Scanning near-field optical microscopy

Ž. Current Opinion in Colloid & Interface Science 4 1999 256264 Scanning near-field optical microscopy Stefan Kirstein Max-Planck-Institut fur Kolloid- und Grenzflachenforschung, Am Muhlenberg 2, D-14476 GolmPotsdam, Germany Abstract Ž. Scanning near-field optical microscopy SNOM has become a widespread technique due to its promising ability of imaging with sub-micron resolution. Despite being developed over more than one decade, SNOM is still not a mature technique, which can be seen from the large number of recent publications describing instrumentational innovations. However, there are also many applications of near-field microscopy to the observation of thin organic film systems, which are supplementary to other techniques and demonstrate the usefulness of the technique. 1999 Elsevier Science Ltd. All rights reserved. Keywords: SNOM; NSOM; Scanning probe microscopy; Two-photon excitation; Langmuir-Blodgett films; Organic films 1. Introduction The principal idea of scanning near-field optical microscopy Ž SNOM. is as simple as it is fascinating: it is based on scanning an arbitrarily small aperture which is illuminated from the backside at a close but constant distance across a sample surface and recording optical information pixel-by-pixel collecting either transmitted, reflected, or fluorescence light to form an image. The resolution of the optical image is solely defined by the size of the aperture due to the strong localization of the light to the aperture size at close distances, which is called the near-field. In conventional or far-field microscopy the resolution is limited by the diffraction at the aperture as described by the Rayleigh-criterion. The idea of near-field microscopy was transformed into real instruments for the first Abbreiations: SNOM, NSOM: Scanning near-field optical microscopy; AFM: Atomic force microscopy Present address: Institut fur Physik, Humboldt-Universitat zu Berlin, Invalidenstr. 110, D-10115 Berlin, Germany. Tel.: 49-30- 2093-8030; fax: 49-30-2093-7682. E-mail address: kirstein@physik.hu-berlin.de Ž S. Kirstein. time by Pohl 1, Betzig 2, and Lewis 3 more than 10 years ago. A comprehensive overview about various techniques and applications of SNOM can be found in the book of Paesler and Mojer 4, and in the book of Fillard 5, which is more concentrated on theory. The most popular instrumental setup which is used by many groups Ž though with several modifications. and which is also the base for most of the commercial instruments, is the fiber-tip SNOM as described in the early work of Betzig 2 and depicted schematically in Fig. 1. A tapered optical fiber coated with aluminum is used to build a tip-like aperture, which is illuminated from behind by coupling laser light Žusually from Ar-laser. into the free end of the fiber. Typical diameters of the apertures obtained by this technique are in the range of 50100 nm, whereas, the total tip diameter typically exceeds 200 nm. The tip-sample distance is controlled by so-called shear-force detection. Therefore, the fiber tip is mechanically excited to transverse vibrations with amplitudes in the range of a few nanometers using a dither piezo and the amplitude is recorded via the induced voltage by a tuning fork-like piezo crystal attached to the fiber 6. 1359-029499$ - see front matter 1999 Elsevier Science Ltd. All rights reserved. Ž. PII: S 1 3 5 9-0 2 9 4 9 9 0 0 0 0 5-9

( ) S. Kirstein Current Opinion in Colloid & Interface Science 4 1999 256264 257 2. Technical aspects Many of the papers describing instrumental innovations have appeared in the special issues of Ultrami- croscopy 7 and Applied Physics A 8, which are conference proceedings. I cannot mention all of them explicitly, but will give reference to them whenever it is appropriate. Before some detailed improvements are discussed, it is worthwhile to emphasize articles that describe newly developed instruments, which provide outstanding mechanical and thermal stability 9, allow efficient collection of reflected light 10,11, or, which are especially designed for polarized measurements 12. The instrument described in 13 allows detection of linear dichroism and birefringence by a modulation method. 