Jing Zhang

This work reports the ratiometric luminescence thermography integrated on micro–nano devices vertically based on 2D van der Waals rare earth material ErOCl. 2D ErOCl endows the thermometer with wide-range temperature probing (300–700 K), high relative sensitivity (2.2% K⁻¹ at 300 K), and high repeatability. This novel approach will shed light on the development of a higher integration of electronics.

Abstract

Remote and real-time thermography integrated on micro–nano devices vertically can promote the integration while estimating their operation, which is hitherto challenging. Here, the ratiometric luminescence thermography integrated on micro–nano devices vertically based on 2D van der Waals (vdW) rare earth (RE) material ErOCl is demonstrated. Ratiometric luminescence intensity varies linearly with temperature deriving from the thermal activation between two thermally coupled levels. Typically, this ratiometric micro–nano thermometer has a wide sensing range (300–700 K), high relative sensitivity (2.2% K⁻¹ at 300 K), and high repeatability. With layered structure and insulated properties, 2D ErOCl can be easily integrated on the target chips vertically without dangling bonds so that a high-density integration can be easily realized. Therefore, 2D ErOCl is employed for thermography of a designed micro–nano device, showing real-time, high-resolution temperature distribution of single device even at electrodes with different morphologies. The feasibility of this temperature sensor is further proved by COMSOL simulation. This novel approach, 2D vdW RE based ratiometric luminescence thermography, provides an excellent platform to the development of high performance eletronics with higher integration.

A red phosphor SrLaScO₄:Eu is successfully synthesized, and the photoluminescence quantum yield (PLQY) is improved by employing (NH₄)₂SO₄-assisted synthesis. The improved PLQY is attributed to reducing Eu³⁺ to Eu²⁺ via a competitive site occupation. This work reveals a reduction strategy to develop Eu²⁺-doped high-efficiency red phosphor for practical applications in solid-state lighting.

Abstract

The discovery of Eu²⁺-doped high-efficiency red phosphors remains a vital challenge for white light-emitting diode (WLED) applications. It is therefore urgent to find effective strategies managing the oxidation state to help reduce Eu³⁺ to Eu²⁺ and accordingly increase the photoluminescence quantum yield (PLQY). Herein, a new red-emitting SrLaScO₄:Eu phosphor is designed, and the PLQY is enhanced from 13% to 67% under 450 nm excitation by employing (NH₄)₂SO₄-assisted sintering. Combined structural analysis, optical spectroscopy, and theoretical calculation reveal that predominant Eu²⁺ prefers to occupy the Sr²⁺ sites in the SrLaScO₄ enabling red emission, and a competitive site occupation of Eu³⁺ in La³⁺ can be restrained, and the reduction mechanism of Eu³⁺ to Eu²⁺ originating from the (NH₄)₂SO₄ addition is analyzed. The fabricated WLED device using red-emitting SrLaScO₄:Eu and yellow-emitting Y₃(Al,Ga)₅O₁₂:Ce³⁺ exhibits a high color-rendering index of 86.7 at a low correlated color temperature of 4005 K. This work provides a feasible reduction strategy for guiding the development of high-efficiency Eu²⁺-doped red phosphor for WLED applications.

2D Janus Monolayers

In article number 2106222, Mete Atature, Sefaattin Tongay, and co-workers use an in situ deterministic plasma technique to enable the synthesis of high-quality excitonic grade 2D SWSe, SMoSe, and other 2D Janus layers. Integrated spectrometers allow for the collection of structural, optical, and phononic properties during the growth. Through time-resolved studies, the team offers the first insights into the growth process and minute control provides the first with excitonic grade Janus layers.

2D materials have spectacular electrical and optical properties but their device fabrication and integration are arduous and expensive. Optical modification methods offer less detrimental processes that can often be done in one optical setup, sometimes even simultaneously with one another, saving time and money. Additionally, the extreme locality of lasers can be utilized in designable laser direct writing processes.

Abstract

2D materials are under extensive research due to their remarkable properties suitable for various optoelectronic, photonic, and biological applications, yet their conventional fabrication methods are typically harsh and cost-ineffective. Optical modification is demonstrated as an effective and scalable method for accurate and local in situ engineering and patterning of 2D materials in ambient conditions. This review focuses on the state of the art of optical modification of 2D materials and their applications. Perspectives for future developments in this field are also discussed, including novel laser tools, new optical modification strategies, and their emerging applications in quantum technologies and biotechnologies.

