Heterostructures produced from nanosheet-based inks.

Rapidly emerging two-dimensional (2D) atomic layer crystals exhibit diverse, tunable electronic properties. They appear to be more flexible than 3D crystals with greater versatility and improved functionality in a wide range of potential applications. Among these 2D materials, metallic crystals are relatively unexplored although two allotropes of gallenene (2D gallium) have been synthesized on a range of substrates. Based on these experimental findings, we investigate systematically the group 13 metals using first-principles density functional theory calculations and an unbiased structural search. In this study, the electronic structure, bonding characteristics, and phonon properties of predicted 2D allotropes of group 13 metals are calculated, including the expected effects of strain induced by substrates on the dynamical stability. Theoretical results predict that most group 13 elements have one or more stable 2D allotropes with the preferred allotrope depending on the cell shape relaxation and strain, indicating that the substrate will determine the overall allotrope preferred. This demonstrates a new avenue for the discovery of thermodynamically stable 2D metallic layers, with properties potentially suitable for electronic and optoelectronic applications.

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.

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.

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.

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 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.

Graphene and other 2D atomic crystals are of considerable interest in catalysis because of their unique structural and electronic properties. Over the past decade, the materials have been used in a variety of reactions, including the oxygen reduction reaction, water splitting and CO2 activation, and have been shown to exhibit a range of catalytic mechanisms. Here, we review recent advances in the use of graphene and other 2D materials in catalytic applications, focusing in particular on the catalytic activity of heterogeneous systems such as van der Waals heterostructures (stacks of several 2D crystals). We discuss the advantages of these materials for catalysis and the different routes available to tune their electronic states and active sites. We also explore the future opportunities of these catalytic materials and the challenges they face in terms of both fundamental understanding and the development of industrial applications.

CONSPECTUS: After the discovery of graphene and the development of powerful exfoliation techniques, experimental preparation of two-dimensional (2D) crystals can be expected for any layered material that is known to chemistry. Besides graphene and hexagonal boron nitride (h-BN), transition metal chalcogenides (TMC) are among the most studied ultrathin materials. In particular, single-layer MoS2, a direct band gap semiconductor with ∼1.9 eV energy gap, is popular in physics and nanoelectronics, because it nicely complements semimetallic graphene and insulating h-BN monolayer as a construction component for flexible 2D electronics and because it was already successfully applied in the laboratory as basis material for transistors and other electronic and optoelectronic devices. Two-dimensional crystals are subject to significant quantum confinement: compared with their parent layered 3D material, they show different structural, electronic, and optical properties, such as spontaneous rippling as free-standing monolayer, significant changes of the electronic band structure, giant spin-orbit splitting, and enhanced photoluminescence. Most of those properties are intrinsic for the monolayer and already absent for two-layer stacks of the same 2D crystal. For example, single-layer MoS2 is a direct band gap semiconductor with spin-orbit splitting of 150 meV in the valence band, while the bilayer of the same material is an indirect band gap semiconductor without observable spin-orbit splitting. All these properties have been observed experimentally and are in excellent agreement with calculations based on density-functional theory. This Account reports theoretical studies of a subgroup of transition metal dichalcogenides with the composition MX2, with M = Mo, or W and X = Se or S, also referred to as "MoWSeS materials". Results on the electronic structure, quantum confinement, spin-orbit coupling, spontaneous monolayer rippling, and change of electronic properties in the presence of an external electric field are reported. While all materials of the MoWSeS family share the same qualitative properties, their individual values can differ strongly, for example, the spin-orbit splitting in WSe2 reaches the value of 428 meV, nearly three times that of MoS2. Further, we discuss the effect of strain on the electronic properties (straintronics). While MoWSeS single layers are very robust against external electric fields, bilayers show a linear reduction of the band gap, even reaching a semiconductor-metal phase transition, and an increase of the spin-orbit splitting from zero to the monolayer value at rather small fields. Strain is yet another possibility to control the band gap in a linear way, and MoWSeS monolayers become metallic at strain values of ∼10%. The density-functional based tight-binding model is a useful tool to investigate the electronic and structural properties, including electron conductance, of large MoS2 structures, which show spontaneous rippling in finite-temperature molecular dynamics simulations. Structural defects in MoS2 result in anisotropy of the electric conductivity. Finally, DFT predictions on the properties of noble metal dichalcogenides are presented. Most strikingly, 1T PdS2 is an indirect band gap semiconductor in its monolayer form but becomes metallic as a bilayer.

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.

The field of two-dimensional (2D) materials has seen remarkable progress, driven by their exceptional electronic, optical, and mechanical properties. In addition to their intrinsic qualities, 2D materials are atomically thin and readily integrated with a wide range of devices, offering immense potential for next-generation on-chip technologies across various domains, beyond just electronics and optics. Central to the fabrication of 2D devices is the development of effective transfer methods, which are crucial for maintaining clean material interfaces and intact material properties. However, as some of the most fundamental techniques, transfer methods are incredibly diverse, making it challenging to navigate the various approaches. This review provides a comprehensive analysis of the state-of-the-art transfer methods, including a preliminary discussion on high-quality 2D material preparation, along with evaluation of the strengths and weaknesses of various transfer techniques. Furthermore, despite being a foundational field, there are still many significant tasks to be undertaken, with several bottlenecks awaiting breakthroughs. We also highlight emerging trends, such as reconfigurable transfer, all-transfer for chip manufacturing, and the application of transfer techniques to low-dimensional materials across broader research fields, such as chemistry, polariton, tribology, haptics, thermal transport, thermodynamic control, quantum science, neuromorphic computing, electrocatalysts, and energy, offering insights into future directions for 2D material integration.

Size-dependent shape characteristics of 2D crystal blisters

A decade of intense research on two-dimensional (2D) atomic crystals has revealed that their properties can differ greatly from those of the parent compound. These differences are governed by changes in the band structure due to quantum confinement and are most profound if the underlying lattice symmetry changes. Here we report a high-quality 2D electron gas in few-layer InSe encapsulated in hexagonal boron nitride under an inert atmosphere. Carrier mobilities are found to exceed 103 cm2 V-1 s-1 and 104 cm2 V-1 s-1 at room and liquid-helium temperatures, respectively, allowing the observation of the fully developed quantum Hall effect. The conduction electrons occupy a single 2D subband and have a small effective mass. Photoluminescence spectroscopy reveals that the bandgap increases by more than 0.5 eV with decreasing the thickness from bulk to bilayer InSe. The band-edge optical response vanishes in monolayer InSe, which is attributed to the monolayer's mirror-plane symmetry. Encapsulated 2D InSe expands the family of graphene-like semiconductors and, in terms of quality, is competitive with atomically thin dichalcogenides and black phosphorus.

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.

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.

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.