Dynamics of Single Fe Atoms in Graphene VacanciesAlex W. Robertson,† Barbara Montanari,‡ Kuang He,† Judy Kim,† Christopher S. Allen,† Yimin A. Wu,†

Jaco Olivier,§ Jan Neethling,§ Nicholas Harrison,‡ Angus I. Kirkland,† and Jamie H. Warner*,†

†Department of Materials, University of Oxford, Parks Road, Oxford, OX1 3PH, United Kingdom‡Rutherford Appleton Laboratory, Didcot, Oxfordshire, OX11 0QX, United Kingdom§Centre for HRTEM, Nelson Mandela Metropolitan University, Port Elizabeth, South Africa

*S Supporting Information

ABSTRACT: Focused electron beam irradiation has been used to create monoand divacancies in graphene within a defined area, which then act as trap sitesfor mobile Fe atoms initially resident on the graphene surface. Aberration-corrected transmission electron microscopy at 80 kV has been used to study thereal time dynamics of Fe atoms filling the vacancy sites in graphene with atomicresolution. We find that the incorporation of a dopant atom results inpronounced displacements of the surrounding carbon atoms of up to 0.5 Å,which is in good agreement with density functional theory calculations. Onceincorporated into the graphene lattice, Fe atoms can transition to adjacentlattice positions and reversibly switch their bonding between four and threenearest neighbors. The C atoms adjacent to the Fe atoms are found to be moresusceptible to Stone-Wales type bond rotations with these bond rotationsassociated with changes in the dopant bonding configuration. These resultsdemonstrate the use of controlled electron beam irradiation to incorporate dopants into the graphene lattice with nanoscalespatial control.

KEYWORDS: Graphene, ACTEM, HRTEM, electron microscopy, defects, TEM

Manipulating the novel properties of graphene,1 such ascarrier concentration2,3 or magnetism,4−6 by utilizing

defects is an area of research with important ramifications in thedeployment of graphene for devices. Impurities, such as metals,often reside on the surface of graphene but they can also beincorporated into the lattice to form a substitutional orinterstitial dopant structure. The magnetic moment oftransition metal atoms incorporated into a graphene sheetthrough covalent bonds to the surrounding C atoms in a monoor divacancy have been theoretically modeled, with somemetals (Cr, Mn) showing paramagnetic moments and others(Zn, Pt) being diamagnetic.6 Fe exhibits a paramagneticmoment when substituting two carbon atoms in a divacancyinterstitial configuration but not when directly substituting asingle carbon atom in a monovacancy arrangement.6 Thisconclusion is supported by studies of Fe implanted graphitethat show paramagnetism.7,8

Available methods for the introduction of defects or dopantsinto graphene with fine spatial control are challenging andlimited but are likely to play an important role in themanipulation of graphene properties. Recent work has shownthat focused electron beam irradiation at an accelerating voltageof 80 kV results in vacancy formation in graphene with 10 × 10nm2 spatial control.9 The increased chemical reactivity of thesecreated defect sites offers the possibility of trapping adatomsresiding on the surface.

Aberration-corrected transmission electron microscopy (AC-TEM), operated at accelerating voltages of 80 kV, allows us toelucidate the behaviors of single atom point defects in agraphene sheet.10−13 These include metal substitutions andinterstitials and their evolution over time under controlledelectron irradiation. Previous work in this area has used high-resolution (HR-) TEM to study the diffusion of metals ongraphene sheets,14−16 and vacancy quenching by metal atoms ingraphene and carbon nanotubes.17,18 Scanning TEM (STEM)combined with electron energy loss spectroscopy (EELS) hasalso been employed to investigate metal−graphene interactionsand has provided information on extended plasmon resonancessurrounding Si atoms in graphene and the oxidation resistanceof Fe atoms incorporated into the graphene lattice.19−23

