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Applied Surface Science 264 (2013) 727– 731 Contents lists available at SciVerse ScienceDirect Applied Surface Science j our nal ho me p age: www.elsevier.com/loc ate/apsusc Effects of surface impurities on epitaxial graphene growth Valeria del Campo∗, Ricardo Henríquez, Patricio Häberle Departamento de Física, Universidad Técnica Federico Santa María, Avenida Espa˜ na 1680, 2390123 Valparaíso, Chile a r t i c l e i n f o Article history: Received 5 August 2012 Received in revised form 17 October 2012 Accepted 22 October 2012 Available online 29 October 2012 Keywords: Graphene Ru(0 0 0 1) Carbon nanodiscs CVD SEM STM a b s t r a c t The focus of this report is to explore the large scale growth of graphene on Ru(0 0 0 1) and verify the possible effects of crystallographic defects and impurities in the quality of the synthesized material. After a Low Pressure Chemical Vapor Deposition (LP-CVD) process we obtained a graphene ﬁlm accompanied by other types of graphitic structures. Impurities on the ruthenium surface behaved as nucleation sites in the formation of carbon islands several micrometers wide. The morphological structure of these islands is constituted by carbon discs with diameters in the range of few to several hundred nanometers and thicknesses always below 1 nm. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Epitaxial graphene growth has some advantages compared to HOPG exfoliation and other chemical methods to produce a high quality material . There is an improved control on ﬁlm size and larger crystalline domains can be obtained. Low Pressure Chemical Vapor Deposition (LP-CVD), using monocrystalline ruthenium as substrate, has allowed the growth of graphene sheets with microm- eter lengths [2,3]. However, graphene transfer from ruthenium to other surfaces is still an open problem, mainly because this substrate is too hard for its use in “roll to roll” methods  and too expensive to release graphene by etching away the substrate . One possible method would be “bubbling transfer”, recently reported by Gao et al.  to release graphene from Pt. Whichever method is used, it is important to gain control of the large scale structure of the carbon ﬁlm. A relevant component for this pur- pose is to verify the effect of defects or impurities on the substrate surface. Graphene growth on Ru(0 0 0 1) through LP-CVD is widely reported in literature and important efforts have been made to understand the mechanism of graphene formation [2,3,7–12]. This process comprises exposure of the substrate to a gaseous carbon source, usually ethylene. Complete dehydrogenation of ethylene on ∗ Corresponding author at: Departamento de Física, Avenida Espa˜na 1680, 2390123 Valparaíso, Chile. Tel.: +56 32 2654021; fax: +56 32 2797656. E-mail addresses: firstname.lastname@example.org, email@example.com (V. del Campo). ruthenium occurs at 700 K. If the substrate temperature exceeds 1360 K [2,7,11] carbon atoms are absorbed in Ru. In this case graphene formation occurs by cooling down the sample, due to a lower C solubility, carbon atoms are forced to diffuse to the surface [2,11]. Other possible way for graphene formation is exposure to ethylene, with substrate temperature low enough to avoid carbon absorption, and then a temperature rise to facilitate surface motion and nucleation of the carbon atoms to form the graphene structure. In this case, substrate temperature was always kept below 1300 K [3,9]. Even though there is much work on graphene on Ru(0 0 0 1), most reports analyze areas smaller than 500 m × 500 m [2,3,9–11], and do not describe the large scale behavior, which could be relevant in technical applications. The aim of this work is to study the formation of carbon struc- tures, in addition to graphene, on Ru(0 0 0 1). For this purpose we analyzed the long range morphology and the effect of surface impu- rities on the carbon ﬁlm formation. During the sample preparation we left some impurities on the substrate. They were predominantly oxidized micrometer size particles composed by copper, aluminum or silicon. After the LP-CVD