  As a new two-dimensional layered material, black phosphorus (BP) is a
promising material for nanoelectronics and nano-optoelectronics. We use Raman
spectroscopy and first-principles theory to report our findings related to
low-frequency (LF) interlayer breathing modes (<100 cm-1) in few-layer BP for
the first time. The breathing modes are assigned to Ag symmetry by the laser
polarization dependence study and group theory analysis. Compared to the
high-frequency (HF) Raman modes, the LF breathing modes are much more sensitive
to interlayer coupling and thus their frequencies show much stronger dependence
on the number of layers. Hence, they could be used as effective means to probe
both the crystalline orientation and thickness for few-layer BP. Furthermore,
the temperature dependence study shows that the breathing modes have a harmonic
behavior, in contrast to HF Raman modes which are known to exhibit
anharmonicity.
