Revealing Surface States in In-Doped SnTe Nanoplates with Low Bulk Mobility

Indium (In) doping in topological crystalline insulator SnTe induces superconductivity, making In-doped SnTe a candidate for a topological superconductor. SnTe nanostructures offer well-defined nanoscale morphology and high surface-to-volume ratios to enhance surface effects. Here, we study In-doped SnTe nanoplates, In_<i>x</i>Sn_1–<i>x</i>Te, with <i>x</i> ranging from 0 to 0.1 and show they superconduct. More importantly, we show that In doping reduces the bulk mobility of In_<i>x</i>Sn_1–<i>x</i>Te such that the surface states are revealed in magnetotransport despite the high bulk carrier density. This is manifested by two-dimensional linear magnetoresistance in high magnetic fields, which is independent of temperature up to 10 K. Aging experiments show that the linear magnetoresistance is sensitive to ambient conditions, further confirming its surface origin. We also show that the weak antilocalization observed in In_<i>x</i>Sn_1–<i>x</i>Te nanoplates is a bulk effect. Thus, we show that nanostructures and reducing the bulk mobility are effective strategies to reveal the surface states and test for topological superconductors

Ambipolar Field Effect in Sb-Doped Bi₂Se₃ Nanoplates by Solvothermal Synthesis

A topological insulator is a new phase of quantum matter with a bulk band gap and spin-polarized surface states, which might find use in applications ranging from electronics to energy conversion. Despite much exciting progress in the field, high-yield solution synthesis has not been widely used for the study of topological insulator behavior. Here, we demonstrate that solvothermally synthesized Bi₂Se₃ nanoplates are attractive for topological insulator studies. The carrier concentration of these Bi₂Se₃ nanoplates is controlled by compensational Sb doping during the synthesis. In low-carrier-density, Sb-doped Bi₂Se₃ nanoplates, we observe pronounced ambipolar field effect that demonstrates the flexible manipulation of carrier type and concentration for these nanostructures. Solvothermal synthesis offers an affordable, facile approach to produce high-quality nanomaterials to explore the properties of topological insulators

Semipolar (202̅1̅) GaN and InGaN Light-Emitting Diodes Grown on Sapphire

We have demonstrated growing uniform and purely nitrogen polar semipolar (202̅1̅) GaN epilayers on 2 in. patterned sapphire substrates. The as-grown surface of (202̅1̅) GaN is composed of two stable facets: (101̅0) and (101̅1̅). A chemical mechanical polishing process was further used to planarize the surface with a final surface root-mean-square roughness of less than 1.5 nm over an area of 10 × 10 μm². InGaN light-emitting diodes were grown on a polished (202̅1̅) GaN/sapphire template with an electroluminescence emission at around 490 nm. Our work exhibits the potential to produce high-quality nitrogen-polar semipolar GaN templates and optoelectronic devices on large-area sapphire substrates with economical feasibility

Metal Seed Layer Thickness-Induced Transition From Vertical to Horizontal Growth of MoS₂ and WS₂

Two-dimensional (2D), layered transition metal dichalcogenides (TMDCs) can grow in two different growth directions, that is, horizontal and vertical. In the horizontal growth, 2D TMDC layers grow in planar direction with their basal planes parallel to growth substrates. In the vertical growth, 2D TMDC layers grow standing upright on growth substrates exposing their edge sites rather than their basal planes. The two distinct morphologies present unique materials properties suitable for specific applications, such as horizontal TMDCs for optoelectronics and vertical TMDCs for electrochemical reactions. Precise control of the growth orientation is essential for realizing the true potential of these 2D materials for large-scale, practical applications. In this Letter, we investigate the transition of vertical-to-horizontal growth directions in 2D molybdenum (or tungsten) disulfide and study the underlying growth mechanisms and parameters that dictate such transition. We reveal that the thickness of metal seed layers plays a critical role in determining their growth directions. With thick (>∼3 nm) seed layers, the vertical growth is dominant, while the horizontal growth occurs with thinner seed layers. This finding enables the synthesis of novel 2D TMDC heterostructures with anisotropic layer orientations and transport properties. The present study paves a way for developing a new class of 2D TMDCs with unconventional materials properties

High-Density Chemical Intercalation of Zero-Valent Copper into Bi₂Se₃ Nanoribbons

A major goal of intercalation chemistry is to intercalate high densities of guest species without disrupting the host lattice. Many intercalant concentrations, however, are limited by the charge of the guest species. Here we have developed a general solution-based chemical method for intercalating extraordinarily high densities of zero-valent copper metal into layered Bi₂Se₃ nanoribbons. Up to 60 atom % copper (Cu_7.5Bi₂Se₃) can be intercalated with no disruption to the host lattice using a solution disproportionation redox reaction

Role of Heterointerface in Lithium-Induced Phase Transition in <i>T</i>_d‑WTe₂ Nanoflakes