Constructing 3D neuromorphic hardware with combined computing and memory functionalities presents the opportunity to surmount the limitation encountered by current 2D artificial circuits. The limitation of 2D in-memory computing architecture is discussed and advantages of 3D neuromorphic circuits along with recent 3D innovations based on two-terminal memristors and multi-terminal transistors are reviewed.

Abstract

Neuromorphic circuits emulating the bio-brain functionality via artificial devices have achieved a substantial scientific leap in the past decade. However, even with the advent of highly advanced bio-inspired algorithms, the artificial intelligence based on current neuromorphic circuits is lagging behind significantly when compared with naturally evolved biological neural circuits. This massive and intriguing discrepancy is partly due to the incomprehensive understanding of bio-brain operating mechanism, which relies heavily on the extremely complexed entangled 3D hierarchical neural networks. Configuring 3D neuromorphic hardware with combined computing and memory functionalities, coupled with compatible progress of software algorithms, can be an inevitable route to surmount the limitation encountered by current 2D artificial circuits. Herein, referring to the neuron configuration in 3D perspective together with detailed signal generation and propagation mechanism, the von Neumann configuration is compared with state-of-the-art in-memory computing architecture, and the development and perspectives of 3D in-memory computing neuromorphic circuits are highlighted.

CrSBr is a 2D semiconductor of interest for its magnetic properties. Temperature- and gate-dependent transport exhibit an extreme anisotropy (ca. 10² to 10⁵)—stronger than in any other 2D semiconductor—and the photocurrent shows clear signatures of 1D behavior. This work shows that CrSBr is a quasi-1D electronic system, a unique characteristic in the panorama of known 2D semiconductors.

Abstract

Electronic transport through exfoliated multilayers of CrSBr, a 2D semiconductor of interest because of its magnetic properties, is investigated. An extremely pronounced anisotropy manifesting itself in qualitative and quantitative differences of all quantities measured along the in-plane a and b crystallographic directions is found. In particular, a qualitatively different dependence of the conductivities σ _a and σ _b on temperature and gate voltage, accompanied by orders of magnitude differences in their values (σ _b /σ _a  ≈ 3 × 10² to 10⁵ at low temperature and negative gate voltage) are observed, together with a different behavior of the longitudinal magnetoresistance in the two directions and the complete absence of the Hall effect in transverse resistance measurements. These observations appear not to be compatible with a description in terms of conventional band transport of a 2D doped semiconductor. The observed phenomenology—and unambiguous signatures of a 1D van Hove singularity detected in energy-resolved photocurrent measurements—indicate that electronic transport through CrSBr multilayers is better interpreted by considering the system as formed by weakly and incoherently coupled 1D wires, than by conventional 2D band transport. It is concluded that CrSBr is the first 2D semiconductor to show distinctly quasi-1D electronic transport properties.

Nature, Published online: 23 February 2022; doi:10.1038/s41586-021-04337-x

Multiple complementary optical signatures confirm the persistence of ferroelectricity and inversion-symmetry-breaking magnetic order down to monolayer NiI2, introducing the physics of type-II multiferroics into the area of van der Waals materials.

Large-angle twisted bilayer graphene (tBLG) is known to be electronically decoupled due to the spatial separation of the Dirac cones corresponding to individual graphene layers in the reciprocal space. The close spacing between the layers causes strong capacitive coupling, opening possibilities for applications in atomically thin devices. Here, we present a self-consistent quantum capacitance model for the electrostatics of decoupled graphene layers, and further generalize it to deal with decoupled tBLG at finite magnetic field and large-angle twisted double bilayer graphene at zero magnetic field. We probe the capacitive coupling through the conductance, showing good agreement between simulations and experiments for all the systems considered. We also propose a new experiment utilizing the decoupling effect to induce a huge and tunable bandgap in bilayer graphene by applying a moderately low bias. Our model can be extended to systems composed of decoupled graphene multilayers a...

Electronic structure of XY haeckelite compounds (X = B, Al, Ga, In, or Tl; Y = N, P, As, or Sb) is studied from the first principles. The calculations reveal these compounds possess fascinating electronic properties with nontrivial band topologies relevant to future optoelectronic and quantum information technologies.

