Prediction of a Cyclic Hydrogenated Boron Molecule as a Promising Building Block for Borophane.

We have extensively searched for a cyclic hydrogenated boron molecule that has a three-center two-electron bond at the center. Using first-principles calculations, we discovered a stable molecule of 2:4:6:8:-2H-1,5:1,5-µH-B8H10 and propose its existence. This molecule can be regarded as a building block for sheets of topological hydrogen boride (borophane), which was recently theoretically proposed and experimentally discovered. The electronic structure of the cyclic hydrogenated boron molecule is discussed in comparison with that of cyclic hydrogenated carbon molecules.

Science of 2.5 dimensional materials: paradigm shift of materials science toward future social innovation.

The past decades of materials science discoveries are the basis of our present society - from the foundation of semiconductor devices to the recent development of internet of things (IoT) technologies. These materials science developments have depended mainly on control of rigid chemical bonds, such as covalent and ionic bonds, in organic molecules and polymers, inorganic crystals and thin films. The recent discovery of graphene and other two-dimensional (2D) materials offers a novel approach to synthesizing materials by controlling their weak out-of-plane van der Waals (vdW) interactions. Artificial stacks of different types of 2D materials are a novel concept in materials synthesis, with the stacks not limited by rigid chemical bonds nor by lattice constants. This offers plenty of opportunities to explore new physics, chemistry, and engineering. An often-overlooked characteristic of vdW stacks is the well-defined 2D nanospace between the layers, which provides unique physical phenomena and a rich field for synthesis of novel materials. Applying the science of intercalation compounds to 2D materials provides new insights and expectations about the use of the vdW nanospace. We call this nascent field of science '2.5 dimensional (2.5D) materials,' to acknowledge the important extra degree of freedom beyond 2D materials. 2.5D materials not only offer a new field of scientific research, but also contribute to the development of practical applications, and will lead to future social innovation. In this paper, we introduce the new scientific concept of this science of '2.5D materials' and review recent research developments based on this new scientific concept.

Wafer-Scale Growth of One-Dimensional Transition-Metal Telluride Nanowires.

The development of bulk synthetic processes to prepare functional nanomaterials is crucial to achieve progress in fundamental and applied science. Transition-metal chalcogenide (TMC) nanowires, which are one-dimensional (1D) structures having three-atom diameters and van der Waals surfaces, have been reported to possess a 1D metallic nature with great potential in electronics and energy devices. However, their mass production remains challenging. Here, a wafer-scale synthesis of highly crystalline transition-metal telluride nanowires is demonstrated by chemical vapor deposition. The present technique enables formation of either aligned, atomically thin two-dimensional (2D) sheets or random networks of three-dimensional (3D) bundles, both composed of individual nanowires. These nanowires exhibit an anisotropic 1D optical response and superior conducting properties. The findings not only shed light on the controlled and large-scale synthesis of conductive thin films but also provide a platform for the study on physics and device applications of nanowire-based 2D and 3D crystals.

Retraction: Pentadiamond: A Hard Carbon Allotrope of a Pentagonal Network of sp

Retraction of DOI: 10.1103/PhysRevLett.125.016001.

Pentadiamond: A Hard Carbon Allotrope of a Pentagonal Network of sp

A pentagonal covalent network consisting of sp^{2} and sp^{3} C atoms has been investigated based on the density functional theory. Our theoretical investigations clarified that the pentagonal covalent network is a metastable three-dimensional carbon allotrope with the Fm3[over ¯]m space group possessing remarkable mechanical properties: relatively high bulk modulus of 381 GPa together with a negative Poisson's ratio of -0.241. Accordingly, the pentagonal covalent network possesses extremely high Young's and shear moduli of 1691 and 1113 GPa, respectively, surpassing those of the diamond. The electronic structure of the pentagonal network is a semiconductor with an indirect band gap of 2.52 eV between L and X points for valence and conduction band edges, respectively, with the relatively small carrier masses.

