Preferred In-plane Orientation Research Articles

Effect of a ZnO buffer layer on the properties of epitaxial ZnO:Ga films deposited on c-sapphire substrate

Bi-layer ZnO films with 2wt.% Al (AZO; ZnO:Al) and 4wt.% Ga-doped (GZO; ZnO:Ga) were deposited on the non-buffered and buffered c(0001)-sapphire(Al2O3) substrates respectively by Pulsed Laser Deposition (PLD). The effect of a ZnO buffer layer on the crystallinity and electrical properties of the GZO thin films was investigated. X-ray Diffraction (XRD) peaks and High Resolution Transmission Electron Microscopy (HRTEM) studies showed that the GZO thin film on a buffered substrate was epitaxially grown with an orientation relationship of (0001) [112¯0]GZO||(0001)[112¯0]Al2O3. However, GZO thin film on a non-buffered substrate was grown as a monocrystalline hexagonal wurtzite phase with c-axis preferred, out-of-plane orientation, and random in-plane orientation. The electrical resistivity of the GZO thin films was improved by introducing a ZnO buffer layer from 2.2×10-4Ωcm to 1.2×10-4Ωcm, respectively. In a word, it was found in the films that more preferred c-axis orientation texture and reduction of the defects such as stacking faults and dislocations, with introducing a ZnO buffer layer.It was seen that the ZnO buffer layer had a great influence on the orientation and defect density of GZO thin films from X-ray Diffraction (XRD) peaks and High Resolution Transmission Electron Microscopy (HRTEM) images.

Orientation of chromonic liquid crystals by topographic linear channels: multi-stable alignment and tactoid structure

We study the alignment behaviour of the chromonic liquid crystal phases of sunset yellow (SSY)/water, disodium cromoglycate (DSCG)/water and their mixtures when confined in cells by polymer films topographically imprinted with linear channels of 250 nm width, depth and spacing. A variety of novel alignment effects are generated by contact with such a patterned surface, as follows. Nematic DSCG and nematic SSY at low concentration and their nematic mixtures orient with the long axes of stacked chromonic aggregates on average parallel to the channels, that is, with the molecular planes normal to the channel axis. These oriented nematics exhibit isotropic/nematic tactoids aligned with their major axes along the channels. Two geometries of the tactoids, elliptic cylinder and rectangular cuboid with hemi cylindrical ends are observed. Additionally, a transition of DSCG tactoids from a three-dimensional (3D) director configuration to a two-dimensional one is observed when the tactoids on one surface of a cell touch the other surface of the cell. In SSY solutions of sufficiently high concentration, multi-stable alignment is found, including preferential in-plane orientation of the director parallel to, perpendicular to and 45° rotated from the channels.

Bulk Heterojunction Nanomorphology of Fluorenyl Hexa-peri-hexabenzocoronene–Fullerene Blend Films

In this study, the nanomorphology of fluorenyl hexa-peri-hexabenzocoronene:[6,6]-phenyl C61-butyric acid methyl ester (FHBC:PC61BM) absorber layers of organic solar cells was investigated. Different electron microscopical techniques, atomic force microscopy, and grazing incidence wide-angle X-ray scattering were applied for a comprehensive nanomorphology analysis. The development of the nanomorphology upon sample annealing and the associated change of the device performance were investigated. It was shown that the annealing process enhances the phase separation and therefore the bulk heterojunction structure. Due to π-π stacking, the FHBC molecules assemble into columnar stacks, which are already present before annealing. While the nonannealed sample consists of a mixture of homogeneously distributed PC61BM molecules and FHBC stacks with a preferential in-plane stack orientation, crystalline FHBC precipitates occur in the annealed samples. These crystals, which consist of hexagonal arranged FHBC stacks, grow with increased annealing time. They are distributed homogeneously over the whole volume of the absorber layer as revealed by electron tomography. The FHBC stacks, whether in the two phase mixture or in the pure crystalline precipitates, exhibit an edge-on orientation, according to results from grazing incidence wide-angle X-ray scattering (GIWAXS), dark-field transmission electron microscopy (DF TEM) imaging and selective area electron diffraction (SAED). The best solar cell efficiencies were obtained after 20 or 40 s sample annealing. These annealing times induce an optimized degree of phase separation between donor and acceptor material.

