Jing Zhang

Micrometer-long graphene nanoribbons (GNRs) with widths of ≈2 nm are directly grown on insulating hexagonal boron nitride (h-BN) substrates through nanoparticle-catalyzed chemical vapor deposition. These ultranarrow GNRs feature a typical bandgap of ≈1 eV, which is suitable for fabricating field-effect devices. Remarkably, the as-grown GNRs are crystallographically aligned with the h-BN substrate, forming 1D moiré superlattices.

Abstract

Graphene nanoribbons (GNRs) with widths of a few nanometers are promising candidates for future nanoelectronic applications due to their structurally tunable bandgaps, ultrahigh carrier mobilities, and exceptional stability. However, the direct growth of micrometer-long GNRs on insulating substrates, which is essential for the fabrication of nanoelectronic devices, remains an immense challenge. Here, the epitaxial growth of GNRs on an insulating hexagonal boron nitride (h-BN) substrate through nanoparticle-catalyzed chemical vapor deposition is reported. Ultranarrow GNRs with lengths of up to 10 µm are synthesized. Remarkably, the as-grown GNRs are crystallographically aligned with the h-BN substrate, forming 1D moiré superlattices. Scanning tunneling microscopy reveals an average width of 2 nm and a typical bandgap of ≈1 eV for similar GNRs grown on conducting graphite substrates. Fully atomistic computational simulations support the experimental results and reveal a competition between the formation of GNRs and carbon nanotubes during the nucleation stage, and van der Waals sliding of the GNRs on the h-BN substrate throughout the growth stage. This study provides a scalable, single-step method for growing micrometer-long narrow GNRs on insulating substrates, thus opening a route to explore the performance of high-quality GNR devices and the fundamental physics of 1D moiré superlattices.

Contrast nonlinear Edelstein magnetization responses in different ferroelectric In₂Se₃ bilayer stacking patterns are demonstrated. The largely tunable electronic band structure of In₂Se₃ bilayers provides a fascinating semiconducting platform for exploring the interplay among spintronics, orbitronics, nonlinear optics, and ferroelectrics in a single platform.

Abstract

Magnetization generation and accumulation in intrinsically nonmagnetic materials is one of the key routes to various exotic applications, especially realizing fast information storage and memory devices. Among the various approaches for magnetization injection, light irradiation is advantageous for its noncontacting and non-invasive nature, and has been receiving great attention during the past few decades. In the current work, a quadratic response theory and first-principles calculations are applied to reveal photoinduced magnetization in recently discovered 2D In₂Se₃ bilayers. The ferroelectric In₂Se₃ bilayers can be (meta-) stabilized in versatile stacking patterns, exhibiting largely tunable electronic band structure, and optical feature. Hence, it is suggested that under circularly polarized light illumination, the system can show different photoinduced magnetization responses with large contrast in different stacking patterns. The magnetizations are composed by both spin and orbital angular momentum contributions, which can reach as large as 1μ_B under a laser with intermediate intensity (≈10⁹ W cm^–2). This magnitude can be easily observed and furthermore manipulated in the state-of-the-art experimental techniques. The proposal suggests a candidate material platform to explore the interplay among spintronics, orbitronics, nonlinear optics, and ferroelectrics in a single platform.

Phase-change materials (PCMs) show a large property contrast that is exploited in memory storage. Fragile-to-strong transitions (FST) aid the application of PCMs, as ultrafast crystallization and amorphous phase stability are simultaneously realized. Here, for the PCM Ge₃Sb₆Te₅ the viscosity is measured over sixteen orders of magnitude, clearly demonstrating an FST.

Abstract

Chalcogenide phase-change materials combine a remarkable set of properties that makes them promising candidates for future non-volatile memory applications. Binary data storage exploits the high contrast in electrical and optical properties between the covalent amorphous and metavalent crystalline phase. Here the authors perform an analysis of the liquid phase kinetics of the phase-change material Ge₃Sb₆Te₅, which is the key to ultrafast switching speeds. By employing four experimental techniques, the viscosity is measured over sixteen orders of magnitude despite its propensity for fast crystallization. These measurements reveal that the liquid undergoes a transition in viscosity–temperature dependence associated with a liquid–liquid phase transition. The system exhibits a shallow viscosity change with temperature near the glass transition which stabilizes the memory cells in the amorphous state and which limits the severity of relaxation processes. Meanwhile, when heated during the writing process, the fragility increases to more than double, causing the viscosity to drop rapidly enabling a nanosecond crystallization speed. This change in viscosity–temperature dependence is highly unusual among glass forming liquids and is reminiscent of the behavior of water. This viscosity transition is also key to the technological success of phase-change materials for computer memory applications.

