Intercalation from the Depths: Growth of a Metastable Chromium

Article pubs.acs.org/JPCC

Intercalation from the Depths: Growth of a Metastable Chromium Carbide between Epitaxial Graphene and Ni(111) by Carbon Segregation from the Bulk A. Picone,* D. Giannotti, M. Finazzi, L. Duò, F. Ciccacci, and A. Brambilla Dipartimento di Fisica, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italy

ABSTRACT: The intercalation of atoms or small molecules underneath graphene epitaxially grown on single metal surfaces is a widely exploited method for modifying the interaction between the carbon monolayer and the substrate. Despite it would be highly desirable to expand the class of the intercalants by including also metal compounds such as oxides, nitrides, or carbides, their use as decoupling layers is a much more challenging task. Here, we demonstrate that it is possible to intercalate an ultrathin layer of Cr-carbide at the graphene/Ni(111) interface by using the carbon dissolved in the bulk of the substrate as a reservoir. Auger electron spectroscopy reveals that Cr deposition on the graphene/Ni(111) interface triggers C segregation from the Ni bulk, while the graphene layer floats on top of the growing film. Scanning tunneling microscopy shows the presence of a periodic superstructure on the surface, due to a coincidence lattice at the graphene/carbide boundary or alternatively to a dislocation network developing at the carbide/Ni(111) interface. Scanning tunneling spectra normalized to the total conductance indicate that the density of states around the Fermi level depends linearly on the energy, suggesting that the graphene layer is electronically decoupled from the Cr-carbide film.

■

INTRODUCTION Graphene, a two-dimensional layer of C atoms arranged in a honeycomb lattice, can be epitaxially stabilized on many single crystal metal surfaces.1,2 During the past decade, graphene layers of exquisite quality have been grown by means of chemical vapor deposition on Ru(0001),3−5 Cu(111),6,7 Pt(111),8,9 Ir(111),10,11 and Ni(111)12,13 surfaces. The investigation of heteroepitaxial graphene deposited on metals is important in order to obtain large area and defect-free graphene sheets14,15 suitable for industrial production. Moreover, graphene interfaced with ferromagnetic layers has been proposed as an efficient spin filter.16 Due to the strong interaction between graphene and the metallic substrate, the electronic properties of the former often strongly deviate from those of a freestanding layer. In order to obtain graphene films accommodated on more inert and technologically relevant substrates, the honeycomb layer can be transferred from the metal to a new support by a number of different techniques, ranging from mechanical exfoliation to polymer-assisted transfer, as recently reviewed by Kang et al.17 However, such a strategy is not always the most efficient one, because a considerable number of defects could be induced in graphene by the transfer process. An alternative approach to © XXXX American Chemical Society

obtain a quasi-freestanding graphitic C layer is to intercalate a third element between the graphene sheet and the metallic substrate. Metallic films,18,19 atomic species,20−23 or organic molecules24 have been successfully intercalated at the graphene/substrate interface, restoring or tailoring the intrinsic electronic properties of the carbon monolayer.25,26 In this framework, much less effort has been devoted to the intercalation of metallic compounds, such as for instance oxides, nitrates or carbides. Such a lack of investigation is mainly due to the experimental drawbacks encountered when one tries to synthesize a complex material underneath a graphitic layer. As a matter of fact, graphene forms a highly impermeable membrane, which hinders the formation of epitaxial and stoichiometric multicomponent systems when the surface of a graphene-covered metal is exposed to a gaseous environment.27 For example, a high partial pressure of oxygen, dosed at high temperature, is required to oxidize a metal covered by graphene, as recently shown in the case of FeO28 and NiO29 intercalated between graphene and either Pt(111) Received: April 26, 2017 Revised: June 29, 2017 Published: July 14, 2017 A

DOI: 10.1021/acs.jpcc.7b03940 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C or Ni(111), respectively. Alternatively, the oxygen intercalation can be assisted by ceria nanoparticles predeposited on the surface, as demonstrated for the graphene/Ru(0001) system.30 A different approach to obtain a binary compound underneath the honeycomb carbon layer could be to exploit elements dissolved in the bulk of the substrate. In this paper we demonstrate that the deposition of Cr onto a graphene-covered Ni(111) single crystal leads to the stabilization of an intercalated crystalline Cr carbide film. Upon Cr deposition, the graphene layer floats on top of the surface, while the Cr film acts as a sink for the C dissolved in the Ni bulk. Thanks to their high melting point and hardness at high temperatures, Cr carbides31 are classified as refractory materials and are widely exploited as coating layers in metallurgy. Moreover, due to their chemical inertness, transition metal carbides are expected to interact weakly with graphene, acting as an ideal decoupling layer. The chemical formula of the stable solid phases of Cr carbides are Cr23C6, Cr7C3, and Cr3C2. Furthermore, a metastable rocksalt CrC phase has been synthesized by carbon-ion implantation into chromium films.32,33 The Cr carbide film discussed in the present work is characterized by a C content remarkably higher than that of the stable or metastable structures reported so far, indicating the development of a new epitaxy-stabilized phase. Hereafter, the chemical, structural and electronic properties of this graphene-covered carbide film are investigated by means of Auger electron spectroscopy (AES) and scanning tunneling microscopy/ spectroscopy (STM/STS), respectively.

