Jiuxiang Dai

In this work, the composition-controllable syntheses of 2D non-layered iron selenide nanosheets (25% Fe-intercalated triclinic Fe₅Se₈ and 50% Fe-intercalated monoclinic Fe₃Se₄), via a robust chemical vapor deposition strategy is presented. Intriguingly, it has been revealed that the 2D Fe₅Se₈ exhibits intrinsic room-temperature ferromagnetic property, and the ultrathin Fe₃Se₄ presents a novel metallic feature.

Abstract

Exploring new-type 2D magnetic materials with high magnetic transition temperature and robust air stability has attracted wide attention for developing innovative spintronic devices. Recently, intercalation of native metal atoms into the van der Waals gaps of 2D layered transition metal dichalcogenides (TMDs) has been developed to form 2D non-layered magnetic TMDs, while only succeeded in limited systems (e.g., Cr₂S₃, Cr₅Te₈). Herein, composition-controllable syntheses of 2D non-layered iron selenide nanosheets (25% Fe-intercalated triclinic Fe₅Se₈ and 50% Fe-intercalated monoclinic Fe₃Se₄) are firstly reported, via a robust chemical vapor deposition strategy. Specifically, the 2D Fe₅Se₈ exhibits intrinsic room-temperature ferromagnetic property, which is explained by the change of electron spin states from layered 1T'-FeSe₂ to non-layered Fe-intercalated Fe₅Se₈ based on density functional theory calculations. In contrast, the ultrathin Fe₃Se₄ presents novel metallic features comparable with that of metallic TMDs. This work hereby sheds light on the composition-controllable synthesis and fundamental property exploration of 2D self-intercalation induced novel TMDs compounds, by propelling their application explorations in nanoelectronics and spintronics-related fields.

2D superlattice materials combining the advantages of 2D materials and advanced composites provide tunable physical and chemical properties to meet diverse requirements in different applications. This review article summarizes the major fabrication methods for preparing 2D superlattice materials and reviews their applications in electrocatalysis involved in emerging energy devices.

Abstract

Exploring low-cost and high-efficient electrocatalyst is an exigent task in developing novel sustainable energy conversion systems, such as fuel cells and electrocatalytic fuel generations. 2D materials, specifically 2D superlattice materials focused here, featured highly accessible active areas, high density of active sites, and high compatibility with property-complementary materials to form heterostructures with desired synergetic effects, have demonstrated to be promising electrocatalysts for boosting the performance of sustainable energy conversion and storage devices. Nevertheless, the reaction kinetics, and in particular, the functional mechanisms of the 2D superlattice-based catalysts yet remain ambiguous. In this review, based on the recent progress of 2D superlattice materials in electrocatalysis applications, the rational design and fabrication of 2D superlattices are first summarized and the application of 2D superlattices in electrocatalysis is then specifically discussed. Finally, perspectives on the current challenges and the strategies for the future design of 2D superlattice materials are outlined. This review attempts to establish an intrinsic correlation between the 2D superlattice heterostructures and the catalytic properties, so as to provide some insights into developing high-performance electrocatalysts for next-generation sustainable energy conversion and storage.

This paper proposes a wide-bandgap zinc oxide (ZnO)−polydimethylsiloxane (PDMS) nanocoating that effectively improves the breakdown strengths and flashover voltages of widely used polymer dielectrics. This is essential for insulation systems in electrical and electronic equipment or energy storage film capacitors.

