Visualizing atoms and discriminating between those of different elements is a goal in many analytical techniques. The use of electron energy-loss spectroscopy (EELS) in such single-atom analyses is hampered by an inherent difficulty related to the damage caused to specimens by incident electrons. Here, we demonstrate the successful EELS single-atom spectroscopy of various metallofullerene-doped single-wall nanotubes (known as peapods) without massive structural destruction. This is achieved by using an incident electron probe with a low accelerating voltage (60 kV). Single calcium atoms inside the peapods were unambiguously identified for the first time using EELS. Elemental analyses of lanthanum, cerium and erbium atoms were also demonstrated, which shows that single atoms with adjacent atomic numbers can be successfully discriminated with this technique.

For potential applications in spintronics and quantum computing, it is desirable to place a quantum spin Hall insulator [i.e., a 2D topological insulator (TI)] on a substrate while maintaining a large energy gap. Here, we demonstrate a unique approach to create the large-gap 2D TI state on a semiconductor surface, based on first-principles calculations and effective Hamiltonian analysis. We show that when heavy elements with strong spin orbit coupling (SOC) such as Bi and Pb atoms are deposited on a patterned H-Si(111) surface into a hexagonal lattice, they exhibit a 2D TI state with a large energy gap of ≥0.5 eV. The TI state arises from an intriguing substrate orbital filtering effect that selects a suitable orbital composition around the Fermi level, so that the system can be matched onto a four-band effective model Hamiltonian. Furthermore, it is found that within this model, the SOC gap does not increase monotonically with the increasing strength of SOC. These interesting results may shed new light in future design and fabrication of large-gap topological quantum states.

Controlling the direction of exciton-energy flow in two-dimensional (2D) semiconductors is crucial for developing future high-speed optoelectronic devices using excitons as the information carriers. However, intrinsic exciton diffusion in conventional 2D semiconductors is omnidirectional, and efficient exciton-energy transport in a specific direction is difficult to achieve. Here we demonstrate directional exciton-energy transport across the interface in tungsten diselenide (WSe2)–molybdenum diselenide (MoSe2) lateral heterostructures. Unidirectional transport is spontaneously driven by the built-in asymmetry of the exciton-energy landscape with respect to the heterojunction interface. At excitation positions close to the interface, the exciton photoluminescence (PL) intensity was substantially decreased in the WSe2 region and enhanced in the MoSe2 region. In PL excitation spectroscopy, it was confirmed that the observed phenomenon arises from lateral exciton-energy transport from WSe2 to MoSe2. This directional exciton-energy flow in lateral 2D heterostructures can be exploited in future optoelectronic devices.

Mesoporous materials suffer from limitations including poor crystallinity and hydrolytic stability, lack of chemical diversity, insufficient pore accessibility, complex synthesis and toxicity issues. Here the association of non-toxic Zr-oxo clusters and feedstock isophthalic acid (IPA) via a Homometallic-Multicluster-Dot strategy results in a robust crystalline mesoporous MOF, denoted as MIP-206, that overcomes the aforementioned limitations. MIP-206, built up from an unprecedented combination of Zr6 and Zr12 oxo-cluster inorganic building units into a single structure, exhibits accessible meso-channels of ca. 2.6 nm and displays excellent chemical stability under different hydrolytic and harsh conditions. Owing to the abundant variety of functionalized IPA linkers, the chemical environment of MIP-206 can be easily tuned without hampering pore accessibility due to its large pore windows. As a result, MIP-206 loaded with palladium nanoparticles acts as an efficient and durable catalyst for the dehydrogenation of formic acid under mild conditions, outperforming benchmark mesoporous materials. This paves the way towards the utilization of MIP-206 as a robust mesoporous platform for a wide range of potential applications. Mesoporous solids, a category of materials possessing structural voids of 2-50 nm in which large guest molecules can be accommodated, are of interest to sustainable development with potentially broad applications in fields related to energy, environment and health. Owing to their high availability and tunable pore size, traditional mesoporous materials, such as silica, metal oxides and activated carbons, are prevalent in current usage, particularly in heterogeneous catalysis 1 , energy conversion/storage 2 , analytical science 3 and medical areas 4 , although they suffer from well-recognized limitations such as lack of crystallinity, chemical diversity, structural uniformity, stability and/or reproducibility. These limitations have inhibited pursuits to improve their performance, extend their use to other applicable fields and accumulate fundamental understanding 5 of these materials. Considerable efforts have been devoted in the past two decades to develop substitutes, promote new strategies and develop alternative candidates 6. Among them, mesoporous metal-organic frameworks (MOFs) are a promising family of materials that exhibits an ordered porosity and successfully addresses some limitations of conventional benchmarks. MOFs can be composed of an almost unlimited selection of inorganic building blocks (metal ions or clusters) and organic linkers in periodic structures which are merged efficiently, exhibiting high crystallinity with atomic precision, good reproducibility of preparation over control, and, most importantly, remarkable structural and chemical tunability 7; 8. Two strategies for preparing mesoporous MOFs are well-documented. Extending the organic linker to construct mesoporous MOFs following the isoreticular chemistry strategy is the most straightforward route and illustrates well the...

