Plastic and elastic strain fields in GaAs/Si core-shell nanowires.

Thanks to their unique morphology, nanowires have enabled integration of materials in a way that was not possible before with thin film technology. In turn, this opens new avenues for applications in the areas of energy harvesting, electronics, and optoelectronics. This is particularly true for axial heterostructures, while core-shell systems are limited by the appearance of strain-induced dislocations. Even more challenging is the detection and understanding of these defects. We combine geometrical phase analysis with finite element strain simulations to quantify and determine the origin of the lattice distortion in core-shell nanowire structures. Such combination provides a powerful insight in the origin and characteristics of edge dislocations in such systems and quantifies their impact with the strain field map. We apply the method to heterostructures presenting single and mixed crystalline phase. Mixing crystalline phases along a nanowire turns out to be beneficial for reducing strain in mismatched core-shell structures.

Nanoskiving core-shell nanowires: a new fabrication method for nano-optics.

This paper describes the fabrication of functional optical devices by sectioning quantum-dot-in-nanowires systems with predefined lengths and orientations. This fabrication process requires only two steps, embedding the nanowires in epoxy and using an ultramicrotome to section them across their axis ("nanoskiving"). This work demonstrates the combination of the following four capabilities: (i) the control of the length of the nanowire sections at the nanometer scale; (ii) the ability to process the nanowires after cutting using wet etching; (iii) the possibility of modifying the geometry of the wire by varying the sectioning angle; and (iv) the generation of as many as 120 consecutive slabs bearing nanowires that have uniform size and approximately reproducible lateral patterns and that can subsequently be transferred to different substrates. The quantum dots inside the nanowires are functional and of a high optical quality after the sectioning process and exhibit photoluminescent emission with wavelengths in the range of 650-710 nm.

Quantum dot opto-mechanics in a fully self-assembled nanowire.

We show that optically active quantum dots (QDs) embedded in MBE-grown GaAs/AlGaAs core-shell nanowires (NWs) are coupled to the NW mechanical motion. Oscillations of the NW modulate the QD emission energy in a broad range exceeding 14 meV. Furthermore, this opto-mechanical interaction enables the dynamical tuning of two neighboring QDs into resonance, possibly allowing for emitter-emitter coupling. Both the QDs and the coupling mechanism, i.e. material strain, are intrinsic to the NW structure and do not depend on any functionalization or external field. Such systems open up the prospect of using QDs to probe and control the mechanical state of a NW, or conversely of making a quantum nondemolition readout of a QD state through a position measurement.

Photonic-plasmonic coupling of GaAs single nanowires to optical nanoantennas.

We successfully demonstrate the plasmonic coupling between metal nanoantennas and individual GaAs nanowires (NWs). In particular, by using dark-field scattering and second harmonic excitation spectroscopy in partnership with analytical and full-vector FDTD modeling, we demonstrate controlled electromagnetic coupling between individual NWs and plasmonic nanoantennas with gap sizes varied between 90 and 500 nm. The significant electric field enhancement values (up to 20×) achieved inside the NW-nanoantennas gap regions allowed us to tailor the nonlinear optical response of NWs by engineering the plasmonic near-field coupling regime. These findings represent an initial step toward the development of coupled metal-semiconductor resonant nanostructures for the realization of next generation solar cells, detectors, and nonlinear optical devices with reduced footprints and energy consumption.

Polarity assignment in ZnTe, GaAs, ZnO, and GaN-AlN nanowires from direct dumbbell analysis.

Aberration corrected scanning transmission electron microscopy (STEM) with high angle annular dark field (HAADF) imaging and the newly developed annular bright field (ABF) imaging are used to define a new guideline for the polarity determination of semiconductor nanowires (NWs) from binary compounds in two extreme cases: (i) when the dumbbell is formed with atoms of similar mass (GaAs) and (ii) in the case where one of the atoms is extremely light (N or O: ZnO and GaN/AlN). The theoretical fundaments of these procedures allow us to overcome the main challenge in the identification of dumbbell polarity. It resides in the separation and identification of the constituent atoms in the dumbbells. The proposed experimental via opens new routes for the fine characterization of nanostructures, e.g., in electronic and optoelectronic fields, where the polarity is crucial for the understanding of their physical properties (optical and electronic) as well as their growth mechanisms.

Three-dimensional multiple-order twinning of self-catalyzed GaAs nanowires on Si substrates.