Crystallization of CO 2 ice and the absence of amorphous CO 2 ice in space

Carbon dioxide (CO2) is one of the most relevant and abundant species in astrophysical and atmospheric media. In particular, CO2 ice is present in several solar system bodies, as well as in interstellar and circumstellar ice mantles. The amount of CO2 in ice mantles and the presence of pure CO2 ice are significant indicators of the temperature history of dust in protostars. It is therefore important to know if CO2 is mixed with other molecules in the ice matrix or segregated and whether it is present in an amorphous or crystalline form. We apply a multidisciplinary approach involving IR spectroscopy in the laboratory, theoretical modeling of solid structures, and comparison with astronomical observations. We generate an unprecedented highly amorphous CO2 ice and study its crystallization both by thermal annealing and by slow accumulation of monolayers from the gas phase under an ultrahigh vacuum. Structural changes are followed by IR spectroscopy. We also devise theoretical models to reproduce different CO2 ice structures. We detect a preferential in-plane orientation of some vibrational modes of crystalline CO2. We identify the IR features of amorphous CO2 ice, and, in particular, we provide a theoretical explanation for a band at 2,328 cm(-1) that dominates the spectrum of the amorphous phase and disappears when the crystallization is complete. Our results allow us to rule out the presence of pure and amorphous CO2 ice in space based on the observations available so far, supporting our current view of the evolution of CO2 ice.

Gas permeability of poly(4-methylpentene-1) in a confined nanolayered film system

In this study, multilayer films of sPS–PPS (syndiotactic Polystyrene and Polyphenylene sulfide blend) against P4MP1 [Poly(4-Methylpentene-1)] with various compositions and layer thicknesses were prepared by a layer multiplying coextrusion process. Incorporating the PPS into sPS layers before coextrusion produced pores in the sPS layers upon a post-extrusion, uniaxial orientation. Confined crystallization of P4MP1 by sPS–PPS was examined before and after the uniaxial orientation. A melt and recrystallization method was employed to achieve the preferred in-plane orientation of P4MP1 crystals. This was characterized by the atomic force microscopy and X-ray spectroscopy. We found that with P4MP1 layer thickness smaller than 400 nm, a confinement effect will induce in-plane P4MP1 crystal orientation where the crystal c-axis is perpendicular to the film. This arrangement of crystals reduced oxygen's tortuosity through the multilayer films. This conclusion was verified by increased oxygen permeability of the stretched–recrystallized multilayer films.

Facile control of long range orientation in mesoporous carbon films with thermal zone annealing velocity

Ordered mesoporous carbons exhibit appealing properties for many applications, but their function and performance can depend critically on their structure. The in-plane orientation of 2D cylinders from the cooperative assembly of Pluronic P123 and resol has been controlled by application of cold zone annealing (CZA). By varying the moving rate, the preferential in-plane orientation of the self-assembled cylinders can be tuned through the entire 180° range possible from ϕ = 50° to ϕ = -130° (relative to the moving direction). At a moving rate of 2 μm s(-1), this simple and easy CZA process leads to cylinders that are well aligned parallel to the moving direction with a high orientational factor of S = 0.98. Moreover, the in-plane oriented cylinders can be nearly perfectly aligned transverse to the moving direction (S = 0.95) by simply decreasing the moving velocity to 0.5 μm s(-1). We attribute the parallel alignment to the flow that develops from the motion of the thermal gradients, while the transverse alignment is related to flow cessation (inertial effect). The preferential orientation is retained through the carbonization process, but there is some degradation in orientation due to insufficient crosslinking of the resol during CZA; this effect is most prominent for the higher moving rates (less time for crosslinking), but can be overcome by post-CZA annealing at uniform elevated temperatures to further crosslink the resol. CZA is a simple and powerful method for fabricating well-aligned and self-assembled mesoporous carbon films over large areas.

