Ab Initio Investigation Research Articles

On the Origin of Raman Activity in Anatase TiO2 (Nano)Materials: An Ab Initio Investigation of Surface and Size Effects.

Titania-based materials are abundant in technological applications, as well as everyday products; however, many of its structure-property relationships are still unclear. In particular, its surface reactivity on the nanoscale has important consequences for fields such as nanotoxicity or (photo)catalysis. Raman spectroscopy has been used to characterize titania-based (nano)material surfaces, mainly based on empirical peak assignments. In the present work, we address the structural features responsible for the Raman spectra of pure, stoichiometric TiO2 materials from a theoretical characterization. We determine a computational protocol to obtain accurate Raman response in a series of anatase TiO2 models, namely, the bulk and three low-index terminations by periodic ab initio approaches. The origin of the Raman peaks is thoroughly analyzed and the structure-Raman mapping is performed to account for structural distortions, laser and temperature effects, surface orientation, and size. We address the appropriateness of previous experimental use of Raman to quantify the presence of distinct TiO2 terminations, and provide guidelines to exploit the Raman spectrum based on accurate rooted calculations that could be used to characterize a variety of titania systems (e.g., single crystals, commercial catalysts, thin layered materials, facetted nanoparticles, etc.).

An ab initio investigation of LiCoBO3 as Li-ion battery cathode material

We give a first principles approach for the structural, electronic and magnetic properties of LiCoBO3 and CoBO3 in the monoclinic lattices. Along [001] direction, based on density functional theory approach and using Full Potential Linear Augmented Plane Wave (FP-LAPW) method, Polarized spin and spin–orbit coupling are included in calculations, and major spin was set up. The calculations were carried out using generalized gradient approximation (GGA) + an on-site Coulomb self-interaction correction potential (GGA + U). The magnetic and orbital moments are estimated. The U value was considered to be equal to 6 eV for Co atom. The average equilibrium voltage over a full cycle, of the LiCoBO3 battery is estimated from our FP-LAPW calculations. The capacity of a cell and energy density of LiCoBO3 were calculated. The compound LiCoBO3 is a semiconductor with average intercalation voltage 4.01 V. The voltage difference help promote electrochemical performance, which opens a window for Li-ion battery marketing application.

Strain effects on the electronic, magnetic and optical properties of CrI3: An ab-initio investigation

Strain effects on the electronic, magnetic and optical properties of CrI3: An ab-initio investigation

Ab Initio Investigations of Gallium Nitride Nanoribbons for Spin Filter and Negative Differential Behavior

This work demonstrates the influence of alternative edge passivation via H/F on the electronic and magnetic characteristics of gallium nitride nanoribbons (GaNNRs). For this study, the first-principles density functional theory (DFT) and non-equilibrium green function (NEGF) frameworks are deployed. Our study demonstrates that the selective edge passivation with F/H zigzag GaN nanoribbon (NR) is a strong contender for spintronics due to its half-metallic property under specific conditions. Various nano-configurations for the H/F ZGaNNRs passivation are investigated here. Thermodynamically, most stable configuration is alternate fluorinated F-GaN-F NR. Further, the negative differential resistance (NDR) behavior is also reported along with the perfect spin-filtering properties. This study paves the way to employ these 2-D nanostructures-based devices for spin filters, spin logic switches, and steep switching nanoelectronic devices.

Ab Initio Investigation of Intramolecular Charge Transfer States in DMABN by Calculation of Excited State X-ray Absorption Spectra

Dual fluorescence in 4-(dimethylamino)benzonitrile (DMABN) and its derivatives in polar solvents has been studied extensively for the past several decades. An intramolecular charge transfer (ICT) minimum on the excited state potential energy surface, in addition to the localized low-energy (LE) minimum, has been proposed as a mechanism for this dual fluorescence, with large geometric relaxation and molecular orbital reorganization a key feature of the ICT pathway. Herein, we have used both equation-of-motion coupled-cluster with single and double excitations (EOM-CCSD) and time-dependent density functional (TDDFT) methods to investigate the landscape of excited state potential energy surfaces across a number of geometric conformations proposed as ICT structures. In order to correlate these geometries and valence excited states in terms of potential experimental observables, we have calculated the nitrogen K-edge ground and excited state absorption spectra for each of the predicted "signpost" structures and identified several key spectral features that could be used to interpret a future time-resolved X-ray absorption experiment.

