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-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 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.

Scheme 1. Synthesis of InAs NCs with Different Combinations of Surfactants

Figure 1

Absorption spectra of InAs NCs made with different combinations of ligands: (a) TOP+OA (reaction time 1 h), (b) DOA+OA (reaction time 2 h), (c) TOA+OA (reaction time 6 h), and (d) HWHM values of the corresponding exciton absorption peaks. TEM images of InAs NCs made with (e) only OA or with TOA:OA volume ratios of (f) 2:3 and (g) 4:1.

Figure 2

(a) Absorption and PL spectra, (b) TEM micrograph, and (c) XRD pattern of tetrapod-shaped InAs NCs. The corresponding reflections of cubic InAs (ICSD number 24518) and hexagonal InAs (ICSD number 190427) are also reported by means of red and green vertical bars, respectively. (d) HRSTEM-EDX elemental maps of two isolated tetrapods. (e) HAADF HRSTEM image of a single tetrapod aligned along the [110] zone axis, with the corresponding fast Fourier transform (FFT) (inset): the arrows indicate the crystallographic directions parallel to the arm’s growth. (f) TEM images of a single tetrapod tilted to different angles around the rotation axis indicated by dashed blue line (scale bars are 5 nm).

Figure 3

(a) Schematic of the InAs NCs shapes used in the k·p calculations: spherical, tetrahedral (with arm length (L_arm) equal to 0), and tetrapod (with L_arm ≠ 0) shapes. (b) Electron (ρ_e) (top) and hole (ρ_h) (bottom) charge densities corresponding to InAs shapes defined in panel a. (c) Calculated bandgap energies of spherical InAs NCs as a function of the core diameter. Blue dots stand for 1-band k·p calculations, and the yellow star represents the experimental value observed in the work of Zhu et al. (∼780 nm). (d) Contour plot of the calculated bandgap energies as a function of the arm length L_arm. The yellow cross shows the bandgap energy observed experimentally in this work, and the dotted lines provide an indication of the tetrapod core size at L_arm ∼ 0.

Figure 4

(a) ¹H NMR spectra in toluene-d of (i) OA (the attribution of ¹H OA signals has been also confirmed by a ¹H–¹³C HSQC spectrum, Figure S21), (ii) “standard” tetrahedral InAs NCs, (iii) TOA (the attribution of ¹H TOA signals has been also confirmed by a ¹H–¹³C HSQC spectrum, Figure S22), (iv) InAs tetrapods, and (v) zoom-in of the CH₂(2) of TOA (∗ = EtOH; # = ethyl acetate). (b) NOESY experiment of tetrapod InAs NCs in toluene-d₈; insets are the zoom-in of the double-bond region from 5.42 to 5.66 ppm, of OA cross-peaks, and diagonal peaks (exchange) and CH₂ of TOA at 2.43 ppm. (c) DOSY experiment with inset that reports the zoom-in of the OA and TOA diagnostic diffusion signal.

Figure 5

(a) Model systems of an InAs NC viewed from (111) facets (i, in orange) and (111) facets (ii, in light blue) as used in the MD simulations. (b, c) Radial distribution functions (RDFs) and cumulative number (CN) of ligands for OA (b) and TOA (c) molecules plotted at different TOA:OA volume ratios. (d, e) Representation of the first (d, at r = 3.0 Å) and second (e, at r = 6.8 Å) ligand shells in the model: OA molecules are depicted in red and TOA in blue. (f) Ligand concentrations of OA on (111) and (111) facets and TOA for all facets at different TOA:OA ratios.