Scanning Near-Field Optical Microscopy and Spectroscopy as a ...

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Optical Near-Field MicroscopyREVIEWSscattering tensor of thefully symmetric vibrationalmodes, a strong decrease insignal intensity is expectedif the polarizers are crossed.In contrast, for anisotropicmodes, some intensity willstill be detected even forcrossed polarizers. This isillustrated in Figure 9. Atan interface, the orientationof the investigated moleculeswill not be randomly oriented and the structure of aninterface can be studied through symmetry considerations.Figure 9 A and 9 B are far-field Raman spectra of liquid p-xylol, while 9 C and 9 D are near-field Raman spectra of thesame. Over the transition from parallel to crossed orientationof the polarizers in the excitation and detection beam path,the expected intensity decrease is observed for many Ramanmodes, for example, the strong aromatic-ring vibrations at 820and 1200 cm 1 . In contrast, the band at about 1600 cm 1remains strong even when crossed polarizers are used,indicating large anisotropic components in its scatteringtensor. Figure 9 shows impressively that the SNOM tip usedfor recording the near-field data preserves the polarizationquite well.Figure 8. Near-field Raman spectrum of a diamond film produced by CVDmethods. The excitation wavelength was 488 nm and the laser line wasfiltered out in front of the spectrograph by a holographic notch filter.Strong bands due to Raman scattering from the SERS tip itself dominatethe spectrum (glass). Inset: the sharp band at 1330 cm 1 is assigned todiamond, and the broad feature at 1600 cm 1 is due partially to glass andpartially to graphitic carbon.1600 cm 1 , but unfortunately a fairly strong residual signalfrom the long progression of glass Raman bands, originatingfrom the tip itself, overlaps with the sp 2 carbon band. This ispartially due to the low Raman intensity of the sample, but theRaman emission from the glass of the SNOM tip dependsstrongly on the glass fiber material used. Our latest generationof SNOM tips exhibit muchlower Raman scattering inthis spectral region, suchthat, in principle, the purityof the film could be measuredat any given location.The exquisite resolutionin the z direction makes theSNOM an attractive tool tostudy the interface betweentwo liquids. In particular,molecular orientation atsuch interfaces may be studiedby the anisotropies ofthe Raman scattering tensorusing polar-filtered excitinglight and detection.For the isotropic part of theFigure 9. Raman spectra recordedin liquid p-xylol using polar-filteredexciting light and detection. A) Farfieldspectrum with parallel polarizers;B) Far-field spectrum withperpendicular polarizers; C) Nearfieldspectrum with parallel polarizers;D) Near-field spectrum withperpendicular polarizers. The absoluteintensities for each spectrumare not directly comparable. Thelaser power was 10 mW (at l ˆ488 nm) and the signal, averagedover 10 min, was used.3.3. Laser Ablation through SNOM TipsThe high transmission SNOM tips that we use permit notonly high resolution optical imaging and spectroscopic investigationsof surfaces, but also optical ªnanosamplingº bypulsed-laser ablation through the tip. [66] The laser-ablatedmaterial can be transported over a considerable distance andanalyzed in a second step by a highly sensitive analyticalmethod such as mass spectrometry. A schematic of thisconcept is shown in Figure 10.Figure 10. Schematic representation of the coupling of laser ablationthrough the SNOM tip with mass-spectrometric analysis of the vaporizedmaterial.This localized ablation has already been described and acrude realization has appeared in the literature. [67] Theauthors guided a sharpened optical fiber over a transmissionelectron microscopy (TEM) grid inside the ion source of amass spectrometer and performed a matrix-assisted laserdesorption/ionization(MALDI) mass-spectrometry experiment.The TEM grid was covered with an acetylcholinesample embedded in a 2,5-dihydroxybenzoic acid matrix. Theadvantage of this experiment over earlier MALDI studies,where the laser light was also brought in by an optical[68, 69]fiber, remains unclear. The spatial resolution was ofseveral micrometers, far greater than what is possible with theSNOM method.Several mechanisms are possible for laser-induced ablationthrough SNOM tips. [70] The photochemical mechanism assumesthat the energy is deposited in the sample via opticalabsorption and ablation follows as a consequence of breakingchemical bonds. The photothermal mechanism is similar butassumes the absorbed optical energy would be converted intoAngew. Chem. Int. Ed. 2000, 39, 1746 ± 1756 1753
