Silver nanowires for highly reproducible cantilever based AFM-TERS microscopy: towards a universal TERS probe

Abstract

Tip-enhanced Raman scattering (TERS) microscopy is a unique analytical tool to provide complementary chemical and topographic information of surfaces with nanometric resolution. However, difficulties in reliably producing the necessary metallized scanning probe tips has limited its widespread utilisation, particularly in the case of cantilever-based atomic force microscopy. Attempts to alleviate tip related issues using colloidal or bottom-up engineered tips have so far not reported consistent probes for both Raman and topographic imaging. Here we demonstrate the reproducible fabrication of cantilever-based high-performance TERS probes for both topographic and Raman measurements, based on an approach that utilises noble metal nanowires as the active TERS probe. The tips show 10 times higher TERS contrasts than the most typically used electrochemically-etched tips, and show a reproducibility for TERS greater than 90%, far greater than found with standard methods. We show that TERS can be performed in tapping as well as contact AFM mode, with optical resolutions around or below 15 nm, and with a maximum resolution achieved in tapping-mode of 6 nm. Our work illustrates that superior TERS probes can be produced in a fast and cost-effective manner using simple wet-chemistry methods, leading to reliable and reproducible high-resolution and high-sensitivity TERS, and thus renders the technique applicable for a broad community.

Controlled Fabrication of Optical Signal Input/Output Sites on Plasmonic Nanowires

“…To date, various metal nanostructures have been investigated for plasmonic waveguiding, including metallic stripes, − nanoparticle arrays, − grooves/slots/teeth, − and tapered plasmonic waveguides. , Chemically synthesized silver nanowires (AgNWs) are especially promising with their well-defined crystallinity (thus smooth surfaces) and low absorption at both visible and near-infrared spectral ranges, affording them excellent plasmonic properties. − Their potential has been demonstrated in applications including plasmonic logic gates, ,, sensors, and lasers. , AgNWs are also excellent tools for nanoscopy, especially as they can allow for the separation in space of the points of optical excitation and probing. Remote excitation of single molecule fluorescence, surface-enhanced Raman scattering (RE-SERS), , and tip-enhanced Raman scattering (TERS) − have all been developed in this context. RE-SERS has even been applied inside single live cells, such that AgNWs become plasmonic endoscopes …”

Advanced Characterization Methods for Electrical and Sensoric Components and Devices at the Micro and Nano Scales

“…As a final consequence, the resolution of all described methods scales with the tip size: the sharper the tip, the better the local resolution and the signal amplification due to increased local fields, which is reflected by the vivid development ongoing in the field of tips and cantilevers …”

Tip Recycling for Atomic Force Microscopy-Based Tip-Enhanced Raman Spectroscopy

“…The heart of every AFM–TERS experiment is an enhancing plasmonic probe; commercially available AFM cantilevers, usually made of Si, need to be metalized to become suitable TERS probes. Many tip fabrication methods have been investigated so far, including electrodeposition, 14 electroless deposition, 15 or the attachment of Ag nanowires to the cantilever, 16 but the most common functionalization method is the deposition of a thin layer of plasmonic metal (generally Ag or Au) using physical vapor deposition (PVD). Despite being widespread and effective, the PVD coating process requires several hours and an expensive vacuum setup.…”