Periodic Superstructures Research Articles

Liquid crystal engineering – new complex mesophase structures and their relations to polymer morphologies, nanoscale patterning and crystal engineering

This critical review focuses on recent progress in the field of T-shaped ternary amphiphiles. These molecules can self-assemble into a series of new liquid crystalline (LC) phases with polygonal cylinder structures, new lamellar phases and LC phases combining columns and layers. These structures are analyzed on the basis of symmetry, net topology and tiling pattern (Laves and Archimedean tilings) and discussed in relation to morphologies of multiblock copolymers, self organized DNA super-lattices, metal-organic frameworks, crystal-engineering and self-assembled periodic superstructures on surfaces (210 references).

Polymer nanostructures in sub-micron lithographically defined channels: film-thickness effects on structural alignment of a small feature size polystyrene-polyisoprene-polystyrene block copolymer.

The use of phase separation in block copolymer systems to generate regular nanopatterns at surfaces may be an alternative to advanced photolithography. Here, substrates with photolithographically defined rectangular channels (of depth 60 nm and widths 166-433 nm) are used to direct nanoscale phase separation of a polystyrene--polyisoprene--polystyrene (PS-PI-PS) block copolymer into aligned periodic superstructures. This nanoscale phase separation results in the rapid formation of parallel and narrow polystyrene (PS) cylinders orientated in a 2D hexagonal arrangement within a polyisoprene (PI) matrix for the polymer composition used here. The PS-PI-PS system is shown to be extremely amenable to simple processing methods allowing precise and homogeneous coating of the substrates. Effects of polymer film thickness are investigated in depth, given that polymer film thickness plays an essential role in the orientation/architecture of the structure formed. It was observed that control of film thickness can determine the orientation of cylinders (parallel or vertical) with respect to the substrate surface. In films where the PS cylinders are aligned parallel to the substrate surface careful control of the processing parameters facilitates the fabrication of regular multi-layer systems ( layers of cylinders) within the channel. The directional effect imposed by the channels is not limited to polymer nanostructures within the channels as long-range order and alignment can be observed at film thicknesses that extend above the channels and onto the outer surface of the substrate. However, as thickness increases, this 'directing' effect conferring alignment is lost.

A growth pathway for highly ordered quantum dot arrays

To realize the desired zero-dimensional behavior of a quantum dot ensemble, the ability to fabricate quantum dots with a high packing density and a high degree of size, shape, and spacing uniformity is crucial. Here we report highly ordered InAs nanodot arrays grown by molecular-beam epitaxy on nonlithographically nanopatterned GaAs. Approximately 20 billion dots are grown in a 1cm2 area with the smallest size dispersion ever reported and forming a lateral superlattice in hexagonal dense packing form. These techniques presage a pathway to controlled growth of periodic quantum dot superstructures, which offer macroscopic spatial coherence in the interaction of quantum dots with radiation.

Supersonic nonlinear dynamics of domain walls in rare-earth orthoferrites

Dynamics of domain walls (DWs) in pulsed magnetic fields as strong as 5.5 kOe at different temperatures in orthoferrites EuFeO 3 (4.2–300 K), TmFeO 3 (100–300 K), LuFeO 3 (300 K), YFeO 3 (460 K), and DyFeO 3 (77–300 K) has been investigated. The domain wall velocity as a function of magnetic field ( V( H)) is nonlinear in all the orthoferrites studied, consisting of discrete intervals (Δ H i ) within which the DW moves at a constant rate. The formation of Δ H i is due to a generation by the DW elastic and bend vibrations in a strongly dissipative and nonlinear medium. The DW motion reveals periodic superstructures at supersonic speeds. At the moment when the DW is overcoming the sound barrier, the nonsteady restructuring of domain structures, accompanied by changes in the fine structure of the DW, has been found.

Dynamical self-organized periodic superstructures in supersonic dynamics of domain wall in orthoferrites

It is found that the domain wall motion at supersonic velocities in orthoferrites is in line with characteristics of dynamic self-organized systems. It is also observed that the transition between two neighbouring ranges Δ H i →Δ H i+1 of a magnetic field on the graph of the domain velocity versus the magnetic field— V( H) is accompanied by a reduction in size of periodic superstructures on the domain wall, λ i → λ i+1 . In this case, the domain wall velocity changing abruptly in the transitions between Δ H i is a bifurcation parameter.

