Change In Band Gap Energy Research Articles

The Optical and Chemical Response of Thermal Neutron-Irradiated CR-39 Polymeric Track Detector after Annealing

In the present work, the aliphatic polycarbonate CR-39 with average thickness 260µm was irradiated with different neutron fluences from the Cirus reactor, Bhabha Atomic Research Center, Mumbai. For all samples, the isochronal annealing was done in the temperature zone 130°–150°C, each for 45 min. The effect of annealing on irradiated samples was studied using UV-Visible and FTIR spectroscopic techniques. With UV-Vis analysis, the change in optical band gap and Urbach's energy with annealing for a particular dose have been revealed. The FTIR analysis of irradiated annealed samples indicates the partial repair of bonds on annealing at high temperatures.

Carrier and Polarization Dynamics in MonolayerMoS2

In monolayer MoS2, optical transitions across the direct band gap are governed by chiral selection rules, allowing optical valley initialization. In time-resolved photoluminescence (PL) experiments, we find that both the polarization and emission dynamics do not change from 4 to 300K within our time resolution. We measure a high polarization and show that under pulsed excitation the emission polarization significantly decreases with increasing laser power. We find a fast exciton emission decay time on the order of 4ps. The absence of a clear PL polarization decay within our time resolution suggests that the initially injected polarization dominates the steady-state PL polarization. The observed decrease of the initial polarization with increasing pump photon energy hints at a possible ultrafast intervalley relaxation beyond the experimental ps time resolution. By compensating the temperature-induced change in band gap energy with the excitation laser energy, an emission polarization of 40% is recovered at 300K, close to the maximum emission polarization for this sample at 4K.

The effect of solvent in the sol–gel synthesis of visible light-activated, sulfur-doped TiO2 nanostructured porous films for water treatment

The effects of solvent on the synthesis of visible light-activated, sulfur-doped TiO2 (S–TiO2) films were studied. Four different polar, protic solvents, isopropanol, 1-butanol, ethanol, and methanol (iPrOH, BtOH, EtOH, and MeOH), were chosen as the solvent in four titania sol–gel preparations. The films were characterized by X-ray diffraction (XRD), high resolution transmission electron microscopy (HR-TEM), atomic force microscopy (AFM), environmental scanning electron microscopy (ESEM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR), Ultraviolet (UV)–vis diffuse reflectance, and porosimetry. The structural, morphological, and porous characteristics of the sulfur-doped TiO2 films were correlated with solvent physical properties such as the dielectric constant (D-value) and the saturated vapor pressure. According to XPS and FT-IR, S6+/S4+ cations replaced Ti4+ ions in the lattice of TiO2, resulting in the formation of localized states within the bandgap of TiO2. The optical absorption edge for all S–TiO2 films was significantly shifted toward the visible light region. The solvent D-value has a negligible effect on the bandgap energy change and the doping states of the prepared S–TiO2 samples. S–TiO2 films synthesized using MeOH (S–TiO2–MeOH), despite their lower Brunauer, Emmett, and Teller (BET) surface area and porosity compared to the other films, showed the highest photocatalytic activity for the degradation of the hepatotoxin microcystin-LR (MC-LR) under visible light irradiation due to their high surface roughness and large pore size. The tailor-designed structure of the S–TiO2–MeOH film contributed to the high photocatalytic degradation rates of MC-LR. The larger pore size of the S–TiO2–MeOH films allowed easier transport of MC-LR inside the porous film, while the higher film surface roughness could increase nano-interfacial interactions between MC-LR and surface active sites. These results indicate that the structural and morphological properties of S–TiO2 photocatalysts can be tailor-designed using different solvents in the sol–gel synthesis, while inducing negligible effects on the sulfur doping and the visible light activation of TiO2. Therefore, the enhancement of photocatalytic activity of S–TiO2 films can be achieved by judicious choice of the main solvent for the sol–gel method.

