Ferroelectric oxides: Enhancement of the anisotropic photocurrent by strain gradients
The polymorphic phase interface of bismuth ferrites subjected to a giant strain gradient exhibits a large enhancement of the anisotropic interfacial photocurrent by two orders of magnitude, as a consequence of the flexoelectric effect.
The flexoelectric effect, which is a dielectric property whereby electric polarization is induced by a strain gradient, is a critical ingredient in understanding mechano-electric phenomena. The flexoelectric effect is ubiquitous in materials subject to inhomogeneous mechanical deformation, whereas the piezoelectric effect appears only in crystalline structures lacking inversion symmetry. Regardless of the universal nature of flexoelectricity, investigations of the flexoelectric effect have focused primarily on such soft materials as liquid crystals, biomaterials, polymers, and elastomeric materials because inorganic materials tend to experience crack-induced structural breaks before significant elastomeric deformations except for ferroelectric compounds with a high degree of electric permittivity. Recent advances in nanoscale characterizations have led researchers to the discovery that large strain gradients are often present in the various self-assembled inorganic nanostructures involved in the strain relaxation of epitaxial films, dislocations, ferroelectric domain walls, and morphotropics phase boundaries. Moreover, such inhomogeneous deformations at the nanoscale have a strain gradient that is approximately a million times larger than that in macroscopic flexural bending, most likely will induce large modulations in electronic behavior and new functionalities. However, electronic conduction in the presence of a giant strain gradient at the nanoscale has received little research attention despite its importance for nanoscale device applications.
Recently, Prof. Chan-Ho Yang and his colleagues have investigated enhanced photocurrent effects observed in some particular regions of the epitaxial bismuth ferrite thin films. The competition between the metastable phases of bismuth ferrite leads to a spatial phase separation and forms a phase boundary where a large strain gradient, as much as ~107 m-1, exists. Remarkably, light-induced short-circuit current was enhanced in proportion to the area of the interface located inside an incident light beam. This could be found through careful analysis of a spatially resolved photocurrent map and a surface morphological image in an integrated manner. In addition, the photocurrent exhibited two-fold azimuthal dependence of light polarization, indicating the anisotropic nature of light-induced electron-hole pair formation is possibly due to the fact that inhomogeneous structural deformation affects the electron orbital structure, and so does the optical transition probability.
These interfaces were carefully investigated by means of nanoscale characterization tools, i.e. piezoresponse force microscopy (PFM) and transmission electron microscopy (TEM). A position-sensitive angle-resolved PFM technique disclosed complicated ferroelectric domain structures in the vicinity of the phase interface, and electron holography was employed to identify the local charge distribution across the interface. In this way, it was clarified that the flexoelectric polarization induced by the strain gradient resulted in a dipole-charged domain wall, and moreover a large built-in electric field created at the interfacial area played an important role in the separation of photo-induced electron-hole pairs. This observation associated with the interfacial flexoelectricity provides a new pathway to improving the efficiency of photovoltaic cells.
Of great importance in this work is the suggestion that a quadratic polarization response with respect to strain gradients can overwhelm the linear response in a large-strain-gradient regime when the system doesn’t have inversion symmetry. This quadratic effect is considered a new symmetry element missing in the electromechanical phenomena; hopefully, it can make fourth position after piezoelectricity, electrostriction, and linear flexoelectric effects.
This study was published online in Nature Nanotechnology (August, 2015).
Kanghyun Chu, Byung-Kweon Jang, Ji Ho Sung, Yoon Ah Shin, Eui-Sup Lee, Kyung Song, Jin Hong Lee, Chang-Su Woo, Seung Jin Kim, Si-Young Choi, Tae Yeong Koo, Yong-Hyun Kim, Sang-Ho Oh, Moon-Ho Jo, and Chan-Ho Yang, “Enhancement of the anisotropic photocurrent in ferroelectric oxides by strain gradients,” Nature Nanotechnology (2015), doi:10.1038/nnano.2015.191
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