Quantum Confined Tomonaga-Luttinger Liquid in Mo6Se6 Nanowires Converted from an Epitaxial MoSe2 Monolayer.

Confining interacting particles in one-dimension (1D) changes the electronic behavior of the system fundamentally, which has been studied extensively in the past. Examples of 1D metallic systems include carbon nanotubes, quasi-1D organic conductors, metal chains, and domain boundary defects in monolayer thick transition-metal dichalcogenides such as MoSe2. Here single and bundles of Mo6Se6 nanowires were fabricated through annealing a MoSe2 monolayer grown by molecular-beam epitaxy on graphene. Conversion from two-dimensional (2D) MoSe2 film to 1D Mo6Se6 nanowire is reversible. Mo6Se6 nanowires form preferentially at the Se-terminated zigzag edges of MoSe2 and stitch to it via two distinct atomic configurations. The Mo6Se6 wire is metallic and its length is tunable, which represents one of few 1D systems that exhibit properties pertinent to quantum confined Tomonaga-Luttinger liquid, as evidenced by scanning tunneling microscopic and spectroscopic studies.

Inversion Domain Boundary Induced Stacking and Bandstructure Diversity in Bilayer MoSe2.

Interlayer rotation and stacking were recently demonstrated as effective strategies for tuning physical properties of various two-dimensional materials. The latter strategy was mostly realized in heterostructures with continuously varied stacking orders, which obscure the revelation of the intrinsic role of a certain stacking order in its physical properties. Here, we introduce inversion-domain-boundaries into molecular-beam-epitaxy grown MoSe2 homobilayers, which induce uncommon fractional lattice translations to their surrounding domains, accounting for the observed diversity of large-area and uniform stacking sequences. Low-symmetry stacking orders were observed using scanning transmission electron microscopy and detailed geometries were identified by density functional theory. A linear relation was also revealed between interlayer distance and stacking energy. These stacking sequences yield various energy alignments between the valence states at the Γ and K points of the Brillouin zone, showing stacking-dependent bandgaps and valence band tail states in the measured scanning tunneling spectroscopy. These results may benefit the design of two-dimensional multilayers with manipulable stacking orders.

Multivalency-Driven Formation of Te-Based Monolayer Materials: A Combined First-Principles and Experimental study.

Contemporary science is witnessing a rapid expansion of the two-dimensional (2D) materials family, each member possessing intriguing emergent properties of fundamental and practical importance. Using the particle-swarm optimization method in combination with first-principles density functional theory calculations, here we predict a new category of 2D monolayers named tellurene, composed of the metalloid element Te, with stable 1T-MoS_{2}-like (α-Te), and metastable tetragonal (ß-Te) and 2H-MoS_{2}-like (γ-Te) structures. The underlying formation mechanism is inherently rooted in the multivalent nature of Te, with the central-layer Te behaving more metal-like (e.g., Mo), and the two outer layers more semiconductorlike (e.g., S). We also show that the α-Te phase can be spontaneously obtained from the magic thicknesses divisible by three layers truncated along the [001] direction of the trigonal structure of bulk Te, and both the α- and ß-Te phases possess electron and hole mobilities much higher than MoS_{2}. Furthermore, we present preliminary but convincing experimental evidence for the layering behavior of Te on HOPG substrates, and predict the importance of multivalency in the layering behavior of Se. These findings effectively extend the realm of 2D materials to group-VI elements.

Graphene-oxide-wrapped ZnMn2O4 as a high performance lithium-ion battery anode.

Cation distribution between tetrahedral and octahedral sites within the ZnMn2O4 spinel lattice, along with microstructural features, is affected greatly by the temperature of heat treatment. Inversion parameters can easily be tuned, from 5%-19%, depending on the annealing temperature. The upper limit of inversion is found for T = 400 °C as confirmed by x-ray powder diffraction and Raman spectroscopy. Excellent battery behavior is found for samples annealed at lower temperatures; after 500 cycles the specific capacity for as-prepared ZnMn2O4 is 909 mAh g-1, while ZnMn2O4 heat-treated at 300 °C is 1179 mAh g-1, which amounts to 101% of its initial capacity. Despite the excellent performance of a sample processed at 300 °C at lower charge/discharge rates (100 mAh g-1), a drop in the specific capacity is observed with rate increase. This issue is solved by graphene-oxide wrapping: the specific capacity obtained after the 400th cycle for graphene-oxide-wrapped ZnMn2O4 heat-treated at 300 °C is 799 mAh g-1 at a charge/discharge rate 0.5 A g-1, which is higher by a factor of 6 compared to samples without graphene -oxide wrapping.

