The Structural Transitions Of 2D TMDs By Transmission Electron Microscopy Studies

In recent years,TMDs have extensive application prospects in the fields of nanoelectronic devices,optoelectronic devices,energy and catalysis due to their unique structure and novel physical properties.However,the structural evolution mechanism of TMDs materials has hardly been explored.In this work,the scanning transmission electron microscopy(STEM)was used to study the structural evolution in PtSe2 and MoS2.The phase transition and interlayer structure transition of bilayer PtSe2 under electron beam irradiation were studied.Combined with the DFT,the differences of electronic structure of different phases and the transition path of interlayer structure transition of bilayer PtSe2 were explored.For MoS2,the structural evolution and stability of 1T'-MoS2 were studied by STEM.In addition,the catalytic performance of 1T'-MoS2 for hydrogen evolution reaction(HER)was investigated,and the catalytic mechanism was revealed by DFT.Furthermore,by in-situ TEM,the microstructure of MoS2 wrinkles and its effects on electrical properties were studied.The details are summarized as follows:PtSe2 thin films were prepared by thermal-assisted conversion method.The phases of T(1T)and H(2H,3R and N)were observed in PtSe2 films.The energy band structures of the different phases were predicted by DFT.By performing in situ experiments in an atomic resolution STEM,it is found that 2H? 1T and 2H? N phase transition can be observed in bilayer PtSe2 under electron beam irradiation.The phase transition can be tracked in real time which enables in-depth analysis the transformation mechanism.It is revealed that both interlayer and intralayer glide occurred during the transition process in bilayer PtSe2.In addition,the intralayer shift requires longer electron beam irradiation time to complete due to the stronger intralayer covalent bond strength.We combine atomic resolution STEM and DFT calculation to study the behavior of interlayer stacking orders and interlayer rotations in bilayer PtSe2.The stacking sequences and twist angle in bilayer PtSe2 was fully investigated by atomic resolution in situ STEM.The results show the diverse and dynamic features of bilayer PtSe2 structures which can be observed and directly tuned under electron beam illumination.DFT calculations reveal a small energy barrier(<0.2 eV per formula unit)for the kinetic evolution of interlayer structures.In conjunction with DFT calculations,the step of transition process has been identified.In the transition process of the interlayer angle,it is found that with increase of the electron beam irradiation time,the twist angle between the two layers of PtSe2 decreased.Furthermore,the linear relationship between twist angle and time was established.The structural evolution of monolayer 1T'-MoS2 were studied.It is found that the variable zigzag chains of molybdenum atoms underwent the recombination of metal-metal bond,leading to the emergence of tetramer clusters and a new oriented zigzag chain.Furthermore,the overpotential and Tafel slope value of 1T'were lower than those of 2H,which means that 1T' has better performance for HER.DFT calculations shown that the ?GH of Volmer and the potential barrier of Heyrovsky of the IT' domain boundary were lower than that of 1T'.More importantly,1T' phase has a large number of domain boundaries under atmospheric pressure.Therefore,the 1T' domain boundary may play a major role in HER.By in-situ TEM,the microstructure of MoS2 wrinkles and its effects on electrical properties were studied.It is found that compressive stress can lead to the formation of wrinkles.In contrast,removal of the stress resulted in the disappearance of wrinkles,and MoS2 nanosheets restored to its initial morphology.Moreover,wrinkles also affect the electrical properties of MoS2 nanosheets.The existence of wrinkles caused an increase of the current,and the disappearance of the wrinkles accompanied by the current decreased,which is due to the piezoresistive effect.This work revealed the phase transition and interlayer structure transition mechanisms of PtSe2 at atomic scale,which may have guiding significance for the use of structural engineering to control the performance of PtSe2.In addition,the structural transformation and stability of 1T'-MoS2 were studied,and a catalytic mechanism of HER was proposed,which may be helpful to further optimize the HER performance of 1T'-MoS2.Moreover,the microstructure of MoS2 wrinkles and its effects on electrical properties were studied,which is helpful to understand the properties of MoS2 nanostructures and is of great significance to the structural design in the electronic field.