Jiuxiang Dai

In this review, the authors summarize the recent progress made in the synthesis and characterization of these 2D metals, so called 2D metals/metallenes, and their oxide forms, 2D metals/metallene oxides as free standing 2D structures formed in situ through the use of TEM and STEM to synthesize these materials.

Abstract

In recent years, two-dimensional (2D) materials have attracted a lot of research interest as they exhibit several fascinating properties. However, outside of 2D materials derived from van der Waals layered bulk materials only a few other such materials are realized, and it remains difficult to confirm their 2D freestanding structure. Despite that, many metals are predicted to exist as 2D systems. In this review, the authors summarize the recent progress made in the synthesis and characterization of these 2D metals, so called metallenes, and their oxide forms, metallene oxides as free standing 2D structures formed in situ through the use of transmission electron microscopy (TEM) and scanning TEM (STEM) to synthesize these materials. Two primary approaches for forming freestanding monoatomic metallic membranes are identified. In the first, graphene pores as a means to suspend the metallene or metallene oxide and in the second, electron-beam sputtering for the selective etching of metal alloys or thick complex initial materials is employed to obtain freestanding single-atom-thick 2D metal. The data show a growing number of 2D metals/metallenes and 2D metal/ metallene oxides having been confirmed and point to a bright future for further discoveries of these 2D materials.

The large-scale intrinsic lattice structure of air-sensitive 1T’-WTe₂ monolayer is directly revealed by atomic scanning transmission electron microscopy imaging with dedicated sample protection. The collective thermal equilibrium lattice distortion, i.e., rippling, is found to be anisotropic and propagate only perpendicular to one of the preferential lattice planes, completely different than monolayer graphene and MoS₂ with hexagonal symmetry.

Abstract

Probing large-scale intrinsic structure of air-sensitive 2D materials with atomic resolution is so far challenging due to their rapid oxidization and contamination. Here, by keeping the whole experiment including growth, transfer, and characterizations in an interconnected atmosphere-control environment, the large-scale intact lattice structure of air-sensitive monolayer 1T’-WTe₂ is directly visualized by atom-resolved scanning transmission electron microscopy. Benefit from the large-scale atomic mapping, collective lattice distortions are further unveiled due to the presence of anisotropic rippling, which propagates perpendicular to only one of the preferential lattice planes in the same WTe₂ monolayer. Such anisotropic lattice rippling modulates the intrinsic point defect (Te vacancy) distribution, in which they aggregate at the constrictive inner side of the undulating structure, presumably due to the ripple-induced asymmetric strain as elaborated by density functional theory. The results pave the way for atomic characterizations and defect engineering of air-sensitive 2D layered materials.

A novel strategy to improve the wetting performance of liquid metal (LM) on metal substrates by means of surface modification method is reported. After the metal substrates are treated with CuCl₂ solution, LM can spontaneously wet the substrates with the assistance of surface topography and ambient environment within a few minutes, via reactive-wetting and metallic bond-enabled wetting.

Abstract

The low melting gallium-based liquid metal (LM) is showing tremendous potential in many technologies due to its unique properties. However, the high surface tensions as well as oxidations result in poor wetting capability on most solid surfaces, which limits its practical application. In this work, a simple chemical method is utilized to enable spontaneous wetting of LM on various metal surfaces, such as copper, nickel, and iron. It is found that LM can spread rapidly on the metal substrates treated with CuCl₂ solution through reactive-wetting and metallic bond-enabled wetting mechanism. The redox reaction between Ga and copper compound and the formation of intermetallic compound via LM phagocytosis help drive LM to wet metal substrates spontaneously. Many factors affecting the wetting behaviors of LM, including surface roughness, crystal size, and ambient environment, are systematically studied. This finding provides a novel strategy to solve the wetting problem of LM and will enable wider applications.

The ultralow infrared emissivity of the 2D Ti₃C₂T_x MXene is experimentally demonstrated. First-principles calculations confirm this discovery and reveal other MXenes such as Ti₂CT_x, Nb₂CT_x, and V₂CT_x are also low-emissivity materials. Such intrinsically visible black but infrared white materials that are rare in nature show great potential in solar–thermal conversion, multispectral camouflage, thermal insulation, and anti-counterfeiting applications.

