Ca intercalated bilayer graphene as a thinnest limit of superconducting C₆Ca

Abstract

Success in isolating a 2D graphene sheet from bulky graphite has triggered intensive studies of its physical properties as well as its application in devices. Graphite intercalation compounds (GICs) have provided a platform of exotic quantum phenomena such as superconductivity, but it is unclear whether such intercalation is feasible in the thinnest 2D limit (i.e., bilayer graphene). Here we report a unique experimental realization of 2D GIC, by fabricating calcium-intercalated bilayer graphene C₆CaC₆ on silicon carbide. We have investigated the structure and electronic states by scanning tunneling microscopy and angle-resolved photoemission spectroscopy. We observed a free-electron–like interlayer band at the Brillouin-zone center, which is thought to be responsible for the superconductivity in 3D GICs, in addition to a large π* Fermi surface at the zone boundary. The present success in fabricating Ca-intercalated bilayer graphene would open a promising route to search for other 2D superconductors as well as to explore its application in devices.

Graphene has attracted considerable attention because it exhibits a variety of unusual physical properties such as the massless charge carriers and the quantum Hall effect (1). Bipolar supercurrent (2) and spin injection at room temperature (3, 4) demonstrate its high potential for application in spintronic devices. To fabricate practical devices based on graphene, it is essential to modify and control electronic parameters such as the sign and concentration of carriers as well as the band gap at the Dirac point. In fact, several attempts have been made to achieve material engineering by introducing an external electric field (5–9) or by depositing atoms or molecules on a graphene sheet (10, 11). In graphite, however, intercalation of guest atoms and molecules into graphite layers (12–15) is known to considerably modify the electronic structure, leading to unique physical properties and technological applications such as superconductivity and rechargeable batteries. It is thus quite challenging to fabricate the thinnest limit of a graphite intercalation compound (GIC), namely, intercalated bilayer graphene, and to investigate its electronic structure to promote graphene engineering.

Results and Discussion

First we demonstrate how to fabricate Ca-intercalated bilayer graphene C₆CaC₆, regarded as the thinnest limit of Ca-intercalated GIC, where Ca atoms are sandwiched between two graphene layers. We used the “atom-replacement method” as explained below. Fig. 1 A–C displays a series of low-energy electron diffraction (LEED) patterns during the fabrication process. First we prepared a high-quality bilayer graphene sheet on a silicon carbide crystal by heating the crystal to 1550 °C in an argon atmosphere (Methods) (16). Then we deposited lithium (Li) atoms using a SAES Getter on the graphene sheet, which led to the emergence of sharp √3×√3R30° LEED spots (Fig. 1A), indicative of intercalation of Li atoms in a regular manner between two adjacent graphene layers (17) with the same structure as in bulk C₆Li. Next, we deposited Ca atoms on this C₆LiC₆ sheet, keeping the substrate temperature at 140 °C, slightly below the Li desorption temperature of 145 °C. Subsequently, we increased the temperature of the substrate to 145 °C, which caused a marked intensity reduction of the √3×√3R30° spot (Fig. 1B). Surprisingly, further heating above 145 °C gradually recovered the intensity of the same √3×√3R30° spot (Fig. 1C). Intriguingly, the intensity of the recovered √3×√3R30° spot was much brighter and sharper than that of C₆LiC₆ prepared at 140 °C (Fig. 1D, line profiles). We observed that the √3×√3R30° spot simply disappeared above 145 °C in the case of C₆LiC₆ without Ca deposition. We further observed that the deposition of Ca on single-layer and bilayer graphene by skipping the Li predeposition process hardly reproduced the √3×√3R30° LEED pattern. These experimental findings suggest that intercalated ordered Li atoms are effectively replaced with Ca atoms at 145 °C, leading to formation of well-ordered Ca-intercalated bilayer graphene C₆CaC₆ on the SiC substrate (Fig. 1E).

Fig. 1.

Fabrication of Ca-intercalated bilayer graphene. (A and B) LEED-pattern evolution of bilayer graphene during the Ca-intercalation process. Heating of Li-intercalated bilayer graphene (C₆LiC₆, A) after Ca deposition leads to the marked suppression of the √3×√3R30° superlattice spots (intermediate state; 145 °C, B), and further heating recovers the same spots (C₆CaC₆, C). (D) Representative intensity line profile of the √3×√3R30° LEED spot in the region denoted by squares in A–C. (E) Schematic view of crystal structure of C₆CaC₆ on SiC substrate.