3. Probe fabrication Fig. 1. Schematic set-up of a typical fiber tip SNOM using shearforce detection to control the tip-sample distance. A dither piezo is used to induce lateral vibration of the fiber tip of its resonance frequency. The amplitude is recorded by a piezo-crystal in the shape of a tuning fork. The amplitude is used as a feedback signal to control the z-position of the sample while scanning in the xy-direction. The transmittance of fluorescence light is detected by a microscope objective. The detector is either a photomuliplier tube or an avalanche photodiobe. The dither amplitude smoothly decreases when the tip approaches the sample surface. The damping is caused by shear-forces acting on the fiber tip due to local contact with the sample surface. This provides a distance-dependent signal which is independent from the optical information, and can be used to operate a feed back loop to maintain constant tip sample distance. The technical problems of SNOM are related to the fabrication of appropriate tips that provide good resolution and high transmittance of light, and on the shear-force distance regulation. That these problems are not solved satisfactory can be seen from the large amount of last year s literature, which was directly related to these topics. Furthermore, a majority of papers describe other instrumentational aspects and most of the remaining references are discussing technical aspects related to their specific experiments. Only a few papers report about SNOM as a tool for high resolution imaging or local spectroscopy. In the first section of this article I will summarize the most important technical improvements and in the second part I will report some applications that demonstrate the capabilities of SNOM in the field of colloid and interface sciences. Various approaches are described to improve the scanning probe quality and functionality by fabrication of tips similar to AFM cantilevers. One concept consists of hollow metal tips integrated in silicon cantilevers 14,15, but other, probably more promising concepts make use of the special electronic and optical properties of semiconductor materials by incorporating Schottky diodes 16 or LEDs 17 into the tip. The most interesting development towards this direction is given by Heisig et al. 18,19, who are using GaAs to fabricate multifunctional cantilever tips. Again, integration of a Schottky diode at the tip was demonstrated, which opens a pathway for the fabrication of various passive and active probes for scanning microscopy due to the outstanding electrical and optical properties of GaAs. The most remarkable improvement of aluminum coated fiber tips was obtained using focused ion-beam etching 20,21. Therefore, a tapered optical fiber was completely covered by aluminum and afterwards the very tip end was cut using a gallium ion beam. This allows the production of very flat tip ends with a well-defined circular aperture down to 20 nm in diameter. By scanning over single molecules a point spread function with FWHM of less than 70 nm was clearly demonstrated. Obviously, this is the ultimate one can do with aluminium coated fiber tips. 4. Apertureless probes All attempts to improve the optical resolution by decreasing the aperture size lead to large technical effort and very low light throughput. A completely different approach was brought up by Zenhausen et

258 ( ) S. Kirstein Current Opinion in Colloid & Interface Science 4 1999 256264 al. 22, namely, apertureless near-field microscopy. Originally, the interference of light reflected from the tip of an AFM and that part of the light reflected from a transparent sample was used for imaging. This concept is currently taken up and adopted to new instrumentation 2326 and also has stimulated theoretical work 27,28. However, this mode is mainly sensitive to contrast in the dielectric constant and thus, it is inapplicable to fluorescence imaging. Another kind of apertureless near-field microscopy has been established by using uncoated fiber tips for both excitation and collection of light. It was shown experimentally by Kaupp et al. 29,30,31 and others 32 that fluorescence image recording with sub-wavelength resolution is possible in this mode of operation. Two conditions have to be fulfilled: first, the tip must be approached very close Ž approx. 5 nm. to the sample surface. In this case a significant amount of backscattered or fluorescent light is collected by the tip. Second, since the light is depolarized in the nearfield of the tip-sample region, one has to detect only the orthogonal polarized part of the backscattered light. The effect of depolarization and the possibility of sub-wavelength resolution were confirmed by theo- retical calculations 33. The ease of use and the increased light throughput may favor this technique for many future applications. A comparison of these three different modes of SNOM operation is given in Fig. 2. 5. Tip sample distance control Although most fiber-tip SNOM setups are using shear-force tip-sample distance control, this mechanism is still a subject of technical innovations 34,35. Also, a comprehensive theoretical description of this dynamic force mode is still not available; however, it is now clearly shown that shear-force is induced by mechanical contact between the vibrating tip and the surface of the sample 36,37. This mechanism is further complicated by a water layer usually found on hydrophilic substrates under ambient conditions 38,39. Due to the water layer, humidity, surface chemistry of the sample, and even time and temperature strongly influence the shear-force signal. This may explain the various and partially contradictory results published throughout the literature. 