Columnar Geometry Research Articles

Contributions of cell behavior to geometric order in embryonic cartilage.

During early development, cartilage provides shape and stability to the embryo while serving as a precursor for the skeleton. Correct formation of embryonic cartilage is hence essential for healthy development. In vertebrate cranial cartilage, it has been observed that a flat and laterally extended macroscopic geometry is linked to regular microscopic structure consisting of tightly packed, short, transversal clonar columns. However, it remains an ongoing challenge to identify how individual cells coordinate to successfully shape the tissue, and more precisely which mechanical interactions and cell behaviors contribute to the generation and maintenance of this columnar cartilage geometry during embryogenesis. Here, we apply a three-dimensional cell-based computational model to investigate mechanical principles contributing to column formation. The model accounts for clonal expansion, anisotropic proliferation and the geometrical arrangement of progenitor cells in space. We confirm that oriented cell divisions and repulsive mechanical interactions between cells are key drivers of column formation. In addition, the model suggests that column formation benefits from the spatial gaps created by the extracellular matrix in the initial configuration, and that column maintenance is facilitated by sequential proliferative phases. Our model thus correctly predicts the dependence of local order on division orientation and tissue thickness. The present study presents the first cell-based simulations of cell mechanics during cranial cartilage formation and we anticipate that it will be useful in future studies on the formation and growth of other cartilage geometries.

Establishment and analysis of numerical model of deuterium fuel ice migration in indirect-drive cryogenic target

A cryogenic target is the core component in inertial confinement fusion. For meeting the experimental requirements, the uniformity of the fuel ice layer in the capsule needs to reach 99% at least. The capsule has a nonuniform temperature distribution natively resulting from the columnar hohlraum geometry, leading to a nonuniform fuel ice layer profile. A proper temperature distribution along the hohlraum can be used to improve the uniformity of the fuel ice layer. A numerical model that can predict the kinetics of the fuel solid-vapor interface during the migration process is proposed by coupling the heat transfer and flow equations with the Stefan condition. This numerical model has good calculational accuracy verified with the pure-ice sublimation experiment. By studying different hohlraum temperatures and fuel ice layer initial eccentricities, the basic characteristic of ice immigration has been figured out. For the manufacture process, a temperature control scheme has been proposed by which the uniformity of the fuel ice layer can reach 99.57%. For the retention process, a perfectly uniform fuel ice layer can be maintained for 2000s before it cannot meet the ignition standard. These results are of significant guidance on cryogenic target experiments.

W fuzz layers: very high resistance to sputtering under fusion-relevant He + irradiations

In this study, we have modeled the sputtering process of energetic He+ ions colliding with W nano-fuzz materials, based on the physical processes, such as the collision and diffusion of energetic particles, sputtering and redeposition. Our modeling shows that the fuzzy nanomaterials with a large surface-to-volume ratio exhibit very high resistance to sputtering under fusion-relevant He+ irradiations, and their sputtering yields are mainly determined by the thickness of fuzzy nano-materials, the reflection coefficients and mean free paths of energetic particles, surface sputtering yields of a flat base material, and the geometry of nano-fuzz. Our measurements have confirmed that the surface sputtering yield of a W nano-fuzz layer with the columnar geometry of nano-fuzz in cross-section is about one magnitude of order lower than the one of smooth W substrates. This work provides a complete model for energetic particles colliding with the nano-fuzz layer and clarifies the fundamental sputtering process occurring in the nano-fuzz layer.

Improving the Room-Temperature Ferromagnetism in ZnO and Low-Doped ZnO:Ag Films Using GLAD Sputtering.

