huangpanqi

The recent progress of pentagonal 2D transition metal dichalcogenides, including preparation, defect engineering, physical and chemical properties, as well as their functionalities for various applications are summarized. The promising functional applications in electronics, optoelectronics, catalysts, and sensors are highlighted. A forward-looking outlook of pentagonal 2D transition metal dichalcogenides is also provided.

Abstract

2D materials with common hexagonal crystal structures, such as graphene, hexagonal boron nitride, and transition metal dichalcogenides have attracted great interest due to their novel physical and chemical properties. Pentagonal transition metal dichalcogenides (TMDs) exhibit distinct optical, electrical, and chemical properties, with valuable functionalities for various applications. This review highlights some of the most important developments in this field, with emphasis on their functionalities for neuromorphic computing, transistors, photodetection, catalysts, etc. Strategies for modifying their physical and chemical properties as well as device performance including defect engineering and interface engineering are presented. Finally, a forward-looking outlook of pentagonal 2D materials is discussed.

First, the anisotropic lattice structures of 2D materials are illustrated. Second, unique experimental methodologies are discussed for characterizing their anisotropic mechanics. Third, recent processes in anisotropic elastic, fracture, friction, and bending properties of 2D materials are reviewed. Subsequently, unique applications of these anisotropic properties are further highlighted. Finally, prospects for the developments of this field are suggested.

Anisotropic mechanics of van der Waals (vdWs) materials offers opportunity to peel off individual atomic layers, initiating a 2D revolution in the fields of materials science, physics, and chemistry. The elasticity, bending, and fracture strength of most of their 2D derivatives are also orientation-dependent, which not only determines the reliability of devices based on 2D materials but also offers a vast playground for atomic manufacturing with tunable functions. Therefore, a comprehensive understanding of the anisotropic mechanical properties of 2D materials is imminent. In this review, the anisotropic mechanical properties of 2D materials are summarized in attempt to capture the current progress in this field, as well as the route toward their applications. Following a brief discussion of the anisotropic lattice structures of 2D materials, unique experimental methodologies that have been developed to characterize their anisotropic mechanics are discussed. Then, the review pivots on recent processes in anisotropic elastic, fracture, friction, and bending properties of 2D materials. Unique applications of these anisotropic properties, such as mechanical fabrication of atomic precision, as well as anisotropic strain-induced piezoelectric and band modulation, are further highlighted. Finally, besides emphasizing the need for breakthrough in anisotropic mechanics, prospects for the developments of this field are suggested.

Here, using four metal halides and two chalcogenides as prototype material systems, their growth process is investigated and it is found that the growth temperature decreases by maximum 35% on van der Waals (vdW) templates compared with non-vdW substrates. This work provides a universal vdW template-assisted method for the low-temperature synthesis of high-crystallinity 2D materials toward applications in flexible electronics and carbon neutralization.

Abstract

To date, the synthesis of high-quality 2D crystals using vapor deposition methods usually requires high temperature, hindering the integration of 2D materials with Si circuits and exacerbating energy consumption. Exploring low-temperature growth strategies and understanding synthesis mechanism are critical for the practical application of 2D materials. Herein, a van der Waals (vdW) template-assisted growth of 2D crystals (including PbI₂, CdI₂, BiI₃, CuI, Sb₂Te_3, and Bi₂Se₃) is reported, the growth temperature decreases by maximum 35% compared with traditional vapor deposition. The low-temperature 2D growth process resulting from the low surface diffusion barrier of precursors on vdW surfaces is proposed, confirmed by the density functional theory and molecular dynamics calculations. Particularly, the grown 2D crystals can be peeled off from vdW templates easily and transferred to arbitrary substrates for functional applications and the exfoliated vdW templates can be reused for another round of growth. Although the growth temperature is reduced greatly, the excellent photoelectric performance of grown 2D crystals is demonstrated, benefitting from high crystalline quality. These findings provide a universal method for the low-temperature synthesis of high-crystallinity 2D materials toward applications in flexible electronics.

A pass-transistor logic configuration based on pseudo-NMOS is realized on a 4-inch high-quality monolayer MoS₂ wafer. Such preliminary integration efforts exhibit a promising future for 2D semiconductors in integrated circuit application.

