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

REVIEWSScanning Near-Field Optical Microscopy and Spectroscopy as a Tool forChemical AnalysisRenato Zenobi* and Volker DeckertResearch into the nanometer-scale regionis currently of relevance in manybranches of modern science and engineering,such as in microelectronics,supramolecular chemistry, and in abiological context. A great deal ofattention is given to the design ofmolecular devices, usually towards understandingthe function of existingªmolecular machinesº. Central to thistask are powerful diagnostic tools capableof recording chemical informationwith spatial resolution in thenanometer range. While elementalanalysis of surfaces with a lateralresolution of a few dozen nanometersis almost routine, analysis of molecularspecies with a resolution of < 1 mm isvery difficult. Scanning-tunneling andatomic-force microscopies usually donot give any chemical information. Bycombining scanning near-field opticalmicroscopy (SNOM)Ðthe ªopticalmemberº of the family of scanningprobemicroscopiesÐwith opticalspectroscopy, it is possible to obtainmolecular information from sampleareas as little as 50 nm in diameter. InSNOM, a light source is scanned abovethe object of interest at a distance of afew nanometers. In the optical nearfield, the illuminated area is not subjectto the Abbe diffraction limit, butmerely by the size of the illuminatingsource. High quality SNOM probes canbe reproducibly prepared by a chemical-etchingmethod. These etchedprobes have an optical transmissionup to 1000-fold higher than commercial(pulled) SNOM tips and can withstandhigher laser power. This lastadvantage allows not only high resolutionoptical imaging, but also localizedspectroscopic investigations ofsurfaces and even optical ªnanosamplingºby pulsed-laser ablation. Theablated material can be transportedover a considerable distance, whichopens the possibility for its subsequentanalysis with a complementary, highlysensitive analytical method, such asmass spectrometry.Keywords: analytical methods ´ massspectrometry ´ optical near-field microscopy´ optical spectroscopy ´ surfaceanalysis1. IntroductionA great breakthrough in surface science was the developmentof the scanning-probe microscopy techniques, led by thedevelopment of the scanning-tunneling microscope (STM),which earned Heinrich Rohrer and Gerd Binnig the 1986Nobel Prize in Physics. [1±3] Soon thereafter, a host of relatedmethods were developed, all based on piezoelectric scanningof a very sharp probe relative to the sample, such as atomicforcemicroscopy (AFM), scanning electrochemical microscopy,scanning magnetic microscopy, and many others. Theyare sometimes collectively referred to as the ªchildren of theSTMº. [4][*] Prof. R. Zenobi, Dr. V. DeckertLaboratorium für Organische ChemieEidgenössische Technische Hochschule (ETH) ZürichUniversitätsstrasse 16, 8092 Zürich (Switzerland)Fax: (‡ 41) 1-632-12-92E-mail: zenobi@org.chem.ethz.chOf particular interest to applications in molecular science isscanning near-field optical microscopy (SNOM), [5±8] theªopticalº version among the children of the STM. SNOMdelivers light to spots of less than 100 nm diameter andthereby holds great promise for molecular analysis on thenanometer scale. An optical resolution of as little as 12 nm hasbeen claimed by Betzig and Trautman [9] but only afterdeconvolution of the size of the near-field optical probe. Thisspatial resolution has not been reproduced lately by otherresearchers. Delivering light to such small spots is not possibleby conventional optical methods, such as by a focussed laserbeam. It is also impossible to resolve features separated byless than l/2 by conventional optical microscopy, where l isthe wavelength of the illuminating light. This limitation isimposed by the Abbe diffraction limit. At visible wavelengths,a resolution approaching 200 ± 250 nm should be theoreticallypossible but, except for confocal schemes employing oilimmersionlenses and therefore subject to other limitations, aresolution better than 0.5 ± 1 mm is rarely achieved. Thisdiffraction limit is the reason that short-wavelength ultra-Angew. Chem. Int. Ed. 2000, 39, 1746 ± 1756 WILEY-VCHVerlag GmbH, D-69451 Weinheim, 2000 1433-7851/00/3910-1747 $ 17.50+.50/0 1747