Crystallization kinetics of nanoconfined GeTe slabs in GeTe/TiTe[Formula: see text]-like superlattices for phase change memories.

Superlattices made of alternating blocks of the phase change compound Sb[Formula: see text]Te[Formula: see text] and of TiTe[Formula: see text] confining layers have been recently proposed for applications in neuromorphic devices. The Sb[Formula: see text]Te[Formula: see text]/TiTe[Formula: see text] heterostructure allows for a better control of multiple intermediate resistance states and for a lower drift with time of the electrical resistance of the amorphous phase. However, Sb[Formula: see text]Te[Formula: see text] suffers from a low data retention due to a low crystallization temperature T[Formula: see text]. Substituting Sb[Formula: see text]Te[Formula: see text] with a phase change compound with a higher T[Formula: see text], such as GeTe, seems an interesting option in this respect. Nanoconfinement might, however, alters the crystallization kinetics with respect to the bulk. In this work, we investigated the crystallization process of GeTe nanoconfined in geometries mimicking GeTe/TiTe[Formula: see text] superlattices by means of molecular dynamics simulations with a machine learning potential. The simulations reveal that nanoconfinement induces a mild reduction in the crystal growth velocities which would not hinder the application of GeTe/TiTe[Formula: see text] heterostructures in neuromorphic devices with superior data retention.

Pressure-induced reversal of Peierls-like distortions elicits the polyamorphic transition in GeTe and GeSe.

While polymorphism is prevalent in crystalline solids, polyamorphism draws increasing interest in various types of amorphous solids. Recent studies suggested that supercooling of liquid phase-change materials (PCMs) induces Peierls-like distortions in their local structures, underlying their liquid-liquid transitions before vitrification. However, the mechanism of how the vitrified phases undergo a possible polyamorphic transition remains elusive. Here, using high-energy synchrotron X-rays, we can access the precise pair distribution functions under high pressure and provide clear evidence that pressure can reverse the Peierls-like distortions, eliciting a polyamorphic transition in GeTe and GeSe. Combined with simulations based on machine-learned-neural-network potential, our structural analysis reveals a high-pressure state characterized by diminished Peierls-like distortion, greater coherence length, reduced compressibility, and a narrowing bandgap. Our finding underscores the crucial role of Peierls-like distortions in amorphous octahedral systems including PCMs. These distortions can be controlled through pressure and composition, offering potentials for designing properties in PCM-based devices.

Expansion of the Materials Cloud 2D Database.

Two-dimensional (2D) materials are among the most promising candidates for beyond-silicon electronic, optoelectronic, and quantum computing applications. Recently, their recognized importance sparked a push to discover and characterize novel 2D materials. Within a few years, the number of experimentally exfoliated or synthesized 2D materials went from a few to more than a hundred, with the number of theoretically predicted compounds reaching a few thousand. In 2018 we first contributed to this effort with the identification of 1825 compounds that are either easily (1036) or potentially (789) exfoliable from experimentally known 3D compounds. Here, we report on a major expansion of this 2D portfolio thanks to the extension of the screening protocol to an additional experimental database (MPDS) as well as the updated versions of the two databases (ICSD and COD) used in our previous work. This expansion leads to the discovery of an additional 1252 monolayers, bringing the total to 3077 compounds and, notably, almost doubling the number of easily exfoliable materials to 2004. We optimize the structural properties of all these monolayers and explore their electronic structure with a particular emphasis on those rare large-bandgap 2D materials that could be precious in isolating 2D field-effect-transistor channels. Finally, for each material containing up to 6 atoms per unit cell, we identify the best candidates to form commensurate heterostructures, balancing requirements on supercell size and minimal strain.

Extreme Bendability of Atomically Thin MoS2 Grown by Chemical Vapor Deposition Assisted by Perylene-Based Promoter.

