Ni(111) System during Vacuum Annealing

Article pubs.acs.org/JPCC

Evolution of CuI/Graphene/Ni(111) System during Vacuum Annealing Alexander V. Generalov,*,†,‡,# Konstantin A. Simonov,§,∥,† Nikolay A. Vinogradov,⊥,† Elena M. Zagrebina,† Nils Mårtensson,§ Alexei B. Preobrajenski,∥,† and Alexander S. Vinogradov† †

V. A. Fock Institute of Physics, St. Petersburg State University, 198504 St. Petersburg, Russia Institut für Festkörperphysik, Technische Universität Dresden, DE-01062 Dresden, Germany § Department of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden ∥ Max IV Laboratory, Lund University, Box 118, 22100 Lund, Sweden ⊥ European Synchrotron Radiation Facility, 6 Rue Jules Horowitz, B.P. 220, FR-38043 Grenoble Cedex, France ‡

ABSTRACT: We present a combined core-level spectroscopy and low-energy electron diffraction study of the evolution of thin CuI layers on graphene/Ni(111) during annealing. It has been found that the annealing of the CuI/graphene/Ni(111) system up to 160 °C results in the formation of an ordered CuI overlayer with a (√3 × √3) R30° structure on top of the graphene surface. At annealing temperatures of about 180 °C or higher, the CuI overlayer decomposes with a simultaneous intercalation of Cu and I atoms underneath the graphene monolayer on Ni(111). Nearly complete intercalation of graphene by Cu and I atoms can be achieved by deposition of about 20 Å of CuI, followed by annealing at 200 °C. The intercalated graphene layer is p-doped due to interfacial iodine atoms.

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INTRODUCTION For the past decade, graphene (monolayer of graphite, MG) has captured the attention of the scientific community due to its extraordinary electronic properties, which have already found application in electronic devices based on this new nonsilicon technology.1−4 The properties of MG can be significantly modified in a controlled and purposeful way using different types of functionalization.3,5 The most popular routes to achieve this modification are chemical functionalization and intercalation.6 They are usually implemented by depositing various atoms, molecules, and compounds on top of a supported graphene layer with a subsequent thermal activation of the system. In the case of chemical functionalization the adsorbed atoms are attached chemically to the graphene monolayer, while during intercalation they penetrate into the space between the graphene and the substrate. Processes of functionalization are often accompanied by charge-transfer to or from the graphene layer, causing changes in its electronic properties and resulting, in particular, in n- or p-doping of graphene. The controlled doping of graphene monolayers is considered to be one of the most important problems in the design of graphene-based electronic devices. Obviously, the electronic properties of the resulting functionalized graphene depend on the structural quality of the original monolayer and the chemistry of the deposited substances, as well as on their changes during the subsequent thermal activation of the system. A graphene monolayer on a chemically active transition metal (TM) surface, such as Ni(111), Co(0001), Rh(111), and Ru(0001), is strongly © XXXX American Chemical Society

coupled to the substrate by a covalent TM nd−C 2p bonding,7−9 which causes significant changes in its electronic structure. On the other hand, the electronic structure of MG undergoes only small changes at interfaces with less active metal surfaces like Cu(111), Pt(111), and Ir(111).9−12 Generally, a material deposited on a graphene monolayer interacts only marginally with MG due to a chemical inertness of the latter. If the material is not desorbed completely during the thermal activation of the system, it can form new structures on the surface of graphene or it can be directly intercalated under the graphene monolayer. Another possibility is that there is a decomposition of the (nonelemental) material followed by the desorption or intercalation of its derivatives. Detailed knowledge of how the interfaces between graphene and adsorbates/intercalants evolve with temperature is a key for understanding the associated variations in the doping level and can help to obtain desired carrier concentrations in graphene. In this context it can be interesting to use metal halides as doping agents in contact with graphene, since these are popular intercalants for preparing graphite intercalation compounds (GIC).13 These intercalants are strong acceptors of electron density that causes the formation of GICs with new electronic properties. Recently, the prospect of using such compounds for graphene doping has been demonstrated by the example of Received: March 11, 2015 Revised: May 13, 2015

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DOI: 10.1021/acs.jpcc.5b02390 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C FeCl3 and AlBr3.14,15 The halogen-containing compounds were deposited on top of weakly bonded graphene on Ir(111) and were then gradually annealed in vacuum. Such a procedure resulted in the decomposition of the initial halogen-rich metal halides (FeCl3 and AlBr3) into halides with lower relative halogen concentration (like FeCl2) and atomic halogens (chlorine or bromine atoms). Atomic halogens were intercalated under graphene and turned out to be the main doping agents, while the metal halide fragments with a lack of halogen atoms usually desorb from the graphene surface14 or constitute only a small fraction of the intercalated material.15 The corresponding graphene Fermi-level lowering (p-doping) due to the charge transfer to the chlorine and bromine atoms was reported to be ∼0.6 and 0.35 eV, respectively. Here it should be mentioned that the largest lowering of the graphene Fermi-level of 0.79 eV was achieved for graphene by intercalation of atomic fluorine produced as a result of gaseous MoF6 decomposition.16 To the best of our knowledge, the use of iodine-containing metal halide for graphene functionalization has not been reported yet. However, it is known that the inner channels of single-walled carbon nanotubes can be filled with onedimensional CuI,17 KI,18 and AgI19 nanocrystals, resulting usually in a p-doping of the nanotubes as revealed by a combination of X-ray spectroscopy techniques.19−21 Therefore, it is of natural interest to investigate graphene functionalization using these iodine-containing compounds. In the present work we follow the temperature-induced evolution of a cuprous iodide (CuI) layer deposited on graphene on Ni(111) substrate by means of high-resolution Xray photoelectron spectroscopy of the core levels (XPS) and the valence band (VB PES),22 near-edge X-ray absorption fine structure (NEXAFS) spectroscopy,23 low-energy electron diffraction (LEED), and auger electron spectroscopy (AES).24 The main idea is to trace the effect of structural changes in CuI on the electronic properties of graphene. MG on Ni(111) is a popular well-studied system where graphene forms large-scale flat domains of high quality without mismatch with the Ni substrate due to the similar lattice parameters of MG and the substrate. The MG/Ni(111) interface is characterized by a strong covalent bonding and a considerable perturbation of the graphene electronic structure.7,25,26 By analogy with other metal halides14,15 it can be expected that annealing of the CuI/ MG/Ni(111) system in vacuum may lead to intercalation, which will ultimately lift the graphene monolayer, breaking the strong covalent bonding of MG to the Ni(111) substrate and, possibly, result in a charge carrier doping of MG.