However, direct observation of the incorporation and migrationof covalently bonded metal atoms in graphene, which wouldprovide insights into the strain and stability, has yet to beobserved.In this paper, we create vacancy defects in graphene in a

defined spatial region using our previously reported techniqueof focused electron beam irradiation at 80 kV.9 We then studiedthe dynamics and the mechanisms by which mobile Fe atomsare trapped by vacancies through covalent bonding to the

Received: December 5, 2012Revised: March 1, 2013

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graphene lattice and the resulting strain induced in thesestructures.Graphene was synthesized by a liquid Cu catalyst chemical

vapor deposition (CVD) method, reported elsewhere,24 and

transferred to Si3N4 TEM grids using a PMMA polymersupporting scaffold. The Cu was removed by using an FeCl3etchant solution. An important stage in this process is theremoval of residual FeCl3 used to etch the Cu, by rinsing inHCl solution. To maximize the possibility that a Fe atom willbe trapped within created vacancy structures the HCl rinseduration was reduced, leaving more Fe residue on the graphenesurface. The nature of the dopants observed by AC-TEM wasconfirmed using annular dark field STEM combined with EELS(Supporting Information Figure S1) along with analysis of thecontrast profiles in AC-TEM images. Defects were created in aselected 10 nm2 area of graphene by localized irradiation as

Figure 1. False color AC-TEM images of (a) a Fe substitutional defectin a graphene monovacancy (Fe@MV) and (b) a Fe interstitial defectoccupying a divacancy (Fe@DV). (c) Smoothed AC-TEM image ofthe Fe@MV shown in (a). (d) DFT optimization of the Fe@MVstructure (the insert shows a side view) and (e) a multislice TEMimage simulation of the system. (f−h) Similar to (c−e) but for a DFT-optimized Fe@DV. Scale bars denote 0.5 nm.

Figure 2. AC-TEM images of (a) Si and (b,c) Fe atoms in graphene. (d−f) The box intensity profiles (a representative box averaging region isshown in panel b), with the red highlighted regions corresponding to the impurity dopant and the yellow highlighted region corresponding to carbonatom contrast. (g−i) “Spectrum” color look-up table (CLUT) applied to the AC-TEM images shown in (a−c), respectively, demonstrating thehigher contrast (red) for the impurity atom.

Figure 3. (a) Smoothed AC-TEM image of a monovacancy ingraphene created by focused electron beam irradiation. (b) Smoothedimage taken a few seconds after (a) showing a Fe atom in theinterstitial Fe@DV configuration. (c) Displacement map obtainedfrom the difference of DFT optimized monovacancy and Fe@DVstructures. (d) Smoothed AC-TEM image of a divacancy created byfocused electron beam irradiation. (e) Image taken 10 s after (d)showing a Fe atom in the substitutional Fe@MV configuration. (f)Displacement map obtained from DFT calculated DV and Fe@MVstructures. The displacement map color LUT scale is from purple; 0 Å,to red; 0.5 Å displacement. Image scale bars are 0.5 nm.

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reported in detail in ref 9. AC-TEM was performed usingOxford’s JEOL 2200MCO fitted with CEOS third order probeand image aberration correctors and operated at an acceleratingvoltage of 80 kV. Atomic models were created using AccelrysDiscovery Studio Visualizer and AC-TEM image simulationswere performed using the multislice algorithm within the JEMSsoftware with parameters set to match our microscope. AC-TEM images are smoothed (nearest neighbor) to reduce noise,while not affecting the interpretation of the atomic positions,and are taken at dark atom contrast. Hybrid exchange densityfunctional theory (DFT) calculations (the B3LYP func-tional)25−27 were performed using the CRYSTAL code.28

The numerical approximations adopted, and the double valencewith polarization local Gaussian basis set, have been describedpreviously.29 The Fe basis set was that used in previous studiesof inorganic materials30 enhanced with a diffused sp-function at0.2 a0

