UV-active plasmons in alkali and alkaline-earth intercalated graphene

The interband $\pi$ and $\pi +\sigma$ plasmons in pristine graphene and the Dirac plasmon in doped graphene are not applicable, since they are broad or weak, and weakly couple to an external longitudinal or electromagnetic probe. Therefore, the ab initio density functional theory is used to demonstrate that the chemical doping of the graphene by the alkali or alkaline-earth atoms dramatically changes the poor graphene excitation spectrum in the ultraviolet frequency range (4–10 eV). Four prominent modes are detected. Two of them are the intralayer plasmons with square-root dispersion, characteristic of the two-dimensional modes. The remaining two are the interlayer plasmons, very strong in the long-wavelength limit but damped for larger wave vectors. The optical absorption calculations show that the interlayer plasmons are both optically active, which makes these materials suitable for small-organic-molecule sensing. This is particularly intriguing because the optically active two-dimensional plasmons have not been detected in other materials.

Figure 1

The intensity of the electronic excitations in (a) ${\mathrm{CsC}}_8$, (b) ${\mathrm{CaC}}_6$, (c) ${\mathrm{LiC}}_6$, and (d) ${\mathrm{LiC}}_2$. The white and green dotted lines in (d) show the boundaries of the $e$-h excitation gaps for the graphene $\pi$ bands around the Dirac point and the Li $\sigma$ bands around the $\mathrm{\Gamma }$ point, respectively.

Figure 2

Spectra of the electronic excitations in ${\mathrm{LiC}}_2$ for the wave vectors (a) $Q=0.17-0.33$ a.u. and (b) $Q=0.016-0.16$ a.u. in the $\mathrm{\Gamma }\mathrm{M}$ direction (solid black lines). (b) Comparison with the doped graphene spectra (dashed red lines) with the matching Fermi level.

Figure 3

(a) The band structure of the ${\mathrm{LiC}}_2$, with the color scheme indicating the predominant origins of particular bands (blue, $\mathrm{Li}\left(\pi \right)$; turquoise, $\mathrm{Li}\left(\sigma \right)$; pink, $\mathrm{C}\left(\sigma \right)$; red, $\mathrm{C}\left(\pi \right)$). (b) Development of the interband plasmons with the increase of the wave vector. Thick solid black lines, $\text{Im}D$; thick dashed black lines, $\text{Re}D$; thin red lines, spectra of the single-particle excitations only. (c) Angle-resolved optical absorption spectra in ${\mathrm{LiC}}_2$, in ultraviolet frequency range.