Jing Zhang

Nature Electronics, Published online: 23 May 2022; doi:10.1038/s41928-022-00764-4

A variety of metal electrodes can be deposited on a graphene substrate, delaminated and transferred onto two-dimensional semiconductors to form high-quality metal–semiconductor interfaces.

Nature Electronics, Published online: 23 May 2022; doi:10.1038/s41928-022-00770-6

Graphene can be used as a donor substrate to create van der Waals contacts between two-dimensional semiconductors and a variety of three-dimensional metal electrodes, including strongly adhering metals.

Taking advantages of miniaturization and integrability, all-dielectric TiO₂ metalenses are designed based on Fresnel diffraction effect and integrated with molybdenum ditelluride (MoTe₂) photodetectors. Owing to the enhancement of photon utilization efficiency, the corresponding integrated devices exhibit a high responsivity and detectivity of 135.0 A W^{− 1} and 4.05 × 10¹² Jones, respectively. These results accelerate the evolution of metalenses in 2D optoelectronics fields.

Abstract

Atomic layers of group-VI 2D transition metal dichalcogenides (TMDCs) with substantial materials system and unique physicochemical properties have exhibited excellent performances in electronics and optoelectronics. However, photodetectors based on TMDCs suffer from poor sensitivity and optical absorption due to the weak phonon conversion efficiency and optical absorption due to their atomically thin feature. Here, taking advantages of miniaturization and integrability, all-dielectric TiO₂ metalenses are designed based on the Fresnel diffraction effect and integrated with molybdenum ditelluride (MoTe₂) photodetectors. Owing to the enhancement of photon utilization efficiency, the corresponding integrated devices exhibit a high responsivity of 135.0 A W^–1, an excellent external quantum efficiency of 202.0%, and a specific detectivity of ≈4.05 × 10¹² Jones. These results indicate that the optoelectrical properties of 2D materials can be significantly optimized by integration with metalens and will accelerate the evolution of metalens in 2D optoelectronics fields.

The developments of circuit-level memory technologies and in-memory computing applications realized experimentally using 2D materials are reviewed. Reports on large-scale material synthesis methods, circuits with different levels of integration, logic-in-memory, and neuromorphic computing applications are systematically summarized. Major challenges and perspectives of large-scale 2D-material-based integrated memory are provided.

Abstract

Memory technologies and applications implemented fully or partially using emerging 2D materials have attracted increasing interest in the research community in recent years. Their unique characteristics provide new possibilities for highly integrated circuits with superior performances and low power consumption, as well as special functionalities. Here, an overview of progress in 2D-material-based memory technologies and applications on the circuit level is presented. In the material growth and fabrication aspects, the advantages and disadvantages of various methods for producing large-scale 2D memory devices are discussed. Reports on 2D-material-based integrated memory circuits, from conventional dynamic random-access memory, static random-access memory, and flash memory arrays, to emerging memristive crossbar structures, all the way to 3D monolithic stacking architecture, are systematically reviewed. Comparisons between experimental implementations and theoretical estimations for different integration architectures are given in terms of the critical parameters in 2D memory devices. Attempts to use 2D memory arrays for in-memory computing applications, mostly on logic-in-memory and neuromorphic computing, are summarized here. Finally, challenges that impede the large-scale applications of 2D-material-based memory are reviewed, and perspectives on possible approaches toward a more reliable system-level fabrication are also given, hopefully shedding some light on future research.

Nature Electronics, Published online: 20 May 2022; doi:10.1038/s41928-022-00775-1

On-chip Floquet photonic topological insulators, which are based on switched-capacitor circulators, could be used to create hybrid electronic–photonic topological integrated circuits for emerging communication technologies.

Long-term memory (LTM) enhanced indium–gallium–zinc oxide (IGZO)-based optical synaptic transistor is introduced to give a clear difference between the LTM and the short-term memory behaviors for optical synapses. This clear difference is achieved by engineering the channel layer of the optical synaptic transistor by inserting mesh-structured titanium dioxide (m-TiO₂) and thin-film structured hafnium dioxide (HfO₂).

