Picometer-Level In Situ Manipulation of Ferroelectric Polarization in Van der Waals layered InSe.

Ferroelectric 2D van der Waals (vdW) layered materials are attracting increasing attention due to their potential applications in next-generation nanoelectronics and in-memory computing with polarization-dependent functionalities. Despite the critical role of polarization in governing ferroelectricity behaviors, its origin and relation with local structures in 2D vdW layered materials have not been fully elucidated so far. Here, intralayer sliding of approximately six degrees within each quadruple-layer of the prototype 2D vdW ferroelectrics InSe is directly observed and manipulated using sub-angstrom resolution imaging and in situ biasing in an aberration-corrected scanning transmission electron microscope.

Van der Waals Ferroelectric Semiconductor Field Effect Transistor for In-Memory Computing.

In-memory computing is a highly efficient approach for breaking the bottleneck of von Neumann architectures, i.e., reducing redundant latency and energy consumption during the data transfer between the physically separated memory and processing units.

High-Performance Photodetectors Based on the 2D SiAs/SnS Heterojunction.

Constructing 2D heterojunctions with high performance is the critical solution for the optoelectronic applications of 2D materials. This work reports on the studies on the preparation of high-quality van der Waals SiAs single crystals and high-performance photodetectors based on the 2D SiAs/SnS heterojunction. The crystals are grown using the chemical vapor transport (CVT) method and then the bulk crystals are exfoliated to a few layers.

An all two-dimensional vertical heterostructure graphene/CuInPS/MoSfor negative capacitance field effect transistor.

As scaling down the size of metal oxide semiconductor field-effect transistors (FETs), power dissipation has become a major challenge. Lowering down the sub-threshold swing (SS) is known as an effective technique to decrease the operating voltage of FETs and hence lower down the power consumption. However, the Boltzmann distribution of electrons (so-called 'Boltzmann tyranny') implements a physical limit to the SS value.

Vapor Deposition of Magnetic Van der Waals NiI Crystals.

The recent discovery of van der Waals magnetic materials has attracted great attention in materials science and spintronics. The preparation of ultrathin magnetic layers down to atomic thickness is challenging and is mostly by mechanical exfoliation. Here, we report vapor deposition of magnetic van der Waals NiI crystals.

Potential 2D Materials with Phase Transitions: Structure, Synthesis, and Device Applications.

Layered materials with phase transitions, such as charge density wave (CDW) and magnetic and dipole ordering, have potential to be exfoliated into monolayers and few-layers and then become a large and important subfamily of two-dimensional (2D) materials. Benefitting from enriched physical properties from the collective interactions, long-range ordering, and related phase transitions, as well as the atomic thickness yet having nondangling bonds on the surface, 2D phase-transition materials have vast potential for use in new-concept and functional devices. Here, potential 2D phase-transition materials with CDWs and magnetic and dipole ordering, including transition metal dichalcogenides, transition metal halides, metal thio/selenophosphates, chromium silicon/germanium tellurides, and more, are introduced.

Phase-selective synthesis of 1T' MoS monolayers and heterophase bilayers.

Two-dimensional (2D) MoS, which has great potential for optoelectronic and other applications, is thermodynamically stable and hence easily synthesized in its semiconducting 2H phase. In contrast, growth of its metastable 1T and 1T' phases is hampered by their higher formation energy. Here we use theoretical calculations to design a potassium (K)-assisted chemical vapour deposition method for the phase-selective growth of 1T' MoS monolayers and 1T'/2H heterophase bilayers.

Chemical Growth of 1T-TaS Monolayer and Thin Films: Robust Charge Density Wave Transitions and High Bolometric Responsivity.

Ultrathin two-dimensional (2D) charge density wave (CDW) materials, with sharp resistance change at the phase-transition temperature, yet with ultrathin thickness, hold great potential for electrical device applications. However, chemical synthesis of high-quality samples and observation of the CDW states down to the monolayer limit is still of great challenge. Chemical vapor deposition of 1T-TaS sheets on hexagonal boron nitride (h-BN) with robust CDW states even down to the monolayer extreme is reported here.

