1. 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

2. Outine Introduction to colloidal CdSe/ZnS QDs
Introduction to single QD detection
Experimental setup
Results and discussion
Summary

3. 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

4. Introduction to colloidal QDs
Rhodamine red
Colloidal QDs
MBP
molecule
Colloidal QDs
Broad absorption with narrow symmetric fluorescence spectra
( FWHM~25-40 )
Large stokes shift
Low photobleaching thresholds
High QYs
Biocompatibility

5. Application of colloidal quantum dots
Quantum dots target breast cancer
Illumination
Fluorescence code

6. 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.

7. 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.

8. Mechanism of time-resolved fluorescence measuerments
Optical excitation of an electron hole pairs
Relaxation by phonon emission (~10 ps) -
phonon emission -
Photon emission
pulsed laser - -
phonon emission - - - - - - - -

9. 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

11. 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

12. 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.
Single QD
4μm by 4μm area
Laser spot

13. How to measure single QD by confocal microscope
Oil-immersion objective N.A.=1.4
pulsed laser
( 400 nm, 50 ps duration time, 10 MHz repetition rate )
Dichroic mirror
Achromatic tube lens
Confocal pinhole
Single photon avalanche photon diodes

14. The photograph of experimental system
Ti:sapphire
Solid State
Laser
spectrometer
Time-resolved
confocal
microscope 2ω
generation

15. TCSPC and time-tag time-resolved techniques

16. Fluorescence intensity imaging of single(cluster) QDs
Streaky feature

17. How to identify the single QD
FWHM : 0.3μm
Multi QDs
Milti- QDs
Single QD
Single QD
2.559μm x 2.559μm

18. P15
2.5μm x 2.5μm
FWHM
0.3μm
lifetime
19 ns

19. 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 ).

20. Decay time fluctuation with photon intensityR ST NR G

21. Localized Surface Plasmon Resonance
Alternative electric fields
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

22. Schematic illustration of sample configuration
CdSe/ZnS QD
Schematic illustration of sample configuration
Gold nanoparticles

23. Low intensity
High intensity
Medium intensity

25. 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.

26. Thank you for your attention

28. 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