Direct Optical Transitions Research Articles

Electronic Structure and Optical Absorption Spectrum of Icosahedral Golden Fullerene Au32

Abstract—The electronic structure of golden fullerene Au32 has been studied using quantum field theory methods in the framework of the Hubbard model. Expressions for the Fourier transforms of a Green’s function, the poles of which determine the energy spectrum of the nanosystem under consideration, have been derived. The energy spectrum of Au32, studied in comparison with the spectrum of icosahedral carbon fullerene C60, indicates the semiconductor state of golden fullerene Au32. The peaks of the density of states correspond to Van Hove singularities. The optical absorption spectra of the neutral and negatively charged fullerene Au32 are presented. The energy of the first direct optical transition of the negatively charged ion of golden fullerene $${\text{Au}}_{{32}}^{ - }$$ is 1.26 eV.

Enhancing and controlling valley magnetic response in MoS2/WS2 heterostructures by all-optical route

Van der Waals heterostructures of transition metal dichalcogenides with interlayer coupling offer an exotic platform to realize fascinating phenomena. Due to the type II band alignment of these heterostructures, electrons and holes are separated into different layers. The localized electrons induced doping in one layer, in principle, would lift the Fermi level to cross the spin-polarized upper conduction band and lead to strong manipulation of valley magnetic response. Here, we report the significantly enhanced valley Zeeman splitting and magnetic tuning of polarization for the direct optical transition of MoS2 in MoS2/WS2 heterostructures. Such strong enhancement of valley magnetic response in MoS2 stems from the change of the spin-valley degeneracy from 2 to 4 and strong many-body Coulomb interactions induced by ultrafast charge transfer. Moreover, the magnetic splitting can be tuned monotonically by laser power, providing an effective all-optical route towards engineering and manipulating of valleytronic devices and quantum-computation.

New window layer of Cu2CdSn3S8 for thin film solar cells

A successful trial has been achieved to prepare a stable quaternary composition Cu2CdSn3S8 (CCTS8). Thin films of the new quaternary CCTS8 have been deposited using spray pyrolysis technique. The crystal structure, morphology, optical and optoelectrical properties have been investigated to understand the films nature. The prepared films reveal n-type semiconducting behaviour. They show optical direct transitions with optical energy gaps in the range (3.79–4.08 eV). The surprising results obtained for the studied films suggests the possibility of utilizing our films as window layers in thin film solar cells or as suitable candidates for various display devices.

Type of Optical Transitions at the Fundamental Absorption Edge in TlGaSe2 and TlInS2 Crystals Subjected to γ-Radiation

The effect of γ-radiation on the optical properties of layered TlGaSe2 and TlInS2 crystals has been studied within a wavelength range of 400–1100 nm at 300 K. By means of analysis of optical absorption spectra, the energies of direct and indirect optical interbend transitions before and after γ-irradiation have been determined. It has been shown that the energies of direct and indirect nonforbidden optical transitions grow with accumulation of γ-radiation dose within 0–25 Мrad in TlGaSe2 and TlInS2 single crystals from Egd = 2.06 eV and Egi = 1.90 eV at D = 0 Мrad to Egd = 2.11 eV and Egi = 1.98 eV at D = 25 Мrad for TlGaSe2 crystals and from Egd = 2.32 eV and Egi = 2.27 eV at D = 0 Мrad to Egd = 2.35 eV and Egi = 2.32 eV at D = 25 Мrad for TlInS2 crystals. A decrease in the transmission coefficient at doses from 0 to 5 Mrad with a further increase in the transmission coefficient at a radiation dose of D = 25 Мrad is observed.

