Indirect Optical Band Gap Energy Research Articles

Physical and optical studies on Li2O-Na2O-WO3-B2O3 glasses

Glasses with composition xLi2O-(30-x)Na2O-10WO3-60B2O3 (where x=0, 5, 10, 15, 20, 25 and 30 mol%) have been prepared using the melt quenching technique. In the present work, the mixed alkali effect (MAE) has been investigated in the above glass system through density and optical studies. The density of the present gasses varies non-linearly, the exhibiting the mixed alkali effect. From the optical absorption studies, the values of direct optical band gap, indirect optical band gap energy (Eo) and Urbach energy(ΔE) have been evaluated. The values of E0 and ΔE vary non-linearly with composition parameter, showing the mixed alkali effect.

Electrical Optical and Structural Properties of p-type Silicon

Electrical, optical and structural properties of p-type single crystal silicon were investigated in this work. Electrical conductivity of p-type silicon was measured in the temperature ranges 190 - 300 K. The acceptor ionization energy (?EA) was between 0.047 - 0.051 eV. Photoconductivity of the material was investigated by varying sample current, light intensity and temperature at a constant chopping frequency of 45.60 Hz. Absorption co-efficient (?) of the material was calculated from optical transmittance and reflectance measurements at room temperature (300 K) in the wavelength range of 300 -2500 nm. The direct optical band gap energy was found between 2.10 - 2.20 eV and the indirect optical band gap energy was found between 0.95 – 1.0 eV. The lattice parameter (a) was found to be 5.419Å from X-ray diffraction method (XRD). DOI: http://dx.doi.org/10.3329/dujs.v63i1.21766 Dhaka Univ. J. Sci. 63(1): 37-41, 2015 (January)

Size Dependent Physical Properties of Nanostructured α-Fe2O3 Thin Films Grown by Successive Ionic Layer Adsorption and Reaction Method for Antibacterial Application

Thin films of nanostructured α-Fe2O3 with thickness of 156, 203 and 251 nm were deposited by successive ionic layer adsorption and reaction (SILAR) method onto glass substrates using FeCl3·6H2O and NaOH as cationic and anionic precursors. The X-ray diffraction studies revealed that, α-Fe2O3 thin films are nanocrystalline in nature with rhombohedral structure. The morphological properties were investigated by field emission scanning electron and atomic force microscopy. The optical studies showed that α-Fe2O3 exhibits direct as well as indirect optical band gap energy. The electrical resistivity of α-Fe2O3 at 305 K decreases from 11.76 × 102 to 9.46 × 102 Ω cm as film thickness increases from 156 to 251 nm. The thermo-emf measurements confirmed that α-Fe2O3 exhibits n-type conductivity. The nanocrystalline α-Fe2O3 exhibits antibacterial character against Staphylococcus aureus and its efficiency increases from 37.50% to 87.50% depending on crystallite size.

Effect of silver nanoparticles incorporated with samarium-doped magnesium tellurite glasses

Silver nanoparticles (Ag NPs) are incorporated in samarium doped tellurite glass of a composition (89−x)TeO2–10MgO–1Sm2O3–xAgCl, where 0.0≤x≤0.6mol%, by a melt quenching technique. It is found that all the glasses are amorphous in nature, and the existence of Ag NPs with an average size of 16.94nm is confirmed by Transmission Electron Microscopy. Meanwhile, their physical properties such as glass density, molar volume and ionic packing density are computed utilizing the normal method. The density and ionic packing density are observed to decrease with increasing Ag NPs, but increase when the Ag NPs are beyond 0.2mol%. On the other hand, the molar volume behaves exactly opposite to the increase in Ag NPs content. It decreases when the Ag NPs content value is more than 0.2mol%. The optical energy band gap and Urbach energy are evaluated from the absorption spectra in the range of 200–900nm at room temperature. It is also observed that the direct and indirect optical energy band gaps reduce with Ag NPs content, but enhance when the Ag NPs are beyond 0.2mol%. Meanwhile, the Urbach energy is found to increase as the Ag NPs content is increased but decreases when Ag NPs is 0.2mol%. The refractive index is deduced from indirect optical energy band gap. Meanwhile, molar refraction and electronic polarizability have been calculated from the Lorentz–Lorentz relation. Refractive index and electronic polarizability are also observed to raise with Ag NPs content, but drop off when Ag NPs content is more than 0.2mol%. In this paper, all properties are discussed with respect to the Ag NPs concentration.