6. SNOM and liquid environments The possibility of imaging within a liquid environment opens the route to the observation of biological samples within its native environment. For such applications, SNOM may have certain advantages against Fig. 2. Sketch of different types of near-field image recording. Ž. a A tapered optical fiber is covered with aluminum to achieve a small aperture. Light is collected in transmission mode or through the fiber itself. Ž b. Uncoated tapered optical fiber; light is collected only through the fiber. Ž. c Interference of light reflected at an AFM tip and the sample surface is recorded Ž apertureless SNOM.. other microscopy techniques, such as confocal microscopy, since it allows the localization of a point source of light to an interface even through solutions, which are very turbid or contain high concentration of fluorescent dyes, which would otherwise result in a large background signal. However, the most striking problems result from the shear-force signal due to the strong damping of tip vibration by the viscosity of the liquid 40. The damping factor is strongly dependent on the geometry of the tip 41 and on the length over which it penetrates into the water. It is, therefore, important to immerse either the complete tip, as it is realized in several setups 40,4244, or a constant fraction independent of the surface topography 45. It was shown that the lower sensitivity of the shearforce control is still sufficient to collect topographic images of very soft samples such as biological cells 43in a liquid. We have been using SNOM to investigate the adsorption of Human Serum Albumin Ž HAS. protein at polystyrene Ž PS. films from the aqueous buffer solution. These investigations were motivated by the question, if lateral surface roughness increases the adsorption. Therefore, HAS was used which was labeled with fluorescent dye molecules and thin films of PS on glass substrates showing lateral structures. In the first attempts, random holes of the PS films served as an indication of surface roughness 40 ; in more recent studies, we used thin PS films with a well-defined surface structure, as shown in Fig. 3. From the comparison of the topographic with the fluorescence image it can be deduced that edges clearly increase the adsorption of the protein. As a curiosity, though important, it is worthwhile to mention the possibility of taking SNOM images at the airwater interface using a fiber tip SNOM. In this case the characteristic interference of the transmitted

( ) S. Kirstein Current Opinion in Colloid & Interface Science 4 1999 256264 259 Fig. 3. Ž. a Topographic image of a polystyrene film showing a topographic structure that consists of regularly ordered hexagons with a height of approximately 100 nm. The image was recorded within the aqueous protein solution using the set-up described in 40. Ž b. Simultaneously recorded fluorescence image of human serum albumin Ž HAS. protein labeled with fluorescene adsorbed at the polystyrene film structure. The noise originates from the large background signal that has been subtracted. The size of the image is 2020 m 2. light due to multiple reflection between the airwater interface and the tip end is used as a signal for distance regulation 46. Thus, fluorescence images of polymerized pentacosadiynioc acid at the airwater interface could be obtained with a resolution below 200 nm. This technique is of special interest for the in situ characterization of Langmuir monalayers. Besides the technical improvements described here, much more developments have been made which are beyond the scope of this article. This includes instruments for experiments at low temperatures 47 as well as Raman 48, infra red 49 and micrometer wave imaging 50. 7. Applications The applications of SNOM described by the papers published during the last year cover various subjects ranging from semiconductor heterostructures to single DNA strands. In this article only those papers are reviewed which are related to organic materials. 8. Thin organic films of conjugated materials Thin films of conjugated organic materials have attracted much interest for their capability in technical applications such as the fabrication of light emitting devices. Three prominent classes of materials are of interest: conjugated polymers; liquid crystals; and dye molecules. For all three systems specific problems could be addressed by SNOM. For example, in blends of a conjugated polymer and an electrical inert matrix, polymer phase separations were observed on a mesoscopic scale by optical as well as topographical contrast 5153. Also, the reorientation dynamics of polymer dispersed liquid crystal films could be imaged utilizing a near-field microscope not only as an optical probe but also as an electrode for the application of local electric fields 54,55. However, the most interesting application is the investigation of Conboy et al. 56 on vacuum deposited organic