  Pentagon is one of the most beautiful geometric structures in nature, but it
is rarely seen simply because five-fold symmetry is mathematically forbidden in
a 2D or 3D periodic lattices. Fortunately, pentagon as a structural element is
allowed in 1D or 0D systems, since translational symmetry is not necessary
there. However, in these systems pentagons only compose a small portion of the
structure. So far, 1D or 0D systems consisting of purely pentagons are still
rare. Here, combing high resolution non-contact atomic force microscopy and
tip-enhanced Raman spectroscopy, we have directly visualized the pentagon-ring
structure in self-assembled Si nanoribbons and magic clusters on Ag(110)
substrate. Moreover, chemical fingerprint of Si pentagon was detected in
individual Si nanoribbon and clusters by tip-enhanced Raman spectroscopy. This
work demonstrates that Si pentagon can be an important element in building
silicon nanostructures, which may find potential applications in future
nanoelectronics based on silicon.
