Decrease In Optical Energy Gap Research Articles

Optical properties and weakening of elastic moduli with increasing glass transition temperature (T g) in (80–x)TeO2-xBaO-20ZnO glasses

Abstract BaO addition to ternary (80–x)TeO2-20ZnO-xBaO (x = 0–20 mol.%) glasses resulted in a decrease in ultrasonic velocities and independent elastic moduli; this result indicated that the rigidity of the glass network weakened possibly because non-bridging oxygen increased. Thermal analysis results showed that glass transition temperature increased as BaO content increased because of the stabilizing effect of Ba2+ on the glass network. Additional analyses using bulk compression and ring deformation models revealed that the ratio between theoretical bulk modulus and experimental bulk modulus increased; this result indicated that the compression mechanism mainly involved isotropic ring compression. Furthermore, the increase in non-bridging oxygen formation with BaO addition caused a decrease in optical energy gap and an increase in refractive index. An increase in Urbach energy indicated that the degree of disorder in the glass system also increased.

UV irradiated PVA–Ag nanocomposites for optical applications

The present paper is focused on the in-situ prepared Poly (vinyl alcohol)–Silver (PVA–Ag) nanocomposites and tailoring their optical properties by means of UV irradiation in such a way that these can be used for anti-reflective coatings and bandpass filters. The reflectance from these irradiated nanocomposites has been found to decrease leading to the increase in refractive index (RI), with increasing UV exposure time, in the entire visible region. Decrease in optical energy gap of PVA film from 4.92 to 4.57eV on doping with Ag nanoparticles has been observed which reduces further to 4.1eV on exposure to UV radiations for 300min. This decrease in optical energy gap can be correlated to the formation of charge transfer complexes within the base polymer network on embedding Ag nanoparticles, which further enhances with increasing exposure time. Such complexes may also be responsible for increased molecular density of the composite films which corresponds to decrease in reflectance corroborating the observed results.

Photoinduced effect in nanostructured InSe thin films for Photonic applications

Nanostructure InSe thin films were prepared using thermal evaporation technique under a vacuum of 10−4Pa onto a cleaned quartz substrate at room temperature, and then exposed to UV illumination. Structural analysis of the films confirmed that the virgin films have an amorphous background with a preferential orientation in the (002) direction. It is found that the UV illumination resulted significant changes in the microstructure of these films. It is observed that the values of crystallite size decrease on increasing illumination time. The optical parameters were obtained using spectrophotometric measurements of the transmittance and reflectance at normal incidence of light in the wavelength range of 200–2500nm. The type of optical transition was found to be an indirect allowed transition and the optical energy gap decreases (photo-darkening) with the increase of UV exposure time. The dispersion of the refractive index has been discussed in terms of Wemple-Didomenico single oscillator model.

The improvement in the electrical properties of nanospherical ZnO:Al thin film exposed to irradiation using a Co-60 radioisotope

Al doped ZnO (ZnO:Al) thin films were prepared using a sol–gel dip coating technique and deposited on borosilicate glass substrates. The irradiation treatment, was conducted using Co-60 radioisotope, played an important role in enhancing electrical properties. The absorbed dose was a key parameter to decrease the electrical resistivity and to increase carrier density and carrier mobility of nanospherical ZnO:Al thin film. ZnO:Al thin film with the doping of Al at 0.8at% had the lowest electrical resistivity and the highest optical transmittance after the irradiation treatment. Optical properties, such as transmittance and reflectance, were affected at an absorbed dose of 0.2Gy. The curves of optical density were improved at ∼380, 420, and 520nm in visible range after the irradiation process. Besides, another characteristic optical density band between∼900 and 1100nm was enhanced by gamma irradiation. It has been suggested that the mechanism of absorption is related to an allowed direct transition at the irradiated ZnO:Al thin film on borosilicate glass. The optical band gap of the ZnO:Al thin films broadened with increasing doping concentration. However, there is a decrease in optical energy gap of ZnO:Al thin film along with the absorbed dose of the film.

