Associated Lattice and Electronic Structural Evolutions in Compressed

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Associated Lattice and Electronic Structural Evolutions in Compressed Multilayer ReS 2

Yalan Yan, Chunlin Jin, Jia Wang, Tianru Qin, Fangfei Li, Kai Wang, Yonghao Han, and Chunxiao Gao J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b01031 • Publication Date (Web): 21 Jul 2017 Downloaded from http://pubs.acs.org on July 24, 2017

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The Journal of Physical Chemistry Letters

Associated Lattice and Electronic Structural Evolutions in Compressed Multilayer ReS2 Yalan Yan1, Chunlin Jin2, Jia Wang1, Tianru Qin1, Fangfei Li1, Kai Wang1*, Yonghao Han1 and Chunxiao Gao1* 1

State Key Laboratory of Superhard Materials, College of Physics, Jilin University, No. 2699

Qianjin Street, Changchun 130012, P.R. China. 2

Electric Power Research Institute, State Grid Jilin Province, Jilin 130000, P.R. China.

Corresponding Author E-mail: [email protected]

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ABSTRACT. The transition metal dichalcogenide (TMD) ReS2 is a promising material for optoelectronic devices because of its remarkable quantum yield. Pressure can effectively tune the optoelectronic properties of TMDs through control of the atomic displacement. Here, we systematically investigated the lattice and electronic structural evolutions of compressed multilayer ReS2. Both Raman spectra and first-principles calculations suggest the occurrence of an intralayer phase transition followed by an interlayer transition. A transition from one indirect to another indirect bandgap at 2.7 GPa was revealed by both high-pressure photoluminescence (PL) measurements and first-principles calculations, this behavior was elucidated by considering the fundamental relationship between lattice variation and electronic evolution. Moreover, by comparing the high-pressure behavior of MoS2 and ReS2, we demonstrated interlayer coupling plays a critical role in determining the lattice and electronic structures in compressed TMDs. Our findings suggest the potential application of ReS2 in fabricating various stacking devices with tailored properties. TOC GRAPHIC


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2H-Transition metal dichalcogenides (TMDs) have attracted extensive attention because of their promising applications in electronic and optoelectronic devices.1-3 However, their optoelectronic properties are strongly dependent on the number of layers. For example, monolayer MoS2 possesses a direct bandgap; however, with more layers it transforms into an indirect-band-gap semiconductor.4 This behavior presents an obstacle for numerous device applications involving light harvesting or detection, for which thicker films with superior quantum yield are desired. Interestingly, the photoluminescence (PL) intensity of the TMD ReS2 has been observed to increase with increasing layer number, making it a potential candidate for such applications.5 The great disparity in the electronic structures of MoS2 and ReS2 can be attributed to the weak coupling of the adjacent layers in ReS 2, with a coupling energy < 8% of that of MoS2.5 This weak coupling arises from the low triclinic symmetry of ReS2 resulting from Peierls distortion5,6 (Figure 1a and b). Previous investigations have reported that multilayer ReS2 exhibits monolayer-like behavior because of its unique electronic decoupling.5 Increasing radiative recombination is thus expected with increasing layer number in ReS2 because of the increasing absorption of photons, which explains the prominent PL intensity observed in its multilayer form. Moreover, the interlayer coupling can also affect the stacking configurations of TMDs. More specifically, ordered stacking is preferred in interlayer-coupled MoS2 because of the significant energy difference among different stacking configurations.7 In contrast, shifting one monolayer over another in interlayer-decoupled ReS2 does not lead to any significant change in total energy; therefore, there is no preference for ordered stacking in this material.5 Briefly, the interlayer coupling in TMDs plays an important role in determining their lattice and electronic structures. However, thus far, studies on the effect of interlayer coupling on the lattice and electronic structures of TMDs have been confined to ambient conditions. It would be of interest to investigate the lattice and electronic structural evolutions of multilayer ReS2 upon compression to further disclose the effect of interlayer coupling, as pressure can effectively 3 ACS Paragon Plus Environment


