Effect of compositional dependence on physical and optical parameters of Te17Se83−xBix glassy system

Accepted Manuscript Effect of compositional dependence on physical and optical parameters of Te17Se83xBix glassy system Pankaj Sharma, M.S. El-Bana, S.S. Fouad, Vineet Sharma PII:

S0925-8388(16)30180-3

DOI:

10.1016/j.jallcom.2016.01.179

Reference:

JALCOM 36530

To appear in:

Journal of Alloys and Compounds

Received Date: 26 November 2015 Revised Date:

16 January 2016

Accepted Date: 22 January 2016

Please cite this article as: P. Sharma, M.S. El-Bana, S.S. Fouad, V. Sharma, Effect of compositional dependence on physical and optical parameters of Te17Se83-xBix glassy system, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.01.179. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Effect of compositional dependence on physical and optical parameters of Te17Se83-xBix glassy system

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Pankaj Sharma1*, M.S. El-Bana2, S.S. Fouad2, Vineet Sharma1

1. Department of Physics & Materials Science, Jaypee University of Information Technology, Waknaghat, Solan, HP-173234, India.

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2. Nano-Science & Semiconductor Laboratories,Department of Physics, Faculty of Education, Ain Shams University, Cairo, Egypt.

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Abstract

In the present paper we have studied the effect of Bi addition on the physical and optical properties of thermally evaporated Te17Se83-xBix thin films. With Bi addition the density, mean coordination number, mechanical constraints, glass transition temperature increases. The other parameters theoretical energy gap, lone pair electron, deviation from stoichiometry decreases. Transmission spectra have been taken in the spectral range 400 nm – 2500 nm using ultraviolet-

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visible-near infrared spectrophotometer. The fundamental absorption edge shifts towards longer wavelength with Bi incorporation. Optical energy gap and linear refractive index have been determined using transmission spectra. A good correlation has been drawn between the optical and theoretical parameters. Using linear optical parameters, the nonlinear optical susceptibility

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and nonlinear refractive index have been estimated. Keywords: Refractive index; Optical energy gap; Nonlinear optical susceptibility; Thin films.

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*Corresponding author: [email protected]

1. Introduction

The chalcogenide glasses occupy a unique place in material science towards the

advancement of science and technology. These glasses show diverse properties as far as the optical behaviour is concerned and have an ability to transmit light in mid to far infrared region. These materials show potential applications in infrared photonics, infrared waveguides, optical

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fiber, thermoelectrics, photosensitizer, superconductors and supercontinuum generation in an ultrafast-laser inscribed chalcogenide glass waveguide etc. [1-8]. The presence of lone pair of electron in chalcogen atoms provides the possibility for a vast spectrum of optical and electrical properties in chalcogenide glasses. Among various

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systems of chalcogenides glasses, Se-Te is an important system as it has wide spread applications depending on their thermo-mechanical, dielectric and nonlinear properties etc. [9-12]. The addition of impurities, like Sn, In, Sb, Pb, Ag, etc. [13-20], to the chalcogenide glasses witness many changes in thermal, electrical and optical properties. The stability and the range of

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applications of Se-Te system has been improved by alloying it with certain elements which increase the difference between the glass transition and crystallization temperature and induce

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structural and configurational changes [21,22].

The choice of Bi element as an additive is an interesting proposition and has been pursued by many researchers for varied applications from switching to nonlinearity in other glassy compositions [10,23-26]. Bi doped chalcogenides are increasingly showing diverse applications. Bismuth telluride glasses and their derivatives possess significant charge transfer which defines their promising thermoelectric and nonlinear optical properties along with some

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other applications like optical recording of information, laser diodes, IR spectroscopy, IR detectors and sensors etc. [27]. The anharmonic electron-phonon interactions in chalcogenide glasses play an important role and are very crucial for nonlinear optical properties. The present manuscript gives an analysis of the effect of adding Bi in place of Se in the

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Te17Se83 glassy alloy. The effect of Bi addition has been studied for various physical properties viz. mean coordination number, density, compactness, free volume percentage, cohesive energy,

