Indirect Transitions Research Articles

Mechanisms of Reactions between HOI and HY (Y = Cl, Br, I) on a Water Nanodroplet Surface.

Iodine chemistry has a broad range of implications for atmospheric processes including new particle formation. Hypoiodous acid (HOI) is a major iodine reservoir species. Its heterogeneous recycling in marine aerosols influences the lifetime of ozone in the troposphere. One important step of such recycling is the reaction between HOI and HY (Y = Cl, Br, I). In this article, we employ ab initio molecular dynamics (AIMD) and quantum chemistry to investigate these reactions at the surface of atmospheric aerosols. Di-halogen (XY) can be formed in a picosecond time scale, with the formation of a loop structure connected by hydrogen and halogen bonds. The photolysis of XY at the surface of an aerosol is faster than in the gas phase. In addition to the formation of di-halogen, a new pathway to forming a [H2O···I···OH2]+ complex by the direct or indirect proton transition is identified. Results presented in this study deepen our understanding of the faster iodine-heterogeneous recycling at the surface of aerosols.

Growth and Liquid-Phase Exfoliation of GaSe1-xSx Crystals.

In recent years, interest in the liquid-phase exfoliation (LPE) of layered crystals has been growing due to the efficiency and scalability of the method, as well as the wide range of practical applications of the obtained dispersions based on two-dimensional flakes. In this paper, we present a comparative study of as-grown and liquid-phase exfoliated GaSe1−xSx flakes. Bulk GaSe1−xSx crystals with x ~ 0, 0.25, 0.5, 0.75, 1 were synthesized by melting stoichiometric amounts of gallium, selenium, and sulfur particles in evacuated ampoules. X-ray diffraction analysis showed that the crystal structure does not change considerably after LPE, while the analysis of the Raman spectra revealed that, after liquid-phase processing in IPA, an additional peak associated with amorphous selenium is observed in selenium-rich GaSeS compounds. Nevertheless, the direct and indirect transition energies determined from the Kubelka-Munk function for LPE crystals correlate with the band gap of the as-grown bulk GaSeS crystals. This finding is also confirmed by comparison with the data on the positions of the photoluminescence peak.

Structural and optical absorption coefficient analysis of barium lead sodium-borate glass doped with graphene nanopowder

In this study, some barium-lead sodium borate graphene-doped glasses have been prepared. The preparation is based on the concept of weight percentage using the fast quenching methodology. The internal phases of the as-prepared solid samples have been characterized using X-ray diffraction (XRD), and Fourier Transform Infrared (FTIR). All characterization processes revealed the amorphous nature of the as-prepared samples, with an increase in the crystallinity degree as the graphene content increased. On the other hand, the optical characterization was achieved using UV–vis, which revealed a decrease in both direct and indirect optical band gaps from 3.5 to 3.32 eV and from 2.95 to 2.66 eV, respectively, as the graphene content increased. The value of Urbach's energy decreased from 0.658 to 0.641 eV, which is an anomalous behavior because it is the same as that of the optical band gaps. At the same time, the decrease in the value of Urbach’s energy confirmed the results of (XRD) about the crystallinity degree. The absorption coefficient spectra have been simulated using the hydrogenic excitonic model (HEM), where some useful parameters were obtained. Such parameters indicate the indirect optical transition as the dominant absorption mechanism, which is a common feature of amorphous solids. The obtained data suggested that graphene nanopowders as a modifier can be used to improve the effectiveness of oxide glasses for UV-blockers, radiation shielding, and greenhouse applications.

The electronic and optical properties of Cs2 Ti1-xBxI6(B=Sn, Te, Se) with first principle method

Lead-free halide perovskite is regarded as the most promising light-harvesting material for solar cells recently. In this work, the properties of CsTiBI (B=Sn, Te, Se) were investigated with first principle method based on density functional theory. The results show that CsTiSnI is an excellent photoelectric material in the sunlight. When the doping concentration was over , CsTiSeI and CsTiTeI have direct–indirect band gap transition. The values of energy gap, density of states and optical absorption coefficients in the visible light region are well matched to each other. Optical results show that absorption peaks locate near 330 nm and have strong ultraviolet absorption capacity. Doping Sn results in the spectrum redshifts and the absorption peak drops. While the peak becomes greater with the increase of tellurium, and the spectrum has obvious blueshift. The spectrum trends of CsTiSeI are more complicated. This work provides theoretical supports for the development of novel optoelectronic materials by means of partial substitution doping.

