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Ru(0001) interface
Surface Science 617 (2013) 81–86
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A comparative study of intercalation mechanism at graphene/Ru(0001) interface Li Jin, Qiang Fu ⁎, Yang Yang, Xinhe Bao ⁎ State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, The Chinese Academy of Sciences, Dalian 116023, PR China
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Article history: Received 31 March 2013 Accepted 8 July 2013 Available online 14 July 2013 Keywords: Graphene LEEM PEEM Intercalation Ru(0001)
a b s t r a c t Both Ni and Pb intercalation reactions at graphene/Ru(0001) interface were studied by low energy electron microscopy (LEEM) and photoemission electron microscopy (PEEM). It is suggested that the Ni intercalation is dominated by an exchange intercalation mechanism, in which Ni adatoms produce transient atomic-scale defects in the graphene lattice and penetrate through the carbon monolayer. In contrast, the Pb intercalation process needs to be facilitated by the diffusion of Pb atoms through extended defect sites of graphene, such as open edges and domain boundaries. The two contrast intercalation mechanisms originate from the different interaction strength of the intercalated elements with carbon. Different responses of the graphene electronic structure to the Ni and Pb intercalation reactions were observed by PEEM. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Epitaxial growth on metals and SiC has been regarded as an important route to prepare high-quality graphene overlayers. However, electronic coupling between the epitaxial graphene structures and the substrate surfaces may damage the characteristic Dirac cone band structure of graphene, which impedes many potential applications of the new material [1–7]. Intercalation of elements or molecules at graphene/ substrate interfaces can decouple the epitaxial graphene overlayers from the substrate surfaces and recover the intriguing electronic properties of freestanding graphene [8–17]. For example, quasi-freestanding epitaxial graphene overlayers can be obtained on SiC(0001) by H intercalation [8]. A linear π-band dispersion near K point has been observed at Au-intercalated graphene/Ni(111) surface [18]. Besides elements, molecules such as H2O and CO can diffuse into the nanospace between graphene and substrate, and chemical reactions may even happen in the two-dimensional (2D) nanospace under graphene sheets [19,20]. The graphene covers can affect the surface chemistry at the metal surfaces [15,17,20,21]. These novel physical and chemical phenomena under graphene overlayers have stimulated extensive studies in intercalation reactions at graphene surfaces in the past few years. Scanning tunneling microscopy (STM) and angle-resolved photoemission spectroscopy (ARPES) are often applied to investigate the intercalation processes. Surface morphology and atomic structure of intercalated graphene layers can be explored by STM. Atomically resolved STM images show that the intercalated graphene surfaces still keep a well-defined graphitic structure but surface moiré patterns ⁎ Corresponding authors. Tel.: +86 411 84379253; fax: +86 411 84694447. E-mail addresses: [email protected] (Q. Fu), [email protected] (X. Bao). 0039-6028/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.susc.2013.07.008
may disappear [17,18,22,23]. ARPES measurements help to reveal the electronic structure of graphene. Linear π bands near Fermi level are often observed at the intercalated graphene surfaces, demonstrating the quasi-freestanding character recovered by the intercalation effect [8,18]. However, these techniques cannot give answers to a question how intercalation happens at a graphene surface. A deep understanding of the intercalation mechanism relies on dynamic characterization methods, by which the surface intercalation processes can be investigated in-situ. Low energy electron microscopy (LEEM) and photoemission electron microscopy (PEEM) are powerful surface imaging techniques for the studies of surface growth, surface phase transition, and surface reactions [24–26]. In recent years, these imaging techniques have been successfully applied to study graphene surfaces [27,28]. The capability of PEEM/LEEM to perform real-time and dynamic imaging enables understanding graphene growth and surface reactions [9,20,29–34]. With the aid of the dynamic characterization of intercalation reactions at graphene surfaces using PEEM/LEEM, the intercalation reactions were assumed to occur via diffusion of intercalants through extended defect sites at graphene surfaces, e.g. island edges and domain boundaries [15,17,29,34–36]. However, our recent study in Si intercalation at graphene/Ru(0001) interface reveals a novel Si–C exchange mechanism, in which surface Si atoms can penetrate through the graphene lattice and form 2D structures between graphene and Ru(0001) surface. We suggest that the exchange intercalation mechanism is driven by the strong interaction of Si with both carbon and Ru, and this mechanism should be active in other intercalation processes [37]. In this work, a comparative study in both Ni and Pb intercalation reactions at graphene/Ru(0001) interface has been performed using in-situ LEEM/PEEM. We found that the Ni intercalation occurs via
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penetration of Ni atoms through the graphene lattice, while graphene domain boundaries play an important role in the Pb intercalation. The two contrast intercalation processes produce different intercalated structures and opposite changes in surface work function. We propose that most intercalation processes at graphene surfaces can be well explained by the two typical mechanisms. 2. Experimental All experiments were conducted in an Elmitec LEEM/PEEM system, which consists of a preparation chamber, a microscopic imaging system equipped with a hemisphere energy analyzer, an aberration corrector, a field emission gun, as well as a vacuum ultraviolet (VUV) laser source (λ = 177.3 nm) [35]. The preparation chamber is equipped with a K-cell evaporation source containing Ni particles. A similar evaporation source is mounted in the imaging chamber for Pb deposition, which can be in-situ monitored by LEEM/PEEM. Ru(0001) surface was prepared by cycled Ar+ sputtering (2.0 keV, 7 × 10− 6 Torr Ar, 10 min), heating in O2 (530 °C, 5 × 10− 7 Torr O2), and annealing in ultrahigh vacuum (UHV) at 1200 °C. The clean Ru(0001) surface was then doped with carbon by exposing to ethylene (C2H4) at 900 °C to have an enough amount of carbon at the near surface region. Ni and Pb were deposited onto the graphene/Ru(0001) surface at room temperature. The background pressure is in a 10−10 Torr range during the metal evaporation processes. In-situ PEEM/LEEM imaging was performed at the Ni- and Pb-covered graphene/Ru(0001) surfaces during UHV annealing at various temperatures. 3. Results and discussion Epitaxial graphene overlayers were obtained on the Ru(0001) surface by the surface segregation process. Each graphene island nucleates from one site and continues to grow in size up to hundreds of micrometers. A typical low energy electron diffraction (LEED) pattern taken from the graphene/Ru(0001) surfaces was shown in Fig. 1a. The pristine graphene surface shows a surface superstructure containing a (12 × 12) lattice of graphene matched to a (11 × 11) lattice of Ru(0001) [38–40]. The quality of the graphene layer is quite high as established by the appearance of high-order diffraction spots in the LEED patterns [41].
