First principles density functional+U pseudopotential study on the electronic structure of layered perovskite LaSr3Fe3O10.

The electronic, magnetic and crystal structures of layered perovskite oxide LaSr3Fe3O10 (LSFO) in the Ruddlesden-Popper structure were studied from first principles using the density functional theory (DFT)+U pseudopotential (PP) method and a self-consistent constrained DFT technique (Hamada and Ohno 2019 J. Phys.: Condens. Matter 31 065501). Using this technique, the magnetic structure of LSFO was determined to be antiferromagnetic and an effective Hubbard on-site interaction parameter for Fe 3d electrons, U eff(Fe3d ) = 6.08 eV was identified for LSFO. The DFT+U PP calculations of LSFO models using this U eff(Fe3d ) value reproduced the experimentally observed metallic characteristics and crystal structure of LSFO, demonstrating the correct determination of the U eff(Fe3d ) value of the large and complex LSFO material. The first-principles DFT+U calculation of large and complex strongly-correlated systems was enabled using the self-consistent constrained DFT technique.

A new constraint DFT technique for self-consistent determination of U values.

A new constraint density functional (DFT) technique workable in combination with the projector augmented wave (PAW) and pseudoptential (PP) methods was developed. This technique calculates the effective on-site-interaction parameter, U eff, of correlated electrons of materials, self-consistently, by using the DFT + U method. The U eff determined by this technique has a clear physical meaning in that it determines the electronic structures of strongly correlated electronic systems (SCESs) and vice versa. The technique was used to determine the U eff of correlated electrons of neodymium sesquioxide (Nd2O3) and iron oxide (FeO), and it was shown to be effective for this purpose. It enables first principles DFT + U PAW and PP calculations of SCESs free from any empirical parameters.

Crystallization behavior of the Li2S-P2S5 glass electrolyte in the LiNi1/3Mn1/3Co1/3O2 positive electrode layer.

Sulfide-based all-solid-state lithium batteries are a next-generation power source composed of the inorganic solid electrolytes which are incombustible and have high ionic conductivity. Positive electrode composites comprising LiNi1/3Mn1/3Co1/3O2 (NMC) and 75Li2S·25P2S5 (LPS) glass electrolytes exhibit excellent charge-discharge cycle performance and are promising candidates for realizing all-solid-state batteries. The thermal stabilities of NMC-LPS composites have been investigated by transmission electron microscopy (TEM), which indicated that an exothermal reaction could be attributed to the crystallization of the LPS glass. To further understand the origin of the exothermic reaction, in this study, the precipitated crystalline phase of LPS glass in the NMC-LPS composite was examined. In situ TEM observations revealed that the ß-Li3PS4 precipitated at approximately 200 °C, and then Li4P2S6 and Li2S precipitated at approximately 400 °C. Because the Li4P2S6 and Li2S crystalline phases do not precipitate in the single LPS glass, the interfacial contact between LPS and NMC has a significant influence on both the LPS crystallization behavior and the exothermal reaction in the NMC-LPS composites.

Two-dimensional Gaussian fitting for precise measurement of lattice constant deviation from a selected-area diffraction map.

Unlike X-ray diffraction or Raman techniques, which suffer from low spatial resolution, transmission electron microscopy can be used to obtain strain maps of nanoscaled materials and devices. Convergent-beam electron diffraction (CBED) and nanobeam electron diffraction (NBED) techniques detect the deviation of a lattice constant (i.e. an indicator of strain) within 0.01%; however, their use is restricted to beam-insensitive samples. Selected-area electron diffraction (SAED) does not have such limitations but has low spatial resolution and precision. The use of a spherical aberration corrector and a nanosized selected-area aperture improves the spatial resolution, but the precision is still low. In this study, a two-dimensional stage-scanning system is used to acquire arrays of diffraction patterns at different positions of the sample under fixed beam conditions. Data processing with iterative nonlinear least-squares fitting enabled the spot displacement for each point of the scan area to be measured with precision comparable to that of the CBED or NBED technique. The precise strain determination, in combination with the simplicity of the measurement process, makes the nanosized SAED technique competitive with other methods for strain mapping at nanoscale dimensions.

Spintronic Transport in Armchair Graphene Nanoribbon with Ferromagnetic Electrodes: Half-Metallic Properties.

Utilizing first-principles theory, we demonstrate that half-metallicity can be realized in a junction composed of non-magnetic armchair graphene nanoribbon (AGNR) and ferromagnetic Ni electrodes. The half-metallic property originates from the AGNR energy gap of the up spin located at the Fermi energy, while large electronic states are generated for the down spin. By altering the interlayer distance and the contact area, namely, the strength of AGNR-Ni interaction, the efficiency of the spin filter becomes lower, since the energy gap moves away from the Fermi energy with the variation of charge transfer intensity.