−2. Defects were represented in a periodic 10 × 10supercell of the graphene primitive unit cell within the Brillouinzone of which the eigensolutions were sampled on a Pack-Monhurst grid of shrinking factor 4 yielding 10 symmetryirreducible k-points.Results and Discussion. We initially discuss the two most

frequently observed atomic configurations for single Fe atomsin a graphene lattice. Figure 1a,b shows raw AC-TEM images(rendered with a fire color look-up table (CLUT) scale appliedto enhance visual contrast) of the two Fe atom point defectconfigurations. Figure 1a demonstrates the substitutionalconfiguration with a Fe atom occupying a monovacancy inthe graphene sheet; this is referred to as Fe@MV, as in ref 6.Figure 1b shows the interstitial configuration, where a Fe atomresides in a graphene divacancy, replacing two C atoms, and isreferred to as Fe@DV. A smoothed AC-TEM image of the

Fe@MV is shown in Figure 1c. DFT calculations for the Fe@MV structure yield the relaxed geometry shown in Figure 1d,and by applying a multislice image simulation to this Figure 1eis produced, showing strong agreement with the AC-TEMimage in Figure 1c. A smoothed AC-TEM image, the DFTrelaxed geometry, and AC-TEM image simulation for the Fe@DV are shown in Figure 1f−h. These image simulationsdemonstrate excellent agreement with the observed structures.Similar results have recently been observed with siliconimpurities by high-resolution STEM.31

To categorically identify the contaminant as Fe it wasnecessary to combine EELS analysis of the sample with furtheradditional experiments. The EELS data, shown in SupportingInformation Figure S1, identifies the presence of Si and C aswell as Fe in the sample. We speculate that the Si originatedfrom the quartz tube of the CVD furnace and the Fe is from theFeCl3 etchant solution. In order to distinguish between Si andFedoped impurity atoms in defects we prepared a second set ofgraphene samples using a different transfer method. Specificallyan electrochemical bubbling transfer method32,33 was used toprepare graphene TEM samples that were not exposed tosources of Fe contamination. It can be inferred that dopantdefects imaged by TEM in the electrochemically transferredsamples were highly likely be Si (i.e., Si@MV or Si@DV), as nosources of Fe were present and thus can provide a suitablereference for comparison to the dopant defects imaged in theFeCl3 transferred samples.In order to distinguish Fe dopants from Si, we used boxed

line profiles taken across the high contrast dopant atom stablyresiding in a monovacancy defect. Multislice image simulationswere used to compare predicted contrast variations for Na, Si,Fe, and Co impurity atoms in a graphene monovacancy (seeSupporting Information). By taking into account variations inimage defocus and image resolution, our image simulationspredict an Fe/C contrast ratio of 1.83, Si/C contrast ratio of1.47, and Na/C ratio of 1.25 for dopants filling themonovacancy position (see Supporting Information) relativeto the C atoms in the graphene lattice. This indicates that Naatoms are barely detectable in the AC-TEM image.For graphene transferred by the bubbling method in NaOH,

we measured line profiles from the same dopant atom in 15different frames and obtained a contrast ratio of 1.48 ± 0.17and then repeated this on another set of images taken from adifferent region of the sample with a dopant atom in themonovacancy (5 frames) and obtained a contrast ratio of 1.45± 0.21. This agrees well with the image simulation predictedvalue of 1.47. For graphene transferred by the FeCl3 etchant,we measured a contrast ratio of 1.68 ± 0.23 (12 frames), 2.10 ±0.22 (6 frames), and 1.89 ± 0.22 (16 frames). The error in allmeasurements is similar giving confidence in the comparison ofeach contrast value. These values are higher than the bubblingtransferred sample and this indicates that it is Fe atoms weprimarily observe for the FeCl3 transfer and Si atoms for thebubbling in NaOH transfer. The number of dopants observedin the FeCl3 transfer was also more than observed in thebubbling transfer process.Figure 2a presents the AC-TEM image with the highest

contrast ratio (1.73) obtained for a Si dopant (Si@MV) fromthe electrochemically transferred sample. Figure 2b,c presentsthe AC-TEM images with the highest contrast ratio (2.23)obtained for Fe dopants from the FeCl3 etched sample. Figure2b,c was taken from two different graphene samples, bothtransferred by FeCl3. By extracting intensity profiles across the

