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NMR studies of surface state of CuInS2 nanocrystals synthesized with dodecanethiol

​Liquid state NMR is used to investigate surface state of CuInS2 nanocrystals (NCs). 1H and 13C NMR studies of NCs dispersed in organic solvent allowed characterizing and quantifying the ligands at the NC surface, while diffusional NMR enabled studying ligand binding dynamics.

Published on 29 January 2018
Ternary metal chalcogenide NCs are explored for various types of applications in the fields of energy conversion (solar cells, photocatalysis, thermoelectrics), light emission and biological detection. Their optical and electronic properties can be tuned by changing the NC size (quantum confinement effect) or by varying the composition. Beside the inorganic core of the NCs, the nature, binding mode, and surface density of the native organic ligands are of paramount importance for their properties and subsequent applications. Within this family, CuInS2 (CIS) NCs are the most widely studied due to their outstanding luminescence properties and their well-established chemical synthesis. It involves mixing the metal precursors (copper iodide and indium acetate) with dodecanethiol (DDT) and heating first to 100°C (30-60 min) for complexation of the precursors and then to 230°C (15-60 min) for the growth of the NCs (cf. Figure 1). Despite the simplicity of this procedure, the surface state of the obtained NCs remains elusive.

Figure 1: Scheme of the synthesis of CIS NCs, starting from the metal precursors heated in dodecanethiol (DDT) at 100°C to generate a 2D polymeric reaction intermediate, to the nucleation and growth of the NCs upon further temperature rise.

Indeed, the precise determination of the surface state of CIS NCs is challenging, due to the fact that the signals of molecules close to the anchoring site on the NC surface are not visible in standard proton and carbon spectra. By means of complementary 1D and 2D NMR techniques (ex. Figure 2), we were able to draw a complete picture of the surface chemistry: (1) The ligand shell is composed of a double layer consisting of dodecanethiolate and didodecyl sulfide molecules in 1:1 ratio, assembled in a head-to-tail manner. This ligand double layer is in full accordance with the observed mass loss in thermogravimetric analysis (∼47%).

Figure 2: (A) 1H NMR spectra of DDT (top) and CIS NCs (bottom) in CDCl3; (B) 1H NMR DOSY spectrum of CIS NCs in CDCl3 and (C) diffusion coefficient measurements as a function of diffusion time Δ: Dself are calculated from the signal attenuation for the peaks at 2.5 and 0.88 ppm.

Furthermore, a high surface ligand density of 3.6 nm-2 has been determined, which has important consequences for the functionalization of the CIS NCs. They have been shown to withstand standard ligand exchange and aqueous phase transfer procedures. Nonetheless, the binding of dodecanethiolate occurs in a dynamic way. Diffusional NMR (Pulsed Gradient Field NMR method, Figure 2 B and C) suggests a fast exchange behavior for ligand desorption/adsorption processes and allows estimating the lower limit of the exchange rate constant as kex >> 50 s−1. These desorption and adsorption processes are accompanied by protonation and deprotonation reactions. Therefore, the use of appropriate protonating agents is a promising strategy for the surface functionalization of CIS NCs obtained with the DDT method.

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