Freestanding ultra-thin silica

Rui Zhao,Hongyeun Kim,Joshua Stapleton,Zi-Kui Liu,Joshua Robinson

doi:10.1063/1.5136232

Abstract

Silica (SiOx) thin films are promising for a wide range of applications, including catalysis, separation technology, biomedicine, or transparent super-hydrophilic films. Here, we present a study demonstrating a unique way of producing ultra-thin, freestanding silica films via silicon etching. This method utilizes silicon wafers with thermally oxidized surfaces and two common inorganic elements (sulfur and tellurium), which leads to high-rate chemical etching of the Si substrate, leaving behind freestanding silica layers. Thermodynamic calculations of the tellurium–silicon–sulfur (Te–Si–S) ternary phase diagram suggest that the removal of the Si substrate from the silica layers is due to chemical reactions that result in liquid/vapor formation of Si–S and Si–Te phases. Importantly, the chemical and physical properties of the silica film post-etch are comparable to those of the starting material. The process described here provides a route to produce large area, flexible glass substrates with widely tunable thicknesses from tens to thousands of nanometers.

Highlights

Silica films1,2 are useful in many industrial applications
This process is based on solid–vapor reactions that lead to chemical vapor etching (CVE) of the silicon substrate, removing the substrate from the SiO2 layer, creating an ultra-thin, freestanding, silica film— akin to creating freestanding graphene grown on copper, where the copper film is etched chemically via solution-based processes
We find that the silicon substrate is dramatically influenced by its surrounding chalcogenide environment [Figs. 1(b)–1(d)]

Summary

Introduction

Silica films1,2 are useful in many industrial applications. For example, it is the most common inorganic filler in fuel cells, which could assist proton exchange, membrane fuel and enhance the performance of fuel cells by improving the membrane properties.3 For instance, the incorporation of silica into the membrane matrices improves mechanical strength,4,5 thermal stability,6,7 and proton conductivity of the membrane.8,9 Silica can be added into different types of membranes or coatings, enhancing their proton conductivity10 and mechanical properties.11,12 In addition, mesoporous silica films enable uniform quantum dot arrays13 and provide growth templates for depositing metallic nanowires.14 Silica films have been used as transition metal templates for photocatalysis of reducing CO2 and H2O.15 Due to its large opening pore size, mesoporous silica thin films have been functionalized to play a critical role in electrochemical sensors.16 freestanding and oriented mesoporous silica films are of interest in various fields, such as metal clusters,17 porous nanoparticles,18 composites,19 patterned self-assembled monolayers,20 liquid crystals,21 and semiconductor devices.22 Among all the aforementioned research and studies, there are only a few methods of producing silica films, including the reaction of the cationic surfactant and the silica source reagent under acidic conditions on supported substrates,23–26 or via sol–gel processes with pre-synthesized molecular precursors.27,28 In the semiconductor industry, the ability to grow a silicon oxide layer from silicon with precise thickness control via thermal oxidation is a key for a variety of technologies, and it has been the focus of many studies since 1950s.29 Here, we elucidate how the amorphous silicon oxide layer can be removed from the silicon substrate with the assistance of two inorganic elements, sulfur (S) and tellurium (Te).

Results

Conclusion