Single-Molecule Fluorescence Blinking and Ultrafast Dynamics in Semiconductor and Metal Nanomaterials

C. T. Yuan, P. T. Tai, P. Yu, D. H. Lee, H. C. Ko, J. Huang, J. Tang*

1. Single-molecule detection. (2)2. Introduction to single colloidal QDs. (4)3. Fluorescence blinking in semiconductor nanostructures. (8)4. Fluorescence properties of noble metal nanoclusters. (8)5. Ultrafast dynamics in metal nanomaterials. (3)

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Single-QD fluorescence images

Single-QD fluorescence time traces Colloidal CdSe/ZnS QDs Fluorescent gold NCs

Why Single-Molecule Detection?

Ensemble measurementsLaser volume~10-6 L

Sample concentration~10-6 MolarTotal measured particles~1011

Single-Molecule DetectionOnly one target is probed at a time

Why Single-Molecule Detection in Nanomaterials?

Sample Heterogeneity (size, shape, local surface)

Time-dependent dynamical fluctuation(intensity, lifetime)

Time

Phys. Rev. Lett. 88, 077402 (2002)

Potential applications based on SMD Protein folding/unfolding dynamics

• Fluorescent labels for SMD• Nontoxic• Small• Biocompatible

Roger Tsien, Nobel Prize in Chemistry in 2008Green fluorescent protein

Colloidal Semiconductor CdSe QDs

Colloidal semiconductor QDs

Glove box

Excellent fluorescence properties1. Photostability2. Broad absorption band 3. Narrow emission band 4. Emission tunability5. Bio-compatibility

Photo-stability and multi-colors labeling

AlexaFluor 488

QDs

3T3 cells

Human epithelial cells

Nature materials 4, 435, 2005

Nature biotechnology 22, 969, 2004

Fluorescence blinking in single CdSe QDs

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On time (ms)

• Single molecules, polymers, Si, PbSe, CdTe NCs……• On the timescales of ms to minutes.• Power-law distribution for on/off-times.• Power-law exponent, 1.1~2.• Modified by surface and environments

On-time

Off-time

tPvs

tAP

AtP

AtP

log.log

logloglog

loglog

On states, neutral QDs Off states, charged QDs

• How the electron is rejected and returned from QDs and traps• Power-law distributions• Timescales (ms~min)

Surface, substrates

Binning-threshold methods

Auger Processes• Long-range Coulomb interactions.• Efficient in 0D QDs due to lack of momentum conservation.• Time-scales of ~ps, depending on size, shape.

Complication for achieving the lasing regime

Fluorescence blinking dark states

radAuger

Nature Physics, 4, 519 (2008)0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

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Diffusion Controlled Electron Transfer (DCET) models

Bright state (neutral QDs)

dark state (charged QDs)

Auger process

Photon emission

Tang and Marcus, Phys. Rev. Lett. 95, 107401 (2005)

Previous workPresent workFuture work

,if)(exp~)(

if~)(

2/2

2/

cc

cc

ttttttP

tttttP

Γ

Power-law behavior with extended time ranges by autocorrelation function analysis

2)(

)()()(

tI

tItIG

Disadvantages for conventional binning-threshold methods-Time resolution is limited by bin sizes (~10 ms).-Bin size is limited by SN ratio.-Pre-defined threshold is affected by human subjectivity.

,)()(1

11

)(21

1

sgsgsssG

,)()(1)(

212

1

sgsgssF

.if~)(

if~)(

2/

2

cc

cc

tttttF

tttttF

The main purpose is to find out the relationship between P(t) and G(t)

Laplace transformation

F(t)=G(t)/G(0)-1

Relationship between power-law blinking statistics P(t) and autocorrelation functions G(t)

63.1 ,37.02

22

m

m

• No requirements of selecting bin times and threshold.• Microsecond time resolution can be achieved.

Interaction between single QDs and Ag NPs

• Energy transfer.• Plasmonic effects.

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Wavelength (nm)

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CdSe/ZnS QDs QDs+triangular Ag NPs

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Lag time (ms)

CdSe/ZnS QDs QDs+triangular Ag NPs

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Lag time (ms)

QDs for former-half part QDs for later-half part

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G()

Lag time (s)

QDs+Ag for former-half part QDs+Ag for later-half part

Fluorescence Lifetime Correlation Spectroscopy (FLCS)

• Fluorescence quenching for individual QDs (uniform quenching).• Improvement of photo-stability.

