Raman Spectroscopy of Lattice-Matched ... - ACS Publications

Raman Spectroscopy of Lattice-Matched Graphene on Strongly Interacting Metal Surfaces Dmitry Yu. Usachov,*,† Valery Yu. Davydov,‡,∞ Vladimir S. Levitskii,‡ Viktor O. Shevelev,† Dmitry Marchenko,¶ Boris V. Senkovskiy,†,§ Oleg Yu. Vilkov,† Artem G. Rybkin,† Lada V. Yashina,∥ Evgueni V. Chulkov,†,⊥ Irina Yu. Sklyadneva,⊥,#,@,△ Rolf Heid,@ Klaus-Peter Bohnen,@ Clemens Laubschat,▽ and Denis V. Vyalikh†,⊥,□ †

St. Petersburg State University, 7/9 Universitetskaya nab., St. Petersburg, 199034, Russia Ioffe Physical Technical Institute, St. Petersburg, 194021, Russia ∞ ITMO University, St. Petersburg, 197101, Russia ¶ Helmholtz-Zentrum Berlin für Materialien und Energie, 14109 Berlin, Germany § II Physikalisches Institut, Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany ∥ M.V. Lomonosov Moscow State University, Leninskie Gory 1/3, 199991 Moscow, Russia ⊥ Departamento de Fisica de Materiales and CFM-MPC UPV/EHU, Donostia International Physics Center (DIPC), 20080 San Sebastian, Spain # Tomsk State University, Lenina Avenue, 36, 634050 Tomsk, Russia @ Institut für Festkörperphysik, Karlsruher Institut für Technologie, D-76021 Karlsruhe, Germany △ Institute of Strength Physics and Materials Science, pr. Academicheskii 2/1, 634021, Tomsk, Russian Federation ▽ Institute of Solid State Physics, Dresden University of Technology, 01062 Dresden, Germany □ IKERBASQUE, Basque Foundation for Science, 48011 Bilbao, Spain ‡

S Supporting Information *

ABSTRACT: Regardless of the widely accepted opinion that there is no Raman signal from single-layer graphene when it is strongly bonded to a metal surface, we present Raman spectra of a graphene monolayer on Ni(111) and Co(0001) substrates. The high binding energy of carbon to these surfaces allows formation of lattice-matched (1 × 1) structures where graphene is significantly stretched. This is reflected in a record-breaking shift of the Raman G band by more than 100 cm−1 relative to the case of freestanding graphene. Using electron diffraction and photoemission spectroscopy, we explore the aforementioned systems together with polycrystalline graphene on Co and analyze possible intercalation of oxygen at ambient conditions. The results obtained are fully supported by Raman spectroscopy. Performing a theoretical investigation of the phonon dispersions of freestanding graphene and stretched graphene on the strongly interacting Co surface, we explain the main features of the Raman spectra. Our results create a reliable platform for application of Raman spectroscopy in diagnostics of chemisorbed graphene and related materials. KEYWORDS: graphene, metal, Raman spectroscopy, electronic structure, oxygen, intercalation

R

aman spectroscopy has become one of the most widely and intensively used tools for characterization of graphene and related materials.1 It has proved to be efficient for the determination of the number of layers in fewlayer-graphene films,2 mechanical strain,3−5 charge doping,6 © 2017 American Chemical Society

Received: April 18, 2017 Accepted: May 11, 2017 Published: May 11, 2017 6336

DOI: 10.1021/acsnano.7b02686 ACS Nano 2017, 11, 6336−6345

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Figure 1. XPS spectra of different graphene/metal systems before and after exposure to air. The insets near the bottom spectra show the LEED patterns of these systems before taking them out of the vacuum chamber.

properties of defects,7 and many other important properties.1,8 The versatility of this method is due to its high sensitivity to tiny changes in the electronic structure and phonon dispersions. The efficiency of Raman spectroscopy greatly depends on the nature of the substrate on which the graphene is supported. When graphene is placed on a thin SiO2 film with properly chosen thickness, the signal is multiplied many times due to the interference of light.9 Thus, SiO2 is one of the most widely used substrates for Raman studies of graphene. However, the possibilities of graphene fabrication directly on SiO2 are very limited. Usually, this process requires the presence of a metal catalyst, which should be removed after synthesis.10 Alternatively, graphene can be transferred from a metal substrate, where it is often grown with a chemical vapor deposition (CVD) method.11 CVD is one of the most popular methods for the synthesis of graphene-based systems. It is well known that the thickness and properties of CVD-grown graphene layers are very sensitive to the synthesis conditions. Thus, it is highly important to have efficient methods for graphene diagnostics on the substrates that are used for CVD. Usually, these are transition metal substrates with Ni and Cu being the most popular. However, the capabilities of Raman spectroscopy for studies of graphene on metals are not so broad as on SiO2. In a recent review of graphene−nickel interfaces12 the authors affirm that no Raman signal is observed for monolayer graphene on nickel. Consequently, determination of the graphene thickness and other characteristics on Ni with Raman spectroscopy13 is not straightforward. The reason is a strong interaction between graphene and metal, accompanied by hybridization of the C 2p and Ni 3d states. As a result, the remarkable feature of the graphene electronic structure known as a Dirac cone becomes essentially modified.14−17 It is generally believed that the absence of the Dirac cone near the Fermi level (EF) leads to loss of the resonant conditions for Raman scattering; therefore the Raman signal is suppressed.18 Similar suppression is observed for graphene on other strongly interacting surfaces, such as Ru(0001).19 In contrast to Ni and Ru, graphene only weakly interacts with Cu. The carbon binding energy to Cu is only 33 meV, and there is no notable hybridization between electronic states of C

