Jiuxiang Dai

SnTe/PbTe monolayer lateral heterostructures are experimentally created through a two-step molecular beam epitaxial growth process. A vortex-oriented quadrant ferroelectric domain configuration is observed in the van der Waals 2D material heterostructure in this work.

Abstract

Heterostructures formed from interfaces between materials with complementary properties often display unconventional physics. Of especial interest are heterostructures formed with ferroelectric materials. These are mostly formed by combining thin layers in vertical stacks. Here the first in situ molecular beam epitaxial growth and scanning tunneling microscopy characterization of atomically sharp lateral heterostructures between a ferroelectric SnTe monolayer and a paraelectric PbTe monolayer are reported. The bias voltage dependence of the apparent heights of SnTe and PbTe monolayers, which are closely related to the type-II band alignment of the heterostructure, is investigated. Remarkably, it is discovered that the ferroelectric domains in the SnTe surrounding a PbTe core form either clockwise or counterclockwise vortex-oriented quadrant configurations. In addition, when there is a finite angle between the polarization and the interface, the perpendicular component of the polarization always points from SnTe to PbTe. Supported by first-principles calculation, the mechanism of vortex formation and preferred polarization direction is identified in the interaction between the polarization, the space charge, and the strain effect at the horizontal heterointerface. The studies bring the application of 2D group-IV monochalcogenides on in-plane ferroelectric heterostructures a step closer.

Publication date: November 2021

Source: Materials Today, Volume 50

Author(s): Hannah-Noa Barad, Mariana Alarcón-Correa, Gerardo Salinas, Eran Oren, Florian Peter, Alexander Kuhn, Peer Fischer

Nanodots derived from layered materials not only exhibit the intriguing properties of traditional nanodots due to the size confinement effect, but also inherit some unique properties of layered materials, making them promising candidates in various applications. The state-of-the-art progress on the preparation and applications of nanodots derived from layered materials is reviewed.

Abstract

Layered 2D materials, such as graphene, transition metal dichalcogenides, transition metal oxides, black phosphorus, graphitic carbon nitride, hexagonal boron nitride, and MXenes, have attracted intensive attention over the past decades owing to their unique properties and wide applications in electronics, catalysis, energy storage, biomedicine, etc. Further reducing the lateral size of layered 2D materials down to less than 10 nm allows for preparing a new class of nanostructures, namely, nanodots derived from layered materials. Nanodots derived from layered materials not only can exhibit the intriguing properties of nanodots due to the size confinement originating from the ultrasmall size, but also can inherit some unique properties of ultrathin layered 2D materials, making them promising candidates in a wide range of applications, especially in biomedicine and catalysis. Here, a comprehensive summary on the materials categories, advantages, synthesis methods, and potential applications of these nanodots derived from layered materials is provided. Finally, personal insights about the challenges and future directions in this promising research field are also given.

Controllable conversion of layered MXene nanosheet to nonlayered metal nitride nanomesh is realized via a nanoconfined topochemical strategy. The prepared carbon-wrapped ultrathin titanium nitride nanomesh exhibits outstanding electrocatalytic activity toward sulfur conversions, leading to greatly improved rate capability, cycling stability, and areal capacity of lithium sulfur batteries.

Abstract

2D non-layered materials (2DNLMs) featuring massive undercoordinated surface atoms and obvious lattice distortion have shown great promise in catalytic/electrocatalytic applications, but their controllable synthesis remains challenging. Here, a new type of ultrathin carbon-wrapped titanium nitride nanomesh (TiN NM@C) is prepared using a rationally designed nano-confinement topochemical conversion strategy. The ultrathin 2D geometry with well-distributed pores offers TiN NM@C plentiful exposed active sites and rapid charge transfer, leading to outstanding electrocatalytic performance tackling the sluggish sulfur redox kinetics in lithium-sulfur batteries (LSBs). LSBs employing TiN NM@C electrocatalyst deliver excellent rate capabilities (e.g., 304 mAh g⁻¹ at 10 C), greatly outperforming that of using TiN nanoparticles embedded in carbon nanosheets (TiN NPs@C) as a benchmark. More impressively, a free-standing electrode for LSBs with a high sulfur loading of 7.3 mg cm⁻² is demonstrated, showing a high peak areal capacity of 5.6 mAh cm⁻² at a high current density of 6.1 mA cm⁻². This work provides a new avenue for the facile and controllable fabrication of 2DNLMs with impressive electrocatalysis for LSBs as well as other energy conversion and storage technologies.

