Tailoring the Optical and Electrical Properties of MoTe₂ via Electrochemical Intercalation of Lithium Ions

Intercalation of lithium (Li) ions is one of the most effective methods to realize structural transformation and to tune the optical and electrical properties of two-dimensional transition metal dichalcogenides (2D TMDCs). Numerous studies have focused on the phase transition from semiconducting 2H phase to metallic 1T (or 1T’) phase in MoS₂ and WS₂ induced by the intercalation of lithium ions. However, few reports explore the effects of lithium intercalation in other TMDCs, such as Mo- or W- ditellurides. In particular, novel electronic and energy devices can be realized using the lithium-intercalated MoTe₂ with its intriguing electrical, topological and catalytic properties.<br/>Here, we report electrochemical lithium intercalation into 1T’- MoTe₂ flakes. The 1T’ phase is stable down to 0.9 V of the applied electrochemical voltage using poly(ethylene glycol) methyl ether methacrylate (PEGMA)/ bisphenol A ethoxylate dimethacrylate (BEMA) solid polymer electrolyte and Li anode, and two new phases are observed at 0.7 V (phase I) and 0.4 V (phase II) of the applied electrochemical voltage. At 0.7 V, lightly intercalated phase I is evidenced by the disappearance of the A_g peak at ~77.7 cm^-1 and appearance of a new peak at ~86.9 cm^-1 in Raman spectroscopy as well as a 10% increase of electrical resistance in two-terminal measurements. For heavily Li-intercalated phase II at 0.4 V, we observe the emergence of additional new peaks at 16.8 cm^-1, 109.0 cm^-1 and 132.8 cm^-1 in Raman spectroscopy and the dramatic increase of electrical resistance for over 8 folds<i>. In situ</i> Hall effect measurements show that the carrier density falls from 10¹⁵ cm^-2 of pristine 1T’-MoTe₂ to 10¹⁴ cm^-2 (phase I) and 10¹² cm^-2 (phase II) despite the supposed electron doping from Li intercalation. Temperature-dependent resistivity measurements further show increasing resistivity with decreasing temperature for phase II, suggesting a gap opening in initially metallic 1T’-MoTe₂. Our results thus demonstrate electrochemical intercalation of lithium ions as a powerful tool to manipulate phase stability and electron density of 2D TMDCs, and report new phases in MoTe₂ induced by lithium intercalation.