In this paper we introduce a new paradigm for nanowire growth that explains the unwanted appearance of parasitic nonvertical nanowires. With a crystal structure polarization analysis of the initial stages of GaAs nanowire growth on Si substrates, we demonstrate that secondary seeds form due to a three-dimensional twinning phenomenon. We derive the geometrical rules that underlie the multiple growth directions observed experimentally. These rules help optimizing nanowire array devices such as solar or water splitting cells or of more complex hierarchical branched nanowire devices.

In(Ga)As quantum dot formation on group-III assisted catalyst-free InGaAs nanowires.

Growth of GaAs and In(x)Ga(1-x)As nanowires by the group-III assisted molecular beam epitaxy growth method on (001)GaAs/SiO(2) substrates is studied in dependence on growth temperature, with the objective of maximizing the indium incorporation. Nanowire growth was achieved for growth temperatures as low as 550 °C. The incorporation of indium was studied by low temperature micro-photoluminescence spectroscopy, Raman spectroscopy and electron energy loss spectroscopy. The results show that the incorporation of indium achieved by lowering the growth temperature does not have the effect of increasing the indium concentration in the bulk of the nanowire, which is limited to 3-5%. For growth temperatures below 575 °C, indium rich regions form at the surface of the nanowires as a consequence of the radial growth. This results in the formation of quantum dots, which exhibit spectrally narrow luminescence.

Antibody CDR loops as ensembles in solution vs. canonical clusters from X-ray structures.

In the past decade, the relevance of antibodies as therapeutics has increased substantially. Therefore, structural and functional characterization, in particular of the complementarity-determining regions (CDRs), is crucial to the design and engineering of antibodies with unique binding properties. Various studies have focused on classifying the CDR loops into a small set of main-chain conformations to facilitate antibody design by assuming that certain sequences can only adopt a limited number of conformations. Here, we present a kinetic classification of CDR loop structures as ensembles in solution. Using molecular dynamics simulations in combination with strong experimental structural information, we observe conformational transitions between canonical clusters and additional dominant solution structures in the micro-to-millisecond timescale for all CDR loops, independent of length and sequence composition. Besides identifying all relevant conformations in solution, our results revealed that various canonical cluster medians actually belong to the same kinetic minimum. Additionally, we reconstruct the kinetics and probabilities of the conformational transitions between canonical clusters, and thereby extend the model of static canonical structures to reveal a dynamic conformational ensemble in solution as a new paradigm in the field of antibody structure design.Abbreviations: CDR: Complementary-determining region; Fv: Antibody variable fragment; PCCA: Perron cluster analysis; tICA: Time-lagged independent component analysis; VH: Heavy chain variable region; VL: Light chain variable region.

Surprisingly Fast Interface and Elbow Angle Dynamics of Antigen-Binding Fragments.

Fab consist of a heavy and light chain and can be subdivided into a variable (V H and V L ) and a constant region (C H 1 and C L ). The variable region contains the complementarity-determining region (CDR), which is formed by six hypervariable loops, shaping the antigen binding site, the paratope. Apart from the CDR loops, both the elbow angle and the relative interdomain orientations of the V H -V L and the C H 1-C L domains influence the shape of the paratope. Thus, characterization of the interface and elbow angle dynamics is essential to antigen specificity. We studied nine antigen-binding fragments (Fab) to investigate the influence of affinity maturation, antibody humanization, and different light-chain types on the interface and elbow angle dynamics. While the CDR loops reveal conformational transitions in the micro-to-millisecond timescale, both the interface and elbow angle dynamics occur on the low nanosecond timescale. Upon affinity maturation, we observe a substantial rigidification of the V H and V L interdomain and elbow-angle flexibility, reflected in a narrower and more distinct distribution. Antibody humanization describes the process of grafting non-human CDR loops onto a representative human framework. As the antibody framework changes upon humanization, we investigated if both the interface and the elbow angle distributions are changed or shifted. The results clearly showed a substantial shift in the relative V H -V L distributions upon antibody humanization, indicating that different frameworks favor distinct interface orientations. Additionally, the interface and elbow angle dynamics of five antibody fragments with different light-chain types are included, because of their strong differences in elbow angles. For these five examples, we clearly see a high variability and flexibility in both interface and elbow angle dynamics, highlighting the fact that Fab interface orientations and elbow angles interconvert between each other in the low nanosecond timescale. Understanding how the relative interdomain orientations and the elbow angle influence antigen specificity, affinity, and stability has broad implications in the field of antibody modeling and engineering.