In article number 2101086, Antony George, Andrey Turchanin, and co-workers present the synthesis of monolayer MoSe₂-WSe₂ lateral heterostructures with the atomically sharp 1D boundaries via a one-pot chemical vapor deposition process. Various p–n junction devices including rectifiers, solar cells, photodetectors, ambipolar transistors, and light emitting diodes are demonstrated using these atomically thin heterostructures.

Boron carbon nitride thin films recently have attracted much interest due to the wide range of application and tunability of these materials. Here, an innovative method is presented to obtain highly conjugated, visible, transparent BCN thin films via chemical vapor deposition. The prepared films show a high refractive index and high oxidation potential.

Abstract

Ternary materials made up only from the lightweight elements boron, carbon, and nitrogen are very attractive due to their tunable properties that can be obtained by changing the relative elemental composition. However, most of the times, the synthesis requires to use up to three different precursor and very high temperatures for the synthesis. Moreover, the low reciprocal solubility of boron nitride and graphene often leads to BN-C composite materials due to phase segregation. Herein, an innovative method is presented to prepare BCN thin films by chemical vapor deposition from a single source precursor, melamine diborate. The deposition occurs homogenously at relatively low temperatures generating very high degree of sp² conjugation. The as-prepared thin films possess high transparency and refractive index values in the visible range that are of interest for reflective mirrors and lenses. Furthermore, they are wide-bandgap semiconductor with very positive valence band, making these materials very stable against oxidation of interest as protective coating and charge transport layer for solar cells. The simple chemical vapor deposition method that relies on commonly available and low-hazard precursor can open the way for application of BCN thin films in optics, optoelectronics, and beyond.

In article number 2101754, Wanli Li, Tomonobu Nakayama, Takeo Minari, and co-workers report an ultrahigh-resolution directed self-assembly strategy based on dual surface architectonics (DSA) for high-performance soft electronics. The DSA endows submicrometer-scale surface regions with strong adsorbing and pinning effect toward metallic nanoparticle inks via photoirradiation and chemical polarization, which enables patterning of electrodes with the resolution of 600 nm and further allows the fabrication of short-channel organic thin-film transistors.

A low-cost post-processing method to etch the defects in phase-transition-assisted CVD-grown 2H-MoTe₂ by using the triiodide ion solution is reported. The etching mechanism is discussed based on the high Te-vacancy densities in defects and 1T′ phase. The etching results are confirmed by electrical measurements and chemical analysis.

Abstract

2D molybdenum ditelluride (MoTe₂) with polymorphism is a promising candidate to developing phase-change memory, high-performance transistors and spintronic devices. The phase-transition-assisted chemical vapor deposition (CVD) process has been used to prepare large-scale 2H-MoTe₂ with large grain size and low density of grain boundary. However, because of the lack of precise control of the growth condition, some defects including the amorphous regions and grain boundaries in 2H-MoTe₂ are hardly avoidable. Here, a facile method of selectively etching defects in large-scale CVD-grown 2H-MoTe₂ by triiodide ion (I₃ ⁻) solution is reported. The defect etching is attributed to the reduced lattice symmetry, high chemisorption activity and high conductivity of the defects due to the high density of Te vacancies. The treated 2H-MoTe₂ shows the suppressed hysteresis in the electrical transfer curve, enhances hole mobility and the higher effective barrier height on the metal contact, suggesting the decreased density of defects. Further chemical analysis indicates that the 2H-MoTe₂ is not damaged or doped by I₃ ⁻ solution during the etching process. This simple and low-cost post-processing method is effective for etching the defects in large-area 2H-MoTe₂ for high-performance device applications.

