First-Principles DFT Studies of the Vibrational Properties of Germanene Nanoflakes

The germanium analogue of graphene, germanene, is a potentially new atomically thin quantum material which theory predicts will possess unique transport and optoelectronic properties. Recently, there have been a number of experimental efforts to successfully grow two-dimensional films of germanene on noble metal substrates using molecular beam epitaxy. In addition to this top-down approach of synthesizing large scale films of germanene, we would like to focus on a bottom-up approach where nanoflakes of germanene could be used as molecular seeds or precursors to grow large films of two-dimensional germanene. A knowledge of their infrared and Raman spectra will be critical for characterizing these germanene nanoflakes in future experiments. In this work we used density-functional theory (DFT) to compute the vibrational spectra of a selected number of lower order germanene nanoflakes (e.g. hexagermabenzene, ${\it germa}$-naphthalene, ${\it germa}$-anthracene, ${\it germa}$-phenanthrene, ${\it germa}$-pyrene, ${\it germa}$-tetracene, and ${\it germa}$-pentacene). Our DFT studies also reveal that these germanene nanoflakes are vibrationally stable with buckling of these molecules from their normal two-dimensional planar forms which exist in graphene nanoflakes.