A simple design tactic for narrowband blue emitter is demonstrated by incorporating sulfone unit into the boron/nitrogen (B/N) embedded polycyclic skeleton. The resulting blue organic light-emitting diodes achieve a high external quantum efficiency of 32.0% and low efficiency roll-off.

Abstract

The blue multi-resonance thermally activated delayed fluorescence materials, simultaneously realizing narrow full-width at half-maximum, high external quantum efficiency (EQE), and low efficiency roll-off, remains a formidable challenge. Herein, three novel emitters, namely PTZBN1, PTZBN2, and PTZBN3, are designed by gradual peripheral modification in boron/nitrogen (B/N) embedded polycyclic skeleton, which exhibit progressively hypsochromic-shifted emission from 490 nm (PTZBN1) to 468 nm (PTZBN3) with photoluminescence quantum yields up to 98%. In particular, the incorporation of sulfone unit in the boron/nitrogen (B/N) embedded polycyclic skeleton provides a simple but effective tactic for narrowband blue emission. The organic light-emitting diodes based on PTZBN2 achieve one of the-state-of-the-art EQEs of 34.8% with electroluminescence (EL) peak at 478 nm. Impressively, PTZBN3-based device exhibits not only a high maximum EQE of 32.0% with EL peak at 468 nm, but also low efficiency roll-off.

Nature Materials, Published online: 12 May 2022; doi:10.1038/s41563-022-01248-8

A new type of translucent Er³⁺−Yb³⁺ doped oxyfluoride precursor glass-ceramic (P-GC) containing large flower-like Ba₂LaF₇ single-crystals is prepared via melt-quenching. The translucent oxyfluoride P-GC becomes transparent with increasing the crystal size and crystallinity as a result of isothermal heat-treatment. This is attributed to the suppression of light scattering. The derived transparent P-GC shows strong up-conversion luminescence.

Abstract

It is known that the optical transparency of an oxide glass decreases with an increase in the size and fraction of crystals in the glass during heat-treatment. Here, the authors report an opposite scenario, where a translucent Er³⁺−Yb³⁺ doped oxyfluoride precursor glass-ceramic (P-GC) becomes transparent with increasing crystal size and crystallinity. Specifically, in the heat-treated P-GC samples, the authors observe that the growth of the existing Ba₂LaF₇ crystals and particularly the formation of small spherical Ba₂LaF₇ crystals greatly enhanced the light transmittance. To reveal the origin of this anomalous phenomenon, the authors perform detailed morphology and structure analyses on both P-GC and the heat-treated P-GC samples and molecular dynamics simulations of the precursor glass. The results show that the composition of the residual glass phase is altered (e.g., depletion of La³⁺) in the way that the differences in refractive index between the glass matrix and the crystals are greatly reduced. As a consequence, the light scattering of the heat-treated P-GC is suppressed, and hence, the derived P-GC becomes transparent. In addition, a proper heat-treatment can also enhance the luminescence of the studied P-GC system.

Huge achievements have been made in graphene chemical vapor deposition (CVD) growth. This review systematical summarizes the current progresses in four research directions, including theoretical study of graphene CVD growth, direct growth on insulating substrates, low temperature growth, layer number, and stacking-angle controlled growth. Finally, the future research directions of graphene are discussed.

