First-principles Total-energy Calculations Research Articles

Towards understanding the dissolving behaviors of oxygen at finite temperature in Au and Ag: First-principles total energy and phonon spectrum calculations

By first-principles total energy and vibration spectrum calculations, we predict the impurity oxygen (O) dissolving behaviors at-finite temperature in Au and Ag. The temperature effect is considered by the lattice expansion and phonon vibration energy. An O atom is invariably preferable to stay at the tetrahedral interstitial site rather than the octahedral interstitial site over the whole temperature regime 200–1200K in two metals. The O solution energy referring to the static O chemical potential μO (T= 0K) decreases with temperature, while the O solution energy in reference to the temperature-dependent O chemical potential μO (T≠0K) increases with the increasing temperature. Meanwhile, phonon vibration energy plays a crucial role in the O dissolving behavior with temperature. Based on the obtained temperature-dependent O solution energy, we predict the O concentration over the temperature range 200–1200K in both metals. Due to that the above data of O dissolving behaviors are rather scarce in both experiment and calculation, our theoretical predictions can provide a very useful reference for purification of Au and Ag as noble metals in industry.

First principles investigation of the mechanical, thermodynamic and electronic properties of FeSn5 and CoSn5 intermetallic phases under pressure

For guidance for developing Fe/Co-Sn-based anode materials for lithium-ion batteries, the mechanical, thermodynamic and electronic properties of FeSn5 and CoSn5 intermetallic phases under pressures ranging from 0 to 30 GPa have been investigated systematically using first-principles total-energy calculations within the framework of the generalized gradient approximation. The pressure was found to have significant effects on the mechanical, thermodynamic and electronic properties of these compounds. In the selected pressure range, CoSn5 has a more negative formation enthalpy than FeSn5. Based on the calculated elastic constants, the bulk modulus, shear modulus, and Young’s modulus were determined via the Viogt-Reuss-Hill averaging scheme. The variations of specific heats at constant volume for FeSn5 and CoSn5 in a wide pressure (0 - 30 GPa) and temperature (0 - 1000 K) range are also predicted from phonon density of states calculation. The calculated results suggested that both FeSn5 and CoSn5 are mechanically stable at pressure from 0 to 30 GPa. FeSn5 is dynamically stable at pressure up to, 30 GPa, at least, however, CoSn5 is dynamically stable no higher than 15 GPa.

Atomic force microscope manipulation of Ag atom on the Si(111) surface

We present first-principles total-energy electronic-structure calculations that provide the microscopic mechanism of the Ag atom diffusion between the half unit cells (HUCs) on the Si(111)-(7x7) surface with and without the tip of the atomic force microscope (AFM). We find that, without the presence of the AFM tip, the diffusions between the two HUCs are almost symmetric with the energy barrier of about 1 eV in the both directions. The diffusion is a two-step process with an intermediate metastable configuration in which the Ag atom is at the boundary of the HUCs. With the presence of the tip, we find that the reaction pathways are essentially the same, but the energy barrier in one direction is substantially reduced to be 0.2 - 0.4 eV by the tip, while that of the diffusion in the reverse direction remains larger than 1 eV. The Si tip reduces the energy barrier more than the Pt tip due to the flexibility of the tip apex structure. In addition to the reduction of the barrier, the tip traps the diffusing adatom preventing the diffusion in the reverse direction. Also we find that the shape of the tip apex structure is important for the trapping ability of the adatom. When the tip apex structure is blunt, the adatom interacts with the tip atom other than the tip apex atom. The bond formation between the AFM tip atom and the surface adatom is essential for the atom manipulation using the AFM tip. Our results show that the atom manipulation is possible with both the metallic and semiconducting AFM tips.

Initial surface silicidation on Ni(110)

Initial silicide formation on a Ni(110) surface was studied by scanning tunneling microscopy (STM) in an ultrahigh vacuum. Less than 0.5ML of Si deposition initiated a Si-Ni mixed layer by displacing substrate Ni, and dark sites were formed in the STM images. A 0.5ML-Si deposited surface showed that Si and Ni were alternately aligned in a close-packed [11¯0] row whereas Si pairs aligned along the [001] direction forming p(1×2), obliquely aligned forming c(2×2), or even straightly-and-obliquely aligned forming c(4×2) superstructures. A first-principles total energy calculation showed that the p(1×2) and c(4×2) structures had almost the same energy while the c(2×2) structure gave 13meV/1×1 higher energy. Because a Si-Si bond in the close-packed [11¯0] row is energetically unfavorable, Si deposition of more than 0.5ML did not further replace the substrate Ni, but silicide islands were nucleated along with a trench structure.