ZnO and doped ZnO films with non-ferromagnetic metal have been widely used as biosensor elements. In these studies, the electrochemical measurements are explored, though the electrical impedance of the system. In this sense, the ferromagnetic properties of the material can be used for multifunctionalization of the sensor element using external magnetic fields during the measurements. Within this context, we investigate the room-temperature ferromagnetism in pure ZnO and Ag-doped ZnO films presenting zigzag-like columnar geometry. Specifically, we focus on the films’ structural and quasi-static magnetic properties and disclose that they evolve with the doping of low-Ag concentrations and the columnar geometry employed during the deposition. The magnetic characterization reveals ferromagnetic behavior at room temperature for all studied samples, including the pure ZnO one. By considering computational simulations, we address the origin of ferromagnetism in ZnO and Ag-doped ZnO and interpret our results in terms of the Zn vacancy dynamics, its substitution by an Ag atom in the site, and the influence of the columnar geometry on the magnetic properties of the films. Our findings bring to light an exciting way to induce/explore the room-temperature ferromagnetism of a non-ferromagnetic metal-doped semiconductor as a promising candidate for biosensor applications.

Microstructural modeling of anisotropic plasticity in large scale additively manufactured 316L stainless steel

In this paper, the anisotropic mechanical response triggered by specific microstructures encountered in wire + arc additively manufactured 316L stainless steels is analyzed. For this purpose, a representative volume element is generated, with an internal geometry that is based on an average periodic fusion zone shape. The large columnar grain geometry with different preferred directions within a single fusion zone is reconstructed with an anisotropic Voronoi algorithm. Grain orientations are extracted from experimental data of different regions within a single sample. Crystal plasticity finite element simulations are performed to predict the macroscopic yield behavior under various loading angles. It is observed that the anisotropic response varies significantly within a single sample, which is essentially caused by differences in the texture. A spatial correlation between the orientation of a grain and its location within the fusion zone is identified, allowing to distinguish clusters of grains with a similar orientation. As a result, strain localizes in a global shear band across the entire height of the fusion zone, which prominently influences the macroscopic response. A comparison of the results relative to a standard isotropic Voronoi grain geometry shows that the elongated geometry of the grains does not influence the macroscopic yield behavior significantly. The local crystallographic orientations are dominating instead, whereby the correlation between location and orientation of grains has to be taken into account.

Pack-boriding of low alloy steel: microstructure evolution and migration behaviour of alloying elements

ABSTRACTLow alloy steel was pack-borided at different processing temperatures (at 850, 950, and 1050°C) and times (2, 4, and 6 h). The microstructural characterisation of boronized steel showed the presence of three zones, namely boronized region containing finer grains and columnar geometry of (Fe, M)2B (where M = Cr, Mn, Mo, and Ni), transition zone, and non-boronized core. The concentrations of the alloying elements in (Fe, M)2B were increased from the surface to the core of the specimen. The pattern of slope variation of boron concentration–depth profile (obtained using GDOES) was linked with the boride morphology and process temperature. Pack-boriding of steel led to the development of systematic trend in slope variation of overall concentration–depth profiles of the alloying elements. The composition and morphology of boride affected the trend of slope variation for the boride-forming alloying elements. However, for Al and Si, the trend of slope variation was connected to the boride morphology and the composition of the matrix. Chemistry of the matrix was strongly dependent on the migration kinetics of the alloying elements during the boride growth. The migration kinetics of Cr, Mn, Mo, and C were found almost equivalent to the rate of boride growth. However, Ni, Al, and Si were migrated at a slower rate. Si showed the lowest migration kinetics among the alloying elements. The concentrations of the alloying elements having higher migration kinetics remained constant in the matrix during the boride growth.

Molecular dynamics simulations of grain interactions in shock-compressed highly textured columnar nanocrystals

