Interpretation of Heteroepitaxy Process on Colloidal III–V/II–VI Nanocrystals

Construction of heterostructures on colloidal nanocrystals (NCs) is one of essential strategies to apply them for various optoelectronic applications such as biomarkers, displays, lightings or solar cells. Many literatures on the heteroepitaxy of NCs have focused on proper processing conditions like ligand types, precursors, or reaction temperatures to accomplish desired morphology and interface. The functionality and its performance of heterostructured NCs are primarily determined by the quality of interface, whether it contains undesired defects or not. In solution phase heteroepitaxy, unfortunately, detailed picture is veiled how elements are participating in epitaxial layer formation and how interfacial defects can be eliminated during the growth. Use of long hydrocarbon ligands may hinder adsorption, surface diffusion, and epilayer formation of metal ions delivered from precursors. We merely perform trial-and-error approaches to find optimal synthetic conditions to realize defect-less heterointerfaces.<br/>In this study, we will propose III-V/II-VI heteroepitaxy on colloidal NCs associated with transfer of surface ligands to alkylphosphine intermediates. Using InP and ZnSe as a III-V seed and a II-VI epitaxy material, we probed association and dissociation of surface ligands as well as organometallic precursors using varied temperature ³¹P, ⁷⁷Se and ¹H nuclear magnetic resonance. We confirmed that trioctylphosphine selenide, representative anion precursor, transfers Se to NCs’ surface by accepting carboxylate ligands to form oleyloxytrioctylphosphonium. And incoming zinc oleate leaves Zn to NCs’ surface by donating oleate ligands to oleyloxytrioctylphosphonium to form dioelyltrioctylphosphorane. Absence of ZnSe vibrational mode in InP NCs covered with Zn and Se adatoms implies that they form loose Zn-Se monomer networks, not crystalline epitaxial layer. We confirmed the presence of energetic barrier to convert the Zn-Se monomer network to the ZnSe epitaxial layer. This transformation barrier is primarily associated with the oxidized sites preventing association of Zn-P and In-Se bonds and the steric hinderance on the adsorption of zinc oleates.<br/>Understanding on surface chemistry of heteroepitaxy allowed us to devise novel heteroepitaxy process quoting uniform and defect-less III-V/II-VI heterointerface. We chose type-I InP/ZnS core/shell NCs as our test bed because of intrinsic difficulty in uniform ZnS epilayer growth originating from large lattice mismatch of 7.8%. We accomplished near unity photoluminescence quantum yield despite very thin, 1~2 ZnS epitaxial layers. Nearly defect-free heterointerface can be inferred from monoexponential photoluminescence decay. We believe that our finding will contribute to realize heterostructure NCs with superb optoelectronic properties.