Multimodal Structural Characterization of Ge-S-I Glasses by

Subscriber access provided by University of Winnipeg Library

C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Multimodal Structural Characterization of Ge-S-I Glasses by Combination of DFT Calculation, IR and Polarized Raman Spectroscopy Matthieu Chazot, Raphael Méreau, Mohammed El Amraoui, Frédéric Adamietz, Younès Messaddeq, and Vincent Rodriguez J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11187 • Publication Date (Web): 14 Jan 2019 Downloaded from http://pubs.acs.org on January 14, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Multimodal Structural Characterization of Ge-S-I Glasses by Combination of DFT Calculation, IR and Polarized Raman Spectroscopy

Matthieu Chazot1, 2, Raphaël Mereau2, Mohammed El Amraoui1, Frédéric Adamietz2, Younès Messaddeq1, Vincent Rodriguez2, *

1Université

de Bordeaux, Institut des Sciences Moléculaires, CNRS UMR 5255, 351 cours de la libération, 33405 Talence cedex, FRANCE.

2Center

for Optics, Photonics, and Lasers (COPL), Université Laval, Québec G1V 0A6, CANADA

*Corresponding author: [email protected]

1 ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 43

Abstract From a dual experimental-theoretical vibrational analysis we propose a new rationalized structural description of Ge-S-I chalcogenides glasses at the nanoscale. A vibrational multipolar approach based on a simultaneous deconvolution of IR and polarized Raman spectra (RS-VV and RS-HV) has been applied on these glasses. According to recent results on the high temperature α-GeS2 crystal structure and to our spectral analyses, we suggest that the local structure of the glass backbone is effectively described by a combination of α-GeS2 nano-layers, Edge-Sharing GeS4 Tetrahedra (ES-Td, ca ~50%) and Corner-Sharing GeS4 Tetrahedra (CS-Td, ca ~50%). We have then compared the experimental spectra to the calculated IR and polarized Raman spectra of some selected GexSyIz structural units obtained by DFT calculation. The stretching modes of the Ge-S-I occurring in the high frequency spectral range (300-450 cm-1) are essentially those of the GeS2 glass backbone and have been revisited. In addition, through a careful analysis of the vibrational multipolar activities of stoichiometric and over-stoichiometric sulfur glasses between 180 and 280 cm-1, we propose new assignments for the seven modes that have been identified by our trimodal spectral analysis. We finally suggest that there is a competition between the insertion of atomic iodine as a glass modifier which involves the Ge-S-I clusters and molecular diiodine as a spectator encaged between two α-GeS2 nanolayers.


Page 3 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. Introduction Chalcogenide glasses (ChGs) are attractive materials for IR applications such as optical signal processing, sensing, thermal imaging, integrated photonics or laser amplifiers military, medical and environment fields

4

1–3.

They are used in

for their particular properties as high linear and nonlinear

refractive index, or high transmission in the Infrared (IR). In this family of materials Ge-S-I ChGs are part of the most promising glasses, particularly due to their wide transparency range extending from the visible around 500 nm, to the infrared at 12 microns 5,6, but also their good ability to dissolve rare earth elements 7,

and their good thermal and chemical stability 8. Considering these properties and the fact that they do

not contain arsenic which is highly toxic 9, make them potential candidates for lasers fibers and amplifiers in the Mid-IR for medical applications. Ge-S-I glasses have been obtained for the first time in 1971 8, but recently, they received a gain of attention with the publication of new works demonstrating the possibility to make high purity glasses by thermal decomposition of Ge2S3I2 10. Since then, many works have been published on the characterization of their thermal, and optical properties 11–14. In a previous work 15, we analyzed the Tg and optical transparency in the visible of Ge-S-I glasses having 5, 10 and 15 % at of Iodine. We pointed out that those glasses exhibit an extremum of their Tg and band-gap near the stoichiometry following the (GeS2)x—(GeI4)1-x composition line, showing that Iodine does act as a network modifier. We also made the link between the glass forming tendency of those glasses and the mean coordination number which is directly related to the composition and the topological arrangement of atoms in the glass structure. In the aim to have a deeper understanding of the correlation between the structure and the properties of those glasses, a vibrational structural characterization has to be considered. So far, two different structural analysis of Ge-S-I ChGs have been reported, using unpolarised Raman and IR spectroscopy 16,17. They pointed out the appearance of absorption bands between 150 and 280 cm-1 with the introduction of Iodine 8. They concluded both that the different vibrational modes are linked to GeSxI4-x 3 ACS Paragon Plus Environment


Page 4 of 43

structural units, where x can take values between 1 and 4. However in those works, the positions and assignments of the bands differ from each other, i.e. they are somehow inconsistent. They were established by making the comparison of the different bands with the ones of Iodo(methyl)germanate groups 16, or AXGeI4-X molecules where A = H, CH3 or CF3 17, where x can take the value 1, 2 or 3. Table 1. summarizes

the different vibrational modes associations, concerning Ge-S-I chalcogenide glasses (unpolarized) Raman spectra, from these two publications 16,17.

Table 1: Assignments of Ge-S-I Raman bands from Ref. 16 and 17 Peak position (cm-1)

Assignments

References

185

υs(GeI3) in SGeI3

17

201

υs(GeI2) in S2GeI2 or υs(GeI3) SGeI3

16

227

υs(GeI) in S3GeI

16

220-230

υs(GeI2) in S2GeI2

17

239

υas(GeI2) in S2GeI2

16

250

υs(GeI) in S3GeI

17

253-261

υas(GeI3) SGeI3

16

Moreover, for concentration in Iodine between 0 to 20%, the main backbone structure of Ge-S-I glasses is the Ge-S network. However, historically we could find at least 3 different models of GeS2 glass network. One of them, based on the analysis of IR, Raman and Mössbauer spectra, proposed a network structure made of two different phases composed in one hand of GeS4 ethane-like (S3Ge-GeS3) fragments, and in the other hand of Outrigger-raft (OR) units

18–20.

Outrigger-raft units originate from clusters having an

internal structure similar to the high-temperature crystal form of GeS2: α-GeS2, where the edge are 4 ACS Paragon Plus Environment

Page 5 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


partially polymerized forming S-S chalcogen dimers (Sketch in Fig 1.a (*), we can see the creation of OR.). The (crystal) structure of α-GeS2 sketched in Figure 1 has been resolved in 1975 21, and is a layered structure composed of slightly distorted GeS4 tetrahedra, in which half of them formed chains of cornersharing tetrahedra and the other half edge-sharing tetrahedra bridging the chains. In contrast, the lowtemperature (LT) crystalline form of GeS2 (β-GeS2) is only composed of corner-sharing GeS4 tetrahedra forming a 3D interconnected network structure 22.

Figure 1: Representations of a) a layer of α-GeS2 along c axis, b) α-GeS2 in three dimension. Sulfur atoms are in yellow, germanium in gray. (*) sketch of network arrangement when the content of sulfur is over the stoichiometry with the formation of OR, by the creation (- - -) of S-S dimer and new link with sulfur atom in excess.



