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Atomic scale graphene landscapes ndash naturaldosed and dopedTo cite this article U Bangert et al 2010 J Phys Conf Ser 241 012098

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Atomic scale graphene landscapes - natural dosed and doped

U Bangert1 M Gass2 A L Bleloch2 and R R Nair13

1 School of Materials The University of Manchester PO Box 88 Manchester M1 7HS UK 2 SuperSTEM STFC Daresbury Laboratory Warrington WA4 4AD UK 3 School of Physics The University of Manchester PO Box 88 Manchester M1 7HS UK

E-mail urselbangertmanchesteracuk

Abstract Graphene surfaces are scrutinised for topographic peculiarities which occur naturally or have been introduced High-angle-annular-dark-field (HAADF) and bright-field lattice images of graphene acquired in an aberration-corrected scanning transmission electron microscope (STEM) in conjunction with electron energy-loss spectroscopy (EELS) show vacancy- and ad-atom-related point defects as well as the presence of hydrogen In boron implanted graphene the edges of graphene sheets appear to be capture centres for boron atoms Furthermore rippling effects in the graphene sheets can be revealed by fast Fourier transform (FFT) procedures these help visualise changes in the bond length projection arising from inclinations of the sheet

1 Introduction In the last five years graphene has become one of the most famous and well researched materials We are interested in atomic detail of these 2-D crystals high-angle-annular-dark-field (HAADF) lattice images of graphene acquired in a scanning transmission electron microscope (STEM) with the electron beam focussed onto the sheet are a direct representation of the ball-and-stick model of the atomic lattice bright contrast corresponds to atoms and dark contrast to the empty space in between In addition variations in brightness can be attributed to ad-atoms The stability of freely suspended extended two-dimensional structures represents a long-standing theoretical debate and it has previously been suggested that graphene is crinkled or that defects might act as stabilisers Here we investigate phenomena related to the stability issue such as the structure of point defects - vacancy related as well as arising from carbon ad-atoms

and also sheet undulations via atomic resolution imaging in HAADF (and simultaneously in BF) in an aberration-corrected 100 keV STEM Doping and dosing of graphene has become an important issue for electronic functionalisation in this context the capability of hydrogen uptake and incorporation of functional impurities (ie boron) is of interest both of which we will address

2 Experimental The experiments reported here were conducted on an aberration-corrected dedicated scanning transmission electron microscope [1-3] with probe size 1Aring (the Daresbury SuperSTEM) equipped with an Enfina electron energy-loss (EEL) spectrometer The operating voltage was mostly 100 keV some spectrum images were obtained at 80 keV The angle of the HAADF detector annulus is 70-210 mrad the instrumental resolution for EEL spectroscopy 028 eV

Electron Microscopy and Analysis Group Conference 2009 (EMAG 2009) IOP PublishingJournal of Physics Conference Series 241 (2010) 012098 doi1010881742-65962411012098

ccopy 2010 Published under licence by IOP Publishing Ltd 1

Graphene samples were obtained by micromechanical cleavage a method which has been extensively reported [4] Freely suspended graphene sheets detailed in figure 2 were exposed to a hydrogen plasma and those detailed in figure 3 were subjected to boron implantation in a low energy ion implanter at 100 eV and a dose of 1014 cm-2 corresponding to one boron atom nm -2

3 Results and discussion Figure 1(a) details an HAADF STEM image of single layer graphene following application of the low-pass filter shown in the top left inset in order to remove noise from the images The chicken-wire contrast arises from carbon atom rings with the atoms positioned at the junctions However the graphene lattice is not of uniform contrast some of the bridges between atoms are significantly brighter The green spots mark such bridges they have been obtained by setting an intensity saturation threshold at twice the intensity of that of the surrounding atoms The blue intensity traces (top curves) in figure 1(c) demonstrate this effect further these traces are acquired along C-C bonds across locations of the highlighted (green) patches (1a) where the HAADF intensity is twice that of the neighbouring carbon atoms This indicates existence of carbon ad-atoms bridging C-C bonds in the 6-rings The red intensity trace (bootom curves) in figure 1(c) is acquired in vacuum and is of a similar level as the intensity in the centre of the 6-rings The arrow in figure 1(a) points to a vacancy The contrast is straight-forwardly interpretable black areas represent empty space and hence missing atoms can be directly seen Vacancies or aggregates thereof are commonly observed in atomic resolution HAADF images [5] and thought to be introduced by the electron beam in the microscope