Reference samples of bulk CuI and metallic copper were prepared by evaporation of CuI (∼250 Å) and metallic Cu (∼300 Å) on a stainless-steel plate.28 The evaporation rate was of the order of 10 Å/min. The NEXAFS spectra at the Cu 2p and C 1s absorption edges were recorded in the total and Auger electron yield modes, respectively.23,29 The energy resolution ΔE of the monochromator at the C 1s and Cu 2p3/2 edges was set to 50 and 300 meV, respectively. The C 1s, Cu 2p3/2 and I 3d,4d XPS and VB PES spectra were recorded in the normal emission geometry22 with a total energy resolution of 100 (C 1s) and 500 meV (Cu 2p and I 3d). The VB PES spectra were measured in the angle-resolved mode with a total energy resolution of the order of 70 meV. The total energy resolution in the I M4,5N4,5N4,5 and Cu L3M4,5M4,5 Auger electron spectra was about 400 and 500 meV, respectively.

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RESULTS AND DISCUSSION

Structural Ordering, Decomposition, and Intercalation. The LEED for the initial MG/Ni(111) system demonstrates a characteristic hexagonal pattern (Figure 1a), which is very similar for MG and Ni(111) due to the lattice match between their crystalline structures. The corresponding C 1s photoemission line is located at a binding energy (BE) of 284.9 eV and has a full width at half-maximum (fwhm) of 0.4 eV (Figure 2a). The obtained value of the BE for the C 1s

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EXPERIMENTAL SECTION All experiments were performed at the Russian−German beamline27 at the BESSY II light source (Helmholtz-Zentrum Berlin, Germany). The initial MG/Ni(111)/W(110) sample was prepared according to a standard procedure,7,26 where the graphene layer was synthesized by thermal cracking of C3H6 molecules on the surface of a thin (∼100 Å) Ni film with the (111) orientation grown on W(110) and kept at 600 °C. For evaporation of CuI at room temperature (RT), a Knudsen-type cell was used. The evaporation rate was about 1 Å/min as monitored by a quartz-microbalance. The annealing of CuI/ MG/Ni(111)/W(110) was performed in steps by thermal radiation from a hot filament under ultrahigh vacuum (UHV) conditions. Each annealing step lasted approximately 10 min. The sample temperature was controlled by a thermocouple, attached to the edge of the tungsten crystal.

Figure 1. LEED patterns for (a) the initial system MG/Ni(111); (b) for CuI/MG/Ni(111) after annealing at 120 °C, (c) after annealing at 180 °C, and (d) after annealing at 200 °C. The yellow arrows point to the arcs from misoriented graphene flakes, centered at azimutal angles ∼±15° with respect to the reflexes from the main graphene domains. The electron beam energy is Ep = 65 eV. (e) A model for the atomic structure of the ordered (√3 × √3) R30 °CuI(111) layer on the graphene surface. The lattice parameters of graphene and the CuI(111) layer are shown by blue and red arrows, respectively. The unit cell of graphene is indicated by the blue dashed rhombus. B


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The Journal of Physical Chemistry C

assigned to the formation of ordered and azimutally disordered domains of CuI(111) on the graphene surface after the annealing, respectively. The presence of disordered CuI domains is indicative of a weak interaction between MG/Ni(111) and CuI, that can likely be related to the chemical inertness of both systems. This is supported by the bright diffuse background inside the ring and the rather prominent 1 × 1 reflexes from MG/Ni(111) (Figure 1b), implying that the CuI layer tends to form small islands.33 The lattice-matched CuI(111)/MG/Ni(111) system may have promising applications in graphene-based dye sensitized solar cells and light emitting diodes, where the wide band gap (Eg = 3.1 eV)34 transparent p-type semiconductor CuI has already demonstrated its high potential.35−37 The stability of the CuI/MG/Ni(111) system up to annealing temperatures of 160 °C is evident from the conservation of the shape and intensity of the Cu 2p3/2 and I 3d5/2 XPS spectra in going from the initial system “as depos” (black lines in Figure 3) to the same system after annealing

Figure 2. C 1s photoelectron spectra (hν = 350 eV) for (a) the initial MG/Ni(111) system, (b) after the deposition of 20 Å of CuI and annealing at 120 °C, (c) after annealing at 180 °C, and (d) after annealing at 200 °C. The spectra are normalized to the intensity of the incident radiation.

