Jing Zhang

Nature Materials, Published online: 14 April 2022; doi:10.1038/s41563-022-01230-4

Cu2O is a promising platform to host Rydberg exciton–polaritons, where excitons strongly couple to cavity photons, however their realization has been elusive. Here, the authors report Rydberg exciton–polaritons with principal quantum numbers up to n = 6.

Nature Physics, Published online: 12 April 2022; doi:10.1038/s41567-022-01560-9

A task group recommends values for many constants in fundamental theories of physics and chemistry. Eite Tiesinga and Peter Mohr tell some of the constants’ stories.

Nature Physics, Published online: 12 April 2022; doi:10.1038/s41567-022-01593-0

No free lunch for Schrödinger’s cat

WSe₂/α-In₂Se₃ heterojunction structure is constructed to enhance the optical absorption and realize a p–n junction photodetector. The photodetector can work efficiently over a broadband spectrum (from 405 to 905 nm) and a rise/fall time of 110/120 µs is achieved. Consequently, the underwater wireless optical communication system is established under λ = 520 nm illumination, which is promising for practical applications.

Abstract

P–n junctions based on 2D materials can be achieved using a selective doping technique, while such a method is challenged by the complex fabrication process. Here, a facile van der Waals (vdWs) structured p–n heterojunction is demonstrated by simply transferring an n-type multilayer α-In₂Se₃ (direct bandgap) on a p-type ultra-thin WSe₂ nanosheet. The vdWs stacked photodetector with an improved type-II band alignment not only realizes a broadband spectral response from visible to near infrared (405–905 nm), but also operates well with a diode-like behavior. This behavior is further confirmed by the high-resolution scanning photocurrent mapping. As a result, the as-fabricated device exhibits a short response time (<120 µs) and a high responsivity of 1.84 A W⁻¹ under 520 nm laser illumination. Accordingly, an underwater optical communication system based on the WSe₂/α-In₂Se₃ p-n heterojunction photodetector is demonstrated, which is promising for next-generation high-performance and low-power detection applications.

Colloidal CdSe/CdSeS core/alloyed-crown nanoplatelets are synthesized using a convenient synthetic protocol. Photoluminescence quantum yield of 100%, broad emission tunability from 502 to 550 nm, narrow emission bandwidth all below 15 nm, and good stability are simultaneously achieved. In addition, the time-resolved photoluminescence spectrum exhibits a single exponential decay characteristic, indicating the suppression of both deep and shallow trap states.

Abstract

Cadmium-based nanoplatelets as optical display and lasing materials are widely explored and exhibit great advantages, owing to their narrow emission linewidths, anisotropic transition-dipole distributions, and low lasing thresholds. However, in the green range, the photoluminescence quantum yield (PLQY) and emission tunability of nanoplatelets are still inferior to that of quantum dots. In this work, a new synthesis protocol is developed, enabling core/crown nanoplatelets to grow continuously from elementary precursors to their final form. A new heterostructure of CdSe/CdSeS core/alloyed-crown nanoplatelets is produced that realizes 100% PLQY, the continuous tunability of emission peaks in between 502 and 550 nm, and low full-width-at-half-maximum (FWHM) of less than 15 nm. Achieving these excellent properties in all three aspects at the same time is unprecedented. In addition, the time-resolved photoluminescence (TRPL) spectra of these nanoplatelets show a mono-exponential decay characteristic, and the nanoplatelet film can also show 100% PLQY and a mono-exponential decay characteristic, indicating the suppression of trap states. The high-quality nanoplatelets achieved in this work provide a solid foundation for developing nanoplatelet-based light sources, like light-emitting diodes and lasers, with much higher efficiency, color purity, and lower working thresholds.

Microscale perovskite quantum dot light-emitting diodes (micro-PeLEDs) are fabricated through an “interface engineering-inkjet printing-plasma etching” strategy using perovskite quantum dot arrays as the emissive layer for full-color displays with the advantages of low cost, free massive transfer, high resolution, and suitability for large-area or flexible displays. This provides a new approach for the development of micro-LEDs.

