Letter pubs.acs.org/NanoLett
Inflating Graphene with Atomic Scale Blisters Alex W. Robertson,* Kuang He, Angus I. Kirkland, and Jamie H. Warner* Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, United Kingdom S Supporting Information *
ABSTRACT: Using 80 kV electron beam irradiation we have created graphene blister defects of additional carbon atoms incorporated into a graphene lattice. These structures are the antithesis of the vacancy defect with blister defects observed to contain up to six additional carbon atoms. We present aberration-corrected transmission electron microscopy data demonstrating the formation of a blister from an existing divacancy, together with further examples that undergo reconfiguration and annihilation under the electron beam. The relative stability of the observed variations of blister are discussed and considered in the context of previous calculations. It is shown that the blister defect is seldom found in isolation and is more commonly coupled with dislocations where it can act as an intermediate state, permitting dislocation core climb without the atom ejection from the graphene lattice required for nonconservative motion. KEYWORDS: Graphene, ACTEM, HRTEM, electron microscopy, defects, TEM, dislocations
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poly(methyl methacrylate) (PMMA) scaffold. TEM data was recorded using the Oxford-JEOL 2200MCO32 fitted with CEOS spherical aberration correctors.33 Several image processing techniques have been used, denoted with an appropriate descriptor in the figure caption. Smoothed images were processed using a 3 × 3 pixel nearest neighbor smoothing operator, implemented with ImageJ. The maximize filter uses an operator in the ImageJ software package that we apply to smoothed TEM images, which sets a pixel value to the maximum of the nearest neighbors (distance of 4−6 pixels).25 The effect of these filters is shown in Supporting Information Figure S1. We also use a Fourier frequency filtering technique, shown in Supporting Information Figure S2, where an image is reconstructed from only the low frequency data. Figure 1 illustrates several different blister defects and the additional carbon atoms required to generate them. We use the notation Cx to denote a blister defect containing x additional carbon atoms, that is, C2, C4, and C6 for the defects shown in Figure 1c,e,g, respectively. The structure in Figure 1b has been previously described as an inverse Stone-Wales (ISW) defect,10,34,35 due to its 7-55-7 structure being the inverse of the 5-77-5 configuration of a Stone-Wales (SW) rotation (inset in Figure 1b). However, it should be emphasized that this structure arises from the inclusion of two additional carbon atoms and is not a simple bond rotation as with the SW defect. For this work, we shall only refer to the 3-fold symmetry two adatom defect (Figure 1c) as a C2 defect and not the ISW defect. The atomic models in Figure 1 are not intended to
anoengineering of arbitrary atomic structures would enable precise tailoring of material properties for specific applications. The simplicity and exceptional properties of the two-dimensional carbon allotrope graphene present an excellent candidate material for developing such techniques,1 where defect structures such as edges,2 extended defects,3 grain boundaries,4 and dislocations5 are all potential building blocks. To date, nanoengineering of defect structures in carbon nanotubes has been previously achieved using scanning tunneling microscopy (STM),6 scanning transmission electron microscopy (STEM),7 and more recently in graphene using transmission electron microscopy (TEM).8,9 A particular defect that has received theoretical consideration but minimal experimental verification is the blister.10−14 These occur through the addition of carbon adatoms to the graphene lattice with a predicted strain-induced out-of-plane distortion.15 It is anticipated that with experimental analysis this defect category will provide an extra building block for the nanoengineering of graphene. Recent advances in TEM, in particular the development of spherical and chromatic aberration correctors (ACTEM),16−18 and monochromated electron sources19 has enabled the imaging of graphene defects with atomic resolution at low voltage.20,21 However, to date the defects analyzed have originated from either the loss,22−25 or rotation26,27 of carbon atoms or from the incorporation of foreign dopants.28,29 In this work, we present 80 kV ACTEM images of carbon adatom blisters. Defects were generated in pristine graphene by focused electron beam exposure of a nanoscale area,8 which enables study without needing an ex situ defect creation28 step or switching to a higher potential.30 Graphene was synthesized by chemical vapor deposition of methane on to a molten copper catalyst31 before transfer to a holey SiN TEM grid via a © XXXX American Chemical Society
Received: November 18, 2013 Revised: January 8, 2014
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Figure 1. (a) Addition of two carbon atoms to a pristine graphene lattice, which subsequently rebonds into the ISW structure shown in (b). The ISW structure is named as it is an inversion of the 5-77-5 ring arrangement in the SW defect (inset in (b)) to a 7-55-7 arrangement. (c) Rotation of the bond indicated by arrows in (b) leads to a C2 defect. (d) Four carbon adatoms reform into a C4 defect shown in (e). (f) Six additional carbon atoms generate a C6 defect shown in (g). Models indicate the additional atoms in the blister defects and do not necessarily indicate the mechanism for their creation. Shading indicates the number of atoms in a ring with blue for 7 and yellow for 5. Carbon adatoms are highlighted in yellow.
