Investigation of the effect of low energy ion beam irradiation on monolayer graphene

In this paper, the effect of low energy irradiation on mono-layer graphene was studied. Mono-layer graphene films were irradiated with B, N and F ions at different energy and fluence. X-ray photoelectron spectroscopy indicates that foreign ions implanted at ion energies below 35 eV could dope into the graphene lattice and form new chemical bonds with carbon atoms. The results of Raman measurement indicate that ion beam irradiation causes defects and disorder to the graphene crystal structure, and the level of defects increases with increasing of ion energy and fluence. Surface morphology images also prove that ion beam irradiation creates damages to graphene film. The experiment results suggest that low-energy irradiation with energies of about 30 eV and fluences up to 5·1014 cm−2 could realize small amount of doping, while introducing weak damage to graphene. Low energy ion beam irradiation, provides a promising approach for controlled doping of graphene.


I. INTRODUCTION
Graphene is a one-atom-thick planar sheet of sp 2 -bonded carbon atoms that are densely packed in a honeycomb crystal lattice.Discovered in 2004, 1 graphene is attracting tremendous interest due to its remarkable electrical, 2 mechanical, 3 and thermal properties. 4Graphene has a wide range of applications, for example it has the potential for technological applications as a successor of silicon in the post Moore's law era, 5,6 as a single-molecule gas sensor 7 in spintronics, [8][9][10] in quantum computing 11 or as a terahertz oscillator. 126][17][18] However, to realize applications of graphene-based electronic devices, it is necessary to control the electronic properties of the graphene samples.Doping with foreign atoms is a standard way to modify the electronic properties of a material.Although it is possible to introduction of foreign atoms into graphene during synthesis, [19][20][21] few studies have investigated the introducing foreign atoms into graphene after growth.Ion beam irradiation is routinely used nowadays when manufacturing conventional semiconductor devices.It can be expected that in addition to substitute foreign atoms, ion beam irradiation would inevitably result in the formation of various irradiation induced defects in graphene.These defects are of great theoretical interest [22][23][24] as a potential source of intervalley scattering, which in principle transforms graphene from a metal to an insulator, 24,25 so it is very important to understand their effect on electronic properties of graphene.However, as a xmwu@suda.deu.cn and hhofsae@uni-goettingen.de  an atomically thin target, graphene has a significantly different response to ion beam irradiation as compared to traditional three-dimensional materials. 26Therefore the study on effects of ion beam irradiation on mono-layer graphene is of particular importance.Recently, some experimental studies of ion beam irradiation on graphene has been reported, [27][28][29] most of them use very high energy ion beam (30 KeV).However, due to high irradiation energy, great damages will be introduced and very few if any ions were directly incorporated to the graphene sheet.Upon this reason, in this paper we investigated the effects of low ion energy (lower than 50 eV) irradiation on graphene.

