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Bulk samples of As-S chalcogenide glasses were prepared by an interaction of vapors of volatile precursors in low-temperature non-equilibrium plasma discharge (plasma enhanced chemical vapor deposition (PECVD process)). Elemental arsenic and sulfur (As + S), and arsenic monosulfide and sulfur (As4S4 + S) were used as initial substances. In parallel, the As-S bulk samples were synthesized by “traditional” melting of the initial substances in the evacuated quartz ampoule from the same precursors. The optical properties of the bulk samples were compared. The exhausted gas mixtures were analyzed to clarify the difference in carbon impurities content. 3D laser ultra microscopy was used to determine the content of heterophase inclusions in the samples.
© 2016 Optical Society of America
1. Introduction
The As-S сhalcogenide glasses are promising material of near and mid-infrared optical range (0.6 to 10 microns) due to their high transparency, stability to crystallization and influence of environment. Moreover, the potential usage of As-S glasses to create a continuous single-stage Raman laser [1], and supercontinuum generation have been recently shown [2]. Purity is the most important parameter of the glasses. In 1980s the theoretical minimum of optical losses for the glasses was estimated on the level of 0.05 - 0.1 dB/km in the wavelength range of 4 to 6 microns [3]. This caused an increasing interest and led to the development of the fundamentals of their preparation, which were not substantially revised during the last two decades. The “traditional” method of As-S glasses preparation is melting of the elements (arsenic and sulfur) in a sealed and evacuated quartz glass ampoule [4]. Loading of the initial elements in the inert atmosphere allows to avoid substantial contamination of the final glasses. Additionally, vacuum sublimation and distillation may be used to deliver arsenic and sulfur into the reactor [5]. The usage of arsenic monosulfide As4S4 and sulfur as the initial substances instead of elements [6] minimizes the impurity content in final glasses as well. As a result, optical losses were reduced from a few decibels per meter in 1980s [7] to several tens of decibels per kilometer in the beginning of 2000s. Despite so promising original dynamics, the limit of real losses of the most studied binary or ternary systems of chalcogenide glasses is hardly better than 40-80 dB/km. Only once in 2009 As-S glasses were prepared with the losses less than 20 dB/km [8]. Even in this case, results seem so far from the theoretically estimated ones.
It should be mentioned, that the traditional physical methods of purification underlying the fundamentals of high-pure glass preparation like distillation, crystallization or sublimation use the coefficient of separation between substances, taking into account the difference of their nature. With transition of impurities size from microns to nanoparticles effectiveness of physical methods tends to zero. In this connection, the particular problem is non-selective absorption or scattering of infrared radiation due to the presence of nanosized inclusions. The barrier of 40-80 dB/km of losses may be substantially caused by this problem. Presumably, most of these nanoparticles are of carbon nature due to the initial substances are originally contaminated by carbon, especially sulfur. It was previously estimated, that the content of carbon in the sulfur designed for synthesis of chalcogenide glasses with optical losses of 10 dB/km should not exceed 10−5 – 10−7 at.% [9]. Partly, the nanoparticles may be of SiO2 matter due to specific interaction of melted batch with materials of the setup at a high temperature and a long duration of the preparation process [10]. Thus, it is actually to develop as a novel method of selective purification of the initial substances from nanosized impurities as well a novel method of preparation of the glasses with a shorter term and a lower temperature.
In order to approach to the solution of the fundamental problem, the aim of this work is to compare the optical properties of the As-S chalcogenide glasses prepared by different methods and study of the heterophase impurities behavior. The “traditional” preparation of the glasses via melting of initial substances in the evacuated quartz glass ampoule is compared with PECVD method, when a batch is formed through interaction of the initial substances in plasma discharge under the dynamic vacuum. The possibilities of usage of plasma discharge as an alternative for the preparation of chalcogenide glasses have been recently demonstrated [11,12].