multilayer films. The crystallization effects within a two-layer film of a titanyl phtalocyanin Ž TiOPc. and a perylene phenethylimide Ž PPEI. were observed by means of AFM and SNOM. Here, the near-field microscope was used to identify the crystallites by their spectroscopic properties. Different types of crystallites within the PPEI layer were observed depending on the presence or absence of the TiOPc layer upon post-deposition solvent vapor annealing. It is shown that the crystal morphology widely effects the charge transfer efficiencies between the layers and thus, the electro-optical properties of the films. This paper should be regarded as eye-opening or at least as a warning to all researchers working in the field of vapor deposited molecular films. 9. Langmuir Blodgett and self-assembled films Two-dimensional phase transitions of Langmuir monolayers of phospholipids at the airwater interface and of films transferred to solid substrates have been investigated intensively by far-field fluorescence microscopy 57. Only recently high-resolution scanning probe microscopy is used to investigate the phase

260 ( ) S. Kirstein Current Opinion in Colloid & Interface Science 4 1999 256264 behavior on a sub-micron scale. As an example, Dunn et al. have studied the coexistence of the liquid expanded and the condensed phase Ž LELC. of L-alpha dipalmitoylphosphatidylcholine Ž DPPC. monolayers transferred to mica by various SXM techniques in- cluding SNOM 58,59. As a fluorescent probe, molecules, which tend to dissolve only in the liquid phase, are doped into the lipid film in a small molar ratio, a technique well established in far-field fluorescence microscopy. Using this technique the authors were able to show that in monolayers as well as in bilayers sub-micron sized LC domains are present in the LE phase, which might result from a dewetting process after film transfer. On mica substrates, these sub-micron-sized domains become mobile above an external humidity of 65% and growing with time in order to reduce the line tension. It could be proven by SNOM that only lateral transport of phospholipid molecules is responsible for this behavior and no collapse of the film occurs 59. Beyond the study of phase separations the distribution of dye molecules or dye crystallites within Langmuir monolayers itself is the subject of SNOM investigation. For example, a complex dyad RhŽ III. - X-RyŽ II. was dissolved in monolayers of a fatty acid or an eicosyl amine 60. These films also show phase separation, and the domains of different phases can be imaged by shear-force detection. The distribution of the dyad within the different phases was carefully studied by the fluorescence images. In this case the relation between fluorescence intensity and dyad density was not trivial, since the dyad shows efficient fluorescence quenching depending on its local environment. Therefore, only the combination of complementary techniques such as AFM and SNOM yielded the topographic and spectroscopic information to reveal the complex structure of this system. Similar investigations were performed by Dutta et al. 61 on sub-micron sized crystallites of a modified perylene molecule mixed with stearic acid. By the combination of topographic and SNOM images recorded at two different wavelengths corresponding to monomers and aggregates, respectively, the full information of the complex film structure was revealed. It could be shown that the dye molecules do not mix with the stearic acid but instead form submicron-sized crystallites, which become three-dimensional upon increasing the molar dye concentration. These crystallites accumulate at the boundaries of the stearic acid islands of the condensed phase. These examples demonstrate the capability of SNOM as an elegant tool to investigate dye distributions within thin films. Energy transfer is used as an additional tool to investigate the mesoscopic structure of self-assembled multilayer films of polyelectrolyte materials 62. They were composed of alternating layers of a zirconiumphosphate Ž ZrP. and dye-labeled polycation polyžal- lylamine hydrochloride.ž dye-pah.. By observation of the fluorescence of the dye with SNOM it could be shown that the ZrP forms platelets with an average size of 150 nm which lay flat on the surface covering nearly 95% of the area. The dye-pah preferentially adsorbs at the ZrP platelets but also spans the space between the platelets, though to a lesser extend. Energy transfer between PAH labeled with different dyes separated by the ZrP plates was investigated locally by the SNOM technique. 