The image in figure 1(b) is obtained of the same area after applying the ring-filter in the inset (top left the procedure and results are detailed in [6]) By imposing a narrow ring mask on the Fourier transform (FFT) of the raw images (with band width corresponding to ~004 Aring) spatial frequencies within ~2 of the frequency corresponding to the real space graphitic a-plane distance are passed The inverse transform (IFFT) reveals locations of atoms with correct lattice spacings although with possible rotations through intense lattice fringes The colour (intensity) coding in figure 1(b) is chosen such that atoms possessing the correct bond length appear yellow (bright) Should the projected bond length change by as little as 2 owing to out-of-plane bending of the atoms the lattice periodicity will become less visible in the IFFT Such regions are made to appear blue (dark) Atomic scale detail can be seen as a fine raster superimposed on the colours Thus undulations of up to 05 nm in amplitude were observed everywhere in freely suspended mono-layer graphene patches in the absence of visible topological defects Figure 1(d) provides a larger area view of this scenario However the ripple patterns can be influenced by point defects as shown in figure 1(b) here the vacancy (arrowed in figure 1(a) causes deformation in an extensive surrounding area

Figure 1 (a) HAADF image of pristine graphene the inset shows the low frequency bandpass filter applied to the FFT of the original raw image to obtain image (a) green spots mark locations of enhanced intensity (b) IFFT obtained from same area as in (a) with the ring filter overlaid on the FFT of the original image the procedure is explained in detail in [6] (c) intensity traces (blue lines top) along sides of hexagons across locations of enhanced intensity (marked green) the red lower trace is of vacuum (d) IFFT as in (b) but of a larger area

a) d)

b)

c)

a) d)

b)

c)

a) d)

b)

c)

a) d)

b)

c)

Electron Microscopy and Analysis Group Conference 2009 (EMAG 2009) IOP PublishingJournal of Physics Conference Series 241 (2010) 012098 doi1010881742-65962411012098

2

manifest in the IFFT (in figure 1(b) by the blue (dark) shadesr which relate to projected bond shortening of gt5 Here the sheet is bent the ripple pattern appears to be the result of local changes in stiffness which is reduced at vacancies (occurring in flanks of ripples) and enhanced in regions with ad-atoms (which are flatter)

Carbon and oxygen ad-atoms have previously been discussed [78] Furthermore the possibility for atomic hydrogen chemi- or physisorbed onto graphite surfaces [9] and graphene [10] has been researched theoretically and experimentally [11] and direct experimental observation has been attempted via transmission electron microscopy imaging [7] In order to assess hydrogen adsorption onto graphene we have carried out electron energy-loss spectroscopy (EELS) We applied the spectrum imaging method [12] and scrutinised the EEL spectrum images for occurrence of the core-level excitation signal of hydrogen (around 13 eV) Figures 2(c) and (f) show low loss spectra extracted from spectrum images (SIs) obtained of the graphene patches shown in figures 2(a) and (d) typically a few 100 nm

2 in size The pixel area in the SIs was ~1 nm2 Spectrum (g) in figure 2 is from a pixel in a hydrogen-free graphene patch All spectra are power-law background subtracted and start with the rise of the +

plasmon at 147 eV [13] the signature of mono-layer

graphene Principal component analysis (PCA) was performed on the spectrum images [eg 14] in order to reduce noise To obtain hydrogen-free areas few layer graphene regions were repeatedly scanned The scan raster was successively decreased in total size and HAADF images in combination with EEL measurements revealed areas being peeled off layer-by-layer until finally a hole appeared Spectra of such freshly revealed areas do not show a bump at 13 eV as can be witnessed in figure 2(g) The dashed line shows a Lorentzian fit to the rise of the +

plasmon this curve presents a perfect fit for hydrogen-free spectra and was used as plasmon background for spectra detailed in figure 2(c) and (f) Curve (f) shows a spectrum from a pixel in the SI-raster detailed in figure 2(e) of pristine graphene exhibiting a bump on the +

plasmon and hence revealing presence of hydrogen The hydrogen seems to be omni-present and possibly marginally accumulated in the vicinity of hydro-carbon contamination (white clouds in the image in figure 2(d) Spectrum (c) is from a pixel in the SI-raster detailed in figure 2(b) of single layer graphene which has been exposed to a hydrogen plasma ie intentionally dosed The bump at 13 eV is more pronounced here This is further illustrated in the hydrogen distribution maps in figure 2(b) and (e) which show relative intensities of the signals in each pixel at ~13eV following subtraction of the Lorentzian background curve The signals were integrated and normalised to the total spectrum intensity These hydrogen distribution maps reveal that the hydrogen-related intensity in the hydrogen -dosed film is about twice that of the pristine film

eV10 15 20 25

g)

c)

f)

