Positron annihilation spectroscopy formation

Numerous applications of positron annihilation spectroscopy (PAS) in investigations of the physico-chemical properties of matter require a precise understanding of the process of Ps formation. Usually it proceeds on a picosecond time scale and is strongly influenced by early (pico- and femtosecond) processes initiated by ionizations in the track of a fast positron. These early intratrack processes initiate all subsequent chemical transformations and, consequently, take a key position in radiation chemistry. 

Since positron annihilation spectroscopy is highly sensitive to atomic defects in solid materials, positron annihilation experiments have been carried out extensively on silicon (Si) and silicon dioxide (Si02), both of which are extremely important for the microelectronic device industry. While several reviews are available [1], those reviews are mainly focused on positron (not positronium) annihilation behavior because positronium (Ps) formation dose not occur in bulk crystalline Si. Recent positron annihilation experimental studies revealed that Ps formation occurs in some Si-based thin films, such as porous Si and hydrogenated amorphous Si furthermore, Ps formation is dominant in high-purity amorphous Si02 thin films. In this chapter, Ps annihilation characteristics in Si and Si02 thin films will be discussed from the experimental point of view. 

For future high-speed microelectronic devices, copper interconnection with low dielectric constant (low-k) interlayer films is required to decrease RC (R interconnect resistance, C interlayer dielectric capacitance) delay. Recently, porous Si02 and silica-based films, developed for low-k films, have been extensively studied by positron annihilation spectroscopy [28], [29], [19]. Since Ps formation occurs with high probability, and the o-Ps annihilate via pick-off process in Si02-based materials, positron annihilation spectroscopy (especially PALS) gives useful information on the size of the pores. 

Positronium formation and annihilation behavior in Si and Si02 thin films are reviewed. Positronium is highly sensitive to pore (or void) sizes, surface properties of pores, defects near pore surfaces, etc., in various Si and Si02 samples. Therefore, not only positron annihilation spectroscopy but also positronium annihilation spectroscopy is useful for characterization of Si and Si02 materials. 

Positron annihilation spectroscopy (PAS) is an excellent technique for investigating vacancy clusters and vacancy—solute complexes behavior during irradiation since positrons are very sensitive to these types of defects. These defects are important for the formation of the feamres responsible for hardening. In this technique, positrons are applied as a probe and positrons are trapped by defects with electron densities different from the bulk materials. These defects can be vacancies, vacancy clusters, interfaces, second-phase particles, dislocations, etc. [69]. Positrons annihilate with a different probability in the defects as compared to the bulk material because of the difference in positron affinity to different atomic species [69]. The advantage of the technique lies in its nondestructiveness, self-seeking nature, and ability to find small defects (>0.1 nm) even in low concentrations (>1 ppm) [69]. PAS can provide information... 

Hikmet RAM, Boots HMJ, Michielsen M (1995) Ferroelectric liquid crystal gels—network stabilized ferroelectric display. Liq Cryst 19 65-74 Inoue T, Higuchi N, Fume H (2008) The effect of pol5mer doping on the formation of helical stmcture in ferroelectric liquid crystals. Ferroelectrics 364 113-120 Jean YC (1990) Positron annihilation spectroscopy for chemical analysis a novel probe for microstmctural analysis of polymers. Microchem J 42 72-102 Jean YC, Nakanishi H, Hao LY, Sandreczki TC (1990) Anisotropy of free-volume hole dimensions in polymers probed by positron annihilation spectroscopy. Phys Rev B 42 9705-9708 Jean YC, Mallon PE, Schrader DM (2003) Principles and application of positron and positronium chemistry. World Scientific, Singapore... 

The micelle formation process and structure can be described by thermodynamic functions (AG°mjc, AH°mjc, AS°mic), physical parameters (surface tension, conductivity, refractive index) or by using techniques such NMR spectroscopy, fluorescence spectroscopy, small-angle neutron scattering and positron annihilation. Experimental data show that the dependence of the aggregate nature, whether normal or reverse micelle is formed, depends on the dielectric constant of the medium (Das et al., 1992 Gon and Kumar, 1996 Kertes and Gutman, 1976 Ward and du Reau, 1993). The thermodynamic functions for micellization of some surfactants are presented in Table 1.1. 

The aim of this chapter is to introduce the reader to the application of positron annihilation techniques to polymers. An extensive review of the large volume of publications related to positron studies in polymers will not be presented. Rather it is intented to introduce the reader to the theory and techniques used in polymer studies and indicate the types of information that can be obtained about different polymer systems. The main focus of this chapter will be on the use of positron annihilation lifetime spectroscopy (PAL) in polymer studies. Chapter 11 discusses the use of monoenergetic slow positron beams used to study polymers surfaces. One of the interesting new developments in the application of positron annihilation techniques in polymers is the positron age-momentum correlation technique (AMOC). This technique promises to shed new light on the mechanisms of positronium formation and annihilation in polymer systems. A more detailed discussion of this technique can be found elswhere in this text. 

Two series of cellulose samples, Avicel and Whatman CFll cellulose ball-milled powders with different crystallinity are studied below Tg temperature by using positron annihilation lifetime spectroscopy. A good correlation is found between ortho-positronium formation probability and crystallinity as measured by Fourier transform -infrared spectroscopy. Sub-nanometer hole distributions are found to be narrowed as a function of milling time. These are interpreted in terms of microstructural changes of cellulose. 

Another possible approach to indirectly characterize the membrane morphology is based on the investigation of the free volume within the matrix. Density measurements [119,120] and positron annihilation lifetime spectroscopy evaluation [47] are common methods. Typically, the comparison between the theoretical density or free volume (calculated by simple additivity rules) and the experimental one can reveal the presence of a good interfacial morphology or the presence of interface voids or clustering formation. Fig. 7.13 shows the influence of filler content on the morphology of poly(trimethylsilyl propyne) (PTMSP)/Ti02 NCMs in terms of the volumetric fraction of interface voids as calculated from a comparison of the expected and measured membrane density [119],... 

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