Removal of Organic Contamination from Graphene with a Controllable

Subscriber access provided by UB + Fachbibliothek Chemie | (FU-Bibliothekssystem)

Article

Removal of Contamination from Graphene with a Controllable Mass-Selected Argon Gas Cluster Ion Beam Bonnie J Tyler, Barry Brennan, Helena Stec, Trupti Patel, Ling Hao, Ian S Gilmore, and Andrew John Pollard J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b03144 • Publication Date (Web): 02 Jul 2015 Downloaded from http://pubs.acs.org on July 4, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Removal of Organic Contamination from Graphene with a Controllable Mass-Selected Argon Gas Cluster Ion Beam Bonnie J. Tyler*, Barry Brennan, Helena Stec, Trupti Patel, Ling Hao, Ian S. Gilmore, Andrew J. Pollard* National Physical Laboratory, Hampton Road, Teddington, TW11 0LW, UK

*[email protected], T: +44 2089437170 [email protected]

Abstract Since the discovery of graphene, organic surface contamination has posed difficulties both for accurate characterization of the material’s intrinsic properties and for development of graphene devices. In this study, we investigate the use of a mass-selected argon gas cluster ion beam for removing organic contaminants, such as residual polymethylmethacrylate (PMMA), from single-layer graphene. The influence of cluster ion size, energy and ion dose has been investigated to identify the important factors for minimizing damage to the graphene layer during the cleaning process. Raman spectroscopy was used to analyse the variation in the D-peak and G-peak intensity ratio, an indicator of damage to the graphene lattice, as a function of ion beam dose and kinetic energy per atom (E/n) in the cluster ions. Using a mass-selected 5.0 keV Ar 5000 beam with a dose of 5.00 ions/nm2, we were able to demonstrate removal of polymer residue and other carbonaceous material from single-layer CVD-grown graphene whilst minimizing the damage to the graphene itself. This demonstrates that the mass-selected argon cluster ion beam is a suitable, industry-relevant technology for use in large scale production of commercially-desirable CVD-grown graphene.

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Keywords: graphene, cleaning, Raman spectroscopy, secondary ion mass spectrometry, contamination, argon gas cluster.

Introduction The exceptional electronic and physical properties of graphene have generated a large amount of scientific investigation over the last decade. Potential applications of this material have been identified in a wide variety of areas including high speed electronics, flexible electronics, touch screen displays, high frequency transistors, interconnects, composite materials and energy storage 1-3. The commercial viability of many of these applications relies on the development of technologies to enable the large scale reproducible production of contamination-free graphene with minimal defect densities. One of the key obstacles to consistent production is the challenge of removing surface contamination from graphene layers. Since the discovery of graphene, surface contamination of single-layer graphene, which by definition has every atom at the surface, has been a persistent problem interfering with accurate characterization and reproducible performance of the material 4-6. Contamination has been observed both from processing steps as well as spontaneous adsorption of atmospheric species under ambient conditions 4, 7. These contamination layers can confound the analysis of the graphene since a single nanometer of organic contamination will contain more carbon atoms than a single-layer of graphene. More significantly, however, surface contaminants can degrade the electrical performance of the graphene by acting as dopants and increasing contact resistance 8-10. These effects are of particular concern for graphene grown by chemical vapor deposition (CVD) . Polymers, used to transfer CVD-grown graphene from the metal catalyst on which they are produced to substrates that are viable for electronic applications, are not entirely removed from the graphene surface using conventional solvent-based methods. The


Page 2 of 22

Page 3 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


electrical properties of the resulting graphene sheets, which have micro-scale polymer deposits, are degraded through contact resistance and charge transfer between the contaminants and the graphene. Similar concerns exist for other graphene production methods such as liquid phase exfoliation11-12. Various methods have been presented in the literature for removing organic contamination layers from graphene, including ultra-violet (UV) ozone treatment, argon plasma etching, vacuum annealing, annealing on a catalyst, contact-mode atomic force microscopy (AFM), and CO 2 cluster jet conditioning 9, 13-18

