High-performance LiNi_(0.8)Mn_(0.1)Co_(0.1)O_(2) cathode by nanoscale lithium sulfide coating via atomic layer deposition

The commercialization of nickel-rich LiNi_(0.8)Mn_(0.1)Co_(0.1)O_(2)(NMC811) has been hindered by its continuous loss of practical capacity and reduction in average working voltage.To address these issues,surface modification has been well-recognized as an effective strategy.Different from the coatings reported in literature to date,in this work,we for the first time report a sulfide coating,amorphous Li_(2)S via atomic layer deposition (ALD).Our study revealed that the conformal nano-Li_(2)S coating shows exceptional protection over the NMC811 cathodes,accounting for the dramatically boosted capacity retention from~11.6%to~71%and the evidently mitigated voltage reduction from 0.39 to 0.18 V after 500 charge–discharge cycles.In addition,the Li_(2)S coating remarkably improved the rate capability of the NMC811 cathode.Our investigation further revealed that all these beneficial effects of the ALD-deposited nano-Li_(2)S coating lie in the following aspects:(i) maintain the mechanical integrity of the NMC811 electrode:(ii) stabilize the NMC electrode/electrolyte interface:and (iii) suppress the irreversible phase transition of NMC structure.Particularly,this study also has revealed that the nano-Li_(2)S coating has played some unique role not associated with traditional non-sulfide coatings such as oxides.In this regard,we disclosed that the Li_(2)S layer has reacted with the released O_(2) from the NMC lattices,and thereby has dramatically mitigated electrolyte oxidation and electrode corrosion.Thus,this study is significant and has demonstrated that sulfides may be an important class of coating materials to tackle the issues of NMCs and other layered cathodes in lithium batteries.

Large scale hybrid Monte Carlo simulations for structure and property prediction

The Monte Carlo method is one of the first and most widely used algorithms in modern computational physics.In condensed matter physics,the particularly popular flavor of this technique is the Metropolis Monte Carlo scheme.While being incredibly robust and easy to implement,the Metropolis sampling is not well-suited for situations where energy and force evaluations are computationally demanding.In search for a more efficient technique,we here explore the performance of Hybrid Monte Carlo sampling,an algorithm widely used in quantum electrodynamics,as a structure prediction scheme for systems with long-range interactions.Our results show that the Hybrid Monte Carlo algorithm stands out as an excellent computational scheme that can not only significantly outperform the Metropolis sampling but also complement molecular dynamics in materials science applications,while allowing ultra-large-scale simulations of systems containing millions of particles.

Ordered SrTiO_3 Nanoripples Induced by Focused Ion Beam

Ordered nanoripples on the niobium-doped SrTiO_3 surfaces were fabricated through focused ion beam bombardment. The surface morphology of the SrTiO_3 nanoripples was characterized using in situ focused ion beam/scanning electron microscopy. The well-aligned SrTiO_3 nanostructures were obtained under optimized ion irradiation conditions. The characteristic wavelength was measured as about 210 nm for different ion beam currents. The relationship between the ion irradiation time and current and SrTiO_3 surface morphology was analyzed. The presented method will be an effective supplement for fabrication of SrTiO_3 nanostructures that can be used for ferroelectric and electronic applications.

Ferroelectricity in Charge-Ordering Crystals with Centrosymmetric Lattices

The switchability between the two ferroelectric(FE)states of an FE material makes FEs widely used in memories and other electronic devices.However,for conventional FEs,its FE switching only occurs between the two FE states whose spatial inversion symmetry is broken.The search for FE materials is therefore subject to certain limitations.We propose a new type of FEs whose FE states still contain spatial inversion centers.The change in polarization of this new type of FEs originates from electronic transfer between two centrosymmetric FE states under an external electric field.Taking BaBiO_(3) as an example,we show that charge-ordering systems can be a typical representative of this new type of FEs.Moreover,unlike traditional ferroelectrics,the change in polarization in this new type of FEs is quantum in nature with the direction dependent on the specific FE transition path.Our work therefore not only extends the concept of FEs but may also open up a new way to find multiferroics.

Effects of Gold Nanorods on Nonlinear Properties of Graphene Films Using Z-Scan Technique

Two dimensional nanomaterials, specifically graphene, can play a significant role in various photonic and electronic devices. This is especially true in handling the enormous heat in high density electronics and in nonlinear optics when using high power lasers. To model these systems it is important to know the thermal-optical properties of graphene. In this paper, we report on the thermal and optical linear and nonlinear properties of graphene materials using Z-scan system. In particular, we explore the thermo-optical properties of graphene, with and without gold nanorods （AuNRs）. The obtained results illustrate that the addition of gold nanorods causes a significant change in thermal nonlinear refractive index coefficients of graphene, due to the plasmonic enhancements.

