First-principles Molecular Orbital Calculations Research Articles

Optimization of First-Principles Calculation Conditions for Ce3+ in Perovskite-Type Oxides

The 4f-5d transition of Ce3+ in crystals is used in a variety of optical materials, including scintillators and phosphors. It also has the advantages that the wavelength can be controlled by the coordination environment and that Ce3+ is relatively inexpensive among rare earth elements. For the theoretical design of novel Ce3+-doped phosphors, prediction of 4f-5d transition energy by first-principles molecular orbital calculations using model clusters is effective. In general, increase of the cluster size improves the accuracy of calculations and reduces the difference between the experimental and calculated values while very large clusters have the disadvantage of requiring a huge amount of calculation time. Hence, understanding the cluster size dependence is important to determine the efficient and effective cluster size for the first-principles calculations.In this study we focused on Ce3+-doped in YAlO3, and performed first-principles calculations of the 4f-5d transition energies for model clusters with different sizes. At first, the local structures around the Y3+ site of YAlO3 were cut out from the crystal structure. Then the central Y3+ ions of these clusters were replaced by Ce3+ ions. Then the transition energy of Ce3+ in YAlO3 was calculated by the relativistic discrete-variational Xα (DV-Xα) molecular orbital calculations using the Slater's transition state method. The detailed comparison of the results showed that for clusters larger than a certain size, the error between the experimental and calculated values became negligibly small. For comparison, the lattice relaxation was also taken into account by modifying the model clusters using two different methods: the method based on the Shannon's crystal radii and the structural optimization using the CASTEP code, and the results were compared in detail.

Change in equilibrium position of misfit dislocations at the GaN/sapphire interface by Si-ion implantation into sapphire. II. Electron energy loss spectroscopic study

In Part I, we have shown that the addition of Si into sapphire by ion implantationmakes the sapphire substrate elastically softer than for the undoped sapphire. The more compliant layer of the Si-implanted sapphire substrate can absorb the misfit stress at the GaN/sapphire interface, which produces a lower threading-dislocation density in the GaN overlayer. Here in Part II, based on experimental results by electron energy loss spectroscopy and a first-principle molecular orbital calculation in the literature, we suggest that the softening effect of Si results from a reduction of ionic bonding strength in sapphire (α-Al2O3) with the substitution of Si for Al.

Remote-Plasma-Assisted Deposition of Pentacene Layer Using Atomic-Hydrogen

Pentacene thin layers were deposited on Si with the native oxide at 80°C by remote-plasma-assisted deposition (RPAD) using hydrogen-plasma cell to supply atomic hydrogen radicals. The deposition rate was increased by RPAD comparing to that by non-excited hydrogen gas supply whereas thermal evaporation rate of pentacene from crucible was same in the both process. DFM and XRD studies showed the grain laterally grew in the thin film phase with the size above 10 μm by RPAD. First-principles molecular orbital calculations suggested pentacene is evaporated from crucible as the trimer or larger cluster but atomic hydrogen penetrated into the cluster enhances cracking of pentacene clusters to the monomer.

Surface Segregation Behavior of B, Ga, Sb, and As Dopant Atoms on Ge(100) and Ge(111) Examined with a First-principles Method

To understand surface segregation behaviour of B, Ga, As, and Sb dopant atoms on Ge (100) and Ge (111) surfaces, the potential energies of these dopant atoms in the five top layers of the surfaces were evaluated by first-principles molecular orbital calculations of model clusters. The obtained potential energy curves of these dopant atoms for Ge (100) closely resemble those for Si (100) surface. On the other hand, Ge (111) surface exhibits different potential profiles as compared with Ge (100) surface. The thermal activation energy between the adsorbed state and sub-surface state was also evaluated for each dopant atom. These energy data determining the driving force and dynamical property of dopant segregation help us to understand the difference in dopant surface segregation phenomena on Ge(100) and Ge(111) surfaces from an atomistic point of view.