126-136 eV

126-136 eV

same intensity scaled)

a)

e)

increasing signal intensity

b)

eV10 15 20 25

g)

c)

f)

126-136 eV

126-136 eV

same intensity scaled)

a)

e)

increasing signal intensity

b)

Figure 2 (a) HAADF image of spectrum image region in hydrogen-dosed graphene The whitish contrast in HAADF images arises from hydro-carbon deposits (b) Integrated intensity of the residual signal after - and

+ -plasmon removal in the energy window 126-136 eV (c) Power-law background-fitted spectrum from pixel in (b) d) HAADF image of pristine graphene with the spectrum image region

region framed (e) Intensity of the residual signal following the same procedures as in (b) (f) Power-law background-fitted spectrum from pixel in (e) and (g) from hydrogen-free graphene The frame width of the HAADF images is 30 nm

Electron Microscopy and Analysis Group Conference 2009 (EMAG 2009) IOP PublishingJournal of Physics Conference Series 241 (2010) 012098 doi1010881742-65962411012098

3

Intentional doping with boron via low energy ion implantation revealed a boron retention of less than 1 at implantation energies of 100 eV Furthermore it occurs that the boron preferably attaches to edges of graphene sheets figure 3(a) shows a BF HREM STEM image of the area from which a SI was acquired The edge of a mono-layer graphene sheet can be seen to run diagonally (right top to left middle) across the image with edges of a further four staggered sheets located inwards from the single sheet edge The black dots are positions of SI pixels where single atom boron K-edge signals (188 eV losses) were detected via EELS (Details of this study are presented elsewhere [15]) Most of these positions line up with the staggered edges Furthermore the boron edge shape (figure 3(b) is not that of substitutional B but rather resembles that of amorphous boron or of B4C This leads us to conclude that ion implantation doping of graphene at energies lt100 eV does not lead to effective substitutional incorporation for electronic modification

4 Conclusion We have demonstrated that free-standing monolayer graphene is remote from the ideal structural model but possesses an intricate surface topology owing to undulations and point defects (ad-atoms and point defects) The former always occur but also specifically arise from the latter Hydrogen is present on all graphene surfaces but the coverage is increased by dosing Intentional boron doping via low-energy ion implantation has so far predominantly led to boron ad-atom incorporation at graphene sheet edges rather than substitution

5 References [1] Krivanek O L Delby N and Lupini A R 1999 Ultramicroscopy 78 1 [2] Howie A 1979 J Microscopy 117 11 [3] Pennycook S and Boatner L A 1988 Nature 336 565 [4] Booth T J Blake P Nair R R Jiang D Hill E W Bangert U Bleloch A Gass M

Novoselov K S Katsnelson M I and Geim A K 2008 Nano Lett 2442 [5] Gass M H Bangert U Bleloch A L Wang P Nair R R and Geim A K 2008 Nature

Nanotech 3 676 [6] Bangert U Gass M H Bleloch A L Nair R R and Geim A K 2009 Phys Stat Sol A

206 1117 [7] Meyer J C Girit C O Crommie M F and Zettl A 2008 Nature 454 319 [8] Nordlund K Keinonen J and Mattila T 1996 Phys Rev Lett 77 699 [9] Jeloaica L and Sidis V 1999 Chem Phys Lett 300 157 [10] Ito A Nakamura H and Takayama A preprint at httparxivorgabscondmat0703377

[11] Elias D C et al 2009 Science 323 610 [12] Jeanguillaume C and Colliex C 1989 Ultramicroscopy 28 252 [13] Eberlein T et al 2008 Phys Rev B 77 233406 [14] Borglund N Astrand P-G and Csillag S 2005 Microsc Microanal 11 88 [15] Bangert U Seepujak A Gass M H and Bleloch A L accepted y Phys Rev B

eV

180 200 220

a)

b)

eV

180 200 220

eV

180 200 220

eV

180 200 220

a)

b)

