Surface-Sensitive Raman Scattering by Transferable Nanoporous Plasmonic Membranes

TL;DR

This paper introduces a transferable nanoporous gold membrane that enhances surface-specific Raman signals by acting as a plasmonic antenna and blocking bulk signals, enabling detailed surface analysis of materials.

Contribution

The study presents a novel nanoporous gold membrane that significantly improves surface sensitivity in Raman spectroscopy, allowing direct surface characterization.

Findings

Surface-to-bulk Raman ratio increased by three orders of magnitude.

90% of Raman enhancement occurs within the top 2.5 nm of the material.

Detected surface-specific Raman mode splitting in LaNiO3 thin film.

Abstract

Raman spectroscopy is a powerful technique to characterize materials. It probes non-destructively chemical composition, crystallinity, defects, strain and coupling phenomena. However, the Raman response of surfaces or thin films is often weak and obscured by dominant bulk signals. Here we overcome this limitation by placing a transferable porous gold membrane (PAuM) on top of the surface of interest. Slot-like nanopores in the membrane act as plasmonic slot antennas and enhance the Raman response of the surface or thin film underneath. Simultaneously, the PAuM suppresses the penetration of the excitation laser into the bulk, efficiently blocking the bulk Raman signal. Using graphene as a model surface, we show that these two simultaneous effects lead to an increase in the surface-to-bulk Raman signal ratio by three orders of magnitude. We find that 90% of the Raman enhancement occurs within the top 2.5 of the material, demonstrating truly surface-sensitive Raman scattering. To validate our approach, we analyze the surface of a LaNiO3 thin film. We observe a Raman mode splitting for the LaNiO3 surface-layer, which is spectroscopic evidence that the surface structure differs from the bulk. This result underpins that PAuM give direct access to Raman signatures of surfaces and their structural properties.