In paper [13] it has been shown that chalcogenide glasses of the As–S system may be synthesized from elementary sulfur and arsenic via plasma-enhanced chemical vapor deposition under reduced pressure at the temperature of the reactor walls about 250°C. The softer conditions of preparation in comparison with the traditional ampoule melting eliminate any contamination as initial substances as final glasses in the synthesis stage. It was shown that the suggested method can be used to obtain As–S glasses in a wide range of content.
2. Experimental
The commercial initial substances all of 4N were used. Before loading bulk arsenic was fractioned into grains with size about 1-3 mm. As4S4 was melt into a bulk matter with constant surface square. Preheating was used to delete the water traces from the initial substances, which were loaded separately in different reservoirs. After loading, the initial substances were preheated at 110 °C during 2 hour under dynamic vacuum conditions. The desorbed water was collected into a trap cooled by liquid nitrogen and analyzed by chromato-mass-spectroscopy technique. Reproducibility in the experiments was reached by following: the initial substances have been loaded one time to make a set of experiments with and without plasma.
The constant temperature, maintained in external furnaces, provided the desired vapor pressure of the reagents. High pure helium was used as plasma feed gas as a carrier gas. It was blown at the constant rate through the quartz reservoirs loaded with the initial substances – arsenic, sulfur and arsenic monosulfide. Averagely, the deposition rate was about 8-10 g/h and duration of the process was 1-2 hours. The batch size was about 10 g. The compact glass samples were the rods with the diameter of 5 mm and the length of 10 mm. The samples with the As40S60 content were prepared. Since all the processes were done in the optimal time-temperature modes there was no influence of the initial substances grains on inhomogemity of compositions of the final glasses.
DSC analysis was made with an STA 409 PC Luxx synchronous thermal analyzer at a heating rate of 10 deg·min–1. The transmission and reflection spectra of the samples were recorded with Perkin Elmer Spectrum BX II FTIR spectrometer. An X-ray fluorescence microanalysis was made with a SEM-515 scanning electron microscope equipped with EDAX-9900 energy-dispersive analyzer. The Raman spectra of the samples were studied with a NTEGRA Spectra complex for Raman Spectroscopy (NT-MDT Company, Zelenograd) with a He–Ne laser (632.8 nm). The Raman spectra were examined in the reflection configuration at 150–1000 cm–1. All the spectra were obtained at room temperature. Analysis of the major reaction products was carried out on gas chromatography-mass spectrometric complex Shimadzu “GCMS - QP2010Plus” (Shimadzu, Japan) with a vacuum sample inlet system through automatic injection valve (Valco Instruments Co Inc, USA) and sampling system, laser ultra microscopy study was done by microscope Axioscop 40 Pol, CCD AxioCam MRc with expose time from 1ms to 20 min. Scattered light was collected by objectives of microscope with numerical aperture 0.12-0.95.
2.1 Preparation of the As40S60 samples by the “traditional” method
The samples were prepared by the “traditional” melting of initial substances into the evacuated quartz ampoule. The initial substances were (As + S) and (As4S4 + S). The precursors were loaded into ampoule by thermal evaporation under dynamic vacuum conditions in the atmosphere of high-pure helium. The synthesis was implemented into rocking furnace at 750 °C for 1 hour. In order to avoid the internal stresses, the glass ampoules with the samples were held at the temperature 120°C – 170 °C for 1 hour with the following cooling down to 80 °C for 3 hours. Cooling to the room temperature was carried out in the “oven off” mode
2.2 Preparation of the As40S60 samples by PECVD method
The PECVD samples were prepared by a set-up with the scheme shown in Fig. 1.
The setup comprises a system for input of the initial mixture of reagents, a glass tube reactor, a trap system, a RF generator and a pumping system. The system also includes reagent gas lines and precision electronic controllers of gas flows. High pure helium was used as plasma feed gas as a carrier gas. Melting of the batch was implemented in the evacuated quartz glass ampoule into the rocking furnace at 750 °C for 1 hour. Quenching and cooling of the samples was done the same way as for the samples prepared by the “traditional” method. The setup and the parameters process of deposition have been described in details in [13].