Figure 4. AC-TEM images showing the mobility of the Fe defect atomand corresponding atomic models. (a,b) Shifting between adjacent MVsites, (c,d) atomic models. (e,f) Chaning between two DV sites, (g,h)atomic models. (i,j) Switching from a DV to a MV site, (k,l) atomicmodels. (m,n) Moving from a MV to a DV site, (o,p) atomic models.Red annotations (circles or arrows) show the location of the defectatom in the previous frame. The accompanying atomic models showthe Fe adatom in blue.

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dopant site, shown in Figure 2d−f, it is possible to compare theintensity of the dopant atom with the mean intensity of thegraphene C atoms. Figure 2g−i use a “spectrum” color look-uptable to replot Figure 2a−c to show the higher contrast values(i.e., more red) from the Fe atom compared to the Si atom.Because of the controlled defect creation procedure used, it is

possible to image the graphene lattice prior to Fe atomincorporation. Theoretical modeling studies of Fe adatoms on agraphene surface predict a migration barrier of <1 eV,suggesting the Fe atoms are relatively mobile on the surfaceof graphene when irradiated,34,35 thus allowing for surfaceadatoms to stochastically occupy created vacancies. Figure 3ashows an AC-TEM image of a vacancy created in the lattice byfocused electron beam irradiation, and Figure 3b shows animage of the same region taken several seconds later, showingan interstitial Fe atom bonded to four adjacent C atoms in theFe@DV configuration. We have measured the position of theatoms of the DFT optimized geometries of the defect structureshown in Figure 3a,b and generated a 2D displacement mapfrom their difference (Figure 3c), similar to previous work oncarbon nanotubes,35 with interpolation used to fill between theshown data points. Figure 3d shows a created divacancy andseveral seconds later (Figure 3e) the divacancy has been filledwith both a C and a Fe atom to form a Fe@MV structure. A

displacement map is shown Figure 3f of the difference betweenthe atomic positions of the DFT optimized structures. Both 2Ddisplacement maps (Figure 3c,f) show appreciable strain arisingfrom the Fe atom incorporation. The displacement maps showatom displacements in the vicinity about the Fe adatom of upto 0.5 Å. Supporting Information movies S1 and S2 show thepair of images used in Figure 3 panels a,b and d,e, respectively,aligned such that the relative change can be visually compared.The movies offer a clear demonstration of the atomdisplacements around the point defects, with a strong differencebetween the two frames in which the nature of thedisplacement can be readily distinguished. SupportingInformation movies S3 and S4 show the quenching effectfrom the equivalent pair of image simulations extracted fromDFT relaxed structures. This observed vacancy quenchingphenomena allows us to rule out the dark contrast spotoriginating from functionalization of a carbon atom with amethyl or other functional group, as suggested in ref 36.Real time monitoring of the Fe atoms reveals they were not

fixed in the MV or DV positions but swap atomic positions andconfigurations. Figure 4 shows the migration of a Fe atombetween each of the four possible permutations. Figure 4a,bshows the Fe atom moving one atomic step in the Fe@MVconfiguration at different graphene sublattice sites and Figure

Figure 5. Atomistic schematics illustrating the four observed types of Fe-vacancy complex migration. Fe@MV to Fe@MV: (a−c) Electron collisionleads to partial ejection of C (yellow highlight) and repositioning of Fe (gray blue) into the adjacent MV position. The displaced C then occupies theprevious Fe lattice site. Fe@DV to Fe@DV: (d−f) C displacement creates a transitional trivacancy occupied by the Fe, with the displaced C in a topsite. Fe@DV to Fe@MV: (g−i) A mobile surface C is localized on the top site of the Fe adatom. This C atom is subsequently perturbed by anelectron collision, leading to the transition into a Fe@MV. Fe@MV to Fe@DV: (j−l) The displacement of the C atom allows the Fe atom to moveinto an interstitial position. The displaced C in the adatom top site subsequently detaches or migrates.

