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High pressure Raman study of layered Mo0.5W0.5S2 ternary compound

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2016 2D Mater. 3 025003

(http://iopscience.iop.org/2053-1583/3/2/025003)

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2DMater. 3 (2016) 025003 doi:10.1088/2053-1583/3/2/025003

PAPER

High pressure Raman study of layeredMo0.5W0.5S2 ternarycompound

Joon-SeokKim1,5, Samuel TMoran1,2,5, Avinash PNayak1,5, Shahar Pedahzur2, Itzel Ruiz2, Gabriela Ponce2,Daniela Rodriguez2, JoannaHenny2, Jin Liu3, Jung-Fu Lin3,4,6 andDeji Akinwande1,2,6

1 Department of Electrical andComputer Engineering, TheUniversity of Texas at Austin, Austin, TX 78712,USA2 NASCENTNSFEngineering ResearchCenter (ERC), Austin, TX 78758,USA3 Department ofGeological Sciences, TheUniversity of Texas at Austin, Austin, TX 78712,USA4 Center forHigh Pressure Science&Technology AdvancedResearch (HPSTAR), Shanghai 201203, People’s Republic of China5 These authors contributed equally to this work.6 Author towhomany correspondence should be addressed.

E-mail: afu@jsg.utexas.edu anddeji@ece.utexas.edu

Keywords: 2Dmaterials, transition-metal dichalcogenide, high pressure, diamond anvil cell, alloy

Supplementarymaterial for this article is available online

AbstractTernary two-dimensional (2D) transitionmetal dichalcogenide compounds exhibit a tunableelectronic structure allowing for control of the interlayer and the intralayer atomic displacement toefficiently tune their physical and electronic properties. Using a diamond anvil cell, hydrostaticpressurewas applied toMo0.5W0.5S2 up to 40 GPa in order to study the optical phonon vibrationalmodes. Analysis of the high-pressure Raman spectra shows that the two in-plane E2gmodesresembling that of pristineMoS2 andWS2, as well as disorder-activated longitudinal acoustic phononmode, are hardened and suppressed as pressure increases. The twoA1gmodes of the ternarycompound that resemble theA1gmodes of pristineMoS2 andWS2, displayed similar Raman shifts tothe pristine compounds as pressure increases. ARaman peak at 470 cm−1 that is close to A1g peaksemerges at∼8 GPa, which represents a disorder-activated pressure-induced out-of-plane Ramanmode observed only in the ternary compound under high pressure. At pressures above∼30 GPa, aRamanpeak at approximately 340 cm−1 is observed, signifying additional disorder-activated vibrationmode.Our results reveal the enhanced interactions in the structural and vibrational behavior of theMoS2 andWS2 domains in theMo0.5W0.5S2 compound under hydrostatic pressure. These resultscould have implications in understanding the electronic, optical, and structural properties of the new2D ternary compoundmaterials under extrememechanical conditions.

1. Introduction

Two-dimensional (2D) materials are considered tohave remarkable potential for applications in futureelectrical and optoelectronic devices. From the adventof graphene [1], several 2D materials have beenisolated, which have diverse properties includinginsulators like hexagonal boron nitride [2, 3], semi-conductors such as the transition metal dichalcogen-ides (TMDs) [4] and other elemental atomic materialssuch as phosphorene and silicone [5, 6]. Of thesematerials, the semiconducting TMDs are of particularinterest for their tunable mechanical and electronicproperties [7, 8]. For several 2D materials, the

electronic bandgap and lattice structure of thematerialis tunable by adjusting the number of layers [9–11],intercalation with foreign molecules [12–14], appliedmechanical strain [15, 16], stacked out-of-plane het-erostructures [17, 18], and in-plane 2D heterostruc-tures [19, 20]. In the case of TMDs, furtherengineering their properties can be achieved by adjust-ing its metal and chalcogen atom contributions toform ternary compoundswith different elements fromthe same element group [8, 21–24]. Only a few TMDternary compounds have been synthesized so far[19, 22, 25–28]. These ternary 2D compounds exhibitdifferent electronic properties depending on theircompositions [8, 22–24, 29–31]. The properties of

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27October 2015

REVISED

24 February 2016

ACCEPTED FOR PUBLICATION

24 February 2016

PUBLISHED

30March 2016

© 2016 IOPPublishing Ltd

these 2D materials can be further altered by strain[16, 32, 33], hydrostatic pressure [34–36], chemicaldoping, and intercalation [13, 37–39]. Here, weexplore the hydrostatic effects on multilayeredMo0.5W0.5S2, an in-plane ternary compound, andelaborate on the comparative differences between theternary compound and the pristine binary MoS2 andWS2 layered compounds.

