Jiuxiang Dai

The significantly suppressed lattice thermal conductivity κ_ph and elevated quality factor B have created high-performance (Bi,Sb)₂Te₃ material. Most noteworthily, the size and mass of the optimal sample are enlarged to Ø40 mm-200 g (industrial grade) with only ≈5% loss of thermoelectric (TE) performance, the resulting 17-couple TE modules exhibit an excellent conversion efficiency up to 6.3% (ΔT = 245 K).

Abstract

As the sole dominator of the commercial thermoelectric (TE) market, Bi₂Te₃-based alloys play an irreplaceable role in Peltier cooling and low-grade waste heat recovery. Herein, to improve the relative low TE efficiency determined by the figure of merit ZT, an effective approach is reported for improving the TE performance of p-type (Bi,Sb)₂Te₃ by incorporating Ag₈GeTe₆ and Se. Specifically, the diffused Ag and Ge atoms into the matrix conduce to optimized carrier concentration and enlarge the density-of-states effective mass while the Sb-rich nanoprecipitates generate coherent interfaces with little loss of carrier mobility. The subsequent Se dopants introduce multiple phonon scattering sources and significantly suppress the lattice thermal conductivity while maintaining a decent power factor. Consequently, a high peak ZT of 1.53 at 350 K and a remarkable average ZT of 1.31 (300–500 K) are attained in the Bi_0.4Sb_1.6Te_0.95Se_0.05 + 0.10 wt% Ag₈GeTe₆ sample. Most noteworthily, the size and mass of the optimal sample are enlarged to Ø40 mm-200 g and the constructed 17-couple TE module exhibits an extraordinary conversion efficiency of 6.3% at ΔT = 245 K. This work demonstrates a facile method to develop high-performance and industrial-grade (Bi,Sb)₂Te₃-based alloys, which paves a strong way for further practical applications.

A generic van der Waals epitaxial approach is demonstrated to synthesize the non-van der Waals 2D ternary chromium tellurium compounds with thicknesses as low as mono-UC. Intrinsic ferromagnetic behavior is observed in the bi-UC, tri-UC, and few-UC nanosheets. Thickness-dependent magnetic textures are studied and applied in neuromorphic computing tasks.

Abstract

2D ferromagnetic chromium tellurides exhibit intriguing spin configurations and high-temperature intrinsic ferromagnetism, providing unprecedented opportunities to explore the fundamental spin physics and build spintronic devices. Here, a generic van der Waals epitaxial approach is developed to synthesize the 2D ternary chromium tellurium compounds with thicknesses down to mono-, bi-, tri-, and few-unit cells (UC). The Mn_0.14Cr_0.86Te evolves from intrinsic ferromagnetic behavior in bi-UC, tri-UC, and few-UC to temperature-induced ferrimagnetic behavior as the thickness increases, resulting in a sign reversal of the anomalous Hall resistance. Temperature- and thickness-tunable labyrinthine-domain ferromagnetic behaviors are derived from the dipolar interactions in Fe_0.26Cr_0.74Te and Co_0.40Cr_0.60Te. Furthermore, the dipolar-interaction-induced stripe domain and field-induced domain wall (DW) motion velocity are studied, and multibit data storage is realized through an abundant DW state. The magnetic storage can function in neuromorphic computing tasks, and the pattern recognition accuracy can reach up to 97.93%, which is similar to the recognition accuracy of ideal software-based training (98.28%). Room-temperature ferromagnetic chromium tellurium compounds with intriguing spin configurations can significantly promote the exploration of the processing, sensing, and storage based on 2D magnetic systems.

A mixture of experimental and theoretical approaches reveals that the ambient stability of 2D In₂Se₃ strongly depends on the surface coordination. Tetrahedral-coordinated surfaces were found to be more stable and suitable for device applications.

Abstract

van der Waals In₂Se₃ has attracted significant attention for its room-temperature 2D ferroelectricity/antiferroelectricity down to monolayer thickness. However, instability and potential degradation pathway in 2D In₂Se₃ have not yet been adequately addressed. Using a combination of experimental and theoretical approaches, we here unravel the phase instability in both α- and β′-In₂Se₃ originating from the relatively unstable octahedral coordination. Together with the broken bonds at the edge steps, it leads to moisture-facilitated oxidation of In₂Se₃ in air to form amorphous In₂Se_3−3x O_3x layers and Se hemisphere particles. Both O₂ and H₂O are required for such surface oxidation, which can be further promoted by light illumination. In addition, the self-passivation effect from the In₂Se_3−3x O_3x layer can effectively limit such oxidation to only a few nanometer thickness. The achieved insight paves way for better understanding and optimizing 2D In₂Se₃ performance for device applications.

