Reversibly switchable materials with applications in plasmonic switching and pressure sensors

Groep: Lumilab
Promotor: Philippe Smet en Dirk Poelman
Begeleiding: Andreas Sousanis

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Probleemstelling:

In this master project, we will fabricate hybrid reversibly switchable materials that show an insulator (I)-metal (M) transition, combined with plasmonic structures. Sm chalcogenides show a pressure-induced insulator-metal transition. These compounds show a volume collapse upon the transition to the metallic state, instead of a crystallographic phase change, which reduces downgrading during the cycling between both states. This is beneficial for e.g. memory applications. SmS is such a switchable material, that shows this transition at a relatively low pressure (0,65 GPa) in comparison with other candidates found in literature. A remaining problem is that to switch back to the insulator state, heating in vacuum, at 400°C, is needed. Some of our preliminary results demonstrate a reversible behaviour of SmS, without heating or tensile stress, by alloying SmS with other rare earth elements, such as Eu, as the energy bands can be slightly shifted.

Figure 1. XRD spectra of the states of SmS.
Figure 1. XRD spectra of the states of SmS.

In this work we will exploit the aforementioned reversible switching of the Sm1-xEuxS alloy to induce a reversible switching of plasmon resonances in Au nanoparticles which are embedded within the matrix of the produced alloy. The thin film structure will be obtained by means of an e-beam evaporator and d.c. sputtering. Post-deposition annealing at proper temperature and atmosphere is performed to enhance the diffusion of Eu into the SmS lattice and for the fabrication of isolated Au nanoparticles within the matrix of the alloy. The optical and structural properties of the thin film parts will be studied via UV/VIS spectroscopy, electron microscopy and X-ray diffraction.

The successful implementation of the final structure will yield a new material with tunable switching threshold for the insulating-metal transition, avoiding heating or tensile stress for the back switching. At the same time the Au nanoparticles support a plasmonic resonance, which can be switched on an off itself, due to the change of the surrounding dielectric function from semiconducting to metallic and vice versa.