Jing Zhang

Nature Materials, Published online: 31 March 2022; doi:10.1038/s41563-022-01220-6

The exceptional quality of hexagonal boron nitride crystals that can be cleaved into few layers provides ultrathin dielectrics, thereby opening a route to ultrasmall capacitors with large capacitances. With such capacitors, the superconducting transmon qubit is scaled down by orders of magnitude.

Nature Physics, Published online: 04 April 2022; doi:10.1038/s41567-022-01557-4

Correlated insulating states are common in twisted bilayer graphene when the density of carriers is close to an integer per moiré unit cell. Now, such states emerge at half-integer fillings and show signs of being spin or charge density waves.

A multiposition controllable gate phototransistor based on the van der Waals heterojunction of WS₂ and MoS₂ is fabricated. A very low subthreshold swing of 47 mV dec⁻¹, highest responsivity of 167.8 A W⁻¹, and maximum detectivity of 5.8 × 10¹² Jones are achieved. In addition, the potential application of this phototransistor in optoelectonic logic operation and biological neural network are also demonstrated.

Abstract

Better control of the channel is crucial to improve the performance of existing electron devices. A phototransistor with a specifically designed dual-gate structure based on a vertical van der Waals heterojunction of WS₂ and MoS₂ is proposed. The top gate modulates the carrier transport in WS₂ at the top of the heterojunction, whereas the back gate can simultaneously control the carrier transport in both MoS₂ and WS₂ regions located on either side of the heterojunction. Therefore, the rectification ratio of the WS₂/MoS₂ heterojunction can be modified from approximately 1 to above 10⁴. A very low subthreshold swing of 47 mV dec⁻¹ is obtained. Optoelectronic characterization shows that the responsivity and detectivity are as high as 167.8 A W⁻¹ and 5.8 × 10¹² Jones at 532 nm, respectively, which are attributed to the combined modulation effect of the WS₂/MoS₂ heterojunction and additional homojunction in WS₂. Moreover, a logic operation between electronic and optical signals can be performed by utilizing only one multiposition controllable gate phototransistor. In addition, the capability of this dual-gate phototransistor to emulate information transmission in neuromorphic architectures is presented. These results demonstrate the potential of this approach for the development of next-generation optoelectronic devices.

Neurochemical Analyzers

In article number 2108203, Chul-Ho Lee, Dong Pyo Jang, Suk-Won Hwang, and co-workers demonstrate a soft, wireless, and bioresorbable neurochemical system that is designed to determine various concentrations of neurotransmitters as well as to monitor peripheral physiologies in a simultaneous manner. Such technology is motivated by the need for assessment and treatment of postoperative complications that may occur over a certain period of time after neurological surgeries.

Advanced Materials, Volume 34, Issue 14, April 7, 2022.

Nature Nanotechnology, Published online: 28 March 2022; doi:10.1038/s41565-022-01095-3

Phase-change memtransistive synapses enable the implementation of biomimetic neural algorithms to perform tasks such as sequential learning and stochastic Hopfield computing networks.

Nature Nanotechnology, Published online: 28 March 2022; doi:10.1038/s41565-022-01080-w

Current DNA computation techniques are slow in generating chemical outputs in response to chemical inputs and rely heavily on fluorescence readouts. Here, the authors introduce a new paradigm for DNA computation where the chemical input is processed and transduced into a mechanical output in the form of macroscopic locomotion using dynamic DNA-based motors.

A two-step process of charge transfer in trilayer-WS₂–MoS₂–WSe₂ is directly observed when WSe₂ is excited. The electrons in WSe₂ are transferred to the high lying electronic state of MoS₂–WS₂, and then electrons eventually relax into the conduction band minimum of MoS₂. In addition, the transfer of interlayer excitons is observed for the first time.

