Deep-UV optical studes of two-dimensional materials

We present the deep-UV optical studies of two-dimensional materials, including graphene, transition metal dichalcogenides, and hexagonal boron nitride (h-BN). Our experimental techniques are deep-UV Raman, photoluminescence (PL), and spectroscopic ellipsometry. First, the Raman intensities from the second-order phonon modes of monolayer graphene and monolayer WS2 are revealed to be decreased or enhanced anomalously by the deep-UV excitation wavelength. We demonstrate theoretically that such resonant behavior correlates with the absorption properties and electron-phonon interactions in these materials. Second, the optical absorption spectra of monolayer and bilayer graphene exhibit two broad bands over the UV spectral region. Based on the first-principles calculations, we assign the observed two absorptions to an excitonic transition at the saddle point (M) in the band structures and collective excitations of the surface plasmons. Third, the deep-UV PL spectra of h-BN single crystal and nanotubes reveal the interactions between the mid-gap defect states and E2g phonon mode. These results advance our understanding of the double resonance Raman scattering process, quasiparticle band structures, and collective excitations of two-dimensional materials.