High-quality GaN films grown on chemical vapor-deposited graphene films

We demonstrate the growth of high-quality GaN films with flat surface and uniform morphology on large-scale polycrystalline chemical vapor-deposited graphene films. The films exhibit stimulated emission even at room temperature, a highly c-axis-oriented crystal structure, and a preferred in-plane orientation. Furthermore, the GaN films grown on the graphene films can be used for fabrication of blue and green light-emitting diodes.

Epitaxy of Orthorhombic BaSi2 with Preferential In-Plane Crystal Orientation on Si(001): Effects of Vicinal Substrate and Annealing Temperature

We attempted to grow orthorhombic BaSi2 epitaxial films having preferential in-plane crystallographic orientation on both exact and vicinal Si(001) substrates with a miscut angle of 2° toward Si[1̄10] by reactive deposition epitaxy (RDE) and subsequent molecular beam epitaxy (MBE). On the vicinal Si(001) substrate, the initial BaSi2 nuclei formed by RDE tended to grow unidirectionally with the [010] direction parallel to Si[110] when the annealing temperature of the Si substrate before the growth was increased from 830 to 1000 °C. Transmission electron microscopy showed that the grain size of the BaSi2 films grown by MBE increased up to approximately 9 µm on the vicinal Si(001) substrate when the substrate annealing temperature was 1000 °C. This is the largest grain size ever obtained for BaSi2. Even in the case of the exact Si(001) substrate, the MBE-grown BaSi2 grains preferentially grew with the [010] direction along Si[110] when the annealing temperature was 1000 °C.

Advanced optical effective medium modeling for a single layer of polydisperse ellipsoidal nanoparticles embedded in a homogeneous dielectric medium: Surface plasmon resonances

We calculate, in the quasistatic coupled dipole approximation, the analytical expressions of the effective dielectric tensor of a single layer of polydisperse ellipsoidal nanoparticles with two of their principal axes in the layer's plane and embedded in a homogeneous dielectric medium. The organization (isotropic or anisotropic) and orientation (without or with a preferential in-plane orientation) of the nanoparticles is taken into account, together with their (possibly correlated) in-plane size, in-plane projected shape, and height distributions. In particular, we propose to describe the response of a layer of nanoparticles presenting a height distribution by using a vertically graded effective medium model. The expressions are tested in the case of finely characterized dielectric/silver/dielectric granular trilayers grown by means of vapor deposition in which the silver coalesced nanoparticles present correlated in-plane size and in-plane projected shape/height distributions and a moderate surface coverage of about 25$%$. A satisfactory quantitative agreement is obtained between the simulated and measured surface plasmon extinction bands of the metal nanoparticles. This agreement is permitted by the capability of the effective medium model of taking into account the ellipsoidal shape of the nanoparticles. The significant role of the size and shape distributions is also demonstrated.

Biaxial texture development in aluminum nitride layers during off-axis sputter deposition

Polycrystalline aluminum nitride (AlN) layers were deposited by pulsed-dc reactive magnetron sputtering from a variable deposition angle α = 0°–84° in 5 mTorr pure N2 at room temperature. X-ray diffraction pole figure analyses show that layers deposited from a normal angle (α = 0°) exhibit fiber texture, with a random in-plane grain orientation and the c-axis tilted by 42° ± 2° off the substrate normal, yielding wurtzite AlN grains with the {101¯2} plane approximately parallel (±2°) to the substrate surface. However, as α is increased to 45°, two preferred in-plane grain orientations emerge, with populations I and II having the c-axis tilted toward and away from the deposition flux, by 53° ± 2° and 47° ± 1° off the substrate normal, respectively. Increasing α further to 65° and 84°, results in the development of a single population II with a 43° ± 1° tilt. This developing biaxial texture is attributed to a competitive growth mode under conditions where the adatom mobility is sufficient to cause intergrain mass transport, but insufficient for the thermodynamically favored low energy {0001} planes to align parallel to the layer surface. Consequently, AlN nuclei are initially randomly oriented and form a kinetically determined crystal habit exposing {0001} and {112¯0} facets. The expected direction of its highest growth rate is 49° ± 5° tilted relative to the c-axis, in good agreement with the 42°–53° measured tilt. The in-plane preferred orientation for α > 0° is well explained by the orientation dependence in the cross section of the asymmetric pyramidal nuclei to capture directional deposition flux. The observed tilt is ideal for shear mode electromechanical coupling, which is maximized at 48°.