II. EXPERIMENTAL DETAILS
Graphene samples are obtained from Graphene-Supermarket, 30 mono-layer graphene is grown via CVD processing on a copper foil and then transferred to a silicon wafer covered with a SiO 2 layer.Each sample is 1 cm × 1 cm in size and graphene coverage is about 90%.Graphene film is polycrystalline which consists with grains with different crystallographic orientation.Graphene samples were irradiated with a world-wide unique mass selected ion beam deposition system. 31A 30 keV mass selected ion beam is homogenized using a beam sweep and cleaned from possibly neutralized ions by electrostatic deflection.Ions are then decelerated down to energies of 35 eV and below and ar incident onto the graphee layer parallel to the surface normal.The chamber vacuum was 10 −6 Pa during ion beam irradiation.Boron and nitrogen, which possess one electron less and more than carbon, respectively, and are also similar in size to carbon atom, may be incorporated into substitutional positions in carbon structures by replacing exactly one carbon atom or taking the vacant site at the edges of graphene films.Furthermore, it has been shown that B-or N-doped graphene displays p-or n-type behavior. 32Therefore, boron and nitrogen are the natural dopants for graphene.Recently, graphene fluoride, one of the thinnest binary compounds attracts enormous interest in new technological applications to batteries or wide-band gap semiconductors. 33,34 ased upon these reasons, boron, nitrogen and fluorine ions were chosen in our experiments.
To investigate the effect of different ions, energy and fluence on graphene samples, boron, nitrogen and fluorine ions with variety energy and fluence were used in our experiment, as shown in Table I.After ion beam irradiation, the graphene samples were transferred into a UHV system for X-ray photoelectron spectroscopy (XPS) measurement, which is directly connected to the irradiation chamber via a UHV transfer system and load-lock valve.Therefore, the samples were not exposed to air between irradiation and XPS measurements.A Mg Ka source (hv = 1253.6eV) was used, binding energies are referred to the Fermi level of a clean Au test sample and were calibrated using the Au 4 f 7/2 core line, for which a binding energy of 84.0 eV is assumed.An energy resolution of about 1 eV is obtained.For a detailed analysis, the core-level lines obtained by XPS were numerically fitted to Doniach-Sunjic functions. 35For studying the crystal structure of pristine and irradiated graphene samples, Raman spectra were collected under ambient conditions via a Raman Horiba HR800 with laser excitation at 633nm.A laser power of 30 mW was used to minimize the damage caused by laser heating.Scanning Electron Microscopy (SEM) LEO spura 35 was utilized to analyze the effect of ion beam irradiation on the surface morphology of the graphene samples.Because of its ultrathin thickness, graphene is more sensitive to low energy electrons than high energy electrons, electron beam of 1 KV was carried out in SEM measurement. 36The irradiation of graphene with low energy B, N and F ions was simulated using the SDTrimSP Monte Carlo program. 37We analyzed the collisions of 35eV and 20 eV N, B, and F ions with a carbon layer consisting of 3 • 10 15 atoms/cm 2 on Si.The results summarized in figure 1(a) to 1(d), take figure 1(a) as example, about 50% of the incident B ions come to stay in the C-layer, 50% in Si.Sputtering is <<1%, a small fraction of <2% per ion of the Si atoms may be recoiled into the C-layer.About 1/3 of the recoiled carbon goes to the Si and about 2/3 remain in the C-layer (15%/50% = 1/3, 35%/50%).All the simulation results show a significant fraction of replacement collisions between ions and carbon atoms of about 50-60% for 35 eV ions and about 80% for 20eV N ions.Therefore, low energy ion irradiation of graphene appears as a very efficient method to replace carbon atoms by dopant ions.