Mn2O3–4TiO2 semiconducting composite thin films for photo-electrochemical water splitting

Mn2O3–4TiO2 composite thin films were deposited on fluorine doped tin oxide (FTO) coated glass substrates from a solution of a newly synthesised hexanuclear single source molecular precursor [Mn2Ti4(TFA)8(THF)6(OH)4(O)2]·0.4THF 1 (where TFA=trifluoroacetato and THF=tetrahydrofuran) by aerosol-assisted chemical vapour deposition (AACVD) technique. The precursor 1 was characterised by melting point, elemental analysis, Fourier transform infra-red (FTIR) spectroscopy, proton nuclear magnetic resonance (1H NMR) spectroscopy, thermogravimetry/derivative thermogravimetry (TG/DTG) and single crystal X-ray analysis. The chemical composition and surface morphology of the fabricated thin film which have been determined by X-ray diffraction, scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX), suggest the formation of impurity-free nanocrystalline mixtures of Mn2O3–4TiO2 composite with well-defined and evenly distributed particles in the size range of 112–308nm. The indirect optical band gap energies of 1.20 and 2.81eV were estimated by UV–Vis spectrophotometry. The current–voltage characterisation proved that the composite Mn2O3–4TiO2 electrodes exhibit n-type semiconducting behaviour. The maximum photocurrent density of 343μA/cm2 at 0.7V versus Ag/AgCl/3M KCl (∼1.23V versus RHE) was obtained for the Mn2O3–4TiO2 photoelectrode deposited at 450°C for 45min from 0.006M solution of 1 in THF.

Double-Layer TiO2 Electrodes with Controlled Phase Composition and Morphology for Efficient Light Management in Dye-Sensitized Solar Cells

The light-scattering effect in the dye-sensitized solar cells (DSCs) was studied by controlling TiO2 phase composition and morphology by fabrication of double-layer cells with different arrangement modes. The starting material for preparation of TiO2 cells was synthesized by an aqueous sol–gel process. X-ray diffraction and field emission scanning electron microscopic analyses revealed that TiO2 nanoparticles had particle size ranging between 18 and 44 nm. The optical property and band gap energy of TiO2 nanoparticles were studied through UV–Vis absorption. The indirect optical band gap energy of anatase and rutile nanoparticles was found to be 3.47 and 3.41 eV, respectively. The double-layer DSC made of nanostructured TiO2 film with phase composition of 78 % anatase and 22 % rutile as the under-layer and mixtures of anatase-nanoparticles and anatase-microparticles as the over-layer (i.e., NM solar cell) was shown the highest power conversion efficiency (PCE) of 6.04 % and open circuit voltage of 795 mV. This was achieved due to the optimal balance between the light scattering effect and dye sensitization parameters. Optimum light scattering of photoanode led to improve the PCE of NM double-layer solar cell which was demonstrated by diffuse reflectance spectroscopy.

The improvement of electron transport rate of TiO2 dye-sensitized solar cells using mixed nanostructures with different phase compositions

Abstract Dye-sensitized solar cells (DSCCs) in the form of mixed nanostructures containing TiO 2 nanoparticles and nanowires with different weight ratios and phase compositions are reported. X-ray diffraction and field emission scanning electron microscopy analyses revealed that the synthesized TiO 2 nanoparticles had average crystallite size in the range 21–39 nm, whereas TiO 2 nanowires showed diameter in the range 20–50 nm. The indirect optical band gap energy of TiO 2 nanowires, anatase- and rutile-TiO 2 nanoparticles was calculated to be 3.35, 3.28 and 3.17 eV, respectively. The power conversion efficiency of the solar cells changed with nanowire to nanoparticle weight ratio, reaching a maximum at a specific value. An increase of 4.3% in cell efficiency was achieved by introducing 10 wt% nanowire into the as-synthesized TiO 2 nanoparticles (WP1 cell). Furthermore, an increase of 27.6% in cell efficiency was achieved by using crystalline anatase-TiO 2 nanoparticles rather than as-synthesized TiO 2 nanoparticles in WP1 solar cell. It was found that the power conversion efficiency and short circuit current of WP1 cell were decreased down to around 30.8% and 39.1%, respectively using rutile nanoparticles rather than anatase nanoparticles. The improvement of cell efficiency was related to rapid electron transport and less recombination of photogenerated electrons, as confirmed by electrochemical impedance spectroscopy.

