Free-Volume Nanostructurization in Ga-Modified As2Se3 Glass

Shpotyuk et al. Nanoscale Research Letters (2016) 11:20 DOI 10.1186/s11671-016-1237-8 NANO EXPRESS Open Access Free-Volume Nanostructurization in Ga-Modified As2Se3 Glass Ya. Shpotyuk1,2,3*, A. Ingram4, O. Shpotyuk5,6, A. Dziedzic2, C. Boussard-Pledel3 and B. Bureau3 Abstract 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. Keywords: Chalcogenides, Nanostructurization, Phase separation, Crystallization, Positron annihilation lifetime spectroscopy Background 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+ [1–9]. 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, 10–16]. In dependence on these pre-requisites, extra Ga additions can result in phase separation, nucleation and, finally, crystal growth, leading to stabilization of different crystalline Ga2Se3 polymorphs. Thus, 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 * Correspondence: 1 Department of Electronics, Ivan Franko National University of Lviv, 107, Tarnavskogo str., 79017 Lviv, Ukraine 2 Centre for Innovation and Transfer of Natural Sciences and Engineering Knowledge, University of Rzeszow, 1, Pigonia str., 35-959 Rzeszow, Poland Full list of author information is available at the end of the article nanometers in sizes) can be displayed, while at more enhanced Ga content reaching 5–10 at. %, this process extends over a microscale, when these crystallites grow to a few micrometers in sizes [8, 13]. In contrast, in Se-rich environment of Ge-based GeSe2–Ga2Se3 glass at heat treatment not too far above Tg, these Ga additions provoke formation of some multication crystallites like GeGa4Se8 [14–16] or Ga2−δGeδSe3 [12]. Crystallite growth and stabilization in ChG matrices is accompanied by complicated changes stretching over both atomistic (atomic-specific) and void (atomic-deficient) structural levels. The latter is related to the evolution of some free-volume entities (typically sub-nanoscale voids, vacancies, vacancylike clusters, etc.), when inner holes are agglomerated to form spaces of reduced electron density available for orientation stabilization of growing crystallites or, conversely, these holes are fragmented on smaller parts ensuring energetically favorable localization for growing crystallites in a predominantly glass environment [13]. In case of technologically controlled crystallization, it is possible to manufacture an important class of glass ceramics transparent in IR region, which possess much better mechanical reliability than their glassy counterparts [17]. But in most cases, these crystallization processes are © 2016 Shpotyuk et al. Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. 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]. In this work, the physical peculiarities of Ga-affected nanostructurization associated with subsequent stages of glass structure modification (phase separation, nucleation, and crystallization), overall described at atomic-deficient void level, are comprehensively studied in g-As2Se3, one of the well-known canonical representatives of functional chalcogenide photonics [19]. Methods The studied samples of Gax(As0.4Se0.6)100−x (x = 0, 1, 2, 3, 4, 5) alloys were prepared from high-purity elemental constituents (5 N or more) by conventional melt-quenching technique as described elsewhere [9, 11]. Total weight of ingredients inserted in silica ampoules of 10 mm in diameter used for melting was 30 g. The ampoule was sealed under a vacuum and heated at 900 °C in a rocking furnace for 10 h, followed by quenching into room temperature water from 700 °C. Then, these alloys were annealed during 5 h at 10 K below glass transition to remove mechanical strains that appeared during quenching, cut into disks of ~2 mm in thickness, and finally, polished to high optical quality. The crystalline state of the samples was controlled with X-ray powder diffraction (XRPD), experimental data being collected in the transmission mode on a STOE STADI P diffractometer (Cu Kα1-radiation). The crystal structures of phases were refined by the Rietveld method with the program FullProf.2k (v. 5.40) [20]. The surface morphology of fresh cut sections of the prepared alloys was tested using scanning electron microscope (SEM) with energy-dispersive spectroscopy (EDS) analyzer FEI QUANTA 3D 200i (Hillsboro, OR, USA). Positron annihilation lifetime (PAL) spectra were registered using fast coincidence system ORTEC of 230 ps resolution (the full width at half maximum) operated at high-stabilized normal measuring conditions. The pair of identical plane-parallel samples of each composition in sandwich geometry was employed for the measurements. The source contribution from 22Na isotope of low activity wa (...truncated)