Surface Defect-Induced Paramagnetism in CdSe Nanocrystals

The magnetic response of undoped semiconductor nanocrystals (NCs) often arises from subtle surface and defect effects that challenge the conventional descriptions of nanoscale magnetism. Here, we show that mild postsynthetic purification has significant but environment-dependent effects on the magnetic response of colloidal CdSe NCs. When purified in air, the saturation magnetization increases monotonically with each washing step, consistent with progressive ligand removal and partial surface oxidation that transform undercoordinated Se sites into spin-active oxide-like surface states. In contrast, purification under N 2 produces a nonmonotonic, oscillatory evolution of the magnetization, number of spins, and effective per-atom moment. This behavior suggests that, in the absence of oxidation, the NC surfaces undergo reversible structural rearrangements that modulate the local surface coordination environment and associated spin-active states. Spin-polarized density functional theory shows that surface vacancy formation and undercoordinated surface environments generate localized, uncorrelated spins associated with partially filled Se 4p states, yielding a weak paramagnetic ground state. These results demonstrate that the magnetism in undoped CdSe NCs originates from surface-localized spin centers whose populations can be reversibly modulated through controlled postsynthetic processing, rather than from magnetic dopants or core defects.