Lifting Epitaxial Graphene by Intercalation of Alkali Metals - The

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Lifting Epitaxial Graphene by Intercalation of Alkali Metals Caio C. Silva, Jiaqi Cai, Wouter Jolie, Daniela Dombrowski, Ferdinand H. Farwick zum Hagen, Antonio J Martínez-Galera, Christoph Schlueter, Tien-Lin Lee, and Carsten Busse J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b02442 • Publication Date (Web): 08 May 2019 Downloaded from http://pubs.acs.org on May 8, 2019

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The Journal of Physical Chemistry

Lifting Epitaxial Graphene by Intercalation of Alkali Metals Caio C. Silva,∗,† Jiaqi Cai,† Wouter Jolie,‡ Daniela Dombrowski,† Ferdinand H. Farwick zum Hagen,‡ Antonio J. Mart´ınez-Galera,‡ Christoph Schlueter,¶ Tien-Lin Lee,¶ and Carsten Busse§,†,‡ †Institut f¨ ur Materialphysik, Westf¨ alische Wilhelms-Universität M¨ unster, Wilhelm-Klemm-Straße 10, 48149 M¨ unster, Germany ‡II. Physikalisches Institut, Universität zu Köln, Z¨ ulpicher Straße 77, 50937 Köln, Germany ¶Diamond Light Source, Didcot OX11 0DE, U.K. §Department Physik, Universität Siegen, Walter-Flex-Straße 3, 57068 Siegen, Germany E-mail: [email protected] Phone: +49 (0)251 8339014

Abstract We study the effect of alkali metal intercalation (Cs and Li) on the geometry of graphene on Ir(111) using the x-ray standing waves technique. For both alkali metals, the increase in the mean height of the carbon layer does not depend on the lateral structure or the density of the intercalated layer. For Li, full delamination of graphene from the metal substrate is found already for a small amount of intercalant. Even though Lithium lifts graphene to a smaller height, it is much more efficient in ironing out the corrugation of pristine graphene on Ir(111).

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Introduction As the number, quality, and complexity of two-dimensional materials (2DMs) prepared by epitaxial growth is constantly increasing, a lot of effort is spend on the modification of the ultrathin sheets after growth. Here, a versatile tool is intercalation, which is the insertion of atoms or molecules between the 2DM and its substrate. On the one hand, this creates a buffer layer by suppressing unwanted interaction with the support, making the film quasifreestanding. 1–5 On the other hand, the intercalant can tailor the electronic, 6 magnetic, 7 and vibrational 8 properties of the 2DM. Regarding graphene (gr), alkali metals are especially useful intercalants as can be exemplified for graphene grown on Ir(111). 9,10 For the pristine system covalent-like carbon-metal bonds are present which induce a moiré superstructure in graphene on the lattice-mismatched substrate. 11 This induces a superperiodic potential, evidenced by the formation of replica cones and mini gaps in graphene’s band structure. 12 Intercalation of Cs and Li breaks these bonds and thereby lifts the superperiodic potential. 13,14 The alkali metals readily donate their single valence electron which is partially transferred to the 2DM and leads to strong doping. However, their orbitals do not overlap significantly with those of the atoms forming the ultrathin sheet and hence no mixing of bands is induced. The effect of alkali intercalation on the π-bands can thus be approximated as a rigid shift 13,15–17 with respect to the freestanding case. Specifically, there is no band gap at the Dirac point. The doping has a strong impact on graphene’s properties. For example, it enables to access the strongly trigonally warped region in the graphene band structure by scanning tunneling spectroscopy, 17 and increases the binding energy of adsorbed naphthalene. 18 In this study, we provide precise structural information on gr/Cs/Ir(111) and gr/Li/Ir(111) in order to understand the mechanisms underlying delamination and doping of graphene. The dependency of the structural decoupling on the amount of intercalant is investigated by means of low-energy electron diffraction (LEED), x-ray photoelectron spectroscopy (XPS), and x-ray standing waves (XSW). 2


