Two haloid borate crystals with large nonlinear optical response.

The photophysical properties of the noncentrosymmetric haloid borates K3B6O10X (X = Cl or Br) are calculated using density functional theory within the recently modified Becke-Johnson potential. The calculated electronic band structure reveals that the theoretical direct band gaps, 5.21 eV (K3B6O10Cl) and 4.85 eV (K3B6O10Br), are in good agreement with the previous calculation for K3B6O10Cl (5.16 eV) and experimental data for K3B6O10Br (4.86 eV). The calculated absorption coefficients, refractive indices, and birefringence are also in good agreement with the experimental data. The calculated nonzero second harmonic generation (SHG) coefficients, d33, d22 (= -d21), and d15(= d32 = d31), show good agreement with the experimental values. Furthermore, we have obtained the microscopic first hyperpolarizability for the dominant tensor component of the SHG.

Role of spin-orbit interaction on the nonlinear optical response of CsPbCO3F using DFT.

We explore the effect of spin-orbit interaction (SOI) on the electronic and optical properties of CsPbCO3F using the full potential linear augmented plane wave method with the density functional theory (DFT) approach. CsPbCO3F is known for its high powder second harmonic generation (SHG) coefficient (13.4 times (d36 = 0.39 pm V-1) that of KH2PO4 (KDP)). Calculations are done for many exchange correlation (XC) potentials. After the inclusion of SOI, the calculated Tran-Blaha modified Becke-Johnson (TB-mBJ) band gap of 5.58 eV reduces to 4.45 eV in agreement with the experimental value. This is due to the splitting of Pb p-states. Importantly, the occurrence of a band gap along the H-A direction (indirect) transforms to the H-H (direct) high symmetry points/direction in the first Brillouin zone. We noticed a large anisotropy in the calculated complex dielectric function, absorption, and refractive index spectra. The calculated static birefringence of 0.1049 and 0.1057 (with SOI) is found to be higher than that of the other carbonate fluorides. From the Born effective charge (BEC) analysis we notice that the Cs atom shows a negative contribution to birefringence whereas Pb, C, and F atoms show a positive contribution. In addition, we have also calculated the nonlinear optical χ(-2ω;ω,ω) dispersion of a CsPbCO3F single crystal. We found that d11 = d12 = 4.35 pm V-1 at 1064 nm, which is 11.2 times higher than d36 of KDP. The origin of the highly nonlinear optical susceptibility dispersion of CsPbCO3F is explained. Overall, our results are in agreement with experiments and it is obvious from the present study that CsPbCO3F is a direct band gap, large second harmonic generation, and good phase matchable NLO crystal in the ultraviolet region.

Luminescence and advanced mass spectroscopic characterization of sodium zinc orthophosphate phosphor for low-cost light-emitting diodes.

A new rare-earth-free NaZnPO4:Mn(2+) (NZP:Mn) phosphor powder has been developed by our group and investigated meticulously for the first time using secondary ion mass spectroscopy and chemical imaging techniques. The studies confirmed the effective incorporation of Mn(2+) into the host lattice, resulting in an enhancement of photoluminescence intensity. Phase purity has been verified and structure parameters have been determined successfully by Rietveld refinement studies. The NZP:Mn phosphor powder exhibits strong absorption bands in the ultraviolet and visible (300-470 nm) regions with a significant broad yellow-green (~543 nm) emission due to the characteristic spin forbidden d-d transition ((4)T1→(6)A1) of Mn(2+) ions, indicating weak crystal field strength at the zinc-replaced manganese site. The decay constants are a few milliseconds, which is a pre-requisite for applications in many display devices. The results obtained suggest that this new phosphor powder will find many interesting applications in semiconductor physics, as cost-effective light-emitting diodes (LEDs), as solar cells and in photo-physics.

Engineering oxygen vacancies towards self-activated BaLuAl(x)Zn(4-x)O(7-(1-x)/2) photoluminescent materials: an experimental and theoretical analysis.