III. RESULTS AND DISCUSSIONS
A. Pristine graphene X-ray photoelectron spectroscopy is a surface chemical analysis technique that can be used to analyze the surface chemical structure of a material.Figure 2(a) shows the XPS spectrum of a pristine graphene sample on SiO 2 substrate, the peaks at binding energy of 104, 153, 284.8, and 531.9 eV correspond to Si 2p, Si 2s of SiO 2 substrate, C 1s of sp 2 -C, and O 1s of oxygen (oxygen from the underlying native SiO 2 layer and possibly also adsorbed oxygen), respectively.No B-C (∼190 eV), N-C (∼399 eV) and F-C (∼688 eV) bond were found.Figure 2(b) is the C 1s core-level spectrum of pristine graphene, the C 1s peak can be fitted into two peaks: a primary peak locates at binding energy of 285.1 eV with 85.28% integrated intensity dues to sp 2 hybridized C-C bonding, 38 the other peak at 286.77 eV with 14.72% integrated intensity dues to C-O bond. 39The much higher intensity of the sp 2 -C bond indicates that most of the carbon atoms in pristine graphene are arranged in honeycomb lattice.Because the samples were not exposed to air between irradiation and XPS measurements, the O peak is most probably due to the SiO 2 native layer on Si substrate.On the other hand, both theoretical and experimental studies reveal that oxygen could adsorb on the surface and edge of graphene.Therefore, in our experiment, the oxygen signal can also relate to oxygen adsorption onto the graphitic layers. 40,41 aman spectroscopy is a sensitive tool for investigating the structure of graphene material. 42Figure 2(c) displays the Raman spectrum of the pristine graphene.Three pronounced peaks of pristine graphene can be observed, which are D, G and 2D peaks appear around 1350, 1580 and 2700 cm −1 , respectively.The D peak dues to the breathing modes of sixatom rings and requires a defect for its activation, the G peak corresponds to the E 2g phonon at the Brillouin zone center. 43The defects in pristine graphene may come from lattice distortion during CVD cooling process or transfer from Cu foil to Si substrate.The 2D peak is the second order of the D peak, which is a single peak in mono-layer graphene. 44,45 he ratio of 2D to G peaks (I 2D /I G ) is 3 and the full-width at half-maximum (FWHM) of the 2D peak is 38 cm −1 , indicating that the graphene is mono-layer. 46,47 he intensity of D to G peaks (I D /I G ) is usually used to study the lattice defects of graphene.The I D /I G of pristine is 0.54, which reveals low level of defects and disorder in graphene lattice.SEM is usually utilized to analysis the surface morphology of graphene.Figure 2(d) displays some typical features of CVD graphene.The pristine graphene is continuous but covered with some ripples, these ripples are associated with the difference of thermal expansion coefficients between graphene and Cu during the CVD process. 44Because Cu has much larger thermal expansion coefficient than graphene, which means Cu shrinks much significantly during the cooling process, which induces mechanical stress on graphene, and this stress is released via the formation of ripples.When graphene films removed from Cu surface to a flat substrate, the graphene film can't fully contact with the substrate, thus resulting in the ridge-like surface structure of graphene.

B. Boron irradiation
Figure 3(a) and 3(c) are the B 1s XPS spectrum of sample S1 and sample S2.For the sample S1 a peak at 193.4 eV accounts for the asymmetry of the B 1s core level, Jimenez et al. 48identified similar states of single crystal boron carbide to B, B 2+ , and B 3+ ionization states, which they attributed to surface oxidation.For the sample S2, the up-shift of binding energy of the B 1s signal compared to that of pure boron (188 eV) indicates boron ions doped into the graphene network. 49The B 1s peak can be fitted into two components, implies that the boron atom exists in two different chemical environments.The peak locates at 189.9 eV is due to the BC 3 bonding environment, which can be attributed to the substitutional configuration of B within the sp 2 carbon network.While the other peak at 191.4 eV is attributes to BC 4 , which is accompanied by defects inside the graphene network and functional edges of graphene sheets. 50The C 1s peak of sample S1 and sample S2 are similar as pristine grpahene, which can be fitted into two peaks: a primary peak locates around binding energy of 285.1 eV and the other peak at 287.04 eV, due to C-C and C-O bond, respectively.No C-B peak which around 281.8 eV is observed in our XPS C 1s spectra. 51This suggests that very small amount B ions doping into graphene lattice after irradiation.Figure 3(e) plots the Raman spectra of the pristine graphene, sample S1 and sample S2.For S1 a very weak D ' peak around 1620 cm −1 , which corresponds to intravalley double-resonance process in the presence of defects, appears. 52The I D /I G of pristine graphene is 0.54, and it slightly increases to 0.55 for S1, which indicates very low level of damage caused to S1 compares with the pristine graphene.After irradiation with the fluence of 2 × 10 15 cm −2 , the I D /I G of the sample S2 increases to 3.3, the D ' peak strongly increases and a D + D ' peak, which is the combination of phonons with different momenta around K and requires a defect for its activation, 52 appears around 2940 cm −1 .All of these results indicate significant damage and disorder caused to sample S2 after higher fluence ion beam irradiation.
After B ion-beam irradiation the graphene surface changed significantly, which can be seen from Figure 3(f) and 3(g).For the sample S1, the ripples are disappeared and the surface is very flat.With the increasing of ion fluence, surface of sample S2 becomes fragmented and the small graphene films are separated by a lot of cracks.We measured 30 different widths of cracks of the irradiated samples and calculated the average crack-width.For the sample S2, the average crack-width is 3.32 μm.We assume during ion irradiation, because ripples were much easier etched than the flat films, B ions break the C-C bonds of ripples firstly and formed dangling bonds in the edge of graphene films.Because dangling bonds were chemical unstable, with the increasing of ion fluence more and more C-C bonds in the edge of graphene were broken and carbon atoms spurted, finally led to wide cracks between graphene films.