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Experimental Details The experiments were carried out at the beamline I09 of the Diamond Light Source (Didcot, UK) in ultra-high vacuum (UHV) (base pressure 2 × 10−10 mbar). A clean surface of Ir(111) was prepared through cycles of ion sputtering and flash-annealing up to 1250 ◦C. This procedure leads to an ordered surface with no contamination as attested by LEED and XPS. A monolayer of graphene was grown by temperature programmed growth followed by chemical vapor deposition: 10 First, the substrate was exposed to 10−7 mbar of ethylene (C2 H4 ) for 100 s at room temperature and subsequently flash-annealed up to 1250 ◦C in UHV. Next, the sample was exposed to 10−7 mbar of C2 H4 at 1000 ◦C for 100 s. The quality of graphene is attested by the moiré superstructure in the LEED pattern. We deposited Cs and Li at room temperature (where both readily intercalate) from alkali metal dispensers until saturation was observed in the LEED pattern. Lower coverages were obtained by subsequent heating steps that remove the intercalant, most probably by desorption. 19 The intercalated systems gr/Cs/Ir(111) and gr/Li/Ir(111) were characterized by LEED, XPS (hν = 2.800 keV for Cs and hν = 0.400 keV for Li), and XSW (EBragg = 2.795 keV). We used a hemispherical electron analyzer (VG Scienta EW4000) with an angular acceptance of 56°. For the photoemission spectra, all components were fitted using the Doniach-Sunjic line shape 20 convolved with a Gaussian function after a Shirley background subtraction. 21 Errors in the binding energy determination were estimated as ∆E = ±0.02 eV. For the corelevels with multiple components, the resulting binding energy differences between individual components and the intensity ratios were fixed in propagating the peak fitting through the photon energy steps in the XSW analysis. The method of XSW 22–24 provides a precise determination of the distance between the adsorbate and the substrate surface and is especially fruitful for epitaxially grown twodimensional materials. 11,25–31 An x-ray standing wave can be formed by coherent superposition of an incoming and a reflected wave at a Bragg condition. Scanning the photon energy 3



through the Bragg reflection shifts the phase of the XSW field and thus alters the spatial variation of the electric field intensity between two Bragg planes. The photoelectron yields of an adsorbed atom induced by such an XSW field exhibit a characteristic photon energy dependence dictated by the adsorption height. In the present work we determine the heights of graphene and intercalated Cs with respect to the ideal lattice termination of the Ir(111) surface by analyzing the photoelectron yields of the core-levels C 1s and Cs 3d3/2 modulated by the Ir(111) XSW as functions of photon energy. Due to the low photoionization cross section of Li 1s at the Bragg energy of 2.795 keV, the height of the intercalated Li cannot be determined. Two structural parameters derive from the fitting of each photoelectron yield curve, namely, the coherent position P H and the coherent fraction f H . P H is the average height of all atoms of the analyzed species in units of the Bragg plane spacing and f H indicates the width of the distribution of the atoms around this average. The coherent fraction can be written as f H = CaH DH , where 1 − C is the fraction of randomly distributed impurities, aH is the geometric factor describing the ordered part, and DH is the Debye-Waller-factor describing thermal vibrations perpendicular to the nodal planes of the XSW. 32 For a completely ordered, perfectly flat layer at 0 K one would find f H = 1. The average height of the species is then given by h = P H + n × dIr(111) , where dIr(111) is the Bragg plane spacing of Ir(111) (dIr(111) = 2.2167 ˚ A). 33 The integer n stems from the fact that the XSW field has the periodicity of dIr(111) , leading to a modulo-dIr(111) ambiguity for the determination of heights using XSW. The value of n can be easily determined from complementary experimental or theoretical results or simply using the corresponding radii of the atoms (atomic, ionic, van der Waals). The XSW analysis was performed using a self-written script based on dynamical diffraction theory of x rays. Non-dipolar effects in the angular distribution of photoelectron emission were taken into account, see. 31,34 Errors were estimated by the method of Mercurio et