Page 6 of 43

A second model was based on a random network composed of GeS4 corner-sharing (CS-Td) and edgesharing tetrahedron (ES-Td), with some ethane-like clusters 23. Finally, a third model considered a network structure similar to α-GeS2 where the glass structure is supposed to be made of CS-Td and ES-Td 24. From that moment, the structural characterization of those glasses remained ambiguous, until recent insights on the network characterization of Ge-S binary glasses system were achieved using X-Ray and neutron diffraction 25,26. Therefore, in the light of these observations, new spectroscopic works are needed to get a better understanding of the network structure of Ge-S-I glasses, taking into account the recent progress on the Ge-S network structure. A multimodal vibrational approach introduced recently by some of us, have shown its impact on the comprehension of Tellurite glasses network to assess and quantify the relation between Raman gain and optical responses 27. In this previous work a simultaneous deconvolution of IR, Raman and Hyper-Raman polarized spectra (pentamodal spectral analyses) enabled to make the link between the structure and the optical polarizability and hyperpolarizability of those glasses. Noteworthy, the more “accessible” combination of IR and polarized Raman spectra is also efficient enough to get structural insights in glasses. It is known that in isotropic media such as glasses, the mutual exclusion rule prevails and thus odd symmetry representation are IR active (dipolar activity) and even representations are Raman active (isotropic and quadrupolar activity) 28. As detailed previously by one of us, polarized Raman spectra enable us to differentiate the isotropic and the quadrupolar contribution of any vibrational modes: the RS-VV spectrum is essentially dominated by the isotropic contributions but contains at a lower extent a quadrupolar contribution, whereas the RS-HV spectrum gives exclusively the quadrupolar contribution of the vibrational modes (Scheme 1, Eq. 1).

1 RS 4 RS 1 RS ( I Isotropic + I Quadrupolar )  I Isotropic 3 10 3 1 RS  + I Quadrupolar 10

RS IVV 

I

RS HV


(1)

Page 7 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Note that the depolarization ratio  (Eq. 2) gives the relative isotropic/quadrupolar contribution of each mode 28. RS I HV 3 0 (isotropic)    RS  (quadrupolar) IVV 4

(2)

This paper is divided as follow. After the experimental and theoretical section, we present distinctively the result and the discussion sections. In the result section, we report the IR and polarized Raman spectra for the different compositions of glasses to give a general overview of the spectral changes that occurs in the Ge-S-I glass system. Then, we present the results of our trimodal approach in term of deconvoluted spectra which fit the experimental spectra for a wide range of composition. The details of that band deconvolution is presented in two parts: the high frequency range between 300 and 500 cm-1, and the low frequency range between 135 and 300 cm-1. Then, we address the general representative elementary structural units (ESUs) of the backbone glass network, composed of germanium and sulfur and corresponding to the high frequency range. Concerning the low frequency range, we present the different representative clusters with Iodine (ESUs), used for the DFT calculations and their simulated activities. Finally, in the discussion section, we propose an interpretation of the observed vibrational peaks by comparing the experimental and the theoretical results in order to build up a global picture of Ge-S-I glass network and we finally conclude.



Page 8 of 43

Scheme 1: Energy diagram representing Infrared (IR) absorption, Raman-Stokes (RS), with their respective multipolar activities.

2. Experimental and theoretical section 2.1.

Glass synthesis:

Ge-S-I glasses were prepared by the melt-quenching technic starting from pure elements, germanium of purity 5N, sulfur of purity 5N and Iodide or purity 3N, following the different steps described elsewhere 15.

Glass samples in the form of small cylinder of 1-2 mm thickness and 10 mm in diameter were polished

with abrasive silicon carbide discs for spectroscopic characterizations.


Page 9 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.2.

Spectroscopic characterizations:

Spontaneous Raman Stokes spectra were obtained at room-temperature using a Horiba Jobin-Yvon (HR800) spectrometer in the back backscattering geometry with a confocal microscope. The incident radiation of 1.8mW at 752 nm is obtained from a Kr+ laser. A rotating polarizer was used to select the polarization state of the incident light from vertical to horizontal. The beam was focused 7 microns under the glass surface with an objective 50X with NA 0.55. The collected scattered light passed through a holographic Notch filter to reject Rayleigh light and a second polarizer in a fixed vertical polarization state to achieve a final VV or HV polarization spectrum. A grating of 900 lines/mm was used, and the light was detected with a CCD air cooled detector. The IR spectra were recorded on a Bruker VERTEX70v spectrophotometer equipped with a DTGS detector. We made a total of 200 scans with a resolution of 4 cm-1. The spectrometer was put under vacuum to minimize atmospheric water vapor and CO2. Reflectance experiments were made using an external reflection set-up at an angle of incidence of 12°. We obtained the complex refractive index of each sample by Kramers-Krönig analysis of its specular reflectance spectrum. In this work the infrared spectra were expressed in term of the imaginary part of the dielectric constant ε”. In the aim to compare Raman and IR spectra, RS-VV and RS-HV spectra have been corrected by the temperature factor and expressed in a reduced form as detailed in ref. 28. The deconvolution of IR and polarized Raman spectra has been simultaneously obtained using the same Gaussian shapes for the three different activities. For a given Gaussian function, the HWHM and positions could be modified during the deconvolution but were constrained to be exactly the same for the three spectra. In the other hand, the intensities were free to adjust each multipolar contribution. A special attention has been paid to fit as well as possible the different bands of the spectra but always with a minimal number of bands. This is a strong



Page 10 of 43

constraint between each modal activity which in some case forces us to introduce additional bands to better reproduce simultaneously the three experimental spectra and with consistency for all the compositions that we studied together. Note that we had to use an additional half-band (wing) in the low frequency region to fit technically the three experimental spectra, but we did not consider these lower frequency contributions in the discussion.

2.3.

DFT calculations:

Density functional theory (DFT) calculations have been performed on a range of clusters selected to represent the various experimental Ge-S-I glasses. After geometry optimizations of the clusters, harmonic vibrational frequency calculations were achieved in order to obtain the IR and Raman spectra of the so obtained stable structures. Prior to any geometry optimizations, the dangling bonds originating from singly bonded sulfur atoms were neutralized by adding hydrogen-like atoms with masses equal to germanium to provide better agreement between calculated and experimental vibrational frequencies. Qualitative vibrational assignments were made by visualizing the atom displacement vectors for each mode likely responsible for experimental IR and Raman bands. In addition, a potential energy distribution (PED) analysis was performed to also quantify the contribution of internal coordinates to a specific vibrational mode. Both geometry optimization and harmonic vibrational frequency calculations were carried out using the three-parameter hybrid B3LYP exchange-correlation functional 29,30, associated with the 6-311++G(3df,2p) basis set. The Gaussian09 software package 31 was used for all computations.


Page 11 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3. Results 3.1.

Experimental IR and Raman spectra: unified trimodal analysis of the vibrational bands

Figure 2 displays the IR, and polarized Raman spectra (VV and HV) of a series of Ge-S-I glasses having 10% of iodide and concentration in germanium varying between 25 and 35 %. Obviously, the change in germanium content is accompanied by strong modifications of the spectral shape indicating important structural transformations. It notably reports the three types of vibrational spectra with their own activity (trimodal representation) for different structural networks: glasses over-stoichiometric in sulfur, glasses at the stoichiometry and glasses under-stoichiometric in sulfur. For purpose of clarity, we restrained this study to glasses stoichiometric or over-stoichiometric in sulfur because of the strong overlap of the bands relied to Ge-Ge entities, Ge-S and Ge-S-I clusters in glasses over-stoichiometric in germanium between 180 and 280 cm-1.



