Whisper Gallery Modes in Monolayer Tungsten Disulfide-Hexagonal Boron Nitride Optical Cavity

There are strong interests in constructing nanolasers using two-dimensional transition metal dichalcogenides (TMDs) due to their strong light–matter interactions and high optical gain. However, most cavity designs based on transfer of exfoliated TMDs on silicon oxide are not optimized since monolayer emitters are located far from where the photonic mode reaches maximum intensity. By taking advantage of the excellent dielectric properties of hexagonal boron nitride (h-BN), we design a new microdisk optical cavity fabricated from a van der Waals (VdW) stacked h-BN/WS₂/h-BN. The heterostructure is patterned into microdisk cavities characterized by whispering gallery modes (WGMs). The emission intensity of the WS₂ trion is enhanced by 2.9 times that of exciton in the heterostructure, giving rise to whisper gallery modes with resonance intensities that show nonlinear power dependence. A Rayleigh scatterer directs the cavity emission to vertical collection. Such VdW heterostructure provides an atomically smooth interface that is ideal for low loss photon propagation, giving a <i>Q</i> factor of 1200

Self-Templated Synthesis of Triphenylene-Based Uniform Hollow Spherical Two-Dimensional Covalent Organic Frameworks for Drug Delivery

Constructing two-dimensional covalent organic frameworks (2DCOFs) with a desirable crystalline structure and morphology is promising but remains a significant challenge. Herein, we report self-templated synthesis of uniform hollow spherical 2DCOFs based on 2,3,6,7,10,11-hexakis(4-aminophenyl) triphenylene. A detailed time-dependent study of hollow sphere formation reveals an intriguing transformation from initial homogeneous solid spheres into uniform hollow spheres with the Ostwald ripening mechanism. Impressively, the resultant spherical 2DCOFs are composed of high crystallinity nanosheets and even hexagonal single crystals, as demonstrated by transmission electron microscopy. Thanks to its uniform morphology and high crystallinity, the pore volume of the obtained 2DCOFs is up to 1.947 cm3 g–1, which makes it function as superior nanocarriers for efficient controlled drug delivery. This result provides an avenue for improving COFs’ performance by regulating their morphology

Fractal Etching of Graphene

An anisotropic etching mode is commonly known for perfect crystalline materials, generally leading to simple Euclidean geometric patterns. This principle has also proved to apply to the etching of the thinnest crystalline material, graphene, resulting in hexagonal holes with zigzag edge structures. Here we demonstrate for the first time that the graphene etching mode can deviate significantly from simple anisotropic etching. Using an as-grown graphene film on a liquid copper surface as a model system, we show that the etched graphene pattern can be modulated from a simple hexagonal pattern to complex fractal geometric patterns with sixfold symmetry by varying the Ar/H₂ flow rate ratio. The etched fractal patterns are formed by the repeated construction of a basic identical motif, and the physical origin of the pattern formation is consistent with a diffusion-controlled process. The fractal etching mode of graphene presents an intriguing case for the fundamental study of material etching

Strain Modulation by van der Waals Coupling in Bilayer Transition Metal Dichalcogenide

Manipulation of lattice strain is emerging as a powerful means to modify the properties of low-dimensional materials. Most approaches rely on external forces to induce strain, and the role of interlayer van der Waals (vdW) coupling in generating strain profiles in homobilayer transition metal dichalcogenide (TMDC) films is rarely considered. Here, by applying atomic-resolution electron microscopy and density functional theory calculations, we observed that a mirror twin boundary (MTB) modifies the interlayer vdW coupling in bilayer TMDC films, leading to the development of local strain for a few nanometers in the vicinity of the MTB. Interestingly, when a single MTB in one layer is “paired” with another MTB in an adjacent layer, interlayer-induced strain is reduced when the MTBs approach each other. Therefore, MTBs are not just 1D discontinuities; they can exert localized 2D strain on the adjacent lattices

Phonon-Mediated Colossal Magnetoresistance in Graphene/Black Phosphorus Heterostructures

