Materials Science (cond-mat.mtrl-sci)

Abstract: In this study, we have employed a DFT+U calculation using quantum-espresso (QE) code to investigate the structural, electronic, optical, and magnetic properties of LiFePO$\rm_{4}$ cathode material for Li-ion batteries. Crystals of LiFePO$\rm_{4}$ and related materials have recently received a lot of attention due to their very promising use as cathodes in rechargeable lithium-ion batteries. The structural optimization was performed and the equilibrium parameters such as the lattice constants, and the bulk modulus are calculated using QE code and found to be a=4.76 {\AA}, b=6.00 {\AA}, c=10.28 {\AA}, B=90.2 GPa, respectively. The projected density of states (PDOS) for the LiFePO$\rm_{4}$ material is remarkably similar to experimental results in literature showing the transition metal $3d$ states forming narrow bands above the O $2p$ band. The results of the various spin configurations suggested that the ferromagnetic configuration can serve as a useful approximation for studying the general features of these systems. In the absence of Li, the majority spin transition metal $3d$ states are well-hybridized with the O 2p band in FePO$\rm_{4}$. The result obtained with a DFT + U showed that LiFePO4 is direct band gap materials with a band gap of 3.82 eV, which is within the range of the experimental values. The PDOS analyses show qualitative information about the crystal field splitting and bond hybridization and help rationalize the understanding of the structural, electronic, optical, and magnetic properties of the LiFePO$\rm_{4}$ as a novel cathode material. On the basis of the predicted optical absorbance, reflection, refractive index, and energy loss function, LiFePO$\rm_{4}$ is demonstrated to be viable and cost-effective, which is very suitable as a cathode material for Li-ion battery.

Abstract: This paper presents the findings of the synthesis of multicomponent (Al, W, Ni, Ti, Nb) alloy coatings from mosaic targets. For the study, a pulsed magnetron sputtering method was employed under different plasma generation conditions: modulation frequency (10 Hz and 1000 Hz), and power (600 W and 1000 W). The processes achieved two types of alloy coatings, high entropy and classical alloys. After the deposition processes, scanning electron microscopy, X-ray diffraction, and energy-dispersive X-ray spectroscopy techniques were employed to find the morphology, thickness, and chemical and phase compositions of the coatings. Nanohardness and its related parameters, namely H3.Er2, H.E, and 1.Er2H ratios, were measured. An annealing treatment was performed to estimate the stability range for the selected coatings. The results indicated the formation of as-deposited coatings exhibiting an amorphous structure as a single-phase solid solution. The process parameters had an influence on the resulting morphology-a dense and homogenous as well as a columnar morphology, was obtained. The study compared the properties of high-entropy alloy (HEA) coatings and classical alloy coatings concerning their structure and chemical and phase composition. It was found that the change of frequency modulation and the post-annealing process contributed to the increase in the hardness of the material in the case of HEA coatings.

Abstract: Highly efficient and sustainable resources of energy are of great demand today to combat with environmental pollution and the energy crisis. In this work, we have examined the novel 2D Janus AsTeX (X = Cl, Br and I) monolayers using first-principles calculations and explore their potential energy conversion applications. We have demonstrated the thermal, energetic, dynamic and mechanical stability of AsTeX (X = Cl, Br, and I) monolayers. Janus AsTeX (X = Cl, Br and I) monolayers are indirect bandgap semiconductors with high carrier mobilities and excellent visible light optical absorption. Our findings demonstrate that the Janus AsTeCl and AsTeBr monolayers exhibits low lattice thermal conductivity and excellent electronic transport properties obtained using semi-classical Boltzmann transport theory including various scattering mechanism. Additionally, the redox potential of water is adequately engulfed by the band alignments of the AsTeCl and AsTeBr monolayers. The water splitting process under illumination can proceeds spontaneously on Janus AsTeBr monolayer, while a minimal low external potential (0.26-0.29 eV) is required to trigger water splitting process on Janus AsTeCl monolayer. A more than 10% STH efficiency of these monolayers indicate their potential practical applications in the commercial production of hydrogen. Thus, our study demonstrates that these monolayers can show potential applications in energy conversion fields.

