Surface Enhanced Raman Spectroscopy (SERS) is a valuable analytical tool for the investigation of molecules adsorbed on roughened noble metal surfaces. The shape, size, and surrounding of the metal protrusions play an important role in the Raman scattering enhancement. By combining scanning near-field optical microscopy (SNOM) with Raman spectroscopy the spatial resolution suffices for investigating isolated silver islands on SERS active substrates. We demonstrate an optical resolution below 70 nm for recording spectra on specifically prepared and fully characterized SERS substrates. For a quantitative evaluation of the SERS signal the spatial distribution of Rhodamine 6G (R6G) deposited on the SERS substrate was determined by friction force measurements. By comparing the Raman intensities of the SERS substrates with those of unmetallized support plates absolute SERS enhancement factors at specific locations on top and in the vicinity of the silver islands were determined directly.

Scanning near-field optical microscopy (SNOM) and Raman scattering were combined to obtain subwavelength molecular resolution. Near-field microscopy allows to overcome the diffraction limit valid for all lens or mirror based optical instruments and lateral resolutions well below 100 nm have been claimed. Raman spectroscopy yields information on the vibrational states of a molecule and therefore allows to distinguish between different chemical compounds easily. This is an advantage to the more widely used near-field fluorescence microscopy. To enhance the notoriously weak near-field Raman signal the sample was brought onto a Raman enhancing surface (silver coated Teflon nanospheres). Additionally brilliant cresyl blue (BCB) acted as a Raman label for our DNA samples. On such samples Raman images with a resolution better than 100 nm have been obtained. A single near-field SERS spectrum was measured in similar to 60 seconds. The acquisition time currently depends critically on the transmission of the near-field probes. Nevertheless, it is possible to measure whole nearfield Raman images in a reasonable time. From these Raman images a preliminary distinction of different constitutions of adsorbed molecules can be done.

We report the combined use of scanning near-field optical microscopy and surface-enhanced Raman scattering (SERS) to obtain spectral, spatial and chemical information of molecular adsorbates with subwavelength lateral resolution. Near-field SERS spectra of cresyl fast violet and rhodamine 6G on silver substrates are obtained with short exposure times (less than 100 s) and a signal-to-noise ratio of >20. Spectra from as few as ≈300 molecules or less than 10−2 monolayers adsorbed on about ten silver nanoparticles can be recorded using 200 nm tip apertures. For the near-field spectra, a local Raman enhancement factor of 1013 or greater can be derived from a comparison with fluorescence measurements.

Scanning probe microscopy has the potential for investigating membranes in a physiological environment. We prepared with a lysis-squirting protocol basal cell membranes, that are suitable for scanning probe microscopy. Investigations using atomic force microscopy under liquid revealed cellular filaments which correlated perfectly with fluorescently stained actin filaments. Globular structures with a diameter as little as 10 nm could be resolved by stripping cytoplasmic components from the membranes. Therefore, cytoplasmic sides of supported basal cell membranes prove useful to gain high resolution with scanning probe microscopy in studies of plasma membrane associated structures and processes under buffer solution.

Silver island films prepared by pulsed laser deposition were tested as substrates for surface enhanced raman spectroscopy. They exhibit a position-dependent thickness and SERS activity distribution. Cresyl Violet and Crystal Violet were chosen as test molecules. The annealing of the films leads to coalescence of the initial island structures and the development of longer ellipsoidal particles, which generally improves the enhancement properties of the substrates. The dimensions of the surface structures depend on the local initial film thickness. The changes in tbe island shape and dimensions manifest themselves in changes in the absorption spectra. (C) 1998 John Wiley & Sons, Ltd.

Monolayers of poly(ethylene glycol) 1500 were grown by dispersing and drying a dilute solution of the polymer on a silica substrate. Atomic force imaging of these thin films reveals self-assembly of vertically oriented polymer chains that eventually leads to dendritic structures on a mesoscopic length scale. Thermal annealing and desorption of such structures were investigated under ultrahigh vacuum using X-ray photoelectron spectroscopy and temperature-programmed desorption. The carbon 1s signal of the polymer starts to decrease before the onset of desorption. This observation can be rationalized by structural reordering of the polymer chains on the surface. Atomic force microscopy images prove that the decrease of the characteristic X-ray photoelectron spectroscopy signal at low temperatures is correlated with the transition of the two-dimensional dendritic structures to three-dimensional microdroplets of the polymer on the surface. As the temperature increases, this surface dewetting process is followed by dissociative desorption of the polymer, shrinking of the microdroplets, and finally complete depletion of the surface.