Combined DFT, Deconvolution Analysis for Structural Investigation of Copper –doped Lead Borate Glasses

Samples from binary lead borate glass system doped with minor quantities of copper have been prepared by conventional melt-annealing technique. X-ray diffraction (XRD) has been used to prove the amorphous nature of prepared glasses. Structural and optical properties were investigated using FTIR and UV-vis. Spectroscopic investigations were ap- proved using density function theory (DFT) calculations. Deconvolution analysis technique (DAT) combined with DFT technique were employed to interpret both of the theoretical and experimental IR data of this glass system and their agreement. Experimental IR data reveal the presence of both triangular and tetrahedral borate groups besides the sharing of Pb-O units. Direct and indirect optical energy band gaps before and after doping with different percents of copper were calculated to evaluate the role of copper in the glassy matrix. Undoped glass sample is observed to exhibit strong UV absorption due to the combined contributions of absorption of both Pb 2+ ions and trace iron impurities. The presence of both Cu 2+ ions to- gether with the other copper valence state Cu + has been proved by the appearance of extra visible absorption bands.

Controlling electron transport rate and recombination process of TiO2 dye-sensitized solar cells by design of double-layer films with different arrangement modes

TiO2 dye-sensitized solar cells (DSSCs) in the form of double-layer films, containing an under-layer and an over-layer, with various crystal structures (i.e., anatase and rutile phases) and morphologies (i.e., nanoparticle and nanowire) were reported. It was found that the photovoltaic performance of TiO2 DSSCs depends on the morphology, crystal structure, light scattering effect, optical band gap energy and arrangement of the under- and over-layer films. The double-layer solar cell made of anatase-TiO2 nanoparticles as the under-layer and anatase-TiO2 nanowires as the over-layer (i.e., AW solar cell) showed the highest power conversion efficiency and fill factor of 6.34% and 62.6%, respectively. High electron lifetime, rapid transportation and less recombination of photogenerated electrons are the factors affect the efficiency improvement of AW film and was demonstrated by electrochemical impedance spectroscopy (EIS). X-ray diffraction (XRD) and field emission scanning electron microscope (FE-SEM) analyses revealed that TiO2 nanoparticles had uniform and nanometer grains with particle size around 20nm, whereas TiO2 nanowires with length of several μm had diameter in the range 20–50nm. The optical properties and band gap energies of TiO2 nanoparticles and nanowires were studied through UV–vis absorption. The indirect optical band gap energy of TiO2 nanowires, anatase-TiO2 and rutile-TiO2 nanoparticles was calculated 3.61, 3.47 and 3.41eV, respectively. The design of double-layer solar cells by manipulation of morphology and crystal structure of TiO2 nanostructures will open a new concept for improvement of power conversion efficiency of dye sensitized solar cells.

Mixed alkali effect in physical and optical properties of Li2O–Na2O–WO3–B2O3 glasses

Glasses with composition xLi2O-(30−x)Na2O–10WO3–60B2O3 (where x=0, 5, 10, 15, 20, 25 and 30mol%) have been prepared using the melt quenching technique. In the present work, the mixed alkali effect (MAE) has been investigated in the above glass system through density and modulated DSC studies. The density and glass transition temperature of the present gasses varies non-linearly, the exhibiting the mixed alkali effect. From the optical absorption studies, the values of direct optical band gap, indirect optical band gap energy (Eo) and Urbach energy(ΔE) have been evaluated. The values of Eo and ΔE vary non-linearly with composition parameter, showing the mixed alkali effect. The electronic polarizability of oxide ions, optical basicity and the Yamashita–Kurosawa's interaction parameter have been examined to check the correlation among them and bond character. Based on good correlation among electronic polarizability of oxide ions, optical basicity and the Yamashita–Kurosawa's interaction parameter, the present Li2O–Na2O–WO3–B2O3 glasses were classified as normal ionic (basic) oxides.

