The polarization response of a material to a strain gradient, known as flexoelectricity, holds great promise for novel electromechanical applications. Despite considerable recent progress, however, the effect remains poorly understood. From both the fundamental and practical viewpoints, it is of crucial importance to know whether the coupling coefficients are primarily governed by the properties of the bulk material or by the details of the sample surface. Here we provide, by means of first-principles calculations, quantitative evidence supporting the latter scenario. In particular, we demonstrate that a SrTiO$_3$ film can yield a positive or negative flexoelectric voltage depending on its surface termination. This result points to a full control of the flexoelectric effect via surface/interface engineering, opening exciting new avenues for device design.

5 pages, 2 figures, 2 tables. Supplementary Information at http://departments.icmab.es/leem/stengel/supp2.pdf