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Cite this: DOI: 10.1039/c4nr03680h

Fullerene growth from encapsulated graphene flakes† Wan Neng,‡a Lei Shuang-ying,‡a Xu Jun,b Martini Matteo,c Zhou Yi-long,a Wan Shu,a Sun Li-tao*a and Huang Qing-an*a The direct in situ observation of fullerene formation encapsulated within a graphene ridge has been made possible using an aberration corrected transmission electron microscope (AC-TEM). An atom-by-atom

Received 2nd July 2014 Accepted 22nd July 2014

mechanism was proposed based on in situ AC-TEM observations. First principle calculations found a continuous energy decrease upon the addition of carbon atoms to the edge of the graphene flakes, which mimics the fullerene growth steps and supports the atom-by-atom mechanism. The ridged

DOI: 10.1039/c4nr03680h

graphene structure worked as a container for pinning small graphene flakes and capturing carbon atoms,

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which increased the growth probability of the fullerene structure within the small encapsulated space.

Introduction The growth of high structure-stable C60 or C60-like fullerene structures has been the object of signicant interest in both material synthesis and mechanistic studies. The routes for the interpretation of the growth mechanism were once limited to mass spectrometry and theoretical calculations, which suffer from the lack of direct and dynamic structural information.1 Key progress was made when it was recently shown that the fullerene structure can be formed on the graphene surface under continuous low voltage (80 keV) e-beam irradiation, where the entire process was directly viewed in an in situ manner.1 Flat graphene akes were observed spontaneously bending with their edges gradually closed to form a sphereshaped fullerene structure. First principles calculations were in accordance with the in situ aberration corrected transmission electron microscope (AC-TEM) observations.1 Bond reconnection along the graphene edge as well as the local structure bending through defect generation included the key processes for the direct graphene–fullerene transformation. Distinct from the direct graphene–fullerene transformation mechanism, starting from the early days of fullerene science, various aspects of fullerene synthesis have been considered in

a

SEU-FEI Nano Pico center, Key Laboratory of MEMS of Ministry of Education, School of Electronics Science and Engineering, Southeast University, Nanjing, 210096, P. R. China. E-mail: [email protected]; [email protected]

order to interpret the growth mechanisms, such as the pentagon road,2,3 fullerene road,4,5 polycyclic folding,6 pyracylene transformation,7 elemental catalysis or even autocatalysis,8 and the ring collapse model.9 As a matter of fact, compared with the direct graphene folding route, several studies have indicated that fullerene structure may grow following an atom-by-atom mechanism initiated from small graphene akes during the conventional experimental fabrication of fullerenes.10–13 Such mechanisms may work in a catalystfree manner due to the intrinsic structural stability of the fullerene. The atom-by-atom mechanism was proven to be very meaningful in the interpretation of the fullerene structures. However, most of these mechanisms were based on theoretical calculations or indirect experimental assumption. Direct experimental evidence was rarely presented, which allowed the thorough understanding of the physical details of fullerene formation to remain under debate. In this study, we observed the transformation of small graphene akes into closed fullerene structures via an atom-byatom mechanism. The transformation process was realized inside a small graphene ridge region and the entire process was recorded by in situ AC-TEM imaging. First principle calculations were performed to verify the atom-by-atom mechanism. Our studies indicate that the graphene ridge region could work as a container for pinning small graphene akes and capturing carbon atoms, which increased the growth probability of the fullerene structure within the small encapsulated space.

b

National Laboratory of Solid State Microstructures, School of Electronic Science and Engineering and School of Physics, Jiangsu Provincial Key Laboratory of Advanced Photonic and Electronic Materials, Nanjing University, 210093 Nanjing, P. R. China

Experiments, results and discussion

c

Laboratoire de Physico-Chimie des Mat´eriaux Luminescents, Universit´e Claude Bernard Lyon 1, UMR 5620 CNRS–UCBL, 69622 Villeurbanne Cedex, France † Electronic supplementary 10.1039/c4nr03680h

information

(ESI)

available.

‡ Authors contributed equally to this work.

