Manu Hegde

Interplay between Size, Composition, and Phase Transition of Nanocrystalline Cr3+-Doped BaTiO3 as a Path to Multiferroism in Perovskite-Type Oxides

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 Cr(3+)-doped BaTiO(3) 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 Cr(3+)-doped BaTiO(3) nanocrystals exhibit robust ferromagnetic ordering (up to ca. 2 μ(B)/Cr(3+)), 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 Cr(3+) 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.

Evidence of charge-transfer ferromagnetism in transparent diluted magnetic oxide nanocrystals: switching the mechanism of magnetic interactions.

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 paramagnetic Mn dopant ions in mixed valence states (Mn(2+) and Mn(3+)) 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.

Origin of size-dependent photoluminescence decay dynamics in colloidal γ-Ga2O3 nanocrystals

We studied size-dependent dynamics of defect-based photoluminescence of colloidal γ-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.

Tuning manganese dopant spin interactions in single GaN nanowires at room temperature.

Control of electron spins in individual magnetically doped semiconductor nanostructures 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.

Electronic structure and magnetism of Mn dopants in GaN nanowires: Ensemble vs single nanowire measurements

We studied the electronic structure and magnetization of Mn dopants in GaN nanowires at the ensemble and single nanowire levels by near edge x-ray absorption fine structure spectroscopies. The results of single nanowire measurements indicate that Mn adopts tetrahedral coordination in GaN nanowires 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 nanowire x-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.

Plasmon-induced carrier polarization in semiconductor nanocrystals

Spintronics1 and valleytronics2 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 nanocrystals3–10 offer an opportunity for interface-free coupling between a plasmon and an exciton. However, plasmon–exciton coupling in single-phase semiconductor nanocrystals remains challenging because confined plasmon oscillations are generally not resonant with excitonic transitions. Here, we demonstrate a robust electron polarization in degenerately doped In2O3 nanocrystals, enabled by non-resonant coupling of cyclotron magnetoplasmonic modes11 with the exciton at the Fermi level. Using magnetic circular dichroism spectroscopy, we show that intrinsic plasmon–exciton coupling allows for the indirect excitation of the magnetoplasmonic modes, and subsequent Zeeman splitting of the excitonic states. Splitting of the band states and selective carrier polarization can be manipulated further by spin–orbit coupling. Our 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 is readily achieved at room temperature, which allows us to harness the magnetoplasmonic mode as a new degree of freedom in practical photonic, optoelectronic and quantum-information processing devices.Cyclotron plasmonic modes can be used to induce carrier polarization in doped semiconductors.

Controlling the Mechanism of Excitonic Splitting in In2O3 Nanocrystals by Carrier Delocalization

Degenerately doped metal oxide nanocrystals have emerged as infrared plasmonic materials with promising applications in optoelectronics, surface-enhanced infrared spectroscopies, and sensing. They also have potential for technological applications in electronics and photonics owing to the possibility of coupling between plasmon and exciton in the absence of a heterojunction. Here, we demonstrate the control of excitonic splitting in In2O3 nanocrystals 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 magnetic circular dichroism spectroscopy, we show that the nanocrystal band splitting has two distinct contributions in plasmonic In2O3 nanocrystals. 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 nanocrystal excited states and the localized defect states. The results of this work demonstrate the ability to control carrier polarization in nonmagnetic metal oxide nanocrystals 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.

Introducing and manipulating magnetic dopant exchange interactions in semiconductor nanowires

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.