Abstract
The structural polymorphism in transition metal dichalcogenides (TMDs) provides exciting opportunities for developing advanced electronics. For example, MoTe2 crystallizes in the 2H semiconducting phase at ambient temperature and pressure, but transitions into the 1T' semimetallic phase at high temperatures. Alloying MoTe2 with WTe2 reduces the energy barrier between these two phases, while also allowing access to the Td Weyl semimetal phase. The WxTe2 alloy system is therefore promising for developing phase change memory technology. However, achieving this goal necessitates a detailed understanding of the phase composition in the MoTe2-WTe2 system. We combine polarization-resolved Raman spectroscopy with x-ray diffraction (XRD) and scanning transmission electron microscopy (STEM) to study bulk
WxTe2 alloys over the full compositional range x from 0 to 1. We identify Raman and XRD signatures characteristic of the 2H, 1T', and Td structural phases that agree with density-functional theory (DFT) calculations, and use them to identify phase fields in the MoTe2–WTe2 system, including single-phase 2H, 1T', and Td regions, as well as a two-phase 1T' + Td region. Disorder arising from compositional fluctuations in
WxTe2 alloys breaks inversion and translational symmetry, leading to the activation of an infrared 1T'-MoTe2 mode and the enhancement of a double-resonance Raman process in
WxTe2 alloys. Compositional fluctuations limit the phonon correlation length, which we estimate by fitting the observed asymmetric Raman lineshapes with a phonon confinement model. These observations reveal the important role of disorder in
WxTe2 alloys, clarify the structural phase boundaries, and provide a foundation for future explorations of phase transitions and electronic phenomena in this system.
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1. Introduction
Transition metal dichalcogenides (TMDs) are van der Waals (vdW) compounds that follow the general formula of MX2, where M is a transition metal from Groups IVB–VIB, and X is a Group VIIA chalcogen, such as S, Se, and Te. This chemical versatility leads to unique electronic properties, such as semiconducting behavior [1], superconductivity [2–4], and topological electronic states [5–8]. Furthermore, two-dimensional (2D) TMD monolayers can be vertically stacked without the need for lattice matching, which allows these dissimilar electronic phases to be combined in a single heterostructure [9].
The chemical versatility intrinsic to TMDs and the novel interactions achieved through vdW stacking are complimented further by the structural polymorphism of TMDs. A prototypical example is molybdenum ditelluride (MoTe2), which can be grown in a semiconducting 2H phase (space group P63/mmc) or a semimetallic 1T' phase (space group P21/m) [10–12]. The hexagonal 2H phase, characterized by a trigonal prismatic coordination, is thermodynamically stable under ambient conditions, while the monoclinic 1T' phase is stable above 900 ∘C. The 1T' phase can be stabilized at room temperature by rapid cooling [13], control of the tellurization rate of Mo films [10], or choosing appropriate precursors for chemical vapor deposition [14, 15]. When cooled below ∼250 K, 1T'-MoTe2 transitions into an orthorhombic Td phase (space group Pmn21) with broken inversion symmetry as evidenced by electrical, structural, and optical measurements [11, 16–18].
Interest in MoTe2 has surged due to the unique electronic properties of its structural phases as well as the possibility of engineering controlled transitions between these phases. For instance, type-II Weyl semimetal states were theoretically predicted and experimentally observed in both Td-MoTe2 and Td-WTe2 [6, 7, 19–23]. The broken inversion symmetry of the Td phase is a necessary condition for the type-II Weyl state [6, 19, 20] and allows for fundamental studies of interesting topological physics. However, efforts to directly observe the Weyl state using angle-resolved photoemission spectroscopy are frustrated by the presence of overlapping band-crossings and insufficient experimental resolution [20]. A more practical application driving investigations of MoTe2 is the development of atomically thin phase change memory. MoTe2 has a small energy difference between the 2H and 1T' phases, making the prospect of engineering on-demand transitions with low power consumption realistic [24–28]. Unfortunately, reversible and rapid phase changes in MoTe2 have yet to be demonstrated. This may suggest that the energy difference between the 2H and 1T' phases must be reduced further in order to successfully perform phase change operations.
The limitations of MoTe2 highlighted above can be addressed by alloy engineering. Substitutional doping of Mo with W atoms results in WxTe2 alloys which have properties advantageous for both fundamental investigations and practical applications.
