R. S. Vemuri

Structural transformation induced changes in the optical properties of nanocrystalline tungsten oxide thin films

Nanocrystalline tungsten oxide (WO3) films were grown by reactive magnetron sputter-deposition. The structure and optical properties of WO3 films were evaluated using grazing incidence x-ray diffraction and optical spectroscopic measurements. The effect of ultramicrostructure was significant on the optical characteristics of WO3 films. The band gap decreases from 3.25 to 2.92 eV with increasing grain-size from ∼9 to 50 nm while the films exhibit a transition from monoclinic to tetragonal phase. A direct microstructure-property relationship found suggests that tuning properties of WO3 films for desired applications can be achieved by tuning the conditions and controlling the phase.

Spectroscopic ellipsometry and x-ray photoelectron spectroscopy of La2O3 thin films deposited by reactive magnetron sputtering

Lanthanum oxide (La2O3) films were grown by the reactive dc magnetron sputtering and studied their structural, chemical and optical parameters. La2O3 films were deposited onto Si substrates by sputtering La-metal in a reactive gas (Ar+O2) mixture at a substrate temperature of 200 °C. Reflection high-energy electron diffraction measurements confirm the amorphous state of La2O3 films. Chemical analysis of the top-surface layers evaluated with x-ray photoelectron spectroscopy indicates the presence of a layer modified by hydroxylation due to interaction with atmosphere. Optical parameters of a-La2O3 were determined with spectroscopic ellipsometry (SE). There is no optical absorption over spectral range λ=250–1100 nm. Dispersion of refractive index of a-La2O3 was defined by fitting of SE parameters over λ=250–1100 nm.

Correlation between microstructure, electrical and optical properties of nanocrystalline NiFe1.925Dy0.075O4 thin films

Dysprosium-doped nickel-ferrite (NiFe1.925Dy0.075O4) thin films were fabricated using sputter-deposition using a stoichiometric bulk target prepared by the solid state chemical reaction. The structural, electrical and optical properties of NiFe1.925Dy0.075O4 thin films were studied in detail. The grain-size (L) and lattice-expansion effects are significant on the electrical and optical properties of NiFe1.925Dy0.075O4 films. Air annealing (Ta) at 450–1000 °C results in the formation of nanocrystalline NiFe1.925Dy0.075O4 films, which crystallize in the inverse spinel structure, with L = 5–40 nm. The lattice constant of NiFe1.925Dy0.075O4 increases compared NiFe2O4 due to Dy-doping. Electrical conductivity of NiFe1.925Dy0.075O4 films (at 300 K) decreases from 1.07 Ω−1 m−1 to 3.9 × 10−3 Ω−1 m−1 with increasing Ta (450 to 1000 °C). Conductivity was found to decrease exponentially with decreasing the temperature from 300 K to 120 K indicating the characteristic semiconducting nature of all the films. Band gap increases from 3.17 to 4.08 eV for NiFe1.925Dy0.075O4 films with increasing Ta from 450 to 1000 °C. A correlation between grain-size, electrical conductivity and optical band gap in nanocrystalline NiFe1.925Dy0.075O4 films is established.

Tungsten oxide (WO3) thin films for application in advanced energy systems

Inherent processes in coal gasification plants produce hazardous hydrogen sulfide (H2S), which must be continuously and efficiently detected and removed before the fuel is used for power generation. An attempt has been made in this work to fabricate tungsten oxide (WO3) thin films by radio-frequency reactive magnetron-sputter deposition. The impetus being the use of WO3 films for H2S sensors in coal gasification plants. The effect of growth temperature, which is varied in the range of 30–500 °C, on the growth and microstructure of WO3 thin films is investigated. Characterizations made using scanning electron microscopy (SEM) and x-ray diffraction (XRD) indicate that the effect of temperature is significant on the microstructure of WO3 films. XRD and SEM results indicate that the WO3 films grown at room temperature are amorphous, whereas films grown at higher temperatures are nanocrystalline. The average grain-size increases with increasing temperature. WO3 films exhibit smooth morphology at growth tempera...