Dimensionality effect can play an important role in thin film properties. Here we show that charge density waves (CDW) can either emerge or change its symmetry in transition metal dichalcogenides (TMD) as the film thickness is reduced to the single-layer limit. Using angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy and spectroscopy (STM/STS), a (2x2) CDW of a Tc=92 K emerges in single-layer 1T-TiTe2 and vanishes in two-layer or multi-layer films [1]. The CDW leads to an unexplained pseudogap and is not reproduced in state-of-the-art ab-initio calculations. The role of impurity on CDW will be discussed. In 1T-VSe2, we observed a dramatic change of the CDW ordering pattern from (4x4) to (root 3 x root 7), and a significant increase of the CDW transition temperature[2]. Furthermore, the CDW symmetry keeps changing up to 4 layer thickness. Effects of K doping on the VSe2 CDW indicates that the driving mechanism is not simply Fermi surface nesting. Both cases indicate that dimensionality is a route of tuning CDW properties.

[1] “Emergence of charge density waves and a pseudogap in single-layer TiTe2”, P. Chen, W. W. Pai et al., Nature communications, (2017)8:516

[2] “Unique gap structure and symmetry of the charge density wave in single-layer VSe2”, P. Chen, W. W. Pai et al., Phys. Rev. Lett., 121, 196402 (2018)