Optical signatures of lattice strain in chemically doped colloidal quantum wells.

Lattice strain plays a vital role in tailoring the optoelectronic performance of colloidal nanocrystals (NCs) with exotic geometries. Although optical identifications of lattice strain in irregular-shaped NCs or hetero-structured NCs have been well documented, less is known about optical signatures of the sparsely distributed lattice mismatch in chemically-doped NCs. Here, we show that coherent acoustic phonons (CAPs) following bandgap optical excitations in Cu-doped CdSe colloidal quantum wells (CQWs) offer a unique platform for indirectly measuring the dopant-induced lattice strain. By comparing the behavior of CAPs in Cu-doped and undoped CQWs (i.e., vibrational phase/lifetime/amplitude), we have revealed the driving force of CAPs related to the optical screening of lattice strain-induced piezoelectric fields, which thus allows to determine the strain-induced piezoelectric field of ~10 V/m in Cu-doped CdSe CQWs. This work may facilitate a detailed understanding of lattice strain in chemically-doped colloidal NCs, which is a prerequisite for the design of favorable doped colloids in optoelectronics.