Prediction of novel ground-state structures and analysis of phonon transport in two-dimensional GeS compounds.

We conducted this study to explore the ground-state structures of two-dimensional (2D) variable-composition GeS compounds, driven by the polymorphic characteristics of bulk germanium sulfides and the promising thermoelectric performance of 2D GeS (2). To accomplish this, we utilized the highly successful evolutionary-algorithm-based code USPEX in conjunction with VASP for total energy calculations, leading to the discovery of three previously unexplored structures of GeS (2/), GeS (3̄1), and GeS (2/). These 2D materials exhibit significantly lower formation energies compared to their reported counterparts.

Enhanced Hydrogen Evolution Performance at the Lateral Interface between Two Layered Materials Predicted with Machine Learning.

While economical and effective catalysts are required for sustainable hydrogen production, low-dimensional interfacial engineering techniques have been developed to improve the catalytic activity in the hydrogen evolution reaction (HER). In this study, we used density functional theory (DFT) calculations to measure the Gibbs free energy change (Δ) in hydrogen adsorption in two-dimensional lateral heterostructures (LHSs) MX/M'X' (MoS/WS, MoS/WSe, MoSe/WS, MoSe/WSe, MoTe/WSe, MoTe/WTe, and WS/WSe) and MX/M'X' (NbS/ZnO, NbSe/ZnO, NbS/GaN, MoS/ZnO, MoSe/ZnO, MoS/AlN, MoS/GaN, and MoSe/GaN) at several different positions near the interface. Compared to the interfaces of LHS MX/M'X' and the surfaces of the monolayer MX and MX, the interfaces of LHS MX/M'X' display greater hydrogen evolution reactivity due to their metallic behavior.

Strain and thickness effects on the electronic structures of low-energy two-dimensional CdTe phases.

Cadmium telluride (CdTe) has prime importance in photovoltaics due to its direct band gap of 1.45 eV. However, its two-dimensional counterparts have not been fully explored due to polymorphism.

Enhanced out-of-plane electromechanical response of Janus ZrSeO.

Two-dimensional piezoelectric materials have attracted great attention as they could play a vital role in nano-electromagnetic systems. Herein, we investigate the compelling piezoelectric properties of Janus ZrSeO in monolayer and bulk structures using density functional theory calculations with a van der Waals correction. One of the two independent out-of-plane piezoelectric coefficients (e31) of the bulk ZrSeO is as high as 287.

Revealing the role of dopants in mitigating degradation phenomena in sodium-ion layered cathodes.

Prevention of the degradation of sodium-based layered cathode materials is the key to developing high-performance and high-stability sodium-ion batteries. In this study, the working mechanism of Mg and Ti dopants in mitigating degradation was investigated through the use of first-principles calculations. More specifically, the effects of each dopant in suppressing the phase transition, lattice expansion and shrinkage, and possible oxygen generation during repeated charging and discharging processes were validated.

The effect of non-analytical corrections on the phononic thermal transport in InX (X = S, Se, Te) monolayers.

We investigate the effect of non-analytical corrections on the phonon thermal transport properties in two-dimensional indium chalcogenide compounds. The longitudinal optical (LO) and transverse optical (TO) branches in the phonon dispersion are split near the Γ-point. The lattice thermal conductivity of monolayer InS is increased by 30.

Ultrahigh and anisotropic thermal transport in the hybridized monolayer (BCN) of boron nitride and graphene: a first-principles study.

Heat removal has become a significant challenge in the miniaturization of electronic devices, especially in power electronics, so semiconducting materials with suitable band gaps and high lattice thermal conductivity are highly desired. Here, through first-principles calculations, we theoretically predict an ultra-high and anisotropic lattice thermal conductivity in monolayer BCN. The predicted values of lattice thermal conductivity at room-temperature are 893.

Stability, spontaneous and induced polarization in monolayer MoC, WC, WS, and WSe.

Ferroelectricity in the ultra-low thin films of the ferroelectric materials is a rare phenomenon due to the depolarization electric field established by the uncompensated surface charge. Using density functional theory based calculations, we design buckled honeycomb binary monolayer MoC, WC, WS, and WSe. The feasibility of their experimental synthesis is evident from their low formation energies, stable lattice vibrations, and high elastic stiffnesses.

Switchable Polarization in Mn Embedded Graphene.

