Shape and size control of colloidal CdSe nanocrystals in the presence of carboxylate and alkylamine ligands

In this thesis, we focus on investigating the shape and size evolution of colloidal CdSe nanomaterials in the presence of carboxylate and alkylamine surface ligands. By elaborating the combination of carboxylates and alkylamines with different hydrocarbon chains, zero-dimensional (0D) quantum dots (QDs), one-dimensional (1D) nanorods, two-dimensional (2D) nanotubes, nanosheets and nanoplatelets were successfully obtained in a controllable way and their growth kinetics and formation mechanism were further explored. First, we study the evolution of shape, size and optical quality of colloidal CdSe NCs upon adding both a carboxylic acid and an amine ligand to the cadmium acetate green chemistry source. Four reaction routes are designed to distinguish the role of the carboxylic acid and the primary amine. An evident synergistic effect is observed by forming QDs with smaller size, more isotropic morphology, less agglomerated behavior and higher PLQY upon concurrent use of a long-chain carboxylic acid and a long-chain amine ligand. Second, we establish the dual role played by oleylamine (OLAm) during the synthesis of CdSe QDs using cadmium oleate as the Cd source. Earlier reports suggested the role of alkylamine either as nucleating or as passivating agent in controlling the growth of CdSe QDs. Remarkably, by exploring four different synthesis routes, in which the reactant addition, timing and concentration are varied, we find that both these two phenomena coexist and control the synthesis. Third, we demonstrate the synthesis of 2D CdSe NCs and nanotubes when short-chain acetate and alkylamines are used as the ligands. Most interestingly, with a combination of acetate and dioctylamine (DOAm), CdSe nanotubes with atomically-flat walls are formed via a template-free protocol.We further elucidate the growth mechanism as consisting of a non-planar 2D oriented attachment approach; that is, simultaneous lateral and angular attachment of early-stage nanoplatelets into curved nanosheets and finally a tubular architecture. Finally, we further elucidate the formation mechanism of anisotropic CdSe NCs in the presence of dioctylamine ligand and carboxylate ligands having different chain length. Both short- and long-chain carboxylate ligands are found to induce the anisotropic growth of CdSe NCs by the attachment of early-formed NC building blocks.A morphological transition from nanotubes and nanosheets to irregular nanorods and finally to QDs is observed when the carbon number of carboxylate ligands increases from 2 to 18. The main reason is that two different NC building blocks, namely kinetically stable primary NCs exposing by {110} and {100} facets and thermodynamically stable primary NCs exposing by {100} and {111} facets, are formed in the presence of short- and long-chain carboxylates, respectively.Density functional theory (DFT) simulation is performed to reveal the surface structure of zincblende CdSe NCs in the absence and presence of carboxylate ligands. We propose that the different local monomer supersaturations caused by different chain-length carboxylates are responsible for the production of intrinsically different building blocks.

Gijs, Martinus
Parashar, Virendra Kumar
Lausanne, EPFL

Note: The status of this file is: EPFL only

 Record created 2019-09-23, last modified 2019-10-29

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