First principles study on the structural, electronic and optical properties of diluted magnetic semiconductors Zn1-xCoxX （X=S, Se, Te）

The electronic and optical properties of zincblende ZnX（X=S, Se, Te） and ZnX：Co are studied from density functional theory （DFT） based first principles calculations. The local crystal structure changes around the Co atoms in the lattice are studied after Co atoms are doped. It is shown that the Co-doped materials have smaller lattice constant （about 0.6%-0.9%）. This is mainly due to the shortened Co-X bond length. The （partial） density of states （DOS） is calculated and differences between the pure and doped materials are studied. Results show that for the Co-doped materials, the valence bands are moving upward due to the existence of Co 3d electron states while the conductance bands are moving downward due to the reduced lattice constants. This results in the narrowed band gap of the doped materials. The complex dielectric indices and the absorption coefficients are calculated to examine the influences of the Co atoms on the optical properties. Results show that for the Co-doped materials, the absorption peaks in the high wavelength region are not as sharp and distinct as the undoped materials, and the absorption ranges are extended to even higher wavelength region.

The First Principles Investigations of the Thermoelectric Properties of GaN with p-and n-Type Doping

We investigate the thermoelectric properties of GaN with p- and n-type doping by the first principles calculation and the semi-classical Boltzmann theory. We find that the power factors （Sacr） of p-type GaN （-3500 μW/mK2） is about twice that of the n-type （-1750 μW/mK2）, which indicates the thermoelectric properties of p-type GaN would be better. Thermal conductivity of GaN crystal decreases rapidly as the temperature increases, but it is still too large for thermoelectric applications. The figure of merit （ZT） estimated at 1500 K is 0.134 for p-type GaN crystal and 0.062 for the n-type.

A review on first principles based studies for improvement of cathode material of lithium ion batteries

Lithium ion batteries（LIBs） are currently best energy storage devices providing rechargeable electrical storage to wide variety of applications – from portable electronics to automobiles. Though, these batteries are fully adopted, widely used and commercialized, but researchers are still extensively working on their constituent materials and developing technology to improve their performance. A major part of related research activities is devoted to the electrode of the battery for improvement in its performance thereby addressing issues like safety, lifetime, specific capacity, energy density and most importantly abundance and cost. There are number of cathode materials that have been proposed and tested at laboratory scale and subsequently utilized in commercialized batteries ever since the appearance of LIBs. Owing to the availability of improved computational resources in the last decade, first principles calculation has become a reliable tool and played a vital role to predict the material properties of electrodes prior to their experimental analysis. This review gives a comprehensive insight and thorough analysis of the global research efforts related to the cathode materials based on first principles framework, sheds light on current status of knowledge and explores the ways forward.

Electrocatalytic and photocatalytic performance of noble metal doped monolayer MoS2 in the hydrogen evolution reaction: A first principles study

To maximize the catalytic performance of MoS_(2) in the hydrogen evolution reaction,we investigate the electrocatalytic and photocatalytic performance of monolayer MoS_(2) doped with noble metal(Ag,Au,Cu,Pd,and Pt)using first principles calculation combined with the climbing image nudged elastic band method.We find the band gap of the monolayer MoS_(2) is reduced significantly by the noble metal doping,which is unfavorable to improving its photocatalytic performance.The optical absorption coefficient shows that the doping does not increase the ability of the monolayer MoS_(2) to absorb visible light.The monolayer MoS_(2) doped with the noble metal is not a potential photocatalyst for the hydrogen evolution reaction because the band edge position of the conduction band minimum is lower than-4.44 eV,the reduction potential of H^(+)/H_(2).Fortunately,the band gap reduction increases the electron transport performance of the monolayer MoS_(2),and the activation energy of water splitting is greatly reduced by the noble metal doping,especially the Pt doping.On the whole,noble metal doping can enhance the electrocatalytic performance of the monolayer MoS_(2).