Carrier Transport and Photoresponse in GeSe/MoS2 Heterojunction p-n Diodes.

Simple stacking of thin van der Waals 2D materials with different physical properties enables one to create heterojunctions (HJs) with novel functionalities and new potential applications. Here, a 2D material p-n HJ of GeSe/MoS2 is fabricated and its vertical and horizontal carrier transport and photoresponse properties are studied. Substantial rectification with a very high contrast (>104 ) through the potential barrier in the vertical-direction tunneling of HJs is observed. The negative differential transconductance with high peak-to-valley ratio (>105 ) due to the series resistance change of GeSe, MoS2 , and HJs at different gate voltages is observed. Moreover, strong and broad-band photoresponse via the photoconductive effect are also demonstrated. The explored multifunctional properties of the GeSe/MoS2 HJs are expected to be important for understanding the carrier transport and photoresponse of 2D-material HJs for achieving their use in various new applications in the electronics and optoelectronics fields.

Band-Gap Engineering of Graphene Heterostructures by Substitutional Doping with B3 N3.

We investigated the energetics and electronic structure of B3 N3 -doped graphene employing density functional theory calculations with the generalized gradient approximation. Our calculations reveal that all of the B3 N3 -doped graphene structures are semiconducting, irrespective of the periodicity of the B3 N3 embedded into the graphene network. This is in contrast to graphene nanomeshes, which are either semiconductors or metals depending on the mesh arrangement. In B3 N3 -doped graphene, the effective masses for both electrons and holes are small. The band gap in the B3 N3 -doped graphene networks and the total energy of the B3 N3 -doped graphene are inversely proportional to the B3 N3 spacing. Furthermore, both properties depend on whether or not the graphene region possesses a Clar structure. In particular, the sheets with a Clar structure exhibit a wider band gap and a slightly lower total energy than those without a Clar structure.

van der Waals interaction-induced photoluminescence weakening and multilayer growth in epitaxially aligned WS2.

Recently, transition metal dichalcogenides (TMDCs) have attracted great interest due to their unique electronic and optical properties. Chemical vapor deposition (CVD) has been regarded as the most promising method for the synthesis of large-area TMDCs with high reproducibility. Having similar hexagonal crystal structures with many TMDCs, c-plane sapphire is commonly used as a growth substrate in CVD. However, few studies have been reported on the influence of the sapphire substrate on the growth behavior and physical properties of TMDCs. In this work, we demonstrate that higher strain is induced in epitaxially grown WS2 grains via van der Waals interactions with sapphire as compared with misaligned WS2 grains. In addition, this strain was found to enhance overlayer deposition on monolayer WS2, while multilayer growth was not observed in non-epitaxial WS2. Photoluminescence (PL) of the epitaxially grown WS2 grains was reduced, reflecting the effective van der Waals interaction with sapphire. Moreover, low-temperature PL measurements revealed strong influence of the c-plane sapphire surface on the optical properties of WS2. Density functional theory (DFT) calculation supports that the aligned WS2 grains are more strongly bound to the sapphire surface, as compared with misaligned WS2. Our work offers a new insight into the understanding of the influence of the substrate on the CVD-grown TMDC materials.

Formation and Characterization of Hydrogen Boride Sheets Derived from MgB2 by Cation Exchange.

