  Extreme nanowires (ENs) represent the ultimate class of crystals: They are
the smallest possible periodic materials. With atom-wide motifs repeated in one
dimension (1D), they offer a privileged perspective into the Physics and
Chemistry of low-dimensional systems. Single-walled carbon nanotubes (SWCNTs)
provide ideal environments for the creation of such materials. Here we present
a comprehensive study of Te ENs encapsulated inside ultra- narrow SWCNTs with
diameters between 0.7 nm and 1.1 nm. We combine state-of-the-art imaging
techniques and 1D-adapted ab initio structure prediction to treat both
confinement and periodicity effects. The studied Te ENs adopt a variety of
structures, exhibiting a true 1D realisation of a Peierls structural distortion
and transition from metallic to insulating behaviour as a function of
encapsulating diameter. We analyse the mechanical stability of the encapsulated
ENs and show that nanoconfinement is not only a useful means to produce ENs,
but may actually be necessary, in some cases, to prevent them from
disintegrating. The ability to control functional properties of these ENs with
confinement has numerous applications in future device technologies, and we
anticipate that our study will set the basic paradigm to be adopted in the
characterisation and understanding of such systems.