Tailoring Structural Properties of Polyethylene Terephthalate (PET) by 200 keV Ar<sup>+</sup> Implantation

In the present work, our aim is to reveal the effect of Ar+implantation on the structural behaviour of Polyethylene terephthalate (PET) using Fourier Transform Infrared and UV-Visible Spectroscopic techniques. PET specimens were implanted with 200 keV Ar+ions in the fluence range of 1x1015to 1x1017ions cm-2. The structural changes have been observed due to change in the position and intensity of the bands in the FTIR spectra of both implanted and unimplanted specimens. A continuous decrease in optical energy gap (from 3.63 eV to 1.48 eV) and enhancement in Urbach energy (from 0.29 eV to 3.70 eV) with increasing ion dose have been observed. The structural changes have been correlated with the optical parameters observed in PET specimens as a result of implantation.

Effects of Surface Disordering on Optical & Mechanical Properties of Ar<sup>+</sup> Implanted Polycarbonate

The surface disordering produced in polycarbonate specimens by 130 keV Ar+ ions has been investigated. The effect of argon ions on the surface structure of polycarbonate specimens has also been studied. The polycarbonate specimens were implanted with 130 keV Ar+ ions in the dose range of 1×1014-1×1016 ions cm-2. The change in the Urbach energy (disorder parameter) after implantation has been estimated using Urbach edge method by applying UV-Visible spectroscopic technique. Optical energy gaps of virgin as well as implanted specimens have also been calculated using UV-Visible spectroscopic technique. A clear enhancement in Urbach energy (disorder parameter) from 0.61 eV (virgin sample) to 1.38 eV (at a fluence of 1×1016 Ar+ cm-2) and a drastic decline in optical energy gap (4.1 eV to 0.63 eV) with increasing implantation dose has been observed. This decrease in optical energy gap has been found to have linear dependence on the increase in the Urbach energy which point towards the formation of disordered structures in the implanted layers of polycarbonate. The structural changes produced due to implantation have been studied using Attenuated Total Reflectance-Fourier Transform Infrared spectroscopic technique. Furthermore, Knoop microhardness has been found to be enhanced 14 times (at a load of 9.8 mN) after implantation. The possible correlation of the increase in Knoop surface hardness with the structural changes observed as a result of implantation has been established and discussed.

γ-Irradiated PVA/Ag nanocomposite films: Materials for optical applications

Polyvinyl alcohol–silver (PVA/Ag) nanocomposites were prepared through in situ reduction of silver nitrate (AgNO3) within PVA hydrogel under continuous stirring at 60°C. The observed characteristic surface plasmon resonance band at around 425nm in the recorded UV–Visible absorption spectrum confirmed the formation of Ag nanoparticles embedded in a dielectric matrix. It was found that pure PVA exhibited indirect optical energy gap (Eg) of 4.92eV, which reduced to 4.57eV after embedding Ag nanoparticles, suggesting the formation of charge transport complexes (CTCs) between the localized highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy bands of PVA. On exposure to gamma (γ) radiations at a dose of 50kGy, the value of Eg was reduced further and found to be 3.72eV. The decrease in optical energy gap and the observed increase in refractive index (n) of PVA/Ag nanocomposites before and after gamma irradiation in comparison with pure PVA suggest the possibility of their use in optical device applications. The induced structural changes in PVA after the formation of embedded Ag nanoparticles and subsequently through gamma irradiation, were revealed through X-ray diffraction (XRD) and Fourier transform infra-red (FTIR) spectroscopy, corroborating the observed changes in their optical behavior.

Variation of lexan polycarbonate properties by electron beam

AbstractThe modifications in microstructural, optical, and photoluminescence properties of the Lexan polycarbonate (bisphenol‐A‐polycarbonate) films exposed to different electron doses have been studied using UV–vis spectroscopy, Fourier transform infrared spectroscopy (FTIR), positron annihilation lifetime spectroscopy, photoluminescence spectroscopy, and scanning electron microscopy (SEM). The obtained UV–vis spectroscopy results showed decrease in optical energy gap, optical activation energy, and increase in number of carbon atoms per cluster with increase in electron dose. The chemical changes in electron irradiated polymers due to chain scission and reconstruction have been observed from FTIR spectroscopy. The correlation of positron lifetime study with optical measurement is obtained, and electron irradiation‐induced microstructural modifications within the polymer is understood. SEM result shows the degradation of Lexan polymer after electron irradiation. The mechanical properties and average molecular weight of Lexan decrease after irradiation, whereas average number of chain scissions per original polymer molecule increases. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Temperature-Induced Irreversible Change of ZnO Optical Energy Gap