electrons in MG/Ni(111) coincides with the one, measured on the same setup in an independent study in ref 30. The deposition of a 20 Å thick layer of CuI on the graphene monolayer at RT results only in a strong decrease in the intensity of the C 1s spectrum and a disappearance of the hexagonal LEED pattern (not shown), indicating the formation of an amorphous film. Thereafter, the system was annealed at temperatures in the range of 100−200 °C with a step of 20 °C. At annealing temperatures up to 160 °C the CuI/MG/ Ni(111) system is stable, as the shape and intensity of the C 1s XPS spectrum and the signals from CuI do not change. However, a new LEED pattern starts to appear. A typical LEED pattern and the C 1s line in this case (T = 120 °C) are presented in Figures 1b and 2b, respectively. Compared to the hexagonal LEED pattern of the initial MG/Ni(111) system (Figure 1a), a new hexagonal (√3 × √3) R30° pattern with a weak ring and diffuse background is observed indicating the formation of a 2D crystalline layer of CuI on MG. Crystalline CuI under normal conditions belongs to the sphalerite structure type with a lattice constant of 6.063 Å.31 In this case the copper cations Cu+ are tetrahedrally surrounded by iodine anions I−, and vice versa.31 This structure can be viewed as a cubic close-packed lattice of iodine anions in which half of the tetrahedral hollows are filled with copper cations. It is equivalent to the stacking of {111} layers of CuI4 tetrahedra along the ⟨111⟩ directions. The interlayer distance is estimated by the height of the tetrahedra which is about 0.35 nm.31 The close-packed iodine {111} layers in CuI have the same hexagonal symmetry as the graphene crystalline film. The lattice parameter of the CuI(111) layer (the distance between iodine atoms along the ⟨110̅ ⟩ direction) aCuI = 4.287 Å31−33 practically coincides with the distance aMG√3 = 4.278 Å along the long diagonal of the MG unit cell.7 As a result, the growth of an epitaxial 2D CuI(111) layer on top of MG/Ni(111) may be expected. A model for the atomic structure of the ordered CuI(111) layer on graphene is presented in Figure 1e, where a single layer of CuI4 tetrahedra is shown. In this case the (√3 × √3) R30° structure and the ring in Figure 1b can be

Figure 3. (a) Cu 2p3/2 and (b) I 3d5/2 XPS spectra (hν = 1100 eV) for the CuI/MG/Ni(111) system at RT (“as depos”) and upon further annealing steps.

below 160 °C (typical spectra at 100 and 120 °C are shown with the green lines in Figure 3a,b). However, it should be noted that already after the first annealing at 100 °C a small high-energy shift of about 0.1 eV for both Cu 2p3/2 and I 3d5/2 spectra relative to their original positions is observed (Figure 3a,b), which is related to the structural ordering of the CuI overlayer. The binding energies of the I 3d5/2 and Cu 2p3/2 electrons for the ordered CuI layer on MG/Ni(111) (Ebind = 619.4 and 932.4 eV, respectively) coincide with those for bulk CuI within the experimental accuracy.28 Similar to the Cu 2p3/2 and I 3d5/2 XPS results, the Cu 2p NEXAFS spectra of the CuI/MG/Ni(111) system do not change upon annealing below 160 °C (Figure 4). The spectrum from the reference sample of bulk CuI is plotted at the top of Figure 4. It is characterized by bands a−h, with the most intense features a−b corresponding to transitions of the Cu 2p3/2 core electrons to unoccupied states of mainly Cu 4s character.28,38 The Cu 2p absorption spectra of CuI on MG/ Ni(111) (T ≤ 160 °C) show general similarity with that of the reference CuI sample. However, the former ones are noticeably noisier due to smaller amount of material, and their bands c−d are broadened and smeared out. In addition, bands c−h for CuI/MG/Ni(111) are shifted to higher photon energies. We attribute these findings to structural differences between the C


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of lattice parameters between graphene and the intercalated layer. It has been shown that for the graphene growth process on a Cu(111) surface, the graphene islands usually grow with rotational disorder.43 The amount of rotational boundaries strongly depends on the surface roughness and the number of surface defects. Herewith, high-angle rotational boundaries are commonly produced when graphene grows over the Cu(111) step bunches.43 Therefore, we believe that the arcs in the LEED pattern of our system (Figure 1c) is an additional proof for intercalation of CuI decomposition products under graphene. The decomposition and the reduced amount of CuI on the graphene surface at this annealing stage is also confirmed by the Cu 2p3/2 NEXAFS spectra (Figure 4). Similar to the Cu 2p3/2 XPS spectra (Figure 3a), the Cu 2p3/2 NEXAFS spectrum decreases significantly in intensity after this annealing (Figure 4, “180 °C”) and simultaneously a low-energy shoulder a* begins to form in the spectrum. The shoulder is located at a photon energy of ∼933 eV, which is very close to the position of the Cu 2p3/2 absorption edge of metallic copper (932.5 eV)44 as seen from the spectrum of copper metal in Figure 4. At the same time, the intensities of bands a−b, related to unoccupied electron states of CuI,28 decrease strongly. These facts, together with the broadening of the Cu 2p3/2 and I 3d5/2 XPS lines, give strong support for the interpretation that CuI decomposes at 180 °C with the formation of metallic copper. The changes observed upon annealing in the Cu L3M4,5M4,5 and I M4,5N4,5N4,5 Auger spectra (Figure 5a,b) also clearly

Figure 4. Cu 2p NEXAFS spectra of CuI/MG/Ni(111): for the initial “as depos” system as well as after annealing at different temperatures in addition to spectra of the reference samples, bulk layers of metallic copper (∼30 nm) and CuI (∼25 nm).

