Our recently developed approach of UHV-tip-enhanced Raman spectroscopy permits us to acquire Raman spectra of a few single brilliant cresyl blue (BCB) molecules and even a single one adsorbed on a Au(111) surface. This is substantiated by simultaneously recorded STM images. Furthermore, because of the reduced photobleaching in UHV, the time frame for spectral acquisition is sufficiently extended to allow tip-enhanced Raman imaging of a single BCB molecule with a lateral resolution of 15 nm.

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Tip-enhanced Raman spectroscopy and microscopy on single dye molecules with 15 nm resolution.

■ Abstract We review recent developments in single-molecule spectroscopy and microscopy. New optical methods provide access to the absorption, emission, or excitation spectra of single nano-objects and can determine either the positions of these objects with subwavelength accuracy or the full three-dimensional orientation of their transition dipole moments. Recent work aims at using single molecules as nanoparts or nanoelements in a variety of molecular-scale devices, from triggered sources of single photons to single-molecular switches. A prominent new direction explores the various interactions between molecules within individual multichromophoric systems obtained by chemical synthesis. These systems are the models for natural self-assembled systems such as the light-harvesting proteins of bacteria and green plants, which are currently studied on a single-molecule basis. Another important class of multichromophoric systems are conjugated polymers. The combination of microscopy with time- and frequency-resolved spectroscopy is opening a wide field of new and exciting applications to individual nano-objects.

Single-molecule optics.

Tip-enhanced Raman spectroscopy (TERS), a combination of Raman spectroscopy and apertureless near-field scanning optical microscopy using a metallic tip which resonates with the local mode of the surface plasmon, can provide a high-sensitive and high-spatial-resolution optical analytical approach. The basic principle of TERS, common experimental setups, various SPM technologies, and excitation/collection configurations are introduced as well as recent research progress with respect to TERS.

http://pubs.acs.org/doi/pdf/10.1021/acs.analchem.6b02093

Tip-Enhanced Raman Spectroscopy

We present a new top-illumination scheme for tip-enhanced Raman spectroscopy (TERS) in a gap-mode configuration with illumination and detection in a straightforward fashion perpendicular to the sample surface. This illumination focuses the light tightly around the tip end, which effectively diminishes far-field background contributions during TERS measurements. The setup maintains the entire functionality range of both the scanning probe microscopy and the confocal optical microscopy of the setup. For the first time, we show large (64 × 64 up to 200 × 200 pixels), high-resolution TERS imaging with full spectral information at every pixel, which is necessary for the chemical identification of sample constituents. With a scanning tunneling microscope tip and feedback, these TERS maps can be recorded with a resolution better than 15 nm (most likely even less, as discussed with Figure 6). An excellent enhancement (∼107×, sufficient for detection of few molecules) allows short acquisition times (≪1 s/pixel) an...

/pdf/nanoscale-chemical-imaging-using-top-illumination-tip-2tt4acw2z1.pdf

Nanoscale chemical imaging using top-illumination tip-enhanced Raman spectroscopy.

An analytical technique operating at the nanoscale must be flexible regarding variable experimental conditions while ideally also being highly specific, extremely sensitive, and spatially confined. In this respect, tip-enhanced Raman scattering (TERS) has been demonstrated to be ideally suited to, e.g., elucidating chemical reaction mechanisms, determining the distribution of components and identifying and localizing specific molecular structures at the nanometre scale. TERS combines the specificity of Raman spectroscopy with the high spatial resolution of scanning probe microscopies by utilizing plasmonic nanostructures to confine the incident electromagnetic field and increase it by many orders of magnitude. Consequently, molecular structure information in the optical near field that is inaccessible to other optical microscopy methods can be obtained. In this general review, the development of this still-young technique, from early experiments to recent achievements concerning inorganic, organic, and biological materials, is addressed. Accordingly, the technical developments necessary for stable and reliable AFM- and STM-based TERS experiments, together with the specific properties of the instruments under different conditions, are reviewed. The review also highlights selected experiments illustrating the capabilities of this emerging technique, the number of users of which has steadily increased since its inception in 2000. Finally, an assessment of the frontiers and new concepts of TERS, which aim towards rendering it a general and widely applicable technique that combines the highest possible lateral resolution and extreme sensitivity, is provided.

Tip-enhanced Raman spectroscopy – from early developments to recent advances

A novel high-resolution stage scanning confocal microscope for fluorescence microscopy and spatially resolved spectroscopy with a high numerical aperture (NA≈1) parabolic mirror objective is investigated. A spatial resolution close to the diffraction limit is achieved. As microscopic fluorescent test objects, dye-loaded zeolite microcrystals (diameter approx. 0.4 µm) and single fluorescent molecules were used. Confocal fluorescence images show a spatial resolution of Δx=0.8·λ both at room temperature and at 1.8 K. Imaging of a quasi-point light source and focusing by the parabolic mirror were investigated experimentally and theoretically. Deviations between the theoretical results for a perfect parabolic mirror and the experimental results can be attributed to small deviations of the mirror profile from an ideal parabola.

Confocal microscopy with a high numerical aperture parabolic mirror

Parabolic mirrors with a high numerical aperture can be conveniently used to produce highly confined optical fields in the focal region. Furthermore, these fields can have interesting polarization behaviour due to the high numerical aperture. In particular, if the mirror is illuminated with a size matched radially polarized or azimuthally polarized doughnut mode, the electric field has in the focal region almost exclusively a longitudinal or a transverse polarization component. Such field distributions are interesting for applications in confocal or near-field optical microscopy. Here we present experimental results where we have probed some of these field distributions by raster scanning a fine gold tip in nanometer steps through the focal region and detecting the scattered light intensity. The measured intensity patterns are compared with corresponding vector-field calculations.

A. Drechsler

Papers

Confocal microscopy with a high numerical aperture parabolic mirror

Probing highly confined optical fields in the focal region of a high NA parabolic mirror with subwavelength spatial resolution.