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Vol. 7. Num. 1.January - March 2018
Pages 1-102
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Vol. 7. Num. 1.January - March 2018
Pages 1-102
Original Article
DOI: 10.1016/j.jmrt.2017.04.008
Effect of laser irradiation on micro-hardness, compactness and Raman spectrum of glassy Se76Te20Sn2Cd2 alloy
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Amit Kumara, Mousa M.A. Imranb, Arvind Sharmaa, Neeraj Mehtaa,
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dr_neeraj_mehta@yahoo.co.in

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a Department of Physics, Institute of Science, Banaras Hindu University, Varanasi 221005, India
b Materials Science Lab, Department of Physics, Faculty of Science, Al-Balqa Applied University, Al-Salt 19117, Jordan
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Table 1. Values of Wm for unexposed and laser-exposed samples of glassy Se76Te20Sn2Cd2 alloy.
Abstract

In present novel work, we have synthesized a novel quaternary Se76Te20Sn2Cd2 glass in bulk form. Four laser light sources from UV-Vis-IR region have been used to investigate the laser-induced effects on the micro-hardness to check the candidature of this material for possible application in fiber optics. The micro-hardness of the quaternary Se76Te20Sn2Cd2 glass is increased appreciably after laser exposure. This result has been analyzed in terms of compactness. The enhancement in both micro-hardness and compactness is also explained by Raman spectroscopy.

Keywords:
Chalcogenide glasses
Laser
Micro-hardness
Raman spectroscopy
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1Introduction

Chalcogenide glasses are well-known for their desirable optical properties such as transmission in the infrared, high refractive index and high nonlinearity. Such properties have enabled many chalcogenide-based applications to be developed in diverse fields including photonics, medicine, environmental sensing and security [1–6]. Conventional methods for depositing these materials such as vacuum coating (thermal evaporation, chemical vapor deposition or sputtering) or pulse laser deposition [7] have been used extensively.

The chalcogenide glasses share two common properties that have a profound impact on their interaction with light; their electronic structure and their phonon vibrations [7]. Electronically, the chalcogenide glasses behave as semiconductors. They possess a bandgap, and consequently they are transparent to a certain range of wavelengths of light. The disorder of the glassy network in these materials creates localized electronic states that extend into the forbidden bandgap. These states have a significant effect on the electrical and optical properties of the chalcogenide glasses. The transparent window extends far into the infrared because the infrared side of the transparent region is determined by the phonon energy of the material. The presence of large, heavy atoms shifts the phonons to lower energy and therefore longer wavelengths. The low phonon energy makes the chalcogenide glasses attractive as infrared optical materials [7].

Laser irradiation to glass has been regarded as a process for structural modification and/or crystallization in glass. Various workers have used laser as a tool for tailoring of different physical properties of chalcogenide glasses. Khan et al. studied laser-induced amorphization and crystallization on Se80Te20−xPbx thin films [8]. Bahishti et al. investigated the Effect of laser irradiation on thermal and optical properties of selenium–tellurium alloy [9]. Lorinczi et al. reported the structural details of the Ag-As2S3 interface obtained by vacuum thermal evaporation followed by green laser irradiation [10]. Al-Ghamdi et al. studied the laser irradiation effects on optical constants in thin films of Ga15Se85−xInx chalcogenide system [11,12]. Naik et al. performed FTIR and XPS measurements to see the laser-induced alterations in the optical properties of Sb10S40Se50 chalcogenide thin films [13]. Bahishti et al. observed the Laser Irradiation Effect on the Optical Band Gap of Se96−xTe4Hgx thin films [14]. Uddin et al. reported the laser irradiation effect on the optical properties of a-Se88Te12−xAlx thin films [15].

In above literature survey, the different groups have observed a common feature that the laser irradiation produces defects in the chalcogenide glasses [16]. The presence of defects and irradiation-induced disorder significantly affects the characteristic features of these materials. The investigation of various properties with laser irradiation is also helpful for understanding of the mechanism of degradation in these materials. This motivated us to start work in this significant and recent on-going research activity. In present work, we have synthesized a novel Se76Te20Sn2Cd2 multi-component glass. The investigations on the micro-hardness of chalcogenide glasses are of straight practical importance since these materials have various potential applications in infrared fiber optics. Keeping in mind this fact, we have planned the micro-hardness measurements in as-prepared sample and laser-exposed samples of this novel glass. Generally, the composition of the glassy alloy is changed for tailoring of properties. We have used a new trend in which lasers of different wavelengths have been used instead of changing the composition. For this purpose, cw-lasers of four different wavelengths (405nm, 532nm, 690nm and 785nm) have been used.

2Theoretical basis

Hardness is a significant mechanical property of materials that is defined as the resistance of a solid object to permanent deformation under pressure. The prediction of hardness has attracted much interest [17,18] since it enables a tailoring of material science to capitulate enhanced mechanical properties. For instance, the hardness of glass is a crucial issue in the development of scratch-resistant glass covers for personal electronic devices.

