We report a colloidal synthesis of gallium oxide (Ga(2)O(3)) nanocrystals having metastable cubic crystal structure (gamma phase) and uniform size distribution. Using the synthesized nanocrystal size series we demonstrate for the first time a size-tunable photoluminescence in Ga(2)O(3) from ultraviolet to blue, with the emission shifting to lower energies with increasing nanocrystal size. The observed photoluminescence is dominated by defect-based donor-acceptor pair recombination and has a lifetime of several milliseconds. Importantly, the decay of this phosphorescence is also size dependent. The phosphorescence energy and the decay rate increase with decreasing nanocrystal size, owing to a reduced donor-acceptor separation. These results allow for a rational and predictable tuning of the optical properties of this technologically important material and demonstrate the possibility of manipulating the localized defect interactions via nanocrystal size. Furthermore, the same defect states, particularly donors, are also implicated in electrical conductivity rendering monodispersed Ga(2)O(3) nanocrystals a promising material for multifunctional optoelectronic structures and devices.

Morphological and phase transformations during the growth of In2O3 nanocrystals are reported. We demonstrate a direct correlation between the nanocrystal size and structure and show that metastable corundum-type In2O3 is a transient structure in the formation of bixbyite-type nanocrystals. Metastable In2O3 can be spontaneously stabilized in nanocrystals below 5 nm.

Doping semiconductor nanocrystals is crucial for enhancing and manipulating their functional properties, but the doping mechanism and the effects of dopants on the nanocrystal growth and structure are not well understood. We show that dopant adsorption to the surfaces of colloidal In 2 O 3 nanocrystals during incorporation inhibits the nanocrystal growth. This phenomenon induces a surface stress which gives rise to a reduction in atomic volume and leads to the formation of metastable corundum-type In 2 O 3 for nanocrystals smaller than 5 nm. The growth beyond the critical size lowers the potential energy barrier height and causes the nanocrystal phase transformation. Direct comparison between Cr 3+ and Mn 3+ dopants indicates that the nanocrystal structure directly determines the dopant incorporation limits and the dopant electronic structure, which can be predicted and controlled. These results enable a new approach to designing multifunctional nanostructures and understanding the early stages of crystal growth in the presence of impurities.

We demonstrate compositionally tunable photoluminescence in complex transparent conducting oxide nanocrystals. Alloyed gallium indium oxide (GIO) nanocrystals with variable crystal structures are prepared by a colloidal method throughout the full composition range and studied by different structural and spectroscopic methods, including photoluminescence and X-ray absorption. The structures and sizes of the GIO nanocrystals can be simultaneously controlled, owing to the difference in the growth kinetics of In(2)O(3) and Ga(2)O(3) nanocrystals and the polymorphic nature of both materials. Using the synthesized nanocrystal series, we demonstrate the structural and compositional dependences of the photoluminescence of GIO nanocrystals. These dependences, induced by the interactions between specific defect sites acting as electron donors and acceptors, are used to achieve broad emission tunability in the visible spectral range at room temperature. The nature of the photoluminescence is identified as donor-acceptor pair recombination and changes with increasing indium content owing to the changes in the energy states of, and interactions between, donors and acceptors. Structural analysis of GIO nanocrystals by extended X-ray absorption fine structure spectroscopy reveals that In(3+) occupies only octahedral, rather than tetrahedral, sites in the spinel-type γ-Ga(2)O(3) nanocrystal host lattice, until reaching the substitutional incorporation limit of ca. 25%. The emission decay dynamics is also strongly influenced by the nanocrystal structure and composition. The oxygen vacancy defects, responsible for the observed photoluminescence properties, are also implicated in other functional properties, particularly conductivity, enabling the application of colloidal GIO nanocrystals as integrated optoelectronic materials.

Colloidal free-standing Cr 3+ -doped In 2 O 3 nanocrystals were synthesized in oleylamine from indium (III) and chromium (III) acetylacetonate precursors. The nanocrystals were treated with trioctylphosphine oxide to remove surface-bound dopant ions and ensure internal doping. The lattice resolved transmission electron microscopy images reveal that nanocrystals are faceted and highly crystalline, with no evidence of a secondary phase formation. The average doping concentration estimated with energy dispersive X-ray spectroscopy at the single nanocrystal level agrees with the average doping concentration from the analogous nanocrystal ensemble measurement. Ligand-field electronic absorption spectroscopy suggests that Cr 3+ dopants are preferentially substituted for In 3+ ions in their trigonally distorted octahedral (b) sites in In 2 O 3 nanocrystals. Nanocrystalline films, prepared under mild conditions using colloidal Cr 3+ -doped In 2 O 3 nanocrystals as building blocks, exhibit robust room temperature ferromagnetism. Structural and compositional analyses combined with the ligand-field spectroscopy indicate intrinsic ferromagnetism in this material. The ability to rationally synthesize and manipulate a new form of transition-metal-doped In 2 O 3 nanocrystals opens up new opportunities for spintronics applications and may provide a framework for understanding the origin of ferromagnetism in transparent conducting oxides.