Abstract

Graphene, since the first successful exfoliation of graphite, has continuously attracted attention due to its remarkable properties and applications. Recently, the research focus on graphene synthesis has been directed to the controllable synthesis of large-area and high-quality graphene. In the last decade, there has been great progress in the chemical vapor deposition (CVD) growth of graphene. Theoretical investigations have led to an enhanced understanding of puzzles on hydrocarbon species stability, key reaction pathways, the role of hydrogen gas, the morphology of graphene islands, and the alignment of graphene on substrates. Experimentally, high-quality graphene is epitaxially grown on both insulating and metal substrates. Progress has also been reported on low-temperature graphene growth and on controlling the thickness and stacking of graphene layers. In this review, the authors summarize the previous theoretical and experimental studies on graphene CVD growth and discuss the future challenges on the growth of graphene i) on insulating substrates, ii) at low temperature, iii) with controllable thickness, and iv) with selected stacking twist angles. The authors assert that the key to the continuous advancement of graphene growth is the synergy of experimental and theoretical investigations.

Single-molecule logic devices are ideal candidates for ultra-minimalistic circuit elements to perform high-density computing. Gated Au/S-(CH₂)₃-Fc-(CH₂)₉-S/Au single-electron transistors not only hold the advantage of controlling molecular orbitals (MOs) via bias and gate voltages, but also exhibit a unique electronic structure with two adjacent MOs. This provides a single-electron logic calculator to implement all universal logic gates within a single-molecule device.

Abstract

Controllable single-molecule logic operations will enable development of reliable ultra-minimalistic circuit elements for high-density computing but require stable currents from multiple orthogonal inputs in molecular junctions. Utilizing the two unique adjacent conductive molecular orbitals (MOs) of gated Au/S-(CH₂)₃-Fc-(CH₂)₉-S/Au (Fc = ferrocene) single-electron transistors (≈2 nm), a stable single-electron logic calculator (SELC) is presented, which allows real-time modulation of output current as a function of orthogonal input bias (V _b) and gate (V _g) voltages. Reliable and low-voltage (ǀV _bǀ ≤ 80 mV, ǀV _gǀ ≤ 2 V) operations of the SELC depend upon the unambiguous association of current resonances with energy shifts of the MOs (which show an invariable, small energy separation of ≈100 meV) in response to the changes of voltages, which is confirmed by electron-transport calculations. Stable multi-logic operations based on the SELC modulated current conversions between the two resonances and Coulomb blockade regimes are demonstrated via the implementation of all universal 1-input (YES/NOT/PASS_1/PASS_0) and 2-input (AND/XOR/OR/NAND/NOR/INT/XNOR) logic gates.

Recent advances in emerging 2D-material-based integrated memory devices are reviewed in terms of working principles, device architectures, array integration, and specific brain-inspired applications. Future challenges and promising research lines toward reliable, practical neuromorphic computing chips are highlighted.

Abstract

With the advent of the Internet of Things and big data, massive data must be rapidly processed and stored within a short timeframe. This imposes stringent requirements on memory hardware implementation in terms of operation speed, energy consumption, and integration density. To fulfill these demands, 2D materials, which are excellent electronic building blocks, provide numerous possibilities for developing advanced memory device arrays with high performance, smart computing architectures, and desirable downscaling. Over the past few years, 2D-material-based memory-device arrays with different working mechanisms, including defects, filaments, charges, ferroelectricity, and spins, have been increasingly developed. These arrays can be used to implement brain-inspired computing or sensing with extraordinary performance, architectures, and functionalities. Here, recent research into integrated, state-of-the-art memory devices made from 2D materials, as well as their implications for brain-inspired computing are surveyed. The existing challenges at the array level are discussed, and the scope for future research is presented.

Nature Nanotechnology, Published online: 12 May 2022; doi:10.1038/s41565-022-01140-1

Nature, Published online: 11 May 2022; doi:10.1038/s41586-022-04588-2

2D materials show great potential to enable the transistor scaling down below 1 nm node. This review focuses on the introduction of the optimization of 2D materials-based transistor, new mechanisms, design flow, and circuits, which indicate the trend of 2D transistors toward very large-scale integrated circuits.

Abstract

2D transition metal chalcogenide (TMDC) materials, such as MoS₂, have recently attracted considerable research interest in the context of their use in ultrascaled devices owing to their excellent electronic properties. Microprocessors and neural network circuits based on MoS₂ have been developed at a large scale but still do not have an advantage over silicon in terms of their integrated density. In this study, the current structures, contact engineering, and doping methods for 2D TMDC materials for the scaling-down process and performance optimization are reviewed. Devices are introduced according to a new mechanism to provide the comprehensive prospects for the use of MoS₂ beyond the traditional complementary–metal–oxide semiconductor in order to summarize obstacles to the goal of developing high-density and low-power integrated circuits (ICs). Finally, prospects for the use of MoS₂ in large-scale ICs from the perspectives of the material, system performance, and application to nonlogic functionalities such as sensor circuits and analogous circuits, are briefly analyzed. The latter issue is along the direction of “more than Moore” research.

