A Bose-Einstein condensate was produced in a vapor of rubidium-87 atoms that was confined by magnetic fields and evaporatively cooled. The condensate fraction first appeared near a temperature of 170 nanokelvin and a number density of 2.5 x 10 12 per cubic centimeter and could be preserved for more than 15 seconds. Three primary signatures of Bose-Einstein condensation were seen. (i) On top of a broad thermal velocity distribution, a narrow peak appeared that was centered at zero velocity. (ii) The fraction of the atoms that were in this low-velocity peak increased abruptly as the sample temperature was lowered. (iii) The peak exhibited a nonthermal, anisotropic velocity distribution expected of the minimum-energy quantum state of the magnetic trap in contrast to the isotropic, thermal velocity distribution observed in the broad uncondensed fraction.

/pdf/observation-of-bose-einstein-condensation-in-a-dilute-atomic-2wpb5ks2pi.pdf

Observation of Bose-Einstein Condensation in a Dilute Atomic Vapor

The emergence of semiconductor nanocrystals as the building blocks of nanotechnology has opened up new ways to utilize them in next generation solar cells. This paper focuses on the recent developments in the utilization of semiconductor quantum dots for light energy conversion. Three major ways to utilize semiconductor dots in solar cell include (i) metal−semiconductor or Schottky junction photovoltaic cell (ii) polymer−semiconductor hybrid solar cell, and (iii) quantum dot sensitized solar cell. Modulation of band energies through size control offers new ways to control photoresponse and photoconversion efficiency of the solar cell. Various strategies to maximize photoinduced charge separation and electron transfer processes for improving the overall efficiency of light energy conversion are discussed. Capture and transport of charge carriers within the semiconductor nanocrystal network to achieve efficient charge separation at the electrode surface remains a major challenge. Directing the future resear...

http://wiki.phy.queensu.ca/shughes/images/d/d1/Quantum_Dot_Solar_Cells._Semiconductor_Nanocrystals_as_Light_Harvesters.pdf

Quantum Dot Solar Cells. Semiconductor Nanocrystals as Light Harvesters

The use of semiconductor nanocrystals (quantum dots) as fluorescent labels for multiphoton microscopy enables multicolor imaging in demanding biological environments such as living tissue. We characterized water-soluble cadmium selenide-zinc sulfide quantum dots for multiphoton imaging in live animals. These fluorescent probes have two-photon action cross sections as high as 47,000 Goeppert-Mayer units, by far the largest of any label used in multiphoton microscopy. We visualized quantum dots dynamically through the skin of living mice, in capillaries hundreds of micrometers deep. We found no evidence of blinking (fluorescence intermittency) in solution on nanosecond to millisecond time scales.

https://science.sciencemag.org/content/sci/300/5624/1434.full.pdf

Water-Soluble Quantum Dots for Multiphoton Fluorescence Imaging in Vivo

The increasing energy demand in the near future will force us to seek environmentally clean alternative energy resources. The emergence of nanomaterials as the new building blocks to construct light energy harvesting assemblies has opened up new ways to utilize renewable energy sources. This article discusses three major ways to utilize nanostructures for the design of solar energy conversion devices:  (i) Mimicking photosynthesis with donor−acceptor molecular assemblies or clusters, (ii) semiconductor assisted photocatalysis to produce fuels such as hydrogen, and (iii) nanostructure semiconductor based solar cells. This account further highlights some of the recent developments in these areas and points out the factors that limit the efficiency optimization. Strategies to employ ordered assemblies of semiconductor and metal nanoparticles, inorganic-organic hybrid assemblies, and carbon nanostructures in the energy conversion schemes are also discussed. Directing the future research efforts toward utiliza...

Meeting the Clean Energy Demand: Nanostructure Architectures for Solar Energy Conversion

A 1,340-fold increase in single-molecule fluorescence has been observed from a lithographically fabricated gold bowtie nanoantenna — approximately an order of magnitude greater than that achieved in previous reports on such structures. The improvement results from an estimated ninefold increase in quantum efficiency, caused by enhanced absorption and an increased radiative emission rate.