Abstract

Amorphous indium–gallium–zinc oxide (IGZO)-based optical synaptic transistor using visible light as the signal that shows a clear difference between long-term memory (LTM) and short-term memory (STM) is introduced in this study. However, since oxide semiconductors, including IGZO, do not sense visible light due to their high band gap energy of around 3.0 eV, defects are generated intentionally within IGZO channel layer to enable sensitivity to the visible light by inserting two layers of mesh-structured titanium dioxide (m-TiO₂) inside the channel layer. Additionally, a layer of thin film-structured hafnium dioxide (HfO₂) is inserted inside the channel to increase the persistent photoconductivity (PPC) effect. This increased PPC effect causes the device to maintain the current long after the light signal is removed. Therefore, the average difference between LTM and STM is achieved ≈110 times, showing a clear difference between LTM and STM. Additionally, 721% of maximum paired-pulse facilitation (PPF) is achieved. Thus, the device introduced in this study is believed to lead the coming technology such as interactive appliances including displays and robots.

Shorter dipole moment in cation MM'X₂ Janus structures leads to a larger built-in electric field, enabling dramatically enhanced out-of-plane second harmonic generation (SHG) response.

Abstract

2D materials are excellent platforms for nonlinear optical (NLO) response, especially second harmonic generation (SHG), due to its large surface to volume ratio and sub-nanometer thickness. The SHG susceptibility strongly relies on the symmetry of materials. Constructing Janus structures can break the out-of-plane mirror symmetry, bringing about asymmetric charge distribution and leading to a built-in electric field. Consequently, SHG response along the out-of-plane direction can be improved. In the current work, combining first-principles calculations with independent particle approximation, the SHG response of nine Janus group-III chalcogenide monolayers (M₂XX', MM'X₂) are systematically evaluated. Both extraordinary in-plane and out-of-plane SHG response are revealed in all the Janus structures. Besides, cation MM'X₂ Janus structures exhibit systemically higher SHG response than that of anion M₂XX’ due to stronger dipole. Among them, GaInTe₂ possesses extremely high out-of-plane SHG response with d ₃₁ up to 10490.4 pm V⁻¹ at photon energy (PE) of 4.7 eV, enabling promising applications in ultraviolet NLO devices. The SHG intensity polar plots from Janus structures display unusual rotational symmetry at different PEs allowed by the out-of-plane SHG components. This work provides theoretical guidelines for further experimental explorations in 2D group-III monochalcogenide Janus structures and paves the way for their utilization in NLO devices.

The 2D transition metal dichalcogenide alloy, Mo_{1− x} W _x S₂, is synthesized to investigate the influence of configurational entropy on the piezoelectrical property. Mo_0.46W_0.54S₂, in which two cations have similar concentrations and the maximum configurational entropy is attained, exhibits the best piezoelectric properties. Combined with excellent mechanical durability, a mechanical sensor based on the Mo_0.46W_0.54S₂ alloy is demonstrated for real-time health monitoring.

Abstract

Piezoelectricity in 2D transition metal dichalcogenides (TMDs) has attracted considerable interest because of their excellent flexibility and high piezoelectric coefficient compared to conventional piezoelectric bulk materials. However, the ability to regulate the piezoelectric properties is limited because the entropy is constant for certain binary TMDs other than multielement ones. Herein, in order to increase the entropy, a ternary TMDs alloy, Mo_{1− x} W _x S₂, with different W concentrations, is synthesized. The W concentration in the Mo_{1− x} W _x S₂ alloy can be controlled precisely in the low-supersaturation synthesis and the entropy can be tuned accordingly. The Mo_0.46W_0.54S₂ alloy (x = 0.54) has the highest configurational entropy and best piezoelectric properties, such as a piezoelectric coefficient of 4.22 pm V⁻¹ and a piezoelectric output current of 150 pA at 0.24% strain. More importantly, it can be combined into a larger package to increase the output current to 600 pA to cater to self-powered applications. Combining with excellent mechanical durability, a mechanical sensor based on the Mo_0.46W_0.54S₂ alloy is demonstrated for real-time health monitoring.

In-Memory Processing

To achieve in-memory processing, the development of an ultrafast and reconfigurable multi-terminal device is required. In article number 2200122, Mohit Kumar, Hyungtak Seo, and co-workers develop a proof-of-concept ultrafast (≈42 ns) and programmable redox thin-film transistor, which is successfully utilized to build in-memory electronics, including logic-in-memory processing, on-demand multiterminal addressable memory, learning, pattern recognition, and classification.

