Jiuxiang Dai

Light: Science & Applications, Published online: 03 April 2023; doi:10.1038/s41377-023-01128-z

Monolithic integration of embedded III-V lasers on SOI

Abstract

Traditional bulk MoS₂ as an effective H₂-evolution cocatalyst is mainly subjected to the weak hydrogen-adsorption ability of high-porpotion saturated S, resulting in a slow interfacial H₂-evolution reaction. In this paper, an efficient strategy for enhancing hydrogen adsorption of saturated S by manipulating electron density through O atoms is proposed to boost photocatalytic performance of CdS. Simultaneously, amorphization of MoS₂ can further increase the unsaturated active S sites. Herein, oxygen-contained amorphous MoS_x (a-MoOS_x) nanoparticles (10–30 nm) were tightly loaded on the CdS surface through a mild photoinduced deposition method by using (NH₄)₂[MoO(S₄)₂] solution as the precursor at room temperature. The photocatalytic H₂-evolution result showed that the a-MoOS_x/CdS performed the superior H₂-production activity (382 µmol·h⁻¹, apparent quantum efficiencies (AQE) = 11.83%) with a lot of visual H₂ bubbles, which was 54.6, 2.5, and 5.1 times as high as that of CdS, MoS_x/CdS, and annealed a-MoOS_x/CdS, respectively. Characterizations and density functional theory (DFT) calculations revealed the mechanism of improved H₂-evolution activity is that the O heteroatom in amorphous MoOS_x can enhance the atomic H-adsorption ability by manipulating the electron density to form electron-deficient S^(2−δ)− sites. This study provides a new idea to improve the efficiency and number of H₂-evolution active sites for developing efficient cocatalysts in the field of photocatalytic hydrogen evolution.

Nature Communications, Published online: 31 March 2023; doi:10.1038/s41467-023-37482-0

The standard topological insulator is characterized by an insulating bulk and a conducting boundary, so a three dimensional insulating bulk of a topological insulator has a conducting surface. Recently, this idea was extended to the edges of the surfaces of the three dimensional material as a new topological phase, referred to as a higher-order topological insulator. Here, the authors find evidence of such higher order topological insulator states in tungsten ditelluride using heterostructures composed of tungsten ditelluride and graphene.

Nature Materials, Published online: 31 March 2023; doi:10.1038/s41563-023-01505-4

A second-harmonic generation approach enables the direct measurement of the potential of zero charge at electrochemical interfaces.

Nature Materials, Published online: 31 March 2023; doi:10.1038/s41563-023-01512-5

Scientists have realized Weyl modes by exposing a topological insulator to large magnetic fields. Their effort enriches the toolbox to design, engineer and manipulate topological materials for physics research and materials applications.

Nature Materials, Published online: 31 March 2023; doi:10.1038/s41563-023-01503-6

The arrangement of magnetic ions between layers of NbS2 affects it as though a giant magnetic field is applied in different directions for electrons moving with opposite velocities. This discovery goes beyond the reach of conventional magnets, and opens up the way to custom-made effective fields engineered to guide materials into new territory.

2D violet phosphorus (VP) nanosheets are utilized as self-powered photoelectrochemical-type photodetector. The results indicate that VP nanosheets achieve preferable photo-response activities from ultraviolet to visible region along with good chemical and environmental stability.

Abstract

Violet phosphorus (VP) is a stable layered van der Waals phosphorus allotrope with unique electronic and optoelectronic properties. In this study, 2D VP nanosheets (NSs) are prepared by liquid phase exfoliation (LPE) method under ambient conditions. The prepared VP NSs are characterized and utilized for photoelectrochemical (PEC)-type photodetector (PD). The systematic PEC measurements reveal that the as-prepared VP-based PD shows tunable photo-response activities from ultraviolet to visible region. A current density of 0.52 µA cm⁻², a photo-responsivity of 31.70 µA W⁻¹, and a detectivity of 7.92 × 10¹⁰ Jones can be obtained in 1.0 m KOH under 350 nm irradiation at 0 V, revealing the good self-powered capability of VP-based PD. Furthermore, the cycle and time stability tests exhibit the good chemical and environmental stability of the as-prepared PD via processing 10 000 s on/off switching and placing after one month. It is expected that this work can offer an understanding of a PEC-type VP NSs-based PD, and highlight the further promising applications of 2D VP NSs for other optoelectronic devices.

A direct handle on the pristine out-of-plane polarization of prototypical ferroelectric Pb(Zr_0.2Ti_0.8)O₃ PZT thin films is shown by tailoring the epitaxial growth environment. When triggered by the substrate temperature and/or the oxygen partial pressure, the changing defect chemistry near the film surface favors a downward-oriented polarization, independent of the polarization preferred by the original electrostatic boundary conditions.

