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The photoluminescence (PL) lifetime and quantum yield (QY) of the two-photon cascade emission (TPCE) provide critical information for assessing the quality of optoelectronic devices. The QY of TPCE has been conventionally evaluated by second-order photon correlation, ${g}^{(2)}$, measurements under weak-excitation conditions in single nanocrystals (NCs). Here, we report that by measuring simultaneously both the ${g}^{(2)}$ and PL-decay curves, the QY of the TPCE can be determined even under strong-excitation conditions. With this technique, the effect of ionization on the QY of the TPCEs in single CdSe/ZnS NCs, which appears under strong-excitation condition, can be revealed. The obtained QYs were compared with the PL lifetime of the first and second photon emissions. We demonstrate that even for ionized NCs, the radiative recombination rates during TPCE are almost proportional to the product of the numbers of electrons and holes.

The sequential two-photon emission process known as biexciton cascade emission is a characteristic phenomenon that occurs in photoexcited semiconductor nanocrystals (NCs). This process occurs when a biexciton state is created in the NCs; thus, the occurrence of the process is related to the photoabsorption properties of the NCs. This paper presents a simple equation that connects the photoabsorption of single NCs and the biexciton cascade emission. The equation is found to be independent of the quantum yields of photoluminescence (PL). With this equation and using an analysis of second-order photon correlation, the absolute absorption cross section $\ensuremath{\sigma}$ of the single NCs can be evaluated, obtaining values on the order of ${10}^{\ensuremath{-}14}\phantom{\rule{4pt}{0ex}}\mathrm{c}{\mathrm{m}}^{2}$. This analysis shows that ionization during PL blinking does not affect the validity of the relation, indicating that the evaluation of $\ensuremath{\sigma}$, based on the equation, is applicable for various NCs with unique structures.

The absorption spectrum represents an important aspect of single semiconductor nanocrystals (NCs), revealing the morphology and orientation of the NCs. By conducting quantitative photoluminescence excitation spectroscopy based on a single-photon counting method, we succeed in clarifying the absorption cross-section spectrum of neutral excitons in single CdSe/ZnS NCs. With the values of the absorption cross section, we reveal that the intensity saturation of the neutral-exciton emission can be explained by considering the Poisson statistics for incident photons and by the small quantum yield of the biexciton emission. We also evaluated quantitatively the biexciton emission contribution and the ionization rate of NCs that appear at increased incident photon fluxes.

Photoluminescence (PL) fluctuations of single semiconductor nanocrystals (NCs), such as PL blinking and spectral diffusion (SD), reflect the quantum nature of charges in the NCs. Through simultaneous measurements of PL spectra and lifetimes on single CdSe/ZnS NCs, PL of neutral excitons is found to exhibit a unique behavior of SD, which is accompanied by changes of radiative recombination lifetime. We find that the SD of neutral excitons originated from the quantum-confined Stark effect, which also affects the SD of charged excitons observed during PL blinking.

CdSe/CdS heterostructured nanocrystals with quasi-type-II band alignments provide an interesting platform for studying the photoluminescence (PL) blinking associated with their unique morphologies. By using simultaneous measurements of the PL intensity, lifetime, and polarization anisotropy, we reveal the role of the electron delocalization during the blinking of single CdSe/CdS dot-in-rods. We found that a significant change in the PL polarization anisotropy distinguishes between two kinds of charged excitons with different electron delocalizations. We report our observation of unique polarized PL blinking governed by the band alignments and the Coulomb interactions between the charges inside and outside the dot-in-rod.

We performed microscopic imaging of photoluminescence (PL) and photocurrent (PC) on InAs nanostructures including disklike structures (nanodisks) and quantum dots (QDs). The correlation between PL and PC images indicates that the major fraction of upconverted carriers originates from nanodisks. By analyzing the excitation spectra, we find evidence that nanodisks and QDs need to be spatially separated to enhance PC generation via upconversion. The efficient simultaneous use of both QDs and nanodisks is an alternative approach to intermediate-band solar cells, where low-energy photons are upconverted in the QDs and high-energy photons are efficiently upconverted in the nanodisks, resulting in enhanced carrier generation yields. With spatially resolved upconverted PL, we show that PC generation in nanodisks is due to ejection of both electrons and holes.

The excitation density dependence of photoluminescence (PL) blinking of single CdSe/ZnS quantum dots was studied by means of single-dot PL spectroscopy. As the excitation density increases, an intermediate state appears in addition to the highly emissive (ON) and nonemissive (OFF) states. The systematic study of the excitation density dependence of PL blinking behavior (PL intensity, probabilities of finding these states, time probability distributions, and PL spectra and lifetimes) suggests that the intermediate state can be attributed to a charged exciton (trion) state generated through biexciton generation and charging (ionization) of a quantum dot under a high-density excitation regime. Our results indicate that the biexciton generation is a precursor of the trion state and not of the OFF state.