Patterning Quasi-Periodic Co 2D-Clusters underneath Graphene on

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Patterning quasi-periodic Co 2D-clusters underneath graphene on SiC(0001) Luis Henrique de Lima, Richard Landers, and Abner de Siervo Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm501976b • Publication Date (Web): 02 Jul 2014 Downloaded from http://pubs.acs.org on July 4, 2014

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Chemistry of Materials

Patterning quasi-periodic Co 2D-clusters underneath graphene on SiC(0001) Luis Henrique de Lima,† Richard Landers,† and Abner de Siervo∗,†,‡ Instituto de F´ısica “Gleb Wataghin”, Universidade Estadual de Campinas, Campinas/SP, 13083-859, Brazil E-mail: [email protected]

Abstract The behavior of Co nanoparticles (NPs) grown on graphene/SiC(0001) after oxygen exposure and heating in ultra-high vacuum is investigated. The results of photoelectron spectroscopy (XPS) show that as grown, the metal is on the surface of the graphene/SiC and suffers oxidation forming a single phase CoO when exposed to O2 , even at low doses. After heating in ultra-high vacuum (UHV), there is a deoxidation of cobalt and intercalation between the graphene (G) and the buffer layer (BL), as indicated by Scanning Tunneling Microscopy (STM) and XPS. Cobalt forms almost regular small 2D clusters between G and BL. Moreover, graphene acts as a barrier to oxidation, preserving the metallic and the magnetic character of the material even when exposed to O2 . This paper shows a method for patterning chemically protected Co NPs on graphene/SiC(0001) which could be used in nanomagnetism based devices. ∗

To whom correspondence should be addressed Universidade Estadual de Campinas - Unicamp ‡ also joint-appointed at Brazilian Synchrotron Light Laboratory. †

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Introduction Graphene (G), a single sheet of carbon atoms bonded via sp2 hybridized orbitals, has been one of the most studied materials, both theoretically and experimentally. This enormous attention occurs due its unusual band structure, with linear dispersion of energy for electrons around the K point of the Brillouin zone 1 and this implies fascinating phenomena such as the exhibition of quantum Hall effect at room temperature. 2 Since the 70’s it is known that graphitization occurs at the surface of SiC when it is heated to high temperatures (above 1000 ◦ C) 3 and performing heating in a controlled way, it is possible to obtain a single or a few layers of graphene on the surface. 4 The SiC is formed by bi-layers of Si and C. When heated in vacuum to temperatures of about 1150 ◦ C, the Si-C bonds are broken and Si atoms are sublimated, leaving C atoms on the surface. The C atoms are bound in a honeycomb-like structure, stable at this temperature and remain on the surface. However, for the SiC(0001) surface, this first layer of C atoms does not exhibit the electronics properties of graphene and is commonly named buffer layer (BL) and this is due its strong interaction with the substrate (SiC) forming covalent bonds. √ √ The BL is characterized by a (6 3 × 6 3)R30◦ reconstruction, although presents a (6 × 6) reconstruction in STM images. The BL remains at the interface with the subsequent growth of the second layer of carbon with this second layer characterized as graphene. The BL occurs only on the (0001) surface and plays a fundamental role in the azimuthal ordering and subsequent growth of graphene layer 4 (epitaxial graphene - EP), also it can be used as a nanometer mesh template for assembling nanostructures on surface. 5 Despite its fascinating characteristics, graphene has a feature that compromises its potential technological application: it is a zero gap semiconductor. Therefore, it is necessary modify the electronic structure according to the intended application and this can be done with the functionalization of the graphene with particles and molecules. There are many examples of graphene functionalization/doping, most of them by alkaline metals. For sodium, 6 the atoms diffuse between the G and BL immediately after deposition and after annealing 2