It is generally recognized that the change of ZnO optical properties due to high-temperature treatment is reversible. However, herein, it is reported that high-temperature treatment in air caused an irreversible decrease in ZnO optical energy gap (Eg). The irreversible decrease in Eg was enhanced with increasing treatment temperature. Furthermore, it was found that the irreversible decrease in Eg can be attributed to the enlargement of ZnO crystal particles.

Thickness Dependence of Structural and Optical Characteristics of AZO Thin Films for Organic Photovoltaic Cells

Aluminum-doped zinc oxide (AZO) thin films with highly (002)-preferred orientation were grown on glass substrates by rf magnetron sputtering. The effect of thickness on structural and optical characteristics of the deposited films were investigated by X-ray diffractometer and spectrophotometer. The results show that the polycrystalline AZO films consist of the hexagonal crystal structures with c-axis as the preferred growth orientation normal to the substrate, and that the thickness significantly affects the crystal structure and optical properties of the thin films. With the increase of thickness, the crystallite size of the films increases, the lattice spacing, dislocation density, micro strain and optical energy gap decrease, and the average transmitance in the wavelength range of the visible spectrum also slightly decreases.

Determination of the optical constants of amorphous As xS 100− x films using effective-medium approximation and OJL model

The transmission spectra were used to obtain an efficient parameterization of the spectral dependences of the optical constants of amorphous As–S thin films by applying a suitable dielectric function model. For studying the compositional dependence of the optical constants, different compositions of As x S 100− x ( x = 10, 15, 20, 25, 30 and 40 at%) thin films were deposited by thermal evaporation technique in a base pressure of 7.5 × 10 −6 Torr at room temperature. The transmission spectra (measured in the wavelength range of 0.2–0.9 μm) were analyzed by applying O’Leary, Johnson, and Lim (OJL) model based on the joint density of states (JDOS) functions. However, the best fit of the optical data was obtained by considering the two-layer configuration film; the top layer was assumed to be consisted of a bulk AsS material embedded in voids (air). Therefore, OJL model along with Bruggeman effective-medium approximation (BEMA) model was used to determine the effective optical constants of the As–S thin films. The photon energy dependence of the dielectric function, ɛ = ɛ r − iɛ i of the investigated As–S films was presented. The film thickness, absorption coefficient α, refractive index n, extinction coefficient k, static refractive index n(0) and optical band gap E g have been deduced. It was found that with the increase in arsenic content up to the stoichiometric As 40S 60, the indirect optical energy gap decreases, while the refractive index increases.

Sonochemical preparation of SbS 1−xSe xI nanowires

A sonochemical method for direct preparation of nanowires of SbS 1− x Se x I solid solution has been established. The SbS 1− x Se x I gel was synthesized using elemental Sb, S, Se and I in the presence of ethanol under ultrasonic irradiation (35 kHz, 2 W/cm 2) at 50 °C for 2 h. The product was characterized by using techniques such as powder X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, energy dispersive X-ray analysis, selected area electron diffraction, and optical diffuse reflection spectroscopy. The SEM and HRTEM investigations exhibit that the as-prepared samples are made up of large quantity nanowires with lateral dimensions of about 10–50 nm and lengths reaching up to several micrometers and single-crystalline in nature. The increase of molar composition of Se affects linear decrease of the indirect forbidden optical energy gap as well as the distance between (121) planes of the SbS 1− x Se x I nanowires.

