Jiuxiang Dai

Nature Communications, Published online: 04 October 2022; doi:10.1038/s41467-022-33654-6

Nature Nanotechnology, Published online: 05 October 2022; doi:10.1038/s41565-022-01207-z

The piezoelectricity in ε-Ga₂O₃ is demonstrated. The piezoelectric coefficient d ₃₃ of ε-Ga₂O₃ (≈10.8–11.2 pm V^–1) is twice larger than that of AlN. For the first time, ε-Ga₂O₃-based surface acoustic wave (SAW) radio-frequency resonators are demonstrated. The wide bandgap and high piezoelectricity make ε-Ga₂O₃ a strong competitor to traditional piezoelectric semiconductors in the communication industry.

Abstract

The explosion of mobile data from the internet of things (IoT) is leading to the emergence of 5G technology with dramatic frequency band expansion and efficient band allocations. Along with this, the demand for high-performance filters for 5G radio frequency (RF) front-ends keeps growing. The most popular 5G filters are constructed by piezoelectric resonators based on AlN semiconductor. However, AlN possesses a piezoelectric constant d ₃₃ lower than 5 pm V⁻¹ and it becomes necessary to develop novel semiconductors with larger piezoelectric constant. In this work, it is shown that strong piezoelectricity exists in ε-Ga₂O₃. High-quality phase-pure ε-Ga₂O₃ thin films with a relatively low residual stress are prepared. A switching spectroscopy piezoelectric force microscope (SS-PFM) measurement is carried out and the piezoelectric constant d ₃₃ of ε-Ga₂O₃ is determined to be ≈10.8–11.2 pm V⁻¹, which is twice as large as that of AlN. For the first time, surface acoustic wave (SAW) resonators are demonstrated on the ε-Ga₂O₃ thin films and different vibration modes resonating in the GHz range are observed. The results suggest that ε-Ga₂O₃ is a great material candidate for application in piezoelectric devices, thanks to its wide bandgap, strong piezoelectric property, small acoustic impedance, and low residual stress.

A WSe₂/WO _x /MoS₂ heterostructure photodetector by using O₂ plasma treatment is fabricated. The obtained results showed that the dark current decreased significantly, while other key photodetection performances such as responsivity, specific detectivity, and noise equivalent power also improved. The plasma-doped photodetector showed more than two orders better photodetection performances compared with the undoped pristine photodetector.

Abstract

2D transition metal dichalcogenides (TMDCs) are anticipated to be the ones of future nano-sized photodevices due to their electronic and optoelectronic properties. They have shown remarkable performances as photodetectors from being fabricated into heterostructures with p–n junction. An oxygen plasma-doped WSe₂/pristine MoS₂-based photodetector with high responsivity and broad detection spectrum ranging from visible to near-infrared (NIR) region is reported. The oxygen plasma treatment forms a WO _x layer on WSe₂ that not only acts as a p-dopant but also an interfacial oxide layer to suppress dark current to as low as ≈pA. Under illumination of visible light (520 nm in this study), greatly enhanced photoresponsivity and specific detectivity of thus fabricated devices are achieved without applying an external bias, in contrast to untreated devices. The devices have also exhibited good photodetection in the NIR region with two orders enhanced photoresponsivity under the illumination of 852 nm light at room temperature. It is confirmed from photomapping measurements that the photocurrent is mainly generated from p–n heterojunction. These results indicate that oxygen plasma-doped WSe₂-based heterojunctions can be used as highly sensitive and self-powered photodetectors.

Artificial optoelectronic synapses based on MoS₂ field-effect transistors (FETs) have been realized by integrating the sensing, memorizing, and preprocessing functions together. Basic opto-synaptic functions, interest-modulate human visual memory, and background noise filtering with a high recognition accuracy ≈85.5% were realized in the device which provide an alternative approach to construct neuromorphic visual systems.

Abstract

Neuromorphic chips show advantages over conventional computing system based on von Neumann architecture when dealing with complex scenes, such as artificial intelligence, automatic driving, image, and speech recognition. However, current neuromorphic circuitry usually suffers from device complexity, low computing speed, and efficiency issues. Here, the fabrication of artificial optoelectronic synapses based on MoS₂ field-effect transistors (FETs) to integrate the sensing, memorizing, and preprocessing functions together is reported. Light-tunable and gate-voltage modulated behaviors enable those devices to implement many opto-synaptic functions, such as paired-pulse facilitation (PPF), short-term memory (STM), long-term memory (LTM), the transition of STM-to-LTM, and interest-modulate human visual memory. Moreover, background noise filtering for image preprocessing with a high recognition accuracy ≈85.5% is realized in MoS₂-based optoelectronic array devices. These simple but prospective opto-synaptic devices provide an alternative approach to construct neuromorphic visual systems.

