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Tip-enhanced Raman spectroscopy and microscopy

Denys Naumenko, Valentinas Snitka, Boris Snopok, Andrius Stogrin

Various modifications of scanning probe microscopies (SPM) are powerful tools for surface visualization at the nanometre scale, but generally lack the ability to perform chemical characterization. Classical Raman spectroscopy is efficiently used for functional mapping of matter (chemical composition, etc) on the micrometer scale. However, the Abbe diffraction limit of far-field optical techniques does not allow extending this method for visualization and local chemical non-destructive analysis of nanometre-sized structures. A practical implementation of near-field optical spectroscopy with ultrahigh spatial resolution has become possible by combining optical spectroscopy and scanning probe microscopy.

The crucial role in near-field Raman spectroscopy is played by the SPM tip as a nanoantenna for illuminating light. In this local excitation-based technique, a tip localizes and enhances the scattered optical radiation due to the excitation of localized Surface Plasmon Resonance in the metal of the tip. In this case, the spatial resolution is determined by the region at the apex of the tip which is in few nanometers in diameter. The enhancement of the Raman scattering from an object near the tip can be as high as 105-108.

TERS principle
SPM tip enhances a scattered optical radiation due to the excitation of localized Surface Plasmon Resonance in the metal tip. The spatial resolution of TERS depends on the tip diameter and the range of tens of nanometers can be achieved (compared to hundreds of nanometers in standard Raman microscopy).
The combination of lateral resolution of SPM with the power of Raman spectroscopy initially proposed in 1985 [1] and was essentially developed by ETH Zurich group in 2000 [2], however, the experimental Raman enhancements have been lower than theoretically predicted. This has hampered the development of TERS to become a robust and commercially useful technique. Some recent reviews in this technology can be found in [3, 4].

In our group, we are developing tip-enhanced Raman scattering (TERS) for characterization of both inorganic and (bio) molecular structures on the nanoscale, using the NTEGRA Spectra platform in both upright and inverted configurations (with 100x high NA objectives) and home-made metal tips fabricated from bulk gold.

TERS scheme
The scheme of the experimental setup.

One of the major difficulties of TERS is the low reproducibility of the tips coated by or fabricated from “plasmon-active” materials (Au or Ag). The enhancement factor and the spatial resolution are both determined by the choice of material and the size and shape of the tip. TERS has the potential to achieve atomic resolution for the tips of 1 nm radius. Obtaining tips with such small radius from metal-coated cantilevers is unrealistic as most of those tips have radius of 20 nm.Alternative way for TERS specific tips fabrication is creation a bulk gold tip which is one of the best candidates for TERS [5]; in contrast to silver, gold is more chemical stability and not compromised by oxidation. Our TERS tips were produced by the electrochemical etching. Before etching, the gold wire was cleaned and glued to the silicon chip (for tapping mode) or to the quartz tuning fork (TF mode) transducers. The fabricated TF probes have a diameter around 100-200 nm and show a Q-factor value of above 500 at a resonant frequency of 186 kHz [6].

TERS probes
SEM images of home-made probes: vertical probe glued to quartz tuning fork and bent probe glued to silicon chip after electrochemical etching in CaCl2 solution.

Application: TERS on graphene

To demonstrate the efficiency of the approach in the tapping mode we evaluate the Raman signal enhancement for the fabricated bulk gold cantilevers used for the graphene investigation [5]. The evaluated Raman signal enhancement factor is around 4x106
TERS of graphene
a) TERS spectrum of the graphene flake on the glass substrate obtained with a bulk gold cantilever in a intermittent contact mode, b) micro- and nano-Raman spectra measured at the same conditions (acquisition time, laser power), demonstrating the enhancement of Raman signal, and c) tip-enhanced Raman spectra at different distances between the tip and the graphene surface.

Application: TERS on cell membrane

To demonstrate the ability of bulk gold cantilevers for tuning fork transducers we analyze the peculiarities of cell membrane of genetically modified yeast cells [6]. It was shown by combined AFM and Raman measurements that genetic modification leads to the changes of topographic and elastic properties of the cell membrane and its protein composition due to producing of glucose dehydrogenase (GDH) protein, which is a transmembrane protein with two transmembrane fragments. Only using TERS the Raman signal from the modified cell membrane was obtained, and the probable fingerprint of GDH protein was extracted.

References

1. J. Wessel, Surface-enhanced optical microscopy, J. Opt. Soc. Am. B, 2 (1985) 1538-1541.
2. R. Stockle, Y.D. Suh, V. Deckert, R. Zenobi, Nanoscale chemical analysis by tip-enhanced Raman spectroscopy, Chem. Phys. Lett., 318 (2000) 131-136.
3. T. Deckert-Gaudig, E. Bailo, V. Deckert, Perspectives for spatially resolved molecular spectroscopy - Raman on the nanometer scale, J. Biophoton., 1 (2008) 377-389
4. K. F. Domke, B. Pettinger, Studying Surface Chemistry beyond the Diffraction Limit: 10 Years of TERS, Chem. Phys. Chem., 11 (2010) 1365-1373
5. V. Snitka, R. D. Rodriguez, V. Lendraitis, Novel gold cantilever for nano-Raman spectroscopy of graphene, Microelectron. Eng., 88 (2011) 2759-2762.
6. D. Naumenko, V. Snitka, E. Serviene, I. Bruzaite, A. Stogrin, V. Lendraitis, B. Snopok, AFM Imaging and Tip-enhanced Raman Fingerprinting of Membrane Proteins of Genetically Modified Yeast Cells, submitted to Australian Journal Chemistry (2011).