In-plane Alignment Research Articles

Optical anisotropy in solution-cast film of cellulose triacetate

The out-of-plane birefringence and its wavelength dispersion are studied employing solution-cast films of cellulose triacetate (CTA). In solution-cast process, CTA molecules are induced to align in the film plane. Although refractive index is the lowest in the oriented direction for the CTA films stretched more than 110 %, refractive index is found to be the lowest in the normal direction for the unstretched cast film. Attenuated total reflection measurements reveal that in-plane alignment of the acetyl group which provides strong polarizability anisotropy is responsible for the phenomenon. Furthermore, the out-of-plane birefringence is found to increase with increasing wavelength, i.e. extraordinary wavelength dispersion, whereas a stretched CTA film shows ordinary wavelength dispersion. The level of the out-of-plane birefringence in cast films depends on the preparation conditions, which is predictable considering the evaporation rate. Moreover, it is demonstrated for the first time that the out-of-plane birefringence and its wavelength dispersion can be modified by addition of a certain plasticizer such as tricresyl phosphate (TCP). During the evaporation, TCP molecules orient in the film plane accompanying the orientation of CTA chains by intermolecular orientation correlation, called nematic interaction. This technique will widen the scope of material design of retardation films because there are numerous liquid compounds having strong polarizability anisotropy.

Solution phase van der Waals epitaxy of ZnO wire arrays

As an incommensurate epitaxy, van der Waals epitaxy allows defect-free crystals to grow on substrates even with a large lattice mismatch. Furthermore, van der Waals epitaxy is proposed as a universal platform where heteroepitaxy can be achieved irrespective of the nature of the overlayer material and the method of crystallization. Here we demonstrate van der Waals epitaxy in solution phase synthesis for seedless and catalyst-free growth of ZnO wire arrays on phlogopite mica at low temperature. A unique incommensurate interface is observed even with the incomplete initial wetting of ZnO onto the substrate. Interestingly, the imperfect contacting layer does not affect the crystalline and optical properties of other parts of the wires. In addition, we present patterned growth of a well-ordered array with hexagonal facets and in-plane alignment. We expect our seedless and catalyst-free solution phase van der Waals epitaxy synthesis to be widely applicable in other materials and structures.

Biaxial alignment of block copolymer-complex lamellae

Microphase-separated lamellar block copolymers can be oriented perpendicular to a substrate. The application of this attractive configuration as a template for nanostructure fabrication requires additionally long-range in-plane alignment of the lamella, for which no simple procedures are available. Here we present a convenient solution to this problem by exploiting the combination of supramolecular liquid-crystalline ordering of the block copolymer in combination with PTFE rubbing technology. The mesogenic ligands incorporated in one of the blocks interact with the substrate and control the self-assembly, rendering a vertical orientation of the microdomains. In addition the vertical lamellae orient parallel to the friction-deposited PTFE layer leading to long-range lateral ordering, providing unique new possibilities for block copolymer nanotechnology.

Lattice Matching in the Epitaxial Formation of Mesostructured Silica Films

Crystallographic orientation of mesostructured silica films on a substrate drastically changes when the substrate is modified with an anisotropic surface. The [01] axis of a two-dimensional (2D) hexagonal structure of the film prepared on a polyimide surface using C(22)EO(20) as a structure-directing agent changes from perpendicular to parallel with respect to the substrate after a rubbing treatment of polyimide, which is accompanied by the simultaneous unidirectional alignment of the cylindrical pores in the plane of the film. The normal direction of the film is [21¯], which has never been observed in the mesostructured silica films reported so far including those with controlled in-plane alignment of the mesochannels. The change of the orientation with respect to the substrate can be explained by the increased lateral distance between the adjacent surface micelles, which is caused by the elongation of the alkyl chains of the surfactant molecules induced by the adsorption onto the polymer surface with a molecular-level anisotropy. These results show that the total structural orientation of the mesostructured silica film is determined by the matching of the intrinsic lattice constant of the mesostructured silica with that of the surface micelle structure on a substrate.

