Low Frequency Vibrational Modes of Two-Dimensional Lead-Free Metal Halide Double Perovskites

2D
layered double perovskites of (S-MPA)4AgBiI8 (MPA-AgBiI8) and (S-MPA)4CuBiI8 (MPA-CuBiI8) (S-MPA, S-β-methylphenethylammonium)
were synthesized with a hydrothermal method. The crystal structure
of MPA-AgBiI8 was determined using single-crystal X-ray
diffraction (scXRD). Powder XRD (pXRD) data suggest that the crystal
structure of MPA-CuBiI8 is more complex than that of MPA-AgBiI8. UV–vis electronic absorption spectra of these perovskites
reveal a bandgap of 2.03 eV for both. Short exciton lifetime from
time-resolved photoluminescence (TRPL) results and low PL intensity
of the Cu-based perovskite suggest a high density of trap states within
the bandgap. Low frequency Raman spectra of both materials show distinct
peaks and a slightly higher frequency for the Cu-based perovskite
than the Ag-based perovskite. Density functional theory (DFT) calculations
were conducted to simulate the low frequency Raman spectra and help
explain the different phonon modes, which are collective vibrations
of metal halide bond bending and stretching within the Ag- and Cu-centered
octahedra coupled with MPA libration and twisting within the inorganic
layer. The DFT results also quantified octahedral distortions in
the two perovskites. This combined experimental and computational
study provides new insights into the low frequency vibrations of 2D
perovskites.