To confirm the intercalation of guest atoms during the fabrication process, we carried out scanning tunneling microscopy (STM) measurements on C₆LiC₆ and C₆CaC₆ samples. The representative topograph images at the sample bias voltage of −100 mV are displayed in Fig. 2 A and B, respectively. In both films, a periodic intensity pattern indicative of the well-ordered surface is observed. Whereas the local atomic position of the bright spot looks different between C₆LiC₆ and C₆CaC₆ (discussed later), their Fourier-transform images commonly show the √3×√3R30° spots, consistent with the LEED measurements (Fig. 1, squares). We confirmed that the surface of these samples is fairly flat, as evidenced by the height profile (Fig. 2 C and D). The local oscillation of height is at most 10 picometer (0.1 Å), which is much smaller than the ionic radius of a Li or Ca atom (0.6–1 Å), suggesting that Li and Ca atoms do not reside on the surface. These results firmly establish that ordered Ca (Li) atoms are indeed intercalated into adjacent graphene layers.

Fig. 2.

STM images of Li- and Ca-intercalated bilayer graphene films. Constant-current STM images (50 × 50 Å, sample bias voltage V_s = −100 mV, and set-point tunneling current I_t = 30 pA) of (A) C₆LiC₆ and (B) C₆CaC₆. Hexagons represent the C honeycomb lattice, and Ca atoms are situated at the center of colored hexagons in B. Inset shows corresponding Fourier-transform images. (C and D) Height profiles along the cut indicated by red lines in A and B, respectively. We have surveyed a several-micrometer-square area by STM and always observed essentially the same image as B. This indicates that Li atoms are successfully replaced with Ca throughout the sample.

It would then be expected that the Ca intercalation causes electron doping from the intercalant (Ca) to the graphene sheet, as in bulk GICs (18–25). Such behavior is vividly seen by a side-by-side comparison of the experimental valence-band structure between pristine bilayer graphene and C₆CaC₆. As seen in Fig. 3A, the band dispersions of the C2p π and σ bands are shifted downward as a whole upon the Ca intercalation, resulting in the movement of the Fermi-level (E_F) crossing point of the π* band away from the K point. We find additional weak features in the band structure of C₆CaC₆ that are not seen in pristine bilayer graphene, for example, a weak X-shaped band around the Γ point at 0–4 eV and a dispersive band at the K point at around 5 eV. All these bands are ascribed to the folded bands due to the superstructure of intercalated Ca atoms, because the K point of the Brillouin zone (BZ) of graphene is folded to the Γ point of the reconstructed BZ in C₆CaC₆.

Fig. 3.

Electronic structure of C₆CaC₆. (A) Comparison of the valence-band dispersion between pristine bilayer graphene (Left) and C₆CaC₆ (Right). (B) Fermi surfaces of C₆CaC₆ determined by plotting the ARPES intensity integrated within ±20 meV with respect to E_F and folded by taking into account the sixfold crystal symmetry. White and red lines correspond to the Brillouin zones of graphene and C₆CaC₆, respectively. (C) Comparison of Fermi vectors k_Fs determined by ARPES (solid blue circles) with calculated FSs (red and green curves in ref. 26). Estimated experimental FS volume by taking into account the contribution from the interlayer band is 2.02 ± 0.05 electrons per C₆CaC₆ unit cell, consistent with the divalent nature of the Ca ions. Note that the charge transfer from the substrate is negligibly small (0.004 electrons per unit cell, as estimated from the FS volume of pristine bilayer graphene) compared with the charge carrier density in Ca-intercalated bilayer graphene (2.02 electrons per unit cell).

To directly clarify the influence of Ca intercalation on the near-E_F electronic states, we have mapped out the Fermi surface (FS) of C₆CaC₆ and compared the result with that of pristine bilayer graphene in Fig. 3B Inset. Obviously, the FS topology of C₆CaC₆ appears to be considerably different from that of the pristine sample. As seen in Fig. 3B, a tiny π* FS at the K point in graphene is significantly expanded upon Ca intercalation, and several FSs with complicated intensity patterns emerged at the same time. From the plot of measured Fermi wave vectors (k_Fs) in Fig. 3C, we have estimated that there are at least three different types of FSs in C₆CaC₆: a triangular hole pocket centered at the K′ point in the reconstructed BZ, a snowflake-like electron pocket at the Γ point, and a hexagonal electron pocket also at the Γ point. The latter two electron pockets are created by the folding of triangular π* FS at the K point of graphene BZ. The good agreement of the experimentally observed FS with the π* FS calculated for C₆CaC₆ (ref. 26) (Fig. 3C) suggests that Ca atoms are certainly intercalated in a well-ordered manner into bilayer graphene and the electronic charge is transferred from the Ca atoms to the graphene layers.