Epitaxial-Strain Effects on Electronic Structure and Magnetic Properties of Hexagonal YMnO3 Thin Films Studied by Femtosecond Spectroscopy

In this paper, (001)-oriented hexagonal YMnO3 (h-YMO) thin films with various nominal strain states were deposited on MgO(100), MgO(111), and YSZ(111) substrates by pulsed laser deposition. The in-plane orientation alignment and substrate-induced epitaxial strain between the h-YMO films and substrates are examined by X-ray diffraction (XRD) θ–2θ, Φ, and ω scans. The effects of epitaxial strain on the on-site Mn d–d transition energy E dd (T), and the Néel temperature (T N) in these thin films are investigated by wavelength-tunable ultrafast pump-probe techniques. We find that the changes of E dd (T) and T N depend on the type and magnitude of the epitaxial strain. The possible mechanisms for the observed strain effect on the electronic structure and associated magnetism are discussed.

YBCO-Buffered NdBCO Film with Higher Thermal Stability in Seeding REBCO Growth and Recycling Failed Bulk YBCO Superconductors

In this work, we report a strengthened super- heating effect caused by a buffering YBa2Cu3Oy (YBCO or Y123) layer in the Nd1+xBa2−xCu3O7−y (NdBCO or Nd123) thin film with MgO substrate (i.e., NdBCO/YBCO/ MgO thin film). In the cold-seeding melt-textured (MT) growth, the NdBCO/YBCO/MgO film presented an even higher superheating level, about 20 °C higher than that of nonbuffered NdBCO film (i.e., NdBCO/MgO film). Pole figure (X-ray Φ-scan) measurement reveals that the Nd- BCO/YBCO/MgO film has better in-plane alignment than the NdBCO/MgO film, which may be the main origin of the enhanced thermal stability. Using this NdBCO/YBCO/MgO film as the seed and under a maximum processing temper- ature (Tmax) up to 1120 °C, we succeeded in growing vari- ous RE1+xBa2−xCu3O7−y (REBCO, RE = Gd, Sm, Nd, RE refers to rare elements) bulk superconductors and recycling the failed REBCO (RE = Y) bulks in a simple and feasible process.

Flux Engineering for Indium Tin Oxide Nanotree Crystal Alignment and Height-Dependent Branch Orientation

Single-crystal indium tin oxide (ITO) nanotrees with engineered trunk and branch orientations are grown using a recently reported technique combining vapor–liquid–solid growth and glancing angle deposition (VLS-GLAD). In this work, three unique capabilities of VLS-GLAD are demonstrated for the first time: (i) nanotrees are aligned without epitaxy, (ii) branches can be placed on select faces of the nanotree trunk, and (iii) branch orientation can be modified along the height of nanotrees. VLS-GLAD uses a collimated obliquely incident vapor flux to place material on desired growth surfaces, resulting in preferential branch growth on the sides of the nanotree trunk exposed to the flux at the time of nucleation. Dynamic control of the azimuthal orientation of the flux relative to a growing nanotree enables the preferential orientation of branches to be modulated along the height of the nanotree, which we have demonstrated with both continuous and discrete substrate rotation schemes. An electron diffraction investigation confirms that the nanotrees can be considered as a single crystal, with continuity of the crystal structure across the trunk–branch interface. Crystal texture of the films is characterized by X-ray diffraction pole figures. By limiting the flux to discrete positions, the films develop both out-of-plane and in-plane crystal alignment (biaxial texture). We interpret the in-plane orientation as the result of competitive growth, which leads to evolutionary selection of similarly oriented structures. Control over in-plane nanotree crystal alignment and height-dependent branch orientation should increase the achievable complexity in three-dimensional nanowire architectures.

Dielectric and magnetic properties of NiCuZn ferrite coated Sendust flakes through a sol–gel approach

Sendust flakes (with 9.59% Si, 5.88% Al and balanced Fe) were coated with NiCuZn ferrite nanoparticles by using a simple sol–gel approach. Both the coated and uncoated flakes were characterized by using scanning electron microscopy, X-ray diffraction and vibrating sample magnetometer. Composites made from the Sendust flakes with the in-plane alignment were fabricated via a vacuum infiltration method. Microwave measurement results indicated that permittivity of the composite made of the coated Sendust flakes was remarkably reduced, with a slight decrease in permeability over 0.1–8.55GHz. Consequently, the sample with a thickness of 6mm showed reflection loss exceeding −20dB at 1.4GHz, while reflection loss of the composite derived from the uncoated Sendust flakes could not reach −10dB at the same sample thickness. Therefore, the Sendust flakes coated with NiCuZn ferrite nanoparticles could be promising candidates to develop microwave attenuation materials.

