Holey 2D Research Articles

Effects of strain on electronic and optic properties of holey two-dimensional C2N crystals

A two-dimensional (2D) material, the holey 2D C2N (h2D-C2N) crystal, has recently been synthesized. Here, we investigate the strain effects on the properties of this material by first-principles calculations. We show that the material is quite soft with a small stiffness constant and can sustain large strains ≥12%. It remains a direct gap semiconductor under strain, and the bandgap size can be tuned in a wide range as large as 1 eV. Interestingly, for biaxial strain, a band crossing effect occurs at the valence band maximum close to a 8% strain, leading to a dramatic increase of the hole effective mass. Strong optical absorption can be achieved by strain tuning with absorption coefficient ∼106 cm−1 covering a wide spectrum. Our findings suggest the great potential of strain-engineered h2D-C2N in electronic and optoelectronic device applications.

Oxidative Etching of Hexagonal Boron Nitride Toward Nanosheets with Defined Edges and Holes.

Lateral surface etching of two-dimensional (2D) nanosheets results in holey 2D nanosheets that have abundant edge atoms. Recent reports on holey graphene showed that holey 2D nanosheets can outperform their intact counterparts in many potential applications such as energy storage, catalysis, sensing, transistors, and molecular transport/separation. From both fundamental and application perspectives, it is desirable to obtain holey 2D nanosheets with defined hole morphology and hole edge structures. This remains a great challenge for graphene and is little explored for other 2D nanomaterials. Here, a facile, controllable, and scalable method is reported to carve geometrically defined pit/hole shapes and edges on hexagonal boron nitride (h-BN) basal plane surfaces via oxidative etching in air using silver nanoparticles as catalysts. The etched h-BN was further purified and exfoliated into nanosheets that inherited the hole/edge structural motifs and, under certain conditions, possess altered optical bandgap properties likely induced by the enriched zigzag edge atoms. This method opens up an exciting approach to further explore the physical and chemical properties of hole- and edge-enriched boron nitride and other 2D nanosheets, paving the way toward applications that can take advantage of their unique structures and performance characteristics.

Holey Graphene Nanomanufacturing: Structure, Composition, and Electrochemical Properties

Topology is critical for properties and function of 2D nanomaterials. Membranes and films from 2D nanomaterials usually suffer from large tortuosity as a result from dense restacking of the nanosheets and thus have limited utility in applications such as electrodes for supercapacitor and batteries, which require ion transport through the nanosheet thickness. In comparison with conventional porous 2D nanomaterials, introducing holes through the nanosheets to create holey 2D nanomaterials with retention of the 2D‐related properties is a more viable approach to improve molecular transport. Here, graphene is used as a model to study the fundamental structure‐property relationship as a result from defect‐enabled hole creation. Specifically, the correlation of electrochemical capacitive properties with structure and composition for holey graphene materials is prepared using a highly scalable controlled air oxidation process. The presence of holes on graphene sheets is not sufficient to account for the observed capacitance improvement. Rather, the improvement is achieved through the combination of an enhanced mesopore fraction with simultaneous oxygen doping while retaining the graphitic carbon network with minimal damage. The detailed understanding might be further applied to other 2D materials toward a broader range of both energy‐related and other applications.

Introduction

Photonic crystal materials attracted much attention during last years with the number of publications and patents increasing exponentially. Two-dimensional (2D) photonic crystal structures, like holey fibers and 2D slab-type photonic crystals, are probably the most advanced and fast developing areas owing to mature fabrication methods and envisioned broad applications. They are now at a critical phase of their development as they move fast from the realm of fundamental studies to photonic devices and commercialization. The idea behind this issue is to look at both fiber and 2D waveguide geometries and explore some of the most important and interesting issues related to implementation of these optical structures as photonic devices. This special issue is inspired by a recent LEOS Topical Meeting on this topic held in Vancouver in July 2003. The meeting was very stimulating and challenged many perceptions of the role of photonic crystal technologies in next generation photonic devices. We have invited a good mix of theoretical, experimental and optical device contributions, both from the photonic crystal community as well as the holey fiber community with the intention that this special issue will catalyze the rapid advance of our subject and bring together these different fields to foster research strength.