Scanning near-field optical microscopy
Introduction
The principal idea of scanning near-field optical microscopy (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 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 (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 (usually from Ar-laser) into the free end of the fiber. Typical diameters of the apertures obtained by this technique are in the range of 50–100 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]. 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 is describing 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.
Section snippets
Technical aspects
Many of the papers describing instrumental innovations have appeared in the special issues of Ultramicroscopy [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
Probe fabrication
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 LED’s [17] into the tip. The most interesting development towards this direction is given by Heisig
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 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 [23], [24], [25]
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
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 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.
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.
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
Langmuir-Blodgett and self-assembled films
Two-dimensional phase transitions of Langmuir monolayers of phospholipids at the air/water 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 behavior on a sub-micron scale. As an example, Dunn et al. have studied the coexistence of the liquid expanded and the condensed phase (LE/LC) of l-alpha dipalmitoylphosphatidylcholine
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
Conclusions
The many papers published on technical issues 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
Acknowledgements
I want to thank all the colleagues who have sent me their recent reprints. I am also grateful to H. Nöhwald for his generous support.
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