Structural and electronic properties of Li intercalated graphene on SiC(0001)

Nuala M. Caffrey,1 Leif I. Johansson,1 Chao Xia,1 Rickard Armiento,1 Igor A. Abrikosov,1, 2, 3 and Chariya Jacobi1

1Department of Physics, Chemistry and Biology (IFM),Linkoping University, SE-581 83, Linkoping, Sweden∗

2Materials Modeling and Development Laboratory, NUST “MISIS”, 119049 Moscow, Russia3LACOMAS Laboratory, Tomsk State University, 634050, Tomsk, Russia

(Dated: February 29, 2016)

We investigate the structural and electronic properties of Li-intercalated monolayer grapheneon SiC(0001) using combined angle-resolved photoemission spectroscopy and first-principles densityfunctional theory. Li intercalates at room temperature both at the interface between the buffer layerand SiC and between the two carbon layers. The graphene is strongly n-doped due to charge transferfrom the Li atoms and two π-bands are visible at the K-point. After heating the sample to 300◦C,these π-bands become sharp and have a distinctly different dispersion to that of Bernal-stackedbilayer graphene. We suggest that the Li atoms intercalate between the two carbon layers withan ordered structure, similar to that of bulk LiC6. An AA-stacking of these two layers becomesenergetically favourable. The π-bands around the K-point closely resemble the calculated bandstructure of a C6LiC6 system, where the intercalated Li atoms impose a super-potential on thegraphene electronic structure that opens pseudo-gaps at the Dirac points of the two π-cones.

I. INTRODUCTION

The epitaxial growth of graphene on SiC substrates isa viable and attractive way of producing large homoge-neous monolayer samples suitable for electronic deviceapplications [1, 2]. When grown on the Si terminatedSiC(0001) surface, the first carbon buffer layer has adetrimental effect on the graphene charge carrier mo-bility and needs to be eliminated [3]. Intercalation ofelements such as hydrogen [4, 5], gold [6], germanium[7], silicon [8, 9], nitrogen [10], fluorine [11], oxygen [12],lithium [13–15] and sodium [16–18] have all been shownto decouple the buffer layer and transform it into a quasifree-standing graphene layer with varying degrees of dop-ing depending on the intercalant. The alkali metals in-duce a strong n-type doping when deposited on graphenesamples. Potassium [19, 20], rubidium and cesium [21]remain on the surface after deposition and do not pen-etrate to the interface even after heating. Lithium [13–15] and sodium [16–18], on the other hand, intercalatereadily at room temperature, decoupling the buffer layer.Increasing the temperature to 300◦C (100◦C) promotesthe complete intercalation of Li (Na) while higher tem-peratures result in the de-intercalation and desorption ofthe metal from the sample. Interest in Li-intercalatedgraphene was originally motivated by the possibility ofusing it to improve the capacity of Li-ion batteries. How-ever, the structural degradation of the carbon layers afterLi deposition renders this unlikely [14]. More recently, su-perconductivity was discovered to occur in Li decoratedmonolayer graphene on SiC(0001) at low temperatures[22, 23]. Detailed studies of the structural and electronicproperties of Li intercalated graphene will be requiredin order to determine the origin and mechanism of thisbehavior.

Low-energy electron microscopy (LEEM) imaging pro-vide evidence that Li atoms form small islands on thegraphene surface directly after deposition [14]. Over

time these islands coalesce, shrink and eventually disap-pear; this process can be accelerated by annealing. Corelevel spectra, combined with angle-resolved photoemis-sion spectroscopy (ARPES) data, show that Li interca-lates both at the interface between the substrate and thebuffer layer and between the carbon layers, with the re-sult that multiple π-bands become visible [13, 15]. A

(√

3 ×√

3)R30◦ diffraction pattern was observed afterLi deposition on monolayer graphene using low-energyelectron diffraction measurements. This pattern did notappear when when Li was deposited on a sample con-taining only the (6

√3 × 6

√3)R30◦ carbon buffer layer,

suggesting that it is due to an ordered intercalated Lilayer between the two graphene sheets. The decouplingof the buffer layer was also shown to occur when Li wasdeposited on samples with an initial coverage of only thebuffer layer. In this case, a single π-band becomes visibledirectly after Li deposition [14], which can only occur ifthe buffer layer has been decoupled.

