Raman spectroscopy remains one of the most vibrant areas of molecular spectroscopy, and the technology has continued to advance with a number of specialized tools. One of the more intriguing possibilities with Raman is tip-enhanced Raman spectroscopy (TERS), which blends together the benefits of both Raman spectroscopy and high spatial-resolution microscopy. In brief, TERS involves combining AFM with a special AFM tip that results in an enhanced Raman signal, making analysis much more practical, as Raman is often bedeviled by low signal strength.
In traditional Raman spectroscopy, when incident laser light scatters from the sample, a small fraction of the scattered light will have a different wavelength, an effect known as Raman scattering, which occurs because rotational and/or vibrational modes of the sample molecules are excited by the incident light. This shift in the wavelength can be used to identify molecules since their rotational and vibrational modes are specific to a particular molecular structure. This allows composition to be determined as the Raman spectra are compared to libraries of known compounds.
Because only some of the incident light undergoes Raman scattering, the resulting signal tends to be weak. However, a certain (somewhat poorly understood) effect can be used to enhance the Raman signal by many orders of magnitude, making analysis less challenging. When sample molecules are in contact with specially prepared metal substrates or nanoengineered surfaces, the Raman signal is greatly strengthened. This surface enhanced Raman spectroscopy (SERS) can then be turned upside-down in TERS. Rather than resting the sample on a prepared substrate, the AFM tip can be prepared with a suitable surface for enhancing the Raman effect. When the tip is brought close to the surface of the sample, the same enhanced Raman signal results.
TERS instruments are generally combined Raman-AFM microscopes, although it is also possible to carry out the technique with a scanning tunneling microscope (STM), which is more suitable for conductive samples. The optical/Raman microscope can be used for locating the precise spot on the sample to be analyzed, and then the AFM is used to position the TERS probe at that spot, so that the spectroscopic analysis can be performed. TERS can have a spatial resolution of a few tens of nanometers, much better than the traditional diffraction limit for light microscopes, and the TERS tip can be scanned across the sample to build up an entire Raman image of the sample.
Applications tend to be highly research oriented, with academia being the principal customer type. TERS can be used to measure all sorts of materials, and is commonly used with new materials like graphene and carbon nanotubues. Polymers and semiconductor materials are also frequently analyzed by TERS. Although somewhat less common, life science applications are also definitely possible, such as studying cell membranes or other protein or cell structures.
Since the base of TERS is AFM and Raman microscopy, the vendors in this market are typically those that specialize in scanning probe and Raman technology. Foremost among them are Bruker, HORIBA and WITec. The component that really enables TERS analysis is the specialized AFM tip with the appropriate coating to enhance the Raman response. These tips cost a few hundred dollars each, and are supplied by both the microscopy vendors and also some firms that specialize in AFM consumables. Other significant vendors in the TERS market include Nanonics, NT-MDT, ostec and ScanSens.
TERS at a Glance