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Unusual Pressure Response of Vibrational Modes in Anisotropic TaS Kedi Wu, Bin Chen, Hui Cai, Mark Blei, Juliana Bennett, Shengxue Yang, David Wright, Yuxia Shen, and Sefaattin Tongay

J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b10263 • Publication Date (Web): 22 Nov 2017 Downloaded from http://pubs.acs.org on November 28, 2017

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The Journal of Physical Chemistry

Unusual Pressure Response of Vibrational Modes in Anisotropic TaS3 Kedi Wu,† Bin Chen,† Hui Cai,† Mark Blei,† Juliana Bennett,† Shengxue Yang,‡ David Wright,§ Yuxia Shen,† and Sefaattin Tongay*,† †

School for Engineering of Matter, Transport and Energy, Arizona State University, Tempe, Arizona 85287, United States

‡

School of Materials Science and Engineering, Beihang University, Beijing 100191, People’s Republic of China

§

LeRoy Eyring Center for Solid State Science, Arizona State University, Tempe, Arizona 85287, United States

Abstract Here, we report on unique vibrational properties of 2D anisotropic tantalum trisulfide (o-TaS3) measured through angle-resolved Raman spectroscopy and high-pressure diamond anvil cell studies. Our broadspectrum Raman measurements first identify optical and low frequency shear modes in pseudo-1D TaS3 for the first time, and introduce their polarization resolved Raman responses to understand atomic vibrations for these modes. Results show that, unlike other anisotropic systems, only S∥ mode at 54cm-1 can be utilized to identify the crystalline orientation of TaS3. More notably, high-pressure Raman measurements reveal previously unknown four distinct types of responses to applied pressure, including positive, negative, and non-monotonic dω/dP behaviors which are found to be closely linked to atomic vibrations for involving these modes. Our results also reveal that the material approaches to isotropic limit under applied pressure as evidenced by much reduced (~70%) degree of anisotropy. Overall, these findings not only significantly advance our understanding of their fundamental properties of pseudo-1D materials, but also our interpretations on vibrational characteristics offer valuable insights about thermal, electrical, as well as optical properties of pseudo-1D material systems.

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Introduction Anisotropic two-dimensional (2D) materials are unique class 2D crystals with reduced crystal symmetry where in-plane atoms are arranged such a way that they form chain-like features extending along specific crystalline direction1-2. As a result of their unique crystalline structures, these material systems carry attractive properties of both 1D (like nanowires and nanotubes) and 2D (graphene, MoS2, etc.) material systems, and in a sense, provide material properties that fall between 1D and 2D materials.

Examples of these anisotropic, also known as pseudo-1D,

materials include black phosphorus3-6 (BPs), group-VII transition metal dichalcogenides7-14 (ReS2 and ReSe2), and group-IV transition metal trichalcogenides15-19 (TiS3 and ZrS3), etc. Owing to their highly anisotropic atomic arrangement, they possess polarization and direction dependent properties such as highly anisotropic electronic mobility20-22 and thermal conduction2324

, polarized excitons25-26, polarization dependent absorption/emission27-28 characteristics are

name to some. Among the pseudo-1D material family, layered tantalum trisulfide (TaS3) is known as a chargedensity-wave (CDW) material29-33 which crystallizes in both monoclinic (m-) and orthorhombic (o-) phase with both undergoing Peierls’ transition at low temperature34-35. Further studies have shown exciting findings on the orthorhombic phase of TaS3

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and presence of a

superconducting dome with a maximum transition temperature of 3.1 K 40. Despite our moderate knowledge on their electrical properties, however, our understanding of their fundamental vibrational properties of o-TaS3 remain at seminal stages. Considering their pseudo-1D highly anisotropic structural arrangement within 2D layers, their vibrational response is anticipated to possess vastly different behavior in comparison to other isotropic 2D sheets such as MoS2, GaSe, and etc. Lack of fundamental understanding of their vibrational properties and characteristics, in return, limits our understanding of their thermal properties, crystalline quality assessment, as well as identification of their anisotropy direction through non-destructive spectroscopy techniques. Here, we report on fundamental vibrational properties of unique pseudo-1D TaS3, and their structural and optical response to hydrostatic pressures through careful angle-resolved Raman spectroscopy (ARS) and diamond anvil cell (DAC) studies. Our studies reveal four distinct

