Jing Zhang

Exceeding the Shockley–Queisser limit for single-junction photovoltaic cells is highly desirable for exploring and discovering new materials and outcome-based approaches. When the threefold rotational and mirror symmetry of 2D material devices is broken by strain-gradient engineering, induced polarization in van der Waals interfaces, or transforming a 2D monolayer into a nanotube, shift-current responses are enabled.

Abstract

It is highly desirable for exploring and discovering new materials and outcome-based approaches to exceed the Shockley–Queisser limit for single-junction photovoltaic cells. Low-dimensional piezoelectric materials have the potential to generate the optoelectronic phenomenon called the bulk photovoltaic effect, which is not limited by the theoretical limit for solar radiation into electricity conversion. The recent development of 2D materials has demonstrated that by using the bulk photovoltaic effect (BPVE) for crystals lacking inversion symmetry, it is possible to overcome this limit. So far, the exploration of p–n junction designs has been addressed in several review articles. However, the mechanism of BPVE differs from traditional p–n junction-based photovoltaics in 2D materials. In this focused review, various concepts regarding the shift-current response are explored, both from theoretical and experimental points of view, which are generated in the framework of deformed 2D materials. Finally, prospective approaches for building BPVE-based next-generation solar cells using ultrathin 2D materials are presented. These materials are expected to work better than current methods of turning energy into electricity.

Free-standing monolayer Si₂Te₂ (MLSi₂Te₂) is theoretically predicted to have a mid-infrared bandgap. However, bulk Si₂Te₂ does not exist naturally. In this work, the successful epitaxial growth of high-quality MLSi₂Te₂ films on a Sb₂Te₃ substrate is reported. Using scanning tunneling spectroscopy, the local electronic structure of MLSi₂Te₂ on Sb₂Te₃ is systematically characterized and compared with density functional theory calculations.

Abstract

The experimental realization of large-scale, homogeneous semiconducting films with a single-layer thickness is of major importance for next-generation devices. Especially in view of the compatibility with state-of-the-art semiconductor technology, Si-based monolayer crystals are of particular interest. Here, the successful epitaxial growth of monolayer Si₂Te₂ (MLSi₂Te₂) films on semiconducting Sb₂Te₃ thin film substrates is reported. High-quality (1 × 1) ML-Si₂Te₂ films with a coverage as high as 95% are obtained as revealed by scanning tunneling microscopy. X-ray photoelectron spectroscopy confirms the existence of the SiTe bonds in the obtained films. By combining scanning tunneling spectroscopy with density functional theory calculations, the existence of a semiconducting bandgap is demonstrated on the order of 370 meV for the MLSi₂Te₂ films which reside in an important mid-infrared spectral range. The results pave the way for practical applications of this novel artificial two-dimensional material.

Small-scale robots (SSRs) have emerged as promising tools in a variety of biomedical, sensing, and decontamination applications. The review encompasses the basic understanding, theory, and up-to-date developments related to SSRs with tailored wettabilities, ranging from hydrophilic to hydrophobic.

Abstract

Small-scale robots (SSRs) have emerged as promising and versatile tools in various biomedical, sensing, decontamination, and manipulation applications, as they are uniquely capable of performing tasks at small length scales. With the miniaturization of robots from the macroscale to millimeter-, micrometer-, and nanometer-scales, the viscous and surface forces, namely adhesive forces and surface tension have become dominant. These forces significantly impact motion efficiency. Surface engineering of robots with both hydrophilic and hydrophobic functionalization presents a brand-new pathway to overcome motion resistance and enhance the ability to target and regulate robots for various tasks. This review focuses on the current progress and future perspectives of SSRs with hydrophilic and hydrophobic modifications (including both tethered and untethered robots). The study emphasizes the distinct advantages of SSRs, such as improved maneuverability and reduced drag forces, and outlines their potential applications. With continued innovation, rational surface engineering is expected to endow SSRs with exceptional mobility and functionality, which can broaden their applications, enhance their penetration depth, reduce surface fouling, and inhibit bacterial adhesion.