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4c,d shows corresponding atomic models. In Figure 4e,f, arotation of the Fe@DV position is observed between the twoframes with the Fe atom changing lattice site, as illustrated inthe atomic models shown in Figure 4g,h. The Fe atom was alsoobserved to change bonding configuration, switching betweenthe Fe@DV and Fe@MV, or from Fe@MV to Fe@DV. Thisimplies that the Fe atom changes coordination from four Catoms in a Fe@DV to three C atoms in the Fe@MV. Figure 4i,jshows a Fe@DV to Fe@MV transition and Figure 4m,n showsa Fe@MV to Fe@DV transition with respective atomic modelsin Figure 4 panels k,l and o,p. Theoretical models suggest thatdriving this migration by thermal activation is prohibitivelyexpensive at room temperature,6 therefore an additional energycontribution from electron irradiation is important. Theorywould also suggest that this transition is associated with aconcomitant switching of magnetic moments, since Fe@DVhas a predicted paramagnetic moment, whereas Fe@MV doesnot.6

The nature of the mechanism behind the migration of metaldefects chemically bonded in graphene has been discussedpreviously in the wider literature, before atomic resolutionACTEM images such as those shown here were available. Inparticular, the conventional HR-TEM work by Gan et al.14 onthe migration of Au atoms incorporated into grapheneprompted a number of further theoretical investigations.37,38

In ref 37, molecular dynamics simulations were employed toelucidate plausible pathways for the migration of an Auinterstitial atom occupying a divacancy with the lowest energypathways involving an out-of-plane translation of either the Auor an adjacent C. In ref 38, the migration of the Au adatomsubstitution was modeled in a similar manner to that of thegraphene monovacancy39 with an intermediate pentagonconfiguration and the Au pushed out of plane. These results,

in conjunction with the imaging data presented here, suggestthat migration is likely to occur as a result of radiation induceddiffusion;40 in which the effect of electron beam irradiation onC atoms surrounding the Fe adatom indirectly facilitates themigration of the Fe. In both refs 37 and 38, the possibility of amore direct, out-of-plane event involving the metal atom is alsodiscussed, but it should be noted that, due to mass of the Featom, it is less likely that the Fe would be significantlyperturbed by the 80 kV electron beam. For comparison, themaximum energy transferred in an 80 kV elastic electron−atomcollision with a C atom is ECmax = 15.8 eV, and with Fe EFemax =3.4 eV; thus there is markedly less energy available for anysubsequent reconfiguration for the Fe atom case.In Figure 5, atomistic models are presented that schemati-

cally illustrate suggested plausible steps in a Fe-vacancycomplex migration for each of the four defect permutationsobserved. In Figure 5a−c, the steps for a lattice switch, or Fe@MV to Fe@MV, are shown, with an electron to carbon collisionimparting energy below the threshold for knock-on displace-ment to the yellow highlighted C atom, adjacent to the blue Fesubstitutional. This subthreshold collision energy ensures thatrather than being ejected, the C remains bonded to the Fe atomin an out-of-plane top site. The Fe atom then follows amigration pathway similar to that of a mobile atom in a MVmigration,39 with the displaced carbon atom occupying thenewly vacated MV site after the Fe has completed themigration. It is also possible that the highlighted C atom isejected outright by the electron collision, although thesputtering cross-section for this at 80 kV, at least for a pristinelattice, is unfavorable.41 For the Fe@DV migration (Figure 5d−f) a C atom is displaced into a top site position, initially forminga trivacancy complex, which the Fe atom subsequently occupiesin a similar way to Pt observed in sputtering experiments,18

Figure 6. A time series of consecutive AC-TEM images of (a) a single, elongated blurred region of dark contrast, (b) a double region of darkcontrast, (c) a point of dark contrast, identified as a Fe@MV. (d−f) Intensity line profiles (as gray scale values) taken across the armchair axis withthe defect at the central position (blue dashed line).

