Type Of Optical Transition Research Articles

Calculation of the Density of the Distribution of Electronic States in the Conduction Band from the Fundamental Absorption Spectra of Amorphous Semiconductors

The region of fundamental absorption in the optical spectra of amorphous semiconductors is theoretically studied using the Davis-Mott approximation according to the Kubo-Greenwood formula. As is known, three types of optical transitions of the electron can be observed in the fundamental absorption region; from the tail of the valence band to the conduction band, from the valence band to the conduction band and from the valence band to the tail of the conduction band. For all these electronic transitions, analytical expressions of the partial absorption spectra are obtained from two different types of the Kubo-Greenwood formula. The width of the optical mobility gap and the proportionality coefficient were determined in the analytical form of the interband absorption spectrum by fitting them to the experimental interband absorption spectrum. A new method is presented for calculating the density of distribution of electronic states in the conduction band of amorphous carbon based on the experimental interband absorption spectrum and the analytical expression of the Kubo-Greenwood formula written for the interband absorption spectrum.

Nano copper (II) complex based on 2 -cyano-N'-((2-hydroxynaphthalen-1-yl) methylene) acetohydrazide: Synthesis, spectroscopic, electrospinning, DFT calculations and optical properties

2-cyano-N'-((2-hydroxynaphthalen-1-yl)methylene)acetohydrazide, (1) and its copper (II) chelate (2) were synthesized and characterized by elemental analyses, TGA, FT-IR and spectroscopic techniques. By using an X-ray diffraction approach, the structural characteristics of the produced powder chelating agent (1) and its copper (II) chelate (2) were identified. The transmission electron microscope technique was utilized to analyze the morphology of the isolated compounds (1 and 2). In DMF solutions at various molar concentrations, the electronic absorption spectra of the chelating agent (1) as well as its copper (II) chelate (2) was examined in the UV– Vis. area. In the area of maximal absorbance, the molar extinction coefficient of the ligand (1) and its copper (II) complex (2) were calculated. For the ligand (1) as well as its copper (II) complex (2) at various molarities, the optical transition type and optical energy band gap values were explored using the absorption coefficient. New electrospun fibers (3) and (4) which fabricated from the prepared ligand (1) and its copper (II) complex (2), respectively. Photoluminescence studies performed for the samples (1, 2, 3 and 4). In addition, computational analyses of the free ligand (1) and its copper complex (II) (2) were carried out using DFT calculations.

Effect of heat treatment on structure, surface morphology, linear and nonlinear optical properties of Na2Pc films for optoelectronic applications

In this paper, we study the influence of thermal annealing on the structural and optical properties of disodium phthalocyanine (Na2Pc) films. Thermal evaporation method under vacuum has been successfully used to deposit thin films of Na2Pc onto quartz and glass substrates. XRD analysis indicated that the pristine film has an amorphous structure and turns into a polycrystalline upon thermal annealing. SEM technique was used to investigate the morphological properties of thin films. The linear and nonlinear optical parameters of pristine and annealed films have been investigated in the wavelength range 200–2500 nm via spectrophotometric measurements of T(λ) and R(λ). The extinction coefficient of Na2Pc films exhibited four electronic transitions, namely, the Q, N, L and C bands. By applying a single oscillator model, many spectral dispersion parameters have been estimated in the transparent region of the spectrum. The dielectric constants, dissipation factor and optical conductivity were calculated and discussed. The optical transition type, energy gaps and Urbach energy of Na2Pc films were determined by the absorption band edges analysis. Also, the nonlinear refractive index and nonlinear optical susceptibility were evaluated from the linear refractive index results. The results of this investigation indicated the possibility of using Na2Pc films in optoelectronic device applications.