Abstract

Van der Waals (vdW) heterostructures (HSs) built on 2D materials provide an ideal platform for research of energy migration at the nanoscale. However, the underlying charge transfer mechanism in type II vdW HSs is still not well understood. Here, ultrafast exciton dynamics are investigated in trilayer-WS₂-MoS₂-WSe₂ and trilayer-MoS₂-WSe₂-WS₂ HSs by broadband pump-probe spectroscopy. A two-step process of exciton transfer in trilayer-WS₂-MoS₂-WSe₂ is directly observed when the band edge exciton of WSe₂ is excited. The electrons in WSe₂ are initially transferred to the high lying electronic state of MoS₂-WS₂ on a time scale of tens of femtoseconds, and then electrons eventually relax into the conduction band minimum of MoS₂ within 1 ps. Furthermore, the transfer of interlayer excitons is observed for the first time in trilayer-MoS₂-WSe₂-WS₂. Both transfer processes can be better understood by the Dexter charge exchange model. Due to the nature of Dexter type transfer that the exchange rate exponentially depends on the donor−acceptor distance, the interlayer exciton transfer rate is nearly a hundred times slower than that of exciton transition in bilayer HSs. The results deepen the understanding of charge transfer in 2D vdW HSs and also indicate that the exciton effect and orbital hybridization make HS a strong coupling system.

A higher lanthanide concentration of 80% sensitizer (Yb³⁺) and 20% activator (Er³⁺) increases the absorption rate of upconversion nanoparticles. A NaYF₄ shell blocks the energy migration pathways to the particle surface, which enhances the upconversion luminescence upon near-infrared excitation significantly. An investigation of particle diameter and shell thickness reveals significantly enhanced luminescence in the red (660 nm) for bioanalytical applications.

Abstract

Lanthanide-doped upconversion nanoparticles (UCNPs) have attracted a lot of interest due to their benefits in biological applications: They are not suffering from intermittence and provide nearly background-free luminescence. The progress in synthesis nowadays enables access to complex core-shell particles of controlled size and composition. Nevertheless, the frequently used doping ratio dates back to where mostly core-only particles of relatively large size have been studied. Especially at low power excitation as needed in biology, a decrease in particle size leads to a drastic decrease in the upconversion efficiency. An enhancement strategy based on an increased absorption rate of near-infrared light provided by an increase of the sensitizer content, together with the simultaneous blocking of the energy migration pathways to the particle surface, is presented. NaYbF₄(20%Er) particles of 8.5 nm diameter equipped with an about 2 nm thick NaYF₄ shell show significantly enhanced upconversion luminescence in the red (660 nm) compared to the most commonly used particles with only 20% Yb³⁺ and 2% Er³⁺. The impact of size, composition, and core-shell architecture on photophysical properties are studied. The findings demonstrate that an increase in doping rates enables the design of small, bright UCNPs useful for biological applications.

Highly luminescent perovskite QDs-composite films (QPFs) with excellent self-healing ability at room temperature are fabricated by incorporation of perovskite QDs into fluoroelastomer polymeric matrix. Moreover, the stretchable QPFs show remarkable environmental and mechanical stabilities that enable their practical applications in flexible white light-emitting devices and wide color gamut displays with high saturations of vivid pictures for the object colors.

Abstract

Metal halide perovskite quantum dots (QDs) and polymer composite films have witnessed extensive investigation in flexible optoelectronic devices, while the unsatisfactory environmental and mechanical properties of the composite films set substantial limitations for practical applications. Herein, highly luminescent perovskite QDs-polymer composite films (QPFs) are fabricated with remarkable environmental and mechanical stabilities by incorporation of perovskite QDs into fluoroelastomer polymeric matrix. The stretchable QPFs show excellent self-healing ability with micron- and centimeter-scale cracks healed in dozens of minutes and hours, respectively, due to the strong dipole–dipole interaction between the CF bonds of fluoroelastomer. After careful optimization of QDs ratios, a flat and smooth film morphology is achieved with a uniform distribution of QDs, which promotes good optical properties with a long PL lifetime of 1213.75 ns and high photoluminescence quantum yields up to 96.1%, and super environmental and mechanical stability These merits of QPFs enable their practical applications in flexible white light-emitting devices and wide color gamut displays with 124% of standard National Television Standards Committee and 96% of Recommendation 2020, respectively, exhibiting vivid pictures with high saturations for the object colors, indicating great potential toward practical applications.