10. Biological systems Biological and medical specimen are extensively investigated by far-field microscopy using fluorescence dye probes. There are various fluorescent dyes available which can be attached with great selectivity to different functional groups of cell constituents and often the dyes show specific changes of their photophysical or spectral properties due to local environmental parameters, such as acidity or polarity of the solvent. However, in conventional fluorescence microscopy, only the dye labeled groups are visible if no autofluorescence of the cell is observed. In this context SNOM techniques are extremely advantageous, since they allow the localization of the fluorescent probes on the topographic image of a cell or tissue. Those images can not only be scanned on dried samples but within the native liquid environment as well 40. Two recent review articles about the application of SNOM to biological samples can be found in 63 and 64. Here, only three very recent experiments will be emphasized. A good example to demonstrate the usefulness of the high resolution of SNOM is given by Hwang et al. in 65. They have investigated the patchy distribution of dye labeled lipids within the plasma membrane of human skin fibroplasts and compared this to the distribution of transmembrane proteins labeled with fluorescent antibody HLA. On fixed dried cells a typical size of the HLA patches of 70 and 600 nm was deduced from spatial correlation analysis of the SNOM fluorescence images. The size of the lipid patches was in accordance with the size of typical lipid domains found by other techniques like fluorescence recovery after photobleaching. It could be shown that the HLA patches are not localized at the same areas as the lipid domains. This is important information, since lateral inhomogeneities, patches and domains in cell surface membranes have been observed only recently and not much is known about their accurate size and distribution 66. The technique of staining can be extended much

( ) S. Kirstein Current Opinion in Colloid & Interface Science 4 1999 256264 261 more if also UV dyes are utilized. Special dyes have been developed for this purpose, like the DNA probe 4,6-diamiddino-2-phenylindol Ž DAPI.. However, UV excitation in combination with SNOM causes several problems, such as auto-fluorescence of the fiber, strong absorption and scattering by the biomaterial, and inexpensive light sources are not available. Therefore, Kirsch and colleagues have found a way to avoid these problems by using two-photon absorption, either in pulsed 67 or continuous wave mode 68,69. In the latter, the 647-nm emission of an ArKr laser was used for excitation of DAPI and uncoated tapered fiber tips served as scanning probes. With this new technique single chromosomes of Drosophila melanogaster labeled with DAPI were imaged. The comparison of topographic and fluorescence images clearly demonstrates the different staining concentration along the chromosome due to AT-rich and AT-poor regions with a resolution below 500 nm, see Fig. 4. In my opinion, this technique is not yet completely developed, but is very promising for two reasons: first, it allows nearly background-free fluorescence recording, which guarantees high signal-tonoise ratios even at low intensities. Second, since two-photon excitation is a non-linear process, it leads to an increased confinement of the excitation within the near-field and, thus, improved resolution. The low intensities Ž 200 mw. and the required spectral range will facilitate the use of laser diodes as an inexpensive light source in the future. Of course, the ultimate goal is the use of near-field microscopy to observe single dye molecules or single proteins on cell constituents. First steps towards this direction have been performed successfully by Van Hulst and colleagues 70. They were imaging single rhodamine or carbocyanine Ž DiI. dye molecules attached to a double-stranded DNA Ž 340 nm in length. with one fluorophore per strand. As can be seen from Fig. 5, the single strands with a typical diameter of 1.4 nm were clearly imaged on mica by the shear-force technique without any deformation induced by the tip. The fluorescence images show resolution of 70 nm at FWHM, well below the optical diffraction limit. In these first experiments the authors have investigated mainly the photobleaching dynamics of the two different dyes and there mobilities. This work is very important, because it demonstrates, that SNOM Žif carefully performed. in combination with appropriate dye probes can serve as a very powerful tool to analyze the behavior of biological samples on a molecular scale. 11. Conclusions The many papers published on technical issues Fig. 4. Topographic Ž. a and fluorescence Ž. b image of Drosphila melanogaster polythene chromosomes labeled with DAPI using continuous mode two-photon excitation. The traces in Ž. c are taken along the dashed lines in Ž. a and Ž. b. Note the inversion of the fluorescence contrast in the shaded region I compared to the region II Žwith permission from 68..

