The last twenty years have witnessed a worldwide interest in the development of colloidal semiconductor nanocrystals (NCs). However, NC synthesis has been performed as an empirical art, lacking mechanistic guidance. With very little insights into the molecular mechanism by which monomers are generated, the current state-of-the-art in the NC synthesis is essentially empirical with a large number of recipes developed. ‘Report the awful truth’ (Nature Nanotech. 8, 693–695 (2013)) clearly commented that many researchers are not aware of the impact of a little impurity in a chemical synthesis. This is the very case in the development of colloidal metal chalcogenide quantum dots (QDs). Since 1993, the wet-chemical synthesis of colloidal metal chalcogenide NCs has been depending on the use of metal salts such as cadmium oleate (Cd(OA)2 or Cd(OOC17H33)2 and tri-n-octylphosphine chalcogenides (E=P(C8H17)3 or ETOP), together with beneficial additives in 1-octadecene (ODE) including amines and thiols. Only recently, dioctylphosphine (HP(C8H17)2) has been identified to be an active impurity in commercial tri-n-octylphosphine (P(C8H17)3 or TOP), leading to low particle yield and low synthetic reproducibility in the NC synthesis which also requires high reaction temperature. To overcome the ever-lasting challenge via the control of precursor reactivity, in-depth understanding of the mechanism of precursor evolution to monomers which undergo nucleation and growth is of critical importance. We took the view and worked on the mechanism of precursor conversions to monomers at low temperature in the reaction of MXn + nE=PPh2H. Based on our extensive experimental (31P NMR) and theoretical (DFT), we propose a general mechanism with the illustration of the structural and electronic models of our proposed intermediates. Our general molecular mechanism of precursor evolution at low temperature to monomers should leads to the advance of the design and synthesis of colloidal semiconductor NCs with high quality, yield and reproducibility.

On the exploration of a general mechanism of precursor evolution at low temperature to colloidal semiconductor nanocrystals

251st National Meeting (2016)

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