Mechanisms of photoinduced fluidity in chalcogenide glasses: Molecular orbital analyses

Abstract

Atomic processes giving rise to the photoinduced fluidity have been studied for amorphous selenium. Ab initio molecular orbital calculations of Se clusters suggest that distorted atomic structures such as curled and intersecting chains have smaller optical gaps. Those atomic sites are likely to be excited by subgap (Urbach-edge) light, which undergo structural relaxation including bond breakages and interchanges. Such photo-electro-structural transformations could originate the macroscopic fluidity. Possibility of electronic glass-transition is also discussed.

Fragility of 1D Floppy Materials

“…One is the interchain slippage, as mentioned previously, and the other is intrabond scission/exchange; the latter being consistent with small bond strengths of E r ≤ 2 eV (Table ) and also with structural models assuming ring‐chain coexistence . (It is also known that the bond scission in Se occurs efficiently under photoexcitation to moderate E a (≈0.2 eV) and high T 0 (< T g ), which may be determined by the two factors: one is the inter‐ and intrabond mixing, in consistency with small R / r , and the other is temperature variations of the chain slippage and breakage.…”

Optically trapped delayed elasticity in germanium selenide glass fibers

“…As a summary, photoinduced deformation should result from cumulative effects, as proposed by Fritzsche . Or, according to the model of Tanaka and Shimakawa, locally entangled chains of Se‐atoms disentangle through PAP and re‐entangle elsewhere, so that the “knots” between chains successively involve different atoms. Since the GeSe 9 glass behaves viscoelastically even in the dark, at room temperature, all these atoms are also involved in STAP.…”

“…Under the irradiation conditions used (subband‐gap, low intensity), the fraction of atoms involved in STE is very small (estimated to be few ppm, using the calculation method of Tanaka). Recent investigations of Tanaka and Shimakawa even show that PAP should be focused on few atoms of specific distorted atomic structures, such as curled and intersecting chains, and that the photofluidity arises from a photoassisted “chain crossing” processes. Should the atoms involved in PAP be always the same, the glass network would consist in few mobile atoms inside a rigid “standing still” network, having consequently a high viscosity (the viscosity is proportional to the average relaxation time).…”

“…Among all the photoinduced effects occurring in chalcogenide glasses, photofluidity is probably the most spectacular one: under irradiation, viscosity may drop by several orders of magnitude without any detectable temperature increase . The phenomenon is one of the few photoinduced effects that occur under subband‐gap irradiation, even though light absorption is very weak.…”

Effect of hot drawing process and carbonization temperature in electrochemical behavior of electrospun carbon nanofibers

“…It is related to the fact that the band-gap photoexcitation and relaxation of carriers in these materials create anisotropic molecular units and defects with higher dielectric polarizabilities. In addition, significant photoinduced softening of the matrix is achieved due to the bond cleavage and stress relaxation [26]. Then, a photoinduced drift of units in the non-uniform electric field of light takes place, which leads to a mass transport and formation of giant relief modulations mostly used for formation of relief diffraction gratings [27].…”