Abstract

The integration of thin-film ferroelectrics with reliable properties into oxide electronics requires accomplishing deterministic polarization states. Since ferroelectricity emerges during thin-film synthesis already, it is essential to elucidate how the interplay of different growth parameters affects the polarization. Here, the polarization of fully strained Pb(Zr_0.2Ti_0.8)O₃ (PZT) films is accessed in situ, during epitaxial growth. Surprisingly, it is found that the orientation of the out-of-plane polarization during growth may differ from the one after growth completion and it strongly depends on the substrate temperature and the oxygen partial pressure. Increasing the growth temperature and/or the oxygen partial pressure favors a uniform downward-oriented polarization, independent of the direction of polarization during growth. Specifically, for films with an emerging upward-oriented polarization, a polarization reversal and a downward-oriented polarization after cool-down is observed. The in situ measurements obtained by optical second harmonic generation (SHG) in conjunction with ex situ piezoresponse force microscopy (PFM) and X-ray diffraction (XRD) measurements point to the temperature- and pressure-dependent formation of a charged Pb defect gradient toward the film surface as the responsible mechanism for the polarization reorientation.

Through cryogenic real-space imaging, exfoliated flakes of the ferromagnetic semiconductor Cr₂Ge₂Te₆ are shown to support a rich phase diagram of magnetic domain structures, including both topologically-protected and topologically-trivial lattices of magnetic bubbles. The type and chirality of these bubble can be controlled by topographic variations in the flake, and magnetoelastic coupling is shown to align and organize both bubble lattices and stripe domains.

Abstract

Ferromagnetic van der Waals (vdW) materials are of large current interest for the fundamental study of low-dimensional magnetism and for potential applications in multilayer heterostructures. Cr₂Ge₂Te₆ (CGT) is particularly exciting because it is a ferromagnetic semiconductor with tunable electronic and magnetic properties. Controlling the magnetic domain structure of CGT is a requirement for understanding its novel interface physics and for tuning behavior for potential devices. Herein, cryo-Lorentz transmission electron microscopy is performed in the temperature range of 12–50K to directly image the magnetic domain structures in CGT. A rich phase diagram of domain structures including stripe domains, magnetic bubble lattices of mixed-chirality, and topologically-protected lattices of homochiral magnetic bubbles is observed. The types and chiralities of the bubbles can be controlled by topographical changes in the CGT flakes. Additionally, it is observed that in-plane strain and magnetoelastic coupling can align and organize both bubble lattices and stripe domains. This study provides insights into creating and controlling complex magnetic domain structures for integration into multilayer heterostructures and for future studies of 2D magnetism.

A controllable synthesis method for high-quality atomically thin PtSe₂ is crucial for both fundamental research and practical applications. In this study, large-scale ultrathin 1T-PtSe₂ ribbons of high quality are synthesized on Au foils and utilized as electrocatalysts. The results provide clear insight into the morphology and structure of PtSe₂ ribbons and offer a new avenue for the rational design and modulation of high-performance 2D catalysts for hydrogen evlution reaction.

Abstract

2D platinum diselenide (PtSe₂) exhibits exceptional layer-dependent electrical properties and high catalytic activity for hydrogen evolution reactions, making it an ideal system for studying structure–activity correlations. However, the synthesis of high-quality atomically thin PtSe₂ materials has proven challenging. This study presents a simple chemical vapor deposition method for synthesizing high-quality ultrathin 1T-PtSe₂ ribbons on Au foils, making it easily applicable. Theoretical and experimental results confirm that these atomically thin 1T-PtSe₂ ribbons possess abundant catalytic sites and can serve as ideal electrocatalysts. This study advances the large-scale synthesis and potential application of ultrathin transition metal disulfides and presents a novel method for designing and synthesizing highly active ultrathin catalysts.

Quasi-1D tellurium nanowires with single-orientation are synthesized via physical vapor deposition on annealed m-plane sapphire. The high crystallinity, narrow bandgap, and inherent structural anisotropy of tellurium nanowire offer its detector high responsivity to short-wave Infrared (SWIR) light with large linear dichroism and high uniformity. This study opens new prospects for building chip-scale multifunctional SWIR optoelectronic devices.

