Low-Temperature Photoluminescence Spectra of Single Lead Halide Perovskite Nanocrystals 

This page is a summary of my research subjects when I was a graduate student in Kanemitsu Lab, Kyoto University. In the following, only the papers I published as the first author are introduced. For a list of publications including co-authored papers, please click here.

I have been working on the low-temperature photoluminescence (PL) spectra of single nanocrystals (NCs) of lead halide perovskite (APbX3, A = Cs, MA (CH3NH3), FA (HC(NH2)2), X = Cl, Br, I), mainly on the following topics.

✓Exciton-Phonon Coupling

  Single photon source is one of the application examples of perovskite NCs. Long exciton coherence time is important for the generation of the indistinguishable single photons, and weaker exciton-phonon coupling is desirable. The observation of phonon sidebands at low temperatures is a powerful method for measuring the strength of exciton-phonon coupling, but many PL peaks appear in the low-temperature PL spectra in single perovskite NCs, and the origin of these peaks has been unclear.

  In this study, we first measured the PL spectra of single CsPbBr3 and FAPbBr3 NCs at 5.5 K, and showed that the PL peaks consist of exciton PL peak, LO-phonon replicas of excitons (four for CsPbBr3 and two for FAPbBr3), and trion and biexciton PL peaks. Compared with first principle calculations, the LO-phonon modes of observed LO-phonon replicas were clarified. We measured the strengths of the exciton-phonon coupling, so called Huang-Rhys factors, by using the PL intensities of LO phonon replicas. We found that they dramatically increases with decreasing NC size. This phenomenon can be explained by the increasing internal electric field with decreasing NC size, which causes a reduction in the spatial overlap between the electron and hole wavefunctions. Furthermore, a comparison of exciton-LO phonon and trion-LO phonon couplings in CsPbBr3 NCs revealed that they show different size dependences of the Huang-Rhys factors each other, reflecting the different charge distributions of the excitons and trions. Two of the papers on this research subject have been published in Nano Letters, a high impact journal in the field of nanotechnology.

  Paper [1] was awarded “Student Presentation Award” (Japanese Only) and “Young Scientist Presentation Award” (Japanese Only) from The Physical Society of Japan and The Japan Society of Applied Physics, respectively. Paper [2] was awarded “ICR Award for graduate students” (Japanese Only) from the president of Institute for Chemical Research, Kyoto University. In addition, Paper [2] was selected for “Most Read Articles” in Nano Letters in October 2022.

  This work was supported by Teranishi Laboratory (Kyoto University), Dr. Terumasa Tadano (National Institute for Materials Science), and Assoc. Prof. Hidekatsu Suzuura (Hokkaido University).

  For more details, please read the following papers.

✓Crystal Phase Transition

  Lead halide perovskite has mainly three crystal structures (orthorhombic, tetragonal, and cubic) and undergoes crystal phase transition as temperature changes. However, the previously reported phase transition temperatures in perovskite NCs vary widely. In this study, we measured the orthorhombic-tetragonal phase transition temperature in a single FAPbBr3 NCs by examining the band gap shift induced by the phase transition. As a result, we found that the phase transition temperature clearly decreases with decreasing NC size. This is because the Gibbs free energy of the entire NC is different for each NC size, which modulates the phase transition temperature. The paper related to this research subject was published in a special topic of The Journal of Chemical Physics entitled "40 Years of Colloidal Nanocrystals in JCP”.

  This work was supported by Teranishi Laboratory (Kyoto University).

  For more details, please read the following paper. It is open access and can be read by anyone.

✓Charged Exciton (Trion) ＆ Biexciton  

  The previously reported binding energies of trions (one electron and two holes) and biexcitons (two electrons and two holes) in perovskite NCs have varied widely due to the difficulty of measuring them at room temperature and for ensemble NCs. In this study, we have determined the binding energies of single perovskite NCs of various compositions (CsPbBr3, CsPbI3, and FAPbBr3) by measuring their PL spectra at 5.5 K, and have succeeded in accurately measuring their size dependences. By normalizing the binding energies and edge length with bulk exciton binding energies and bulk exciton Bohr radii, respectively, we showed that the size dependence of the trion and biexciton binding energies are not strongly affected by their compositions, but only by the quantum confinement effect. By comparing with theoretical calculations using effect mass aproximation, we also showed that the dynamic screening effect has a large influence on the binding energies. The paper on this research subject was published in ACS Nano, a high-impact journal in the field of nanotechnology.

This work was supported by Teranishi Laboratory (Kyoto University), and by Associate Professor Hidekatsu Suzuura and graduate student Takao Sato (Hokkaido University).

For more details, please read the following paper.