Anomalous moiré pattern of graphene investigated byscanning tunneling microscopy: Evidence of graphenegrowth on oxidized Cu(111) Nicolas Reckinger, Eloise Van Hooijdonk, Frédéric Joucken, Anastasia V. Tyurnina, Stéphane Lucas, and
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Nano Research DOI 10.1007/s12274-013-0382-y
———————————— Address correspondence to Jean-François Colomer, [email protected]
Nicolas Reckinger, Eloise Van Hooijdonk, Frédéric
Joucken, Anastasia V. Tyurnina, Stéphane Lucas,
Jean-François Colomer*
University of Namur, Belgium
Page Numbers. The font is
ArialMT 16 (automatically
inserted by the publisher)
Anomalous moiré patterns composed of well-defined linear periodic
modulations have been observed on graphene grown on (111) copper.
The explanation is that graphene grows on the oxygen-induced
reconstructed copper surface and not on the oriented (111) copper, as
ABSTRACT The growth of graphene on oriented (111) copper films has been achieved by atmospheric pressure chemical vapor deposition. The structural properties of as-produced graphene have been investigated by scanning tunneling microscopy. Anomalous moiré superstructures composed of well-defined linear periodic modulations have been observed. We report here on comprehensive and detailed studies of these particular moiré patterns present in the graphene topography revealing that, in certain conditions, the growth can occur on the oxygen-induced reconstructed copper surface and not directly on the oriented (111) copper film, as expected. KEYWORDS Graphene – scanning tunneling microscopy – Cu(111) – copper oxidation – atmospheric pressure chemical vapor deposition
1. Introduction The growth of graphene by chemical vapor deposition (CVD) using single-crystal copper has been first reported very recently to meet the challenge of growing graphene of higher quality
than the one obtained from polycrystalline copper (Cu) surfaces (foils or thin films) [1]. Due to the low solubility of carbon atoms in Cu, it is considered to be an excellent substrate for making graphene [2]. Moreover, Cu single crystals offer two distinctive advantages providing a more appropriate template
Nano Res DOI (automatically inserted by the publisher) Research Article
for graphene growth: the same hexagonal crystalline symmetry as graphene and a small lattice mismatch. Another important point is that metal thin films of Cu [3], and also other suitable metals for graphene growth such as Co [4], Ni [5], etc. can be easily grown heteroepitaxially on single-crystal sapphire (c-plane -Al2O3) substrates or on oriented (111) MgO substrates [6]. Focusing on oriented Cu, the previously reported results indicate that high-quality graphene can be synthesized [1]. Another study compares the growth of graphene on Cu(111) and Cu(100). The authors show that the Cu(111) films give graphene with a single orientation consistent with the underlying Cu lattice while graphene grown on Cu(100) exhibits a multidomain structure [6]. The same group also evidenced that the single orientation of the graphene film was favored if the CVD synthesis was performed at a higher temperature (1000 °C) [7]. Moreover, twin boundaries were observed in the Cu films (sputter-deposited at room temperature) and are naturally detrimental to the production of single-orientation graphene sheets [7]. The occurrence of twin boundaries can be avoided by using a higher temperature during the sputtering (500 °C), leading to Cu films with a single orientation on the sapphire substrates [8]. Graphene has been extensively characterized by the scanning tunneling microscopy (STM) technique. Moiré patterns, due to a mismatch between two periodic lattices, have been observed with STM in different graphitic systems. In particular, such superstructures arising from a misorientation between the graphene layers of multilayer films have been extensively studied on the carbon face of SiC [9-11] or on highly ordered pyrolytic graphite (HOPG) [12-13]. On metallic substrates, moiré patterns caused by the misalignment between the substrate and the graphene layer have also been investigated [14]. In this work, we present STM images of as-grown
