Spherical aberration is probably the most important factor limiting the practical performance of a confocal Raman microscope. This paper suggests some simple samples that can be readily fabricated in any laboratory to test the performance of a confocal Raman microscope under realistic operating conditions (i.e., a deeply buried interface, rather than the often-selected alternative of a bare silicon wafer or a thin film in air). The samples chosen were silicon wafers buried beneath transparent polymeric or glass overlayers, and a polymer laminate buried beneath a cover glass. These samples were used to compare the performance of three types of objectives (metallurgical, oil immersion, and dry corrected) in terms of depth resolution and signal throughput. The oil immersion objective gave the best depth resolution and intensity, followed by a dry corrected (60x, 0.9 numerical aperture) objective. The 100x metallurgical objective was the worst choice, with degradations of approximately 5x and 8x in the depth resolution and signal from a silicon wafer, comparing a bare wafer with one buried under a 150 microm cover glass. In particular, the high signal level obtained makes the immersion objective an attractive choice. Results from the buried laminate were even more impressive; a 30x improvement in spectral contrast was obtained using the oil immersion objective to analyze a thin (19 microm) coating on a PET substrate, buried beneath a 150 microm cover glass, compared with the metallurgical objective.

Raman spectroscopy is steadily growing in importance in the industrial analytical laboratory. The nature of the equipment, its ease of use and its cost are making the method more acceptable to non-experts. This paper illustrates the capabilities of a recent commercialized Raman system which fulfils the criteria of ease of use, speed and low cost. The system is designed for Raman microscopy and uses a single spectrograph and CCD detector. The combination provides confocal microscopy, high throughput and optimum sensitivity, to the extent that it only requires the use of a low-powered laser to provide high-quality spectral data. The additional feature of direct Raman imaging is seen as being beneficial in the industrial environment in that it provides spatial information over large surface areas quickly and without the need for excessive amounts of data processing. The performance of the instrument is illustrated with applications taken from the industrial environment.

Raman microscopy has been attractive because of its ability to characterize materials on a spatial scale commensurate with optical microscopy. Typically the lateral spatial resolution is quoted as determined by the Airy disc[1] which is 1.22λ/NA where λ is the wavelength of the illuminating light, and NA is the numerical aperture which is equal to nsinθ, where n is the index of refraction of the medium (1.0 in the case of air) and  is the angle subtended by the optics. However, the Airy disc description cannot be correct for a Raman microscope. The Airy disc assumes uniform illumination of the focusing optic, and the laser profile is anything but. In addition, in some instruments the Gaussian laser profile is not well matched to the aperture of the focusing objective. At any rate, this article is going to concentrate on the depth resolution of the Raman microscope. Optical calculations for depth resolution of an optical microscope state that the it is proportional to λ/(NA) 2 . The essential point to recognize is that the spatial resolution of any Raman microscope depends on the detection optics as well as the focusing optics. How effectively does the optical system collect the Raman signal excited in the laser focal spot, and reject the signal from the surrounding volume that is illuminated by the laser but not in focus?Then the essential question becomes how to evaluate the depth resolution experimentally. Historically people have used a piece of a polished Si wafer for these tests. But this material was originally chosen more for the repeatability of any measurement of its signal rather than its appropriateness for answering the questions of depth resolution. The problem with using silicon is that when performing a depth profile, as the sample surface is moved away from the focal plane, the laser-illuminated area is increased; the final signal is a convolution of the losses because the laser illuminated area is not passed efficiently through the confocal hole, and the increase in signal because of the increased excitation volume. In addition, the depth of penetration of the laser into the crystal is not necessarily negligible. At 633nm, it will penetrate 3µm, at 785nm 12 µm. In trying to determine a better way to determine the confocal properties of a Raman system microscopic polymer beads were selected. With such a sample, when the sample is defocused, the Raman volume cannot be larger than the volume of the bead. Comparison of depth profiles of 2µm and 0.5µm beads and silicon will be shown to provide insight into the confocal behavior of the Raman microscope. These measurements are done using the 532nm excitation wavelength whose depth of penetration into silicon is about 0.7µm. This avoids the complications of volume effects. Figure 1 shows depth profiles of the 2µm and 0.5µm spheres of polystyrene recorded while varying the confocal hole. As the confocal hole is increased, the signal strength increased because light from more of the bead volume is transmitted. However, the full 360

N-type multi-walled nanotubes were synthesized by nitrogen doping using pyridine and pyridine-melamine mixtures in chemical vapor deposition, and their donor states were verified by Scanning Tunneling Spectroscopy. Tunneling Electron Microscopy reveals small amounts of residual catalyst and Scanning Electron Microscopy show well aligned mats of the Nitrogen doped nanotubes. Nitrogen is present in the lattice of these MWNTs as pyridine structures and CNx structures. Raman scattering measurements were performed as a function of increasing growth temperature and the results compared to previously studied boron doped multiwalled nanotubes.

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