262 ( ) S. Kirstein Current Opinion in Colloid & Interface Science 4 1999 256264 Fig. 5. Shear-force Ž. a and SNOM fluorescence Ž. b image of DNA fragments labeled with rhodamine 6G molecules. The scan size is 1.51.5 2 m. The DNA strands were deposited on freshly cleaved mica Žwith permission from 70.. demonstrate that SNOM is a lively and rapidly developing field. New trends may be seen in the case of apertureless probes. The applications cover various fields from semiconducting hetero-structures to biological samples. In all fields SNOM becomes a desirable instrument whenever it serves to improve the lateral resolution of the images, or profits from the simultaneous recording of optical and topographic images, or circumvents other problems like background fluorescence, which appear in conventional light microscopy. Acknowledgements I want to thank all the colleagues who have sent me their recent reprints. I am also grateful to H. Nohwald for his generous support. References and recommended reading of special interest of outstanding interest 1 Pohl DW, Denk W, Lanz M. Optical stetoscopy: image recording with resolution 20. Appl Phys Lett 1984; 44:651. 2 Betzig E, Trautmann JK, Harris TD, Weiner JS, Kostalek RL. Breaking the diffraction barrier: optical microscopy on a nanometric scale. Science 1991;251:14681470. 3 Harootunian A, Betzig E, Isaacson M, Lewis A. Super-resolution fluorescence near-field scanning optical microscopy. Appl Phys Lett 1986;49Ž 11.:674676. 4 Paesler MA, Moyer PJ. Near-field optics theory, instrumentation, and applications. New York: John Wiley & Sons Inc, 1996. 5 Fillard JP. Near field optics and nanoscopy. Singapore: World Scientific, 1995. This book gives a very detailed introduction into the theory of near-field optics. Various modes of operation are explained and discussed. 6 Karrai K, Grober RD. Piezoelectric tip-sample distance control for near field optical microscopes. Appl Phys Lett 1995;66Ž 14.:18421844. Ž. 7 Ultramicroscopy 1998;71 14. This volume is a special issue, which contains the Proceedings of the Near-Field Optics Conference. Jerusalem, 1997 Ž NFO-4.. 8 Applied Physics a Ž Materials Science Processing. 66 ŽPart 1 Suppl S.. This volume contains the Proceedings of the 9th International Conference on Scanning Tunneling MicroscopySpectroscopy and Related Techniques. Hamburg 1997. 9 Merritt G, Monson E, Betzig E, Kopelman R. A compact fluorescence and polarization near-field scanning optical microscope. Rev Sci Instruments 1998;69Ž. 7 :26852690. 10 Cricenti A, Generosi R, Barchesi C, Luce M, Rinaldi M. A multipurpose scanning near-field optical microscope-reflectivity and photocurrent on semiconductor and biological samples. Rev Sci Instrum 1998;69Ž. 9 :32403244. 11 Stranick SJ, Richter LJ, Cavanagh RR. High efficiency, dual collection made near-field scanning optical microscope. J Vacuum Sci Technol B 1998;16Ž. 4 :19481952. 12 Mitsuoka Y, Nakajima K, Homma K et al. Polarization properties of light emitted by a bent optical fiber probe and polarization contrast in scanning near-field optical microscopy. J Appl Phys 1998;83Ž. 8 :39984003. 13 Mcdaniel EB, Mcclain SC, Hsu JWP. Nanometer scale polarimetry studies using a near-field scanning optical microscope. Appl Opt 1998;37Ž. 1 :8492. 14 Werner S, Rudow O, Mihalcea C, Oesterschulze E. Cantilever probes with aperture tips for polarization-sensitive scanning near-field optical microscopy. Appl Phys A ŽMater Sci Process. 1998;66ŽŽ Part 1 Suppl..:S367S370. 15 Oesterschulze E, Rudow O, Mihalcea C, Scholz W, Werner S. Cantilever probes for SNOM applications with single and double aperture tips. Ultramicroscopy 1998;71Ž 14.:8592. 16 Leinhos T, Stopka M, Oesterschulze E. Micromachined fabrication of si cantilevers with Schottky diodes integrated in the tip. Appl Phys A Ž Mater Sci Process. 1998;66 ŽPart 1 Suppl.:S65S69. 17 Gottlich H, Heckl WM. A novel probe for near field optical microscopy based on luminescent silicon. Ultramicroscopy 1995;61Ž 14.:145153. 18 Heisig S, Oesterschulze E. Gallium arsenide probes for scanning near-field probe microscopy. Appl Phys A ŽMater Sci Process. 1998;66 Ž Part 1 Suppl.: S385S390. The use of GaAs enables the fabrication of various multifunctional tips for sensing optical, electrical, and topographical surface

( ) S. Kirstein Current Opinion in Colloid & Interface Science 4 1999 256264 263 properties. This and the following paper describe the first steps towards this direction. 19 Heisig S, Danzebrink HU, Leyk A, Mertin W, Munster S, Oesterschulze E. Monolithic gallium arsenide cantilever for scanning near-field microscopy. Ultramicroscopy 1998;71 Ž 14.:99105. 20 Pilevar S, Edinger K, Atia W, Smolyaninov I, Davis C. Focused ion-beam fabrication of fiber probes with well-defined apertures for use in near-field scanning optical microscopy. Appl Phys Lett 1998;72Ž 24.:31333135. 21 Veerman JA, Otter AM, Kuipers L, van Hulst NF. High definition aperture probes for near-field optical microscopy fabricated by focused ion beam milling. Appl Phys Lett 1998;72Ž 24.:311517. Here one can find presentations of the best aluminum-coated fiber tips that have ever been described. 