Jiuxiang Dai

Light: Science & Applications, Published online: 02 January 2023; doi:10.1038/s41377-022-01051-9

Light: Science & Applications, Published online: 02 January 2023; doi:10.1038/s41377-022-01047-5

Evidence of lattice transformation of exfoliated ZrX₂ materials from 2D to 1D is reported, mimicking ZrX₃ materials. The thermal properties are investigated using Raman spectroscopy to further elucidate on the origin of this lattice transformation.

Abstract

Recent reports on thermal and thermoelectric properties of emerging 2D materials have shown promising results. Among these materials are Zirconium-based chalcogenides such as zirconium disulfide (ZrS₂), zirconium diselenide (ZrSe₂), zirconium trisulfide (ZrS₃), and zirconium triselenide (ZrSe₃). Here, the thermal properties of these materials are investigated using confocal Raman spectroscopy. Two different and distinctive Raman signatures of exfoliated ZrX₂ (where X = S or Se) are observed. For 2D-ZrX₂, Raman modes are in alignment with those reported in literature. However, for quasi 1D-ZrX₂, Raman modes are identical to exfoliated ZrX₃ nanosheets, indicating a major lattice transformation from 2D to quasi-1D. Raman temperature dependence for ZrX₂ are also measured. Most Raman modes exhibit a linear downshift dependence with increasing temperature. However, for 2D-ZrS₂, a blueshift for A1g mode is detected with increasing temperature. Finally, phonon dynamics under optical heating for ZrX₂ are measured. Based on these measurements, the calculated thermal conductivity and the interfacial thermal conductance indicate lower interfacial thermal conductance for quasi 1D-ZrX₂ compared to 2D-ZrX₂, which can be attributed to the phonon confinement in 1D. The results demonstrate exceptional thermal properties for Zirconium-based materials, making them ideal for thermoelectric device applications and future thermal management strategies.

A one-pot molten salt electrochemical etching (E) method is proposed to achieve Ti₂C MXene directly from elemental substances (Ti, Al, and C), which greatly simplifies the preparation process. By using carbon sources with different morphologies, such as carbon nanotubes and reduced graphene oxide, MAX and MXene with tuned morphology are prepared based on the “carbon-template-growth” mechanism.

Abstract

MXenes, a rapidly growing family of 2D transition metal carbides, carbonitrides, and nitrides, are one of the most promising high-rate electrode materials for energy storage. Despite the significant progress achieved, the MXene synthesis process is still burdensome, involving several procedures including preparation of MAX, etching of MAX to MXene, and delamination. Here, a one-pot molten salt electrochemical etching (E) method is proposed to achieve Ti₂C MXene directly from elemental substances (Ti, Al, and C), which greatly simplifies the preparation process. In this work, different carbon sources, such as carbon nanotubes (CNT) and reduced graphene oxide (rGO), are reacted with Ti and Al micro-powders to prepare Ti₂AlC MAX with 1D and 2D tuned morphology followed by in situ electrochemical etching from Ti₂AlC MAX to Ti₂CT _x MXene in low-cost LiCl-KCl. The introduction of the O surface group via further ammonium persulfate (APS) treatment can act in concert with Cl termination to activate the pseudocapacitive redox reaction of Ti₂CCl _y O _z in the non-aqueous electrolyte, resulting in a Li⁺ storage capacity of up to 857 C g⁻¹ (240 mAh g⁻¹) with a high rate (86 mAh g⁻¹ at 120 C) capability, which makes it promising for use as an anode material for fast-charging batteries or hybrid devices in a non-aqueous energy storage application.

A method to scale up freestanding synthesis of MoS₂ sheets on 100 mm-diameter silicon wafers that have hundreds of nanoapertures is proposed. The MoS₂ sheets cover the apertures to form ultrathin membranes that are useful in nanopore sensing.

