Jiuxiang Dai

To epitaxially grow large area single crystals of hBN and other 2D materials with lower symmetries, using a high-index substrate is highly favorable. The above figure illustrates the alignments of hBN islands on various Cu surfaces, from which we can clearly see that the unidirectional alignment of hBN islands can be achieved on various high-index Cu surfaces.

Abstract

Recently, the successful synthesis of wafer-scale single-crystal graphene, hexagonal boron nitride (hBN), and MoS₂ on transition metal surfaces with step edges boosted the research interests in synthesizing wafer-scale 2D single crystals on high-index substrate surfaces. Here, using hBN growth on high-index Cu surfaces as an example, a systematic theoretical study to understand the epitaxial growth of 2D materials on various high-index surfaces is performed. It is revealed that hBN orientation on a high-index surface is highly dependent on the alignment of the step edges of the surface as well as the surface roughness. On an ideal high-index surface, well-aligned hBN islands can be easily achieved, whereas curved step edges on a rough surface can lead to the alignment of hBN along with different directions. This study shows that high-index surfaces with a large step density are robust for templating the epitaxial growth of 2D single crystals due to their large tolerance for surface roughness and provides a general guideline for the epitaxial growth of various 2D single crystals.

Monolayer MoSe₂-WSe₂ lateral heterostructures with atomically precise 1D boundaries are synthesized using a one-pot chemical vapor deposition process. Their functional properties are demonstrated in various electronic, optoelectronic, photovoltaic, and light-emitting devices.

Abstract

Lateral heterostructures of dissimilar monolayer transition metal dichalcogenides provide great opportunities to build 1D in-plane p–n junctions for sub-nanometer thin low-power electronic, optoelectronic, optical, and sensing devices. Electronic and optoelectronic applications of such p–n junction devices fabricated using a scalable one-pot chemical vapor deposition process yielding MoSe₂-WSe₂ lateral heterostructures are reported here. The growth of the monolayer lateral heterostructures is achieved by in situ controlling the partial pressures of the oxide precursors by a two-step heating protocol. The grown lateral heterostructures are characterized structurally and optically using optical microscopy, Raman spectroscopy/microscopy, and photoluminescence spectroscopy/microscopy. High-resolution transmission electron microscopy further confirms the high-quality 1D boundary between MoSe₂ and WSe₂ in the lateral heterostructure. p–n junction devices are fabricated from these lateral heterostructures and their applicability as rectifiers, solar cells, self-powered photovoltaic photodetectors, ambipolar transistors, and electroluminescent light emitters are demonstrated.

Nature Nanotechnology, Published online: 06 May 2021; doi:10.1038/s41565-021-00887-3

Graphene promises long-distance transfer of spin information with concomitant high charge carrier mobility. Proximity coupling of bilayer graphene with the 2D interlayer antiferromagnetic CrSBr now enables active generation of spin currents in graphene both electrically and thermally.

Nature Materials, Published online: 06 May 2021; doi:10.1038/s41563-021-00979-4

The optically detected magnetic resonance of a single defect in hexagonal boron nitride is reported.

In article number 2005735, Jiaxu Yan, Wei Huang, and co‐workers systematically review the progress of stacking engineering of transition metal dichalcogenide hetero‐bilayers: from controllable fabrication methods to routine characterization, then to the dependence of interlayer coupling on stacking configurations/angles, and lastly the current challenges and possible future strategies.

Two-dimensional (2D) materials offer opportunities to explore both fundamental science and applications in the limit of atomic thickness. Beyond the prototypical case of graphene, other 2D materials have recently come to the fore. Of particular technological interest are 2D semiconductors, of which the family of materials known as the group-VI transition metal dichalcogenides (TMDs) has attracted much attention. The presence of a bandgap allows for the fabrication of high on–off ratio transistors and optoelectronic devices, as well as valley/spin polarized transport. The technique of chemical vapor deposition (CVD) has produced high-quality and contiguous wafer-scale 2D films, however, they often need to be transferred to arbitrary substrates for further investigation. In this review, the various transfer techniques developed for transferring 2D films will be outlined and compared, with particular emphasis given to CVD-grown TMDs. Each technique suffers undesirable process-relat...