and Cu. Thus, the Dirac cone remains almost unperturbed near EF.20 Consequently, the Raman spectra taken from graphene on copper21 are rather similar to those of freestanding graphene. Nevertheless, a certain influence of the Cu substrate on the spectral features can be clearly observed.21 In the intermediate case of graphene on the Ir(111) substrate the interaction is considered to be quite weak, despite the fact that the carbon binding energy reaches the value of 50 meV.22 For this system the Raman spectrum is very sensitive to the orientation of the graphene lattice relative to the substrate. The useful signal may even disappear at certain orientations.23 Thus, the previous studies of graphene/metal systems obviously indicate that Raman scattering is extremely sensitive to the peculiarities of the respective interface. At the same time, it is also clear that the relationship between the interface properties and the features of Raman spectra are currently not well understood. Appropriate objects for investigation of Raman scattering in graphene on strongly interacting surfaces are the well-known graphene/Ni(111) and graphene/Co(0001) systems. The lattice mismatch between graphene and the close-packed Ni and Co surfaces is in the range of 1−2%, and covalent bonding of carbon with metal allows formation of lattice-matched (1 × 1) structures due to the stretching of graphene.24−27 In this work, we demonstrate successful observation of the G band in the Raman spectra of the graphene/Ni and graphene/Co systems. Of course, the signal is much weaker in comparison to that of the classical graphene/SiO2 system, but it is obviously resolvable. We compare the lattice-matched systems with polycrystalline graphene on Co and analyze the possibilities for oxygen intercalation into the interface at ambient conditions. After comprehensive characterization with electron diffraction and photoelectron spectroscopy we demonstrate that Raman spectroscopy provides useful information that is fully consistent with other methods. Further, we explore theoretically the phonon dispersions of freestanding stretched graphene and graphene on Co. These results demonstrate that Raman spectroscopy remains a helpful method for characterization of graphene even on strongly interacting transition metal surfaces. 6337


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RESULTS AND DISCUSSION Photoemission Study. In most of the Raman studies, the investigated graphene-based system is kept at ambient conditions. Therefore, it becomes essential to explore first how the freshly prepared in ultrahigh vacuum (UHV) graphene/metal sample behaves when it is exposed to air. Following a well-known recipe,28 a single-layer latticematched graphene can be formed on the Ni(111) surface within the synthesis temperature range of ∼450−650 °C. Since the quality of graphene is improved at higher temperatures,28,29 we performed its synthesis at 620 °C. Strict orientation of graphene is unambiguously proven by the low-energy electron diffraction (LEED) pattern shown in the inset of Figure 1a. Immediately after synthesis in UHV the sample was controlled by X-ray photoemission spectroscopy (XPS) and the respective spectrum of C 1s core level exhibits a single peak labeled as C0. This is in full agreement with the previous studies of the graphene/Ni(111) system.28 Then, after exposure of the sample to air for a few hours, it was checked again by XPS. The survey spectra reveal the appearance of oxygen and extra carbon atoms, which belong to molecules adsorbed on the graphene surface. However, subsequent short-term annealing of the sample at a temperature of 300 °C leads to desorption of most of the carbonaceous contaminants and the C 1s line shape is almost completely restored. Thus, we can conclude that no significant changes occurred in the graphene/Ni(111) interface when the sample was transferred from UHV to the air and back. A rather similar behavior with minor differences was observed also for the graphene/Co(0001) sample. The lattice-matched graphene monolayer on the Co(0001) surface can be formed in a quite narrow range of temperatures close to 650 °C. Note that lower temperatures usually lead to the synthesis of polycrystalline graphene, which consists of misoriented domains. The LEED pattern of the lattice-matched graphene/Co system is shown in the inset of Figure 1b and demonstrates a perfectly oriented hexagonal lattice. The C 1s XPS spectrum taken from this system clearly shows two peaks, Ch and Ct. Recently, we have shown27 that such spectral shape reflects two carbon sublattices of graphene. One of them is located atop Co atoms, while the other one occupies the hollow sites. It should be noted that such a splitting of the C 1s spectrum is not observed for the related graphene/Ni(111) system probably because of coexistence of areas with different types of interface structure.25 The survey XPS spectrum taken from the graphene/Co(0001) sample exposed to air also suggests the appearance of extra oxygen and carbonaceous species on the surface. They can be almost completely removed by means of short annealing of the sample at 420 °C in UHV. After that, the shape of the C 1s XPS spectrum is restored. Thus, the graphene/Co interface also remains unaffected by the contact with air. However, it was found that the latter is true only for the lattice-matched interface, while for the polycrystalline graphene on cobalt the situation is rather different. Figure 1c shows the results of LEED and XPS measurements for such a polycrystalline graphene sample, which was grown on a Co(0001) substrate at a temperature of 620 °C. The LEED pattern demonstrates the misoriented graphene domains. The XPS spectrum shows a relatively broad peak C0, originating from multiple random locations of carbon atoms at the Co surface.27 After exposure to air a new peak C1 appears in the XPS spectrum besides a broad peak of carbonaceous adsorbates. Also the concentration of oxygen reaches a much