. al 35 resulting in ∆P H = ±0.02, ∆f H = ±0.04 and ∆h = ±0.03 A 4


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Results The experimental results for gr/Cs/Ir(111) are summarized in Tab. 1 and Fig. 1. Fig. 1a shows the photoemission spectra of Cs 3d3/2 , Fig. 1b presents the LEED patterns related to different amounts of intercalated Cs, and Fig. 1c shows the photoemission spectra of C 1s. Table 1: Summary of experimental results for Cs-intercalated gr/Ir(111). Θ: Coverage of Cs (in ML, see text), Egr , Egr/Cs , ECs : binding energy of main component (in eV) for the three species pristine graphene (Egr ), intercalated graphene (Egr/Cs , component with higher binding energy) and Cs (ECs ) as determined by XPS, Igr , Igr/Cs , ICs : integral intensity of the photoemission peak (for graphene relative to the sum of both species, for Cs relative H H H H H to the intensity for Θ = 0.93 ML), PgrH , Pgr/Cs , PCs and fgr , fgr/Cs , fCs : structural parameters determined by XSW, hgr , hgr/Cs , hCs : height of species (in ˚ A) with respect to the substrate surface. Structural parameters for Θ = 0 ML are taken from Ref. 11 Errors: ∆E = ±0.02 eV, ∆P H = ±0.02, ∆f H = ±0.04, ∆h = ±0.03A Θ 0.00 0.24 0.86 0.93 1.00

Egr 284.10 284.12 284.09 -

C 1s (pristine gr) H Igr PgrH fgr 1.00 0.52 0.74 0.64 0.59 0.68 0.06 0.58 0.71 -

hgr 3.38 3.52 3.50 -

Egr/Cs 284.85 284.88 284.87 284.88

C 1s (intercalated gr) H H Igr/Cs Pgr/Cs fgr/Cs 0.00 0.36 0.92 0.52 0.94 0.94 0.78 1.00 0.93 0.69 1.00 0.93 0.77

hgr/Cs 6.47 6.51 6.50 6.50

Cs 3d3/2 (intercalated Cs) H H ECs ICs PCs fCs hCs 0.00 725.28 0.26 0.47 0.55 3.26 725.27 0.92 0.46 0.68 3.24 725.29 1.00 0.48 0.60 3.28 725.28 0.72 0.47 0.68 3.26

Figure 1: Evolution of the intercalated system gr/Cs/Ir(111) for different amounts of Cs (coverage Θ indicated in the subfigures). a) Stacked Cs 3d3/2 core-level emission spectra measured with hν = 2.800 keV. b) LEED patterns (inverted contrast). The red lines and circles √ indicate √ the superstructure of Cs (p(2 × 2) with respect to graphene for 0.86 ML and p( 3 × 3)R30° with respect to Ir(111) for 1.00 ML). The blue circles mark zoomed-in images of the diffraction peaks. c) Stacked C 1s core-level emission spectra (hν = 2.800 keV). The LEED of gr/Ir(111) (0 ML) exhibits the characteristic moiré superstructure. The 5



photoemission spectrum of C 1s shows a narrow, slightly asymmetric peak at a binding energy of 284.10 eV, which is typical for gr/Ir(111), 36 while no Cs is detected in Fig. 1a. With increasing amount of intercalated Cs the C 1s peak characteristic for pristine gr/Ir(111) shrinks and then vanishes, while a new, asymmetrically broadened peak with a shoulder at high binding energies attributed to intercalated graphene grows at a higher binding energy. At the same time, a single component of Cs 3d3/2 with increasing intensity is observed. At the highest coverage, a new Cs component appears on the lower binding energy side as a shoulder, which we identify as Cs adsorbed on top of gr/Cs/Ir(111) (see below). In LEED the intensity of the moiré spots gradually diminishes and new spots corresponding to Cs superstructures appear. Guided by previous studies 13 we identify them as √ √ a p(2 × 2) structure with respect to graphene ((2 × 2)gr in short) and a p( 3 × 3)R30° √ √ structure with respect to Ir(111) ( 3Ir in short). For a surface fully covered by 3Ir we define the Cs coverage as Θ√3Ir = 1 ML. In consequence, for a full (2 × 2)gr we have √

