Columnar Domains Research Articles

Alignment Change of Discotic Liquid Crystal Domains with Wavelength Tunable CO<sub>2</sub> Laser Irradiation

Infrared-induced alignment change with wavelength tunable CO2 laser irradiation for columnar liquid crystal domains was investigated for a triphenylene-based discotic liquid crystal. A uniformly aligned alignment change of domains was observed when a plused linearly polarized infrared laser light corresponding to the wavelength of the aromatic C-O-C stretching vibration band (9.65µm) was irradiated while it was not re-aligned uniformly with continious wave irradiation. The results strongly imply that the infrared irradiation is a possible technique for device fabrication by use of columnar mesophase as a liquid crystalline semiconductor.

Mechanism of compositional modulations in epitaxial InAlN films grown by molecular beam epitaxy

A mechanism for compositional modulations in InxAl1−xN films is described which considers growth kinetics during molecular beam epitaxy. InAlN crystalline films with various indium contents, grown on GaN or AlN buffer layers to create a variation in lattice mismatch conditions, were studied by transmission electron microscopy. Films comprise of columnar domains which are observed regardless of mismatch, with increasing indium concentration toward domain edges. We propose that indium is incorporated preferentially between adjacent dynamical InAlN platelets, owing to tensile strain generated upon platelet coalescence. The resulting In-rich boundaries are potential minima for further indium adatoms, creating a permanent indium composition gradient.

Nanostructured Materials through Orthogonal Self-Assembly in a Columnar Liquid Crystal

This paper describes a system in which an acid functionalized discotic molecule and poly(propylene-imine) dendrimer self-assemble into a new type of oblique columnar liquid crystalline (LC) phase that displays a well-ordered superlattice. The orthogonal combination of hydrogen bonding in the columnar direction and ionic interaction in the plane perpendicular to the columns gives rise to a structure in which the dendrimer is confined to separate columnar domains. The structure of the mesophases formed in the mixed system has been elucidated by infrared spectroscopy and X-ray diffraction. Investigation by differential scanning calorimetry and polarizing optical microscopy has shown that the LC phase is most stable in an 8:1 molar mixture but remains stable over a wide temperature and composition range. In dendrimer enriched mixtures the lattice swells to take up more dendrimer, while discotic enriched mixtures show the appearance of lamellar phases with a columnar structure that is probably closely related to the oblique superlattice. Additionally, the structure of the oblique superlattice can be covalently stabilized at elevated temperature via amidation of the ionic carboxylic acid−amine complexes. The results show the potential of orthogonal self-assembly in columnar LC phases to obtain nanostructured materials with a periodicity of 2–10 nm.

Cross-sectional analysis of ferroelectric domains in PZT capacitors via piezoresponse force microscopy

Ferroelectric domains have been investigated on the cross-section of Pb(Zr0.55Ti0.45)O3 (PZT) thin film capacitors by scanning probe microscopy. The static domain images on the cross-section were obtained by the lateral piezoresponse force microscopy (LPFM) method, in which the ac voltage used to induce the converse piezoelectric effect was applied between the conductive tip and the bottom electrode. The polarization component normal to the substrate could be characterized via both d33 and d15 piezoelectric coefficients, which resulted in a high resolution of LPFM images. After a variable dc bias was applied between the top and the bottom electrodes, the variations of domain image on the cross-section were recorded by the LPFM immediately. Upon the application of low bias, new domain sites appeared near the PZT/Pt interface opposite to the initial polarization. Forward stretch of new domains was facilitated under the dc field approaching the coercive field Ec. Under a very high field (about three times of the Ec), the sidewise expansion of columnar domains was observed. However, the domains were only partially switched even though a very high field was applied. The observed domain growth process indicated a lower energy barrier for nucleation compared with that of domain wall motion. Possible reasons for the incomplete switching are the substantial influences of the interface and depolarization in thin film capacitors.

Alignment control of columnar liquid crystals with wavelength tunable CO 2 laser irradiation

Infrared-induced alignment change with wavelength tunable CO 2 laser irradiation for columnar liquid crystal domains was investigated for a liquid crystalline triphenylene derivative. A uniformly aligned alignment change of domains was observed when a chopped linearly polarized infrared laser light corresponding to the wavelength of the aromatic C–O–C stretching vibration band (9.65 μm) was irradiated. The results strongly imply that the infrared irradiation is a possible technique for device fabrication by use of columnar mesophase as a liquid crystalline semiconductor.

