Should the T-phase of transition metal dichalcogenide be metallic?

Kevin Chen^1*, Yueh-Chiang Yang², Shang-Hsien Hsieh³, Po-Wen Chiu², Yuan-Chieh Tseng⁴, Chia-Hao Chen³

¹Program for Science and Technology of Accelerator Light Source, National Yang Ming Chiao Tung University, Hsinchu, Taiwan
²Department of Electrical Engineering, National Tsing Hua University, Hsinchu, Taiwan
³National Synchrotron Radiation Research Center, Hsinchu, Taiwan
⁴Department of Materials Science and Engineering, National Yang Ming Chiao Tung University, Hsinchu, Taiwan

* Presenter:Kevin Chen, email:kevin.als07g@nctu.edu.tw

It is generally assumed that H-phase is a direct bandgap semiconductor in transition metal dichalcogenide, such as WS₂. On the other hand, for T-phase, despite of theory predicted metallic T-phase, there are controversies about the electronic/optical properties of T-phase. Therefore, it is necessary to provide more evidences on the relationship between phases and electronic structures. In this work, we come across an ideal of using scanning photoelectron microscopy (SPEM) to clarify the H/T-phase controversies in WS₂. SPEM can simultaneously resolve electronic and phase structures with the advantage of being surface sensitive and non-destructive.
The phase information of H/T phases are distinguishable in SPEM images due to the photoelectron diffraction (XPD) effect, atomic symmetry (D₃h/D₃d) causes different periodic changes in the XPD result for H and T phase.
However, we prove experimentally that T-phase WS₂ show non-zero valence band maximum value, which is identical from the H-phase in both core-level and valence band photoelectron spectrum. In other words, the H/T-phases WS₂ can only differentiate by phase structure, but electronic structure is indistinguishable.
Although our experimental results are challenging most of the theoretical predictions, this work may provide further discussion and insight for the structure isomer of TMDC material.

Keywords: TMDC, WS2, T/H phase, SPEM, XPD