Subtractive Low-Temperature Preparation Route for Porous SiO2 Used for the Catalyst-Assisted Growth of ZnO Field Emitters.

Stefanie Haugg,Robert Zierold,Carina Hedrich,Robert H Blick

doi:10.3390/nano11123357

Stefanie Haugg, Robert Zierold + Show 2 more

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https://doi.org/10.3390/nano11123357

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Abstract

The possibility to gradually increase the porosity of thin films facilitates a variety of applications, such as anti-reflective coatings, diffusion membranes, and the herein investigated tailored nanostructuring of a substrate for subsequent self-assembly processes. A low-temperature (<160 °C) preparation route for porous silicon oxide (porSiO2) thin films with porosities of about 60% and effective refractive indices down to 1.20 is tailored for bulk as well as free-standing membranes. Subsequently, both substrate types are successfully employed for the catalyst-assisted growth of nanowire-like zinc oxide (ZnO) field emitters by metal organic chemical vapor deposition. ZnO nanowires can be grown with a large aspect ratio and exhibit a good thermal and chemical stability, which makes them excellent candidates for field emitter arrays. We present a method that allows for the direct synthesis of nanowire-like ZnO field emitters on free-standing membranes using a porSiO2 template. Besides the application of porSiO2 for the catalyst-assisted growth of nanostructures and their use as field emission devices, the herein presented general synthesis route for the preparation of low refractive index films on other than bulk substrates—such as on free-standing, ultra-thin membranes—may pave the way for the employment of porSiO2 in micro-electro-mechanical systems.

Highlights

Porous silicon oxide is often investigated as an anti-reflective coating (ARC)since the low-cost material can be generated by a multitude of techniques and its refractive index (RI) can be tailored over a wide range below 1.46, which is the typical RI of denseSiO2 [1,2,3,4,5]
For the synthesis of porSiO2 on free-standing membranes, the sample was placed on a tailor-made PTFE holder without clamping to avoid any mechanical stress applied to the fragile substrate, and the sample was heated within the H3 PO4 (30 mL)
Both approaches are based on the effective medium theory that assumes the porous material as an optically isotropic medium that has an effective RI and that is constituted of two components, namely the carcass of a specific material and of the pores [2,37]

Summary

Introduction

Porous silicon oxide (porSiO2 ) is often investigated as an anti-reflective coating (ARC)since the low-cost material can be generated by a multitude of techniques and its refractive index (RI) can be tailored over a wide range below 1.46, which is the typical RI of denseSiO2 [1,2,3,4,5]. Porous silicon oxide (porSiO2 ) is often investigated as an anti-reflective coating (ARC). Porous films are reported as thermal insulation layers in uncooled infrared detectors and as biocompatible substrates because their roughness can enhance the growth and proliferation of cells [8,9,10]. A frequently used synthesis method for porSiO2 is the simple sol-gel technique. It requires a calcination step at temperatures well above 150 ◦ C for the complete removal of the solvents, which provokes cracking of the remaining porous layer and prohibits the coating of temperature-sensitive substrates. Plasma-enhanced chemical vapor deposition (PECVD) was Nanomaterials 2021, 11, 3357.

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