O.Y. Khyzhun

Synthesis, Structural, Thermal, and Electronic Properties of Palmierite-Related Double Molybdate α-Cs2Pb(MoO4)2

Cs2Pb(MoO4)2 crystals were prepared by crystallization from their own melt, and the crystal structure has been studied in detail. At 296 K, the molybdate crystallizes in the low-temperature α-form and has a monoclinic palmierite-related superstructure (space group C2/m, a = 2.13755(13) nm, b = 1.23123(8) nm, c = 1.68024(10) nm, β = 115.037(2)°, Z = 16) possessing the largest unit cell volume, 4.0066(4) nm3, among lead-containing palmierites. The compound undergoes a distortive phase transition at 635 K and incongruently melts at 943 K. The electronic structure of α-Cs2Pb(MoO4)2 was explored by using X-ray emission spectroscopy (XES) and X-ray photoelectron spectroscopy methods. For α-Cs2Pb(MoO4)2, the photoelectron core-level and valence-band spectra and the XES band representing the energy distribution of Mo 4d and O 2p states were recorded. Our results allow one to conclude that the Mo 4d and O 2p states contribute mainly to the central part and at the top of the valence band, respectively, with also significant contributions throughout the whole valence-band region of the molybdate under consideration.

Huge operation by energy gap of novel narrow band gap Tl1−x In1−x B x Se2 (B = Si, Ge): DFT, x-ray emission and photoconductivity studies

It is shown that narrow band gap semiconductors Tl1−x In1−x GexSe2 are able effectively to vary the values of the energy gap. DFT simulations of the principal bands during the cationic substitutions is done. Changes of carrier transport features is explored. Relation with the changes of the near the surface states is explored . Comparison on a common energy scale of the x-ray emission Se Kβ 2 bands, representing energy distribution of the Se 4p states, indicates that these states contribute preliminary to the top of the valence band. The temperature dependence of electrical conductivity and spectral dependence photoconductivity for the Tl1−x In1−x Ge x Se2 and Tl1−x In1−x Si x Se2 single crystals were explored and compared with previously reported Tl1−x In1−x Sn x Se2. Based on our investigations, a model of centre re-charging is proposed. Contrary to other investigated crystals in Tl1−x In1−x Ge x Se2 single crystals for x = 0.1 we observe extraordinarily enormous photoresponse, which exceed more than nine times the dark current. X-ray photoelectron core-level and valence-band spectra for pristine and Ar+-ion irradiated surfaces of Tl1−x In1−x GexSe2 (x = 0.1 and 0.2) single crystals have been studied. These results indicate that the relatively low hygroscopicity of the studied single crystals is typical for the Tl1−x In1−x Ge x Se2 crystals, a property that is very important for handling these quaternary selenides as infrared materials operating at ambient conditions.

Manifestation of Anomalous Weak Space-Charge-Density Acentricity for a Tl4HgBr6 Single Crystal

Density functional theory (DFT) calculations within the concept of the MBJ+U+SO (modified Becke-Johnson potential + U + spin orbit) approach were performed for a Tl4HgBr6 single crystal for the first time assuming weak noncentrosymmetry (space group P4nc). Excellent agreement was achieved between the calculated and experimental band-gap-energy magnitudes as well as the density of electronic states measured by the X-ray photoelectron spectroscopy method. It is a very principal result because usually the DFT calculations underestimate the energy-gap values. In the present study, we carry out calculations of the optical properties (absorption coefficient, real and imaginary parts of the dielectric function, electron energy-loss spectrum, refractive index, extinction coefficient, and optical reflectivity dispersions). It has been established that the principal origin of the observed weak acentricity is determined by delocalized band states at the top of the valence band originating from the p states of the Br atoms.

Synthesis and structure of novel Ag2Ga2SiSe6 crystals: promising materials for dynamic holographic image recording