Abstract

2D semiconductors, such as molybdenum disulfide (MoS₂), have attracted tremendous attention in constructing advanced monolithic integrated circuits (ICs) for future flexible and energy-efficient electronics. However, the development of large-scale ICs based on 2D materials is still in its early stage, mainly due to the non-uniformity of the individual devices and little investigation of device and circuit-level optimization. Herein, a 4-inch high-quality monolayer MoS₂ film is successfully synthesized, which is then used to fabricate top-gated (TG) MoS₂ field-effect transistors with wafer-scale uniformity. Some basic circuits such as static random access memory and ring oscillators are examined. A pass-transistor logic configuration based on pseudo-NMOS is then employed to design more complex MoS₂ logic circuits, which are successfully fabricated with proper logic functions tested. These preliminary integration efforts show the promising potential of wafer-scale 2D semiconductors for application in complex ICs.

Abstract

Two-dimensional anisotropic materials have been widely concerned by researchers because of their great application potential in the field of polarized detector devices and optical elements, which is a very important and popular research direction at present. As a IV–V two-dimensional material, silicon phosphide (SiP) has obvious in-plane anisotropy and exhibits excellent optical and electrical anisotropy properties. Herein, the optical anisotropy of SiP is studied by spectrometric ellipsometry measurements and polarization-resolved optical microscopy, and its electrical anisotropy is tested by SiP-based field-effect transistor. In addition, the normal and anisotropic photoelectric performance of SiP is shown by fabricating a photodetector and measuring it. In various measurements, SiP exhibits obvious anisotropy and good photoelectric performance. This work provides basic optical, electrical, and photoelectric performance information of SiP, and lays a foundation for further study of SiP and applications of SiP-based devices.

Hexagonal boron nitride (h-BN) has attracted great interest motivated by fascinating properties in the fields of quantum optics, electronics, and optoelectronics. The most recent discoveries of structural, optical, and electrical properties of h-BN and advancements in emerging photonic and electronic applications are reviewed.

Abstract

Hexagonal boron nitride (h-BN), an insulating 2D layered material, has recently attracted tremendous interest motivated by the extraordinary properties it shows across the fields of optoelectronics, quantum optics, and electronics, being exotic material platforms for various applications. At an early stage of h-BN research, it is explored as an ideal substrate and insulating layers for other 2D materials due to its atomically flat surface that is free of dangling bonds and charged impurities, and its high thermal conductivity. Recent discoveries of structural and optical properties of h-BN have expanded potential applications into emerging electronics and photonics fields. h-BN shows a very efficient deep-ultraviolet band-edge emission despite its indirect-bandgap nature, as well as stable room-temperature single-photon emission over a wide wavelength range, showing a great potential for next-generation photonics. In addition, h-BN is extensively being adopted as active media for low-energy electronics, including nonvolatile resistive switching memory, radio-frequency devices, and low-dielectric-constant materials for next-generation electronics.

Nature Materials, Published online: 23 June 2022; doi:10.1038/s41563-022-01301-6

A general method by controlling reaction kinetics is proposed to synthesize 67 kinds of two-dimensional crystal with custom-made phases and compositions, in particular, Fe- and Cr-based (layered and non-layered) chalcogenides and phosphorous chalcogenides, which show interesting ferromagnetism and superconductivity properties.

Nature Materials, Published online: 23 June 2022; doi:10.1038/s41563-022-01291-5

A competitive-chemical-reaction-based growth mechanism by controlling the kinetic parameters can easily realize the growth of transition metal chalcogenides and transition metal phosphorous chalcogenides with different compositions and phases.

An erbium-doped optical amplifier is fabricated on a silicon-nitride-based optical platform.

Defect-rich MoS₂ QDs are obtained via a mild biomineralization-assisted bottom-up strategy to show enhanced photoluminescence and reactive oxygen species (ROS) generation due to great defect/active sites elevation.

Abstract

Transition metal dichalcogenide (TMD) quantum dots (QDs) with defects have attracted interesting chemistry due to the contribution of vacancies to their unique optical, physical, catalytic, and electrical properties. Engineering defined defects into molybdenum sulfide (MoS₂) QDs is challenging. Herein, by applying a mild biomineralization-assisted bottom-up strategy, blue photoluminescent MoS₂ QDs (B-QDs) with a high density of defects are fabricated. The two-stage synthesis begins with a bottom-up synthesis of original MoS₂ QDs (O-QDs) through chemical reactions of Mo and sulfide ions, followed by alkaline etching that creates high sulfur-vacancy defects to eventually form B-QDs. Alkaline etching significantly increases the photoluminescence (PL) and photo-oxidation. An increase in defect density is shown to bring about increased active sites and decreased bandgap energy; which is further validated with density functional theory calculations. There is strengthened binding affinity between QDs and O₂ due to lower gap energy (∆E _ST) between S₁ and T₁, accompanied with improved intersystem crossing (ISC) efficiency. Lowered gap energy contributes to assist e⁻–h⁺ pair formation and the strengthened binding affinity between QDs and ³O₂. Defect engineering unravels another dimension of material properties control and can bring fresh new applications to otherwise well characterized TMD nanomaterials.