higher value than in the previous systems. Upon annealing in UHV at 340 °C, the carbonaceous contaminants are desorbed, while the intensity of the peak C1 is notably increased. Its strong chemical shift with respect to the peak C0 indicates that a significant part of the graphene layer became decoupled from the Co surface. The observed spectral changes can be readily explained with oxygen intercalation underneath graphene.30 Angle-resolved XPS measurements (not shown) prove that the oxygen atoms are preferentially located under graphene after annealing. The shape of the O 1s XPS spectrum (see Figure 1c) can be described with two components. We suppose that the narrow peak OA at lower binding energy can be assigned to stoichiometric (ordered) cobalt surface oxide, while the broad peak OB may correspond to a nonstoichiometric (disordered) surface oxide.31 Further annealing leads to rearrangement of oxygen at the metal surface. This is indicated by the increased intensity of the OA and C1 components. The phenomenon of oxygen intercalation was also observed for graphene on the Ir(111)32 and Ru(0001)33 surfaces at elevated temperatures. In our case this process occurs already at room temperature. The XPS data in Figure 1c suggest that oxygen intercalation is nonuniform and a notable part of the polycrystalline graphene layer remains tightly bonded to the Co substrate. In order to visualize the regions of intercalated oxygen, we have used photoemission electron microscopy (PEEM). Figure 2a−c show PEEM images of misoriented graphene on Co, recorded with the use of a Hg discharge lamp as an excitation source. In this case the contrast is caused mainly by the lateral variations of the work function. After synthesis in UHV the graphene/Co sample has a quite uniform work function, while after exposure to ambient environment the PEEM image reveals a strong contrast. The appearance of dark areas can be attributed to graphene intercalation with oxygen, which leads to a charge transfer from graphene to oxygen and causes an increase of the work function. The most striking proof of the oxygen intercalation comes from the angle-resolved photoemission spectroscopy (ARPES) data shown in Figure 2d−i. Before contact with air the π states of graphene are hybridized with the 3d states of Co. As a consequence, the Dirac cone of graphene at the K point is destroyed.26 The Dirac point becomes shifted away from EF and appears at the binding energy of 2.8 eV,16,26 as shown with dashed lines in Figure 2d. It should be noted that the two observed Dirac cones originate from the two most probable orientations of graphene domains. Further oxygen intercalation passivates the bonds of Co; therefore graphene becomes quasifreestanding. This is clearly confirmed by the appearance of Dirac cones at EF in Figure 2f. The Dirac point of intercalated graphene is shifted by ∼0.3 eV above EF. This observation confirms p-type doping of graphene accompanied by the work function increase detected with PEEM. The p-type doping of oxygen-intercalated graphene is in agreement with other experimental and theoretical studies.32−34 The π band at the Γ point (Figure 2g−i) also exhibits notable modifications. After exposure to air the parabolic π band is smeared over the binding energy range of 5−11 eV. This indicates an enhanced electron scattering as a result of structural inhomogeneities. Further annealing leads to a better decoupling of graphene from cobalt, and the parabolic shape of the π band is restored. Thus, the experimental results obtained by means of XPS, PEEM, and ARPES are self-consistent and allow constructing the complete picture of the formation of polycrystalline 6338


