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The photoluminescence (PL) spectra of CsPbBr3 perovskite nanocrystals grown in a fluorophosphate glass matrix exhibit phonon replicas of the exciton line. The dependence of intensity of the phonon replica on its number is simulated taking into account the difference in the curvature of the excited and ground adiabatic potentials. The Raman spectra of CsPbBr3 nanocrystals are measured. Calculations based on the density functional theory is performed to obtain the spectrum of phonon states of these crystals in the orthorhombic phase. The phonon frequencies observed in the PL and Raman spectra are compared with the calculation results. [ABSTRACT FROM AUTHOR]

Proton irradiation of β-Ga2O3 crystals has been established to lead to a significant increase in the amount of Cr3+ ions being active in luminescence. Using angle-resolved luminescence, the features of the spectra of Cr3+ ions were studied. The high sensitivity of photoluminescence spectra and the related selection rules to the local symmetry of Cr3+ ions in the β-Ga2O3 matrix has been found. The results obtained indicate the potential possibility of using β-Ga2O3 crystals as optical dosimeters of proton irradiation. [ABSTRACT FROM AUTHOR]

At present, the available experimental data on the optical properties of layered III–VI monochalcogenide compounds are scattered and somewhat contradictory, although they are the parent materials for promising two-dimensional (2D) structures. This work is devoted to optical studies of bulk GaSe crystals, whose perfect structural properties are confirmed by Raman studies, observation of singlet-triplet splitting of 1.5 meV, and the polarized photoluminescence measurements from the sample edge. We analyze the band structure of GaSe, namely the sequence and energies of direct and indirect exciton transitions, using cw and time-resolved micro-photoluminescence measurements with variation of temperature. It turns out that the direct band gap in bulk GaSe is at 2.13 eV, close to calculated values. The indirect exciton transition is located ∼15 meV below the direct exciton (2.11 eV). Its intensity quickly quenches and characteristic decay time strongly shortens with increasing temperature, while the contribution of the direct exciton is relatively enhanced. [ABSTRACT FROM AUTHOR]

A technology for obtaining graphene by means of ultrasonic dispersion of thermally expanded graphite in the presence of a surface-active polymer Nation is presented. The technology makes it possible to obtain large amounts of low-layer (1–3 layers) graphene in a relatively short time. An approach to control the dispersion process based on UV spectroscopy of dispersions is described. A mechanism is proposed for the effect of a surface-active polymer on the production of low-layer graphene by ultrasonic dispersion. [ABSTRACT FROM AUTHOR]

The influence of manganese atom intercalation on the electronic structure of graphene grown on Au/Co(0001)/W(110) and SiC(0001) substrates is experimentally studied by angle-resolved photoelectron spectroscopy. Dispersion dependences of the graphene π states at the point of the Brillouin zone are obtained for both systems using ultraviolet photoelectron spectroscopy, and these dependences exhibit shifts of the Dirac cone induced by manganese intercalation followed by annealing. The structure of the near-surface layers of the systems is investigated by X-ray photoelectron spectroscopy. An analysis of the core level lines indicates the possibility of formation of a thin Mn2Au layer on the graphene–Mn–Au interface and a quasi-two-dimensional Mn layer under graphene on the SiC substrate. A comprehensive Raman spectroscopy study of graphene on SiC demonstrates that the buffer layer remains coupled to the substrate and does not transform into an additional graphene layer after the Mn intercalation of this system. After intercalation, the amount of lattice defects in graphene increases. [ABSTRACT FROM AUTHOR]

We report the results of systematic experimental and theoretical studies of structural and dynamical properties of short-period GaN/AlN superlattices. The multilayer structures with the thicknesses of the constituent layers varying from two to several monolayers are grown using the submonolayer digital molecular beam epitaxy technique. In the framework of density functional theory, the lattice dynamics properties of the superlattices are studied. Good agreement between the experiment and theory is found, which made it possible to establish unambiguously a relationship between the features observed in the Raman spectra and the microscopic nature of the acoustic and optical phonon modes. The results obtained enhance the capabilities of Raman spectroscopy as a fast and non-destructive characterization method of short-period GaN/AlN superlattices and can be used to optimize growth process parameters for the fabrication of structurally perfect low-dimensional heterostructures. [ABSTRACT FROM AUTHOR]

Raman spectroscopy is used to evaluate the structural perfection of epitaxial graphene films before and after deposition of a Ni layer to their surface by magnetron sputtering. Two deposition modes with different gas pressures and deposition times are investigated. It is found that Ni deposition under low pressure combined with long deposition time does not lead to the separation of graphene/Ni film. On the other hand, higher pressure and shorter deposition time results in successful but uncontrollable exfoliation of graphene together with the Ni film. The results obtained will serve as the basis for the optimization of Ni deposition modes, in order to achieve complete exfoliation of the graphene film from the SiC substrate without damaging the graphene layer. [ABSTRACT FROM AUTHOR]

The structural and some other characteristics of quasi-freestanding single-layer graphene obtained by annealing of the buffer layer in the flow of hydrogen are studied in comparison with those of conventional epitaxial graphene. The high structural quality and good lateral uniformity of the thus-obtained graphene film are checked and confirmed by the use of such techniques as Raman spectroscopy, atomic force, and Kelvin probe force microscopies. The confirmation of its single-layer and freestanding character is obtained via the analysis of respective data of X-ray photoelectron spectroscopy. [ABSTRACT FROM AUTHOR]