Abstract

The family of III–V element compounds (i.e., XY compounds; X = B, Al, Ga, In, or Tl; Y = N, P, As, or Sb) have been intensively investigated for several decades because of their enormous applications for many optoelectronic devices. Here, by employing first-principles calculations, the electronic structures of bulk XY haeckelite compounds are examined. It is identified that InSb (TlN and TlP) is Dirac semimetal (are strong topological insulators). The other fifteen XY compounds are semiconducting. The effect of biaxial and uniaxial tensile and compressive strains on the electronic structures are studied. These materials offer diverse topological orders. The semiconducting band gaps are mainly found between the bonding and antibonding states of the mixed X(p)–Y(p) orbitals at the top of the valence band and the bottom of the conduction bands, respectively. The topological insulating nature of the XY compounds is explained based on the degenerate p _x  + p _y orbitals and their orbital energies relative to the p _z orbitals near the Fermi energy. The nontrivial band topologies of TlN and TlP are confirmed by calculating the Z ₂ (1;000) index, surface states, and Wilson loop calculations. The bands split into two branches by including spin-orbit interaction. The results demonstrate that haeckelite compounds are fascinating materials with broad potential applications in optoelectronics and possessing the possibility of hosting emergent physical phenomena.

The lattice mismatch between a monolayer of MoS 2 and its Au(111) substrate induces a moiré superstructure. The local variation of the registry between sulfur and gold atoms at the interface leads to a periodic pattern of strongly and weakly interacting regions. In consequence, also the electronic bands show a spatial variation. We use scanning tunneling microscopy and spectroscopy (STM/STS), x-ray photoelectron spectroscopy (XPS) and x-ray standing wave (XSW) for a determination of the geometric and electronic structure. The experimental results are corroborated by density functional theory. We obtain the geometric structure of the supercell with high precision, identify the fraction of interfacial atoms that are strongly interacting with the substrate, and analyze the variation of the electronic structure in dependence of the location within the moiré unit cell and the nature of the band.

The novel thermal properties of the Janus MoSSe/WSSe heterostructure are manipulated using thermo-mechanical coupling.

Abstract

2D Janus transition metal dichalcogenide (TMD) semiconductor materials have attracted great interest for their potential applications. Because of the increased requirement for thermal management in 2D devices with single-atom thickness, a fundamental understanding of interfacial thermal conduction (ITC) has emerging significance. In this work, the ITC of in-plane heterostructures constructed using MoSSe and WSSe is reported. In addition to the interface connected normally by MoSSe and WSSe with the same type of chalcogen atoms are on the same side of left and right sections, inversional interface by rotation of 180° of WSSe is also considered, in which S atoms are on the opposite side of the left and right sections. Interestingly, the ITC in the normally connected heterostructure is found to be almost twice as much as that in the inversely connected heterostructure. The unusually large change in ITC is attributed to the bending curvature and additional discontinuity in the inversely connected heterostructure. Euler–Bernoulli beam model gives further insight into the origin of such interface bending. The findings offer the very first insight into the phonon transport in Janus heterostructures, and benefit thermal management of 2D devices based on Janus monolayers.

A series of layered oxyselenides Bi₂LnO₄Cu₂Se₂ (Ln = Nd, Sm, Eu, Gd, Tb, Dy, Ho, and Er) are synthesized by a time-saving method. The origin of the excellent thermoelectric performance of Bi₂LnO₄Cu₂Se₂ is thoroughly investigated. A maximum ZT value of ≈0.27 at 923K is achieved in Bi₂DyO₄Cu₂Se₂, which proves to be a potential thermoelectric system for further investigation.

Abstract

Layered oxyselenides have been widely investigated as promising thermoelectric materials due to their unique merits such as super-lattice structural features and intrinsic complexity, which contributes to low thermal conductivity and easily controllable electrical properties. Newly developed Bi₂LnO₄Cu₂Se₂ (Ln stands for lanthanide) oxyselenides are found to be potential thermoelectric systems since they have excellent electrical conductivity over 10³ S cm⁻¹. In this work, unique energy and time-saving method combined self-propagating high-temperature synthesis (SHS) with spark plasma sintering (SPS) is adopted to successfully prepare a highly pure Bi₂LnO₄Cu₂Se₂ instead of a traditional solid-state reaction. To explore the most suitable lanthanide for Bi₂LnO₄Cu₂Se₂, thermoelectric performance in a wide temperature range (300 to 923 K) of Bi₂LnO₄Cu₂Se₂ (Ln = Nd, Sm, Eu, Gd, Tb, Dy, Ho, and Er) is deeply evaluated and studied. Ultimately, with a relatively high electrical conductivity, moderate Seebeck coefficient, and extremely low thermal conductivity, a maximum ZT value of ≈0.27 at 923K is achieved in Bi₂DyO₄Cu₂Se₂, which is 4 times larger than that of the ever-reported Bi₂YO₄Cu₂Se₂ and proves a potential thermoelectric system for further investigation. This work may provide some enlightenment and broaden the horizon in finding new thermoelectric materials, especially for complex layered compounds.