In this article, Bouaziz and co-workers present a procedure for the transfer of epitaxial oxide films where a rigid bond to the final host holder is obtained via a metallic Au/Ag bonding layer. Cross section analysis and X-ray diffraction show straight interfaces without plastic deformation of the transferred membranes.

Abstract

In the past 5 years, the transfer of epitaxial oxide thin films has drawn a renewed interest in the scientific community. The major challenge in this technology is to minimize the appearance of extended bulk defects such as plastic deformations, cracks, and delamination, which are induced by the transfer process to a new host substrate. In this work, a procedure for the transfer of epitaxial oxide films where a rigid bond to the final host holder is obtained via a metallic Au/Ag bonding layer is presented. Here, the transfer of SrRuO₃ (SRO) and SrRuO₃/SrTiO₃ (STO) epitaxial films grown on a water-soluble Sr₃Al₂O₆ sacrificial layer is reported. These epitaxial films are grown on a STO substrate and transferred onto a Si host substrate. Roughness values lower than 1nm are observed for the transferred SRO membranes. Cross-section analysis shows straight interfaces without plastic deformation of the membranes. X-ray diffraction rocking-curve analysis evidences that mechanical damage is minimized and the membranes remain close to their initial quality. This procedure represents an important step forward in the development of advanced technologies for membrane transfer of epitaxial oxides and superstructures.

A light-tunable tunneling mechanism photodetector through the photogating effect of black phosphorus induced localized holes in the SnS₂/WSe₂-BP heterojunction. Base on this light-tunable mechanism, a super-linear photocurrent-power dependence is discovered. This work provides a feasible strategy for both the photon-generated carriers high gain and the fast extraction in 2D van der Waals heterojunction.

Abstract

Van der Waals (vdW) heterojunctions composed of 2D layered materials exhibit novel physical phenomena that can power a range of electronic and photonic applications. Here, the first demonstration of the light switchable mechanism between the rectifier diode and Esaki diode in SnS₂/WSe₂-black phosphorus (BP) vdW heterojunctions is presented. In this design, the photogating effect of WSe₂-BP junction is employed as the switch toggle between rectifier diode and Esaki diode. Based on this light switchable mechanism, a super-linear photocurrent-power dependence is observed and a high-performance photodetector with high detectivity (5.1 × 10¹¹ Jones) and rapid response speed (<112 us) is obtained. A feasible strategy for a well-balanced photodetector with high detectivity and response speed in the vdW heterojunction is provided.

It remains unknown to what extent the interlayer twist between two semiconductor monolayers affects their minority carrier lifetimes, diffusion lengths, and light absorbance properties. This work sheds light on this fundamental question. It explores the effects of various twist angles between vertically stacked p-n-like MoS₂/WS₂ heterobilayers on their fundamental optical properties.

Abstract

Atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDs) are promising materials for photovoltaic (PV) applications. Their self-terminated nature and strong absorption characteristics introduce an unprecedented possibility for high voltages to bandgap ratios, with secondary benefits including the potential for high internal quantum efficiencies/low recombination and strong absorption coefficients coupled with stability in a range of environments. However, despite the promise of such material systems, their PV performances still lag behind the conventional 3D materials. In principle, one possible way to manipulate the behavior of a 2D heterobilayer structure is to change its interlayer twist angle. In this study, the effects of twist angle between vertically stacked type-II MoS₂/WS₂ heterobilayers on fundamental optical properties such as light absorbance, excess carrier lifetime, and diffusion are reported. These properties can have a direct effect on the final PV performance of these heterobilayers. It is found that the interlayer twist in MoS₂/WS₂ heterobilayers does not affect their absorbance. However, the carrier lifetime and photon emission across the heterobilayers are modulated with the interlayer twist. These findings could be useful to facilitate the optimization of monolayer TMD-based optoelectronic devices.

Generating and controlling ferromagnetism in two-dimensional electron gases (2DEGs) at oxide interfaces would lead to new fundamental understanding and functional applications. 2DGEs generated at LaTiO₃ on EuTiO₃ heterostructures show a clear ferromagnetic ordering. They display distinctive behaviors in magnetotransport which are tunable by the carrier density and by the thickness of the LaTiO₃ overlayer.