Catalyst-free nanowires with axial InxGa1-xAs/GaAs heterostructures.

Self-catalyzed growth of axial In(x)Ga(1-x)As/GaAs heterostructures has been realized by molecular beam epitaxy. The growth of the wires is achieved from gallium/indium alloy droplets that are nucleated in situ. By variation of the In/Ga beam flux during the growth it was possible to vary the effective indium content up to x = 5%, as deduced from photoluminescence measurements. We have analyzed the dependence of the alloy concentration on the growth conditions and present a simple model for the growth. The heterostructures grown with the method presented were spatially mapped along the wires with confocal microphotoluminescence and cathodoluminescence. It was found as expected that the emission of GaAs/In(x)Ga(1-x)As/GaAs heterostructures is localized. This work is important for the use of an external catalyst-free growth of complex axial heterostructures and related opto-electronic devices that facilitates its possible integration in the device or system fabrication processes.

Single crystalline and core-shell indium-catalyzed germanium nanowires-a systematic thermal CVD growth study.

Germanium nanowires were synthesized using thermal chemical vapor deposition (CVD) and indium as a catalyst. The process parameter space for successful growth was studied. By optimizing the growth temperature and gas pressure, high aspect ratio germanium nanowires have been obtained. Scanning electron microscopy investigations indicate that the final diameter of the nanowires is strongly influenced by the growth temperature and the germane partial pressure. High resolution transmission electron microscopy reveals that nanowires grow either as high quality single crystalline, or with a high quality single-crystalline core and a concentric amorphous shell. The occurrence of these two morphologies is found to only depend on the wire diameter. Chemical analysis of the nanowire tip indicates the presence of indium, validating its role as a catalyst. Raman spectroscopy measurements reveal a higher incidence of core-shell structures for nanowires synthesized at 30 Torr and indicate the presence of tensile strain. These results are important towards obtaining high quality germanium nanowires without the use of gold as a catalyst, which is known to degrade the wires' electrical and optical properties.

Antibody humanization-the Influence of the antibody framework on the CDR-H3 loop ensemble in solution.

Antibody engineering of non-human antibodies has focused on reducing immunogenicity by humanization, being a major limitation in developing monoclonal antibodies. We analyzed four series of antibody binding fragments (Fabs) and a variable fragment (Fv) with structural information in different stages of humanization to investigate the influence of the framework, point mutations and specificity on the complementarity determining region (CDR)-H3 loop dynamics. We also studied a Fv without structural information of the anti-idiotypic antibody Ab2/3H6, because it completely lost its binding affinity upon superhumanization, as an example of a failed humanization. Enhanced sampling techniques in combination with molecular dynamics simulations allow to access micro- to milli-second timescales of the CDR-H3 loop dynamics and reveal kinetic and thermodynamic changes involved in the process of humanization. In most cases, we observe a reduced conformational diversity of the CDR-H3 loop when grafted on a human framework and find a conformational shift of the dominant CDR-H3 loop conformation in solution. A shallow side minimum of the conformational CDR-H3 loop ensemble attached to the murine framework becomes the dominant conformation in solution influenced by the human framework. Additionally, we observe in the case of the failed humanization that the potentially binding competent murine CDR-H3 loop ensemble in solution shows nearly no kinetical or structural overlap with the superhumanized variant, thus explaining the loss of binding.

Polarization response of nanowires à la carte.

Thanks to their special interaction with light, semiconductor nanowires have opened new avenues in photonics, quantum optics and solar energy harvesting. One of the major challenges for their full technological deployment has been their strong polarization dependence in light absorption and emission. In the past, metal nanostructures have been shown to have the ability to modify and enhance the light response of nanoscale objects. Here we demonstrate that a hybrid structure formed by GaAs nanowires with a highly dense array of bow-tie antennas is able to modify the polarization response of a nanowire. As a result, the increase in light absorption for transverse polarized light changes the nanowire polarization response, including the polarization response inversion. This work will open a new path towards the widespread implementation of nanowires applications such as in photodetection, solar energy harvesting and light emission.

Hybrid axial and radial Si-GaAs heterostructures in nanowires.

