Band Gap Energy Region Research Articles

Probing the defects states in MAPbI3 perovskite thin films through photoluminescence and photoluminescence excitation spectroscopy studies

The luminescence properties of methylammonium lead iodide (MAPbI3) perovskite thin films deposited on corning 1737 glass substrates are studied using photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopy. From the PL spectra analysis, it is realized that there are two emission peaks, a high-intensity peak at 1.58 eV (~783 nm) and a smaller shoulder peak at 1.64 eV (~754 nm) in the bandgap energy region. The peak at ~1.64 eV corresponds to the bandgap of MAPbI3, whereas the high-intensity peak at photon energy ~1.58 eV is due to the presence of shallow trap states. The calculated optical bandgap energy 1.58 ± 0.01 eV is close to the reported bandgap energy 1.60–1.61 eV and consistent with the PL peak position. The slight difference in bandgap value is due to shallow defect states in MAPbI3 films. The slow decay of photocurrent observed in the transient photocurrent measurements further confirms the presence of shallow trap states in MAPbI3 perovskite.

Spectroscopic insight of low energy electron emission from diamond surfaces

Low work function materials are desirable in many applications such as electron emission and photocatalysis. We have studied low energy electron emission from low work function hydrogen terminated diamond surfaces via electron spectroscopy to gain insight into the mechanisms involved during electron excitation and emission. Electron emission was found to be dominated by electrons within the band gap energy region, allowed due to the negative electron affinity diamond surface, while sub-bandgap illumination was able to significantly increase emission current. Substantial upward surface band bending greater than 2 eV was observed for the diamond samples, which affect electron accumulation at the surfaces. Intra-bandgap states are shown to strongly influence electron emission behavior, which can have great implications for various energy conversion devices.

Optical properties of (Se80Te20)100-xZnx (2 ≤ x ≤ 6) amorphous thin films

The (Se80Te20)100-xZnx (2 ≤ x ≤ 6)thin film compositions have been prepared by melt quenching technique followed by electron-beam physical vapour deposition. X-ray diffraction pattern and SEM micrographs ascertain the amorphous nature of prepared thin films. The transmission spectra from UV/VIS/NIR double beam spectrophotometer (500 ≤ λ (nm) ≤ 2600) is used for optical characterization of thin films. The absorption coefficient and energetic parameter (A) for various compositions indicate that the absorption mechanism in Se-Te-Zn glassy system occur via allowed indirect transitions. The optical band gap for thin films lies at the edge of near infrared region in A-band (λ~900 nm).The average heat of atomization has no essential influence on the optical band gap energy values. The refractive index (n) for thin films exhibit normal dispersion in the wavelength range 800 nm ≤ λ ≤ 1100 nm (IR-A), and is nearly constant at longer wavelength λ ≥ 1400 nm (IR-B). The dispersion of refractive index below the band gap energy region is studied using the Wemple-DiDomenico (WDD) single oscillator model. It is found that, to a good approximation the single-oscillator energy scales with the optical band gap (Eo≈1.4Egopt). The deduced value of parameter β = 0.24 ± 0.01 corresponds to ionic character of bonds in the Se-Te-Zn system. The dielectric parameters (ε’, ε”,ɛ∞, ɛL), optical conductivity (σ1,σ2), ratio of carrier concentration to the effective mass (N/m*) have also been deduced for thin films. The calculated values of plasma frequency ωP and Nopt are in good agreement with carrier concentration of semiconductors. The energy loss occurred in the interior of the Se-Te-Zn thin films is found to be due to the interband electronic transitions.

Spherical and Dumbbell Shape Biphasic Paramagnetic ZnS:Fe Nanoparticles on Ferromagnetic ZnS Host Background

Spherical and dumbbell shape ZnS:Fe nanoparticles have been synthesized by the co-precipitation method with polyethylene glycol as a capping ligand. Biphasic formation from ternary chalcogenide ZnS:Fe is synthesized at different dopant concentrations (1–5 wt.%) which is confirmed by electron paramagnetic resonance (EPR) spectra. The effect of the doping on the structural, optical and magnetic properties is investigated. The x-ray diffractometer (XRD) result reveals the coexistence of wurtzite and zinc blend structures of ZnS:Fe. The x-ray photoelectron spectroscopy (XPS) studies confirm the presence of coexisting Fe3+ and Fe2+ ions in host ZnS. Undoped ZnS nanoparticles reveal the ferromagnetism whereas Fe doped ZnS nanoparticles exhibit paramagnetic nature in the presence of a magnetic field. The defects formation is confirmed by photoluminescence (PL) measurements. The Fe3+ doping enhances the absorbance of ZnS at the bandgap energy region (330 nm) and a wide absorbance is observed in the visible wavelength region (400–800 nm) that reveals the sub-bandgap formation.