WxTe2 alloys have been theoretically predicted [20] and experimentally confirmed [29] to be type-II Weyl semimetals. Importantly, the separation of the Weyl nodes in the alloys can be tuned with composition [20], which facilitates the observation of topological electronic states. Additionally, the ground state energy difference between the semiconducting 2H and semimetallic 1T' or Td phases in
WxTe2 alloys can also be tuned with composition [25, 27], thereby reducing the energy required to trigger a semiconductor-semimetal phase transformation. The desirable combination of tunable phase transitions with the low-dimensionality of TMDs makes
WxTe2 highly promising for phase change memory applications.
Application of the WxTe2 alloy system in the 2D limit necessitates an understanding of the compositional dependence of phase transformations and the impact of disorder upon the bulk material properties. The literature on this subject is very limited. In a pioneering work on
WxTe2 polycrystalline powders, Revolinsky and Beerntsen found that the alloys crystallize in the 2H phase for
, the Td phase for
, and a two-phase region of 2H + Td in between [30]. On the contrary, Champion detected a two-phase 2H + Td region only for
composition, whereas a higher (lower) W content resulted in
WxTe2 powders in a Td (2H) phase, respectively [31]. However, no detailed structural studies were reported in these two papers to shed light on the co-existence of the 2H and Td phases, especially considering a noticeable difference in their symmetry. Recently, Rhodes et al reported a simplified phase diagram without two-phase regions for single-crystalline
WxTe2 alloys grown by the chemical vapor transport method (CVT) with iodine or TeCl4 as the transport agent [32]. Finally, Lv et al and Yan et al suggested a more complicated phase diagram with a mixed 1T' + Td region that exists for compositions
in
WxTe2 alloys also grown using CVT, but with Br2 as the transport agent [33, 34]. This significant disagreement between the studies about phase boundaries between the phases in the MoTe2-WTe2 system, combined with the absence of thorough studies of compositional disorder on optical properties, calls for a fresh look at this alloy system.
Here, we provide a comprehensive examination of the WxTe2 alloy system by combining x-ray diffraction (XRD), scanning transmission electron microscopy (STEM), density-functional theory (DFT), and polarization-resolved Raman spectroscopy to explore the properties of the 1T', Td, and 2H structural phases in bulk
WxTe2 crystals grown by iodine-assisted CVT. Measurement of bulk flakes has the added advantage of minimizing the well-known degrading oxidation effects of Te-based TMDs [35]. XRD and STEM measurements indicate that the
WxTe2 alloys with the 1T' crystal structure are stable at elevated temperatures with W content
, while alloys with the Td structure are stable for
. The alloys with intermediate compositions
form a two-phase, 1T' + Td, mixture. Polarized Raman measurements offer further insight into the transition from 1T' to the two-phase, 1T' + Td, field and ultimately to the Td single-phase region as a function of x. We use Raman tensor analysis to assign the phonon mode symmetry for all compositions and find that the tensor elements must be complex in order to capture the polarization dependence. This observation is consistent with prior studies of MoTe2 [18, 36] as well as studies of other layered TMD materials [37]. The Raman peaks for certain phonon modes show particular sensitivity to x and lattice symmetry. For example, the Raman peak at 128
for the 1T'-Mo
WxTe2 alloys broadens at
and splits into a doublet for compositions
, which implies a loss of inversion symmetry [18] due to the substitution of Mo by W. We also observe the activation of a new Raman mode at 178
that is unique to
alloys. Based on our DFT calculations, we assign this feature as a disorder-activated infrared mode in MoTe2. Furthermore, the separation between the two modes near 260
in MoTe2 is highly composition-dependent and can be used to infer x. In
WxTe2, we observe minor changes in the
and
mode frequencies, linewidths, and relative intensities. We also identify a new Raman mode at 202
that originates from a double-resonance Raman process [38] and is apparently enhanced by alloy disorder. The comprehensive structural and spectroscopic data assembled here provide an important roadmap for the future study and application of 2D
WxTe2 alloys.
2. Methods
Polycrystalline WxTe2 alloys (
) were prepared by reacting stoichiometric amounts of molybdenum (99.999%), tungsten (99.9%), and tellurium (99.9%) powders at 750
C in vacuum-sealed quartz ampoules.