Nanoclusters of CaSe in calcium-doped Bi2Se3 grown by molecular-beam epitaxy.

In calcium (Ca) doped Bi2Se3 films grown by molecular beam epitaxy, nanoclusters of CaSe are revealed by high-angle annular dark field imaging and energy dispersive x-ray spectroscopy analysis using a scanning transmission electron microscope. As the interface between the ordinary insulator CaSe and topological insulator, Bi2Se3, can host topological nontrivial interface state, this represents an interesting material system for further studies. We show by first principles total energy calculations that aggregation of Ca atoms in Bi2Se3 is driven by energy minimization and a preferential intercalation of Ca in the van der Waals gap between quintuple layers of Bi2Se3 induces reordering of atomic stacking and causes an increasing amount of stacking faults in film. The above findings also provide an explanation of less-than-expected electrical carrier (hole) concentrations in Ca-doped samples.

In situ synthesis of TiO2(B) nanotube/nanoparticle composite anode materials for lithium ion batteries.

Titania nanotubes were prepared by a simple hydrothermal route. Their electrochemical performance has been examined in detail and compared to TiO2(B) nanoparticles, TiO2 anatase and P25 titania nanoparticles. The cycling and rate performance of TiO2 nanotubes is superior to both types of nanoparticles, and it can be further improved by an in situ titanium precursor treatment, which results in the formation of TiO2 nanoparticles on/between the nanotubes. The obtained specific capacity after 200 cycles at 0.2 A g(-1) charge/discharge rate remained above 130 mAh g(-1). The enhanced lithium storage properties of these samples can be attributed to their unique morphology and crystal structure.

Crossover from 3D to 2D quantum transport in Bi2Se3/In2Se3 superlattices.

The topological insulator/normal insulator (TI/NI) superlattices (SLs) with multiple Dirac channels are predicted to offer great opportunity to design novel materials and investigate new quantum phenomena. Here, we report first transport studies on the SLs composed of TI Bi2Se3 layers sandwiched by NI In2Se3 layers artificially grown by molecular beam epitaxy (MBE). The transport properties of two kinds of SL samples show convincing evidence that the transport dimensionality changes from three-dimensional (3D) to two-dimensional (2D) when decreasing the thickness of building block Bi2Se3 layers, corresponding to the crossover from coherent TI transport to separated TI channels. Our findings provide the possibility to realizing "3D surface states" in TI/NI SLs.

Dense network of one-dimensional midgap metallic modes in monolayer MoSe2 and their spatial undulations.

We report the observation of a dense triangular network of one-dimensional (1D) metallic modes in a continuous and uniform monolayer of MoSe(2) grown by molecular-beam epitaxy. High-resolution transmission electron microscopy and scanning tunneling microscopy and spectroscopy studies show that these 1D modes are midgap states at inversion domain boundaries. Scanning tunneling microscopy and spectroscopy measurements further reveal intensity undulations of the metallic modes, presumably arising from the superlattice potentials due to the moiré pattern and the quantum confinement effect. A dense network of the metallic modes with a high density of states is of great potential for heterocatalysis applications. The interconnection of such midgap 1D conducting channels may also imply new transport behaviors distinct from the 2D bulk.

Atomistic Insight into the Epitaxial Growth Mechanism of Single-Crystal Two-Dimensional Transition-Metal Dichalcogenides on Au(111) Substrate.

A mechanistic understanding of interactions between atomically thin two-dimensional (2D) transition-metal dichalcogenides (TMDs) and their growth substrates is important for achieving the unidirectional alignment of nuclei and seamless stitching of 2D TMD domains and thus 2D wafers. In this work, we conduct a cross-sectional scanning transmission electron microscopy (STEM) study to investigate the atomic-scale nucleation and early stage growth behaviors of chemical vapor deposited monolayer (ML-) MoS2 and molecular beam epitaxy ML-MoSe2 on a Au(111) substrate. Statistical analysis reveals the majority of as-grown domains, i.e., ∼88% for MoS2 and 90% for MoSe2, nucleate on surface terraces, with the rest (i.e., ∼12% for MoS2 and 10% for MoSe2) on surface steps. Moreover, within the latter case, step-associated nucleation, ∼64% of them are terminated with a Mo-zigzag edge in connection with the Au surface steps, with the rest (∼36%) being S-zigzag edges. In conjunction with ab initio density functional theory calculations, the results confirm that van der Waals epitaxy, rather than the surface step guided epitaxy, plays deterministic roles for the realization of unidirectional ML-MoS2 (MoSe2) domains on a Au(111) substrate. In contrast, surface steps, particularly their step height, are mainly responsible for the integrity and thickness of MoS2/MoSe2 films. In detail, it is found that the lateral growth of monolayer thick MoS2/MoSe2 domains only proceeds across mono-Au-atom high surface steps (∼2.4 Å), but fail for higher ones (bi-Au atom step and higher) during the growth. Our cross-sectional STEM study also confirms the existence of considerable compressive residual strain that reaches ∼3.0% for ML-MoS2/MoSe2 domains on Au(111). The present study aims to understand the growth mechanism of 2D TMD wafers.