Abstract

Tellurium (Te), an elemental van der Waals semiconductor, has intriguing anisotropic physical properties owing to its inherent quais-1D crystal structure. Synthesizing ultrathin Te crystal with uniform orientation is important to its large-scale device applications, but that remains a great challenge. Herein, the nanoscale grooves-induced unidirectional epitaxy growth of 1D Te nanowires via physical vapor deposition on the annealed m-plane sapphire is demonstrated. By enhancing the annealing temperature from 1000 to 1300 °C, nanoscale grooves on m-plane sapphire arising along the [101¯$\overline 1 $0] direction and gradually distinct, and the corresponding Te nanowires grown on them turns from random to uniform, finally achieving nearly 95% unidirectional Te nanowires. The as-grown Te nanowires possess high crystallinity with clearly chiral helical chains along the c-axis direction and reveal thickness-tunable bandgap with prominent linear-dichroic. As results, the Te nanowire-based photodetectors demonstrate a broadband photoresponse from visible (532 nm) to short-wave infrared (2530 nm), with high responsivity of 327 A W⁻¹ as well as strong and uniform polarization sensitivity (anisotropic ratio = 2.05) to 1550 nm light. The high crystallinity and superior anisotropy of Te nanowires, combined with the orientation-controlled preparation endows it with great feasibility for constructing chip-scale multifunctional optoelectronic devices.

A ferroelectric polarization strategy is presented for enhancing the performance of 2D piezoelectric nanogenerator (PENG), and the output currents of the polarized CuInP₂S₆ (CIPS)-based PENG are enhanced significantly. In addition to collecting biomechanical energy such as wrist joints movement, the CIPS-based PENG combined with deep learning models can be further used as an intelligent voice recognition system.

Abstract

2D piezoelectric materials have strong intrinsic piezoelectricity and superior flexibility, which are endowed with huge potential to develop piezoelectric nanogenerators (PENGs). However, there are few attempts to investigate the energy harvesting of 2D ferroelectric materials. Herein, an enhanced output performance is reported by ferroelectric polarization in a PENG with exfoliated 2D ferroelectric CuInP₂S₆ (CIPS). Specifically, the polarized CIPS-based PENG produces a short-circuit current of 760 pA at 0.85% tensile strain, which is 3.8 times higher than that of unpolarized CIPS-based PENG. Systematical PFM and Raman analysis reveal that the ferroelectric polarization remarkably reinforces the effective piezoelectric constant of CIPS nanoflakes and boosts the in-plane migration and out-of-plane hopping of copper ions, which is the main reason for the enhancement of output performance. Furthermore, the CIPS-based PENG can not only be utilized to harvest biomechanical energy such as wrist joints movement, but also exhibits a potential for a voice recognition system integrated with deep learning technology. The classification accuracy of a series of letter sounds is as high as 96%. This study commendably broadens the application scope of 2D materials in micro-nano energy and intelligent sensors, which will have profound implications for exploring wearable nanoelectronic devices.

Nature Communications, Published online: 01 April 2023; doi:10.1038/s41467-023-37500-1

The optical quality of large-area transition metal dichalcogenide (TMD) monolayers is usually limited by surface defects and inhomogeneities. Here, the authors report a method based on 1-dodecanol encapsulation to improve the optical properties of TMD monolayers over mm-scale, enabling the fabrication of an array of polariton photonic crystal cavities.

3R-MoS₂ based solid mounted resonators with the resonant frequency up to 27 GHz and the electromechanical coupling coefficient ≈47.6% are demonstrated. Successful implantation of 2D piezoelectric 3R-MoS₂ flakes into acoustic resonators suggests that ultrathin 2D piezoelectric nanoflakes can break through the frequency limitation of conventional acoustic resonators. 2D piezoelectric materials can be promising for next generation acoustic devices.

Abstract

Conventional bulk and thin piezoelectric materials based film bulk acoustic resonators (FBARs) are facing an insurmountable challenge for millimetric frequency applications due to the poor piezoelectric properties of the materials when their thickness reaches the sub-micron regime. Novel FBARs for ultra-high working frequencies are in urgent demand to meet the requirements of the fast-growing 5/6G telecommunication techniques. Recent advances in 2D piezoelectric nanomaterials create an opportunity in this perspective. Here, the first FBAR chip based on 2D 3R-MoS₂ ultrathin piezoelectric flakes with a solidly mounted resonator (SMR) architecture is reported. The typical resonant frequency for an SMR device based on ≈200 nm 3R-MoS₂ flake reaches over 25 GHz with high reproducibility. Theoretical and finite element analysis suggest that the observed resonance is of longitudinal acoustic modes. This study demonstrates for the first time that the access to 2D piezoelectric nanomaterials makes high performance piezoelectric devices feasible for various promising applications including high-speed telecommunication, acousto-optic, and sensor fields,etc.

A comprehensive overview on the state-of-the-art progress of the emerging 2D MA₂Z₄ family is presented. The versatile properties including mechanics, optics, thermal transport, piezoelectrical effect, electronics, and magnetics are elaborated. The property tunability is also revealed by substitution/doping, strain engineering, and layered strategy. Theoretical and experimental attempts or advances in applying 2D MA₂Z₄ to transistors/spintronics, photocatalysts, batteries, and gas sensors are then reviewed to show its prospective applications over a vast territory.

