Article pubs.acs.org/JPCC
Influence of the Rotational Domain in the Growth of Transition Metal Clusters on Graphene Antonio J. Martínez-Galera,*,†,‡ Iván Brihuega,†,§,∥ and José M. Gómez-Rodríguez†,§,∥ †
Departamento de Física de la Materia Condensada, C−III, §Condensed Matter Physics Center (IFIMAC), and ∥Instituto Nicolás Cabrera (INC), Universidad Autónoma de Madrid, E-28049, Madrid, Spain ABSTRACT: The influence of the relative orientation between graphene monolayers and the underlying substrate on the growth of transition metal clusters over epitaxial graphene on Ir(111) surfaces is investigated by scanning tunneling microscopy (STM) in ultrahigh vacuum (UHV). This experimental study has been carried out for W and Ir clusters and, in both cases, the results revealed the existence of noticeable differences in the size and distribution of the clusters formed over areas where graphene and substrate lattices are aligned with respect to those grown on regions where both lattices present a relative rotation angle. In particular, while over aligned domains, in a consistent way with previous findings, the formation of ordered arrays of monodisperse clusters exhibiting a great structural perfection is observed, on the rotated ones it takes place the formation of larger size isolated clusters scattered around the surface. Moreover, the boundaries between different rotational domains are found to be decorated by these larger clusters. This disparity observed in the growth of clusters is explained in terms of the differences in the graphene−substrate interaction existing on aligned and rotated domains.
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INTRODUCTION The growth of aggregates composed of a few atoms of metallic elements on graphene surfaces is a topic of great importance. On the one hand, aimed to harness the potential of graphene in technological applications they could be extremely useful for the construction of metal contacts in a future generation of small size electronic circuits based on graphene.1−3 On the other hand, from a fundamental point of view, the study of the formation as well as of the properties of small size clusters on graphene surfaces is also a relevant subject. Specifically, it is well-known that, due to quantum confinement effects, the aggregates formed by a reduced number of atoms, exhibit some interesting properties that are not present in the bulk of the material.4,5 However, because of the importance of the surface with respect to the volume in clusters of small dimensions, their properties could depend strongly on the environment where they are located. For this reason, in principle, it should be expected that the graphene surface, due to its low reactivity, would be an ideal support for studying the intrinsic properties of these atomic aggregates. Therefore, special attention should be devoted to the study of the growth of small clusters on graphene surfaces. The formation of aggregates composed by a few atoms has been recently observed in some experimental studies related to the adsorption of different materials on graphene surfaces epitaxially grown on various metal substrates. In all these works it was demonstrated that the size and the distribution of the aggregates on the graphene surface depends on the adsorbate as well as on the graphene-metal system. More specifically, it has been reported that, while the adsorption of Ni on graphene/ Rh(111),6 Ru on graphene/Ru(0001)7 or Eu on graphene/ Ir(111)8 gives rise to aggregates randomly distributed on the © XXXX American Chemical Society
surface without any defined order, in other cases it takes place the formation of ordered arrays of monodisperse clusters. In particular, these cluster superlattices have been observed for the adsorption of Pt on graphene/Ru(0001)9 as well as for the adsorption of Ir,10−12 Rh,13 Pt,11 and W11 on areas of graphene/Ir(111) surfaces where graphene and substrate lattices are aligned. Among them, the graphene/Ir(111) interface is particularly relevant since, as a consequence of the weak interaction between graphene and the Ir(111) surface, most of the electronic properties of ideal graphene are kept.12,14 Precisely due to this weak interaction, graphene monolayers can adopt several rotational orientations with respect to the Ir(111) substrate.15,16 Thus, a comprehensive study of the growth of small size clusters on this graphene-metal interface requires the analysis over different rotational domains. Nevertheless, until now both theoretical17 and experimental10−12 studies related to the formation of clusters have been restricted to regions of the graphene/Ir(111) surface in which the atomic periodicities of graphene and substrate are aligned. Here, we present the first experimental study focused on the analysis of the influence of the rotational orientation between graphene monolayers and the metallic substrate underneath on the growth of transition metal clusters. In particular, we have investigated by means of Scanning Tunneling Microscopy (STM) the growth of W and Ir clusters over different rotational domains of the graphene/Ir(111) interface. Our experimental results have demonstrated that despite the inert character of the graphene surface, the relative orientation between the graphene Received: November 21, 2014 Revised: January 16, 2015
