Jiuxiang Dai

Nature Materials, Published online: 07 December 2022; doi:10.1038/s41563-022-01441-9

Highlights

• Two-dimensional transition metal borides have high mechanical stability, high charge carrier mobility and great electrochemical performance.

• The potential applications of two-dimensional transition metal borides in the direction of energy conversion and storage have not been systematically reviewed.

• We summarize the research on the role of two-dimensional transition metal borides in catalysis and ion batteries, and put forward the new opportunities in preparation and biotechnology.

Nature Electronics, Published online: 05 December 2022; doi:10.1038/s41928-022-00877-w

Abstract

Small contact resistance and low Schottky barrier height (SBH) are the keys to energy-efficient electronics and optoelectronics. Two-dimensional (2D) semiconductors-based field effect transistors (FETs), holding great promise for next-generation information circuits, still suffer from poor contact quality at the metal—semiconductor junction interface, which severely hinders their further applications. Here, a novel contact strategy is proposed, where Bi₂Te₃ nanosheets with high conductivity were in-situ epitaxially grown on MoS₂ as van der Waals contacts, which can effectively avoid the damage to MoS₂ caused during the device manufacturing process, leading to a high-performance MoS₂ FET. Moreover, the small work function difference between Bi₂Te₃ and MoS₂ (Bi₂Te₃: 4.31 eV, MoS₂: 4.37 eV, measured by Kelvin probe force microscopy (KPFM)), enables small band bending and Ohmic contact at the junction interface. Electrical characterizations indicate that the MoS₂ FET device with Bi₂Te₃ contacts possesses a high current on/off ratio (5 × 10⁷), large effective carrier mobility (90 cm²/(V·s)), and low flat-band SBH (60 meV), which is favorable as compared with MoS₂ FET with traditional Cr/Au electrodes contacts, and superior to the vast majority of the reported chemical vapor deposition (CVD) MoS₂-based FET device. The demonstration of epitaxial van der Waals Bi₂Te₃ contacts will facilitate the application of 2D MoS₂ nanosheet in next-generation low-power consumption electronics and optoelectronics.

Abstract

Piezoelectricity is the electric charge which accumulates in certain materials in response to mechanical stimuli, while piezoelectric nanogenerators (PENGs) converting mechanical energy into electricity can be widely used for energy harvesting and self-powered systems. The group IV–VI monochalcogenides may exhibit strong piezoelectricity because of their puckered C_2v symmetry and electronic structure, making them promising for flexible PENG. Herein, we investigated the synthesis and piezoelectric properties of multilayer SnSe nanosheets grown by chemical vapor deposition (CVD). The SnSe nanosheets exhibited high single-crystallinity, large area, and good stability. The strong layer-dependent in-plane piezoelectric coefficient of SnSe nanosheets showed a saturated trend to be ∼ 110 pm/V, which overcomes the weak piezoelectric response or odd-even effects in other layered nanosheets. A high energy conversion efficiency of 9.3% and a maximum power density of 538 mW/cm² at 1.03% strain have been demonstrated in a SnSe-based PENG. Based on the enhanced piezoelectricity of SnSe and attractive output performance of the nanogenerator, a self-powered sensor for human motion monitoring is further developed. These results demonstrate the strong piezoelectricity in high quality CVD-grown SnSe nanosheets, allowing for application in flexible smart piezoelectric sensors and advanced microelectromechanical devices.

Abstract

Two-dimensional (2D) layered materials have attracted extensive research interest in the field of high-performance photodetection due to their high carrier mobility, tunable bandgap, stability, and other excellent properties. Herein, we propose a gate-tunable, high-performance, self-driving, and wide detection range phototransistor based on a 2D PtSe₂ on silicon-on-insulator (SOI). Benefiting from the strong built-in electric field of the PtSe₂/Si heterostructure, the phototransistor has a fast response time (rise/fall time) of 36.7/32.6 µs. The PtSe₂/Si phototransistor exhibits excellent photodetection performance over a broad spectral range from ultraviolet to near-infrared, including a responsivity of 1.07 A/W and a specific detectivity of 6.60 × 10⁹ Jones under 808 nm illumination at zero gate voltage. The responsivity and specific detectivity of PtSe₂/Si phototransistor at 5 V gate voltage are increased to 13.85 A/W and 1.90 × 10¹⁰ Jones under 808 nm illumination. Furthermore, the fabricated PtSe₂/Si phototransistor array shows excellent uniformity, reproducibility, and long-term stability in terms of photoresponse performance with negligible variation between pixel cells. The architecture of present PtSe2/Si on SOI platform paves a new way of a general strategy to realize high-performance photodetectors by combining the advantages of both 2D materials and conventional semiconductors which is compatible with current Si-complementary metal oxide semiconductor (CMOS) process.

Copper iodide (CuI) is a high-performance p-type transparent semiconductor. Yet, CuI lacks processes for scalable and conformal thin film deposition. In this work, this issue is tackled by a novel approach relying on atomic layer deposition of CuO and its conversion into CuI at room temperature, yielding phase-pure, uniform, and high purity CuI thin films.