The Vickers hardness test method is one of the most frequent and consistent methods for hardness measurements. It provides helpful information concerning the mechanical behavior of brittle solids [19]. Moreover, the indentation micro-hardness is important for understanding the mechanisms of deformation and fracture of materials [20]. Studies on the indentation fracture in brittle materials have also shown that indentation testing is a straightforward technique for characterizing the fracture behavior of glass and ceramics [21]. The indentation is generally carried out using sharp indenters such as a cone or pyramid, because of the geometrical similarity of the residual impressions. The contact pressure with such geometry is independent of indent size and thus affords a convenient measure of hardness [19]. The Vickers hardness test method consists of indenting the test material with a diamond indenter, in the form of a right pyramid with a square base and an angle of 136° between opposite faces subjected to a load (≤100kgf). The schematic diagram for understanding the geometry of indenter is shown in Fig. 1.

Fig. 1.
(0.11MB).

Geometry of diamond indenter used as a probe of micro-hardness.

3Experimental

Bulk sample of quaternary Se76Te20Sn2Cd2 alloy in glassy form was prepared by recognized quenching technique. The accurate proportions of high purity 5N (99.999%) elements were weighed in accordance with their atomic percentages. An electronic balance having least count 10−4g was employed for this purpose. The materials were then sealed in evacuated quartz tubes. A high Vacuum pumping system (Hind Hivac, Model: VS65D) with facility of a liquid nitrogen trap was used to achieve suitable vacuum (∼10−6Torr). The tube was tucked with a ceramic rod and was heated inside a muffle furnace at an appropriate temperature. During heating, the tube was frequently rocked by rotating the ceramic rod. This was done to make the melt homogeneous. After rocking for about 12h, the obtained melts were cooled rapidly by removing the tube from the furnace and dropping to ice-cooled water swiftly. The ingots of the sample were then taken out by breaking the quartz tube. The glassy nature of the present glass was confirmed by using XRD and SEM techniques. The SEM photographs and XRD pattern of as-prepared sample of Se76Te20Sn2Cd2 are shown in Fig. 2. On alert inspection of SEM image, the existence of homogeneous clusters of hierarchical surface morphology can be viewed. Fig. 2 also shows the XRD pattern in lower panel of same sample. The absence of any sharp peak also confirms the non-crystalline structure of present sample.

Fig. 2.
(0.24MB).

SEM image and XRD pattern of glassy Se76Te20Sn2Cd2 alloy.

Disk shaped samples (pellets) of identical geometry (diameter ∼6mm and thicknesses ∼0.5mm) were used for micro-indentation testing. Micro-indentation measurements were performed on the specimen using an automated digital Vickers micro-hardness tester (Vaiseshika Electron Devices, Model: DHV-1000). The four pellets are irradiated uniformly using four pulsed lasers of different wavelengths (405nm, 532nm, 690nm and 785nm) in UV, visible and IR region at fixed intensity (∼50mW) and exposure time (∼3min). The experimental photograph of irradiation of a sample by green diode laser in laboratory is shown in Fig. 3.

Fig. 3.
(0.13MB).

Experimental demonstration of illumination of sample of glassy Se76Te20Sn2Cd2 alloy by green diode laser having wavelength 532nm.

4Results and discussion

Traditional hardness property is measured using a static machine and involves the measurement of the diagonal of the impression after unloading. The Vickers hardness number (VHN)Hv is obtained as the ratio of the applied load (P) to the superficial area of the impression, in kgmm−2. The Vickers indenter is a square based diamond pyramid with included angle between opposite faces of 136°. The perfect impression is square and has a superficial area of d2/2 sin 68°.

The formula for calculation of VHN is as follows:

Here Hv is Vickers hardness number, P is load in kgf and d is the average diagonal of the impression in mm and θ is the included angle between the faces of the square pyramid (θ=136° for a Vickers indenter). The diagonals of the resulting indentations were measured on all specimens directly by using a micrometer eyepiece that was calibrated using a ERMA disk at each magnification.

Fig. 4 shows typical photographs of Vickers indented marks obtained from optical microscope for virgin and exposed samples of quaternary Se76Te20Sn2Cd2 alloy at 100g load. In the present study, the hardness behavior due to load was investigated by instrumented indentation with 100g load examined optically and computer operator software. Each measurement was carried out at least ten times and the mean value of the hardness number was determined at room temperature. The values of micro-hardness Hv for as-prepared and laser-exposed samples of glassy Se76Te20Sn2Cd2 alloy are given in Table 1 (here we have labeled 0nm wavelength for the unexposed sample). From this table, it is clear that the value of Hv is increased after laser exposure. The maximum increment in micro-hardness is observed in case of red laser of 690nm.

Fig. 4.
(0.3MB).

Impression of the Vickers indentation on as-prepared and laser-exposed samples of glassy Se76Te20Sn2Cd2 alloy.

Table 1.

Values of Wm for unexposed and laser-exposed samples of glassy Se76Te20Sn2Cd2 alloy.