graphene synthesized by atmospheric pressure chemical vapor deposition (APCVD) on oriented (111) Cu films at elevated temperatures. Large scale images display hexagonal moiré superstructures with different periodicities, indicating different orientations of the graphene grains with respect to the underlying substrate lattice. In certain growth conditions, anomalous moiré patterns were observed with well-defined linear periodic modulations. The alternating bands have a 2.3 nm spatial modulation. These anomalous moiré superstructures will be analyzed and explained, revealing that graphene has grown on the oxygen-induced reconstructed copper surface and not directly on the oriented (111) Cu film. 2. Experimental 2.1. Graphene synthesis. 700-nm-thick Cu films were deposited onto c-plane -Al2O3 substrates by DC magnetron sputtering with a power of 150 W in Ar atmosphere (2 mTorr) at 500 °C. For CVD, the Cu/-Al2O3 sample was placed on a quartz boat and then introduced into a horizontal quartz reactor at room temperature. The reactor can be inserted/extracted into/from the furnace’s hot zone rapidly. After sealing the reactor, ultrapure Ar (99.9999% purity) was flowed for 15 min under atmospheric pressure. Next, the quartz reactor was inserted into the furnace (kept at 700 °C), and 100 sccm of H2 (99.9% purity) were added, the temperature was raised to 1050 °C, and the copper piece was annealed in these conditions for 1 h. Then, two conditions of graphene synthesis were used: (i) graphene was grown by admitting 0.2 sccm of CH4 (99.5% purity) for 15 min (noted sample 1); the cooling was rapidly performed under a mixture of Ar/H2 (the tube is extracted manually outside the furnace); (ii) graphene was grown by admitting 2 sccm of C2H4 (99.5% purity) for 5 min (noted sample 2); the cooling was rapidly performed under Ar (the tube was extracted manually outside the furnace). The main differences between the two conditions
were the use of different hydrocarbons sources (CH4 and C2H4) and the cooling conditions with the addition of H2 in the Ar flow in the first case. Note that, in order to be complete, in the case of sample 1, a supplementary precaution was taken to etch the superficial copper oxide (of the Cu films), that is always present due to the storage conditions in ambient air, by acetic acid (99.5% purity) at 35 °C for 10 min [15] just before the synthesis. 2.2. Characterization. The STM images were acquired with a VP2 microscope from Park Instruments operating in ultrahigh vacuum (UHV, P < 2.0 10-10 mbar) at room temperature with electrochemically etched tungsten tips; this UHV system is equipped with a spectaLEED and an Auger spectrometer from Omicron. The crystallinity and crystallographic orientation of Cu films were analyzed by X-ray diffraction (XRD) (Siemens D5000). The hard-sphere atomic model for each crystal lattice used in this manuscript is detailed in Fig. S-1 in the Electronic Supplementary Material (ESM).
3. Results and discussion
After the deposition, the Cu films have been characterized by several techniques in order to unambiguously determine the crystallographic orientation of the Cu films. The physical properties of Cu film play indeed an essential role in the catalytic growth of graphene as reported in the introduction. The XRD pattern (Fig. 1a) recorded from the as-deposited Cu film shows a single sharp diffraction peak at 2 = 43.5°, related to the (111) direction. No other peak can be observed, demonstrating that only the (111) orientation of Cu is present. The XRD analysis is confirmed by the low energy electron diffraction (LEED) pattern (Fig. 1b) recorded at 100 eV whose 6-fold symmetry corresponds to the hexagonal lattice symmetry of Cu(111). In summary, the epitaxial relationship of Cu(111) with the c-plane sapphire is well confirmed by XRD and LEED measurements. Graphene grown
on these films by APCVD under different conditions described in the previous experimental section has been studied at the nanometer scale by STM.
Figure 1 (a) XRD and (b) LEED patterns recorded at E = 100 eV
of the Cu (111)/c-plane sapphire catalyst as deposited.
In Fig. 2a, the moiré pattern of sample 1 appears as a hexagonal superlattice (lattice constant 2.6 nm) which results both from the misalignment of the hexagonal lattices of the graphene layer and the Cu(111) substrate and from the mismatch of their lattice constants (2.46 Å and 2.56 Å, respectively) [16]. An atomic resolution image of the graphene sheet is displayed in Fig. 2b, revealing the honeycomb structure of the carbon atoms’ arrangement.