Metallic Li in the form of ultrathin 2D nanosheet is synthesized through electrochemical reactions inside transmission electron microscope. The real-time dynamic growth process together with density-functional theory calculations clarifies the critical role of preferential surface passivation of oxygen in dictating 2D anisotropic growth. 2D Li nanosheets exhibit pronounced plasmonic properties with photon emissions at visible range.

Abstract

As the lightest solid element and also the simplest metal, lithium (Li) is one of the best representations of quasi-free electron model in both bulk form and the reduced dimensions. Herein, the controlled growth of 2D ultrathin Li nanosheets is demonstrated by utilizing an in situ electrochemical platform built inside transmission electron microscope (TEM). The as-grown freestanding 2D Li nanosheets have strong structure-anisotropy with large lateral dimensions up to several hundreds of nanometers and thickness limited to just a few nanometers. The nanoscale dynamics of nanosheets growth are unraveled by in situ TEM imaging in real-time. Further density-functional theory calculations indicate that oxygen molecules play an important role in directing the anisotropic 2D growth of Li nanosheets through controlling the growth kinetics by their facet-specific capping. The plasmonic optical properties of the as-grown Li nanosheets are probed by cathodoluminescence spectroscopy equipped within TEM, and a broadband visible emission is observed that contains contributions of both in-plane and out-of-plane plasmon resonance modes.

Publication date: September 2021

Source: Materials Today, Volume 48

Author(s): Dmitrii V. Semenok, Ivan A. Troyan, Anna G. Ivanova, Alexander G. Kvashnin, Ivan A. Kruglov, Michael Hanfland, Andrey V. Sadakov, Oleg A. Sobolevskiy, Kirill S. Pervakov, Igor S. Lyubutin, Konstantin V. Glazyrin, Nico Giordano, Denis N. Karimov, Alexander L. Vasiliev, Ryosuke Akashi, Vladimir M. Pudalov, Artem R. Oganov

Nature Nanotechnology, Published online: 01 July 2021; doi:10.1038/s41565-021-00934-z

A single or multilayer graphene veil grown by chemical vapour deposition can be used to protect artworks against colour fading, with a protection factor of up to 70%.

2D transition metal borides (MBenes) have emerged as promising post-MXene materials. This work provides signposts for the rational design and development of novel MBene-based biotechnological solutions. Latest milestones are discussed as an inspiration for future research approaches using MBenes. A detailed understanding of MBenes’ bio-recognition, responses, and material-property relationship is essential to explore their biological potential and ensure their safe application.

Abstract

2D transition metal borides (MBenes) have emerged as promising post-MXene materials with potential application in various biotechnological fields. Although they possess prospective bioactive properties due to boron in their structure, the experience gained from MXenes shows that an in-depth understanding of their biological recognition and response as well as the exploration of their biological applications are highly challenging. This makes the identification of the most promising 2D MBenes for future biological research and final industrial applications rather complicated. Herein, MBenes are differentiated from MXenes and further untangled for their bioactivity-generating features. It is expected that MBenes’ positive or negative biological impact on living organisms and different types of cells connect with their morphological, structural and physicochemical features in the context of relevant environments. Necessary toxicological data are also highlighted, which are key aspects to enable MBenes’ safe application in biotechnology and nanomedicine. Furthermore, a perspective for the rational development and design of novel biotechnological solutions based on MBenes is provided, which will meet the legal safety requirements for nanomaterials. In this regard, this work is an unprecedented contribution toward strategies for regulatory development for MBene/MXene-type nanomaterials. It provides an inspiration for future biotechnological and nanotoxicological approaches using MBenes.

We report a chiral mesostructured BiOBr film exhibiting a chirality-dependent circularly polarized colour response due to the structural-handedness matching and synergistic effect of multiple optical activities.