Abstract

Group III–V semiconductor-based microscale light-emitting diode (micro-LED) displays are recognized as one of the most promising display technologies with superhigh brightness and resolution, strong contrast, and superlarge color gamut but currently suffer from low efficiency, low production yield, and high cost. Here, it is proposed to realize cost-effective and full-color micro-LED displays by using microscale perovskite quantum dot LEDs (micro-PeLEDs) as emitters. Bright and uniform red-, green-, and blue-emitting (RGB) micro-PeLED arrays with each pixel size as small as 45 µm are fabricated by in situ inkjet printing a perovskite quantum dot (PeQD) emissive layer in the structure of indium tin oxide/poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS)/poly(9-vinylcarbazole) (PVK)/sodium dodecyl sulfate (SDS)/PeQD/1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene (TPBi)/LiF/Al. These RGB micro-PeLED arrays display an external quantum efficiency of 0.832%, 0.419%, and 0.052% for red, green, and blue colors, a resolution of 210 PPI, and a wide color gamut (135% of the National Television Standards Committee standard). The authors further demonstrate flexible and full-color active-matrix micro-PeLED displays, which have potential applications in future ultrahigh-definition displays, light communication, and artificial intelligence.

A large zero-bias tunneling magnetoresistance (TMR) and giant energy-dependent TMR can be attainable through a functional van der Waals interface and the doubly spin-filtering effect. Also, the two contrasting photoelectric responses can be implemented for encoding the digital information as “1” and “0” via flipping the interlayer magnetic states. These findings facilitate the future development of atomically thin magnetic information storage.

Abstract

Van der Waals (vdW) magnetic heterostructures can offer much improved performance in logical operation and information storage technology compared with conventional ones. However, it is still challenging to achieve the perfect spin-filtering capability in the vdW magnetic devices, a major obstacle to the advancement of low-dimensional magnetic information storage. Herein, this study reports two newly designed vdW magnetic multilayers, ZrTe₂/CrOCl/CrOX/ZrTe₂ (X = Cl, Br), where the CrOCl/CrOX bilayer acts as the spin-filter tunnel barriers. With the vdW interfacial engineering and the doubly spin-filtering effect of the CrOCl/CrOX bilayer, the vdW four-layer heterostructures can potentially function as a perfect zero-bias spin filter with giant spin-filter energy-dependent tunnel magnetoresistance up to 48 000%. Importantly, these calculations also show that the CrOCl/CrOBr bilayer without the 1T-ZrTe₂ stacking can produce intensive and weak photo-carrier transmission at parallel and antiparallel magnetization states, respectively. As such, the two contrasting photoelectric responses can be implemented for encoding the digital information as “‘1”’ and “‘0”’ via flipping the interlayer magnetic states. These novel functionalities not only endow the CrOCl/CrOX bilayer as a promising candidate for spin-based vdW devices but also facilitate the future development of atomically thin magnetic information storage.

The hybridization of 2D nanomaterials with 3D graphene architectures has offered a promising strategy to prepare high-performance hybrid materials for electrochemical energy storage and conversion. This review summarizes the typical methods to hybridize 2D nanomaterials with 3D graphene architectures, as well as their application in rechargeable batteries, supercapacitors, and electrocatalytic water splitting.

Abstract

Since the discovery of graphene, diverse kinds of 2D nanomaterials have been explored and exhibited great promise for application in electrochemical energy storage and conversion. However, the restacking of 2D nanomaterials severely reduces their exposed active sites and thus impairs their electrochemical performance. Moreover, except for graphene, a large number of 2D nanomaterials normally possess unsatisfactory electrical conductivity. One of the effective strategies to address the aforementioned shortcomings is to hybridize 2D nanomaterials with 3D graphene architectures since large specific surface area and rapid transport pathways for electrons, ions, and mass can be achieved in the obtained hybrid materials. This review summarizes the typical strategies to hybridize 2D nanomaterials with 3D graphene architectures and then highlights the application of these hybrid materials in rechargeable batteries, supercapacitors, and electrocatalytic water splitting. The challenges and future research directions in this research area are also discussed.

