Fernandes, B.; Stanish, P. C.; Mišković, Z. L.; Radovanovic, P. V. “Photoluminescence Decay Dynamics in γ-Ga2O3 Nanocrystals: the Role of Exclusion Distance at Short Time Scales” Chem. Phys. Lett. 2017, 684, 135–140
We developed a comprehensive theoretical model describing the photoluminescence decay dynamics at short and long time scales based on the donor-acceptor defect interactions in γ-Ga2O3 nanocrystals, and quantitatively determined the importance of exclusion distance and spatial distribution of defects. We allowed for donors and acceptors to be adjacent to each other or separated by different exclusion distances. The optimal exclusion distance was found to be comparable to the donor Bohr radius and have a strong effect on the photoluminescence decay curve at short times. The importance of the exclusion distance at short time scales was confirmed by Monte Carlo simulations.
Fang, H.; Hegde, M.; Yin, P.; Radovanovic, P. V. “Tuning Plasmon Resonance of In2O3 Nanocrystals throughout the Mid-Infrared Region by Competition between Electron Activation and Trapping” Chem. Mater. 2017, 29, 4970–4979
Controlling plasmonic properties of aliovalently doped semiconductor nanocrystals in mid-infrared (MIR) spectral region is of a particular current interest, because of their potential application in heat-responsive devices and near-field enhanced spectroscopies. However, a lack of detailed understanding of the correlations among the electronic structure of the host lattice, dopant ions, and surface properties hampers the development of MIR-tunable plasmonic nanocrystals (NCs). In this article, we report the colloidal synthesis and spectroscopic properties of two new plasmonic NC systems based on In2O3, antimony- and titanium-doped In2O3 NCs, and comparative investigation of their electronic structure using the combination of the Drude–Lorenz model and density functional theory. The localized surface plasmon resonances (LSPRs) lie at lower energies and have smaller bandwidths for Ti-doped than for Sb-doped In2O3 NCs with similar doping levels, indicating lower free electron density. Surprisingly, the Fermi level is found to be higher in Ti-doped In2O3 than in Sb-doped In2O3, suggesting the formation of electron trap states on nanocrystal surfaces, which reduce carrier density without significantly impacting their mobility. Controlling the competition between doping concentration and electron trapping allowed us to generate LSPR in Ti-doped In2O3 nanocrystals deep in the MIR region, and tune the absorption spectra from 650 cm–1 to 8000 cm–1. We also demonstrated the possibility to enhance the intensity of LSPR in these new plasmonic NCs by adjusting the synthesis and post-synthesis treatment conditions. The results of this work allow for an expansion of the tuning range of LSPR of colloidal metal oxide NCs by controlling the electronic structure of aliovalent dopant and charge carrier trapping.
Ghodsi, V.; Jin, S.; Byers, J. C.; Pan, Y.; Radovanovic, P. V. “Anomalous Photocatalytic Activity of Nanocrystalline γPhase Ga2O3 Enabled by Long-Lived Defect Trap States” J. Phys. Chem. C 2017, 121, 9433-9441.
Semiconductor photocatalysis has emerged as an efficient and sustainable advanced oxidation process for wastewater treatment and other environmental remediation and forms the basis for water splitting and solar-to- fuel conversion. Nanocrystalline metal oxides are particularly promising photocatalysts because of their efficiency, stability, and low toxicity. However, the influence of the crystal structure and defects on the photocatalytic activity of these polymorphic materials is still poorly understood. In this work we investigated the structural dependence of the photocatalytic activity of nanocrystalline Ga2O3. We demonstrate that metastable cubic-phase γ-Ga2O3 prepared from colloidal nanocrystals exhibits an anomalously high photocatalytic activity, which rapidly decreases upon thermally induced transformation to monoclinic β-Ga2O3 . Using steady-state and time-resolved photoluminescence measurements we showed that the reduction in photocatalytic activity upon annealing is accompanied by a decrease in native defect (i.e., oxygen vacancy) concentration and interactions. Trapping charge carriers in defect-induced states in γ-Ga2O3 nanocrystals results in a reduced rate of charge recombination and enhanced interfacial charge transfer, which has been unambiguously confirmed by comparative measurements using In3+-doped Ga2O3. These phenomena are enabled by the unique character of defect states in γ-Ga2O3 nanocrystals which have much longer lifetime than typical metal oxide surface states. Using various scavengers, we demonstrated that reactive radicals (OH• and O2•−) formed by photogenerated charge carriers play a key role in the mechanism of photocatalytic degradation by Ga2O3. The results of this work demonstrate how manipulation of the location and electronic structure of defect sites in nanostructured metal oxides can be effectively used to control charge carrier separation and enhance photocatalytic activity, without a detriment to high surface-to- volume ratio.
Garnet, N. S.; Ghodsi, V.; Hutfluss, L. N.; Yin, P.; Hegde, M.; Radovanovic, P. V. “Probing the Role of Dopant Oxidation State in the Magnetism of Diluted Magnetic Oxides Using Fe-Doped In2O3 and SnO2 Nanocrystals” J. Phys. Chem. C, 2017, 121, 1918-1927.
Investigation of the origin of high-Curie temperature ferromagnetism in diluted magnetic oxides has become one of the focal points of research on solid-state magnetism. While several possible mechanisms have been proposed theoretically, broader experimental evidence is still lacking. Here we report a comparative study of the electronic structure and magnetic properties of colloidal Fe-doped In2O3 and SnO2 nanocrystals, as building blocks for grain boundary-rich diluted magnetic oxide films. The dopant ions in both nanocrystal host lattices are principally in 3+ oxidation state, with possibly a minor presence of Fe2+ in In2O3, and no conclusive evidence of the presence of Fe2+ in SnO2 nanocrystals. Subsequently, we found that Fe-doped In2O3 nanocrystalline films exhibit only minor ferromagnetic ordering (with the magnetic moment of less than ca. 0.1 μB/Fe) and decreasing saturation magnetization with increasing doping concentration at room temperature. The saturation magnetic moment of Fe-doped SnO2 nanocrystalline films is insignificant or below the detection limit. These results contrast previous findings for analogous Mn-doped nanocrystals, which contain mixed oxidation states (Mn2+ and Mn3+), and exhibit a robust ferromagnetism at room temperature. The correlation between the mixed dopant oxidation states and the observed magnetic properties implies that ferromagnetism in these systems is of a Stoner type, enabled by electron transfer between dopant ions and the local defect states arising from the grain boundaries within a nanocrystalline film. These results suggest the prospect of probing and manipulating ferromagnetism in non-magnetic oxides by simultaneous control of the transition metal dopant oxidation states and extended structural defects.
Stanish, P. C.; Radovanovic, P. V. “Surface-Enabled Energy Transfer in Ga2O3–CdSe/CdS Nanocrystal Composite Films: Tunable All-Inorganic Rare Earth Element-Free White-Emitting Phosphor” J. Phys. Chem. C, 2016, 120, 19566–19573.
