Jiuxiang Dai

The great application potentials and challenges in high-performance devices inspire the search for new two-dimensional (2D) van der Waals (vdW) materials. Via molecular beam epitaxy, the vdW SnI₂ monolayer is successfully fabricated with a new structure. This SnI₂ monolayer is a 2D semiconductor with thickness-dependent properties, which is promising as a building block in future electronics/optoelectronics.

Abstract

Two-dimensional (2D) van der Waals (vdW) materials have garnered considerable attention for their unique properties and potentials in a wide range of fields, which include nano-electronics/optoelectronics, solar energy, and catalysis. Meanwhile, challenges in the approaches toward achieving high-performance devices still inspire the search for new 2D vdW materials with precious properties. In this study, via molecular beam epitaxy, for the first time, the vdW SnI₂ monolayer is successfully fabricated with a new structure. Scanning tunneling microscopy/spectroscopy characterization, as corroborated by the density functional theory calculation, indicates that this SnI₂ monolayer exhibits a band gap of ≈2.9 eV in the visible purple range, and an indirect- to direct-band gap transition occurs in the SnI₂ bilayer. This study provides a new semiconducting 2D material that is promising as a building block in future electronics/optoelectronics.

The transition metal carbides and nitrides (MXenes) are among the largest 2D material families. MXenes’ unique properties, such as their metal-like electrical conductivity, render them quite useful in a large number of applications including energy storage, optoelectronic, biomedical, communications, and environmental. A brief historical overview of the first 10 years of MXene research and a perspective on their synthesis and future development are provided.

Abstract

Since their discovery in 2011, the number of 2D transition metal carbides and nitrides (MXenes) has steadily increased. Currently more than 40 MXene compositions exist. The ultimate number is far greater and in time they may develop into the largest family of 2D materials known. MXenes’ unique properties, such as their metal-like electrical conductivity reaching ≈20 000 S cm⁻¹, render them quite useful in a large number of applications, including energy storage, optoelectronic, biomedical, communications, and environmental. The number of MXene papers and patents published has been growing quickly. The first MXene generation is synthesized using selective etching of metal layers from the MAX phases, layered transition metal carbides and carbonitrides using hydrofluoric acid. Since then, multiple synthesis approaches have been developed, including selective etching in a mixture of fluoride salts and various acids, non-aqueous etchants, halogens, and molten salts, allowing for the synthesis of new MXenes with better control over their surface chemistries. Herein, a brief historical overview of the first 10 years of MXene research and a perspective on their synthesis and future development are provided. The fact that their production is readily scalable in aqueous environments, with high yields bodes well for their commercialization.

Platinum atomic layer deposition and thermal assisted conversion are combined to conformally and selectively coat structured substrates with layered platinum diselenide. The viability of the approach to controllably fabricate hybrid devices with a 2D material is demonstrated by the manufacture of a structured gas sensor and a fully integrated infrared photodetector on a Si-waveguide.

Abstract

2D materials display very promising intrinsic material properties, with multiple applications in electronics, photonics, and sensing. In particular layered platinum diselenide has shown high potential due to its layer-dependent tunable bandgap, low-temperature growth, and high environmental stability. Here, the conformal and area selective (AS) low-temperature growth of layered PtSe₂ is presented defining a new paradigm for 2D material integration. The thermally-assisted conversion of platinum which is deposited by AS atomic layer deposition to PtSe₂ is demonstrated on various substrates with a distinct 3D topography. Further the viability of the approach is presented by successful on-chip integration of hybrid semiconductor devices, namely by the manufacture of a highly sensitive ammonia sensors channel with 3D topography and fully integrated infrared-photodetectors on silicon photonics waveguides. The presented methodologies of conformal and AS growth therefore lay the foundation for new design routes for the synthesis of more complex hybrid structures with 2D materials.

Publication date: September 2021

Source: Materials Today, Volume 48

Author(s): Laurie Donaldson

Nature, Published online: 18 August 2021; doi:10.1038/d41586-021-02191-5

Supersolids are exotic materials whose constituent particles can simultaneously form a crystal and flow without friction. The first 2D supersolid has been produced using ultracold gases of highly magnetic atoms.

The dynamics of suspended two-dimensional (2D) materials has received increasing attention during the last decade, yielding new techniques to study and interpret the physics that governs the motion of atomically thin layers. This has led to insights into the role of thermodynamic and nonlinear effects as well as the mechanisms that govern dissipation and stiffness in these resonators. In this review, we present the current state-of-the-art in the experimental study of the dynamics of 2D membranes. The focus will be both on the experimental measurement techniques and on the interpretation of the physical phenomena exhibited by atomically thin membranes in the linear and nonlinear regimes. We will show that resonant 2D membranes have emerged both as sensitive probes of condensed matter physics in ultrathin layers, and as sensitive elements to monitor small external forces or other changes in the environment. New directions for utilizing suspended 2D membranes for material characte...

