Periodic Superstructures Research Articles

Imaging surface structure and premelting of ice Ih with atomic resolution.

Ice surfaces are closely relevant to many physical and chemical properties, such as melting, freezing, friction, gas uptake and atmospheric reaction1-8. Despite extensive experimental and theoretical investigations9-17, the exact atomic structures of ice interfaces remain elusive owing to the vulnerable hydrogen-bonding network and the complicated premelting process. Here we realize atomic-resolution imaging of the basal (0001) surface structure of hexagonal water ice (ice Ih) by using qPlus-based cryogenic atomic force microscopy with a carbonmonoxide-functionalized tip. We find that the crystalline ice-Ih surface consists of mixed Ih- and cubic (Ic)-stacking nanodomains, forming periodic superstructures. Density functional theory reveals that this reconstructed surface is stabilized over the ideal ice surface mainly by minimizing the electrostatic repulsion between dangling OH bonds. Moreover, we observe that the ice surface gradually becomes disordered with increasing temperature (above 120 Kelvin), indicating the onset of the premelting process. The surface premelting occurs from the defective boundaries between the Ih and Ic domains and can be promoted by the formation of a planar local structure. These results put an end to the longstanding debate on ice surface structures and shed light on the molecular origin of ice premelting, which may lead to a paradigm shift in the understanding of ice physics and chemistry.

Nanoparticle assembly by transient topography induced by applying soft lithography to block copolymer films.

We present a simple approach for patterning metal nanoparticles into periodic superstructures on flat films spanning centimeter-square areas. Our approach is based on capillary force lithography, a soft lithography method that is used to impart topography to molten polymer films, and applies it to block copolymer films to obtain substrates featuring both topographic and chemical contrasts that can serve as templates for the selective deposition of nanoparticles. Here we show that flattening the films by exposure to solvent vapour prior to nanoparticle deposition not only retains chemical heterogeneity but also provides access to unique hierarchically-organized nanoparticle superstructures that are unattainable by other methods. Such structures could be useful for optical, sensor, and catalytic applications.

Unsupervised machine learning to classify the confinement of waves in periodic superstructures.

We propose a rigorous method to classify the dimensionality of wave confinement by utilizing unsupervised machine learning to enhance the accuracy of our recently presented scaling method [Phys. Rev. Lett.129, 176401 (2022)10.1103/PhysRevLett.129.176401]. We apply the standard k-means++ algorithm as well as our own model-based algorithm to 3D superlattices of resonant cavities embedded in a 3D inverse woodpile photonic band gap crystal with a range of design parameters. We compare their results against each other and against the direct usage of the scaling method without clustering. Since the clustering algorithms require the set of confinement dimensionalities present in the system as an input, we investigate cluster validity indices (CVIs) as a means to find these values. We conclude that the most accurate outcome is obtained by first applying direct scaling to find the correct set of confinement dimensionalities, and subsequently utilizing our model-based clustering algorithm to refine the results.

Periodic anti-phase boundaries and crystal superstructures in PtCu3 nanoparticles as fuel cell electrocatalysts

To reduce the cost of the electrocatalyst for the oxygen reduction reaction in proton exchange membrane fuel cells, supported metal alloy nanoparticles are commonly used to decrease the amount of the scarce and expensive Pt. Since the structure of the nanoparticles has a crucial influence on the catalytic properties, advanced characterization methods help elucidate these relationships and ultimately enable more rational synthesis. This work focuses on investigating the presence of one particular planar defect, namely periodic anti-phase boundaries in carbon-supported PtCu3 nanoparticles. The studying of this structural phenomenon was approached with several characterization tools. Ex-situ X-ray diffractograms were used in conjunction with computer simulations and Rietveld analyses to reliably determine the modulated unit cell containing anti-phase boundaries, while their temperature-dependent formation and disappearance were observed with high-temperature X-ray powder diffraction. In addition, their presence was also confirmed by electron diffraction and atomically resolved scanning transmission electron microscopy. Furthermore, the average neighborhood of Cu and Pt atoms was confirmed using extended X-ray absorption fine structure. Finally, the electrocatalytic activity for the oxygen reduction reaction for composites with and without anti-phase boundaries was determined using a thin-film rotating disk electrode, and the performance was found to correlate with the degree of alloy ordering but not with the presence of anti-phase boundaries. Overall, this study represents a significant step towards better control over the atomically precise synthesis of advanced functional metallic materials by providing valuable insight into the formation of planar defects in metallic alloy nanoparticles and their impact on the structure-property relationships of electrocatalysts.

