Jing Zhang

Nature, Published online: 27 March 2023; doi:10.1038/s41586-023-05973-1

Hybrid 2D/CMOS microchips for memristive applications

Nature Electronics, Published online: 27 March 2023; doi:10.1038/s41928-023-00939-7

Oxide-based solid-state protonic electrochemical transistors that have symmetric operation and are compatible with CMOS technology can be used to create crossbar arrays for deep learning applications.

This review outlines a unique category of van der Waals materials that support novel devices for high-performance detection. Based on the insight into the unique properties of the material systems and the underlying microscopic mechanisms, emerging trends in junction devices are discussed, a new morphology of photodetectors is proposed, and some potential innovative directions in the subject area are suggested.

Abstract

With the continuous advancement of nanofabrication techniques, development of novel materials, and discovery of useful manipulation mechanisms in high-performance applications, especially photodetectors, the morphology of junction devices and the way junction devices are used are fundamentally revolutionized. Simultaneously, new types of photodetectors that do not rely on any junction, providing a high signal-to-noise ratio and multidimensional modulation, have also emerged. This review outlines a unique category of material systems supporting novel junction devices for high-performance detection, namely, the van der Waals materials, and systematically discusses new trends in the development of various types of devices beyond junctions. This field is far from mature and there are numerous methods to measure and evaluate photodetectors. Therefore, it is also aimed to provide a solution from the perspective of applications in this review. Finally, based on the insight into the unique properties of the material systems and the underlying microscopic mechanisms, emerging trends in junction devices are discussed, a new morphology of photodetectors is proposed, and some potential innovative directions in the subject area are suggested.

A new 2D perovskite scintillator, Cs₂CdBr₂Cl₂:5%Mn²⁺, is reported. With near-unity PLQY and negligible self-absorption, Cs₂CdBr₂Cl₂:5%Mn²⁺ exhibits excellent X-ray scintillation performance, including high light yield (64 950 photons MeV⁻¹) and low detection limits (17.82 nGy_air s⁻¹). A flexible scintillator screen combining Cs₂CdBr₂Cl₂:5%Mn²⁺ and PDMS enables low-dose X-ray imaging with a high spatial resolution of 12.3 line pairs (lp) mm⁻¹.

Abstract

High-performance X-ray scintillators with low detection limits and high light yield are of great importance and are a challenge for low-dose X-ray imaging in medical diagnosis and industrial detection. In this work, the synthesis of a new 2D perovskite, Cs₂CdBr₂Cl₂, via hydrothermal reaction is reported. By doping Mn²⁺ into the perovskite, a yellow emission located at 593 nm is obtained, and the photoluminescence quantum yield (PLQY) of Cs₂CdBr₂Cl₂:5%Mn²⁺ perovskite reaches the highest value of 98.52%. The near-unity PLQY and negligible self-absorption of Cs₂CdBr₂Cl₂:5%Mn²⁺ enable excellent X-ray scintillation performance with a high light yield of 64 950 photons MeV⁻¹ and low detection limit of 17.82 nGy_air s⁻¹. Moreover, combining Cs₂CdBr₂Cl₂:5%Mn²⁺ with poly(dimethylsiloxane) to fabricate a flexible scintillator screen achieves low-dose X-ray imaging with a high resolution of 12.3 line pairs (lp) mm⁻¹. The results suggest that Cs₂CdBr₂Cl₂:5%Mn²⁺ is a promising candidate for low-dose and high-resolution X-ray imaging. The study presents a new approach to designing high-performance scintillators through metal-ion doping.

Exquisite improvements are witnessed for MXene memristors. Conventional memory cells confront the demands of future data-intensive computing applications. This sortation has led the research on constructing the memristors with novel 2D functional materials for advanced applications of next-generation memory technology. MXene for high-density computing and synapse functionality at low power (among all Ti₃C₂) with edge detection applicability are manifested.

