Direct Optical Band Gap Energy Research Articles

Optical and electrical properties of β-Ni (OH)2/reduced graphene oxide nanocomposite

Structural and optical properties of β-Ni(OH)2/reduced graphene oxide nanocomposite which was obtained by ultrasound dispersion of hydrothermally synthesized β-Ni(OH)2 and reduced graphene oxide were investigated. Frequency dependencies of conductivity for initial components and composite materials were studied. Specific surface areas of reduced graphene oxide, β-Ni(OH)2 and β-Ni(OH)2/reduced graphene oxide are 402, 20 and 88 m2/g, respectively. It was established that initial ultrafine β-Ni(OH)2 and β-Ni(OH)2/reduced graphene oxide composite have a percolation electrical conductivity. The enlarging of β-Ni(OH)2/reduced graphene oxide optical direct band gap energy compared to the initial components due to O/C contents changes was observed.

FTIR, electronic polarizability and shielding parameters of B2O3 glasses doped with SnO2

In this work, six glass samples with nominal compositions (35Li2O–10ZnO–55B2O3) + (xSnO2: 0 ≤ x ≤ 3 wt%) have been prepared by solid state reaction method. Spectra of UV–visible absorption for these glasses have been performed in wavelength within the range 200–1100 nm. FTIR has been recorded in the range of 4000–400 cm−1 to estimate the vibrational modes in the samples. The direct and indirect optical energy band gap ( $$E_{{\text{g}}}^{{{\text{ASF}}}}$$ ) and the corresponding refractive index (n) have been calculated by absorption spectrum fitting model. Molar refraction (Rm), polarizability (αm), reflection loss (RL), and optical transmission (T) for the glass samples have been evaluated. Moreover, the mass attenuation coefficients (µ/ρ) have been evaluated using the Monte Carlo code (MCNPX, version 2.6.0) in the energy range 0.356–1.33 MeV to understand the radiation shielding properties for the prepared glasses. From the µ/ρ values, we have calculated some other parameters such as effective atomic number (Zeff), the half value layer, and the mean free path for the present glass samples. The results revealed that the investigated glass samples are promising for the laser stimulated nonlinear optics and the composition with the highest value of SnO2 content (3.0 wt%) is encouraging candidate for nuclear radiation shielding.

Physical, structural and optical studies on magnesium borate glasses doped with dysprosium ion

Intense visible emissions from dysprosium (Dy3+) ions doped glasses became prospective for diverse technological applications. In this paper, physical, optical and structural properties of magnesium borate glasses doped with varied concentrations of Dy2O3 were examined. Such glasses were synthesised by melt quenching method and characterized at room temperature using several analytical techniques. Luminescence and absorption spectra (in the visible region) of as-quenched samples were used to evaluate the physical and optical properties. XRD pattern confims the amorphous state of as-quenched samples. The Fourier transform infrared (FTIR) spectra of glasses reveal various bonding vibrations assigned to different functional groups. UV-vis-NIR spectra disclose eight absorption bands accompanied by a band for hypersensitive transition positioned at 1260 nm (6H15/2 → 6F11/2). The values of direct and indirect optical energy band gap of the studied glasses are decreased with the increase of Dy3+ ion contents. The photoluminescence spectra of all glasses under the excitation of 380 nm display two prominent emission bands centred at 497 nm (4F9/2 → 6H15/2, blue) and 587 nm (4F9/2 → 6H13/2, green). The achieved intense luminescence from the proposed glass composition may be beneficial for solid-state laser applications.

Effects of GNP addition on optical properties and band gap energies of PMMA films

In this study, the effects of graphene nanoplatelet (GNP) doping on the optical parameters (absorption coefficient, α and extinction coefficient, k) and the optical transition energies (optical band gap and Urbach energies) of poly(methyl methacrylate)/graphene nanoplatelet (PMMA/GNP) composite films were studied. PMMA/GNP composite films with various GNP mass fractions were prepared by spin coating technique. The absorbance (A) changes of the prepared composites were measured by using UV–Vis technique. The α and k values of the composites were obtained from UV–Vis data. The observed rapid increase in A values in UV‐region were associated with the optical transitions of GNP electrons from valence to conduction band. The direct (Ed) and indirect (Ei) optical band gap energies of the composites were determined by using Tauc method. Both of the band gap energies were decreased when the GNP content in the PMMA matrix is increased. The decrease in the band gap energies was interpreted as evidence of increased conductivity of the composites. Additionally, energies of the band tails (Urbach energy) were calculated. It was seen that the Urbach energy levels were increased with GNP content. POLYM. COMPOS., 40:1862–1869, 2019. © 2018 Society of Plastics Engineers

