Jing Zhang

Nature Materials, Published online: 02 May 2022; doi:10.1038/s41563-022-01239-9

Highly stretchable organic electrochemical transistors with stable charge transport under severe tensional strains are demonstrated using a honeycomb semiconducting polymer morphology, thereby enabling controllable signal output for diverse stretchable bioelectronic applications.

Nature Materials, Published online: 02 May 2022; doi:10.1038/s41563-022-01247-9

Bioelectronics demand stretchable devices with steady performance under deformation. By combining an amphiphilic organic semiconducting polymer with tailored film processing, highly stretchable organic electrochemical transistors are demonstrated.

Nature Electronics, Published online: 02 May 2022; doi:10.1038/s41928-022-00755-5

Photodetectors that offer broadband imaging from ultraviolet to mid-infrared can be created by using a silicon depletion well for charge integration, single-layer graphene for non-destructive direct readout and multilayer graphene for infrared photocharge injection.

Through a combined experimental and theoretical investigation, it is demonstrated that the reflection phase of hyperbolic surface polaritons propagating in hexagonal boron nitride (hBN) microstructures can be systematically altered by varying the corner angle of the hBN structure, and that it experiences a π jump around a specific angle.

Abstract

Polaritons—confined light–matter waves—in van der Waals (vdW) materials are a research frontier in light–matter interactions with demonstrated advances in nanophotonics. Reflection, as a fundamental phenomenon involving waves, is particularly important for vdW polaritons, predominantly because it enables the investigation of polariton standing waves using the scanning probe technique. While previous works demonstrate a rigid phase ≈π/4 for the polariton reflection, herein is reported the altering of the polariton reflection phase by varying the geometry of polaritonic microstructures for the case study of hyperbolic surface polaritons (HSPs) in hexagonal boron nitride (hBN). Specifically, it is demonstrated that the polariton reflection phase can be systematically altered by varying the corner angle of the hBN microstructures, and that it experiences a π jump around a specific angle. This behavior, which is a consequence of the mathematical nature of the reflection coefficient, is therefore expected in other physical phenomena.

Magnetic metal Co is successfully introduced into 2D layered halide perovskite by solution method. It is found that doped perovskite has excellent circularly polarized light (CPL) emission characteristics with left handed and right handed circularly polarized light excitation. Moreover, CPL detectors with good selectivity are demonstrated based on the as-synthesized doped perovskites.

Abstract

The generation, manipulation, and detection of polarized light are the foundations of spin optoelectronics. Polarized light has a wide range of applications in new-generation information transmission, storage, and quantum communications. Hybrid organic–inorganic halide perovskites have attracted extensive attention recently because of their strong spin–orbit coupling, Rashba splitting, spin-dependent optical selection, and have been regarded as promising candidates for application in spin optoelectronic devices. Herein, the authors report the successful synthesis of 2D layered lead halide perovskites (PEA)₂PbI₄ (PEI) doped with magnetic metal Co²⁺ and the observation of strong circularly polarized photoluminescence and photoresponse. The introduction of magnetic metals in perovskites can induce an analogous Zeeman effect in the system, which can increase its energy degeneracy, giving rise to an imbalanced population of electronic spin states, thereby enhancing the spin polarization in doped samples. Thus, a maximum circularly polarized PL of 35% is observed at room temperature. Moreover, the achieved Co²⁺-doped (PEA)₂PbI₄ also exhibits a selective photoresponse with circularly polarized light (CPL) emission, and an outstanding anisotropy factor of 0.41 for photocurrent is achieved. The experimental results lay the foundation for the controlled synthesis of magnetically doped perovskites as well as applications for direct CPL detection in spin optoelectronics.

Chromium acting as a bipolar active dopant for forming n- and p-ReSe₂ is presented. A p–n homojunction light-emitting diode made by stacking multilayered n- and p-ReSe₂ is first fabricated to emit electroluminescence of ≈946 nm at the band edge. Another stacked p–n homojunction solar cell is also manufactured to exhibit maximum axial conversion efficiency along b-axis.

