Fundamental Absorption Region Research Articles

A Full‐Scale Analysis of Absorption Edges in Dye‐Doped Nonlinear Optical Polymers

AbstractA full‐scale analysis of the absorption edges by modified Tauc‐Lorentz models is essential in determining the optical bandgap and Urbach energy of semiconductors, transparent conductors, ionic compounds, and dielectric materials. This technique has not yet been applied to analyzing organic nonlinear optical (NLO) materials. This problem is tackled by preparing high‐quality films of guest–host NLO polymers with a wide thickness range from sub‐micron to 200 microns, allowing accurate measurement of full‐spectral absorption coefficients of NLO materials over four orders of magnitude by the UV‐VIS‐NIR spectroscopy. The Tauc model and a new Monolog–Lorentz model are used to study the optical absorption edge of guest‐host NLO polymers containing various push‐pull chromophores and the dependence of optical bandgap and Urbach edge on the structure and composition of materials is analyzed. The results reveal the critical transition of the Urbach exponential tail to a low energy tail that overlaps with vibrational overtones of materials at the telecom wavelengths. Determining the fundamental absorption region of organic NLO films in this study provides quantitative insight into the research to harness the resonance‐enhanced nonlinear coefficients of materials by operating at the wavelengths near the band edge with the control of optical loss.

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.

New optical dispersion models for the accurate description of the electrical permittivity in direct and indirect semiconductors

We propose new optical dispersion models to describe the imaginary part of the electrical permittivity of dielectric and semiconductor materials in the fundamental absorption region. We work out our procedure based on the well-known structure of the semi-empirical Tauc–Lorentz dispersion model and the band-fluctuations approach to derive a five-parameter formula that describes the Urbach, Tauc and high-absorption regions of direct and indirect semiconductors. Main features of the dispersion models are the self-consistent generation of the exponential Urbach tail below the bandgap and the incorporation of the Lorentz oscillator behavior due to electronic transitions above the fundamental region. We apply and test these models on optical data of direct (MAPbI3, gallium arsenide and indium phosphide), indirect (gallium phosphide and crystalline silicon), and amorphous hydrogenated silicon semiconductors, accurately describing the spectra of the imaginary part of the electrical permittivity. Lastly, we compare our results with other similarly inspired dispersion models to assess the optical bandgap, Urbach tail and oscillator central resonance energy.

Dispersion of optical constants of Si:PbGeO crystal in the terahertz range

Objectives. Advances in laser physics over the last decade have led to the creation of sources of single-period electromagnetic pulses having a duration of about 1 ps, corresponding to the terahertz (THz) frequency range and a field amplitude of several tens of MV/cm. This allows the electrode-free application of an electric field to a ferroelectric for observing not only the excitation of coherent phonons, but also ultrafast (at the sub-picosecond timescale) dynamic polarization switching. To detect polarization switching, a pump-probe technique is used in which a THz pulse is used with an optical probe. Since its intensity is proportional to the square of the polarization, the signal of the optical second harmonic is used to measure polarization switching under the action of a THz pulse. To evaluate switching efficiency, both linear (refractive index and absorption coefficient) and non-linear optical characteristics (quadratic and cubic susceptibilities) are required. For any application of ferroelectric crystals in the THz range, knowledge of the relevant linear optical characteristics is also necessary.Methods. The technique of THz spectroscopy in the time domain was used; here, a picosecond THz pulse transmitted through the crystal is recorded by strobing the detector with a femtosecond optical pulse. The THz-induced dynamics of the order parameter in a ferroelectric was studied by detecting the intensity of a nonlinear optical signal at the frequency of the second optical harmonic.Results. The transmission of a THz wave and the intensity of second harmonic generation on a lead germanate crystal doped with silicon in the time and spectral domains were measured. On this basis, the absorption coefficient dispersion and cubic nonlinear susceptibility were calculated in the range of 0.5-2.0 THz. The presence of a region of fundamental absorption near the phonon modes was confirmed along with a resonant enhancement of the cubic nonlinear susceptibility for two phonon modes Ω1 = 1.3 THz and Ω2 = 2.0 THz.Conclusions. The proposed technique is effective for analyzing the dispersion of the optical characteristics of ferroelectric crystals. The significantly improved spectral resolution (0.1 THz) increases the accuracy of determining nonlinear susceptibility due to the detailed analysis of the linear and nonlinear contributions to the second harmonic intensity.