Abstract

Copper iodide (CuI) is a high-performance p-type transparent semiconductor that can be used in numerous applications, such as transistors, diodes, and solar cells. However, the lack of conformal and scalable methods to deposit CuI thin films limits its establishment in applications that involve complex-shaped and/or large substrate areas. In this work, atomic layer deposition (ALD) is employed to enable scalable and conformal thin film deposition. A two-step approach relying on ALD of CuO and its subsequent conversion to CuI via exposure to HI vapor at room temperature is demonstrated. The resulting CuI films are phase-pure, uniform, and of high purity. Furthermore, CuI films on several substrates such as Si, amorphous Al₂O₃, n-type TiO₂, and γ-CsPbI₃ perovskite are prepared. With the resulting n-TiO₂/p-CuI structure, the easy and straightforward fabrication of a diode structure as a proof-of-concept device is demonstrated. Moreover, the successful deposition of CuI on γ-CsPbI₃ proves the compatibility of the process for using CuI as the hole transport layer in perovskite solar cell applications in the nip-configuration. It is believed that the ALD-based approach described in this work will offer a viable alternative for depositing transparent conductive p-type CuI thin films in applications that involve complex high aspect ratio structures and large substrate areas.

Fifty-one types of MoSi₂N₄ and WSi₂N₄-based van der Waals heterostructures (vdWHs) are systematically classified. Multiple candidates with exceptional excitonic solar cell power conversion efficiency and strong optical absorption in the UV regimes are identified. The findings unravel MoSi₂N₄-and WSi₂N₄-based vdWHs as a compelling material platform for designing high-performance excitonic solar cells and photonics devices.

Abstract

2D materials van der Waals heterostructures (vdWHs) provide a revolutionary route toward high-performance solar energy conversion devices beyond the conventional silicon-based pn junction solar cells. Despite tremendous research progress accomplished in recent years, the searches of vdWHs with exceptional excitonic solar cell conversion efficiency and optical properties remain an open theoretical and experimental quest. Here, this study shows that the vdWH family composed of MoSi₂N₄ and WSi₂N₄ monolayers provides a compelling material platform for developing high-performance ultrathin excitonic solar cells and photonics devices. Using first-principle calculations, 51 types of MoSi₂N₄ and WSi₂N₄-based [(Mo,W)Si₂N₄] vdWHs composed of various metallic, semimetallic, semiconducting, insulating, and topological 2D materials are constructed and classified. Intriguingly, MoSi₂N₄/(InSe, WSe₂) are identified as Type II vdWHs with exceptional excitonic solar cell power conversion efficiency reaching well over 20%, which are competitive to state-of-the-art silicon solar cells. The (Mo,W)Si₂N₄ vdWH family exhibits strong optical absorption in both the visible and UV regimes. Exceedingly large peak UV absorptions over 40%, approaching the maximum absorption limit of a freestanding 2D material, can be achieved in (Mo,W)Si₂N₄/α₂-(Mo,W)Ge₂P₄ vdWHs. The findings unravel the enormous potential of (Mo,W)Si₂N₄ vdWHs in designing ultimately compact excitonic solar cell device technology.

By combination of vacuum annealing and fast cooling of epitaxial MoTe₂ ultrathin films, Mo₆Te₆ nanowires stacking on varying van der Waals surface are obtained. Through scanning tunneling microscopy measurement, local defects in forms of single Te vacancies or axial twist are observed. Moreover, the 1D electronic properties of pristine and defective Mo₆Te₆ nanowires are confirmed.

Abstract

The sub-nanometer-diameter transitional-metal chalcogenides (M₆X₆) molecular wires are ideal 1D quantum systems. The electronic properties of such system are very sensitive to the interface interaction and local imperfection. Here, the atomic structure and local electronic structure of van der Waals (vdW) stacking Mo₆Te₆ nanowires fabricated by using molecular beam epitaxy are reported. Atomic-resolution scanning tunneling microscopy measurement shows that the vdW interface distance varies from 0.71 to 1.05 nm when Mo₆Te₆ wires stacked on graphite, MoTe₂, and Mo₆Te₆ surfaces. Scanning tunneling spectroscopy confirmed the 1D quantum effect of van Hove singularity and Tomonaga–Luttinger liquid behavior at 77.8 K. Single Te vacancies and their effect to the local structure distortion are observed as well. These observations shed light on the local structure and quantum effects of the M₆X₆ nanowire materials, which may find applications in future electronic devices.

Giant tunability of charge transport in two-dimensional inorganic molecular crystals, from an insulator to a semiconductor, is revealed for the first time through in situ high-pressure experimental characterizations.

