Abstract
Group IV nonmetal elements consist of C, Si, and Ge. They can each form a 2D monolayer called graphene, silicene, and germanene on metal surfaces. Multilayer graphene can be grown with weak Van der Waals-type interlayer coupling. Naturally, it is also desirable to grow multilayer silicene and germanene and heterostructure of the two 2D materials. To this end, a good start is to study and understand the adsorption and growth mechanism of Ge and Si on the monolayer silicene on metal surfaces such as the Ag(111) surface. Si MBE on Ag(111)-supported silicene monolayer results in multilayer 2D islands with the same (√3 × √3) surface structure above 1.0 ML. It is possible that Ag acts as a surfactant to foster the low-temperature growth of metastable silicene multilayers, which would be an analog of graphite and a new state of Si. However, several experiments, including the deuterium atomic etching, have confirmed that the multilayer Si films grown on Ag(111) have a bulk Si structure, not a stack of silicene. By contrast, Ge MBE on top of silicene monolayer on the Ag(111) surface results in either a dispersed adlayer or an ordered adlayer depending on the silicene phases. The ordered adsorbed Ge atoms on the (3 × 3) domains occupy directly on top of down-atoms in the buckled silicene layer, similar to the adatom positions on the Ge(111)-c(2 × 8) surface. Experimental results suggest that deposited Ge atoms tend to build an additional 3D bulk layer on silicene monolayer and that the growth of the 2D germanene/silicene heterostructure may not be achieved in a straightforward manner due in part to their dangling bond characteristics, which are originated from sp -sp hybridization.