A liquid-metal-assisted chemical vapor deposition method is proposed to achieve the controlled growth of large-sized monolayer and few-layer GaTe with high quality, high phase-selectivity, chemical uniformity, and good reproducibility. The method can be extended to grow Ga-based 2D materials and their alloys, including GaSe, GaS, GaTe_{1- x} Se _x , and even InSe, which significantly speed up the technological applications of 2D materials.

Abstract

GaTe has recently attracted significant interest due to its direct bandgap and unique phase structure, which makes it a good candidate for optoelectronics. However, the controllable growth of large-sized monolayer and few-layer GaTe with tunable phase structures remains a great challenge. Here the controlled growth of large-sized GaTe with high quality, chemical uniformity, and good reproducibility is achieved through liquid-metal-assisted chemical vapor deposition method. By using liquid Ga, the rapid growth of 2D GaTe flakes with high phase-selectivity can be obtained due to its reduced reaction temperature. In addition, the method is used to synthesize many Ga-based 2D materials and their alloys, showing good universality. Raman spectra suggest that the as-grown GaTe own a relatively weak van der Waals interaction, where monoclinic GaTe displays highly-anisotropic optical properties. Furthermore, a p-n junction photodetector is fabricated using GaTe as a p-type semiconductor and 2D MoSe₂ as a typical n-type semiconductor. The GaTe/MoSe₂ heterostructure photodetector exhibits large photoresponsivity of 671.52 A W⁻¹ and high photo-detectivity of 1.48 × 10¹⁰ Jones under illumination, owing to the enhanced light absorption and good quality of as-grown GaTe. These results indicate that 2D GaTe is a promising candidate for electronic and photoelectronic devices.

Multiple magnetic phases emerge in single-crystal SnS₂ layered semiconductors. Two ferromagnetic (FM) transitions with Curie temperatures dependent on Mn-doping concentration can be distinguished based on magnetic measurements. The positive-to-negative crossover and anisotropy in magnetoresistance further confirm the FM semiconducting behavior. Mn-SnS₂ is expected to enlarge the scope of layered FM semiconductors towards practical applications in the field of spintronics.

Abstract

2D van der Waals magnetic semiconductors have emerged along with the possibilities of achieving an efficient gate tunability and a proximity effect with a high magnetic anisotropy compared with 3D counterparts. Little explored are multiple magnetic phases with a single crystallographic phase. Herein, the multiple magnetic phases in a Mn-doped SnS₂ single crystal with different doping concentrations using a one-step self-flux method are reported. Two ferromagnetic phases with a canted spin direction exist regardless of the Mn-doping concentration at up to 5 at%. Antiferromagnetism coexists with the ferromagnetic order and strengthens at high Mn-doping concentrations. A magnetoresistance measurement conducted on a 2 at% Mn-SnS₂ flake exhibits a positive-to-negative crossover with a value of as high as 50% and clear anisotropy, confirming the presence of ferromagnetic order in the material. By revealing multiple magnetic phases in Mn-doped SnS₂, the study broadens the scope of state-of-the-art research on layered magnetic semiconductors.

The electronic and magnetic properties of a 2D monolayer ferromagnet on a layered superconducting substrate are studied using different experimental techniques and theoretical calculations. It is confirmed that the chromium tribromide monolayer retains its ferromagnetic order and induces proximitized magnetism on the underlying superconductor niobium diselenide. The results contribute to the broader framework of exploiting proximity effects to realize novel phenomena in 2D heterostructures.

Abstract

The ability to imprint a given material property to another through a proximity effect in layered 2D materials has opened the way to the creation of designer materials. Here, molecular-beam epitaxy is used for direct synthesis of a superconductor–ferromagnet heterostructure by combining superconducting niobium diselenide (NbSe₂) with the monolayer ferromagnetic chromium tribromide (CrBr₃). Using different characterization techniques and density-functional theory calculations, it is confirmed that the CrBr₃ monolayer retains its ferromagnetic ordering with a magnetocrystalline anisotropy favoring an out-of-plane spin orientation. Low-temperature scanning tunneling microscopy measurements show a slight reduction of the superconducting gap of NbSe₂ and the formation of a vortex lattice on the CrBr₃ layer in experiments under an external magnetic field. The results contribute to the broader framework of exploiting proximity effects to realize novel phenomena in 2D heterostructures.