Ni was deposited onto the graphene/Ru(0001) surface at room temperature. The micro-region LEED (μ-LEED) pattern recorded from the Ni-covered graphene surface shows little change except for slight diffusing of the background, which may be induced by scattering from the deposited Ni clusters. When the surface was heated up to 470 °C, the intensity of the (0, 1) diffraction spots corresponding to Ru(0001) surface (marked by green arrows in Fig. 1a and b) is much weaker than that from the pristine graphene/Ru(0001) surface. The similar results have been reported at intercalated graphene/SiC(0001) surfaces, in which the diffraction spots from the SiC substrate get weakened or even disappear after intercalation [8,29]. As shown in Fig. 1b, the second- and higher-order diffraction spots around the (0, 0) and (0, 1) diffraction spots disappeared but the first-order diffraction spots are still observed. For comparison, μ-LEED was performed on the bare Ru(0001) surface regions covered by Ni. The first-order diffraction spots around the (0, 0) and (0, 1) spots were also observed in the pattern (Fig. 1c). It has been reported that relaxed epitaxial Ni films form on Ru(0001) surface, showing a hexagonal moiré superstructure, and LEED pattern from the Ni films consists of satellite spots [42,43]. Since the difference of lattice constant between Ni and graphene is less than 1.2% [44,45], both graphene/Ru(0001) and Ni/Ru(0001) surfaces should have the similar periodicity in the moiré superstructure. Accordingly, we infer that the first-order diffraction spots around the (0, 0) and (0, 1) spots observed in Fig. 1b can be attributed to the superstructure of Ni layer, which intercalates under graphene and epitaxially grows on Ru(0001). This conclusion can be further confirmed by recent STM results showing that the Ni-intercalated graphene structures display the same moiré patterns as the pristine graphene surfaces [16,22]. The similar LEED study has been performed for Pb intercalation at the graphene/Ru(0001) surface. Deposition of Pb at room temperature also results in the diffusing of the LEED pattern background. Annealing the Pb-covered graphene surface around 150 °C causes a big change in the LEED pattern (Fig. 1d). First, the intensity of the (0, 1) diffraction spots from the Ru(0001) surface has been largely suppressed, similar to the case of Ni-intercalated surfaces. Second, all the high-order diffraction spots around the (0, 0) and (0, 1) spots disappear, suggesting that the Pb intercalation makes the graphene overlayers freestanding. This is fully consistent with our previous work [35]. Finally, additional diffraction spots as marked by arrows in Fig. 1d appear, which can be attributed to the intercalated Pb layer formed between the topmost
Fig. 1. LEED patterns recorded from pristine graphene/Ru(0001) surface (a), Ni-intercalated graphene/Ru(0001) surface regions (b), Ni-covered Ru(0001) surface regions (c), Pb-intercalated Ru(0001) surface regions (d), and Pb-covered Ru(0001) surface regions (e). The LEED patterns of Ru(0001) substrate (Ru), graphene (G), Ni overlayers on Ru, and Pb overlayers on Ru are marked by arrows in the figures. The electron beam energy for the LEED measurement is 50 eV.
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graphene and the Ru(0001) surface, since a similar diffraction structure has been observed in the μ-LEED pattern recorded from the Pb-covered Ru(0001) surface regions (Fig. 1e). LEEM images acquired from the Ni and Pb intercalated graphene surfaces are shown in Fig. 2a and b, respectively. For the Ni-intercalated surface, the most important feature is that the intercalated islands with the bright contrast are distributed on the graphene surface randomly. Some islands have a well-defined shape, and the faceted structures are believed to be related to a strong substrate symmetry effect [22,23]. Highly dispersed intercalated islands under graphene have been observed by STM in the cases of Ni intercalation at graphene/Ru(0001) and graphene/Rh(111) surfaces. The LEEM and STM results from the Ni-intercalated graphene surfaces are also similar to our observations at the Si-intercalated graphene/Ru(0001) surface. On the contrary, the Pb intercalation produces different surface structures. Our previous work has shown that the Pb intercalation starts from the edge sites of each graphene island [35]. A clear reaction frontier is present, which moves forwards from the island edges to the center. A LEEM image which contains a Pb-intercalated graphene region (with dark contrast) and a pristine graphene region (with bright contrast) is shown in Fig. 2b, where the intercalation process was quenched deliberately before its completion. We can see that the intercalated region is uniform in the surface structure, indicating the occurrence of Pb-intercalation everywhere in this region. In a LEEM imaging mode, the intensity of a reflected electron beam (noted as I) varies with the energy of an incident electron beam (noted as V). When the electron beam energy exceeds the work function difference between the electron gun and the sample surface, the
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reflected electron beam intensity decreases due to electron–surface interactions [46]. Thus, the surface work function change can be derived from the I–V curves by measuring the shift of the energy position where the reflected intensity decreases by 10% [46–50]. The I–V curves were recorded at the pristine graphene surface, the metal-covered graphene surfaces, and the metal-intercalated graphene surface regions (marked by squares in the LEEM images). As shown in Fig. 2c and d, the surface work function presents an increase of 1.2 and 0.1 eV in the cases of Ni and Pb deposition at room temperature, respectively. The large increase in the surface work function of the Ni-covered graphene surface is due to the much large work function of Ni (5.04–5.35 eV) compared to that of Pb (4.25 eV) [51]. Upon annealing, the occurrence of intercalation leads to the variation of the surface work function. The surface work function decreased by 0.9 eV after the Ni intercalation, while it increased by 0.6 eV after the Pb intercalation. Overall, compared to the pristine graphene/Ru(0001) surface the Ni intercalation does not produce a significant change in surface work function, while the Pb intercalation results in an increase of the surface work function by 0.7 eV. Density functional theory (DFT) calculations revealed that many metals can transfer charge to graphene and, thus, decrease the surface work function [3]. The stronger the metal–graphene interaction, the lower the work function of the graphene/metal surface. Both Ni and Ru present strong interaction with graphene [5,52], and thus the graphene/ Ni and graphene/Ru structures may present a similar lower work function than the free graphene structure. Therefore, a structural change from the graphene/Ru(0001) surface to the graphene/Ni/Ru(0001) surface does not produce obvious variation in the surface work function (Fig. 2c). The surface work function of graphene/Pb structure should be
Fig. 2. LEEM images from a Ni-intercalated graphene/Ru(0001) surface region (a) and a Pb-intercalated graphene/Ru(0001) surface region (b). The start voltage was set at 1.3 and 2.0 V, respectively for images (a) and (b). I–V curves were acquired for the Ni (c) and Pb (d) intercalation. The curves are from the pristine graphene/Ru(0001) surface, Ni and Pb-covered graphene surfaces, and Ni and Pb-intercalated surfaces.
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similar to that of the free graphene because of the weak interaction of Pb with graphene [53]. It can be understood that the transformation of the graphene/Ru(0001) surface into the graphene/Pb/Ru(0001)surface results in an increase in the work function (Fig. 2d). As shown above, the intercalation reactions were accompanied by changes in the surface work function. Therefore, these surface processes can be effectively investigated by in-situ PEEM. For the graphene/ Ru(0001) surface, graphene domains are bright and the bare Ru(0001) surface regions are dark in the PEEM images excited by Hg lamp [54]. After the Ni deposition at room temperature, the whole surface becomes dark due to the increase in the surface work function, which has been confirmed by the I–V curve study as well (Fig. 2c). When the surface was stepwise annealed, a series of PEEM images were recorded and shown in Fig. 3a–d. During the UHV annealing, the contrast of each graphene island increases gradually. At about 229 °C, the edge of the graphene island becomes bright, indicating that Ni atoms start to intercalate at the edge sites. However, at much higher temperatures the whole island changes its image contrast uniformly. Around 445 °C the graphene surface has similar image contrast as the pristine graphene/Ru(0001) surface. The uniform contrast change in the PEEM image during the Ni intercalation process is consistent with the LEEM data, showing that the intercalated Ni structures form at the graphene surface randomly. The in-situ PEEM images of the Pb-intercalation process were displayed in Fig. 3e–h. It has been found that Pb starts to intercalate at the edges of the graphene islands, and then moves to the centers. After the reaction is complete, all graphene islands become darker than the pristine graphene/Ru(0001) surfaces because of an increase in the surface work function (Fig. 2d). At the same time, Pb islands on the bare Ru(0001) surface regions become visible. Upon annealing Pb atoms on Ru(0001) surface tend to aggregate and form large islands. Interestingly, the Pb island density is much lower in the regions surrounding the graphene islands, where the surface Pb adatoms may be sucked to the graphene/Ru(0001) interfaces to form the intercalated Pb structures. The results show that the presence of graphene island edges is a prerequisite for the Pb intercalation, which is not necessary for the Ni intercalation. This conclusion can be further supported by carrying out the intercalation experiments over the complete graphene surfaces. As shown in Fig. 4, the Ru(0001) surface has been fully covered by graphene overlayers, which consists of monolayer (bright regions)
and bilayer (dark regions) graphenes. Below 266 °C the PEEM images present little change in the gray intensity. Annealing the Ni-covered graphene surface at much higher temperatures has increased the image brightness gradually (Fig. 4a–d), indicating the occurrence of the Ni intercalation. In contrast, the graphene surface keeps intact when exposing to Pb flux from room temperature to 210 °C (Fig. 4e–h). Since both the adsorption energy and diffusion barrier of Pb atom on graphene are quite low [55,56], Pb atoms desorb from graphene surface and/or diffuse quickly to defect sites at much higher temperatures. The comparative studies in Ni and Pb intercalation reactions at the graphene/Ru(0001) surface illustrate different intercalation channels for the two intercalants. The Pb intercalation needs extended defect sites at graphene surface, such as open edges. The Ni atoms can penetrate through the graphene lattice and no extended defects are needed for the intercalation reaction. The Ni intercalation is similar to what we have observed for Si intercalation at the graphene/Ru(0001) surface [37]. Based on in-situ surface imaging experiments and DFT calculations, we proposed that Si can activate C\C bonds transiently and form a transition state of Si–C dimer. Through a Si–C atom exchange process, the C\C bonds are re-established such that the graphene lattice is healed. Consequently, the Si atom penetrates through the lattice to bond with Ru surface. We suggested that elements which can bond with carbon strongly might adopt the same mechanism. The present Ni intercalation results fully support this hypothesis. Recently, Sicot et al. also explained the Fe and Ni intercalation processes at graphene/ Rh(111) surface using the same mechanism [22]. The Pb atoms, which are not sufficient to activate C\C bonds, need to intercalate graphene layers through extended defect sites of graphene. The similar mechanism is active for O intercalation at graphene/Ru(0001) surfaces [15,34,57] and CO intercalation at graphene/Pt(111) surface [20]. The two contrast intercalation mechanisms can be attributed to the different interaction strength between intercalant and carbon. Ni is known to bond with carbon strongly [58]. For example, Lahiri et al. have found that surface Ni deposit forms a stable surface-carbide on a graphene/Ni(111) surface even at 100 °C [59]. DFT calculations also demonstrate a strong charge transfer from Ni to graphene [56]. The graphene lattice may get destructed when in contact with Ni adatoms. The Ni intercalation can be described in Fig. 5a. Ni adatoms anchoring on the graphene produce transient atomic-scale defects at the graphene lattice, through which Ni can diffuse through the carbon layer. After the
Fig. 3. (a–d): a series of PEEM images from a Ni-covered graphene/Ru(0001) surface annealed at 48, 229, 278, and 445 °C, respectively. (e–h): a series of PEEM images from a Pb-covered graphene/Ru(0001) surface annealed at 53, 131, 145, and 163 °C, respectively. The field of view (FoV) is 50 μm.
