The Thinnest Carpet on the Smallest Staircase: The Growth of

Article pubs.acs.org/JPCC

The Thinnest Carpet on the Smallest Staircase: The Growth of Graphene on Rh(533) B. Casarin,† A. Cian,† Z. Feng,† E. Monachino,† F. Randi,† G. Zamborlini,† M. Zonno,† E. Miniussi,†,‡ P. Lacovig,§ S. Lizzit,§ and A. Baraldi*,†,‡ †

Physics Department and CENMAT, University of Trieste, Via Valerio 2, I-34127 Trieste, Italy IOM-CNR, Laboratorio TASC, S.S. 14 Km 163.5, I-34149 Trieste, Italy § Elettra - Sincrotrone Trieste S.C.p.A., S.S. 14 Km 163.5, 34149 Trieste, Italy ‡

S Supporting Information *

ABSTRACT: We here discuss the growth process and the properties of single-layer graphene on a vicinal Rh(533) surface. The structural anisotropy of the substrate leads to a moirè cell with nonequivalent lattice vectors in the directions parallel and orthogonal to the steps. Our results indicate that the high structural quality of the carbon layer, combined with the weaker interaction with the substrate and the higher thermal stability with respect to graphene on Rh(111), is strongly influenced by the presence of the surface steps, which play a fundamental role in the defect-healing mechanism first predicted by earlier theoretical calculations.

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INTRODUCTION The capability of modifying the structure, and hence the electronic properties, of graphene (GR) is among the key goals of the scientific community interested in the application of this allotropic form of carbon in a range of nanotechnologically relevant fields. Not surprisingly, chemical vapor deposition (CVD), which is commonly believed to be one of the most effective methods in synthesizing high-quality single layers of graphene, is currently employed, along with other deposition techniques, to achieve a fine-tuning of the electronic properties of graphene, driven by the variation of the interaction strength with the substrate on which it is grown. The doping of epitaxial graphene with selected elemental species,1−6 the intercalation of light,7−13 alkali,14−19 noble or transition metal atoms,20−22 the growth of oxides,8,23 the preparation of bimetallic alloy substrates,24,25 and the fabrication of nanopatterned surfaces26 are a few examples of the success of this approach. Surfaces with different crystallographic orientations from hexagonal close-packed have also been widely employed to modify the morphology of the carbon layer,27−31 which in turn leads to significant changes in the electronic properties of the system. An alternative procedure to modify the band structure of epitaxial graphene is by growing it on strongly anisotropic substrates, i.e., substrates with different lattice parameters along different crystallographic directions. In this sense, one of the most meaningful results has been obtained by Vinogradov et al., who used the Fe(110) surface to grow a graphene layer with a “washboard”-like, one-dimensional corrugation (with a periodicity of 2.6 nm) and an impressive degree of long-range order.32 It is therefore clear that the use of surfaces with © 2014 American Chemical Society

structural regularities, like high-Miller index surfaces, offers an innovative and widely applicable solution for the fine-tuning of the structural properties of graphene. The study of Šrut et al.33 has shown how the Ir(332) surface can be used to grow single layers of graphene with a local one-dimensional corrugation. Due to the weak graphene−Ir interaction, however, the surface is covered with differently oriented graphene domains, with a wide angular spread and with structural properties that strongly depend on the growth temperature. In this work we show, instead, how a vicinal surface of Rh, which is commonly considered a strongly interacting metal,3,21,34−36 can be used to grow single-domain graphene layers with a moirè cell whose lattice vector is identical to that of GR/Rh(111) in one crystallographic direction, while in the other direction it is specifically determined by the periodicity of the stepped substrate. The graphene layer exhibits a substantially modified interaction with the substrate and a different thermal stability from that of the C layer grown on the flat Rh(111) surface.

