Crystallization of structured silica

Soft-Chemistry-Based Routes to Epitaxial a-Quartz Thin Films with Tunable Textures.

by Adrian Carretero-Genevrier, Martí Gich, Laura Picas, Jaume Gazquez, Glenna L. Drisko, Cedric Boissiere, David Grosso, Juan Rodriguez-Carvajal, Clément Sanchez

The piezoelectric properties of α-quartz (SiO2) are currently used in resonators, sensors, high accuracy timers, transformers and actuators. Despite being the second most abundant mineral found in Earth’s crust, high purity α-quartz is relatively hard to come by, thus most devices are fabricated by slicing hydrothermally synthesized α-quartz crystals. This process is incompatible with nanostructuration and the obtainable thinness is limited by the cutting utensils.

Adrian Carretero-Genevrier, et al. have developed a method for synthesizing structured thin films of epitaxial α-quartz on silicon wafers (A. Carretero-Genevrier, M. Gich, L. Picas, J. Gazquez, G. L. Drisko, C. Boissiere, D. Grosso, J. Rodriguez-Carvajal, C. Sanchez Science, 2013, 340, 827-831). An amorphous silica layer was deposited using sol-gel chemistry and evaporation-induced self-assembly. Nanostructures between 40 nm and 1 micron were maintained after crystallization by using Ba2+ and Sr2+ salts to catalyze crystallization. The mono-orientation of the crystals was generated by the nucleation and competitive growth of α-quartz from the silicon substrate, due to the favorable mismatch between these crystals.

Dense α-quartz was also grown via the infiltration of a ~3 nm mesoporous silica film with Sr(NO3)2 solution. The thickness of the quartz layer was controlled through the length of the heat treatment and the initial thickness of the sol-gel layer. These films were found to demonstrate piezoelectricity.

The flexibility and tunability of this technique may be extended to a multitude of other structures for integration into microelectromechanical systems.