of plasmonic and magnetic properties and the manipulation of light with external magnetic fields. In this research, indium oxide (In2O3) as a typical n-type semiconductor with high mobility and carrier concentration is selected as the host lattice for doping, and molybdenum (Mo) and tungsten (W) which are transition metal elements from the same group as dopants. Colloidal molybdenum-doped indium oxide (IMO) NCs and tungsten-doped indium oxide (IWO) NCs with varying doping concentrations have been successfully synthesized, and their plasmonic and magneto-optical properties have been explored. Similarities and differences between IMO NCs and IWO NCs were discussed. Both IMO and IWO NCs have shown good tunability of plasmon resonance in the MIR range approximately from 0.22 eV to 0.34 eV. 9.2 % IMO NCs show the strongest LSPR at 0.34 eV and the maximum free electron concentration of 1.1x1020 cm-3, and 1.5 % IWO NCs exhibit the strongest LSPR at 0.33 eV with the free electron concentration of 0.94x1020 cm-3. The magneto-optical properties were studied by magnetic circular dichroism (MCD) spectroscopy. The variable-temperature-variable-field MCD spectra that coincide with the band gap absorption, indicate the excitonic splitting in the NCs. A robust MCD intensity at room temperature suggests intrinsic plasmon-exciton coupling and carrier polarization induced by plasmon, which might be phonon-mediated. A decrease in MCD signal with temperature and the saturation-like field dependence of MCD intensity for IMO and IWO NCs may related to the different oxidation states of the dopant ions since the reduced 5+ oxidation states can exhibit the Curie-type paramagnetism. IMO and IWO NCs show the coupling between exciton and plasmon in a single-phase which opens a possibility for their application in electronics and photonics. Moreover, magnetoplasmonic modes provide a new degree of freedom for controlling carrier polarization at room temperature in practical photonic, optoelectronic and quantum-information processing devices.

Currently used technologies are reaching the natural performance limit, invigorating the development of different quantum technologies. Spintronics and valleytronics are emerging quantum electronic technologies that rely on using electron spin and multiple extrema of the band structure (valleys), respectively, as additional degrees of freedom. There are also collective properties of electrons in semiconductor nanostructures that potentially could be exploited in multifunctional quantum devices. Specifically, plasmonic semiconductor nanocrystals (NCs) offer an opportunity for interface-free coupling between a plasmon and an exciton. However, plasmon-exciton coupling in single-phase semiconductor NCs remains challenging because confined plasmon oscillations are generally not resonant with excitonic transitions. In this thesis, using magnetic circular dichroism (MCD) spectroscopy, I examined the electron polarization in plasmonic semiconductor NCs, and the effect of electron localization, plasmon oscillator strength and damping, as well as NC morphology on carrier polarization. The results effectively open up the field of plasmontronics, which involves the phenomena that arise from intrinsic plasmon-exciton and plasmon-spin interactions. Furthermore, the dynamic control of carrier polarization allows us to harness the magnetoplasmonic mode as a new degree of freedom in practical photonic, optoelectronic and quantum-information processing devices. First, we demonstrated the control of excitonic splitting in In2O3 NCs upon excitation with circularly polarized light in an external magnetic field by simultaneous control of the electronic structure of donor defects and the nanocrystal host lattice. Using variable-temperature− variable-field MCD spectroscopy, we show that the NC band splitting has two distinct contributions in plasmonic In2O3 NCs. Temperature-independent splitting arises from the cyclotron magnetoplasmonic modes, which impart angular momentum to the conduction band excited states near the Fermi level, and increases with the intensity of the corresponding plasmon resonance. Temperature-dependent splitting is associated with the localized electron spins trapped in defect states. The ratio of the two components can be controlled by the formation of oxygen vacancies or introduction of aliovalent dopants. Using these experimental results in conjunction with the density functional theory modeling, relative contribution of the two mechanisms is discussed in the context of the perturbation theory taking into account energy separation between the NC excited states and the localized defect states. To implement such opportunities it is essential to develop robust understanding of the parameters that influence magnetoplasmon-induced carrier polarization. I investigated comparatively the plasmonic properties of Mo-doped In2O3 (IMO) and W-doped In2O3 (IWO) NCs, with a particular emphasis on the role of plasmonic properties on excitonic splitting. In contrast to tungsten dopants, which are predominantly in 6+ oxidation state, molybdenum coexists as Mo5+ and Mo6+, resulting in a lower dopant activation in IMO compared to IWO NCs. By manipulating the plasmonic properties of these two NC systems, such as localized surface plasmon resonance energy, intensity, and damping, we identified two opposing influences determining the excitonic Zeeman splitting induced by magnetoplasmonic modes. Localized surface plasmon resonance oscillator strength, commensurate with free carrier density, increases while electron damping, caused by ionized impurity scattering, decreases the transfer of the angular momentum from the magnetoplasmonic modes to the conduction band electronic states. The results contribute to fundamental understanding of the mechanism of non-resonant plasmon-exciton coupling and magnetoplasmon-induced Zeeman splitting in degenerate semiconductor NCs, allowing for the rational design of multifunctional materials with correlated plasmon and charge degrees of freedom. The magnetoplasmon-induced carrier polarization was further attested in oxygen-deficient TiO2 NCs of which the excitonic MCD band has an opposite sign compared to that observed for plasmonic In2O3 NCs indicating the plasmon-induced carrier polarization can be controlled by the electronic properties of NC host lattice. In addition, further manipulation of excitonic splitting in colloidal TiO2 NCs was demonstrated by simple control of their faceting. By changing NC morphology via reaction conditions, I controlled the concentration and location of oxygen vacancies, which can generate localized surface plasmon resonance and foster the reduction of lattice cations leading to the emergence of individual or exchange-coupled Ti(III) centers with high net-spin states. These species can all couple with the nanocrystal lattice under different conditions resulting in distinctly patterned excitonic Zeeman splitting and selective control of conduction band states in an external magnetic field. These results demonstrate that the combination of redox-active vacancy sites and nanocrystal morphology can be used to control quantum states in individual NCs using both localized and collective electronic properties, representing a new approach to complex multifunctionality in reduced dimensions. The results of this work demonstrate the ability to control carrier polarization in nonmagnetic metal oxide NCs using both individual and collective electronic properties, and allow for their application as an emerging class of multifunctional materials with strongly interacting degrees of freedom.

