First-principles Analysis Research Articles

Design principles for NASICON super-ionic conductors

Na Super Ionic Conductor (NASICON) materials are an important class of solid-state electrolytes owing to their high ionic conductivity and superior chemical and electrochemical stability. In this paper, we combine first-principles calculations, experimental synthesis and testing, and natural language-driven text-mined historical data on NASICON ionic conductivity to achieve clear insights into how chemical composition influences the Na-ion conductivity. These insights, together with a high-throughput first-principles analysis of the compositional space over which NASICONs are expected to be stable, lead to the successful synthesis and electrochemical investigation of several new NASICONs solid-state conductors. Among these, a high ionic conductivity of 1.2 mS cm−1 could be achieved at 25 °C. We find that the ionic conductivity increases with average metal size up to a certain value and that the substitution of PO4 polyanions by SiO4 also enhances the ionic conductivity. While optimal ionic conductivity is found near a Na content of 3 per formula unit, the exact optimum depends on other compositional variables. Surprisingly, the Na content enhances the ionic conductivity mostly through its effect on the activation barrier, rather than through the carrier concentration. These deconvoluted design criteria may provide guidelines for the design of optimized NASICON conductors.

Exploring the Optoelectronic Potential of M2SnX3F2 (M = Sr, Ba; X = S, Se) Compounds Through First-Principles Analysis of Structural, Electronic, and Optical Properties

Exploring the Optoelectronic Potential of M2SnX3F2 (M = Sr, Ba; X = S, Se) Compounds Through First-Principles Analysis of Structural, Electronic, and Optical Properties

Manipulating Quantum Interference between σ and π Orbitals in Single-Molecule Junctions via Chemical Substitution and Environmental Control.

Understanding and manipulating quantum interference (QI) effects in single molecule junction conductance can enable the design of molecular-scale devices. Here we demonstrate QI between σ and π molecular orbitals in an ∼4 Å molecule, pyrazine, bridging source and drain electrodes. Using single molecule conductance measurements, first-principles analysis, and electronic transport calculations, we show that this phenomenon leads to distinct patterns of electron transport in nanoscale junctions, such as destructive interference through the para position of a six-membered ring. These QI effects can be tuned to allow conductance switching using environmental pH control. Our work lays out a conceptual framework for engineering QI features in short molecular systems through synthetic and external manipulation that tunes the energies and symmetries of the σ and π channels.

A 99.74% Efficient Capacitor-Charging Converter Using Partial Power Processing for Electrocalorics

This work combines a 99.2% efficient GaN-based low-voltage fast-switching half-bridge converter with a Si-based high-voltage slow-switching and almost lossless switched capacitor multiplexer using a partial power processing approach to a six-level prototype with 99.74% efficiency. A loss breakdown shows how the ideal partial power processing efficiency is theoretically increased to 99.84% by using four additional voltage levels, but then reduced to the measured efficiency by the additional static and dynamic losses of the multiplexer. For electrocaloric heat pumps, an emerging technology for cooling and heating applications with zero global warming potential, such high charging efficiencies enable a high heat pump system coefficient of performance (COP). Based on available data of electrocaloric lead magnesium niobate (PMN)-based samples, and based on a first-principle and best-case analysis for Carnot-like cycles, it is predicted that the 99.74% electrical charging efficiency in combination with the electrocaloric material data enables to surpass 50% of the thermal Carnot limit (for cooling with a heat pump). Ultra-high efficiency of power converters thus pave the way towards future electrocaloric heat pumps of competitive system performance.

Nonlinear saturation of whistler modes driven by runaway electrons

Runaway electrons exhibit kinetic instabilities with potentially beneficial consequences. The anomalous Doppler resonance between the electrons and whistler modes is a primary underlying mechanism. These instabilities require a first-principle nonlinear theoretical analysis, which would ultimately enable assessment of their impact on disruption mitigation and diagnostics, especially in ITER-relevant conditions. This paper presents recent progress in developing the required theoretical framework for isolated nonlinear resonances. By employing the generic bump-on-tail model, we predict the wave saturation levels in both near-threshold and strongly driven regimes. Remarkably, for the waves of interest, the parallel component of the wave vector dictates the parallel momentum of the resonant particles. This feature, together with the expected wave saturation level, provides a complete description of the resonance impact on the runaway electron distribution function. We show that the parametric dependence of the nonlinearly saturated mode provides a way to recover certain features of the runaway momentum distribution function.

