Ying Hao Chu

In conventional solid-state photovoltaics, electron-hole pairs are created by light absorption in a semiconductor and separated by the electric field spaning a micrometre-thick depletion region. The maximum voltage these devices can produce is equal to the semiconductor electronic bandgap. Here, we report the discovery of a fundamentally different mechanism for photovoltaic charge separation, which operates over a distance of 1-2nm and produces voltages that are significantly higher than the bandgap. The separation happens at previously unobserved nanoscale steps of the electrostatic potential that naturally occur at ferroelectric domain walls in the complex oxide BiFeO 3. Electric-field control over domain structure allows the photovoltaic effect to be reversed in polarity or turned off. This new degree of control, and the high voltages produced, may find application in optoelectronic devices. © 2010 Macmillan Publishers Limited. All rights reserved.

Domain walls may play an important role in future electronic devices, given their small size as well as the fact that their location can be controlled. Here, we report the observation of room-temperature electronic conductivity at ferroelectric domain walls in the insulating multiferroic BiFeO 3. The origin and nature of the observed conductivity are probed using a combination of conductive atomic force microscopy, high-resolution transmission electron microscopy and first-principles density functional computations. Our analyses indicate that the conductivity correlates with structurally driven changes in both the electrostatic potential and the local electronic structure, which shows a decrease in the bandgap at the domain wall. Additionally, we demonstrate the potential for device applications of such conducting nanoscale features.

The antiferromagnetic domain structure of BiFeO ₃ films was imaged and changes induced in the antiferromagnetic domains on switching the ferroelectric polarization was recorded. Ferroelectric measurements confirm a large polarization value along the surface normal and magnetic measurements show a weak, saturated magnetic moment. The ferroelectric domain structure is both imaged and switched using piezoelectric force microscopy (PEM). The antiferromagnetic domain structure is studied before and after electrical poling using photoemission electron microscopy (PEEM) based on X-ray linear dichroism (XLD). Bulk BiFeO ₃ is a room temperature ferroelectric with a spontaneous electric polarization directed along one of the axes of the perovskite structure. Coupling between ferroelectricity and antiferromagnetism in BiFeO ₃ thin film is a result from the coupling of both antiferromagnetic and ferroelectric domains to the underlying ferroelastic domain structure.