Several recent investigations have attempted to elu-cidate the exact nature of the Li intercalation on bothzero- and mono-layer graphene and its temperature de-pendence. Bisti et al. determined that the Li atoms oc-cupy the T4 site of the topmost SiC bilayer after they in-tercalate underneath the buffer layer, thereby decouplingit from the substrate [24]. Sugawara et al. deposited Lion bilayer graphene at 30 K and reported the appearanceof a sharp (

√3 ×√

3)R30◦ diffraction pattern [25], butdid not observe this pattern after the deposition of Li onmonolayer graphene. They suggested Li intercalates be-tween the top two adjacent carbon layers and takes thesame well-ordered superstructure as in bulk C6Li. How-ever, the band structure around the K-point for bothLi-deposited monolayer and bilayer graphene containedonly a single π-band (see Figs. 2(e) and (f) in Ref. [25]).This contrasts significantly with earlier findings [13–15]where at least two π-bands appear around the K-pointafter Li deposition on monolayer graphene.

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We present here detailed ARPES data collected be-fore and after Li deposition on monolayer graphene,both at room temperature and after heating to 300◦C,in order to unambiguously determine the number of π-band branches present and their dispersions. Directlyafter deposition, we show clearly the presence of multi-ple π-bands around the K-point. After heating the sam-ple to 300◦C, these bands become considerably sharperand their dispersions do not resemble that of Bernalstacked bilayer graphene [26, 27]. To understand theseobservations, tight-binding and density functional the-ory band structure calculations were performed for Liintercalated free-standing bilayer graphene, as well as forLi-intercalated graphene systems that explicitly take theSiC(0001) surface into account.

Our combined experimental and theoretical resultslead us to suggest the following: Directly after depo-sition, Li intercalates both underneath the buffer layerand between the two carbon layers. Heating the samplepromotes the complete intercalation of the Li atoms tothe interface, as well as the development of an orderedLi configuration between the two carbon layers. In theprocess, it becomes energetically more favorable for thecarbon layers to become AA- rather than Bernal-stacked.We show that the π-bands visible around the K-pointclosely resemble the band structure of a C6LiC6 systemas calculated with density functional theory. The peri-odic perturbation of the graphene electronic structure bythe ordered Li layer induces pseudo-‘gaps’ to open at theDirac point of each of the π-cones. Tight-binding calcu-lations show that the interlayer coupling is enhanced bythe presence of the Li atoms beyond what is typical inclean bilayer graphene, and the impact of the enhancedKekule-textured skew-coupling terms on the band disper-sion is verified by ARPES spectra.

II. METHODOLOGY

A. Experimental Details

Monolayer graphene samples were grown in-situ onn-type wafers of 4H-SiC(0001) purchased from SiCrys-tal, which were specified to have a misorientation errorwithin 0.05◦. ARPES measurements were performed us-ing Beamline I4 at the MAX IV Laboratory, which isequipped with a spherical grating monochromator anda PHOIBOS 100 mm CCD analyzer from SPECS. Thewide angular dispersion mode was selected, providing anacceptant angle of ±14◦. Each ARPES spectrum was col-lected parallel to the A-K-A′ direction of the Brillouinzone of graphene, in steps of 0.25◦ along the Γ-K-M di-rection, using photon energies of 33 eV and 70 eV. Fromthis data, the π-band structure along certain directionsin the Brillouin zone can be determined, as well as theangular distribution pattern, Ei(kx,ky), at selected ini-tial state energies. A Ta foil was used as a reference todetermine the Fermi level.