Shaping two-dimensional (2D) materials in arbitrarily complex geometries is a key to designing their unique physical properties in a controlled fashion. This is an elegant solution, taking benefit from the extreme flexibility of the 2D layers but requiring the ability to force their spatial arrangement from flat to curved geometries in a delicate balance among free-energy contributions from strain, slip-and-shear mechanisms, and adhesion to the substrate. Here, we report on a chemical vapor deposition approach, which takes advantage of the surfactant effects of organic molecules, namely the tetrapotassium salt of perylene-3,4,9,10-tetracarboxylic acid (PTAS), to conformally grow atomically thin layers of molybdenum disulphide (MoS2) on arbitrarily nanopatterned substrates. Using atomically resolved transmission electron microscope images and density functional theory calculations, we show that the most energetically favorable condition for the MoS2 layers consists of its adaptation to the local curvature of the patterned substrate through a shear-and-slip mechanism rather than strain accumulation. This conclusion also reveals that the perylene-based molecules have a role in promoting the adhesion of the layers onto the substrate, no matter the local-scale geometry.

Geometry of tellurene adsorbed on the Si(111)-R30°-Sb surface from first principles calculations.

The 2D form of tellurium, named tellurene, is one of the latest discoveries in the family of 2D mono-elemental materials. In a trilayer configuration, free-standing tellurene was predicted theoretically to acquire two crystallographic forms, the α and ß phases, corresponding to either a 1T-MoS2-like geometry or a trilayer slab exposing the Te(101̄0) surface of bulk Te with helical chains lying in-plane and further reconstructed due to the formation of interchain bonds. Either one or the other of the two phases was observed experimentally to prevail depending on the substrate they were grown onto. In the perspective to integrate tellurene on silicon, we here report an ab initio study of the adsorption of tellurene on the Si(111)-R30° surface passivated by antinomy. According to the literature, this substrate is chosen for the growth of several tellurides by molecular beam epitaxy. The calculations reveal that on this substrate the adsorption energy mostly compensates the energy difference between the α and ß phases in the free-standing configuration which suggests that the prevalence of one phase over the other might in this case strongly depend on the kinetics effects and deposition conditions.

Prediction of Phonon-Mediated Superconductivity with High Critical Temperature in the Two-Dimensional Topological Semimetal W2N3.

Two-dimensional superconductors attract great interest both for their fundamental physics and for their potential applications, especially in the rapidly growing field of quantum computing. Despite intense theoretical and experimental efforts, materials with a reasonably high transition temperature are still rare. Even more rare are those that combine superconductivity with a nontrivial band topology that could potentially give rise to exotic states of matter. Here, we predict a remarkably high superconducting critical temperature of 21 K in the easily exfoliable, topologically nontrivial 2D semimetal W2N3. By studying its electronic and superconducting properties as a function of doping and strain, we also find large changes in the electron-phonon interactions that make this material a unique platform to study different coupling regimes and test the limits of current theories of superconductivity. Last, we discuss the possibility of tuning the material to achieve coexistence of superconductivity and topologically nontrivial edge states.

Atom-surface van der Waals potentials of topological insulators and semimetals from scattering measurements.

The phenomenology of resonant scattering has been known since the earliest experiments upon scattering of atomic beams from surfaces and is a means of obtaining experimental information about the fundamentals of weak adsorption systems in the van der Waals regime. We provide an overview of the experimental approach based on new experimental data for the He-Sb2Te3(111) system, followed by a comparative overview and perspective of recent results for topological semimetal and insulator surfaces. Moreover, we shortly discuss the perspectives of calculating helium-surface interaction potentials from ab initio calculations. Our perspective demonstrates that atom-surface scattering provides direct experimental information about the atom-surface interaction in the weak physisorption regime and can also be used to determine the lifetime and mean free path of the trapped atom. We further discuss the effects of elastic and inelastic scattering on the linewidth and lifetime of the trapped He atom with an outlook on future developments and applications.

2-D Materials for Ultrascaled Field-Effect Transistors: One Hundred Candidates under the Ab Initio Microscope.

Due to their remarkable properties, single-layer 2-D materials appear as excellent candidates to extend Moore's scaling law beyond the currently manufactured silicon FinFETs. However, the known 2-D semiconducting components, essentially transition metal dichalcogenides, are still far from delivering the expected performance. Based on a recent theoretical study that predicts the existence of more than 1800 exfoliable 2-D materials, we investigate here the 100 most promising contenders for logic applications. Their current versus voltage characteristics are simulated from first-principles, combining density functional theory and advanced quantum transport calculations. Both n- and p-type configurations are considered, with gate lengths ranging from 15 down to 5 nm. From this large collection of electronic materials, we identify 13 compounds with electron and hole currents potentially much higher than those in future Si FinFETs. The resulting database widely expands the design space of 2-D transistors and provides original guidelines to the materials and device engineering community.

