Electron-beam induced transformations provide unprecedented opportunities for shaping materials atom-by-atom, and for studying the underlying processes in real time using in-situ microscopy. High-energy electrons can drive materials transformations via different mechanisms. Knock-on displacement of atoms has been used extensively to induce defects, such as vacancies in 2D materials. Here, we discuss this process in the context of structural transformations between different stable phases of few-layer metal chalcogenide semiconductors, which are initiated by the electron-induced generation of chalcogen vacancies and provide access to layered crystals with unique optoelectronic properties. While direct atomic displacements have long been explored, other interactions of relativistic electrons with matter are only now being recognized. For example, a focused electron beam represents a localized evanescent source of supercontinuum light. We can use this effect to stimulate and image the plasmon-mediated growth of anisotropic metal nanoprisms in solution. This novel approach can support the development of non-thermal chemical processes that depend on plasmonic light harvesting and the transfer of non-equilibrium charge carriers.