Xingxing Zhang

Recent advances in the fabrication of 2D metal oxides (2DMOs) via liquid metals are comprehensively reviewed. The introduction of progress in fabricating 2DMOs by virtue of the features of liquid metals improves diversity in the development and applications of 2DMOs. The current challenges concerning the fabrication of 2DMOs are discussed to promote future research.

Abstract

2D metal oxides (2DMOs) have been widely applied in the fields of electronic, magnetic, optical, and catalytic materials, owing to their rich surface chemistry and unique electronic structures. However, their further development faces challenges such as the difficulty in fabricating 2DMOs with unstable surface induced by strong surface polarizability, or the high cost and limited yield of the fabrication process. Recently, liquid metals have shown great potential in the fabrication of 2DMOs. The native oxide skin formed on the surface of liquid metals can be considered as a perfect 2D planar material. Due to the solubility, fluidity, and reactivity of liquid metals, they can act as the solvent, reactant, and interface in the fabrication of 2DMOs. Moreover, liquid metals undergo a liquid–solid phase transition, enabling them to be a symmetric matched substrate for growing high‐quality 2DMOs. An insightful survey of the recent progress in this research direction is presented. The features of liquid metals including good solubility, chemical reactivity, weak interface force, and liquid–solid phase transitions are introduced in detail. Furthermore, strategies for the fabrication of 2DMOs by virtue of these features are summarized comprehensively. Finally, current challenges and prospects regarding the future development in the fabrication of 2DMOs via liquid metals are highlighted.

A comprehensive overview of hetero‐MXenes, from theoretical predictions to experimental investigations, is presented. The reviewed theoretical calculations focus on three sites of MXenes and the reviewed metal‐/nonmetal‐doped/substituted MXenes are experimentally synthesized by in situ/ex situ strategies. It is believed that this work will be beneficial to bridge the predictions and experiments, further promoting the development of hetero‐MXenes.

Abstract

Since their discovery in 2011, MXenes (abbreviation for transition metal carbides, nitrides, and carbonitrides) have emerged as a rising star in the family of 2D materials owing to their unique properties. Although the primary research interest is still focused on pristine MXenes and their composites, much attention has in recent years been paid also to MXenes with diverse compositions. To this end, this work offers a comprehensive overview of the progress on compositional engineering of MXenes in terms of doping and substituting from theoretical predictions to experimental investigations. Synthesis and properties are briefly introduced for pristine MXenes and then reviewed for hetero‐MXenes. Theoretical calculations regarding the doping/substituting at M, X, and T sites in MXenes and the role of vacancies are summarized. After discussing the synthesis of hetero‐MXenes with metal/nonmetal (N, S, P) elements by in situ and ex situ strategies, the focus turns to their emerging applications in various fields such as energy storage, electrocatalysts, and sensors. Finally, challenges and prospects of hetero‐MXenes are addressed. It is anticipated that this review will be beneficial to bridge the gap between predictions and experiments as well as to guide the future design of hetero‐MXenes with high performance.

In article number 2005254, Xiaoyan Zhang and co‐workers summarize the recent progress in the construction of heterostructures based on black phosphorus nanosheets, ranging from 2D hybrid structures to 3D networks. The preparation methods, optical and electronic properties, and various applications of these heterostructures are discussed. Critical challenges and future directions in this area are also highlighted.

Recent advances in the fabrication of 2D metal oxides (2DMOs) via liquid metals are comprehensively reviewed. The introduction of progress in fabricating 2DMOs by virtue of the features of liquid metals improves diversity in the development and applications of 2DMOs. The current challenges concerning the fabrication of 2DMOs are discussed to promote future research.

