Purification/annealing of graphene with 100-MeV Ag ion irradiation
© Kumar et al.; licensee Springer. 2014
Received: 15 January 2014
Accepted: 2 March 2014
Published: 17 March 2014
Studies on interaction of graphene with radiation are important because of nanolithographic processes in graphene-based electronic devices and for space applications. Since the electronic properties of graphene are highly sensitive to the defects and number of layers in graphene sample, it is desirable to develop tools to engineer these two parameters. We report swift heavy ion (SHI) irradiation-induced annealing and purification effects in graphene films, similar to that observed in our studies on fullerenes and carbon nanotubes (CNTs). Raman studies after irradiation with 100-MeV Ag ions (fluences from 3 × 1010 to 1 × 1014 ions/cm2) show that the disorder parameter α, defined by ID/IG ratio, decreases at lower fluences but increases at higher fluences beyond 1 × 1012 ions/cm2. This indicates that SHI induces annealing effects at lower fluences. We also observe that the number of graphene layers is reduced at fluences higher than 1 × 1013 ions/cm2. Using inelastic thermal spike model calculations, we estimate a radius of 2.6 nm for ion track core surrounded by a halo extending up to 11.6 nm. The transient temperature above the melting point in the track core results in damage, whereas lower temperature in the track halo is responsible for annealing. The results suggest that SHI irradiation fluence may be used as one of the tools for defect annealing and manipulation of the number of graphene layers.
KeywordsGraphene Ion irradiation Annealing Disorder parameter Inelastic thermal spike model
Graphene is a new member of the carbon family that has two-dimensional honeycomb lattice structure [1, 2]. It is a basic building block for other carbon materials of different dimensionalities. It can be wrapped into zero-dimensional fullerenes, rolled up into one-dimensional carbon nanotube and stacked together to form three-dimensional graphite . The high mechanical stability together with ballistic electron transport properties makes graphene a strong replacement for silicon-based semiconductor technology. At present, electron beam lithography (EBL) is widely used to fabricate nanoelectronic devices. For example, EBL is often adapted for fabrication of drain source and gate electrodes in graphene field-effect transistor (GFET) structure, while graphene channel is fabricated by alternate means. Similarly, the use of scanning electron microscope (SEM) for observation and testing of GFET structures is very common . Electron beam itself has also been used for the formation of epitaxial graphene on the surface of a 6H-SiC substrate by irradiation . Hence, in conventional EBL, the electron beam irradiation of graphene and consequent damage become unavoidable. For this reason, the electron beam irradiation-induced modifications in graphene [4, 6–11], especially in the energy regime relevant for lithography and microscopy, are widely being studied.
The graphene properties are sensitive to defects. For example, it has been reported that metal to insulator transition occurs with small introduction of defects in graphene perfect lattice . Hence, for the proper use of graphene-based devices, the studies on defect control and behaviour of graphene in defect environment are very important. Low-energy (keV to few MeV) ion beam irradiation is one of the important tools to engineer defects in a controlled manner. Though the effect of ion irradiation and collision cascades in solid have been known for a long time , there are only few studies on the interaction of ions with two-dimensional (2D) crystals such as graphene  and how the irradiation-induced defects can be used to engineer their electronic properties . It has been theoretically shown that vacancies can induce 2D magnetic ordering in graphene  besides modifying the electronic states near the Fermi level originating from the shape of electronic bands [17–19] near the Dirac point [20, 21]. Ripple formation induced by 1-keV Ar ion irradiation, showing a possibility of tailoring of few-layer graphene electronic band structure with inter-layer coupling, has been demonstrated . Irradiation study of graphene with 90-eV Ar+ ions has shown that the disorder produced in graphene leads to two competing processes resulting in an initial increase in the formation of defects as depicted by the increase in ID/IG ratio followed by its decrease at higher fluence . The defect studies with 500-keV C+ ions  and 2-MeV protons  show that a single-layer graphene is more sensitive to irradiation damage than a multilayer graphene, and its damage follows the behaviour proposed by Tuinstra and Koenig for nanographite . Structural transformation from nanocrystalline graphene to amorphous carbon and corresponding electrical transition from Boltzmann diffusion to carrier hopping transport induced by 30-keV Ga ions have been demonstrated by Zhou et al. . Graphene is also supposed to be mechanically stable under irradiation. Atomistic simulations have demonstrated that irradiated graphene, even with a high vacancy concentration, does not exhibit any sign of instability, thus showing its applicability as robust windows . Zhao et al. studied the defect production in graphene, induced directly by incident ion and also indirectly by backscattered and sputtered particle with the help of molecular dynamics method . Yang et al. proposed the thinning of graphene layer by the use of nitrogen plasma irradiation and post-annealing treatments .