2. Experimental

The Mo1−xWxS2 ternary compound with x=0.5 andpurity 99.9995%was purchased from2D semiconduc-tors (SKU: BLK-MOWS2) and was used for the high-pressure diamond anvil cell (DAC) experiments. TheFEI Quanta 650 scanning electron microscope (SEM)equipped with Bruker energy dispersive x-rayspectroscopy (EDX) system was used to analyze thechemical composition of the ternary compound. TheJEOL 2010F transmission electron microscope (TEM)was used to evaluate the atomic crystal structure of theternary compound. All Raman spectroscopy andphotoluminescence (PL) measurements were doneusing the Renishaw InVia Raman system coupled witha continuous 532 nm green laser. The ruby PL (forin situ pressure determination) was measured using0.000 001 mW for 1 s at a ∼0.5 nm resolution with agrating of 1200 line mm−1, while the Ramanmeasure-ments were taken for 60 s for each given spectrum (6accumulations of 10 s) with a laser power of 1 mWwith ∼1 cm−1 resolution at a grating of 3000line mm−1. These experimental settings as well asrelative polarization of the incident light in the Xdirection were maintained throughout all measure-ments. The extreme hardness and optical transparencyof the diamond anvils in a DAC, together with the useof a hydrostatic Ne pressure medium, allow forhydrostatic pressures to be exerted onto the sample,and offer in situ optical Raman measurements to becarried out up to the maximum pressure of 40 GPa inthis study. An initially 250 μm thick Re gasket sheetwas pre-indented using a pair of 350 μm diamondsand drilled in the middle of pre-indented area to forma sample chamber size of diameter 200 μm andthickness 40 μm. A Mo0.5W0.5S2 ternary compoundsample cleaved to size ∼100 μm in diameter and∼10 μm in thickness was placed onto the very centerof the sample chamber along with two pressureindicator ruby spheres (figure S1). Neon pressuremedium gas was loaded into the sample chamberusing the gas loading system; potential chemicalreactions of the sample with pressure medium gasunder high pressure was inhibited by use of inert Nemedium. Before loading gas, the sample chamber wasvacuumed for 30 min to remove any potential waterand volatile contamination in the sample.

3. Results and discussion

The isomorphismof TMDmaterialsmakes themgreatcandidates to form ternary van der Waals (vdW)compounds. Two or more elements with similarproperties can be alloyed into a new layered material,such as the M0.5W0.5S2 ternary TMD (figure 1(a)).Characterizing the Mo0.5W0.5S2 with SEM–EDXshows the elemental composition of the ternary TMDstructure as well as the amounts of Mo, W, and Spresent in the sample (figure 1(b)); the uniformity ofthe EDX spectra indicates homogeneous chemicalcomposition along the surface of the Mo0.5W0.5S2(figure S2). To further characterize the ternary com-pound, analysis of the TEM images shows the hexago-nal lattice of the material, without any major disorderof the atomic structure (figure 1(c)). Although thechemical composition is static like that in the pristineMoS2 or WS2, slight difference in the lattice constantsof the unit cell and arbitrary distribution of metalatoms might cause internal stress and local perturba-tion in vibrational modes. Previous studies on theRaman spectra of Mo1−xWxS2 have reported a con-volution of the two pristine Ramanmodes observed inMoS2 and WS2, but slight shifts of the peak positionsare observed when the chemical composition of agiven ternary compounds changes [23, 40]. Theseperturbations of the phonon modes suggest thatdistortion of the lattice constants and the disorderednature at the atomic scale have a large effect on thelattice and optoelectronic properties of the ternarycompound material. Applying hydrostatic pressureonto the ternary compound can further distort thelattice structure and its electronic properties, and canbe an effective means to investigate the optoelectronicand structural properties of the ternary TMDs underhigh strain conditions. Here, we apply hydrostaticpressure on the Mo0.5W0.5S2 ternary compound tounderstand its lattice phonon vibrational behaviorusing Raman spectroscopy in aDAC.

