Two-dimensional cadmium selenide electronic and optical properties: first principles studies

Structural, electronic and optical properties of two-dimensional (2D) cadmium selenide (CdSe) structures with \(2\times 2\) periodicities are investigated. First principles total energy calculations are performed within the periodic density functional theory. Initially, the structural properties are determined using the local density approximation as implemented in the PWscf code of quantum ESPRESSO package. To investigate the electronic properties, the GW method is applied to determine the energy gap within the plasmon pole and the random phase approximations. Optical properties are investigated to determine the dielectric constant and the Bethe–Salpeter theory is used to calculate the exciton binding energies. Zinc blende and wurtzite phases are considered to calculate the bulk energy gaps, which are compared to the experimental values, finding good agreement. The 2D structure exhibits an energy gap larger than that of the bulk, indicating the effects of reduction in dimensionality; these changes can be attributed to the dangling bonds that are present in the 2D layer.     

Linear and nonlinear optical properties of borate crystals as calculated from the first principles

With the development of the state-of-the-art band calculation scheme and massively parallel processing in the high performance computing, we are now able to calculate all important physical properties, including (i) the nonlinear susceptibility; (ii) the multiphoton absorption rate; (iii) the birefringence; and (iv) the energy gap, from the first principles for complex practical nonlinear optical crystals, such as the borate crystal series, with an accuracy acceptable for materials development/design, and answer the questions often raised by the material scientists.     

Dielectric properties of organosilicons from first principles

Density functional perturbation theory calculations have been performed to determine the dielectric constant of Si “doped” polyethylene (PE). Substitution of C atoms in PE by Si ranging from 0 to 100% has been considered. Both the electronic and ionic contributions to the dielectric constant increase with increasing Si content. These increases are attributed, respectively, to enhanced σ conjugation and increased IR vibrational intensity of modes involving Si containing bonds (owing to their softness and polarity).     

SiC陶瓷与Zn界面结合特性的第一性原理研究

低温焊接SiC陶瓷是金属/陶瓷连接领域非常重要的研究方向,而与之相关的理论研究相对匮乏,同时,通过实验手段难以描述金属/陶瓷界面原子之间的相互作用。为研究低温Zn基钎料与SiC陶瓷的界面结合方式,采用第一性原理方法,计算了Zn(0001)和SiC(0001)的表面能,6种不同堆垛方式的Zn(0001)/SiC(0001)界面模型的分离功,并分析了其中最稳定两种模型的电荷密度图、电荷密度差分图和Mulliken布局。结果表明:Zn/SiC界面只形成了Zn-Si离子键,Si终端孔穴型界面的Zn-Si键结合强度高于C终端孔穴型。     

OEDIPE: a new graphical user interface for fast construction of numerical phantoms and MCNP calculations

Although great efforts have been made to improve the physical phantoms used to calibrate in vivo measurement systems, these phantoms represent a single average counting geometry and usually contain a uniform distribution of the radionuclide over the tissue substitute. As a matter of fact, significant corrections must be made to phantom-based calibration factors in order to obtain absolute calibration efficiencies applicable to a given individual. The importance of these corrections is particularly crucial when considering in vivo measurements of low energy photons emitted by radionuclides deposited in the lung such as actinides. Thus, it was desirable to develop a method for calibrating in vivo measurement systems that is more sensitive to these types of variability. Previous works have demonstrated the possibility of such a calibration using the Monte Carlo technique. Our research programme extended such investigations to the reconstruction of numerical anthropomorphic phantoms based on personal physiological data obtained by computed tomography. New procedures based on a new graphical user interface (GUI) for development of computational phantoms for Monte Carlo calculations and data analysis are being developed to take advantage of recent progress in image-processing codes. This paper presents the principal features of this new GUI. Results of calculations and comparison with experimental data are also presented and discussed in this work.     

