Controlling edge morphology in graphene layers using electron irradiation: from sharp atomic edges to coalesced layers forming loops

Effects of vacancy defects on the interfacial thermal resistance of partially overlapped bilayer graphene

Graphene has been extensively applied in composite materials due to its high thermal conductivity. Multi-layered graphene has great potential in the construction of a continuous filler network but is restricted by the high interfacial thermal resistance between adjacent graphene layers. This paper investigates the effects of the overlapping area and interlayer sp³ bonding of partially overlapped bilayer graphene on the interfacial thermal resistance using molecular dynamic simulations. The results show the linear relationship between the interfacial thermal resistance and the overlapping area. Then, identical vacancy defects of the same plane coordinates were added to each of the two graphene sheets, and it was found that the presence of an armchair edge restricted the formation of interface sp³ bonding to some extent, while the zigzag edge did not. However, their similar bond length and the phonon density of state of bonded atoms in the models with different edges indicated their similar effects on the heat transfer. Therefore, the thermal resistance of all single sp³ bonds in different models could be approximated to 14.3 × 10^-9 m² KW^-1. A formula is proposed to describe the inverse relationship between the number of sp³ bonds and the interfacial thermal resistance. Finally, the vacancy defect on the upper graphene sheet was moved to stagger the two vacancies. The length of sp³ bonds was changed slightly due to the staggered arrangement, and the interfacial thermal resistance was found to be positively correlated with the bond length. This allows valuable interfacial heat-transfer properties of the partially overlapped bilayer graphene to contribute to the thermal management of the 3D filler network.

Local Carbon Concentration Determines the Graphene Edge Structure

Although the structures and properties of various graphene edges have attracted enormous attention, the underlying mechanism that determines the appearance of various edges is still unknown. Here, a global search of graphene edge structures is performed by using the particle swarm optimization algorithm. In addition to locating the most stable edges of graphene, two databases of graphene armchair and zigzag edge structures are built. Graphene edge self-passivation plays an important role in the stability of the edges of graphene, and self-passivated edge structures that contain both octagons and triangles are found for the first time. The obvious "apical dominance" feature of armchair edges is found. The appearance of the experimentally observed ac(56), ac(677), and Klein edges can be explained by the local carbon concentration. Additionally, the graphene edge database is also significant for the study of the open end of nanotubes or fullerenes.

Tailoring nonlinear optical properties of Bi2Se3 through ion irradiation

The nonlinear optical property of topological insulator bismuth selenide (Bi₂Se₃) is found to be well-tailored through ion irradiation by intentionally introducing defects. The increase of the optical modulation depth sensitively depends on the careful selection of the irradiation condition. By implementing the ion irradiated Bi₂Se₃ film as an optical saturable absorber device for the Q-switched wave-guide laser, an enhanced laser performance has been obtained including narrower pulse duration and higher peak power. Our work provides a new approach of tailoring the nonlinear optical properties of materials through ion irradiation, a well-developed chip-technology, which could find wider applicability to other layered two-dimensional materials beyond topological insulators, such as graphene, MoS₂, black phosphours etc.

Correlating atomic structure and transport in suspended graphene nanoribbons

Graphene nanoribbons (GNRs) are promising candidates for next generation integrated circuit (IC) components; this fact motivates exploration of the relationship between crystallographic structure and transport of graphene patterned at IC-relevant length scales (<10 nm). We report on the controlled fabrication of pristine, freestanding GNRs with widths as small as 0.7 nm, paired with simultaneous lattice-resolution imaging and electrical transport characterization, all conducted within an aberration-corrected transmission electron microscope. Few-layer GNRs very frequently formed bonded-bilayers and were remarkably robust, sustaining currents in excess of 1.5 μA per carbon bond across a 5 atom-wide ribbon. We found that the intrinsic conductance of a sub-10 nm bonded bilayer GNR scaled with width as GBL(w) ≈ 3/4(e(2)/h)w, where w is the width in nanometers, while a monolayer GNR was roughly five times less conductive. Nanosculpted, crystalline monolayer GNRs exhibited armchair-terminated edges after current annealing, presenting a pathway for the controlled fabrication of semiconducting GNRs with known edge geometry. Finally, we report on simulations of quantum transport in GNRs that are in qualitative agreement with the observations.

Edge-edge interactions in stacked graphene nanoplatelets

High-resolution transmission electron microscopy studies show the dynamics of small graphene platelets on larger graphene layers. The platelets move nearly freely to eventually lock in at well-defined positions close to the edges of the larger underlying graphene sheet. While such movement is driven by a shallow potential energy surface described by an interplane interaction, the lock-in position occurs via edge-edge interactions of the platelet and the graphene surface located underneath. Here, we quantitatively study this behavior using van der Waals density functional calculations. Local interactions at the open edges are found to dictate stacking configurations that are different from Bernal (AB) stacking. These stacking configurations are known to be otherwise absent in edge-free two-dimensional graphene. The results explain the experimentally observed platelet dynamics and provide a detailed account of the new electronic properties of these combined systems.