Abstract

The formation of p- or n-type material via impurity doping should be crucial and essentially prior to the establishment of junction devices in semiconductor processing. Especially in a 2D transition-metal dichalcogenide (TMD), dopant selection for growing p- and n-type TMD semiconductors may suffer much higher difficulty and complexity than conventional Si and III–V compounds owing to the complicated valences occurred in transition metals. Different amount of chromium doped in ReSe₂ interestingly showing dissimilar carrier types of p-ReSe₂ with Cr 10% and 20% doping and n-ReSe₂ with Cr 0%, 1%, and 5% doping, respectively, is presented here. According to structural and optical characterization, the crystal structure and bandgap of the Cr-doped ReSe₂ remain unchanged in which a deeper donor of Cr⁶⁺ can exist in n-ReSe₂ (1% and 5%) while a shallow acceptor of Cr³⁺ may appear in p-ReSe₂ multilayer (10% and 20%). A p–n homojunction light-emitting diode made by stacking multilayered n-ReSe₂ (Cr 0%) and p-ReSe₂ (Cr 20%) is first fabricated, and it emits an electroluminescence of ≈946 nm from the band edge. Another p–n homojunction solar cell is also manufactured to exhibit a maximum axial conversion efficiency of η ≈ 1.05% along with the b-axis polarization of ReSe₂.

A facile solution growth method for fabricating single-crystalline flakes of layered-perovskite quantum wells of (PEA)₂(MA) _{n −1}Pb _n I_{3 n +1} (PEA = phenylethylammonium, <n> = 1, 2, and 3) is reported. These single-crystalline flakes show the biexciton amplified spontaneous emission at 541, 588, and 627 nm with threshold of 35.4, 15.1, and 5.8 μJ cm⁻², respectively, for (PEA)₂(MA) _{n −1}Pb _n I_{3 n +1} (n = 1, 2, and 3) single crystals in cryogenic condition. The exfoliated smaller flakes show multimode lasing at 586 and 627 nm for <n> = 2 and 3 with threshold of 10.5 and 4.8 μJ cm⁻², respectively.

Abstract

The biexciton is a quasi-particle created from two free excitons and can be applied for basic quantum operations, entangled light sources, and lasers. 2D lead-halide layered perovskites (LPs) naturally form organic–inorganic quantum wells (QWs), leading to high exciton binding energy and tunable band-gaps dependent on the QW thickness of <n>. Although excitonic lasing has been reported for thicker LP-QWs, biexciton emissions and lasers from phase-pure LP-QWs as a function of <n> remain largely unexplored. Here, a facile solution growth method for fabricating single-crystalline flakes (SCFs) of LP-QWs of (PEA)₂(MA) _{n −1}Pb _n I_{3 n +1} (PEA = phenylethylammonium, <n> = 1, 2, and 3) is reported. Further, multicolor amplified spontaneous emissions from biexciton states of (PEA)₂(MA) _{n −1}Pb _n I_{3 n +1} (<n> = 1, 2, and 3) SCFs are demonstrated at 541 (green), 588 (yellow), 627 nm (red) with thresholds of 35.4, 15.1, and 5.8 µJ cm⁻² at 20 K, respectively. In order to offer cavities for these single-crystalline flakes, they are exfoliated into smaller flakes. Multimode lasing is shown at 586 and 627 nm for <n> = 2 and 3 with threshold of 10.5 and 4.8 µJ cm⁻², respectively. The results of this study not only enrich the basic understanding of many-body excitonic interactions in 2D LPs but also provide insights for developing solution-processed multicolor lasers.

In-depth phase analysis of developed Na_{1− x} TMO₂ cathode materials, NFMO with P2- and O3-type phases (NFMO-P2/O3) is offered. As a result, the synergetic effect of the simultaneous existence of P- and O-type phases with their unique structures allows an extraordinary level of capacity retention in a wide range of voltage (1.5–4.5 V).

Abstract

The layered sodium transition metal oxide, NaTMO₂ (TM = transition metal), with a binary or ternary phases has displayed outstanding electrochemical performance as a new class of strategy cathode materials for sodium-ion batteries (SIBs). Herein, an in-depth phase analysis of developed Na_1−x TMO₂ cathode materials, Na_0.76Ni_0.20Fe_0.40Mn_0.40O₂ with P2- and O3-type phases (NFMO-P2/O3) is offered. Structural visualization on an atomic scale is also provided and the following findings are unveiled: i) the existence of a mixed-phase intergrowth layer distribution and unequal distribution of P2 and O3 phases along two different crystal plane indices and ii) a complete reversible charge/discharge process for the initial two cycles that displays a simple phase transformation, which is unprecedented. Moreover, first-principles calculations support the evidence of the formation of a binary NFMO-P2/O3 compound, over the proposed hypothetical monophasic structures (O3, P3, O′3, and P2 phases). As a result, the synergetic effect of the simultaneous existence of P- and O-type phases with their unique structures allows an extraordinary level of capacity retention in a wide range of voltage (1.5–4.5 V). It is believed that the insightful understanding of the proposed materials can introduce new perspectives for the development of high-voltage cathode materials for SIBs.