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EXPERIMENTAL SECTION The Rh(533) surface consists of (111) terraces formed by four atomic rows and separated by monatomic steps oriented in the (100) direction (see Supporting Information). Alternatively, also the notation 4(111) × (100) can be used for this surface. The terrace width in the (111) plane is ≃0.85 nm, while the Received: November 25, 2013 Revised: January 22, 2014 Published: January 27, 2014 6242

dx.doi.org/10.1021/jp411582a | J. Phys. Chem. C 2014, 118, 6242−6250

The Journal of Physical Chemistry C

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Figure 1. (a) Time-lapsed sequence of C 1s spectra acquired during GR growth on Rh(533) at 770, 970, and 1020 K, respectively. (b) Evolution of the intensity curves vs C coverage for the different carbon species (Cs, S, and W), as obtained from an accurate analysis of the integrated photoemission intensity of the spectra. The intensity ratio between the strongly and the weakly interacting C 1s components (S/W, gray line in (b)) is plotted against the right axis. (c) Corresponding intensity evolution of the various C species as a function of the C2H4 exposure. In the bottom graph the pressure curve vs exposure (which was the same in all experiments) is also shown superimposed.

step height of 0.22 nm corresponds to the interlayer distance between the (111) planes of Rh. The low-energy electron diffraction (LEED) data were acquired at the Surface Science Laboratory of Elettra Sincrotrone Trieste (Italy), while the photoemission experiments were performed at the SuperESCA beamline of the thirdgeneration synchrotron light source Elettra. The ultrahigh vacuum chamber of the Surface Science Laboratory, which is primarily devoted to LEED and X-ray photoelectron spectroscopy/X-ray photoelectron diffraction (XPS/XPD) measurements, houses a VG LEED optics with a transfer width of 150 Å, two conventional Al and Mg Kα X-ray sources, and a hemispherical electron energy analyzer and is also equipped with a quadrupole mass spectrometer and standard sample preparation facilities. The ultrahigh vacuum chamber of the SuperESCA beamline, besides the standard surface science facilities for sample cleaning and characterization, is equipped with a Phoibos hemispherical electron energy analyzer from SPECS (mean radius of 150 mm), combined with a custom homemade delay-line detection system. The energy tunability and the high brilliance of synchrotron radiation allow in situ monitoring the evolution of the atomic core levels of the sample under CVD conditions. The overall experimental

resolution attained in our experiments for the C 1s and Rh 3d5/2 signals was of the order of 40 meV. The Rh(533) single crystal, mounted on a manipulator with 4 degrees of freedom, was heated either by direct irradiation or by electron bombardment from three tungsten filaments mounted behind it. Liquid nitrogen was used to cool the sample down to 80 K, to reduce the phonon contribution in both LEED and XPS experiments. The temperature of the crystal was always monitored in both chambers by means of two K-type thermocouples directly spotwelded on one side of the sample. The surface was cleaned by repeated cycles of Ar+ sputtering followed by annealing up to 1275 K. The surface subsequently underwent a series of heating−cooling cycles in oxygen (p(O2) = 1 × 10−7 mbar, Tmax = 1000 K) and hydrogen atmosphere (p(H2) = 5 × 10−8 mbar, Tmax = 750 K). In this way, the residual carbon impurities were removed by the oxygen and the latter reacted out with hydrogen. The oxidation−reduction treatment was followed by a final flash annealing to 650 K. The cleanliness was checked by means of photoemission and by inspecting the LEED pattern, which shows, besides the features due to the (111) nanofacets, the spots induced by the step periodicity (see Supporting Information). In the following, we will adopt the definition of 1 monolayer (ML) as the surface 6243