For 2Dmaterials, room temperature in situmicro-Raman spectroscopy has been used to study the effectsof the number of layers [9, 22], unwanted byproducts[41], functional groups [42], structural damage [43],and chemical modifications [44]. Along with manyother TMD materials, both 2H-MoS2 and 2H-WS2exhibit D4

6h symmetry (space group P63/mmc, No.194) with specific Raman active phonon modesincluding out-of-plane A1g, in-plane E ,1

2g rigid E ,22g

and E1g modes. We note that since E22g rigid mode

appears at∼35 cm−1 forMoS2 and∼27 cm−1 forWS2,respectively, which is below the cut-off frequency ofthe Raman system used in this study, and E1g mode isonly activated with specialized polarization environ-ment, we restrict our analysis to A1g and E1

2g (noted asE2g mode) modes in this study [45–47]. The con-voluted Raman peaks for the Mo0.5W0.5S2 compoundclearly shows the two in-plane E2g modes from pris-tine MoS2 and WS2, and the two A1g modes

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superimposed at wavenumber 420–430 cm−1. Furtheranalysis of the Raman spectrum of the Mo0.5W0.5S2compound studied here shows additional hybridizedRaman peaks that are not present in the individualMoS2 and WS2 compounds at ambient conditions(figure 1(d)).

Using a DAC, hydrostatic pressure can be appliedto the material which causes compression along alldirections (both c-axis and a-axis), allowing for in-plane and out-of-plane strains to be studied[34, 48, 49]. Due to the anisotropic nature of the vdWcompounds, it is expected that applied pressure willresult in more significant compression in the out-of-plane direction than in the in-plane direction. Pre-vious high pressure studies on multilayered MoS2 andWS2 clearly demonstrated that a metallization trans-ition arises from strong interlayer interactionsbetween S–S atoms due to the shortened interlayer dis-tance at applied pressures [35, 36, 50]. Studying thein situ change in the hybridized A1g and E2g modeswith hydrostatic pressure has been shown to be usefulin deciphering the electronic isostructural phase tran-sitions and the disorder-activated phonon behavior[34, 36]. In our study on the ternary compound here,both the E2g and the A1g modes from the MoS2 andWS2 are convoluted in the Mo0.5W0.5S2 compound

(figure 2(a)). The prominence of the A1g mode is sig-nificant in comparison to the E2g modes since the A1g

modes for the MoS2 and WS2 are separated only byΔ≈8 cm−1, while the E2g modes are further sepa-rated at Δ≈34 cm−1. The asymmetric shape of theA1g peak, with a shoulder on lower-wavenumber side,also implies that the two A1g peaks are superimposedwithin the prominent peak. Note that a small peak at∼370 cm−1 at 2.3 GPa between the two E2gmodes cor-responds to the disorder-activated longitudinal acous-tic (DALA) phonon mode [22–24], that represents thenature of the lattice disorder in the ternary compound.Further analysis of the measured Raman spectra usingthe Lorentzian curve fitting shows this peak to be pro-minent up to>35 GPa (figure S3).

Further increasing the pressure on the material, apeak, namely A* emerged at ∼8 GPa (near the A1g

peaks) and continued to grow in intensity (figures 2(a)and (b)). The fact that neither MoS2 nor WS2 havebeen reported to has a new peak develop at∼470 cm−1

at such a pressure range suggests that the A* peak isrelated to disorder of the metal atoms at pressures.Moreover, pressure-related activation and enhance-ment of the peak further implies that its origin is rela-ted to the disorder-activated out-of-plane vibrationmode. Increasing hydrostatic pressure is expected to