WxTe2 crystals were then grown by the CVT method using approximately 1.5 g of poly-Mo
WxTe2 charge and a small amount of iodine (99.8%, 5 mg
3) sealed in evacuated quartz ampoules. It was found that the temperature required for high-yield synthesis of
WxTe2 crystals is lower for higher x. Therefore, the growth temperatures used in this study were 1000
C for
, 950
C for
, and 900
C for
. The ampoules were ice-water quenched after 7 d of growth. To study phase transformation in
WxTe2, as-grown crystals were vacuum-sealed in small ampoules (internal volume
cm3) and annealed at 750
C for 72 h followed by cooling to room temperature at a rate of 10
C/hr.
Chemical compositions with an accuracy of 0.01 were determined by energy-dispersive x-ray spectroscopy (EDS) using a JEOL JSM-7100F field emission scanning electron microscope (FESEM) equipped with an Oxford Instruments X-Max 80 EDS detector6. We examined the θ– XRD patterns derived from a Norelco Philips Diffractometer with the Bragg-Brentano geometry. Lattice parameters were refined using the MDI-JADE 6.5 software package. For the powder XRD study,
WxTe2 crystals were finely ground using an agate mortar. An Aberration-Corrected High Angle Annular Dark Field Scanning Transmission Electron Microscopy (Cs-corrected HAADF-STEM) FEI Titan 80-300 TEM/STEM operating at 300 kV was employed for the characterization of
WxTe2 samples. The flakes were crushed in ethanol and a drop of solution was deposited onto an amorphous Carbon (a-C) coated TEM grid (Agar Inc.). HAADF-STEM images were collected at a camera length of 100 mm corresponding to inner and outer collection angles of 70.6 and 399.5 mrad respectively.
For Raman measurements, the as-grown WxTe2 crystals were mechanically exfoliated onto Si/SiO2 substrates. Polarization-dependent Raman measurements were performed on bulk flakes in a back-scattering geometry at room temperature in atmosphere using a linearly-polarized 532 nm continuous wave laser. The polarization of the excitation beam was controlled with a motorized achromatic half-wave plate and was focused onto the sample using a 0.75 NA microscope objective. The back-scattered Raman emission was collected by the same objective, and then sent through a motorized analyzer and a long-pass filter. The excitation polarization and collection analyzer were oriented in both co-polarized (
) and cross-polarized (
) configurations, and then rotated together while the sample remained fixed. The filtered light was focused into a multimode fiber to scramble polarization and then directed to a spectrometer for analysis. Raman peaks were fit to Lorentzian functions to extract mode frequencies, linewidths, and amplitudes. For angle-dependent Raman maps, measurements were normalized by the feature with the greatest intensity, the ∼163
peak. The angle-dependent peak intensities were fit using the Raman tensors to assign peak symmetries.
All simulations were based on density-functional theory (DFT) using the projector-augmented wave method as implemented in the plane-wave code VASP [39]. The simulations were performed using the vdW-DF-optB88 exchange-correlation functional [40], which provides an excellent description of the lattice constants of bulk 1T'-MoTe2, Td-MoTe2, and Td-WTe2. An energy cutoff of 600 eV and k-point mesh of 10 × 18 × 5 for the 1 × 1 × 1 unit cells of bulk 1T'-MoTe2, Td-MoTe2, and Td-WTe2 resulted in an accuracy of the total energies of 1 meV/unit cell. The 5s25p4 and 4d55s electrons were considered as the valence electrons for Te and Mo, respectively. Including the semicore 4s2p6 electrons for Mo had a negligible effect on the results, as, for instance, the lattice parameters of bulk 1T'-MoTe2 changed by less than 10%. -point phonon frequencies of bulk 1T'-MoTe2, Td-MoTe2, and Td-WTe2 were estimated from density-functional perturbation theory simulations of the
unit cells of respective materials. Irreducible representations of normal modes were obtained from the PHONOPY program [41] and the Bilbao Crystallographic Server [42]. The phonon dispersion of Td-WTe2 in the entire Brillouin zone was estimated by computing normal mode frequencies on a uniform three-dimensional mesh of
-points between (0, 0, 0) and (
,
,
) (figure S1 in the supporting information (stacks.iop.org/TDM/4/045008/mmedia)). The phonon dispersion of Td-WTe2 in the entire Brillouin zone was computed using the finite difference method on the 106 atom 3 × 3 × 1 supercell.