Development of inorganic phosphors capable of generating white light in a homogeneous and reproducible fashion without the use of rare earth elements can lead to an efficient, long-lasting, and sustainable solid state lighting. The design of such phosphors requires that different inorganic components emitting in complementary spectral ranges are electronically coupled to avoid the challenges associated with multicomponent approach, such as inhomogeneity, poor chromaticity control, and low color rendering index. Here we demonstrate coupling between electronically-excited blue-emitting Ga2O3 and orange-red emitting CdSe/CdS core/shell nanocrystals by surface-enabled Förster resonance energy transfer. This energy transfer process is evident from quenching of Ga2O3 (donor) and an enhancement of CdSe/CdS (acceptor) nanocrystal emission, and is further confirmed through diminished lifetime of Ga2O3 and significantly extended lifetime of CdSe/CdS nanocrystals in the composite films. Controlling the energy transfer efficiency by adjusting the separation and distribution of codeposited CdSe/CdS and Ga2O3 nanocrystals allows for tuning of the emission color. White light is reproducibly generated for [CdSe/CdS]:[Ga2O3] ≈0.5 by tuning energy transfer efficiency to be ca. 25 %, using 4.5±0.3 nm Ga2O3 and 6.4±0.3 nm CdSe/CdS NCs. The main goal of this work is to quantitatively explore the energy transfer coupling between heterogeneous nanocrystals having complementary optical properties, anchored without the application of organic linkers. These broadly-relevant results are applied to demonstrate a path to all-inorganic rare earth element-free nanocrystal phosphors for potential application in white light-emitting diodes, and other light emitting devices.
Ghodsi, V.; Layek, A.; Hegde, M.; Yildirim, B.; Radovanovic, P. V. “Native Defects Determine Phase-Dependent Photoluminescence Behavior of Eu2+and Eu3+in In2O3Nanocrystals” Chem. Comm. 2016, 52, 4353-4356.
We demonstrate the coexistence of Eu2+ and Eu3+ in corundum and bixbyite-type colloidal In2O3 nanocrystals. The emission properties of dopants in both oxidation states are determined by their inter- action with native defects, and are dramatically different in the two nanocrystal phases. This difference arises from the smaller nano- crystal size and higher defect density in metastable corundum-type nanocrystals.
Stanish, P. C.; Radovanovic, P. V. “Energy Transfer between Conjugated Colloidal Ga2O3and CdSe/CdS Core/Shell Nanocrystals for White Light Emitting Applications”, Nanomaterials, 2016, 6, 32. Invited paper for the Special Issue “Current Trends in Colloidal Nanocrystals”
Developing solid state materials capable of generating homogeneous white light in an energy efficient and resource-sustainable way is central to the design of new and improved devices for various lighting applications. Most currently-used phosphors depend on strategically important rare earth elements, and rely on a multicomponent approach, which produces sub-optimal quality white light. Here, we report the design and preparation of a colloidal white-light emitting nanocrystal conjugate. This conjugate is obtained by linking colloidal Ga2O3 and II–VI nanocrystals in the solution phase with a short bifunctional organic molecule (thioglycolic acid). The two types of nanocrystals are electronically coupled by Förster resonance energy transfer owing to the short separation between Ga2O3 (energy donor) and core/shell CdSe/CdS (energy acceptor) nanocrystals, and the spectral overlap between the photoluminescence of the donor and the absorption of the acceptor. Using steady state and time-resolved photoluminescence spectroscopies, we quantified the contribution of the energy transfer to the photoluminescence spectral power distribution and the corresponding chromaticity of this nanocrystal conjugate. Quantitative understanding of this new system allows for tuning of the emission color and the design of quasi-single white light emitting inorganic phosphors without the use of rare-earth elements.
Layek, A.; Yildrim, B.; Ghodsi, V.; Hutfluss, L. N.; Hegde, M.; Radovanovic, P. V. “Dual Europium Luminescence Centers in Colloidal Ga2O3Nanocrystals: Controlled In Situ Reduction of Eu(III) and Stabilization of Eu(II)”, Chem. Mater., 2015, 27, 6030-6037.
Introducing multiple luminescent centers into colloidal nanocrystals is an attractive way to impart new optical properties into this class of materials. Doping disparate ions into specific nanocrystals is often challenging, due to the preferential incorporation of one type of dopant. Here, we demonstrate the coexistence of europium dopants as divalent and trivalent ions in colloidal Ga2O3 nanocrystals, achieved by controlled in situ reduction of Eu3+ to Eu2+. The two dopant species exhibit distinctly different steady-state and time-resolved photoluminescence, and their ratio can be modified via doping concentration, reaction temperature, or thermal treatment of as-synthesized NCs. The Eu2+ ions are proposed to be stabilized internally owing to the attractive interaction with oxygen vacancies, while Eu3+ dopants partly reside in the nanocrystal surface region. The relationship between the electronic structure of the native defects and the dopant centers is discussed in the context of the overall emission properties. The exposure of these samples to X-ray radiation leads to the reduction of Eu3+ to Eu2+, demonstrating an alternative way of manipulating the oxidation state and suggesting the potential application of this material as an X-ray storage phosphor. The coexistence of Eu2+ and Eu3+ and the ability to control their relative fraction over the full oxidation state range in group III oxide nanocrystals allow for the design and preparation of new photonic and light emitting materials.
Hegde, M.; Hosein, I. D.; Radovanovic, P. V. “Molecular Origin of Valence Band Anisotropy in Single β-Ga2O3 Nanowires Investigated by Polarized X-Ray Absorption Imaging”, J. Phys. Chem. C, 2015, 119, 17450-17457.
The origin of anisotropy at the molecular and electronic structure levels in individual β-Ga2O3 nanowires grown along the crystallographic b direction was studied using linearly polarized X-ray absorption imaging at both gallium L3 and oxygen K edges. The O K-edge spectrum shows significant linear dichroism for the electric field vector of the X-rays oriented parallel and perpendicular to the nanowire long axis. The contributions from the three nonequivalent oxygen sites to the observed spectral anisotropy were elucidated by using FDMNES calculations in the framework of the multiple scattering theory. The role of relevant O and Ga orbitals in the linearly polarized O K-edge absorption was determined based on the point group symmetry arguments. The results of this work suggest mixed covalent and ionic character of the Ga−O bond in individual nanowires, with the dominant contribution of O 2pz orbitals to the absorption spectrum for the electric field vector oriented perpendicular, and O 2px,y orbitals for the electric field vector oriented parallel to the nanowire long axis. Gallium 4p and d orbitals were found to contribute mostly to the antibonding states. These results improve the understanding of the origin of anisotropy in complex transparent metal oxide nanostructures, and could lead to the prediction of physical properties for different nanowire growth directions.
Chirmanov, V.; Stanish, P.; Layek, A.; Radovanovic, P. V. “Distance-Dependent Energy Transfer between Ga2O3 Nanocrystal Defect States and Conjugated Organic Fluorophores in Hybrid White Light-Emitting Nanophosphors”, J. Phys. Chem. C, 2015, 119, 5687-5696.
We report a quantitative analysis and development of hybrid white-light-emitting nanoconjugates, prepared by functionalizing colloidal γ-Ga2O3 nanocrystals with selected organic fluorophores. Using the Förster resonance energy transfer (FRET) formalism, we studied the coupling of native defect states in Ga2O3 nanocrystals, as energy donors, with different orange-red-emitting fluorophores bound to nanocrystal surfaces, as energy acceptors. Variations in the average nanocrystal size and dye surface coverage were used to characterize the efficiency of the energy transfer process and the corresponding donor−acceptor separations. The results show that for approximately three rhodamine B molecules per nanocrystal the energy transfer efficiency increases from 23% to 49% by decreasing the NC size from 5.3 to 3.6 nm. These FRET efficiencies correspond to the estimated donor−acceptor distances of 3.55 ± 0.02 and 2.99 ± 0.03 nm, respectively. Similar trends were observed for ATTO 590-conjugated Ga2O3 nanocrystals, although ATTO 590 proved to be a more effective energy acceptor owing to a larger molar extinction coefficient in the conjugated form. The size-dependent luminescence of Ga2O3 nanocrystals and the control of FRET parameters through the variations in the bound dye molecules allow for the generation of tunable blue-orange emission, ultimately resulting in white light with targeted chromaticity and high color rendition.
