Jiuxiang Dai

2D material-based helical screws with an Archimedean spiral arrangement are naturally assembled via spontaneous rotation generated from anisotropic internal stress. Thermodynamic and kinetic control influence the resultant morphology of helical screws by manipulating the pitch and helical angle. Helix formation induces unique properties in helical screws, including solvent-driven actuator performance and electrical and electrothermal properties.

Abstract

Helix structures, which are frequently observed in nature, act as versatile structural templates for complex functionalities with asymmetry and anisotropy. However, atomically thin 2D materials, including graphene, transition metal dichalcogenides (TMDs), and MXenes, do not have inherent chirality in their planar geometry and cannot easily form such a structure. This study presents the macroscopic self-assembly of 2D materials for helical screws with an Archimedean spiral arrangement. The naturally triggered spontaneous rotation upon the 1D fiber assembly of 2D materials forms helical screws consisting of multiple helices and perversions. For a clear understanding of the morphological evolution of helical screws, variations in the helical pitch and angle are systematically analyzed considering thermodynamic and kinetic conditions. Subsequently, the influence of spontaneous helix formation on the properties of the 2D assembled fibers is investigated in terms of the solvent-driven actuator performance and electrical and electrothermal properties. The suggested approach provides a new perspective on the directed self-assembly of inherently achiral 2D materials toward chiral helix formation.

The terminal atom-controlled etching of 2D-TMDs through defects introduced by laser irradiationand subsequent thermal etching in a controlled etching atmosphere are presented. The forward etched hole arrays are etched in Ar/H₂ atmosphere, the hexagonal hole arrays are etched in pure Ar atmosphere, and the reverse etched hole arrays are etched in Ar/sulfur vapor atmosphere.

Abstract

The controlled etching of 2D transition metal dichalcogenides (2D-TMDs) is critical to understanding the growth mechanisms of 2D materials and patterning 2D materials but remains a major comprehensive challenge. Here, a rational strategy to control the terminal atoms of 2D-TMDs etched holes is reported. Using laser irradiation combined with an improved anisotropic thermal etching process under a determined atmosphere, terminal atom-controlled etched hole arrays are created on 2D-TMDs. By adjusting the gas atmosphere during the thermal etching stage, triangular etched hole arrays terminated by the tungsten zigzag (W-ZZ) edge (in an Ar/H₂ atmosphere), hexagonal etched hole arrays terminated alternately by the W-ZZ edge and sulfur (selenium) zigzag (S-ZZ or Se-ZZ) edge (in a pure Ar atmosphere), and triangular etched hole arrays terminated by the S-ZZ (Se-ZZ) edge (in an Ar/sulfur [selenium] vapor atmosphere) can be obtained. Density functional theory reveals the forming energy of different edges and the different activities of metal atoms and chalcogenide atoms under different atmospheres, which determine the terminal atoms of the holes. This work may enhance the understanding of the etching and growth of 2D-TMDs. The 2D-TMDs hole arrays constructed by this work may have important applications in catalysis, nonlinear optics, spintronics, and large-scale integrated circuits.

A straightforward strategy to rapidly and directly crystallize luminescent metal halide nanocrystals into close-packed face-centered-cubic (FCC) superlattices during the liquid-phase synthesis is demonstrated. The concomitant nanocrystal growth and superlattice formation process is governed by the interplay between the nanocrystal size and surface-coating ligand.

Abstract

The crystallization of nanocrystal building blocks into artificial superlattices has emerged as an efficient approach for tailoring the nanoscale properties and functionalities of novel devices. To date, ordered arrays of colloidal metal halide nanocrystals have mainly been achieved by using post-synthetic strategies. Here, a rapid and direct liquid-phase synthesis is presented to achieve a highly robust crystallization of luminescent metal halide nanocrystals into perfect face-centered-cubic (FCC) superlattices on the micrometer scale. The continuous growth of individual nanocrystals is observed within the superlattice, followed by the disassembly of the superlattices into individually dispersed nanocrystals owing to the highly repulsive interparticle interactions induced by large nanocrystals. Transmission electron microscopy characterization reveals that owing to an increase in solvent entropy, the structure of the superlattices transforms from FCC to hexagonal close-packed (HCP) and the nanocrystals disassemble. The FCC superlattice exhibits a single and slightly redshifted emission, due to the reabsorption-free property of the building block units. Compared to individual nanocrystals, the superlattices have three times higher quantum yield with improved environmental stability, making them ideal for use as ultrabright blue-light emitters. This study is expected to facilitate the creation of metamaterials with ordered nanocrystal structures and their practical applications.

