Influence of the layer number and stacking order on out-of-plane phonons in few-layer graphene

Abstract

We investigate the stacking-order dependence of the double-resonant LO–ZO Raman combination mode, also called N mode, in ABA- and ABC-stacked trilayer graphene. By tuning the laser energy, we observe different N mode dispersions for both stackings and show that these shifts are indicative for the stacking type. We explain the different shifts with stacking-related changes in the electronic bands and the phonon dispersion. Additionally, we performed simulations of the double-resonant Raman spectra of the N mode in bilayer graphene. Our calculations predict a splitting of the N mode for laser excitation energies above 2.6 eV due to different contributions from both bands. We also analyzed the subpeaks of the N mode in few-layer graphene and explain their appearance only at even numbers of graphene layers. Graphical illustration of the in-plane LO and out-of-plane ZO -point vibrations in Bernal-stacked (AB-stacked) bilayer graphene. The unit-cell atoms are highlighted in blue. The dashed vertical lines denote the unit cell atoms that are directly above each other.