Optical and Vibrational Spectra of CsCl-Enriched GeS2-Ga2S3 Glasses

Klym et al. Nanoscale Research Letters (2016) 11:132 DOI 10.1186/s11671-016-1350-8 NANO EXPRESS Open Access Optical and Vibrational Spectra of CsClEnriched GeS2-Ga2S3 Glasses Halyna Klym1*, Ivan Karbovnyk2, Mariangela Cestelli Guidi3, Oleksandra Hotra4 and Anatoli I. Popov5 Abstract Optical and FTIR spectroscopy was employed to study the properties of 80GeS2-20Ga2S3-CsCl chalcohalide glasses with CsCl additives in a temperature range of 77–293 K. It is shown that CsCl content results in the shift of fundamental absorption edge in the visible region. Vibrational bands in FTIR spectra of (80GeS2-20Ga2S3)100 − х(СsCl)x (x = 5, 10, and 15) are identified near 2500 cm−1, 3700 cm−1,, around 1580 cm−1, and a feature at 1100 cm−1. Low energy shifts of vibrational frequencies in glasses with a higher amount of CsCl can be caused by possible thermal expansion of the lattice and nanovoid agglomeration formed by CsCl additives in the inner structure of the Ge-Ga-S glass. Keywords: Chalcogenide, Chalcohalide glass, Optical spectra, Vibrational properties, Modification PACS Numbers: 61.43.Fs, 71.23.Cq, 81.70Pg, 82.56Ub, 78.70.Bj Background Modern IR photonics emphasizes a significant importance of glassy functional materials with improved exploitation characteristics [1–3]. Among the promising media for applications of photonics are specific glasses, such as non-oxide glassy-like materials with a high content of chalcogens (S, Se, Te), which are also widely known as the chalcogenide glasses (ChGs) [4, 5]. Main developments concerning the preparation of such materials include different methods of their technological and post-technological structural modification using external influences, such as thermal annealing, high-energy irradiation, and laser beam treatment [5–8]. Technical possibilities of these modification methods, however, are, to a large extent, restricted by peculiarities of a vitreous state with characteristic effects of natural physical aging, functional non-reproducibility, and thermodynamic instability in view of high affinity to chemical reactivity. That is why the commonly used optimization of ChG is connected with traditional chemical compositional modification based on doping possibilities with additional components introduced into the glass matrix to attain new, sometimes unusual, properties. The principal functionality of ChG is determined by their excellent IR * Correspondence: 1 Lviv Polytechnic National University, 12 Bandery str., Lviv 79013, Ukraine Full list of author information is available at the end of the article transparency. A wide range including both commercially important atmospheric telecommunication windows at 3–5 and 8–12 μm up to a space telecommunication domain at 20–25 μm can be effectively combined with the transparency of halide compounds in a visible range by developing mixed chalcogenide-halogenide glasses such as Ge-Ga-S-CsCl systems [9, 10]. The mix of unique optical properties with high flexibility in composition and fabrication methodology makes these ChG systems compelling for IR photonics [11]. The exceptional IR transparency associated with suitable viscosity/temperature dependence creates a good opportunity for developing ChG-based molded optics for IR devices. In [12, 13], we studied the influence of CsCl amount on an atomic-deficit sub-system (void- or pore-type structure formed due to the lack of atoms at some of glassy network sites) in Ge-Ga-S-CsCl chalcohalide composition. In this work, we analyze the CsCl effect on the optical and vibrational properties of (80GeS220Ga2S3)100 − x(CsCl)x glasses with x = 5, 10, and 15. Methods GeS2-Ga2S3-CsCl chalcogenide glasses were sintered from Ge, Ga, S, and CsCl compounds (99.999 % purity), as described in details elsewhere [14–16]. Raw materials were melted at 850 °C in a silica tube for several hours. The (80GeS2-20Ga2S3)100 − х(СsCl)x (x = 5, 10, and 15) © 2016 Klym 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. Klym et al. Nanoscale Research Letters (2016) 11:132 Page 2 of 6 glasses were annealed at 15 °C below a glass transition temperature (Tg) for each of the glasses [16] to minimize inner strains. Such amount of CsCl additives is optimal in order to modify Ge-Ga-S glasses before future doping of these materials by rare-earth ions. For the purpose of convenience, the obtained glasses of (80GeS2-20Ga2S3)100(СsCl)0, (80GeS2-20Ga2S3)95(СsCl)5, (80GeS2-20Ga2S3)90(СsCl)10, and (80GeS2-20Ga2S3)85(СsCl)15 hereafter are referred to as (CsCl)0, (CsCl)5, (CsCl)10, and (CsCl)15, respectively. Optical spectra were measured using a Cary5 (Varian) short-wavelength spectrophotometer. A Bruker Vector 22 instrument was exploited to record the spectra in the mid and far-infrared regions [16]. IR spectroscopy at different temperatures was carried out at the infrared beamline SINBAD of the Daphne Light synchrotron IR facility [17–21]. The infrared transmission spectra were collected using a Vertex 70V FTIR spectrometer equipped with a Janis ST-100-FTIR (Janis Research Company, LLC, Woburn, MA) continuous flow cryostat and a room temperature DTGS detector. The outer cryostat windows were made of CaF2. The temperature points (293, 220, 150, and 77 K) were set with a LakeShore 331 temperature controller and kept constant, controlling the flux of liquid nitrogen and heating power during the necessary measurement procedure. The heating/cooling rate in experiments was set to 10 K min−1. The transmission spectra were acquired in the vacuum between 4500 and 900 cm−1 with a spectral resolution of 4 cm−1, performing 128 scans. Results and Discussion The transmission in the visible and infrared region of spectra for ChG under study measured at 293 K is shown in Fig. 1. Samples with x = 0 and x = 5 are essentially transparent down to 500 nm. A further increase of the CsCl content in the base GeS2-Ga2S3 glassy matrix results in a shift of the absorption edge towards shorter wavelengths which is in line with the earlier reports [14, 16]. The transmission increases with a CsCl concentration from 60 % in CsCl0 to 80 % in CsCl15. As was shown in [16], by adding up to 15 mol% of the alkali halide in the glassy matrix, the bandgap energy evolves from 2.64 to 2.91 eV. From a structural point of view, the addition of less than 15 % of CsCl in GeS2-Ga2S3 glasses is characterized by the formation of GaS4 − xClx tetrahedra that are dispersed in the glass network. Hence, the average number of Ga–S bonds decreases in favor of the average number of Ga–Cl bonds. One of the observed features for (80GeS2-20Ga2S3)100 − х(СsCl)x (x = 0, 5, 10, and 15) gl (...truncated)