High electron mobility, quantum Hall effect and anomalous optical response in atomically thin InSe.

Two-dimensional (2D) atomic crystals have extraordinary electronic and photonic properties and hold great promise in the applications of photonic and optoelectronics. Here, we review some of our works about the spin-orbit interaction of light on the surface of 2D atomic crystals. First, we propose a general model to describe the spin-orbit interaction of light of the 2D free standing atomic crystal, and find that it is not necessary to involve the effective refractive index to describe the spin-orbit interaction. By developing the quantum weak measurements, we detect the spin-orbit interaction of light in 2D atomic crystals, which can act as a simple method for defining the layer numbers of graphene. Moreover, we find the transverse spin-dependent splitting in the photonic spin Hall effect exhibits a quantized behavior. Furthermore, the spin-orbit interaction of light for the case of air-topological insulator interface can be routed by adjusting the strength of the axion coupling. These basic finding may enhance the comprehension of the spin-orbit interaction, and find the important application in optoelectronic.

Three-dimensional (3D) III-V semiconductors including indium pnictides are widely used in optoelectronic devices, such as light-emitting diodes, laser diodes and photodetectors, in their bulk or thin-film geometries. On the other hand, two-dimensional (2D) atomic crystals such as graphene, phosphorene, and transition metal dichalcogenides are promising candidates for next generation optoelectronic technologies. Here, we designed a type of III-V indium pnictide 2D material that can be exfoliated and rebuilt from bulk wurtzite structures, which show benign stability and intriguing physical properties, including in-plane ferroelectricity/antiferroelectricity with low transition barriers (0.01-0.31 eV f.u.-1), direct/quasi-direct band gaps (HSE + SOC: 1.498-2.852 eV), ferroelasticity (2.86-11.90% elastic deformation), switchable hidden spin polarization and spin splitting (31 meV), as well as controllable in-plane negative Poisson's ratio (∼-0.51). Our study suggests a new class of optoelectronic materials that combines the advantages of the well-studied 3D III-V semiconductors and 2D atomic crystals and offers a platform to study the interplay of optoelectronic properties with multiferroic, spintronic, and mechanical properties for the development of miniaturized multifunctional optoelectronic devices.

H2 atmosphere is often involved in growth and application of two-dimensional (2D) atomic crystals, and it is of great importance to understand interaction of the 2D materials with H2 molecules. Here, a full graphene layer and a full hexagonal boron nitride (h-BN) layer grown on Pt(111) were exposed to H2 atmosphere, which were investigated by in situ near ambient pressure X-ray photoelectron spectroscopy and quasi in situ ultraviolet photoelectron spectroscopy. We confirm the occurrence of hydrogen intercalation of the graphene and h-BN overlayers in ambient pressure H2. The hydrogen intercalation in 0.1 Torr H2 at room temperature and hydrogen desorption in 0.1 Torr H2 at 200 °C are fully reversible on the graphene/Pt(111) and h-BN/Pt(111) surfaces. Furthermore, hydrogen desorption on the graphene/Pt(111) and h-BN/Pt(111) surfaces was found to happen at lower temperature than that on the Pt(111) surface due to the graphene and h-BN cover effect.

Stacking order critically influences the optoelectronic properties of 2D van der Waals materials. Here, first-principles calculations were performed to study the geometries, band structures, and second-harmonic generation (SHG) of hexagonal boron nitride (h-BN) bilayers constructed by the relative shifts and rotations between h-BN layers. Our results indicate that the stability, interlayer coupling, and band structures of h-BN bilayers are sensitive to the stacking orders. For interlayer sliding, the direction and size of lateral displacement obviously affect the band gap and components at the band edge. By contrast, the band structure of twisted h-BN bilayers is highly angle-dependent, and when the sum of twist angles in two moiré superlattices is 60°, they have similar band structures due to identical band folding. As for the second-order susceptibility, interlayer sliding tends to enhance the SHG intensity compared to that of the original AA stacking. When the incident angle of the pump light deviates from the normal direction of the h-BN bilayer, the change in lattice symmetry induced by interlayer sliding results in distinct variations in SHG patterns, thereby facilitating identification of the corresponding structures through polarization-resolved SHG spectroscopy. For twisted configurations, as the rotation angle increases from 0 to 60°, the evolution of SHG intensity departs significantly from the coherent superposition model due to the strong exciton effects in h-BN bilayers. Although the interlayer rotation preserves the SHG polarization image, the experimental measurement of relative SHG intensity enables the determination of the rotation angle, which allows for distinguishing structures of twisted h-BN bilayers.

The electronic and structural properties of a material are strongly determined by its symmetry. Changing the symmetry via a photoinduced phase transition offers new ways to manipulate material properties on ultrafast timescales. However, to identify when and how fast these phase transitions occur, methods that can probe the symmetry change in the time domain are required. Here we show that a time-dependent change in the coherent phonon spectrum can probe a change in symmetry of the lattice potential, thus providing an all-optical probe of structural transitions. We examine the photoinduced structural phase transition in VO(2) and show that, above the phase transition threshold, photoexcitation completely changes the lattice potential on an ultrafast timescale. The loss of the equilibrium-phase phonon modes occurs promptly, indicating a non-thermal pathway for the photoinduced phase transition, where a strong perturbation to the lattice potential changes its symmetry before ionic rearrangement has occurred.