Influence of atomic hydrogen annealing on the optical properties of vacuum-deposited a-Si:H films

The influence of atomic hydrogen annealing on the optical parameters of a-Si:H films was studied using spectrophotometric measurements of the film transmittance and reflectance in the wavelength range 200–3000 nm. In this annealing, the deposition of a thin layer and treatment with atomic hydrogen were repeated alternately, where the thickness of the thin cyclic layer, d cyc, and the treatment time of each cycle, t ca, were kept fixed for each sample. A series of different samples with average thickness of 0.5 μm and different d cyc and t ca were prepared. It was found that the refractive index, n, and the optical energy gap, E g, increase as the treatment time, t ca, increases from 0 to 60 s, while at t ca=120 s both n and E g decrease. Also, both the refractive index and the optical energy gap decrease with increasing the relative diffusion length of hydrogen, √ t ca/ d cyc from 0.39 to 0.77. The widening of E g is due to the structural relaxation resulting from impingement of atomic hydrogen on the growing surface. Thus, a good-quality a-Si:H with Urbach parameter 65 mev and optical energy gap of 1.78 eV was successfully prepared.

IR and UV spectral studies for rare earths-doped tellurite glasses

Abstract Infrared absorption spectra were measured for pure TeO 2 glasses and doped with 10-wt% of different rare-earth oxides, La 2 O 3 , CeO 2 , Pr 2 O 3 , Sm 2 O 3, Nd 2 O 3 , and Yb 2 O 3 . The recorded bands were attributed to the different modes of vibrations of Te–O band. Rare earths connected to the chains of TeO 4 groups are identified on the basis of the simultaneous presence of the bands at ∼660 to 625 cm −1 . Quantitative justification of these absorption bands shows that the values of the experimental wave number for most recorded absorption bands agree well with the theoretical ones. Optical absorption spectra show that the absorption edge has a tail extending towards lower energies. The edge shifts towards higher energies for all rare earths-doped glasses. The degree of the edge shift was found to be depending on the ionic radius of the rare earths and is mostly related to the structural rearrangement and the relative concentrations of the glass basic units. By increasing the ionic radius of the rare earths, the optical energy gap decreases, while the width of the localized states is increased.

Microstructural modifications in swift ion irradiated PET

Polyethylene terephthalte (PET) was irradiated with carbon (70 MeV) and copper (120 MeV) ions to analyze the induced modifications with respect to optical, structural and thermal properties. In the present investigation, the fluence for carbon irradiation was varied from 1×10 11 to 1×10 14 ions cm −2, while that for copper beam was kept in the range of 1×10 11 to 1×10 13 ions cm −2. UV–vis, FTIR, XRD and DSC techniques were utilized to study the induced changes. The analysis of UV–vis absorption studies reveals that there is decrease of optical energy gap up to 10% on carbon ion irradiation (at 1×10 14 ions cm −2), whereas the copper beam (at 1×10 13 ions cm −2) leads to a decrease of 49%. FTIR analysis indicated the formation of alkyne end groups along with the overall degradation of polymer with copper ion irradiation. X-ray diffraction analysis revealed that the semi-crystalline PET losses its crystallinity on swift ion irradiation. It was found that the carbon beam (1×10 14 ions cm −2) decreased the crystallite size by 16% whereas this decrease is of 12% in case of the copper ion irradiated PET at 1×10 13 ions cm −2. The loss in crystallinity on irradiation has been supported by DSC thermograms.

Proton (3 MeV) and copper (120 MeV) ion irradiation effects in low-density polyethylene (LDPE)

Low-density polyethylene (LDPE) was irradiated with proton (3 MeV) and copper (120 MeV) ions to analyze the induced modifications with respect to optical and structural properties. In the present investigation, the fluence for proton irradiation was varied up to 2×10 15 protons cm −2, while that for copper beam was kept in the range of 1×10 1 to 1×10 13 ions cm −2 to study the swift heavy ion-induced modifications in LDPE. Ultraviolet–visible (UV–vis), FTIR and X-ray diffraction (XRD) techniques were utilized to study the induced changes. The analysis of UV–vis absorption studies reveals that there is decrease of optical energy gap up to 43% on proton irradiation (at 2×10 15 ions cm −2), whereas the copper beam (at 1×10 13 ions cm −2) leads to a decrease of 51%. FTIR analysis indicated the presence of unsaturations due to vinyl end groups in the irradiated sample. The formation of O H and C O groups has also been observed. XRD analysis revealed that the semi-crystalline LDPE losses its crystallinity on swift ion irradiation. It was found that the proton beam (2×10 15 ions cm −2) decreased the crystallite size by 23% whereas this decrease is of 31% in case of the copper ion-irradiated LDPE at 1×10 13 ions cm −2.

