LETTER pubs.acs.org/JPCL
Graphene on Ni(111): Coexistence of Different Surface Structures Wei Zhao,† Sergey M. Kozlov,‡ Oliver H€ofert,† Karin Gotterbarm,† Michael P. A. Lorenz,† Francesc Vi~nes,*,‡ Christian Papp,*,† Andreas G€orling,‡,§ and Hans-Peter Steinr€uck†,§,^ †
Lehrstuhl f€ur Physikalische Chemie II, ‡Lehrstuhl f€ur Theoretische Chemie, §Interdisciplinary Center for Interface Controlled Processes, and ^Erlangen Catalysis Resource Center (ECRC), Universit€at Erlangen-N€urnberg, Egerlandstrasse 3, 91058 Erlangen, Germany
bS Supporting Information ABSTRACT: A combined high-resolution X-ray photoelectron spectroscopy (HR-XPS) and ab initio density functional theory study on graphene on Ni(111) shows the coexistence of two structures, a bridge-top and a top-fcc structure, that have almost identical energies according to DFT calculations. Consequently, both geometries are detected simultaneously on the Ni(111) surface by HR-XPS, while their relative fractions depend on minor surface defect concentrations (pinning sites). The two structures are identified due to their different core level shifts that are in line with DFT calculations. SECTION: Surfaces, Interfaces, Catalysis
contradictory results coming both from experiments11,26-30 and from density functional theory (DFT) calculations employing various approaches.31,32 All of the calculations agree in predicting that an almost planar carbon sheet is formed on Ni(111). This surface is a particularly interesting substrate because its surface lattice constant (249 pm) almost perfectly fits that of graphene (246 pm). Thus, a graphene layer, containing two carbon atoms per unit cell, could attach in perfect registry with the substrate; this behavior has indeed been proposed from the observed (1 1) LEED pattern.14 The position of the two carbon atoms in the unit cell, that is, their “adsorption site” relative to the Ni substrate atoms, is, however, a matter of debate. Six different structures, depicted in Figure 1, can be considered. Early experimental results11,26-30 discuss mainly the hcpfcc and the top-fcc geometries (see Figure 1e and a, respectively); the latter was favored later on by DFT calculations within the local density approximation (LDA).32 These DFT calculations, however, also point to a second possible low-energy conformation, namely, the bridge-top geometry (Figure 1b).32 It is noteworthy that in this DFT study, van der Waals (vdW) interactions that are likely to be responsible for the binding of graphene to Ni were not taken into account. In fact, LDA calculations resulted in bound situations due to the well-known overbinding by the LDA exchange-correlation functional. Note that calculations carried out using the PerdewBurke-Ernzerhof (PBE) exchange-correlation functional without taking into account vdW interactions resulted mostly in unbound situations, and in discrepancy with LDA calculations, only one stable conformation, namely, hcp-fcc, was found. Interestingly, the
G
raphene is one of the most promising candidates for future carbon-based electronics. Its outstanding properties include a remarkable electrical conductivity due to high carrier mobility,1 a half integer quantum hall effect, and also interesting magnetic properties.2,3 Possible applications range from gas sensors to ballistic electronic devices, and one might envisage even its use as a hightemperature superconductor.4-6 This high potential is due to the special electronic structure of free-standing graphene, which shows a linear dispersion at the Dirac point, indicating the existence of relativistic quasiparticles.7 However, the interaction of graphene with adsorbates or with a substrate, in the case of supported graphene, can shift and modify its electronic bands and thereby change its physical and chemical properties.8,9 For supported graphene, the structure on a particular substrate and the respective interactions are thus of fundamental interest. The growth of graphene on metal surfaces is especially interesting as it could be a low-energy route for its production; the growth temperatures are significantly lower compared to, for example, SiC,10 and nevertheless high-quality, large-area graphene sheets can be formed. Consequently, in recent years, a lot of effort has been dedicated to the study of graphene grown on various metal surfaces.11-16 On single-crystal surfaces, not only the electronic structure is of interest,17 but also the chemical modification of graphene sheets and their growth mechanism are the focus of research efforts.18-23 In addition, the growth of graphene on polycrystalline thin-film substrates has been investigated. Nickel has been studied as an easily accessible substrate for the production of graphene, which can be removed after the growth process, leading to free-standing graphene.24,25 The growth behavior and the structure of graphene on a Ni(111) single-crystal surface are presently heavily debated, with still r 2011 American Chemical Society
Received: January 10, 2011 Accepted: March 1, 2011 Published: March 14, 2011 759
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Figure 1. Different adsorption geometries for graphene on Ni(111): (a) top-fcc, (b) bridge-top, (c) top-hcp, (d) bridge-hcp, (e) hcp-fcc, and (f) bridgefcc. The geometries evolve from each other by a shift of the graphene layer against the Ni(111) surface in a direction along a carbon-carbon bond.
