Tip enhanced Raman spectroscopy imaging of opaque samples in organic liquid

Near-field scanning optical microscopies (NSOM) associating vibrational spectroscopies (Raman, infrared: IR) and Scanning Probe Microscopies (SPMs) stand as powerful techniques to provide unambiguous and precise chemical... [ view full abstract ]

Near-field scanning optical microscopies (NSOM) associating vibrational spectroscopies (Raman, infrared: IR) and Scanning Probe Microscopies (SPMs) stand as powerful techniques to provide unambiguous and precise chemical identification with high sensibility and high spatial resolution. The specific development of Raman-SNOM, i.e. Tip Enhanced Raman Spectroscopy (TERS), has necessitated to tailor specific probes which support localized surface plasmon resonance close to the apex of the tip, at the so-called "hot spot”. When a gold or silver tip is shinned with a visible laser light of suitable wavelength, the electric field at the hot spot is considerably amplified and translates into a strong near field Raman scattering signal enhancement at the tip/sample interface. Low laser excitation power can be therefore applied, minimizing considerably the far-field contribution to the scattered signal. The strong signal enhancement compensates for the low efficiency of the inelastic scattering process of many compounds, enabling detection and imaging of single molecules. While silver provides the largest enhancement, gold is less prone to oxidation and therefore more adapted to liquids. Implementation of Tip Enhanced Raman Spectroscopy in liquid is still a challenge. We demonstrate herein its feasibility in an upright illumination/collection configuration. Through a thin layer of organic solvent covering the sample, laser focusing on the tip is possible, enabling TERS imaging in liquid.