Increase In Optical Band Gap Research Articles

Moss-Burstein effect in stable, cubic ZrO2: Eu+3 nanophosphors derived from rapid microwave-assisted solution-combustion technique

A single step, rapid (<1min), microwave driven, solution combustion technique is used to obtain luminescent, cubic ZrO2: Eu+3nanophosphors. The high temperature cubic ZrO2 phase is obtained directly using L-serine amino acid as the fuel. The phase is stable, and shows no transformation on calcination (upto 800°C, 2h). The method presented here results in a final product with low concentrations of unintentional impurities (carbon<0.65 at%, N<0.09 at%, and no S); desirable for a luminescence host. Kisielowski’s model based analysis suggests that Eu3+ is present in the lattice in concentrations as high as 2.5 mol%. The effect of calcination temperature (400, 600 and 800°C) on optical and luminescent properties is investigated. Interestingly calcination of this material results in an increase in optical band gap (ΔEg=0.12eV), strongly suggesting defect-induced Moss-Burstein (M-B) effect in as-synthesized sample. It is noteworthy that, this is the first report demonstrating defect induced M-B effect in microwave derived ZrO2. Using photoluminescence spectra based asymmetric ratio based analysis, it is observed that the increase in symmetry around the Eu3+ ion is concomitant with the increase in overall point-defect density (which in turn results in M-B effect). There is an increase in PL intensity with calcination, which is attributed to increased crystallinity.

Effect of impurity concentration on optical and magnetic properties in ZnS:Cu nanoparticles

To obtain enhanced room temperature ferromagnetism (RTFM) along with the increase in optical bandgap in the compound semiconductors has been an interesting topic. Here, we report RTFM along with increase in energy bandgap in chemically synthesized Zn1−xCuxS (0 ≤ x ≤ 0.04) DMS nanoparticles. Structural properties of the synthesized samples studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) show the formation of cubic phase Cu doped ZnS nanoparticles of ~3–5nm size. An intrinsic weak ferromagnetic behavior was observed in pure ZnS sample (at 300K) which got increased in Cu doped samples and was understood due to defect induced ferromagnetism. UV–vis measurement showed increase in the energy bandgap with the increase in Cu doping. The PL study suggested the presence of sulfur and zinc vacancies and surface defects which were understood contributing to the intrinsic FM behavior.

Enhanced dielectric and ferroelectric properties of PVDF-BiFeO3 composites in 0–3 connectivity

Some composites fabricated with PVDF (Poly-Vinyl-difluoride) and BFO (BiFeO3) in 0–3 connectivity have been fabricated by a solution-casting method. The existence of all three polymorphic phases (α-, β- and γ-) of PVDF in the composites is confirmed by analysing x-ray diffraction (XRD) and Fourier Transform Infrared (FTIR) spectra. The intensity of the XRD peaks of the crystalline phase of the BFO ceramics increases with the increase of BFO filler concentration in the composites. Studies of micrographs recorded by scanning electron microscopy (SEM) technique reveal that the filler-particles have homogeneously been dispersed in the polymer matrix. The dielectric properties of the composites are very much dependent on both volume fraction of the filler and frequency of the applied field. The percolation threshold of electric field is observed in the composite at 50 vol percentage of BFO filler. A large enhancement in the dielectric constant and ferroelectric polarization in the composites is observed as compared to those of pure PVDF. With increase in BFO content in the composites, the leakage current density improves. Some IR inactive vibrational mode frequencies is found to be disappeared in the range of 400–800 cm−1 in FTIR spectra indicating a possible interaction between some molecules of PVDF and BFO. Incorporation of BFO filler in the polymer matrix has resulted in increase of optical band gap.