Effect of substrate temperature on the structural, morphological and optical properties of copper bismuth sulfide thin films deposited by electron beam evaporation method

Copper bismuth sulfide thin films were deposited at 200 °C, 300 °C, 400 °C and 500 °C on the glass substrates by electron beam evaporation method. X-ray diffraction study revealed that the copper bismuth sulfide films of single and mixed phases were formed as a function of substrate temperatures. Substrate temperature of 300 °C and 400 °C formed single phase Cu4Bi4S9 and Cu4Bi5S10 films respectively whereas substrate temperature of 500 °C formed mixed phases of Cu4Bi4S9 and Cu4Bi5S10 film. Crystallite size, dislocation density and microstrain of the films were modified by the various substrate temperatures. Surface morphology of the film Cu4Bi5S10 deposited at 400 °C examined by scanning electron microscopy showed the distribution of spherical shaped particles on the film surface. The presence of copper, bismuth and sulfur elements in the deposited films was confirmed using energy dispersive spectral studies. The calculated direct optical band gap energy of the films deposited at different substrate temperature varied from 1.47 to 1.64 eV and the absorption coefficient is in the order of 106 cm−1.

Effect of asymmetrical peripheral substitution of sulfonic acid group on the geometric and electronic structures and vibrations of copper phthalocyanine studied by computational and experimental techniques

The optimized geometry and vibrational frequencies of an organic compound copper phthalocyanine tetrasulfonic acid tetra sodium salt (CuPcTS) have been investigated using density functional theory. We have also optimized the structures of copper phthalocyanine (CuPc) and its substituted complex and simulated their vibrational spectra to study the substitution effects of sulfonic acid group at the peripheral sites of copper phthalocyanine. Experimentally, vibrational frequencies of these molecules have been studied using infrared absorption, Raman spectroscopic techniques. It has been found that the vibrational frequencies obtained from the theoretical studies generally agree with the experiment. Most of the bands show red shift in CuPcTS due to decrease of stretching force constant of CC and reduction in the contribution from in-plane CH bending vibrations. New bands have been observed due to SO vibrations. Degeneracy of IR bands is removed because of change of symmetry from D4h in CuPc to C1 in substituted CuPc. Effect of substitution on the structural parameters and mode compositions of the vibrational bands have also been studied. The electronic absorption spectra of the substituted copper phthalocyanine in different solvents have been studied using UV–Vis spectroscopy. It has been found that the substituted copper phthalocyanine shows aggregation effect in water because of greater extent of hydrogen bonding. The direct optical band gap energy and refractive index of CuPcTS have also been calculated in the different solvents. We have also plotted the IV characteristic for both the molecules and it has been found that electrical and photoconductivity of CuPcTS is more as compared to CuPc. The intermolecular charge transfer interactions have been studied by HOMO-LUMO and natural bonding orbital (NBO) analysis. The values of first order hyperpolaizability for both the molecules indicate that substituted copper phthalocyanine acquires excellent non-linear optical (NLO) properties. The interaction sites of the both the molecules have been studied by the molecular electrostatic potential (MEP) and solvent accessible surface area (SASA) analysis.

The effect of Sn on the physical and optical properties of (Se0.6As0.1Ge0.3)100-xSnx glasses

Chalcogenide glasses are significant materials for semiconducting and infrared imaging because of their infrared (IR) transparency. In the present study, Se-Ge-As based chalcogenide glasses with chemical composition of (Se0.6As0.1Ge0.3)100-x Snx doped with different amounts of Sn were prepared by conventional melting-quenching method. Physical properties of glasses were investigated by studying density and molar volume. Based on differential scanning calorimetry (DSC) results and density measurements, the binary role of Sn was proved. Fourier transform infrared spectroscopy (FT-IR) study of the samples approved presence of impurities. Using the UV–Vis spectra, optical properties including Fermi energy level, direct and indirect optical band gap and Urbach energy were determined. The results showed a decrease in density of glasses with the increase of Sn from 2 to 6 mol%. Increasing Sn in the glassy microstructure of samples provides a semiconducting character to Se based chalcogenide glass by reducing the direct and indirect optical band gaps of glass samples from 1.29 to 1.15 (eV) and 1.16 to 1.01 (eV), respectively.