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comparable with the typical values obtained from graphene on SiC substrates. Such broadening is probably related to the tiny variations of strain at the nanoscale. For the graphene/Ni(111) system the width is notably larger. We attribute this to the inhomogeneities of the interface structure, observed with scanning tunneling microscopy.25 In contrast to the lattice-matched systems, the spectra of misoriented graphene/Co show strong variations upon scanning along the surface. Figure 3b demonstrates the changes of Raman spectra along a straight arbitrary path on the surface. It can be seen that at the region of 3−9 μm the spectra are similar to those of the weakly bonded graphene on Cu or SiO2. An example of such a spectrum is shown in the upper part of Figure 3a (spectrum no. 4). It contains G, D, and 2D bands and corresponds to decoupled graphene on the oxidized Co surface. At the region of 19−23 μm the Raman bands of graphene are strongly suppressed. This segment corresponds to spectrum no. 3 in Figure 3a. The suppression of signal does not imply the absence of graphene at the respective regions. We have checked and confirmed the integrity of graphene with Auger electron microscopy. Later on we will show that these regions correspond to the misoriented graphene domains with no significant amount of oxygen intercalated underneath. Another interesting feature is a strong variation of the positions of G and 2D bands. Usually, such variations are explained with mechanical strain and charge transfer.1,8 Figure 3c demonstrates certain correlation between the positions of main bands. The straight line shows graphene deformation without charge doping, while the cross point of the dashed lines corresponds to unstrained graphene. The fact that almost all experimental values fall above this line indicates the presence of charge transfer, which is known to increase the G-band frequency.6 This is in line with our ARPES data, which show pdoping of intercalated graphene. Notable scattering of the data points indicates that the doping is not uniform and also agrees with the ARPES results. The deviations of the 2D peak position from its value in unstrained graphene reaches 90 cm−1 toward the lower frequencies. This value is too large for doping and points to a notable stretching of graphene. The maximal stretching estimated from the 2D band position is about 0.56%. This value can be compared to the LEED data shown in Figure 3d, where one can see the reflexes of misoriented graphene and of the Co(0001) surface. From the intensity profile maxima we have determined the relative amount of rotated graphene domains (Figure 3e) and the relative changes of their lattice constant as a function of rotation angle (Figure 3f). Obviously, there is a difference of ∼1.2% between the lattice constants of rotated and latticematched graphene domains. Keeping in mind that the lattice mismatch between graphite and the Co(0001) surface is 1.8%, we can easily estimate that the stretching of rotated graphene domains directly after CVD synthesis on Co should reach 0.6%. This is fully consistent with the maximal stretching of 0.56% derived from the Raman data. Further inhomogeneous oxygen intercalation leads to partial graphene decoupling from cobalt. Some domains are well decoupled; therefore their lattices shrink to the unstrained state. Other domains are poorly decoupled and remain stretched. This explains the large spread of the 2D band position. In order to provide further support for our interpretation of Raman spectra, we have performed additional experiments aimed at decoupling of the lattice-matched graphene from strongly interacting Ni and Co surfaces. We have found that

Figure 2. (a−c) Photoemission electron microscopy (PEEM) images of misoriented graphene on the Co(0001) surface before and after exposure to air. Bright colors correspond to high photoemission intensity. (d−i) Respective angle-resolved photoemission spectroscopy (ARPES) maps aquired near the Γ and K points of the Co(0001) surface Brillouin zone. Dashed lines show Dirac cones of graphene domains.

graphene on Co(0001) and subsequent intercalation of atmospheric oxygen. Raman Study. Having the knowledge on how the latticematched and mismatched graphene on metals behave at ambient conditions, we turn now to the Raman study of the same set of samples. Figure 3a shows the respective set of Raman spectra. In the cases of lattice-matched systems with well-oriented graphene on the Ni(111) and Co(0001) surfaces the spectra (no. 1 and 2) were always the same independently from the place on the sample. A closer inspection of these spectra reveals several striking features: (i) the 2D band, which is expected at ∼2670 cm−1, is absent (therefore its region is not shown), (ii) the D line with the typical shift of ∼1340 cm−1 is not detected either, (iii) the G band is shifted by more than 100 cm−1 from its position in free graphene, shown with the dashed line at 1581 cm−1, and (iv) the intensity of spectral features is a few hundred times smaller than in the case of the graphene monolayer on SiO2 or ∼20 times less than on SiC. Due to the rather weak intensity of the graphene-related features, the peak of atmospheric oxygen is clearly visible at 1556 cm−1. Usually, the intensity of this peak is below the noise level in most of the graphene studies. The measured parameters of the graphene G band are summarized in Table 1. For the graphene/Co system the full width at half-maximum (fwhm) of the G band exceeds the usual values observed for the free graphene, but it is 6339


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Figure 3. (a) Raman spectra of graphene on the Ni(111) and Co(0001) surfaces acquired at ambient conditions using the laser wavelength of 532 nm and a power of 4 mW. Dashed lines indicate positions of D, G, and 2D bands of free graphene. (b) Raman map of misoriented graphene on the Co(0001) surface measured along a randomly selected straight path. (c) Correlation plot of the positions of G and 2D Raman peaks for misoriented graphene/Co. The linear dependence for strained graphene is adopted from ref 5. (d) LEED intensity for misoriented graphene/Co derived from the LEED image shown as the inset in Figure 1c; the line indicates the positions of intensity profile maxima. (e) Corresponding intensity of LEED reflexes as a function of crystal lattice rotation angle. (f) Relative change of the lattice period derived from the LEED data; the color intensity shows reliability of the obtained values.