Optical constants and electrical conductivity of polysilanes: Effects of substituents and iodine doping

This work deals with the effect of chemical structure and iodine doping on electro-optical characteristics of different polysilanes in the solid state. The properties of polymethylphenylsilane homopolymer were compared with those of polysilane copolymers containing diphenylsilylene structural units associated with either methylhydrogen- or methylphenylsilylene moieties. Their optical parameters, such as Urbach energy and optical band gap, were evaluated using the method proposed by Tauc. The forbidden gap values for the studied polysilanes are reduced after doping from 3.22–3.46 eV to 2.47–2.64 eV, while the electrical conductivities were found in the range of 6.2 10−11 – 1.2 10−8 S/cm for pristine samples, and increased by 2–3 orders of magnitude following their doping, depending on the nature/content of the organic substituents attached to the silicon main chain. A simple mathematical approach starting from Tauc classical theory was used to establish allowed direct optical transitions for both undoped and doped polysilane samples. The obtained data indicated that the polysilane films with a high content of phenyl side groups presented the best properties for their use in electro-optical applications.

Circular Photocurrent in Weyl Semimetals with Mirror Symmetry

We have studied theoretically the Weyl semimetals the point symmetry group of which has reflection planes and which contain equivalent valleys with opposite chiralities. These include the most frequently studied compounds, namely the transition metals monopnictides TaAs, NbAs, TaP, NbP, and also Bi$_{1-x}$Sb$_x$ alloys. The circular photogalvanic current, which inverts its direction under reversal of the light circular polarization, has been calculated for the light absorption under direct optical transitions near the Weyl points. In the studied materials, the total contribution of all the valleys to the photocurrent is nonzero only beyond the simple Weyl model, namely, if the effective electron Hamiltonian is extended to contain either an anisotropic spin-dependent linear contribution together with a spin-independent tilt or a spin-dependent contribution cubic in the electron wave vector $\bf{k}$. With allowance for the tilt of the energy dispersion cone in a Weyl semimetal of the $C_{4v}$ symmetry, the photogalvanic current is expressed in terms of the components of the second-rank symmetric tensor that determines the energy spectrum of the carriers near the Weyl node; at low temperature, this contribution to the photocurrent is generated within a certain limited frequency range $\Delta $. The photocurrent due to the cubic corrections, in the optical absorption region, is proportional to the light frequency squared and generated both inside and outside the $\Delta$ window.

Growth, structure, optical and optoelectrical characterizations of the Cu2NiSnS4 thin films synthesized by spray pyrolysis technique

Earth-abundant Kesterite materials are very important to fabricate low-cost solar cells. Here, good quality Cu2NiSnS4 (CNSS) thin films were successively manufactured on a glass substrate at 350 °C using a simple spray pyrolysis system. The structural characterization of the sprayed Cu2NiSnS4 thin films was examined by the FE-SEM and XRD techniques. X-ray diffraction patterns indicate that the sprayed Cu2NiSnS4 films are single phase and having polycrystalline structure. The elemental composition analysis of the sprayed Cu2NiSnS4 thin films confirmed that the Cu2NiSnS4 film is near stoichiometric in compound. Our optical observations indicate that the refractive index (n) of the sprayed Cu2NiSnS4 films was increased by increasing the film thickness. Moreover, the sprayed Cu2NiSnS4 films exhibit a direct optical transition and the magnitudes of the energy gap have been decreased from 1.28 to 1.14 eV with the increase of thickness. The optoelectrical parameters of the Cu2NiSnS4 films, like optical conductivity, optical mobility, optical resistivity, optical carrier concentration and relaxation time were estimated with different thickness. Additionally, the nonlinear optical parameters of the Cu2NiSnS4 films were estimated. The fabricated CNSS/n-Si heterojunction achieved a conversion efficiency of 11.34%.

