Article pubs.acs.org/Langmuir
Localized Deoxygenation and Direct Patterning of Graphene Oxide Films by Focused Ion Beams Derrek E. Lobo,†,‡ Jing Fu,‡ Thomas Gengenbach,§ and Mainak Majumder*,†,‡ †
Nanoscale Science and Engineering Laboratory (NSEL) and ‡Mechanical and Aerospace Engineering, Monash University, Clayton, Victoria, Australia § CSIRO Materials Science and Engineering, Bayview Avenue, Clayton, Victoria, Australia S Supporting Information *
ABSTRACT: Exposure to controlled doses (∼4.65 × 10−3 to 2.79 × 10−2 nC/μm2 ion fluence) of Ga ions via a focused ion beam (FIB) deoxygenates graphene oxide (GO) and increases the electrical conductivity in 100 × 100 μm2 patches by several orders of magnitude compared to that in unexposed GO. Raman spectra and the carbon/oxygen ratio in exposed areas are indicative of chemically reduced graphene oxide (rGO). This novel FIB-induced conversion technique is harnessed for the direct imprinting of complex micrometer-scale shapes and sub-20-nm lines of rGO in insulating films and flakes of GO establishing the capability of generating features varying in size from approximately tens of nanometers to approximately hundreds of micrometers in a maskless, efficient manner.
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INTRODUCTION Graphene has risen to prominence as a scientifically intriguing yet versatile material that may shape technologies in electronics and energy storage.1,2 It has remarkable material properties, including chemical and mechanical stability, high electron mobility, and zero-gap semimetal characteristics.3−7 Graphene oxide (GO), a variant of this material, is synthesized by the oxidative exfoliation of graphite and contains oxygenated functional groups8,9 in the basal and edge planes of graphene. These functionalities, while allowing GO to solubilize in different solvents including water, decrease the electrical conductivity of GO to the extent that it becomes insulating.10 Chemical and direct heat treatment can decrease the proportion of these oxygenated groups to form reduced GO (rGO), and in doing so, partially restores the electrical conductivity.11−14 Graphene patterning is being considered as a potential route to opening band gaps and for applications in nanoelectronics4 and the fabrication of planar microsupercapacitors.15 Features that are ∼100 nm in size can be patterned in graphene films by the removal of graphene layers by the lithographically defined masking and plasma etching of exposed regions.16,17 In a recent study, Dimiev et al. reports a method to form ∼100 nm features. The method relies on the patterned deposition of metal films of Zn on graphene multilayers and the subsequent layer-by-layer removal of single graphene layers based on a strong Zn−graphene interaction by acid etching.18 Alternatively, selective functionalization with organic moieties through scanning tunneling microscope lithography19 and an electrically biased atomic force microscope (AFM)20 to form feature sizes in the 5−20 nm range are also reported. While © 2012 American Chemical Society
these methods have focused on graphene, the spatially selective deoxygenation of GO is an alternative strategy for patterning graphene-based materials. Relying on local thermal reduction, networks of conducting pathways in insulating GO films have been reported by utilizing the heating power of lasers15,21 and the heated tip of an AFM.22 Although the direct laser-writing methods are simple and can be implemented with a DVD burner,23 they have been limited to the generation of micrometer-scale features; however, smaller (hundreds of nanometers) features can be produced by coupling interference techniques.24 The hot AFM tip,22 however, can imprint nanometer-scale (∼15 nm minimum) features but is technically complicated and requires special modifications of commercially available apparatuses. A focused ion beam (FIB), an integral part of most modernday electron microscopy facilities, has been used extensively to reveal micro- and nanostructures by selective ablation (in conjunction with scanning electron microscopes), the deposition of materials by chemical vapor deposition, and the maskless patterning and repair of integrated circuits.25 FIB operates by the use of a liquid metal source, typically gallium (Ga), is ionized at the tip, and then is focused by electromagnetic lenses to bombard and remove materials or induce chemical reactions.25,26 When the ability of energetic He ions to remove materials selectively on fine scales has been used, sub-20-nm features in graphene samples have been produced.27 More recently, molecular dynamics simulations Received: June 2, 2012 Revised: September 19, 2012 Published: September 21, 2012 14815