3. Results and discussions
3.1 Investigation of the glassy batch obtained by plasma discharge
In order to prove the idea that formation of glassy matter takes place just in plasma discharge and the only homogenization is required the study of the batch was done. During the process of plasma-chemical deposition the precipitated material was gradually distributed with different color intensity varying with the increasing of the distance from the beginning of the reactor. The total length of the deposition zone was equal to the length of the discharge zone initiated by the external inductor which in its turn was 90 mm (Fig. 2). The reactor was cut into four zones with different colors.
The chemical consistence of each zone was determined by X-ray microanalysis. The Table 1 shows that with increasing of the distance from the beginning of the reactor the sulfur content in the probes increases. Portion 1 was taken from the beginning of the reactor; portions 2 and 3 were taken from the middle part of it, and portion 4 – from the final part of the reactor.
Table 1. Chemical consistence of the batch into 1, 2, 3 and 4 zones of precipitation for the As36S64 bulk sample.
The Raman spectroscopy study was done for each part of the reactor. Respectively, four points were investigated (Fig. 3). The first zone is characterized by excess of arsenic and the present of As4S4 crystal phase (185 cm−1, 273 cm−1 and 228 cm−1 bands) structural unit of glass net AsS3/2 (340 cm−1 and 367 cm−1) and linear sulfur (485 cm−1) [4, 14, 15].
Fig. 3 Raman profile of the glassy batch deposited. 1 – As62S38, 2 – As52S48, 3 – As39S61, and 4 – As31S69.
The second zone includes more amount of sulfur which leads to decreasing of intensity of As4S4 bands. For the third zone content may be closely referred to the stoichiometry of As2S3. Finally, the fourth zone is the glassy batch with engaging of substantial sulfur overbalance in the form of S8 rings (150, 220 and 475 cm−1) [4, 15]. According to the data obtained the arsenic content decreases with increasing of the distance from the beginning of the plasma zone. It is worse to be studied additionally in order to correlate the plasma parameters with the batch precipitated chemical content, but at the first glance the rise of the temperature of the gas mixture flowing on the length of the reactor leads to the amend of the batch content.
The differential scanning calorimetry (DSC) analysis was done for each zone of precipitation for determination of the phase transitions in the interval 20-400 °C at the heating rate of 10 deg/min (Fig. 4). The main difference of the DSC curves (1-4) in Fig. 4 from the same ones of the bulk glasses is the absence of the glass crystallization exothermic peaks before the melting peaks, pointing out that the crystalline phase was formed directly in the gas phase by plasma discharge and the batch presepitated seems to be the formation including a glassy matrix with the impregnation of a crystalline phase. The presence of a few transition processes for each curve proves the strong inhomogeneity of the solid phase.
Fig. 4 The DSC curves of the batch deposited. 1 – As62S38, 2 – As52S48, 3 – As39S61, and 4 – As31S69.
For each zone of deposition the vitrification transitions (Tg1 = 201 °C, Tg2 = 198 °C, Tg3 = 207°C, and 219°C) are observed. For the zones (1-3) the melting process take place at 305 °C and 327 °C regarding to As4S4 and As2S3 crystals melting, respectively. The assumption about the presence of two phases in the batch - the glassy composition close to stoichiometry to As2S3 and the crystal one, which is similar to arsenic monosulfide As4S4 may be done. Additionally, on the DSC curves 3 and 4 the endothermic effect of melting of excess of sulfur at 130 °C arises. All the DSC results are in a good matching with the data of the Raman spectra and the chemical content determined.
3.2 Raman spectra and thermal properties of the As2S3 bulk glasses prepared from different precursors by different techniques
In order to determine the relationship between the structural units of the glass net and the method of glass preparation the Raman spectra were investigated (Fig. 5).
Fig. 5 Raman spectra of As2S3 bulk glasses prepared from different precursors by different techniques.