Temperature behavior of the near band edge luminescence in CsPbBr3 single crystal and nanoparticle ensemble

The temperature dependences of the luminescence spectra for CsPbBr3 single crystal and nanoparticle ensemble are presented. The emission bands of single crystal are interpreted in terms of the direct and indirect exciton transitions. An attempt to estimate the contribution of the direct and indirect transitions for wide luminescence band inherent for CsPbBr3 nanoparticles is made. The temperature dependence of the luminescence band position is analyzed by the taking into account the thermal expansion and exciton-phonon interaction. For both the single crystal and the ensemble of nanoparticles, main contribution to the shift of the luminescence bands of the direct and indirect excitons to the low-energy region was revealed to be caused by the interaction with optical phonons. This interaction is predominant for the indirect excitons and can be used as a sign of Rashba nature of the exciton.

Investigation of Band Structure in Strained Single Crystalline Si1-X Sn X

We evaluated the optical properties and the band structure of strained single crystalline Si1-x Sn x using spectroscopic ellipsometry. The results suggest a reduction of the band gap at the Γ point and the formation of an optical transition by Van-Hove singularity with higher Sn fraction. In addition, since the reduction of the band gap with increasing Sn fraction at the Γ point is larger than at other points, it is expected the indirect transition type Si1-x Sn x used in this study may eventually change to the direct transition type. Although higher Sn fraction are required to achieve direct transition type Si1-x Sn x , the band structure of strained single crystal Si1-x Sn x was experimentally clarified.

Studying spectroscopic, cyclic voltammetry, and electrical properties of novel 4-amino antipyrine derivative for photonic applications

The present work aims to synthesize a new antipyrine derivative and then study its optical properties in the form of thin films with the aim of using the obtained results in the fabrication and designing of new optoelectronic devices. A N1,N3-bis(1,5-dimethyl-3-oxo-2-phenyl-2,3-dihydro-1H-pyrazol-4-yl)malonamide, dpdp novel organic compound has been successfully synthesized in dimethylformamide (DMF) medium by the chemical reaction between 4-aminoantipyrine and malonyl chloride. The obtained dpdp compound is characterized quantitatively and qualitatively by using micro elemental analysis, FTIR spectroscopy, 1H– & 13C–NMR, and thermogravimetric measurements. Two steps weight loss of 84 % (calcd. mass loss 37.25 %) is recorded in the temperature range of 25 °C to 800 °C. Thin films of the novel organic dpdp were successfully grown by the thermal evaporation technique. FSEM images show the surface topology of the as-deposited thin film which is characterized by high quality, continuous, and crystalline granular surface with an average particle size of 70 nm. The cyclic voltammetric analysis showed that the energy difference between the HOMO and LUMO equals 2.43 eV. The calculated values of the optical band gap transitions deduced from the diffuse reflectance spectra are found to be 2.27 eV, and 3.44 eV for the powder and the as-deposited dpdp thin films, respectively. The photoluminescence spectra analysis showed that the as-deposited film dpdp is characterized by a main green peak centered at 530 nm. The optical transflection spectra showed that the calculated values of the indirect optical band gap transitions are found to be 3.53 eV for the as-deposited thin film which slightly increased to 3.63 eV after annealing at 373 K and 423 K for 4 h. Spectral variations of the refractive index and the specific polarizability showed one peak around 400 nm which fades by annealing treatment at 373 K and 423 K. The dc electrical resistivity activation energy of the free charge carriers was estimated.

Is the Bandgap of Bulk PdSe 2 Located Truly in the Far‐Infrared Region? Determination by Fourier‐Transform Photocurrent Spectroscopy

Is the Bandgap of Bulk PdSe ₂ Located Truly in the Far‐Infrared Region? Determination by Fourier‐Transform Photocurrent Spectroscopy

Multi-frequency complete optical isolation based on indirect interband transition

Indirect interband photonic transition is an important approach to achieve on-chip complete optical signal isolation. We demonstrate that the operational isolation bandwidth based on this effect can be increased by utilizing non-interfering transition channels obtained from the process of transition channel engineering. Multiple optical modes can be excited through non-interfering channels and be absorbed by matched filters without interfering with each other, thus achieving multi-frequency complete optical isolation. With two non-interfering transition channels, an optical isolator design that achieves complete isolation at two frequencies is proposed as an example. The potential design of multi-frequency isolator is also discussed. All the results are verified with finite-difference time-domain simulation.