Θ(2×2)gr =

√ 3× 3 2×2

×

a2Ir a2gr

˚ 33 ML = 0.92 ML, where the lattice constants are aIr = 2.715 A

and agr = 2.452 ˚ A. 9 We determine the coverage Θ of intercalated Cs using the integrated intensity ICs of the corresponding peak in XPS. Until adsorption of Cs on top of graphene sets in (which attenuates the photoelectrons emitted from intercalated Cs), we expect Θ = αICs , where the proportionality constant α has to be determined experimentally. However, this determination √ is not straightforward: When we observe 3Ir in LEED, adsorbed Cs is already present. The system with the (2 × 2)gr pattern is not phase-pure as we observe both pristine and intercalated graphene in XPS. For the intermediate coverage, we find a mixture of the (2×2)gr √ and the 3Ir phase. We solve this problem using the following model for the coverage leading to the the (2 × 2)gr pattern: The ratio of the pristine and intercalated graphene areas is given by the ratio of the intensities of the corresponding XPS signals Igr and Igr/Cs . Pristine graphene is free of Cs (Θgr = 0) whereas for the intercalated graphene a Cs coverage of Θ(2×2)gr = 0.92 ML is assumed. The total coverage is then obtained by:

6


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Θ=

Igr × Θgr + Igr/Cs × Θ(2×2)gr = 0.86. Igr + Igr/Cs

By this we can derive α =

Θ ICs

(1)

= 0.93 and thus determine Θ for all other experiments

with the exception of the highest coverage due to the aforementioned attenuation. However, in this case saturation of the intercalated layer is obviously reached and thus we assign Θ = 1 ML. An interesting detail is that for 0.24 ML, the Cs coverage of the intercalated area is found as Θinter = Θ/Igr/Cs = 0.67 ML, i.e. lower than for the (2 × 2)gr . It seems that for low coverage another Cs phase with reduced density exists. Tentatively, one could propose a (3 × 3)Ir -phase with two Cs per unit cell which would indeed lead to Θ = 0.67 ML. However, there is no direct evidence for such a phase as of yet. Fig. 2 shows the results for 0.86 ML as an example for coexistence between pristine gr/Ir(111) and (2 × 2)gr . The C 1s signal in XPS (see Fig. 2a) is fitted by three components: Two marked in blue with binding energies of 285.63 eV and 284.88 eV are assigned to (2×2)gr and one in olive at 284.10 eV corresponds to pristine graphene.

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Figure 2: Gr/Ir(111) intercalated by 0.86 ML of Cs. a) Photoemission spectrum of C 1s measured with hν = 2.800 keV. Circles: Data points after background subtraction. Colored areas: Fits of the indicated components. Solid black line: Sum of fits. b) XSW analysis for intercalated (blue) and pristine (olive) graphene showing the variation in total photoelectron yield (black line) as function of the photon energy scan along the reflectivity curve (red line). The values of coherent position (P H ) and coherent fraction (f H ) are shown to each species analysis. c) Photoemission spectrum of Cs 3d3/2 measured with hν = 2.800 keV. Circles: Data points after background subtraction. Solid purple line: Fit. d) XSW analysis for the Cs 3d3/2 peak.

In Fig. 2b the photoemission yield of C1s and the reflectivity of Ir(111) are plotted as functions of photon energy. In a preliminary XSW analysis (not shown) we evaluated the two blue components separately. However, identical structural parameters were found, indicating that the intercalated graphene contains just one chemical species. In consequence, we used H the sum of both components in further analysis. Based on the values of Pgr/Cs shown in

. This value Fig. 2b and Tab. 1, we determine the height of intercalated graphene as 6.50 A is independent of the amount of intercalated Cs, and, thus, is the same for (2 × 2)gr and

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√ which is slightly increased with respect to 3Ir . For pristine graphene we find hgr = 3.50 A gr/Ir(111) without the presence of Cs. 11 Fig. 2c shows the photoemission spectrum of Cs 3d3/2 which can be fitted by a single component with a binding energy of 725.27 eV. The XSW analysis for this coverage (Fig. 2d) combined with the full range of coverages studied (Tab. 1) indicates that the height of

. As the height of intercalated graphene, this value intercalated Cs is approximately 3.26 A does (within error bars) not depend on the Cs coverage, and, thus, the superstructure. For the fully saturated sample we observe an additional component in the Cs 3d3/2 signal (724.30 eV) that we attribute to Cs atoms adsorbed on top of graphene. A similar shift towards lower binding energy was also seen for Cs on top of epitaxial hBN/Ir(111). 34 XSW

. analysis yields P H = 0.63 corresponding to a height of 10.26 A Similar to the case of intercalation with Cs, we summarize the main findings on gr/Li/Ir(111) in Tab. 2 and Fig. 3. Fig. 3a shows the photoemission spectra of Li 1s, Fig. 3b presents the LEED patterns for different amounts of Li intercalated between the graphene layer and the substrate, and Fig. 3c shows the photoemission spectra of C 1s . The coverage of Li is estimated from the area of the Li 1s peak in Fig. 3a.

