Radiative recombination in BiI₃(Mn) and BiI₃(Cr) layered single crystals

The photoluminescence spectra of BiI₃(Mn) and BiI₃(Cr) single crystals were studied. The new bands at λ = 1.1 and λ = 1.83 μm were registered at the temperature 4.2 K. It was shown that the first and second bands arise due to Mn²⁺ and Cr³⁺ ions, accordingly. The experimental results were explained using the configuration model. The energies between minima of configuration curves of excited and basic states for both centres were estimated

Effect of temperature variation on shift and broadening of exciton band in Cs₃Bi₂I₉ layered crystals

The exciton reflection spectra of Cs₃Bi₂I₉ layered crystals is investigated in the temperature region 4.2–300 K with light polarization E ⊥ c. It is estimated that the energy gap Eg equals 2.857 eV (T = 4.2 K) and the exciton binding energy Ry is 279 meV. A nontraditional temperature shift of Eg(T) for the layered substances is found for the first time. It is learned that this shift is described very well by the Varshni formula. A transition region in the temperature broadening of the half-width H(T) of the exciton band with the increase of temperature is registered in the interval between 150 and 220 K. It is shown that this region may be identified as the heterophase structure region where ferroelastic and paraelastic phases coexist. A surge of H(T) at the point of the ferroelastic phase transition (Tc = 220 K) is also observed

Study of supercapacitors with a double electrical layer based on activated carbon materials

It is proposed ecologically pure technology to obtain activated carbon. On the base of this carbon the supercapacitors were manufactured. Their characteristics were determined and compare with analogs obtained using ecologically dangerous materials

Influence of Mn and Cr impurities on quasi-surface exciton states of BiI₃ layered single crystals

The influence of Mn and Cr impurities on quasi-surface excitons of BiI3 single crystals was studied. The broadening of their band was found which may be caused by ionization of quasi-surface excitons by the electric field of Mn²+ and Cr³+ ions

Saddle point excitonic resonances in BiI3 layered single crystals

Excitonic resonances near the critical saddle point of M₁ sort by van Hove have been revealed for the first time in the BiI₃ layered semiconductor. Their main parameters are estimated

Phase transition in Cs₃Bi₂I₉ ferroelastic: investigation by Raman scattering technique

Raman scattering in unpolarized light has been studied for the first time for Cs₃Bi₂I₉ layered ferroelastic at temperatures from 5 to 300 К in the heating mode. Neither soft mode nor frequency softening was observed. For the low-frequency lines 37.0, 45.0, 61.4, 68.3 and 97.4 cm⁻¹, the doublets were detected. Those may be due to Davydov splitting or splitting of degenerate vibrations of E-symmetry. The intensity equalization effect has been found for 114.8 and 125.3 cm⁻¹ lines in the phase transition region. It is shown that the 150.6 and 131.2 cm⁻¹ lines can be explained by vibrations of Bi⁻¹ bonds in the [Bi1]³⁻ octahedrons. The Raman spectra in z(y,y)z and z(y,x)z polarizations taken at 100 К were found to be similar to those in unpolarized light. An analysis of both known and our results enabled us to conclude that the ferroelastic phase transition at Тᶜ = 220 К belongs to those of the first order but close to the second one.Впервые в неполяризованном свете изучено комбинационное рассеяние (КР) в слоистом сегнетоэластике Cs₃Bi₂I₉ при температурах 5-300 К в режиме нагревания. Не обнаружено ни мягкой моды, ни смягчения частот. 3арегистрированы дублеты у низкочастотных линий 37,0, 45,0, 61,4, 68,3 и 97,4 см⁻¹, обусловленных давыдовским расщеплением, либо расщеплением вырожденных колебаний симметрии Е. Обнаружен эффект выравнивания интенсивностей линий 114,8 и 125,3 см⁻¹ в области фазового перехода. Показано, что линии 150,6 и 131,2 см-1 можно объяснить колебаниями связей Bi⁻¹ в октаэдрах [Bi1 ]³⁻. Установлено, что при 100 К спектры КР в поляризации света z(y,y)z и z(y,x)z подобны спектрам КР в неполяризованном свете. На основе анализа известных и полученных нами данных сделан вывод о принадлежности сегнетоэластичного фазового перехода при Тᶜ = 220 К к фазовым переходам первого рода, близких ко второму.Вперше у неполяризованому свiтлi вивчено комбiнацiйне розсiювання (КР) у шаруватому сегнетоеластику Cs₃Bi₂I₉ при температурах 5-300 К у режимi нагрiвання. Не виявлено анi м'якої моди, анi пом'якшення частот. 3ареєстровано дублети у низькочастотних лiнiй 37,0, 45,0, 61,4, 68,3 i 97,4 см⁻¹, обумовлених давидiвським розщепленням, або ж розщепленням вироджених коливань симетрiї Е. Виявлено ефект вирiвнювання iнтенсивностей лiнiй 114,8 i 125,3 см⁻¹ в областi фазового переходу. Показано, що лiнii 150,6 i 131,2 см⁻¹ можна пояснити коливаннями зв'язкiв Bi⁻¹ в октаедрах [Bi16]³⁻. Встановлено, що при 100 К спектри КР у поляризацii свiтла z(y,y)z та z(y,x)z є подiбними до спектрiв КР у неполяризованому свiтлi. Проаналiзовано вiдомi та отриманi нами данi i зроблено висновок про належнiсть сегнетоеластичного фазового переходу при Тᶜ = 220 К до фазових переходiв першого роду, що є близькими до другого

Analysis of exciton reflection spectrum of 2H-PbI₂ layered single crystals with atomically clean surface

We performed the computer modelling of the dispersion dependences of real ε₁(E) and imaginary ε₂(E) parts of complex dielectric function ε(E) for 2H-PbI₂ crystals with atomically clean surface at the temperature 5 K and the light polarization E⊥C and determined the energy position of the exciton bands and the parameters of the critical points. The obtained data allowed us to conclude the exciton spectra of this semiconductor can be described in terms of a single Wannier series with a large ground-state anomaly caused by a repulsive central-cell correction due to the cationic character of the exciton