Jiuxiang Dai

The design of artificial complex van der Waals 2D superlattices with unit cell repetitions reaching n > 10 is introduced. A fully automated, rapid, and relatively simple in situ pulsed laser deposition approach provides cm² scale homogeneous superlattices with on-demand material parameters tailoring (layer number, order, and composition). The potential for designing properties by shaping the band-structure landscape is highlighted.

Abstract

Van der Waals heterostructures are set as strong contenders for post-CMOS quantum materials engineering. A major step for their systematic exploration and exploitation of technological component demonstrators resides in their eased large-scale design. In this direction, the growth of artificial van der Waals 2D superlattices is presented here such as (MoS₂/WS₂)_n, (WS₂/WSe₂)_n, and (MoS₂/WSe₂)_n with unit cells repetitions reaching n > 10. The fabrication of these materials is enabled by a fully automated in-situ pulsed laser deposition (PLD) tool. This approach provides cm² scale homogeneous superlattices with on-demand material parameters tailoring (layer number, order, and composition). The process is rapid and simple compared to manual pickup exfoliation methods or to sequential transfers of single layers grown by techniques such as chemical vapor deposition, allowing a large repetition of the unit cells in a “mille-feuille” cake configuration. The computational exploration of this family of superlattice materials sheds light on the potential for optoelectronic property design by shaping the band-structure landscape while taking into account the influential effects induced by proximity. Overall, this large-area approach is proposed as an entry point for the systematic design of complex van der Waals heterostructures.

Significant improvements and enhanced gate-voltage tunability of spin–orbit torque are achieved in CoFe/Pt films interfaced with WS₂/WS₂ moiré superlattices. The findings are attributed to the moiré magnetic field-modulated absorption of spin-Hall current from Pt through the magnetic proximity effect. This study highlights the potential of moiré physics as a novel building block for the design of advanced spintronic devices.

Abstract

Artificial moiré superlattices created by stacking 2D crystals have emerged as a powerful platform with unprecedented material-engineering capabilities. While moiré superlattices are reported to host a number of novel quantum states, their potential for spintronic applications remains largely unexplored. Here, the effective manipulation of spin–orbit torque (SOT) is demonstrated using moiré superlattices in ferromagnetic devices comprised of twisted WS₂/WS₂ homobilayer (t-WS₂) and CoFe/Pt thin films by altering twisting angle (θ) and gate voltage. Notably, a substantial enhancement of up to 44.5% is observed in SOT conductivity at θ ≈ 8.3°. Furthermore, compared to the WS₂ monolayer and untwisted WS₂/WS₂ bilayers, the moiré superlattices in t-WS₂ enable a greater gate-voltage tunability of SOT conductivity. These results are related to the generation of the interfacial moiré magnetic field by the real-space Berry phase in moiré superlattices, which modulates the absorption of the spin-Hall current arising from Pt through the magnetic proximity effect. This study highlights the moiré physics as a new building block for designing enhanced spintronic devices.

Guided by the quasi-spherical theory, we successfully introduced two cage-like carborane molecular ferroelectric crystals: 9-OH/SH-o-carborane. This discovery marks the first appearance of ferroelectric crystals composed of carborane molecules, paving the way for exploration of multi-axis ferroelectric crystals in various carborane cage derivatives.

Abstract

Carborane compounds, known for their exceptional thermal stability and non-toxic attributes, have garnered widespread utility in medicine, supramolecular design, coordination/organometallic chemistry, and others. Although there is considerable interest among chemists, the integration of suitable carborane molecules into ferroelectric materials remains a formidable challenge. In this study, we employ the quasi-spherical design strategy to introduce functional groups at the boron vertices of the o-carborane cage, aiming to reduce molecular symmetry. This approach led to the successful synthesis of the pioneering ferroelectric crystals composed of cage-like carboranes: 9-OH-o-carborane (1) and 9-SH-o-carborane (2), which undergo above-room ferroelectric phase transitions (T _c) at approximately 367 K and 347 K. Interestingly, 1 and 2 represent uniaxial and multiaxial ferroelectrics respectively, with 2 exhibiting six polar axes and as many as twelve equivalent polarization directions. As the pioneering instance of carborane ferroelectric crystals, this study introduces a novel structural archetype for molecular ferroelectrics, thereby providing fresh insights into the exploration of molecular ferroelectric crystals with promising applications.