Birefringence and Optical Waveguiding Losses in Preferentially c-Axis Oriented LiNbO3 Thin Films on SiO2 Produced by Electron Cyclotron Resonance Plasma Sputtering

LiNbO3 (LN) films deposited on SiO2 substrate by electron cyclotron resonance plasma sputtering and thermal treatment were structurally and optically characterized. Birefringence between 60 and 70% for single crystal was confirmed for c-axis oriented films on both Si and SiO2, whereas non-annealed amorphous film was optically isotropic. Crystallization during sputtering produced less than a 10% volume fraction of c-axis oriented spherical domains associated with the interface roughness layer. This sample had a large propagation loss of 32 dB/cm at 632.8 nm due to light scattering at the grain boundaries. LN films subjected to solid phase crystallization in a vacuum produced highly c-axis oriented columnar texture domains, which yielded propagation losses between 5 and 19 dB/cm, depending on the thickness (0.64–1.35 µm) and the mode order. The bulk and surface origins for waveguiding losses are discussed based on the film structure.

Columnar Structures and Stress Relaxation in Thick GaN FilmsGrown on Sapphire by HVPE

Thick GaN films with high quality are directly grown on sapphire in ahome-built vertical hydride vapour phase epitaxy (HVPE) reactor. Theoptical and structural properties of large scale columnar domains nearthe interface are studied using cathodoluminescence and micro-Ramanscattering. These columnar domains show a strong emission intensity dueto extremely high free carrier concentration up to 2×1019 cm−3, which are related with impurities trapped instructural defects. The compressive stress in GaN film clearlydecreases with increasing distance from interface. The quasi-continuouscolumnar domains play an important role in the stress relaxation forthe upper high quality layer.

Effects of side-chain length on the magnetic response of discotic metallomesogens

The magnetic responses of columnar superstructures of discotic metallomesogens with different alkyl side-chain lengths, cobalt octa(n-alkylthio)porphyrazine (CoSx, where x is the peripheral n-alkyl chain length, x = 10, 12, 14), have been investigated by small-angle neutron scattering (SANS). CoSx (x = 10, 12, 14) were heated to their isotropic phases and cooled down to their columnar mesophases under various external magnetic fields (0.2–1.1 T) and the orientational orderings of the columnar mesophases were measured by SANS. The SANS patterns showed clear anisotropies indicating the alignment of columnar domains with their columnar directors perpendicular to the applied magnetic field. The annularly averaged SANS data, I(Φ), of CoSx (x = 10, 12, 14) under various magnetic field strengths were fitted with Lorentzian functions. The full width at half maximum (FWHM) of the I(Φ) of CoS10 and CoS12 rapidly decreases with increasing applied magnetic field and then saturates to about 42° at ~0.5 T and 50° at ~0.6 T, respectively. In the case of CoS14, however, the diffraction anisotropy was very weak even at field strengths as high as 1.13 T and the FWHM was very broad, ca. 120°. The dramatic decrease of magnetic field sensitivity of CoS14 may be attributed to the entropy increase with the side-chain length.

Epitaxial growth and orientation of AlN thin films on Si(001) substrates deposited by reactive magnetron sputtering

Epitaxial domain formation and textured growth in AlN thin films deposited on Si(001) substrates by reactive magnetron sputtering was studied by transmission electron microscopy and x-ray diffraction. The films have a wurtzite type structure with a crystallographic orientation relationship to the silicon substrate of AlN(0001)‖Si(001). The AlN film is observed to nucleate randomly on the Si surface and grows three dimensionally, forming columnar domains. The in-plane orientation reveals four domains with their a axes rotated by 15° with respect to each other: AlN⟨112¯0⟩‖Si[110], AlN⟨011¯0⟩‖Si[110], AlN⟨112¯0⟩‖Si[100], and AlN⟨011¯0⟩‖Si[100] An explanation of the growth mode based on the large lattice mismatch and the topology of the substrate surface is proposed.

Orientation control of a synthetic columnar perfluorinated supramolecular dendrimer: Surface anchoring and magnetic-field induced alignments

Orientation ordering of a synthetic perfluorinated supramolecule containing a hydrophilic core group and perfluorinated tails is strongly affected by the functionality, molecular shape, surface anchoring and magnetic field. Small-angle neutron scattering (SANS), synchrotron X-ray diffraction, polarized light microscopy (PLM) and transmission electron microscopy (TEM) results show that the molecule exhibits hexagonal columnar mesophase upon cooling from isotropic phase. The orientation of the columns was controlled by surface anchoring; the columnar axes were perpendicular to the hydrophobic carbon substrates, while planar alignment is favored on hydrophilic surfaces. Furthermore, the columnar domains align with the magnetic field lines, which is due to diamagnetism of these fan-shaped molecules containing aromatic rings. We show that the magnetic-induced alignment is much effective for the large-scale control of the orientation of the perfluorinated columnar mesophase.