Two-dimensional (2D) materials are promising for applications in a wide range of fields because of their unique properties. Hydrogen boride sheets, a new 2D material recently predicted from theory, exhibit intriguing electronic and mechanical properties as well as hydrogen storage capacity. Here, we report the experimental realization of 2D hydrogen boride sheets with an empirical formula of H1B1, produced by exfoliation and complete ion-exchange between protons and magnesium cations in magnesium diboride (MgB2) with an average yield of 42.3% at room temperature. The sheets feature an sp2-bonded boron planar structure without any long-range order. A hexagonal boron network with bridge hydrogens is suggested as the possible local structure, where the absence of long-range order was ascribed to the presence of three different anisotropic domains originating from the 2-fold symmetry of the hydrogen positions against the 6-fold symmetry of the boron networks, based on X-ray diffraction, X-ray atomic pair distribution functions, electron diffraction, transmission electron microscopy, photo absorption, core-level binding energy data, infrared absorption, electron energy loss spectroscopy, and density functional theory calculations. The established cation-exchange method for metal diboride opens new avenues for the mass production of several types of boron-based 2D materials by countercation selection and functionalization.

Out-of-Plane Strain Induced in a Moiré Superstructure of Monolayer MoS2 and MoSe2 on Au(111).

Making contact of transition metal dichalcogenides (TMDCs) with a metal surface is essential for fabricating and designing electronic devices and catalytic systems. It also generates strain in the TMDCs that plays significant role in both electronic and phonon structures. Therefore, detailed understanding of mechanism of the strain generation is important to fully comprehend the modulation effect for the electronic and phonon properties. Here, MoS2 and MoSe2 monolayers are grown on Au surface by chemical vapor deposition and it is demonstrated that the contact with a crystalline Au(111) surface gives rise to only out-of-plane strain in both MoS2 and MoSe2 layers, whereas no strain generation is observed on polycrystalline Au or SiO2 /Si surfaces. Scanning tunneling microscopy analysis provides information regarding consequent specific adsorption sites between lower S (Se) atoms in the SMoS (SeMoSe) structure and Au atoms via unique moiré superstructure formation for MoS2 and MoSe2 layers on Au(111). This observation indicates that the specific adsorption sites give rise to out-of-plane strain in the TMDC layers. Furthermore, it also leads to effective modulation of the electronic structure of the MoS2 or MoSe2 layer.

Polarity control of h-BN nanoribbon edges by strain and edge termination.

We studied the polarity of h-BN nano-flakes in terms of their edge geometries, edge hydrogen termination, and uniaxial strain by evaluating their electrostatic potential using density functional theory. Our calculations have shown that the polarity of the nanoribbons is sensitive to their edge shape, edge termination, and uniaxial tensile strain. Polarity inversion of the ribbons can be induced by controlling the hydrogen concentration at the edges and the uniaxial tensile strain. The polarity inversion indicates that h-BN nanoribbons can exhibit non-polar properties at a particular edge hydrogen concentration and tensile strain, even though the nanoribbons essentially have polarity at the edge. We also found that the edge angle affects the polarity of nanoribbons with hydrogenated edges.

Na-ion diffusion in a NASICON-type solid electrolyte: a density functional study.

Based on density functional theory, we have systematically studied the crystal and electronic structures, and the diffusion mechanism of the NASICON-type solid electrolyte Na3Zr2Si2PO12. Four possible elementary processes are addressed: three inner-chain and one inter-chain processes. In inner-chain processes, Na tends to move inside the Na diffusion chain, while Na moves across the Na diffusion chain in the inter-chain process. The activation energies for the inner-chain and inter-chain processes are 230 meV and 260 meV, respectively. By combining possible elementary processes, three preferable pathways along a, b, and c directions are found.

Tuning localized transverse surface plasmon resonance in electricity-selected single-wall carbon nanotubes by electrochemical doping.

Localized surface-plasmon resonance affects the optical absorption and scattering of nanosized materials. The intensities and peak energies of the surface plasmons strongly depend on the carrier density; thus, the optical absorption peaks originating from the surface-plasmon resonance can be manipulated by the density of injected carriers. In single-wall carbon nanotubes (SWCNTs), the correct identification of surface-plasmon resonance modes is of great interest due to their emerging plasmonic and optoelectronic applications. Here, we demonstrate that high-carrier injection by electric double layers can induce a transverse surface-plasmon peak in aggregated, electricity-selected SWCNTs. In contrast to the well-discussed surface-plasmon resonance mode, whose polarization is parallel to the axis and whose resonance frequency is located in the THz region, our identified mode, which was normal to the axis, was located in the near-infrared range. In addition, our mode's peak position and intensities were tunable by carrier injections, indicating a route to control plasmonic optical processes by electric double-layer carrier injections using ionic liquid.