Figure 3: Evolution of the intercalated system gr/Li/Ir(111) for different amounts of Li (coverage Θ indicated in the subfigures). a) Stacked Li 1s core-level emission spectra measured with hν = 0.150 keV. b) LEED patterns (inverted contrast). c) Stacked C 1s core-level emission spectra measured with hν = 0.400 keV. 9



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Table 2: Summary of experimental results for Li-intercalated gr/Ir(111). Θ: Coverage of Li (in ML) as determined by the area of Li 1s, Egr/Li (component with higher binding energy), ELi : binding energy (in eV) of main component as determined by XPS, Igr/Li , ILi : relative integral intensity of component (relative to the full graphene signal for C 1s and relative to H H H H the saturated signal for Li), Pgr/Li , PLi and fgr/Li , fLi : structural parameters determined by XSW, hgr/Li , hLi : height of component (in ˚ A) with respect to the substrate surface. Errors: H H ∆E = ±0.02 eV, ∆P = ±0.02, ∆f = ±0.04, ∆h = ±0.03A Θ 0.00 0.10 0.15 0.28 0.40 0.50 0.80 1.00

Egr/Li 285.09 285.12 284.90 284.86 284.89 284.90 284.84

C 1s (intercalated gr) H H Igr/Li Pgr/Li fgr/Li 0.00 0.62 0.94 0.41 0.71 0.95 0.74 1.00 0.96 0.95 1.00 0.97 0.94 1.00 0.97 0.94 1.00 0.97 0.93 1.00 0.94 0.92

hgr/Li 4.30 4.32 4.35 4.37 4.37 4.37 4.30

Li ELi 55.53 55.46 55.49 55.45 55.42 55.35 55.65

1s ILi 0.10 0.15 0.28 0.40 0.50 0.80 1.00

Prior to the intercalation with Li, we observe a typical moiré pattern in LEED (Fig. 3b) and the C 1s core-level spectrum is characteristic for pristine gr/Ir(111). 36 Fig. 3a shows no sign of Li 1s before the intercalation. After Li deposition, the satellites spots and the spot connected to the Ir lattice present a decrease of intensity while the amount of intercalated Li increases. However, at 1 ML, the intensity of the diffraction spots at the Ir lattice positions increases due to the formation of the (1 × 1) phase. This behavior was seen before by Pervan et al. 14 C 1s spectra in Fig. 3c show that already at the smallest Li coverage no pristine graphene remains. From 0.28 ML onward, a line shape similar to the one found for gr/Cs/Ir(111) is ob-

independent served. C 1s analysis yields approximately a constant graphene height of 4.35 A of the amount of Li. A more complicated behavior is found for the lowest coverages (0.10 and 0.15 ML), where the C 1s peak is formed with additional components. Fig. 4a presents a line-shape analysis of the C 1s spectrum after intercalation of 0.1 ML of Li. The C 1s peak is fitted with four components centered at 285.72 eV (blue in Fig. 4a), 285.72 eV (blue), 284.75 eV (orange) and 10


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284.40 eV (green). Similar to gr/Cs/Ir(111), the two components depicted in blue exhibit an identical behavior in the XSW analysis and thus share the same adsorption height and belong to a single species. Therefore, in the following we use three species to describe the system that we label as I (two blue components), II (orange component) and III (green component). Fig. 4b shows the XSW analysis of the three species. We determine the carbon height to be hgrI /Li = 4.30 ˚ A, hgrII /Li = 4.08 ˚ A and hgrIII /Li = 3.39 ˚ A. At the Li coverage of 0.15 ML, the green component becomes significantly weaker while the orange one remains.