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Fig. 4. (a–d): a series of PEEM images from a Ni-covered complete graphene layer annealed at 266, 348, 431, and 463 °C, respectively. (e–h): a series of PEEM images taken from a complete graphene layer exposed to Pb flux at 98, 101, 198, and 210 °C, respectively. The white regions are monolayer graphene and the dark regions are bilayer graphene. FoV of the PEEM images is 50 μm.
Ni atoms bond to the substrate surface, the transient defects are healed and a perfect graphene surface is reformed. Although Ni atoms can form clusters on graphene at elevated temperatures, the Ni intercalation should occur via single-atom penetration, which has lower barrier compared to the penetration of Ni clusters. Some metals including Pb are believed not to form any carbides. The electronic interaction between graphene and Pb is also quite weak. Furthermore, Pb atoms can diffuse fast from the graphene surface to the bare Ru surface regions due to its low diffusion energy of 0.09 kcal/mol, in comparison to 5.12 kcal/mol for Ni diffusion on graphene [56]. Once the Pb-covered graphene surface is heated, Pb atoms can transport from graphene island surface to island edges. In case the bare Ru(0001) surface region has been fully covered by Pb, the additional Pb atoms can diffuse to the graphene/Ru(0001) interface driven by the strong interaction of Pb with Ru (Fig. 5b). Although Ni atoms were observed to intercalate the graphene/Ru(0001) interface at
the edge sites (Fig. 3b), the low mobility of Ni atoms at Ru surface impedes the further growth of the Ni-intercalated regions. Instead, the intercalation channel for Ni penetration through graphene lattice becomes active at higher temperature such that the exchange mechanism becomes dominant in the Ni intercalation process. We believe that the two mechanisms can explain most of the intercalation reactions at graphene surfaces. The elements which can interact strongly with carbon are supposed to intercalate at graphene/substrate interfaces via the exchange mechanism, and the Si, Ni, and Fe intercalation at graphene/Ru(0001) surface has been demonstrated to occur through this channel by our experiments. Furthermore, we suggest that most early transition metals and Al could penetrate through graphene lattice and form the intercalated structures. These processes have to be driven by the strong interaction of the elements with carbon and the substrate elements. Most molecules are not able to activate C\C bonds at a relatively low temperature range, and they are supposed to
Fig. 5. Schemes for the Ni (a) and Pb (b) intercalation reactions at the graphene/Ru(0001) surface.
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intercalate at the graphene/substrate interfaces through extended defects. Moreover, elements which either weakly interact with carbon or are too large to penetrate through small size defects are suggested to diffuse under graphene via open edges or domain boundaries. This mechanism may work for Pb, Au, and alkali metals. 4. Conclusion The occurrence of Ni intercalation has been observed at the submonolayer graphene/Ru(0001) surface and the complete graphene/ Ru(0001) surface, while the Pb intercalation was observed only at the submonolayer graphene/Ru(0001) surface. The intercalated Ni islands were distributed randomly under the graphene, and the intercalated Pb atoms formed a continuous interfacial layer between graphene and Ru surfaces. Accordingly, two contrast intercalation mechanisms have been suggested to explain intercalation reactions at graphene/metal surfaces. Elements, which interact with carbon strongly, are supposed to produce transient atomic-scale defects at the graphene lattice, and the intercalant atoms can penetrate through the carbon graphene lattice. This exchange intercalation mechanism is feasible for elements, such as Si, Ni, and Fe. On the other hand, those elements and molecules which are not able to activate C\C bonds need to diffuse under graphene through extended defects, such as domain boundaries and island edges. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (No. 21222305, No. 21073183, and No. 21033009), and the Ministry of Science and Technology of China (No. 2011CB932704, and No. 2013CB834603). References [1] E. Voloshina, Y. Dedkov, Phys. Chem. Chem. Phys. 14 (2012) 13502. [2] S.Y. Zhou, G.H. Gweon, A.V. Fedorov, P.N. First, W.A. De Heer, D.H. Lee, F. Guinea, A.H.C. Neto, A. Lanzara, Nat. Mater. 6 (2007) 916. [3] G. Giovannetti, P.A. Khomyakov, G. Brocks, V.M. Karpan, J. van den Brink, P.J. Kelly, Phys. Rev. Lett. 101 (2008) 026803. [4] P. Mallet, F. Varchon, C. Naud, L. Magaud, C. Berger, J.Y. Veuillen, Phys. Rev. B: Condens. Matter Mater. Phys. 76 (2007) 041403. [5] B. Wang, M.L. Bocquet, S. Marchini, S. Gunther, J. Wintterlin, Phys. Chem. Chem. Phys. 10 (2008) 3530. [6] F. Varchon, R. Feng, J. Hass, X. Li, B.N. Nguyen, C. Naud, P. Mallet, J.Y. Veuillen, C. Berger, E.H. Conrad, L. Magaud, Phys. Rev. Lett. 99 (2007) 126805. [7] J. Wintterlin, M.L. Bocquet, Surf. Sci. 603 (2009) 1841. [8] C. Riedl, C. Coletti, T. Iwasaki, A.A. Zakharov, U. Starke, Phys. Rev. Lett. 103 (2009) 246804. [9] C. Virojanadara, S. Watcharinyanon, A.A. Zakharov, L.I. Johansson, Phys. Rev. B: Condens. Matter Mater. Phys. 82 (2010) 205402. [10] S. Watcharinyanon, L.I. Johansson, A.A. Zakharov, C. Virojanadara, Surf. Sci. 606 (2012) 401. [11] S.L. Wong, H. Huang, Y.Z. Wang, L. Cao, D.C. Qi, I. Santoso, W. Chen, A.T.S. Wee, ACS Nano 5 (2011) 7662. [12] E.N. Voloshina, A. Generalov, M. Weser, S. Bottcher, K. Horn, Y.S. Dedkov, New J. Phys. 13 (2011) 113028. [13] M. Weser, E.N. Voloshina, K. Horn, Y.S. Dedkov, Phys. Chem. Chem. Phys. 13 (2011) 7534. [14] C. Enderlein, Y.S. Kim, A. Bostwick, E. Rotenberg, K. Horn, New J. Phys. 12 (2010) 033014.
