Direct Forbidden Transitions Research Articles

Optical analysis of Cr-doped ITO films deposited by double-target laser ablation

We investigate the optical properties of ITO and Cr-doped ITO films deposited at room temperature by pulsed laser deposition onto amorphous SiO2 substrates. Our analysis approach is based on the Tauc's plot method applied to the absorption coefficient estimated by a route realistically describing the film structural features and including the contribution of the non-measurable film–substrate interface. Going beyond the conventional application of the Tauc's plot method, we quote two different transition energies for ITO and Cr-doped ITO and discuss their origin in the framework of a band-structure picture as a function of film thickness, Cr changes of the host ITO dispersion and Cr-doping content. In contrast to the conventional optical ITO description, we account for the existence of direct dipole forbidden transitions between the ITO fundamental band edges, involving different electronic and optical band gaps. Our results and discussion demonstrate that disregarding this theoretically established picture, as occurs in the experimental literature, would lead to conclusions inconsistent with the Cr-induced band occupation and effects on ITO dispersions.Preliminary optical (based on transmittance and reflectance spectra as well as band-tailing effects), electrical and structural inspection of the samples are also considered to check reliability and consistency of our discussion.

Synthesis, conductivity and photophysical properties of soluble low bandgap poly{(3-butyryl)pyrrole-[2,5-diyl(p-hydroxybenzylidene)

In this paper, a novel soluble low bandgap π-conjugated polymer, poly{(3-butyryl)pyrrole-[2,5-diyl(p-hydroxybenzylidene)]} (PBPDHBE), was synthesized and characterized by 1H NMR, FT-IR, EA and GPC. The glass transition temperature (Tg) of PBPDHBE was found to be 272.11°C that was about 70°C larger than that of PBPDHBA. PBPDHBE was decomposed mainly by two steps and the decomposition temperature of backbone chain was up to 467.16°C. The conductivities of PBPDHBE doped with FeCl3 and H2SO4, were measured to be 8.1×10−7 and 6.7×10−6Scm−1, respectively. The absorption peaks of PBPDHBE in NMP solution and film were located at around 462nm and 482nm, respectively, and the emission wavelength maximum at 529nm was also observed. PBPDHBE had a low optical bandgap and the optical bandgap of PBPDHBE film for indirect allowed and direct forbidden transitions were measured to be 1.15eV and 1.42eV, respectively. The resonant third-order nonlinear optical susceptibility of PDPBHBE measured by DFWM at 532nm was in the order of 10−8esu, which was about 1–3 orders of magnitude larger than that of the other ordinary π-conjugation polymers, because of its low bandgap and the substituent groups on the main chain of PBPDHBE.

Band structure and optical transitions in LaFeO3: theory and experiment

The optical absorption properties of LaFeO3 (LFO) have been calculated using density functional theory and experimentally measured from several high quality epitaxial films using variable angle spectroscopic ellipsometry. We have analyzed the calculated absorption spectrum using different Tauc models and find the model based on a direct-forbidden transition gives the best agreement with the ab initio band gap energies and band dispersions. We have applied this model to the experimental data and determine the band gap of epitaxial LFO to be ∼2.34 eV, with a slight dependence on strain state. This approach has also been used to analyze the higher indirect transition at ∼3.4 eV. Temperature dependent ellipsometry measurements further confirm our theoretical analysis of the nature of the transitions. This works helps to provide a general approach for accurate determination of band gaps and transition energies in complex oxide materials.

Effect of Ga addition on optical properties of crystalline Ga20Se80 system

GaxSe100−x (20≤x≤50) in polycrystalline form was prepared by direct fusion of stoichiometric proportions of pure elements. The spectral behavior of transmittance (T) and the reflectance (R) in the wavelength range 400–2500nm for all films of different thicknesses were measured to obtain different optical parameters (refractive index, n, and absorption index, k). The study of inter-band transitions indicates that the existence of direct forbidden transitions and indirect forbidden transitions with energy gaps decrease with increasing Ga percentage.