Abstract

Black inorganic materials with low infrared absorption/emission (or IR white) are rare in nature but highly desired in numerous areas, such as solar–thermal energy harvesting, multispectral camouflage, thermal insulation, and anti-counterfeiting. Due to the lack of spectral selectivity in intrinsic materials, such counter-intuitive properties are generally realized by constructing complicated subwavelength metamaterials with costly nanofabrication techniques. Here, the intrinsically low mid-IR emissivity (down to 10%) of the 2D Ti₃C₂T_x MXene is reported. Associated with a high solar absorptance (up to 90%), it embraces the best spectral selectivity among the reported intrinsic black solar-absorbing materials. Its appealing potential in several of the aforementioned areas is experimentally demonstrated. First-principles calculations reveal that the IR emissivity of MXene relies on both the nanoflake orientations and terminal groups, indicating great tunability. The calculations also suggest more potential low-emissivity MXenes including Ti₂CT_x, Nb₂CT_x, and V₂CT_x. This work opens the avenue to further exploration of a family of intrinsically low-emissivity materials with over 70 members.

A systematic study of electric transport in Sb_{2− x} V _x Te₃ films demonstrates the coexistence of topological surface states and long-range ferromagnetism evidenced by Shubnikov–de Haas oscillations. Vanadium doping acts as an efficient tuning parameter for the Fermi surface size. This opens up a new route for the investigation of the crossover between different topological states based on this material class.

Abstract

An effective way of manipulating 2D surface states in magnetic topological insulators may open a new route for quantum technologies based on the quantum anomalous Hall effect. The doping-dependent evolution of the electronic band structure in the topological insulator Sb_{2− x} V _x Te₃ (0 ≤ x ≤ 0.102) thin films is studied by means of electrical transport. Sb_{2− x} V _x Te₃ thin films were prepared by molecular beam epitaxy, and Shubnikov–de Hass (SdH) oscillations are observed in both the longitudinal and transverse transport channels. Doping with the 3d element, vanadium, induces long-range ferromagnetic order with enhanced SdH oscillation amplitudes. The doping effect is systematically studied in various films depending on thickness and bottom gate voltage. The angle-dependence of the SdH oscillations reveals their 2D nature, linking them to topological surface states as their origin. Furthermore, it is shown that vanadium doping can efficiently modify the band structure. The tunability by doping and the coexistence of the surface states with ferromagnetism render Sb_{2− x} V _x Te₃ thin films a promising platform for energy band engineering. In this way, topological quantum states may be manipulated to crossover from quantum Hall effect to quantum anomalous Hall effect, which opens an alternative route for the design of quantum electronics and spintronics.

A powerful one-step puffing carbonization technology is proposed to synthesize versatile multiscale carbon-based composites (e.g. carbon/metal oxides, carbon/metal carbides, carbon/metal sulfides, carbon/metals, metal/semiconductors, etc.) with controlled morphology, adjustable dimensions and tunable composition on a large scale. As a representative example, a sulfur-doped cross-linked puffed carbon (SCPC)/Li₂S electrode shows extraordinary performance with superior rate capability and ultrastable cycling life.

Abstract

Carbon materials play a critical role in the advancement of electrochemical energy storage and conversion. Currently, it is still a great challenge to fabricate versatile carbon-based composites with controlled morphology, adjustable dimension, and tunable composition by a one-step synthesis process. In this work, a powerful one-step maltose-based puffing carbonization technology is reported to construct multiscale carbon-based composites on large scale. A quantity of composite examples (e.g., carbon/metal oxides, carbon/metal nitrides, carbon/metal carbides, carbon/metal sulfides, carbon/metals, metal/semiconductors, carbon/carbons) are prepared and demonstrated with required properties. These well-designed composites show advantages of large porosity, hierarchical porous structure, high conductivity, tunable components, and proportion. The formation mechanism of versatile carbon composites is attributed to the puffing-carbonization of maltose plus in situ carbothermal reaction between maltose and precursors. As a representative example, Li₂S is in situ implanted into a hierarchical porous cross-linked puffed carbon (CPC) matrix to verify its application in lithium–sulfur batteries. The designed S-doped CPC/Li₂S cathode shows superior electrochemical performance with higher rate capacity (621 mAh g^–1 at 2 C), smaller polarization and enhanced long-term cycles as compared to other counterparts. The research provides a general way for the construction of multifunctional component-adjustable carbon composites for advanced energy storage and conversion.