Abstract

With the demand for low-power-operating artificial intelligence systems, bio-inspired memristor devices exhibit potential in terms of high-density memory functions and the emulation of the synaptic dynamics of the human brain. The 2D material MXene attracts considerable interest for use in resistive-switching memory and artificial synapse devices owing to its excellent physicochemical properties in memristor devices. However, few memristive and synaptic MXene devices that display increased switching performances are reported, with no significant results. Herein, the conductivity of MXene (Ti₃C₂T_x) is engineered via etching and oxidation to enhance the switching performance of the device. The exceptional properties of partially oxidized MXene memristors include large memory windows and low threshold biases, and the complex spike-timing-dependent plasticity synaptic rules are also emulated. The low threshold potential distribution, reliable retention time (10⁴ s), and distinct resistance states with a high ON–OFF ratio (>10⁴) are the main memory-related features of this device. The experimentally determined switching potentials of the optimized device are also uniformly distributed, according to a statistical probability-based approach. This investigation may promote the essential material properties for use in high-density non-volatile memory storage and artificial synapse systems in the field of innovative nanoelectronic devices.

Nature Communications, Published online: 24 March 2023; doi:10.1038/s41467-023-37337-8

Sliding and twisting of van der Waals layers can produce fascinating physical phenomena. Here, authors show that moiré polar domains in bilayer hBN give rise to a topologically non-trivial winding of the polarization field, forming networks of merons and antimerons.

Nature Communications, Published online: 24 March 2023; doi:10.1038/s41467-023-37076-w

Berry curvature sits at the heart of both the anomalous hall effect and topological hall effect, with the former arising from a momentum space berry curvature, while the latter arises from a real space berry curvature. Here, Li et al present an intriguing example of a combined real and reciprocal space berry curvature in the kagome material Mn3Sn, resulting in a large field linear anomalous Hall effect.

Room-temperature skyrmions are realized in a non-collinear antiferromagnet, Mn₃Sn, capped with a Pt overlayer via tuning the magnitude of the interfacial Dzyaloshinskii–Moriya interaction. Besides, the temperature-induced novel transition from skyrmions to antiferromagnetic meron-like spin textures at ≈220 K is also observed in the Mn₃Sn/Pt samples. This may stimulate the discovery of new topological spin textures in antiferromagnets.

Abstract

Topologically protected magnetic “whirls” such as skyrmions in antiferromagnetic materials have recently attracted extensive interest due to their nontrivial band topology and potential application in antiferromagnetic spintronics. However, room-temperature skyrmions in natural metallic antiferromagnetic materials with merit of probable convenient electrical manipulation have not been reported. Here, room-temperature skyrmions are realized in a non-collinear antiferromagnet, Mn₃Sn, capped with a Pt overlayer. The evolution of spin textures from coplanar inverted triangular structures to Bloch-type skyrmions is achieved via tuning the magnitude of interfacial Dzyaloshinskii–Moriya interaction. Beyond that, the temperature can induce an unconventional transition from skyrmions to antiferromagnetic meron-like spin textures at ≈220 K in the Mn₃Sn/Pt samples. Combining with the theoretical calculations, it is found that the transition originates from the temperature dependence of antiferromagnetic exchange interaction between kagome sublayers within the Mn₃Sn crystalline unit-cell. These findings open the avenue for the development of topological spin-swirling-based antiferromagnetic spintronics.

Ferroelectric materials are great candidates for nonlinear optics and electro-optic modulators. A giant second-harmonic generation effect is reported in physical vapor-deposited ferroelectric material, SnSe. Nanoscale in-plane ferroelectric domains are revealed, those with ferroelectric stacking exhibit ≈100 times higher nonlinear optical efficiency than monolayer TMDs, due to a parallel stacking structure where nonlinear dipoles in each vdW ferroelectric layer add constructively.

Abstract

Thin ferroelectric materials hold great promise for compact nonvolatile memory and nonlinear optical and optoelectronic devices. Herein, an ultrathin in-plane ferroelectric material that exhibits a giant nonlinear optical effect, group-IV monochalcogenide SnSe, is reported. Nanometer-scale ferroelectric domains with ≈90°/270° twin boundaries or ≈180° domain walls are revealed in physical-vapor-deposited SnSe by lateral piezoresponse force microscopy. Atomic structure characterization reveals both parallel and antiparallel stacking of neighboring van der Waals ferroelectric layers, leading to ferroelectric or antiferroelectric ordering. Ferroelectric domains exhibit giant nonlinear optical activity due to coherent enhancement of second-harmonic fields and the as-resulted second-harmonic generation was observed to be 100 times more intense than monolayer WS₂. This work demonstrates in-plane ferroelectric ordering and giant nonlinear optical activity in SnSe, which paves the way for applications in on-chip nonlinear optical components and nonvolatile memory devices.