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glass transition temperature etc. The optical parameters (the refractive index, optical band gap, non-linear refractive index etc.) have been determined using the transmission curves of the Te17Se83-xBix thin films to get an idea of the effect of alloying Bi to the Te17Se83 glassy. 2. Experimental details Four compositions of Te17Se83-xBix (where x = 0, 2, 4, 6) bulk glasses were prepared from

high purity (99.999 %) elements using melt-quenching technique. The elements were weighed according to their atomic weight percentage and then heated together in evacuated (10-4 Pa) silica ampoules up to 730 K at a heating rate of 2 K/min. During the heating process, the ampoules were frequently rocked to ensure homogeneity of the melt. The melt was kept at 730 K for 10h 2

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then quenched into ice-cooled water to avoid crystallization. Thin films of Te17Se83-xBix bulk samples were deposited by vacuum evaporation. The vacuum evaporation process (at a pressure ~ 10-4 Pa) was carried out inside a coating (HINDHIVAC 12A4D) system. The amorphous state of the films was checked using x-ray diffractometer. The absence of crystalline peaks confirmed

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the amorphous state of the prepared samples. A double beam (Perkin Elmer Lambda 750) spectrophotometer was used to obtain the transmission spectra of films in 400 nm to 2500 nm wavelength range with spectral resolution of 2 nm and an accuracy of 0.0001% transmittance at

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room temperature (295 K). 3. Results

3.1 Mean coordination number and network topology

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The continuous random network model (CRN model) provides the most satisfactory description to get an idea about the atomic structure of the chalcogenides glassy systems. In this regard the parameter, mean coordination number () plays a very important role [28]. The mean coordination number signifies and gives an indication of the nearest neighbors. The mean coordination number for Te17Se83-xBix (x = 0, 2, 4, 6) system has been calculated using the

< m > = (aNSe + bNTe + cNBi ) / 100,

where a, b and c are the atomic % of Se, Te

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equation;

and Bi respectively and NSe = 2, NTe = 2 and NBi = 3 are their respective coordination numbers. There is cross linking of two fold Se chains and Se-rings by Te atoms and then by three fold Bi atoms. The addition of three fold Bi atoms leads to an increase in the mean coordination number.

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The values of changes from 2.00 at x = 0 to 2.06 at x = 6 (Table 1). There are two types of constraints in the covalent bonded glassy network viz. the bond stretching (NS) and the bond bending (NB). These constraints mechanically strain the glassy system. The average bond

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bending constraints per atom has been calculated using; NB = 2 < m > − 3 , and average bond stretching number of constraints per atom has been calculated using; NS = < m > / 2 (Table 1). The sum of average constraints arising from bond stretching and bond bending gives the average number of total constraints (NT) [29]. According to Thorpe [30], a system has floppy region (mean coordination number < 2.4) and rigid region (mean coordination number > 2.4) for a glass forming composition. Therefore, the glass compositions under investigation are in floppy region as indicated by the value of the mean coordination number.

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3.2 Density, molar volume, free volume percentage and compactness Archimedes principle has been used to measure the density of the samples. The distilled water has been taken as the reference liquid. The densities of samples have been calculated using

ρ = {wa /(wa − wl )}× ρl , where wa and wl are the weights of samples in air and in

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the relation;

the reference liquid respectively, while ρl (= 1gcm-3 at 20oC) is the liquid density. The measurements of density have been done five times and their average values are reported in Table 1. The value of density has been observed to increase with Bi addition.

Using the density of investigated samples, the molar volume (Vm) has been calculated

∑pm) i

i

ρ , where mi is molecular weight of ith element and pi is the

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from the relation; Vm = (

atomic percentage of same element. With increase in Bi content from x = 0 to x = 6, the molar

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volume increases as the mean coordination number increases from = 2.0 to = 2.06. The free volume percentage (‫ )ܸܲܨ‬for the Te17Se83-xBix (x = 0, 2, 4, 6) system has been calculated by employing the relation;

FVP = {(Vm − VT ) Vm }× 100 , where VT is the theoretically

obtained molar volume. The theoretical molar volume has been calculated for Te17Se83-xBix(x = 0, 2, 4, 6) samples using the additive formula given elsewhere [31]. The FVP increases with the