The results of experiments aimed at fabricating and studying the properties of photodetector structures based on single-layer graphene produced by chemical vapor deposition are presented. The configuration of a Ta2O5 vertical microcavity with a resonance wavelength of about 850 nm and a lower dielectric SiO2/Ta2O5 distributed Bragg reflector is taken as the base structure. The conditions for the transfer and fabrication of mesas in the graphene layer on the microcavity surface are optimized. The diagnostics by Raman spectroscopy of the structural quality of graphene after fabrication of the mesas in the graphene layer and contact pads are indicative of the single-layer structure of graphene with a low intensity of features in its spectrum, responsible for imperfection of the structure. The photocurrent is measured under local optical pumping. [ABSTRACT FROM AUTHOR]

Systematic studies of the effect of the electron concentration on the Raman spectra of single-layer graphene films have been carried out. The samples were grown by thermal destruction of the Si-face of the 4H-SiC substrate. Analysis of the results led us to the conclusion that for the correct estimation of the electron concentration and strain values in graphene using Raman spectroscopy data it is necessary to take into account the value of the Fermi velocity in the graphene layer. This conclusion is valid for graphene on any other substrate as well, since the Fermi velocity in graphene depends on the dielectric constant of the substrate. [ABSTRACT FROM AUTHOR]

We have studied the effect of temperature and etching duration of the 4H-SiC (0001) surface in hydrogen on the structural perfection of graphene films grown by thermal destruction. Several technological modes have been identified that enable etching of the substrate without changing the stoichiometric composition of the surface. It has been demonstrated that pregrowth etching in hydrogen at T = 1600°C with a duration of 1 min makes it possible to obtain a more uniform and structurally perfect graphene than etching at T = 1300°C with a 30 min duration. [ABSTRACT FROM AUTHOR]

We report the results of systematic Raman spectroscopy studies of AlxGa1 –xN (x ~ 0.75) layers grown using plasma-assisted molecular beam epitaxy at various stoichiometric conditions and growth fluxes. The high-intensity asymmetric low-frequency peak obeying Bose statistics is discovered in Raman spectra of the layers grown by temperature-modulated epitaxy at strongly Ga-enriched conditions. Theoretical model is developed to explain the origin and the high intensity of the observed low-frequency peak, which is attributed to the presence of excessive metallic gallium in AlGaN layers and can be explained by vibrations of gallium clusters with a diameter of ~1 nm. The nature of the low-frequency peak is similar to that of the boson peak in glasses, which occupies the same frequency range in Raman spectra. We demonstrate the capabilities of Raman spectroscopy as an express and non-destructive technique for optimization of growth conditions of AlGaN layers to achieve simultaneously the atomically-smooth droplet-free surface morphology and the high structural quality. [ABSTRACT FROM AUTHOR]

Abstract: The structural and electronic characteristics of epitaxial graphene films grown by thermal decomposition of the Si face of a semi-insulating 4H-SiC substrate in an argon environment have been studied by a large set of analytical techniques. It is shown that the results of a complex study make it possible to optimize the growth parameters and develop a reliable technology for the growth of high-quality single-layer graphene films. The charge-carrier concentration in the graphene layer was within 7 × 1011-1 × 1012 cm-2, and the maximum mobility of electrons at room temperature approached 6000 cm2/(V s). [ABSTRACT FROM AUTHOR]

We present the results of investigations of the transport properties of graphene films obtained by thermodestruction of a 4H-SiC (0001) surface in argon. The charge-carrier concentration in the graphene layer was within 7 × 10-1 × 10 cm, and the maximum mobility of electrons approached 6000 cm/(V · s). The achieved parameters of mobility are close to theoretical values calculated for graphene films with intrinsic conductivity on the Si face of SiC at Т = 300 К in the absence of intercalated hydrogen. [ABSTRACT FROM AUTHOR]

The structural, chemical, and electronic properties of epitaxial graphene films grown by thermal decomposition of the Si-face of a semi-insulating 6 H-SiC substrate in an argon environment are studied by Raman spectroscopy, atomic-force microscopy, the low-energy electron diffraction method, X-ray photoelectron spectroscopy, angle-resolved photoemission spectroscopy and X-ray absorption spectroscopy at the carbon K edge. It is shown that the results of a systematic integrated study make it possible to optimize the growth parameters and develop a reliable technology for the growth of high-quality single-layer graphene films with a small fraction of bilayer graphene inclusions. [ABSTRACT FROM AUTHOR]

Proton irradiation of β-Ga2O3 crystals has been established to lead to a significant increase in the amount of Cr3+ ions being active in luminescence. Using angle-resolved luminescence, the features of the spectra of Cr3+ ions were studied. The high sensitivity of photoluminescence spectra and the related selection rules to the local symmetry of Cr3+ ions in the β-Ga2O3 matrix has been found. The results obtained indicate the potential possibility of using β-Ga2O3 crystals as optical dosimeters of proton irradiation. [ABSTRACT FROM AUTHOR]