Abstract

The unique intermolecular van der Waals force in emerging two-dimensional inorganic molecular crystals (2DIMCs) endows them with highly tunable structures and properties upon applying external stimuli. Using high pressure to modulate the intermolecular bonding, here we reveal the highly tunable charge transport behavior in 2DIMCs for the first time, from an insulator to a semiconductor. As pressure increases, 2D α-Sb₂O₃ molecular crystal undergoes three isostructural transitions, and the intermolecular bonding enhances gradually, which results in a considerably decreased band gap by 25 % and a greatly enhanced charge transport. Impressively, the in situ resistivity measurement of the α-Sb₂O₃ flake shows a sharp drop by 5 orders of magnitude in 0–3.2 GPa. This work sheds new light on the manipulation of charge transport in 2DIMCs and is of great significance for promoting the fundamental understanding and potential applications of 2DIMCs in advanced modern technologies.

A space-confined CVD strategy is designed to synthesize atomically thin ε-Fe₂O₃ single crystals and disclose the room-temperature ferrimagnetism. The strong ferroelectricity and its switching behavior are discovered in atomically thin ε-Fe₂O₃, accompanied with an anomalous thickness-dependent coercive voltage. The robust room-temperature magnetoelectric coupling is uncovered by controlling the magnetism with electric field and verifies the multiferroic feature of ε-Fe₂O₃.

Abstract

2D multiferroics with magnetoelectric coupling combine the magnetic order and electric polarization in a single phase, providing a cornerstone for constructing high-density information storages and low-energy-consumption spintronic devices. The strong interactions between various order parameters are crucial for realizing such multifunctional applications, nevertheless, this criterion is rarely met in classical 2D materials at room-temperature. Here an ingenious space-confined chemical vapor deposition strategy is designed to synthesize atomically thin non-layered ε-Fe₂O₃ single crystals and disclose the room-temperature long-range ferrimagnetic order. Interestingly, the strong ferroelectricity and its switching behavior are unambiguously discovered in atomically thin ε-Fe₂O₃, accompanied with an anomalous thickness-dependent coercive voltage. More significantly, the robust room-temperature magnetoelectric coupling is uncovered by controlling the magnetism with electric field and verifies the multiferroic feature of atomically thin ε-Fe₂O₃. This work not only represents a substantial leap in terms of the controllable synthesis of 2D multiferroics with robust magnetoelectric coupling, but also provides a crucial step toward the practical applications in low-energy-consumption electric-writing/magnetic-reading devices.

Grain boundaries (GBs) provide a fascinating degree of freedom for modulating and converting the properties of polycrystalline 2D materials into versatile applications. This review summarizes the current understanding of diverse GB microstructures and functionalities in a wide range of 2D materials, and highlights the opportunities and challenges in establishing GB engineering for practical applications of 2D materials.

Abstract

The coalescence of randomly distributed grains with different crystallographic orientations can result in pervasive grain boundaries (GBs) in 2D materials during their chemical synthesis. GBs not only are the inherent structural imperfection that causes influential impacts on structures and properties of 2D materials, but also have emerged as a platform for exploring unusual physics and functionalities stemming from dramatic changes in local atomic organization and even chemical makeup. Here, recent advances in studying the formation mechanism, atomic structures, and functional properties of GBs in a range of 2D materials are reviewed. By analyzing the growth mechanism and the competition between far-field strain and local chemical energies of dislocation cores, a complete understanding of the rich GB morphologies as well as their dependence on lattice misorientations and chemical compositions is presented. Mechanical, electronic, and chemical properties tied to GBs in different materials are then discussed, towards raising the concept of using GBs as a robust atomic-scale scaffold for realizing tailored functionalities, such as magnetism, luminescence, and catalysis. Finally, the future opportunities in retrieving GBs for making functional devices and the major challenges in the controlled formation of GB structures for designed applications are commented.

This work demonstrates reversible phase transition in ultrathin molybdenum ditelluride (MoTe₂) controlled by thermal and mechanical mechanisms on the van der Waals (vdW) nanoelectromechanical systems (NEMS) platform. Benefiting from efficient electrothermal heating and straining effects in the suspended vdW heterostructures, reversible MoTe₂ phase transition is controlled by regulating temperature and strain level.

Abstract

This work reports experimental demonstrations of reversible crystalline phase transition in ultrathin molybdenum ditelluride (MoTe₂) controlled by thermal and mechanical mechanisms on the van der Waals (vdW) nanoelectromechanical systems (NEMS) platform, with hexagonal boron nitride encapsulated MoTe₂ structure residing on top of graphene layer. Benefiting from very efficient electrothermal heating and straining effects in the suspended vdW heterostructures, MoTe₂ phase transition is triggered by rising temperature and strain level. Raman spectroscopy monitors the MoTe₂ crystalline phase signatures in situ and clearly records reversible phase transitions between hexagonal 2H (semiconducting) and monoclinic 1T′ (metallic) phases. Combined with Raman thermometry, precisely measured nanomechanical resonances of the vdW devices enable the determination and monitoring of the strain variations as temperature is being regulated by electrothermal control. These results not only deepen the understanding of MoTe₂ phase transition, but also demonstrate a novel platform for engineering MoTe₂ phase transition and multiphysical devices.