Scanning tunneling microscopy is used to study the vicinity of defects in the Kondo lattice of samarium hexaboride (SmB6).

A bismuth samarium oxide thin film on a substrate maintains its ferroelectricity at a thickness of only 1 nanometer.

Solid-state reactions formed vertical carpets of 2D metal carbides and nitrides on metal substrates.

A bismuth samarium oxide thin film on a substrate maintains its ferroelectricity at a thickness of only 1 nanometer.

The performance of electronic and optoelectronic devices is dominated by charge carrier injection through the metal–semiconductor contacts. Therefore, creating low-resistance electrical contacts is one of the most critical challenges in the development of devices based on new materials, particularly in the case of two-dimensional semiconductors. Herein, we report a strategy to reduce the contact resistance of MoS2 via local pressurization. We fabricated electrical contacts using an atomic force microscopy tip and applied variable pressure ranging from 0 to 25 GPa. By measuring the transverse electronic transport properties, we show that MoS2 undergoes a reversible semiconducting-metallic transition under pressure. Planar devices in field effect configuration with electrical contacts performed at pressures above ∼15 GPa show up to 30-fold reduced contact resistance and up to 25-fold improved field-effect mobility when compared to those measured at low pressure. Theoretical simulations show that this enhanced performance is due to improved charge injection to the MoS2 semiconductor channel through the metallic MoS2 phase obtained by pressurization. Our results suggest a novel strategy for realizing improved contacts to MoS2 devices by local pressurization and for exploring emergent phenomena under mechano-electric modulation.

Tetragonal phase LiErF₄:0.5%Tm³⁺@LiYF₄ is tailored as local structure-sensitive upconversion hosts, and achieves a unique excitation wavelength-dependent enhancement of upconversion luminescence by local structural engineering-pressure. The present work not only offers new insights into the understanding of the relationship between the local structure and optical properties of upconversion nanoparticles, but also suggests prospects for the future exploration of advanced luminescent materials.

Abstract

Local structural engineering is an endogenous approach to modulate upconversion luminescence (UCL) from upstream to meet the needs of specific application scenarios. Herein, high pressure is utilized as a means to modulate the local structure, and the designed LiErF₄:0.5%Tm³⁺@LiYF₄ (Er:Tm@Y) nanoparticles with fast energy transfer rates, abundant cross-relaxation processes, and multiple near-infrared wavelengths (808, 980, 1530 nm) excitation properties are tailored as local structure-sensitive hosts. A unique excitation wavelength-dependent UCL enhancement of Er:Tm@Y upconversion nanoparticles is observed by pressure-induced local structure distortions. When the pressure of ≈6 GPa is applied, the UCL is enhanced by a factor of 2.6 at 980 nm excitation only. After pressure release, the luminescence diminishes and recovers. Density functional theory calculations show that the symmetry distortion of the LiErF₄ crystal reaches a maximum at pressurization to 6 GPa, while a new Er-4f state emerges, greatly reducing the bandgap from 8.3 to 5.7 eV. Comparative experiments demonstrate that the local symmetry distortion caused by 0.5%Tm³⁺ doping and the different energy transfer patterns of Er³⁺ to Tm³⁺ at different excitations are responsible for this wavelength-dependent luminescence enhancement.

This review summarizes the recent progress in grown methods of perovskite single crystals (SCs), and then gives a detailed demonstration of the intrinsic properties of perovskite SCs such as optical properties, defects, charge carrier dynamics, ion migration and stability. On these base, the applications of perovskite SCs in various optoelectronic devices including solar cells, photodetectors, and X-ray detectors are discussed.

Abstract

Metal halide perovskite have shown great potential for applications in photovoltaics, light-emitting diodes and photon detectors, mainly owing to their superb optoelectronic properties, low-cost raw materials and facile fabrication process. Although, polycrystalline perovskite thin-films have been actively investigated for preparing various optoelectronic devices, the presence of detrimental defects at grain boundaries, serious ion migration and limited stability unfortunately hinder their device performance and practical application. As a contrast, perovskite single crystals (SCs) exhibit no grain boundaries, much lower trap density and much improved stability, hence providing a more attractive choice for not only optoelectronic device applications but also fundamental research. In this review, recent progress in the growth methods of perovskite SCs is summarized, followed by giving a detailed introduction of the intrinsic properties of perovskite SCs including optical properties, defects, charge carrier dynamics, ion migration and stability. On these base, the applications of perovskite SCs in various optoelectronic devices like solar cells, photodetectors, and radiation detectors are discussed, where the relationship between the composition, device architecture and device performance is highlighted. Finally, a tentative discussion on the current challenges and future opportunities in the development of perovskite SCs and optoelectronic devices is presented.

