Functional nanomaterials not only promise potential applications, but also offer a platform where intrinsic or new physical effects can be exposed, discovered and understood. In single-domain functional materials, a wide range of processes, such as phase transition, carrier transport and energy exchange, can be directly investigated in situ without the restriction and complications in the bulk.

In this talk I will primarily discuss two examples from our research that highlight the significance of approaching the single domain level. The first example is on the metal-insulator and ferroelastic phase transition of vanadium dioxide. By probing the phase transition at the single-domain level along individual vanadium dioxide nanostructures, we uncover intrinsic physical properties that are previously hidden by phase inhomogeneities. Nanoscale power-meter and thermometer are developed from these materials which enable direct quantification of thermal energy exchange at the nanoscale. In the second example I will discuss a new photovoltaic effect which occurs in bismuth ferrite with periodic ferroelectric domain structures. Under sufficiently strong illumination, domain walls function as nanoscale generators of the photovoltaic current. In open circuit, photovoltages from the periodically ordered domain walls are additive and voltages much larger than the bandgap can be generated.