Novel self-activated yellow-emitting BaLuAlxZn4-xO7-(1-x)/2 photoluminescent materials were investigated by a combined experimental and theoretical analysis. The effects of Al/Zn composition modulation, calcination atmosphere and temperature on the crystal structure and photoluminescence properties have been studied via engineering oxygen vacancies. Accordingly, BaLuAl0.91Zn3.09O7 prepared in an air atmosphere was found to be the stable crystalline phase with optimal oxygen content and gave a broad yellow emission band with a maximum at 528 nm. The self-activated luminescence mechanism is ascribed to the O-vacancies based on the density functional theory (DFT) calculation. A theoretical model originating from the designed oxygen vacancies has been proposed in order to determine the influence of O-vacancies on the band structure and self-activated luminescence. Therefore, the appearance of a new local energy level in the band gap will cause the wide-band optical transitions in the studied BaLuAlxZn4-xO7-(1-x)/2 materials.

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.

Dispersion of linear and nonlinear optical susceptibilities and the hyperpolarizability of 3-methyl-4-phenyl-5-(2-pyridyl)-1,2,4-triazole.

As a starting point for our calculation of 3-methyl-4-phenyl-5-(2-pyridyl)-1,2,4-triazole we used the XRD data obtained by C. Liu, Z. Wang, H. Xiao, Y. Lan, X. Li, S. Wang, Jie Tang, Z. Chen, J. Chem. Crystallogr., 2009 39 881. The structure was optimized by minimization of the forces acting on the atoms keeping the lattice parameters fixed with the experimental values. Using the relaxed geometry we have performed a comprehensive theoretical investigation of dispersion of the linear and nonlinear optical susceptibilities of 3-methyl-4-phenyl-5-(2-pyridyl)-1,2,4-triazole using the full potential linear augmented plane wave method. The local density approximation by Ceperley-Alder (CA) exchange-correlation potential was applied. The full potential calculations show that this material possesses a direct energy gap of 3.4 eV for the original experimental structure and 3.2 eV for the optimized structure. We have calculated the complex's dielectric susceptibility ε(ω) dispersion, its zero-frequency limit ε(1)(0) and the birefringence. We find that a 3-methyl-4-phenyl-5-(2-pyridyl)-1,2,4-triazole crystal possesses a negative birefringence at the low-frequency limit Δn(0) which is equal to about -0.182 (-0.192) and at λ = 1064 nm is -0.193 (-0.21) for the non-optimized structure (optimized one), respectively. We also report calculations of the complex second-order optical susceptibility dispersions for the principal tensor components: χ(ω), χ(ω) and χ(ω). The intra- and inter-band contributions to these susceptibilities are evaluated. The calculated total second order susceptibility tensor components at the low-frequency limit |χ(0)| and |χ(ω)| at λ = 1064 nm for all the three tensor components are evaluated. We established that the calculated microscopic second order hyperpolarizability, ß(ijk), the vector component along the dipole moment direction, at the low-frequency limit and at λ = 1064 nm, for the dominant component |χ(ω)| is 4.99 × 10(-30) esu (3.4 × 10(-30) esu) and 7.72 × 10(-30) esu (5.1 × 10(-30) esu), respectively for the non-optimized structure (optimized structure).

Density functional calculations of the electronic structure of 3-phenylamino-4-phenyl-1,2,4-triazole-5-thione.