Abstract: Bio-spinterfaces present numerous opportunities to study spintronics across the biomolecules attached to (ferro)magnetic electrodes. While it offers various exciting phenomena to investigate, it's simultaneously challenging to make stable bio-spinterfaces, as biomolecules are sensitive to many factors that it encounters during thin-film growth to device fabrication. The chirality-induced spin-selectivity (CISS) effect is an exciting discovery demonstrating an understanding that a specific electron's spin (either UP or DOWN) passes through a chiral molecule. The present work utilizes Ustilago maydis Rvb2 protein, an ATP-dependent DNA helicase (also known as Reptin) for the fabrication of bio-spintronic devices to investigate spin-selective electron transport through protein. Ferromagnetic materials are well-known for showing spin-polarization, which many chiral and biomolecules can mimic. We report spin-selective electron transmission through Rvb2 that exhibits 30% spin polarization at a low bias (+ 0.5 V) in a device configuration, Ni/Rvb2 protein/ITO measured under two different magnetic configurations. Our findings demonstrate that biomolecules can be put in circuit components without any expensive vacuum deposition for the top contact. Thus, it holds a remarkable potential to advance spin-selective electron transport in other biomolecules such as proteins, and peptides for biomedical applications.

Abstract: Identification of novel multiferroic materials with high-ordering temperatures remains at the forefront of condensed matter physics research. In this regard, the antiferromagnetic RCrO$_3$ compounds (like GdCrO$_3$) constitute a promising class of multiferroic compounds, which, however, mostly become ferroelectric concomitant with the antiferromagnetic ordering much below room-temperature, arising from a subtle competition between the ferroelectric off-centering mode and a non-polar antiferrodistortive rotation mode that inhibits ferroelectricity. Recently, room-temperature ferroelectricity of structural origin, arising from off-centering displacements of R and Cr ions, has been identified in spark-plasma sintered GdCrO$_3$ [Suryakanta Mishra et al., Phys. Rev. B 104, L180101 (2021)]. Interestingly, some of the experimentally observed non-ferroelectric RCrO$_3$ compounds have been theoretically predicted to host similar ferroelectric instabilities. Here, we have identified two such non-ferroelectric RCrO3 compounds, one DyCrO$_3$ (which is reported as a quantum paraelectric) and another LaCrO$_3$ (which is paraelectric), and using a modified synthesis protocol involving spark-plasma-sintering (SPS), we have been successful in engineering an intrinsic room-temperature ferroelectricity in the paramagnetic state, driven by noncentrosymmetric structural phase in both SPS sintered DyCrO$_3$ and LaCrO$_3$, in contrast to room-temperature paraelectricity in solid-state synthesized DyCrO$_3$ and LaCrO$_3$. While the ferroelectricity in SPS-prepared DyCrO$_3$ and LaCrO$_3$ is stable at room-temperature, it undergoes an irreversible transition from a ferroelectric (Pna2$_1$) phase to a paraelectric (Pbnm) phase at 440 K. Significantly, SPS-sintered LaCrO$_3$, which undergoes antiferromagnetic ordering at 290 K, emerges as a promising near room-temperature multiferroic material.

Abstract: The highly efficient photocatalytic water splitting to produce clean energy requires novel semiconductor materials to achieve high solar-to-hydrogen energy conversion efficiency. Herein, the photocatalytic properties of anisotropic $\beta$-PtX$_2$ (X=S, Se) and Janus $\beta$-PtSSe monolayers are investigated based on density functional theory. Small cleavage energy for \{beta}-PtS2 (0.44 J/m2) and $\beta$-PtSe$_2$ (0.40 J/m$^2$) endorses the possibility of their mechanical exfoliation from respective layered bulk material. The calculated results find \{beta}-PtX2 monolayers to have an appropriate bandgap (~1.8-2.6 eV) enclosing the water redox potential, light absorption coefficients (~104 cm$^{-1}$), and excitons binding energy (~0.5-0.7 eV), which facilitates excellent visible-light driven photocatalytic performance. Remarkably, an inherent structural anisotropy leads to the anisotropic and high carrier mobility (up to ~5 x 10$^3$ cm$^2$ V$^{-1}$ S$^{-1}$) leading to fast transport of photogenerated carriers. Notably, the small required external potential to derive hydrogen evolution reaction and oxygen evolution reaction processes with an excellent solar-to-hydrogen energy conversion efficiency of $\beta$-PtSe$_2$ (~16%) and $\beta$-PtSSe (~18%) makes them promising candidates for solar water splitting applications.