. None of these methods are optimal for large scale micro-electronics manufacturing. AFM is not

suited to cleaning larger than millimeter-scale areas. Annealing can leave behind islands of reduced amorphous carbon. UV ozone treatment, argon plasma etching and CO 2 cluster jet conditioning all lead to defect formation in graphene. In this study, we have investigated the use of mass-selected large argon cluster ion sputtering as a means for removing organic residues from graphene. Large Ar clusters were introduced for the nondestructive removal of organic matter by the group of Isao Yamada 19 and have become well established for the large-scale surface processing of silicon wafers in industry 20. More recently, Ar cluster beams have been adopted for sputter depth profiling of organic films 21-23. The mechanism of sputtering with large Ar cluster differs substantially from Ar plasma cleaning or sputtering with a monatomic Ar+beam. Etching with an Ar plasma exposes the surface to EM radiation, free radicals, excited state atoms, monatomic ions, radical species, and free electrons with a broad energy distribution. In contrast, an ion beam contains solely ions with a narrow and controllable energy distribution and direction. Gas Cluster ion sources have been shown to produce synergistic sputtering effects that cannot be produced with low energy monatomic species. The effective of cluster size and energy on sputtering of both organic and inorganic materials has been extensively studied 24. Sputtered volume per cluster atom is a function of the energy per cluster atom (E/n) for a wide range of materials. These beams are of particular interest



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

for cleaning graphene because of the more than two order-of-magnitude difference in sputter yield observed between organic and inorganic materials 24-25, which is a result of the much higher surface binding energy in inorganic materials. Because this difference between the sputter yields increases as the energy-per-atom in the clusters decreases, prospects are good for finding beam conditions that will remove organic contaminants without significantly damaging the graphene layer. Raman spectroscopy 26-28 has been used to investigate defect formation in mechanically exfoliated graphene as a function of argon cluster size, kinetic energy and ion dose, i.e the number of ions per unit area that impact on the surface. Conditions which minimize the graphene damage during the removal of a patchy PMMA layer have been identified and tested on CVD-grown graphene. At an energy per atom (E/n) of 1 eV/atom there is almost 4 orders of magnitude difference in sputtering yield between PMMA and SiO 2 [21, 26]. The sputtering yield of graphene is expected to be similar to inorganic materials, such as SiO 2 .

Methods 300 nm silicon oxide layers on silicon substrates (SiO 2 /Si) were first patterned with a grid system using a focused bismuth ion beam to allow for co-location of sample regions in the confocal Raman microscope and secondary ion mass spectrometry (SIMS) instruments. Graphene flakes were then produced through mechanical exfoliation and transferred onto the patterned SiO 2 /Si substrate (commonly referred to as the ‘Scotch tape’ method). These initially pristine graphene flakes were therefore effectively disorderfree, as indicated by Raman spectroscopy measurements. After the exfoliation process, the samples were introduced into an ultrahigh vacuum (UHV) time-of-flight secondary ion mass spectrometry (TOFSIMS IV) instrument (ION-TOF GmbH, Muenster, Germany) equipped with an argon gas cluster ion gun beam (GCIB) mounted to impact the sample at 45 degrees. The ION-TOF Ar GCIB gun provides a mass


Page 4 of 22

Page 5 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


filtered cluster beam with cluster size, n, from 500 to 10,000 atoms, and energy, E, from 2.5 to 20.0 keV. This GCIB source generates a narrow size distribution beam (see Figure 1) with the full-width-at-halfmaximum (FWHM) of the distribution varying from 700 atoms/cluster for 2.5 keV Ar 5000 to 1000 atoms/cluster for 5.0 keV Ar 5000 . For all experiments, the ion beam current was measured in a Faraday cup by recording the beam on and off currents for extended periods of time and calculating the difference between the two readings. The associated uncertainty in the current measurement was less than 5%. Selected regions of the samples were exposed to a defined dose of Ar clusters ranging from 10-2 ions/nm2 to 10 ions/nm2.

