Tip-Induced Switch of Germanene Atomic Structure

Abstract

A new germanene crystallographic structure is investigated by scanning tunnelling microscopy and density functional theory calculations. We found that germanene can crystallize in two stable but different structures when grown on Al(111) at the same temperature. These structures are evidenced in scanning tunnelling images by a honeycomb contrast and by a hexagonal contrast. These contrasts are relevant of a Ge network with one (hexagonal) or two (honeycomb) Ge atoms per unit cell shifted upward with respect to the other Ge atoms. These structures appear alternatively and can be turned on and off by a tip-induced process.

Account of the diversity of tunneling spectra at the germanene/Al(1 1 1) interface

“…Although the occurrence of such events is the exception rather than the rule, they emphasize on the fragility of the germanene adlayer. This is consistent with the weak adsorption energy calculated for Ge atoms on the Al(111) surface [33,35]. As an additional hint for the key role of adsorbed species at the tip apex in STS, we show CITS result in Figure 9.…”

Kagome-like silicene: A novel exotic form of two-dimensional epitaxial silicon

“…The bright protrusions are still dominating, but perusal reveals less bright features and dark holes in a rhombic 3´3 supercell (black rhombus). This indicates that the observed 3´3 superstructure is made of Si atoms at different heights, as in the case of the Al(111)3´3-Ge system [14,25,26].…”

“…Nevertheless, even though the system Si on Al(111) exhibits the same 3´3 surface periodicity with respect to Al(111) substrate, the STM images do not resemble those obtained for the Ge on Al(111) system. As a matter of fact, they do not exhibit a clear honeycomb pattern (corresponding to a 2´2 germanene sheet with two protruding Ge atoms matching the Al(111)3´3 supercell [14]), nor just a simple hexagonal pattern with only one protruding Ge atom, with a frequent tip-induced switch between the two patterns [26]. They also differ from the hexagonal structure obtained by Endo et al, which corresponds to a √3´√3 germanene sheet matching a different Al(111)√7´√7 crystallographic supercell [27].…”

“…3c) it confirms that Si atoms form an overlayer on top of the Al(111) surface, as in case of Ge deposition. [14,25,26]). The Al 2p spectrum from the clean bare Al(111) surface acquired in highly surface sensitive conditions (136 eV photon energy) appears nearly symmetric.…”

Tracing the structural evolution of quasi-freestanding germanene on Ag(111)

“…Moreover, the magnitude of the buckling in germanene strongly depends on the on-growing carrier substrate 17 . Ag(111) turned out to be a promising candidate for synthesizing freestanding germanene by considering the encouraging predictions of Dirac cones in germanene on silver 18 , as well as moderate interaction and poor charge transfer at the interface compared to other substrates 19 – 21 . But even germanium alloy phases on Ag(111) feature Dirac signatures in their electronic structure 22 .…”