Figure 2 STM images of sample 1 showing (a) the hexagonal
moiré pattern due to the interactions of graphene with the
underlying oriented (111) Cu film and (b) the corresponding
honeycomb structure of graphene.
The LEED pattern (Fig. 3) gives supplementary information about the synthesized graphene.
Indeed, the presence of a diffuse ring indicates single-layer graphene domains’ rotational disorder or/and multi-layer graphene with each layer independently stacked (although some orientations are apparently preferred). The six more intense spots slightly closer to the center are related to the hexagonal lattice symmetry of Cu(111), similar to the LEED pattern of the Cu films before graphene synthesis (Fig. 1). Raman spectroscopy performed on the two samples (not shown) confirms the conclusions drawn previously from LEED. The samples consist of a patchwork of mono-, bi-, or few-layer graphene. The presence of this patchwork corroborates the observations of diffuse ring in the LEED pattern. The comparison between the two spectra recorded for a graphene bilayer for both samples shows only the presence of higher degree of defects in the sample 2, as testified by the much higher D peak and the appearance of the D’ peak (Fig. S-2).
Figure 3 LEED pattern of sample 1 recorded at E = 75 eV.
The experimental moiré patterns can be accounted for from hard-sphere atomic model of the two superposed lattices by varying their relative angle of rotation (Fig. 4). The resulting perception of the atomic arrangement consists in a hexagonal pattern of spots. The higher the angle of rotation between lattices, the smaller the spot diameter and the periodicity of the hexagonal pattern, until it is no longer visible for angles greater than 30° (Fig. 4d).
The best agreement between the presented experimental picture and the hard-sphere atomic model occurs for an angle of 6° (Fig. 4b). The moiré patterns can also be observed for other angles of lattice mismatch as exemplified for the angle of 0° (Fig. 4c). Note that, as is the case for the LEED pattern (Fig. 3), the moiré observed with STM can also be explained if we consider several graphene layers rotationally disordered (Fig. S-3). Considering now sample 2, STM images exhibit an anomalous moiré superstructure. Indeed, in this case, the observed moiré superstructure is composed of well-defined linear periodic modulation with a wavelength of 2.3 nm (Fig. 5).
Figure 4 (a) Experimental STM image with the moiré
superstructure of sample 1. (b-d) Hard-sphere atomic model:
superposition of the graphene lattice on a Cu(111) surface for
various angles between the a1 vector of the graphene unit cell
and the a1 vector of the Cu(111) unit cell. Given the periodicity
of the graphene layer and the Cu(111) substrate, the pattern
obtained for a certain angle θ is repeated for the angles θ + n
60°, n being an integer.
The LEED pattern of sample 2 (not shown) presents the same characteristics as those of sample 1 with diffuse rings related to the produced graphene (although some orientations are apparently preferred similarly to sample 1). What are the assumptions that may be made to interpret this
peculiar moiré pattern? The first possible explanation is that the Cu surface is not oriented along the (111) direction but locally along the (100) direction. Along this direction, it is known that a moiré superstructure appears with a linear periodic modulation having a period depending on the graphene orientations with respect to the underlying Cu(100) lattice [17]. The linear modulations in this case are due to the square lattice symmetry of the Cu(100) surface. However, experimentally, the previous LEED analysis has shown a 6-fold symmetry corresponding to the hexagonal Cu (111) surface, and not a 4-fold symmetry, as expected in the hypothesis of a cubic (100) Cu surface. In addition, the maximum period of the linear moiré superstructure in the case of graphene on Cu(100) is 1.6 nm (Fig. S-4), well below the 2. 3nm observed in our case. As a consequence, the moiré fringes we observe can certainly not be explained by a growth on the Cu(100) surface.
Figure 5 STM images of sample 2 showing (a) the linear moiré
pattern, and (b) the corresponding honeycomb structure of
graphene.