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responses that are vastly different from other isotropic 2D systems: (1) Systematic ARS measurements establish phonon vibration modes involving atomic vibrations along the chain direction and establish Raman intensity variation with the polarization direction, enabling us to identify anisotropy direction through ARS measurements. (2) TaS3 under hydrostatic pressure display peak splitting at 3-4 GPa which is well below the pressure required for phase transition. This suggests that a variety of energetically degenerate Raman modes with different pressure coefficients (dω/dP) exist at atmospheric pressures. (3) Whist all Raman peaks stiffen (dω/dP > 0) with applied hydrostatic pressure, the high frequency peaks at ω0 = 484 cm-1 and 498 cm-1 exhibit previously unobserved non-monotonic behavior between 0 and 15.3 GPa. (4) Interestingly, the degree of anisotropy significantly decreases by ~ 70 % with applied pressure. Our systematic studies shed light on the origin of such anisotropic-to-isotropic crossover. This work not only marks the first investigation of the vibrational characteristics of pseudo-1D o-TaS3, but also provides us with a valuable fundamental insight into their pressure dependent response of its novel Raman modes.

Methods Material growth and characterization: The o-TaS3 whiskers were grown via chemical vapor transport (CVT) method. 1.5 grams of tantalum (Sigma Aldrich, ~ 325 mesh, 99.9%) and sulfur (Sigma Aldrich, 99.998%) precursors were loaded in a quartz ampule following stoichiometric ratio with 25 mg I2 added as the transport agent. The ampule was sealed under vacuum and placed in a two-zone furnace where the temperature of hot zone and cold zone was kept at 650 °C and 550 °C, respectively. After 7 days, the ampule was cooled down to room temperature naturally, and shiny ribbon-like TaS3 whiskers were found across the quartz ampule. The crystallinity of the as-grown o-TaS3 was further proved by the powder X-ray diffraction (XRD) technique using Cu Ka irradiation on Siemens D5000 X-Ray diffractometer. Scanning electron microscopy (SEM) was performed on AMRAY 1910 and Hitachi S4700 ﬁeld-emission SEM with a working distance of 12.7 mm and an acceleration voltage of 15 kV. The transmission electron microscopy (TEM) image and the diffraction pattern were recorded by FEI Titan 80300 TEM at 300 keV with a spherical aberration corrector. The thickness of exfoliated TaS3 was measured using atomic force microscopy (AFM), Dimension Multimode 8, under tapping mode,

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and the scanning rate was set to 1.0 Hz with a resolution of 256 × 256. Angle-resolved Raman spectroscopy: Polarized Raman spectra of exfoliated TaS3 were measured under the backscattering geometry, and the incident light and scattered light were aligned parallel to each other. The Raman spectra using blue laser (λ=488 nm) were performed in a Renishaw InVia spectroscopy system with a 50× objective lens, and the spectra using green laser (λ=532 nm) and red laser (λ=633 nm) were collected using an Acton 300i spectrograph and a back thinned Princeton Instruments liquid nitrogen cooled CCD detector. The sample was rotated every 15° from 0° to 360°. High-pressure Raman spectroscopy: The hydrostatic pressure was applied to TaS3 to 15.3 GPa using a diamond anvil cell (DAC). The TaS3 micro-crystal was placed in the pinhole of a gasket made of stainless steel T301. In the experiments, cesium iodide (CsI) was used as the pressure transmitting medium, and the pressure gauge was a small ruby crystal. The Raman spectra with increasing pressure were measured under long working distance 50× optical lens (numerical aperture 0.42) using green laser (532 nm) as the radiation source with a power of 6 mW. Results and discussion Tantalum trisulfide crystals in both monoclinic and orthorhombic phase, and the lattice parameters have been determined to be a = 9.52 Å, b = 3.34 Å, c = 14.91 Å, and β = 109.99° for m-TaS3, and a = 36.80 Å, b = 15.17 Å, and c = 3.34 Å for o-TaS3. Although the full atomic structure of the o-TaS3 remains vague, its unit cell has been described as stacking four slabs of m-TaS3 unit cells35, 39 (Figure S1), giving rise to as many as 96 atoms and 24 Ta chains. In our work, the o-TaS3 was grown via chemical vapor transport (CVT) method using tantalum and sulfur as precursors and I2 as the transport agent in an evacuated ampule at the 650 °C for 7 days with the temperature gradient kept at 100 °C. As shown in Figure 1a, typical growth yields shiny ribbon-like TaS3 whiskers across the quartz ampule because of its crystalline anisotropy. The crystal size of each individual whisker ranges from hundreds of microns to macroscopic centimeter-scale in length, and only a few microns in width, indicating its pseudo-1D nature.