Figure 4: Gr/Ir(111) intercalated by 0.10 ML of Li. a) Photoemission spectrum of C 1s measured with hν = 0.400 keV. Circles: Data points after background subtraction. Colored areas: Fits of the indicated components. Solid black line: Sum of fits. b) XSW analysis for the blue, orange and green components showing the variation in total photoelectron yield (black line) and the reflectivity (red line) as function of the photon energy. The values of coherent position (P H ) and coherent fraction (f H ) are shown to each component analysis. 11



Discussion Our main experimental findings are: (i) The corrugation of graphene is only slightly reduced upon Cs intercalation. A fingerprint for the amplitude of the corrugation is the coherent H H fraction f H which only increases from fgr = 0.74 for pristine graphene to fgr/Cs = 0.77 for the

fully intercalated layer. The effect of Li (see below) is much stronger. (ii) The intercalated H Cs has a broad height distribution indicated by fCs = 0.68 for the fully intercalated layer,

which implies a width of the distribution similar to the one for corrugated, pristine graphene. This is unexpected for strongly bound atoms with defined lateral registry. (iii) The binding distance of graphene intercalated with Cs (hgr/Cs ≈ 6.50 ˚ A) does not strongly depend on the intercalated amount, and, in consequence, does not depend on the Cs superstructure (within the limits of experimental error). (iv) For the case of Li, already a small intercalated coverage leads to a fully decoupled graphene sheet. (v) The height of the intercalated graphene (hgr/Li ≈ 4.35 ˚ A) does not depend on Li coverage. (vi) Li irons out the corrugation H of graphene efficiently (fgr/Li = 0.92 for the fully intercalated layer). Summarizing the

structure of both gr/Cs/Ir(111) and gr/Li/Ir(111) after the full intercalation, Fig. 5 presents a model with the relative positions found in the XSW analysis.

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√ Figure 5: Schematic side view of gr/ 3Ir -Cs/Ir(111) (top panel), gr/(2 × 2)gr -Cs/Ir(111) (middle panel) and gr/Li/Ir(111) (bottom panel). We show a cut perpendicular to the surface in the [¯1¯12]-direction of Ir(111) which goes along the main diagonal of the simplified commensurate 10-on-9 unit cell (see 11 ). Only atoms intersected by this plane are shown. On the right, the main results for the heights of intercalated graphene and the intercalant (where possible) are indicated. The values are averaged over all relevant coverages. We do not include speculations on the remaining corrugation. Parameters hC , hhollow , htop , hhcp are indicated as defined in the text. The sphere sizes indicates the ionic radii of Cs and , purple and rion,Li = 0.90 A , brown 37), the atomic radii of C and Ir Li (rion,Cs = 1.74 A , black and ratomic,Ir = 1.35 A , grey 38) and the van der Waals radius of C (ratomic,C = 0.70 A 39 , dark grey ). (rvdW,C = 1.77 A

These observations can be explained in a model that takes the incommensurability between graphene and Ir(111) into account: The intercalated alkali atoms are frustrated as they cannot find the optimum binding position with respect to graphene and to the substrate simultaneously. To go into more detail: Different experiments agree that the threefold-hollow hcp-site is √ √ always favored for the ( 3× 3)R30◦ phase on the close packed surfaces of elements from the platinum group (for a review see Ref. 40 ), which is corroborated by a recent DFT calculation for Li/Ir(111) and Cs/Ir(111). 16 Hence, we assume that at this site a short vertical bond distance hhcp is found (all the parameters used in the following are depicted in Fig. 5). Referring to symmetry, we assume that adsorption on top of a metal atom is least favorable with htop > hhcp . For the case of free-standing graphene, DFT calculations 41 predict that 13