Abstract

As the core components of insulation systems or film capacitors, polymer dielectrics are widely used in electrical and electronic equipment and for energy storage. However, the electrical strength is always the bottleneck that limits further application. Therefore, a coating with a wide bandgap comprising of zinc oxide and polydimethylsiloxane is designed in this paper. Experiments show that the coating effectively improves the breakdown strength and flashover voltage. A Kelvin probe force microscope is used to determine the microscopic mechanism for performance improvement, the charge dissipation characteristics and the changes in trap parameters in the nanoscale regime. This shows that many shallow traps are introduced on the polymer surface after applying the coating, which dissipate the surface charges and increase the flashover voltage. First-principles calculations indicate that the band gap of the coating is wider than that of the polymer dielectric, which explains the enhancement in breakdown field strength. This paper presents an efficient method for enhancing the electrical strengths of polymer dielectrics, which is crucial for their application. More importantly, the experimental and theoretical methods used to determine the microscopic mechanism and the conclusions obtained in this paper provide guidance for future research on polymer dielectrics.

Nature Communications, Published online: 21 October 2022; doi:10.1038/s41467-022-34104-z

Nature Energy, Published online: 19 October 2022; doi:10.1038/s41560-022-01151-1

This work reports (010) specific surface-regulated Bi₂MoO₆ nanosheets. The modified surface exhibits boosted CO₂ photoreduction because of increased activation/adsorption of CO₂ molecules. A combination of advanced characterizations and theoretical simulation confirmed performance and mechanism.

Abstract

Photocatalytic performance can be optimized via introduction of reactive sites. However, it is practically difficult to engineer these on specific photocatalyst surfaces, because of limited understanding of atomic-level structure-activity. Here we report a facile sonication-assisted chemical reduction for specific facets regulation via oxygen deprivation on Bi-based photocatalysts. The modified Bi₂MoO₆ nanosheets exhibit 61.5 and 12.4 μmol g⁻¹ for CO and CH₄ production respectively, ≈3 times greater than for pristine catalyst, together with excellent stability/reproducibility of ≈20 h. By combining advanced characterizations and simulation, we confirm the reaction mechanism on surface-regulated photocatalysts, namely, induced defects on highly-active surface accelerate charge separation/transfer and lower the energy barrier for surface CO₂ adsorption/activation/reduction. Promisingly, this method appears generalizable to a wider range of materials.

2D monolayers and thin film WO₃ are accomplished via magnetron sputtering. 2D monolayers are only formed on a cooled substrate. Resistive switching device and photocatalysis of 2D WO₃ are discussed. The failure of resistive switching device is closely related with the 2D structure of WO₃. In photocatalysis, the enhancements of photocatalysis are obtained with 2D WO₃ after their exfoliation.

Abstract

2D oxide materials have gained tremendous attention in the applications. Herein, a synthesis route of 2D WO₃ materials via magnetron sputtering is reported. A deposition between 2D monolayers and thin film structure are accomplished according to the temperature of substrate. And 2D monolayers are only formed on a cooled substrate. Ag doping helps to exfoliate 2D WO₃ into freestanding monolayers, in which the thickness of 2D monolayer is only ≈3 nm. The ultralarge size of 2D WO₃ shows unique features from the traditional 2D material. Resistive switching device and photocatalysis are discussed as examples of application. There is a clear intermediate resistance state in the device with 2D structure. And the failure of resistive switching device is closely related with the 2D structure of WO₃. In the application of photocatalysis, an improved two-step exfoliation, achieving a stack of WO₃ monolayers with a large internal volume, is utilized. The enhancements of photocatalysis are obtained with 2D WO₃ after exfoliation.

Large work function modifications of thin exfoliated α-RuCl₃ flakes by argon sputtering are reported in this study. The value of the work function after treatment depends on the thickness of the flakes. From photoemission spectroscopy it is found that the chlorine concentration after sputtering strongly depends on the flake thickness as well.

Abstract

α-RuCl₃ is a candidate material for the realization of a Kitaev spin liquid with envisioned applications for quantum computing. It is a van-der-Waals material with in-plane honeycomb lattice equivalent to the CrX ₃ (X= Cl, Br, I) type 2D magnets. Here, possibilities of defect engineering and surface modification of thin crystals are explored by Ar⁺ dosing and vacuum annealing. Chlorine is easily removed from the surface, which reduces the Ru valence and eventually leaves a Ru rich surface layer behind. A peculiar thickness dependence of the work function emerges after Ar⁺ sputtering, which is ascribed to the remaining chlorine concentration. This work elucidates material properties of thin α-RuCl₃ and introduces concepts of property engineering to create homojunctions and control level alignment by standard in situ methods.

