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Size selected Gd nanoparticles and Gd-Pd pair nanoparticles for hydrogen induced switching

Final Activity Report Summary - NANOSWITCH (Size selected Gd nanoparticles and Gd-Pd pair nanoparticles for hydrogen induced switching)

The switchable mirror effect is based on the continuous changes in the optical properties from mirror like reflecting metallic state to transparent semiconductor state in rare earth hydride (REHx) films on increasing the hydrogen concentration from x = 2 to x = 3. A thin palladium layer is essentially required to serve two purposes. First, it prevents the rare earth film form oxidation and second, acts as a catalyst during hydrogenation and dehydrogenation. Fabrication of switchable mirror devices based on rare earth metals or alloys in thin film or multi-layer forms have been proposed and reported in literature. Realisation of the potential applications of rare earth based switchable mirror requires large improvements in the colour neutrality, switching / recovery time and optical contrast between the REH2 and REH3 states than those normally observed in the conventional devices. Use of rare earth nanoparticle can significantly improve the switchable mirror parameters due to quantum confinement and enhanced surface area effects at nano dimensions. Due to quantum confinment effect, the absorption edge of the trihydride state can be increased at nanodimensions and thus the colour neutrality can be achieved without changing the material chemistry. Increased surface to volume ratio at small dimensions is advantageous for increased H-rare earth interaction.

The advantages of using nanoparticle have been demonstrated by growing nanocrystalline layers by a simple method of inter gas condensation. The advantages of using Pd nanoparticles in place of ultra thin films of Pd as over layers has also been demonstrated by using Pd nanocrystalline layers. The synthesis of rare earth and Pd nanoparticles with controllable-sizes and well-defined size distribution to replace nanocrystalline layers (which are composted of nanocrystallites having a large and mostly uncontrolled size distribution) is the next important step crucial for realising the size-dependent advantages mentioned above.

The central objectives of the proposed work has been to synthesise size-selected Pd and rare earth nanoparticles for preparing nanoparticle-thin film, thin film-nanoparticle and pair nanoparticle structures. Rare earth metals are highly susceptible to oxygen. Protection of rare earth metal from oxidation during synthesis is one of the nagging problems encountered during the synthesis. In practical terms, this was observed to the major challenge for realising the goals of the present project.

The initial experiments carried out to synthesise rare earth nanoparticle by the conventional method of furnace evaporation yielded fully or partially oxidised nanoparticles. The deposition set up was modified by incorporating a spark generator having Pr or Gd rods for growing rare earth nanoparticle in a high purity carrier gas without oxidation. A dual deposition set up has been fabricated to prepare nanoparticle-thin film composite structures for post-synthesis protection of the rare earth nanoparticles from oxidation.

The successful synthesise of spherical, mono crystalline and size-selected Pd and Pr nanoparticles in the size range 10-30 nm without oxidation was one of the most useful result of the present project. The novel deposition set ups used to prepare:
i) size selected rare earth nanoparticles; and
ii) dual deposition set up used to prepare thin film -nanoparticle hybrid structure are shown in the attached files.
A detailed characterisation of the Pd, Pr nanoparticle and hybrid structures has been carried out (and is in progress) using high resolution electron microscopy, optical absorption photometry and electrical measurements.