Jing Zhang

Van der Waals (vdW) material Fe 5 GeTe 2 , with its long-range ferromagnetic ordering near room temperature, has significant potential to become an enabling platform for implementing novel spintronic and quantum devices. To pave the way for applications, it is crucial to determine the magnetic properties when the thickness of Fe 5 GeTe 2 reaches the few-layers regime. However, this is highly challenging due to the need for a characterization technique that is local, highly sensitive, artifact-free, and operational with minimal fabrication. Prior studies have indicated that Curie temperature T C can reach up to close to room temperature for exfoliated Fe 5 GeTe 2 flakes, as measured via electrical transport; there is a need to validate these results with a measurement that reveals magnetism more directly. In this work, we investigate the magnetic properties of exfoliated thin flakes of vdW magnet Fe 5

The ability of the perovskite CsPbBr₃ spectroscopic-grade detector for continuum hard X-ray detection in energy-discrimination configuration (single photon mode), as well as in photoconductive modes is comparatively demonstrated. Blocking contacts are used to achieve energy determination of the incident X-ray photons and to achieve a low detectable dose rate limit of 0.02 nGy_air s⁻¹.

Abstract

Spectroscopic-grade single crystal detectors can register the energies of individual X-ray interactions enabling photon-counting systems with superior resolution over traditional photoconductive X-ray detection systems. Current technical challenges have limited the preparation of perovskite semiconductors for energy-discrimination X-ray photon-counting detection. Here, this work reports the deployment of a spectroscopic-grade CsPbBr₃ Schottky detector under reverse bias for continuum hard X-ray detection in both the photocurrent and spectroscopic schemes. High surface barriers of ≈1 eV are formed by depositing solid bismuth and gold contacts. The spectroscopic response under a hard X-ray source is assessed in resolving the characteristic X-ray peak. The methodology in enhancing X-ray sensitivity by controlling the X-ray energies and flux, and voltage, is described. The X-ray sensitivity varies between a few tens to over 8000 μC Gy_air ⁻¹ cm⁻². The detectable dose rate of the CsPbBr₃ detectors is as low as 0.02 nGy_air s⁻¹ in the energy discrimination configuration. Finally, the unbiased CsPbBr₃ device forms a spontaneous contact potential difference of about 0.7 V enabling high quality of the CsPbBr₃ single crystals to operate in “passive” self-powered X-ray detection mode and the X-ray sensitivity is estimated as 14 μC Gy_air ⁻¹ cm⁻². The great potential of spectroscopic-grade CsPbBr₃ devices for X-ray photon-counting systems is anticipated in this work.

Multiferroic Materials

In article number 2109144, Jae Hoon Kim, Je-Geun Park, and co-workers report a new exciton in multiferroic NiI₂, consisting of a 2D triangular lattice. This newly discovered exciton is not only magnetic but also quantum entangled. When an electron and hole combine via a transition between two quantum-entangled states, it produces a bright reddish light of 1.384 eV.

Bioprogrammable Lasers

Inspired by natural responsivity of biomaterials, in article number 2107809, Yu-Cheng Chen and co-workers demonstrate the first laser encoding by exploiting enzymatic hydrogels in a droplet cavity. The cover illustrates tunable lasing wavelengths achieved by manipulating biological activities and 3D networks in a microgel laser array. The biological encoded laser will provide a new insight into the development of bioprogrammable laser devices, offering new opportunities for secure communication and smart sensing.

Single-crystalline nanotetrapod transits from a single to multiphoton emitter upon tuning its shape, which is revealed by single-particle spectroscopy. Calculation and time-resolved spectroscopic results prove that the precise changeover of the exciton confinement dimension is driven by the geometry of the particle shape. Interactions between multiple excitons allow a useful and direct manipulation of single-nanotetrapod luminescence.

Abstract

As the properties of a semiconductor material depend on the fate of the excitons, manipulating exciton behavior is the primary objective of nanomaterials. Although nanocrystals exhibit unusual excitonic characteristics owing to strong spatial confinement, studying the interactions between excitons in a single nanoparticle remains challenging due to the rapidly vanishing multiexciton species. Here, a platform for exciton tailoring using a straightforward strategy of shape-tuning of single-crystalline nanocrystals is presented. Spectroscopic and theoretical studies reveal a systematic transition of exciton confinement orientation from 3D to 2D, which is solely tuned by the geometric shape of material. Such a precise shape-effect triggers a multiphoton emission in single nanotetrapods with arms longer than the exciton Bohr radius of material. In consequence, the unique interplay between the multiple quantum states allows a geometric modulation of the quantum-confined Stark effect and nanocrystal memory effect in single nanotetrapods. These results provide a useful metric in designing nanomaterials for future photonic applications.