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See

DOI:

An aberration-corrected TEM (AC-TEM, Titan 80-300, FEI Company) operating at 80 keV was used for in situ HRTEM observations. Cs was tuned to be 12 micron and the other astigmatism was minimized. The e-beam intensity used during

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the in situ observations was typically on the order of 105 e cm2 s1. Chemical vapor deposition (CVD) multilayer graphene grown on a copper mesh was purchased from Graphene SuperMarket. Several-layer graphene (typically 5–20 layers) was found in some regions on the grid. Ridges were found on the surface with amorphous carbonate structures or small encapsulated graphene akes (Fig. 1), as demonstrated by the schematic gure in the lower right part of Fig. 1.14–16 The ridge structure sets up a channel in which tubular structures were formed during the e-beam irradiation. The direction of the experimental e-beam project (as indicated in the right lower part of Fig. 1) enables the visualization of graphene akes inside the graphene edge. Small curved graphene akes, with the size of 0.7  0.7 nm2 (as indicated by the green arrows) were found inside of a graphene ridge structure with the width of 1.5 nm as indicated by the green arrows shown in Fig. 2(a) [see also the rst atomic model in (i)]. The identication of the fullerene ake structures, distinguished from a confounding kink structure that is formed on the encapsulating CNT, can be conrmed by an analysis of the TEM images. Typically two types of geometry are observed in kink structures: (1) rst, the kinked side is parallel to the e-beam project direction. In this geometry, kinks formed on CNT will show a dented structure towards the CNT center. Such morphology was indeed observed as denoted by the arrows in Fig. S1.† However, they are evidently different from those indicated as graphene akes in Fig. 2(a). Second, the kinked side is perpendicular to the e-beam project direction. In this case, without image simulation, it is not hard to interpret that a kink structure will result in two contrasting features (similar to that of a graphene ridge), whereas a single graphene structure results in only one considering the project geometry.17–20 The TEM images show the features of a single layered graphene structure, which is in accordance with our previous

The TEM morphology of the graphene-encapsulated structure. The red zone focuses on the analysis area. Right-lower part shows the schematic structure of the sample. Dashed cyan lines indicate the position of the graphene layer without the ridge structure. The shaded blue region indicates a channel for graphene flake encapsulation and fullerene growth. The e-beam is projected to be nearly perpendicular to the surface of the sample during in situ observations.

Fig. 1

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assumptions and excludes the possibility of a kink structure forming in the encapsulating CNT. In the sections highlighted by the dashed yellow rectangles in Fig. 2(a), two well-resolved graphene akes (as indicated by the green arrows) were found encapsulated inside a double layer structure, for which the outer layer is a single layer graphene ridge and the inner layer is determined to be a carbon nanotube structure. The determination of a CNT structure was realized by image simulation presented in the ESI (see Fig. S1†). A pair of short cyan lines indicates the two sides of the graphene ridge. As a reference, a pre-existing fullerene structure, pinned onto the side wall, was also observed inside the graphene ridge as indicated by the dashed red circle. This fullerene structure, as a matter of fact, was also formed during the AC-TEM observations. A series of images in Fig. 2(b)–(h) shows the evolution of structure under continuous e-beam irradiation. In Fig. 2(b) (aer several minutes e-beam irradiation), one of the graphene akes was eliminated, leaving a single curved graphene ake [see also the second atomic model in (i) and the simulated TEM images in Fig. 2(j) “I”] attached on the inner wall of the carbon nanotube structure. The elimination of the graphene akes [as can be seen in Fig. 2(b)] was conrmed by long term in situ observations [several minutes time interval in Fig. 2(a)–(c)]. The interwall thickness between the curved graphene akes and the carbon nanotube wall was measured to be 0.33 nm [see Fig. S3(a) and (b)†], which is consistent with the layer spacing in graphite. The long-term e-beam irradiation did not change this van der Waals-type attachment geometry. The projected length (0.75 nm) of the curved fullerene ake was maintained nearly unchanged during this section [Fig. 2(a) to (c)]. Persistent e-beam irradiation caused structural improvements of the curved graphene akes. In Fig. 2(d), the projected length of the curved fullerene ake increased to 0.94 nm, indicating possible add-atom mechanism. Structural improvements can be observed much more clearly from the region of contrast enhancement as indicated by the red arrow in Fig. 2(e), in which a fullerene structure is observed with some defect sites [indicated by the yellow arrows in Fig. 2(e) and (f), also see Fig. 2(j) “II”]. Such structural characteristics were well preserved and further improved upon as shown in Fig. 2(f) and (g) [as the third atomic model in (i), also see Fig. 2(j) “III”]. A contrast analysis of Fig. 2(g) [see Fig. S3(c) and (d)†] indicates that a 0.65 nm diameter fullerene structure formed inside the space in the graphene ridge. The 0.33 nm inter-layer spacings between the fullerene and the inner wall are similar to the typical multilayered fullerene structures.9,21 Fig. 2(h) shows the nal complete structure of the fullerene [as the fourth atomic model in (i)]. The stable fullerene structure was veried using our longterm AC-TEM observation (see Fig. S2†), which show a similar contrast distribution with the pre-existing fullerene structure on its right side [dashed red circle in Fig. 2(h)], but evidently a different contrast distribution with the graphene akes on its le side [as those indicated by the green arrows in Fig. 2(a)]. Contrast analysis of sequential TEM images [Fig. 2(k), see Fig. S3(c)† for the position of contrast prole] was compared with the contrast prole of the simulated TEM images shown in