Figure 3 (a) HREM BF STEM image of boron implanted (~1 at nm-2) staggered graphene sheets (leftmost edge is of a single sheet edges of subsequent sheets occur as frayed andor parallel lines) black dots are locations of pixels with B K-edge signals in the corresponding SI (b) examples of signals extracted from pixels of black (B-K edge black line) and unmarked (no signal grey line) locations

Electron Microscopy and Analysis Group Conference 2009 (EMAG 2009) IOP PublishingJournal of Physics Conference Series 241 (2010) 012098 doi1010881742-65962411012098

4

Graphene samples were obtained by micromechanical cleavage a method which has been extensively reported [4] Freely suspended graphene sheets detailed in figure 2 were exposed to a hydrogen plasma and those detailed in figure 3 were subjected to boron implantation in a low energy ion implanter at 100 eV and a dose of 1014 cm-2 corresponding to one boron atom nm -2

3 Results and discussion Figure 1(a) details an HAADF STEM image of single layer graphene following application of the low-pass filter shown in the top left inset in order to remove noise from the images The chicken-wire contrast arises from carbon atom rings with the atoms positioned at the junctions However the graphene lattice is not of uniform contrast some of the bridges between atoms are significantly brighter The green spots mark such bridges they have been obtained by setting an intensity saturation threshold at twice the intensity of that of the surrounding atoms The blue intensity traces (top curves) in figure 1(c) demonstrate this effect further these traces are acquired along C-C bonds across locations of the highlighted (green) patches (1a) where the HAADF intensity is twice that of the neighbouring carbon atoms This indicates existence of carbon ad-atoms bridging C-C bonds in the 6-rings The red intensity trace (bootom curves) in figure 1(c) is acquired in vacuum and is of a similar level as the intensity in the centre of the 6-rings The arrow in figure 1(a) points to a vacancy The contrast is straight-forwardly interpretable black areas represent empty space and hence missing atoms can be directly seen Vacancies or aggregates thereof are commonly observed in atomic resolution HAADF images [5] and thought to be introduced by the electron beam in the microscope

The image in figure 1(b) is obtained of the same area after applying the ring-filter in the inset (top left the procedure and results are detailed in [6]) By imposing a narrow ring mask on the Fourier transform (FFT) of the raw images (with band width corresponding to ~004 Aring) spatial frequencies within ~2 of the frequency corresponding to the real space graphitic a-plane distance are passed The inverse transform (IFFT) reveals locations of atoms with correct lattice spacings although with possible rotations through intense lattice fringes The colour (intensity) coding in figure 1(b) is chosen such that atoms possessing the correct bond length appear yellow (bright) Should the projected bond length change by as little as 2 owing to out-of-plane bending of the atoms the lattice periodicity will become less visible in the IFFT Such regions are made to appear blue (dark) Atomic scale detail can be seen as a fine raster superimposed on the colours Thus undulations of up to 05 nm in amplitude were observed everywhere in freely suspended mono-layer graphene patches in the absence of visible topological defects Figure 1(d) provides a larger area view of this scenario However the ripple patterns can be influenced by point defects as shown in figure 1(b) here the vacancy (arrowed in figure 1(a) causes deformation in an extensive surrounding area

Figure 1 (a) HAADF image of pristine graphene the inset shows the low frequency bandpass filter applied to the FFT of the original raw image to obtain image (a) green spots mark locations of enhanced intensity (b) IFFT obtained from same area as in (a) with the ring filter overlaid on the FFT of the original image the procedure is explained in detail in [6] (c) intensity traces (blue lines top) along sides of hexagons across locations of enhanced intensity (marked green) the red lower trace is of vacuum (d) IFFT as in (b) but of a larger area

a) d)

b)

c)

a) d)

b)

c)

a) d)

b)

c)

a) d)

b)

c)

Electron Microscopy and Analysis Group Conference 2009 (EMAG 2009) IOP PublishingJournal of Physics Conference Series 241 (2010) 012098 doi1010881742-65962411012098

2

manifest in the IFFT (in figure 1(b) by the blue (dark) shadesr which relate to projected bond shortening of gt5 Here the sheet is bent the ripple pattern appears to be the result of local changes in stiffness which is reduced at vacancies (occurring in flanks of ripples) and enhanced in regions with ad-atoms (which are flatter)