First-Principles Analysis of CuMg2InS4: Insights into Optical, Piezoelectric, and Thermoelectric Properties

First-Principles Analysis of CuMg2InS4: Insights into Optical, Piezoelectric, and Thermoelectric Properties

Excellent Switchable Properties, Broad-Band Emission, Ferroelectricity, and High Tc in a Two-Dimensional Hybrid Perovskite: (4,4-DCA)2PbBr4 Exploited by H/F Substitution.

Switchable materials have gained significant attention due to their potential applications in data storage, sensors, and switching devices. Two-dimensional (2D) hybrid perovskites have demonstrated promising prospects for designing switchable materials, where the dynamic motion of the organic components coupled with the distortion of the inorganic framework provides the driving force for triggering multifunctional switchable properties. Herein, through the H/F substitution strategy, we report a polar 2D hybrid lead-based perovskite, (4,4-DCA)2PbBr4 (4,4-DCA = 4,4-difluorocyclohexylammonium) (1), which exhibits dual-stable behavior in a dielectric and second harmonic generation (SHG) response during the reversible phase transition process near the high Curie temperature Tc ∼ 409 K. The phase transition temperature is significantly increased by 41 K compared to the corresponding non-fluorinated (CHA)2PbBr4 (CHA = cyclohexylammonium). Remarkably, the material shows rare broad-band yellow emission under UV excitation, attributed to the induction of self-trapped exciton emission by the distortion of the [PbBr6]4- octahedra, as confirmed by the first-principles analysis. 1 also exhibited ferroelectricity with a saturation polarization value and a small coercive field. This study provides a new insight into the modification of multifunctional switchable materials through the H/F substitution strategy.

First-Principles Analysis of the Effects of Covalency and Ionicity on the 4f-5d Transition Energy of Ce3+ in Garnet-Type Oxides

Abstract A blue light-emitting diode (LED) and a yellow phosphor are frequently combined to create white LEDs, with (Ce3+)-doped YAG as a common phosphor utilized in this process. Yellow light is produced when Ce3+ ions, excited by blue LEDs, are combined with direct blue light to form white light. This study investigated the effects of electronic characteristics, such as covalency and ionicity, on the 5d and 4f level energies of Ce3+ in garnet-type crystals using relativistic discrete variational-Xα (DV-Xα) molecular orbital (MO) calculations. The purpose of this study is to elucidate a detailed mechanism for the centroid shift of the 5d level energies of Ce3+ in crystals based on the MO theory. The theoretical 4f-5d transition energies agreed well with experimental ones, and electronic structure analysis revealed a high correlation between the centroid shift and the net charge of Ce3+. The primary cause of the centroid shift of the 5d level energies relative to the lowest 4f level of Ce3+ in crystals is an increase of the 4f level energies caused by a reduction of the net charge of Ce3+. These results provide a theoretical foundation for the creation of novel Ce3+-doped garnet phosphors for use in displays and solid-state lighting.

Efficient Photocatalytic Degradation of Co Doped MoS2 Assembled by Nano-sheet and First-Principles Analysis

Efficient Photocatalytic Degradation of Co Doped MoS2 Assembled by Nano-sheet and First-Principles Analysis

Stabilizing the Ferroelectric Phase in HfAlO Ferroelectric Tunnel Junction With Different Bottom Electrodes

To satisfy the demand of high-speed and low-power non-volatile memory in the field of integrated circuits for in-memory computing, HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -based ferroelectric tunnel junctions have been development with different film thickness and bottom electrodes. In this study, we investigated the influence of bottom electrode on the ferroelectric characteristics of HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -based devices by the first-principles calculations and experimental results. First-principles analysis verified that the device with GaAs bottom electrode showed robust ferroelectric properties with remanent polarization of about 20 μC/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> due to high O-phase proportion. Meanwhile, the samples with GaAs and Pt bottom electrode display excellent retention properties owing to the high system energy. These results pave the way the development of HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -base ferroelectric tunnel junctions.