number of additional lattice planes are created and terminated. A similar symmetrical configuration exists for the other two armchair directions (not shown). Figure 2f shows a C2 defect with the corresponding model shown in Figure 2g. This defect surrounds a central hexagon, rotated by 30° with respect to the bulk lattice and, as for the C6 structure in Figure 2a, can also be considered as a self-contained triad of dislocation cores (Figure 2h). This particular blister defect has been previously studied in the theoretical literature and is reported to have an out-of-plane perturbation.10,11,13 We have therefore investigated the effect that the out-of-plane distortion has on the graphene lattice using geometric phase analysis (GPA)37,38 of the images of the three blister defects, shown in Supporting Information Figure S3. The strain component maps do not show significant strain beyond ca. 0.5 nm from the defect and thus any distortions in the surrounding graphene must be less than the noise levels in the strain maps at ca. 10%. Because this analysis does not indicate any large displacement of the surrounding lattice we conclude that the strain arising from the incorporation of additional atoms is relieved by out-of-plane buckling consistent with the previously reported modeling studies.10,11,13 From our large set of defective graphene images, we were only able to find three separate occurrences of isolated blister systems and thus conclude that either the blister defects do not
indicate the mechanism by which the blisters are formed but to show the additional atoms required. Results and Discussion. ACTEM images of three adatom defect structures are shown in Figure 2. Figure 2a shows a maximized filter ACTEM image of the C6 carbon adatom defect, where a single rotationally misaligned carbon hexagon is separated from the bulk crystal lattice by four 5- and four 7membered rings. An atomistic model of this structure is shown in Figure 2b. The central 7-55-6-55-7 structure resembles two merged 7-55-7 ISW defects, which can be more clearly seen by comparing Figure 1f,g to Figure 1a,b. Figure 2c shows an image of a C4 defect with the corresponding atomic model shown in Figure 2d. Edge dislocations in graphene are generally considered as terminating at each end with a 5-7 dislocation core with each dislocation defect consisting of a pair of these cores.36 However, the defect shown in Figure 2c demonstrates the existence of a matched triad of dislocation cores, each oriented in the three armchair directions such that they mutually eliminate extra lattice planes, limiting the number of additional atoms in the crystal to four. To illustrate this Figure 2e shows an atomic model where each dislocation core has been moved to an adjacent lattice plane, increasing the length of each dislocation. The annotation in Figure 2e indicates the armchair axis for one rotation and it is evident that an equal B
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Figure 2. (a) A maximize filtered ACTEM image of a C6 adatom defect and (b) atomic model. (c) ACTEM image of a C4 adatom defect and (d) atomic model. (e) Annotated atomic model emphasizing the dislocation triad generated by displacing each dislocation core. The annotations are shown along one set of the three armchair lattice planes. (f) ACTEM image of a C2 adatom structure and (g) atomic model of the C2 defect. (h) Annotated atomic model demonstrating the dislocation triad generated by expanding the 5-7 dislocation core of the C2 defect. Scale bar for images is 0.5 nm. Color scheme for the atomic models denotes the number of carbon atoms in the ring with 5 as yellow, 6 as red, and 7 as blue.