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increase in Bi content and is maximum for x = 6 (Table 1). The increase in the value of FVP on addition of Bi may be attributed to the change in interatomic spacing which alters the number of bonds per unit volume in the glassy network. As bismuth concentration is enhanced there is an increase in total number of constraints with increase in coordination number. This clearly

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indicates an enhancement in FVP. Since in our system the average coordination number is well below the threshold value i.e. 2.4, therefore, the system is in floppy mode and is loosely

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connected [32]. The values of compactness of the Te17Se83-xBix (x = 0, 2, 4, 6) samples have been calculated using the relation given elsewhere [31] and are given in Table 1.The compactness of the samples increases with increasing Bi content. This can be justified on the basis of the increase in the molar volume values with Bi content. 3.3 Heat of atomization, average single bond energy and theoretical energy gap The heat of atomization (HS ) for the Te17Se83-xBix (x = 0, 2, 4, 6) samples has been calculated using the relation given elsewhere [33]. The values of heat of atomization for Se, Te and Bi elements taken for calculations are 46.0 kcal/mol, 49.4 kcal/mol, 49.1 kcal/mol

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respectively. The average heat of atomization HS and average single bond energy (HS / < m >) have been calculated and given in Table 1. The average heat of atomization decreases with an increase in Bi concentration. Similarly the average single bond energy (H S / < m >) decreases

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with an increase in Bi content which indicates a decrease in cohesive energy as well as in the optical energy gap (calculated later).

The energy gap (Egth) for Te17Se83-xBix (x = 0, 2, 4, 6) samples has been theoretically calculated using the relation given elsewhere [34]. The energy gap decreases with the addition of

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Bi to Te-Se system (Table 1). This can be explained considering that the average single bond energy of the system decreases on Bi addition. Further, the decrease in energy gap has been

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correlated with the decrease in overall bond energy of the system. 3.4 Cohesive energy and lone pair electron

The heteropolar bonds form more easily in comparison to homopolar bonds according to the chemical bond approach [35]. Considering this, the bond energy of the possible bonds have been calculated using Pauling’s relation; EA−B = (EA−A × EB−B )

0.5

+ 30 (χ A − χB ) , where EA-A and EB-B 2

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are the bond energies of the homopolar bonds and χA and χB are the electronegativities of the atoms involved (χTe = 2.10, χSe = 2.55, χBi = 2.02). The different types of bonds possible in the present system are Se-Se (44.00 kcal/mol), Te-Te (33.00 kcal/mol), Bi-Bi (25.00 kcal/mol), Se-Te (44.18 kcal/mol), Se-Bi (41.59 kcal/mol) and Te-Bi (28.91 kcal/mol). The bonds are supposed to

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be formed in the sequence of the decrease in their bond energy which may provide maximum chemical ordering. Consequently, Se-Te bonds will be formed first followed by Se-Bi. The uncompensated Se will form Se-Se homopolar bonds in the last. The cohesive energy has been

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calculated by summing the bond energies of overall expected bonds for the Te17Se83-xBix (x = 0, 2, 4, 6) samples. The cohesive energy has been observed to decrease with an increase in Bi content (Table 1).

The non-bonding pair of electrons in the valence band of chalcogenides glasses is termed

as lone pair of electrons (L). The presence of lone pair electrons eases out the strain during the formation of amorphous materials. The lone pair present on chalcogens provides the chances of flexible bonding. Therefore, larger concentration of lone pair electrons enhances the glass formation. The number of lone pair electrons in the investigated system has been calculated

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using the difference of all valence electrons of the glassy system and the mean coordination number. The number of lone pair electrons has been found to decrease with Bi addition (Table 1). This is due to the interaction of Bi ions with the bridging Se atom lone-pair electrons. So, the lone-pair electrons are not effectively able to participate in the glass formation. The glass

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formation in chalcogenides can be easily analyzed using a simple criterion [36], i.e., for a binary system L > 2.6 and for ternary system L > 1. Since L > 3 for samples under investigation, this reveals that our samples are good glass formers.

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3.5 Deviation from stoichiometry and prediction of glass transition temperature

The ratio of covalent bonding possibilities of chalcogen atom to that of non-chalcogen atom is used to calculate the deviation from stoichiometry and is indicated by parameter R. The

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composition of the glass is considered as chalcogen-rich if the value of R is more than unity and is considered to be chalcogen poor if value of R is less than unity. The calculated values of R are given in Table 2. The values of R > 1 for the studied samples indicate that the glasses are rich in chalcogen.

