Colloidal Thallium Halide Nanocrystals with Reasonable Luminescence, Carrier Mobility and Diffusion Length
Colloidal lead halide based perovskites nanocrystals (NCs) have been recently established as an interesting class of defect-tolerant NCs with potential for superior optoelectronic applications. Electronic band structure of thallium halides (TlX, where X = Br, I) show a strong resemblance to lead halide perovskites, where both Pb2+ and Tl+ exhibit 6s2 inert pair of electrons and strong spin-orbit coupling. Though the crystal structure of TlX is not perovskite, but the similarities of its electronic structure with lead halide perovskites have motivated us to prepare colloidal TlX NCs. These TlX NCs exhibit a wider bandgap (>2.5 eV or < 500 nm) along with a potential to exhibit reduced density of deep defect states. Optical pump terahertz (THz) probe spectroscopy with the excitation fluence in the range of 0.85-5.86 x 1013 photons/cm2 on NC films showed high effective carrier mobility (~ 220 to 329 cm2V-1s-1, long diffusion length (~ 0.77 to 0.98 μm), along with reasonably high photoluminescence efficiency (~ 10%) for TlBr NCs. This combination of properties is remarkable compared to other wide-bandgap (>2.5 eV) semiconductor NCs, suggesting the reduction of deep-defect states in our TlX NCs. Furthermore, ultrafast carrier dynamics and temperature-dependent reversible structural phase transition along with its influence on optical properties of TlX NCs have been studied.