Feb 12, 2019 | |
Phase transition dynamics in two-dimensional materials(Nanowerk News) Scientists from National University of Singapore have discovered the mechanism involved when transition metal dichalcogenides on metallic substrates transform from the semiconducting 1H-phase to the quasi-metallic 1T’-phase. |
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Two-dimensional transition metal dichalcogenides (2D-TMDs) such as monolayer molybdenum disulphide (MoS2) are atomically thin semiconductors in which a layer of transition metal atom is sandwiched between two layers of chalcogen atoms, in the form MX2. | |
They can exist in both a semiconducting 1H-phase and a quasi-metallic 1T’-phase, with each having a different crystal structure. The 1T’-phase is particularly interesting as theoretical predictions show that it has potential to be used in less conventional applications, such as super capacitor electrodes and hydrogen evolution reaction catalysts. | |
However, the quantity of 1T’-phase 2D-TMDs that can be obtained by converting them from the 1H-phase through a phase transition process is low. This potentially limits the use of such novel materials for a wide range of applications. | |
Molecules of monolayer molybdenum disulphide (MoS2.) and tungsten diselenide (WSe2) on top of a metal substrate. (Image: National University of Singapore) | |
A research team led by Professor Andrew Wee from the Department of Physics at the National University of Singapore’s (NUS) Faculty of Science has discovered that while different 2D-TMD materials have their own intrinsic energy barriers when transiting from the 1H to the 1T’ structural phase, the use of a metallic substrate with higher chemical reactivity can significantly increase the 1H- to 1T’- phase transition yield (Advanced Science, "Unravelling high-yield phase-transition dynamics in transition metal dichalcogenides on metallic substrates"). | |
This is a convenient and high-yielding method to obtain 2D-TMD materials in their 1T’ metallic phase. When the 2D-TMD material is placed in contact with the metal substrate, such as gold, silver and copper, electric charges are transferred from the metal substrate to the 2D-TMD material. | |
Furthermore, it weakens the bond strength of the 2D-TMD structure significantly, and increases the magnitude of the interfacial binding energy. This in turn increases the susceptibility of the 1H-1T' structural phase transition. As a result, this enhanced interfacial hybridisation at the interface of the two materials makes the 1H-1T’ structural phase transition much easier to achieve. | |
The NUS research team combined multiple experimental techniques and first-principles calculations in their research work. These includes optical spectroscopies, high resolution transmission electron microscopy and density functional theory based first-principles calculations to identify the phase changes - both 1H- and 1T'-phases - of the 2D-TMDs in the samples. | |
This study provides new insights on the influence of interfacial hybridisation affecting the phase transition dynamics of 2D-TMDs. The findings can potentially be used in a model system for the controlled growth of 2D-TMDs on metallic substrates, creating possibilities for new 2D-TMDs-based device applications. | |
Prof Wee said, “The controllability of the semiconductor to metal phase transition at the 2D-TMD and metal interfaces can enable new device applications such as low contact resistance electrodes.” |
Source: National University of Singapore | |
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