The preparation of (sub)monolayers of small and short-chain organic molecules on oxide-covered silicon is
described. The molecular end groups and their chemical reactions were characterized by attenuated total
reflection Fourier transform infrared (ATR-FTIR) spectroscopy, spectroscopic ellipsometry (IR-UV), laserinduced
desorption of monolayers (LIDOM), X-ray photoelectron spectroscopy (XPS), and contact-angle
experiments. Surface species were identified and their reactions were monitored by FTIR analysis of
characteristic vibrational modes. This includes bottom-up synthesis of siloxane chains, diverse reactions of
double bonds, and specific molecular transformations such as the Diels-Alder reaction. Layer thicknesses
could be estimated with a sensitivity of ~0.02 nm and accuracy of ~0.05 nm by oxidation of the hydrocarbons.
This was achieved by in situ real-time detection of the corresponding thickness changes by spectroscopic
ellipsometry. From time-of-flight (TOF) experiments, which provided the desorption temperature and mass of
the emitted species, the thermal stability, chemical transformation, and fragmentation pattern of chemisorbed
species could be extracted. To analyze the hydrophilic or hydrophobic nature of functionalized surfaces the
surface energy and wettability were determined.
VUV-laser-induced oxidation of Si(111)-(1×1):H, Si(100):H, and a-Si:H at 157 nm (F2 laser) in pure O2 and pure H2O
atmospheres was studied between 30°C and 250°C. The oxidation process was monitored in real time by spectroscopic
ellipsometry (NIR-UV) and FTIR spectroscopy. The ellipsometric measurements could be simulated with a three-layer
model, providing detailed information on the variation of the suboxide interface with the nature of the silicon substrate
surface. Besides the silicon-dioxide and suboxide layer, a dense, disordered, roughly monolayer thick silicon layer was
included, as found previously by molecular dynamics calculations. The deviations from the classical Deal-Grove
mechanism and the self-limited growth of the ultrathin dioxide layers (<6 nm) are described by different kinetic models
for O2 and H2O. The tailored modification of silicon surfaces by functionalization with organic end groups was studied
by silylation of oxidized silicon surfaces with terminating trimethylsilyl (TMS) groups and n-alkylthiol monolayers on
gold-coated silicon. The C-H stretching vibrations of the methylene and methyl groups could be identified by FTIR
spectroscopy and IR ellipsometry.
In this paper the recent developments in the field of diode-laser photoacoustics will be discussed. Photoacoustic detector designs with high sensitivity and signal-to-noise ratio will be presented. The advantages and disadvantages of different modulation and measurement techniques will be discussed and compared. Simple expressions will be given for an estimation of the sensitivity achievable with an optimized photoacoustic detector. Recent results will be presented for a state-of-the-art dual-resonator differential cell suitable for sensitive trace gas analysis with low electronic and acoustic noise. With this resonant photoacoustic cell and a near-infrared DFB diode laser with 42 mW power at 1.53 mm polar ammonia molecules could be detected with a sensitivity of 200 parts-per-billion volume (ppbv) under flow conditions used to reduce the adsorption problem. By excitation of fundamental vibrations in methane using a pulsed optical parametric oscillator (OPO) with 60 mW output power a sensitivity of 1.2 ppbv has been achieved using this resonant cell. This setup allows sub-ppbv detection of the greenhouse gas methane with a concentration of about 1.7 ppmv in ambient air.
Nondestructive evaluation of elastic properties of advanced materials was performed with short surface acoustic wave (SAW) pulses in the 10 MHz-300 MHz range. The elastic surface pulses were launched thermoelastically with pulsed laser radiation and detected with a piezoelectric foil transducer. This technique was used, for example, to determine the mechanical and elastic properties of superhard materials, such as microcrystalline CVD diamond films and submicrometer thick nanocrystalline films of cubic boron nitride. These results are compared with the properties of the corresponding single-crystal material, In layered systems or graded materials the introduction of a length scale leads to dispersion of the surface acoustic waves (SAWs), which allows the simultaneous determination of several properties, such as the density, Young's modulus, and Poisson ratio. In free-standing polycrystalline CVD diamond plates dispersion of elastic surface pulses was observed. This material is neither isotropic nor homogeneous because the grain size and structure vary from nucleation side to the growth side. In some samples anomalous dispersion of SAWs was observed on the nucleation side with the finer grains. Amorphous SiCxNy films with various compositions were investigated to compare the microscopic bonding characteristics, determined by molecular dynamics (MD) simulations, with the macroscopic mechanical properties obtained by surface acoustic wave spectroscopy (SAWS).
This volume, the last in a four-volume series on Photothermal and Photoacoustic Science and Technology (PPST), presents a comprehensive review of the diverse progress made in PPST of semiconductors and electronic materials during the 1990s. As with other volumes in the series, this text is useful as a reference for practicing scientists and engineers and as a supplement to upper level graduate courses in various areas of PPST and its subfields.
parameters of laser induced tissue ablation. Threshold fluence for ablation is related to some of the optical and thermal properties of the tissue. For this a necessary condition of ablation is imposed that the rate of deposition of laser energy should be greater than the rate of expending energy for vaporization and at threshold they should be equal. A simple expression for resulting depth of the ablation crater is derived assuming total conversion of the optical energy above threshold into kinetic energy of the ablation products. Evaporation that should follow the ablation in the superheated exponential tail in the tissue is also considered for contributing to the crater depth. The assumption of total conversion of optical energy above threshold into kinetic energy of the ablation products is also used to find the average velocity of the ablation products ejected from the tissue. The laser induced ablation is shown to produce a pressure wave which has two components. The first component originates because of the static pressure arising due to the conversion of condensed state of the tissue into a super heated fluid which remained confined to its original volume because of its kinetics and inertia. This component has been worked out using the ideal gas laws. The second component is the recoil pressure of the ejected ablation products which at any instant of time is governed by the instantaneous intensity of the laser beam and it is calculated using Rocket effect. The values of the aforesaid parameters calculated from this model are compared with our and others experimental results and are found to be in agreement with reasonable accuracy.
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