thin layers and bulk CuI. In the thin (quasi 2D) CuI layer on MG/Ni(111), the CuI4 tetrahedra may be distorted along the surface normal, which probably explains the smearing of some bands in the spectrum of the CuI/MG/Ni(111) system.38 Annealing at 180 °C leads to a considerable increase in the total intensity of the C 1s photoemission signal with the appearance of a new low-energy component at 284.2 eV, which is shifted by about 0.7 eV from the original C 1s component for MG/Ni(111) (Figure 2c). The low-energy component is dominating the C 1s spectrum with an intensity just slightly smaller than the total C 1s intensity for the initial MG/Ni(111) system (Figure 2a). The appearance of this low-energy component indicates an onset of intercalation of CuI and/or its derivatives under graphene at 180 °C,14,39 which is accompanied by a decoupling of MG from Ni(111). In line with this interpretation is also the smaller fwhm of the low-energy C 1s component (0.3 eV) as compared to that for strongly bonded graphene on Ni(111) (0.4 eV).9 The corresponding LEED pattern (Figure 1c) shows an increase in intensity of the 1 × 1 graphene reflexes, while the (√3 × √3) R30° reflexes from CuI become diffuse and strongly attenuated. Besides, arcs centered at azimuthal angles of about 15° relative to the main graphene reflexes (shown by yellow arrows) emerge. The ring and the diffuse background, related to cuprous iodide, are considerably weakened and have almost vanished in the LEED pattern. At the same time, the Cu 2p3/2 and I 3d5/2 XPS signals become weaker and broader (magenta lines “180 °C” in Figure 3a,b). Their FWHMs increase from 1.0 eV (Cu 2p3/2) and 1.1 eV (I 3d5/2) to 1.05 and 1.35 eV, respectively, as a result of annealing at 180 °C. All these facts point to a gradual CuI decomposition and its removal from the graphene surface at 180 °C as a consequence of intercalation and probably desorption processes. The appearance of the arcs in the LEED pattern is indicative of the formation of laterally misoriented graphene domains in the system. Similar arcs were observed for MG/Ni(111) after intercalation of Cu and Al atoms.40−42 They were related to a weaking of the bonding between graphene and the nickel substrate as a consequence of intercalation and to a mismatch

Figure 5. (a) Cu L3M4,5M4,5 (hν = 960 eV) and (b) I M4,5N4,5N4,5 (hν = 740 eV) Auger spectra for the initial CuI/MG/Ni(111) system (“as depos”) as well as after different annealing stages. The spectra are normalized to the intensity of the incident radiation.

indicate CuI decomposition starting from 180 °C. Indeed, in addition to the Auger lines from pristine CuI, new high-energy components appear in both spectra, indicating the presense of new copper and iodine phases. Some of the Auger lines from initial CuI and their high-energy counterparts from decomposed CuI are marked by dashed lines in Figure 5. On the whole, we can conclude that upon annealing at 180 °C an active decomposition of CuI starts followed by intercalation of elemental Cu and I under graphene. The final annealing at 200 °C results in a further weakening of the (√3 × √3) R30° reflexes in the LEED pattern, while the 1 × 1 graphene reflexes recover their intensities almost completely (Figure 1d). The arcs from misoriented graphene domains gain in intensity as well. The intensity of the D


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reduced down to 1.15 for the MG/Cu+I/Ni(111) system. Taking into consideration that the surface sensitivity for the copper spectrum (mean-free path for the 170 eV photoelectrons is around 5 Å)22 is higher than for the iodine one (mean-free path for the 480 eV photoelectrons is around 10 Å)22 we can conclude that the amount of iodine relative to copper is reduced after the last annealing. It is natural to assign this reduction to a partial desorption of iodine atoms, produced in the process of CuI decomposition. Desorption of copper atoms at 200 °C is unlikely. The reduction of the amount of iodine in the system most likely occurs due to the desorption of molecular iodine which may be produced during CuI decomposition along with “ionic” iodine. Indeed, it is known that molecular iodine adsorbed on the external walls of carbon nanotubes desorbes already at room temperature.46 On the other hand, the deintercalation of iodine from inner channels of single-walled carbon nanotubes, where iodine is supposed to be in ionic state, only takes place at temperatures higher than 500 °C.46 Thus, our conclusion about the ionic character of iodine in the MG/Cu+I/Ni(111) system is in accordance with the results of the above-mentioned work in ref 46. Let us now consider the I 3d5/2 and I 4d3/2,5/2 XPS spectra from the CuI/MG/Ni(111) system after two annealings at 180 and 200 °C. In Figure 6 these spectra are presented along with their peak fit analysis. Based on the discussion above, the I 3d5/2 (Ebind = 619.05 eV) and I 4d3/2,5/2 (Ebind = 49.1 eV for the