Table 1. Measured Parameters of the Raman G Band for the Lattice-Matched Graphene/Metal Interfaces substrate

position, cm−1

fwhm, cm−1

asymmetry

Ni(111) Co(0001)

1478 ± 2 1452 ± 2

53 ± 6 21 ± 3

no yes

oxygen intercalation into the graphene/Co(0001) system is not homogeneous, while for reliable analysis it is always better to have homogeneous samples. This can be achieved by intercalation of different materials, and we have chosen gold and silicon. Graphene on the Ni(111) surface intercalated with Au is a well-studied subject. It is known that the electronic structure of graphene near EF in this system is similar to the Dirac cone of free graphene, but with a tiny amount of pdoping35 and Rashba-type splitting of π states.36 The typical Raman spectrum of the graphene/Au/Ni(111) system is shown in the lower part of Figure 4. The spectra acquired at different areas of the sample surface were almost identical. Negligible scatter of band frequencies and intensities demonstrated sample homogeneity at the scale of more than one micrometer. Before Au intercalation all spectra taken at different points of the surface were similar to spectrum no. 1 in Figure 3a and showed no 2D band. It can be seen that after Au intercalation the 2D band appeared in the Raman spectrum, while the G band was shifted close to its position in free graphene. These spectral transformations indicate the transition of graphene from strongly bonded to a quasi-freestanding state. Similar spectral changes are observed when Si atoms are intercalated into the graphene/Co(0001) system. The Raman spectrum transforms from spectrum no. 2 in Figure 3a to the upper spectrum in Figure 4 any place on the surface. This indicates that graphene is transformed to a quasi-freestanding state in agreement with previous XPS and ARPES studies.15,37 The appearance of an

Figure 4. Raman spectra of a quasi-freestanding single graphene layer produced by intercalation of Au and Si atoms under the lattice-matched graphene on the Ni(111) and Co(0001) surfaces, respectively. Dashed lines indicate positions of D, G, and 2D bands of free graphene.

intensive D band in the spectra may indicate that the intercalation process produces a lot of defects. The shift of the G band to its position in free graphene is caused by shrinking of the initially stretched graphene lattice. This should inevitably lead to formation of cracks in the graphene layer and may also produce structural inhomogeneities at the nanoscale. Significant decrease of the graphene quality is reflected in the large width of all Raman bands after intercalation. In order to gain further insight into the Raman data obtained from the lattice-matched systems, we have calculated the phonon dispersions. We have considered (i) freestanding 6340


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Figure 5. (a, b) Calculated phonon dispersions of free graphene with a lattice constant of 2.4600 Å (blue lines), strained graphene with a calculated lattice constant of Co(0001), namely, 2.4977 Å (red lines), and graphene on the Co(0001) surface (black dotted lines). (c) Simplified scheme of the electronic band structure for the lattice-matched graphene on the Co(0001) and Ni(111) surfaces.

strongly dependent on the graphene adsorption geometry. If the two carbon sublattices occupy symmetric positions with respect to the metal atoms of the topmost layer, the minicone is expected to be almost gapless.27,39 However, in the case of sublattice asymmetry (when one sublattice is positioned atop metal atoms and the other one occupies the hollow sites) a notable gap appears at EF. This is indeed the case for the latticematched graphene/Co system.27 The absence of carbon states at EF is the probable reason for the vanishing of the Kohn anomaly. Figure 5c also helps to explain the main features of the Raman spectra of lattice-matched systems. Let us consider the two processes that may lead to the appearance of the G band. These may be resonant or nonresonant processes.1 Although it was proposed that the resonant contribution vanishes in free graphene due to a special symmetry of the Hamiltonian,40 this may be not applicable to strongly bonded graphene. For the particular photon wavelength of 532 nm the resonant process may occur only if the laser excites an electron from the occupied metal majority spin states into the unoccupied majority graphene states (see Figures S1 and S2 of the Supporting Information). This is because there are no available occupied graphene states just below the minicone. Such excitation may be possible because the carbon states are mixed with metal states and partially penetrate into the substrate. However, in the particular case when the hole is created in the metal it will have a very short lifetime. Therefore, no further radiative decay with well-defined photon energy is expected. The nonresonant process may occur if the electron is excited from the minicone states just below EF into a virtual state. The created hole may live a sufficiently long time for a successful Raman process, because the minicone is located in the local band gap of the metal. This is supported with our ARPES data for the graphene/Co system, exhibiting a narrow line width of the quasi-particle spectral function of the minicone.27 Thus, we believe that formation of the G band is likely governed by a nonresonant process. The 2D band is known to appear due to a double-resonant intervalley scattering.1 Figure 5c shows an example of twophonon scattering in analogy to the most intensive doubleresonant Raman process, which takes place in freestanding graphene. As mentioned above, in our case of strongly bonded graphene the resonant excitation should involve creation of a very short-lived hole in the metal states. Additionally, such a