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EXPERIMENTAL SECTION The experiments were carried out in an ultrahigh vacuum system with a base pressure of 1 × 10−10 Torr equipped with a home-built Variable Temperature Scanning Tunneling Microscope (VT-STM).18,19 Ir(111) surfaces were prepared by two cycles of Ar+ bombardment (E = 1 kV), the first one with the sample at RT and the second one at 850 °C. After the second sputtering cycle the sample was flashed at 1200 °C. The growth of graphene monolayers on Ir(111) surfaces was performed by low pressure UHV-CVD. More specifically, clean Ir(111) substrates kept at 1050 °C were exposed to an ethylene partial pressure of 3 × 10−7 Torr during 1 min. The quality of the samples was checked by Low Energy Electron Diffraction (LEED) and STM. W and Ir were sublimated onto the graphene/Ir(111) surfaces at room temperature from homebuilt evaporators consisting in current heated wires made out of each material. A precise calibration of the deposition rate for both materials as a function of the filament temperature measured with a digital infrared pyrometer was performed by means of STM images (e.g., typical values were 8 × 10−3 ML/s and 2400 °C for W). The deposition rate of W and Ir was calibrated over bare Ir(111) areas uncovered by graphene and the surface coverages given in the present work are referred to the atomic density of the close packed Ir(111) surface. Before the deposition of both materials on the graphene/Ir(111) surfaces, the filaments were previously degassed at the sublimation temperature. It allowed that the pressure in the system was in the range of 10−10 Torr during the sublimation onto the sample. Finally, STM data acquisition and analysis was performed by means of the WSxM software.20
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RESULTS AND DISCUSSION As expected for weakly coupled graphene-metal interfaces, on the monolayers epitaxially grown on Ir(111), domains with different orientations of graphene relative to the underlying substrate coexist.15,16,21 However, as observed in Figure 1a, it is possible to obtain graphene monolayers perfectly aligned with respect to the Ir(111) substrate by performing a previous step consisting on the exposure of the sample at room temperature to 10 L ethylene and annealing at 1050 °C prior to the CVD process.15 Specifically, in the LEED pattern displayed in Figure 1a the existence of spots associated with the graphene periodicity, which are aligned with the ones of the Ir(111) substrate, as well as of six satellite spots around them can be observed (see dotted lines and circle, respectively). The presence of these satellite spots will be explained latter. In contrast, after the growth of graphene by CVD at 1050 °C without the previous step above mentioned different rotational domains are present as it can be noticed in the LEED pattern shown in Figure 1b. In this case, in addition to the spots related to the Ir(111) surface, the graphene and the six satellite spots around them, the LEED pattern shown in Figure 1b also exhibits arcs with the periodicity of graphene subtending an angle of roughly 10° located between 10° and 20° with respect to the Ir(111) lattice (see solid lines). An interesting property of this kind of interfaces is that the superposition of graphene and substrate lattices under specific orientations gives rise to superstructures, referred to as moiré
Figure 1. (a, b) LEED patterns acquired with an electron beam energy of 62 eV on different graphene/Ir(111) samples. (a) The graphene dots aligned with those of Ir(111) and the six satellite spots associated with the moiré are observed (see dotted line and circle, respectively). (b) Together with the spots of the Ir(111), those of graphene and the six satellite spots related to the moiré pattern, arcs with the periodicity of graphene subtending an angle between 10° and 20° with respect to the Ir(111) lattice are also visible (see solid lines). (c−e) (Left) Atomically resolved STM images exhibiting the moire patterns most frequently found during our experiments. (c) Tunneling parameters: Vs = +0.12 V, IT = 3.6 nA, size: 6 × 6 nm2; (d) Tunneling parameters: Vs = +0.58 V, IT = 1.8 nA, size: 4 × 4 nm2; (e) Tunneling parameters: Vs = +0.07 V, IT = 1.1 nA, size: 4 × 4 nm2. (c−e) (Right) Schematic representations of the superstructures observed in the STM images in (c)−(e).