Starting with the X-ray diffraction data on the 3-phenylamino-4-phenyl-1,2,4-triazole-5-thione compound obtained by Wang et al. [Molecules, 2009, 14, 608], we have optimised the atomic positions by minimization of the forces acting on the atoms using a full potential linear augmented plane wave method within density-functional theory along with the generalized gradient approximation (GGA) by Perdew, Burke and Ernzerhof (PBE) exchange-correlation potential. In addition, for the electronic band-structure and the calculation of the gap the Engel-Vosko (EV-GGA) scheme has applied. The full potential calculations show that the valence band maximum (VBM) is located at the Gamma point and the conduction band minimum (CBM) is located at the Z point of the Brillouin zone resulting in an indirect energy gap of 3.1 eV. The upper VBM is mainly formed by S-p states while the lower CB has mainly C-p states character. Thus the S-p states in the upper valence band and the C-p states in the lower conduction band have a significant effect on the energy band gap. We present an analysis of the partial densities of states which gives useful information on hybridization and the orbital character of the states. We have calculated the bond lengths and bond angles and find better agreement with the experimental data than the density functional theory calculations at B3LYP/6-311G** and PBE1PBE/6-311G** levels of theory by the Berny method within the Gaussian 03 software package. This is attributed to the fact that the values obtained by these methods belong to the isolated molecule in the gas-phase whereas the experimental and the full potential linear augmented plane wave method values are attributed to the molecule in the solid state.

Band structure, density of states, and optical susceptibilities of a novel lithium indium orthoborate Li3InB2O6.

By use of the structural parameters of the single crystal lithium indium orthoborate obtained by Penin et al. (Solid State Sci. 2001, 3, 461-468), from X- ray diffraction data, we present a first-principle study of the electronic structure and the linear optical properties for the novel lithium indium orthoborate Li3InB2O6. A full-potential linear augmented plane wave method within density functional theory with the Engel-Vosko exchange correlation was used. This compound has a wide direct energy band gap of about 3.8 eV with both the valence band maximum and conduction band minimum located at the center of the Brillouin zone. Our calculations of the partial density of states shows that the upper valence band originates predominantly from the O-p, B- p, and In-p states, and the lower conduction band is dominated by the O-s/p, In-p, and B-p states. Thus the O-p states in the upper valence band and lower conduction band has a significant effect on the energy band gap dispersion. The uniaxial anisotropy [deltaepsilon=(epsilon0zz-epsilon0xx)/epsilon0tot] is about -0.041.

X-ray diffraction, crystal structure, and spectral features of the optical susceptibilities of single crystals of the ternary borate oxide lead bismuth tetraoxide, PbBiBO4.

The all-electron full-potential linearized augmented plane-wave method has been used for an ab initio theoretical study of the band structure, the spectral features of the optical susceptibilities, the density of states, and the electron charge density for PbBiBO4. Our calculations show that the valence-band maximum (VBM) and conduction-band minimum (CBM) are located at the center of the Brillouin zone, resulting in a direct energy gap of about 3.2 eV. We have synthesized the PbBiBO4 crystal by employing a conventional solid-state reaction method. The theoretical calculations in this work are based on the structure built from our measured atomic parameters. We should emphasize that the observed experimental X-ray diffraction (XRD) pattern is in good agreement with the theoretical one, confirming that our structural model is valid. Our calculated bond lengths show excellent agreement with the experimental data. This agreement is attributed to our use of full-potential calculations. The spectral features of the optical susceptibilities show a small positive uniaxial anisotropy.

X-ray diffraction, X-ray photoelectron spectra, crystal structure, and optical properties of centrosymmetric strontium borate Sr2B16O26.

We report results of X-ray diffraction (XRD) and valence band X- ray photoelectron (VB-XPS) spectra for strontium borate Sr(2)B(16)O(26). The X-ray structural analysis shows that the single crystals of Sr(2)B(16)O(26) crystallize in the monoclinic space group P2(1)/c with a = 8.408(1) A, b = 16.672(1) A, c = 13.901(2) A, beta = 106.33(1) degrees , and Z = 4. The crystal structure consists of a 3D network of the complex borate anion [B(16)O(20)O(12/2)](4-), formed by 12 BO(3) triangles and four BO(4) tetrahedra, which can be viewed as three linked [B(3)O(3)O(4/2)](-) triborate groups bonded to one pentaborate [B(5)O(6)O(4/2)](-) group and two BO(3) triangles. Using this structure, we have performed theoretical calculations using the all-electron full potential linearized augmented plane wave (FP-LAPW) method for the band structure, density of states, electron charge density, and the frequency-dependent optical properties. Our experimental VB-XPS of Sr(2)B(16)O(26) is compared with results of our FP-LAPW calculations. Our calculations show that the valence band maximum (VBM) and conduction band minimum (CBM) are located at Gamma of the Brillouin zone (BZ) resulting in a direct energy gap of about 5.31 eV. Our measured VB-XPS show reasonable agreement with our calculated total density of states for the valence band that is attributed to the use of the full potential method.

