We report on the observation of an intermediate state in the blinking of single CdSe/CdS core-shell nanocrystals. This state has a low quantum yield and connects the "on" and "off" states commonly observed in the photoluminescence blinking of individual nanocrystals. We find that the transitions between these two emitting states follow nearly single-exponential statistics. The transitions from the "on" state to this intermediate state result from changes in the surface passivation of the nanocrystal. The data are consistent with photoinduced, adsorption/desorption events that take place at the surface of the nanocrystals. The trion state leads to a reduction in photoluminescence in nanocrystals.

We report direct observation of the lowest two states of the band-edge exciton fine structure in the photoluminescence from single CdSe/ZnS core/shell nanocrystals at cryogenic temperatures. The temperature dependence of this spectral fingerprint reveals exciton spin relaxation rates as low as 10 micros(-1). The fine structure is also dependent on the nanocrystal charge state facilitating the identification of a bright negatively charged trion state with a quantum yield comparable to that of neutral emission.

We use a combination of low-temperature magneto-optical and lifetime spectroscopies to study the band-edge exciton fine structure of highly photostable single CdSe/ZnS nanocrystals (NCs). Neutral NCs displaying multiline emission spectra and multiexponential photoluminescence (PL) decays are studied as a function of temperature and external magnetic fields. Three different fine structure regimes are identified as a function of the NC aspect ratio. In particular, we identify an optically inactive ground exciton state, whose oscillator strength is tuned up under magnetic field coupling to bright exciton states, and attribute it to the zero angular momentum ground exciton state of elongated NCs. We also show evidence for highly efficient biexciton emission in these NCs, with radiative yields approaching unity in some cases.

Charged quantum dots provide an important platform for a range of emerging quantum technologies. Colloidal quantum dots in particular offer unique advantages for such applications (facile synthesis, manipulation and compatibility with a wide range of environments), especially if stable charged states can be harnessed in these materials. Here we engineer the CdSe nanocrystal core and shell structure to efficiently ionize at cryogenic temperatures, resulting in trion emission with a single sharp zero-phonon line and a mono exponential decay. Magneto-optical spectroscopy enables direct determination of electron and hole g-factors. Spin relaxation is observed in high fields, enabling unambiguous identification of the trion charge. Importantly, we show that spin flips are completely inhibited for Zeeman splittings below the low-energy bound for confined acoustic phonons. This reveals a characteristic unique to colloidal quantum dots that will promote the use of these versatile materials in challenging quantum technological applications.

We have investigated a series of branched fluorescent sensing compounds with thiophene units in the arms and triphenylamine centers for the detection of nitrated model compounds for 2,4,6-trinitrotoluene (TNT) and the plastic explosives taggant 2,3-dimethyl-2,3-dinitrobutane (DMNB). Stern−Volmer measurements in solution show that the fluorescence is more efficiently quenched by nitroaromatic compounds when compared to a non-nitrated quencher, benzophenone. Simple modification of the structure of the sensing compound was found to result in significant changes to the sensitivity and selectivity toward the nitrated analytes. A key result from time-resolved fluorescent measurements showed that the chromophore−analyte interaction was primarily a collisional process. This process is in contrast to conjugated polymers where static quenching dominates, a difference that could offer a potentially more powerful detection mechanism.

Narrow zero-phonon emission lines are observed in single CdSe/CdZnS core/shell colloidal nanocrystals over a range of cryogenic temperatures up to 40 K. These nanocrystals display dramaticaly improved spectral stability enabling the observation of acoustic phonon sidebands accompanying most zero-phonon lines. A discrete phonon mode is attributed to the electron coupling to the l ) 0 acoustic breathing mode via the deformation coupling. The Huang-Rhys parameter, S ac , for this interaction is found to vary from 0.0016 to 0.09, demonstrating a wide dispersion in exciton-phonon coupling between different nanocrystals. Indeed, we observe single nanocrystals in which all acoustic phonon sidebands vanish, in close agreement with theoretical predictions that there should be negligible acoustic phonon coupling in an ideal spherical CdSe nanocrystal. Such nanocrystals are virtually decoupled from their environment, which is potentially useful for quantum technologies, such as single photon sources and quantum computing. In general, the ability to detect and quantify phonon interactions within single nanocrystals will provide significant insight into energy relaxation and dephasing processes in these systems.

We report that high quality PbS nanocrystals, synthesized in the strong quantum confinement regime, have quantum yields as high as 70% at room temperature. We use a combination of modelling and photoluminescence up-conversion to show that we obtain a nearly monodisperse size distribution. Nevertheless, the emission displays a large nonresonant Stokes shift. The magnitude of the Stokes shift is found to be directly proportional to the degree of quantum confinement, from which we establish that the emission results from the recombination of one quantum confined charge carrier with one localized or surface-trapped charge carrier. Furthermore, the surface state energy is found to lie outside the bulk bandgap so that surface-related emission only commences for strongly quantum confined nanocrystals, thus highlighting a regime where improved surface passivation becomes necessary.

As colloidal semiconductor nanocrystals are developed for a wider range of diverse applications, it becomes more important to gain a deeper understanding of their properties in order to direct synthetic efforts. While most synthetic developments are guided by changes in ensemble properties, certain applications such as those in nano-electronics and nano-photonics rely on properties of nanocrystals at the individual level. For such applications and even for a more detailed understanding of the ensemble behavior, single nanocrystal spectroscopy becomes a vital tool. This review looks at how single nanocrystal spectroscopy has been applied to materials based on modern synthetic techniques and how these studies are elucidating properties that remain hidden at the ensemble level. First, recent theoretical models that are important for understanding many observed phenomena are explored. Then the review highlights new insights into many of the photophysical properties that are of interest in semiconductor nanocrystal materials, such as the ubiquitous spectral instability, magneto-optical identification of the band-edge exciton fine structure, emission from multi-excitons, and the spectroscopic properties of charged nanocrystals that challenge long standing theories on photoluminescence blinking behavior. To date most of the research has been conducted on materials based on cadmium selenide primarily due to its many years of development as a prototypical nanocrystal system. The review ends with a discussion of new materials that would also benefit from a detailed photophysical understanding afforded by single nanocrystal spectroscopy.