Preparation of Macroporous Epitaxial Quartz Films on Silicon by Chemical Solution Deposition

Summary

A protocol is presented for the preparation of piezoelectric macroporous epitaxial films of quartz on silicon by solution chemistry using dip-coating and thermal treatments in air.

Abstract

This work describes the detailed protocol for preparing piezoelectric macroporous epitaxial quartz films on silicon(100) substrates. This is a three-step process based on the preparation of a sol in a one-pot synthesis which is followed by the deposition of a gel film on Si(100) substrates by evaporation induced self-assembly using the dip-coating technique and ends with a thermal treatment of the material to induce the gel crystallization and the growth of the quartz film. The formation of a silica gel is based on the reaction of a tetraethyl orthosilicate and water, catalyzed by HCl, in ethanol. However, the solution contains two additional components that are essential for preparing mesoporous epitaxial quartz films from these silica gels dip-coated on Si. Alkaline earth ions, like Sr²⁺ act as glass melting agents that facilitate the crystallization of silica and in combination with cetyl trimethylammonium bromide (CTAB) amphiphilic template form a phase separation responsible of the macroporosity of the films. The good matching between the quartz and silicon cell parameters is also essential in the stabilization of quartz over other SiO₂ polymorphs and is at the origin of the epitaxial growth.

Introduction

When a piezoelectric material like α-quartz is submitted to a voltage bias it undergoes a mechanical deformation. If this material is porous, these volume changes can lead to pore expansion or contraction, creating a responsive system similar to what may be observed in living biological organelles.¹ Deformable porous α-quartz has been produced using microfabrication,² but such techniques cannot yet produce 3-D pore structures, and pore diameters are on the order of hundreds of nanometers. Crystallization of structured amorphous silica has been hindered by inhomogeneous nucleation caused by high surface energies and architectural deforma....

Protocol

1. Preparation of the Sol

1. Prepare a solution of prehydrolyzed tetraethyl orthosilicate (TEOS) the day before the preparation of the gel films in a fume hood in which a lab balance and a magnetic stirrer are placed. In this step and throughout the protocol wear a lab coat, gloves and safety goggles.
   1. In a 50 ml beaker containing a Teflon coated magnetic stirring bar weigh 1.68 g of CTAB, add 48.13 ml of ethanol and 3.00 ml of HCl 35%, cover the beaker with a watch glass and stir until the CTAB is completely dissolved.
   2. Add 7.37 ml of TEOS to the beaker dropwise (at a rate of about 1-2 drops per second) while stirring, cover the flask wi....

Results

The progress of the material synthesis was controlled by monitoring different aspects. After the dip-coating process one can observe the aspect of the films, the eventual appearance of diffraction structures in the reflected spot of a green laser and the Scanning Electron Microscopy (SEM) images in backscattered electrons mode (Figure 1A-B). After the crystallization process it is important to record Atomic Force Microscopy (AFM) topographic images (Figure 1C

Discussion

The presented method is a bottom-up approach to produce macroporous quartz films on Si. Compared to the standard method of production of quartz films, a top down technology based on cutting and polishing of large hydrothermally grown crystals, the method described in the protocol allows obtaining much thinner films with thicknesses between 150 and 450 nm which can be controlled with the withdrawal rate. All experimental details regarding the control of quartz films thickness, and piezoelectric response are reported in re.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Dip coater	Nadetech	ND-DC 11/150
Furnace	Nabertherm	R 50/250/12
Atomic Force Microscope	Agilent	5500 LS
Silicon wafers	SHE Europe Ltd.
SrCl2·6H2O	Aldrich	13909
CTAB	Aldrich	H5582
Ethanol Absolute	Aldrich	161086
HCl 35% solution	PanReac	721019
TEOS	Aldrich	131903