Phase diagrams of the AgGaSe2–SiSe2 system were explored by differential thermal analysis (DTA) and X-ray diffraction (XRD) analysis methods for the first time. It was demonstrated that the investigated system forms quaternary compounds of compositions Ag2Ga2SiSe6 and AgGaSiSe4. Ag2Ga2SiSe6 melts at 1042 K and exists in two polymorphous modifications. The crystal structure of the low-temperature modification was determined by the single crystal method (space group I2d (122) and lattice parameters a = 5.9021(1) A, b = 5.9021(1) A, and c = 10.4112(10) A). Additional details (CIF file) regarding the crystal structure investigations are available at the Fachinformationszentrum Karlsruhe. The band gap (Eg) of the Ag2Ga2SiSe6 system was estimated from the fundamental absorption edge and we showed that it decreases with increasing temperature (100–300 K) from 2.13 eV to 1.97 eV. The compound is photosensitive and its spectral dependence on the photoconductivity has two maxima: at λmax1 = 640 nm and λmax2 = 900 nm. For the pristine Ag2Ga2SiSe6 crystal surface, X-ray photoelectron core-level and valence-band spectra were obtained. The X-ray photoelectron valence-band spectrum of Ag2Ga2SiSe6 was compared on a common energy scale with the X-ray emission Se Kβ2 and Ga Kβ2 bands, representing peculiarities of the energy distribution of the Se 4p and Ga 4p states, respectively. The comparison revealed that the principal contributions of the valence Se p and Ga p states occur in the upper and central parts of the valence band, respectively, with significant contributions to other valence band regions. The illumination by the bicolour coherent pulses of the Er:glass nanosecond lasers at different angles led to the formation of the gratings, which are sensitive to the irradiation time.

Systematic synthesis and analysis of change in morphology, electronic structure and photoluminescence properties of 2,2′-dipyridyl intercalated MoO3 hybrid nanostructures and investigation of their photocatalytic activity

An organic–inorganic hybrid structure was synthesized by using 2,2′-dipyridyl and MoO3 nanorods through a simple hydrothermal method. The as-prepared dipyridyl–MoO3 hybrid samples looked like rod shaped micro crystals. The starting material used for this work was MoO3 nanorods which had a width of 150 nm and a length of several microns, whereas the resulting hybrid structure had a width of one micron and a length of 10 to 30 microns. Here, dipyridyl has acted as a stretching molecule and bonded the MoO3 nanorods together along their length to form hybrid micro crystals. By calcinating the hybrid sample at 400 °C, intercalated dipyridyl was removed, while maintaining the microscale morphology. These deintercalated MoO3 samples looked like micro slabs having a width of 5 micrometers. The presence and intercalation of dipyridyl was confirmed by the change in the XRD [0 k 0] peak positions. As the cross sectional size of the dipyridyl is close to the van der Waals gap of the orthorhombic MoO3 crystal, this space was effectively used for this intercalation process. The deintercalation process, i.e. the removal of dipyridyl was confirmed by TGA, and XRD measurements. The influence of dipyridyl in the valence band electronic density of states (DOS) of MoO3 was also analyzed by XPS and XES methods. A photoluminescence study was also conducted, reflecting the intercalation effect on the emission characteristics of the MoO3 nanostructures. A photodegradation study on Procion Red MX 5B was also carried out, showing that the dipyridyl deintercalated MoO3 micro slab like samples had the highest photodegradation efficiency.

Crystal structure and electronic properties of facile synthesized Cr2O3 nanoparticles

We report on a facile method of synthesis of Cr2O3 nanoparticles by hydrothermal method. Chromium sulfate was used as a starting material whereas urea was used as a strong reducing agent. Cr2O3 nanoparticles, with rhombohedral crystal structure, have been synthesized, when the reaction solution was treated under hydrothermal condition at high pH (10). At pH = 8 amorphous Cr2O3 powders were obtained. Chromium oxide could not be synthesized in the absence of urea. Two different Raman modes have been detected for the final products synthesized at the high pH value. As-prepared Cr2O3 nanoparticles reveal agglomeration as evidenced from scanning electron microscope (SEM) images. Flake-like Cr2O3 nanoparticles, 20 to 50 nm in size, show clear lattice fringes through the high resolution transmission electron microscope (HRTEM) images. The electronic structure of the Cr2O3 nanoparticles has been studied employing x-ray photoelectron spectroscopy (XPS) and x-ray emission spectroscopy (XES) methods.

Graphene Quantum Dot Solid Sheets: Strong blue-light-emitting & photocurrent-producing band-gap-opened nanostructures

Graphene has been studied intensively in opto-electronics, and its transport properties are well established. However, efforts to induce intrinsic optical properties are still in progress. Herein, we report the production of micron-sized sheets by interconnecting graphene quantum dots (GQDs), which are termed ‘GQD solid sheets’, with intrinsic absorption and emission properties. Since a GQD solid sheet is an interconnected QD system, it possesses the optical properties of GQDs. Metal atoms that interconnect the GQDs in the bottom-up hydrothermal growth process, induce the semiconducting behaviour in the GQD solid sheets. X-ray absorption measurements and quantum chemical calculations provide clear evidence for the metal-mediated growth process. The as-grown graphene quantum dot solids undergo a Forster Resonance Energy Transfer (FRET) interaction with GQDs to exhibit an unconventional 36% photoluminescence (PL) quantum yield in the blue region at 440 nm. A high-magnitude photocurrent was also induced in graphene quantum dot solid sheets by the energy transfer process.

