Jiuxiang Dai

A comprehensive overview of applications on low-melting-temperature liquid metals in next-generation high-energy-density batteries is presented. Starting from the theoretical basis of materials properties and electrochemical features with liquid metals and alloys, reported applications are summarized by working mechanisms, critically evaluated with strengths and issues, and proposed with future development directions in the field.

Abstract

Increasing need for the renewable energy supply accelerated the thriving studies of Li-ion batteries, whereas if the high-energy-density Li as well as alkali metals should be adopted as battery electrodes is still under fierce debate for safety concerns. Recently, a group of low-melting temperature metals and alloys that are in liquid phase at or near room-temperature are being reported for battery applications, by which the battery energy could be improved without significant dendrite issue. Besides the dendrite-free feature, liquid metals can also promise various high-energy-density battery designs on the basis of unique materials properties. In this review, the design principles for liquid metals-based batteries from mechanical, electrochemical, and thermodynamical aspects are provided. With the understanding of the theoretical basis, currently reported relevant designs are summarized and analyzed focusing on the working mechanism, effectiveness evaluation, and novel application. An overview of the state-of-the-art liquid metal battery developments and future prospects is also provided in the end as a reference for further research explorations.

How to achieve simulated microgravity conditions on Earth? The art of growing and processing 2D porous crystalline molecular frameworks in simulated microgravity is presented.

Abstract

To date, crystallization studies conducted in space laboratories, which are prohibitively costly and unsuitable to most research laboratories, have shown the valuable effects of microgravity during crystal growth and morphogenesis. Herein, an easy and highly efficient method is shown to achieve space-like experimentation conditions on Earth employing custom-made microfluidic devices to fabricate 2D porous crystalline molecular frameworks. It is confirmed that experimentation under these simulated microgravity conditions has unprecedented effects on the orientation, compactness and crack-free generation of 2D porous crystalline molecular frameworks as well as in their integration and crystal morphogenesis. It is believed that this work will provide a new “playground” to chemists, physicists, and materials scientists that desire to process unprecedented 2D functional materials and devices.

NbOI₂ has a strong in-plane structural anisotropy, where the [NbO₂I₄] octahedra are connected through I–I edges along the c-axis and cornered O atoms along the b-axis. The I anions with weaker electronegativity contribute to the electrons near the Fermi surface, leading to the highly anisotropic dispersion of band structures along the c-axis. Hence, NbOI₂ exhibits large anisotropic factor of 1.75 and 1.7 for optical absorbance and photoresponsivity, which makes it a promising platform for novel polarization-sensitive photodetection applications.

Abstract

Exploring in-plane anisotropic 2D materials is of great significance to the fundamental studies and further development of polarizationsensitive optoelectronics. Herein, chiral niobium oxide diiodide (NbOI₂) is introduced into the intriguing anisotropic 2D family with the experimental demonstration of anisotropic optical and electrical properties. 2D NbOI₂ crystals exhibit highly anisotropic dispersed band structures around the Fermi surface and strong in-plane anisotropy of phonon vibrations owing to the different bonding modes of Nb atoms along the b- and c-axes. Consequently, the anisotropic factors of optical absorbance and photoresponsivity in 2D NbOI₂ crystals reach up to 1.75 and 1.7, respectively. These anisotropic properties make 2D NbOI₂ an interesting platform for novel polarization-sensitive optoelectronic applications.

Monolayer graphene is stronger than bulk graphite: the elastic properties of graphene are such that it could hypothetically withstand an elephant balancing on a pencil. A contactless optical technique, reported by Bartlomiej Graczykowski and co-workers in article number 2008614 now shows that this is not the case for MoSe₂, an attractive semiconducting 2D material. This material becomes significantly softer by reducing its thickness from bulk to a few layers.

MoO _x species on MoS₂ flakes are investigated. For samples oxidatively etched at 350–370 °C in air MoO₃ species are detected as particles via atomic force microscopy (AFM), X-ray photoelectron spectroscopy and X-ray absorption spectroscopy. For flakes heated at 220 °C at 10% relative humidity, 2 nm thick MoO _x layer is observed via AFM and Auger photoelectron spectroscopy.

Abstract

The chemical presence of the MoO _x species on single microscopic MoS₂ flakes is shown at two conditions, which are of interest for future MoS₂-based devices and where their presence is not previously confirmed. First, the case of thick MoS₂ flakes oxidatively etched at 350–370 °C in air is treated. Atomic force microscopy (AFM), high resolution X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy are combined to unambiguously confirm the chemical presence of the α-MoO₃ species on such samples, mostly in the form of loose particles. Second, it is shown that MoS₂ flakes heated at temperatures of only 220 °C display a quite uniform ≈2 nm thick MoO _x layer at already 10% relative humidity. The presence of such MoO _x oxide layers is confirmed by scratching the sample with AFM tips and performing comparative Kelvin probe force microscopy and Auger photoelectron spectroscopy on scratched-out and untouched parts of the flakes.

State-of-the-art progress in controllable synthesis of wafer-scale graphene films by chemical vapor deposition is summarized according to application requirements. Synthesis challenges of graphene on metal wafers deal with wrinkle formation, difficulties in few-layer growth and transfer-related issues, while those on insulating wafers are mainly concerned with massive grain boundaries, layer inhomogeneity, and low growth rates.