Nature Communications, Published online: 27 August 2021; doi:10.1038/s41467-021-25262-7

Electrostatically defined quantum confinement in large-area chemical vapor deposition-grown 2D WS₂ with HfO₂ dielectrics grown by atomic layer deposition is reported. This marks a key milestone in scalable approaches toward 2D-semiconductor-based quantum devices, which has hitherto only been demonstrated with micrometer-sized exfoliated flakes. The measurements show that low-temperature carrier mobility is not charge impurity limited as commonly thought, but is due to another important but commonly overlooked factor: interface roughness.

Abstract

Temperature-dependent transport measurements are performed on the same set of chemical vapor deposition (CVD)-grown WS₂ single- and bilayer devices before and after atomic layer deposition (ALD) of HfO₂. This isolates the influence of HfO₂ deposition on low-temperature carrier transport and shows that carrier mobility is not charge impurity limited as commonly thought, but due to another important but commonly overlooked factor: interface roughness. This finding is corroborated by circular dichroic photoluminescence spectroscopy, X-ray photoemission spectroscopy, cross-sectional scanning transmission electron microscopy, carrier-transport modeling, and density functional modeling. Finally, electrostatic gate-defined quantum confinement is demonstrated using a scalable approach of large-area CVD-grown bilayer WS₂ and ALD-grown HfO₂. The high dielectric constant and low leakage current enabled by HfO₂ allows an estimated quantum dot size as small as 58 nm. The ability to lithographically define increasingly smaller devices is especially important for transition metal dichalcogenides due to their large effective masses, and should pave the way toward their use in quantum information processing applications.

Scratches on amorphous glass and silicon substrates, made in a few seconds by mechanical polishing with diamonds, are used for the guided growth of single-crystal semiconductor nanowires. Control over the morphology of the features is gained by the abrasive size and polishing load. Photoluminescence measurements reveal that materials maintain their pristine optoelectronic nature on both glass and silicon.

Abstract

Aligned growth of planar semiconductor nanowires (NWs) on crystalline substrates has been widely demonstrated during the past two decades and was used for the fabrication of a large variety of devices. However, the dependence on single-crystal substrates is a major obstacle in the way of implementing NW-based applications in today's silicon- and glass-based technologies. Here, the guided growth of semiconductor NWs is demonstrated along nanoscale-depth scratches, created in a nonlithographic process on amorphous oxidized silicon wafers and soda-lime glass. Scratches are created on the substrates in a few seconds using a robust and scalable mechanical polishing process. Growth of planar NWs of different materials (CdS, CdSe, ZnSe, and ZnO) guided by scratches on Si/SiO₂ wafers and glass is demonstrated and studied. Photoluminescence measurements from individual NWs grown along scratches show that the interaction with the substrate preserves the optical properties of the material. Crystallographic analysis indicates that all materials grow as single crystals, and the influence of the scratches on the different materials is discussed in terms of morphology, crystallinity, and crystallographic orientations. This process opens the way to large-scale integration of NWs into functional devices by guided growth for various applications including displays, polarized light sensors, and smart windows.

Ionic gating can suppress the Verwey state in magnetite (Fe₃O₄) by metalizing the trimeron insulating state with gated-induced proton addition and oxygen vacancy. Tuning the trimeron state also causes the sign reversal in the anomalous Hall effect, indicating that the ionic gating can create a new type of carrier carrying competing spins.

Abstract

Understanding the Verwey transition in magnetite (Fe₃O₄), a strongly correlated magnetic oxide, is a one-century-old topic that recaptures great attention because of the recent spectroscopy studies revealing its orbital details. Here, the modulation of the Verwey transition by tuning the orbital configurations with ionic gating is reported. In epitaxial magnetite thin films, the insulating Verwey state can be tuned continuously to be metallic showing that the low-temperature trimeron states can be controllably metalized by both the gate-induced oxygen vacancies and proton doping. The ionic gating can also reverse the sign of the anomalous Hall coefficient, indicating that the metallization is associated with the presence of a new type of carrier with competing spin. The variable spin orientation associated with the sign reversal is originated from the structural distortions driven by the gate-induced oxygen vacancies.

Nature Materials, Published online: 25 August 2021; doi:10.1038/s41563-021-01059-3