Jiuxiang Dai

In this study, it fingerprints local electronic properties, thickness, and strain fields in twisted WS₂ flakes. Advanced imaging techniques and computational analysis reveal that twisted flakes display non-uniform strain distribution, leading to a remarkable 20% increase in bandgap energy attributed to deformation angle influence. This discovery illuminates twisted 2D materials' electronic behavior and introduces a novel framework with twistronics implications.

Abstract

Twisted 2D materials present an enticing platform for exploring diverse electronic properties owning to the tunability of their bandgap energy. However, the intricate relationship between local heterostrain fields, thickness, and bandgap energy remains insufficiently understood, particularly at the nanoscale. Here, it presents a comprehensive nanoscale study elucidating the remarkable sensitivity of the bandgap energy to both thickness and heterostrain fields within twisted WS₂ nanostructures. This approach integrates electron energy-loss spectroscopy (EELS) enhanced by machine learning with 4D scanning transmission electron microscopy (STEM). Through this synergistic methodology, enhancements up to 20% in the bandgap energy is unveiled depending on the specimen thickness. This phenomenon is traced back to sizable deformation angles present within individual layers, which can be directly linked to distinct variations in local heterostrain fields. The findings represent a significant advancement in comprehending the electronic behavior of twisted 2D materials and introduce a novel methodological framework with far-reaching implications for twistronics and the investigation of other materials within the nanoscience domain.

Volatile and nonvolatile memristors based on 2D hexagonal boron nitride are fabricated and the leaky-integrate-and-fire neuron and synaptic functions are demonstrated. The network connectivity relationships using 2D hBN-based artificial neuron and synaptic devices in hardware are investigated. The connection strength between the artificial neurons is well modulated via the different synaptic weights of the artificial synaptic device.

Abstract

Brain-inspired neuromorphic computing has been developed as a potential candidate for solving the von Neumann bottleneck of traditional computing systems. 2D materials-based memristors have been exponentially investigated as promising building blocks of neuromorphic computing because of their excellent electrical performance, simple structure, and small device scale. However, while many researchers have focused on looking into individual artificial neuromorphic devices based on memristors, only few studies on the integration of artificial neuron and synaptic devices have been reported. In this work, both volatile and nonvolatile memristors are fabricated by using a 2D hexagonal boron nitride film for artificial neuron and synaptic devices, respectively. The leaky-integrate-and-fire neuron performance and synaptic functions (e.g., synaptic weight plasticity and spike-timing-dependent plasticity) are well emulated with the fabricated volatile and nonvolatile devices. The MNIST image classification is conducted based on the experimental data. For the first time, an artificial neuron-synapse-neuron neural network is physically constructed using the artificial neuron and synaptic devices to mimic the biological neural networks. The synaptic connection strength modulation is experimentally demonstrated between the neurons depending on the conductance state of the synapse, paving the way for the development of large-scale neural network hardware.

The first tantalum-based deep-UV NLO crystals A₅Ta₃OF₁₈ (A = K, Rb) were designed and synthesized by a newly developed de-covalency band gap engineering strategy. With the optimized combination of two types of 5d ⁰-Ta-centered polyhedra, they simultaneously realized strong SHG efficiencies and record-short phase-matching wavelength.

Abstract

The development of urgently-needed ultraviolet (UV)/deep-UV nonlinear optical (NLO) materials has been hindered by contradictory requirements of the microstructure, in particular the need for a strong second-harmonic generation (SHG) response as well as a short phase-matching (PM) wavelength. We herein employ a “de-covalency” band gap engineering strategy to adjust the optical linearity and nonlinearity. This has been achieved by assembling two types of transition-metal (TM) polyhedra ([TaO₂F₄] and [TaF₇]), affording the first tantalum-based deep-UV-transparent NLO materials, A₅Ta₃OF₁₈ (A = K (KTOF), Rb (RTOF)). Experimental and theoretical studies reveal that the highly ionic bonds and strong electropositivity of tantalum in the two oxyfluorides induce record short PM wavelengths (238 (KTOF) and 240 (RTOF) nm) for d ⁰-TM-centered oxides, in addition to strong SHG responses (2.8 × KH₂PO₄ (KTOF) and 2.6 × KH₂PO₄ (RTOF)), and sufficient birefringences (0.092 (KTOF) and 0.085 (RTOF) at 546 nm). These results not only broaden the available strategies for achieving deep-UV NLO materials by exploiting the currently neglected d ⁰-TMs, but also push the shortest PM wavelength into the short-wavelength UV region.