Abstract

The availability of high-quality, large-scale, and single-crystal wafer-scale graphene films is fundamental for key device applications in the field of electronics, optics, and sensors. Synthesis determines the future: unleashing the full potentials of such emerging materials relies heavily upon their tailored synthesis in a scalable fashion, which is by no means an easy task to date. This review covers the state-of-the-art progress in the synthesis of wafer-scale graphene films by virtue of chemical vapor deposition (CVD), with a focus on main challenges and present status. Particularly, prevailing synthetic strategies are highlighted on a basis of the discussion in the reaction kinetics and gas-phase dynamics during CVD process. Perspectives with respect to key opportunities and promising research directions are proposed to guide the future development of wafer-scale graphene films.

A new strategy using mixed salts for chemical vapor deposition of tunable Re- and V-doped 2D transition metal dichalcogenides (TMDCs) is realized by Shisheng Li, Takaaki Taniguchi, and co-workers, as described in article number 2004438. By applying the highly conductive V-doped WSe₂ as van der Waals contacts for 2D WSe₂ based field-effect transistor, the on-state current and on/off ratio are substantially improved, compared to the metal contacts.

Nature Nanotechnology, Published online: 03 June 2021; doi:10.1038/s41565-021-00916-1

Nature Communications, Published online: 02 June 2021; doi:10.1038/s41467-021-23869-4

The electronic properties of stacked few-layer 2D materials depend strongly on their precise structural arrangement. In this paper, a 4D scanning transmission electron microscopy technique is described that utilizes interference between Bragg disks from separate layers of bilayer and trilayer 2D materials, e.g., to enable measurement of average interlayer spacings and nanometer-scale mapping of local structural reconstructions.

Abstract

Van der Waals materials composed of stacks of individual atomic layers have attracted considerable attention due to their exotic electronic properties that can be altered by, e.g., manipulating the twist angle of bilayer materials or the stacking sequence of trilayer materials. To fully understand and control the unique properties of these few-layer materials, a technique that can provide information about their local in-plane structural deformations, twist direction, and out-of-plane structure is needed. In principle, interference in overlap regions of Bragg disks originating from separate layers of a material encodes 3D information about the relative positions of atoms in the corresponding layers. Here, an interferometric 4D scanning transmission electron microscopy technique is described that utilizes this phenomenon to extract precise structural information from few-layer materials with nm-scale resolution. It is demonstrated how this technique enables measurement of local pm-scale in-plane lattice distortions as well as twist direction and average interlayer spacings in bilayer and trilayer graphene, and therefore provides a means to better understand the interplay between electronic properties and precise structural arrangements of few-layer 2D materials.

2D materials exhibit unique physical phenomena and features that are absent in conventional bulk semiconductors. The theoretical and ab initio methods that yield the optically excited states in 2D materials are reviewed. Several analytical and numerical approaches are introduced and their results are compared with experiments.

Abstract

Recent studies of the optical properties of 2D materials have reported unique phenomena and features that are absent in conventional bulk semiconductors. Many of these interesting properties, such as enhanced light-matter coupling, gate-tunable photoluminescence, and unusual excitonic optical selection rules arise from the nature of the two- and multi-particle excited states such as strongly bound Wannier excitons and charged excitons. The theory, modeling, and ab initio calculations of these optically excited states in 2D materials are reviewed. Several analytical and ab initio approaches are introduced. These methods are compared with each other, revealing their relative strength and limitations. Recent works that apply these methods to a variety of 2D materials and material-defect systems are then highlighted. Understanding of the optically excited states in these systems is relevant not only for fundamental scientific research of electronic excitations and correlations, but also plays an important role in the future development of quantum information science and nano-photonics.

Nature, Published online: 02 June 2021; doi:10.1038/s41586-021-03541-z

The patterned synthesis of high-quality 2D PdTe₂ is demonstrated. The as-grown PdTe₂ exhibits high electrical and thermal conductivities, showing potential as an ideal contact material in nanoelectronics. Based on the nondestructive synthesis of PdTe₂ patterns directly on various low-dimensional semiconductors, high-performance field-effect transistors (FETs) with reduced contact barriers are chemically constructed.

Abstract

Low-dimensional semiconductors provide promising ultrathin channels for constructing more-than-Moore devices. However, the prominent contact barriers at the semiconductor–metal electrodes interfaces greatly limit the performance of the obtained devices. Here, a chemical approach is developed for the construction of p-type field-effect transistors (FETs) with low contact barriers by achieving the simultaneous synthesis and integration of 2D PdTe₂ with various low-dimensional semiconductors. The 2D PdTe₂ synthesized through a quasi-liquid process exhibits high electrical conductivity (≈4.3 × 10⁶ S m⁻¹) and thermal conductivity (≈130 W m⁻¹ K⁻¹), superior to other transition metal dichalcogenides (TMDCs) and even higher than some metals. In addition, PdTe₂ electrodes with desired geometry can be synthesized directly on 2D MoTe₂ and other semiconductors to form high-performance p-type FETs without any further treatment. The chemically derived atomically ordered PdTe₂–MoTe₂ interface results in significantly reduced contact barrier (65 vs 240 meV) and thus increases the performance of the obtained devices. This work demonstrates the great potential of 2D PdTe₂ as contact materials and also opens up a new avenue for the future device fabrication through the chemical construction and integration of 2D components.

Nature Communications, Published online: 01 June 2021; doi:10.1038/s41467-021-23359-7