Structural, optical and DFT studies of disodium phthalocyanine thin films for optoelectronic devices applications

Theoretical and experimental investigations have been carried out on the structural and optical properties of disodium phthalocyanine (Na2Pc) thin films prepared using the thermal evaporation method. The optimized geometry structure, vibrational bands and HOMO-LUMO gap were predicted by density functional theory (DFT) computations. FTIR and XRD techniques were utilized to analyze the structural characteristics of Na2Pc thin films. XRD exhibited that the Na2Pc powder has α-polycrystalline nature with a triclinic structure, whereas the Na2Pc thin films have an amorphous structure. The optical properties of different thicknesses films were studied in the wavelength range of 200–2500 nm using the UV–Vis-NIR spectrophotometer. The absorption coefficient of the Na2Pc molecules showed four absorption bands in the UV–Vis region. The types of optical transitions were determined by applying the energy band model as an indirect allowed transition. The onset gap, optical gap and Urbach energy were estimated to be 1.31, 2.25 and 0.41 eV, respectively. The results of absorption coefficient and energy gap showed that the Na2Pc thin films can be utilized in optoelectronic device applications. The absorption index, k, and refractive index, n, were determined and found to be independent of the film thickness. The dispersion parameters have been evaluated according to a single oscillator model in the non-absorbing region of the spectrum.

Optical response of two-dimensional Dirac materials with a flat band

Two-dimensional Dirac materials with a flat band have been demonstrated to possess a plethora of unusual electronic properties, but the optical properties of these materials are less studied. Utilizing $\alpha$-$\mathcal{T}_3$ lattice as a prototypical system, where $0\le \alpha \le 1$ is a tunable parameter and a flat band through the conic intersection of two Dirac cones arises for $\alpha > 0$, we investigate the conductivity of flat-band Dirac material systems analytically and numerically. Motivated by the fact that the imaginary part of the optical conductivity can have significant effects on the optical response and is an important factor of consideration for developing $\alpha$-$\mathcal{T}_3$ lattice based optical devices, we are led to derive a complete conductivity formula with both the real and imaginary parts. Scrutinizing the formula, we uncover two phenomena. First, for the value of $\alpha$ in some range, two types of optical transitions coexist: one between the two Dirac cones and another from the flat band to a cone, which generate multi-frequency transverse electrical propagating waves. Second, for $\alpha=1$ so the quasiparticles become pseudospin-1, the flat-to-cone transition can result in resonant scattering. These results pave the way to exploiting $\alpha$-$T_3$ lattice for optical device applications in the terahertz frequency domain.

Межзонный двухфотонный линейно-циркулярный дихроизм в полупроводниках в приближении Кейна

Interband two-photon optical transitions are classified and expressions are obtained for the matrix elements in a narrow-gap semiconductor depending on the band parameters, degree of polarization, and light frequency. It is shown that the main contribution to two-photon linear-circular dichroism in narrow-gap semiconductors is made by optical transitions proceeding from the subband of light holes to the conduction band. The dependences of the partial coefficients of interband two-photon absorption of light, which differ from each other by the types of optical transitions, are analyzed depending on the degree of polarization of the light, and a quantitative analysis of the coefficient of linear-circular dichroism of two-photon absorption of light is carried out. Expressions are obtained for the spectral dependence of the coefficient of interband two-photon absorption of light in narrow-gap semiconductors in the Kane model.

Interband two-photon linear-circular dichroism in semiconductors in the Kane approximation

The dependences of the partial coefficients of interband two-photon absorption of light, which differ from each other by the types of optical transitions, are analyzed depending on the degree of polarization of the light, and a quantitative analysis of the coefficient of linear-circular dichroism of two-photon absorption of light is carried out. Expressions are obtained for the spectral dependence of the coefficient of interband two-photon absorption of light in narrow-gap semiconductors in the Kane model. Keywords: initial, virtual and final states, interband two-photon absorption of light, Kane's approximation.

