ABSTRACTS


Internal Field Effect on the Photoluminescence Intensity Fluctuations of Single CdSe Nanocrystals

So-Jung Park, Stephan Link, William Miller, Andre Gesquiere, and
Paul F. Barbara
Department of Chemistry and Biochemistry and the Center for Nano- and Molecular Science and Technology, University of Texas, Austin,
TX 78712, USA

An investigation of the effect of applied electric field on the photoluminescence (PL) intensity of single CdSe nanocrystals has revealed a measurable field induced PL modulation for a large fraction of the nanocrystals studied. The field induced intensity modulation characteristics (i.e. modulation sign and depth) were observed to vary from particle to particle, and even for different time periods for the same particle in some cases. Simultaneous intensity and frequency resolved PL measurement show the PL intensity modulation is in fact due to a field effect on the PL quantum yield. The results are consistent with a model in which the energy of surface charge trapping sites are modulated by the electric field, causing in turn a modulation of the rates of exciton quenching by these sites. The complex observed field effects can be explained by the superposition of the applied field and the internal field due to deeply trapped charges on the surface of the nanoparticle.

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Statistics of Single Molecule Time Series

Jianshu Cao
Department of Chemistry, MIT, Cambridge, MA 02139 (jianshu@mit.edu)

(1) A transfer matrix formalism is developed to calculate the probability of single molecule kinetic time series and to analyze memory effects in on-off blinking traces and photon emission traces. An analogy to optical spectroscopy helps understand the information content of single molecule time series.
(1) A transfer matrix formalism is developed to calculate the probability of single molecule kinetic time series and an analogy to optical spectroscopy helps understand the information content of time series.
(2) Detailed balance violations are essential for biological functions and are shown to result in detectable signatures in single molecule event histograms
(3) Extracting reliable information from time series is generally not reliable because of the noise and the binning of the data. A candidate to overcome this practical difficulty is informational theorybased methods, which avoid the data inversion problem inherent in traditional numerical methods.
(4) Analysis of the photon emission data reported in single protein experiments shows universal and non-universal relaxation dynamics on the millisecond time-scale. Analysis of quantum dots blinking sequences reveals unexpected behavior due to the power-law waiting time distribution.

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Fluorescence blinking statistics from CdSe core and
core-shell nanorods

Catherine Crouch and Marija Drndic

This talk will present fluorescence blinking statistics measured from single CdSe nanorods (NRs) of seven different sizes with aspect ratio ranging from 3 to 11. Our study included core/shell CdSe/ZnSe NRs and core NRs with two different surface ligands producing different degrees of surface passivation. We compare the findings for NRs to blinking statistics from spherical CdSe core and CdSe/ZnS core/shell nanocrystals (NCs). We find that for both NRs and spherical NCs, the off-time probability distributions are well described by a power law, while the on-time probability distributions can be described well by a truncated power law, .
A power law crossing over to a stretched exponential, , can also be used. The measured crossover time determined from truncated power law fitting is indistinguishable within experimental uncertainty for core and core/shell NRs, and for core NRs with different ligands, indicating that surface passivation does not affect the blinking statistics significantly. We find that at fixed excitation intensity,
1/t_c increases approximately linearly with increasing NR aspect ratio; for a given sample, 1/t_c increases very gradually with increasing excitation intensity. Examining 1/t_c vs. single-particle photon absorption rate for all samples indicates that the change in NR absorption cross-section with sample size can account for some but not all of the differences in crossover time.

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Time-Dependent Quenching of Photoluminescence in Single Nanocrystals and Nanowires