patterns. The moiré patterns most frequently found during our measurements performed on graphene/Ir(111) surfaces can be observed with atomic resolution in the STM images displayed in Figure 1c−e. Additionally, according to our experimental findings, we have elaborated ball models of their respective unit cells, which are compatible with some of the superstructures previously reported.21−23 The moiré pattern observed in Figure 1c is the one most commonly found over this interface and it corresponds to an incommensurate superstructure, where 10 interatomic distances of carbon atoms approximately coincide with 9 of the Ir(111) lattice. This superstructure is originated B
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Figure 2. (a) STM image acquired after the adsorption of 0.50 ± 0.05 ML of W on a region of the graphene/Ir(111) surface in which graphene and Ir(111) lattices are aligned. It can be observed how the resulting superlattice of W clusters has a high degree of perfection over large regions of the sample. Tunneling parameters: Vs = +1.5 V, IT = 50 pA, size: 250 × 250 nm2. (b) Zoom-in of the area indicated by the brown square in (a). (c) 3D representation of an STM image acquired after adsorption at room temperature of 0.50 ± 0.05 ML of W on a graphene sample epitaxially grown on an Ir(111) surface. On the bottom part of the image the cluster network formed over regions where the graphene lattice is aligned with respect to the Ir(111) substrate is present. However, on the upper part, the formation of larger aggregates without any apparent order is observed. Tunneling parameters: Vs = +1.5 V, IT = 50 pA, size: 250 × 250 nm2. (d) 3D representation of an STM image acquired over the area of 4 × 4 nm2 indicated by the green square in (c). This image shows the same moiré superstructure as that observed in Figure 1e, which is compatible with a relative orientation of 18.5° between graphene and Ir(111). Tunneling parameters: Vs = +71 mV, IT = 1.1 nA. (e) Topography profile along the green line displayed in (c).
referred to graphene is almost coincident with a √13 one of the Ir(111) surface. The moiré pattern observed in Figure 1e is associated with a superstructure which roughly coincides with a 3 × 3 supercell referred to the lattice parameter of graphene. This moiré pattern could be ascribed to the superposition of graphene and Ir(111) lattices with a relative rotation of around 18.5°. According to this model graphene and Ir(111) lattices roughly coincide every three times the periodicity of the graphene lattice and √7 times the interatomic distance of the Ir(111) surface. This fact explains the presence of the moire pattern with approximately 3 × 3 periodicity with respect to the graphene lattice observed in the STM image displayed in Figure
when graphene and the underlying Ir(111) substrate are aligned (see the schematic drawing shown in Figure 1c). This fact explains the presence of the spots with the periodicity of graphene aligned with those related to the Ir(111) surface observed in the LEED patterns displayed in Figure 1a,b. Likewise, the satelite spots are associated with the moiré pattern resulting from the alignment of graphene and Ir(111). A moiré pattern nearly coincident with a 4 × 4 superstructure referred to the graphene lattice is displayed in Figure 1d. This superstructure can be explained by a rotation between graphene and Ir(111) lattices of around 14°, as observed in the schematics shown in Figure 1d. In this case, a 4 × 4 supercell C
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Figure 3. (a−f) Representative STM images illustrating the cluster growth (a) on aligned areas with a W coverage of 0.32 ± 0.02 ML, (b) on aligned areas for 0.73 ± 0.06 ML, (c and d) on rotated areas by 14° for 0.32 ± 0.02 ML and 0.73 ± 0.06 ML coverages, respectively, (e and f) on rotated areas by 18.5° for 0.32 ± 0.02 ML and 0.73 ± 0.06 ML. Tunneling parameters: (a) Vs = +1.5 V, IT = 50 pA, (b) Vs = +1.5 V, IT = 50 pA, (c) Vs = +1.5 V, IT = 50 pA, (d) Vs = +1.5 V, IT = 50 pA, (e) Vs = +1.5 V, IT = 50 pA, (f) Vs = +1.5 V, IT = 50 pA. The size is 80 × 80 nm2 for all the images. (g) Graph showing the evolution of the average apparent height of clusters as a function of the W coverage for each one of the three orientations of the graphene layer with respect to the Ir(111) substrate studied in this work.