Figure 2. Potential energy surface for the structures of Figure 1, solid line. The distance of graphene to the topmost Ni layer is also shown, dashed line.
published atomic-resolved STM results11,26,27 show discrepancies with respect to the symmetry (two-fold versus three-fold) of the observed features, which have not been understood in detail yet. Furthermore, an X-ray photoelectron spectroscopy (XPS) study on the growth of graphene on a Ni film on a W(110) crystal was recently conducted; however, the local geometry was not addressed in detail.14 In this Letter, we studied the structure of a graphene layer on Ni(111) with a combination of in situ high-resolution XPS (HRXPS) and ab initio DFT calculations with dispersion correction33 to take into account vdW interactions. Our results indicate that graphene on Ni(111) has two energetically favored geometries, namely, the top-fcc and the bridge-top structures displayed in Figure 1a and b; the corresponding layers grow with an almost stochastic distribution, leading to a coexistence of these two adsorption modes on the surface. We start out with our results from the DFT calculations employing the PBE functional plus dispersion correction to take into account vdW interactions. A cut of the potential energy surface along a coordinate that describes the movement of the graphene sheet relative to the surface along one of the C-C bond directions is shown in Figure 2 (solid line). It contains the adsorption geometries of Figure 1. The most stable conformation of graphene on the Ni(111) surface is the bridge-top adsorption mode, which already was found to be energetically favorable in earlier calculations within the LDA approximation.32 The top-fcc geometry shows a very
similar energy, again in line with the earlier LDA results.32 However, it is important to mention that the present calculations result in a bonding of the graphene layer only due to the dispersion, that is, vdW interactions, the pure PBE surface-graphene interaction is repulsive. The energy difference between bridge-top and top-fcc lies within the accuracy of DFT calculations, especially in view of the large contributions from vdW forces. In this sense, both conformations are energetically equivalent and equally accessible according to the DFT calculations for graphene on Ni(111). Note here that the other structures were found not be minima of the potential energy surface; they spontaneously evolved into either bridge-top or top-fcc when no constraints were imposed. Last but not least, it is worth highlighting that, in principle, the bonding energy per C atom to the Ni(111) surface is small (about 12-13 kJ mol-1) and thus can be interpreted as a physisorption situation mainly due to vdW interactions. However, although these interactions are weak, they add up along the graphene sheet, resulting in a strong total interaction of a strength approaching that of chemisorption. In addition, estimates of carbon core level shifts have been obtained for all of the structures depicted in Figure 1, as shown in Table 1. The values are given relative to a free-standing graphene layer. In both the bridge-top and the top-fcc geometries, a shift to larger binding energies is observed. For the bridge-top geometry, almost identical shifts of -0.40 and -0.36 eV are predicted for the two carbon atoms labeled C1 and C2; for the top-fcc 760
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Table 1. Calculated Core Level Shifts (CLS) of the Carbon Atoms in the (1 1) Unit Cell for the Two Different Adsorption Geometries of Graphene on Ni(111) with Respect to Free-Standing Graphene and Differential Core Level Shifts with a Comparison from Theory and Experiment DFT calculations
a
experimental measurements a
differential shifts relative to bridge-top
differential shifts relative to bridge top
CLS (C1) in eV
CLS (C2) in eV
bridge-top
-0.40
-0.36
0.00/0.00
0.00/0.00
top-fcc
-0.63
-0.19
-0.25/þ0.19
-0.42/þ0.16
top-hcp
þ0.30
þ0.31
þ0.68/þ0.69
-/-
bridge-fcc
-0.20
-0.56
þ0.18/-0.26
-/-
bridge-hcp hcp-fcc
-1.46 þ0.32
-1.46 þ0.31
-1.08/-1.08 þ0.70/þ0.71
-/-/-
The bridge-top shifts from calculations are set to an average value of -0.38 eV.