In this research, high-quality epitaxial films of VO₂ (020) are directly grown on fluorophlogopite substrates, and the relationship between phase-transition temperature and external strains is revealed. Based on experimental results and density functional theory calculations, it is speculated that lattice constant and bandgap between d_// and π * are strongly affected by external strains, which allow for more effective dynamic modulation of phase-transition process.

Abstract

Unique metal–insulator transition behaviors of strongly correlated electronic materials, vanadium dioxide (VO₂), and their wide potential applications have gained much attention for investigation. In this research, high-quality epitaxial films of VO₂ (020) are directly grown on fluorophlogopite (001) substrates, and the relationship between phase-transition temperature and external strains is revealed. After verifying the like-freestanding property and low intrinsic resistance changing of VO₂/fluorophlogopite, variable phase-transition temperatures under different external strains with a tuning rate of 5.37 K per 0.1% strain are obtained. Based on experimental results and theoretical calculation, it is speculated that lattice constant and bandgap between d_// and π * are strongly affected by the external strains, which allow for more effective dynamic modulation of phase-transition process. This research provides a comprehensive understanding of strain engineering on phase-transition properties and also broadens the possibility of potential applications of certain optoelectronic devices for strain modulation.

Nature Communications, Published online: 20 October 2022; doi:10.1038/s41467-022-33965-8

An on-chip electrocatalytic microdevice platform is established for the microscopic investigation of the electrocatalytic oxygen reduction reaction (ORR) of single 2D nanosheets, which is successfully used to demonstrate that Ar-plasma treatment is effective in enhancing the ORR activity of MoS₂ nanosheets at the individual nanosheet level.

Abstract

The on-chip electrocatalytic microdevice (OCEM) is an emerging platform specialized in the electrochemical investigation of single-entity nanomaterials, which is ideal for probing the intrinsic catalytic properties, optimizing performance, and exploring exotic mechanisms. However, the current catalytic applications of OCEMs are almost exclusively in electrocatalytic hydrogen/oxygen evolution reactions with minimized influence from the mass transfer. Here, an OCEM platform specially tailored to investigate the electrocatalytic oxygen reduction reaction (ORR) at a microscopic level by introducing electrolyte convection through a microfluidic flow cell is reported. The setup is established on gold microelectrodes and later successfully applied to investigate how Ar-plasma treatment affects the ORR activities of 2H MoS₂. This study finds that Ar-plasma treatment significantly enhances the ORR performance of MoS₂ nanosheets owing to the introduction of surface defects. This study paves the way for highly efficient microscopic investigation of diffusion-controlled electrocatalytic reactions.

This work reports room-temperature ultra-narrow linewidth photo-emitters in selenium nanoflakes (SeNFs), fabricated via a unique hot-pressing strategy. A process-induced crystal symmetry breaking in SeNFs results in a localized polymorphic transition from the trigonal to orthorhombic phase, causing photo-emitters with extremely narrow linewidth. Considering the increasing demand for the lack in single-photon sources in 2D materials, the current report brings quasi-2D selenium into the limelight.

