Biomedical subjects

S N Rashkeev

Publications and source records attributed to S N Rashkeev.

3 recordsLinked to original sources

PubMed2003

Role of electronic versus atomic relaxations in Stokes shifts at defects in solids.

Redshifts of luminescence relative to optical absorption bands (Stokes shifts) of molecules and of defects in solids are universally attributed to slow atomic relaxations on the grounds that electronic transitions are fast (Franck-Condon principle). Here we report a novel phenomenon that can occur only in the solid state: Stokes shifts caused by slow electronic relaxations. We demonstrate that the phenomenon occurs in the nonbridging oxygen defect in amorphous SiO2. We predict that another defect (OH group), which can exist in either crystalline or amorphous SiO2, has a similar Stokes shift, but it arises from a mix of lattice and electronic relaxations with manifest differences in the two phases.

Journal Article↗

PubMed2002

Reactions and diffusion of water and oxygen molecules in amorphous SiO2.

Water and oxygen molecules determine many of the properties of amorphous SiO2 used in several technologies, but the underlying atomic-scale processes remain unresolved. We report results of first-principles calculations showing that a wide range of behavior is possible in an amorphous environment, including diffusion of the molecule as a whole and various reactions with the network. Experimental data including oxygen exchange reaction and radiation sensitivity are accounted for. The possibility of H3O+ formation as a source of positive charge is discussed.

Journal Article↗

PubMed2001

Defect generation by hydrogen at the Si- SiO(2) interface.

Hydrogen is known to passivate Si dangling bonds at the Si-SiO(2) interface, but the subsequent arrival of H(+) at the interface causes depassivation of Si-H bonds. Here we report first-principles density functional calculations, showing that, contrary to conventional assumptions, depassivation is not a two-step process, namely, neutralization of H(+) by a Si electron and subsequent formation of an H(2) molecule. Instead, we establish that H(+) is the only stable charge state at the interface and that H(+) reacts directly with Si-H, forming an H(2) molecule and a positively charged dangling bond (P(b) center). As a result, H-induced interface-trap formation does not depend on the availability of Si electrons.

Journal Article↗