  Cutting-edge research in the band engineering of nanowires at the ultimate
fine scale is related to the minimum scale of a nanowire-based device. The
fundamental issue at the subnanometre scale is whether angle-resolved
photoemission spectroscopy (ARPES) can be used to directly measure the
momentum-resolved electronic structure of a single wire because of the
difficulty associated with assembling single wire into an ordered array for
such measurements. Here, we demonstrated that the one-dimensional (1D)
confinement of electrons, which are transferred from external dopants, within a
single subnanometre-scale wire (subnanowire) could be directly measured using
ARPES. Convincing evidence of 1D electron confinement was obtained using two
different gold subnanowires with characteristic single metallic bands that were
alternately and spontaneously ordered on a stepped silicon template, Si(553).
Noble metal atoms were adsorbed at room temperature onto the gold subnanowires
while maintaining the overall structure of the wires. Only one type of gold
subnanowires could be controlled using external noble metal dopants without
transforming the metallic band of the other type of gold subnanowires. This
result was confirmed by scanning tunnelling microscopy experiments and
first-principles calculations. The selective control clearly showed that
externally doped electrons could be confined within a single gold subnanowire.
This experimental evidence was used to further investigate the effects of the
disorder induced by external dopants on a single subnanowire using ARPES.