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exposed to a controlled fluence of Ga ions as described in the article to reduce the GO selectively. Further details can be found in sections S2 and S3 of the SI. Energy-Dispersive Spectroscopy. EDS measurements were used to probe the relative amounts of carbon and oxygen present in the sample both before and after FIB exposure. To achieve this, a 2 keV electron beam was used. A low-energy beam was used because the GO layer was only 0.8 μm thick and low energy (2 keV) reduces the interaction volume of the sample, minimizing the interference from the Si substrate while still being high enough to stimulate a response from carbon and oxygen species in the sample, for which a representative spectrum can be seen in section S4.1 in Figure S5 of the SI. EDS was also used to probe the sample for the presence of Ga in which a 15 keV electron beam was used to probe the FIB exposed samples with the detector recording data until the elemental peaks stabilized, which equated to a full-scale value of 16 000 counts. The samples were fabricated as 2 μm × 2 μm squares with ion fluences ranging from 5.00 × 10−3 to 3.00 × 10−1 nC/μm2. With EDS characterization carried out on each of the samples, the presence of Ga was not detected in our samples at the ion fluence range to be studied (5.00 × 10−3− 2.79 × 10−2 nC/μm2). The presence of Ga was first detected in samples when the ion flux was 6.00 × 10−2 nC/μm2 or greater, indicating that the amount is below the detection limit for our conversion experiments (max ion fluence ∼2.79 × 10−2 nC/ μm2). A representative spectrum can be seen in section S4.1 (Figure S6), with further details of this study also discussed in section S4.1 X-ray Photoelectron Spectroscopy. XPS analysis was performed using an Axis Ultra DLD spectrometer (Kratos Analytical Inc., Manchester, U.K.) with a monochromatic Al Kα source at a power of 180 W and the standard aperture (analysis area: 0.3 mm × 0.7 mm) at a pressure of less than 10−8 mbar. Survey spectra were acquired at a pass energy of 160 eV. To obtain more detailed information about the chemical structure, high-resolution carbon 1s spectra were recorded at a 20 eV pass energy (yielding a typical peak width for polymers of 1.0 eV). Analysis was performed at an emission angle of 0° as measured from the surface normal. Assuming typical values for the electron attenuation length of relevant photoelectrons, we find that the XPS analysis depth (from which 95% of the detected signal originates) ranges from 5 to 10 nm. Conductivity Measurements. Two-point conductivity measurements were conducted using an Agilent B2900 series precision source/measuring unit wired through an EmCal Genelyte probe station with 5 μm tipped tungsten probes. Measurements were made by varying the applied voltage between −4.5 V and +4.5 V, and this range was deemed acceptable because studies have shown that the 0−30 V range is required to stimulate the electrically induced reduction of GO.48 Added to this, we observed no temporal changes in conductivity when the measurements were run on the GO or exposed samples, even with the measurements run multiple times. The patterns studied were 100 × 100 μm2 squares, and the probes were placed 50 μm apart. The scan rate used in the measurements was 0.8 V/s with a measurement taken every 0.008 V. The samples thickness was determined from SEM images and a profilometer (Dektak 150 surface profilometer with a stylus radius of 12.5 μm and a depth resolution of 0.07 μm) and was shown to be 0.8 μm. Further details can be found in section S4.2 of the SI.
unraveling the effects of bombarding graphene sheets with Ar ions have been reported.28 However, studies exploring the effects of ion bombardment of GO as opposed to graphene are limited, with the most relevant being the reported effect of a high-energy Ti ion on films of GO. The extremely large dose of ions (4.80 × 10−1 nC/μm2) in the study, although it deoxygenates GO, also leads to amorphization and loss of the sheetlike structure of GO.29 For the present problemthe patterning of GO filmsthe approach described above was not spatially selective because the Ti ions were not focused, hence it cannot be harnessed for imprinting GO with conductive pathways. In this article, we present the selective, patterned reduction of GO through a low dose of focused Ga ions and also elucidate a plausible mechanism for this interesting phenomenon. Graphite was converted to graphene oxide by the modified Hummer method. GO films were then deposited on Si substrates (with a 100 nm thermally grown oxide layer) having an inherent electrical conductivity on the order of 10−16 S/ m30,31 by spin coating. Ga ions impinged on GO films set at an operating voltage of 30 kV with low ion fluence. The ion fluence (nC/μm2) can be well controlled by parametrizing the beam current, irradiated area, and time of exposure, each of which can be measured independently during the experiment. The effect of ion bombardment was investigated by electrical conductivity measurements on irradiated patterns of 100 μm × 100 μm size and was supplemented with micro-Raman spectroscopy, energy-dispersive spectroscopy (EDS), and Xray photoelectron spectroscopy (XPS) to confirm the conversion process. Complex structures such as a map of Australia and lines down to 15 nm composed of rGO were patterned directly by this proposed novel technique. To the best of our knowledge, this is the earliest proof of concept for patterning electrically conducting pathways of rGO in GO films by FIB.