■

METHODS The experiments were performed in a ultrahigh vacuum (UHV) system with a base pressure of 2 × 10−10 mbar. Ni(111) single crystal substrates were treated with several cycles of sputtering with Ar+ ions accelerated at 1.5 keV, followed by annealing at 500 °C for 10 min. The clean Ni(111) was exposed at 500 °C to 3000 L of ethylene (C2H4) at a pressure of 2 × 10−5 mbar. After dosing C2H4, the sample was kept at 500 °C in UHV for 5 min. This procedure induces the stabilization of an epitaxial layer of graphene on the Ni(111) surface. Figure 1 displays the low energy electron diffraction (LEED) patterns acquired on the clean [panel (a)] and on the graphene covered Ni(111) [panel (b)] surfaces. On the graphene-covered Ni(111) surface the LEED pattern retains an hexagonal symmetry with a 1 × 1 periodicity with respect to the substrate, indicating that the surface is free from Ni carbide species, which would produce a (√39 × √39)R16.1° reconstruction.34 The quality of the graphene layer is confirmed also by the atomically resolved scanning tunnelling microscopy (STM) image displayed in Figure 1c, where the honeycomb lattice formed by the carbon atoms is free from clusters and vacancies, indicating that the annealing temperature is sufficiently high to obtain a highly ordered layer.35 Cr was evaporated by electron bombardment of a metallic rod. The average deposition rate, as measured by a quarz microbalance, was 1 ML/min, where 1 ML is defined as the number of atoms present in the topmost layer of Ni(111). The Cr coverage was crosschecked by evaluating the attenuation of the Ni peaks corresponding to the LMM transitions, finding an agreement within 35% with the microbalance calibration. During the Cr deposition the substrate was kept at 400 °C, as measured by a thermocouple positioned close to the sample. The AES data were collected by means of a Omicron SPECTALEED with a retarding field analyzer (total acceptance

Figure 1. Panels (a) and (b) display the LEED patterns acquired at a beam energy of 150 eV on the clean and on the graphene-covered Ni(111) surfaces, respectively. Panel (c) shows an atomically resolved STM image of the graphene layer. Image size is 3.7 × 3.7 nm2, tunneling parameters are V = 0.01 V and I = 50 nA.

angle 102°). A 30 μA electron beam accelerated by a potential difference of 3 kV was used as an excitation source. STM images and STS data were acquired at room temperature by an Omicron variable temperature microscope. The STM tip was obtained by electrochemical etching of a tungsten wire. STM topography were acquired in the constant tunneling current mode, while STS spectra were collected by switching off the feedback loop. A modulation voltage with a root-mean-square amplitude of 30 mV was superimposed to the applied sample bias V and the dI/dV signal was detected by a lock-in amplifier.

■

RESULTS Chemical Composition. Figure 2a displays the evolution of the carbon (C) KVV and chromium (Cr) LMM Auger peaks upon the deposition of Cr on the graphene/Ni(111) substrate. The line shape of the C KVV transition strongly depends on the chemical environment of carbon, allowing one to discern between atoms holding sp2 hybridized orbitals, characteristic of graphene, from those forming carbidic compounds with metallic elements.36,37 The shape and the intensity (with respect to that of the Ni LMM transition at 850 eV) of the C B


Article

The Journal of Physical Chemistry C

In fact, ΔH0f is slightly positive for Ni3C, that is, the reaction is endothermic and the compound is metastable, while the system releases energy after the formation of Cr7C3, being in this case ΔH0f = −14.1 kJ/mol of atoms.39 In order to isolate the carbidic component from the C KVV transition, the spectrum of graphene/Ni(111) was subtracted from those acquired after the Cr deposition. The results of such a subtraction for 2 and 8 ML thick films are displayed in spectra (iii) and (v), respectively. Figure 2b provides a quantitative analysis of AES spectra. The red circles display the evolution with the coverage of INi , where INi and ICr are the intensities of ICr

the main Ni LMM and Cr LMM peaks, respectively. The black squares correspond to ICr , where IC is the intensity of the C IC