This review first summarizes the synthesis strategies including chemical vapor deposition, intercalation, and so on. Then, the novel atomic structures of 2D materials are analyzed, followed by the fascinating physical properties including ferroelectricity, ferromagnetism, and superconductivity. Finally, the conclusion and outlook are offered including the challenges and future prospects of 2D materials with different compositions and phases.

Abstract

In recent years, 2D materials—M _a X _b with different compositions and phases have attracted tremendous attention due to their diverse structures and electronic features. The common thermodynamically stable 2H and metastable 1T phases have been extensively studied, however, there are many unusual compositions and phases with novel physical properties that have yet to be explored. Therefore, summarization of the synthesis strategies, atomic structures, and the unique physical properties of 2D materials with different compositions and phases is very important for their development. In this review, the strategies including chemical vapor deposition, intercalation, atomic layer deposition, chemical vapor transport, and electrostatic gating for synthesizing various 2D materials with different phases and compositions are first summarized. Specially, the intercalation strategies including heterogeneous- and self-intercalation for controllable phases and compositions fabrication are mainly discussed. Then, the novel atomic structures of 2D materials are analyzed, followed by the fascinating physical properties including ferroelectricity, ferromagnetism, superconductivity, and so on. Finally, the conclusion and outlook are offered including the challenges and future prospects of 2D materials with different compositions and phases.

Understanding size effects in antiferroelectrics provides important information for optimizing the performance of energy-storage devices at small scales. Direct experimental evidence for intrinsic size-driven scaling is demonstrated in lead-free antiferroelectric NaNbO₃ membranes, in which an intriguing antiferroelectric-to-ferroelectric transition occurs upon reducing thickness, leading a ferroelectric phase in thin membranes and coexistence of ferroelectric and antiferroelectric phases in thick membranes.

Abstract

Despite extensive studies on size effects in ferroelectrics, how structures and properties evolve in antiferroelectrics with reduced dimensions still remains elusive. Given the enormous potential of utilizing antiferroelectrics for high-energy-density storage applications, understanding their size effects will provide key information for optimizing device performances at small scales. Here, the fundamental intrinsic size dependence of antiferroelectricity in lead-free NaNbO₃ membranes is investigated. Via a wide range of experimental and theoretical approaches, an intriguing antiferroelectric-to-ferroelectric transition upon reducing membrane thickness is probed. This size effect leads to a ferroelectric single-phase below 40 nm, as well as a mixed-phase state with ferroelectric and antiferroelectric orders coexisting above this critical thickness. Furthermore, it is shown that the antiferroelectric and ferroelectric orders are electrically switchable. First-principle calculations further reveal that the observed transition is driven by the structural distortion arising from the membrane surface. This work provides direct experimental evidence for intrinsic size-driven scaling in antiferroelectrics and demonstrates enormous potential of utilizing size effects to drive emergent properties in environmentally benign lead-free oxides with the membrane platform.

Nature Communications, Published online: 04 February 2023; doi:10.1038/s41467-023-36044-8

Hydrogen spillover is a surface phenomenon encountered in catalytic reactions. Here the authors show that the surface-lattice-confinement effect can regulate the hydrogen spillover directions and accelerate spillover rates. It is intrinsically related to the local surface geometries and coordination numbers of surface O sites.

Nature Communications, Published online: 04 February 2023; doi:10.1038/s41467-023-36201-z

Magnet/superconductor hybrids have been explored for the realization of topological superconductivity but have mainly focused on ferromagnets with full gaps. Here, the authors find that the antiferromagnet/superconductor heterostructure of monolayer Mn on a Nb(110) surface is a topological nodal-point superconductor.