Epitaxial relationships of ZnO nanostructures grown by Au-assisted pulsed laser deposition on c- and a-plane sapphire

We report on the epitaxial growth of ZnO nanosheets and nanowires on a- and c-plane sapphire substrates by Au-assisted pulsed laser deposition. The epitaxial relationship of the nanostructures was determined by x-ray diffraction (XRD) pole figure measurements. On c-plane sapphire, the ZnO nanowires grew along the ZnO c-axis and were inclined to the substrate surface normal with an angle of about 37°. The ZnO(0001) plane of the wires aligned with Al2O3(101¯4) of the sapphire substrate via two degenerate in-plane configurations, promoted by low lattice mismatch (0.05%). ZnO nanosheets grown on c-plane sapphire exhibited no preferential orientation on the substrate and no epitaxial relationship could be unambiguously identified. On a-plane sapphire, ZnO nanowires grew vertically along the ZnO c-axis with a single epitaxial configuration, whereas ZnO nanosheets seemed to grow along ZnO[101¯0] in two preferred in-plane orientations, 72°–74° apart. These configurations could be explained by two distinct alignments of the ZnO(101¯1) plane on the a-plane sapphire substrate surface, promoted by low lattice mismatches.

Electrical conductivity of compacts of graphene, multi-wall carbon nanotubes, carbon black, and graphite powder

The electrical conductivity of different carbon materials (multi-walled carbon nanotubes, graphene, carbon black and graphite), widely used as fillers in polymeric matrices, was studied using compacts produced by a paper preparation process and by powder compression. Powder pressing assays show that the bulk conductivity depends not only on the intrinsic material properties but is also strongly affected by the number of particle contacts and the packing density. Conductivities at high pressure (5MPa) for the graphene, nanotube and carbon black show lower values (~102S/m) as compared to graphite (~103S/m). For nanotube, graphene and graphite particles, the conductive behavior during compaction is governed by mechanical particle arrangement/deformation mechanisms while for carbon black this behavior is mainly governed by the increasing particle contact area. The materials resulting from the paper preparation process for carbon black and graphite showed similar conductivity values as for the compacts, indicating a limited effect of the surfactant on the conductivity. The paper preparation process for the large surface area nanotube and graphene particles induces a highly preferred in-plane orientation, thereby yielding largely the single particle intrinsic conductivity for the in-plane direction, with values in the order of 103S/m.

The Kinetics and Mechanism of Long-Range Pore Ordering in Anodic Films on Aluminum

Anodic aluminum oxide has unique and highly attractive properties, including self-ordering of porous structure during anodization. Although anodization regimes leading to formation of highly ordered porous structures had been found experimentally, many aspects of the self-organization mechanism remain unsolved. Here, the detailed in situ small-angle X-ray diffraction study of the self-ordering in porous alumina films is reported. Structure evolution kinetics was deduced by a quantitative analysis of diffraction patterns combined with electron microscopy. The rate of pore ordering is shown to have maximal value at the initial anodization stage and rapidly decreases inversely proportional to t0.2. Self-organization is shown to occur via growth of domains possessing preferential in-plane orientation and “death” of other domains, similar to Ostwald ripening governed by difference in pore growth rates for domains of different orientations. The process is accompanied by pore death and splitting making a significant impact on anodic oxides utilization in any mass-transport issues. This finding opens a novel approach for growth of highly ordered porous anodic oxide films.