Alexander Efros
Naval Research Laboratory, Washington DC

Spectroscopy of single nanostructures has advanced our knowledge of physical processes in ensembles, where contributions from slightly different nanostructures may obscure the underlying processes. One such effect, recently observed in spectroscopy of single nanocrystals and nanowires, is the random intermittency of the photoluminescence intensity. The resulting sequence of “on” and “off” states resembles a random telegraph signal.
We propose a model of Auger quenching, in which the time dependence of the photoluminescence intensity of a single nanostructure, under CW excitation conditions, exhibits just such a random telegraph sequence [1]. In our model the “off” state occurs when the dot is ionized. Immediately after ionization, nonradiative Auger recombination is completely dominant over radiative recombination, and the luminescence is quenched. The duration of this “off” state is controlled by the time required for the charge to return to the nanostructure. The duration of the “on” state depends on the rate of ionization (via thermal and tunneling processes or Auger autoionization), and on the intensity of excitation. In nanowires the size of the “dark spot”, where the PL is quenched, depends on the collective efficiency of the ionized center, and is expected to be a strong function of temperature [2].
The rates of Auger processes have been calculated as a function of energy-band parameters and nanocrystal size. The abrupt potential barrier at the surface of the nanostructure is responsible for the increased nonradiative rate in small nanocrystals, and removes the low-temperature energy threshold known for bulk semiconductors.
1. Al. L. Efros and M. Rosen, Phys. Rev. Lett. 78 , 1110 (1997).
2. Al. L. Efros and V. N. Prigodin Appl. Phys. Lett. 62 , 3013 (1993).

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Quantum dot blinking without charging

Pavel A. Frantsuzov
Notre Dame

Celebrated model of Efros and Rosen [1] relates fluctuations of the colloidal quantum dot fluorescence intensity (blinking) to charging and neutralization events. According to the model the relaxation time of the quantum dot excitation makes sudden jumps between two extremal values: small for the charged dot (Auger process) and high for the neutral dot (radiative process). Recent single photon counting measurements [2-4] have demonstrated that the single quantum dot has a continuous spectrum of the relaxation times. It is a convincing evidence that the molecular mechanism of the intermittency differs from the charging/neutralization, and it relies on the internal dynamics of the quantum dot. Conceivable models of this kind, that explain the power-law statistics of the blinking times, will be discussed including the 1Pe state diffusion model [5].
1. Al.L. Efros and M. Rosen, Phys. Rev. Lett., 78 (1997) 1110.
2. G. Schlegel, J. Bohnenberger, I. Potapova, and A. Mews, Phys. Rev. Lett. 88 (2002) 137401.
3. B.R. Fisher, H.-J. Eisler, N.E. Scott, and M.G. Bawendi, J. Phys. Chem. B 108 (2004) 143.
4. K. Zhang, H.Y. Chang, A.H. Fu, A.P. Alivisatos, H. Yang, Nano Lett., 6, 843-847 (2006)
5. P.A. Frantsuzov, R.A. Marcus, Phys. Rev. B, 72 (2005) 155321.

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Switching luminescence of colloidal quantum dots by charge injection

Philippe Guyot-Sionnest
The University of Chicago

The electrochemical injection of electrons in thin films of quantum dots has led to a number of observations including the electrochromic response of the films in the visible and IR due to quantum state ocupation, the appearance of ohmic conductivity due to partial band occupation, and, of relevance to this workshop, changes in intensity and spectral characteristics of the luminescence. Following the original proposal by Efros and Rosen that an ionizing Auger process leaving a electron or hole in the quantum dot could be at the origin of the PL blinking phenomena, it is generally accepted that an ionized quantum dot is a dark dot. The experiments here on ensemble thin films stress the more important role of reduced surface states in inducing nonradiative recombination. Other time-resolved experiments also raise question on the accepted Auger mechanism.

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Power-law blinking in the fluorescence of single organic molecules

Jacob P. Hoogenboom
ICFO – Institut de Ciències Fotòniques, 08860 Castelldefels (Barcelona), Spain

Dark states in molecular fluorescence occur as a result of reversible photoblinking and irreversible photobleaching. In a strongly coupled, superradiant perylene trimer molecule, bleaching of a perylene unit is accompanied by a change in fluorescence intensity, lifetime, and emission spectra. Blinking on the other hand is evidenced by a collective dark state of the entire trimer molecule, which allows us to unambiguously discriminate between bleaching and blinking events. We find that the collective dark states form an important intermediate state for molecular photobleaching[1]. The duration of dark states as well as on states follows power-law statistics. This power-law blinking is also observed for the perylene monomer molecule, under entirely different excitation conditions. Using a maximum likelihood estimator, power-law exponents can be extracted from single molecule intensity traces even though data size is limited due to photobleaching[2]. For both systems, the on exponent distributions appear similar, whereas the off exponent distributions are markedly different and display a large spread from molecule to molecule[3].