Abstract

Achieving strong and broadband circularly polarized colour responses in chiral inorganic materials is challenging. Here, we fabricated chiral mesostructured bismuth oxybromide (BiOBr) films (CMBFs) via hydrothermal growth using chiral sugar alcohols as symmetry-breaking agents. The layered slabs of BiOBr crystals with weak van-der-Waals interactions are prone to mismatching due to the chiral driving force, resulting in hierarchically chiral arrangements of fine size. Three levels of chirality exist in the CMBFs: primary, helical distortion crystal lattices of a nanoflake, secondary, helical stacking of nanoflakes to form nanoplates, and tertiary, chiral vortexes arranged by nanoplates. The CMBFs displayed optical activities (OAs) over a wide wavelength range of 350–2500 nm with an anisotropic factor of up to 0.99, which led to a significant chirality-dependent colour response to circularly polarized light. The high selectivity can be considered as the result of enhanced resonance due to structural-handedness matching and the synergistic effect of multiple OAs.

Nature, Published online: 30 June 2021; doi:10.1038/s41586-021-03590-4

The signature of a Wigner crystal—the analogue of a solid phase for electrons—is observed via the optical reflection spectrum in a monolayer transition metal dichalcogenide.

The magnetic proximity effect in topological insulator–magnetic materials heterostructures is a promising strategy to optimize new topological phases with potential applications in low-energy electronics. Progress in this field is summarized with an introduction to the underlying physical mechanisms, a review of emerging material systems for magnetic proximity in topological insulators, and the status and prospects for obtaining new topological phases.

Abstract

Inducing long-range magnetic order in 3D topological insulators can gap the Dirac-like metallic surface states, leading to exotic new phases such as the quantum anomalous Hall effect or the axion insulator state. These magnetic topological phases can host robust, dissipationless charge and spin currents or unique magnetoelectric behavior, which can be exploited in low-energy electronics and spintronics applications. Although several different strategies have been successfully implemented to realize these states, to date these phenomena have been confined to temperatures below a few Kelvin. This review focuses on one strategy: inducing magnetic order in topological insulators by proximity of magnetic materials, which has the capability for room temperature operation, unlocking the potential of magnetic topological phases for applications. The unique advantages of this strategy, the important physical mechanisms facilitating magnetic proximity effect, and the recent progress to achieve, understand, and harness proximity-coupled magnetic order in topological insulators are discussed. Some emerging new phenomena and applications enabled by proximity coupling of magnetism and topological materials, such as skyrmions and the topological Hall effect, are also highlighted, and the authors conclude with an outlook on remaining challenges and opportunities in the field.

Stress transfer has been investigated for exfoliated hexagonal boron nitride (hBN) nanosheets (BNNSs) through the use of Raman spectroscopy. Single BNNSs of different thicknesses of up to 100 nm (300 layers) were deposited upon a poly(methyl methacrylate) (PMMA) substrate and deformed in unixial tension. The Raman spectra from the BNNSs were relatively weak compared to graphene, but the in-plane E 2g Raman mode (the G band) could be distinguished from the spectrum of the PMMA substrate. It was found that G band down-shifted during tensile deformation and that the rate of band shift per unit strain decreased as the thickness of the BNNSs increased, as is found for multi-layer graphene. The efficiency of internal stress transfer between the different hBN layers was found to be of the order of 99% compared to 60%–80% for graphene, as a result of the stronger bonding between the hBN layers in the BNNSs. The reduction in bandshift rate can be related to the effective Young’...

Author(s): Martin F. Jakobsen, Arne Brataas, and Alireza Qaiumzadeh

We report generic and tunable crossed Andreev reflection (CAR) in a superconductor sandwiched between two antiferromagnetic layers. We consider recent examples of two-dimensional magnets with hexagonal lattices, where gate voltages control the carrier type and density, and predict a robust signature...

[Phys. Rev. Lett. 127, 017701] Published Tue Jun 29, 2021

npj 2D Materials and Applications, Published online: 28 June 2021; doi:10.1038/s41699-021-00242-z

Ambient effect on the Curie temperatures and magnetic domains in metallic two-dimensional magnets