Unusual surface and edge morphologies, sp2 to sp3 hybridized transformation and electronic damage after Ar+ ion irradiation of few-layer graphene surfaces
© Al-Harthi et al.; licensee Springer. 2012
Received: 16 July 2012
Accepted: 5 August 2012
Published: 19 August 2012
Roughness and defects induced on few-layer graphene (FLG) irradiated by Ar+ ions at different energies were investigated using X-ray photoemission spectroscopy (XPS) and atomic force microscopy techniques. The results provide direct experimental evidence of ripple formation, sp2 to sp3 hybridized carbon transformation, electronic damage, Ar+ implantation, unusual defects and edge reconstructions in FLG, which depend on the irradiation energy. In addition, shadowing effects similar to those found in oblique-angle growth of thin films were seen. Reliable quantification of the transition from the sp2-bonding to sp3-hybridized state as a result of Ar+ ion irradiation is achieved from the deconvolution of the XPS C (1s) peak. Although the ion irradiation effect is demonstrated through the shape of the derivative of the Auger transition C KVV spectra, we show that the D parameter values obtained from these spectra which are normally used in the literature fail to account for the sp2 to sp3 hybridization transition. In contrast to what is known, it is revealed that using ion irradiation at large FLG sample tilt angles can lead to edge reconstructions. Furthermore, FLG irradiation by low energy of 0.25 keV can be a plausible way of peeling graphene layers without the need of Joule heating reported previously.
Ion irradiation of materials subjected to different ion energies, ion doses and irradiation geometries such as angle of incidence of the beam, sample tilt angle and sample rotation has been widely studied [1, 2]. In addition, most of the underlying ion-matter interaction mechanisms, cascade collisions and irradiation-induced defects have been theoretically explained by Monte Carlo and classical molecular dynamic simulations  and density functional theory total energy calculations . However, the discovery of the 2-D crystals such as graphene  has introduced new challenges which need new insights. An approach employing transport of ions in matter (TRIM) simulations which is successfully used for bulk material analysis was found not necessarily to work for 2-D crystals . This is due to the fact that in TRIM calculations, the sample is treated as an amorphous matrix with a homogenous mass density neglecting the atomic structure that reflect the nature of atomically flat targets such as graphene. However, a code based on analytical potential molecular dynamic simulations with much more accurate capabilities than TRIM has been developed to account for amorphizations, defects, and single, double and complex vacancies in graphene under ion irradiation as functions of angle of incidence and ion energy .
Experimentally, Lopez et al.  in their study of a single-layer graphene grown on a SiO2/Si substrate exposed to 30 keV Ga+ ion irradiation pointed out the requirements for developing a more thorough understanding of graphene's ability to withstand prolonged ion irradiation. These include: (1) the absence of cascade collisions due to the 2-D graphene nature, (2) the possibility of C atoms not to be displaced from the lattice as the ion energy is completely transferred to the substrate underneath, (3) graphene open structure which facilitates the ion channeling, hence limiting the number of ion/target collisions and ejected carbon atoms, and (4) implanted ions which might cause a net positive charge to build up in graphene and electrostatically repel the subsequent incoming incident ions. In addition to the above issues, the importance of this study helps to understand the damage production mechanisms and types of defects created by the energetic ions in the sample for the efficient use of ion beams and optimization of the graphene cutting process . Furthermore, this study suggests the possibility of using graphene membranes in ion beam analysis , elimination of surface contaminants from ex situ prepared graphene layers  and graphene defect-based applications. Although, un-optimized ion irradiation is expected to breakdown the graphene 2-D network, destroy its sp2 bonding configurations and affect the graphene carrier mobility, this study shade light on defects associated with this process which are equally important. In this respect, applications and mechanisms based on defects in graphene such ferromagnetism , creation of metallic wires , porous graphene for DNA detection , atmospheric pollutant filtration  and graphene hydrophobicity enhancement can be realized and understood.
Here, we have employed low energy Ar+ with energies from 0.25 to 5 keV at incidence angle of 45° and sample tilt angle of 66° to study the irradiation effects in FLG. In addition to defects found on flat irradiated areas, we show that FLG edge defects associated with sp2 to sp3 hybridized carbon transformation are most typical in the low energy range and irradiation geometry used. In addition, we show that the reliable quantification of the sp2/sp3 hybridized state ratio as a function of irradiation energy can be achieved from the deconvolution of the XPS C 1s envelope. This ratio, together with newly developed features in the Auger transition C KVV spectra and π − π* transition behavior as a function of irradiation energy provide insights on the FLG structural and electronic damage. Furthermore, we point out the possibility of a combined effect of ion implantation and ion reflection to be responsible for the low rate of sputtering of FLG.
FLG samples were obtained by peeling layers from highly oriented pyrolytic graphite (HOPG ZYA) using adhesive tape. They were then fixed on steel substrates by double sided stick carbon tape. These substrates were then subjected to Ar+ ion irradiation for 30 min for energies ranging from 0.25 to 5 keV at incidence angle of 45° and sample tilt angle of 66° in the ultra high vacuum conditions.
