Symmetry Dictated Grain Boundary State in a Two-Dimensional Topological Insulator.

Grain boundaries (GBs) are ubiquitous in solids and have been of central importance in understanding the nature of polycrystals. In addition to their classical roles, topological insulators (TIs) offer a chance to realize GBs hosting distinct topological states that can be controlled by their crystal symmetries. However, such roles of crystalline symmetry in two-dimensional (2D) TIs have not been definitively measured yet. Here, we present the first direct evidence of a symmetry-enforced metallic state along a GB in 1T'-MoTe2, a prototypical 2D TI. Using scanning tunneling microscopy, we show a metallic state along a GB with nonsymmorphic lattice symmetry and its absence along another boundary with symmorphic symmetry. Our atomistic simulations demonstrate in-gap Weyl semimetallic states for the former, whereas they demonstrate gapped states for the latter, explaining our observation well. The observed metallic state, tightly linked to its crystal symmetry, can be used to create a stable conducting nanowire inside TIs.

Two-dimensional Dirac fermions on oxidized black phosphorus.

We explore the oxidation of a single layer of black phosphorus using ab initio density functional theory calculations. We search for the equilibrium structures of phosphorene oxides, POx with various oxygen concentrations x (0 ≤ x ≤ 1). By evaluating the formation energies with diverse configurations and their vibrational properties for each of various x values, we identify a series of stable oxidized structures with x and confirm that the oxidation occurs naturally. We also find that oxidation makes some modes from the P-O bonds and P-P bonds IR-active implying that the infrared spectra can be used to determine the degree of oxidation of phosphorene. Our electronic structure calculations reveal that the fully oxidized phosphorene (PO) has a direct band gap of 0.83 eV similar to the pristine phosphorene. Intriguingly, the PO possesses two nonsymmorphic symmetries with the inversion symmetry broken, guaranteeing symmetry-protected band structures including the band degeneracy and four-fold degenerate Dirac points. Our results provide an important guide in the search for the rare example of a Dirac semimetal with a higher level of degeneracy, giving significant insight into the relations between the symmetry of the lattice and band topology of electrons.

A "non-dynamical" way of describing room-temperature paramagnetic manganese oxide.

We present a new approach based on static density functional theory (DFT) to describe paramagnetic manganese oxides, representative paramagnetic Mott insulators. We appended spin noncollinearity and a canonical ensemble to the magnetic sampling method (MSM), which is one of the supercell approaches based on the disordered local moment model. The combination of the noncollinear MSM (NCMSM) with DFT+U represents a highly favorable computational method called NCMSM+U to accurately determine the paramagnetic properties of MnO with moderate numerical cost. The effects of electron correlations and spin noncollinearity on the properties of MnO were also investigated. We found that the spin noncollinearity plays an important role in determining the detailed electronic profile and precise energetics of paramagnetic MnO. Our results illustrate that the NCMSM+U approach may be used for insulating materials as an alternative to the ab initio framework of dynamic mean field theory based on DFT in the simulation of the room-temperature paramagnetic properties.

Enhanced Thermoelectric Properties in a New Silicon Crystal Si24 with Intrinsic Nanoscale Porous Structure.

Thermoelectric device is a promising next-generation energy solution owing to its capability to transform waste heat into useful electric energy, which can be realized in materials with high electric conductivities and low thermal conductivities. A recently synthesized silicon allotrope of Si24 features highly anisotropic crystal structure with nanometer-sized regular pores. Here, based on first-principles study without any empirical parameter we show that the slightly doped Si24 can provide an order-of-magnitude enhanced thermoelectric figure of merit at room temperature, compared with the cubic diamond phase of silicon. We ascribe the enhancement to the intrinsic nanostructure formed by the nanopore array, which effectively hinders heat conduction while electric conductivity is maintained. This can be a viable option to enhance the thermoelectric figure of merit without further forming an extrinsic nanostructure. In addition, we propose a practical strategy to further diminish the thermal conductivity without affecting electric conductivity by confining rattling guest atoms in the pores.