Following exposure to the GCIB, samples were removed from vacuum and transferred to the Raman spectroscopy system (LabRAM HR Evolution, Horiba Scientific, UK). Graphene single-layer flakes in a region with a designated ion dose, cluster size and cluster energy were then located using a combination of optical microscopy and Raman spectroscopy. Raman spectra were collected from individual flakes using a 532 nm (2.33 eV) laser source with a total laser spot power of less than 1 mW so that the measurement technique used to determine the disorder in the graphene lattice did not itself create further defects. For each GCIB exposure condition, confocal Raman spectroscopy measurements were performed on 3-7 flakes within a sputtered region. Additionally, samples of CVD-grown graphene 29 were grown on copper and transferred onto a quartz substrate using a wet transfer method. To provide support during the transfer process a polymer, PMMA, was first spin-coated onto the graphene face of the copper, to produce a PMMA layer thickness of approximately 400 nm. When the polymer had dried and hardened, ammonium persulphate solution was used to remove the Cu, with nitric acid used as the catalyst in the etching process. The sample was then rinsed twice in a bath of deionised (DI) water, to reduce the level of impurities. The final stage



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

involved transferring the PMMA/graphene sandwich, graphene face down, onto the polished quartz substrate (either 10 mm × 10 mm or 3 mm × 3 mm). The sample was left to dry and the PMMA was removed through a two minute acetone soak, followed by further rinsing in DI water. A 5 mm × 5 mm region of one CVD-grown graphene sample (CVD1) was cleaned using 5.0 keV Ar 5000 clusters and a dose of 1.00 ions/nm2 using the stage raster mode on the ION-TOF SIMS instrument described above. Subsequently, a ToF-SIMS image was collected over a 7 mm × 7 mm region of this sample using a high current bunched Bi 3 + primary ion beam. A dual beam depth profile of the PMMA residue was then collected from a 200 µm × 200 µm region in the unsputtered portion of the sample using a 5.0 keV Ar 5000 sputter beam and a high current bunched Bi 3 + analysis beam. A second CVDgrown graphene sample (CVD2) on a ~ 3 mm × 3 mm quartz substrate was sputter cleaned using 5.0 keV Ar 5000 clusters and a dose of 5.00 ions/nm2.. Confocal Raman spectroscopy analysis was performed on selected regions of the sample both before and after the Ar cluster cleaning. Results and Discussion Preliminary studies using a range of cluster sizes and energies at a dose of 1.00 ion/nm2 showed significant damage to the graphene for clusters with an energy greater than 1 eV per atom. Figure 2 shows representative Raman spectra from mechanically exfoliated graphene flakes exposed to a dose of 1.00 ions/nm2 of 10.0 keV Ar 2000 (5 eV/atom), 2.25 ions/nm2 of 5.0 keV Ar 5000 (1 eV/atom), and 2.00 ions/nm2 of 2.5 keV Ar 5000 (0.5 eV/atom). A Raman spectrum of pristine exfoliated graphene is included for reference. The Raman spectrum of the pristine single-layer graphene contains two distinctive peaks; the G-peak at ~1585 cm-1, corresponding to the E 2g phonon, and the 2D-peak at ~2680 cm-1. Defects in graphene give rise to two additional peaks, the D-peak (at ~1350 cm-1), which is attributed to breathing modes of the hexagonal sp2 carbon rings, and the D’-peak (at ~1620 cm-1). The ratio of the D-peak and G-peak intensities (I D /I G ) is well established as an indicator of disorder in the graphene matrix. A