Photoelectrochemical performance of dye and semiconductor sensitization on 1-D hollow hexagonal ZnO rods: A comparative study

One-dimensional hollow hexagonal ZnO rods (1-D HHZRs) were grown onto the SnO2: F (FTO) coated glass substrates by using two-step deposition techniques. Initially, the ZnO seed layer was coated onto the FTO which was followed by hydrothermal route in order to grow 1-D HHZRs. These 1-D HHZRs were decorated with Eosin-Y dye and CdS semiconductor nanoparticles (NPs) by using a chemical routes like dip coating and successive ionic layer absorption reaction (SILAR) technique, respectively to improve its photoelectrochemical (PEC) performance. The structural, morphological, optical and electrochemical characterizations of the thin films were analyzed by various sophisticated instruments. X-ray diffraction (XRD) pattern corroborated the phase formation of ZnO and CdS with the hexagonal and cubic crystal structure, respectively. 1-D HHZRs films were obtained by scanning electron microscopy (SEM) with rod diameter of about 1.47 μm. Furthermore, SEM image clearly showed the CdS NPs covered 1-D HHZRs. The direct optical bandgap energy of the samples were estimated to be 3.28, 3.24 and 3.03 eV. CdS NPs-sensitized 1-D HHZRs samples showed hydrophilic nature for water contact angle, which is advantageous for the better improvement in the PEC performance as compared with pristine and dye-sensitized 1-D HHZRs.

ZnO nanoparticles dispersed PVA–PVP blend matrix based high performance flexible nanodielectrics for multifunctional microelectronic devices

Polymer nanocomposite (PNC) films based on the blend matrix of poly(vinyl alcohol) (PVA) and poly(vinyl pyrrolidone) (PVP) (50/50 wt%) incorporated with zinc oxide (ZnO) nanoparticles (i.e., (PVA–PVP)–x wt% ZnO; x = 0, 1, 3 and 5) were prepared by solution-cast method. The behaviour of polymer-polymer and polymer-nanoparticle interactions in the PNC films was ascertained by employing X-ray diffraction, energy dispersive X-ray, and Fourier transform infra-red spectroscopies. Scanning electron microscopy and atomic force microscopy were performed for the morphological characterization, whereas the thermal and optical properties of the PNC films were investigated by using differential scanning calorimetry and ultraviolet–visible spectroscopy, respectively. The dielectric and electrical behaviour of these PNC materials were determined by employing the dielectric relaxation spectroscopy over the frequency range from 20 Hz to 1 MHz. The influence of ZnO concentration on the degree of PVA crystalline phase and the crystallite size, surface morphology and roughness of the films, the glass phase transition and melting phase transition temperatures, direct and indirect optical energy band gap, refractive index, complex permittivity, electrical conductivity, activation energy and the structural dynamics of these PNC materials were explored. The investigated properties of the PNC films were credited to an innovation and engineering of novel high performance flexible nanodielectrics in the area of advanced functional materials for their promising applications especially in the next generation optoelectronic, gas sensor and microelectronic devices.

Physical, structural and luminescence investigation of Eu3+-doped lithium-gadolinium bismuth-borate glasses for LEDs

The aim of the current report is to fabricate Eu3+-doped glasses with the chemical composition of 50Li2O-15Gd2O3-5Bi2O3-(30-x)B2O3-xEu2O3 (where x = 0.5, 1.0, 1.5, 2.0 and 2.5 mol%), with the help of conventional melt quenching technique. The fabricated glasses have been studied with help of physical, structural and luminescence properties for application of LEDs. The structural properties were investigated by XRD and FTIR spectra. Physical properties have been measured. Direct and indirect optical energy band gap (Eg) have been calculated and found to be increasing with Eu2O3 concentration. Luminescence spectra have been observed from photo and radioluminescence spectra and found in good agreement with each other, however the concentration quenching was not determined for the samples. The high-covalence and asymmetric nature was confirmed from Photoluminescence emission and RL emission transition as well as from the higher values of luminescence intensity ratio. The JO parameters have been found for the better performance of lasing materials. The lifetime's data have been found to be decreasing from 1.64 to 1.50 ms, which is the confirmation of energy transfer in Eu3+ ions through cross relaxations. From the calculated properties it has been suggested that the present glass samples might be good for red-light emitting devices.