Carbon and oxygen ad-atoms have previously been discussed [78] Furthermore the possibility for atomic hydrogen chemi- or physisorbed onto graphite surfaces [9] and graphene [10] has been researched theoretically and experimentally [11] and direct experimental observation has been attempted via transmission electron microscopy imaging [7] In order to assess hydrogen adsorption onto graphene we have carried out electron energy-loss spectroscopy (EELS) We applied the spectrum imaging method [12] and scrutinised the EEL spectrum images for occurrence of the core-level excitation signal of hydrogen (around 13 eV) Figures 2(c) and (f) show low loss spectra extracted from spectrum images (SIs) obtained of the graphene patches shown in figures 2(a) and (d) typically a few 100 nm

2 in size The pixel area in the SIs was ~1 nm2 Spectrum (g) in figure 2 is from a pixel in a hydrogen-free graphene patch All spectra are power-law background subtracted and start with the rise of the +

plasmon at 147 eV [13] the signature of mono-layer

graphene Principal component analysis (PCA) was performed on the spectrum images [eg 14] in order to reduce noise To obtain hydrogen-free areas few layer graphene regions were repeatedly scanned The scan raster was successively decreased in total size and HAADF images in combination with EEL measurements revealed areas being peeled off layer-by-layer until finally a hole appeared Spectra of such freshly revealed areas do not show a bump at 13 eV as can be witnessed in figure 2(g) The dashed line shows a Lorentzian fit to the rise of the +

plasmon this curve presents a perfect fit for hydrogen-free spectra and was used as plasmon background for spectra detailed in figure 2(c) and (f) Curve (f) shows a spectrum from a pixel in the SI-raster detailed in figure 2(e) of pristine graphene exhibiting a bump on the +

plasmon and hence revealing presence of hydrogen The hydrogen seems to be omni-present and possibly marginally accumulated in the vicinity of hydro-carbon contamination (white clouds in the image in figure 2(d) Spectrum (c) is from a pixel in the SI-raster detailed in figure 2(b) of single layer graphene which has been exposed to a hydrogen plasma ie intentionally dosed The bump at 13 eV is more pronounced here This is further illustrated in the hydrogen distribution maps in figure 2(b) and (e) which show relative intensities of the signals in each pixel at ~13eV following subtraction of the Lorentzian background curve The signals were integrated and normalised to the total spectrum intensity These hydrogen distribution maps reveal that the hydrogen-related intensity in the hydrogen -dosed film is about twice that of the pristine film

eV10 15 20 25

g)

c)

f)

126-136 eV

126-136 eV

same intensity scaled)

a)

e)

increasing signal intensity

b)

eV10 15 20 25

g)

c)

f)

126-136 eV

126-136 eV

same intensity scaled)

a)

e)

increasing signal intensity

b)

Figure 2 (a) HAADF image of spectrum image region in hydrogen-dosed graphene The whitish contrast in HAADF images arises from hydro-carbon deposits (b) Integrated intensity of the residual signal after - and

+ -plasmon removal in the energy window 126-136 eV (c) Power-law background-fitted spectrum from pixel in (b) d) HAADF image of pristine graphene with the spectrum image region

region framed (e) Intensity of the residual signal following the same procedures as in (b) (f) Power-law background-fitted spectrum from pixel in (e) and (g) from hydrogen-free graphene The frame width of the HAADF images is 30 nm

Electron Microscopy and Analysis Group Conference 2009 (EMAG 2009) IOP PublishingJournal of Physics Conference Series 241 (2010) 012098 doi1010881742-65962411012098

3

Intentional doping with boron via low energy ion implantation revealed a boron retention of less than 1 at implantation energies of 100 eV Furthermore it occurs that the boron preferably attaches to edges of graphene sheets figure 3(a) shows a BF HREM STEM image of the area from which a SI was acquired The edge of a mono-layer graphene sheet can be seen to run diagonally (right top to left middle) across the image with edges of a further four staggered sheets located inwards from the single sheet edge The black dots are positions of SI pixels where single atom boron K-edge signals (188 eV losses) were detected via EELS (Details of this study are presented elsewhere [15]) Most of these positions line up with the staggered edges Furthermore the boron edge shape (figure 3(b) is not that of substitutional B but rather resembles that of amorphous boron or of B4C This leads us to conclude that ion implantation doping of graphene at energies lt100 eV does not lead to effective substitutional incorporation for electronic modification

