Thiam Teck Tan

Doping concentration dependence of microstructure and magnetic behaviours in Co-doped TiO2 nanorods

Co-doped titanium dioxide (TiO2) nanorods with different doping concentrations were fabricated by a molten salt method. It is found that the morphology of TiO2 changes from nanorods to nanoparticles with increasing doping concentration. The mechanism for the structure and phase evolution is investigated in detail. Undoped TiO2 nanorods show strong ferromagnetism at room temperature, whereas incorporating of Co deteriorates the ferromagnetic ordering. X-ray photoelectron spectroscopy (XPS) and electron spin resonance (ESR) results demonstrate that the ferromagnetism is associated with Ti vacancy.

Tunable thermopower and thermal conductivity in Lu doped In2O3

Lu-doped polycrystalline (In1−xLux)2O3 (x = 0, 0.025, 0.05, 0.10, 0.15) materials were prepared by co-precipitation method followed by spark plasma sintering processing. The large and heavy Lu3+ was strategically selected to replace In3+ in order to lower the thermal conductivity while minimizing changes to the electronic structure of In2O3. The resulting samples had an average grain size of 200–400 nm at a density of ∼90% of the theoretical mass density. Lu doping was found to increase the thermopower of In2O3, however the electrical resistivity was also increased, primarily due to the remarkable decrease in carrier concentration. The thermal conductivity of In2O3 was significantly reduced by the substitution of In by Lu.

Tunable resistance switching in solution processed chromium-doped strontium titanate nanoparticles films

In this work, resistance switching behaviours in solution processed chromium (Cr)-doped strontium titanate (SrTiO3) films have been investigated. Undoped SrTiO3 film shows I-V characteristics of typical nonlinear resistor and no resistance hysteresis loops are observed. On the contrary, Cr-doped SrTiO3 films show stable and reversible hysteresis loops, which can be controlled by applying different voltage bias. Based on a series of characterization results, including X-ray diffraction (XRD), Raman and X-ray photoelectron spectroscopy (XPS), we infer that Ti4+ is substituted by Cr3+, giving rise to increased concentration of oxygen vacancies. Therefore, the observed resistance switching phenomenon is attributed to voltage driven oxygen vacancy migration. Furthermore, gradually decreased overall resistance is also realized under repeated sweeping cycles.

Key Role of Bismuth in the Magnetoelastic Transitions of Ba3BiIr2O9 and Ba3BiRu2O9 As Revealed by Chemical Doping

The key role played by bismuth in an average intermediate oxidation state in the magnetoelastic spin-gap compounds Ba3BiRu2O9 and Ba3BiIr2O9 has been confirmed by systematically replacing bismuth with La(3+) and Ce(4+). Through a combination of powder diffraction (neutron and synchrotron), X-ray absorption spectroscopy, and magnetic properties measurements, we show that Ru/Ir cations in Ba3BiRu2O9 and Ba3BiIr2O9 have oxidation states between +4 and +4.5, suggesting that Bi cations exist in an unusual average oxidation state intermediate between the conventional +3 and +5 states (which is confirmed by the Bi L3-edge spectrum of Ba3BiRu2O9). Precise measurements of lattice parameters from synchrotron diffraction are consistent with the presence of intermediate oxidation state bismuth cations throughout the doping ranges. We find that relatively small amounts of doping (∼10 at%) on the bismuth site suppress and then completely eliminate the sharp structural and magnetic transitions observed in pure Ba3BiRu2O9 and Ba3BiIr2O9, strongly suggesting that the unstable electronic state of bismuth plays a critical role in the behavior of these materials.

Subtle interplay between localized magnetic moments and itinerant electrons in LaAlO3/SrTiO3 heterostructures

Clarification of the role of magnetic ordering and scattering in two-dimensional electron gas has become increasingly important to understand the transport and magnetic behavior in the LaAlO3 (LAO)/SrTiO3 (STO) heterostructures. In this work, we report the sheet resistance of the LAO/STO heterostructures as functions of temperature, magnetic field, and field orientation. An unexpected resistance minimum was discovered at ∼10 K under a sufficiently high in-plane magnetic field. An anisotropic magnetoresistance (MR) is clearly identified, indicating the presence of magnetic scattering which may be related to the interaction between itinerant electrons and localized magnetic moments in the LaAlO3/SrTiO3 heterostructures. It is believed that the high concentration of oxygen vacancies induced by the ultralow oxygen partial pressure during the deposition process plays a predominant role in the occurrence of the anisotropic MR.

The effect of annealing oxygen concentration in the transformation of CaxCoO2 to thermoelectric Ca3Co4O9

Calcium cobalt oxide thin films were deposited using radio-frequency (RF) sputtering on single crystal c-axis sapphire substrates. The as-deposited calcium cobaltite crystallized in the form of CaxCoO2, which has a slight calcium deficiency compared to the annealed Ca3Co4O9 phase. When the films have a slight calcium deficiency, transformations to the Ca3Co4O9 phase proceed with Co3O4 as a by-product. It was discovered that this transformation can take place with a relatively low oxygen concentration in the annealing gas, as low as 5% in this study. However, optimum thermoelectric and electrical properties were achieved in the films annealed with an oxygen concentration of about 20%. There was no significant change in such properties when the films were annealed at higher oxygen concentrations. This suggests that the optimum defect concentration induced via the transformation occurs with an oxygen concentration of about 20% in the annealing atmosphere.

Largely Enhanced Mobility in Trilayered LaAlO3/SrTiO3/LaAlO3 Heterostructures

LaAlO3 (LAO)/SrTiO3 (STO)/LaAlO3 (LAO) heterostructures were epitaxially deposited on TiO2-terminated (100) SrTiO3 single-crystal substrates by laser molecular beam epitaxy. The electron Hall mobility of 1.2 × 104 cm2/V s at 2 K was obtained in our trilayered heterostructures grown under 1 × 10-5 Torr, which was significantly higher than that in single-layer 5 unit cells LAO (∼4 × 103 cm2/V s) epitaxially grown on (100) STO substrates under the same conditions. It is believed that the enhancement of dielectric permittivity in the polar insulating trilayer can screen the electric field, thus reducing the carrier effective mass of the two-dimensional electron gas formed at the TiO2 interfacial layer in the substrate, resulting in a largely enhanced mobility, as suggested by the first-principle calculation. Our results will pave the way for designing high-mobility oxide nanoelectronic devices based on LAO/STO heterostructures.