Characterization of optical properties of Porous Silicon using Photoacoustic Technique

Photoacoustic absorption spectra of the porous silicon samples (P-Si) of different thickness and porosity percentage were measured at different modulation frequency. The absorption edge of the P-Si layer for all samples shows a blue shift from that of crystalline silicon (C-Si) at 1.1 eV. At low modulation frequency the estimated energy gap (1.88 eV) is almost the same for all samples and the PA signal increases as the porosity percentage increases. At the higher modulation frequency, the spectra show an increase in the energy gap indicating the effect of quantum confinement as the porosity increasing. The Raman shifts of the P-Si samples are correlated with the particle size leading to an estimated average particle size. The quantum confinement interpretation of the PA results is in agreement with the Raman measurements that indicate the presence of such nanostructure in the P-Si layer

Photoacoustic and electron microscopic studies of reminerlized artificially carious enamel and dentin

Photoacoustic (PA) spectroscopy and scanning electron microscopy (SEM) have been employed to investigate remineralized artificially carious enamel as well as dentin. The remeniralization process was carried out by applying bioactive glass particles and mineralizing solutions either directly or after the specimen was first treated with Zeolite powder. The PA was carried out to evaluate the change in the thermal parameters (diffusivity, effusivity and conductivity) quantitatively. The remineralization process was evaluated by comparing the thermal parameters to those of the healthy tissue. PA measurements show that; the values of thermal conductivity of the normal tissues (enamel: 0.9 W m$^{-1}$ K$^{-1}$ and dentin: 0.58 W m$^{-1}$ K$^{-1}$) were altered by caries to be (enamel: $\approx$ 0.72 W m$^{-1}$ K$^{-1}$ and dentin: $\approx$ 0.44 W m$^{-1}$ K$^{-1}$) and they approach their normal values after the application of the different mineralizing agents. Also the results indicate that; Bioactive glass enhances the thermal parameters of artificially carious enamel and dentin (considering the healthy values as a reference), slightly more than the mineralizing solution. Furthermore, PA measurements show that the application of Zeolite powder enhances the effect of the mineralizing agent. In order to augment the PA results, SEM was used to depict surface topography, for each treated tissue

Determination of thickness and porosity of porous silicon layer using photoacoustic technique

The thickness of thin porous layers of silicon samples and their varying porosity have been determined using photoacoustic technique (PA). The measured values of the effective thermal diffusivity ($\alpha$$_{\rm eff}$) and effective thermal effusivity (e$_{\rm eff}$) were exploited to determine the thickness of porous silicon (p-Si) film using the effective layer model. Also the determined $\alpha$$_{\rm eff}$ together with the two-layer model were used to obtain the thermal diffusivity of the p-Si layer only. Using Maxwell- Rayleigh model, the porosity percentage for the different samples were determined and compared to the results obtained by scanning electron microscope (SEM) with 10 % variations

Photoacoustic characterization of optical and thermal properties of CdSe quantum dots

We report on the optical absorption properties of as prepared CdSe quantum dots (QDs) measured by the photoacoustic (PA) method. CdSe QDs were fabricated by the chemical solution deposition (CD) technique. With increasing growing time, the redshift of the PA spectra can be clearly observed and optical absorption in the visible region due to CdSe Q-dots is demonstrated. The average diameters of the CdSe QDs for each growth time interval is estimated using the effective mass approximation giving diameters ranging from 2.6 nm to 3.4 nm. These values are comparable to those obtained by scanning tunnelling microscope (STM). Thus, PA spectroscopy is useful to obtain the QDs sizes as grown and with no further preparation. In addition, PA measurements provide also the thermal diffusivity of samples of different sizes which in this case show an increase by at least an order of magnitude than the bulk value

Photoacoustic spectroscopy characterization of CdSe quantum rods

The optical absorption properties of as prepared CdSe quantum rods (QRs) are measured by the photoacoustic (PA) method. CdSe QRs were fabricated by the chemical solution deposition (CD) technique. Decreasing the rod size (width and length) that is determined by scanning tunneling microscope (STM), causes blue-shift of the PA spectra that can be clearly observed. The exciton energy of the CdSe QRs as estimated using the effective mass approximation (EMA) model and the dimensions determined by STM shows a reasonable agreement with that predicted by the PA measurements. The exciton energies calculated by EMA model show deviations from the experimental data for samples having smaller diameters. The thermal diffusivity of samples of different sizes shows an increase by at least an order of magnitude than the bulk value

Shape and size dependence of the surface plasmon resonance of gold nanoparticles studied by Photoacoustic technique

We report on the optical absorption properties of as prepared gold naoparticles of different shapes and sizes measured by photoacoustic (PA) method. The gold nanoparticles of two different shapes (dots, rods) have been prepared using the seed mediated growth method. The shape and the size of these different nanoparticles were determined by STM measurements. PA spectra show the splitting of the surface plasmon resonance (SPR) into two modes (transverse and longitudinal) in case of gold nanorods. The increase in the aspect ratio of the nanorods leads to clear redshifts of the longitudinal SPR. These shifts were used to determine the dielectric constant of the surrounding medium and its variation with the aspect ratios