The Raman spectra of As2S3 bulk glasses prepared by the plasma method as well as the glass samples of As2S3 prepared by the “traditional” way from different initial substances ((As + S) or (As4S4 + S)) look the same. The vibration peaks on 340 cm−1 corresponding to the structural unit AsS3/2 and the peaks on 485 cm−1 corresponding to linear sulfur of the glass net [4, 14, 15] are presented in all spectra. There is not obvious impact of the raw materials and glass preparation technique on the structure of the glasses.
Tg values of all obtained glasses are in the range of 183-185 °C with accuracy ± 0.6°. There is no impact of the initial substances or method of preparation on Tg of the final glasses as well.
3.3 IR absorption spectra of the As-S bulk glasses
The infrared absorption spectra of the As-S bulk samples are shown in Fig. 6(a) and 6(b).
Fig. 6 (a). The infrared absorption spectra of As-S bulk samples obtained via elemental As and S. 1 – prepared by the “traditional” method, 2 – sample, prepared in plasma. (b). The infrared absorption spectra of As-S bulk samples obtained via As4S4 and S. 1 – sample, prepared by the “traditional” method, 2 – sample, prepared in plasma.
The IR spectra contain the absorption bands of different functional groups. The bands located near 2.8 μm and 6.3 μm are the absorption lines of molecular H2O; 2.9 μm is the band of O-H groups, 3.1 μm and 4.0 μm bands refer to S-H - groups; 4.3 μm band refers to CO2 molecular. 7.6 μm and 10.6 μm absorption lines point at the presence of As4O6 (oxide form I) and As2O3 (oxide form III), respectively. The absorption band in the region of 10.1 μm is the intrinsic absorption of the S-S bonds [14].
On Fig. 6(a) where arsenic and sulfur are the initial substances the intensity of absorption bands of CO2 and H2O of the thermal samples (curve 1) is substantially higher, than that of the samples, prepared by plasma (curve 2). CO2 is formed due to interaction of carbon-containing impurities with traces of oxygen. It was described below, plasma preparation is divided on two independent stages – preparation of the glassy batch and the following homogenization. Since the batch formation in plasma takes place under continuous pumping, there is a constant evacuation of CO2 and H2O from glassy matter. In its turn, in the “traditional” method the ampoule is sealed and gas-forming impurities have no way to leave out.
On Fig. 6(b) when the arsenic monosulfide and sulfur are the initial substances distinction of intensities of CO2 lines between samples prepared by different methods is not so drastic (curves 1 and 2). Presumably, during the intermediate synthesis of As4S4 most of carbon-containing impurities have been deleted. The water absorbance lines and S-H lines are more intensive for the sample, prepared by traditional method (curve 1). The traces of water enter the samples from setup walls, but in the case of plasma (curve 2) water traces were gradually evacuated during the process of batch preparation.
3.4 Exhausted gas mixture chromate-mass-spectrometry analysis
In order to clarify the data of IR-spectroscopy, the exhausted gas mixtures were analyzed by chromate-mass-spectrometry (Table 2). In the case of the “traditional” method the gas mixture was collected into the trap cooled by liquid nitrogen during the process of loading of the initial substances into the quartz ampoule by thermal evaporation under dynamic vacuum conditions. In the case of plasma preparation, the same loading took place, but through the plasma discharge.
Table 2. Contents of the exhausted gas mixtures.
Concentration of H2S and SO2 in the gas mixture without plasma is substantially higher than that with plasma. But the opposite situation with concentration of CO2, which is much greater after plasma discharge. Concentrations of COS and CS2 are approximately the same.