Spin–phonon coupling in monolayer chromium tribromide is investigated via Raman spectroscopy in combination with first principle calculations. The experimental Curie temperature and phonon shifts are in good agreement with numerical simulations. This work demonstrates how magnetic exchange interactions affect the phonon vibrations and establishes the missing guidelines for the design of the 2D magnetic materials and related devices.

Abstract

Novel 2D magnets exhibit intrinsic electrically tunable magnetism down to the monolayer limit, which has significant value for nonvolatile memory and emerging computing device applications. In these compounds, spin–phonon coupling (SPC) typically plays a crucial role in magnetic fluctuations, magnon dissipation, and ultimately establishing long-range ferromagnetic order. However, a systematic understanding of SPC in 2D magnets that combines theory and experiment is still lacking. In this work, monolayer chromium tribromide is studied to investigate SPC in 2D magnets via Raman spectroscopy and first principle calculations. The experimental Curie temperature and phonon shifts are found to be in good agreement with the numerical simulations. Specifically, it is demonstrated how magnetic exchange interactions affect phonon vibrations, which helps establish design fundamentals for 2D magnetic materials and other related devices.

Nature Nanotechnology, Published online: 10 March 2022; doi:10.1038/s41565-022-01077-5

Reply to: Detectivities of WS₂/HfS₂ heterojunctions

Nature, Published online: 22 December 2021; doi:10.1038/s41586-021-04151-5

Inelastic neutron scattering measurements show that superconductivity in UTe2 is associated with a resonance near antiferromagnetic order that suggests an unexpected spin-singlet component to the electron pairing.

Nature Electronics, Published online: 25 November 2021; doi:10.1038/s41928-021-00682-x

Transistors made from two-dimensional materials have been around for a decade, but do the devices have a realistic future in integrated circuits?

Nature Chemistry, Published online: 07 March 2022; doi:10.1038/s41557-022-00900-9

The electronic structure of an electrode can affect the electron transfer rate of electrochemical processes at its surface. Now, it has been shown that varying the ‘twist’ angle between two stacked layers of graphene modifies the bilayer electronic structure and provides a new dimension to control interfacial redox activity.

MXene and silicon wafers are used to construct a real-time tribovoltaic nanogenerator (named MS-TVNG), which presents outstanding wear-resistance and energy output stability over those made by traditional metals. The output current of the MS-TVNG increases linearly when increasing the applied pressure and sliding speed. A novel principle for self-powered speed, displacement, tension, oscillation angle, and vibration sensors is proposed.

Abstract

Tribovoltaic nanogenerators (TVNGs), an emerging high-entropy energy harvesting technique, present great features such as low matching resistance, high current density, and continuous output performance. Here, an MXene layer and a semiconducting silicon wafer are assembled into a tribovoltaic nanogenerator (named MS-TVNG). The output peak current of the MS-TVNG reaches up to 22 µA for a P-type (0.1–0.5 Ω cm) silicon wafer under a normal force of 4.56 N and a sliding speed of 2 m s⁻¹. Owing to the unique metal characteristics of the MXene layer, the performance is superior to those previously reported TVNGs using traditional metals. The layered structure of MXene endows the real-time MS-TVNG with outstanding wear-resistance and stable output properties. The performance of the MS-TVNG can be tuned by the doping type and concentration of the silicon wafer, as well as by the pressure and the relative sliding speed between two friction surfaces. The MS-TVNG has proven to be a solid foundation for high-performance self-powered speed sensors and has excellent potentials for applications in displacement, tension, oscillation angle, and vibration detection.

Advanced Functional Materials, Volume 32, Issue 10, March 2, 2022.

The past decade has witnessed the emergence of topological materials in condensed matter physics. This comprehensive review surveys recent advances in functional optoelectronic devices based on topological materials. Perspectives and challenges in using topological materials in next-generation optoelectronic devices are discussed.

Abstract

The recent realization of topology as a mathematical concept in condensed matter systems has shattered Landau's widely accepted classification of phases by spontaneous symmetry breaking as he famously said, “a particular symmetry property exists or does not exist.” Topological materials (TMs) such as topological insulators and topological semimetals, are characterized by properties that depend on the topology of the band structure. Such dependence has drastic implications on the optical, electrical, and thermal properties of the material. Fundamental physics of TMs is currently under active research in condensed matter, materials science, and high energy physics. In this review, recent advances in exploiting the unique properties of TMs to realize functional optoelectronic devices are surveyed. Current and future applications that are, or may be, enabled by their unique properties are discussed. Although many theoretical ideas have been proposed over the past decade or so on using TMs in optoelectronic applications, the focus will be on experimentally realized devices.

