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The energy positions and wave function shapes of the ground and excited states of impurities, including resonance states, are studied using the expansion of the impurity wave function in basis functions. The structures under study are rectangular GaAs/AlGaAs quantum wells with four different widths. In all cases, the impurity binding energy (relative to the corresponding sub-band) has a maximum at or near the center of the quantum well, decreases as the heterointerface is approached, and apparently has a limit of 0 if the impurity moves deeper into the barrier. If the impurity moves away from the center of the quantum well, then the "center of mass" of the electron charge of non-resonant impurity states follows the impurity atom, and the "center of mass" of the electron charge of the resonant impurity states moves away from it. The effect is more pronounced for the ground and first resonance states for wider quantum wells, and the shifts reach a maximum when the impurity atom is positioned near the midpoint of the path between the quantum well center and the heterointerface. [ABSTRACT FROM AUTHOR]

Cu2ZnSn(S,Se)4 (CZT(S,Se)) thin films exhibit the characteristics necessary to be effective absorbers in solar cells. In this report, the room temperature experimental Raman scattering spectra, recorded at different excitation wavelengths, are systematically analyzed theoretically using the results of DFT harmonic frequencies calculations at the Γ-point for various modifications of kesterite (KS), stannite (ST), and pre-mixed Cu-Au (PMCA) crystal structures. The specific anharmonism‐induced features in the spectra of CZT(S,Se) crystals are identified, and the spectral lineshapes at varied strengths of anharmonic interaction are simulated. A robust agreement between the experimental Raman spectrum and the theoretical results is demonstrated, ensuring the reliability of estimating parameters related to anharmonic effects. Therefore, our findings show that incorporating anharmonism as an additional contribution to the phonon spectra, particularly in non-resonant cases, allows for a more accurate description of the vibrational properties of CZT(S,Se). This could play a crucial role in distinguishing between different phases of CZT(S,Se) materials and open new possibilities for the fabrication of solar cells with enhanced characteristics. [ABSTRACT FROM AUTHOR]

Boronate esters are a class of compounds containing a boron atom bonded to two oxygen atoms in an ester group, often being used as precursors in the synthesis of other materials. The characterization of the structure and properties of esters is usually carried out by UV-visible, infrared, and nuclear magnetic resonance (NMR) spectroscopic techniques. With the aim to better understand our experimental data, in this article, the density functional theory (DFT) is used to analyze the UV-visible and infrared spectra, as well as the isotropic shielding and chemical shifts of the hydrogen atoms 1H, carbon 13C and boron 11B in the compound 4-(4,4,5,5-tetramethyl-1,3,2-dioxoborolan-2-yl)benzaldehyde. Furthermore, this study considers the change in its electronic and spectroscopic properties of this particular ester, when its boron atom is coordinated with a fluoride anion. The calculations were carried out using the LSDA and B3LYP functionals in Gaussian-16, and PBE in CASTEP. The results show that the B3LYP functional gives the best approximation to the experimental data. The formation of a coordinated covalent B–F bond highlights the remarkable sensitivity of the NMR chemical shifts of carbon, oxygen, and boron atoms and their surroundings. Furthermore, this bond also highlights the changes in the electron transitions bands n → π* and π → π* during the absorption and emission of a photon in the UV-vis, and in the stretching bands of the C=C bonds, and bending of BO2 in the infrared spectrum. This study not only contributes to the understanding of the properties of boronate esters but also provides important information on the interactions and responses optoelectronic of the compound when is bonded to a fluorine atom. [ABSTRACT FROM AUTHOR]

In recent years, two‐dimensional (2D) materials with unique mechanical, optical, and electrical properties have attracted extensive attention. In terms of mechanical properties, 2D molybdenum disulfide (MoS2) can perform larger strains than traditional semiconductor materials. In this contribution, the chemical vapor deposition technique to grow MoS2 films on a Si wafer and transfer them onto a flexible substrate is used. The controlled deformation of 2D MoS2 samples is realized by encapsulating them with a flexible acrylate film via successive spin‐coating and photopolymerization. Improved strain control is achieved due to the perfect integration of different components (MoS2/substrate) and the high adhesion of polymers. This approach provides a better detection of the changing structure of the MoS2 monolayer on the flexible substrate during tensile. It is noted that the crystal symmetry damage caused by strain is reflected in the redshift of the characteristic bands of MoS2. Hence, an effective way for strain regulation of MoS2 for future applications in flexible devices is provided. [ABSTRACT FROM AUTHOR]