C. Nitrogen irradiation
In the pristine graphene, the N 1s peak is absent, while it is different for the N ion beam irradiated graphene.Figure 4(a) and 3(c) are the N 1s spectra for sample S3 and sample S4, which indicates N ions doped into graphene film.Both N 1s peak for sample S3 and S4 have two components, which are center around 398.7 and 401 eV, corresponding to graphite-N and pyridinic-N bonds, respectively. 53A nitrogen atom could be incorporated into a graphene network having a variety of different bonding environments. 53,54 he graphite-N bond refers to a N atom substitute one C atom in the graphene honeycomb lattice without creating any defects.While for the pyridinic-N bond, which means a N atom replacing one C atom in the graphene lattice and also creating one vacancy.For the sample S4, the C 1s peak can be fitted into three small peaks, as shown in Figure 4(d).The peak centers at 285.1eV is corresponds to sp 2 C bond, which is also found in pristine graphene.In addition, for the sample S4 two extra peaks which are locate at 285.85 and 287 eV, could be attribute to N-sp 2 C and N-sp 3 C (or C-O bond) bonds, respectively. 53The N-sp 2 C and N-sp 3 C bonds may originate from substitution of the N atoms, defects or the edge of the graphene sheets. 53However no carbide nitrogen bond can be found in C 1s peak of sample S3, which may be due to the very small amount of N ions doped into graphene lattice was.The nitrogen-carbide bond detected in both C 1s and N 1s peaks indicates that N ions were successfully doped into graphene by very low energy ion irradiation.
Raman spectra of the pristine graphene, sample S3 and sample S4 are shown in Figure 4(f).After irradiation with 35 eV N ion beam at fluence of 10 14 cm −2 , the intensity of D peak increases and I D /I G increases to 1.27, a very weak D ' peak appears, showing the increase of defects with increasing fluence of N ion beam irradiation.The defect signal of sample S3 is much weaker compared with Guo et al., 29 who irradiated mono-layer graphene with 30 KeV N ions at a fluence of 10 14 cm −2 .If the ion beam fluence is further increased to 6 × 10 15 cm −2 , both D, G and 2D peaks of sample S4 become broader and overlap in parts.The Raman spectrum of sample S4 presents a typical feature of amorphous carbon or carbon nitride, where either carbon or nitrogen atoms are completely in a disordered structure.However, no carbon nitride related peaks are observed.This clearly reveals that the mono-layer graphene transformed from a high quality crystal structure to an amorphous material after irradiation at the high fluences >10 15 cm −2 .When N ions are implanted into the graphene lattice and replace the carbon atoms, point defects can also be created.With increasing ion fluence, a large number of point defects, which could either be C vacancies or N interstitials/substitutes, were produced and accumulate, finally leading to disorder and amorphization.Therefore, N ion beam irradiation induces structural defects in graphene, and the graphene lattice gradually evolves from a high quality crystal to a nanographitic structure and finally to amorphous carbon.
The surface of sample S3 is flat and uniform, as shown in Figure 4(f).However, after irradiation at high fluence of 6 × 10 15 cm −2 , sample S4 becomes fragmented, the uniform graphene film is separated by a lot of cracks.It is obvious that high fluence irradiation causes significant damage to graphene, similar to B ion irradiation.   .The pronounced F 1s peaks in Figure 5(a) and 5(c) are the proof of fluorine ions doping into graphene lattice and forming C-F bond.The F 1s peak of sample S5 can be fitted into two components, locating around 686.9 eV and 687.7 eV which are related to F 2 -C and F-C bond, respectively. 55For the sample S6, when the F ion beam was increases to 2 × 10 15 cm −2 , besides the F-C bond at 688 eV, another F-Si peak locates at 686.4 eV could also be found in Figure 5(c). 56F ion beam irradiation also cause defect production.Also parts of the ions penetrate through graphene film and react with the Si atoms from silicon substrate and form F-Si chemical bonds.The C 1s peak of sample S5 and sample S6 are shown in Figure 5(b) and 3(d), two peaks could be fitted, the primary peak centers at the binding energy of 285.1 eV corresponds to sp 2 -C bond, and the small peak centers at 286.77 eV may attribute to C-F or C-O bond. 38The much higher intensity of sp 2 -C peak indicates most of the C atoms were maintained in the honeycomb lattice after F ion beam irradiation.SEM images are another proof of higher fluence causing more damages to graphene.As shown in Figure 5(e) and 5(f), F ion beam irradiated graphene are fragmented and covered with many cracks, the average crack-width increases from 3.1 μm to 3.46μm with the fluence increasing from 1 × 10 14 to 2 × 10 15 cm −2 .
XPS results indicate that B, N and F ions were successfully doped into graphene films, but damage such as vacancies were also introduced and the level of damage is different depending on ion species and ion fluence.For graphene irradiated at fluence of 10 14 cm −2 , the I D /I G of B, F and N irradiated samples is 0.55, 0.69 and 1.27.If the fluence increases to 10 15 cm −2 , the I D /I G of B and F ion beam irradiated samples is 3.3 and 4.96, while the N ion beam irradiated sample transforms to an amorphous carbon film.For both lower and higher-fluence ion beam irradiation, B ion beam irradiation causes fewest damages to graphene among three of ions.For the heavier N and F, if taken ion mass into consideration, F ion should create more defects than N ion due to its heavier mass, but the fact is opposite.For low energy ion beam irradiation, physical collision effects are weak and chemical interaction must be taken into consideration.The bond energy of C-N is 293 kJ/mol, weaker than C-C bond (348 kJ/mol) and C-F bond (485 kJ/mol).Therefore C-F bonds are chemically very stable compared to C-N bonds which may explains that more defects and vacancies are created after N ion beam irradiation.