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before switching to a more stable DV interstitial position withthe C atom occupying the residual vacancy position. DFToptimization found that this Fe occupied trivacancy (Fe@TV)structure’s energy is 4.9 eV higher than the initial Fe@DVconfiguration, which is beneath the maximum energy of 15.8 eVthat can be transferred in an 80 kV electron-carbon collision.The Fe@DV to Fe@MV switch proposed in Figure 5g−i canbe simply explained by the incorporation of a surface C atominto the graphene lattice. However, it is possible that this switchis facilitated by either an electron collision perturbing the Cfrom the top site or a displacement of the Fe. In Figure 5h weillustrate a suggested intermediate C top site configuration,which resembles a five-coordinate Fe-porphyrin structure,42

which is stable,43 and that exists as a transitional step. The

opposite switch (Figure 5j−l) requires a collision event with anadjacent C atom that moves it into an out-of-plane position,opening up a DV that the Fe moves to occupy. The nowsurplus C atom either remains in the top site position on the Feor it may be displaced by a subsequent electron collision. Thetop site configuration of carbon on Fe@DV was modeled tohave an energy 3.8 eV higher than that of the initial Fe@MVconfiguration, which is again beneath the maximum energytransfer from the 80 kV electron beam.We have found that, as in previous reports,15,38 the Fe atoms

exhibit a preference for bonding along the edges of thegraphene sheets. The Fe atoms on the graphene edge exhibitsubstantially more mobility than those trapped in vacancy sites,with the atoms moving up to several nanometers between

Figure 7. (a−c) AC-TEM image series of (a) a Fe@DV, (b) Fe@DV with a nearby bond that undergoes a SW bond rotation, and (c) subsequentrerotation back into a switching Fe@MV state. (d−g) Corresponding atomistic models of (a−c), the black arrow in (e) indicating an additional Cadatom and (f,g) showing the two configurations that are recorded during (c). (h−j) A similar AC-TEM images series with (h) showing a Fe@DVthat is subject to a proximal SW rotation in (i) and which relaxes back to a switching Fe@MV in (j). (k−n) Corresponding atomistic models of (h−j), with (m,n) illustrating the two Fe@MV configurations that switch during the image captures in (j). In the schematics red corresponds to C andblue to Fe with the gold highlighted atoms indicating the C atoms that undergo SW rotations. Blue arrows indicate the larger C rings that form dueto SW rotations that are predisposed to C adatom addition.

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exposures, which are typically of the order of several seconds(Supporting Information Figure S2). The calculated diffusioncoefficient for these edge bonded Fe atoms is 7 ± 1 × 10−21 m2

s−1, with corresponding activation energy of 0.8 eV (seeSupporting Information). In contrast, as discussed, a Fe adatomincorporated into the graphene bulk was found to only migrateto adjacent atomic positions between exposures, implying a farsmaller diffusion coefficient.The four switching permutations observed (Figure 4) and

modeled (Figure 5) do not include the possible occurrence ofincomplete steps, for instance where the displaced C atom inFigure 5b moves back into its previously occupied position.Because of the temporal resolution limits of our AC-TEMexperiments it is not possible to resolve these intermediatestates in the defect transitions. The relatively long exposuretime (∼2−3 s) can however lead to features in some frames(Figure 6) that are consistent with a defect captured betweentwo states, which appears as either a blurred dark contrastfeature (Figure 6a), or as two separate features (Figure 6b). Wecan discount the possibility that these features originate fromthe presence of an additional Fe atom along the defect as asubsequent frame (Figure 6c) from the same time series asFigure 6a,b shows a single darker contrast atom. It is moreplausible that these images are the result of oscillations betweenstable Fe@MV configurations that switch at a frequency that isfaster than the exposure time. However, an alternative structurethat is consistent with the image shown in Figure 6a is that theFe atom is located in the out-of-plane bridge position betweentwo C atoms in standard graphene lattice positions. This wouldthen require a mechanism that would allow for the perturbationof the Fe atom out of the plane of the graphene lattice butremaining localized at the same defect site for a prolongedperiod in order to corroborate with the TEM image series.Furthermore, DFT calculations have demonstrated that whenthe Fe atom is located out-of-plane then its most energeticallyfavorable position is above the center of a hexagonal carbonring, known as the hole site, equidistant from the nearest six Catoms, rather than at a bridge site, as would be required to fitthis alternative explanation.44,45 Therefore it is reasonable todiscount this alternative bridge site explanation.During the acquisition of two separate Fe adatom formation