Applied Physics Letters, Volume 122, Issue 5, January 2023.
The development of the epitaxial lift-off technique using a pseudoperovskite Sr3Al2O6 sacrificial layer has unleashed latent physical properties emerging in freestanding membranes, mainly composed of lattice-matched perovskite-type complex oxides. Here, we report the superconductivity in a freestanding single-crystalline FeSe membrane prepared using a SrTiO3 capped water-soluble Sr3Al2O6 sacrificial layer, which serves as an ex situ growth template. The FeSe membrane is synthesized by etching the sacrificial layer and transferred on a SiO2/Si substrate. X-ray diffraction pattern and scanning transmission electron microscopy reveal that the FeSe membrane is fully relaxed with minimum degradation of its structural properties during the lift-off process. A superconductivity with zero resistance below 4.2 K is exhibited in the freestanding FeSe membrane, while it is not observed in a compressed thin-film form by an in-plane tensile strain. In addition, critical magnetic field and critical current density of the FeSe membrane are comparable to those of the bulk single crystal. Our demonstration of the superconducting FeSe membrane ensures a high utility of the epitaxial lift-off technique for various thin-film materials grown on SrTiO3. This study paves the way for functional applications using ex situ thin-film growth and lift-off technique with an expanded selection from an inventory of various materials.

Applied Physics Letters, Volume 122, Issue 5, January 2023.
The surface activated bonding (SAB) technique enables room temperature bonding of metals, such as Au, by forming metal bonds between clean and reactive surfaces. However, the re-adsorption on the activated surface deteriorates the bonding quality, which limits the applicability of SAB for actual packaging processes of electronics. In this study, we propose and demonstrate the prolongation of the surface activation effect for room temperature bonding of Au by utilizing a self-assembled monolayer (SAM) protection. While the bonding without SAM fails after exposure of the activated Au surface to ambient air, the room temperature bonding is achieved using SAM protection even after 100 h exposure. The surface analysis reveals that the clean and activated Au surface is protected from re-adsorption by SAM. This technique will provide an approach of time-independent bonding of Au at room temperature.

Nature Communications, Published online: 03 February 2023; doi:10.1038/s41467-023-36286-6

Recent studies have reported the growth of 2D non-centrosymmetric single crystals on substrates with surface steps, but the mechanisms are still unclear. Here, the authors demonstrate a method to grow unidirectionally aligned transition metal dichalcogenide grains on various types of substrates, showing the importance of the simultaneous formation of grain nuclei and substrate steps.

Nature Electronics, Published online: 03 February 2023; doi:10.1038/s41928-023-00919-x

Conventional dielectric layers used in stretchable electronics are solution-processed, thick and have poor electrical performance compared with rigid, inorganic dielectrics. A stretchable nanometre-thick gate dielectric layer has been produced using large-area vacuum deposition. This material has excellent electrical, mechanical and chemical properties and could facilitate the development of high-performance wearable devices.

Nature Electronics, Published online: 02 February 2023; doi:10.1038/s41928-023-00918-y

A thin and stretchable polymer layer can be fabricated over large areas with high uniformity using a vacuum-deposition method and used as the gate dielectric in stretchy carbon-nanotube-based transistors and circuits that can function at 40% strain.

The piezo-phototronic effect shows promise with regards to improving the performance of 2D semiconductor-based flexible optoelectronics, which will potentially open up new opportunities in the electronics field.

Abstract

The piezo-phototronic effect shows promise with regards to improving the performance of 2D semiconductor-based flexible optoelectronics, which will potentially open up new opportunities in the electronics field. Mechanical exfoliation and chemical vapor deposition (CVD) influence the piezo-phototronic effect on a transparent, ultrasensitive, and flexible van der Waals (vdW) heterostructure, which allows the use of intrinsic semiconductors, such as 2D transition metal dichalcogenides (TMD). The latest and most promising 2D TMD-based photodetectors and piezo-phototronic devices are discussed in this review article. As a result, it is possible to make flexible piezo-phototronic photodetectors, self-powered sensors, and higher strain tolerance wearable and implantable electronics for health monitoring and generation of piezoelectricity using just a single semiconductor or vdW heterostructures of various nanomaterials. A comparison is also made between the functionality and distinctive properties of 2D flexible electronic devices with a range of applications made from 2D TMDs materials. The current state of the research about 2D TMDs can be applied in a variety of ways in order to aid in the development of new types of nanoscale optoelectronic devices. Last, it summarizes the problems that are currently being faced, along with potential solutions and future prospects.

Due to the strong electron-hole asymmetry in the electronic structure, intrinsic excitation benefits the high-temperature thermoelectric transport of the narrow band-gap semiconductor SmS. Consequently, a high-power factor of 1.41 mW K^–2 m^–1 and a peak zT value of 1.1 at 1123 K are attained, which is among the best n-type high-temperature thermoelectrics.

