Article pubs.acs.org/JPCC
Filling the Gap: Li-Intercalated Graphene on Ir(111) Johannes Halle,* Nicolas Néel, and Jörg Kröger Institut für Physik, Technische Universität Ilmenau, D-98693 Ilmenau, Germany ABSTRACT: Graphene on Ir(111) was intercalated by Li. The intercalant formed a variety of superstructures with increasing coverage. For all superstructures, the moiré pattern of pristine graphene on Ir(111) was discernible. At low coverage, the intercalant exhibited a disperse pattern of elementary assemblies that served as building blocks for Li-induced structures at higher coverage. In contrast to previous reports on other intercalants, Li intercalation gave rise to isotropic assemblies in the lowcoverage range. Pattern formation was qualitatively rationalized in terms of different energy costs and gains and a corresponding ranking of preferred intercalation regions. While for intermediate Li coverage an ordered intercalant superstructure was observed, higher coverage led to the decoration of step edges with adsorbed Li clusters.
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fied, which served as building blocks for intercalation structures at higher coverage. An ordered phase of the intercalant was reflected by the atomically resolved p(√3 × √3)R 30° superstructure at monolayer coverage. The influence of the graphene moiré lattice was clearly discernible for all superstructures. Based on the observed intercalant structures, a ranking of preferred intercalation regions was proposed. At elevated exposures, Li formed clusters adsorbed atop graphene in the vicinity of step edges.
INTRODUCTION Pristine graphene exhibits a wealth of intriguing properties that render the sp2-bonded two-dimensional lattice of C atoms interesting for fundamental research and applications alike. The exceptionally high mechanical strength,1 ballistic electron transport at room temperature,2,3 and presence of massless Dirac Fermions4 represent only a few of the appealing characteristics. Excellent review articles thoroughly summarize the hallmarks of pristine and adsorbed graphene.5−8 Chemical modifications of graphene open the alley for tailoring additional properties and for novel applications. Epitaxially grown graphene on surfaces led to site-specific chemical reactivity.9−12 Doping graphene with donors or acceptors is one possible route to controlled modification of the electronic structure.13,14 Local pn junctions were realized in that way.15 Intercalation, or the transfer of material between epitaxial graphene and a substrate, is an emerging procedure to engineer the bonding to the substrate16−18 or to open a band gap.19 Intercalation is likewise relevant to the use of graphene in spin-electronic devices. Ferromagnetic materials,20−22 alkali metals, alkaline earth metals, and lanthanides23 were employed to tune the magnetic properties of graphene. In this context, graphite intercalation compounds are noteworthy since addition of alkali metals or rare earths was reported to induce superconductivity.24 Lithium adsorbed to pristine, or freestanding, graphene has been predicted to become superconducting at ∼8 K.25 Here a low-temperature scanning tunneling microscope (STM) was used to unravel structural aspects of Li-intercalated graphene on Ir(111). The main impetus to our investigations was the small size of Li compared to the size of other intercalants reported so far. Deviations from already observed intercalation structures were therefore expected and awaited their experimental verification. At the lowest Li coverage, small and isotropically shaped intercalation assemblies were identi© XXXX American Chemical Society
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EXPERIMENTAL PROCEDURES Experiments were performed with an STM operated at 6 K and in ultrahigh vacuum (10−9 Pa). Clean Ir(111) was prepared by Ar+ bombardment and annealing. Each cycle was succeeded by O2 exposure at 10−5 Pa partial pressure and a sample temperature of 1270 K. A single layer of graphene was fabricated by exposing Ir(111) to C2H2 (purity 99.99%) with a partial pressure of 10−4 Pa and a sample temperature of 1470 K.26−29 While Li intercalation was observed already at room temperature30−32 the graphene-covered Ir(111) sample was exposed to Li at 630 K in the present experiments in order to ensure that the intercalant explores the entire surface for finding the energetically favored adsorption site. Higher sample temperatures were not used in order to avoid desorption of Li. For intercalated Li, desorption was reported for a temperature of 870 K.31 The coverage was increased by successively depositing Li for ∼4 min. Coverage was calibrated by assigning a coverage of 100% to the occupation of each graphene C6 ring with a single Li atom. Accordingly, the closed Li layer exhibiting the p(√3 × √3)R 30° superstructure reflects a coverage of 33%. All other coverage levels were then determined by Received: January 22, 2016 Revised: February 11, 2016
