Phase engineering and optical properties of 2D MoSe2: Promise and pitfalls

Phase engineering monolayer MoS2, selectively controlling the MoS2 2H to 1T′ transition via lithium intercalation, has driven recent excitement in the nanoscale electronics field, due to resultant MoS2 contact resistance reduction and the compatibility of MoS2 with CMOS device architecture. Here, we report the “on-chip” 2H to 1T′ transition for the related MoSe2 system, which has a smaller 1.55 eV 2H bandgap, and for which the 1T′ phase transformation should be more energetically favorable. We report the first on-chip 2H to 1T′ transformation of monolayer MoSe2 on both SiO2 and sapphire substrates. The on-chip 1T′MoSe2 shows higher transparency despite an increased number of metallic states, indicating tunable optoelectronic properties with potential applications in transparent electrodes and energy harvesting. We also describe the challenges introduced by on-chip phase engineering via n-butyllithium exposure. Density functional theory (DFT) calculations indicate that Li+ ions are required on both sides of the MoSe2 monolayer to create a strong thermodynamic driving force for the 1T′ transformation. We observe that patterned n-butyllithium exposures can be inconsistent, with widely variable kinetics. Due to manifest n-butyllithium-engineered 1T′ MoSe2 stability concerns we propose the process is an unreliable processing technique for 2D electronics.