Metals enhance the Raman response. However, they can also promote photochemical or photo-thermal reactions. This can severely affect the system under analysis and cause it to suffer from low reproducibility. Moreover, the extension of near-field enhancement vanishes when the distance between the analyte and metal exceeds 10 nm. This represents a further limitation to the use of plasmonic nanoparticles in the analysis of thin films and heterojunctions.

The promise of semiconductors

On the other hand, semiconductor-based Raman-active substrates can be used as an effective alternative to metals. One of the most striking advantages offered by semiconductors is the spatial extension of the near-field enhancement. High-density silicon nanowires (Si NWs) are intensively investigated not only as Raman active substrates, but also as Raman lasers, anti-reflective materials, and thermoelectric and photovoltaic components.

Long-distance excitation

There is a complex interplay between the light-trapping due to multiple scattering within and among NWs and morphology-dependent resonances. This has been predicted to excite electromagnetic surface waves over long distances. However, no experimental proof of the spatial extension of the near-field excitation have been reported so far.

Extending the near-field

Here, a research team led by Ivano Alessandri demonstrates for the first time that the near-field can be extended over a distance of 50 nm. This originates from the excitation of light-trapping Si NWs/TiO2 heterostructures. Such a long-range extension of the near-field makes these nanostructured materials promising alternatives to plasmonic metals for SERS analysis of thin films and interfacial processes. These results can have an impact not only for Raman spectroscopy, but also in energy conversion devices (e.g. artificial leaves, photovoltaics) and semiconductor-based photocatalysts.

More information can be found in the journal Nanotechnology 25 465705.

Further reading

Ring-like assembly of gold nanoparticles for enhanced SERS detection (Sept 2014)
Gold nanodisks for highly sensitive SERS substrates (May 2014)
Nanostars for hot-spot engineering (May 2014)