Influence of ultraviolet irradiation on the optical properties of amorphous Sb10Se90 thin films

Amorphous Sb10Se90 thin films were prepared by thermal evaporation of the bulk glass. The changes in the optical properties (transmittance, optical gap, absorption coefficient, refractive index and extinction coefficient) have been measured in the wavelength range 500–900 nm of virgin and ultraviolet (UV) illuminated films. Analysis of the optical absorption data shows that the rule of non-direct transitions predominates. It is found that the optical energy gap decreases (photo-darkening) and the refractive index increases with the increase of UV exposure time. The dispersion of the refractive index (n) has been discussed in terms of Wemple–Didomenico single oscillator model. The oscillator energy E0 and the dispersion energy Ed have been determined and discussed in terms of UV exposure time. The photo-darkening was discussed in terms of some of the current literature models.

Preparation and properties of polycrystalline CdSxxTe1-x1-x thin films for solar cells

The structural, optical and electrical properties of CdSxxTe1-x1-x thin films prepared by co_evaporation of powders of CdS and CdTe ha ve been studie d by x_ray diffraction, atomic force microscopy and optical transmittance spectr a measurements. Results show that the as_deposited CdSxxTe1-x1 -x thi n films are homogeneous, adherent and compact on the glass slides substrates wit hout pinhole and that their conductivity is n_type for x≥05 and p_type fo r xxTe1-x1-x thin f ilms are pol ycrystalline and show highly preferential orientation. The predominant direct op tical transitions were observed and the variation of the optical energy gap E gg is nonlinear with x. Given the values of lattice parameter and opt ical energy gap, the exact composition of the phase change at x=025 has been determined. The films are of a cubic phase for x025. After annealing there is no change in the structure of the films with a slight decrease in optical energy gap. Finally, a new structure of CdS/CdTe/Z nTe/ZnTe:Cu/Au solar cells with a CdSxxTe1-x1-x buffer l ayer has been proposed.

Swift heavy ion induced modifications in polypropylene

The samples of polypropylene were irradiated with 58Ni 7+ (86 MeV) ion beam to analyze the swift heavy ion (SHI) induced modifications as a function of ion fluence ranging from 1 × 10 11 to 1 × 10 13 ions cm −2. Modifications in optical, chemical and structural properties of the polymer were evaluated by UV–VIS, FTIR and XRD techniques. UV–VIS absorption studies have revealed that optical energy gap decreases by 51% at the highest fluence of 1 × 10 13 ions cm −2. Analysis of FTIR spectra of the virgin and irradiated samples revealed the appearance of new bands in the spectra of irradiated samples, which are attributable to O–H and CO functions. This indicates the possible creation of simple aliphatic ketone and/or carboxylic acid moieties on irradiation. XRD spectra revealed that the virgin polypropylene is semi-crystalline in nature and there is an average decrease of ∼24% in the crystallite size on irradiation at the highest fluence of 1 × 10 13 ions cm −2.

Effect of composition and deposition parameters on optical properties of Ge 25Sb 15− xBi xS 60 thin films

The optical constants of the Ge 25Sb 15− x Bi x S 60 (0⩽ x⩽15) chalcogenide films, either as-deposited or after being annealed at various temperatures have been computed in the spectral wavelength range 400–2400 nm from the transmittance and reflectance measurements of normally-incident light. With the increase in bismuth content, the optical energy gap (which is indirect) decreases, while the refractive index increases. The effects of film thickness, substrate type, deposition rate and γ-radiation on optical properties have been studied. The effect of thermal annealing on the growth characteristics and stability of the films has been studied using X-ray diffraction and scanning electron microscopy. The dispersion of the refractive index is discussed in terms of the single-oscillator Wemple–DiDomenico model.