Jiuxiang Dai

The existence of a liquid crystal (LC) mesophase in metallic materials is counterintuitive. Typical LCs are made of covalently bonded organic molecules. This study reports on the observation of LC mesophases in supercooled liquid gallium and eutectic gallium–indium alloy. An LC texture of the lamellar structure similar to that of the smectic phase is unveiled at room temperature.

Abstract

Understanding the origin of structural ordering in supercooled liquid gallium (Ga) has been a great scientific quest in the past decades. Here, reflective polarized optical microscopy on Ga sandwiched between glasses treated with rubbed polymers reveals the onset of an anisotropic reflection at 120 °C that increases on cooling and persists down to room temperature or below. The polymer rubbing usually aligns the director of thermotropic liquid crystals (LCs) parallel to the rubbing direction. On the other hand, when Ga is sandwiched between substrates that align conventional LC molecules normal to the surface, the reflection is isotropic, but mechanical shear force induces anisotropic reflection that relaxes in seconds. Such alignment effects and shear-induced realignment are typical to conventional thermotropic LCs and indicate a LC structure of liquid Ga. Specifically, Ga textures obtained by atomic force and scanning electron microscopy reveal the existence of a lamellar structure corresponding to a smectic LC phase, while the nanometer-thin lamellar structure is transparent under transmission polarized optical microscopy. Such spatial molecular arrangements may be attributed to dimer molecular entities in the supercooled liquid Ga. The LC structure observation of electrically conductive liquid Ga can provide new opportunities in materials science and LC applications.

Monochromated electron energy-loss spectral maps reveal four distinct localized surface plasmon resonance (LSPR) components and intensity distributions of all LSPR modes in a hexagonal gold nanoplate (NPL). Strain maps provide experimental evidence that the tensile strain field induced by a Z-shaped faulted dipole is responsible for the asymmetric distribution of the LSPR intensity in a hexagonal gold NPL.

Abstract

Anisotropic gold nanoplates (NPLs) have raised the interesting possibility that their reduced geometrical symmetry allows fine tuning of their optical properties associated with the excitation of localized surface plasmon resonances (LSPRs). Recent developments have greatly improved LSPR tunability by utilizing the spatial distribution of LSPR modes. However, the nanoscale interplay between defect-induced mechanical strain and the spatial variation of LSPR modes remains poorly understood. In this work, the combination of high spatial- and spectral-resolution mapping of LSPR modes and nanoscale strain mapping using aberration-corrected transmission electron microscopy are applied to investigate the nanoscale distribution of LSPR modes in an ultrathin single hexagonal gold NPL and the effect of defect-induced strains on its LSPR properties. The electron energy-loss spectral maps reveal four distinct LSPR components and intensity distributions of all LSPR modes in a hexagonal gold NPL. Furthermore, the strain maps provide experimental evidence that the tensile strain field induced by a Z-shaped faulted dipole is responsible for the asymmetric distribution of LSPR intensity in a hexagonal gold NPL.

Light: Science & Applications, Published online: 04 August 2021; doi:10.1038/s41377-021-00599-2

This review focuses on the advance of optical design from nano/micro to macro in SERS and SEIRA, especially on the optical coupling between nano/micro and macro scales.

Nature Communications, Published online: 03 August 2021; doi:10.1038/s41467-021-24531-9

Bound states in superconducting vortices are expected to exhibit an electron-hole asymmetry, but it is usually tiny and can be easily washed out. Here, the authors show that the vortex bound states coupling to magnetic impurities provides an axial electron-hole asymmetry on a much longer scale, and that the direction of the asymmetry depends on the band character of the superconducting material.

Nature Materials, Published online: 02 August 2021; doi:10.1038/s41563-021-01064-6

SnSe has a very high thermoelectric figure of merit ZT, but uncommonly polycrystalline samples have higher lattice thermal conductivity than single crystals. Here, by controlling Sn reagent purity and removing SnOx impurities, a lower thermal conductivity is achieved, enabling ZT of 3.1 at 783 K.

Author(s): Tatiana Latychevskaia

Conventional three-dimensional (3D) imaging methods require multiple measurements of the sample in different orientation or scanning. When the sample is probed with coherent waves, a single two-dimensional (2D) intensity measurement is sufficient as it contains all the information of the 3D sample d...