Light absorption mechanisms in sodium vanadium bronze thin films in electrochromic cells

Light absorption mechanisms in electrochromic sodium vanadium bronze thin films presenting a phase mixture of hydrated NaV6O15 (predominant component) and Na1.1V3O7.9 were studied in this work. Light absorption spectra were analyzed by using the combination of our method of successive extraction of contributions in the weak absorption range and the standard method for determination of the optical bandgap in the strong absorption region, which is related to direct allowed transitions of electrons. Bandgaps connected to different contributions of indirect allowed, direct forbidden and direct allowed transitions have been determined for three values of the voltage (−2 V, 0 V and +2 V) applied on an electrochromic cell containing the studied sodium vanadium bronze thin film. Optical bandgap (direct allowed transitions) at zero voltage has been determined to be 2.986 eV. Three indirect allowed transitions with bandgap values of 1.398 eV, 2.377 eV and 2.605 eV have been identified, involving absorption/emission of phonons with energy of about 50 meV. Another one direct forbidden transition between points separated for 2.279 eV has been found. Positive voltage applied on the cell decreases all the bandgap values, while the negative voltage increases them.

A Study of the Structural and Optical Properties of SnS:F prepared by chemical spray pyrolysis technique

Thin films of pure tin mono-sulfide SnS and tin mono-sulfide for (1,2,3,4)% fluorine SnS:F with Thicknesses of (0.85 ±0.05) ?m and (0.45±0.05) ?m respectively were prepared by chemical spray pyrolysis technique. the effect of doping of F on structural and optical properties has been studied. X-Ray diffraction analysis showed that the prepared films were polycrystalline with orthorhombic structure. It was found that doping increased the intensity of diffraction peaks. Optical properties of all samples were studied by recording the absorption and transmission spectrum in range of wave lengths (300-900) nm. The optical energy gap for direct forbidden transition and indirect allowed transition were evaluated It is found that for doping less than 2% the optical energy gap increases as the percentage of doping increases in the samples while for doping more than 2% the values of the optical energy gap decreases as the percentage of doping increases.

A Study of the Structural and Optical Properties of SnS:F prepared by chemical spray pyrolysis technique

Thin films of pure tin mono-sulfide SnS and tin mono-sulfide for (1,2,3,4)% fluorine SnS:F with Thicknesses of (0.85 ±0.05) ?m and (0.45±0.05) ?m respectively were prepared by chemical spray pyrolysis technique. the effect of doping of F on structural and optical properties has been studied. X-Ray diffraction analysis showed that the prepared films were polycrystalline with orthorhombic structure. It was found that doping increased the intensity of diffraction peaks. Optical properties of all samples were studied by recording the absorption and transmission spectrum in range of wave lengths (300-900) nm. The optical energy gap for direct forbidden transition and indirect allowed transition were evaluated It is found that for doping less than 2% the optical energy gap increases as the percentage of doping increases in the samples while for doping more than 2% the values of the optical energy gap decreases as the percentage of doping increases.

Effect of Heat Treatment on Structural and Electronic Transition of Nano-Crystalline Titanium Dioxide Film

Nanocrystalline of titanium dioxide nano films were deposited by spin coating method on silicon and ITO-glass substrates at 2000 rpm, in which starting material of Titanium Isopropoxide (C12H28O4Ti).X-ray diffraction (XRD) patterns confirmed thatpolycrystalline TiO2 anatase phase formation. The intensity of XRD peaks increases with the increase in heat treatment and better crystallinity takes place at higher temperature. The morphology of deposited films were characterized by atomic force microscope (AFM); with increasing heat treatment, both the particle size and surface roughness increased .The particle size value were 2.184,2.374,4.834,5.125, and 8.336 nm and RMS roughness values were 0.161, 0.223,0.552,0.810 and 1.494 nm for films deposited at 250, 350, 450, 550 and 650 o C respectively. Optical properties measurements (Transmittance ,T , and Absorptance , A ,) of (TiO2) films showed high transparency. It is observed that maximum transmittance at 250 o C for wavelength range 320-900 nm. The optical band gap of the films has been found to be in the range 3.28-3.62 eV for the forbidden direct electronic transition and 3.45-3.75 eV for the allowed direct transition for the different heat treatment. Keywords - Nanoanatase TiO2, Sol-Gel, XRD,AFM,Transmittance, Absorptance