Thermal evolution of physical properties of evaporated CdS thin films for perovskite solar cell applications

The thermal evolution to microstructural and optoelectrical properties of CdS thin films have been investigated in this short communication where the films were grown on glass and indium-doped tin oxide coated substrates using electron-beam evaporation. The pristine (as-deposited) films were annealed in air atmosphere within temperature range of 150–450 °C. The XRD patterns indicated that the formation of cubic phase with F4¯3m space group of deposited films along with (111) preferential orientation. The transmittance and direct optical energy band gap were observed to increase with annealing and the current-voltage characteristics showed ohmic behaviour while conductivity is decreased with annealing. The films are found to be homogenous, well covered and without any voids and cracks and the EDS analysis confirmed the deposition of CdS films. The experimental results warrant that the annealed films may be used in perovskite solar cells as electron transport layer.

Structural, Optical and Mössbauer Study of Ba1 − xCuxFe12O19 (0.5 ≤ x) Nano Hexaferrites

In this study, a novel Ba1−xCuxFe12O19 (0.5 ≤ x) nano-hexaferrites were prepared by a simple and cost-effective sol–gel auto-combustion method using barium nitrates, iron nitrate, copper (II) acetate monohydrate and citric acid, and its structural, optical properties and hyperfine interactions were reported. Structural properties were analyzed through XRD (X-ray diffraction), Scanning electron microscopy (SEM), and TEM (Transmission electron microscopy), while percent diffuse reflectance spectroscopy (DRS) and Mossbauer spectrometer were used for analyzing the optical and magnetic properties of the resultant products. The observed Mossbauer studies proved the ferromagnetic nature of nanoparticles (NPs) samples. The crystallite size (XRD) varies in a range of (23.30–35.12) nm. The direct optical energy band gap (E g ) of all samples were calculated by Tauc plots where the E g values are found in a small range of 1.97–2.15 eV. The experimental evidences signify the promising use of newly prepared nano-hexaferrites in the development of materials in various industrial devices and far better than the conventional available hexaferrites materials.

Ternary transition titanium-niobium trisulfide as photoanode for assisted water splitting

Ternary titanium-niobium trisulfide, [TixNb1-xS3 (Nb rich) and NbxTi1-xS3 (Ti rich), x<(1-x)], is explored as photoanode in photoelectrochemical cells (PEC) and compared with their respectives binary sulfides, TiS3 and NbS3. Investigated samples were successfully synthesized by solid-gas reaction of metal and sulfur vapor in a vacuum-sealed ampoule at 550 °C during 20h. They were characterized by X-ray diffraction (XRD), energy dispersive analysis of X-ray (EDX), Raman spectroscopy (RS) and scanning electron microscopy (SEM). Whereas samples of binary trisulfides were univocally identified as polycrystalline nanoribbons of monoclinic TiS3 (P21/m) and triclinic NbS3 (P1¯2), respectively, the structure of the ternary trisulfide depends on the dominant presence of one of the metals: Ti (monoclinic (P21/m)) or Nb (triclinic (P1¯)). Optical absorption coefficients were determined from reflectance and transmittance measurements. The TixNb1-xS3 and NbxTi1-xS3 direct optical bandgap energies obtained are 1.0±0.1 eV. Photoelectrochemical measurements in 0.5 M Na2SO3 with a platinum counter electrode have been carried out to characterize the semiconductor/electrolyte interface. The flat band potentials (TiS3-Vfb = -0.64 ± 0.02V NHE, NbS3-Vfb = -0.73 ± 0.02V NHE, NbxTi1-xS3-Vfb = -0.63 ± 0.05V NHE and TixNb1-xS3V-fb = -0.72 ± 0.05V NHE) have been measured by Electrochemical Impedance Spectroscopy (EIS) and they have been used, beside the bandgap energy, to depict the energy band levels scheme. Hydrogen fluxes (photogenerated under 200 mW/cm2 white light illumination in a PEC at 0.3 V bias potential) have been quantified as a function of illumination time by quadrupole mass spectrometry (QMS) reaching values up to 2.2±0.1 μmolH2/min⋅cm2 in the best scenario.