Hybrid functional study of the NASICON-type Na3V2(PO4)3: crystal and electronic structures, and polaron-Na vacancy complex diffusion.

The crystal and electronic structures, electrochemical properties and diffusion mechanism of NASICON-type Na3V2(PO4)3 have been investigated based on the hybrid density functional Heyd-Scuseria-Ernzerhof (HSE06). A polaron-Na vacancy complex model for revealing the diffusion mechanism is proposed for the first time in the field of Na-ion batteries. The bound polaron is found to favorably form at the first nearest V site to the Na vacancy. Consequently, the movement of the Na vacancy will be accompanied by the polaron. Three preferable diffusion pathways are revealed; these are two intra-layer diffusion pathways and one inter-layer pathway. The activation barriers for the intra-layer and inter-layer pathways are 353 meV and 513 meV, respectively. For further comparison, the generalized gradient approximation with an onsite Coulomb Hubbard U (GGA+U) is also employed.

Nanoporous carbon tubes from fullerene crystals as the π-electron carbon source.

Here we report the thermal conversion of one-dimensional (1D) fullerene (C60) single-crystal nanorods and nanotubes to nanoporous carbon materials with retention of the initial 1D morphology. The 1D C60 crystals are heated directly at very high temperature (up to 2000 °C) in vacuum, yielding a new family of nanoporous carbons having π-electron conjugation within the sp(2)-carbon robust frameworks. These new nanoporous carbon materials show excellent electrochemical capacitance and superior sensing properties for aromatic compounds compared to commercial activated carbons.

Superatomic Layer of Cubic Mo4S4 Clusters Connected by Cl Cross-Linking.

Superatomic clusters - assemblies of atoms with various sizes, shapes, and compositions - can form hierarchical architectures that exhibit emergent electronic properties not found in their individual units. In particular, cubic M4X4 clusters of chalcogenides (M = transition metal; X = chalcogen) are recognized as versatile building blocks for 3D structures with tunable morphologies and electronic properties. However, tetrahedral M4X4 clusters rarely assemble into 2D architectures, which could offer a distinct class of functional materials from their 3D analogues. Here, this work reports the preparation of 2D Mo8S8Cl11, a superatomic layer with a sandwich structure consisting of Mo4S4 clusters interconnected through Cl cross-linking. The vapor-phase reaction inside nanotubes promotes the selective growth of Mo8S8Cl11 nanoribbons, allowing detailed characterization via transmission electron microscopy. This methodology can be applied to the growth of layered structures containing Mo8S8Cl11 at the micrometer scale. This work has demonstrated that mono- and few-layer Mo8S8Cl11 can be prepared by exfoliation of parent solids. Electronic structure calculations indicate that the 2D monolayer has quasi-flat bands, giving rise to an indirect-to-direct bandgap transition under mechanical strain. Furthermore, scanning electrochemical microscopy reveals the potential of the layered structures as highly efficient catalysts for the hydrogen-evolution reaction.

Nanoscrolls of Janus Monolayer Transition Metal Dichalcogenides.