While experimental and computational studies abound demonstrating the diverse range of phenomena caused by grain interactions under quasistatic loading conditions, far less attention has been given to these interactions under the comparatively dramatic conditions of shock compression. The consideration of grain interactions is essential within the context of contemporary shock-compression experiments that exploit the distinctive x-ray diffraction patterns of highly textured (and therefore strongly anisotropic) targets in order to interrogate local structural evolution. We present here a study of grain interaction effects in shock-compressed, body-centered cubic tantalum nanocrystals characterized by a columnar geometry and a strong fiber texture using large-scale molecular dynamics simulations. Our study reveals that contiguous grains deform cooperatively in directions perpendicular to the shock, driven by the gigapascal-scale stress gradients induced over their boundaries by the uniaxial compression, and in so doing are able to reach a state of reduced transverse shear stress. We compare the extent of this relaxation for two different columnar geometries (distinguished by their square or hexagonal cross-sections), and quantify the attendant change in the transverse elastic strains. We further show that cooperative deformation is able to replace ordinary plastic deformation mechanisms at lower shock pressures, and, under certain conditions, activate new mechanisms at higher pressures.

Supramolecular Polymerization in Liquid Crystalline Media: Toward Modular Synthesis of Multifunctional Core-Shell Columnar Liquid Crystals.

Recently, we discovered a modular synthetic approach for constructing core-shell columnar liquid crystals (LCs) by supramolecular polymerization in LC media. In the present work, we successfully confirmed that our modular synthetic approach has the potential to be widely extended to the development of multifunctional columnar LCs. Herein, we constructed the first core-shell columnar LC that was proved to be orientable by both electric and magnetic fields by the supramolecular polymerization of NODiskNH* in a nematic LC medium of 4-cyano-4'-pentyloxybiphenyl (5OCB). NODiskNH* is a chiral benzenetricarboxamide derivative bearing 2,2,6,6-tetramethylpiperidine 1-oxyl termini, which is known to form a helical supramolecular polymer via a triple hydrogen-bonding array. NODiskNH* alone formed a hydrogen-bonded liquid phase without any long-range structural ordering. However, a nematic LC medium of 5OCB, when mixed with NODiskNH* at a molar ratio of 1:3, underwent a "structural order-increasing" mesophase transition, affording an optically active single LC phase with a hexagonally arranged core-shell columnar geometry in a temperature range from 113 to 51 °C. Unprecedentedly, this core-shell columnar LC can orient its columns both electrically and magnetically, resulting in unidirectional columnar ordering.

Columnar liquid crystalline assembly of a U‐shaped molecular scaffold stabilized by covalent or noncovalent incorporation of aromatic molecules

ABSTRACTWe report on our serendipitous finding of the anomalous behavior of a novel liquid crystalline (LC) molecule UH/H, containing a U‐shaped handle. While UH/H itself shows a bicontinuous cubic (Cubbi) phase, UH/H in the presence of pyrene molecules as an external guest forms a hexagonal columnar (Colh) phase without phase separation. Interestingly, even when the pyrene molecules were covalently attached to UH/H (UPy/Py), the molecule exhibited a Colh phase, with a similar columnar geometry to that of UH/H mixed with pyrene molecules (1–4 equiv.). This result means that no matter how the pyrene moieties are incorporated in the LC system, the resultant material exhibited a similar molecular assembly with a columnar geometry. This finding is interesting because UH/H and pyrene molecules do not seem to have specific interactions or shape complementarity to form a columnar assembly, as in the case of UPy/Py. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 342–351

A room-temperature heptazine core discotic liquid crystal

ABSTRACTHeptazine is the fundamental structural and functional unit of graphitic carbon nitrides and has a π-conjugated planar symmetry with semiconducting properties. Molecular self-assembly is one of the key factors to drive the electronic behaviour of π-conjugated organic molecules. To enhance the semiconductivity, heptazine is decorated at its active sites with 2,3,5-tris(dodecyloxy)aniline to get well-organised columnar packing. A novel heptazine-core room-temperature discotic liquid crystal (HDLC) with hexagonal columnar geometry is discovered. The molecular structure and mesomorphic properties of HDLC are investigated by 1H-NMR, 13C-NMR, IR, polarised optical microscopy, differential scanning calorimetry and X-ray diffraction. Photophysical and electrochemical properties of HDLC are studied by UV–vis/fluorescence spectroscopy and cyclic voltammetry (CV), respectively. The energy band gap of HDLC estimated from CV at room temperature is 1.63 eV, which is much narrower than the previously reported band gap for heptazine derivatives. This decrease in the energy band can be attributed to the particular designing of HDLC for columnar packing. As a columnar liquid crystal providing a smooth path to the charge transport, HDLC with such a narrow energy band gap may find applications in organic electronics.

