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

Two-Dimensional CH₃NH₃PbI₃ Perovskite: Synthesis and Optoelectronic Application

Hybrid organic–inorganic perovskite materials have received substantial research attention due to their impressively high performance in photovoltaic devices. As one of the oldest functional materials, it is intriguing to explore the optoelectronic properties in perovskite after reducing it into a few atomic layers in which two-dimensional (2D) confinement may get involved. In this work, we report a combined solution process and vapor-phase conversion method to synthesize 2D hybrid organic–inorganic perovskite (<i>i.e.</i>, CH₃NH₃PbI₃) nanocrystals as thin as a single unit cell (∼1.3 nm). High-quality 2D perovskite crystals have triangle and hexagonal shapes, exhibiting tunable photoluminescence while the thickness or composition is changed. Due to the high quantum efficiency and excellent photoelectric properties in 2D perovskites, a high-performance photodetector was demonstrated, in which the current can be enhanced significantly by shining 405 and 532 nm lasers, showing photoresponsivities of 22 and 12 AW^–1 with a voltage bias of 1 V, respectively. The excellent optoelectronic properties make 2D perovskites building blocks to construct 2D heterostructures for wider optoelectronic applications

Graphene–Bi₂Te₃ Heterostructure as Saturable Absorber for Short Pulse Generation

Rapid progresses have been achieved in the photonic applications of two-dimensional materials such as graphene, transition metal dichalcogenides, and topological insulators. The strong light–matter interactions and large optical nonlinearities in these atomically thin layered materials make them promising saturable absorbers for pulsed laser applications. Either Q-switching or mode-locking pulses with particular output characteristics can be achieved by using different saturable absorbers. However, it remains still very challenging to produce saturable absorbers with tunable optical properties, in particular, carrier dynamics, saturation intensity as well as modulation depth, to suit for self-starting, high energy or ultrafast pulse laser generation. Here we report a new type of saturable absorber which is a van der Waals heterostructure consisting of graphene and Bi₂Te₃. The synergetic integration of these two materials by epitaxial growth affords tunable optical properties, that is, both the photocarrier dynamics and the nonlinear optical modulation are variable by tuning the coverage of Bi₂Te₃ on graphene. We further fabricated graphene–Bi₂Te₃ saturable absorbers and incorporated them into a 1.5 μm fiber laser to demonstrate both Q-switching and mode-locking pulse generation. This work provides a new insight for tailoring two-dimensional heterostructures so as to develop desired photonic applications