Nature Nanotechnology, Published online: 02 May 2022; doi:10.1038/s41565-022-01115-2

A ferromagnetic tunnelling contact enables electrically controlled valley polarization in monolayer WSe2.

Nature Physics, Published online: 28 April 2022; doi:10.1038/s41567-022-01597-w

A theoretical analysis shows how a person’s location in space could be verified by the transmission of single photons. A vital application of quantum networks may be within reach.

Additive manufacturing of 3D luminescent microarchitectures with light emission in the visible range is presented. The 3D europium-doped zirconia structures (ZrO₂:Eu³⁺) are produced using two-photon lithography and tailor-made resin. The orange−red emission of ZrO₂:Eu³⁺ microstructures with sub-micrometer features is showcased using fluorescence microscopy and studied with cathodoluminescence. The presented structuring technology provides a platform for developing 3D luminescent microdevices.

Abstract

Implementation of more refined structures at the nano to microscale is expected to advance applications in optics and photonics. This work presents the additive manufacturing of 3D luminescent microarchitectures emitting light in the visible range. A tailor-made organo-metallic resin suitable for two-photon lithography is developed, which upon thermal treatment in an oxygen-rich atmosphere allows the creation of silicon-free tetragonal (t-) and monoclinic (m-) ZrO₂. The approach is unique because the tailor-made Zr-resin is different from what is achieved in other reported approaches based on sol−gel resins. The Zr-resin is compatible with the Eu-rich dopant, a luminescent activator, which enables to tune the optical properties of the ZrO₂ structures upon annealing. The emission characteristics of the Eu-doped ZrO₂ microstructures are investigated in detail with cathodoluminescence and compared with the intrinsic optical properties of the ZrO₂. The hosted Eu has an orange−red emission showcased using fluorescence microscopy. The presented structuring technology provides a new platform for the future development of 3D luminescent devices.

Here the authors show how to prepare subtractive plasmonic structural colors with good quality over large areas, using colloidal self-assembly for lithography. Using a nanodisk−insulator−mirror configuration with silver as metal, they produce subtractive colors in reflection mode and show that this improves brightness compared to nanostructures utilizing the primary (RGB) color scheme.

Abstract

Plasmonic metasurfaces for color generation are emerging as important components for next generation display devices. Fabricating bright plasmonic colors economically and via easily scalable methods, however, remains difficult. Here, the authors demonstrate an efficient and scalable strategy based on colloidal lithography to fabricate silver-based reflective metal–insulator–nanodisk plasmonic cavities that provide a cyan-magenta-yellow (CMY) color palette with high relative luminance. With the same basic structure, they exploit different mechanisms to efficiently produce a complete subtractive color palette. Finite-difference time-domain simulations reveal that these mechanisms include gap surface plasmon modes for thin insulators and hybridized modes between disk plasmons and Fabry–Pérot modes for thicker systems. To produce yellow hues, they take advantage of higher-energy gap surface plasmon modes to allow resonance dips in the blue spectral region for comparably large nanodisks, thereby circumventing difficult fabrication of nanodisks less than 80 nm. It is anticipated that incorporation of these strategies can reduce fabrication constraints, produce bright saturated colors, and expedite large-scale production.

Specially designed polymer composite with homogeneously distributed perovskite nanocrystals and a halogen precursor can be used to control an in situ anion exchange reaction. This procedure is compatible with direct laser patterning and can be applied to fabricate light-emitting micropixel arrays.

Abstract

Cesium lead halide perovskite nanocrystals (NCs) have gained enormous attention as promising light-emitting and light-converting materials. Most of their applications require embedding NCs in various matrices, which is a challenging task due to their low stability, especially in the case of red-emitting CsPbI₃ NCs. In this work, a new approach is proposed allowing the formation of red-emitting perovskite NCs by anion exchange induced directly inside a solid polymer matrix using green-emitting CsPbBr₃ NCs as templates and iodododecane as an iodine source. Moreover, a simple and efficient route to photo-assisted termination of the anion exchange reaction in the polymer composite after reaching desired optical properties is demonstrated. The findings allow the authors to pattern a thin composite film with an ultrashort UV laser resulting in a selective generation of green- and red-emitting features with a 15 µm resolution.

2D MoSe₂ is discovered with intrinsic low infrared emissivity and is further exfoliated into adequately aminated nanosheets with nice thermal camouflage performance. Experimental and density functional theory calculations together reveal mechanisms to affect infrared emissivity as electron–photon reflection and phonon–photon absorption. This work offers inspiration for two-dimensional materials selection and preparation toward desired infrared radiation properties in certain wavebands to satisfy numerous applications.