Enhancing the Rashba spin splitting at X/Au/Si(111) surface by induced intrinsic spin-polarization

Adsorbed/modified surface is usually treated as a prototype to study Rashba spin splitting. Using first-principles calculations and analyses, we first explain the Rashba spin splitting of monovalent alkali metal adsorbed on Au/Si(111) surface as realized in experiment. We clearly identify that Na adatom donates one electron to pristine Au/Si(111) surface, leading to the Fermi level shift upward to the parabolic range of Au d and Si p hybridized states and thus leading to large Rashba spin splitting in Na/Au/Si(111) system. However, for the larger intra-atomic spin-orbit coupling Cs adsorbate, it not only plays the identical role to Na adatom, but induces intrinsic spin-polarization surprisingly, which further enhances the Rashba spin splitting on the Cs/Au/Si(111) surface up to ∼200 meV. Our proposed mechanism that intrinsic spin-polarization significantly enhancing the Rashba spin splitting is generally applicable and can be extended readily to alkali-earth metal as epitaxial layer grown on Au/Si(111) surface.

A First-principles analysis of Structural, Electronic, Elastic, and vacant properties of BCC, FCC and HCP tungsten at different electronic temperatures

Since tungsten (W) was considered as the most promising plasma facing materials (PFMs) in fusion reactors, there has been extensive research on the physical performance of W-PFMs. It is found that under the extreme conditions in a fusion reactor, W-PFMs should be in a nonequilibrium state of high electronic temperature and low ionic temperature. This leads to the possibility of non-thermal phase transitions, where the crystal structure of the tungsten material may change from body-centered cubic (bcc) phase to hexagonal close-packed (hcp) phase or face-centered cubic (fcc) phase. Consequently, it is necessary to investigate the relevant physical properties of hcp-W and fcc-W under the electron-excited state. In this work, the fundamental physical properties, including atomic structures, electronic structures, elastic constants, and vacancy formation energies, of bcc-W, hcp-W and fcc-W, were theoretically calculated at various electronic temperatures. The mechanical stability of these three phases was also systematically analyzed under varying electronic temperatures. The results of this research are expected to provide a certain guidance in the optimization of W-PFMs in future fusion reactors.

Enhanced xylene detection performance of colloidal quantum dots modified tungsten oxide composites

Surface modification of sensitive materials is an effective strategy for building high-performance sensors. In this work, NH4Cl acted as structure-directing agent and corrosive was utilizing in the fabrication of WO3 nanoplates with neat grooves. The prepared Co3O4 colloid quantum dots were uniformly dispersed on the WO3 surface to construct the gas sensor with excellent sensing performance. The intrinsic properties, such as crystal structures, surface adsorption states, and chemical states of all samples, were analyzed. Compared with the sensor without Co3O4, the sensing performance of the Co3O4 functionalized sensor towards xylene was significantly improved. Compared with the pristine WO3 sensor, the optimal working temperature of the Co3O4 functionalized sensor (S4) was low (240 ℃), and the response value was as high as 24.63. Combined with material characterization, device testing, and the first-principles analysis, the improved sensor performance is mainly attributed to the following points: the oxygen-adsorbing capacity of grooved WO3 is stronger than smooth WO3 nanoplates, the formation of the p-n junction between Co3O4 and WO3 increased the width of the depletion layer, and oxygen vacancy-rich Co3O4 have preferential adsorption on xylene.