[15] P.W. Sutter, J.T. Sadowski, E.A. Sutter, J. Am. Chem. Soc. 132 (2010) 8175. [16] L. Huang, Y. Pan, L.D. Pan, M. Gao, W.Y. Xu, Y.D. Que, H.T. Zhou, Y.L. Wang, S.X. Du, H.J. Gao, Appl. Phys. Lett. 99 (2011) 163107. [17] H. Zhang, Q. Fu, Y. Cui, D.L. Tan, X.H. Bao, J. Phys. Chem. C 113 (2009) 8296. [18] A. Varykhalov, J. Sanchez-Barriga, A.M. Shikin, C. Biswas, E. Vescovo, A. Rybkin, D. Marchenko, O. Rader, Phys. Rev. Lett. 101 (2008) 157601. [19] X. Feng, S. Maier, M. Salmeron, J. Am. Chem. Soc. 134 (2012) 5662. [20] R.T. Mu, Q. Fu, L. Jin, L. Yu, G.Z. Fang, D.L. Tan, X.H. Bao, Angew. Chem. Int. Ed. 51 (2012) 4856. [21] E. Granas, J. Knudsen, U.A. Schroder, T. Gerber, C. Busse, M.A. Arman, K. Schulte, J.N. Andersen, T. Michely, ACS Nano 6 (2012) 9951. [22] M. Sicot, P. Leicht, A. Zusan, S. Bouvron, O. Zander, M. Weser, Y.S. Dedkov, K. Horn, M. Fonin, ACS Nano 6 (2012) 151. [23] T. Gao, Y.B. Gao, C.Z. Chang, Y.B. Chen, M.X. Liu, S.B. Xie, K. He, X.C. Ma, Y.F. Zhang, Z.F. Liu, ACS Nano 6 (2012) 6562. [24] F.J.M.Z. Heringdorf, M.C. Reuter, R.M. Tromp, Nature 412 (2001) 517. [25] M. Kim, M. Bertram, M. Pollmann, A. von Oertzen, A.S. Mikhailov, H.H. Rotermund, G. Ertl, Science 292 (2001) 1357. [26] E. Bauer, M. Mundschau, W. Swiech, W. Telieps, Ultramicroscopy 31 (1989) 49. [27] K.L. Man, M.S. Altman, J. Phys. Condens. Matter 24 (2012) 314209. [28] N.C. Bartelt, K.F. McCarty, MRS Bull. 37 (2012) 1158. [29] K.V. Emtsev, A.A. Zakharov, C. Coletti, S. Forti, U. Starke, Phys. Rev. B: Condens. Matter Mater. Phys. 84 (2011) 125423. [30] P.W. Sutter, J.I. Flege, E.A. Sutter, Nat. Mater. 7 (2008) 406. [31] E. Loginova, N.C. Bartelt, P.J. Feibelman, K.F. McCarty, New J. Phys. 10 (2008) 093026. [32] P.W. Sutter, J.T. Sadowski, E.A. Sutter, Phys. Rev. B: Condens. Matter Mater. Phys. 80 (2009) 245411. [33] Y. Cui, Q. Fu, X.H. Bao, Phys. Chem. Chem. Phys. 12 (2010) 5053. [34] L. Jin, Q. Fu, H. Zhang, R.T. Mu, Y.H. Zhang, D.L. Tan, X.H. Bao, J. Phys. Chem. C 116 (2012) 2988. [35] L. Jin, Q. Fu, R.T. Mu, D.L. Tan, X.H. Bao, Phys. Chem. Chem. Phys. 13 (2011) 16655. [36] Y.C. Li, G. Zhou, J. Li, J. Wu, B.L. Gu, W.H. Duan, J. Phys. Chem. C 115 (2011) 23992. [37] Y. Cui, J.F. Gao, L. Jin, J.J. Zhao, D.L. Tan, Q. Fu, X.H. Bao, Nano Res. 5 (2012) 352. [38] A.L.V. de Parga, F. Calleja, B. Borca, M.C.G. Passeggi, J.J. Hinarejos, F. Guinea, R. Miranda, Phys. Rev. Lett. 100 (2008) 056807. [39] S. Marchini, S. Gunther, J. Wintterlin, Phys. Rev. B: Condens. Matter Mater. Phys. 76 (2007) 075429. [40] Y. Pan, H.G. Zhang, D.X. Shi, J.T. Sun, S.X. Du, F. Liu, H.J. Gao, Adv. Mater. 21 (2009) 2777. [41] E. Loginova, N.C. Bartelt, P.J. Feibelman, K.F. McCarty, New J. Phys. 11 (2009) 063046. [42] J. Kolaczkiewicz, E. Bauer, Surf. Sci. 423 (1999) 292. [43] J.A. Meyer, P. Schmid, R.J. Behm, Phys. Rev. Lett. 74 (1995) 3864. [44] G. Odahara, S. Otani, C. Oshima, M. Suzuki, T. Yasue, T. Koshikawa, Surf. Sci. 605 (2011) 1095. [45] J.B. Sun, J.B. Hannon, R.M. Tromp, P. Johari, A.A. Bol, V.B. Shenoy, K. Pohl, ACS Nano 4 (2010) 7073. [46] Y. Murata, E. Starodub, B.B. Kappes, C.V. Ciobanu, N.C. Bartelt, K.F. McCarty, S. Kodambaka, Appl. Phys. Lett. 97 (2010) 143114. [47] C.M. Yim, K.L. Man, X.D. Xiao, M.S. Altman, Phys. Rev. B: Condens. Matter Mater. Phys. 78 (2008) 155439. [48] B. Unal, Y. Sato, K.F. McCarty, N.C. Bartelt, T. Duden, C.J. Jenks, A.K. Schmid, P.A. Thiel, J. Vac. Sci. Technol. A 27 (2009) 1249. [49] S. Nie, A.L. Walter, N.C. Bartelt, E. Starodub, A. Bostwick, E. Rotenberg, K.F. McCarty, ACS Nano 5 (2011) 2298. [50] J.B. Hannon, J. Sun, K. Pohl, G.L. Kellogg, Phys. Rev. Lett. 96 (2006) 246103. [51] D. Lide, Handbook of Chemistry and Physics, CRC, 2008. [52] P.A. Khomyakov, G. Giovannetti, P.C. Rusu, G. Brocks, J. van den Brink, P.J. Kelly, Phys. Rev. B: Condens. Matter Mater. Phys. 79 (2009) 195425. [53] H.G. Gao, J.A. Zhou, M.H. Lu, W. Fa, Y.F. Chen, J. Appl. Phys. 107 (2010) 114311. [54] Y. Cui, Q. Fu, H. Zhang, D.L. Tan, X.H. Bao, J. Phys. Chem. C 113 (2009) 20365. [55] M. Hupalo, X.J. Liu, C.Z. Wang, W.C. Lu, Y.X. Yao, K.M. Ho, M.C. Tringides, Adv. Mater. 23 (2011) 2082. [56] X.J. Liu, C.Z. Wang, M. Hupalo, W.C. Lu, M.C. Tringides, Y.X. Yao, K.M. Ho, Phys. Chem. Chem. Phys. 14 (2012) 9157. [57] E. Starodub, N.C. Bartelt, K.F. McCarty, J. Phys. Chem. C 114 (2010) 5134. [58] D.W. Boukhvalov, M.I. Katsnelson, Appl. Phys. Lett. 95 (2009) 023109. [59] J. Lahiri, M. Batzill, Appl. Phys. Lett. 97 (2010) 023102.
