Temperature Dependence of Urbach Energy in Non-Crystalline Semiconductors

Until now, no analytical relationships have been derived for the temperature dependence of the Urbach energy in non-crystalline semiconductors. Consequently, the problem associated with the theoretical study of the temperature dependence of this energy has not been solved. This paper presents the results of theoretical calculations and attempts to establish the temperature dependence of the Urbach energy in non-crystalline semiconductors. A linear increase in the Urbach energy with increasing temperature is shown.

Effect of post-thermal annealing on the structural and optical properties of ZnO thin films prepared from a polymer precursor

ZnO thin films were synthesised by a new method which uses polyvinyl alcohol （PVA） as the polymer precursor. The films are annealed at different temperatures and for different annealing times. The structural parameters, like grain size, lattice constants, optical band gap, and Urbach energy, depend on the annealing temperature and time. All the films possess tensile strain, which relaxes as the annealing temperature and time increase. The photoluminescence （PL） spectra contain only ultraviolet （UV） peaks at low temperature, but as the annealing temperature and time increase, we observe peaks at the blue and green regions with a variation in the intensities of these peaks with annealing temperature and time.

The Signatures of Acid Concentration on the Optical Band Gap and Associated Band Tails of Chitosan from Shrimp for Application in Optoelectronic Devices

Over the recent years, the global increase of electronic wastes from electrical and electronic devices (e-wastes) has been on an alarming trend in quantity and toxicity and e-wastes are non-biodegradable resulting in its cumulative increase over time. Changes in technology and unrestricted regional movement of electrical devices have facilitated the generation of more e-wastes leading to high levels of air, soil and water pollution. To address these challenges, biodegradable organic components such as chitosan have been used to replace their inorganic counterparts for optoelectronic device applications. However, in-depth knowledge on how such materials can be used to tune the optical properties of their hybrid semiconductors is unrivaled. Thus, systematic studies of the interplay between the preparation methods and optical band gap and Urbach energy of such organic components are vital. This study has thus been dedicated to map out the effect of acid concentrations during chitosan extraction on the corresponding optical band gap and Urbach energy with a view to improving its applications in optoelectronic devices. The, 1.0 to 2.5 molar hydrochloric acid (HCl) was used for 12 hours at room temperature during demineralization and 2.0 molar sodium hydroxide (NaOH) during deprotonation processes. The absorbance spectrum of the samples was collected by UV-Vis spectrophotometer and band gap energies were analyzed by performing Tauc’s plot. This study revealed that the energy band gap of chitosan extracted from 1 M HCl, 1.5 M HCl, 2.0 M HCl and 2.5 M HCl were 3.72 eV, 3.50 eV, 3.45 eV and 3.36 eV respectively. Furthermore, the Urbach energy of chitosan extracted from 1 M HCl, 1.5 M HCl, 2.0 M HCl and 2.5 M HCl were 0.60496 eV, 0.5292 eV, 4724 eV and 0.2257 eV, respectively.

Synthesis and Characterization of Copper Doped Zinc Oxide Thin Films Deposited by RF/DC Sputtering Technique

Undoped and copper(Cu)doped zinc oxide(Zn_(1-x)Cu_(x)O,where x=0-0.065)nano crystal thin films have been deposited on glass substrate via RF/DC reactive co-sputtering technique.The aim of this work is to investigate the crystal structure of ZnO and Cu doped ZnO thin films and also study the effect of Cu doping on optical band gap of ZnO thin films.The identification and confirmation of the crystallinity,film thickness and surface morphology of the nano range thin films are confirmed by using X-ray diffractometer(XRD),scanning electron microscope and atomic force microscope.The XRD peak at a diffractive angle of 34.44°and Miller indices at(002)confirms the ZnO thin films.Crystallite size of undoped ZnO thin films is 27 nm and decreases from 27 nm to 22 nm with increasing the atomic fraction of Cu(x_(Cu))in the ZnO thin films from 0 to 6.5%respectively,which is calculated from XRD(002)peaks.The different bonding information of all deposited films was investigated by Fourier transform infrared spectrometer in the range of wave number between 400 cm^(-1) to 4000 cm^(-1).Optical band gap energy of all deposited thin films was analyzed by ultraviolet visible spectrophotometer,which varies from 3.35 eV to 3.19 eV with the increase of x_(Cu) from 0 to 6.5%respectively.Urbach energy of the deposited thin films increases from 115 meV to 228 meV with the increase of x_(Cu) from 0 to 6.5% respectively.