Wafer-sized α-Ag₂S thin films on substrates with high mobility of ≈150 cm² V⁻¹ s⁻¹ and long diffusion length exceeding 500 nm are produced by a solution-processed approach. Mechanically flexible, free-standing α-Ag₂S films are developed by the addition of poly(vinyl alcohol) into precursors. The resulting free-standing films possess good carrier mobility of ≈35 cm² V⁻¹ s⁻¹, relevant for flexible optoelectronics.

Abstract

Monoclinic α-Ag₂S is an intriguing member of transition metal sulfides with great potential for all-inorganic flexible optoelectronics and thermoelectrics. Fabrication of large-area, high-quality α-Ag₂S thin films and understanding their charge transport properties are critical for device operations yet have remained largely unexplored. Here, a novel two-step, the solution-processed approach is reported to produce wafer-sized, highly crystalline α-Ag₂S thin films. Ultrafast terahertz (THz) conductivity measurements reveal that photogenerated charge carriers undergo efficient band transport, with room-temperature mobility of ≈150 cm² V^-1 s^-1 and a diffusion length exceeding 500 nm. Furthermore, introducing poly(vinyl alcohol) (PVA) as the rigid component into the aqueous silver thiolate precursor enables the synthesis of free-standing α-Ag₂S thin films with a mobility of ≈35 cm² V^-1 s^-1, demonstrating their potential for flexible optoelectronics. This study provides a facile synthesis route for high-quality, large-area α-Ag₂S thin films with good charge transport properties, relevant for their integration into optoelectronics and wearable electronics.

By using atomic force microscopy (AFM)-based infrared spectroscopy as a nano-optical writing/readout tool, a novel optical/ferroelectric multiplexing poly(vinylidene fluoride) memory with independent four-level optical and bilevel ferroelectric signals is demonstrated. The different temperature tolerance of phase change and polarization switching enables an integration of optical read-only memory and ferroelectric random access. This work opens up new opportunities for high density and security data storage.

Abstract

Multiplexing physical dimensions to realize multidimensional storage in a single material has been a goal to increase storage density and data security. Multidimensional storage is only achieved in optical storage material (OSM) by far. Poly(vinylidene fluoride) (PVDF), a semicrystalline polymer, is widely studied as a candidate for ferroelectric random access (FeRAM). Herein, the atomic force microscopy (AFM)-based infrared spectroscopy techniqueis used to induce multilevel phase transformations in PVDF ultrathin film on nanometric scales and for writing/readout of IR signals. An optical/ferroelectric multiplexing PVDF memory, where information can be coded with independent four-level optical IR and bilevel ferroelectric signals, is demonstrated. High data security and a storage density up to 180 GBit in.⁻² are achieved simultaneously. Owing to the different critical temperature for phase transformation (optical data, <167 °C) and polarization switching (ferroelectric data, <100 °C), the multiplexing memory can function both as optical read-only memory and FeRAM. This work expands material supporting physical dimensions multiplexing beyond OSM for the first time, opening up new opportunities for future high-capacity, multifunctional nano-memory. The strategy proposed here enables on-demand and tunable programming on IR waves, offering prospects for fabrication of active nano-optical devices.

Ultrathin CrTe₃ films are successfully synthesized. Monolayer (ML) CrTe₃ represents a unique van der Waals (vdW) magnet with intralayer antiferromagnetism, which is identified by scanning probe microscopy. A facile method is developed to fabricate CrTe₃/CrTe₂ magnetic heterostructures from ML CrTe₃ with an atomically sharp and seamless interface, which is significant for the development of miniaturized spintronic devices based on vdW magnets.

Abstract

Ultrathin van der Waals (vdW) magnets are heavily pursued for potential applications in developing high-density miniaturized electronic/spintronic devices as well as for topological physics in low-dimensional structures. Despite the rapid advances in ultrathin ferromagnetic vdW magnets, the antiferromagnetic counterparts, as well as the antiferromagnetic junctions, are much less studied owing to the difficulties in both material fabrication and magnetism characterization. Ultrathin CrTe₃ layers have been theoretically proposed to be a vdW antiferromagnetic semiconductor with intrinsic intralayer antiferromagnetism. Herein, the epitaxial growth of monolayer (ML) and bilayer CrTe₃ on graphite surface is demonstrated. The structure, electronic and magnetic properties of the ML CrTe₃ are characterized by combining scanning tunneling microscopy/spectroscopy and non-contact atomic force microscopy and confirmed by density functional theory calculations. The CrTe₃ MLs can be further utilized for the fabrication of a lateral heterojunction consisting of ML CrTe₂ and ML CrTe₃ with an atomically sharp and seamless interface. Since ML CrTe₂ is a metallic vdW magnet, such a heterostructure presents the first in-plane magnetic metal–semiconductor heterojunction made of two vdW materials. The successful fabrication of ultrathin antiferromagnetic CrTe₃, as well as the magnetic heterojunction, will stimulate the development of miniaturized antiferromagnetic spintronic devices based on vdW materials.