Uniform monolayer MoS₂ on a large scale is achieved by the delicate control of gas flows of precursors through a well-designed perforated carbon nanotube film, which functions as both precursor carrier and gas modulator. The as-grown monolayer MoS₂ shows quite good uniformity in geometry, density, structure, and electrical properties across the entire substrate.

Abstract

Scaling up the chemical vapor deposition (CVD) of monolayer transition metal dichalcogenides (TMDCs) is in high demand for practical applications. However, for CVD-grown TMDCs on a large scale, there are many existing factors that result in their poor uniformity. In particular, gas flow, which usually leads to inhomogeneous distributions of precursor concentrations, has yet to be well controlled. In this work, the growth of uniform monolayer MoS₂ on a large scale by the delicate control of gas flows of precursors, which is realized by vertically aligning a well-designed perforated carbon nanotube (p-CNT) film face-to-face with the substrate in a horizontal tube furnace, is achieved. The p-CNT film releases gaseous Mo precursor from the solid part and allows S vapor to pass through the hollow part, resulting in uniform distributions of both gas flow rate and precursor concentrations near the substrate. Simulation results further verify that the well-designed p-CNT film guarantees a steady gas flow and a uniform spatial distribution of precursors. Consequently, the as-grown monolayer MoS₂ shows quite good uniformity in geometry, density, structure, and electrical properties. This work provides a universal pathway for the synthesis of large-scale uniform monolayer TMDCs, and will advance their applications in high-performance electronic devices.

Predeposition of NaCl and MoO₃ by thermal evaporation leads to the separate deposition of NaCl and MoO₃. The formation of the intermediates follows a dual chemical reaction path in which NaCl reacts with MoO₃ and S separately. The self-assembly of grains on the liquid intermediates forms large single-crystalline monolayer MoS₂.

Abstract

The recent introduction of alkali metal halide catalysts for chemical vapor deposition (CVD) of transition metal dichalcogenides (TMDs) has enabled remarkable two-dimensional (2D) growth. However, the process development and growth mechanism require further exploration to enhance the effects of salts and understand the principles. Herein, simultaneous predeposition of a metal source (MoO₃) and salt (NaCl) by thermal evaporation is adopted. As a result, remarkable growth behaviors such as promoted 2D growth, easy patterning, and potential diversity of target materials can be achieved. Step-by-step spectroscopy combined with morphological analyses reveals a reaction path for MoS₂ growth in which NaCl reacts separately with S and MoO₃ to form Na₂SO₄ and Na₂Mo₂O₇ intermediates, respectively. These intermediates provide a favorable environment for 2D growth, including an enhanced source supply and liquid medium. Consequently, large grains of monolayer MoS₂ are formed by self-assembly, indicating the merging of small equilateral triangular grains on the liquid intermediates. This study is expected to serve as an ideal reference for understanding the principles of salt catalysis and evolution of CVD in the preparation of 2D TMDs.

Abstract

Due to the high theoretical capacity and energy density, lithium-sulfur (Li−S) batteries have good commercial prospects. However, shuttle effect of soluble lithium polysulfides (LiPSs) formed by sulfur reduction has severely limited the further development of Li−S batteries. In this work, the two-dimensional (2D) MXene-metal-organic framework (MOF) (Ti₃C₂T_x-CoBDC (BDC: 1,4-benzenedicarboxylate)) heterostructures were employed to modify the separator to inhibit the shuttle effect and facilitate the conversion of the soluble polysulfides. Firstly, a bottom-up synthesis strategy was adopted to synthesize the 2D MXene-MOF heterogeneous layered structure. With high specific surface area, in which the catalytic metal atoms not only restrain the shuttle effect of polysulfides but also exhibit excellent redox electrocatalytic performance. The cell with Ti₃C₂T_x-CoBDC@PP (PP: polypropylene) separator has a high initial capacity of 1255 mAh·g⁻¹ at 0.5 C. When the current density is 2 C, the battery has a capacity retention rate of 94.4% after 600 cycles, with the fading rate of only 0.01% per cycle. Besides, with a sulfur loading of 7.5 mg·cm⁻², the battery shows the discharge capacity of 1096 mAh·g⁻¹ at 0.2 C and exhibits excellent cycling stability. This work offers novel insights into the application of MOF and MXene heterostructures in Li−S batteries.

Highlights

• Large-area production of self-standing graphene nanofilm (~ 20 cm) through a clean ‘substrate replacement’ strategy.

• Realizing highly crystalline graphene nanofilms without micro-gasbags by introducing polymers.