22 Zenhausen F, Martin Y, Wickramasinghe HK. Scanning interferometric apertureless microscopy: optical imaging at 10 Angstrom resolution. Science 1995;269:10831085. 23 Wurtz G, Bachelot R, Royer P. A reflection-mode apertureless scanning near-field optical microscope developed from a commercial scanning probe microscope. Rev Sci Instrum 1998;69Ž. 4 :17351743. 24 Laddada R, Adam PM, Royer P, Bijeon JL. Apertureless near-field optical microscope in reflection and transmission modes. Opt Eng 1998;37Ž. 7 :21422147. 25 Adam PM, Royer P, Laddada R, Bijeon JL. Polarization contrast with an apertureless near-field optical microscope. Ultramicroscopy 1998;71Ž 14.:327331. 26 Hatano H, Inouye Y, Kawata S. A near-field scanning optical microscope which measures both constant-height and constant-gap images. Jpn J Appl Phys Part 2-Lett 1998;37Ž 8B.: L1008L1010. 27 Cory H, Boccara AC, Rivoal JC, Lahrech A. Electric field intensity variation in the vicinity of a perfectly conducting conical probe application to near-field microscopy. 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Sub-wavelength imaging by depolarization in a reflection near-field optical microscope using an uncoated fiber probe. Opt Commun 1998; 146Ž 16.:277284. 33 Vonfreymann G, Schimmel T, Wegener M. Computer simu- lations-subwavelength resolution with an apertureless SNOM. Appl Phys AŽ Mater Sci Process. 1998;66ŽPart 2 Suppl S.:S93942. This contribution will be helpful for all working with apertureless fiber tips. 34 Pedarnig JD, Goettlich H, Heckl WM. Calibration and set-up of 100 khz shear-force distance control for near-field optical microscopy. Probe Microsc 1998;1:239246. 35 Naber A, Maas HJ, Razavi K, Fischer UC. A dynamic force distance control suited to various probes for scanning nearfield optical microscopy. Rev Sci Instrum submitted 36 Bruckl H, Matthes F, Reiss G. Direct measurement of the oscillation amplitude and criteria for high-quality images in shear force microscopy. Appl Phys A Ž Mater Sci Process. 1998;66Ž Part 1 Suppl.:S345S348. 37 Smolyaninov II, Atia WA, Pilevar S, Davis CC. Experimental study of probe-surface interaction in near-field optical microscopy. Ultramicroscopy 1998;71Ž 14.:177182. 38 Davy S, Spajer M, Courjon D. Influence of the water layer on the shear force damping in near-field microscopy. Appl Phys Lett 1998;73Ž 18.:25942596. 39 Okajima T, Hirotsu S. Study of probe-surface interaction in shear-force microscopy: effects of humidity and lateral spring constant. Opt Rev 1998;5Ž. 5 :303309. 40 Mertesdorf M, Schonhoff M, Lohr F, Kirstein S. Scanning near-field optical microscope designed for operation in liquids. Surf Interface Anal 1997;25Ž 10.:755759. 41 Kirstein S, Mertesdorf M, Schonhoff M. The influence of a viscous fluid on the vibration dynamics of scanning near-field optical microscopy fiber probes and atomic force microscopy cantilevers. J Appl Phys 1998;84Ž. 4 :17821790. 42 Hollricher O, Brunner R, Marti O. Piezoelectrical shear-force distance control in near-field optical microscopy for biological applications. Ultramicroscopy 1998;71Ž 14.:143147. 43 Gheber LA, Hwang J, Edidin M. Design and optimization of a near-field scanning optical microscope for imaging biological samples in liquid. Appl Opt 1998;37Ž 16.:35743581. 44 Talley CE, Lee MA, Dunn RC. Single molecule detection and underwater fluorescence imaging with cantilevered nearfield fiber optic probes. Appl Phys Lett 1998;72Ž 23.: 29542956. 45 Lambelet P, Pfeffer M, Sayah A, Marquisweible F. Reduction of tip-sample interaction forces for scanning near-field optical microscopy in a liquid environment. Ultramicroscopy 1998;71Ž 14.:117121. 46 Kramer A, Hartmann T, Eschrich R, Guckenberger R. Scanning near-field fluorescence microscopy of thin organic films at the waterair interface. Ultramicroscopy 1998;71: 12332. The possibility of taking SNOM-images at the waterair interface may be stimulating for many researchers working with Langmuir monolayers. 47 Durand Y, Woehl JC, Viellerobe B, Gohde W, Orrit M. New design of a cryostat-mounted scanning near-field optical microscope for single molecule spectroscopy. Rev Sci Instrum 1999;70Ž. 2 :13181325. 48 Deckert V, Zeisel D, Zenobi R, Vodinh T. Near-field surface enhanced Raman imaging of dye-labeled DNA with 100-nm resolution. Anal Chem 1998;70Ž 13.:26462650. 49 Knoll B, Keilmann F. Sanning microscopy by mid-infrared near-field scattering. Appl Phys A Ž Mater Sci Process. 1998; 66Ž. 5 :477481. 50 Lann AF, Golosovsky M, Davidov D, Frenkel A. Combined millimeter-wave near-field microscope and capacitance distance control for the quantitative mapping of sheet resistance of conducting layers. Appl Phys Lett 1998;73Ž 19.:28322834. 51 Hsu JH, Wei PK, Fann WS, Chuang KR, Chen SA. The inhomogeneity in conjugated polymer blend films. Ultramicroscopy 1998;71Ž 14.:263267. 52 Webster S, Smith DA, Batchelder DN, Lidzey DG, Bradley DDC. Application of fluorescence scanning near-field optical microscopy to the study of phase-separated conjugated polymers. Ultramicroscopy 1998;71Ž 14.:275279.

