Jiuxiang Dai

Light: Science & Applications, Published online: 18 October 2023; doi:10.1038/s41377-023-01299-9

Orientation-patterned GaAs waveguide and mid-infrared picosecond pump laser are used in tandem to produce octave-spanning quadratic supercontinuum.

Abstract

Layered trihalides exhibit distinctive band structures and physical properties due to the sixfold coordinated 3d or 4d transition metal site and partially occupied d orbitals, holding great potential in condensed matter physics and advanced electronic applications. Prior research focused on trihalides with highly symmetric honeycomb-like structures, such as CrI₃ and α-RuCl₃, while the role of crystal anisotropy in trihalides remains elusive. In particular, the trihalide MoCl₃ manifests strong in-plane crystal anisotropy with the largest difference in Mo-Mo interatomic distances. Research on such material is imperative to address the lack of investigations on the effect of anisotropy on the properties of trihalides. Herein, we demonstrated the anisotropy of MoCl₃ through polarized Raman spectroscopy and further tuned the phonon frequency via strain engineering. We showed the Raman intensity exhibits twofold symmetry under parallel configuration and fourfold symmetry under perpendicular configuration with changing the polarization angle of incident light. Furthermore, we found that the phonon frequencies of MoCl₃ decrease gradually and linearly with applying uniaxial tensile strain along the axis of symmetry in the MoCl₃ crystal, while those frequencies increase with uniaxial tensile strain applied perpendicularly. Our results shed light on the manipulation of anisotropic light-matter interactions via strain engineering, and lay a foundation for further exploration of the anisotropy of trihalides and the modulation of their electronic, optical, and magnetic properties.

Nature, Published online: 18 October 2023; doi:10.1038/s41586-023-06572-w

Orbital multiferroicity reported in pentalayer rhombohedral graphene features ferro-orbital-magnetism and ferro-valleytricity, both of which can be controlled by an electric field.

Nature Communications, Published online: 18 October 2023; doi:10.1038/s41467-023-42131-7

There are many potential applications for non-centrosymmetric molecular crystals, but due to their typical brittle nature, efficiency of applications declines on prolonged use. Here, the authors report an autonomous self-healing ability of dibenzoate derivative single crystals that can retain its non-linear optical response.

Highlights

• Compared to the n-type two-dimensional (2D) semiconductors, the family of p-type 2D semiconductors is relatively small, which limits the broad integration of 2D semiconductors in potential applications. Here, the discovery and preparation of p-type 2D semiconductors are very important and meaningful.

• This review presents a timely and in-depth overview on the preparation and applications of p-type 2D semiconductors, which would help the related researchers to grasp the dynamics of this field and thus lay the foundations for their potential application in electronics and optoelectronics.

Violet phosphorus undergoes sequential polymorphic transitions under high pressure. Concomitant with the first phase transition, violet phosphorus exhibits emergent insulator–metal transition and dramatic switching from positive to negative photoconductivity. Intriguingly, the sample irreversibly transforms into black phosphorus with pronouncedly different physical properties from the pristine sample after decompression.

Abstract

Polymorphic phase transition is an essential phenomenon in condensed matter that the physical properties of materials may undergo significant changes due to the structural transformation. Phase transition has thus become an important means and dimension for regulating material properties. Herein, this study demonstrates the pressure-induced multi-transition of both structure and physical properties in violet phosphorus, a novel phosphorus allotrope. Under compression, violet phosphorus undergoes sequential polymorphic phase transitions. Concomitant with the first phase transition, violet phosphorus exhibits emergent insulator–metal transition, superconductivity, and dramatic switching from positive to negative photoconductivity. Remarkably, the resistance of violet phosphorus shows a sudden drop of around 10⁷ along with the phase transition. In addition, piezochromism from translucent red to opaque black and suppression of photoluminescence are observed upon compression. Of particular interest is that the sample irreversibly transforms into black phosphorus with a pronounced discrepancy in physical properties from the pristine violet phosphorus after decompression. The abundant polymorphic transitions and property changes in violet phosphorus have significant implications for designing novel pressure-responsive electronic/optoelectronic devices and exploring concealed polymorphic transition materials.