differences in the local geometrical arrangements of the carbon atoms.34,35 Although this study focuses on the identification of the structure of graphene by means of XPS, we also calculated STM images for the two possible structures of graphene on the Ni(111) surface. The calculated STM images clearly show three-fold symmetry for the top-fcc structure, as discussed in ref 11, and two-fold symmetry for the bridge-top structure, as seen in refs 26 and 27; see the Supporting Information for the calculated STM images and further details. Experimentally, the growth and structure of graphene on Ni(111) was investigated by in situ HR-XPS. In Figure 3a, the XP spectra taken during the growth of a graphene layer are shown. The Ni(111) crystal was exposed to a pressure of 1 10-6 mbar of propene at a temperature of 900 K until saturation of the C 1s intensity was observed. The spectra consist of one main peak at 285.0 eV and a minor contribution due to carbidic carbon at 283.6 eV. This observation is in good agreement with the previous study by Gr€uneis et al.14 of graphene on a Ni layer on W(110), where comparable peaks were observed during growth at 900 K (note that in the latter study, no XPS measurements at low temperatures were presented). In the inset of Figure 3a the total carbon intensity is shown, which reaches a maximum corresponding to two carbon atoms ((10%) per Ni atom (i.e., 2.0 ( 0.2 ML) after 1200 s; this coverage corresponds exactly to one monolayer of graphene. The calibration √ √ was done by comparison to a saturated well-ordered ( 7 7)R19.1 benzene layer with a carbon coverage of 0.84 ML.34 For comparative purposes with previous studies, especially angle-resolved UV photoelectron spectroscopy of the valence bands,11,36,37 the graphene layer was cooled to temperatures as low as 150 K. During the cooling process, XP spectra were continuously recorded (see Figure 3b); in Figure 3c, the same data are shown in a color-coded density plot to visualize the temperature-induced changes. Upon cooling, a second spectral feature evolved at lower binding energies, ∼284.46 ( 0.05 eV, while the main peak shifted to higher binding energies by ∼0.05 eV. The total carbon intensity remained unchanged in this experiment to within less than (1%. Interestingly, yet surprisingly at first sight, several preparations under identical experimental conditions yielded differing results. Typical examples of the corresponding XP spectra obtained at 200 K are shown in Figure 4. In the topmost spectrum, the line shape is dominated by one peak (dashed, green line), whereas the bottom spectrum is dominated by two separated peaks (solid, blue lines) of identical intensity. A large number of preparation procedures always led to spectral shapes, which are composed of
Figure 3. (a) In situ XP spectra of the C 1s region during the growth of graphene at 900 K; the inset shows the carbon coverage as function of time. (b) Temperature evolution of the C 1s region during cooling. (c) Color-coded density plot of (b). (hν = 380 eV; all spectra were taken at 45 off normal emission.)
geometry, shifts of -0.63 (fcc hollow site) and -0.19 eV (on top site) are obtained, reflecting the strongly differing chemical surrounding; thus, for this geometry, two well-separated peaks are expected in the XP spectra. Note that the bridge-fcc structure exhibits core level shifts similar to those of the top-fcc structure due to its proximity to the Ni(111) surface (see the graphene-nickel distance in Figure 2). The bridge-fcc structure, however, has not been considered in the further discussion because it represents no stationary point of the potential energy surface. The reliability of chemical shifts, in particular, those of carbon, when treating core electrons in a projector augmented wave approach is limited. However, here, only relative values of carbon atoms in comparable chemical environments are considered. Moreover, the calculated relative chemical shifts are plausible because they reflect the 761
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Table 2. Frequencies (ν, in cm-1) and Mean Atomic Displacements (in pm) of Γ-Point Vibrations for Graphene Adsorbed in Bridge-Top Adsorption Mode in a p(1 1) Supercell of Ni(111)a C1 vibration name
ν
C2
Ni
0 200 800 0 200 800 0 200 800
C-C stretch
1533 2
2
2
2
2
2 0
0
0
C-C stretch
1530 2
2
2
2
2
2 0
0
0
693 3
3
4
3
3
4 0
0
0
C-Ni asymmetric out of plane 249 4
C-C out of plane
5
8
4
5
9 2
3
5
C-Ni symmetric out of plane
141 5
7 13
5
7 13 3
5 10
C-Ni assymmetric in plane
139 5
8 15
5
8 15 3
4
C-Ni assymmetric in plane C-Ni in plane symmetric
81 4 8 16 54 7 17 33
C-Ni in plane symmetric
47 11 26 53 11 26 53 4
8
4 8 16 5 10 20 7 17 33 6 13 25 9 17
a
Vibrations of the carbon atoms C1 and C2 and of Ni atoms of the first layer are considered. Estimates of mean atomic displacements for temperatures of 0, 200, and 800 K are given.
