A Gapped Phase in Semimetallic T_d-WTe₂ Induced by Lithium Intercalation

<br/>Lithium intercalation in two-dimensional (2D) materials can introduce useful electronic and structural phase transitions, as enabled by its high electron doping power on the order of 10¹⁴e/cm². For example, lithium intercalation can transform semiconducting 2H-MoS₂ into metallic 1T’-MoS₂. Despite the extensive studies of lithium intercalation in MoS₂and WS₂, the effects of lithium intercalation in 2D transition metal tellurides and thickness effects on intercalation-induced phase transitions have rarely been explored. Herein, we report a reversible structural phase transition in WTe₂ from 2D Weyl semimetal T_d phase to semiconducting T_d’ phase by electrochemical lithium intercalation.^[1]We systematically investigated the phase transition dynamics as a function of electrochemical intercalation voltage and layer thickness using combined <i>in situ</i> Raman spectroscopy, <i>in situ</i> x-ray diffraction, and <i>in situ</i> electron transport characterization. Structurally, the new T_d’ phase has an in-plane structure resembling the theoretically predicted 2x2 charge density wave (CDW) of monolayer WTe₂ with electron doping.^[2,3] Electronically, the new T_d’ phase has an increased longitudinal resistance and a reduction of carrier density both by two orders of magnitude, probed by <i>in situ</i> Hall measurements. The more resistive T_d’ phase also exhibits an increasing resistance with decreasing temperature, suggesting that this T_d’ phase is a semiconductor with a gap opening at the Fermi level. Our <i>ab initio</i> calculations further predict that this T_d’ phase has an indirect band gap of 0.14 eV and a direct band gap of 0.66 eV, which supports our transport data. Our finding of a new gapped phase, potentially a CDW phase, in a two-dimensional (2D) semimetal demonstrates electrochemical intercalation as a powerful tuning knob for modulating electron density and phase stability in 2D materials. The greatly expanded electronic and structural phase diagram of 2D materials accessible using lithium intercalation will stimulate the research for novel quantum phases, such as topological superconductivity.<br/><br/><br/><br/>[1] M. Wang, A. Kumar, H. Dong, J. M. Woods, J. V. Pondick, S. Xu, D. J. Hynek, P. Guo, D. Y. Qiu, J. J. Cha, <i>Advanced Materials</i> <b>2022</b>, 2200861.<br/>[2] P. K. Muscher, D. A. Rehn, A. Sood, K. Lim, D. Luo, X. Shen, M. Zajac, F. Lu, A. Mehta, Y. Li, X. Wang, E. J. Reed, W. C. Chueh, A. M. Lindenberg, <i>Advanced Materials</i> <b>2021</b>, <i>33</i>, 2101875.<br/>[3] J.-H. Lee, Y.-W. Son, <i>Physical Chemistry Chemical Physics</i> <b>2021</b>, <i>23</i>, 17279.