Mikko Nisula 05.05.2011. Overview Introduction Plasmonics Theoretical modeling Influence of particle properties Applications.

Optical properties of metal nanoparticles

Mikko Nisula05.05.2011

OverviewIntroductionPlasmonicsTheoretical modelingInfluence of particle propertiesApplications

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

ApplicationsOptoelectronics

Solar cellsBiomedical

BiolabellingCure for cancer!

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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