Nazar Abbas Shah

Effects of Mg doping on optical and CO gas sensing properties of sensitive ZnO nanobelts

We report the synthesis, optical characterization and enhanced carbon monoxide (CO) gas sensing properties of magnesium (Mg) doped 1D zinc oxide (ZnO) nanobelts obtained via a vapor transport method. The structural, morphological and compositional properties of the samples were investigated by powder X-ray diffraction (p-XRD), field emission scanning electron microscopy (FESEM) and energy dispersive X-ray (EDX) analysis. Optical characterization was carried out using Raman spectroscopy, Photoluminescence (PL), diffuse reflectance spectroscopy (DRS), UV sensing and CO gas sensing. Crystalline nanobelts were obtained with average thickness of about 34 nm, width of 290 nm, and length of 3.25 μm. Significant changes in the bandgap energy due to Mg doping were observed. The gas sensing properties of undoped and Mg doped ZnO nanostructures were tested based on the resistance change upon exposure to air and CO gas. The ability of Mg doped ZnO nanobelts to sense 20 ppm of CO gas at 350 °C was enhanced fivefold, with good stability, indicating that Mg doping is very effective in improving the CO sensing of ZnO nanobelts. In addition, a model that describes the CO gas sensing mechanism of both undoped and Mg doped ZnO nanostructures is presented.

Synthesis of ZnO nanostructures for low temperature CO and UV sensing.

In this paper, synthesis and results of the low temperature sensing of carbon monoxide (CO) gas and room temperature UV sensors using one dimensional (1-D) ZnO nanostructures are presented. Comb-like structures, belts and rods, and needle-shaped nanobelts were synthesized by varying synthesis temperature using a vapor transport method. Needle-like ZnO nanobelts are unique as, according to our knowledge, there is no evidence of such morphology in previous literature. The structural, morphological and optical characterization was carried out using X-ray diffraction, scanning electron microscopy and diffused reflectance spectroscopy techniques. It was observed that the sensing response of comb-like structures for UV light was greater as compared to the other grown structures. Comb-like structure based gas sensors successfully detect CO at 75 °C while other structures did not show any response.

Fabrication and thermal characteristics of functionalized carbon nanotubes impregnated polydimethylsiloxane nanocomposites

Multiwalled carbon nanotubes (MWCNTs) were modified to covalently attach the carboxylic moiety with their surfaces. Variant concentrations of functionalized multiwalled carbon nanotubes (F-MWCNTs) were introduced into polydimethylsiloxane (PDMS) adopting solution mixing technique. Fourier transform infrared spectroscopy (FTIR) confirms the carboxy functionalization presence on the surface of the nanotubes. X-ray diffraction (XRD) patterns for both MWCNTs and F-MWCNTs illustrate that the crystallinity does not alter with surface modification of the nanotubes. Experimental results simulated that electrical conductivity of the nanocomposites was augmented with increasing filler concentration in the host matrix. Thermal conductivity and thermal impedance of the nanocomposite specimens were evaluated according to developed methodologies and the accumulative data revealed the nanocomposites thermal transport dependence on the F-MWCNTs doping concentration in the host polymer matrix. Thermal stability enhancement with increasing filler incorporation into the polymer matrix was observed in thermogravimetric/differential thermal analyzer (TG/DTA) contours. Crystallization, glass transition, and melting temperatures were examined using differential scanning calorimeter (DSC) and it was observed that phase transition temperatures of the composite specimens can be tuned by varying the nanotubes to matrix ratio. Scanning electron microscopy and energy dispersive x-ray spectroscopy were carried out to analyze the surface morphology/composition of the fabricated nanocomposites and dispersion of functionalized and pristine MWCNTs in the polymer matrix.

Investigation of Cu-containing low resistivity CdTe thin films deposited by the two-source evaporation technique

Cadmium telluride (CdTe) thin films were deposited onto scratch-free transparent glass substrates by the two-source evaporation technique, using Cd and Te as two different evaporants. In the next step, films were heated under vacuum at 500 ?C for 1 h and dipped in Cu(NO3)2?H2O solution at room temperature. These films were again heated under vacuum for 1 h at 500 ?C to obtain maximum Cu diffusion. The samples were characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM), optically by a Lambda 900 UV/VIS/NIR spectrophotometer and electrically, i.e. dc electrical resistivity, by the van der Pauw method at room temperature, dark conductivity, and activation energy analysis as a function of temperature by the two-probe method under vacuum. The EDX results showed an increase of Cu content in the samples by increasing the immersion time of the CdTe films in the solution. The layer thickness of diffused Cu atoms in CdTe is determined by comparing the difference of absorption between as-deposited and immersed films with the absorption graph of Cu films of varying thicknesses.

Bipolar tri-state resistive switching characteristics in Ti/CeOx/Pt memory device

Highly repeatable multilevel bipolar resistive switching in Ti/CeOx/Pt nonvolatile memory device has been demonstrated. X-ray diffraction studies of CeO2 films reveal the formation of weak polycrystalline structure. The observed good memory performance, including stable cycling endurance and long data retention times (> 104 s) with an acceptable resistance ratio (~102), enables the device for its applications in future non-volatile resistive random access memories (RRAMs). Based on the unique distribution characteristics of oxygen vacancies in CeOx films, the possible mechanism of multilevel resistive switching in CeOx RRAM devices has been discussed. The conduction mechanism in low resistance state is found to be Ohmic due to conductive filamentary paths, while that in the high resistance state was identified as Ohmic for low applied voltages and a space-charge-limited conduction dominated by Schottky emission at high applied voltages.

