Fig. 1. (a) Schematic of (PEA)2PbI4 crystal structure. (b) Optical microscope image of the as-synthesized crystal. Scale bar: 2 mm. (c) Powder XRD pattern of as-synthesized crystals. (d) Optical microscope image of an exfoliated flake. Scale bar: 10 µm.

Fig. 2. PL spectra of a (PEA)2PbI4 crystal under different excitation at 80 K.

Fig. 3. PL spectra of a (PEA)2PbI4 crystal excited by (a) 2.33 eV and (b) 1.96 eV lasers at different temperatures.

Fig. 4. Raman spectra of a (PEA)2PbI4 crystal under 2.33 eV (black) and 1.96 eV (red) laser excitation at 80 K. The positions of all peaks are 76cm−1(P1), 102.5cm−1(P2), 134cm−1(P3) (non-excited state), and 79.6cm−1(P1), 108.5cm−1(P2), 134cm−1(P3), 160.8cm−1(P4) (excited state).

Fig. 5. Temperature-dependent Raman spectra of a (PEA)2PbI4 crystal under non-excited states excited by (a) 1.96 eV and (b) 2.33 eV lasers, respectively.

Fig. 6. Raman peak versus temperature extracted from Fig. 5. The gray dot lines are used to guide the eye.

Fig. 7. Raman spectra of (PEA)2PbBr4 under 2.33 eV (black) and 1.96 eV (red) laser excitation at 80 K.