Directional Anisotropy of the Vibrational Modes in 2D-Layered Perovskites

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 with respect 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 to 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 us 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. Softer molecules
such as BA result in lower vibrational frequencies and splitting into
fewer modes, while more rigid molecules such as PEA lead to higher
frequency oscillations and larger number of Raman peaks at low temperature.
Interestingly, in distinct bands the number of peaks in the Raman
bands is doubled for the rigid PEA compared to the soft BA linkers.
Our work shows that the coupling to specific vibrational modes can
be controlled by the incident light polarization and choice of the
organic moiety, which could be exploited for tailoring exciton–phonon
interaction, and for optical switching of the optoelectronic properties
of such 2D layered materials.