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migrate to the BL-SiC interface, decoupling the BL from the SiC substrate. In the case of lithium, 7 the decoupling starts even at room temperature. For Rubidium and Cesium, 8 no intercalation occurs even after annealing. For transition metals, there are examples of decoupling of BL by gold, 9 copper 10 and manganese 11 atoms among others and by molecules, for example, hydrogen, 12 fluorine 13 and oxygen. 14 However, the studies involving intercalation of magnetic metals or oxides in this system are scarce. Here, we have studied the behavior of Co nanoparticles grown on graphene/SiC(0001) with subsequent formation of CoO NPs by exposure to oxygen. Upon annealing there is a complete deoxidation of CoO to a metallic state with intercalation between graphene and BL on SiC(0001). The intercalated cobalt atoms form a quasi-periodic pattern of 2D metallic Co clusters. From our STM measurements we have evidences that such a pattern was induced by the BL reconstruction that acts as a nanotemplate. The intercalated Co clusters are not re-oxidized if exposed to O2 because they are protected by the capping graphene layer. Moreover, for sufficient amounts of intercalated Co atoms they display ferromagnetic behavior as observed in our preliminary X-ray Magnetic Circular Dichroism (XMCD) investigation discussed below. We demonstrate a method to prepare immobilized Co NPs (and probably other metals) by using the graphene/SiC(0001) substrate as a template to produce cluster arrays of magnetic Co NPs protected by a single graphene layer. Since it is very difficult to preserve the metallic character of very small Co NPs or even films, such intercalation could be used to define a sharp interface between ferromagnetic nanodots/films and another material, for instance a CoO antiferromagnetic layers which could have applications in nanomagnetism devices.

Results and discussion The SiC used in this study was 6H-type obtained from MTI corp. The surface was etched in 1 atm of H2 at 1100 ◦ C during 10 minutes. After this, the crystal was annealed in UHV for 1 hour at 600 ◦ C and finally heated between 1200-1300 ◦ C for 10 minutes to produce the

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CoO. In addition, the presence of large shake up satellite peaks (S) also corroborates the stable state of cobalt oxide (II), 17 since the second most common state for cobalt oxide, Co3 O4 does not show these satellites. In this process, we used a partial pressure of O2 of 8 × 10−7 mbar during 30 minutes at room temperature. It is important to mention that repeating the experiment with the same pressure and lower exposition time (∼ 5 min) we have obtained the same final oxidation state. Due to relatively low pressure of O2 used and room temperature, no changes were observed for the carbon and silicon core levels before and after the O2 dosing, as expected. The oxygen intercalation between the graphene layers and BL only occurs for higher values of pressure and temperature, as previously reported. 14,18 Again, it is important to stress that if part of the cobalt was intercalated at RT, we should observe the metallic state coexisting with the oxide one after the first oxidation as will be clear in the discussion. In order to deoxidize the cobalt, the sample was annealed at different temperatures (400, 500 and 650 ◦ C), being monitored by XPS. After annealing at 650 ◦ C (10 min, red curve), it is possible to observe a transition to metallic cobalt, with the Co 2p3/2 main peak at 778.3 eV, the decrease in the spin-orbit splitting to 15.1 eV and the correspond suppression of the shake-up satellites. Thus, the most obvious explanation would be a simple deoxidation of the cobalt oxide present over the graphene back to the previous metallic state. In the intermediate temperatures, a metal/oxide mixed state was observed (not shown), with higher predominance of the oxide component. The next step was to again expose the surface to O2 under the same conditions used before, but no oxidation of the cobalt was observed. We have tried further oxidation even at higher pressures of oxygen (10−5 mbar) and the intercalated cobalt still remained metallic. The explanation is that after the deposition the metallic cobalt remains on the surface, as observed by STM images (fig. 1), and suffers almost instantaneous oxidation if exposed to O2 . After annealing at 650 ◦ C, besides the deoxidation, the cobalt intercalates between the graphene and the BL where graphene acts as a protecting membrane.

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chemical shift, would be expected in XPS for such annealing temperature. 21 Also, a possible formation of cobalt silicide or carbide would affect the magnetic properties of cobalt, which will be discussed later on. For the graphene to maintain the same reconstruction observed for the clean case with the cobalt intercalated between graphene and BL, the Co atoms cannot form a homogeneous monolayer, but should be in the form of 2D clusters. In fact, the ripple in the BL, at the interface between SiC and graphene, works as a template for the accommodation of molecules, atoms, and clusters. 5 Figure 5 shows STM images of Co clusters intercalated between graphene and BL, for other sample with less Co deposited compared to the previous case. The images show a structure very similar to the case of intercalation of gold between graphene/SiC. 22 The intercalation of Au forms 2D structures that generate a superlattice of resonators, evidenced by the pattern of standing waves that is generated in the Fourier transform of the STM images. 23 These Au clusters have been shown to modify the band structure of graphene around the Van Hove singularity (M point of BZ), with possible application of graphene as high-temperature superconductor. 24 The main mechanism for the intercalation should be the migration of Co atoms through point defects and domain boundaries of graphene. However, since the intercalation is followed by desorption of oxygen, we cannot exclude the formation of new lattice vacancies by formation of CO or CO2 , but a possible C loss during Co intercalation is below our XPS detection limit and we have not been able to observe in STM images due the bright spots shown in the region with the intercalated Co. In a recent work involving the oxygen intercalation/deintercalation on Graphene/Ir(111) 25 the authors observed the formation of pores after oxygen deintercalation, which were attributed to oxygen reaction with the graphene. Such a mechanism might also be present here. Figure 5(a) and 5(b) show STM images of intercalated Co clusters, it is also possible to visualize the carbon atoms (honeycomb). The clusters have two predominant shapes triangles and lozenges that have two preferential directions of alignment, rotated 60◦ relative

