Two-dimensional layered materials have been at the forefront of material research ever since the discovery of graphene about a decade ago. The dangling-bond-free lattice has made it feasible to integrate these materials with highly disparate atomic layers, such as industrial silicon substrates, to create a wide spectrum of van der Waals heterostructures without the constraints of lattice matching and processing compatibility.
Van der Waals (vdW) thio- and seleno-phosphates, belonging to the transitional metal thio/seleno-phosphates (TPS) family, have recently gained considerable attention for the use as ‘active’ dielectrics in two-dimensional or quasi-two-dimensional electronic devices. Bulk ionic conductivity in these materials has been identified as a key factor for the control of their electronic properties. However, direct evidence of specific ion species’ migration at the nanoscale, particularly under electric fields, and its impact on material properties has been elusive.
Here, we report on direct evidence of a phase-selective anisotropic Cu ion hopping mechanism in copper indium thiophosphate (CuInP2S6) through detailed scanning probe microscopy measurements. A two-step Cu-hopping path including a first intralayer hopping (in-plane) and second interlayer hopping (out-of-plane) crossing the vdW gap is unveiled. Evidence of electrically controlled Cu ion migration is further verified by nanoscale energy-dispersive X-ray spectroscopy (EDS) mapping. These findings offer new insight into anisotropic ionic manipulation in layered vdW ferroelectric/dielectric materials for emergent vdW electronic device design.
About the presenter
Dawei Zhang is a PhD student working with CI Jan Seidel at UNSW.