The best of both worlds

Abstract

This year marks 80 years since Chandrasekhara Venkata Raman was awarded the Nobel Prize for his investigations on the molecular scattering of light [1], work inspired during a trip to Europe by his first glimpse of the 'wonderful blue opalescence of the Mediterranean Sea' [2]. These studies led to the discovery of Raman scattering, now widely exploited for the unique spectral Raman 'fingerprint' associated with substances that facilitate their identification. However, one of the drawbacks of Raman spectroscopy has always been the low Raman scattering cross section, typically more than a 1000 times weaker than the Rayleigh scattering cross section, resulting in an extremely weak signal. Progress in nanotechnology revealed ways of enhancing Raman signals using metal nanoparticles, resulting in optical detection and spectroscopy at the level of a single molecule [3]. Surface plasmon resonances in metal nanoparticles have demonstrated great potential in a range of applications, including data storage, light generation, nonlinear optics, microscopy and biophotonics, and this has motivated many investigations aimed at optimising plasmonic properties. Researchers in Japan and China demonstrated how the self-assembly of gold nanoparticles can be used to tune plasmonic responses [4], and more recently researchers in America have demonstrated how nanocrescent structures can be tuned to respond in the infrared part of the electromagnetic spectrum, lending these nanostructures to applications in cellular imaging in vivo [5]. At the time that Nanotechnology was launched 20 years ago, nanoscale research had been galvanized by developments in scanning probe techniques that pushed microscopic resolutions to unprecedented scales, enabling people to `see' atoms for the first time. The intrinsic awe of such images and the potency of these investigative tools naturally drove further research into refining techniques in scanning, tunnelling and atomic force microscopy [6, 7]. However, the data from these scanning probe techniques are traditionally limited in their ability to retrieve spectral details, thus inhibiting optical characterization.Scanning optical microscopy looked set to commandeer the best of both worlds, when a team of researchers at Bell Laboratories in the USA retrieved optical information with nanometre resolution [8]. Since then other methods have developed to overcome the spectral bottle neck in progressing scanning probe techniques. Recently in Nanotechnology, a team of scientists in the UK reported the fabrication of a coaxial tip for scanning probe energy loss spectroscopy [9]. The outer sheath is grounded to shield the field between the tip and substrate, thus reducing distortions to the trajectory of the electrons. In this issue, researchers in Illinois, USA, report improvements to a method incorporating an atomic force microscopy tip in infrared spectroscopy that offers benefits in terms of sensitivity and speed [10]. They obtain infrared spectra containing details of the molecular structure of materials with nanoscale resolution.There are many instances when circumstances enforce a choice between two equally desirable resources. The latest developments in scanning probe spectroscopy are an encouragement to abandon the compromise of spectral detail for nanoscale resolution, inspiring further endeavours toward technological progress.