Enhancement In Optical Band Gap Research Articles

Ferromagnetic ordering in half metallic Fe doped CdS diluted magnetic semiconductor: A DFT study

A detailed experimental and theoretical investigations on the origin of room temperature ferromagnetism in Fe doped CdS diluted magnetic semiconductor is reported in the present study. X-ray diffraction results reveal the cubic zinc blende CdS structure with an average crystallite size of 2 nm–3 nm which is in concurrence with the Transmission electron microscopy results. UV–visible spectroscopy depict the enhancement of optical band gap of Fe doped CdS nanoparticles compared to the bulk CdS as a result of the quantum confinement effect. Photoluminescence spectra revealed the presence of Cd2+ and S2− defect states in the nanoparticles in addition to the fluorescence quenching effect. FTIR results demonstrate the stretching vibrations of CdS bond. Magnetic study confirms the existence of room temperature ferromagnetic ordering in the nanoparticles for the lower concentration of Fe, as a result of spin polarization induced by the Fe atoms to the nearest neighboring S atoms. At higher concentration of Fe, the enhanced antiferromagnetic interaction between short range Fe ions suppresses the ferromagnetic ordering. Theoretical calculations demonstrate the half metallic nature of Fe doped CdS system with strong p-d hybridization between the Fe-d states and the S-p states which results the room temperature ferromagnetism.

All p-i-n hydrogenated amorphous silicon oxide thin film solar cells for semi-transparent solar cells

We focused on fabricating all p-i-n layer hydrogenated amorphous silicon oxide (a-SiOx:H) thin film solar cells in order to improve their transmittance in visible ranges of 500–800 nm for application in building integrated photovoltaics system. We varied CO2/SiH4 (R) gas flow ratio from 0 to 0.6 for i-layer to investigate an effect of oxygen addition. When the R ratio increased, the transmittance of devices improved due to the enhancement in optical bandgap of the absorber. However, power conversion efficiency (PCE) decreased owing to the increase in defects and recombination rate which might be ascribed to back bonding of oxygen atoms with Si atoms. Unlike other R flow ratio, however, the PCE of a-SiOx:H solar cell at the R ratio of 0.2 was slightly improved by the increase in open circuit voltage as indicated by the wide band gap and the increase in quantum efficiency within short-wavelengths (300–500 nm). We introduced a figure of merit (FOM), multiplication of PCE and average transmittance in range of 500–800 nm, in order to assess the performance of transparent solar cells. The a-SiOx:H solar cell at the R ratio of 0.2 achieved the highest FOM, which was better than conventional amorphous silicon solar cell.

Investigation of structural, morphological and optical properties of Zn doped CdS nanostructures synthesized via co-precipitation method

The present work reported the Zn doped CdS nanostructures synthesized via using co-precipitation method from cadmium and sulphide particle arrangements with no ligands at different temperature arrangement and amalgamation time. The as synthesized CdS nanostructures were characterized by XRD, SEM, PL, Raman, FTIR and UV–vis spectroscopy to investigate the structural, morphological, and optical properties. XRD analysis revealed that all the samples were acknowledged the cubic crystal structure and crystallite size is found in 3–9 nm range. SEM analysis exhibits that synthesized material is in nano size range. PL, Raman and FTIR analysis also revealed that the inorganic component in all the synthesized samples is a CdS phase with a cubic crystal structure. An enhancement in optical band gap is observed with increasing Zn doping concentration in CdS nanostructures. The investigation revealed that Zn doping plays a significant role to tuning structural and optical properties of CdS nanostructures, which influence the particle size, band gap and morphology. Therefore, the synthesized CdS nanostructures at different doping concentration of Zn can be used as a window layer for solar cell applications and optoelectronic devices.

Effect of calcination temperature on phase transformation, structural and optical properties of sol–gel derived ZrO2 nanostructures

Zirconia (ZrO2) nanostructures of various sizes have been synthesized using sol–gel method followed by calcination of the samples from 500 to 700°C. The calcined ZrO2 powder samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier-transform infra-red spectroscopy (FT-IR), UV–visible spectroscopy (UV–vis.), Raman spectroscopy (RS) and thermogravimetric analysis (TGA). The phase transformation from tetragonal (t) to monoclinic (m) was observed. The average diameter of the ZrO2 nanostructures calcined at 500, 600 and 700°C was calculated to be 8, 17 and 10nm, respectively. The ZrO2 sample calcined at 500°C with tetragonal phase shows a direct optical band gap of 5.1eV. The value of optical band gap is decreased to 4.3eV for the ZrO2 calcined at 600°C, which contains both tetragonal (73%) and monoclinic (27%) phases. On further calcination at 700°C, where the ZrO2 nanostructures have 36% tetragonal and 64% monoclinic phases, the optical band gap is calculated to be 4.8eV. The enhancement in optical band gap for ZrO2 calcined at 700°C may be due to the rod like shape of ZrO2 nanostructures. The tetragonal to monoclinic phase transformation was also confirmed by analyzing Raman spectroscopic data. The TG analysis revealed that the ZrO2 nanostructure with dominance of monoclinic phase is found to be more stable over the tetragonal phase. In order to confirm the phase stability of the two phases of ZrO2, single point energy is calculated corresponding to its monoclinic and tetragonal structures using density functional theory (DFT) calculations. The results obtained by theoretical calculations are in good agreement with the experimental findings.