Figure 3. (a) Smoothed ACTEM of an amorphous defect structure consisting of an additional six carbon adatoms imaged immediately after high current density defect creation. (b) Image taken 25 s later showing an evolved structure following two Stone-Wales rotations. (c) A further SW rotation transforms the structure to a new metastable C6 adatom defect state imaged after a further 22 s. (d−f) Annotated Fourier frequency filtered images of (a−c), respectively. (g−i) Atomic models of (a−c) with blue arrows indicating bonds that undergo a SW rotation in the next frame and blue circles indicating the atoms that have been switched. Scale bar is 0.5 nm. Color scheme for the atomic models denotes the number of carbon atoms in the ring with blue as 7, red as 6, and yellow as 5 carbon atoms.
readily form under electron beam irradiation due to their being energetically unfavorable or any created are rapidly sputtered to reform pristine graphene. To address these points we have considered both results from previously reported density functional theory (DFT) calculations and the observed lifetimes of the defects recorded. DFT calculations of carbon blisters taken from the literature have focused on either the ISW or the C2 structure shown in Figure 2f with the other observed adatom morphologies not discussed.10−14 The formation energy for the C2 defect has been calculated to be 6.07 eV and can be viewed as an ISW defect (Ef = 6.2 eV) subjected to a single SW switch (as in Figure 1b,c).11 Two separate isolated occurrences of the C2 defect were recorded in these studies. In the first instance, the defect was stable for at least 3 min. However, this is a lower bound estimate for the lifetime, as unfortunately the ultimate annealing of this particular defect was not captured. The other example oscillated between various structures on a ∼30 s time scale, before reverting to pristine graphene after 139 s irradiation. The C6 and C4 defects exhibited relatively short life times of approximately 30 s (a few image exposures). In contrast, the lifetime under electron beam irradiation for the C2 defect is comparable to divacancy structures,8 suggesting that the rarity of this defects is not due to rapid annealing but rather to a significant energy barrier to formation. We now discuss the evolution of adatom defects under electron irradiation together with data on the formation and annihilation of these structures. The C6 defect was found to undergo limited evolution under the electron beam. Immediately after high current density defect creation an irregular structure was imaged (Figure 3a) containing six additional carbon atoms. This structure subsequently relaxed to a
symmetric C6 configuration shown in Figure 3c via the intermediate state shown in Figure 3b. The sequence of four SW rotations necessary for this transformation are illustrated schematically in the atomic models in Figure 3g-i. The C6 defect shown in Figure 3c subsequently undergoes carbon dimer sputtering after a ca. 39 s period of stability, as shown in Figure 4. The atomic model in Figure 4d illustrates the C6 adatom defect corresponding to the TEM image in Figure 4a with the highlighted carbon dimer sputtered to form the 3-fold symmetric C4 defect shown in Figure 4b. This adatom defect has a lifetime of ca. 45 s before four further carbon atoms are sputtered to produce a pristine graphene sheet (Figure 4c). In Figure 4e,f, we show our proposed order of atom removal with the carbon dimer highlighted in Figure 4e ejected first, as this yields the energetically favored ISW structure, before the final additional carbon pair is sputtered. While we refer to the sputtering of carbon dimers in our discussion of proposed mechanisms, in reality the removal of carbon atoms should occur through sequential single atom sputtering events. However, a structure with an odd number of carbon adatoms that possesses one under-coordinated carbon atom with a single dangling bond23 will have a lower threshold for subsequent sputtering and so is likely to soon be ejected, resulting in the removal of atoms in pairs. One instance of a C2 adatom defect was observed to form from a divacancy by a mechanism that is similar to that suggested in ref 11 and is shown in Figure 5. The morphology of the defect in Figure 5b suggests that two atoms have been incorporated into the right-hand seven-membered ring of the initial 555-777 divacancy (Figure 5a), although the temporal C
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Figure 6. Evolution of the C2 defect. (a) Maximize-filtered ACTEM image of a C2 defect. (b) The C2 defect just before a SW rotation switches it to (c) an ISW structure. (d) The defect is finally annealed to pristine graphene lattice. Time values indicate the elapsed time from (a). (e−h) Atomic models corresponding to (a−d). The arrow indicates a bond that undergoes a SW rotation in the next frame. Color scheme for the atomic models and annotations denotes the number of carbon atoms in the ring with blue as 7, red as 6, and yellow as 5 carbon atoms.
Figure 4. (a) Frequency filtered ACTEM image of a C6 adatom defect. (b) After 39 s two carbon atoms are ejected, yielding a C4 blister defect. (c) After 45 s, all four extra adatoms are sputtered, leaving pristine graphene. (d) Atomic model of the C6 adatom defect shown in (a). The dotted blue circle denotes the carbon dimer that is removed. (e) Atomic model of the C4 adatom defect shown in (b). We suggest that carbon dimer removal occurs in two steps with the first yielding an ISW defect, shown in (f). (g) Pristine graphene. Color scheme for the atomic models denotes the number of carbon atoms in the ring with blue as 7, red as 6, and yellow as 5 carbon atoms. Scale bar is 0.5 nm.
