Jiuxiang Dai

A full 2D van der Waals heterostructure ferroelectric semimetal junction is fabricated to unravel the conductive mechanism of CuInP₂S₆, and a detailed phase diagram of the competing mechanisms and transition boundaries with temperature and an external electric field is presented. Based on this understanding, an artificial synapse with automatic gain control is also demonstrated.

Abstract

The recently unfolded ferroionic phenomena in 2D van der Waals (vdW) copper–indium–thiophosphate (CuInP₂S₆ or CIPS) have received widespread interest as they allow for dynamic control of conductive switching properties, which are appealing in the paradigm-shift computing. The intricate couplings between ferroelectric polarization and ionic conduction in 2D vdW CIPS facilitate the manipulation and dynamic control of conductive behaviors. However, the complex interplays and underlying mechanisms are not yet fully explored and understood. Here, by investigating polarization switching and ionic conduction in the temperature and applied electric field domains, it is discovered that the conducting mechanisms of CIPS can be divided into four distinctive states (or modes) with transitional boundaries, depending on the dynamics of Cu ions in the material. Further, it demonstrates that dynamically-tunable synaptic responsive behavior can be well implemented by governing the working-state transition. This research provides an in-depth, quantitative understanding of the complex phenomena of conductive switching in 2D vdW CIPS with coexisting ferroelectric order and ionic disorder. The developed insights in this work lay the ground for implementing high-performance, function-enriched devices for information processing, data storage, and neuromorphic computing based on the 2D ferroionic material systems.

Hybrid metasurfaces of plasmonic lattices and 2D materials provide a versatile platform for both fundamental and practical studies because of their unprecedented ability for precise manipulation of light at the nanoscale. This review summarizes how the structure design and nanofabrication led to application advances of enhanced photoluminescence, quantum emission, optoelectronic detection, nonlinear process, and valleytronics in hybrid metasurfaces.

Abstract

Plasmonic metasurfaces can significantly enhance the interaction between light and 2D materials. Hybrid structures of plasmonic lattices and 2D materials show great promise for both fundamental and practical studies because of their unprecedented ability for precise manipulation of light at the nanoscale. This review starts with an overview of the basic concepts of plasmonic lattices and optical properties of 2D materials, as well as fabrication strategies for hybrid metasurfaces. Then, the enhanced photoluminescence, quantum emission, optoelectronic detection, nonlinear process, and valleytronics in hybrid metasurfaces are summarized, and their development for nanophotonic functional devices are reviewed. Further, several compelling topics are also outlined that provide outlooks for future directions of hybrid metasurfaces such as novel structural design and high-quality fabrication, all-dielectric metasurfaces, dynamic metasurfaces, and plasmonic mediation of chemical reactions and physical processes. It is believed that hybrid metasurfaces of plasmonic lattices and 2D materials can open prospects for versatile platforms for light-matter interactions and contribute to the revolutions on nanophotonic devices.

Ferromagnetism is imprinted in van der Waals (vdW) SnS by Co doping through a bottom-up reaction. A low Gilbert damping of 1.69 × 10⁻³ is obtained in vdW Co _x Sn_{1− x} S. In addition, photodetectors based on Co _x Sn_{1− x} S are demonstrated. Co _x Sn_{1− x} S is an emerging semiconductor with both ferromagnetic ordering and photoelectric response, which provides unprecedented opportunities in spintronic-photonic integrated applications.