Page 12 of 43

Figure 2: Experimental IR, and polarized Raman spectra of Ge-S-I glasses having 10% of iodide and Ge content varying between 25 and 35%. To summarize the vibrational spectral features of those glasses observed in Figure 2, we find in the highfrequency part the strongest peaks both in the IR and Raman spectra. The observed bands are commonly ascribed to Ge-S entities between 300 and 430 cm-1 and to S-S modes near 470 cm-1. Despite the 12 ACS Paragon Plus Environment

Page 13 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


assignment of some of these modes seems to be well established in the literature (i.e. the modes at 370 cm-1 in IR spectrum and the modes at 340cm-1 and 430 cm-1 in the Raman spectra), the assignment of the whole bands including less intense ones is not so clear and sometimes some contradiction may be noticed. Hence it is necessary to clarify our understanding of the vibrational spectra of the Ge-S-I glass system. Figure 3 reports the consistent (but complex) results of the unified deconvolution of the IR, and polarized Raman spectra on two extreme compositions: Ge25S65I10 ( = -40) and Ge31.7S58.3I10 ( = +0.2). The specific choice of the intermediate composition Ge31.7S58.3I10 is related to the parameter Δ that we introduced in a previous publication 15. This parameter points out the deviation from the stoichiometry for Ge-S-I glasses, taking into account the role of Iodide as a modifier in the structure. In contrast to the other composition, Ge31.7S58.3I10 presents an extremum in Tg and band-gap properties for the series of glasses containing 10% of iodide (see Fig. 3 photographies). For a purpose of clarity, we will divide our analysis of the spectra in two spectral ranges: the high frequency one between 300 and 500 cm-1 and the low frequency one between 135 and 300 cm-1.


The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Figure 3: Experimental IR and polarized Raman (VV and HV) spectra with their deconvolution of Ge25S65I10 (A:  = -40) and Ge31.7S58.3I10 (B:  = 0.2); a photograph of each composition is also provided. The red vertical line which is a guide for the eye at 370 cm-1 nicely underlines the complementarity of the trimodal spectral activities.


Page 14 of 43

Page 15 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


High frequency range Figure 4 presents the effect of iodide content on the spectra for three compositions in the high frequency range, while keeping the stoichiometric ratio almost constant ( = -4). As the iodide content increases, we can observe a slightly decrease of the overall intensity in this region. Still, we do not observe a strong transformation of the spectra with the introduction of Iodine. We note that all samples exhibit peaks between 300 and 430 cm-1 (Figure 4). Basically, they are linked to the GeS4 tetrahedral units which are the irreducible ESUs that compose the glass network. To fit the experimental spectra in this region, it was necessary to introduce six bands for the IR and five bands for the two polarized Raman spectra VV and HV. An additional band at 313 cm-1 was introduced to fit technically the IR spectra between the low and the high frequency region, but was not included in the discussion. The peak positions and their normalized intensities are reported on Table 2. The most intense peak in the RS-VV spectra (labelled b) at 344 cm-1 is commonly recognized as the symmetrical stretching mode (s) of the distorted GeS4 tetrahedron. Following G. Lucovsky, et al. 32, the strongest mode in the IR spectra (labelled d) at 369 cm-1 is assigned to the asymmetrical stretching mode (as) of the GeS4 tetrahedral unit. The IR spectra exhibit also a shoulder around 340 cm-1 which is generally assigned to (s) of GeS4. However, in this work two set of bands at 335 and 349 cm-1 (peaks labelled a and c), which are independent of the major mode in RS-VV at 344 cm-1, were necessary to reproduce the experimental IR spectra. The assignment of the other modes both in IR and Raman spectra is more tedious. The mode at 369 cm-1 in Raman has been assigned to a variety of entities’ vibrational modes, such as ethane-like fragments (S3Ge-GeS3) or Outrigger-raft (OR) units introduced by Griffiths et al 20. However, nowadays this mode is preferentially associated to edgesharing clusters. The peaks between 370 and 430 cm-1 (labelled e and f) are assigned to corner-shared tetrahedra, while the peak at 436 cm-1 (labelled g) is assigned to a vibrational mode of edge-shared tetrahedral units. On the other hand, we see on Figure 3a that glasses over-stoichiometric in sulfur are 15 ACS Paragon Plus Environment


Page 16 of 43

identified by the appearance of Raman peaks (VV and HV) ca 480-500 cm-1. In particular, modes at 465 and 475 cm-1 start to become predominant for highest sulfur content. The main feature in this part of the spectra is the peak at 475 cm-1 which is assigned to an S-S stretch mode in S8 rings. The mode at 465 cm-1 is generally assigned to S-S vibrations in free chains, while the peak observed at 490 cm-1 is generally associated to S-S vibrations in OR.


Page 17 of 43

Table 2: Summary of the peaks in the region 300-460 cm-1, obtained by spectral deconvolution of IR, RS-VV and RS-HV for three compositions: Ge31.6S63.4I5, Ge31S59I10 and Ge30S55I15 (=-4). All the intensities have been normalized to the most intense peak in the region for each activity. , the depolarization ratio, is defined by the ratio of the experimental Raman intensities (not normalized) IHV/IVV.

0.22

0

0

0

1

0.88

0.20

0

0

1

0.23 0.30

0.38

0.19

0

0

0

1

0.78

0

1

0.71

0.20

0

0

0.19

0

0

0.09

1

0.25

0.28

0.08

1

0.25

0.28

0.09

0.19 0.56

0.29

0.44

0.14

0.62

0.30

0.56

0.15

0.61

0.32

0.49

0.20 1

0.34

0.54

0.21

1

0.33

0.64

0.22

1

0.35

0.08

0.11 0.41

0.26

0.05

0.1

0.36

0.25

0.04

0.09

0.28

0.24



RS-HV

0

RS-HV

RS-VV

0

Ge30S55I15 RS-VV

IR

0.19



RS-HV

a/

RS-VV

Position

Ge31S59I10

IR

Ge31.6S63.4I5



Peak/

IR

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


334 cm-1 b/

0.05

0.05

0.05

344 cm-1 c/ 348 cm-1 d/ 369 cm-1 e/ 383 cm-1 f/ 409 cm-1 g/ 436 cm-1



Figure 4: Deconvolution of IR and polarized Raman spectra (VV and HV) of glasses of composition: Ge31.6S63.4I5 (left:  = -4), Ge31S59I10 (middle:  = -4) and Ge30S55I15 (right:  = -4), in the 300-460 cm-1 spectral range. 18 ACS Paragon Plus Environment

Page 18 of 43

Page 19 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Low frequency range Figure 5 focuses on the effect of iodine content while keeping the Ge/S ratio almost constant (=-4) but in the low frequency range. As the iodine content increases, we can observe a general increase of the intensity in the region 180-280 cm-1, mostly for the VV and HV polarized Raman spectra. This direct correlation underlines the link between these seven bands and the introduction of Iodine. From figure 5, we see that five bands denoted 3,4,5,6 and 7, respectively at 227, 235, 244, 258 and 266 cm-1, were necessary to fit simultaneously the IR, RS-VV and RS-HV experimental spectra in the 220-270 cm-1 range. In addition, in the 180-220 cm-1 range, one band denoted “1-2” at 200 cm-1 was introduced to fit the three spectra for the glass with 5% of iodide, whereas two distinct bands (numbered as 1 and 2) at 190 and 210 cm-1 were used for glass composition with 10 and 15 at% of Iodine. These results unravelled peaks which had never been observed in previous works and it was impossible to link the peaks observed here to the literature. As a consequence, a new interpretation of the modes in this low frequency range was needed. Going back to Figure 3a in the low frequency range, we also observe for the composition Ge25S65I10 additional sharp (molecular-like) Raman lines at 150 and 220 cm-1, which are assigned to S-S vibrational modes of S8 rings and free chains. These modes are characteristic of sulfur excess in Ge-S and Ge-S-I glasses. Additionally, at high Iodine content (at least 15%), it has to be mentioned the occurrence of a new peak at 154 cm-1 in Ge30S55I15 which is absent in Ge31.6S63.4I5 and Ge31S59I10 (not shown). Actually, this peak cannot be related to S-S vibrational modes because of the lack of the others distinctive “companion” modes at 220 and 475 cm-1. Indeed, it is probably the symmetric stretch (s) of GeI4 moieties which occurs around 148 cm-1.