Abstract

Photoluminescence (PL) in state-of-the-art 2D materials suffers from narrow spectral coverage, relatively broad linewidths, and poor room-temperature (RT) functionality. The authors report ultra-narrow linewidth photo-emitters (ULPs) across the visible to near-infrared wavelength at RT in polymorphic selenium nanoflakes (SeNFs), synthesized via a hot-pressing strategy. Photo-emitters in NIR exhibit full width at half maximum (Γ) of 330 ± 90 µeV, an order of magnitude narrower than the reported ULPs in 2D materials at 300 K, and decrease to 82 ± 70 µeV at 100 K, with coherence time (τ_c) of 21.3 ps. The capping substrate enforced spatial confinement during thermal expansion at 250 °C is believed to trigger a localized crystal symmetry breaking in SeNFs, causing a polymorphic transition from the semiconducting trigonal (t) to quasi-metallic orthorhombic (orth) phase. Fine structure splitting in orth-Se causes degeneracy in defect-associated bright excitons, resulting in ultra-sharp emission. Combined theoretical and experimental findings, an optimal biaxial compressive strain of −0.45% cm⁻¹ in t-Se is uncovered, induced by the coefficient of thermal expansion mismatch at the selenium/sapphire interface, resulting in bandgap widening from 1.74 to 2.23 ± 0.1 eV. This report underpins the underlying correlation between crystal symmetry breaking induced polymorphism and RT ULPs in SeNFs, and their phase change characteristics.

By utilizing the monolayer thickness of graphene in Nanoelectromechanical system (NEMS) switches, sub-1 V switching characteristics are achieved. The irreversible static friction at the switch contact is overcome by employing the weak van der Waals (vdW) bonding of graphene-hexagonal boron nitride. These Graphene NEMS vdW switches show sub-0.5 V switching voltage, 10⁵ ON/OFF ratio, and nearly zero hysteretic window characteristics.

Abstract

The Nanoelectromechanical (NEM) switches are a promising candidate to overcome the physical limitations of the complementary metal-oxide-semiconductor (CMOS) switches due to their quasi-zero leakage behavior, sub-thermal switching, and suitability to operate in harsh environments. The main obstacles affecting NEM switches are their irreversible switch-contact stiction, the large switching voltage, and its hysteretic loop. In this study, the irreversible static friction is overcome by employing the weak van der Waals (vdW) bonding of graphene-hexagonal boron nitride (hBN) contact in the Graphene NEM (GNEM) switches. These vdW switches show sub-0.5 V switching voltage with an ON/OFF ratio higher than 10⁵ and nearly zero hysteretic window characteristics with a high endurance of over 50 000 switching cycles. These remarkable performances are achieved by exploiting graphene's monolayer thickness, high Young's modulus, cubic mechanical restoring force, and low vdW binding energy characteristics. As chemical vapor deposition graphene and hBN are used in these GNEM switches, it exhibits the prospect for large-scale graphene NEM system applications. These GNEM switches can be potentially used in ultralow-power CMOS integrated circuits, hybrid NEM-CMOS systems, logic devices, NEM resonator mass sensing, and single-molecule sensors.

This work presents simply fabricated black silicon heterojunction photodiodes with near-100% broadband quantum efficiency. Nanostalagmites are fabricated for an easy-to-passivate surface with high optical absorption. Nanocrystal indium tin oxide contact and implementation of a deep-well depletion region enable effective photocarrier transport and extraction. The self-biased device exhibits >98% average external quantum efficiency from 570 to 925 nm wavelength.

Abstract

Photodiodes are fundamental components in modern optoelectronics. Heterojunction photodiodes, simply configured by two different contact materials, have been a hot research topic for many years. Currently reported self-biased heterojunction photodiodes routinely have external quantum efficiency (EQE) significantly below 100% due to optical and electrical losses. Herein, an approach that virtually overcomes this 100% EQE challenge via low-aspect-ratio nanostructures and drift-dominated photocarrier transport in a heterojunction photodiode is proposed. Broadband near-ideal EQE is achieved in nanocrystal indium tin oxide/black silicon (nc-ITO/b-Si) Schottky photodiodes. The b-Si comprises nanostalagmites which balance the antireflection effect and surface morphology. The built-in electric field is explored to match the optical generation profile, realizing enhanced photocarrier transport over a broadband of photogeneration. The devices exhibit unprecedented EQE among the reported leading-edge heterojunction photodiodes: average EQE surpasses ≈98% for wavelengths of 570–925 nm, while overall EQE is greater than ≈95% from 500 to 960 nm. Further, only elementary fabrication techniques are explored to achieve these excellent device properties. A heart rate sensor driven by nanowatt faint light is demonstrated, indicating the enormous potential of this near-ideal b-Si photodiode for low power consuming applications.

