Page 70 - Book Hosokawa Nanoparticle Technology Handbook

1.13 OPTICAL PROPERTY OF NANOPARTICLE FUNDAMENTALS
oscillations of electrons to shield polarization caused by means, and to measure light intensity with a detector.
external fields such as light and electron rays, is referred Especially for electron rays, their irradiation focused
to as plasma oscillation. This oscillatory wave is into a space of the order of tens of nanometers at low
expressed as a particle and referred to as a plasmon. The voltage has been realized, corresponding to the
contribution of a conduction electron is interpreted progress of the development of modern field emission
using the classical Drude model, which regards elec- display panels. Therefore, their significance as an exci-
trons as traveling within a stationary field with an ionic tation source in the emission spectrometry of nanopar-
core resembling that of a gas, but it assumes their inter- ticles has been increasing [5]. Since nanoparticles and
action with other electrons and ions as a mere damping nanostructured materials emit very little light as a
term. An electric field, generated when an electronic result of their small size, the analytical system should
gas deviates from a stationary ionic field as a whole, have sufficient capability in emission collection, spec-
acts as the restoring force. This is the principle by which tral efficiency and detector sensitivity. In emission
plasma oscillations occur. Interband transition also has measurements, an appropriate spectral method enables
a certain contribution in real particles. Therefore, opti- us to eliminate excitation light with deviated energy
cal response is described as their mixed state. from the emissions, so that weak emission can be
It is not often the case that real metal nanoparticles detected without being influenced by scattered light.
are completely isolated one by one in a vacuum: they Nevertheless, it is difficult to control the size, shape
are surrounded by a medium with different dielectric and composition of each nanoparticle precisely.
constants as a practically important morphology. Light Statistical measurement of many particles would there-
incidence onto transparent insulators such as metal fore necessarily measure the spread of emission based
nanoparticle-dispersed glass brings about a mode of on their inhomogeneity. Although the spread enables us
vibration called a surface plasmon that is localized on to analyze characteristics such as their size distribution,
the metal nanoparticle surface. A dielectric function it is difficult to measure the optical properties of a sin-
varies at the interface of different substances to try to gle nanoparticle with light. For example, the applica-
be continuous. Consequently, a mode of vibration with tion of useful spectroscopic analytical methods, such as
its resonance frequency that is deviated from the ordi- the fluorescence analysis, UV-Vis-IR spectroscopy, and
nary plasma frequency occurs, which creates a surface Raman spectroscopy, to an aggregate of nanoparticles
plasmon. Beautiful colors appear when incident light merely provides the average spectrum of nanoparticles
and surface plasmons resonate. This phenomenon has with a distribution of size and composition. The optical
been known and used commonly for many years, as in measurement of a nanoparticle requires mapping of
optical glasses such as stained glass in European spatially limited spectroscopic information in combi-
cathedrals since the Middle Ages. The effect of surface nation with spatial information obtained with an opti-
plasmons is manifested brilliantly in ultrafine particles cal microscope. However, the diffraction limit ( /2sin )
with a large surface fraction. Recently, metal nanopar- of an optical microscope based on the wave nature of
ticle systems have been attracting attention [3, 4] light prevents imaging of light far smaller than its
because of their enhanced nonlinear optics properties wavelength. Although light of a shorter wavelength can
by single electronic electrical conduction phenomenon improve spatial resolution, the wavelength is typically
because of Coulomb blockade and surface plasmon dependent upon the measurement method and cannot
mode. The nonlinear optical effect originates from be shortened easily. An alternative to shorten the wave-
nonlinear polarization that is proportional to the power length without varying energy is to let the light pass
of order. Therefore, a nonlinear optical effect can be through transparent matter of a refractive index n to
implemented at high efficiency by reinforcing a local convert the wavelength into /n. The oil immersion lens
field remarkably by surface plasmons. method using oil follows this principle, and is therefore
adopted in biological observation with an optical
microscope. A solid immersion lens (SIL) replaces oil
1.13.2 Measurement method of optical properties of with a transparent solid lens [6, 7]. However, the refrac-
nanoparticles tive index of transparent materials between visible radi-
ation is about n 2; there are also other restrictions,
The optical properties of these nanoparticles are meas- i.e., the necessity of evenness of the sample.
ured with spectrometry systems that are generally used Near-field scanning optical microscopy (NSOM) is
for ordinary powder particles. The wavelength of light attracting attention [8, 9]. It enables formation of a
is far greater than the nanoparticle size. Consequently, fine spot below the diffraction limit of ordinary light
it is likely that many particles are measured as a group. using near-field light. Its fundamental concept was
Luminescence measurement is a particularly effective proposed by Synge in 1928 [10].
measurement method for the optical measurement of The diffraction limit of ordinary light is derived by
nanoparticles. The basic procedures of the measure- solving Maxwell’s equation under the assumption that
ment are to excite a sample with excitation sources like light intensity follows a Gaussian distribution in the
light or electron rays, to carry out spectral analysis of radial direction, which reflects the oscillatory nature of
the luminescence of the sample with appropriate an electromagnetic wave that propagates in space.

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