A method called tube etching for the fabrication of near-field optical probes is presented. Tip formation occurs inside a cylindrical cavity formed by the polymer coating of an optical fiber which is not stripped away prior to etching in hydrofluoric acid. The influence of temperature, etchant concentration, and fiber type on the tip quality is studied. A tip formation mechanism for the given geometry is proposed. The procedure overcomes drawbacks of the conventional etching techniques while still producing large cone angles: (i) tips with reproducible shapes are formed in a high yield, (ii) the surface roughness on the taper is drastically reduced, and (iii) the tip quality is insensitive to vibrations and temperature fluctuations during the etching process. After aluminum coating, optical probes with well-defined apertures are obtained. Due to the smooth glass surface the aluminum coating is virtually free of pinholes.
We describe an atmospheric pressure nanosampling interface for mass spectrometry based on near-field laser ablation. Pulsed laser radiation is delivered to the sample surface through a near-field optical probe, and the ablation plume is sampled through a capillary orifice and analyzed by standard MS methods. A spatial resolution of less than 200 nm and a sensitivity below 2 amol is demonstrated.
Rough silver surfaces enhance the Raman scattering of adsorbed molecules by many orders of magnitude. The size, shape, and surroundings of the metal particles greatly influence the resulting Raman intensities. With the controlled formation of well-separated silver islands, direct investigation of the local origin of the Raman enhancement is greatly facilitated. We summarize the effects of experimental parameters during and after vapor deposition of silver on the resulting surface morphology. With the selection of suitable experimental conditions, particle sizes and shapes, as well as the interparticle distances, can be controlled so that well-separated silver islands are formed that still show high Raman enhancement. The surfaces were characterized morphologically by means of atomic force microscopy (AFM) and scanning electron microscopy (SEM). Optical characterization was carried out by using ultraviolet-visible and Raman spectroscopy.
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