Page 6 of 22

Page 7 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


detailed explanation of the defects in graphene caused by monatomic ion bombardment and the effect on the Raman spectra is given by Pollard et al30. The Raman spectrum from the graphene exposed to 1.00 ions/nm2 of 10.0 keV Ar 2000 in Figure 2 shows an intense D -peak, indicating significant damage has been done to the graphene layer, with an estimated inter-defect distance, L D , of 8 nm – 10 nm from Ref. 28. In contrast, the Raman spectra from the graphene irradiated with 5.0 keV or 2.5 keV Ar 5000 show only small levels (estimated L D of 30 nm – 100 nm [28]) of damage to the graphene. Figure 3a shows the ratio of the D- and G-peak intensities as a function of ion dose for graphene flakes exposed to 10.0 keV Ar 2000 , 5.0 keV Ar 5000 and 2.5 keV Ar 5000 . Each data point represents measurements on an individual graphene flake. For 10.0 keV Ar 2000 (5 eV/atom), the damage to the graphene layer, as indicated by I D /I G , increases rapidly above an ion dose of 10-1 ions/nm2. At an ion dose of 1.00 ion/nm2, the I D /I G value of ~2 is suggestive of significantly damaged graphene (estimated L D of 7 nm – 9 nm [28]). In contrast, when sputtering with 5.0 keV Ar 5000 (1 eV/atom) and 2.5 keV Ar 5000 (0.5 eV/atom), I D /I G remains below 0.6 (estimated L D of 15 nm – 20 nm [28]) up to an ion dose of 10.00 ions/nm2, indicating only modest damage to the graphene for a much greater ion dose. For 10.0 keV Ar2000, the energy per atom in the clusters (5.0 keV/atom) is just above the C-C bond energy in graphene which has been estimated at ~4.9 eV; whereas the energy per atom is well below the C-C bond energy for both the 5 keV Ar 5000 clusters (1.0 eV atom) and the 2.5 keV Ar 5000 (0.5 eV/atom). Although the damage decreases significantly for the lower E/n clusters some limited damage is still observed when E/n is below the C-C bond energy. Although the damage is measurable, it remains low relative to that observed for other comparable cleaning methods of I D /I G = 0.5-1.5 13. For further comparison, electrons of energy 0.1 – 1.0 keV show a similar I D /I G ratio to the GCIB of ~0.531, whilst Mn ions of 25keV show -an I D /I G ratio of 4.3 for a dose reduced by one hundred times 30. The cross-sectional area of a 5000 atom argon cluster is



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

approximately 40 nm2, so at a dose of 10.00 ions/nm2 every area in the graphene sheet has been struck approximately 400 times. Considering the exceedingly high ion dose, the level of damage is surprisingly low. When sputtering with monatomic Mn+ ions, similar changes in the Raman I D /I G ratio have been observed at an ion dose of 0.002 ions/nm2, nearly four orders of magnitude lower dose.30 Although the level of damage to the graphene is lower for the 2.5 keV Ar 5000 (0.5 eV/atom) than for the 5.0 keV Ar 5000 , sputter yields for organic species drop precipitously in this energy range. Figure 3b shows the same I D /I G data shown in Figure 3a but plotted against the equivalent PMMA sputter depth for the given ion dose, which was estimated using Seah’s Universal equation for Ar cluster sputtering yields 24. The relationship between sputter yield for PMMA and level of damage produced in graphene is of particular importance for removing a non-uniform layer of polymer residue. During sputter cleaning, graphene that is covered by only a thin organic layer will be quickly exposed to the cluster beam before the patches of thicker PMMA material are removed. In order to remove all of the polymer residue, the ion dose received for graphene in regions with only a thin initial organic coating will be determined by the dose required to remove the thicker PMMA islands. From Figure 3b, it can be seen that an optimum regime exists around 1 eV/atom (i.e. 5.0 keV Ar 5000 ) which permits efficient removal of organic contamination while minimizing damage to the graphene. Figures 4 and 5 show ToF-SIMS images of the PMMA residue on graphene sample CVD1. Figure 4 shows the PMMA distribution on a 200 µm × 200 µm area of the sample prior to argon cluster cleaning. The data is 3D-ToF-SIMS image obtained in dual beam mode, using 5keV Ar 5000 + as the sputter beam and 25 keV Bi 3 + as the analytical beam. Figure 4a shows the image for the sum of the C 3 O 2 H+ and C 4 OH 5 + ions (characteristic PMMA peaks) integrated through the depth profile and Figure 4b shows a Z-corrected 3D image 32 of the same data. Figure 4d shows the sputter depth profile through the selected regions shown in red and blue in Figure 4c, which corresponds to Figure 4a. Prior to Ar cluster sputter cleaning,