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moiré lattice is 25.3 ± 0.2 Å, which indicates the alignment of ⟨112̅0⟩ graphene directions with crystallographic ⟨11̅0⟩ Ir(111) directions.35 On the basis of previous findings,34 we assign the highest protrusions [yellow dot in Figure 1a, yellow hexagon in Figure 1b] to C6 rings of the graphene lattice residing at facecentered cubic (fcc) Ir(111) sites. Slightly lower protrusions (red) correspond to C6 rings at hexagonal close-packed (hcp) Ir(111) sites. C6 rings at on-top Ir(111) sites give rise to depressions (blue). The sketch depicted in Figure 1b will hereafter be used to characterize the Li intercalation structures. Exposing graphene-covered Ir(111) to Li at 630 K led to STM images presented in Figure 2a,b. At low Li coverage of 3.7% (Figure 2a) and 6.1% (Figure 2b), a regular array of protrusions appeared in STM images. These protrusions exhibited an apparent height of 42 ± 3 pm and a diameter of 9.2 ± 1.0 Å, as extracted as the full width at half-maximum (fwhm) from cross-sectional profiles. The regular array of protrusions showed the same periodicity and orientation as the moiré lattice of pristine graphene. We assign these protrusions to intercalated Li at one specific moiré site and refer to them as dots in the following. It is reasonable to assume that this intercalation site is preferred over other regions of the moiré lattice. However, from STM images alone, the identification as fcc, hcp, or on-top region is difficult with present knowledge. Recent density functional calculations of Li-intercalated graphene on Ir(111) unraveled Ir(111) hcp hollow sites as preferred Li adsorption sites.30 In addition, these calculations showed that C6 rings of the graphene lattice are centered atop Li atoms,30 in agreement with predictions for freestanding graphene25 and graphite intercalation compounds.36,37 This arrangement corresponds to the hcp stacking region in the moiré unit cell. Therefore, we identify the moiré hcp regions as the preferred intercalation site. Figure 2c illustrates this assignment by enframing the occupied hcp sites (red) with thick lines. The number of atoms in an intercalant dot was estimated by multiplying the total number of carbon rings in the moiré unit cell by the determined coverage. As a result, dots at Li coverage of 3.7% contain assemblies with ∼4 Li atoms. The inset to Figure 2a shows the proposed arrangement of Li atoms (solid circles) beneath C6 rings (circles). Based on the atomic Li superstructure at monolayer coverage, 12 Li atoms represent the maximum occupation of a dot. Increasing the Li coverage led to an increase in lateral dot size (Figure 2b). The observed compact assemblies at this coverage exhibited a diameter of 11.5 ± 0.2 Å (fwhm of crosssectional profiles) and an apparent height of 44 ± 2 pm. The estimated average number of Li atoms occupying the dots at this coverage is ∼7. In addition, intercalant structures coalesced at step edges and gave rise to elongated Li accumulation regions at the lower sides of the steps. In contrast, pairs of dots and cloverlike assemblies formed at the upper side of step edges. The different Li intercalation behavior observed at lower and upper step edges may be due to the different sign of the graphene curvature in both regions.38 In particular, upward bending of graphene at lower edges facilitates accumulation of Li due to the interplay of different energy contributions as explained below. An additional asymmetry between these two regions is caused by the Smoluchowski effect,39 which leads to a charge accumulation (depletion) at lower (upper) step edges. Alkali metals are known to exhibit ionic bonding to the substrate surface; that is, the weakly bonded valence electron is readily transferred to the substrate.40 Indeed, recent calculations showed that Li intercalation is accompanied by
calculating the fraction of surface areas occupied by Li over the total area multiplied by 33%. For Li intercalation at room temperature, a (1 × 1) superstructure with respect to Ir was reported.31 This superstructure reflects Li coverage up to 100%. All STM images were recorded at constant current and with the bias voltage applied to the sample. STM data were processed by use of WSxM.33
