S. Negm

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.

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.

Photomodulation raman spectroscopy as a pump-probe technique for surfaces and interfaces of semiconductors

Photomodulation Raman spectroscopy (PM-RS) has been employed to study the surface depletion electric field Es and to monitor the change in surface charge density in n-type GaAs, using the forbidden LO phonon scattering for low doping samples and coupled plasmon–LO phonon modes for high doping samples. In PM-RS, the photomodulating pumping beam (PB) is incident on the sample while the Raman measurements are in progress hence PM-RS can be viewed as a pump-probe technique. The photogenerated carriers partly neutralize the surface charges. Two different GaAs surfaces (011) with low and moderate doping density and (001) with high doping density were used. The total surface charge density has been obtained as a function of the PB intensity considering a constant depletion electric field for the lower doping sample of (011) surface and using the dependence of the unscreened LO phonon on the depletion width for the higher doping samples of (001) surface. The minority carriers lifetime was also determined through dynamical measurements for the PM-RS of the low doping sample as ≈ 21 s, in a good agreement with other techniques

Photoacoustic measurements of excitons in cdse nanorods

Photoacoustic (PA) spectra were obtained for CdSe nanorods (NRs) of different aspect ratios, prepared via the organometallic synthesis. The second derivative spectra were used to have an accurate determination of the different excitonic transitions. Using the lowest transition energy (band gap) 1S(e)-1S1/2(h) and applying the effective mass model, the NRs diameters were determined. The obtained diameters were then compared to direct measurements of scanning tunneling microscopy (STM) and XRD. It is observed that the band gap depends on the diameter of the rods due to quantum confinement effect, since diameters are of the order of the bulk CdSe Bohr radius. The second derivative of the PA spectra for CdSe NRs also shows clearly a second excitonic transition1P(e)-1P1/2(h) in contrast to UV-Vis absorption spectra carried out for colloidal samples. The thermal parameters for samples were also measured and compared to the bulk values.

Use Of Surface Enhanced Raman Scattering To Characterize Metal Substrate

It is proposed that surface enhanced Raman scattering (SERS) of a monolayer of a dye deposited on a rough metal surface can be used to characterize the film roughness. The advantage of these techniques is the use of a very sensitive methods (Raman scattering signal) to measure microscopic roughness in metal substrate. The Raman enhancement is due to the amplification by the Ag core of the electric field associated with the driven modes of the dye layer. In the present case the due is crystal violet (CV) and the measured enhancements are calculated to be of the order of 104 to 106. Since, the enhancement of the Raman spectra depends on the structure of the metal island film and specially the roughness, this effect can be applied to investigate the surface roughness of the substrate and characterize its structure.