Nature, Published online: 22 March 2023; doi:10.1038/s41586-023-05797-z

The epitaxial synthesis of high-density, vertically aligned arrays of two-dimensional (2D) fin-oxide heterostructures is described, enabling the fabrication of 2D fin field-effect transistors with high electron mobility and desirable low-power specifications.

Nature, Published online: 22 March 2023; doi:10.1038/s41586-023-05819-w

A two-dimensional field-effect transistor made of indium selenide is shown to outperform state-of-the-art silicon-based transistors, operating at lower supply voltage and achieving record high transconductance and ballistic ratio.

An ultrathin crystalline ferroelectric polymer based on trichloromethyl (CCl₃)-terminated poly(vinylidene difluoride-co-trifloroethylene) (P(VDF-TrFE)) is developed on AlO _X to realize negative-capacitance metal–oxide–semiconductor field-effect transistors (MOSFETs) with a MoS₂ channel. Systematic adjustment of the thickness ratio between the polymer and dielectric AlO _X achieves a hysteresis-free FET with a minimum subthreshold swing of 28 mV dec⁻¹, operating at less than 2 V.

Abstract

Negative-capacitance field-effect transistors (NC-FETs) have gathered enormous interest as a way to reduce subthreshold swing (SS) and overcome the issue of power dissipation in modern integrated circuits. For stable NC behavior at low operating voltages, the development of ultrathin ferroelectrics (FE), which are compatible with the industrial process, is of great interest. Here, a new scalable ultrathin ferroelectric polymer layer is developed based on trichloromethyl (CCl₃)-terminated poly(vinylidene difluoride-co-trifloroethylene) (P(VDF-TrFE)) to achieve the state-of-the-art performance of NC-FETs. The crystalline phase of 5–10 nm ultrathin P(VDF-TrFE) is prepared on AlO _X by a newly developed brush method, which enables an FE/dielectric (DE) bilayer. FE/DE thickness ratios are then systematically tuned at ease to achieve ideal capacitance matching. NC-FETs with optimized FE/DE thickness at a thickness limit demonstrate hysteresis-free operation with an SS of 28 mV dec⁻¹ at ≈1.5 V, which competes with the best reports. This P(VDF-TrFE)-brush layer can be broadly adapted to NC-FETs, opening an exciting avenue for low-power devices.

Ti₃C₂T_x MXene films are integrated into GaN high electron mobility transistors (HEMTs) as gate contacts, wherein van der Waals heterojunctions are formed between MXene films and GaN without direct chemical bonding. The Schottky gate GaN HEMTs with enhanced gate controllability exhibit a record high I _ON/I _OFF current ratio of ≈10¹³ and a near-ideal subthreshold swing of 61 mV dec⁻¹.

Abstract

Gate controllability is a key factor that determines the performance of GaN high electron mobility transistors (HEMTs). However, at the traditional metal-GaN interface, direct chemical interaction between metal and GaN can result in fixed charges and traps, which can significantly deteriorate the gate controllability. In this study, Ti₃C₂T_x MXene films are integrated into GaN HEMTs as the gate contact, wherein van der Waals heterojunctions are formed between MXene films and GaN without direct chemical bonding. The GaN HEMTs with enhanced gate controllability exhibit an extremely low off-state current (I _OFF) of 10⁻⁷ mA mm⁻¹, a record high I _ON/I _OFF current ratio of ≈10¹³ (which is six orders of magnitude higher than conventional Ni/Au contact), a high off-state drain breakdown voltage of 1085 V, and a near-ideal subthreshold swing of 61 mV dec⁻¹. This work shows the great potential of MXene films as gate electrodes in wide-bandgap semiconductor devices.