Influence of post-annealing on electrical, structural and optical properties of vanadium oxide thin films

Vanadium oxide thin films were grown onto quartz substrates using the pulsed DC reactive magnetron sputtering technique at room temperature and afterwards post annealed under vacuum conditions in the temperature range from 75 to 230°C. The electrical resistance, temperature coefficient of resistance (TCR), optical energy gap and structural properties were investigated. The films are amorphous, nanoscale grained V2O5 phase and the mean grain size increases with increasing temperature. Additionally, the post-annealing at 230°C induces formation of both V2O5 and V4O9 phases and pinholes on the film surface. The temperature dependent variation of the electrical resistance indicates two activation energy areas corresponding to two TCR values for the films post annealed up to 180°C, but only one activation area was found after annealing at 230°C. Analyses of the absorption coefficient versus photon energy revealed a direct forbidden transition. The mean grain size and TCR values increase with increasing post-annealing temperature, whereas the optical energy gap and electrical resistance do not follow this tendency. The evolution of the structure and its correlation to the optical energy gap, electrical resistance, activation energy and TCR were discussed by means of the results obtained in the present study.

Isolation, structural, spectral, and thermal studies of imperatorin micro-crystals from Prangos pabularia

Imperatorin (mol. formula = C16H14O4, mol. mass = 270) micro-crystals were isolated from ethyl acetate extract of the root parts of the plant Prangos pabularia. The composition related structural, spectral, and thermal properties were investigated. All the crystals were found to have single phase with orthorhombic structure. UV–Visible spectrum showed both direct forbidden transition and indirect allowed transition. The calculated optical band gaps (Eg) were 3.64 and 3.70 eV for direct forbidden and indirect allowed transitions, respectively. From the thermal studies of these crystals, melting point, stability, and other relevant information was obtained, ensuring the suitability of the material for application in any device where the crystals may have to withstand this range of temperature.

Study of optical properties of highly oriented nanocrystalline V2O5·nH2O films doped with K ions

Optical transmission and reflection measurements of highly oriented nanocrystalline KxV2O5·nH2O films (0 ≤ x < 0.01) were studied. The optical constants such as, refractive index, the extinction coefficient, absorption coefficient, optical band gap have been calculated. The optical spectra of all samples exhibited two distinct regions of optical gap, Eop1 suggesting a direct allowed transition with optical gap ranging from 0.37 up to 0.42 eV and Eop2 suggesting a direct forbidden transition with optical gap ranging from 2.02 up to 2.23 eV. This indicates that KxV2O5·nH2O films have more than one type of conduction mechanism.

Optical properties of N-doped Cu2O films and relevant analysis with first-principles calculations

N-doped Cu2O films are deposited at different temperatures by sputtering a CuO target in the mixture of Ar and N2. By the analysis of transmission spectra, it is found that the N-doped Cu2O films are changed into a direct allowed band-gap semiconductor and the optical band gap energy is enlarged to 2.52±0.03 eV for the films deposited at different temperatures. The first-principles calculations indicate that the energy band gap increase by 25%, which is in good agreement with the experimental result. The change from a direct forbidden band-gap transition to a direct allowed band-gap transition can be attributed to the occupation of 2p electrons of N at the top of valence band in the N-doped Cu2O film.

Absorption coefficient of bulk and thin film Cu 2O

The optical absorption coefficient of thin film and bulk Cu 2O at room temperature is obtained from an accurate analysis of their transmittance and reflectance spectra. These absorption spectra are modeled, together with the low temperature data reported in the literature, using an analytical expression to assess and quantify the role of the different absorption mechanisms. The results suggest that direct forbidden transitions and indirect transitions play an almost equally relevant role. A table of the optical constants of Cu 2O single crystal is given for reference.