also observed that, on this rotated area, the folded graphene along monatomic steps in the underlying Ir(111) substrate as well as the boundary between the aligned and the rotated domains are found to be decorated by the larger clusters. Finally, it should be mentioned that the apparent height of clusters grown over this rotational domain is larger than that of clusters grown over the aligned area (see Figure 1f). Aimed to investigate the possible influence of rotational domains on the growth of W clusters on the graphene/Ir(111) interface, the adsorption of W has been studied for several coverages in more than 200 areas exhibiting different orientations. The obtained results are summarized in Figure 3. Figure 3a−f shows STM images acquired over areas where graphene and Ir(111) lattices are rotated by 0°, 14° and 18.5° and for W coverages of 0.32 and 0.73 ML. A comparison of the images shown in Figure 3a,c,e for a W coverage of 0.32 ML as well as of those in Figure 3b,d,f for 0.73 ML clearly reveals that, regardless of coverage, whereas over aligned areas clusters form an ordered array, on the rotated ones clusters present larger sizes and they are randomly distributed over the surface. In contrast, there are no significant differences on the size and distribution of clusters between rotated areas with different angle. The differences in the growth of clusters over aligned and rotated domains can be visualized in a more quantitative way in the STM study of the apparent heights shown in the plot in Figure 3g. The apparent height of clusters as a function of the W coverage has been analyzed over areas exhibiting three different orientations of the graphene layer with respect to the underlying Ir(111) substrate. For all these three orientations, a monotonic increase of the apparent height with the coverage of W is observed in the range 0−1.5 ML. It is interesting to note that clusters grown over rotated domains exhibit significantly larger apparent heights than those formed over aligned areas.
1e. Finally, it should be noted that the relative orientations between graphene and Ir(111) of around 14° and 18.5° proposed in the schematic drawings exhibited in Figure 1d,e for the rotational variants are within the angle subtended by the arcs observed in the LEED pattern shown in Figure 1b. Figure 2a shows a large scale STM image obtained after the adsorption of 0.50 ML ± 0.05 ML of W on a graphene surface epitaxially grown on an Ir(111) substrate. In a consistent way with previous experimental reports,10,11 in this image it is observed a cluster superlattice with nearly perfect order extending over large areas of the sample. This superstructure of W clusters can be visualized in more detail in Figure 2b, where it is shown a zoom-in of the area indicated by the brown square in Figure 2a. Similar clusters superlattices have been previously reported after the adsorption of Ir, Pt, and W on regions where the graphene layer and the Ir(111) substrate are aligned.10,11 In addition, as shown in Figure 2c, during our STM measurements performed on graphene/Ir(111) samples after the adsorption of W we have observed some regions where the cluster distribution is completely different. In those regions, the clusters are larger and are randomly distributed over the surface (see the upper part in Figure 2c). A zoom-in STM image (Figure 2d) allows to observe in the region between clusters that the graphene surface presents a moiré pattern associated with a superstructure which approximately coincides with a 3 × 3 supercell referred to the graphene lattice. Then, this moiré pattern is the same one observed in Figure 1e which is compatible with a relative orientation between graphene and the Ir(111) substrate of around 18.5°. Therefore, this fact suggests that the adsorption of W atoms on the graphene/Ir(111) surface could produce different results depending on the rotational orientation between the graphene layer and the Ir(111) substrate. Additionally, in Figure 2c it is D
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Figure 4. (a, b) 3D representation of STM images acquired after adsorption of 0.50 ± 0.05 ML Ir on a graphene/Ir(111) sample. (a) The cluster superlattice formed on regions where the graphene layer and the Ir(111) surface are aligned is observed. Tunneling parameters: Vs = +1.5 V, IT = 0.15 nA; size: 80 × 80 nm2. (b) Together with the cluster superlattice, an area located in the upper left part of the STM image, where clusters are larger and they possess no apparent order, is observed. By STM imaging we have found that the moiré pattern present in that region is the one observed in Figure 1d, which is compatible with a relative orientation between graphene and Ir(111) of 14°. Tunneling parameters: Vs = +1.5 V, IT = 0.15 nA; Size: 77 × 77 nm2. (c, d) Topography profiles along the lines indicated, respectively, in black and blue in (b).
