Jiuxiang Dai

Light: Science & Applications, Published online: 21 April 2023; doi:10.1038/s41377-023-01134-1

We theoretically and experimentally show a 900-fold boosting of optical THG emission in silicon metasurfaces using a novel type of nonlinear multi-mode Fano mechanism.

Nature Communications, Published online: 21 April 2023; doi:10.1038/s41467-023-38024-4

The microscopic mechanism of the metal-insulator transition in 2D disordered semiconductors is not fully understood. Shin et al. propose a universal mechanism due to curvature-induced band gap fluctuations in a structurally disordered system, based on gate-tunable scanning tunneling microscopy on monolayer MoS2.

The merged water molecules into Zn(AOT)₂ help construct a layered structure of the resultant plastic crystal. The water molecules play key roles in this process, which not only position AOT anions regularly through the ion-dipole interaction, but also activate Zn²⁺ conduction relying on heteroleptic coordination.

Abstract

Miniaturized solid zinc-ion batteries that are safe, environmentally friendly, and low-cost are ideal candidates for powering emerging microelectronics. However, sluggish Zn²⁺ mobility in solid phases hampers the viability of solid Zn²⁺ electrolytes and hence their practicability. Here, nanoscale Zn²⁺ channels are successfully engineered in a plastic-crystal electrolyte, thus activating fast Zn²⁺ solid-state transport. The ion-dipole interaction exerted by water molecules orients amphiphilic anions in bilayers, further forming a layered architecture backed by long-range van der Waals attractive forces. In the interlayer, the heteroleptic coordination contributed by the water molecule and anion frees the Zn²⁺ from anionic traps, leading to a high Zn²⁺ conductivity of 2.2 × 10⁻³ S cm⁻¹. This elaborately tailored texture confers a combination of robust mechanical characteristics and outstanding electrochemical performance upon the resultant electrolyte. The applicability is demonstrated by the high Zn²⁺ platting/stripping efficiency (99.6%), durable longevity of symmetric Zn-Zn and Zn-MnO₂ cells, as well as the engineering of versatile micro batteries (MBs). This work provides new perspectives for developing super multivalent ion conductors through the innovative design of ion-conducting nanochannels.

The adsorption of molecular electron donors and acceptors allows controlled modification of the In₂O₃ surface electron accumulation layer by interfacial charge transfer. This enables tuning the 2D electron gas at the In₂O₃ surface for advanced applications.

Abstract

In₂O₃, an n-type semiconducting transparent transition metal oxide, possesses a surface electron accumulation layer (SEAL) resulting from downward surface band bending due to the presence of ubiquitous oxygen vacancies. Upon annealing In₂O₃ in ultrahigh vacuum or in the presence of oxygen, the SEAL can be enhanced or depleted, as governed by the resulting density of oxygen vacancies at the surface. In this work, an alternative route to tune the SEAL by adsorption of strong molecular electron donors (specifically here ruthenium pentamethylcyclopentadienyl mesitylene dimer, [RuCp*mes]₂) and acceptors (here 2,2′-(1,3,4,5,7,8-hexafluoro-2,6-naphthalene-diylidene)bis-propanedinitrile, F₆TCNNQ) is demonstrated. Starting from an electron-depleted In₂O₃ surface after annealing in oxygen, the deposition of [RuCp*mes]₂ restores the accumulation layer as a result of electron transfer from the donor molecules to In₂O₃, as evidenced by the observation of (partially) filled conduction sub-bands near the Fermi level via angle-resolved photoemission spectroscopy, indicating the formation of a 2D electron gas due to the SEAL. In contrast, when F₆TCNNQ is deposited on a surface annealed without oxygen, the electron accumulation layer vanishes and an upward band bending is generated at the In₂O₃ surface due to electron depletion by the acceptor molecules. Hence, further opportunities to expand the application of In₂O₃ in electronic devices are revealed.

A vertical GaN nanodiode with a 50 nm air channel is reported, fabricated using IC-compatible technologies, with a record field emission current of 11 mA@10 V. Notably, the device displays outstanding stability and fast switching characteristics with a sub-10 ns response time. Additionally, the temperature-dependent performance can guide the design of GaN NACTs for applications in extreme conditions.