Graphene, despite its many unique properties, is neither intrinsically polar due to inversion symmetry nor magnetic. However, based on density functional theory, we find that Mn, one of transition metals, embedded in single or double vacancy (Mn@SV and Mn@DV) in a graphene monolayer induces a dipole moment perpendicular to the sheet, which can be switched from up to down by Mn penetration through the graphene. Such switching could be realized by an external stimuli introduced through the tip of a scanning probe microscope, as already utilized in the studies of molecular switches.

Strain engineering of phonon thermal transport properties in monolayer 2H-MoTe.

The effect of strain on the phonon properties such as phonon group velocity, phonon anharmonicity, phonon lifetime, and lattice thermal conductivity of monolayer 2H-MoTe is studied by solving the Boltzmann transport equation based on first principles calculations. The phonon thermal transport properties of the unstrained monolayer 2H-MoTe are compared to those of the strained case under different biaxial tensile strains. One of the common features of two-dimensional materials is the quadratic nature near the Γ point of the out-of-plane phonon flexural mode that disappears by applying tensile strain.

MoS@VS Nanocomposite as a Superior Hybrid Anode Material.

Using density functional theory, MoS@VS nanocomposite is reported as a hybrid anode with upgraded electronic conductivity and Li/Na storage capacity. The chemically active monolayer VS can be stabilized in energy and phonon vibrations by using the monolayer MoS as a substrate. The stability of the chemically active monolayer VS is attributed to the interfacial charge accumulation between the monolayer MoS and VS.

Ultra low lattice thermal conductivity and high carrier mobility of monolayer SnS and SnSe: a first principles study.

Using density functional theory, we systematically investigate the lattice thermal conductivity and carrier mobility of monolayer SnX (X = S, Se). The room-temperature ultra low lattice thermal conductivities found in monolayer SnS (6.41 W m K) and SnSe (3.

Thermoelectric and phonon transport properties of two-dimensional IV-VI compounds.

We explore the thermoelectric and phonon transport properties of two-dimensional monochalcogenides (SnSe, SnS, GeSe, and GeS) using density functional theory combined with Boltzmann transport theory. We studied the electronic structures, Seebeck coefficients, electrical conductivities, lattice thermal conductivities, and figures of merit of these two-dimensional materials, which showed that the thermoelectric performance of monolayer of these compounds is improved in comparison compared to their bulk phases. High figures of merit (ZT) are predicted for SnSe (ZT = 2.

Adsorption and diffusion of mono, di, and trivalent ions on two-dimensional TiS.

A comparative study of the monovalent (Li, Na, and K) and multivalent (Be, Mg, Ca, and Al) metal ion adsorption and diffusion on an electronically semi-metallic two-dimensional nanosheet of 1T structured TiS is presented here to contribute to the search for abundant, cheap, and nontoxic ingredients for efficient rechargeable metal ion batteries. The total formation energy of the metal ion adsorption and the Bader charge analysis show that the divalent Mg and Ca ions can have a charge storage density double that of the monovalent Li, Na, and K ions, while the Be and Al ions form metallic clusters even at a low adsorption density because of their high bulk energies. The adsorption of Mg ions shows the lowest averaged open circuit voltage (0.

Switchable polarization in an unzipped graphene oxide monolayer.

Ferroelectricity in low-dimensional oxide materials is generally suppressed at the scale of a few nanometers, and has attracted considerable attention from both fundamental and technological aspects. Graphene is one of the thinnest materials (one atom thick). Therefore, engineering switchable polarization in non-polar pristine graphene could potentially lead to two-dimensional (2D) ferroelectric materials.

First-Principles Study of the α-β Phase Transition of Ferroelectric Poly(vinylidene difluoride): Observation of Multiple Transition Pathways.

Transition routes from the α (nonpolar) phase to the β (polar) phase of polyvinylidene difluoride (PVDF) are investigated by first-principles simulation methods. Among various possible routes, including complex torsional and rotational motions, we propose two prototypical transition routes and identify important intermediate structures along each transition pathway using the generalized solid-state nudged elastic band (G-SSNEB) method. The effect of the external electric field and mechanical drawing on the transition behavior is investigated by estimating electric enthalpy and stress tensors.

Dipolar polarization and piezoelectricity of a hexagonal boron nitride sheet decorated with hydrogen and fluorine.