Intrinsic threshold breakdown voltage with an ultrahigh gain is observed in an avalanche photodetector (APD) based on a graphite/InSe Schottky junction, which is attributed to the high ionization rate due to the reduced dimensionality of electron–phonon scattering in layered InSe. This work opens up a new avenue for future APDs with both low energy consumption and high sensitivity.

Abstract

Realizing both ultralow breakdown voltage and ultrahigh gain is one of the major challenges in the development of high-performance avalanche photodetector. Here, it is reported that an ultrahigh avalanche gain of 3 × 10⁵ can be realized in the graphite/InSe Schottky photodetector at a breakdown voltage down to 5.5 V. Remarkably, the threshold breakdown voltage can be further reduced down to 1.8 V by raising the operating temperature, approaching the theoretical limit of 1.5 Eg\[{{\cal E}_{\bf g}}\]/e, with Eg${{\cal E}_{\bf g}}$ the bandgap of semiconductor. A 2D impact ionization model is developed and it is uncovered that observation of high gain at low breakdown voltage arises from reduced dimensionality of electron–phonon scattering in the layered InSe flake. These findings open up a promising avenue for developing novel weak-light detectors with low energy consumption and high sensitivity.

Nature Materials, Published online: 15 September 2022; doi:10.1038/s41563-022-01354-7

The authors use circularly polarized light pulses to trigger all-optical magnetization switching in an atomically thin ferromagnetic semiconductor. The switching process is related to spin angular momentum transfer from photoexcited carriers to local magnetic moments.

Nature Electronics, Published online: 15 September 2022; doi:10.1038/s41928-022-00824-9

The ultraviolet-assisted intercalative oxidation of high-mobility two-dimensional semiconductor Bi2O2Se can be used to create a single-crystalline native oxide dielectric—β-Bi2SeO5—that can yield top-gated transistors with an equivalent oxide thickness of 0.41 nm.

The underlying mechanism of how the phonon transport in layered material is affected by twisting is revealed with machinelearning based potential. The huge reduction of the thermal conductivity after twisting arises from the almost flat acoustic phonon branches and the enhanced third- and fourth-order phonon anharmonicity due to the strong coupling between the twisted layers.

Abstract

Although the twisting strategy has provided great opportunities to tune the electronic and optical properties of materials, little research has been done on how twisting affects phonon properties. Using machine-learning-based interatomic potentials within DFT-level quality and the perturbation theory to the fourth-order anharmonicity, the phonon transport properties are studied and the phonon behaviors of layered material Bi₂O₂Se when twisting is applied. It is found that the phonons of Bi₂O₂Se exhibit hardening effects at finite temperature, and the intrinsic lattice thermal conductivity along the out-of-plane (in-plane) direction is reduced to 3.21 (3.42) W/mK from 3.69 (4.55) W/mK at 300 K by including the four-phonon scattering. When introducing the twisting between the layers, the out-of-plane thermal conductivity can be further reduced by 83% as compared to that of the twist-free configuration. Such huge reduction of the thermal conductivity arises from the nearly flat acoustic phonon branches and the enhanced third- and fourth-order phonon anharmonicity due to the strong coupling between the twisted layers. These findings unravel that twisting is an effective strategy for tuning phonon band structure and phonon-phonon interactions, leading to ultralow lattice thermal conductivity of materials.

MX₂-based devices for high-performance circuits are expected to be introduced in production after the Si-sheet-based CFET. A stacked-sheet MX₂ device will look like that prepared by process simulation. The status of the process development needed to produce such a device in a fab environment, and the gaps to be overcome are described.

