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1 doi: /nature11562 Figure S1 Distinction of graphene and copper grain boundaries. a-b, SEM images of oxidised graphene/cu at different magnifications. The white dotted lines indicate the oxidised copper that was expanded after UV treatment. The regions of different contrast visible in the SEM images represent the different orientation of the copper grains as well as the twin-boundary of the Cu. These grain boundaries are denoted by red dotted lines. The graphene grain boundaries (coincident with oxidised copper in white dotted lines) clearly cross over the copper grain boundaries. The graphene grain size was also much smaller than that of the copper. 1

2 Weight % Cu O Inside grain On boundaries Figure S2 EDS mapping of the oxygen content of oxidised graphene/cu. a-b, SEM images of oxidised copper after UV treatment. The graphene grain boundary denoted by the red square was more severely oxidised than the others. Consequently, this graphene grain boundary was cracked, as observed in b. The existence of different types of graphene grain boundaries is consistent with different defect characters at the grain boundaries 1,2. Cu step lines were also visible in b. c, EDS analysis of the oxygen content. The inset presents the 2D oxygen mapping of the SEM area located within the red square. The oxygen distributions determined using EDS and SEM were in good agreement. d, The oxygen content of the white regions at the boundary was greater than that within the grain. In addition, numerous separated white regions were observed to originate from the point defects of the graphene. 2

3 Composition (atomic percentage) Pure graphene Oxidized graphene C O Cu O/(C+Cu) Figure S3 Full XPS spectra and carbon, copper, and oxygen content before and after UV treatment of the graphene/cu sample. The concentration of oxygen increased from 2.8 to 14.2 at% after UV treatment. The small concentration of oxygen atoms in the pristine graphene originated from the oxidisation of graphene under ambient conditions 3,4,5. Both the graphene and the underlying Cu layer were oxidised. 3

4 Figure S4 Raman mapping of the pristine graphene on copper foil. D-band or G-band patterns were not observed in the samples prior to UV treatment. The small bright dot in the G- band mapping originated from the multilayer portion formed at the nucleation seeds. 4

5 Figure S5 Graphene grain boundaries and defects under UV exposure at 37% of humidity for 10 minutes. Graphene grain boundaries were clearly visualised by optical microscope and AFM. The AFM line profiles 1 across the graphene grain boundary line before and after UV oxidation were shown in the top panel d1. The oxidised copper line width was about 300 nm, which was visible also by an optical microscope. Another AFM line profile 2 inside grain was also shown in the bottom panel d2. Here, the point defects were detected by AFM. The widths of dots were around 50 nm, which were indicated by the repeatable peaks in d2 (red line). These dots were not visualised by optical microscope due to the resolution limit. Stronger exposure conditions are required to enlarge these point defects to be detected by optical microscope. 5

6 Figure S6 Graphene grain boundaries and defects under UV exposure at 37% of humidity for 10 minutes at another position. This was similar to Fig. S5 but the width of oxidised copper line was narrower. The width of measured oxidised copper line was about 200 nm. It was also interesting to see that some grains showed an abundance of defect population while others did not reveal the defects. Further investigation was required for studying the dependence of defect population on different types of graphene grains. 6

7 Figure S7 Effects of the oxidation conditions. a-d, Graphene was oxidised by UV irradiation for 5 to 30 minutes (from left to right) at a fixed humidity level of 25%. The graphene grain boundary began to be visible at longer exposure times and was saturated at 30 min. Only partial grain boundary lines were observed. e-h, Graphene/Cu oxidised at humidity levels from 25 to 66% with a fixed exposure time of 10 min. The graphene grain boundaries became more distinct at high humidity levels. The scale bar is 10 μm. The graphene grain boundary lines did not increase with longer exposure times. This result strongly suggests that the presence of OH radicals as a result of the moisture was a key factor in the oxidisation of the graphene by UV irradiation. 7