Characterization of the B4 phase by dielectric and AFM measurements

Dielectric measurements on a sample consisting of banana-shaped molecules were carried out in a frequency range between 0.1 Hz and 10 MHz. The sample exhibited B2 and B4 phases. As usual, two ranges of relaxation were detected in the B2 phase, the fast reorientation about the long axes of the molecules and a slow collective process. In the B4 phase, only one dielectric active process at low frequencies was found; this does not differ from the low frequency relaxation of the B2 modification. This relaxation is probably related to the dynamics of superstructures. Crystallization could be observed after keeping the sample for a longer time at higher temperature. Thus, it was possible to differentiate clearly between the crystalline and the B4 phases. AFM investigations prove the existence of focal-conic domains and periodic superstructures in the B4 phase; then do not appear in the crystalline state. For this reason the B4 phase is regarded as different from a classical crystalline phase.

A new way to the “knitting pattern” via blending of ABC triblock copolymers

The “knitting pattern” morphology was obtained via blending of two polystyrene-block-polybutadiene-block-poly(methyl methacrylate) triblock copolymers. This demonstrates that blending of block copolymers is a useful way to design periodic superstructures by taking advantage of the self-assembling nature of microphase separated block copolymers.

Long range ordering in the graphite intercalation compounds

Graphite surfaces treated with NaOH, LaCl 3 and CeCl 3 aqueous solutions were studied by scanning tunneling microscopy. These metals in the compounds are known to have poor lattice matching with graphite. In the presence of low densities of intercalants, which are not enough to form a stage structure, the existence of large periodic superstructures was confirmed. Since the observed structures could not be interpreted by Moiré pattern hypothesis, they might be directly related to the intercalant arrangement. The obtained results directly show the importance of long-range interactions for the understanding of the graphite intercalation compound structures as predicted by theoretical studies.

Nature of the atomic-scale restructuring of Pt(100) electrode surfaces as evidenced by in-situ scanning tunneling microscopy

Elucidating the atomic and nanoscale structures of single-crystal metal electrodes is of central importance in electrochemistry. Until recently, most information along those lines was obtained by low-energy electron diffraction and related methods following transfer into ultrahigh vacuum (uhv) [1,2]. The recent advent of scanning tunneling microscopy (STM) as an in-situ probe is offering important new insight into the structural properties of the electrode substrates themselves as well as ordered adsorbate layers [3]. Examinations of surface restructuring, i.e. the occurrence of metal atomic arrangements which differ from the ideal bulktermination structure, are forming an important part of current research efforts [2,3]. Most studies along these lines are concerned with metal reconstructions, whereby the top layer of surface atoms rearrange to form ordered terraces with unit cells that are quite different from the (1 X 1) symmetry. A related, yet quite distinct, form of restructuring entails longer-range disruption of the surface lattice, possibly to form periodic superstructures, but perhaps involving retention of the local unit-cell geometry. While examples are as yet less plentiful, the local-probe nature of the STM makes the technique well suited to the examination of the latter form of restructuring, which can escape detection when using diffraction-based methods. A recent example of restructuring with retention of the terrace (1 X 1) geometry is seen by STM on Au(ll0) electrode in iodide media, where periodic cleavage along the (li0) direction is observed so to yield terrace “channels” bordered by monoatomic steps at high potentials, where an ordered iodine adlayer is present 141. Another case, examined initially by Vitus et al. [5& involves the formation of dense arrays of 20-40 A substrate “islands” on initially uniform Pt(100) terraces upon in-situ replacement of an iodine adlayer, formed

Periodic superstructures in tetrahedrally bonded homopolymers.

Using a lattice sum over single bipolaron potentials displaced by periodicity d, we have analytically obtained a solution for a bipolaron lattice for a tetrahedrally bonded homopolymer within the continuum model of Rice and Phillpot. This solution is used to derive the band structure, which consists of two bipolaron bands symmetrically located about the middle of the band gap in addition to the conduction and valence bands. The electronic density of states, chemical potential \ensuremath{\mu}, and the energy of formation of a bipolaron lattice are also calculated as a function of the bipolaron density ${\ensuremath{\rho}}_{b}$. The bipolaron chemical potential lies between the conduction-band edge and the upper edge of the upper bipolaron band, indicating that the bipolaron lattice is energetically the most favorable charge configuration at low ${\ensuremath{\rho}}_{b}$. In the strict weak-coupling limit (infinite momentum cutoff \ensuremath{\Lambda}) the bipolaron-bipolaron interaction is found to be repulsive and varies with bipolaron density as (1/${\mathrm{\ensuremath{\rho}}}_{\mathit{b}}$)exp(-2/${\mathrm{\ensuremath{\rho}}}_{\mathit{b}}$${\ensuremath{\xi}}_{\mathit{p}}$), ${\ensuremath{\xi}}_{\mathit{p}}$ being the bipolaron characteristic length. Thus, the bipolaron lattice is stable only in the range 0${\ensuremath{\rho}}_{b}$2/${\ensuremath{\xi}}_{p}$. This suggests the possibility of a phase separation of a doped homopolymer into conducting bipolaron droplets at ${\ensuremath{\rho}}_{b}$=2/${\ensuremath{\xi}}_{p}$, while the rest of the system is insulating. Our results apply to polysilylenes, polygermylenes, and their derivatives, as well as to a wide class of carbon-based polymers.