Abstract

Low infrared emissivity materials play a key role in thermal camouflage or retardation. Among these, fillers can be easily shaped into various flexible forms and normally provide an omnidirectional and polarization-insensitive emissivity. However, conventional fillers suffer from drawbacks of full-waveband ultrahigh reflectivity, unsatisfactory thermal camouflage performances, or poor chemical/thermal stabilities. Herein, 2D MoSe₂ is discovered as a new semiconductor with intrinsic low infrared emissivity after first-principle density functional theory calculation and experimental demonstration on eight types of two-dimensional materials (2DMs). Mechanisms of electron–photon reflection and phonon–photon absorption for the low infrared emissivity are proposed. A two-step microwave-assisted amination process is developed to exfoliate the nanosheets and obtain a desired low infrared emissivity. The as-obtained chitosan modified MoSe₂ (CS@MoSe₂) has an ultrahigh spectral reflectivity of 78%–86% in 8–14 µm, and its resin-based coating still exhibits a low infrared emissivity of 0.32 and shows a dramatic reduction in radiation temperature of 28 °C for a hot object at 70 °C. Besides, CS@MoSe₂ can endure a high temperature of 220 °C and is demonstrated with a long-term thermal camouflage efficiency in hot environments. This work will guide 2DMs selection and preparation toward desired infrared radiation properties to satisfy numerous applications.

This article comprehensively reviews the progress of chemical vapor deposition growth, characterization, and electrical properties of graphene depending on layer number and twist angle. The characterization methods for measuring the layer number and twist angle are summarized. Electrical properties and applications of graphene, particularly magic-angle twist bilayer graphene, are briefly introduced. Outlooks and challenges are presented.

Abstract

Numerous studies conducted on the layered graphene family—including the monolayer, bilayer, trilayer, few-layer, and multilayer—draw plenty of attention to stacking modes and twist angles, which are extensively explored for its controlled growth, properties, and applications. This review provides a comprehensive overview of current challenges and opportunities for the chemical vapor deposition (CVD) growth, characterization, and electrical properties of graphene depending on the layer number and twist angles. Various state-of-the-art innovations using the CVD method, which incorporates graphene synthesis through the control of metal substrates, layer numbers, and twist angles, are presented. The underlying growth mechanisms are discussed in terms of the interactions among graphene substrates/layers and its dynamic process. The characterization methods for determining the layer number and twist angle of graphene layers are summarized. Furthermore, the electrical properties and applications of graphene, particularly magic-angle twist bilayer graphene, are briefly introduced. Finally, outlooks and perspectives for the engineering of CVD graphene layers are discussed.

An atomic threshold switch composed entirely of 2D material is composed using single-layer hBN (≈0.33 nm) and single-layer graphene (≈0.34 nm). The insertion of a graphene layer on top of hBN allows the controlled injection of metal ions that form atomically thin conductive filaments. Atomic defects in graphene control the injection of metal ions, whereas B defects in hBN affect the formation of conductive filaments. The hBN-graphene atomic switch shows low operating voltage, low leakage current (<1 pA), high on/off ratio (>10⁸), high endurance (>10⁴ cycles), and fast switching (≈70 ns).

Abstract

This report demonstrates that atomic-level controlled formation/rupture of conductive filaments using all 2D heterostructure of hBN-graphene is a feasible way to achieve excellent switching characteristics for ideal atomic switches. At a threshold voltage, graphene with stable ion migration routes forms a few atom comprising Ag filaments in hBN, which subsequently spontaneously break as the applied voltage lowers, resulting in optimal threshold switching behavior in an hBN atomic switch. The hBN-graphene atomic threshold switch maintains volatile threshold switching behavior in a wide range of operating currents (10 nA – 100 µA), has an extremely high on/off ratio > 10⁸, ultralow leakage current < 1 pA and exceptionally rapid switching (≈70 ns). Ionic transport of Ag⁺ through graphene can avoid the migration of excessive Ag atoms into single layer hBN during operations. As a result, the growth of conductive filaments is stable and protected from atomic defects widening in hBN. The novel findings of this study show that the scaling layer thickness can reach the physical limit of vertical thickness scaling to accommodate the single-atom volume of silver (≈0.257 nm) when using hBN, and a single-atom-thick ionic barrier made of graphene can be used to control switching.

2D TaSe₂-MoSe₂ metal–semiconductor heterostructures are successfully achieved usin an edge-induced epitaxial growth mode. The unique contact potential and strong current rectification behavior will facilitate the development high-performance transition metal dichalcogenide-based electronic devices.