A new family of 2D transition metal carbo-chalcogenides (TMCCs) can be considered an atomic-level combination of TM carbides (MXenes) and TM dichalcogenides (TMDCs). Realized by electrochemical-assisted delamination, Nb₂S₂C and Ta₂S₂C are the first examples of TMCCs to be delaminated into single layers. These newly developed 2D TMCCs are 50% stronger than TMDC counterparts and have a wide range of applications.

Abstract

Here, a new family of 2D transition metal carbo-chalcogenides (TMCCs) is reported, which can be considered a combination of two well-known families, TM carbides (MXenes) and TM dichalcogenides (TMDCs), at the atomic level. Single sheets are successfully obtained from multilayered Nb₂S₂C and Ta₂S₂C using electrochemical lithiation followed by sonication in water. The parent multilayered TMCCs are synthesized using a simple, scalable solid-state synthesis followed by a topochemical reaction. Superconductivity transition is observed at 7.55 K for Nb₂S₂C. The delaminated Nb₂S₂C outperforms both multilayered Nb₂S₂C and delaminated NbS₂ as an electrode material for Li-ion batteries. Ab initio calculations predict the elastic constant of TMCC to be over 50% higher than that of TMDC.

Multifunctional Logic Circuits

In article number 2109491, Yutaka Wakayama and co-workers demonstrate electrically reconfigurable organic logic circuits using a dual-gate anti-ambipolar transistor. The transistor shows a negative differential transconductance even at room temperature. This unique feature achieves five two-input logic gate operations with “only a transistor”. Such logic operations are not obtainable in conventional transistors. The device concept will provide a way of realizing multifunctional organic logic circuits using a simple circuit design.

Ferroelectric Field-Effect Transistors

In article number 2200032, Hang Luo, Jian Sun, and co-workers report how a ferroelectric field-effect transistor (FeFET) can work as both a hysteresis-free low-power-consumption negative-capacitance field-effect transistor and a memory device for neural computing. The functions are reconfigured and controlled by modulating the behaviors of the interfacial oxygen vacancies. The frontispiece shows the chips consisting of the reconfigurable FeFET devices.

Exciton Linewidth

Due to strong multiparticle interactions in 2D transition metal dichalcogenides at room temperature, their homogeneous exciton linewidths are significantly broadened, degrading the quality of their excitonic mode and emission. In article number 2108721, Yuebing Zheng and co-workers achieve near-intrinsic exciton linewidth in monolayer WS₂ at room temperature, approaching the theoretical limit at 0 K. A dielectric nanosphere is designed to boost the dynamic competition between exciton and trion decay channels, rebuilding the excitonic relaxation processes with suppressed exciton nonradiative recombination.

The fabrication of centimeter-scale, crack-free, freestanding Hf_0.5Zr_0.5O₂ nanomembranes is reported, and their robust ferroelectricity is confirmed. By transferring freestanding Hf_0.5Zr_0.5O₂ membranes to transmission electron microscopy grids, the phases, orientations, grain-size distributions, and grain boundaries are atomically characterized from plan-view. This work opens a new paradigm for exploring the complex structures and unconventional ferroelectricity in polymorphic Hf_0.5Zr_0.5O₂ using plan-view imaging.

