In nature, organisms utilizes amorphous phases in order to achieve controlled mineralization. One of the main advantages of this strategy, is that it enables the organism to control over the resulting crystalline structure – which is not necessarily the thermodynamic-stable one. This control is achieved by manipulating the short-range order in the amorphous phase (usually, using different additives).
In our research we draw inspiration from this phenomenon and study the ability to control various structural aspects of amorphous materials by nanometer size effects.
We chose the Atomic Layer Deposition (ALD) as our material deposition technique; this method enables the production of conformal, pinhole-free films with high control over the thickness, by sequential exposure of the substrate to the precursors and it’s chemisorption onto the surface.
Figure 1 – The description of one ALD cycle; (a) functionalized substrate (b) exposure of the surface to the first precursor which adsorbs on the surface (c) purging the excess of the precursor out of the chamber (d) exposure to the seconds precursor, which reacts on the surface, creating one monolayer for the desired material (e) purging the excess of the precursor out of the chamber (f) repeating steps b-e, according to the desired final thickness.
It was shown in our group, that size effects indeed change the short-range ordering in Aluminum-Oxide (Al2O3) nano-layers; thinner films exhibit higher fraction of 4-coordinated Al sites.5 This result was proven experimentally via various characterization methods. These structural changes encouraged us to further study how different physical properties of amorphous Al2O3 are affected due to size effects and it was found that the density of these nano-films changes gradually with the size; thinner films has lower density than thicker ones. We expect that additional physical properties will be size-dependent as well, which can open new possibilities for materials selections and applications.
- Etinger-Geller, Y.; Katsman, A.; Pokroy, B. Density of Nanometrically Thin Amorphous Films Varies by Thickness. Chemistry of Materials 2017, 29, 4912-4919.
- Bloch, L.; Kauffmann, Y.; Pokroy, B. Size Effect on the Short Range Order and the Crystallization of Nanosized Amorphous Alumina. Crystal Growth & Design 2014, 14, 3983-3989.