XPS measurements were carried out using an Omicron Nanotechnology XPS system (Omicron NanoTechnology GmbH, Taunusstein, Germany) using a monochromatic Al Kα radiation (hν = 1,486.6 eV). The source voltage and emission current were 15 kV and 20 mA, respectively. The base pressure at which the scans were done was 10−10 mbar. The chemical composition of the sample was extracted from the wide scan using CASA XPS software (Fairley, N. CASA XPS, version 2.0; CASA Software Ltd., Devon, U.K.). Short scans were recorded at pass energy of 20 eV. In order to avoid charging effect during the scans, an electron gun flooding was used for charge compensation.
The nanoscale images presented were performed using Nanoscope V atomic force microscope (AFM) obtained in tapping mode using ultra high resolution cantilevers made of tungsten having radius of less than 1 nm and force constant of 46 N/m. During the imaging, both the scan rate and the imaging resolution were set at 0.5 Hz and 512 × 512 pixels, respectively.
Results and discussion
In order to further shed light on the morphology of the two layers after the 5-keV ion irradiation, the roughness exponent α has been determined from the 5-keV PSD spectrum in the high frequency region. It is evident that the spectrum shows bimodal intensity trend (i.e., two slopes as shown in Figure 4) which obeys the inverse power law K f −β where β and K are the spectral index and spectral strength, respectively . The two slopes of the PSD in this region were found to be β1 = 6.0 and β2 = 6.5. Since β is related to α by the equation α = (β − d)/2 , where the line scan dimension d is 2, then α1 = (6–2)/2 = 2 and α2 = (6.5 − 2)/2 = 2.25. Here, α1 attributes to the roughness exponent associated to the first FLG layer and α2 could be assigned to roughness exponent of the second FLG layer.
Also, variation in the effect of ion irradiation can be observed from the newly developed shoulder (shoulder labeled A) in the XPS spectra along with the increasing of the irradiation energy. The origin of shoulder A is unclear at the moment; however, complex defects previously theoretically predicted  to be associated with Ar+ irradiation might be the reason. In addition, the commonly observed π − π* transition in graphene at binding energy of 290 eV  seems to gradually disappear as the irradiation energy is increased, indicating the FLG electronic damage. This is supported by the following further XPS investigations carried out to determine the transition from the sp2-bonding to sp3-hybridized state as a result of Ar+ ion irradiation.
It can be concluded that the resilience (i.e., only two layers being sputtered with Ar+ at 5 keV for 30 min) of FLG to the Ar+ ion irradiation stems from the combined effect of the Ar+ implantation as evident from Figure 5 and back scattering or reflection of the incoming ions due to the sample tilting. As the implanted Ar+ ions build up, they electrostatically repel subsequent incoming incident ions, therefore reducing number of ions involved in collisions. This charge repulsion can be understood from the ion concentration saturation observed in the inset of Figure 5 at energies ≥3 keV; a caution should be taken here as re-sputtering of the implanted Ar+ ions can also take place and lead to the saturation observed. Auxiliary experiments at constant irradiation energy and at different tilt sample angles were carried out to confirm the effect of the reflection of the incoming ions on the resilience of FLG. Indeed, the results (to be reported elsewhere) obtained from the ion irradiation of FLG samples at constant energy of 1 keV, and different sample tilt angles (Φ) from 0° to 80° show that the amount of implanted Ar+ ions seems to decrease as the tilt angle is increased.
On the other hand, the initial irradiation of the FLG causes fast transformation of sp2 to sp3 hybridization as seen in region I of Figure 7. The irradiation energies in this region induce irradiation damage dominated by large number of small pits (see AFM images in Figure 1) which give less sp2 character due to the enhanced possibility of the damage of the edge of the graphene structure, consequently the formation of dangling bonds. An amorphous stable mixture containing equal sp2 and sp3 hybridizations (i.e., sp2/sp3 = 1) is observed (region II) when employing irradiation energy in the range from 1.5 to 4 keV. At high irradiation energies above 4 keV, the number of transformed sp2 bonds seems to be slightly less as depicted in region III of Figure 7 compared to those of region II. This observation can be understood in two different ways by considering the morphology formed after the irradiation in the vertical and the lateral directions. First, in the vertical direction, the observation is supported with the conclusion of the depth distribution analysis where the third graphene layer was just found to be exposed at these high irradiation energies. Therefore, XPS is expected to directly detect the sp2 contribution from this layer and the layers beneath up to approximately 2 nm of the expected XPS sampling depth in the geometry used. In the lateral direction and as seen from the AFM image at 5 keV in Figure 1, few large pits are formed due to the irradiation damage; hence, large untouched islands on the surface of the samples with better sp2 character are expected to exist. The reduced number and the large size of these pits are reflected from the long periodicity of λ = 72 nm and two orders of magnitude increase in PSD intensity found in Figure 3, respectively.