Abstract

Hafnia-based compounds have considerable potential for use in nanoelectronics due to their compatibility with complementary metal–oxide–semiconductor devices and robust ferroelectricity at nanoscale sizes. However, the unexpected ferroelectricity in this class of compounds often remains elusive due to the polymorphic nature of hafnia, as well as the lack of suitable methods for the characterization of the mixed/complex phases in hafnia thin films. Herein, the preparation of centimeter-scale, crack-free, freestanding Hf_0.5Zr_0.5O₂ (HZO) nanomembranes that are well suited for investigating the local crystallographic phases, orientations, and grain boundaries at both the microscopic and mesoscopic scales is reported. Atomic-level imaging of the plan-view crystallographic patterns shows that more than 80% of the grains are the ferroelectric orthorhombic phase, and that the mean equivalent diameter of these grains is about 12.1 nm, with values ranging from 4 to 50 nm. Moreover, the ferroelectric orthorhombic phase is stable in substrate-free HZO membranes, indicating that strain from the substrate is not responsible for maintaining the polar phase. It is also demonstrated that HZO capacitors prepared on flexible substrates are highly uniform, stable, and robust. These freestanding membranes provide a viable platform for the exploration of HZO polymorphic films with complex structures and pave the way to flexible nanoelectronics.

Nature Physics, Published online: 11 April 2022; doi:10.1038/s41567-022-01556-5

The presence or absence of a strange metal phase in twisted bilayer graphene has been controversial. Now, measurements over a wide range of temperature and doping give much stronger evidence for its existence.

Polarization-resolved second harmonic generation imaging in ultrathin orthorhombic 2D SnS is performed. By fitting the experimental data with a developed nonlinear optics model, the armchair/zigzag crystallographic directions are mapped with high resolution, and the relative strength of the second-order nonlinear susceptibility tensor in different directions is calculated, offering quantitative information on the in-plane anisotropy in SnS.

Abstract

Two-dimensional (2D) tin(II) sulfide (SnS) crystals belong to a class of orthorhombic semiconducting materials with remarkable properties, such as in-plane anisotropic optical and electronic response, and multiferroic nature. The 2D SnS crystals exhibit anisotropic response along the in-plane armchair (AC) and zigzag (ZZ) crystallographic directions, offering an additional degree of freedom in manipulating their behavior. Here, advantage of the lack of inversion symmetry of the 2D SnS crystal, that produces second harmonic generation (SHG), is taken to perform polarization-resolved SHG (P-SHG) nonlinear imaging of the in-plane anisotropy. The P-SHG experimental data are fitted with a nonlinear optics model, allowing to calculate the AC/ZZ orientation from every point of the 2D crystal and to map with high resolution the AC/ZZ direction of several 2D SnS flakes belonging in the same field of view. It is found that the P-SHG intensity polar patterns are associated with the crystallographic axes of the flakes and with the relative strength of the second-order nonlinear susceptibility tensor in different directions. Therefore, the method provides quantitative information of the optical in-plane anisotropy of orthorhombic 2D crystals, offering great promise for performance characterization during device operation in the emerging optoelectronic applications of such crystals.

Multisensory artificial synapses based on electrolyte-gated vertical organic field-effect transistors are first developed. The artificial synapse can emulate human multiple perceptions, such as image learning and memorizing, sound localization, and taste detection. The minimum energy consumption (E _c) of one synaptic event is 0.06 fJ, which is significantly lower than that of biological synapses (1–10 fJ).

Abstract

Bioinspired electronics have shown great potential in the field of artificial intelligence and brain-like science. Low energy consumption and multifunction are key factors for its application. Here, multisensory artificial synapse and neural networks based on electrolyte-gated vertical organic field-effect transistors (VOFETs) are first developed. The channel length of the organic transistor is scaled down to 30 nm through cross-linking strategy. Owing to the short channel length and extremely large capacitance of the electric double layer formed at the electrolyte–channel interface, the minimum power consumption of one synaptic event is 0.06 fJ, which is significantly lower than that required by biological synapses (1–10 fJ). Moreover, the artificial synapse can be trained to learn and memory images in a 5 × 5 synapse array and emulate the human brain's spatiotemporal information processing and sound azimuth detection. Finally, the artificial tongue is designed using the synaptic transistor that can discriminate acidity. Overall, this study provides new insights into realizing energy-efficient artificial synapses and mimicking biological sensory systems.