At glass transition temperature (Tg ) the network in glass structure is always of dynamic

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nature and the viscosity decreases. This is due to slow breakdown of network in glasses. According to Tichý and Tichá [37,38], the mean bond energy ( E ) requires to be considered for analyzing the total network. They considered a covalent bond approach for chalcogenide glasses providing a correlation between glass transition temperature and the mean bond energy with an

[

]

using;

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empirical relation; Tg = 311 E − 0.9 . The mean bond energy ( E ) of the system is calculated

< E > = EC + Erm , where Ec is the overall contribution towards bond energy from strong

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heteropolar bonds and Erm is the contribution arising from weaker bonds that remain after the number of strong heteropolar bonds become maximum i.e. average bond energy per atom of the 'remaining matrix'. The calculated values of the mean bond energy and glass transition temperature are listed in Table 2. 3.6 Transmission spectra, refractive index and dispersion parameters Optical parameters i.e. refractive index and optical band gap can be evaluated using the single transmission spectrum. Fig. 1 shows the transmission spectra for Te17Se83-xBix (x = 0, 2, 4, 6) thin films under investigation. The transparent region of spectra has been analyzed to evaluate 6

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the refractive index by using the envelope method proposed by Swanepoel [39]. For the calculation of optical parameters the film thickness d and the complex refractive index

( n * = n − ik ) , where n is the linear refractive index and k is extinction coefficient of the film,

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have been considered. The elaborated method for the determination of refractive index can be seen elsewhere [40]. In the interference free region, two-constant Cauchy’s relation has been used. Fig. 2 shows the variation of refractive index with wavelength. It can be seen that refractive index decreases with an increase in wavelength while it increases with an increase in Bi content. Thin films thicknesses have been calculated from their spectra using the relation

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given in [40] and have been found to be approximately 1µm (Table 2).The calculated values of thicknesses are in good agreement with the observed values from thickness monitor during

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deposition of thin films. The percentage fluctuation in thickness is up to 5%, so one can neglect the effect of thickness in examining the optical parameters.

Wemple – DiDomenico (WDD) model [41,42] has been employed to study the dispersion of

refractive

index

both

in

visible

and

near

IR

region.

WDD

relation

is;

n 2 − 1 = E d E0 {E02 − (hν ) 2 } , where E0 is the oscillator energy also called average energy gap, Ed is the dispersion energy and hυ is the photon energy. Ed is independent of E0, because Ed is

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dependent on dielectric loss (εi) whereas E0 is not. Fig. 3 shows a plot of (n2 −1)−1 as a function of (hν)2. The values of E0 and Ed have been determined from the slope and intercept of characteristics in Fig. 3, and have been reported in Table 2. The values of E0 have been found to

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decrease with an increase of Bi content whereas the parameter Ed increases. The static refractive index (n0) has been calculated from the values of E0 and Ed. The static refractive index has been calculated by establishing hν → 0 in WDD relation, then the relation reduces to

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2 n0 − 1 = E d E0 . The values of n0 have been reported in Table 2 and are found to increase with

increasing Bi content. Dielectric constant (ε ∞ ) at infinite frequency has been calculated using the relation ε ∞ = n0 2 . Also from the electron excitation spectrum, the frequency-dependent complex dielectric constant can be described as ε * = ε 1 + ιε 2 . All the coveted consequences can be determined either from the real part of dielectric constant (ε1) or the imaginary part of dielectric constant (ε2). Thus, the WDD parameters (Ed and Eo), and can be represented in term of the moments of ε2 as [41,42]: E02 = M −1 M −3 and Ed2 = M −31 M −3 , where M-1 and M-3 are the

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moments (Table 2).The parameter E0 is independent of ε2 , whereas Ed depends on the scale of ε2 and hence provides an interband strength parameter. The dependence of the refractive index (n) on the lattice dielectric constant εL and

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wavelength given in [43] is n 2 = ε L − (e 2 4π 2ε o c 2 ) ( N m* )λ2 , where e is electronic charge, (N/m*) is the ratio of free carrier concentration to the electron effective mass and c is the velocity of light. The dependence of n2 on λ2 is linear at longer wavelength and is shown in Fig. 4. By extrapolating plot to λ 2 → 0 and from the slope the values of εL and (N/m*) have been calculated

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and are found to increase with increasing Bi content (Table 2).