Tuning the Optical Band Gap of Graphene Oxide By Ozone Treatment

Graphene oxide (GO) inherits high transparency, substantial conductivity, high tensile strength from its parent materials graphene. Apart from these properties, it emits fluorescence which makes it a potential material to use in optoelectronics and bio-sensing applications. In this work, we have utilized systematic ozone treatment to alter the optical band gap of single-layered graphene oxide in aqueous suspensions. Due to controlled ozonation, additional functionalization takes place in GO graphitic sheet which changes GO electronic structure. This is confirmed by the increase in vibrational transitions of a number of oxygen-containing functional groups with treatment and the appearance of the prominent carboxylic group feature at c.a. 1700 cm-1. Albeit, timed ozone induction introduces only slight change in color and absorption spectra of GO samples, the emission spectra show a gradual increase in intensity with a significant blue shift up to 100 nm from deep red to green. This large blue shift suggests an increase in optical band gap with additional functionalization introduced by ozone treatment. We utilize a semi-empirical theoretical approach to describe the effects of functionalization-induced changes. This model attributes the origins of fluorescence emission to the quantum confined sp² carbon islands in GO encircled by the functional groups. As we decrease the graphitic carbon cluster size on the GO sheet, the optical bandgap calculated via HyperChem molecular modeling increases, which supports the experimentally observed blue shifts in emission. This theoretical result is further supported by the TEM of ozone-treated samples, which shows a decreasing trend of average ordered graphitic carbon cluster size on GO sheets with treatment time. Theoretical modeling, as well as the experimental results, indicate that the optical bandgap and emission intensity of GO are alterable with controlled ozone treatment, which allows tailoring the optical properties of GO for specific applications in optoelectronics and bio-sensing.

Optical band gap tuning of Ag doped Ge2Sb2Te5 thin films

Thin films of (Ge2Sb2Te5)100−xAgx (x = 0, 1, 3, 5 and 10) were deposited using thermal evaporation technique. X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy was used to confirm the amorphous nature, uniformity and chemical compositions of deposited films respectively. Transmission spectra divulged the highly transparent nature of films in near infra red region. The average transmission in near infra red region and optical band gap (estimated by Tauc’s plot) was increased with Ag doping upto x = 3 while it decreased for higher values of x. The increase in transmission and optical band gap was attributed to the reduction in density of localized states and vacancies. However, the decrease in the transmission and optical band gap is due to the increase in distortion of the host Ge2Sb2Te5 lattice because Ag is doped at the expense of Ge, Sb and Te. The increased optical band gap could be utilized to reduce threshold current which enhances switching speed in phase change materials.

Electronic structure and vacancy formation in photochromic yttrium oxy-hydride thin films studied by positron annihilation

In order to investigate the mechanism of the photochromic effect in yttrium oxy-hydride (YOxHy) thin films, Doppler broadening positron annihilation spectroscopy (PAS) was applied to probe the electronic structure and the presence of vacancies in YOxHy and related materials as a function of composition, UV illumination and thermal annealing. The Doppler S and W parameter depth profiles of a series of Y, yttrium di-hydride YH1.9+δ and Y2O3 thin films show strong systematic changes caused by the distinct differences in electronic structure of the metals Y, YH1.9+δ and the wide band gap insulator Y2O3. The Doppler broadening parameters of photochromic YOxHy (a semiconductor with a band gap of ~2.6eV) are intermediate to those of YH1.9+δ and Y2O3. In order to probe the nanostructural changes related to the photochromic effect, the S parameter of YOxHy was monitored during in-situ UV illumination. A small but systematic increase of the S parameter was observed, possibly induced by generation of cation mono-vacancies or small vacancy clusters involving generated anion vacancies. The changes did not relax during bleaching under dark conditions, showing that the structural changes are not directly responsible for the photochromic mechanism. For temperatures above around 90°C, thermal annealing leads to a substantial increase in the Doppler S parameter, pointing to the formation of vacancies by local removal of hydrogen. Simultaneously, the optical band gap increases, consistent with an increase in O:H ratios.

