Highly-enhancing TERS tips

What is it about?

In this manuscript, we discuss the functionalization of atomic force microscopy (AFM) probes obtained by virtue of self-assembling monolayers of block copolymer (BCP) micelles. The BCP micelles are loaded with highly enhancing, geometrically confined clusters of silver nanoparticles (AgNPs) or with bimetallic structures of AgNPs and gold nanoparticles (AuNPs) in a three-step procedure. These tips are utilized for tip-enhanced Raman scattering (TERS) and scattering-type near-field scanning optical microscopy (s-SNOM). Changing the loading composition allows tuning the plasmonic properties of the structured tip, which is of central interest for increasing the performance of TERS probes. In addition, BCP protects silver from oxidation and can be removed before measurements by UV irradiation and represents a novelty with respect to previous works. The nanoisland is a multiscale heterostructure that promotes large local field amplification. Gold is intended as a biofriendly contact layer while silver promotes the largest amplification. As a result, a high scattering efficiency is experimentally demonstrated and supported by our numerical simulations.

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Why is it important?

This provides a first important point of innovation for the following reason. In literature, usually TERS experiments have been carried out in combination with a bottom plasmonic mirror in the so-called gap-mode configuration, which limits TERS applicability to thin molecular layers. Our experimental results demonstrate a spatially average surface enhanced Raman scattering (SERS) enhancement factor of 10^6 near the tip even without a bottom plasmonic mirror, which enables non-gap TERS on thick samples like spores, as here experimentally shown on Bacillus subtilis spores. Furthermore, we apply the tip in a gap-TERS experiment combining the coated tip with a planar, transparent SERS substrate made of the same kind of coating used for the tip. In a previous work, we demonstrated the high spatial reproducibility of this kind of SERS substrate [Nanoscale, 2015, 7, 8593-8606]. Here, we show, for the first time, an experiment that proves single-molecule sensitivity with a bi-analyte, nanoscale spatially resolved TERS characterization. The outcomes point out a spatial resolution of approximately 12 nm and a gap-TERS enhancement factor of 10^9. With respect to previous works on single-molecule TERS, the innovation of our approach relies on the fact that our nanoscale spatially resolved version of bi-analyte method unambiguously allows one to assess TERS detection in single-molecule regime, since accounting for experimental issues related to the statistics of the enhancement factor at the gap hot spots. The tips are also applied for scattering type near field scanning optical microscopy (s-SNOM) on several lithographic structures, confirming the results of spatial resolution below 12 nm. The possibility to use the probes also for s-SNOM is a result of the large scattering efficiency even in a non-gap-mode configuration.

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