Pulsed laser deposition of BaFe2(As,P)2 superconducting thin films with high critical current density

Superconducting BaFe2(As0.6P0.4)2 (Ba122:P) thin films were fabricated on MgO(100) substrates by a pulsed laser deposition (PLD) method using a second-harmonic Nd:YAG (yttrium aluminum garnet) laser. Structural investigation by means of x-ray diffraction confirmed both c-axis orientation and in-plane alignment, or epitaxial growth on the substrate. The film exhibited Tc(onset) = 26.5 K and Tc(zero) = 24.0 K. High Jc values of 3.5 MA cm−2 at 4.2 K in the self-field and over 1 MA cm−2 at 10 K under 1 T were also obtained. Fabrication of Ba122:P film by Nd:YAG PLD seems to be a promising approach for preparing superconducting tapes, since a high-Tc and high-Jc film can be relatively easily obtained.

PB38 導電性高分子活性層を用いた面内螺旋配向コレステリック液晶レーザの発振特性(フォトニクス・光デバイス,ポスター発表,2012年日本液晶学会討論会)

PB38 導電性高分子活性層を用いた面内螺旋配向コレステリック液晶レーザの発振特性(フォトニクス・光デバイス,ポスター発表,2012年日本液晶学会討論会)

Single Molecule Tracking Studies of Flow-Aligned Mesoporous Silica Monoliths: Aging-Time Dependence of Pore Order

Single molecule tracking (SMT) methods are employed to characterize the in-plane alignment and order of cylindrical mesopores in flow-aligned surfactant-templated silica monoliths prepared within glass microfluidic channels. The majority of dye molecules observed in wide-field fluorescence videos of these samples exhibit one-dimensional (1D) diffusive motions. Orthogonal regression analysis of these motions provides a measure of the mesopore orientation distribution function, which in turn is used to quantify the mesopore order via a two-dimensional orientational order parameter, <P>. Mesopore organization is explored as a function of aging time between sol preparation and filling of the microfluidic channels. Channels filled well before gelation of the sol are shown to incorporate large monodomains having average pore alignment within a few degrees of the flow direction. These monodomains extend over several millimeters and yield aging-time-independent <P> values larger than ~0.80. In contrast, channels filled near the time of sol gelation yield monoliths with misaligned pores that are also more disordered, having <P> ≈ 0.35. The SMT results are compared to those from small-angle X-ray scattering anisotropy experiments; these data are consistent across the range of samples investigated. A model describing the aging-time dependence of sol organization is presented. These studies demonstrate that well-aligned mesoporous silica monoliths can be obtained by simple flow alignment procedures but that short sol aging times are required in order to achieve optimum pore organization.

SIMPLE: Software for ab initio reconstruction of heterogeneous single-particles

The open source software suite SIMPLE: Single-particle IMage Processing Linux Engine provides data analysis methods for single-particle cryo-electron microscopy (cryo-EM). SIMPLE addresses the problem of obtaining 3D reconstructions from 2D projections only, without using an input reference volume for approximating orientations. The SIMPLE reconstruction algorithm is tailored to asymmetrical and structurally heterogeneous single-particles. Its basis is global optimization with the use of Fourier common lines. The advance that enables ab initio reconstruction and heterogeneity analysis is the separation of the tasks of in-plane alignment and projection direction determination via bijective orientation search - a new concept in common lines-based strategies. Bijective orientation search divides the configuration space into two groups of paired parameters that are optimized separately. The first group consists of the rotations and shifts in the plane of the projection; the second group consists of the projection directions and state assignments. In SIMPLE, ab initio reconstruction is feasible because the 3D in-plane alignment is approximated using reference-free 2D rotational alignment. The subsequent common lines-based search hence searches projection directions and states only. Thousands of class averages are analyzed simultaneously in a matter of hours. Novice SIMPLE users get a head start via the well documented front-end. The structured, object-oriented back-end invites advanced users to develop new alignment and reconstruction algorithms. An overview of the package is presented together with benchmarks on simulated data. Executable binaries, source code, and documentation are available at http://simple.stanford.edu.