Ab-initio investigations for structural, optoelectronic and thermoelectric properties of Li2BeXSe4 (X = Ge, Si, Sn) compounds

Motivated by photovoltaic activities, we study certain aspects of the stannite structure (ST) of the Li2BeXSe4 (X = Ge, Si, Sn) compounds using the full potential augmented plane wave approach implemented in the Wien2k code. Concretely, we use the generalized gradient approximation (GGA) and the modified Becke–Johnson exchange potential (mBJ) to evaluate in a precise way, the band gaps, the total and the partial densities, as well as the optical and the thermoelectric properties. Among others, we show that the Li2BeXSe4 (X = Ge, Si, Sn) semiconductors exhibit a direct band gap in the Γ point of the Brillouin Zone (BZ). Furthermore, the optical properties of the three compounds show a strong optical conductivity and a good optical absorption. As a result, we reveal that the proposed materials are suitable for the application in photovoltaic solar cells. Using the BoltzTrap package, we also examine and discuss the thermoelectric properties such as the Seebeck coefficient (S), the electrical conductivity, and the thermal conductivity as a function of the temperature. We find that the Li2BeXSe4 (X = Ge, Si, Sn) compounds exhibit good thermoelectric transport properties, making them a good candidates for clean energy applications.

Atomic Ordering and Interfacial Interaction at Liquid-Mg/SiC{0 0 0 1} Interfaces: An Ab Initio Molecular Dynamics Study

We present the results of ab initio molecular dynamics investigations on the atomic ordering and chemical interactions at the interfaces between liquid Mg and SiC{0 0 0 1} interfaces. The simulations reveal distinct borders between the SiC substrates and liquid Mg. The liquid Mg atoms adjacent to the substrates are bonded to the outmost C/Si atoms and are positively charged. The terminating Mg layers contain a variety of atomic vacancies, being topologically rough. The liquid Mg atoms adjacent to the substrates display unusual prenucleation phenomenon with strong layering but weak in-plane ordering. The obtained information here is helpful to get insight into the formation and interfacial interactions in the SiC joined nano-sized magnesium matrix composites and the role of SiC particles as potential nucleation sites during solidification, and further helps understand interfacial interactions at the grain boundaries in ceramic/metal composites and welded parts, etc. in general.

Computational studies of HCCCCS isomers

The recent identification of interstellar molecules HCCCHCCC and HCCCCS in TMC-1 has prompted an ab initio investigation of these isomers. The isomers of HCCCHCCC have been studied previously but not so much HCCCCS. In this paper we carry out high-level CCSD(T) calculations to characterise the lowest eleven isomers of HCCCCS. We also apply complete basis set extrapolation including core correlation and relativistic effects to further investigate the key isomers and provide high-precision rotational constants which will hopefully aid astronomers in their assignments. Source: This image has been adapted from http://www.eso.org/public/images/eso1209d/ licensed under CC BY-SA 3.0.

Cubic and tetragonal phases competition of magnetic Ru2VZ(Si;Ge) Heusler compounds: ab initio investigation

First-principles calculations, were carried out to study the structural stability of the cubic [(Fm3‾m),(F4‾3m)] and tetragonal [(I4/mmm),(I4‾m2)] phases, of Ru2VZ(Si;Ge) full Heusler compounds; where Energy of formation, elastic, Mechanical, phonon and thermodynamic properties and electronic density of states were determined.Through our results, all ferromagnetic phases exhibit a total magnetic moment of 1μB for both Ru2VSi and Ru2VGe compounds, except the tetragonal (I4‾m2) phase that manifest a zero (0μB) total magnetic moment.Further, the antiferromagnetic state of the cubic phase Fm3‾mRu2VZ(Si;Ge) was found energetically more favorable than the ferromagnetic one.The analysis of the calculated elastic and dynamic properties, revealed that only the regular cubic phase (Fm3‾m) of the Ru2VSi and the regular tetragonal phase (I4/mmm) of the Ru2VGe are the most stable among the considered phases.The calculated electronic density of states for the ferromagnetic stable phases, confirm the existence of a half-metal character with high spin polarization (Pn) of: ∼91% and ∼100% for the Ru2VSi and Ru2VGe respectively.