Deposition of Li was performed during a five minute in-terval using a commercial alkali metal source (from SAESGetters) and with the sample at room temperature. Fur-ther details can be found in Refs. [13–15]. Subsequentheating was carried out for four minutes at each selectedtemperature. The sample temperature was determinedusing optical pyrometers. ARPES measurements weretaken after the sample had cooled to room temperature.The Si 2p and Li 1s core level spectra were recorded be-fore and after deposition and heating. They showed thesame features and changes upon heating as reported ear-lier [13] and therefore no core level data is included below.

B. Computational Details

Density functional theory calculations were performedusing vasp-5.3 [28–30]. The Perdew-Burke-Ernzerhof(PBE) [31] parametrization of the generalized gradientapproximation (GGA) was employed. The plane wavebasis set was converged using an 800 eV energy cutoff.Structural relaxations of the C6LiC6 cell were carried outusing a 9 × 9 × 1 k-point Monkhorst-Pack mesh [32] tosample the Brillouin zone. A 24×24×1 mesh was used todetermine the total energies. The Tkatchenko-Schefflermethod was used to describe van der Waals interactions[33]. The free-standing bilayer structures were optimized

until all residual forces were less than 0.001 eV A−1

The SiC(0001) substrate was modelled using an asym-metric slab consisting of 6 bilayers of SiC(0001), arrangedin the ABCACB stacking associated with the 6H poly-type. A bulk termination was assumed. The GGA cal-culated lattice constant of bulk SiC is 3.09 A, in goodagreement with the experimental value of 3.08 A. A vac-uum layer of at least 15 A was included in the directionnormal to the surface to ensure no spurious interactionsbetween repeating slabs and the dipole correction was ap-plied where appropriate. The dangling C bonds on theSiC(0001) surface were passivated with H atoms. Thepositions of the top two bilayers of SiC(0001), as well asthe H-terminating atoms, the Li atoms, and all carbonlayers, were optimized until all residual forces were less

than 0.01 eV A−1

. The remaining atoms were held fixedat their bulk positions. The (6

√3×6√

3)R30◦ surface re-construction [34–37] of the buffer layer on SiC(0001) was

modelled using the simplified (√

3×√

3)R30◦ cell whichcorresponds to a 2 × 2 graphene cell. Such an approxi-mation was shown to be adequate to correctly describethe interaction between the SiC(0001) surface and thecarbon layers [38].

3

III. RESULTS

A. Experimental ARPES Spectra

The band structure obtained from the as-grown mono-layer graphene sample, along the Γ-K-M direction of theBrillouin zone of graphene, is displayed in Fig. 1(a). Thecontribution from the π-band dominates, with branchesvisible at both the first and second K point. The pho-toelectron angular distribution pattern, Ei(kx, ky), ob-tained at the Fermi energy is shown in Fig. 1(b). Theimage has been overexposed to show the presence of sixreplica π-cones surrounding the Dirac cone at the first Kpoint, as well as the presence of the Dirac cone at thesecond K point. The Dirac cone at the second K pointappears, as it should, as a mirror image of the one atthe first K point. The lower intensity obtained aroundthe second K-point is attributed to photoelectron matrixeffects [39, 40]. The π-band structure obtained along theA-K-A′ direction around the first K point is shown inFig. 1(c). A single, linearly dispersing π-band is visiblewith the Dirac point located approximately 0.45 eV be-low the Fermi level. Taken together, the ARPES spectrain Figs. 1(a), (b) and (c) demonstrate that high quality,monolayer graphene is present on the SiC(0001) surface[26].

FIG. 1. (Color online) Band dispersion recorded from mono-layer graphene samples before (a – c) and after (d – f) Lideposition, and after heating to 300◦C (g – i). The dashedlines are guides to the eye for illustrating the location of thefirst and second K points or an initial state energy of 0 eVand −1.4 eV. The Fermi energy is located at 0 eV. A denotesthe direction perpendicular to the Γ-K-M direction in the 2DBrillouin zone of graphene.

