  The vibrational modes in organic/inorganic layered perovskites are of
fundamental importance for their optoelectronic properties. The hierarchical
architecture of the Ruddlesden-Popper phase of these materials allows for
distinct directionality of the vibrational modes withrespect to the main axes
of the pseudocubic lattice in the octahedral plane. Here, we study the
directionality of the fundamental phonon modes in single exfoliated
Ruddlesden-Popper perovskite flakes with polarized Raman spectroscopy at
ultralow-frequencies. A wealth of Raman bands is distinguished in the range
from 15-150 cm-1 (2-15 meV), whose features depend on the organic cation
species, on temperature, and on the direction of the linear polarization of the
incident light. By controlling the angle of the linear polarization of the
excitation laser with respect to the in-plane axes of the octahedral layer, we
gain detailed information on the symmetry of the vibrational modes. The choice
of two different organic moieties, phenethylammonium (PEA) and butylammonium
(BA) allows to discern the influence of the linker molecules, evidencing strong
anisotropy of the vibrations for the (PEA)2PbBr4 samples. Temperature dependent
Raman measurements reveal that the broad phonon bands observed at room
temperature consist of a series of sharp modes, and that such mode splitting
strongly differs for the different organic moieties and vibrational bands.