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A comparative study of intercalation mechanism at graphene/Ru(0001) interface Li Jin, Qiang Fu ⁎, Yang Yang, Xinhe Bao ⁎ State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, The Chinese Academy of Sciences, Dalian 116023, PR China
a r t i c l e
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Article history: Received 31 March 2013 Accepted 8 July 2013 Available online 14 July 2013 Keywords: Graphene LEEM PEEM Intercalation Ru(0001)
a b s t r a c t Both Ni and Pb intercalation reactions at graphene/Ru(0001) interface were studied by low energy electron microscopy (LEEM) and photoemission electron microscopy (PEEM). It is suggested that the Ni intercalation is dominated by an exchange intercalation mechanism, in which Ni adatoms produce transient atomic-scale defects in the graphene lattice and penetrate through the carbon monolayer. In contrast, the Pb intercalation process needs to be facilitated by the diffusion of Pb atoms through extended defect sites of graphene, such as open edges and domain boundaries. The two contrast intercalation mechanisms originate from the different interaction strength of the intercalated elements with carbon. Different responses of the graphene electronic structure to the Ni and Pb intercalation reactions were observed by PEEM. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Epitaxial growth on metals and SiC has been regarded as an important route to prepare high-quality graphene overlayers. However, electronic coupling between the epitaxial graphene structures and the substrate surfaces may damage the characteristic Dirac cone band structure of graphene, which impedes many potential applications of the new material [1–7]. Intercalation of elements or molecules at graphene/ substrate interfaces can decouple the epitaxial graphene overlayers from the substrate surfaces and recover the intriguing electronic properties of freestanding graphene [8–17]. For example, quasi-freestanding epitaxial graphene overlayers can be obtained on SiC(0001) by H intercalation [8]. A linear π-band dispersion near K point has been observed at Au-intercalated graphene/Ni(111) surface [18]. Besides elements, molecules such as H2O and CO can diffuse into the nanospace between graphene and substrate, and chemical reactions may even happen in the two-dimensional (2D) nanospace under graphene sheets [19,20]. The graphene covers can affect the surface chemistry at the metal surfaces [15,17,20,21]. These novel physical and chemical phenomena under graphene overlayers have stimulated extensive studies in intercalation reactions at graphene surfaces in the past few years. Scanning tunneling microscopy (STM) and angle-resolved photoemission spectroscopy (ARPES) are often applied to investigate the intercalation processes. Surface morphology and atomic structure of intercalated graphene layers can be explored by STM. Atomically resolved STM images show that the intercalated graphene surfaces still keep a well-defined graphitic structure but surface moiré patterns ⁎ Corresponding authors. Tel.: +86 411 84379253; fax: +86 411 84694447. E-mail addresses: [email protected] (Q. Fu), [email protected] (X. Bao). 0039-6028/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.susc.2013.07.008
may disappear [17,18,22,23]. ARPES measurements help to reveal the electronic structure of graphene. Linear π bands near Fermi level are often observed at the intercalated graphene surfaces, demonstrating the quasi-freestanding character recovered by the intercalation effect [8,18]. However, these techniques cannot give answers to a question how intercalation happens at a graphene surface. A deep understanding of the intercalation mechanism relies on dynamic characterization methods, by which the surface intercalation processes can be investigated in-situ. Low energy electron microscopy (LEEM) and photoemission electron microscopy (PEEM) are powerful surface imaging techniques for the studies of surface growth, surface phase transition, and surface reactions [24–26]. In recent years, these imaging techniques have been successfully applied to study graphene surfaces [27,28]. The capability of PEEM/LEEM to perform real-time and dynamic imaging enables understanding graphene growth and surface reactions [9,20,29–34]. With the aid of the dynamic characterization of intercalation reactions at graphene surfaces using PEEM/LEEM, the intercalation reactions were assumed to occur via diffusion of intercalants through extended defect sites at graphene surfaces, e.g. island edges and domain boundaries [15,17,29,34–36]. However, our recent study in Si intercalation at graphene/Ru(0001) interface reveals a novel Si–C exchange mechanism, in which surface Si atoms can penetrate through the graphene lattice and form 2D structures between graphene and Ru(0001) surface. We suggest that the exchange intercalation mechanism is driven by the strong interaction of Si with both carbon and Ru, and this mechanism should be active in other intercalation processes [37]. In this work, a comparative study in both Ni and Pb intercalation reactions at graphene/Ru(0001) interface has been performed using in-situ LEEM/PEEM. We found that the Ni intercalation occurs via
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penetration of Ni atoms through the graphene lattice, while graphene domain boundaries play an important role in the Pb intercalation. The two contrast intercalation processes produce different intercalated structures and opposite changes in surface work function. We propose that most intercalation processes at graphene surfaces can be well explained by the two typical mechanisms. 2. Experimental All experiments were conducted in an Elmitec LEEM/PEEM system, which consists of a preparation chamber, a microscopic imaging system equipped with a hemisphere energy analyzer, an aberration corrector, a field emission gun, as well as a vacuum ultraviolet (VUV) laser source (λ = 177.3 nm) [35]. The preparation chamber is equipped with a K-cell evaporation source containing Ni particles. A similar evaporation source is mounted in the imaging chamber for Pb deposition, which can be in-situ monitored by LEEM/PEEM. Ru(0001) surface was prepared by cycled Ar+ sputtering (2.0 keV, 7 × 10− 6 Torr Ar, 10 min), heating in O2 (530 °C, 5 × 10− 7 Torr O2), and annealing in ultrahigh vacuum (UHV) at 1200 °C. The clean Ru(0001) surface was then doped with carbon by exposing to ethylene (C2H4) at 900 °C to have an enough amount of carbon at the near surface region. Ni and Pb were deposited onto the graphene/Ru(0001) surface at room temperature. The background pressure is in a 10−10 Torr range during the metal evaporation processes. In-situ PEEM/LEEM imaging was performed at the Ni- and Pb-covered graphene/Ru(0001) surfaces during UHV annealing at various temperatures. 3. Results and discussion Epitaxial graphene overlayers were obtained on the Ru(0001) surface by the surface segregation process. Each graphene island nucleates from one site and continues to grow in size up to hundreds of micrometers. A typical low energy electron diffraction (LEED) pattern taken from the graphene/Ru(0001) surfaces was shown in Fig. 1a. The pristine graphene surface shows a surface superstructure containing a (12 × 12) lattice of graphene matched to a (11 × 11) lattice of Ru(0001) [38–40]. The quality of the graphene layer is quite high as established by the appearance of high-order diffraction spots in the LEED patterns [41].