Abstract

Large-area 2D-material-based devices may find applications as sensor or photonics devices or can be incorporated in the back end of line (BEOL) to provide additional functionality. The introduction of highly scaled 2D-based circuits for high-performance logic applications in production is projected to be implemented after the Si-sheet-based CFET devices. Here, a view on the requirements needed for full wafer integration of aggressively scaled 2D-based logic circuits, the status of developments, and the definition of the gaps to be bridged is provided. Today, typical test vehicles for 2D devices are single-sheet devices fully integrated in a lab environment, but transfer to a more scaled device in a fab environment has been demonstrated. This work reviews the status of the module development, including considerations for setting up fab-compatible process routes for single-sheet devices. While further development on key modules is still required, substantial progress is made for MX₂ channel growth, high-k dielectric deposition, and contact engineering. Finally, the process requirements for building ultra-scaled stacked nanosheets are also reflected on.

Negative Differential Resistance

In article number 2202799, a reconfigurable, multiple negative differential resistance (m-NDR) device, which features electric-field-induced tunability of multiple threshold voltages, is presented by Jin-Hong Park and co-workers. Its reconfigurability is verified in terms of the function of a multi-valued logic circuit composed of a reconfigurable m-NDR device and a load transistor; staggered-type and broken-type double peak-NDR device operations are adopted for ternary inverter and latch circuits, respectively.

Polycrystalline ErMnO₃ displays an anomalous domain size/grain size scaling behavior in comparison to classical ferroelectric materials, such as BaTiO₃ or Pb(Zr,Ti)O₃. The fundamental difference is due to the formation of topologically protected vortex/anti-vortex pairs in polycrystalline ErMnO₃ and their interaction with elastic strain fields.

Abstract

The research on topological phenomena in ferroelectric materials has revolutionized the way people understand polar order. Intriguing examples are polar skyrmions, vortex/anti-vortex structures, and ferroelectric incommensurabilties, which promote emergent physical properties ranging from electric-field-controllable chirality to negative capacitance effects. Here, the impact of topologically protected vortices on the domain formation in improper ferroelectric ErMnO₃ polycrystals is studied, demonstrating inverted domain scaling behavior compared to classical ferroelectrics. It is observed that as the grain size increases, smaller domains are formed. Phase field simulations reveal that elastic strain fields drive the annihilation of vortex/anti-vortex pairs within the grains and individual vortices at the grain boundaries. The inversion of the domain scaling behavior has far-reaching implications, providing fundamentally new opportunities for topology-based domain engineering and the tuning of the electromechanical and dielectric performance of ferroelectrics in general.

Nature Communications, Published online: 05 September 2022; doi:10.1038/s41467-022-32884-y

Silicon is an abundant element on earth and is perfectly compatible with the well-established CMOS processing industry. Here, Sun et al. demonstrate multifunctional neuromorphic devices based on silicon nanosheet stacks, bringing silicon back as a potential material for neuromorphic devices.

Nature Materials, Published online: 08 September 2022; doi:10.1038/s41563-022-01352-9

The variation in the properties of rare earth (RE) steels is shown to stem from the presence of oxygen-based inclusions, and only under very-low-oxygen conditions can RE elements perform a vital role in purifying, modifying and micro-alloying steels.

Nature, Published online: 07 September 2022; doi:10.1038/s41586-022-05024-1

Excitons in the electronvolts range are found to couple strongly to coherent magnons in hundreds of microelectronvolts in an atomically thin two-dimensional antiferromagnetic semiconductor.

Nonlinear anomalous (NLA) Hall effect is the Berry curvature dipole induced second-order Hall voltage or temperature difference induced by a longitudinal electric field or temperature gradient. These are the prominent Hall responses in time-reversal symmetric systems. These band-geometry induced responses in recently realized twistronic platforms can probe their novel electronic band structure and topology. Here, we investigate the family (electrical, thermoelectric, and thermal) of second-order NLA Hall effects in the moiré system of twisted double bilayer graphene (TDBG). We combine the semiclassical transport framework with the continuum model of TDBG to demonstrate that the NLA Hall signals can probe topological phase transitions in moiré systems. We show that the whole family of NLA Hall responses undergo a sign reversal across a topological phase transition. Our study establishes a deeper connection between valley topology and nonlinear Hall effects in time-reversal symmetric systems.