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modify the interlayer coupling in TMDs. However, few works have investigated highpressure ReS2. A high-pressure X-ray diffraction (XRD) study revealed a phase transition in ReS2 at 11.3 GPa, but identification of phase II remained unclear because of the peaks broadening and overlapping at high pressures.8 Recent ab initio crystal structure searching by Zhou et al. predicted that the ambient “distorted-3R” phase transforms into a “distorted-1T” structure upon compression.9 They also confirmed that the “distorted-1T” structure undergoes a semiconductor–metal transition at approximately 70 GPa based on high-pressure transport measurements. Moreover, above 90 GPa, a metallic phase with tetragonal I41/amd symmetry and a critical superconducting temperature of approximately 2 K at 102 GPa was predicted. In this study, high-pressure Raman and PL measurements and first-principles calculations of multilayer ReS2 were used to further explore the evolution of its lattice and electronic structures under compression. The abnormal Raman behavior observed in compressed ReS2 revealed an intralayer and subsequent interlayer transition. High-pressure PL spectra and firstprinciples calculations demonstrated the occurrence of an indirect to another indirect interband transition at 2.7 GPa, which was elucidated by considering the fundamental relationship between lattice variation and electronic evolution in compressed multilayer ReS 2. In addition, the lattice and electronic structures of compressed MoS2 and ReS2 were compared. Our findings demonstrate that interlayer coupling plays a critical role in the evolution of the lattice and electronic structures in compressed TMDs and suggest the potential of multilayer ReS2 for application in electronic and optoelectronic devices with tailored properties. Figure 1c presents the Raman spectrum of multilayer ReS2 at ambient conditions. “A1g” and “Eg” represent the dominant out-of-plane and in-plane vibrations, respectively, and “Cp” represents the in-plane and out-of-plane coupled mode. The modes above 250 cm−1 are mainly attributed to S motions; whereas those below mainly originate from Re vibrations. 10 The pressure-dependent Raman spectra of multilayer ReS2 are presented in Figure 2a–c. Tiny shoulders emerged on the left side of Eg-1 and on the right side of Eg-2 at 1.1 GPa. The Eg-1 4 ACS Paragon Plus Environment

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and Eg-2 modes can be attributed to in-plane vibrations, which are degenerate in frequency along the a- and b-axis.10 Therefore, the application of finite pressure would break these degeneracies, resulting in the observed splitting. In a previous report, when strain was applied to monolayer ReSe2 placed on an elastomeric substrate, splitting of the Raman mode at 125 cm−1 was observed, which resembled the Eg-2 mode in ReS2.11 Thus, it can be concluded that the application of both pressure and strain are effective methods to induce lattice distortion in Re dichalcogenides.

Figure 1. (a) Top and (b) side view of the crystalline structure of distorted-3R ReS2. (c) Raman spectrum of multilayer ReS2 at ambient conditions. Multiple peak fitting with Lorentz profile curves is shown using different colors. (d) PL spectra of multilayer ReS2 at ambient conditions. Multiple peak fitting with Gaussian profile curves is shown using different colors.



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The dashed blue curve represents the contribution to the PL from an indirect optical gap. (e) Calculated bandgap of ReS2 at ambient conditions.

Figure 2. Selected high-pressure Raman spectra of multilayer ReS2 in the regions of (a) 125– 190 cm−1, (b) 205–270 cm−1, and (c) 265–600 cm−1. Multiple peak fitting with Lorentz profile curves is shown using different colors in (a) and (b). The emerging splitting mode and depleted mode under high pressure are indicated by solid and dashed red arrows in (c), respectively. (d, e) Pressure-dependent Raman modes in multilayer ReS2. Linear fits to the observed frequencies are indicated by solid lines. (f) Pressure-dependent intensity ratio between Eg-3 and Eg-4 modes. The dashed red lines serve as visual guides for the intensity 6 ACS Paragon Plus Environment