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Figure 1 Material growth and characterization. a. As grown o-TaS3 whiskers in the sealed quartz ampule. b. Scanning electron microscopy (SEM) image of o-TaS3 whiskers showing its pseudo-1D nature. c. Transmission electron microscopy (TEM) and corresponding electron diffraction pattern (inset) of a single o-TaS3 nanoribbon. d. The optical image of an exfoliated o-TaS3 nanoribbon and height profile measured by atomic force microscopy (AFM). e. Raman spectra of o-TaS3 nanoribbon at selected alpha angle (α).

The as-grown TaS3 whiskers were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD, shown in Figure S2). The SEM image in Figure 1b shows ribbon-like structures of the as-grown TaS3, which further proves the highly anisotropic structure of the material. The bright field TEM image of a single TaS3 whisker is shown in Figure 1c, and the crystal directions can be determined from the electron diffraction pattern (inset), which indicates that the longitudinal direction of the whisker is aligned with [001] axis. In contrast to the monoclinic phase of TaS3 where the Ta chains propagates along its b-axis, the chain direction (also known as the whisker growth direction) of the o-TaS3 extends along the c-axis, resulting in the strong in-plane anisotropy. In the out-ofplane direction (a-axis), the TaS3 layers made of repeating parallel Ta chains are weakly coupled to each other via van der Waals interaction. Similar to MoS2 and graphene, TaS3 can be exfoliated down to thin layers as shown in Figure 1d optical image of o-TaS3 nanoribbon on 285 nm SiO2/Si wafers (AFM images in the inset). Due to its higher mosh hardness scale as well as

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geometrical anisotropy, however, typical exfoliation process only yields pseudo-1D ribbons that measure between 5 to 15 nm in thickness. Even though, our studies are in the few-layer limit, conclusions extend to monolayers since Raman peak position shifts by ~ 5cm-1 by thickness variation which is a negligible shift for the studies presented in this work.

Figure 2 Anisotropic Raman responses of o-TaS3 under excitation wavelength of λ = 532 nm. Angleresolved Raman spectroscopy (polar plots) for selected the Raman peaks at a. 54 cm-1, b. 58 cm-1, c. 159 cm-1, d. 276 cm-1, e. 337 cm-1, and f. 498 cm-1 at an increment of 15°. The data points show the Raman intensity at different angles (α), and the solid curves represent best fitting results.

Anisotropic Raman response. Due to their crystalline anisotropy, Raman / vibrational properties of pseudo-1D systems such as TiS3 and ReS2 exhibit high sensitivity to angle between chain direction and excitation polarization. Although the lack of atomic structure of TaS3 limits the ability to depict a full picture of the phonon vibrational modes and it warrants future theoretical and experimental studies, the angle-resolved Raman polar plots, however, allow one to identify the fundamental origin of variety of atomic vibrations of o-TaS3. In our studies, Raman spectrum of o-TaS3 was measured at different angles, and we find that intensity of the observed Raman peaks displays strong dependence on α (the angle between excitation E and caxis) and hence the crystalline orientation (Figure 1e). These angular Raman measurements were performed at 15° increments, and the Raman intensity of six characteristic Raman modes 6