Ab-initio investigation of structural, optoelectronic, and transport properties of metal-alkali-based binary chalcogenides, X2Te [X = Na, K, Rb]: Rb2Te a potential candidate for UV-shielding and thermoelectric devices

Ab-initio investigation of structural, optoelectronic, and transport properties of metal-alkali-based binary chalcogenides, X2Te [X = Na, K, Rb]: Rb2Te a potential candidate for UV-shielding and thermoelectric devices

Experimental and Ab-Initio Investigation of the Electrical Conductivity of Emeraldine Salt

We present an experimental and first-principles study to describe the changes in the electrical conductivity properties of the Emeraldine Salt (ES) form of polyaniline when using two different synthesis methods. The ES powders obtained by the interfacial synthesis method (PANI-I) exhibit higher electrical conductivity than that of the powders obtained by the conventional method (PANI-C). Investigation of the calculated band structure and density of states together with experimentally obtained optical-absorption spectra and the magnetic measurements indicate that PANI-I differs from PANI-C with respect to its localized defect state type which significantly alters the intrinsic conductivity. Furthermore, comparative studies of bond length, dihedral angles, and relative stabilities of Leucomeraldine Base, Emeraldine Base, ES Bipolaron state (ESB), and Polaron state (ESP) indicate that ESB and ESP states might coexist. Additionally, we confirm that increasing the length of the polymer chain to octamer in the unit cell does not influence the relative stability between ESB and ESP defect states.

Structural classification of boron nitride twisted bilayers and ab initio investigation of their stacking-dependent electronic structure

Since the discovery of superconductive twisted bilayer graphene which initiated the field of twistronics, moiré systems have not ceased to exhibit fascinating properties. We demonstrate that in boron nitride twisted bilayers, for a given moiré periodicity, there are five different stackings which preserve the monolayer hexagonal symmetry (i.e. the invariance upon rotations of 120°) and not only two as always discussed in literature. We introduce some definitions and a nomenclature that identify unambiguously the twist angle and the stacking sequence of any hexagonal bilayer with order-3 rotation symmetry. Moreover, we employ density functional theory to study the evolution of the band structure as a function of the twist angle for each of the five stacking sequences of boron nitride bilayers. We show that the gap is indirect at any angle and in any stacking, and identify features that are conserved within the same stacking sequence irrespective of the angle of twist.

Disentangling photoexcitation and photoluminescence processes in defective MgO

Oxygen vacancies are ubiquitous in oxides and strongly influence the material's electronic structure and catalytic and transport properties. Here we focus on a seemingly simple defective oxide, MgO, and on the electronic properties of oxygen vacancies, which remain controversial in spite of numerous studies. We present an ab initio investigation of the photoexcitation and photoionization process of these defects, using a newly developed embedding Bethe-Salpeter equation approach, implemented in the WEST code. We find absorption and emission energies in good agreement with experiments. Our results provide a detailed, microscopic understanding of the absorption and emission processes of the neutral and positively charged oxygen vacancy, reconciling different views present in the chemistry and condensed-matter physics communities.

Microstructure control and property switching in stress-free van der Waals epitaxial VO2 films on mica

Realizing stress-free inorganic epitaxial films on weakly bonding substrates is of importance for applications that require film transfer onto surfaces that do not seed epitaxy. Film-substrate bonding is usually weakened by harnessing natural van der Waals layers (e.g., graphene) on substrate surfaces, but this is difficult to achieve in non-layered materials. Here, we demonstrate van der Waals epitaxy of stress-free films of a non-layered material VO2 on mica. The films exhibit out-of-plane 010 texture with three in-plane orientations inherited from the crystallographic domains of the substrate. The lattice parameters are invariant with film thickness, indicating weak film-substrate bonding and complete interfacial stress relaxation. The out-of-plane domain size scales monotonically with film thickness, but the in-plane domain size exhibits a minimum, indicating that the nucleation of large in-plane domains supports subsequent island growth. Complementary ab initio investigations suggest that VO2 nucleation and van der Waals epitaxy involves subtle polarization effects around, and the active participation of, surface potassium atoms on the mica surface. The VO2 films show a narrow domain-size-sensitive electrical-conductivity-temperature hysteresis. These results offer promise for tuning the properties of stress-free van der Waals epitaxial films of non-layered materials such as VO2 through microstructure control.