Interplay between Kitaev interaction and single ion anisotropy in ferromagnetic CrI_(3) and CrGeTe_(3) monolayers

Magnetic anisotropy is crucially important for the stabilization of two-dimensional(2D)magnetism,which is rare in nature but highly desirable in spintronics and for advancing fundamental knowledge.Recent works on CrI3 and CrGeTe_(3) monolayers not only led to observations of the long-time-sought 2D ferromagnetism,but also revealed distinct magnetic anisotropy in the two systems,namely Ising behavior for CrI3 versus Heisenberg behavior for CrGeTe_(3).Such magnetic difference strongly contrasts with structural and electronic similarities of these two materials,and understanding it at a microscopic scale should be of large benefits.Here,firstprinciples calculations are performed and analyzed to develop a simple Hamiltonian,to investigate magnetic anisotropy of CrI3 and CrGeTe_(3) monolayers.The anisotropic exchange coupling in both systems is surprisingly determined to be of Kitaev-type.Moreover,the interplay between this Kitaev interaction and single ion anisotropy(SIA)is found to naturally explain the different magnetic behaviors of CrI3 and CrGeTe_(3).Finally,both the Kitaev interaction and SIA are further found to be induced by spin–orbit coupling of the heavy ligands(I of CrI3 or Te of CrGeTe_(3))rather than the commonly believed 3d magnetic Cr ions.

Strain engineering of electro-optic constants in ferroelectric materials

Electro-optic effects allow control of the ow of light using electric fields,and are of utmost importance for today’s information and communication technologies,such as TV displays and fiber optics.The search for large electro-optic constants in films is essential to the miniaturization and increased efficiency of electro-optic devices.In this work,we demonstrate that strain-engineering in PbTiO_(3) films allows to selectively choose which electro-optic constant to improve.Unclamped electro-optic constants larger than 100 pm V^(−1) are predicted,either by driving the softening of an optical phonon mode at a phase transition boundary under tensile strain,or by generating the equivalent of a negative pressure via compressive strain to obtain large piezoelectric constants.In particular,a r_(13) electro-optic coefficient twice as large as the one of the commonly used LiNbO_(3) electro-optic material is found here when growing PbTiO_(3) on the technologicallyimportant Si substrate.

Coherent-interface-induced strain in large lattice-mismatched materials:A new approach for modeling Raman shift

Strain engineering as one of the most powerful techniques for tuning optical and electronic properties of III-nitrides requires reliable methods for strain investigation.In this work,we reveal,that the linear model based on the experimental data limited to within a small range of biaxial strains(<0.2%),which is widely used for the non-destructive Raman study of strain with nanometer-scale spatial resolution is not valid for the binary wurtzite-structure group-III nitrides GaN and AlN.Importantly,we found that the discrepancy between the experimental values of strain and those calculated via Raman spectroscopy increases as the strain in both GaN and AlN increases.Herein,a new model has been developed to describe the strain-induced Raman frequency shift in GaN and AlN for a wide range of biaxial strains(up to 2.5%).Finally,we proposed a new approach to correlate the Raman frequency shift and strain,which is based on the lattice coherency in the epitaxial layers of superlattice structures and can be used for a wide range of materials.

Tailoring properties of hybrid perovskites by domain-width engineering with charged walls

Charged ferroelectric domain walls are fascinating electrical topological defects that can exhibit unusual properties.Here,in the search for novel phenomena,we perform and analyze first-principles calculations to investigate the effect of domain width on properties of domains with charged walls in the photovoltaic material consisting of methylammonium lead iodide hybrid perovskite.We report that such domains are stable and have rather low domain wall energy for any investigated width(that is,up to 13 lattice constants).Increasing the domain width first linearly decreases the electronic band gap from≃1.4 eV to about zero(which therefore provides an efficient band-gap engineering),before the system undergoes an insulator-to-metal transition and then remains metallic(with both the tail-to-tail and head-to-head domain walls being conductive)for the largest widths.All these results can be understood in terms of:(i)components of polarization along the normal of the domain walls being small in magnitude;(ii)an internal electric field that is basically independent of the domain width;and(iii)rather negligible charge transfer between walls.These findings deepen the knowledge of charged ferroelectric domain walls and can further broaden their potential for applications,particularly in the context of halide perovskites for photovoltaics.