ISW defect (Figure 6c) before a sputtering event removed the two adatoms, leaving pristine graphene. The maximum filtered defect shown in Figure 6c is not as clearly defined as for the other images; specifically the 5-membered ring on the right side lacks sufficient contrast to confidently resolve its bonding structure. Performing further image analysis (see Supporting Information Figure S5) does suggest the model presented in Figure 6g is the correct interpretation, however it is not completely unambiguous. It is possible that because the subsequent frame (Figure 6d) is of pristine graphene, the image in Figure 6c exposed both the ISW defect and the subsequent sputtering event, resulting in a superposition of the two states being captured. In addition to the isolated blisters already discussed, we have found evidence for the formation of blisters associated with dislocations in the graphene lattice. This is not surprising, as it is known that the addition of a significant number of defects into graphene generates out of plane distortions,25,39 and it is reasonable to speculate that the incorporation of blister defects would reduce the overall strain in the system. Figure 7a (annotated in Figure 7b) shows a dislocation core pair, indicated by white arrows, associated with part of a blister
resolution is insufficient to resolve individual mechanistic steps. Figure 5d,e shows atomic models illustrating this inclusion of the carbon dimer into the 7-membered ring of the divacancy. A single SW rotation transforms this defect into a 585 divacancy and an inverse SW defect pair, shown in Figure 5g with the corresponding structural model shown in Figure 5i. We propose that the divacancy in the 555-777 morphology is less able then the 585 structure to incorporate two carbon atoms and anneal to a pristine lattice, hence the formation of the structure in Figure 5b. The 585 divacancy shown in Figure 5g subsequently evolves into a C2 defect through the separate incorporation of two further carbon atoms as shown in Figure 5h. The process through which the defect structure shown in Figure 5h was observed to anneal back to pristine lattice is shown in Figure 6. After being stable for 29 s (Figure 6a,b) the C2 blister was observed to unwind via a SW rotation into an
Figure 5. The incorporation of surplus carbon atoms to form a blister. (a) Maximize-filtered ACTEM image of a divacancy defect, which includes two additional atoms in the following image, shown in (b). (c−f) Atomic models showing the probable sequence for the carbon dimer (green) incorporation observed in (a) and (b). The arrow indicates a bond that undergoes a SW rotation in the next frame. (g,h) ACTEM images showing the further incorporation of an additional two carbon atoms, yielding a C2 adatom blister. (i,j) Atomic models corresponding to the ACTEM images shown in (g,h), respectively. The dotted circles in (i) highlight an ISW defect (blue) and a 585 divacancy (red). The red emphasized atoms in (j) indicate two carbon atoms that were added to anneal the circled 585 divacancy shown in (i). Time values indicate elapsed time from image (a). Color scheme for the atomic models denotes the number of carbon atoms in the ring with purple as 8, blue as 7, red as 6, and yellow as 5 carbon atoms. D
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Figure 7. (a) Maximize-filtered ACTEM image and (b) annotation showing a pair of dislocation cores and attached blister structures. White dashed lines indicate the zigzag lattice planes. (c) Maximize-filtered ACTEM image and (d) annotation showing the bottom defect in (a) evolving via a SW rotation to a structure similar to that of the upper defect. (e) Maximize-filtered ACTEM image and (f) annotation showing another example of a blister-dislocation core structure, which subsequently loses two atoms to form the dislocation core pair shown in (g) and annotated in (h). (i−l) Corresponding atomic models of the images shown in (e−h). SW rotations are highlighted in yellow and indicated with an arrow, and the red circle in (k) indicates the carbon atom pair that is sputtered. The TEM annotations use white arrows to indicate the dislocation cores. Color scheme for the atomic models and annotations denotes the number of carbon atoms in the ring with 5 (yellow), 6 (red), 7 (blue), and 8 (purple) carbon atoms.