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RESULTS AND DISCUSSION Pristine Graphene and Low Li Coverage. Figure 1a shows an STM image of clean graphene on Ir(111). Due to the mismatch of lattice constants [graphene 2.452 Å, Ir(111) 2.715 Å],34 a moiré superstructure is formed whose lattice vectors are indicated as m1 and m2 in Figure 1a. The periodicity of the
Figure 1. (a) STM image of graphene on Ir(111), showing the moiré lattice (−0.2 V, 0.1 nA, 20 × 20 nm2) with lattice vectors m1 and m2. Yellow, red, and blue dots indicate fcc, hcp, and on-top regions of the moiré pattern, respectively. (b) Sketch of graphene on Ir(111). C6 rings are indicated as orange circles. Ir atoms appear as light gray (first layer) and dark gray (second layer) dots. Colored hexagons indicate high-symmetry regions of the moiré lattice. Crystallographic directions of Ir and graphene are shown in the bottom left corner. B
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Figure 2. (a, b) STM images of Li-intercalated graphene at Li coverage of (a) 3.7% and (b) 6.1% (0.1 V, 0.1 nA, 40 × 40 nm2). Regular arrays of dots have the same periodicity and orientation as the moiré pattern of pristine graphene on Ir(111) and are assigned to Li assemblies residing at hcp sites of the moiré lattice. White arrows indicate the moiré ⟨112̅0⟩ direction. (a, inset) Suggested arrangement of Li atoms (solid circles) beneath C6 rings (open circles) in dots. The suggestion is based on the Li superstructure determined at higher coverage. (c) Sketch of moiré lattice (colored hexagons); hcp regions occupied by Li assemblies are indicated by thick lines.
substantial charge transfer from the intercalant to the Ir substrate as well as to graphene.30 The resulting partial positive charge of the intercalated Li atoms may therefore explain their preferred diffusion to lower step edges. Similar findings were reported for molecule adsorption on noble metal surfaces.41,42 Intermediate Li Coverage. In the coverage range presented next, the formation of cloverlike intercalation assemblies and the pairing of dots to elongated stripes was observed on terraces (Figure 3a). At lower coverage, they occurred solely at the upper side of step edges (Figure 2b). The occupation of the moiré lattice at intermediate Li coverage is illustrated in Figure 3b. The cloverlike assemblies exhibited an apparent height of their central part of 77 ± 2 pm and a distance between dots at their periphery of 24.8 ± 0.7 Å. Figure 4a demonstrates the preference for a uniform orientation. Only in rare cases was the opposite orientation observed, as shown by the close-up view in Figure 4b. Here, two cloverlike assemblies that are rotated by 180° with respect to the prevailing orientation are presented. Li atoms at the center of these assemblies are proposed to occupy a second moiré lattice region. As illustrated in the inset to Figure 4b, the orientation of such an assembly may be rotated by 180° when the central moiré site changes from fcc to on-top. According to the previously determined orientation of the moiré lattice (Figure 1) and the prevailing orientation of the cloverlike structures (Figure 4), the second occupied moiré region corresponds to the moiré fcc site. The observed stripes displayed an apparent height of 54 ± 4 pm, and the fwhm of cross-sectional profiles was 9.2 ± 0.2 Å. Stripe orientations correspond to all three high-symmetry directions of the moiré lattice. Occasionally coalesced stripes enclosed an angle of 120°, while angles of 60° were not observed. In the intermediate coverage range, stripes preponderantly formed in the vicinity of defects, which likely induced an anisotropic strain distribution in graphene. In Figure 4a, the top left corner of the STM image shows a graphene blister, which led to intercalation stripes oriented along the moiré lattice directions and pointing toward the blister. This indicates a directional influence of anisotropic strain on stripe orientation. At distances from the defect exceeding ∼30 nm, the cloverlike assemblies prevailed again, indicating isotropically strained areas. Similar conclusions were drawn for intercalation of Eu under graphene on Ir(111), where the density of intercalation stripes proved to be an indicator of preexisting strain in the
Figure 3. (a) STM image of Li-intercalated graphene at 14.8% Li coverage (0.1 V, 0.1 nA, 80 × 80 nm2). The central region of cloverlike assemblies is the moiré fcc site, which represents the second favorable Li intercalation site.