Abstract

High-temperature thermoelectric (TE) materials are common wide-gap semiconductors that are used in order to prevent the bipolar effect. Here, a potential high-temperature n-type TE material SmS with a simple NaCl structure that demonstrates a narrow band gap of ≈0.25 eV is reported. As expected, a temperature-dependent carrier concertation is observed, which is attributed to the thermal activation of electrons from valence band edge to conduction band. Interestingly, the intrinsic activation does not cause any sign of a bipolar effect. Density functional theory calculations suggest that the phenomenon originates from the strong electron-hole asymmetry in the electronic structure and the electron-to-hole conductivity ratio is as high as 700–900. As a result, the activated minority carriers barely participate in the TE transport and the maximum power factor reaches 1.41 mW K⁻² m⁻¹ at 1123 K. By further alloying with Se to reduce lattice thermal conductivity, a peak zT of ≈1.1 is obtained in Sm_1.08S_0.78Se_0.22 at 1123 K, which is among the best n-type high-temperature thermoelectrics. This study proves high-temperature TE materials can be found in narrow-gap semiconductors, which significantly enriches the scope of possibilities for novel TE materials.

Nature Materials, Published online: 02 February 2023; doi:10.1038/s41563-022-01413-z

When BiFeO3 layers are confined between TbScO3 layers in an epitaxial superlattice, crystallographically orthogonal voltages can induce reversible, non-volatile switching between polar and antipolar states in BiFeO3. This symmetry switch also leads to marked changes in the nonlinear optical response, piezoresponse and resistivity of the system.

This study reports the synthesis of palladium sulfides with centimeter scale and uniform stoichiometric ratio via controlling the sulfurization temperature of palladium thin films. The PdS₂ films show an n-type semiconducting behavior with high mobility of 10.4 cm² V⁻¹ s⁻¹ and present a broadband photoresponse from 450 to 1550 nm.

Abstract

2D materials with mixed crystal phase will lead to the nonuniformity of performance and go against the practical application. Therefore, it is of great significance to develop a valid method to synthesize 2D materials with typical stoichiometry. Here, 2D palladium sulfides with centimeter scale and uniform stoichiometric ratio are synthesized via controlling the sulfurization temperature of palladium thin films. The relationship between sulfurization temperature and products is investigated in depth. Besides, the high-quality 2D PdS₂ films are synthesized via sulfurization at the temperature of 450–550 °C, which would be compatible with back-end-of-line processes in semiconductor industry with considering of process temperature. The PdS₂ films show an n-type semiconducting behavior with high mobility of 10.4 cm² V⁻¹ s⁻¹. The PdS₂ photodetector presents a broadband photoresponse from 450 to 1550 nm. These findings provide a reliable way to synthesizing high-quality and large-area 2D materials with uniform crystal phase. The result suggests that 2D PdS₂ has significant potential in future nanoelectronics and optoelectronic applications.

Nature Communications, Published online: 02 February 2023; doi:10.1038/s41467-023-35972-9

Weak interlayer van der Waals (vdW) bonding has significant impact on the structure and properties of vdW layered materials. Here authors use in-situ aberration-corrected ADF-STEM for an atomistic insight into the cation diffusion in the vdW gaps and the etching of vdW surfaces at high temperatures.

Nature, Published online: 01 February 2023; doi:10.1038/s41586-022-05612-1

We report full-colour, vertically stacked µLEDs that achieve exceptionally high array density (5,100 pixels per inch) and small size (4 µm) via a 2D material-based layer transfer technique, allowing the creation of full-colour µLED displays for augmented and virtual reality.

Tapered Cross Section Photoelectron Spectroscopy (TCS-PES), a promising method to investigate the chemistry and electronic structure at buried interfaces in semiconductor devices, is for the first time used on III–V architectures. PES yielded insight into the chemistry at the buried interfaces, but polishing induced alterations limit the information on interface energetics.

Abstract

Interfaces are key elements that define electronic properties of the final device. Inevitably, most of the active interfaces of III–V semiconductor devices are buried and it is therefore not straightforward to characterize them. The Tapered Cross Section Photoelectron Spectroscopy (TCS-PES) approach is promising to address such a challenge. That the TCS-PES can be used to study the relevant heterojunction in epitaxial III–V architectures prepared by metalorganic chemical vapor deposition is demonstrated here. A MULTIPREP polishing system that enables controlling the angle between the sample holder and the polishing plate has been employed to improve the reproducibility of the polishing procedure. With this procedure, that preparing the TCS of III–V semiconductor devices with tapering angles lower than 0.02° is possible is demonstrated. The PES provides then information about the buried interfaces of Ge|GaInP and GaAs|GaInP layer stacks. Both, chemical and electronic properties have been measured by PES. It evidences that the preparation of the TCSs under an uncontrolled atmosphere modifies the pristine properties of the critical buried heterointerfaces. Surface states and reaction layers are created on the TCS surface, which restrict unambiguous conclusions on buried interface energetics.