Abstract

Van der Waals (vdWs) heterostructures based on 2D metals and semiconductors have attracted considerable attention due to their excellent properties and great application potential in next-generation electronic and optoelectronic devices. To obtain such vdWs heterostructures, the conventional approach with artificial exfoliation and stacking of 2D metals onto 2D semiconductors in the vertical direction is still far from satisfactory, because of the low yield and impurity-involved transfer process. Here, two-step vapor deposition growth of 2D TaSe₂-MoSe₂ metal–semiconductor heterostructures is reported. Raman maps confirm the precise spatial modulation of the as-grown 2D TaSe₂-MoSe₂ heterostructures. Structural analysis reveals that the upper 1T-TaSe₂ is formed heteroepitaxially on/around the presynthesized 2H-MoSe₂ monolayers with an epitaxial relationship of (10-10)_TaSe2//(10-10)_MoSe2 and [0001]_TaSe2//[0001]_MoSe2. Based on the detailed characterizations of morphology, structure, and composition, an edge-induced growth mechanism is proposed to illustrate the formation process of the 2D heterostructures, confirmed by first-principle calculations. In addition, Kelvin probe force microscope characterizations and electrical transport measurements confirm that the 2D metal–semiconductor heterostructures exhibit typical rectification characteristics with a contact potential height of ≈431 mV. The direct growth of high-quality 2D metal–semiconductor heterostructures marks an important step toward high-performance integrated optoelectronic devices.

A gapped phase in Weyl semimetal T_d-WTe₂ induced by lithium intercalation is reported. The new phase features an increasing resistivity with decreasing temperature from in situ transport measurements and is identified as a charge density wave (CDW) state with a bandgap of 0.14 eV by theoretical calculations. The findings thus provide the first experimental evidence of CDW in T_d-WTe₂.

Abstract

The Weyl semimetal WTe₂ has shown several correlated electronic behaviors, such as the quantum spin Hall effect, superconductivity, ferroelectricity, and a possible exciton insulator state, all of which can be tuned by various physical and chemical approaches. Here, a new electronic phase in WTe₂ induced by lithium intercalation is discovered. The new phase exhibits an increasing resistivity with decreasing temperature and its carrier density is almost two orders of magnitude lower than the carrier density of the semimetallic T_d phase, probed by in situ Hall measurements as a function of lithium intercalation. The theoretical calculations predict the new lithiated phase to be a potential charge density wave (CDW) phase with a bandgap of ≈0.14 eV, in good agreement with the in situ transport data. The new phase is structurally distinct from the initial T_d phase, characterized by polarization-angle-dependent Raman spectroscopy, and large lattice distortions close to 6% are predicted in the new phase. This finding of a new gapped phase in a 2D semimetal demonstrates electrochemical intercalation as a powerful tuning knob for modulating electron density and phase stability in 2D materials.

Nature Nanotechnology, Published online: 29 April 2022; doi:10.1038/s41565-022-01100-9

A new excitation scheme broadens the choice of colours for the near-infrared excitable photon avalanching nanoscale labels for super-resolution imaging.

Nature Electronics, Published online: 21 April 2022; doi:10.1038/s41928-022-00745-7

This Review examines the development of perovskite light-emitting diodes, exploring the key challenges involved in creating efficient and stable devices.

Nature Electronics, Published online: 28 April 2022; doi:10.1038/s41928-022-00761-7

Room-temperature skyrmions in 2D ferromagnets

Recent research progress on the use of liquid-phase exfoliation beyond layered van der Waals (vdW) crystals is summarized. Questions remain unanswered on how these 3D strongly bonded nonlayered crystals exfoliate into nanoplatelets, and on the insight of the exfoliation mechanism. The answer to the questions helps in revealing the future direction of this newly grown sub-family of 2D-class of materials.

Abstract

For nearly 15 years, researchers have been using liquid-phase exfoliation (LPE) to produce 2D nanosheets from layered crystals. This has yielded multiple 2D materials in a solution-processable form whose utility has been demonstrated in multiple applications. It was believed that the exfoliation of such materials is enabled by the very large bonding anisotropy of layered materials where the strength of intralayer chemical bonds is very much larger than that of interlayer van der Waals bonds. However, over the last five years, a number of papers have raised questions about our understanding of exfoliation by describing the LPE of nonlayered materials. These results are extremely surprising because, as no van der Waals gap is present to provide an easily cleaved direction, the exfoliation of such compounds requires the breaking of only chemical bonds. Here the progress in this unexpected new research area is examined. The structure and properties of nanoplatelets produced by LPE of nonlayered materials are reviewed. A number of unexplained trends are found, not least the preponderance of isotropic materials that have been exfoliated to give high-aspect-ratio nanoplatelets. Finally, the applications potential of this new class of 2D materials are considered.