| AVS 65th International Symposium & Exhibition | |
| Spectroscopic Ellipsometry Focus Topic | Monday Sessions |
| Session EL+AS+EM-MoM |
| Session: | Application of SE for the Characterization of Thin Films and Nanostructures |
| Presenter: | Michele Magnozzi, Università di Genova, Italy |
| Authors: | M. Magnozzi, Università di Genova, Italy M. Ferrera, Università di Genova, Italy M. Canepa, Università di Genova, Italy F. Bisio, CNR-SPIN, Italy |
| Correspondent: | Click to Email |
Metal nanoparticles (NPs) have the interesting property of behaving as efficient converters of EM radiation into heat. While this can occur via interband photoexcitation, the presence of a Localized Surface Plasmon Resonance provides an extra degree of freedom to tune and optimize the heating [1].
Assessing the temperature of plasmonic NPs during or immediately after illumination is not an easy task, and typically involves the use of models that necessarily have to simplify the complex temperature-dependent dielectric and thermodynamic response of nanosystems; for this reason, a measurement of the T-dependent optical behavior of the NPs at well-defined, externally controlled T would greatly contribute towards a better understanding of the thermoplasmonic properties of metal NPs.
Spectroscopic ellipsometry (SE), being a high-sensitive and non-destructive technique, is an ideal tool to investigate the optical response of NPs systems, provided that a proper model is used for data analysis.
We report a T-dependent investigation of the optical response of densely-packed 2D arrays of gold nanoparticles supported on an insulating nanopatterned substrate [2]. SE measurements were acquired in the 245-1450 nm spectral range, under high-vacuum conditions and in the 25-350 °C temperature interval [3]. Using a dedicated effective medium approximation developed for this kind of systems [2], we are able to reproduce the complex anisotropic optical response of this system employing morphological parameters deduced by ex-post AFM analysis; the temperature-dependent dielectric functions of Au, required as input in the model, was obtained in a dedicated SE measurement. The model yields a very good agreement with experimental data at relatively low T; however, though the appropriate T-dependent dielectric function of Au is systematically employed, the model is no longer able to reproduce the data obtained at the highest T. Indeed, a satisfactory agreement is attained introducing an effective correction to the Drude term of the dielectric function of Au, that keeps into account morphological effects affecting the NPs surface - such as softening or melting - that enhance the surface electron scattering rate. Our analysis thus shows that the T-dependent optical properties of metal NPs deviate from simplified expectations, and validate SE as valuable tool to study the complex, anisotropic properties of plasmonic NPs systems.
References
[1] A.O. Govorov and H.H. Richardson. Nano Today 1:30-38, 2007
[2] L. Anghinolfi, R. Moroni, L. Mattera, M. Canepa, F. Bisio. J. Phys. Chem. C, 115: 14036–14043, 2011
[3] M. Magnozzi, F. Bisio, M. Canepa, Appl. Surf. Sci., 421:651-655, 2017