Nature Nanotechnology, Published online: 20 March 2023; doi:10.1038/s41565-023-01340-3

Multiple alternating layers of two-dimensional materials and epilayers are grown on III–N and III–V substrates in a single growth run. Then, each epilayer is harvested by mechanical exfoliation, producing multiple freestanding membranes from a single wafer.

Nature Nanotechnology, Published online: 20 March 2023; doi:10.1038/s41565-023-01349-8

Differential phase contrast scanning transmission electron microscopy probes the electric field distribution across a GaN-based semiconductor heterointerface.

Heterosynaptic MoS₂ memtransistors are developed to enable tunable synaptic plasticity by applying paired voltage pulses at the drain and gate terminals serving as presynaptic neurons and neuromodulators, respectively, for energy-efficient neuromorphic electronics. Consequently, a SET voltage of 2.0 V and sub-1 fJ energy consumption are achieved, which are the best recorded values for three-terminal synaptic devices reported so far.

Abstract

Heterosynaptic neuromodulation is a key enabler for energy-efficient and high-level biological neural processing. However, such manifold synaptic modulation cannot be emulated using conventional memristors and synaptic transistors. Thus, reported herein is a three-terminal heterosynaptic memtransistor using an intentional-defect-generated molybdenum disulfide channel. Particularly, the defect-mediated space-charge-limited conduction in the ultrathin channel results in memristive switching characteristics between the source and drain terminals, which are further modulated using a gate terminal according to the gate-tuned filling of trap states. The device acts as an artificial synapse controlled by sub-femtojoule impulses from both the source and gate terminals, consuming lower energy than its biological counterpart. In particular, electrostatic gate modulation, corresponding to biological neuromodulation, additionally regulates the dynamic range and tuning rate of the synaptic weight, independent of the programming (source) impulses. Notably, this heterosynaptic modulation not only improves the learning accuracy and efficiency but also reduces energy consumption in the pattern recognition. Thus, the study presents a new route leading toward the realization of highly networked and energy-efficient neuromorphic electronics.

An Fe-assisted epitaxial strategy to synthesize 4 in. length transition-metal dichalcogenides (TMDCs) single crystals on c-plane sapphire is designed, ultrahigh mobility and remarkable on/off current ratio are discovered due to the ultralow contact resistance. The introduction of Fe decreases the formation energy of parallel steps on sapphire surfaces and contributes to the edge-nucleation of unidirectional alignment of TMDCs domains.

Abstract

Epitaxial growth and controllable doping of wafer-scale atomically thin semiconductor single crystals are two central tasks to tackle the scaling challenge of transistors. Despite considerable efforts are devoted, addressing such crucial issues simultaneously under 2D confinement is yet to be realized. Here, an ingenious strategy to synthesize record-breaking 4 in. length Fe-doped transition-metal dichalcogenides (TMDCs) single crystals on industry-compatible c-plane sapphire without special miscut angle is designed. Atomically thin transistors with high electron mobility (≈146 cm² V⁻¹ s⁻¹) and remarkable on/off current ratio (≈10⁹) are fabricated based on 4 in. length Fe-MoS₂ single crystals, due to the ultralow contact resistance (≈489 Ω µm). In-depth characterizations and theoretical calculations reveal that the introduction of Fe significantly decreases the formation energy of parallel steps on sapphire surfaces and contributes to the edge-nucleation of unidirectional alignment TMDCs domains (>99%). This work represents a substantial leap in terms of bridging synthesis and doping of wafer-scale 2D semiconductor single crystals, which should promote the further device downscaling and extension of Moore's law.

Nature Electronics, Published online: 20 March 2023; doi:10.1038/s41928-023-00941-z

The Curie temperature of Fe5+xGeTe2 thin films can be modulated from 260 to 380 K via iron doping, allowing the two-dimensional material to be used to create planar spiral inductors and low-pass Butterworth filters.

Nature Electronics, Published online: 20 March 2023; doi:10.1038/s41928-023-00940-0

Micro-light-emitting-diode display applications are growing quickly as technology companies begin to use them in a range of products. Key to the development of these applications was the miniaturization of gallium nitride light-emitting diodes. Hongxing Jiang and Jingyu Lin recount how this was achieved.

Nature Materials, Published online: 20 March 2023; doi:10.1038/s41563-023-01492-6

This Review discusses the field of antiferromagnetic spintronics with a focus on coherent effects.