Abstract

Nanoscale air channel transistors (NACTs) have received significant attention due to their remarkable high-frequency performance and high switching speed, which is enabled by the ballistic transport of electrons in sub-100 nm air channels. Despite these advantages, NACTs are still limited by low currents and instability compared to solid-state devices. GaN, with its low electron affinity, strong thermal and chemical stability, and high breakdown electric field, presents an appealing candidate as a field emission material. Here, a vertical GaN nanoscale air channel diode (NACD) with a 50 nm air channel is reported, fabricated by low-cost IC-compatible manufacturing technologies on a 2-inch sapphire wafer. The device boasts a record field emission current of 11 mA at 10 V in the air and exhibits outstanding stability during cyclic, long-term, and pulsed voltage testing. Additionally, it displays fast switching characteristics and good repeatability with a response time of fewer than 10 ns. Moreover, the temperature-dependent performance of the device can guide the design of GaN NACTs for applications in extreme conditions. The research holds great promise for large current NACTs and will speed up their practical implementation.

This review aims to provide comprehensive knowledge of grain size engineering research of CVD-grown large-area graphene films with nanocrystalline, polycrystalline, and single-crystal structures. The main strategies and underlying growth mechanisms are summarized. In addition, the scaling law of physical properties as a function of the grain sizes is briefly discussed and the challenges for future development are also provided.

Abstract

Graphene, a single atomic layer of graphitic carbon, has attracted much attention because of its outstanding properties hold great promise for a wide range of technological applications. Large-area graphene films (GFs) grown by chemical vapor deposition (CVD) are highly desirable for both investigating their intrinsic properties and realizing their practical applications. However, the presence of grain boundaries (GBs) has significant impacts on their properties and related applications. According to the different grain sizes, GFs can be divided into polycrystalline, single-crystal, and nanocrystalline films. In the past decade, considerable progress has been made in engineering the grain sizes of GFs by modifying the CVD processes or developing some new growth approaches. The key strategies involve controlling the nucleation density, growth rate, and grain orientation. This review aims to provide a comprehensive description of grain size engineering research of GFs. The main strategies and underlying growth mechanisms of CVD-grown large-area GFs with nanocrystalline, polycrystalline, and single-crystal structures are summarized, in which the advantages and limitations are highlighted. In addition, the scaling law of physical properties in electricity, mechanics, and thermology as a function of grain sizes are briefly discussed. Finally, the perspectives for challenges and future development in this area are also presented.

Nature Photonics, Published online: 20 April 2023; doi:10.1038/s41566-023-01198-w

The dipole-dependent propagation of hybrid excitons in a van der Waals heterostructure containing a WSe2 bilayer is characterized by modulating the layer hybridization and interplay between many-body interactions of excitons with an applied vertical electric field.

Nature Physics, Published online: 20 April 2023; doi:10.1038/s41567-023-02043-1

Time crystals are a new state of matter. Conventional crystal properties are periodic in space, while the properties of a time crystal are periodic in time. A continuous quantum time crystal has recently been realized, and now, using optically driven many-body interactions in a nano-mechanical photonic metamaterial, a classical continuous time crystal has been created.

Nature Physics, Published online: 20 April 2023; doi:10.1038/s41567-023-02016-4

Glasses relax internally even when their structure is frozen. Observations of a two-dimensional glass former now show that although structure relaxation freezes with the glass transition, non-constrained bonds survive; this accounts for persisting internal relaxation.

Nature Physics, Published online: 20 April 2023; doi:10.1038/s41567-023-02017-3

The spontaneous topological Hall effect, combining non-coplanar antiferromagnetic order with finite scalar spin chirality in the absence of a magnetic field, is now experimentally demonstrated for the triangular lattice compounds CoTa3S6 and CoNb3S6.

Nature Communications, Published online: 20 April 2023; doi:10.1038/s41467-023-38012-8

Metal free’ materials offer a cheap and chemical benign platform for magnetism, however, the typical source of magnetism are unpaired electrons of a metal, thus designing ‘metal free’ magnetic materials represents a significant challenge. Here, Du et al present a strategy for enhancing the magnetism in carbon nitride using boron bridges.