Highlights

• KTiNbO₅ and KTiNbO₅/reduced graphene oxide (rGO) nanocomposites were successfully synthesised via solvothermal methods. Optimising the rGO wt% yielded a composite with 12 wt% (KTNO/rGO-12).

• KTNO/rGO-12 was tested for its potassium storage performance, achieving a first charge capacity of 128.1 mAh g⁻¹ and retaining 76.1% over 500 cycles at 20 mAh g⁻¹.

• The mechanism of intercalation was examined, suggesting a potentially low-strain material, with both titanium and niobium redox activity contributing to the charge storage.

Nature Physics, Published online: 06 November 2023; doi:10.1038/s41567-023-02259-1

A detailed analysis of inelastic neutron scattering data, including the evaluation of entanglement witnesses used in quantum information theory, supports the proposal that the triangular-lattice antiferromagnet KYbSe2 is close to a spin-liquid phase.

Nature Physics, Published online: 06 November 2023; doi:10.1038/s41567-023-02266-2

Interactions between excitons and correlated electrons can lead to the formation of interesting states. Now, evidence suggests that these interactions can give rise to a Mott insulator of excitons.

In this study, the spray pyrolysis technique, a novel approach for large-area manufacturing of 2D ferroelectric-semiconductor In₂Se₃, is proposed. It demonstrates excellent uniform growth with a 15 cm × 15 cm sized glass substrate. Excellent electrical characteristics such as a large memory window and high on/off current ratio exceeding 10⁷ are achieved with the fabricated In₂Se₃ ferroelectric-semiconductor field-effect transistor.

Abstract

In₂Se₃, 2D ferroelectric-semiconductor, is a promising candidate for next-generation memory device because of its outstanding electrical properties. However, the large-area manufacturing of In₂Se₃ is still a big challenge. In this work, spray pyrolysis technique is introduced for the growth of large-area In₂Se₃ thin film. A polycrystalline γ-In₂Se₃ layer can be grown on 15 cm × 15 cm glasss at the substrate temperature of 275 °C. The In₂Se₃ ferroelectric-semiconductor field effect transistor (FeS-FET) on glass substrate demonstrates a large hysteresis window of 40.3 V at the ±40 V of gate voltage sweep and excellent uniformity. The FeS-FET exhibits an electron field effect mobility of 0.97 cm² V⁻¹ s⁻¹ and an on/off current ratio of >10⁷ in the transfer curves. The memory behavior of the large-area, In₂Se₃ FeS-FETs for next-generation memory is demonstrated.

A resistless scanning probe-based nanolithography method is presented, which allows the controllable patterning of down to single atomic layers of MoTe₂. Through this, it is possible to demonstrate for the first time an anisotropic instability of 1T’-MoTe₂ to aqueous environments. It is furthermore used to fabricate a nanoribbon transistor out of a three atomic layer thick 2H-MoTe₂ nanosheet.

Abstract

An unconventional approach for the resistless nanopatterning 2H- and 1T’-MoTe₂ by means of scanning probe lithography is presented. A Fowler–Nordheim tunneling current of low energetic electrons (E = 30–60 eV) emitted from the tip of an atomic force microscopy (AFM) cantilever is utilized to induce a nanoscale oxidation on a MoTe₂ nanosheet surface under ambient conditions. Due to the water solubility of the generated oxide, a direct pattern transfer into the MoTe₂ surface can be achieved by a simple immersion of the sample in deionized water. The tip-grown oxide is characterized using Auger electron and Raman spectroscopy, revealing it consists of amorphous MoO₃/MoO _x as well as TeO₂/TeO _x . With the presented technology in combination with subsequent AFM imaging it is possible to demonstrate a strong anisotropic sensitivity of 1T’-/(T_d)-MoTe₂ to aqueous environments. Finally the discussed approach is used to structure a nanoribbon field effect transistor out of a few-layer 2H-MoTe₂ nanosheet.

Ag (2D) and 2D Ga are initially intercalated into epitaxial graphene, and the de-intercalation processes are markedly different from each other as followed by photoemission electron microscopy. Molecular dynamic simulations and calculations provide insight into the role of the intercalant—they induce different interactions with (defective) graphene with implications to defect healing and kinetics of the (de)intercalation process.

