Research on surface plasmons (SPs), one of the major field in nanophotonics,
explores the properties of a confined electromagnetic field over dimensions in the
order of or smaller than the working wavelength. SPs resonance (SPR) is an optical
resonance phenomenon where conduction electrons are coupled in metallic
nanostructures with an external electromagnetic field. The distinct resonance
condition is characteristic of an SPR, and an associated enhanced optical near-field is
formed in the SPR. This partially confined optical near-field provides an effective
route to design SPs-based materials and devices in several fields such as molecular
sensors, organic solar cells, light emitting diodes and etc. Many innovative studies
have been conducted over the years. However, there is still a need for a more in-depth
comprehension of SPs for additional practical applications.
The present work focused on the impact of SPs on the photophysical properties
of chromophore, especially in photoluminescence (PL) enhancement. An optical
illumination on metal/dielectric interfaces agitates surface waves (SPs), resulting in an
enhanced optical near-field in the sub-wavelength dimension. This enhanced field can
simultaneously quench or enhance PL of nearby chromophore (competing process),
depending on the spacing between metal and chromophore as well as the spectral
overlap between the emission band of the chromophore and the SPR band. PL
enhancement can be achieved using a delicate design, in which either propagating SPs
in planar waveguide or localized SPs (LSPs) are in the metallic nanostructure. Given
these conditions, the present thesis emphasizes the adoption of known numerical
modeling methods, including the quasi-static method for very small nanoparticles, the
Mie theory for nanoparticles with regular spherical shapes, and discrete dipole
approximation for nanoparticles with arbitrary shapes.
Considering the working wavelength in the visible and near-infrared (NIR)
region, silver (Ag) and gold (Au) were chosen as research materials. With instructions
for numerical calculation, a variety of different contexts for fabrication were introduced. For instance, an Ag film sustaining the propagating SPs was deposited on
a dielectric substrate via thermal evaporation, whereas Ag/Au nanoparticles with
various shapes, such as spherical Ag nanoparticles, Ag nanorods, and Ag nanoprism,
among others, were synthesized using the wet chemical method. The synthetic
mechanism of Ag/Au nanoparticles was in accordance with the classical
thermodynamic theory. To enhance the PL of chromophore PL at concerning working
frequency, samarium (Sm) and cadmium chalocogenides were chosen as active
chromophores.
An Sm3+-SU8/Ag/SiO2 configuration was fabricated to investigate the
SPs-assisted PL enhancement. The sharp and distinct spectra of Sm3+ (562, 598, and
644 nm corresponding to 4G5/4 → 6H5/2, 6H7/2, and 6H9/2, respectively) facilitated the
investigation on the role of SPs during the PL enhancement process. The PL
intensities between the sample with and without an Ag film were compared. In
addition, the different PL enhancement factors were also determined by varying the
thickness of the Ag film. For instance, the PL enhancement factor at room
temperature was 8.3 for the sample with a 30 nm Ag film, whereas this factor dropped
to 6.6 for the sample with a 70 nm Ag film. Moreover, to eliminate unwanted
influence from lattice vibration, the PL enhancement factor was measured at a very
low temperature (10 K). At 10 K, the PL enhancement factor was 2.3 for the sample
with a 30 nm Ag film, whereas this factor increased to 3.7 for the sample with a 70
nm Ag film.
Numerical analysis revealed for the first time that the electron density in metallic
nanostructures is the key component in tunneling the PL enhancement factor. This
discovery provided a simple and versatile way to tunnel the emission spectral
performance of the Sm3+-complex in an Sm3+-SU8/Ag/Au/SiO2 multilayer because of
the different work functions of Au (5.10 eV) and Ag (4.52 eV). The dispersion
relation of SPs in such a multilayer configuration was deduced, and the relation
validity was proved with the experimental results, in which the PL peak intensities of the Sm3+-complex centering at 562 and 598 nm changed without any modification in
the molecular structure of the Sm3+-complex.
To verify the SPs that were active in the PL enhancement process in the tri-layer
system, the Ag film was printed with a periodic pattern, which can convert the
propagating SPs into detectable scattering light. A self-assembly monolayer (SAM)
pattern was formed by employing polystyrene nanosphere (NS), a cheap and
conventional method. However, the hydrophobicity of the Ag film and the
hydrophilicity of the surfactant of the polystyrene NS hindered the formation of the
SAM pattern. Triton X-100 was used for the pre-decoration of the Ag film due to its
capacity to change the hydrophobic characteristic of the Ag film to hydrophilic. A
large scale SAM pattern was then fabricated, and the image of the SPs from the
pattern was observed during the PL enhancement process. The present study would
open up new opportunities for highly efficient and wavelength-selective
electroluminescent devices.
Moreover, the thesis investigated the emission performance of europium (Eu3+)
ions in an Ag nanoparticles system. The Ag nanoparticles were synthesized either via
the Ag film annealing method or the polyol reduction method. These methods showed
potential as good LSPs hosts for PL enhancement of Eu3+ ions. The PL enhancement
factor and lifetime of Eu3+ ions influenced by LSPs were measured to study the
influences of LSPs on radiative and nonradiative decay process.
Furthermore, considering the applications of SPs in bio-labeling or solar cells,
the modulation of active nanoparticles with noble metallic nanoparticles was a
prerequisite and had provided fundamental research interest. The modulation of
cadmium (Cd) chalocogenides nanoparticles with Ag nanoparticles emerged against a
number of technical issues, such as forward cationic ion exchange in mutual
nanoparticles, conjugation efficiency, and occurrence of PL quenching in very close
mutual distance (smaller than 5 nm), among others. To overcome these challenges,
the present study is the first to propose the synthesis of CdS and Ag nanoplate (CdS-Ag NP) hybrid by a forward-reverse cation exchange. The morphology,
crystallinity, and atomic composition of the CdS-Ag NPs were investigated using
energy dispersive X-Ray spectroscopy and high resolution transmission electron
spectroscopy. The CdS-Ag NPs were Cd2+-rich CdS nanoparticles covalently bonded
to the surfactant of the Ag NPs. PL enhancement of measured CdS was attributed to
the matching of the emission bands of CdS and the tailor-made LSPR bands provided
by the Ag NP. In addition, the application of the CdS-Ag NP in HeLa cell imaging
was also demonstrated. The present results provided a simple and flexible
methodology for conjugating complex nanoparticles, thus offering promising practical
applications in nanotechnology.
In conclusion, the present thesis presented a systematic investigation on the
properties of SPs and their applications. Following the essential elements on SP
research, the experimental designs of SPs applications were proposed using a
top-down methodology.
| Date of Award | 3 Oct 2011 |
|---|
| Original language | English |
|---|
| Awarding Institution | - City University of Hong Kong
|
|---|
| Supervisor | Yue Bun Edwin PUN (Supervisor) |
|---|