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(a–h) Sequence of HRTEM images depicting the growth of a fullerene inside a graphene ridge, as highlighted by the dashed yellow rectangle. Bar ¼ 2 nm. The cyan lines in (a) delimit the two sides of the graphene ridge. (i) Atomic model for demonstrating the detail of the growth process of the fullerene structure. Note that the fullerene structure was drawn as a C60 structure, while the actual structure may not be exactly 60 carbon atoms. Also note that the fullerene flakes were drawn for demonstration, which may not be their actual shape and size. (j) HRTEM simulation for the growth of fullerene structure from graphene flakes. The upper part shows the possible atomic structure models, while the lower part shows the simulated HRTEM images at different levels of defocus, as indicated on the right side. The images at a defocus ¼ 8 nm were used for comparison with the experimental images. (k) TEM image contrast profiles [labels correspond to TEM images (a) to (h)] show the processes of fullerene formation. (l) The contrast profiles of simulated images. Dashed black lines indicate the position of the fullerene wall. Fig. 2

Fig. 2(l).10,22 For open structures observed in Fig. 2(a) and (b), a similar featureless contrast [highlighted in the shadow of Fig. 2(k) and (l)] was observed both in the experimental [“a” and “b” in Fig. 2(k)] and simulated TEM images [“I” and “II” in Fig. 2(l)]. With gradual structure perfection, evident contrast arose from the fullerene wall as indicated by the dashed black lines (“c” to “h”). A contrast dip was also shown at the center of the fullerene as indicated by the blue arrows. In particular, this dark contrast was clearly observed in each simulated TEM image corresponding to the fullerene structure [Fig. 2(j), “III” and “IV”], which was in line with the experimental contrast prole shown in Fig. S3(d),† as indicated by the blue arrows. Such a contrast cannot be observed in at graphene structures, supporting the observation of the fullerene structures. Moreover, detailed TEM image simulations including the zone-axis tilted defective fullerene (Fig. S4†) and the fullerene on layered graphene (Fig. S5†) were performed in order to check the formation of the fullerene structure. Reasonable accordance between the simulated and the experimental TEM images was observed, which supports the gradual growth of the fullerene structures as demonstrated in Fig. 2(i). It is also worth noting that the observed fullerene structure is different from the previously reported hump structure (or a fused fullerene).23 First, the hump structure shows no carbon

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wall distance of 0.33 nm (the graphite VdW layer distance), which is different from the situation of the encapsulated fullerene. The latter case should show a carbon wall distance of no less than 0.33 nm, which is in accordance with our results. Second, from the graphene ake precursor, there is a possibility that they may fuse onto the carbon wall and form the hump structure. If this is true, graphene akes will prefer to attach to the carbon wall with their open edges. However, this was not observed. Third, according to our long-term in situ observations, the fullerene structure was observed occasionally pinning onto the encapsulating wall with possible defect sites and covalent carbon bond(s); however, a well separated fullerene structure was ultimately observed. In contrast, a hump structure will pin in a stable manner with a denite site (for example, with its side wall). Also, fullerenes fused onto CNT appeared to be unstable and it is still not clear under which condition a hump structure will form. In the previously reported hump structure,23 well-separated fullerenes were nally observed in each case, which indicates a more stable fullerene structure. According to our in situ data, isolated fullerenes were found aer even 2 h of e-beam irradiation (see Fig. S2†). As no clear evidence indicated an intermediated hump structure, we suggest a direct graphene– fullerene transition process.