photoelectron C 1s signal from intercalated graphene increases even more after this annealing (Figure 2d) and becomes comparable with the intensity of this signal for the initial MG/ Ni(111) system (Figure 2a). Moreover, the fwhm of the C 1s line decreases to 0.27 eV, which is significantly smaller than for MG/Ni(111) (0.4 eV). All these findings indicate a complete graphene intercalation and an absence of noticeable amounts of CuI on its surface. The C 1s XPS line from MG/Ni(111) (Figure 2d) also increases slightly in intensity. It is most likely related to the desorption of CuI residues from the surface of not intercalated MG/Ni(111) areas. It is important to note that the energy separation of 0.7 eV between the low- and highenergy components does not change during the different annealing stages. In the corresponding Cu 2p3/2 and I 3d5/2 XPS spectra, noticeable changes are also observed (Figure 3a,b, “200 °C”). The FWHMs of the Cu 2p3/2 and I 3d5/2 XPS lines are reduced considerably to ∼0.95 eV (Cu 2p3/2) and ∼1.0 eV (I 3d5/2), which are even smaller than those for initial CuI (1.0 and 1.1 eV, respectively). At the same time, the energy separation of 313.2 eV between the Cu 2p3/2 (Ebind = 932.25) and I 3d5/2 (Ebind = 619.05) XPS lines is noticeably larger than that for cuprous iodide (313.0 eV). These facts support the formation of well-defined copper and iodine phases, distinct from the initial CuI phase. The latter conclusion is further supported by the comparison of the Cu L3M4,5M4,5 and I M4,5N4,5N4,5 Auger spectra (Figure 5a,b, “200 °C”). After the last annealing at 200 °C the spectra look similar to the spectra of initial CuI (Figure 5a,b, “as depos”), except for the fact that they are shifted to higher kinetic energies (by 2.15 and 1.6 eV for the Cu and I Auger spectra, respectively) and that they are noticeably narrower. The electron kinetic energy of the most intense line in the Cu L3M4,5M4,5 Auger spectrum (1G-term) ∼918.6 eV is very close to the energy of the corresponding line in the spectrum of metallic copper (∼918.4 eV).44 This fact is in agreement with our conclusion, based on the analysis of Cu 2p3/2 NEXAFS spectra, that the CuI decomposition is accompanied by the formation of metallic copper. The I M4,5N4,5N4,5 Auger spectra now look very similar to those from an iodine overlayer chemisorbed on the Cu(111) surface.31 The observed increase in kinetic energy of the I M4,5N4,5N4,5 Auger electrons and the narrowing of the corresponding Auger lines can be assigned to a change of the chemical state of iodine from a covalently bonded one in the initial CuI to a more ionic state.45 This scenario is also confirmed by the 0.35 eV shift to lower BEs of the I 3d5/2 line upon the last annealing (Ebind = 619.05 eV, Figure 3b, “200 °C”) relative to the case of ordered CuI on graphene (Ebind = 619.4 eV, Figure 3b, “120 °C”). Such a decrease of the I 3d5/2 BE may be attributed to an increase of negative charge on the iodine-ion as a consequence of a change in chemical state from covalent to more ionic. Thus, we have demonstrated that, after annealing at 200 °C, the process of CuI decomposition with the formation of metallic copper and ionic iodine I− followed by their intercalation under graphene is completed. We will denote the obtained system as “MG/Cu+I/ Ni(111)”. One more evidence of the ionic character of the iodine atoms in the MG/Cu+I/Ni(111) system can be obtained from the following considerations. The ratio between the total intensities of the I 3d5/2 and Cu 2p3/2 XPS lines is about 1.75 for the CuI/ MG/Ni(111) system after annealing at 120 °C. This ratio is

Figure 6. I 4d3/2,5/2 (hν = 245 eV) (a) and I 3d5/2 (hν = 1100 eV) (b) photoelectron spectra with fitted high- and low-energy components for the CuI/MG/Ni(111) system after annealing at 180 and 200 °C. The fit of the spectrum is shown by the solid blue line, the linear background by the dashed green line, and the component from CuI by the gray solid line with pink filling. The components with a green and a pink filling are interpreted as due to an “ionic” iodine phase and CuI phase, respectively. E


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The Journal of Physical Chemistry C I 4d5/2) low-energy components (with a green filling) correspond to the “ionic” iodine phase, and the high-energy components (Ebind = 619.4 and 49.6 eV, respectively; shown with a pink filling) are related to initial CuI on the graphene surface. One can see from the figure that, after annealing at 180 °C, the high-energy component has a noticeably lower intensity in the I 3d5/2 spectrum in comparison with the I 4d3/2,5/2 spectrum. Considering the higher surface sensitivity of the I 4d3/2,5/2 XPS spectra compared to the I 3d5/2 spectra, the given result allows us to conclude that the “ionic” iodine is placed under the CuI layer. This is in agreement with the assumption about intercalation of “ionic” iodine under graphene while retaining residues of CuI on the graphene surface. Finally, the structure of the intercalated “Cu+I” layer shall be discussed. First of all, any noticeable amount of intact CuI in the sample after the last annealing can be ruled out, because all the Cu and I spectra (XPS as well as Auger) can be reasonably explained for decomposition products only (namely, metallic Cu and “ionic” iodine). Furthermore, one can clearly see from the Cu and I Auger spectra (Figure 5) that the total intensities of the spectra are significantly reduced for MG/Cu+I/Ni(111) as compared to those for initial CuI/MG/Ni(111). Taking into account relatively large mean-free path for corresponding Auger electrons (10−15 Å),22 the amount of Cu and I atoms in the MG/Cu+I/Ni(111) system must be reduced after intercalation. The thickness of the intercalated layer composed of Cu and I atoms can be estimated as 4−5 Å from the attenuation of the Ni 2p photoemission lines for MG/Cu+I/Ni(111) as compared to their intensities for MG/Ni(111) (not shown). The much smaller thickness of the intercalated layer in comparison with the initial CuI overlayer (20 Å) should mostly be attributed to the partial CuI desorption and the molecular iodine I2 desorption upon annealing. The above-mentioned reduction of the ratio between total intensities of the I 3d5/2 and Cu 2p3/2 XPS lines from 1.75 for the CuI/MG/Ni(111) system to about 1.15 for MG/Cu+I/Ni(111) implies that the I:Cu atomic ratio is close to 0.65 in the intercalated layer (or the number of iodine atoms is 39% of total number of intercalated atoms). The Cu phase is likely to consist of Cu islands to account for the metallic-like absorption edge in the Cu 2p3/2 NEXAFS spectrum of the MG/Cu+I/Ni(111) system (Figure 4). Taking into account the rather large ionic radius of the iodine atom (2.06 Å)47 and that one atomic layer of Cu atoms has a thickness of about 2.09 Å (interlayer distance between (111) atomic planes in metallic Cu), we come to the conclusion that a rather large percentage of iodine ions should be spatially separated from the Cu metallic phase to accommodate in the estimated 4−5 Å thickness of intercalated layer. However, since the iodine Auger spectra for MG/Cu+I/Ni(111) look very similar to those for the iodine layer chemisorbed on the Cu(111) surface,31 the presence of areas of intercalated Cu layers with iodine ions on top cannot be excluded. Clearly, this issue cannot be resolved by spectroscopic techniques only, and additional studies involving microscopy techniques are required. Graphene Hole Doping by CuI. As mentioned above, the BE of the C 1s electrons in graphene is considerably reduced upon intercalation (from 284.9 eV for MG/Ni(111) to 284.2 eV for MG/Cu+I/Ni(111)). Interestingly, the BE of the C 1s electrons in graphene on Ni(111), intercalated with copper atoms only, is larger, 284.65 eV.48 A similar value for the BE of C 1s electrons in graphene, grown on copper films, can be