graphene, (ii) stretched but still freestanding graphene with a lattice constant of the Co(0001) surface, and (iii) graphene/ Co(0001) with a (1 × 1) interface structure. The results are shown in Figure 5. The stretching of free graphene affects mainly longitudinal (LO) and transverse optical (TO) phonon modes by shifting them to lower frequencies. Binding to cobalt strongly affects shear-vertical modes ZO and ZA. The ZO mode is downshifted by about 160 cm−1 all over the Brillouin zone (BZ) due to the hybridization of the metal 3d band with the graphene π states. The acoustic vibrations ZA, LA, and TA of graphene are mixed with vibrations of cobalt below 300 cm−1. As a consequence of the symmetry reduction of adsorbed graphene, the ZA/ZO degeneracy at the K point is lifted and a “gap” of 98 cm−1 is opened between these branches. These observations are very similar to the case of the graphene/ Ni(111) system, which is described in detail in ref 18; therefore, we do not provide a detailed description herein. It is readily apparent from Figure 5b that the presence of cobalt has a minor influence on the energy of LO (TO) phonons at the Γ point. Only a tiny downshift of ∼9 cm−1 relative to the stretched graphene is predicted. These particular phonons are related to the G band in the Raman spectra. Thus, the observed giant shift of the G band is related mainly to the change of the graphene lattice parameter. An interesting consequence of the graphene bonding to the Co(0001) surface is a suppression of the Kohn anomalies in the LO and TO phonon branches at the Γ and K points of the BZ. The Kohn anomaly at the K point is completely vanished, as highlighted with circles in Figure 5a,b. This was also predicted for the graphene/Ni(111) system.18 Vanishing of the Kohn anomaly indicates suppression of the electron−phonon interaction for electronic states near EF. The respective reason can be understood from the electronic band structure of the lattice-matched interfaces. The calculated bands are shown in Figures S1 and S2 of the Supporting Information; they agree with previous experimental and theoretical studies.16,17,26,38 The electronic structure is quite complicated; however, the main features of the calculated bands can be explained using the simplified scheme shown in Figure 5c. The strong mixing of graphene π states with 3d orbitals of the metal leads to splitting of the Dirac cone into two parts. One part is shifted below the 3d band, and its apex is located at the BE of 2.8 eV,16,26 while the other part (a so-called minicone) is located near EF.26 The minicone dispersion is 6341


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ACS Nano hole will not be localized at the surface. This makes a welldefined Raman scattering very unfavorable. For these reasons the 2D band is not observed in the Raman spectra of chemisorbed graphene. The D band appears in the spectrum of freestanding graphene due to a defect-induced double-resonant scattering.41 Since this band is related to defects, its absence indicates a high crystalline quality of graphene. However, in the case of graphene on Ni and Co the absence of the D band is not related to the structural perfection. Indeed, it can be explained by the same reasons as the absence of the 2D band. Let us now consider spectrum no. 3 in Figure 3a. It was recorded in the region of misoriented graphene on Co where no significant oxygen intercalation is observed, as evidenced by a very low intensity of the peak at 1590 cm−1, related to a G band of quasi-freestanding graphene. This spectrum demonstrates no D and 2D bands; however a well-defined G band is observed. It is shifted by 68 cm−1 relative to the G band of the lattice-matched graphene/Co system. We attribute this peak to the mismatched graphene/Co interface. The observed shift corresponds to the difference of 1.2% in the lattice periods of matched and mismatched graphene domains. This value is in perfect agreement with the LEED data shown in Figure 3f. It should be noted that the G-band intensity of the mismatched graphene is different for various regions on the surface, as shown in Figure S3 of the Supporting Information. Usually the intensity is smaller than in Figure 3a, while at some points it is hardly detectable. We attribute these regions with strongly suppressed G band to the graphene domains rotated through a large angle relative to the substrate lattice. Looking at Figure S4 of the Supporting Information one can see that there is a region of bulk Co states between the two K points in k-space. Thus, when graphene is rotated far away from its position in a (1 × 1) structure, the carbon states near EF should move from the local gap into the region of bulk metal states. This probably leads to stronger hybridization between carbon and metal states and even more suppresses the Raman scattering. In Figure 6 the positions of the G band are compared with the frequencies of LO/TO phonons at the Γ point obtained from density functional theory (DFT) calculations, highresolution electron energy loss spectroscopy (HREELS), and Raman results related to the frequencies of LO/TO phonons with zero momentum in graphene on different strongly and weakly interacting substrates. The solid straight line in Figure 6 is derived from the Raman study of stretched graphene.5 It perfectly fits to the results of our DFT calculations, shown with black crosses. Our Raman results for the graphene/Ni(111) system are in a good agreement with frequencies calculated in ref 18; however, there is a notable discrepancy with the HREELS data presented in refs 42 and 43. An explanation of such difference requires further combined Raman and HREELS studies. The G band of the lattice-matched graphene/Co(0001) system is shifted by nearly 130 cm−1 from its position in the free graphene. This is probably the largest G-band shift ever observed for any graphene-based system. For both graphene/ Ni(111) and graphene/Co(0001) interfaces the Raman data fall below the line of stretched graphene. This indicates valuable influence of the carbon−metal bonds on the G-band position. This influence is probably underestimated in our DFT calculation of the graphene/Co interface. Relatively large shifts of LO/TO phonon frequencies have been observed also for graphene on the (111) surfaces of transition metal carbides.47,49 For these systems the LEED