One-dimensional (1D) transition metal chalcogenides (TMCs) have recently attracted much attention because of their atomically thin, wire structures and superior conducting properties. These wires interact via van der Waals forces, aggregating into 1D crystals of different shapes with desired properties. However, relevant studies on their transport properties remain limited because of the lack of high-quality samples. Herein, we report the formation of a two-dimensional (2D) carrier gas in thin, ribbon-shaped bundles of laterally assembled WTe nanowires grown by chemical vapor deposition. Magnetoresistance measurements reveal that a single WTe bundle exhibits weak antilocalization and Shubnikov-de Haas (SdH) oscillations at low temperatures. Angle-dependent SdH oscillations serve as evidence of the realization of a 2D carrier gas in the WTe bundle. The present findings indicate the versatility of TMC nanowires as building blocks to produce electronic systems of desired dimensionality for future functional electronic and energy-harvesting devices.

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In-plane heterostructures of transition metal dichalcogenides (TMDCs) have attracted much attention for high-performance electronic and optoelectronic devices. To date, mainly monolayer-based in-plane heterostructures have been prepared by chemical vapor deposition (CVD), and their optical and electrical properties have been investigated. However, the low dielectric properties of monolayers prevent the generation of high concentrations of thermally excited carriers from doped impurities. To solve this issue, multilayer TMDCs are a promising component for various electronic devices due to the availability of degenerate semiconductors. Here, we report the fabrication and transport properties of multilayer TMDC-based in-plane heterostructures. The multilayer in-plane heterostructures are formed through CVD growth of multilayer MoS2 from the edges of mechanically exfoliated multilayer flakes of WSe2 or Nb x Mo1–x S2. In addition to the in-plane heterostructures, we also confirmed the vertical growth of MoS2 on the exfoliated flakes. For the WSe2/MoS2 sample, an abrupt composition change is confirmed by cross-sectional high-angle annular dark-field scanning transmission electron microscopy. Electrical transport measurements reveal that a tunneling current flows at the Nb x Mo1–x S2/MoS2 in-plane heterointerface, and the band alignment is changed from a staggered gap to a broken gap by electrostatic electron doping of MoS2. The formation of a staggered gap band alignment of Nb x Mo1–x S2/MoS2 is also supported by first-principles calculations.

Using the metal−organic chemical vapor deposition technique, we synthesized a bilayer lateral heterostucture of MoS 2 and WS 2 , each of which layers consist of alternately arranged nanostrips of MoS 2 and WS 2 . Transmission electron microscope images exhibit a checked pattern contrast, reflecting the three distinct interlayer metal arrangements of Mo/Mo, W/Mo (Mo/W), and W/W stacking. Theoretical calculations based on the density functional theory elucidated that the bilayer lateral heterostructure of MoS 2 and WS 2 exhibits complexed type-II band edge alignments depending on the interlayer metal arrangement: The conduction band edge is located at the Mo atom in a Mo/Mo sector, while the valence band edge is located at the W atom in a W/W sector. According to the complexed band edge alignment, the field-induced carrier injection behavior in the bilayer heterosheet composed of MoS 2 and WS 2 strips shows gate-induced transdimensionality, where the accumulated carrier distributions continuously vary from zero-to two-dimensional based on the applied gate voltage. Tunable carrier dimensionality can be applicable for wide areas of electronics, such as quantum dot arrays with tunable dot−dot interaction.