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Tip-Enhanced
Raman Spectroscopy
Raman
Spectroscopy is a powerful analytical tool due to the wealth of
information contained in Raman spectra. However, Raman signals
are relatively weak compared to techniques such as fluorescence.
Surface enhanced Raman scattering (SERS) has
helped overcome this problem, offering signal enhancements of
several orders of magnitude over conventional Raman scattering.
However, the heterogeneity of metallic SERS substrates creates
variable electromagnetic field enhancement across the surface.
These enhancing 'hot spots' limit the utility of the technique
and render quantitative measurements unreliable.
As an alternative to conventional SERS, apertureless scanning
near-field optical microscopy can be employed, in which a modified
AFM tip can be brought into contact with a sample surface. This
approach, which was pioneered in our laboratory, provides highly
localized enhancements and offers more uniform enhancement when
scanning over the sample.
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Normal
Raman |
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Tip
Enhanced Raman |
The
instrument combines a laser scanning confocal microscope and an
AFM capable of both tip and sample scan modes. The confocal microscope
provides rapid optical alignment of the laser focus with the AFM
tip. Through scanning the sample relative to the tip and optics,
the piezo sample stage (x-y-z PI) ensures constant alignment of
the laser beam with the AFM tip.
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The
example presents enhanced Raman spectra of a flat layer
of brilliant cresyl blue dispersed on a glass slide with
(a) the tip retracted from the sample and (b) the tip in
contact with the sample. Experimental conditions: silver
coated tip (10-15 nm thickness); 488 nm excitation with
a power of 1.588 mW; 60 s acquisition time. Adapted from
Stöckle et al., Chem. Phys. Lett. 2000, 318,
131. |
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. We have been actively
involved in the fabrication and design of various TERS probes
based on AFM tips, which are intimately connected to Raman enhancements
possible in an experiment. A contributing factor has been identified
as being caused by the effect of the AFM tip material on the wavelength
of the surface plasmon resonance of the attached Ag particles.
Materials with low refractive indices such as SiO2,
SiOx and AlF3 have
been found suitable as supporting platforms for Ag films at 488
nm illumination to give strong TERS signals. Huge observed enhancements
of 70-80x, corresponding to net enhancements of >104 have been achieved for brilliant cresyl blue test analyte using
Ag-coated tips made from or pre-coated with low refractive index
materials. The yield of tips giving significant enhancement to
the Raman signals is found to be close to 100%. These findings
are crucial steps towards the use of TERS as a robust technique
for rapid chemical imaging with nanometer spatial resolution.
Left:
The Raman spectra of brilliant cresyl blue thin film acquired
with tip in contact
(black) and retracted (green traces) are shown for Ag coated (a)
SiO2, (b) SiOx/SiN
and (c) AlF3/SiN tips. The orange traces
are the tips' spectra collected after the experiments.
Right: High quality SEM image of a Ag-coated tip used in our experiments.
Besides
the transmission mode TERS, a side-illumination mode TERS setup,
that is suitable for opaque samples has also been built up in
our lab. Especially, when the substrate is metal, a highly localized
surface plasmon mode will be excited in the tip-sample gap, and
a signal enhancement more than 106 times can be achieved. With
such a giant enhancement, single molecule sensitivity can be reached.
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Tip-enhanced
Raman spectra from a few (<10) BCB molecules. The peak at
585 cm-1 was greatly enhanced after the tip approached the
sample surface (a). The collection time was 10 seconds.
To make sure that it is a pure near-field effect, Raman
intensity as a function of tip-sample distance was measured
(b). The signal is enhanced only if the tip-sample distance
is smaller than 30 nm. Time-resolved TERS measurements have
also been done. A tip-enhanced spectral series of one hundred
exposures from a BCB sub-monolayer made by spin coating
a 10-5 M solution on an Au substrate (c) were collected
continuously with an exposure time of 5 second per spectrum.