KVV peak after the subtraction of the graphene component. The INi value decreases exponentially with the Cr coverage, as ICr

expected for the growth of a structurally smooth and chemically sharp interface. On the other hand, the ICr ratio remains nearly IC

constant at about 0.6 for the coverages investigated. In AES spectra the concentration of an element x is proportional to Ix , Sx

where I and S are the Auger peak intensity and the elemental sensitivity factor, respectively.40 By considering SCr = 0.3 and SC = 0.2,40 the AES data suggest that the stoichiometric composition of the carbide film is close to CrC2. It is important to notice that, up to the highest coverage investigated of 8 ML, the Ni LMM transitions, reduced by a factor of about 5 with respect to those measured on the graphene/Ni(111) surface, are still well visible in AES spectra. Despite the inelastic mean free path of electrons at the kinetic energies of the Cr LMM (λ531 = 1.1 nm) and C KVV (λ275 = 0.7 nm) transitions is lower than that of the Ni LMM (λ850 = 1.5 nm),41 we can consider that the Auger spectroscopy probes the whole Cr carbide layer even at a thickness of 8 ML. Such an observation, combined with the fact that the ICr signal does not depend on the Cr

Figure 2. Panel (a) displays AES curves acquired in the energy window of C KVV and Cr LMM transitions. In the graphene/Ni(111) substrate (i) the C peak contains only the contribution of the sp2 hybridized orbitals. Spectra (ii) and (iv) have been acquired after deposition of 2 and 8 ML of Cr, respectively. Spectra (iii) and (v) are obtained by subtracting the spectrum (i) from spectra (ii) and (iv), respectively. Note the residual C peak due to the carbide phase. Panel (b): the red circles indicate the Cr coverage dependent ratio between the intensity of the Ni (850 eV) and the Cr (531 eV) peaks. Black squares represent the ratio between the Cr (531 eV) peak and the carbidic component of the C KVV transition.

IC

coverage, suggests that the stoichiometric composition of the Cr carbide layer is nearly the same for every thickness considered in this paper. Figure 3 demonstrates that, upon Cr deposition, the graphene layer floats on top of the surface. Figure 3a displays AES spectra acquired on a 4 ML Cr thick film at different emission angles with respect to the surface normal. The larger the emission angle, the more sensitive AES is to the topmost layers, since the electrons run through a longer path inside the solid before being emitted, resulting in a reduced escape depth. For increasing emission angles, the high energy carbide peak of the C KVV transition is strongly attenuated and the shape of the spectrum approaches that of the graphene/Ni(111) substrate. Such a result indicates that the Cr carbide film is encapsulated by the graphene layer. Figure 3b displays the results of O2 titration experiments performed at room temperature on 1 ML Cr deposited on graphene/Ni(111) and on Ni(111). It is well-known that the graphitic layer is highly impermeable to gases, thus, the resistance of the sample toward oxidation can be taken as a fingerprint for the presence of a compact graphene layer covering the film. Comparing the spectra obtained before (i) and after (ii) exposure to 100 L of O2 reveals that oxygen does not adsorb on the graphene/Cr-carbide sample. A further comparison can be made with the case of Cr directly grown at

peak on the graphene/Ni(111) substrate [curve (i)] is in good agreement with previously published results,38 indicating that, before Cr deposition, a single layer of graphitic carbon is present on the surface. Spectra acquired after the deposition of 2 ML (ii) and 8 ML (iv) of Cr reveal that the amount of carbon in the surface region increases with the Cr coverage. Moreover, the shape of the C KVV curve is modified with respect to that of graphene, developing the three peaks profile characteristic of the carbidic phase.36−38 These data reveal that the deposition of chromium on the graphene/Ni(111) surface induces the segregation of a considerable quantity of C from the bulk of the substrate. The incorporation of carbon into the growing film can be rationalized by considering that C atoms lower their chemical potential when moving from Ni to Cr. An estimate of the energy balance driving the formation of the Cr carbide can be obtained by comparing the standard formation enthalpy ΔH0f per mole of atoms of the stable Ni and Cr carbide bulk phases. C


Article


Figure 3. (a) AES data acquired at a different emission angles θ on a 4 ML Cr film grown on graphene/Ni(111). Spectrum (v) is acquired at normal emission on graphene/Ni(111). (b) Spectra (i) and (ii) refer to 1 ML Cr deposited on graphene/Ni(111) before and after the exposure to 100 L of O2 at room temperature, respectively. Spectra (iii) and (iv) refer to 1 ML Cr deposited on graphene-free Ni(111) before and after the exposure to 100 L of O2 at room temperature, respectively.

