Jing Zhang

Nature Photonics, Published online: 06 March 2023; doi:10.1038/s41566-023-01155-7

X-ray photons emitted by free electrons travelling in van der Waals materials show energy shifts induced by quantum recoil, thus offering a viable route to generating tailored and tunable single X-ray photons.

A synergistical optimization of layered architecture on half-Heusler alloys is demonstrated for high conversion efficiency thermoelectric power generation applications. The (Nb, Hf)FeSb alloy exhibits a superior figure of merit of 1.5 at 973 K. A comprehensive coupling of the thermoelectric parameters with temperature gradient is realized, and a remarkable conversion efficiency of ≈15% is achieved under ΔT of 670 K.

Abstract

Waste-heat electricity generation using high-efficiency solid-state conversion technology can significantly decrease dependence on fossil fuels. Here, a synergistical optimization of layered half-Heusler (hH) materials and module to improve thermoelectric conversion efficiency is reported. This is realized by manufacturing multiple thermoelectric materials with major compositional variations and temperature-gradient-coupled carrier distribution by one-step spark plasma sintering. This strategy provides a solution to overcome the intrinsic concomitants of the conventional segmented architecture that only considers the matching of the figure of merit (zT) with the temperature gradient. The current design is dedicated to temperature-gradient-coupled resistivity and compatibility matching, optimum zT matching, and reducing contact resistance sources. By enhancing the quality factor of the materials by Sb-vapor-pressure-induced annealing, a superior zT of 1.47 at 973 K is achieved for (Nb, Hf)FeSb hH alloys. Along with the low-temperature high-zT hH alloys of (Nb, Ta, Ti, V)FeSb, the single stage layered hH modules are developed with efficiencies of ≈15.2% and ≈13.5% for the single-leg and unicouple thermoelectric modules, respectively, under ΔT of 670 K. Therefore, this work has a transformative impact on the design and development of next-generation thermoelectric generators for any thermoelectric material families.

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.

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.

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.

Hybrid organic/inorganic Van der Waals heterostructures have emerged recently with enormous potential applications in nanotechnology and industrial areas. In these heterostructures, the interfacial effects can modulate the final properties, creating further possibilities in the design and operation of innovative devices. With this perspective in mind, we report on an experimental investigation of a hybrid organic/inorganic heterostructure of coronene and a few-layers . We observe a local enhancement of optical properties using both far-field and near-field Raman scattering and photoluminescence. Mainly located at edges and defects, the local enhancement is due to the assembling of coronene molecules in , as confirmed by atomic force microscopy. Quantum semi-empirical and fully atomistic molecular dynamics simulations were also used to gain further insights into these phenomena. Our results pave the way to engineer molecules in two-dimensional (2D) layered nanomaterials and control and modulate optical phenomena.

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 Communications, Published online: 01 March 2023; doi:10.1038/s41467-023-36799-0

Interfacing superconducting quantum information processors with long-distance optical networks would require coherent interfacing between microwave and optical photons. Here, the authors show a chip-integrated microwave-to-optical transducer based on rare earth ion ensembles.

Nature, Published online: 01 March 2023; doi:10.1038/s41586-022-05669-y

Chemistry governs water organization at a graphene electrode

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.

Proximity-induced fine features and spin-textures of the electronic bands in graphene-based van der Waals heterostructures can be explored from the point of tailoring a twist angle. Here we study spin–orbit coupling and exchange coupling engineering of graphene states in the proximity of 1T-TaS2 not triggering the twist, but a charge density wave (CDW) in 1T-TaS2—a realistic low-temperature phase. Using density functional theory and effective model we found that the emergence of the CDW in 1T-TaS2 significantly enhances Rashba spin–orbit splitting in graphene and tilts the spin texture by a significant Rashba angle—in a very similar way as in the conventional twist-angle scenarios. Moreover, the partially filled Ta d-band in the CDW phase leads to the spontaneous emergence of the in-plane magnetic order that transgresses via proximity from 1T-TaS2 to graphene, hence, simultaneously superimposing along the spin–orbit also the exchange coupling proximity effect. To describe this intricate proximity landscape we have developed an effective model Hamiltonian and provided a minimal set of parameters that excellently reproduces all the spectral features predicted by the first-principles calculations. Conceptually, the CDW provides a highly interesting knob to control the fine features of electronic states and to tailor the superimposed proximity effects—a sort of twistronics without twist.