The impact of sodium chloride as dopant on optical and electrical properties of polyvinyl alcohol

Solid polymer electrolytes based on polyvinyl alcohol (PVA) complexed with different amounts of sodium chloride (NaCl) (0.1, 0.2, 0.3, 0.4 and 0.5 g) were prepared using solution cast method. The effect of NaCl dopant amount on the optical properties was characterized using UV-Vis spectroscopy. Absorbance spectra, direct and indirect optical band gap energies of PVA/NaCl samples were studied. These band gap energies decreased as NaCl amount in PVA matrix increased. The Urbach energy of samples was also calculated using the Urbach relation which increased significantly as the NaCl amount increased due to the increase in the disorder of the polymer occurred by doping. The electrical conductivity of PVA/NaCl electrolyte samples has been estimated by DC conductivity as function in the NaCl amount at room temperature. DC conductivity of prepared electrolytes containing different amounts of NaCl salt increased from 2.06 × 10⁻² S/m at doped amount 0.1 g of NaCl to 4.60 × 10⁻² S/m at doped amount 0.5 g of NaCl. This increasing was due to the increase in the degree of conjugation of π-orbitals, thus, the increase in the polarons amount in PVA matrix.

Structure, surface topography and optical characterization of Ag co – doped Cd1-xCuxO nanostructure thin films

Cd1-xCuxO and Ag co-doped Cd1-xCuxO nanostructure thin films with x = 0.2 and different Ag content (0-8%) were deposited by chemical spray pyrolysis technique on glass substrates at a temperature of 350 oC. The XRD results showed that the prepared films have polycrystalline with low crystallinity nature for CdO cubic structure. The preferential orientation of all films was absorbed along (111) plane. CuO monoclinic phase has appeared with low intensity while the Ag cubic phase appeared only in 8% of Ag content. Structural parameters such as average crystallite size, dislocation density and micro-strain were also investigated. SEM images revealed that the surface morphology of the films consists of spherical shaped grains uniformly distributed without detectable micro-cracks and improved by Ag Co-doping. EDXS spectrum analysis confirmed purity and stoichiometry of the prepared compositions. AFM results showed that the surface topography and the surface quality of the deposited thin films can be controlled by the variation of the Ag co-doping concentration. Optical absorbance and transmittance of Ag co-doped Cd1-xCuxO thin films has high values in the visible and near infrared regions respectively and varied with Ag co-doping content. Direct optical energy band gap of Cd1-xCuxO exhibits a blue shift with Ag co-doping, due to the quantum size and Burstein–Moss (BM) effects. The increasing of optical energy gap was confirmed by the decrease in the Urbach tails energy EU after Ag co-doping..

CATHODIC ELECTRODEPOSITION OF Cu 4 SnS 4 THIN FILMS FROM ACIDIC SOLUTION

In this work the synthesis of copper tin sulfide thin films by electrodeposition is carried out. The films were deposited onto ITO glass substrates from an aqueous solution bath containing copper sulfate, tin chloride and sodium thiosulfate at pH 1 and room temperature. Prior to the deposition, a cyclic voltammetry experiment was carried out between two potential limits (+1000 to -1000 mV versus Ag/AgCl) to probe the effect of the applied potential and to determine the most likely suitable electrodeposition potential for the deposition of copper tin sulfide. The deposition was attempted at various cathodic potentials such as -400, -600, -800, -1000 mV to determine the optimum deposition potential. The films have been characterized by techniques such as optical absorption, Xray diffraction and atomic force microscopy. The XRD patterns show that the films are polycrystalline with orthorhombic structure. The AFM studies reveal the electrodeposited films were smooth, compact and uniform at deposition potentials of –600 mV versus Ag/AgCl. The direct optical band-gap energy was obtained to be 1.58 eV.