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have been selected to generate a series of full 360° rotation polar plots (Figure 2), and a schematic diagram for these Raman modes is shown in Figure S3. At low frequency regime, o-TaS3 display prominent peaks located at 54 and 58 cm-1 which is well below anticipated optical branches. These peaks originate from shear displacements between adjacent TaS3 layers, and appear at low frequencies due to weak interlayer coupling. Out of these three modes, the 54 cm-1 mode exhibits a two-lobed feature with maximum value found at 0° and 180°, indicating enhanced Raman scattering when the E-field is along the chain direction. Since the Raman intensity correlates to the polarizability tensor, we conclude that 54 cm-1 peak is related to shear mode displacement of individual TaS3 layers along the chain direction. Thus, this peak is identified as S∥ and enables us to identify the Ta-chain direction through quick and non-destructive optical methods without any need for advanced microscopy techniques. We note that these shear vibrations can also take place in other directions. Thus, we observe two shear (S) Eigen modes with interlayer displacement parallel (S∥ ~54 cm-1) and perpendicular/non-parallel (S⊥~58 cm-1) to the Ta-chain direction with peaks at slightly different locations (∆ω(⊥-∥) ~ 4 cm-1). Since the chemical and atomic environment change is greater for displacement non-parallel to Ta-chain direction, S⊥ vibration frequency is blue shifted (higher) by ∆ω and its polar response in Figure 2b reflects displacement in other directions. The ARS measurements on the Raman peaks that are associated with optical phonon modes display either four-lobed (such as 159 cm-1 in Figure 2c) or two-lobed response with orthogonal relation to the c-axis peaks. For example, both Ag - like mode at 337 cm-1 (Figure 2e) and the Ags-s mode at 498 cm-1 (Figure 2f) exhibit polarization in 900 angle, suggesting strong atomic vibrations perpendicular to the Ta-chain direction, and cannot be utilized to assess crystalline direction. In addition to the selected Raman modes shown in Figure 2, the polar plots for all other the Raman peaks of o-TaS3 can be found in Figure S4. However, we note that many of the observed angle dependent responses and complex anisotropic Raman response are often attributed to the equally complex Raman tensor of each mode as well as the absorption of the incident laser. Due to the anisotropic light-matter interactions, Raman features can show strong variation with respect to laser wavelength such as black phosphorus (BP) and gallium telluride (GaTe). In our studies, we have focused our Raman measurements on green laser (λ=532 nm) since the Raman intensity is much enhanced compared to other excitation wavelengths due to 7



various reasons including larger absorption coefficient, larger (induced) polarization, and/or larger Raman scattering cross-section at that particular wavelength. For completeness, however, the angular dependence of different Raman modes measured using blue laser (λ=488 nm) and red laser (λ=633 nm) can be found in the supporting information (Figure S5). It is noteworthy that unlike other pseudo-1D materials, such as GaTe41-42, ReS243-45, ZrTe315, and TiS346, angleresolved Raman response of optical modes cannot be correlated to the Ta-chain direction. Instead, the crystalline orientation can only be assessed from low-frequency Raman shear mode, in particular the S∥ peak (Figure 2a).

Figure 3 High-pressure Raman spectroscopy measured using a diamond anvil cell (DAC). Raman spectra of o-TaS3 at selected pressures measured at two configurations with the excitation laser polarization a. parallel (α = 0°) and b. perpendicular (α = 90°) to the chain. c. Pressure dependent Raman peak position measured from ambient pressure to 15.3 GPa. d. The fitted dω/dP values of all the Raman modes. e. Four types of characteristic pressure dependent Raman response represented by peaks located at ω0 = 58 cm-1 for type I, ω0 = 404 cm-1 for type II, ω0 = 484 cm-1 for type III, and ω0 = 498 cm-1 for type IV. The experimental error range is within the size of the symbols.

