Mixed-cation perovskite quantum dots: from single to multi-photon emission

• Main Author: Tan, Qi Ying
• Other Authors: Cesare Soci
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2024
• Subjects: Physics; Mixed-cation perovskites; Quantum dots; Color tunability
• Online Access: https://hdl.handle.net/10356/174388
• Institution: Nanyang Technological University

Perovskites, a family of materials with crystalline structure similar to the mineral calcium titanate (CaTiO3), were first discovered in 1839 and named after the mineralogist Lev Perovski. These materials have a general chemical formula ABX3 where A, B, and X refer to the organic/inorganic cations, divalent metal cations, and halide anions, respectively. Due to the versatility of their synthesis, different degrees of quantum confinement are achievable in perovskite material micro- and nano-structures of various dimensionalities. The zero-dimensional form, or quantum dots - semiconductor nanoparticles with diameters of 2 to 10 nm - are of particular significance for light-emitting devices (LEDs, lasers, and single photon sources), because of their absolute photoluminescence quantum yield, narrow linewidth, size, compositionally-tunable emission, and excellent charge characteristics. The ability to exert precise control over the spectral characteristics of the emitted light is highly crucial in many fields, such as bio-imaging, remote sensing, and quantum optical communications. Spectral tunability can be achieved either by acting directly on the constituent material properties or by carefully engineering optical cavities. While the manipulation of optical cavities has been extensively explored for various purposes, achieving precise control directly over the materials’ spectral properties has remained a challenge. The state-of-the-art method for tuning the emission of perovskite quantum dots mostly relies on the mixed halides (the X term in the ABX3 chemical formula). This approach is, however, highly uncontrollable, and susceptible to the adverse effect such as ion segregation, limiting the stability of the devices. In the work covered in this thesis, I depart from the conventional mixed-halide approach to explore a different route for tuning the emission wavelength of perovskite quantum dots within the visible spectral region and demonstrate its applicability to single photon emission and lasing. By adopting a Cs1-xFAx mixed-cation (the A term in the ABX3 chemical formula), I realize compositionally-tunable Cs1-xFAxPbBr3 perovskite quantum dots, through a colloidal method. The synthesized quantum dots show high crystallinity and mono-dispersity, with an average size of ~10 nm. By tailoring the stoichiometry of the cations, I can controllably tune emissions across the green regime (500–535 nm). Contrary to the mixed-halide quantum dots, these mixed-cation ones show minimal spectral drift and are less susceptible to the effect of ion segregation. By tailoring their chemical composition, I was able to demonstrate the first compositionally-tunable and photo-stable single photon emission, from individual Cs1-xFAx mixed-cation quantum dots, and the first room-temperature tunable lasing, in the case of Cs1-xFAx mixed-cation quantum dot films embedded in optical cavities.