[Phys. Rev. Lett. 127, 063601] Published Mon Aug 02, 2021

Scanning tunneling microscopy and spectroscopy (STM/STS) are critical to research on semiconductor heterojunctions (SHs). Recent progress on the study of SHs by using STM/STS is reviewed. Beginning with the basics of STM/STS, both common and advanced STM/STS methods are introduced for research on SHs. Three types of SHs (i.e., lateral, vertical and bulk SHs) investigated with STM/STS are concerned.

Abstract

The band alignment, interface states, interface coupling, and carrier transport of semiconductor heterojunctions (SHs) need to be well understood for the design and fabrication of various important semiconductor structures and devices. Scanning tunneling microscopy (STM) with high spatial resolution and scanning tunneling spectroscopy (STS) with high energy resolution are significantly contributing to the understanding on the important properties of SHs. In this work, the recent progress on the use of STM and STS to study lateral, vertical and bulk SHs is reviewed. The spatial structures of SHs with atomically flat surface have been examined with STM. The electronic band structures (e. g., the band offset, interface state, and space charge region) of SHs are measured with STS. Combined with the spatial structures and the tunneling spectra features, the mechanism for the carrier transport in the SH may be proposed.

npj 2D Materials and Applications, Published online: 02 August 2021; doi:10.1038/s41699-021-00250-z

Spatially controlled epitaxial growth of 2D heterostructures via defect engineering using a focused He ion beam

Few-layer SnS thin films are grown by molecular beam epitaxy and the reversible conversion between top layer of SnS and monolayer SnS₂ is realized via annealing the sample with sulfur supply, which produces a high-quality SnS₂/SnS 2D heterojunction with band alignment. This work provides a promising approach to construct artificial 2D heterojunctions with desired properties.

Abstract

2D van der Waals heterojunction provides an attractive opportunity for realizing novel electronic or optoelectronic devices. It remains challenging to realize high-quality heterostructures through scalable methods such as molecular epitaxy growth (MBE). Here, growth of few-layer SnS thin films is reported on mica and Nb-doped SrTiO₃(100) substrates by MBE. Then the top layer of SnS film is uniformly sulfurized to monolayer SnS₂ in a sulfur atmosphere, resulting in a high-quality SnS₂/SnS 2D heterojunction. Furthermore, the SnS₂ layer can be recovered to SnS by annealing SnS₂/SnS without sulfur supply, indicating the heterojunction formation is reversible. The scanning tunneling spectroscopy measurements on SnS₂/SnS heterostructure indicate the type-II band alignment in SnS₂/SnS. The work provides a promising approach to construct artificial 2D heterojunctions with desired properties, which could be extended to other sulfide and selenide systems.

Kinetics and dynamics of the crystallization of thermally evaporated amorphous Te films are studied. Single-crystalline tellurium arrays are demonstrated by controlling the crystallization pathway of the evaporated Te films. Field-effect transistors based on the evaporated Te single crystals exhibit an average effective hole mobility of ≈100 cm² V⁻¹ s⁻¹ and on/off ratio of ≈3 × 10⁴.

Abstract

Thermally evaporated tellurium possesses an intriguing crystallization behavior, where an amorphous to crystalline phase transition happens at near-ambient temperature. However, a comprehensive understanding and delicate control of the crystallization process for the evaporated Te films is lacking. Here, the kinetics and dynamics of the crystallization of thermally evaporated Te films is visualized and modeled. Low-temperature processing of highly crystalline tellurium films with large grain size and preferred out-of-plane orientation ((100) plane parallel to the surface) is demonstrated by controlling the crystallization process. Tellurium single crystals with a lateral dimension of up to 6 µm are realized on various substrates including glass and plastic. Field-effect transistors based on 5 °C crystallized Te single grains (6-nm-thick) exhibit an average effective hole mobility of ≈100 cm² V⁻¹ s⁻¹, and on/off current ratio of ≈3 × 10⁴.

Monolayer transition metal dichalcogenides (TMDs) show promising potential for next-generation optoelectronics due to excellent light capturing and photodetection capabilities. Photodetectors, as important com...