Another possible explanation is the presence of a copper oxide layer on top of the Cu(111) film due to the experimental conditions. Indeed, it was already reported that oxidizing impurities are inevitably present in the reactor’s atmosphere for the APCVD [18]. Moreover, as a reminder, sample 2 was grown on a Cu film where the superficial copper oxide layer was not removed by acidic pretreatment. Analyzing in details all of the experimental data, paramount information has been recorded by
Auger electron spectroscopy (AES) on the graphene/Cu films (Fig. 6). Comparing the AES spectra recorded for both samples 1 and 2, exhibiting respectively the “normal” hexagonal and the “anomalous” moiré superstructures, the main difference found is the presence of oxygen on the surface of sample 2. Indeed, in the Auger electron spectrum of sample 2, the KLL peak of oxygen is clearly visible whereas in the spectrum of sample 1, no oxygen peak is visible. There is no apparent relative shift of the carbon KLL Auger peaks, for the two spectra, the kinetic energy value for both peaks is around 262 eV. By checking the Cu LMM region to get more information on the oxidation state of Cu for each sample, a small shift in the kinetic energy of copper L3M4,5M4,5 Auger electron is observed.
Figure 6 Comparison of the Auger electron spectra of sample 1
and sample 2 with the C KLL and the Cu LMM Auger peaks in
insets.
Indeed, the kinetic energy value of the Cu LMM peak for sample 1 is higher than for sample 2. This shift can be attributed to the presence of oxidized Cu on the surface of Cu (111) for sample 2 [19-20]. In effect, the AES technique allows distinguishing metallic Cu from the oxide, where the values of the Cu LMM Auger transition kinetic energy decrease from metallic copper to CuO2 via CuO [19-20]. To find an explanation for the anomalous moiré pattern recorded for sample 2, hard-sphere atomic models have been constructed for the Cu2O(111)
(Fig. S-5) and Cu2O(100) (Fig. S-6) lattices. They confirm that, with an underlying lattice having a hexagonal symmetry, it is impossible to observe linear periodic modulations in the moiré patterns but only hexagonal moiré superstructures, as in the case of Cu(111). In other words, this means that if Cu2O(111) – a lattice with a hexagonal symmetry – lies on top of Cu(111), its presence cannot explain our experimental observations. On the contrary, using Cu2O(100) in hard-sphere atomic models, linear moiré superstructures can be observed due to the cubic symmetry of this underlying lattice. Moreover, the spatial modulation values are in good agreement with the observed experimental value of 2.3 nm for different rotation angles of the graphene lattice with respect to the Cu2O(100) lattice (Fig. S-6). In conclusion, the presence of Cu2O(100) on the top of Cu(111) seems to be a good explanation of observed anomalous moiré superstructures. However, two caveats are to be made about this conclusion. The LEED pattern symmetry does not correspond to the cubic symmetry of Cu2O(100). Moreover, the oxidation of copper in the (111) orientation has been well documented for the last twenty years [21-27] and the first evidence reported from these studies is the following: the thin film of bulk cuprous oxide grown on a Cu(111) film has been identified to be Cu2O(111) and not Cu2O(100) [21]. Accordingly, the experimentally observed anomalous moiré pattern cannot be explained by the presence of an underlying Cu2O(100) lattice. A thorough investigation of the literature about the oxidation of Cu(111) gives us a possible explanation. The reconstruction of the Cu(111) surface induced by the chemisorption of oxygen has been reported [21-27] with a penetration of the oxygen into the topmost Cu(111) surface layers. The induced superstructures have been identified by STM [22-23]. Two oxygen-induced reconstructions of Cu(111) have been distinguished (in Wood’s
notation): (i) the 73 5.8 21 10.9R R lattice
structure – also named ‘44’-structure because its unit cell is 44 times larger than the 11 surface unit
cell – and (ii) the 13 46.1 7 21.8R R lattice
structure – also named ‘29’-structure for a similar reason [21-22]. The formation of these two structures is still discussed in the literature, depending on the experimental conditions [25-27]. Some authors reported the following cycle: formation of a ‘44’-structure at 300 °C and at low pressure of O2; its conversion into a ‘29’-structure via an annealing treatment at 400 °C; and the ‘44’-structure restoring via another annealing treatment at 500 °C in vacuum [25]. In parallel, other authors claimed the formation of a ‘29’-structure at 300 °C under O2 exposure [26]; contradicted by others who reported the formation of a ‘44’-structure in these same conditions [27]. From the current literature, we can argue that the identified rearrangement of the Cu(111) surface is due to the mobility of the atoms at elevated temperature, leading to the formation of a well-ordered ‘oxide-related’ superstructure. In our CVD conditions, elevated temperatures are obviously used and the presence of oxygen in the CVD reactor has been reported [18]. Based on these considerations, a hard-sphere atomic model has been constructed for the ‘44’-structure of the oxidized Cu(111) surface (Fig. S-1). When different rotation angles between the graphene lattice and the underlying ‘44’-structure lattice are taken into consideration, moiré superstructures can appear for certain angles around 50°, presenting the expected linear periodic spatial modulations of 2.3 nm, in good agreement with the experimentally observed moiré pattern (Fig. 7). Other superposition angles of the graphene lattice on the surface of a ‘44’-structure have been explored, but the best fit corresponds to the angles close to 50° (Fig. S-7).
