Structural and Plasmonic Evolution in Mixed-Dimensionality Bismuth/Graphene Heterostructures

Mixed-dimensionality heterostructures of low-dimensional bismuth (Bi) with two-dimensional (2D) graphene are of interest in a variety of application fields ranging from nanoelectronics, next-generation batteries, and (photo)catalysis to plasmonics. We here explore the evolution of the morphology and structure of low-dimensional Bi/graphene heterostructures by high-resolution (scanning) transmission electron microscopy ((S)TEM). To this end, we deposit low-dimensional Bi nanostructures onto suspended monolayer graphene membranes via physical vapor deposition (PVD). This enables us to study intrinsic Bi-graphene interactions, in contrast to prior work that utilized Bi on supported graphene. We find that Bi deposited onto room temperature graphene consists of grains formed by irregularly shaped β-Bi crystals with a β-Bi[001]⊥graphene(001) texture and β-Bi nanorods with a β-Bi[2-21]⊥graphene(001) texture. Importantly, both texture types show rotational van der Waals epitaxy with the supporting graphene. The room temperature depositions grow via an initial amorphous β-Bi[2-21]-like state into a closed film of β-Bi structure. For higher graphene temperatures of 150 to 250 °C during deposition, we find the formation of amorphous Bi nanoparticles (NPs) at much reduced coverage due to Bi reverse desorption at these temperatures. While the room temperature deposited Bi films remain static under the electron beam in (S)TEM, the amorphous Bi NPs from higher temperature depositions exhibit electron beam induced in situ crystallization in TEM. In parallel to observing their structural evolution during this crystallization, this also enables us to probe the evolution of plasmonic features of Bi NPs via (valence) electron energy loss spectroscopy ((V)EELS), suggesting a link between crystallization state and Bi NP surface plasmon (SP) energy.