Optical properties of metal nanoparticles

1. Opticalproperties of metalnanoparticles Mikko Nisula 05.05.2011

2. Overview • Introduction • Plasmonics • Theoretical modeling • Influence of particle properties • Applications

3. Introduction • Metal nanoparticles interact with light more strongly than any other chromophore • Optical cross-section is greater than the geometrical cross-section

4. Plasmon resonance • Oscillating electric field causes the conduction electrons to oscillate coherently. • Oscillation frequency determined by • Density of electrons • Effective electron mass • Shape and size of the charge distribution

5. Localized surface plasmons • Limited dimension of an nanoparticle prohibit the plasmon waves from propagating. • The excited state is not stable and decays • Radiatively -> Scattering of photons • Non-radiatively -> Absorption and conversion to heat • Scattering + Absorption = Extinction • All conductive materials support LSPs • Ag, Au and Cu most studied as their plasmon resonance frequency is near to that of visible light.

6. Mie-theory • Exact solution to Maxwell’s equations for the case of a sphere • Input: 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

7. Mie-theory • No intrinsic restriction on particle size or wave length, however: • d < 10 nm -> Surface scattering must be taken into account • d < 1 nm -> Classical electrodynamics no longer valid • Experimentally derived coefficients -> No information on the underlying mechanism i.e. LSPs • For particles with arbitrary shapes, computationally demanding numerical methods are needed

8. Size • Two types of size effects, threshold for the two regimes dependent on the metal • Extrinsic (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.

9. Size • With increasing sizes, the retardation effect may lead to higher-order oscillations -> additional peaks at shorter wavelengths

10. Shape • Peak position shift correlates with the increased number of sharp tips or edges • Surface roughness results in redshifting

11. Shape • More complex shapes can feature distinct LSPs on different surfaces • Core-shell NPs, Nanorings • Coupling of two surfaces leads to alteration of the overall optical response

12. Environment • The 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

13. Interparticle coupling • Properties of a group of NPs can differ from a single one even if the group is homogenous • Closely spaced particle pairs exhibit a strong polarization sensitivity • Polarization of the incidence light perpendicular to the center-to-center line -> Blueshift • P0larization along the line -> Redshift • Periodically ordered NPs act as a grating

14. Characterization • Geometrical measurements • SEM • TEM • AFM • Optical properties • Spectrophotometry • LSP resonance maximum at transmission minimum • Near and far field optical microscopy for single particles

15. Applications • Optoelectronics • Solar cells • Biomedical • Biolabelling • Cure for cancer!

16. Summary • The 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.

17. 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 • Kelly, K.L., Coronado, E., Zhao, L.L., Schatz, The Optical Properties of Metal Nanoparticles: The Influence of Size, Shape, and Dielectric Environment, J. Phys. Chem. B, 107, 2003, 668-667 • Sosa, I.O., Noguez, C., Barrera, G.R., Optical Properties of Metal Nanoparticles with Arbitrary Shapes, J. Phys. Chem. B, 107, 2003, 6269-6275 • Lin, Q., Sun, Z., Study on optical properties of aggregated ultra-small metal nanoparticles, J. Light Electron Opt. 2010, Article in press • Biswas, A., Wang, T., Biris, A. S., Single metal nanoparticle spectroscopy: Optical characterization of individual nanosystems for biomedical applications, Nanoscale, 2, 2010, 1560-1572 • Peng, H.-I., Miller, B. L., Recent advancements in optical DNA biosensors: Exploiting the plasmonic effects of metal nanoparticles, Analyst, 136, 2011, 436-447 • Biju, V., Itoh, T., Anas, A., Sujith, A., Ishikawa, M., Semiconductor quantum dots and metal nanoparticles: Syntheses, optical properties, and biological applications, Anal. Bioanal. Chem., 391, 2008, 2469-2495