Figure 7 (a) Experimental STM image with the moiré
superstructure of sample 2. (b) Hard-sphere atomic model:
superposition of the graphene lattice on the surface of a
‘44’-structure for an angle of 50° between the a1 vector of the
graphene unit cell and the a1 vector of the ‘44’-structure unit
cell.
In the model, the relative orientation of graphene on the ‘44’-structure surface that gives the most visible lines on the moiré pattern makes an angle of around 30° with those lines. This fact is in agreement with the experimental observations. Can this surprising result be corroborated by our experimental data? Looking back at the LEED analysis (Fig. 1b), we can see that only the six spots related to Cu(111) with the 6-fold symmetry show up. The spots of the oxidized surface can be visualized for the same range of energy but the observation conditions must be done in order to decrease the intensity of the central beam and therefore increase the contrast close to the central spot (the larger dimensions of the ‘44’-structure give spots at closer distance to the central beam in the LEED pattern). Complementary experimental results were obtained by studying the oxidation of Cu(111) under the synthesis conditions of sample 2. We recorded a second experimental LEED pattern where the observed spots are arranged in a relatively complex pattern, resembling a six-pointed star (Fig. 8d). In addition, at the border of the pattern, we can identify six sharp and more intense spots from the Cu(111) surface. To decrypt this observed complex pattern, we compared it to data reported by Matsumoto et al. [25], explaining in details such structures (Fig. 8). In their paper, the
authors calculated the LEED pattern of a ‘44’-structure for six equivalent domains with the structure represented by
73 5.8 21 10.9R R in Wood’s notation (Fig.
8a). This pattern presents more spots compared to our experimental picture (Fig. 8d). Another point is that the experimental pattern reported by the author in their work [25] does not seem to correspond to our recorded pattern (not shown). Nevertheless, a detailed two-steps analysis of our image allows extracting some similarities with the calculated LEED pattern (Fig. 8a). The first step consists in selecting particular spots in the calculated pattern. In the experimental image, the intensity of the spots related to the oxide layer is weak in comparison to that of the Cu(111) spots; and some spots related to the oxide layer cannot be perceived. This is certainly due to the small thickness of the oxide layer. Hence, we extracted from the calculated pattern the spots that, due to the vicinity of other spots, could be merged into a unique spot in accordance with the experimental pattern; and we remove the spots occurring alone and apart from each other. Fig. 8b is the resulting image, where a six-pointed star pattern appears. The second step consists in grouping isolated – but very close – spots into a single point, Fig. 8c being the resulting image. The comparison of Fig. 8c (the “simplified” calculated pattern) with Fig. 8d (the experimental image) shows a very nice agreement. In conclusion, the experimentally obtained pattern is explained when taking into account the presence of a ‘44’-structure. A clarification is needed to account for the difference between the six-fold symmetry of the LEED pattern and the two-fold symmetry of the STM image. Indeed, the detailed analysis of the experimental LEED pattern is based on the calculated pattern [25] for the ‘44’-structure for six equivalent domains varying by rotation and reflection, in direct relation with the Cu(111) 6-fold symmetry. In the experimental STM image, only
one domain is scanned because of the scale of the analysis, a few tens of nanometers whereas the electron spot size in the LEED experiment is about 1 mm², probing a large number of domains with different orientations. This means that the LEED pattern is an average of all of the 6 possible orientations the '44'-structure can take, displaying a six-fold symmetry [25]. This is the reason why the experimental pattern matches very well the calculated one but that the symmetry of the pattern appears different from those of the STM image. This fact is confirmed by other STM images where the moiré patterns are rotated by 60° (not shown).