Optical transitions and photoluminescence of fluorine-doped zinc tin oxide thin films prepared by ultrasonic spray pyrolysis

Abstract Thin films of fluorine-doped zinc tin oxide (FZTO) were deposited on borosilicate glass by ultrasonic spray pyrolysis. Structural and elemental investigations revealed that all films have a single-phase polycrystalline tetragonal rutile structure and proved the successful incorporation of zinc ions into tin sites and the fluorine ions into the sites of oxygen vacancies. A comprehensive analysis of the optical transitions in FZTO films was presented. All films show high optical transparency (≥85%) in the visible range. Four direct optical transitions were observed in the range 3.8 - 5.1 eV by spectrophotometric measurements, in addition to three transitions identified by photoluminescence. From these transitions observed for the first time in FZTO, three states of point defects were determined in the bandgap and attributed to the neutral, singly- and doubly-ionized oxygen vacancies. The ascription of these states to oxygen vacancies was supported by the x-ray photoelectron spectroscopy measurements. The band diagram of FZTO was sketched, demonstrating the observed transitions. The obtained results are expected to have their imprint on understanding the optical processes in FZTO and evaluating the future utility of it in various optoelectronic applications.

Optical characterization of nano-structured Cu2ZnSnS4 thin films deposited by GLAD technique

In this paper, Cu2ZnSnS4 (CZTS) thin films were elaborated at room temperature by thermal evaporation method using Glancing Angle Deposition (GLAD) technique at different incident angles γ = 00°, 20°, 40°, 60°, 75° and 85°. XRD, Raman scattering analysis, (SEM) and UV-Visible-NIR spectroscopy were used to characterize the crystalline structure, morphology and optical properties of CZTS samples. The results have showed that the ellipsometric analysis leaded to an optical anisotropy due to the structural anisotropy for CZTS samples deposited at γ = 85°. All Cu2ZnSnS4 samples exhibited a high absorption coefficient (α > 104 cm−1) and a direct optical transition varied between 1.48 eV and 2.05 eV for CZTS thin films deposited at γ = 00° and 85°, respectively. The value of the Urbach energy increased with incident angle, indeed, its value increased from 58 meV (γ = 00°) to 604 meV (γ = 75°) and decrease to be 368 meV for γ = 85°. This result is correlated with the Raman analysis. From transmittance data of CZTS thin films deposited at γ = 00°, 20° and 40° Swanepoel's method was used, to estimate the refractive index n. It allows us, using the Wemple-DiDomenico and Spitzer-Fan models, to calculate other optical parameters such as the oscillator energy E0, dispersion energy Ed, zero frequency refractive index n0, high frequency dielectric constant ε∞ and the electric susceptibility χe. On the other hand, to have an idea about the evolution of the nonlinear optical character, the nonlinear susceptibility χ(3) and the nonlinear refractive index n2 of CZTS thin films deposited at γ = 00°, 20° and 40° were investigated. Ellipsometric measurements of CZTS thin films has leaded to an optical anisotropy for γ = 85°. In addition, the generalized ellipsometry in Jones formalism have proved this property, which can be related to the nano-columnar slanted structure as revealed by (SEM) analysis.

Optical and NIR luminescence spectral studies: Nd3+-doped borosilicate glasses

Nd3+-doped borosilicate glasses with molar composition 20B2O3+ (45-x) SiO2 +20K2CO3+5Na2CO3+10LiF + xNd2O3 were prepared by conventional melt-quenching technique. Structural and optical properties of the proposed glasses were studied using XRD, FTIR, UV–Visible absorption, excitation and NIR emission spectroscopy. Judd-Ofelt theory was applied to the absorption spectrum of 1.0 mol% Nd3+-doped borosilicate glass to estimate the JO intensity parameters (Ω2, Ω4, Ω6). From this theory, various radiative properties like radiative transition probabilities (AT), branching ratios (βr) and stimulated emission cross section (σemi) were determined and reported. The measured lifetimes for theF3/24 level were decreased with increase in Nd3+ concentrations. Moreover, the optical energy gap was calculated for direct and indirect optical transitions. The higher stimulated emission cross section (5.84622 × 10−20 cm2), optical gain (142.64 × 10−25 cm2s), quantum efficiency (77%), and low threshold (1.71 W/m2) indicate that 1.0 mol% Nd3+-doped borosilicate glass could be potential gain medium for solid state NIR laser application at 1.06 μm.

