Evolution of the Single-Nanocrystal Photoluminescence Linewidth with Size and Shell: Implications for Exciton–Phonon Coupling and the Optimization of Spectral Linewidths

Abstract

The optimization of photoluminescence spectral linewidths in semiconductor nanocrystal preparations involves minimizing both the homogeneous and inhomogeneous contributions to the ensemble spectrum. Although the inhomogeneous contribution can be controlled by eliminating interparticle inhomogeneities, far less is known about how to synthetically control the homogeneous, or single-nanocrystal, spectral linewidth. Here, we use solution photon-correlation Fourier spectroscopy (S-PCFS) to measure how the sample-averaged single-nanocrystal emission linewidth of CdSe core and core/shell nanocrystals change with systematic changes in the size of the cores and the thickness and composition of the shells. We find that the single-nanocrystal linewidth at room temperature is heavily influenced by the nature of the CdSe surface and the epitaxial shell, which have a profound impact on the internal electric fields that affect exciton-phonon coupling. Our results explain the wide variations, both experimental and theoretical, in the magnitude and size dependence in previous reports on exciton-phonon coupling in CdSe nanocrystals. Moreover, our findings offer a general pathway for achieving the narrow spectral linewidths required for many applications of nanocrystals.

Low‐Threshold Amplified Spontaneous Emission and Lasing from Thick‐Shell CdSe/CdS Core/Shell Nanoplatelets Enabled by High‐Temperature Growth

“…Moreover, the emission spectra of CdSe/CdS NPLs could remain the narrower full‐width at half maximum (FWHM) of ≈24 nm. The broadening of emission linewidth in these core/shell heterostructures is likely ascribed to the enhanced exciton–phonon coupling in the shell and the increased heterogeneity at the core/shell interface by atomic diffusion . The corresponding linewidths of single CdSe/CdS NPL (≈22 nm) shows a comparable value with the ensemble measurement (see Figure S1, Supporting Information), indicating the minimal heterogeneous broadening among NPLs.…”

Excited-State Dynamics in Colloidal Semiconductor Nanocrystals

“…Coupling of photogenerated carriers to phonons provides an important energy relaxation pathway, thus being essential to a number of photophysical processes in semiconductor NCs (e.g., exciton relaxation dynamics, carrier cooling, thermal transport) [42, 50–53]. Moreover, coupling to acoustic phonon modes determines the homogeneous linewidths of optical transitions [54, 55], while coupling to optical phonon modes has been observed to relax selection rules at low temperatures, yielding distinct phonon-assisted transitions (the so-called phonon replicas) [56, 57]. …”

Excited-State Dynamics in Colloidal Semiconductor Nanocrystals

“…Coupling of photogenerated carriers to phonons provides an important energy relaxation pathway, thus being essential to a number of photophysical processes in semiconductor NCs (e.g., exciton relaxation dynamics, carrier cooling, thermal transport) [42,[50][51][52][53]. Moreover, coupling to acoustic phonon modes determines the homogeneous linewidths of optical transitions [54,55], while coupling to optical phonon modes has been observed to relax selection rules at low temperatures, yielding distinct phonon-assisted transitions (the so-called phonon replicas) [56,57].…”