Strain-Induced 2H to 1T′ Phase Transition in Suspended MoTe2 Using Electric Double Layer Gating

MoTe2 can be converted from the semiconducting (2H) phase to the semimetallic (1T′) phase by several stimuli including heat, electrochemical doping, and strain. This type of phase transition, if reversible and gate-controlled, could be useful for low-power memory and logic. In this work, a gate-controlled and fully reversible 2H to 1T′ phase transition is demonstrated via strain in few-layer suspended MoTe2 field effect transistors. Strain is applied by the electric double layer gating of a suspended channel using a single ion conducting solid polymer electrolyte. The phase transition is confirmed by simultaneous electrical transport and Raman spectroscopy. The out-of-plane vibration peak (A1g)—a signature of the 1T′ phase—is observed when VSG ≥ 2.5 V. Further, a redshift in the in-plane vibration mode (E2g) is detected, which is a characteristic of a strain-induced phonon shift. Based on the magnitude of the shift, strain is estimated to be 0.2–0.3% by density functional theory. Electrically, the temperature coefficient of resistance transitions from negative to positive at VSG ≥ 2 V, confirming the transition from semiconducting to metallic. The approach to gate-controlled, reversible straining presented here can be extended to strain other two-dimensional materials, explore fundamental material properties, and introduce electronic device functionalities.


Difference map overlaid on AFM scan
As mentioned in the main text, the difference map reveals that the intensities of the 1T ′ are stronger in the suspended region indicating that the phase transition happens mainly near the suspended region.In supplementary figure S8), AFM topography scans for devices 1 and 2 are overlaid with the difference map to strengthen the claim of location-specific strain.after the MoTe 2 transfer; the depth after transfer is 5 times smaller than before which proves that the flake is suspended over both the trenches.Note that the appearance of sagging in the AFM image is exaggerated because the length scale of the X axis is ∼1000× larger than Y axis.

Raman mapping device 2
Measurement reported in Figure 5 for device 1 are repeated here on device 2.

Reversible switching
To demonstrate repeated switching between the phases, the gate voltage was switched between V SG = 3 V and 0 V and Raman spectra were acquired.No evidence of 1T ′ phase retention was found at V SG = 0 V during six consecutive 2H−1T ′ phase transitions (Shown in Supplementary Figure S11).12. Raman spectra of suspended MoTe 2 FET before and after mapping Raman maps reported in the manuscript are collections of three spectral acquisitions (each of 10 s) at every pixel in the map.The mapping exposes the MoTe 2 flake to a high intensity laser (power density <∼ 1 mW/µm 2 ) for few hours which can induce a permanent phase transition in 2H-MoTe 2 . 1 The possibility that the phase transition occurs due to exposure to the laser is disproven by the Raman spectra below that show no change before and after mapping.

Phase transition dynamics
Time-dependent Raman spectroscopy is performed to calculate the time required for the phase transition to be detected.A gate voltage of +3 V is applied to an intrinsic 2H phase MoTe 2 device and Raman spectra were acquired (each spectrum for 30 s) in a continuous mode for 500 s at three different locations on the flake.Time-dependent difference between the intensities of out-of-plane vibration mode of 1T ′ and 2H phase is reported in Supple-

EDL formation dynamics
As reported in section 9, the intrinsic phase transition time of the MoTe 2 is ∼ 200 s.The phase transition time will be a combination of the time required to either form or dissipate the double layer and the time for the atoms in the MoTe 2 to rearrange.Here, we show that the EDL formation on a supported MoTe 2 FET gated using a single-ion conductor is ∼ 20 s (shown in Supplementary Figure S14), which is an order magnitude smaller than the observed phase transition time of ∼ 200 s.Thus, the transition time is not limited by ion dynamics.

Figure S1 :
Figure S1: Top and cross-sectional views of the suspended MoTe 2 FET (a) exfoliation, (b) e-beam lithography followed by plasma etching, (c) metal evaporation with metal thickness equal to the h-BN thickness, (d) e-beam lithography followed by plasma etching to create the suspension area holes, (e) dry-flake transfer using PC/PMMA stamp, (f) deposition of single-ion conductor by drop-casting.
Figure S3: Raman spectra of the supported MoTe 2 device without the single-ion conductor.

Figure S4 :
Figure S4: Raman spectra of the single-ion conductor excited using 633 nm wavelength at three different locations on SiO 2 .There are no Raman active modes in the range of 130 − 270 cm −1 , which means there is no contribution to the MoTe 2 spectrum from the single-ion conductor.

7.
Electrical measurements as a function of V SG for the suspended MoTe 2 FET (Device 1)

Figure S7 :
Figure S7: Electrical measurements as a function of gate voltage for device 1 (suspended MoTe2 with single-ion conducting gate) (a) Resistance extracted from the output measurements reported in Figure 6.(b) Transfer characteristics and gate leakage current, (I SG ).No indication of electrochemical reaction was detected in the I SG .

Figure S8 :
Figure S8: The difference between out-of-phase Raman vibration intensities of 1T ′ and 2H phase overlaid with AFM topology scans for (a) device 1 and (b) device 2. The maps are generated by subtracting the signal intensities of the 2H phase from the 1T ′ phase at every pixel, which are taken from the same spectral acquisition.Baseline correction was performed before subtracting the intensities to make the difference more accurate.

Figure S11 :
Figure S11: Raman spectra of suspended MoTe 2 FET after repeatedly applying 3 V to the side gate (red spectra) and removing the gate voltage (blue spectra).No evidence of 1T ′ was retained after removing the gate voltage suggesting that the phase transition is fully reversible.

Figure S12 :
Figure S12: Raman spectra of a suspended MoTe 2 FET (device 2) with ∼ 1 µm film of the single-ion conductor before and after the mapping.
mentary figure S13).Positive values on the Y axis represent the 1T ′ phase whereas negative values represent 2H phase.It takes ∼ 200 s for both the 1T ′ → 2H and 2H → 1T ′ phase transition.

Figure S13 :
Figure S13: Dynamics of phase transition: time dependent Raman spectroscopy of (a) intrinsic 2H phase MoTe 2 device by applying V SG = +3 V and (b) transformed 1T ′ phase by applying V SG = 0 V.Note that the three different datasets indicate three different locations on the same flake

Figure S14 :
Figure S14: Time-dependent current of a supported MoTe 2 FET gated using a single-ion conductor at V SG = +2 V and V D = 50 mV.