Enhancing charge-density-wave order in 1T-TiSe2 nanosheet by encapsulation with hexagonal boron nitride

Layered transition metal dichalcogenides (TMDs) provide an ideal platform for exploring the effects of dimensionality on correlated electronic phases such as charge density wave (CDW) order. When TMDs are reduced in thickness to the 2-D limit, it is expected that the substrates will exert a considerable influence on the electron states. Here, we report a study of the charge density wave state in 1T-TiSe2 nanosheets of different thicknesses when the sheets are encapsulated by hexagonal Boron Nitride or supported on the SiO2 substrate. Our results show that dimensionality reduction results in an enhancement of CDW order and that disorder and substrate phonons tend to destroy CDW order, preventing observation of intrinsic CDW transition in ultrathin samples. Encapsulated 10 nm thick 1T-TiSe2 samples exhibit intrinsic CDW with transition temperature as high as 235 K. Our study points out that choosing the right substrate is important in the search for room temperature CDW materials.