Figure 6. Calculated and measured frequencies of graphene LO and TO phonons at the Γ point. The HREELS data for graphene on the surfaces of Ni,42,43 Cu,44 Pt,45 NbC,45 ZrC,46 TaC,47 TiC,47 and HfC47 are taken from refs 42−47. Typical error bars for the HREELS data are shown only by the example of TiC(111) for simplicity. The Raman data for stretched graphene,5 graphene on Pt(111),48 and graphene on Cu21 are taken from refs 5, 21, and 48. The DFT results for graphene/Ni(111) are adopted from ref 18.

images revealed notable graphene stretching up to 3 ± 1% in the case of graphene/TaC(111). Unfortunately, no Raman studies were performed for these interfaces with strongly bonded graphene. In the case of weakly bonded graphene on Cu and Pt the Raman data are in good agreement with HREELS.

CONCLUSIONS In summary, we have demonstrated that Raman spectroscopy remains a powerful tool for the studies of graphene even when it strongly interacts with transition metal surfaces. Applying photoemission and Raman spectroscopy to single-layer graphene on Ni(111) and Co(0001) surfaces, we show that the properties of these systems essentially depend on the conditions of CVD synthesis. In particular, graphene may form either a lattice-matched (1 × 1) interface or a polycrystalline layer. In the case of the lattice-matched structure the Raman G band exhibits a giant shift toward lower frequencies due to significant stretching of the graphene lattice. The hybridization between graphene and metal electronic states leads to the loss of conditions for efficient double-resonant Raman scattering. Consequently, the D and 2D bands are absent in the Raman spectra. Using photoemission spectroscopy and microscopy, it was shown that the lattice-matched systems remain stable at ambient conditions; however in the case of polycrystalline graphene oxygen intercalation under graphene takes place. It results in partial decoupling of graphene from the metal surface. The Raman spectra taken from the places of intercalated oxygen look qualitatively similar to those of freestanding graphene. The areas of misoriented graphene without intercalated oxygen demonstrate a rather weak G band with intensity depending on the lattice orientation. Finally, we would like to emphasize that the results obtained with photoemission and Raman spectroscopies are fully self-consistent. We believe that this example will initiate further Raman spectroscopy studies of such nontrivial objects as graphene on strongly interacting metallic surfaces. 6342


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the MB scheme is described in ref 53. Phonons were calculated on a 9 × 9 × 1 grid (q vectors) and then interpolated in reciprocal space to calculate phonon dispersions.

METHODS Experiment. Single-layer graphene samples were synthesized under UHV conditions by CVD on crystalline Ni(111) and Co(0001) films with a thickness of ∼10 nm, deposited on a clean W(110) surface. The base pressure in the UHV chamber was 2 × 10−10 mbar. The LEED patterns of the metal films always showed a sharp (1 × 1) hexagonal pattern, indicating their good crystallinity. The graphene synthesis was carried out as follows: the substrate was heated to the synthesis temperature; then propylene (C3H6) with a pressure of 10−6 mbar was introduced into the UHV chamber for 15 min. Under these conditions graphene growth starts immediately on the hot metal surface, and the reaction is self-limited to a single layer. After the monolayer formation the growth is stopped as the catalytically active metal surface is passivated with graphene.28 By using scanning Auger spectroscopy with a resolution of ∼30 nm we have confirmed the thickness uniformity of graphene and proved the absence of bilayer or multilayer regions in the samples. Gold intercalation under graphene on the Ni(111) surface was performed by deposition of 3 Å of Au and annealing at a temperature of 500 °C. Decoupling of graphene from Ni was confirmed by XPS and ARPES spectra, which were in agreement with previous studies.35,36 Silicon intercalation into the graphene/Co(0001) system was achieved by deposition of 5 Å of Si followed by annealing at 550 °C. This procedure was repeated several times until homogeneous decoupling of graphene from Co was reached. This was evidenced by a single C 1s peak in the XPS spectrum and a Dirac cone in the ARPES spectra, in agreement with the published data.15,37 The XPS measurements were carried out at the Russian−German beamline (RGBL) of the BESSY II synchrotron radiation facility (HZB Berlin). The PEEM measurements were conducted using an Omicron FOCUS IS-PEEM microscope equipped with an imaging energy filter at the RGBL-2 beamline of BESSY II. The Hg discharge lamp was used as an excitation source (4.9 eV). The ARPES measurements were performed using He II radiation (40.8 eV) at the Resource Center “Physical Methods of Surface Investigation” (RC PMSI) of the Research Park of Saint Petersburg State University. The Raman spectra were measured at room temperature using a LabRam HR 800 spectrometer equipped with a confocal microscope, a liquid-nitrogencooled CCD, and a 600 gr/mm grating (blazed at 500 nm). A frequency-doubled Nd:YAG laser operating at a wavelength of 532 nm was used as an excitation source. The scattered light was collected in the backscattering geometry, and the spectral resolution was 4 cm−1. The spectra were measured with the spot size on the sample of ∼1 μm for a 100× (NA = 0.9) objective lens. A typical acquisition time was 10 min with a power at the sample equal to 4.0 mW. Calculations. The calculations were carried out within the density functional formalism and the generalized gradient approximation using a mixed-basis pseudopotential approach. The electron−ion interaction was represented by norm-conserving pseudopotentials in the form proposed by Vanderbilt.50 Wave functions were expanded in a mixed basis (MB) of local functions and plane waves with a kinetic energy cutoff of 36 Ry. The Fourier expansion of the crystal potential and charge density was truncated at 50 Ry. We used supercell geometry with five layers of hexagonal cobalt terminated on both sides by a graphene layer to avoid a dipole formation. A vacuum of 8 Å separates the slabs to avoid an artificial interaction between neighboring slabs. Lateral atomic positions are fixed at the optimized lattice parameter of Co, a = 2.4977 Å, so that the C−C bonds of the graphene layer are stretched by 1.5%. Forces between the atomic layers were minimized to obtain the relaxed positions. The geometry optimization yields a C−Co bond length of 2.09 Å and a small inward buckling of 0.015 Å of the C atoms on the hollow sites. Integrations over the surface Brillouin zone were performed by sampling a uniform 18 × 18 × 1 k-point mesh corresponding to 37 special points in the irreducible BZ combined with a Gaussian broadening with a smearing parameter of 0.1 eV. All results were checked for convergence with respect to both the number of special points and the plane-wave energy cutoff. The dynamical matrices were calculated with linear response technique.51,52 Its implementation in