Permian–Triassic boundary microbialites at Zuodeng Section, Guangxi Province, South China: Geobiology and palaeoceanographic implications

A previously unknown microbialite bed in the Permian–Triassic (P–Tr) boundary beds of Zuodeng section, Tiandong County, Guangxi, South China comprises a thin (5cm maximum thickness) stromatolite in the lower part and the remaining 6m is thrombolite. The Zuodeng microbialite has a pronounced irregular contact between the latest Permian bioclastic limestone and microbialite, as in other sites in the region. The stromatolite comprises low-relief columnar and broad domal geometries, containing faint laminations. The thrombolite displays an irregular mixture of sparitic dark coloured altered microbial fabric and light coloured interstitial sediment in polished blocks. Abundant microproblematic calcimicrobe structures identified here as Gakhumella are preserved in dark coloured laminated areas of the stromatolite and sparitic areas in thrombolites (i.e. the calcimicrobial part, not the interstitial sediment) and are orientated perpendicular to stromatolitic laminae. Each Gakhumella individual has densely arranged segments, which form a column- to fan-shaped structure. Single segments are arch-shaped and form a thin chamber between segments. Gakhumella individuals in the stromatolite and thrombolite are slightly different from each other, but are readily distinguished from the Gakhumella- and Renalcis-like fossils reported from other P–Tr boundary microbialites in having a smaller size, unbranching columns and densely arranged, arch-shaped segments. Renalcids usually possess a larger body size and branching, lobate outlines. Filament sheath aggregates are also observed in the stromatolite and they are all orientated in one direction. Both Gakhumella and filament sheath aggregates may be photosynthetic algae, which may have played an important role in constructing the Zuodeng microbialites. Other calcimicrobes in the Zuodeng microbialite are spheroids, of which a total of five morphological types are recognized from both stromatolite and thrombolite: (1) sparry calcite spheroid without outer sheaths, (2) a large sparry calcite nucleus coated with a thin sparry calcite sheath, (3) a large nucleus of micrite framboid aggregates rimmed by a thin sparry calcite sheath (bacterial clump-like spheroids), (4) a large nucleus of micrite framboid aggregates coated with a thin micritic sheath, and (5) a small sparry nuclei rimmed by coarse-grained, radiated euhedral rays. The irregular contact beneath the Zuodeng microbialites is interpreted as a subaerial exposure surface due to regional regression in South China. The demise of the Zuodeng microbialites may have been due to rapid rise in sea-level because they grew in relatively shallow marine conditions and are overlain by muddy limestones containing pelagic conodonts. Also siliciclastic content increases above the microbialite, suggesting a possible climate-related increase in weathering as the transgression progressed.

Investigation of nucleation processes during dynamic recrystallization of ice using cryo-EBSD.

Nucleation mechanisms occurring during dynamic recrystallization play a crucial role in the evolution of microstructures and textures during high temperature deformation. In polycrystalline ice, the strong viscoplastic anisotropy induces high strain heterogeneities between grains which control the recrystallization mechanisms. Here, we study the nucleation mechanisms occurring during creep tests performed on polycrystalline columnar ice at high temperature and stress (T=-5°C;σ=0.5 MPa) by post-mortem analyses of deformation microstructures using cryogenic electron backscatter diffraction. The columnar geometry of the samples enables discrimination of the nuclei from the initial grains. Various nucleation mechanisms are deduced from the analysis of the nuclei relations with the dislocation sub-structures within grains and at grain boundaries. Tilt sub-grain boundaries and kink bands are the main structures responsible for development of polygonization and mosaic sub-structures. Nucleation by bulging at serrated grain boundaries is also an efficient nucleation mechanism near the grain boundaries where strain incompatibilities are high. Observation of nuclei with orientations not related to the 'parent' ones suggests the possibility of 'spontaneous' nucleation driven by the relaxation of the dislocation-related internal stress field. The complexity of the nucleation mechanisms observed here emphasizes the impact of stress and strain heterogeneities on dynamic recrystallization mechanisms.This article is part of the themed issue 'Microdynamics of ice'.

