Ultrafast laser spectroscopy of novel fluorescent nanocrystals

Optical properties of colloidal semiconductor nanocrystals (NCs) have been widely investigated using optical spectroscopy techniques since their birth. In particular, low-temperature spectroscopy minimizes the additional complexity induced by thermal effects, and therefore has been extensively used to investigate the temperature dependent excitonic behavior of various semiconductor materials. It has been well established that the composition, structure and the nature of the NCs surface strongly influence their optical properties. Present PhD dissertation focuses on two main generations of semiconductor colloidal NCs, i.e. 2D metal chalcogenide nanoplatelets (NPLs) and lead halide perovskite NCs. The former generation of material demonstrate remarkable optoelectronic properties, with a narrow and homogeneously broadened emission linewidth (at room temperature), fast exciton recombination and high fluorescence quantum efficiency. These advantageous properties can be further tuned in heterostructures by coating them with another semiconductor materials for instance CdSe/CdS/CdTe core/crown/barrier NPLs. Due to the staggered band offset between CdSe and CdTe, we observed emission from an indirect transition around 650 nm. As CdS forms a barrier for hole relaxation between crown and core regions, the CdSe/CdS/CdTe yielded an additional emission peak from the CdSe core, in contrast with CdSe/CdTe core/crown nanoplatelets without a barrier. The resulting dual emission was investigated as a function of temperature. The different nature of both emission peaks (direct in CdSe vs. indirect across the CdSe/CdTe interface) yielded a spectrally and temporally stable indirect transition as a function of temperature, while the emission rate of the CdSe emission increased at lower temperatures, and the spectral position shifted to shorter wavelengths. The second generation of material studied here i.e “lead halide perovskite” NCs is one of the most investigated semiconductor material in the last decade due to their ease of preparation, broadly tunable band gap, near unity fluorescence quantum efficiency and excellent color purity. We carried out a comprehensive study of size, composition and surface functionalization dependent optical properties of lead halide perovskite NCs. Contrary to most of the previous findings, we observe a single, narrow emission peak at low temperature for NCs with various sizes, compositions and surface coatings. Temperature-dependent photoluminescence (PL) and PL-lifetime data for different compositions (APbBr3, A=Cs, MA, FA) reveal that MA-based NCs were the most sensitive to temperature variations with least preservation of PL, featuring the highest thermal broadening of PL and longest lifetimes, whereas FA based NCs were the most resilient. Furthermore, a comparison of the photophysical properties of NCs having different surface coatings shows that their optical properties are strongly influenced by surface chemistry, with quaternary bromide capped NCs being the most stable samples at elevated temperature, as they retained the highest PL intensity. Considering all these results together, we provide unequivocal evidence that lead halide perovskite NCs exhibit no inhomogeneity in their PL and additionally their optical properties are strongly surface functionalization dependent. These fundamental insight into the optical properties of both generation of materials would be key for the development of future photonic devices.