Na-ion diffusion in a NASICON-type solid electrolyte: a density functional study.

Based on density functional theory, we have systematically studied the crystal and electronic structures, and the diffusion mechanism of the NASICON-type solid electrolyte Na3Zr2Si2PO12. Four possible elementary processes are addressed: three inner-chain and one inter-chain processes. In inner-chain processes, Na tends to move inside the Na diffusion chain, while Na moves across the Na diffusion chain in the inter-chain process. The activation energies for the inner-chain and inter-chain processes are 230 meV and 260 meV, respectively. By combining possible elementary processes, three preferable pathways along a, b, and c directions are found.

First-principles analysis on role of spinel (111) phase boundaries in Li4+3xTi5O12 Li-ion battery anodes.

The practical anode material Li4+3xTi5O12 is known to undergo a two-phase separation into Li7Ti5O12 and Li4Ti5O12 during charging/discharging. This phase-separated Li4+3xTi5O12 exhibits electron conduction, although individual phases are expected to be insulators. To elucidate the role played by spinel (111) phase boundaries on these physical properties, first principles calculations were carried out using the GGA+U method. Two-phase Li7Ti5O12/Li4Ti5O12 models are found to exhibit metallic characteristics near their phase boundaries. These boundaries provide conduction paths not only for electrons, but also for Li ions. Judging from the formation energy of Li vacancies/interstitials, the phase boundaries preferentially uptake or release Li via in-plane conduction and then continuously shift in a direction perpendicular to the phase boundary planes. The continuous phase boundary shift leads to a constant electrode potential. A three-dimensional network of cubic {111} planes may contribute to smooth electrochemical reactions.

Multi-spin-state at a Li3PO4/LiCoO2 (104) interface.

We have found the disproportion between the intermediate spin (IS) and low spin (LS) configurations of Co atoms at a Li3PO4/LiCoO2 (104) interface through density functional molecular dynamics (DF-MD). The manifold of the spin state at the interface, however, does not affect the band alignment between the Li3PO4 and LiCoO2 regions.

Interactions between C and Cu atoms in single-layer graphene: direct observation and modelling.

Metal doping into the graphene lattice has been studied recently to develop novel nanoelectronic devices and to gain an understanding of the catalytic activities of metals in nanocarbon structures. Here we report the direct observation of interactions between Cu atoms and single-layer graphene by transmission electron microscopy. We document stable configurations of Cu atoms in the graphene sheet and unique transformations of graphene promoted by Cu atoms. First-principles calculations based on density functional theory reveal a reduction of energy barrier that caused rotation of C-C bonds near Cu atoms. We discuss two driving forces, electron irradiation and in situ heating, and conclude that the observed transformations were mainly promoted by electron irradiation. Our results suggest that individual Cu atoms can promote reconstruction of single-layer graphene.

Conservation of the pure adiabatic state in Ehrenfest dynamics of the photoisomerization of molecules.

We examined real-time-propagation time-dependent density functional theory (rtp-TDDFT) coupled with molecular dynamics (MD), which uses single-particle representation of time-evolving wavefunctions allowing exchange of orbital characteristics between occupied and empty states making the effective Kohn-Sham Hamiltonian dependent on the potential energy surfaces (PESs). This scheme is expected to lead to mean-field average of adiabatic potential energy surfaces (PESs), and is one of Ehrenfest (mean-field) approaches. However, we demonstrate that the mean-field average can be absent in simulating photoisomerization of azobenzene and ethylene molecules. A transition from the S2 to the S1 excited state without the mean- field average was observed after examining several rtp-TDDFT-MD trajectories of a photoexcited azobenzene molecule. The subsequent trans-cis isomerization was observed in our simulation, which is consistent with experimental observation and supported by previous calculations. The absence of the mean-field average of PESs was also observed for the transition between the S1 and S0 states, indicating that the MD simulation was on a single PES. Conversely, we found no transition to the ground state (S0 state) when we performed a MD simulation of an S1 excited ethylene molecule owing to the constraint on the occupation number of each molecular orbital. Thus, we conclude that, at least for azobenzene and ethylene molecules, the rtp-TDDFT-MD is an on-the-fly simulation that can automatically see the transition among the PESs of excited states without the mean-field average unless the simulation reaches the PES of the S0 state.

Hybrid functional study of the NASICON-type Na3V2(PO4)3: crystal and electronic structures, and polaron-Na vacancy complex diffusion.