Investigation of the Optical Properties for Quaternary Se60-xGe35Ga5Sbx (x = 0, 5, and 10) Chalcogenide Glass.

A quenching technique was used to prepare the chalcogenide system of the (x = 0, 5, and 10 at. %), which was deposited as thin films onto glass substrates using a thermal evaporation technique. X-ray diffraction patterns were used for structure examination of the fabricated compositions, which exposes the amorphous nature of the deposited samples. Meanwhile, the chemical compositions of the prepared samples were evaluated and calculated via the energy-dispersive X-ray spectroscopy (EDX), which was in agreement with the measured compositional element percentages of the prepared samples. Based on the optical reflectance R and transmittance T spectra from the recorded spectrophotometric data ranging from 350 to 2500 nm, the influence of the Sb element on the thin films’ optical properties was studied. The film thickness and the refractive index were calculated via Swanepoel’s technique from optical transmittance data. It has been observed that the films’ refractive index increases with increasing x value over the spectral range. The refractive index data were used to evaluate the dielectric constants and estimate dispersion parameters and using the Wemple–DiDomenico model. The optical energy gap was calculated for the tested compositions. The result of the optical absorption analysis shows the presence of allowed direct and indirect transitions.

Synthesis and characterization g‐C3N4‐doped PMMA polymeric nanocomposites films for electronic and optoelectronic applications

AbstractSolution casting was used to synthesis polymethyl methacrylate (PMMA) nanocomposites with graphitic carbon nitride (g‐C3N4) nanoparticles in various percentages (0.033, 0.166, 0.33, 1.665, and 3.3 wt%). XRD patterns confirmed the amorphous nature of PMMA/C3N4 nanocomposites. Optical constants are obtained at room temperature in the 200–2500 nm wavelength range. The optical band gaps of PMMA/C3N4 doped films, both direct and indirect, are measured and have values change from 4.8 to 2.32 eV for and changes from 5 to 3.7 eV for . The refractive index values of the materials were also calculated using Moss, Reddy, Anani, and Kumar‐Singh relationships with average values vary from (2.109 to 2.529) and vary from 2.155 to 2.32 for indirect and direct transition, respectively. The dielectric constant, dielectric loss, and AC electrical conductivity at RT and the g‐C3N4 doping level was examined as frequency variables from 100 to 1 MHz. In consideration of Jonscher's universal power law, the variation of σAC with frequency was examined. The addition of C3N4 appears to raise the σAC of the studied films, making them attractive for semiconductor applications. According to the optical limiting results, the samples doped with g‐C3N4 are severely attenuated for the laser beam of 638.2 nm. PMMA doped with g‐C3N4 is a viable contender for electronic and optoelectronic applications, particularly laser power reduction.

Enhancement of structural and optical properties of synthesized Dy3+ - Mn2+ doped and co-doped magnesium borate MgB4O7

In the present work, magnesium borate (MgB4O7) was synthesized via sol-gel process. Mn2+ and Dy3+ were added as a dopants and co-dopants in the prepared material at concentrations of 0.01 g. X-ray diffraction (XRD) studies checked the nature of the synthesized materials. Fourier Transform Infrared (FTIR) was investigated to find the effect of dopants on the absorption spectra in comparison with that of pure sample (without dopants). The particles sizes were calculated using Scherrer's equation and found to be in the range of 9.37–13.78 nm and 24.75–32.15 nm at 700 and 900 °C respectively. The energy band gap (Eopt) of MgB4O7 increased (from 3.78 eV to 3.83 eV) with doping for direct transitions and ranged between 3.63 and 3.66 eV for indirect transitions. In the same time, Urbach energy values are found to be in the range of 0.070–0.175 eV. Results of the structural and optical properties can help increase utilization of the correct form of magnesium borate MgB4O7 in different applications in industry.

Preparation Structural and Optical Analysis of Tungsten Sulphide (WS<sub>2</sub>) Thin Film by DC-Sputtering

Tungsten Sulfide (WS2) has been appeared as visible range semi-conductors with having substantial direct band gap. In two-dimensional (2D) transition metal dichalcogenides (TMDC) i.e.WS2 has been growing research attention over more than the past twenty years in low cost, energy-efficient, adaptable or environment-friendly material, which is crucial part of optoelectronic emission devices. WS2 semiconductor thin films were grown on glass and Si substrates by using DC sputtering method. During deposition the substrate were kept at temperature of 200°C. The thin films were studied by X-ray diffraction, UV-Vis spectrometry and Spectrofluorometer (FS5). XRD analysis revealed the nature of grown film effected by temperature of substrate results in to be amorphous. Photoluminescence analysis at 532nm and optical study proved the direct to indirect band transition in WS2.