Figure 4. (a) STM constant current image acquired on 1.9 ML Cr grown on graphene/Ni(111). Image size is 120 × 120 nm2, tunneling parameters are V = 0.5 V and I = 500 pA. (b) Close up of the nanometric periodic pattern. Image size is 10 × 10 nm2, tunneling parameters are V = 0.01 V and I = 50 nA. Panels (c) and (d) display topographic profiles corresponding to the line scans indicated in panels (a) and (b), respectively. (e) LEED pattern acquired at a primary beam energy of 20 eV on the 1.9 ML thick sample, where the diffraction spots due to the nanometric superstructure are visible. (f) Hexagonal LEED pattern acquired at 150 eV.

400 °C on the pristine Ni(111) surface. This is reported in spectrum (iii), where only the carbidic component is visible in the C peak. This spectrum reveals that Cr carbide is formed also upon the deposition of Cr on the graphene-free Ni(111) surface, confirming that C atoms segregate from the Ni(111) bulk. Upon oxygen exposure of the latter graphene-free surface, a strong attenuation of the C peak is observed in spectrum (iv), likely due to the formation of volatile compounds like CO or CO2. Furthermore, the development of the O KLL peak and the modification of the low kinetic energy Cr peak (not shown) reveal that the Cr film gets oxidized.42 The previous AES data demonstrate the stabilization of a Cr carbide film perfectly intercalated between the graphene overlayer and the Ni(111) film. The C rich chemical composition of the film does not resemble any reported stoichiometry of stable Cr carbides in the bulk form. However, it is well-known that, due to the epitaxial constraints imposed by the substrate, nanometer thick films can crystallize also as phases, which are different from their parent bulk materials.43−46 Structural and Electronic Properties. The microscopic analysis performed with STM provides further insights into the crystal structure of the graphene-covered Cr carbide. Figure 4a displays the sample topography of a 1.9 ML thick Cr film. The surface is characterized by atomically flat terraces separated by 200 pm high steps (see line profile in Figure 4c), corresponding to the interlayer spacing of face centered cubic Ni in the (111) direction. Such an observation suggests that the carbide film is pseudomorphic with the substrate. A closer look to the surface

morphology reveals the presence of randomly distributed patches covered by a nanometer-sized hexagonal superstructure. Figure 4b displays a highly resolved STM image of such a superstructure, from which it is possible to infer a periodicity of about 1.9 nm. The line scan drawn in Figure 4b corresponds to the sinusoidal profile displayed in Figure 4d, which indicates a continuous variation of the topographic height across the surface. The nanometric periodicity is also visible on the LEED image of Figure 4e, acquired with a primary beam energy of 20 eV. At a beam energy of 150 eV, the LEED image displayed in Figure 4f is dominated by the main diffraction peaks arranged in a hexagonal pattern, unreconstructed with respect to that of the substrate. At a coverage of 4 ML the surface roughness increases, due to the nucleation of clusters with average height of about 2 nm. However, the hexagonal superstructure is still clearly visible and completely covers the surface, as displayed in Figure 5a. The fast Fourier transform in Figure 5b exhibits a hexagonal superlattice with 0.6 nm−1 long basis vectors, corresponding to the same real space periodicity measured at lower coverages. The observation of a periodic pattern on the surface demonstrates that the Cr carbide phase is crystalline, with a hexagonal surface mesh. The surface undulations could be due D


Article


where aCrC is the lattice constant of the Cr carbide, while aNi is the lattice constant of the Ni(111) surface (aNi = 2.49 Å). By using the experimental value of Λ = 1.9 nm, it possible to extract a lattice constant for the Cr carbide surface of about aCrC = 2.8 Å. We investigated the electronic properties of the graphene overlayer by means of STS acquired at constant tip−surface separation. In such spectra the first derivative of the tunneling current I with respect to the tip−sample bias V is related to the sample local density of states ρs through the approximated formula dI/dV ∝ T(z, V) × ρs(V) × ρt, where ρt is the tip density of states and T(z, V) is the tunnel transmission coefficient, depending on the sample-tip distance z. The continuous blue curve in Figure 6a represents the average of 30

Figure 5. (a) STM topography acquired on 4 ML Cr grown on graphene/Ni(111). Image size is 150 × 150 nm2, tunneling parameters are V = 1.5 V and I = 1 nA. (b) Fast Fourier transform of the STM image of panel (a). The profile shown in panel (c) corresponds to the line drawn in panel (b).

