Nanoscale eletromechanical behavior

In this dissertation, we try to address some of the questions which arise while understanding the electromechanical behavior at the nanoscale. (1) Metals exhibit a size-dependent hardening when subjected to indentation. Mechanisms for this phenomenon have been intensely researched in recent times. Does such a size-effect also exist in the electromechanical behavior of ferroelectrics? If yes, what are the operative mechanisms? Experiments on BaTiO3 indeed suggest an elastic electromechanical size-effect. We argue, through theoretical calculations and differential experiments on another non-ferroelectric piezoelectric (Quartz), that the phenomenon of flexoelectricity (as opposed to dislocation activity) is most likely responsible for our observations. (2) Using a combination of a theoretical framework and atomistic calculations, we highlight the concept of surface piezoelectricity that can be used to interpret the piezoelectricity of nanostructures. Focusing on three specific material systems (ZnO, SrTiO3 and BaTiO3), we discuss the renormalization of apparent piezoelectric behavior at small scales. In a rather interesting interplay of symmetry and surface effects, we show that nanostructures of certain non-piezoelectric materials may also exhibit piezoelectric behavior. For the case of ZnO, using a comparison with first principles calculations, we also comment on the fidelity of the widely-used core-shell interatomic potentials to capture non-bulk electro-mechanical response. (3) Building entire devices with multiple components on single nanowires will lead to the ultimate miniaturization promised by nanotechnology. The capacitance measured from a single coaxial nanowire capacitor Cu-Cu2O-C device corresponds to ∼294microF/cm2, which considerably exceeds previously reported values for metal-insulator-metal micro-capacitors and is nearly fifty times larger than what is predicted by classical electrostatics. Our quantum mechanical calculations indicate that this unusually high capacitance value is attributed to negative quantum capacitance of the dielectric-metal interface. Also, we argue through first principle calculations on Graphene-Boron Nitrate-Graphene capacitors that quantum capacitance plays a key role in decreasing the total effective capacitance.