Figure 8. (a) Maximize-filtered ACTEM image of a dislocation pair and accompanying GPA strain map components (εxx, εxy, εyy, and rotation). (b) Maximize-filtered ACTEM image of a dislocation pair with one dislocation core part of a C2 blister (magnified view inset) and GPA strain fields. Unfiltered versions of the TEM images shown were used to obtain the strain maps (see Supporting Information).
shown in Figure 8a,b, respectively. It can be seen that there is no significant difference in the calculated strain between the systems, thus discounting the idea that these blisters are generated to relieve strain. Instead, we suggest that the larger number of blisters found near dislocations is due to them aiding with nonconservative dislocation climb, that is, the movement of a dislocation core along the zigzag axis without the ejection of the necessary carbon atom pair.5 This additional atom pair are instead incorporated into a blister defect. Evidence to support this suggestion is presented in Figure 9. In Figure 9a, we show a TEM image of a dislocation core pair with the top dislocation being adjacent to an ISW adatom defect. In the subsequent TEM image (Figure 9b), we found that the ISW defect had merged with the dislocation, resulting
defect. The lower structure in Figure 7a shows a C2 555-777 blister with a merged dislocation core. The upper defect in Figure 7a shows a C2 555-777 defect with an additional SW rotation, again merged with a dislocation core. The lower C2 defect subsequently evolves into a similar configuration as the top core, shown in Figures 7c. A second example of a dislocation core merged with a C2 blister is shown in Figure 7e,f, which subsequently anneals to form a dislocation core through the removal of the two additional atoms. This leads to a normal dislocation core pair shown in Figure 7g,h, illustrated schematically in Figure 7i−l. In order to evaluate whether the blisters act to relieve the strain caused by dislocations we have carried out a GPA analysis of the image shown in Figure 7e−h, the results of which are E
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Figure 9. A dislocation core climb that conserves the total atom count of the system, achieved by the incorporation of an inverted SW defect into the dislocation. (a) Maximize-filtered ACTEM image of a dislocation pair with the top dislocation core adjacent to an ISW adatom defect and the bottom next to a C2 defect. (b) A subsequent image showing the merging of the ISW defect into the dislocation, resulting in the left dislocation core climbing to an adjacent lattice plane. (c) Atomic model corresponding to the TEM image shown in (a). The dislocation is indicated by the white Ts connected by a black line, and the ISW defect is circled in red. (d−f) The proposed mechanism by which the ISW merges with the dislocation, via four SW bond rotations (arrows). (g) Atomic model corresponding to the TEM image shown in (b).
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ACKNOWLEDGMENTS J.H.W. thanks the support from the Royal Society and Balliol College, Oxford. A.W.R. has been supported by EPSRC (Platform Grants EP/F048009/1 and EP/K032518/1). Financial support from EPSRC (Grants EP/H001972/1, EP/ F028784/1, and EP/F048009/1) is acknowledged.
in a dislocation climb. A proposed route for this process is illustrated by atomic models in Figure 9c−g. A total of four SW bond rotations, indicated by the arrows, are needed for the complete incorporation of the ISW defect into the dislocation and permits the dislocation core climb without the need for further atom incorporation. Conclusion. Using ACTEM imaging of selectively irradiated graphene we have demonstrated the formation of atomic scale blister defects that include up to six additional atoms. Negligible in-plane lattice strain accompanies their formation confirming previous reports that the strain is removed by buckling of the graphene lattice. These adatom defects seldom formed in isolation even under extensive electron irradiation, suggesting that there is a large energetic barrier inhibiting their formation. A C2 defect was observed to form by the incorporation of two carbon atoms adjacent to a divacancy leading to a divacancy-ISW pair, followed by the addition of another two atoms to remove the divacancy, suggesting a possible pathway for blister defect formation. We have also shown image sequences demonstrating the elimination of adatom defects. Importantly, the relatively short lifetimes of these structures suggest that ultimately they may not be sufficiently stable to serve as candidates for graphene defect nanoengineering. Finally, we have explored the prevalence of blister defects associated with dislocation cores, which permit dislocation climb without carbon atom ejection.
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ASSOCIATED CONTENT
* Supporting Information S
Detailed methods and equipment descriptions, discussions of the image processing techniques employed, and additional image and strain analysis are provided. This material is available free of charge via the Internet at http://pubs.acs.org.
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REFERENCES
AUTHOR INFORMATION
Corresponding Authors
*E-mail: (J.H.W.)
[email protected]. *E-mail: (A.W.R.)
[email protected]. Notes
The authors declare no competing financial interest. F
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