Bulk and 2D SnTe are synthesized using flame melting and liquid phase exfoliation. Nanostructured 2D SnTe shows a Seebeck coefficient of 280 µV K⁻¹, four times higher than bulk SnTe. This is due to increased DOS and bandgap. Enhanced interface scattering reduces thermal conductivity, yielding a high ZT of 0.17, promising wearable self-charging devices.

Abstract

A lot of experimental studies are conducted on theoretically predicted thermoelectric 2D materials. Such materials can pave the way for charging ultra-thin electronic devices, self-charging wearable devices, and medical implants. This study systematically explores the thermoelectric attributes of bulk and 2D nanostructured Tin Telluride (SnTe), employing experimental investigations and theoretical analyses based on semiclassical Boltzmann transport theory. The bulk SnTe is synthesized through flame melting, while the 2D SnTe is produced via liquid phase exfoliation. The comprehensive assessment of thermoelectric properties integrated experimental measurements utilizing a Physical Property Measurement System and theoretical calculations from the BoltzTraP code. Experimental thermoelectric studies show a high ZT of 0.17 for 2D SnTe when compared to bulk (0.005) at room temperature. This rise in ZT is due to the high Seebeck coefficient and low thermal conductivity of nanostructured 2D SnTe. Density functional theory (DFT) studies reveal the contribution of the density of states (DOS) and energy bandgap in enhancing the Seebeck coefficient and lowering thermal conductivity by interface scattering.

The oxygen-defective nanostructures-based thin film miniaturized energy storage devices provide several benefits for social and economic development, such as avionics, health, and the environment.

Abstract

For the initial instance, oxygen deficiency-enriched vanadium pentoxide (O─V₂O₅@500) thin film electrodes are tuned by the Pulsed Laser Ablation technique. The O─V₂O₅@500 thin film electrode shows remarkable electrochemical performances confirming the greater potential window of −0.4 to 0.9 V versus Hg/HgO in an alkaline electrolyte; also, the O─V₂O₅@ 500 thin film electrode exhibits a noteworthy volumetric capacity of 167.7 mAh cm⁻³ (areal capacity of 73.3 µAh cm⁻²). Additionally, Density Functional Theory (DFT) theory calculations are carried out for oxygen-deficient V₂O₅. From the partial density of states (pDOS) and partial charge density analysis, it is clear that oxygen vacancy improves the electrical conductivity due to the higher degree of electron delocalization of V─O─V near the vacancy and enhances the redox properties due to the formation of in-gap states. Further, it is reported that a O─V₂O₅@ 500 ||PVA-KOH|| Bi₂O₃ A-650 thin film supercapbattery (TFSCB) device attains an exceptional discharge volumetric capacitance of 182.85 F cm⁻³ (equal volumetric capacity of 124.5 mAh cm⁻³). Furthermore, the TFSCB device exhibits an extraordinary maximum volumetric energy (power) density of 14.28 mWh cm⁻³ (1.66 W cm⁻³); TFSCB succeeds in supreme capacity retention of 86% with outstanding coulombic efficiency of 94.4% after 21 000 cycles.

A GeTe/MoTe2 heterostructure cross-point memristor is fabricated using RF magnetron sputtering for nonvolatile memory and neuromorphic-computing applications. This memristor exhibits resistive switching at remarkably low switching voltages, making it highly energy efficient. Additionally, it can efficiently mimic various synaptic functions of a biological synapse, indicating that the GeTe/MoTe2 HS can be used for energy-efficient neuromorphic-computing applications.

Abstract

Advanced electronic semiconducting Van der Waals heterostructures (HSs) are promising candidates for exploring next-generation nanoelectronics owing to their exceptional electronic properties, which present the possibility of extending their functionalities to diverse potential applications. In this study, GeTe/MoTe₂ HS are explored for nonvolatile memory and neuromorphic-computing applications. Sputter-deposited Ag/GeTe/MoTe₂/Pt HS cross-point devices are fabricated, and they demonstrate memristor behavior at ultralow switching voltages (V_SET: 0.15 V and V_RESET: −0.14 V) with very low energy consumption (≈30 nJ), high memory window, long retention time (10⁴ s), and excellent endurance (10⁵ cycles). Resistive switching is achieved by adjusting the interface between the Ag top electrode and the heterojunction switching layer. Cross-sectional transmission electron microscope images and conductive atomic force microscopy analysis confirm the presence of a conducting filament in the heterojunction switching layer. Further, emulating various synaptic functions of a biological synapse reveals that GeTe/MoTe₂ HS can be utilized for energy-efficient neuromorphic-computing applications. A multilayer perceptron is implemented using the synaptic weights of the Ag/GeTe/MoTe₂/Pt HS device, revealing high pattern accuracy (81.3%). These results indicate that HS devices can be considered a potential solution for high-density memory and artificial intelligence applications.