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ratio. Above 6.8 GPa, a shoulder emerged on the right side of the Eg-4 mode (Figure 2b). Considering that the Eg-4 mode mainly originates from the in-plane vibration of Re atoms along the diagonal line of the Re4 unit,10 the splitting should be related to changes of the force constants of the Re atoms. Specifically, with increasing pressure, intralayer distortion of the Re4 unit occurs in ReS2, resulting in the splitting. Crucial details about the evolution of interlayer coupling upon compression were obtained by integrating our spectroscopic results with an analysis of the displacement patterns. The pressure-dependent Raman modes are shown in Figure 2d and e. Despite all the vibration modes exhibiting hardening, the pressure coefficients for the A1g-3 and A1g-4 modes were obviously higher. As the A1g-3 and A1g-4 modes involve the out-of-plane vibrations of S atoms or ReS2 slabs expanding against each other,10 the apparent larger pressure coefficient for A1g-3 and A1g-4 modes indicates that the out-of-plane modes of S atoms are much stiffer than the other modes, signifying more effective compression along the c-axis than along the aor b-axis. With increasing pressure, the layers have increasingly less room to expand, resulting in stronger interlayer coupling. We also traced the evolution of the pressure coefficients of each mode: distinct inflection points were observed at 8.0 and 19.6 GPa (Figure 2d and e). These anomalies in the Raman spectra are expected for materials undergoing a pressure-induced phase transition, and have been observed in numerous TMDs.12,13 Referring to previous high-pressure XRD results and the prediction of a high-pressure phase for ReS2,8,9 we attributed the anomalous Raman behavior between 8.0 and 19.6 GPa to the intralayer transition and a subsequent interlayer transition, as discussed below. The phase transition was further confirmed by the pressure-dependent intensity ratio between the Eg-3 and Eg-4 modes (Figure 2f). The intensity ratio rapidly decreased with increasing pressure followed by a decline in the rate of decrease after 8.0 GPa. It is well 7 ACS Paragon Plus Environment


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known that symmetry is crucial to the Raman intensity, because it directly affects the matrix elements of the polarizability tensors.14 Thus, the inflection points at 8.0 GPa are a qualitative indicator of the onset of the phase transition. Above 19.6 GPa, an inversion of the peak intensities for the Eg-3 and Eg-4 modes was observed (Figure 2b), and the intensity ratio remained almost constant (Figure 2f), suggesting the completion of the phase transition at 19.6 GPa and the existence of a stable high-pressure phase up to 33.7 GPa. The effect of pressure on the crystallographic structure of ReS2 was further elucidated using first-principles calculations. The calculated unit cell lattice parameters of distorted-3R ReS2 upon compression are listed in Table 1 (in Supporting Information). The lattice length ratios (a/a0, b/b0, c/c0) and cell angle ratios (α/α0, β/β0, and γ/γ0) as a function of pressure are shown in Figure 3; these results indicate that the c-axis was clearly more compressive than the a- or b-axis, which is consistent with the Raman spectra.

Figure 3. Calculated (a) cell parameter and (b) cell angle ratios of ReS2 as a function of pressure, where a0, b0, c0, α0, β0, and γ0 represent the unit cell parameters at ambient conditions. In addition, β and γ (α) decreased (increased) upon compression compared with those of the initial structure (0 GPa), (Figure 3b) which signifies a bending (stretching) effect for β and γ (α) under high pressure. With increasing pressure, γ changed much more slowly than α or β; therefore, the stress effects of the combination of α and β play an important role in 8 ACS Paragon Plus Environment

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determining the structure of ReS2. This finding indicates a counterclockwise rotation of the S atoms around the chain of Re4 atoms along with a slight distortion of the Re4 units, thus explaining the sharp Raman anomalies at 8.0 GPa. In contrast with the prediction of a highpressure phase for ReS2,9 a counterclockwise rotation of the S atoms around the chain of Re4 atoms is indeed required to transform the initial distorted 3R into distorted 1T ReS2. With increasing pressure, both the Eg-3 and A1g-4 modes split at 15.4 GPa (Figure 2b and c). Previous reports have suggested that the Eg-3 mode involves out-of-plane vibrations of the S atoms along with in-plane vibrations of Re atoms and that the A1g-4 mode results from the out-of-plane vibrations of S atoms.10,15 As the distortion of Re4 units occurs at a much lower pressure (6.8 GPa), as discussed above, the splitting phenomena at 15.4 GPa should be attributed to the changes of force constants of S atoms. Previous theoretical calculations have indicated that ambient ReS2 is randomly stacked, because shifting one ReS2 monolayer over another does not lead to any significant change in total energy owing to its weak interlayer coupling.5 However, the stacking configuration upon compression should be quite different from that at ambient conditions as the stronger interlayer coupling under high pressure would prompt an ordered stacking;16,17 thus, an interlayer transition from disordered to ordered stacking was expected at 15.4 GPa. At this point, the interlayer force constants change, resulting in the split. In fact, the effect of the changes in layer stacking configurations of TMDs on the out-of-plane vibrations of S atoms has been previously demonstrated by various groups.18,19 Notably, our results are consistent with those of Zhou et al., who also predicted the pressure-induced interlayer sliding of ReS2 from the distorted-3R to distorted-1T phase, where all Re atoms in the distorted-1T phase share the same (x, y) coordinate.9 Further compression led to the disappearance of separate peaks for the Eg-1 and Eg-2 modes at 18.7 and 19.6 GPa, respectively (Figure 2a). Moreover, the Cp-2 mode was significantly depleted at 17.5 GPa and finally disappeared at 19.6 GPa (Figure 2c), further signifying the 9 ACS Paragon Plus Environment