In the past decade, two-dimensional (2D) atomic crystals have stimulated great interest in the electronic device community for their interesting physics and diverse applications. With the rapid growing market of wearable electronics and internet-of-things (IoTs), flexible nanoelectronics with high operation speed and low power consumption has become one of the most researched topics. In this regard, 2D atomic van der Waals crystals has emerged as promising candidates for such applications owing to their compelling capabilities including ultimate thickness scalability, superior mechanical flexibility and highly tunable electronic performance. We describe state-of-the-art advancements in 2D flextronics based on chemical vapor deposition (CVD) monolayer graphene, CVD monolayer MoS2, and few-layer black phosphorus with specific emphasis on radio frequency (RF) applications.

For decades, two-dimensional electron gases (2DEG) have allowed important experimental discoveries and conceptual developments in condensed-matter physics. When combined with the unique electronic properties of two-dimensional crystals, they allow rich physical phenomena to be probed at the quantum level. Here, we create a 2DEG in black phosphorus--a recently added member of the two-dimensional atomic crystal family--using a gate electric field. The black phosphorus film hosting the 2DEG is placed on a hexagonal boron nitride substrate. The resulting high carrier mobility in the 2DEG allows the observation of quantum oscillations. The temperature and magnetic field dependence of these oscillations yields crucial information about the system, such as cyclotron mass and lifetime of its charge carriers. Our results, coupled with the fact that black phosphorus possesses anisotropic energy bands with a tunable, direct bandgap, distinguish black phosphorus 2DEG as a system with unique electronic and optoelectronic properties.

Resonant tunneling diodes with negative differential resistance (NDR) have attracted significant attention due to their unique quantum resonant tunneling phenomena and potential applications in terahertz emission/detection and high‐density logic/memory. In this paper, resonant tunneling devices, where the carriers tunnel through a hexagonal boron nitride (hBN) barrier sandwiched by two black phosphorus (BP) layers, are explored. The resonance occurs when the energy bands of the two black phosphorus layers are aligned. The conductive atomic force microscopy (CAFM) measurements reveal prominent NDR peaks with large peak‐to‐valley ratios at room temperature. It is found that the positions of the NDR peaks are very sensitive to the amplitude and the shape of the voltage waveform used in CAFM, which can be explained by the charge trapping effect. Furthermore, resonant tunneling transistors are demonstrated based on BP/hBN/BP stacks in which the locations of the NDR peaks are tunable by the electrostatic gating. As compared to the traditional tunneling diodes based on bulk materials, the tunneling devices based on thin boron nitride tunneling barrier and high mobility black phosphorus offer ultra‐high‐speed response. This feature, together with the NDR characteristics, provides the potential for applications in THz oscillators and multi‐value logic devices.

Layered black phosphorus (BP) has recently emerged as a promising semiconductor because of its tunable band gap, high carrier mobility and strongly in-plane anisotropic properties. One-dimensional (1D) BP materials are attractive for applications in electronic and thermal devices, owing to their tailored charge and phonon transports along certain orientations. However, the fabrication of 1D BP materials still remains elusive thus far. We herein report the successful synthesis and characterization of nanotube-like BP for the first time by a selective composite with hexagonal boron nitride (h-BN) nanotubes under high pressure and high temperature conditions. The produced 1D BP/h-BN composites possess flexible diameter, length and thickness by adjusting the experimental synthesis parameters. Interestingly, it is important to notice that the stability of our BP sample has been significantly improved under the formation of heterostructures, which can actively promote their commercial applications. Our experimental work, together with first-principles calculations, presents a new scalable strategy of designing 1D tube-like BP/h-BN heterostructures that are promising candidates for flexible and high efficiency electronic platform.

In this paper, we study the Imbert-Fedorov shifts of anisotropic two-dimensional atomic crystal. This beam shift is an out-plane shift which can manifests as spin-dependent splitting. The Imbert-Fedorov shift related to the crystal axis direction, conductance, and doping concentration of two-dimensional atomic crystal is discussed. In particular, our study found that the shifts at the black phosphorus interface are sensitive to the change of its optical axis angle, doping concentration and frequency. We believe the precise measurement of the sensitive parameters through quantum weak measurement technology would provide important theoretical guidance for the electromagnetic characterization of two- dimensional atomic crystals.