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findings have more recently been confirmed also by the theoretical work of Gao and Zhao about C cluster nucleation on the steps of the Rh(433) surface during the early stage of graphene growth.42 Despite the different periodicity of the two vicinal Rh surfaces considered in the cited works, the identical nature of the (111) terraces and of the (100) step sites is a good reason to believe the same configuration is also present on Rh(533). It is also useful to note that the BE of Cs is quite close to the DFT-calculated C 1s core level BE of the C monomer sitting on the steps of Ru(0001) (283.55 vs 283.83 eV).25 In addition, we can safely rule out the identification of Cs as surface carbide species, which are normally found at BEs between 283.5 and 282 eV.43−45 An important result that emerges from the observation of the plots in Figure 1(c) is that, even under a constant supply of atomic C, the population of C atoms at the steps remains approximately constant over a large period of time, in particular at the highest temperature. This can be ascribed to the presence of two competing mechanisms against GR growth: (i) C bulk dissolution, whose contribution is especially important in the early stages of C2H4 CVD, and (ii) the high formation energy of a C dimer at low C coverage.42 Some important insight into the mechanisms of GR growth on the Rh(533) surface prior to cluster nucleation comes from the theoretical results reported by Nykänen et al. for C adsorption on the vicinal Pd(211) surface.46 In fact, besides the well-known affinity between the two metals in terms of their electronic structure, the Rh(533) and Pd(211) surfaces are also geometrically very similar, since both of them have (111) terraces and (100)-terminated steps. On Pd(211), Nykänen et al. found that, at low C coverage, the octahedral interstitial site is energetically more favorable than the on-surface fcc and hcp sites (although less stable than the step sites), and since the energy barrier for C2H4 dissociation and C subsurface penetration is lower at the step edges47 because of the increased surface openness, C can easily migrate from the steps into the first interlayer region. We can reasonably infer that, also on Rh(533), C incorporation into the subsurface region takes place through the step edges of the substrate. It is important to underline that the process of C bulk dissolution we revealed with our spectroscopic approach is not specific to the case of stepped Rh but has also been observed by STM on GR/Rh(111) by Dong et al.,36,48 who found that the dissolved C partly resegregates to the surface and participates in the formation of the C network. Nevertheless, the presence of steps, which, according to the theoretical calculations, represent a preferential place for C subsurface migration, indicates that the steps play a key role in the initial nucleation of the GR network. Above a certain C concentration, on the other hand, it becomes energetically more favorable to accommodate part of the C atoms also on the surface, so that an equilibrium is finally established between the subsurface-segregated and the surfaceadsorbed C phase. Only once GR has started to nucleate under C supersaturation conditions the rate of C−C bond formation exceeds that of carbon bulk dissolution. On a similar note, theoretical calculations for the vicinal Rh(433) surface42 predict that, at low C coverage, the energy cost of forming a C dimer at the steps is significantly (0.35 eV) higher than the total energy of two isolated C monomers and that the sp to sp2 bond rehybridization becomes energetically favorable only for C clusters of at least 10 atoms.

coverage yielded by a complete, defect-free single layer of GR extending over the whole substrate. All the core level spectra presented in this work were fitted to Doniach−Šunjić (DS)37 functions convoluted with a Gaussian distribution. The DS profile combines a Lorentzian width Γ, which is related to the finite core-hole lifetime, with the Anderson singularity index α, which reflects the probability of electron−hole pair excitations. The Gaussian width accounts for the phonon broadening, the instrumental resolution, and any source of inhomogeneous broadening. The background was assumed to be linear. The reported binding energy (BE) positions of the spectra were offset by the corresponding Fermi level, which was measured under the same conditions.

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RESULTS Carbon atoms were deposited on Rh(533) by ethylene (C2H4) chemical vapor deposition in the temperature range 770−1020 K, while the C 1s core level spectrum of the system was monitored by fast XPS38 (see Figure 1(a)). At the lowest deposition temperature (770 K), we observe three C 1s components, characterized by a different evolution at increasing coverage (see top row of Figure 1). The first C species that appears during the uptake shows up as a peak at 283.83 eV BE (Cs in Figure 1(a)), which grows linearly at the beginning, reaches a maximum around 0.03 ML, and subsequently stabilizes around a steady-state value at higher exposures. After the initial intensity maximum of Cs, also the S and W components, centered at 284.80 and 284.31 eV BE, respectively, start growing at similar, constant rates, with an intensity ratio skewed in favor of S (Figure 1(b), top panel). At an intermediate temperature of 970 K, we observe that the intensity of Cs monotonically decreases after its initial maximumalthough not vanishing completelywhile the S and W spectral components grow linearly, keeping an approximate 1:1 ratio up to saturation (Figure 1(b), central panel), and exhibit a significantly narrower width than at 770 K. The C saturation coverage, 0.7 ML, is nearly 20% higher than at 770 K. At the highest growth temperature we used (1020 K), S and W become even sharper and gain intensity at a similar, constant rate (Figure 1(b), bottom panel); Cs, on the other hand, quickly loses intensity after the initial stage of the exposure and disappears around 1 ML coverage. The final saturation carbon coverage, 1 ML, is 65% higher than the one obtained in the lowest temperature experiments. While the analysis of the curves reported in Figure 1(b) indicates that the populations of C atoms associated with S and W grow at the same ratea clear sign of GR formation, as we will discuss in detail laterthe same coverage curves plotted vs exposure (Figure 1(c)) provide information on the initial stage of the growth process. As the temperature increases from 770 to 1020 K, in fact, we notice that the appearance of the S and W components is preceded by a long induction period, which gets larger as the deposition temperature increases and during which only the Cs component is present on the surface (blue peak in Figure 1). On the basis of earlier DFT calculations, we interpret this core level component as due to C atoms at the steps. In fact, Saad et al.39 proved that carbon atoms on Rh(211) prefer to adsorb at the (100) steps. In addition, a systematic study carried out by McCarty et al. on Ru(0001) and on Ir(111) revealed that, at low C concentration, graphene nucleation preferentially occurs in the proximity of a substrate step edge, while at higher C concentrations it is simultaneously observed both near the step edges and on the terraces.40,41 These 6244