A novel approach is introduced for the production of integrated networks of graphene pn-junctions at the nanoscale. Via hydrogen intercalation, one-dimensionally confined graphene nanoribbons are transformed into a single 2D graphene layer rolling over 6H-SiC mesa structures. Due to the different surface terminations of the vicinal and basal SiC planes, different carrier types are induced into graphene, leading to an array of spatially well-defined pn-junctions.

Abstract

Graphene pn-junctions offer a rich portfolio of intriguing physical phenomena. They stand as the potential building blocks for a broad spectrum of future technologies, ranging from electronic lenses analogous to metamaterials in optics, to high-performance photodetectors important for a variety of optoelectronic applications. The production of graphene pn-junctions and their precise structuring at the nanoscale remains to be a challenge. In this work, a scalable method for fabricating periodic nanoarrays of graphene pn-junctions on a technologically viable semiconducting SiC substrate is introduced. Via H-intercalation, 1D confined armchair graphene nanoribbons are transformed into a single 2D graphene sheet rolling over 6H-SiC mesa structures. Due to the different surface terminations of the basal and vicinal SiC planes constituting the mesa structures, different types of charge carriers are locally induced into the graphene layer. Using angle-resolved photoelectron spectroscopy, the electronic band structure of the two graphene regions are selectively measured, finding two symmetrically doped phases with p-type being located on the basal planes and n-type on the facets. The results demonstrate that through a careful structuring of the substrate, combined with H-intercalation, integrated networks of graphene pn-junctions could be engineered at the nanoscale, paving the way for the realization of novel optoelectronic device concepts.

CsPbBrCl₂: Ln³⁺ PQDs are employed in a perovskite solar cell to achieve a “lattice to lattice” doping effect and passivate the intrinsic defects in MAPbI₃-based PSCs. CsPbBrCl₂: Ln³⁺ PQDs can adjust work function, optimize bandgap alignment, and form stronger Ln-X bonds, and displays a power conversion efficiency of 22.52% and a high V _oc of 1.20 V.

Abstract

The passivation effect of inorganic perovskite quantum dots (PQDs) is a promising method to attain outstanding performance in perovskite solar cells (PSCs), which has ignited widespread interest recently. Lanthanides (Ln) doped PQDs demonstrate unique properties, but nevertheless, are not explored in PSCs. In this work, four kinds of Ln³⁺ doped CsPbBrCl₂ PQDs (Ln³⁺ = Yb³⁺, Ce³⁺, Eu³⁺, Sm³⁺) are firstly introduced into PSCs, which displays the synergistic effect of composition engineering and defect engineering. The results indicate that the introduction of CsPbBrCl₂: Ln³⁺ can not only improve the crystallinity and passivate the intrinsic and surface defects of the MAPbI₃ layer through ion and ligand passivation, but also form a stronger LnI bond than PbI, adjust work function (W _F), and optimize band alignments. CsPbBrCl₂:Sm³⁺ PQDs possess the best performance and exhibit remarkable promotions of open-circuit voltage (V _oc) from 1.13 to 1.20 V and power conversion efficiency from 18.54% to 22.52%. The humid-resist, thermal-resist abilities, and the long-term stability of PSCs are energetically improved due to enhanced structure stability by Sm³⁺ doping and the hydrophobic characteristic. The strategy of Ln³⁺ doped PQDs applied to PSCs provide an approach to achieve high-performance PSCs.

A novel MoS₂ coating co-deposited with tantalum shows adaptable lubricating mechanisms that enable it to excel in both terrestrial and deep-space environments. Addition of tantalum within the MoS₂ structure shows equal stability in intercalated or substituted states of Ta in the MoS₂, which favors “adaptative” tribochemical response and tribofilm formation.