3.7 Optical energy gap, optical conductivity, SELF and VELF

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The fundamental absorption edge data has been used to examine the kind of optical band transitions. The optical band transitions have been examined using the Tauc’s relation [44]

αhν = B(hν − Egopt ) p , where B is constant, Egopt is optical band gap and p gives the type of optical transition. After fitting different values of p [44], the nature of transitions in these samples has been observed to be indirect. Fig. 5 shows the variation of (α hν ) 0.5 as a function of

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hν . By extrapolating (α hν ) 0.5 → 0 the optical band gap has been estimated. The values of E gopt

have been given in Table 2. The optical band gap has been found to decrease with Bi content. Same trend has also been observed in WDD parameter Eo. Similar results have also been reported in literature [23].

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Optical conductivity (σ) [45] is related to real part of dielectric constant (ε1) as

ε 1 = σ (ω ) ωε o . The optical conductivity has been determined from the relation; σ = αnc , 4π

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where α is absorption coefficient, n is linear refractive index and c is velocity of light. Fig. 6 shows the variation of optical conductivity as a function of photon energy. The rate of energy loss of energetic electrons while passing through the sample has been

estimated from the volume energy loss function (VELF) and surface energy loss function (SELF) [46]. The SELF and VELF are dependent on real and imaginary parts of dielectric constants as VELF =

ε2

(ε − ε ) 2 1

2 2

and SELF = ε 2

((ε 1 + 1) 2 + ε 22 )

. Fig. 7 shows the variation of VELF and

SELF as a function of photon energy for Te17Se83-xBix system. The nature of curves is similar but VELF is more prominent than SELF for Te17Se83-xBix system.

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3.8 Nonlinear optical parameters To measure nonlinear optical parameters there are various experimental techniques which give different values [47-49]. The variation in values may be avoided on the basis of linear optical parameters by using semiempirical relations for the nonlinear refractive index and third

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order nonlinear susceptibility. Ticha et al [50] used Miller’s generalized rule along with WDD parameters. The third-order nonlinear optical susceptibility ( χ (3) ) can be estimated by using E0, Ed, and Miller’s generalized rule in the limit hν → 0 . The relation χ (3) = 4.02 ×10 −15 ( E d E0 ) 4

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esu, has been applied to calculate χ (3) . The nonlinear refractive index has been calculated using the relation n2 =12πχ (3) no , where no is the static refractive index. The calculated values have been given in Table 3. Our results are consistent with the earlier reported results [10,51]. The

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reported results are well supported by the observations made by Derkowska et al [52] where they have observed an increase in nonlinear susceptibility on replacing Zn (atomic radius = 134 pm) with Mg (atomic radius = 160 pm) in Zn1-xMgxSe thin films. This leads to conclude that with the increase in atomic radius of dopant there is an increase in nonlinear behaviour of refractive index in chalcogenide glasses. In Table 3, a comparison has been given for the third order

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susceptibility and the nonlinear refractive index of our compositions with some other S, Se and Te based chalcogenides [50]. Believing this we may expect the usage of the samples under investigation in non-linear devices. 4. Discussion

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The mean bond energy of the system increases with increase in Bi content. The values of < E > have been used to calculate the glass transition temperature of the Te17Se83-xBix (x = 0, 2, 4,

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6) samples. The glass transition temperature also increases with increase in Bi concentration and mean coordination number. There is probability of formation of the heteropolar Se-Bi bonds (41.59 kcal/mol) at the cost of Se-Se homopolar bonds (44.00 kcal/mol).The increase of glass transition temperature as well as the mean bond energy with increase in Bi content could be attributed to the increase in the concentration of heteropolar bonds on Bi addition. There may be an increase in the rigidity of the system as the structure of the TeSe glasses undergoes a transition from two dimensional to three dimensional state with increasing Bi content. The mean coordination number and the connectedness of the system also show an increase with Bi addition. 9

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There is an increase in the density of Te17Se83-xBix compositions with Bi addition due to the possibility of low density Se constituent being replaced by high density Bi element. Moreover, the increase in density can be explained on the basis of chemical bond formation. Since, heteropolar Bi-Se bonds have large bond formation probability as compared to homopolar

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Se-Se bonds; this causes an increase in the number of bonds per unit volume enhancing the average cross linking density with Bi addition.