Hybrid structures are formed from materials of different families. Traditionally, group IV and III-V semiconductors have not been integrated together in the same device or application. In this work we present a new approach for obtaining Si-GaAs hybrid heterostructures in nanowires based on a combination of molecular beam epitaxy and plasma enhanced chemical vapor deposition. Crystalline Si segments are integrated into GaAs nanowires grown by the Ga-assisted growth method at temperatures as low as 250 °C. We find that one of the most important factors leading to the successful growth of Si segments on GaAs is the silane-hydrogen dilution, which affects the concentration of silicon and hydrogen-based radicals (SiHx with x < 3) in the plasma, and determines if the Si shell is amorphous, polycrystalline or crystalline, and also if the growth takes place in the axial and/or radial directions. This work opens the path for the successful integration of silicon and III-V materials in one single nanowire.

Magnetic states of an individual Ni nanotube probed by anisotropic magnetoresistance.

Defined magnetization states in magnetic nanotubes could be the basic building blocks for future memory elements. To date, it has been extremely challenging to measure the magnetic states at the single-nanotube level. We investigate the magnetization states of an individual Ni nanotube by measuring the anisotropic magnetoresistance effect at cryogenic temperature. Depending on the magnitude and direction of the magnetic field, we program the nanotube to be in a vortex- or onion-like state near remanence.

Suppression of three dimensional twinning for a 100% yield of vertical GaAs nanowires on silicon.

Multiple seed formation by three-dimensional twinning at the initial stages of growth explains the manifold of orientations found when self-catalyzed GaAs nanowires grow on silicon. This mechanism can be tuned as a function of the growth conditions by changing the relative size between the GaAs seed and the Ga droplet. We demonstrate how growing under high V/III ratio results in a 100% yield of vertical nanowires on silicon(111). These results open up the avenue towards the efficient integration of III-V nanowire arrays on the silicon platform.

Vertical "III-V" V-shaped nanomembranes epitaxially grown on a patterned Si[001] substrate and their enhanced light scattering.

We report on a new form of III-V compound semiconductor nanostructures growing epitaxially as vertical V-shaped nanomembranes on Si(001) and study their light-scattering properties. Precise position control of the InAs nanostructures in regular arrays is demonstrated by bottom-up synthesis using molecular beam epitaxy in nanoscale apertures on a SiO(2) mask. The InAs V-shaped nanomembranes are found to originate from the two opposite facets of a rectangular pyramidal island nucleus and extend along two opposite <111> B directions, forming flat {110} walls. Dark-field scattering experiments, in combination with light-scattering theory, show the presence of distinctive shape-dependent optical resonances significantly enhancing the local intensity of incident electromagnetic fields over tunable spectral regions. These new nanostructures could have interesting potential in nanosensors, infrared light emitters, and nonlinear optical elements.

Untangling the electronic band structure of wurtzite GaAs nanowires by resonant Raman spectroscopy.

In semiconductor nanowires, the coexistence of wurtzite and zinc-blende phases enables the engineering of the electronic structure within a single material. This presupposes an exact knowledge of the band structure in the wurtzite phase. We demonstrate that resonant Raman scattering is a important tool to probe the electronic structure of novel materials. Exemplarily, we use this technique to elucidate the band structure of wurtzite GaAs at the Γ point. Within the experimental uncertainty we find that the free excitons at the edge of the wurtzite and the zinc-blende band gap exhibit equal energies. For the first time we show that the conduction band minimum in wurtzite GaAs is of Γ(7) symmetry, meaning a small effective mass. We further find evidence for a light-hole-heavy-hole splitting of 103 meV at 10 K.

EOS calculations for hydrothermal diamond anvil cell operation.

The hydrothermal diamond anvil cell (HDAC) is an excellent tool for high-temperature, high-pressure (hydrothermal) experiments. For an accurate determination of pressure induced by a certain temperature in an isochoric sample chamber volume, an equation of state (EOS) of water can be used instead of direct measurement. This paper reviews the theoretic background and provides all needed equations for the application of EOS of water to HDAC experiments summarizing state-of-the-art knowledge and incorporating up-to-date thermodynamic data. The p-T conditions determined using the IAPWS-95 formulation for the thermodynamic properties of ordinary water are in agreement with values obtained from direct methods or other established EOS formulations. In particular, (1) the calculation of density through the (a) melting point or (b) homogenization method along with determining (2) pressure as a function of density and temperature or (3) density as a function of pressure and temperature is explained. As a new aspect in the context of HDAC operations, the critical influence of nucleation and a strategy to overcome this problem are discussed. Furthermore, we have derived new polynomial equations, which allow the direct calculation of the fluid phase's density from the melting temperature. These are implemented in a spreadsheet program, which is freely available for interested users.