between aligned and rotated domains. On the one hand, the superstructure resulting from the superposition of graphene and Ir(111) lattices on the aligned domains presents a chemical modulation via the formation of weak covalent bonds between two C atoms and the Ir ones placed just below them.24 In particular, these two atoms belong to regions inside the superstructure where the center of C hexagons are respectively placed above 3-fold coordinated hcp and fcc sites. Then, the characteristic sp2 hybridization of graphene is locally modified in a such way that the carbon atoms involved in the weak covalent bonds with the substrate are rehybridized to sp3. As a consequence, it becomes energetically favorable for the three neighbors of the C atoms, which are weakly covalently bonded to the substrate, to bind to metal atoms adsorbed on the surface.17,25 On the other hand, when the lattices are rotated, the size of the moiré superstructure is decreased and it reduces the possibility of finding carbon atoms placed on top of Ir ones. In other words the registry of atomic positions of carbon atoms on the graphene lattice with respect to those of the underlying Ir(111) substrate becomes unfavorable for atomic rehybridization. This circumstance dramatically diminishes the interaction of metal adsorbates with the graphene surface. For this reason, as a consequence of the high structural quality of graphene, metal adatoms and clusters can diffuse long distances over the surface due to the absence of strong bonds between the metal and the substrate. In contrast, the interaction of metal adsorbates with steps and other defects is stronger and they can act as nucleation sites for cluster growth. This fact together with the stronger metal−metal bonds in the cases of W and Ir compared to the metal−substrate interaction results in the formation of 3D metal clusters randomly distributed over the surface, according to a Volmer−Weber growth mode, with a higher density along domain boundaries and steps. Here it is interesting to note that the STM images shown in Figure 3c-f illustrating the growth of W clusters over rotated domains are quite similar to those previously reported on other weakly interacting substrate−adsorbate systems. For example, our STM images of W and Ir clusters grown on the rotated domains resemble to those reported for the adsorption of some metals as Fe,26 Pt,27 Ru,28 Ag,29,30 and W31 over HOPG (highly oriented pyrolitic graphite) surfaces. Therefore, it can be concluded that the cluster growth of transitions metals over
However, for each value of the coverage, no significant differences are observed in the apparent height of the clusters grown over the different rotated domains. From the data shown in Figures 2 and 3, it was concluded that, while on areas where graphene and Ir(111) lattices are aligned a superlattice of W clusters exhibiting high degree of perfection is formed, over areas where graphene and Ir(111) are rotated the resulting clusters present a larger apparent height and they are scattered all over the surface without any apparent order. Here it is interesting to point out that cluster superlattices similar to the one observed in Figure 2a have been observed also for other materials as Ir and Pt adsorbed over aligned regions.10,11 But, what happens when these materials are adsorbed over rotated domains? To provide an answer to this question we have studied the growth of clusters from a different material (Ir) on both aligned and rotated regions. Figure 4a shows an STM image acquired over a 30 × 30 nm2 area of a graphene/Ir(111) sample after the adsorption of 0.5 ML of Ir. The cluster network with the periodicity of the moire pattern resulting from the alignment of graphene and Ir(111) is observed. This superlattice is also present in Figure 4b, where, additionally, an area located in the upper left part of the image reveals