Ion insertion in between the 2D layers of WTe₂ is shown to induce an exotic crystallographic phase. This novel phase is linked to uniquely large uniaxial in-plane strain. On-chip electrochemical control over the lithium content in single flakes of WTe₂ is demonstrated as an efficient, fast, and reversible way to control the strain, making Li _x WTe₂ a promising microscale actuator.

Abstract

On-chip dynamic strain engineering requires efficient micro-actuators that can generate large in-plane strains. Inorganic electrochemical actuators are unique in that they are driven by low voltages (≈1 V) and produce considerable strains (≈1%). However, actuation speed and efficiency are limited by mass transport of ions. Minimizing the number of ions required to actuate is thus key to enabling useful “straintronic” devices. Here, it is shown that the electrochemical intercalation of exceptionally few lithium ions into WTe₂ causes large anisotropic in-plane strain: 5% in one in-plane direction and 0.1% in the other. This efficient stretching of the 2D WTe₂ layers contrasts to intercalation-induced strains in related materials which are predominantly in the out-of-plane direction. The unusual actuation of Li _x WTe₂ is linked to the formation of a newly discovered crystallographic phase, referred to as Td', with an exotic atomic arrangement. On-chip low-voltage (<0.2 V) control is demonstrated over the transition to the novel phase and its composition. Within the Td'-Li_{0.5− δ} WTe₂ phase, a uniaxial in-plane strain of 1.4% is achieved with a change of δ of only 0.075. This makes the in-plane chemical expansion coefficient of Td'-Li_0.5−δWTe₂ far greater than of any other single-phase material, enabling fast and efficient planar electrochemical actuation.

Examining the chemical bonding in rock-salt-type IV–VI functional materials by projected force constants as derived from ab initio thermochemistry yields unusually strong long-range forces along the cubic lattice vectors, impossible to account for by either two-center covalent or ionic bonding. The full analysis, including orbital-based bonding descriptors, points toward hyperbonding, thereby explaining certain unique properties of this material class.

Abstract

Chemical bonding in main-group IV chalcogenides is an intensely discussed topic, easily understandable because of their remarkable physical properties that predestine these solid-state materials for their widespread use in, for instance, thermoelectrics and phase-change memory applications. The atomistic origin of their unusual property portfolio remains somewhat unclear, however, even though different and sometimes conflicting chemical-bonding concepts have been introduced in the recent years. Here, it is proposed that projecting phononic force-constant tensors for pairs of atoms along differing directions and ranges provide a suitable and quantitative descriptor of the bonding nature for these materials. In combination with orbital-based quantitative measures of covalency such as crystal orbital Hamilton populations (COHP), it is concluded that the well-established many-center and even n-center bonding is an appropriate picture of the underlying quantum-chemical bonding mechanism, supporting the recent proposal of hyperbonded phase-change materials.

Recent progress in black phosphorus/polymer (BP/polymers) research is highlighted, including the preparation methods, a comprehensive introduction of their prop erties, and a detailed account of their applications in optoelectronics, biomedicine, energy-storage devices, flame retardancy, and information storage. Forward-looking perspectives on the current challenges are also provided.

Abstract

As a newly emerged mono-elemental nanomaterial, black phosphorus (BP) has been widely investigated for its fascinating physical properties, including layer-dependent tunable band gap (0.3–1.5 eV), high ON/OFF ratio (10⁴), high carrier mobility (10³ cm² V⁻¹ s⁻¹), excellent mechanical resistance, as well as special in-plane anisotropic optical, thermal, and vibrational characteristics. However, the instability caused by chemical degradation of its surface has posed a severe challenge for its further applications. A focused BP/polymer strategy has more recently been developed and implemented to hurdle this issue, so at present BP/polymers have been developed that exhibit enhanced stability, as well as outstanding optical, thermal, mechanical, and electrical properties. This has promoted researchers to further explore the potential applications of black phosphorous. In this review, the preparation processes and the key properties of BP/polymers are reviewed, followed by a detailed account of their diversified applications, including areas like optoelectronics, bio-medicine, and energy storage. Finally, in accordance with the current progress, the prospective challenges and future directions are highlighted and discussed.

npj 2D Materials and Applications, Published online: 29 July 2021; doi:10.1038/s41699-021-00252-x

Exciton dynamics in monolayer graphene grown on a Cu(111) surface

Microgravity has proved to be an ideal condition to grow crystals. In article number 2101777, Raphael Pfattner, Tiago Sotto Mayor, Daniel Ruiz-Molina, Josep Puigmartí-Luis, and co-workers demonstrate how to generate simulated microgravity on Earth to grow 2D porous crystalline molecular frameworks such as 2D metal–organic frameworks and 2D covalent organic frameworks.