Pressure dependence of direct optical transitions in ReS2 and ReSe2

The ReX2 system (X = S, Se) exhibits unique properties that differ from other transition metal dichalcogenides. Remarkably, its reduced crystal symmetry results in a complex electronic band structure that confers this material in-plane anisotropic properties. In addition, multilayered ReX2 presents a strong 2D character even in its bulk form. To fully understand the interlayer interaction in this system, it is necessary to obtain an accurate picture of the electronic band structure. Here, we present an experimental and theoretical study of the electronic band structure of ReS2 and ReSe2 at high-hydrostatic pressures. The experiments are performed by photoreflectance spectroscopy and are analyzed in terms of ab initio calculations within the density functional theory. Experimental pressure coefficients for the two most dominant excitonic transitions are obtained and compared with those predicted by the calculations. We assign the transitions to the Z k-point of the Brillouin zone and other k-points located away from high-symmetry points. The origin of the pressure coefficients of the measured direct transitions is discussed in terms of orbital analysis of the electronic structure and van der Waals interlayer interaction. The anisotropic optical properties are studied at high pressure by means of polarization-resolved photoreflectance measurements.

Photovoltage spectroscopy of direct and indirect bandgaps of strained Ge1-xSnx thin films on a Ge/Si(001) substrate

The near-bandgap optical properties of Ge1-xSnx alloys were characterized by photovoltage spectroscopy and spectral ellipsometry measurements. Contributions of Urbach tailing as well as direct and indirect optical transitions were observed. The compositional dependence of direct bandgaps of strained GeSn films grown on a Ge buffered Si substrate was studied for up to 15% Sn content. The contribution to the photovoltage spectra of Ge1-xSnx alloys (x < 6%) from indirect optical transitions was observed at lower energies than from direct bandgaps. Using bowing parameters of bGeSnΓ = 3.16–0.5x and bGeSnL = 1.93 eV, a correlation was detected between calculated and measured indirect and direct bandgaps at 82 K. As the Sn content was increased, the difference between the energies of the indirect and direct bandgaps decreased, resulting in a smaller contribution of the indirect transitions due to competition with direct transitions and Urbach tails. Two sublayers with different Sn content, strain values and bandgaps were observed for samples with x ∼12%. The results indicated that strain relaxation in films with thicknesses exceeding a critical value occurs via formation of a Sn-rich top layer with higher direct bandgap. These findings have important implications when designing IR photodetectors or solar cells.

Direct and indirect optical transitions in bulk and atomically thin MoS2 studied by photoreflectance and photoacoustic spectroscopy

Optical transitions in atomically thin MoS2 samples made by sulfidation of a metallic molybdenum layer have been studied by photoreflectance (PR) and photoacoustic (PA) spectroscopy. The obtained spectra are compared with PR and PA spectra of bulk MoS2. It is shown that the absorption edge observed in the PA spectrum shifts to blue when moving from the bulk MoS2 to the atomically thin MoS2 layers, whereas the direct optical transitions at the K point of the Brillouin zone (A and B transitions), which are observed in the PR spectrum, do not shift spectrally in a significant manner. On the other hand, the AH transition, which is related to the direct optical transition at the H point of the Brillouin zone and is typical of bulk MoS2, is not observed for atomically thin MoS2 layers. Moreover, a strong and broad PR resonance related to the band nesting (C transition) is identified in the PR spectra of studied samples. In this case, C and CH transitions are observed for bulk MoS2, while only a C transition is observed for atomically thin MoS2.

