  Porous materials provide a large surface to volume ratio, thereby providing a
knob to alter fundamental properties in unprecedented ways. In thermal
transport, porous nanomaterials can reduce thermal conductivity by not only
enhancing phonon scattering from the boundaries of the pores and therefore
decreasing the phonon mean free path, but also by reducing the phonon group
velocity. Here we establish a structure-property relationship by measuring the
porosity and thermal conductivity of individual electrolessly etched single
crystalline silicon nanowires using a novel electron beam heating technique.
Such porous silicon nanowires exhibit extremely low diffusive thermal
conductivity (as low as 0.33 Wm-1K-1 at 300K for 43% porosity), even lower than
that of amorphous silicon. The origin of such ultralow thermal conductivity is
understood as a reduction in the phonon group velocity, experimentally verified
by measuring the Young modulus, as well as the smallest structural size ever
reported in crystalline Silicon (less than 5nm). Molecular dynamics simulations
support the observation of a drastic reduction in thermal conductivity of
silicon nanowires as a function of porosity. Such porous materials provide an
intriguing platform to tune phonon transport, which can be useful in the design
of functional materials towards electronics and nano-electromechanical systems.
