Non-blinking, Wide-bandgap Single Photon Sources with Simultaneously High Brightness and Single Photon Purity Enabled by Surface Rigidity Engineering of Strongly Confined Perovskite Quantum Dots

Single photon sources are essential elements of photonic based devices for quantum computing and quantum information sciences that can operate at room temperature. Current single-photon emitters (SPEs) are limited to specific materials, such as color centers and epitaxial quantum dots (QDs) that have well-spaced energy levels, and isolated luminescence centers to regulate emission dynamics under random classical light excitations. As a result, current SPEs material lack tunability to their emission energies and operating conditions. To date, most single photon emitters are limited to small band gap materials. Developing a wide-bandgap material emitting blue single photons are therefore highly desired for applications like free-space and underwater quantum communications. Additionally, a bright blue SPE can act as quantum excitation sources to enable passive SPEs using conventional fluorescence materials that are incapable of regulating emission dynamics and otherwise good SPE materials.<br/>Colloidal QDs have emerged as promising SPE materials due to their high luminescence efficiency at room temperature and facile and scalable synthesis. However, traditional wide-bandgap II-VI QDs are subjective to low fluorescence efficiencies even with epitaxially coated shells. Lead halide perovskites emerge as a promising alternative QD material due to their unique defect tolerance, which allows perovskite nanocrystals to exhibit a high luminescence quantum yield without shells. Despite limited success on red and green perovskite SPEs, blue perovskite SPEs have not been demonstrated because of the challenges in modifying the surface of blue perovskite QDs such as strongly confined CsPbBr₃perovskite QDs (SCPQDs) with sizes ranging from 4 nm to 7 nm.<br/>Perovskite QD surface ions are extremely dynamic. When making SPE samples, the colloidal QDs must be diluted, and dried onto the substrate. During these processes, the surface ligand will detach from the QD, leading to the loss of surface halide ions which is detrimental to the photoluminescence quantum yield. Additionally, the labile surface of SCPQDs has profound effects on their photophysical properties such as fluorescence intermittency (also referred as “blinking”). This phenomenon reduces the emission stability and efficiency of the light source, posing a significant barrier to the development of QD SPEs.<br/>In our pioneering work we have developed a method to embed SCPQDs in an organic halide salt matrix to inhibit the surface ion loss. Using this method, we have revealed a new blinking mechanism with in SCPQDs that happens when exciton is self-trapped by the soft surface lattices albeit being well-passivated. By using various conjugated organic ligands in the salt matrix and adjust the conjugations and crystallinities of organic molecules, we can tune the surface rigidity of SCPQDs. We found that the blinking can be strongly suppressed when the surface rigidity is increased. Additionally, the emission spectral linewidth of single SCPQD narrows down to &lt; 50 meV when the surface ions are anchored with matrix that has high crystallinity. By introducing inorganic halide slat in matrix such as sodium bromide, we can further fix the surface lattices of SCPQDs and obtain essentially non-blinking SCPQDs. The new QD-in-matrix SPE material have enhanced photo-stability. Taking advantage of the unique ultrafast bi-exciton annihilation process in SCPQDs, high single photon purities (&gt; 95%) can be achieved under high excitation rates that can usually compromise single photon purity in normal QDs. Our work will accelerate the adoption of photonic based quantum communication devices, and accommodate the application of SPEs that require tunable emission properties.