Page 8 of 22

Page 9 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


most of the imaged area (blue region) is covered with a thin layer of PMMA of ~2 to 3 nm, but several 10 – 20 µm patches with a thickness > 20 nm are observed (red regions). Layer thicknesses were calculated from the estimated PMMA sputtering yield 24. As can be seen in Figure 4d, the PMMA has been removed from the blue regions with an ion dose of less than 0.5 ions/nm2 but 2.5 ions/nm2 are required to remove the thick patches of PMMA in the red regions. Argon clusters have a penetration depth on the order of 1 nm and so the graphene will not be exposed to the Ar cluster ions until the PMMA is removed. This is of importance because after a dose of 0.5 ions/nm2, bare graphene will be exposed in the blue regions that were covered with only a thin PMMA layer. These regions will then be directly exposed to the Ar cluster beam during the time in which the PMMA is removed from the thicker patches. Thus, graphene surface damage will be dependent on the thickness and uniformity of initial PMMA coverage. Figure 5 shows a 7 mm x 7 mm SIMS image of PMMA on graphene sample CVD1. Although there are no characteristic peaks for the graphene, its location can be inferred based on the peak shifts due to differential charging of the sample and are shown on in Figure 5a. The area outlined in blue has been etched with 1.00 ion/nm2 5 keV Ar 5000 . The PMMA distribution is shown in Figure 5b, and an overlay of the graphene (red) and PMMA (green) is shown in Figure 5c. This dose has removed the majority of the PMMA but patches of thicker residue are observed to remain. Figures 6 and 7 show measurements of the I D /I G ratio for graphene sample CVD2 before and after Ar GCIB cleaning. Prior to argon cluster cleaning (Figure 6a), the I D /I G ratio is low except for artefacts where there are patches of carbonaceous material (white areas). Following 5 keV Ar 5000 sputtering with a dose of 5.00 ions/nm2 (Figure 6b), the patches of carbonaceous material have been removed. Low levels of damage to the graphene layer, as indicated by the increase in the I D /I G ratio, is detected over



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

much of the graphene surface. This damage is lowest in the regions where the islands of amorphous material were originally observed. Figure 7a shows histograms of the I D /I G data from Figure 6. Variation in the extent of damage after cleaning with 5 keV Ar 5000 suggests that regions that were covered with only a few nanometers of organic contamination were quickly exposed to the sputter beam and so received a larger ion dose than areas which had a thicker layer of contamination. Figure 7b shows the Raman spectra from the mode of the distribution before and after Ar GCIB sputter cleaning of the CVD-grown graphene. A small D-peak is typically present for the CVD-grown graphene prior to Ar GCIB sputtering and the D-peak is significantly more pronounced following Ar GCIB cleaning of the sample. This increase in the I D /I G in the CVD-grown graphene is greater than was observed for the pristine mechanically exfoliated graphene for the same sputtering conditions. This difference in ∆I D /I G arises because, unlike the exfoliated graphene, there was significant disorder in the CVD-grown graphene we studied, before exposure to the Ar GCIB. The greater increase in the ∆ I D /I G for the CVD-grown graphene suggests that disordered regions of the graphene may be more susceptible to damage from the cluster beam. However, the larger ∆ I D /I G does not necessarily indicate that more defects have been introduced because the relationship between defect density and I D /I G is not linear, as shown previously24-25, 28. It is also likely that the CVD-grown graphene contains regions of graphene with more than one layer present and thickness of the graphene will have an influence on the Raman I D /I G value after exposure to the ion beam33. Further investigation is needed to better understand the relationship between the change in the Raman peak intensity cha (∆ I D /I G ) and the nature and density of the defects generated by the Ar cluster beam.