Abstract

Quantum phenomena such as superconductivity usually emerge in strongly correlated oxide systems as the result of the interplay of spin, charge, and orbital degrees of freedom. Adding ferromagnetism to two-dimensional electron gases (2DEGs) at oxide interfaces is intriguing due to their scientifically exotic and technically valuable properties, which may lead to new fundamental understanding and multifunctional applications. Here, ferromagnetic 2DEGs at the interface of polar antiferromagnetic LaTiO₃ and nonpolar antiferromagnetic EuTiO₃ are generated. The magnetotransport properties of these 2DEGs depend on the thickness of LaTiO₃ that determines the carrier concentration, with all showing robust ferromagnetism up to 5.5 K. This magnetism is intrinsic to the strongly-correlated 2D electron system and is highly sensitive and tunable based on the sample configuration. Thus, a prototype oxide system with magnetic functionality for spintronics and ferromagnetic semiconductors has been developed.

A flexible polydimethylsiloxane (PDMS)/Ti₃C₂ pressure sensor is fabricated by the dip-coating method to study the influence of low temperature on bending endurance. Decreasing the fracture rate of van der Waals interactions by increasing the strain energy stored by PDMS substrate during bending cycles plays a key role in improving the fatigue resistance for such sensors.

Abstract

The flexible and wearable Ti₃C₂ (MXene)-based sensors are attracting wide attention in various applications owing to high sensitivity and flexibility. However, as a key characteristic, the bending endurance of such sensors under low temperature is yet to be explored. Herein, the flexible MXene/polydimethylsiloxane (PDMS) pressure sensor is fabricated by dip coating, which exhibits good mechanical stability over 10 000 bending cycles above 10 °C. When the temperature decreases to −40 °C, a terrible deterioration of bending endurance is observed. The finite element analysis results and theoretical calculation suggest that the accelerated rupture of van der Waals (VDW) interactions between MXene nanosheets and that at the MXene/PDMS interface is responsible for such observation. At temperature above 10 °C, owing to the absorption of strain energy by PDMS, the rupture rate of VDW forces is relatively low. However, the decrease of temperature leads to the reduce of the strain energy stored in PDMS and thus accelerates the rupture of VDW interactions, which significantly deteriorates the bending endurance for the sensor.

In this review, an overview and future perspective on 1D nanomaterials are introduced. Specifically, various synthetic routes of 1D nanomaterials, analysis of 1D nanomaterials, and application of 1D nanomaterials in energy storage system are highlighted, which broaden the readership. Last, future research directions and perspectives of 1D nanomaterials are presented, which provide an outstanding pathway for more advanced 1D nanomaterials.

Abstract

Due to the unique properties which are considerably different from macro-scale or bulk materials, 1D) nanostructures have received great interests. In particular, they have greatly contributed to building next-generation batteries by providing beneficial features such as short ion/electron pathways, structural versatility (i.e., formation of 3D current collectors, free-standing electrodes, and separators), and excellent stress relaxation. Owing to these definite advantages, 1D nanostructure is often followed by discovery of new electrode materials. This review provides a systematic overview of the state-of-the-art research progresses on 1D nanostructures, which are extensively used for rechargeable batteries. Specifically, a brief introduction of some important 1D nanostructuring methods is started and then, in situ structural characterizations using 1D nanostructures are summarized, which allow a great step forward in atomic-scale monitoring of reaction kinetics and observing dynamic structural evolution of electrode materials. 1D nanstructuring in solid-state electrolytes and stabilization of metal anodes are also highlighted which are not only substantially important in current research trends for future batteries but also rarely discussed in previous reviews. At the end, critical perspective and future research direction of 1D nanoengineering for future batteries are suggested.

Abstract

Oxygen evolution reaction (OER) plays an important role in many energy conversions and storage technologies, such as water splitting, rechargeable metal air batteries, renewable fuel cells, and electrocatalytic carbon dioxide reduction and nitrogen reduction, but its slow kinetics and high overpotential seriously affect the energy efficiency. Fabrication of high-performance and well-stocked OER catalysts is the key to the large-scale implementation of these energy-related technologies. Two-dimensional (2D) materials get a lot of attention as OER catalysts due to their large specific surface area, abundant active sites, and adjustable structures and compositions. Here, an overview is presented for the latest achievements in design and synthesis of 2D materials (including layered double hydroxides, metal-organic frameworks and their derivatives, covalent-organic frameworks, graphene, and black phosphorus) for the OER, emphasizing novel strategies (including metal/nonmetal doping, defect engineering, interface engineering, lattice strain, and fabrication of heterojunction) for achieving high electrocatalytic activity. Peculiarly, the structure-function relationship is analyzed in detail to gain deeper insight into the reaction mechanism, which is crucial to rational design of more high-performance 2D materials for the OER. Finally, the remaining challenges to improve the OER performance of 2D electrocatalysts are put forward to indicate possible future development of 2D materials.