to either a Moiré pattern or to a network of misfit dislocations developing at the graphene/Cr carbide or at the Cr carbide/ Ni(111) interface, respectively. In the former case, the variation of the vertical displacement of surface atoms would depend on the different registry of graphene with respect to the Cr carbide,3 while in the latter it would be due to the elastic deformations induced by a dislocation network developing at the buried Cr carbide/Ni(111) interface.47,48 We suggest that the latter option is the most probable one, for the following reasons: (i) generally the presence of a Moiré pattern is related to a strong coupling between the graphene and the substrate, such as in the case of graphene/Ru(0001),49 while the intercalation of a decoupling element underneath graphene typically lifts the Moiré pattern.49 Since we do not have any experimental evidence of a strong coupling between graphene and Cr carbide (see below), it is likely that the surface undulations are due to the strain field originating at the buried Cr carbide/Ni(111) interface.50 (ii) The periodic pattern covers only a fraction of the surface for a low thickness of Cr carbide and is fully developed for coverages exceedings 4 ML. This observation is in agreement with a gradual release of the elastic energy accumulated on the Cr carbide film, as observed in other cases of strained epitaxial films.51 The nanometric superstructure can be interpreted as the superposition between two hexagonal lattices with different lattice constants. Assuming that the nanometer sized pattern is associated with the development of misfit dislocations at the Cr carbide/Ni(111) interface, its periodicity is given by Λ = aCrCaNi/|aCrC − aNi|,

Figure 6. (a) Dash-dot red and dot black curves represent the average of 30 dI/dV tunneling spectra acquired at constant tip−surface separation on the corrugated and on the atomically flat surface regions, respectively. (b) Tunneling spectra normalized to the total conductance I/V. For the sake of clarity, the spectrum acquired on the corrugated region has been translated upward by 0.5. For comparison purposes, the pristine and normalized dI/dV spectra collected on the graphene/Ni(111) sample are displayed as a continuous blue curves in panels (a) and (b), respectively.

spectra collected on the graphene/Ni(111) system before Cr deposition. Due to the hybridization of the Ni 3d and π graphene electronic states, the band structure of graphene is strongly modified by the interaction with the substrate. As a matter of fact, the dI/dV spectrum acquired on the graphene/ Ni(111) sample reveals a high ρs value around the Fermi energy (EF = 0 eV), in good agreement with previous results.52 On the other hand, dI/dV at V = 0 V nearly vanishes on the intercalated system, both on the corrugated (dash-dot red line) and on the atomically flat surface regions (dot-black line), suggesting that the energy of the graphene Dirac point ED coincides with EF. A better approximation of ρs can be obtained by normalizing the dI/dV curves to the total conductance I/V, since in this case the influence of T(z, V) on the spectra is mitigated.53 The normalized spectra (see Figure 6b) acquired on the graphene/CrC2/Ni(111) system are linear with respect to the applied bias in the 500 meV wide energy window E