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completion of the phase transition at 19.6 GPa. After removing the pressure, the frequencies of all the Raman modes returned to their initial values (Figure 2a–c), demonstrating the reversible character of the lattice structure. As the lattice structure of multilayer ReS2 was successfully modulated by pressure, the electronic-related behavior should also be sensitive to pressure; thus, PL spectra were obtained and analyzed. A previous report claimed that ReS2 is a direct-band-gap semiconductor with a 1.55-eV Γ→Γ bandgap.5 However, a later investigation reported an optical transition in ReS2 exhibiting indirect character with lower energy than the direct gap based on PL and reflectivity contrast measurements.20 To clarify this discrepancy, Ignacio et al. used ionic liquid field-effect transistors to confirm that ReS2 is an indirect-bandgap semiconductor with a bandgap of 1.41 eV.21 Moreover, recently, both high-resolution angleresolved

photoemission

spectroscopy

(unpublished

results)

and

theoretical

calculations22,(unpublished results) have confirmed the indirect character of the bandgap in ReS2. It is worth mentioning that the indirect (1.41 eV) and direct (1.5 eV) bandgap are almost degenerate.21 In this case, hot carriers are expected to occupy transiently the available states around the Γ symmetry point, leading to a prominent hot PL. This hot PL mode has been proposed and justified by Mak et al.23 Figure 1d presents the PL spectrum of multilayer ReS2 at ambient conditions. Previous PL and reflectance spectra of ReS2 have demonstrated that peak I can be attributed to an indirect optical transition, whereas peak B can be attributed to three optical transitions, which can be fitted by three peaks with Gaussian profiles, as illustrated in Figure 1d.20 According to the calculated bandgap of ReS2 at ambient conditions (Figure 1e), peak I at approximately 1.49 eV is assigned to the indirect interband transition from the G point to the valence band maximum (VBM) along the G–Y line. In contrast, the interband transition in peak B is difficult to assign owing to the rather complex optical transition and the unavoidable errors in the multiple peak fitting and theoretical calculation. Therefore, we focused on the evolution of 10 ACS Paragon Plus Environment

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the indirect bandgap upon compression, which is essential for revealing the electronic evolution in compressed multilayer ReS2. To clarify the contribution from the indirect optical transition, multiple peak fitting with Gaussian profile curves was performed for the PL spectra of multilayer ReS2 under different pressures (Figure S1). As Figure 4a shows, our PL spectra suggest the occurrence of a clearly pressure-induced changeover of peak I at approximately 2.7 GPa. For 0.2 < P < 2.7 GPa, peak I showed a blue shift at a rate of 5.2 meV/GPa. However, for 2.7 < P < 5.0 GPa, peak I* showed a red shift at a rate of –12.2 meV/GPa. This blue–red shift changeover was supported by further first-principles calculations (Figure 4b and c), which showed that the indirect bandgap of ReS2 slowly increased with increasing pressure until the valley along the T–Z line became the conduction band minimum (CBM) at 4 GPa, because the compressive strain induced a downward shift of the valley along the T–Z line (Figure 4b). After 4 GPa, the indirect bandgap of ReS2 rapidly decreased with increasing pressure (Figure 4c). That is, peak I related to the indirect interband transition from point G to the VBM along the G–Y line at ambient conditions became peak I*, which is associated with the interband transition from the CBM along the T–Z line to the VBM along the G–Y line after 2.7 GPa. Thus, a changeover from one indirect to another indirect interband transition occurs at approximately 2.7 GPa in multilayer ReS2. As demonstrated in Figure 4c, both the high-pressure PL spectra and first-principles calculations confirm that the energy of peak I* is more sensitive to pressure than that of peak I. To understand further the underlying micromechanism for this behavior, the partial density of states (DOS) of ReS2 at 0 and 6 GPa was calculated. Peak I is associated with the separation between the VBM and CBM before 2.7 GPa, where both the VBM and CBM derive from the Re d states, as shown in the insert in Figure 4d at 0 GPa. That is, peak I originates from the transition from Re d state to Re d state. In contrast, peak I* is related to the gap between the VBM and CBM after 2.7 GPa, where VBM derives from the S p state and CBM derives from 11 ACS Paragon Plus Environment