Abstract: In recent years, researchers have manifested their interest in the two-dimensional (2D) mono-elemental materials of group-VI elements because of their excellent optoelectronic, photovoltaic and thermoelectric properties. Despite the intensive recent research efforts, there is still a possibility of novel 2D allotropes of these elements due to their multivalency nature. Here, we have predicted a novel 2D allotrope of tellurium (ring-Te) using density functional theory. Its stability is confirmed by phonon and ab-initio molecular dynamics simulations. The ring-Te has an indirect band gap of 0.69 eV (1.16 eV) at PBE (HSE06) level of theories and undergoes an indirect-direct band gap transition under the tensile strain. The higher carrier mobility of holes (~103cm$^2$V$^{-1}$s$^{-1}$), good UV-visible light absorption ability and low exciton binding (~0.35 eV) of ring-Te gives rise to its potential applications in optoelectronic devices. Further, the electrical contact of ring-Te with topological Dirac semimetal (sq-Te) under the influence of electric field shows that the Schottky barriers and contact types can undergo transition from p-type to n-type Schottky contact and then to ohmic contact at higher electric field. Our study provides an insight into the physics of designing high-performance electrical coupled devices composed of 2D semiconductors and topological semimetals.

Abstract: Highly efficient, environmental friendly and renewable sources of energy are of great need today to combat with increasing energy demands and environmental pollution. In this work, we have investigated the novel 2D allotropes i.e., $\beta$-Te$_2$X (X = S, Se) using first-principles calculations and study their potential applications in light harvesting devices. Both the monolayers possess to have the high stability and semiconducting nature with an indirect band gap. The high carrier mobilities and excellent optical absorption of these monolayers make them potential candidates for solar conversion applications. We have proposed the type-II heterojunction solar cells and calculated their power conversion efficiencies (PCEs). The small conduction band offset and appropriate band gap of donor material in case of $\beta$-Te$_2$S(S-Side)/$\alpha$-Te$_2$S(Te-Side) heterojunction results in the PCE of ~ 21%. In addition to that, the band alignments of these monolayers properly engulf the redox potentials of the water. The overpotentials required to trigger the hydrogen reduction (HER) and water oxidation (OER) half reactions reveal that HER and OER preferred the acidic and neutral mediums, respectively. The calculated solar-to-hydrogen (STH) efficiencies of $\beta$-Te$_2$S ($\beta$-Te$_2$Se) monolayers come out to be ~ 13 % (~12 %), respectively, which implies their practical applications in water splitting. Thus, our work provides strong evidence regarding the potential applications of these materials in the field of light harvesting devices.

Abstract: Metal halide perovskites have shown extraordinary performance in solar energy conversion. They have been classified as "soft semiconductors" due to their flexible corner-sharing octahedral networks and polymorphous nature. Understanding the local and average structures continues to be challenging for both modelling and experiments. Here, we report the quantitative analysis of structural dynamics in time and space from molecular dynamics simulations of perovskite crystals. The descriptors cover a wide variety of properties, including octahedral tilting and distortion, local lattice parameters, molecular orientations, as well as the spatial correlation of these properties. To validate our methods, we have trained a machine learning force field (MLFF) for methylammonium lead bromide (CH$_3$NH$_3$PbBr$_3$) using an on-the-fly training approach with Gaussian process regression. The known stable phases are reproduced and we find an additional symmetry-breaking effect in the cubic and tetragonal phases close to the phase transition temperature. To test the implementation for large trajectories, we also apply it to 69,120 atom simulations for CsPbI$_3$ based on an MLFF developed using the atomic cluster expansion formalism. The structural dynamics descriptors and Python toolkit are general to perovskites and readily transferable to more complex compositions.

