80kV- High Resolution Transmission Electron Microscopy (HRTEM) characterization of micromechanically cleaved graphene shows in real- time atomically resolved dynamics of the carbon atoms in graphene constrictions. During electron irradiation, processes of hole formation generate graphene constrictions and eventually lead to stable single carbon atom chains. Carbon chains also occur between carbonaceous adsorbates and looping along graphene edges.

Imaging-side Aberration Corrected TEM

Since the discovery of graphene, HRTEM has been used to reveal the structural properties of the two-dimensional carbon allotrope [1] and moreover has demonstrated to be a reliable tool to study the dynamics of this two dimensional carbon structure [2, 3, 4]. Our experiment using an imaging-side aberration corrected TEM operating at 80kV shows a reliable method to fabricate single carbon chains from free hanging graphene membrane [5, 6].

Sample Preparation and Instrumentation

Graphene was fabricated by mechanical cleavage [7] of natural graphite crystals (Dragon Seal), using Nitto Denko tape, then placed on to 300 nm SiO₂/Si wafer. The graphene flake was transferred to a TEM grid (Quantifoil) with 1.2 µm holes diameter as described previously [8]. The TEM investigations were performed using an imaging-side aberration corrected FEI TITAN 80-300 at 80 kV with an electron current beam density of about 3 x 10⁷ e^- nm^-2 s^-1. The spherical aberration coefficient was set to 20 µm and images were recorded at Scherzer defocus. To reduce the energy spread of the source, the extraction voltage of FEG was reduced from 3.8 to 1.7 kV. This allowed pushing information limit of the instrument, otherwise limited by chromatic aberration, to point-to-point resolution defined by Cs-corrector. Thus, dark contrast can be directly interpreted in terms of the atomic structure.
Dynamics of graphene during electron beam interaction were recorded by taking sequences of images with one second exposure and four seconds intervals.

Observations

Under the electron beam, contamination on the graphene promoted the formation of holes.

The atoms at the edges of graphene were removed by the 80 keV electrons, while the hexagonal membrane remained stable. We centered our observations where two adjacent holes formed. In the course of carbon atom removal from the edges of the holes, the graphene membrane between them became narrower, maintaining the perfect hexagonal structure until it reached ~1 nm width, i.e. 6 carbon atoms wide bridge. At this width the graphene nanobridge experienced deviations from the hexagonal structure such as: short carbon chains, higher order polygon atom arrangements and pentagon-heptagon pairs. Such deviations in the nanobridge preceded the formation of single carbon atom chains. The carbon atom chains bind between the adjacent graphene membranes until the chains are detached and disappeared. This process provides the possibility of site-specific fabrication of 1D carbon species for nano-engineering and functionalization of thin graphite membranes.
The constriction of a graphene membrane, experienced deviations from the hexagonal structure. One of the most stable configurations observed was a pentaheptite-like arrangement where pairs of pentagons and heptagons formed a carbon mesh with the carbon atoms remaining planar with sp2 configuration. Such structure was predicted by Crespi et al. [9]. Figure 1 (a) shows an image of the pentaheptite-like bridge by averaging 10 CCD frames. Fig. 1 (b) shows a geometrical model of the atom arrangement in the bridge, while Fig. 1 (c) shows the atomistic model of the pure carbon-planar pentaheptite with the corresponding atoms found in our experiment marked in pink.
The single atom chains that formed from the nanobridge consist of carbon atoms, as we assert the formation by analyzing the atom by atom observation in our sequence. To confirm that such chains are linear arrangement of carbon atoms we model such structures and calculated the image by multislice calculation. The result coincides with the width and contrast of the experimental micrographs. Fig. 2 shows the experimental image, model and image calculation of the formation of chains in time-sequence. The bond configuration of the chains cannot be resolved in this experiment due to the low signal to noise ratio obtained. According to calculations, linear carbon chains would be polyyne (alternate single-triple C-C bonding) or cumulene type (double-double C-C bonding). Their structure depends on even or odd number of atoms in the chains [10]. Consequently, further experiments should be performed to gain insight on the structure of these chains. Regardless the type of structure and the oscillation of the chains from atom to atom on the closest available site in the graphene edges, they remained stable up to 2 minutes under the electron beam.
Apart from the latter we recorded two other processes of carbon chain formation. In one case two patches of carbonaceous contamination were thining down under the electron beam until the formation of chains. The chains bridged between carbonaceous adsorbates and were supported by the graphene membrane. Figure 3 (left) shows a carbon chain formed on top of the graphene layer. In Fig. 3 (right) hexagonal structure of supporting graphene was filtered out to demonstrate a linear chain which binds to the patch of carbonaceous contamination. Another type of chain formation process occured when carbon chains appear looping at the edges of graphene membrane when observing graphene edges for long time. Fig. 4 shows a long chain that loops at the edge of a graphene hole.