Exciton Luminescence and Optical Properties of Nanocrystalline Cubic Y2O3 Films Prepared by Reactive Magnetron Sputtering.

This paper presents a comprehensive study of the energy structure, optical characteristics, and spectral-kinetic parameters of elementary excitations in a high-purity nanocrystalline cubic Y2O3 film synthesized by reactive magnetron sputtering. The optical transparency gaps for direct and indirect interband transitions were determined and discussed. The dispersion of the refractive index was established based on the analysis of interference effects. It was found that the refractive index of the Y2O3 film synthesized in this study is higher in order of magnitude than that of the films obtained with the help of other technologies. The intrinsic emission of Y2O3 film has been discussed and associated with the triplet–singlet radiative relaxation of self-trapped and bound excitons. We also studied the temperature behavior of the exciton luminescence of Y2O3 for the first time and determined thermal activation barriers. The optical energy and kinetic parameters of cubic Y2O3 films were analyzed in comparison with those of the monoclinic films of yttrium oxide. The main difference between the optical properties of cubic and monoclinic Y2O3 films was established, which allowed for a supposition of their application prospects.

Zinc gallate (ZnGa2O4) epitaxial thin films: determination of optical properties and bandgap estimation using spectroscopic ellipsometry

Electronic grade ZnGa2O4 epitaxial thin films were grown on c-plane sapphire substrates by metal-organic chemical vapor deposition and investigated using spectroscopic ellipsometry. Their thickness, roughness and optical properties were determined using a Multiple Sample Analysis based approach by the regression analysis of optical model and measured data. These samples were then compared to samples which had undergone ion etching, and it was observed that etching time up to four minutes had no discernible impact on its optical properties. Line shape analysis of resulting absorption coefficient dispersion revealed that ZnGa2O4exhibited both direct and indirect interband transitions. The modified Cody formalism was employed to determine their optical bandgaps. These values were found to be in good agreement with values obtained using other popular bandgap extrapolation procedures.

KARAKTERISTIK SIFAT OPTIK NANOPARTIKEL KARBON (CARBON DOTS) DENGAN METODE UV-VIS DRS (ULTRAVIOLET-VISIBLE DIFFUSE REFLECTANCE SPECTROSCOPY)

Research on the synthesis of Carbon Dots using the laser ablation method has been carried out at a time duration of 1, 2, and 3 hours (energy 60 mJ, wavelength 1064 nm and frequency 10 Hz). This research uses organic material from Tea and Toluene as the carbon solvent. The characterization method uses a UV-Vis Diffuse Reflectance Spectroscopy (UV-Vis DRS) spectrum to measure the reflectance value and uses the Kubelka-Munk equation to determine the relationship between the absorbance coefficient parameter (s) and the scattering reflectance coefficient (k). The size and morphological characterization methods used Particle Size Analyzer (PSA) and Transmission Electron Microscopy, while the functional group characteristics used the Fourier Transform InfraRed (FTIR) tool. FTIR spectra show the O-H group which is a hydroxyl bond and N-H is a carboxylic acid. The results of measurement and analysis of bandgap energy at different time durations for direct transition (n=2) were 1 hour (3.62 eV, 342.49 nm), 2 hours (3.24 eV, 380.61 nm) and 3 hours (2.74, 451.82 nm). Indirect transitions (n=1/2) were 1 hour (3.17 eV, 391.02 nm), 2 hours (2.50 eV, 495.36 nm) and 3 hours (2.21 eV, 559.04).

Low-temperature NIR-VUV optical constants of (001) LaAlO3 crystal

The optical constants and dielectric function of (001) LaAlO3 crystal were investigated at low temperatures down to 10 K in the NIR-VUV spectral range (photon energies 0.8-8.8 eV). Reflection variable angle spectroscopic ellipsometry and transmission spectroscopy were applied. Interband transitions were examined using the Tauc plots and the critical-point analysis. At room temperature, the indirect bandgap of 5.6 ± 0.01 eV and the lowest-energy direct transition at 7.2 ± 0.03 eV were detected. On cooling to 10 K, a blueshift of ∼0.2 eV and ∼0.1 eV was observed for the indirect and direct transitions, respectively. In the transparency spectral range, the index of refraction was found to be nearly temperature-independent and vary with photon energy from 2.0 (1 eV) to 2.5 (5.5 eV). It was suggested that the excellent thermal stability of the index of refraction may be related to the revealed thermally stable interband transitions. The results are of importance for modeling and design of modern optical devices.