Is hexagonal boron nitride always good as a substrate for carbon nanotube-based devices?

Hexagonal boron nitride sheets have been noted especially for their enhanced properties as substrates for sp(2) carbon-based nanodevices. To evaluate whether such enhanced properties would be retained under various realistic conditions, we investigate the structural and electronic properties of semiconducting carbon nanotubes on perfect and defective hexagonal boron nitride sheets under an external electric field as well as with a metal impurity, using density functional theory. We verify that the use of a perfect hexagonal boron nitride sheet as a substrate indeed improves the device performances of carbon nanotubes, compared with the use of conventional substrates such as SiO2. We further show that even the hexagonal boron nitride with some defects can show better performance as a substrate. Our calculations, on the other hand, also suggest that some defective boron nitride layers with a monovacancy and a nickel impurity could bring about poor device behavior since the imperfections impair electrical conductivity due to residual scattering under an applied electric field.

Distance-Dependent Evolution of Electronic States in Kagome-Honeycomb Lateral Heterostructures in FeSn.

In this work, we demonstrate the formation and electronic influence of lateral heterointerfaces in FeSn containing Kagome and honeycomb layers. Lateral heterostructures offer spatially resolved property control, enabling the integration of dissimilar materials and promoting phenomena not typically observed in vertical heterostructures. Using the molecular beam epitaxy technique, we achieve a controllable synthesis of lateral heterostructures in the Kagome metal FeSn. With scanning tunneling microscopy/spectroscopy in conjunction with first-principles calculations, we provide a comprehensive understanding of the bonding motif connecting the Fe3Sn-terminated Kagome and Sn2-terminated honeycomb surfaces. More importantly, we reveal a distance-dependent evolution of the electronic states in the vicinity of the heterointerfaces. This evolution is significantly influenced by the orbital character of the flat bands. Our findings suggest an approach to modulate the electronic properties of the Kagome lattice, which should be beneficial for the development of future quantum devices.

Low Ohmic contact resistance and high on/off ratio in transition metal dichalcogenides field-effect transistors via residue-free transfer.

Beyond-silicon technology demands ultrahigh performance field-effect transistors. Transition metal dichalcogenides provide an ideal material platform, but the device performances such as the contact resistance, on/off ratio and mobility are often limited by the presence of interfacial residues caused by transfer procedures. Here, we show an ideal residue-free transfer approach using polypropylene carbonate with a negligible residue coverage of ~0.08% for monolayer MoS2 at the centimetre scale. By incorporating a bismuth semimetal contact with an atomically clean monolayer MoS2 field-effect transistor on hexagonal boron nitride substrate, we obtain an ultralow Ohmic contact resistance of ~78 Ω µm, approaching the quantum limit, and a record-high on/off ratio of ~1011 at 15 K. Such an ultra-clean fabrication approach could be the ideal platform for high-performance electrical devices using large-area semiconducting transition metal dichalcogenides.

Interplay between Topological States and Rashba States as Manifested on Surface Steps at Room Temperature.

The unique spin texture of quantum states in topological materials underpins many proposed spintronic applications. However, realizations of such great potential are stymied by perturbations, such as temperature and local fields imposed by impurities and defects, that can render a promising quantum state uncontrollable. Here, we report room-temperature scanning tunneling microscopy/spectroscopy observation of interaction between Rashba states and topological surface states, which manifests local electronic structure along step edges controllable by the layer thickness of thin films. The first-principles theoretical calculation elucidates the robust Rashba states coexisting with topological surface states along the surface steps with characteristic spin textures in momentum space. Furthermore, the Rashba edge states can be switched off by reducing the thickness of a topological insulator Bi2Se3 to bolster their interaction with the hybridized topological surface states. The study unveils a manipulating mechanism of the spin textures at room temperature, reinforcing the necessity of thin film technology in controlling the quantum states.

