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 Li ions. However, few reports explore the effects of Li intercalation in other TMDCs, such as Mo- or W- ditellurides. In particular, novel electronic and energy devices can be achieved using the Li-intercalated MoTe₂ with its intriguing electrical, topological and catalytic properties.<br/>Here, we report electrochemical Li intercalation into 1T’- MoTe₂ flakes. The 1T’ phase is stable down to 0.9 V of the applied electrochemical voltage, and two new phases are observed at 0.7 V (phase I) and 0.4 V (phase II), respectively. The lightly Li-intercalated phase I is evidenced by the disappearance of the A_g peak at ~77.7 cm^-1 and the appearance of a peak at ~86.9 cm^-1 in Raman spectroscopy and a 10% increase of electrical resistance in two-terminal measurements. For the heavily Li-intercalated phase II, we observe a lattice expansion of ~7% in (001) direction in single-crystal X-ray diffraction, the emergence of new Raman peaks at 16.8 cm^-1, 109.0 cm^-1 and 132.8 cm^-1 in Raman spectroscopy and increase of electrical resistance for over 8 folds<i>. In situ</i> Hall effect measurements confirm the decrease in conductivity, which also decreases with decreasing temperature for the phase II, suggesting a semiconducting phase. The Hall carrier density falls from 10¹⁵ cm^-2 in pristine 1T’-MoTe₂ to 10¹⁴ cm^-2 in phase I and to 10¹² cm^-2 in phase II. Our results highlight the importance of electrochemical intercalation of Li ions as a powerful tool to manipulate phase stability and electron density of 2D TMDCs.