Water-soluble, luminescent ZnTe quantum dots: supersaturation-controlled synthesis and self-assembly into nanoballs, nanonecklaces and nanowires

A supersaturation-controlled aqueous synthesis route has been developed for ZnTe quantum dots (QDs) with high monodispersity, size tunability, stability, band-edge luminescence (full-width at half-maximum (FWHM) 10-12 nm) and negligibly small Stokes' shift (2-4 nm). The degree of supersaturation of the initial reaction mixture was varied by increasing the reagent concentration, but keeping the molar ratio Zn2+ : thioglycolic acid : Te2- constant at 1 : 2.5 : 0.5. For a 10x increase in supersaturation, the photoluminescence (PL) peak underwent a 50 nm blue shift from 330 to 280 nm at pH 6. The effect was more pronounced at pH 12, where the PL peak blue-shifted by 100 nm from 327 to 227 nm. Concomitantly, the FWHM was also reduced to a low value of 10 nm, indicating high monodispersity. For a 10x change in supersaturation, the particle size decreased by 63% (from 2.2 to 0.8 nm) at pH 12, whereas it changed by 19% (from 2.1 to 1.7 nm) at pH 6. High-resolution transmission electron microscopy and selected area electron diffraction data further revealed that the QDs synthesized at higher supersaturation had a better crystallinity. These QDs exhibited the unique property of undergoing isotropic and anisotropic self-assembly, which resulted in a blue shift and a red shift in the absorption and PL spectra, respectively. Isotropic assembly into spherical nanoballs (100 nm diameter, 1 nm inter-QD separation) occurred when the QDs were stored at pH 12 for 3 weeks at room temperature. The nanoballs further self-assembled into a 'pearl necklace' arrangement. On the partial removal of the capping agents, the QDs self-organized anisotropically into nanowires (1.3 mu m long and 4.6 nm in diameter). The self-assembled nanostructures showed exciton-exciton coupling and excellent PL properties, which may be useful in enhanced opto-electronics, photovoltaics and biochemical sensing.