Three-dimensional graphoepitaxial alignment resulting from solid-state dewetting of Au films on surfaces with monoperiodic topography

Polycrystalline gold films were deposited and annealed on oxidized silicon with flat surfaces and surfaces modified to have saw-tooth surface topography. Annealing led to solid-sate dewetting to form discrete islands. As-deposited films on flat surfaces had a strong (111) texture that became still stronger during dewetting. Films deposited on saw-tooth structures had a weak (100) texture with a weak [110] in-plane alignment along the grooves. This in-plane alignment became much more uniform during dewetting. It is proposed that graphoepitaxial alignment observed in both the as-deposited and annealed films on saw-tooth structures is the result of surface-energy-driven grain growth.

Architecture, microstructure and Jc anisotropy of highly oriented biaxially textured Co-doped BaFe2As2 on Fe/IBAD-MgO-buffered metal tapes

Optimized, biaxially textured BaFe1.8Co0.2As2 thin films with an in-plane alignment of 1.7° have been realized on high-quality IBAD-textured MgO-coated technical substrates utilizing additional Fe buffer layers. High critical current densities (Jc) were achieved, comparable to films on single crystalline MgO (Jc ≥ 1 MA cm−2 at 4 K, self-field). Transmission electron microscopy investigations reveal a small number of c-axis correlated defects introduced by the MgO template. The effect of these defects on the Jc anisotropy was determined in angular-dependent electronic transport measurements.

Light exposure dependence of field-effect mobility of pentacene thin films deposited on very thin polyimide photo-alignment layers

We report the anisotropic field-effect hole mobility of 30 nm-thick pentacene films deposited on 1.6 nm-thick polyimide photo-alignment layers with different in-plane anisotropy. To induce different in-plane anisotropy, the light exposure was varied up to 16 J/cm2. In this light exposure range, the long molecular axis of pentacene was standing with respect to the substrate surface (c*-axis oriented “thin film phase”), and the pentacene molecular plane was oriented on average parallel to the average alignment direction of the underlying polyimide backbone structures. For light exposures of 6 and 11 J/cm2, the field-effect mobility for the current flow direction parallel (perpendicular) to the alignment direction of pentacene molecular plane was enhanced (suppressed) compared to that of the pentacene film deposited on non-aligned polyimide film. This field-effect mobility enhancement shows a positive effect of the in-plane alignment control of pentacene molecules. However, the field-effect mobility decreased at a light exposure of 16 J/cm2, even though the in-plane orientation order of the pentacene molecular plane slightly increased. The causes for the mobility enhancement at 6 and 11 J/cm2 and the mobility decrease at 16 J/cm2 were discussed.

Preparation of Epitaxial YBa&lt;sub&gt;2&lt;/sub&gt;Cu&lt;sub&gt;3&lt;/sub&gt;O&lt;sub&gt;7−y&lt;/sub&gt; Films and CeO&lt;sub&gt;2&lt;/sub&gt; Buffer Layer on Ni-5at.%W Substrates by MOD Method

Biaxially textured CeO2 films were deposited on Ni–5at.%W (Ni–5W) tapes by a metal-organic deposition (MOD) method. Subsequent YBa2Cu3O7-y (YBCO) films were prepared using the (MOD) method leading to a simplified coated conductor architecture of YBCO/CeO2/Ni–5W. X-ray diffraction measurements revealed an epitaxial growth of the CeO2 buffer layer the 7.06°, 5.16°, for the out-of-plane and in-plane alignment, respectively. The superconducting coated film with a texture spread down to the out-of-plane texture is ω = 6.2° and the in-plane texture is φ = 7.6°. The results indicate that single CeO2-buffered Ni–5W tapes are suitable for the epitaxial growth of YBCO higher cube texture and the epitaxial YBCO film was obtained. To microstructural investigations, the film’s SEM results showed a dense, smooth and crack-free surface morphology. The superconducting properties were measured by four probe method. The superconducting transition temperature (Tc) was about 90 K with a narrow transition of 0.8 K and the critical current density (Jc) was about 1.5×104 A/cm2 at 77 K, 4T.