Correlation between interfacial structure and c-axis-orientation of LiNbO3 films grown on Si and SiO2 by electron cyclotron resonance plasma sputtering

Thin films of crystalline lithium niobate (LN) grown on Si(100) and SiO2 substrates by electron cyclotron resonance plasma sputtering exhibit distinct interfacial structures that strongly affect the orientation of respective films. Growth at 460–600°C on the Si(100) surface produced columnar domains of LiNbO3 with well-oriented c-axes, i.e., normal to the surface. When the SiO2 substrate was similarly exposed to plasma at temperatures above 500°C, however, increased diffusion of Li and Nb atoms into the SiO2 film was seen and this led to an LN–SiO2 alloy interface in which crystal-axis orientations were randomized. This problem was solved by solid-phase crystallization of the deposited film of amorphous LN; the degree of c-axis orientation was then immune to the choice of substrate material.

Electron cyclotron resonance plasma sputtering growth of textured films of c-axis-oriented LiNbO3 on Si(100) and Si(111) surfaces

Textured films of c-axis-oriented lithium niobate (LN) in a single Li:Nb=1:1 phase have been grown on Si(100) and Si(111) substrates by electron cyclotron resonance (ECR) plasma sputtering. The O2 partial pressure is critical in determining the composition of the film, whereas the substrate temperature mainly affects crystallinity. At O2 flow rates between 1 and 1.5 sccm and growth temperature between 510 and 550 °C, the refractive indices of the films are within their minimum range. A crystal image of the LN film revealed columnar textured domains, about 40% of which were c-axis oriented. The plasma-enhanced migration of surface atoms is responsible for the polarized grain growth. The amorphous interlayer formed between the LN film and the Si substrate consisted of either a single layer (2 nm) or double component layers (4 nm). The orientation of the LN crystalline overlayer has some correlation with the structure of the underlying amorphous interlayer.

Transfer of π-Conjugated Columnar Stacks from Solution to Surfaces

Three hydrogen-bonded oligo(p-phenylenevinylene)s, OPV3, OPV4, and OPV5, that differ in conjugation length have been synthesized and fully characterized. All three compounds contain chiral side chains, long aliphatic chains, and a ureido-s-triazine hydrogen bonding unit. (1)H NMR and photophysical measurements show that the OPV oligomers grow hierarchically in an apolar solvent; initially, dimers are formed by hydrogen bonds that subsequently develop into stacks by pi-pi interactions of the phenylenevinylene backbone with induced helicity via the chiral side chains. SANS measurements show that rigid cylindrical objects are formed. Stacks of OPV4 have a persistence length of 150 nm and a diameter of 6 nm. OPV3 shows rigid columnar domains of 60 nm with a diameter of 5 nm. Temperature and concentration variable measurements show that the stability of the stacks increases with the conjugation length as a result of more favorable pi-pi interactions. The transfer of the single cylinders from solution to a solid support as isolated objects is only possible when specific concentrations and specific solid supports are used as investigated by AFM. At higher concentrations, an intertwined network is formed, while, at low concentration, ill-defined globular objects are observed. Only in the case of inert substrates (graphite and silicium oxide) single fibers are visible. In the case of the repulsive surfaces (mica and glass), clustering of the stacks occurs, while, at attractive surfaces (gold), the stacks are destroyed.

Inversion domains in AlN grown on (0001) sapphire

Al-polarity inversion domains formed during AlN layer growth on (0001) sapphire were identified using transmission electron microscopy (TEM). They resemble columnar inversion domains reported for GaN films grown on (0001) sapphire. However, for AlN, these columns have a V-like shape with boundaries that deviate by 2°±0.5° from the c axis. TEM identification of these defects agrees with the post-growth surface morphology as well as with the microstructure revealed by etching in hot aqueous KOH.

High resolution X-ray diffraction study of the Si doping influence on columnar crystal growth of GaN layers

The morphology of hexagonally shaped coherent domains in undoped and Si-doped GaN has been characterised by X-ray determination of lateral and vertical correlation lengths and the twist ( α Φ ) and tilt ( α Ω ) angles. In the present paper all measured symmetrical ω scans have Gaussian shape. The extrapolation method based on theoretically derived expression for ω scan diffraction profile measured in the triple axis mode is presented. The method was applied for determination of values of L lateral correlation length and α Ω . In order to examine the shape and spatial misorientations of coherent domains the measurements were performed for scattering vectors q x parallel to all [ hk0] azimuthal directions. From a detailed analysis of plot L against q x we came to the conclusion that both in undoped and Si-doped samples the coherent domains in GaN layer are of quasi hexagonal shape. This is the first evidence of hexagonal shape of coherent domains concluded from X-ray data. However, the coherent domains of Si-doped layer exhibit a higher regularity of geometrical shape. The extrapolation method developed in the present work for determination of in-plane size of coherent domains and their tilt angle is applicable to a strong texture (about growth direction) of columnar coherent domains.