Colloidal Metal Oxide Nanoparticles: Synthesis, Characterization and Applications is a one-stop reference for anyone with an interest in the fundamentals, synthesis and applications of this interesting materials system. The book presents a simple, effective and detailed discussion on colloidal metal oxide nanoparticles. It begins with a general introduction of colloidal metal oxide nanoparticles, then delves into the most relevant synthesis pathways, stabilization procedures, and synthesis and characterization techniques. Final sections discuss promising applications, including bioimaging, biosensing, diagnostic, and energy applications—i.e., solar cells, supercapacitors and environment applications—i.e., the treatment of contaminated soil, water purification and waste remediation. Provides the most comprehensive resource on the topic, from fundamentals, to synthesis and characterization techniques Presents key applications, including biomedical, energy, electronic and environmental Discusses the most relevant techniques for synthesis, patterning and characterization

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.

Controlling the crystal structure of transparent metal oxides is essential for tailoring the properties of these polymorphic materials to specific applications. Structural control is usually achieved via solid state phase transformation at high temperature or pressure. The first half of this work is a kinetic study of in situ phase transformation of In2O3 nanocrystals from metastable rhombohedral phase to stable cubic phase during their colloidal synthesis. By examining the phase content as a function of time using the model fitting approach, two distinct coexisting mechanisms are identified - surface and interface nucleation. It is shown that the mechanism of phase transformation can be controlled systematically through modulation of temperature and precursor to solvent ratio. The increase in both of these parameters leads to gradual change from surface to interface nucleation, which is associated with the increased probability of nanocrystal contact formation in the solution phase. The activation energy for surface nucleation is found to be 144±30 kJ/mol, very similar to that for interface nucleation. In spite of the comparable activation energy, interface nucleation dominates at higher temperatures due to increased nanocrystal interactions. The results of this work demonstrate enhanced control over polymorphic nanocrystal systems, and contribute to further understanding of the kinetic processes at the nanoscale, including nucleation, crystallization, and biomineralization. The ability to further modify the properties of transparent metal oxides through doping of transition metal ions into the host lattice offers a world of possibilities in terms of viable systems and applications. In particular, the use of transition metal dopants to induce room temperature ferromagnetic behaviour in non-magnetic transparent metal oxides is highly desirable for applications such as spintronics. Thus, the second half of this study is concerned with the doping of Fe into nanocrystalline In2O3 via colloidal synthesis and the fundamental characterization of the nanocrystals in anticipation of further development of these materials for potential spintronics applications. Focus is placed on the relationship between the doping concentration, observed phase of the host lattice, and nanocrystal growth and properties. Structural characterizations determine that Fe as a dopant behaves quite unlike previously studied dopants, Cr and Mn, establishing a positive correlation between increasing nanocrystal size and increasing doping concentration; the opposite was observed in the aforementioned previous systems. Through analysis of X-ray absorption near edge structure spectra and the pre-edge feature, it is found that ca. 10% of the assimilated Fe is reduced to Fe2+ during synthesis. Magnetization measurements reveal that these nanocrystals are weakly ferromagnetic at room temperature, suggesting the possibility of an interfacial defect mediated mechanism of magnetic interactions. With increasing doping concentration, the decrease in saturation magnetization suggests a change in the magnetic exchange interaction and a consequential switch from ferromagnetic to antiferromagnetic behaviour. It is clear from this work that colloidal Fe-doped In2O3 nanocrystals are a promising species, prompting further investigation using additional spectroscopic and magneto-optical techniques to increase understanding of the origin of the observed properties. A thorough understanding of this system in conjunction with other transition metal doped transparent conducting oxides will enable enhanced control in the materials design process and effectively allow tailoring of these materials for specific applications, such as spintronics.

A physics book that covers the optical properties of quantum-confined semiconductor nanostructures from both the theoretical and experimental points of view together with technological applications. Topics to be reviewed include quantum confinement effects in semiconductors, optical adsorption and emission properties of group IV, III-V, II-VI semiconductors, deep-etched and self assembled quantum dots, nanoclusters, and laser applications in optoelectronics.