Evolution of crystallinity during nanoparticle formation
Besides the classical nucleation pathway based on supersaturation, non-classical nucleation pathways have been established for various nanoparticle systems (ZnO, CdS, CdTe,…). Crystalline particles do not directly nucleate, but only form after several steps.
For ZnO, we have shown that initial, tetrahedrally coordinated precursor complexes form in solution, which assemble to internally disordered particles. Only gradually those particles become crystalline, see scheme. The crystallization kinetics depend on the organic ligands, which stabilize the final particle sizes.
The interesting applicational properties, i.e. photoluminescence in case of CdS, result from its internal structure. Quite often, final products are well characterized, but the evolution of internal particle structures is insufficiently understood.
SAXS – SANS – free-film sample environment
We run in-situ nucleation experiments to follow the particle formation and watch the crystallization as it happens. To understand the role of organic ligands during the nucleation, we carry out small angle neutron scattering (SANS) experiments in which we can selectively follow the dynamics of either the nanoparticle or the ligand shell around it.
To enhance signal-noise, we developed a novel sample environment, which literally is a free liquid film. It provides an average film thickness of 0.49 mm as determined from incoherent scattering and a sample area with a width of 7 mm and a height of up to 20 mm. The instrumental background is reduced by 37% compared with standard Hellma cells.
For further insights, we combine this method with small angle X-ray scattering (SAXS), transmission electron microscopy (TEM) and dynamic light scattering (DLS). Combining these methods, we are able to access the size of the CdS cores, the ligand shell and the kinetics of core-shell evolution.
Krauss, S. W., Schweins, R., Magerl, A., & Zobel, M. Free-film small-angle neutron scattering: a novel container-free in situ sample environment with minimized H/D exchange. Journal of Applied Crystallography, 52(2) (2019)
Zobel, M., Windmüller, A., Schmidt, E., et al. The evolution of crystalline ordering for ligand-ornamented zinc oxide nanoparticles, CrystEngComm 18 (2016) 2163-2172.
Zobel, M., Chatterjee, H., Matveeva, G., et al. Room-temperature sol-gel synthesis of organic ligand-capped ZnO nanoparticles, Journal of Nanoparticle Research 17 Issue 5 (2015)