Nanoplasmonics for Absorption Engineering and Hydrogen Sensing

Licentiate thesis, 2012

When light interacts with metallic nanoparticles smaller than its wavelength it can excite a collective oscillation of the conduction electrons that gives rise to efficient light absorption and scattering. Moreover, locally strongly enhanced electric fields are created around the particles. This phenomenon, called localized surface plasmon resonance (LSPR), has been applied for centuries for staining glass in medieval church windows. Nevertheless, it is only within the last hundred years that fundamental physical understanding of its origin and potential has been created. The latter has finally coined one of the most vibrant sub-areas of modern nanoscience - nanoplasmonics. Palladium (Pd) is a noble metal whose main current use is in the three-way catalytic converter for car exhaust gas cleaning. However, Pd is likely to play an important role in many future technological applications related to a hydrogen economy, where hydrogen will take the role as carbon free energy carrier. Both the extraordinary catalytic properties of Pd and its unique ability to spontaneously dissociate and absorb hydrogen atoms into interstitial sites in its crystal lattice are expected to be of key importance. In this thesis nanoplasmonics in combination with Pd nanoparticles was used for two different purposes. In the newly developed optical absorption engineering application, plasmonic Au nanoantennas were used to enhance light absorption in an adjacent Pd nanoparticle up to 8 times. This novel absorption enhancement principle is envisioned to boost catalytic reactions on the surface of the Pd by either directly harvesting energetic electron-hole pairs or by utilizing local heating caused the non-radiative decay of the LSPR. In the nanoplasmonic hydrogen sensing application, the size-dependent hydrogen sorption thermodynamics of Pd nanoparticles in the sub-10 nm size range was investigated by means of indirect nanoplasmonic sensing (INPS). We found that the combination of two contributing effects, that is surface strain and hydrogen sorption in subsurface sites, basically cancel out each other, yielding no pronounced effect of particle size on the hydride formation thermodynamics.