Band Gap Engineering and Trap Depths of Intrinsic Point Defects in RAlO3 (R = Y, La, Gd, Yb, Lu) Perovskites

The work was supported by the Polish National Science Centre (Project No. 2018/31/B/ST8/00774), by the NATO SPS Project G5647, and by the Ministry of Education and Science of Ukraine (Project DB/Kinetyka no. 0119U002249). L.V. acknowledges support of the National Research Foundation of Ukraine under Grant No. 2020.02/0373 “Crystalline phosphors’ engineering for biomedical applications, energy saving lighting and contactless thermometry”. Researchers from Tartu were supported by the ERDF fundings in Estonia granted to the Centre of Excellence TK141 “Advanced materials and high-technology devices for sustainable energetics, sensorics and nanoelectronics (HiTechDevices)” (Grant No. 2014-2020.4.01.15-0011) and Estonian Research Council Grant PRG-629. The Institute of Solid State Physics, University of Latvia as the Center of Excellence acknowledges funding from the H2020-WIDESPREAD-01-2016-2017-Teaming Phase2 under Grant Agreement No. 739508, Project CAMART2. N.K. was supported by the National long-term project No. WQ20142200205 (Recruitment Program of Global Experts, PRC). Authors are thankful to George Loutts from Norfolk State University, United States, and Dorota Pawlak from Institute of Electronic Materials Technology, Poland for providing some single crystals studied in the work, as well as to Kirill Chernenko from FinEstBeAMS of MAX IV for his assistance with synchrotron experiments.The possibility of band gap engineering (BGE) in RAlO3(R = Y, La, Gd, Yb, Lu) perovskites in the context of trap depths of intrinsic point defects was investigated comprehensively using experimental and theoretical approaches. The optical band gap of the materials,Eg, was determined via both the absorption measurements in the VUV spectral range and the spectra of recombination luminescence excitation by synchrotron radiation. The experimentally observed effect ofEgreduction from ∼8.5 to ∼5.5 eV in RAlO3perovskites with increasing R3+ionic radius was confirmed by the DFT electronic structure calculations performed for RMIIIO3crystals (R = Lu, Y, La; MIII= Al, Ga, In). The possibility of BGE was also proved by the analysis of thermally stimulated luminescence (TSL) measured above room temperature for the far-red emitting (Y/Gd/La)AlO3:Mn4+phosphors, which confirmed decreasing of the trap depths in the cation sequence Y → Gd → La. Calculations of the trap depths performed within the super cell approach for a number of intrinsic point defects and their complexes allowed recognizing specific trapping centers that can be responsible for the observed TSL. In particular, the electron traps of 1.33 and 1.43 eV (in YAlO3) were considered to be formed by the energy level of oxygen vacancy (VO) with different arrangement of neighboring YAland VY, while shallower electron traps of 0.9-1.0 eV were related to the energy level of YAlantisite complexes with neighboring VOor (VO+ VY). The effect of the lowering of electron trap depths in RAlO3was demonstrated for the VO-related level of the (YAl+ VO+ VY) complex defect for the particular case of La substituting Y. © 2021 The Authors. Published by American Chemical SocietyNATO SPS G5647; National Research Foundation of Ukraine 2020.02/0373; Polish National Science Centre 2018/31/B/ST8/00774; Eesti Teadusagentuur PRG-629; Latvijas Universitate 739508, WQ20142200205; Institute of Solid State Physics, Chinese Academy of Sciences; Ministry of Education and Science of Ukraine 0119U002249; European Regional Development Fund 2014-2020.4.01.15-0011, TK14

Luminescence of praseodymium doped perovskite - like SrLa4_4Ti5_5O17_{17} under excitation with VUV and UV synchrotron radiation

The wide-band complex spectra of intrinsic luminescence of undoped SrLa$_4$Ti$_5$O$_{17}$ compounds are found in range of 350-1100 nm under excitation by the light from ultraviolet and vacuum ultraviolet range (50-334 nm) of synhrotron radiation. The possible origines of the observed luminescence are associated with peculiarities of the structure of the layered perovskite-like SrLa$_4$Ti$_5$O$_{17}$ compound. So, three luminescence components of the intrinsic luminescence peaked near ~ 530, ~ 830, and ~ 1100 nm have been attributed to the radiation decay of excitons localized on (TiO$_6$)$^{8-}$ molecular groups of three ypes in the lattice of SrLa$_4$Ti$_5$O$_{17}$. The luminescent features related with $4f→4f$ and $4f5d→4f$ radiation transitions in Pr$^{3+}$ ions are found in addition to mentioned intrinsic emission for the SrLa$_4$Ti$_5$O$_{17}$ compound doped with Pr$^{3+}$ ions. Obtained results show the prospects of the SrLa$_4$Ti$_5$O$_{17}$ compounds doped with Pr$^{3+}$ ions to be used as luminophores of wide emission spectra