Localized excitons in ZnMnO

Lines a, b, c, d and impurity absorption edge of ZnMnO for sigma- and π-polarizations of light in temperature interval of 7-300 K were registered in this paper. Intensive lines aπ and asigma are clearly observed in interval of 7-100 K, while other lines are observed only at low temperatures. For determination of type of optical transitions, to which excitonic lines a, b, c, d are corresponding the calculation of oscillator strength of most intensive lines was made. Lines aπ and asigma have Lorentz form, parameters of this form were calculated with OriginPro 9.1 program. The energy of impurity absorption edge was determined. Lines a, b, c and d were analyzed in model of Mn2+-4O2- cluster. Optical transitions take place from antibonding (p+d5)*-states in forbidden gap to state, which is splitted from the bottom of conduction band of ZnMnO. Electron-hole pairs which are localized inside the cluster are called as local excitons. Keywords: zinc oxide, localized excitons, antibonding states.

Локализованные экситоны в ZnMnO

Lines a, b, c, d and impurity absorption edge of ZnMnO for σ-and π-polarizations of light in temperature interval of 7 – 300 К were registered in this paper. Intensive lines aπ and aσ are clearly observed in interval of 7-100 K, while other lines are observed only at low temperatures. For determination of type of optical transitions, to which excitonic lines a, b, c, d are corresponding the calculation of oscillator strength of most intensive lines was made. Lines aπ and aσ have Lorentz form, parameters of this form were calculated with OriginPro 9.1 program. The energy of impurity absorption edge was determined. Lines a, b, c и d were analyzed in model of Mn2+ - 4O2- cluster. Optical transitions take place from antibonding (p+d5)*-states in forbidden gap to state, which is splitted from the bottom of conduction band of ZnMnO. Electron-hole pairs which are localized inside the cluster are called as local excitons.

Determination of the types of optical transitions and concentrations of donors and acceptors in GaN by the dependence of photoluminescence intensity on the excitation power

The results of the calculated and experimental depen-dence of the photoluminescence intensity on the excitation power density for silicon-doped GaN layers grown by molecular beam epitaxy are presented. A model was constructed for transitions in a compensated semiconductor upon interband generation of electron-hole pairs. It is shown that the dependence of the photoluminescence intensity on the excitation power density can be used to determine the recombination mechanism and concentrations of donors and acceptors in semiconductor. Keywords: GaN, ammonia-MBE, photoluminescence, heterostructures, point defects.

Pressure‐Tailored Band Engineering for Significant Enhancements in the Photoelectric Performance of CsI3 in the Optical Communication Waveband

AbstractThe bandgap and type of optical transition are key factors in determining the functionalities and applications of photoelectric materials. However, it is extremely difficult to modulate the bandgap and indirect‐direct bandgap transition for most materials. This study reports significant enhancements in photocurrents and an extended detection bandwidth resulting from pressure‐regulated indirect–direct bandgap transition in hypervalent CsI3. Furthermore, this study achieves an increase in the photocurrent by almost five orders of magnitude under visible‐light illumination. Impressively, the detection band‐edge shows a successive redshift from visible light to 1650 nm (optical communication waveband) upon compression. And high pressure is conducive to CsI3 operating at an ultralow bias input. Extensive high‐pressure spectroscopy analyses and theoretical calculations suggest that changes in the photoelectric properties of CsI3 are associated with enhanced I–I interactions along the quasi‐endless linear chain directions under compression. These findings offer an effective band engineering strategy for achieving broadband spectral response and high gains with an ultralow bias in photoelectric detectors.

Effect of CuCl2 powder on the optical characterization of Methylcellulose (MC) polymer composite