Twisted bilayer graphene (tBLG) is prepared by chemical vapor deposition on liquid Cu. When the growth temperature exceeds a certain critical value, the state of aligned high-quality single-layer graphene domains grown on liquid Cu is broken. Then, tBLG with twisted double-layer regions is obtained in situ by rotating and intercalating between graphene domains.

Abstract

Twisted bilayer graphene (tBLG) possesses various novel physical properties. Most of tBLG are fabricated by using artificial methods of stacking or folding single-layer graphene. Chemical vapor deposition (CVD) has been verified that it holds great potential for preparation of large-size high-quality graphene. Therefore, it is significant for preparing tBLG in situ by using CVD technology. In this work, a novel approach is developed to directly prepare tBLGs on liquid Cu substrate. When the growth temperature exceeds a certain critical value, the state of aligned high-quality single-layer graphene domains grown on liquid Cu will be broken. Then, tBLG with twisted double-layer regions is prepared in situ by rotating and intercalating between graphene domains. Experimental observations suggest that the liquid phase of Cu substrate and gas flow play a crucial role for the formation of tBLGs. These results demonstrate that the liquid Cu is an ideal potential substrate for preparing tBLGs with a full range of twisted angles and studying the formation mechanism of layer-stacked materials.

The thickness-dependent magnetic order in 2D Cr₅Si₃ nanosheets is herein studied. The thickness-dependent topological Hall effect and its relation to the observed perpendicular magnetic anisotropy are investigated. At the same time, Cr₅Si₃ nanosheets that are compatible with the complementary metal-oxide-semiconductor technology are synthesized through the chemical vapor deposition setup. It can be used for reference for the growth of other 2D silicides.

Abstract

Antiferromagnets with noncollinear spin order are expected to exhibit unconventional electromagnetic response, such as spin Hall effects, chiral abnormal, quantum Hall effect, and topological Hall effect. Here, 2D thickness-controlled and high-quality Cr₅Si₃ nanosheets that are compatible with the complementary metal-oxide-semiconductor technology are synthesized by chemical vapor deposition method. The angular dependence of electromagnetic transport properties of Cr₅Si₃ nanosheets is investigated using a physical property measurement system, and an obvious topological Hall effect (THE) appears at a large tilted magnetic field, which results from the noncollinear magnetic structure of the Cr₅Si₃ nanosheet. The Cr₅Si₃ nanosheets exhibit distinct thickness-dependent perpendicular magnetic anisotropy (PMA), and the THE only emerges in the specific thickness range with moderate PMA. This work provides opportunities for exploring fundamental spin-related physical mechanisms of noncollinear antiferromagnet in ultrathin limit.

A lateral p–n homojunction with outstanding photovoltaic performance is built by simply contacting CVD-grown multilayer WSe₂ nanosheet with two different metals of TiN/Ni, and it can work efficiently as superb self-powered photodetector for visible and near-infrared lights, with responsivity of over 0.5 A W⁻¹ and fast photoresponse speed of 10 µs under 520 nm illumination.

Abstract

Here an IR-heating chemical vapor deposition (CVD) approach enabling fast 2D-growth of WSe₂ thin films is reported, and the great potential of metal contact doping in building CVD-grown WSe₂-based lateral homojunction is demonstrated by contacting with TiN/Ni metals in favor of holes/electrons injection. Shortening nanosheet channel to ≈2 µm leads to pronounced enhancement in the performance of diode. The fabricated WSe₂-based diode exhibits high rectification ratios without the need of gate modulation and can work efficiently as photovoltaic cell, with maximum open circuit voltage reaching up to 620 mV and a high power conversion efficiency over 15%, empowering it as superb self-powered photodetector for visible to near-infrared lights, with photoresponsivity over 0.5 A W⁻¹ and a fast photoresponse speed of 10 µs under 520 nm illumination. It is of practical significance to achieve well-performed photovoltaic devices with CVD-grown WSe₂ using fab-friendly metals and simple processing, which will help pave the way toward future mass production of optoelectronic chips.