Structural Modification of Single-Layer Graphene Under Laser Irradiation Featured by Micro-Raman Spectroscopy

Abstract Confocal micro-Raman spectroscopy is used as a sensitive tool to study the nature of laser-induced defects in single-layer graphene. Appearance and drastic intensity increase of D- and D′ modes in the Raman spectra at high power of laser irradiation is related to generation of structural defects. Time- and power-dependent evolution of Raman spectra is studied. The dependence of relative intensity of defective D- and D′ bands is analyzed to relate the certain types of structural defects. The surface density of structural defects is estimated from the intensity ratio of D- and G bands using the D-band activation model. Unusual broadening of the D band and splitting of the G band into G− and G+ components with redistribution of their intensities is observed at high laser power and exposition. Position of the G+ band is discussed in relation with nonuniform doping of graphene with charge impurities. Simultaneous presence in the Raman spectra of heavily irradiated graphene of rather narrow G band and broaden D band is explained by coexistence within the Raman probe of more and less damaged graphene areas. This assumption is additionally confirmed by confocal Raman mapping of the heavily irradiated area

Luminescence mechanisms in the 2V2_2O5_5-xLi2_2O-(98-x)B2_2O3_3 glass matrices developed for creation of glass-ceramic materials

The oxide glass-ceramics is a promising class of solid state materials because they are based on thermally stable and chemically inert glass oxide matrices. Development of such efficient glass matrices suitable for creation of glass-ceramic materials for several purposes is an important practical task. The xLi$_2$O-yV$_2$O$_5$-(100-x-y)B$_2$O$_3$ undoped glass and 47Li$_2$O-$_2$V$_2$O$_5$-50B$_2$O$_3$-1La$_{0.3}$Eu$_{0.7}$VO$_4$ glass samples with crystalline nanoinclusions were synthesized and investigated using XRD, IR and UV-Vis spectroscopy and UV band-to-band excitation of luminescence. The synthesized glass samples are characterized by wide band photoluminescence emission with maximum at 570 nm and intensity increased with increase of Li$_2$O concentration. The excitation spectra consist of three bands with maxima located at 270, 320 and 365 nm. The observed concentration dependencies of spectral distributions in the absorption and excitation spectra are explained by influence of the lithium ions on a ratio between triborate and tetraborate groups in the glass networks. The assumption is made that the observed wide band photoluminescence emission of the glass matrix can appear as a result of recombination processes between the defects in borate networks and the broken vanadate groups. The crystalline component in the doped glass samples is found to not affect the luminescence properties of the glass matrix. Intensity of narrow band photoluminescence emission of the crystalline component is up to 10 times more intense than that of the glass matrix wide band emission. The synthesized type of the glass matrices has promising characteristics for the use of developed materials in lighting devices, as it allows improving the spectral distribution of light emission towards the white light

Influence of Gold Nanoparticles on Luminescence of Eu3+\mathrm{Eu^{3+}} Ions Sensitized by Structural Defects in Germanate Films

This investigation shows new possibilities of gold nanoparticles using for rise of Ln$^{3+}$ ions luminescence in oxide matrixes at ultraviolet synhrotron’s excitation on example of germanate films doped with Eu and Au. It has been established that formation of gold nanoparticles (AuNPs) in such films leads to the enhancement of the Eu$^{3+}$ ions luminescence intensity by many times at excitation in broad absorption bands having λ$_{max}$ about 120 and 180 nm. The discovered effect is explained by joint influence of the following factors: (1) rise in the nonbridging oxygen hole centers (NBOHCs) concentration as a result of breakage of the O–Eu bonds in the ≡Ge–O–Eu= bridges by the forming AuNPs; (2) nonradiative transfer of excitations from excitons to these centers; (3) effective sensitization of the Eu$^{3+}$ ions luminescence by the centers via the charge transfer (CT) state of the O$^{2–}$ + Eu$^{3+}$ complexes; (4) increase in the optical path of the exciting radiation due to the intensification of light scattering by the AuNPs, and, possibly, (5) promoting by the AuNPs of excitations transfer in the exciton −NBOHC–(O$^{2–}$ + Eu$^{3+}$) system in the nanoparticles field as well as weakening of reverse passivation of the hole centers by the hydrogen atoms formed on the breakage of the end OH– groups under the action of UV radiation