Synthesis of Direct Bandgap ZnS/GaP Colloidal Quantum Well

Gallium Phosphide is an indirect bandgap material with a zinc blende structure, and thus it is difficult to achieve high photoluminescence (PL) efficiency. However, it has been predicted that the indirect bandgap characteristics can be alleviated through the quantum confinement effect. In particular, there was a theoretical prediction that GaP nanoparticles turn into a direct bandgap material when the size of 2 nm or less. However, the difficulty of synthesizing GaP NPs smaller than 2 nm has hindered its experimental confirmation.<br/>In this study, the strong PL characteristics of GaP were achieved through a unique structure, that is ZnS/GaP reverse type I quantum dot. In detail, we synthesized a GaP colloidal quantum well, in which ZnS nanocrystal is used as a seed and GaP is stacked on the ZnS surface with a very thin thickness of 2 nm or less through. This structure provides distinct advantages in terms of energy level and synthesis. First, nanocrystal ZnS seed has a larger bandgap than GaP, and therefore conduction band minimum (CBM) and valence band maximum (VBM) are located higher and lower than GaP, respectively. As a result, electrons and holes are confined to the GaP quantum well and do not interfere with exciton recombination in GaP. In addition, since the lattice parameter difference is very small as 0.05A (5.40A for ZnS and 5.45A for GaP), GaP can be uniformly stacked on the ZnS surface. Because of these advantages, ZnS/GaP achieved a high PL quantum yield of 77% even after several purification processes. Furthermore, the GaP quantum well showed a change in the PL peak from 398 nm to 412 nm depending on the synthesis temperature and time change. This work goes beyond confirming the strong PL characteristics due to the direct bandgap of GaP and suggests a new possibility that GaP can also be used as a non-Cd-based quantum dot material alternative to InP or ZnSe.