Tubular structures of transition metal dichalcogenides (TMDCs) have attracted attention in recent years due to their emergent physical properties, such as the giant bulk photovoltaic effect and chirality-dependent superconductivity. To understand and control these properties, it is highly desirable to develop a sophisticated method to fabricate TMDC tubular structures with smaller diameters and a more uniform crystalline orientation. For this purpose, the rolling up of TMDC monolayers into nanoscrolls is an attractive approach to fabricating such a tubular structure. However, the symmetric atomic arrangement of a monolayer TMDC generally makes its tubular structure energetically unstable due to considerable lattice strain in curved monolayers. Here, we report the fabrication of narrow nanoscrolls by using Janus TMDC monolayers, which have an out-of-plane asymmetric structure. Janus WSSe and MoSSe monolayers were prepared by the plasma-assisted surface atom substitution of WSe2 and MoSe2 monolayers, respectively, and then were rolled by solution treatment. The multilayer tubular structures of Janus nanoscrolls were revealed by scanning transmission electron microscopy observations. Atomic resolution elemental analysis confirmed that the Janus monolayers were rolled up with the Se-side surface on the outside. We found that the present nanoscrolls have the smallest diameter of about 5 nm, which is almost the same as the value predicted by the DFT calculation. The difference in work functions between the S- and Se-side surfaces was measured by Kelvin probe force microscopy, which is in good agreement with the theoretical prediction. Strong interlayer interactions and anisotropic optical responses of the Janus nanoscrolls were also revealed by Raman and photoluminescence spectroscopy.

Charge manipulation in molecules encapsulated inside single-wall carbon nanotubes.

We report clear experimental evidence for the charge manipulation of molecules encapsulated inside single-wall carbon nanotubes (SWCNTs) using electrochemical doping techniques. We encapsulated ß-carotene (Car) inside SWCNTs and clarified electrochemical doping characteristics of their Raman spectra. C=C streching modes of encapsulated Car and a G band of SWCNTs showed clearly different doping behaviors as the electrochemical potentials were shifted. Electron extraction from encapsulated Car was clearly achieved. However, electrochemical characteristics of Car inside SWCNTs and doping mechanisms elucidated by calculations based on density-functional theory indicate the difficulty of charge manipulation of molecules inside SWCNTs due to the presence of strong on-site Coulomb repulsion energy at the molecules.

Experimental verification of SO2 and S desorption contributing to defect formation in MoS2 by thermal desorption spectroscopy.

The defect-free surface of MoS2 is of high importance for applications in electronic devices. Theoretical calculations have predicted that oxidative etching could be responsible for sulfur vacancy formation. No direct experimental evidence, however, points out the role of adsorbed oxygen on sulfur vacancy formation for MoS2, especially on an insulating SiO2/Si substrate. Herein, by applying thermal desorption spectroscopy, we found that sulfur loss can be tightly coupled to adsorbed oxygen, as confirmed by observation of SO2 desorption. With annealing MoS2, even under ultrahigh vacuum, oxygen molecules adsorbed on MoS2 assist the sulfur atom in dissociating from MoS2, and thus, defects are formed as the result of SO2 desorption from 200 °C to 600 °C. At higher temperatures (over 800 °C), on the other hand, direct sulfur desorption becomes dominant. This finding can be well explained by combining the morphology investigation enabled by atomic layer deposition at defective sites and optical transitions observed by photoluminescence measurements. Moreover, a preannealing treatment prior to exfoliation was found to be an effective method to remove the adsorbed oxygen, thus preventing defect formation.

Observation of the photovoltaic effect in a van der Waals heterostructure.

van der Waals (vdW) heterostructures, which can be assembled with various two-dimensional materials, provide a versatile platform for exploring emergent phenomena. Here, we report an observation of the photovoltaic effect in a WS2/MoS2 vdW heterostructure. Light excitation of WS2/MoS2 at a wavelength of 633 nm yields a photocurrent without applying bias voltages, and the excitation power dependence of the photocurrent shows characteristic crossover from a linear to square root dependence. Photocurrent mapping has clearly shown that the observed photovoltaic effect arises from the WS2/MoS2 region, not from Schottky junctions at electrode contacts. Kelvin probe microscopy observations show no slope in the electrostatic potential, excluding the possibility that the photocurrent originates from an unintentionally formed built-in potential.