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METHODOLOGY Simulations. Monte Carlo simulations were carried out to determine the effect of the bombardment of GO with Ga ions. This was carried out using the stopping range of ions in matter (SRIM)31,32 software, which makes use of the collision cascade model45,46 to describe the interactions between the ions and the target sample atoms. The incident energy was varied from 1 to 60 keV, the common range for the FIB, and the target was composed of three layers. The uppermost layer was a GO film with 1 μm thickness and a carbon to oxygen ratio indicative of the modified Hummer method of 1.8.8,33 The next layer was composed of 100 nm of SiO2, and finally, the base was 1 mm of Si. The sample was then bombarded with 3000 Ga ions, and the results of the ejected species were recorded. For a detailed explanation, please refer to section S1 in the Supporting Information (SI). Sample Preparation. GO was synthesized from graphite powder (SP-1 grade 325 mesh, Bay Carbon Inc.) using a modified Hummer method.47 It was then used to fabricate samples via spin coating on Si to make a continuous GO film that was 0.8 μm in depth, and a dilute 0.0009 wt % GO solution was used to deposit individual GO flakes by dip coating for 20 s, followed by air drying. The samples were then placed into the chamber of an FEI Helios Nanolab 600 FIB-SEM and pumped down to a vacuum level of below 1 × 10−3 Pa. Markers were milled into the surface using a 30 kV beam, 9.3 nA current with a fluence of 2.79 nC/μm2, and the area within the markers was 14816
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Raman Analysis. Raman spectra were obtained using a Renishaw confocal micro-Raman spectrometer equipped with a HeNe (632.8 nm) laser operating at 10% power. Extended scans (10 s) were performed between 100 and 3200 cm−1 with a laser spot size of 1 μm. The apparatus was calibrated on the silicon peak, and this spectrum was removed from our results because the samples were fabricated on silicon wafers. Once the background was removed in this manner, the intensity of each spectrum was normalized by dividing the data by the maximum intensity. The peak position was found by using the full width at half-maximum as is common practice in analyzing spectral data.
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RESULTS AND DISCUSSIONS The Ga ions bombard the surface of GO with a significant kinetic energy and are dispersed into the GO lattice. This energy, if large enough, will break the covalent bonds between the atoms, and material will be ejected. Given that oxygen atoms have a lower surface binding energy or a higher volatility than carbon atoms, they have a higher probability of being ejected, and this simple principle provides the basis for selective, tunable reduction. The concept is illustrated in Figure 1. Monte Carlo simulations were conducted a priori to confirm the novel concept of the preferential removal of oxygen from GO using the stopping range of ions in matter (SRIM) software.32,33 The incident ions are set as Ga, and the accelerating voltage was varied between 1 and 60 kV. The target was composed of three layers: the uppermost was a film of graphene oxide (∼1 μm thick) with a carbon to oxygen ratio of 1.8,8,34 indicative of GO produced by the modified Hummer method. The middle layer was composed of 100 nm of SiO2, and finally the base was 1 mm of Si. Standard values of the displacement energies of Si and O (15 and 20 eV, respectively) in the Si and SiO2 lattices were utilized, and the binding energy for the GO layer was determined from an approximate composition of GO and the strength of the bonds. It is assumed that GO was composed of 66.7% sp3 bonds (bond energy 3.8 eV) and 33.3% sp2 bonds9,35 (bond energy ∼7.05 eV). For the carbon to oxygen bonds, hydroxyl groups (C− OH, bond energy ∼3.73 eV) made up 36.6%, epoxies (C−O− C, bond energy ∼3.82 eV) were about 53.3%, and the remaining 10% were carbonyl groups (CO, bond energy ∼7.67 eV).36 From this composition, it is apparent that in a GO system the sp2 C atoms and the CO bonds are the strongest and most difficult to disrupt. Given that the bond-breaking process is probabilistic and influenced by the relative amounts of each material and their bond strengths, we estimated that mean effective bond strength of the C−O bonds is ∼4.17 eV whereas the C−C bond energy is ∼4.90 eV. Although the bond energies indicate their propensity to be broken, whether the atom is actually ejected is to a large extent determined by the surface binding energy. Only if an atom at the surface has a kinetic energy greater than that of the surface binding energy is it able to leave the solid. Exact numerical values of the surface binding energy for most materials are difficult to obtain; however, the physically analogous heat of sublimation is a reasonable approximation33 for analyzing a sputtering process. Most importantly for our case, carbon and oxygen have significantly different heats of sublimation: carbon ∼7.5 eV37 and oxygen ∼2 eV.38 In the simulation, the composite target was then bombarded with 3000 Ga ions at different accelerating voltages, and the details of the ejected species were recorded.