4 Conclusion We have demonstrated that free-standing monolayer graphene is remote from the ideal structural model but possesses an intricate surface topology owing to undulations and point defects (ad-atoms and point defects) The former always occur but also specifically arise from the latter Hydrogen is present on all graphene surfaces but the coverage is increased by dosing Intentional boron doping via low-energy ion implantation has so far predominantly led to boron ad-atom incorporation at graphene sheet edges rather than substitution

5 References [1] Krivanek O L Delby N and Lupini A R 1999 Ultramicroscopy 78 1 [2] Howie A 1979 J Microscopy 117 11 [3] Pennycook S and Boatner L A 1988 Nature 336 565 [4] Booth T J Blake P Nair R R Jiang D Hill E W Bangert U Bleloch A Gass M

Novoselov K S Katsnelson M I and Geim A K 2008 Nano Lett 2442 [5] Gass M H Bangert U Bleloch A L Wang P Nair R R and Geim A K 2008 Nature

Nanotech 3 676 [6] Bangert U Gass M H Bleloch A L Nair R R and Geim A K 2009 Phys Stat Sol A

206 1117 [7] Meyer J C Girit C O Crommie M F and Zettl A 2008 Nature 454 319 [8] Nordlund K Keinonen J and Mattila T 1996 Phys Rev Lett 77 699 [9] Jeloaica L and Sidis V 1999 Chem Phys Lett 300 157 [10] Ito A Nakamura H and Takayama A preprint at httparxivorgabscondmat0703377

[11] Elias D C et al 2009 Science 323 610 [12] Jeanguillaume C and Colliex C 1989 Ultramicroscopy 28 252 [13] Eberlein T et al 2008 Phys Rev B 77 233406 [14] Borglund N Astrand P-G and Csillag S 2005 Microsc Microanal 11 88 [15] Bangert U Seepujak A Gass M H and Bleloch A L accepted y Phys Rev B

eV

180 200 220

a)

b)

eV

180 200 220

eV

180 200 220

eV

180 200 220

a)

b)

Figure 3 (a) HREM BF STEM image of boron implanted (~1 at nm-2) staggered graphene sheets (leftmost edge is of a single sheet edges of subsequent sheets occur as frayed andor parallel lines) black dots are locations of pixels with B K-edge signals in the corresponding SI (b) examples of signals extracted from pixels of black (B-K edge black line) and unmarked (no signal grey line) locations

Electron Microscopy and Analysis Group Conference 2009 (EMAG 2009) IOP PublishingJournal of Physics Conference Series 241 (2010) 012098 doi1010881742-65962411012098

4

manifest in the IFFT (in figure 1(b) by the blue (dark) shadesr which relate to projected bond shortening of gt5 Here the sheet is bent the ripple pattern appears to be the result of local changes in stiffness which is reduced at vacancies (occurring in flanks of ripples) and enhanced in regions with ad-atoms (which are flatter)

Carbon and oxygen ad-atoms have previously been discussed [78] Furthermore the possibility for atomic hydrogen chemi- or physisorbed onto graphite surfaces [9] and graphene [10] has been researched theoretically and experimentally [11] and direct experimental observation has been attempted via transmission electron microscopy imaging [7] In order to assess hydrogen adsorption onto graphene we have carried out electron energy-loss spectroscopy (EELS) We applied the spectrum imaging method [12] and scrutinised the EEL spectrum images for occurrence of the core-level excitation signal of hydrogen (around 13 eV) Figures 2(c) and (f) show low loss spectra extracted from spectrum images (SIs) obtained of the graphene patches shown in figures 2(a) and (d) typically a few 100 nm

2 in size The pixel area in the SIs was ~1 nm2 Spectrum (g) in figure 2 is from a pixel in a hydrogen-free graphene patch All spectra are power-law background subtracted and start with the rise of the +

plasmon at 147 eV [13] the signature of mono-layer

graphene Principal component analysis (PCA) was performed on the spectrum images [eg 14] in order to reduce noise To obtain hydrogen-free areas few layer graphene regions were repeatedly scanned The scan raster was successively decreased in total size and HAADF images in combination with EEL measurements revealed areas being peeled off layer-by-layer until finally a hole appeared Spectra of such freshly revealed areas do not show a bump at 13 eV as can be witnessed in figure 2(g) The dashed line shows a Lorentzian fit to the rise of the +