Publication date: October 2022

Source: Progress in Materials Science, Volume 130

Author(s): Bo Li, Yue Luo, Yufeng Zheng, Xiangmei Liu, Lei Tan, Shuilin Wu

A layered air-stable topological crystalline insulator (TCI) candidate, ErAsS, is designed and synthesized, which maps the atomic layers in real space to the band topology in momentum space. The distorted As atomic layer and magnetic order of ErAsS induce both the hourglass fermion surface state and the magnetic-tuned exotic phases including the possible magnetic TCI.

Abstract

Topological crystalline insulators (TCIs) with hourglass fermion surface state have attracted a lot of attention and are further enriched by crystalline symmetries and magnetic order. Here, the emergence of hourglass fermion surface state and exotic phases in the newly discovered, air-stable ErAsS single crystals are shown. In the paramagnetic phase, ErAsS is expected to be a TCI with hourglass fermion surface state protected by the nonsymmorphic symmetry. Dirac-cone-like bands and nearly linear dispersions in large energy range are experimentally observed, consistent well with theoretical calculations. Below T_N  ≈ 3.27 K, ErAsS enters a collinear antiferromagnetic state, which is a trivial insulator breaking the time-reversal symmetry. An intermediate incommensurate magnetic state appears in a narrow temperature range (3.27–3.65 K), exhibiting an abrupt change in magnetic coupling. The results reveal that ErAsS is an experimentally available TCI candidate and provide a unique platform to understand the formation of hourglass fermion surface state and explore magnetic-tuned topological phase transitions.

Nature Electronics, Published online: 06 June 2022; doi:10.1038/s41928-022-00769-z

Transistors based on two-dimensional semiconductors suffer from electrical instabilities because charges readily get trapped in the gate oxides. As charge trapping is sensitive to the energetic alignment of the channel Fermi level to the defect bands in the oxide, the number of electrically active traps can be reduced by tuning the channel Fermi level.

2D magnetic semiconductor α-MnSe flakes are synthesized by space-confined CVD. Impressively, the spin-ordering-related magnons and defects confirmed by low-temperature photoluminescence spectra confer themselves with a broadband luminescence from 550 to 880 nm, an ultraviolet–near-infrared photoresponse from 365 to 808 nm, and enhanced photon-to-electron conversion performance at 80 K.

Abstract

Two-dimensional (2D) magnetic semiconductors are considered to have great application prospects in spintronic logic devices, memory devices, and photodetectors, due to their unique structures and outstanding physical properties in 2D confinement. Understanding the influence of magnetism on optical/optoelectronic properties of 2D magnetic semiconductors is a significant issue for constructing multifunctional electronic devices and implementing sophisticated functions. Herein, the influence of spin ordering and magnons on the optical/optoelectronic properties of 2D magnetic semiconductor α-MnSe synthesized by space-confined chemical vapor deposition (CVD) is explored systematically. The spin-ordering-induced magnetic phase transition triggers temperature-dependent photoluminescence spectra to produce a huge transition at Néel temperature (T _N  ≈ 160 K). The magnons- and defects-induced emissions are the primary luminescence path below T _N and direct internal ⁴ _aT_1g→⁶A_1g transition-induced emissions are the main luminescence path above T _N . Additionally, the magnons and defect structures endow 2D α-MnSe with a broadband luminescence from 550 to 880 nm, and an ultraviolet–near-infrared photoresponse from 365 to 808 nm. Moreover, the device also demonstrates improved photodetection performance at 80 K, possibly influenced by spin ordering and trap states associated with defects. These above findings indicate that 2D magnetic semiconductor α-MnSe provides an excellent platform for magneto-optical and magneto-optoelectronic research.

Nature Photonics, Published online: 02 June 2022; doi:10.1038/s41566-022-01012-z

The strongly temperature-dependent band-edge absorption from gallium arsenide enables an optical thermometer with nanokelvin temperature resolution and microscale spatial resolution.