Developing new ways of generating white light is of paramount importance for the design of the next generation of smart, energy-efficient lighting sources. Here we report tunable white light emission of hybrid organic− inorganic nanostructures based on colloidal ZnO nanocrystals conjugated with organic fluorophores. These materials act as single nanophosphors owing to the distance-dependent energy transfer between the two components. The defect-based sizetunable ZnO nanocrystal blue-green emission coupled with complementary color emission from different fluorophores allows for the generation of white light with targeted chromaticity, color temperature, and color rendering index. We further show that silane layer-protected nanoconjugates result in increased stability of white light emission over a long period of time. The results of this work demonstrate an inexpensive, green, and sustainable approach to general solid-state lighting, without the use of rare earth or heavy metals. Colloidal form of the reported hybrid nanoconjugates allows for their further functionalization or incorporation into light-emitting devices. More broadly, size dependence of the electronic structure of native defects in transparent metal oxide nanocrystals and their electronic coupling with conjugated organic species could also represent a vehicle for introducing and manipulating new properties in these hybrid nanostructures.
Controlling the crystal structure of transparent metal oxides is essential for tailoring the properties of these polymorphic materials to specific applications. The structural control is usually done via solid state phase transformation at high temperature or pressure. Here, we report the 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, we identified two distinct coexisting mechanisms, 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. Despite 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.
Anode materials of rechargeable lithium-ion batteries have been developed towards the aim of high power density, long cycle life, and environmental benignity. As a promising anode material for high power density batteries for large scale applications in both electric vehicle and large stationary power supplies, the spinel Li4Ti5O12 anode has become more attractive for alternative anodes for its relatively high theoretical capacity (175 mA h g1), stable voltage plateau of 1.5 V vs. Li/Li+, better cycling performance, high safety, easy fabrication, and low cost precursors. This perspective first introduces recent studies on the electronic structure and performance, synthesis methods, and strategies for improvement including carbon-coating, ion-doping, surface modifications, nano-structuring and optimization of the particle morphology of the Li4Ti5O12 anode. Furthermore, practical applications of the commercial spinel lithium-ion batteries are demonstrated. Finally, the future research directions and key developments of the spinel Li4Ti5O12 anode are pointed out from a scientific and an industrial point of view. In addition, the prospect of the synthesis of graphene–Li4Ti5O12 hybrid composite anode materials for next-generation lithium-ion batteries is highlighted.
Farvid, S. S.; Sabergharesou, T.; Hutfluss, L. N.; Hegde, M.; Prouzet, E.; Radovanovic, P. V. “Evidence of Charge-Transfer Ferromagnetism in Transparent Diluted Magnetic Oxide Nanocrystals: Switching the Mechanism of Magnetic Interactions”, J. Am. Chem. Soc., 2014, 136, 7669–7679.
We report the experimental evidence of a new form of room-temperature ferromagnetism in high surface area nanocrystalline manganese-doped In2O3, prepared from colloidal nanocrystals as building blocks. The nanocrystal structure (bixbyite or corundum) and assembly were controlled by their size, and the type and concentration of dopant precursors. The existence of substitutional para-magnetic Mn dopant ions in mixed valence states (Mn2+ and Mn3+) was confirmed and quantified by different spectroscopic methods, including X-ray absorption and magnetic circular dichroism. The presence of different oxidation states is the basis of ferromagnetism induced by Stoner splitting of the local density of states associated with extended structural defects, due to charge transfer from the Mn dopants. The extent of this charge transfer can be controlled by the relationship between the electronic structures of the nanocrystal host lattice and dopant ions, rendering a higher magnetic moment in bixbyite relative to corundum Mn-doped In2O3. Charge-transfer ferromagnetism assumes no essential role of dopant as a carrier of the magnetic moment, which was directly confirmed by X-ray magnetic circular dichroism, as an element-specific probe of the origin of ferromagnetism. At doping concentrations approaching the percolation limit, charge-transfer ferromagnetism can switch to a double exchange mechanism, given the mixed oxidation states of Mn dopants. The results of this work enable the investigations of the new mechanisms of magnetic ordering in solid state and contribute to the design of new unconventional magnetic and multifunctional materials.
Sun, X.; Hegde, M.; Wang, J.; Zhang, Y.; Liao, J.; Radovanovic, P. V.; Cui, B. “Structural Analysis and Electrochemical Studies of Carbon Coated Li4Ti5O12 Particles Used as Anode for Lithium Ion Battery”, ECS Transactions, 2014, 58, 79-88.
Carbon coated Li4Ti5O12 (C-LTO) particles have been synthesized by hydrothermal method and high-temperature calcination process. Nanostructure and carbon-coating has been characterized in detail by X- ray diffraction (XRD), high resolution TEM (HR-TEM), selected electron diffraction (SAED), and scanning transmission X-ray microscopy (STXM) combined with X-ray absorption near edge structure (XANES) spectroscopy. The prepared particles are comprised of highly crystalline spinel-type Li4Ti5O12 with the size range of 20-70 nm. HR-TEM imaging and STXM-XANES spectromicrocopy confirmed the amorphous carbon layer uniformly covered on the surface of single LTO particles with optimized content and coating thickness (~5 nm). The electrochemical performance of C-LTO particles as an anode in lithium-ion batteries is evaluated, demonstrating both improved rate capability and cycling performance, which was attributed to the enhanced electron transport/high electrical conductivity and fast lithium-ion insertion/extraction, as a result of uniform and optimized amorphous carbon coating on the C-LTO particles.
Hosein, I. D.; Hegde, M.; Jones, P. D.; Chirmanov, V.; Radovanovic, P. V. ” Evolution of the Faceting Morphology and Aspect Ratio of Galium Oxide Nanowires Grown by Vapor-Solid Deposition”, J. Cryst. Growth, 2014, 396, 24-32.
Gallium oxide nanostructures with high aspect ratio and variable faceting were synthesized by the chemical vapor deposition method via vapor–solid growth mechanism. Systematic investigation of the growth conditions revealed that these nanowires can be produced under the conditions of high temperature and low precursor flow. The nanowires crystalize as the β-phase Ga2O3, which has the monoclinic crystal structure. Preferred growth was along the [0 1 0] direction, as corroborated with lattice-resolved imaging and crystal plane models. The high degree of faceting is discussed in terms of the evolution of the nanowire cross section morphology, based on the growth rate of the facet boundaries relative to the nanowire surface planes. The obtained nanowires show intense blue emission, characterized by a broad-band photoluminescence spectrum with a maximum at 430 nm and long decay time. This emission arises from the defect-related donor-acceptor pair recombination mechanism, and depends on the nanostructure dimensionality and morphology. The possible influence of controlled nanowire faceting on the observed optical properties is also discussed. Owing to their morphology- dependent optical properties, these nanowires are promising building blocks for electronic and optoelectronic structures and devices.
Sun, X.; Hegde, M.; Zhang, Y.; He, M.; Gu, L.; Wang, Y.; Shu, J.; Radovanovic, P. V.; Cui, B. “Structure and Electrochemical Properties of Spinel Li4Ti5O12 Nanocomposites as Anode for Lithium-Ion Battery”, Int. J. Electrochem. Sci., 2014, 9, 1583-1596.