Metal-like electrical conductivity and a large MXene surface area are desirable characteristics for alternative sensor material. An overview of recent advances in MXene-based sensor technology that utilize the beneficial properties of MXenes is offered. Insights into low-cost, high-performance MXene-based sensors for next generation soft-electronics applications are also provided.

Abstract

Various fields of study consider MXene a revolutionary 2D material. Particularly in the field of sensors, the metal-like high electrical conductivity and large surface area of MXenes are desirable characteristics as an alternative sensor material that can transcend the boundaries of existing sensor technology. This critical review provides a comprehensive overview of recent advances in MXene-based sensor technology and a roadmap for commercializing MXene-based sensors. The existing sensors are systematically categorized as chemical, biological, and physical sensors. Each category is then classified into various subcategories depending on the electrical, electrochemical, structural, or optical sensing mechanism, which are the four fundamental working mechanisms of sensors. Representative structural and electrical approaches for boosting the performance of each category are presented. Finally, factors that hinder commercializing MXene-based sensors are discussed, and several breakthroughs in realizing commercially available MXene-based sensors are suggested. This review provides broad insights pertaining to previous and existing MXene-based sensor technology and perspectives on the future generation of low-cost, high-performance, and multimodal sensors for soft-electronics applications.

A pressing need in low energy spintronics is two-dimensional (2D) ferromagnets with Curie temperature above the liquid-nitrogen temperature (77 K), and sizeable magnetic anisotropy. We studied Mn3Br8 monolayer wh...

We combine synchrotron-based near-field infrared spectroscopy and first principles lattice dynamics calculations to explore the vibrational response of CrPS 4 in bulk, few-, and single-layer form. Analysis of the mode pattern reveals a C 2 polar + chiral space group, no symmetry crossover as a function of layer number, and a series of non-monotonic frequency shifts in which modes with significant intralayer character harden on approach to the ultra-thin limit whereas those containing interlayer motion or more complicated displacement patterns soften and show inflection points or steps. This is different from MnPS 3 where phonons shift as 1/size 2 and are sensitive to the three-fold rotation about the metal center that drives the symmetry crossover. We discuss these differences as well as implications for properties such as electric polarization in terms of presence or absence of the P–P dimer and other aspects of local structure, sheet density,...

Nature, Published online: 28 April 2021; doi:10.1038/s41586-021-03428-z

Direct experimental evidence of chemical short-range atomic-scale ordering (CSRO) in a VCoNi medium-entropy alloy is provided via diffraction and electron microscopy, analysed from specific crystallographic directions.

We combine synchrotron-based near-field infrared spectroscopy and first principles lattice dynamics calculations to explore the vibrational response of CrPS 4 in bulk, few-, and single-layer form. Analysis of the mode pattern reveals a C 2 polar + chiral space group, no symmetry crossover as a function of layer number, and a series of non-monotonic frequency shifts in which modes with significant intralayer character harden on approach to the ultra-thin limit whereas those containing interlayer motion or more complicated displacement patterns soften and show inflection points or steps. This is different from MnPS 3 where phonons shift as 1/size 2 and are sensitive to the three-fold rotation about the metal center that drives the symmetry crossover. We discuss these differences as well as implications for properties such as electric polarization in terms of presence or absence of the P–P dimer and other aspects of local structure, sheet density,...

Single-crystal copper substrates have gained importance for the preparation of high-quality graphene and hexagonal boron nitride monolayer films by chemical vapor deposition (CVD). Especially, large-scale single-crystal copper foils with high-index planes are synthesized recently and attract great interests. However, the current synthesis methods of single-crystal copper foils and films are energy and time-consuming. Here, we show a rapid and efficient approach for the preparation of centimeter-scale single-crystal copper foils by making small incisions at the edges of polycrystalline copper foils before high-temperature annealing. 1.5 cm × 4 cm pieces of grain-boundary-free copper foils can be prepared by annealing at 1080 °C for 60 min. The annealed copper foil manifests a single high-index plane and is grain-boundary-free over the whole area. We also show that CVD of graphene on the high-index single-crystal copper affords a higher growth rate than on low-index copper substra...