Thickness-dependent surface proton conduction in (111) oriented yttria-stabilized zirconia thin film

Polycrystalline Zr0.92Y0.08O2 (YSZ) thin films were prepared by RF magnetron sputtering. 80- and 160-nm thin films exhibited (111) orientation and a polycrystalline structure, respectively. The 80-nm thin film had larger distortion than the 160-nm thin film. While the 80-nm thin film and a 120-nm thin film exhibited proton conduction, the 160-nm thin film did not, indicating that surface proton conduction can depend on film thickness. The activation energy of the 80- and 120-nm thin films measured in a wet atmosphere (0.52eV) was about half of that measured in a dry atmosphere. The enhancement of conductivity for the thin films may be attributed to distortion, which may change the structure around an oxygen vacancy at the YSZ grain surface, accompanied by possible enhancement of H2O adsorption. H2O-annealed thin film had a hydrogen-induced level in the band gap energy region. This is the first observation of hydrogen-induced level at the surface state of the YSZ thin film obtained by X-ray absorption spectroscopy.

The interface of SrTiO3 and H2O from density functional theory molecular dynamics.

We use dispersion-corrected density functional theory molecular dynamics simulations to predict the ionic, electronic and vibrational properties of the SrTiO3/H2O solid-liquid interface. Approximately 50% of surface oxygens on the planar SrO termination are hydroxylated at all studied levels of water coverage, the corresponding number being 15% for the planar TiO2 termination and 5% on the stepped TiO2-terminated surface. The lateral ordering of the hydration structure is largely controlled by covalent-like surface cation to H2O bonding and surface corrugation. We find a featureless electronic density of states in and around the band gap energy region at the solid-liquid interface. The vibrational spectrum indicates redshifting of the O-H stretching band due to surface-to-liquid hydrogen bonding and blueshifting due to high-frequency stretching vibrations of OH fragments within the liquid, as well as strong suppression of the OH stretching band on the stepped surface. We find highly varying rates of proton transfer above different SrTiO3 surfaces, owing to differences in hydrogen bond strength and the degree of dissociation of incident water. Trends in proton dynamics and the mode of H2O adsorption among studied surfaces can be explained by the differential ionicity of the Ti-O and Sr-O bonds in the SrTiO3 crystal.

A comparative run for visible-light-driven photocatalytic activity of anionic and cationic S-doped TiO2 photocatalysts: A case study of possible sulfur doping through chemical protocol

Photocatalytic degradation of pollutant molecules delivered great challenges for sustainable development of green energy in the field of environmental science. For this purpose, growing interests have been made to witness the fast development of new photocatalysts with improved catalytic efficiency and to monitor the reaction mechanism at the atomic and molecular levels. Therefore, doping of semiconductor metal oxide (e.g. TiO2) with main group elements, especially with sulfur atoms (S), has gained much interest due to the introduction of a localized band between the conduction bands (CB) and valence bands (VB) that increase the absorbance in the visible light region. More interestingly, photocatalytic practices of the S-doped TiO2 material are of pronounced significance due to photon-to-carrier conversion ability of S-doping beneath the band gap energy region of pure TiO2. Therefore, visible-light-activated S-doped TiO2 photocatalysts were prepared via template free and low-temperature oxidant peroxide method (OPM) assisted hydrothermal treatments. Experimental findings have revealed the successful incorporation of sulfur atoms into TiO2 crystal lattice and, as a result, substitution of Ti4+ by S6+ to form TiOS bonds for cationic S-doping was observed. Whereas, in the case of anionic S-doping, substitution of S2− by O2− to form OTiS bonds was achieved. More evidence was observed for the presence of chemisorbed sulfate groups on the surface of S-doped TiO2 samples and inhibition of crystallite size growth by S-doping, and obviously, upsurge the absorbance in the visible light region. The photocatalytic activity of as-prepared 1-D nanorods shaped photocatalysts and the mechanism involved for the photodegradation of organic molecules (methyl orange and phenol) under visible-light irradiation were investigated by adding different scavengers into the system solution to capture active species. It was found that in cationic S-doped TiO2 photocatalysts, chemisorbed hydroxyls (OHads−) and photoinduced holes (h+) played a major role in photocatalysis. Whereas, in the case of anionic S-doped TiO2 photocatalysts, electrons (e−) and photoinduced holes (h+) played the nearly equal role in photocatalysis.