Depositing Li on the sample at room temperature in-duces significant changes in the band structure, as illus-

trated in Figs. 1(d), (e) and (f). Fig. 1(d) shows thatthe entire π-band structure, here displayed in the Γ-K-M direction, is rigidly shifted downwards by about 1 eV;the position of the π-band at the Γ-point is now locatedat −10 eV, whereas for the clean sample it is located atapproximately −9 eV [c.f. Fig. 1(a)]. A second π-bandis now visible close to the Fermi energy and the disper-sion of these bands is no longer linear. The lower branchis instead quite flat and located below the Fermi level.This can be seen clearly in the Fermi surface [Fig. 1(e)]where the π-bands are shown to be occupied all the waybetween the first and second K-points. Again, the Fermisurface near the second K-point is a mirror image of thatnear the first. The π-bands in the A-K-A′ direction areshown in Fig 1(f). The Dirac point has now shifted downin energy to approximately 1.4 eV below the Fermi level.This is due to the strong n-type electron doping providedby the Li atoms to the graphene layers.

Heating the sample to 300◦C induces further changesin the band structure, as illustrated in Figs. 1(g), (h) and(i). The entire band structure, displayed in Fig. 1(g), isnow rigidly shifted by about 0.8 eV back towards theFermi level. This can be seen by comparing the π-bandsat the Γ-point in (g) and (d). Two π-band branches arenow clearly resolved in an energy window between theFermi level and −6 eV. The Fermi surface in Fig. 1(h)shows that neither of these π-band branches is continu-ously located below the Fermi level between the first andsecond K-point. Again, the Fermi surfaces at the firstand second K-points are mirror images of each other.

Comparing Figs. 1(i) and (f), one can see that the π-bands around the K-point are considerably sharper, andtheir dispersions distinctly different, after heating thandirectly after Li deposition at room temperature. TheDirac point has moved to approximately 0.6 eV belowthe Fermi level with two distinct π-bands visible aboveand below it. The bands show neither the parabolic dis-persion of Bernal stacked bilayer graphene nor the lineardispersion predicted for AA-stacking [26, 27].

Photoemission angular distribution patterns extractedat several fixed energies above and below the Dirac pointare shown in Fig. 2. At energies far from the Dirac point,namely at −0.1 eV and −1.5 eV from the Fermi energy,two ring-like patterns corresponding to the two π-bandsare present. A strong triangular deformation, or trigo-nal warping, of the outer Dirac cone is clearly visible atthese energies. At energies closer to the Dirac point, forexample at −0.4 eV and −1.1 eV, the trigonal warping isconsiderably reduced and the inner ring has been reducedto a point. Finally, the angular distribution at energiesof −0.6 eV and −0.8 eV are not comprised of ring-likesegments but of two and three points of varying intensity,respectively. In all cases, the intensity anisotropy of theangular maps does not correspond to that predicted forBernal-stacked bilayer graphene [40, 41].

4

FIG. 2. (Color online) Band dispersion recorded in the vicin-ity of the K-point (top two panels) and angular distributionsextracted at fixed energies close to the Dirac point. The sixenergies are shown as white dashed lines through the banddispersions in the top panels. The Fermi energy is located at0 eV. An incident photon energy of 33 eV was used.

B. DFT Band Structures

The ARPES spectra presented in the previous sectionshows clearly the presence of two π-bands after Li depo-sition, which implies that Li has intercalated to the inter-face and decoupled the buffer layer from the substrate.This has previously been corroborated by both experi-mental core-level spectra and DFT calculations and ver-ified in the current work (see Appendix). We will nowaddress the relationship between the presence of Li be-tween the two carbon layers and its effect on the disper-sion by calculating the band structure of free-standingbilayer graphene with and without intercalated Li.

The first possibility is that Li does not intercalate be-tween the two carbon layers. These two layers then re-main Bernal stacked and the resulting band structure willhave a symmetric, parabolic band dispersion around the

K-point, as shown in Fig. 3(a). The band structure ofclean AA-stacked bilayer graphene, shown in Fig. 3(b),is comprised of two Dirac cones with linear dispersion,separated in energy by 0.5 eV. We find an interlayer dis-tance of 3.52 A for clean bilayer AA-stacked graphene,close to the experimentally found value for AA-stackedgraphite [42].