Abstract

Molybdenum disulfide coatings have been employed as lubricants for spacecraft since the 1950s but continue to face major engineering challenges including performance in both terrestrial air and deep space vacuum environments and service lifetimes on the order of decades without maintenance. Co-deposition of MoS₂ with additive compounds provide enhancements in some circumstances but a lubricant which can perform in all space-facing environments with long lifetimes remains an ongoing problem. Herein, it is demonstrated the multi-environment adaptable performance of a novel MoS₂ + tantalum lubricant coating, which excels as a lubricant in both terrestrial and space environments while the benchmark space-qualified commercial MoS₂ lubricants do not. It is noted that the 10% tantalum additive exhibits preferential oxidation in air to preserve the lubricating ability of MoS₂ while forming phases of TaS₂, which aid in the exceptional lubrication of MoS₂ in ultra-high vacuum. Additionally, completely different tribofilms of small particles and compact sheets are noted for air and vacuum environments, respectively, which allows for adaptable lubricating mechanisms from a single coating depending on the environment. This novel coating sets the benchmark as the first demonstrated instance of a fully versatile space lubricant which offers high-performance in both terrestrial and deep space environments.

It is demonstrated in the present study that efficient grain boundaries engineering via laser generated nanocrystals with tailored surface states for improved carriers dynamics can lead to the construction of perovskite solar cells with pronounced environmental stability and increased champion power conversion efficiency up to 23.74%.

Abstract

Grain boundaries (GBs) engineering of hybrid perovskite films is of significance for accessing high performance perovskite solar cells (PSCs), owing to the abundant defect states existed therein originating from the low temperature film processing. Nanocrystals embedding at GBs has shown profound advantages in carrier dynamics modulation, while the surface defects on nanocrystals in turn lead usually to the trapping of carriers at GBs. The authors herein demonstrate the efficient GBs engineering via laser generated nanocrystals with tailored surface states for improved carriers dynamics and environmental stability of PSCs. The embedding of La doped BaSnO₃ (LBSO) nanocrystals with bare surfaces in perovskite provides an additional channel to facilitate the effective carrier extraction and reduce the carrier recombination, leading to a maximum power conversion efficiency (PCE) of 21.11% with negligible hysteresis for the mixed-cation PSCs. To clarify the influence of surface defect states of the laser generated nanocrystals on the performance of PSCs, 1H,1H-perfluorooctylamine is grafted on LBSO nanocrystals during the laser irradiation, resulting in improved champion PCE up to 21.65% and pronounced environmental stability. The universal embedding of the LBSO nanocrystals with tailored surface states in different perovskite by fabricating FAPbI₃ PSCs with a champion PCE of 23.74% is further demonstrated.

Ln³⁺-based halide Cs₃TbCl₆ QDs are synthesized and introduced to the interface of perovskite films to promote better bandgap alignment and reduce interface defects density. The Cs₃TbCl₆ QDs modified device has achieved a super high open-voltage of 1.235 V. Cs₃TbCl₆ QDs and black phosphorus dual modified device has yielded a champion photoelectric conversion efficiency of 23.49% and a filling factor of 80.32%.

Abstract

Interfacial engineering is one of the most effective means to improve the photoelectric conversion efficiency (PCE) and stability of perovskite solar cells (PSCs). In this work, Ln³⁺-based halide Cs₃TbCl₆ quantum dots (QDs) are synthesized through a modified hot-injection method, which displays an excitonic emission centered at 431 nm and the characteristic emission peaks of Tb³⁺ ions. Then, the Ln³⁺-based halide Cs₃TbCl₆ QDs are introduced to the interface of Cs_0.05(FA_0.83MA_0.17)_0.95Pb(I_0.83Br_0.17)₃ perovskite films in the PSCs, which can regulate the energy levels, fill the grain boundaries and remove the ionic defects. Surprisingly, the Cs₃TbCl₆ QDs modified devices achieve a champion PCE of 22.89% with a super high open-voltage of 1.235 V. The high open-voltage can be mainly attributed to the better bandgap alignment, enhanced interface, and reduced defects density. Afterward, the hole transport layer (HTL) is modified by the black phosphorus QDs (BPQDs), yielding a champion PCE of 23.49% and a filling factor of 80.32%. The Cs₃TbCl₆ QDs modified unencapsulated device possesses well environmental stability and humidity stability. This work demonstrates a new kind of Ln³⁺-based metal QDs and explores a new approach to fabricate the PSCs with high open-voltage, high efficiency, and good stability through the QD-based passivation techniques.

