Micro/Nanostructure Engineering of Epitaxial Piezoelectric α-Quartz Thin Films on Silicon.

The monolithic integration of sub-micron quartz structures on silicon substrates is a key issue for the future development of piezoelectric devices as prospective sensors with applications based on the operation in the high-frequency range. However, to date, it has not been possible to make existing quartz manufacturing methods compatible with integration on silicon and structuration by top-down lithographic techniques. Here, we report an unprecedented large-scale fabrication of ordered arrays of piezoelectric epitaxial quartz nanostructures on silicon substrates by the combination of soft-chemistry and three lithographic techniques: (i) laser interference lithography, (ii) soft nanoimprint lithography on Sr-doped SiO2 sol-gel thin films, and (iii) self-assembled SrCO3 nanoparticle reactive nanomasks. Epitaxial α-quartz nanopillars with different diameters (from 1 µm down to 50 nm) and heights (up to 2 µm) were obtained. This work demonstrates the complementarity of soft-chemistry and top-down lithographic techniques for the patterning of epitaxial quartz thin films on silicon while preserving its epitaxial crystallinity and piezoelectric properties. These results open up the opportunity to develop a cost-effective on-chip integration of nanostructured piezoelectric α-quartz MEMS with enhanced sensing properties of relevance in different fields of application.

Tailoring the crystal growth of quartz on silicon for patterning epitaxial piezoelectric films.

Epitaxial films of piezoelectric α-quartz could enable the fabrication of sensors with unprecedented sensitivity for prospective applications in electronics, biology and medicine. However, the prerequisites are harnessing the crystallization of epitaxial α-quartz and tailoring suitable film microstructures for nanostructuration. Here, we bring new insights into the crystallization of epitaxial α-quartz films on silicon (100) from the devitrification of porous silica and the control of the film microstructures: we show that by increasing the quantity of devitrifying agent (Sr) it is possible to switch from an α-quartz microstructure consisting of a porous flat film to one dominated by larger, fully dense α-quartz crystals. We also found that the film thickness, relative humidity and the nature of the surfactant play an important role in the control of the microstructure and homogeneity of the films. Via a multi-layer deposition method, we have extended the maximum thickness of the α-quartz films from a few hundreds of nm to the µm range. Moreover, we found a convenient method to combine this multilayer approach with soft lithography to pattern silica films while preserving epitaxial crystallization. This improved control over crystallization and the possibility of preparing patterned films of epitaxial α-quartz on Si substrates pave the path to future developments in applications based on electromechanics, optics and optomechanics.

Fabrication of α-Fe2O3/In2O3 composite hollow microspheres: A novel hybrid photocatalyst for toluene degradation under visible light.

The α-Fe2O3/In2O3 composite hollow microspheres were first synthesized through a well-designed two-step hydrothermal approach with an aim to promote the photocatalytic activity of the pure In2O3. The morphologies, phase structures, and optical properties of the resultant samples were systematically characterized by field-emission scanning electron microscopy, transmission electron microscopy, X-ray powder diffraction, X-ray photoelectron spectroscopy, UV-vis diffuse-reflectance, and photoluminescence spectroscopy. The α-Fe2O3 nanoparticles acted as visible-light sensitizer, which were well-decorated on the surface of the In2O3 hollow microspheres. Meanwhile, the investigation of photocatalytic performance confirmed that the visible-light induced photocatalytic degradation rate of gaseous toluene was improved after the introduction of α-Fe2O3 component, which was about 1.6 times higher than that of pure In2O3 sample under identical conditions. Furthermore, some intermediates formed during the photocatalytic oxidation process were also indentified by in situ FTIR spectroscopy. The enhanced photocatalytic performance of the α-Fe2O3/In2O3 composites mainly stemmed from the strong visible-light-harvesting ability and the efficient spatial separation of photo-generated electron-hole pairs.