K. C. George

Correlated barrier hopping in ZnO nanorods

The ac conduction in ZnO nanorods has been investigated in the frequency range of 100 Hz−1 MHz and at various temperatures between 303 and 543 K. An agreement between experimental and theoretical results suggests that the ac conduction in ZnO nanorods can be successfully explained by Correlated Barrier Hopping model. PL spectrum gives the evidence of surface defects in ZnO nanorods which contribute to this type of conduction mechanism. The different physical parameters such as hopping distance, density of charged defect states, etc. are calculated from the experimental data.

Correlated barrier hopping in CdS nanoparticles and nanowires

The ac conduction in CdS nanoparticles and nanowires was investigated in the frequency range 102 to 106 Hz and in the temperature range 303–573K. The behavior of ac conductivity was found to agree with that reported for amorphous materials and doped semiconductors. The values of ac conductivities were found to be higher than those reported for bulk CdS. The experimental results were analyzed with the CBH model proposed by Elliott. This model provided reasonable values for the maximum barrier height and characteristic relaxation time. The electrical properties of CdS nanowires and nanoparticles were compared. It was also found that the concentration of charged defect centers was large in nanowires and nanoparticles and it tended to decrease with an increase in temperature. The density of neutral defect, formed by the conversion of charged defects was also calculated. The defects in both types of nanostructures were studied by photoluminescence spectroscopy. It was found that nanoparticles possessed higher defect density than nanowires.The ac conduction in CdS nanoparticles and nanowires was investigated in the frequency range 102 to 106 Hz and in the temperature range 303–573K. The behavior of ac conductivity was found to agree with that reported for amorphous materials and doped semiconductors. The values of ac conductivities were found to be higher than those reported for bulk CdS. The experimental results were analyzed with the CBH model proposed by Elliott. This model provided reasonable values for the maximum barrier height and characteristic relaxation time. The electrical properties of CdS nanowires and nanoparticles were compared. It was also found that the concentration of charged defect centers was large in nanowires and nanoparticles and it tended to decrease with an increase in temperature. The density of neutral defect, formed by the conversion of charged defects was also calculated. The defects in both types of nanostructures were studied by photoluminescence spectroscopy. It was found that nanoparticles possessed higher ...

Phase instability and defect induced evolution of optical properties in Cd rich-CdS nanoparticles

Although semiconductor nanostructures exhibit improved physical properties, their structural instability is be a problem for technological applications. This paper reports a temperature-induced low energy phase transformation that occurs in CdS nanoparticles which is ascribed to thermally generated stacking-faults. The phase transformation is found to change the nature of lattice strain in the samples. The formation of this defect-induced lattice strain and its influence on the physical properties of the samples are investigated. The modified optical absorption and emission properties of the samples are thoroughly studied. Using resonance Raman spectroscopy, the evolution of electron-phonon (e-p) coupling strength with structural changes is studied. An explanation to the contradictory nature of e-p coupling strength observed in the nano-regime is given.

Correlated barrier hopping of CuO nanoparticles

The ac conduction mechanism in copper oxide nanoparticles with 8 nm size, synthesized by a precipitation method was studied by analyzing ac conductivity in the frequency range of 50 Hz–1 MHz and in the temperature range of 373–573 K. X-ray diffraction and transmission electron microscopy (TEM) were employed for the structural and morphological characterization of CuO nanoparticles. The experimental and theoretical investigations suggested that the ac conduction mechanism in CuO nanoparticles can be successfully explained by a correlated barrier hopping model, which provided reasonable values for the maximum barrier height and characteristic relaxation time. It was also found that bipolaron hopping become prominent up to a particular temperature and beyond that single polaron hopping predominates. Physical parameters such as hopping distance and density of defect states were also calculated. Photoluminescence studies confirm the presence of a surface defect in CuO nanoparticles.

Optical Properties of CuO Nanoparticles

CuO nanoparticles were successfully prepared by a chemical method which has been widely studied as photothermal and photo conductive materials. The samples were characterized by X‐ray diffraction (XRD), Scanning Electron Microscopy (SEM), UV‐Visible spectroscopy and Photoluminescence Spectroscopy. The SEM image showed that the prepared product consists of spherical nanoparticles with narrow size distribution. The average crystallite size of CuO nanoparticles calculated from XRD pattern was 8 nm. The UV‐Visible absorption spectrum of CuO nanoparticles shows a strong blue shift compared to that of bulk. The band gap energy was calculated from the absorption spectra. Due to the high surface to volume ratio, CuO nanoparticles have high density of surface defects, interstitials and oxygen vacancies. Photo luminescence spectroscopy has been employed in order to explore the optical emission properties of CuO nanoparticles. The emission at 535 nm corresponds to the band edge emission and that at 600 nm is due to ...

Optical properties of SnO2 nanoparticles

SnO2 nanoparticles were successfully prepared by a sol-gel technique. The samples were analyzed by XRD, SEM, TEM, UV, Photoluminescence (PL) and Raman studies. The obtained product has a particle size of 12 nm with absorption peak at 278 nm. The absorption peak shows a blue shift when compared to the bulk due to quantum confinement. The FTIR spectrum of the prepared SnO2 nanoparticles exhibits a broad absorption band between 3100 and 3400 cm−1 as well as a narrower peak at 1600 cm−1. The PL spectrum shows two strong peaks at 420 and 484 nm and broad peak between 430 and 470 nm.

Correlated Barrier Hopping in ZnO Nanotubes

The ac conduction in ZnO nanotubes has been investigated in the frequency range 100 Hz–1 MHz and at temperatures between 303 and 543 K. An agreement between experimental and theoretical results suggests that the electron transport in ZnO nanotubes can be successfully explained by Correlated Barrier Hopping model. PL spectrum gives the evidence of surface defects in ZnO nanotubes which contribute to this type of conduction mechanism.

Gaussian Confinement of Phonons in CdS Nanoparticles

The Raman spectrum of CdS nanoparticles prepared by precipitation technique was studies in the light of Gaussian confinement model. The values of FWHM, asymmetry ratio and peak position were calculated BOTH experimentally and theoretically. It was found that all these parameters corresponding to 1LO and 2LO optical phonons vary from their bulk counterparts due to the quantum confinement effect.

Optical Properties of Al Doped ZnO Nanorods

Aluminium doped Zinc Oxide nanorods Zn (1− x ) Al x O ( x = 0,0.02,0.05,0.08) were synthesized by hydrothermal process. XRD, TEM, SEM‐EDS and SAED pattern were used to characterise the crystalline structure, size and morphology of the samples. Results showed that the aluminium ions replace Zn 2+ ions into the ZnO lattice without changing its wurtzite structure. The optical properties of as‐synthesised Al doped ZnO nanorods were investigated in detail by UV‐vis absorption, Photoluniniscence and Raman spectra. No apparent changes in the band gap energies were observed upto the doping concentration of 5 mol%. The Al doped ZnO nanorods with 8 mol% showed a strong exciton absorption peak at 360 nm and there was a sudden increase in the band gap energy. The perfect wurtzite structure of Al doped samples were verified by the intense E 2 (high) Raman mode. The broad band in the range 535–545 cm −1 was associated with intrinsic lattice defects arised by the doping which was absent in the Raman spectra of pure ZnO nanorods. Because of the anharmonicity effect, some overtone and combination modes were observed besides the fundamental phonon modes. The PL spectra showed that the ratio of ultraviolet to visible emission peak ( I UV / I Vis ) decreased till the doping concentration of 5 mol%. The strong ultraviolet emission and high I UV / I Vis ratio were observed in the ZnO nanorods with doping concentration of 8 mol%.