Solidlike and liquidlike behavior in monolayers and multilayers of metal-bearing amphiphiles

Atomic force microscopy (AFM) of cadmium stearate (CdSt) and cobalt stearate (CoSt) Langmuir-Blodgett films show differences in their in-plane morphologies. CdSt films, with a huge number of in-plane "pinhole" defects, follow self-affine behavior, whereas CoSt films, which are almost void of such in-plane defects, show deviation from self-affinity especially at small length scales, suggesting liquidlike behavior, imparting flexibility to the system, in plane. Phase images of CoSt obtained from tapping mode AFM show gentle undulations or hemispherelike features in contrast to its smooth topography, unlike the CdSt system where both height and phase images show self-affine domains. Near edge x-ray absorption fine structure spectroscopy indicates no preferred in-plane orientation of the head group in CoSt films. The undulating features in CoSt is explained by invoking a radially symmetric orientational distribution in the tilt of adjacent hydrocarbon tails, causing a small in-plane density variation which shows up in the phase image. These orientational disorders in adjacent tails probably allow "filling up" of in-plane defects thereby giving rise to its excellent in-plane coverage and hence a "liquidlike" behavior in CoSt. Brewster angle microscopy shows that parent Langmuir monolayers of stearic acid in the presence of Cd and Co ions in the aqueous subphase behave as two-dimensional "solids" and "liquids," respectively, suggesting the phenomena to be inherent in the amphiphiles and probably independent of their organization as monolayers and multilayers.

Electric field inside a “Rossky cavity” in uniformly polarized water

Electric field produced inside a solute by a uniformly polarized liquid is strongly affected by dipolar polarization of the liquid at the interface. We show, by numerical simulations, that the electric "cavity" field inside a hydrated non-polar solute does not follow the predictions of standard Maxwell's electrostatics of dielectrics. Instead, the field inside the solute tends, with increasing solute size, to the limit predicted by the Lorentz virtual cavity. The standard paradigm fails because of its reliance on the surface charge density at the dielectric interface determined by the boundary conditions of the Maxwell dielectric. The interface of a polar liquid instead carries a preferential in-plane orientation of the surface dipoles thus producing virtually no surface charge. The resulting boundary conditions for electrostatic problems differ from the traditional recipes, affecting the microscopic and macroscopic fields based on them. We show that relatively small differences in cavity fields propagate into significant differences in the dielectric constant of an ideal mixture. The slope of the dielectric increment of the mixture versus the solute concentration depends strongly on which polarization scenario at the interface is realized. A much steeper slope found in the case of Lorentz interfacial polarization also implies a higher free energy penalty for polarizing such mixtures.

Broad-band electrical conductivity of carbon nanofibre-reinforced polypropylene foams

The influence of foaming a semi-crystalline polymer reinforced with different concentrations of carbon nanofibres (0–20 wt.%) on the formation of an electrically conductive network was studied at room temperature using an impedance analyzer over a wide interval of frequencies (from 10 −2 to 10 6 Hz). Composites were prepared by melt-compounding using a twin-screw extruder, and later chemically foamed. Although composite materials displayed lower conductivities than expected, assuming a percolative behavior, foaming promoted a tunnel-like conduction at lower CNF concentrations than in the solids. At higher CNF concentrations, no great improvements were achieved as tunneling conduction decreased with increasing local crystallinity. Foams showed electrical conduction characteristics typical of a conductive random-distributed fibre-like system, while the behavior of the solids was closer to a system of spherical particles, related to CNF aggregation. The anisotropic cellular structure of the 20 wt.% CNF composite foamed by a physical foaming process disrupted the preferential in-plane CNF orientation attained during solid preparation, with these foams showing higher through-plane conductivity and more isotropic electrical properties than the chemically-foamed ones. It has been demonstrated that foaming PP–CNF composites resulted in the formation of a conductive network at lower CNF concentrations than in the solids, with foams showing the potential for use in conductive high-performance lightweight composite systems.

Effects of deposition temperature and elemental layer thickness on the properties of Fe/Co multilayers grown by molecular beam epitaxy

Fe/Co multilayers were grown at different temperatures up to 200 °C onto MgO-(1 0 0) monocrystalline substrates, varying both the Co and Fe elemental layer thicknesses. All samples show a multilayered structure where the elemental layers are separated by a thin interfacial region which, for high-temperature growths, is constituted by an equiatomic FeCo compound. The multilayers grown at room temperature show an in-plane uniaxial magnetic anisotropy. The increase in the growing temperature determines an improvement of the sample smoothness and the appearance of two preferred in-plane orientations of the easy magnetization axis having different strengths. By increasing the Co layer thickness, the in-plane magnetic behaviour becomes isotropic, while by increasing the Fe layer thickness, an out-of-plane contribution to the magnetization vector is established, which is responsible for the appearance of a well-defined magnetic morphology constituted by stripe-like domains.