and evolution time series it was apparent that Stone-Wales(SW) bond rotations occurred in the bonds proximal to the Featom. In Figure 7a−c, a bond at the top right (Figure 7a, goldhighlight in Figure 7d) adjacent to the Fe in a standard Fe@DVundergoes a SW rotation, leading to the defect state captured inFigure 7b (modeled in Figure 7e). In addition to the bondrotation it is apparent from the TEM image that another C,marked by an arrow, has entered the graphene lattice adjacentto the Fe. This rotated bond subsequently relaxes back,however the TEM images following this show an elongatedregion of dark contrast, indicative of a Fe@MV oscillatingbetween two positions, as discussed earlier and shownschematically in Figure 7f,g. Subsequently within the timeseries a Fe@MV is clearly resolved, supporting our oscillatingsingle Fe@MV hypothesis. Figure 7h−j shows a similar SWrotation adjacent to a Fe interstitial, ultimately leading to atransition from a Fe@DV to an oscillating Fe@MV state. Fromthese two examples it seems likely that the SW rotation assistsin the transformation from Fe@DV to Fe@MV, by increasingthe susceptibility of the incorporation of an extra C atom intothe structure. This could be simply due to the SW rotationincreasing the size of a ring of atoms adjacent to the Fe. The

blue arrows in Figure 7e,l indicate the larger ring that is formedby the bond rotation, which in the case of Figure 7e alreadycontains an additional C adatom (black arrow). Importantly wenote that subjecting pristine graphene to 80 kV electronirradiation at the imaging current densities used did not resultin significant numbers of bond rotations, suggesting that thestructure of the Fe-vacancy lends itself to more energeticallyfavorable SW rotations.

Conclusion. By manipulating a focused electron beam at 80kV, we have been able to locally control the formation of monoand divacancies in graphene. These have been subsequentlyused to trap single dopant atoms to form covalently bondedFe@MV and Fe@DV metal−defect complexes with apronounced effect on the displacement of neighboring carbonatoms of up to 0.5 Å, supported by DFT calculations. These Fecontaining defect structures are stable in comparison to Featoms incorporated into the graphene edge. However, they areobserved to undergo limited migration to adjacent lattice sites,with four distinct categories of movement observed. Models areproposed that explain the mechanism for these in terms ofelectron beam mediated migration. We suggest that the abilityto alter the bonding state of Fe in Fe@MV and Fe@DVconfigurations could be used to select magnetic and non-magnetic states in doped graphene.

■ ASSOCIATED CONTENT*S Supporting InformationMethod details, equipment details, elemental analysis, calcu-lation of the Fe diffusion coefficient at the graphene edge, andmultislice image simulations of dopant atoms filling graphenemonovacancies. This material is available free of charge via theInternet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: Jamie.warner@materials.ox.ac.uk.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSJ.H.W. thanks the support from the Royal Society and BalliolCollege, Oxford. A.W.R. has been supported by an EPSRCstudentship. Financial support from EPSRC (Grants EP/H001972/1, EP/F028784/1, and EP/F048009/1) is acknowl-edged.

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Nano Letters Letter

dx.doi.org/10.1021/nl304495v | Nano Lett. XXXX, XXX, XXX−XXXH