mikko nisula 05.05.2011. overview introduction plasmonics theoretical modeling influence of particle...
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IntroductionMetal nanoparticles interact with light more
strongly than any other chromophoreOptical cross-section is greater than the
geometrical cross-section
Plasmon resonanceOscillating electric field causes the conduction
electrons to oscillate coherently.Oscillation frequency determined by
Density of electronsEffective electron massShape and size of the charge distribution
Localized surface plasmonsLimited dimension of an nanoparticle prohibit
the plasmon waves from propagating.The excited state is not stable and decays
Radiatively -> Scattering of photonsNon-radiatively -> Absorption and conversion
to heatScattering + Absorption = Extinction
All conductive materials support LSPsAg, Au and Cu most studied as their plasmon
resonance frequency is near to that of visible light.
Mie-theoryExact solution to Maxwell’s equations for the case of a
sphereInput: Wavelength, particle radius, particle’s dielectric
function and the dielectric function of the environment.
Output: Exact extinction, scattering and absorption cross-sections, internal and external field intensities
The theory states that the scattering cross-section varies with r6 while absorption cross-section varies with r3
-> Absorption becomes more dominant as particle size decreases
Mie-theoryNo intrinsic restriction on particle size or
wave length, however:d < 10 nm -> Surface scattering must be taken
into accountd < 1 nm -> Classical electrodynamics no
longer validExperimentally derived coefficients -> No
information on the underlying mechanism i.e. LSPs
For particles with arbitrary shapes, computationally demanding numerical methods are needed
SizeTwo types of size effects, threshold for the
two regimes dependent on the metalExtrinsic (Above threshold): Related to the
diameter and bulk dielectric function. Redshifting and broadening of the resonance peak with increasing particle sizes
Intrinsic (Below threshold): Attenuation and broadening of the resonance peak due to surface scattering of electrons.
SizeWith increasing sizes, the retardation effect
may lead to higher-order oscillations -> additional peaks at shorter wavelengths
ShapePeak position shift correlates with the
increased number of sharp tips or edgesSurface roughness results in redshifting
ShapeMore complex shapes can feature distinct
LSPs on different surfacesCore-shell NPs, NanoringsCoupling of two surfaces leads to alteration of
the overall optical response
EnvironmentThe scattering spectrum redshifts as the
refractive index of the surrounding medium increases
NPs often deposited on a substrate prior to analysis -> May distort the results.A transition metal substrate dampens LSPs
Interparticle couplingProperties of a group of NPs can differ from a
single one even if the group is homogenousClosely spaced particle pairs exhibit a strong
polarization sensitivityPolarization of the incidence light
perpendicular to the center-to-center line -> Blueshift
P0larization along the line -> RedshiftPeriodically ordered NPs act as a grating
CharacterizationGeometrical measurements
SEMTEMAFM
Optical propertiesSpectrophotometry
LSP resonance maximum at transmission minimumNear and far field optical microscopy for single
particles
SummaryThe optical properties of metal nanoparticles arise
from localized plasmon resonance.Spherical particles can be modeled analytically
with Mie-theory, other shapes require numerical methods.
The optical properties are influenced by particle material, size, shape, environment and interaction with other particles.
Characterization with spectrophotometry and optical microscopy.
Applications range from optoelectronics to biomedicine.
References Temple, T.L., Optical properties of metal nanoparticles and their influence on
silicon solar cells, University of Southampton, School of Electronics and Computer Science, PhD Thesis, 2009
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Lin, Q., Sun, Z., Study on optical properties of aggregated ultra-small metal nanoparticles, J. Light Electron Opt. 2010, Article in press
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