Large single-molecule fluorescence enhancements produced by a bowtie nanoantenna

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 (ie, 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, ie, 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

Nonexponential “blinking” kinetics of single CdSe quantum dots: A universal power law behavior

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 a...

``On''/``off'' fluorescence intermittency of single semiconductor quantum dots

A detailed modeling of recently observed nonexponential fluorescence intermittency in colloidal semiconductor quantum dots (QDs) is presented. In particular, experiments have shown that both ``on''-time and ``off''-time probability densities generated from single-QD fluorescence trajectories follow an inverse power law, $P({\ensuremath{\tau}}_{\mathrm{o}\mathrm{n}/\mathrm{o}\mathrm{f}\mathrm{f}})\ensuremath{\propto}1/{\ensuremath{\tau}}_{\mathrm{o}\mathrm{n}/\mathrm{o}\mathrm{f}\mathrm{f}}^{1+\ensuremath{\alpha}},$ over multiple decades in time, where the exponent $1+\ensuremath{\alpha}$ can, in general, differ for ``on'' versus ``off'' episodes. Several models are considered and tested against their ability to predict inverse power law behavior in both $P({\ensuremath{\tau}}_{\mathrm{on}})$ and $P({\ensuremath{\tau}}_{\mathrm{off}}).$ A physical picture involving electron tunneling to, and return from, traps located several nanometers away from the QD is found to be consistent with the observed $P({\ensuremath{\tau}}_{\mathrm{off}})$ but does not yield the inverse power-law behavior seen in $P({\ensuremath{\tau}}_{\mathrm{on}}).$ However, a simple phenomenological model based on exponentially distributed and randomly switched on and off decay rates is analyzed in detail and shown to yield an inverse power-law behavior in both $P({\ensuremath{\tau}}_{\mathrm{on}})$ and $P({\ensuremath{\tau}}_{\mathrm{off}}).$ Monte Carlo calculations are used to simulate the resulting blinking behavior, and are subsequently compared with experimental observations. Most relevantly, these comparisons indicate that the experimental $\mathrm{o}\stackrel{\ensuremath{\rightarrow}}{n}\mathrm{off}$ blinking kinetics are independent of excitation intensity, in contradiction with previous multiphoton models of on/off intermittency based on an Auger-assisted ionization of the QD by recombination of a second electron-hole pair.

Modeling distributed kinetics in isolated semiconductor quantum dots

The emission profiles of the cesium resonance lines broadened by collisions with inert gases have been measured from about 50-1000 ${\mathrm{cm}}^{\ensuremath{-}1}$ from line center. The emission is observed from optically excited Cs in a cell whose temperature is varied from about 300-800\ifmmode^\circ\else\textdegree\fi{}K. By measuring the wing intensity relative to the entire line intensity from optically thin Cs, the profiles can be related to theoretical models without knowledge of the cesium density. The quasistatic theory of line broadening, extended to include the distribution of perturber positions about the Cs*, is used to analyze the data. The observed temperature dependence of the emission profiles is associated with the temperature dependence of the perturber distribution in the Cs*-inert-gas adiabatic potential. The quasistatic spectrum depends on the difference between excited- and ground-state adiabatic potentials, so each potential is thereby separately determined from the data. The $X\ensuremath{\Sigma}$, $A\ensuremath{\Pi}$, and $B\ensuremath{\Sigma}$ potentials for the 3.5-5-\AA{} region are given.

Extreme-Wing Line Broadening and Cs-Inert-Gas Potentials

The total and partial electron collisional ionization cross sections for CH4, C2H6, SiH4, and Si2H6 have been measured for electron energies from threshold to 300 eV. Comparisons are made to earlier measurements.

Alan Gallagher

Papers

Nonexponential “blinking” kinetics of single CdSe quantum dots: A universal power law behavior

``On''/``off'' fluorescence intermittency of single semiconductor quantum dots

Modeling distributed kinetics in isolated semiconductor quantum dots

Extreme-Wing Line Broadening and Cs-Inert-Gas Potentials

Total and partial electron collisional ionization cross sections for CH4, C2H6, SiH4, and Si2H6