8 Cu2p 3/2 Before oxidization After oxidization Cu(OH) Cu Cu(OH) 2 /Cu C1s Before oxidization After oxidization C=C (sp2) C-OH (sp3) C-O-C (sp3) COOH (sp3) sp3/sp Figure S8 XPS C1s, O1s, and Cu2p peaks before and after UV treatment. The C1s peak exhibited tails related to epoxide, carboxyl, and hydroxyl groups. The Cu2p 1/2 and Cu2p 3/2 peaks for graphene/cu before and after oxidisation exhibited a Cu(OH) 2 peak at and ev, respectively. Only the Cu(OH) 2 peak appeared at the Cu2p peaks. These results provide further evidence that the OH radicals (and not the pure O radicals) play a role in the Cu oxidation, although the effect did not seem to be obvious. The Cu 0 peak was also not changed at ev after UV treatment. A CuO peak was not observed at approximately and ev for Cu2p 3/2 and Cu2p 1/2, respectively 6. Because the region of oxidised copper was very narrow, located at the graphene grain boundaries, the nominal increase in the oxygen content by XPS analysis was not evident, as illustrated in the table. 8

9 Figure S9 Geometries of Stone-Wales (SW) defect with and without functionalisation by adatom or admolecules in ball-and-stick model. The 1 st and 2 nd rows showed the atomic geometries with top and side view, respectively. The 3 rd and 4 th rows showed Fukui fields with an isovalue of e/å 3. F(-) indicated the relative reactivity with respect to electrophilic attack while F(+) indicated that to electrophobic attack. Functionalisation of carbon atoms at SW defect pulled out the carbon atoms normal to the surface, resulting in considerable amount of buckling of graphene, as shown in the 2 nd row. In the case of OH-functionalised SW, the defect was also expanded to 1.75 Å (ideal one, 1.42 Å) of C-C bond length due to this deformation (first row). The adsorption of a single adatom/admolecule was calculated to be barrier-less. Fukui fields showed that the SW defect was reactive for any radical, and was inactive independent of electrophilicity after functionalisation, as shown in the 3 rd and the 4 th rows. Therefore, an open space in heptagons of SW defect can be used for adatom diffusion pathway with a minimum diffusion barrier height if it exists. 9

10 Figure S10 The atomic geometries of transition states for diffusion of various adatoms or admolecules in ball-and-stick model. The related diffusion barriers are listed in Table S1. The local geometries of carbon atoms of ideal SW defect were strongly modified, hence increasing diffusion barrier height. On the other hand, the local geometries of carbon atoms near the functionalised SW defects were not altered much due to inactiveness of the functionalised SW defects shown by Fukui functions in Fig. S9. In the case of OH-functionalised SW defect, the upper heptagon was taken for adatom diffusion pathway because of the steric hindrance by the presence of OH group inside the bottom heptagon. 10

11 Penentrating atom or molecule SW (ev) H-covered SW (ev) O-covered SW (ev) OH-covered SW (ev) Mixed adatom covered SW (ev) H (0.45) 1.77 O (2.86) 5.26 OH (4.49) 9.44 Table S1 The diffusion energy barriers of H atom, O atom, and OH molecule through Stone-Wales defect functionalised by adatom or admolecules. We considered the diffusion pathways through the centre of heptagon, starting from 4 Å above the top surface graphene to 1 Å below the graphene, as shown in Fig. S10. The transition state geometries have been found by using QST transition search routine. Since the SW defect size was expanded and deactivated by functional groups, the diffusion barrier height was lowered. Numbers in parenthesis in the 5th column indicate the penetration energy barrier when Cu (111) surface was placed below graphene layer. It is obvious to see that OH-terminated SW defect gives the minimum diffusion barrier height for incoming molecules compared to the H and O-terminated SW defect. The diffusion barrier height was further reduced when Cu surface was introduced. Thus, oxidation of Cu substrate was promoted by the functionalisation of SW defects with OH groups and further assisted by the copper substrate itself by lowering the diffusion barrier height of the transition state. The barrier heights could be further lowered by optimizing the separation distance of Cu substrate and more importantly UV-assisted excited states of graphene. This requires further investigations. The fact that OH functionalisation is necessary to facilitate diffusion of radicals is well corroborated with experimental observations that humidity is required to oxidise Cu substrate. 11