The structure and elasticity of phase B silicates under high pressure by first principles simulation

The structures and elasticities of phase B silicates with different water and iron（Fe） content are obtained by firstprinciples simulation to understand the effects of water and Fe on their properties under high pressure.The lattice constants a and b decrease with increasing water content.On the contrary,c increases with increasing water content.On the other hand,the b and c decrease with increasing Fe content while a increases with increasing Fe content.The decrease of M（metal）–O octahedral volume is greater than the decrease of SiO polyhedral volume over the same pressure range.The density,bulk modulus and shear modulus of phase B increase with increasing Fe content and decrease with increasing water content.The compressional wave velocity（Vp） and shear wave velocity（Vs） of phase B decrease with increasing water and Fe content.The comparisons of density and wave velocity between phase B silicate and the Earth typical structure provide the evidence for understanding the formation of the X-discontinuity zone of the mantle.

Applying“First Principles of Instruction”as a Design Theory of the EFL Classroom:Findings from an Empirical Study

How to benefit the learners learning English as a foreign language(EFL)from the aspect of instructional design aroused the concerns of many teachers and researchers.The main purpose of this study is to address the question of how teachers can design and implement EFL classroom that benefit the learners.Specifically,the“First Principles of Instruction,”advocated by Merrill(2002),was applied by the English teachers to conduct their day-to-day pedagogical practices.The corresponding effects of this theory in designing EFL classroom activities were reported in this study.Two research questions are formulated as follows:(1)Does the“First Principles of Instruction”benefit the EFL classroom designing in the aspect of student achievement?If yes,what is the effect?(2)How do the English teachers perceive the“First Principles of Instruction”to design EFL classroom?While observing the EFL classroom activities and interviewing the college English teachers,an empirical study was done to perceive how college English teachers could best apply these principles to design EFL classroom activities.

Ground-State Structure and Physical Properties of NB_2 Predicted from First Principles

Using the newly developed particle swarm optimization algorithm on crystal structural prediction, we predict a new class of boron nitride with stoicMometry of NB2 at ambient pressure, which belongs to the tetragonal 14m2 space group. Then, its structure, elastic properties, electronic structure, and chemical bonding are investigated by first-principles calculations with the density functional theory. The phonon calculation and elastic constants confirm that the predicted NB2 is dynamically and mechanically stable, respectively. The large bulk modulus, large shear modulus, large Young＇s modulus, and small Poisson＇s ratio show that the 14m2 NB2 should be a new superhard material with a calculated theoretical Vickers hardness value of 66 GPa. Further analysis on density of states and electron localization function demonstrate that the strong B B and 13 N covalent bonds are the main reason for its high hardness in 14m2 NB2.

First principles study on effect of lanthanum on nitrogen diffusion in ε-Fe_3N phase

The effect of lanthanum(La) on nitrogen atom diffusion in ε-Fe3N phase produced during nitriding with and without rare earth addition were studied based on the density function theory of first principles and the quantum mechanics program CASTEP code of Material Studio software.Calculated results show that the presence of La in ε-Fe3N phase can lead to the shrinks of a and b axis and the elongation of c axis of ε-Fe3N crystal cell.The total energy and the stability of the crystal cell with La are lower and higher than that without La,respectively.Diffusion activation energy of nitrogen atom in ε-Fe3N phase with La is 1.01 eV smaller than that in ε-Fe3N phase without La.This is the main reason why rare earth elements diffusing into compound layer produced during nitriding can accelerate the nitriding process or shorten the nitriding duration.

First principles study on the electronic structure and magnetism of Fe1-xCoxSi alloys

Electronic and magnetic properties of Fe1-xCoxSi alloys were investigated by using a full-potential linear augmented-plane-wave method based on density functional theory. Electronic structure calculation demonstrates that half-metallic property appears in the Fe-rich region of 0 〈 x ≤ 0.25, while the alloys turn out to be a magnetic metal for x 〉 0.25. The concentration dependence of the magnetic moment of the alloys can be understood by the fixed Fermi level at minority band in Fe-rich region, as well as at the majority band in Co-rich region. In Fe-rich alloys, the electronic structure and the magnetic properties at Fe site depend mainly on the spin-polarization of nearest neighbouring Co atoms, while in Co-rich alloys, these features at Co site arise mainly from the neighbours of Fe atoms.