Possible routes to superconductivity in the surface layers of V-doped Mg1−δTi2O4 through multiple charge transfers and suppression of Jahn-Teller activity

Superconductivity in the family of spinel oxides is very rare owing to their robust Mott-insulating nature. About half a century ago, ${\mathrm{LiTi}}_{2}{\mathrm{O}}_{4}$ became the first reported spinel compound to show superconductivity with a 12 K transition temperature. Since then, several unsuccessful attempts were made to enhance the ${T}_{\mathrm{c}}$ of this family of materials. However, a very recent experiment [A. Rahaman et al., arXiv:2209.02053] has reported superconductivity at a higher temperature (below $\ensuremath{\sim}16$ K) in the V-doped ${\mathrm{Mg}}_{1\ensuremath{-}\ensuremath{\delta}}{\mathrm{Ti}}_{2}{\mathrm{O}}_{4}$ thin surface layer while its bulk counterpart remains Mott insulating. The superconducting ${T}_{\mathrm{c}}$ of this material is significantly higher compared to other engineered ${\mathrm{MgTi}}_{2}{\mathrm{O}}_{4}$ thin films grown on different substrates. From our first-principles analysis, we have identified that Mg depletion significantly reduces Jahn-Teller (JT) activity and antiferromagnetic superexchange at the surface layer of V-doped ${\mathrm{Mg}}_{1\ensuremath{-}\ensuremath{\delta}}{\mathrm{Ti}}_{2}{\mathrm{O}}_{4}$ due to considerable charge transfer between various ions. The combined effect of a degraded antiferromagnetic order and reduced JT activity weakens the ``Mottness'' of the system, leading to the emergence of superconductivity at higher temperatures.

Structural evolution of MgO layer in Mg-based composites reinforced by Metallic Glasses during the SPS sintering process

The SPS sintering process is used in this study to create 50 wt % micro-sized spherical Fe-based metallic glass powders reinforced magnesium matrix composites (FMGP). The release of trace amounts of Cr in Fe-based metallic glass particles due to a positive mixing enthalpy with Mg not only improves the mechanical suitability of the interface between the metallic glass and the matrix but also contributes to the solution to the increasing problem of fracture toughness of the composite. Following the inclusion of metallic glass particles (50 wt %), the tensile strength (23%), yield strength (50%), and plasticity (more than 8%) were significantly increased. The Cr diffused to the interface and dissolved into the MgO in the form of replacement, forming the oxidation transition layer with the MgO and α-Mg at the interface and distributed among a discontinuous form, according to the analysis of the high-resolution transmission diagram of the composites and the calibration of the diffraction spots of the magnesium matrix at the interface. This enhances mechanical interface matching and transforms the composite's fracture process from inter-crystalline to quasi-destructive, increasing the material's fracture toughness. Once established, the model's first-principles computation and analysis are consistent with the test results.

Energy Bandgap of Cd1−xZnxTe, Cd1−xZnxSe and Cd1−xZnxS Semiconductors: A First-Principles Analysis Based on Tran–Blaha–Modified Becke–Johnson Exchange Potential

Energy Bandgap of Cd1−xZnxTe, Cd1−xZnxSe and Cd1−xZnxS Semiconductors: A First-Principles Analysis Based on Tran–Blaha–Modified Becke–Johnson Exchange Potential

Quantum statistical mechanics near a black hole horizon

We undertake a first-principles analysis of the thermodynamics of a small body near a black hole horizon. In particular, we study the paradigmatic system of a quantum ideal gas in a small box hovering over the Schwarzschild horizon. We describe the gas in terms of free quantum fields, bosonic and fermionic, massive and massless. We identify thermodynamic properties through the microcanonical distribution. We first analyse the more general case of a box in Rindler spacetime, and then specialize to the black hole case. The physics depends strongly on the distance of the box from the horizon, which we treat as a macroscopic thermodynamic variable. We find that the effective dimension of the system transitions from three-dimensional to two-dimensional as we approach the horizon, that Bekenstein's bound fails when the box is adiabatically lowered towards the black hole, and that the pressure is highly anisotropic. The pressure difference between the upper and lower wall leads to an effective force that must be added to the gravitational acceleration. We also show that the approximation of quantum fields propagating on a fixed background for matter breaks down when the system is brought to microscopic distances from the horizon, in which case backreaction effects must be included.