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The growth of the double peak structure in the C 1s spectrum, accompanied by the simultaneous depletion of the Cs component, is a fingerprint of the formation of GR. In the GR− transition metal (TM) systems characterized by a strong interaction between the carbon layer and the substrate, like Rh,35 Re,43 and Ru,35 the origin of the S and W components is conventionally attributed to the presence of two main populations of carbon atoms, characterized by a different degree of interaction with the substrate underneath. In this simplified picture, the component at lower binding energy (W) is assigned to the elevated, nonbonding parts of the graphene sheet, whose electronic properties are similar to those of freestanding graphene, while the high-BE peak (S) originates from the strongly interacting parts, where the C atoms are almost covalently bonded to the metal substrate underneath. The fact that the intensity of the S and W components increases at the same rate at all temperatures indicates that GR growth proceeds through a homogeneous expansion of the GR islands via the addition of whole moirè unit cells. We believe the expansion of the atomic-thick GR film on the surface occurs in a continuous, carpet-flow mode, covering the substrate steps, which form like a nanometric staircase. A similar growth mechanism has in fact been reported for substrates like Ru(0001) and Ir(111),49−51 where the C layer is seen to expand like a “rolling carpet” on the terraces, overgrowing the stepsnormally in the downhill direction. The observed narrowing of both the S and W peaks at increasing temperaturealthough the opposite effect would be expected, due to the larger phonon contributionis a clear sign of the improved long-range order of the C layer, which is instead not attainable at lower temperatures. The progressive increase of the C coverage with increasing the CVD temperature clearly indicates a reduction in the density of surface defects: the filling of mono- and divacancies and, above all, of domain boundaries, where the local coverage is expected to be lower than in a defect-free GR layer, clearly increases the average C concentration on the substrate. To get further insight into the properties of the GR layer obtained at high temperature, we compared the line shape of the C 1s and Rh 3d5/2 core level spectra with those of graphene grown by CVD on the clean Rh(111) surface. Figure 2 reports the high-resolution C 1s and Rh 3d5/2 spectra of clean and GRcovered Rh(111) and Rh(533), acquired at low temperature (80 K) and in the normal emission geometry. The Rh 3d5/2 spectra of the clean (111) and (533) substrates show some differences, owing to the particular morphology of the vicinal surface, which leads to the appearance of spectral components which are absent in the spectrum of the flat Rh(111) surface. Following the same assignment proposed by Gustafson et al. for the analogous clean Rh(553) surface,52 we can identify four populations of nonequivalent Rh atoms, characterized by different coordination numbers (CN): bulk (B) and terrace (T) atoms (with CN = 12 and 9, respectively), which are basically equivalent to the bulk and surface atoms of a Rh(111) single crystal, underlayer (U) atoms (CN = 11), and step (S) atoms (which have the lowest coordination, CN = 7). The BEs of the associated spectral components show a clear dependence on CN, in that a lower coordination is reflected into a reduced core level BE, as found in previous coordinationdependent Rh 3d5/2 surface core level shift (SCLS) measurements.52,53 Besides final state effects, this trend is explained in terms of the d-band narrowing at the surface, which becomes even more evident for undercoordinated atoms. The SCLSs we