Figure 4. C 1s spectra of different graphene layers on Ni(111) (hν = 380 eV; TS = 200 K; spectra were taken at 45 off normal emission) and the two most stable structures according to DFT calculation. Shown are preparations with (a) mainly graphene in bridge-top geometry, (b) a mixture of graphene in bridge-top and top-fcc geometries, and (c) mainly graphene in top-fcc geometry.
the second peak at 284.46 evolves again. This observation allows one to rule out segregation effects, which typically are not reversible upon heating and are not likely to occur at such low temperatures. This conclusion is also strongly supported by the constant coverage during cooling and heating cycles (see inset in Figure 3b). The observed changes with temperature can be explained as follows. The calculated potential energy surface profile is relatively flat in the region between top-fcc and bridge-top, as can be seen in Figure 2. This leads to lateral vibrations with large amplitudes at high temperatures (see estimates at the Γ point in Table 2), which imply an average lateral displacement of C atoms well beyond the saddle point connecting the conversion of top-fcc to bridge-top situations. In summary, in the experiment, an average of many local adsorption sites is monitored. Note that, in principle, also the different thermal expansion coefficients for nickel and graphene38 might lead to a loss of registry to the substrate and, consequently, to an averaging of the different positions of the carbon atoms relative to the Ni surface atoms at higher temperatures; this would lead to a broadened peak that has no distinct contributions in the C 1s region at high temperatures. A similar situation was discussed for graphene on Ir(111), where a Moire pattern is observed in STM.39 However, calculations accounting for the Ni thermal expansion yielded the same qualitative results and relative energies of the different graphene adhesion conformations. We also considered other possible reasons for the appearance of two peaks and the changes in the line shape upon heating: The minor amount of carbidic carbon (peak at 283.6) in the layers varied slightly but showed no correlation with the observed structures. From this observation and the growth temperature of 900 K, the growth mechanism of graphene from either a nickel carbide phase, which was reported for T < 730 K,40 and/or carbon dissolved in the Ni bulk cannot be determined; nevertheless, it might have an influence on the different structures formed. In addition, we found evidence in our calculations that the coexistence of “carbidic” subsurface carbon does not influence the relative stability of the graphene structures discussed. Moreover, possible surface contaminations are ruled out from XP spectra, and the formation of bilayer graphene is excluded from the coverage calibration. Finally, the formation of defects in the graphene sheet or segregation of carbon
a combination of these single-peak and double-peak spectra. These results can be understood in light of the above-discussed DFT calculations for graphene on Ni(111). They yield two energetically equivalent structures, the top-fcc structure (Figure 1a) and the bridge-top structure (Figure 1b). According to the CLS shifts collected in Table 1, we expect that the former geometry leads to two C 1s peaks with identical intensity and the latter to only one C 1s peak (the predicted minor difference of 0.04 eV in the core level shifts of the latter structure cannot be resolved in the experiment). It is worth mentioning that the estimated separation between the two spectral peaks of graphene in the top-fcc geometry is 0.44 eV, which is in good agreement with the value of 0.58 ( 0.05 eV found in the experiment; furthermore, the peak position from the bridge-top geometry, 284.88 ( 0.05 eV, lies between the two peaks from the top-fcc geometry, again in line with the calculations. In Figure 4, three different cases that were found are shown; in (a), the bridge-top geometry is the dominating species, in (b), the two geometries coexist, and in (c), the top-fcc geometry dominates the spectrum. The analysis of 20 independent preparations measured at 200 K yielded 40% bridge-top geometries and 60% top-fcc geometries on average, showing no clear preference for one of the structures. With the energies for the two structures being nearly degenerate, both geometries coexist on the surface, with the relative fractions probably depending on minor defect concentrations (pinning sites) on the surface that, at present, cannot be identified experimentally. One interesting aspect is the observation that the change of the shape of the spectra upon cooling (i.e., the evolution of the low binding energy feature) is completely reversible upon heating (not shown), that is, when increasing the temperature again to temperatures above 750 K, the spectral shape is the same as that observed during the growth of graphene, that is, it shows one main peak located at 285.04 eV. Upon cooling to low temperatures, again, the same spectral shape is found as that before the heating step, that is, 762