A thermal shock annealing method that enables kinetics-controlled epitaxial growth of graphene on SiC within 10 s is reported, which efficiently fulfills the requirements for producing high-quality, few-layer, and low-cost graphene on SiC.

Abstract

The direct epitaxial growth of graphene on semi-insulating SiC presents significant potential for a variety of technologically important applications, including next-generation electronics, photonics, and quantum metrology. However, this approach also poses a competitive disadvantage in terms of quality and cost, primarily due to the uncontrollable and time-consuming nature of the annealing process. Herein, a thermal shock annealing (TSA) method is reported that enables kinetics-controlled epitaxial growth of graphene on SiC within 10 s, which efficiently fulfills the requirements for producing high-quality, few-layer, and low-cost graphene on SiC. The epitaxial graphene (EG) grown on both β-SiC nanoparticles (SiC@EG NPs) and centimeter-scale α-SiC wafer (EG/SiC) exhibits mono- or bi-layer features with negligible structural defects. Moreover, the findings indicate that the TSA method can efficiently mitigate the persistent issue of step bunching conundrum and improve the flatness of EG/SiC. As an application demonstration, the significant enhancement of surface-enhanced infrared absorption (SEIRA) by SiC@EG NPs is exhibited. The graphene plasmon arising on SiC@EG NPs enables SEIRA detection sensitivity of up to a monolayer of p-nitrobenzenethiol (p-NTP). Consequently, the precise regulation and comprehensive comprehension of TSA afford an exceedingly desirable approach to produce cost-effective, high-quality EG growth on SiC for diverse emerging application scenarios.

npj 2D Materials and Applications, Published online: 19 October 2023; doi:10.1038/s41699-023-00434-9

Transfer-free rapid growth of 2-inch wafer-scale patterned graphene as transparent conductive electrodes and heat spreaders for GaN LEDs

Nature Electronics, Published online: 19 October 2023; doi:10.1038/s41928-023-01047-2

Industry compatible solid-state doping of regions between the channel and contacts in carbon nanotube transistors can be used to control device polarity and improve device performance.

Nature Materials, Published online: 19 October 2023; doi:10.1038/s41563-023-01667-1

The authors report the emergence of quadrupolar excitons in WS2/WSe2/WS2 trilayer heterostructures where the electron is layer-hybridized in WS2 layers and the hole localizes in WSe2. Quadrupolar excitons exhibit distinct behaviour under electric fields, enriching exciton–exciton interactions.

As electronic devices continue to shrink to just a few nanometers, enabling the integration of billions of devices in computers, power consumption has surged beyond control, exceeding thermal limits and leading to failure. To address this issue, ...

MnO nanoparticles embedded in a carbon layer are synthesized and found to spontaneously transform to Mn₃O₄ under ambient conditions, which can be reversed by heating in vacuum. The transformation is investigated in detail with in situ transmission electron microscopy, giving important insights into the stability of MnO nanoparticles used in, for example, battery technology.

Abstract

Manganese is an attractive element for sustainable solutions. It is largely available in the earth's crust, making it ideal for cost-effective and large-scale applications. Especially MnO nanoparticles have recently received attention for applications in battery technology. However, manganese has many oxidation states that are energetically very similar, indicating that they may easily transform from one to the other. Herein, the reversible oxidation of MnO nanoparticles to Mn₃O₄ studied with in situ transmission electron microscopy is shown. The oxygen sublattices of MnO and Mn₃O₄ are found to be perfectly aligned, and an atomic mechanism where the transformation is facilitated by the migration of Mn cations on the shared O sublattice is proposed. Even when protected with an amorphous carbon layer, MnO particles are highly unstable and oxidize to Mn₃O₄ in ethanol. The poor stability of MnO lacks discussion in many battery-related works, and strategies aimed at avoiding this should be developed.

A 2D van der Waals heterostructure-based multifunctional floating gate memory device is fabricated, which exhibits superior nonvolatile electronic/optoelectronic memory behaviors when subjected to intense stimuli and volatile memory behaviors after weakening the input stimuli. Leveraging its rich dynamics, a multimodal reservoir computing system, which can pave the way for future smart computing systems at the edge, is demonstrated.