The progress made toward the development of a modular compact modeling technology for graphene field-effect transistors under DC, transient, AC, and noise analysis, is reported. This includes non-idealities such as extrinsic-, short-channel-, traps-, self-heating-, and non-quasi-static-effects. An overview of the challenges ahead in graphene transistor modeling to enable computer-aided circuit design is also provided.

Abstract

The progress made toward the definition of a modular compact modeling technology for graphene field-effect transistors (GFETs) that enables the electrical analysis of arbitrary GFET-based integrated circuits is reported. A set of primary models embracing the main physical principles defines the ideal GFET response under DC, transient (time domain), AC (frequency domain), and noise (frequency domain) analysis. Another set of secondary models accounts for the GFET non-idealities, such as extrinsic-, short-channel-, trapping/detrapping-, self-heating-, and non-quasi static-effects, which can have a significant impact under static and/or dynamic operation. At both device and circuit levels, significant consistency is demonstrated between the simulation output and experimental data for relevant operating conditions. Additionally, a perspective of the challenges during the scale up of the GFET modeling technology toward higher technology readiness levels while drawing a collaborative scenario among fabrication technology groups, modeling groups, and circuit designers, is provided.

Van der Waals gap-free 1D contacts to 2D materials, toward their large-scale application to 2D electronic and quantum devices, are reviewed. 1D edge contacts enable low contact resistivity, scattering-free carrier transport, high-density transistor integration, and double-gate transistor operation. In addition, an outlook of the 1D contact methods is presented.

Abstract

Recent studies have intensively examined 2D materials (2DMs) as promising materials for use in future quantum devices due to their atomic thinness. However, a major limitation occurs when 2DMs are in contact with metals: a van der Waals (vdW) gap is generated at the 2DM-metal interfaces, which induces metal-induced gap states that are responsible for an uncontrollable Schottky barrier (SB), Fermi-level pinning (FLP), and high contact resistance (R _C), thereby substantially lowering the electronic mobility of 2DM-based devices. Here, vdW-gap-free 1D edge contact is reviewed for use in 2D devices with substantially suppressed carrier scattering of 2DMs with hexagonal boron nitride (hBN) encapsulation. The 1D contact further enables uniform carrier transport across multilayered 2DM channels, high-density transistor integration independent of scaling, and the fabrication of double-gate transistors suitable for demonstrating unique quantum phenomena of 2DMs. The existing 1D contact methods are reviewed first. As a promising technology toward the large-scale production of 2D devices, seamless lateral contacts are reviewed in detail. The electronic, optoelectronic, and quantum devices developed via 1D contacts are subsequently discussed. Finally, the challenges regarding the reliability of 1D contacts are addressed, followed by an outlook of 1D contact methods.

A large majority of materials in the Inorganic Crystal Structure Database have at least one topological band.

Nature Physics, Published online: 19 May 2022; doi:10.1038/s41567-022-01606-y

Transport experiments highlight a technique to detect transitions in the topological state of two-dimensional materials, with possible applications in memory devices.

This work develops a high-speed multifunctional artificial vision system capable of recognizing, memorizing, and actuating self-protection by combining a Sb₂Se₃/CdS-core/shell nanorod arrays optoelectronic memristor, a threshold-switching memristor, and an electrochemical actuator. When an optoelectronic memristor is activated, it can simulate the eye muscle contraction and reproduce the self-protection response of closing eyes when human eyes are injured by intense light.

Abstract

More than 80% of biological learning information is received through the visual system; therefore, artificial vision systems have garnered continual interest in the field of artificial intelligence technologies. Simulating the activities of a range of human vision systems, such as discrimination, memory, and induced muscular activity, which still remains a challenge. The authors develop a high-speed multifunctional artificial vision system capable of recognizing, memorizing, and actuating self-protection by combining a Sb₂Se₃/CdS-core/shell (SC) nanorod array optoelectronic memristor, a threshold-switching memristor, and an electrochemical actuator. When an optoelectronic memristor is activated, it can cause an electrochemical actuator to move, simulating the eye muscle contraction and reproducing the self-protection response of closing eyes when the human eyes are injured by intense light. Light absorption and charge carrier extraction are advantages of optoelectronic memristors with high-quality SC nanorod arrays. The device achieves a fast response speed and a large response current of up to 40 µs and 0.8 µA. Artificial vision systems offer a potential technique for bionanotechnology, particularly in the domain of artificial intelligence simulation of biosensor systems.

