Near-Infrared-Emitting CuInS2/ZnS Dot-in-Rod Colloidal Heteronanorods by Seeded Growth

Abstract

Synthesis protocols for anisotropic CuInX₂ (X = S, Se, Te)-based heteronanocrystals (HNCs) are scarce due to the difficulty in balancing the reactivities of multiple precursors and the high solid-state diffusion rates of the cations involved in the CuInX₂ lattice. In this work, we report a multistep seeded growth synthesis protocol that yields colloidal wurtzite CuInS₂/ZnS dot core/rod shell HNCs with photoluminescence in the NIR (∼800 nm). The wurtzite CuInS₂ NCs used as seeds are obtained by topotactic partial Cu⁺ for In³⁺ cation exchange in template Cu_{2- x}S NCs. The seed NCs are injected in a hot solution of zinc oleate and hexadecylamine in octadecene, 20 s after the injection of sulfur in octadecene. This results in heteroepitaxial growth of wurtzite ZnS primarily on the Sulfur-terminated polar facet of the CuInS₂ seed NCs, the other facets being overcoated only by a thin (∼1 monolayer) shell. The fast (∼21 nm/min) asymmetric axial growth of the nanorod proceeds by addition of [ZnS] monomer units, so that the polarity of the terminal (002) facet is preserved throughout the growth. The delayed injection of the CuInS₂ seed NCs is crucial to allow the concentration of [ZnS] monomers to build up, thereby maximizing the anisotropic heteroepitaxial growth rates while minimizing the rates of competing processes (etching, cation exchange, alloying). Nevertheless, a mild etching still occurred, likely prior to the onset of heteroepitaxial overgrowth, shrinking the core size from 5.5 to ∼4 nm. The insights provided by this work open up new possibilities in designing multifunctional Cu-chalcogenide based colloidal heteronanocrystals.

Figure 1

(a,b) TEM images and corresponding size histograms of CIS NCs (b) obtained by partial Cu⁺ for In³⁺ CE in template Cu_2–xS NCs (a). The size histograms are constructed by measuring over 200 NCs and are fitted to a Gaussian distribution function. (c) X-ray diffraction patterns of template Cu_2–xS NCs and product CIS NCs. The gray lines indicate the high chalcocite Cu₂S diffraction pattern (JCPDS Card 00-026-1116). The orange lines indicate the wurtzite CIS diffraction pattern (JCPDS Card 01-077-9459).

Figure 2

Schematic illustration of four seeded injection methods. All the experimental conditions are the same unless otherwise specified. (I) S/ODE (control experiment, no CIS NC seeds were added), (II) a mixture of S/ODE and CIS NC seeds is injected, (III) CIS NC seeds followed by S/ODE (injection interval time, 20 s), (IV) S/ODE followed by CIS NC seeds (injection interval time, 20 s). The corresponding TEM images of purified products are displayed on the right side of the panel. Scale bars are 50 nm.

Figure 3

(a,b) TEM image and corresponding size histogram of CIS/ZnS HNCs prepared by injection of wurtzite CIS NC seeds and S/ODE into a solution of Zn(oleate)₂ and HDA in ODE at 210 °C following injection protocol IV described above. The size histograms were constructed by measuring the diameter (4.2 nm with a polydispersity of 17%) and length (79 nm with a polydispersity of 14%) of over 200 HNCs and are independently fitted to Gaussian distribution functions. (c) Absorption (dashed lines) and PL (solid lines) spectra of the template Cu_2–xS NCs, the product CIS NCs obtained by CE, and the final CIS/ZnS HNCs obtained by seeded injection and shown in (a). As the emission of CIS/ZnS HNCs (780 nm) is at the limit of both the UV–vis and the NIR detector, the full PL spectra were acquired by a combination of the two detectors (excitation wavelength 450 nm). (d) PL decay curve of the CIS/ZnS HNCs shown in (a). The detected wavelength was set at 780 nm. The data is best fit by a triple exponential decay (τ₁ = 7.5 ns (1.85%), τ₂ = 107 ns (18.93%), τ₃ = 410 ns (79.22%)) (see Supporting Information, Figure S14 for details). Inset shows a digital image of a CIS/ZnS HNCs suspension in toluene illuminated by a 405 nm diode laser.

Figure 4

(a) High-resolution TEM image of the CIS seed NCs. The Fourier transform (FT) analysis shows that the CIS NCs have the wurtzite crystal structure (see details in the Supporting Information, Figure S16). (b) Overview HAADF-STEM image of the CIS/ZnS HNCs. The line profile in the inset reveals that the intensity is higher at one end of the nanorods. (c–f) High resolution HAADF-STEM images. FT analyses of the regions indicated with red squares are shown in the insets. Cell views of the CIS/ZnS HNCs are given in (d,f). The FT patterns in (c) and (e) are consistent with the [420] and the [100] zone axis of the wurtzite, respectively. For clarity, the cell views simulated from FT patterns in (c) and (e) are presented in (d) and (f), respectively (red represents Cu/In/Zn atoms while yellow denotes S atoms).

Figure 5

Electron tomography reconstruction of a single CIS/ZnS HNCs. The core appears brighter in the 2D image (a) and the orthoslice (b). (c) A threshold is used to distinguish the core from the shell in 3D. Additional measurements are provided in the Supporting Information (Figures S21–S24).

Figure 6

Schematic illustration of the mechanism proposed for the multistep seeded growth protocol used in this work to synthesize colloidal CIS/ZnS dot core/rod shell HNCs.