Using molecular vibrations to probe exciton delocalization in films of perylene diimides with ultrafast mid-IR spectroscopy.

Ultrafast vibrational spectroscopy in the mid-infrared was used to directly probe the delocalization of excitons in two different perylenediimide (PDI) derivatives that are predicted to preclude the formation of excimers, which can act as trap sites for excited state energy in organic semiconductors. We identified vibrational modes within the conjugated C-C stretch modes of PDI molecules whose frequencies reported the interactions of molecules within delocalized excitonic states. The vibrational linewidths of these modes, which we call intermolecular coordinate coupled (ICC) modes, provided a direct probe of the extent of exciton delocalization among the PDI molecules, which was confirmed using X-ray diffraction and electro-absorption spectroscopy. We show that a slip-stacked geometry among the PDI molecules in their crystals promotes delocalized charge-transfer (CT) excitons, while localized Frenkel excitons tend to form in crystals with helical, columnar stacking geometries. Because all molecules possess vibrational modes, the use of ultrafast mid-infrared spectroscopy to measure ICC vibrational modes offers a new approach to examine exciton delocalization in a variety of small molecule electron acceptors for optoelectronic and organic photovoltaic applications.

Seismic characteristics of fluid escape pipes in sedimentary basins: Implications for pipe genesis

Fluid escape pipes were first documented from 3D seismic data over a decade ago, and have subsequently been identified in many petroliferous basins worldwide. They are characterized on seismic data by vertical to sub-vertical zones of reduced reflection continuity that have a columnar geometry in three-dimensions. The upper terminations of these pipes commonly coincide with pockmarks or palaeo-pockmarks, signifying a close connection of pipe formation with a high flux fluid expulsion process. Dimensions range from tens to hundreds of metres in diameter, and hundreds to over a thousand metres in height, and the slenderness ratio, defined as height/diameter (Ω), ranges from 0.8 to over 20. Pipes are frequently associated with sub-vertical clustering of amplitude anomalies on seismic data, related either to the presence of free gas, or to cementation linked to the passage of hydrocarbons.Three mechanisms have been suggested to explain pipe genesis: (1) hydraulic fracturing, (2) erosional fluidisation, and (3) capillary invasion. We suggest a further two possible mechanisms in the form of localised collapse by volume loss and synsedimentary flow localisation. We review all five mechanisms and conclude that it is unlikely that a single mechanism applies but that combinations of these processes may all occur in particular contexts. Fluid escape pipes may be far more widespread that currently appreciated, and they may play a critical role in secondary hydrocarbon migration and in providing leakage pathways for trapped hydrocarbons through overlying seals.

Slanted Columnar Thin Films Prepared by Glancing Angle Deposition Functionalized with Polyacrylic Acid Polymer Brushes

Three-dimensional inorganic nanostructured thin films with slanted columnar morphologies are functionalized with organic polymer brushes to fabricate a nanohybrid functional material. Nanostructured thin films are fabricated by glancing angle deposition of silicon onto silicon or gold to produce slanted columnar thin films (SCTFs). Polymer brushes are regarded as very promising nanomaterials for surface coatings because these systems are capable of responding to external stimuli such as temperature or pH, generally by reversible swelling/deswelling behavior. The fabrication of the SCTF as well as the stepwise reactions of poly(acrylic acid) guiselin polymer brushes to the SCTF nanocolumns are characterized with generalized ellipsometry and scanning electron microscopy. This study demonstrates that SCTFs are capable of withstanding the polymer brush grafting-to process and that both ellipsometry and electron microscopy indicate polymer brush immobilization within the void spaces of the SCTF. Furthermore, in situ combinatorial ellipsometry and quartz crystal microbalance with dissipation both reveal that the brushes retain their ability to swell/deswell with changes in pH of buffer solution. These tunable nanohybrid functional materials with increased surface area, complex columnar geometries, and stimuli-responsive characteristics provide novel material surfaces for nanoelectronics, biotechnology, and a variety of other advanced material applications.