The variation of refractive index with both wavelength and Bi content is presented in Fig. 2. The increase in the value of refractive index with increasing wavelength could be attributed in ∞

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accordance with the fundamental Kramers–Kronig relation ( n ( 0 ) = 1 + (1 2π ) ∫ α d λ ) . The 0

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observed increase in the refractive index with Bi content may be explained on the basis of density (Table 1). The relation between density and refractive index is nearly linear, so with an increase in Bi content the refractive index increases. The increase in refractive index may also be correlated to atomic polarizability of constituents. Since the atomic size of Bi (146 pm) is much more than the atomic size of Se (116 pm) and Te (135 pm) so according to Lorentz-Lorenz

refractive index [53].

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relation the atomic polarizability will increase with an increase of Bi content and hence the

It can also be easily observed from Fig. 1 that the high absorption region in the transmission spectra shifts towards the longer wavelength. The decrease in optical band gap with Bi content (Fig. 5) has been correlated with the decrease of bond energy as per chemical bond

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approach [35]. The bond energies for various bonds in the system are Se-Se (44.00 kcal/mol), SeTe (44.18 kcal/mol), Se-Bi (41.59 kcal/mol). The bond energy of Se-Bi is the least. Therefore, on

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replacing Se with Bi the overall bond energy of system become less and hence leads to a decrease in optical band gap. The decrease in optical band gap has also been related with the decrease in average single bond energy measured from the heat of atomization (Fig. 8). The cohesive energy physically implies the stabilization energy of an infinitely large cluster of material per atom, and hence it can also be correlated with the energy gap. From Table 1, we have found that cohesive energy decreases with the addition of Bi content. Therefore, the decrease of energy gap can also be explained with the decrease in cohesive energy. VELF and SELF are quantities of interest in studying the rate of energy loss of high energy electrons passing through material, and from Fig. 7 it is clear that VELF is more

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prominent than SELF, this means that loss by the free charge carriers when traversing through the bulk material is more than travelling through the film surface. This could be attributed to the fact that charge carriers will travel long distance through the bulk material leading to more collisions with the charges inside the material. This causes more loss in energy of the high

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energy electrons that pass through the material. However, VELF increases more significantly in comparison to SELF for x = 2 and x = 0 samples compared to the other two compositions. This may be attributed to the high transmission for x = 2 and x = 0 as compared with samples x = 4 and x = 6. From the transmission spectra (Fig. 1), it has been observed that x = 2 sample has

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highest transmission.

The nonlinear refractive index may be defined as the disproportionate response of the

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material to the applied electric field. The polarization and the electric susceptibility also give nonlinear response under these conditions. The first, second and third order corrections terms have to be added to justify the nonlinear response of polarization to the applied electric filed. The nonlinear refractive index and the third order susceptibility have been observed to increase with increase in Bi content indicating an increase in the nonlinear response of bound electrons of the

5. Conclusions

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Te17Se83-xBix glassy system to the applied electric field.

Physical and optical parameters have been analyzed for ternary Te17Se83-xBix glassy system. The addition of Bi to Te-Se leads to the increase in coordination number, number of constraints and density. From the density calculations we have found that the free volume

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percentage also increases with Bi content. Average heat of atomization and cohesive energy decreases with an increase in Bi content which successfully explains the decrease of optical band

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gap. Refractive index has been observed to increase with the addition of Bi content and this behavior has been explained with increasing density as well as increase in polarizability. The third order susceptibility and nonlinear refractive index show an increase with increase in Bi content.

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List of Table and Figure captions Table 1 Values of mean coordination number (), constraints due to bond stretching forces (ܰௌ ) and bond bending forces (ܰ஻ ), density (ρ), free volume percentage (FVP), compactness (δ), pair electrons (L) for Te17Se83-xBix (x = 0, 2, 4, 6) compositions.