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the Re d state, as shown in the inset in Figure 4d at 6 GPa. That is, peak I* originates from the transition from Re d state to S p state. With increasing pressure, the S atoms rotate around the chain of Re atoms, and the interlayer coupling increases in multilayer ReS 2. Therefore, the optical transition associated with the S atoms should be more sensitive to pressure than that related to the Re atoms, because the Re atoms are insensitive to interlayer coupling due to their position in the middle of the sandwich structure. This difference explains the different energy evolution of peaks I and I*, which can be mechanistically elucidated by considering the correlations between structure and bandgap. In fact, this correlation between lattice and electronic evolution was also observed in multilayer MoS2, with an abrupt decrease of resistivity of approximately three orders of magnitude accompanied by an anomalous behavior in Raman spectra confirmed at approximately 10 GPa.24


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Figure 4. (a) Representative PL spectra as a function of pressure. (b) Calculated band structure of multilayer ReS2 at different pressures. (c) Energies of peaks I and I* at different pressures. (d) Calculated partial DOS for the distorted-3R ReS2 at 0 and 6 GPa. The insets show enlarged images of the partial DOS at 0 and 6 GPa. Additionally, the symmetry of the PL profiles changed at 7.1 GPa (Figure S1). Below 7.1 GPa, the PL spectra were asymmetric due to various optical transitions occurring in different polarization directions within the in-plane crystal,25 as ReS2 possesses a low-symmetry triclinic structure resulting from Peierls distortion.5,8,20,26 Nevertheless, above 7.1 GPa, the PL spectra were well fitted by a single peak with Gaussian profile curves. As grain boundaries (defects) may be introduced in compressed multilayer ReS2, the possibility of defect-induced PL emission above 7.1 GPa should also be considered; thus, high-pressure adsorption spectra for multilayer ReS2 were obtained. The adsorption spectra and derived optical absorption edge as a function of pressure in multilayer ReS2 are presented in Figure S2. Figure S2b shows that above 7.3 GPa, the optical absorption edge decreased more rapidly with increasing pressure. On the one hand, the rapidly decreasing optical absorption edge may be associated with the pressure-induced phase transition at 8.0 GPa, as previous first-principles calculations have demonstrated the continuously decreased bandgap of multilayer ReS2 (without defects) after a phase transition.9 On the other hand, grain refinement is unavoidable in a compressed single crystal; therefore, the generation of grain boundaries after 7.1 GPa in multilayer ReS 2 may also be attributed to the rapidly decreasing energy of the optical absorption edge. Previous optical absorption measurements have demonstrated that the increasing volume of grain boundaries in nanocrystalline ZnO thin films can induce a decrease of the bandgap, because the varying electrostatic potential at the grain boundaries can enhance band bending, thereby altering the optical absorption edge.27 To explore further the derivation of the optical absorption edge, we compared the energy differences between the optical absorption edge and PL emission in multilayer ReS 2 after 7.1 13 ACS Paragon Plus Environment


