Awais Ali

Improvement of (004) texturing by slow growth of Nd doped TiO2 films

In this work, we present preferred (004) texturing of sputter deposited titanium dioxide (TiO2) films on glass substrates as a consequence of Nd doping at very slow growth rate. Nd concentration was varied from 0.0 to 2.0 atomic percent (at. %) in TiO2 thin films deposited under identical growth conditions, i.e., the growth rate of 0.6 A/s and at 500 °C. At 2 at. % Nd, complete texturing along (004) plane was observed, as indicated by the X-ray diffraction analyses. Morphology of the deposited TiO2 films from pure to 2 at. % Nd doped TiO2 films showed faceted to planar growth. Visualization of electrical and structural analysis (VESTA) software revealed that the change in the morphology was due to preferential growth in (004) orientation. Raman spectroscopy highlighted the phonon confinement in the Eg mode, and a red shift was observed due to an increase in the anti-symmetry in bonding with increased Nd concentration. XPS results confirmed the variation in oxygen vacancy concentration along with the reduc...

Temperature dependent tuning of the flat band voltages of TiO2/Si interfaces

In this work, we present study of charge accumulation at the TiO2/Si interfaces and its variation in the TiO2 thin films sputter-deposited on n-Si, p-Si, and B-implanted Si substrates at various growth temperatures. TiO2 films, deposited in an oxygen deficient environment, showed significant growth of rutile phase and absence of anatase phase in the as-grown films. Annealing in air resulted in emergence of anatase phase, thus improving the ratio of anatase to rutile phase in TiO2 films. The flatband voltages determined from capacitance-voltage measurements were observed to be high in the rutile phase TiO2 and dropped considerably on annealing, due to formation of the anatase phase. The drop in the flatband voltages on annealing was also observed to depend strongly on the initial growth temperature. Films grown at high temperatures showed relatively low flatband voltages as compared to the films grown at room temperature. It is demonstrated that VFB strongly depends on the interface traps, and oxide-relate...

Role of Ti phases in the modulation of border traps at the TiO2/n-Si interfaces

Charge accumulation at interfaces is a key issue for the use of high dielectric constant materials in nanoelectronics. In this work, we report the charge accumulation behavior at the TiO2/n-Si interfaces formed at various growth temperatures. Growth of TiO2 in an oxygen deficient environment led to the formation of rutile phase in the as-grown films. The anatase phase was recovered by annealing in air and the ratio of anatase to rutile phase in the TiO2 films improved considerably. The amount of charge accumulation and the direction of charge injection were studied by obtaining C–V hysteresis curves in the as-grown and the annealed TiO2 films. It was observed that the amount of accumulated charges decreased as the density of interface border traps dropped considerably due to annealing. X-ray photo-electron spectroscopy revealed the presence of two major phases corresponding to TiO2 and Ti2O3, due to the existence of oxygen deficiencies. The annealing in air resulted in appreciable increase in the weight p...

Effect of Cr-N codoping on structural phase transition, Raman modes, and optical properties of TiO2 nanoparticles

Noncompensated cation-anion codoping in TiO2 nanoparticles has been achieved by a chemical synthesis route. Significant reduction in the optical bandgap and enhancement in the absorption of visible light have been observed. Structural phase transformation has been tracked in detail as a function of doping and heat treatment temperature. Anatase to rutile phase transition temperature for doped samples was higher in comparison to the pure TiO2 nanoparticles. Nitrogen and chromium addition increases the phase transformation barrier, where the effect of the former dopant is of more significance. The Raman results showed an increase in the oxygen content with higher post annealing temperatures. With Cr incorporation, the peak associated with the Eg mode has been found to shift towards a higher wave number, while with nitrogen incorporation, the shift was towards a lower wave number. A decrease in reflectance with N co-doping for all samples, irrespective of phase and annealing temperatures, has been observed. ...

Facile synthesis of gold nanostars over a wide size range and their excellent surface enhanced Raman scattering and fluorescence quenching properties

Surface enhanced Raman scattering (SERS) is an excellent technique for detecting the trace concentration of targets. However, SERS based detection of fluorescent targets is tricky as the fluorescence signal can severely interfere with the SERS signal upon resonance excitation. This can limit the trace detection capability of SERS. Herein, the authors demonstrate that Au nanostars (NSs) are excellent for fluorescence quenching and trace detection by SERS. The NS based SERS substrate was used to measure the R6G concentrations as low as 10 pM upon laser excitation that are in resonance with the fluorescence absorption. The authors used a simple seed-mediated and surfactant assisted method to synthesize AuNSs in a wide size range. The size of the NSs can be tuned from 55 nm to about 1 μm simply by varying the seed to HAuCl4 ratio in the growth solution.Surface enhanced Raman scattering (SERS) is an excellent technique for detecting the trace concentration of targets. However, SERS based detection of fluorescent targets is tricky as the fluorescence signal can severely interfere with the SERS signal upon resonance excitation. This can limit the trace detection capability of SERS. Herein, the authors demonstrate that Au nanostars (NSs) are excellent for fluorescence quenching and trace detection by SERS. The NS based SERS substrate was used to measure the R6G concentrations as low as 10 pM upon laser excitation that are in resonance with the fluorescence absorption. The authors used a simple seed-mediated and surfactant assisted method to synthesize AuNSs in a wide size range. The size of the NSs can be tuned from 55 nm to about 1 μm simply by varying the seed to HAuCl4 ratio in the growth solution.