Luminescent Colloidal InSb Quantum Dots from In Situ Generated Single-Source Precursor

Abstract

Despite recent advances, the synthesis of colloidal InSb quantum dots (QDs) remains underdeveloped, mostly due to the lack of suitable precursors. In this work, we use Lewis acid-base interactions between Sb(III) and In(III) species formed at room temperature in situ from commercially available compounds (viz., InCl₃, Sb[NMe₂]₃ and a primary alkylamine) to obtain InSb adduct complexes. These complexes are successfully used as precursors for the synthesis of colloidal InSb QDs ranging from 2.8 to 18.2 nm in diameter by fast coreduction at sufficiently high temperatures (≥230 °C). Our findings allow us to propose a formation mechanism for the QDs synthesized in our work, which is based on a nonclassical nucleation event, followed by aggregative growth. This yields ensembles with multimodal size distributions, which can be fractionated in subensembles with relatively narrow polydispersity by postsynthetic size fractionation. InSb QDs with diameters below 7.0 nm have the zinc blende crystal structure, while ensembles of larger QDs (≥10 nm) consist of a mixture of wurtzite and zinc blende QDs. The QDs exhibit photoluminescence with small Stokes shifts and short radiative lifetimes, implying that the emission is due to band-edge recombination and that the direct nature of the bandgap of bulk InSb is preserved in InSb QDs. Finally, we constructed a sizing curve correlating the peak position of the lowest energy absorption transition with the QD diameters, which shows that the band gap of colloidal InSb QDs increases with size reduction following a 1/d dependence.

Keywords: III−V semiconductors; colloidal quantum dots; indium antimonide; near-infrared emission; semiconductor nanocrystals; single-source precursor.

Figure 1

¹H NMR spectra of the precursor solution, before and after the addition of InCl₃. Spectra of the pure components are included for reference (DMA, Sb[NMe₂]₃, ODA). The peak at 2.11 ppm in the four top spectra originates from residual undeuterated toluene and is absent in the two bottom spectra because the concentration of these solutions was substantially higher. Additional ¹H and ¹³C NMR spectra are provided in the Supporting Information (sections S1 and S2).

Figure 2

Overview TEM images of representative InSb QD samples after postsynthetic size fractionation: (a) 2.8 ± 0.3 nm, (b) 5.3 ± 0.5 nm, (c) 10.3 ± 1.2 nm, and (d) 14.9 ± 1.0 nm. High-resolution TEM (a–c) and STEM (d) images of one exemplary QD for each ensemble are shown in the insets. The scalebars correspond to 50 nm. Overview TEM images of 18.2 ± 1.6 nm diameter InSb QDs are provided in the Supporting Information (Figure S9).

Figure 3

(a) Representative TEM image of a colloidal InSb QD sample prepared using a delay time of 10 min between the addition of superhydride to OLA at 240 °C and the injection of the InSb precursor solution. (b) Schematic illustration of the aggregative growth mechanism.

Figure 4

Crystal structure analysis of representative InSb QDs. (a, b) TEM images and corresponding azimuthally integrated ED patterns of InSb QD samples of average diameter (a) 5.3 ± 1.2 nm and (b) 14.9 ± 0.9 nm. Original ED patterns are also included as insets. (c) Representative HRTEM images of InSb QDs from the sample shown in (a) and corresponding FFT. (d) Representative HAADF-STEM images of InSb QDs from the sample shown in (b) and corresponding FFT. For each crystallographic orientation, a schematic of the lattice projection and a simulated single-crystal diffraction pattern (DP) along the given zone axis are shown (for wurtzite the [uvw] notation is used). Selected lattice planes contributing to the pattern are highlighted. Additional HAADF-STEM images of the sample shown in (b), in different crystallographic orientations, as well as the images shown in (d) are reported in full size in the Supporting Information (Figure S20).

Figure 5

Absorption (dashed lines) and photoluminescence (solid lines) spectra of colloidal suspensions of 2.9 ± 0.3 (blue), 3.7 ± 0.5 (green), and 4.2 ± 0.5 (red) nm diameter InSb QDs in toluene. Inset: Photoluminescence decay trace of a colloidal suspension of 3.7 ± 0.5 nm diameter InSb QDs in toluene. The solid line is a single-exponential fit. The measurements were carried out at room temperature.

Figure 6

(a) Near-infrared absorption spectra of colloidal suspensions of InSb QDs with diameters ranging from 2.8 ± 0.3 to 6.0 ± 0.7 nm. The ensemble size polydispersity is ∼10% for all samples. The arrow highlights the shift of the lowest energy exciton absorption transition (i.e., the band gap) to higher energies with decreasing QD diameter. (b) Size dependence of the band gap (E₁) of colloidal InSb QDs showing our own experimental data (solid circles, color code is the same used in panel a), as well as experimental data from previous works (Liu et al. ∼5% polydispersity, Yarema et al. ∼15% polydispersity, Zhao et al. ∼10% polydispersity, and Maurice et al. ∼13% polydispersity, solid blue symbols) and theoretically calculated values reported by Efros and Rosen (eight-band effective-mass approximation approach), Sills et al. (semiempirical pseudopotential approach), and Sukkabot (atomistic tight-binding approach) (empty symbols). The green solid line is a fit to our experimental data based on eq 2 above.