3. Results and discussion
Figure 1(a) summarizes heat-treatment schedules, compositions, and crystal phases of WxTe2 (
) samples examined in this study. The high-temperature phases of the alloys were preserved by quenching of the growth ampoules in an ice-water bath. This process is known to prevent reversal of the 1T' phase to the 2H phase, which is thermodynamically stable in MoTe2 under ambient conditions [43]. Notably, XRD θ-2θ scans from the as-grown, un-milled
WxTe2 flakes produce only 00l-type reflections and miss all asymmetric reflections, thus limiting the ability to reliably determine phase composition in the alloys. Therefore, we collected the scans from finely ground flakes to register all possible hkl reflections to distinguish 1T', Td, and 2H phases and their mixtures. For example, R. Clarke et al have established that a 1T' → Td transition in MoTe2 and the two-phase region can be observed by specifically monitoring h0l reflections as a function of temperature, e.g.
0 12 and 1 0 12 reflections of 1T'-MoTe2 coalesce into a single 1 0 12 reflection of the low-temperature Td phase [17]. A similar approach was applied to construct a phase diagram of
NbxTe2 alloys that undergo an orthorhombic to monoclinic phase transition with increasing x [44].
Figure 1. (a) Temperature-composition map of crystal phases in the MoTe2-WTe2 system, identified from the XRD analysis. Circles correspond to the samples that were quenched from the growth temperature indicated on the y-axis (for example, 3 adjacent circles in the upper-left corner represent WxTe2 alloys with
, 0.01, and 0.02 that were quenched from the growth temperature of 1000 °C), and squares correspond to the samples that were additionally annealed in vacuum at 750 °C after growth and slowly cooled to room temperature. Blue circles correspond to 1T' single-phase alloys, yellow circles and squares correspond to Td single-phase alloys, half-filled blue + yellow circles correspond to two-phase 1T' + Td mixture, and black squares are single-phase 2H alloys. The pink area at the bottom-left schematically denotes the single-phase 2H region, which extends from
to
at room temperature. (b) XRD θ–2θ scans of three most representative
WxTe2 samples with
(single-phase 1T' alloy),
(two-phase 1T' + Td mixture), and
(single-phase Td alloy). (c,d) Enlarged XRD scans near 006 and 008 reflections that compare single-phased
(1T') and
(Td) alloys with
two-phase mixture. Reflections position and hkl(s) for figures 1(b) and (d) are listed in table S2 in the supporting information. (e) and (f) HAADF-STEM images of
WxTe2 samples with (e)
in 1T' phase and (f)
in Td phase, together with fast fourier transforms (FFT) in the insets. Atomic models are superimposed on the images where Te atoms are blue spheres and Mo/W atoms are red spheres.
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Standard image High-resolution imageThe θ–2θ scans of three representative WxTe2 samples with
, 0.33, and 0.71, produced by milling as-grown flakes in an agate mortar, are shown in figure 1(b). Figures 1(c) and (d) show enlarged portions of the scans around 2θ angles of 35° and 48°, respectively, which illustrate the distinct changes in the lineshape with increasing x. The scans for
and 0.71 were unambiguously assigned to the 1T' and Td phase, respectively. The
scan can only be fitted by combining reflections from both 1T' and Td phases, which indicates a two-phase coexistence. Calculated lattice parameters, Bragg reflection angles for the three samples, and a detailed explanation of the protocol used for identifying structural phases are presented in tables S1, S2, and supporting note 1 in the supporting information. By analysis of the powder XRD scans, we established that the quenched Mo
WxTe2 alloy samples synthesized in this study are in the monoclinic 1T' phase for
, the orthorhombic Td phase for
, and in the 1T' + Td two-phase state for the compositions x between 0.04 and 0.63.
We further verified these observations by performing HAADF-STEM measurements of 1T'-Te2 and Td-
Te2 crystals, shown in figures 1(e) and (f) with the overlapped atomic models and their corresponding fast fourier transforms (FFT) in the insets. Both the 1T' and the Td phases exhibit a 'buckled' structure with visible shifts for Te atoms and a zig-zag pattern for Mo/W atoms. The presence of the two phases was observed on a sample with
, proving the 1T' + Td coexistence in Mo
WxTe2 alloys for
, although we were not able to map the spatial distribution of 1T' and Td phases.