Effect of charged metal nanoparticles on carrier injection in graphene by an external electric field

First-principles total-energy calculations clarify the effect of charged Al nanoparticles on carrier accumulation in graphene by an external electric field. Carrier injection in graphene with Al nanoparticles is sensitive to the relative position of the Al nanoparticles to the gate electrode. The nanoparticles sandwiched between graphene and an electrode prevent carrier injection in graphene, while the nanoparticles adsorbed on the opposite side do not affect the Dirac point shift, resulting in successive carrier injection in graphene. Because of the density of the state difference, the capacitance of graphene with Al nanoparticle also depends on the electrode position.

Theoretical study of multiatomic vacancies in single-layer hexagonal boron nitride

The physical properties of multiatomic vacancies are investigated by first-principles total-energy calculations. The formation energies of various vacancies as functions of chemical potential and charge states are calculated. The relationship between optimized atomic structures and charge states is analyzed. On the basis of the results, it is confirmed that the variations of formation energies and atomic structures are closely related to the changes in electronic states. In addition, the stabilities of generally large multiatomic vacancies are estimated on the basis of edges and corner energies. It is found that larger vacancies are not stable and have lower densities than smaller ones. The results are also compared with previous theoretical and experimental results.

Band engineering and relative stabilities of hexagonal boron nitride bilayers under biaxial strain

We perform first-principles total-energy calculations to investigate stabilities and electronic properties of hexagonal boron nitride ($h$-BN) bilayers under biaxial tensile strains. The possible stacking patterns of $h$-BN bilayers are investigated in detail. We show that the interlayer distances between two layers in $h$-BN bilayers can be changed under applied strains, and furthermore, they can decrease and increase depending on the stacking patterns of $h$-BN bilayers. We find that the band gaps are tunable by applying strains. We also find that tensile strains can give rise to a transformation from an indirect- to a direct-gap semiconductor in the case of the most stable stacking bilayer. These results indicate the high importance of $h$-BN bilayers as future electronic and optoelectronic device materials.

Ru2NbGa : A Heusler-type compound with semimetallic characteristics

The Heusler-type compound of ${\mathrm{Ru}}_{2}\mathrm{NbGa}$ has been successfully synthesized. X-ray analysis confirms that ${\mathrm{Ru}}_{2}\mathrm{NbGa}$ crystallizes in a cubic $L{2}_{1}$ structure. The electronic properties of ${\mathrm{Ru}}_{2}\mathrm{NbGa}$ have been characterized by means of the transport and nuclear magnetic resonance (NMR) measurements. The temperature dependence of the electrical resistivity exhibits a typical semimetallic behavior. The NMR spin-lattice relaxation rate shows activated behavior at higher temperatures, attributing to the thermally excited carriers across a pseudogap. We have also deduced a low Fermi-level density of states (DOS), being consistent with the semimetallic characteristic for ${\mathrm{Ru}}_{2}\mathrm{NbGa}$. In addition, we have performed first-principles total-energy calculations including ${G}_{0}{W}_{0}$ and $G{W}_{0}$ corrections for band gaps to investigate the electronic band structure of ${\mathrm{Ru}}_{2}\mathrm{NbGa}$. The calculated result reveals an indirect overlap between electron and hole pockets that leads to a residual DOS at the Fermi level, providing a consistent explanation for the experimental observations.

(Invited) First-Principles Study on Electron Conduction at 4H-SiC(0001)/SiO2 Interface

The thermal oxidation process of a hydrogen-terminated 4H-SiC(0001) substrate is examined by performing the first-principles total-energy calculations based on the density functional theory. It is found that the oxidation process of a SiC(0001) substrate is different from that of a Si substrate, which proceeds in a layer-by-layer manner. CO2 molecules are mainly emitted from the SiC surface although CO2 emission becomes less favorable and CO molecules are emitted from the SiC/SiO2 interface. Except for the oxidation energy, the situation is the same with the case of a bare 4H-SiC(0001) surface. We conclude that the effect of the dangling bonds on the surface is marginal and the stress at the interface is responsible for the removal of C during the oxidation.