The excessive formation of H2S and SO2 in the gas mixture without plasma may be explained due to intensive interaction of sulfur with water traces at high temperature (200-600 °C):
Equation (1) is the source of excessive formation of S-H groups in the glasses due to appearance of H2S in a sealed ampoule (curve 1 on Fig. 6(a) and 6(b)). The temperature of the reactor`s wall in plasma is usually in the interval of 160-200 °C, which hinders the Eq. (1). At the same time, water is hard to be converted in plasma and becomes possible to be evacuated. Sulfur presents in the vapor phase in the form of agglomerates of different compositions - from S2 to S12 [17, 18]. It was shown that the most thermodynamically preferable form of sulfur in vapor phase at the conditions of the experiments is S8 ring [17, 18].Concentration of CO2 after plasma is noticeably higher, than that without plasma due to cracking of S8 rings, releasing of carbon nanoparticles and their following selective burning out by oxygen traces [Eq. (3)]:
Without plasma non-selective oxidation carries on: Thus, without plasma there are two sources of SO2 formation – Eq. (1) and Eq. (5). The lower ratio of the impurities content in the exhausted gas mixture, when arsenic monosulfide and sulfur were used as the initial substances, may be explained intermediate formation and distillation of As4S4 where the impurities were partly released and deleted.3.5 3D laser ultramicroscopy
In order to prove the statement about cracking of S8 rings, releasing of carbon nanoparticles and their following selective burning out by oxygen traces, microinclusions in glasses were studied by the method of laser ultramicroscopy (LUM) based on the determination of individual inclusions by laser radiation of He–Ne laser at 632.8 nm wavelength scattered by them in the direction orthogonal to the incident beam [16]. In this method the image of a laser beam (in the form of a strip), which contained the image of circles of scattering (Tyndall cones) from suspended particles against the background of molecular scattering, was observed in the field of view of the microscope. The observed particles were much smaller than the resolving power of the objective lens of the microscope. The sizes of submicron inclusions were determined by brightness of digital images of diffraction spots finding the solution to the inverse problem of light-scattering on the basis of Mie theory.
The sizes of larger inclusions were determined by the microscopic method at staining using the method of dark field. The concentration of inclusions in the volume of the sample was determined by counting their pieces. Microphotographs and histograms of inclusion distribution on sizes in As-S glass samples, prepared by the “traditional” method (a) and PECVD method (b) from As and S are represented in Fig. 7 and 8. The accuracy of histogram information strongly depends on the statistical methods of data processing and the assumption of what is the nature of heterophase inclusions. The accuracy of the measurements in the present paper was estimated from 30 to 50% and depends on the size range of the particles.
Fig. 7 Microphotographs and histograms of inclusion distribution on sizes in As-S glass samples, prepared by the “traditional” method (a) and PECVD method (b) from As and S.
Fig. 8 Microphotographs and histograms of inclusion distribution on sizes in As-S glass samples, prepared by the “traditional” method (a) and PECVD method (b) from As4S4 and S.
The concentrations of large inclusions with the diameter more than 90 nm in both samples are approximately the same. But the concentration of inclusions with sizes 50-90 nm is substantially less in case of plasma preparation. The lower impurities content in the case of arsenic monosulfide utilization as the precursor may be explained that commercial As4S4 is originally purer then commercial As. Nevertheless, the general tendency of their distribution looks the same – smaller particles present in less concentration in the sample obtained via plasma.
Previously, the same behavior of carbon-containing impurities was observed during the process of isotopically enriched sulfur preparation from its hexafluoride [19]. Earlier the behavior of carbon containing impurities was studied during the process of plasma-chemical conversion of silicon chlorides [20].
4. Summary
Bulk samples of As-S chalcogenide glasses were prepared as by interaction of vapors of volatile precursors in low-temperature non-equilibrium plasma discharge (PECVD process) as by the “traditional” melting of initial substances in the evacuated quartz ampoule. The optical properties of the bulk samples were studied and compared. It was observed, that the samples obtained via plasma occurred purer in terms of carbon impurities content. The data of the analysis of exhausted gas mixtures and 3D laser ultramicroscopy proof the intensive conversion of carbon-containing impurities into CO2 due to interaction with traces of oxygen during the plasma preparation.
Funding
Russian Scientific Foundation (15-19-00147).
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