Electronic structure and optical properties of the Ag3SbS3 crystal: experimental and DFT studies.

A high-quality Ag3SbS3 single crystal was grown by the Bridgman-Stockbarger method and its crystalline structure and homogeneity were investigated. The fundamental absorption edge of Ag3SbS3 was studied. The value of the band gap of the studied compound was obtained at the level of 1.91 eV at T = 300 K. The structural, electronic, and optical properties of the Ag3SbS3 crystal were considered within the framework of first-principles calculations using density functional theory (DFT). The structure of the crystal lattice was optimized and its closeness to the experimental one is shown. The band-energy structure of the crystal was calculated revealing that the crystal has a band gap of indirect type with Eg = 0.88 eV for GGA (0.35 eV for LDA). The origin of the energy bands in the crystal was clarified and the nature of the fundamental absorption edge was analyzed using the calculated density of electronic states. The dielectric function (real part ε1(ω) and imaginary part ε2(ω)) and absorption coefficient α(ω) were calculated for two independent directions in the crystal and compared with experimental data. The character and anisotropy of optical functions are analyzed. The high value of the absorption coefficient of the Ag3SbS3 crystal is shown, which makes it a promising material for use as an absorbing layer in photovoltaics.

Specific features of the electronic structure of a novel ternary Tl3PbI5 optoelectronic material.

A novel Tl3PbI5 crystal has been studied both experimentally and theoretically. Complex measurements of the X-ray photoelectron core-level and valence-band spectra for the pristine and Ar(+)-ion irradiated surfaces of a Tl3PbI5 single crystal grown by the Bridgman-Stockbarger method were performed in order to clarify their principal properties (charge carriers mobility, effective inter-band distances, effective absorption etc.) relevant for optoelectronic applications. The principal role of two heavy cations - Tl and Pb - is explored. The X-ray photoelectron spectroscopy results reveal a high chemical stability of the Tl3PbI5 single crystal surface which makes it very promising for technological applications. Theoretical band-structure calculations for the Tl3PbI5 compound reveal that the I 5p states dominate in the top of the valence band and play a crucial role in the formation of the optical features and charge carrier mobility. The bottom of the Tl3PbI5 valence band is formed mainly by the admixture of Tl 6s and Pb 6s states, while the unoccupied Pb 6p and Tl 6p states dominate at the bottom of the conduction band. The band energy dispersion related to effective masses and the charge carrier mobility is studied in detail. Crucially, the theoretical calculations reveal an indirect band gap for Tl3PbI5, which indicates a strong influence of the electron-phonon interaction on the observed optoelectronic features. The temperature measurements of the fundamental absorption have shown that the band energy gap of Tl3PbI5 increases from 2.29 to 2.39 eV when the temperature changes from 300 to 100 K.

Photoelectrical properties and the electronic structure of Tl(1-x)In(1-x)Sn(x)Se2 (x = 0, 0.1, 0.2, 0.25) single crystalline alloys.

Photoelectrical properties of Tl1-xIn1-xSnxSe2 single crystalline alloys (x = 0, 0.1, 0.2, 0.25) grown using the Bridgman-Stockbarger method were studied. The temperature dependence of electrical and photoconductivity for the Tl1-xIn1-xSnxSe2 single crystals was explored. It has been established that photosensitivity of the Tl1-xIn1-xSnxSe2 single crystals increases with x. The spectral distribution of photocurrent in the wavelength spectral range 400-1000 nm has been investigated at various temperatures. Photoconductivity increases in all the studied crystals with temperature. Therefore, thermal activation of photoconductivity is caused by re-charging of the photoactive centers as the samples are heated. Based on our investigations, a model of center re-charging is proposed that explains the observed phenomena. X-ray photoelectron valence-band spectra for pristine and Ar(+)-ion irradiated surfaces of the Tl1-xIn1-xSnxSe2 single crystals have been measured. These results reveal that the Tl1-xIn1-xSnxSe2 single-crystal surface is sensitive to the Ar(+) ion irradiation that induced structural modification in the top surface layers. Comparison on a common energy scale of the X-ray emission Se Kß2 bands representing energy distribution of the Se 4p-like states and the X-ray photoelectron valence-band spectra was done.