High pressure Raman spectroscopy. To further understand the vibrational properties of o-TaS3, we have performed pressure dependent Raman studies on TaS3 under hydrostatic pressure using a diamond anvil cell (DAC) where the pressure gauge was a small piece of ruby. Cesium iodide (CsI) served as the pressure transmitting medium47. Since Raman intensity maxima of most

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peaks are seen when the excitation is either parallel (α = 0°) or perpendicular (α = 90°) to the chain, we focus on these two specific configurations and the pressure dependent Raman spectra are shown in Figure 3a and 3b. Upon application of pressure, Raman modes located at ω0 = 123, 144, 276, and 373 cm-1 start to split into two or more peaks above 3.7 GPa. In principle, the Raman peak splitting or the presence of new peaks can be attributed to phase transition48. For o-TaS3, however, peak splitting occurs only for select type of modes as opposed to peaks splitting for all Raman peaks due to Brilliouin zone changes during phase transition. Considering that Raman spectrum does not undergo sudden and drastic changes, and peak splitting is only valid for certain set of peaks at rather low pressures (3.7 GPa), we eliminate the possibility of phase transition. Instead, it is likely that complex atomic structure of o-TaS3 leads to Raman peaks made of multiple degenerate Raman modes at ambient pressure with different dω/dP pressure coefficients which results in peak splitting behavior at finite pressure values. Here, the peak position (ωp) as a function of pressure (Figure 3c) can be described as ωp = ω0 + (dω/dP)P with ω0 representing the peak position in ambient, and the fitted dω/dP values of each mode are given in Figure 3d. To further understand the pressure effect, we also define the (∆ω/∆P)Pi value as the pressure dependence (slope) between two successive data points as (∆ω/∆P)Pi = (ωi-ωi-1)/(Pi-Pi-1) with the positive integer i representing the ith pressure point in Figure 3e. Close inspection of Figure 3c-e show that observed Raman peaks display four distinct pressure dependence: Type-I with saturating ∆ω/∆P response with ∆ω/∆P approaching ~ 0 cm-1 GPa-1 above critical pressure (Pcritical ~ 8.5 GPa), type-II with positive ∆ω/∆P response across the entire pressure range, type-III modes with unique negative ∆ω/∆P response, and lastly type-IV behavior where modes first possess negative ∆ω/∆P but crossover to positive values above Pcritical. Raman modes of typical materials -including 2D material systems-, however, blue-shift monotonically upon application of pressure due to lattice stiffening effects. Such conventional stiffening response was also observed for most of the prominent optical modes (type II response) in the frequency range spanning from 100 to 450 cm-1 with positive and constant ∆ω/∆P >0 values. Here, observed type-II behavior is a conventional response observed for most (if not all) material systems including 2D materials and will not be further discussed.

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Unlike type-II response observed for optical Raman peaks, ultra-low frequency shear modes display positive but suturing pressure dependence (type I response). This is observed in the lowfrequency regime (ω < 100 cm-1) for shear modes located at S∥ ~ 54 and S⊥ ~ 58 cm-1, while the modes at 77 and 88 cm-1 did not yield sufficient Raman signal to reach any conclusion. From ambient to 8.5 GPa, shear Eigen modes of S∥ and S⊥ both exhibit stiffening behavior with positive dω/dP values of 1.01 ± 0.06 and 1.52 ± 0.02 cm-1/GPa (Figure 3d), respectively. Since S⊥ shear mode involves displacement perpendicular to Ta-chain, we attribute larger dω/dP value for S⊥ to high sensitivity to applied hydrostatic pressure caused by larger interlayer coupling strength. Interestingly, however, these peaks remain nearly unchanged above Pcritical~8.5 GPa, and dω/dP value thus the Gruneisen parameter (-dlnω/dlnV=(ωχT)dω/dP) approaches to zero. We note that type-IV modes at high frequencies exhibit negative to positive dω/dP crossover (Figure 3d) at similar hydrostatic pressure threshold (Pcritical). These results will be discussed in greater detail later in the article. The type III behavior represented by the mode at ω0 = 484 cm-1 gives negative values ranging from -1.5 to -3.2 cm-1/GPa without any monotonic trend. Since 484 cm-1 peak involves sulfursulfur (S-S) atomic vibrations46, increased pressure enhances the orbital interaction between atoms localized within adjacent layers, and effectively increases the distance (and coupling strength) between S-S by increasing the angle formed along S-Ta-S. We note that type-III peak at 484 cm-1 is not detectable above 8 GPa, thus this peak may also display negative to positive pressure coefficient cross-over. Interestingly, the type IV mode (ω0 = 498 cm-1) first displays negative pressure coefficient but crossover to positive above Pcritical ~ 8.5 GPa (Figure 3e, gray pentagons). Even though, this nonmonotonic response is not fully understood, it is possible that two competing effects, namely S-S softening and interlayer stiffening responses, play a vital role in determining their overall pressure responses. At low pressures, S-S vibration softens due to reduction in interlayer spacing and consequent repulsion between S-S pairs. As the pressure is increased, second and third order interlayer coupling effects start to dominate the overall response with positive pressure dependence, giving overall non-monotonic response. The degree of anisotropy under pressure. The applied hydrostatic pressure has shown

