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

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