GGA+U method from first principles: application to reduction–oxidation properties in ceria-based oxides

We implement DFT calculations by a Hubbard-like correction for localized strongly correlated electrons, coupled with a generalized gradient approximation to the exchange-correlation functional to elucidate the role of the ceria based catalytically active supports for the chemical reactions involving reduction–oxidation processes. These catalytic processes are relevant for many industrial applications, such as catalytic converters in automotive applications, solid oxide fuel cells, and hydrogen production from biomass. The Hubbard-like correction U is computed from first principles as physical property of the system. We find that the high performance of ceria-based oxides as an active support for noble metals in catalysis relies on an efficient supply of lattice oxygen at reaction sites governed by oxygen vacancy formation.     

Experiments and first principles calculations on the effects of hydrogen on the optical properties of ferroelectric materials

The effects of hydrogen on the optical properties of lead zirconate titanate (PZT) ceramics and BaTiO₃ single crystals have been investigated. The experimental results showed that the color of BaTiO₃ single crystals changed from light to dark after charging of hydrogen in NaOH solution at room temperature and in H₂ gas at high temperature, and then, restored to light color after outgassing in the air, which were the same as PZT-5H ceramics reported earlier. Measurement of the absorbance indicated that hydrogen increased the absorbance within the visible spectrum for both PZT ceramics and BaTiO₃ single crystals. The first principles calculations for Pb(Zr_0.5Ti_0.5)O₃ and BaTiO₃ showed that hydrogen increased the absorbance within the infrared and visible spectral region and the absorbance increased with the rise of hydrogen concentration. Density of states (DOS) researches indicated that hydrogen changed the DOS of the outer shell electrons, which caused the change of absorbance spectrum.     

Mechanical and electronic properties of diamond nanowires under tensile strain from first principles

The atomic and electronic structures, heat of formation, Young's modulus, and ideal strength of hydrogenated diamond nanowires (DNWs) with different cross-sections (from 0.06 to 2.80 nm(2)) and crystallographic orientations ((100), (110), (111), and (112)) have been investigated by means of first-principles simulations. For thinner DNWs (cross-sectional area less than 0.6 nm(2)), preferential growth orientation along (111) is observed. The Young's modulus and ideal strength of these DNWs decrease with decreasing cross-section and show anisotropic effects. Moreover, the band gap of DNWs is sensitive to the size, crystallographic orientation and tensile strain, implying the possibility of a tunable gap. The effective mass at the edges of the conduction band and valence band are also obtained. These theoretical results are helpful for designing novel optoelectronic devices and electromechanical sensors using diamond nanowires.     

Dependence of pressure on elastic,electronic and optical properties of CeO2 and ThO2: A first principles study

The phase transformation of CeO₂ and ThO₂ from fluorite to cotunnite-type structure under pressure is predicted within the density functional theory implemented with the GGA-PW91 method, the pressure induced structural phase transition occurs at 28.9 GPa for CeO₂ and 29.8 GPa for ThO₂. These values are in excellent agreement with the experimentally measured data. The elastic, electronic and optical properties at normal as well as for high-pressure phase have been calculated, particular attention is devoted to the cotunnite phase. Further, the dependence of the elastic constants, the bulk modulus B, the energy band gaps and the dielectric function on the applied pressure are presented.     

The structural and mechanical properties of CdN compound: A first principles study

First principles calculations are performed to investigate the structural, elastic, and mechanical properties of CdN for various structures: NaCI, CsCl, ZnS, wurtzite, WC, CdTe, NiAs, and CuS. The local density and generalized gradient approximations are used for modeling exchange–correlation effects. Our calculations indicate that CuS (B18) structure is energetically the most stable among the considered structures. The some basic physical properties such as lattice parameters, bulk modulus, and second-order elastic constants are calculated. We have also predicted the shear modulus, Young’s modulus, Poison’s ratio, Debye temperature, and sound velocities. Our structural and some other results are consistent with the available theoretical data.     