Validation of columnar CsI x‐ray detector responses obtained with hybridMANTIS, a CPU‐GPU Monte Carlo code for coupled x‐ray, electron, and optical transport

hybridMANTIS is a Monte Carlo package for modeling indirect x-ray imagers using columnar geometry based on a hybrid concept that maximizes the utilization of available CPU and graphics processing unit processors in a workstation. The authors compare hybridMANTIS x-ray response simulations to previously published MANTIS and experimental data for four cesium iodide scintillator screens. These screens have a variety of reflective and absorptive surfaces with different thicknesses. The authors analyze hybridMANTIS results in terms of modulation transfer function and calculate the root mean square difference and Swank factors from simulated and experimental results. The comparison suggests that hybridMANTIS better matches the experimental data as compared to MANTIS, especially at high spatial frequencies and for the thicker screens. hybridMANTIS simulations are much faster than MANTIS with speed-ups up to 5260. hybridMANTIS is a useful tool for improved description and optimization of image acquisition stages in medical imaging systems and for modeling the forward problem in iterative reconstruction algorithms.

Self-radiolysis of tritiated water. 1. A comparison of the effects of 60Co γ-rays and tritium β-particles on water and aqueous solutions at room temperature

Monte Carlo simulations were used to investigate the chemistry of pure water and aqueous solutions after irradiation with different kinds of radiation: tritium β-rays and high-energy electrons or 60Co γ-rays. The objective of this work was to elucidate the mechanisms involved in the self-radiolysis of tritiated water, and to examine the importance of the effects of higher “linear energy transfer” (LET) by comparing 3H β-electrons (mean initial energy of ∼5.7 keV) with 60Co γ-rays (∼1-MeV electrons). We considered several chemical systems for which experimental data were available. These included pure water, aqueous solutions of sulfuric acid, and aqueous ferrous sulfate solutions in aerated 0.4 M H2SO4 (Fricke dosimeter). Simulations clearly showed quantitatively different yields of radical and molecular products produced by the radiolysis of water with tritium β− particles compared with corresponding yields from γ or energetic electron radiolysis. As a rule, lower radical and higher molecular yields were observed for 3H β-rays. These differences in yields are completely consistent with differences in the nonhomogeneous distribution of primary transient species (i.e., the structure of electron tracks) in the two cases. In the “short-track” (columnar) geometry of tritium β-electron radiolysis, radicals were formed in much closer initial proximity than in the “spur” (spherical) geometry of γ radiolysis. The “short-track” geometry favors radical-radical reactions in the diffusing tracks, which increases the proportion of molecular products at the expense of the radical products. The same trend in yields of radical and molecular products was also found under acidic conditions as well as in the aerated Fricke dosimeter. Unfortunately, comparison with experimental data was rather limited due to the paucity of experimental information for the radiolysis of water by 3H β-particles. Despite this deficiency, our simulations reproduced very well the significant increase observed in the yield of H2 at the microsecond time scale for 3H β-electrons (∼0.6 molecule/100 eV) compared to 60Co γ-rays (0.45 molecule/100 eV). Furthermore, our predicted yield of Fe3+ ions for tritium β-electron radiolysis of Fricke (acidic ferrous sulfate) solutions compared well with the literature values (∼11.9–12.9 molecules/100 eV). In particular, it was shown that the measured yield of the Fricke dosimeter was best reproduced if a single, “mean” or “equivalent” electron energy of ∼7.8 keV was used to mimic the energy deposition by the tritium β-particles (rather than the commonly used mean of ∼5.7 keV that mimics the tritium beta energy spectrum), in full accordance with a recommendation of ICRU Report 17. This decrease in G(Fe3+) compared to the value observed for 60Co γ-rays (15.5 ± 0.2 molecules/100 eV) was mostly due to the decrease in the yield of the escape radical products. Such results, even if fragmentary, corroborate very well with previous experimental and theoretical work, and support a model of tritium β radiolysis mainly driven by the chemical action of short tracks of high local LET.