The use of computational methods that allow us to perform characterization on new compounds is not a novelty; nevertheless, the degree of complexity of the structures makes their study more challenging since new techniques and methods are required to adjust to the new structural model. The case of nuclear magnetic resonance characterization of boronate esters is fascinating because of its widespread use in materials science. In this paper, we use density functional theory to characterize the structure of the compound 1-[5-(4,5-Dimethyl-1,3,2-dioxaborolan-2-yl)thiophen-2-yl]ethanonea by means of nuclear magnetic resonance. We studied the compound in its solid form with the PBE–GGA and PBEsol–GGA functionals, with a set of plane wave functions and an augmented wave projector, which included gauge in CASTEP and its molecular structure with the B3LYP functional using the package Gaussian 09. In addition, we performed the optimization and calculation of the chemical shifts and isotropic nuclear magnetic resonance shielding of 1 H, 13 C, and 11 B. Finally, we analyzed and compared the theoretical results with experimental diffractometric data observing a good approximation. [ABSTRACT FROM AUTHOR]

A theoretical investigation on electronic states in triangular MoS2 quantum dots of different sizes is performed within density functional theory first‐principles formalism. Herein, the associated interband optical response from real and imaginary parts of the dielectric function is calculated. The study considers both undoped and Al‐, Si‐, and P‐doped systems. Spin‐related magnetic properties are considered through the evaluation of total magnetic moment. It is revealed that small enough structures have a semiconductor character, but evolve to seemingly half‐metallic with increasing dot size. Nonzero magnetic response is attributed to edge effects. Magnetic behavior of doped systems deviates from the linear dependence of the total magnetic moment with size occurring in the undoped case, producing situations with higher total momentum values. It is found that increasing the doping density may have certain influence over total magnetic moment response of the structures, although edge dominance is kept all the way. Optical properties are not as sensitive to incident light polarization, but they actually do with regard to the particular spin orientation. Calculation for the refraction index in the triangular quantum dots shows values still below the accepted in the MoS2 monolayer case, although a correct trend of variation is reported in this sense. [ABSTRACT FROM AUTHOR]

This theoretical study is devoted to the effects of pressure and temperature on the optoelectronic properties assigned to the first lowest transition of the (D + , X) excitonic complex (exciton-ionized donor) inside a single A l A s / G a A s / A l A s spherical quantum dot. Calculations are performed within the effective mass approximation theory using the variational method. Optical absorption and refractive index as function of the degree of confinement, pressure, and temperature are investigated. Numerical calculation shows that the pressure favors the electron-hole and electron-ionized donor attractions which leads to an enhancement of the binding energy, while an increasing of the temperature tends to reduce it. Our investigations show also that the resonant peaks of the absorption coefficient and the refractive index are located in the terahertz region and they undergo a shift to higher (lower) therahertz frequencies when the pressure (temperature) increases. The opposite effects caused by temperature and pressure have great practical importance because they offer an alternative approach for the adjustment and the control of the optical frequencies resulting from the transition between the fundamental and the first excited state of exciton bound to an ionized dopant. The comparison of the optical properties of exciton, impurity and (D + , X) facilitates the experimental identification of these transitions which are often close. Our investigation shows that the optical responses of (D + , X) are located between the exciton (high energy region) and donor impurity (low energy region) peaks. The whole of these conclusions may lead to the novel light detector or source of terahertz range. [ABSTRACT FROM AUTHOR]

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Following the chronological stages of calculations imposed by the WIEN2K code, we have performed a series of density functional theory calculations, from which we were able to study the effect of strain on the kesterite structures for two quaternary semiconductor compounds Cu 2 ZnGeS 4 and Cu 2 ZnGeSe 4 . Remarkable changes were found in the electronic and optical properties of these two materials during the application of biaxial strain. Indeed, the band gap energy of both materials decreases from the equilibrium state, and the applied strain is more pronounced. The main optical features are also related to the applied strain. Notably, we found that the energies of the peaks present in the dielectric function spectra are slightly shifted towards low energies with strain, leading to significant refraction and extinction index responses. The obtained results can be used to reinforce the candidature of Cu 2 ZnGeX 4 (X = S, Se) in the field of photovoltaic devices. [ABSTRACT FROM AUTHOR]