Ph.D. Thesis
Title Spectral Properties and Relaxation Dynamics
of Surface Plasmon Electronic Oscillations in Gold and Silver Nanodots
and Nanorods
Adviser Professor Mostafa A. El-Sayed
Thesis Committee Z. L. Wang, School of Materials Science &
Engineering; R. L. Whetten, School of Chemistry and Biochemistry;
J. Zhang, School of Chemistry and Biochemistry; R. Hernandez, School
of Chemistry and Biochemistry, Georgia Tech.
Essay
Noble metals and especially gold in colloidal form have
fascinated scientists since the Middle Ages because of its intense
red color originating from the surface plasmon absorption. The red
color of stained glass windows as used in old churches and cathedrals
originates from nanoparticles of metallic gold in its neutral valence
state, which was first recognized by Faraday. Recently, the study
of small metal particles of nanometer size has found tremendous interest
because of interesting new material properties due to the reduction
of the particle size and because of possible applications of nanotechnology
in such diverse fields as (photo)catalysis, (opto)electronics, magnetic
devices and sensors.
A strong visible absorption appears when the size of
the gold decreases to the nanometer length scale. It results from
the coherent oscillation of he free electrons in the conduction band.
This is called the surface plasmon oscillation and the resulting absorption
is called the surface plasmon absorption. Both the radiative and nonradiative
relaxation properties of this unique motion were focus of the thesis
research. 16 publications with two invited review articles have resulted
from this research. [1,2, and references
therein]
Spherical gold nanoparticles in aqueous solution having
different sizes with mean diameters between 10 and 100 nm have been
synthesized and the size and temperature dependence of the surface
plasmon absorption has been studied. The band width of the surface
plasmon absorption is related to the dephasing time (T2) of
the coherent plasmon electronic oscillation which is found to be on
the order of a few femtoseconds. From its weak temperature dependence
it is concluded that the dephasing results from mainly electron-electron
repulsion and not electron phonon coupling.
Gold-silver nanoparticle alloys with varying compositions
have been prepared similar to pure gold nanoparticles. The formation
of homogeneously mixed alloy nanoparticles is concluded from high
resolution transmission electron microscopy (TEM), energy dispersive
spectrometry (EDS) and optical absorption spectroscopy. The latter
reveals the presence of only one plasmon absorption band whose maximum
shifts linearly with the gold mole fraction between the values of
the pure silver and gold nanoparticles enabling an easy spectral tuning
of the surface plasmon band.
For gold nanorods, the surface plasmon absorption
splits into two bands corresponding to the electron oscillations along
(longitudinal mode) and perpendicular (transverse mode) to the long
axis of the nanorods. The wavelength of the maximum absorption intensity
of the longitudinal plasmon absorption increases linearly with increasing
nanorod aspect ratio. This relationship is examined theoretically
by using the Gans theory.
The same gold nanorods encapsulated in micelles in aqueous
solution fluoresces with a quantum yield, which is over a million
times larger than that of the bulk gold films. The origin of this
enhancement is treated theoretically and is found to be caused by
the local field effect ("lightning" gold nanorods) similar to the
previously proposed fluorescence and Raman enhancement on noble metal
rough surfaces.
Enhancement factors of 107 are calculated,
in agreement with experimental results. Simulations can furthermore
reproduce the linear dependence of the fluorescence maximum on the
gold nanorod aspect ratio and the quadratic dependence of the fluorescence
quantum yield on the aspect ratio. Enhanced electron-surface scattering
in nanoparticles smaller than the electron mean free path (~ 50 nm
in gold) has been suggested to influence not only the dephasing time
of the coherent plasmon oscillation (T2) but also the energy
relaxation (T1) of photoexcited hot electrons into lattice
excitation. Excitation with femtosecond laser pulses leads to a transient
broadening of the surface plasmon band(s), which results in a transient
bleach signal caused by a hot electron gas. It is found that the excited
electrons couple to the lattice vibrations by electron-phonon coupling
establishing a thermal equilibrium between electron and phonon subsystem
within a few picoseconds. The deposited laser energy is then released
to the surrounding medium which is found to take place in about 100
ps. An enhanced electron-surface scattering should cause a decrease
in the electron-phonon relaxation time. However, no size and shape
dependence of the electron-phonon relaxation dynamics are found for
particles in the size range between 10 and 100 nm.
The effect of the surrounding medium on the cooling
dynamics of spherical gold nanoparticles is examined by comparing
the dynamics of gold nanoparticles in solution with the same particles
embedded in MgSO4 powder. A slower electron-phonon relaxation
time is found and the results are attributed to a hindered heat exchange
between the gold nanoparticles and the surrounding medium for the
particles in the MgSO4 powder.
Depending on the femtosecond laser pulse energy, irradiation
of gold nanorods in colloidal solution is found to lead to a shape
transformation into spherical nanoparticles of comparable volume or
into smaller nanodots by fragmentation. Femtosecond pulses are found
to be more gentle and effective in transforming the gold nanorods
into nanodots. High energy nanosecond laser pulses cause fragmentation
of the nanorods into smaller nanodots by the absorption of additional
photons while the nanoparticle lattice is still hot.
The time required to melt a gold nanorod in solution
is investigated by femtosecond transient absorption spectroscopy.
This allows one to follow the rise of the permanent bleach of the
longitudinal surface plasmon absorption due to the transformation
of the nanorods into spheres of comparable volume. The dynamics of
this shape transformation is found to occur in 35 ps for nanorods
with aspect ratios between 1.9 and 3.7, at laser pump power at the
energy threshold for the complete melting of the gold nanorods. The
minimum absorbed energy necessary for this shape transformation is
further determined to be about 60 femtojoule per nanorod.
The mechanism of the nanorod-to-nanodot shape transformation
is examined by high resolution TEM. The as-prepared gold nanorods
are single crystals and are found to be defect-free. However, after
excitation with femtosecond and nanosecond laser pulses with energies
below the energy required for the complete melting different internal
defect structures appear in the middle of the rod. It is therefore
concluded that the initial step in the shape photothermal transformation
involves the creation of defect structures inside the rods followed
by surface diffusion and reconstruction of the unstable {110} surface
of the gold nanorods.
The knowledge of the cooling dynamics is also important
for any possible future optical applications since the heat absorbed
by laser light needs to be dissipated without structural damage to
the nanostructural device itself on a time scale faster than its duty
cycle. On the other hand, the controlled shape control of chemically
prepared metallic nanoparticles can be achieved with pulsed laser
light of the appropriate pulse width and energy.
[1] S. Link, M. A. El-Sayed, Steady-state
and time-resolved optical properties of metallic nanoparticles: The
surface plasmon absorption as an analytical tool to investigate particle
properties. Int. Rev. Phys. Chem. 19, 409 (2000).
[2] S. Link, M. A. El-Sayed, Spectral properties and
relaxation dynamics of surface plasmon electronic oscillations in
gold and silver nanodots and nanorods. J. Phys. Chem. B 103,
8410 (1999).