Spectral dependence of UV light penetration into powder TiO2 anatase

The penetration depth of UV monochromatic light into deposited TiO2 anatase powder was investigated. The light penetration depth was considered in units of numbers of surface hole, Ns, and electron, NO2, centers activated during irradiation. The numbers of hole centers Ns were obtained from photostimulated oxygen isotope exchange reaction registered by means of mass spectrometry. The numbers of electron centers NO2 were obtained from subsequent thermo-programmed desorption spectroscopy of photoadsorbed oxygen O2-. It was shown that the depth D of UV light penetration into UV absorbing TiO2 powder coat depends on the wavelength and intensity I0 of incident UV light and obeys to the equation DI0=1kefflg(I0/Im) obtained from the Beer–Lambert–Bouguer law with effective extinction coefficient keff and minimal value of light intensity Im, below which the photoactivation is considered negligible. Commonly, the value Im has an order of a few microwatts/cm2 for activation of electron and hole type centers except for a case of 334 nm falling into the band of the first direct band-to-band transition of anatase, where relatively high threshold value Im=0.063 mW/cm2 for photoactivation of hole centers was revealed. Effective extinction coefficient keff may be considered proportional to the absorption coefficient of TiO2 in the region of fundamental absorption, whereas, in the non-fundamental absorption region, keff exceeds significantly that of TiO2 due to, probably, dominating scattering processes. At small intensities of the incident light, there is a strong sensitivity of the penetration depth (and activated surface) to the intensity.

Luminescent properties of colloidal Ag-=SUB=-2-=/SUB=-S quantum dots for photocatalytic applications

The structural and optical properties of colloidal Ag2S quantum dots in various environments are investigated. With the help of transmission electron microscopy, X-ray diffraction and energy-dispersive X-ray analysis the formation of colloidal Ag2S quantum dots with an average size of 2-3 nm with a monoclinic crystal lattice, and Ag2S/SiO2 core-shell systems based on them, has been established. The change in the luminescence quantum yield of quantum dots with the change of the surface environment state is shown. The decoration of TiO2 nanoparticles of 10-15 nm in size with Ag2S quantum dots was performed and the influence of the structure of the interfaces of quantum dots and their environment (2-mercaptopropionic acid, water, ethylene glycol, SiO2 dielectric shell with a thickness of 0.6 nm and 2.0 nm) on the formation of TiO2-Ag2S heterosystems was analyzed. For Ag2S quantum dots passivated with 2-mercaptopropionic acid, signs of charge phototransfer after adsorption on TiO2 nanoparticles surface have been established. Signs of reactive oxygen species appearance due to charge phototransfer in heterosystem are enstablished, based on methylene blue photobleaching under excitation of heterosystem outside of TiO2 fundamental absorption region, Keywords: Quantum dots, photocatalysis, luminescence, titanium dioxide, silver sulfide.

Luminescent properties of colloidal Ag-=SUB=-2-=/SUB=-S quantum dots for photocatalytic applications

The structural and optical properties of colloidal Ag2S quantum dots in various environments are investigated. With the help of transmission electron microscopy, X-ray diffraction and energy-dispersive X-ray analysis the formation of colloidal Ag2S quantum dots with an average size of 2-3 nm with a monoclinic crystal lattice, and Ag2S/SiO2 core-shell systems based on them, has been established. The change in the luminescence quantum yield of quantum dots with the change of the surface environment state is shown. The decoration of TiO2 nanoparticles of 10-15 nm in size with Ag2S quantum dots was performed and the influence of the structure of the interfaces of quantum dots and their environment (2-mercaptopropionic acid, water, ethylene glycol, SiO2 dielectric shell with a thickness of 0.6 nm and 2.0 nm) on the formation of TiO2-Ag2S heterosystems was analyzed. For Ag2S quantum dots passivated with 2-mercaptopropionic acid, signs of charge phototransfer after adsorption on TiO2 nanoparticles surface have been established. Signs of reactive oxygen species appearance due to charge phototransfer in heterosystem are enstablished, based on methylene blue photobleaching under excitation of heterosystem outside of TiO2 fundamental absorption region, Keywords: Quantum dots, photocatalysis, luminescence, titanium dioxide, silver sulfide.