Investigation of optical and thermally stimulated properties of SiO2 nanoparticles‐filled polycarbonate

AbstractPolycarbonate nanocomposite containing silicon oxide nanoparticles average size of 5 nm at different weight ratio has been prepared by solution mixing method. The dispersion of nanoparticles in polymer matrix was studied by transmission electron microscopy (TEM). The optical and thermally stimulated behavior of nanocomposites were analyzed by energy dispersive X‐ray spectra (EDX), X‐ray diffraction pattern (XRD), UV–vis spectroscopy, differential scanning calorimetry (DSC), and thermally stimulated discharge current (TSDC). TEM images show the dispersion and size of the nanoparticles, however, EDX indicate the presence of SiO2 on the surface of the nanocomposite film. An XRD result reveals that the crystallinity increases with increase in concentration of SiO2 nanoparticles in polymer matrix. The direct and indirect optical energy band gaps decreased and number of carbon atom increased with concentration of SiO2 nanoparticles. We have observed that the increase of SiO2 nanoparticles in PC significantly reduces the refractive index. DSC and TSDC show that glass transition temperature increases according to SiO2 weight ratio. The TSDC of nanocomposites samples could be understand in terms of non‐Debye theory of charge relaxation and co‐tunneling mechanism of charge transport. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Phase transformation and disorder effect on optical and electrical properties of Zn3P2 thin films

The phase transformation of zinc phosphide (Zn3P2) thin films was detected through isochronal annealing process. The effects on isochronal annealing on the internal structural, optical and electrical properties of deposited Zn3P2 thin films have been discussed. The films were prepared by thermal evaporation under constant preparation conditions of vacuum 1.3×10−5Torr, substrate temperature (300K), rate of deposition (∼1nm/s) and film thickness (480nm). The annealing process was carried out under vacuum for 2h at different temperatures ranging from 373 to 623K. X-ray diffraction patterns showed that the as-deposited films and those annealed at temperatures less than 623K exhibit amorphous structure, while the films annealed at 623K showed tetragonal polycrystalline structure. The optical transmission and reflection spectra were measured at the wavelength range of 190–2500nm. The absorption coefficient spectra and the degree of disorder as measured from the absorption edge were determined. The indirect and direct optical energy band gaps were evaluated for indirect allowed and direct allowed transitions for amorphous and polycrystalline films, respectively. The refractive index no increases with raising the annealing temperature which refers to more condensation in the material. The electrical resistivity for Zn3P2 films decreases exponentially with raising the annealing temperature up to 623K as influenced by structure transformation and decreasing the degree of disorder in the films.

Phase change and optical band gap behavior of Ge–Te–Ga thin films prepared by thermal evaporation

The amorphous Ge11.4Te86.4Ga2.2 chalcogenide thin films were prepared by thermal evaporation onto chemically cleaned glass substrates. Properties measurements include X-ray diffraction (XRD), Scanning electron microscopy (SEM), Differential scanning calorimetry (DSC), Four-point probe and VIS–NIR transmission spectra. The allowed indirect transition optical band gap and activation energy of samples were calculated according to the classical Tauc equation and Kissinger's equation, respectively. The results show that there is an amorphous-to-crystalline phase transition of Ge11.4Te86.4Ga2.2 thin film. The investigated film has high crystallization temperature (∼200 °C) and activation energy (2.48 eV), indicating the film has good amorphous stability. The sheet resistance of the crystalline state is ∼10 Ω/and the amorphous/crystalline resistance ratio is about 105. Besides, a wide optical band gap (0.653 eV) of Ge11.4Te86.4Ga2.2 is obtained, indicating that the material possesses a low threshold current from amorphous-to-crystalline state for phase-change memory application.

