Profiling the Electron Trap States of II–VI Chalcogenide Colloidal Quantum Dots

Profiling sub-bandgap density of states (DOS) remains critical yet challenging for colloidal quantum dots (CQDs) due to methodological limitations and structural complexity. We address this through systematic capacitance studies on ZnCdSe/ZnSe/ZnS CQDs with mature synthetic protocols, selectively modifying either the intermediate shell or surface ligands. Comparative analyses reveal that the conventional zeroth-order capacitance method is limited by interface state interference, whereas drive-level capacitance profiling (DLCP) provides reliable trap-state quantification with resolvable DOS spectra. Key findings include the following: (1) State-of-the-art CQDs exhibit unexpectedly high electron trap densities (≥1.5 × 10¹⁷ cm^-3), despite superior electroluminescence. (2) Gradient shell alloying and surface ligand engineering demonstrate complementary functionality in DOS modulation, achieving an order-of-magnitude reduction in deep- and shallow-level traps, respectively. These results establish mechanistic guidelines for synthesizing low-trap CQDs through targeted interface and surface engineering, providing design strategies for high-performance optoelectronic devices.