Enhanced Photoelectrocatalytic Water Splitting at Hierarchical Gd3+:TiO2 Nanostructures through Amplifying Light Reception and Surface States Passivation

The influence of rare earth gadolinium (Gd3+) ion doping on optical and photoelectrochemical properties of TiO2 is studied. The hierarchical clump-type TiO2 nanostructure was fabricated using poly-vinyl acetate as soft-template. The optical absorbance quantity of TiO2 was strikingly promoted at bandgap energy region (380 nm) by Gd3+ doping, as well as it extend a wide absorbance in visible wavelength region (400 – 800 nm) elucidating the sub-bandgap formation. As a result, Gd3+:TiO2 exhibits high photocurrent density than undoped TiO2 in photoelectrocatalytic experiments. Another plausible reason for enhancing the photocurrent density at Gd3+:TiO2 was analyzed through electrochemical impedance spectroscopy. The underlying mechanism of surface states controlled charge transfer at TiO2/electrolyte interfaces affected the photoelectrocatalytic hydrogen fuel generation, and compete with Gd3+ ion doping through bottlenecking of photoelectrons trapping at surface states. The improved charge separation (e−/h+) at Gd3+:TiO2 result effective photoelectron collection and thus yield 180 % higher hydrogen gas (∼ 2.34 mL.h−1.cm−2 ) generation compare to pristine TiO2 (1.28 mL.h−1.cm−2) under UV light irradiation. The improved optical and charge transfer characteristics of hierarchical TiO2 by Gd3+ ions can be implemented to wide range of other metal oxide based photocatalytic fuel generation.

Growth of TiO2−δ thin film by RF magnetron sputtering using oxygen radicals and Ti metal

TiO2−δ thin films have been deposited by RF magnetron sputtering using oxygen radicals. The deposition rate of the thin films largely increases with oxygen radical irradiation during the deposition in the metal mode. A as-deposited TiO2−δ thin film on a Pt substrate prepared at the substrate temperature of 300 °C crystallizes without postannealing. The Ti 2p X-ray absorption spectrum indicates that the Ti 3d electron enters the bottom of the conduction band. The Ti 3d states at the Fermi level (EF) and at ∼1.5 eV from EF in the band gap energy region are closely related to the electrical conductivity.

Exciton binding energies in chalcopyrite semiconductors

The optical spectra of Cu-III-VI${}_{2}$ chalcopyrite compounds display rich excitonic features in the fundamental direct bandgap energy region. The energy structure of excited excitonic states reported in the literature are reexamined using a calculation of the eigenstates of the hydrogenic problem in the context of the anisotropic band structure and the anisotropy of the dielectric constant. We find some remarkable agreements as well as inconsistencies in the literature that we attribute to the following reasons: (i) the difficulty to interpret fine structure-splitting data in noncubic semiconductors, and (ii) the more severe difficulty growing these materials with high enough quality. We finally propose some values that match very well with recent proposals and integrate the trend between Rydberg energies and bandgap values for the binary inorganic zincblende and wurtzite semiconductors.

Electronic structure of protonic conductor SrCeO 3 by soft-X-ray spectroscopy

The electronic structure of protonic conductor SrCe 1− x Y x O 3− δ ( x=0, 0.05) has been studied by resonant-photoemission spectroscopy (RPES) and X-ray absorption spectroscopy (XAS) in the soft-X-ray region. The RPES spectra exhibit that the O 2p state strongly hybridizes with Ce 4f state in the valence band. The valence band consists of the mixed-valent states of the 4f 0 (Ce 4+) and 4f 1 L (Ce 3+) configurations. The XAS spectra in the band gap energy region show a hole state at the top of the valence band and an acceptor-induced level just above the Fermi level ( E F). The energy position of the hole state accords with that of the Ce 3+ state. The energy separation between the hole state and E F is in good agreement with the activation energy.

Effect of voltage treatment applied on photoacoustic and photoelectrochemical current spectra in final preparation processes of porous TiO2 electrodes

We report the effect of different voltages in a concentrated KCl electrolyte applied to highly porous, polycrystalline TiO2 films during their final preparation processes on photoacoustic (PA) and photoelectrochemical (PEC) current spectra. Below the band-gap energy region, the PA signal intensities of the TiO2 films with different voltage treatment applied are higher than those without treatment and they increase with an increase of applied voltage. These experimental results suggest an innerband transition from an increase in carrier concentration due to the formation of donor levels by partially reduced Ti ions (Ti4+→Ti3+) by the voltage treatment. The PEC current spectra for applied voltage treatment over −1.5 V show two peaks above the band-gap energy region whose intensities increase rapidly above applied voltage of −1.5 V (eight times larger for voltage of −3.5 V than those without treatment), which is similar to that of the PA intensity at a photon energy of 2.0 eV. The modulation frequency dependence of the PEC current intensity at the peak position shows different characteristics depending on the voltage treatment applied, indicating the difference in PEC current response due to the formation of donor levels by the treatment.