Raman chemical imaging on a scale of 100 nm is demonstrated for the first time. This is made possible by the combination of scanning near-field optical microscopy (SNOM or NSOM) and surface-enhanced Raman scattering (SERS), using brilliant cresyl blue (BCB)-labeled DNA as a sample. SERS substrates were produced by evaporating silver layers on Teflon nanospheres. The near-field SERS spectra were measured with an exposure time of 60 s and yielded good signal-to-noise ratios (25:1). The distinction between reflected light from the excitation laser and Raman scattered light allows the local sample reflectivity to be separated from the signal of the adsorbed DNA molecules. This is of general importance to correct for topographic coupling that often occurs in near-field optical imaging. The presented data show a lateral dependence of the Raman signals that points to special surface sites with particularly high SERS enhancement.

Scanning near-field optical microscopy (SNOM) is an optical microscopy whose resolution is not bound to the diffraction limit. It provides chemical information based upon spectral, polarization and/or fluorescence contrast images. Details as small as 20 nm can be recognized. Photophysical and photochemical effects can be studied with SNOM on a similar scale. This article reviews a good deal of the experimental and theoretical work on SNOM in Switzerland.

Optical tiber tips for scanning near-held optical microscopy (SNOM or NSOM) produced by etching with the protection layer method and subsequent metalization show an optical transmission of up to 0.5%. This throughput is 2-4 orders of magnitude higher than that of conventional pulled fiber tips. The high light transmission permits SNOM-based surface analytical and spectroscopic applications with high spatial resolution (<100 nm) and a high chemical information content. Fluorescence imaging of dye-labeled polystyrene spheres is demonstrated. Fast and irreversible photobleaching is shown to take place in the near held with enhanced efficiency. Surface-enhanced Raman spectra of cresol fast violet and p-aminobenzoic acid on a silver substrate are obtained in the optical near held with a signal-to-noise ratio of >10. Rapid heating of chemically etched probes with pulsed laser radiation can be used for thermal desorption of molecules from organic crystals and for modification of polymer surfaces. A lateral resolution of as little as 75 mn fwhm is achieved in the latter experiments.

Laser-induced desorption on a nanometer scale using scanning near-field optical microscopy (SNOM) is demonstrated. Pulsed laser-induced desorption of anthracene with completely metallized SNOM fiber tips resulted in a lateral resolution of 70 nm. This became possible due to the use of chemically etched tips with a taper region smaller than 200 μm and cone angles varying from 6 to 30 degrees. These tips are further useful for optical imaging with a very high optical transmission coefficient (up to 10−3), achieving a lateral resolution of 80 nm.

Scanning near-field microscopy (SNOM or NSOM) is a versatile and attractive scanning probe technique for imaging with subdiffraction-limited spatial resolution using visible light. At least three different types of images can be recorded simultaneously of the selected sample area, such as the topography, the near-field optical transmission, and the fluorescence from excited chromophores. We have built such a microscope, especially designed for achieving the high resolution and the sensitivity needed for single molecule detection. We report on optical near-field investigations of surface structures and thin polymer films that are doped with fluorescent dye molecules. The effective aperture diameters of the fiber tips used in the SNOM experiments were determined by a photon-scanning tunneling microscope (PSTM) giving values between 70 and 160 nm, The transmission imaging of transparent polymer phase gratings reveals the existence of different contrast mechanisms, which are either based on the inherent distance dependence of the optical near field or on the periodic change of boundary conditions for the electric field component of the light between the aperture and the sample. Furthermore, we demonstrate selective irreversible photobleaching of dye molecules at moderate concentration (10(-5) M) induced locally by the subwavelength-sized probe tip. Finally, we present fluorescence images showing single molecule detection in a thin solid film. The chromophores (rhodamine 6G) were embedded at low concentration (10(-7) M) in a 25-nm thin polyvinylbutyral film, A lateral resolution of 160 nm was achieved. We find that the signal strengths of the brightest fluorescent features vary considerably in a sequence of images (a typical single-molecule behavior), whereas the fluorescence background exhibits the usual photobleaching behavior of a large ensemble.

A scanning near-field optical microscope (SNOM) for fluorescence imaging of single molecules requires efficient optical signal detection and background rejection combined with long-term stability and high spatial resolution. These requirements are dictated by the extremely low fluorescence signal of an individual dye molecule. We have built a SNOM that meets these requirements by combining a rigid and versatile near-field optical scanner with confocal detection optics. The relevant design parameters are discussed in detail. The near-field part of the microscope is based on a commercially available fiber aligner for coarse approaching the sample with respect to the tip. It also permits us to select a specific sample area in a range of 3 mm×3 mm with a nominal resolution of 10 nm. The tip–sample separation is probed by shear-force detection using a fiber-optical interferometer, which gives an excellent signal-to-noise ratio. The high versatility of this SNOM is demonstrated with measurements of a transparent dielectric grating and by imaging the fluorescence from individual rhodamine-6G molecules with a spatial resolution of ≊160 nm.