Graphene Intercalation

Over the past few years, it has been discovered that a number of elements will "intercalate" between graphene and its substrate support, or between graphene layers. This suggests an analogy to the well-known Graphite Intercalation Compounds (GIC's) that have been an important material class in condensed matter physics for many decades [1]. By intercalating different atoms between layers of graphite, it is possible to dramatically tune physical properties. For example, intercalating FeCl₃ leads to interesting magnetic ordering phenomena [2] and intercalating Ca leads to an unusual superconductor (with stoichiometry CaC₆) that has a relatively high transition temperature [3].

Intercalation of atoms beneath graphene to make graphene intercalation compounds could be a more kinetically efficient process than bulk graphite intercalation. In fact, a large number of elements have already been observed to intercalate graphene on substrates including H [4], Au [5,6], and Ge [7]. Alkalai metal intercalation has been particularly important in revealing unique aspects of the band structure of graphene such as evidence for "plasmaron" quasiparticles [8] and an extended van Hove singularity [9]. In the simplest view, the alkalai's ionize upon interacting with the surface and "dope" the graphene with their outer valence s electron.

Our group has achieved the first high resolution STM imaging of Na intercalation on epitaxial graphene on SiC(0001) [10]. We found that 2 different Na intercalation structures can coexist upon Na deposition at room temperature (Figure 1) and characterized these structures by combing local tunneling spectra of image-potential-derived states with DFT calculations [10]. The initial intercalation process occurs primarily by insertion of Na between single layer graphene and the interfacial buffer layer. With sufficient time (accelerated by annealing), Na tends to move deeper down by one layer to decouple the interfacial buffer layer. Understanding the complex coexistence between these structures is crucial to interpreting spatially-averaged measurements like angle-resolved photoemission and to assessing the versatility of graphene intercalation.

Figure 1. Intercalation structures of Na at the epitaxial graphene-SiC interface as reported in Ref. [10]. The top panel shows a structure prevalent at room temperature where substrate corrugation is visible in the Na-intercalated domains. The bottom panel shows a denser intercalation of Na where the substrate corrugation is no longer seen.

In addition, Na intercalates between the 6 root 3 buffer layer and the SiC substrate, resulting in the formation of an single electron-doped graphene sheet that is different from the typical single layer graphene on SiC [10]. This idea of buffer layer decoupling by intercalant atoms represents a versatile approach to fine tuning electronic properties of supported graphene on SiC.

References

[1] M.S. Dresselhaus and G. Dresselahaus, Adv. in Phys. 51, 1 (2002).

[2]Simon et al., Synthetic Metals 8, 53 (1983).

[3]Weller et al., Nat. Phys. 1, 39 (2005).

[4] Riedl et al., Phys. Rev. Lett. 103, 246804 (2009).

[5] Premlal et al., Appl. Phys. Lett. 94, 263115 (2009).

[6] Gierz et al., Phys. Rev. B 81, 235408 (2010).

[7] Emstev et al. Phys. Rev. B 84, 125423 (2011).

[8] Bostwick et al., Science 328, 999 (2010).

[9] McChesney et al., Phys. Rev Lett 104, 136803 (2010).

[10] Sandin et al., Phys. Rev. B 85, 125410 (2012).