Fundamental Limit of the Emission Linewidths of Quantum Dots: An Ab Initio Study of CdSe Nanocrystals.

ABSTRACT

The emission linewidth of a semiconducting nanocrystal (NC) significantly affects its performance in light-emitting applications, but its fundamental limit is still elusive. Herein, we analyze the exciton-phonon coupling (EPC) from Huang-Rhys (HR) factors using ab initio calculations and compute emission line shapes of CdSe NCs. When surface traps are absent, acoustic modes are found to dominate EPC. The computed linewidths are mainly determined by the size of NCs, being largely insensitive to the shape and crystal structure. Linewidths obtained in this work are much smaller than most measurements on homogeneous linewidths, but they are consistent with a CdSe/CdxZn1-xSe (core/shell) NC [Park, Y.-S.; Lim, J.; Klimov, V. I. Nat. Mater. 2019 18, 249-255]. Based on this comparison, it is concluded that the large linewidths in most experiments originated from internal fields by surface (or interface) traps or quasi-type II band alignment that amplifies EPC. Thus, the present results on NCs with ideal passivation provide the fundamental minimum of homogeneous linewidths, indicating that only the CdSe/CdxZn1-xSe NC has achieved this limit through well-controlled synthesis of shell structures. To further verify the role of internal fields, we model NCs with charged surface defects. We find that the internal field significantly increases HR factors and linewidths, in reasonable agreement with experiments on single cores. By revealing the fundamental limit of the emission linewidths of quantum dots, this work will pave the way for engineering quantum dots with an ultrasharp spectrum.