Electronic structure analysis of Mn- and Fe-codoped In2O3 by photoemission yield measurements

The valence band electronic structures of Mn- and/or Fe-doped In2O3, i.e., In2O3:Mn, In2O3:Fe, and In2O3:(Mn, Fe), are investigated by photoemission yield measurements. Significant changes are observed in the threshold energy of photoemission, depending on the doped magnetic ions, which indicates that an additional occupied band appears above the top of the valence band of In2O3 owing to doping with Mn and/or Fe ions. It is confirmed that the order of the threshold energies of photoemission, EPET, is EPET(In2O3:Mn)<EPET(In2O3:(Mn, Fe))<EPET(In2O3:Fe)<EPET(In2O3). To gain a better understanding of these results, first-principles molecular orbital calculations are also carried out, which successfully explain the observed changes in the photoemission threshold energies.

Study of the Electronic Structure and Magnetic Properties of ε-Fe2O3by First-Principles Calculation and Molecular Orbital Calculations

ε-Fe2O3 is known to exhibit a large coercive field of 20 kOe at room temperature. In this work, we examine the electronic structure and magnetic properties of ε-Fe2O3 using first-principles calculation and discrete variational (DV)-Xα molecular orbital calculation. The first-principles calculation shows that ε-Fe2O3 is a charge-transfer type insulator with a valence band of O2p and a conduction band of Fe3d. The optical transition is an indirect transition from Γ to S point. The density of states (DOS) of the four nonequivalent Fe sites (FeA, FeB, FeC, and FeD) indicates that ε-Fe2O3 has ferrimagnetic ordering due to α spins on FeB and FeC and β spins on FeA and FeD. The charge density map of the occupied Fe3d band displays a strong hybridization between Fe3d and O2p. Molecular orbital calculation for each Fe site also supports the existence of a strong Fe3d–O2p hybridization. Such a strong hybridization induces nonzero orbital angular momentum L on Fe3d through the partial charge transfer from O2p to Fe3d. The appearance of L causes a large magnetic anisotropy through spin–orbit interaction, which induces the large coercive field.

Interaction of Metallic Nanoparticles with a Biologically Active Molecule, Dopamine

We present the results of first-principles molecular orbital calculations describing the interaction of metallic nanoparticles, represented by Mn(13), Ag(13), and Al(13) atomic clusters, with a biologically active molecule, dopamine. The interaction strength, determined in terms of the nanoparticle-molecule complex binding energy, is found to be higher for Mn than either Ag or Al and can be explained in terms of the degree of the hybridization of the (metal) atomic orbitals with the molecular orbitals in the complex. Furthermore, smaller interaction strength of these metallic nanoparticles with water compared to that with dopamine predicts the preference of forming a complex of dopamine with the metallic nanoparticles in the aqueous solution. The calculated results may therefore suggest that the presence of these metallic nanoparticles could induce different levels of dopamine depletion in solution.

Ionic conductivity of tetragonal ZrO 2 polycrystal doped with TiO 2 and GeO 2

The effects of the co-doping and the resultant co-segregation of 2 mol% TiO 2 and 2 mol% GeO 2 on the ionic conductivity and on the chemical bonding state in a tetragonal ZrO 2 polycrystal were investigated. The conductivity data and grain boundary microstructure showed that the doped Ti 4+ and Ge 4+ cations segregate along the grain boundary, and this segregation causes a reduction in the conductivity of both the grain interior and grain boundary and an increase in the activation energy of the grain boundary conductivity. Overall, the data indicate that the segregation retards the diffusion of oxygen anions. A first-principle molecular orbital calculation explains the retarded diffusion of the oxygen anion from a change in the covalent bonds around the dopant cations; an increase in the strength of the covalent bond between the oxygen and doped cation should work to suppress the diffusion of the oxygen anion.