process, we used Low Energy Elec- tron Diffraction (LEED) and conﬁrmed that a graphene monolayer was formed on Ru . Nevertheless, Scanning Electron Microscopy (SEM) showed in addition, the formation of large carbon islands. These islands grew only around the impurities and their structure was mainly composed by carbon nanodiscs. Sizes and morphol- ogy were characterized with Scanning Tunneling Microscopy (STM). The diameter of the observed discs varied from less than 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.10.109 728 V. del Campo et al. / Applied Surface Science 264 (2013) 727– 731 Fig. 1. LEED patterns after the LP-CVD process. In the marked regions diffraction spots were enhanced to overcome lack of focus of our electron diffractometer and photographic issues. Relative intensities of satellites spots change with energy. (a) 150 eV and (b) 170 eV. 20 to more than 700 nm, while their thickness did not exceed 1 nm. 2. Materials and methods The Ru(0 0 0 1) crystal, 99.99% purity, was commercially obtained (Mateck). First we performed an elemental analysis of the crystal as delivered with Energy Dispersive Spectroscopy (EDS). From this study we found surface impurities present in the form of particles. They were mainly composed of Cu, Si, O, Al, N and Ca. Also Zn, P and K were present in very low concentrations. The pres- ence of copper and calcium is reported in the Ru(0 0 0 1) data sheet as impurities. The presence of silicon, nitrogen and oxygen as con- taminants in ruthenium crystals was previously reported by Grantt and Haas . In their work they explored a cleaning procedure for Ru(0 0 0 1), that has become standard for graphene growth on Ru. It consists on cycles of sample annealing and argon-ion sputtering. In order to keep some of the impurities on the surface of our substrate, the cleaning procedure consisted on heating the sample to 1300 K followed by argon-ion sputtering (1 kV) for 15 min at room temper- ature and ﬁnal heating up to 900 K. We performed this process only once before the LP-CVD. The in situ LEED obtained after the cleaning procedure was consistent with sharp Ru(0 0 0 1) diffraction spots. The LP-CVD process was performed in an ultrahigh vacuum (UHV) chamber at a base pressure of 2 × 10−10 Torr. Initially, the substrate was heated up to 980 K, at this temperature Ru was exposed to 23 L of ethylene. After exposure, the sample was annealed at 1100 K for 40 min. The resulting carbon structure was characterized by LEED, SEM–EDS and STM. LEED measurements were performed in situ with the electron gun 5 cm away from the samples. We used ex situ High Vacuum SEM with low electron energies (∼5 kV) to study the morphology of the carbon ﬁlm across the entire substrate. This measurement was complemented with Energy Dispersive Spectroscopy (EDS). The analysis of the samples morphology, in the nanoscale, was performed with STM at room temperature in UHV (1 × 10−10 Torr). This characterization was ex situ, after exposure to air, nevertheless the graphene covered samples are fairly inert to air exposure, as was veriﬁed by the robustness of the LEED patterns after air exposure. To correlate SEM and STM information, zones of the samples were marked previous to both characterizations. 3. Results Fig. 1 shows LEED patterns taken in situ, at two different ener- gies, after the LP-CVD process. Diffraction spots from Ru(0 0 0 1) are surrounded by satellite spots. This pattern is linked to a Moiré structure induced by the mismatch between the graphene lattice and the Ru surface [2,3]. SEM–EDS analysis of the same sample show large carbon islands nucleated only around impurity particles on the ruthenium sur- face. The diameters of these islands vary from 40 m to more than 500 m. SEM images show them to be homogeneous in the inside but marbled on the edges (Fig. 2). This characteristic structure, plus Fig. 2. Scanning Electron Microscopy (SEM) images taken at 5 kV. (a) Carbon islands of different sizes, some of them have diameters larger than 500 m. All islands are roughly centered on a bright spot (Ru surface impurity). (b) Larger magniﬁcation in which every islands seem