Synaptic Devices

With lithography-free, directlaser-writing-controlled MoS₂/MoS_2−x O_δ grain boundaries, synaptic devices fabricated by Kai Liu and co-workers, as described in article number 2102435, exhibit short-term and long-term plasticity characteristics that are responsive to electric and light stimulation simultaneously; they also exhibit a low energy consumption that is over 40 times lower than that of conventional complementary metal–oxide–semiconductors

Germanene

Several layers of germanene with Dirac characteristics and ultrafast carrier dynamics are produced by Guanyu Liu, Yi Du, Qiaoliang Bao, and co-workers, as reported in article number 2101042. As a new group-IV member material, germanene is further demonstrated as a promising saturable absorber to construct ultrafast mode-locked lasers and generate sub-picosecond pulses in the telecommunication band.

To address the demand for ubiquitous, energy-neutral light sensing, this study investigates for the first time self-powered photodetectors based on 2D antimony-based perovskite-inspired materials. The higher structural dimensionality of these materials enables cutting-edge performance for antimony- and bismuth-based, perovskite-inspired, self-powered photodetectors. Detailed characterization reveals that this performance boost originates from the enhanced optoelectronic properties associated with the 2D structure.

Abstract

The ongoing Internet of Things revolution has led to strong demand for low-cost, ubiquitous light sensing based on easy-to-fabricate, self-powered photodetectors. While solution-processable lead-halide perovskites have raised significant hopes in this regard, toxicity concerns have prompted the search for safer, lead-free perovskite-inspired materials (PIMs) with similar optoelectronic potential. Antimony- and bismuth-based PIMs are found particularly promising; however, their self-powered photodetector performance to date has lagged behind the lead-based counterparts. Aiming to realize the full potential of antimony-based PIMs, this study examines, for the first time, the impact of their structural dimensionality on their self-powered photodetection capabilities, with a focus on 2D Cs₃Sb₂I_{9− x} Cl _x and Rb₃Sb₂I₉ and 0D Cs₃Sb₂I₉. The 2D absorbers deliver cutting-edge self-powered photodetector performance, with a more-than-tenfold increase in external quantum efficiency (up to 55%), speed of response (>5 kHz), and linear dynamic range (>four orders of magnitude) compared to prior self-powered A₃M₂X₉ implementations (A⁺: monovalent cation; M³⁺: Sb³⁺/Bi³⁺; X⁻: halide anion). Detailed characterization reveals that such a performance boost originates from the superior carrier lifetimes and reduced exciton self-trapping enabled by the 2D structure. By delivering cutting-edge performance and mechanistic insight, this study represents an important step in lead-free perovskite-inspired optoelectronics toward self-powered, ubiquitous light sensing.

2D semiconductors based on transition metal dichalcogenides are highly promising for ultrathin photodetectors due to their thickness in the nanometer range and their exceptional light absorption properties. To enable efficient separation of optically generated electron–hole pairs heterostructures have to be implemented, which are usually prepared by poorly controlled mechanical steps such as exfoliation, transfer and stacking processes that prevent industrial upscaling. Here, semitransparent photodetectors in the mm 2 range based on MoS 2 /WS 2 heterostructures are presented that are realized without any transfer step by a scalable metal-organic chemical vapor deposition process on a sapphire substrate in a continuous growth run. The heterostructure device exhibits a responsivity, which is enhanced by about 5–6 orders of magnitude with respect to reference devices based on either MoS 2 or WS 2 monolayers only. The large gain enhan...

The field of two-dimensional (2D) materials has been developing at an impressive pace, with atomically thin crystals of an increasing number of different compounds that have become available, together with techniques enabling their assembly into functional heterostructures. The strategy to detect these atomically thin crystals—based on optical contrast enhanced by Fabry–Pérot interference—has however remained unchanged since the discovery of graphene. Such an absence of evolution is starting to pose problems, because for many of the 2D materials of current interest the optical contrast provided by the commonly used detection procedure is insufficient to identify the presence of individual monolayers, or to determine unambiguously the thickness of atomically thin multilayers. Here we explore an alternative detection strategy, in which the enhancement of optical contrast originates from the use of optically inhomogeneous substrates, leading to diffusively reflected light. Owing to...

Hyperspectral imaging at cryogenic temperatures is used to investigate exciton and trion propagation in MoSe 2 monolayers (ML) encapsulated with hexagonal boron nitride (hBN). Under a tightly focused, continuous-wave laser excitation, the spatial distribution of neutral excitons and charged trions strongly differ at high excitation densities. Remarkably, in this regime the trion distribution develops a halo shape, similar to that previously observed in WS 2 ML at room temperature and under pulsed excitation. In contrast, the exciton distribution only presents a moderate broadening attributed to a much less pronounced halo. Spatially and spectrally resolved luminescence spectra reveal the buildup of a significant temperature gradient at high excitation power, that is attributed to the energy relaxation of photoinduced hot carriers. We show, via a numerical resolution of the transport equations for excitons and trions, that the halo can be interpreted as therma...