There is a huge demand for magnetoresistance (MR) sensors with high sensitivity, low energy consumption, and room temperature operation. It is well-known that spatial charge inhomogeneity due to impurities or defects introduces mobility fluctuations in monolayer graphene and gives rise to MR in the presence of an externally applied magnetic field. However, to realize a MR sensor based on this effect is hampered by the difficulty in controlling the spatial distribution of impurities and the weak magnetoresistance effect at the monolayer regime. Here, we fabricate a highly stable monolayer graphene-on-black phosphorus (G/BP) heterostructure device that exhibits a giant MR of 775% at 9 T magnetic field and 300 K, exceeding by far the MR effects from devices made from either monolayer graphene or few-layer BP alone. The positive MR of the G/BP device decreases when the temperature is lowered, indicating a phonon-mediated process in addition to scattering by charge impurities. Moreover, a nonlocal MR of >10 000% is achieved for the G/BP device at room temperature due to an enhanced flavor Hall effect induced by the BP channel. Our results show that electron–phonon coupling between 2D material and a suitable substrate can be exploited to create giant MR effects in Dirac semimetals

Two-Dimensional Polymer Synthesized <i>via</i> Solid-State Polymerization for High-Performance Supercapacitors

Two-dimensional (2-D) polymer has properties that are attractive for energy storage applications because of its combination of heteroatoms, porosities and layered structure, which provides redox chemistry and ion diffusion routes through the 2-D planes and 1-D channels. Here, conjugated aromatic polymers (CAPs) were synthesized in quantitative yield <i>via</i> solid-state polymerization of phenazine-based precursor crystals. By choosing flat molecules (2-TBTBP and 3-TBQP) with different positions of bromine substituents on a phenazine-derived scaffold, C–C cross coupling was induced following thermal debromination. CAP-2 is polymerized from monomers that have been prepacked into layered structure (3-TBQP). It can be mechanically exfoliated into micrometer-sized ultrathin sheets that show sharp Raman peaks which reflect conformational ordering. CAP-2 has a dominant pore size of ∼0.8 nm; when applied as an asymmetric supercapacitor, it delivers a specific capacitance of 233 F g^–1 at a current density of 1.0 A g^–1, and shows outstanding cycle performance

Lateral Epitaxy of Atomically Sharp WSe₂/WS₂ Heterojunctions on Silicon Dioxide Substrates

Lateral Epitaxy of Atomically Sharp WSe₂/WS₂ Heterojunctions on Silicon Dioxide Substrate

Chemical Vapor Deposition of Large-Size Monolayer MoSe₂ Crystals on Molten Glass

We report the fast growth of high-quality millimeter-size monolayer MoSe₂ crystals on molten glass using an ambient pressure CVD system. We found that the isotropic surface of molten glass suppresses nucleation events and greatly improves the growth of large crystalline domains. Triangular monolayer MoSe₂ crystals with sizes reaching ∼2.5 mm, and with a room-temperature carrier mobility up to ∼95 cm²/(V·s), can be synthesized in 5 min. The method can also be used to synthesize millimeter-size monolayer MoS₂ crystals. Our results demonstrate that “liquid-state” glass is a highly promising substrate for the low-cost growth of high-quality large-size 2D transition metal dichalcogenides (TMDs)

<i>In Situ</i> Observation and Electrochemical Study of Encapsulated Sulfur Nanoparticles by MoS₂ Flakes

Sulfur is an attractive cathode material for next-generation lithium batteries due to its high theoretical capacity and low cost. However, dissolution of its lithiated product (lithium polysulfides) into the electrolyte limits the practical application of lithium sulfur batteries. Here we demonstrate that sulfur particles can be hermetically encapsulated by leveraging on the unique properties of two-dimensional materials such as molybdenum disulfide (MoS₂). The high flexibility and strong van der Waals force in MoS₂ nanoflakes allows effective encapsulation of the sulfur particles and prevent its sublimation during <i>in situ</i> TEM studies. We observe that the lithium diffusivities in the encapsulated sulfur particles are in the order of 10^–17 m² s^–1. Composite electrodes made from the MoS₂-encapsulated sulfur spheres show outstanding electrochemical performance, with an initial capacity of 1660 mAh g^–1 and long cycle life of more than 1000 cycles