Optical properties of single CdSe/ZnS colloidal QDs on a glass cover

slip and gold colloid surface

C. T. Yuan, W. C. Chou, Y. N. Chen, D. S. Chuu

Department of electrophysics, National Chiao Tung university

Outine Introduction to colloidal CdSe/ZnS QDs

Introduction to single QD detection

Experimental setup

Results and discussion

Summary

Introduction to CdSe/ZnS colloidal quantum dots

Diameter about 1~10 nm( aB of CdSe about 6 nm )

Enhancement of fluorescence QYs by ZnS overcoated

High QYs ( 50~85 % )Detective fluorescence at RT

Emission color ranging from red to violet

Introduction to colloidal QDs

Rhodamine red

Colloidal QDs

Colloidal QDsMBP

molecule

• Broad absorption with narrow symmetric fluorescence spectra ( FWHM~25-40 )• Large stokes shift• Low photobleaching thresholds• High QYs• Biocompatibility

Application of colloidal quantum dots

Quantum dots target breast cancer Fluorescence code

Illumination

Formation of CdSe colloidal QDs

• Tuning size by changing the growth conditions.

•To enhance quantum yield, we can over-coat a high energy gap ZnS layers around the QDs.

• Formation of colloidal QDs with hydrophobic TOPO ligands.

• For biological application, we need to modify TOPO surfactant by use of thiol-carboxyl ligands (HS-(CH2)-CooH) to form a water soluble QDs.

The fundamental concept of CdSe nanocrystals

• Emission color is sensitive to size of QDs.

•Energy separation between intra-level is much large than thermal energy (~meV to 25 meV).

• Ground state emission can be seen.

• Surface to volume ratio is very high( 30% surface atom for 4 nm QDs )

• Surface states attributed to defects, dangling bonds, adsorbate.

Mechanism of time-resolved fluorescence measuerments

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Optical excitation of an electron hole pairs

pulsed laser

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Relaxation by phonon emission (~10 ps)

phonon emission

phonon emission

Photon emission

Fluorescence of ensemble QDs

In general case-Concentration:10-6 M-Laser volume:10-6 L-Total numbers of QDs:1011 , ( size distribution 5% )

cuvette

40 50 60 70 800

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Co

un

t

Time ( S )

-20 0 20 40 60 80 100 120 140 160

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Co

un

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Events

0 2 4 6 8 10

5x104

105

1.5x105

2x105

2.5x105

3x105

1=7.42 ns

2=1.06 ns

Inte

nsity

( a

.u.

)

Time ( S )

Why do we need to measure single QD

In experiment and analysis

• Size and surface effect is a crucial issue

• Nominal uniform size distribution, 5% size variation

• Optical properties are sensitive to size and surface of colloidal QDs

• Specific phenomena of single QD can be seen (spectra diffusion, intermittency)

In physical and biological application

• Single photon emitter at room temperature

• Quantum information process

• Single QD device

Shuming Nie et. al. Science

Preparation of single CdSe/ZnS QD onto glass or quartz cover-slip

To dilute CdSe QDs solution to ~nano-Molar concentration

( a drop involved 108 QDs).

To uniformly disperse QDs onto clean glass or quartz of 2cm by 2cm area by spin coated.

Isolated single QD onto 4μm by 4μm area.

Single QD can be detected by far field optical microscopy.

Diffraction limited laser spot size of 0.3 μm can be obtained by use of high N.A. oil-immersion objective.

Laser spot

Single QD

4μm by 4μm area

How to measure single QD by confocal microscope

Oil-immersion objective N.A.=1.4

Dichroic mirror

Achromatic tube lens

Confocal pinhole

Single photon avalanche photon diodes

pulsed laser

( 400 nm, 50 ps duration time, 10

MHz repetition rate )

The photograph of experimental system

spectrometer

Ti:sapphire

2ω generation

Time-resolvedconfocal microscope

Solid State Laser

TCSPC and time-tag time-resolved techniques

t1 t2

Fluorescence intensity imaging of single(cluster) QDs

4.7 *4.7μm2

7 *7μm2Streaky feature

How to identify the single QD

Single QD

Milti- QDs

Single QD

Multi QDs

2.559μm x 2.559μm

FWHM : 0.3μm

P15

FWHM 0.3μm

lifetime 19 ns

2.5μm x 2.5μm

Schematic illustration of non-radiative Auger recombination

• Two electron-hole pairs.

• Non-radiative recombination.

• Fast decay process(~ps) than radiative recombination(~ns)

• Energy from electron-hole recombination transfer to third particle either an electron or a hole.

• Energy from Auger recombination can re-excited the third particle to eject outside the QDs.

• Ionized the QDs ( off time ).

Decay time fluctuation with photon intensity

35 40 45 50 55 60

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Inte

nsity

(C

ount

s/m

s)

Observed time ( S )

0 20 40 60 80 1000.01

0.1

1

Nor

mal

ized

inte

nsity

( a

.u.

)

Decay time ( ns )

19.54 ns

-5 0 5 10 15 20 25 30 35 400.01

0.1

1

Nor

mal

ized

inte

nsity

( a

.u.

)

Decay time ( ns )

5 ns

0.5 ns

obs rad nrad

rad

rad nrad

Q

G

E

STRNR

fluctuation

Localized Surface Plasmon Resonance

• Resonance phenomena can occur at specific wavelength of optical excitation• Strong light scattering• Intense plasmon absorption bands-size, size distribution, shape, environment • Enhancement of local electrical field• Enhancement of emitter

Alternative electric fields

Schematic illustration of sample configurationCdSe/ZnS QD

Gold nanoparticles

0 20 40 60 80 100

0

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100

Inte

nsity

( c

ount

s/m

s )

Observed time ( ns )

-20 0 20 40 60 80 100

0.01

0.1

1

Nor

mal

ized

inte

nsity

( a

.u.

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Decay time ( ns )

0.6 ns

9.3 ns

0 20 40 60 80 1000.01

0.1

1

Inte

nsity

( a

.u.

)

Decay time ( ns )

19.56 ns

0 20 40 60 80

0.1

1

Nor

mal

ized

inte

nsity

( a

.u.

)

Decay time ( ns )

25 ns

High intensity

Low intensity

Medium intensity

0 20 40 60 80 100 120 140 160 18010

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Dec

ay li

fetim

e (

ns )

Excitation power ( a.u. )

QD Au/QD

Summary

Fluorescence intermittency of single QD can be observed.

Fluctuation of decay lifetime of single QD is attributed to non-radiative contribution.

Fluorescence intensity and lifetime of single QD can be enhanced by incorporating gold nano-particles.

Thank you for your attention

Comparison of electron dynamics between bulk materials and nano-particles

- DOS for electron and phonon decrease with size- Weaker electron phonon interaction - Less non-radiative decay process- Longer lifetime

- Enhancement of spatial confinement from bulk to nanoparticles- Stronger electron-hole interaction- Increasing electron hole recombination- Shorter lifetime