As mentioned above, most of the irradiation studies on graphene are focused on damage by electron and low-energy ions, and there are only few reports available on the ion irradiation studies of graphene at high energy . The study of high-energy ion irradiations, such as mega-electron volt protons, is also important due to the potential use of graphene devices in space applications, specially solar cells . Though the irradiation-induced damage is very common, it has been demonstrated that swift heavy ions (SHI), with carefully selected ranges of fluence, can also be used for annealing of defects in graphene. We have earlier observed the annealing/ordering effect in fullerenes and multiwalled carbon nanotubes (MWCNT) at lower fluences after irradiation with 200-MeV Au, 60-MeV Ni and 55-MeV C ions [33, 34].
In the present work, graphene flakes grown with chemical vapour deposition (CVD) technique on Si/SiO2/Ni substrate from Graphene Laboratories Inc., USA were used. Though mechanically cleaved graphene samples generally provide better quality single-layer graphene, we have used commercially available CVD-grown samples which are available in larger size, with industrial applications in mind. Irradiation is carried out with 100-MeV Ag7+ ions using 15-MV Pelletron accelerator at IUAC, New Delhi, India, at fluences of 3 × 1010, 1 × 1011, 3 × 1011, 1 × 1012, 3 × 1012, 1 × 1013, 3 × 1013 and 1 × 1014 ions/cm2. The pressure in the irradiation chamber was of the order of 10−6 mbar, and the average ion current was 1 particle nanoampere (pnA; 1 pnA = 6.25 × 109 ions/s). The sample was cut into pieces of 1 × 1 cm2 size for irradiation. Half of the area of each sample was masked with thin aluminium foil during irradiation to keep half the portion unirradiated for comparing the results.
The surface topography was studied using Digital Instruments Nanoscope IIIa atomic force microscope (AFM; Santa Barbara, CA, USA) in the tapping mode. Raman measurements were carried out using Renishaw inVia microRaman set-up (Gloucestershire, UK). Initially, we performed Raman measurements on pristine sample at several spots with different laser powers to check the self-heating effect. We took 10% (2 mW) of the laser power, which is below the threshold value for laser-induced heating, for measurement on all the samples. Therefore, self-annealing effect during Raman measurement is ruled out.
Raman spectra of pristine (unirradiated) as well as irradiated portions of each sample were recorded at four different spots (flakes) using Ar ion laser with excitation wavelength of 514.5 nm and averaged out for further analysis. The size of the laser spot is approximately 2 μm with × 50 optical magnification of the attached optical microscope. The peak intensities have been obtained by fitting the peaks assuming Lorentzian distribution.
Results and discussion
The film thickness shows a linear relation of h ~ 0.35n + t0, where h is the measured height, t0 is the approximate distance between the first graphene layer and the substrate and n is the number of layers. Substrate to the first layer height t0 is approximately 0.6 to 0.7 nm .
The Raman spectra of unirradiated and irradiated graphene at different fluences with 100-MeV Ag ions are shown in Figure 2. The spectra have been normalized to keep the height of G peak the same. The spectra have also been staggered in the y-axis for clarity. It has been observed that D peak areal intensity first decreases up to a fluence of 3 × 1011 ions/cm2 and then starts to increase with increasing fluences up to 1 × 1014 ions/cm2.
