Jiuxiang Dai

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

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

Nature Photonics, Published online: 17 March 2022; doi:10.1038/s41566-022-00971-7

Researchers demonstrate electrically controllable chirality by exploiting doping-dependent valley polarization of excitonic states in monolayer tungsten diselenide.

Abstract

Regarding the reverse process of materials growth, etching has been widely concerned to indirectly probe the growth kinetics, offering an avenue in governing the growth of two-dimensional (2D) materials. In this work, interface-driven anisotropic etching mode is demonstrated for the first time to be generally applied to 2D heterostructures. It is shown that the typical in-plane graphene and hexagonal boron nitride (h-BN) heterostructures follow a multi-stage etching behavior initiated first along the interfacial region between the two materials and then along edges of neighboring h-BN flakes and finally along central edges of h-BN. By accurately tuning etching conditions in the chemical vapor deposition process, series of etched 2D heterostructure patterns are controllably produced. Furthermore, scaled formation of graphene and h-BN heterostructures arrays has been realized with full assist of as-proposed etching mechanism, offering a direct top-down method to make 2D orientated heterostructures with order and complexity. Detection of interface-driven multi-staged anisotropic etching mode will shed light on understanding growth mechanism and further expanding wide applications of 2D heterostructures.

Hierarchically assembled nanosheets with ultimate molecular thickness, bearing functional cobalt(III) complexes are investigated by X-ray photoelectron spectroscopy and tip-enhanced Raman scattering. The combination of both techniques provides detailed insights into the structure of the formed 2D materials down to the nanoscale.

Abstract

Chemical functionalization of molecular 2D materials toward the assembly of hierarchical functional nanostructures is of great importance for nanotechnology including areas like artificial photocatalytic systems, nanobiosensors, or ultrafiltration. To achieve the desired functionality of 2D materials, these need to be characterized down to the nanoscale. However, obtaining the respective chemical information is challenging and generally requires the application of complementary experimental techniques. Here, the synthesis and chemical characterization of hierarchically assembled molecular nanosheets based on ≈1 nm thin, molecular carbon nanomembrane (CNM) and covalently grafted, single-molecule layer cobalt(III) catalysts for the light-driven hydrogen evolution reaction (HER) is demonstrated. X-ray photoelectron spectroscopy (XPS) and tip-enhanced Raman spectroscopy (TERS) to access both the transversal and longitudinal chemical information of the synthesized nanosheets with nanometer resolution are employed. TERS and XPS data provide detailed information on the average and local surface distribution of the catalyst as well as mechanistic details of the grafting reaction. The proposed approach represents a general route toward a nanoscale structural analysis for a variety of molecular 2D materials—a rapidly growing materials class with broad prospects for fundamental science and applications.

Nonlayered Te/In₂S₃ tunneling heterojunctions possess type-II band alignment and can transfer to type-I or III depending on the electric field applied, exhibiting reverse rectification ratio exceeding 10⁴ and ultralow forward current of 10⁻¹² A. A photodetector based on heterojunctions shows greatly-improved photoresponsivity of 146 A W⁻¹ and response time of 5 ms compared to its constituent materials.

Abstract

A photodetector based on 2D non-layered materials can easily utilize the photogating effect to achieve considerable photogain, but at the cost of response speed. Here, a rationally designed tunneling heterojunction fabricated by vertical stacking of non-layered In₂S₃ and Te flakes is studied systematically. The Te/In₂S₃ heterojunctions possess type-II band alignment and can transfer to type-I or type-III depending on the electric field applied, allowing for tunable tunneling of the photoinduced carriers. The Te/In₂S₃ tunneling heterojunction exhibits a reverse rectification ratio exceeding 10⁴, an ultralow forward current of 10⁻¹² A, and a current on/off ratio over 10⁵. A photodetector based on the heterojunctions shows an ultrahigh photoresponsivity of 146 A W⁻¹ in the visible range. Furthermore, the devices exhibit a response time of 5 ms, which is two and four orders of magnitude faster than that of its constituent In₂S₃ and Te. The simultaneously improved photocurrent and response speed are attributed to the direct tunneling of the photoinduced carriers, as well as a combined mechanism of photoconductive and photogating effects. In addition, the photodetector exhibits a clear photovoltaic effect, which can work in a self-powered mode.

The memristor based on 2D BiOI exhibits high-performance memristive behaviors with an ultralow SET voltage of ≈0.05 V, which is one order of magnitude lower than that of most reported memristors based on 2D materials. The memristor demonstrates electrical and light-induced synaptic plasticity eminently suitable for low-power optoelectronic synapses, which can be used to simulate the “learning experience” behaviors.