The universal growth of ultrathin perovskite single crystals is realized by designing an oriented solvent microenvironment induced by the interfacial electric field originated from the charge separation between solid and liquid phases. Such a strategy can fabricate a wide range of high-quality ultrathin perovskite single crystals, from layered to nonlayered, organic to inorganic, and toxic to low-toxic lead-free perovskite. Notably, the realization of high quality and diversity of ultrathin perovskites will facilitate both fundamental studies and optoelectronic applications.

Abstract

Perovskites have engaged significant attention owing to rich species and remarkable physical properties as well as optoelectronic applications. Compared to bulk counterparts, ultrathin perovskites exhibit more available compositions due to the breaking of bulk lattice limitation. Coupled with crystal lattice relaxation and quantum confinement, infinite intriguing properties of ultrathin perovskites deserve to be explored. Developing ultrathin perovskites with alterable composition and structure is a necessity to fully explore this versatile family. Herein, a universal strategy is conceived via constructing oriented solvent microenvironment induced by the interfacial electric field originated from the charge separation between solid and liquid phases, which is conducive to controlling the precursor distribution and makes crystals preferentially nucleate and grow in the preferentially lateral mode. From layered to nonlayered, organic to inorganic, and toxic to low-toxic lead-free perovskite, a full-range synthesis is achieved of ultrathin perovskites. This work opens up opportunities both for ultrathin perovskite exploration through compositional engineering and for device miniaturization in energy conversion applications.

Atomically resolved multiple 2D phase transformations (MoS₂ → Mo₄S₆, MoSe₂ → L-, Z-Mo₆Se₆) is observed in monolayer transition metal dichalcogenides under in situ heating with stoichiometry control by electron beam irradiation. Through chalcogen sliding and reconstruction mechanisms, phase transformations are well manipulated to fabricate diphase heterostructures with atomically sharp interfaces, which will pave the way to phase engineered optoelectronics.

Abstract

Phase transformation lies at the heart of materials science because it allows for the control of structural phases of solids with desired properties. It has long been a challenge to manipulate phase transformations in crystals at the nanoscale with designed interfaces and compositions. Here in situ electron microscopy is employed to fabricate novel 2D phases with different stoichiometries in monolayer MoS₂ and MoSe₂. The multiphase transformations: MoS₂ → Mo₄S₆ and MoSe₂ → Mo₆Se₆ which are highly localized with atomically sharp boundaries are observed. Their atomic mechanisms are determined as chalcogen 2H ↔ 1T sliding, cation shift, and commensurate lattice reconstructions, resulting in decreasing direct bandgaps and even a semiconductor–metal transition. These results will be a paradigm for the manipulation of multiphase heterostructures with controlled compositions and sharp interfaces, which will guide the future phase engineered electronics and optoelectronics of metal chalcogenides.

Two bistable and reversibly controllable antiferromagnetic states in strained BFO film are discovered. These two non-volatile antiferromagnetic states are successfully patterned with a non-contact approach combining both optical and magnetic methods. The written antiferromagnetic pattern is electrically readable with at least 30% signal difference. This work promises an alternative route toward practical applications of antiferromagnetic spintronics.

Abstract

Antiferromagnetic spintronics is an emerging field of non-volatile data storage and information processing. The zero net magnetization and zero stray fields of antiferromagnetic materials eliminate interference between neighbor units, leading to high-density memory integrations. However, this invisible magnetic character at the same time also poses a great challenge in controlling and detecting magnetic states in antiferromagnets. Here, two antiferromagnetic spin states close in energy in strained BiFeO₃ thin films at room temperature are discovered. It can be reversibly switched between these two non-volatile antiferromagnetic states by a moderate magnetic field and a non-contact optical approach. Importantly, the conductivity of the areas with each antiferromagnetic textures is drastically different. It is conclusively demonstrated the capability of optical writing and electrical reading of these newly discovered bistable antiferromagnetic states in the BiFeO₃ thin films.

Optimized tandem device of the multi-resonance type blue emitter achieves high external quantum efficiency over 25% and extremely long device lifetime of over 500 h at 1000 cd m⁻² and 30000 h at 100 cd m⁻² up to 95% of initial luminance.