The crystal and electronic structures, electrochemical properties and diffusion mechanism of NASICON-type Na3V2(PO4)3 have been investigated based on the hybrid density functional Heyd-Scuseria-Ernzerhof (HSE06). A polaron-Na vacancy complex model for revealing the diffusion mechanism is proposed for the first time in the field of Na-ion batteries. The bound polaron is found to favorably form at the first nearest V site to the Na vacancy. Consequently, the movement of the Na vacancy will be accompanied by the polaron. Three preferable diffusion pathways are revealed; these are two intra-layer diffusion pathways and one inter-layer pathway. The activation barriers for the intra-layer and inter-layer pathways are 353 meV and 513 meV, respectively. For further comparison, the generalized gradient approximation with an onsite Coulomb Hubbard U (GGA+U) is also employed.

Correlation between the surface electronic structure and CO-oxidation activity of Pt alloys.

The surface electronic structure and CO-oxidation activity of Pt and Pt alloys, Pt3T (T = Ti, Hf, Ta, Pt), were investigated. At temperatures below 538 K, the CO-oxidation activities of Pt and Pt3T increased in the order Pt < Pt3Ti < Pt3hHf < Pt3Ta. The center-of-gravity of the Pt d-band (the d-band center) of Pt and Pt3T was theoretically calculated to follow the trend Pt3Ti < Pt3Ta < Pt3Hf < Pt. The CO-oxidation activity showed a volcano-type dependence on the d-band center, where Pt3Ta exhibited a maximum in activity. Theoretical calculations demonstrated that the adsorption energy of CO on the catalyst surface monotonically decreases with the lowering of the d-band center because of diminished hybridization of the surface d-band and the lowest-unoccupied molecular orbital (LUMO) of CO. The observed volcano-type correlation between the d-band center and the CO oxidation activity is rationalized in terms of the CO adsorption energy, which counterbalances the surface coverage by CO and the rate of CO oxidation.

Effect of contact area on electron transport through graphene-metal interface.

We perform first-principles investigations of electron transport in armchair graphene nanoribbons adsorbed on Cu(111) and Ni(111) surfaces with various contact areas. We find that the contact area between metals and graphene has different influences on the conductance. The Cu-graphene system shows an increase in differential conductance for more contact area at a low bias voltage, primarily originating from the shift of transmission peaks relative to the Fermi energy. As the bias increases, there is an irregular change of conductance, including a weak negative differential conductance for more contact area. In contrast, the conductance of the Ni-graphene junction is monotonically enhanced with increasing overlap area. The minority spin which shows a broad transmission is responsible for the conductance increase of Ni-graphene. These behaviors can be attributed to different mechanisms of the interfacial electron transport: Charge transfer between graphene and Cu largely dominates the transmission enhancement of Cu-graphene, whereas hybridization between graphene and Ni states plays a more important role in the transmission enhancement of Ni-graphene. The different behaviors of transmission increase correlate with not only the strength of the graphene-metal interaction but also the location of metal d states.

A hybrid density functional study on the electron and hole trap states in anatase titanium dioxide.

We present a theoretical study on electron and hole trap states in the bulk and (001) surface of anatase titanium dioxide using screened hybrid density functional calculations. In both the bulk and surface, calculations suggest that the neutral and ionized oxygen vacancies are possible electron traps. The doubly ionized oxygen vacancy is the most stable in the bulk, and is a candidate for a shallow donor in colorless anatase crystals. The hole trap states are localized at oxygen anions in both the bulk and surface. The self-trapped electron centered at a titanium cation cannot be produced in the bulk, but can be formed at the surface. The electron trap level at the surface oxygen vacancy is consistent with observations by photoelectron spectroscopy. The optical absorptions and luminescence in UV-irradiated anatase nanoparticles are found to come from the surface self-trapped hole and the surface oxygen vacancy.

Tautomers of extended reduced pyrazinacenes: a density-functional-theory based study.

We present a structural and electronic inspection of reduced pyrazinacenes within the DFT framework. Our analysis provides a clear indication that compounds in which reduced pyrazine rings are well separated from each other are rather stable. Conversely, if the reduced pyrazine rings approach each other or cluster together, the compounds become increasingly unstable. The tautomers analyzed are likely to possess properties suitable for application as proton transport materials due to protic isomerism processes. On the basis of our findings, we propose that protic transport should occur through a concerted proton transfer without involving intramolecular aggregation of the dihydropyrazine groups. Furthermore, the electronic structure analysis shows that this class of compounds can be classified as small bandgap semiconducting materials, possessing even metallic character depending on the tautomeric structure, and with potential nanotechnological applications in molecular electronics and fuel cells.

Ferroelectric mobile water.