Figure 1.Characterization of structural and vibrational properties ofMo0.5W0.5S2 ternary compound at ambient conditions. (a)Arendering depicting the ternary compoundMo0.5W0.5S2 layered structure. Tungsten andmolybdenum atoms are randomlydistributed. (b) SEM–EDX spectrum showing fluorescence characteristics ofMo,W, and S elements. The peaks at energy range of7–12 keV associatedwith theWfluorescence signals are enlarged. (c)TEM image of aMo0.5W0.5S2 thinfilm showing the layeredpattern of thematerial. The inset shows the selected area electron diffraction (SAED)pattern exhibiting the diffraction pattern of theMo0.5W0.5S2 compound. The brighter spots correspond to the reflection of theWandMo atoms in the lattice, although they areindistinguishable in the pattern. The sulfur atoms are not resolved. (d)Comparison of the Raman spectra ofWS2,MoS2 andMo0.5W0.5S2 compound at ambient conditions.Mo0.5W0.5S2 compound has a convolution of the Raman bands observed in theMoS2andWS2materials. The inset shows the latticemovements for the E2g andA1g Raman vibrationalmodes.MoS2 andWS2 samples usedfor comparisonwere purchased fromSPI supplies and 2D semiconductors, respectively.

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decrease the inter-layer distance of the multi-layerMo0.5W0.5S2, and therefore would increase the inter-action between neighboring layers, which have a non-uniform distribution of Mo and Wmetal atoms alongthe c-axis. It is worthwhile to make a comparison withthe DALA, which is related to in-plane metal–metaldisorder and therefore observed even without externalpressure. Pressure dependence of A* peak similar tothat of A1g peaks further supports that the peak origi-nated from the out-of-plane disorder, rather than thein-plane vibration (figure 2(c)). The E2g modes andDALA mode stay convoluted to form a plateau near350–400 cm−1, which evolved together to higherwavenumber with increasing pressure (figures 2(a),and S4). Preserved shape of the plateau suggests theDALAmode survives up to higher pressure.

At ∼30 GPa, another peak (denoted here as A†)was observed at 350 cm−1 suggesting that theMo0.5W0.5S2 compound has further developed anadditional disorder-activated vibrational mode at thispressure (figure 2(b)). The A† peak continued toevolve to higher wavenumber at a rate of approxi-mately 1.4 cm−1 GPa−1 that is higher compared toneighboring E2g modes or DALA peak, but is rathercloser to that of A1gmodes or A* peak (figures 2(b) and(c)). This higher pressure dependence suggests that A†peak has closer lattice movement to A* peak ratherthan DALA peak. Therefore the origin of A† peakcould be speculated as another form of the disorder-activated vibrational mode [23, 24, 40]. However, wenote that other explanation for the origin of A† peakmay be possible. For example, considering the wave-number and pressure dependence of A†, E1g modes at286 cm−1 (MoS2) and 305 cm−1 (WS2) observedunder certain polarization [51–53] could be activatedby lattice distortion. Further experiments involvinginfrared, absorption spectroscopy, and x-ray diffrac-tion (XRD) as well as theoretical calculation are nee-ded to fully understand the origin of the A† peak. Note

that at 30.7 GPa, A† peak and E2g peaks are at slightlylower wavenumber and the intensity of A† is sig-nificantly larger compared to other pressures (opensymbols in figures 2(b), and S4 and S5). Other than theA† and E2g peaks, however, other Raman peaks are notaffected. We speculate that this phenomenon is corre-lated to lattice sliding like those observed in otherTMDmaterials [35, 48, 54, 55].

To further confirm the phonon evolution and itsorigin, the Raman shifts of the Mo0.5W0.5S2 com-pound are compared with Raman shifts of pristineMoS2 and WS2 at high pressures [34, 35, 50]. TheRaman frequencies of both A1g modes of Mo0.5W0.5S2as a function of pressure are in good agreement withthat of the A1g modes of MoS2 and WS2 (figure 3(a)).WS2-like A1g mode showed similar behavior to the A1g

mode inWS2.MoS2-like A1g mode, on the other hand,shows no phonon softening at 10–20 GPa where pureMoS2 experiences phonon softening during metalliza-tion [35]. Slight softening of phonon was observed atrather higher pressure of 25–35 GPa (figure 3(a)),speculating possible isostructural phase transition ormetallization similar to those of MoS2 end member,which requires further electrical conductivity mea-surements to confirm. The attenuated phonon soft-ening might be attributed to the random distributionof Mo/W atoms, or effect of localized metallizationcentered at Mo atom sites. The in-plane Raman modecomparison also provided a fairly goodmatch betweenMo0.5W0.5S2, MoS2, and WS2 (figure 3(b)), but withsome blue-shift in case of Mo0.5W0.5S2. These com-parisons suggest that out-of-plane vibration modes(A1g) are less affected by the disorder, due to the chal-cogen-sandwich structure screening effects of disorderin metal atoms. Note that the pressure dependence ofthe MoS2 and WS2 compounds is adopted from pre-viouswork [35, 50].

