Ligand Steric Profile Tunes the Reactivity of Indium Phosphide Clusters

Abstract

Indium phosphide quantum dots have become an industrially relevant material for solid-state lighting and wide color gamut displays. The synthesis of indium phosphide quantum dots from indium carboxylates and tris(trimethylsilyl)phosphine (P(SiMe 3 ) 3 ) is understood to proceed through the formation of magic-sized clusters, with In 37 P 20 (O 2 CR) 51 being the key isolable intermediate. The reactivity of the In 37 P 20 (O 2 CR) 51 cluster is a vital parameter in controlling the conversion to quantum dots. Herein, we report structural perturbations of In 37 P 20 (O 2 CR) 51 clusters induced by tuning the steric properties of a series of substituted phenylacetate ligands. This approach allows for control over reactivity with P(SiMe 3 ) 3 , where meta-substituents enhance the susceptibility to ligand displacement, and para-substituents hinder phosphine diffusion to the core. Thermolysis studies show that with complete cluster dissolution, steric profile can modulate the nucleation period, resulting in a nanocrystal size dependence on ligand steric profile. The enhanced stability from ligand engineering also allows for the isolation and structural characterization by single-crystal X-ray diffraction of a new III−V magic-sized cluster with the formula In 26 P 13 (O 2 CR) 39 . This intermediate precedes the In 37 P 20 (O 2 CR) 51 cluster on the InP QD reaction coordinate. The physical and electronic structure of this cluster are analyzed, providing new insight into previously unrecognized relationships between II−VI and III−V materials and the discrete growth of III− V cluster intermediates.

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

“…The carboxylate ligands show a binding mode distribution of nine chelating, three symmetric bridging, and 12 asymmetric bridging carboxylates. This affinity for asymmetric bridging carboxylates is consistent with other III–V cluster structures but the InAs seems to show an augmented ratio of the chelating binding mode in comparison to these other structures. , It is interesting to note that the L-type binding of water in those previously reported InP clusters forces bridging ligands into a monodentate binding mode. However, in the InAs cluster, the L-type binding of phosphines is to otherwise 3-coordinate indium atoms, none of which are simultaneously ligated by carboxylates (Figure S10).…”

“…We note that the experimental absorption doublet is substantially red-shifted from the computationally predicted absorbance. However, the mismatch in energy is similar to that previously reported for comparable materials. , Finally, the simulated Raman modes show good agreement with those characterized experimentally, falling in the range of 160–280 cm –1 (Figures S21, S22, S23). These energies are similar to those reported for the TO, SO, and LO modes of InAs nanowires at approximately 218, 237, and 239 cm –1 , respectively …”

“…The structure of this In 26 As 18 cluster appears to be related to a motif previously observed in InP, CdSe, and CdS. ,,,, We observe that our recently reported In 26 P 13 structure can be superimposed onto the In 26 As 18 structure, showing that the anion sublattice has a high degree of overlap (RMS = 0.323 Å), however the indium sublattice is substantially different (Figure S14). Observing the similarity in the pnictogen substructure leads us to hypothesize that the early stage InAs-395 intermediate likely has an As 13 sublattice that matches both the shape and stoichiometry of the P 13 sublattice of In 26 P 13 .…”

“…Removal of the surface indium results in a nonstoichiometric core of [In 23 As 18 ] 15+ , which stands as a stark comparison to the previously reported core stoichiometries [In 21 P 20 ] 3+ and [In 14 P 13 ] 3+ in InP. , It becomes evident that the drastic change in core stoichiometry is driven by the abundant 3-coordinate As atoms that make up nearly half the number of As present in the structure. We have previously concluded that in InP, there is a 4-coordinate requirement for the pnictide in cluster materials that forces a cation-rich stoichiometry through a large number of surface In atoms, passivating the otherwise 3-coordinate P. We see here that this is not the case in InAs.…”

“…This method has been used extensively in the characterization of metallic nanoclusters to unveil a deep, refined understanding of intermediate structures, growth pathways, and surface chemistry in those systems . While the number of reported semiconductor clusters is not as substantial, crystallography has demonstrated unprecedented phases that are not observable in the bulk, precise surface analysis of ligand binding modes and stoichiometries, and a more nuanced insight into the differences between clusters made up of different semiconducting materials. ,,, With a tenacious approach to structural characterization, the QD community will benefit from the same insights that have allowed great progress in the field of metallic nanoclusters. Toward that end, the structures of clusters as QD intermediates have been determined for a wide variety of the most common semiconducting QD materials, such as CdSe, CdS, and InP. ,,, However, many other material systems have identified the presence of magic-sized clusters spectroscopically but still lack full structural characterization of those intermediates. ,,− …”