Quantum-Well Bound States in Graphene Heterostructure Interfaces

We present experimental evidence of electronic and optical interlayer resonances in graphene van der Waals heterostructure interfaces. Using the spectroscopic mode of a low-energy electron microscope (LEEM), we characterized these interlayer resonant states up to 10 eV above the vacuum level. Compared with nontwisted, AB-stacked bilayer graphene (AB BLG), an $\approx 0.2\AA$ increase was found in the interlayer spacing of 30° twisted bilayer graphene (30°-tBLG). In addition, we used Raman spectroscopy to probe the inelastic light-matter interactions. A unique type of Fano resonance was found around the D and G modes of the graphene lattice vibrations. This anomalous, robust Fano resonance is a direct result of quantum confinement and the interplay between discrete phonon states and the excitonic continuum.

Figure 1

Low-energy electron microscopy of graphene grown via CVD on a Ni–Cu gradient alloy foil. (a) Schematics of electron and photon scattering experimental setup. (b) Atomic model of the dodecagonal pattern formed by the 30°-tBLG crystal structure. (c) Bright-field LEEM image of a typical sample area, incident electron energy $E=5.6\mathrm{eV}$; the scale bar is $5\mu \mathrm{m}$. (d) $\mu$-LEED on a 30°-tBLG area taken at an electron energy of $E=42\mathrm{eV}$.

Figure 2

Dynamical low-energy electron reflectivity of graphene systems on ${\mathrm{TiO}}_x$ substrate. (a) Bright-field LEEM image of a typical 30°-tBLG sample on ${\mathrm{TiO}}_x$, $E=2.3\mathrm{eV}$, scale bar is $5\mu \mathrm{m}$. (b) Schematics of side view of presented graphene interface material system. (c) LEEM-IV spectra of 30°-tBLG, AB BLG, and SLG samples on ${\mathrm{TiO}}_x$.

Figure 3

(a) Raman spectrum of a transferred SLG, AB BLG, and 30°-tBLG on ${\mathrm{TiO}}_x$ substrate. (b)–(d) Scanning Raman image using integrated peak intensity centered around D peak, G peak, and 2D peak, respectively; integration window is $100{\mathrm{cm}}^{-1}$. (e)–(g) Fano model fitting of G peak for SLG, AB BLG, and 30°-tBLG, respectively.