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Moreover, our observation show distinctly different characteristics in comparison with the direct graphene–fullerene selfassembling processes previously observed.1 The entire process of fullerene formation observed by AC-TEM assays proves that once some of the graphene akes were eliminated [Fig. 2(a)–(c)], the residual ones grow inside the graphene ridge region until (i) the enlargement of the graphene akes [Fig. 2(c)–(e)] and (ii) the nal structure perfection [Fig. 2(e)–(h)]. Once a fullerene structure was formed, it is very stable under continuous e-beam irradiation, indicating that the close-structured fullerene can be more stable than graphene akes. A model based on the atom-by-atom growth of fullerene has been proposed to support the experimental TEM observations. First principle energy calculations were carried out by using density functional theory (DFT), which is implemented in the Vienna ab initio simulation package (VASP).24,25 The generalized gradient approximation (GGA) is applied for the exchange correlation by using the PW91 function.26,27 The ultraso pseudopotential is used28,29 and the plan wave cutoff energy is set to be 400 eV. The convergence criterion for energy and force ˚ 1, respectively. is set to be 104 eV and 0.02 eV A The atomic model [Fig. 3(a)] was simplied with a graphene ake (with the size of 1.5 nm  0.7 nm) placed inside a carbon nanotube (CNT, diameter D ¼ 1.2 nm) test tube, which was used to mimic the encapsulation effect of the graphene ridge. The open end of CNT was saturated with hydrogen in order to eliminate the possible effect of dangling bonds. The graphene ake was decorated with defect structures (5-carbon circle)

Fig. 3 (a) Graphene flake encapsulated inside a CNT test tube was used to calculate the formation of fullerene structure within graphene ridge. Upper part: side view. Lower part: perspective view. (b) Atomby-atom process for the growth of fullerene structure from a graphene flake. Note the outside CNT was removed for better view. (c) The total energy during the atom-by-atom growth process.

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similar to a fragile C60. Carbon atoms were added one-by-one to the edges of the graphene ake [Fig. 3(b)]. Aer the addition of each carbon atom, the structure was optimized and the total energies were calculated [Fig. 3(c)]. The optimized structure showed a distance of 0.36 nm between the CNT wall and the graphene ake, which is roughly in accordance with the van der Waals interactions observed in Fig. 2. We found a gradual decrease in energy upon the addition of carbon atoms as shown in Fig. 3(c). An energy decrease of 8 eV upon the addition of each carbon atom was observed. This nding directly supports the atom-by-atom mechanism of the graphene ake-fullerene growth. More importantly, the detailed atomic structure during the addition of carbon atoms [Fig. 3(b)] also suggests the tendency to form a fullerene structure because imperfect benzene rings [6C ring], such as the 5C or 7C rings, are necessary for constructing the fullerene structure.30–32 Upon the addition of the rst carbon atom (the “+1C” geometry), a 4C ring was formed around the new incoming carbon atom, which induced an increase in the bending of the initial structure. The “+2C” geometry transformed the 4C ring into a more stable 5C ring, with the curved structure being well reserved. The “+3C” geometry generated three connecting 5C (5C–5C–5C) rings and it was then transformed to a more stable 5C–6C–5C connecting structure in the “+4C” geometry. The atomic geometries from the initial to the “+4C” structure suggested the high possibly of continuous growth towards the nal fullerene structure. We also tried to add a carbon atom to a slightly different position as in the “+5C” situation. The added carbon atom had bonded with an edge carbon atom in a 5C ring. Adding the 6th carbon atom to the side, however, did not introduce a bond between the two atoms. Instead, it transformed a 6C ring into a 7C ring (see the “+6C” geometry). A more stable structure was formed aer the 7th carbon atom was added, as shown in the “+7C” geometry. The 7th carbon atom attached to a 5C ring with the 5th carbon atom adjacent to the 7C ring. Such a defective structure induced the bending of the graphene structure and promoted the formation of the fullerene structure. The atom-by-atom mechanism effectively works by enlarging the graphene ake and bending the graphene akes. Indeed, atoms that were added at the edge of the graphene ake caused an evident energy decrease, which is the driving force for the continuous growth. Due to the van de Waals interaction nature between the graphene ake and the graphene wall, as shown in Fig. 3, a relative small effect could be applied by the graphene wall. However, the side-walls of the ridge may guide the addatom mechanism. Furthermore, the graphene ridge supplied a conned region, which increased the atom resting time and increased the probability of growth. The effect of the 80 keV e-beam irradiation is also discussed to clarify the in situ TEM conditions where the fullerene growth was observed. According to Bethe's theory, the energy transfer by the energetic electron follows: T(q) ¼ 2E(E + 2mec2)cos2(q)/ (Mc2)33 where q is the scattering angle, me is the electron mass, M is the mass of the nucleus, c is the speed of the light, and E is the energy of the incident primary electron. The energy transfer from the primary electron beam is distributed in the range of 0– T(q)max depending on the scattering angle q. If carbon atoms This journal is © The Royal Society of Chemistry 2014

