Raman Spectroscopy
Tip Enhanced Raman Scattering (TERS) or nano-Raman
Raman microscopy has greatly improved in terms of mapping speed thanks to higher performance detectors and new fast hyper-spectral imaging technologies like SWIFT, allowing the quick acquisition of high-spatial-resolution images.
Raman confocal imaging, however, is still limited by Abbe’s light diffraction limit principle just like all conventional confocal techniques.
Super-resolution techniques (like PALM or STORM) manage to break this spatial resolution limit in confocal fluorescence imaging. However these methods rely heavily on labeling the chemical compounds of interest. The beauty of Raman spectroscopy is that it provides label-free chemically specific information, and without labels those super-resolution methods cannot be applied.
The development of Scanning Probe Microscopy (SPM) and especially Atomic Force Microscopy (AFM) has made visualization of the nano-world easy and affordable compared to techniques like Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). It has also brought new ways to probe sub-diffraction limited light, with the development of near-field optical microscopy techniques (SNOM, NSOM).
Near-field transmission and reflection microscopy are easily performed with tapered optical fibers or hollow cantilevers, while near-field fluorescence is more challenging because of light conversion efficiency. SNOM-Raman (or NSOM-Raman) however, has never really been successful except for strongly resonant molecules, because spontaneous Raman is a very weak scattering. As a rule of thumb, only one-in-a-million excitation photon is scattered back.
Therefore near-field spontaneous Raman of non-resonant molecules is close to impossible even with the best possible optics and detection hardware.
Surface-Enhanced Raman Scattering has flourished in the past 30 years as it brought the promise of single molecule detection with Raman, through the use of surface plasmon resonance effects. Tip-Enhanced Raman Scattering essentially takes advantage of the same effect, but the SPM probe tip is the enhancing surface promoting Raman scattering only from a nanoscale area.
Successful TERS measurement usually involves properly crafted probe-tips, which must be coated with a rough noble-metal layer in order to allow coupling of light into surface plasmons. Proper excitation wavelength selection to match possible sample resonance and probe-tip peak plasmon coupling efficiency also helps generate extremely high enhancement of the local Raman signal so that when scanning the probe, spatial resolution in the order of 10nm can be achieved and best resolution of 2nm has been reported using HORIBA equipment.
Over the years, HORIBA Scientific has developed an optimized optical, mechanical and software SPM coupling platform to provide the best possible light collection efficiency with the ability to easily align the Raman laser on the SPM tip for efficient and stable TERS imaging. HORIBA Scientific leadership and innovation in Raman spectroscopy lets you approach this novel technique with confidence, focusing on sample measurement, trusting the equipment is optimized for this purpose.
TERS is being explored for many different application areas, and opens up new possibilities in nanoscale chemical analysis and imaging.
To learn more about this technology and how you can integrate all your needs into one powerful instrument, please fill out the contact form or call or email your local HORIBA Scientific representative.