Abstract: Low-temperature neutron diffraction experiments at P = 8 GPa have been conducted to investigate the magnetic structures of metallic Holmium at high pressures by employing a long d-spacing highflux diffractometer and a Paris-Edinburgh press cell inside a cryostat. We find that at P = 8 GPa and T = 5 K, no nuclear symmetry change is observed, keeping therefore the hexagonal closed packed (hcp) symmetry at high pressure. Our neutron diffraction data confirm that the ferromagnetic state does not exist. The magnetic structure corresponding to the helimagnetic order, which survives down to 5 K, is fully described by the magnetic superspace group formalism. These results are consistent with those previously published using magnetization experiments.

Abstract: Strong light-matter interaction constitutes the bedrock of all photonic applications, empowering material elements with the ability to create and mediate interactions of light with light. Amidst the quest to identify new agents facilitating such efficient light-matter interactions, a class of promising materials have emerged featuring highly unusual properties deriving from their dielectric constant {\epsilon} being equal, or at least very close, to zero. Works so far have shown that the enhanced nonlinear optical effects displayed in this 'epsilon-near-zero' (ENZ) regime makes it possible to create ultrafast albeit transient optical switches. An outstanding question, however, relates to whether one could use the amplification of light-matter interactions at the ENZ conditions to achieve permanent switching. Here, we demonstrate that an ultrafast excitation under ENZ conditions can induce permanent all-optical reversal of ferroelectric polarization between different stable states. Our reliance on ENZ conditions that naturally emerge from the solid's ionic lattice, rather than specific material properties, suggests that the demonstrated mechanism of reversal is truly universal, being capable of permanently switching order parameters in a wide variety of systems.

Abstract: The conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) is one of the most highly researched materials, yet electronic structure investigations of conducting polymers are still uncommon. The bipolaron model has traditionally been the dominant attempt to explain the electronic structure of PEDOT. Though recent theoretical studies have begun to move away from this model, some aspects remain commonplace, such as the concepts of bipolarons or polaron pairs. In this work, we use density functional theory to investigate the electronic structure of undoped and AlCl4- doped PEDOT oligomers. By considering the influence of oligomer length, oxidation or doping level and spin state, we find no evidence for self-localisation of positive charges in PEDOT as predicted by the bipolaron model. Instead, we find that a single or twin peak structural distortion can occur at any oxidation or doping level. Rather than representing bipolarons or polaron pairs, these are electron distributions driven by a range of factors, which also disproves the concept of polaron pairs. Localisation of distortions does occur in the doped case, although distortions can span an arbitrary number of nearby anions. Furthermore, conductivity in conducting polymers has been experimentally observed to reduce at very high doping levels. We show that at high anion concentrations, the non-bonding orbitals of the anions cluster below the HOMO-LUMO gap and begin to mix into the HOMO of the overall system. We propose that this mixing of highly localised anionic orbitals into the HOMO reduces the conductivity of the polymer and contributes to the reduced conductivity previously observed.

Abstract: Abundant chemical diversity and structural tunability make organic-inorganic hybrid perovskites (OIHPs) a rich ore for ferroelectrics. However, compared with their inorganic counterparts such as BaTiO$_3$, their ferroelectric key properties, including large spontaneous polarization ($P_s$), low coercive field ($E_c$), and strong second harmonic generation (SHG) response, have long been great challenges, which hinder their commercial applications. Here, a quasi-one-dimensional OIHP DMAGeI$_3$ (DMA=Dimethylamine) is reported, with notable ferroelectric attributes at room temperature: a large $P_s$ of 24.14 $\mu$C/cm$^2$ (on a par with BaTiO$_3$), a low $E_c$ below 2.2 kV/cm, and the strongest SHG intensity in OIHP family (about 12 times of KH$_2$PO$_4$ (KDP)). Revealed by the first-principles calculations, its large $P_s$ originates from the synergistic effects of the stereochemically active $4s^2$ lone pair of Ge$^{2+}$ and the ordering of organic cations, and its low kinetic energy barrier of small DMA cations results in a low $E_c$. Our work brings the comprehensive ferroelectric performances of OIHPs to a comparable level with commercial inorganic ferroelectric perovskites.