The low-energy and long-lived edge states of 2D lead halide perovskites arise from the rotational symmetry elevation at the crystal layer edges, which are enabled by dangling PbBr₆ octahedra at the lattice terminals. These dangling octahedra with enhanced atomic vibrations can give rise to localized electronic states that enable an effective electron transport from the interior to layer edges.

Abstract

The structural reconstruction at the crystal layer edges of 2D lead halide perovskites (LHPs) leads to unique edge states (ES), which are manifested by prolonged carrier lifetime and reduced emission energy. These special ES can effectively enhance the optoelectronic performance of devices, but their intrinsic origin and working mechanism remain elusive. Here it is demonstrated that the ES of a family of 2D Ruddlesden–Popper LHPs [BA₂CsPb₂Br₇, BA₂MAPb₂Br₇, and BA₂MA₂Pb₃Br₁₀ (BA = butylammonium; MA = methylammonium)] arise from the rotational symmetry elevation of the PbBr₆ octahedra dangling at the crystal layer edges. These dangling octahedra give rise to localized electronic states that enable an effective transport of electrons from the interior to layer edges, and the population of electrons in both the interior states and the ES can be manipulated via controlling the external fields. Moreover, the abundant phonons, activated by the dangling octahedra, can interact with electrons to facilitate radiative recombination, counterintuitive to the suppressive role commonly observed in conventional semiconductors. This work unveils the intrinsic atomistic and electronic origins of ES in 2D LHPs, which can stimulate the exploration of ES-based exotic optoelectronic properties and the corresponding design of high-performance devices for these emergent low-dimensional semiconductors.

The construction of a superhydrophilic bionic interface layer (Bio-IL) significantly suppresses the coffee-ring effect during printing of large-area perovskite films via regulating the nucleation rate (υ_N) and radial transport rate (υ_RT) of the perovskite precursor, which induces a homogeneous large-area flexible perovskite film and high-performance inverted flexible perovskite solar cells with an efficiency of 21.08%.

Abstract

The inhomogeneity, poor interfacial contact, and pinholes caused by the coffee-ring effect severely affect the printing reliability of flexible perovskite solar cells (PSCs). Herein, inspired by the bio-glue of barnacles, a bionic interface layer (Bio-IL) of NiO _x /levodopa is introduced to suppress the coffee-ring effect during printing perovskite modules. The coordination effect of the sticky functional groups in Bio-IL can pin the three-phase contact line and restrain the transport of perovskite colloidal particles during the printing and evaporation process. Moreover, the sedimentation rate of perovskite precursor is accelerated due to the electrostatic attraction and rapid volatilization from an extraordinary wettability. The superhydrophilic Bio-IL affords an even spread over a large-area substrate, which boosts a complete and uniform liquid film for heterogeneous nucleation as well as crystallization. Perovskite films on different large-area substrates with negligible coffee-ring effect are printed. Consequently, inverted flexible PSCs and perovskite solar modules achieve a high efficiency of 21.08% and 16.87%, respectively. This strategy ensures a highly reliable reproducibility of printing PSCs with a near 90% yield rate.

D. Yagodkin, K. Greben, A. Eljarrat, S. Kovalchuk, M. Ghorbani-Asl, M. Jain, S. Kretschmer, N. Severin, J. P. Rabe, A. V. Krasheninnikov, C. T. Koch, K. I. Bolotin

A new excitonic state is observed in 2D semiconductors after electron beam functionalization. The state is bright, has a very narrow photoluminescence peak at cryogenic temperatures, and survives up to room temperature. It is shown that this state is not related to intrinsic defects in transition metal dichalcogenides but is associated with molecules on the surface of the material.

