In-situ Investigation Of The Growth And Sublimation Of PbSe Nanocrystals By Transmission Electron Microscope

In-situ thermal investigation as an important part of in-situ TEM experiment is widely applied in many nanocrystal structural studies.Combining the TEM imaging technique and thermodynamic research method,the in-situ thermal investigation can directly show the heat-initiated structural variations of nanocrystals.Nowadays,many in-situ thermal investigations of nanocrystals have been reported,and many important phenomena and conclusions have been revealed.However,there are still deficiencies in recent in-situ heating experiments on nanocrystals.For instance,most studies focus on the low to medium temperature zones during characterization,and the high-temperature zone is neglected;There are many reports on nanocrystal growth and phase transition,but the other physical phenomena such as nanocrystal sublimation are relatively underestimated.The relatively stable PbSe nanocrystals are systematically studied by the In-situ heating technique,the processes of nanocrystal growth and sublimation are directly observed,and the related nanocrystal structures,interface structures and dislocations are careful analyzed.The main results are shown as following:1.High-quality and small-sized PbSe nanocrystals with diameter less than 10 nm were prepared by solvothermal method,based on which the transformation process following the growth modes of oriented attachment and grain boundary migration at low to medium temperatures are careful investigated.In the original state,the PbSe nanocrystals with high degree of freedom grow up following the oriented attachment growth mode.As the temperature increases,the change of geometrical structure of the nanocrystals inhibits the oriented rotation,and the growth mode of nanocrystal switches into grain boundary migration.The small-sized PbSe nanocrystals grow into large-sized nanocrystals during the heating process,the increase in nanocrystal size and the decrease in nanocrystal quantity change the corresponding degree of freedom,which leads to a hybrid growth mode of PbSe nanocrystals with oriented attachment and grain boundary migration.2.The behaviors of atomic motion in nanocrystals with different growth modes are analyzed in details.During the process of oriented attachment growth,the PbSe nanocrystals rotate themselves to reduce the interfacial lattice mismatch,and the surface/ interfacial atoms diffuse to lower the system energy.The process of grain boundary migration is mainly realized through the interfacial atomic reconstruction,due to the local effects of stress and high interfacial energy,the interfacial atoms reconstruct gradually along the interface,which leads to the directional migration of nanocrystal boundaries,and at last leads to the growth of PbSe nanocrystals.3.The in-situ sublimation of PbSe nanocrystals at high temperature is also studied,and effects of nanocrystal size,interface,and dislocations are careful analyzed.The organic ligands of PbSe nanocrystal would deteriorate under high intensity electron beam irradiation,which would inhibit the growth of nanocrystals.As a result,nanocrystals maintain their original sizes and morphologies at high temperature,which is beneficial for the in-situ study of sublimation of PbSe nanocrystals with smaller size.The sublimation velocities of large-sized nanocrystals are isotropic in all directions.However,when the size of nanocrystals decreases to about 10 nm,the influences of stress and large crystal surface energy lead to an obvious change in sublimation directions.While the stress induced by the crystal lattice mismatch promotes the sublimation,the movements of dislocations at interfacial regions consume part of the system energy,which obviously reduces the sublimation velocity at the interface,which leads to the deviation of the contact interface from the fracture interface.