REVIEWSheat, which causes ablation from the locally heated spot. Aballistic mechanism is also possible, in which the materialfrom the tip is sputtered onto the sample and material ablatesfrom the surface. A ballistic mechanism is the most likelyexplanation for an experiment carried out on an anthracenesurface using a completely metallized SNOM tip, [41] since, inthis case, no photons reach the sample and therefore a sputterprocess, either by metal atoms from the tip or by materialadsorbed on its surface, was believed to be the reason for theablation process. Fourth, one also has to take transientthermal expansion [71±73] into account as a possible mechanismfor the creation of surface indentations, but it is likely to benegligible. [70]We now discuss an experiment that gave clear results aboutthe ablation mechanism of a rhodamine B film. The samplewas irradiated with laser pulses either at its optical adsorbtionmaximum (ªon resonanceº, 532 nm) or nearby this point (ªoffresonanceº, 650 nm).SNOM tips with aperturesof < 100 nm diameterwere used. No significantdifference inthe optical transmissionof the SNOM tip overthe 400 to 650 nmrange, although somedecrease is expectedwith an increasing differencebetween thewavelength and tipaperture size. Figure 11shows shear-force topographicimages recordedafter several laserpulses had been firedonto the sample. Noablation was observedfor the off-resonanceFigure 11. Shear-force topographic imagesof a rhodamine B film after trans-11 A; l ˆ 650 nm,wavelength (Figuremitting laser pulses through the SNOM 2.1 mJ per pulse). Intip at A) 2.1 mJ atl ˆ 650 nm, far fromcontrast, well defined,the maximum absorption of rhodamineB, and B) 1.4 mJ atl ˆ 532 nm (B), a 70 nm diameterwavelength near the maximum absorption.The total topographic contrast in holes were created(FWHM), 5 nm deepthe z direction is A) 10 nm andwhen the laser operatedB) 30 nm.at a wavelength that isstrongly absorbed bythe sample (Figure 11 B; l ˆ 532 nm, 1.4 mJ per pulse). Thisclearly points to an optical ablation mechanism, eitherphotochemical or photothermal. Energy transfer by absorptionof photons appears to be more efficient than by a ballisticprocess. The photon energy used here is not sufficient forinducing bond dissociation in rhodamine molecules, in contrastto ablation experiments with ultraviolet laser radiation,where photochemical dissociation is often used to rationalizethe observed phenomena. [74] We therefore suggest that aphotothermal mechanism is responsible for the ablation of therhodamine film: The absorbed photon energy is converted toR. Zenobi and V. Deckertvibrational excitation, which causes a rapid temperatureincrease and leads to thermal desorption of the molecules.Another interesting observation in Figure 11 is that theablated material is redeposited on the sample surface close tothe ablation crater. The distribution of redeposited materialwas not symmetric in this experiment but was always to theleft side of the crater and independent of the scan direction. Inother SNOM ablation experiments, the distribution was moresymmetric. The origin of the directionality appears to be thetip itself: The apex of a SNOM tip is frequently not symmetricand the ablated material is ejected in a specific direction.Transport over micrometer distances was observed inseveral cases. The result of such an experiment is shown inFigure 12. [70] Here, an anthracene crystal surface was continuouslyirradiated with a pulsing laser (6 ns, 532 nm, 50 mJ)operated at a 20 Hz repetition rate over a 1 2 mm area. Ashear-force image of a larger area was then recorded withoutFigure 12. Transport of material over many micrometers: A topographicimage of a 5 8 mm area of an anthracene film crystal surface, previouslyirradiated in a 1 2 mm trench in the upper left corner with ablating laserpulses (pulse: 6 ns pulse; wavelength: 532 nm; pulse energy: 50 mJ;repetition rate: 20 Hz). Total topographic contrast in the z direction is100 nm. Adapted from ref. [70].irradiation. An approximately 1 mm wide trench with welldefined edges could be detected in the upper-left corner of theimage, while a 2 mm wide, 60 nm high mound was found about5 mm away running roughly parallel to the trench. Weinterpret this again as vaporization followed by redepositionof material in a preferred direction. The direction ofdeposition is most likely determined by an asymmetry of thetip. For collection of ablated molecules, for example by avacuum interface to a mass spectrometer, this asymmetrycould be an advantage if the SNOM tip could be arranged todirect the ablated material towards the collector.From Figure 12 we also estimate that only a few, largerchunks of material are ablated. Most of the anthracene isfound in the form of a diffuse and structureless mound, whichis expected if the molecules are redeposited from the gasphase. We have also shown that the laser-ablated moleculesdo not decompose. [70] We conclude that material transportdoes not take place by a mechanical dragging from the SNOMtip, but instead by vaporization and redeposition in a directiondictated by the assymetry of the tip.4. Conclusions and OutlookNear-field imaging based on aperture probes can nowprovide spectroscopic data with a lateral resolution on the1754 Angew. Chem. Int. Ed. 2000, 39, 1746 ± 1756
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