FIG. 3. Calculated band structure of (a) AB-stacked cleanbilayer graphene, (b) AA-stacked clean bilayer graphene (c)AB-stacked C8LiC8, (d) AA-stacked C8LiC8, (e) AB-stackedC6LiC6 and (f) AA-stacked C6LiC6. The high-symmetry la-bels are those of the graphene unit cell.

A second possibility is that Li intercalates between thetwo carbon layers, which remain AB-stacked, such thata Li atom sits on top of the centre of a hexagon for onegraphene layer and directly underneath a carbon atomof the second carbon layer. The band structure asso-ciated with such a scenario is shown in Fig. 3(c) for aC8LiC8 concentration. The structural asymmetry is ev-ident in the resulting ‘Mexican hat’ band dispersion. Agap opens at the K-point as a result of the asymmet-ric doping of the two carbon layers. For this particularLi concentration a gap of 0.13 eV is opened. The thirdpossibility is that the registry between the two graphenelayers switches from AB- to AA-stacking. A sufficientlyhigh concentration of intercalated Li atoms is alreadyknown to shift the stacking pattern of bilayer grapheneand Li-graphite intercalation compounds (Li-GICs) to anAA-stacking of the carbon sheets [43, 44]. The resultingband structure is shown in Fig. 3(d). It has the lineardispersion typical of AA-stacked bilayer graphene. Theeffect of the intercalated Li can be seen in the increasedenergy separation in the two Dirac cones, from 0.5 eV to

5

0.6 eV.

The highest concentration of Li found in GICs at am-bient conditions is LiC6. The (

√3×√

3)R30◦ diffractionpattern observed after Li deposition at room temperature[13] would suggest that this Li concentration is also themost favourable in bilayer graphene. If we consider AB-stacked graphene with a C6LiC6 concentration, we findthree π-bands are visible, due to the asymmetric dopingof the two graphene layers. Finally, we consider AA-stacked graphene at the same concentration. The energydispersion, shown in Fig. 3(f), is comprised of two Diraccones shifted relative to one another by the interlayercoupling and with a pseudo-gap opened in both at theK-point. We find that AA-stacked C6LiC6 is 36 meV/Clower in energy than the equivalent AB-stacked config-uration. As a comparison, Bernal stacked clean bilayergraphene is 6 meV/C lower in energy than AA-stackedbilayer graphene.

IV. DISCUSSION

Comparing the band structures presented in Fig. 3with the experimental ARPES spectra in Fig. 1(i), wefind that agreement is only achieved if we assume aC6LiC6 concentration of Li atoms between AA-stackedbilayer graphene, as shown in Fig. 4. The developmentof severe cracks and wrinkles after Li deposition [14] fur-ther supports this postulation. We suggest that this isdue to the strain induced in the system when the topcarbon layer attempts to shift to an AA-stacking. Thesecracks do not appear when Li is deposited on the bufferlayer only.

FIG. 4. (Color online) Band structure of free-standingC6LiC6 as calculated with DFT (red solid lines) and usinga tight-binding model (blue dashed lines), compared to theexperimental ARPES spectra [c.f. Fig. 1(i)]. The calculatedband structures have been shifted rigidly in energy to matchthe top π∗-band.

A. Tight-binding Model

We will now discuss the unusual shape of the electronicband dispersion close to the Dirac point, and in par-ticular the opening of the two pseudo-gaps of differingmagnitudes at the K-point, using a simple tight-bindingmodel. The Li atoms, which intercalate in an orderedfashion, cause a periodic modification of the grapheneelectronic structure. The effect can be described by aperiodic potential U(r) = U(r + l1R1 + l2R2) whereR1 = n1a1 +m1a2, R2 = n2a1 +m2a2, and a1 and a2

are the graphene lattice vectors [45]. l, n and m are inte-gers. In our case, R1 and R2 describe the lattice vectors