plasmon this curve presents a perfect fit for hydrogen-free spectra and was used as plasmon background for spectra detailed in figure 2(c) and (f) Curve (f) shows a spectrum from a pixel in the SI-raster detailed in figure 2(e) of pristine graphene exhibiting a bump on the +

plasmon and hence revealing presence of hydrogen The hydrogen seems to be omni-present and possibly marginally accumulated in the vicinity of hydro-carbon contamination (white clouds in the image in figure 2(d) Spectrum (c) is from a pixel in the SI-raster detailed in figure 2(b) of single layer graphene which has been exposed to a hydrogen plasma ie intentionally dosed The bump at 13 eV is more pronounced here This is further illustrated in the hydrogen distribution maps in figure 2(b) and (e) which show relative intensities of the signals in each pixel at ~13eV following subtraction of the Lorentzian background curve The signals were integrated and normalised to the total spectrum intensity These hydrogen distribution maps reveal that the hydrogen-related intensity in the hydrogen -dosed film is about twice that of the pristine film

eV10 15 20 25

g)

c)

f)

126-136 eV

126-136 eV

same intensity scaled)

a)

e)

increasing signal intensity

b)

eV10 15 20 25

g)

c)

f)

126-136 eV

126-136 eV

same intensity scaled)

a)

e)

increasing signal intensity

b)

Figure 2 (a) HAADF image of spectrum image region in hydrogen-dosed graphene The whitish contrast in HAADF images arises from hydro-carbon deposits (b) Integrated intensity of the residual signal after - and

+ -plasmon removal in the energy window 126-136 eV (c) Power-law background-fitted spectrum from pixel in (b) d) HAADF image of pristine graphene with the spectrum image region

region framed (e) Intensity of the residual signal following the same procedures as in (b) (f) Power-law background-fitted spectrum from pixel in (e) and (g) from hydrogen-free graphene The frame width of the HAADF images is 30 nm

Electron Microscopy and Analysis Group Conference 2009 (EMAG 2009) IOP PublishingJournal of Physics Conference Series 241 (2010) 012098 doi1010881742-65962411012098

3

Intentional doping with boron via low energy ion implantation revealed a boron retention of less than 1 at implantation energies of 100 eV Furthermore it occurs that the boron preferably attaches to edges of graphene sheets figure 3(a) shows a BF HREM STEM image of the area from which a SI was acquired The edge of a mono-layer graphene sheet can be seen to run diagonally (right top to left middle) across the image with edges of a further four staggered sheets located inwards from the single sheet edge The black dots are positions of SI pixels where single atom boron K-edge signals (188 eV losses) were detected via EELS (Details of this study are presented elsewhere [15]) Most of these positions line up with the staggered edges Furthermore the boron edge shape (figure 3(b) is not that of substitutional B but rather resembles that of amorphous boron or of B4C This leads us to conclude that ion implantation doping of graphene at energies lt100 eV does not lead to effective substitutional incorporation for electronic modification

4 Conclusion We have demonstrated that free-standing monolayer graphene is remote from the ideal structural model but possesses an intricate surface topology owing to undulations and point defects (ad-atoms and point defects) The former always occur but also specifically arise from the latter Hydrogen is present on all graphene surfaces but the coverage is increased by dosing Intentional boron doping via low-energy ion implantation has so far predominantly led to boron ad-atom incorporation at graphene sheet edges rather than substitution

5 References [1] Krivanek O L Delby N and Lupini A R 1999 Ultramicroscopy 78 1 [2] Howie A 1979 J Microscopy 117 11 [3] Pennycook S and Boatner L A 1988 Nature 336 565 [4] Booth T J Blake P Nair R R Jiang D Hill E W Bangert U Bleloch A Gass M

Novoselov K S Katsnelson M I and Geim A K 2008 Nano Lett 2442 [5] Gass M H Bangert U Bleloch A L Wang P Nair R R and Geim A K 2008 Nature

Nanotech 3 676 [6] Bangert U Gass M H Bleloch A L Nair R R and Geim A K 2009 Phys Stat Sol A

206 1117 [7] Meyer J C Girit C O Crommie M F and Zettl A 2008 Nature 454 319 [8] Nordlund K Keinonen J and Mattila T 1996 Phys Rev Lett 77 699 [9] Jeloaica L and Sidis V 1999 Chem Phys Lett 300 157 [10] Ito A Nakamura H and Takayama A preprint at httparxivorgabscondmat0703377

