Monolayer germanene, a novel graphene-like germanium allotrope akin to silicene has been recently grown on metallic substrates. Lying directly on the metal surfaces the reconstructed atom-thin sheets are prone to lose the massless Dirac fermion character and unique associated physical properties of free standing germanene. Here, we show that few layer germanene, which we create by dry epitaxy on a gold template, possesses Dirac cones thanks to a reduced interaction. This finding established on synchrotron-radiation-based photoemission, scanning tunneling microscopy imaging and surface electron diffraction places few layer germanene among the rare two-dimensional Dirac materials. Since germanium is currently used in the mainstream Si-based electronics, perspectives of using germanene for scaling down beyond the 5 nm node appear very promising. Other fascinating properties seem at hand, typically the robust quantum spin Hall effect for applications in spintronics and the engineering of Floquet Majorana fermions by light for quantum computing.

Carbon and silicon pentagonal low-dimensional structures attract a great interest as they may lead to new exotic phenomena such as topologically protected phases or increased spin–orbit effects. However, no pure pentagonal phase has yet been realized for any of them. Here we unveil through extensive density functional theory calculations and scanning tunnelling microscope simulations, confronted to key experimental facts, the hidden pentagonal nature of single- and double-strand chiral Si nano-ribbons perfectly aligned on Ag(110) surfaces whose structure has remained elusive for over a decade. Our study reveals an unprecedented one-dimensional Si atomic arrangement solely comprising almost perfect alternating pentagons residing in the missing row troughs of the reconstructed surface. We additionally characterize the precursor structure of the nano-ribbons, which consists of a Si cluster (nano-dot) occupying a silver di-vacancy in a quasi-hexagonal configuration. The system thus materializes a paradigmatic shift from a silicene-like packing to a pentagonal one.

The charge inhomogeneity at the overlayer and the origin of the two $\mathrm{Sn}4d$ core-level components present at the $\mathrm{Sn}$-covered $\mathrm{Ge}\left(111\right)$ and $\mathrm{Si}\left(111\right)$ $\alpha$ phases have been investigated. We probe comparatively by high-resolution core-level spectroscopy, the prototypical tin on silicon or germanium $\left(111\right)$ systems upon dosing these with a donor (potassium) and an acceptor (iodine). Giant effects are noticed which reveal the bimodal behavior of two types of adatoms with two well-defined, but inequivalently charged, states. We find that the relative weights of these two distinctive $\mathrm{Sn}$ atoms, in inverse proportions on $\mathrm{Si}\left(111\right)$ and $\mathrm{Ge}\left(111\right)$, can be extrinsically and completely tuned by electron or hole donation. These results shed light on the phase-transition driving force for $\mathrm{Sn}∕\mathrm{Ge}\left(111\right)$ and $\mathrm{Sn}∕\mathrm{Si}\left(111\right)$ $\alpha$ phases.