Time resolved in situ transmission electron microscopy shows that the reactivity of defects and unsaturated edges plays an integral role in the growth of highly defective graphene formed by the catalyst-free thermal formation of freestanding polymer films. In addition to the observed migration and merging of nanostructures at high temperatures, graphene nanoflakes are highly unstable and tend to loose atoms or groups of atoms to adjacent larger domains indicating an Ostwald-like ripening active in these 2D materials. Beam-off heating experiments were carried out to understand the effect of the electron beam on the observed processes and to separate out the inherent temperature-driven mechanisms. All of the processes observed during continuous imaging (beam on) were also observed during beam-off experiments. This confirms that the observed dynamics are inherently temperature-driven and that the electron beam is only providing additional activation energy, thereby increasing the reaction kinetics. Atomistic simulations were carried out to estimate the activation energy for the different processes and confirm that the observed dynamics are thermally accessible at the experimental temperature.
It is well known that defects greatly influence the properties and fundamental dynamics of graphene and other 2D materials . These deviations from the perfect structure can also be used to tailor the properties, thus enabling new functionalities. However, these defects not only influence the physical properties, but can also influence the structural dynamics and growth mechanisms in these materials . Recent advances in graphitization of polymer thin films led to the successful synthesis of a wide variety of graphenoid (graphene-like) materials with high defect density [3,4]. Pyrolysis of thin polymer films results in a material with domain sizes on the order of a few nanometers, which has been termed nanocrystalline graphene (ncg). This catalyst-free reaction results in a material with a lot of defects, active edges and consisting of a variety of carbon nanostructures. High-temperature studies of these materials with high defect density provide the opportunity to understand the role of defects/active edges on the fundamental growth mechanisms and the interaction of nanostructures with defects in the substrate.
This highly defective structure greatly influences the dynamics and reactions of the carbon nanostructures and the growth of domains. Apart from the reported migration and merging of carbon nanoflakes/nanostructures on top of graphitic substrates [5–8], in the presence of a large number of defects and active edges, Ostwald-like ripening is also observed as a mechanism contributing to the growth of the graphene as well as vertical merging between partially overlapping graphene sheets. These observed dynamics have been confirmed to be inherent temperature driven processes with the help of beam-off experiments and atomistic simulations.
Commercially available photoresist, microposit S1805, was spin coated onto Aduro heating chips to form freestanding thin polymer films across the holes. After an initial heating to 600°C inside a Gatan pumping station, the films were graphitized inside the TEM to form free standing thin nanocrystalline films. An aberration corrected (image) Titan 80-300 TEM (FEI Company) operated at 80 kV equipped with a US1000 slowscan CCD (Gatan Inc.) camera was used for imaging. The in situ heating experiments were carried out at a heating rate of 10°C per minute. Images were acquired with an interval of 5 s with a 1 s exposure time. The dose for each single high-resolution TEM image varied between 2.0 x 107 e/nm2 and 4.1 x 108 e/nm2 depending on the magnification. Further details are available elsewhere [9,10].
Results and Discussion
Graphitization and Domain Growth
Figure 1 shows the graphitization and domain growth in freestanding nanocrystalline graphene observed by in situ TEM techniques. The in situ studies showed that the graphitization process is highly dynamic with a number of intermediate reactions leading to the formation of different carbon nanostructures. The structure consists of curved and wrinkled crystallites at 600°C. With increasing temperature, the ordering of the domains and their further growth can be seen from the bright field images in figure 1a. In agreement, crystallite growth, calculated from the SAED patterns, can be seen in figure 1b. The sp2 content at each temperature was calculated from the carbon core loss edge by comparing it with a fully graphitic sample. The sp2 content increased from around 70% at 600°C to completely graphitic at 1000ºC. This shows that the graphitization proceeds via a two-step growth mechanism, where at intermediate temperatures (600-1000ºC) crystallite growth occurs by consuming amorphous carbon around the crystallites and at high temperatures (1000- 1200ºC) the growth proceeds by merging of crystallites. The amorphous carbon is transformed in two ways, either by attaching to the active edges of domains or by a catalyst-free transformation on the top of graphitic layers. This catalyst-free transformation forms both mobile and stationary (pinned) carbon nanostructures with varying size and shape . The dynamics and interaction of these structures with the graphitic substrate differ significantly. To understand the high-temperature dynamics of the graphene flakes in more detail, we use time resolved HRTEM, which provided new insights in to the fundamental processes controlling graphene growth.
Growth of Graphene Flakes by Migration and Merging
In situ HRTEM investigations revealed the formation of graphene nanoflakes and cage-like nanostructures during graphitization. Figure 2 shows the migration and merging of graphitic flakes at 1200ºC. The flake marked by a blue arrow is migrating into the field of view after 590 sec. (fig. 2b). At first, the flake is pinned, then it moves to a nearby spot and comes back to the original position in the next two frames. (fig. 2c,d). Again, the flake is pinned at this position before it moves and attaches to an edge around 10 nm away (fig. 2f). The nanostructure completely merges with this edge, thus extending it. This facilitates a lateral material transfer. Another flake indicated by a red arrow sitting at the top of flake edge rearranges and merges with the flake edge below facilitating a vertical material transfer. This vertical transfer is slow compared to the fast lateral merging. These are two examples for the prominent mechanisms responsible for high-temperature growth of ncg: migration and merging of nanoflakes. Furthermore, it is important to note that the flake edges are highly dynamic and are constantly rearranging. The merging of a flake leads to a big step in the edge of the graphene crystallite (fig. 1f,g), which is quickly redistributed, thus reducing the overall curvature of the edge (fig. 1h). This illustrates the highly dynamic and reactive nature of edges that is critical for the high-temperature processes in nanocrystalline graphene.
