The self-trapped state (STS), originating from strong electron-phonon coupling, is promising in white light luminance owing to its broadband emission. Colloidal CdSe nanoplatelets (NPL) have substantial potential in light-emitting applications owing to their quantum-well-like characteristic. However, achieving STS in CdSe NPLs is extremely challenging because of their intrinsic weak EPC nature.
One possible method is to increase the fluctuation of a rigid lattice by reducing the geometric scale of CdSe colloidal nanocrystals into the atom-cluster regime. However, it suffers from its structural stability.
Researchers from the National Center for Nanoscience and Technology (NCNST) of the Chinese Academy of Sciences (CAS), have successfully achieved the stable bright self-trapped state emission based on CdSe nanoplatelets by building self-assembly superlattice structures. This work was recently published in Nano Letters.
In this work, researchers demonstrate the strong STS emission in the spectral range of 450-600 nm by building hybrid SL structures of colloidal CdSe NPLs. Through systematic optical spectroscopy studies, it is discovered that STS can be generated by the strong coupling of excitons and zone-folded longitudinal acoustic phonons (ZFLAPs) in SLs. ZFLAPs are formed by bulk acoustic phonon folding into a mini-Brillouin zone of the SL. Additionally, researchers revealed that the STS is formed in a time scale of ~450 fs and localized in a spatial extent of ~0.56 nm. The Huang–Rhys parameter, which describes the EPC strength in SL structure, is estimated to be ~19.9, which is much larger than that (~0.1) of mono-dispersed CdSe NPL. The in-depth understanding of the STS emission mechanism in this study bears important implications for manipulating STS through optimal design of SL structures.
The work was financially supported by Strategic Priority Research Program of Chinese Academy of Sciences, the Ministry of Science and Technology, the CAS Instrument Development Project, the National Natural Science Foundation of China and Open Research Fund Program of the State Key Laboratory of Low-Dimensional Quantum Physics.