Jing Zhang

Nature Photonics, Published online: 27 April 2023; doi:10.1038/s41566-023-01205-0

Spatial light modulator-based lithography-free programmable light transmission through optical gain medium is demonstrated for optical switching and a rudimentary photonic neural network.

Nature Materials, Published online: 27 April 2023; doi:10.1038/s41563-023-01544-x

Using direct laser writing with a nanosecond pulsed laser operating at above-bandgap photon energies, we demonstrate the selective formation of spin defects in photonic crystal cavities in 4H-silicon carbide and their in situ characterization.

Nature Electronics, Published online: 27 April 2023; doi:10.1038/s41928-023-00955-7

Incorporation of the alkali metal salt caesium chloride into a hole injection layer can improve the efficiency of charge carrier injection and induce the growth of quasi-two-dimensional perovskite layers with improved emissive properties, resulting in blue perovskite light-emitting diodes with an external quantum efficiency of up to 16.1%.

Nature Electronics, Published online: 27 April 2023; doi:10.1038/s41928-023-00963-7

Memristors level up

The moiré exciton states and their dynamics in a twisted MoSe₂/WSe₂ heterobilayer are studied by photoluminescence spectroscopy and rate-equation analysis. The existence of a dark moiré exciton state ≈8 meV above the bright moiré exciton state and their dynamics determined by phonon-assisted transition between bright and dark state are clarified.

Abstract

Moiré potential by stacking two monolayers with slightly different lattice mismatches acts as periodic quantum confinement for optically generated excitons and provides spatially ordered 0D quantum systems. Fundamental studies on intrinsic optical phenomena in the moiré exciton are conducted; however, the excitonic states and, particularly, the dynamic properties of moiré excitons are underexplored. In this study, the unrevealed structures of moiré exciton states and their intriguing dynamics in twisted MoSe₂/WSe₂ heterobilayers are experimentally investigated by photoluminescence spectroscopy. Phonon-mediated momentum dark exciton state above bright exciton state with splitting energy of ≈8 meV inside moiré potential is observed. Additionally, it is demonstrated that the dynamics of moiré excitons are determined by the radiative recombination process of bright moiré excitons at low temperatures (<20 K) and phonon-assisted non-radiative processes from the lowest bright to dark moiré exciton state at high temperatures (>30 K). Furthermore, additional peaks at the high-energy side under high-power excitation conditions are observed, which indicates emission from the triplet bright moiré exciton state with a longer decay time of 700 ns. Experimental evidence of the bright and dark exciton states within the moiré potential might provide novel platforms for quantum optics applications using moiré superlattices.

A planar optical waveguide integrated with isolation-layer microlens array (WIMLA) with double isolation is realized by combining SU-8 self-writing optical waveguide with a black photoresist isolation layer. This WIMLA can improve the resolution of imaging and contrast of images, and combine with optimized image stitching algorithm to achieve panoramic stitching.

Abstract

The compound eyes of insects in nature provide many innovative perspectives for the development of bionic imaging devices. The construction of thousands of ommatidia favors compound eye imaging, and the close arrangement between each small eye generates a significant amount of optical crosstalk. This work reports an optical waveguide integrated with isolation-layer microlens array (WIMLA) with high-contrast, high-resolution imaging inspired by the compound eyes of insects. This WIMLA is mainly fabricated by self-written optical waveguide formed under ultraviolet exposure and the introduction of a black photoresist under the action of capillary force to achieve the effect of double isolation. The experimental results demonstrate that image contrast increases by 59.8% with double-layer isolation and the full width at half-maximum reduces by 27.7% in comparison with those of the MLA without any isolation layer. As a result, a more focused optical path can be confined inside the microlens for transmission. Panoramic stitching is accomplished by traversing all potential matching subimages with the edge detection and breadth-first search algorithms. The combination of the prepared WIMLA with commercial cameras to capture faces will open up new possibilities for mobile navigation, face recognition, and medical monitoring.

Owing to its large bandwidth, we identify layered semiconducting antiferromagnet CrPS₄ as an ideal candidate to realize field-effect transistors based on 2D magnetic materials. Low-temperature measurements exhibit a gate-tunable magnetoconductance exceeding 5000%, as well as the ability to continuously tune electrostatically the boundaries between different states in the magnetic phase diagram of the material.