Abstract

The demand for economical and efficient data processing has led to a surge of interest in neuromorphic computing based on emerging two-dimensional (2D) materials in recent years. As a rising van der Waals (vdW) p-type Weyl semiconductor with many intriguing properties, tellurium (Te) has been widely used in advanced electronics/optoelectronics. However, its application in floating gate (FG) memory devices for information processing has never been explored. Herein, an electronic/optoelectronic FG memory device enabled by Te-based 2D vdW heterostructure for multimodal reservoir computing (RC) is reported. When subjected to intense electrical/optical stimuli, the device exhibits impressive nonvolatile electronic memory behaviors including ≈10⁸ extinction ratio, ≈100 ns switching speed, >4000 cycles, >4000-s retention stability, and nonvolatile multibit optoelectronic programmable characteristics. When the input stimuli weaken, the nonvolatile memory degrades into volatile memory. Leveraging these rich nonlinear dynamics, a multimodal RC system with high recognition accuracy of 90.77% for event-type multimodal handwritten digit-recognition is demonstrated.

A self-limited epitaxial growth strategy is developed for large-area patterned growth of monolayer organic crystals. This approach confines the molecular nucleation and crystallization in defined wetting patterns. Intermolecular lattice mismatch between C₈-BTBT and C₁₀-BTBT inhibits the vertical molecular stacking, promoting the monolayer assembly. Polarization-sensitive phototransistors based on the monolayer organic crystals exhibit an ultrahigh dichroic ratio of 4.6 × 10³.

Abstract

Monolayer organic crystals provide an unprecedented opportunity for studies on the intrinsic charge transport of organic semiconductors because of their 2D nature that is free of grain boundaries with fewer defects. However, due to the spatially stochastic molecular nucleation and the difficulty in controlling monolayer assembly, large-scale growth of monolayer organic crystals remains a formidable challenge. Here, a self-limited epitaxial growth strategy is proposed to realize large-area patterned growth of monolayer organic crystals via physical vapor deposition process. Specifically, this approach confines the molecular nucleation and crystallization in defined wetting patterns, whose sizes are smaller than the mean free path of the molecules on substrate, enabling the formation of a single nucleus in wetting pattern. Meanwhile, the intermolecular lattice mismatch between 2,7-dialkyl[1]benzothieno[3,2-b][1]benzothiophene (C _n -BTBT, n = 8 and 10) derivatives inhibits vertical molecular stacking, thereby promoting the monolayer assembly of organic molecules. Using this approach, centimeter-sized patterned growth of mixed C _n -BTBT monolayer organic crystals is achieved. Polarization-sensitive phototransistors based on the monolayer organic crystals exhibit ultrahigh dichroic ratio up to 4.6 × 10³, on par with the commercial polarizers (10³). This work sheds light on large-scale patterned growth of monolayer organic crystals toward high-performance optoelectronic devices.

The freestanding form brings unique structural characteristics beyond bulk and epitaxial films, like declamping effect, extreme strain tunability and free stacking ability, providing a fertile playground for emergent primary ferroic/multiferroic properties and electronic applications. Here, the recent progress is briefly reviewed and the promising outlook for future research in freestanding ferroic perovskite oxide membranes and their applications is discussed.

Abstract

Transition metal perovskite oxide membranes and their unique properties have attracted great attention recently and have been developed into one of the research frontiers in condensed matter physics and materials science. Free of constraint imposed by the underlying substrate, freestanding membranes exhibit extraordinary structural tunability and flexibility far exceeding the bulk materials and epitaxial films clamped on substrates, which substantially extends the explorable regime in the phase diagrams. Moreover, high-quality oxide membranes, even down to a single unit cell, can be synthesized and stacked/integrated with any materials for novel artificial heterostructures and electronic applications. The exceptional structural tunability and stacking ability in oxide membranes provide new knobs to explore the spontaneous symmetry breaking in oxides, providing a fertile playground for emergent primary ferroic/multiferroic properties and electronic applications. Here, the recent progress is briefly reviewed and the promising outlook for future research in ferroic perovskite oxide membranes and their applications is discussed.