High-quality 2D SnSe/Ge heterostructure with abrupt rectifying interface is grown. A significant photoluminescence quenching behavior is observed due to spontaneous photoexciton transfer at heterointerface. It makes transient photoexcited carriers easier to manipulate and explore self-driven optoelectronic devices. The self-powered photodetector is then developed, which shows ultrasensitive performance including the maximum photocurrent, detectivity, and operating speed.

Abstract

Binary chalcogenide semiconductor of SnSe with layer-by-layer phase opens up new opportunities for construction of versatile nanoelectronics and optoelectronics devices. The superior crystal properties make the layered SnSe compatible with traditional microfabrication techniques. Although several significant dynamic processes of carriers on 2D SnSe/Ge interface can be responsible for low-dissipation devices with higher quantum efficiency, the controlled growth and photodetection application of SnSe/Ge heterojunction have rarely been investigated. Herein, the high-quality 2D SnSe/Ge heterostructure with electronically abrupt interface is designed for the first time. A prominent photoluminescence quenching behavior is observed attributing to spontaneously interlayer transfer of long-lived carriers under optical excitation. It is confirmed to be attractive for development of self-powered photodetectors. Then, the ultrasensitive SnSe/Ge photodetector is explored, which shows a higher photocurrent up to 1.22 mA under 1064 nm illumination at 0.0 V. Meanwhile, it presents an obtainable responsivity of 0.51 A W⁻¹, larger detectivity of 1.51 × 10¹¹ Jones, and ultrashort response time of 5.47 µs in self-driven mode. The study provides a profound understanding of interlayer excitation transfer mechanisms of long-lived carriers inside SnSe/Ge heterojunction. The newly-developed photodetector provides an essential step toward meeting the ever-increasing demand for ultrafast and low-dissipation optoelectronic devices.

Publication date: Available online 17 October 2022

Source: Joule

Author(s): Thomas J. Macdonald, Adam J. Clancy, Rebecca R.C. Shutt, Christopher A. Howard

npj 2D Materials and Applications, Published online: 14 October 2022; doi:10.1038/s41699-022-00349-x

A micrometer-scale, fast aquisition speed, Faraday rotation spectroscopy technique is demonstrated. It is based on charge-coupled device (CCD) detection, works over a broadband wavelength range, and offers a high sensitivity. Measurements of the exciton g-factor in atomically thin semiconductors and hysteresis loops of thin magnetic films are presented as proofs of the concept.

Abstract

A Faraday rotation spectroscopy (FRS) technique is presented for measurements on the micrometer scale. Spectral acquisition speeds of about two orders of magnitude faster than state-of-the-art modulation spectroscopy setups are demonstrated. The experimental method is based on charge-coupled-device detection, avoiding speed-limiting components, such as polarization modulators with lock-in amplifiers. At the same time, FRS spectra are obtained with a sensitivity of 20 µrad (0.001°\[0.001{\bm{^\circ }}\]) over a broad spectral range (525–800 nm), which is on par with state-of-the-art polarization-modulation techniques. The new measurement and analysis technique also automatically cancels unwanted Faraday rotation backgrounds. Using the setup, Faraday rotation spectroscopy of excitons is performed in a hexagonal boron nitride-encapsulated atomically thin semiconductor WS₂ under magnetic fields of up to 1.4 T at room temperature and liquid helium temperature. An exciton g-factor of −4.4 ± 0.3 is determined at room temperature, and −4.2 ± 0.2 at liquid helium temperature. In addition, FRS and hysteresis loop measurements are performed on a 20 nm thick film of an amorphous magnetic Tb₂₀Fe₈₀ alloy.