Ni was deposited onto the graphene/Ru(0001) surface at room temperature. The micro-region LEED (μ-LEED) pattern recorded from the Ni-covered graphene surface shows little change except for slight diffusing of the background, which may be induced by scattering from the deposited Ni clusters. When the surface was heated up to 470 °C, the intensity of the (0, 1) diffraction spots corresponding to Ru(0001) surface (marked by green arrows in Fig. 1a and b) is much weaker than that from the pristine graphene/Ru(0001) surface. The similar results have been reported at intercalated graphene/SiC(0001) surfaces, in which the diffraction spots from the SiC substrate get weakened or even disappear after intercalation [8,29]. As shown in Fig. 1b, the second- and higher-order diffraction spots around the (0, 0) and (0, 1) diffraction spots disappeared but the first-order diffraction spots are still observed. For comparison, μ-LEED was performed on the bare Ru(0001) surface regions covered by Ni. The first-order diffraction spots around the (0, 0) and (0, 1) spots were also observed in the pattern (Fig. 1c). It has been reported that relaxed epitaxial Ni films form on Ru(0001) surface, showing a hexagonal moiré superstructure, and LEED pattern from the Ni films consists of satellite spots [42,43]. Since the difference of lattice constant between Ni and graphene is less than 1.2% [44,45], both graphene/Ru(0001) and Ni/Ru(0001) surfaces should have the similar periodicity in the moiré superstructure. Accordingly, we infer that the first-order diffraction spots around the (0, 0) and (0, 1) spots observed in Fig. 1b can be attributed to the superstructure of Ni layer, which intercalates under graphene and epitaxially grows on Ru(0001). This conclusion can be further confirmed by recent STM results showing that the Ni-intercalated graphene structures display the same moiré patterns as the pristine graphene surfaces [16,22]. The similar LEED study has been performed for Pb intercalation at the graphene/Ru(0001) surface. Deposition of Pb at room temperature also results in the diffusing of the LEED pattern background. Annealing the Pb-covered graphene surface around 150 °C causes a big change in the LEED pattern (Fig. 1d). First, the intensity of the (0, 1) diffraction spots from the Ru(0001) surface has been largely suppressed, similar to the case of Ni-intercalated surfaces. Second, all the high-order diffraction spots around the (0, 0) and (0, 1) spots disappear, suggesting that the Pb intercalation makes the graphene overlayers freestanding. This is fully consistent with our previous work [35]. Finally, additional diffraction spots as marked by arrows in Fig. 1d appear, which can be attributed to the intercalated Pb layer formed between the topmost
Fig. 1. LEED patterns recorded from pristine graphene/Ru(0001) surface (a), Ni-intercalated graphene/Ru(0001) surface regions (b), Ni-covered Ru(0001) surface regions (c), Pb-intercalated Ru(0001) surface regions (d), and Pb-covered Ru(0001) surface regions (e). The LEED patterns of Ru(0001) substrate (Ru), graphene (G), Ni overlayers on Ru, and Pb overlayers on Ru are marked by arrows in the figures. The electron beam energy for the LEED measurement is 50 eV.
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graphene and the Ru(0001) surface, since a similar diffraction structure has been observed in the μ-LEED pattern recorded from the Pb-covered Ru(0001) surface regions (Fig. 1e). LEEM images acquired from the Ni and Pb intercalated graphene surfaces are shown in Fig. 2a and b, respectively. For the Ni-intercalated surface, the most important feature is that the intercalated islands with the bright contrast are distributed on the graphene surface randomly. Some islands have a well-defined shape, and the faceted structures are believed to be related to a strong substrate symmetry effect [22,23]. Highly dispersed intercalated islands under graphene have been observed by STM in the cases of Ni intercalation at graphene/Ru(0001) and graphene/Rh(111) surfaces. The LEEM and STM results from the Ni-intercalated graphene surfaces are also similar to our observations at the Si-intercalated graphene/Ru(0001) surface. On the contrary, the Pb intercalation produces different surface structures. Our previous work has shown that the Pb intercalation starts from the edge sites of each graphene island [35]. A clear reaction frontier is present, which moves forwards from the island edges to the center. A LEEM image which contains a Pb-intercalated graphene region (with dark contrast) and a pristine graphene region (with bright contrast) is shown in Fig. 2b, where the intercalation process was quenched deliberately before its completion. We can see that the intercalated region is uniform in the surface structure, indicating the occurrence of Pb-intercalation everywhere in this region. In a LEEM imaging mode, the intensity of a reflected electron beam (noted as I) varies with the energy of an incident electron beam (noted as V). When the electron beam energy exceeds the work function difference between the electron gun and the sample surface, the
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reflected electron beam intensity decreases due to electron–surface interactions [46]. Thus, the surface work function change can be derived from the I–V curves by measuring the shift of the energy position where the reflected intensity decreases by 10% [46–50]. The I–V curves were recorded at the pristine graphene surface, the metal-covered graphene surfaces, and the metal-intercalated graphene surface regions (marked by squares in the LEEM images). As shown in Fig. 2c and d, the surface work function presents an increase of 1.2 and 0.1 eV in the cases of Ni and Pb deposition at room temperature, respectively. The large increase in the surface work function of the Ni-covered graphene surface is due to the much large work function of Ni (5.04–5.35 eV) compared to that of Pb (4.25 eV) [51]. Upon annealing, the occurrence of intercalation leads to the variation of the surface work function. The surface work function decreased by 0.9 eV after the Ni intercalation, while it increased by 0.6 eV after the Pb intercalation. Overall, compared to the pristine graphene/Ru(0001) surface the Ni intercalation does not produce a significant change in surface work function, while the Pb intercalation results in an increase of the surface work function by 0.7 eV. Density functional theory (DFT) calculations revealed that many metals can transfer charge to graphene and, thus, decrease the surface work function [3]. The stronger the metal–graphene interaction, the lower the work function of the graphene/metal surface. Both Ni and Ru present strong interaction with graphene [5,52], and thus the graphene/ Ni and graphene/Ru structures may present a similar lower work function than the free graphene structure. Therefore, a structural change from the graphene/Ru(0001) surface to the graphene/Ni/Ru(0001) surface does not produce obvious variation in the surface work function (Fig. 2c). The surface work function of graphene/Pb structure should be
Fig. 2. LEEM images from a Ni-intercalated graphene/Ru(0001) surface region (a) and a Pb-intercalated graphene/Ru(0001) surface region (b). The start voltage was set at 1.3 and 2.0 V, respectively for images (a) and (b). I–V curves were acquired for the Ni (c) and Pb (d) intercalation. The curves are from the pristine graphene/Ru(0001) surface, Ni and Pb-covered graphene surfaces, and Ni and Pb-intercalated surfaces.
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similar to that of the free graphene because of the weak interaction of Pb with graphene [53]. It can be understood that the transformation of the graphene/Ru(0001) surface into the graphene/Pb/Ru(0001)surface results in an increase in the work function (Fig. 2d). As shown above, the intercalation reactions were accompanied by changes in the surface work function. Therefore, these surface processes can be effectively investigated by in-situ PEEM. For the graphene/ Ru(0001) surface, graphene domains are bright and the bare Ru(0001) surface regions are dark in the PEEM images excited by Hg lamp [54]. After the Ni deposition at room temperature, the whole surface becomes dark due to the increase in the surface work function, which has been confirmed by the I–V curve study as well (Fig. 2c). When the surface was stepwise annealed, a series of PEEM images were recorded and shown in Fig. 3a–d. During the UHV annealing, the contrast of each graphene island increases gradually. At about 229 °C, the edge of the graphene island becomes bright, indicating that Ni atoms start to intercalate at the edge sites. However, at much higher temperatures the whole island changes its image contrast uniformly. Around 445 °C the graphene surface has similar image contrast as the pristine graphene/Ru(0001) surface. The uniform contrast change in the PEEM image during the Ni intercalation process is consistent with the LEEM data, showing that the intercalated Ni structures form at the graphene surface randomly. The in-situ PEEM images of the Pb-intercalation process were displayed in Fig. 3e–h. It has been found that Pb starts to intercalate at the edges of the graphene islands, and then moves to the centers. After the reaction is complete, all graphene islands become darker than the pristine graphene/Ru(0001) surfaces because of an increase in the surface work function (Fig. 2d). At the same time, Pb islands on the bare Ru(0001) surface regions become visible. Upon annealing Pb atoms on Ru(0001) surface tend to aggregate and form large islands. Interestingly, the Pb island density is much lower in the regions surrounding the graphene islands, where the surface Pb adatoms may be sucked to the graphene/Ru(0001) interfaces to form the intercalated Pb structures. The results show that the presence of graphene island edges is a prerequisite for the Pb intercalation, which is not necessary for the Ni intercalation. This conclusion can be further supported by carrying out the intercalation experiments over the complete graphene surfaces. As shown in Fig. 4, the Ru(0001) surface has been fully covered by graphene overlayers, which consists of monolayer (bright regions)
and bilayer (dark regions) graphenes. Below 266 °C the PEEM images present little change in the gray intensity. Annealing the Ni-covered graphene surface at much higher temperatures has increased the image brightness gradually (Fig. 4a–d), indicating the occurrence of the Ni intercalation. In contrast, the graphene surface keeps intact when exposing to Pb flux from room temperature to 210 °C (Fig. 4e–h). Since both the adsorption energy and diffusion barrier of Pb atom on graphene are quite low [55,56], Pb atoms desorb from graphene surface and/or diffuse quickly to defect sites at much higher temperatures. The comparative studies in Ni and Pb intercalation reactions at the graphene/Ru(0001) surface illustrate different intercalation channels for the two intercalants. The Pb intercalation needs extended defect sites at graphene surface, such as open edges. The Ni atoms can penetrate through the graphene lattice and no extended defects are needed for the intercalation reaction. The Ni intercalation is similar to what we have observed for Si intercalation at the graphene/Ru(0001) surface [37]. Based on in-situ surface imaging experiments and DFT calculations, we proposed that Si can activate C\C bonds transiently and form a transition state of Si–C dimer. Through a Si–C atom exchange process, the C\C bonds are re-established such that the graphene lattice is healed. Consequently, the Si atom penetrates through the lattice to bond with Ru surface. We suggested that elements which can bond with carbon strongly might adopt the same mechanism. The present Ni intercalation results fully support this hypothesis. Recently, Sicot et al. also explained the Fe and Ni intercalation processes at graphene/ Rh(111) surface using the same mechanism [22]. The Pb atoms, which are not sufficient to activate C\C bonds, need to intercalate graphene layers through extended defect sites of graphene. The similar mechanism is active for O intercalation at graphene/Ru(0001) surfaces [15,34,57] and CO intercalation at graphene/Pt(111) surface [20]. The two contrast intercalation mechanisms can be attributed to the different interaction strength between intercalant and carbon. Ni is known to bond with carbon strongly [58]. For example, Lahiri et al. have found that surface Ni deposit forms a stable surface-carbide on a graphene/Ni(111) surface even at 100 °C [59]. DFT calculations also demonstrate a strong charge transfer from Ni to graphene [56]. The graphene lattice may get destructed when in contact with Ni adatoms. The Ni intercalation can be described in Fig. 5a. Ni adatoms anchoring on the graphene produce transient atomic-scale defects at the graphene lattice, through which Ni can diffuse through the carbon layer. After the
Fig. 3. (a–d): a series of PEEM images from a Ni-covered graphene/Ru(0001) surface annealed at 48, 229, 278, and 445 °C, respectively. (e–h): a series of PEEM images from a Pb-covered graphene/Ru(0001) surface annealed at 53, 131, 145, and 163 °C, respectively. The field of view (FoV) is 50 μm.