to grow around an impurity and (c) Detailed SEM image of the C-island edges, where the surface looks marbled or rariﬁed. V. del Campo et al. / Applied Surface Science 264 (2013) 727– 731 729 Fig. 3. (a) SEM image of a particle inside a carbon island and (b) Energy Dispersive Spectroscopy (EDS) spectrum captured from the particle shown in (a). the fact the islands’ edges percolate, conﬁrm that carbon preferen- tially migrates away from the impurities. Extensive EDS analysis was performed to conﬁrm the car- bon content of the islands and to discard the possibility that the nucleation centers were amorphous carbon particles instead of ruthenium impurities. EDS spectra from the nucleation centers showed most of them were composed by Al, Si and Ca or their corresponding oxides. A characteristic EDS spectrum of these par- ticles is presented in Fig. 3. Particles composed only by Cu (with a small amount of oxygen) were also present, an example is shown in Fig. 4, where an EDS map distinguishes the Cu particle from the Ru substrate. EDS spectra from the islands conﬁrm they are only composed by carbon (Fig. 4). There was no evidence of particles containing Zn, P, K and N, elements present on the sample before Fig. 4. EDS characterization of a carbon island on the Ru surface. (a) SEM image of a carbon island around its nucleating particle. (b) EDS spectrum captured from the selected region shown in (a). (c) SEM image of the center of the island in (a). (d) EDS-Cu map from the selected region shown in (c), lighter section corresponds to Cu and (e) EDS-Ru map of the region shown in (c), gray area corresponds to Ru. The complementary nature of images (d) and (e), indicates the particle is indeed formed by Cu. 730 V. del Campo et al. / Applied Surface Science 264 (2013) 727– 731 Fig. 5. STM images taken with Vsample = 0.2 V. (a) Carbon discs of different sizes shown in a 2000 nm × 2000 nm, It = 0.22 nA. (b) A smaller area scan conﬁrms the circular shape of the small discs. 400 nm × 400 nm, It = 0.22 nA and (c) Larger section of the sample away from the island center. Ru terrace steps appear decorated with the circular structures. 1500 nm ×1500 nm, It = 0.35. the cleaning procedure. The diameters of the impurities remained almost unaffected by the sputtering annealing cycles. Their sizes vary from tenths of a micron to no more than 40 m. There is no correlation between the particle size and the islands diameter (Fig. 2b). The structure of these carbon islands was studied with STM. Fig. 5(a) and (b) shows measurements performed at the inner parts of the islands, while Fig. 5(c) is representative of STM images taken away from the corresponding “centers”. These STM images indicate the islands are made up by a superposition of discs with differ- ent sizes. The diameters of the discs vary from less than 20 nm to more than 700 nm. A statistical analysis of the diameter distribution (more than 300 discs were measured) resulted in two main popula- tions. The ﬁrst group had far more discs than the second one. Their mean diameter was 42 nm with a standard deviation of 4 nm. The second group presented a mean diameter of 292 nm with a standard deviation of 32 nm. Besides these two groups, there were few larger discs with diameters varying between 500 and 700 nm. Discs of less than 20 nm were found by moving the tip away from the carbon islands to regions where Ru terrace steps were observed. The steps were decorated by these small discs (Fig. 5c). Discs height was mea- sured with STM, and we found that independent of diameters, discs height varied between 3 and 10 ˚A. 4. Discussion LEED results conﬁrm the formation of graphene on the Ru(0 0 0 1) surface. Graphene formation at 1100 K seems to occur by diffusion of the carbon atoms on the ruthenium surface . We observed a larger accumulation of C atoms around impurities on the surface. This preferential nucleation around surface impurities is expected as a mechanism to reduce surface energy. A similar effect is observed at the surface step edges, where we also found carbon discs. Marchini et al.  reported exclusive graphene nucleation at the steps when