Colloidal InAs Tetrapods: Impact of Surfactants on the Shape Control

We have approached the synthesis of colloidal InAs nanocrystals(NCs) using amino-As and ligands that are different from the commonlyemployed oleylamine (OA). We found that carboxylic and phosphonicacids led only to oxides, whereas tri-n-octylphosphine,dioctylamine, or trioctylamine (TOA), when employed as the sole ligands,yielded InAs NCs with irregular sizes and a broad size distribution.Instead, various combinations of TOA and OA delivered InAs NCs withgood control over the size distribution, and the TOA:OA volume ratioof 4:1 generated InAs tetrapods with arm length of 5-6 nm.Contrary to tetrapods of II-VI materials, which have a zinc-blendecore and wurtzite arms, these NCs are entirely zinc-blende, with armsgrowing along the & LeftAngleBracket;111 & RightAngleBracket; directions. They feature a narrowexcitonic peak at & SIM;950 nm in absorption and a weak photoluminescenceemission at 1050 nm. Our calculations indicated that the bandgap ofthe InAs tetrapods is mainly governed by the size of their core andnot by their arm lengths when these are longer than & SIM;3 nm.Nuclear magnetic resonance analyses revealed that InAs tetrapods aremostly passivated by OA with only a minor fraction of TOA. Moleculardynamics simulations showed that OA strongly binds to the (111) facetswhereas TOA weakly binds to the edges and corners of the NCs and theircombined use (at high TOA:OA volume ratios) promotes growth alongthe & LeftAngleBracket;111 & RightAngleBracket; directions, eventually forming tetrapods.Our work highlights the use of mixtures of ligands as a means of improvingcontrol over InAs NCs size and size distribution.