Single molecule confocal microscopy is used to investigate the detailed kinetics of fluorescence intermittency in colloidal II–VI (CdSe) semiconductor quantum dots. Two distinct modes of behavior are observed corresponding to (i) sustained “on” episodes (τon) of rapid laser absorption/fluorescence cycling, followed by (ii) sustained “off” episodes (τoff) where essentially no light is emitted despite continuous laser excitation. Both on-time and off-time probability densities follow an inverse power law, P(τon/off)∝1/τon/offm, over more than seven decades in probability density and five decades in time. Such inverse power law behavior is an unambiguous signature of highly distributed kinetics with rates varying over 105-fold, in contrast with models for switching between “on” and “off” configurations of the system via single rate constant processes. The unprecedented dynamic range of the current data permits several kinetic models of fluorescence intermittency to be evaluated at the single molecule level and indicate the importance of fluctuations in the quantum dot environment.

Single molecule confocal microscopy is used to study fluorescence intermittency of individual ZnS overcoated CdSe quantum dots (QDs) excited at 488 nm. The confocal apparatus permits the distribution of “on” and “off” times (i.e., periods of sustained fluorescence emission and darkness) to be measured over an unprecedentedly large dynamic range (109) of probability densities, with nonexponential behavior in τoff over a 105 range in time scales. In dramatic contrast, these same τoff distributions in all QDs are described with remarkable simplicity over this 109-fold dynamic range by a simple inverse power law, i.e., P(τoff)∝1/τoff1+α. Such inverse power law behavior is a clear signature of distributed kinetics, such as predicted for (i) an exponential distribution of trap depths or (ii) a distribution of tunneling distances between QD core/interface states. This has important statistical implications for all previous studies of fluorescence intermittency in semiconductor QDs and may have broader implications for other systems such as single polymer molecules.

It is known that light can be slowed down in dispersive materials near resonances. Dramatic reduction of the light group velocity-and even bringing light pulses to a complete halt-has been demonstrated recently in various atomic and solid state systems, where the material absorption is cancelled via quantum optical coherent effects. Exploitation of slow light phenomena has potential for applications ranging from all-optical storage to all-optical switching. Existing schemes, however, are restricted to the narrow frequency range of the material resonance, which limits the operation frequency, maximum data rate and storage capacity. Moreover, the implementation of external lasers, low pressures and/or low temperatures prevents miniaturization and hinders practical applications. Here we experimentally demonstrate an over 300-fold reduction of the group velocity on a silicon chip via an ultra-compact photonic integrated circuit using low-loss silicon photonic crystal waveguides that can support an optical mode with a submicrometre cross-section. In addition, we show fast (approximately 100 ns) and efficient (2 mW electric power) active control of the group velocity by localized heating of the photonic crystal waveguide with an integrated micro-heater.

Thermal rate constants of the complex-forming bimolecular reaction HO+CO■HOCO→H+CO2 were measured between 90 and 830 K in the bath gas He over the pressure range 1–700 bar. In addition, the vibrational relaxation of HO in collisions with CO was studied between 300 and 800 K. HO was generated by laser photolysis and monitored by saturated laser-induced fluorescence. The derived second-order rate coefficients showed a pronounced pressure and complicated non-Arrhenius temperature dependence. Above 650 K, the disappearance of HO followed a biexponential time law, indicating thermal instability of collisionally stabilized HOCO. By analyzing the corresponding results, an enthalpy of formation of HOCO of ΔHof,0=−(205±10) kJ mol−1 was derived. On the basis of energy- and angular-momentum-dependent rates of HOCO formation, activated complex properties for the addition reaction HO+CO→HOCO were derived from the limiting high-pressure rate constants; with the limiting low-pressure rate constants, activated complex properties for the dissociation HOCO→H+CO2 could be fitted as well. The observed transitions between low- and high-pressure limiting rate constants were well reproduced with these molecular parameters and collisional contributions; some evidence for rotational effects in collisional energy transfer was found. The surprisingly successful theoretical modeling of all available experimental data (80–2800 K, 0.0001–700 bar) allows for a satisfactory data representation of the rate coefficients over very wide ranges of conditions.

Monodisperse 4 nm FePt magnetic nanoparticles were synthesized by superhydride reduction of FeCl2 and Pt(acac)2 at high temperature, and thin assemblies of FePt nanoparticles with controlled thickness were formed via polymer mediated self-assembly. Adding superhydride (LiBEt3H) to the phenyl ether solution of FeCl2 and Pt(acac)2 in the presence of oleic acid, oleylamine, and 1,2-hexadecanediol at 200 °C, followed by refluxing at 263 °C, led to monodisperse 4 nm FePt nanoparticles. The initial molar ratio of the metal precursors was retained during the synthesis, and the final FePt composition of the particles was readily tuned. Alternately, adsorbing a layer of polyethylenimine (PEI) and the FePt nanoparticles onto a solid substrate resulted in nanoparticle assemblies with tunable thickness. Chemical analysis of the assemblies revealed that more iron oxide was present in the thinner assemblies annealed at lower temperature or for shorter time. Thermal annealing induced the internal particle structure change from chemically disordered fcc to chemically ordered fct and transformed the thin assembly from superparamagnetic to ferromagnetic. This controlled synthesis and assembly can be used to fabricate FePt nanoparticle-based functional devices for future nanomagnetic applications.

Phase-change storage is widely used in optical information technologies (DVD, CD-ROM and so on), and recently it has also been considered for non-volatile memory applications. This work reports advances in thermal data recording of phase-change materials. Specifically, we show erasable thermal phase-change recording at a storage density of 3.3 Tb inch(-2), which is three orders of magnitude denser than that currently achievable with commercial optical storage technologies. We demonstrate the concept of a thin-film nanoheater to realize ultra-small heat spots with dimensions of less than 50 nm. Finally, we show in a proof-of-concept demonstration that an individual thin-film heater can write, erase and read the phase of these storage materials at competitive speeds. This work provides important stepping stones for a very-high-density storage or memory technology based on phase-change materials.

Saturated laser induced fluorescence is used for the sensitive detection of radicals in high pressure gases. The method and its application to a series of addition reactions of HO radicals in the high pressure regime are described. Experiments between 1 and 150 bar of the bath gas He allow for falloff extrapolations to the high pressure limit of the recombination reactions. Limiting rate constants (in cm3 molecule−1 s−1) of 2.2×10−11 for HO+HO→H2O2, of 3.3×10−11 for HO+NO→HONO, of 7.5×10−11 for HO+NO2→HONO2, and of 9.7×10−13 for HO+CO→HOCO (and H+CO2) are derived at 298 K.

We present a simple polymer-mediated process of assembling magnetic FePt nanoparticles on a solid substrate. Alternatively absorbing the PEI molecule and FePt nanoparticles on a HO-terminated solid surface leads to a smooth FePt nanoparticle assembly with controlled assembly thickness and dimension. Magnetic measurements show that the thermally annealed FePt nanoparticle assembly as thin as three nanoparticle layers is ferromagnetic. The magnetization direction of this thin FePt nanoparticle assembly is readily controlled with the laser-assisted magnetic writing. The reported process can be applied to various substrates, nanoparticles, and functional macromolecules and will be useful for future magnetic nanodevice fabrication.