extracted from ref 49. This implies that there is an additional shift of the C 1s level to lower BE in our MG/Cu+I/Ni(111) system relative to the reference MG/Cu/Ni(111) system. This may be explained by a Fermi-level lowering in graphene due to the electron charge transfer from carbon to the iodine atoms, which are formed as a result of CuI decomposition. The Fermilevel lowering in graphene for our system is further supported by the lower value of C 1s electrons as compared to that in graphite (284.5 eV).30 In the framework of a rigid band model13,49 the Fermi-level lowering of graphene results in a decrease of the C 1s BE. In this connection the observed formation of “ionic” iodine in the system can be regarded as a consequence of this charge transfer. The assumption about p-doping of graphene by iodine atoms in the MG/Cu+I/Ni(111) system is directly proven by considering the angle-resolved valence band photoemission (VB PES) and the C 1s NEXAFS spectra. The VB PES and the C 1s NEXAFS spectra recorded in the normal emission geometry for the pristine MG/Ni(111) system and the CuI/ MG/Ni(111) system after a step-by-step annealing are presented in Figures 7 and 8, respectively. All the spectra were normalized to the intensity of the incident radiation. In the VB PES spectrum of the initial MG/Ni(111) system the signals from the graphene π and Ni 3d states are dominant (Figure 7a). The former are situated at a BE of ∼10.1 eV. This is considerably larger than in graphite (∼8.0 eV)30,39 due to the strong C 2pzπ−Ni 3dπ-bonding with the substrate.7,39,48,50 The Ni 3d states are located in the BE range of 0−3 eV. The C 1s NEXAFS spectra of MG/Ni(111) (Figure 8a) are characterized by two low-energy bands A and A″ at hν ≈ 285 eV and ∼287 eV and by a broad band B″−C″.51 The bands A and A″ are attributed to the transitions of C 1s core electrons to unoccupied C 2pzπ states of graphene, strongly bonded with the Ni substrate, while the band B″−C″ is associated with

Figure 7. Angle resolved VB photoemission spectra (hν = 75 eV), recorded in the normal emission mode for (a) the initial MG/Ni(111) system, (b) the CuI/MG/Ni(111) system (“as depos”), (c) after annealing at 180 °C, and (d) after the final annealing at 200 °C. F


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high-energy side of the new band. The appearance of this new band of graphene π-states indicates the formation of graphene areas which are weakly bonded to Ni(111). This provides evidence for the onset of an intercalation process and, as a consequence, for the breaking of the strong Ni 3dπ−C2pzπ bonding.48,50,55 Clearly, the increase of the Ni 3d photoemission signal along with the decrease of the signal from CuI indicates a partial desorption of CuI from the MG/Ni(111) surface at these annealing stages. In the corresponding C 1s NEXAFS spectra (Figure 8 (c)) strong changes are also observed. The total intensity of the spectra increases in accordance with the decrease of the CuI amount on top of graphene. Further, the relative intensity of band A increases, while band A″, characteristic of strongly bonded graphene on Ni(111), decreases in intensity. An additional shoulder A′ emerges on the low-energy side of the πresonance A. A new band B appears at hν ≈ 292 eV and the σband B″ (hν ≈ 291 eV) decreases in relative intensity. Based on the direct comparison with the spectrum of the initial graphene, it is clear that the shoulder B″ reflects the presence of small residues of the MG/Ni(111) phase. It is important to note that in the VB PES spectrum (Figure 7c) a new peak appears at an energy of ∼2.65 eV. It coincides in energy with the Cu 3d states of intercalated copper (∼2.6 eV) for the MG/Cu/Ni(111) system.50 This observation correlates well with our previous conclusion about CuI decomposition at this temperature and the intercalation of copper and iodine atoms under graphene. Furthermore, this observation may also be regarded as evidence for the lack of significant interaction between the intercalated Cu and I atoms. Otherwise, a difference in energy position of the Cu 3d states in the MG/Cu+I/Ni(111) system relative to the reference MG/ Cu/Ni(111) system would be observed. As seen from Figures 7d and 8d, the VB PES and the C 1s NEXAFS from the CuI/MG/Ni(111) system after annealing at 200 °C clearly demonstrate a nearly complete decoupling of graphene from Ni. Indeed, the absorption spectrum becomes very similar to that of graphite.7 The low-energy shoulder B″ (hν ≈ 291 eV) at the σ-resonance B of intercalated graphene (hν ≈ 292 eV), attributed to the initial MG/Ni(111) phase and which was still visible in the spectrum at the previous annealing step (Figure 8c), is now absent. This is due to the formation of graphene, which is fully intercalated by copper and iodine atoms. The main difference from the absorption spectrum of graphite is the appearance of an additional weak shoulder A′ at the low-energy side of the π-resonance A. The observation of A′ is an important evidence for p-doping of graphene. The formation of a similar low-energy shoulder in the C 1s NEXAFS spectrum is usually associated with empty C 2pzπ states of graphene appearing as a result of partial transfer of the C 2pzπ valence electron density onto the halogen atoms.14,56,57 The reason for this in the present case is a charge transfer from the carbon atoms of graphene to iodine atoms, formed as a result of CuI decomposition. Another experimental confirmation of the carbon-to-iodine charge transfer is a shift of the atomic-like band C in the VB PES spectra (Figure 7d), previously attributed to photoemission mainly from the I 5p states of CuI, by about 0.3 eV to lower binding energies relative to its position in CuI (Ebind ≈ 4.5 eV). The lower BE of the I 5p electrons in the MG/Cu+I/ Ni(111) system is a consequence of the increased electron density on the iodine ion. Note that the band C, located in the range of 4−5 eV cannot be related to photoemission from Cu

