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 MBP molecule Colloidal QDs 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 Illumination Fluorescence code

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 Optical excitation of an electron hole pairs Relaxation by phonon emission (~10 ps) - phonon emission - Photon emission pulsed laser - - phonon 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

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. Single QD 4μm by 4μm area Laser spot

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

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

TCSPC and time-tag time-resolved techniques

Fluorescence intensity imaging of single(cluster) QDs Streaky feature

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

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

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 R ST NR G

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

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

Low intensity High intensity Medium intensity

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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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