Stacking Faults and Topological Properties in MnBi2Te4: Reconciling Gapped and Gapless States.

Despite theoretical predictions of a gapped surface state for the magnetic topological insulator MnBi2Te4 (MBT), there has been a series of experimental evidence pointing toward gapless states. Here, we theoretically explore how stacking faults could influence the topological characteristics of MBT. We envisage a scenario that a stacking fault exists at the surface of MBT, causing the uppermost layer to deviate from the ground state and its interlayer separation to be expanded. This stacking fault with modulated interlayer couplings hosts a nearly gapless state within the topmost layer due to charge redistribution as the outermost layer recedes. Furthermore, we find evidence of spin-momentum locking and preservation of weak band inversion in the gapless surface state, suggesting the nontrivial topological surface states in the presence of the stacking fault. Our findings provide a plausible elucidation to the long-standing conundrum of reconciling the observation of gapped and gapless states on MBT surfaces.

Theoretical Investigation of Delafossite-Cu2ZnSnO4 as a Promising Photovoltaic Absorber.

In the quest for efficient and cost-effective photovoltaic absorber materials beyond silicon, considerable attention has been directed toward exploring alternatives. One such material, zincblende-derived Cu2ZnSnS4 (CZTS), has shown promise due to its ideal band gap size and high absorption coefficient. However, challenges such as structural defects and secondary phase formation have hindered its development. In this study, we examine the potential of another compound, Cu2ZnSnO4 (CZTO), with a similar composition to CZTS as a promising alternative. Employing ab initio density function theory (DFT) calculations in combination with an evolutionary structure prediction algorithm, we identify that the crystalline phase of delafossite structure is the most stable among the 900 (meta)stable CZTO. Its thermodynamic stability at room temperature is also confirmed by the molecular dynamics study. Excitingly, this new phase of CZTO displays a direct band gap where the dipole-allowed transition occurs, making it a strong candidate for efficient light absorptions. Furthermore, the estimation of spectroscopic limited maximum efficiency (SLME) directly demonstrates the high potential of delafossite-CZTO as a photovoltaic absorber. Our numerical results suggest that delafossite-CZTO holds promise for future photovoltaic applications.

Oxidation-enhanced thermoelectric efficiency in a two-dimensional phosphorene oxide.

We performed density functional theory calculations to investigate the thermoelectric properties of phosphorene oxide (PO) expected to form by spontaneous oxidation of phosphorene. Since thermoelectric features by nature arise from the consequences of the electron-phonon interaction, we computed the phonon-mediated electron relaxation time, which was fed into the semiclassical Boltzmann transport equation to be solved for various thermoelectric-related quantities. It was found that PO exhibits superior thermoelectric performance compared with its pristine counterpart, which has been proposed to be a candidate for the use of future thermoelectric applications. We revealed that spontaneous oxidation of phosphorene leads to a significant enhancement in the thermoelectric properties of n-doped phosphorene oxide, which is attributed to the considerable reduction of lattice thermal conductivity albeit a small decrease in electrical conductivity. Our results suggest that controlling oxidation may be utilized to improve thermoelectric performance in nanostructures, and PO can be a promising candidate for low-dimensional thermoelectric devices.

Low Lattice Thermal Conductivity of a Two-Dimensional Phosphorene Oxide.

A fundamental understanding of the phonon transport mechanism is important for optimizing the efficiency of thermoelectric devices. In this study, we investigate the thermal transport properties of the oxidized form of phosphorene called phosphorene oxide (PO) by solving phonon Boltzmann transport equation based on first-principles density functional theory. We reveal that PO exhibits a much lower thermal conductivity (2.42-7.08 W/mK at 300 K) than its pristine counterpart as well as other two-dimensional materials. To comprehend the physical origin of such low thermal conductivity, we scrutinize the contribution of each phonon branch to the thermal conductivity by evaluating various mode-dependent quantities including Grüneisen parameters, anharmonic three-phonon scattering rate, and phase space of three-phonon scattering processes. Our results show that its flexible puckered structure of PO leads to smaller sound velocities; its broken-mirror symmetry allows more ZA phonon scattering; and the relatively-free vibration of dangling oxygen atoms in PO gives rise to additional scattering resulting in further reduction in the phonon lifetime. These results can be verified by the fact that PO has larger phase space for three-phonon processes than phosphorene. Furthermore we show that the thermal conductivity of PO can be optimized by controlling its size or its phonon mean free path, indicating that PO can be a promising candidate for low-dimensional thermoelectric devices.