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GPa. Notably, above 7.1 GPa, the optical absorption edge (Figure S2b) was obviously smaller than the energy of PL emission at the same pressure (Figure S2c), indicating that the rapidly decreasing optical absorption edge after 7.1 GPa does not originate from the phase transition in compressed multilayer ReS2. Otherwise, the energy of the optical absorption edge should have been larger than that of PL emission at approximately 1.3 eV according to the theory of interband optical transitions. A reasonable explanation is that grain refinement is unavoidable in a compressed single crystal; therefore, the generation of grain boundaries after 7.1 GPa in multilayer ReS2 results in a rapidly decreasing optical absorption edge. Thus, if radiative combination occurs at the grain boundaries, their PL emission energy should be smaller than that of the absorption edge; therefore, the observed PL emission at approximately 1.3 eV after 7.1 GPa can be attributed to the radiative recombination in grains rather than at grain boundaries. In addition, it is difficult to confirm the combination form at the grain boundaries using Raman spectroscopy owing to its finite measurement range; therefore, further investigation is needed. The micromechanisms of the radiation transition for single-crystal multilayer ReS2 and compressed multilayer ReS2 with grain boundaries (defects) are schematically illustrated in Figure S3a and b, respectively. In general, pressure tends to destroy the Peierls distortion and leads to a high-symmetry structure.9 The more symmetrical PL signal after 7.1 GPa may indicate a reduction of the in-plane anisotropy in the grain upon compression, further confirming the pressure-induced intralayer transition discussed above.

Figure 5. Calculated (a, b) in-plane and (c, d) out-of-plane ELF of distorted-3R ReS2. 14 ACS Paragon Plus Environment

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After 7.1 GPa, red shifting of the PL peak was observed with quenching above 9.5 GPa (Figure 4a). It is speculated that further compression would lead to the movement of Re-d orbitals and S-p orbitals toward the Fermi level, with the separation between this bandgap continuously decreasing and finally resulting in metallization.9 Upon compression, the interlayer spacing decreases and the intra- and interlayer interactions are expected to alter the electronic structure of ReS2. To elucidate further the effect of intra- and interlayer interactions on metallization, both in-plane and out-of-plane electron location functions (ELFs) of distorted-3R ReS2 at 0 and 6 GPa were calculated (Figure 5). Upon compression, the interlayer S–S interactions increased more obviously than the intralayer Re–S interactions, because the c-axis was more compressible than the a- or b-axis. Thus, delocalization of the electrons between the ReS2 layers is expected with further compression, which would result in metallization. That is, the metallization can be attributed to the closure of the bandgap owing to the stronger S–S interactions with decreasing interlayer distance. Notably, the emission intensity of the PL peak significantly decreased with increasing pressure and was difficult to detect after 9.5 GPa (Figure 4a), indicating that multilayer ReS2 retains its indirect bandgap character after the phase transition. In general, such quenching can be attributed to decreased excitons in the radiative transition induced by Brillouin zone deformation. In detail, the monolayer behavior in multilayer ReS2 should be confined under high pressure because the interlayer coupling is strengthened with increasing pressure, which leads to a decreased PL intensity. The effect of interlayer coupling was also demonstrated in MoS2, where the PL intensity decreased with increasing layer number.28,29 In addition, the formation of grain boundaries in compressed multilayer ReS2 may significantly weaken the PL intensity of the hosting material because of the presence of a significant amount of nonradiative recombination,30 which reduces the radiative recombination probability through



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interaction at the defect sites. The PL signal reappeared upon quenching to ambient conditions (Figure 4a), demonstrating the reversible electronic character. The lattice and electronic structure evolution of multilayer MoS2 clearly differs from that of multilayer ReS2. Their pressure-induced transition sequences are quite different. A previous report has demonstrated that multilayer MoS2 undergoes interlayer sliding from 2Hc to 2Ha stacking at approximately 20 GPa.24 Our investigation confirmed the intralayer phase transition followed by an interlayer transition from disordered to ordered stacking in compressed multilayer ReS2, this obvious difference in the interlayer transitions of MoS2 and ReS2 originates from the adjacent layers in ReS2 only being weakly coupled to each other at ambient conditions, with a coupling energy < 8% of that of MoS2.5 The interlayer transition from disordered to ordered stacking is preferred in interlayer decoupled TMDs such as ReS 2, because the increased interlayer coupling upon compression should promote ordered stacking.16,17 In contrast, interlayer sliding is expected in interlayer coupled TMDs, such as MoS2, because of the significant energy difference in different stacking configurations. 7 It is expected that further compression would drive interlayer sliding in multilayer ReS2; however, further investigation is required. In addition, the pressure-induced electronic structure evolution of multilayer ReS2 is quite different from that of multilayer MoS2. Multilayer MoS2 (or ReS2) undergoes an electronic transition from a semiconducting to metallic state at approximately 19 (70) GPa with an average pressure coefficient of –63 (–21) meV/GPa for the bandgap. The more easily approached metallization in multilayer MoS2 can also be attributed to its much stronger interlayer coupling compared with that of ReS2.5 As discussed above, the metallization of multilayer MoS224 and ReS2 arises from the closure of the bandgap because of the increasing interlayer S–S interactions with decreasing interlayer distance. The interlayer overlap of wave functions is more prominent in MoS2 thanks to its stronger interlayer coupling compared with 16 ACS Paragon Plus Environment