A new polytype of WTe2 with a bandgap has been recently discovered through the intercalation of lithium into the van der Waals gaps of Td-WTe2. Here, we report the effects of reduced thicknesses and heterointerfaces on the intercalation-induced phase transition in WTe2. Using in situ Raman spectroscopy during the electrochemical lithiation of WTe2 flakes as a function of flake thickness, we observe that additional electrochemical energy is required for the phase transition of WTe2 from the Td phase to the new lithiated Td′ phase, going from 0.8 V of the applied electrochemical voltage for a thick flake to 0.5 V and 0.3 V for 7- and 5-layered samples, respectively. We ascribe this suppression of the phase transition to the interfacial interaction between the nanoflake and SiO2/Si substrate, which plays an increasing role as the sample thickness is reduced. The suppressed kinetics of the phase transition can be mitigated by placing the WTe2 flake on a hexagonal boron nitride (hBN) flake, which facilitates the release of the in-plane strain induced by the phase transition. Our study underscores the significance of interfacial effects in modulating phase transitions in two-dimensional (2D) materials, suggesting heterogeneous transition pathways, as well as interfacial engineering to control these phase transitions

Synthesis of MoS₂ and MoSe₂ Films with Vertically Aligned Layers

Layered materials consist of molecular layers stacked together by weak interlayer interactions. They often crystallize to form atomically smooth thin films, nanotubes, and platelet or fullerene-like nanoparticles due to the anisotropic bonding. Structures that predominately expose edges of the layers exhibit high surface energy and are often considered unstable. In this communication, we present a synthesis process to grow MoS₂ and MoSe₂ thin films with vertically aligned layers, thereby maximally exposing the edges on the film surface. Such edge-terminated films are metastable structures of MoS₂ and MoSe₂, which may find applications in diverse catalytic reactions. We have confirmed their catalytic activity in a hydrogen evolution reaction (HER), in which the exchange current density correlates directly with the density of the exposed edge sites

Chemical Intercalation of Zerovalent Metals into 2D Layered Bi₂Se₃ Nanoribbons

We have developed a chemical method to intercalate a variety of zerovalent metal atoms into two-dimensional (2D) layered Bi₂Se₃ chalcogenide nanoribbons. We use a chemical reaction, such as a disproportionation redox reaction, to generate dilute zerovalent metal atoms in a refluxing solution, which intercalate into the layered Bi₂Se₃ structure. The zerovalent nature of the intercalant allows superstoichiometric intercalation of metal atoms such as Ag, Au, Co, Cu, Fe, In, Ni, and Sn. We foresee the impact of this methodology in establishing novel fundamental physical behaviors and in possible energy applications

Synthesis of SnTe Nanoplates with {100} and {111} Surfaces

SnTe is a topological crystalline insulator that possesses spin-polarized, Dirac-dispersive surface states protected by crystal symmetry. Multiple surface states exist on the {100}, {110}, and {111} surfaces of SnTe, with the band structure of surface states depending on the mirror symmetry of a particular surface. Thus, to access surface states selectively, it is critical to control the morphology of SnTe such that only desired crystallographic surfaces are present. Here, we grow SnTe nanostructures using vapor–liquid–solid and vapor–solid growth mechanisms. Previously, SnTe nanowires and nanocrystals have been grown [Saghir et al. <i>Cryst. Growth Des.</i> <b>2014</b>, <i>14</i>, 2009–2013; Safdar et al. <i>Cryst. Growth Des.</i> <b>2014</b>, <i>14</i>, 2502–2509; Safdar et al. <i>Nano Lett.</i> <b>2013</b>, <i>13</i>, 5344–5349; Li et al. <i>Nano Lett.</i> <b>2013</b>, <i>13</i>, 5443–5448]. In this report, we demonstrate the synthesis of SnTe nanoplates with lateral dimensions spanning tens of micrometers and thicknesses of a few hundred nanometers. The top and bottom surfaces are either (100) or (111), maximizing topological surface states on these surfaces. Magnetotransport on these SnTe nanoplates shows a high bulk carrier density, consistent with bulk SnTe crystals arising due to defects such as Sn vacancies. In addition, we observe a structural phase transition in these nanoplates from the high-temperature rock salt to a low-temperature rhombohedral structure. For nanoplates with a very high carrier density, we observe a slight upturn in resistance at low temperatures, indicating electron–electron interactions

General Facet-Controlled Synthesis of Single-Crystalline {010}-Oriented LiMPO₄ (M = Mn, Fe, Co) Nanosheets

Facet-controlled synthesis of phospho-olivine (LiMPO₄, M = Mn, Fe, Co) cathode materials is of particular interest to manipulate their electrochemical properties because of their anisotropic ionic transport behavior. This study provides a general facet-controlled synthesis of single-crystalline LiMPO₄ (M = Mn, Fe, Co) nanosheets with significantly large exposure of (010)-facets, which has not been readily achieved by conventional solution-based coprecipitation or solid-reaction methods. The as-obtained nanosheets show controllable thickness with the thinnest thickness down to 15–20 nm and lateral dimension up to ∼5 μm. Due to the shortened lithium ion diffusion pathway and high ratio of active surface enabled by the thin thickness, the as-prepared LiFePO₄ nanosheets, as a model material, demonstrate greatly improved rate capability and cycling stability, with a reversible capacity of ∼80 mA h g^–1 at a current rate of 30 C and a stable capacity retention of ∼93% after 500 cycles at a current rate of 5 C. Further electrochemical analysis reveals an enhanced interfacial lithium ion diffusion of the nanosheets, suggesting that facet-controlled 2D LiMPO₄ nanosheets are a promising material platform for next-generation high-rate lithium-ion batteries