the existence of randomly distributed clusters of a larger apparent height as observed in the topographic profiles shown in Figure 4c,d. Inside this area, clusters are also found decorating the graphene layer above the monatomic step of Ir(111). In addition, these higher clusters are also adsorbed along the boundary between the region exhibiting the superlattice and that with larger and scattered clusters. By means of STM measurements, the moire pattern shown in Figure 1d was observed in this region. The apparent heights of Ir clusters in the superlattice as well as of those grown on the rotated area are similar to the average apparent heights of clusters grown after the adsorption of 0.5 ML of W on, respectively, aligned and rotated areas. Therefore, it is possible to infer from our STM measurements performed after the adsorption of Ir that the differences observed in the growth of W clusters over aligned and rotated areas of the graphene/ Ir(111) interface could be extended to other transition metals. The different behavior observed in the cluster growth over aligned and rotated areas can be explained in terms of the differences in the graphene-Ir(111) interaction existing E
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(4) Deheer, W. A. The Physics of Simple Metal-Clusters Experimental Aspects and Simple-Models. Rev. Mod. Phys. 1993, 65, 611−676. (5) Bose, S.; Garcia-Garcia, A. M.; Ugeda, M. M.; Urbina, J. D.; Michaelis, C. H.; Brihuega, I.; Kern, K. Observation of Shell Effects in Superconducting Nanoparticles of Sn. Nat. Mater. 2010, 9, 550−554. (6) Sicot, M.; Bouvron, S.; Zander, O.; Rudiger, U.; Dedkov, Y. S.; Fonin, M. Nucleation and Growth of Nickel Nanoclusters on Graphene Moireacute on Rh(111). Appl. Phys. Lett. 2010, 96, 093115. (7) Sutter, E.; Albrecht, P.; Wang, B.; Bocquet, M. L.; Wu, L. J.; Zhu, Y. M.; Sutter, P. Arrays of Ru Nanoclusters with Narrow Size Distribution Templated by Monolayer Graphene on Ru. Surf. Sci. 2011, 605, 1676−1684. (8) Foerster, D. F.; Wehling, T. O.; Schumacher, S.; Rosch, A.; Michely, T. Phase Coexistence of Clusters and Islands: Europium on Graphene. New J. Phys. 2012, 14, 023022. (9) Donner, K.; Jakob, P. Structural Properties and Site Specific Interactions of Pt with the Graphene/Ru(0001) Moire Overlayer. J. Chem. Phys. 2009, 131, 164701. (10) N’Diaye, A. T.; Bleikamp, S.; Feibelman, P. J.; Michely, T. TwoDimensional Ir Cluster Lattice on a Graphene Moire on Ir(111). Phys. Rev. Lett. 2006, 97, 215501. (11) N’Diaye, A. T.; Gerber, T.; Busse, C.; Myslivecek, J.; Coraux, J.; Michely, T. A Versatile Fabrication Method for Cluster Superlattices. New J. Phys. 2009, 11, 103045. (12) Rusponi, S.; Papagno, M.; Moras, P.; Vlaic, S.; Etzkorn, M.; Sheverdyaeva, P. M.; Pacile, D.; Brune, H.; Carbone, C. Highly Anisotropic Dirac Cones in Epitaxial Graphene Modulated by an Island Superlattice. Phys. Rev. Lett. 2010, 105, 246803. (13) Cavallin, A.; Pozzo, M.; Africh, C.; Baraldi, A.; Vesselli, E.; Dri, C.; Comelli, G.; Larciprete, R.; Lacovig, P.; Lizzit, S.; et al. Local Electronic Structure and Density of Edge and Facet Atoms at Rh Nanoclusters Self-Assembled on a Graphene Template. ACS Nano 2012, 6, 3034−3043. (14) Pletikosic, I.; Kralj, M.; Pervan, P.; Brako, R.; Coraux, J.; N’Diaye, A. T.; Busse, C.; Michely, T. Dirac Cones and Minigaps for Graphene on Ir(111). Phys. Rev. Lett. 2009, 102, 056808. (15) van Gastel, R.; N’Diaye, A. T.; Wall, D.; Coraux, J.; Busse, C.; Buckanie, N. M.; zu Heringdorf, F. J. M.; von Hoegen, M. H.; Michely, T.; Poelsema, B. Selecting a Single Orientation for Millimeter Sized Graphene Sheets. Appl. Phys. Lett. 2009, 95, 121901. (16) Hattab, H.; N’Diaye, A. T.; Wall, D.; Jnawali, G.; Coraux, J.; Busse, C.; van Gastel, R.; Poelsema, B.; Michely, T.; Heringdorf, F.; et al. Growth Temperature Dependent Graphene Alignment on Ir(111). Appl. Phys. Lett. 2011, 98, 