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that of ReS2, which results in metallization being more easily attained in multilayer MoS2 than in ReS2. As discussed above, interlayer coupling plays a critical role in determining the lattice and electronic structural evolutions in compressed TMDs, which suggests the potential application of multilayer ReS2 in designing and fabricating different stacking nanometer devices with tailored properties for electronic and optoelectronic applications because of its more adjustable range of interlayer coupling. In summary, the evolution of the lattice and electronic structures of multilayer ReS2 was investigated by combining first-principles calculations with high-pressure Raman and PL spectroscopy measurements. An intralayer transition was confirmed at 8.0 GPa, which was associated with the rotation of S atoms around the chain of Re atoms along with a slight distortion in Re4 units, followed by an interlayer transition from disordered to ordered stacking at 15.4 GPa. Both the high-pressure PL measurements and first-principles calculations demonstrated an indirect to another indirect interband transition at approximately 2.7 GPa, which was elucidated by considering the fundamental relationship between lattice variation and electronic evolution in compressed multilayer ReS2. By comparing the lattice and electronic structures of compressed MoS2 with those of ReS2, we demonstrated the critical role of interlayer coupling in determining the evolution of the lattice and electronic structures in compressed TMDs. These findings suggest the promising potential applicability of multilayer ReS2 in the design and fabrication of various stacking nanometer-scale devices with tailored properties for electronic and optoelectronic applications. ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 11374121 and 11404133). SUPPORTING INFORMATION 17 ACS Paragon Plus Environment


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Supporting

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(a) Top and (b) side view of the crystalline structures of distorted-3R ReS2. (c) The Raman spectrum of multilayer ReS2 at ambient conditions. Multiple peak fitting with Lorentz profile curves is shown with different colors. (d) The PL spectra of multilayer ReS2 at ambient conditions. Multiple peak fitting with Gauss profile curves is shown with different colors. The dashed blue curve is the contribution to the PL from an indirect optical gap. (e) Calculated band gap of ReS2 at ambient condition. 150x130mm (300 x 300 DPI)

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Selected high pressure Raman spectra of multilayer ReS2 in the region of (a) 125-190 cm-1 (b) 205-270 cm1 . Multiple peak fitting with Lorentz profile curves is shown with different colors. (c) 265-600 cm-1. The emerging splitting mode and the depleted mode under high pressure are indicated by solid and dash red arrows, respectively. (d, e) Pressure-dependent Raman modes in multilayer ReS2. Linear fits to the observed frequencies are indicated by solid lines. (f) The pressure-dependent intensity ratio between Eg-3 and Eg-4 mode. The dash red lines serve as visual guides for the intensity ratio. 143x152mm (300 x 300 DPI)



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Figure 3. Calculated (a) cell parameter and (b) cell angle ratio of ReS2 as a function of pressure, where a0, b0, c0, α0, β0, and γ0 represent the unit cell parameters at ambient condition. 80x52mm (300 x 300 DPI)


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(a) Representative PL spectra as a function of pressure. (b) Calculated band structure of multilayer ReS2 at different pressures. (c) Energies of peaks I and I* at different pressures. (d) Calculated partial DOS for the distorted-3R ReS2 at 0 and 6 GPa. The insets show enlarged images of the partial DOS at 0 and 6 GPa. 150x137mm (300 x 300 DPI)



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Figure 5. Calculated (a, b) in-plane and (c, d) out-of-plane electron localization function (ELF) of distorted3R ReS2 155x51mm (300 x 300 DPI)


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Associated Lattice and Electronic Structural Evolutions in Compressed

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