The peak intensity ~585 cm-1 shows a random fluctuation
(d). Panel (e) is the histogram of (d). It shows a broad
distribution of the intensity. The same experiment was done
with a sample made by spin coating a 10-4 M BCB solution
on an Au substrate and its histogram of the peak intensity
(f) |
Contact:
Dr. Thomas Schmid, schmid@org.chem.ethz.ch
Lothar Opilik, opilik@org.chem.ethz.ch
Carolin Blum, blum@org.chem.ethz.ch
Roman M. Balabin, balabin@org.chem.ethz.ch
Johannes Stadler, stadler@org.chem.ethz.ch
Prof. Dr. Renato Zenobi, zenobi@org.chem.ethz.ch
Literature:
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J. Stadler, T. Schmid, L. Opilik, P. Kuhn, P. S. Dittrich, R. Zenobi, Tip-enhanced Raman spectroscopic imaging of patterned thiol monolayers. Beilstein J. Nanotechnol., (2011), accepted. |
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J. Stadler, T. Schmid, R. Zenobi, Chemical Imaging on the Nanoscale – Top-Illumination Tip- Enhanced Raman Spectroscopy.Chimia, 65, (2011), 235–239. |
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J. Stadler, T. Schmid, R. Zenobi, Nanoscale Chemical Imaging Using Top-Illumination Tip- Enhanced Raman Spectroscopy, Nano Lett. 10, (2011), 4514–4520. |
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Stadler J., Schmid T., & Zenobi R., Nanoscale Chemical Imaging Using Top-Illumination Tip-Enhanced Raman Spectroscopy. Nano Lett.,10, (2010), 4514 -4520. |
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Yeo B.-S., Amstad E., Schmid T., Stadler J., & Zenobi R., Nanoscale Probing of a Polymer-Blend Thin Film with Tip-Enhanced Raman Spectroscopy. Small, 5, (2009), 952-960. |
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Schmid T., Yeo B.-S., Leong G., Stadler J., & Zenobi R., Performing tip-enhanced Raman spectroscopy in liquids. J. Raman Spectr., 40, (2009), 1392–1399 |
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Yeo B. S., Stadler J., Schmid T., Zenobi R., & Zhang W. H., Tip-enhanced Raman Spectroscopy – Its status, challenges and future directions. Chemical Physics Letters, 472, (2009), 1–13. |
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Zhang, W. H., Yeo, B. S., Schmid, T., & Zenobi, R., Near-Field Heating, Annealing, and Signal Loss in Tip-Enhanced Raman Spectroscopy. J. Phys. Chem. C , 112, (2008), 2104-2108. |
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W. Zhang, X. D. Cui, B.-S. Yeo, T. Schmid, C. Hafner, and R. Zenobi, Nanoscale Roughness on Metal Surfaces Can Increase Tip-Enhanced Raman Scattering by an Order of Magnitude, Nano Lett. 7, (2007), 1401 - 1405. |
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Zhang, W. H.; Yeo, B. S.; Schmid, T.; Zenobi, R., Single molecule tip-enhanced Raman spectroscopy with silver tips. Journal of Physical Chemistry C, 111, (2007), 1733-1738. |
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Yeo, B. S., Maedler, S., Schmid, T., Zhang, W. H., & Zenobi, R., Tip-Enhanced Raman Spectroscopy Can See More: The Case of Cytochrome c. J. Phys. Chem. C , 112, (2008), 4867-4873. |
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Boon-Siang Yeo, Thomas Schmid, Weihua Zhang and Renato Zenobi, Towards rapid nanoscale chemical analysis using tip-enhanced Raman spectroscopy with Ag-coated dielectric tips, Analytical and Bioanalytical Chemistry,387, (2007), 2655-2662. |
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Schmid, T.; Schmitz, T. A.; Setz, P. D.; Yeo, B. S.; Zhang, W. H.; Zenobi, R., Methods for molecular nanoanalysis. Chimia 2006, 60, (11), A783-A788 |
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C. Vannier, J. Boon-Siang Yeo, J. Melanson and R. Zenobi, Multifunctional microscope for far-field and tip-enhanced Raman spectroscopy, Rev. Sci. Instr. 77, 023104 (2006) |
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R. Stöckle, Y. D. Suh, V. Deckert, and R. Zenobi, Nanoscale chemical analysis by Tip-enhanced Raman Scattering, Chem. Phys. Lett. 318, 131 - 136 (2000). |
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