An investigation of the bandgap and Urbach tail of vacuum-evaporated SnO2 thin films

Abstract Tin oxide (SnO 2 ) thin films with thickness in the range 100–600 nm were prepared by vacuum evaporation on glass substrates at ambient temperature. X-ray diffractograms showed that the films have poor crystallinity. The transmittance of the films was measured at room temperature in the wavelength range 290–1100 nm. The absorption coefficient was plotted as a function of photon’s energy for films of different thickness and oscillatory behaviour was observed in the high energy side. This oscillatory behaviour which was found to increase with film thickness is evidence on the presence of nanoparticles. Direct and indirect optical bandgap energies were estimated and found to be decreasing with film thickness. Urbach tailing in the bandgap was observed and the width of the tail which is related with disorder and localized states was estimated and found to be decreasing with film thickness.

γ-Ray modifications of optical/chemical properties of a PVC polymer

Polyvinyl chloride (PVC) films are subjected to high doses of γ -radiation up to 800 kGy. The modifications in the optical properties, the structural and chemical changes are investigated by recording the ultraviolet–visible (UV–VIS) and the Fourier transform infrared (FTIR) spectra of unirradiated and irradiated PVC films. The UV–VIS data show an increase in absorbance and shift of the absorption edge towards the visible region from the UV region with the increase in the γ -radiation dose. This may be attributed to the formation of conjugated system of bonds. The direct and the indirect optical energy band gaps are determined as a function of the γ -exposure dose. The results show that both the direct and the indirect energies of transitions decrease with the increasing γ -absorbed dose. Also the direct energy band gap has higher values when compared with the corresponding values of the indirect energy band gap. The intensity of the different infrared bands in the FTIR spectra of irradiated PVC indicates the presence of the hydroxyl group, release of volatile gases and polyene formation. A total destruction of the structure is observed at a very high γ -radiation dose.

Optical and shielding behavior studies of vanadium-doped lead borate glasses

A glass sample of composition PbO 42.1%– B2O3 57.9 mol% B2O3 together with samples of the same composition doped with varying V2O5 contents was prepared via melt annealing technique. X-ray diffraction technique was applied to prove the amorphous nature of the prepared glasses. UV–visible absorption spectra were measured in the range 200–1500 nm before and after successive gamma irradiation, which were applied to evaluate optical properties, including direct and indirect optical energy band gaps before and after gamma irradiation. Undoped glass sample is observed to exhibit strong UV absorption; due to the combined contributions of absorption of both Pb2+ ions and trace iron impurities. The presence of V3+ ions together with the other vanadium valence states (V4+, V5+) was proved by the appearance of extra visible absorption bands. Infrared absorption spectra were measured in the range 4000–400 cm−1 which revealed the presence of both triangular and tetrahedral borate groups besides the sharing of lead ions.

An investigation of the bandgap and Urbach tail of spray-deposited SnO2:F thin films

Fluorine-doped tin oxide (SnO2:F) thin films with thickness 100±5 and 200±10 nm were prepared by the spray pyrolysis technique on glass substrates at two different substrate temperatures: 400±5 and 480±5 °C. One set of films was etched by HNO3, another set of films was annealed in nitrogen atmosphere at 400±5 °C and yet another set was first etched by HNO3 and then annealed in nitrogen atmosphere at 400±5 °C. X-ray diffraction patterns and scanning electron microscope images showed that the films are polycrystalline. Energy dispersive analysis by x-rays (EDAX) revealed that the films are oxygen rich. Despite the fact that the same solution was used to prepare all the films, EDAX revealed that the films prepared at the higher substrate temperature contain more fluorine and less oxygen. The transmittance of the films was measured at room temperature in the wavelength range 290–1100 nm. Direct and indirect optical bandgap energies were estimated and showed a tendency to decrease with heat treatment. Urbach tailing in the bandgap was observed, and the width of the tail, which is related to disorder and localized states, was estimated and showed a tendency to increase with heat treatment.