Applied Physics Letters, Volume 122, Issue 16, April 2023.
Both two-dimensional (2D) transitional metal dichalcogenides (TMDs) and III–V semiconductors have been considered as potential platforms for quantum technology. While 2D TMDs exhibit a large exciton binding energy, and their quantum properties can be tailored via heterostructure stacking, TMD technology is currently limited by the incompatibility with existing industrial processes. Conversely, III-nitrides have been widely used in light-emitting devices and power electronics but not leveraging excitonic quantum aspects. Recent demonstrations of 2D III-nitrides have introduced exciton binding energies rivaling TMDs, promising the possibility to achieve room-temperature quantum technologies also with III-nitrides. Here, we discuss recent advancements in the synthesis and characterizations of 2D III-nitrides with a focus on 2D free-standing structures and embedded ultrathin quantum wells. We overview the main obstacles in the material synthesis, vital solutions, and the exquisite optical properties of 2D III-nitrides that enable excitonic and quantum-light emitters.

Nitrogen-doped BiOCl atomic layers can directly split carbon-sequestrated natural seawater into stoichiometric CO and HClO under visible light with selectivities greater than 90 %. The produced HClO can effectively kill typical bacteria in the ballast water of ocean-going cargo ships.

Abstract

Seawater is one of the most important CO₂ sequestration media for delivering value-added chemicals/fuels and active chlorine; however, this scenario is plagued by sluggish reaction rates and poor product selectivity. Herein, we first report the synthesis of nitrogen-doped BiOCl atomic layers to directly split carbon-sequestrated natural seawater (Yellow Sea, China) into stoichiometric CO (92.8 μmol h⁻¹) and HClO (83.2 μmol h⁻¹) under visible light with selectivities greater than 90 %. Photoelectrons enriched on the exposed BiOCl{001} facet kinetically facilitate CO₂-to-CO reduction via surface-doped nitrogen bearing Lewis basicity. Photoholes, mainly located on the lateral facets of van der Waals gaps, promote the selective oxidation of Cl⁻ into HClO. Sequestrated CO₂ also maintains the pH of seawater at around 4.2 to prevent the alkaline earth cations from precipitating. The produced HClO can effectively kill typical bacteria in the ballast water of ocean-going cargo ships, offering a green and safe way for onsite sterilization.

Distinct from the established spin-flop type metamagnetism, this work reports the unconventional spin-lattice interlocked metamagnetism in few-layer CrOCl down to the monolayer thickness. These unconventional metamagnetic transitions produce a unique field-tunable ferrimagnetic phase with the magnetization axis freely rotated by magnetic field within an easy plane.

Abstract

The long-range magnetic ordering in frustrated magnetic systems is stabilized by coupling magnetic moments to various degrees of freedom, for example, by enhancing magnetic anisotropy via lattice distortion. Here, the unconventional spin-lattice coupled metamagnetic properties of atomically-thin CrOCl, a van der Waals antiferromagnet with inherent magnetic frustration rooted in the staggered square lattice, are reported. Using temperature- and angle-dependent tunneling magnetoconductance (TMC), in complementary with magnetic torque and first-principles calculations, the antiferromagnetic (AFM)-to-ferrimagnetic (FiM) metamagnetic transitions (MTs) of few-layer CrOCl are revealed to be triggered by collective magnetic moment flipping rather than the established spin-flop mechanism, when external magnetic field (H) enforces a lattice reconstruction interlocked with the five-fold periodicity of the FiM phase. The spin-lattice coupled MTs are manifested by drastic jumps in TMC, which show anomalous upshifts at the transition thresholds and persist much higher above the AFM Néel temperature. While the MTs exhibit distinctive triaxial anisotropy, reflecting divergent magnetocrystalline anisotropy of the c-axis AFM ground state, the resulting FiM phase has an a-c easy plane in which the magnetization axis is freely rotated by H. At the 2D limit, such a field-tunable FiM phase may provide unique opportunities to explore exotic emergent phenomena and novel spintronics devices.