In order to study temperature-induced phase transformations in MoWxTe2 alloys, the samples were annealed in vacuum-sealed ampoules at 750
for 72 h followed by slow cooling to room temperature (squares in figure 1(a)). We found that alloys with
could be converted to the hexagonal 2H phase, as schematically depicted in figure 1(a) by the pink-colored area. HAADF-STEM and XRD data of the
Te2 sample converted from 1T' into 2H phase are provided in figure S2 of the supporting information. The vacuum annealing did not change the crystal structures of the alloys with larger x. Thus, an upper limit for Mo
WxTe2 alloys to experience a reversible phase transformation between semiconducting 2H and metallic 1T' phases is
. Approximately the same boundary between metallic and semiconducting phases was recently reported for Mo
WxTe2 alloys [32, 33]. Additional studies are required to determine compositional dependencies of the phase transition temperatures in Mo
WxTe2 alloys, which are beyond the scope of this paper.
We now investigate the impact of composition, disorder, and crystal structure on the Raman-active phonon modes of MoWxTe2 alloys. We first examine the 1T' → Td phase transition in Mo
WxTe2, and then explore the impact of alloy potential fluctuations on the 2H phase. In order to minimize rapid degradation of the detected Raman signal due to surface oxidation of Te-based TMD layers in air [35], we focus exclusively on bulk Mo
WxTe2 flakes that are mechanically exfoliated onto Si substrates with a 285 nm SiO2 layer. Future measurements will assess layer-dependent properties, but will require exfoliation and encapsulation in an inert environment. The home-built confocal Raman microscope used in these measurements is oriented in a backscattering geometry and operated in two polarization configurations: one with the excitation polarization and analyzer co-polarized (
) and the other with them cross-polarized (
). The excitation/analyzer orientation is fixed and the two are rotated together relative to the crystal lattice. All measurements are performed at room temperature in atmosphere on bulk flakes. By acquiring a series of these spectra at different orientations, we assemble polarized Raman maps that provide a concise visualization of the angle-dependent Raman spectra as a function of x, which are shown for all 1T' and Td samples in figure S3 of the supporting information. Due to our experimental geometry, only Ag (A1) and Bg (A2) symmetry modes are accessible for the 1T' (Td) crystal structure. These modes have distinct dependencies on laser-analyzer orientation and the orientation relative to the crystal axes [36, 45]. In short, the polarized Raman signal is given by
, where
and
are the incident and scattered fields and
is the Raman tensor. In bulk MoTe2 and WTe2,
is complex-valued for all modes, suggesting that optical absorption is significant [36, 46, 47]. We summarize the results of the Raman tensor analysis fitting to the Raman peaks of 1T'-MoTe2 and WTe2 in figures S4, S5, and supporting note 2 of the supporting information. In table 1 we summarize the experimentally-determined mode assignments for 1T'-MoTe2 and Td-WTe2, as well as the results of our DFT calculations of the 1T'-MoTe2, Td-MoTe2, and Td-WTe2. In the 1T' phase, the Ag and Bg modes are Raman-active while the Au and Bu modes are only infrared-active. Interestingly, all modes are Raman active for the Td phase.
Table 1. Theoretically predicted and experimentally measured wavenumbers, and
, respectively, alongside their associated symmetries.
1T'-MoTe2 | Td-MoTe2 | Td-WTe2 | |||||
---|---|---|---|---|---|---|---|
![]() ![]() |
![]() ![]() |
Symmetry | ![]() ![]() |
Symmetry | ![]() ![]() |
![]() ![]() |
Symmetry |
76.81 | 78 | Ag | 76.96 | A1 | 79.0 | 80 | A1 |
85.56 | Ag | 85.74 | B1 | 84.6 | 91 | A2 | |
88.20 | Bg | 88.09 | B2 | 84.7 | B2 | ||
90.88 | Bg | 90.68 | A2 | 86.2 | B1 | ||
104.90 | 107 | Bg | 104.88 | A2 | 107.7 | B2 | |
105.61 | 111 | Bg | 105.52 | B2 | 107.7 | B2 | |
108.37 | Au | 108.37 | A2 | 110.9 | 111 | A2 | |
108.67 | Ag | 108.71 | A1 | 111.5 | 117 | A1 | |
110.80 | Au | 110.76 | B2 | 112.6 | B2 | ||
113.60 | 111 | Ag | 113.61 | B1 | 115.3 | B1 | |
115.38 | Bu | 115.53 | B1 | 120.6 | B1 | ||