12 Figure S11 Confocal Raman mapping and SEM images to confirm non-destructiveness of our technique. Samples were UV-treated at 37% of humidity for 10 minutes and transferred on SiO 2 /Si substrate. a-b, Optical images of graphene on copper and transferred on SiO 2. The optical micrograph showed graphene grain boundary patterns were removed after transfer. No crack formation was visible after transfer. c, Raman spectra of graphene on graphene grain boundaries and inside grain after transfer. D-band intensity was higher at the boundary than inside grain. No G -band position shift was observed, implying that the strain was released after transfer. d-f, Raman mapping of D-band intensity, G-band intensity, and G -band position, respectively. No morphological correlation with D-band mapping was observed after transfer, in good contrast with those on copper. g, SEM images of UV-treated graphene after transfer. No scratch formation was further check in SEM. 12

13 Figure S12 Optical micrographs with the samples UV-treated at 42% of humidity for 15 minutes. Optical micrographs of graphene on copper (left) and on SiO 2 (right). Graphene grain boundary lines were more clearly observed at this condition, similar to previous observation (Fig. S11). No crack was formed. The sheet resistance of pure graphene without UV treatment was 400 ohm/sq (PMMA removed). The sheet resistance of UV-treated graphene was 644 ohm/sq and was recovered to 426 ohm/sq after heat treatment in vacuum (10-5 Torr) at 600 o C for 4 hours. Figure S13 Estimate of graphene grain sizes using SAED. The SAED pattern was collected using a 1.2 μm aperture at different locations: centre, bottom, and top (from left to right). There were two different orientations of the graphene grains at the centre position that created two slightly misoriented hexagonal spots in the SAED pattern (red and blue). The single grains in the bottom and top positions reproduced the same red and blue hexagonal rings in the centre position, respectively. This result implies that the SAED region consisted of two grains with different orientations: one was located at the bottom and another at the top. The size of the graphene grain of a single orientation should be on the order of micrometres. 13

14 Figure S14 The effect of copper substrate annealing and growth temperature on the sheet resistance. In the first scheme, the copper foil was heated to the designated temperature and annealed for 30 minutes. CH 4 was then flown into the chamber, and the graphene was grown at the same temperature. The resultant curve (shown in green) demonstrates that the sheet resistance decreased with increasing temperature. In the second scheme, the copper foil was heated and annealed at 1060 o C for 30 minutes. Thus, the copper morphology was not different in these samples. During the growth stage, the temperature was decreased to 1045, 1030, 1015 and 1000 o C and added CH 4 to assess the effect of gas dissociation. The black curve in the graph demonstrates that only a small variation in the sheet resistance of these samples was observed. Therefore, it is concluded that only a minor effect of CH 4 dissociation occurred at temperatures greater than 1000 o C. In the third scheme, the annealing temperature was changed from 1000 to 1060 o C and the growth temperature was fixed at 1000 o C; the variation in the sheet resistance of this scheme was similar to that of the first scheme. Therefore, we concluded that the sheet resistance was strongly correlated with the copper morphology. A rougher copper morphology 14

15 gave rise to higher resistance due to abundant nucleation seeds, thereby resulting in rich graphene grain boundaries. A rougher copper morphology also gave rise to a larger number of nucleation seeds, which reduced the graphene grain size. This observation is in excellent agreement with a previous report 7. It is concluded that there is a direct correlation between graphene grain size and the sheet resistance of graphene. The sheet resistance was measured without removing PMMA on the top. 15

