Optically-selective window coatings of precious metal nanoparticles
- Publication Type:
- Thesis
- Issue Date:
- 2010
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Energy-efficient window coatings limit the transfer of energy from one side of a
window to the other. Their use has the potential to significantly reduce the electrical
energy consumed by air conditioning, heating and lighting the interior space of a
building. In particular, the use of spectrally selective coatings allows for more control
over both the internal climate, the colour of the transmitted light, and the overall energy
efficiency of the window coating. Gold nanoparticles, especially the optically
anisotropic gold nanorods, offer a uniquely stable and long lasting solution to obtaining
tuneable absorption of electromagnetic radiation for a wide range of potential
applications. In this thesis the potential of gold nanorods as a spectrally selective
coating for window glass is examined, the possible means of securing the nanorods to
the glass are explored and their effectiveness as an optical coating compared to current
commercially available coatings of similar design. The optical properties of these
nanorods and some other shapes with potentially interesting properties are
computationally modelled and are compared to the properties of samples fabricated
using electron beam lithography and to samples made by wet chemistry. The work
shows that gold nanostructures could serve as the basis for a new generation of
spectrally-selective coatings for architectural glass.
Chapter 1 of this thesis reviews the fabrication, optical properties and potential
applications of gold nanostructures. The fabrication methods reviewed include reported
lithographic procedures as well as wet chemical techniques for the formation of gold
nanostructures. The optical properties of the gold nanostructures are examined, and the
phenomenon of surface plasmon resonance is analysed. Some potential applications for
the optical properties of gold nanostructures are explored, and an appraisal is made of
window glazing technologies including their operating mechanism. Also, the
advantages of the various technologies are reviewed.
In Chapter 2, computational modelling of the optical properties of gold nanoparticles
of rod, X and V shapes is carried out. The study shows the surface plasmon resonances
of the gold nanoparticles are influenced by the dimensions, spacings and angles of the
structures. The polarisation of the incoming light also affects the optical extinction
properties of the investigated nanoparticles. When the structures are aligned in a 1D
array, the main surface plasmon resonance peaks are red-shifted compared to the
isolated structure. An analysis of the electric field intensity as a function of distance
from the structure is conducted to determine the potential use of these structures as an
amplifier for two-photon fluorescence.
Chapter 3 explores the electron beam lithography fabrication process, including an
examination of the effects of different substrates, chamber pressure, deposition
parameters, dose characteristics and post-exposure processes. Due to the reduction in
exposure time for conducting glass over insulating glass, further experimentation is
conducted with a transparent conducting glass substrate. The formation of large arrays
of high quality gold nanostructures is achieved through the comprehensive investigation
of deposition conditions and in particular the addition of a short burst of plasma
cleaning after the development step. Sufficient areas for rod and X arrays are fabricated
to enable the measurement of the optical properties.
The optical properties of the electron beam lithographically fabricated structures are
addressed in Chapter 4. A comparison is undertaken between the measured optical
properties from structures produced in Chapter 3 to the modelled structures investigated
in Chapter 2. Surface plasmon resonance peaks are found on each gold nanostructured
sample investigated. However, the optical properties of a single layer of gold
nanostructures are insufficient to produce an effective window coating.
Chapter 5 investigates the attachment of gold nanorods onto glass through an
application of a molecular binding layer and immersion of the nanorods into a polymer.
The use of 3-mercaptopropyltrimethoxysilane to bind gold to glass is found to increase
the amount of gold nanorods attaching to glass compared to samples without the
binding molecule. However, the immersion of gold nanorods into a polymer layer is
found to have the most potential for use as a window coating, as careful control over the
gold nanorod concentration could be used to adjust the required optical properties.
Mixtures of different aspect ratio gold nanorods combine the different longitudinal
plasmon resonances to absorb light over a larger spectral range. Control over the aspect
ratio of the nanorods included in the film, ensured control over optical properties. The
optical transmission properties of the large area films of gold nanorod mixtures
provided superior window transmission properties in the spectral range 700 – 900 nm
compared to a currently commercially available solar laminate window coating.
In Chapter 6 the effect of an electric field on gold nanorods in solution is
investigated. Prior reports suggested that the gold nanorods could be aligned under the
influence of an AC electric field. No alignment was observed here for wet-chemically
produced gold nanorods, with a CTAB stabiliser, when under the influence of an AC or
DC electric field. A DC electric field of sufficient strength is found to strip the gold
nanorods of the CTAB stabilising agent, which results in gold depositing on the positive
electrode.
The final chapter of this thesis, Chapter 7, summarises the work presented and
highlights the significant conclusions. It also outlines some directions for possible
further research built on and extending from the work presented here.
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