Conclusions


Page 10 of 22

Page 11 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


We have investigated the potential of using argon gas clusters to remove organic contaminants from graphene. Raman spectroscopy was used to assess the damage to the graphene layers as a function of Ar cluster ion size, kinetic energy and dose. We have shown that minimal damage is incurred for cluster ions with a kinetic energy at or below 1 eV/atom for doses up to 1 ion/nm2. Clusters with greater than 1 eV/atom create increasingly higher levels of disorder in single-layer graphene. This necessitates the use of a mass-selected cluster beam with a narrow size distribution in order to achieve adequate surface cleaning while minimizing damage. Although damage to the graphene layer is very low for clusters with kinetic energy at or below 1 eV/atom, there does not appear to be an energy threshold below which no damage is generated. Although our Raman spectroscopy measuments indicate a low defect density in the cleaned graphene, improvements in the electronic properties of the material what will be the determining factor for in industrial adoption of this cleaning techniqe. Further research is warranted to provide a full understanding of the influence of GCIB sputter cleaning on the materials electronic performance. We have demonstrated removal of PMMA residue using a 5 keV Ar 5000 cluster ion beam with minimal damage to the graphene layer. Since the introduction of lattice defects in the graphene layer does not begin until the PMMA layer is removed, the ability to non-destructively monitor the sputtering process could further reduce damage done to the graphene during the cleaning procedure. Although this is not possible in the ToF-SIMS instrument used for this study, new instrument designs should make in-situ monitoring of the cleaning process routine. Although our experiments focused on the removal of PMMA form CVD-grown graphene, argon gas cluster have also been shown to be efficient at sputtering a wide range of organic compounds similar to those used in the liquid phase exfoliation processes and thus this cleaning technique would also be applicable for this top-down type of large-scale graphene production. This technique is very promising for application in the industrial manufacture of graphene devices. Since industry-scale argon cluster sputter cleaning is already used for 300 mm silicon wafers it may readily be



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

adopted in the fabrication process. Further investigation to better understand the nature of the defects generated by the argon cluster cleaning process and any influence on the electronic properties of the graphene is warranted.

Acknowledgements The authors would like to acknowledge the National Measurement Office (NMO) for funding through the Innovation, Research and Development (IRD) programme (Project Numbers 115948 and 118616) and both Dr. Debdulal Roy and Dr. Alexander G. Shard for discussions related to the manuscript.

References 1. Allen, M. J.; Tung, V. C.; Kaner, R. B., Honeycomb Carbon: A Review of Graphene. Chem. Rev. 2009, 110, 132-145. 2. Siochi, E. J., Graphene in the Sky and Beyond. Nat Nano 2014, 9, 745-747. 3. Novoselov, K. S.; Falko, V. I.; Colombo, L.; Gellert, P. R.; Schwab, M. G.; Kim, K., A Roadmap for Graphene. Nature 2012, 490, 192-200. 4. Martinez-Martin, D.; Longuinhos, R.; Izquierdo, J. G.; Marele, A.; Alexandre, S. S.; Jaafar, M.; Gomez-Rodriguez, J. M.; Banares, L.; Soler, J. M.; Gomez-Herrero, J., Atmospheric Contaminants on Graphitic Surfaces. Carbon 2013, 61, 33-39. 5. Gass, M. H.; Bangert, U.; Bleloch, A. L.; Wang, P.; Nair, R. R.; Geim, A. K., Free-Standing Graphene at Atomic Resolution. Nat Nano 2008, 3, 676-681. 6. Dan, Y.; Lu, Y.; Kybert, N. J.; Luo, Z.; Johnson, A. T. C., Intrinsic Response of Graphene Vapor Sensors. Nano Lett 2009, 9, 1472-1475. 7. Li, Z. T., et al., Effect of Airborne Contaminants on the Wettability of Supported Graphene and Graphite. Nature Materials 2013, 12, 925-931. 8. Wehling, T. O.; Yuan, S.; Lichtenstein, A. I.; Geim, A. K.; Katsnelson, M. I., Resonant Scattering by Realistic Impurities in Graphene. Phys Rev Lett 2010, 105, 056802. 9. Li, W., et al., Ultraviolet/Ozone Treatment to Reduce Metal-Graphene Contact Resistance. Appl Phys Lett 2013, 102. 10. Lee, J.; Kim, Y.; Shin, H. J.; Lee, C.; Lee, D.; Moon, C. Y.; Lim, J.; Jun, S. C., Clean Transfer of Graphene and Its Effect on Contact Resistance. Appl Phys Lett 2013, 103, 5. 11. Hernandez, Y., et al., High-Yield Production of Graphene by Liquid-Phase Exfoliation of Graphite. Nat Nano 2008, 3, 563-568. 12. Pupysheva, O. V.; Farajian, A. A.; Knick, C. R.; Zhamu, A.; Jang, B. Z., Modeling Direct Exfoliation of Nanoscale Graphene Platelets. J. Phys. Chem. C 2010, 114, 21083-21087.