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tremendous stiffening and softening effects on vibrational properties of pseudo-1D o-TaS3, but how does the pressure influence its Raman anisotropy?

Figure 4 Raman anisotropy at high-pressure. Logarithmic scale (radial axis is set to natural log for better assessment) angle-resolved Raman spectra (polar plots) measured at ambient, 5.9 GPa, and 11.0 GPa for Raman modes located at a. ω0 = 54 and 337 cm-1 and b. ω0 = 337 cm-1 with Raman intensity converted to reader friendly natural logarithmic scale. c. Losing the degree of anisotropy (η) as a function of pressure. The experimental error range is within the size of the symbols.

Here, we define the degree of anisotropy (η) as the ratio between the maximum intensity over the minimum as ηω0 = Iω0, max / Iω0, min. This notation previously has been shown to be effective by our team as well as others in other systems. The peaks located at ω0 = 54 and 337 cm-1 (Figure 2a and 2e) have been selected to show this effect owing to their two-lobed features in the polar plots with the intensity maxima found either parallel or perpendicular to the chain direction (0° and 180°), and the polar plots for peak intensities in ambient, 5.9 GPa, and 11.0 GPa are given in Figure 4a and 4b where the intensity values are converted to a reader friendly natural logarithmic scale. It is noteworthy that the mode at ω0 = 54 cm-1 (Figure 4a) retain the two-lobed feature as well as the intensity maxima found along the chain direction, whereas the intensity minima found at 90° and 270° show obvious enhancement as pressure increases, indicating a reduced anisotropic Raman response. Similar response can be observed in the mode at ω0 = 337 cm-1 (Figure 4b) despite a 90° offset of the maximum intensity axis with respect to the chain direction. Therefore, the pressure effect on the degree of Raman anisotropy is consistent in these Raman modes regardless whether the intensity maxima are aligned parallel or perpendicular to the chain. To further understand the pressure effect, we have also investigated degree of anisotropy (η) of 11



these two modes as a function of pressure. As shown in Figure 4c, the η values of the ω0 = 54 cm-1 mode is greatly reduced by ~ 70% in the entire pressure range, and the mode at ω0 = 337 cm-1 also demonstrates a ~ 75 % drop in η with pressure increasing from ambient to 11.0 GPa. We argue that observed reduction in η values might be related to higher degree of coupling (stronger force constant) between neighboring pseudo-1D chains caused by applied hydrostatic applied pressure acting both in the c- and b-axis directions. However, further studies are necessary to shed light on fundamental mechanisms governing this phenomenon. Conclusion Our results introduce the very first detailed vibrational response of pseudo-1D orthorhombic phase of TaS3 through SEM, TEM, AFM, angle-resolved Raman, and high-pressure DAC Raman studies. We have identified low-frequency inter-layer rigid vibrational mode as sole Raman mode for identification of crystalline axis of pseudo-1D nanosheets for fast and nondestructive optical methods. Furthermore, pressure dependent studies have revealed four vastly different pressure coefficients for different modes which are explained within the tight-binding model. Lastly, our DAC studies have demonstrated the very first pressure induced anisotropic to isotropic crossover in pseudo-1D materials. Overall, findings significantly advance our understanding of their fundamental properties of pseudo-1D materials, and our interpretations on vibrational characteristics offer valuable insights that are related to their thermal, electrical, as well as optical properties. ASSOCIATED CONTENT Supporting information: Figure S1 Two potential unit cell structures of orthorhombic-TaS3; Figure S2 X-ray diffraction (XRD) pattern of o-TaS3; Figure S3 Schematic diagram for S∥ , S⊥, Ag and Ags-s modes; Figure S4 Angle-resolved Raman spectroscopy (polar plots) for additional the Raman peaks of o-TaS3 under excitation wavelength of λ = 532 nm; Figure S5 Comparison between the angle-resolved Raman response (polar plots) of o-TaS3 under three different excitation wavelengths: λ = 488 nm, 532 nm, 633 nm.

