  Halide perovskites have attracted increasing research attention regarding
their outstanding optoelectronic applications. Owing to its low activation
energy, ion migration is implicated in the long-term stability and many unusual
transport behaviors of halide perovskite devices. However, precise control of
the ionic transport in halide perovskite crystals remains challenging. Here we
visualized and quantified the electric-field-induced halide ion migration in an
axial CsPbBr$_3$-CsPbCl$_3$ nanowire heterostructure and demonstrated a
solid-state ionic rectification, which is due to the non-uniform distribution
of the ionic vacancies in the nanowire that results from a competition between
electrical screening and their creation and destruction at the electrode
interface. The asymmetric heterostructure characteristics add an additional
knob to the ion-movement manipulation in the design of advanced ionic circuits
with halide perovskites as building blocks.