The Post‐Perovskite Transition in Fe‐ and Al‐Bearing Bridgmanite: Effects on Seismic Observables

AbstractThe primary phase of the Earth’s lower mantle, (Al, Fe)‐bearing bridgmanite, transitions to the post‐perovskite (PPv) phase at Earth’s deep mantle conditions. Despite extensive experimental and ab initio investigations, there are still important aspects of this transformation that need clarification. Here, we address this transition in (Al3+, Fe3+)‐, (Al3+)‐, (Fe2+)‐, and (Fe3+)‐bearing bridgmanite using ab initio calculations and validate our results against experiments on similar compositions. Consistent with experiments, our results show that the onset transition pressure and the width of the two‐phase region depend distinctly on the chemical composition: (a) Fe3+‐, Al3+‐, or (Al3+, Fe3+)‐alloying increases the transition pressure, while Fe2+‐alloying has the opposite effect; (b) in the absence of coexisting phases, the pressure‐depth range of the Pv‐PPv transition is likely too broad to cause a sharp D” discontinuity (<30 km); (c) the average Clapeyron slope of the two‐phase regions are consistent with previous measurements, calculations in MgSiO3, and inferences from seismic data. In addition, (d) we observe a softening of the bulk modulus in the two‐phase region. The consistency between our results and experiments gives us the confidence to proceed and examine this transition in aggregates with different compositions computationally, which will be fundamental for resolving the most likely chemical composition of the D" region by analyses of tomographic images.

Calculation of dislocation binding to helium-vacancy defects in tungsten using hybrid ab initio-machine learning methods

Calculations of dislocation-defect interactions are essential to model metallic strength, but the required system sizes are at or beyond ab initio limits. Current estimates thus have extrapolation or finite size errors that are very challenging to quantify. Hybrid methods offer a solution, embedding small ab initio simulations in an empirical medium. However, current implementations can only match mild elastic deformations at the ab initio boundary. We describe a robust method to employ linear-in-descriptor machine learning potentials as a highly flexible embedding medium, precisely matching dislocation migration pathways whilst keeping at least the elastic properties constant. This advanced coupling allows dislocations to cross the ab initio boundary in fully three dimensional defect geometries. Investigating helium and vacancy segregation to edge and screw dislocations in tungsten, we find long-range relaxations qualitatively change impurity-induced core reconstructions compared to those in short periodic supercells, even when multiple helium atoms are present. We also show that helium-vacancy complexes, considered to be the dominant configuration at low temperatures, have only a very weak binding to screw dislocations. These results are discussed in the context of recent experimental and theoretical studies. More generally, our approach opens a vast range of mechanisms to ab initio investigation and provides new reference data to both validate and improve interatomic potentials.

An ab initio investigation of electronic, elastic, mechanical and vibrational properties of Co2VX (X = Al, Ga)

An ab initio investigation of electronic, elastic, mechanical and vibrational properties of Co2VX (X = Al, Ga)

Ab-initio investigation of the structural stability, electronic and optical properties of the LiBO2 compound by using the G0W0+BSE approach

This article investigates the structural stability, and electronic and optical properties of the LiBO2 compound using density functional theory (DFT) and solving the many-body Bethe-Salpeter equation within the G0W0+BSE approach. The structural properties showed that the monoclinic structure of LiBO2 is thermodynamically and dynamically stable within both LDA and GGA approximations. The band structure results indicated that the direct band gap of LiBO2 at the Γ point is 6.05 eV, which increased to 8.95 eV by considering the quasi-particle effects in G0W0@LDA calculations. The maximum reflectance in the x, y, and z directions is 0.48, 0.86, and 0.54, respectively, with the corresponding static refractive index values of 1.52, 1.61, and 1.54. The direct optical band gap obtained from the imaginary part of the dielectric function is 5.95 eV, 5.95 eV, and 6.10 eV for the various directions x, y, and z respectively. Considering the electron-hole effects in the G0W0+BSE approach, the optical gap showed more accurate results, and the value of 7.54 eV was obtained in the x and z-directions and 8.10 eV in the y-direction. Therefore, excitonic effects within the G0W0+BSE approximation, in comparison to G0W0+RPA, leads to the redshift of the optical absorption spectrum. The calculated exciton binding energy is 1.41 eV for the x and z-directions and 0.85 eV for the y-direction. Finally, it can be stated that LiBO2 is a wide-gap insulator with a gap greater than 7 eV in the monoclinic structure. This indicates the high transparency of the LiBO2 crystal, with the prospect of application in optical instruments.