In this paper, polymer composite (PC) film based on Methylcellulose (MC) has been prepared by solution cast technique and its optical bandgap was discussed in detail. X-ray diffraction (XRD) method , UV–Vis absorption spectroscopy and Fourier transforms infrared spectroscopy (FTIR) technique were used to study the effect of different concentrations of copper chloride (CuCl2) salt on the structural and optical properties of the MC polymer. Scanning electron microscopy (FESEM) is used to examine the effect of CuCl2 on films morphology. The fundamental optical parameters, such as optical energy gap (Eg), optical dielectric constant (εr) and dielectric loss (εi) have been measured in which the amount of CuCl2 has significant effectiveness. Tauc’s relation was used to specifying the direct and indirect bandgap energy. The Eg of pure MC was considerably reduced from 6.21 to 2.68 eV by the addition of 20 wt% of CuCl2 salts. To specify the optical transition type, the exponent value of Tauc's law (γ) was identified by using the plot of εi against photon energy (hυ) to estimate the Eg value and specify the electronic transition type. For the composite samples, the Urbach energy (E0) increased, which can be evidence for a large possible number of tail-to-tail transitions. There is an increasing the refractive index (n) from 1.44 for pure MC to 2.04 upon the addition of 20wt.%CuCl2, indicating the occurrence of interaction between electrons and photons.

One-step hydrothermally assisted synthesis of CuZnSe thin film: photovoltaic application

Herein, a one-step hydrothermal route was directly employed to synthesize copper-doped zinc selenide (CuZnSe) thin film. The synthesized CuZnSe thin film was analyzed for its optostructural, morphological, compositional and photovoltaic characteristics. The optical studies revealed that the deposited CuZnSe thin film exhibited a direct allowed type of optical transition with 2·32 eV bandgap energy. The structural studies demonstrated the formation of a pure cubic CuZnSe crystal phase structure with 57 nm crystallite size. The low- and high-resolution scanning electron microscopy images showed the formation of nanospherical surface morphology. The energy-dispersive spectroscopic analysis confirmed the presence of the elements copper (Cu), zinc (Zn) and selenium (Se) in the deposited thin film. Lastly, the photoelectrochemical properties were investigated using a two-electrode configuration in the dark and under light illumination from a 500 W tungsten filament lamp with an intensity of 30 mW/cm2 on an electrochemical workstation. In the dark condition, the current density–voltage (J–V) curve shows diode-like rectifying characteristics, and under illumination of light, it shows photovoltaic performance with 0·73% photoconversion efficiency. The photovoltaic results indicated that employment of one-step hydrothermally synthesized CuZnSe thin films has great potential for fabrication of photoelectrochemical solar cells.

OPTICAL PROPERTIES OF OF ZnS:Fe NANOCHRYSTALLS OBTAINED BY COLLOIDAL METHOD

Iron doped zinc sulfide nanocrystals were obtained by colloidal synthesis using gelatin, lactose or polyvinyl alcohol as a stabilizing matrix. The structure of the nanocrystals was determined using X-ray diffraction (XRD). The influence of the concentration effect on the size and properties of ZnS nanocrystals, the optical absorption spectra and photoluminescence spectra were studied, and the types of optical transitions in these nanocrystals were determined.

Development of an Innovative and Eco-Friendly UV Radiation Absorber, Based on Furan Moieties

The increasing concern about the environmental impact of ingredients largely used as sunscreens today is pushing research towards new frontiers for the synthesis and degradation potential of innovative sun protection structures. This study deals with the design, synthesis, efficacy evaluation, and environmental impact of new photo-stable molecules at extended electronic conjugation and which have the ‘charge-transfer’ type of optical transition (push and pull). They efficiently perform absorption of electro-magnetic radiation in the UVB and UVA regions. A preliminary screening, dealing with a whole series of aromatic donors, led to the selection of a key substrate derived from waste material, provided with low environmental impact and largely available on the market. As far as the acceptor was concerned, an ester with cyan-acrylic structure and functionalized with aliphatic chains emerged as the most suitable. This allowed a shift of the λ of absorption in the range of the ultraviolet wavelengths. The synthesis procedures include an acid-catalyst-fitted esterification, condensation, and cold crystallization. It was continuously monitored with suitable characterization instruments (GC-MS, 1H NMR, 13C NMR), in order to avoid by-products and parasitic reactions. The resulting structures (patent pending) are composed by a furan core condensed with cyano-acrylic esters, derivatized with branched chains of hydrocarbons of medium length. The obtained structures are capable of good absorption in the range of wavelengths between 300 and 400 nm. Because of their structures, the new molecules are largely soluble in many common oils and cosmetic ingredients. The new molecules have been successively evaluated in terms of photo-stability, biodegradability, compatibility, and UVB-UVA protection capability, both in-vivo and in-vitro, in comparison with existing sunscreens.