Charge Density Modulation and the Luttinger Liquid State in MoSe2 Mirror Twin Boundaries.

A mirror twin-domain boundary (MTB) in monolayer MoSe2 represents a (quasi) one-dimensional metallic system. Its electronic properties, particularly the low-energy excitations in the so-called 4|4P-type MTB, have drawn considerable research attention. Reports of quantum well states, charge density waves, and the Tomonaga-Luttinger liquid (TLL) have all been made. Here, by controlling the lengths of the MTBs and employing different substrates, we reveal by low-temperature scanning tunneling microscopy/spectroscopy, Friedel oscillations and quantum confinement effects causing the charge density modulations along the defect. The results are inconsistent with charge density waves. Interestingly, for graphene-supported samples, TLL in the MTBs is suggested, whereas that grown on gold, an ordinary Fermi liquid, is indicated.

Room-temperature ferroelectricity in MoTe2 down to the atomic monolayer limit.

Ferroelectrics allow for a wide range of intriguing applications. However, maintaining ferroelectricity has been hampered by intrinsic depolarization effects. Here, by combining first-principles calculations and experimental studies, we report on the discovery of robust room-temperature out-of-plane ferroelectricity which is realized in the thinnest monolayer MoTe2 with unexploited distorted 1T (d1T) phase. The origin of the ferroelectricity in d1T-MoTe2 results from the spontaneous symmetry breaking due to the relative atomic displacements of Mo atoms and Te atoms. Furthermore, a large ON/OFF resistance ratio is achieved in ferroelectric devices composed of MoTe2-based van der Waals heterostructure. Our work demonstrates that ferroelectricity can exist in two-dimensional layered material down to the atomic monolayer limit, which can result in new functionalities and achieve unexpected applications in atomic-scale electronic devices.

Multifarious Interfaces, Band Alignments, and Formation Asymmetry of WSe2-MoSe2 Heterojunction Grown by Molecular-Beam Epitaxy.

Monolayer (ML) transition-metal dichalcogenides (TMDs) continue to attract research attention, and the heterojunctions formed by vertically stacking or laterally stitching two different TMDs, e.g., MoSe2 and WSe2, may have many interesting electronic and optical properties and thus are at the center stage of current research. Experimentally realizing such heterojunctions with desired interface morphologies and electronic properties is of great demand. In this work, we report a diverse interface structure in molecular-beam epitaxial WSe2-MoSe2 heterojunction. The corresponding electronic bands show type-II band alignment for both monolayer ML-ML and ML-bilayer lateral junctions irrespective of the presence or absence of step states. Interestingly, a strong anisotropy in lateral heterojunction formation is observed, where sharp interfaces are obtained only when WSe2 deposition precedes MoSe2. Reversing the deposition order leads to alloying of the two materials without a notable boundary. This is explained by a step segregation process as suggested by the first-principles total energy calculations.

Ultrathin ß-tellurium layers grown on highly oriented pyrolytic graphite by molecular-beam epitaxy.

Monolayer tellurium (Te) or tellurene has been suggested by a recent theory as a new two-dimensional (2D) system with great electronic and optoelectronic promises. Here we present an experimental study of epitaxial Te deposited on highly oriented pyrolytic graphite (HOPG) by molecular-beam epitaxy. Scanning tunneling microscopy of ultrathin layers of Te reveals rectangular surface cells with the cell size consistent with the theoretically predicted ß-tellurene, whereas for thicker films, the cell size is more consistent with that of the [101[combining macron]0] surface of the bulk Te crystal. Scanning tunneling spectroscopy measurements show that the films are semiconductors with the energy band gaps decreasing with increasing film thickness, and the gap narrowing occurs predominantly at the valence-band maximum (VBM). The latter is understood by strong coupling of states at the VBM but a weak coupling at conduction band minimum (CBM) as revealed by density functional theory calculations.

Quantum Effects and Phase Tuning in Epitaxial Hexagonal and Monoclinic MoTe2 Monolayers.