Structural, morphological and optical study of Li doped ZnO thin films on Si (100) substrate deposited by pulsed laser deposition

Structural, morphological and optical properties of Zn1−xLixO thin films grown on Si (100) substrates by pulsed laser deposition at different Li doping concentrations (x=0.0, 0.05, 0.07 and 0.09) are reported. The XRD spectra indicate the formation of single crystalline ZnO thin films preferentially grown along (002) crystallographic orientation with c-axis perpendicular to the substrate surface. FESEM analysis shows the average size of Zn1−xLixO nanostructures to decrease from 42 to 30nm and AFM analysis also shows the average size of Zn1−xLixO nanostructures to decrease from 30 to 05nm with increasing lithium concentrations from x=0 to 0.09. Moreover, Raman and Photoluminescence spectra also confirm the single-phase formation of ZnO thin films. A significant enhancement in optical band gap is found from 3.35 to 3.63eV by Li doping from x=0 to 0.09. It may be due to the increased carrier concentrations by Li doping that blocks the lowest states in the conduction band and results in the increase in band gap. The obtained Zn1−xLixO thin films with c-axis oriented along (002) plane are useful for device fabrication in broadband UV photo-detectors for high tunable wavelength resolution.

Structural, Optical, and Ferroelectric Behaviors of Cu1−x Li x O (0 ≤ x ≤ 0.09) Nanostructures

We report structural, optical, and ferroelectric behaviors of lithium-doped copper oxide (Cu1−x Li x O with x = 0.0, 0.05, 0.07, and 0.09) nanostructures synthesized by hydrothermal method. The XRD pattern indicates the pure phase formation of CuO without any impurity, and the crystallite size is found to be increases for x = 0–0.07 and decreases for x = 0.09. FESEM analysis shows that the average size of Cu1−x Li x O nanostructures increases with the increasing the Li-doping concentrations up to 7% and then decreases for 9% Li doping concentration. Moreover, Raman and photoluminescence spectrum also confirm the phase formation of CuO. A significant reduction in optical band gap is observed up to x = 0.07, and then band gap increases for x = 0.09 due to segregation of the impurities on the surface or grain boundaries, which may suppress the grain growth and results the enhancement in optical band gap. Moreover, a weak ferroelectricity is observed in CuO nanostructures for pure and 9% Li doping through polarization versus electric field (P–E).

Structural, optical, vibrational, and magnetic properties of sol-gel derived Ni doped ZnO nanoparticles

With a view to study structural, optical, vibrational, and magnetic properties of solgel derived Zn1−xNixO (x = 0.02, 0.04, and 0.06) nanoparticles, systematic investigations have been carried out. The Rietveld refinement of X-ray powder diffraction data revealed a single hexagonal phase with space group P63mc. The secondary phase of NiO appeared only in 6% Ni doped sample. Phonon modes in Ni doped ZnO nanoparticles were studied through Fourier transform infrared measurements. Furthermore, the enhancement in optical band gap with Ni doping from 3.29 to 3.32 eV has been observed through UV-visible spectroscopic analysis. Photoluminescence spectra of Zn1−xNixO show the UV-emission peak showing the blue shift with increase in doping concentration followed by broad visible (blue) emission corresponding to the defect emission whose intensity decreased with increasing Ni concentration. A clear room temperature ferromagnetism is observed in all samples but saturation magnetization decreased with increasing Ni content. The suitability of bound magnetic polarons (BMP) model is checked and numbers of BMPs are found to be of the order 1015 per cm3, which is very small for the percolation in ZnO. In the present case, oxygen rich stoichiometry with enhanced Zn-O bonding favours the indirect Ni-O-Ni ferromagnetic exchange coupling and reduction of oxygen vacancies leading to strong hybridization of Ni in ZnO host matrix responsible for room temperature ferromagnetism.