Electroreflectance of Cu2(Cd1−x,Znx)SnS4 thin film solar cells

The effect of Cu off-stoichiometry and Zn alloying on the fundamental absorption region of ${\mathrm{Cu}}_{2}\mathrm{CdSn}{\mathrm{S}}_{4}$ (CCTS) absorbers in complete solar cells has been investigated using electroreflectance (ER) spectroscopy at room temperature. It is found that ER spectra consist of contributions from two different sources, one of which corresponds to band gap transition in the absorber layer and the other to the interference effect in the window layer. ER measurements on CCTS samples reveal a near-constant band gap energy of 1.37--1.38 eV and a relatively small broadening of 60--90 meV in the probed $0.8<\mathrm{Cu}/(\mathrm{Cd}+\mathrm{Sn})<0.89$ compositional range, in contrast to related kesterites ${\mathrm{Cu}}_{2}\mathrm{ZnSn}{(\mathrm{S},\mathrm{Se})}_{4}$. The analysis of the band gap in ${\mathrm{Cu}}_{2}({\mathrm{Cd}}_{1\ensuremath{-}x},{\mathrm{Zn}}_{x})\mathrm{Sn}{\mathrm{S}}_{4}$ alloys yields a quadratic dependence on Zn content with a bowing parameter of 0.4 eV. Finally, the broadening parameters of the band gap transitions as well as their compositional dependence are evaluated and discussed.

Study of the parameters of molybdenum oxide films by optical methods

In this work, the physical properties of molybdenum oxide films obtained by reactive magnetron sputtering were studied. Based on the experimental transmission and reflection spectra for samples at different currents in the fundamental absorption region, the optical band gap was determined, and in the visible range, the thickness of the films was defined. The studied films had the values of optical parameters that were observed by other authors. However, the dependence of the optical band gap width on the thickness of the films is not found in the literature and can be associated with the influence of the flux density of the substance sputtered from the target surface on the structure of the film.

Люминесцентные свойства коллоидных квантовых точек Ag-=SUB=-2-=/SUB=-S для фотокаталитических приложений

The structural and optical properties of colloidal Ag2S quantum dots in various environments are investigated. With the help of transmission electron microscopy, X-ray diffraction and X-ray energy dispersion analysis, the formation of colloidal Ag2S quantum dots with an average size of 2-3 nm with a monoclinic crystal lattice, and Ag2S/SiO2 core-shell systems based on them, has been established. The change in the luminescence quantum yield of quantum dots with the change of the surface environment state is shown. The decoration of TiO2 nanoparticles of 10-15 nm in size with Ag2S quantum dots was performed and the influence of the structure of the interfaces of quantum dots and their environment (2-mercaptopropionic acid, water, ethylene glycol, SiO2 dielectric shell with a thickness of 0.6 nm and 2.0 nm) on the formation of TiO2-Ag2S heterosystems was analyzed. For Ag2S quantum dots passivated with 2-mercaptopropionic acid, signs of charge phototransfer after adsorption on TiO2 nanoparticles surface have been established. Signs of reactive oxygen species appearance due to charge phototransfer in heterosystem are enstablished, based on methylene blue photobleaching under excitation of heterosystem outside of TiO2 fundamental absorption region,

Bandgap engineering of hydrogenated a-SiC:H thin films for photoelectrochemical water splitting applications

Bandgap engineering of a-SiC:H thin films was carried out to assess the material light absorption without compromising its photoelectrochemical water splitting capabilities. The tailoring was performed by varying the hydrogen concentration in the semiconductor and by post-deposition isochronical annealing treatments from 100 °C to 700 °C. Bandgap values were obtained by fitting the fundamental absorption region of the absorption coefficient using three different models. Differences among bandgap values extracted by these methods and its correlation with the a-SiC:H structure, demonstrate that structural features, rather than a hydrogen rearrangement or depletion, would be responsible for annealing induced optical bandgap increment. These features are taking in advantage for the bandgap engineering of a-SiC:H without changing Si-C stoichiometry. Optical bandgap values for p-doped a-SiC:H samples gradually increased from 2.59 to 2.76 eV upon performing each annealing step until 600 °C. Temperature at which an enhancement in the electric performance is observed. We believe, these results will help on the design of monolithic tandem solar cells for water splitting applications.