Abstract

Intercalation forms heterostructures, and over 25 elements and compounds are intercalated into graphene, but the mechanism for this process is not well understood. Here, the de-intercalation of 2D Ag and Ga metals sandwiched between bilayer graphene and SiC are followed using photoemission electron microscopy (PEEM) and atomistic-scale reactive molecular dynamics simulations. By PEEM, de-intercalation “windows” (or defects) are observed in both systems, but the processes follow distinctly different dynamics. Reversible de- and re-intercalation of Ag is observed through a circular defect where the intercalation velocity front is 0.5 nm s⁻¹ ± 0.2 nm s.⁻¹ In contrast, the de-intercalation of Ga is irreversible with faster kinetics that are influenced by the non-circular shape of the defect. Molecular dynamics simulations support these pronounced differences and complexities between the two Ag and Ga systems. In the de-intercalating Ga model, Ga atoms first pile up between graphene layers until ultimately moving to the graphene surface. The simulations, supported by density functional theory, indicate that the Ga atoms exhibit larger binding strength to graphene, which agrees with the faster and irreversible diffusion kinetics observed. Thus, both the thermophysical properties of the metal intercalant and its interaction with defective graphene play a key role in intercalation.

A nanobelt-assisted transfer strategy is introduced for the universal transfer of over 20 kinds of electrodes. Contacts with Fermi energy level pinning factor S = 0.99 are realized. Remarkably low Schottky barriers of 4.2 meV for electrons and 11.2 meV for holes are observed. This approach is applied to create a doping-free WSe₂ inverter with a static power consumption of 58 pW, offering a versatile method for electrode preparation in the development of high-performance post-Moore's law electronic devices.

Abstract

Metal–semiconductor contacts are crucial components in semiconductor devices. Ultrathin two-dimensional transition-metal dichalcogenide semiconductors can sustain transistor scaling for next-generation integrated circuits. However, their performance is often degraded by conventional metal deposition, which results in a high barrier due to chemical disorder and Fermi-level pinning (FLP). Although, transferring electrodes can address these issues, they are limited in achieving universal transfer of full-class metals due to strong adhesion between pre-deposited metals and substrates. Here, we propose a nanobelt-assisted transfer strategy that can avoid the adhesion limitation and enables the universal transfer of over 20 different types of electrodes. Our contacts obey the Schottky–Mott rule and exhibit a FLP of S = 0.99. Both the electron and hole contacts show record-low Schottky barriers of 4.2 and 11.2 meV, respectively. As a demonstration, we construct a doping-free WSe₂ inverter with these high-performance contacts, which exhibits a static power consumption of only 58 pW. This strategy provides a universal method of electrode preparation for building high-performance post-Moore electronic devices.

A dual-logic-in-memory device–a device that can simultaneously perform two logic operations using four states–is demonstrated through a single bidirectional polarization-integrated ferroelectric field-effect transistor by integrating the in-plane vdW ferroelectric semiconductor SnS and the out-of-plane vdW ferroelectric gate dielectric material CuInP2S6. To overcome complexity and energy consumption issues, we demonstrate dual-logic-in-memory computing for high-density and high-efficiency programmable logic implementations in memory.

Abstract

The rapid advancement of AI-enabled applications has resulted in an increasing need for energy-efficient computing hardware. Logic-in-memory is a promising approach for processing the data stored in memory, wherein fast and efficient computations are possible owing to the parallel execution of reconfigurable logic operations. In this study, a dual-logic-in-memory device, which can simultaneously perform two logic operations in four states, is demonstrated using van der Waals ferroelectric field-effect transistors (vdW FeFETs). The proposed dual-logic-in-memory device, which also acts as a two-bit storage device, is a single bidirectional polarization-integrated ferroelectric field-effect transistor (BPI-FeFET). It is fabricated by integrating an in-plane vdW ferroelectric semiconductor SnS and an out-of-plane vdW ferroelectric gate dielectric material—CuInP₂S₆. Four reliable resistance states with excellent endurance and retention characteristics were achieved. The two-bit storage mechanism in a BPI-FeFET was analyzed from two perspectives: carrier density and carrier injection controls, which originated from the out-of-plane polarization of the gate dielectric and in-plane polarization of the semiconductor, respectively. Unlike conventional multilevel FeFETs, the proposed BPI-FeFET does not require additional pre-examination or erasing steps to switch from/to an intermediate polarization, enabling direct switching between the four memory states. To utilize the fabricated BPI-FeFET as a dual-logic-in-memory device, two logical operations were selected (XOR and AND), and their parallel execution was demonstrated. Different types of logic operations could be implemented by selecting different initial states, demonstrating various types of functions required for numerous neural network operations. The flexibility and efficiency of the proposed dual-logic-in-memory device appear promising in the realization of next-generation low-power computing systems.