graphene sheet.43 However, in that case isotropically shaped intercalant assemblies were not observed, even in isotropically C
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Figure 4. (a) STM image of Li-intercalated graphene at Li coverage of 16.7% (0.1 V, 0.1 nA, 80 × 80 nm2). The top left corner shows a graphene blister, which represents a defect of the regular moiré lattice. In its vicinity, that is, within distances of ∼30 nm, stripe patterns along moiré lattice directions formed, which indicate the locally anisotropic strain in graphene. The strain becomes isotropic again in regions where cloverlike intercalation assemblies dominate. (b) STM image of Li-intercalated graphene at Li coverage of 14.8% (0.1 V, 0.1 nA, 25 × 25 nm2). Two cloverlike intercalation assemblies that are rotated by 180° with respect to the prevailing orientation are marked by dashed white circles. (Inset) Sketch of two cloverlike Li intercalation assemblies with opposite orientation. By exchanging the central region of the assembly from fcc (yellow, right) to on-top (blue, left) the orientation of the assembly rotates by 180°. White arrows indicate the moiré ⟨112̅0⟩ direction.
Figure 5. (a) STM image of Li-intercalated graphene at Li coverage of 25.1% (0.2 V, 10 pA, 100 × 100 nm2). An Ir(111) step edge separates the upper (left) from the lower (right) terrace. Dark lines are due to regions beneath graphene that are not occupied by Li intercalants at this coverage. The regular array of circular protrusions is due to the graphene moiré lattice. (b) Close-up view of an intercalated region (−0.1 V, 1 μA, 5.2 × 5.2 nm2). The ⟨112̅0⟩ direction is indicated by the white arrow. Ordered patterns with different periodicities are visible. The smallest periodicity is due to the graphene atomic lattice. The hexagonal arrangement of depressions indicated by white circles is a Li-induced superstructure explained in the text. The large circular protrusions are due to the graphene moiré pattern. (c) Two-dimensional Fourier transform of the STM image in panel b. Fourier maxima marked with orange circles are due to the graphene lattice. Additional spots (dashed red circles) are due to the Li superstructure. Both lattices enclose an angle of 30°.
local strain relief, Er, and an energy cost due to graphene bending, Eb:
strained areas. Here, the small size of Li leads to a decrease in the energy gained due to strain relief at edges of intercalated assemblies. Therefore, formation of isotropic assemblies with cloverlike shape is enabled. The observed formation of dots, pairs of dots, stripes, and cloverlike intercalation structures may be explained in terms of different energy contributions.43 To this end, we define a site energy per intercalated Li atom:
Ee = Er − E b
With the help of the orientation preference of cloverlike Li assemblies, a ranking of energetically favored intercalation regions is suggested. The peripheral dots correspond to the initially occupied moiré sites at low Li coverage and thus represent the most favorable intercalation site. On the basis of recent calculations,30 this intercalation region is identified as the moiré hcp site. The center of the cloverlike arrangement is then the second favorable intercalation region and corresponds to the moiré fcc site, leaving the remaining on-top site as the least favorable intercalation region. In terms of Es, this ranking on‑top > Efcc . corresponds to Ehcp s s > Es
Es = Ea − Ed which includes the adsorption energy, Ea, as an energy gain and the delamination energy, Ed, as an energy cost. The delamination energy takes into account the lifting of the graphene sheet due to intercalation. In addition, intercalation creates graphene edges surrounding the Li assemblies. The edge energy per edge length, Ee, includes an energy gain due to D
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Figure 6. STM image of adsorbed Li clusters at the lower edge of a step at (a) 15% coverage (0.3 V, 0.4 nA, 26 × 26 nm2) and (b) 32% coverage (0.1 V, 0.1 nA, 26 × 26 nm2). (b) The clusters grow with the same periodicty and orientation as the moiré lattice. Compared to the moiré lattice on the flat terrace, whose unit cells are indicated by the white lines, the cluster lattice is shifted by two moiré sites (≈29 Å) in ⟨011̅0⟩ direction, indicated by the dashed arrow. (Inset) Illustration of the shift from preferred intercalation (hcp) to preferred adsorption (fcc) site. Solid arrows indicate the moiré ⟨112̅0⟩ direction.