Nanocomposite particles of amorphous carbon-Li4Ti5O12 (C-LTO) and carbon nanotube-Li4Ti5O12 (CNT-LTO) were synthesized by solvothermal method and subsequent high-temperature calcination. X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HR-TEM), and selected area electron diffraction (SAED) were applied to characterize the phase structure, particle morphology, and the coating structure. XRD analysis, TEM micrographs, HR-TEM images and SAED analysis revealed that both LTO particles exhibited a welldeveloped spinel nanocrystal structure with average sizes between 20-70 nm. The C-LTO particles exhibited roughly spherical shape coated by an amorphous carbon layer up to 10 nm in thickness. The CNT-LTO samples showed uniform square nanocrystals with edge length around 20 nm and nanoscale graphitic layers covering the surface, revealing the carbon nanotubes interconnection networks among the particle assemblies. Electrochemical studies of lithium insertion/extraction performance are evaluated by the galvanostatic charge/discharge tests, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Both LTO particles showed the superior initial discharge capacity of more than 200 mAh/g at 1/10C rate. The irreversible capacity of the C-LTO particles at more cycles was due to large polarization resulted from excessive carbon and possible residual precursors. The CNT-LTO particles show larger reversible capacity and enhanced electrochemical Li+ insertion/extraction kinetics at different cycling rates. The comparative structural and electrochemical analyses demonstrated that both nanoscale graphitic covering layers and the CNT interconnection networks increase the electronic conductivity and improve the kinetics rates of lithium insertion/extraction in the CNT-LTO particles.
Wang, T.; Layek, A.; Hosein, I. D.; Chirmanov, V.; Radovanovic, P. V. “Correlation between Native Defects and Dopants in Colloidal Lanthanide-Doped Ga2O3 Nanocrystals: A Path to Enhancing Functionality and Controlling Optical Properties”, J. Mater. Chem. C, 2014, 2, 3212-3222. Invited paper. “
We report the synthesis and study of the photoluminescence properties of colloidal lanthanide(III)-doped Ga2O3 nanocrystals. The Ga2O3 nanocrystal host lattice acts as a sensitizer of the Eu3+ dopant red emission arising from intra-4f orbital transitions, and concurrently exhibits a strong blue photoluminescence originating from defect-based donor–acceptor pair (DAP) recombination. The Eu3+ sensitization, enabled by the energy transfer from the nanocrystal host lattice to the dopant centers, allows for the generation of dual blue-red emission. Increasing doping concentration leads to a decrease in the donor activation energy allowing for simultaneous control of the optical and electrical properties of these multifunctional nanocrystals. Analyses of the steady-state and time-resolved photoluminescence spectra suggest that Eu3+ ions occupy at least two different sites, which were tentatively designated as the six-coordinate internal and surface-related dopants. Uniquely, both DAP and Eu3+ emissions have long lifetimes (in milliseconds), although Eu3+ luminescence has a slower decay rate. These phenomena enable a temporal modulation of the dual emission and photoluminescence chromaticity on the millisecond timescale. The generality of these findings was demonstrated by preparing Tb3+-doped Ga2O3 nanocrystals, as a blue-green dual emitter. Owing to their optical transparency, electrical properties, emission color versatility, robustness, and fabricability, colloidal lanthanide(III)-doped Ga2O3 nanocrystals are a promising class of multifunctional materials and complex phosphors
Hegde, M.; Hosein, I. D.; Sabergharesou, T.; Farvid, S. S.; Radovanovic, P. V. “Introducing and Manipulating Magnetic Dopant Exchange Interactions in Semiconductor Nanowires”, Proc. of SPIE, 2013, 8813, 88132S. Invited paper. “
The ability to control both spin and charge degrees of freedom in semiconductor nanostructrures is at heart of spintronic and quantum information technologies. Magnetically-doped semiconductor nanowires have emerged as a promising platform for spintronics, which warrants the exploration of their synthesis, electronic structure, and magnetic properties. Here we demonstrate the preparation of manganese-doped GaN and SnO2 nanowires by chemical vapor deposition and solvothermal methods, respectively. The investigation of both systems by electron microscopy and x-ray absorption spectroscopy at ensemble and single nanowire levels indicates that manganese dopants exist in a dual oxidation state, Mn2+ and Mn3+, with Mn2+ being the majority species. X-ray magnetic circular dichroism studies of individual nanowires suggest ferromagnetic interactions of manganese dopants, and the nanowire orientation-dependent magnetization owing to the magnetocrystalline anisotropy. The results of these studies demonstrate quantitative determination of the dopant electronic structure at the molecular level, and allow for a prediction of the magnetic properties of diluted magnetic semiconductor nanowires based on their orientation and geometry.
Wang, T.; Chirmanov, V.; Chiu, W. H. M.; Radovanovic P. V. “Generating Tunable White Light by Resonance Energy Transfer in Transparent Dye-Conjugated Metal Oxide Nanocrystals”, J. Am. Chem. Soc., 2013, 135, 14520−14523. “
We report the design and properties of hybrid white-light-emitting nanophosphors obtained by electronic coupling of defect states in colloidal Ga2O3 nanocrystals emitting in blue-green with selected organic molecules emitting in orange-red. Coupling between the two components is enabled by the nanocrystal’s sizedependent resonance energy transfer, allowing the photoluminescence chromaticity to be precisely tuned by changing the nanocrystal size and selecting the complementary organic dye molecule. Using this approach, we demonstrate the generation of pure white light with quantum yield of 30%, color rendering index up to 95, and color temperature of 5500 K. These results provide a guideline for the design of a new class of hybrid whitelight-emitting nanophosphors and other multifunctional nanostructures based on transparent metal oxides.
Sun, X.; Bai, X.; Wang, Y.; Hegde, M.; Hosein, I. D.; Radovanovic P. V.; Guo, Y. G.; Cui, B. “Comparison of Structural Analysis and Electrochemical Studies of C-Li4Ti5O12and CNT-Li4Ti5O12 Nanocomposites Particles Used as Anode for Lithium Ion Battery”, MRS Proc., 2013, 1541, mrss13-1541-f09-01.
Carbon-Li4Ti5O12 (C-LTO) and carbon nanotube-Li4Ti5O12 (CNT-LTO) nanocomposite particles have been synthesized by hydrothermal method and a following high-temperature calcinations using a mixture of micro-size Li-Ti-O precursors and conducting black and carbon nanotubes, respectively. Two different types of coating layers have been characterized and analyzed on two kinds of Li4Ti5O12 particles surface by high resolution transmission electron microscopy images (HR-TEM) and selected area electron diffraction (SAED). Typical HR-TEM images and SAED patterns at nano-scale confirmed and showed that both particles exhibited a well-developed spinel nanocrystal with average sizes around 20-50 nm. The C-LTO particles exhibited the roughly spherical shape with more than 5 nm graphitic coating uniformly on the spherical surfaces; however, the CNT-LTO particles showed uniform square nanocrystal with edge length around 30 nm and a few layers of graphene covering the surface.
Electrochemical studies of galvanostatic discharge/charge cycling capacity testing indicated that both Li4Ti5O12particles showed the superior initial discharge capacity of more than 200 mA·h/g at 0.1C rate, and also the CNT-LTO particles show much improved specific capacity than that of the C-LTO particles during different cycling processing. It has been proposed that, grephene covering layers and the CNT interconnection networks are prove to increase electronic conductivity and improve the kinetics of Li4Ti5O12 toward fast lithium insertion/extraction. The comparative experimental results demonstrated that both nanoscale grephene layer and CNT inter-networks among particles is highly effective in improving the electrochemical properties of the CNT-LTO particles.
