'Cold' crystallization in nanostructurized 80GeSe2-20Ga2Se3 glass

Klym et al. Nanoscale Research Letters 'Cold' crystallization in nanostructurized 80GeSe -20Ga Se glass 2 2 3 Halyna Klym 0 5 Adam Ingram 2 5 Oleh Shpotyuk 1 4 5 Laurent Calvez 3 5 Elena Petracovschi 3 5 Bohdan Kulyk 5 7 Roman Serkiz 5 7 Roman Szatanik 5 6 0 Lviv Polytechnic National University , 12 Bandera str, Lviv 79013 , Ukraine 1 Lviv Institute of Materials of SRC 'Carat' , 202 Stryjska str, Lviv 79031 , Ukraine 2 Physics Faculty of Opole University of Technology , 75 Ozimska str, Opole 45370 , Poland 3 Equipe Verres et et Ceramiques 4 Institute of Physics of Jan Dlugosz University , 13/15 al. Armii Krajowej, Czestochowa 42201 , Poland 5 UMR-CNRS 6226, Institute des Sciences chimiques de Rennes, Universite de Rennes 1 , Rennes, Cedex 35042 , France 6 Opole University , 11a Kopernika sq, Opole 45040 , Poland 7 Ivan Franko National University of Lviv , 1 Universytetska str, Lviv 79000 , Ukraine 'Cold' crystallization in 80GeSe2-20Ga2Se3 chalcogenide glass nanostructurized due to thermal annealing at 380C for 10, 25, 50, 80, and 100 h are probed with X-ray diffraction, atomic force, and scanning electron microscopy, as well as positron annihilation spectroscopy performed in positron annihilation lifetime and Doppler broadening of annihilation line modes. It is shown that changes in defect-related component in the fit of experimental positron lifetime spectra for nanocrystallized glasses testify in favor of structural fragmentation of larger free-volume entities into smaller ones. Nanocrystallites of Ga2Se3 and/or GeGa4Se8 phases and prevalent GeSe2 phase extracted mainly at the surface of thermally treated samples with preceding nucleation and void agglomeration in the initial stage of annealing are characteristic features of cold crystallization. Chalcogenide glass; Crystallization; Annealing; Positron annihilation; Trapping - Background The Se-based chalcogenide glasses (ChGs) possessing good transparency in 0.8 to 16 m spectral range are widely used in optoelectronic systems exploring thermal and optical imaging effects in both atmospheric telecommunication windows (3 to 5 and 8 to 12 m) [1,2]. They also possess an excellent glass-forming ability, mechanical and chemical stability, which makes them one of the most unprecedented media for different IR fiber-optic applications [3,4]. It is known that crystallization of such ChG can improve their physical, mechanical, and thermal properties considerably, but it is difficult to produce IR-transmitting glass-ceramics properly because growing crystals is generally out of control during heat treatment, which makes the material opaque [5,6]. Such crystallization processes can be adequately studied at the level of atomistic structural arrangement using numerous experimental measuring techniques, such as IR vibrational and Raman scattering spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy, atomic force microscopy (AFM) and scanning electron microscopy (SEM), nuclear magnetic resonance, etc. [7-12]. However, the row of experimental probes available to study atomic-deficient void structure of such materials is rather limited, especially at nanometer and sub-nanometer scale. One of the best techniques capable to identify such finest free-volume voids is positron annihilation lifetime (PAL) spectroscopy, the method grounded on physical phenomenon of electron interaction with its antiparticle (positron) in a matter [13-15]. In application to semiconductor materials, this method is used to identify intrinsic free volumes owing to simple models considering competitive channels of positron trapping from delocalized defect-free bulk states, deep ground states of positron traps (extended free-volume defects), and decaying of bounded positron-electron (positronium (Ps)) states [13,16]. In the measuring mode of Doppler broadening of annihilation line (DBAL), this technique allows additional identification of dominant positron trapping sites in the tested objects [13,17]. So, combined PAL-DBAL measurements are expected to be useful to study atomicdeficient void structure of solids affected by different nanosctructurization treatments, in part those producing nanosized inhomogeneities like extractions of segregated inner phases, nucleates, agglomerates, and fragments of crystallites, vacancy clusters and freevolume voids, etc. In this work, we analyze evolution of free volume in glassy 80GeSe2-20Ga2Se3 caused by crystallization treatment at relatively low temperatures (so-called 'cold' crystallization) using combined PAL-DBAL, as well as XRD, AFM, and SEM measuring probes. Methods The ChG of 80GeSe2-20Ga2Se3 composition was prepared from melting mixture of highly pure raw materials (Ge, Ga, and Se of 99.999% purity) in a sealed silica ampoule kept under 106 Pa vacuum [5,18]. The ampoule of 9-mm inner diameter was placed in a rocking furnace. The raw materials were heated from 20 to 850C using 2C/min rate and maintained at this temperature for 12 h at least. Then, the silica tube was quenched in water, annealed at 30C below glass transition temperature (Tg = 370C) for 3 h to minimize inner strains, and slowly cooled down to room temperature. The obtained glass rods were cut into slices of 1 mm in thickness and polished for further optical measurements. The 'cold' crystallization of 80GeSe2-20Ga2Se3 glass was performed with a single step of thermal treatment at (Tg + 10)C. This temperature was chosen as an optimal one for ceramization allowing control simultaneous nucleation and growth of nanoparticles within a glassy matrix in dependence on heat treatment duration. Thus, the glass samples were placed in a ventilated furnace for different times varying from 10 to 100 h, the temperature being kept with an accuracy of 2C. The PAL spectra were recorded with fast coincidence system ORTEC of 230 ps resolution (the full width at half maximum (FWHM) of a single Gaussian determined by measuring 60Co isotope) at the temperature of T = 22C and relative humidity of RH = 35%, provided by special climatic installation [18-20]. Two identical samples were used to build a sandwich geometry needed for PAL measurements. Two independent PAL experiments were assembled with each sample of the same thermal prehistory, the obtained results agreeing well with each other within an experimental error bar. Each PAL spectrum was measured with a channel width of 6.15 ps (the number of channels was 8000) and contained no less than 106 coincidences in total, which can be considered as conditions of improved measurement statistics. The 22Na isotope of slight activity (approximately 50 kBq) prepared from aqueous solution of 22NaCl wrapped by Kapton foil of 12-m thickness and sealed was used as a source of positrons. The measured PAL spectra were processed with LT 9.0 program [21]. In our previous work [18], we applied a two-component fitting procedure to reconstruct the measured PAL spectra, this being achieved by corresponding choice (...truncated)