Figure 8 (a) Calculated LEED pattern for the ‘44’-structure
(from [25]); (b-c) modified calculated LEED pattern to evidence
the experimental pattern shown in (d) (E = 51.1 eV).
The other questions that now come to mind are the following: (1) where does the oxygen come from and (2) when does the oxidation occur? Two reasonable explanations to the first question are possible: the occurrence of oxidizing impurities in the reactor’s atmosphere [18] and the presence of a superficial copper oxide layer (not removed by acidic pretreatment) on the top of copper film are inevitable. To provide an answer to the second question, i.e., to know when the copper film’s surface is oxidized, we have to examine the synthesis conditions (cf. the experimental section). One of the differences between the two samples is
the hydrocarbon source: methane for sample 1 and ethylene for sample 2, which certainly does not play any role in the oxidation of the Cu surface. The second difference is the use of hydrogen during the cooling of sample 1, and not for sample 2. In this context, it could be argued that, during the fast cooling without protective hydrogen, graphene could be very likely oxidized and therefore, could not preserve the underlying Cu. In a previous study [18], we have indeed evidenced the presence of residual oxidizing impurities in the furnace’s atmosphere. But we have also shown that no oxidation of graphene occurs during fast cooling, as used in the present study, even if no hydrogen is flowed during this step. The conclusion is that the oxidation of the surface must probably have occurred before or during the growth of graphene. Still, the presence of hydrogen, all along the annealing treatment and the graphene growth, should ensure the complete reduction of the oxidized copper surface and should prevent its oxidation by the oxidizing contaminants. In consequence, based on these arguments, a plausible hypothesis is that the superficial copper oxide is not totally reduced by the hydrogen annealing at high temperature without acidic pretreatment. The pretreatment of the Cu films by acetic acid to etch the copper oxide seems to be essential to ensure that the oxidized layer is removed from the top of the copper surface or more drastic conditions during the annealing step under hydrogen must be used. The presence of superficial oxide is apparently not an obstacle to the growth of graphene, contrary to the admitted prescription that the Cu surface must be purely metallic. Indeed, copper oxide reduces the catalytic activity of Cu, as mentioned in the literature [28] but appears not to prevent the formation of graphene as shown here (certainly due to the small thickness of the oxide superficial layer).
4. Conclusions
In this work, we explained the anomalous moiré
pattern observed in certain growth conditions of graphene on Cu(111) films. The considered moiré pattern consists of well-defined linear periodic modulation with a 2.3 nm spatial modulation. These observed anomalous moiré superstructures have been analyzed and interpret showing that graphene has grown on copper oxide (‘44’-structure) and not on the oriented (111) Cu film as expected. It is the first evidence of the graphene growth on the oxidized copper surface. Acknowledgements
This work is supported by the Belgian Fund for Scientific Research (FRS-FNRS) under FRFC contract “Chemographene” (convention N°2.4577.11). E. Van Hooijdonk and J.-F. Colomer are supported by the Belgian Fund for Scientific Research (FRS-FNRS) as Postdoctoral Researcher and Research Associate, respectively. The authors acknowledge C. N. Santos and B. Hackens for their help with the Raman measurements. Electronic Supplementary Material: Supplementary material (details of the construction of hard-sphere atomic model of each crystallographic structure, Raman spectra of samples and superposition of graphene and Cu surface models to image the moiré superstructures) is available in the online version of this article at http://dx.doi.org/10.1007/s12274-***-****-* (automatically inserted by the publisher). References [1] Reddy, K. M.; Gledhill, A. D.; Chen, C-H.; Drexler, J. M.;
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