Article


(9) Kim, H. W.; Takemoto, S.; Minamitani, E.; Okada, T.; Takami, T.; Motobayashi, K.; Trenary, M.; Kawai, M.; Kobayashi, N.; Kim, Y. Confinement of the Pt(111) Surface State in Graphene Nanoislands. J. Phys. Chem. C 2016, 120, 345−349. (10) N’Diaye, A. T.; Coraux, J.; Plasa, T. N.; Busse, C.; Michely, T. Structure of Epitaxial Graphene on Ir(111). New J. Phys. 2008, 10, 043033. (11) Fei, X.; Zhang, X.; Lopez, V.; Lu, G.; Gao, H.-J.; Gao, L. Strongly Interacting C60/Ir(111) Interface: Transformation of C60 into Graphene and Influence of Graphene Interlayer. J. Phys. Chem. C 2015, 119, 27550−27555. (12) Dedkov, Y. S.; Fonin, M.; Laubschat, C. A possible Source of Spin-Polarized Electrons: The Inert Graphene/Ni(111) System. Appl. Phys. Lett. 2008, 92, 052506. (13) Tamtögl, A.; Bahn, E.; Zhu, J.; Fouquet, P.; Ellis, J.; Allison, W. Graphene on Ni(111): Electronic Corrugation and Dynamics from Helium Atom Scattering. J. Phys. Chem. C 2015, 119, 25983−25990. (14) Zhao, L.; Rim, K.; Zhou, H.; He, R.; Heinz, T.; Pinczuk, A.; Flynn, G.; Pasupathy, A. Influence of Copper Crystal Surface on the CVD Growth of Large Area Monolayer Graphene. Solid State Commun. 2011, 151, 509−513. (15) Ago, H.; Ohta, Y.; Hibino, H.; Yoshimura, D.; Takizawa, R.; Uchida, Y.; Tsuji, M.; Okajima, T.; Mitani, H.; Mizuno, S. Growth Dynamics of Single-Layer Graphene on Epitaxial Cu Surfaces. Chem. Mater. 2015, 27, 5377−5385. (16) Cho, Y.; Choi, Y. C.; Kim, K. S. Graphene Spin-Valve Device Grown Epitaxially on the Ni(111) Substrate: A First Principles Study. J. Phys. Chem. C 2011, 115, 6019−6023. (17) Kang, J.; Shin, D.; Bae, S.; Hong, B. H. Graphene Transfer: Key for Applications. Nanoscale 2012, 4, 5527−5537. (18) jun Wang, Z.; Dong, A.; Wei, M.; Fu, Q.; Bao, X. Graphene as a Surfactant for Metal Growth on Solid Surfaces: Fe on Graphene/ SiC(0001). Appl. Phys. Lett. 2014, 104, 181604. (19) Brede, J.; Sławińska, J.; Abadia, M.; Rogero, C.; Ortega, J. E.; Piquero-Zulaica, I.; Lobo-Checa, J.; Arnau, A.; Cerdá, J. I. Tuning the Graphene on Ir(111) Adsorption Regime by Fe/Ir Surface-Alloying. 2D Mater. 2017, 4, 015016. (20) Voloshina, E. N.; Generalov, A.; Weser, M.; Böttcher, S.; Horn, K.; Dedkov, Y. S. Structural and Electronic Properties of the Graphene/Al/Ni(111) Intercalation System. New J. Phys. 2011, 13, 113028. (21) Nagashima, A.; Tejima, N.; Oshima, C. Electronic States of the Pristine and Alkali-Metal-Intercalated Monolayer Graphite/Ni(111) systems. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 50, 17487− 17495. (22) Bussetti, G.; Yivlialin, R.; Alliata, D.; Li Bassi, A.; Castiglioni, C.; Tommasini, M.; Casari, C. S.; Passoni, M.; Biagioni, P.; Ciccacci, F.; et al. Disclosing the Early Stages of Electrochemical Anion Intercalation in Graphite by a Combined Atomic Force Microscopy/ Scanning Tunneling Microscopy Approach. J. Phys. Chem. C 2016, 120, 6088−6093. (23) Halle, J.; Néel, N.; Kröger, J. Filling the Gap: Li-Intercalated Graphene on Ir(111). J. Phys. Chem. C 2016, 120, 5067−5073. (24) Monazami, E.; Bignardi, L.; Rudolf, P.; Reinke, P. Strain Lattice Imprinting in Graphene by C60 Intercalation at the Graphene/Cu Interface. Nano Lett. 2015, 15, 7421−7430. (25) Sung, S. J.; Yang, J. W.; Lee, P. R.; Kim, J. G.; Ryu, M. T.; Park, H. M.; Lee, G.; Hwang, C. C.; Kim, K. S.; Kim, J. S.; et al. SpinInduced Band Modifications of Graphene Through Intercalation of Magnetic Iron Atoms. Nanoscale 2014, 6, 3824−3829. (26) Cattelan, M.; Peng, G. W.; Cavaliere, E.; Artiglia, L.; Barinov, A.; Roling, L. T.; Favaro, M.; Pis, I.; Nappini, S.; Magnano, E.; et al. The Nature of the Fe-Graphene Interface at the Nanometer Level. Nanoscale 2015, 7, 2450−2460. (27) Dedkov, Y. S.; Fonin, M.; Rüdiger, U.; Laubschat, C. GrapheneProtected Iron Layer on Ni(111). Appl. Phys. Lett. 2008, 93, 022509. (28) Dahal, A.; Batzill, M. Growth from Behind: IntercalationGrowth of Two-Dimensional FeO Moiré Structure Underneath of Metal-Supported Graphene. Sci. Rep. 2015, 5, 11378.

centered around EF. Such trend is the one expected for the graphene density of states (ρ g ) around E D , that is, ρg ∝| E − ED |.54 On the other hand, the normalized spectra measured on the graphene/Ni(111) sample strongly deviate from a linear trend, confirming the remarkable influence of the Ni(111) substrate on the electronic structure of graphene. In conclusion, the STS data suggest that the intercalation of CrC2 between Ni(111) and graphene effectively decouples the latter from the substrate.

■

CONCLUSIONS Upon Cr deposition on the graphene passivated Ni(111) surface, a crystalline Cr carbide develops underneath the graphitic layer. The analysis of AES spectra indicates that the Cr carbide stoichiometry is close to CrC2. Atomically resolved STM constant-current images reveal the development of a highly ordered nanometric pattern on the surface, possibly related to the mismatch at the Cr carbide/Ni(111) interface. Differential STS spectra display an almost linear dependence on the applied bias, suggesting that the graphene layer is electronically decoupled from the Cr carbide film. As both graphene and metal carbides are used as protective layers against metals corrosion, the superlattice obtained by their combination could be considered as a very effective nanocoating. In a broader context, our results indicate that foreign species dissolved in the bulk of single crystal metals can be exploited to stabilize crystalline metal compounds underneath graphene, paving the way for obtaining new grapheneintercalated systems.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

A. Picone: 0000-0001-7920-6893 Notes

The authors declare no competing financial interest.