Electronic and magnetic properties of Fe3?x Cr x Si ordered alloys from first principles

Density functional theory calculations are performed to investigate the electronic and magnetic properties of Fe_3−x Cr _x Si alloys with Cr concentration in the range of 0.25 ≤ x ≤ 2.75. The L2₁ phase is found to be a more stable one in comparison with the A15 phase for x ≤ 1.50 beyond which the A15 phase becomes more stable. Alloys with the stable L2₁ phase are found to be metallic for x ≤ 0.75, however, a half metallic behavior is found at x = 1.00, 1.25, and 1.50 with band gaps of 0.60, 0.24, and 0.21 eV, respectively. In contrast, all A15 structures are found to be metallic. The total magnetic moments are found to decrease for L2₁ phase from 14.4 μ_B/cell at x = 0.25 to zero at x = 2.00 with non-integer values for the metallic structures and integer values for the half-metallic. However, a monotonic decrease is found for the case of A15 phase with values larger than those of L2₁ phase.     

水热法制备不同形貌纳米ZnO阵列及光学性能的研究

采用简单水热法制备得到棒状、铅笔状和塔状不同形貌的纳米ZnO阵列。采用扫描电镜(SEM)、X射线衍射(XRD)、光致发射光谱对样品结构、形貌、光学性能和催化性能进行表征;结果表明,样品晶型比较完整,PL谱图表明样品在380nm左右都出现了强烈的紫外发射峰。同时,以甲基橙为模拟污染物,通过光催化测试表明,产品均具有良好的光催化性能,其中塔状纳米ZnO的光催化性能较高。     

Linear and nonlinear optical properties of organic crystals: MBANP and its derivatives

In addition to possessing strong optical nonlinearity, the (−)-2-(α-methylbenzylamino)-5-nitropyridine (MBANP) crystal has been shown to have unusual linear optical properties. Two methyl derivatives of MBANP have now been synthesized and grown as large single crystals; their refractive indices have been measured as a function of frequency and second-harmonic generation, including the determination of the phase matching loci has been investigated. Data is also available on the racemic form of crystalline MBANP. The properties of this family of crystals have been reviewed and an attempt is made to relate the experimental responses of the crystals to their molecular structure and the calculated properties of the molecules.     

Formation of tetrapod ZnO nanowhiskers and its optical properties

Nanopowder containing ZnO nanowhiskers and nanoparticles was synthesized by the oxidation of Zn vapor, at temperatures from 1050 to 1450 °C in a flow of Ar and O₂ gas mixture. The morphology and structure of the nanowhiskers were dependent on the synthesis parameters such as evaporation temperature and the gas pressure. The nanowhiskers formed had a tetrapod shape with needle-like feet that become shorter and fatter on increasing temperature under a certain pressure range. The amount of Zn phase inside the nanowhiskers increased with increasing temperature. The Zn phase melted at temperature of 420 °C, which was confirmed by differential scanning calorimetry (DSC). The nanopowders showed strong sintering activity after they were pressed. The far infrared light was intensely absorbed by the ZnO tetrapod nanowhiskers.     

Mechanical properties of U-0.95 mass fraction of Ti alloy quenching and aging treatment:a first principles study

First principles plane wave pseudopotential method was executed to calculate the mechanical properties with respect to the uranium-0.95 mass fraction of titanium(U-0.95 mass fraction of Ti) alloy for quenching and aging,including the elastic modulus,the value of shear modulus to bulk modulus(G/B) and the ideal tensile strength.The further research has also been done about the crack mechanism through Griffith rupture energy.These results show that the elastic moduli are 195.1 GPa for quenching orthorhombic a phase and 201.8 GPa for aging formed Guinier-Preston(G.P) zones,while G/B values are0.67 and 0.56,respectively.With the phase change of uranium-titanium(U-Ti) alloy via the quenching treatment,the ideal tensile strength is diverse and distinct with different crystal orientations of the anisotropic α phase.Comparison of quenching and short time aging treatment,both of the strength and toughness trend to improve slightly.Further analysis about electronic density of states(DOS) in the electronic scale indicates that the strength increases continuously while toughness decreases with the aging proceeding.The equilibrium structure appears in overaging process,as a result of decomposition of metastable quenching a phase.Thereby the strength and toughness trend to decrease slightly.Finally,the ideal fracture energies of G.P zones and overaging structure are obtained within the framework of Griffith fracture theory,which are 4.67 J/m2 and 3.83 J/m2,respectively.These results theoretically demonstrate strengthening effect of quenching and aging heat treatment on U-Ti alloy.     