CATHODIC ELECTRODEPOSITION OF Cu 4 SnS 4 THIN FILMS FROM ACIDIC SOLUTION

In this work the synthesis of copper tin sulfide thin films by electrodeposition is carried out. The films were deposited onto ITO glass substrates from anaqueous solution bath containingcopper sulfate, tin chloride and sodium thiosulfate at pH 1 and room temperature. Prior to the deposition, a cyclic voltammetry experiment was carried out between two potential limits (+1000 to -1000 mV versus Ag/AgCl) to probe the effect of the applied potential and to determine the most likely suitable electrodeposition potential for the deposition of copper tin sulfide. The deposition was attempted at various cathodic potentials such as -400, -600, -800, -1000 mV to determine the optimum deposition potential. The films have been characterized by techniques such as optical absorption, X-ray diffraction and atomic force microscopy. The XRD patterns show that the films are polycrystalline with orthorhombic structure. The AFMstudies reveal the electrodeposited films were smooth, compact and uniform at deposition potentials of –600 mV versus Ag/AgCl. The direct optical band-gap energy was obtained to be 1.58 eV.

Effect of thermal annealing on structural, optical and electrical properties of transparent Nb2O5 thin films

We report on the structural, optical and electrical properties of RF sputtered Niobium pentoxide (Nb2O5) thin films. The structural studies have been carried out using XRD, and AFM techniques. Optical constants such as optical band gap energy, absorption coefficient, refractive index, complex dielectric constant and optical conductivity have been estimated for as-deposited and annealed Nb2O5 thin films. The estimated direct optical band gap energy values were found to be Egd=3.62 eV for the as deposited which decreases to Egd=3.07 eV for the annealed films at 973K. The dispersion curves of the refractive index of Nb2O5 thin films in the optical transparency region are explained by using single oscillator and Drude models. The correlations between optical parameters and the annealing temperature of the Nb2O5 thin films are discussed. The DC activation energy has been estimated by using two probe technique.Mott’s variable range hopping conduction process (VRH) and small-polaron hopping have been used to understand DC electrical conductivity.

Structural, optical and vibrational properties of Cr2O3 with ferromagnetic and antiferromagnetic order: A combined experimental and density functional theory study

Chromium oxide (Cr2O3) thin films have been synthesized on glass substrates by the spray pyrolysis technique. The structural, morphological and optical properties of the sample have been studied by X-ray diffraction (XRD), Raman spectroscopy, FTIR spectroscopy, scanning probe microscopy and UV–vis spectroscopy respectively. X-ray diffraction results reveal that as deposited film is polycrystalline with a rhombohedral corundum structure and a preferential orientation of the crystallites along the (104) direction. IR and Raman spectra were recorded in the 100–900cm−1 range and the observed modes were analysed and assigned to different normal modes of vibration. The direct optical band gap energy value calculated from the transmittance spectra of as-deposited thin film is about 3.38eV. We employ first principles calculations based on density functional theory (DFT) with the B3LYP hybrid functional and a coupled perturbed Hartree–Fock/Kohn–Sham approach (CPHF/KS). We study the electronic structure, optimum geometry, and IR and Raman spectra of ferromagnetically and antiferromagnetically ordered Cr2O3. The computed results are consistent with the experimental measurements, and provide complete vibrational assignment, for the characterization of Cr2O3 thin film materials which can be used in photocatalysis and gas sensors.

Study of the crystalline and optical properties of lithium tantalate thin films deposited by high power RF magnetron sputtering

Lithium tantalate (LiTaO3) thin films were deposited by high power (250 W) RF magnetron sputtering on Pt/Ti/SiO2/Si(100) substrates and were annealed from 600 °C–700 °C in an O2 or N2 environment. The crystallinity of those films were examined by x-ray diffraction and Raman spectra to study the effects of annealing temperature and annealing gas on the crystal properties of LiTaO3 thin films. LiTaO3 thin films annealing at 650 °C in O2 have a better crystalline quality in a (012) preferential orientation. However, LiTaO3 thin films annealed at 700 °C in O2 contain impurities with titanium oxide (Ta2O5). This is attributable to the fact that the crystal structure is unstable above the Curie temperature and the tantalum reacts with oxygen in the annealing process. At the same time, the result of the Raman spectra suggests that LiTaO3 thin films annealed at 600 °C and 650 °C are closer to near-stoichiometric LiTaO3 and have less Li diffusivity and Li-deficient phases. Finally, optical constants of LiTaO3 films are also primarily probed by spectroscopic ellipsometry. For LiTaO3 thin films annealed at 650 °C in O2, the refractive index is 2.33, the extinction coefficient is 0.002 34 and the direct optical band gap energy is estimated to be minimal at 3.66 eV.