■

REFERENCES

(1) Dedkov, Y.; Voloshina, E. Graphene Growth and Properties on Metal Substrates. J. Phys.: Condens. Matter 2015, 27, 303002. (2) Batzill, M. The Surface Science of Graphene: Metal Interfaces, CVD Synthesis, Nanoribbons, Chemical modifications, and Defects. Surf. Sci. Rep. 2012, 67, 83−115. (3) Marchini, S.; Günther, S.; Wintterlin, J. Scanning Tunneling Microscopy of Graphene on Ru(0001). Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 76, 075429. (4) Martoccia, D.; Björck, M.; Schlepütz, C. M.; Brugger, T.; Pauli, S. A.; Patterson, B. D.; Greber, T.; Willmott, P. R. Graphene on Ru(0001): a Corrugated and Chiral Structure. New J. Phys. 2010, 12, 043028. (5) Politano, A.; Slotman, G. J.; Roldán, R.; Chiarello, G.; Campi, D.; Katsnelson, M. I.; Yuan, S. Effect of Moiré Superlattice Reconstruction in the Electronic Excitation Spectrum of Graphene-Metal Heterostructures. 2D Mater. 2017, 4, 021001. (6) Gao, L.; Guest, J. R.; Guisinger, N. P. Epitaxial Graphene on Cu(111). Nano Lett. 2010, 10, 3512−3516. (7) Reddy, K. M.; Gledhill, A. D.; Chen, C.-H.; Drexler, J. M.; Padture, N. P. High Quality, Transferrable Graphene Grown on Single Crystal Cu(111) Thin Films on Basal-Plane Sapphire. Appl. Phys. Lett. 2011, 98, 113117. (8) Sutter, P.; Sadowski, J. T.; Sutter, E. Graphene on Pt(111): Growth and Substrate Interaction. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 80, 245411. F


Article


Fe(001) Surfaces. Phys. Rev. B: Condens. Matter Mater. Phys. 2015, 92, 115434. (49) Cui, Y.; Gao, J.; Jin, L.; Zhao, J.; Tan, D.; Fu, Q.; Bao, X. An Exchange Intercalation Mechanism for the Formation of a TwoDimensional Si Structure Underneath Graphene. Nano Res. 2012, 5, 352−360. (50) Springholz, G. Strain Contrast in Scanning Tunneling Microscopy Imaging of Subsurface Dislocations in Lattice-Mismatched Heteroepitaxy. Appl. Surf. Sci. 1997, 112, 12−22. (51) Brambilla, A.; Picone, A.; Giannotti, D.; Riva, M.; Bussetti, G.; Berti, G.; Calloni, A.; Finazzi, M.; Ciccacci, F.; Duò, L. Self-Organized Nano-Structuring of CoO Islands on Fe(001). Appl. Surf. Sci. 2016, 362, 374−379. (52) Garcia-Lekue, A.; Balashov, T.; Olle, M.; Ceballos, G.; Arnau, A.; Gambardella, P.; Sanchez-Portal, D.; Mugarza, A. Spin-Dependent Electron Scattering at Graphene Edges on Ni(111). Phys. Rev. Lett. 2014, 112, 066802. (53) Tromp, R. M. Spectroscopy with the Scanning Tunnelling Microscope: a Critical Review. J. Phys.: Condens. Matter 1989, 1, 10211. (54) Andrei, E. Y.; Li, G.; Du, X. Electronic Properties of Graphene: a Perspective from Scanning Tunneling Microscopy and Magnetotransport. Rep. Prog. Phys. 2012, 75, 056501.

