Atomic resolution imaging of graphene by transmission electron microscopy

Stability and dynamics of the tetravacancy in graphene

The relative prevalence of various configurations of the tetravacancy defect in monolayer graphene has been examined using aberration corrected transmission electron microscopy (TEM). It was found that the two most common structures are extended linear defect structures, with the 3-fold symmetric Y-tetravacancy seldom imaged, in spite of this being a low energy state. Using density functional theory and tight-binding molecular dynamics calculations, we have determined that our TEM observations support a dynamic model of the tetravacancy under electron irradiation, with Stone-Wales bond rotations providing a mechanism for defect relaxation into lowest energy configurations. The most prevalent tetravacancy structures, while not necessarily having the lowest formation energy, are found to have a local energy minimum in the overall energy landscape for tetravacancies, explaining their relatively high occurrence.

Vacancy diffusion and coalescence in graphene directed by defect strain fields

The formation of extended defects in graphene from the coalescence of individual mobile vacancies can significantly alter its mechanical, electrical and chemical properties. We present the results of ab initio simulations which demonstrate that the strain created by multi-vacancy complexes in graphene determine their overall growth morphology when formed from the coalescence of individual mobile lattice vacancies. Using density functional theory, we map out the potential energy surface for the motion of mono-vacancies in the vicinity of multi-vacancy defects. The inhomogeneous bond strain created by the multi-vacancy complexes strongly biases the activation energy barriers for single vacancy motion over a wide area. Kinetic Monte Carlo simulations based on rates from ab initio derived activation energies are performed to investigate the dynamical evolution of single vacancies in these strain fields. The resultant coalescence processes reveal that the dominant morphology of multi-vacancy complexes will consist of vacancy lines running in the two primary crystallographic directions, and that more thermodynamically stable structures, such as holes, are kinetically inaccessible from mono-vacancy aggregation alone.

Inflating graphene with atomic scale blisters

Using 80 kV electron beam irradiation we have created graphene blister defects of additional carbon atoms incorporated into a graphene lattice. These structures are the antithesis of the vacancy defect with blister defects observed to contain up to six additional carbon atoms. We present aberration-corrected transmission electron microscopy data demonstrating the formation of a blister from an existing divacancy, together with further examples that undergo reconfiguration and annihilation under the electron beam. The relative stability of the observed variations of blister are discussed and considered in the context of previous calculations. It is shown that the blister defect is seldom found in isolation and is more commonly coupled with dislocations where it can act as an intermediate state, permitting dislocation core climb without the atom ejection from the graphene lattice required for nonconservative motion.

Sensitivity of graphene edge states to surface adatom interactions

Electron beam irradiation at 60 kV is used to open holes in graphene and expose fresh clean edges for further examination by electron energy loss spectroscopy (EELS) at the single atom level combined with scanning transmission electron microscopy (STEM). We show that light element surface adatoms attached on top of the edges of graphene influence the carbon K-edge EELS. A single nitrogen adatom on graphene was imaged by STEM and chemically identified by EELS. We also extend this study to small graphene nanoribbons, termed nanoconstrictions. The arrival of surface adatoms disrupt the detection of unique carbon edge states present in both single edges and in the nanoconstrictions. The spatial distribution of the EELS signals is also examined. These results show that edge states in graphene are highly sensitive to single atom functionalization and sheds light on their long-term stability.

Cytotoxicity assessment of MDA-MB-231 breast cancer cells on screen-printed graphene-carbon paste substrate

Graphene is a novel carbon-based material widely studied in bio-electrochemical fields because of its high electrical conductivity and excellent electrocatalytic activity. However, its biological applications have been limited due to the lack of understanding of its compatibility with numerous biological entities. In this paper, cytoxicities of MDA-MB-231 breast cancer cells (MDA cells) on carbon paste (CP) and graphene-carbon paste (GCP) substrates are assessed. GCP was prepared by mixing graphene powder into carbon paste with different graphene contents. Cytotoxic effect was evaluated from cell viability, cell adhesion, ROS production and fluorescence staining studies. Cell viability on GCP substrate was found to initially increase as graphene content increases from 0 to 2.5 wt% but then decrease as the content increases further. In addition, the viability decreases with time for all substrates. Similarly, graphene concentration affected the number of adherent cells in the same manner as the cell viability. Likewise, reactive oxygen species (ROS) induced by carbon substrate increased with time and decreased with small graphene inclusion, confirming that low graphene content led to lower cytotoxicity. Moreover, confluence of MDA cells on substrate evaluated using Hoechst 33342 fluorescence staining was also found to be enhanced at low graphene concentration. Therefore, low-content graphene incorporation can effectively improve biocompatibility of carbon-based materials with MDA-MB-231 breast cancer cells, enabling potential applications such as electrochemical electrode for cell study.