The increasing interest in ferroelectric tunnel junctions (FTJ) driven by ferroelectricity in fluorite-structured oxides such as HfO₂ and ZrO₂ offers advantages in thickness scaling, complementary metal-oxide-semiconductor compatibility, and 3D integration over perovskite-based FTJs. This review covers recent developments in FTJs with fluorite structures, explores their transport mechanisms, discusses applications, presents structural approaches, and reviews materials and integration challenges.

Abstract

The interest in ferroelectric tunnel junctions (FTJ) has been revitalized by the discovery of ferroelectricity in fluorite-structured oxides such as HfO₂ and ZrO₂. In terms of thickness scaling, CMOS compatibility, and 3D integration, these fluorite-structured FTJs provide a number of benefits over conventional perovskite-based FTJs. Here, recent developments involving all FTJ devices with fluorite structures are examined. The transport mechanism of fluorite-structured FTJs is explored and contrasted with perovskite-based FTJs and other 2-terminal resistive switching devices starting with the operation principle and essential parameters of the tunneling electroresistance effect. The applications of FTJs, such as neuromorphic devices, logic-in-memory, and physically unclonable function, are then discussed, along with several structural approaches to fluorite-structure FTJs. Finally, the materials and device integration difficulties related to fluorite-structure FTJ devices are reviewed. The purpose of this review is to outline the theories, physics, fabrication processes, applications, and current difficulties associated with fluorite-structure FTJs while also describing potential future possibilities for optimization.

A novel hexagonal boron nitride (hBN) synthesis method is demonstrated for two very different application classes: (i) single-crystal electronic grade hBN with a controllable number of layers for emerging 2D devices and (ii) protection of industrial alloys with extremely large-scale usage such as low-carbon and stainless steels, cupronickels and Inconels. The results suggest that steels protected by hBN can be used in very different industries.

Abstract

While hexagonal boron nitride (hBN) has been widely used as a buffer or encapsulation layer for emerging electronic devices, interest in utilizing it for large-area chemical barrier coating has somewhat faded. A chemical vapor deposition process is reported here for the conformal growth of hBN on large surfaces of various alloys and steels, regardless of their complex shapes. In contrast to the previously reported very limited protection by hBN against corrosion and oxidation, protection of steels against 10% HCl and oxidation resistance at 850 °C in air is demonstrated. Furthermore, an order of magnitude reduction in the friction coefficient of the hBN coated steels is shown. The growth mechanism is revealed in experiments on thin metal films, where the tunable growth of single-crystal hBN with a selected number of layers is demonstrated. The key distinction of the process is the use of N₂ gas, which gets activated exclusively on the catalyst's surface and eliminates adverse gas-phase reactions. This rate-limiting step allowed independent control of activated nitrogen along with boron coming from a solid source (like elemental boron). Using abundant and benign precursors, this approach can be readily adopted for large-scale hBN synthesis in applications where cost, production volume, and process safety are essential.

This work illustrates a thermal-assisted non-planar nanostructure transfer printing strategy for multiscale patterning of polystyrene nanosphere. The printing mechanism is the drop of Young's modulus, giving rise to an increased contact area, self-adhesive effect, and inter-particle necking. The multiscale colloidal nanostructure patterns on a 4-inch wafer with over 2750 pixels per inch resolution enable a new concept of multi-modal anti-counterfeiting.

Abstract

Nanotransfer printing of colloidal nanoparticles is a promising technique for the fabrication of functional materials and devices. However, patterning nonplanar nanostructures pose a challenge due to weak adhesion from the extremely small nanostructure-substrate contact area. Here, the study proposes a thermal-assisted nonplanar nanostructure transfer printing (NP-NTP) strategy for multiscale patterning of polystyrene (PS) nanospheres. The printing efficiency is significantly improved from ≈3.1% at low temperatures to ≈97.2% under the glass transition temperature of PS. Additionally, the arrangement of PS nanospheres transitioned from disorder to long-range order. The mechanism of printing efficiency enhancement is the drastic drop of Young's modulus of nanospheres, giving rise to an increased contact area, self-adhesive effect, and inter-particle necking. To demonstrate the versatility of the NP-NTP strategy, it is combined with the intaglio transfer printing technique, and multiple patterns are created at both micro and macro scales at a 4-inch scale with a resolution of ≈2757 pixels per inch (PPI). Furthermore, a multi-modal anti-counterfeiting concept based on structural patterns at hierarchical length scales is proposed, providing a new paradigm of imparting multiscale nanostructure patterning into macroscale functional devices.