Abstract

2D metal oxides (2DMOs) have been widely applied in the fields of electronic, magnetic, optical, and catalytic materials, owing to their rich surface chemistry and unique electronic structures. However, their further development faces challenges such as the difficulty in fabricating 2DMOs with unstable surface induced by strong surface polarizability, or the high cost and limited yield of the fabrication process. Recently, liquid metals have shown great potential in the fabrication of 2DMOs. The native oxide skin formed on the surface of liquid metals can be considered as a perfect 2D planar material. Due to the solubility, fluidity, and reactivity of liquid metals, they can act as the solvent, reactant, and interface in the fabrication of 2DMOs. Moreover, liquid metals undergo a liquid–solid phase transition, enabling them to be a symmetric matched substrate for growing high‐quality 2DMOs. An insightful survey of the recent progress in this research direction is presented. The features of liquid metals including good solubility, chemical reactivity, weak interface force, and liquid–solid phase transitions are introduced in detail. Furthermore, strategies for the fabrication of 2DMOs by virtue of these features are summarized comprehensively. Finally, current challenges and prospects regarding the future development in the fabrication of 2DMOs via liquid metals are highlighted.

The single‐step growth of alloy/alloy (MoS_{2(1‐ x )}Se_{2 x} /SnS_{2(1‐ y )}Se_{2 y} ) vertical heterostructures is demonstrated and the heterostructure exhibits nearly intrinsic Van der Waals (vdW) interface in terms of a near‐atomically sharp and defect‐free boundary along the interface as well as a well‐aligned epitaxial relationship. The almost‐ideal interface enables the identification of the intrinsic behaviors of the heterostructures such as the band alignment, charge transfer, and carrier transport.

Abstract

The interfacial tunable band alignment of heterostructures is coveted in device design and optimization of device performance. As an intentional approach, alloying allows band engineering and continuous band‐edge tunability for low‐dimensional semiconductors. Thus, combining the tunability of alloying with the band structure of a heterostructure is highly desirable for the improvement of device characteristics. In this work, the single‐step growth of alloy‐to‐alloy (MoS_{2(1‐ x )}Se_{2 x} /SnS_{2(1‐ y )}Se_{2 y} ) 2D vertical heterostructures is demonstrated. Electron diffraction reveals the well‐aligned heteroepitaxial relationship for the heterostructure, and a near‐atomically sharp and defect‐free boundary along the interface is observed. The nearly intrinsic van der Waals (vdW) interface enables measurement of the intrinsic behaviors of the heterostructures. The optimized type‐II band alignment for the MoS_{2(1‐ x )}Se_{2 x} /SnS_{2(1‐ y )}Se_{2 y} heterostructure, along with the large band offset and effective charge transfer, is confirmed through quenched PL spectroscopy combined with density functional theory calculations. Devices based on completely stacked heterostructures show one or two orders enhanced electron mobility and rectification ratio than those of the constituent materials. The realization of device‐quality alloy‐to‐alloy heterostructures provides a new material platform for precisely tuning band alignment and optimizing device applications.

The library of atomically thin 2D materials featuring non‐volatile resistive switching can provide a promising and broad platform for exploring the sub‐nanometer scaling limit, which is beneficial for emerging device concepts. A dissociation–diffusion–adsorption (DDA) model is proposed to describe the common conductive‐point mechanism behind 2D‐materials‐based universal resistive switching and supported by systematic density functional theory (DFT) calculations showing favorable adsorption of metal into native defects.

Abstract

Non‐volatile resistive switching (NVRS) is a widely available effect in transitional metal oxides, colloquially known as memristors, and of broad interest for memory technology and neuromorphic computing. Until recently, NVRS was not known in other transitional metal dichalcogenides (TMDs), an important material class owing to their atomic thinness enabling the ultimate dimensional scaling. Here, various monolayer or few‐layer 2D materials are presented in the conventional vertical structure that exhibit NVRS, including TMDs (MX₂, M = transitional metal, e.g., Mo, W, Re, Sn, or Pt; X = chalcogen, e.g., S, Se, or Te), TMD heterostructure (WS₂/MoS₂), and an atomically thin insulator (h‐BN). These results indicate the universality of the phenomenon in 2D non‐conductive materials, and feature low switching voltage, large ON/OFF ratio, and forming‐free characteristic. A dissociation–diffusion–adsorption model is proposed, attributing the enhanced conductance to metal atoms/ions adsorption into intrinsic vacancies, a conductive‐point mechanism supported by first‐principle calculations and scanning tunneling microscopy characterizations. The results motivate further research in the understanding and applications of defects in 2D materials.