Abstract

Using field-effect transistors (FETs) to explore atomically thin magnetic semiconductors with transport measurements is difficult, because the very narrow bands of most 2D magnetic semiconductors cause carrier localization, preventing transistor operation. Here, it is shown that exfoliated layers of CrPS₄—a 2D layered antiferromagnetic semiconductor whose bandwidth approaches 1 eV—allow the realization of FETs that operate properly down to cryogenic temperature. Using these devices, conductance measurements as a function of temperature and magnetic field are performed to determine the full magnetic phase diagram, which includes a spin-flop and a spin-flip phase. The magnetoconductance, which depends strongly on gate voltage, is determined. reaching values as high as 5000% near the threshold for electron conduction. The gate voltage also allows the magnetic states to be tuned, despite the relatively large thickness of the CrPS₄ multilayers employed in the study. The results show the need to employ 2D magnetic semiconductors with sufficiently large bandwidth to realize properly functioning transistors, and identify a candidate material to realize a fully gate-tunable half-metallic conductor.

Two distinct requirements of various nanomaterials, including from a single semiconductor or with van der Waals heterostructures, are summarized, including where strain must be avoided, such as solutions to produce strain-insensitive devices, and where strain is required, such as pressure-sensitive outcomes.

Abstract

2D materials with dangling-bond-free surfaces and atomically thin layers have been shown to be capable of being incorporated into flexible electronic devices. The electronic and optical properties of 2D materials can be tuned or controlled in other ways by using the intriguing strain engineering method. The latest and encouraging techniques in regard to creating flexible 2D nanoelectronics are condensed in this review. These techniques have the potential to be used in a wider range of applications in the near and long term. It is possible to use ultrathin 2D materials (graphene, BP, WTe₂, VSe₂ etc.) and 2D transition metal dichalcogenides (2D TMDs) in order to enable the electrical behavior of the devices to be studied. A category of materials is produced on smaller scales by exfoliating bulk materials, whereas chemical vapor deposition (CVD) and epitaxial growth are employed on larger scales. This overview highlights two distinct requirements, which include from a single semiconductor or with van der Waals heterostructures of various nanomaterials. They include where strain must be avoided and where it is required, such as solutions to produce strain-insensitive devices, and such as pressure-sensitive outcomes, respectively. Finally, points-of-view about the current difficulties and possibilities in regard to using 2D materials in flexible electronics are provided.

Ultralong deep potassium storage enabled by a BiOCl/MXene van der Waals heterostructure is attained for conversion/alloying-type anodes, delivering 225 mAh g⁻¹ after 10 month cycling at 100 mA g⁻¹. The stabilized conversion and alloying/de-alloying reactions can be attributed to the improved dispersion of potassium ions during the initial optimized intercalation and facilitated ion diffusion kinetics.

Abstract

Conversion/alloying-type anodes are drawing attention due to their high theoretical capacities, but inferior reversibility, especially under low current densities, has hampered potential applications. Conventional strategies mainly focus on conversion/alloying processes, whereas the intercalation process is rarely analyzed. Herein, the intercalation process is correlated with conversion/alloying processes by ion dispersion states. BiOCl/Ti₃C₂T _x MXene van der Waals heterostructure is selected as a proof-of-concept system. Multifunctional MXenes not only contribute to atomic dispersion and boosted ion diffusion at the first cycle by constructing a novel heterostructure but serve as supporting frameworks to sustain long-term structural stability. Consequently, a cell with BiOCl/MXene anode delivers an ultralong cycle-life of running over ten months, maintaining a high capacity of 225 mAh g⁻¹ over 1300 cycles at 100 mA g⁻¹ and a retention of 81.3%. These findings verify that enhanced initial intercalation can facilitate higher reversibility and shed light on developing high-performance conversion/alloying-type anodes.