Nature Electronics, Published online: 01 September 2022; doi:10.1038/s41928-022-00808-9

A method that minimizes strain and doping can be used to fabricate metal contacts to encapsulated ultraclean tungsten diselenide monolayers with contact resistances of 5 kΩ μm and transfer lengths of 1 μm.

Herein, it is demonstrated that hybrid biological bots based on pollen microparticles (SFPµP-BioBots) are able to attract, manipulate, and kill ovarian cancer cells. These electrostatic forces are generated by the negative and positive surface charge of cancer cells and SFPµP-BioBots, respectively. As a result, the strong attraction between them allows the doxorubicin delivery close to the surrounding of cancer cells.

Abstract

Naturally occurring micro/nanoparticles provide an incredible array of potential sources when preparing hybrid micro/nanorobots and their intrinsic properties can be exploited as multitasking functionalities of modern robotics as well as ensuring their mass production availability. Herein, magnetic biological bots (BioBots) prepared from defatted sunflower pollen microparticles by ferromagnetic metal layer evaporation on one side of its surface are described. It is demonstrated that the methodology employed introduces magnetic properties to sunflower pollen microparticles-based BioBots and enable their magnetic actuation. Interestingly, as-prepared magnetic sunflower pollen-based BioBots can naturally attract cancer cells due to their opposite charges (positive and negative, respectively). Such attracted cancer cells can then be transported by microrobots. This strong attraction also allows the delivery of drugs intended to kill the cancer cells. Sunflower-based BioBots can be fabricated in large quantities, and are naturally programmable, making them promising candidates for cancer cell therapy.

Well-controlled van der Waals trilayer MoS₂ is used as the semiconductor channel, which can successfully address the long-standing issue of performance degradation from physical-vapor deposited metal contact, leading to the demonstration of high-speed MoS₂ transistors with a high drain current (589 µA µm⁻¹ at V _ds = 1 V) as well as record-high saturation velocity (4.2 × 106 cm s⁻¹) at room temperature.

Abstract

Two-dimensional MoS₂ field-effect transistors (FETs) have great potential for next-generation electronics due to their excellent electronic properties with an atomic thin channel. However, multiple challenges exist for the monolayer MoS₂ channel, including interface scattering and ohmic contact. In this work, well-controlled trilayer MoS₂ with high mobility and large single crystals is successfully grown on soda-lime glass substrates using chemical vapor deposition, with a lateral size of up to 148 µm, which is the largest reported size to date. A record high on/off ratio of ≈10¹² and a high carrier mobility of 62 cm² V⁻¹ s⁻¹ of trilayer MoS₂ FETs are demonstrated, showing notable advantages compared with the monolayer counterpart. The long-standing issue of monolayer MoS₂ performance degradation from physical vapor deposited metal contact can be mitigated by the trilayer MoS₂ channel, achieving the lowest contact resistance of 350 Ω µm using the common method of e-beam evaporated Ni. Moreover, 40-nm channel-length trilayer MoS₂ FETs using ultrathin HfLaO dielectrics exhibit a high current of 589 µA µm⁻¹ at a supply voltage of 1 V at room temperature, which increases to 1162 µA µm⁻¹ at 4.3 K, the highest among those using commonly evaporated metal. Record high electron saturation velocity of 4.2 × 10⁶ cm s⁻¹ can be achieved at room temperature.

Tuning the carrier polarity of 2D semiconductors is a prerequisite for thedesign of advanced functional devices. In this work, quasi-continuous tuning of carrier polarity of monolayered MoS₂ from intrinsic n-type to p-type via ambipolarity by substituting molybdenum atoms with vanadium atoms is realized, which can be extended to other monolayered semiconducting 2D TMDs.