Using the long-range nature of electrostatic forces to create defined lateral molecular distributions in langmuir-blodgett films

Fluorescence microscopic observations with lipid monolayers in the fluid/ordered phase coexistence range are reviewed to conclude on the interplay of nucleation kinetics, line tension and long-range electrostatic forces. The latter arise from the orientation of molecular dipoles at the interface and cause periodic lamellar or hexagonal superstructures. They enable manipulation of lipid domains in electric fields and may be responsible for instabilities of phase boundaries. Due to these forces also J-aggregates formed by adsorption from the subphase are shown to form periodic superstructures. For a ternary system existing of the protein concanavalin A and two lipid phases a phase separation is demonstrated with the lateral distribution of the phases determined by electrostatic forces. The ordered lipid phases are shown to exhibit long-range orientational and short-range positional order.

Current induced periodic ferroelectric domain structures in LiNbO3 applied for efficient nonlinear optical frequency mixing

Periodic laminar ferroelectric domain superstructures in chromium-doped LiNbO3 have been grown by modulating the bias current during Czochralski growth. Periodic domain patterns stacked along the y axis with domain thicknesses as low as 8 μm have been prepared. The alternating sign of the effective nonlinear optical (nlo) coefficient enables efficient frequency mixing independent of the constraints of birefringence. This is demonstrated by realizing frequency doubling of 1.06-μm YAG : Nd laser radiation using the largest nlo coefficient, d33. For domain thicknesses equal to an odd multiple of the coherence length, d=(2m+1)lc, the second harmonic intensity varies as the square of the domain number. For the optimum case, d=lc, an improvement of the nlo efficiency, as compared to the 90° phase-matched process, by the factor [(2/π)d33/d31]2=15 is inferred.

Adsorbed layers on (111)InAs faces in contact with In-As-Cl-H gas phase, and the possibility of phase transitions in the adsorbed layers

Adsorption of various species existing in the In-As-Cl-H CVD gaseous system on both InAs (111) faces is considered. Arsenic is supposed to be adsorbed in the form of triangles As 3 and tetrahedrons As 4, each of them occupying 3 atomic sites above In or As atoms on (111)In or (111)As, respectively. The system of polyatomic adsorption equations was used to find the coverages of the faces by various species. Admolecule-surface bond strengths are taken to be equal to the ones for the single bonds in molecules. Pauling electronegativities were used to find the effective charges of the atoms in the adsorption layer. Thus, the dipole moments of adsorbed molecules which arise are directed along the In-As bonds in the InAs lattice. With this geometry, the calculated electrostatic dipole-dipole attraction between InCl molecules forming a dense layer on (111)As exceeds 12 kcal/mol. Thus, condensation of the two-dimensional gas of adsorbed InCl molecules should be expected. Corresponding S-shape isotherms θ( P) are calculated for different As 3 vapor pressures, θ and P being the surface coverage and bulk vapor pressure of InCl. Intervals of InCl 3 H 2 ratios at different temperatures where the two-dimensional condensation may occur, are presented for realistic CVD conditions. Two-dimensional condensation may result in sharp changes in kinetic coefficient and thus in autho-oscillations in growth rate and doping level creating periodic superstructures. Nucleation and CVD growth processes are discussed.

Stacking variations in cancrinite minerals

TEM reveals (0001) stacking faults and periodic superstructures in cancrinite-related minerals, afghanite and franzinite, which are interpreted as aperiodic or periodic displacements of the hexagonal [(Si,AI)60~2] ring units. Franzinite also shows a superstructure when the structure is projected onto the xy plane; satellites occur in the hk0 diffraction patterns.