Laser-Induced Carbonization

Laser-induced carbonization is a viable method for creating anticounterfeiting tags on flexible printed circuit boards. In article number 2211762, Sunkook Kim, Hogun Park, and co-workers illustrate that using a laser, a pattern can be configured in any desired shape, such as a fingerprint. The laser beam is used to create random size carbonized spots that are distributed according to the pattern inputted into the laser. The pattern's outer circles can be extracted and digitized for authentication purposes.

Lower power, continuous wave illumination in a confocal microscope provides convenient direct write patterning of conductive, reduced graphene oxide, from the sub-micron to millimetre scales, in both 2D and 3D. Correlative transmission, reflection, and fluorescence images reveal the photothermal reduction mechanism in detail.

Abstract

Graphene oxide (GO) films can be readily prepared at wafer scale, then reduced to form graphene-based conductive circuits relevant to a range of practical device applications. Among a variety of reduction methods, laser processing has emerged as an important technique for localized reduction and patterning of GO films. In this study, the novel use of confocal microscopy is demonstrated for high-resolution characterization, in situ laser reduction, and versatile patterning of GO films. Multi-modal imaging and real-time tracking are performed with 405 and 488 nm lasers, enabling large-area direct observation of the reduction progress. Using image analysis to cluster flake types, the different stages of reduction can be attributed to thermal transfer and accumulation. Delicate control of the reduction process over multiple length scales is illustrated using millimeter-scale stitched patterns, micropatterning of single flakes, and direct writing conductive 2D wires with sub-micrometer resolution (530 nm). The general applicability of the technique is shown, allowing fabrication of both conductive reduced graphene oxide (rGO) films (sheet resistance: 2.5 kOhm sq⁻¹) and 3D microscale architectures. This simple and mask-free method provides a valuable tool for well-controlled and scalable fabrication of reduced GO structures using compact low-power lasers (< 5 mW), with simultaneous in situ monitoring and quality control.

Ferroelectric control of polarity of the spin-polarized current can be achieved in van der Waals multiferroic heterostructures via reversible FE switch of the internal electric field and the FE control of the interfacial effect. This opens up new opportunities for constructing low-energy-dissipation, non-volatile, and high-sensitive spintronic devices such as multiferroic spin field-effect transistors.

Abstract

Ferroelectric (FE) control of magnetism at nanoscale, for instance, FE control of the polarity of spin-polarized current is crucial for technological advances in magnetoelectric and spintronic applications. However, this fascinating functionality has not been reported in nanoscale systems yet. Herein, a new class of FE/A-type antiferromagnetic heterobilayer/FE van der Waals (vdW) multiferroic structures is found, in which the FE control of polarity of spin-polarized current is found possible. Take Sc₂CO₂/CrSiTe₃/CrGeTe₃/Sc₂CO₂ heterostructure as a successful example. First-principles calculations reveal that its polarity of half-metallicity can be switched by flipping the FE polarization orientation. Meanwhile, device transport simulation shows that its up/down spin current transmission ratio is as large as 0.1 × 10³ at P→↑↑\[\mathop {\rm{P}}\limits^ \to \uparrow \uparrow \] Sc₂CO₂ configuration and is only 2.6 × 10⁻³ at P→↓↓$\mathop {\rm{P}}\limits^ \to \downarrow \downarrow $ Sc₂CO₂ configuration in the vdW multiferroic heterostructures. Essentially, it stems from the reversible FE switch of the internal electric field across the CrSiTe₃/CrGeTe₃ heterobilayer and the FE control of the interfacial effect between Sc₂CO₂ and Cr(Si/Ge)Te₃ layers. This work opens a direction for constructing low-energy-dissipation, non-volatile, and high-sensitive spintronic devices such as spin field-effect transistors.

Room temperature spin-phonon coupling in nanocrystals of antiferromagnetic Cr₂O₃ is investigated via Raman spectroscopy in combination with magnetic measurements. This study demonstrates the interplay between the crystalline and magnetic structures in these 3D antiferromagnets when varying the surface-to-volume ratio and helps establish the fundamentals of coupling phononic excitations with the magnetization dynamics at room temperature, offering a highly prospective nanomaterial for the design of novel magnonic devices.