of C6LiC6 as shown in Fig. 5(a). Here, a1 = (√

32 ,−

12 )a0,

a2 = (√

32 ,

12 )a0, R1 = (

√3, 0)a0 and R2 = (

√3

2 ,32 )a0,

where a0 is the graphene lattice constant. A band gapwill open at the K-point when the periodic potential inreciprocal space, U(K), is not equal to zero. This con-dition will be satisfied when (n1 −m1) mod 3 = 0 and(n2 −m2) mod 3 = 0. This rule is reminiscent of thatdescribing the gap chirality in carbon nanotubes [46] andthe effect has already been harnessed to open bandgapsin monolayer and bilayer graphene through the periodicpattering of antidot lattices [47, 48]. The triangular sym-

metry associated with a (√

3 ×√

3)R30◦ Li adsorptionpattern has already been shown to satisfy the conditionsnecessary to open a gap in monolayer graphene at theDirac point [49, 50].

FIG. 5. (Color online) Structure of C6LiC6. (a) Top viewshowing the graphene lattice vectors a1 and a2 and the latticevectors of C6LiC6, R1 and R2. R1 and R2 are rotated by30◦ with respect to a1 and a2 and are

√3 longer. The two

carbon sublattices are denoted with gray and white circles andLi atoms are shown as green circles. The two types of bondsassociated with the Kekule textured graphene are shown asthin and thick solid lines. The two different intralayer hoppingparameters between carbon atoms associated with these twobonds are denoted t and t′. The 6 atoms belonging to theC6LiC6 unit cell of one of the graphene layers are labeled 1-6, and are used to generate the Hamiltonian of the system.(b) Side view showing the interlayer coupling parameters, γ1between carbon atoms directly on top of one another, and γ2and γ′2 describing the textured skew coupling terms.

We will now extend the model of Ref. [50] from Li deco-rated monolayer graphene to AA-stacked bilayer C6LiC6

graphene using a tight-binding model that takes the effect

6

of the intercalated Li into account via a Kekule textur-ing of the nearest neighbour hopping parameters. The12×12 matrix that describes the Hamiltonian of such assystem has the following form:

H =

(HT HTB

HBT HB

)where HT(B) is the 6×6 matrix describing intralayer hop-ping in the top (bottom) graphene layer and HTB is the6×6 matrix describing interlayer coupling. The intralayerHamiltonian, HT = HB, is given by:

ε t 0 t′eik.τ1 0 tt ε t 0 t′e−ik.τ2 00 t ε t 0 t′eik.τ3

t′e−ik.τ1 0 t ε t 00 t′eik.τ2 0 t ε tt 0 t′e−ik.τ3 0 t ε

where t and t′ are the textured nearest neighbour car-bon hoppings (see Fig. 5(a)), ε is the on-site energy and

τ1,2 = a0

(√3

2 ,∓32

)and τ3 = a0

(−√

3, 0). The inter-

layer coupling matrix, HTB = HBT, is given by:γ1 γ2 0 γ′2e

ik.τ1 0 γ2

γ2 γ1 γ2 0 γ′2e−ik.τ2 0

0 γ2 γ1 γ2 0 γ′2eik.τ3

γ′2e−ik.τ1 0 γ2 γ1 γ2 00 γ′2e

ik.τ2 0 γ2 γ1 γ2

γ2 0 γ′2e−ik.τ3 0 γ2 γ1

where γ1 is the vertical interlayer coupling between twocarbon atoms directly on top of one another and γ2

and γ′2 are the textured skew hopping terms. Theseare sketched in Fig. 5(b). The resulting band struc-ture is shown by blue dashed lines in Fig. 4. We findthat, as a result of the intercalation symmetry, a pseudo-gap opens at the Dirac point of each cone. These twocones are then shifted in energy relative to one anotherwith an energy equal to the vertical interlayer coup-ing, γ1 = 0.34 eV. The in-plane hopping parameters,t = 2.79 eV and t′ = 2.59 eV, agree with those found inin Ref. [50] to describe Li-adsorbed monolayer graphene.The values of γ1 = −0.34 eV and γ′2 = 0.04 eV are char-acteristic of AA-stacked bilayer graphene and graphite[51].