Large-scale synthesis of crystalline g-C3N4 nanosheets and high-temperature H2 sieving from assembled films.

Poly(triazine imide) (PTI), a crystalline g-C3N4, hosting two-dimensional nanoporous structure with an electron density gap of 0.34 nm, is highly promising for high-temperature hydrogen sieving because of its high chemical and thermal robustness. Currently, layered PTI is synthesized in potentially unsafe vacuum ampules in milligram quantities. Here, we demonstrate a scalable and safe ambient pressure synthesis route leading to several grams of layered PTI platelets in a single batch with 70% yield with respect to the precursor. Solvent exfoliation under anhydrous conditions led to single-layer PTI nanosheets evidenced by the observation of triangular g-C3N4 nanopores. Gas permeation studies confirm that PTI nanopores can sieve He and H2 from larger molecules. Last, high-temperature H2 sieving from PTI nanosheet-based membranes, prepared by the scalable filter coating technique, is demonstrated with H2 permeance reaching 1500 gas permeation units, with H2/CO2, H2/N2, and H2/CH4 selectivities reaching 10, 50, and 60, respectively, at 250°C.

Relative Abundance of [Formula: see text] Topological Order in Exfoliable Two-Dimensional Insulators.

Quantum spin Hall insulators make up a class of two-dimensional materials with a finite electronic band gap in the bulk and gapless helical edge states. In the presence of time-reversal symmetry, [Formula: see text] topological order distinguishes the topological phase from the ordinary insulating one. Some of the phenomena that can be hosted in these materials, from one-dimensional low-dissipation electronic transport to spin filtering, could be promising for many technological applications in the fields of electronics, spintronics, and topological quantum computing. Nevertheless, the rarity of two-dimensional materials that can exhibit nontrivial [Formula: see text] topological order at room temperature hinders development. Here, we screen a comprehensive database we recently identified of 1825 monolayers that can be exfoliated from experimentally known compounds to search for novel quantum spin Hall insulators. Using density-functional and many-body perturbation theory simulations, we identify 13 monolayers that are candidates for quantum spin Hall insulators including high-performing materials such as AsCuLi2 and (platinum) jacutingaite (Pt2HgSe3). We also identify monolayer Pd2HgSe3 (palladium jacutingaite) as a novel Kane-Mele quantum spin Hall insulator and compare it with platinum jacutingaite. A handful of promising materials are mechanically stable and exhibit [Formula: see text] topological order, either unperturbed or driven by small amounts of strain. Such screening highlights a relative abundance of [Formula: see text] topological order of around 1% and provides an optimal set of candidates for experimental efforts.

Equipartition of Energy Defines the Size-Thickness Relationship in Liquid-Exfoliated Nanosheets.

Liquid phase exfoliation is a commonly used method to produce 2D nanosheets from a range of layered crystals. However, such nanosheets display broad size and thickness distributions and correlations between area and thickness, issues that limit nanosheet application potential. To understand the factors controlling the exfoliation process, we have liquid-exfoliated 11 different layered materials, size-selecting each into fractions before using AFM to measure the nanosheet length, width, and thickness distributions for each fraction. The resultant data show a clear power-law scaling of nanosheet area with thickness for each material. We have developed a simple nonequilibrium thermodynamics-based model predicting that the power-law prefactor is proportional to both the ratios of in-plane-tearing/out-of-plane-peeling energies and in-plane/out-of-plane moduli. By comparing the experimental data with the modulus ratio calculated from first-principles, we find close agreement between experiment and theory. This supports our hypothesis that energy equipartition holds between nanosheet tearing and peeling during sonication-assisted exfoliation.

Valley-Engineering Mobilities in Two-Dimensional Materials.

Two-dimensional materials are emerging as a promising platform for ultrathin channels in field-effect transistors. To this aim, novel high-mobility semiconductors need to be found or engineered. Although extrinsic mechanisms can in general be minimized by improving fabrication processes, the suppression of intrinsic scattering (driven, for example, by electron-phonon interactions) requires modification of the electronic or vibrational properties of the material. Because intervalley scattering critically affects mobilities, a powerful approach to enhance transport performance relies on engineering the valley structure. We show here the power of this strategy using uniaxial strain to lift degeneracies and suppress scattering into entire valleys, dramatically improving performance. This is shown in detail for arsenene, where a 2% strain stops scattering into four of the six valleys and leads to a 600% increase in mobility. The mechanism is general and can be applied to many other materials, including in particular the isostructural antimonene and blue phosphorene.

