Narrow Results

The parameters of monoclinic unit cell of TlInSeS layered crystals were determined from X-ray powder diffraction study. The optical properties of TlInSeS have been investigated by means of transmission and reflection measurements in the wavelength range of 500–1100 nm. The optical indirect transitions with band gap energy of 2.05 eV and direct transitions with band gap energy of 2.21 eV were found by means of the analysis of the absorption data at room temperature. Transmission measurements carried out in the temperature range of 10–300 K revealed that the rate of change of the indirect band gap with temperature is γ = -4.7 × 10^-4 eV/K. The dispersion of the refractive index is discussed in terms of the single oscillator model. The refractive index dispersion parameters: oscillator wavelength and strength were found to be 2.78 × 10^-7 m and 1.21 × 10¹⁴m^-2, respectively.

GaSe nanoflakes on silicon substrates covered by SiO₂ films are prepared by mechanical exfoliation from the bulk Bridgman-grown GaSe crystals using a scotch tape. The thickness of SiO₂ films on Si substrates providing the highest optical contrast for observation of GaSe flakes is estimated by taking into account the spectral sensitivity of a commercial CMOS camera and broadband visible light illumination. According to our estimations, the optimal SiO₂ thickness is $\sim$126 nm for the visualization of GaSe flakes of 1–3 layers and $\sim$100 nm for the flakes of 40–70 layers. The obtained nanoflakes are investigated by optical and atomic force microscopy and Raman spectroscopy. The observed spectral positions of the Raman peaks are in agreement with the positions of the peaks known for bulk and nanolayered GaSe samples. It is found that the 50 nm thick flakes are stable but are covered by oxide structures with lateral size about 100 nm and height $\sim$5 nm after $\sim$9 months exposure to ambient atmosphere.

In this study, we experimentally study the dependence of the intensity of laser radiation incident on and transmitted through a layered InSe crystal. A picosecond YAG:Nd³⁺ laser with a parametric light generator was used as a light source, making it possible to generate light pulses with a duration of $\sim$30 ps, tunable in frequency within the range of 0.75–1.5 μm. Optical bistability in the region of exciton resonance was detected in an InSe crystal at room-temperature under the picosecond YAG:Nd laser. It is shown that the decrease in absorption in the exciton resonance region is associated with the process of exciton–exciton interaction, shielding of excitons, and the Mott transition.