Electronic structures of protonic conductors SrTiO 3 and SrCeO 3 by O 1 s X-ray absorption spectroscopy

The electronic structures of protonic conductors SrTi 1− x Sc x O 3 and SrCe 1− x Yb x O 3 have been studied by O 1 s X-ray absorption spectroscopy (XAS). In SrTi 1− x Sc x O 3, hole state and acceptor-induced level are observed in the band gap energy region. Such a structure is also observed in the O 1 s XAS spectra of SrCe 1− x Yb x O 3. The XAS structures and their temperature dependence reflect the activation energy estimated from electrical conductivity. These facts indicate that the conductivity is achieved by deep acceptor-induced level lying in the middle of the band gap.

Electronic structure in the band gap of lightly dopedSrTiO3by high-resolution x-ray absorption spectroscopy

The electronic structure of lightly Nb-doped ${\mathrm{SrTiO}}_{3}$ has been investigated using high resolution x-ray absorption spectroscopy (XAS). Below the O $1s$ threshold, XAS spectra show two features due to empty states whose energy positions match with those of the $3d$ photoemission spectra in the band gap energy region of the parent undoped ${\mathrm{SrTiO}}_{3}.$ Similar features are also observed in lightly La-doped ${\mathrm{SrTiO}}_{3}.$ These features exhibit systematic temperature dependence, which is well explained by the Fermi-Dirac distribution function. This indicates that the two features in the band gap are real bulk states such as simple donor states.

SCATTERING-THEORETICAL METHOD FOR THE RECONSTRUCTED β-SiC(100) SURFACE

By using scattering-theoretical method, we have calculated the surface-band structure of ideal and （２×１） reconstructed β-SiC(100) surface, and the lay-, atom-and orbital-resolved densities of states. The results show that the major effects of reconstruction occur in the band-gap energy region, and the reconstructed Si and C terminated surfaces are semiconducting. Our calculated surface-band structure are in good agreement with experiments. According to the bonding states and localized state densities of two dimerized atoms, the dimeric symmetry of Si atoms and the dimeric asymmetry of C atoms have qualitatively been explained.

Optical study of the strain effect in pseudomorphic In1-xGaxAs-InP heterostructures.

Photoreflectance (PR) and optical absorption (OA) have been used to study the energy-gap dependence on temperature in ${\mathrm{In}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ga}}_{\mathit{x}}$As thin layers, metalorganic vapor-phase epitaxy grown on InP substrates under nearly lattice-matched conditions (\ensuremath{\Vert}\ensuremath{\Delta}a/${\mathit{a}}_{0}$\ensuremath{\Vert}\ensuremath{\le}${10}^{\mathrm{\ensuremath{-}}3}$). Both PR and OA spectra show a split structure in the band-gap energy region. Moreover, the energy-gap values at all temperatures, obtained by Franz-Keldysh-oscillations analysis of PR line shapes, are blueshifted with respect to the values of the perfectly lattice-matched alloy (x=0.472). The results are related to the valence-band splitting at k=0 generated by the biaxial strain in ${\mathrm{In}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ga}}_{\mathit{x}}$As, due to the small lattice mismatch. We calculated the splitting and shift values using the orbital-strain Hamiltonian and accounting for compositional effects. The values are in agreement with the experimental results giving optical evidence of coherent growth of the alloy film.

A study on the optical non-linearity near the band gap energy region of CuCl by the non-degenerate reflected four-wave mixing

We have demonstrated three beam non-degenerate reflected four-wave mixing (reflected FWM) in which two driving fields have the same photon energy (◑ω 1) and the monitor and the resulting mixed light fields have another equal photon energy (◑ω 2). Mixed light has been observed with the driving field's energy from Z 3 exciton up to band gap energy when ◑ω 2 is fixed in the Z 3 and Z 1,2 exciton resonance region. The mechanism by which such non-degenerate optical non-linearity occurs is discussed according to a calculation by a simple model.

Investigation of oxidized aIIIBv surfaces by photoresponse

The wavelength dependence of photoresponse was studied, observed in the sub band gap energy region of Au−GaP Schottky barriers. Anomalously high response was measured in this energy region if the metal layer was prepared immediately after chemical cleaning, just after etching. The high response diminished pronouncedly when electrodes were made onto surfaces oxidized during storage in air. The presence of a transparent layer on the surface could be established by electron diffraction and ellipsometry as well. The layer structure depended on surface treatment. It was concluded that the high response is located in the interface region, and is characteristic of the slightly oxidized surface.