Page 12 of 22

Page 13 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


13. Choi, H.; Kim, J.; Cho, Y.; Hwang, T.; Lee, J.; Kim, T., Conditioning of Graphene Surface by Co 2 Cluster Jet. Rsc Adv 2014, 4, 41922-41926. 14. Longchamp, J. N.; Escher, C.; Fink, H. W., Ultraclean Freestanding Graphene by Platinum-Metal Catalysis. J Vac Sci & Tech B (Microelectronics and Nanometer Structures) 2013, 31, 020605 (3 pp.)020605 (3 pp.). 15. Gong, C., et al., Rapid Selective Etching of Pmma Residues from Transferred Graphene by Carbon Dioxide. J Phys Chem C 2013, 117, 23000-23008. 16. Lin, Y. C.; Lu, C. C.; Yeh, C. H.; Jin, C. H.; Suenaga, K.; Chiu, P. W., Graphene Annealing: How Clean Can It Be? Nano Lett 2012, 12, 414-419. 17. Lemme, M. C.; Bell, D. C.; Williams, J. R.; Stern, L. A.; Baugher, B. W. H.; Jarillo-Herrero, P.; Marcus, C. M., Etching of Graphene Devices with a Helium Ion Beam. Acs Nano 2009, 3, 2674-2676. 18. Pirkle, A.; Chan, J.; Venugopal, A.; Hinojos, D.; Magnuson, C. W.; McDonnell, S.; Colombo, L.; Vogel, E. M.; Ruoff, R. S.; Wallace, R. M., The Effect of Chemical Residues on the Physical and Electrical Properties of Chemical Vapor Deposited Graphene Transferred to Sio2. Appl Phys Lett 2011, 99, 122108. 19. Yamada, I.; Matsuo, J.; Toyoda, N.; Kirkpatrick, A., Materials Processing by Gas Cluster Ion Beams. Materials Sci & Eng: R: Reports 2001, 34, 231-295. 20. Kirkpatrick, A., Gas Cluster Ion Beam Applications and Equipment. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 2003, 206, 830-837. 21. Ninomiya, S.; Nakata, Y.; Ichiki, K.; Seki, T.; Aoki, T.; Matsuo, J., Measurements of Secondary Ions Emitted from Organic Compounds Bombarded with Large Gas Cluster Ions. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 2007, 256, 493-496. 22. Lee, J. L. S.; Ninomiya, S.; Matsuo, J.; Gilmore, I. S.; Seah, M. P.; Shard, A. G., Organic Depth Profiling of a Nanostructured Delta Layer Reference Material Using Large Argon Cluster Ions. Analytical Chem 2010, 82, 98-105. 23. Shard, A. G., et al., Argon Cluster Ion Beams for Organic Depth Profiling: Results from a Vamas Interlaboratory Study. Analytical Chem 2012, 84, 7865-7873. 24. Seah, M. P., Universal Equation for Argon Gas Cluster Sputtering Yields. J Phys Chem C 2013, 117, 12622-12632. 25. Toyoda, N.; Yamada, I. In Cluster Size Dependence of Etching by Reactive Gas Cluster Ion Beams, AIP Conference Proceedings, 2008. 26. Ferrari, A. C.; Basko, D. M., Raman Spectroscopy as a Versatile Tool for Studying the Properties of Graphene. Nat Nano 2013, 8, 235-246. 27. Lucchese, M. M.; Stavale, F.; Ferreira, E. H. M.; Vilani, C.; Moutinho, M. V. O.; Capaz, R. B.; Achete, C. A.; Jorio, A., Quantifying Ion-Induced Defects and Raman Relaxation Length in Graphene. Carbon 2010, 48, 1592-1597. 28. Cançado, L. G.; Jorio, A.; Ferreira, E. H. M.; Stavale, F.; Achete, C. A.; Capaz, R. B.; Moutinho, M. V. O.; Lombardo, A.; Kulmala, T. S.; Ferrari, A. C., Quantifying Defects in Graphene Via Raman Spectroscopy at Different Excitation Energies. Nano Lett 2011, 11, 3190-3196. 29. Shaforost, O.; Wang, K.; Adabi, M.; Guo, Z.; Hao, L.; Gallop, J.; Klein, N. In Microwave Characterization of Large Area Graphene Using a Te01δ Dielectric Resonator, Physics and Engineering of Microwaves, Millimeter and Submillimeter Waves (MSMW), 2013 International Kharkov Symposium on, 23-28 June 2013; 2013; pp 427-429. 30. Pollard, A. J.; Brennan, B.; Stec, H.; Tyler, B. J.; Seah, M. P.; Gilmore, I. S.; Roy, D., Quantitative Characterization of Defect Size in Graphene Using Raman Spectroscopy. Appl Phys Lett 2014, 105, -. 31. Murakami, K.; Kadowaki, T.; Fujita, J.-i., Damage and Strain in Single-Layer Graphene Induced by Very-Low-Energy Electron-Beam Irradiation. Appl Phys Lett 2013, 102, 043111. 32. Graham, D. J.; Castner, D. G., Image and Spectral Processing for Tof-Sims Analysis of Biological Materials. Mass Spectrometry 2013, 2.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

