Twinning superlattices in indium phosphide nanowires

Here, we show that we control the crystal structure of indium phosphide (InP) nanowires by impurity dopants. We have found that zinc decreases the activation barrier for 2D nucleation growth of zinc-blende InP and therefore promotes the InP nanowires to crystallise in the zinc blende, instead of the commonly found wurtzite crystal structure. More importantly, we demonstrate that we can, by controlling the crystal structure, induce twinning superlattices with long-range order in InP nanowires. We can tune the spacing of the superlattices by the wire diameter and the zinc concentration and present a model based on the cross-sectional shape of the zinc-blende InP nanowires to quantitatively explain the formation of the periodic twinning.Comment: 18 pages, 4 figure

Generic nano-imprint process for fabrication of nanowire arrays

A generic process has been developed to grow nearly defect free arrays of (heterostructured) InP and GaP nanowires. Soft nanoimprint lithography has been used to pattern gold particle arrays on full 2 inch substrates. After lift-off organic residues remain on the surface, which induce the growth of additional undesired nanowires. We show that cleaning of the samples before growth with piranha solution in combination with a thermal anneal at 550 C for InP and 700 C for GaP results in uniform nanowire arrays with 1% variation in nanowire length, and without undesired extra nanowires. Our chemical cleaning procedure is applicable to other lithographic techniques such as e-beam lithography, and therefore represents a generic process.Comment: 12 pages, 4 figures, 2 table

Bio-inspired broadband and omni-directional antireflective surface based on semiconductor nanorods

Bio-inspired layers of semiconductor nanorods increase light coupling into a high refractive index substrate. Reflection and transmission measurements show unambiguously, that the reduced reflection is due to optical impedance matching at the interfaces

Mimicking moth's eyes for photovoltaic applications with tapered GaP nanorods

We demonstrate experimentally that ensembles of conically shaped GaP nanorods form layers of graded refractive index due to the increased filling fraction of GaP from the top to the bottom of the layer. Graded refractive index layers reduce the reflection and increase the coupling of light into the substrate, leading to broadband and omnidirectional antireflection surfaces. This reduced reflection is the result of matching the refractive index at the interface between the substrate and air by the graded index layer. The layers can be modeled using a transfer-matrix method for isotropic layered media. We show theoretically that the light coupling efficiency into silicon can be higher than 95% over a broad wavelength range and for angles up to 60° by employing a layer with a refractive index that increases parabolically. Broadband and omnidirectional antireflection layers are specially interesting for enhancing harvesting of light in photovoltaics

III-Phospide Nanowires

International audienc

Design of light scattering in nanowire materials for photovoltaic applications

We experimentally investigate the optical properties of layers of InP, Si, and GaP nanowires, relevant for applications in solar cells. The nanowires are strongly photonic, resulting in a significant coupling mismatch with incident light due to multiple scattering. We identify a design principle for the effective suppression of reflective losses, based on the ratio of the nondiffusive absorption and diffusive scattering lengths. Using this principle, we demonstrate successful suppression of the hemispherical diffuse reflectance of InP nanowires to below that of the corresponding transparent effective medium. The design of light scattering in nanowire materials is of large importance for optimization of the external efficiency of nanowire-based photovoltaic device

Crystal structure transfer in core/shell nanowires

Contains fulltext : 91678.pdf (publisher's version ) (Closed access)5 p

Classical density-functional theory studies of fluid adsorption on nanopatterned planar surfaces

This contribution is based on our talk at the BIRS Workshop on “Coupled Mathematical Models for Physical and Biological Nanoscale Systems and Their Applications”. Our aim here is to summarize and bring together recent advances in wetting of nanostructured surfaces, using classical density-functional theory (DFT). Classical DFT is an ab initio theoretical-computational framework with a firm foundation in statistical physics allowing us to systematically account for the fluid spatial inhomogeneity, as well as for the non-localities of intermolecular fluid-fluid and fluid-substrate interactions. The cornerstone of classical DFT, is to express the grand free energy of the system as a functional of its one-body density, thus generating a hierarchy of N-body correlation functions. Unconstrained minimization of a properly approximated free-energy functional with respect to the one-body density then yields the basic DFT equation. And since most macroscopic quantities of interest can often be cast as averages over a one-body distribution, this equation provides a very useful and accessible computational tool. Indeed, there has been a rapid growth of classical DFT applications across a broad variety of fields, including phase transitions in solutions of macromolecules, interfacial phenomena, and even nucleation. Here we attempt to give a taste of what simple equilibrium DFT models look like, and what they can and cannot capture, as far as wetting on chemically heterogeneous substrates is concerned. We review recent progress in the understanding of planar prewetting and interface unbending on such substrates and compute substrate-fluid interfaces and wetting isotherms