123.82 | Bu | 123.23 | A1 | 127.2 | 132 | A1 | |
125.52 | Ag | 126.23 | A1 | 127.4 | B1 | ||
128.25 | 128 | Ag | 128.07 | B1 | 128.5 | 134 | A1 |
129.60 | Bu | 129.92 | B1 | 130.4 | 137 | A1 | |
134.80 | Bu | 134.84 | A1 | 131.3 | B1 | ||
155.54 | Ag | 155.59 | B1 | 146.3 | A2 | ||
159.24 | 163 | Ag | 159.35 | A1 | 146.6 | A2 | |
176.52 | Au | 176.64 | A2 | 152.4 | B1 | ||
176.97 | Au | 176.87 | B2 | 153.7 | A2 | ||
187.99 | Bg | 188.27 | A2 | 155.4 | A1 | ||
189.20 | 192 | Bg | 189.29 | B2 | 156.2 | B2 | |
192.13 | Bu | 192.11 | B1 | 165.6 | B1 | ||
192.33 | Bu | 192.25 | A1 | 166.2 | 164 | A1 | |
247.13 | Ag | 247.24 | A1 | 201.2 | A1 | ||
249.03 | Ag | 248.99 | B1 | 201.5 | B1 | ||
251.58 | 251 | Ag | 251.48 | A1 | 204.6 | 212 | A1 |
254.14 | 260 | Ag | 253.95 | B1 | 206.1 | B1 | |
265.96 | Bu | 266.14 | B1 | 227.9 | A1 | ||
267.37 | Bu | 267.42 | A1 | 228.4 | B1 |
The polarized Raman maps presented in figure S3 of the supporting information are instructional for an overview of the composition-induced evolution of the vibrational modes from the 1T' phase to the Td phase. However, the sensitivity of the Raman spectrum to the orientation of the excitation and analyzer complicates further interpretation of the data in this form. To eliminate this orientation-dependence, we sum the parallel and perpendicular data over all angular orientations (figures 2(e) and (f)). The composition-dependent Raman spectra of the alloys show several important features. Most modes are present in all compositions and exhibit only small frequency shifts due to the similarity between the 1T' and Td lattices (figures 2(a) and (d)). However, certain Raman modes (identified with boxes in figures 2(e) and (f)) exhibit unique behavior that is dependent on structural symmetry and composition x in the alloys.
Figure 2. (a) and (b) Three layer 1T'-MoTe2 and (c) and (d) Td-WTe crystal structure diagrams. The red boxes indicate the unit cells. (e) and (f) Polarized raman spectra for MoWxTe2 alloys with all crystal orientations summed for co-polarized (
) and cross-polarized (
) configurations, respectively. The x value for each alloy is labeled. Boxed regions are shown in figure 3.
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Standard image High-resolution imageWe direct our attention first to the Raman peak observed at 128 (box i of figures 2(e) and 3(a)). For
, this feature corresponds to an Ag symmetry mode in MoTe2, as demonstrated by our polarization-resolved measurements, DFT calculations [36], and other literature observations [10, 18, 48]. This mode is a single peak for compositions
, which is consistent with the inversion symmetric 1T' phase. For compositions
and 0.12, however, the 128
mode broadens and is best fit by a pair of Lorentzian functions. Finally, for
, the 128
mode splits into two well-resolved peaks. The separation between these two peaks is presented versus composition in figure 3(c) and illustrates the appearance and evolution of the doublet, which persists into the
(pure WTe2) case but with a smaller peak separation. Temperature-dependent electrical and XRD measurements have previously shown that MoTe2 undergoes a temperature-induced phase transition from the 1T' to Td crystal structure when cooled below 250 K [16, 17]. Recent temperature-dependent Raman measurements in [18] have also demonstrated that the Ag mode at 128
in MoTe2 splits into a doublet with A1 mode symmetry due to inversion-symmetry breaking upon transitioning into the Td phase at low temperatures. Our XRD measurements identify the
region as two-phase, and therefore we cannot interpret the appearance of the doublet as signifying a phase transition from 1T' to Td. Instead, we attribute the doublet to the breakdown of inversion symmetry in the 1T'-Mo
WxTe2 alloys, which originates not from a 1T' → Td phase transition, but instead from the random substitution of Mo atoms with W atoms. Alloying therefore provides a means of destroying inversion symmetry without eliminating the 1T' phase. From these observations, it is apparent that
W concentration is sufficient to drive a breakdown of inversion symmetry and suggests that Weyl physics may be observable even in this two-phase regime [6, 19].
