Direct Synthesis and Characterization of Hydrophilic Cu-Deficient Copper Indium Sulfide Quantum Dots

Copper indium sulfide (CIS) nanocrystals constitute a promising alternative to cadmium- and lead-containing nanoparticles. We report a synthetic method that yields hydrophilic, core-only CIS quantum dots, exhibiting size-dependent, copper-deficient composition and optical properties that are suitable for direct coupling to biomolecules and nonradiative energy transfer applications. To assist such applications, we complemented previous studies covering the femtosecond-picosecond time scale with the investigation of slower radiative and nonradiative processes on the nanosecond time scale, using both time-resolved emission and transient absorption. As expected for core particles, relaxation occurs mainly nonradiatively, resulting in low, size-dependent photoluminescence quantum yield. The nonradiative relaxation from the first excited band is wavelength-dependent with lifetimes between 25 and 150 ns, reflecting the size distribution of the particles. Approximately constant lifetimes of around 65 ns were observed for nonradiative relaxation from the defect states at lower energies. The photoluminescence exhibited a large Stokes shift. The band gap emission decays on the order of 10 ns, while the defect emission is further red-shifted, and the lifetimes are on the order of 100 ns. Both sets of radiative lifetimes are wavelength-dependent, increasing toward longer wavelengths. Despite the low radiative quantum yield, the aqueous solubility and long lifetimes of the defect states are compatible with the proposed role of CIS quantum dots as excitation energy donors to biological molecules.