In this work, using a combination of a theoretical framework and atomistic calculations, we highlight the concept of “surface piezoelectricity,” which 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. Finally, 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.

Building entire multiple-component devices on single nanowires is a promising strategy for miniaturizing electronic applications. Here we demonstrate a single nanowire capacitor with a coaxial asymmetric Cu-Cu 2 o-C structure, fabricated using a two-step chemical reaction and vapour deposition method. The capacitance measured from a single nanowire device corresponds to ~140 µF cm − 2 , exceeding previous reported values for metal-insulator-metal micro-capacitors and is more than one order of magnitude higher than what is predicted by classical electrostatics. Quantum mechanical calculations indicate that this unusually high capacitance may be attributed to a negative quantum capacitance of the dielectric-metal interface, enhanced significantly at the nanoscale.

We examine the behavior of spherical silica particles trapped at an air-nematic liquid crystal interface. When a strong normal anchoring is imposed, the beads spontaneously form various structures depending on their area density and the nematic thickness. Using optical tweezers, we determine the pair potential and explain the formation of these patterns. The energy profile is discussed in terms of capillary and elastic interactions. Finally, we detail the mechanisms that control the formation of an hexagonal lattice and analyze the role of gravity for curved interfaces. PACS numbers: 61.30.Jf,61.30.Hn,64.75.Xc Colloidal particles confined at liquid interfaces display rich two-dimensional (2D) phase properties [1,2]. The spontaneous formation of ordered structures such as microcrystals has been mainly studied in simple fluids [3][4][5] where the self-arrangement is controlled by direct colloidal interactions (electrostatic [6,7], magnetic [1]...) and possible capillary effects. The latter might come from the anisotropic shape [8] or the roughness of the particles [9]. It is only recently that an interest [10][11][12][13] has developed in the behavior of particles trapped at an ordered fluid interface. In bulk liquid crystals (LC), additional long-range interactions between particles are present because of the partial order and elasticity. Colloidal suspensions [14,15] in a nematic matrix are thus qualitatively different from their isotropic analogues. They display rich self-ordering phenomena involving particles and topological defects. At LC interfaces, complex ordered structures were also observed in several cases: glycerin droplets [10,11] or solid beads [16] at nematic/air interfaces or microparticules at nematic/water interface [12,13]. All these systems display 2D hexagonal crystals that were ascribed to the competition between a repulsion due to the bulk liquid crystal elasticity and a capillary attraction resulting from the interface distortions caused by the "nematic elastic pressure". This new type of capillary interaction is however thoroughly discussed in two recent works [17,18] and its role is not clearly established. To clarify the respective role of the elastic and capillary force a direct force measurement between trapped particles coupled with a careful control of the LC anchoring on the beads as well as of the flatness of interface would be suitable.In this work, we present a simple technique for trapping colloids at the flat interface of an aligned thin layer of nematic liquid crystal. By controlling the beads density, the interface curvature and the LC anchoring, we were then able to establish their respective role in the formation of the colloidal structures. A direct measurement of the pairwise interaction has been obtained with optical tweezers, which allowed us to discuss the respective roles played by LC elasticity and capillarity. FIG. 1: a) Deposition of colloids at the air liquid crystal interface. The LC texture is either hybrid due to the strong planar anchoring on polymide and ho...

We exploit the long-ranged elastic fields inherent to confined nematic liquid crystals (LCs) to assemble colloidal particles trapped at the LC interface into reconfigurable structures with complex symmetries and packings. Spherical colloids with homeotropic anchoring trapped at the interface between air and the nematic LC 4-cyano-4′-pentylbiphenyl create quadrupolar distortions in the director field causing particles to repel and consequently form close-packed assemblies with a triangular habit. Here, we report on complex open structures organized via interactions with defects in the bulk. Specifically, by confining the nematic LC in an array of microposts with homeotropic anchoring conditions, we cause defect rings to form at well-defined locations in the bulk of the sample. These defects source elastic deformations that direct the assembly of the interfacially trapped colloids into ring-like assemblies, which recapitulate the defect geometry even when the microposts are completely immersed in the nematic. When the surface density of the colloids is high, they form a ring near the defect and a hexagonal lattice far from it. Because topographically complex substrates are easily fabricated and LC defects are readily reconfigured, this work lays the foundation for a versatile, robust mechanism to direct assembly dynamically over large areas by controlling surface anchoring and associated bulk defect structure.lassically, the bulk of a material system is where the action is, and the interface is often relegated to a set of "boundary conditions." However, crystal faceting (1), the quantum hall effect (2), and even the anti-de Sitter space-conformal field theory (AdS-CFT) correspondence (3) fundamentally reverse this relationship: The bulk properties can be read off from their effects on the boundaries. In this contribution, we demonstrate migration and organization of colloids constrained to a liquid crystal (LC)-air interface, driven remotely by the elastic distortion created by the presence of topological defects in the liquid crystalline bulk. Just as phantoms are used in MRI (4), it is necessary for us to prepare bulk defects in known configurations to verify our bulk/ boundary connection. To do this, we prepare a substrate patterned with microposts that, with appropriate surface treatment, seed a reproducible defect complexion. Colloidal spheres on the interface experience an attraction to the regions above the submerged defects, as well as an elastic repulsion from each other, leading to complex new assemblies. The long range of these elastic interactions allows defects in the bulk nematic phase far below the interface to direct assembly at the interface. Other recent work on producing ordered arrangements of particles at LC interfaces beyond simple triangular lattices, such as chains (5), stripes (6), and dense quasihexagonal lattices (7), has focused on confining the nematic in thin film or droplet geometries and on varying the surface coverage fraction. Our sensitive control over substrate topography provid...