Linear, non-linear optical susceptibilities and the hyperpolarizability of the mixed crystals Ag(0.5)Pb(1.75)Ge(S(1-x)Se(x))4: experiment and theory.

As the starting point for a comprehensive theoretical investigation of the linear and nonlinear optical susceptibilities, we have used our experimental crystallographic data for Ag0.5Pb1.75GeS3Se (Ag2Pb7Ge4S12Se4) reported. The experimental crystallographic positions were optimized by minimizing the forces acting on each atom to get meaningful theoretical predictions of the optical properties. The linear optical susceptibilities are calculated. We find that the optical band gap shows very good agreement with our measured gap. The second-order nonlinear optical (NLO) susceptibilities dispersion namely the optical second harmonic generation (SHG) is calculated and compared with our experimental measurements. The microscopic first order hyperpolarizability, ß123, vector component along the principal dipole moment directions for the χ((2))(123)(ω) component was obtained theoretically and compared with our measured values at different temperatures. The dependence of the two-photon absorption (TPA) for the pump-probing at SHG of the microsecond CO2 laser was measured. In addition we explored the linear electro-optical effect in these crystals. This effect is described by the third rank polar tensors similarly to the SHG. However, for the Pockels effect besides the electronic contribution, the phonon subsystem also begins to play a principal role. As a consequence we study the dispersion of the linear electro-optical effects in the mentioned crystals.

Electronic and optical properties of thiogermanate AgGaGeS4: theory and experiment.

The electronic and optical properties of an AgGaGeS4 crystal were studied by first-principles calculations, where the full-potential augmented plane-wave plus local orbital (APW+lo) method was used together with exchange-correlation pseudopotential described by PBE, PBE+U, and TB-mBJ+U approaches. To verify the correctness of the present theoretical calculations, we have measured for the AgGaGeS4 crystal the XPS valence-band spectrum and the X-ray emission bands representing the energy distribution of the electronic states with the biggest contributions in the valence-band region and compared them on a general energy scale with the theoretical results. Such a comparison indicates that, the calculations within the TB-mBJ+U approach reproduce the electron-band structure peculiarities (density of states - DOS) of the AgGaGeS4 crystal which are in fairly good agreement with the experimental data based on measurements of XPS and appropriate X-ray emission spectra. In particular, the DOS of the AgGaGeS4 crystal is characterized by the existence of well-separated peaks/features in the vicinity of -18.6 eV (Ga-d states) and around -12.5 eV and -7.5 eV, which are mainly composed by hybridized Ge(Ga)-s/p and S-p state. We gained good agreement between the experimental and theoretical data with respect to the main peculiarities of the energy distribution of the electronic S 3p, Ag 4d, Ga 4p and Ge 4p states, the main contributors to the valence band of AgGaGeS4. The bottom of the conduction band is mostly donated by unoccupied Ge-s states, with smaller contributions of unoccupied Ga-s, Ag-s and S-p states, too. The AgGaGeS4 crystal is almost transparent for visible light, but it strongly absorbs ultra-violet light where the significant polarization also occurs.

Searching for better X-ray and γ-ray photodetectors: structure-composition properties of the TlPb2Br5-x I x quaternary system.