Abstract

Artificial optoelectronic synapses with both electrical and light-induced synaptic behaviors have recently been studied for applications in neuromorphic computing and artificial vision systems. However, the combination of visual perception and high-performance information processing capabilities still faces challenges. In this work, the authors demonstrate a memristor based on 2D bismuth oxyiodide (BiOI) nanosheets that can exhibit bipolar resistive switching (RS) performance as well as electrical and light-induced synaptic plasticity eminently suitable for low-power optoelectronic synapses. The fabricated memristor exhibits high-performance RS behaviors with a high ON/OFF ratio up to 10⁵, an ultralow SET voltage of ≈0.05 V which is one order of magnitude lower than that of most reported memristors based on 2D materials, and low power consumption. Furthermore, the memristor demonstrates not only electrical voltage-driven long-term potentiation, depression plasticity, and paired-pulse facilitation, but also light-induced short- and long-term plasticity. Moreover, the photonic synapse can be used to simulate the “learning experience” behaviors of human brain. Consequently, not only the memristor based on BiOI nanosheets shows ultra-low SET voltage and low-power consumption, but also the optoelectronic synapse provides new material and strategy to construct low-power retina-like vision sensors with functions of perceiving and processing information.

Nature Nanotechnology, Published online: 04 April 2022; doi:10.1038/s41565-022-01102-7

A two-dimensional transition metal dichalcogenide-on-compound-semiconductor fabrication method enables the realization of an active matrix micro-LED display.

Arbitrary Substrates

In article number 2113211, Xuelin Yang, Bing Huang, Bo Shen, and co-workers propose a strategy of polarization-driven-orientation selective growth and demonstrate that single-crystalline GaN can in principle be achieved on polycrystalline diamond or other substrates by utilizing a composed buffer layer consisting of graphene and polycrystalline physical-vapor-deposited AlN. This strategy can be extended to the growth of any emergent single-crystalline semiconductor films on any arbitrary freestanding substrates by choosing appropriate 2D materials with matched crystal structures.

Flexible Optoelectronic Memristors

In article number 2110900, Cong Ye, Jiahong Wang, and co-workers present a topochemical method to transform crystalline phosphorus nanoribbons into copper phosphide nanoribbons (Cu₃P NRs) with well-maintained morphology. The flexible optoelectronic memristor constructed with Cu₃P NRs exhibit excellent performance, and the memristor-based synapse arrays mimic the memory backtracking and pattern recognition.

2D Materials

In article number 2110119, Bin Wang, Shaojuan Li, Dabing Li, and co-workers discuss the state of the art of using 2D materials in optoelectronic integration. They also provide general advice for future development in this field.

Atomically thin 2D materials exhibit outstanding mechanical robustness as compared to 3D crystals. This feature and the exceptional tunability of the optical and electronic properties of 2D crystals under strain has boosted the emerging fields of flextronics and straintronics. Knowledge of the adhesion and elasticity properties is thus pivotal to the manufacturing and integration of 2D crystal-based devices.

Abstract

2D materials, such as graphene, hexagonal boron nitride (hBN), and transition-metal dichalcogenides (TMDs), are intrinsically flexible, can withstand very large strains (>10% lattice deformations), and their optoelectronic properties display a clear and distinctive response to an applied stress. As such, they are uniquely positioned both for the investigation of the effects of mechanical deformations on solid-state systems and for the exploitation of these effects in innovative devices. For example, 2D materials can be easily employed to transduce nanometric mechanical deformations into, e.g., clearly detectable electrical signals, thus enabling the fabrication of high-performance sensors; just as easily, however, external stresses can be used as a “knob” to dynamically control the properties of 2D materials, thereby leading to the realization of strain-tuneable, fully reconfigurable devices. Here, the main methods are reviewed to induce and characterize, at the nm level, mechanical deformations in 2D materials. After presenting the latest results concerning the mechanical, elastic, and adhesive properties of these unique systems, one of their most promising applications is briefly discussed: the realization of nano-electromechanical systems based on vibrating 2D membranes, potentially capable of operating at high frequencies (>100 MHz) and over a large dynamic range.

Nature Electronics, Published online: 06 April 2022; doi:10.1038/s41928-022-00738-6

Quantum computing has attracted attention owing to its potential to solve problems that are intractable with traditional computing technologies; however, a scalable scheme for producing millions of qubits remains elusive. A new effort demonstrates a milestone to achieving this by fabricating qubits in the same factory where state-of-the-art semiconductor chips are manufactured.

Nature, Published online: 06 April 2022; doi:10.1038/s41586-022-04425-6

In the standard Si transistor gate stack, replacing conventional dielectric HfO2 with an ultrathin ferroelectric–antiferroelectric HfO2–ZrO2 heterostructure exhibiting the negative capacitance effect demonstrates ultrahigh capacitance without degradation in leakage and mobility, promising for ferroelectric integration into advanced logic technology.