Abstract

2D ferromagnetic semiconductors are key to next-generation spintronic devices in the post-Moore era. The combination of ferromagnetic and optoelectronic properties offers exciting opportunities for advanced multifunctional devices in spin-optoelectronic applications. Herein, the authors synthesize 2D van der Waals (vdW) Co _x Sn_{1- x} S with ferromagnetism and photoresponse through a bottom-up reaction, which has a high yield compared to typical mechanical exfoliation. Ferromagnetic ordering is realized in 2D vdW semiconductor SnS by Co doping at the Sn sites. Magnetic properties are thoroughly studied at different doping concentrations, and first-principles calculations are further performed to reveal the magnetism origin and spin interactions. In particular, a low Gilbert damping of 1.69 × 10⁻³ is obtained in vdW Co _x Sn_{1− x} S through ferromagnetic resonance. In addition, photodetectors based on Co _x Sn_{1− x} S quantum dots are demonstrated. These studies establish a promising semiconductor with both ferromagnetic ordering and photoelectric response, which provides unprecedented opportunities in spintronic-photonic integrated applications.

The unexpected discovery of ferroelectricity in functional oxides such as HfO₂- and AlN- has aroused significant interest in computing device applications. Herein, the fundamental science on the manner in which the “hidden” ferroelectricity can be achieved in the ferroelectric (Hf,Zr)O₂ and (Al,Sc)N and their practical applications are thoroughly discussed.

Abstract

Ferroelectric materials are considered ideal for emerging memory devices owing to their characteristic remanent polarization, which can be switched by applying a sufficient electric field. However, even several decades after the initial conceptualization of ferroelectric memory, its applications are limited to a niche market. The slow advancement of ferroelectric memories can be attributed to several extant issues, such as the absence of ferroelectric materials with complementary metal–oxide–semiconductor (CMOS) compatibility and scalability. Since the 2010s, ferroelectric memories have attracted increasing interest because of newly discovered ferroelectricity in well-established CMOS-compatible materials, which are previously known to be non-ferroelectric, such as fluorite-structured (Hf,Zr)O₂ and wurtzite-structured (Al,Sc)N. With advancing material fabrication technologies, for example, accurate chemical doping and atomic-level thickness control, a metastable polar phase, and switchable polarization with a reasonable electric field can be induced in (Hf,Zr)O₂ and (Al,Sc)N. Nonetheless, various issues still exist that urgently require solutions to facilitate the use of the ferroelectric (Hf,Zr)O₂ and (Al,Sc)N in emerging memory devices. Thus, ferroelectric (Hf,Zr)O₂ and (Al,Sc)N are comprehensively reviewed herein, including their fundamental science and practical applications.

Abstract

Carbon source precursor is a critical factor governing chemical vapor deposition growth of graphene films. Methane (CH₄), has been the most commonly used precursor in the last decade, but it presents challenges in terms of decomposition efficiency and growth rate. Here we thoroughly evaluated acetylene (C₂H₂), a precursor that is probably for providing carbon dimer (C₂) species, for fast growth of large-scale graphene films. We find that the graphene growth behaviors fueled by C₂H₂ exhibit unconventional localized growth behavior with significant advantages in terms of high growth rate, which mainly ascribe to the as-decomposed C₂ species. Therefore, a C₂-fueled scanning growth strategy is proposed, and the fast scanning growth rate of 40 cm/min was experimentally demonstrated. This growth strategy is compatible with the approach of unidirectional growth of single-crystal graphene films, and the as-grown graphene films are of high-quality. This work demonstrates a reliable and promising strategy for the rapid synthesis of high-quality graphene film and may pave the avenue to cost-effective mass production of graphene materials in the roll-to-roll system.

Nature Materials, Published online: 22 June 2023; doi:10.1038/s41563-023-01585-2

FeSe does not exhibit magnetic order and lacks a nematic quantum critical point coinciding with optimal superconductivity, suggesting that an orbital mechanism drives nematicity, but direct evidence is lacking. Here, combining X-ray linear dichroism with in situ uniaxial stress, the role of spontaneous orbital polarization in nematic-phase FeSe is determined.

Nature Materials, Published online: 22 June 2023; doi:10.1038/s41563-023-01580-7

The authors demonstrate strong coupling in bound state in the continuum metasurfaces on nanostructured bulk WS2 and exhibiting sharp resonances with tailored linewidths and controllable light-matter coupling strength.

