Phase Engineering In Monolayer MoS₂ And Its Applications

Monolayer molybdenum disulfide（ML-MoS₂）,a typical type of 2D transition metal dichalcogenides（TMDs）,has been widely applied in the field of electronic transistors,optoelectrical devices,catalysis,resulting from its variable structural and electronic properties.ML-MoS₂ consists of a slab of S-Mo-S sandwich layer and two phases exist,the so-called 2H and 1T phases.The 2H phase corresponds to a trigonal prismatic coordination of the metal atoms,the sulfur atoms are vertically aligned and stacking sequence is AbA.The 1T phase corresponds to an octahedral coordination of the metal atoms,one of the sulfur layers is shifted compared to the other leading to an AbC stacking.So the 2H phase is semiconducting with a band gap of 2.2 eV,while the1T phase is metallic.The two phases can be principally converted from each other by structural change.Tailoring the properties of ML-MoS₂ by controlled phase engineering is technically important and can be extended to wider application fields.The thesis is mainly constituted by the following three aspects:⒈Argon plasma induced phase transition in monolayer MoS2 and its application in electrical device.We report a controllable,clean,facile phase engineering technique for ML-MoS₂.Local 2H→1T phase transitions in ML-MoS₂can be achieved using weak Ar-plasma.These phase transitions are stabilised by few point defects,the sizes of induced 1T phase domains are only a few nanometers.Based on a selected-area phase patterning process,we fabricated ML-MoS₂ FETs with 1T-phase contacts,which show substantially improved device performance.Our results present an important advance towards phase engineering and open up new possibility for controlled modification of MoS₂ properties.⒉The catalytic activity in basal plane of heterophase monolayer MoS2.It is generally believed that the catalytic activity of MoS₂ originates from its edges while the basal plane is rather inert.We report the domain boundary in basal plane of the as-grown polycrystalline heterophase monolayer MoS₂ can serve as new active sites for HER.Both the 2H-2H domain boundaries and 2H-1T phase boundaries were investigated.And the 2H-1T phase boundaries are more efficient for hydrogen evolution reaction as active sites than 2H-2H domain boundaries.⒊Polycrystalline heterophase MoS₂ for enhanced hydrogen evolution reaction.We report that the polycrystalline heterophase MoS₂ with multilevel hierarchy design of these two types of domain boundaries show superior HER catalytic activity.Our result further show the ease of preparation for large-scale monolayer MoS₂catalysts in combination with excellent electrocatalytic activity and remarkable long-term operation stability.We believe that these results will provide new option for the design of MoS₂ electrodes for industrial-scale HER applications.