They propose a unique concept to realize originally unstable two-dimensional materials by using moiré potential. This is demonstrated from their finding that octahedral MoTe₂ which is unstable in the bulk form is stabilized on graphene due to the electronic-energy gain associated with moiré superlattice. This concept would be useful to explore new crystal phases in various two-dimensional materials.

Abstract

A current key challenge in 2D materials is the realization of emergent quantum phenomena in hetero structures via controlling the moiré potential created by the periodicity mismatch between adjacent layers, as highlighted by the discovery of superconductivity in twisted bilayer graphene. Generally, the lattice structure of the original host material remains unchanged even after the moiré superlattice is formed. However, much less attention is paid for the possibility that the moiré potential can also modify the original crystal structure itself. Here, it is demonstrated that octahedral MoTe₂ which is unstable in bulk is stabilized in a commensurate MoTe₂/graphene hetero-bilayer due to the moiré potential created between the two layers. It is found that the reconstruction of electronic states via the moiré potential is responsible for this stabilization, as evidenced by the energy-gap opening at the Fermi level observed by angle-resolved photoemission and scanning tunneling spectroscopies. The present results provide a fresh approach to realize novel 2D quantum phases by utilizing the moiré potential.

Monolayer graphdiyne is synthesized by in situ acetylenic homocoupling of hexaethynylbenzene within the sub-nanometer interlayer space of Mxene. Free-standing monolayer graphdiyne flakes with micrometer-scale lateral are exfoliated from MXene interlayer assisted with ion intercalation. The free-standing monolayer graphdiyne presents excellent electronic properties which are superior to various previously reported multilayer graphdiyne materials.

Abstract

Graphdiyne (GDY) is an artificial carbon allotrope that is conceptually similar to graphene but composed of sp- and sp ²-hybridized carbon atoms. Monolayer GDY (ML-GDY) is predicted to be an ideal 2D semiconductor material with a wide range of applications. However, its synthesis has posed a significant challenge, leading to difficulties in experimentally validating theoretical properties. Here, it is reported that in situ acetylenic homocoupling of hexaethynylbenzene within the sub-nanometer interlayer space of MXene can effectively prevent out-of-plane growth or vertical stacking of the material, resulting in ML-GDY with in-plane periodicity. The subsequent exfoliation process successfully yields free-standing GDY monolayers with micrometer-scale lateral dimensions. The fabrication of field-effect transistor on free-standing ML-GDY makes the first measurement of its electronic properties possible. The measured electrical conductivity (5.1 × 10³ S m⁻¹) and carrier mobility (231.4 cm² V⁻¹ s⁻¹) at room temperature are remarkably higher than those of the previously reported multilayer GDY materials. The space-confined synthesis using layered crystals as templates provides a new strategy for preparing 2D materials with precisely controlled layer numbers and long-range structural order.

This article delves into the latest developments in 2D-memtransistor devices, shedding light on the essential operational mechanisms that allow for multiple non-volatile memory states. Following this, the article offers an in-depth review of how these devices are applied in neuromorphic, probabilistic, information security, and edge-sensing areas. The discussion further addresses current challenges and presents a forward-looking strategy for incorporating them into large-scale technological applications.

Abstract

The increased demand of high-performance computing systems has exposed the inherent limitations of the current state-of-the-art von Neumann architecture. Therefore, developing alternate computing primitives that can offer faster computing speed with low energy expenditure is critical. In this context, while several non-volatile memory (NVM) devices such as synaptic transistors, spintronic devices, phase change memory (PCM), and memristors have been demonstrated in the past, their two-terminal nature necessitates additional peripheral elements that increase area and energy overhead. Recently, a new multiterminal device prototype known as a memtransistor has shown tremendous potential to overcome these limitations through exceptional control of the gate electrostatics as enabled by 2D channel materials. In this perspective, a brief overview of recent developments in 2D-memtransistor devices is provided, including their fundamental operational mechanisms and the role of defects in enabling multiple NVM states and optical photoresponse. An overview of their implementation in the context of neuromorphic, probabilistic, information security, and edge-sensing primitives is also provided. Finally, a futuristic perspective is provided looking toward their successful large-scale technological integration.