Effect of Mn2+ and Cu2+ co-doping on structural and luminescent properties of ZnS nanoparticles

Undoped and transition metal (Cu, Mn, Cu:Mn) doped ZnS nanoparticles are synthesized by chemical co-precipitation method via an aqueous synthesis route. Synthesized samples are characterized by various techniques for their structural and optical properties. Crystallite size obtained from X-Ray Diffraction (XRD) is 1.68, 1.87, 1.50, 1.42nm for undoped, Cu, Mn, Cu:Mn doped ZnS nanoparticles. The XRD, High Resolution Transmission Electron Microscopy, and Selected Area Electron Diffraction confirm the evolution of stable hexagonal phase of ZnS nanoparticles at low temperature. Energy Dispersive Spectroscopy confirms the doping of nanoparticles. Blue shift in UV absorbance shows the increase in optical bandgap with decrease in particle size. The Photoluminescence studies exhibit blue, yellow and red emission in visible region. Surface functionalization of nanoparticles is confirmed from Fourier Transform Infra Red spectroscopy. The present samples are tunable in wider range of emission and are prospective candidates for biological labels due to their fluorescent properties.

Transparent conductive polymers for laminated multi-wire metallization of bifacial concentrator crystalline silicon solar cells with TCO layers

Replacing expensive silver with inexpensive copper for the metallization of silicon wafer solar cells can lead to substantial reductions in material costs associated with cell production. A promising approach is the use of multi-wire design. This technology uses many wires in the place of busbars, and the copper wires are “soldered” during the low-temperature lamination process to the fingers (printed or plated) or to the transparent conductive oxide (TCO) layer, e.g. in the case of the a-Si:H/c-Si heterojunction cells. Here we describe a solar cell design in which wires are attached to TCO layers using transparent conductive polymer (TCP) films. To this end, we have synthesized a number of thermoplastics, poly(arylene ether ketone) copolymers (co-PAEKs), containing phthalide in their main chain. The fraction of phthalide-containing units in the copolymers was p=3, 5, 15, and 50mol%. With increasing p, the peak strain temperature of the co-PAEKs rises from 205 to 290°C and their optical band gap and refractive index increase from 3.12 to 3.15eV and from 1.6 to 1.614, respectively. The copolymers have a negligible absorption coefficient in the wavelength range 400–1100nm. When exposed to an excess pressure of 1atm or above, co-PAEK films less than 30µm in thickness undergo a transition from a dielectric to a conductive state. The resistivity (ρС) of wire/TCP/TCO (ITO=In2O3:Sn and IFO=In2O3:F) contacts ranges from 0.37 to 1.43mΩcm2. The polymer with the highest phthalide content (p=50mol%) has the lowest ρС. The average work of adhesion per unit area determined by pulling off the wires from the polymer surface depends on both the phthalide content of the co-PAEKs and their reduced viscosity, ranging from 14.3 to 43.5N/cm. The highest value was obtained for the co-PAEK with p=50mol%. We have fabricated low-concentration bifacial IFO/(n+pp+)Cz-Si/ITO solar cells with a wire contact grid attached to IFO and ITO using a co-PAEK film. The efficiency of the best cell under 1× to 7× front/rear illumination was determined to be 18.3–18.9%/15.0–15.6%.

Effect of sputtering power on optical properties of prepared TiO2 thin films by thermal oxidation of sputtered Ti layers

In this research, TiO2 thin films prepared via thermal oxidation of Ti layers were deposited by RF-magnetron sputtering method at three different sputtering powers. The effects of sputtering power on structure, surface and optical properties of TiO2 thin films grown on glass substrate were studied by X-ray diffraction (XRD), atomic force microscopic (AFM) and UV–visible spectrophotometer. The results reveal that, the structure of layers is changed from amorphous to crystalline at anatase phase by thermal oxidation of deposited Ti layers and rutile phase is formed when sputtering power is increased. The optical parameters: absorption coefficient, dielectric constants, extinction coefficient, refractive index, optical conductivity and dissipation factor are decreased with increase in sputtering power, but increase in optical band gap is observed. The roughness of thin films surface is affected by changes in sputtering power which is obtained by AFM images.

Double Charged Surface Layers in Lead Halide Perovskite Crystals.