[1] J. P. Hoogenboom, E. M. H. P. van Dijk, et al., Phys. Rev. Lett. 95, 097401 (2005)

[2] J. P. Hoogenboom, W. K. den Otter, H.L. Offerhaus, J. Chem. Phys. 125, 204713 (2006)

[3] J. P. Hoogenboom, J. Hernando, et al., ChemPhysChem (accepted, 2007)

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Real-time imaging of single events with quantum dots

Jyoti Jaiswal

Complexity of biological systems is due to their ability to rapidly respond and adapt to its surroundings. This results in spatial and temporal heterogeneity in the response from individual cells and molecules. Averaging causes loss of information regarding the behavior of individual molecular or cellular response. This necessitates use of approaches that allow monitoring individual events, often many of them simultaneously, in live cells and organisms. A prerequisite for this is to establish approaches for labeling single cells and molecules in a way that labeling is stable and does not alter the behavior of the labeled entity. Quantum dots have many desirable optical properties that make them suitable for such purpose. We have developed approaches for labeling cells and molecules with QDs and tested the suitability of these approaches for imaging live cells.

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On the Theory of Intermittent Fluorescence of Quantum Dots

Rudy Marcus
California Institute of Technology
Noyes Laboratory of Chemical Physics, Pasadena, California 91125

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Stochastic and Physical Models of Blinking Nano-Crystals

G. Margolin, V. Protasenko, M. Kuno, E. Barkai

We discuss the non-reproducible nature of data analysis of blinking quantum dots, and its relation to the theory of weak ergodicity breaking. Both stochastic theory and experimental results show that time averages of physical observables, for example the time averaged intensity and the intensity correlation function remain random even in the limit of long measurement time. Analysis of the distribution of the time averaged intensities of blinking CdSe-ZnS nano-crystals reveals an interesting symmetry between on and off times. Relation between single particle information and anomalous photon statistics of an ensembles of dots is discussed. Finally we critically examine physical models of blinking dots suggested in the literature, in particular a variation of Onsager's electron-hole recombination model.

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Fluorescence intermittency as a tool for tracking nanoparticle surface modification and breakdown in living cells

Jay Nadeau
McGill University

The conjugation of CdSe/ZnS particles to electron donors, such as dopamine, alters the nanoparticle fluorescence in proportion to the number of conjugates attached. Steady-state fluorescence, time-resolved fluorescence, and fluorescence intermittency ("blinking") are all affected; however, the latter may give the greatest resolution, enabling us to distinguish the loss of tens of molecules from the particle surface. Extension of this analysis to live-cell imaging would be of great value for tracking nanocrystal breakdown in biological systems and thus leading to a greater understanding of the factors involved in uptake by cells, penetration into organelles, and toxicity.

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Combined Trapping and Single-molecule FRET

Matthew Lang

The ability to combine optical trapping and single molecule FRET measurements in the most common coincident arrangement has been a long standing goal of the single molecule biophysics community because it offers control over a mechanical reaction coordinate with the ability to simultaneously watch structures using fluorescence. We demonstrate this measurement with a model system consisting of a classic DNA hairpin opening mechanical transition assay controlled with force on the optical trap. A FRET pair placed at the base of the hairpin, consisting of a favored Cy3 fluorophore as the donor molecule and an Alexa acceptor molecule, reports the conformational state of the hairpin as open or closed. We show reversible mechanical control over the state of the hairpin while simultaneously watching the hairpin conformation through both mechanical displacement of the trapped bead and FRET reporting identifying the precise location of the transition. A technical solution to the photobleaching problem, outlined in a paper by Brau et al, makes this measurement possible. Additional advances in single molecule biophysics will be presented such as our use of the M13 bacteriophage to genetically engineer a single molecule assay.