A Plasmon-Enhanced Fluorescence Protein Microchip with High-Sensitivity for Multiple Islet Autoantibodies Measurements in Autoimmune Diabetes Diagnosis.

Correct classification diagnosis of diabetes is critical for timely and appropriate treatment, which partially depends on the multiple islet autoantibodies measurement and combined analysis. This study reports plasmon-enhanced fluorescence protein microchip method, a fast, high-throughput, and high sensitive method, to measure circulating islet autoantibodies. This method can achieve islet autoantibodies combined analysis within a few hours by using a few microliters of serum.

Anisotropic Strain Relaxation of Graphene by Corrugation on Copper Crystal Surfaces.

Corrugation is a ubiquitous phenomenon for graphene grown on metal substrates by chemical vapor deposition, which greatly affects the electrical, mechanical, and chemical properties. Recent years have witnessed great progress in controlled growth of large graphene single crystals; however, the issue of surface roughness is far from being addressed. Here, the corrugation at the interface of copper (Cu) and graphene, including Cu step bunches (CuSB) and graphene wrinkles, are investigated and ascribed to the anisotropic strain relaxation.

Wrinkle-Free Single-Crystal Graphene Wafer Grown on Strain-Engineered Substrates.

Wrinkles are ubiquitous for graphene films grown on various substrates by chemical vapor deposition at high temperature due to the strain induced by thermal mismatch between the graphene and substrates, which greatly degrades the extraordinary properties of graphene. Here we show that the wrinkle formation of graphene grown on Cu substrates is strongly dependent on the crystallographic orientations. Wrinkle-free single-crystal graphene was grown on a wafer-scale twin-boundary-free single-crystal Cu(111) thin film fabricated on sapphire substrate through strain engineering.

Anomalous Polarized Raman Scattering and Large Circular Intensity Differential in Layered Triclinic ReS.

The Raman tensor of a crystal is the derivative of its polarizability tensor and is dependent on the symmetries of the crystal and the Raman-active vibrational mode. The intensity of a particular mode is determined by the Raman selection rule, which involves the Raman tensor and the polarization configurations. For anisotropic two-dimensional (2D) layered crystals, polarized Raman scattering has been used to reveal the crystalline orientations.

In-Plane Uniaxial Strain in Black Phosphorus Enables the Identification of Crystalline Orientation.

Identification of the crystalline axis of anisotropic black phosphorus (BP) is important for investigating its physical properties, as well as for optical and electronic applications. Herein, it is showed that by applying in-plane uniaxial strain and measuring the changes of the Raman shifts, the crystalline axis of BP can be reliably determined. The strain effects on the Raman shifts are angle-dependent, and they can be expressed as a combination of the Raman responses under zigzag and armchair strain.

Controlled growth of large-area anisotropic ReS atomic layer and its photodetector application.

As an anisotropic 2D layered material, rhenium disulfide (ReS) has attracted much attention because of its unusual properties and promising applications in electronic and optoelectronic devices. However, the low lattice symmetry and interlayer decoupling of ReS make asymmetric growth and out-of-plane growth occur quite easily; therefore, thick flake, dendritic and flower-like structures of ReS have mostly been obtained previously. Here, we report on an approach based on space-confined epitaxial growth for the controlled synthesis of ReS films.

Birefringence-Directed Raman Selection Rules in 2D Black Phosphorus Crystals.

The incident and scattered light engaged in the Raman scattering process of low symmetry crystals always suffer from the birefringence-induced depolarization. Therefore, for anisotropic crystals, the classical Raman selection rules should be corrected by taking the birefringence effect into consideration. The appearance of the 2D anisotropic materials provides an excellent platform to explore the birefringence-directed Raman selection rules, due to its controllable thickness at the nanoscale that greatly simplifies the situation comparing with bulk materials.

Optical Anisotropy of Black Phosphorus in the Visible Regime.