Physical and photo-electrochemical properties of MgFe2O4 prepared by sol gel route: application to the photodegradation of methylene blue

The spinel MgFe2O4 synthesized by sol gel method, was characterized by physical and electrochemical methods. The thermal analysis (TG) gives an optimal synthesis temperature of 650 °C where the X-ray diffraction pattern exhibits a single phase crystallizing in a cubic symmetry (Space Group: Fd\(\stackrel{-}{3}\)m) with spherical crystallites (~ 0.45 µm). The morphology observed through the SEM image shows elliptical and spherical grains of ~ 0.8 µm. The spinel possesses a direct optical transition (2.16 eV) due to electrons transfer Fe3+: d−d transition, coming from the crystal field splitting in agreement with the red color. The low electrons mobility is assigned to a narrow conduction band of Fe3+: 3d parentage with activation energy (0.14 eV) in conformity with a conduction mechanism by small polaron hopping. The semi-logarithmic plot confirms the chemical stability of oxide. Unlike other spinels, the capacitance plot exhibits n type conduction confirmed by chrono-amperometry with a flat band potential (Efb) of 0.27 V. The electrochemical impedance spectroscopy displays a semi-circle attributed to the bulk behavior. As application, the photodegradation of methylene blue (10 mg L− 1) was successfully on the spinel suspension; 87% are eliminated after 2 h of exposure to sunlight irradiation.

High‐Frequency Nonlinear Transport and Photogalvanic Effects in 2D Topological Insulators

AbstractExcitation of a topological insulator by a high‐frequency electric field of a laser radiation leads to a dc electric current in the helical edge channel whose direction and magnitude are sensitive to the radiation polarization and depend on the physical properties of the edge. An overview of theoretical and experimental studies of such edge photoelectric effects in 2D topological insulators based on semiconductor quantum wells is presented. First, a phenomenological description is given of edge photocurrents, which may originate from the photogalvanic effects or the photon drag effects, for edges of all possible symmetry. Then, microscopic mechanisms of photocurrent generation for different types of optical transitions involving helical edge states are discussed. They include direct and indirect optical transitions within the edge channel and edge‐to‐bulk optical transitions.

Синтез и оптические свойства пленок оксида титана, модифицированных кобальтом

Using X-ray phase analysis, scanning electron microscopy atomic force and magnetic force microscopy, and IR spectroscopy the properties of polycrystalline TiO2 films modified by cobalt during magnetron sputtering and subsequent pulsed photon processing in air have been investigated. It has been found that in the course of the modification, a nanocrystalline (with a grain size of ~ 50 nm) film consisting of cobalt and titanium oxides is formed. Their surface exhibits magnetic properties. In the IR reflection spectra obtained at different incident angle of beam, two of the transverse optical (TO) phonons and their corresponding longitudinal (LO) phonons above 500–600 cm–1 were observed, which identify the formation of Co3O4 in the spinel structure. The study of optical absorption indicates the predominant existence in the films of phases with direct optical transitions. The optical band gap value was 1.43 and 1. 83 eV for Co3O4 and 2.65 eV for the cubic phase of CoO.

Тип оптических переходов на краю фундаментального поглощения кристаллов TlGaSe-=SUB=-2-=/SUB=- и TInS-=SUB=-2-=/SUB=-, подвергнутых &amp;gamma;-облучению

The effect of gamma irradiation on the optical properties of layered TlGaSe2 and TlInS2 crystals in the wavelength range of 400–1,100 nm at 300 K is studied. From the analysis of optical absorption spectra, the energies of direct and indirect optical interband transitions before and after gamma irradiation are determined. It is shown that as the gamma-irradiation dose is accumulated in the range of 0–25 Mrad by TlGaSe2 and TlInS2 single crystals, the energies of direct and indirect allowed optical transitions increase from Egd = 2.06 eV and Egi = 1.90 eV at D = 0 Mrad to Egd = 2.11 eV and Egi = 1.98 eV at D = 25 Mrad for TlGaSe2 and Egd = 2.32 eV crystals and Egi = 2.27 eV at D = 0 Mrad to Egd = 2.35 and Egi = 2.32 eV at D = 25 Mrad for TlInS2 crystals. A decrease in the transmittance at doses from 0 to 5 Mrad and a further increase in the transmittance at the radiation dose D = 25 Mrad are observed.