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Blinking of Nanocrystal Quantum Dots
On Time Scales from Microseconds to Seconds

Matt Pelton
Argonne National Laboratory

We have characterized fluctuations in the fluorescence of individual core-shell semiconductor nanocrystals over seven decades in time, from microseconds to several seconds. We find a change in the blinking statistics on sub-microsecond time scales. This may provide an important constraint on models of quantum-dot blinking.

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Dispersive Kinetics from Single Molecules Oriented in Single Crystals of Potassium Acid Phthalate

Philip J. Reid, Kristin L. Wustholz, Eric D. Bott, and Bart Kahr
Department of Chemistry, Box 351700, University of Washington, Seattle, WA 98195. email: preid@chem.washington.edu

The intermittent emission or “blinking” of single violamine R (1) and 2’,7’-dichlorofluorescein (2) molecules incorporated into single crystals of potassium acid phthalate (KAP) is studied using confocal fluorescence microscopy. Blinking dynamics are quantified in terms of switching rates and on- and off-length probability distributions. Mixed crystals of KAP/1 and KAP/2 consist of photophysical sub-populations with ~40% and ~20% exhibiting persistent emission, respectively, and the remainder demonstrating a broad range of blinking behavior that is well described by a power-law distribution. The dependence of the power-law exponent on chromophore, experimental bin time, intensity threshold, and excitation power is examined. The blinking dynamics are also modeled using Monte Carlo simulations based on a three-level electronic system with the rate constants for population and depopulation of the “dark” state being distributed. No correlation between molecular orientation and blinking dynamics is observed, suggesting that intermolecular electron-transfer is not the origin of the power-law behavior. Alternative origins for this behavior (e.g., conformational flexibility and spectral diffusion) are explored using a combination of experimental and computational techniques.

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Photoluminescence blinking of colloidal quantum dots: residual memory and deviations from power-law distributions

Fernando Stefani

Two aspects of the emission blinking of colloidal quantum dots will be discussed based on experimental results. First, two independent processes are identified and characterized: one being responsible of the power-law distribution of on- and off-times and another one introducing a cut off for the maximum observed on/off times. Second, it is observed that the lengths of the on- and off-times are not random. Instead, a residual memory lasting about 40 on/off periods is observed.”

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Deciphering the Origin of Dispersed Kinetics of Blinking in Single Quantum Dots

X. Sunney Xie and Peter A. Sims
Department of Chemistry and Chemical Biology
Harvard University, Cambridge, MA 02138

Based on several of our new experimental observations, we propose a model that explains the suppression of single QD blinking by chemical reagents, offers a molecular mechanism that contributes to the power law distributions of on- and off-times of blinking, and accounts for a memory effect in the on- and off-times, whose autocorrelation functions are dependent on the excitation power and suppressant concentration.

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Beyond a Simple Two-State Model: Characterization of Intermediate; Emission States in Single CdSe/ZnS Core-Shell Colloidal Nanoparticles

Haw Yang
Department of Chemistry, University of California at Berkeley and
Physical Bioscience Division, Lawrence Berkeley National Laboratory
Berkeley, CA 94720

Time-correlated single-photon counting was used to study the emission intermittency of individual CdSe/Zns core-shell colloidal nanoparticles. Both the arrival time and the excitation-emission delay of every detected photon were recorded for off-line statistical analysis. The former gives information about the emission rate of the particle whereas the latter gives information about the excited state lifetime. A recently developed statistical method, photon-by-photon change-point analysis,1 was used to quantitatively locate the intensity change points in a single-molecule data stream. Since the change-point analysis does not require any presumed kinetic model, it is particular powerful in the analysis of such complex phenomena as quantum-dot blinking. It was found that a simple two-state model was not sufficient to describe the intermittent behavior. Further, the dynamics of luminescence lifetime fluctuations do not follow those of intensity fluctuations.2 If time permits, new experimental results that include correlated wavelength data will also be presented.
1
L. P. Watkins and H. Yang, Journal of Physical Chemistry B 109, 617 (2005).
2
K. Zhang, H. Chang, A. Fu, et al., Nano Letters 6, 843 (2006).

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