Ab initio calculation of the electronic band structure, density of states and optical properties of alpha-2-methyl-1-nitroisothiourea.

The electronic and optical properties of alpha-2-methyl-1-nitroisothiourea have been studied using a full potential linear augmented plane wave method within density-functional theory along with the Engel-Vosko exchange correlation function. The structural data obtained by Vasil'ev et al. [Vasil'ev, A. D.; Astakhov, A. M.; Gelemurzina, I. V.; Stepanov, R. S. Dokl. Chem. 2001, 379, (4-6), 232-235; translated from Dokl. Akad. Nauk 2001, 379 (6), 781-784] from X-ray diffraction was used. Our calculations show that the valence band maximum (VBM) is located at the T and the conduction band minimum (CBM) is located at the R point of the Brillouin zone, resulting in an indirect energy gap of 3.1 eV. The calculated partial density of states shows that the upper valence band and the lower conduction band arise predominantly from the O-p, N-p and S-p states. The compound has a large uniaxial dielectric anisotropy and a large negative birefringence.

X-ray photoelectron spectrum and electronic properties of a noncentrosymmetric chalcopyrite compound HgGa(2)S(4): LDA, GGA, and EV-GGA.

An all electron full potential linearized augmented plane wave method has been applied for a theoretical study of the band structure, density of states, and electron charge density of a noncentrosymmetric chalcopyrite compound HgGa(2)S(4) using three different approximations for the exchange correlation potential. Our calculations show that the valence band maximum (VBM) and conduction band minimum (CBM) are located at Gamma resulting in a direct energy gap of about 2.0, 2.2, and 2.8 eV for local density approximation (LDA), generalized gradient approximation (GGA), and Engel-Vosko (EVGGA) compared to the experimental value of 2.84 eV. We notice that EVGGA shows excellent agreement with the experimental data. This agreement is attributed to the fact that the Engel-Vosko GGA formalism optimizes the corresponding potential for band structure calculations. We make a detailed comparison of the density of states deduced from the X-ray photoelectron spectra with our calculations. We find that there is a strong covalent bond between the Hg and S atoms and Ga and S atoms. The Hg-Hg, Ga-Ga, and S-S bonds are found to be weaker than the Hg-S and Ga-S bonds showing that a covalent bond exists between Hg and S atoms and Ga and S atoms.

Dispersion of linear and nonlinear optical susceptibilities in calcium neodymium oxyborate Ca4NdO(BO3)3-LDA versus GGA.

We have performed ab inito theoretical calculations of the electronic structure and the linear and nonlinear optical susceptibilities for calcium neodymium oxyborate Ca4NdO(BO3)3 using two approximations for the exchange correlation (XC) potentials, the local density approximation (LDA) and the generalized gradient approximation (GGA). Our calculations show that this compound is metallic-like, with density of states at the Fermi energy E(F), N(E(F)), of 5.95 and 10.33 states/Ry-cell or bare electronic specific heat coefficients of 1.03 and 1.79 mJ/mol-K2 for LDA and GGA, respectively. The overlap between the valence and conduction bands is strong, resulting in metallic behavior. We found that Nd-s/p/d, Ca-s/p, B-p, and O-s/p states controlled the overlapping around E(F). The effect of LDA and GGA on the band structure, density of states, and linear optical properties is very small, while for the nonlinear optical properties, it is very pronounced. Our calculations show that χ(111)(2)(ω) is the dominant component for both LDA and GGA. We find opposite signs of the contributions of the 2ω and 1ω inter/intraband to the real and imaginary parts for the dominant component throughout the wide optical frequency range.