• The graphene nanofilms demonstrate a solid light–matter interaction (photoelectric conversion in the mid-infrared and electromagnetic interference (EMI) shielding in X-band) with performance beyond state-of-the-art graphene/silicon diodes and EMI materials.

npj 2D Materials and Applications, Published online: 04 March 2023; doi:10.1038/s41699-023-00375-3

Field-controlled quantum anomalous Hall effect in electron-doped CrSiTe₃ monolayer

In a high-performance semiconductor Bi₂O₂Se, a strain-induced ferroelectric transition is demonstrated by the appearance of butterfly loops and 180° phase switching in the piezoelectric force microscopy measurements, together with the evolution of optical second-harmonic generation. Materials with paraelectrics at ambient pressure and ferroelectrics under strain are rare.

Abstract

Phase engineering by strain in 2D semiconductors is of great importance for a variety of applications. Here, a study of the strain-induced ferroelectric (FE) transition in bismuth oxyselenide (Bi₂O₂Se) films, a high-performance (HP) semiconductor for next-generation electronics, is presented. Bi₂O₂Se is not FE at ambient pressure. At a loading force of ≳400 nN, the piezoelectric force responses exhibit butterfly loops in magnitude and 180° phase switching. By carefully ruling out extrinsic factors, these features are attributed to a transition to the FE phase. The transition is further supported by the appearance of a sharp peak in optical second-harmonic generation under uniaxial strain. In general, solids with paraelectrics at ambient pressure and FE under strain are rare. The FE transition is discussed using first-principles calculations and theoretical simulations. The switching of FE polarization acts as a knob for Schottky barrier engineering at contacts and serves as the basis for a memristor with a huge on/off current ratio of 10⁶. This work adds a new degree of freedom to HP electronic/optoelectronic semiconductors, and the integration of FE and HP semiconductivity paves the way for many exciting functionalities, including HP neuromorphic computing and bulk piezophotovoltaics.

Nature Communications, Published online: 02 March 2023; doi:10.1038/s41467-023-36709-4

Polymeric nanofibers are attractive nanomaterials owing to their high surfacearea- to-volume ratio and superior flexibility but designing durable and recyclable polymer nanofibers is challenging. Here, the authors integrate the concept of covalent adaptable networks to produce dynamic covalently crosslinked nanofibers via electrospinning.

An optoelectronic/electronic memory device with both the long-term potentiation/depression triggered by electrical pulses and short-term potentiation induced by 1550-nm laser pulses is fabricated from Te-based 2D van der Waals heterostructure, which holds great promise for fully memristive in-sensor RC at the optical communication band.

Abstract

Although 2D materials are widely explored for data storage and neuromorphic computing, the construction of 2D material-based memory devices with optoelectronic responsivity in the short-wave infrared (SWIR) region for in-sensor reservoir computing (RC) at the optical communication band still remains a big challenge. In this work, an electronic/optoelectronic memory device enabled by tellurium-based 2D van der Waals (vdW) heterostructure is reported, where the ferroelectric CuInP₂S₆ and tellurium channel endow this device with both the long-term potentiation/depression by voltage pulses and short-term potentiation by 1550 nm laser pulses (a typical wavelength in the conventional fiber optical communication band). Leveraging the rich dynamics, a fully memristive in-sensor RC system that can simultaneously sense, decode, and learn messages transmitted by optical fibers is demonstrated. The reported 2D vdW heterostructure-based memory featuring both the long-term and short-term memory behaviors using electrical and optical pulses in SWIR region has not only complemented the wide spectrum of applications of 2D materials family in electronics/optoelectronics but also paves the way for future smart signal processing systems at the edge.

Nature Electronics, Published online: 02 March 2023; doi:10.1038/s41928-023-00930-2

This Review examines the development of cryogenic memory technologies—including non-superconducting memories, superconducting memories and hybrid memories—and their potential application in superconducting single-flux quantum circuits and quantum computers.

Nature Synthesis, Published online: 01 March 2023; doi:10.1038/s44160-023-00269-8

Late-stage transfer

Nature, Published online: 01 March 2023; doi:10.1038/s41586-022-05617-w

Varying growth temperatures enables the tuning of the degree of disorder, which is fully described by the absence/presence of medium-range order and temperature-dependent densities of nanocrystallites, and electrical conductivity in amorphous monolayer carbon films.

Nature Communications, Published online: 01 March 2023; doi:10.1038/s41467-023-36823-3

Obtaining good initial structures is the main challenge for the computational study of transition states. Here, fast and accurate predictions for transition state of gas phase reactions are achieved by machine learning based on interatomic distances.

Ultrathin inorganic perovskite nanosheets are prepared by liquid-air interfacial synthesis by adding only iodine into the precursor solution. The thickness, lateral size, defects, optical and optoelectronic properties of these freestanding materials are optimized by simply adjusting the iodine concentration of the solution.

