Phase transformation at controlled locations in nanowires by in situ electron irradiation

Solid state phase transtormations have drawn great attention because they can be effectively exploited to control the microstructure and property of materials.Understanding the physics of such phase transformation processes is critical to designing materials with controlled structure and with desired properties.However,in traditional ex situ experiments,it is hard to achieve position controlled phase transformations or obtain desirable crystal phase on nanometer scale.Meanwhile the underlying mechanisms of the reaction processes are not fully understood due to the lack of direct and real-time observation.In this paper,we observe phase transformation from body-centered tetragonal PX PbTiO3 to monclinic TiO2(B)on the atomic scale by in situ electron iradiation during heat treatment in transmission electron microscope,at pre-defined locations on the sample.We demonstrate that by controlling the location of the incident electron beam,a porous TiO2(B)crystal structure can be formed at the desired area on the nanowire,which is dificult to achieve by traditional synthesis methods.Upon in situ heating,the Pb atoms in the crystal migrate out of the pristine nanowire through inelastic scattering under incident electrons while high temperature(>400℃)provides energy for the crytalization of TiO2(B)and the vlatlization of a substantial number of Pb atoms,which makes the resultingTiO2(B)nanowires to be porous.In contrast,at temperatures<400℃,the segregated Pb atoms form Pb particles and the TiO、nanowires remain in the amorphous state.This work not only provides in situ visualization of the phase transition from the PX-PbTiO3 to monoclinic TiO2(B),but also suggests a crystallography engineering strategy to obtain the desired crystal phase at controlled locations on the nanometer scale.