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case the Co 2p core levels have a metallic character. To further check the metallic character and the magnetic properties of the intercalated Co on Graphene/SiC(0001) a preliminary investigation using x-ray magnetic circular dichroism (XMCD) was performed. XMCD is a well-established magnetic-element/chemical specific technique which is based in the measurement of the x-ray absorption spectroscopy (XAS) for core-levels using circularly polarized light. 28 The measurements were performed using the planar grating monochromator beamline (PGM) at the Brazilian Synchrotron Light Source (LNLS) 29 using an UHV-XMCD chamber 30 with facilities for sample heating, cooling and application of moderate magnetic field for absorption measurements in the total electron yield mode. An equivalent sample to the one presented in figure 2 and 3 was prepared with cobalt intercalation displaying the same behavior of the previous samples as detected by XPS. The sample was then stored in air for a few hours before been transferred to the UHV XMCD chamber. Figure 6(a) shows the XMCD measurements for the as transferred sample. The fine structure in the region of the Co L3 edge is clearly resolved showing the peaks named A, B and C at 777.0 eV, 778.3 eV and 778.7eV, respectively, which are characteristic for CoO. 31 The peak D at 779.5 eV is much smaller than the characteristic one for a thick CoO film, indicating that the intercalated Co has not coalesced into large islands, corroborating the STM findings. Due to the long exposure to atmosphere, a major part of the intercalated cobalt was re-oxidized, thus the XMCD signal (blue line) was almost zero, since CoO has anti-ferromagnetic ordering. This result is very interesting and indeed corroborates the XPS and STM measurements that suggest Co intercalating between graphene and BL. The graphene membrane can protect against oxidation under vacuum at moderate oxygen pressures and temperatures, however, oxygen intercalation at high pressures is expected as already reported. 14,18 On other hand, if Co would have formed carbides or silicides after intercalation we should not observe any re-oxidation, since oxidation of Co silicides or carbides are energetically and thermodynamically unfavorable at RT. Once again, the deoxidation of the already intercalated CoO was promoted by annealing the sample to about 450 ◦ C un-

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der UHV. Thereafter, XMCD was measured at 13 K with 0.7 T applied magnetic field and in remanence. XAS clearly shows the characteristic L2,3 spectrum of metallic Co without any extra component which could be attributed to oxides, silicide 32 or carbide. Moreover, cobalt silicide does not show any dichroic signal as reported. 32 Despite the signal to noise produced by the low amount of cobalt and vibrations from the close-cycle cryostat, a clear ferromagnetic ordering is observed in the XMCD signal. Further and systematic studies are necessary, for example, to establish the limit for blocking temperature, anisotropy and transition from superparamagnetic to ferromagnetic ordering as function of the particle/cluster size and density. However, this study demonstrate the possibilities to create magnetic ordering for periodic clusters or nanoislands of Co templated in the BL/SiC(0001) through intercalation, which uses the graphene as a single protecting layer, opening a huge field of possibilities to engineering magnetic based devices.

Conclusions To summarize, after the deposition the Co atoms are subject to oxidation because they are unprotected. The possibility of assembling Co NPs and them CoO NPs onto G/SiC(0001) is demonstrated. Further evaporation would produce an ultrathin film that, if oxidized, would form a conventional CoO antiferromagnetic film. By annealing a proper coverage of CoO film or NPs, a process of deoxidation with the intercalation of Co between the graphene and the buffer layer is induced, forming 2D clusters that are immobilized and preserve the metallic and magnetic character protected against oxidation by the single layer graphene. Such a method would allow for example the production of very well defined and protected interface between antiferromagnetic/ferromagnetic junctions. The possibility to produce large-scale graphene-capped ferromagnetic clusters (or films) assembled on a semiconductor substrate has enormous potential for technological applications, for example, in magnetic data storage media or other devices.

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Patterning Quasi-Periodic Co 2D-Clusters underneath Graphene on

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