Optical Manipulation of the Rashba Effect in Germanium Quantum Wells

AbstractThe Rashba effect in Ge/Si0.15Ge0.85 multiple quantum wells embedded in a p‐i‐n diode is studied through polarization and time‐resolved photoluminescence. In addition to a sizeable redshift arising from the quantum‐confined Stark effect, a threefold enhancement of the circular polarization degree of the direct transition is obtained by increasing the pump power over a 2 kW cm−2 range. This marked variation reflects an efficient modulation of the spin population and is further supported by dedicated investigations of the indirect gap transition. This study demonstrates a viable strategy to engineer the spin‐orbit Hamiltonian through contactless optical excitation and opens the way toward the electro‐optical manipulation of spins in quantum devices based on group‐IV heterostructures.

Linear optical characteristics as well as gamma-ray shielding capabilities of quaternary lithium-zinc borate glasses with Y3+ ions

Transparent quaternary lithium-zinc borate glasses with Y3+ ions with chemical formula 15Li2O–15ZnO-(70-x)B2O3-xY2O3, where x = 0.0 (BLZY0.0), 0.5 (BLZY0.5), 1.0 (BLZY1.0), 1.5 (BLZY1.5), 2.0 (BLZY2.0), and 2.5 (BLZY2.5) mol% has been prepared. The linear optical properties as well as gamma-ray attenuation capabilities of these glasses have been examined. The direct band gap (Eopticalgapdirect) was reduced from 4.3 to 3.35 (eV) and the indirect band gap (Eopticalgapindirect) was reduced from 3.09 to 2.82 (eV) by insertion of Y3+ ions. The recorded values of Urbach's energy (ΔE) were 0.718, 0.45, 0.37, 0.36, 0.33, and 0.26 eV for BZLY0.0, BZLY0.5, BZLY1.0, BZLY1.5, BZLY2.0, and BZLY2.5 respectively. The refractive index (n) values varied between 2.11 and 2.308 for direct transition, while between 2.37 and 2.45 for indirect transition. Numerically, the linear attenuation coefficient (μ) was found equal to 33.573, 34.417, 35.221, 36.095, 37.012, and 37.918 cm−1 at 0.015 MeV, while equal to 0.050, 0.051, 0.052, 0.054, 0.055, and 0.057 cm−1 at 15 MeV for BLZY0.0, BLZY0.5, BLZY1.0, BLZY1.5, BLZY2.0, and BLZY2.5, respectively. The half value layer (HVL) and mean free path (MFP) are found to take the order BLZY2.5 < BLZY2.0 < BLZY1.5 < BLZY1.0 < BLZY0.5 < BLZY0.0.In conclusion, the prepared glasses can be applied for optical and gamma-ray shielding applications.

Temperature and doping-dependent interplay between the direct and indirect optical response in buffer-mediated epitaxial germanium

The structural and optical properties of buffer mediated epitaxial germanium (Ge) layer were investigated and compared with bulk n-type and p-type Ge substrates. An interconnected dual-chamber molecular beam epitaxy (MBE) system was used to grow a 280 nm thin Ge epilayer on (100)GaAs substrate with an intermediate AlAs buffer layer. The lattice-matched, abrupt Ge/AlAs heterointerface was analyzed using cross-sectional transmission electron microscopic analysis, and no elemental interdiffusion was detected via secondary ion mass spectrometry. A strong direct gap transition, compared to the indirect gap transition, and a series of phonon-assisted transitions was observed by photoluminescence (PL) spectroscopy. In addition, the intensity of the direct gap recombination decreases with decreasing PL measurement temperatures, which was ascribed to the reduced density of Γ-valley electrons available for recombination at lower temperature. Furthermore, the intensity ratio between the direct and indirect optical transition drastically decreases with decreasing temperature in both n-type epitaxial and p-type bulk Ge. An empirical relation in both direct and indirect peak position with temperature was established. The observed strong luminescence in 280 nm thick epitaxial Ge at room temperature is vital for Ge-based photonic devices. In addition, the quality of the epitaxial Ge layer grown via MBE is on par with bulk Ge substrates.