Trioctylamine in the Synthesis of Tris(trimethylsilyl)arsine-Based InAs Quantum Dots Prevents the Formation of Si-Based Byproducts

To date, the best reported strategy to synthesize large colloidal indium arsenide (InAs) quantum dots (QDs) with precise control over size and size distribution consists of a seeded-growth synthesis that uses dioctylamine coupled with tris(trimethylsilyl)arsine as arsenic precursor and oleic acid as ligand. Here, we demonstrate through nuclear magnetic resonance studies that, in such an approach, dioctylamine and oleic acid condense at the high temperatures required for synthesis (>250 °C), releasing water as a byproduct. The released water, in turn, leads to the formation of trimethylsilanol, which subsequently condenses to form hexamethyldisiloxane and trimethylsilyl oleate. As a result, dioctylamine-based InAs QDs are contaminated, even after multiple washing steps, by both unbound trimethylsilyl oleate and bound trimethylsilyl-derived species. We further show that these issues can be solved by replacing dioctylamine with a tertiary amine, for example tri-n-octylamine, which prevents the formation of water and leads instead to clean InAs QDs. We also demonstrate that this modified procedure delivers InAs QDs with excellent control over their optical features, with excitonic absorption peaks as narrow as 50 meV (half-width at half-maximum) and peak-to-valley ratios (an important parameter for optoelectronic applications) as high as ∼2, representing a record value for InAs QDs.