Strain-Induced Semiconducting to Semi-Metallic Phase Transition in MoTe₂ Using a Single-Ion Conductor

Commonly studied transition metal dichalcogenide (TMD) crystals exhibit polymorphism, where the electronic structure of the material changes significantly with a change in the crystal structure. MoTe₂ has gained particular interest because the potential energy difference between the semiconducting 2H (trigonal prismatic coordination) and semi-metallic 1T' phase (distorted octahedral coordination) is the lowest (40 meV) among TMD polymorphs, making it promising for low voltage switching. Although the 2H phase is the most stable form, it can be transformed to 1T' by 0.3 - 3% by tensile strain thereby causing a large change in the electronic conductivity. Recent studies have experimentally shown this phase transition by mechanically stretching the entire substrate or applying local mechanical strain using atomic force microscopy (AFM) tip, but both strain methods would be difficult to implement in CMOS. What is needed is a straining mechanism driven by field-effect, where a single device can be controlled electrically by a nearby gate.<br/>Here, we employ an ‘ionomer’ or single-ion conductor to impart strain, which has been used by the polymer physics community for artificial muscles and actuators. An ionomer contains mobile cations but has anions that are covalently bonded to a polymer backbone. Under an applied electric field, the cations accumulate at the MoTe₂ surface, effectively controlling electron transport in the material, while anions maintain their position in the polymer backbone creating a charge imbalance. The imbalance causes the ionomer to bend, which then induces strain in the MoTe₂.<br/>In this work, the electrical and structural properties of a suspended MoTe₂ FET are measured simultaneously using a home-built set-up combining electrical measurements with Raman spectroscopy. With no gate voltage (V_G) applied, the insulating 2H phase is confirmed. For V_G &gt; 2.5 V, the 1T’ phase is detected by a significant decrease in the electrical resistance accompanied by a characteristic 1T’ peak in the Raman spectra. To our knowledge, this is the first demonstration of using a gate to induce the phase transition; moreover, the transition occurs at a voltage that is ~ 1-2 V lower than phase transitions induced by electrostatic and electrochemical mechanisms. However, because a large portion of the flake is supported and not suspended, a contribution from the 2H phase persists. Mapping the 2H and 1T’ peaks across the entire flake reveals that the phase transition is reversible (i.e., the flake reverts to the semiconducting 2H phase when the voltage is removed), which is an essential feature of a switch. These proof-of-concept results are encouraging because there is plenty of opportunities to optimize the device structure and the ionomer to engineer a complete transition and enable a true low-voltage electronic switch.