Doping induces charge density wave in two-dimensional semiconductor

Researchers at the National University of Singapore (NUS) have observed a doping-tunable charge density wave (CDW) in a single-layer semiconductor, Chromium(III) selenide (Cr₂Se₃), extending the CDW phenomenon from metals to doped semiconductors.

CDWs are intriguing electronic patterns widely observed in metallic two-dimensional (2D) transition metal chalcogenides (TMCs). The study of CDW provides insights into emergent orders in quantum materials, where electron correlations play a non-negligible role. However, most reported TMCs exhibiting CDW are intrinsic metals, and tuning their carrier density is predominantly accomplished through intercalation or atomic substitution. These approaches may introduce impurities or defects that complicate the understanding of the underlying mechanisms.

A research team led by Professor CHEN Wei from the NUS Department of Physics and the Department of Chemistry, synthesised single-layer semiconducting Cr₂Se₃ and demonstrated the CDW phenomenon using scanning tunnelling microscopy (STM). Contrary to the previously reported 2D metallic TMCs, CDW in semiconducting Cr₂Se₃ can be cleanly and reversibly tuned via surface charge transfer doping, without introducing foreign impurities. This research work was carried out in collaboration with Professor ZHENG Feipeng from Jinan University, China.

The research findings were published in the scientific journal Science Advances on 3 September 2025.

At very low temperatures, electrons in this material naturally arrange themselves into tiny “patches” of higher and lower density, a pattern called charge ordering, similar to ripples or a checkerboard of charge. The researchers saw these patterns using scanning tunnelling microscopy and showed that the crystal lattice slightly distorts when the pattern appears, which are hallmarks of a charge-ordered state. They found that the pattern can be controlled by “doping”. Removing electrons (hole doping) turns the pattern off while adding electrons (electron doping) makes the pattern stronger and more regular, forming a neat, repeating 3√3-by-3√3 pattern.

One of the authors, Dr DUAN Sisheng said, “Due to its semiconducting nature, the CDW in 2D Cr₂Se₃ can be tuned through both electron and hole doping. This suggests potential device applications for extrinsic manipulation of electronic correlated phases.”

“In fact, 2D Cr₂Se₃ is also antiferromagnetic. A doping tunable CDW in a 2D antiferromagnet offers a platform for understanding the interplay between magnetism, charge order, and electron correlation,” added Prof Chen.