  We reveal through extensive DFT calculations, confronted to key experimental
facts, the hidden penta-silicene nature of single and double strand chiral Si
nanoribbons perfectly aligned on Ag(110) surfaces, that were discovered in
2005, but whose structure remained elusive. We thus simultaneously demonstrate
the existence of penta-silicene, a recently conjectured novel pentagonal
silicon allotrope, which remained unveiled for 11 years, and which materializes
a paradigmatic shift from normal hexagonal silicene.