Multistep atomic reaction enhanced by an atomic force microscope probe on Si(111) and Ge(111) surfaces

We present first-principles total-energy electronic-structure calculations that provide the microscopic mechanism of the adatom interchange reaction on the Sn- and Pb-covered Ge(111)-(2x8) and the Sb-covered Si(111)-(7x7) surfaces with and without the tip of the atomic force microscope (AFM). We find that, without the presence of the AFM tip on the Ge surface, the adatom interchange occurs through the migration of the adatom, the spontaneous formation of the dimer structures of the two adatoms, the dimer-dimer structural transitions that induce the exchange of the positions of the two adatoms, and then the backward migration of the adatom. We also find that the dimer structure is unfeasible at room temperature on the Si surface and the adatom interchange are hereby unlikely. With the presence of the tip, we find that the reaction pathways are essentially the same for the Ge surface but that the energy barriers of the migration and the exchange processes are substantially reduced by the AFM tip. We further find that the AFM tip induces the spontaneous formation of the dimer structure even on the Si surface, hereby opening a channel of the interchange of the adatoms. Our calculations show that the bond formation between the AFM tip atom and the surface adatom is essential for the atom manipulation using the AFM tip.

Interface and Doping Effect on the Electrochemical Property of Graphene/LiFePO4

Surface properties of olivine phosphate LiFePO4, as cathodes in lithium ion batteries, are of importance for overall performance as the nanoscale of particles has become indispensable. Using the first-principles total energy calculations, the effects of graphene or Mn dopant on the electrochemical properties of graphene/LiFePO4 have been comprehensively investigated. It is revealed that the interfacial binding between graphene and LiFePO4 in parallel orientation is stable and improved in the process of doping. The Li adsorption energy at different sites elucidates the core–shell model in Li extraction/insertion process and indicates the anomalous Li storage in the interface between graphene and LiFePO4. Mechanisms underlying influences of adsorption site, Mn dopant, and graphene modification on the Li adsorption energy are discussed through edge effect, doping stability, and interfacial binding strength, respectively. The surface conductivity is improved in the presence of graphene or Mn dopant with respe...

Effect of an intersection of carbon nanotubes on the carrier accumulation under an external electric field

We studied the electronic structure of semiconducting carbon nanotube (CNT) thin films, in which CNTs intersect each other, under an external electric field, using first-principles total-energy calculations within the framework of the density functional theory. Our calculations show that the distribution of accumulated carriers strongly depends on the CNT species, their mutual arrangement with respect to the electrode, and carrier concentrations. Under particular conditions, an induced electric field between the CNTs is opposite to the applied field. We also showed that the quantum capacitance of the CNT thin films depends on the arrangement of the CNTs relative to the electrode.

Structural and mechanical properties of transition metal borides Nb2MB2 (M=Tc, Ru, and Os) under pressure

Abstract First-principle total energy calculations are employed to provide a fundamental understanding of the structural, mechanical, and electronic properties of transition metal borides Nb 2 MB 2 (M=Tc, Ru, and Os) within the tetragonal superstructure P 4/ mnc structure. The mechanically and dynamically stabilities of three borides have been demonstrated by the elastic constants and phonons calculations under pressure. Among these three compounds, Nb 2 TcB 2 exhibits the biggest bulk and Young's modulus, smallest Poission's ratio, and highest harness. Density of states of them revealed that the strong B–B, Nb–B and M–B covalent bonds are major driving forces for their high bulk and shear moduli as well as small Poisson's ratio.

Crystal structure and electronic properties of CrAlGe

First-principles total energy calculations indicate that CrAlGe prefers a TiSi2-type structure to a Cu2 Sb-type one and that its magnetic ground state is ferromagnetic. These predictions are in agreement with experimental observation. As a result of consideration of the disorder between Al and Ge, it is found that the magnetic moment on Cr locally varies from site to site, but a magnetization per formula unit is almost constant. The analysis of electronic structure indicates a possibility that a ferromagnetic CrAl2−xGex (0 ≤x≤ 1) becomes half-metallic and that this nature is impervious to the disorder between Al and Ge.

Electrostatic properties of fullerenes under an external electric field: First-principles calculations of energetics for all IPR isomers from C60 to C78

Based on first-principles total energy calculations, we analyze the energetics of the fullerene isomers from C60 to C78, all of which satisfy the isolated pentagon rule, under a parallel electric field. Our calculations show that the total energy of the fullerene is proportional to the square of the external electric field. On the other hand, the coefficient of the quadratic energy profile is sensitive to the fullerene species and their orientation. Furthermore, fullerenes possessing lower symmetry exhibit asymmetric quadratic energy profiles with respect to the field, indicating that they possess intrinsic polarization along particular molecular orientations.