Effect of deposition time on structural, electrical, and optical properties of SnS thin films deposited by chemical bath deposition

The effect of deposition time on the structural, electrical and optical properties of SnS thin films deposited by chemical bath deposition onto glass substrates with different deposition times (2, 4, 6, 8 and 10 h) at 60 °C were investigated. The obtained films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and optical absorption spectra. All deposited films were polycrystalline and had orthorhombic structure with small crystal grains. Their microstructures had changed with deposition time, and their compositions were nearly stoichiometric. Electrical parameters such as resistivity and type of electrical conduction were determined from the Hall Effect measurements. Hall Effect measurements show that obtained films have p-type conductivity and resistivity values of SnS films have changed with deposition time. For allowed direct, allowed indirect, forbidden direct and forbidden indirect transitions, band gap values varied in the range 1.30–1.97 eV, 0.83–1.36 eV, 0.93–1.49 eV and 0.62–1.23 eV, respectively.

Research on the direct doping effect of silicon on cubic boron nitride ceramics by UV–VIS diffuse reflectance

Cubic boron nitride (cBN) micro-powders mixed with 1 wt% silicon were sintered at 1450 °C under a pressure of 5.0 GPa. The grain boundaries and silicon distribution in Si-cBN ceramics were studied by scanning electronic microscope (SEM) and energy-dispersive spectrometer (EDS). Optical properties of the ceramics were investigated by UV–VIS diffuse reflectance and photoluminescence spectra at room temperature. Some important parameters of studied ceramics such as absorption coefficient and defect levels were identified from reflection spectra by intercept method. The experimental results indicated the direct n-doped effect of silicon on cubic boron nitride ceramics. With a direct forbidden transition characteristic, the donor energy level of Si in forbidden zone of cBN ceramics was found to be 2.82 eV. The phonon energy related to the direct forbidden transition was 0.235 eV.

Spectroscopic study of nanocrystalline V 2O 5·nH 2O films doped with Li ions

Optical properties of nanocrystalline, Li x V 2O 5· nH 2O films (0< x<22 mol%), are explored in the present work. These films have been produced by the sol–gel technique (colloidal route), which was used for the preparation of high purity and homogeneity films. Optical measurements were carried out using a double-beam spectrophotometer. The optical constants such as refractive index n, the extinction coefficient k, absorption coefficient α, and optical band gap of the films material have been evaluated. The optical absorption coefficient was calculated from the measured normal reflectance, R, and transmittance, T, spectra. The optical spectra of all samples exhibited two distinct regions: at high energy, which suggests a direct forbidden transition with optical gap ranging from 1.75 to 2.0 eV and increases with increase in Li-content. On the other hand a second low-energy band suggests a direct allowed transition with optical gap ranging from 0.40 to 0.42 eV. The width of the localized states (band tail) E e was also estimated for all samples. Additional calculations applying the real part of the optical dielectric function led to the evaluation of the charge carrier concentration and their effective mass.

The temperature behavior of smart windows under direct solar radiation

The purpose of this paper was to investigate the variation in temperature of electrochromic devices under direct solar radiation and to compare the results with double-glazed glass. The devices consisted of a V 2O 5 layer as an ion storage film and a WO 3 layer as an electrochromic layer. The V 2O 5 and WO 3 films were prepared by thermal and electron beam evaporation, respectively. The optical properties and structures of these films were investigated. Both the ion storage film and the electrochromic layer were amorphous. The optical absorption was caused by a direct-forbidden transition in V 2O 5 and by an indirect-allowed transition in WO 3. The maximum temperatures under solar radiation were measured for colored and bleached devices, double glass and air, they were found to be approximately 63, 63, 53 and 36 °C, respectively. The rates of increasing temperature to the incident power density for colored, bleached devices and double glass were 0.051, 0.049 and 0.041 °C/(W/m 2), respectively.