Estimation of Grain Boundary Diffusivity in Cation-Doped Polycrystalline Alumina

was estimated from the densification behavior during the final stage sintering. The densification rate was monitored from room temperature to the sintering temperature using a laser-scanning micrometer, which allows in-situ, non-contact measuring of the specimen's dimensions. The grain boundary diffusivity in Al 2 O 3 is sensitively affected by the doped cation which segregates at the grain boundaries. A first-principle molecular orbital calculation indicates that the doping effect on the grain boundary diffusivity is related to the ionicity in Al 2 O 3 ; the grain boundary diffusivity correlates well with the net charge of the Al and O ions in the cation-doped Al 2 O 3 .

Calculation of X-ray absorption near edge structure of CeO 2 using a model cluster

We report result of a first-principle molecular orbital calculation using discrete-variational (DV)-Xα method on a model of CeO 2 ([CeO 8] 12−), and compare them with experimental date on X-ray absorption-near-edge structure. Even using a small cluster model, we can reproduce the two-peak structure near edge and explain the origin of the peaks. The two-peak structure is relially interpreted from the viewpoint of interactions between atomic orbitals. The theoretical spectra are obtained with the dipole approximation. In addition, we calculate the wave functions, which indicate that the low-energy peak in the two-peak structure originates from a quasi-bound state composed of localized Ce d and O component. The orgin of the high-energy peak is the phase shift between localized Ce d orbital and that of the delocalized standing wave of O atomic orbitals.

Luminescence properties of YAl 3(BO 3) 4:Gd 3+ phosphors substituted with Sc 3+ ions

YAl 3(BO 3) 4:Gd 3+ is an ultraviolet phosphor giving 3.97-eV emission line under the excitation by vacuum ultraviolet light. On the basis of a theoretical material design using first-principles molecular orbital calculations, Sc 3+ ions were proposed as a promising dopant to improve the luminescent properties of the YAl 3(BO 3) 4:Gd 3+ phosphor. In this work, YAl 3(BO 3) 4:Gd 3+ phosphors substituted with Sc 3+ ions were synthesized and their luminescence and excitation spectra were measured at room temperature. The observed excitation spectra of Y(Al,Sc) 3(BO 3) 4 :Gd 3+ and (Y,Sc)Al 3(BO 3) 4 :Gd 3+ phosphors actually show the additional absorption structure on the lower-energy side of the fundamental absorption edge of YAl 3(BO 3) 4:Gd 3+ phosphors and the associated luminescent intensity is significantly enhanced. From the results, we confirmed that the emission peak intensity of (Y,Sc)Al 3(BO 3) 4:Gd 3+ phosphors is stronger by a factor of about 1.5 compared to that of YAl 3(BO 3) 4:Gd 3+ phosphor to date.

The origin of n-type conductivity in undoped In2O3

This study explores the origin of the native donor in undoped In2O3. The electronic structure of various point defects in In2O3 clusters is studied using the first-principles molecular orbital calculation. The results show that an oxygen vacancy cannot act as a native donor, because the defect level formed is much lower than the bottom of the conduction band. However, interstitial indium can generate a shallow donor level, close to the conduction band, and an even shallower donor level is formed when it associates with an oxygen vacancy. It is concluded that the origin of the native donor in undoped In2O3 is interstitial indium, but also that the existence of an oxygen vacancy is absolutely essential for carrier generation.

GeO2-TiO2添加TZPの超塑性変形における高温延性と粒界化学結合状態

Temperature and strain rate dependence on high temperature elongation to failure in fine-grained ceramics is phenomenologically explained from grain growth behavior during deformation and the superplastic flow behavior. The elongation to failure at temperatures between 1573 and 1773 K was analyzed for 2 mol%TiO2 and 2 mol%GeO2 co-doped tetragonal zirconia polycrystal (TZP), which exhibits excellent high temperature ductility. The improvement in the high temperature ductility in TZP is attributed to dopant cation segregation in the vicinity of the grain boundaries. The phenomenological analysis revealed that co-doping of Ti and Ge cations increases the grain size at the time of failure, as a parameter to describe a limit of an accommodation process for superplastic flow. The parameter of the critical grain size at the time of failure correlates well with the value of overlap population in cation-doped TZP model cluster obtained from a first-principle molecular orbital calculation. The covalent bond at the grain boundaries plays a critical role in the high temperature tensile ductility of TZP.