Understanding defect chemistry, particularly ion migration, and its significant effect on the surface's optical and electronic properties is one of the major challenges impeding the development of hybrid perovskite-based devices. Here, using both experimental and theoretical approaches, we demonstrated that the surface layers of the perovskite crystals may acquire a high concentration of positively charged vacancies with the complementary negatively charged halide ions pushed to the surface. This charge separation near the surface generates an electric field that can induce an increase of optical band gap in the surface layers relative to the bulk. We found that the charge separation, electric field, and the amplitude of shift in the bandgap strongly depend on the halides and organic moieties of perovskite crystals. Our findings reveal the peculiarity of surface effects that are currently limiting the applications of perovskite crystals and more importantly explain their origins, thus enabling viable surface passivation strategies to remediate them.

Correlation between structural and optoelectronic properties of tin doped indium oxide thin films

Spin coating method was employed to deposit the thin films using redispersed nanoparticle suspensions of tin doped indium oxide (ITO). These films were then annealed in air at 450, 500, 550, 600, and 650°C for 1h. The effect of annealing temperature on the structural, optical, morphological, and electrical properties were studied by XRD, UV–vis spectroscopy, SEM, and four point probe technique, respectively. XRD analysis revealed that all thin films have cubic bixbyite structure with dominant (2 2 2) plane. Williamson-Hall analysis of XRD patterns showed that crystallite size increases with the rise in annealing temperature. SEM images of ITO thin film depicted spherical and non-spherical shaped particles of uneven size. The transmission spectra obtained from UV–vis spectroscopy reveals that transparency of thin films undulate with annealing temperature. With the rise in annealing temperature the optical band gap increases, while the Urbach energy and electrical resistivity decreases. The optical band gap, Urbach energy, and electrical resistivity were found to have systematic dependence on the crystallite size. The figure of merit is maximum for ITO thin film annealed at 550°C.

Optical and electrical properties of In4Se96-xSx chalcogenide thin films

The objective is to study the physical, structural, compositional, electrical and optical changes in InSeS alloy. In4Se96-xSx (where x = 0, 4, 8, 12) chalcogenide semiconductors has been deposited on non-conducting glass substrate by thermal evaporation technique at a pressure of approximately 10−6 Torr. The thin films were characterized by XRD, SEM, UV spectroscopy and electrical measurement. XRD shows the amorphous nature of the prepared thins films. Surface morphological analysis were carried out by scanning electron microscope (SEM), which shows the grain development of the prepared samples. Energy dispersive analysis by X-ray (EDAX) was used to check the compositional elements of deposited films. The analysis of the absorption spectra show indirect band gap, the magnitude of optical band gap increases and the corresponding absorption coefficient decreases with Sulfur concentration. The temperature dependent DC Conductivity (σdc) decreases and the corresponding activation energy increases with increase of Sulfur content for the In4Se96-xSx. High field conduction study shows the space charge limited conduction (SCLC) is dominated from where the density of states has been evaluated, density of state decreases with increases of sulfur concentration which may be due to decrease in disorderness or decrease in defects states of the system.

Fluoride-assisted synthesis of anatase TiO2 nanocrystals with tunable shape and band gap via a solvothermal approach

A simple solvothermal approach employing oleic acid has been developed to prepare anatase TiO2 nanocrystals with different shapes, which were tuned from nanorods to nano-ellipsoids by increasing the amount of NaF from 0 to 0.5mmol, and the optical band gap decreased from 3.47eV to 3.29eV accordingly. However, when the fluoride was changed to NH4F, the resultant TiO2 nanocrystals possessed an anatase phase but were made up of smaller-sized nanocrystals and nanorods, and the band gap was increased to 3.53eV. The X-ray photoelectron spectroscopy (XPS) results illustrated an increase of fluorine content with an increasing amount of NaF could account for the variation of the shape and optical band gap of TiO2 nanocrystals. Moreover, the absence of fluorine content brought about less change of shape and increase of optical band gap of the product synthesized in the presence of NH4F. This result may offer another way to alter the shape and band gap of metal oxide nanocrystals with the assistance of fluoride.