Ionicity and birefringence of α-LiNH4SO4 crystals: ab initio DFT study, X-ray spectroscopy measurements

The structural, electronic properties and ionicity of the lithium ammonium sulfate dielectric crystals are examined using a complex approach including experimental studies of X-ray spectroscopy and the first principles band structure techniques within a framework of local electron density functional theory (DFT). Band energy dispersion, density of electronic states and dielectric function dispersion in the wide spectral range corresponding to electronic excitations were calculated using the plane wave basis and Vanderbilt ultra-soft pseudopotentials. The origin of the energy bands are estimated using density of states diagrams and the band gap magnitudes for different exchange correlation functions. To verify the data of the performed band structure calculations, the X-ray photoelectron spectroscopy (XPS) and X-ray emission spectroscopy (XES) are used. The XPS core-level and valence-band spectra as well as the XES bands representing the energy distribution of the O 2p and N 2p states are studied. Theoretical refractive indices dispersion for the main crystallographic directions (na, nb and nc) as well as birefringence spectral dependences (Δna, Δnb and Δnc) in the visible spectral range are obtained. All the calculated properties are compared with the available experimental results and good agreement between both sets of data is demonstrated.

Electronic structure of KTiOAsO4, a novel material for non-linear optical applications

A high-quality KTiOAsO4 (KTA) single crystal has been successfully synthesized employing the high temperature solution growth technique with rotating and pooling. The XPS valence-band spectra have been measured for pristine and the 1.5 keV Ar+ ion-bombarded thin (001)KTA plate cut from the crystal part that was without any optical inhomogeneities or domain boundaries. The present XPS measurements have revealed the existence of two O 2s subbands on the XPS spectrum of the pristine (001)KTA surface. It has been established that 1.5 keV Ar+ ion bombardment of the (001)KTA surface causes the complete elimination of the O 2s sub-band related to oxygen atoms involved in the formation of Ti–O–As bonds in KTA. In addition, the XPS results reveal that such a treatment leads to a significant decrease of the relative intensity of the XPS As5+ 3d core-level spectrum and causes the formation of the additional As0 3d core-level spectrum in the topmost layer of the (001)KTA surface. The experimental data are compared to the results of the first-principles band-structure calculations of KTA.

Electronic band-structure and optical constants of Pb2GeS4: Ab initio calculations and X-ray spectroscopy experiments

The electronic band-structure of the ternary sulfide Pb2GeS4 was investigated by combining experimental and theoretical methods. Binding energy (BE) values of core electrons of Pb2GeS4 are measured employing X-ray photoelectron spectroscopy (XPS) for as-synthesized and treated with Ar+-ions crystal surfaces. The XPS measurements indicate that Ar+-ion treatment does not change the BE values of the core-level electrons of atoms constituting the Pb2GeS4 single crystal as well as peculiarities of the XPS valence band (VB) spectrum. The treatment does not cause changes in the crystal surface stoichiometry. The band-structure calculations based on density functional theory (DFT) reveal total density of states and partial densities of states of Pb2GeS4 within different exchange–correlation approximations. The best fit with the experiment is derived when the DFT calculations of Pb2GeS4 employ modified Becke-Johnson potential with Hubbard-corrected functional and taking into account spin–orbit (SO) interaction. The calculations indicate that top and upper portion of the VB is composed mainly by S 3p states, its central portion is formed by Ge 4p and S 3p states, while contributions of Pb 6s states dominate at its bottom with slightly smaller contributions of Ge 4s states as well. Contributions of unoccupied Pb 6p states dominate at the conduction band (CB) bottom. Regarding the occupation of the VB by Ge 4p and S 3p states, the theoretical data are confirmed experimentally by matching the XPS VB spectrum on a common energy scale with the X-ray emission spectra representing the valence S p and Ge p states. The present calculations yield that the VB maximum is positioned at the Y point, while the CB minimum at the Г point; this fact indicates that Pb2GeS4 sulfide is an indirect-gap material. The principal optical constants are also elucidated using the ab initio DFT calculations.