Molecular Layer Deposition of Crosslinked Polymeric Lithicone for Superior Lithium Metal Anodes

In this work,we for the first time developed a novel lithium-containing crosslinked polymeric material,a lithicone that enables excellent protection effects over lithium(Li)metal anodes.This new lithicone was synthesized via an accurately controllable molecular layer deposition(MLD)process,in which lithium tert-butoxide(LTB)and glycerol(GL)were used as precursors.The resultant LiGL lithicone was analyzed using a suite of characterizations.Furthermore,we found that the LiGL thichicone could serve as an exceptional polymeric protection film over Li metal anodes.Our experimental data revealed that the Li electrodes coated by this LiGL lithicone can achieve a superior cycling stability,accounting for an extremely long cyclability of>13,600 Listripping/plating cycles and having no failures so far in Li/Li symmetric cells at a current density of 5 mA/cm^(2)and an areal capacity of 1 mAh/cm^(2).We found that,with a sufficient protection by this LiGL coating,Li electrodes could realize long-term stable cyclability with little formation of Li dendrites and solid electrolyte interphase.This novel LiGL represents a facile and effective solution to the existing issues of Li anodes and potentially paves a technically feasible route for lithium metal batteries.

Author Correction:Strain engineering of electro-optic constants in ferroelectric materials

After publishing this article,we realized that the calculation of the piezoelectric constants dij by Density Functional Perturbation Theory(DFPT),as explained in the methods described in the original version of the Supplementary Information,are incorrect in the case of a partially clamped situation.The correct methodology is to inverse the subspace matrix of elastic constants.

Creating multiferroic and conductive domain walls in common ferroelastic compounds

Domain walls in ferroelectrics and ferroelastics often present peculiar functional properties,offering an intriguing route toward the design of nano-devices.Here we use first-principles simulations to illustrate an approach for engineering such walls,working with representative ferroelastic perovskites LaGaO_(3) and CaTiO_(3)(insulating,non-magnetic,non-polar).We show that a wide range of substitutional dopants can be used to create long-range-ordered structures confined within the walls of these compounds,yielding functional interfaces with tailor-made properties.We thus identify clear-cut strategies to produce metallic walls within an insulating matrix.Further,we find ways to create magnetic walls that also display ferroelectric order(proper or improper),thus providing an original route to obtain magnetoelectric multiferroics.Given the recent developments on the preparation of high-density domain structures in perovskite films,our results suggest a definite path toward new functional nano-materials.

Nanodots of multiferroic oxide material BiFeO_3 from the first principles

Multiferroic nanodots can be harnessed to aid the development of the next generation of nonvolatile data storage and multi-functional devices. In this paper, we review the computational aspects of multiferroic nanodot materials and designs that hold promise for the future memory technology. Conception, methodology, and sys- tematical studies are discussed, followed by some up-to-date experimental progress towards the ultimate limits. At the end of this paper, we outline some challenges remaining in multiferroic research, and how the first principles based approach can be employed as an important tool providing critical information to understand the emergent phenomena in multiferroics.

Prediction of room-temperature half-metallicity in layered halide double perovskites

Half-metallic ferromagnets(HMFs)that possess intriguing physical properties with completely spin-polarized current are key candidates for high-efficiency spintronic devices.However,HMFs that could simultaneously have high Curie temperature(Tc),wide half-metallic gap(ΔHM),and large bulk magnetocrystalline anisotropy energy(MAE)are very rare,which significantly restrict their room-temperature(RT)applications.In this article,through materials screening in layered halide double perovskites(LHDPs).

Dynamics of antipolar distortions

Materials possessing antipolar cation motions are currently receiving a lot of attention because they are fundamentally intriguing while being technologically promising.Most studies devoted to these complex materials have focused on their static properties or on their zone-center phonons.As a result,some important dynamics of antipolar cation distortions,such as the temperature behavior of their phonon frequencies,have been much less investigated,despite the possibility to exhibit unusual features.Here,we report the results and analysis of atomistic simulations revealing and explaining such dynamics for BiFeO3 bulks being subject to hydrostatic pressure.It is first predicted that cooling such material yields the following phase transition sequence:the cubic paraelectric Pm3m state at high temperature,followed by an intermediate phase possessing long-range-ordered in-phase oxygen octahedral tiltings,and then the Pnma state that is known to possess antipolar cation motions in addition to in-phase and antiphase oxygen octahedral tiltings.Antipolar cation modes are found to all have high phonon frequencies that are independent of temperature in the paraelectric phase.On the other hand and in addition to antipolar cation modes increasing in number,some phonons possessing antipolar cation character are rather soft in the intermediate and Pnma states.Analysis of our data combined with the development of a simple model reveals that such features originate from a dynamical mixing between pure antipolar cation phonons and fluctuations of oxygen octahedral tiltings,as a result of a specific trilinear energetic coupling.The developed model can also be easily applied to predict dynamics of antipolar cation motions for other possible structural paths bringing Pm3m to Pnma states.

