H. Talaat

Micro-Raman analysis of polysilicon membranes deposited on porous silicon channels

Abstract Local stress distribution is determined in polycrystalline silicon using micro-Raman spectroscopy. The microstructures are thin membranes over a thick porous silicon channel locally grown on bulk silicon. The effect of the width of the membrane on the distribution of the local stress is measured. The results show that annealing reduces the residual stress by an order of magnitude and that the tensile stress in the membrane has a sinusoidal dependence for the relatively narrow channels.

Thermal properties of hydrated cement pastes studied by the photoacoustic technique

Photoacoustic (PA) technique has been applied to measure the effective thermal diffusivity (αeff) of hydrating cement pastes with a varying water to -cement ratio (w/c) and for variable duration (d) of hydration. Four samples with w/c = 0.3, 0.4, 0.5 and o.6 were prepared. The frequency variation of the PA signal for each sample was recorded at the begining (0 d), as well as one week and one month of hydration. The effective thermal effusivity (eeff) was obtained by measuring the variation of the signal with modulation frequency and the corresponding values of the effective thermal conductivity (keff) were calculated. The results for keff show a decrease at higher w/c (0.6), no change for other samples has been observed. The thickness of the duplex film of Ca(OH)2 and C-S-H formed on the surface of the samples of w/c = 0.5 were determined using the effective layer model in the 0 d and after one month of hydration; a remarkable increase was observed in the last case.

Determination of thermophysical parameters of porous silicon using a photothermal technique

The effective thermophysical properties, diffusivity (α), effusivity (e), and thermal conductivity (k) have been measured by photoacoustic (PA) and photothermal interferometry (PTI) for silicon samples of varying porosities. The experimental results show a reduction in the thermal conductivity with increasing the porosity, by two orders of magnitude. The results obtained by PTI are compared to those obtained by the PA technique.

Radiative and non-radiative decay of surface plasmons in thin metal films

A comparative study of surface plasmon polaritons (SPP) on thin Ag films of varying intrinsic (as evaporated) surface roughness was carried out using three well known complementary spectroscopies; the Attenuated Total Reflection (ATR), the Forward Scattering (FS) and the Photoacoustic Attenuated Total Reflection (PA-ATR). The differences in the resonance coupling halfwidth obtained in the three methods, have been analyzed to account for the different decay processes involved in each spectroscopy. An estimate for the attenuation length of the multiple scattering and directional scattering processes is obtained.

Laser Induced Potential Energy Surface Crossing in Ion-Molecule Reactions: Application to Li++CH4

Abstract Laser ion-molecule reaction interaction through long range dipole moments leads to potential energy surface crossings. We will show here by using gauge representation (electric field gauge) for wave length λ = 20.6 μm, Intensity I = 5×1012 W/cm2, I = 1×1013 W/cm2, I = 3×1013 W/cm2, that we can create laser induced potential energy surface crossing in the virtual transition state region (4.a.u.) where the dipole moment changes sign. We illustrate such effects for the Li H + CH3+ ↔ Li+ + CH4 reaction which takes the form of inverted Morse (without a barrier) using ab-initio methods for calculating the reaction path and the permanent dipole moment of this ion-molecule reaction.

Raman microprobe analysis of strained polysilicon deposited layers

Abstract Microscopic strain distributions in varying thicknesses of polysilicon layers deposited as bridges over silicon substrates were determined by high resolution micro-Raman spectroscopy. In particular, we mapped the dependence of the first order Raman spectrum as a function of the position across polysilicon bridges (approximately 550 × 230 μm2), over tunnels etched in the silicon substrate. Shifts in Raman band frequencies as a function of position on the bridge structures were observed to be dependent upon the thickness of the membrane layer (between 1 and 3 μm) as well as the annealing conditions. Assuming a simplified uni-axial stress parallel to the surface, the effective tensile stress at the center of the bridge of thickness 2.5 μm is reduced by annealing from 2.5 GPa to 0.18 GPa.

Effect of intrinsic surface roughness and other decay processes on surface plasmon polariton resonance halfwidth

A comparative study of surface plasmon polaritons (SPPs) on thin Ag films of varying intrinsic surface roughness is conducted using three complementary spectroscopies: the attenuated total reflection (ATR), the forward scattering (FS) and the photoacoustic attenuated total reflection (PA-ATR). The differences in the resonance coupling obtained in the three methods, particularly the resonance halfwidth, are analyzed to account for the decay processes involved in each spectroscopy. The role played by surface roughness interaction in either decreasing or increasing the surface plasmon polariton wavevector is discussed. The PA-ATR is employed to obtain an estimate of the contribution of the multiple scattering and directional scattering processes to the total attenuation of the SPP.<<ETX>>

Quantum dynamics of heavy light heavy reactions: Application to (F + CH4 → FCH3 + H) reaction

We apply the time-dependent theory to the collinear exchange reaction F + CH4 → FCH3 + H. We have performed detailed calculations on two-dimensional potential surfaces representing the ground electronic potentials of the collinear F + CH4 → FCH3 + H reaction, at incident energies. Transmission coefficients range from zero to unity, depending upon the incident energy. Normal modes of vibrations are displayed along the reaction path.

Photomodulation of the coupled plasmon-LO phonon of GaAs surfaces

Photomodulation of the coupled plasmon–LO phonon modes has been employed to determine the change in both the surface charge density and the depletion electric field as a function of photomodulation beam (PMB) intensity. The samples are two pieces of highly doped (001) n-type GaAs. The total surface charge density has been obtained as a function of the photomodulating intensity using the dependence of the unscreened LO phonon on the depletion width. We are able to separate the impact of the PMB on the surface electric field from the impact on the depletion width. This allows a separate determination of the change in depletion electric field, which reaches ~73% of its original value at the highest intensity used for PMB.

Characterization of core/shell (Ag/CdSe) nanostructure using photoacoustic spectroscopy

Photoacoustic (PA) technique has been used to study the optical and thermal properties of core/shell (Ag/CdSe) nanostructure. Core shell Ag/CdSe nanostructure particles were prepared using organometalic method, having a core of (12 -14 nm) and CdSe shell thickness ranging from (2.59 nm to 5 nm) as determined by TEM. The PA spectra were compared with regular UV-Vis absorption which gave comparable results, though the UV-Vis is for colloidal form and the PA spectra is for powder form. The obtained spectra are combination of the surface plasmon (SP) absorption bands of the Ag core and the band gap of CdSe shell. Second derivative fitting method and Gaussian peak fitting were used to determine precise absorption peaks and band of the PA spectra. The SP bands of the Ag core show a decrease in amplitude, broadening of the width and red shift as the CdSe shell thickness increases. The CdSe absorption band edge also shows an increasing red shift from (517 nm to 604) nm with the increases in the thickness. The thermal diffusivity of the Ag/CdSe core shell samples increase by an order of magnitude larger than the CdSe bulk value which is caused by Ag core.