First-principles analysis of the physical properties of XAcTe2 (X = Li, Na) Heusler alloys for optoelectronic and thermoelectric devices

Materials with diverse properties are in high demand due to their utilization in multipurpose devices. The current work presents the systematical analysis of electronic structure, mechanical, optoelectronic and transport properties of new Heusler XAcTe2 (X = Li, Na) alloys. Density functional theory (DFT) based calculations are carried out first time to compute the physical properties of the alloys. The exchange–correlation potential is solved with modified Becke-Johnson (mBJ) approximation. Structural stability is obtained with ground state optimized lattice parameters, negative formation energy values, and phonon dispersion curves. A brittle nature with an anisotropic character is observed for both alloys. The band structures show a direct bandgap for both alloys, which is also reflected in the density of states. The evaluated optoelectronic response shows the high absorption in the ultraviolet region, confirming their ability to use them in optoelectronic devices. The analysis of thermoelectric parameters shows the high electrical and minimum thermal conductivities for both alloys. The electronic and lattice vibrations dependent high ZT values are achieved for LiAcTe2 and NaAcTe2 alloys. The optoelectronic and thermoelectric responses of the alloys show that they could be promising candidates for use in optoelectronic and renewable energy applications.

First-principles analysis of electrochemical hydrogen storage behavior for hydrogenated amorphous silicon thin film in high-capacity proton battery

Silicon material electrodes as proton carriers for high-capacity proton battery have only been proposed for such a short period of time that their physicochemical properties and electrochemical hydrogen storage behavior during charge and discharge processes remain nearly uncharted territory. Herein, the hydrogenated amorphous silicon (a-Si:H) thin film electrodes are prepared by radio frequency sputtering followed by ex-situ hydrogenation. The electrochemical properties of a-Si:H electrodes are tested experimentally, and the electrochemical hydrogen storage behaviors of a-Si:H electrodes are analyzed by first-principles calculations. The results show that the hydrogenation process significantly increases the electrochemical capacity of the electrodes and reduces the band gap of the electrode structure. The electrode exhibits weak conductivity during the initial charging, but the instability of the electrode electronic structure during the later charging results in a slight fluctuation of the electrochemical charging process. The a-Si:H electrode have better electrochemical hydrogen storage/release reversibility than non-hydrogenated electrodes, but this reversibility is weakened by oxygen atoms covered on the electrode surface. The electrochemical hydrogen storage process is easier to accomplish than the electrochemical desorption process of hydrogen evolution reaction for the a-Si:H electrodes. The a-Si:H thin film electrode is more stable on the Ni(111) substrate surface and the good conductivity of the electrode/substrate interface provides convenient conditions for the free transport of electrons in the electrochemical charge/discharge processes. We believe that these results perfectly explain the microscopic mechanisms responsible for the electrode reaction and electrochemical behavior of a-Si:H electrodes in this type of proton battery, and have a certain reference value in understanding the physicochemical properties and electrochemical hydrogen storage behavior of silicon material electrodes applied to other types of batteries during charge/discharge processes.

Quantitative Evaluation of the Carrier Separation Performance of Heterojunction Photocatalysts: The Case of g-C3N4/SrTiO3

Heterojunction photocatalysts are of great interest in the energy and environmental fields, because of their potential to significantly increase the efficiency of harvesting solar energy. Advances in design have been hampered by the continued use of only qualitative analyses. Quantitative evaluation of the carrier separation performance is urgently needed for the design and application of heterojunction photocatalysts. Taking the g-C3N4/SrTiO3 heterojunction as an example, we address the conventional energy band and electronic structure issues by first-principles analysis. After interface coupling, the band edge alignment reverses from that of the respective isolated states of the heterojunction components, suggesting new ways of thinking about the catalytic mechanism of the heterojunction. More significantly, we show the carrier separation performance of heterojunction photocatalysts can be quantitatively predicted by the nonadiabatic molecular dynamics method, enabling more precisely directed research and promoting the quantified design and application of heterojunction photocatalysis, making a contribution of great scientific significance.