Growth by Ostwald-like Ripening
While looking carefully at these migrating flakes, one can observe a constant reduction in size during migration. This is illustrated by the dynamics of a nanoflake heated to 1200°C indicated by the red arrow in figure 3a-d. The nanoflake is shrinking in size before it migrates and merges with a large flake. This is due to the detachment of atoms from the nanoflake when pinned to the substrate. At the same time, one can also detect an active region in a larger flake, where the addition of atoms is resulting in a growth of the larger flake (indicated by blue arrow). This means that atoms or groups of atoms are detaching from the small flake and attach to the larger ones, resulting in an Ostwald-like ripening in these highly defective graphene systems that has not been observed in coarse-grained graphene.
If the pinning is sufficiently strong, the nanostructure can completely shrink and disappear by detachment of atoms as an alternative mechanism to the observed migration/merging of flakes. This can be observed in figure 3e-f, where a pinned nanostructure is completely disappearing during the heating. Simultaneously, a nearby edge shows an active region where the edge seems to open up receiving atoms. The high-resolution image in figure f shows a connecting edge between the pinned nanoflake and the growing edge. This serves as a possible pathway for an atom transfer from the small flake to the large one. Here, two possible transfer mechanisms can be imagined: during pinning, the bond formation between the flake edge and substrate defect can result in an edge atom transferring to the defect in the substrate, thus healing the defect or, if the defect in the substrate is a continuous, like-line defect or an active edge, an atom can be transferred along the flake edge acting as a pathway between the shrinking and growing flakes.
Beam off heating experiments have been carried out to understand the effect of the electron beam to separate beam induced transformations and inherent temperature-driven processes. All structural changes observed during continuous imaging were also observed during beam-off condition . This shows that the basic structural changes are inherent in high-temperature growth of nanocrystalline graphene, whereas the electron beam is only providing an additional activation energy, accelerating the process.
To have a better understanding of the processes, activation energies were calculated for the observed edge dynamics and atom removal processes. The calculation shows that edge diffusion (EA = 0.58 eV) and edge reconstructions (EA = 1.76 eV) are thermally accessible at the experimental temperatures. This confirms that the highly dynamic nature of flake edges are inherent temperature driven and not beam induced. However, the activation energy for single atom removal form a flake edge is very high (EA = 9.17 eV) and not thermally accessible at this experimental temperature. Nevertheless, the activation energy for an atom removal is reduced drastically (EA,depinning = 2.47 eV) when there is a reactive site nearby the flake edge, which makes this process thermally accessible. Possible reactive sites are vacancies in the substrate or unsaturated edges. The activation energy values show that in highly defective graphene systems, nanoflakes can lose atoms to reactive sites and if this reactive site is an active edge, the atoms can further migrate along the edge by diffusion or edge reconstructions to other growing flakes.
Combining time resolved HRTEM and atomistic simulations, it has been shown that highly defective graphene systems can show Ostwald-like ripening as a prominent growth mechanism, occurring simultaneously with migration and merging of nanoflakes. In the case of coarse-grained polycrystalline graphene, where flakes exhibit a high mobility, the flakes can migrate and merge, and this will be the dominating mechanism. However, with increasing defect density, the migrating nanoflakes are pinned to the substrate. As soon as the flakes are pinned, the migration is hindered and the instability of the nanoflakes results in a shrinking by the detachment of atoms. The activation energy calculations confirm that nanoflakes can lose atoms to reactive sites and if the reactive site is an active edge or an extended defect, the atom can further migrate making this a possible migration pathway between shrinking and growing flakes. Overall, this study shows that the high-temperature dynamics of highly defective graphene systems is fundamentally different from coarse-grained graphene and shows the critical role of defects and active edges in the observed dynamics.
CNSK gratefully acknowledges the PhD funding from the Deutscher Akademischer Austauschdienst (DAAD) and MK gratefully acknowledges support by the HPC 2 program of the Baden-Württemberg Stiftung (Project MSMEE) and by the GRK 2450.
C. N. Shyam Kumar1,2, Manuel Konrad1, Wolfgang Wenzel1, Ralph Krupke1,2, Christian Kübel1,2,3,4
1 Institute of Nanotechnology, Karlsruhe Institute of Technology, Karlsruhe, Germany
2 Department of Materials and Earth Sciences, Technical University Darmstadt, Darmstadt, Germany
3 Helmholtz Institute Ulm, Karlsruhe Institute of Technology, Karlsruhe, Germany
4 Karlsruhe Nano Micro Facility, Karlsruhe Institute of Technology, Karlsruhe, Germany
Dr. Christian Kübel
Deputy Head of the Karlsruhe Nano Micro Facility
Karlsruhe Institute of Technology (KIT)
Institute of Nanotechnology (INT)
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