Quantum dots, semiconductor nanoparticles with size-sensitive electronic and optical properties, have been synthesized for numerous materials in recent years.  An archetypal example is the facile synthesis of the CdSe quantum dots in 1-octadecene, a non-coordinating solvent, which involves the high-temperature reaction of co-dissolved Cd and Se monomers, fatty acid Cd salts and trioctylphosphine selenide.  Once the CdSe seeds form, fatty acid carboxylate ions coordinate to the surface and provide a capping layer.  The delivery of the Cd and Se monomers to the surface through this layer determines the quantum dot growth rate.  We explored how the fatty acid structure affects the CdSe particle growth in this reaction.  The effect of the hydrocarbon chain unsaturation was examined by comparing three C18 acids: stearic (no double bonds), oleic (one double bond), and linoleic (two double bonds).  Stearic acid provided the least resistance to the monomer delivery resulting in the largest particles, 4-7 nm, depending on the time of the sample withdrawal.  For oleic acid, the corresponding sizes were significantly smaller 2-5 nm.  Between oleic and linoleic acids, the difference was less significant.  We propose that double bonds introduce the capping layer disorder, whereas fully saturated chains pack to form compact domains with accessible breaches between them.  The effect of the chain length was examined by using fully saturated acids: octanoic, decanoic, dodecanoic, hexadecanoic, octadecanoic (stearic).  As the chain shortens from C18 to C12, the protective action decreases and the particle growth accelerates. For C10 and C8 chains, the growth accelerates significantly, especially in certain crystallographic directions; the particles develop protrusions and grow as tetrapods.  The effect of the acid chain branching is also discussed.

Effects of the carboxylic capping ligand structure on the growth of the CdSe quantum dots

246th National Meeting (2013)

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