Fundamentals of Hierarchical Nanostructures - Hierarchical Nanostructures for Energy Devices - page 17

in crystal color and may also lead to spectacular changes in
luminescence.
(d) These effects mean that nanocrystals of almost any material will have
unique physical and optical properties that are determined by the
nanocrystal size and shape. In this article, we concentrate on gold as
just one example of a material that can be synthesized with a range of
sizes and optical properties.

d n 3 r 4 n g | 7

One of the key concepts in the field of nanomaterial optical properties is
the localized surface plasmon (LSP) resonance, which can be understood as
a collective resonant oscillation of all of the conduction electrons of the
nanoparticle in response to an incident optical field. 81 A nanoparticle sup-
ports a range of LSPs, but the optical response is usually dominated by di-
polar modes. The oscillation frequencies are highly dependent on the shape
of the nanoparticle, but they also crucially depend on the absolute size and
dielectric environment. 82 The oscillation can be localized on a single
nanoparticle, or it can involve many coupled nanoparticles. Because of the
strong and tunable particle-light interaction, several interesting effects and
applications are possible. 82 Examples include surface-enhanced Raman
scattering (SERS), 83,84 optical waveguides 85,86 and biochemical sensors. 87-90
Figure 2.4 shows the size-dependent optical property of single nanoparticles.
Figure 2.4(a) shows scattering spectra of single isolated silver particles with
diameters of D ¼ 50, 100, 150, and 200 nm, together with Lorentzian curve
fits and calculations. 81 The intensity axis in Figure 2.4(a) is proportional to
scattering eciency. This allows for convenient comparisons of particles of
different sizes and scattering cross sections. Figure 2.4(b) shows that there is
a linear shift of the resonance as a function of particle size, as expected from
theory. From the slope of the line, it is found that the peak position shift is
approximately 27 nm per 10 nm change in diameter for the geometry used.
The experimental and theoretical slopes agree very well (26.4 and 27.5 nm,
respectively). 81
Recent advances in particle synthesis and nanofabrication technology
have made it possible to produce well-defined metal nanostructures,
enabling systematic studies of their optical characteristics. 81 A number of
groups have studied the optical properties of gold and silver nano-
particle arrays. 91-95 The results show that interparticle coupling effects
give rise to pronounced shifts of the LSP resonance frequency compared
to isolated particles. Competing factors that give rise to either blue or red
shifts with decreasing interparticle distances have been identified. 91,96,97
The electromagnetic coupling between particles involves both very short-
distance interactions, due to evanescent fields, and long-range interactions,
mainly due to propagating dipolar fields. 91 A thorough understanding
of these local field and coupling effects is crucial for nanoparticle
wave-guiding applications, where one wants to achieve ecient energy
transport between particles while minimizing far-field scattering losses. 91

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