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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Kedi Wu: 0000-0003-4160-2457 Bin Chen: 0000-0002-2106-7664 Shengxue Yang: 0000-0002-3417-9702 Sefaattin Tongay: 0000-0001-8294-984X ACKNOWLEDGMENTS S. T. acknowledges support from NSF DMR-1552220 and Army Research Office, Materials Science, Physical Properties of Materials Program. K.W. acknowledges helpful discussions with Dr. Emmanuel Soignard. We gratefully acknowledge the use of facilities within the LeRoy Eyring Center for Solid State Science at Arizona State University. REFERENCES 1. Rahman, M.; Davey, K.; Qiao, S. Z., Advent of 2d Rhenium Disulfide (ReS2): Fundamentals to Applications. Adv. Funct. Mater. 2017, 27, 1606129. 2. Joshua, O. I.; Aday, J. M.-M.; Mariam, B.; Robert, B.; Eduardo, F.; José, M. C.; José, R. A.; Carlos, S.; Herre, S. J. v. d. Z.; Roberto, D. A., et al., Electronics and Optoelectronics of Quasi-1d Layered Transition Metal Trichalcogenides. 2D Mater. 2017, 4, 022003. 3. Ling, X.; Huang, S.; Hasdeo, E. H.; Liang, L.; Parkin, W. M.; Tatsumi, Y.; Nugraha, A. R. T.; Puretzky, A. A.; Das, P. M.; Sumpter, B. G., et al., Anisotropic Electron-Photon and Electron-Phonon Interactions in Black Phosphorus. Nano Lett. 2016, 16, 2260-2267. 4. Ribeiro, H. B.; Pimenta, M. A.; de Matos, C. J. S.; Moreira, R. L.; Rodin, A. S.; Zapata, J. D.; de Souza, E. A. T.; Castro Neto, A. H., Unusual Angular Dependence of the Raman Response in Black Phosphorus. ACS Nano 2015, 9, 4270-4276. 5. Gong, P.-L.; Deng, B.; Huang, L.-F.; Hu, L.; Wang, W.-C.; Liu, D.-Y.; Shi, X.-Q.; Zeng, Z.; Zou, L.-J., Robust and Pristine Topological Dirac Semimetal Phase in Pressured Two-Dimensional Black Phosphorus. J. Phys. Chem. C 2017, 121, 20931-20936. 6. Łapińska, A.; Taube, A.; Judek, J.; Zdrojek, M., Temperature Evolution of Phonon Properties in Few-Layer Black Phosphorus. J. Phys. Chem. C 2016, 120, 5265-5270. 7. Wu, K.; Chen, B.; Yang, S.; Wang, G.; Kong, W.; Cai, H.; Aoki, T.; Soignard, E.; Marie, X.; Yano, A., et al., Domain Architectures and Grain Boundaries in Chemical Vapor Deposited Highly Anisotropic ReS2 Monolayer Films. Nano Lett. 2016, 16, 5888-5894. 8. Tongay, S.; Sahin, H.; Ko, C.; Luce, A.; Fan, W.; Liu, K.; Zhou, J.; Huang, Y.-S.; Ho, C.-H.; Yan, J., et al., Monolayer Behaviour in Bulk ReS2 Due to Electronic and Vibrational Decoupling. Nat. Commun. 2014, 5, 3252. 9. Chenet, D. A.; Aslan, B. O.; Huang, P. Y.; Fan, C.; van der Zande, A. M.; Heinz, T. F.; Hone, J.

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