Amorphization and disorder of PrFeO3 thin films after heavy ion irradiation

We present structural, morphological, optical absorption and Raman study of 200MeV Ag15+ ion irradiated PrFeO3 thin film of around 200nm thickness deposited on LaAlO3 [001] substrate by using the pulsed laser deposition technique. X-ray diffraction studies shows that it has an orthorhombic structure with Pbnm space group and is grown with (001)-orientation. The substrate-induced strain is found to be small and is found to increase for ion fluence. UV–visible spectra shows enhancement of optical band gap with ion fluence. At room temperature Raman spectra, the different Raman modes observed are classified according to the orthorhombic structure. All the observed modes show variation in Raman shift with rise in ion fluences. The line width and position variations with fluence for observed modes are correlated with octahedral distortion of FeO6, Fe–O bond length and as well as O–Fe–O bond angle. Surface morphology and grain size also get affected with ion fluences. Many possibilities have been explored to explain the observed results.

Investigation of phase segregation in Zn1−xMgxO systems

The present work reports on the synthesis of the Zn1−xMgxO (x = 0, 0.02, 0.05, 0.10, 0.15 and 0.20) samples by sol–gel method and the investigations on their structural, morphological and optical properties. X-ray diffraction (XRD) data analysis confirms the formation of pure ZnO phase below 10% Mg doping and MgO related phases appears in 10% doped sample indicating that phase segregation of MgO starts at x ≥ 0.10 samples. The phase segregation observed through XRD analysis is also supported by results from Scanning Electron Microscopy (SEM), Raman spectroscopy and photoluminescence studies. Furthermore, the enhancement in optical band gap, with Mg doping, from 3.1 ± 0.1 eV to 3.5 ± 0.1 eV has been observed through UV–Vis spectroscopic analysis. Above results have been discussed on the basis of defects level observed through Raman and photoluminscence studies.

Dielectric and transport properties of carbon nanotube-CdS nanostructures embedded in polyvinyl alcohol matrix

Multiwalled carbon nanotube-CdS/polyvinyl alcohol (MWCNT-CdS/PVA) composites have been grown by a simple chemical process on one-dimensional templates. The plane-view transmission electron micrographs clearly indicate the formation of nanocrystalline CdS on the nanotube surfaces. The superior dielectric behavior of the MWCNT-CdS nanostructures over MWCNT and PVA host matrices has been demonstrated. The dc and ac transport properties of CdS carbon nanotube-insulating polymer nanocomposites have been studied using impedance spectroscopy. An enhancement in optical band gap of nanocomposites over the bulk CdS has been observed due to the quantum confinement effect in CdS nanocrystals.

The improvement in gas barrier performance and optical transparency of DLC-coated polymer by silicon incorporation

In this study, induced modification caused by silicon incorporation (up to 21 at.%) in the composition and microstructure of hydrogenated amorphous carbon (a-C:H) thin films synthesized by plasma-enhanced chemical vapour deposition (PECVD) technique has been studied. The effect of this new microstructure on gas barrier performance and optical properties has been studied. A small amount of silicon incorporation (8.5 at.%) into the a-C:H coatings significantly improves the barrier performance, giving a gas reduction factor of 98%. Moreover, silicon incorporation results in an enhancement of optical band gap (doubled its value) and in a reduction of film density, stress, and refractive index. The silicon inclusion in the a-C:H structure breaks down large size sp 2 carbon clusters and enhances the sp 3 bonding; however, the use of tetramethylsilane (TMS) precursor gas in the PECVD process also leads to the development of a more polymer-like structure due to the increasing hydrogen content. The development of sp 3 bonding and the reduction of large sp 2 graphitic clusters are responsible for the enhancement of the optical band gap. We have demonstrated that Si- a-C:H thin film coatings can be used as a transparent barrier coating on polymer substrates with barrier performance comparable to that of conventional metal oxides coatings.

Fluorine-induced suppression of disorder effects of carbon in the hydrogenated amorphous silicon-carbon alloy thin films

We report here the Raman scattering studies of modifications in the structural disorder due to fluorine incorporation in the hydrogenated amorphous silicon-carbon (a-Si1−xCx:H) alloys. The structural disorder in the unfluorinated a-Si1−xCx:H alloys has been observed to increase continuously with the increasing carbon content, whereas the incorporation of fluorine appears to substantially suppress disordering effects of carbon, irrespective of its concentration in the alloy. Additional support for this suppression of disorder is sought from the enhancement of optical band gap after fluorine incorporation in these alloys. The fluorine-induced suppression of disorder is attributed to the increase in the fraction of sp3 bonded carbon in the fluorinated alloys compared to that of the unfluorinated alloys. The fluorine as well as hydrogen appear to have a preferential attachment with the carbon atoms that tend to form higher hydride and fluoride species which in turn constrain the carbon atoms to form sp3 hybridized C-C or Si-C bonding in the alloys.