Figure 5: Deconvolution of IR and polarized Raman spectra (VV and HV) of glasses of composition: Ge31.6S63.4I5 (left:  = -4), Ge31S59I10 (middle:  = -4) and Ge30S55I15 (right:  = -4), in the 180-280 cm-1 spectral range 20 ACS Paragon Plus Environment

Page 20 of 43

Page 21 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3.2.

Representative GeS2 ESUs of the backbone glass network for DFT calculationhigh frequency range

It has been hard to depict a fine picture of the glass network until recent advancement with new structural characterizations based on Neutron and X-ray diffraction of GeS2 glass materials 25,26. It was found that the GeS2 glass network is composed of 47% of edge-sharing tetrahedra and the rest of corner-sharing tetrahedra. They also showed that there were not any evidence of the presence of GeGe and S-S homopolar bonds in high proportions, thus opposing the model in which the network structure is built of ethane-like and outrigger-raft fragments 18. These observations are important as they pave the way to draw a better overview of amorphous GeS2 network. We can then propose a model for GeS2 glass network, composed of GeS4 tetrahedron in which nearly half of them are linked by two common sulfur (ES-Td), and the other half by only one sulfur (CS-Td), in which the presence of S-S or Ge-Ge “wrong” bonds can be present but at the level of defects. This structure is indeed very similar to the high-temperature (HT) α-GeS2 crystal network. A link between amorphous a-GeS2 and α-GeS2 structures had already been observed by comparing Raman spectra of GeS2 glass and HT crystal

24.

In this work, the Raman peaks of HT α-GeS2 were artificially broaden to mimic the

amorphous states, assuming a weak phonon dispersion between the center and the edge of Brillouin zone in such crystal. These analyses revealed tremendous similarities between both spectra, unraveling yet strong resemblance between both networks. Supporting then, the hypothesis that a-GeS2 network structure can be made of α-GeS2 nano-layers interconnected by corner-sharing tetrahedron in a random manner, rendering a non-periodic structure at a large scale. On the basis of these observations we propose three clusters depicted in Figure 6 (labelled 1, 2 and 3), to reproduce spectrally the Ge-S backbone glass structure. These structural units consist of CS-Td (cluster 2) and ES-Td (cluster 3) and a medium-range construction of combined CS-Td and ES-Td



Page 22 of 43

(cluster 1). Cluster 1 derives from the α-GeS2 layer structure, and has been obtained as represented on Figure 6 by taking three rings composed of 6, 4 and 6 alternating (Ge, S) atoms.

Figure 6: Sketch of the obtaining of a) clusters 1 (6-4-6) from α-GeS2 layer structure. See text for more details. and b) cluster number 2 and c) number 3 corresponding to CS-Td and ES-Td respectively. Sulfur atoms are in yellow, germanium in gray.


Page 23 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7.a displays the simulated unpolarized Raman and IR spectra of clusters 1, 2, and 3. The unpolarised spectra are the sum of RS-VV and RS-HV, which are highly dominated by isotropic contributions and are sufficient here to demonstrate the choice of these three clusters. The experimental unpolarized Raman (VV+HV) and IR spectra of a glass of composition Ge31.6S63.4I5 (with low content in iodine) in the same frequency region are reported in Figure 7.b for comparison purpose. Basically, one can observe that clusters 2 and 3 could be sufficient to describe the experimental Raman feature with a dominating strong band around 345 cm-1 (red line) and a broad wing (vibrational density of state, V-DOS) at higher frequency with two emerging bands ca 370 cm-1 (blue line) and 435 cm-1 (green line). Cluster 1 that we have introduced does not show any other specific band, i.e. it gives a strong band at 345 cm-1 and a large V-DOS in the 380-440 cm-1 range as does cluster 2 for example. Notably, only cluster 3, the ES-Td cluster, is able to reproduce the shoulder observed experimentally ca 370 cm-1 (blue line). In contrast, the dipolar IR spectra nicely demonstrates that cluster 1, which is an extension of cluster 3 including part of cluster 2 in the two three–rings (Ge-S) cycles, is necessary to reproduce, first, the observed strong IR band at ~370 cm-1 (blue line) and, second, also the two bands with medium intensity below 350 cm-1. Experimentally, in the IR-spectrum reported in figure 7b, we clearly observe the low frequency mode as a shoulder at ~335 cm-1 (orange line). Beside this, the three clusters provide several bands with no clear distinction in the 380-425 cm-1 range which are compatible with the experimental IR spectrum. Finally, both clusters 1 and 3 better reproduce the final wings observed at 435 cm-1 (green line). As a matter of fact, these three clusters seems to reproduce in a satisfactory way the different spectral activities of the GeS2 glass structure. Further details will be discussed in section 4.1.



Page 24 of 43

Figure 7: a) Simulated Raman (VV+HV) and IR spectra of cluster 1 (6-4-6), cluster 2 (CS-Td), cluster 3 (ES-Td) and their sum; b) experimental Raman (VV+HV) and IR spectra of a glass of composition Ge31.6S63.4I5 between 300 and 460 cm-1. Orange (short dash), Red (full), Blue (long dash) and green (dotted) lines are guide to the eye pointing at 335, 345, 370 and 435 cm-1.

3.3.

Representative GexSyIz ESUs for DFT calculation - low frequency range

Figure 8 presents the different clusters that were considered for the assignment of the bands relied to the introduction of Iodine in the low frequency region. These clusters were constructed on the basis of the ESUs (clusters 1, 2 and 3) on which we add progressively Iodine. The clusters are classified in 24 ACS Paragon Plus Environment

Page 25 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 8 in terms of their expected occurrence in function of Iodine glass content. Clusters 4 to 6 are expected to be present at low Iodine concentration (only one Iodine on each clusters), while clusters 7 to 10 are expected to appear at intermediated Iodine concentration (with two Iodine on each clusters) higher than 5% and around 10%. Cluster 11 should occur at high Iodine concentration (15% and more) since it contains three Iodine on one germanium. Cluster 5 represents edge-sharing tetrahedra with one sulfur substituted by one Iodine atom. Cluster 4, 10 and 11 corresponds to CS-Td with one, two and three sulfur atoms respectively, substituted by Iodine. Clusters 6, 7, 8 and 9 have the same main structure as cluster 1 but are characterized by the substitution of sulfur atoms by Iodine on cornershared GeS4 tetrahedra. These germaniums are embedded in two 6 atoms ring (6-ring) separated by edge-sharing GeS4 tetrahedra. We did not consider the case where Iodine is replacing sulfur atom in ES-Td as it will induce the destruction of one ring and i.e. of the 6-4-6 cluster. Cluster 6 contains only one Iodine atom whereas clusters 7, 8 and 9 have two Iodine atoms. For clusters 7 and 8, iodine atoms are bonded to different germanium atoms. The main differences between these clusters are the positions of the Iodine regarding the ES-Td in the center of the cluster. In cluster 7, the two iodine are in Trans configuration, facing each other regarding a pseudo-inversion center localized on the ES-Td. In contrast, in cluster 8, the two iodine are on the same side (Cis configuration) with one iodine in the plane of a 6 ring and the other one out of plane. Finally, Cluster 9 contains also two iodine but bonded to the same germanium atom.



