Positron annihilation and infrared spectroscopy studies of porous silicon

Abstract

Measurements of the angular correlation of annihilation radiation and infrared absorption spectra were conducted with porous silicon samples, containing capillary macropores with a diameter of about 1 µm. The set of data shows that a high proportion of Si -O bonds contribute to positron annihilation and IR absorption for porous silicon. Annihilation parameters and estimated values of the specific surface area point to the availability of a nanoporous system in the macroporous silicon. Most likely the macropore surface is covered by the nanoporous material to a thickness of 100 -200 nm. The characteristic size of the nanopores is estimated at 1 -2 nm. Si -O bonds are located at the nanopore surface and do not exceed one monolayer.1 Introduction The positron diagnostics of structural defects in solids is based on the phenomenon of positron capture by vacancy type voids, which are defined as energy wells [1]. As a consequence of the reduced electron density, the lifetime of captured positrons is longer in comparison with bulk annihilation. Other annihilation characteristics also vary: the angular and energy distributions of annihilation quanta become narrower for localized positrons. Positronium atoms (Ps) are generated in many substances. They have two spin states: singlet p-Ps (S = 0, statistical weight 2S + 1 = 1) and triplet o-Ps (S = 1, 2S + 1 = 3). A p-Ps self-annihilation has lifetime τ p = 0.125 ns and gives the so-called narrow component in angular correlation of annihilahion radiation (ACAR). If p-Ps atoms are free, the full width at half-maximum (FWHM) of this component is determined by apparatus resolution and, as a rule, is not more than 1 mrad. For captured p-Ps atoms, broadening of the narrow component takes place, and in this case the FWHM is determined by void size. Bulk annihilation of o-Ps atoms produces a lifetime component of 1-2 ns and, as well as positrons, a broad component (FWHM = 10 -15 mrad) in ACAR. However, the lifetime of o-Ps atoms captured in voids depends on the void size and may be as much as several tens of nanoseconds.The first measurements of positron lifetime in porous silicon [2] displayed the intermediate component of ~0.5 ns, while there is a single one of τ = 0.22 ns in defectless silicon crystals. The subsequent investigations [3 -6] revealed the presence of very long positron lifetime components (up to ~50 ns [5]). This indicates that a major fraction of positrons are localized before annihilation in nanopores, partially forming positronium atoms. It was found that the average nanopore radius is on the order of 1 nm [3] and

The lives and deaths of positrons in the interstellar medium

“…Guessoum, Ramaty and Lingenfelter (1991) added spectral considerations to the positron-dust process but kept a simplified picture of the dust and showed that unless the abundance of dust in the annihilation regions is increased many times compared to "normal" quantities, grains change the spectrum of the line mostly in the hot phase. Astrophysical knowledge of grains has since increased considerably (see the reviews by Draine 2003, Li 2004, and Krugel 2003 and although a huge body of work exists on positron interactions with solids and solid surfaces (Nieminen & Oliva 1980;Huttunen et al 1990;Dannefaer et al 1996;Dupasquier & Mills 1995;Puska & Nieminen 1994;Weiss et al 1994;Suzuki et al 2003;Mills et al 1989;Mokrushin et al 2003;Pi et al 2003;Weber & Lynn 2000), experiments dealing with positrons and dust like materials remain scarce. We here summarize our best understanding of the relevant information we have collected on dust grains in our context.…”

Silicon Structures with Variable Morphology of Pores Methods of Obtaining Physical and Optical Properties

“…13,14,19 A disadvantage of electrodes based on macroporous GPS with a porosity of 50-60% is the low specific surface area of the working surface. Despite the presence of nanoscale (1-2 nm) pores on the surface of the wall in a layer with a thickness of ∼100 nm, 21,22 no significant increase in the active surface area is observed. Efforts have been made to increase the specific surface area by depositing a layer of carbon nanotubes 20-50 nm in diameter on the inner surface of the pore walls.…”