Figure 2. High-resolution C 1s (red) and Rh 3d5/2 (black) core level spectra of clean and GR-covered Rh(533). A schematic model of the four systems is reported on the right.

obtain for the (533) surface are: −90 ± 20 meV (underlayer), −430 ± 10 meV (terrace), and −690 ± 25 meV (steps), in good agreement with the results obtained for the Rh(553) surface.52 The Rh 3d5/2 spectrum of GR-covered Rh(533) is quite similar to that of Rh(111). In both cases, the component due to Rh atoms on the terraces is significantly suppressed, and for Rh(533), the same effect is observed in particular for the undercoordinated step atoms, thus indicating that the graphene−metal interaction induces a modification of the electronic structure of Rh atoms. Similarly to what was observed on Rh(111), a spectral modification, reflecting the coupling of the carbon atoms with the substrate beneath, appears at higher BEs than the S and T components. More useful information is obtained from the analysis of the C 1s core level spectrum of GR on the two Rh surfaces: despite their apparently similar double-peak structure, in fact, there are a few relevant differences. To highlight the differences between GR on Rh(111) and on Rh(533), on each C 1s spectrum, represented by red circles, is superimposed as a gray shade the C 1s spectrum of GR obtained on the other surface. In the case of Rh(533), the spectral weight is skewed in favor of the weakly interacting component (W), and the whole spectrum is significantly broader than in the case of Rh(111). The larger population of weakly interacting C atoms on Rh(533) is most likely due to the presence of the steps, where the C−substrate separation is larger and the coupling is accordingly weaker. In addition, the shift of both S and W to lower BEs (ΔE ≃ 300 meV) is an indication of the reduced charge transfer, which results in a decreased strength of the covalent interfacial bonding. Finally, the increased complexity of the GR/substrate matchingdue to the broken in-plane crystal symmetry of the flat Rh(111) surfaceand interaction is responsible for the overall Gaussian broadening of the S and W components, an indication of the wider range of nonequivalent C atomic configurations. 6245



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Rh(111) were also reported by Voloshina et al. in a later work.21 These trends are accompanied by a broadening of the d band structure of the substrate. This effect can be appreciated in the Rh 3d5/2 core level spectra where the GR-induced surface components (orange peak in Figure 2, right panel) at low BE move up in energy and the d band narrowing due to the reduced coordination is counteracted by the d band broadening induced by the GR−metal bond formation. The question naturally arises to what extent the morphology of the Rh(533) substrate is affected by graphene growth. The answer, provided by LEED experimental observations, is that the surface structure is largely preserved. The LEED pattern of the clean Rh(533) surface shows a hexagonal atomic arrangement of the surface, with an additional splitting of the diffraction spots due to the periodicity of the ordered arrays of steps (see Supporting Information). The LEED pattern of GR/Rh(533) (Figure 4(a)) shows a moirè periodicity, which is, however, different from the one observed on the 3-fold (111) termination. In the case of GR/ Rh(111), in fact, it is known that the modulus of the two vectors of the moirè cell is identical in both crystallographic directions and is in a 12:11 ratio with the substrate lattice parameter.34−36 It is important to notice that the LEED pattern of GR/ Rh(533) cannot be interpreted in terms of a single-scattering model. In the present case, in fact, the spot intensity changes with the diffraction order and with the electron energy in a strongly nonlinear way, with some missing or strongly suppressed spots. In addition, the diffraction maxima are found at positions given by the linear combination of a reciprocal lattice vector of the substrate with a lattice vector of the overlayer. This behavior is typical of multiple scattering mechanisms, which, actually, are not uncommon in GR-based interfaces and have been observed, e.g., for epitaxial GR on the reconstructed SiC(0001) surface.55 With respect to the case of GR on the 3-fold Rh(111) surface, the periodicity of the moirè cell of GR/Rh(533) is different in the crystallographic directions parallel and orthogonal to the steps. A thorough analysis of the LEED line profiles (Figure 4(b)) reveals that, while the stoichiometry in the direction parallel to the steps (left panel) is 1.07, very close to the 12:11 ratio found on flat Rh(111), the periodicity in the direction orthogonal to the steps (right panel) is different. On the basis of our LEED analysis, we propose a model moirè cell with lattice vectors perpendicular to the corresponding vectors in the reciprocal space and with the smallest periodicity that enables matching the GR layer with the substrate with limited distortions. The so-modeled moirè cell is shown in Figure 5. It has a dimension of 29.56 Å in the direction parallel to the steps, while the other vector, which extends over two terraces of the substrate, measures 19.37 Å and forms an angle of 65.5° with the first. To match the nearest integer number of Rh atoms underneath, the GR layer has been compressed by 0.9% in the direction parallel to the steps and stretched by 2.3% in the orthogonal direction. It is important to notice that, despite the very complex LEED pattern of the GR/Rh(533) system, we still observe a shift along k// of the diffracted intensity as a function of the electron kinetic energy (see waterfall plots in Figure 4(c), showing the LEED line profile of clean and GR-covered Rh(533) as a function of the electron energy). The modulation of the diffracted intensity upon changing the energy is very similar in both cases, which indicates that the structure of the steps is