Photoelectrochemical Study of the Band Structure of Zn2SnO4 Prepared by the Hydrothermal Method

It is fundamentally interesting to study the photoelectrochemical properties of complex oxides for applications in photovoltaics and photocatalysis. In this paper, we study the band gap (E(g)) and energetics of the conduction band (CB) and valence band (VB) for films of zinc stannate (Zn(2)SnO(4)) nanoparticles (ca. 25 nm) of the inverse-spinel structure prepared by the hydrothermal method. UV-vis spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), electrochemistry, and photoelectrochemistry were used to study the films. The fundamental E(g) for Zn(2)SnO(4) is proposed to be 3.6-3.7 eV with a direct-forbidden transition. The position of the CB was approximated from the flat band potential, E(fb), measured by the photocurrent onset potential. In aqueous and nonaqueous solutions the E(fb) of n-Zn(2)SnO(4) was found to be more positive than TiO(2) anatase in the electrochemical scale. In aqueous solutions E(fb) of Zn(2)SnO(4) was found to follow a 59 mV/pH slope with E(fb) extrapolated at pH 0 of 0.08 V vs NHE. In acetonitrile solutions that simulate the electrolyte for dye-sensitized solar cells (DSCs) the E(fb) of Zn(2)SnO(4) was found to be strongly dependent on electrolyte composition and more positive than TiO(2) vs the I(-)/I(3)(-) couple. The reverse trend observed for the open-circuit voltage in certain DSC electrolytes is explained in terms of the higher rates of electron-triiodide recombination of TiO(2) despite the lower position of the Zn(2)SnO(4) CB in the vacuum scale.

Valence-band characterization of AgGaSe2 thin films

AgGaSe2 thin films were prepared at post-deposition annealing temperatures from 100 to 350 °C for 15 min onto glass substrates by the stacked elemental layer deposition technique in vacuum. The structural properties of the films were ascertained by the x-ray diffraction method. The atomic compositions and optical properties of the films were measured by energy dispersive analysis of x-ray and UV–VIS–NIR spectrophotometry, respectively. The lattice parameters are independent and the grain size varies directly with post-deposition annealing temperatures. The films demonstrate compositional uniformity. The nature and extent of the band gap energy of the films on various post-deposition annealing temperatures have been analysed. Two types of electronic transitions are observed. Direct allowed and direct forbidden transitions vary from 1.67 eV to 1.75 eV and from 2.05 eV to 2.08 eV, respectively. The splitting of the valence band is proposed to be due to spin–orbit interaction. The spin–orbit splitting is 0.33 eV at 300 °C, which is nearest to the single crystal value. The p–d hybridizations vary directly while spin–orbit splitting varies inversely with post-deposition annealing temperature. The admixture of Ag 4d levels with otherwise p-like valence bands is calculated to be 13%.

Structural characterization and optical properties of Cd (1− x) Mn xSe thin films

Stoichiometric bulk ingot materials of the ternary mixture Cd (1− x) Mn x Se (0.05 ≤ x ≤ 0.9) were prepared by direct fusion of the constituent elements in vacuum sealed silica tubes. X-ray diffraction studies indicate that the investigated samples exhibited a hexagonal structure. The lattice parameters varied linearly with Mn content, following Vegard's law. Thin films were deposited by thermal evaporation from the pre-synthesized ingot material, onto glass substrates. X-ray and electron diffraction studies on the as-deposited and annealed films revealed an amorphous-to-crystalline phase transition at T a ≈ 423 K. EDAX studies on the prepared films show that the as-deposited films are nearly stoichiometric. The transmittance and reflectance of the deposited Cd (1− x) Mn x Se films were measured at normal light incident in the wavelength spectral range 500–2500 nm. Analysis of the transmittance spectra in the entire wavelength range allowed the determination of the refractive index. The dispersion parameters have been calculated, from which the static refractive index as well as static dielecric constant were calculated. Analysis of the absorption coefficient of the investigated films revealed the existence of both the allowed direct and forbidden direct optical transition mechanisms. The corresponding energies were estimated.