Abstract

2D materials are ideal for nanopores with optimal detection sensitivity and resolution. Among these, molybdenum disulfide (MoS₂) has gained traction as a less hydrophobic material than graphene. However, experiments using 2D nanopores remain challenging due to the lack of scalable methods for high-quality freestanding membranes. Herein, a site-directed, scaled-up synthesis of MoS₂ membranes on predrilled nanoapertures on 4-inch wafer substrates with 75% yields is reported. Chemical vapor deposition (CVD), which introduces sulfur and molybdenum dioxide vapors across the sub-100 nm nanoapertures results in exclusive formation of freestanding membranes that seal the apertures. Nucleation and growth near the nanoaperture edges is followed by nanoaperture decoration with MoS₂, which proceeds until a critical flake curvature is achieved, after which fully spanning freestanding membranes form. Intentional blocking of reagent flow through the apertures inhibits MoS₂ nucleation around the nanoapertures, promoting the formation of large-crystal monolayer MoS₂ membranes. The in situ grown membranes along with facile membrane wetting and nanopore formation using dielectric breakdown enables the recording of dsDNA translocation events at an unprecedentedly high 1 MHz bandwidth. The methods presented here are important steps toward the development of scalable single-layer membrane manufacture for 2D nanofluidics and nanopore applications.

High-quality and nonlayered WO₂ nanoplates with tunable thickness and lateral dimension by a WSe₂-assisted strategy are reported. Electrical measurements demonstrate that the WO₂ nanoplates exhibit metallic behavior with excellent conductivity, ultrahigh breakdown current density, and strong anisotropic resistance. Magnetotransport studies show quantum-interference-induced weak-localization behavior. These studies demonstrate that 2D metal oxides are promising candidates for interconnecting and other novel electronic devices.

Abstract

2D metal oxides (2DMOs) have stimulated tremendous attention due to their distinct electronic structures and abundant surface chemistry. However, it remains a standing challenge for the synthesis of 2DMOs because of their intrinsic 3D lattice structure and ultrahigh synthesis temperature. Here, a reliable WSe₂-assisted chemical vapor deposition (CVD) strategy to grow nonlayered WO₂ nanoplates with tunable thickness and lateral dimension is reported. Optical microscopy and scanning electron microscopy studies demonstrate that the WO₂ nanoplates exhibit a well-faceted rhombic geometry with a lateral dimension up to the sub-millimeter level (≈135 µm), which is the largest size of 2DMO single crystals obtained by CVD to date. Scanning transmission electron microscopy studies reveal that the nanoplates are high-quality single crystals. Electrical measurements show the nanoplates exhibit metallic behavior with strong anisotropic resistance, outstanding conductivity of 1.1 × 10⁶ S m⁻¹, and breakdown current density of 7.1 × 10⁷ A cm⁻². More interestingly, low-temperature magnetotransport studies demonstrate that the nanoplates show a quantum-interference-induced weak-localization effect. The developed WSe₂-assisted strategy for the growth of WO₂ nanoplates can enrich the library of 2DMO materials and provide a material platform for other property explorations based on 2D WO₂.

Abstract

van der Waals heterostructures (vdWHs) based on two-dimensional (2D) materials without the crystal lattice matching constraint have great potential for high-performance optoelectronic devices. Herein, a WS₂/InSe vdWH photodiode is proposed and fabricated by precisely stacking InSe and WS₂ flakes through an all-dry transfer method. The WS₂/InSe vdWH forms an n—n heterojunction with strong built-in electric field due to their intrinsic n-type semiconductor characteristics and energy-band alignments with a large Fermi level offset between WS₂ and InSe. As a result, the device displays excellent photovoltaic behavior with a large open voltage of 0.47 V and a short-circuit current of 11.7 nA under 520 nm light illumination. Significantly, a fast rising/decay time of 63/76 µs, a large light on/off ratio of 10⁵, a responsivity of 61 mA/W, a high detectivity of 2.5 × 10¹¹ Jones, and a broadband photoresponse ranging from ultraviolet to near-infrared (325–980 nm) are achieved at zero bias. This study provides a strategy for developing high-performance self-powered broadband photodetectors based on 2D materials.

Author(s): Xiang Gao, Michael Urbakh, and Oded Hod

A new frictional mechanism, based on collective stick-slip motion of moiré superstructures across polycrystalline two-dimensional material interfaces, is predicted. The dissipative stick-slip behavior originates from an energetic bistability between low- and high-commensurability configurations of l…

[Phys. Rev. Lett. 129, 276101] Published Thu Dec 29, 2022

Spatial self-phase modulation (SSPM), a third-order nonlinear optical response is observed in a Weyl semimetal TaAs. This extends previous SSPM discoveries in 0D, 1D, and 2D materials into 3D materials. Also amazing is that it reveals the long-sought laser-induced hole coherence. Furthermore, SSPM in a topological semimetal is reported.