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Fig. 4. (a–d): a series of PEEM images from a Ni-covered complete graphene layer annealed at 266, 348, 431, and 463 °C, respectively. (e–h): a series of PEEM images taken from a complete graphene layer exposed to Pb flux at 98, 101, 198, and 210 °C, respectively. The white regions are monolayer graphene and the dark regions are bilayer graphene. FoV of the PEEM images is 50 μm.
Ni atoms bond to the substrate surface, the transient defects are healed and a perfect graphene surface is reformed. Although Ni atoms can form clusters on graphene at elevated temperatures, the Ni intercalation should occur via single-atom penetration, which has lower barrier compared to the penetration of Ni clusters. Some metals including Pb are believed not to form any carbides. The electronic interaction between graphene and Pb is also quite weak. Furthermore, Pb atoms can diffuse fast from the graphene surface to the bare Ru surface regions due to its low diffusion energy of 0.09 kcal/mol, in comparison to 5.12 kcal/mol for Ni diffusion on graphene [56]. Once the Pb-covered graphene surface is heated, Pb atoms can transport from graphene island surface to island edges. In case the bare Ru(0001) surface region has been fully covered by Pb, the additional Pb atoms can diffuse to the graphene/Ru(0001) interface driven by the strong interaction of Pb with Ru (Fig. 5b). Although Ni atoms were observed to intercalate the graphene/Ru(0001) interface at
the edge sites (Fig. 3b), the low mobility of Ni atoms at Ru surface impedes the further growth of the Ni-intercalated regions. Instead, the intercalation channel for Ni penetration through graphene lattice becomes active at higher temperature such that the exchange mechanism becomes dominant in the Ni intercalation process. We believe that the two mechanisms can explain most of the intercalation reactions at graphene surfaces. The elements which can interact strongly with carbon are supposed to intercalate at graphene/substrate interfaces via the exchange mechanism, and the Si, Ni, and Fe intercalation at graphene/Ru(0001) surface has been demonstrated to occur through this channel by our experiments. Furthermore, we suggest that most early transition metals and Al could penetrate through graphene lattice and form the intercalated structures. These processes have to be driven by the strong interaction of the elements with carbon and the substrate elements. Most molecules are not able to activate C\C bonds at a relatively low temperature range, and they are supposed to
Fig. 5. Schemes for the Ni (a) and Pb (b) intercalation reactions at the graphene/Ru(0001) surface.
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intercalate at the graphene/substrate interfaces through extended defects. Moreover, elements which either weakly interact with carbon or are too large to penetrate through small size defects are suggested to diffuse under graphene via open edges or domain boundaries. This mechanism may work for Pb, Au, and alkali metals. 4. Conclusion The occurrence of Ni intercalation has been observed at the submonolayer graphene/Ru(0001) surface and the complete graphene/ Ru(0001) surface, while the Pb intercalation was observed only at the submonolayer graphene/Ru(0001) surface. The intercalated Ni islands were distributed randomly under the graphene, and the intercalated Pb atoms formed a continuous interfacial layer between graphene and Ru surfaces. Accordingly, two contrast intercalation mechanisms have been suggested to explain intercalation reactions at graphene/metal surfaces. Elements, which interact with carbon strongly, are supposed to produce transient atomic-scale defects at the graphene lattice, and the intercalant atoms can penetrate through the carbon graphene lattice. This exchange intercalation mechanism is feasible for elements, such as Si, Ni, and Fe. On the other hand, those elements and molecules which are not able to activate C\C bonds need to diffuse under graphene through extended defects, such as domain boundaries and island edges. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (No. 21222305, No. 21073183, and No. 21033009), and the Ministry of Science and Technology of China (No. 2011CB932704, and No. 2013CB834603). References [1] E. Voloshina, Y. Dedkov, Phys. Chem. Chem. Phys. 14 (2012) 13502. [2] S.Y. Zhou, G.H. Gweon, A.V. Fedorov, P.N. First, W.A. De Heer, D.H. Lee, F. Guinea, A.H.C. Neto, A. Lanzara, Nat. Mater. 6 (2007) 916. [3] G. Giovannetti, P.A. Khomyakov, G. Brocks, V.M. Karpan, J. van den Brink, P.J. Kelly, Phys. Rev. Lett. 101 (2008) 026803. [4] P. Mallet, F. Varchon, C. Naud, L. Magaud, C. Berger, J.Y. Veuillen, Phys. Rev. B: Condens. Matter Mater. Phys. 76 (2007) 041403. [5] B. Wang, M.L. Bocquet, S. Marchini, S. Gunther, J. Wintterlin, Phys. Chem. Chem. Phys. 10 (2008) 3530. [6] F. Varchon, R. Feng, J. Hass, X. Li, B.N. Nguyen, C. Naud, P. Mallet, J.Y. Veuillen, C. Berger, E.H. Conrad, L. Magaud, Phys. Rev. Lett. 99 (2007) 126805. [7] J. Wintterlin, M.L. Bocquet, Surf. Sci. 603 (2009) 1841. [8] C. Riedl, C. Coletti, T. Iwasaki, A.A. Zakharov, U. Starke, Phys. Rev. Lett. 103 (2009) 246804. [9] C. Virojanadara, S. Watcharinyanon, A.A. Zakharov, L.I. Johansson, Phys. Rev. B: Condens. Matter Mater. Phys. 82 (2010) 205402. [10] S. Watcharinyanon, L.I. Johansson, A.A. Zakharov, C. Virojanadara, Surf. Sci. 606 (2012) 401. [11] S.L. Wong, H. Huang, Y.Z. Wang, L. Cao, D.C. Qi, I. Santoso, W. Chen, A.T.S. Wee, ACS Nano 5 (2011) 7662. [12] E.N. Voloshina, A. Generalov, M. Weser, S. Bottcher, K. Horn, Y.S. Dedkov, New J. Phys. 13 (2011) 113028. [13] M. Weser, E.N. Voloshina, K. Horn, Y.S. Dedkov, Phys. Chem. Chem. Phys. 13 (2011) 7534. [14] C. Enderlein, Y.S. Kim, A. Bostwick, E. Rotenberg, K. Horn, New J. Phys. 12 (2010) 033014.