Electronically inverted structures with switched crystallographic positions of cations and anions from the original structure allow an exotic phenomenon to be accessed. Antiperovskite Gd₃SnC exhibits unusual coexistence of ferromagnetism and heavy fermions, contradicting the common belief that heavy-fermion behavior cannot emerge in ferromagnetic 4f-block elements. These unusual phenomena originate from the mixed bonding nature of antiperovskite structure.

Abstract

Inverted structures of common crystal lattices, referred to as antistructures, are rare in nature due to their thermodynamic constraints imposed by the switched cation and anion positions in reference to the original structure. However, a stable antistructure formed with mixed bonding characters of constituent elements in unusual valence states can provide unexpected material properties. Here, a heavy-fermion behavior of ferromagnetic gadolinium lattice in Gd₃SnC antiperovskite is reported, contradicting the common belief that ferromagnetic gadolinium cannot be a heavy-fermion system due to the deep energy level of localized 4f-electrons. The specific heat shows an unusually large Sommerfeld coefficient of ≈1114 mJ mol⁻¹ K⁻² with a logarithmic behavior of non-Fermi-liquid state. It is demonstrated that the heavy-fermion behavior in the non-Fermi-liquid state appears to arise from the hybridized electronic states of gadolinium 5d-electrons participating in metallic GdGd and covalent GdC bonds. These results accentuate the unusual chemical bonds in CGd₆ octahedra with the dual characters of gadolinium 5d-electrons for the emergence of heavy fermions.

In this review, the exfoliated method and application of MXene are summarized, especially the ionic liquid exfoliation of MXene, which is a new green, nontoxic and safe method and has great application potential in the medical field, such as photothermal therapy, photodynamic therapy, drug delivery, bioimaging, and antibacterial activity. Therefore, MXene materials have prospects in the field of medicine.

Abstract

Research on 2D nanomaterials is still in its early stages. Most studies have focused on elucidating the unique properties of the materials, whereas only few reports have described the biomedical applications of 2D nanomaterials. Recently, important questions about the interaction of 2D MXene nanomaterials with biological components have been raised. 2D MXenes are monolayer atomic nanosheets derived from MAX phase ceramics. As a new type of inorganic nanosystems, they are being widely used in biology and biomedicine. This review introduces the latest developments in 2D MXenes for the most advanced biomedical applications, including preparation and surface modification strategies, treatment modes, drug delivery, antibacterial activity, bioimaging, sensing, and biocompatibility. Besides, this review also discusses the current development trends and prospects of 2D inorganic nanosheets for further clinical applications. These emerging 2D inorganic MXenes will play an important role in next-generation cancer treatments.

The recent advances of the integrated quantum dot (QD)/photonic structures in many optoelectronic and optical devices for performance enhancement and new functionalities are summarized in this review. The use of four typical photonic structures applied in either modulating the light absorption or light emission properties of QD-based devices is discussed, and the innovative QD-based on-chip photonic circuit is briefly overviewed.

Abstract

Colloidal quantum dot (QD), a solution-processable nanoscale optoelectronic building block with well-controlled light absorption and emission properties, has emerged as a promising material system capable of interacting with various photonic structures. Integrated QD/photonic structures have been successfully realized in many optical and optoelectronic devices, enabling enhanced performance and/or new functionalities. In this review, the recent advances in this research area are summarized. In particular, the use of four typical photonic structures, namely, diffraction gratings, resonance cavities, plasmonic structures, and photonic crystals, in modulating the light absorption (e.g., for solar cells and photodetectors) or light emission (e.g., for color converters, lasers, and light emitting diodes) properties of QD-based devices is discussed. A brief overview of QD-based passive devices for on-chip photonic circuit integration is also presented to provide a holistic view on future opportunities for QD/photonic structure-integrated optoelectronic systems.