Project description:Two-dimensional (2D) molybdenum telluride (MoTe2) is attracting increasing attention for its potential applications in electronic, optoelectronic, photonic and catalytic fields, owing to the unique band structures of both stable 2H phase and 1T' phase. However, the direct growth of high-quality atomically thin MoTe2 with the controllable proportion of 2H and 1T' phase seems hard due to easy phase transformation since the potential barrier between the two phases is extremely small. Herein, we report a strategy of the phase-controllable chemical vapor deposition (CVD) synthesis for few-layer (<3 layer) MoTe2. Besides, a new understanding of the phase-controllable growth mechanism is presented based on a combination of experimental results and DFT calculations. The lattice distortion caused by Te vacancies or structural strain might make 1T'-MoTe2 more stable. The conditions for 2H to 1T' phase conversion are determined to be the following: Te monovacancies exceeding 4% or Te divacancies exceeding 8%, or lattice strain beyond 6%. In contrast, sufficient Te supply and appropriate tellurization velocity are essential to obtaining the prevailing 2H-MoTe2. Our work provides a novel perspective on the preparation of 2D transition metal chalcogenides (TMDs) with the controllable proportion of 2H and 1T' phase and paves the way to their subsequent potential application of these hybrid phases.
Project description:Exposing and stabilizing undercoordinated platinum (Pt) sites and therefore optimizing their adsorption to reactive intermediates offers a desirable strategy to develop highly efficient Pt-based electrocatalysts. However, preparation of atomically controllable Pt-based model catalysts to understand the correlation between electronic structure, adsorption energy, and catalytic properties of atomic Pt sites is still challenging. Herein we report the atomically thin two-dimensional PtTe2 nanosheets with well-dispersed single atomic Te vacancies (Te-SAVs) and atomically well-defined undercoordinated Pt sites as a model electrocatalyst. A controlled thermal treatment drives the migration of the Te-SAVs to form thermodynamically stabilized, ordered Te-SAV clusters, which decreases both the density of states of undercoordinated Pt sites around the Fermi level and the interacting orbital volume of Pt sites. As a result, the binding strength of atomically defined Pt active sites to H intermediates is effectively reduced, which renders PtTe2 nanosheets highly active and stable in hydrogen evolution reaction.
Project description:A complete phase diagram and its corresponding physical properties are essential prerequisites to understand the underlying mechanism of iron-based superconductivity. For the structurally simplest 11 (FeSeTe) system, earlier attempts using bulk samples have not been able to do so due to the fabrication difficulties. Here, thin FeSe(x)Te(1-x) films with the Se content covering the full range (0 ≤ x ≤ 1) were fabricated by using pulsed laser deposition method. Crystal structure analysis shows that all films retain the tetragonal structure in room temperature. Significantly, the highest superconducting transition temperature (T(C) = 20 K) occurs in the newly discovered domain, i.e., 0.6 ≤ x ≤ 0.8. The single-phased superconducting dome for the full Se doping range is the first of its kind in iron chalcogenide superconductors. Our results present a new avenue to explore novel physics as well as to optimize superconductors.
Project description:Electron band topology is combined with intrinsic magnetic orders in MnBi2Te4, leading to novel quantum phases. Here we investigate collective spin excitations (i.e. magnons) and spin fluctuations in atomically thin MnBi2Te4 flakes using Raman spectroscopy. In a two-septuple layer with non-trivial topology, magnon characteristics evolve as an external magnetic field tunes the ground state through three ordered phases: antiferromagnet, canted antiferromagnet, and ferromagnet. The Raman selection rules are determined by both the crystal symmetry and magnetic order while the magnon energy is determined by different interaction terms. Using non-interacting spin-wave theory, we extract the spin-wave gap at zero magnetic field, an anisotropy energy, and interlayer exchange in bilayers. We also find magnetic fluctuations increase with reduced thickness, which may contribute to a less robust magnetic order in single layers.
Project description:Owning to their unusual optical properties, such as electrical tunability and strong spatial confinement, two-dimensional surface polaritons (2DSPs) hold great promise for deep sub-wavelength manipulation of light in a reduced low-dimensional space. Control of 2DSPs is possible by using their interaction with a boundary between two media, similar to how light behaves in three-dimensional (3D) space. The understanding of the interaction in the 2D case is still in its early stages, unlike the 3D case, as in-depth investigations are only available in a few cases including the interaction of 2DSPs with structured 2D crystals. Here, we extend the scope of our understanding to the interaction of 2DSPs with metallic nano-plates on 2D crystals, focusing on the reflection of 2DSPs. Through our rigorous model, we reveal that, for strongly confined 2DSPs having much larger momentum than free space photons, the interaction results in almost total internal reflection of 2DSPs as the radiative coupling of the 2DSPs to free space is negligible. We also find that the reflection involves an anomalous phase shift dependent on the thickness of the nano-plate, due to the temporary storing of electromagnetic energy in the evanescent waves induced near the edge of the nano-plate. Our theory predicts that the phase shift saturates to an anomalous value, 0.885π, as the nano-plate becomes thicker. Our work provides a detailed understanding of how to manipulate the 2DSPs by using one of the simplest nanostructures, essential for the further development of nanostructure-integrated low-dimensional devices for polariton optics.