annealing temperature is 1000 K, while for an annealing temperature higher than 1470 K the surface is fully cov- ered by graphene. Our annealing temperature is 1100 K; therefore growth in our samples is closer to the low temperature regime where nucleation commonly occurs on step edges. Preferential nucleation around impurity particles and steps allowed the growth of the observed carbon structures, as an alternative to an exclusive graphene formation. Most studies on graphene growth on Ru(0 0 0 1) seems to be limited to small areas, STM images do not exceed 200 nm × 200 nm [3,9,10], SEM images show areas as large as 500 m × 500 m  and LEEM images are not larger than 30 m × 30 m . One exception is the report of Pan et al.  in which a millimeter-scale crystalline graphene monolayer was observed on Ru(0 0 0 1) and characterized by LEED and STM. However, they reported the forma- tion of large clusters several nanometers high almost all over the graphene layer. These clusters were removed by ﬂash annealing in an oxygen atmosphere. To the best of our knowledge, no fur- ther attention has been given to the formation of carbon clusters on Ru(0 0 0 1). We analyzed the structure of the carbon islands formed around impurities. These islands were mainly composed by carbon discs with diameters varying from a few tens to several hundreds nanometers. The uniform height and low thickness of these discs (between 3 and 10 ˚A) leads us to propose they have a graphitic arrangement, but further studies are required to describe their atomic structure in detail. Literature on carbon nanodiscs (CNDs) or graphene discs [14–19], report discs of diameters in the range of 100 nm to 3 m and thicknesses varying from 3 to 70 nm. In none of these cases the discs were obtained through a CVD process. This method may present some advantages for CNDs growth. First, allows the growth of small discs, less than 10 ˚ A thickness and diameters in the nanometer scale. Second, regions of the substrate for discs growth could be selected by a careful implantation of particles onto the ruthenium surface. Also, since CVD conditions control size of graphene layers  we believe the size of the discs could be con- trolled by a proper combination of annealing time and temperature. 5. Conclusions We grew a graphene monolayer on Ru(0 0 0 1) through LP-CVD. Large areas of the surface were analyzed with SEM–EDS. Through this study we veriﬁed the effect of impurities on the resulting car- bon ﬁlm. Large carbon islands were found to grow around the impurity particles. 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Rutt, Tunable trans- mission in a graphene photonic crystal in mid-infrared, in: Graphene 2011 Conference, Bilbao, Spain, 11–14 April, 2011. f impurities on the resulting car- bon ﬁlm. Large carbon islands were found to grow around the impurity particles. The formation of carbon structures, different from graphene, is a subject relevant in large scale or industrial graphene fabrication based on ruthenium. We observed the formation of carbon nanodiscs on Ru(0 0 0 1) through LP-CVD. This is an alternative procedure to grow thin (below 1 nm) carbon nanodiscs and it may provide a way to control discs diameters in the range of a few to several hundred nanome- ters. Acknowledgements This research was partially supported by FONDECYT grant no. 1110935 and CENAVA grant no. 791100037. V. del Campo et al. / Applied Surface Science 264 (2013) 727– 731 731 References  C. Soldano, A. Mahmood, E. Dujardin, Production, properties and potential of graphene, Carbon 48 (2010) 2127–2150.  P.W. Sutter, J.I. Flege, E.A. Sutter, Epitaxial graphene on ruthenium, Nature Materials 7 (2008) 406–411.  S. Marchini, S. Günther, J. Wintterlin, Scanning tunneling microscopy of graphene on Ru(0 0 0 1), Physical Review B 76 (2007) 075429–75439.  Z.Y. Juang, C.Y. Wu, A.Y. Lu, C.Y. Su, K.C. Leou, F.R. Chen, C.H. Tsai, Graphene synthesis by chemical vapor deposition and transfer by a roll-to-roll process, Carbon 48 (2010) 3169–3174.  K.S. Kim, Y. Zhao, H. Jang, S.Y. Lee, J.M. Kim, K.S. Kim, J.H. Ahn, P. Kim, J.Y. Choi, B.H. Hong, Large-scale pattern growth o