The recent development of novel atomically thin 2D materials, including graphene, hexagonal boron nitride, and transition metal dichalcogenides, offers great opportunities for potential breakthroughs in the membrane separation field. Recent research advances, challenges, and potential future directions in the application of nanoporous atomically thin membranes for gas separation are summarized.

Abstract

Porous graphene and other atomically thin 2D materials are regarded as highly promising membrane materials for high-performance gas separations due to their atomic thickness, large-scale synthesizability, excellent mechanical strength, and chemical stability. When these atomically thin materials contain a high areal density of gas-sieving nanoscale pores, they can exhibit both high gas permeances and high selectivities, which is beneficial for reducing the cost of gas-separation processes. Here, recent modeling and experimental advances in nanoporous atomically thin membranes for gas separations is discussed. The major challenges involved, including controlling pore size distributions, scaling up the membrane area, and matching theory with experimental results, are also highlighted. Finally, important future directions are proposed for real gas-separation applications of nanoporous atomically thin membranes.

A low-voltage memristor array based on ultrathin PdSeO _x /PdSe₂ heterostructure is demonstrated using controllable ultraviolet–ozone treatment. By confining the formation of conductive filaments in the heterostructure, the memristor achieves a remarkable uniform switching with low set and reset voltage variability of 4.8% and −3.6%, respectively. The crossbar array further enables multiple convolutional image processing with a high image-recognition accuracy of ≈93.4%.

Abstract

In-memory computing based on memristor arrays holds promise to address the speed and energy issues of the classical von Neumann computing system. However, the stochasticity of ions’ transport in conventional oxide-based memristors imposes severe intrinsic variability, which compromises learning accuracy and hinders the implementation of neural network hardware accelerators. Here, these challenges are addressed using a low-voltage memristor array based on an ultrathin PdSeO _x /PdSe₂ heterostructure switching medium realized by a controllable ultraviolet (UV)–ozone treatment. A distinctively different ions’ transport mechanism is revealed in the heterostructure that can confine the formation of conductive filaments, leading to a remarkable uniform switching with low set and reset voltage variability values of 4.8% and −3.6%, respectively. Moreover, convolutional image processing is further implemented using various crossbar kernels that achieve a high recognition accuracy of ≈93.4% due to the highly linear and symmetric analog weight update as well as multiple conductance states, manifesting its potential beyond von Neumann computing.

Quantum conductance effects in memristive devices are reviewed, from fundamentals of electrochemical phenomena underlying memristive functionalities to ballistic electronic conduction transport in atomic-sized conductive filaments. Related challenges in nanoscale metrology for the characterization of memristive phenomena at the nanoscale are analyzed together with opportunities and envisioned applications for memristive devices working in the quantum regime.

Abstract

Quantum effects in novel functional materials and new device concepts represent a potential breakthrough for the development of new information processing technologies based on quantum phenomena. Among the emerging technologies, memristive elements that exhibit resistive switching, which relies on the electrochemical formation/rupture of conductive nanofilaments, exhibit quantum conductance effects at room temperature. Despite the underlying resistive switching mechanism having been exploited for the realization of next-generation memories and neuromorphic computing architectures, the potentialities of quantum effects in memristive devices are still rather unexplored. Here, a comprehensive review on memristive quantum devices, where quantum conductance effects can be observed by coupling ionics with electronics, is presented. Fundamental electrochemical and physicochemical phenomena underlying device functionalities are introduced, together with fundamentals of electronic ballistic conduction transport in nanofilaments. Quantum conductance effects including quantum mode splitting, stability, and random telegraph noise are analyzed, reporting experimental techniques and challenges of nanoscale metrology for the characterization of memristive phenomena. Finally, potential applications and future perspectives are envisioned, discussing how memristive devices with controllable atomic-sized conductive filaments can represent not only suitable platforms for the investigation of quantum phenomena but also promising building blocks for the realization of integrated quantum systems working in air at room temperature.

Nature Nanotechnology, Published online: 04 April 2022; doi:10.1038/s41565-022-01102-7

A two-dimensional transition metal dichalcogenide-on-compound-semiconductor fabrication method enables the realization of an active matrix micro-LED display.