| With the continuous development of material processing technology,the size of materials in devices is decreasing.The physical and chemical properties of materials will change obviously when the material size is decreased to the nanometer level.As one of the most important properties of materials,phase transition has attracted extensive attention.At present,it is generally accepted that the phase transition point of nanomaterials decreases with the decrease of size.By contrast,limited effort has been directed towards the size-dependent dynamic phase transition behavior of nanomaterials,due to the lack of proper characterization method with atomic resolution,which not only enables the manipulation between different phases,but also allows for in situ imaging of the atomic structural change during phase transformation.Taking advantage of recent advances in transmission electron microscopy(TEM)and microelectromechanical system(MEMS)technology,we investigated the phase transition behavior and the shape change caused by phase transition of Ag2S at nanoscale.The main research contents and conclusions are listed below.1.For the first time,it's directly proved that Ag atoms in ?-Ag2S are distributed in 6(b),12(d)and 24(h)interstices of S framework.Atomic-resolution high-angle annular dark field(HAADF)images along[100]?,[110]?,[210]p and[111]?zone axes were acquired from single crystalline Ag2S nanoparticles.The experimental HAADF images and the simulated HAADF images indicate that Ag atoms occupied 6(b),12(d)and 24(h)interstices of S framework.In situ heating experiments showed that the site occupation of Ag atoms kept unchanged even when heated to 300 ?.This work provides new critical information for thoroughly understanding the structure ?-Ag2S.2.A shape memory semiconductor is reported for the first time.An intrinsic reversible-shape-memory effect was unveiled for the first time.The in-situ atomic scale STEM&TEM observations revealed that single-crystalline Ag2S can remember its shape by memorizing the shortest zone axis of the low temperature phase for hundreds of thermal cycles,during the order to disorder transformation between the monoclinic phase and BCC phase.Theoretical analysis and calculations suggest that such memory effect can be ascribed to the time-consuming homogenization of Ag in BCC phase Ag2S.Intriguingly,by altering the dwell time of heating,we demonstrate the ability to quasi-control the directions of the memorized zone axis of Ag2S and then its shape.This work offers a pathway to develop reversible shape memory effect without training and may broaden our current knowledge of the solid-solid phase transition.3.The contribution of the curved surface to the phase transition process was revealed.By quantitatively characterizing the phase transition behavior of nanoparticles,it was found that the disordering behavior is different from the traditional model that the disordered layer follows a logarithmic thickness dependence with temperature.The correlation length of disordered phase was found to be several times larger than the typical value for bulk surfaces.The onset temperature of disordered phase of the small nanoparticle was found to be lower than that of the big nanoparticle.In addition,it was found that the disordered phase thickness of small nanoparticle is always thicker than that of big nanoparticle.By considering surface and interface free-energy,a phenomenological model based on the minimization of system free-energy was established,which could well explain our experimental results.Finally,we investigated the phase transition process of heterogeneous morphology samples by characterize the disordering phase transition of Ag2S nanowires.Since the approximate curvature of thenanowire tip is larger than the middle region of the nanowire,the phase transition of the nanowire starts from the tip,which is consistent with the experimental results in nanoparticles.These discoveries extend our understanding of size-dependent phase transition mechanism. |
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