Optical Spectroscopic Analysis of High Density Lead Borosilicate Glasses

Lead borosilicate glasses, of chemical composition 20SiO2-xPbO-(15 + x)B2O3-5WO2-10ZnO-(50-2x) Na2O (where x = 5, 10, 15, 20, 25) were prepared using the normal melt-quenching technique. The samples were examined using a Philips Analytical X-ray diffraction system in order to check their amorphous nature. The effect of increasing B2O3 and PbO content on glass transition temperature was examined using Differential Thermal Analysis measurements (DTA). The results of DTA showed that both melting and glass transition temperatures decrease with increase of lead and boron oxides. Density and its related parameters have been determined to study the effect of lead-boron content on the structural properties of the prepared samples. Based on the density and DTA results, the network forming role of Pb and B ions was proved. The optical properties of the glass samples have been obtained using UV-VIS measurements. The optical parameters, such as optical band gap, Urbach energy, refractive index, and electronic polarizability were deduced based on the optical data. The observed increase in optical band gap and decrease in Urbach energy as well as the red shift in the absorption spectra arise due to the formation of non-bridging oxygen.

Effect of Hf doping on the structural, dielectric and optical properties of CaCu3Ti4O12 ceramic

Ceramic samples of CaCu3(Ti1−xHfx)4O12 (CCTHO) were prepared by solid state reaction method. The influences of Hf+4 ions substitution on the structural, optical and dielectric properties of the prepared samples were investigated using X-ray diffraction, diffuse reflectance spectroscopy and impedance spectroscopy respectively. The solubility limit of Hf in CaCu3Ti4O12 (CCTO) is found to be ~2 %. The optical spectroscopy data reveals that with Hf doping the optical band gap increases. Further we observed that with Hf substitution in CCTO at Ti site improves the dielectric properties, i.e. the value of dielectric constant (e′) increases and that of dielectric loss (tanδ) decreases with respect to parent un-doped CCTO sample. Two dielectric relaxations are observed in Hf doped sample, whereas only one relaxation is observed in CCTO. The characteristic dielectric relaxation frequency shifts towards higher values with Hf doping. Further from the fitting of e′ versus frequency data it appears that the relaxation mechanism shifts from Debye to Cole–Cole type.

Preparation and properties of Si/SiCxOy multilayer films containing Si quantum dots

Si/SiCxOy multilayers have been prepared by magnetron sputtering technique. Effects of annealing temperature (T a ) on the microstructural and electrical properties of Si/SiCxOy multilayer films were systematically investigated using high resolution transmission electron microscope, grazing incident X-ray diffraction, Raman spectroscopy, UV–Vis–NIR spectroscopy, and Hall measurements. The T a has great effects on the optical properties of Si/SiCxOy multilayer thin films. It is found that the onset of nanocrystalline Si phase (Si quantum dots) formation occurs at 800 °C. SiC crystalline phases are not formed in Si/SiCxOy multilayer film even at the T a 1000 °C. It is indicated that the method of depositing a Si/SiCxOy multilayer followed by thermal induced crystallization gives flexibility to suppress the crystallization of an a-SiC phase. With increasing T a the grain size, dot density, and optical bandgap increase, the conductivity also increase. These results demonstrate that high quality Si QDs embedded in Si/SiCxOy multilayer films deposited by magnetron sputtering can be obtained by properly controlling the annealing condition.