In situ real-time spectroscopic ellipsometry study of HfO2 thin films grown by using the pulsed-source metal-organic chemical-vapor deposition

The HfO2 thin-film growth process is investigated by using in situ real-time spectroscopic ellipsometry (SE) technique combined with the first-principles molecular-orbital (MO) calculations of the electronic states. The HfO2 films are grown on the silicon substrate by using the pulsed-source metal-organic chemical-vapor deposition method. Particular attention is paid to the formation of an interfacial layer at the early stage of the growth process by monitoring energy-dependent dielectric constants of the film. The energy dependence of the electronic polarizabilities and dielectric constants is calculated for the amorphous HfO2, SiO2, and HfSiO4 films based on the electronic states and density of states obtained using the discrete-variational Xα MO method with the unit cluster model. The measured SE spectra show that the average dielectric constants of the film vary gradually from those for SiO2 to those for HfO2 when the number of deposition cycles increases. By comparing the varied dielectric constants during the film growth with the calculated results, we find that the HfO2 film growth process can be divided into two stages with different growth mechanisms: SiO2 and HfxSiyOz layers are grown at the first stage, which are regarded as the interfacial layers, and the HfO2 layer formation becomes predominant at the second stage.

Dopant effect on grain boundary diffusivity in polycrystalline alumina

The densification behavior during sintering in 0.1 mol% MgO-, MnO-, SrO-, LuO 1.5-, TiO 2-, ZrO 2- or PtO 2-doped Al 2O 3 was investigated at the sintering temperature of 1300–1500 °C in order to systematically examine the dopant effect on grain boundary diffusivity in Al 2O 3. The densification behavior was monitored from room temperature to the sintering temperature using a laser-scanning system, which allows in situ, non-contact measuring of the specimen’s dimensions. The grain boundary diffusivity in Al 2O 3 is sensitively affected by the dopant cation which segregates at the grain boundaries. The dopant effect on the grain boundary diffusivity is related to the ionicity in Al 2O 3; a lower energy level of the dopant element’s outer shells provides a higher value of diffusivity in the divalent or tetravalent cation-doped Al 2O 3. A first-principle molecular orbital calculation revealed that the grain boundary diffusivity correlates well with the net charge of the Al and O ions in the cation-doped Al 2O 3. The ionic bond strength in the vicinity of the grain boundaries dominates the high-temperature grain boundary diffusion in polycrystalline Al 2O 3.

Digital magnetic moment of tetrahedrally bonded half-metallic nanoclusters

We investigate the half metallicity and localized spin magnetic moments for individual nanoclusters of zinc-blende transition-metal pnictides and chalcogenides TX (T= V, Cr, and Mn; X= N, P, As, Sb, S, Se, and Te) using first-principles molecular-orbital calculations. These nanoclusters, as well as the bulk, show half-metallic ferromagnetic behavior for a wide range of bond lengths; otherwise, an antiferromagnetic arrangement is stabilized. The total magnetic moment of an isolated half-metallic nanocluster TkX, turns out to be sZtot ˛8 dk + s 4˛ DZdsk ˛ ,d in units of mB, where Ztot is the total number of valence electrons per formula unit, DZ =1 for pnictides, and DZ= 2 for chalcogenides. This , dependence results from anion dangling hybrids on the cluster surface. Induced antiparallel magnetic moments at anion sites are interpreted in terms of a bond-orbital model; the hybridization effect between cation d states and anion p states creates holes in the majority-spin states of the bond orbitals. When the nanocluster is embedded in a lattice-matched compound semiconductor with a common anion, the total moment approaches sZtot ˛8 dkmB. Half metallicity is maintained at the boundary sites of the nanocluster without any sign of interface states, suggesting the Ohmic nature of the contact.