CeO2 nanoscale particles:Synthesis,characterization and photocatalytic activity under UVA light irradiation

CeO2nanoparticles(NPs) were synthesized in alkaline medium via the homogeneous precipitation method and were subsequently calcined at 80 ℃/24 h(assigned as CeO2-80) and 500 ℃/2 h(assigned as CeO2-500). The as-prepared materials and the commercial ceria(assigned as CeO2-com) were characterized using TGA-MS, XRD, SEM-EDX, UV-vis DRS and IEP techniques. The photocatalytic performances of all obtained photocatalysts were assessed by the degradation of Congo red azo-dye(CR) under UVAlight irradiation at various environmental key factors(e.g., reaction time and calcination temperature).Results reveal that CeO2compounds crystalize with cubic phase, CeO2-500 exhibits smaller crystallite size(9 nm vs 117 nm) than that of bare CeO2-com. SEM analysis shows that the materials are sphericallike in shape NPs with strong assembly of CeO2NPs observed in the CeO2-500 NPs. EDX analysis confirms the stoichiometry of CeO2NPs. UV-vis DRS measurement reveals that, CeO2-500 NPs exhibits a red-shift of absorption band and a more narrow bandgap(2.6 eV vs 3.20 eV) than that of bare CeO2-com. On the contrary, Urbach energy of Eu is found to be increased from 0.12 eV(CeO2-com) to 0.17 eV(CeO2-500),highlighting an increase of crystalline size and internal microstrain in the CeO2-500 NPs sample. Zeta potential(IEP) of CeO2-500 NPs is found to be 7.2. UVA-light-responsive photocatalytic activity is observed with CeO2-500 NPs at a rate constant of 10×10-3min-1, which is four times higher than that of CeO2-com(Kapp=2.4×10-3min-1) for the degradation of CR. Pseudo-first-order kinetic model gives the best fit. On the basis of the energy band diagram positions, the enhanced photocatalytic performance of CeO2-500 nano-catalyst can be ascribed to O2-, ’OH and R’+as the primary oxidative species involved in the degradation of RC under UVA-light irradiation.

Incorporation of Ag into Cu(In,Ga)Se_(2) films in low-temperature process

Chalcopyrite Cu(In,Ga)Se_(2)(CIGS) thin films deposited in a low-temperature process(450℃) usually produce fine grains and poor crystallinity. Herein, different Ag treatment processes, which can decrease the melting temperature and enlarge band gap of the CIGS films, were employed to enhance the quality of thin films in a low-temperature deposition process. It is demonstrated that both the Ag precursor and Ag surface treatment process can heighten the crystallinity of CIGS films and the device efficiency. The former is more obvious than the latter. Furthermore, the Urbach energy is also reduced with Ag doping. This work aims to provide a feasible Ag-doping process for the high-quality CIGS films in a low-temperature process.

Comprehensive study of electronic polarizability and band gap of B_(2)O_(3)-Bi_(2)O_(3)-ZnO-SiO_(2)glass network

Quaternary glasses were successfully fabricated using melt quenching technique based on the chemical compound composition(x)Bi_(2)O_(3)–(0:5-x)ZnO-(0.2)B_(2)O_(3)–(0.3)SiO_(2),where(x=0.1,0.2,0.3,0.4,0.45)mole.The sources of SiO_(2)was produced from rice husk ash(RHA)at 99.36%of SiO_(2).The Urbach energy was increased from 0.16 eV to the 0.29 eV as the mole of Bi_(2)O_(3)increased in the glass structure.The indirect energy band gap is indicated in decrement pattern with 3.15 eV towards 2.51 eV.The results of Urbach energy and band gap energy that were obtained are due to the increment of Bi^(3+)ion in the glass network.The refractive indexes for the prepared glasses were evaluated at 2.36 to 2.54 based on the Lorentz–Lorentz formulation which correlated to the energy band gap.The calculated of molar polarizability,electronic polarizability and optical basicity exemplify fine complement to the Bi_(2)O_(3)addition in the glass network.The glass sample was indicated in amorphous state.