Radovanovic, P. V. “Defect-Induced Optical and Magnetic Properties of Colloidal Transparent Conducting Oxide Nanocrystals”, In Functional Metal Oxides: New Science and Novel Applications, Ogale, S. B.; Venkatesan, T. V.; Blamire, M. (Editors); Wiley-VCH: Weinheim, 2013, Chapter 5, 161-192. Invited book chapter.
Sabergharesou, T.; Wang, T.; Ju, L.; Radovanovic, P. V. “Electronic Structure and Magnetic Properties of sub-3 nm Diameter Mn-Doped SnO2 Nanocrystals and Nanowires”, Appl. Phys. Lett., 2013, 103, 012401.
Manganese-doped SnO2 nanocrystals and nanowires with diameters below SnO2 Bohr radius were synthesized by solution methods. X-ray absorption studies reveal that dopant ions are substitutionally incorporated as Mn2þ and Mn3þ. Mn2þ is the dominant species at low doping levels, but the fraction of Mn3þ increases with doping concentration. Room-temperature ferromagnetism with the saturation moment of 0.27 lB/Mn is observed for nanocrystalline films containing high fraction of Mn2þ dopant, which is associated with hybridization of Mn2þ d-levels with a donor-impurity band. These results imply the possibility of manipulating magnetic interactions via dopant electronic structure and quantum confinement of the host lattice.
Farvid, S. S.; Hegde, M.; Radovanovic, P. V. “Influence of the Host Lattice Electronic Structure on Dilute Magnetic Interactions in Polymorphic Cr(III)-Doped In2O3 Nanocrystals”, Chem. Mater., 2013, 25, 233-244.
The effect of the host lattice structure on the spectroscopic and magnetic properties of Cr3+-doped In2O3 nanocrystals is reported. The influence of the dopant ions on the nanocrystal growth allows for the solution-phase stabilization and separation of doped colloidal In2O3 nano-crystals having different crystal structures − stable cubic phase (bcc-In2O3) and metastable rhombohedral (rh-In2O3) phase − and comparative study of the electronic structure and magnetic properties of Cr3+ in both polymorphs. Investigations by a range of complementary spectroscopic techniques, including Raman, X-ray absorption and magnetic circular dichroism spectroscopies, revealed that the change in the In2O3 phase leads to distinctly different electronic structure of Cr3+ dopants, associated with a different nature of the substitutional doping sites and different electronic structure of the nanocrystal host lattice. Nanocrystalline films prepared from colloidal nanocrystals exhibit ferromagnetism at room temperature, although the average magnetic moment of Cr3+ in rh-In2O3 is an order of magnitude smaller than that in bcc-In2O3 samples. This difference in magnetization is associated with wider band gap of rh-In2O3 nanocrystals, which prevents effective hybridization of the defect donor band, as a mediator of the Cr3+ magnetic exchange interactions, and the Cr3+ 3d states at the Fermi level. The results of this work demonstrate that a change in the defect and electronic structures of the same semiconductor host lattice by nanocrystal phase control in solution allows for tuning of the magnetic properties of diluted magnetic semiconducting oxides.
Sun, X.; Iqbal, A.; Hosein, I. D.; Yacaman, M. J.; Tang, Z. Y.; Radovanovic, P. V.; Cui, B. “Structure Characterization and Electrochemical Characteristics of Carbon Nanotube-Spinel Li4Ti5O12 Nanoparticles”, MRS Proc., 2012 1440, mrss12-1440-o09-34.
Carbon nanotube-spinel lithium titanate (CNT-Li4Ti5O12) nanoparticles have been synthesized by hydrothermal reaction and higher-temperature calcinations with LiOH·H2O and TiO2 precursors in the presence of carbon nanotubes sources. The CNT-Li4Ti5O12 nanoparticles have been characterized by X-ray diffraction (XRD), high angle annular dark field (HAADF) images, and selected area electron diffraction (SAED). The particles exhibited a spinel cubic crystal phase and homogenous size distribution, with sizes around 50-70 nm. HAADF imaging confirmed that carbon content exists on the surface of the CNT-Li4Ti5O12 nanoparticles with graphitic carbon coating of 3-5 nm thickness under 800oC in the Ar gas. The graphitic carbon phase was further confirmed with Raman spectroscopy analysis on powder samples. Electrochemical characteristics were evaluated with galvanostatic discharge/charge tests, which showed that the initial discharge capacity is 172 mA·h/g at 0.1C. The nanoscale carbon layers uniformly coated the particles, and the interconnected carbon nanotube network is responsible for the improved charge rate capability and conductivity.
The kinetics of phase transformation of colloidal In2O3 nanocrystals (NCs) during their synthesis in solution was explored by a combination of structural and spectroscopic methods, including X-ray diffraction, transmission electron microscopy, and extended X-ray absorption fine structure spectroscopy. Johnson−Mehl−Avrami−Frofeyev−Kholmogorov (JMAEK) and the interface nucleation models were used to analyze the isothermal kinetic data for the phase transformation of NCs in the temperature range of 210−260 °C. The results show that NCs are initially stabilized in the metastable corundum (rh-In2O3) phase. The phase transformation occurs via nucleation of cubic bixbyite (bcc-In2O3) phase at the interface between contacting rh-In2O3 NCs, and propagates rapidly throughout the NC volume. The activation energy of the phase transformation was determined from the Arrhenius expression to be 152 ± 60 kJ/ mol. The interface nucleation rate is maximal at the beginning of the phase transformation process, and decreases over the course of the reaction due to a decrease in the concentration of rh-In2O3 NCs in the reaction mixture. In situ high-temperature XRD patterns collected during nonisothermal treatment of In2O3 NCs reveal that phase transformation of smaller NCs occurs at a faster rate and lower temperature, which is associated with their higher packing density and contact formation probability. Because NC surfaces and interfaces play a key role in phase transformation, their control through the synthesis conditions and reaction kinetics is an effective route to manipulate NC structure and properties.
We studied size-dependent dynamics of defect-based photoluminescence of colloidal c-Ga2O3 nanocrystals in the framework of the donor-acceptor pair model. Two theoretical models were developed based on relative positioning of donor and acceptor sites: (1) for random distribution of defects throughout the nanocrystal volume and (2) for surface segregation of defects. The results of the modeling indicate that defect sites are predominantly located in the vicinity of nanocrystal surfaces and that the density of defects increases with decreasing nanocrystal size. The donor Bohr radius obtained as a fitting parameter suggests an increase in the donor binding energy with decreasing nanocrystal size. VC 2012 American Institute of Physics.
Ju, L.; Sabergharesou, T.; Stamplecoskie, K. G.; Hegde, M.; Wang, T.; Combe, N. A.; Wu, H.; Radovanovic, P. V. “Interplay between Size, Composition and Phase Transition of Nanocrystalline Cr3+-Doped BaTiO3 as a Path to Multiferroism in Perovskite-Type Oxides”, J. Am. Chem Soc., 2012, 134, 1136-1146.
