Solution-state NMR studies of the surface structure and dynamics of semiconductor nanocrystals

H-1 and C-13 nuclear magnetic resonance (NMR) relaxation studies of thiophe nol-capped CdS nanocrystals are presented. The transverse and longitudinal relaxation times were investigated as a function of nanocrystal radius, and the transverse relaxation time was also studied as a function of temperatu re. Both proton and carbon T-2 values were found to increase with nanocryst al radius, contrary to initial expectations. This effect is explained in te rms of motion of the thiophenol with respect to the nanocrystalline surface . Theoretical expressions for relaxation due to anisotropic motion are deve loped based on both bridging and terminal bonding configurations of the thi ophenol ligands, and the data are fit to these models. The data are found t o be consistent with thiophenol ligands bound in a terminal fashion to a si ngle Cd atom. The temperature dependence of the proton T-2 value is also su prising. T-2 is found to decrease with increasing temperature, and the size of this change scales with the nanocrystal radius. This is explained in te rms of an extra component of relaxation due to thermally excited electrons.