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b02686. Additional results of the Raman and DFT studies (PDF) Calculated phonon dispersions of free graphene (TXT) Calculated phonon dispersions of graphene on Co(0001) (TXT)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Dmitry Yu. Usachov: 0000-0003-0390-0007 Valery Yu. Davydov: 0000-0002-5255-9530 Vladimir S. Levitskii: 0000-0002-7877-1329 Viktor O. Shevelev: 0000-0002-0065-0631 Dmitry Marchenko: 0000-0003-1496-4161 Boris V. Senkovskiy: 0000-0003-1443-6780 Oleg Yu. Vilkov: 0000-0002-8984-8790 Artem G. Rybkin: 0000-0002-8237-4959 Lada V. Yashina: 0000-0002-8370-9140 Irina Yu. Sklyadneva: 0000-0002-4651-8281 Rolf Heid: 0000-0002-2144-1417 Denis V. Vyalikh: 0000-0001-9053-7511 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS L.V.Ya., D.Yu.U., O.Yu.V., and V.O.S. acknowledge RSF (Grant No. 16-42-01093). E.V.Ch. and I.Yu.S. acknowledge support by the Spanish Ministry of Science and Innovation (Grant No. FIS2016-75862-P). C.L. acknowledges DFG (Grant Nos. LA655-17/1 and LA655-19/1). E.V.Ch., D.Yu.U., D.V.V., V.O.S., and B.V.S. acknowledge Saint Petersburg State University (SPbU) for research Grant Nos. 11.65.42.2017 and 15.61.202.2015 and RFBR (Grant No. 17-02-00427). We thank Helmholtz-Zentrum Berlin für Materialien und Energie for support within the bilateral Russian−German Laboratory program. REFERENCES (1) Ferrari, A. C.; Basko, D. M. Raman Spectroscopy as a Versatile Tool for Studying the Properties of Graphene. Nat. Nanotechnol. 2013, 8, 235−246. (2) Li, X.-L.; Han, W.-P.; Wu, J.-B.; Qiao, X.-F.; Zhang, J.; Tan, P.-H. Layer-Number Dependent Optical Properties of 2D Materials and Their Application for Thickness Determination. Adv. Funct. Mater. 2017, 1604468, 1−23. (3) Bousige, C.; Balima, F.; Machon, D.; Pinheiro, G. S.; Torres-Dias, A.; Nicolle, J.; Kalita, D.; Bendiab, N.; Marty, L.; Bouchiat, V.; Montagnac, G.; Souza Filho, A. G.; Poncharal, P.; San-Miguel, A. Biaxial Strain Transfer in Supported Graphene. Nano Lett. 2017, 17, 21−27. (4) Androulidakis, C.; Koukaras, J.; Emmanuel, N.; Parthenios; Kalosakas, G.; Papagelis, K.; Galiotis, C. Graphene Flakes under Controlled Biaxial Deformation. Sci. Rep. 2016, 5, 18219. 6343


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