Figure 1. Schematic of the sequences of selective patterning of GO using the FIB: (a) Graphene oxide prior to FIB exposure shows the hexagonal lattice structure of the carbon atoms, with the red spheres representing the oxygenated functional groups of GO. (b) Displays of the sample during FIB exposure. The FIB tip can be seen in the top left corner with a blue cone representing the gallium ions, which when incident on the substrate have enough energy to break bonds and create a collision cascade. However, the fact that oxygen is more likely to be ejected results in the reduction of graphene oxide as seen in image c.
The simulation results are plotted in Figure 2a, and detailed simulation methodology is presented in SI section S1. Accurate only to the extent of suggesting trends, the simulations predict that the oxygen atoms are more likely to be sputtered than carbon from GO, and this preferential removal is particularly effective at lower energies of the incident ion beams. Most importantly, even at 30 keV (a standard FIB operating energy of Ga ions) the simulations indicate the preferential removal of five oxygen atoms for every two carbon atoms. To generate direct experimental evidence, samples of GO films and flakes were prepared (the method of GO film and flake preparation is described in SI section S2) and irradiated with beams of 30 keV Ga ions having an effective fluence of between 4.65 × 10−3 and 2.79 × 10−2 nC/μm2 with a current varying between 93 pA and 2.1 nA and exposure times of 10 to 120 s. Details of the methodology for the exposure of GO to 14817
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Figure 2. Selective reduction of GO: (a) results from the simulation of Ga ions incident on a film of GO showing the preferential removal of oxygen over carbon. There is a relative error of ±2% in the simulations. (b) Snapshot of the displacements caused by 30 keV Ga ions in a GO film of 1 μm thickness with the majority of the ions reaching a depth of only 30 nm, indicating that the process is limited to the surface and upper region of the film and not the bulk of the film. (c) SEM image showing the positions for EDS measurements and (d) EDS results illustrating a decrease in oxygen content in a 0.8-μm-thick GO film after exposure to the ion beam, with the atomic C/O ratio changed from 1.8 ± 0.1 to 2.9 ± 0.1.
focused ion beams can be found in SI section S3. This exposed region was then investigated with EDS (experimental details and typical spectra are available in SI section S4.1) and compared to the unexposed region (Figure 2c,d). Areas of GO films unexposed to ion beams had a carbon content of 64 ± 2% and an oxygen content of 36 ± 2%. This changed to 74.5 ± 2.5% for carbon and 25.5 ± 2.5% for oxygen in the exposed regions, clearly demonstrating deoxygenation. The C/O ratio in the exposed sample is similar to that reported for the reduction of GO via hydrazine reduction.39,40 X-ray photoelectron spectroscopy (XPS) was used to characterize the chemical structure of exposed (2.79 × 10−2 nC/μm2) versus unexposed regions of GO films further (Figure 3). XPS clearly confirmed the EDS results showing a localized, marked reduction in oxygen content after ion bombardment (Figure 3a). C 1s highresolution spectra recorded prior to ion exposure reveal a high concentration of C−O-based functional groups (peak at 286.5−287 eV)) as well as other C, O species such as carbonyls (288 eV) and O−CO-based moieties (289−290 eV). The reduction in intensity seen between 286 and 290 eV relative to the intensity at around 285 eV (C bonded to only C or H) after exposure to the ion beam is clear evidence of the localized deoxygenation of the GO film (Figure 3b). Unlike laser-based approaches, additional radiation effects such as heating are presumably negligible in FIB studies. An understanding of how ions transfer energy to the target materials is still far from established, and current models including Coulomb exploration and thermal spikes offer no clear consensus and lack experimental validation;41 however, even for ice from water molecules milled by FIB, temperatureinduced devitrification was not observed.42,43 With the fine
Figure 3. XPS analysis of a region exposed to ion bombardment (600 μm × 800 μm). (a) Elemental map (2.5 mm × 2.5 mm) displaying relative concentrations of oxygen (green) and carbon (red). (b) Corresponding C 1s high-resolution spectra recorded outside (preexposure) and inside (postexposure) the exposed region.