Thermoelasticity of CaO from first principles

The thermoelastic properties of CaO over a wide range of pressure and temperature are studied using density functional theory in the generalized gradient approximation. The transition pressure taken from the enthalpy calculations is 66.7 GPa for CaO, which accords with the experimental result very well. The athermal elastic moduli of the two phases of CaO are calculated as a function of pressure up to 200 GPa. The calculated results are in excellent agreement with existing experimental data at ambient pressure and compared favourably with other pscudopotential predictions over the pressure regime studied. It is also found that the degree of the anisotropy rapidly decreases with pressure increasing in the B1 phase, whereas it strongly increases as the pressure increases in the B2 phase. The thermodynamic properties of the B1 phase of CaO are predicted using the quasi-harmonic Debye model; the heat capacity and entropy arc consistent with other previous results at zero pressure.

First principles study of post-boron carbide phases with icosahedra broken

Boron carbide (B4C) is a rhombic structure composed of icosahedra and atomic chains, which has an important application in armored materials. The application of B4C under super high pressure without failure is a hot spot of research. Previous studies have unmasked the essential cause of B4C failure, i.e., its structure will change subjected to impact, especially under the non-hydrostatic pressure and shear stress. However, the change of structure has not been clearly understood nor accurately determined. Here in this paper, we propose several B4C polymorphs including B4C high pressure phases with non-icosahedra, which are denoted as post-B4C and their structures are formed due to icosahedra broken and may be obtained through high pressure and high temperature (HPHT). The research of their physical properties indicates that these B4C polymorphs have outstanding mechanical and electrical properties. For instance, aP10, mC10, mP20, and oP10-B4C are conductive superhard materials. We hope that our research will enrich the cognition of high pressure structural deformation of B4C and broaden the application scope of B4C.

Hydrogen Incorporation in Crystalline Jadeite: Insight from First Principles Calculations

Hydrogen incorporation is critical for explaining defect energies, structure parameters and other physical characteristics of minerals and understanding mantle dynamics. This work analyzed the hydrogen complex defects in jadeite by the plane-wave pseudo-potential method based on density functional theory, and optimized the atomic positions and lattice constants in all configurations （different defective systems）. Incorporation mechanisms considered for hydrogen （H） in jadeite include： （1） hydrogen incorporating with the 02 site oxygen and coexisting with M2 vacancy; （2） one H atom combined with an AI atom replacing Si in tetrahedron; （3） 4H atoms directly replacing Si in tetrahedron and （4） 3H atoms replacing Al on the M1 site. The four incorporation mechanisms mentioned above form the corresponding VNa-Hi, Alsi-Hi, Vsi-4Hi and VAr3Hi point defects. The molecular dynamics simulation to the ideal, VNa-Hi, Alsi-Hi, Vsi-4Hi and VAr3Hi point defects under the P-T conditions of 900 K, 2 GPa, the Vsa-Hi and Alsi-Hi point defects under different pressures at T = 900 K, and Alsj-Hj point defects under different temperatures at P = 3 GPa was performed to examine the preferential mode of hydrogen incorporation in jadeite by means of first-principles calculations. The calculations show that the averaged O-H bond-length in the hydrogen point defects system decreased in the order of Alsi-Hi, VNa-HI, Vsl-4Hf and VAI-3Hi. VNa-HI complex defects result in a contraction of the jadeite volume and the presence of Alsi-Hi, Vsi-4H~ and VAI-3Hi defects could increase the superceli volume, which is the most obvious in the VAt-3Hi defects. The energy of formation of Also-HI and VA[-3HI complex defects was much lower than that of other defect systems. The VAI-3Hi defects system has the lowest energy and the shortest O-H bond-length, suggesting that this system is the most favorable. The analytical results of vacancy formation energy, O-H bond- length, and the stability of the hydrogen defects in jadeite have suggested that the preferred hydration incorporation mode in jadeite is VAI-3Hi complex defect.