with Li atoms. An almost closed intercalation film was formed at a coverage of ∼33%. The intercalation film was imaged with atomic resolution. Figure 5b shows a close-up view of a representative region. The pattern with the smallest spatial periodicity (circular protrusions) is due to the atomic graphene lattice, while the structure with the largest periodicity (bright circular regions) is due to the moiré lattice. An additional superstructure is visible as depressions, which are indicated by the white circles in Figure 5b. With respect to the graphene lattice, this superstructure is described by p(√3 × √3)R 30°. The lattice vectors spanning the unit cell exhibit a length of 4.3 ± 0.1 Å, which deviates by 1% from the expected p(√3 × √3)R 30° lattice constant of 4.24 Å. The rotation angle was best determined from the Fourier transform of the STM image (Figure 5c). Graphene and superstructure Fourier spots are indicated by solid and dashed circles, respectively. The p(√3 × √3)R 30° superstructure was retained for the intercalated Li monolayer at ∼33% coverage. The same superstructure was reported for Li intercalation of graphene on SiC32 and graphite intercalation compounds with alkali metal interlayers.37 In addition, the corrugation of graphene in these intercalated regions is 9 ± 1 pm only, which is about half the value obtained for pristine graphene on Ir(111), 19 ± 1 pm.47 Previously, the moiré corrugation was identified as an appropriate measure of graphene−substrate hybridization.17,48 Therefore, the observed low corrugation hints at a low hybridization of graphene with the Li intercalation layer. This is in contrast to many other metal intercalants and substrates,17,20−22 which showed large moiré corrugations of 60−180 pm. In these reports the intercalants adjusted their lattice constants to the substrate lattice constant, giving rise to pseudomorphic growth accompanied by a (1 × 1) superstructure with respect to the substrate lattice, as opposed to the nonpseudomorphic p(√3 × √3)R 30° superstructure with respect to graphene reported here. Starting from Li coverage of ∼15%, regularly spaced protrusions of 100−150 pm apparent height and average
Our observations suggest that intercalation sites and orientation of Li assemblies in the absence of anisotropic strain are determined by Es, while Ee defines size and shape of the intercalated assemblies. In areas with large anisotropic strain, Er forces the intercalated assemblies into stripe shape,43 with an orientation given by the strain distribution. The findings presented here deviate from observations for Eu43 and CO44 intercalation under graphene on Ir(111), where moiré on-top sites were identified as preferred intercalation regions. This conclusion was based on the derived ranking of hcp 43 < Efcc Consistently, delamination energies Eon‑top d d < Ed . density functional calculations of graphene on Ir(111) showed that fcc and hcp regions are characterized by weak covalent bonds, while moiré on-top regions exhibit repulsive interaction.45 In the case of Li intercalation, however, it was suggested that the energy costs for delamination, Ed, are significantly reduced,31 owing to the small size of Li compared to most intercalants examined so far.17,21,22,31,43,44,46 Thus, a larger influence of the adsorption energy Ea on Es may be expected and, as shown by our intercalation experiments, leads to a different ranking of preferred moiré lattice sites for intercalation. For the same reason, Li requires less energy Eb for bending graphene, which leads to the appearance of disperse intercalation patterns consisting of many elementary Li assemblies at low to intermediate coverage, rather than to compact islands.43,44 High Li Coverage. Increasing Li coverage led to the coalescence of dots, stripes, and cloverlike assemblies. The resulting intercalation structure at a coverage of 25.1% is shown in the STM image of Figure 5a. It appears rather flat with embedded elongated depressions, which exhibit a width of 14.7 ± 0.1 Å, corresponding approximately to the average width of a moiré site (14.6 Å). We assign the elongated depressions to nonintercalated regions. Some direction preference was observed, which is probably due to the influence of anisotropic strain again. Upon further increase of Li coverage, the number of elongated depressions was reduced due to their occupation E