Improvement on the Electron Transfer of Dye-Sensitized Solar Cell Using Vanadium Doped TiO2

In dye-sensitized solar cells, nanoporous structure of TiO2 is very important for efficient cell because lots of dye molecules are adsorbable and they are the source of the photocurrent. However, the internal impedance of TiO2 is relatively large and it limits the performance. For better performance, vanadium was doped into TiO2 in this work. Doping different material generally improves the characteristics and functions of original materials. Vanadium doping has some advantages such as the reduction of internal resistance, the improvement of chemical stability and high absorption. Especially, reduced internal resistance is so helpful for better electron transfer in TiO2 network. Various amounts of vanadium were applied and photovoltaic performance, internal impedance and absorbance were measured in order to verify the effect of vanadium doping. As a result, vanadium doping improved the overall performance from 6.01 to 6.81% with decreased internal resistance although adsorbed dye amount was reduced by decreased surface area and open circuit voltage was also decreased by the change of band-gap energy.

Effect of valence band energy on the photocatalytic performance of N-doped TiO2 for the production of O2 via the oxidation of water by visible light

Five types of N-doped TiO2 (TiO2−xNx, x=0, 0.005, 0.015, 0.03, and 0.06) nanoparticles were prepared and evaluated for use as a catalyst for production of O2 via the oxidation of water by irradiation with visible light. The resulting materials were characterized using ultraviolet diffuse reflectance spectroscopy (UV-DRS), electrochemical impedance spectroscopy (EIS), and photoluminescence (PL) techniques. The results indicate that a change in band gap energy (Eg) as a function of N-doping level in the materials resulted in a change in the valence band (VB) level, but not the conduction band (CB) level. The very small upward shift in VB enhanced the photocatalytic formation of OH radicals produced by irradiation with visible light. In addition, the small amount N-dopant (up to 3at%) incorporated into the samples accelerated the evolution of O2, rather than the formation of OH species. The incorporation of larger amounts of N-dopant in the TiO2−xNx reduced the rates of both O2 evolution and OH formation, due to the small energy level of VB and the short life time of the photo-generated charges. The performance of TiO1.97N0.03 nanoparticles with an Eg value of 2.8eV was outstanding for the evolution of O2 from the water under the experimental conditions used.

Study the Optical properties of Poly (vinyl alcohol) Doped Cupper Chloride

The change in the optical band gap and optical activation energy have been investigated for pure and doped Poly (vinyl alcohol) films with different cupper chloride concentrations. The optical properties were measured in the wave length range from (200-800) nm at room temperature. The optical band gap (Eg) for allowed direct transition decrease with increase the concentration of cupper chloride. The optical activation energy for allowed direct transition band gap was evaluated using Urbach- edges method.

Temperature dependent optical properties of Si nanocrystals embedded in SiO2 matrix

The optical properties of Si nanocrystals (nc-Si) with different sizes (2∼5 nm) embedded in a SiO2 matrix synthesized by the SiOx/SiO2 superlattice approach were studied in a temperature range from room temperature to 600 K by spectroscopic ellipsometry. The Maxwell–Garnett effective medium approximation and Lorentz oscillator model were employed to extract the dielectric function of nc-Si. The results show that the temperature dependence of optical properties of nc-Si is sensitive to its size. An empirical expression of Varshni approximation was obtained to characterize the band-gap energy change of nc-Si at different temperatures.

Band gap change induced by defect complexes in Cu2ZnSnS4

Understanding the impact on the electronic and optical properties of kesterite Cu2ZnSn(S,Se)4 due to the presence of native defects is crucial for designing and manufacturing efficient solar cells. In this work, we complement earlier published theoretical studies by investigating the change in the Cu2ZnSnS4 band gap energy induced by the dominating isovalent (ZnCu+CuZn) and (ZnCu+VCu) defect complexes and the corresponding larger [(ZnCu+CuZn)+(ZnCu+VCu)] complex. The study is based on the generalized gradient approximation with corrections for on-site Coulomb interactions (GGA+U) and the Heyd–Scuseria–Ernzerh hybrid functional method (HSE06) within the first-principles density functional theory. We find that (ZnCu+CuZn) and (ZnCu+VCu) affect the band gap energies in an opposite way: (ZnCu+CuZn) decreases the gap whereas (ZnCu+VCu) increases the gap. However, there is a strong correlation between these two defect complexes. The presence of the Cu-poor (ZnCu+VCu) defect complex lowers the formation energy of the antisite (ZnCu+CuZn) defect complex in close configuration, and vice versa. Thereby, and considering the opposite effect on the band gap for (ZnCu+CuZn) and (ZnCu+VCu), our results indicate that the band gap energy will be compensated and stabilized in Cu-poor Cu2ZnSnS4. We discuss the underlying mechanisms behind the band gap physics of the considered isovalent defect complexes.