Abstract

In this study, nanocrystals of antiferromagnetic Cr₂O₃ are shown via Raman spectroscopy to display peculiar lattice dynamics in terms of phonon softening and the occurrence of an exceptionally strong spin-phonon coupling. This effect, which is observed to persist well above the onset of the antiferromagnetic ordering temperature, is ascribed to locally correlated spin fluctuations due to the modulation of the magnetic exchange interactions as the chromium atoms oscillate about their equilibrium position. It is found that the spin-phonon coupling strength is governed by the competing antiferromagnetic and ferromagnetic interactions, where changes in the surface spin configuration can also play a crucial role. Overall, this work proves the size dependence of the interplay between the crystalline and magnetic structures in 3D antiferromagnets varying the surface-to-volume ratio and helps establish the fundamentals for a spin-phonon coupling engineering at the nanoscale via a simple route in a very stable and easy to synthesize material. More importantly, it demonstrates the possibility of coupling phononic excitations with the magnetization dynamics at room temperature, offering a highly prospective nanomaterial for the design of novel magnonic devices.

A direct optical writing approach is utilized to deterministically realize polymorphic 2D materials by locally inducing metallic 1T′-MoTe₂ on the semiconducting 2H-MoTe₂ host layer. A seven-fold enhancement in third harmonic generation intensity in 1T′-MoTe₂ compared to 2H-MoTe₂ is observed with telecom-band ultrafast pump laser. A Schottky photodiode with high optoelectronic performance is realized with polymorphic MoTe₂.

Abstract

Developing selective and coherent polymorphic crystals at the nanoscale offers a novel strategy for designing integrated architectures for photonic and optoelectronic applications such as metasurfaces, optical gratings, photodetectors, and image sensors. Here, a direct optical writing approach is demonstrated to deterministically create polymorphic 2D materials by locally inducing metallic 1T′-MoTe₂ on the semiconducting 2H-MoTe₂ host layer. In the polymorphic-engineered MoTe₂, 2H- and 1T′- crystalline phases exhibit strong optical contrast from near-infrared to telecom-band ranges (1–1.5 µm), due to the change in the band structure and increase in surface roughness. Sevenfold enhancement of third harmonic generation intensity is realized with conversion efficiency (susceptibility) of ≈1.7 × 10⁻⁷ (1.1 × 10⁻¹⁹ m² V⁻²) and ≈1.7 × 10⁻⁸ (0.3 × 10⁻¹⁹ m² V⁻²) for 1T′ and 2H-MoTe₂, respectively at telecom-band ultrafast pump laser. Lastly, based on polymorphic engineering on MoTe₂, a Schottky photodiode with a high photoresponsivity of 90 AW⁻¹ is demonstrated. This study proposes facile polymorphic engineered structures that will greatly benefit realizing integrated photonics and optoelectronic circuits.

In situ uniaxial tensile strain is applied to a freestanding BiFeO₃ film by stretching an organic substrate. A scanning nitrogen-vacancy microscopy is applied to image the nanoscale magnetic order. The strain is continuously increased to 1.5% and a spin cycloid tilting ≈12.6° is observed. A first principle calculation is processed to show that the tilting is energetically favorable.

Abstract

Bismuth ferrite (BiFeO₃) possesses a non-collinear spin order while the ferroelectric order breaks space inversion symmetry. This allows efficient electric-field control of magnetism and makes it a promising candidate for applications in low-power spintronic devices. Epitaxial strain effects have been intensively studied and exhibit significant modulation of the magnetic order in bismuthBiFeO₃, but tuning its spin structure with continuously varied uniaxial strain is still lacking at this moment. Herein, in situ uniaxial tensile strain is applied to a freestanding BiFeO₃ film by mechanically stretching an organic substrate. A scanning nitrogen-vacancy (NV) microscopy is applied to image the nanoscale magnetic order in real space. The strain is continuously increased from 0% to 1.5% and four images under different strains are acquired during this period. The images show that the spin cycloid tilts by ≈12.6° when strain approaches 1.5%. A first principle calculation is processed to show that the tilting is energetically favorable under such strain. The in situ strain applying method in combination with scanning NV microscope real-space imaging ability paves a new way in studying the coupling between magnetic order and strain in BiFeO₃ films.