Nature Materials, Published online: 22 June 2023; doi:10.1038/s41563-023-01573-6

High-quality AlN heteroepitaxial films are obtained by controllable discretization and coalescence of columns on nano-patterned AlN/sapphire templates with hexagonal holes, where the density of dislocation etch pits is greatly reduced to ~10 × 104 cm−2.

Nature Electronics, Published online: 22 June 2023; doi:10.1038/s41928-023-00981-5

Strontium titanate two-dimensional electron gas channels that have a thin hafnium oxide barrier layer between the channel and an ionic liquid gate can have ballistic constrictions and clean normal-state conductance quantization.

Author(s): L. Facheris, S. D. Nabi, A. Glezer Moshe, U. Nagel, T. Rõõm, K. Yu. Povarov, J. R. Stewart, Z. Yan, and A. Zheludev

High-resolution neutron and THz spectroscopies are used to study the magnetic excitation spectrum of Cs2CoBr4, a distorted-triangular-lattice antiferromagnet with nearly XY-type anisotropy. What was previously thought of as a broad excitation continuum [L. Facheris et al., Phys. Rev. Lett. 129, 0872…

[Phys. Rev. Lett. 130, 256702] Published Thu Jun 22, 2023

A review describes the challenges facing the search for an elusive class of quasiparticles.

Over 100-fold enhancement of upconversion emission is achieved in cryogenic Er³⁺-rich nanosystems. The role of temperature in emission is quantitatively revealed, which obeys an exponential relationship.

Abstract

Recently high doping of lanthanide ions (till 100 %) is realized unprecedentedly in nanostructured upconversion (UC) phosphors. However, oddly enough, this significant breakthrough did not result in a corresponding UC enhancement at ambient temperature, which hinders the otherwise very interesting applications of these materials in various fields. In this work, taking the Er³⁺-rich UC nanosystem as an example, we confirm unambiguously that the phonon-assisted cross relaxation (CR) is the culprit. More importantly, combining the theoretical modeling and experiments, the precise roles of different CR channels on UC energy loss are quantitatively revealed. As a result, lowering the temperature can exponentially enhance the relevant UC luminescence by more than two orders of magnitude. Our comprehension will play an important role in promoting the UC performance and further application of high doping rare earth materials. As a proof of concept, an Er³⁺-rich core/multi-shell nanophosphor is exploited which demonstrates the great potential of our finding in the field of ultra-sensitive temperature sensing.

Nature Reviews Materials, Published online: 23 June 2023; doi:10.1038/s41578-023-00578-6

An article in Nature Nanotechnology reports ferroelectric field-effect transistors that are compatible with Si complementary metal–oxide–semiconductor back-end-of-line processes.

This review summarizes recent advancements in atomically-thin layered ferroelectrics, focusing on the fundamentals of intrinsic out-of-plane ferroelectricity in van der Waals materials at 2D limit, the emergence of artificial 2D ferroelectricity by utilizing the layer degree of freedom, and related ferroelectric device applications in non-volatile storage, non-linear computation, and optoelectronics.

Abstract

With the advent of the post Moore era, modern electronics require further device miniaturization of all electronic components, particularly ferroelectric memories, due to the need for massive data storage. This demand stimulates the exploration of robust switchable ferroelectric polarizations at the atomic scale. In this scenario, van der Waals ferroelectrics have recently gained increasing attention because of their stable layered structure at nanometer thickness, offering the opportunity to realize two-dimensional ferroelectricity that is long-sought in conventional thin film ferroelectrics. In this review, recent advancements are summarized in layered ferroelectrics with highlights of the fundamentals of intrinsic two-dimensional ferroelectricity, the emergence of artificial stacking ferroelectricity, and related protype devices with exotic functions. In addition, the unique polarization control in van der Waals ferroelectrics is discussed. Although great challenges remain unsolved, these studies undoubtedly advance the integration of 2D ferroelectrics in electronics.