Enhancement in dielectric and optical properties of La1-xCexFeO3 nanoparticles

La1-xCexFeO3; 0≤×≤0.15 nanoparticles were prepared in single phase using flash combustion technique. The crystalline structure and morphology of the investigated samples were researched through XRD and HRTEM. As a result, the dielectric constant of LaFeO3 was enhanced by Ce3+ ion substitution due to some the Ce3+ ions changes its valence to Ce4+ ions. The conduction mechanism in La1-xCexFeO3 samples is the small polaron (SP) tunneling. The type of optical transition is direct allowed transition and the band gap values are in the range of (2.8–1.89eV). ε/ values of La0.90Ce0.10FeO3 decreases by increasing the external applied pressure at different frequencies.

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 parameters of epoxy-CoSO4.7H2O polymer hybrid material

ABSTRACTEpoxy-CoSO4.7H2O polymer hybrid films were prepared with various loading levels (from 0.28 wt% up to 5.00 wt%) of inorganic component (CoSO4.7H2O). UV-vis absorption spectra of these hybrid films, in the wavelength range starting from 200 nm up to 800 nm were analyzed to estimate their band gap. Interestingly, drastic reduction in the band gap was observed for hybrid film containing 0.56 wt% of CoSO4.7H2O, compared to pure epoxy film. Various optical parameters such as transmittance (T’), reflectance (R), absorption coefficient (α), extinction coefficient (k), refractive index (n), real and imaginary parts of optical dielectric constant (εr and εi), skin depth (δ) and optical conductivity (σ) for these films are determined and analyzed. Variation of εi with respect to energy is also employed to confirm the types of optical transitions in the material. Single oscillator model was also adopted to obtain various optical parameters of importance.

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.

The promotion of Indeno [1, 2-b] flourene-6, 12 dione thin film to be changed into stable aromatic compound under the effect of annealing treatment

Thin films of Indeno [1, 2-b] flourene-6,12-dione (I[1, 2-b]F dione) were performed by the thermal vacuum evaporation technique. The identifications of a thin film such as structure, molecular structure and surface morphological were determined before and after annealing temperature using Fourier-transform infrared (FTIR), X-Ray diffreaction (XRD) and Scan electron microscopy (SEM). FTIR spectrum showed a diminishing of C═O bonds after annealing treatment. The analysis of XRD clarified that I[1, 2-b]F dione in its powder form is orthorhombic with space group P222. On the other hand, the as-deposited thin films were realized by amorphous nature, while the annealed I[1, 2-b]F dione thin films showed polycrystalline nature. The calculation of the average crystallite sizes proved that the annealed I[1, 2-b]F dione thin films were characterized by their nanostructure property. The SEM images illustrated that the aggregation of the particles has the formation of the nano-rod shape associated with average width in the range of 120–150 nm for the as-deposited thin film, while the average width of the branched nano-rod shape for the annealed thin films was observed to be in the range of 150–200 nm. The transmittance and reflectance spectrum at normal incidence of light in the range 190–2500 nm were employed to discuss the optical properties of as-deposited and the annealed I[1, 2-b]F dione thin films. Our conceptions elucidated that the optical transition type of I[1, 2-b]F dione films is the indirect allowed transition. The values of the fundamental energy gap for the as-deposited and the annealed films at 473 K were calculated out be 3.41 and 3.70 eV respectively. The dispersion of the refractive index was studied according to the single oscillator model. Moreover, the third-order nonlinear susceptibility and the nonlinear refractive index for I[1, 2-b]F dione thin films were calculated.