Investigations on the effect of complexation of NaF salt with polymer blend (PEO/PVP) electrolytes on ionic conductivity and optical energy band gaps

Sodium ion conducting polymer blend electrolyte films, based on polyethylene oxide (PEO) and polyvinyl pyrrolidone (PVP) complexed with NaF salt, were prepared using solution casting technique. The complexation of the salt with the polymer blend was confirmed by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and UV–vis spectroscopy. Electrical conductivity of the films was measured with impedance analyzer in the frequency range of 1Hz to 1MHz and in the temperature range of 303–348K. It was observed that the magnitude of conductivity increased with the increase in the salt concentration as well as the temperature. UV–vis absorption spectra in wavelength region of 200–800nm were used to evaluate the optical properties like direct and indirect optical energy band gaps, optical absorption edge. The optical band gaps decreased with the increase in Na+ ion concentration. This suggests that NaF, as a dopant, is a good choice to improve the electrical properties of PEO/PVP polymer blend electrolytes.

CORRELATION BETWEEN WATER-BATH TEMPERATURE, SOLUTION VISCOSITY AND OPTICAL PROPERTIES OF SOL–GEL DERIVED V2O5 THIN FILMS

Thin films of V 2 O 5 were deposited on glass substrates by dip-coating technique using sol–gel route. The vanadium pentoxide sol was prepared by dissolving V 2 O 5 powder in H 2 O 2 solution. The solutions were kept in water bath and heated at five different temperatures for 30 min. The samples were annealed at 150°C for 60 min. The optical, structural, and morphological measurements were performed for characterization of samples. The effect of viscosity on optical constants (n, k, and Eg) of V 2 O 5 were studied. The indirect optical band gap energy of sol–gel V 2 O 5 thin films are about 2.33 eV and independent of viscosity within the error limits. The results show that the water-bath temperature strongly influences the viscosity of sol. The viscosity of sol was changed from 12.5 to 75 m Pa s. with increasing water-bath temperature from 50 to 90°C. The XRD measurements show that the structure of the V 2 O 5 films and powders is polycrystalline after annealing at 450°C. A correlation exists between the viscosity, morphology, and optical characteristics.

Dose Response and Optical Properties of Dyed Poly Vinyl Alcohol-Trichloroacetic Acid Polymeric Blends Irradiated with Gamma-Rays

Problem statement: The effects of gamma irradiation on optical properties of cresol-red dyed Poly Vinyl Alcohol (PVA) blended with Trichloroacetic Acid (TCA) for possible use in dosimetry and measurement of radiation dose in gamma rays have been studied using both Raman spectroscopy and UV-Visible spectrophotometer method. Approach: The dosimeters are composed of Poly Vinyl Alcohol (PVA), Trichloroacetic Acid (TCA) at various concentrations are 20, 25, 30 and 35%, and acid-base indicator cresol-red dyed. Results: The dosimeters were irradiated to doses up 12 kGy using 60Co gamma ray source at a constant dose rate. The polymeric films undergo color change from purple to yellow due to radiation-induced acid formation. The molecular vibrational spectra were measured using Raman spectroscopy, resulting in a decrease of the Raman intensity inelastic scattering of C-Cl molecular stretching from TCA with increasing dose. The absorption spectra were measured using UV-visible spectrophotometer in the wavelength range 350-700 nm, resulting in a decrease of the absorbance at 575 nm band peak with increasing dose. The dose sensitivity D0 increases with increasing TCA concentration for both scattering and absorption methods. The optical absorption studies show that the direct and indirect optical energy band gaps and optical activation energies are dependent on dose and TCA concentration. Conclusion: The shift in the optical band gap Eg values towards lower energy with radiation dose leads to a shift of the optical activation energy DeltaE value towards the lower energy region with increasing dose. The optical band gap (Eg) and the absorption edge decrease with increasing dose attributed to the structural disorder of polymer blends due to dehydrochlorination of trichloroacetic acid with increasing dose. The energy width of the tail of localized state in the forbidden band gap was evaluated using the Urbach-edges method. It was found that the activation energy (DeltaE) is less dependent of radiation dose but strongly dependent on concentration of blends.