4C15(Purpose)

Hnin YuYuKo

Nanoparticles have a variety of unique spectroscopic, electronic, and chemical properties

that arise from their small sizes and high surface/volume ratios1. In particular, noble metal clusters andcolloids with nanometer dimensions have attracted a great deal of interest recently. The quantum size
effect, nonlinear optical effect, and electron dynamics of noble metal nanoparticles are being
extensively investigated, because they can be applied in catalysts, sensors, drug delivery,
optoelectroelectric, and magnetic devices.
ExperimentalWe have prepared three kinds of Ag colloidal nanoparticles: (1) dodecanethiol-

derivatized Ag nanoparticle (sample-1), pure Ag nanoparticles dissolved in water (sample-2), andlong-chain carboxylate acid protected Ag nanoparticles (sample-3), respectively. Sample-1 is preparedby using two-phase liquid-liquid system2. Sample-2 is synthesized by the direct adding of reduction
agent (NaBH4) to the precursor solution of AgNO3. Sample-3 is made by solvent free preparation
method with treating the mole ratio (1:10) of silver actate and oleic acid at 150°C-1h. The
excited-state dynamics of samples are studied by using femtosecond laser spectroscopy andpicosecond single-photon timing spectroscopy. The average particles of samples are preformed by theTEM observation. The photoexcitation in the interband leads to the heating of conduction electron gas
and its subsequent thermalization through electron-electron and electron-phonon interaction. The
results are compared to the dynamics of Ag nanoparticles with changing the laser intensity and solvent
effects.
Results and Discussion The absorption spectra

1.2
sample-1
sample-2
sample-3

Intensity (norm.)

1of samples are shown in Fig. 1. The surface
plasmon peaks of sample-1, 2, and 3 are observed
at about 440, 390, and 420 nm, respectively. The

0.8

difference peak position can be considered on thechange in refractive index of surrounding

0.6

mediums. The TEM observation revealed that
0.4

homogeneous dispersion and the average particle
size of samples are 4~10 nm, respectively.

0.2

0Optical excitation into the plasmon band creates
300

400

500

600

sample-1,2,700

Wavelength (nm)Fig. 1. Absorption spectra of Ag colloidal
nanoparticles
respectively.

forand3,

hot electrons as manifested by bleaching of thisband. The bleach recovery can be described by abiexponential. The first component with a decay
time between about 1 and 10 ps is attributed toelectronic energy relaxation through electron-phonon coupling. The second component with a decay time of several tens of picoseconds originatesfrom the subsequent cooling of the lattice by phonon-solvent interaction3.
The transient absorption spectra and their temporal profiles for sample-1, 2, and 3 are shown in Fig. 2,

30 ps2.8 ns500 ps2 ps

12 ps
3.5 ps

OD

1 ps

OD

100 ps00 (0.1/div)

00 (0.01/div)

10 ps

0 ps

2 ps
0 ps

base line
450500

550

600

650

base line
700

Wavelength (nm)Fig. 2. Transient absorption spectra ofsample-1 dissolved in toluene at different
time delay, λex=390 nm.
1.5 ns300 ps400440480520560

600

640

680

Wavelength (nm)Fig. 3. Transient absorption spectra ofsample-2 dissolved in water at different timedelay, λex=360 nm.

3, and 4, respectively. The strong bleaching (around410~440 nm) and a positive transient absorption

00 (0.05/div)

above 480 nm are observed. For sample-1, the10 psbleaching peaks are recovered within a few

3 ps

picosecond time scales. The positive absorption
peak exists until a few hundred picosecond time

1 ps

ODscales. In the case of sample-2, the bleaching peaks
become weak in a few ten picosecond and than0 psstrong bleaching peak with large width appear
again. This peak is shifted to longer wavelength.

base line
400440480520560600

640

Both bleach and positive absorption remain until a680

Wavelength (nm)Fig. 4. Transient absorption spectra ofsample-3 dissolved in toluene at different
time delay, λex=360 nm.

few nanosecond time scales. For sample-3, thebleaching peaks are recovered within a few
picosecond time scales. The positive absorption

peak remains until a few hundred picosecond time scales. This phenomenon is very similar to that ofsample-1. From the results, the behavior of excited state dynamics of samples is different with andwithout capping (stabilizing) agent. Therefore, we tried to prepare sample-3 dissolved in varioussolvents (toluene, cyclohexane and NaOH solution) with different silver acetate/oleic acid molar ratios
and examined their excited-state dynamics. It is clear that the phonon-phonon coupling time depends
on the surrounding mediums. We will also report on the dependence of laser power intensity on therelaxation dynamics and luminescence of Ag nanoparticles.
References(1) S. L. Longunov, T. S. Ahmadi, M. A. El-Sayed, J. T. Khoury and R. L. Whetten, J. Phys.

Chem. B, 1997, 101, 3713. (2) M. Brust, M. Walker, D. Bethell, D. J. Schiffrin, R. Whyman, J. Chem. Soc, Chem.
Comm., 1994, 801. (3) T. W. Roberti, B. A. Smith, and J. Z. Zhang, J. Chem. Phys., 1995, 102, 3860.