Developing X-ray and γ-ray detectors with stable operation at ambient temperature and high energy resolution is an open challenge. Here, we present an approach to search for new detector materials, combining binary photodetector compounds. More specifically, we explore quaternary TlPb2Br5-x I x compositions, relying on materials synergy between TlBr, TlI, and PbI2 photodetectors. We discover a broad solid solution in the TlPb2Br5-'TlPb2I5' section, which can be derived from a new quaternary compound, TlPb2BrI4, by partial substitution of Br by I atoms on the 4c site or by replacement of I by Br atoms on the 16l site. We carry out a thorough crystallographic analysis of the new TlPb2BrI4 compound and prepare a high-quality standardized structure file. We also complete the phase diagram of the TlPb2Br5-'TlPb2I5' section, based on 21 alloys. Furthermore, we synthesize a series of high quality centimeter-sized TlPb2Br5-x I x single crystals (x = 2, 2.5, 3, 3.5, 4, 4.5) by the Bridgman-Stockbarger method and study their structure and properties using a combination of experimental techniques (X-ray diffraction, X-ray photoelectron spectroscopy, and absorption spectroscopy) and theoretical calculations.

Photoinduced features of energy bandgap in quaternary Cu2CdGeS4 crystals.

The quaternary chalcogenide crystal Cu2CdGeS4 was studied both experimentally and theoretically in the present paper. Investigations of polarized fundamental absorption spectra demonstrated a high sensitivity to external light illumination. The photoinduced changes were studied using a cw 532 nm green laser with energy density about 0.4 J cm(-2). The spectral maximum of the photoinduced anisotropy was observed at spectral energies equal to about 1.4 eV (energy gap equal to about 1.85 eV) corresponding to maximal density of the intrinsic defect levels. Spectroscopic measurements were performed for polarized and unpolarized photoinducing laser light to separate the contribution of the intrinsic defect states from that of the pure states of the valence and conduction bands. To understand the origin of the observed photoinduced absorption near the fundamental edge, the benchmark first-principles calculations of the structural, electronic, optical and elastic properties of Cu2CdGeS4 were performed by the general gradient approximation (GGA) and local density approximation (LDA) methods. The calculated dielectric function and optical absorption spectra exhibit some anisotropic behavior (shift of the absorption maxima in different polarizations) within the 0.15-0.20 eV energy range not only near the absorption edge; optical anisotropy was also found for the deeper inter-band transition spectral range. Peculiar features of chemical bonds in Cu2CdGeS4 were revealed by studying the electron density distribution. Possible intrinsic defects are shown to affect the optical absorption spectra considerably. Pressure effects on the structural and electronic properties were modeled by optimizing the crystal structure and calculating all relevant properties at elevated hydrostatic pressure. The first estimations of the bulk modulus (69 GPa (GGA) or 91 GPa (LDA)) and its pressure derivative for Cu2CdGeS4 are also reported.

Absorption and photoconductivity spectra of Ag2GeS3 crystal: experiment and theory.

Spectral features of polycrystalline Ag(2)GeS(3) samples synthesized from high-purity elements (at least 99.99 wt.% purity) in quartz ampoules evacuated to 0.1 Pa were explored. The band energy gap of Ag(2)GeS(3) crystals estimated from the fundamental absorption edge and photoconductivity spectra were found to be equal to 1.98 eV and 2.16 eV, respectively. Simultaneously we have performed calculations of the band structure, total and partial density of states and the electron charge density using the ab initio FP-LAPW method. All the calculations were performed with four different exchange-correlation (xc) potentials. It was found that the effect of using different xc is very marginal on the valence band maximum (VBM) while it is dramatically shifted the conduction band minimum (CBM) towards higher energies with respect to the Fermi energy position. Our theoretical results have given a band energy gap equal to 0.40 eV (for LDA), 0.42 eV (GGA), 1.03 eV (EVGGA) and 1.30 eV (mBJ) xc potentials. Thus the underestimation of the energy gap in LDA and GGA was partially corrected in EVGGA and mBJ model. As a remarkable fact mBJ did not bring the energy band gap very close to the experimental once. We have discovered that the Ag-s states have only a small effect on the conduction bands shifts whereas Ge-s states have a strong effect in extending of the gap, while remaining the valence bands unchanged.