Composition dependence of the charge-driven phase transition in group-VI transition metal dichalcogenides

Abstract

Materials exhibiting multiple stable phases can be used as functional components in electronic and optical applications if the phase transition is controllable. Group-VI transition metal dichalcogenides (TMDs, MX2, where M=Mo, W and X = S, Se) are known to undergo charge induced transitions from semi-conducting H phases to metallic T phases. This occurs, for example, when bulk TMDs are exfoliated with the aid of alkali ion intercalants. However, it is difficult to experimentally decouple the effect of composition-dependent phase transition barriers from indirect effects related to the exfoliation process. Here, using first-principles calculations, we study the energetics of transition between the different structural polytypes of four group-VI TMDs upon lithium adsorption. We find that both the activation barrier from the H phase to the metallic phase in charged monolayers and the reverse barrier in neutral monolayers are required to explain experimental results. We show that the high proportion of metallic phase found in WS2 monolayers after alkali treatment can be explained by high transition barriers to revert back to the H phase once in a neutral state. The calculated barriers however cannot explain the low proportion of metallic phase found in MoS2 monolayers in some experiments and so non-electronic effects must also play a role. arXiv:1901.02697v3 [cond-mat.mtrl-sci]

Overcoming Old Scaling Relations and Establishing New Correlations in Catalytic Surface Chemistry: Combined Effect of Charging and Doping

“…Note that besides our own report 17 on the consequences of these unphysical approaches to electrostatics for chemisorption in the literature, similar observations were recently made in the context of electrochemical barriers 32 and chargedriven phase transitions. 33…”

Metallic Transition Metal Dichalcogenides of Group VIB: Preparation, Stabilization, and Energy Applications

“…Note that this process is reversible, those intercalated metallic TMDs tend to back to 2H phase as the intercalants are removed through annealing or aging. Previous work has calculated the transition barriers of MoS 2 , MoSe 2 , WS 2 , and WSe 2 with different lithium ions intercalation concentration, [ 37 ] and it indicates that there is a distinct decrease of transition barriers for all the four materials after lithium‐ion intercalation, which may be the reason why lithium ions intercalation could trigger the phase transition. However, lithium ions intercalation cannot eliminate transition barrier completely but to decrease it and there is still a barrier of 0.24 eV for the smallest distance from H‐phase to T′‐phase (Li 1 WSe 2 ).…”

Revisiting Intercalation‐Induced Phase Transitions in 2D Group VI Transition Metal Dichalcogenides

“…[ 4,16,17 ] For most group VI TMDCs, with the exception of WTe 2 , the trigonal prismatic 2H phase is thermodynamically more stable than the octahedral 1T or distorted 1T′ phase with the energy difference (Δ E ) ranging from 0.84 eV in monolayer MoS 2 to 0.04 eV in monolayer MoTe 2 per unit formula. [ 4,16,17 ] While the exact values of the energy difference vary among different DFT calculations, all of the calculations show the same trend: the energy difference between the two phases is the largest for the sulfides and smallest for the tellurides. Thus, it might be expected that the 2H → 1T′ phase transition is most difficult for the sulfides and easiest for the tellurides.…”

“…They found that Li adsorption leads to a decreased energy barrier for the 2H → 1T′ phase transition, and the energy difference, E 2H − E 1T′ , becomes positive above the threshold Li loading. [ 17 ] Another theoretical work showed that the undistorted 1T phase is an intermediate phase on the pathway from 2H to 1T′ in monolayer MoS 2 , and the transition from 1T to 1T′ occurs spontaneously with an almost zero activation energy, as shown in Figure 2B. [ 14 ]…”

“…For example, potential ordering of the lithium atoms in multilayer TMDC might significantly affect the phase transition, which cannot be captured in calculations based on monolayer TMDCs. [ 14,17 ] A recent article by Dongare and coworkers reported a simulation of Li intercalation dynamics in a ten‐layered MoS 2 and showed a layer‐by‐layer occupation of Li in MoS 2 , which subsequently induces a layer‐by‐layer nucleation of the 1T phase. [ 25 ] Thus, this theoretical work highlights the importance of studying intercalation in multilayer TMDCs to capture the intercalation pathways and phase transition pathways, step by step.…”