The direct observation of self-hybridized exciton-polaritons in a bare WS₂ layer is reported. The polariton emission is demonstrated to be controllable by varying the thickness of the sample or excitation power. It is also shown that the polariton inherits the valley degree of freedom from excitons and exhibits high valley polarization (P _v = 0.18) at room temperature.

Abstract

The strong excitonic properties of transition metal dichalcogenides (TMD) have led to the successful demonstration of exciton-polaritons (EPs) in various optical cavity structures. Recently, self-hybridized EPs have been discovered in a bare TMD layer, but experimental investigation is still lacking because of their nonradiative nature. Herein, the direct observation of self-hybridized EPs in a bare multilayer WS₂ via the evanescent field coupling technique is reported. Because of the thickness-dependent Rabi splitting energy, the dispersion curves of the EPs change sensitively with sample thickness. Moreover, continuous tuning of EP dispersion curves is demonstrated by controlling the excitation laser power. Lastly, it is observed that guided EPs retain valley polarization up to 0.2 at room temperature, representing a valley-preserved strong coupling regime. It is believed that the high tunability and valley polarization properties of the guided EPs in bare TMD layers can facilitate new nanophotonic and valleytronic applications.

Magnetic skyrmions and large topological Hall effect are demonstrated in chromium telluride. The Curie temperature and magnetic anisotropy in Cr_{1+ x} Te₂ can be controlled by the self-intercalate concentration x. In Cr_1.53Te₂, which has a Curie temperature of 295 K and a relatively weak perpendicular magnetic anisotropy, room-temperature skyrmions and topological Hall resistivity as large as ≈106 nΩ cm are observed.

Abstract

Room-temperature magnetic skyrmion materials exhibiting robust topological Hall effect (THE) are crucial for novel nano-spintronic devices. However, such skyrmion-hosting materials are rare in nature. In this study, a self-intercalated transition metal dichalcogenide Cr_{1+ x} Te₂ with a layered crystal structure that hosts room-temperature skyrmions and exhibits large THE is reported. By tuning the self-intercalate concentration, a monotonic control of Curie temperature from 169 to 333 K and a magnetic anisotropy transition from out-of-plane to the in-plane configuration are achieved. Based on the intercalation engineering, room-temperature skyrmions are successfully created in Cr_1.53Te₂ with a Curie temperature of 295 K and a relatively weak perpendicular magnetic anisotropy. Remarkably, a skyrmion-induced topological Hall resistivity as large as ≈106 nΩ cm is observed at 290 K. Moreover, a sign reversal of THE is also found at low temperatures, which can be ascribed to other topological spin textures having an opposite topological charge to that of the skyrmions. Therefore, chromium telluride can be a new paradigm of the skyrmion material family with promising prospects for future device applications.

2D semiconductors represent one of the most promising candidates to extend Moore's law. However, their full potential has been greatly hindered by the limited p-type ones with high performance and stability. This review offers a comprehensive discussion of the fundamentals, current status, and outlook of p-type 2D semiconductors, aiming to inspire relevant research for advanced future electronics.

Abstract

2D semiconductors represent one of the best candidates to extend Moore's law for their superiorities, such as keeping high carrier mobility and remarkable gate-control capability at atomic thickness. Complementary transistors and van der Waals junctions are critical in realizing 2D semiconductors-based integrated circuits suitable for future electronics. N-type 2D semiconductors have been reported predominantly for the strong electron doping caused by interfacial charge impurities and internal structural defects. By contrast, superior and reliable p-type 2D semiconductors with holes as majority carriers are still scarce. Not only that, but some critical issues have not been adequately addressed, including their controlled synthesis in wafer size and high quality, defect and carrier modulation, optimization of interface and contact, and application in high-speed and low-power integrated devices. Here the material toolkit, synthesis strategies, device basics, and digital electronics closely related to p-type 2D semiconductors are reviewed. Their opportunities, challenges, and prospects for future electronic applications are also discussed, which would be promising or even shining in the post-Moore era.