Phase engineering of two-dimensional materials for electronic applications

Abstract

Two-dimensional (2D) van der Waals (vdWs) materials, including graphene and transition metal dichalcogenides (TMDs), opened a new research field in future electronics and optoelectronics due to their novel physical, electrical and optical properties. Particularly, 2D materials can be engineered in ultra-scaled dimension to modulate its functionality. Phase transition is an interesting approach to change properties of 2D materials. To engineering the phase of 2D materials, many strategies have been introduced such as heat, electrostatic doping, tensile strain, plasma treatment, and electric field. From the changed various polymorphs of 2D materials, many practical applications have been demonstrated including monolithic electrical and optical devices, and electrocatalysis. However, since these techniques could induce distortion of layered structure or irregular phase transition, it has been hard to the physics in structural changes of ultrathin layered structures (e.g., layer number and stacking angle dependence). Moreover, there have been a lack of studies on phase and composition chances of 2D materials during heat-mediated phase change process due to their lower thermal stability. In this thesis, most of works are focused on the phase engineering of 2D materials to tailor their physical properties. Prior to introduce the detailed studies, brief general background about 2D materials including structural modification methods and vdW heterostructures are given in chapter 2. Then, to engineer the structural phase of TMDs, especially Molybdenum dichalcogenide (MoTe2), analysis on dimensionality and heat distribution of MoTe2 are described in chapter 3. Because the MoTe2 has a smaller phase transition barrier energy than other TMDs, I easily transited its phase by encapsulation annealing. I observed stepwise increase of phase transition temperature in 2H-to-Td phase transition of ultrathin MoTe2: higher transition temperature for smaller number of layers. From this, I showed that it is possible to precisely control the phase transition of MoTe2 by using of dimensionality, which enables the demonstration of vertical and lateral phase patterning of MoTe2. Also, I modulated the phase and composition of MoTe2 in various hBN-MoTe2 vdW heterostructures by controlling the heat dissipation and evaporation of MoTe2. When the vdW heterostructure of hBN-encapsulated MoTe2 was irradiated by laser, the 2H-MoTe2 transformed into 1T′ phase. Meanwhile, MoTe2 flakes opened to air or covered only one side by hBN transformed into other stoichiometric structures such as Te and Mo3Te4. From these systematic studies, I revealed that control both temperature distribution and evaporation are crucial to engineer the phase of MoTe2 under laser irradiation. Finally, I reversibly transited phase of MoTe2 from 1T′ phase to 2H phase. Laser-induced polycrystalline 1T′ grain was changed to single-crystalline 2H grain by annealing process. By using in-situ heating transmission electron microscopy (TEM), I studied the propagation mechanism of the 1T′-2H phase transition, which revealed that the newly generated 2H-phase is anisotropically and layer-independently propagated. I observed that 1T′-to-2H phase fronts travel primarily along the b-axis of the 1T′ phase with a layer-by-layer phase transformation. Moreover, I observed that the phase transition was initiated at the 1T′-2H phase boundaries. The nucleation-less phase transition reduces the energetic barriers for phase transition, which leads to low-temperature phase transition even at 225 °C. These results describe the microscopic picture of phase transition in MoTe2, which can be able to expand the same principle on other 2D materials. Our findings can provide a better understanding of unique properties in phase transition of 2D materials, which can be applied to design a new device based on phase engineering of 2D materials.