The artificial Wnt3a niche with concentration gradient is fabricated by Multiphoton, Microfabrication, and Micropatterning (MMM) technology. The resulted Wnt3a niche is bio-functional to promote the proliferation and maintain the inductive phenotype of human hair dermal papilla cells (HHDPC) in a Wnt3a-gradient dependent manner, demonstrating its utility in cell niche studies and drug screening studies for hair follicle inductivity and regeneration.

Abstract

The Wingless/Integrated (Wnt) signaling is a central player in regulating multiple cellular events ranging from developmental process, adult homeostasis to tumorigenesis. In native microenvironments, Wnt signals often function through the concentration gradients between the secreting site and the targeting site. Hence, presenting Wnt molecules in a concentration gradient in vitro in a controllable manner recapitulates the native Wnt niche. Nevertheless, most in vitro Wnt niche models, where the ligands are homogeneously immobilized on material surface, fail in reconstituting the concentration gradient of Wnt as that in vivo. Herein, using a previously established Multiphoton Microfabrication and Micropatterning (MMM) technology, a concentration gradient of Wnt3a is immobilized on a protein substrate via neutravidin-biotin interactions, in a quantitatively and spatially controllable manner. Such Wnt3a gradient is bioactive as it triggers a dose- and time-dependent upregulation of β-catenin and cell proliferation, in model mouse L cells. Most importantly, human hair dermal papilla cells (HHDPCs) cultured in a cell niche with Wnt3a gradient proliferate in a Wnt3a-gradient-dependent manner and exhibit a Wnt3a-gradient-dependent hair-inductive phenotype including Wnt/β-catenin signaling activation and alkaline phosphatase (ALP) expression. This work contributes to the future development of a hair follicle inductivity biochip for screening of drugs and therapeutics on hair regeneration.

Nature Nanotechnology, Published online: 27 April 2023; doi:10.1038/s41565-023-01376-5

Intrinsic ferroelectricity in bilayer WTe2 can be used for electrical switching of the centred-rectangular moiré potential in WTe2/WSe2 heterostructures.

Nature Nanotechnology, Published online: 27 April 2023; doi:10.1038/s41565-023-01390-7

Growth of molybdenum disulfide at 275 °C and monolithic integration of 2D transistors with silicon complementary metal oxide semiconductor circuits have been demonstrated.

Nature Communications, Published online: 24 April 2023; doi:10.1038/s41467-023-37887-x

The integration of high-κ dielectric layers with 2D semiconductors is essential for electronic applications, but remains challenging. Here the authors report a dry transfer method of wafer-scale Al2O3 and HfO2 thin films for the realization of top-gated monolayer MoS2 transistors and logic gates.

Nature Physics, Published online: 24 April 2023; doi:10.1038/s41567-023-02039-x

Spin liquids are predicted to emerge in materials that combine strong electronic correlations with geometric frustration. Evidence has now been found for a spin liquid state in the triangular-lattice material NaRuO2.

Two-dimensional (2D) materials have received extensive research attentions over the past two decades due to their intriguing physical properties (such as the ultrahigh mobility and strong light–matter interaction at atomic thickness) and a broad range of potential applications (especially in the fields of electronics and optoelectronics). The growth of single-crystal 2D materials is the prerequisite to realize 2D-based high-performance applications. In this review, we aim to provide an in-depth analysis of the state-of-the-art technology for the growth and applications of 2D materials, with particular emphasis on single crystals. We first summarize the major growth strategies for monolayer 2D single crystals. Following that, we discuss the growth of multilayer single crystals, including the control of thickness, stacking sequence, and heterostructure composition. Then we highlight the exploration of 2D single crystals in electronic and optoelectronic devices. Finally, a perspective is given to outline the research opportunities and the remaining challenges in this field.

Continuous-wave optically pumped perovskite sub-micrometer lasers are demonstrated. Their thresholds are comparable to or even lower than those of several-micrometer-sized counterparts owing to their enhanced exciton–photon coupling strength. The potential of exciton–polaritons in laser miniaturization is unveiled experimentally and theoretically to develop low-threshold semiconductor lasers with device sizes approaching the optical diffraction limit.

