ALD Applied to Conformal Rare-Earth Coating of ZnO Nanoparticles for Low Temperature Thermal Imaging Applications

Nanostructured materials can be challenging to coat. Amongst the various deposition processes atomic layer deposition (ALD) has been recognized to be the most promising method for coating nanomaterials and more specifically nanoparticles.^1,2 Engineering of materials on the nanometer scale to control size and shape is of fundamental importance for tuning physical properties. Amongst oxide nanoparticles, zinc oxide is one of the most studied compound semiconductors due to its wide direct bandgap (Eg ≈3.37eV) and high potential in many nano-electronic, magnetic and optical applications. Research targeting preparation and characterization of ZnO based materials on a nano-scale has gained particular importance over the past decade.³

In this paper, ZnO nanoparticles were dispersed on the surface of a silicon substrate and then coated to be embedded inside an oxide rare-earth thin film deposited using ALD. Films were deposited using Eu(thd)₃ or Nd(thd)₃ and ozone as precursors at 200ºC.  The ZnO nanoparticles were synthesized via sol-gel method⁴using zinc acetylacetonate hydrate or zinc acetate with benzylamine. The mixture was put into a stainless steel autoclave and then heated in a furnace at 200ºC for 2 days.

The structural properties of the nanostructured thin films were studied by XRD, SEM and HRTEM. XRD measurements shows that all ZnO NPs have the hexagonal wurtzite-analogous (P6₃mc) crystal structure (a=3.25Å and c=5.20Å) and are highly crystalline. The rare earth oxide thin films are amorphous. The TEM studies highlight a very low roughness and a homogeneous distribution and coating of the ZnO nanoparticles inside the rare-earth oxide thin film.

Photoluminescence (PL) studies show that pure rare-earth oxide thin films deposited under the same conditions do not present any PL under UV excitation. The PL study of the nanocomposite thin films shows two apparent emission bands centered at around 370nm and a broad peak around 500 nm (See figure). Pure ZnO nanoparticles did not exhibit red photoluminescence contrary to the embedded ones (See figure). The nature of these emission peaks will be discussed considering the interaction between the ZnO nanoparticles and the rare earth matrix.⁵ This red PL emission can be attributed to indirect excitation of europium, commonly termed as sensitization or “antenna” effect.⁶ The influence of the nature of the matrix (Eu₂O₃ and Nd₂O₃) will be compared and discussed. A comparison of ZnO nanoparticles embedded in oxide-rare-earth matrix with other oxide perovskite nanoparticles like CaHfO₃ and SrHfO₃embedded in similar matrix will be addressed to identify the probable nature of the photoluminescence. Finally, the variation of the photoluminescence with the temperature (See figure) will be presented demonstrating a potential for applications in the field of thermal imaging and optoelectronics.

Acknowledgements

Financial support from Marie Curie (PERG05-GA-2009-249243) and the Research Council Norway project 176740/130 is acknowledged.

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Figure: Temperature-dependent PL spectra (form 10K to 300K of ZnO nanoparticles embedded in a Eu₂O₃ matrix. (a)Image of the comparison of pure ZnO nanoparticles spread on glass substrate (left) and 3 different amounts of ZnO nanoparticles dispersed on silicon substrate and coated with Eu₂O₃ (right) under UV-light illumination at room temperature. Only coated nanoparticles exhibit red photoluminescence. (b)ZnO nanoparticles embedded in Eu₂O₃ matrix (1mmx1mm) under UV excitation at 8K.