  Black phosphorus (BP) has emerged as a promising candidate for next
generation electronics and optoelectronics among the 2D family materials due to
its extraordinary electrical/optical/optoelectronic properties. Interestingly,
BP shows strong anisotropic transport behaviour because of its puckered
honeycomb structure. Previous studies have demonstrated the thermal transport
anisotropy of BP and theoretically attribute this to the anisotropy in both
phonon dispersion relation and phonon relaxation time. However, the exact
origin of such strong anisotropy lacks clarity and has yet to be proven
experimentally. In this work, we probe the thermal transport anisotropy of BP
nanoribbons (NRs) by an electron beam technique. We provide direct evidence
that the origin of this anisotropy is dominated by the anisotropic phonon group
velocity for the first time, verified by Young modulus measurements along
different directions. It turns out that the ratio of thermal conductivity
between zigzag (ZZ) and armchair (AC) ribbons is almost same as that of the
corresponding Young modulus values. The results from first-principles
calculation are consistent with this experimental observation, where
anisotropic phonon group velocity between ZZ and AC is shown. Our results
provide fundamental insight into the anisotropic thermal transport in low
symmetric crystals.