Figure 8. C 1s NEXAFS spectra, recorded in the normal emission geometry for (a) the initial MG/Ni(111) system, (b) the CuI/MG/ Ni(111) system (multiplied by a factor of 5), (c) after annealing at 180 °C, and (d) the final annealing at 200 °C.

electron transitions to unoccupied states of σ-symmetry.7,42,51 The band A″ and the blurred structure B″−C″ reflect the presence of strong chemical bonding between the graphene monolayer and the Ni(111) substrate. A deposition of 20 Å, CuI on graphene results in an almost total disappearance of the photoelectron signal from the πstates of graphene and a strong weakening of the Ni 3d emission: only a weak Fermi edge is observed (Figure 7b). On the other hand, new intense bands A−D show up in the BE range of 1−6 eV. These are related to valence electron states of cuprous iodide, which have Cu 3d−I 5p hybridized character.28,52−54 The bands A and D correspond to Cu 3d−I 5p mixed states with the main contribution from the Cu 3dt2 and I 5pt2 orbitals, respectively. The bands B and C are atomiclike Cu 3de and I 5pt1,e states, respectively. The shape of the C 1s NEXAFS spectrum does not change after the CuI deposition. However, its intensity is significantly reduced due to the attenuation by the CuI overlayer (Figure 8b). The conservation of the C 1s NEXAFS shape is again reflecting the chemical inertness of CuI in contact with graphene. An annealing of the CuI/MG/Ni(111) system below 160 °C does not change the VB photoemission or the C 1s absorption spectra considerably (not shown). Only a small high-energy shift of about 0.1 eV for all cuprous iodide states is observed in the valence band spectrum, which can be assigned to the structural ordering in the CuI film on graphene, similarly to the shifts of the I 3d5/2 and Cu 2p3/2 XPS lines (Figure 3a,b). Upon annealing at 180 °C and above, the decomposition of CuI results in noticeable changes in all spectra. The annealing is accompanied by a significant decrease in intensity of all CuI bands A−D in the VB PES spectrum, an emergence of a new band of graphene π-states at a BE of 8.2 eV and an increase of the Ni 3d photoemission signal in the vicinity of the Fermi level (Figure 7c). The graphene π-states from the initial MG/ Ni(111) system are now visible as a broad shoulder on the G


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consider the concentrations of Cu and I atoms in the system and account for the charge-transfer per Cu and I atom, which are unknown quantities for that particular system. Besides, one should take into account the possibility of a chemical interaction between the intercalated Cu and I atoms as well as the likely band gap opening in graphene, which cannot be ruled out completely based only on the analysis of the experimental data obtained in the present work. As a first approximation, for the resulting Fermi-level lowering in graphene for the MG/Cu+I/Ni(111) system we may take the value of 0.3 eV for the shift of the C 1s core-level (Ebind = 284.2 eV) to the lower BEs relative to that for graphite (Ebind = 284.5 eV), obtained under similar experimental conditions, see ref 30. If we assume no differences between the band structure of graphene in MG/Cu+I/Ni(111) and that of free-standing graphene, the corresponding net hole concentration nnet h in graphene can be calculated using the formula:60