1D p-Te/2D n-Bi₂O₂Se heterodiodes with rationally designed nanoscale ultra-photosensitive channels are fabricated via dangling bond-free mixed-dimensional van der Waals integration. Except for the excellent photodetection performance, the type-II diodes exhibit a superlinear photoelectric conversion phenomenon, with a model based on the in-gap trap-assisted recombination proposed for this superlinearity.

Abstract

Continuous miniaturization of semiconductor devices is the key to boosting modern electronics development. However, this downscaling strategy has been rarely utilized in photoelectronics and photovoltaics. Here, in this work, a full-van der Waals (vdWs) 1D p-Te/2D n-Bi₂O₂Se heterodiode with a rationally designed nanoscale ultra-photosensitive channel is reported. Enabled by the dangling bond-free mixed-dimensional vdWs integration, the Te/Bi₂O₂Se type-II diodes show a high rectification ratio of 3.6 × 10⁴. Operating with 100 mV reverse bias or in a self-power mode, the photodiodes demonstrate excellent photodetection performances, including high responsivities of 130 A W⁻¹ (100 mV bias) and 768.8 mA W⁻¹ (self-power mode), surpassing most of the reports of other heterostructures. More importantly, a superlinear photoelectric conversion phenomenon is uncovered in these nanoscale full-vdWs photodiodes, in which a model based on the in-gap trap-assisted recombination is proposed for this superlinearity. All these results provide valuable insights in light–matter interactions for further performance enhancement of photoelectronic devices.

Inspired by the structure of cell membranes, including aquaporins for fast water transport and hydrophilic polymers for fouling resistance, a membrane is fabricated by assembling chitosan-modified graphene nanomesh. This cell membrane-inspired functionalized graphene nanomesh membrane is endowed with high water-permeance and superior antifouling when separating surfactant-stabilized oil-in-water emulsions.

Abstract

Graphene exhibits fascinating prospects for preparing high-performance membranes with fast water transport, due to its low friction with water and extreme thinness. However, for graphene-assembled membranes, each molecule passing through the membrane should bypass many graphene sheets, which lengthens the molecular pathways and increases the mass transfer resistance. Herein, a graphene nanomesh (GNM) membrane is fabricated that is inspired by cell membranes, including aquaporins with their hydrophilic gate for selective transport and hydrophobic channel for low friction with water, thus resulting in fast water transport, as well as hydrophilic polymer brushes on the membrane surface for fouling resistance. GNM is synthesized by etching nanopores on graphene oxide (GO) nanosheets to significantly shorten the water transport channels, whereas the hydrophobic graphene sheets lead to low water friction; in combination, ultra-fast, selective water flux is achieved. Also, hydrophilic polymer chitosan is utilized to modify GNM to construct a hydration layer, which suppresses foulants from touching the membrane surface. Accordingly, the permeance of the cell membrane-inspired graphene nanomesh membrane reaches almost 4000 L m^–2 h^–1 bar^–1, which is about 260 times the permeance in a GO membrane, and the membranes show superior antifouling properties for separating various surfactant-stabilized oil-in-water emulsions.

A general electrostatic self-assembly strategy is proposed for synthesizing nano-cabbages like MnO anchored on reduced graphene oxide (rGO/MnO). Benefited from the strong interfacial interactions, fast Li⁺ diffusion kinetics, and high Li-adsorption ability, the rGO/MnO heterostructure possesses impressive capacity and rate performances. Further coupled with activated carbon to assemble high-performance flexible solid-state lithium-ion capacitors, demonstrating its feasibility for practical applications.