33. Martins Ferreira, E. H.; Moutinho, M. V. O.; Stavale, F.; Lucchese, M. M.; Capaz, R. B.; Achete, C. A.; Jorio, A., Evolution of the Raman Spectra from Single-, Few-, and Many-Layer Graphene with Increasing Disorder. Phys Rev B 2010, 82, 125429.


Page 14 of 22

Page 15 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1: Cluster size distributions for the three Ar cluster beam conditions. The 10 keV Ar 2000 clusters have a full-width-at-half-maximum (FWHM) of 800 atoms. The 5 keV Ar 5000 clusters have a FWHM of 1000 atoms and the 2.5 keV Ar 5000 clusters have a FWHM of 700 atoms.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2: Raman spectra for mechanically exfoliated graphene flakes exposed to 1.00 ion/nm2 of 10 keV Ar 2000 , 2.25 ions/nm2 of 5 keV Ar 5000 and 2.00 ions/nm2 of 2.5 keV Ar 5000 . A Raman spectrum of pristine graphene is included for comparison.


Page 16 of 22

Page 17 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. Ratio of Raman D- and G-peak intensities, as a function of Ar cluster ion dose for three GCIB beam conditions. The top plot (a), shows the intensity ratio as a function of ion dose and the lower plot (b) shows the intensity ratio as a function of equivalent PMMA sputter depth, with broken lines included as a guide for the reader.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 4. SIMS sputter depth profile of PMMA residue on a 200 µm × 200 µm area of CVD-grown graphene sample prior to Argon cluster cleaning. An image of the PMMA characteristic peak, integrated through the sputter depth, is shown in (a) and a 3D reconstruction of the PMMA overlayer is shown in (b). The depth profiles as a function of sputter ion dose which are plotted in graph (d), are for selected regions of (a) and correspond to the red and blue regions in (c).


Page 18 of 22

Page 19 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. 7 mm × 7 mm ToF-SIMS image of PMMA residue on CVD1. The location of the graphene, inferred based on differential charging of the sample, is shown in (a), and the region outlined in blue has been cleaned with a dose of 1.00 ion/nm2 of 5 keV Ar 5000 . An overlay of the graphene and PMMA signals (from (a) and (b) respectively) are shown in (c) with some regions of thicker PMMA residue (green signal) remaining on the surface after ion dosing.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 6. Lateral maps of D- and G-peak intensity ratio for the same area of CVD-grown graphene sample CVD2, before (a) and after (b) cleaning with 5.00 ions/nm2 5 keV Ar 5000 . The white regions observed in image (a) before cleaning are areas where the graphene is covered with a thick layer of carbonaceous material.


Page 20 of 22

Page 21 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7. Detailed Raman spectroscopy analysis of CVD2 region in Figure 6, before and after cleaning with 5 keV Ar 5000 . A histogram of the D- to G-peak intensity ratio is shown in (a) and represented spectra from a selected pixels are shown in (b).



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

221x89mm (72 x 72 DPI)


Page 22 of 22

Removal of Organic Contamination from Graphene with a Controllable

Recommend Documents