Nature Communications, Published online: 18 April 2023; doi:10.1038/s41467-023-37828-8

A challenge in making a flexible mold stamp using roll-to-roll nanoimprint lithography is to increase area while minimizing perceptible seams. Here, based on Fourier spectral analysis of moiré patterns resulting from superposed identical patterns, a method that enables the fabrication of scalable, quasi-seamless functional surfaces without the use of alignment marks is proposed.

Highlights

• The progression of MXene's oxidation stability, the techniques available to monitor the phenomenon as well as the variables that contribute to its oxidation rate are discussed.

• Comprehensive aspects of the oxidation process in various storage settings and the debated oxidation mechanism along with the most effective antioxidation strategies are addressed in conjunction with current challenges to the air stability of MXenes.

Light: Science & Applications, Published online: 19 April 2023; doi:10.1038/s41377-023-01137-y

Near-field terahertz nonlinear optics with blue light

Two-dimensional (2D) sheets of a conjugated polymer were prepared via a space-confined polymerization method that uses ice as a template. The key step of this strategy is the transportation of monomer to ice crystal boundaries using micelles as carriers, followed by oxidative polymerization in a frozen state. After characterization, the 2D sheets were assembled to self-standing films and found to exhibit excellent performance when used as electrodes in supercapacitors.

Abstract

Despite significant progress in the preparation and characterization of two-dimensional (2D) materials, the synthesis of 2D organic materials remains challenging. Here, we report a novel space-confined polymerization method that enables the large-scale synthesis of 2D sheets of a functional conjugated polymer, namely, poly(3,4-ethylenedioxythiophene) (PEDOT). A key step in this method is the confinement of monomer to the boundaries of ice crystals using micelles. This spatial confinement directs the polymerization to form 2D PEDOT sheets with high crystallinity and controlled morphology. Supercapacitors prepared from the 2D PEDOT sheets exhibit outstanding performance metrics. In aqueous electrolyte, a high areal specific capacitance of 898 mF cm⁻² at 0.2 mA cm⁻² along with an excellent rate capability is achieved (e.g., capacitance retention of 67.6 % at a 50-fold higher current). Moreover, the 2D PEDOT-based supercapacitors exhibit outstanding cycling stability (capacitance retention of 98.5 % after 30,000 cycles). Device performance is further improved when an organic electrolyte is used.

N-type single walled carbon nanotube thin film transistors fabricated using an environmentally friendly tri-layer dielectric.

Abstract

The proliferation of disposable, wearable, and implantable printable electronics requires the development of high-performance biodegradable, and sustainable electronic components. Often green materials don't have the necessary properties for high-performance electronics, therefore obtaining the ideal properties requires a combination of multiple green materials. A tri-layer dielectric is reported using poly(lactic acid) (PLA), poly(vinyl alcohol)/cellulose nanocrystals (PVAc), and toluene diisocyanate terminated poly(caprolactone) (TPCL), which is integrated into semiconducting single-walled carbon nanotube (sc-SWCNT) based thin film transistors (TFTs) in a top gate bottom contact architecture. The PVA provides a high dielectric constant due to the hydroxy groups, the cellulose is used to optimize the viscosity, the TPCL layer provides a robust hydrophobic surface, and the PLA eliminates the interfacial charge traps present in the PVAc and improves the adhesion between PVAc and the substrate. This leads to a decrease in leakage currents and reduces the polarity at the dielectric/semiconductor interface. The TFTs fabricated using tri-layer dielectrics led to air-stable n-type devices with higher overall performance when compared against the PVAc/TPCL bilayer devices.

PEDOT:PSS/Al₂O₃/n-ZnO tri-layer metal-oxide–semiconductor structure on rigid ITO/glass substrate and flexible ITO/PET substrate are prepared and the effect of positive and negative piezo-charges on the tunneling characteristics of metal-oxide–semiconductor structures at forward and reverse bias is systematically investigated. This work provides an in-depth understanding for the piezo-phototronic effect on the Fowler–Nordheim tunneling mechanism.