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The Journal of Physical Chemistry C diameter of 15.6 ± 0.4 Å began to decorate the lower side of step edges (Figure 6a). Their mutual distance was virtually identical with the graphene moiré periodicity. With increasing coverage, the protrusions formed a hexagonal lattice with the moiré periodicity and orientation (Figure 6b). Compared to the moiré superstructure on the flat (lower) terrace, the superimposed new lattice was laterally shifted by two moiré sites (≈29 Å) in ⟨011̅0⟩ direction (dashed arrow in Figure 6b). Due to their variation in size and shape, we assign the protrusions to nonintercalated, i.e., adsorbed Li clusters. An adsorption phase was likewise reported for Eu23,43 and Cs23,31 on graphene-covered Ir(111). On the basis of calculations and photoelectron spectroscopy data for Li-intercalated graphene on Ir(111), however, adsorbed Li was not indicated.31 Since the calculations did not consider step edges, and due to the laterally averaged photoemission data, we think that this previous report31 is not necessarily at odds with our findings, which concern adsorbed Li clusters in the vicinity of step edges. The observed lateral shift of the array of clusters with respect to the moiré lattice (Figure 6) implies that adsorbed Li clusters preferably occupy fcc regions of the moiré superstructure. Ordered adsorption phases were likewise reported for Ir49 and H50 on nonintercalated graphene on Ir(111). For Ir clusters, hcp moiré regions were identified as preferred adsorption sites. In summary, the main findings of our paper reveal that, at low coverage, the small size of the Li intercalant leads to a disperse distribution of intercalation assemblies. In addition, the energy gain due to strain relief upon edge formation is low in isotropically strained areas, which enables isotropic cloverlike shapes. This observation is in contrast to previously studied larger intercalants, such as Eu23,43 and Cs.23,31 In accordance with recent calculations30 our results are compatible with Li preferably occupying hcp stacking regions of the moiré lattice. Based on the prevailing orientation of intercalated assemblies, fcc and on-top regions represent the second and third favorite intercalation sites, respectively. Larger intercalants were reported to adsorb at on-top regions first and then at fcc and hcp regions.43,44 For Li the graphene delamination energy is reduced,31 owing to the small size of the intercalant, which leads to adsorption energy rather than delamination energy being decisive for the different ranking of preferred adsorption regions. Similar to results reported for Li intercalation of graphene-covered SiC32 and for graphite intercalation compounds with alkali metal interlayers,37 a p(√3 × √3)R 30° superstructure was found with respect to graphene. However, the graphene corrugation in this elevated-coverage range is nearly an order of magnitude lower than for intercalants displaying pseudomorphic growth.17,21,22,46 Low hybridization of graphene with the Li layer and/or the absence of pseudomorphic Li growth may be at the origin of this observation.17 At high coverage, Li additionally exhibits an adsorbed phase, which is reflected by the decoration of step edges with regularly spaced Li clusters residing at fcc stacking regions of the moiré lattice.
leads to a low edge energy of intercalated assemblies reflected by their isotropic shape at low coverage. Moreover, our observations evidence that an adsorbed phase may form preferably at step edges. The presented findings may serve as a structural basis for forthcoming experimental and theoretical investigations of electronic and dynamic properties of Li-intercalated graphene as well as intercalation of other two-dimensional materials. Disperse distributions of intercalated assemblies could be of interest for nanopatterning of an adsorbed phase. The emerging low hybridization of graphene with the Li intercalant layer may render Li-intercalated graphene on Ir(111) particularly appealing for the efficient decoupling of adsorbed molecules.
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AUTHOR INFORMATION
Corresponding Author
*E-mail
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support by the Deutsche Forschungsgemeinschaft through Grant KR 2912/10-1 is acknowledged. REFERENCES
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CONCLUSIONS Our results show that the size of an intercalant matters for pattern formation and selection of intercalation sites. A reduction of graphene delamination energy costs for small intercalants like Li enhances the influence of the adsorption energy gain. Thus, the adsorption geometry with respect to graphene and substrate becomes the main factor in determining preferred intercalation regions. In addition, the small size of Li F
DOI: 10.1021/acs.jpcc.6b00729 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.jpcc.6b00729 J. Phys. Chem. C XXXX, XXX, XXX−XXX