IV. CONCLUSIONS
In summary, the effect of low energy ion beam irradiation on mono-layer graphene was investigated.Mono-layer graphene films were irradiated with mass selected Boron, Nitrogen and Fluorine ions at energies of 35 eV and 20 eV and different ions fluences.The ions could be incorporated into graphene films.Irradiation induced damage is also inevitably caused to graphene and the amount of defects and disorder increases with the increasing ion energy and fluence.B ion beam irradiation causes the fewest damage to graphene because of its lightest mass.For the same ion energy and fluence N ions cause more damage than F ions, the reason may be explained with chemical effects.Low energy ion beam irradiation provides an attractive method for controlled doping graphene.

FIG. 1 .FIG. 2 .
FIG. 1.(a) to (d): Atomic transport calculated with SDTrimSP for irradiation of a thin a-C layer on Si with low energy B, N and F ions, given in % per incident fluence.

Figure 5 (
e) shows the Raman spectra of pristine and F ion beam irradiated graphene samples.Both sample S5 and sample S6 are irradiated with the ion energy of 35 eV.For sample S5, after irradiation with lower-fluence, I D /I G increases to 0.69, suggesting some damage of graphene after F ion beam irradiation.With further increasing fluence, I D /I G increases dramatically to 4.96 and two other defect related peaks D ' and D + D ' appear, indicating severe damage caused to sample S6.

TABLE I .
Ions with different energy and fluence used in the experiments.