Colloidal InAs Tetrapods: Impact of Surfactants on the Shape Control

Abstract

We have approached the synthesis of colloidal InAs nanocrystals (NCs) using amino-As and ligands that are different from the commonly employed oleylamine (OA). We found that carboxylic and phosphonic acids led only to oxides, whereas tri-noctylphosphine, 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 with good control over the size distribution, and the TOA:OA volume ratio of 4:1 generated InAs tetrapods with arm length of 5−6 nm. Contrary to tetrapods of II− VI materials, which have a zinc-blende core and wurtzite arms, these NCs are entirely zinc-blende, with arms growing along the ⟨111⟩ directions. They feature a narrow excitonic peak at ∼950 nm in absorption and a weak photoluminescence emission at 1050 nm. Our calculations indicated that the bandgap of the InAs tetrapods is mainly governed by the size of their core and not by their arm lengths when these are longer than ∼3 nm. Nuclear magnetic resonance analyses revealed that InAs tetrapods are mostly passivated by OA with only a minor fraction of TOA. Molecular dynamics simulations showed that OA strongly binds to the (111) facets whereas TOA weakly binds to the edges and corners of the NCs and their combined use (at high TOA:OA volume ratios) promotes growth along the ⟨111⟩ directions, eventually forming tetrapods. Our work highlights the use of mixtures of ligands as a means of improving control over InAs NCs size and size distribution.

Synthesis and Single Crystal X-ray Diffraction Structure of an Indium Arsenide Nanocluster

“…One of the QD material systems known to form clusters but lacks structural characterization is InAs. The synthesis of high-quality InAs QDs has been reported and improved upon for more than 20 years, with much attention paid to the final monodispersity and less to the formation mechanisms. ,,,,− , Most reported syntheses use indium carboxylate, a tertiary alkylphosphine, and a reactive silylarsine to nucleate and grow InAs QDs. In studying this precursor system, magic-sized clusters were observed as an intermediate with a characteristic absorption doublet with maxima at 425 and 460 nm. ,,, Even when investigating the usage of germylarsines to improve nanocrystal monodispersity, this characteristic cluster persists .…”

Synthesis and Single Crystal X-ray Diffraction Structure of an Indium Arsenide Nanocluster

“…The synthesis of high quality InAs QDs has been reported and improved upon for more than 20 years with much attention paid to the final monodispersity and less towards the formation mechanisms. 21,26,40,41,[45][46][47][48][49][50] The vast majority of the reported syntheses use indium carboxylate, a tertiary alkylphosphine and a reactive silylarsine to nucleate and grow InAs QDs. In the process of studying this precursor system, magic-sized clusters were observed as an intermediate with a characteristic absorption doublet with maxima at 425 and 460 nm.…”

Surface-Originated Weak Confinement in Tetrahedral Indium Arsenide Quantum Dots

“…The recent colloidal InAs nanocrystal synthesis can be categorized based on two precursor species: aminoarsines (As­(NR 2 ) 3 ) and silylarsines (As­(SiR 3 ) 3 ). The majority of silylarsine-based InAs QDs synthesized with carboxylic acids are considered as “spherical”, even though the shape deviates from a sphere in some reports due to irregularities. , In this work, we chose tris-dimethylaminoarsine (As­(NMe 2 ) 3 ) as an As source since various shapes such as tetrahedrons and tetrapods have been reported with this classification of precursors in amine or phosphine ligands due to the significantly stabilized (111) surface by halide/amine copassivation. − The crystal shape diversity was obtained with a minor modification of the synthesis by Srivastava et al: a typical hot-injection step with lithium bis­(trimethylsilyl)­amide (LiHMDS) additives followed by the injection of reducing agent (diisobutyl aluminumhydride, DIBAL-H) (Figure a). To get various sized tetrahedral InAs QDs without forming alloying with phosphorus atoms, a hydride-based and mild reducing agent, DIBAL-H, was selected instead of aminophosphines.…”