Nature Communications, Published online: 23 October 2023; doi:10.1038/s41467-023-42447-4

The authors study Josephson junctions where the superconductors are Fe(Te,Se) flakes and the weak link is just a 0.36 nm van-der-Waals gap between the two stacked flakes. They report global device-level transport signatures of interfacial ferromagnetism.

Nature Communications, Published online: 24 October 2023; doi:10.1038/s41467-023-42567-x

van der Waals materials are usually characterized by a significant out-of-plane optical anisotropy, but in-plane birefringence is also necessary for photonics applications. Here, the authors report the presence of broadband optical anisotropy in a layered material, Ta2NiS5, showing in-plane birefringence of ~2 and ~0.5 in the visible and mid-infrared range, respectively.

A proton-reservoir type TPPS molecule is designed and synthesized to deliver organic synapses exhibiting 64-state conductance modulation characteristic with ultra-low power consumption of 6.5 fW to 0.83 pW and retention over 30 min. The critical contradiction between low-power operation and long-term stability of organic synapses is solved with the present TPPS molecule-based devices.

Abstract

High-performance artificial synapse with nonvolatile memory and low power consumption is a perfect candidate for brainoid intelligence. Unfortunately, due to the energy barrier paradox between ultra-low power and nonvolatile modulation of device conductances, it is still a challenge at the moment to construct such ideal synapses. Herein, a proton-reservoir type 4,4′,4″,4'''-(Porphine-5,10,15,20-tetrayl) tetrakis (benzenesulfonic acid) (TPPS) molecule and fabricated organic protonic memristors with device width of 10 µm to 100 nm is synthesized. The occurrence of sequential proton migration and interfacial self-coordinated doping will introduce new energy levels into the molecular bandgap, resulting in effective and nonvolatile modulation of device conductance over 64 continuous states with retention exceeding 30 min. The power consumptions of modulating and reading the device conductance approach the zero-power operating limits, which range from 16.25 pW to 2.06 nW and 6.5 fW to 0.83 pW, respectively. Finally, a robust artificial synapse is successfully demonstrated, showing spiking-rate-dependent plasticity (SRDP) and spiking-timing-dependent plasticity (STDP) characteristics with ultra-low power of 0.66 to 0.82 pW, as well as 100 long-term depression (LTD)/potentiation (LTP) cycles with 0.14%/0.30% weight variations.

Publication date: 15 November 2023

Source: Joule, Volume 7, Issue 11

Author(s): Alessandro Virtuani, Alejandro Borja Block, Nicolas Wyrsch, Christophe Ballif

“High-quality pulsed laser deposited barium titanate (BTO) thin films on magnesium oxide substrates are studied. Key findings include measurements of Pockels coefficient and relative permittivity at varying frequencies, revealing the potential for high-speed BTO applications. Additionally, the crystalline quality and domain distribution are characterized. This research paves the way for advanced BTO-based components in sensing and communications.”

Abstract

The study demonstrates high-quality pulsed-laser-deposited (PLD) barium titanate (BTO) thin-films on a magnesium oxide substrate. The frequency response of the relative permittivity (dielectric constant) and the linear electro-optical coefficient (Pockels coefficient) are measured. At 0.2 GHz, the Pockels coefficient is fitted to be r ₄₂ ≈ 1030 pm V⁻¹. It decreases to ≈390 pm V⁻¹ at 10 GHz after which it remains constant up to 70 GHz. The unbiased BTO permittivity is measured to be ε _a ≈ 7600 at 0.2 GHz, dropping to ≈1100 at 67 GHz, while the biased BTO  had a permittivity ε _a ≈ 2000 at 0.2 GHz, dropping to ≈500 at 67 GHz. These results fill an important experimental characterization gap for high-speed BTO applications and show the high quality of PLD-grown BTO films. Lastly, the material's crystalline quality is characterized and the domain distribution is imaged. The findings enable the design and fabrication of a new generation of BTO-based components for sensing and communications.