Darshan C. Kundaliya

Niobium doped TiO2: Intrinsic transparent metallic anatase versus highly resistive rutile phase

We report on the structural, electrical, and optical properties of 5% niobium doped TiO2 thin films grown on various substrates by pulsed laser deposition. The epitaxial anatase Nb:TiO2 film on LaAlO3 is shown to be an intrinsic transparent metal and its metallic property arises from Nb substitution into Ti site as evidenced by the Rutherford backscattering channeling result. In contrast, the rutile Nb:TiO2 thin films show insulating behaviors with 2–3 orders higher room temperature electrical resistivity and ∼30 times lower mobility. A blueshift in the optical absorption edge is observed in both phases, though of differing magnitude.

Protein and polymer immobilized La0.7Sr0.3MnO3 nanoparticles for possible biomedical applications

La0.7Sr0.3MnO3 (LSMO) is a mixed-valent room temperature ferromagnet with properties that are attractive for their applicability in biomedicine. We report, for the first time, immobilization of commonly used biocompatible molecules on LSMO nanoparticles, namely bovine serum albumin and dextran. The former was conjugated to LSMO using 1-ethyl-3-(3-dimethyl aminopropyl)-carbodiimide (CDI) as a coupling agent while the latter was used without any coupler. These bioconjugated nanoparticles exhibit several properties that suggest their applicability in the field of biomedicine, namely (a) no changes in the Curie temperature at ~360 K after conjugation with biomolecules, (b) rapid attainment of the desired temperature (48 °C) at low concentration (e.g. fluidized dextran-coated system at 80 µg ml−1) upon exposure to 20 MHz radio-frequency, (c) extremely low cytotoxicity in skin carcinoma, human fibrosarcoma and neuroblastoma cell lines and (d) high stability of the LSMO system with negligible leaching of ionic manganese into the delivery medium, indicating their safety in possible human applications.

Anomalous ferromagnetism in TbMnO3 thin films

Magnetometry, x-ray, and neutron scattering have been used to study the structural and magnetic properties of a TbMnO3 thin film grown on a [001] SrTiO3 substrate by pulsed laser deposition. Although bulk TbMnO3 is a low temperature antiferromagnet, magnetometry measurements indicate the presence of low temperature ferromagnetism. Depth profiling by x-ray and polarized neutron reflectometry reveals a net sample magnetization that is commensurate with the film thickness, indicating that the observed ferromagnetism is not due to an altered surface phase (such as Mn3O4), or external impurities that might give rise to an artificial magnetic signal. Instead, these results show that the ferromagnetism is an intrinsic property of the TbMnO3 film.

La0.7Sr0.3MnO3 nanoparticles coated with fatty amine

We report on the synthesis of La0.7Sr0.3MnO3 (LSMO) nanoparticles having perovskite structure and particle size of the order of 30nm. The process involves citrate-gel synthesis, size filtering, and surface coating with a shell of octadecyl amine (ODA) using electrostatic interaction-assisted novel chemical route. Magnetic measurements show the Curie temperature of ∼360K establishing the desired stoichiometry and phase. Fourier transform infrared studies bring out that the amine group of ODA interacts with the LSMO surface. Refluidization yields uniform redispersion of the coated and dried powder.

Search for ferromagnetism in undoped and cobalt-doped HfO2−δ

We report on the search for ferromagnetism in undoped and cobalt-doped high-k dielectric HfO2 films. Over a broad range of growth conditions, we do not observe ferromagnetism in undoped HfO2 films. On the other hand, we do observe room temperature ferromagnetism in dilutely Co-doped HfO2 films, but the origin of the same appears extrinsic (a Co rich surface layer) at least for the regime of growth conditions explored.