Wavelength (nm)  Hv (kgf/mm2δ 
49.4  −0.263 
405  51.5  −0.202 
532  61.6  −0.122 
690  68.7  −0.103 
785  56.8  −0.140 

Since there is a need of critical number of constraints for a material to display mechanical resistance, therefore the hardness of material is governed by the number of network constraints at room temperature. One of the significant constraints which play crucial role in deciding the hardness of the solid is its compactness. It is associated with the free volume and the flexibility of the network. The compactness can assume negative values, which correspond to larger free volumes and flexibilities. The compactness of the structure of the glasses is determined from the experimentally measured value of density ρ of the glasses.

The compactness (δ) was calculated by the formula [22,23]:

Here xi, Ai and ρi are the respective atomic fraction, the atomic weight and the atomic density of the ith element of the glass and ρ is the measured density of the glass. The compactness is a measure of the normalized change of the mean atomic volume due to chemical interactions of the elements forming the network of a given solid [24]. Consequently, it is more sensitive to changes in the structure of the glass network as compared to the mean atomic volume. The calculated values of δ for as-prepared and laser-exposed samples of glassy Se76Te20Sn2Cd2 alloy are also listed in Table 1. From this table, it is clear that the value of δ is also increased after laser exposure. The maximum increase in compactness is also observed in case of red laser of 690nm. Thus, one can expect that the micro-hardness of the samples is increased with increase in its compactness. To verify this fact, we have plotted a graph between VHN and compactness in Fig. 5. From this figure, one can see that the plot of Hv versus δ is almost a straight line having adequate co-relation coefficient.

Fig. 5.
(0.06MB).

Plot of micro-hardness versus compactness for glassy Se76Te20Sn2Cd2 alloy.

Chalcogenide glasses are soft semiconductors due to the two-fold coordinated chalcogen atoms. Consequently, they behave as a flexible electron-lattice coupling system when they are subject to exhibit electro-atomic responses [3]. Therefore, the enhancement in Vickers hardness of present sample after laser exposure can also be understood as a consequence of a fact that the anharmonic phonons are very sensitive to the increased temperature for the chalcogenide glasses [3,25]. However, it will be a subject of a separate work in future.

As a part of the efforts to understand the structure of chalcogenide glasses network as well the structural change which can occur after illumination, Raman scattering continues to provide useful insights into the local structures of the vitreous state. The shift of the main Raman peaks can be attributed to the changed chemical environment. Therefore, we have used Raman spectroscopy to determine as far as possible the structural change which can occur in this glass after laser exposure. Fig. 6 presents Raman spectra for as-prepared and laser-exposed samples of glassy Se76Te20Sn2Cd2 alloy in wavenumber range 100–900cm−1. It is interesting to note that we have found two significant peaks around 198cm−1 and 275cm−1 [see Fig. 7(a) and (b)].

Fig. 6.
(0.09MB).

Raman spectra of as-prepared and laser-exposed samples of glassy Se76Te20Sn2Cd2 alloy.

Fig. 7.
(0.19MB).

Observation of Raman peaks around (a) 198cm−1 and (b) 275cm−1.

The Raman band corresponding to 275cm−1 is generally assigned to the characteristic vibrational mode of the Selenium atoms in amorphous phase that consists of Se chains and rings as well as other mixed units. The Raman band corresponding to 198cm−1 is assigned to the bending modes of Se units. The vibration band at 198cm−1 is of special interest. Its intensity decreases for samples exposed by diode lasers; irrespective of wavelength. This new feature can be attributed to the enhancement in Sn(Se1/2)4 tetrahedral units [26]. However, we cannot suggest more rigorous and detailed structure model of the present multi-component glass because of the absence of more detailed experimental data for quaternary glasses. Future experiments may reveal the exact knowledge of molecular structure in such complex glassy networks.

The careful inspection of the afore said Raman peaks shown in Fig. 7(a) confirm that these peaks slightly move toward smaller wavenumber for laser exposed samples. A possible reason of the peak displacement can be expressed in terms of consequences occurred due to local heating of glassy network under laser exposure. When we expose the sample by the laser, the sample is warmed up a little bit locally. This local heating causes stress in the amorphous network inducing increase of the spring force and thus the vibrational modes shifts to smaller wavenumbers. This is probably the reason of increase of compactness and hence the increase in micro-hardness after laser exposure. Thus, the Raman analysis supports the observed increment in the values of Hv and δ after laser exposure.

5Conclusions

Hardness and density measurement as well as Raman analysis has been done in samples of as-prepared novel multi-component Se76Te20Sn2Cd2 glass and its laser-exposed samples. The hardness of the exposed samples is increased after illumination by four laser light sources having wavelengths in UV-Vis-IR region. The enhancement of micro-hardness is increased in terms of raise in the compactness of the exposed samples. The VHN is increased almost linearly with compactness. Analysis of Raman spectra shows the evidence of increase in the spring force owing to stress generation in the amorphous network after laser exposure. This provides the basis for the observed enhancement in micro-hardness Hv and compactness δ.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgement

NM and AK are thankful to the Department of Science and Technology (DST), Delhi, India for providing financial assistance under Fast Track Young Scientists Scheme [Scheme No. SR/FTP/PS-054/2010].

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Journal of Materials Research and Technology

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