beam size and ultralow dose in the proposed approach, the structural change in the target GO is expected to be limited and confined to a volume of tens of nanometers. Indeed, XPS analysis indicated that deoxygenation is limited to just the top few nanometers to tens of nanometers at the surfaces of the GO films. To ascertain this ion-beam-assisted conversion process, electrical conductivity measurements on areas impinged with varying doses of ion fluence were performed (Figure 4). Instead of GO flakes, which may not form continuous films, relatively thick GO films and large patterns were utilized for direct measurements without any lithographically defined electrical pads. Typically, the patterns were 100 × 100 μm2 with a 14818
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Figure 4. Analysis of exposed regions: (a) SEM image of patterned areas color coded to the conductivity data. (b) Raman spectra (obtained from a 632.8 nm laser operating at 10% power) of GO after exposure to ion beams (2.79 × 10−2 nC/μm2) indicating an increase in the ID/IG ratio and a shift in the G peak analogous to spectral changes observed during the chemical reduction of GO.23,39,41,42,44−46 Also presented are the results of exposure to a 10-fold increase in ion fluence (3.00 × 10−1 nC/μm2) displaying amorphization of the sample.29,47−49 (c) Increasing conductivity of the patterns with increasing ion fluence: (green line) ion fluence of 2.79 × 10−2 nC/μm2 and conductivity of 4.0 ± 0.2 × 10−2 S/m; (dark-blue line) ion fluence of 1.40 × 10−2 nC/μm2 and conductivity of 2.0 ± 0.5 × 10−2 S/m; and (red line) ion fluence of 4.65 × 10−3 nC/μm2 and conductivity of 0.46 ± 0.01 × 10−2 S/m; and (light-blue line) as-synthesized GO with a conductivity below the detection limit of our measurements.
thickness of 0.8 μm, and the electrodes were contacted inside the pattern using micromanipulators. (For more details regarding the methodology, please refer to SI section S4.2.) Areas exposed to the ion beam demonstrate an increase in conductivity in comparison to the unexposed GO, and the conductivity increases with increased ion fluence (within the range of ∼4.65 × 10−3 to 2.79 × 10−2 nC/μm2 ion fluence that was investigated). We also noticed that under these conditions of conversion the content of Ga implantation was below the detection limit. The SEM was operated at 15 kV, and the EDS detector was left running until elemental peak stability was achieved. Only when the ion fluence was increased to around 6.00 × 10−2 nC/μm2 (i.e., twice the maximum value of ionbeam-assisted conversion experiments) were we able to detect the first traces of Ga (as shown in Figure S6 of SI section S3) indicating that the increase in conductivity arises from the large change in the concentration of oxygen in the films. Further increases in ion fluence to ∼3.00 × 10−1 nC/μm2 resulted in the conventional milling of the sample as observed in Figure S7 of SI section S3. For comparison, the highest conductivity of 4.0 ± 0.2 × 10−2 S/m in these patterns is significantly larger than the reported conductivity of GO (∼10−5−10−3 S/m).41 In our experiments, the conductivity of GO was below the
detection limit of our instrument. Given that our instrument is capable of measuring currents on the order of picoamps, the conductivity of the heavily oxidized GO sheets is likely to be on the order of 10−5 S/m, indicating an increase by about 3 orders of magnitude in the exposed regions. However, by no means is this conversion complete because it is still 2 to 3 orders of magnitude lower than in previous reports on rGO via chemical and thermal methods.11−14 We note that this conversion is likely to be limited to the surface of the GO films because our simulations indicate that Ga ions at 30 keV penetrate only to a depth of 30 nm (i.e. ∼3.75% of a 0.8-μm-thick GO film). Additionally, the majority of the incident ions may lose energy upon initial contact with the substrate, and each subsequent collision has less energy available. This would mean that the cross-sectional thickness of the current-carrying layers measured by profilometry (0.8 μm) is an overestimation, and the conductivity values reported here are an underestimation of the actual values. It may also suggest that the FIB-induced conversion, unlike chemical or thermal techniques, can be utilized for the surface conversion of relatively thick GO films. The FIB-induced conversion is also supported by changes in the Raman spectra where the ratio of the D to G peak increases upon irradiation (from 1 to 1.11) and the G peak shifted from 14819