])exp(1

1[)1()1(1

)(

blinking QDsdiffusion 3D

)]exp(1

1[)1()1(1

)(

blinking statetriplet diffusion 3D

)1()1(1

)(

diffusion 3D

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tdd

tdd

dd

T

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z

r

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11~

8.2~

4~)(

)(

AgQDs

QDs

QDs

Ag

AgQDs

QDs

F

F

N

N

QYs

QYs

Fluorescence decay profiles

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QDs QDs coupled triangular Ag

AgQDsQDs

AgQDs

QDs

AgQDs

QDs

AgQDs

QDs

AgQDs

QDs

AgQDs

QDs

flrnrr

r

kk

F

F

k

k

F

F

kkk

kF

~

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Brightness per QDs(FCS)

Measured lifetimes(TCSPC)

• No significant effect on radiative decay rates.• Enhancing nonradiative decay rates.

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Observation time (s)

CdSe/ZnS QDs CdSe/ZnS QDs+triangular Ag NPs

Fluorescence Time Traces and Intensity Distribution for Immobilized QDs

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Fluorescence lifetime

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Nontoxic, Water-soluble, Tiny, Fluorescent Gold Nanoclusters

Three regimes for gold NPs

R>>λ R~50 nm, electron mean free path

R<2 nm, electron Fermi-wavelength

bulkNanoparticle (scattering light)

Nanocluster (fluorescence)

Why fluorescent gold nanoclusters?

• CdSe QDs, toxic precursor• Gold NPs, scattering signal is too weak for <10 nm particles

Fluorescent, nontoxic, nanometer-sized materials gold nanoclusters

Dickson et al, Phys. Rev. Lett. 93, 077402 (2004)

Absorption~R3

Scattering~R6

useless

Robert M. Dickson

• Encapsulating Au clusters by PMAMA dendrimers• QYs~50%

History of Fluorescence from Gold Materials

• Size, 30*300 nm• Similar behavior to SPR• Orientation dependent emission

Synthesis and Characterization of Gold NCs

• NP fragmentation (6 nm-2 nm).• DHLA ligands for water soluble.• QYs~1 %.• Good colloidal stability.

Collaborator: Prof. Chang, in CYCU

Optical Properties of Ensemble Au NCs

• No surface plasmon resonance features.• Broad band emission.

Fluorescence properties of single gold NCs

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Single Au NCs10 ms bin time

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Observation time (s)

blinking behavior

Single-step photobleaching

Incomplete shape : photobleaching phenomenon

Streaky pattern : blinking behavior

On/off-time distribution

• Power-law distribution for on/off-times• Power-law exponents for on/off-times are 2, 1.8, respectively

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Fluorescence Lifetime Image Microscopy (FLIM)

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Single NCs ns

Specific labeling and nonspecific uptake

• Human hepatoma cells for specific labeling.• Streptavidin-biotin pairs.• Human aortic endothelial cells for nonspecific uptake.

Scale bar : 50 micron

Ultrafast Dynamics in Metal NPs

(http:www.nims.go.jp)

fs (10-15 sec) ~ ps (10-12 sec)

mm (10-6 m) ~ cm (10-2 m)

Pump-Probe Techniques – to achieve ~fs resolution

Rod Disc Triangularpyramid

Thin film Prism Sphere

A

B

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Relative surface energy: γ111 < γ100 < γ110

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Delay Time / ps

Resi

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Oscillation component zN(t)-z

1(t), 0.04

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H = 31.4 nm, T = 8.5 nm

H = 31.4 nm, T = 8.5 nm

H = 31.6 nm, T = 7.8 nm

Silver Nanoprisms

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References

• Y. C. Yeh, C. T. Yuan, C. C. kang, P. T. Chou, J. ang, Appl. Phys. Lett. 93, 223110 (2008).

• P. Yu, J. Tang, S. H. Lin, J. Phys. Chem. C 112, 17133 (2008).

• J. Tang, Y. C. Yeh, P. T. Tai, Chem. Phys. Lett. 463, 134 (2008).

• J. Tang, J. Chem. Phys. 129, 084709 (2008).

• C. T. Yuan et al, Appl. Phys. Lett. 92, 183108 (2008).

• D. H. Lee, J. Tang, J. Phys. Chem. C 112, 15665 (2008).

• J. Tang, Chem. Phys. Lett. 458, 363 (2008).

• J. Tang, J. Chem. Phys. 128, 164702 (2008).

• J. Tang, Appl. Phys. Lett. 92, 011901 (2008).

Thank you for your attention