Displacement Current in Domain Walls of Bismuth Ferrite

In 1861,Maxwell conceived the idea of the displacement current,which then made laws of electrodynamics more complete and also resulted in the realization of devices exploiting such displacement current.Interestingly,it is presently unknown if such displacement current can result in large intrinsic ac current in ferroic systems possessing domains,despite the flurry of recent activities that have been devoted to domains and their corresponding conductivity in these compounds.Here,we report firstprinciples-based atomistic simulations that predict that the transverse(polarization-related)displacement currents of 71°and 109°domains in the prototypical BiFeO_(3) multiferroic material are significant at the walls of such domains and in the GHz regime,and,in fact,result in currents that are at least of the same order of magnitude than previously reported dc currents(that are likely extrinsic in nature and due to electrons).Such large,localized and intrinsic ac currents are found to originate from low-frequency vibrations at the domain walls,and may open the door to the design of novel devices functioning in the GHz or THz range and in which currents would be confined within the domain wall.

Dielectric response of BaZrO_(3)/BaTiO_(3) superlattice

We use the first-principles-based molecular dynamic approach to simulate dipolar dynamics of BaZrO_(3)/BaTiO_(3)superlattice,and obtain its dielectric response.The dielectric response is decomposed into its compositional,as well as the in-plane and out-of-plane parts,which are then discussed in the context of chemical ordering of Zr/Ti ions.We reveal that,while the in-plane dielectric response of BaZrO_(3)/BaTiO_(3)superlattice also shows dispersion over probing frequency,it shall not be categorized as relaxor.

Strain-induced resonances in the dynamical quadratic magnetoelectric response of multiferroics

For the last few years,the research interest in magnetoelectric(ME)effect,which is the cross-coupling between ferroelectric and magnetic ordering in multiferroic materials,has experienced a significant revival.The extensive recent studies are not only conducted towards the design of sensors,actuators,transducers,and memory devices by taking advantage of the cross-control of polarization(or magnetization)by magnetic(or electric)fields,but also aim to create a clearer picture in understanding the sources of ME responses and the novel effects associated with them.Here we derive analytical models allowing to understand the striking and novel dynamics of ME effects in multiferroics and further confirm it with atomistic simulations.Specifically,the role of strain is revealed to lead to the existence of electroacoustic magnons,a new quasiparticle that mixes acoustic and optical phonons with magnons,which results in resonances and thus a dramatic enhancement of magnetoelectric responses.Moreover,a unique aspect of the dynamical quadratic ME response under a magnetic field with varying frequencies,which is the second harmonic generation(SHG),has not been discussed prior to the present work.These SHGs put emphasis on the fact that nonlinearities should be considered while dealing with such systems.

Strain control of layer-resolved negative capacitance in superlattices

Negative capacitance in BaTiO_(3)/SrTiO_(3) superlattices is investigated by Monte Carlo simulations in an atomistic effective Hamiltonian model,using fluctuation formulas for responses to the local macroscopic field that incorporates depolarizing fields.We show that epitaxial strain can tune the negative capacitance of the BaTiO_(3) ferroelectric layer and the overall capacitance of the system.In addition,we predict and explain an original switching of the negative capacitance from the BaTiO3 layer to the SrTiO_(3) layer at low temperatures for intermediate strains.

Structural transitions in Pb(In_(1/2)Nb_(1/2))O_(3) under pressure

Room-temperature Raman scattering and x-ray difraction measurements together with first principles calculations were employed to invetigate the behavior of disordered Pb(In_(1/2)Nb_(1/2))O_(3)(PIN)under pressure up to 50GPa.Raman spectra show broad bands but a peak near the 380cm^(-1) increases its intensity with pressure.The linewidth of the band at 550cm^(-1) also increases with pressure,while two of the Raman peaks merge above 6GPa.Above 16 GPa,we observe additional splitting of the band at 50cm^(-1).The pressure evolution of the diffraction patterns for PIN shows obvious Bragg peaks splitting above 16GPa;consistent with a symmetry lowering transition.The transition at 0.5 GPa is identified as a pseudo-cubic to orthorhombic(Pbam)structural change whereas the transition at 16GPa is istructure and associated with changes in linear compresibility and octahedral titling,and the transition at 30 GPa is associated to an orthorhombic to monoclinic change.First-principles calculations indicate that the Pbam structure is ground state with antiferrodisdortion consistent with experiment.