From Chip Size to Wafer-Scale Nanoporous Gold Reliable Fabrication Using Low Currents Electrochemical Etching.

Pericle Varasteanu,Mihaela Kusko,Oana Tutunaru,Cosmin Romanitan,Gabriel Craciun,Antonio Radoi,Iuliana Mihalache,Alexandru Bujor

doi:10.3390/nano10112321

Abstract

We report a simple, scalable route to wafer-size processing for fabrication of tunable nanoporous gold (NPG) by the anodization process at low constant current in a solution of hydrofluoric acid and dimethylformamide. Microstructural, optical, and electrochemical investigations were employed for a systematic analysis of the sample porosity evolution while increasing the anodization duration, namely the small angle X-ray scattering (SAXS) technique and electrochemical impedance spectroscopy (EIS). Whereas the SAXS analysis practically completes the scanning electronic microscopy (SEM) investigations and provides data about the impact of the etching time on the nanoporous gold layers in terms of fractal dimension and average pore surface area, the EIS analysis was used to estimate the electroactive area, the associated roughness factor, as well as the heterogeneous electron transfer rate constant. The bridge between the analyses is made by the scanning electrochemical microscopy (SECM) survey, which practically correlates the surface morphology with the electrochemical activity. The results were correlated to endorse the control over the gold film nanostructuration process deposited directly on the substrate that can be further subjected to different technological processes, retaining its properties. The results show that the anodization duration influences the surface area, which subsequently modifies the properties of NPG, thus enabling tuning the samples for specific applications, either optical or chemical.

Highlights

IntroductionNanoporous gold (NPG) has been known since the ancient times; the South American population and eastern part of Europe throughout the centuries used to etch (dealloy) and polish the surface of gold–copper alloys to create the illusion of bulk gold of shininess, a process known as depletion gilding or gold colouration (mise-en-couleur) [1]
Nanoporous gold (NPG) has been known since the ancient times; the South American population and eastern part of Europe throughout the centuries used to etch and polish the surface of gold–copper alloys to create the illusion of bulk gold of shininess, a process known as depletion gilding or gold colouration [1]
As for plasmonic sensing, NPG provides simple excitation schemes of strong plasmonic resonances localized on the nanoporous structure ligaments, with the resulting plasmon-induced electric field being the primary mechanism in surface-enhanced spectroscopy, including surface-enhanced Raman scattering (SERS), surface-enhanced infrared absorption (SEIRA), and surface-enhanced fluorescence (SEF) [10]

Summary

Introduction

Nanoporous gold (NPG) has been known since the ancient times; the South American population and eastern part of Europe throughout the centuries used to etch (dealloy) and polish the surface of gold–copper alloys to create the illusion of bulk gold of shininess, a process known as depletion gilding or gold colouration (mise-en-couleur) [1]. NPG preserves the properties of gold, such as high conductivity, good chemical stability, as well as biocompatibility, and presents significant advantages, mainly determined by the large surface to volume ratio. It becomes a good candidate for improving generation chemical/biochemical sensors [3,4,5]. Taking into account that the upper limit of gold nanoparticles that could sustain catalytic activity is 5 nm [16], the ligament size larger than 10 nm seems to be too large for catalytic performances, but it is successfully compensated by the presence of steps, kinks, surface defects, twin boundaries, and dislocations in the NPG architecture [17,18,19]

Methods

Results

Conclusion