Сравнительный анализ люминесценции слоев Ge : Sb, выращенных на подложках Ge(001) и Si(001)

Comparative studies of the luminescent properties of Sb doped Ge layers grown on Si (001) and Ge (001) substrates have been carried out. It is shown that in Ge:Sb/Ge(001) layers, in contrast to the Ge:Sb layers grown on silicon, indirect optical transitions make a significant contribution to the photoluminescence signal. This fact is associated with a longer lifetime of charge carriers in homoepitaxial Ge:Sb/Ge structures due to the absence of crystal lattice defects associated with the relaxation of elastic strain. It is shown that the significant increase in the contribution of direct optical transitions to the total photoluminescence signal observed at higher doping levels of Ge:Sb/Ge(001) layers is caused by an increase in the population of electronic states in the Г valley. The luminescent properties of Ge:Sb/Ge(001) and Ge:Sb/Si(001) layers with Sb concentration significantly exceeding its equilibrium solubility are strongly affected by the nonradiative recombination centers, which may be clusters of impurity atoms.

Changes of optical transition models caused by crystal structural changes in CaSe2O5

The present work concerns calcium pyroselenites CaSe2O5 with two independent crystal structures, their difference physical properties and phase transformations. α-CaSe2O5 in orthorhombic form shows 2D calcium oxide anionic layers whereas new compound β-CaSe2O5 crystallizes in monoclinic space group C2/c. It's a new structure type with 1D calcium oxide thick chains which are further connected by Se2O5 dimers. Rietveld refinement indicates that the weight fraction for α/β-CaSe2O5 mixture phase was found to be 84.5% and 15.5% for α- and β-CaSe2O5, respectively. Optical diffuse reflectance spectrum coupled with Kubelka–Munk fit gives a result that α-CaSe2O5 adopts direct optical transition model while β-CaSe2O5 is an indirect semiconductor. The experimental result matches well with the prediction of theory calculation. The optical bandgap values were fitted to be 4.74 and 4.12 eV for α- and β-CaSe2O5, respectively. Thermal analysis coupled with variable temperature powder XRD indicates the phase transformation between these two phases.

Nanostructured copper (II) oxide and its novel reduction to stable copper nanoparticles

In this study, we developed a facile approach for synthesizing CuO nanostructures and their conversion into stable nanostructured Cu via a green method. The release of hydroxyl radicals by the phytochemicals present in Psidium guajava leaves was exploited in the synthesis process. The structural, morphological, thermal, and optical characteristics of the CuO nanostructures were determined by X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, thermogravimetric and differential thermal analysis, ultraviolet–visible spectroscopy, and photoluminescence spectroscopy. The complete phase formation of the CuO nanostructures occurred above 554 °C. The single phase, monoclinic CuO with an average crystallite size of 35.2 nm exhibited strong absorption at about 275 nm with a prominent greenish yellow emission at 604 nm. The possibilities of both direct (2.35 eV) and indirect (1.01 eV) optical transitions were confirmed by optical studies. Elongated spherical particles with an average size of 36.6 nm were observed by electron microscopy. The magnetic properties of the synthesized nanostructures were probed using the vibrating sample magnetometer technique and weak ferromagnetism was observed at room temperature. The coercivity forces for the CuO nanostructures at T = 20 K and T = 300 K were estimated at 2945.2 G and 153.8 G, respectively. The bifurcation in the field cooled and zero-field cooled curves had a Curie temperature above 280 K, and the presence of a ferromagnetic shell and antiferromagnetic core were demonstrated in the synthesized sample. Very high enhancement of the thermal conductivity was observed for the CuO/water and CuO/ethylene glycol nanofluids systems. We also demonstrated the reduction of the synthesized CuO nanostructures using hydrazine hydrate to stable Cu nanoparticles at 303 K (which is much lower than previous findings) with an average size of 32.2 nm. The reaction parameters for the reduction process were optimized and the reduced particles were found to be highly stable under ambient conditions. The biosynthesized nanostructures are promising candidates for biomedical applications.