Tip-enhanced near-field Raman spectroscopy applied to nano-composite materials

Abstract

Vibrational spectroscopy, including Raman spectroscopy can be used for identifying molecular species, which is not possible by a scanning probe microscopy or an electron microscopy. Moreover, vibrational spectra contain structural information, such as intermolecular interactions, molecular orientations, and symmetry distortions of each species. Therefore, Raman spectroscopy is a powerful tool for studying the chemical composition of matter. By employing Tip-enhanced Raman spectroscopy (TERS), we can perform Raman spectroscopy with nano-scale spatial resolution. Our approach relies on the enhanced filed near a laser irradiated metal tip which works as the Raman excitation source. We have investigated nano-composite materials by TERS. Near-field Raman spectra revealed the nano-scale properties of molecules encapsulated in single-wall carbon nanotubes (SWNT). The enhanced field act on encapsulated molecules through the wall of SWNT to extract chemical information inside. b-carotene which has strong Raman intensities under visible light illuminations is used as an encapsulated molecule. The advantage of Raman spectroscopy is that the information of both SWNT and b-carotene can be obtained at the same time. So, it is possible to discuss the interaction between SWNT and the encapsulated molecules. Near-field Raman spectra measured at several different positions on SWNT bundle show that b-carotenes inside the tube are not uniformly distributed. We also find that the filling rates and the peak positions of the radial breathing mode of SWNT are linearly correlated.