The D values are within the reported values of 21 to 23 eV for HOPG (multi-layer graphene) , but not sensitive to the irradiation energy. However, comparison between the intensities of shoulder A and peak B reflects the sp2 to sp3 transition trend similar to that observed in Figure 7. Here, the intensity difference between A and B reaches the minimum and stabilizes after ion irradiation in energy range between 1.5 and 4 keV. Slight increase in the intensity difference is found at energies above 4 keV. Although, the shape of derivative of Auger transition C KVV spectra gives the indication of the ion irradiation effect, the D parameter values obtained from these spectra fail to account for sp2/sp3 hybridization transition, and reliable quantification of sp2/sp3 ratio is only obtained from the deconvolution of the XPS C (1s) envelope as explained before and shown in Figure 6. Our preliminary results to exploit the roughness created by ion irradiation (surfaces shown in Figure 1) for nanobubble formation and, consequently, creating graphene surfaces with less friction for different applications are promising. Supplement data shown in Figure S1 in Additional file 1 support this claim where observed contact angle, size and density of nanobubbles deposited on the sputtered samples are observed to increase with surface roughness.
From all the above findings, it is clear that more than one factor can contribute to the sp2 to sp3 transformation mechanism. For example, irradiation energy will cause lattice displacement in the FLG which affects the FLG layer cross-linking, therefore contributing in the formation of new sp3 bonds. In addition, bonds can be broken to form sp3 due to the collision impact. This is evident from the XPS reduction of π electron (π bond) intensity as the irradiation energy is increased. Furthermore, bending (to be discussed later) and breaking of the graphene planes are likely to contribute to the sp2 to sp3 transformation observed.
In this case, the edges are seen to be lifted up and shadow the regions behind them; shadowing effects are seen from the cross section profile taken at a position shown by a solid line in Figure 9b and illustrated in Figure 9c. (3) The lower edges act as weak points where more carbon sputtering takes place, and (4) new features (shown by arrow in Figure 9b) at an angle of 53° from the edges are formed as the irradiation energy is increased. All irradiation features and their roughness amplitudes after 3 keV ion irradiation can be seen in the supplement data shown in Figure S2a,b in Additional file 2. The presence of these features is detected by the fast Fourier transform (FFT) analysis shown in the inset of Figure S2b in Additional file 2.
This type of folding has been observed on the surface of FLG resembling single wall nanotubes or double wall nanotubes due to the single- or double-graphene layer folding back to itself . The folding observed might be due to the electron beam damage associated with scanning electron microscope and transmission electron microscope used while characterizing the samples in these studies. In our case, the folding is rather attributed to the irradiation process-induced damage which includes multi-layers of graphene as inferred from the thickness of folded layers ranging from 2 to 4 nm. Intense irradiation of the multi-folded layers can induce isolated particles (enclosed by a rectangle) and initiate wrinkles (indicated by black arrow) as seen in Figure 10b. Similar edge reconstructions have been reported by Huang et al. , which are due to the graphene layer-by-layer peeling as a result of void formations and their migration towards the graphene edges. This process was associated with atomic sublimation caused by Joule heating and facilitated by atomic displacement caused by high-energy electron irradiation. Despite the serious electronic and morphological damages at high energies, roughness, PSD, XPS, π − π* transition, sp2/sp3 ratio, sp2 and sp3 FWHM values and Auger transition C KVV spectra data analysis reveal that, using low energy of 0.25 keV Ar+ ion irradiation at large sample tilt angle at room temperature, is an alternative way of peeling graphene layers without the need of Joule heating.
Ar+ ion irradiation of FLG has been investigated in which ripple structure formation with varying amplitude and periodicity predominantly in the first two graphene layers was observed. In addition, sp2 to sp3 bonding transformation through the quantification of the deconvolution of the XPS C (1s) envelope along with electronic damage as deduced from the gradual disappearance of the π − π* transition in graphene, as a function of irradiation energy, was found. Although the shape of derivative of Auger transition C KVV spectra provided an indication of the ion irradiation effect, the D parameter values obtained from these spectra failed to account for sp2 to sp3 hybridization transition. Among others, energy-dependent edge wiggles, shadowing effects, defects (such as holes and FLG edge reconstructions) and edge folding which depend on the sputtering conditions were all found. The existence of these defects suggests the possibility of creating nanoporous FLG by ion irradiation and the development of high density nanobubble FLG surfaces for various applications. Furthermore, the combined effect of Ar+ implantation and sample tilt angle on FLG irradiation can explain the ion irradiation resilience of FLG. Although we looked specifically at Ar+ irradiation, our results can also provide insights into the response of FLG to irradiation by other noble gasses or other species. Despite the mixing of sp2 and sp3 is not desirable for the electronic properties of FLG, coming up with applications which depend on tuning sp2/sp3 ratio will be essential to fully appreciate the importance of the ion irradiation of FLG. In addition, future investigations at atomic scale for the edge reconstructions and other defects will be essential to support the results of this study.
SHA is an associate professor in Nanotechnology and Surface Science. ME is a professor in Theoretical Physics, doing first principle calculations. MA is an associate professor in Theoretical Physics and Computational Simulation Physics. AK is a masters degree student. TH is a Ph.D. student working on swift ions in materials. MTZM is a Ph.D. student working on nanotechnology. MRA is a professor in magnetism and head of the department.
Authors would like to thank Sultan Qaboos University for providing experimental facilities used in this study.
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