Figure 3. (a) Magnified Raman spectra from box i of figure 2(e) highlighting the splitting of the 128 peak. (b) Magnified Raman spectra from box ii of figure 2(e). The 1T'-MoTe2 mode at 192
decreases with x while the Td-WTe2 mode at 212
increases. A new 'disorder peak' exclusive to the alloys appears at 178
. (c) Separation between the doublet in (a) versus x. (d) Normalized intensity versus x for the 192
1T'-MoTe2 mode (black squares), 212
WTe2 mode (red circles), and the 178
mode (blue triangles) identified in (b). (e) Magnified Raman spectra from box iii of figure 2(f). Example fits are shown as shaded blue and red Lorentzian functions. (f) The peak separation of the doublet in (e),
, versus x extracted from fits to the data in (e).
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Standard image High-resolution imageWe find that other modes also display sensitivity to compositional disorder and the substitution of Mo for W atoms. Box ii of figure 2(e) isolates MoTe2 and WTe2 Raman modes that evolve with changing composition, as well as a mode at 178 that is not present in pure 1T'-MoTe2 or Td-WTe2. We summarize the composition-dependent relative intensities for these three peaks in figure 3(d). The 'MoTe2 peak' refers to the feature at 192
(black squares) that is present only in Mo-rich compositions (small x) and is assigned as a Bg symmetry mode in MoTe2. The 'WTe2 peak' is the large 212
feature (red circles) present only in W-rich compositions (large x) and is assigned as an A1 symmetry mode in WTe2. Finally, the 'disorder peak' refers to the 178
mode unique to the alloys. The polarization dependence of the disorder peak in the
composition can be seen in figure S6 of the supporting information. The MoTe2 and WTe2 peaks appear to faithfully track the removal and addition of each atomic species, while the disorder mode appears at
, peaks at
, and disappears at
. The observed frequency agrees with an infrared-active, but Raman-forbidden, Au mode at 177
predicted by our DFT calculations (table 1). We therefore suggest that the disorder mode originates from an infrared mode that is activated by the loss of translation symmetry in the lattice. The combined effects of lattice disorder and reduced Mo content at large x values drives the mode to reach its maximum intensity at
, which also is the point where the ratio of the normalized intensities of the MoTe2 peak to the WTe2 peak approach unity. We note that similar activations of infrared modes by alloy disorder have been previously observed, particularly in GaxAl1-xAs [49].
Given the non-destructive nature and wide-spread use of Raman spectroscopy, it is desirable to determine alloy composition using a Raman-based method. The MoTe2 Raman modes present near 260 (box iii, figure 2(f)) provide a potential measure of the alloy composition, which we demonstrate in figures 3(e) and (f). We observe a pair of broad Raman modes near 260
in MoTe2 that are assigned as Ag modes (box iii of figures 2(f) and 3(e)) and have been seen in prior studies [36, 48]. By fitting these two peaks in each spectrum to Lorentzians, we can track the peak separation with composition. We find that the separation between these two features increases with increasing x, and that we can use it to estimate global W content in a Mo
WxTe2 crystal (figure 3(f)). Our results indicate that this method will be effective for
, and is therefore most appropriate for higher W concentrations.
Finally, we examine the primary WTe2 peak at 212 which is broad and asymmetric upon its appearance at
, but sharpens as
(figure 3(b)). We magnify this feature in figure 4(a) for select compositions. The asymmetric lineshape of the 212
peak provides valuable information regarding the incorporation of W into the Mo
W
Te2 lattice. We find that the asymmetry of this feature and its evolution with x can be well understood in the context of the phonon confinement model, also referred to as the spatial correlation model [49, 50]. The phonon confinement model accounts for relaxation of the
Raman selection rule by multiplying the Lorentzian function, used to represent standard Raman peaks in a pure crystal, with a Gaussian function of the form
. Thus, the intensity I of Raman peaks in the phonon confinement model is given by [49]
![Equation (1)](https://content.cld.iop.org/journals/2053-1583/4/4/045008/revision2/tdmaa7a32eqn001.gif)
where is in units of
,
,
, and
are the DFT-relaxed lattice parameters of WTe2,
is the full width at half maximum of the W peak for composition
,
is the dispersion relation which we calculate from DFT and shift to match the experimental value of
(figure S1 of the supporting information), and LC is the phonon correlation length. In a pure crystal, LC is infinite due to the translational symmetry of the lattice and results in plane wave eigenstates. The Gaussian factor in equation (1), in this case, is zero for all
except the
point, and therefore the
Raman selection rule is preserved. However, Mo
WxTe2 alloys exhibit potential fluctuations due to the substitutional doping on the transition metal sublattice. The random positioning of the dopant atoms destroys translational symmetry in the crystal, thereby yielding a finite LC and relaxing the
Raman selection rule. We fit the 212
W peak for
in background-subtracted Raman spectra with this model (red lines in figure 4(a)) using experimentally-derived parameters and the DFT calculated phonon dispersion. The extracted phonon correlation length LC is plotted versus x in figure 4(b), and is found to increase rapidly with x.