Abstract

The piezo-phototronic effect can modulate the dynamics of photo-generated carriers by utilizing external-strain–induced piezoelectric charges (piezo-charges). Most of the current researches focus on the modulation effect on the optoelectronic properties of p–n junctions. There are only a few studies focusing on the metal-oxide–semiconductor (MOS) structures. In this work, a PEDOT:PSS/Al₂O₃/n-ZnO tri-layer MOS tunneling junction is fabricated and the piezo-phototronic effect on its photo-carriers dynamic and performances is systematically investigated. The photoresponsivity to 365 nm laser illumination is reduced because of the negative piezo-charges generated at the Al₂O₃/n-ZnO interface. Meanwhile but unexpectedly, the similar phenomenon utilizing the positive piezo-charges at the Al₂O₃/n-ZnO interface is observed. The in-depth working mechanisms are carefully investigated by analyzing the effect of piezo-charges on the energy band diagram. Strain induced piezo-charges could feasibly adjust the intermediate oxide layer's barrier height and width so that the tunneling effect can be efficiently modulated by the piezo-phototronic effect, leading to the effective control over the MOS tunneling junction's photo-carriers dynamic and corresponding photoresponses. This work provides an in-depth understanding for the piezo-phototronic effect on the MOS tunneling junction and provides guidance for the subsequent researches on the coupling of piezo-phototronic effect and tunneling effect.

An approach for spatially resolved X-ray reflectivity (XRR) is proposed. By taking advantage of the liquid surface curvature at synchrotron conditions, it allows to visualize in situ and in real-time along a given direction the micrometer-scale domains of a 2D material—graphene, during growth and to reconstruct XRR curves containing information about both covered and noncovered parts of the surface.

Abstract

Here the possibility of utilizing X-ray reflectivity for visualization with ≈µm spatial resolution of a surface with a heterogeneous electron density due to a partial coverage by another nanometrically thin material is demonstrated. It requires the sample to be convexly bent, thus reflecting the collimated incident beam onto a magnified image recorded by a position-sensitive detector. By the use of a small, intense, and parallel beam such as provided by the most recent synchrotron sources, one can record such spatially resolved X-ray reflectivity with 0.1–1 kHz frame rate. The use of the method for in situ, time-resolved characterization of single-layer graphene domains during their chemical vapor deposition on a naturally curved surface of a liquid copper drop is demonstrated. This method can follow the growth kinetics, including the coverage ratio, 2D crystal (flake) sizes, and distances between flakes. By taking a single scan, the individual X-ray reflectivity curves can be reconstructed, of both covered and noncovered parts of the surface, allowing to deduce the corresponding electron density profiles perpendicular to the surface. The technique has a promising perspective for in situ study of 2D materials, ultrathin films, and self-assemblies on liquid as well as solid surfaces.

Abstract

Although photodetection based on two-dimensional (2D) van der Waals (vdWs) P—N heterojunction has attracted extensive attention recently, their low responsivity (R) due to the lack of carrier gain mechanism in reverse bias or zero bias operation hinders their applications in advanced photodetection area. Here, a black phosphorus/rhodamine 6G/molybdenum disulfide (BP/R6G/MoS₂) photodiode with high responsivity at reverse bias or zero bias has been achieved by using interfacial charge transfer of R6G molecules assembled between heterojunction layers. The formed vdWs interface achieves high performance photoresponse by efficiently separating the additional photogenerated electrons and holes generated by R6G molecules. The devices sensitized by the dye molecule R6G exhibit enhanced photodetection performance without sacrificing the photoresponse speed. Among them, the R increased by 14.8–20.4 times, and the specific detectivity (D*) increased by 24.9–34.4 times. The strategy based on interlayer assembly of dye molecules proposed here may pave a new way for realizing high-performance photodetection based on 2D vdWs heterojunctions with high responsivity and fast response speed.

Nature Communications, Published online: 19 April 2023; doi:10.1038/s41467-023-37635-1

The lack of a standardized approach for the characterization of the performance of 2D photodetectors represents an important obstacle towards their industrialization. Here, the authors propose practical guidelines to characterize their figures of merit and analyse common situations where their performance can be misestimated.