adsorption of an alkali atom in the hollow site (hhollow ) is more favorable than adsorption on top of a carbon atom (hC ). We thus assume hhollow < hC . Cs can form two densely-packed intercalation phases, the (2 × 2)gr aligned with the √ graphene lattice and the 3Ir aligned with Ir(111). This is depicted in the side view presented √ in Fig. 5 where for 3Ir (top panel) the Cs atoms are in registry with the Ir atoms, whereas for (2 × 2)gr (middle panel) they are in registry with the C atoms. In both phases, intercalated alkali atoms with minimum frustration (smallest possible vertical bond distance to both sides) and maximum frustration (shortest bond to one side, longest to the other) will be found. The minimally frustrated atoms determine the minimum height of graphene as hmin = hhcp + hhollow , whereas the maximally frustrated ones determine hmax,√3Ir = hhcp + htop or hmax,(2×2)gr = htop + hhollow . For all other registries, the graphene height will be found in between these values. This explains the low coherent fraction of the Cs-intercalated graphene. As a counterpart of the vertical relaxation of the carbon atoms, also the alkali atoms can gain binding energy by shifting slightly perpendicular to the substrate. For example, in the √ 3Ir -structure, Cs atoms can move upwards from the threefold-hollow site to improve their binding with graphene. The resulting vertical disorder shows as the low coherent fraction H . fCs

The finding that hgr/Cs does not depend on the superstructure implies hmax,(2×2)gr ≈ hmax,√3Ir (hmin is necessarily the same). This requires htop + hhollow ≈ hhollow + htop . The exact agreement that we find here may be partially fortuitous, but is shows that the inplane variation of the optimal vertical binding distance is a rather smooth function both for graphene an for Ir(111). More insight can be inferred from a comparison between fully intercalated gr/Cs/Ir(111) and Cs-intercalated hBN on Ir(111): 34 The hBN layer is slightly closer (hhBN/Cs = 6.44 ˚ A) to the substrate than graphene (hgr/Cs = 6.50 ˚ A). In contrast, the binding distance of Cs to

vs 3.26 A ), and the Cs the substrate is significantly smaller when it intercalates hBN (3.15 A 14


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H atoms are more aligned vertically (fCs = 0.93). We attribute this to the fact that there is

only weak binding of Cs to hBN (which has no states at the Fermi level and is presumably charge neutral) as compared to graphene (states at Fermi level, negatively charged). As a result, Cs always adopts the threefold-hollow hcp-site of Ir(111) surface when intercalating

hBN. For Cs adsorbed on top of the intercalated 2D-layer, similar values are found (9.52 A on top of hBN, 10.26 ˚ A on top of graphene). Like Cs, intercalation with Li also leads to a constant height of the carbon. The lower height of the delaminated graphene is as expected from the large size difference between Cs and Li. However, delamination of the entire graphene layer is completed at a much earlier stage, as shown in Fig. 6, where the fraction of lifted graphene is plotted versus the intercalant coverage for Li and Cs intercalation. We attribute this finding to the formation of a Li-adatom gas at room temperature with the atoms in continuous random motion, based on the previous observation that intercalated Li forms periodic structures at 6 K, 42 but not at room temperature 14 (for Θ < 1 ML), when intercalating graphene on Ir(111). The independence of hgr/Li on coverage in combination with the flatness of intercalated graphene indicates that the potential energy surface is even more smooth for Li than for Cs, which is also evidenced by the high mobility of Li at room temperature.

Figure 6: Fraction of lifted graphene as a function of the intercalant coverage for gr/Cs/Ir(111) (purple) and gr/Li/Ir(111) (wine). 15



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) is in agreement with the correThe height of graphene in gr/Li/Ir(111) for 1 ML (4.30 A 16 or hgr/Li = 4.24 A . 13 The discrepancies sponding values from DFT studies of hgr/Li = 4.32 A between the studies (and our experiment) are due to the different approaches to simplify the incommensurate gr/Ir(111) in order to make the calculations feasible. Regarding Cs, in the

for 1 ML Cs was found, 13 which study presented by Petrović et al., a value of hgr/Cs = 6.71 A is also in rough agreement with experiment. Despite partial agreement, it is obvious that a full theoretical study of gr/Li/Ir(111) would require the use of a realistically large unit cell. From the coherent fraction f H , upper limits for the height distribution of the carbon atoms in intercalated graphene can be derived. Starting from f H = CaH dH , we make the conservative assumption of C = 1. Using for simplicity a Gaussian distribution of heights 2π 2 σ 2 H . with a standard deviation σ, the coherent fraction can be expressed as f = exp − d2 Ir(111)

