Free-Volume Nanostructurization in Ga-Modified As2Se3 Glass.

Ya Shpotyuk,O Shpotyuk,B Bureau,A Ingram,A Dziedzic,C Boussard-Pledel

doi:10.1186/s11671-016-1237-8

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Different stages of intrinsic nanostructurization related to evolution of free-volume voids, including phase separation, crystalline nuclei precipitation, and growth, were studied in glassy As2Se3 doped with Ga up to 5 at. %, using complementary techniques of positron annihilation lifetime spectroscopy, X-ray powder diffraction, and scanning electron microscopy with energy-dispersive X-ray analysis. Positron lifetime spectra reconstructed in terms of a two-state trapping model testified in favor of a native void structure of g-As2Se3 modified by Ga additions. Under small Ga content (below 3 at. %), the positron trapping in glassy alloys was dominated by voids associated with bond-free solid angles of bridging As2Se4/2 units. This void agglomeration trend was changed on fragmentation with further Ga doping due to crystalline Ga2Se3 nuclei precipitation and growth, these changes being activated by employing free volume from just attached As-rich glassy matrix with higher content of As2Se4/2 clusters. Respectively, the positron trapping on free-volume voids related to pyramidal AsSe3/2 units (like in parent As2Se3 glass) was in obvious preference in such glassy crystalline alloys.

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Ga-modified chalcogenide glasses (ChG) are known to be of high importance in view of their perspectives for modern IR photonics as active media with improved optical functionality, revealed, in part, when these glasses are doped with rare earth (RE) activators such as Pr3+, Dy3+, Tb3+, Er3+, and Nd3+ [1,2,3,4,5,6,7,8,9]
At higher Ga content, some crystallites of low-temperature α-Ga2Se3 phase appear in glassy crystalline g/c-Ga4(As0.4Se0.6)96 alloy, the crystallized phase being well identified due to six sharp X-ray powder diffraction (XRPD) peaks corresponding to one of cubic Ga2Se3 polymorphs with a space group of F43m (JCPDS ICDD card no. 89-7201) [9, 27,28,29]
The most pronounced crystallization effect is revealed in g/cGa5(As0.4Se0.6)95, where ten separate XRPD peaks centered near 28.5°, 33°, 47°, 56°, 69°, 76°, 87.5°, 94.5°, 106°, and 113° 2Θ are detected (Fig. 1)

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Ga-modified chalcogenide glasses (ChG) are known to be of high importance in view of their perspectives for modern IR photonics as active media with improved optical functionality, revealed, in part, when these glasses are doped with rare earth (RE) activators such as Pr3+, Dy3+, Tb3+, Er3+, and Nd3+ [1,2,3,4,5,6,7,8,9] Such ChG demonstrates an obvious tendency to nanostructurization by forming intrinsic inhomogeneities because of strong Ga affinity to chemical interaction with chalcogens, this process being governed by Ga content and preferential type of its environment in parent glass matrix [4, 10,11,12,13,14,15,16]. Under small Ga content (2–3 at. %) added in mixed Se–Te environment of TAS235 glass (e.g., glassy g-As30Se50Te20 alloy), the nanoscale droplets of dominated γ-Ga2Se3 phase (a few hundreds of Shpotyuk et al Nanoscale Research Letters (2016) 11:20 undesirable, especially when ChG should be doped with RE ions to get tunable, high-power, secondary remote mid-IR sources [9], or drawn into fiber to produce active media for optical biosensing [18].

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