[15] P.W. Sutter, J.T. Sadowski, E.A. Sutter, J. Am. Chem. Soc. 132 (2010) 8175. [16] L. Huang, Y. Pan, L.D. Pan, M. Gao, W.Y. Xu, Y.D. Que, H.T. Zhou, Y.L. Wang, S.X. Du, H.J. Gao, Appl. Phys. Lett. 99 (2011) 163107. [17] H. Zhang, Q. Fu, Y. Cui, D.L. Tan, X.H. Bao, J. Phys. Chem. C 113 (2009) 8296. [18] A. Varykhalov, J. Sanchez-Barriga, A.M. Shikin, C. Biswas, E. Vescovo, A. Rybkin, D. Marchenko, O. Rader, Phys. Rev. Lett. 101 (2008) 157601. [19] X. Feng, S. Maier, M. Salmeron, J. Am. Chem. Soc. 134 (2012) 5662. [20] R.T. Mu, Q. Fu, L. Jin, L. Yu, G.Z. Fang, D.L. Tan, X.H. Bao, Angew. Chem. Int. Ed. 51 (2012) 4856. [21] E. Granas, J. Knudsen, U.A. Schroder, T. Gerber, C. Busse, M.A. Arman, K. Schulte, J.N. Andersen, T. Michely, ACS Nano 6 (2012) 9951. [22] M. Sicot, P. Leicht, A. Zusan, S. Bouvron, O. Zander, M. Weser, Y.S. Dedkov, K. Horn, M. Fonin, ACS Nano 6 (2012) 151. [23] T. Gao, Y.B. Gao, C.Z. Chang, Y.B. Chen, M.X. Liu, S.B. Xie, K. He, X.C. Ma, Y.F. Zhang, Z.F. Liu, ACS Nano 6 (2012) 6562. [24] F.J.M.Z. Heringdorf, M.C. Reuter, R.M. Tromp, Nature 412 (2001) 517. [25] M. Kim, M. Bertram, M. Pollmann, A. von Oertzen, A.S. Mikhailov, H.H. Rotermund, G. Ertl, Science 292 (2001) 1357. [26] E. Bauer, M. Mundschau, W. Swiech, W. Telieps, Ultramicroscopy 31 (1989) 49. [27] K.L. Man, M.S. Altman, J. Phys. Condens. Matter 24 (2012) 314209. [28] N.C. Bartelt, K.F. McCarty, MRS Bull. 37 (2012) 1158. [29] K.V. Emtsev, A.A. Zakharov, C. Coletti, S. Forti, U. Starke, Phys. Rev. B: Condens. Matter Mater. Phys. 84 (2011) 125423. [30] P.W. Sutter, J.I. Flege, E.A. Sutter, Nat. Mater. 7 (2008) 406. [31] E. Loginova, N.C. Bartelt, P.J. Feibelman, K.F. McCarty, New J. Phys. 10 (2008) 093026. [32] P.W. Sutter, J.T. Sadowski, E.A. Sutter, Phys. Rev. B: Condens. Matter Mater. Phys. 80 (2009) 245411. [33] Y. Cui, Q. Fu, X.H. Bao, Phys. Chem. Chem. Phys. 12 (2010) 5053. [34] L. Jin, Q. Fu, H. Zhang, R.T. Mu, Y.H. Zhang, D.L. Tan, X.H. Bao, J. Phys. Chem. C 116 (2012) 2988. [35] L. Jin, Q. Fu, R.T. Mu, D.L. Tan, X.H. Bao, Phys. Chem. Chem. Phys. 13 (2011) 16655. [36] Y.C. Li, G. Zhou, J. Li, J. Wu, B.L. Gu, W.H. Duan, J. Phys. Chem. C 115 (2011) 23992. [37] Y. Cui, J.F. Gao, L. Jin, J.J. Zhao, D.L. Tan, Q. Fu, X.H. Bao, Nano Res. 5 (2012) 352. [38] A.L.V. de Parga, F. Calleja, B. Borca, M.C.G. Passeggi, J.J. Hinarejos, F. Guinea, R. Miranda, Phys. Rev. Lett. 100 (2008) 056807. [39] S. Marchini, S. Gunther, J. Wintterlin, Phys. Rev. B: Condens. Matter Mater. Phys. 76 (2007) 075429. [40] Y. Pan, H.G. Zhang, D.X. Shi, J.T. Sun, S.X. Du, F. Liu, H.J. Gao, Adv. Mater. 21 (2009) 2777. [41] E. Loginova, N.C. Bartelt, P.J. Feibelman, K.F. McCarty, New J. Phys. 11 (2009) 063046. [42] J. Kolaczkiewicz, E. Bauer, Surf. Sci. 423 (1999) 292. [43] J.A. Meyer, P. Schmid, R.J. Behm, Phys. Rev. Lett. 74 (1995) 3864. [44] G. Odahara, S. Otani, C. Oshima, M. Suzuki, T. Yasue, T. Koshikawa, Surf. Sci. 605 (2011) 1095. [45] J.B. Sun, J.B. Hannon, R.M. Tromp, P. Johari, A.A. Bol, V.B. Shenoy, K. Pohl, ACS Nano 4 (2010) 7073. [46] Y. Murata, E. Starodub, B.B. Kappes, C.V. Ciobanu, N.C. Bartelt, K.F. McCarty, S. Kodambaka, Appl. Phys. Lett. 97 (2010) 143114. [47] C.M. Yim, K.L. Man, X.D. Xiao, M.S. Altman, Phys. Rev. B: Condens. Matter Mater. Phys. 78 (2008) 155439. [48] B. Unal, Y. Sato, K.F. McCarty, N.C. Bartelt, T. Duden, C.J. Jenks, A.K. Schmid, P.A. Thiel, J. Vac. Sci. Technol. A 27 (2009) 1249. [49] S. Nie, A.L. Walter, N.C. Bartelt, E. Starodub, A. Bostwick, E. Rotenberg, K.F. McCarty, ACS Nano 5 (2011) 2298. [50] J.B. Hannon, J. Sun, K. Pohl, G.L. Kellogg, Phys. Rev. Lett. 96 (2006) 246103. [51] D. Lide, Handbook of Chemistry and Physics, CRC, 2008. [52] P.A. Khomyakov, G. Giovannetti, P.C. Rusu, G. Brocks, J. van den Brink, P.J. Kelly, Phys. Rev. B: Condens. Matter Mater. Phys. 79 (2009) 195425. [53] H.G. Gao, J.A. Zhou, M.H. Lu, W. Fa, Y.F. Chen, J. Appl. Phys. 107 (2010) 114311. [54] Y. Cui, Q. Fu, H. Zhang, D.L. Tan, X.H. Bao, J. Phys. Chem. C 113 (2009) 20365. [55] M. Hupalo, X.J. Liu, C.Z. Wang, W.C. Lu, Y.X. Yao, K.M. Ho, M.C. Tringides, Adv. Mater. 23 (2011) 2082. [56] X.J. Liu, C.Z. Wang, M. Hupalo, W.C. Lu, M.C. Tringides, Y.X. Yao, K.M. Ho, Phys. Chem. Chem. Phys. 14 (2012) 9157. [57] E. Starodub, N.C. Bartelt, K.F. McCarty, J. Phys. Chem. C 114 (2010) 5134. [58] D.W. Boukhvalov, M.I. Katsnelson, Appl. Phys. Lett. 95 (2009) 023109. [59] J. Lahiri, M. Batzill, Appl. Phys. Lett. 97 (2010) 023102.