Controlling the Charge Density Wave Transition in Monolayer TiSe₂: Substrate and Doping Effects

Abstract

TiSe2 is an exciting material because it can be tuned between superconducting and charge density wave (CDW) transitions. In the monolayer limit, TiSe2 exhibits a sizable energy gap in the CDW phase that makes it a promising quantum material. It is shown that interfacing a single layer of TiSe2 with dissimilar van der Waals materials enables control of its properties. Using angle‐resolved photoemission spectroscopy, the energy gap opening is analyzed as a function of temperature for TiSe2 monolayers supported on different van der Waals substrates. A substantial increase in the CDW transition temperature of ≈45 K is observed on MoS2 compared to graphite (highly oriented pyrolytic graphite) substrates. This control of the CDW in monolayer TiSe2 is suggested to arise from varying charge screening of the unconventional CDW of TiSe2 by the substrate. In addition, the suppression of CDW order and a complete closing of the energy gap by electron doping of monolayer TiSe2 is demonstrated. Regulating the many‐body physics phenomena in monolayer TiSe2 lays the foundation of modifying TiSe2 in, for example, artificial van der Waals heterostructures and thus creates a new approach for utilizing the quantum states of TiSe2 in device applications.

Strong-coupling charge density wave in monolayer TiSe2

“…Moreover, the CDW persists in monolayers grown on different substrates; here we have considered only SiC/graphene, but similar data including sharp signatures of the backfolded bands can be obtained on graphite (HOPG, see Supplementary Fig. S3), while CDW signatures have also been reported for TiSe 2 grown on MoS 2 [15]. The CDW also seems to be relatively insensitive to the higher levels of disorder present in MBE-grown films compared to high-quality single crystals.…”

“…The three-dimensional character of the CDW is linked to the finite k z -dispersions of both the holeand electron-like states [9]. However, a similar 2a×2a charge instability is known to occur in the 2D limit of monolayer TiSe 2 [10][11][12][13][14][15][16]. In particular, angle-resolved photoemission spectroscopy (ARPES) measurements on monolayer samples have shown prominent temperature-dependent signatures of the CDW [11][12][13], but the nature and origin of the instability remains controversial.…”

Compositional Phase Change of Early Transition Metal Diselenide (VSe2 and TiSe2) Ultrathin Films by Postgrowth Annealing

“…This has been shown by several reports of the successful MBE-synthesis of VSe 2 and TiSe 2 in both monolayer as well as few layer materials. 11,12,13,14,15,16,17,18,19,20 Here we show that by post-growth annealing these TMDCs can be transformed into metal rich intercalation compounds.…”

“…11 While bulk materials have been studied extensively, recent interest has been on the properties of these materials when reduced to only a single-or a few-molecular layers. 12,13,14,15,16,17,18,19,20 This has been achieved by growing these compounds by molecular beam epitaxy on van der Waals substrates, 21 such as graphene, HOPG, or MoS 2 . The expected structure for these early transition metal dichalcogenides (TMDCs) is the octahedral coordination of the metals to the chalcogen atoms, also known as the 1Tstructure.…”

Spectroscopic photoemission and low-energy electron microscopy studies of the surface and electronic structure of two-dimensional materials

“…However, how the reduced dimensionality and how substrate affect these cooperative orders remain outstanding questions [14,132]. ARPES investigation have been carried out in MBE-grown thin films such as NbSe 2 [133], TaS 2 [134], TiSe 2 [135][136][137][138] and VSe 2 [139][140][141] to address these questions. And even more interestingly, VSe 2 on certain vdW substrates also exhibits 2D magnetic order [142].…”