Silicon Photonic Infrared-Wave Emitter

This study proposes an approach for building high quality cheap emitters in mid-wave (MW) and long-wave (LW) infrared spectrum bands. A photonic infrared-wave emitter based on light down conversion from the region of fundamental absorption by semiconductor to infrared region has been proposed. The efficiency of such conversion does not depend on quantum yield of interband recombination. It increases with the rise of emitter temperature and can be optically controlled. Such device has a large working surface area with spectral characteristics that do not depend on the forbidden bandwidth in semiconductor. The calculated and experimental characteristics of the power of silicon photonic emitter in 3–5 μm and 8–12 μm wavelength bands as a function of the temperature and intensity of exciting radiation are also presented. The parameters of the known emitters and proposed one are compared, and the technological description of the proposed device is presented.

Structural transformation of energetic cyclo-pentazolate salt under the pressure

The density functional theory (DFT) was applied to investigate the structure, optical properties, and non-bonded interactions of the cyclo-pentazolate salt [Mg(H2O)6(N5)2]·4H2O in the pressure range of 0–100 GPa. The results show that there is a structural transition at the pressure of 64 GPa, which has not been found in the experiments. As the pressure increases, the band gap gradually decreases with a sudden increase at the structural transition point. The analysis of DOS presents that the electronic localization in the crystal gradually weakens under the pressure. The structural transition point also exists in the hydrogen bonds and the Hirshfeld surfaces under compression. The absorption spectra of the cyclo-pentazolate salt at high pressures show some strong bands in the fundamental absorption region. The increasing pressure helps to improve the photoelectric performances of the crystal. Our studies are expected to provide a new understanding of the structural transition of the novel energetic materials under high pressures.

Tailoring of electrical properties of Cu2O thin films fabricated by oxygen injection after argon plasma reduction of CuO films

Cu2O thin films with tailored electrical properties were produced by 25 s plasma reduction of CuO films and immediate oxidation under oxygen flows of 0, 3, 6, 9, 12, 15, and 18 sccm. CuO target films were easily prepared by the dip-coating method and annealing at 400 °C. CuO was transformed to Cu2O by employing an argon plasma treatment devised in our laboratory. Cu2O was produced as the only crystalline phase detected, and the films exhibited smooth surfaces with roughness around 5 nm. All films presented high reflectance in the fundamental absorption region (30%) and high transmittance in the transparency region of Cu2O (65%). The good optical behavior permitted the accurate calculation of optical constants by fitting reflectance and transmittance to an optical model designed with the help of Film Wizard™ software. Variations with oxygen injection were detected for the components of the dielectric function, index of refraction, and absorption coefficient. More importantly, the electrical properties of Cu2O showed a clear dependence on O2 injection. Even though resistivity was on average 80 Ω∙cm, hole mobility varied in the span of 0.75–3.2 cm2/V s, and hole density in the range of 1016–1017/cm3. Upon storage in open-air conditions, the mobility of Cu2O films was increased to 3.8 cm2/V s. The Cu2O films produced show similar quality to those obtained by a sputtering deposition method. However, simpler equipment was employed. The plasma method studied is promising for fast mass production of Cu2O thin films.

Електронні характеристики квантових точок СdS, що містять дефекти

Using the density functional theory and the generalized gradient approximation, we calculated the atomic structure, the density of electronic states, and the optical absorption spectra of CdS quantum dots containing intrinsic defects — a cadmium vacancy VCd and an interstitial sulfur atom SI, and substitutional impurities — zinc and copper in place of the atom cadmium — ZnCd and CuCd, respectively. The calculations were performed for the Cd33S33 cluster corresponding to the so-called “magic” size of the quantum dot. This size has a minimum of dangling bonds at the surface and allows the using of such a cluster without the passivation. The structural relaxation during the formation of such defects and the distribution of the wave function of the state corresponding to the top of the valence band are analyzed in details. It has been shown that the cadmium vacancy forms local states in the band gap of CdS nanocrystals, and can serve as centers of radiative recombination. Other defects form energy levels in the depths of the valence band or near its top, but whose energy positions do not correspond to the band maxima in the experimental photoluminescence spectra of CdS quantum dots, both undoped and doped with zinc. The calculated optical absorption spectra demonstrate a strong peak in the region of fundamental absorption of CdS for a cluster containing a substitutional impurity of CuCd, in contrast to other systems where no such peaks are observed. In addition, the replacement of the cadmium atom with copper leads to a decrease in the number of chemical bonds to three and, accordingly, to the largest relaxation among the systems studied. This feature is caused by the crystal structure inhomogeneity of copper sulfide CuxS, which, depending on stoichiometry, can be either a semiconductor or a metal.

