Quantum dots have been studied intensely in the past years due to their unique fluorescent properties which are controllable by the size of the particle. For online measurements often UV-Vis is used in order to gain access to particle size evolution during quantum dot synthesis. We now want to explore whether the two-photon fluorescence (2-PF) of the quantum dots can be used instead of UV-Vis as fluorescence is expected to be more sensitive to shape, surface termination and defects inside the quantum dot. Compared to one-photon fluorescence (1-PF), 2-PF uses longer wavelengths for excitation, thus reducing scattering losses. In order to determine a possible advantage of 2-PF we investigated the sensitivity of 2-PF in terms of concentration and compared it with UV-Vis and 1-PF sensitivity. We found that for CdS and CdSe quantum dots, the respective 2-PF is still detectable at concentrations which are not accessible with UV-Vis or 1-PF. This can be highly advantageous for detection of earlier stages of particle formation for 2-PF. Preliminary results show that the synthesis of ZnO quantum dots can be followed in situ with 2-PF. To improve spectral resolution the synthesis will be transferred into a t-mixer in order to spatially resolve the different stages of particle formation which then allows for higher integration times at each stage. For ZnO quantum dots we moreover found that ligand exchange of acetate with catechol takes place within 15 min for UV-Vis and 2 s for 2-PF. This suggests that the two techniques indeed probe different states or sites of the quantum dots.

2-photon fluorescence of quantum dots for investigations of nanoparticle formation and growth

251st National Meeting (2016)

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