Multiferroics, materials that exhibit coupling between spontaneous magnetic and electric dipole ordering, have significant potential for high-density memory storage and the design of complex multistate memory elements. In this work, we have demonstrated the solvent-controlled synthesis of Cr3+-doped BaTiO3 nanocrystals and investigated the effects of size and doping concentration on their structure and phase transformation using X-ray diffraction and Raman spectroscopy. The magnetic properties of these nanocrystals were studied by magnetic susceptibility, magnetic circular dichroism (MCD), and X-ray magnetic circular dichroism (XMCD) measurements. We observed that a decrease in nanocrystal size and an increase in doping concentration favor the stabilization of the paraelectric cubic phase, although the ferroelectric tetragonal phase is partly retained even in ca. 7 nm nanocrystals having the doping concentration of ca. 5%. The chromium(III) doping was determined to be a dominant factor for destabilization of the tetragonal phase. A combination of magnetic and magneto-optical measurements revealed that nanocrystalline films prepared from as-synthesized paramagnetic Cr3+-doped BaTiO3 nanocrystals exhibit robust ferromagnetic ordering (up to ca. 2 μB/Cr3+), similarly to magnetically doped transparent conducting oxides. The observed ferromagnetism increases with decreasing constituent nanocrystal size because of an enhancement in the interfacial defect concentration with increasing surface-to-volume ratio. Element-specific XMCD spectra measured by scanning transmission X-ray microscopy (STXM) confirmed with high spatial resolution that magnetic ordering arises from Cr3+ dopant exchange interactions. The results of this work suggest an approach to the design and preparation of multiferroic perovskite materials that retain the ferroelectric phase and exhibit long-range magnetic ordering by using doped colloidal nanocrystals with optimized composition and size as functional building blocks.
Farvid, S. S.; Hegde, M.; Hosein, I. D.; Radovanovic, P. V. “Electronic structure and magnetism of Mn dopants in GaN nanowires: ensemble vs single nanowire measurements”, Appl. Phys. Lett., 2011, 99, 222504.
We studied the electronic structure and magnetization of Mn dopants in GaNnanowires at the ensemble and single nanowire levels by near edge x-ray absorption fine structurespectroscopies. The results of single nanowiremeasurements indicate that Mn adopts tetrahedral coordination in GaNnanowires and has mixed oxidation state (Mn2+/Mn3+), with Mn2+ being in relative majority. Ensemble nanowire spectra suggest co-deposition of Mn secondary phases alongside nanowires. Single nanowirex-ray magnetic circular dichroism indicates intrinsic magnetic ordering of Mn dopants at 300 K. In contrast, as-grown nanowire samples show only residual magnetization, due to nanowire orientation dependence of magnetization.
Understanding the mechanisms of defect-related photoluminescence in colloidal transparent conducting oxide nanocrystals is important for the development of new multifunctional nanostructures and devices. Here we report a study of the role of NC size, structure, defects, and surface capping on the photoluminescence energy, efficiency, and dynamics of colloidal γ-Ga2O3 nanocrystals. A strong blue emission (quantum yield 25%) is associated with the presence of the vacancy-defect sites, and assigned to the donor acceptor pair (DAP) recombination. The emission energy and lifetime are generally determined by the donor and acceptor binding energies (which are dependent on the NC structure) and the attractive Coulombic interactions between charged donor and acceptor sites (which are dependent on the defect concentration). Variable temperature photoluminescence measurements reveal that binding energies of the donor and acceptor levels are also size- dependent; in 6.0 ( 1.0 nm γ-Ga2O3 nanocrystals donor binding energy was determined to be 205 meV, increasing by ca. 30 meV in 3.3 ( 0.5 nm nanocrystals. The defect sites on nanocrystal surfaces (OH_ or O2_) also influence DAP recombination by trapping photogenerated valence band holes. Removal of the surface defect sites by capping ligands (dodecylamine and tri-n-octylphosphine oxide) is shown to eliminate this hole-trapping pathway, enhancing a hole capture by the acceptor sites, and increasing the DAP emission intensity. The results of the mechanistic study of the DAP recombination in this work serve as a useful guideline for introducing and manipulating PL properties in oxide nanostructures by controlling the native defect interactions.
Control of electron spins in individual magnetically doped semiconductor nano- structures has considerable potential for quantum information processing and storage. The manipulations of dilute magnetic interactions have largely been restricted to low temperatures, limiting their potential technological applications. Among the systems predicted to be ferromagnetic above room temperature, Mn-doped GaN has attracted particular attention, due to its attractive optical and electrical properties. However, the experimental data have been inconsistent, and the origin of the magnetic interactions remains unclear. Furthermore, there has been no demonstration of tuning the dopant exchange interactions within a single nanostructure, which is necessary for the design of nanoscale spin-electronic (spintronic) devices. Here we directly show for the first time intrinsic magnetization of manganese dopants in individual gallium nitride nanowires (NWs) at room temperature. Using high-resolution circularly polarized X-ray microscopy imaging, we demonstrate the dependence of the manganese exchange interactions on the NW orientation with respect to the external magnetic field. The crystalline anisotropy allows for the control of dilute magnetization in a single NW and the application of bottom-up approaches, such as in situ nanowire growth control or targeted positioning of individual NWs, for the design of networks for quantum information technologies.
Wang, T.; Radovanovic, P. V. “In-situ enhancement of the blue photoluminescence of colloidal Ga2O3 nanocrystals by promotion of defect formation in reducing conditions”, Chem. Comm., 2011, 47, 7161-7163. “
We demonstrate redox control of defect-based photoluminescence efficiency of colloidal c-Ga2O3 nanocrystals. Reducing environment leads to an increase in photoluminescence intensity by enhancing the concentration of oxygen vacancies, while the blue emission is suppressed in oxidative conditions. These results enable optimization of nanocrystal properties by in situ defect manipulation.
Farvid, S. S.; Wang, T.; Radovanovic, P. V. “Colloidal Gallium Indium Oxide Nanocrystals: A Multifunctional Light Emitting Phosphor Broadly Tunable by Alloy Composition”, J. Am. Chem Soc., 2011, 133, 6711-6719.”
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 In2O3 and Ga2O3 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 In3þ occupies only octahedral, rather than tetrahedral, sites in the spinel-type γ-Ga2O3 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 photolumines- cence properties, are also implicated in other functional properties, particularly conductivity, enabling the application of colloidal GIO nanocrystals as integrated optoelectronic materials.
We report the synthesis and separation of colloidal indium tin oxide (ITO) nanocrystals in the stable cubic bixbyite (bcc-ITO) and metastable corundum (rh-ITO) phase under identical conditions, based on the size-structure correlation. Both phases are obtained in the same reactions, with nanocrystals below ca. 5 nm in size having corundum crystal structure. This bimodal size distribution allows for the separation of the nanocrystal phases by size selective precipitation. A comparative study of bcc-ITO and rh-ITO nanocrystals reveals a dramatic difference in their optical and electrical properties. Unlike smaller rh-ITO nanocrystals, bcc-ITO nanocrystals exhibit a strong absorption in the near-infrared (NIR) region arising from the plasmon oscillations due to the presence of free electrons. The difference in the free electron concentration in bcc-ITO and rh-ITO nanocrystals is related to the different electronic structure of the donor states, associated with Sn4+ dopants, in these two nanocrystal allotropic modifications. The donor activation energy is significantly higher in rh-ITO NCs, prohibiting any appreciable concentration of free electrons in the conduction band. The increased replacement of organic protective ligands by anions in the solution leads to the oriented attachment of larger sized bcc-ITO nanocrystals and the formation of flowerlike clusters. These results demonstrate tuning of the optical and electrical properties of complex oxide nanocrystals by selecting their crystal and electronic structures through size and composition and allow for a designed preparation and controlled self-assembly of ITO nanocrystals.
Farvid, S. S.; Wang, T.; Radovanovic, P. V. “Spectroscopic and Magnetic Properties of Colloidal Transition Metal-Doped Transparent Conducting Oxide Nanocrystals as Building Blocks for Spintronic Material”, Proc. SPIE, 2010, 7760, 77600B.”