One-pot synthesis of co-substituted manganese oxide nanosheets and physical properties of lamellar aggregates

Co-substituted manganese oxide nanosheets, (Mn1−xCox)O2 have been synthesized in the form of a colloidal suspension via a simple one-pot method. Substitution effects on the structural, optical absorption, and magnetic properties are investigated for the nanosheets and their lamellar aggregates. The composition of the (Mn1−xCox)O2 nanosheets can be controlled continuously by adjusting the molar ratio of the starting materials. The solubility limit is x∼0.20 based on the cell volume. In the 0.00≤x≤0.20 range, the band gap energy, magnetic moment, and Weiss temperature change systematically with x. The charge density of the (Mn,Co)O2 layer is independent of x (i.e., [(Mn,Co)O2]0.2−) and the cobalt ions are trivalent in low-spin state.

Studies on phase transformation and molecular orientation in nanostructured zinc phthalocyanine thin films annealed at different temperatures

Studies on the electronic and optical properties of thin films of organometallic compounds such as phthalocyanine are very important for the development of devices based on these compounds. The nucleation and grain growth mechanism play an important role for the final electronic as well as optoelectronic properties of the organic and organometallic thin films. The present article deals with the change in the film morphology, grain orientation of nanocrystallites and optical properties of zinc phthalocyanines (ZnPc) thin films as a function of the post deposition annealing temperature. The effect of annealing temperature on the optical and structural property of vacuum evaporated ZnPc thin films deposited at room temperature (30°C) on quartz glass and Si(100) substrates has been investigated. The thin films have been characterized by the UV–vis optical absorption spectra, X-ray diffraction (XRD), atomic force microscopy (AFM), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy. From the studies of UV–vis absorption spectra and XRD data, a metastable α to β-phase transformation has been observed when the thin films were annealed at a temperature greater than about 250°C. The FESEM images have shown the particlelike structure at room temperature and the structure became rodlike when the films were annealed at high temperatures. TEM image of ZnPc film dissolved in ethanol has shown spectacular rod-shaped crystallites. High resolution transmission electron microscopy image of a single nanorod has shown beautiful “honey-comb” like structure. Particle size and root mean square roughness were calculated from AFM images. The changes in band gap energy with increase in annealing temperature have been evaluated.

Processing dependence of structural and physical properties of Mg2Ge thin films prepared by pulsed laser deposition

Magnesium germanide (Mg2Ge) thin films were deposited on MgO (001) substrates using pulsed laser deposition technique. The films were deposited at various substrate temperatures, ranging from 300 to 600°C. The effects of substrate temperature on structural, electrical and optical properties were studied. All the films, except the samples prepared at 300°C, were polycrystalline with major diffraction from (200) plane. The highest electrical conductivity of 141.86Ω−1m−1 measured at room temperature was observed for the sample deposited at the highest temperature, with the corresponding charge carrier mobility and concentration of 2.62cm2V/s and 8.66×1018cm−3, respectively. The carrier concentration dependence of the optical absorption edge energy is accounted for by the Burstein-Moss shift. The variation of strain value may have also contributed to the change in bandgap energy. The reduction in direct bandgap energy was found to vary from 2.20 to 2.00eV with increasing the deposition temperature.

Determination of optical parameters of Ga0.75In0.25Se layered crystals

AbstractThe optical properties of the Ga0.75In0.25Se crystals have been investigated by means of transmission and reflection measurements in the wavelength range of 380–1100 nm. The analysis of the results performed at room temperature revealed the presence of optical indirect transtions with band gap energy of 1.89 eV. The variation of the band gap energy as a function of temperature was also studied in the temperature range of 10–300 K. The rate of change of band gap energy (γ = –6.2 × 10–4 eV/K) and absolute zero value of the band gap (Egi(0) = 2.01 eV) were reported. The wavelength dependence of the refractive index was analyzed using Wemple and DiDomenico, Sellmeier and Cauchy models to find the oscillator energy, dispersion energy, oscillator strength and zero‐frequency refractive index values. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Structural and optical studies on La doped CdS nanocrystalline films