Abstract

Semiconducting 2D transition metal dichalcogenides (2D TMDs) with tunable electronic properties are a fundamental prerequisite for the development of next generation advanced electronic/optoelectronic devices. However, controllable and quasi-continuous tuning carrier polarity of monolayered MoS₂ ranging from intrinsic n-type to p-type via ambipolarity still remains a challenge. Herein, quasi-continuous tailoring of carrier polarity of monolayered MoS₂ through substitutional doping of molybdenum (Mo) with vanadium (V) atoms is presented. Atomic distribution in real space characterized by spherical aberration-corrected scanning transmission electron microscopy (Cs-STEM) reveals that the V atoms randomly substitute Mo in monolayered MoS₂, and its doping concentration can be tuned in a wide range from 0.7 to ≈10 at.%. Electrical measurements confirm that the carrier polarity of the monolayered MoS₂ can be tuned from intrinsic n-type to p-type via ambipolarity depending on the V doping degree, consistent with the density functional theory calculations. Moreover, this doping strategy is demonstrated to extend to other monolayered 2D TMDs by using MoSe₂ as a model material, owing to a good universality.

Nature Nanotechnology, Published online: 12 September 2022; doi:10.1038/s41565-022-01204-2

A van der Waals magnetic insulator, NiPS3, hosts a type of polaritonic quasiparticle that emerges from the strong coupling between an optical microcavity mode and spin-correlated excitons.

The current state of thermal management of neuromorphic computing technology is described and the challenges and opportunities of energy efficient implementation of neuromorphic devices are addressed. The fundamental features of the brain's thermal regulation and their further physical implementation are discussed. It is believed that the underlined importance of thermal management of neuromorphic computing technology will guide researchers in this field.

Abstract

Machine learning has experienced unprecedented growth in recent years, often referred to as an “artificial intelligence revolution.” Biological systems inspire the fundamental approach for this new computing paradigm: using neural networks to classify large amounts of data into sorting categories. Current machine-learning schemes implement simulated neurons and synapses on standard computers based on a von Neumann architecture. This approach is inefficient in energy consumption, and thermal management, motivating the search for hardware-based systems that imitate the brain. Here, the present state of thermal management of neuromorphic computing technology and the challenges and opportunities of the energy-efficient implementation of neuromorphic devices are considered. The main features of brain-inspired computing and quantum materials for implementing neuromorphic devices are briefly described, the brain criticality and resistive switching-based neuromorphic devices are discussed, the energy and electrical considerations for spiking-based computation are presented, the fundamental features of the brain's thermal regulation are addressed, the physical mechanisms for thermal management and thermoelectric control of materials and neuromorphic devices are analyzed, and challenges and new avenues for implementing energy-efficient computing are described.

Light: Science & Applications, Published online: 02 September 2022; doi:10.1038/s41377-022-00956-9

This special issue covers a series of cutting-edge works on exploring novel rare earth luminescent materials and their applications in lighting, display, information storage, sensing, and bioimaging as well as therapy.

Nature Communications, Published online: 02 September 2022; doi:10.1038/s41467-022-32730-1

Author Correction: Human eye-inspired soft optoelectronic device using high-density MoS₂-graphene curved image sensor array

The optical and optoelectronic properties of 2D Xenes and their applications in photonic devices including all-optical modulators, wavelength converters, ultrafast lasers, and photodetectors are comprehensively reviewed. The article mainly focuses on five most extensively studied Xenes: graphene, black phosphorus, antimonene, bismuthene, and tellurenene. The properties and applications of other Xenes are also briefly introduced.

Abstract

In recent years, tremendous attention has been paid to the investigation of single-element 2D materials. These 2D materials mainly consist of elements from group IV and group V such as silicene, phosphorene, and antimonene. Together with other four elements from groups III and VI, they are classified as 2D Xenes and exhibit rich optical and optoelectronic properties such as broadband optical response, strong nonlinearity, ultrafast recovery time, and layer-dependent bandgap. 2D Xenes can be easily integrated with microfibers and other optical platforms. On the basis of their attracting characteristics, 2D Xenes have been utilized in various functional devices. In this review, the optical and optoelectronic properties of the most intensively studied 2D Xenes are introduced. Their applications in photonic devices including all-optical modulators, wavelength converters, ultrafast lasers, and photodetectors are explicitly explored. Finally, the challenges and future perspectives of photonic devices based on 2D Xenes are discussed.