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received energies larger than a critical threshold energy T(q) > Tc, the atoms will be knocked-off. In the case of bulk carbon atoms in the graphene structure, the threshold primary e-beam energy is 86 keV;34 for the edge carbon atoms, the threshold e-beam energy can be much smaller (e.g. 2/3 of the bulk threshold35). This means the edge atom sputtering occurs much easily under the 80 keV e-beam irradiation.36,37 The elimination of graphene akes [Fig. 2(a)–(c)] can be attributed to the e-beam edge sputtering effect. This mechanism also works for supplying carbon atoms for fullerene growth, that is, the sputtered carbon atoms can be recaptured by the graphene edges and contribute to fullerene growth. From another viewpoint, for cases of sub-threshold energy transfer (with T(q) < Tc), energies transferred by primary electrons can contribute to the local structure reconstruction during fullerene formation,38,39 for example, to break an unstable weak bond, to overcome the energy barrier for bond formation, and to activate atom hopping for defect repairing.40 Comparing with high energy e-beam conditions (conventional 200 keV or 300 keV) where the etching of carbon atoms becomes a dominant effect, the 80 keV e-beam is adequate for both the generation of free carbon atoms and for local structure reconstruction. This can be a crucial factor for fullerene growth under the current in situ AC-TEM conditions. It should be also stressed that the applied observational conditions are different from those in the chemical vapor deposition or the electric arc chamber. The in situ TEM conditions includes no high temperature and additional carbon source as both of these can be replaced by e-beam excitation of the target material, as discussed above. Processes, such as precursor decomposition and catalyst activation were not incorporated in this process. This makes the in situ TEM observation a relatively pure process. Although such conditions are different from the typical realistic fullerene growth conditions, they provide meaningful physical insights with direct experimental evidence about fullerene structure formation.

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Acknowledgements This work is supported by the National Basic Research Program of China (Grant no. 2013CB632101, 2011CB707601 and 2009CB623702), the National Natural Science Foundation of China (Grant no. 61370042, 51071044, 61001044, 60976003 and 61006011), Specialized Research Fund for the Doctoral Program of Higher Education (Grant no. 20100092120021 and 20100092110014). One of the authors (W. N.) would like to thank F. LIN for help with the TEM image simulations.

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Summary In summary, we directly observed the formation of fullerene structures from graphene akes via an atom-by-atom mechanism under a graphene ridge encapsulated structure by in situ AC-TEM. Our observations conrmed the gradual growth of graphene akes until structure perfection. The rst principle calculations veried the atom-by-atom growth mechanism through an add-atom mechanism from the graphene edge. A continuous energy decrease upon the addition of each atom was calculated. The formation of 5C and 7C rings induces the bending of the graphene plane and promotes the formation of the fullerene structure. Graphene ridges provide a conned space that increases the probability of growth and may guide the formation of the fullerene structure. The 80 keV e-beam supplies carbon atoms through graphene edge atom sputtering and local structure reconstruction, which activates the fullerene growth.

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Fullerene growth from encapsulated graphene flakes.

The direct in situ observation of fullerene formation encapsulated within a graphene ridge has been made possible using an aberration corrected transm...
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