This paper reports the synthesis of pure and lanthanum (La) doped nanocrystalline CdS semiconducting films on the glass substrate by means of chemical bath deposition (CBD), with Cd(CH_3COO)_2 as Cadmium (Cd) ion source, (NH_2)_2CS as sulphur (S) ion source and Thioglycerol as capping agent. Long, rod like structures is observed in SEM micrographs of the films. XRD studies show the existence of both hexagonal and cubic phases. Increase in FWHM is observed for smaller particle due to nanocrystalline effect. UV-VIS absorption studies show shift of absorption edge towards lower wavelength side. Corresponding change in band gap energy results in shift of photoluminescence edge emission spectra. Also emission peaks near 366 nm are observed under 230 nm excitation due to transitions at higher energy levels.

Sulphur doped DLC films deposited by DC magnetron sputtering

Diamond-like carbon (DLC) and sulphur doped diamond-like carbon (S-DLC) films were synthesised at different sulphur molar percentage of 0%, 2%, 5%, 8% and 10% by direct current (DC) magnetron sputtering process using novel compressed sulphur-graphite targets at relatively low power density. Films were characterised for their morphologies, structural, electrical and optical properties. Scanning electron microscope images reveal changes in the quality of the obtained films shown by the denser packing of DLC grains at different sulphur percentage. The conductivities of S-DLC films were found to be in the range of 6.0 × 10−3–0.6/Ω cm. The optical band gap energies were found to be in the range of ∼1.4–2.0 eV. Both electrical and optical measurements exhibit nonlinear responses with optimum at around 5% sulphur molar percentage (minimum for conductivity and maximum for optical band gap energy). These trends of change in both conductivity and optical band gap energy are consistent with the variation in bond characters of the films indicated by Raman spectroscopy. © 2011 Canadian Society for Chemical Engineering

Size Regulation of Si Nanoparticles through Photon-enhanced Chemical Etching

Abstract A Si nanoparticle surface was etched in an aqueous HF solution under monochromatic light irradiation in the infrared-to-visible region. The average size of the nanoparticles was controlled by changing the photon energy of the irradiating light. After prolonged photoetching, the size distribution of the nanoparticles was narrowed significantly. The correlation between the photon energy and the final size of the nanoparticles indicated that the band gap energy change due to the quantum size effect played an important role in controlling nanoparticle size.

Studying of the optical properties of poly (vinyl alcohol) films using Aluminum sulphate as additive by measuring allowed direct transition energy gap

The change in the optical band gap and optical activation energy have been investigated for pure Poly (vinyl alcohol)and Poly (vinyl alcohol) doped with Aluminum sulphate to proper films from their optical absorption spectra. The absorption spectra were measured in the wave range from (200-700) nm at temperature range (25-140) 0C. The optical band gap (Eg) for allowed direct transition decrease with increase the concentration of Aluminum sulphate. The optical activation energy for allowed direct transition band gap was evaluated using Urbach- edges method. It was found that ?E increases with increasing the concentration of Al2 (SO4)3 and decreases when temperature increases.

Studying of the optical properties of poly (vinyl alcohol) films using Aluminum sulphate as additive by measuring allowed direct transition energy gap

The change in the optical band gap and optical activation energy have been investigated for pure Poly (vinyl alcohol)and Poly (vinyl alcohol) doped with Aluminum sulphate to proper films from their optical absorption spectra. The absorption spectra were measured in the wave range from (200-700) nm at temperature range (25-140) 0C. The optical band gap (Eg) for allowed direct transition decrease with increase the concentration of Aluminum sulphate. The optical activation energy for allowed direct transition band gap was evaluated using Urbach- edges method. It was found that ?E increases with increasing the concentration of Al2 (SO4)3 and decreases when temperature increases.