The synthesis of colloidal Cr3+-doped In2O3 NCs with the body-centered cubic bixbyte-type crystal structure, and Cr3+– doped SnO2 NCs with the rutile crystal structure was described. Ligand-field electronic absorption spectroscopy suggests that Cr3+ dopants have quasi-octahedral coordination in both In2O3 and SnO2 NC host lattices. Unlike free-standing nanocrystals, the nanocrystalline films fabricated from colloidal Cr3+-doped In2O3 and SnO2 nanocrystals exhibit room temperature ferromagnetism. Analogous magnetic behavior suggests the same origin of ferromagnetic ordering in both materials. The observed ferromagnetism has been related to the existence of extended structural defects, formed at the interfaces between nanocrystals in nanocrystalline films. These structural defects are likely responsible for the formation of charge carriers which mediate the dopant magnetic moment ordering.
We report a colloidal synthesis of gallium oxide (Ga2O3) nanocrystals having metastable cubic crystal structure (γ phase) and uniform size distribution. Using the synthesized nanocrystal size series we demonstrate for the first time a size-tunable photoluminescence in Ga2O3 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 phosphores- cence 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 monodis- persed Ga2O3 nanocrystals a promising material for multifunc- tional optoelectronic structures and devices.
Dave, N.; Pautler, B. G.; Farvid, S. S.; Radovanovic, P. V. “Synthesis and Surface Control of Colloidal Cr3+-Doped SnO2 Transparent Magnetic Semiconductor Nanocrystals”, Nanotechnology, 2010, 21, 134023.”
The synthesis of colloidal Cr3+-doped SnO2 nanocrystals prepared under mild conditions via a hydrolysis method is described. We show by means of nanocrystal surface ligand exchange that even under mild reaction conditions a significant fraction of the dopant ions reside on the nanocrystal surfaces. Two different approaches aimed at achieving internal dopant incorporation—surface-bound dopant complexation and isocrystalline shell growth—are described and compared. While free-standing nanocrystals are paramagnetic, the films prepared from the same nanocrystals exhibit ferromagnetic ordering at room temperature. The measured magnetization is associated with structural defects formed at the interfaces of nanocrystals in their films, and discussed in terms of the defect-related itinerant-electron-mediated mechanism. The observed ferromagnetism is compared to ferromagnetism in Cr3+-doped In2O3 nanocrystalline films. These results demonstrate the possibility of controlling surface structure and composition of doped oxide nanocrystals using different approaches. Furthermore, this work emphasizes the importance of surface structure and composition in tailoring properties of doped multifunctional transparent conducting oxide nanostructures.
Farvid, S. S.; Dave, N.; Radovanovic, P. V. “Phase-Controlled Synthesis of Colloidal In2O3 Nanocrystals via Size-Structure Correlation”, Chem. Mater., 2010, 22, 9-11. (Highlighted in Angew. Chem. 2010, 49, 5610-5612)”
Control and manipulation of crystal structures has important implications for the design and preparation of new solid-state materials. The ability to obtain meta- stable high-energy structures in a controlled way requires fundamental understanding of the phase transformation mechanisms.1,2 The kinetics of phase transitions in nano- crystals (NCs) are generally less complex than those in bulk because of their large surface areas and a low number of crystal lattice defects.2 As such, colloidal NCs offer a unique opportunity to control metastability and manipulate structural transformations in solutions.
Farvid, S. S.; Dave, N.; Wang, T.; Radovanovic, P. V. “Dopant-Induced Manipulation of the Growth and Structural Metastability of Colloidal Indium Oxide Nanocrystals”, J. Phys. Chem. C, 2009, 113, 15928-15933. (Highlighted in Angew. Chem. 2010, 49, 5610-5612)”
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 In2O3 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 In2O3 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 Cr3+ and Mn3+ 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.
Semiconductor nanowires need to be doped before they can be used for many applications, but this process is not well understood. A laser-based approach has now shed new light on the doping of nanowires.
Doping the intentional incorporation of atomic impurities in a material is routinely used to control and manipulate the electrical, optical and magnetic properties of semiconductors. However, it has proved challenging to dope semiconductor nanostructures such as nanocrystals because, unlike bulk samples, they are usually prepared under non-equilibrium conditions. Kinetic models of nanocrystal doping have shown that nanocrystal morphology, surface structure and the presence of surfactants are all relevant factors.
Farvid, S. S.; Ju, L.; Worden, M.; Radovanovic, P. V. “Colloidal Chromium-Doped In2O3 Nanocrystals as Building Blocks for High-TC Ferromagnetic Transparent Conducting Oxide Structures”, J. Phys. Chem. C, 2008, 112, 17755-17759.”
Colloidal free-standing Cr3+-doped In2O3 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 Cr3+ dopants are preferentially substituted for In3+ ions in their trigonally distorted octahedral (b) sites in In2O3 nanocrystals. Nanocrystalline films, prepared under mild conditions using colloidal Cr3+-doped In2O3 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 In2O3 nanocrystals opens up new opportunities for spintronics applications and may provide a framework for understanding the origin of ferromagnetism in transparent conducting oxides.
Stamplecoskie, K. G.; Ju, L.; Farvid, S. S.; Radovanovic, P. V. “General Control of Transition-Metal-Doped GaN Nanowire Growth: Toward Understanding the Mechanism of Dopant Incorporation”, Nano Lett., 2008, 8, 2674-2681.”
We report the first synthesis and characterization of cobalt- and chromium-doped GaN nanowires (NWs), and compare them to manganese- doped GaN NWs. Samples were synthesized by chemical vapor deposition method, using cobalt(II) chloride and chromium(III) chloride as dopant precursors. For all three impurity dopants hexagonal, triangular, and rectangular NWs were observed. The fraction of NWs having a particular morphology depends on the initial concentration of the dopant precursors. While all three dopant ions have the identical effect on GaN NW growth and faceting, Co and Cr are incorporated at much lower concentrations than Mn. These findings suggest that the doping mechanism involves binding of the transition-metal intermediates to specific NW facets, inhibiting their growth and causing a change in the NW morphology. We discuss the doping concentrations of Mn, Co, and Cr in terms of differences in their crystal-field stabilization energies (∆CFSE) in their gas-phase intermediates and in substitutionally doped GaN NWs. Using iron(III) chloride and cobalt(II) acetate as dopant precursors we show that the doping concentration dependence on ∆CFSE allows for the prediction of achievable doping concentrations for different dopant ions in GaN NWs, and for a rational choice of a suitable dopant-ion precursor. This work further demonstrates a general and rational control of GaN NW growth using transition-metal impurities.
The prospect of coexistence and control of multiple functional properties in materials is of both scientific and technological importance. Materials that combine electrical and magnetic proper- ties have attracted much attention for their promise in spin- electronics, or spintronics.1 Semiconductor nanowires (NWs) have emerged as key components for future photonic and electronic devices.2 Imparting magnetic properties into semiconductor NWs by controlled doping with transition-metal ions is therefore of a particular interest.3-7 These materials, known as diluted magnetic semiconductor NWs (DMS-NWs), are seen as prime candidates for spintronics applications.3-7 Synthesis of DMS-NWs is a challenging task, however, because of difficulties in doping control during quasi-one-dimensional (1-D) crystal growth. Little is known about the mechanism of dopant ion incorporation and the effects of dopants on the growth, structure, and properties of DMS-NWs. Moreover, directional growth and well-defined faceting make NWs an excellent model system for studies of crystal growth in the presence of impurities.