The Role of M3+ Substitutional Doping (M = In, Sb, Bi) in the Passivation of the α-CsPbI3(100) Surface

Photovoltaic applications based on perovskite materials still face several challenges to deliver high power conversion efficiency, in particular the chemical passivation of surfaces, which is required to remove surface electronic states. In this work, we propose a surface passivation route for the α-CsPbI3(100) surface through substitutional doping at the Pb sites by trivalent cations such as In, Sb, or Bi. Our ab initio investigation was based on density functional theory calculations. We found a preferential doping position at surface octahedrons by more than 0.35 eV/dopant in comparison with inner layers as well as a bandgap decrease due to surface states and optical bandgaps wider than fundamental ones as a result of dark transitions. The surface passivation with Bi atoms demonstrates to be the most effective one, with enhanced sunlight absorption when compared to the 3D bulk α-CsPbI3. Furthermore, we changed the doping concentration and demonstrate the possibility to combine trivalent cations with other passivating mechanisms. Therefore, our computational results disclose a promising route for saturating perovskite surfaces.

High-resolution rovibrational spectroscopy of trans-formic acid in the v1 OH stretching fundamental: Dark state coupling and evidence for charge delocalization dynamics

High-resolution infrared (IR) reduced-Doppler absorption spectra of jet-cooled gas phase trans-formic acid in the v1 OH stretching fundamental region are reported for the first time, obtained by supersonically expanding trans-formic acid/Ar mixtures through a slit jet nozzle source and rotationally cooling to Trot ≈ 10.9(5) K, with absorption signals recorded by high-resolution difference-frequency IR absorption spectroscopy. Two a/b-type rovibrational bands of comparable intensity, one ∼10-fold weaker b-type band, and one ∼6-fold weaker a-type band are observed, with vibrational band origins at 3570.493(5), 3566.793(5), 3560.032(9), and 3534.6869(2) cm−1, respectively. Based on previous Raman jet spectroscopic work by Nejad and Sibert [A. Nejad, E.L. Sibert III, The Raman jet spectrum of trans-formic acid and its deuterated isotopologs: Combining theory and experiment to extend the vibrational database, J. Chem. Phys. 154(6) (2021) 064301.], these four rovibrational bands have been assigned to v1, (v2 + v7), (v6 + 2v7 + 2v9), and 2v3, respectively. Specifically, two of the three upper dark states (2171 (a′) and 617292 (a′)) are close enough to the “bright” 11 (a′) state to facilitate strong anharmonic resonance interactions, which results in intensity mixing into the two zero-order bands that would otherwise be “dark”. Furthermore, our high-resolution spectral analysis reveals that there are local rotational crossings between these zero-order 11 and 2171 states resulting in extra lines (i.e., some upper levels in the nominally v1 band have majority zero-order 2171 state character). This motivates development of a 3 coupled state (11, 2171, and 617292) picture to aid in the spectral analysis, which is able to match all 3 observed band origins and relative band intensities, as well as indicate the necessity of multistate (> 2) coupling. Though limited by range of J and Ka levels (J' ≤ 9 and Ka' ≤ 3) populated at supersonic jet temperatures, this work offers first precision spectroscopic analysis of trans-formic acid in the v1 OH stretch region, which should aid in assignment of the more complete yet highly congested room temperature FTIR spectra [D. Hurtmans, F. Herregodts, M. Herman, J. Liévin, A. Campargue, A. Garnache, A. Kachanov, Spectroscopic and ab initio investigation of the νOH overtone excitation in trans-formic acid, J. Chem. Phys. 113(4) (2000) 1535–1545.]. Finally, and in sharp contrast to the spectral complexity in the three predominantly b-type bands, the lone a-type 2v3 rovibrational band at 3534.6869(2) cm−1 is well described by a simple, rigid asymmetric top Hamiltonian.