(29) Dedkov, Y.; Klesse, W.; Becker, A.; Späth, F.; Papp, C.; Voloshina, E. Decoupling of Graphene from Ni(111) via Formation of an Interfacial NiO Layer. Carbon 2017, 121, 10−16. (30) Novotny, Z.; Netzer, F. P.; Dohnálek, Z. Cerium Oxide Nanoclusters on Graphene/Ru(0001): Intercalation of Oxygen via Spillover. ACS Nano 2015, 9, 8617−8626. (31) Li, Y.; Gao, Y.; Xiao, B.; Min, T.; Yang, Y.; Ma, S.; Yi, D. The Electronic, Mechanical Properties and Theoretical Hardness of Chromium Carbides by First-Principles Calculations. J. Alloys Compd. 2011, 509, 5242−5249. (32) Liu, B. X.; Cheng, X. Y. A Metastable Cr Carbide of NaCl Formed by Carbon-Ion Implantation into Chromium Films. J. Phys.: Condens. Matter 1992, 4, L265. (33) Kavitha, M.; Priyanga, G. S.; Rajeswarapalanichamy, R.; Iyakutti, K. Structural Stability, Electronic, Mechanical and Superconducting Properties of CrC and MoC. Mater. Chem. Phys. 2016, 169, 71−81. (34) Klink, C.; Stensgaard, I.; Besenbacher, F.; LÃ eģ sgaard, E. An STM Study of Carbon-Induced Structures on Ni(111): Evidence for a Carbidic-Phase Clock Reconstruction. Surf. Sci. 1995, 342, 250−260. (35) Niu, T.; Zhou, M.; Zhang, J.; Feng, Y.; Chen, W. Growth Intermediates for CVD Graphene on Cu(111): Carbon Clusters and Defective Graphene. J. Am. Chem. Soc. 2013, 135, 8409−8414. (36) Lahiri, J.; Miller, T.; Adamska, L.; Oleynik, I. I.; Batzill, M. Graphene Growth on Ni(111) by Transformation of a Surface Carbide. Nano Lett. 2011, 11, 518−522. (37) Rosei, R.; Ciccacci, F.; Memeo, R.; Mariani, C.; Caputi, L.; Papagno, L. Kinetics of Carbidic Carbon Formation from CO in the 10−6 Torr Range on Ni(110). J. Catal. 1983, 83, 19−24. (38) Lahiri, J.; Miller, T. S.; Ross, A. J.; Adamska, L.; Oleynik, I. I.; Batzill, M. Graphene Growth and Stability at Nickel Surfaces. New J. Phys. 2011, 13, 025001. (39) Meschel, S.; Kleppa, O. Standard Enthalpies of Formation of Some 3d Transition Metal Carbides by High Temperature Reaction Calorimetry. J. Alloys Compd. 1997, 257, 227−233. (40) Davis, L. E.; MacDonald, N. C.; Palmberg, P. W.; Riach, G. E.; Weber, R. E. Handbook of Auger Electron Spectroscopy; Perkin-Elmer Corporation: MN, 1976. (41) Powell, C. J.; Jablonski, A. NIST Electron Inelastic - Mean - Free Path Database, Version 1.2; National Institute of Standards and Technology: Gaithersburg, MD, 2010. (42) Riva, M.; Picone, A.; Bussetti, G.; Brambilla, A.; Calloni, A.; Berti, G.; Duò, L.; Ciccacci, F.; Finazzi, M. Oxidation Effects on Ultrathin Ni and Cr Films grown on Fe(001): A Combined Scanning Tunneling Microscopy and Auger Electron Spectroscopy Study. Surf. Sci. 2014, 621, 55−63. (43) Rodriguez, J. A.; Liu, P.; Graciani, J.; Senanayake, S. D.; Grinter, D. C.; Stacchiola, D.; Hrbek, J.; Fernández-Sanz, J. Inverse Oxide/ Metal Catalysts in Fundamental Studies and Practical Applications: A Perspective of Recent Developments. J. Phys. Chem. Lett. 2016, 7, 2627−2639. (44) Picone, A.; Fratesi, G.; Riva, M.; Bussetti, G.; Calloni, A.; Brambilla, A.; Trioni, M. I.; Duò, L.; Ciccacci, F.; Finazzi, M. Selforganized Chromium Oxide Monolayers on Fe(001). Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 87, 085403. (45) Surnev, S.; Fortunelli, A.; Netzer, F. P. Structure-Property Relationship and Chemical Aspects of Oxide-Metal Hybrid Nanostructures. Chem. Rev. 2013, 113, 4314−4372. (46) Calloni, A.; Berti, G.; Bussetti, G.; Fratesi, G.; Finazzi, M.; Ciccacci, F.; Duò, L. Electronic Structure and Magnetism of Strained bcc Phases across the fcc to bcc Transition in Ultrathin Fe Films. Phys. Rev. B: Condens. Matter Mater. Phys. 2016, 94, 195155. (47) Jnawali, G.; Hattab, H.; Meyer zu Heringdorf, F.; Krenzer, B.; Horn-von Hoegen, M. Lattice-Matching Periodic Array of Misfit Dislocations: Heteroepitaxy of Bi(111) on Si(001). Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 76, 035337. (48) Riva, M.; Picone, A.; Giannotti, D.; Brambilla, A.; Fratesi, G.; Bussetti, G.; Duò, L.; Ciccacci, F.; Finazzi, M. Mesoscopic Organization of Cobalt Thin Films on Clean and Oxygen-saturated G


Intercalation from the Depths: Growth of a Metastable Chromium

Recommend Documents