High resolution environmental TEM investigation of the catalytic channeling of few‐layer graphene

Abstract

As the 2D graphene flakes are zero‐gap semiconductors, one strategy proposed for the fabrication of graphene structures with a finite bad gap was its patterning in quasi‐one dimensional (1D) shapes called graphene nanoribbons (GNRs) 1 . Several methods have been developed to fabricate GNRs, such as direct chemical routes 2 , unzipping carbon nanotubes 3 or graphene/FLG flakes nanopatterning 4 . The catalytic nanopatterning of FLG using metallic nanoparticles (MNPs) as “nanoscissors” is one of the most promising methods. Under well‐controlled conditions, a number of MNPs supported on graphene‐based structures act as mobile nanoreactors able to pattern the graphene support with a nanometer precision. At the origin of the nanoparticles' “graphene cutting” lies their capacity to dissolve carbon and catalyze, at high temperatures, gas‐carbon reactions. Transmission electron microscopy (TEM) plays an important role in the investigation of the metal‐catalyzed gas‐graphite reactions. However, ex‐situ TEM approach offers a limited perspective over the dynamic behavior of the catalyzed reaction. Environmental TEM (ETEM) is a more appropriate technique for investigating dynamical processes, offering in real‐time imaging and chemical analyses at atomic resolution. In this study we present a complete ETEM investigation of the catalytic nanopatterning of FLG by metallic iron nanoparticles (FeNPs) performed in a dedicated environmental Atmosphere (Protochips) cell. At first the influence of a number of parameters, such as the structure of the initial magnetite nanoparticles on the channeling activity is carefully investigated. This analysis shows that incompletely reduced FeNPs cannot sustain a well‐defined channeling activity that is specific only for pure metallic FeNPs for which a specific crystallographic orientation formed between the metallic nanoparticle and the graphene edge is controlling their motion. In the second part, the ETEM reaction setup is controlled at such an extent that it allows the real‐time imaging of nanoparticles at high resolution. We show that the nanoparticles' frontal facets, i.e. the facets sustaining the carbon dissolution from the graphene edges, presents a continuous nanometer‐sized “waving” during the channeling activity as an effect of the constant carbon dissolution and its diffusion from the contact area to the rear facets of the nanoparticle. The real‐time imaging of the rear facet of the nanoparticle shows an even more interesting phenomenon, e. g. the formation of graphitized nanosized carbon structure with a very short lifetime as well as of amorphous carbon tails. Their location together with their evolution during channeling reveals important aspects concerning catalytic channeling mechanism. The last part deals with a number of aspects related with the nanoparticles motion on the graphene subtracted such as the changes in the channeling direction or the channeling rates. As shown in Fig. 1 , the changes in the cutting directions are a very complex phenomenon where the restructuration of the nanoparticle and especially the rearmament of the frontal faceting geometry play the most important role. The causes of the cutting directions changes can be separated in two categories, one related with a gradient of the carbon dissolved in the nanoparticle and one caused by “external” uncontrolled factors as the position fluctuations of the carbon tail.