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1594 to 1588 cm−1 as shown in Figure 4b. The changes are consistent with chemically reduced GO,23,39,41,42,44−46 where the slight increase in the D peak intensity has been linked to the creation of smaller but numerous graphene domains. Acknowledging the fact that higher doses of ion-beam irradiation may lead to amorphization, we determined through systematic experiments that a 10-fold increase over the maximum dosage of the GO conversion process (i.e., ∼3 × 10−1 nC/μm2) leads to distinct changes in the Raman spectra by the appearance of a single broad peak in the region of D and G peaks indicative of amorphization29,47−49 (Figure 4b). This is consistent with the hypothesis that an excess dose of ions leads to the amorphization whereas a controlled minimal dose can selectively remove the oxygen species while retaining the structural integrity of the GO sheets. The collective evidence of (i) decreasing oxygen concentration, (ii) increasing electrical conductivity, and (iii) changing Raman spectra consistent with chemically reduced graphene conclusively supports the spatially selective reduction of GO by a controlled dose of focused ion beams. Given the relatively low dosage required to form the conductive patterns, the higher-resolution SEM images even retained folds in graphene sheets that extend from the unexposed to the exposed regions with only a change in contrast as shown in Figure 5a. This microstructure is in sharp
GO structures through well-directed experiments and simulations. Nevertheless, a possible application of this technique is to pattern complex shapes, an example of which is the map of Australia (Figure 5b). Given the ability to focus ion beams to spot sizes as small as 5 nm, the fabrication of nanoscale features is feasible. Demonstrated here are lines as small as 15 nm. Although the tip of the focused ion beam can be as small as 5 nm, the feature size is limited by its longitudinal beam profile, which follows a Gaussian distribution. This distribution would effectively give us a full width at half-maximum radius that was 2 to 3 times the radius at the tip, and this plays an important role in interactions with the substrate and results in a feature size larger than the nominal beam radius as seen in Figure 5c. The patterns fabricated with the FIB and imaged with an SEM can also be viewed in reflected light microscopes as seen in Figure 5d. In conclusion, we demonstrate a method for the localized deoxygenation of GO and direct writing of structures on the length scale of tens of nanometers to hundreds of micrometers using a Ga-based FIB. The ability to reduce and selectively pattern GO with complex, conductive patterns by FIB, a tool that is readily available with most micro/nanofabrication facilities, opens up the possibility of maskless fabrication of microcircuits in graphene electronics. Additionally, when fine tuned, this technique could be a potential tool for the fabrication of structures to explore quantum effects50 and nanoscale capacitors for energy storage.
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ASSOCIATED CONTENT
S Supporting Information *
Monte Carlo simulations of Ion implantation and sputtering. Sample preparation. Focused ion beam patterning. Energy dispersive spectroscopy. Electrical conductivity measurements. Raman spectroscopy. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
Figure 5. Application in patterning: (a) High resolution SEM displaying the edge of a pattern shows features crossing the sharp boundary of exposed and unexposed regions. The features are structurally intact, with the changes limited to the contrast. (b) SEM image of GO patterned into the Australian continent. (c) A 15 nm line drawn on a GO flake. (d) optical image of the GO patterned into the map of Australia with a reflecting light microscope.
The authors declare the following competing financial interests. We have recently filed an Australian Patent on the process of the reduction of GO to rGO, Majumder et al. “Conductive Portions in Insulating Materials”, application number 2012902606.
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ACKNOWLEDGMENTS We acknowledge funding from the Australian Research Council (LP 110100612 and DP 1101100082), Monash University, and Strategic Energy Resources. We also acknowledge the use of the facilities in the Melbourne Centre for Nanofabrication. D.E.L. acknowledges a graduate student scholarship (ERLA) from Monash University. We also thank Phillip A. Sheath for his synthesis of the GO and Rachel Tkacz who performed the profilometry.
contrast to the formation of “whirlpool-like” structures on the surface as reported by Chen et al.29 in their article on the amorphization of GO by ion-beam implantation. A previous discussion of the Raman spectroscopy results corroborates the observation of minimal damage to the graphene sheets. However, the simulations suggest the preferential removal of oxygen over carbon, never indicating that carbon is not ejected. Therefore, it is rather intriguing that GO retains the sheetlike structure and does not undergo amorphization as observed by Chen et al.29 The results of this investigation thus beg the question, do the GO sheets retain the sheetlike structure after deoxygenation or do they recrystallize into graphene lattices during low-dose ion-beam exposure? Our current aim is to understand these fundamental questions and the evolution of
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