Two-dimensional (2D) atomic crystals have triggered significant excitement due to their rich physics as well as potential industrial applications. The possibility of a molecular counterpart with scalable processability and superior performance is intriguing from both fundamental and applied perspectives. Here, we present the freestanding 2D molecular charge-transfer bis(ethylenedithio)tetrathiafulvalene-C60 crystals prepared by a modified Langmuir–Blodgett method, with precisely controlled few-layer thickness and centimeter-scale lateral dimension. The interconversion of intrinsic excited process, the long-range ordering and anisotropic stacking arrangement of the molecular layered crystals generate external stimuli responsive behaviors and anisotropic spin-charge conversion with magnetic energy conversion ability, as well as a superior UV photosensitivity. Moreover, the 2D freestanding crystals demonstrate superior magneto-electrical properties. These results suggest that a new class of 2D atomically thin molecular crystals with novel electronic, optical and magnetic properties have great potential for spintronic, energy and sensor applications.

Phosphorene, a monolayer of elemental phosphorus, has emerged as one of the most significant two-dimensional (2D) atomic crystals in the post-graphene era, as a potential candidate for semiconductor industry, nanotechnology, optoelectronics, and nanomedicine. However, the toxicological effects of this material toward different biomolecules remain elusive. In this article, we perform all-atom molecular dynamics simulations to decipher the effect of interaction and adsorption of two dimeric proteins, namely the HIV-1 integrase and the λ6–85 repressor protein on black phosphorene (BPn). It is revealed that upon purely noncovalent adsorption “on the top” of the BPn surface, the secondary structure of the proteins remains conserved, maintaining all the inter- and intraresidue interactions. In addition, the dimeric structure of the proteins does not dissociate into individual monomeric units, thereby inflicting insignificant to no nanotoxicity. However, if the protein–nanomaterial interaction occurs from an orientation where the edge of the 2D material is directed toward the dimer interface and the axis of dimerization of the protein is perpendicular to the plane of the material, a “clean cut” of the dimer is highly probable, wherein individual monomers are structurally not perturbed. The process of BPn edge-induced dimer cleavage is highly favorable as the Gibbs free energy change accompanying the process is negative for both of the dimeric proteins. Thus, BPn is expected to show orientation-dependent nanotoxicity toward dimeric proteins. For monomeric proteins, however, the extent of nanotoxicity would be significantly milder, and the 2D material may be used as a “molecular scissor” for the nondisruptive cleavage of dimeric proteins.

Two-dimensional (2D) atomic crystals are considered to be very promising for nanoelectronics and spintronics applications. It provides a large class of materials proposed to be important for long distance spin transport, spin polarized tunneling and large spin-orbit coupling. Here I will discuss spintronic aspects of these 2D materials and their heterostructures. Graphene is considered to be an ideal material for spin transport due to the high mobility and long spin lifetime of the carriers. Recently, we demonstrated spin transport over distances of 16 µm and spin lifetimes up to 1.2 ns in large area CVD graphene on SiO 2 /Si substrate at room temperature [1]. Subsequently, using insulating h-BN as a tunnel barrier on graphene, we observe an enhancement in spin polarized tunneling [2], and its negative sign for thicker h-BN layers [3]. These signatures provide an experimental evidence of the spin filtering across the ferromagnet/hBN-graphene van der Waals heterostructures. We also employed 2D materials such as h-BN and MoS 2 in ferromagnetic tunnel junctions for observation of tunnel magnetoresistance up to room temperature [4]. We further aim to address the issue of spin manipulation in graphene by employing heterostructures with other 2D semiconductors [5], topological insulators [6] and materials with novel spin textures. I will present both electronic and spintronic properties of these 2D materials and their heterostructures. These findings open a platform for exploring novel spin functionalities in 2D crystals and understanding the basic phenomenon that control their behavior.

Large-area uniform of single-crystal tungsten disulfide (WS2) is important for advanced optoelectronics based on two-dimensional (2D) atomic crystals. However, difficulties in controlling the interrelated growth parameters restrict its development in devices. Herein, we present the synthesis of triangular monolayered WS2 flakes with good uniformity and single crystal by adjusting the introduction time of sulfur precursor and the distances between the sources and substrates to control the nucleation density. Investigation of the morphology and structure by transmission electron microscopy and Raman spectroscopy indicates that a series of triangular (side length of 233 μm) monolayered WS2 flakes shows high-quality structure and homogenous crystallinity. Field-effect transistors based on the fabricated triangular monolayered WS2 with single crystal demonstrate environmentally stable charge transport with a field-effect mobility of 50.5 cm2/V s and current modulation Ion/ Ioff of ∼107. The results of this study pave the way for the application of monolayered WS2 in a multitude of 2D-material-based devices.

The new paradigm of heterostructures based on two-dimensional (2D) atomic crystals has already led to the observation of exciting physical phenomena and creation of novel devices. The possibility of combining layers of different 2D materials in one stack allows unprecedented control over the electronic and optical properties of the resulting material. Still, the current method of mechanical transfer of individual 2D crystals, though allowing exceptional control over the quality of such structures and interfaces, is not scalable. Here we show that such heterostructures can be assembled from chemically exfoliated 2D crystals, allowing for low-cost and scalable methods to be used in device fabrication.