The WSe₂ monolayer in 1T’ phase is a large‐gap quantum spin Hall insulator, but is thermodynamically metastable. Thanks to the enhanced interface interaction, a single‐phase 1T'‐WSe₂ monolayer is grown on SrTiO₃(100) substrate using the molecular beam epitaxial method, and the interlayer in‐plane strain also drives the 1T'‐WSe₂ into a semimetallic phase.

Abstract

The WSe₂ monolayer in 1T’ phase is reported to be a large‐gap quantum spin Hall insulator, but is thermodynamically metastable and so far the fabricated samples have always been in the mixed phase of 1T’ and 2H, which has become a bottleneck for further exploration and potential applications of the nontrivial topological properties. Based on first‐principle calculations in this work, it is found that the 1T’ phase could be more stable than 2H phase with enhanced interface interactions. Inspired by this discovery, SrTiO₃ (100) is chosen as substrate and WSe₂ monolayer is successfully grown in a 100% single 1T’ phase using the molecular beam epitaxial method. Combining in situ scanning tunneling microscopy and angle‐resolved photoemission spectroscopy measurements, it is found that the in‐plane compressive strain in the interface drives the 1T'‐WSe₂ into a semimetallic phase. Besides providing a new material platform for topological states, the results show that the interface interaction is a new approach to control both the structure phase stability and the topological band structures of transition metal dichalcogenides.

In article number 2005612, Pai‐Chun Wei, Hsin‐Jay Wu, and co‐workers leverage the interplay between the spinodal decomposition and athermal phase transition in GeTe‐based thermoelectric materials. The exquisite microstructure, featured by strong composition fluctuations and the coexistence of rhombohedral‐ and cubic‐GeTe, yield high zT values of >2.6. This work provides a new thermodynamic route to developing higher‐performance thermoelectric materials in general.

Higher‐order exchange and quantum effects are widely known to play an important role in nanomagnets. However, it is unclear whether they are important in recently discovered 2D magnets. It is shown that biquadratic exchange interactions and quantum‐rescaling corrections are the keys on the long‐range ordering of 2D CrI₃ layers. Metastability induces domain walls of hybrid types with different chirality.

Abstract

Higher‐order exchange interactions and quantum effects are widely known to play an important role in describing the properties of low‐dimensional magnetic compounds. Here, the recently discovered 2D van der Waals (vdW) CrI₃ is identified as a quantum non‐Heisenberg material with properties far beyond an Ising magnet as initially assumed. It is found that biquadratic exchange interactions are essential to quantitatively describe the magnetism of CrI₃ but quantum rescaling corrections are required to reproduce its thermal properties. The quantum nature of the heat bath represented by discrete electron–spin and phonon–spin scattering processes induces the formation of spin fluctuations in the low‐temperature regime. These fluctuations induce the formation of metastable magnetic domains evolving into a single macroscopic magnetization or even a monodomain over surface areas of a few micrometers. Such domains display hybrid characteristics of Néel and Bloch types with a narrow domain wall width in the range of 3–5 nm. Similar behavior is expected for the majority of 2D vdW magnets where higher‐order exchange interactions are appreciable.

The excellent metallic conductivity of MXenes has elevated them to the forefront of a wide range of applications. The performance of highly conductive MXenes is comparable to that of conductive metals, and superior to many carbon‐based nanomaterials for electromagnetic shielding, conducting electrodes, sensors, and thermal heaters. A brief overview of the mentioned application areas is presented.

Abstract

Since their discovery in 2011, 2D transition metal carbides, nitrides, and carbonitrides, known as MXenes, have attracted considerable global research interest owing to their outstanding electrical conductivity coupled with light weight, flexibility, transparency, surface chemistry tunability, and easy solution processability. Here, the promising abilities of 2D MXenes, from both experimental and theoretical perspectives, for designing conductive materials for a range of applications, including electromagnetic interference shielding, flexible optoelectronics, sensors, and thermal heaters, are presented.