X-ray absorption fine-structure study on the fine structure of lutetium segregated at grain boundaries in fine-grained polycrystalline alumina

Local atomic structures around the Lu3+ ion in a 0.1 mol% LuO1.5-doped fine-grained Al2O3, in which the doped Lu3+ ions segregate to the grain boundaries, was characterized by Lu L3-edge X-ray absorption fine structure. Structural parameters in LuO1.5-doped Al2O3 were determined by a curve-fitting method, and the results showed that six O2− ions coordinate with the Lu ion in LuO1.5-doped Al2O3. In addition, it was also revealed that the Lu–O interatomic distance in Lu-doped Al2O3 was close to that in Lu2O3, which was about 19% longer than the Al–O interatomic distance in undoped Al2O3. The present results indicated that the local atomic structures around Lu in Al2O3 are close to that in Lu2O3. It is thus supposed that atomic distances between Al3+ and O2− ions in the vicinity of Lu-segregated grain boundaries are shortened in comparison with that in undoped Al2O3. A first-principles molecular orbital calculation was performed for the [Al2O9]−12 model cluster, and the shortening of the Al–O interatomic distance was found to have an effect of increasing the ionicity in Al3+ ions.

Criterion for High Temperature Failure and Grain Boundary Chemistry in Superplastic TZP

Temperature and strain rate dependence on high temperature elongation to failure in fine-grained ceramics is phenomenologically explained from grain growth behavior during deformation and the superplastic flow behavior. The elongation to failure at temperatures between 1573 and 1773 K was analyzed for 2 mol% TiO2 and 2 mol% GeO2 co-doped tetragonal zirconia polycrystal (TZP), which exhibits excellent high temperature ductility. The improvement in the high temperature ductility in TZP is attributed to dopant cation segregation in the vicinity of the grain boundaries. The phenomenological analysis revealed that co-doping of Ti and Ge cations increases the grain size at the time of failure, as a parameter to describe a limit of an accommodation process for superplastic flow. The parameter of the critical grain size at the time of failure correlates well with the value of overlap population in cation-doped TZP model cluster obtained from a first-principle molecular orbital calculation. The covalent bond at the grain boundaries plays a critical role in the high temperature tensile ductility of TZP.

Improvement of High-temperature Creep Resistance in Polycrystalline Al&lt;SUB&gt;2&lt;/SUB&gt;O&lt;SUB&gt;3&lt;/SUB&gt; by Cations Co-doping

High-temperature creep resistance in cations co-doped polycrystalline Al 2 O 3 was examined by uniaxial compression creep test at 1250°C. The dopant oxides used in this study were 0.1 mol% of YO 1 . 5 , ZrO 2 , SrO, MgO and TiO 2 . The creep rate in Al 2 O 3 was significantly changed by cations co-doping. For instance, Zr/Y co-doping suppressed the creep rate in Al 2 O 3 by a factor of about 400. A high-resolution transmission electron microscopy (HREM) and nano-probe energy dispersive X-ray spectroscopy (EDS) analysis revealed that Y and Zr cations segregate along grain boundaries. The grain boundary diffusion in Al 2 O 3 was supposed to be retarded by the segregation of Y and Zr cations. A first-principle molecular orbital calculation was made for cations co-doped Al 2 O 3 and cation singly doped Al 2 O 3 model cluster. The creep rate was correlated with the value of net charge in oxygen anion. The net charge of oxygen anion was one of the most important factors to determine the creep resistance in Al 2 O 3 .

Bonding nature and wave function around the Fermi level of MgB2-related compounds

We present first-principles molecular-orbital calculations for the 39 K superconductor MgB2 and related diborides using the discrete variational (DV)-Xα method. The electronic structure of a slab cluster embedded in the Madelung potential is determined self-consistently in terms of charge transfer between Mg and B. In contrast with the simplest ionic picture Mg2+B−2, the negative charge of B is estimated to be 0.39. Our results, which include total and partial density of states, overlap population in the neighboring B–B and Mg–B bonds and wave functions around the Fermi energy EF consisting of B 2px,y orbitals in the B planes, indicate the incomplete filling of the σ orbitals in MgB2. We discuss possible ways to achieve higher transition temperatures in non-periodic systems such as surfaces, interfaces and artificially layered hetero-structures based on this approach.