Abstract: Thermoelectric devices can directly convert waste heat into electricity, which makes them an important clean energy technology. The underlying materials performance can be evaluated by the dimensionless figure of merit ZT. Metal halides are attractive candidates due to their chemical flexibility and ease of processing; however, the maximum ZT realized (ZT = 0.15) falls far below the level needed for commercialization (ZT > 1). Using a first-principles procedure we assess the thermoelectric potential of copper halide CsCu2I3, which features 1D Cu-I connectivity. The n-type crystal is predicted to exhibit a maximum ZT of 2.2 at 600 K along the b-axis. The strong phonon anharmonicity of this system is shown by locally stable non-centrosymmetric Amm2 structures that are averaged to form the observed centrosymmetric Cmcm space group. Our work provides insights into the structure-property relations in metal halide thermoelectrics and suggests a path forward to engineer higher-performance heat-to-electricity conversion.

Abstract: Van der Waals (vdW) Dirac magnon system CrI$_3$, a potential host of topological edge magnons, orders ferromagnetically (FM) (T$_C=61$ K) in the bulk, but antiferromagnetic (AFM) order has been observed in nanometer thick flakes, attributed to monoclinic (M) type stacking. We report neutron scattering measurements on a powder sample where the usual transition to the rhombohedral (R) phase was inhibited for a majority of the structure. Elastic measurements (and the opening of a hysteresis in magnetization data on a pressed pellet) showed that an AFM transition is clearly present below $\sim$50 K, coexisting with the R-type FM order. Inelastic measurements showed a decrease in magnon energy compared to the R phase, consistent with a smaller interlayer magnetic coupling in M-type stacking. A gap remains at the Dirac point, suggesting that the same nontrivial magnon topology reported for the R phase may be present in the M phase as well.

Abstract: $ $Dual-phase $\gamma$-TiAl and $\alpha_2$-Ti$_{3}$Al alloys exhibit high strength and creep resistance at high temperatures. However, they suffer from low tensile ductility and fracture toughness at room temperature. Experimental studies show unusual plastic behaviour associated with ordinary and superdislocations, making it necessary to gain a detailed understanding on their core properties in individual phases and at the two-phase interfaces. Unfortunately, extended superdislocation cores are widely dissociated beyond the length scales practical for routine first-principles density-functional theory (DFT) calculations, while extant interatomic potentials are not quantitatively accurate to reveal mechanistic origins of the unusual core-related behaviour in either phases. Here, we develop a highly-accurate moment tensor potential (MTP) for the binary Ti-Al alloy system using a DFT dataset covering a broad range of intermetallic and solid solution structures. The optimized MTP is rigorously benchmarked against both previous and new DFT calculations, and unlike existing potentials, is shown to possess outstanding accuracy in nearly all tested mechanical properties, including lattice parameters, elastic constants, surface energies, and generalized stacking fault energies (GSFE) in both phases. The utility of the MTP is further demonstrated by producing dislocation core structures largely consistent with expectations from DFT-GSFE and experimental observations. The new MTP opens the path to realistic modelling and simulations of bulk lattice and defect properties relevant to the plastic deformation and fracture processes in $\gamma$-TiAl and $\alpha_2$-Ti$_{3}$Al dual-phase alloys.

Abstract: The Joint Automated Repository for Various Integrated Simulations (JARVIS) infrastructure at the National Institute of Standards and Technology (NIST) is a large-scale collection of curated datasets and tools with more than 80000 materials and millions of properties. JARVIS uses a combination of electronic structure, artificial intelligence (AI), advanced computation and experimental methods to accelerate materials design. Here we report some of the new features that were recently included in the infrastructure such as: 1) doubling the number of materials in the database since its first release, 2) including more accurate electronic structure methods such as Quantum Monte Carlo, 3) including graph neural network-based materials design, 4) development of unified force-field, 5) development of a universal tight-binding model, 6) addition of computer-vision tools for advanced microscopy applications, 7) development of a natural language processing tool for text-generation and analysis, 8) debuting a large-scale benchmarking endeavor, 9) including quantum computing algorithms for solids, 10) integrating several experimental datasets and 11) staging several community engagement and outreach events. New classes of materials, properties, and workflows added to the database include superconductors, two-dimensional (2D) magnets, magnetic topological materials, metal-organic frameworks, defects, and interface systems. The rich and reliable datasets, tools, documentation, and tutorials make JARVIS a unique platform for modern materials design. JARVIS ensures openness of data and tools to enhance reproducibility and transparency and to promote a healthy and collaborative scientific environment.