Studying the full-width-half-maximum (FWHM)change with pressure can clarify electronic changes in

Figure 2.Evolution of Raman bands ofMo0.5W0.5S2 compound under high pressure. (a)Representative Raman spectra withincreasing pressure.Most Raman bands exhibit blue-shift (phonon hardening) as a function of pressure. Appearance of A* andA†peaks are observed at high pressures. (b)The evolution of the Raman frequency shifts reveals the comprehensive pressure-effect onMo0.5W0.5S2. The twoA1g peaks and twoE2g peaks, as well as disorder-activated longitudinal acoustic (DALA) peak experience blue-shift with increasing pressure. The appearance of the A* andA† peaks, first observed at∼8 GPa and∼30 GPa, respectively, suggestincreasing disorder of the lattice. TheA†,WS2-like E2g, andDALApeaks at pressure 30.7 GPa aremarked as open symbols to note itsdeviation toward lowerwavenumber. (c)Pressure dependent Raman coefficient of the vibrational peaks using the linear-fitting to theRaman shifts as a function of pressure. A1g peaks andA

* peak showhigh pressure dependence compared to E2g peaks andA† peak.

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thematerial (figure 4) [35, 56]. In theMoS2, FWHMofboth the E2g and A1g peak slowly increase up to∼17 GPa where a significant drop is observed; thisimplies isostructural phase transition and metalliza-tion. For the Mo0.5W0.5S2 compound, the FWHM ofthe peaks monotonically increases without any abruptdrop up to∼40 GPa. Absence of such FWHMdecreasemay be attributed to suppression of isostructural

phase transition due to metal–metal disorder. Thebehavior of the DALA peak and the A* peak are quitedifferent compared to E2g and A1g peaks. DALA peakFWHM is not significantly affected by pressure. A*

peak FWHM decreased from its first observation at∼8 GPa through ∼30 GPa. The decrease of FWHMsupports that the A*mode activates and develops intoan explicit phonon mode as pressure increases and

Figure 3.Comparison of Raman shifts ofMoS2,WS2, andMo0.5W0.5S2 compound as a function of pressure. The pressure dependenceof the out-of-plane A1g phononmodes (a) and the in-plane E2g phononmodes (b) of the ternary compoundwere comparedwith thatof pristineWS2 andMoS2.Overall trends of the Raman shifts of the ternary compound are in good agreementwith the pristinematerials, but the E2gmodes show relatively less agreement compared to theA1gmodes suggesting a stronger effect of disorder.

Figure 4.Pressure-dependent full-width half-maximum (FWHM) of Raman peaks forMoS2,WS2, andMo0.5W0.5S2 ternarycompound. FWHMof (a) in-plane E2gmodes and (b) out-of-planeA1gmodes fromMoS2,WS2, andMo0.5W0.5S2 was compared tofurther understand the pressure dependence of each phononmode. The E2g andA1gmodes ofMo0.5W0.5S2 experienced an increase ofFWHMwith pressure, with the exception of theMoS2-like E2gmodewhich shows an undefined dependence at approximately 30 GPa.The FWHMof theDALA andA*modes did not appear to show any consistent pressure-induced changes.

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thus inter-layer distance decreases. After 30 GPa, theA* peak also widens due to pressure effect. The shift inthe FWHM could suggest Davydov splitting observedinGaSe [57], GaS [58], as well asMoS2 [59]. The effectsof pressure on increasing the FWHMcan be visualizedinfigure S4.

4. Conclusion

Hydrostatic pressure up to 40 GPa was applied toMo0.5W0.5S2 layered ternary compound to study thevibrational modes and understand the phonon andlattice distortion for high compressive forces. Our highpressure results show that with increasing pressure, thein-plane E2g modes and out-of-plane A1g modes followclosely to that of pristine MoS2 or WS2. MoS2-like A1g

peak experiences slight phonon softening at 25–35 GPa,where pure MoS2 experiences at 10–20 GPa withmetallization.Apeak (denotedA*) enhances as pressureincreases to ∼8 GPa, with similar pressure dependenceto A1g modes. The FWHM of the A* peak showed nonotable increase as increasing pressure, suggesting thatthis peak originates from the out-of-plane disorder ofmetal atoms. Another new peak (denoted A†) firstobserved at∼30 GPa, could signify a further developingdisorder in the lattice. To fully comprehend the originof newly reported A† peak, as well as phonon softeningof MoS2-like A1g mode, future study of the crystalstructure using XRD, and electrical conductivity mea-surementswill be of vital importance.