Abstract

In this study, a multiple resonance (MR) type blue emitter is synthesized, characterized, and evaluated for highly efficient and stable blue fluorescent organic light-emitting diodes (OLEDs). The MR blue fluorescent emitter has a di-tert-butyl benzene substituent in the MR core structure to minimize quenching mechanisms by intermolecular interaction. The emitter shows a high photoluminescence quantum yield and small full width at half maximum of 22 nm, which realize high external quantum efficiency (EQE) of 11.4% in the single unit OLED and device lifetime up to 95% of the initial luminance (LT₉₅) of 208 h at 1000 cd m⁻² and over 10 000 h at 100 cd m⁻². The optimized tandem device of the new blue emitter achieves high EQE over 25% and extremely long LT₉₅ of over 500 h at 1000 cd m⁻² and 30 000 h at 100 cd m⁻². The lifetime of this work is one of the best data of blue OLED lifetime reported in the literature.

Semiconducting two-dimensional (2D) polymers feature synthetically tailored porosity and thus show both efficient electronic and ion transport. It is shown that two-dimensional polymers (2DPs) can be patterned on the micrometer scale, and can be used as the active material for organic electrochemical transistors.

Abstract

Organic electrochemical transistors (OECTs) are devices with broad potential in bioelectronic sensing, circuits, and neuromorphic hardware. Their unique properties arise from the use of organic mixed ionic/electronic conductors (OMIECs) as the active channel. Typical OMIECs are linear polymers, where defined and controlled microstructure/morphology, and reliable characterization of transport and charging can be elusive. Semiconducting two-dimensional polymers (2DPs) present a new avenue in OMIEC materials development, enabling electronic transport along with precise control of well-defined channels ideal for ion transport/intercalation. To this end, a recently reported 2DP, TIIP, is synthesized and patterned at 10 µm resolution as the channel of a transistor. The TIIP films demonstrate textured microstructure and show semiconducting properties with accessible oxidation states. Operating in an aqueous electrolyte, the 2DP-OECT exhibits a device-scale hole mobility of 0.05 cm² V^–1 s^–1 and a µC* figure of merit of 1.75 F cm^–1 V^–1 s^–1. 2DP OMIECs thus offer new synthetic degrees of freedom to control OECT performance and may enable additional opportunities such as ion selectivity or improved stability through reduced morphological modulation during device operation.

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.

N-doped graphene/WSe₂ (NG/WSe₂) superlattice obtained by a cost-effective synthetic strategy is used as a sulfur host to regulate lithium polysulfide (LiPS) conversion reaction. The increased interlayer spacing (1.04 nm) and enhanced conductivity guarantee the ion and electron transfer in Li–S reaction process, and the heterogeneous interface probes strong lithio/sulfiphilic dual-adsorption sites to confine the LiPS shuttle effect. Consequently, robust lithium–sulfur batteries are obtained.

Abstract

Superlattices are rising stars on the horizon of energy storage and conversion, bringing new functionalities; however, their complex synthesis limits their large-scale production and application. Herein, a simple solution-based method is reported to produce organic–inorganic superlattices and demonstrate that the pyrolysis of the organic compound enables tuning their interlayer space. This strategy is exemplified here by combining polyvinyl pyrrolidone (PVP) with WSe₂ within PVP/WSe₂ superlattices. The annealing of such heterostructures results in N-doped graphene/WSe₂ (NG/WSe₂) superlattices with a continuously adjustable interlayer space in the range from 10.4 to 21 Å. Such NG/WSe₂ superlattices show a metallic electronic character with outstanding electrical conductivities. Both experimental results and theoretical calculations further demonstrate that these superlattices are excellent sulfur hosts at the cathode of lithium–sulfur batteries (LSB), being able to effectively reduce the lithium polysulfide shuttle effect by dual-adsorption sites and accelerating the sluggish Li–S reaction kinetics. Consequently, S@NG/WSe₂ electrodes enable LSBs characterized by high sulfur usages, superior rate performance, and outstanding cycling stability, even at high sulfur loadings, lean electrolyte conditions, and at the pouch cell level. Overall, this work not only establishes a cost-effective strategy to produce artificial superlattice materials but also pioneers their application in the field of LSBs.

Analog Resistive Memories

In article number 2109970, Armantas Melianas, Armin VahidMohammadi, Alberto Salleo, Mahiar Max Hamedi, and co-workers present the world's first electrochemical transistor memory based on 2D materials (MXene). These transistors can be used for neuromorphic computers, where they are a thousand times faster than previous ionic memories and other state-of-the-art technologies like resistive- or phase-change memristors (Image credit: Armin VahidMohammadi).