The formation of GR was also confirmed by the analysis of the valence band (VB) spectra acquired during C2H4 CVD at T = 1020 K and hv = 100 eV in the normal emission configuration, corresponding to the Γ point in the reciprocal space (see Figure 3). Figure 3 gives an overview of the valence

Figure 3. (a) 2D waterfall plot showing the evolution of the VB spectrum during C2H4 CVD at 1020 K as a function of the exposure (vertical axis). (b) VB spectrum of the clean (black) and GR-covered (red) Rh(533) surface acquired at hv = 100 eV in the normal emission configuration (corresponding to the Γ point in k-space). (c) Integrated intensity of the carbon π-band signal associated with GR growth as a function of the exposure.

band modifications. Figure 3(b) shows a comparison of the high-energy resolution VB spectrum of rhodium before (black curve) and after (red curve) graphene growth, which clearly highlights the appearance of the π-band peak associated with GR formation. Finally, panel (c) reports the intensity evolution of the π-band signal as a function of the C2H4 exposure. Notice that, also in this case, GR nucleation starts only above ∼20 L exposure, with an initial delay which is, however, shorter compared to the induction time observed in the XPS experiments (Figure 1), due to the higher C2H4 partial pressure used at the beginning of the uptake. The signal associated with the bonding C p states is centered at about 7.8 eV below the Fermi level, in fairly good agreement with the theoretically predicted results for the analogous GR/Rh(111) system. Our data should be compared, in particular, with the calculated Catom projected density of states (DOS) in the valence band for the four high-symmetry configurations in the moirè cell of GR/ Rh(111). In their theoretical paper on the electronic structure of GR on Rh(111),54 Iannuzzi and Hutter found that the energetically most stable C atomic configuration in the case of GR/Rh(111) is the bridge one. The projected density of states on the C pz orbitals shows an evident splitting between occupied and unoccupied p states and a significant energy down-shift of the bonding orbitals, to about −7 eV relative to the Fermi Level. This phenomenon is caused by the orbital hybridization between the C p states and the metal d bands and by the ensuing charge transfer between GR and the substrate. Similar conclusions on the valence band structure of GR/ 6246



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Figure 4. (a) LEED pattern of GR/Rh(533) at Ek = 104 eV. (b) LEED line profile of GR/Rh(533) along the direction parallel (left) and orthogonal (right) to the steps, at Ek = 98 and Ek = 128 eV. (c) Waterfall plots of the electron energy-dependent LEED line profiles of the (left) clean and (right) GR-covered Rh(533) surface.