Page 26 of 43

Figure 8: Structural models of GexSyIz representative ESUs (clusters) used for DFT calculations, classified in terms probability of occurrence. Germanium are in grey, sulfur in yellow, Iodine in violet and hydrogens atoms in white.

We report on Table 3 the more significant vibrational bands calculated in the range 180-280 cm-1, from the GexSyIz structural units. Cluster 4 presents one mode at 227 cm-1 relied to ν S3GeI in CS-Td, with medium IR and strong Raman intensities. We can observe that Cluster 5 is characterized by one mode 26 ACS Paragon Plus Environment

Page 27 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


at 243 cm-1 due to S3GeI in ES-Td, with a medium IR intensity and a weak Raman intensity, with respect to the other vibrational activities of the cluster. In cluster 6 we observe one peak at 252.5 cm-1 relied to vibrational modes of S3GeI unit in CS-Td embedded in 6 ring. This mode has a strong IR intensity and a medium Raman intensity. Clusters 7 exhibits two important modes at 248 and 253 cm-1. These modes are both ascribed to νs S3GeI and labelled (1) and (2) respectively. The νs S3GeI (1) vibrational mode of cluster 7 is an out-of-phase stretching of the two Iodines in the S3GeI tetrahedron, and νs S3GeI (2) is the in-phase stretching mode of Iodines. The mode at 248 cm-1 has a very strong IR intensity and is almost Raman inactive. Actually, the opposite is observed for the mode at 253 cm-1 which is strongly Raman active and almost IR inactive. Cluster 8 shows two vibrational activities labelled (3) and (4) at 223 and 253 cm-1 which are also relied to νs S3GeI. The mode νs S3GeI (3) at 223 cm-1 is linked to the vibration of S3GeI where the Iodine atom is in the plane of the 6 ring where his germanium is embedded. This mode has a medium IR and Raman intensity whereas the mode νs S3GeI (4) at 253 cm-1 is relied to the vibration of S3GeI with the Iodine atom out of the plane of his 6 ring, and is strongly IR active but weakly Raman active. We can mention that the peaks positions and intensities of the symmetrical modes of cluster 7 and 8 are different depending on the positions of the two Iodine compared to the central ES-Td. For instance, their modes at the same frequency of 253 cm-1 is stronger in IR for cluster 8 than for cluster 7, while this is the opposite for Raman activity. In contrast the mode has no IR intensity in cluster 7. Indeed, we can see on Figure 8 that cluster 7 includes two Iodines in Trans configuration and possess then a pseudo inversion center in the middle of the ES-Td. So, because of the mutual exclusion rule, one expects effectively this complementarity between IR and Raman activity. For the same reason, the mode at 248 cm-1 of cluster 7 is very strong in IR and not active in Raman. This effect cannot be observed for cluster 8 because the Cis position of the two Iodine breaks the pseudo center of symmetry of the skeleton. Clusters 9 and 10 exhibit both two modes which are very close in frequency (around 220 and 255 cm-1) and similar in nature as they both implies S2GeI2 vibrations in CS-Td. The intensity of the νs S2GeI2 mode around 220 cm-1 is moderate in IR and Raman



Page 28 of 43

for both clusters, while the mode at 252.5 cm-1 for cluster 10 is strong in IR and medium in Raman; the mode at 258 cm-1 for cluster 9 is strong in IR and weak in Raman. For cluster 11, which represents a CS-Td with 3 Iodides, we could find three peaks at 193.5, 249.5 and 252 cm-1. The latest modes at 249.5 and 252 cm-1 imply two different asymmetrical deformations of SGeI3 atoms, while the mode at 193.5 cm-1 is relied to a symmetric deformation of SGeI3 with a shear of the Ge-S-Ge bond. Furthermore, one has to mention a general mode around 236 cm-1, which is present on all clusters based on 6-4-6 structure. It exhibits a weak intensity in Raman in cluster 1 but it becomes significant when Iodine is introduced. This mode corresponds to a cluster deformation involving the ES-Td.


Page 29 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 3: Summary of the most significant bands of clusters 4 to 11 calculated in the frequency range 180-280 cm-1 Frequency (cm-1)

Assignment

IR intensity

Raman Intensity VV

(km/mol)

+ HV (Å4/AMU)

Clusters 4

227

ν S3GeI

10

11

5

243

ν S3GeI

18

3

6

252.5

ν S3GeI

36

6.1

248

νs S3GeI (1)

89

0.2

253

νs S3GeI (2)

1.4

13.4

223

νs S3GeI (3)

17

13

253

νs S3GeI (4)

36

6.4

220

νs S2GeI2

14

11

258

νas S2GeI2

38

6

219

νs S2GeI2

17

13

252.5

νas S2GeI2

28

3.6

193.5

δcis S3GeI

1.2

10

249.5

δas S3GeI

43.5

4.1

252

δas S3GeI

36.5

4

7

8

9

10

11

4. Discussion 4.1.

Structural analysis of the high frequency range

We have seen in Figure 4 that the introduction of Iodine does not induce strong modification of the spectra in the high frequency region. We only observe a decrease of the overall intensity with the 29 ACS Paragon Plus Environment


Page 30 of 43

increase of iodine concentration which can be relied to the modification of Ge-S backbone clusters due to the introduction of Iodine. It is then possible to discuss of this spectral range which is more representative of the Ge-S backbone structure, independently of the introduction of Iodine. From the experimental side, the trimodal procedure combining IR and polarized Raman VV and HV is successful to describe a large set of glass composition using a minimal list of bands of seven bands (denoted from a to g) reported in Table 2. The link between these experimental bands and the local structure of the glass is not so straightforward. Obviously, it is not enough to consider the unique GeS4 moiety, as originally proposed by Lucovsky and Galeener 33, to properly explain the complex Raman and IR spectra. We have thus proposed in section 3.2 to combine molecular superstructures involving CS-Td (cluster 2) and ES-Td (cluster 3) ESUs and at least a combination of both (cluster 1). This set of three clusters first mimics the local structure of the HT-GeS2 crystals and second fits properly the spectral responses based on the occurrence of the four remarkable bands represented by vertical colored lines in Figure 7. Table 4 summarizes the assignment of the experimental bands (in the high frequency range) regarding those obtained from DFT calculations (section 3.2) and based on clusters 1, 2 and 3. In particular, we see that five experimental bands (a, b, c, d and g) among the seven from Table 2 are reported, including a nearly “double” peak (peaks b and c) close to 345 cm-1 which corresponds to the vertical red line in figure 7, but clearly with different IR and Raman activities. These five remarkable bands actually fit quite nicely, in terms of energy (cm-1) and intensity, the experimental IR and Raman spectra. As reported in Table 4, they could be qualified as “core” vibrational modes since they do not fully involve the motion of the external atoms, which are artificial hydrogen atoms but with the mass of germanium. Two experimental bands, denoted e and f in Table 2 that occur in the 380-430 cm-1 range, are not included in Table 3 since their specific assignment is less conclusive. As a matter of fact, the calculated modes in the 380-430 cm-1 range, whatever the considered cluster, provide a bunch of modes with frequencies distributed in that range that could coincide with the two average bands (e and f). However, in terms of intensity these three molecular clusters fail to reproduce


