Structural Phase Transition and Interlayer Coupling in Few-Layer 1T′ and T_d MoTe₂

Abstract

We performed polarized Raman spectroscopy on mechanically exfoliated few-layer MoTe2 samples and observed both 1T′ and Td phases at room temperature. Few-layer 1T′ and Td MoTe2 exhibited a significant difference especially in interlayer vibration modes, from which the interlayer coupling strengths were extracted using the linear chain model: strong in-plane anisotropy was observed in both phases. Furthermore, temperature-dependent Raman measurements revealed a peculiar phase transition behavior in few-layer 1T′ MoTe2. In contrast to bulk 1T′ MoTe2 crystals where the phase transition to the Td phase occurs at ~250 K, the temperature-driven phase transition to the Td phase is increasingly suppressed as the thickness is reduced, and the transition and the critical temperature varied dramatically from sample to sample even for the same thickness. Raman spectra of intermediate phases that correspond to neither 1T′

Persistence of Monoclinic Crystal Structure in 3D Second‐Order Topological Insulator Candidate 1T′‐MoTe2 Thin Flake Without Structural Phase Transition

“…Raman spectroscopy is an experimental technique which is directly sensitive to the crystal structure. [ 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 , 52 ] Previous Raman spectroscopy studies of the bulk 1 T ′‐MoTe 2 crystals indicate that in the high‐temperature 1 T ′ phase at T ≥ 250 K upon cooling (or at T ≥ 260 K upon warming), the out‐of‐plane vibration mode D around 125 cm –1 is Raman in‐active (i.e., only infrared active) and is absent in the Raman spectra due to the centrosymmetry of the monoclinic 1 T ′ structure, while in the low‐temperature T d phase at T < 250 K upon cooling (or at T < 260 K upon warming), the out‐of‐plane vibration mode D becomes both Raman‐ and infrared‐active and can be probed by Raman spectroscopy owing to the centrosymmetry breaking in the orthorhombic T d structure (see the two vibration modes D and E in the Raman spectra of the MoTe 2 bulk crystal with the orthorhombic T d structure measured at T = 80 K in Figure 2 a , the vibration mode e in the Raman spectra of the MoTe 2 bulk crystal with the monoclinic 1 T ′ structure at T = 300 K in Figure 2b , and the Raman spectra of the MoTe 2 bulk crystal in the energy range from 60 to 300 cm –1 in Figure S1 , Supporting Information). [ 35 , 36 , 37 , 38 , 39 , 40 ] Thus, the presence of the out‐of‐plane vibration mode D can be regarded as a spectroscopic signature of the temperature‐driven structural phase transition in MoTe 2 from the high‐temperature monoclinic 1 T ′ structure to the low‐temperature orthorhombic T d structure.…”

“…[ 53 ] Therefore, the doping type and the doping level at room temperature are expected to induce different behaviors of the structural phase transition in MoTe 2 , which can be supported by the observation of the different crystal structures of the thin flakes with the same thickness and the existence of the intermediate phase corresponding to neither the monoclinic phase nor the orthorhombic phase. [ 43 , 44 , 45 , 46 , 47 , 48 ] It is worth noticing that the phonon energies of the transition metal dichalcogenides can show significant dependence on the doping level and the doping type. [ 49 ] For 2 H ‐MoTe 2 , the out‐of‐plane vibration mode A 1g exhibits a blue (or red) shift with the enhancement of the hole (or electron) concentration, while the A 1g mode shows a red shift with the decrease in the flake thickness.…”

Anomalous Dimensionality‐Driven Phase Transition of MoTe2 in Van der Waals Heterostructure

“…To verify the formation of the T d phase, linear polarized Raman spectroscopy was used in a low-frequency range (Figure 1d,e). [17,18] The upper and lower insets in Figure 1b show the Raman peak intensity maps for E 2g of 2H and A 1 of T d , respectively. This indicates that 2H-MoTe 2 uniformly transformed into the T d phase.…”

Large change of interlayer vibrational coupling with stacking in Mo$_{1-x}$W$_{x}$Te$_{2}$

“…It is known that the transition of MoTe 2 is broadened or suppressed for thin samples [16,38,39]. There is some evidence for weaker interlayer vibrational coupling for few-layer samples; the interlayer force constants K x from Raman measurements on ≤8-layer MoTe 2 are 0.673 (11) and 0.604 (15) for T d -and 1T -MoTe 2 , respectively [17], both substantially smaller than our values of 0.919 (24) and 0.760 (15) for bulk Mo 0.91 W 0.09 Te 2 . Also, bilayer WTe 2 shows signs of a transition above ∼340 K (in the disappearance of a second harmonic generation signal [40]); if the intralayer positions are unchanged, then the only explanation for the arrival of inversion symmetry in a bilayer structure would be a structure with δ = 0.5 (analogous to the hypothetical T 0 phase discussed in Ref.…”

“…The a-axis interlayer shear mode (ISM) has been studied for its relevance in identifying the T d phase and in modulating its Weyl semimetal properties. These studies include Raman spectroscopy in MoTe 2 [13][14][15][16][17] and WTe 2 [18][19][20], and various ultrafast spectroscopy techniques in MoTe 2 [21][22][23][24] and WTe 2 [23,[25][26][27][28][29][30]. Raman spectroscopy, however, is limited to measuring the zonecenter energy hω m (i.e., the maximum of the ISM dispersion), and only in the T d phase (for bulk samples) is this mode Raman-active.…”