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 ~102 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. Chemical doping induces lattice strain in semiconductor nanocrystals which modulates their optical properties. Here, the authors show that the study of coherent acoustic phonons can explain this behavior in Cu-doped colloidal quantum wells.