In contrast to graphene, a hexagonal boron nitride (h-BN) monolayer is piezoelectric because it is non-centrosymmetric. However, h-BN shows neither in-plane nor out-of-plane dipole moments due to its three-fold symmetry on the plane and the fact that it is completely flat. Here, we show that the controlled adsorption of hydrogen and/or fluorine atoms on both sides of a pristine h-BN sheet induces flatness distortion in a chair form and an out-of plane dipole moment.

Four-states multiferroic memory embodied using Mn-doped BaTiO3 nanorods.

Multiferroics that show simultaneous ferroic responses have received a great deal of attention by virtue of their potential for enabling new device paradigms. Here, we demonstrate a high-density four-states multiferroic memory using vertically aligned Mn-doped BaTiO3 nanorods prepared by applying the dip-pen nanolithography technique. In the present nanorods array, the polarization (P) switching by an external electric field does not influence the magnetization (M) of the nanorod owing to a negligible degree of the P-M cross-coupling.

High-mobility graphene nanoribbons prepared using polystyrene dip-pen nanolithography.

Graphene nanoribbons (GNRs) are fabricated by dip-pen nanolithography and polystyrene etching techniques on a SrTiO(3)/Nb-doped SrTiO(3) substrate. A GNR field-effect transistor (FET) shows bipolar FET behavior with a high mobility and low operation voltage at room temperature because of the atomically flat surface and the large dielectric constant of the insulating SrTiO(3) layer, respectively.

A nonvolatile memory device made of a ferroelectric polymer gate nanodot and a single-walled carbon nanotube.

We demonstrate a field-effect nonvolatile memory device made of a ferroelectric copolymer gate nanodot and a single-walled carbon nanotube (SW-CNT). A position-controlled dip-pen nanolithography was performed to deposit a poly(vinylidene fluoride-ran-trifluoroethylene) (PVDF-TrFE) nanodot onto the SW-CNT channel with both a source and drain for field-effect transistor (FET) function. PVDF-TrFE was chosen as a gate dielectric nanodot in order to efficiently exploit its bipolar chemical nature.

NiO resistive random access memory nanocapacitor array on graphene.

In this study, a NiO RRAM nanocapacitor array was fabricated on a graphene sheet, which was on a Nb-doped SrTiO(3) substrate containing terraces with a regular interval of about 100 nm and an atomically smooth surface. For the formation of the NiO RRAM nanocapacitor (Pt/NiO/graphene capacitor) array, an anodic aluminum oxide (AAO) nanotemplate with a pore diameter of about 30 nm and an interpore distance of about 100 nm was used. NiO and Pt were subsequently deposited on the graphene sheet.

Self-formed exchange bias of switchable conducting filaments in NiO resistive random access memory capacitors.

We report on the ferromagnetism of conducting filaments formed in a NiO thin film, which exhibited a typical bistable resistive switching characteristic. The NiO thin film showed an antiferromagnetic hysteresis loop for a high resistive state (R(OFF)). However, for a low resistive state (R(ON)), the conducting filaments exhibited a ferromagnetic hysteresis loop for the field cooling.

Molecular dynamics study of dielectric response in a relaxor ferroelectric.

We use atomistic molecular dynamics simulations to study relaxor behavior in the 0.75 PbMg(1/3)Nb(2/3)O(3)-0.25 PbTiO(3) material.

Dip-pen lithography of ferroelectric PbTiO(3) nanodots.

Dip-pen nanolithography of ferroelectric PTO nanodots is described. This position-controlled dip-pen nanolithography using a silicon nitride cantilever produced an array of ferroelectric nanodots with a minimum lateral dimension of approximately 37 nm on a Nb-doped SrTiO(3) substrate. This minimum-sized PTO dot is characterized by single-domain epitaxial growth with an enhanced tetragonality (c/a ratio) of 1.

Atomistic modeling of III-V nitrides: modified embedded-atom method interatomic potentials for GaN, InN and Ga(1-x)In(x)N.

Modified embedded-atom method (MEAM) interatomic potentials for the Ga-N and In-N binary and Ga-In-N ternary systems have been developed based on the previously developed potentials for Ga, In and N. The potentials can describe various physical properties (structural, elastic and defect properties) of both zinc-blende and wurtzite-type GaN and InN as well as those of constituent elements, in good agreement with experimental data or high-level calculations. The potential can also describe the structural behavior of Ga(1-x)In(x)N ternary nitrides reasonably well.