  The ability to engineer the thermal conductivity of materials allows us to
control the flow of heat and derive novel functionalities such as thermal
rectification, thermal switching, and thermal cloaking. While this could be
achieved by making use of composites and metamaterials at bulk scales,
engineering the thermal conductivity at micro- and nano-scale dimensions is
considerably more challenging. In this work we show that the local thermal
conductivity along a single Si nanowire can be tuned to a desired value
(between crystalline and amorphous limits) with high spatial resolution through
selective helium ion irradiation with a well-controlled dose. The underlying
mechanism is understood through molecular dynamics simulations and quantitative
phonon-defect scattering rate analysis, where the behavior of thermal
conductivity with dose is attributed to the accumulation and agglomeration of
scattering centers at lower doses. Beyond a threshold dose, a
crystalline-amorphous transition was observed.