Search for ferromagnetism in conductive Nb:SrTiO3 with magnetic transition element (Cr, Co, Fe, Mn) dopants

Thin films of (0.5%, 1%) Nb:SrTiO3 dilutely doped with (2at.%) magnetic transition elements (Cr, Co, Fe, Mn) are examined for ferromagnetism. X-ray diffraction, Rutherford backscattering ion channeling, scanning transmission electron microscopy Z-contrast imaging, and electron energy loss spectroscopy techniques establish high crystalline quality of the films with no impurity phase(s) and highly uniform dopant distribution. Although the film conductivity improves dramatically by Nb doping, no ferromagnetism is found in any of our samples over the temperature range of 365 down to 5K. This is contrasted to the case of ferromagnetism reported in cobalt doped (La,Sr)TiO3.

Defects in Co-doped and (Co, Nb)-doped TiO2 ferromagnetic thin films

We have investigated the defect structure in pure Co-doped and (Co, Nb)-doped TiO2 (anatase) thin films grown by pulsed laser deposition on (001) single crystal LaAlO3. Although both films exhibit room temperature ferromagnetism, dilute Nb doping significantly improves the conductivity and microstructure of the TiO2 (anatase) thin film at the cost of a lower saturation magnetization. Z-contrast imaging and electron-energy-loss-spectroscopy study in the scanning transmission electron microscope show cluster-free microstructure in the (Co, Nb)-doped thin film although Co is enriched into the surface and forms a CoxTi1−x−yNbyO2−δ phase. In contrast, metallic Co, rutile TiO2, and cobalt oxide nanoparticles combined with some crystallographic shear defect structures are observed in the pure Co-doped thin film. The magnetic behavior in each film can be explained with the presence of these defect states or second phases.

Substrate-induced strain effects on the transport properties of pulsed laser-deposited Nb-doped SrTiO3 films

Thin films of Nb-doped SrTiO3 (NSTO) are grown via pulsed laser deposition (PLD) on LaAlO3 (LAO,001), MgAl2O4 (MAO,001), SrTiO3 (STO,001), and Y-stabilized ZrO2 (YSZ,001) substrates. The effects of the film-substrate lattice mismatch, film thickness, and substrate temperature during growth on the film properties are investigated. The electrical transport in NSTO films is shown to exhibit a strong sensitivity to strain, which is suggested to arise from the dependence of carrier mobility on bond distortions/stretching and related changes in phonon modes.

Consequences of niobium doping for the ferromagnetism and microstructure of anatase Co:TiO2 films

It is shown that dilute niobium doping has a significant effect on the ferromagnetism and microstructure of dilutely cobalt-doped anatase TiO2 films. Epitaxial films of anatase TiO2 with 3% Co, without and with 1% niobium doping were grown by pulsed-laser deposition at 875°C at different oxygen pressures. For growth at 10−5Torr niobium doping suppresses the ferromagnetism, while it enhances the same in films grown at 10−4Torr. High-resolution Z-contrast scanning transmission electron microscopy and electron energy loss spectroscopy show uniform surface segregation of cobalt-rich Ti1−x−yCoxNbyO2−δ phase, but without cobalt metal clusters.

Interfacial characteristics of a Fe3O4∕Nb(0.5%):SrTiO3 oxide junction

The temperature dependent Schottky diode characteristics of epitaxial junctions between Nb:SrTiO3 (Nb concentrations: 0.5%) and Fe3O4 are studied. Epitaxial thin films of Fe3O4 were grown on Nb:SrTiO3 substrates by pulsed laser deposition technique. The films and heterointerfaces were characterized by x-ray diffraction, Z-contrast transmission electron microscopy, magnetic susceptibility, four-probe in-plane resistivity, and the temperature dependent junction current-voltage (I‐V) characteristics. The nonlinear nature of the characteristics is analyzed within the framework of thermionic emission theory. Junction parameters such as the Schottky barrier height (ϕB) and ideality factor (η) are extracted. The temperature evolution of these parameters shows interesting and systematic trends, with remarkable changes near the Verwey transition (TV=120K). The magnetic field dependence of I‐V characteristic data is also recorded and a spin polarization of ∼80% is estimated for the magnetite electrode.