Inspired by cephalopod skin, an MXene robotic skin with tunable infrared emission is developed with four tightly integrated functions, including dynamic thermal camouflage, temperature sensation, strain sensation, and wireless communication. Equipped with the conformal MXene skins, “all-in-one” soft robots can perform adaptive thermal camouflage through actively monitoring environmental temperature changes and remotely sending/receiving crucial guidance information.

Abstract

Cephalopod skin, which is capable of dynamic optical camouflage, environmental perceptions, and herd communication, has long been a source of bio-inspiration for developing soft robots with incredible optoelectronic functions. Yet, challenges still exist in designing a stretchable and compliant robotic skin with high-level functional integration for soft robots with infinite degrees of freedom. Herein, an emerging 2D material, Ti₃C₂T_x MXene, and an interfacial engineering strategy are adopted to fabricate the soft robotic skin with cephalopod skin-inspired multifunctionality. By harnessing interfacial instability, the MXene robotic skin with reconfigurable microtextures demonstrates tunable infrared emission (0.30–0.80), enabling dynamic thermal camouflage for soft robots. Benefiting from the intrinsic Seebeck effect, crack propagation behaviors as well as high electrical conductivity, the MXene robotic skins are tightly integrated with thermal/strain sensation capabilities and can serve as a deformable antenna for wireless communication. Without additional electronics installed, the soft robots wearing the conformal MXene skins perform adaptive thermal camouflage based on the thermoelectric feedback in response to environmental temperature changes. With built-in strain sensing and wireless communication capabilities, the soft robot can record its locomotion routes and wirelessly transmit the key information to the following soft robot to keep both in disguise under thermographic cameras.

2D layered nanosheet MXene has been widely studied in pressure sensing due to the excellent mechanical and electrical properties, outstanding hydrophilicity, and extensive modifiability. To further promote the application of MXene in pressure sensors, the preparation of MXene, MXene-based microstructures, MXene pressure-sensor mechanisms, and the integration of multiple devices are reviewed.

Abstract

Flexible pressure sensors are one of the most important components in the fields of electronic skin (e-skin), robotics, and health monitoring. However, the application of pressure sensors in practice is still difficult and expensive due to the limited sensing properties and complex manufacturing process. The emergence of MXene, a red-hot member of the 2D nanomaterials, has brought a brand-new breakthrough for pressure sensing. Ti₃C₂T _x is the most popular studied MXene in the field of pressure sensing and shows good mechanical, electrical properties, excellent hydrophilicity, and extensive modifiability. It will ameliorate the properties of the sensitive layer and electrode layer of the pressure sensor, and further apply pressure sensing to many fields, such as e-skin flexibility. Herein, the preparation technologies, antioxidant methods, and properties of MXene are summarized. The design of MXene-based microstructures is introduced, including hydrogels, aerogels, foam, fabrics, and composite nanofibers. The mechanisms of MXene pressure sensors are further broached, including piezoresistive, capacitive, piezoelectric, triboelectric, and potentiometric transduction mechanism. Moreover, the integration of multiple devices is reviewed. Finally, the chance and challenge of pressure sensors improved by MXene smart materials in future e-skin and the Internet of Things are prospected.

2D perovskite is applied as a charge-trapping and photosensitive layer in floating-gate non-volatile optoelectronic memory, demonstrating superior performance and multi-bit negative photoconductivity (NPC)/positive photoconductivity (PPC) behavior with low optical switching power.