Acknowledgments

Research at The University of Texas at Austin wassupported in part by a Young Investigator Award (DA)from the Defense Threat Reduction Agency (DTRA).SP, IR, GP, DR, and JH acknowledges support fromthe Young Scholars program of the NASCENT Engi-neering Research Center (Cooperative Agreement No.EEC-1160494). JFL acknowledges the support ofNSAF (Grant No: U1530402). We also thank TianshuLi for his thoughtful discussions, and Jo Wozniak forher contribution tofigure illustrations.

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2DMater. 3 (2016) 025003 J-SKim et al

Supplementary Information

High Pressure Raman Study of Layered Mo0.5W0.5S2 Ternary Compound

Joon-Seok Kim1,*, Samuel T. Moran1,2,*, Avinash P. Nayak1,*, Shahar Pedahzur2, Itzel Ruiz2,

Gabriela Ponce2, Daniela Rodriguez2, Joanna Henny2, Jin Liu3, Jung-Fu Lin3,4, † and Deji

Akinwande1,2, †

1 Department of Electrical and Computer Engineering, The University of Texas at Austin, Austin, Texas

78712, USA

2 NASCENT NSF Engineering Research Center (ERC), Austin, Texas 78758, USA

3 Department of Geological Sciences, The University of Texas at Austin, Austin, Texas 78712, USA

4 Center for High Pressure Science & Technology Advanced Research (HPSTAR), Shanghai 201203,

People’s Republic of China

* These authors contributed equally to this work

† Corresponding authors

Figure S1. High-pressure Raman Experimental Setup Coupled with a DAC. (a) The DAC

setup along with the laser Raman spectroscopy used for the Raman vibrational experiments. (b)

A representative optical image of the sample chamber with the Mo0.5W0.5S2 ternary compound,

Ne pressure medium (transparent area), and a ruby pressure calibrant. A ruby sphere (red circle)

is used to measure the pressure inside the sample chamber of the DAC.

Figure S2. SEM-EDX Images of Mo0.5W0.5S2 Ternary Compound. The energy dispersive x-

ray spectroscopy (EDX) of a part of Mo0.5W0.5S2 sample with molybdenum atoms indicated in

red, sulfur atoms in blue, and tungsten atoms in green.

Figure S3. Lorentzian Fitting of Raman Spectra of Mo0.5W0.5S2 Ternary Compound at

High Pressure. Raman spectra, consisting of multiple Raman modes, were fitted using the

Lorentzian curve fitting method. Representative Raman spectra at (a) 2.3 GPa, (b) 10.6 GPa with

A* peak, and (c) 36.5 GPa with A† peak, are presented. Good fit of Lorentzian sum to the

measured spectra suggests reliable representation of the fits to the evolution of the Raman peaks

as a function of pressure.

Figure S4. Evolution of Raman Spectra for Mo0.5W0.5S2 Ternary Compound. Full stack of

Raman spectra from 2.3 GPa (lowermost) to 41.3 GPa (uppermost), measured every 1-2 GPa in a

diamond anvil cell. These spectra highlight the occurrence of the phonon hardening (blue-shift),

and enhancement of the ternary compound at applied pressures.

Figure S5. Representative Raman Spectra of the Mo0.5W0.5S2 Ternary Compounds at

Pressures Between 30 GPa and 41 GPa in the Lower Wavenumber Region. These spectra

were collected using similar experimental conditions including laser power and collection time.

At 30.7 GPa, WS2-like E2g, DALA, and developing A† peak experienced sudden deviation to

lower wavenumber for ~10 cm-1. Also, the intensity of A† peak is notably large at 30.7 GPa

compared to any other pressures. This suggests potential lattice distortion at the pressure.

However, thorough examination of structure is needed to fully understand the origin of this

deviation and strong peak intensity.