3d states from the intercalated Cu atoms, since these are located between 2 and 4 eV.50 In the VB PES spectrum (Figure 7d) only one low-energy peak from graphene π-states is present now, with the same total intensity as for the initial graphene on Ni(111) (Figure 7a). This provides additional evidence for a complete intercalation of the CuI decomposition products under graphene. The Ni 3d signal as compared to the one from the MG/Ni(111) reference system (Figure 7a) is not fully recovered which also supports the fact that there is an intercalation of CuI decomposition products into the space between graphene and the nickel substrate. As stated above, graphene in the MG/Cu+I/Ni(111) system is weakly bonded to Ni(111). Therefore, the electronic band structure of MG in this system may be approximated by that of nearly free-standing graphene,3 with the band occupation modified by charge transfer between C atoms and intercalated Cu and I atoms. Note that the low-energy shift of the graphene π-states in the case of MG/Cu+I/Ni(111) is 1.9 eV, while it is only 1.5 eV for the case of pure Cu intercalation.48,50 In the framework of the rigid band model13,49 this larger shift in our system can be assigned to the p-doping of graphene by iodine atoms. Indeed, the doping should lead to a Fermi level lowering for graphene in MG/Cu+I/Ni(111) in comparison with MG/ Cu/Ni(111) resulting in a lower BE for the graphene π-states. The amount of p-doping by iodine can be estimated from the difference in shift of the π-states for the systems without and with additional iodine, i.e. 1.9 − 1.5 = 0.4 eV. Here it should be noted that this estimation is justified by the applicability of the rigid band model to the reference MG/Cu/Ni(111) system, where the graphene layer is also considered to interact only weakly with the substrate.10,48 Nearly the same value (0.45 eV) for the Fermi level lowering of graphene due to iodine doping is obtained from the difference of C 1s binding energies for the MG/Cu+I/Ni(111) system (∼284.2 eV) and for the MG/Cu/Ni(111) system (∼284.65 eV).48 This fact supports the applicability of the rigid band model for the description of the shifts of the C 1s core level and the valence graphene π-states in the MG/Cu+I/ Ni(111) system, justifying the analysis above. Thus, the shifts to lower energies of the graphene π-states and the C 1s line (0.4 and 0.45 eV, respectively), observed in the spectra of the MG/Cu+I/Ni(111) system in comparison with the MG/Cu/Ni(111) system, are related to the graphene Fermi level lowering due to p-doping by iodine atoms. Here, it should also be noted that graphene on Ni(111) intercalated by Cu atoms10 or graphene grown on a Cu(111) single crystal11 and Cu foils49,58 is commonly considered to be n-doped by Cu atoms. The n-doping in this case is manifested in an upward shift of the Fermi level relative to the Dirac point of graphene by up to ∼0.3 eV, although a small band gap-opening has also been reported.10,11,59 The lower absolute value of the latter shift in comparison with the 0.4 eV shift of the graphene valence states due to p-doping by iodine atoms should result in a pdoped graphene layer in the MG/Cu+I/Ni(111) system which is explicitly confirmed by the observation of the shoulder A′ in the C 1s absorption spectrum (Figure 8d). However, it should be noted that the resulting Fermi-level lowering in graphene relative to the Dirac point (if we assume that there is no gapopening in the graphene band structure in the MG/Cu+I/ Ni(111) system) cannot be evaluated by a simple difference of the shifts due to n- and p-doping by Cu and I atoms, respectively. To make it in a more correct way one should

n(E F − E D) =

1 (|E F − E D|)2 π (ℏυF)

(1)

where υF ≈ 106 m/s is the Fermi velocity of graphene electrons and EF and ED are the energy position of the Fermi level and Dirac point, correspondingly. This yields the value of nnet h ≈ 6.6 × 1012 cm−2, which is smaller than those for F- and Cl-doped graphene (nh ≈ 4.5 × 1013 cm−2 and nh ≈ 3 × 1013 cm−2, respectively)14,16 and comparable to that for Br-doped graphene (nh ≈ 9 × 1012 cm−2).15 It is worth noting that the obtained values for the Fermi level lowering of graphene by iodine atoms are in agreement with recent results on the iodine doping of graphene on a SiO2 substrate obtained by means of Raman spectroscopy.61 The authors of this work estimated the graphene Fermi level lowering to be 0.43 eV. In addition, ref 61 has generally confirmed the results of many other studies devoted to iodine interaction with graphite13,62 and carbon nanostructures such as fullerenes63−65 and carbon nanotubes,46,66−73 where it is stated that the main p-doping species at iodination are actually the short polyiodides I3− and I5−. The latter are the energetically favorable associations of the iodine anion I− with one and two I2 molecules, respectively.74 For our system MG/Cu+I/ Ni(111), we observe only a single component in the I 3d XPS spectrum at a BE of 619.05 eV, which is close to the value of 619.1 eV obtained for I3− polyiodide in the SWCNTs.46

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CONCLUSIONS We have studied the evolution of a thin CuI layer on MG/ Ni(111) under annealing up to 200 °C using various X-ray spectroscopies (XPS, VB PES, NEXAFS, and AES) and lowenergy electron diffraction. The analysis of the Cu 2p and C 1s NEXAFS spectra, the C 1s, Cu 2p3/2, I 3d, 4d XPS and VB PES spectra as well as the LEED patterns leads to the conclusion that deposition of 20 Å CuI and postannealing at temperatures up to 160 °C first lead to structural ordering of the CuI overlayer on MG/Ni(111) with the (√3 × √3) R30° structure. At an annealing temperature of 180 °C the CuI layer starts to decompose and partially desorb. The copper and iodine atoms produced during the decomposition are intercalated under the graphene layer. Postannealing of the system at 200 °C results in a graphene layer which is almost fully decoupled from the Ni(111) substrate by the intercalated copper and iodine atoms. H


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Moreover, the graphene layer is p-doped due to the intercalated iodine. The corresponding net hole concentration, induced by the intercalated iodine atoms, was estimated to be nnet h ≈ 6.6 × 1012 cm−2. As a general conclusion, it has been demonstrated that CuI can be effectively used for the hole doping in graphene.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address

# Max IV Laboratory, Lund University, Box 118, 22100 Lund, Sweden.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by St. Petersburg State University (a research Grant No. 11.38.638.2013), the Russian Foundation for Basic Research (Project Nos. 12-02-00999 and 15-0206369), the bilateral Program Russian-German Laboratory at BESSY, and the Swedish Research Council. A.V.G. is grateful to the German Research Foundation (Project DE1679/2-1). N.A.V. gratefully acknowledges the financial support from KAW Foundation.

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