In contrast to clean AA-stacked graphene or graphite,however, the effect of the skew coupling terms cannot beneglected. The effect of their textured nature on the bandstructure can be seen in the breaking of the equivalence ofthe two pseudo-gaps. If γ2 and γ′2 were equal, the pseudo-gap opened in each cone would also be equal. Insteadwe have that one gap, Eg1 , is 0.20 eV wide, while theother, Eg2 , is 2.3 times larger, at 0.46 eV. This ratio isapproximately equal to that between γ2 and γ′2 which are0.10 eV and 0.04 eV, respectively.

To conclude, we have shown that Li intercalation servesto switch the stacking in a graphene bilayer from AB

to AA. The highly ordered Li layer generates a periodicperturbation of the graphene electronic structure suchthat a pseudo-gap opens in each cone at the Dirac point.The increase in magnitude of the skew coupling terms issignificant and its effects are visible in the experimentalARPES spectra.

B. Reduction in Doping after Heating

Finally, we discuss the observation that the graphenedoping level decreases significantly, from −1.4 eV to−0.6 eV, after heating to 300◦C [c.f. Fig. 1(f) and (i)].Core-level spectra shows that this dramatic reduction indoping occurs despite evidence that almost half of the Liatoms have not deintercalated or desorbed from the sam-ple. The Li 1s core level spectrum shows that the com-ponent assigned to surface-adsorbed Li essentially dis-appears while the components assigned to intercalatedLi remain, in agreement with earlier observations [13–15, 24]. We therefore suggest that some Li atoms whichwere initially adsorbed on the surface, or between thetwo graphene layers, intercalate to the interface betweenSiC and graphene after heating. In doing so, part of thecharge which was previously transferred to the carbonatoms is now transferred to the SiC substrate, thus re-ducing the graphene doping level. If we consider a 2× 1SiC(0001) unit cell, that includes two graphene layers andthree Li atoms as sketched in Fig. 6, we find that it isenergetically more favourable, by 0.54 eV, for one of theLi atoms to move from between the two graphene layersto the interface between the substrate and the bottomgraphene layer.

To visualize the charge transfer, we show in Fig. 6(a)the charge density difference (CDD) that occurs when aLi atom is inserted between the top two carbon layers.The CDD is defined as ∆ρ = ρfinal − ρinital − ρLi , whereρfinal, ρinital and ρLi are the charge densities of the fi-nal system, the initial system that includes only two Liatoms, and an isolated Li atom, respectively. The chargeis transferred equally between the two carbon layers. Incontrast, Fig. 6(b) shows how the charge is distributedfrom the Li atom when it is placed at the interface be-tween SiC and the buffer layer. In this case, the majorityof the charge is transferred to the substrate, with only asmall amount transferred to the decoupled buffer layer.This process can explain the shifting of the Dirac conestowards the Fermi level after heating as observed in theARPES spectra.

V. CONCLUSION

We have carried out detailed ARPES (Angle ResolvedPhotoelectron Spectroscopy) studies of the band struc-ture of monolayer graphene samples, before and after Lideposition at room temperature as well as after subse-quent heating to 300◦C. Li intercalation is shown to have

7

FIG. 6. (Color online) Side view of a 2x1 unit cell ofSiC(0001) that includes two layers of graphene (16 C atoms ineach) and three Li atoms. Isosurfaces of charge density differ-ence show how charge is transferred from a Li atom inserted(a) between the two carbon layers and (b) at the interfacebetween SiC and the bottom carbon layer. A purple (orange)isosurface refers to a gain (loss) in charge density. The iso-surface value is 0.0005 e/a.u.3.

