Alpenglow under the microscope

Improved optical microscope provides molecular images with color

No mountains, but a molecular landscape with five brilliant cresyl blue molecules shows this scanning tunneling microscope image of the TERS microscope. © Fritz Haber Institute of the Max Planck Society
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Classic scanning tunneling microscopes deliver extremely sharp images of atomic landscapes, but unfortunately only in black and white: one atom looks like the other. Optical microscopes, on the other hand, distinguish substances by their color spectrum, but so far can not distinguish individual atoms or individual molecules. The new TERS optical microscope now achieves a resolution that was unthinkable for optical microscopes for a long time. It images individual molecules with the help of their radiated Raman light and provides them with color.

Its resolution of 15 nanometers is comparable to that of atomic force microscopes. This is due to the innovation of the researchers: As they report in the journal "Physical Review Letters", the light beam of the microscope is bundled with the help of a parabolic mirror and a very sharp gold tip, thus illuminating the smallest possible portion of their sample.

When the Max Planck physicist Bruno Pettinger from the Fritz Haber Institute observes one of his microscope images on his computer, the picture is a little reminiscent of an alpine landscape. The three-dimensional image shows a rugged plain with several seemingly snow-capped mountains. Only they are not mountains, but scattered molecules of the color brilliant cresyl blue on a gold surface. Such images of scanning tunneling microscopes are known in abundance, but what is new is that such a measurement provides yet another image. It shows the glow of the molecules.

Molecules identified by the Raman spectrum

With the colors of this light, the so-called Ramans spectrum, Pettinger determines which molecule sits where - as if he called the individual mountains by their names. The researchers improved the resolution of this spectroscopy to 15 nanometers in their microscope and thus for the first time directly identified individual molecules based on their Raman spectrum.

In the future, for example, this technique could map nanoparticles that populate the surface of catalysts and trap foreign molecules there. With a picture of the different shapes of these particles and the molecules attached to them, it is possible to determine how their catalytic properties develop. display

New microscope takes advantage of two techniques

The intensity of the Raman signal of a single brilliant cresyl molecule (over a surface area of ​​12 x 12 nanometers). From the Raman signal, the researchers determine the color spectrum of the molecule and, if necessary, can determine the type of molecule. Fritz Haber Institute of the Max Planck Society

Currently available optical microscopes, with a few, still immature exceptions, do not simultaneously indicate the location of the molecules in addition to the type. Thus, with Raman spectroscopy, two different neighboring molecules can not be distinguished, even if they are 200 nanometers (nm) apart. In the landscape, this would mean that the individual peaks in the mountains would be too hazy to distinguish them. The image would show a barely textured, large plane in a mixed color. In contrast, a classical scanning tunneling microscope could even image individual rocks - ie atoms - but all of them only in gray. What elements occur in it, the stones do not reveal.

The microscope of the working groups around Alfred Meixner at the University of T bingen and Pettinger combines the advantages of both techniques. The two research teams initially worked independently, however. During a visit to the lab, the fellow scientists shared their ideas and eventually came to similar conclusions using parabolic mirrors instead of lenses. "It's convenient that we were competitors for a while, " says Meixner. The principle worked in both laboratories. So we know it keeps what it promises

Gold tips as a tool

The mentioned principle combines the two techniques of scanning tunneling microscopy and Raman spectroscopy (TERS) - with an exposed, sharp tip. In scanning tunneling microscopes, the metal needle scans the surface of the sample, in TERS microscopes it amplifies the desired optical effects. The researchers have produced very fine gold tips for this purpose, whose diameter is about 15 nanometers, less than one-thousandth of a human hair.

For the height profile of the sample, the gold tip traces the structure of the surface as in a scanning tunneling microscope. As soon as she finds a survey, a molecule, the innovation of the T binger and Max Planck researchers comes into play. Because the Raman scattering is very weak. For an evaluable signal of only one molecule, it must be enormously enhanced. To do this, the researchers focus laser beams perfectly on the needle with a parabolic concave mirror.

The electromagnetic field of the beams is thereby aligned along the tip and sets the charge carriers in metal tip and sample surface in vibration. The tip is only a nanometer from the sample, so the light concentrates on an extremely small space in the gap. Not only the gold tip shines then, but also the elevations on the surface directly below. There, molecules produce millions of times more Raman radiation than farther from the tip.

Diffraction limit outlined

The glowing gold needle in the microscope acts like an antenna. The laser beams stimulate their extremely sharp tip to emit light waves of a frequency. On the way through the gap between the tip and the sample surface, however, the waves are disturbed by small obstacles: the molecules that sit on the surface of the sample. They tap waves of very specific amounts of energy to vibrate themselves, leaving a kind of chemical fingerprint in the rays. This phenomenon is called Raman scattering.

The parabolic mirror collects as much of the scattered radiation around the gap as possible. The researchers are now able to evaluate and identify the previously unknown substances under their microscope using the Raman spectrum.

30 times higher resolution

With the TERS microscope, the researchers achieve a resolution 30 times higher than that of classical, diffraction-limited optical microscopes, which bundle light rays with lenses, for example. For the latter, the resolution is limited to the size of half a wavelength of the light, here it would be about 300 nm. For a long time, this was regarded as the absolute upper limit of the resolution of light microscopes. However, the resolution of the TERS microscope is limited differently here, namely the size of the gold tip used. Thus, the diffraction limit is outwitted, since the individual pixels are not imaged directly by light, but by the rastering of the gold tip over the surface.

Because of its high resolution and detection sensitivity, this type of microscopy is not only interesting for the investigation of catalysts, but also, for example, for the sequencing of DNA or for the imaging of nanostructures in semiconductors. The resolution of their TERS microscope Pettinger and Meixner want to further improve by using even sharper tips. "With the help of our TERS microscope, we will also drive research in the young but extremely interesting field of single-molecule spectroscopy, " says Pettinger.

(idw - MPG, 24.06.2008 - DLO)