To see more clearly the complicated electronic states around the Γ point, we plot in Fig. 4B the experimentally determined band dispersions near E_F along the K′ΓK′ high-symmetry line, compared with the calculation (Fig. 4A) (26). We observe a couple of π* bands: The inner band consists of two nearly degenerated π* bands, forming the snowflake-like and hexagonal electron pockets, respectively (Fig. 3C), whereas the outer π* band is responsible for the triangular hole pocket. In addition to these π* bands, we recognize in Fig. 4B a parabolic band that has a bottom of dispersion at 0.5 eV and merges into the inner π band around E_F. A similar parabolic band dispersion is also observed in another high-symmetry line M′ΓM′ (Fig. 4C), indicating its isotropic nature. By taking into account the good agreement of its dispersive feature with the calculated band (green curve in Fig. 4A), this band is assigned to the interlayer (IL) band with a nearly free-electron–like character (26–28), as also reported in the previous angle-resolved photoemission spectroscopy (ARPES) study of bulk C₆Ca (18). It is notable that the parabolic band is completely absent in C₆LiC_6, as shown in Fig. 4D, whereas the two linearly dispersive π* bands are clearly observed, as in the case of C₆CaC₆. The appearance and absence of the interlayer band in C₆CaC₆ and C₆LiC₆, respectively, is also clearly seen in the ARPES spectrum itself in Fig. 4E. This indicates that emergence of the interlayer band is not a common feature for all 2D GICs. Intriguingly, the appearance of the interlayer band near E_F in C₆CaC₆ is also recognized in its STM image with the bias voltage near E_F. As seen in Fig. 2, the Ca site in C₆CaC₆ has a relatively high intensity compared with the Li site in C₆LiC₆, in good agreement with the theoretical prediction that the interlayer band near E_F is substantially contributed by the Ca 3d atomic orbital (29). It is noted that the ARPES data of C₆CaC₆ show no evidence of contamination from C₆LiC₆, suggesting the perfect replacement of Li with Ca during the sample fabrication.

Fig. 4.

Interlayer band in C₆CaC₆. Comparison of (A) the band calculation (26) with experimental band dispersion near E_F around the Γ point in C₆CaC₆ along (B) the K′ΓK′ and (C) the M′ΓM′ cuts. Experimental band dispersion was obtained by taking the second derivative of ARPES spectra. Estimated effective mass of the interlayer band (IL) m* is 60% of the free electron mass m₀. (D) Same as B but for C₆LiC₆. (E) ARPES spectra of C₆CaC₆ and C₆LiC₆ at the Γ point (points α and β in C and D, respectively).

The present success in fabricating the thinnest limit of GICs, C₆CaC₆, by the atom-replacement method opens a promising pathway to the investigation of unique physical properties of 2D GICs as well as to their technological applications. The observation of the interlayer band near E_F in C₆CaC₆, as in 3D C₆Ca (ref. 18), would accelerate systematic studies on low-temperature properties such as superconductivity and/or competing order. If superconductivity is realized in 2D GICs, it would be quite challenging to study the similarities and differences between 2D and 3D superconducting mechanisms. The present results demonstrate that the interlayer electrons also participate in the transport properties in 2D GICs, in addition to the π* electrons. Such a situation adds functionality to graphene engineering, which essentially relies on only the π* electrons. By controlling the type and concentration of interlayer electrons as well as the number of graphene layers in 2D GICs, it would be possible to achieve high conductivity and fine-tune it. This advantage is particularly useful in fabricating graphene-based nanodevices such as field-effect transistors. In addition to electronic devices, ultrathin battery and microchemical catalysis are also highly potential targets for 2D GICs. Thus, intercalated graphene provides a promising platform for devices and technological applications of graphene-based nanomaterials.

Methods

Bilayer graphene was prepared by heating a n-type Si-rich 6H-SiC(0001) single crystal to 1550 °C with resistive heating under Ar gas at 0.1 MPa (16, 17, 30). The number of graphene layers was conformed by the number of π bands in ARPES measurements (10, 16, 17). Atomic force microscopy measurements confirmed the typical terrace size of ∼5 μm. Deposition of lithium was carried out using a lithium dispenser (SAES Getters), and calcium was deposited with a Knudsen cell under ultrahigh vacuum of 5 × 10⁻¹⁰ torr. LEED measurements were performed with the primary electron energy of 150 eV. After the fabrication of C₆CaC₆ and its characterization by LEED, we transferred the sample to the ARPES spectrometer chamber or STM system without breaking vacuum and performed in situ ARPES or STM measurements. The STM system was operated using a Unisoku USM-1300S at 78 K under ultrahigh vacuum below 2.0 × 10⁻¹⁰ torr. All STM images were obtained in a constant current mode. ARPES measurements were carried out using a VG-Scienta SES2002 spectrometer with a high-flux helium discharge lamp and a toroidal grating monochromator at Tohoku University. The He IIα (photon energy hν = 40.814 eV) resonance line was used to excite photoelectrons. The energy and angular (momentum) resolutions were set at 16 meV and 0.2° (0.01Å⁻¹), respectively. The temperature of the samples was kept at 30 K during the ARPES measurements. The Fermi level of sample was referenced to that of a gold film deposited onto the sample substrate. No degradation of the sample surface was observed during the measurement period of 48 h. We have checked the reproducibility of the data by measuring more than 10 samples.