Texture and microstructure in co-sputtered Mg-M-O (M = Mg, Al, Cr, Ti, Zr, and Y) films

Mg-M-O solid solution films (M = Mg, Al, Cr, Ti, Zr, and Y) with various M contents are grown employing reactive co-sputtering by varying the target-to-substrate distance. It is shown that all films are biaxially aligned. When the two cathodes are equipped with the same target material (Mg), the in-plane alignment is determined by the cathode closest to the substrate, i.e., by the largest material flux. In the case of nearly equal material fluxes from the two cathodes, double in-plane orientation is observed. This is also the case for the Mg-Al-O and Mg-Cr-O films, while the Mg-Ti-O, Mg-Zr-O and Mg-Y-O films exhibit single in-plane orientation. Pole figures indicate that the grains in Mg-M-O (M different than Mg) are titled; in the Mg-Al-O, Mg-Cr-O, and Mg-Ti-O films, the grains tilt towards the Al, Cr, and Ti metal flux, respectively, while the grain tilt of the Mg-Zr-O and Mg-Y-O films is found to be towards the Mg metal flux. Furthermore, SEM cross-sectional images of the Mg-M-O films reveal columnar microstructure with columns tilted to the same direction as the grains. A mechanism which is based on the cation radius change upon the incorporation of an M atom in the MgO lattice is proposed to explain the tilting.

Superconductivity in Layered Organic Metals

In this short review, I will give an overview on the current understanding of the superconductivity in quasi-two-dimensional organic metals. Thereby, I will focus on charge-transfer salts based on bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF or ET for short). In these materials, strong electronic correlations are clearly evident, resulting in unique phase diagrams. The layered crystallographic structure leads to highly anisotropic electronic as well as superconducting properties. The corresponding very high orbital critical field for in-plane magnetic-field alignment allows for the occurrence of the Fulde–Ferrell– Larkin–Ovchinnikov state as evidenced by thermodynamic measurements. The experimental picture on the nature of the superconducting state is still controversial with evidence both for unconventional as well as for BCS-like superconductivity.

Epitaxial II–VI Tripod Nanocrystals: A Generalization of van der Waals Epitaxy for Nonplanar Polytypic Nanoarchitectures

We report for the first time the synthesis of nonplanar epitaxial tripod nanocrystals of II-VI compounds (ZnO, ZnS, ZnSe, ZnTe, CdS, CdSe, and CdTe) on muscovite mica substrate. With CdS as a case study, we conclude via Raman spectroscopy and electron microscopy studies that the tripods, which are found to be polytypic, followed a seeded growth mechanism. The epitaxy, manifested by the in-plane alignment of the legs of the tripods within a substrate, is attributed to the van der Waals interaction between the tripod bases and the mica surface, instead of to the covalent chemical bond which would require lattice matching between the epilayer and the substrate. The results demonstrated herein could have widespread immediate implications, including the potential of van der Waals epitaxy to be applicable in producing ordered arrays of more complex nanoarchitectures from various classes of compounds toward a broad range of technological applications.

Reflective type Solar-LCDs by using polarizing polymer solar cells

We present herein the results of a study of the reflective polarizing photovoltaic (PV) effects in an aligned polymer bulk-heterojunction PV layer. The PV layer consisted of a composite of regioregular poly(3-hexylthiophene) and methanofullerene (P3HT:PCBM) and the fairly uniform in-plane alignment of the P3HT:PCBM PV layer was achieved by means of a simple rubbing technique. The macroscopic axial orientation of the P3HT polymer in the aligned PV layer was observed to be significantly increased in the direction of rubbing with an axial orientational order parameter of 0.40. Moreover, it was also found that the reflective polarizing polymer solar cells (PSCs) that contained the aligned P3HT:PCBM layers exhibited a greater degree of anisotropy of 1.60 for the PV efficiencies under polarized illumination along the two principal axes. These reflective polarizing PSCs were applied to new reflective type solar cell-liquid crystal displays (Solar-LCDs), which exhibited a contrast ratio of 1.7. These results form a promising foundation for various energy-harvesting polarization-dependent opto-electrical Solar-LCD device applications.