Unusual terahertz-wave absorptions in δ/α-mixed-phase FAPbI3 single crystals: interfacial phonon vibration modes

The terahertz (THz)-wave absorption properties in organic-inorganic hybrid perovskite (OHP) materials are investigated with the in-depth development of OHP-based THz applications. In the THz range from 0.5 to 3 THz, OHPs typically show several interesting phonon modes such as transverse, longitudinal, and halogen self-vibrations. To modulate these frequencies, the density changes in defect-incorporated structures and element mixtures were tested and confirmed. In the literature, the origin of phonon modes in OHP materials have been mostly explained. However, we found new phonon vibration modes in formamidinium (FA)-based hybrid perovskite structures. FAPbI3 single crystals, organic–inorganic hybrid perovskites, of the δ-, δ/α-mixed-, and α-phases were prepared. We intriguingly found that the δ/α-mixed-phase exhibited significant THz-wave absorption peaks at 2.0 and 2.2 THz that were not related to any phonon modes from either the δ- or α-phases, although the δ/α-mixed-phase sample was confirmed to be formed by a physical combination of the δ- and α-phases without the creation of any new chemical states. Our theoretical study performed with ab initio calculations provides an explanation for these unusual THz-wave absorption behaviors; they originate from the novel vibration modes excited at the seamless interfaces in the mixed phase of FAPbI3. We found new phonon vibration modes in formamidinium (FA)-based hybrid perovskite structures. We intriguingly found that the δ/α-mixed-phase exhibited significant THz-wave absorption peaks at 2.0 and 2.2 THz that were not related to any phonon modes from either the δ- or α-phases, although the δ/α-mixed-phase sample was confirmed to be formed by a physical combination of the δ- and α-phases without the creation of any new chemical states. From the theoretical study, they originate from the novel vibration modes excited at the seamless interfaces in the mixed phase of FAPbI3. New vibrational modes generated by low-cost organic–inorganic crystals could be of value for terahertz devices used for non-invasive cell and tissue imaging. Vibrationally excited molecules can often be identified from terahertz energy signals, but these signals are hard to detect with conventional electronics. Researchers led by Young-Kyun Kwon from Kyung Hee University in Seoul, South Korea, and Min-Cherl Jung at the University of Tsukuba in Ibaraki, Japan, report insights into how optically active materials known as hybrid perovskites can be engineered to improve terahertz detection. The team’s analysis of formamidinium-lead-iodide perovskites showed that new terahertz-sensitive modes could be seen when the material solidified into a crystal containing two different structural arrangements known as phases. Theoretical simulations revealed the modes appear because of vibrations generated at the phase interfaces.