In-situ TEM Studies Of The Nanoscale Phase Transitions In Cu₂Se And VO₂

With the wide study and application of nanomaterials,the features of nanoscale materi-als which differ from the bulk features are of great importance.As one of the significant characters of materials,phase transition has been researched for a long time.However,there are still a lot of unveiled characters about the phase transition behaviours at nanoscale.Along with the rapid development of in-situ electron microscopy,detailed analyses on the influences induced by the experimental method are urgently needed.Here in this thesis,we report our studies about the nanoscale phase transition behaviours of Cu2Se and VO2(A)using in-situ heating transimission electron microscopy(TEM),the theoretical modelling analysis on Cu2Se,the discussion about the crystal structure of Cu2Se low-temperature phase,and effects induced by the in-situ TEM experiments.The details are listed below.1.For the first time,the coexistence of the two phases in wedge-shaped nanocrystal made of second-order phase transition material Cu2Se is discovered,and the atomic scale manipula-tion of phase boundary is revealed First,high-quality nano-sized single crystal specimens are prepared by focused ion beam(FIB)milling method.Then the phenomena of the thermody-namically stable phase coexistence in a wedge-shaped nanocrystal made of second-order phase transition materials,and the phase transition’s sensitive response to the temperature,are re-vealed in in-situ heating TEM experiments,which are different from the bulk materials.The thermodynamic model(Chapter 4)based on the above phenomena matches the experimental data well,and verifies the second-order nature of phase transition in bulk Cu2Se.Based on these discoveries,the accurate and sensitive reversible manipulation of the phase boundary in wedge-shaped Cu2Se nanocrystal is realized.Finally,the possible influences induced by the in-situ electron microscope experiment,including the electron irradiation induced heating effect(Chapter 5),are investigated systematically and proved to be insignificant in this experiment.2.Thermodynamic model which describes the phenomena of phase coexistence and con-trollable phase transition behaviour in a wedge-shaped nanocrystal is established.The phenom-ena above are impossible in bulk second-order phase transition materials.However,because the decrease in size and the increase in specific surface area at nanoscale,the surface effect can no longer be ignored.By combining the Landau theory which is suitable for the bulk materials,and the contribution of surface effect to the system total Gibbs free energy at nanoscale,a ther-modynamic model is established,and a semiquantitative explanation of the phenomena in Chapter 3 is presented.Also,the case of first-order phase transition materials is discussed.3.Modelling and quantitative analyses on the heating effect induced by the electron ir-radiation in the in-situ TEM experiments are revealed.To investigate the heating effect of electron irradiation in the in-situ heating experiments of Cu2Se,a model which describes the temperature distribution of a wedge-shaped specimen under electron irradiation is established.By combining the experimental data,the temperature inhomogeneity induced by the electron irradiation in the in-situ characterization of wedge-shaped Cu2Se nanocrystal are investigated.The results indicate that even under a strong dose rate,the temperature inhomogeneity in-duced by the electron irradiation in the high-resolution observed area can be ignored.4.The characters of multiformity and ordered fluctuation in Cu2Se low-temperature phase are unveiled for the first time.In oriented high-quality Cu2Se nano-sized single crystal specimens prepared by FIB milling method,the multiple structural characters of Cu2Se low-temperature phase are analyzed according to the electron diffraction data and the data ac-quired along the[112]c zone axis by spherical aberration(Cs)-corrected scanning transimis-sion electron microscope(STEM).Models of multiple structure variations of Cu2Se low-temperature phase are established by the first principle calculations.The comparison between the simulated high-angle annular dark field images and the experimental results indicates that these models and the assembly of them can reflect the structural characters well.5.The transient states during the phase transition process of VO2(A)nanowires are discovered.The phase transition process of VO2(A)nanowires are characterized in in-situ heating experiments by Cs-corrected STEM.According to the results,the mechanism that the inlayer transition is prior to the interlayer transition in the(001)atomic layers during the phase transition of VO2(A)are proposed.In this thesis,a preliminary theoretical model of the phase transition behaviors of na-nomaterials different from the bulk materials is provided,which is verified using Cu2Se as an example.An example is provided to investigate the influences induced by in-situ heating TEM experiments.Experimental foundations of microstructures are given to better under-stand the excellent performance of Cu2Se during the phase transition,and the phase transi-tion process of VO2(A).This thesis expands the understandings of phase transition at na-noscale from several aspects,and offers experimental and theoretical foundations to a fur-ther study of the phase transition behaviours.