The Ni-PTFE mold is a nanocomposite mold fabricated by one-pot electroforming using anodic aluminum oxide (AAO) foil as a template. It has low surface energy and low surface wettability, enabling precise, defect-free nanopatterns down to 100 nm over 20 cycles of thermal nanoimprinting (T-NIL). UV-NIL also succeeded, with the nanocomposite mold exhibiting superior demolding compared to a silane-coated Ni mold.

Abstract

Nanoimprinting large-area structures, especially high-density features like meta lenses, poses challenges in achieving defect-free nanopatterns. Conventional high-resolution molds for nanoimprinting are often expensive, typically constructed from inorganic materials such as silicon, nickel (Ni), or quartz. Unfortunately, replicated nanostructures frequently suffer from breakage or a lack of definition during demolding due to the high adhesion and friction at the polymer-mold interface. Moreover, mold degradation after a limited number of imprinting cycles, attributed to contamination and damaged features, is a common issue. In this study, a disruptive approach is presented to address these challenges by successfully developing an anti-sticking nanocomposite mold. This nanocomposite mold is created through the co-deposition of nickel atoms and low surface tension polytetrafluoroethylene (PTFE) nanoparticles via electroforming. The incorporation of PTFE enhances the ease of polymer release from the mold. The resulting Ni-PTFE nanocomposite mold exhibits exceptional lubrication properties and a significantly reduced surface energy. This robust nanocomposite mold proves effective in imprinting fine, densely packed nanostructures down to 100 nm using thermal nanoimprinting for at least 20 cycles. Additionally, UV nanoimprint lithography (UV-NIL) is successfully performed with this nanocomposite mold. This work introduces a novel and cost-effective approach to reusable high-resolution molds, ensuring defect-reduction production in nanoimprinting.

2D nanomaterials, with their ultrathin structures and unique properties, have revolutionized hydrogel-based therapies for bone regeneration and disease treatment. This review discusses advancements in integrating 2D materials like graphene and MXene into hydrogels, enhancing their mechanical properties, drug release capabilities, and therapeutic efficacy in bone repair, cancer treatments, and antibacterial applications.

Abstract

This review highlights recent advancements in the synthesis, processing, properties, and applications of 2D-material integrated hydrogels, with a focus on their performance in bone-related applications. Various synthesis methods and types of 2D nanomaterials, including graphene, graphene oxide, transition metal dichalcogenides, black phosphorus, and MXene are discussed, along with strategies for their incorporation into hydrogel matrices. These composite hydrogels exhibit tunable mechanical properties, high surface area, strong near-infrared (NIR) photon absorption and controlled release capabilities, making them suitable for a range of regeneration and therapeutic applications. In cancer therapy, 2D-material-based hydrogels show promise for photothermal and photodynamic therapies, and drug delivery (chemotherapy). The photothermal properties of these materials enable selective tumor ablation upon NIR irradiation, while their high drug-loading capacity facilitates targeted and controlled release of chemotherapeutic agents. Additionally, 2D-materials -infused hydrogels exhibit potent antibacterial activity, making them effective against multidrug-resistant infections and disruption of biofilm generated on implant surface. Moreover, their synergistic therapy approach combines multiple treatment modalities such as photothermal, chemo, and immunotherapy to enhance therapeutic outcomes. In bio-imaging, these materials serve as versatile contrast agents and imaging probes, enabling their real-time monitoring during tumor imaging. Furthermore, in bone regeneration, most 2D-materials incorporated hydrogels promote osteogenesis and tissue regeneration, offering potential solutions for bone defects repair. Overall, the integration of 2D materials into hydrogels presents a promising platform for developing multifunctional theragenerative biomaterials.

Through the integration of the custom-designed glove-box interconnection system with atomic-level scanning transmission electron microscopy (STEM) characterization and scanning tunneling microscopy/spectroscopy, this work has successfully identified, for the first time, three intrinsic grain boundary structures within the air-sensitive single-layer two-dimensional (2D) topological insulator-WTe₂. Additionally, this investigation has revealed heightened electron state densities at these grain boundaries.