Polytypism inLaOBiS2-type compounds based on different three-dimensional stacking sequences of two-dimensionalBiS2layers

LaOBiS2-type materials have drawn much attention recently because of various interesting physical properties, such as low-temperature superconductivity, hidden spin polarization, and electrically tunable Dirac cones. However, it was generally assumed that each LaOBiS2-type compound has a unique and specific crystallographic structure (with a space group P4/nmm) separated from other phases. Using first-principles total energy and stability calculations we confirm that the previous assignment of the centrosymetric P4/nmm structure to LaOBiS2 is incorrect as a phonon instability renders this structure impossible. Furthermore, we find that the unstable structure is replaced by a family of energetically closely spaced modifications (polytypes) differing by the layer sequences and orientations. We find that the local Bi-S distortion leads to three polytypes of LaOBiS2 with different stacking patterns of the distorted BiS2 layers. The energy difference between the polytypes of LaOBiS2 is merely ~1 meV/u.c., indicating the possible coexistence of all polytypes in the real sample and that the particular distribution of polytypes may be growth-induced. The in-plane distortion can be suppressed by pressure, leading to a phase transition from polytypes to the high-symmetry P4/nmm structure with a pressure larger than 2.5 GPa. In addition, different choices of the intermediate atoms (replacing La) or active atoms (BiS2) could also manifest different ground state structures. One can thus tune the distortion and the ground state by pressure or substituting covalence atoms in LaOBiS2-family.

Formation of an N-pair at the Si(001)/α–quartz interface

We conduct a first-principles total energy calculation of the atomic and electronic structures of the N defects near the Si(001)/α–quartz interface. We find that the N atoms preferentially form a pair structure at the Si surface immediately below the interface to minimize the system energy. A silicon-induced gap state (SIGS), which is predominantly localized at the interface, is formed just above the valence band edge of bulk Si. The energy level of the SIGS shifts toward the bulk Si valence band edge as the N pairs accumulate at the interface, indicating that the energy gap in the system increases. The atomic and electronic structures of the Np defects is consistent with the previous core-level spectroscopy and spectroscopic charge pumping results.

Energetics and electronic structure of tubular Si vacancies filled with carbon nanotubes

We studied the energetics and electronic structure of tubular Si vacancies incorporating a carbon nanotube (CNT), using first-principles total-energy calculations based on the density functional theory. Our calculations show that the incorporated CNT into a Si nanotunnel acts as an atom-thickness liner providing the electrostatically flat nanoscale space inside them by shielding the dangling bond states of tubular Si vacancies. The incorporation of the CNT into the tubular Si vacancies is exothermic with an energy gain up to 7.4 eV/nm depending on the diameters of the vacancy and encapsulated CNT. The electronic states of the vacancy substantially hybridize with those of the CNT, leading to the complex electronic energy band near the Fermi level.

Equation of state for technetium from X‐ray diffraction and first-principle calculations

The ambient temperature equation of state (EoS) of technetium metal has been measured by X-ray diffraction. The metal was compressed using a diamond anvil cell and using a 4:1 methanol-ethanol pressure transmitting medium. The maximum pressure achieved, as determined from the gold pressureEquation of state for technetium from X-ray diffraction and first-principle calculations scale, was 67GPa. The compression data shows that the HCP phase of technetium is stable up to 67GPa. The compression curve of technetium was also calculated using first-principles total-energy calculations. Utilizing a number of fitting strategies to compare the experimental and theoretical data it is determined that the Vinet equation of state with an ambient isothermal bulk modulus of B0T=288GPa and a first pressure derivative of B′=5.9(2) best represent the compression behavior of technetium metal.

Automating impurity-enhanced antiphase boundary energy calculations from ab initio Monte Carlo

The effect of impurities on antiphase boundary (APB) energies is studied using cluster expansion and Monte Carlo (MC) techniques from first-principles total-energy calculations. We present a code that automates the generation of APB structures for MC sampling within the Alloy Theoretic Automated Toolkit software package. The functionalities of the code is demonstrated by a case study on Ni3Al with Ti impurities.