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average heat of atomization (HS ), theoretical energy gap (Egth) , cohesive energy (CE) and lone

Table 2 Values of deviation from stoichiometry (R), mean bond energy (), glass transition temperature (Tg), and some optical parameters for Te17Se83-xBix (x = 0, 2, 4, 6) compositions. Table 3 Values of non-linear optical susceptibility (χ(3)), non-linear refractive index (n2) for

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Te17Se83-xBix (x = 0, 2, 4, 6) compositions and a comparison of the same with other chalcogenide compositions from literature [50].

edge of Te17Se83-xBix thin films.

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Fig. 1 Transmission spectra of Te17Se83-xBix thin films. Inset shows red shift in the absorption Fig. 2Plot of refractive index versus wavelength for Te17Se83-xBix thin films. Fig. 3Plot of (n2 −1)−1 as a function of (hν)2 for Te17Se83-xBix thin films. Fig. 4Variation of n2 as a function of λ2 for Te17Se83-xBix thin films.

Fig. 5 Plot of (α hν ) 0.5 as a function of hν for Te17Se83-xBix thin films.

thin films.

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Fig. 6 Variation of optical conductivity (σ) as a function of photon energy (hν) for Te17Se83-xBix Fig. 7 Variations of VELF and SELF as a function of photon energy for Te17Se83-xBix thin films. Fig. 8 Comparative plot showing the variation of theoretical energy gap, optical energy gap

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(Tauc Gap), average single bond energy (Hs/) and cohesive energy as a function of Bi

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content for Te17Se83-xBix thin films.

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Table 1 x



Ns

NB

FVP

ρ

HS

HS / < m >

Egth

CE

(kcal/mol)

(kcal/mol)

(eV)

(kcal/mol)

δ

-3

0

2.00

1.00 1.00

4.981

2.49

-0.0249

48.822

24.411

1.577

44.036

4.0

2

2.02

1.01 1.04

5.094

2.60

-0.0260

48.816

24.166

1.532

43.948

3.9

4

2.04

1.02 1.08

5.187

3.05

-0.0305

48.810

23.926

1.488

43.855

3.8

6

2.06

1.03 1.12

5.293

3.22

-0.0322

48.804

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(gcm )

L

23.691

1.444

43.758

3.7

N/m*

Egopt

(g-1cm-3)

(eV)

R

--



Tg

d

Eo

Ed

(eV)

(K)

(µm)

(eV)

(eV)

no

ε∞

M-3

M-1

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x

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Table 2

εL

×10

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1.920 317.32 1.064 4.52

18.67 2.265 5.129 4.13 0.2021 4.782

0.513

1.57

2 27.34 1.922 317.72 1.033 4.32

18.73 2.311 5.339 4.34 0.2329 4.853

0.798

1.55

4 13.50 1.925 318.78 1.043 3.19

22.22 2.824 7.974 6.97 0.6870 6.821

1.921

1.52

30.16 3.617 13.08 12.1 1.9379 8.271

6.164

1.46

0

8.89

6

1.930 320.47 1.034 2.50

χ(3) (esu)

0

1.169 × 10-12

2

1.425× 10-12

4

9.507× 10-12

6

8.561× 10-11

n2 (esu)

Composition

n2 (esu)

χ(3) (esu)

1.945 × 10-11

Ge15As25S60 [50]

3.11× 10-11

1.86× 10-12

2.324 × 10-11

Ge36As4Se60[50]

7.88× 10-11

5.23× 10-12

1.269 × 10-10

Ge10Sb30Se60[50]

1.1× 10-10

7.58× 10-12

As16Te84 [50]

1.99× 10-10

2× 10-9

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x

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Table 3

AC C

8.919 × 10-10

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Fig. 1

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Fig. 2

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Fig. 3

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Fig. 4

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Fig. 5

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Fig. 6

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Fig. 8

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Fig. 7

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Highlights •

Physical and optical parameters have been analyzed for Te17Se83-xBix glassy alloys. The addition of Bi leads to decrease of average heat of atomization and cohesive energy.

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• •

The optical band gap decreases with increasing Bi content.

•

The third order susceptibility and nonlinear refractive index show an increase

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with increase in the Bi content.