Abstract

Laser-induced electron coherence is a fascinating topic in manipulating quantum materials. Recently, it has been shown that laser-induced electron coherence in 2D materials can produce a third-order nonlinear optical response spatial self-phase modulation (SSPM), which has been used to develop a novel all-optical switching scheme. However, such investigations have mainly focused on electron coherence, whereas laser-induced hole coherence is rarely explored. Here, the observation of the optical Kerr effect in 3D Weyl semimetal TaAs flakes is reported. The nonlinear susceptibility (χ⁽³⁾) is obtained, which exhibits a surprisingly high value (with χone−layer(3)\[{\bm{\chi }}_{{\bf one}{\bm{ - }}{\bf layer}}^{{\bf (3)}}\] = 9.9 × 10⁻⁹ e.s.u. or 1.4 × 10⁻¹⁶ m² V⁻² at 532 nm). This cannot be explained by the conventional electron mobility, but can be well understood by the unique high anisotropic hole mobility of TaAs. The wind-chime model and χ⁽³⁾ carrier mobility correlation adequately explain the results, suggesting the crucial role of laser-induced nonlocal ac hole coherence. These observations extend the understanding of SSPM from 2D to 3D quantum materials with anisotropic carrier mobility and from electron coherence to hole coherence.

This work reports a new class of honeycomb-layered oxides (Ag₂ M ₂TeO₆ (M is a transition or alkaline-earth metal)) comprising unconventional sub-valent bilayered silver-rich domains (Ag₆ M ₂TeO₆), as elucidated via aberration-corrected transmission electron microscopy. Stability of the bilayers is rationalized by a new model— SU(2)× U(1) interactions between three degenerate states of silver metal—responsible for all the observed unconventional features.

Abstract

Honeycomb-layered oxides with monovalent or divalent, monolayered cationic lattices generally exhibit myriad crystalline features encompassing rich electrochemistry, geometries, and disorders, which particularly places them as attractive material candidates for next-generation energy storage applications. Herein, global honeycomb-layered oxide compositions, Ag₂ M ₂TeO₆ (M=Ni,Mg,etc$M = \rm Ni, Mg, etc$.) exhibiting Ag$\rm Ag$ atom bilayers with sub-valent states within Ag-rich crystalline domains of Ag₆ M ₂TeO₆ and Ag$\rm Ag$-deficient domains of Ag2−xNi2TeO6${\rm Ag}_{2 - x}\rm Ni_2TeO_6$ (0<x<2$0 < x < 2$). The Ag$\rm Ag$-rich material characterized by aberration-corrected transmission electron microscopy reveals local atomic structural disorders characterized by aperiodic stacking and incoherency in the bilayer arrangement of Ag$\rm Ag$ atoms. Meanwhile, the global material not only displays high ionic conductivity but also manifests oxygen-hole electrochemistry during silver-ion extraction. Within the Ag$\rm Ag$-rich domains, the bilayered structure, argentophilic interactions therein and the expected Ag$\rm Ag$ sub-valent states (1/2+,2/3+$1/2+, 2/3+$, etc.) are theoretically understood via spontaneous symmetry breaking of SU(2)× U(1) gauge symmetry interactions amongst 3 degenerate mass-less chiral fermion states, justified by electron occupancy of silver 4dz2$4d_{z^2}$ and 5s orbitals on a bifurcated honeycomb lattice. This implies that bilayered frameworks have research applications that go beyond the confines of energy storage.

In this review, the most recent progress on low-dimensional II–VI semiconductors based flexible photodetectors and their application in wearable electronics are summarized, including wearable monitoring sensors, image sensors, and self-powered integrated wearable electronics. Meanwhile, the challenges and outlook of flexible photodetectors in the future integration of wearable electronic are also discussed.

