Apr 6, 2011
Commercial upright microscope integrated with AFM for TERS study
Tip-enhanced Raman mapping is a very powerful and promising tool for performing material analysis and provides a wealth of chemical information with nanometre spatial resolution. One of the main requirements for tip-enhanced Raman scattering (TERS) experiments is to bring a sharp metal tip (usually coated with gold or silver) to a focused laser "hotspot". This is often achieved by adapting an inverted microscope. Here a laser is focused on the sample from below using a high numerical aperture objective while the metal tip approaches from above. Although this type of TERS arrangement is more efficient in collecting the Raman signal, its application is limited to the study of transparent or very thin samples. For opaque or thick samples, the tip and the laser will have to be introduced from the same side, which often requires an upright microscope with a specific in-house design or modifications to the optics used.
Researchers from Imperial College London have demonstrated the possibility of obtaining TERS images with nanometre spatial resolution by using a commercially available upright microscope (top-illumination) and a gold-coated silicon AFM cantilever working in contact mode.
They obtained images of single-walled carbon nanotubes (SWCNTs) with nanometre spatial resolution by using this approach. One of main obstacles for this type of TERS study is the shadowing and the background noise generated from the cantilever. The team overcame these difficulties by using an optical mask to create side-illumination and employing a "tip at the end of the cantilever" type AFM probe to minimize the shadowing effect.
The spatial resolution achieved with this system in an upright configuration was ~20–50 nm, which demonstrates the potential of this technique for studying non-transparent materials at the nanoscale without imposing limitations on samples.
The researchers presented their work in the journal Nanotechnology.
About the author
Sergei Kazarian is professor of physical chemistry at the Department of Chemical Engineering, Imperial College London, UK. His current research focuses on the application of advanced spectroscopic imaging to materials, biomedical samples and pharmaceutical formulations. Dr Andrew Chan is a research associate working in Kazarian's group.