Synthesis, IR, UV-vis spectra, x-ray diffraction and band structure of a non-centrosymmetric borate single-crystal CaBiGaB(2)O(7).

A non-centrosymmetric borate, CaBiGaB(2)O(7), has been grown by a solid-state reaction method at a temperature below 700 °C. The single-crystal x-ray structural analysis has shown that it crystallizes in the tetragonal space group [Formula: see text] with a = 0.7457(1) nm, c = 0.4834(1) nm, Z = 2. It has a three-dimensional (3D) structure in which [B(2)O(7)](8-) groups are bridged by [GaO(4)](5-) tetrahedra through shared O atoms to form 2D [Formula: see text] layers that are further linked by Bi(3+)/Ca(2+) cations giving rise to the final 3D framework. The IR spectrum confirms the presence of [BO(4)](5-) groups and the UV-vis diffuse reflectance spectrum shows that the optical band gap is about 2.9 eV. This value is compared with our band structure calculations using the full potential linearized augmented plane wave approach within the framework of the Engel-Vosko GGA formalism.

Spin-dependent scattering induced negative magnetoresistance in topological insulator Bi2Te3 nanowires.

Studies of negative magnetoresistance in novel materials have recently been in the forefront of spintronic research. Here, we report an experimental observation of the temperature dependent negative magnetoresistance in Bi2Te3 topological insulator (TI) nanowires at ultralow temperatures (20 mK). We find a crossover from negative to positive magnetoresistance while increasing temperature under longitudinal magnetic field. We observe a large negative magnetoresistance which reaches -22% at 8 T. The interplay between negative and positive magnetoresistance can be understood in terms of the competition between dephasing and spin-orbit scattering time scales. Based on the first-principles calculations within a density functional theory framework, we demonstrate that disorder (substitutional) by Ga+ ion milling process, which is used to fabricate nanowires, induces local magnetic moments in Bi2Te3 crystal that can lead to spin-dependent scattering of surface and bulk electrons. These experimental findings show a significant advance in the nanoscale spintronics applications based on longitudinal magnetoresistance in TIs. Our experimental results of large negative longitudinal magnetoresistance in 3D TIs further indicate that axial anomaly is a universal phenomenon in generic 3D metals.

Electronic structure and optical properties of 1T-TiS2 and lithium intercalated 1T-TiS2 for lithium batteries.

We report results of first-principles calculations of electronic and optical properties of pristine 1T-TiS(2) and 1T-TiS(2) intercalated with lithium. Calculations have been performed using the full-potential linearized augmented plane wave method based on density functional theory together with the local density approximation for the exchange correlation energy functional. We have calculated the band structure, density of states, and the linear optical properties. We compare our results of the intercalated 1T-LiTiS(2) with the host 1T-TiS(2) to ascertain the effect of Li intercalation on the electronic and optical properties. The Li-s and Li-p bands are very broad and do not contribute much to the density of states. Our calculations show that the electronic and optical properties are influenced significantly when TiS(2) is intercalated with lithium.

X-ray diffraction and optical properties of a noncentrosymmetric borate CaBiGaB(2)O(7).

Single crystals of the noncentrosymmetric borate CaBiGaB(2)O(7) were synthesized by conventional solid state reaction. The purity of the crystal was checked by x-ray powder diffraction. The optical properties were measured by analyzing the diffuse reflectance data obtained with a Shimadzu UV-3101PC double-beam, double-monochromator spectrophotometer. We find a steep absorption edge confirming its semiconducting nature. The optical band gap obtained by extrapolation of a linearlike absorption edge was roughly 2.9 eV consistent with the observed pale yellow color of the sample. Theoretical calculations based on the structural model built from our measured atomic parameters have been performed using the all-electron full potential linearized augmented plane wave method. The generalized gradient approximation (GGA) of the exchange correlation potential as given by Engel-Vosko GGA is used. The frequency-dependent complex dielectric function was calculated and the origin of some of the spectral peaks is discussed. The linear optical properties show strong uniaxial anisotropy and birefringence that favors large second order susceptibility. Our calculations show that the complex second order nonlinear optical susceptibility tensor chi(322) ((2))(omega) is the dominant component having the largest total Re chi(322) ((2))(0) value of about 1.8 pm/V.