Influence of urea doping on optical, thermal, mechanical and electrical properties of l-arginine phosphate monohydrate crystals for NLO applications

In the present investigation, the influence of urea doping on the optical, structural, thermal, mechanical and dielectric properties of l-arginine phosphate monohydrate (LAP) crystal were investigated. Pure and doped LAP crystals were grown from aqueous solution by evaporating solvent at a constant temperature. The presence of dopant was confirmed qualitatively by FT-IR spectroscopy. Powder X-ray diffraction study reveals slight modification in lattice parameters of doped crystal. Ultraviolet–visible–near infrared transmission study reveals increase in optical transparency and band gap of doped crystals with doping concentration. Other optical parameters such as refractive index, extinction coefficient, reflectance, optical and electrical conductivity of pure and doped crystals were evaluated from transmission spectrum only. Noticeable enhancement in second harmonic generation efficiency (1.67 times more) has been observed for 1mol percent urea doped LAP crystal as compare to pure one. Doped LAP crystals show higher mechanical and thermal stabilities than pure LAP crystal. Electrical properties like as dielectric constant, dielectric loss, ac conductivity and activation energy were also studied.

Room temperature ferromagnetism in undoped and Mn doped t-ZrO2 nanostructures originated due to oxygen vacancy and effect of Mn doping on its optical properties

In the present report, we have synthesized undoped and Mn doped (1, 5, and 10 at.%) t-ZrO2 nanostructures by sol–gel method and their magnetic and optical properties have been investigated. UV–VIS. spectroscopy revealed the increase in band gap energy with Mn doping in ZrO2 matrix. The increase of optical band gap is attributed to Moss–Burstein effect. Photoluminescence (PL) spectroscopy revealed the presence of oxygen vacancies in undoped and Mn doped ZrO2 nanostructures. The shifting of peak position of the emission band to higher wavelength region for Mn doped ZrO2 nanostructures is attributed to the quantum size effects. The possible PL mechanism in undoped and Mn doped ZrO2 nanostructures is also discussed. In the present report, we have observed room temperature ferromagnetism (RTFM) for undoped and Mn doped t-ZrO2 nanostructures. The value of saturation magnetization (Ms) is decreased with the increase of Mn content into t-ZrO2 matrix. Oxygen vacancies are found responsible for RTFM in undoped ZrO2. The paramagnetic nature of Mn doped (10 at.%) t-ZrO2 nanostructures is explained in terms of anti-ferromagnetic coupling of Mn–Mn at high doping concentration.

Studies on the influence of sputtering power on amorphous carbon films deposited by pulsed unbalanced magnetron sputtering

Abstract Amorphous carbon (a-C) films were deposited by pulsed unbalanced magnetron sputtering technique under different sputtering powers from 100 to 220 W. The film thickness, bonding configuration, mechanical and optical properties of these films were investigated by various techniques. It is found that the film thickness of a-C films increases with the increase of sputtering power. The results of X-ray photoelectron spectroscopy analysis suggest that the sp 3 /sp 2 ratio in the films increases with increasing sputtering power from 100 to 180 W, and then decreases with increasing sputtering power from 180 to 220 W. Mechanical and optical properties measurements show that the nanohardness, refractive index and optical band gap increase with increasing sputtering power from 100 to 180 W, and then decreases with the further increase of sputtering power; the extinct coefficient decreases first and then increases with increasing sputtering power. The results above indicate that sputtering power has a significant influence on the microsturture and properties of a-C films deposited by pulsed unbalanced magnetron sputtering technique.

Effect of gamma irradiation on the structural and optical properties of thin films of a-CdSe

Thermal quenching technique was used for the preparation of bulk sample of amorphous CdSe (50:50 composition) (a-CdSe) and the thin films were fabricated by thermal evaporation technique on glass substrates. The thin films of a-CdSe were irradiated by 60Co gamma-rays of different doses. XRD patterns of as-deposited and gamma irradiated thin films show phase transformation from amorphous to crystalline state after gamma irradiation. FT-IR spectrum shows strong asymmetric stretch and indicates that the bond strength increases after gamma irradiation. PL study confirms the defect concentration decreases and also supports the change in optical properties occurs due to gamma irradiation. The optical characterization of as deposited and gamma-ray irradiated thin films of CdSe were carried out by using UV-vis-spectrophotometer. It was found that the value of optical band gap increases and other optical parameters like extinction co-efficient (K), refractive index (n), Urbach's energy (Eu) and absorption co-efficient (α) changes accordingly.