Unraveling exciton photo-physics in low-dimensional perovskites towards classical and quantum light emissions

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Organic-inorganic (hybrid) halide perovskite, an arising class of low-cost semiconductor materials, has gained great research interest due to the intriguing photo-physical properties as well as great potentials in photovoltaics and light emitting applications. Such chemically-altered semiconductor platforms enable physical tunability of charge carriers in different dimensionalities, ranging from 3D bulk materials to 0D quantum dots, resulting in novel physical behaviors and light emission properties. Despite great research attention and wide applications in opto-electronics, the fundamental physical properties of the electron-hole pair quasiparticles termed excitons, such as exciton-phonon interactions in 2D perovskites, and the fine structures in perovskite nanocrystals, are still underexplored. In this thesis, we will focus on the exciton properties in two specific type of perovskites systems: two-dimensional perovskites and zero-dimensional perovskite quantum dots. Using a series of spectroscopic and structural characterizations, we will investigate how excitons in 2D perovskites interact with the lattice vibrations and structural dynamics, and the intrinsic exciton behaviors in buried quantum dots, such as fine structure splitting and quantum light emissions at the single-dot level. In the first part, we study the exciton-phonon coupling and carrier dynamics using the ultrafast spectroscopy and stead-state cryogenic spectroscopies, which provides a correlated prospective of the light-induced structural dynamics and origin of exciton-phono couplings in multi-layered 2D perovskites. We suggest that the creation of a dense electron–hole plasma triggers the relaxation of lattice distortion at shorter timescales by modulating the crystal cohesive energy. We also demonstrate close to 3D like exciton-LO phonon coupling, as well as unique light-matter interactions such as exciton-polaritons and lasing properties in 2D perovskites. In the second part, we will demonstrate a novel material platform of perovskite-based quantum emitters, by embedding FAPbI3 based perovskite quantum dots (QDs) into the wide-bandgap 3D perovskite FAPbBr3 using one-step solution processed technique. Spectroscopic characterizations reveal the quantum nature of light emission from the buried QDs, as well as rich exciton fine structures such as triplets and singlets with assistance of magneto-spectroscopy. Such buried QDs exhibit a clear photon-antibunching signature, with second-order correlation function g2(0) to be ~0.15 at T = 6K. Photoluminescence suggests ultra narrow emissions lines with 130 μeV FWHM. High-resolution transmission electron microscope (HR-TEM) confirms the presence of nanometer-sized domains, which indicates the formation of quantum dots during the rapid crystallization of the precursor solvent. The embedded emitters exhibit a mono-exponential radiative decay (τ = 300 ps), with additional multi-exciton states from bi-excitons and trions, as well as temperature-dependent linewidth broadening and phonon sidebands, expected for colloidal FA-based nanocrystals. In addition, direct spectroscopic signatures of the exciton fine states - such as triplets splitting and singlet states brightening - are clearly resolved under magnetic field, revealing the spectral origin and rich photo-physics from the embedded QDs. Furthermore, we have firstly demonstrated the capability of electrical-driven single-photon emission in perovskite by sandwiching the system between electron-transport and hole-transport layers. Our results may pave the pathway of on-chip integration of low-cost single-photon sources for quantum optical systems.

Doctor of Philosophy
halide perovskites, 2D halide perovskites, quantum dots, spectroscopy, single photon emission
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