Abstract

Two-dimensional (2D) tungsten selenide (WSe₂) is promising candidate material for future electronic applications, owing to its potential for ultimate device scaling. For improving the electronic performance of WSe₂-based field-effect transistors (FETs), the modification of surface properties is essential. In this study, the seamless structural phase transition in WSe₂ lattice is achieved by soft oxygen plasma, regulating the electrical conductance of WSe₂-based FETs. We found that during the soft oxygen plasma treatment with optimal processing time, the generated oxygen ions can substitute some selenium atoms and thus locally modify the bond length, inducing 2H → 1T phase transition in WSe₂ with seamless interfaces. The mosaic structures have been proven to tailor the electronic structure and increase the hole carrier concentration inside WSe₂, significantly increasing the channel conductance of WSe₂ FETs. With the further increase of the oxygen plasma treatment time, the creation of more selenium vacancy defects leads to the electronic doping, resulting in the reduction of conductance. Benefiting from the hexagonal boron nitride (h-BN) encapsulation to interrupt the partial structural relaxation from 1T to 2H phase, our WSe₂ FET exhibits high electronic stability with conductance of 6.8 × 10⁻⁴ S, which is about four orders of magnitude higher than 2H WSe₂ (5.8 × 10⁻⁸ S). This study could further broaden the WSe₂ FETs in applications for functionalization and integration in electronics.

Abstract

A variety of out-of-plane deformation patterns have been observed for two-dimensional (2D) materials including ripples, wrinkles, buckles, scrolls, folds, tents, and bubbles due to their extra-low bending rigidity. Among them, the micro- and nanoscale bubbles arising from the deformation of the atomically thin membrane by gases, liquids, and solids trapped underneath 2D materials were frequently observed. On the one hand, the presence of bubbles may severely deteriorate the performance of 2D material devices because of the obstructed charge, photon, and phonon transport across the interface. On the other hand, these bubbles offer a novel avenue to explore the intrinsic mechanical parameters (e.g., Young’s modulus and bending rigidity) of 2D materials as well as their interfacial properties (e.g., shear stress and adhesion energy). Furthermore, these bubbles with stable and controllable morphology also act as effective knobs to tune the electronic and photonic performance of various 2D materials. This review highlights the recent progress on the 2D material bubbles, which will be helpful for measurement of the mechanical properties of ultrathin 2D materials and the applications of developing 2D material devices.

A solution-processed copper monoiodide (CuI) transistor with boosted mobility through vacancy-engineering on sodium-embedded alumina dielectrics is reported. The optimized CuI thin film transistors show high hole mobility of 21.6 ± 4.5 cm² V⁻¹ s⁻¹ and successfully operate indium gallium zinc oxide-CuI complementary logic gates, demonstrating the device's applicability.

Abstract

Development of a novel high performing inorganic p-type thin film transistor could pave the way for new transparent electronic devices. This complements the widely commercialized n-type counterparts, indium-gallium-zinc-oxide (IGZO). Of the few potential candidates, copper monoiodide (CuI) stands out. It boasts visible light transparency and high intrinsic hole mobility (>40 cm² V⁻¹ s⁻¹), and is suitable for various low-temperature processes. However, the performance of reported CuI transistors is still below expected mobility, mainly due to the uncontrolled excess charge- and defect-scattering from thermodynamically favored formation of copper and iodine vacancies. Here, a solution-processed CuI transistor with a significantly improved mobility is reported. This enhancement is achieved through a room-temperature vacancy-engineering processing strategy on high-k dielectrics, sodium-embedded alumina. A thorough set of chemical, structural, optical, and electrical analyses elucidates the processing-dependent vacancy-modulation and its corresponding transport mechanism in CuI. This encompasses defect- and phonon-scattering, as well as the delocalization of charges in crystalline domains. As a result, the optimized CuI thin film transistors exhibit exceptionally high hole mobility of 21.6 ± 4.5 cm² V⁻¹ s⁻¹. Further, the successful operation of IGZO-CuI complementary logic gates confirms the applicability of the device.

Controlled growth of enriched nanoscale mix-phase (2H/1T') MoTe_{2(1− x )}S_{2 x} alloy with coherent atomic structures by composition-triggered. Coherent phase interfaces of monolayer MoTe_{2(1− x )}S_{2 x} alloys exhibit a wide photo-response range from 532 to 1550 nm with dark current ≈10⁻¹⁰ A at 100 mV bias voltage. The responsivity and response time are 101 mA W⁻¹ and 258 ms under 1550 nm illumination, respectively.