Study of cadmium sulfide thin films as a window layers

Cadmium sulfide (CdS) thin films of different thicknesses were deposited on corning glass by closed space sublimation (CSS) technique in a vacuum. Structural, electrical, optical, Fourier transform infrared (FTIR) with Raman spectrometric properties were investigated. X-rays diffraction used to identify the preferred orientation of CdS thin films as well as crystalline size, dislocation density and strain vs thickness was estimated. The electrical properties like resistivity, mobility vs thickness was calculated at room temperature. Optical properties were investigated from UV-VIS-NIR ranges which were used to calculate band gap, thickness of the films, also samples were placed at different angles in spectrophotometer to investigate the effect of angles variation on %T and band gap. Raman spectroscopy for identification of vibration modes in thin films by varying thickness.

Optical and gas sensing properties of SnO2 nanowires grown by vapor–liquid–solid mechanism

SnO2 nanowires were synthesized via vapor transport method by modulating the thickness of the Gold (Au) catalyst. The effect of morphology and photoluminescence properties of nanowires on gas sensing was investigated. The structural and morphological studies reveal that the synthesized nanowires are crystalline in nature with high density. The nanowires were evenly spread on the surface of the substrate. These nanowires were tested for gas sensing properties based on change in resistance under exposure to air and gases (CO, CH4, Methanol). The results showed an improved response as compared to the previous studied. These sensors have potential applications in advanced sensing devices.

Synthesis of SnO2 nanowires forCO, CH4 and CH3OH gases sensing

Synthesis of one-dimensional nanostructures, such as nanowires, is of vigorous significance for achieving the desired properties and fabricating functional devices. In this work, we report the synthesis of tin oxide (SnO2) nanowires on gold-catalyzed silicon substrate by carbothermal reduction process. SnO2 nanowires were synthesized with SnO2 and graphite powders as the source materials at atmospheric pressure and temperature of 900°C in the ambience of nitrogen (N2) gas. First, the effect of source material ratio SnO2:C on growth of SnO2 nanowires was studied. The structural, morphological and compositional properties of the samples were investigated by X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy. The scanning electron microscopy investigation reveals that uniform dense nanowires of SnO2 (diameter ~127 nm and length ~40 µm) were synthesized with vapour–liquid–solid mechanism. Ultraviolet–visible spectra estimate that the optical band gap of the synthesized SnO2 nanowires was 3.72 eV. Second, the gas sensing performance of synthesized SnO2 nanowires was investigated by testing with carbon monoxide (CO), Methane (CH4) and methanol (CH3OH) gases at different operating temperatures and concentrations. Results indicate that the synthesized SnO2 nanowires are highly promising for gas sensing applications.

Effects of CdCl2 Treatment on Physical Properties of CdTe/CdS Thin Film Solar Cell

We report CdTe, CdS, and ITO thin films on glass substrates for solar cell fabrication by closed space sublimation and chemical bath deposition. CdTe and CdS thin films were sublimated to chemical treatment at 25°C in a saturated CdCl2 solution (1.04 g/100 ml methanol) and heat treated at 400°C for 30 minutes. Indium tin oxide and tellurium films were analyzed by spectrophotometer and scanning electron microscopy. It has been observed that solar cell performance can be improved by depositing a CdCl2 layer on the CdTe/CdS layers. The optical, structural, and morphological changes of CdTe and CdS surfaces on CdTe/CdS/ITO/glass solar cells due to CdCl2 solution treatment followed by annealing for 400°C were studied. Optical analysis showed about 15% decrease in transmittance after CdCl2 heat treatment in case of CdTe thin film, whereas CdS thin film demonstrated an increase of about 10–15% transmittance after CdCl2 heat treatment. Similarly, a decrease in band gap values was found for both CdTe and CdS thin films after CdCl2 heat treatment. XRD and SEM results of CdCl2 heat‐treated CdTe and CdS samples showed recrystallization, reorientation, and progressive increase in grain size. The grain sizes of CdTe and CdS samples demonstrated an increase of about 0.2 μm.

Comparative Study of CdS Nano Films Deposited by Chemical Bath and Close Spaced Sublimation Techniques

Thin films of Cadmium Sulfide (CdS) are deposited on microscope glass slides by two techniques, chemical bath deposition (CBD) and close spaced sublimation (CSS). Cadmium Sulfide thin films fabricated by both procedures are characterized by spectrophotometer, scanning electron microscopy (SEM) and X-ray diffraction (XRD). Optical analysis of CdS thin films shows changes in transmittance, refractive Index and energy band gap by using two different techniques. XRD and SEM results show CdS thin film samples with better crystalline structure for close spaced sublimation technique and grain size is also found different for these two methods. CdS thin films fabricated by any of these two methods are good window layers with small thickness and suitable for CdTe/CdS/ITO based solar cells.