In molecular dynamics simulations single-domain ferroelectric water is produced under ordinary ambient conditions utilizing carbon nanotubes open to a water reservoir. This ferroelectric water diffuses while keeping its proton-ordered network intact. The mobile/immobile water transitions and the step-wise changes in net polarization of water are observed to occur spontaneously. The immobile water becomes mobile by transforming into the single-domain ferroelectric water. Our general notion of relating a more highly ordered structure with a lower temperature has so far restricted researchers' attention to very low temperatures when experimenting on proton-ordered phases of water. The present study improves our general understanding of water, considering that the term 'ferroelectric water' has so far practically stood for 'ferroelectric ice,' and that single-domain ferroelectric water has not been reported even for the ice nanotubes.

Hydration effect on the optical property of a DNA fiber: first-principles and molecular dynamics studies.

We have studied the theoretical hydration effect on the optical property of a deoxyribonucleic acid (DNA) double helix fiber. First-principles electronic structure and molecular dynamics simulations reveal that the electronic structure of the DNA fiber varies according to the amount of hydration or the relative humidity. We show that ultraviolet optical conductivity is influenced by hydration structure and DNA deformation, and our findings agree with other theoretical results and experimental observations. Infrared (IR) optical conductivity is estimated by the molecular dynamics approach. The humidity dependence of optical conductivity due to dipole relaxation of water is in close agreement with experimental observations. The theoretical IR absorption spectrum due to DNA vibrations agrees with the experimental spectrum. We discuss deformation and screening effects of the DNA fiber on humidity dependence of the optical spectra.

Tautomerism in Reduced Pyrazinacenes.

Monoprotic and diprotic NH tautomerism in reduced oligoazaacenes, the pyrazinacenes, was studied by using first principles simulations. Stepwise reductions in the metadynamics-sampled free energy profile were observed during consecutive monoprotic tautomerizations, with energy barriers gradually reducing with increasing proton separation during monoprotic processes. This is accompanied by an increasing contribution from the quinoidal electronic structure, as evidenced by the computed highest occupied molecular orbital (HOMO) structure. An unusual odd-even effect in the free energy profiles is also observed upon changing the length of the pyrazinacene. Calculated HOMO structures reveal an increasing tendency for delocalization of pyrazine lone pairs with an increasing number of ring annelations. The influence of tautomerism on the pyrazine lone pair delocalization was also observed. Tautomers with protons situated centrally on the pyrazinacene backbone are predicted to be more stable due to a combination of (enamine) delocalization and a loss of Clar sextet resonance stabilization in tautomers with protons at terminal pyrazine rings. Experimental evidence suggesting the structure of pyrazinacene tautomers is included and discussed as a support to the calculation.

A mechanism of adsorption of beta-nicotinamide adenine dinucleotide on graphene sheets: experiment and theory.

Beta-nicotinamide adenine dinucleotide (NAD(+)) and its reduced form (NADH) play major roles in the development of electrochemical enzyme biosensors and biofuel cells. Unfortunately, the oxidation of NADH at carbon electrodes suffers from passivation of the electrodes and a decrease in passing currents. Here, we investigate experimentally and theoretically the reasons for such passivation. High-resolution X-ray photoelectron spectroscopy (HR-XPS), voltammetry, and amperometry show that adsorption occurs on the edges and "edge-like" defects of graphene sheets. HR-XPS and ab initio molecular dynamics show that the adsorption of NAD(+) molecules on the edges of graphene happens due to interaction with oxygen-containing groups such as carboxylic groups, while graphene edges substituted only with hydrogen are prone to passivation.

Ultrafast permeation of water through protein-based membranes.

Pressure-driven filtration by porous membranes is widely used in the production of drinking water from ground and surface water. Permeation theory predicts that filtration rate is proportional to the pressure difference across the filtration membrane and inversely proportional to the thickness of the membrane. However, these membranes need to be able to withstand high water fluxes and pressures, which means that the active separation layers in commercial filtration systems typically have a thickness of a few tens to several hundreds of nanometres. Filtration performance might be improved by the use of ultrathin porous silicon membranes or carbon nanotubes immobilized in silicon nitride or polymer films, but these structures are difficult to fabricate. Here, we report a new type of filtration membrane made of crosslinked proteins that are mechanically robust and contain channels with diameters of less than 2.2 nm. We find that a 60-nm-thick membrane can concentrate aqueous dyes from fluxes up to 9,000 l h(-1) m(-2) bar(-1), which is approximately 1,000 times higher than the fluxes that can be withstood by commercial filtration membranes with similar rejection properties. Based on these results and molecular dynamics simulations, we propose that protein-surrounded channels with effective lengths of less than 5.8 nm can separate dye molecules while allowing the ultrafast permeation of water at applied pressures of less than 1 bar.