Experimental observations and density functional simulations on the structural transition behavior of a two-dimensional transition-metal dichalcogenide

Abstract

In this work, we show an obvious evidence of nondestructive Raman spectra for the structural transition, i.e., the existence of a charge density wave (CDW) in monolayer 2H-TaS2, which can exhibit a much higher transition temperature than bulk and results in additional vibrational modes, indicating strong interactions with light. Furthermore, we reveal that the degenerate breath and wiggle modes of 2H-TaS2 originated from the periodic lattice distortion can be probed using the optical methods. Since recently several light-tunable devices have been proposed based on the CDW phase transition of 1 T-TaS2, our study and in particular, the theoretical results will be very helpful for understanding and designing electronic devices based on the CDW of 2H-TaS2.

A theoretical study of Lifshitz transition for 2H-TaS2

“…In fact, after injecting holes, the total energy of some CDW images (called the X image) in Table 1 is significantly reduced. Large geometric structural difference and small energy difference between T-Ts and X configurations can reasonably explain the wider-temperature thermal hysteresis effect 16 at low temperature for mono-layer TaS 2 .…”

“…Our theoretical PT temperature T CDW from the CDW to the metal phase is roughly estimated as approximately 68 K for mono-layer 2H-TaS 2 after hole doping. Recent temperature-dependent resistance experiments 16 indicate that mono-layer 2H-TaS 2 on SiO 2 /Si substrates undergoes a PT at 140 K upon heating which is far above that (75 K) of bulk samples. Its higher T CDW was attributed to the reduced dimensionality and enhancement of electron–phonon coupling.…”

“…7,12,13 However, the CDW phase occurs at around 55-140 K, while its switch of p-n carrier sign is absent when reducing the sample thickness toward the 2D limit. [14][15][16] Early theoretical studies 17,18 proposed a two-band model for the normal state of some superconductors with a b-tungsten structure to explain the anomalous temperature dependence of a variety of properties. Their model requires two bands that slightly overlap or almost overlap.…”

“…7,12,13 However, the CDW phase occurs at around 55–140 K, while its switch of p–n carrier sign is absent when reducing the sample thickness toward the 2D limit. 14–16…”