16 Figure S15 Mapping of different graphene grain boundaries by conductance AFM. a-f, Morphology and conductance mapping of graphene on copper before UV exposure. The morphology and conductance mapping of position 1 and position 2 were zoomed in and shown in c-d and e-f, respectively. The conductance along graphene grain boundary lines was more severely contrasted in f than in d. This implies the existence of different types of graphene grain boundaries. g-i, Morphology and conductance mapping of graphene on copper at the same position as (a) after UV exposure. The morphology and conductance mapping of marked position was zoomed in and shown in k-i. After UV, topological and conductance images were strongly correlated to each other. Oxidation of copper through graphene grain boundary lines scatters like points, which was similarly observed from SEM images. The graphene grain boundary lines which were ambiguously observed particularly in (d), were more clearly visible in g an h after UV exposure. This is again a merit of using optical microscopy with UV oxidation to visualize graphene grain boundary. 16

17 Figure S16 Effects of growth temperature to the number of nucleation seeds. Graphene was initially grown for 3 seconds at different temperatures. The formation of nucleation seeds relies on morphological change of copper substrate and carbon diffusion length on copper substrate. At low growth temperature, the formation of nucleation seeds was governed by adsorption, i.e., adsorption-dominant. Abundance of small size nucleation seeds was observed. On the other hand, at high temperature, diffusion of carbon atoms was dominant, diffusiondominant. The number of nucleation seeds decreased, whereas the sizes were enlarged. The effect of substrate morphology to form nucleation seeds was also checked further by maintaining longer annealing time at 1060 o C in inert gases (H 2 and Ar), followed by a gas flow of active gases (CH 4 ) for 3 seconds. In this case, the number of nucleation seeds decreased, strongly suggesting that morphological change of the copper substrate played an important role in determining formation of nucleation seeds. Therefore the growth conditions strongly influenced graphene grain sizes and subsequently the sheet resistance, as shown in Fig. 4e-f. 17

18 Figure S17 Graphene grown on nickel before and after UV exposure. Multilayer graphene grown on nickel substrate was UV-treated at 70% of humidity for 30 minutes. No grain boundaries and point defects were visualised due to the limited diffusion of radicals through multilayer graphene 8, in good contrast with radical diffusion through grain boundary of single layer graphene on copper. Figure S18 Boron nitride (BN) on copper before and after UV exposure. BN on copper substrate was UV-treated at 70% of humidity for 30 minutes. Grain boundary of BN was not clearly visualised due to three BN layers similar to multilayer graphene on Ni but point defects were visualised in this case 9. The underlying mechanism of point defect formation is not clear at this moment and requires further investigation. 18

19 References. 1. An, J. et al., Domain (Grain) Boundaries and Evidence of Twinlike Structures in Chemically Vapor Deposited Grown Graphene. ACS Nano, 5(4), 2433 (2011). 2. Huang, P. Y. et al. Grains and grain boundaries in single-layer graphene atomic patchwork quilts. Nature. 469, (2011). 3. Gunes, F. et al. UV-Light-Assisted Oxidative sp 3 Hybridization of Graphene. NANO 6, ( 2011). 4. Aguirre, C. M. et al. The Role of the Oxygen/Water Redox Couple in Suppressing Electron Conduction in Field-Effect Transistors. Adv. Mater. 21, (2009). 5. Duong, D. L., Lee, S. M. and Lee, Y. H. Origin of unipolarity in carbon nanotube field effect transistors. J. Mater. Chem. 22, (2012) Chen, S. et al. Oxidation Resistance of Graphene-Coated Cu and Cu/Ni Alloy. ACS Nano 5, (2011). 7. Han, G. H. et al. Influence of Copper Morphology in Forming Nucleation Seeds for Graphene Growth. Nano Lett. 11, (2011). 8. Chae, S. J. et al. Synthesis of Large-Area Graphene Layers on Poly-Nickel Substrate by Chemical Vapor Deposition: Wrinkle Formation. Adv. Matter. 21, (2009). 9. Lee, K. H. et al. Large-scale synthesis of high-quality hexagonal boron nitride nanosheets for large-area graphene electronics. Nano Lett. 12, (2012) 19