Closing the Surface Bandgap in Thin Bi2Se3/Graphene Heterostructures.

Topological insulator (TI), a band insulator with topologically protected edge states, is one of the most interesting materials in the field of condensed matter. Bismuth selenide (Bi2Se3) is the most spotlighted three-dimensional TI material; it has a Dirac cone at each top and bottom surface and a relatively wide bandgap. For application, suppression of the bulk effect is crucial, but in ultrathin TI materials, with thicknesses less than 3 QL, the finite size effect works on the linear dispersion of the surface states, so that the surface band has a finite bandgap because of the hybridization between the top and bottom surface states and Rashba splitting, resulting from the structure inversion asymmetry. Here, we studied the gapless top surface Dirac state of strained 3 QL Bi2Se3/graphene heterostructures. A strain caused by the graphene layer reduces the bandgap of surface states, and the band bending resulting from the charge transfer at the Bi2Se3-graphene interface induces localization of surface states to each top and bottom layer to suppress the overlap of the two surface states. In addition, we verified the independent transport channel of the top surface Dirac state in Bi2Se3/graphene heterostructures by measuring the magneto-conductance. Our findings suggest that the strain and the proximity effect in TI/non-TI heterostructures may be feasible ways to engineer the topological surface states beyond the physical and topological thickness limit.

Latent Order in High-Angle Grain Boundary of GaN.

We report the existence of latent order during core relaxation in the high-angle grain boundaries (GBs) of GaN films using atomic-resolution scanning transmission electron microscopy and ab initio density functional theory calculations. Core structures in the high-angle GBs are characterized by two pairs of Ga-N bonds located next to each other. The core type correlates strongly with the bond angle differences. We identify an order of core relaxation hidden in the high-angle GBs by further classifying the 5/7 atom cores into a stable 5/7 core (5/7(S)) and a metastable 5/7 core (5/7(M)). This core-type classification indicates that metastable cores can exist at real high-angle GBs under certain circumstances. Interestingly, 5/7(M) exhibits distinct defect states compared to 5/7(S), despite their similar atomic configurations. We investigate the reconstruction of defect states observed in 5/7(M) by analyzing the real-space wave functions. An inversion occurred between two localized states during the transition from 5/7(S) to 5/7(M). We suggest an inversion mechanism to explain the formation of new defect states in 5/7(M).

Extremely high electrical conductance of microporous 3D graphene-like zeolite-templated carbon framework.

We report the remarkably high electrical conductance of microporous 3D graphene-like carbons that were formed using lanthanum (La)-catalyzed synthesis in a Y zeolite (LaY) template investigated using conductive atomic force microscopy (C-AFM) and theoretical calculations. To uncover the relation between local electrical conductance and the microporous structures, we tuned the crystallographic ordering of LaY-templated carbon systems by changing the heating temperature. The structure of the LaY-templated carbon prepared at the higher temperature has graphene-like sp 2 hybridized bonds, which was confirmed using high-resolution transmission electron microscopy and X-ray diffraction measurements. C-AFM current-voltage spectroscopy revealed that the local current flow in the LaY-templated carbon depends on the quantity of C-C bonds within the narrow neck between the closed supercages (i.e. there are three types of carbon: carbon with heat treatment, carbon without heat treatment, and carbon synthesized at low temperature). The difference in electrical conductance on the LaY-templated carbon was also confirmed via theoretical computation using the Boltzmann transport theory and the deformation potential theory based on the density functional theory. These results suggest that the degree of order of the pores in the 3D zeolite-templated carbon structures is directly related to electrical conductance.