Excitons in thin layers of semiconducting transition metal dichalcogenides are highly subject to the strongly modified Coulomb electron–hole interaction in these materials. Therefore, they do not follow the model system of a two-dimensional hydrogen atom. We investigate experimentally and theoretically excitonic properties in both the monolayer (ML) and the bilayer (BL) of MoSe2 encapsulated in hexagonal BN. The measured magnetic field evolutions of the reflectance contrast spectra of the MoSe2 ML and BL allow us to determine g-factors of intralayer A and B excitons, as well as the g-factor of the interlayer exciton. We explain the dependence of g-factors on the number of layers and excitation state using first principles calculations. Furthermore, we demonstrate that the experimentally measured ladder of excitonic s states in the ML can be reproduced using the approach with the Rytova–Keldysh potential that describes the electron–hole interaction. In contrast, the analogous calculation for the BL case requires taking into account the out-of-plane dielectric response of the MoSe2 BL.

Nature Photonics, Published online: 27 February 2023; doi:10.1038/s41566-023-01167-3

Single-crystal perovskite LEDs exhibit reduced ion migration and Auger recombination and increased device lifetime. Perovskite single-crystals-based LEDs exhibit a maximum brightness of 86,000 cd m−2, a peak EQE of 11.2% and T50 lifetime of 12,500 h at an initial luminance of 100 cd m−2.

Nature, Published online: 27 February 2023; doi:10.1038/s41586-023-05859-2

Continuous Symmetry Breaking in a Two-dimensional Rydberg Array

Recently, sharp exciton resonances in antiferromagnet NiPS₃ have been reported to correlate with magnetic order. It is found that the polarization of maximal exciton emission in NiPS₃ rotate locally, revealing three possible spin chain directions. This discovery establishes a new understanding of the antiferromagnetic order hidden in previous neutron scattering and optical experiments.

Abstract

A long-standing pursuit in materials science is to identify suitable magnetic semiconductors for integrated information storage, processing, and transfer. Van der Waals magnets have brought forth new material candidates for this purpose. Recently, sharp exciton resonances in antiferromagnet NiPS₃ have been reported to correlate with magnetic order, that is, the exciton photoluminescence intensity diminishes above the Néel temperature. Here, it is found that the polarization of maximal exciton emission rotates locally, revealing three possible spin chain directions. This discovery establishes a new understanding of the antiferromagnet order hidden in previous neutron scattering and optical experiments. Furthermore, defect-bound states are suggested as an alternative exciton formation mechanism that has yet to be explored in NiPS₃. The supporting evidence includes chemical analysis, excitation power, and thickness dependent photoluminescence and first-principles calculations. This mechanism for exciton formation is also consistent with the presence of strong phonon side bands. This study shows that anisotropic exciton photoluminescence can be used to read out local spin chain directions in antiferromagnets and realize multi-functional devices via spin-photon transduction.

Ab initio quantum transport simulation reveals that giant asymmetry between the n- and p-type devices in bulk InAs field-effect transistors (FETs) is significantly reduced in the sub-2-nm-diameter gate-all-around InAs nanowire FETs. The reason lies in the band inversion and quantum confinement effect.

Abstract

Complementary metal-oxide-semiconductor (CMOS) field-effect transistors (FETs) are the key component of a chip. Bulk indium arsenide (InAs) owns nearly 30 times higher electron mobility µ _e than silicon but suffers from a much lower hole mobility µ _h (µ _e/µ _h = 80), thus unsuited to CMOS application with a single material. Through the accurate ab initio quantum-transport simulations, the performance gap between the NMOS and PMOS is significantly narrowed is predicted and even vanished in the sub-2-nm-diameter gate-all-around (GAA) InAs nanowires (NW) FETs because the inversion of the light and heavy hole bands occurs when the diameter is shorter than 3 nm. It is further proposed several feasible strategies for further improving the performance symmetry in the GAA InAs NWFETs. Short-channel effects are effectively depressed in the symmetric n- and p-type GAA InAs NWFETs till the gate length is scaled down to 2 nm according to the standards of the International Technology Roadmap for Semiconductors. Therefore, the ultrasmall GAA InAs NWFETs possess tremendous prospects in CMOS integrated circuits.

A new self-assembly method can increase the information storage density in ferroelectric memories. A scroll-like 3D memory is fabricated by self-rolling-up freestanding single-crystalline ferroic oxide membranes. The information storage density can be enhanced at least one order of magnitude (experimentally 45.7 times) and 100–450 times (theoretically) than planar thin films.