Photoluminescence properties of ZnSe1−x Te x thin films on GaAs/ITO substrates by electron beam evaporation technique

Zinc chalcogenide which includes zinc selenide, zinc sulphide, zinc telluride and mixed crystals of these shows a great potential as an optoelectronic device material. Zinc selenotelluride is a suitable material for visible light emitting devices which are expected to cover the spectral range from yellow to blue. In our present study the composition controlled ZnSe1−xTex films with different Te content x = 0, 0.2, 0.4, 0.6, 0.8 and 1.0 were deposited by electron beam (EB) evaporation technique. GaAs films were deposited by vacuum evaporation route on indium tin oxide (ITO) substrates which were used as base for depositing the ZnSe1−xTex film. The band-gap energy change in the entire composition range was determined at room temperature by photoluminescence (PL) spectroscopy. The peak observed at about 2.56 eV shows the effect of solid solution formation between ZnSe and ZnTe which modifies the lattice and consequently the band edge emission characteristics. The heterostructures showed three peaks in the visible region of white light spectrum.

Yb3+Er3+ions 동시도핑에 의한 TiO2분말의 열적 안정성 증가효과와 upconversion 특성 연구

Sol-gel method로 합성한 anatase상의 <TEX>$TiO_2$</TEX>와 Yb와 Er을 codoping한 <TEX>$TiO_2:Yb^{3+},\;Er^{3+}$</TEX> 파우더의 열적안정성과 이에 따른 광촉매 특성의 변화를 비교하였다. XRD 분석을 통하여 <TEX>$TiO_2$</TEX> 결정성 및 상변화에 <TEX>$Yb^{3+},\;Er^{3+}$</TEX> ions이 미치는 영향을 온도에 따라 비교하였으며 anatase <TEX>$TiO_2$</TEX>와 rutile <TEX>$TiO_2$</TEX>의 광학적 성질을 UV-Vis을 통하여 비교하였다. UV-Vis 분석결과 anatase <TEX>$TiO_2:Yb^{3+},\;Er^{3+}$</TEX> ions도핑에 의하여 <TEX>$TiO_2$</TEX>의 밴드갭이 미세하게 감소함을 확인하였고 이를 통해 광촉매 작용을 향상시킬 수 있음을 확인하였다. <TEX>$Yb^{3+}$</TEX> 이온과 <TEX>$Er^{3+}$</TEX> 이온이 도핑된 <TEX>$TiO_2:Yb^{3+},\;Er^{3+}$</TEX> 파우더를 980 nm에서 PL 분석하여 녹색 및 적색 형광을 하는 up-converting 형광특성을 분석하였다. Thermal stabilizing effect of <TEX>$Yb^{3+},\;Er^{3+}$</TEX> codoping into <TEX>$TiO_2$</TEX> powder prepared by sol-gel method and its upconversion characteristics were analyzed. The effect of <TEX>$TiO_2:Yb^{3+},\;Er^{3+}$</TEX> ions on crystallinity and phase transition was studied by X-ray diffraction (XRD). The change of band-gap energy induced from Yb and Er codoping was analyzed by UV-Vis. The band-gap energy of <TEX>$TiO_2$</TEX> have been slightly narrowed by <TEX>$Yb^{3+},\;Er^{3+}$</TEX> codoping, which indicated that the <TEX>$Yb^{3+},\;Er^{3+}$</TEX> ions can enhance the photo-catalytic property of <TEX>$TiO_2$</TEX>. green and red up-conversions of <TEX>$Yb^{3+}$</TEX> and <TEX>$Er^{3+}$</TEX> co-doped <TEX>$Y_2O_3:Yb^{3+},\;Er^{3+}$</TEX> phosphor were analyzed by PL equipped with 980 nm laser.

Medium-term thermal stability of amorphous Ge 2Sb 2Te 5 flash-evaporated thin films with regards to change in structure and optical properties

The stability of flash-evaporated amorphous Ge 2Sb 2Te 5 thin films has been studied under medium-term temperature treatment (30 - 80 °C, with a step of 10 °C) in ten subsequent heating and cooling cycles. The significant changes in structure and optical properties are reported. The temperature cycling of the films resulted in formation of an isolated 5 - 7 nm nano-crystalline phase in the amorphous phase. The corresponding increase in refractive index and change in optical bandgap energy and sheet resistance are also presented. The formation of Ge 2Sb 2Te 5 nano-crystals (~ 5 - 7 nm) even under temperature below 80 °C could contribute to the explanation of mechanism of resistivity fluctuation (drift) of the “amorphous phase” films. We also show that the optical and electrical properties of flash evaporated Ge 2Sb 2Te 5 thin films are very similar to those reported for sputtered films.