Abstract

Monolayer WTe₂ has attracted significant attention for its unconventional superconductivity and topological edge states. However, its air sensitivity poses challenges for studying intrinsic defect structures. This study addresses this issue using a custom-built inert gas interconnected system, and investigate the intrinsic grain boundary (GB) structures of monolayer polycrystalline 1T’ WTe₂ grown by nucleation-controlled chemical vapor deposition (CVD) method. These findings reveal that GBs in this system are predominantly governed by W-Te rhombi with saturated coordination, resulting in three specific GB prototypes without dislocation cores. The GBs exhibit anisotropic orientations influenced by kinks formed from these fundamental units, which in turn affect the distribution of grains in various shapes within polycrystalline flakes. Scanning tunneling microscopy/spectroscopy (STM/S) analysis further reveals metallic states along the intrinsic 120° twin grain boundary (TGB), consistent with computed band structures. This systematic exploration of GBs in air-sensitive 1T’ WTe₂ monolayers provides valuable insights into emerging GB-related phenomena.

Amorphous glass to crystalline nepheline phase in Eu²⁺-doped Na₂O-Al₂O₃-SiO₂ system is identified, and persistent luminescence (PersL) colors ranging from blue to yellow are achieved. A high-security optical information storage model is constructed and X-ray luminescence time-lapse imaging is achieved. These findings advance the development of multi-source excitation PersL materials, and provide spectral manipulation strategies for PersL materials.

Abstract

Multi-mode luminescence tuning through in situ crystallization of topological glass holds significance for photonics applications. Here, NaAlSiO₄:Eu²⁺-based glass ceramics are presented as stable persistent luminescence (PersL) materials by controlling the phase transformation in Eu²⁺ doped Na₂O-Al₂O₃-SiO₂ from amorphous glass to crystalline nepheline phase. X-ray and UV excited PersL colors from blue to yellow are realized by adjusting the crystallization duration time, and trap statuses reveal the dynamic PersL process with a trap redeployment from 0.83 to 1.09 eV during the transformation. Optical information storage model is constructed with a structure of stacked emitting layers to enhance storage capacity. X-ray excited luminescence time-lapse imaging is further demonstrated with optical memory longer than 5 days. These findings advance the development of superior stable PersL glass ceramics and offer optical manipulation tactics for multi-mode optical information storage applications.

Nature Communications, Published online: 19 June 2024; doi:10.1038/s41467-024-49107-1

The authors present an equimolar-ratio element high-entropy strategy for designing high-performance dielectric ceramics and uncover the immense potential of tetragonal tungsten bronze-type materials for advanced energy storage applications.

With the unique device structures and the development of 2D materials, many transistors with ultrasmall feature-sizes are successfully fabricated. This review summarizes the novel, advanced methods for fabricating ultrasmall nanoscale transistors, and discusses the challenges and future trends of ultrasmall devices.

Abstract

For several decades after Moore's Law is proposed, there is a continuous effort to reduce the feature-size of transistors. However, as the size of transistors continues to decrease, numerous challenges and obstacles including severe short channel effects (SCEs) are emerging. Recently, low-dimensional materials have provided new opportunities for constructing small feature-size transistors due to their superior electrical properties compared to silicon. Here, state-of-the-art low-dimensional materials-based transistors with small feature-sizes are reviewed. Different from other works that mainly focus on material characteristics of a specific device structure, the discussed topics are utilizing device structure design including vertical structure and nano-gate structure, and nanofabrication techniques to achieve small feature-sizes of transistors. A comprehensive summary of these small feature-size transistors is presented by illustrating their operation mechanism, relevant fabrication processes, and corresponding performance parameters. Besides, the role of small feature-size transistors based on low-dimensional materials in further reducing the small footprint is also clarified and their cutting-edge applications are highlighted. Finally, a comparison and analysis between state-of-art transistors is made, as well as a glimpse into the future research trajectory of low dimensional materials-based small feature-size transistors is briefly outlined.

Molecular-beam epitaxy of 2D chalcogenides typically yields small, disconnected islands, with premature onset of multilayer formation. This work reports how utilizing excited ions of a sacrificial species during the growth can dramatically enhance nucleation of the epitaxial layer, enabling growth of large-area monolayers with enhanced carrier lifetimes and facilitating the fabrication of all-epitaxial van der Waals heterostructures.

