Internal structure of InP/ZnS nanocrystals unraveled by high-resolution soft X-ray photoelectron spectroscopy

High-energy resolution photoelectron spectroscopy (ΔF < 200 meV) is used to investigate the internal structure of semiconductor quantum dots containing low Z-contrast elements. In InP/ZnS core/shell nanocrystals synthesized using a single-step procedure (core and shell precursors added at the same time), a homogeneously alloyed InPZnS core structure is evidenced by quantitative analysis of their In3d_5/2 spectra recorded at variable excitation energy. When using a two-step method (core InP nanocrystal synthesis followed by subsequent ZnS shell growth), XPS analysis reveals a graded core/shell interface. We demonstrate the existence of In-S and S _x-In-P_1-x bonding states in both types of InP/ZnS nanocrystals, which allows a refined view on the underlying reaction mechanisms.