interacting graphene components. To reduce the number of degrees of freedom, the BE shifts of the two peaks were fixed to the values found for the high-resolution C 1s spectra acquired at low temperature. Although the absolute BE of the peaks was allowed to evolve as a function of the temperature, the thermally induced shift is negligible, as previously proved by accurate investigations of GR/transition metal interfaces.56 From the intensity evolution of the S and W components, we derived an activation temperature for the thermally induced C− C bond breaking of 1100 ± 10 K (Figure 6(a),(b)). Above this temperature, we observe a dramatic drop in the C 1s photoemission intensity curves, along with a Gaussian broadening of the spectral line shapes. Since C desorption from the surface is very unlikely, due to the high C−metal adsorption energy, we can reasonably conclude that the C atoms dissolve into the bulk. This statement is further supported by the fact that also the reverse process (i.e., C segregation from the bulk to the surface of the substrate) is observed upon cooling of the Rh(533) surface after saturating the bulk of the crystal with C. The process of C−C bond breaking and C diffusion into the subsurface region leaves some of the C atoms in the moirè layer with nonsaturated bonds and hence favors the formation of C− metal bonds with the substrate. The redistribution of the C atoms inside the moirè cell and the rearrangement of the C−C and C−metal bonds into new, nonequivalent local configurations produces a modification of the overall electronic structure of the system, leading to the overall broadening of the C 1s spectrum at the destruction temperature. Our observations for the GR/Rh(533) system should be compared with the findings of Dong et al.36 for the analogous GR/Rh(111) system. The authors observed that, at temperatures above 1053 K, GR becomes unstable on the surface and

Figure 5. Cannon ball atomic model of GR/Rh(533), as obtained on the basis of our LEED analysis. The Rh atoms at the steps are highlighted in blue, while the moirè cell is marked by a yellow line. In the inset (top left) is shown the calculated LEED pattern produced by multiple scattering mechanisms.

largely preserved. Nevertheless, on the sole basis of our data, we cannot exclude a partial GR-induced structural modification and reshaping of the substrate steps, as revealed by STM measurements carried out by Dong et al.36 on Rh(111). On the other hand, our LEED analysis provides compelling evidence that the steps are still present on the GR-covered Rh(533) surface and that the morphology of the clean substrate is not significantly altered. To further explore the differences between GR on Rh(533) and on the flat Rh(111) surface, we decided to investigate the thermal stability of the C network. Starting from the GRcovered sample, prepared by C2H4 CVD at 1020 K, the temperature was linearly increased from 470 to 1170 K at a rate of 1 K s−1, while the C 1s core level spectrum (hν = 400 eV) was monitored in real time by fast XPS. The time-lapsed sequence of C 1s core level spectra of GR/Rh(533), shown in Figure 6(a), was then analyzed using a two-peak fitting procedure to investigate the thermal behavior of the intensities and line shape parameters of the weakly (W) and strongly (S) 6247



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Figure 6. (a) 2D plot of the thermal evolution of the C 1s core level photoemission signal of GR/Rh(533) during annealing to 1100 K. (b) Corresponding C 1s intensity curve vs temperature. (c) C 1s intensity curves vs time during GR formation by C bulk-to-surface segregation at 948, 973, and 998 K, respectively. (d) Arrhenius plot of the data reported in (c).

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CONCLUSIONS Our findings ultimately evidence some clear differences between GR grown on Rh(533) and on the flat Rh(111) surface. In particular, the high-resolution C 1s spectrum of GR/ Rh(533) shows a smaller intensity ratio between the S and W components than in the case of GR/Rh(111) and a shift to lower BEs. This can be explained by considering the presence of the steps, which weaken the coupling between the C atoms and the metal substrate due to the increased average C−metal distance. The reduced bonding with the substrate can also explain the higher thermal stability of GR/Rh(533) with respect to what was previously found on Rh(111). What remains unexplained is the origin of the excellent quality of the GR layer, as judged from the sharp LEED spots induced by the moirè cell. Our tentative interpretation is based on the possible healing effect of the substrate steps on the GR layer defects. This mechanism has been first described in the theoretical work of Meng et al.,57 who found that the presence of substrate steps on a Ni surface is beneficial to the removal of lattice defects formed in the GR layer during hydrocarbon CVD. The authors showed, in particular, that GR growth is often accompanied by the formation of defects caused by a Ni atom being pulled out of a (111) terrace; however, although this mechanism is active on both flat and stepped surfaces, the defects are energetically more difficult to be healed on the terraces, while they can be more easily removed with the assistance of the step atoms on a vicinal surface. The steps, in fact, significantly lower the energy barrierand thus the activation temperaturefor defect healing. We can reasonably believe this healing mechanism is triggered during hightemperature CVD and helps remove the defects formed during the growth of the C network. Above a certain temperature, of course, this mechanism alone is not sufficient to counteract the breakup of the GR network, and the process rapidly completes above 1100 K. The very high quality of the GR layer grown on the vicinal Rh(533) surface can be interpreted as the result of the favorable combination of two factors: (i) the strong adhesion between GR and Rh surfaces in general, which forces the C layer into registry with the substrate and prevents the formation of multiple rotational domains (as instead observed in the case of