[11] Elias D C et al 2009 Science 323 610 [12] Jeanguillaume C and Colliex C 1989 Ultramicroscopy 28 252 [13] Eberlein T et al 2008 Phys Rev B 77 233406 [14] Borglund N Astrand P-G and Csillag S 2005 Microsc Microanal 11 88 [15] Bangert U Seepujak A Gass M H and Bleloch A L accepted y Phys Rev B

eV

180 200 220

a)

b)

eV

180 200 220

eV

180 200 220

eV

180 200 220

a)

b)

Figure 3 (a) HREM BF STEM image of boron implanted (~1 at nm-2) staggered graphene sheets (leftmost edge is of a single sheet edges of subsequent sheets occur as frayed andor parallel lines) black dots are locations of pixels with B K-edge signals in the corresponding SI (b) examples of signals extracted from pixels of black (B-K edge black line) and unmarked (no signal grey line) locations

Electron Microscopy and Analysis Group Conference 2009 (EMAG 2009) IOP PublishingJournal of Physics Conference Series 241 (2010) 012098 doi1010881742-65962411012098

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Intentional doping with boron via low energy ion implantation revealed a boron retention of less than 1 at implantation energies of 100 eV Furthermore it occurs that the boron preferably attaches to edges of graphene sheets figure 3(a) shows a BF HREM STEM image of the area from which a SI was acquired The edge of a mono-layer graphene sheet can be seen to run diagonally (right top to left middle) across the image with edges of a further four staggered sheets located inwards from the single sheet edge The black dots are positions of SI pixels where single atom boron K-edge signals (188 eV losses) were detected via EELS (Details of this study are presented elsewhere [15]) Most of these positions line up with the staggered edges Furthermore the boron edge shape (figure 3(b) is not that of substitutional B but rather resembles that of amorphous boron or of B4C This leads us to conclude that ion implantation doping of graphene at energies lt100 eV does not lead to effective substitutional incorporation for electronic modification

4 Conclusion We have demonstrated that free-standing monolayer graphene is remote from the ideal structural model but possesses an intricate surface topology owing to undulations and point defects (ad-atoms and point defects) The former always occur but also specifically arise from the latter Hydrogen is present on all graphene surfaces but the coverage is increased by dosing Intentional boron doping via low-energy ion implantation has so far predominantly led to boron ad-atom incorporation at graphene sheet edges rather than substitution

5 References [1] Krivanek O L Delby N and Lupini A R 1999 Ultramicroscopy 78 1 [2] Howie A 1979 J Microscopy 117 11 [3] Pennycook S and Boatner L A 1988 Nature 336 565 [4] Booth T J Blake P Nair R R Jiang D Hill E W Bangert U Bleloch A Gass M

Novoselov K S Katsnelson M I and Geim A K 2008 Nano Lett 2442 [5] Gass M H Bangert U Bleloch A L Wang P Nair R R and Geim A K 2008 Nature

Nanotech 3 676 [6] Bangert U Gass M H Bleloch A L Nair R R and Geim A K 2009 Phys Stat Sol A

206 1117 [7] Meyer J C Girit C O Crommie M F and Zettl A 2008 Nature 454 319 [8] Nordlund K Keinonen J and Mattila T 1996 Phys Rev Lett 77 699 [9] Jeloaica L and Sidis V 1999 Chem Phys Lett 300 157 [10] Ito A Nakamura H and Takayama A preprint at httparxivorgabscondmat0703377

[11] Elias D C et al 2009 Science 323 610 [12] Jeanguillaume C and Colliex C 1989 Ultramicroscopy 28 252 [13] Eberlein T et al 2008 Phys Rev B 77 233406 [14] Borglund N Astrand P-G and Csillag S 2005 Microsc Microanal 11 88 [15] Bangert U Seepujak A Gass M H and Bleloch A L accepted y Phys Rev B

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Figure 3 (a) HREM BF STEM image of boron implanted (~1 at nm-2) staggered graphene sheets (leftmost edge is of a single sheet edges of subsequent sheets occur as frayed andor parallel lines) black dots are locations of pixels with B K-edge signals in the corresponding SI (b) examples of signals extracted from pixels of black (B-K edge black line) and unmarked (no signal grey line) locations

Electron Microscopy and Analysis Group Conference 2009 (EMAG 2009) IOP PublishingJournal of Physics Conference Series 241 (2010) 012098 doi1010881742-65962411012098

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