Journal of Nano Research Vol. 5

Abstract: Cathodoluminescence (CL) quenching was observed in ZnO nanostructures when doped with Yb by both chemical and physical methods. CL spectra of the samples revealed a defect emission at 2.25 eV in samples prepared by the chemical method, and an emission at 2.5 eV in samples prepared by the physical method. From the thermal treatment studies, it was found that oxygen vacancies are responsible for the 2.5 eV emission. Observed CL quenching in ZnO is explained through the participation of point defects in the energy transfer process from ZnO to Yb3+.

Abstract: In this work we report synthesis and characterization of hydrogenated nanocrystalline silicon (nc-Si:H) thin films by plasma chemical vapor deposition (P-CVD) method at 200 0C on glass substrates. Film properties are carefully and systematically investigated as a function of argon (Ar) flow rate. Characterization of these films with Raman spectroscopy revealed that the addition of Ar into SiH4-H2 plasma endorses the growth of crystallinity in the films. The Fourier transform infrared (FTIR) spectroscopic analysis showed that with increasing Ar flow rate the hydrogen bonding in the films shifts from mono-hydride (Si-H) to di-hydride (Si-H2) and (Si-H2)n complexes. The hydrogen content in the films was found < 7 at. % over the entire range of studied Ar flow rate. The band gap of nc-Si:H films was found to be higher than hydrogenated amorphous silicon (a-Si:H) films (> 2 eV). The nc-Si:H films with dark conductivity 1.3x10-7 S/cm having deposition rate as high as 2.5 Å/s and of crystalline fraction 98 % have been obtained.

Abstract: Developing new semiconductor materials with improved photocatalytic activity is a promising technology for the remedy of environmental pollution. Here we report on the synthesis of Yb containing TiO2 nanoparticles and their catalytic activity under visible light. Highly monodispersed, spherical TiO2 and TiO2 :Yb nanoparticles of 27- 40 nm size range were prepared through controlled hydrolysis of titanium tetrabutoxide (TTB) and characterized by X-ray diffraction (XRD), energy dispersion spectroscopy (EDS), high-resolution transmission electron microscopy (HRTEM), high angle annular dark field (HAADF), and ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS) techniques. Average size of the nanoparticles was found to decrease with the increase of Yb doping concentration. The photocatalytic activity of Yb doped TiO2 was evaluated by measuring the degradation rates of methylene blue (MB) under UV and visible lights. Doping with ytterbium ions enhanced significantly the photocatalytic activity of TiO2 nanoparticles for MB oxidation under visible light.

Abstract: In this theoretical study, the H2 adsorption is considered in a novel system formed by a (6,6) carbon nanotube fragment and a planar graphene layer portion, when they are separated 4.72 a.u., with both subsystems inside a cubic supercell box of 25 a.u. side. There is greater H2 adsorption inside the carbon nanotube than in the interstitial space between the graphene layer and the carbon nanotube. The results are compared with the way the H2 molecule is adsorbed upon a lonely graphene layer and inside or outside the (6,6) carbon nanotube. It is studied if in the interstitial space close to the middle point between the wall and the graphene layer the hydrogen molecule could be adsorbed with a greater binding energy that in any other case. This was not possible for the selected supercell, but there are given some suggestions to be explored (using a nanotube with smaller radii or increasing the size of the supercell), that perhaps can optimize the binding energy for H2 adsorption. A general result is that the size of the cubic supercell can be selected to confine not only hydrogen inside the carbon nanotube but also in the interstitial space between the carbon nanotube wall and the graphene layer. It is believed that the studied system or a modification of this could sooner or later be used in a competitive way in comparison with other H2 storage materials respect to the hydrogen adsorption and desorption process.

Abstract: Powders of elemental Mg, Zn, Al and Ag were milled in order to produce nanocrystalline alloys with nominal composition Mg98M2 (M=Zn, Al and Ag). Pure Mg was also mechanically milled without any additions. Single-phase nanocrystalline (crystal size 24-26 nm) Mg98M2 alloys were produced after 216 ks of milling. A passivity procedure was followed immediately after milling, by gradually exposing the alloy powders to air (~ 12 hrs). After this procedure, the mechanically alloyed powders were kept under argon atmosphere before being hydrided at 200 and 300 °C under 0.5 and 3 MPa P for 10 min. Previously milled (~ 1.5 years before) and passivated powder alloys (stored in air and referred to as “AE” samples) were also hydrided under the same conditions. No hydriding was observed in the as-received Mg powders (crystal size >> 100 nm), but the as-milled, passivated nanocrystalline alloys were partially hydrided (even the AE samples). The amounts of the MgH2 phase in the hydrided samples were larger in the Ar-stored than in the AE samples under all hydriding conditions. The possible role of MgO and Mg hydroxides, as well as of the alloying elements, on the hydriding behavior of the nanostructured, mechanically alloyed powder alloys is discussed.

Abstract: ZnO nanorods (NRs) have been synthesized by a chemical bath deposition (CBD) method on simple glass substrate that had been precoated by successive ionic layer absorption and reaction (SILAR) with a thin ZnO film. ZnO NR array was obtained by using zinc acetate and hexamethylenetetramine as aqueous solutions at optimized pH concentration and deposition time. X-ray diffraction (XRD) and SEM analysis were used to confirm the growth of ZnO nanorods. The pH and deposition time of the solution was found to influence the growth behavior of ZnO NRs. PL analysis also reflected the growth behavior of ZnO NRs.

Abstract: Field emission from Al-doped ZnO nanostrcutures has been investigated in planar diode configuration under ultra high vacuum conditions. The Al-doped ZnO nanostructures were synthesized by co-precipitation method with varying aluminium concentrations. The as- synthesized product was characterized by x-ray diffraction, scanning electron microscope and energy dispersive x-ray analysis. The threshold field required to draw a current density of ~ 1 μA/cm2 was observed to be ~ 2.0 V/μm and ~ 2.3 V/μm for Al-doped ZnO nanostructures synthesized with aluminium concentrations of 1% and 3%, respectively. The Fowler- Nordheim (F-N) plots for both the specimens exhibit non-linear behaviour, which is observed to be specimen dependent. The non-linearity observed in the F-N plots has been interpreted on the basis of the theory of electron emission from semiconductor emitters. The field enhancement factors, estimated from the slope of the F-N plots, are found to be ~ 9.3 x 103 and 3.9 x 103 for 1% and 3% Al-doped ZnO emitters, respectively. The high values of the field enhancement factor suggest that the emission is from the nanostructures. The emission current stability measured at the preset value of ~ 2 μA over a period of more than three hours is found to be fairly stable. The results indicate use of Al-doped ZnO nanostructures as promising emitters for field emission based devices.