Nanoscale surface dynamics of Bi2Te3(111): observation of a prominent surface acoustic wave and the role of van der Waals interactions.

We present a combined experimental and theoretical study of the surface vibrational modes of the topological insulator Bi2Te3. Using high-resolution helium-3 spin-echo spectroscopy we are able to resolve the acoustic phonon modes of Bi2Te3(111). The low energy region of the lattice vibrations is mainly dominated by the Rayleigh mode which has been claimed to be absent in previous experimental studies. The appearance of the Rayleigh mode is consistent with previous bulk lattice dynamics studies as well as theoretical predictions of the surface phonon modes. Density functional perturbation theory calculations including van der Waals corrections are in excellent agreement with the experimental data. Comparison of the experimental results with theoretically obtained values for films with a thickness of several layers further demonstrate, that for an accurate theoretical description of three-dimensional topological insulators with their layered structure the inclusion of van der Waals corrections is essential. The presence of a prominent surface acoustic wave and the contribution of van der Waals bonding to the lattice dynamics may hold important implications for the thermoelectric properties of thin-film and nanoscale devices.

Prediction of a Large-Gap and Switchable Kane-Mele Quantum Spin Hall Insulator.

Fundamental research and technological applications of topological insulators are hindered by the rarity of materials exhibiting a robust topologically nontrivial phase, especially in two dimensions. Here, by means of extensive first-principles calculations, we propose a novel quantum spin Hall insulator with a sizable band gap of ∼0.5 eV that is a monolayer of jacutingaite, a naturally occurring layered mineral first discovered in 2008 in Brazil and recently synthesized. This system realizes the paradigmatic Kane-Mele model for quantum spin Hall insulators in a potentially exfoliable two-dimensional monolayer, with helical edge states that are robust and that can be manipulated exploiting a unique strong interplay between spin-orbit coupling, crystal-symmetry breaking, and dielectric response.

Two-dimensional materials from high-throughput computational exfoliation of experimentally known compounds.

Two-dimensional (2D) materials have emerged as promising candidates for next-generation electronic and optoelectronic applications. Yet, only a few dozen 2D materials have been successfully synthesized or exfoliated. Here, we search for 2D materials that can be easily exfoliated from their parent compounds. Starting from 108,423 unique, experimentally known 3D compounds, we identify a subset of 5,619 compounds that appear layered according to robust geometric and bonding criteria. High-throughput calculations using van der Waals density functional theory, validated against experimental structural data and calculated random phase approximation binding energies, further allowed the identification of 1,825 compounds that are either easily or potentially exfoliable. In particular, the subset of 1,036 easily exfoliable cases provides novel structural prototypes and simple ternary compounds as well as a large portfolio of materials to search from for optimal properties. For a subset of 258 compounds, we explore vibrational, electronic, magnetic and topological properties, identifying 56 ferromagnetic and antiferromagnetic systems, including half-metals and half-semiconductors.

Ordered Peierls distortion prevented at growth onset of GeTe ultra-thin films.

Using reflection high-energy electron diffraction (RHEED), the growth onset of molecular beam epitaxy (MBE) deposited germanium telluride (GeTe) film on Si(111)-(√3 × âˆš3)R30°-Sb surfaces is investigated, and a larger than expected in-plane lattice spacing is observed during the deposition of the first two molecular layers. High-resolution transmission electron microscopy (HRTEM) confirms that the growth proceeds via closed layers, and that those are stable after growth. The comparison of the experimental Raman spectra with theoretical calculated ones allows assessing the shift of the phonon modes for a quasi-free-standing ultra-thin GeTe layer with larger in-plane lattice spacing. The manifestation of the latter phenomenon is ascribed to the influence of the interface and the confinement of GeTe within the limited volume of material available at growth onset, either preventing the occurrence of Peierls dimerization or their ordered arrangement to occur normally.

Unveiling the Mechanisms Leading to H2 Production Promoted by Water Decomposition on Epitaxial Graphene at Room Temperature.