Abstract

The delicate structural engineering is widely acknowledged as a powerful tool for boosting the electrochemical performance of conversion-type anode materials for lithium storage. Here, a general electrostatic self-assembly strategy is proposed for the in situ synthesis of MnO nano-cabbages on negatively charged reduced graphene oxide (rGO/MnO). The strong interfacial heterostructure and robust lithium storage mechanism related to fast Li⁺ diffusion kinetics and high Li-adsorption ability of rGO/MnO heterostructure are confirmed through operando experimental characterizations and theoretical calculation. Owing to the rapid charge transfer, enriched reaction sites, and stable heterostructure, the as-synthesized rGO/MnO anode delivers a high capacity (860 mAh g⁻¹ at 0.1 A g⁻¹), superior rate capability (211 mAh g⁻¹ at 10 A g⁻¹), and cycle stability. Notably, the as-assembled flexible pouch cell of activated carbon//rGO/MnO solid-state lithium-ion capacitors (LICs) possesses an exceptional energy density of 194 Wh kg⁻¹ and power density of 40.7 kW kg⁻¹, both of which are among the highest flexible solid-state LICs reported so far. Further, the LICs possess an ultralong life span with ≈77.8% retention after 10 000 cycles and extraordinary safety, demonstrative of great potential for practical applications.

Comprehensive understanding of interaction between photovoltaic and photothermoelectric effects is demonstrated via a time-resolved photocurrent (TRPC) measurement technique. Compared to MoS₂/multilayer WSe₂ p–n junction having a conventional TRPC dip, MoS₂/1L WSe₂ n–n junction processes a distinct TRPC peak, which is attributed to the opposite polarity between photovoltaic and photothermoelectric currents and can be further modulated via an external bias.

Abstract

Self-powered ultrafast 2D photodetectors have demonstrated great potential in imaging, sensing, and communication. Understanding the intrinsic ultrafast charge carrier generation and separation processes is essential for achieving high-performance devices. However, probing and manipulating the ultrafast photoresponse is limited either by the temporal resolution of the conventional methods or the required sophisticated device configurations. Here, van der Waals heterostructure photodetectors are constructed based on MoS₂/WSe₂ p–n and n–n junctions and manipulate the picosecond photoresponse by combining photovoltaic (PV) and photothermoelectric (PTE) effects. Taking time-resolved photocurrent (TRPC) measurements, a TRPC peak at zero time delay is observed with decay time down to 4 ps in the n–n junction device, in contrast to the TRPC dip in the p–n junction and pure WSe₂ devices, indicating an opposite current polarity between PV and PTE. More importantly, with an ultrafast photocurrent modulation, a transition from a TRPC peak to a TRPC dip is realized, and detailed carrier transport dynamics are analyzed. This study provides a deeper understanding of the ultrafast photocurrent generation mechanism in van der Waals heterostructures and offers a new perspective in instruction for designing more efficient self-powered photodetectors.

Carbon nanocoil-based magnetic helical nanorobots (C-HNR) are developed through a facile and economical pathway. Taking advantage of torque-driven corkscrew motion, the C-HNR can effectively penetrate the plasma membrane and even nuclear envelope at the subcellular level, on the basis of which it is used to collect intracellular surface enhanced Raman scattering signals and conduct photothermal therapy against targeted single cancer cell.

Abstract

Penetration of cell membrane at micro/nano-scale with untethered probe is of great scientific significance, yet still challenging. Here, a carbon nanocoil based magnetic nanorobot, which can precisely target single cell and perform cell membrane penetration, is reported. The nanorobots are as-synthesized by chemical vapor deposition with high yield, followed by physical deposition of Ni and Au nanofilms. Rotating electromagnetic fields are used to steer the nanorobots to navigate following any pre-designed path. The helical nanorobots can realize mechanical perforation of the plasma membrane and even nuclear envelope with precise position at the subcellular level. With the aid of the externally deposited Au nanofilm, surface enhanced Raman scattering bio-sensing signals can be collected from cellular plasma and nucleus. Furthermore, the nanorobots demonstrate capability of efficient photothermal therapy against targeted cancer cells and serve as an integrated theranostic platform, providing promising prospects in future precision medicine.

Nature Communications, Published online: 06 May 2022; doi:10.1038/s41467-022-30383-8

Nature Nanotechnology, Published online: 04 May 2022; doi:10.1038/s41565-022-01128-x