Bioinspired MXene-reinforced soft tubular actuators with omnidirectional light-tracking and adaptive phototropism are demonstrated through in situ photopolymerization of a judiciously designed photopolymerizable MXene nanomonomer into reversible shape-morphing crosslinked main-chain liquid crystal elastomers. Like the hollow stem of plants, the resulting actuators show a high light-tracking accuracy and are capable of quickly sensing, continuously tracking, and adaptively interacting with the incident light in all zenithal and azimuthal angles of 3D space.

Abstract

Endowing artificial advanced materials and systems with biomimetic self-regulatory intelligence is of paramount significance for the development of somatosensory soft robotics and adaptive optoelectronics. Herein, a bioinspired phototropic MXene-reinforced soft tubular actuator is reported that exhibits omnidirectional self-orienting ability and is capable of quickly sensing, continuously tracking, and adaptively interacting with incident light in all zenithal and azimuthal angles of 3D space. The novelty of the soft tubular actuator lies in three aspects: 1) the new polymerizable MXene nanomonomer shows high compatibility with liquid crystal elastomer (LCE) matrices and can be in situ photopolymerized into the polymer networks, thus enhancing the mechanical and photoactuation properties; 2) the distinct hollow and radially symmetrical structure facilitates the actuator with fast photoresponsiveness and phototropic performance through retarding the heat conduction along the radial direction; 3) the MXene-LCE soft tubular actuator simultaneously integrates sensing, actuation, and built-in feedback loop, thus leading to a high light-tracking accuracy and adaptive phototropism like a hollow stem of plants in nature. As a proof-of-concept demonstration, an adaptive photovoltaic system with solar energy harvesting maximization is illustrated. This work can provide insights into the development of artificial intelligent materials toward adaptive optoelectronics, intelligent soft robotics, and beyond.

Coexisting charge density wave (CDW) and nontrivial topology is predicted in monolayer NbTe₂. In its CDW phase, monolayer NbTe₂ changes from a quantum spin Hall insulator to a quantum anomalous Hall insulator with increasing electronic correlations. This finding establishes NbTe₂ as a promising candidate to explore exotic quantum states at the confluence of nontrivial topology, electronic correlation, and CDW.

Abstract

The combination of nontrivial topology and charge density wave (CDW) has been proposed as a powerful resource for realizing novel quantum phenomena such as axion electrodynamics and the anomalous Hall effect. Hence, topological materials with CDW states attract great interest, yet they are still very rare, particularly in the 2D limit. Here, it is predicted that monolayer NbTe₂ in its high-symmetry 1T phase stabilized by anharmonicity at room temperature exhibits nontrivial topology sensitive to electronic interactions: it changes from a quantum spin Hall (QSH) state to a quantum anomalous Hall (QAH) state when the Hubbard potential U exceeds a critical value. At low temperature, a 4 × 4 CDW order emerges and coexists with the nontrivial topology. Meanwhile, the critical U increases because CDW reduces density of states at Fermi level. More interestingly, in contrast to the high-symmetry structure that actually is a topologically nontrivial metal, the CDW structure shows an insulating nontrivial gap either in the QSH or the QAH phase, indicating CDW is an effective means to modulate the topological state for developing new functions and devices. These discoveries establish NbTe₂ as a promising candidate to explore exotic quantum states at the confluence of nontrivial topology, electronic correlation, and CDW.

This work demonstrates light absorption enhancement by slow light using a fully organic photonic structure by doping some layers with a J-aggregate dye. This dye creates new photonic bandgaps with increased strength. The structure is considered a biomimetic of a natural photonic structure, consolidating the hypothesis of the use of photonics by nature to enhance light capture.

Abstract

Natural photosynthetic photonic nanostructures can show sophisticated light–matter interactions including enhanced light absorption by slow light even for highly pigmented systems. Beyond fundamental biology aspects, these natural nanostructures are very attractive as blueprints for advanced photonic devices. But the soft-matter biomimetic implementations of such nanostructures is challenging due to the low refractive index contrast of most organic photonic structures. Excitonic organic materials with near-zero index (NZI) optical properties allow overcoming these bottlenecks. Here, it is demonstrated that the combination of NZI thin films with photonic multilayers like the ones found in nature enables broadband tunable strong reflectance as well as slow light absorption enhancement and tailored photoluminescence properties in the full VIS spectrum. Moreover, it is shown that this complex optical response is tunable, paving the way toward the development of active devices based on all-polymer and near-zero index materials photonic structures.