Topographic and spectroscopic characterization of electronic edge states in CVD grown graphene nanoribbons

We used scanning tunneling microscopy and spectroscopy (STM/S) techniques to analyze the relationships between the edge shapes and the electronic structures in as-grown chemical vapor deposition (CVD) graphene nanoribbons (GNRs). A rich variety of single-layered graphene nanoribbons exhibiting a width of several to 100 nm and up to 1 μm long were studied. High-resolution STM images highlight highly crystalline nanoribbon structures with well-defined and clean edges. Theoretical calculations indicate clear spin-split edge states induced by electron-electron Coulomb repulsion. The edge defects can significantly modify these edge states, and different edge structures for both sides of a single ribbon produce asymmetric electronic edge states, which reflect the more realistic features of CVD grown GNRs. Three structural models are proposed and analyzed to explain the observations. By comparing the models with an atomic resolution image at the edge, a pristine (2,1) structure was ruled out in favor of a reconstructed edge structure composed of 5-7 member rings, showing a better match with experimental results, and thereby suggesting the possibility of a defective morphology at the edge of CVD grown nanoribbons.

Stability of graphene edges under electron beam: equilibrium energetics versus dynamic effects

Electron beam of a transmission electron microscope can be used to alter the morphology of graphene nanoribbons and create atomically sharp edges required for applications of graphene in nanoelectronics. Using density-functional-theory-based simulations, we study the radiation hardness of graphene edges and show that the response of the ribbons to irradiation is not determined by the equilibrium energetics as assumed in previous experiments, but by kinetic effects associated with the dynamics of the edge atoms after impacts of energetic electrons. We report an unexpectedly high stability of armchair edges, comparable to that of pristine graphene, and demonstrate that the electron energy should be below ~50 keV to minimize the knock-on damage.

Low temperature edge dynamics of AB-stacked bilayer graphene: naturally favored closed zigzag edges

Closed edges bilayer graphene (CEBG) is a recent discovered novel form of graphene structures, whose regulated edge states may critically change the overall electronic behaviors. If stacked properly with the AB style, the bilayer graphene with closed zigzag edges may even present amazing electronic properties of bandgap opening and charge separation. Experimentally, the CEBG has been confirmed recently with HRTEM observations after extremely high temperature annealing (2000 °C). From the application point of view, the low temperature closing of the graphene edges would be much more feasible for large-scale graphene-based electronic devices fabrication. Here, we demonstrate that the zigzag edges of AB-stacked bilayer graphene will form curved close structure naturally at low annealing temperature (< 500 °C) based on Raman observation and first principles analysis. Such findings may illuminate a simple and easy way to engineer graphene electronics.

Structural defects in graphene

Graphene is one of the most promising materials in nanotechnology. The electronic and mechanical properties of graphene samples with high perfection of the atomic lattice are outstanding, but structural defects, which may appear during growth or processing, deteriorate the performance of graphene-based devices. However, deviations from perfection can be useful in some applications, as they make it possible to tailor the local properties of graphene and to achieve new functionalities. In this article, the present knowledge about point and line defects in graphene are reviewed. Particular emphasis is put on the unique ability of graphene to reconstruct its lattice around intrinsic defects, leading to interesting effects and potential applications. Extrinsic defects such as foreign atoms which are of equally high importance for designing graphene-based devices with dedicated properties are also discussed.

Electronic Functionality in Graphene-Based Nanoarchitectures: Discovery and Design via First-Principles Modeling

Graphene has promised many novel applications in nanoscale electronics and sustainable energy due to its novel electronic properties. Computational exploration of electronic functionality and how it varies with architecture and doping presently runs ahead of experimental synthesis yet provides insights into types of structures that may prove profitable for targeted experimental synthesis and characterization. We present here a summary of our understanding on the important aspects of dimension, band gap, defect, and interfacial engineering of graphene based on state-of-the-art ab initio approaches. Some most recent experimental achievements relevant for future theoretical exploration are also covered.

Graphene edges: a review of their fabrication and characterization

The current status of graphene edge fabrication and characterization is reviewed in detail. We first compare different fabrication methods, including the chemical vapor deposition method, various ways of unzipping carbon nanotubes, and lithographic methods. We then summarize the different edge/ribbon structures that have been produced experimentally or predicted theoretically. We discuss different characterization tools, such as transmission electron microscopy and Raman spectroscopy, that are currently used for evaluating the edge quality as well as the atomic structures. Finally, a detailed discussion of defective and folded edges is also presented. Considering the short history of graphene edge research, the progress has been impressive, and many further advances in this field are anticipated.