Abstract

Flexible photodetectors exhibit many advantages such as a good bendability, foldability, and even stretchability as well as weight light, which have triggered a widely concerned in wearable electronics including wearable monitoring, wearable image sensing, self-powered integrated electronics, etc. Recently, various II–VI semiconductor nanostructures have become promising candidates in flexible photodetectors due to their unique characteristics, such as direct bandgap semiconductors, excellent optical and electric properties, high quantum efficiency, and inherent mechanical flexibility. Herein, the most recent progress on low-dimensional (0D, 1D, 2D, and related heterostructures) II–VI semiconductors based flexible photodetectors and their application in wearable electronic is reviewed. First, a brief introduction of the main sensing mechanisms and key figures of merits for photodetectors is presented. Then, the recent progresses on flexible photodetectors are provided, in which the functional materials synthesis methods are also discussed. More importantly, the applications of the flexible photodetectors are summarized, including wearable monitoring sensors, image sensors, and self-powered integrated wearable electronics. Finally, the challenges and the future research direction of the flexible photodetectors are discussed, meanwhile the outlook for the development of flexible photodetectors in the future integration of wearable electronic is also provided.

Publication date: March 2023

Source: Materials Today, Volume 63

Author(s): Xiaoyue Wang, Chi Liu, Yuning Wei, Shun Feng, Dongming Sun, Huiming Cheng

Atomic-scale precision epitaxy and microscopic observation let us customize the synthetic magnetic order, useful for spintronic applications. However, conventional approaches have been limited to metallic heterostructures. Here, atomic-scale modulation of the synthetic magnetic order is reported across the insulating spacer, observed by a polarized neutron reflectometer. This approach can yield novel controllability of the magnetic order across insulating spacers.

Abstract

Atomic-scale precision control of magnetic interactions facilitates a synthetic spin order useful for spintronics, including advanced memory and quantum logic devices. Conventional modulation of synthetic spin order has been limited to metallic heterostructures that exploit Ruderman–Kittel–Kasuya–Yosida interaction through a nonmagnetic metallic spacer; however, they face issues arising from Joule heating and/or electric breakdown. The practical realization and observation of a synthetic spin order across a nonmagnetic insulating spacer will lead to the development of spin-related devices with a completely different concept. Herein, the atomic-scale modulation of the synthetic spiral spin order in oxide superlattices composed of ferromagnetic metal and nonmagnetic insulator layers is reported. The atomically controlled superlattice exhibits an oscillatory magnetic behavior, representing the existence of a spiral spin structure. Depth-sensitive polarized neutron reflectometry evidences modulated spiral spin structures as a function of the nonmagnetic insulator layer thickness. Atomic-scale customization of the spin state can move the field one step further to actual spintronic applications.

A review of hardware security solutions using novel electronics, optical, and material properties of 2D materials and their corresponding heterostructure device configurations for Internet of Things platforms. The five quintessential security domains for discussion are physically unclonable functions, true random number generators, logic locking, hardware watermarking and anticounterfeiting, and hardware camouflaging.

Abstract

Hardware security is a major concern for the entire semiconductor ecosystem that accounts for billions of dollars in annual losses. Similarly, information security is a critical need for the rapidly proliferating edge devices that continuously collect and communicate a massive volume of data. While silicon-based complementary metal-oxide-semiconductor technology offers security solutions, these are largely inadequate, inefficient, and often inconclusive, as well as resource intensive in time, energy, and cost, leading to tremendous room for innovation in this field. Furthermore, silicon-based security primitives have shown vulnerability to machine learning (ML) attacks. In recent years, 2D materials such as graphene and transition metal dichalcogenides have been intensely explored to mitigate these security challenges. In this review, 2D-materials-based hardware security solutions such as camouflaging, true random number generation, watermarking, anticounterfeiting, physically unclonable functions, and logic locking of integrated circuits (ICs) are summarized with accompanying discussion on their reliability and resilience to ML attacks. In addition, the role of native defects in 2D materials in developing high entropy hardware security primitives is also examined. Finally, the existing challenges for 2D materials, which must be overcome for large-scale deployment of 2D ICs to meet the security needs of the semiconductor industry, are discussed.

npj 2D Materials and Applications, Published online: 24 December 2022; doi:10.1038/s41699-022-00362-0