Revealing unexpected intercalation phenomena in layered materials

Woei Wu Larry Pai^1,2*

¹Center for condensed matter sciences, National Taiwan University, Taipei, Taiwan
²Department of physics, National Taiwan University, Taipei, Taiwan

* Presenter:Woei Wu Larry Pai, email:wpai@ntu.edu.tw

Properties of layered two-dimensional materials can be modulated by intercalation. However, microscopic details of intercalation are often ambiguous. Here we present several cases to illustrate the effects of intercalation. In transitional metal dichalcogenides (TMDs), metal self-intercalation is common. For TiTe2, a (2x2) charge density wave (CDW) has been reported in monolayers but not in bulk. We show this CDW is a consequence of tensile strain due to Ti intercalation and not an intrinsic property of pristine TiTe2 monolayers. For VTe2, V intercalation can be controlled through the V:Te flux growth ratio. The often-observed periodicity doubling in VTe2 bilayers is shown to be a result of a new V-intercalated phase, e.g., V5Te8. This phase can be distinguished from the pristine VTe2 bilayers through a spectroscopic fingerprinting feature due to dz2 hybridization. Finally, alkali intercalation is an important topic related to energy storage. On bulk VSe2, Na atoms intercalate easily to form embedded irregular islands, which exhibit expected energy level downshift except at the island edges. The CDW state of Na-intercalated regions becomes the monolayer type. Interestingly, 3D Na islands with well-defined facets also form and cause atomically sharp bents of VSe2. K atoms marginally intercalate into VSe2 and show similar characteristics as Na, but with an unexpected full gap at 4 K. All other alkali atoms such as Li, Rb, Cs, and K (mostly) do not intercalate into VSe2 through surface doping, and form adsorption structure commensurate with the VSe2 (4x4) CDW. Our works show even simple intercalation can lead to rich behaviors. Use of other intercalation species (e.g. magnetic, spacer layers) can greatly expand the functionalities of layered materials.

Keywords: 2D layered materials, intercalation, STM/STS, alkali, VSe2