Polarization, piezoelectric properties, and elastic coefficients of In x Ga1−x N solid solutions from first principles

III-nitrides GaN and InN, and In _x Ga_1−x N solid solutions are polarizable semiconductors that crystallize in the prototypical wurtzite (W) structure. We present here the results of a density functional theory study carried out to determine the spontaneous polarization, piezoelectric coefficients, and elastic coefficients of In _x Ga_1−x N alloys as a function of In the concentration x. To calculate these properties, we construct three distinct hexagonal/orthorhombic equivalent In _x Ga_1−x N supercells that are derived from the disordered W unit cell of GaN and InN. These include an ordered W lattice (P6₃ mc/Pmc2₁) and orthorhombic O-16 and O-32 lattices with Pmn2₁/Pna2₁ or P2₁ symmetry, respectively. Depending on the crystal structure, spontaneous polarization as a function of the In concentration x shows a downward bowing (W), a linear interpolation (O-16), and an upward bowing (O-32) between −0.033 C/m² and −0.043 C/m², the spontaneous polarizations of the end components GaN and InN, respectively. The composition dependence of the effective basal plane and out of plane (along the [0001] direction) piezoelectric coefficients (e _// and e ₃₃, respectively) in the W and O-16 structure is non-linear and varies between e _// = −0.287 C/m² and e ₃₃ = 0.598 C/m² for GaN, and e _// = −0.455 C/m² and e ₃₃ = 1.044 C/m² for InN. While the bulk modulus of In _x Ga_1−x N in the W and O-16 structures follows Vegard’s law from 170 GPa (x = 0) to 124 GPa (x = 1), in the O-32 structure it shows a strong downward bowing for compositions 0 < x < 0.5.     

Extended multiscale finite element method: its basis and applications for mechanical analysis of heterogeneous materials

Extended multiscale finite element method (EMsFEM) has been proved to be an efficient method for the mechanical analysis of heterogeneous materials. The key factor for efficiency and accuracy of EMsFEM is the numerical base functions (NBFs). The paper summarizes the general method for constructing NBFs and proposes a generalized isoparametric interpolation based on the rigid displacement properties (RDPs) of NBFs. We prove that the NBFs constructed by linear, periodic and rotational angle boundary conditions satisfy the RDPs, which is independent with the shape and material properties of unit cells. The properties of NBFs for oversampling technique are also comprehensively discussed. The algorithm complexity is discussed in theoretical and numerical aspects, which concludes that the computation quantity of EMsFEM is much smaller than the direct solutions. The algorithm is validated by linear analysis of the materials with random impurities and holes and the efficiency is improved further by parallel computing.     

Multiple polynomial regression method for determination of biomedical optical properties from integrating sphere measurements

We present a new, to our knowledge, method for extracting optical properties from integrating sphere measurements on thin biological samples. The method is based on multivariate calibration techniques involving Monte Carlo simulations, multiple polynomial regression, and a Newton-Raphson algorithm for solving nonlinear equation systems. Prediction tests with simulated data showed that the mean relative prediction error of the absorption and the reduced scattering coefficients within typical biological ranges were less than 0.3%. Similar tests with data from integrating sphere measurements on 20 dye-polystyrene microsphere phantoms led to mean errors less than 1.7% between predicted and theoretically calculated values. Comparisons showed that our method was more robust and typically 5-10 times as fast and accurate as two other established methods, i.e., the inverse adding-doubling method and the Monte Carlo spline interpolation method.