Page 31 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the experimental IR and Raman V-DOS. Notably, whatever the considered cluster, the calculated modes in the 380-430 cm-1 range are rather “border modes”, i.e. they involve much more the motion of border hydrogen-like atoms and very less the motion of central atoms; this is even truer for the biggest cluster. This is clearly a limitation of our clusters approach to mimic spectral quantities of glasses. Despite the fact that these terminal hydrogen atoms have a mass equal to Ge, the equilibrium distance is still of a S-H bond so too much shorter than a Ge-S one. Nevertheless, a change of the length of the terminal S-H bond to the S-Ge length (which does not correspond to a minimum of energy) allow to check that the relative intensities of those “border modes” weaken significantly (not reported). This indicate that some methodologies have to be developed for that but this is actually out of the purpose of that work. Furthermore, we would like to highlight from Figure 3 the evolution of the different modes in the Raman spectra within the 430-520 cm-1 spectral range. They are relied to S8 rings, free-chains and also S-S dimers in Outrigger-raft (OR) units. The first mode appearing with the increase of sulfur is at 490 cm-1 and corresponds to S-S dimers in OR. This is consistent with the fact that the increase of sulfur can induce a restructuration of the network composed of α-GeS2 layered-like fragments. A link can be made with our previous results 15, revealing a rapid increase of the absorptions in the weak-absorption tail region with the increase of Sulfur content. Indeed OR are characterized by their low difference in energy between the HOMO and LUMO orbitals (0.8 eV) 34, and their appearance is intrinsically linked to the increase of sulfur over the stoichiometry (even at small content). This results in a dark-yellow coloration of Ge-S-I glasses. A further increase of sulfur is followed by the appearance of new bands at 465 and 475 cm-1 which are assigned to S-S vibrational modes in free chains and S8 rings. This is in agreement with the chain crossing model proposed by Lucovsky and Galeener in Ge-S glasses 32, and the idea that no more than two sulfur can be incorporated between two Ge atoms. This can also be linked to the increase of scattering losses in the visible for high Sulfur content in Ge-S-I ChGs, due to Sulfur phase separation. 31 ACS Paragon Plus Environment


Page 32 of 43

Table 4: Summary of the assignment of the main highlighted bands in the frequency range 300450 cm-1. The corresponding colored lines reported in figure 7 are also included. Experimental peaks

Assignment (Cluster)

Vibrational Activities

(see Table 2) a / 334 cm-1

Asymmetric equatorial breathing mode of 6- Strong IR activity

(orange line)

rings and ES-Td in Cluster 1

b / 344 cm-1

Symmetric breathing mode of 6-rings in Very

(red line)

Cluster 1

activity

νs GeS4 in Cluster 2

Strong Raman activity

strong

Raman

c / 348 cm-1

Asymmetric axial breathing mode of 6-rings Strong IR activity

(red line)

and ES-Td in Cluster 1

d / 369 cm-1

Symmetric breathing mode of ES-Td with Strong Raman activity

(blue line)

𝑆 scissoring of 𝐺𝑒 < 𝑆 in Cluster 3 Out of phase asymmetric vibration of the two Very strong IR activity 6-rings in Cluster 1

g / 436 cm-1 (green line)

4.2.

𝑆 Symmetric breathing mode of 𝐺𝑒 < 𝑆 > 𝐺𝑒 Medium IR and Raman activity in Cluster 3

Structural analysis of the low frequency range

The present trimodal spectral analysis enable us to distinguish seven vibrational modes in the 180-280 cm-1 range (Figure 5). The vibrational bands in the low frequency range have been generally ascribed to GeSxI4-x structural units, where 4 ≥ x ≥ 0 and to S-S vibrational modes for compositions having sulfur in excess, as indicated in Table 1. Actually, the positions and intensities of the involved structural units, could not fit the different vibrational activities that we observe. Then, there is some inconsistency with the previous assignments and we need to revisit the assignments in that range, based on DFT calculations developed in section 3.3. Our assumptions follow: the main network structure consist of a combination of α-GeS2 layered-like fragments (clusters 1), CS-Td (cluster 2) and ES-Td (cluster 3) 32 ACS Paragon Plus Environment

Page 33 of 43

on which we substituted sulfur atoms by Iodine as detailed in Figure 8. Table 5 reports the normalized integrated intensities observed on three glass samples having Iodine concentration of 5, 10 and 15% and a similar deviation from stoichiometry close to zero. Based on a comparison of the experimental intensities with our simulated results obtained by DFT calculation, we are able to propose an assignment of the observed peaks in table 5.

Table 5: Summary of the peaks assignments in the region 180-280 cm-1, obtained by spectral deconvolution of IR, RS-VV and RS-HV for three compositions: Ge31.6S63.4I5, Ge31S59I10 and Ge30S55I15. All the intensities have been normalized by the most intense peak in the region for each activity. (R) and (IR) underline the dominant spectral activity. Peak number / wavenumber

Ge31.6S63.4I5

200 cm-1

I2 between nano-layers

(broad

Intermolecular

mode)

mode (R)

RS-VV

RS-HV

IR

RS-VV

RS-HV

/ 1 / 190 cm-1

IR

“1-2”

RS-HV

main assignment (see Table 3)

Ge30S55I15

RS-VV

and

Ge31S59I10

IR

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

0.77

0.96

0

0.12

0.15

0

0.3

0.06

I-S

2 / 210 cm-1

0

0.19

0.12

0

0.28

0.04

0.32

0.31

0.17

0.24

0.36

0.11

I2 between nano-layers Intramolecular

I-I

stretching mode (R) 3 / 227 cm-1

0.28

0.19

0.05

νs S2GeI2 in cluster 9 (R) and 10 (R) 33 ACS Paragon Plus Environment


4 / 235 cm-1

Page 34 of 43

0.23

0.64

0.48

0.31

0.67

0.32

0.3

0.77

0.36

0.87

1

0.81

0.75

0.94

0.68

0.7

1

0.65

1

0.91

1

1

1

1

1

0.92

0.95

0.46

0.13

0.51

0.62

0.34

0.81

0.56

0.5

1

ν S3GeI (CS-Td) in cluster 4 (R) νs S3GeI (3) in cluster 8 (R) 5 / 244 cm-1 ν S3GeI (ES-Td) in cluster 5 (IR) ν

s

𝑆 𝐺𝑒 < 𝑆 > 𝐺𝑒 in all 6-4-6

clusters with Iodine (R) 6 / 258 cm-1 ν S3GeI (CS-Td) in cluster 6 (IR/R) νs S3GeI (1) in cluster 7 (IR) νs S3GeI (4) in cluster 8 (IR/R) νas S2GeI2 in cluster 10 (IR) δas SGeI3 in cluster 11 (IR) 7 / 266 cm-1 νs S3GeI (2) in cluster 7 (R) νas S2GeI2 in cluster 9 (IR)

The assignments that we propose in Table 5 fit quite fairly with the simulated bands (Table 3), not only in term of frequency but also in terms of their respective activity and intensity. Due to the complexity of the model it is not possible to describe each of these assignments. However, as an example, we can discuss of the case where the concentration of Iodine is low in Ge-S systems. In this case we expect to see first the formation of units having only one Iodine atom linked to germanium (cluster 4, 5 and 6). Clusters 4 and 6 present one peak at 227 and 252.5 cm-1 respectively (Table 3),


