Deconstructing the room-temperature emission spectra of nanocrystals using Photon-Correlation Fourier Spectroscopy

Abstract

The photoluminescence spectrum of an ensemble of emitters is the result of the homogeneous "natural" spectra of single emitters subjected to interparticle inhomogeneities and perturbations from the environment. For semiconductor nanocrystals (NCs), efforts to tune ensemble linewidths for optical applications have focused primarily on eliminating sample inhomogeneities because conventional single-molecule methods cannot reliably build accurate ensemble-level statistics for single-particle linewidths. Photon-correlation Fourier spectroscopy in solution (S-PCFS) offers a unique approach for investigating single-nanocrystal spectra with large sample statistics, without user selection bias, with high signal-to-noise ratios, and at fast timescales. With S-PCFS, we directly and quantitatively deconstruct the ensemble spectra of nanocrystals into contributions from the average single-NC homogeneous linewidth, spectral dynamics, and sample inhomogeneity. First, we discovered that single NCs at room temperature, in contrast to cryogenic temperatures, do not exhibit spectral dynamics on sub-millisecond timescales. Second, the linewidths of these homogeneous spectra were found to vary significantly from batch to batch and subject to synthetic control. Our findings crystallize our understanding of the synthetic challenges facing underdeveloped nanomaterials such as InP and InAs nanocrystals and introduce new avenues for the synthetic optimization of fluorescent nanoparticles. Finally, we have made strides toward understanding the underlying physical processes responsible for the homogeneous spectra of single nanocrystals at room temperature. Through careful synthetic control over the nanocrystal structure and composition, we have been able to understand changes in the homogeneous spectral linewidth in terms of exciton-phonon coupling. Combined with a simple spectral lineshape model, we have worked towards quantitatively understanding exciton-phonon coupling with respect to specific nanocrystal structural and composition parameters.