Band structure of silicon and germanium thin films based on first principles

In nanomaterials, optical anisotropies reveal a fundamental relationship between structural and optical properties, in which directional optical properties can be exploited to enhance the performance of optoelectronic devices. First principles calculation based on density functional theory （DFT） with the generalized gradient approximation （GGA） are carried out to investigate the energy band gap structure on silicon （Si） and germanium （Ge） nanofilms. Simulation results show that the band gaps in Si （100） and Ge （111） nanofilms become the direct-gap structure in the thickness range less than 7.64 nm and 7.25 nm respectively, but the band gaps of Si （111） and Ge （110） nanofilms still keep in an indirect-gap structure and are independent on film thickness, and the band gaps of Si （110） and Ge （100） nanofilms could be transferred into the direct-gap structure in nanofilms with smaller thickness. It is amazing that the band gaps of Si（1-x）/ZGexSi（1-x）/2 sandwich structure become the direct-gap structure in a certain area whether （111） or （100） surface. The band structure change of Si and Ge thin films in three orientations is not the same and the physical mechanism is very interesting, where the changes of the band gaps on the Si and Ge nanofilms follow the quantum confinement effects.

First principles studies on the electronic structures of LiMxFe1-xPO4 （M = Co, Ni and Rh）

The local crystal structures and electronic structures of LiMxFe1-xPO4 （M = Co, Ni, Rh） are studied through first-principles calculations. The lattice constants and unit cell volumes are smaller for the Co and Ni doped materials than for pure LiFePO4, while larger than for the Rh doped material. The local structures around M atoms in the doped materials are studied in details. The total density of states （DOS） and atomic projected DOS （PDOS） are all calculated and analysed in detail. The results give a reasonable prediction to the improvement of electronic conductivity through Fe-site doping in LiFePO4 material.

Elastic properties of CaCO3 high pressure phases from first principles

Elastic properties of three high pressure polymorphs of CaCO_3 are investigated based on first principles calculations.The calculations are conducted at 0 GPa–40 GPa for aragonite, 40 GPa–65 GPa for post-aragonite, and 65 GPa–150 GPa for the P2_1/c-h-CaCO_3 structure, respectively. By fitting the third-order Birch–Murnaghan equation of state（EOS）, the values of bulk modulus K_0 and pressure derivative K~＇_0 are 66.09 GPa and 4.64 for aragonite, 81.93 GPa and 4.49 for post-aragonite, and 56.55 GPa and 5.40 for P2_1/c-h-CaCO_3, respectively, which are in good agreement with previous experimental and theoretical data. Elastic constants, wave velocities, and wave velocity anisotropies of the three highpressure CaCO_3 phases are obtained. Post-aragonite exhibits 25.90%–32.10% V_P anisotropy and 74.34%–104.30% V_S splitting anisotropy, and P2_1/c-h-CaCO_3 shows 22.30%–25.40% V_Panisotropy and 42.81%–48.00% V_S splitting anisotropy in the calculated pressure range. Compared with major minerals of the lower mantle, CaCO_3 high pressure polymorphs have low isotropic wave velocity and high wave velocity anisotropies. These results are important for understanding the deep carbon cycle and seismic wave velocity structure in the lower mantle.

Search for potential K ion battery cathodes by first principles

An important challenge facing K-ion batteries lies in exploring earth-abundant and safe cathode materials that can provide high capacity with high migration rate of K ions.Here,we propose a simple and efficient method for searching potential K cathode materials with first principles calculations.Our screening is based on combinations of weight capacity,K ion occupation ratio,volume change per K,and valence limit.With this screening method we predicted a series of potential K ions cathodes with favorable electrochemical performance,such as K_(2)VPO_(4)CO_(3)-like structures with 1 D diffusion channels,3 D channel structures K_(2)CoSiO_(4),layered materials KCoO_(2),KCrO_(2),KVF_(4) and K_(5)V_(3)F_(14),and others.These potential cathodes have small volume changes,suitable voltage,and high capacity,with small diffusion barriers.They may be useful in K-ion batteries with high energy density and rate performance.