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from the bulk would be inconsistent with the reversibility of the spectral shape and the constant coverage upon heating and cooling. In conclusion, here, we report on two coexisting structures of graphene on a Ni(111) surface, namely the bridge-top geometry with the carbon atoms adsorbed equivalently off-center on top of surface Ni atoms and the top-fcc geometry with one carbon atom adsorbed at the on-top position and the other on the hollow-fcc site. A combination of HR-XPS and ab initio DFT calculations was employed to elucidate this controversially discussed topic. In the XP spectra, a core level shift of the different carbon atoms was observed, which was reproduced by DFT calculations, enabling an assignment of the spectral features. The two graphene structures show very similar energies according to DFT calculations. As a consequence, both geometries coexist on the surface, with the relative fractions probably depending on minor defect concentrations (pinning sites) on the surface.
’ AUTHOR INFORMATION
’ EXPERIMENTAL AND COMPUTATIONAL DETAILS The XPS experiments were performed in a transportable UHV apparatus, equipped with an electron energy analyzer (Omicron EA 125), using synchrotron radiation from beamline U 49/2 at BESSY II, Berlin. The combined resolution in the C 1s region was ∼200 meV, at a photon energy of 380 eV. The typical measuring time for one XP spectrum was 10 s. Using a threestage supersonic molecular beam, a local pressure of up to ∼1 10-6 mbar of propylene was achieved on the sample. For a more detailed description of the chamber, see ref 41. The DFT calculations were carried out with the program package VASP42 and employed the PBE exchange-correlation functional. An empirical vdW correction33 was used in the geometry optimizations with the parameters optimized for graphite. The adsorption of graphene on a Ni(111) surface was investigated employing a small p(1 1) supercell containing one layer with two C atoms and a sixlayer Ni slab per supercell. The three top Ni layers were fully relaxed during the optimizations. The three bottom layers were kept fixed at the bulk geometry with a lattice parameter of 248.9 pm that corresponds to the PBE bulk-optimized Ni-Ni distance.43 A vacuum layer of more than 1 nm was added between the slabs. Calculations were carried out in a spin-polarized fashion without any constraints on the magnetization. Interactions of valence electrons with the atomic cores were described by the projector augmented wave method.44 A kinetic energy cutoff of 415 eV for the plane-wave basis set was employed throughout, guaranteeing convergence of energies to more than 0.01 kJ mol-1. Geometry optimizations were performed using a conjugate gradient algorithm until forces acting on each atom became less than 0.01 eV Å-1. For the construction of the potential energy surface in Figure 2, points were calculated by fixing the lateral position of the C atoms while letting the vertical distance from the Ni(111) plane relax. An optimized MonkhorstPack k-points grid of 17 17 1 points has been used in all of the calculations. This guarantees convergence of the energy to values within 0.1 kJ mol-1, as checked by calculations using denser grids. Core level shifts have been calculated following a procedure described in the past in the final state approximation.45,46
’ REFERENCES
Corresponding Author
*E-mail:
[email protected] [email protected] (F.V.).
(C.P.);
’ ACKNOWLEDGMENT The authors gratefully acknowledge the funding of the BMBF through Grant 05 ES3XBA15 and the German Research Council (DFG), which, within the framework of its “Excellence Initiative”, supports the Cluster of Excellence “Engineering of Advanced Materials” (www.eam.uni-erlangen.de) at the University of ErlangenNuremberg. F.V. thanks the Alexander von Humboldt Foundation for financing his postdoctoral grant. We thank BESSY staff for their support during beamtime.
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’ ASSOCIATED CONTENT
bS
Supporting Information. Calculated STM images and further details. This material is available free of charge via the Internet at http://pubs.acs.org. 763
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The Journal of Physical Chemistry Letters
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