By means of a combination of surface-science spectroscopies and theory, we investigate the mechanisms ruling the catalytic role of epitaxial graphene (Gr) grown on transition-metal substrates for the production of hydrogen from water. Water decomposition at the Gr/metal interface at room temperature provides a hydrogenated Gr sheet, which is buckled and decoupled from the metal substrate. We evaluate the performance of Gr/metal interface as a hydrogen storage medium, with a storage density in the Gr sheet comparable with state-of-the-art materials (1.42 wt %). Moreover, thermal programmed reaction experiments show that molecular hydrogen can be released upon heating the water-exposed Gr/metal interface above 400 K. The Gr hydro/dehydrogenation process might be exploited for an effective and eco-friendly device to produce (and store) hydrogen from water, i.e., starting from an almost unlimited source.

Vibrational dynamics and band structure of methyl-terminated Ge(111).

A combined synthesis, experiment, and theory approach, using elastic and inelastic helium atom scattering along with ab initio density functional perturbation theory, has been used to investigate the vibrational dynamics and band structure of a recently synthesized organic-functionalized semiconductor interface. Specifically, the thermal properties and lattice dynamics of the underlying Ge(111) semiconductor crystal in the presence of a commensurate (1 × 1) methyl adlayer were defined for atomically flat methylated Ge(111) surfaces. The mean-square atomic displacements were evaluated by analysis of the thermal attenuation of the elastic He diffraction intensities using the Debye-Waller model, revealing an interface with hybrid characteristics. The methyl adlayer vibrational modes are coupled with the Ge(111) substrate, resulting in significantly softer in-plane motion relative to rigid motion in the surface normal. Inelastic helium time-of-flight measurements revealed the excitations of the Rayleigh wave across the surface Brillouin zone, and such measurements were in agreement with the dispersion curves that were produced using density functional perturbation theory. The dispersion relations for H-Ge(111) indicated that a deviation in energy and lineshape for the Rayleigh wave was present along the nearest-neighbor direction. The effects of mass loading, as determined by calculations for CD3-Ge(111), as well as by force constants, were less significant than the hybridization between the Rayleigh wave and methyl adlayer librations. The presence of mutually similar hybridization effects for CH3-Ge(111) and CH3-Si(111) surfaces extends the understanding of the relationship between the vibrational dynamics and the band structure of various semiconductor surfaces that have been functionalized with organic overlayers.

The interaction of organic adsorbate vibrations with substrate lattice waves in methyl-Si(111)-(1 × 1).

A combined helium atom scattering and density functional perturbation theory study has been performed to elucidate the surface phonon dispersion relations for both the CH3-Si(111)-(1 × 1) and CD3-Si(111)-(1 × 1) surfaces. The combination of experimental and theoretical methods has allowed characterization of the interactions between the low energy vibrations of the adsorbate and the lattice waves of the underlying substrate, as well as characterization of the interactions between neighboring methyl groups, across the entire wavevector resolved vibrational energy spectrum of each system. The Rayleigh wave was found to hybridize with the surface rocking libration near the surface Brillouin zone edge at both the M̄-point and K̄-point. The calculations indicated that the range of possible energies for the potential barrier to the methyl rotation about the Si-C axis is sufficient to prevent the free rotation of the methyl groups at a room temperature interface. The density functional perturbation theory calculations revealed several other surface phonons that experienced mode-splitting arising from the mutual interaction of adjacent methyl groups. The theory identified a Lucas pair that exists just below the silicon optical bands. For both the CH3- and CD3-terminated Si(111) surfaces, the deformations of the methyl groups were examined and compared to previous experimental and theoretical work on the nature of the surface vibrations. The calculations indicated a splitting of the asymmetric deformation of the methyl group near the zone edges due to steric interactions of adjacent methyl groups. The observed shifts in vibrational energies of the -CD3 groups were consistent with the expected effect of isotopic substitution in this system.

Evidence of confinement of the π plasmon in periodically rippled graphene on Ru(0001).

High-resolution electron energy loss spectroscopy has been used to study the electronic response of periodically rippled monolayer graphene grown on Ru(0001). A plasmonic mode, assigned to the π plasmon, has been observed at around 6 eV. The dispersion curve of this collective mode indicates plasmon confinement within the hills of the ripples. Moreover, we found that the corrugation of the graphene sheet also significantly affects the damping processes of the π plasmon.