The determining factor of a preferred orientation of GaN domains grown on m-plane sapphire substrates.

Epitaxial lateral overgrowth in tandem with the first-principles calculation was employed to investigate the determining factor of a preferred orientation of GaN on SiO2-patterned m-plane sapphire substrates. We found that the (1100)-orientation is favored over the (1103)-orientation in the region with a small filling factor of SiO2, while the latter orientation becomes preferred in the region with a large filling factor. This result suggests that the effective concentration determines the preferred orientation of GaN: the (1100)- and (1103)-orientations preferred at their low and high concentrations, respectively. Our computational study revealed that at a low coverage of Ga and N atoms, the local atomic arrangement resembles that on the (1103) surface, although the (1100) surface is more stable at their full coverage. Such a (1103)-like atomic configuration crosses over to the local structure resembling that on the (1100) surface as the coverage increases. Based on results, we determined that high effective concentration of Ga and N sources expedites the growth of the (1103)-orientation while keeping from transition to the (1100)-orientation. At low effective concentration, on the other hand, there is a sufficient time for the added Ga and N sources to rearrange the initial (1103)-like orientation to form the (1100)-orientation.

Liquid metal nanodroplet dynamics inside nanocontainers.

Here we report direct observations of spatial movements of nanodroplets of Pb metal trapped inside sealed carbon nanocontainers. We find drastic changes in the mobility of the liquid droplets as the particle size increases from a few to a few ten nanometers. In open containers the droplet becomes immobile and readily evaporates to the vacuum environment. The particle mobility strongly depends on confinement, particle size, and wetting on the enclosed surface. The collisions between droplets increase mobility but the tendency is reversed if collisions lead to droplet coalescence. The dynamics of confined nanodroplets could provide new insights into the activity of nanostructures in spatially constrained geometries.

Adsorption properties of chalcogen atoms on a golden buckyball Au16(-) from first principles.

Using first-principles density functional theory, we investigate the adsorption properties of chalcogen elements (oxygen and sulfur) on an anionic golden nanocage Au(16)(-) and its effects on the structural and electronic properties of the golden cage. In particular, we find that when a sulfur atom is encapsulated inside Au(16)(-), its bonding character with Au atoms appears ionic due to electron transfer from sulfur to the gold nanocage. In contrast, the exohedrally adsorbed S atom tends to have strong orbital hybridization with the golden nanocage. For an oxygen adsorption case, electrons from the golden cage tend to be shared with the adsorbed O atom exhibiting strong orbital hybridization, regardless of its adsorption sites. To investigate the transition behaviors between the most stable exohedral and endohedral adsorption configurations, we calculate the activation and reaction energies in the transition. The oxygen atom experiences a lower energy barrier than the sulfur atom due to its smaller atomic radius. Finally, we explore the vibrational properties of S- or O-adsorbed Au(16)(-) buckyballs by calculating their infrared spectra.

Interplay between structural and electronic properties of bundled Mo6S(9-x)I(x) nanowires.

Using first principles density functional theory, we investigate the structural, electronic and magnetic properties of isolated and bundled Mo(6)S(9 - x)I(x) nanowires with x = 3, 4.5, and 6. The unit cell of each system contains two Mo(6) octahedra decorated with S and I atoms and two S(3) linkages. Due to the bistability of each sulfur linkage, finite-length nanowires or nanowire bundles exhibit many stable structural minima. We explore the structural stability, elastic behavior and electronic structure at all these minima for each composition x. We find that the axial strain and inter-wire interaction in bundles significantly modify the electronic structure. The most intriguing changes occur in nanowires with x = 4.5 and 6, which change from metal to semiconductor or undergo a magnetic transition upon axially stretching or compressing the nanowires or upon changing the inter-wire separation.