Page 35 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


relied to S3GeI vibrations in CS-Td. On the other hand, cluster 5 displays one band at 243 cm-1 due to S3GeI vibration in ES-Td. If we look to the experimental spectra of the Ge31.6S63.4I5 glass composition, for which the concentration in Iodine is very low, we effectively observe three major bands at 235, 244 and 258 cm-1, which are the first one to increase with the introduction of Iodide. We ascribed then those bands labelled 4, 5, and 6 on Figure 7 to ν S3GeI in CS-Td (cluster 4), ν S3GeI in ES-Td (cluster 5) and ν S3GeI in CS-Td embedded in a 6 ring (cluster 6) respectively. Let us now focus on IR inactive modes “1-2”, 1 and 2 at 200, 190 and 210 cm-1, respectively. Mode 2 intensity starts to merge in the RS-VV spectra of glasses containing at least 10% of Iodide whereas the RS-HV intensity remains very weak or weak. Actually molecular diiodine is known to have a RS-VV active mode (IR inactive) at 210 cm-1 in the liquid phase 35. We assign mode 2 to the molecular diiodine streching mode localized between two α-GeS2 layers, i.e. between at least two clusters 1, following our cluster description. Mode 1 is assigned to the intermolecular (ligand-like) mode of I2 with the Sulfur atoms from the two layers. The observed IR inactivity of that intermolecular mode is probably due to the very weak polarity of that ligand interaction between the two nanolayers. Mode “1-2” appears as a broad mode because the intensity of these two modes are too weak to be resolved. Due to the strong bond-energy difference between Ge-I and Ge-S (268 and 534 kJ.mol-1 respectively 36), we think that there is a competition between the insertion of molecular Iodine among the glass network (as a spectator ) and the insertion of atomic iodine into the glass network (as a modifier now) involving thus the formation of clusters of type 4 to 11 involving the main modes reported in Table 5. Finally, for composition having sulfur in excess (Figure 3.A), we observe additional bands in the low frequency region at 155 and 220 cm-1, relied to sulfur in free chains or rings. This confirm the presence of Sulfur in chains and rings in the structure, which has already been discussed.



Page 36 of 43

5. Conclusion The goal of that paper was to propose a consistent structural model of Ge-S-I ChGs. To do so, we have performed an extended experimental vibrational analysis (IR and polarized Raman) on Ge-S-I glasses and Density Functional Theory (DFT) calculations on different Ge-S-I clusters to calculate the IR and polarized Raman spectra of representative elementary structural units (ESUs). We have specifically performed a unified deconvolution of the experimental IR and polarized (VV and HV) Raman spectra (trimodal spectral analyses) on five compositions representing the different type of structures we can find in stoichiometric or over-stoichiometric in sulfur glasses in that ternary Ge-S-I system. We have selected first two compositions having the same Iodine content, but different Ge/S ratios. Then, we choose three compositions having the same deviation from the stoichiometry of the (GeS2)x—(GeI4)1-x composition line but with different Iodine content. In a final step, we have compared the calculated spectra to the de-summarized experimental ones based on their respective multipolar activities. This methodology enabled us to depict a realistic model describing the complex Ge-S-I glass network. This structure is assumed to be made of a mixture of three clusters which are the ESUs of the glass network forming an α-GeS2 layered like nanostructure interconnected by CS-Td and ES-Td clusters, considering that nearly 50% of GeS4 Td are ES-Td and the other half are CS-Td. Thus the high frequency range (300-450 cm-1), weakly perturbed by the iodine, has been revisited on the basis of the three GeS2 clusters for the DFT calculations and we have highlighted five bands with specific IR and Raman intensities. In addition, we have proposed a new assignment of the vibrational modes involving Iodine in the low frequency range. The use of a unified deconvolution revealed for the first time seven bands in that region 180-280 cm-1. We assigned these bands involving additional Ge-S-In clusters (with n =1, 2, 3) derived from the three GeS2 ESUs composing the backbone structure. We have then suggested that there is a competition between the insertion of atomic iodine as a glass


Page 37 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


modifier which involves the Ge-S-In clusters and molecular diiodine as a spectator encaged between two α-GeS2 nanolayers.

Acknowledgments This study has been carried out with financial support from the French Nouvelle Aquitaine region [Grant 2015-1R10205] and by the Canadian Excellence Research Chair program (CERC) in Photonics Innovations. This work has been done in the framework of “the Investments for the future” Programme IdEx Bordeaux-LAPHIA (ANR-10-IDEX-03-02) and in the framework of the Laboratoire International Associé (LIA) LuMAQ. The authors are also grateful to the Natural Sciences and Engineering Research Council of Canada (NSERC), the Fonds de Recherche Québecois sur la Nature et les Technologies (FRQNT) and the Canadian Foundation for Innovation (CFI) for their financial support. The authors would like to thank Steeve Morency for his technical support. Computer time was provided by the Pôle Modélisation HPC facilities of the Institut des Sciences Moléculaires UMR 5255 CNRS − Université de Bordeaux, co-funded by the Nouvelle Aquitaine region.



Page 38 of 43

References (1)

Tao, G.; Ebendorff-Heidepriem, H.; Stolyarov, A. M.; Danto, S.; Badding, J. V.; Fink, Y.; Ballato, J.; Abouraddy; F., A. Infrared Fibers. Adv. Opt. Photonics 2015, 7, 379–458. https://doi.org/10.1364/AOP.

(2)

Eggleton, B. J.; Luther-Davies, B.; Richardson, K. Chalcogenide Photonics. Nat. Photonics 2011, 5, 141–148. https://doi.org/10.1038/nphoton.2011.309.

(3)

Sanghera, J. S.; Shaw, L. B.; Aggarwal, I. D. Chalcogenide Glass-Fiber-Based Mid-IR Sources and Applications. IEEE J. Sel. Top. Quantum Electron. 2009, 15 (1), 114–119. https://doi.org/10.1109/JSTQE.2008.2010245.

(4)

Adam, J.-L.; Zhang, X. Chalcogenide Glasses: Preparation, Properties and Application; Woodhead publishing series, 2014.

(5)

Heo, J.; Mackenzie, J. D. Chalcohalide Glasses for Infrared Fiber Optics. Opt. Eng. 1991, 30 (4), 470–479.

(6)

Heo, J.; Mackenzie, J. D. Chalcohalide Glasses I . Synthesis and Properties of Ge-S-Br and Ge-S-I Glasses. J. Non. Cryst. Solids 1989, 111, 29–35.

(7)

Oswald, J.; Kuldová, K.; Frumarová, B.; Frumar, M. Near and Mid-Infrared Luminescence of New Chalcohalide Glasses Doped with Pr3+ Ions. Mater. Sci. Eng. B Solid-State Mater. Adv. Technol. 2008, 146, 107–109. https://doi.org/10.1016/j.mseb.2007.07.053.

(8)

Sanghera, J. S.; Heo, J.; Mackenzie, J. D. Chalcohalide Glasses. J. Non. Cryst. Solids 1988, 103, 155–178.

(9)

Snopatin, G. E.; Shiryaev, V. S.; Plotnichenko, V. G.; Dianov, E. M.; Churbanov, M. F. HighPurity Chalcogenide Glasses for Fiber Optics. Inorg. Mater. 2009, 45 (13), 1439–1460. 38 ACS Paragon Plus Environment

Page 39 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


https://doi.org/10.1134/S0020168509130019. (10)

Velmuzhov, А. P.; Sukhanov, М. V; Plekhovich, А. D.; Suchkov, А. I.; Potapov, А. М.; Shiryaev, V. S.; Churbanov, М. F. New Method for Preparation of Specially Pure Glasses in the Ge–S–I System by Melting the Products of Thermal Decomposition of Ge2S3I2. J. Non. Cryst. Solids 2015, 429, 178–182. https://doi.org/10.1016/j.jnoncrysol.2015.09.006.

