Synthesis and Post-synthesis Transformations of Colloidal Semiconductor Nanocrystals
Creators: name: IMRAN, MUHAMMAD
Publisher: name: Universit� degli studi di Genova
Year: 2018

description: The present PhD thesis focuses on two main classes of semiconductor colloidal nanocrystals, i.e. lead halide perovskite and copper chalcogenides. The former class of semiconductor NCs are promising materials for many high performance optoelectronics applications, as they exhibit a tunable band gap in the range of 1.4 to 2.9 eV and an efficient photoluminescence characterized by narrow emission linewidths and have been explored the most in the last years. Following the standard hot injection based synthesis and selecting a combination of short chain acid (octanoic acid or hexanoic acid) together with alkyl amines (octylamine and oleylamine) we prepared strongly fluorescent CsPbBr3 perovskite nanowires with tuneable width, in the range from 20 nm (exhibiting no quantum confinement, hence emitting in the green) to 3 nm (in the strong quantum-confinement regime, emitting in the blue) for the first time. However the main limitation of the colloidal synthesis protocols that was followed in aforementioned case including the ligand assisted reprecipitation routes which is the second most frequently used method for preparation of LHPs, is that they employ PbX2 (X= Cl, Br, or I) salts as both lead and halide precursors which consequently limit the precise tunability of the amount of reaction species such as metals or halides precursors and are not applicable to entire family of APbX3 (A=FA, MA and Cs; X=Cl, Br, I). To overcome this issue we developed benzoyl halide based colloidal synthesis route i.e broadly applicable to the entire family of LHP NCs and not only ensures the independent tunability of reaction precursors but also maintain the overall integrity of the NCs such as phase purity and high PLQY. Despite the significant advances in synthesis procedures, the control over size monodispersity, shape and phase purity remains another long standing challenge. This is in fact due to the tendency of primary alkyl amine in the form of alkylammonium ions that could compete with Cs+ ions and leads to the anisotropic growth such as NPLs or their use in excess permotes the Pb-depleted Cs4PbX6 phases. We develop here a strategy to achieve size, shape and phase pure CsPbBr3 nanocubes by substituting primary alkyl amines with secondary alkyl amines. We attributed this excellent control over the shape and phase purity to the inability of secondary amines to find the right steric conditions at the surface of the nanocrystals which consequently limits the formation of low dimensional structures. The shape purity and narrow size distribution leads to their ease of self-assembly in superlattices reaching up to 50 microns in lateral dimensions, which are the largest dimensions reported to date for superlattices of LHP NCs. The second class of materials studied here, i.e. copper chalcogenides, are mainly attractive due to their tunable composition via post synthesis chemical transformations, plasmonic properties, low toxicity and environmental friendliness. Taking the advantage of colloidal synthesis and using Cu2S as a template we develop a strategy to obtain novel AuCuS-Cu2S heterostructure through cation exchange, which cannot be realized through conventional synthesis approaches. We further investigated the stability of Cu2S NCs with different dimensionalities and their thermal evolution subsequent to the metal decoration. Interestingly the presence of additional metallic NCs, such as Au and Pt not only improves their thermal stability but also leads to the formation of bi-metallic alloys semiconductor heterostructure.