Atomically resolved in situ TEM studies of the heat-induced evolution of hydrocarbons on graphene.

October 24, 2011 – After the dramatic improvements in the correction of lens aberrations [1-5] scientists now report that they observed the transformation of amorphous carbon to the crystalline phase at atomic resolution on top of a graphene substrate. They obtained stable atomic-resolution images of the light-element material carbon, by reducing low acceleration voltages (see, e.g., refs. 6-9). The understanding of carbon systems, such as graphene or carbon nanotubes, has already significantly benefitted from the recent instrumental developments in TEM. When the instrument is operated in HRTEM mode, in situ experimentation allow the study of dynamic phenomena on the single-atom level also with high temporal resolution [10]. The technical challenge that the scientists needed to solve was that owing to practical limitations, such investigations have been limited so far to temperatures below about 1500 K [11]. Now graphene, which is already widely used as sample support in low-voltage TEM, has made it possible to achieve far higher temperatures at which the crystallization of carbon could be observed.

Using graphene to obtain unprecedented temperature in the TEM

One reason for the temperature limitation of the experiments is that conventional materials could not provide enough mechanical, thermal and chemical stability to reach temperatures above 1500 K. Now using graphene, the scientists could for the first time follow the heat-induced transformations of carbon adsorbates at temperatures up to 2000 K in detail. The quasi-transparent substrate for transmission electron microscopy of sensitive materials [12-15] can withstand unprecedented temperatures [16] owing to its high mechanical [17-18] and thermal [16] stability as well as chemical inertness. It can serve as a very effective in-situ heater [19-21] when an electrical current is passed through it. To reveal the local temperature, gold nano-islands were deposited on the graphene by thermal evaporation (Fig. 1). Above a certain temperature, the first particles form liquid drops and begin to evaporate (Figure 1b,c) [22-23].

According to theoretical predictions and experimental findings, the melting temperature depends strongly on the particle size, with gold particles ranging in diameter from 3 to 20 nm to melt between 800 and 1300 K [24]. With the finite-element method (FEM) simulations the scientists estimated the entire temperature profile in the region of free-standing graphene (Figure 1d). The local temperature can also be determined by observing the transition from amorphous to crystalline silicon nitride of the SiN of the sample carrier (1600 K), as well as the evaporation of SiN (2000 K) [22].

After the evaporation of the gold particles, a carbon shell with the shape of the original gold particle becomes visible. Such shells typically consist of one to five graphitic layers (Figure 1c) [25]. Although the shells appear at first glance to be closed graphitic layers, the TEM images reveal also a large concentration of amorphous structure (Figure 1c). Furthermore, the irradiation during the observation with the e-beam is accompanied by an electron-beam-induced immobilization and enrichment of hydrocarbons [26]. Under electron irradiation at 80 kV, mobile hydrocarbon deposits are converted to amorphous carbon, increasing the amount of amorphous carbon already present from the gold nanoparticle shells.

“The setup corresponds to an in-situ TEM observation of amorphous adsorbates on a graphene substrate where the temperature of the substrate can be exactly tuned” explains Ute Kaiser, director of the SALVE project. “If the temperature is high enough, it should allow to study the transition from the amorphous to the crystalline phase.”

Reorganization of atoms under extreme heat

This is a major extend of the in-situ capabilities of the transmission electron microscope. The resulting amorphous carbon adsorbates are comparably stable under further electron-beam irradiation. The atomic structure of the adsorbates can be resolved (Figure 2b), which appears to be amorphous, consisting of a seemingly random arrangement of carbon pentagons, hexagons, heptagons, and other (less-frequently observed) carbon polygons.

When a temperature of ~1000 K is applied, the adsorbates indeed reorganize into structures characterized by large areas consisting of single-layer amorphous carbon containing some crystallized domains (Figure 3). Gold atoms originating from the surrounding nanoislands contain are incorporated into the carbon matrix, as marked by the arrows in Figure 3b. Crystalline and amorphous domains are observed, like those obtained by Turchanin et al. for annealed carbonaceous samples [27].

When the temperature is increased to the extreme of 2000 K, the Fourier-filtered HRTEM micrograph shows fully developed graphene grains (Fig. 4). In contrast to previous room-temperature HRTEM observations of beam-induced holes in graphene [6, 15], most of the annealed edges exhibit an armchair-type configuration (Figure 4c). In a statistical analysis of the edge configurations (see Supporting Information), 58% of all visible edges could be assigned clearly to one of the geometries calculated in the literature [28]. Among the classified edges, a dominant fraction—83%—exhibits the armchair conformation, 14% manifest the 5–7 reconstructed zigzag edge structure, and 3% are found in the unreconstructed zigzag geometry.

Stability of the graphene edges

At the end of the transformation, the observed graphene structures are those that would be expected from the theory (Table 1 in ref [28]). The armchair edge is the lowest energy configuration when considering the energy per atom. Furthermore, the armchair and 5–7 reconstructed zigzag is the two lowest-energy edge configuration when considering edge energy per length. It is surprising that AC-HRTEM characterizations of free-standing graphene samples at room temperature [6, 15] observe a slight preponderance of the unreconstructed zigzag configuration, which have an about 0.33 eV/Å higher energy than that of the armchair edge [28]. It has been speculated previously that this finding arises from the unreconstructed zigzag edges being less sensitive to radiation damage [6]. In the case of high-temperature annealing, however, the thermodynamically preferred armchair configuration appears to be most stable, as also recently observed by Song et al. [29].

“After overcoming the temperature limitation by using graphene as an in-situ heater in the TEM,” says Kaiser. “We could finally observe the crystallization of carbon and confirm the theoretical results that were based on the minimization of the energy per atm of the edge structure. The study contributes to our understanding of crystallization reactions in general, which can be exceptionally well observed for two-dimensional structures in the TEM.”

Resource: Westenfelder, B., Meyer, J. C., Biskupek, J., Kurasch, S., Scholz, F., Krill III, C. E., & Kaiser, U. A. (2011). Transformations of carbon adsorbates on graphene substrates under extreme heat. Nano letters, 11, 5123-5127, doi: 10.1021/nl203224z, [PDF], see also the supporting information.

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