The upper limits for the width of the height distributions for the fully intercalated layers are

for Li and σ = (0.26 ± 0.03) A for Cs. then σ = (0.14 ± 0.04) A There are two contributions to the width of the height distribution, thermal broadening p 2 2 + σst . As (with standard deviation σth ) and the structural corrugation (σst ) with σ = σth the amplitude of the thermal vibrations of a graphene layer bounded to a substrate in the direction perpendicular to the plane is not known, we refer to our experiments to given an estimation: The highest coherent fraction we observe is f H = 0.95 for 0.28 ML of Li. Under the assumption that this value is caused by thermal vibrations only (σst = 0) we obtain

. This means that the the upper limit for the thermal disorder of σth,max = (0.11 ± 0.05) A thermal broadening is the main effect for a fully Li-intercalated layer, whereas for a fully Cs-intercalated layer thermal broadening is negligible compared with the corrugation. In a wide coverage range our photoemission spectra for gr/Li/Ir(111) are virtually identical to previous work. 15 However, for the lowest coverages, additional C 1s species are found. We attribute this to the different preparation procedures: We obtained the lowest coverages by heating a fully intercalated sample, whereas Schröder et al. deposited the final amounts at room temperature. We assume that in our approach, graphene relamination can take place

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(as found for Cs intercalation 13 ). This can trap Li atoms underneath the graphene, potentially in a stripe-like pattern as observed by Halle et al. 42 We tentatively attribute species II (orange component) in Fig. 4a) to these stripes, and species III (green component) to the relaminated graphene between the stripes. This could explain why the carbon atoms of species III reside at a height characteristic for pristine graphene, but show a small but significant shift in binding energy (300 meV) caused by doping due to the charge spill-out from the neighboring, intercalated region. Another possibility would be that Li moves to a subsurface position upon heating, so that the graphene is bound to Ir, but still obtains electrons form Li. Lastly, we discuss the changes intercalation induces in the C 1s photoemission peak which are (i) a shift to higher binding energy, (ii) an asymmetric broadening towards higher binding energies, and (iii) the emergence of a shoulder at higher binding energies (see also Ref. 15 for an in-depth discussion). The shift is a consequence of the n-type doping of graphene induced by the presence of Cs or Li atoms. The asymmetric broadening is caused by electron-hole pair formation and 2D plasmon losses as theoretically shown by Sernelius. 43 Both processes become more relevant when doping increases the density of states at the Fermi level. The shoulder could be tentatively attributed to a second species of carbon atoms, formed for example by C atoms closest to the intercalated ions that presumably receive more charge than others. This hypothesis can, however, be rejected by three arguments: (a) In our XSW analysis of the structural parameters, no differences between the main peak and the shoulder are found. (b) Breaking the symmetry between the carbon atoms should open up a band gap in graphene, in contrast to observations. (c) As explicitly shown for gr/Li/Ir(111), 15 the ratio of the peak areas for the main peak and the shoulder is not a simple function of Θ. In consequence, the shoulder cannot be due to a second chemical species, but must be caused in the photoemission process itself. Furthermore, as we only find it for C 1s, is must be an intrinsic effect related to the core hole. As a possible loss mechanism, Nagashima et al. 44 suggested interband transitions involving the alkali metal’s s-Orbital.

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Conclusions In summary, we find that for Cs as well as for Li intercalation the distance of the graphene sheet to the metal substrate does not depend on the structure of the alkali metals in between, albeit for different reasons: For Cs, it is the frustration of alkali atoms that can not optimize their registry with both Ir(111) and graphene at the same time, leading to a vertically disordered intercalated layer. For Li, it is the gas-like behavior of the inserted ions. The change in height of graphene (≈ 3 ˚ A for Cs, ≈ 1 ˚ A for Li) follows the size difference between the intercalants. Even though Li lifts graphene to a smaller height as compared with Cs, it irons out its corrugation much more effectively.

Acknowledgement We acknowledge support from DFG through project BU2197/4-1 (part of SPP 1459 ’Graphene’) and from the University of Cologne via the Advanced Postdoc Grant ’2D materials beyond graphene’ in the framework of the Excellence Initiative. We appreciate experimental support by the staff at I09. C.C.S. acknowledges funding via CAPES Foundation, Ministry of Education of Brazil.

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Lifting Epitaxial Graphene by Intercalation of Alkali Metals - The

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