Twist angle induces various Moiré-related properties in 2D materials, but most studies only focus on bilayer systems. Here, via a folding strategy, multilayer MoS₂ Moiré superlattices are fabricated whose interlayer coupling, indirect bandgap, and degree of circular polarization (DOCP) are tunable by twist angle. The highest DOCP for folded bilayer MoS₂ can reach 86% above liquid nitrogen temperature.

Abstract

Twist angle provides a new degree of freedom for 2D material modifications. In principle, the intrinsic properties of twisted multilayers can be regulated by twist angle between each adjacent layer and thus have greater tunability than widely studied bilayer structures. Considering its complexity, it is important to first investigate the simplest twisted multilayers with only one interface twisted. In this work, multilayer Moiré superlattices with only one twisted interface via paraffin-assisted folding of non-twisted stacked (highly symmetrically stacked) multilayer MoS₂ are successfully fabricated, and their twist-angle dependent optical properties are systematically studied. Compared to non-twisted stacked multilayer MoS₂, the one-interface-twisted multilayers show a 2–3.5 times higher PL intensity, and their interlayer coupling, indirect bandgap, and degree of circular polarization (DOCP) are tunable by twist angle. Notably, the DOCP for the one-interface-twisted four-layer (folded bilayer) can reach 86%, which is the highest value ever reported for transition metal dichalcogenide homostructures above liquid nitrogen temperature. This work provides a solid base for understanding twist-angle dependent properties of twisted multilayer 2D-materials.

Scalable deposition of high-quality Dion–Jacobson perovskite films via tailoring crystallization kinetics is reported. Notably retarded crystallization is realized by using a ternary solvent, which yields a film with improved crystallinity, highly vertical orientation, and graded phase distribution. The prepared solar devices exhibit an impressive open-circuit voltage of 1.21 V and remarkable stability under stimuli of light, heat, and humidity.

Abstract

Two-dimensional perovskites have attracted substantial attention for solar cell applications because of their higher stability as compared to their 3D analogs. To achieve efficient charge transport in thin-film devices, obtaining high crystalline perovskite crystals perpendicularly aligned to the substrate is of great importance. This article reports the scalable printing of high-quality Dion–Jacobson (DJ) perovskite thin films via tailoring crystallization kinetics. Introducing a small amount of 1-methyl-2-pyrrolidinone to the conventional N,N-dimethylformamide:dimethyl sulfoxide-based precursor, the strong coordination with ammonium spacers enables a notably retarded crystallization, which results in perovskite films with distinctly enhanced crystallinity, highly vertical orientation, and graded phase distribution. Accordingly, efficient charge generation and ultrafast interphase charge transfer are realized. The champion DJ perovskite device delivers a high current density of 17.10 mA cm^–2, an impressive open-circuit voltage of 1.21 V, leading to a stabilized efficiency of 16.19%. In addition, the devices processed from the ternary solvent exhibit remarkably improved stability under stimuli with light, heat, and humidity, benefiting from their superb phase stability. This work demonstrates an important advancement in scalable deposition of DJ perovskite thin films for efficient and stable photovoltaic devices.

Superior nonlinear optical responses from the near-infrared to visible range in non-centrosymmetric stacking spiral MoTe₂ nanopyramids are realized, enabled by their broken inversion symmetry, weak interlayer coupling, and strong light–matter interaction from the edge-rich quasi-multilayer structure. Moreover, the second-order nonlinear susceptibility of the spiral MoTe₂ is estimated to be around 1–2 order(s) of magnitude larger than those of most reported transition metal dichalcogenides.

Abstract

Transition metal dichalcogenides (TMDs) are of great promise for various nonlinear optical (NLO) applications due to their unique electronic and optoelectronic properties, such as tunable optical bandgap, strong spin–orbit coupling, and exciton effects. However, the desired NLO performances of regular 2H-TMDs are usually restricted by their limited absorption at atomic thickness. With this regard, a structurally novel spiral MoTe₂ (s-MoTe₂) nanopyramids is reported with unique and superior NLO response, enabled by their broken inversion symmetry, weak interlayer coupling, exciton resonance, and strong light–matter interaction from the edge-rich 3R-like quasi-multilayer structure. The excellent NLO response over a wide spectral range from the near-infrared to visible region is demonstrated, where second- and third-order NLO responses have been simultaneously observed. Moreover, the second-order nonlinear susceptibility of s-MoTe₂ is estimated to be around 1–2 order(s) of magnitude larger than those of most reported TMDs. The demonstration of a superior NLO response in such s-MoTe₂ not only paves a new way for designing the best NLO TMD structures, but also greatly prompts their practical applications in micro–nano NLO devices on chips in future.