Project description:Atomically thin transition metal dichalcogenides (TMDCs) present a promising platform for numerous photonic applications due to excitonic spectral features, possibility to tune their constants by external gating, doping, or light, and mechanical stability. Utilization of such materials for sensing or optical modulation purposes would require a clever optical design, as by itself the 2D materials can offer only a small optical phase delay - consequence of the atomic thickness. To address this issue, we combine films of 2D semiconductors which exhibit excitonic lines with the Fabry-Perot resonators of the standard commercial SiO2/Si substrate, in order to realize topological phase singularities in reflection. Around these singularities, reflection spectra demonstrate rapid phase changes while the structure behaves as a perfect absorber. Furthermore, we demonstrate that such topological phase singularities are ubiquitous for the entire class of atomically thin TMDCs and other high-refractive-index materials, making it a powerful tool for phase engineering in flat optics. As a practical demonstration, we employ PdSe2 topological phase singularities for a refractive index sensor and demonstrate its superior phase sensitivity compared to typical surface plasmon resonance sensors.
Project description:The field of two-dimensional (2D) ferromagnetism has been proliferating over the past few years, with ongoing interests in basic science and potential applications in spintronic technology. However, a high-resolution spectroscopic study of the 2D ferromagnet is still lacking due to the small size and air sensitivity of the exfoliated nanoflakes. Here, we report a thickness-dependent ferromagnetism in epitaxially grown Cr2Te3 thin films and investigate the evolution of the underlying electronic structure by synergistic angle-resolved photoemission spectroscopy, scanning tunneling microscopy, x-ray absorption spectroscopy, and first-principle calculations. A conspicuous ferromagnetic transition from Stoner to Heisenberg-type is directly observed in the atomically thin limit, indicating that dimensionality is a powerful tuning knob to manipulate the novel properties of 2D magnetism. Monolayer Cr2Te3 retains robust ferromagnetism, but with a suppressed Curie temperature, due to the drastic drop in the density of states near the Fermi level. Our results establish atomically thin Cr2Te3 as an excellent platform to explore the dual nature of localized and itinerant ferromagnetism in 2D magnets.
Project description:ABSTRACT Rational design and synthesis of catalytically active two-dimensional (2D) materials with an abundance of atomically precise active sites in their basal planes remains a great challenge. Here, we report a ligand exchange strategy to exfoliate bulk [Cu4(OH)6][O3S(CH2)4SO3] cuprate crystals into atomically thin 2D cuprate layers ([Cu2(OH)3]+). The basal plane of 2D cuprate layers contains periodic arrays of accessible unsaturated Cu(II) single sites (2D-CuSSs), which are found to promote efficient oxidative Chan-Lam coupling. Our mechanistic studies reveal that the reactions proceed via coordinatively unsaturated CuO4(II) single sites with the formation of Cu(I) species in the rate-limiting step, as corroborated by both operando experimental and theoretical studies. The robust stability of 2D-CuSSs in both batch and continuous flow reactions, coupled with their recyclability and good performance in complex molecule derivatization, render 2D-CuSSs attractive catalyst candidates for broad utility in fine chemical synthesis. A ligand exchange strategy exfoliates bulk cuprate crystals into atomically thin 2D cuprate layers with periodic unsaturated copper single sites, which are found to promote efficient oxidative Chan-Lam coupling.
Project description:The quaternary topological insulator (Bi,Sb)2(Te,Se)3 has demonstrated topological surface states with an insulating bulk. Scientists have identified an optimized composition of Bi(1.5)Sb(0.5)Te(1.7)Se(1.3) with the highest resistivity reported. But the physics that drive to this composition remains unclear. Here we report the crystal structure and the magneto-transport properties of Bi(1.5)Sb(0.5)Te(3-x)Se(x) (BSTS) series. A correlation between the structure and the physical properties has been revealed. We found out that within the rhombohedral structure, the composition with most Te substituting Se has the highest resistivity. On the other hand, segregation of other composition phases will introduce much higher bulk concentration.
Project description:Upconverting infrared light into visible light via the triplet-triplet annihilation process in solid state is important for various applications including photovoltaics, photodetection, and bioimaging. Although inorganic semiconductors with broad absorption and negligible exchange energy loss have emerged as promising alternative to molecular sensitizers, currently, they have exclusively suffered from low efficiency and contained toxic elements in near-infrared (NIR)-to-visible upconversion. Here, we report an ultrathin bilayer film for NIR-to-visible upconversion based on atomically thin two-dimensional (2D) monolayer semiconductors. The atomic flatness and strong light absorption of 2D monolayer semiconductors enable ultrafast energy transfer and robust NIR-to-visible emission with a high upconversion quantum yield (1.1 ± 0.2%) at modest incident power (260 mW cm-2). Increasing layer thickness adversely quenches the upconversion emission, highlighting the 2D advantage. Considering the whole library of 2D semiconductors, the facile large-scale production and the ultrathin solid-state architecture open a new research field for solid-state upconversion applications.