Abstract

Ferroelectric memory is one of the most attractive emerging nonvolatile memory. Conventional methods to increase storage density in ferroelectrics include reducing the storage bit size or fabricating 3D stacks. However, the former will face a physical limit finally, and the integration of single-crystalline ferroelectric oxide following the latter still remains a great challenge. Here, a new method is introduced to construct a scroll-like 3D memory structure by self-rolling-up single-crystalline ferroelectric oxides. PbZr_0.3Ti_0.7O₃ single-crystalline thin film is chosen as a prototype and epitaxially grown on another oxide stressor layer with a few lattice-mismatch. Releasing such “Pb(Zr, Ti)O₃/stressor” bilayered structure from the substrate induces self-rolling-up due to the internal stress from the lattice-mismatch. High-density information can be written in the form of switched ferroelectric domains on those flat “Pb(Zr, Ti)O₃/stressor” membranes via piezoelectric force microscopy. In self-rolling-up membranes, information density can be experimentally enhanced up to 45 times. Theoretically, the freestanding “Pb(Zr, Ti)O₃/stressor” membranes have a strongly driven force to self-rolling-up, and the area ratio can enhance 100–450 times, corresponding to an ultra-high density information storage of 10² Tbit In⁻². This study provides a new and general method to develop compact, high-density, and 3D memories from oxide materials.

Atomic-level regulated ReSe₂ nanosheets serve as the general platform to significantly advance the photocatalytic H₂ evolution on various semiconductors, such as TiO₂, CdS, ZnIn₂S₄, and C₃N₄. The outstanding activity is attributed to the existence of abundant Re/Se active sites and strongly coupled interface between atomic-level regulated ReSe₂ and the semiconductor photocatalyst.

Abstract

Solar hydrogen (H₂) generation via photocatalytic water splitting is practically promising, environmentally benign, and sustainably carbon neutral. It is important therefore to understand how to controllably engineer photocatalysts at the atomic level. In this work, atomic-level engineering of defected ReSe₂ nanosheets (NSs) is reported to significantly boost photocatalytic H₂ evolution on various semiconductor photocatalysts including TiO₂, CdS, ZnIn₂S₄, and C₃N₄. Advanced characterizations, such as atomic-resolution aberration-corrected scanning transmission electron microscopy (AC-STEM), synchrotron-based X-ray absorption near edge structure (XANES), in situ X-ray photoelectron spectroscopy (XPS), transient-state surface photovoltage (SPV) spectroscopy, and transient-state photoluminescence (PL) spectroscopy, together with theoretical computations confirm that the strongly coupled ReSe₂/TiO₂ interface and substantial atomic-level active sites of defected ReSe₂ NSs result in the significantly raised activity of ReSe₂/TiO₂. This work not only for the first time realizes the atomic-level engineering of ReSe₂ NSs as a versatile platform to significantly raise the activities on different photocatalysts, but, more importantly, underscores the immense importance of atomic-level synthesis and exploration on 2D materials for energy conversion and storage.

A dry patterning approach together with the electrode transfer method for 2D solution-processed organic single-crystal field-effect transistors via bottom-up fabrication is demonstrated. High-performance flexible monolayer OFETs and high-speed organic rectifiers are fabricated based on these strategies.

Abstract

Organic field-effect transistors (OFETs) based on 2D monolayer organic semiconductors (OSC) have demonstrated promising potentials for various applications, such as light emitting diode (LED) display drivers, logic circuits, and wearable electrocardiography (ECG) sensors. To date, the fabrications of this class of highly crystallized 2D organic semiconductors (OSC) are dominated by solution shearing. As these organic active layers are only a few molecular layers thick, their compatibilities with conventional thermal evaporated top electrodes or sophisticated photolithography patterning are very limited, which also restricts their device density. Here, an electrode transfer stamp and a semiconductor patterning stamp are developed to fabricate OFETs with channel lengths down to 3 µm over a large area without using any chemicals or causing any damage to the active layer. 2D 2,9-didecyldinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (C₁₀-DNTT) monolayer OFETs developed by this new approach shows decent performance properties with a low threshold voltage (V _TH) less than 0.5 V, intrinsic mobility higher than 10 cm² V⁻¹ s⁻¹ and a subthreshold swing (SS) less than 100 mV dec⁻¹. The proposed patterning approach is completely comparable with ultraflexible parylene substrate less than 2 µm thick. By further reducing the channel length down to 2 µm and using the monolayer OFET in an AC/DC rectifying circuit, the measured cutoff frequency is up to 17.3 MHz with an input voltage of 4 V. The newly proposed electrode transfer and patterning stamps have addressed the long-lasting compatibility problem of depositing electrodes onto 2D organic monolayer and the semiconductor patterning. It opens a new path to reduce the fabrication cost and simplify the manufacturing process of high-density OFETs for more advanced electronic or biomedical applications.