The optimal functionalities of materials often appear at phase transitions involving simultaneous changes in the electronic structure and the symmetry of the underlying lattice. It is experimentally challenging to disentangle which of the two effects––electronic or structural––is the driving force for the phase transition and to use the mechanism to control material properties. In this talk, we present a concurrent pumping and probing of Cu2S nanoplates using an electron beam to directly manipulate the transition between two phases with distinctly different crystal symmetries and charge-carrier concentrations, and show that the transition is the result of charge generation for one phase and charge depletion for the other. We demonstrate that this manipulation is fully reversible and nonthermal in nature. Our observations reveal a phase-transition pathway in materials, where electron-induced changes in the electronic structure can lead to a macroscopic reconstruction of the crystal structure. This control method is in contrast to conventional chemical doping, which is irreversible and often introduces unwanted lattice distortions .
On the other hand, Cu2S material recently has attracted considerable attention for its ionic and thermoelectric properties. Unveiling the structural origin of this material's functionality is of great interest. Taking advantage of in situ electron diffraction and aberration-corrected imaging techniques, we further reveal the structural characteristics of Cu2S, including the Cu arrangement in the S framework through different thermal processes and the strain mapping at the phase boundaries during phase transitions.
 J. Tao et al., PNAS 114, 9832 (2017)