Unexplored photoluminescence from bulk and mechanically exfoliated few layers of Bi2Te3.

We report the exotic photoluminescence (PL) behaviour of 3D topological insulator Bi2Te3 single crystals grown by customized self-flux method and mechanically exfoliated few layers (18 ± 2 nm)/thin flakes obtained by standard scotch tape method from as grown Bi2Te3 crystals. The experimental PL studies on bulk single crystal and mechanically exfoliated few layers of Bi2Te3 evidenced a broad red emission in the visible region from 600-690 nm upon 375 nm excitation wavelength corresponding to optical band gap of 2 eV. These findings are in good agreement with our theoretical results obtained using the ab initio density functional theory framework. Interestingly, the observed optical band gap is several times larger than the known electronic band gap of ~0.15 eV. The experimentally observed 2 eV optical band gap in the visible region for bulk as well as for mechanically exfoliated few layers Bi2Te3 single crystals clearly rules out the quantum confinement effects in the investigated samples which are well known in the 2D systems like MoS2,WS2, WSe2, and MoSe2 for 1-3 layers.

Optical spectra and band structure of Ag(x)Ga(x)Ge(1-x)Se2 (x = 0.333, 0.250, 0.200, 0.167) single crystals: experiment and theory.

Theoretical and experimental studies of the Ag(x)Ga(x)Ge(1-x)Se2 (x = 0.333, 0.250, 0.200, 0.167) single crystals are performed. These crystals possess a lot of intrinsic defects which are responsible for their optoelectronic features. The theoretical investigations were performed by means of DFT calculations using different exchange-correlation potentials. The experimental studies were carried out using the modulated VUV ellipsometry for dielectric constants and birefringence studies. The comparison of the structure obtained from X-ray with the theoretically optimized structure is presented. The crucial role of the intrinsic defect states is manifested in the choice of the exchange correlation potential used. The data may be applicable for a large number of the ternary chalcogenides which are sensitive to the presence of the local disordered states near the band edges.

Influence of replacing Si by Ge in the chalcogenide quaternary sulfides Ag2In2Si(Ge)S6 on the chemical bonding, linear and nonlinear optical susceptibilities, and hyperpolarizability.

The linear and nonlinear optical properties of Ag2In2SiS6 and Ag2In2GeS6 are calculated so as to obtain further insight into the electronic properties. The influence of using different exchange correlation potentials and the effect of replacing Si by Ge on the geometry, chemical bonding, and on the optical properties are presented. There is notable increasing in the energy band gap when moving from LDA to GGA, EVGGA then to mBJ. The effect of replacing Si by Ge atom causes a geometric change, which leads to large changes in the linear as well as the nonlinear optical susceptibilities. For the linear optical properties, it causes to increase the amplitude of the left-hand hump of ε(2)(average)(ω) as well as a small shift of the main peak to lower energies. We have evaluated ε(1)(average)(0) and find that a smaller energy gap yields a larger ε1(0) value. From the calculated refractive indices we obtained the birefringence, which is important for second harmonic generation (SHG) and optical parametric oscillation (OPO) as it is defined by the phase-matching condition. The second-order nonlinear optical susceptibilities, namely, the SHG are investigated for χ(111)(2)(ω), χ(122)(2)(ω), χ(133)(2)(ω), χ(221)(2)(ω), and χ(331)(2)(ω). We find that χ(111)(2)(ω) is the dominant component. The microscopic second order hyperpolarizability, ß111, for the dominant component χ(111)(2)(ω) was obtained. We should emphasize that replacing Si by Ge enhances the linear and nonlinear optical susceptibilities so that Ag2In2GeS6 shows higher values of the linear and nonlinear optical susceptibilities and ß111 in comparison to Ag2In2SiS6.