Abstract

The transition-metal chalcogenides include some of the most important and ubiquitous families of 2D materials. They host an exceptional variety of electronic and collective states, which can in principle be readily tuned by combining different compounds in van der Waals heterostructures. Achieving this, however, presents a significant materials challenge. The highest quality heterostructures are usually fabricated by stacking layers exfoliated from bulk crystals, which – while producing excellent prototype devices – is time consuming, cannot be easily scaled, and can lead to significant complications for materials stability and contamination. Growth via the ultra-high vacuum deposition technique of molecular-beam epitaxy (MBE) should be a premier route for 2D heterostructure fabrication, but efforts to achieve this are complicated by non-uniform layer coverage, unfavorable growth morphologies, and the presence of significant rotational disorder of the grown epilayer. This work demonstrates a dramatic enhancement in the quality of MBE grown 2D materials by exploiting simultaneous deposition of a sacrificial species from an electron-beam evaporator during the growth. This approach dramatically enhances the nucleation of the desired epi-layer, in turn enabling the synthesis of large-area, uniform monolayers with enhanced quasiparticle lifetimes, and facilitating the growth of epitaxial van der Waals heterostructures.

Ferromagnetic Josephson junction is important for understanding the interplay between superconductivity and ferromagnetism. The NbSe₂/Cr₂Ge₂Te₆/NbSe₂ ferromagnetic Josephson junction shows an unconventional dual-peak feature in the magnetic-field-dependent critical supercurrent due to the multidomain structure of the Cr₂Ge₂Te₆ tunnel barrier. This work helps researchers explore the interactions between Ising Cooper pairs and magnetic domains and realize practical magnetic Josephson junction devices.

Abstract

Ferromagnetic Josephson junctions play a key role in understanding the interplay between superconductivity and ferromagnetism in condensed matter physics. The magnetic domain structures of the ferromagnet in such junctions can significantly affect the tunneling of the superconducting Cooper pairs due to the strong interactions between Cooper pairs and local magnetic moments in the ferromagnetic tunnel barrier. However, the underlying microscopic mechanism of relevant quasiparticle tunneling processes with magnetic domain structures remains largely unexplored. Here, the manipulation of Cooper-pair tunneling in the NbSe₂/Cr₂Ge₂Te₆/NbSe₂ ferromagnetic Josephson junction is demonstrated by using a multidomain ferromagnetic barrier with anisotropic magnetic moments. The evolution of up-, down-magnetized domain and Bloch domain structures in Cr₂Ge₂Te₆ barrier under external magnetic fields leads to the enhancement of the critical tunneling supercurrent and an unconventional dual-peak feature with two local maxima in the field-dependent critical current curve. The phenomenon of magnetic-field-modulated critical tunneling supercurrent can be well explained by the competition between the coherence length of tunneling Cooper pairs and the size of magnetic domain walls in Cr₂Ge₂Te₆ barrier. This kind of ferromagnetic Josephson junction provides an intriguing material system for manipulating Cooper-pair tunneling by tuning the local magnetic moments within magnetic Josephson junction devices.

Nature Photonics, Published online: 18 June 2024; doi:10.1038/s41566-024-01435-w

Spatial distribution of the photoluminescence of interlayer excitons in van der Waals heterostructures comprising MoSe2 and WSe2 monolayers and encapsulated in rather thick hexagonal boron nitride is investigated, revealing interlayer exciton long-range transport with 1/e decay distances reaching and exceeding 100 μm.

Nature Photonics, Published online: 13 June 2024; doi:10.1038/s41566-024-01444-9

Using the 3R phase of molybdenum disulfide nanodisks with various radii, more than 100-fold enhancement of second-harmonic generation can be obtained in a single resonant nanodisk compared with an unpatterned flake of the same thickness.

npj 2D Materials and Applications, Published online: 13 June 2024; doi:10.1038/s41699-024-00475-8

Full phonon dispersion along the stacking direction in nanoscale van der Waals materials by picosecond acoustics

Author(s): Rodolfo Rodriguez, Mikhail Cherkasskii, Rundong Jiang, Ritwik Mondal, Arezoo Etesamirad, Allison Tossounian, Boris A. Ivanov, and Igor Barsukov

A universal formalism for the critical spin dynamics of auto-oscillations based on an isomorphism between spin dynamics in ferrimagnets and inertial ferromagnets could guide the development of numerous applications involving spin-transfer torque physics.

[Phys. Rev. Lett. 132, 246701] Published Thu Jun 13, 2024

Author(s): Elliot Snider, Nathan Dasenbrock-Gammon, Raymond McBride, Xiaoyu Wang, Noah Meyers, Keith V. Lawler, Eva Zurek, Ashkan Salamat, and Ranga P. Dias

[Phys. Rev. Lett. 132, 249901] Published Thu Jun 13, 2024