It is clear from Figure 3 that a small decrement in the value of α is observed up to the fluence of 3 × 1011 ions/cm2, and with further increase in the fluence, there is an increase in its value. This initial decrease signifies ordering or purification of graphene at low fluences and is induced by energy dissipated by the incident energetic ion. To the best of our knowledge, this is the first kind of report on the ordering or purification of graphene. Electron beam irradiation studies [9, 38–40] have only shown increase in defects, whereas we have observed decrease in defects at lower fluences. The data with electron beam irradiation is available at a higher fluence only (more than 600 s). It is possible that measurements at a lower fluence with electron beam irradiation (say approximately 10 s) may show annealing. We have also irradiated samples for 10 s at the lowest fluence.
The damage and annealing occurs simultaneously. However, because the ratio of the core and halo radii is approximately 1:4 (from Figure 7b), annealing of defect occurs in much larger area rather than irreversible damage of graphene by the core region. Therefore, the damage is insignificant at low-fluence irradiation. At higher fluences, the core region areas start to overlap, and the damaged area increases. The halo regions are not capable to anneal the damage created in the core regions. From these values of radii for the core and halo, fluences when the core and halo regions are expected to start overlapping are obtained as 5.5 × 1012 and 3.4 × 1011 ions/cm2, respectively. Hence, we can expect that annealing will increase up to a fluence of 3.4 × 1011 ions/cm2 when halos cover the sample. Beyond this fluence, the region damaged by the track core starts to dominate. Kumar et al. have observed similar behaviour in carbon nanotubes due to ion irradiation . The purification of carbon nanotube is previously reported by laser irradiation and heat treatment also [45, 46].
σ1 and σ2, respectively, represent the cross-sectional area of cylindrical regions which are annealed and damaged by an incident ion. The annular halo and core radii estimated from the value of σ1 and σ2 are 5.7 and 1.4 nm, respectively. The overlapping fluence calculated with this core radius is 1.6 × 1013 ions/cm2. The halo and core radii observed for graphene are found to be smaller in comparison to radii 11.6 and 2.6 nm obtained from thermal spike calculations. The smaller core and halo radii can be explained in terms of difference in the strength of electron-phonon coupling (g value) of graphene and graphite. Larger thermal conductivity and mean free path may lead to a lower value of g in the case of graphene as compared to graphite (g = 3 × 1013 W/cm3/K) and subsequent estimated lower value for core and halo radii.
It is expected that the intensity ratio I2D/IG would increase at higher fluence when the number of layers decreases [35, 37, 50], but at the same time due to higher defect density, the ratio I2D/IG decreases much more rapidly . Therefore, overall effect is decreased in I2D/IG ratio as shown in Figure 4. It is also known from the reported work that the FWHM of 2D peak increases with the increase of defects in graphene [52, 53]. Hence, at a higher fluence above 1 × 1013 ions/cm2, the expected decrease in FWHM of 2D peak due to layer number reduction is not observed.
In summary, the annealing effect of swift heavy ion irradiation on graphene in low-fluence regime has been observed. Raman studies show that 100-MeV Ag ion irradiation induced annealing up to a fluence of 3 × 1011 ions/cm2 after which a rapid increase in disorder is observed. The effect is similar to that shown earlier in fullerenes and CNTs. It is concluded that graphene can be purified by the use of SHI irradiation which is of particular importance for using CVD-grown graphene in device applications. Our results suggest that swift heavy ion with selected ranges of fluence may be used as one of the tools for defect annealing and manipulation of the number of graphene layers.
swift heavy ion
electron beam lithography
scanning electron microscope
multiwalled carbon nanotubes
chemical vapour deposition
atomic force microscope
full width at half maxima
inelastic thermal spike.
Sunil Kumar is thankful to the Council of Scientific and Industrial Research (CSIR), New Delhi, India for providing fellowship. We are also thankful to the Pelletron group of IUAC New Delhi for providing stable beam and Dr. M. Toulemonde (CIMAP, Caen) for providing inelastic thermal spike code.
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