Abstract

Continuous-wave (CW) pumped lasers with device areas below 1 µm² constitute a key step to meeting the energy efficiency requirement for on-chip optical communications. However, a debate about whether a sub-micrometer device size and low threshold can be simultaneously satisfied has persisted owing to insurmountable radiation losses when approaching the optical diffraction limit. Herein, a record-small CW optically pumped perovskite laser with a device area of 0.65 µm² is demonstrated. The thresholds of sub-micrometer lasers can be found lower than those of several-micrometer counterparts, and are ascribed to the enlarged group refractive index and modal confinement resulting from the enhanced exciton–photon coupling. Moreover, the operation temperature is elevated to 150 K through the reduction in heat generation. These findings unveil the potential of exciton–polaritons in laser miniaturization, providing an alternative for developing low-threshold semiconductor lasers without artificial optical cavities, to approach the optical diffraction limit.

A life-cycle assessment (LCA) of Ti₃C₂T _x MXene as an EMI-shielding coating is presented. MXene synthesis, including laboratory-scale systems and precursor generation utilizing TiO₂, is examined for energy demand and environmental impacts. Findings demonstrate that laboratory electricity usage has the greatest impact on MXene production. Sustainable MXene synthesis toward the large-scale manufacturing of this intriguing 2D material is discussed.

Abstract

MXenes, 2D transition metal carbides, nitrides, and carbonitrides, have been investigated for diverse applications since their discovery; however, their life-cycle assessment (LCA) has not been studied. Here, a “cradle to gate” LCA is performed to assess the cumulative energy demand (CED) and environmental impacts of lab-scale synthesis of Ti₃C₂T _x , the most researched MXene composition. Electromagnetic interface (EMI) shielding is selected as it is one of MXenes' most promising applications and LCA of Ti₃C₂T _x synthesis is compared to aluminum and copper foils, two typical EMI-shielding materials. Two laboratory-scale MXene synthesis systems—gram and kilogram batches—are examined. The CED and environmental implications of Ti₃C₂T _x synthesis are investigated based on its precursor production, selective etching, delamination processes, laboratory location, energy mix, and raw material type. These results show that laboratory electricity usage for the synthesis processes accounts for >70% of the environmental impacts. Manufacturing 1.0 kg of industrial-scale aluminum and copper foil releases 23.0 kg and 8.75 kg of CO₂, respectively, while 1.0 kg of lab-scale MXene synthesis releases 428.10 kg. Chemical usage is less impactful than electricity, which suggests that recycled resources and renewable energy can make MXene synthesis more sustainable. Understanding MXene LCA helps the industrialization of this material.

This review provides a critical summary on the recent development of substrate engineering strategies, in specific, surface-stepping approaches, chemical-functionalization methods, substrate seeding strategies and so on, with the aim of the growth of large-scale 2D transition metal dichalcogenides (TMDs). The contribution might guide the deep understanding on the controlled growth of high-quality 2D TMDs toward their industrial-scale practical applications.

Abstract

The large-scale production of 2D transition metal dichalcogenides (TMDs) is essential to realize their industrial applications. Chemical vapor deposition (CVD) has been considered as a promising method for the controlled growth of high-quality and large-scale 2D TMDs. During a CVD process, the substrate plays a crucial role in anchoring the source materials, promoting the nucleation and stimulating the epitaxial growth. It thus significantly affects the thickness, microstructure, and crystal quality of the products, which are particularly important for obtaining 2D TMDs with expected morphology and size. Here, an insightful review is provided by focusing on the recent development associated with the substrate engineering strategies for CVD preparation of large-scale 2D TMDs. First, the interaction between 2D TMDs and substrates, a key factor for the growth of high-quality materials, is systematically discussed by combining the latest theoretical calculations. Based on this, the effect of various substrate engineering approaches on the growth of large-area 2D TMDs is summarized in detail. Finally, the opportunities and challenges of substrate engineering for the future development of 2D TMDs are discussed. This review might provide deep insight into the controllable growth of high-quality 2D TMDs toward their industrial-scale practical applications.