(11)

Velmuzhov, A. P.; Sukhanov, M. V; Plekhovich, A. D.; Suchkov, A. I.; Shiryaev, V. S.; Churbanov, M. F. Preparation and Investigation of Glasses in the GeS2–GeI4 System. Opt. Mater. (Amst). 2015, 42, 340–344. https://doi.org/10.1016/j.optmat.2014.12.048.

(12)

Zhu, M.; Wang, X.; Jiang, C.; Xu, H.; Nie, Q.; Zhang, P.; Dai, S.; Shen, X.; Xu, T.; Tao, G.; et al. Freely Adjusted Properties in Ge–S Based Chalcogenide Glasses with Iodine Incorporation. Infrared Phys. Technol. 2015, 69, 118–122. https://doi.org/10.1016/j.infrared.2015.01.015.

(13)

Velmuzhov, A. P.; Sukhanov, M. V; Plekhovich, A. D.; Snopatin, G. E.; Churbanov, M. F. Preparation and Investigation of Ge–S–I Glasses for Infrared Fiber Optics. Opt. Mater. (Amst). 2016, 52, 87–91. https://doi.org/10.1016/j.optmat.2015.12.019.

(14)

Mochalov, L. A.; Churbanov, M. F.; Velmuzhov, A. P.; Lobanov, A. S.; Kornev, R. A.; Sennikov, G. P. Preparation of Glasses in the Ge–S–I System by Plasma-Enhanced Chemical Vapor Deposition. Opt. Mater. (Amst). 2015, 46, 310–313. https://doi.org/10.1016/j.optmat.2015.04.037.

(15)

Chazot, M.; El Amraoui, M.; Morency, S.; Messaddeq, Y.; Rodriguez, V. Investigation of the Drawing Region in the Production of Ge-S-I Optical Fibers for Infrared Applications. J. Non. Cryst. Solids 2017, 476, 137–143. https://doi.org/10.1016/j.jnoncrysol.2017.09.037.

(16)

Heo, J.; Mackenzie, J. D. Chalcohalide Glasses III. Vibrational Spectra of Ge-S-I Glasses. J. Non. Cryst. Solids 1989, 113, 246–252.



(17)

Page 40 of 43

Koudelka, L.; Pisacik, M. Raman Study of Short-Range Order in GexSyIz Glasses. J. Non. Cryst. Solids 1989, 113, 239–245.

(18)

Boolchand, P.; Grothaus, J.; Tenhover, M.; Hazle, M. A.; Grasseli, R. K. Structure of GeS2 Glass: Spectroscopic Evidence for Broken Chemical Order. Phys. Rev. B 1986, 33 (8), 5421– 5434.

(19)

Bridenbaugh, P. M.; Espinosa, G. P.; Griffiths, J. E.; Phillips, J. C.; Remeika, J. P. Microscopic Origin of the Companion A1 Raman Line in Glassy Ge(S,Se)2. Phys. Rev. B 1979, 20 (10), 4140–4144.

(20)

Griffiths, J. E.; Phillips, J. C.; Espinosa, G. P.; Remeika, J. P.; Bridenbaugh, P. M. Assignment of the Companion A1c Line in Gex(S, Se)1-x, Glasses. phys. stat. sol 1984, 122, 11–15.

(21)

Dittmar, V. G.; Schäfer, H. Die Kristallstruktur von H.T.-GeS2. Acta Cryst. 1975, No. B31, 2060–2064.

(22)

Dittmar, V. G.; Schäfer, H. Die Kristallstruktur von L.T-GeS2. Acta Cryst. 1976, B32, 1188– 1192.

(23)

Porezag, D. V; Pederson, M. R. Raman-Active Modes of a-GeSe2 and a-GeS2: A FirstPrinciples Study. Phys. Rev. B 1999, 60 (22), 985–989.

(24)

Kawamoto, Y.; Kawashima, C. Infrared and Raman Spectroscopic Studies on Short-Range Structure of Vitreous GeS2. Mat. Res. Bull 1982, 17 (7), 1511–1516.

(25)

Zeidler, A.; Drewitt, J. W. E.; Salmon, P. S.; Barnes, A. C.; Crichton, W. A.; Klotz, S.; Fischer, H. E.; J Benmore, C.; Ramos, S.; Hannon, A. C. Establishing the Structure of GeS2 at High Pressures and Temperatures: A Combined Approach Using X-Ray and Neutron Diffraction. J. Phys. Condens. Matter 2009, 21 (474217), 1–22. https://doi.org/10.1088/09538984/21/47/474217. 40 ACS Paragon Plus Environment

Page 41 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(26)


Durandurdu, M. High-Density Amorphous Phase of GeS2 Glass under Pressure. Phys. Rev. B 2009, 79 (205202), 1–7. https://doi.org/10.1103/PhysRevB.79.205202.

(27)

Rodriguez, V.; Guery, G.; Dussauze, M.; Adamietz, F.; Cardinal, T.; Richardson, K. Raman Gain in Tellurite Glass: How Combination of IR, Raman, Hyper-Raman and Hyper-Rayleigh Brings New Understandings. J. Phys. Chem. C 2016, 210, 23144–23151. https://doi.org/10.1021/acs.jpcc.6b07627.

(28)

Rodriguez, V. New Structural and Vibrational Opportunities Combining HyperRayleigh/hyper-Raman and Raman Scattering in Isotropic Materials. J. Raman Spectrosc. 2012, 43 (5), 627–636. https://doi.org/10.1002/jrs.3132.

(29)

Becke, A. D. Density-Functional Thermochemistry . III . The Role of Exact Exchange. J. Chem. Phys. 1993, 98 (7), 5648–5652. https://doi.org/10.1063/1.464913.

(30)

Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti Correlation-Energy Formula into a Functional of the Electron Density. Phys. Rev. B 1988, 37 (2).

(31)

Gaussian 09 (Revision A.2 Ed.), M. J. Frisch, et Al., Inc., Wallingford CT, 2009.

(32)

Lucovsky, G.; Galeener, F. L.; Keezer, R. C.; Geils, R. H.; Six, H. A. Structural Interpretation of the Infrared and Raman Spectra of Glasses in the Alloy System Ge1-xSx. Phys. Rev. B 1974, 10 (12), 5134–5146.

(33)

Lucovsky, G.; DeNeufville, J. P.; Galeener, F. L. Study of the Optic Modes of Ge30S70 Glass by Infrared and Raman Spectroscopy. Phys. Rev. B 1974, 9 (4), 1591–1597.

(34)

Holomb, R.; Mitsa, V.; Johansson, P. Localized States Model of GeS2 Glasses Based on Electronic States of GenSm Clusters Calculated by Using TD-DFT Method. J. Optoelectron. Adv. Mater. 2005, 7 (4), 1881–1888.

(35)

Strauss, H. L. The Resonance Raman Spectrum of I2 in Solution. J. Indian Inst. Sci. 1988, 68, 41 ACS Paragon Plus Environment


Page 42 of 43

493–504. (36)

Luo, Y.-R. Chapter Nine in Comprehensive Handbook of Chemical Bond Energies; Press, C., Ed.; Press, CRC, 2007.


Page 43 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC graphic


Multimodal Structural Characterization of Ge-S-I Glasses by

Recommend Documents