Abstract

The development of floating-gate nonvolatile memory (FGNVM) is limited by the charge storage, retention and transfer ability of the charge-trapping layer. Here, it is demonstrated that due to the unique alternate inorganic/organic chain structure and superior optical sensitivity, an insulating 2D Ruddlesden–Popper perovskite (2D-RPP) layer can function both as an excellent charge-storage layer and a photosensitive layer. Optoelectronic memory composed of a MoS₂/hBN/2D-RPP (MBR) van der Waals heterostructure is demonstrated. The MBR device exhibits unique light-controlled charge-storage characteristics, with maximum memory window up to 92 V, high on/off ratio of 10⁴, negligible degeneration over 10³ s, >1000 program/erase cycles, and write speed of 500 µs. Dependent on the initial states, the MBR optoelectronic memory can be programmed in both positive photoconductivity (PPC) and negative photoconductivity (NPC) modes, with up to 11 and 22 distinct resistance states, respectively. The optical program power for each bit is as low as 36/10 pJ for PPC/NPC. The results not only reveal the potential of 2D-RPP as a superior charge-storage medium in floating-gate memory, but also provides an effective strategy toward fast, low-power and stable optical multi-bit storage and neuromorphic computing.

Structural Color

A view of the large-scale assembly of structural colors by different nanoresonators is shown, as described by Andrea Fratalocchi and co-workers in article number 2108013. Each nanoresonator is deformed in all dimensions, including the vertical axis, and supports competing mechanisms of scattering and resonant light–matter interactions that yield in high-resolution, a complete gamut of colors even exceeding the red, green, and blue spectrum.

By structurally-functionally modularizing Cu₂Te with multiple point defects, mosaic nanograins, and crystal-amorphicity duality, high thermoelectric performance is obtained near the Mott–Ioffe–Regel limit. This work presents a case of paradigm shift from the band edge to the mobility edge in thermoelectric materials research.

Abstract

To date, thermoelectric materials research stays focused on optimizing the material's band edge details and disfavors low mobility. Here, the paradigm is shifted from the band edge to the mobility edge, exploring high thermoelectricity near the border of band conduction and hopping. Through coalloying iodine and sulfur, the plain crystal structure is modularized of liquid-like thermoelectric material Cu₂Te with mosaic nanograins and the highly size mismatched S/Te sublattice that chemically quenches the Cu sublattice and drives the electronic states from itinerant to localized. A state-of-the-art figure of merit of 1.4 is obtained at 850 K for Cu₂(S_0.4I_0.1Te_0.5); and remarkably, it is achieved near the Mott–Ioffe–Regel limit unlike mainstream thermoelectric materials that are band conductors. Broadly, pairing structural modularization with the high performance near the Mott–Ioffe–Regel limit paves an important new path towards the rational design of high-performance thermoelectric materials.

Ferroelastic–ferroelectric multiferroic materials can switch orientations of crystal structure and polarization with external strain. 2D ferroelastic–ferroelectric multiferroicity in single-crystalline rhenium dichalcogenides is discovered. Reorientation of the physical properties based on reversible bond switching between the rhenium atoms provides insights for 2D multiferroic phase transitions and opens up new opportunities for applications such as multilevel memory.

Abstract

2D multiferroics with combined ferroic orders have gained attention owing to their novel functionality and underlying science. Intrinsic ferroelastic–ferroelectric multiferroicity in single-crystalline van der Waals rhenium dichalcogenides, whose symmetries are broken by the Peierls distortion and layer-stacking order, is demonstrated. Ferroelastic switching of the domain orientation and accompanying anisotropic properties is achieved with 1% uniaxial strain using the polymer encapsulation method. Based on the electron localization function and bond dissociation energy of the Re–Re bonds, the change in bond configuration during the evolution of the domain wall and the preferred switching between the two specific orientation states are explained. Furthermore, the ferroelastic switching of ferroelectric polarization is confirmed using the photovoltaic effect. The study provides insights into the reversible bond-switching process and potential applications based on 2D multiferroicity.

Nature Nanotechnology, Published online: 14 March 2022; doi:10.1038/s41565-022-01073-9

Nonlinear optical parametric polaritons are observed in a WS2 monolayer microcavity, opening the way for all-optical valley polariton nonlinear devices.

Nature Nanotechnology, Published online: 14 March 2022; doi:10.1038/s41565-021-01068-y

This Review discusses the recent progress in the emerging field of exciton phenomena in semiconductor moiré superlattices.

Nature Nanotechnology, Published online: 17 March 2022; doi:10.1038/s41565-021-01038-4

The implementation of parity–time symmetry in a complementary metal–oxide–semiconductor process technology enables the realization of wide-band high-quality microwave generation and broadband strong microwave isolation at gigahertz frequencies.