Metal Ion Dynamic Nuclear Polarization in Mn(II)-Doped CdS Nanocrystals: Atomic-Scale Investigation of the Dopant and Its Host.

Development of functional nanocrystals requires precise control over their composition and structure. Particularly, surface composition, defects, and doping play a central role in our ability to develop functional nanomaterials. As such, there is great interest in capturing these properties. Solid-state NMR spectroscopy is a powerful tool for probing structural and compositional features at the atomic scale, in particular, when it is coupled with the high sensitivity gained by dynamic nuclear polarization (DNP). DNP enhances NMR sensitivity by transferring high electron spin polarization to the surrounding nuclear spins. This dramatically improves the signal intensity, making it a valuable tool for detecting subtle structural features. Utilizing metal ion dopants as polarization agents for DNP has been shown to be an excellent approach to increasing ssNMR sensitivity in the bulk of inorganic solids. Here, we demonstrate the implementation of this approach to nanocrystals, focusing on Mn(II)-doped CdS, where homogeneous doping is known to be challenging while being critical for the DNP process. The intricate nature of the doping was elucidated by quantitative electron microscopy and electron paramagnetic resonance spectroscopy. We confirmed that Mn(II) doping is confined to the core of the nanocrystals and that statistically dopants are homogeneously distributed within each nanocrystal. DNP from Mn(II) dopants is then shown to increase Cd NMR sensitivity by an order of magnitude, enabling distinction between core and surface environments as well as the detection of defects in the bulk of the nanocrystals. We expect that the approach can be extended to other nanocrystals, providing an efficient route for characterizing their bulk and surface properties.