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

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Optical properties of single CdSe/ZnS colloidal
QDs on a glass cover slip and gold colloid
surfaceC. T. Yuan, W. C. Chou, Y. N. Chen, D.
S. ChuuDepartment of electrophysics, National
Chiao Tung university
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Outine

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

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Introduction to CdSe/ZnS colloidal quantum dots
Diameter about 110 nm ( aB of CdSe about 6 nm
) Enhancement of fluorescence QYs by ZnS
overcoated High QYs ( 5085 ) Detective
fluorescence at RT
Emission color ranging from red to violet
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Introduction to colloidal QDs
Rhodamine red
Colloidal QDs
MBP molecule
Colloidal QDs

• Broad absorption with narrow symmetric
  fluorescence spectra
• ( FWHM25-40 )
• Large stokes shift
• Low photobleaching thresholds
• High QYs
• Biocompatibility

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

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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.

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Mechanism of time-resolved fluorescence
measuerments
Optical excitation of an electron hole pairs
Relaxation by phonon emission (10 ps)
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phonon emission
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Photon emission
pulsed laser
phonon emission
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Fluorescence of ensemble QDs
In general case -Concentration10-6 M -Laser
volume10-6 L -Total numbers of QDs1011 , ( size
distribution 5 )
cuvette
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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
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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 4痠 by 4痠 area.
• Single QD can be detected by far field optical
  microscopy.
• Diffraction limited laser spot size of 0.3 痠 can
  be obtained by use of high N.A. oil-immersion
  objective.

Single QD
4痠 by 4痠 area
Laser spot
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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
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The photograph of experimental system
Tisapphire
Solid State Laser
spectrometer
Time-resolved confocal microscope
2? generation
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TCSPC and time-tag time-resolved techniques
t1
t2
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Fluorescence intensity imaging of single(cluster)
QDs
4.7 4.7痠2
7 7痠2
Streaky feature
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How to identify the single QD
FWHM 0.3痠
Multi QDs
Milti- QDs
Single QD
Single QD
2.559痠 x 2.559痠
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P15
2.5痠 x 2.5痠
FWHM 0.3痠
lifetime 19 ns
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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 ).

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Decay time fluctuation with photon intensity
fluctuation
E
R
ST
NR
G
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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

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Schematic illustration of sample configuration
CdSe/ZnS QD
Gold nanoparticles
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Low intensity
High intensity
Medium intensity
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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.

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Thank you for your attention
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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