141903. (17) Feibelman, P. J. Onset of Three-Dimensional Ir Islands on a Graphene/Ir(111) Template. Phys. Rev. B 2009, 80, 085412. (18) Custance, O.; Brochard, S.; Brihuega, I.; Artacho, E.; Soler, J. M.; Baró, A. M.; Gómez-Rodríguez, J. M. Single Adatom Adsorption and Diffusion on Si(111)-(7 × 7) Surfaces: Scanning Tunneling Microscopy and First-Principles Calculations. Phys. Rev. B 2003, 67, 235410. (19) Martinez-Galera, A. J.; Gomez-Rodriguez, J. M. Nucleation and Growth of the Prototype Azabenzene 1,3,5-Triazine on Graphite Surfaces at Low Temperatures. J. Phys. Chem. C 2011, 115, 11089− 11094. (20) Horcas, I.; Fernandez, R.; Gomez-Rodriguez, J. M.; Colchero, J.; Gomez-Herrero, J.; Baro, A. M. WSXM: A Software for Scanning Probe Microscopy and a Tool for Nanotechnology. Rev. Sci. Instrum. 2007, 78, 013705. (21) Loginova, E.; Nie, S.; Thurmer, K.; Bartelt, N. C.; McCarty, K. F. Defects of Graphene on Ir(111): Rotational Domains and Ridges. Phys. Rev. B 2009, 80, 085430. (22) Coraux, J.; N’Diaye, A. T.; Busse, C.; Michely, T. Structural Coherency of Graphene on Ir(111). Nano Lett. 2008, 8, 565−570. (23) Coraux, J.; N’Diaye, A. T.; Engler, M.; Busse, C.; Wall, D.; Buckanie, N.; Heringdorf, F.; van Gastei, R.; Poelsema, B.; Michely, T. Growth of Graphene on Ir(111). New J. Phys. 2009, 11, 023006.
rotated areas follows a similar pattern to that previously reported over the flat graphite surface where the metal adatoms present a weak interaction with the surface. However, over aligned areas cluster superlattices are formed due to the fact that the adatom−graphene interaction is enhanced in some regions of the moire superstructure as a straightforward consequence of the particular graphene−substrate interaction existing in this case. This statement is also consistent with the results by Granas et al.32 In the STM images reported by these authors it is observed that clusters grown over aligned areas after the graphene−Ir(111) interaction was decreased by intercalating an oxygen film of monatomic thickness32 present a similar appearance to the STM images displayed in Figure 3c−f and in the upper left part in Figure 4b.
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CONCLUSIONS To summarize, the present work reports on a comparative study of the growth of transition metal clusters over different rotational domains of the graphene/Ir(111) interface. In particular, it was found that, in a consistent way with previous findings, the adsorption of W and Ir over regions where graphene and Ir(111) lattices are aligned results in the formation of cluster superlattices exhibiting a high structural quality over large areas of the sample. However, our STM analysis has demonstrated that on rotational variants the clusters grown present larger apparent heights and they are found to be randomly distributed over the flat terraces as well as decorating the steps edges and domain boundaries. This disparity observed in the growth of clusters is attributed to the differences in the graphene−substrate interaction existing for aligned.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +49 2214703599. E-mail:
[email protected]. Present Address
‡ II. Physikalisches Institut, Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support from the CAM, under Contract No. CPI/ 0256/2007, and from the Spanish MINECO under Grant Nos. MAT2013-41636-P and CSD2010-00024, as well as from the European Commission through a Marie Curie Fellowship (Proposal 332214 ELECTROMAGRAPHENE), is gratefully acknowledged. We thank prof. Thomas Michely for fruitful discussions and critical reading of the manuscript.
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REFERENCES
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DOI: 10.1021/jp511652f J. Phys. Chem. C XXXX, XXX, XXX−XXX
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DOI: 10.1021/jp511652f J. Phys. Chem. C XXXX, XXX, XXX−XXX