C dissolves into the bulk. The slightly higher activation temperature for C−C bond breaking and C bulk dissolution on Rh(533) can be interpreted as a consequence of the on-average weaker interaction of the C layer with the vicinal surface. As previously found for strongly interacting GR/substrate systems, in particular for GR/Re(0001),43 there is a direct correlation between the interaction of the C layer and its high-temperature thermal stability. In the cited work, it was found that the process of C−C bond breaking is more likely to start in the buckled, strongly interacting regions of the moirè cell, where the C atoms are closer to the substrate, although it requires the presence of diffusing GR layer monovacancies. By the same token, we can expect that the larger population of strongly interacting C atoms in the case of GR/Rh(111) will partly reduce the activation temperature for C−C bond breaking. Our picture of C bulk segregation/antisegregation is further supported by a separate experiment, aimed at estimating the activation energy for C−C bond formation on Rh(533). In this case, the sample was first exposed to a C2H4 flux at high temperature and then slowly cooled from 1020 down to 920 K in steps of 25 K (approximately 500 s each), while the intensity of the GR-elated C 1s signal was monitored as a function of time. The coverage curves vs time were subsequently fitted to an exponential to extract the temperature-dependent C bond formation rate (Figure 6(c)). We focused our analysis on three specific temperatures: 948, 973, and 998 K, and from an Arrhenius plot of the so-obtained decay time constants (vs 1/ kBT), we estimated an overall activation energy of 0.95 ± 0.15 eV (Figure 6(d)). This value actually corresponds to the highest energy barrier of the different elementary processes involved in the formation of the GR layer: carbon bulk to surface diffusion, lateral surface diffusion, new C−C bond formation. Interestingly, the calculated energy barrier (between 0.76 and 0.95 eV, depending on the specific pathway46) for the diffusion of a C atom from an interstitial octahedral site in the interlayer region to a step edge site on the Pd(211) surface turns out to be in excellent agreement with our experimentally estimated activation energy. 6248



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weakly interacting substrates, like Pt(111)58,59 or Ir(332)33) and (ii) the healing effect of the steps, a thermally activated mechanism which plays a key role during CVD and possibly hampers the formation of C layer defects during annealing. Our study therefore suggests that the presence of surface steps may be highly beneficial to the growth of GR layers with excellent, tunable structural properties and an improved thermal stability. In addition, the use of a versatile and experimentally reproducible method like CVD to grow epitaxial GR could be readily extended to a wide range of high Miller index surfaces, with different geometries and step periodicities. Our findings suggest the possibility of synthesizing high-quality, thermally stable GR layers with a moirè cell whose periodicity in the direction orthogonal to the steps can be tailored by selecting a TM vicinal surface with the desired morphology, while the periodicity in the parallel direction is kept fixed.

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ASSOCIATED CONTENT

S Supporting Information *

Additional structural data for the clean Rh(533) surface, including an atomic ball model and a LEED pattern of the substrate. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +39 040 3758719. Fax: +39 040 3758776. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.B. would like to thank Elettra-Sincrotrone Trieste for providing the access to Elettra and for the support received during the beamtime allocated at the SuperESCA beamline, the Physics Department, University of Trieste, for financial support for the course ‘Laboratorio di Fisica della Materia’, and the MIUR for financial support within the program PRIN20102011 for the project entitled ‘GRAF. Frontiers in Graphene Research: understanding and controlling Advanced Functionalities’.

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