Abstract

The discovery of 2D layered MoSi₂N₄ and WSi₂N₄ without knowing their 3D parents by chemical vapor deposition in 2020 has stimulated extensive studies of 2D MA₂Z₄ system due to its structural complexity and diversity as well as versatile and intriguing properties. Here, a comprehensive overview on the state-of-the-art progress of this 2D MA₂Z₄ family is presented. Starting by describing the unique sandwich structural characteristics of the emerging monolayer MA₂Z₄, their versatile properties including mechanics, piezoelectricity, thermal transport, electronics, optics/optoelectronics, and magnetism is summarized and anatomized. The property tunability via strain engineering, surface functionalization and layered strategy is also elaborated. Theoretical and experimental attempts or advances in applying 2D MA₂Z₄ to transistors, photocatalysts, batteries and gas sensors are then reviewed to show its prospective applications over a vast territory. New opportunities are further discussed and prospects are suggested for this emerging 2D family. The overview is anticipated to guide the further understanding and exploration on 2D MA₂Z₄.

Advanced X-ray diffraction analysis shows that in thin films of some non-collinear antiferromagnets the magnetic atoms can be displaced away from high-symmetry positions. The resulting spin canting allows for control of domain structure. In Mn₃SnN this allows for the observation of an unusual temperature-dependent anomalous Hall effect that indicates the presence of a coherent spin-rotational non-collinear antiferromagnetic phase.

Abstract

Antiferromagnets with non-collinear spin structures display various properties that make them attractive for spintronic devices. Some of the most interesting examples are an anomalous Hall effect despite negligible magnetization and a spin Hall effect with unusual spin polarization directions. However, these effects can only be observed when the sample is set predominantly into a single antiferromagnetic domain state. This can only be achieved when the compensated spin structure is perturbed and displays weak moments due to spin canting that allows for external domain control. In thin films of cubic non-collinear antiferromagnets, this imbalance is previously assumed to require tetragonal distortions induced by substrate strain. Here, it is shown that in Mn₃SnN and Mn₃GaN, spin canting is due to structural symmetry lowering induced by large displacements of the magnetic manganese atoms away from high-symmetry positions. These displacements remain hidden in X-ray diffraction when only probing the lattice metric and require measurement of a large set of scattering vectors to resolve the local atomic positions. In Mn₃SnN, the induced net moments enable the observation of the anomalous Hall effect with an unusual temperature dependence, which is conjectured to result from a bulk-like temperature-dependent coherent spin rotation within the kagome plane.

The fundamental role played by the electrical double-layer capacitance (EDLC) of few layers graphene (FLG)-electrodes, in rising the electrical power output of triboelectric nanogenerators (TENGs, 530 mW m⁻²) is unveiled. Highly capacitive FLG electrodes are obtained by integrating electrolyte gel composites based on 2D-transition metal dichalcogenides. This work introduces a novel design concept leading to sustainable electrochemical-TENGs.

Abstract

The integration of 2D materials in triboelectric nanogenerators (TENGs) is known to increase the mechanical-to-electrical power conversion efficiency. 2D materials are used in TENGs with multiple roles as triboelectric material, charge-trapping fillers, or as electrodes. Here, novel TENGs based on few-layers graphene (FLG) electrodes and stable gel electrolytes composed of liquid phase exfoliated 2D-transition metal dichalcogenides and polyvinyl alcohol are developed. TENGs embedding FLG and gel composites show competitive open-circuit voltage (≈ 300 V), instant peak power (530 mW m⁻²), and stability (> 11 months). These values correspond to a seven-fold higher electrical output compared to TENGs embedding bare FLG electrodes. It is demonstrated that such a significant improvement depends on the high electrical double-layer capacitance (EDLC) of FLG electrodes functionalized with the gel composites. The wet encapsulation of the TENGs is shown to be an effective strategy to increase their power output further highlighting the EDLC role. It is also shown that the EDLC is dependent upon the transition metal (W vs Mo) rather than the relative abundance of 1T or 2H phases. Overall, this work lays down the roots for novel sustainable electrochemical-(e)-TENGs developed exploiting strategies typically used in electrochemical capacitors.

Nature Energy, Published online: 30 March 2023; doi:10.1038/s41560-023-01221-y

Laboratory innovations in energy research do not necessarily transfer into commercial success due to scale-up and other related issues. Here the authors review scientific challenges in realizing large-scale battery active materials manufacturing and cell processing, trying to address the important gap from battery basic research.