Figure 4. (a) Composition-dependence of the 212 cm Td-
mode. The red lines are fits to the phonon confinement model in equation (1). (b) LC extracted from the fits in (a) versus composition.
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Standard image High-resolution imageWe now comment on the Raman spectra of 2H-MoWxTe2 (
), which are shown in figure 5(a). The
(173
),
(234
), and
(289
) modes are visible in all compounds (figure 5(a)) and exhibit only small changes. The shifts in mode frequency and linewidth for the
and
modes are summarized in figures 5(b) and (c). For
, we find that the
and
modes develop asymmetric tails on the low and high energy sides of the peak, respectively. This asymmetry originates from a finite phonon correlation length as discussed previously and we note that the direction of the tail for each mode is consistent with the phonon dispersions of 2H-MoTe2 [51]. In addition, we identify a feature appearing at 202
for
alloys which we assign as a double-resonance Raman mode originating from the scattering of two longitudinal acoustic phonons from the M-point or an
and a transverse acoustic mode, both also from the M point [38]. This feature has only been observed in few-layer 2H-MoTe2 under resonant excitation [38], and its appearance in the bulk alloy samples is believed to originate from an enhancement in
Raman scattering processes by compositional disorder in the lattice.
Figure 5. (a) Normalized Raman spectra of 2H-Mo alloys measured in the co-polarized configuration. The double-resonance Raman mode is indicated with the black arrow. Inset: crystal structure of 2H-Mo
. (b) and (c) illustrate the evolution in frequencies (black, left axis) and linewidths (red, right axis) of the
and
modes, respectively.
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Standard image High-resolution image4. Conclusion
We have used XRD, STEM, DFT, and Raman spectroscopy to characterize the different crystal phases spanned by the MoWxTe2 alloy system. XRD and STEM measurements determined that 1T'/Td-Mo
WxTe2 alloys are in the 1T' phase for
and the Td phase for
. For compositions
, Mo
WxTe2 exists in a 1T' + Td two-phase mixture. Raman measurements enable the assignment of phonon mode symmetries across the compositional phase space and permit the observation of a new disorder-activated mode unique to Mo
WxTe2 alloys. Furthermore, we find that inversion symmetry breaking in the 1T' phase can occur without transitioning to an orthorhombic configuration by monitoring the splitting of the 128
peak. Finally, we find that the asymmetry of the primary WTe2 peak can be captured by the phonon confinement model, which in turn allows for the determination of the phonon correlation length. Our studies of the 2H phase show small changes in mode frequencies with x and provide evidence for disorder enhancement of double-resonance Raman scattering processes. These measurements are foundational for future studies seeking to explore the electronic, vibrational, or topological properties of Mo
WxTe2 alloys.
Acknowledgments
SMO, JJ, IRS, and PMV acknowledge support from the Office of Naval Research through Grant No. N-00014-15-1-2357, the George Mason University OSCAR Program, and the George Mason University Presidential Scholarship Program. RB thanks the National Research Council Research Associateship Programs for its support. SK acknowledges support from the US Department of Commerce, National Institute of Standards and Technology under the financial assistance award 70NANB16H043. AKS is funded by the Professional Research Experience Postdoctoral Fellowship under award No. 70NANB11H012. This research used computational resources provided by the Texas Advanced Computing Center under Contract TG-DMR150006. This work used the Extreme Science and Engineering Discovery Environment (XSEDE), which was supported by National Science Foundation grant number ACI-1053575. AVD, SK, IK, and AB acknowledge the support of Material Genome Initiative funding allocated to NIST.
Footnotes
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Disclaimer: certain commercial equipment, instruments, or materials are identified in this paper in order to specify the experimental procedure adequately. Such identification is not intended to imply recommendation or endorsement by the National Institute of Standards and Technology, nor is it intended to imply that the materials or equipment identified are necessarily the best available for the purpose.