264 ( ) S. Kirstein Current Opinion in Colloid & Interface Science 4 1999 256264 53 Nagahara LA, Nakamura M, Tokumoto H. Investigation of mesoscopic domains in thin organic films using near-field optical absorption mapping. Ultramicroscopy 1998;71 Ž 14.:281285. 54 Mei EW, Higgins DA. Near-field scanning optical microscopy studies of electric-field-induced molecular reorientation dynamics. J Phys Chem 1998;102Ž 39.:75587563. 55 Mei E, Higgins DA. Local dynamics in polymer-dispersed liquid crystals studied by near-field scanning optical microscopy. Appl Phys Lett 1998;73Ž 24.:35153517. 56 Conboy JC, Olson EJC, Adams DM, et al. Impact of solvent vapor annealing on the morphology and photophysics of molecular semiconductor thin films. J Phys Chem B 1998;102Ž 23.:451625. In this paper the topography and fluorescence behavior of evaporated thin films of two organic molecules Ža phthalocyanine and a perylene compound. are investigated. Very nice pictures of crystallites are presented which appear after postdeposition solvent vapor annealing. This paper is highly recommended to all researchers exploring the electronic and optical properties of thin organic films prepared by vacuum deposition. 57 Mohwald H. Phospholipid and phospholipid-protein monolayers at the airwater interface. Annu Rev Phys Chem 1990;41:441476. 58 Hollars CW, Dunn RC. Submicron structure in L-alpha-dipalmitoylphosphatidylcholine monolayers and bilayers probed with confocal, atomic force, and near-field microscopy. Biophys J 1998;75Ž. 1 :342353. 59 Shiku H, Dunn RC. Direct observation of DPPC phase domain motion on mica surfaces under conditions of high relative humidity. J Phys Chem B 1998;102Ž 19.:37913797. 60 Kirsch AK, Schaper A, Huesmann H, Rampi MA, Mobius D, Jovin TM. Scanning force and scanning near-field optical microscopy of organized monolayers incorporating a nonamphiphilic metal dyad. Langmuir 1998;14Ž 14.:38953900. 61 Dutta AK, Vanoppen P, Jeuris K, Grim PCM, Pevenage D, Salesse C, De Schryver FC. Spectroscopic, AFM, and NSOM studies of 3d crystallites in mixed Langmuir-Blodgett films of n,n-bisž 2,6-dimethylphenyl. 3,4,9,10-perylenetetracarboxylic dimide and stearic acid. Langmuir 1999;15Ž. 2 :607612. 62 Kerimo J, Adams DM, Barbara PF, Kaschak DM, Mallouk TE. NSOM investigations of the spectroscopy and morphology of self-assembled multilayered thin films. J Phys Chem B, 102Ž 47.:945160 63 Meixner AJ, Kneppe H. Scanning near-field optical microscopy in cell biology and microbiology. Cell Mol Biol 1998; 44Ž. 5 :673688. 64 Subramaniam V, Kirsch AK, Jovin TM. Cell biological applications of scanning near-field optical microscopy Ž SNOM.. Cell Mol Biol 1998;44Ž. 5 :689700. 65 Hwang J, Gheber LA, Margolis L, Edidin M. Domains in cell plasma membranes investigated by near-field scanning optical microscopy. Biophys J 1998;74Ž. 5 :21842190. 66 Edidin M. Lipid microdomains in cell surface membranes: transience and stability in the lateral organization of phospholipid bilayers. Curr Opin Struct Biol 1998;7:528532. 67 Jenei A, Kirsch AK, Subramaniam V, Arndt-Jovin DJ, Jovin TM. Picosecond multiphoton scanning near-field optical microscopy. Biophys J 1999;76Ž. 2 :10921100. 68 Kirsch AK, Subramaniam V, Striker G, Schnetter C, Arndtjovin DJ, Jovin TM. Continuous wave two-photon scanning near-field optical microscopy. Biophys J 1998;75 Ž. 3 :151321. In this article a detailed description of the SNOM setup and detailed analysis of the two-photon excitation mechanism are presented. As a first application topographic and fluorescence images of dye labeled polythene chromosomes of Drosophila melanogaster are shown. The contrast of the fluorescence images is outstanding due to the low background noise. 69 Hell SW, Booth M, Wilms S et al. Two-photon near- and far-field fluorescence microscopy with continuous-wave excitation. Opt Lett 1998;23Ž 15.:12381240. 70 Garciaparajo MF, Veerman JA, Vannoort SJT, Degrooth BG, Greve J, Vanhulst NF. Near-field optical microscopy for DNA studies at the single molecular level. Bioimaging 1998;6Ž. 1 :4353. In this article topographic images of double stranded DNA fragments deposited on mica are presented that are recorded using shear-force distance control. Although the average diameter is less than 15 A the strands are clearly visible and no distortions are found from the fiber tip. Single rhodamine molecules attached to the strands can easily be identified on the simultaneously recorded fluorescence images. This paper demonstrates very clearly the usefulness of SNOM for fundamental studies of biological probes on a single molecular level. However, it is also emphasized, that the instrument has to be designed very carefully to obtain pictures of this outstanding quality.