Spatiotemporal Information Processing

In article number 2108440, Ye Zhou and co-workers design a 2D heterostructure-based memtransistor to emulate the Bienenstock–Cooper–Munro theory, since such structures not only induce a spontaneous forgetting process but also offer another gate-tunable forgetting effect. The BCM learning rule is perfectly demonstrated on this memtransistor using triplet-STDP. Furthermore, high-order spatiotemporal recognition is achieved in a feedforward neuron network.

High-performance Ni/epoxy/Bi_0.5Sb_1.5Te₃ magnetic flexible thermoelectric films are prepared by incorporating Ni nanoparticles. Te vacancies are induced by the orientation reaction between Ni-NPs and Te from Bi_0.5Sb_1.5Te₃, which triggers the presence of negatively charged V‴Bi(V‴Sb) and Bi′Te(Sb′Te) anti-site defects with atomic-sized electric field and further causes a novel acceleration movement and hopping migration of carriers.

Abstract

Improving the thermoelectric (TE) performance of Bi₂Te₃-based flexible films remains a huge challenge. Herein, high-performance Ni/epoxy/Bi_0.5Sb_1.5Te₃ magnetic flexible TE films are prepared by incorporating Ni nanoparticles (Ni-NPs). Atomic-resolution STEM investigation demonstrates that Te vacancies induced by the orientation reaction between Ni-NPs and Te from Bi_0.5Sb_1.5Te₃ trigger the presence of negatively charged V‴Bi(V‴Sb) and Bi′Te(Sb′Te) anti-site defects and atomic-sized electric field in the magnetic flexible TE films and further cause the acceleration movement and hopping migration of carriers. The transport measurements indicate an increased carrier concentration due to the anti-site defects, while the significant increase of carrier mobility originates from the acceleration movement of carriers. The magnetic scattering and hopping migration of carriers are responsible for maintaining large Seebeck coefficient. As compared to epoxy/Bi_0.5Sb_1.5Te₃ flexible TE film, the maximum power factor of the magnetic flexible TE film with 0.1% Ni-NPs reaches 2.74 mW m⁻¹ K⁻² at 300 K and increases by 61%, while the cooling temperature difference increases by 250%.

A single-crystal Pd(111) quantum dots on 2D graphdiyne (GDY) catalysts are used for large current density hydrogen evolution reactions. The GDY acts as an ideal platform for constructing highly selective and active electrocatalysts. The catalyst prepared by this method is environmental protection, uniform size, simple synthesis method, and the potential to achieve large-scale preparation without pollution.

Abstract

The key issue for industrial large-scale hydrogen production by water electrolysis is developing environment-friendly electrocatalysts that can work well at large current densities and low overpotentials. Thanks to the superior advantages of 2D graphdiyne (GDY) on chemical structure and the alkyne bond strong reductivity, the highly selective, in situ growth of the single-crystal Pd (111) quantum dots is achieved. The metal dots distribute uniformly and densely on the GDY surface (GDY-Pd1) in a controllable way at low temperatures without adding additional reductive agents. Experimental and theoretical results show that the 2D GDY affords an ideal platform to construct highly selective and active electrocatalysts with accurate structures, defined valence states, facilitated charge transfer ability, and enhanced electric conductivity for hydrogen evolution reaction. Remarkably, the electrocatalyst can reach 500 and 1000 mA cm⁻² at small overpotentials of only 201 and 261 mV, with high long-term stability, which are better than most of the reported ones. The results demonstrate that 2D DGY is an excellent support in the controllable synthesis of metal quantum dots with well-defined surface and structure and the potential to achieve large-scale preparation. This study takes a critical step toward industrial hydrogen production.

Nature Nanotechnology, Published online: 03 February 2022; doi:10.1038/s41565-021-01060-6

Graphene has a centrosymmetric crystal symmetry, which prohibits second-order effects in transport experiments. Yet, giant second-order nonlinear transports can emerge in graphene moiré superlattices at zero magnetic field, originating from the skew scattering of chiral Bloch electrons in the superlattice and giving rise to both longitudinal and transverse nonlinear conductivities under time-reversal symmetry.