A general approach for the synthesis of manganese-doped II−VI and III−V nanowires based on metal nanocluster-catalyzed chemical vapor deposition has been developed. High-resolution transmission electron microscopy and energy-dispersive X-ray spectroscopy studies of Mndoped CdS, ZnS, and GaN nanowires demonstrate that the nanowires are single-crystal structures and homogeneously doped with controllable concentrations of manganese ions. Photoluminescence measurements of individual Mn-doped CdS and ZnS nanowires show characteristic pseudo-tetrahedral Mn2+ (4T1 f 6A1) transitions that match the corresponding transitions in bulk single-crystal materials well. Photoluminescence studies of Mn-doped GaN nanowires suggest that manganese is incorporated as a neutral (Mn3+) dopant that partially quenches the GaN band-edge emission. The general and controlled synthesis of nanowires doped with magnetic metal ions opens up opportunities for fundamental physical studies and could lead to the development of nanoscale spintronic devices.
Archer, P. I.; Radovanovic, P. V.; Heald, S. M.; Gamelin, D. R. “Low-Temperature Activation and Deactivation of High-Curie-Temperature Ferromagnetism in a New Diluted Magnetic Semiconductor: Ni2+-Doped SnO2” J. Am. Chem. Soc., 2005, 127, 14479-14487.”
We report the synthesis of colloidal Ni2+-doped SnO2 (Ni2+:SnO2) nanocrystals and their characterization by electronic absorption, magnetic circular dichroism, X-ray absorption, magnetic susceptibility, scanning electron microscopy, and X-ray diffraction measurements. The Ni2+ dopants are found to occupy pseudooctahedral Sn4+ cation sites of rutile SnO2 without local charge compensation. The paramagnetic nanocrystals exhibit robust high-Curie-temperature (TC) ferromagnetism (Ms(300 K) ) 0.8 µB/Ni2+, TC . 300 K) when spin-coated into films, attributed to the formation of interfacial fusion defects. Facile reversibility of the paramagnetic-ferromagnetic phase transition is also observed. This magnetic phase transition is studied as a function of temperature, time, and atmospheric composition, from which the barrier to ferromagnetic activation (Ea) is estimated to be 1200 cm-1. This energy is associated with ligand mobility on the surfaces of the Ni2+:SnO2 nanocrystals. The phase transition is reversed under air but not under N2, from which the microscopic identity of the activating defect is proposed to be interfacial oxygen vacancies.
Ferromagnetism with T(c)>350 K is observed in the diluted magnetic semiconductor Ni(2+):ZnO synthesized from solution. Whereas colloidal Ni(2+):ZnO nanocrystals are paramagnetic, their aggregation gives rise to robust ferromagnetism. The appearance of ferromagnetism is attributed to the increase in domain volumes and the generation of lattice defects upon aggregation. The unusual temperature dependence of the magnetization coercivity is discussed in terms of a temperature-dependent exchange interaction involving paramagnetic Ni2+ ions.
Methods for introducing new magnetic, optical, electronic, photophysical, or photochemical properties to semiconductor nanocrystals are attracting intense applications-oriented interest. In this communication, we report the preparation and electronic absorption spectroscopy of colloidal ZnO DMS-QDs. Our synthetic procedure involves modification of literature methods known to yield highly crystalline and relatively monodisperse nanocrystals of pure ZnO to allow introduction of transition-metal dopants. We use ligand-field electronic absorption spectroscopy as a dopant-specific optical probe to monitor dopant incorporation during nanocrystal growth and to verify internal substitutional doping in Co2+:ZnO and Ni2+:ZnO DMS-QDs. To the best of our knowledge, these are the first free-standing oxide DMS-QDs reported. The synthesis of colloidal oxide DMS-QDs introduces a new category of magnetic semiconductor materials available for detailed physical study and application in nanotechnology.
Radovanovic, P. V.; Gamelin, D. R. “Magnetic Circular Dichroism Spectroscopy of Co2+:CdS Diluted Magnetic Semiconductor Quantum Dots ” Proc. SPIE-Int. Soc. Opt. Eng., 2002, 4809, 51-61. ”
We report the use of electronic absorption and magnetic circular dichroism (MCD) spectroscopies to probe the magneto-optical properties of Co2+ dopant ions in diluted magnetic semiconductor quantum dots. Emphasis is placed on observation and analysis of the ligand field transitions of the Co2+ ions. Because the ligand field transitions may be observed in an energy region where the semiconductor host is transparent, ligand field absorption and MCD spectroscopies serve as excellent site-specific spectroscopic methods for studying the dopant ions within DMS nanocrystals. Cobalt-doped CdS nanocrystals (Co2+CdS) prepared in solution by the isocrystalline core/shell method are shown by high-resolution TEM to be of high crystallinity. The ligand field spectroscopy demonstrates substitutional doping of Co2+ at Cd2+ sites. The MCD spectra show a 103 enhancement in sensitivity for the Co2+ ligand field transitions relative to the CdS bandgap transitions. Saturation magnetization experiments yield optically detected ground state magnetization data for these materials, and show that both the ligand field and bandgap MCD intensities follow S = 3/2 Brillouin saturation behavior associated with the isolated Co2+ ions. The 4 A2→4 T1(P) ligand field bandshape and the sign of the bandgap MCD feature are analyzed in terms of electronic structural parameters for this material.
Radovanovic, P. V.; Gamelin, D. R. “Isocrystalline Core/Shell Synthesis of High Quality Diluted Magnetic Semiconductor Quantum Dots: Ligand-Field Spectroscopic Studies” Proc. SPIE-Int. Soc. Opt. Eng., 2002, 4807, 223-231. “
Ligand field electronic absorption spectroscopy has been applied as a direct probe of Co2+ dopant ions in II-VI based diluted magnetic semiconductor quantum dots. Synthesis of Co2+-doped CdS (Co2+:CdS) quantum dots by simple coprecipitation in inverted micelle solutions has been found to yield predominantly surface bound dopant ions, which are unstable with respect to solvation in a coordinating solvent (pyridine). The solvation kinetics are biphasic, involving two transient intermediates. In contrast, Co2+ ions are doped much more isotropically in ZnS QDs, and this difference is attributed to the similar ionic radii of Co2+ and Zn2+ ions (0.74 Å), as opposed to Cd2+ ions (0.97 Å). We have developed an isocrystalline core/shell synthetic methodology that enables us to synthesize high quality internally doped Co2+:CdS quantum dots. The effect of Co2+ binding on the surface energies of CdS and ZnS quantum dots is discussed and related to the growth mechanism of diluted magnetic semiconductor quantum dots.
Radovanovic, P. V.; Gamelin, D. R. “Electronic Absorption Spectroscopy of Cobalt Ions in Diluted Magnetic Semiconductor Quantum Dots: Demonstration of an Isocrystalline Core/Shell Synthetic Method” J. Am. Chem. Soc. 2001, 123, 12207-12214.”
This paper reports the application of ligand-field electronic absorption spectroscopy to probe Co2+dopant ions in diluted magnetic semiconductor quantum dots. It is found that standard inverted micelle coprecipitation methods for preparing Co2+-doped CdS (Co2+:CdS) quantum dots yield dopant ions predominantly bound to the nanocrystal surfaces. These Co2+:CdS nanocrystals are unstable with respect to solvation of surface-bound Co2+, and time-dependent absorption measurements allow identification of two transient surface-bound intermediates involving solvent−cobalt coordination. Comparison with Co2+:ZnS quantum dots prepared by the same methods, which show nearly isotropic dopant distribution, indicates that the large mismatch between the ionic radii of Co2+ (0.74 Å) and Cd2+ (0.97 Å) is responsible for exclusion of Co2+ ions during CdS nanocrystal growth. An isocrystalline core/shell preparative method is developed that allows synthesis of internally doped Co2+:CdS quantum dots through encapsulation of surface-bound ions beneath additional layers of CdS.