Monolayer (ML) transition-metal dichalcogenides exist in different phases, such as hexagonal (2H) and monoclinic (1T') structures. They show very different properties: semiconducting for 2H-MoTe2 and semimetallic for 1T'-MoTe2. The formation energy difference between 2H- and 1T'-phase MoTe2 is small, so there is a high chance of tuning the structures of MoTe2 and thereby introducing applications of phase-change electronics. In this paper, we report the growth of both 2H- and 1T'-MoTe2 MLs by molecular-beam epitaxy (MBE) and demonstrate its tenability by changing the conditions of MBE. We attribute the latter to an effect of Te adsorption. By scanning tunneling microscopy and spectroscopy, we reveal not only the atomic structures and intrinsic electronic properties of the two phases of MoTe2 but also quantum confinement and quantum interference effects in the 2H- and 1T'-MoTe2 domains, respectively, as effected by domain boundaries in the samples.

Line and Point Defects in MoSe2 Bilayer Studied by Scanning Tunneling Microscopy and Spectroscopy.

Bilayer (BL) MoSe2 films grown by molecular-beam epitaxy (MBE) are studied by scanning tunneling microscopy and spectroscopy (STM/S). Similar to monolayer (ML) films, networks of inversion domain boundary (DB) defects are observed both in the top and bottom layers of BL MoSe2, and often they are seen spatially correlated such that one is on top of the other. There are also isolated ones in the bottom layer without companion in the top-layer and are detected by STM/S through quantum tunneling of the defect states through the barrier of the MoSe2 ML. Comparing the DB states in BL MoSe2 with that of ML film reveals some common features as well as differences. Quantum confinement of the defect states is indicated. Point defects in BL MoSe2 are also observed by STM/S, where ionization of the donor defect by the tip-induced electric field is evidenced. These results are of great fundamental interests as well as practical relevance of devices made of MoSe2 ultrathin layers.

Observation of intervalley quantum interference in epitaxial monolayer tungsten diselenide.

The extraordinary electronic structures of monolayer transition metal dichalcogenides, such as the spin-valley-coupled band edges, have sparked great interest for potential spintronic and valleytronic applications based on these two-dimensional materials. In this work, we report the experimental observation of quasi-particle interference patterns in monolayer WSe2 using low-temperature scanning tunnelling spectroscopy. We observe intervalley quantum interference involving the Q valleys in the conduction band due to spin-conserving scattering processes, while spin-flipping intervalley scattering is absent. Our results establish unequivocally the presence of spin-valley coupling and affirm the large spin splitting at the Q valleys. Importantly, the inefficient spin-flipping scattering implies long valley and spin lifetime in monolayer WSe2, which is a key figure of merit for valley-spintronic applications.

In situ synthesis of CuxO/SnOx@CNT and CuxO/SnOx@SnO2/CNT nanocomposite anodes for lithium ion batteries by a simple chemical treatment process.

SnO2-based electrodes for lithium ion batteries (LIBs) typically exhibit high initial specific capacity but poor cycling performance. A possible strategy to improve the cycling performance is to prepare nanocomposites containing SnO2. Here we demonstrate a straightforward method to prepare composites containing SnOx and CuxO by a simple chemical treatment of the LIB electrode on copper foil. The in situ formation of a multiphase composite results in a dramatic improvement in the cycling performance, so that specific capacities exceeding 580 and 800 mA·h/g can be obtained after 70 charge/discharge cycles for CuxO/SnOx@CNT and CuxO/SnOx@SnO2/CNT electrodes, respectively (compared to <100 mA·h/g for pure SnO2). The capacity retention achieved at the 70th cycle compared to the 2nd cycle was 96% for the CuxO/SnOx@CNT electrode. The mechanisms responsible for the formation of a composite material and the improvement in the performance are discussed.

Anisotropic topological surface states on high-index Bi2Se3 films.

A high-index topological insulator thin film, Bi2 Se3 (221), is grown on a faceted InP(001) substrate by molecular-beam epitaxy (see model in figure (a)). Angle-resolved photoemission spectroscopy measurement reveals the Dirac cone structure of the surface states on such a surface (figure (b)). The Fermi surface is elliptical (figure (c)), suggesting an anisotropy along different crystallographic directions. Transport studies also reveal a strong anisotropy in Hall conductance.

Diameter control of tungsten oxide nanowires as grown by thermal evaporation.

Tungsten oxide nanowires with controllable diameter were synthesized on Si substrates by thermal evaporation of tungsten trioxide powder in a tube furnace. Depending on the temperature of the source (900-1000 °C), tungsten oxide W(18)O(49) nanowires with diameters ranging from 10 to 100 nm are obtained with high yield. The exponential dependence of the nanowire diameter on the source temperature leads to an energy of about 2.0 eV. The growth process is discussed; it is believed to be a kinetic effect.