Toward Ultralow‐Bandgap Liquid Crystalline Semiconductors: Use of Triply Fused Metalloporphyrin Trimer–Pentamer as Extra‐large π‐Extended Mesogenic Motifs

In contrast with their dimeric homologue, triply fused zinc porphyrin trimer-pentamer, as extra-large π-extended mesogens, assemble into columnar liquid crystals (LCs) when combined with 3,4,5-tri(dodecyloxy)phenyl side groups (3 P(Zn) -5 P(Zn) , Figure 1). Their LC mesophases develop over a wide temperature range, namely, 41-280 °C (on heating) for 5 P(Zn) , and all adopt an oblique columnar geometry, typically seen in columnar LC materials involving strong mesogenic interactions. These LC materials are characterized by their wide light-absorption windows from the entire visible region up to a near infrared (NIR) region. Such ultralow-bandgap LC materials are chemically stable and serve as hole transporters, in which 5 P(Zn) gives the largest charge carrier mobility (2.4×10(-2) cm V(-1) s(-1) ) among the series. Despite a big dimensional difference, they coassemble without phase separation, in which the resultant LC materials display essentially no deterioration of the intrinsic conducting properties.

Inhomogeneous electron distribution in InN nanowires: Influence on the optical properties

AbstractIn this work, we study theoretically and experimentally the influence of the surface electron accumulation on the optical properties of InN nanowires. For this purpose, the photoluminescence and photoluminescence excitation spectra have been measured for a set of self‐assembled InN NWs grown under different conditions. The photoluminescence excitation experimental lineshapes have been reproduced by a self‐consistent calculation of the absorption in a cylindrical InN nanowires. With the self‐consistent model we can explore how the optical absorption depends on nanowires radius and doping concentration. Our model solves the Schrödinger equation for a cylindrical nanowire of infinite length, assuming a parabolic conduction band. The columnar geometry introduces effects in both the electron density and in the self‐consistent conduction band profile, with no equivalence in planar layer. On the other hand, the differences in the photoluminescence excitation spectra are related to the inhomogeneous electron distribution inside the nanowires, caused by a bulk donor concentration and a two‐dimensional density of ionized surface states. For nanowire radii larger than 30 nm, such concentrations modify the absorption edge and the lineshape, respectively, and can be determined from the comparison with the experimental data (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Carrier dynamics in coalescence overgrowth of GaN nanocolumns

Coalescence overgrowth of pattern-grown GaN nanocolumns on c-plane sapphire substrate with metal organic chemical vapor deposition is demonstrated. The subsequent coalescence overgrowth opens a possibility for dislocation reduction due to the lateral strain relaxation in columnar geometry. We present further growth optimization and innovative characterization of metal organic chemical vapor deposition layers, overgrown on the columnar structure with varying diameters of columns. Nano-imprint lithography was applied to open circular holes of 250, 300, 450, and 600 nm diameter on the SiO 2 layer, deposited on the GaN layer on the c-plane sapphire template. After the growth of ~ 1 μm high GaN nanocolumns, the further coalescence conditions led to an overgrown layer ~2 μm thickness. Photoelectrical and optical properties of the overgrown layers and a reference sample were investigated by time-resolved picosecond transient grating and time-integrated photoluminescence. We note a 3–4 fold increase in carrier lifetime in the overgrown epilayers when the diameter of columns increased from 250 to 450 nm. This feature is a clear indication of an ~4-fold reduced defect density.