Piezoelectricity is a unique property of materials that permits the conversion of mechanical stimuli into electrical and vice versa. On the basis of crystal symmetry considerations, pristine carbon nitride (C 3 N 4 ) in its various forms is non-piezoelectric. Here we find clear evidence via piezoresponse force microscopy and quantum mechanical calculations that both atomically thin and layered graphitic carbon nitride, or graphene nitride, nanosheets exhibit anomalous piezoelectricity. Insights from ab inito calculations indicate that the emergence of piezoelectricity in this material is due to the fact that a stable phase of graphene nitride nanosheet is riddled with regularly spaced triangular holes. These non-centrosymmetric pores, and the universal presence of flexoelectricity in all dielectrics, lead to the manifestation of the apparent and experimentally verified piezoelectric response. Quantitatively, an e 11 piezoelectric coefficient of 0.758 C m À 2 is predicted for C 3 N 4 superlattice, significantly larger than that of the commonly compared a-quartz.

Conventional wisdom suggests that decreasing dimensions of dielectric materials (e.g., thickness of a film) should yield increasing capacitance. However, the quantum capacitance and the so-called "dead-layer" effect often conspire to decrease the capacitance of extremely small nanostructures, which is in sharp contrast to what is expected from classical electrostatics. Very recently, first-principles studies have predicted that a nanocapacitor made of graphene and hexagonal boron nitride (h-BN) films can achieve superior capacitor properties. In this work, we fabricate the thinnest possible nanocapacitor system, essentially consisting of only monolayer materials: h-BN with graphene electrodes. We experimentally demonstrate an increase of the h-BN films' permittivity in different stack structures combined with graphene. We find a significant increase in capacitance below a thickness of ∼5 nm, more than 100% of what is predicted by classical electrostatics. Detailed quantum mechanical calculations suggest that this anomalous increase in capacitance is due to the negative quantum capacitance that this particular materials system exhibits.

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The structure of dislocation cores in elastically anisotropic materials is considered. A definition of the dislocation core radius is introduced and used to demonstrate that the elastic anisotropy that develops near a composition driven phase transition, such as that predicted for the Ti-Nb based alloys known as gum metals, can drive dislocation core radii to infinity. Under these circumstances, dislocation cores necessarily overlap. The atomic scale structures predicted to arise from core overlap in Ti-V alloys are reminiscent of nanodisturbances observed in gum metals.

The complex interplay between the various attractive and repulsive forces that mediate between biological membranes governs an astounding array of biological functions: cell adhesion, membrane fusion, self-assembly, binding-unbinding transition among others. In this work, the entropic repulsive force between membranes-which originates due to thermally excited fluctuations-is critically reexamined both analytically and through systematic Monte Carlo simulations. A recent work by Freund [1] has questioned the validity of a well-accepted result derived by Helfrich [2]. We find that, in agreement with Freund, for small inter-membrane separations (d), the entropic pressure scales as p ∼ 1/d, in contrast to Helfrich's result: p ∼ 1/d 3 . For intermediate separations, our calculations agree with that of Helfrich and finally, for large inter-membrane separations, we observe an exponentially decaying behavior.