Effect of a ZnO buffer layer on the properties of Ga-doped ZnO thin films grown on Al 2O 3 (0 0 0 1) substrates at a low growth temperature of 250 °C

4 wt% Ga-doped ZnO (GZO) thin films have been prepared on the buffered and non-buffered Al 2O 3 (0 0 0 1) substrates by RF magnetron sputtering technique at a low growth temperature (250 °C). The effect of a ZnO buffer layer on the crystallinity, optical, and electrical properties of the GZO thin films is investigated. X-ray diffraction and transmission electron microscopy studies showed that the GZO thin film on a buffered substrate was epitaxially grown at a low growth temperature of 250 °C with an orientation relationship of ( 0001 ) [ 11 2 ¯ 0 ] GZO | | ( 0001 ) [ 11 2 ¯ 0 ] Al 2 O 3 . However, the GZO thin film on a non-buffered substrate was grown as a polycrystalline hexagonal wurtzite phase with c-axis preferred, out-of-plane orientation, and random in-plane orientation. The optical transmittance in the visible region and the electrical resistivity of the GZO thin film was improved by introducing a ZnO buffer layer from 70% to 80% and from 4.69×10 −3 to 1.01×10 −3 Ω cm, respectively.

Effects of thermal annealing on the photophysical properties of bisfluorene-cored dendrimer films

Annealing is widely used in the processing of organic semiconductors, and can modify their film morphology and photophysical properties. A study of the effect of annealing on films made from a blue emitting bisfluorene-cored dendrimer is reported. Annealing causes a 15 nm blue-shift in the photoluminescence (PL) spectrum and an 11 nm blue-shift in the amplified spontaneous emission (ASE) spectrum. It causes the PL efficiency to decrease only slightly from 0.92 to 0.83. The radiative decay rate of 1.3 × 10 9 s −1, the ASE threshold of 1.5 × 10 18 cm −3 and the singlet–singlet exciton annihilation rate of 5.5 × 10 −10 cm 3 s −1 are unaffected by annealing. The results indicate a scope for colour adjustment of dendrimer light-emitting diodes and lasers without affecting their efficiencies. Investigation by spectroscopic ellipsometry shows that on annealing, the films become anisotropic, with larger values of the refractive index and extinction coefficient observed for light polarised in the plane of the film than the corresponding out-of-plane values in the absorption region of the bisfluorene core. This anisotropy indicates a preferential in-plane orientation of bisfluorene cores upon annealing.

Effect of film thickness on the structural and electrical properties of Ga-doped ZnO thin films prepared on glass and Al2O3 (0001) substrates by RF magnetron sputtering method

Thin films of Ga-doped ZnO (GZO) were prepared on glass and Al2O3 (0001) substrates by using RF magnetron sputtering at a substrate temperature of 350 °C, RF power of 175 W, and working pressure of 6 mTorr. The effect of film thickness and substrate type on the structural and electrical properties of the thin films was investigated. X-ray diffraction study showed that GZO thin films on glass substrates were grown as a polycrystalline hexagonal wurtzite phase with a c-axis preferred, out-of-plane orientation and random in-plane orientation. However, GZO thin films on Al2O3 (0001) substrates were epitaxially grown with an orientation relationship of . The structural images from scanning electron microscopy and atomic force microscopy showed that the GZO thin films on glass substrates had a rougher surface morphology than those on Al2O3 (0001) substrates. The electrical resistivity of 1000 nm-thick GZO thin films grown on glass and Al2O3 (0001) substrates was 3.04 × 10−4 Ωcm and 1.50 × 10−4 Ωcm, respectively. It was also found that the electrical resistivity difference between the films on the two substrates decreased from 9.48 × 10−4 Ωcm to 1.45 × 10−4 Ωcm with increasing the film thickness from 100 nm to 1000 nm.