Abstract

2D transitional metal dichalcogenides (TMDs) have attracted great interest for their advantageous application in room-temperature broadband photodetectors. Developing effective strategies to optimize the photo-carrier dynamical process of monolayer TMDs is still urgently necessary to extend wavelength range and reduce dark current due to the theoretical limitation of their intrinsic band structure. Herein, an interesting approach is reported to realize broadband photodetection from 532 to 1550 nm with low dark current for the first time by using composition-triggered growth of coherent atomic structures of enriched nanoscale mix-phase (2H/1T') monolayer MoTe_{2(1− x )}S_{2 x} alloys. The morphology and phase evolution at the nanoscale of monolayer MoTe_{2(1− x )}S_{2 x} alloys are elucidated as affected by tiny formation energy (ΔE) by the chemical composition of S/Te atoms triggered. As-grown enriched nanoscale mix-phase (2H/1T') of monolayer MoTe_{2(1− x )}S_{2 x} alloys devices exhibit typical n-type conductivity properties. More interestingly, the devices show an extended photo-response range from 532 to 1550 nm with reduced dark current to 10⁻¹⁰ A at 100 mV bias voltage. This work demonstrates that coherent atomic structure of enriched nanoscale mix-phase (2H/1T′) monolayer TMDs alloys can be an alternative approach to obviously extend photo-response wavelength range without increasing dark current for room temperature broadband photodetection.

Publication date: November 2023

Source: Materials Today, Volume 70

Author(s): BingJin Chen, Minggang Zeng, Khoong Hong Khoo, Debasis Das, Xuanyao Fong, Shunsuke Fukami, Sai Li, Weisheng Zhao, Stuart S.P. Parkin, S.N. Piramanayagam, Sze Ter Lim

CoCl₂/Co₃O₄ heterojunctions are obtained on Si/SiO₂ substrates with the assistance of supercritical CO₂ (SC CO₂), and the prepared samples have significantly higher coercivity and saturation magnetization intensity.

Abstract

As a remarkable structure, 2D magnetic heterojunctions have attracted researchers’ attention owing to their controlled manipulation in the electronic device. However, successful fabrication as well as modulation of their structure and compound remain challenging. Herein, a novel method is designed to obtain a CoCl₂/Co₃O₄ heterojunction on Si/SiO₂ substrate with the assistance of supercritical CO₂ (SC CO₂), and the as-fabricated sample has significantly increased coercivity and saturation magnetization, which is 11 times higher than pure Co₃O₄. Further, it can be found that the CO₂ pressure has the decisive effect on the saturation magnetization of the sample. Therefore, it suggests that the tunable electronic-magnetic device can be anticipated to be obtained in the future.

AI and IoT are driving the rapid development of microelectronic devices based on self-powered technology, which profoundly affects the development of the world. This work comprehensively reviews the overall research progress, energy sources, principles and characteristics of energy harvesting technologies, and typical applications covering humans, animals, and the environment of self-powered microelectronics.

Abstract

With the rapid development of the Internet of Things (IoTs), numerous distributed wide-area low-power electronic devices have been utilized in various fields, such as wireless monitoring sensors and wearable electronics. Due to the dispersion and mobility of microelectronic devices, their energy supply faces serious challenges. The inconvenience and non-environmental friendliness of using traditional centralized low entropy energy and chemical batteries to power distributed microelectronic devices are becoming increasingly prominent. Environmental energy harvesting technology with high entropy characteristics is considered an effective solution for low-power electronic devices. This paper comprehensively reviews the recent progress in microelectronic technologies based on energy harvesting and signal sensing. First, state-of-the-art micro-power electronic devices in humans, animals, and the environment are introduced. Secondly, the available micro-energy sources in the environmentare elaborated and summarized. Then, the principles and characteristics of ambient microenergy harvesting technologies based on different mechanisms are classified, summarized, and analyzed. In addition, this work comprehensively summarizes the applications of self-powered micro-electronics technology in 11 different fields, including human, animal, and environment. Finally, research challenges, technical difficulties, and research gaps in self-powered microelectronics based on micro-energy harvesting technology are discussed and summarized.

Nature Photonics, Published online: 02 November 2023; doi:10.1038/s41566-023-01318-6

Three-dimensional nonlinear optical metamaterials are realized by directly engineering the symmetries of electronic wavefunctions at the atomic scale by stacking individual two-dimensional van der Waals interfaces into a precisely designed three-dimensional configuration.

Nature Communications, Published online: 02 November 2023; doi:10.1038/s41467-023-42781-7

The authors study the layered superconductor AuSn4 (Tc = 2.4 K) and reveal a two-fold symmetric angular dependence, consistent with unconventional pairing. They argue that the two-fold symmetry results from the Rashba-driven mixture of p-wave surface and s-wave bulk contributions.