A new localized excitonic state is demonstrated in patterned monolayer 2D semiconductors. The signature of an exciton associated with that state is observed in the photoluminescence spectrum after electron beam exposure of several 2D semiconductors. The localized state, which is distinguished by non-linear power dependence, survives up to room temperature and is patternable down to 20 nm resolution. The response of the new exciton to the changes of electron beam energy, nanomechanical cleaning, and encapsulation via multiple microscopic, spectroscopic, and computational techniques is probed. All these approaches suggest that the state does not originate from irradiation-induced structural defects or spatially non-uniform strain, as commonly assumed. Instead, it is shown to be of extrinsic origin, likely a charge transfer exciton associated with the organic substance deposited onto the 2D semiconductor. By demonstrating that structural defects are not required for the formation of localized excitons, this work opens new possibilities for further understanding of localized excitons as well as their use in applications that are sensitive to the presence of defects, e.g. chemical sensing and quantum technologies.

Space Lubricants

In article number 2110429, Aurélien Saulot, Tobin Filleter, Guillaume Colas, and co-workers engineer a high-performance space lubricant for the extreme environments faced by spacecrafts. The co-deposition of MoS₂ with tantalum produces a synergistic effect where the tantalum preferentially oxidizes in terrestrial environments and enhances lubrication in deep space vacuum, resulting in low friction and wear despite the harsh conditions seen by spacecrafts.

This manuscript reports dynamic, unclonable cryptographic primitives based on plasmonic fluorescence blinking that can generate a large number of temporally varying codes in a single device—a key requirement toward their applications in modern anti-counterfeiting and communication systems. It opens a new field of investigation into dynamic encryption and provides new insight for future anti-counterfeiting research.

Abstract

Merging cryptographic primitive technologies and physical unclonable functions (PUFs) have become a new paradigm of one-way encryption. Herein, the authors report a dynamic PUF cryptographic primitive based on plasmonic fluorescence blinking from single or a few dye molecules embedded within the nanogaps of plasmonic patch nanoantennas. This cryptographic primitive carries two sets of high-capacity optical codes: the fluorescence blinking of the embedded dye molecules and multi-color light scattering enabled by the plasmonic nanoantennas. The former allows the generation of temporal binary codes from a large number of individual plasmonic patch nanoantennas by holding either “1” (bright state) or “0” (dark state), while the latter provides a permanent color-based novenary code that acts as a decryption channel for authentication. Benefiting from the high electromagnetic field localized within the nanogaps and the large Purcell enhancement of the plasmonic nanoantennas, the fluorescence blinking is readily detectable by a common fluorescence microscope with a mercury arc lamp as a low-power excitation source. The developed dynamic PUF codes are robustly and accurately authenticated by a self-programmed computer vision algorithm. This study revolutionizes the conventional static PUF encryption to nanophotonics-based dynamic encryption, opening a new avenue for next-generation advanced anti-counterfeiting.

Micro-nano structures are crucial to high-performance perovskite optoelectronics. The working mechanisms of micro-nano structures in optics, electricity, and mechanics are provided. A systemic understanding of these functionalities applied in perovskite-based optoelectronics is presented, and promising guidelines are proposed to promote the multifunctional commercial applications of micro-nano structured perovskite optoelectronics.

Abstract

Metal halide perovskite, an emerging photosensitive semiconductor, has been widely employed in solar cells, light-emitting diodes, photodetectors, and lasers owing to its excellent photophysical properties and simple solution preparation processing. However, as a photoactive layer, the higher refractive index and thinner thickness of perovskite film can cause reflection and transmission at the interface, and confine the emitted light within devices, resulting in the poor incident photon absorption and emitted photon extraction. In addition, the intrinsic brittleness of perovskite material restricts its potential applications in flexible optoelectronics. Therefore, great effort has been put into micro-nano structured perovskite optoelectronics, and the reported reviews mainly focus on the fabrication process of micro-nano patterned perovskite. Herein, the functionalities of micro-nano structures in optoelectronics, including improving the light trapping, light extraction, light modulation, carrier dynamics, mechanical robustness, and other novel functionalities, are comprehensively reviewed. The specific applications of these functionalities in perovskite-based optoelectronic devices are then discussed in detail to provide a better understanding of the photophysical properties of micro-nano structure functionalized optoelectronics. Finally, promising strategies to promote the multifunctional commercial applications of micro-nano structured perovskite optoelectronics are provided.