The striking in-plane anisotropy remains one of the most intriguing properties for the newly rediscovered black phosphorus (BP) 2D crystals. However, because of its rather low-energy band gap, the optical anisotropy of few-layer BP has been primarily investigated in the near-infrared (NIR) regime. Moreover, the essential physics that determine the intrinsic anisotropic optical property of few-layer BP, which is of great importance for practical applications in optical and optoelectronic devices, are still in the fancy of theory.

Growth of MoS(2(1-x))Se(2x) (x = 0.41-1.00) Monolayer Alloys with Controlled Morphology by Physical Vapor Deposition.

Transition-metal dichalcogenide (TMD) monolayer alloys are a branch of two-dimensional (2D) materials which can have large-range band gap tuning as the composition changes. Synthesis of 2D TMD monolayer alloys with controlled composition as well as controlled domain size and edge structure is of great challenge. In the present work, we report growth of MoS2(1-x)Se2x monolayer alloys (x = 0.

Identifying the crystalline orientation of black phosphorus using angle-resolved polarized Raman spectroscopy.

An optical anisotropic nature of black phosphorus (BP) is revealed by angle-resolved polarized Raman spectroscopy (ARPRS), and for the first time, an all-optical method was realized to identify the crystal orientation of BP sheets, that is, the zigzag and armchair directions. We found that Raman intensities of Ag(1), B2g, and Ag(2) modes of BP not only depend on the polarization angle α, but also relate to the sample rotation angle θ. Furthermore, their intensities reach the local maximum or minimum values when the crystalline orientation is along with the polarization direction of scattered light (es).

Selective scission of C-O and C-C bonds in ethanol using bimetal catalysts for the preferential growth of semiconducting SWNT arrays.

For the application of single-walled carbon nanotubes (SWNTs) to electronic and optoelectronic devices, techniques to obtain semiconducting SWNT (s-SWNT) arrays are still in their infancy. We have developed herein a rational approach for the preferential growth of horizontally aligned s-SWNT arrays on a ST-cut quartz surface through the selective scission of C-O and C-C bonds of ethanol using bimetal catalysts, such as Cu/Ru, Cu/Pd, and Au/Pd. For a common carbon source, ethanol, a reforming reaction occurs on Cu or Au upon C-C bond breakage and produces C(ads) and CO, while a deoxygenating reaction occurs on Ru or Pd through C-O bond breaking resulting in the production of O(ads) and C2H4.

Growth of close-packed semiconducting single-walled carbon nanotube arrays using oxygen-deficient TiO2 nanoparticles as catalysts.

For the application of single-walled carbon nanotubes (SWNTs) in nanoelectronic devices, techniques to obtain horizontally aligned semiconducting SWNTs (s-SWNTs) with higher densities are still in their infancy. We reported herein a rational approach for the preferential growth of densely packed and well-aligned s-SWNTs arrays using oxygen-deficient TiO2 nanoparticles as catalysts. Using this approach, a suitable concentration of oxygen vacancies in TiO2 nanoparticles could form by optimizing the flow rate of hydrogen and carbon sources during the process of SWNT growth, and then horizontally aligned SWNTs with the density of ∼ 10 tubes/μm and the s-SWNT percentage above 95% were successfully obtained on ST-cut quartz substrates.

Two-dimensional molybdenum tungsten diselenide alloys: photoluminescence, Raman scattering, and electrical transport.

Two-dimensional transition-metal dichalcogenide alloys have attracted intense attention due to their tunable band gaps. In the present work, photoluminescence, Raman scattering, and electrical transport properties of monolayer and few-layer molybdenum tungsten diselenide alloys (Mo1-xWxSe2, 0 ≤ x ≤ 1) are systematically investigated. The strong photoluminescence emissions from Mo1-xWxSe2 monolayers indicate composition-tunable direct band gaps (from 1.

Growth of large-area 2D MoS₂(₁-x) Se₂x semiconductor alloys.

Semiconducting MoS₂(₁-x) Se₂x mono-layers where x = 0-0.40 are successfully grown over large areas. A random arrangement of the S and Se atoms and a tunable bandgap photoluminescence are observed.