a large impact on the dispersion of the bands at energiesclose to the Dirac point, as well as donating charge tothe graphene layers. Directly after Li deposition we ob-serve the appearance of a second π-band. Upon heatingto 300◦C the electronic band structure changes signifi-cantly: The π-bands become significantly sharper andhave a distinctly different dispersion to that of Bernalstacked bilayer graphene. The position of the Diracpoint is also shifted closer to the Fermi energy to ap-proximately −0.6 eV. To understand these observations,we performed density functional theory band structurecalculations for Li intercalation in free-standing bilayergraphene for different stacking of the layers, as well asfor Li-intercalated graphene on the SiC(0001) surface.Our combined experimental and theoretical results showthat, after Li deposition and subsequent heating, Li bothintercalates underneath the buffer layer – decoupling itfrom the substrate – and between the two carbon layers.In the process, it becomes energetically more favorablefor the carbon layers to become AA-stacked rather thanBernal stacked. We show that the π-bands around theK-point closely resemble the calculated band structure ofa C6LiC6 system, where the intercalated Li atoms intro-duce a periodic perturbation to the graphene electronicstructure that opens pseudo-gaps in the π-bands at theDirac point.

ACKNOWLEDGMENTS

The authors wish to acknowledge the Swedish Re-search Council (VR) grants No. 621-2011-4252 and 621-2011-4426, the Swedish Foundation for Strategic Re-search (SSF) program SRL Grant No. 10-0026, the Euro-pean Union Seventh Framework Programme under grantagreement no. 604391 Graphene Flagship. R.A. acknowl-edges financial support from VR grant No. 621-2011-

4249 and the Linnaeus Environment at Linkoping onNanoscale Functional Materials (LiLi-NFM) funded byVR. I.A.A. acknowledges the support from the Grant ofMinistry of Education and Science of the Russian Feder-ation (Grant No. 14.Y26.31.0005) and Tomsk State Uni-versity Academic D. I. Mendeleev Fund Program (projectNo. 8.1.18.2015). All calculations were performed usingthe supercomputer resources of the Swedish National In-frastructure for Computing (SNIC) National Supercom-puting Center (NSC) and the PDC Centre for High Per-formance Computing (PDC-HPC).

Appendix: Li-intercalated graphene on SiC(0001)

To determine the effect of doping due to the substrate,and also to verify that Li intercalation is sufficient to de-couple the carbon buffer layer from the Si surface, wenow take the SiC(0001) surface into account explicitly.Due to computational constraints we could not model aC6LiC6 bilayer graphene on top of SiC and therefore can-not reproduce the pseudo-gaps that open as a result ofthis periodic potential. Instead we model a C8LiC8 con-centration. In agreement with the results of Refs. [24]and [52] we find that Li intercalation at the interface issufficient to decouple the carbon buffer layer from thesubstrate. The Li atom is located at the T4 site on theSiC(0001) surface, breaking the Si – C bonds. The de-coupled graphene sheet is then free to move laterally sothat the centre of the carbon hexagon is directly over theLi atom beneath it.

FIG. 7. (Color online) Electronic band structure of (a) theLi intercalated zero layer graphene on SiC(0001) system and(b) Li intercalated monolayer graphene on SiC(0001). TheLi concentration corresponds to LiC8 and LiC8LiC8, respec-tively. The insets show the relaxed structure of both configu-rations. The Fermi energy is located at 0 eV.

The band structure for the configuration that includesonly zero layer graphene is shown in Fig. 7(a). A singleπ-band, associated with the now decoupled carbon layer,is visible. The Dirac point is located 0.93 eV below theFermi energy due to the strong doping from the Li atom.Fig. 7(b) then shows the band structure of the config-uration that includes a second carbon layer. Li atomsare now present both underneath the buffer layer and

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between the two carbon layers. As in the free-standingcase, the AA-stacked configuration is energetically pre-ferred. Two Dirac cones are now visible, separated inenergy by 0.7 eV. This would suggest an interlayer cou-pling, γ1, of approximately 0.35 eV, similar to what we

found for the free-standing intercalated bilayer. As a re-sult of the doping due to these Li atoms, the Dirac pointsof the two cones are located 1.5 eV and 0.8 eV below theFermi level, respectively.

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