Abstract

The liquid-air interface offers a platform for the in-plane growth of free-standing materials. However, it is rarely used for inorganic perovskites and ultrathin non-layered perovskites. Herein the liquid-air interfacial synthesis of inorganic perovskite nanosheets (Cs₃Bi₂I₉, Cs₃Sb₂I₉) is achieved simply by drop-casting the precursor solution with only the addition of iodine. The products are inaccessible without iodine addition. The thickness and lateral size of these nanosheets can be adjusted through the iodine concentration. The high volatility of the iodine spontaneously drives precursors that normally stay in the liquid to the liquid-air interface. The iodine also repairs in situ iodine vacancies during perovskite growth, giving enhanced optical and optoelectronic properties. The liquid-air interfacial growth of ultrathin perovskites provides multi-degree-of-freedom for constructing perovskite-based heterostructures and devices at atomic scale.

A chemical protocol is established for free-standing molecularly thin nanosheets; solid-state surfactant lamellae can serve as templates guiding amorphous silica nanosheets. Delamination of the lamellar hybrids with ordered alkyl-chain arrangement allows for free-standing amorphous silica nanosheets with 0.9 nm in thickness. The nanosheets show high colloidal stability that enables atomic layer engineering of film with a highly insulating property.

Abstract

Recent progress in 2D materials has initiated new fields of molecularly thin amorphous materials with mysterious properties and structures. However, designed synthesis of molecularly thin amorphous silica still remains a challenge; whether free-standing molecularly thin amorphous silica nanosheets can exist is unclear. Here, this issue is addressed by using a new chemical protocol; solid-state surfactant lamellae with ordered alkyl-chain arrangements can serve as superior templates guiding free-standing amorphous silica nanosheets. Simple sonication of the lamellar hybrids allows exfoliation into monolayer amorphous silica nanosheets with 0.9 nm thickness. In addition, the nanosheets show the distinctive feature of high colloidal stability that enables atomic layer engineering of silica nanocoatings and dielectric nanofilms. The approach may shed new light on the properties and applications of old silica.

The band structure of SrTiO₃ can be altered by surface engineering, revealed by angle-resolved photoemission spectroscopy, X-ray photoemission spectroscopy, and ab initio calculation. The purified 2D electron states emerge on the Sr-enriched surface, while the light irradiation tunes the band renormalization and electron-phonon interaction.

Abstract

Developing reliable methods for modulating the electronic structure of the 2D electron gas (2DEG) in SrTiO₃ is crucial for utilizing its full potential and inducing novel properties. Herein, it is shown that relatively simple surface preparation reconstructs the 2DEG at the SrTiO₃ (STO) surface, leading to a Lifshitz-like transition. Combining experimental methods, such as angle-resolved photoemission spectroscopy (ARPES) and X-ray photoemission spectroscopy with ab initio calculations, that the modulation of the surface band structures can be effectively achieved via transforming the chemical composition at the atomic scale is found. In addition, ARPES experiments demonstrate that vacuum ultraviolet light can be efficiently employed to alter the band renormalization of the 2DEG system and control the electron-phonon interaction . This study provides a robust and straightforward route to stabilize and tune the low-dimensional electronic structure via the chemical degeneracy of the STO surface.

A novel concept of nanodevice for microwave detection exploits the unique transport features associated to quantum confined energy levels in zero-dimensional semiconductors. A calibration-free and high-sensitivity microwave detector, with nanometer scale spatial resolution, based on InAs/InP nanowire double quantum dots is demonstrated, fostering the development of novel high-performance microwave circuitry.

Abstract

At the cutting-edge of microwave detection technology, novel approaches which exploit the interaction between microwaves and quantum devices are rising. In this study, microwaves are efficiently detected exploiting the unique transport features of InAs/InP nanowire double quantum dot-based devices, suitably configured to allow the precise and calibration-free measurement of the local field. Prototypical nanoscale detectors are operated both at zero and finite source-drain bias, addressing and rationalizing the microwave impact on the charge stability diagram. The detector performance is addressed by measuring its responsivity, quantum efficiency and noise equivalent power that, upon impedance matching optimization, are estimated to reach values up to ≈2000 A W⁻¹, 0.04 and ≈10−16W/Hz${10^{ - 16}}{\rm{W}}/\sqrt {Hz} $, respectively. The interaction mechanism between the microwave field and the quantum confined energy levels of the double quantum dots is unveiled and it is shown that these semiconductor nanostructures allow the direct assessment of the local intensity of the microwave field without the need for any calibration tool. Thus, the reported nanoscale devices based on III-V nanowire heterostructures represent a novel class of calibration-free and highly sensitive probes of microwave radiation, with nanometer-scale spatial resolution, that may foster the development of novel high-performance microwave circuitries.