Plasmonic Gold Helices for the visible range fabricated by oxygen plasma purification of electron beam induced deposits

TL;DR

This study demonstrates a room-temperature oxygen plasma treatment to purify gold-containing EBID nanostructures, creating plasmonically active gold shells suitable for visible-range applications.

Contribution

It introduces a novel oxygen plasma purification process that enhances the metallic quality of EBID gold nanostructures without shape distortion.

Findings

Structures retain shape after plasma treatment

Gold shell thickness of about 20 nm achieved

Optical response comparable to pure gold nanostructures

Abstract

Electron beam induced deposition (EBID) currently provides the only direct writing technique for truly three-dimensional nanostructures with geometrical features below 50 nm. Unfortunately, the depositions from metal-organic precursors suffer from a substantial carbon content. This hinders many applications, especially in plasmonics where the metallic nature of the geometric surfaces is mandatory. To overcome this problem a post-deposition treatment with oxygen plasma at room temperature was investigated for the purification of gold containing EBID structures. Upon plasma treatment, the structures experience a shrinkage in diameter of about 18 nm but entirely keep their initial shape. The proposed purification step results in a core-shell structure with the core consisting of mainly unaffected EBID material and a gold shell of about 20 nm in thickness. These purified structures are plasmonically active in the visible wavelength range as shown by dark field optical microscopy on helical nanostructures. Most notably, electromagnetic modeling of the corresponding scattering spectra verified that the thickness and quality of the resulting gold shell ensures an optical response equal to that of pure gold nanostructures.