Temperature Dependence of the Band Gap of MnAgIn7S12 Single Crystals

MnAgIn7S12 single crystals 16 mm in diameter and ~40 mm in length are grown by planar crystallization of the melt. It is shown that the material grown crystallizes with the formation of the cubic spinel structure. From the transmittance spectra recorded in the region of fundamental absorption in the temperature range 10–320 K, the band gap Eg of the single crystals and its temperature dependence are determined. The dependence has a shape typical of most semiconductor materials: as the temperature is lowered, the band gap Eg increases. A calculation is carried out, and it is shown that the calculated values are in agreement with the experimental data.

Electrodeposition, optical and photoelectrochemical properties of BiVO4 and BiVO4/WO3 films

BiVO4 films and BiVO4/WO3 heterostructure have been obtained by electrosynthesis method with the use of interferometric control of the thickness of films during their electrodeposition. The electrodeposition of BiVO4 films involves the electrochemical stage of oxidation of VO2+ to H2VO4− and the chemical interaction of BiO+ with H2VO4−. It has been established that the as-deposited film contains up to 25% water by means of two-beam interferometry, DTA and TGA. The films containing BiVO4 and WO3 after annealing at 500 °C are crystallized to a monoclinic scheelite structure. It is found that the photoelectrochemical efficiency of crystalline BiVO4 films depends on their thickness. It is shown that the quantum efficiency (η = 0.30–0.65) at λ = 400–450 nm is observed for BiVO4/WO3 heterostructure with a BiVO4 film thickness of 140 nm and WO3 layer of 500 nm. The WO3 layer makes an additional contribution to photocurrent in the fundamental absorption region and increases the overall photoelectrochemical efficiency in the UV and the near-visible regions.

Band-fluctuations model for the fundamental absorption of crystalline and amorphous semiconductors: a dimensionless joint density of states analysis

We develop a band-fluctuations model which describes the absorption coefficient in the fundamental absorption region for direct and indirect electronic transitions in disordered semiconductor materials. The model accurately describes both the Urbach tail and absorption edge regions observed in such materials near the mobility edge in a single equation with only three fitting parameters. An asymptotic analysis leads to the universally observed exponential tail below the bandgap energy and to the absorption edge model at zero Kelvin above it, for either direct or indirect electronic transitions. The latter feature allows the discrimination between the absorption edge and absorption tails, thus yielding more accurate bandgap values when fitting optical absorption data. We examine the general character of the model using a dimensionless joint density of states formalism with a quantitative analysis of a large amount of optical absorption data. Both heavily doped p-type GaAs and nano-crystalline Ga1-xMnxN, as examples for direct bandgap materials, as well as amorphous Si:Hx, SiC:Hx and SiNx, are modeled successfully with this approach. We contrast our model with previously reported empirical models, showing in our case a suitable absorption coefficient shape capable of describing various distinct materials while also maintaining the universality of the exponential absorption tail and absorption edge.

Температурная зависимость ширины запрещенной зоны монокристаллов MnAgIn-=SUB=-7-=/SUB=-S-=SUB=-12-=/SUB=-

AbstractMnAgIn_7S_12 single crystals 16 mm in diameter and ~40 mm in length are grown by planar crystallization of the melt. It is shown that the material grown crystallizes with the formation of the cubic spinel structure. From the transmittance spectra recorded in the region of fundamental absorption in the temperature range 10–320 K, the band gap E _ g of the single crystals and its temperature dependence are determined. The dependence has a shape typical of most semiconductor materials: as the temperature is lowered, the band gap E _ g increases. A calculation is carried out, and it is shown that the calculated values are in agreement with the experimental data.

Investigation of optical spectra of dielectric crystals

Optical spectra of crystals oxyhalides of bismuth are investigated in this work. When measuring the transmission spectra of single crystals of oxyhalides of bismuth in the direction perpendicular to the layers, a large number of peaks associated with the occurrence of the interference pattern when light is reflected from the layers. The intense absorption band at 4.0 eV for BiOCl crystal has a doublet structure, in the case of the BiOBr crystal splitting is less pronounced. Upon excitation in the fundamental absorption region there photoluminescence spectra are investigated. The spectrum of luminescence of BiOCl crystal is not elementary and it consists of two bands of radiation. The presence in the excitation spectra and luminescence shortwave bands strongly depends on the degree of purity of the bismuth oxide is used in the synthesis oxyhalides of bismuth crystal. Thus, it allows to link the short-wave emission band with uncontrolled impurity, and long-wave – with its own glow crystal substrate.