Evaluation of activation parameters of molecular mobility of parylene C using differential scanning calorimetry, dielectric spectroscopy, and thermally stimulated depolarization currents.

The molecular kinetics of α-relaxation were studied by dielectric spectroscopy (DS) and thermally stimulated depolarization currents (TSDC) for the thermoplastic semicrystalline parylene C (-H2C-C6H3Cl-CH2-)n thin film. Activation energy of the α-relaxation process brings very close values with both methods (155 ± 2 kJ mol(-1)). Dielectric spectroscopy carried out in a wide temperature range allowed one to determine the α-relaxation time against the temperature and then to estimate the Kauzmann and Angell temperatures for parylene C. From these temperatures and the determination of kinetics and thermodynamics parameters from DSC analysis, it was possible to investigate the cooperative rearranging region (CRR) in parylene C. CRR is quantified in terms of characteristic length (ζCRR), volume (VCRR), number z of rearranging units, and height barrier Δµ of the rearranging unit. Moreover, the fragility index m, calculated from these three methods, is discussed. A value of m = 94 is obtained by DSC confirming a "strong" character of parylene C. A lowest value of 35 ± 4 obtained from electric analyses (DS and TSDC) must be considered with caution because these methods of characterizations bring into play only dipolar relaxations. Last, the kinetic and thermodynamic parameters for parylene C highlighted in this work are positioned in comparison with other organic and inorganic materials.

Morphology and microstructure at different scales of porous silicon prepared by a nonconventional technique.

In this work we present first results concerning the detailed structure of porous silicon (PS) layers prepared by a new method using a vapour-etching (VE)-based technique. Studies of the photoluminescence properties of VE-based PS show that the visible emission occurs at high energies as compared with PS prepared by conventional techniques. To understand the VE-based PS features, we need to point out the PS microstructure throughout its general morphology. For this purpose a microscopy multiscale study was done. Scanning, conventional transmission, and high-resolution transmission electron microscopes were employed. The investigations were made on PS films prepared from moderately and heavily doped n- and p-type silicon. SEM images show that VE-based PS layers are essentially formed of clusters like interconnected structures. TEM studies show that these clusters are composed of nanocrystallites with different shapes. The effect of the doping type of the starting Si substrate on the characteristics of the PS layers was examined (thickness, porosity, behavior). Pore propagation was found to depend on doping type. The crystallinity of the PS layers was also locally studied in depth.