First principles study of hafnium intercalation between graphene and Ir(111)substrate

The intercalation of heteroatoms between graphene and metal substrates is a promising method for integrating epitaxial graphene with functional materials.Various elements and their oxides have been successfully intercalated into graphene/metal interfaces to form graphene-based heterostructures,showing potential applications in electronic devices.Here we theoretically investigate the hafnium intercalation between graphene and Ir(111).It is found that the penetration barrier of Hf atom is significantly large due to its large atomic radius,which suggests that hafnium intercalation should be carried out with low deposition doses of Hf atoms and high annealing temperatures.Our results show the different intercalation behaviors of a large-size atom and provide guidance for the integration of graphene and hafnium oxide in device applications.

Coadsorption of gold with chlorine on CeO_2(111) surfaces:A first principles study

To investigate the effects of chlorine on the Au/ceria catalysts,the adsorption of gold or chlorine and their coadsorpiton on the stoichiometric and partially reduced CeO2（111） surfaces are studied from the first principles.It is found that the adsorption of Au is significantly enhanced by the chlorine preadsorption on the stoichiometric CeO2（111） surface;while on the partially reduced CeO2（111） surface,the preadsorbed chlorine inhabits the oxygen vacancy（which is the preferred adsorption site for gold）,leading to a CeOCl phase and the dramatical weakening of the Au adsorption.Therefore,chlorine on the CeO2（111） surface can affect the Au adsorption thus the activity of the Au/CeO2 catalyst.

High-throughput identification of one-dimensional atomic wires and first principles calculations of their electronic states

Low dimensional materials are suitable candidates applying in next-generation high-performance electronic,optoelectronic,and energy storage devices because of their uniquely physical and chemical properties.In particular,one-dimensional(1D)atomic wires(AWs)exfoliating from 1D van der Waals(vdW)bulks are more promising in next generation nanometer(nm)even sub-nm device applications owing to their width of few-atoms scale and free dandling bonds states.Although several 1D AWs have been experimentally prepared,few 1D AW candidates could be practically applied in devices owing to lack of enough suitable 1D AWs.Herein,367 kinds of 1D AWs have been screened and the corresponding computational database including structures,electronic structures,magnetic states,and stabilities of these 1D AWs has been organized and established.Among these systems,unary and binary 1D AWs with relatively small exfoliation energy are thermodynamically stable and theoretically feasible to be exfoliated.More significantly,rich quantum states emerge,such as 1D semiconductors,1D metals,1D semimetals,and 1D magnetism.This database will offer an ideal platform to further explore exotic quantum states and exploit practical device applications using 1D materials.The database are openly available at http://www.dx.doi.org/10.11922/sciencedb.j00113.00004.

First principles study of behavior of helium at Fe(110)-graphene interface

Recently,metal-graphene nanocomposite system has aroused much interest due to its radiation tolerance behavior.However,the related atomic mechanism for the metal-graphene interface is still unknown.Further,stainless steels with Fe as main matrix are widely used in nuclear systems.Therefore,in this study,the atomic behaviors of point defects and helium(He) atoms at the Fe(110)-graphene interface are investigated systematically by first principles calculations.The results indicate that graphene interacts strongly with the Fe(110) substrate.In comparison with those of the original graphene and bulk Fe,the formation energy values of C vacancies and Fe point defects decrease significantly for Fe(110)-graphene.However,as He atoms have a high migration barrier and large binding energy at the interface,they are trapped at the interface once they enter into it.These theoretical results suggest that the Fe(110)-graphene interface acts as a strong sink that traps defects,suggesting the potential usage of steel-graphene with multiply interface structures for tolerating the radiation damage.