Layer-by-layer assembled films of a rigid poly(phenyl-ethynylene) and alternate poly(phenyl-ethynylene)/poly(aniline)

A new main chain segmented rigid rod-like polymer, poly[2,5-dodecyloxy-1,4-bis(ethylene glycol benzoate)ethynylene], hereafter called pPEG2 has been synthesized by a palladium catalyzed polycondensation. The interactions within the pPEG2 molecules are strong enough to give self-assembled multilayers (SAM), (pPEG2/pPEG2) m from diluted solutions. The films show emission in the green region at 512 nm. pPEG2 alternated with enzymatically synthesized poly(aniline) (PANi) as polycation provides supramolecular layered assemblies (pPEG2/PANi) n , which will be of great interest for the future realization of light emitting diodes (LEDs) or biological detectors. The thickness of the films can be controlled by the number of absorbed layers as stated by UV-Vis spectroscopy. The morphology of (pPEG2/pPEG2) m assemblies, analyzed by AFM, consists of columnar domains coexisting with fine and homogenously distributed needles, while for the alternated (pPEG2/PANi) n assemblies, the SAM are rather of granular type.

Structural Study of V-like Columnar Inversion Domains in AlN Grown on Sapphire

ABSTRACTV-like columnar inversion domains with a divergence angle of about 4.5° ± 1° grown in AlN films with N-polarity were studied using transmission electron microscopy (TEM) and atomic force microscopy. Such domains emerge at the surface forming a small islands in form of hexagonal, truncated pyramids. A model of such pyramid was proposed. TEM studies indicate a displacement of c/2 along the [0001] direction at the inversion domain boundary. A boundary itself is composed of long segments located on the {1100} planes, which are alternated by short segments on some inclined planes.

Laser Ablation Deposition of CeO2−x Epitaxial Domains on Glass

Laser ablation of CeO2 target in vacuum (5×10−4 Pa) was used to produce nanometer-size condensates, which deposited as yellowish top coating and whitish bottom coating on a soda-lime glass substrate. The top coating consists of optically anisotropic columnar domains conformable to monoclinic Ce6O11 phase coexisting with cubic (c) CeO2−x, whereas the bottom coating is optically isotropic c-CeO2−x due to oxygen uptake from the substrate. Transmission electron microscopy indicated that the columnar domains are made up of defective fluorite-type nanoparticles, which tended to coalesce over (111) plane to form dislocations and (111)-preferred orientation, an artificial epitaxy owing to rotation–coalescence of (111) faceted CeO2−x condensates on the amorphous substrate and/or within the coating.

Influence of GaN domain size on the electron mobility of two-dimensional electron gases in AlGaN/GaN heterostructures determined by x-ray reflectivity and diffraction

X-ray reflectivity and diffraction measurements were performed on Ga-face AlGaN/GaN heterostructures to determine the influence of interface roughness scattering and GaN domain boundaries scattering on the electron mobility of polarization induced two-dimensional electron gases. From simulations of the specular reflectivity, the root-mean-square roughness of the AlGaN/GaN interfaces was obtained. In reciprocal space maps, laterally elongated streaks passing through the Bragg peaks have been observed, which are attributed to column-like domains in the GaN buffer layers. The relationship between electron mobility measured by Hall effect and the interface roughness on one hand, and the column domain size on the other hand, demonstrates that the interface roughness scattering is not limiting the electron mobility, whereas the transport properties of the two-dimensional electron gas degrade with decreasing size of columnar domains in the GaN layer.

Misfit defect formation in thick GaN layers grown on sapphire by hydride vapor phase epitaxy

The nucleation and microstructure of large-scale columnar domains present in hydride vapor phase epitaxial (HPVE)-GaN layers grown directly on sapphire have been studied using cathodoluminescence and transmission electron microscopy. The domains are distributed in a quasicontinuous layer close to the GaN/sapphire interface. The domain boundaries are found to be associated with stacking mismatch defects. They are initiated at steps on the sapphire surface and are formed between nucleation islands growing on adjacent terraces. The formation of these domains in the initial stages of HVPE-GaN heteroepitaxial growth is proposed to play an important role in the strain relaxation mechanism.