Extracting the average single-molecule biexciton photoluminescence lifetime from a solution of chromophores.

We present a method for obtaining the average single-molecule biexciton lifetime from an ensemble of chromophores in solution. We apply this analysis to a series of core/shell CdSe/CdS quantum dot heterostructures with increasing shell thickness and find that the lifetime of the biexciton increases with increasing shell thickness, consistent with a simultaneous measurement of biexciton quantum yield.

PbS Nanocrystal Emission Is Governed by Multiple Emissive States.

Lead chalcogenide colloidal nanocrystals (NCs) are promising materials for solution processable optoelectronics. However, there is little agreement on the identity and character of PbS NC emission for different degrees of quantum confinement-a critical parameter for realizing applications for these nanocrystals. In this work, we combine ensemble and single NC spectroscopies to interrogate preparations of lead sulfide NCs. We use solution photon correlation Fourier spectroscopy (S-PCFS) to measure the average single NC linewidth of near-infrared-emitting PbS quantum dots and find it to be dominated by homogeneous broadening. We further characterize PbS NCs using temperature-dependent linear and time-resolved emission spectroscopy which demonstrate that a kinetically accessed defect state dominates room temperature emission of highly confined emitting NCs. These experiments, taken together, demonstrate that the linewidth and Stokes shift of PbS NCs are the result of emission from two states: a thermally accessed defect-with an energetically pinned charge carrier-and an inhomogeneously broadened band-edge state.

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

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.

Sample-averaged biexciton quantum yield measured by solution-phase photon correlation.

The brightness of nanoscale optical materials such as semiconductor nanocrystals is currently limited in high excitation flux applications by inefficient multiexciton fluorescence. We have devised a solution-phase photon correlation measurement that can conveniently and reliably measure the average biexciton-to-exciton quantum yield ratio of an entire sample without user selection bias. This technique can be used to investigate the multiexciton recombination dynamics of a broad scope of synthetically underdeveloped materials, including those with low exciton quantum yields and poor fluorescence stability. Here, we have applied this method to measure weak biexciton fluorescence in samples of visible-emitting InP/ZnS and InAs/ZnS core/shell nanocrystals, and to demonstrate that a rapid CdS shell growth procedure can markedly increase the biexciton fluorescence of CdSe nanocrystals.

Deconstructing the photon stream from single nanocrystals: from binning to correlation.

Prior to the advent of single-molecule fluorescence spectroscopy, many of the fundamental optical properties of colloidal semiconductor nanocrystal quantum dots were obscured by ensemble averaging over their inherent inhomogeneities. Single quantum dot spectroscopy has become a leading technique for the unambiguous determination of the governing excitonic physics of these quantum-confined systems. The analysis and interpretation of the timing and energies of photons emitted from individual nanocrystals have uncovered unexpected and fundamental electronic processes at the nanoscale. We review several different paradigms for deconstructing the photon stream from single nanocrystals, ranging from intensity "binning" techniques to more sophisticated methods based on single-photon counting. In particular, we highlight photon correlation - a powerful developing paradigm in single-nanocrystal studies. The application of photon-correlation techniques to single nanocrystals is changing the study of multiexcitonic recombination dynamics, uncovering the basic processes governing spectral linewidths and spectral diffusion, and enabling the extraction of single-nanocrystal properties directly from an ensemble with high statistical significance. These single-molecule techniques have proven invaluable for understanding the physics of nanocrystals and can provide unique insight into other heterogeneous and dynamical systems.

Direct observation of rapid discrete spectral dynamics in single colloidal CdSe-CdS core-shell quantum dots.

We measure the anomalous spectral diffusion of single colloidal quantum dots over eight temporal decades simultaneously by combining single-molecule spectroscopy and photon-correlation Fourier spectroscopy. Our technique distinguishes between discrete and continuous dynamics and directly reveals that the quasicontinuous spectral diffusion observed using conventional spectroscopy is composed of rapid, discrete spectral jumps. Despite their multiple time scales, these dynamics can be captured by a single mechanism whose parameters vary widely between dots and over time in individual dots.

Direct probe of spectral inhomogeneity reveals synthetic tunability of single-nanocrystal spectral linewidths.

The spectral linewidth of an ensemble of fluorescent emitters is dictated by the combination of single-emitter linewidths and sample inhomogeneity. For semiconductor nanocrystals, 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 to investigating single-nanocrystal spectra with large sample statistics and high signal-to-noise ratios, without user selection bias and at fast timescales. With S-PCFS, we directly and quantitatively deconstruct the ensemble linewidth into contributions from the average single-particle linewidth and from sample inhomogeneity. We demonstrate that single-particle linewidths vary significantly from batch to batch and can be synthetically controlled. These findings delineate the synthetic challenges facing underdeveloped nanomaterials such as InP and InAs core-shell particles and introduce new avenues for the synthetic optimization of fluorescent nanoparticles.