Wavelength‐Tunable Circularly Polarized Laser Arrays for Multidimensional Information Encryption

AbstractInformation security has attracted particular attention along with the exponential growth of information communications. Stimuli‐responsive microlasers show considerable potential for anti‐counterfeiting and encryption technologies due to their distinguishable signal readout, superior responsiveness, and distinct polarization states, yet their application is frequently restricted by the low security level due to the visualization of information recorded solely in emission wavelength. Here, a novel strategy is proposed to realize high‐security multidimensional information encryption based on wavelength‐tunable circularly polarized (CP) microlaser arrays by introducing additional covert polarization states as coding elements. CP laser emissions are obtained in cholesteric liquid crystal (CLC) arrays due to well‐organized periodic helical superstructures, and the lasing wavelength is thermally tailored across a broad range. Based on the synergistic use of wavelength and polarization state as cryptographic primitives, the multidimensional information encryption and decryption in thermo‐responsive CP microlaser arrays is realized. These results provide valuable enlightenment to the development of stimuli‐responsive CP laser for information security and anti‐counterfeiting applications.

Effect of the Processes of Microsegregation on the Formation of Periodic Doping Superstructures in Binary Alloys

Effect of the Processes of Microsegregation on the Formation of Periodic Doping Superstructures in Binary Alloys

Densely packed tetrahedra clusters displaying diamond-like superstructures

Two special tetrahedra packing clusters were analytically calculated and discussed. Diamond and double-diamond, periodic superstructures emerge. Both structures exemplify an efficient, congruent tetrahedra space tiling strategy. The diamond superstructure exhibits a remarkable packing density of φ∼0.76 exceeding maximal density for the packing of equal spheres. The presented architectures display long range order with specific, repetitive structural motifs. Due to their simple tetrahedral symmetry, the investigated clusters have the potential to become interesting, unique sub-units both in construction technology and in self-assembled systems design. The modified structure of presented cluster can entirely tile three-dimensional Euclidean space ℝ3 through periodic tessellations. Moreover, detailed analysis of the cluster enables definition of a new plesiohedron.

Влияние процессов микросегрегации на формирование периодических примесных сверхструктур в бинарных сплавах

The influence of the trapping of mobile impurity atoms by immobile defects on superstructures is analyzed. Superstructures are formed as a result of solid-solid phase transitions like the directional crystallization. A capillary wave model was used for the analysis. It was shown that the conditions for the appearance of periodic superstructures are facilitated when the time of the diffusion jump of impurity atoms is much shorter than the characteristic time of their segregation at defects.

Conditions of Formation of Doping Superstructures at Phase Transitions

This work is aimed at general analytical representation of the conditions necessary for generation of layered periodic doping superstructures resulting from the solid-state phase transition accompanied by variation in admixture concentration. Using the capillary-wave model, the explicit expression for the region of temperature gradient and averaged interface velocity, inside which the interface velocity transits to the regime of periodic oscillations, is obtained. The formula for the spatial period of the doping superstructure whose appearance is caused by these oscillations is derived.

Soliton superlattices in twisted hexagonal boron nitride

Properties of atomic van der Waals heterostructures are profoundly influenced by interlayer coupling, which critically depends on stacking of the proximal layers. Rotational misalignment or lattice mismatch of the layers gives rise to a periodic modulation of the stacking, the moiré superlattice. Provided the superlattice period extends over many unit cells, the coupled layers undergo lattice relaxation, leading to the concentration of strain at line defects – solitons - separating large area commensurate domains. We visualize such long-range periodic superstructures in thin crystals of hexagonal boron nitride using atomic-force microscopy and nano-infrared spectroscopy. The solitons form sub-surface hexagonal networks with periods of a few hundred nanometers. We analyze the topography and infrared contrast of these networks to obtain spatial distribution of local strain and its effect on the infrared-active phonons of hBN.

Self-Assembly of Liquid-Crystal Droplets in Cells With Patterned Indium Tin Oxide

Self-assembly of colloids into superstructures is very important for fabricating functional devices. This study demonstrates that in a colloidal system of anisotropic liquid-crystal droplets dispersed in isotropic glycerol, the droplets self-assemble into periodic superstructures in cells with patterned indium tin oxide (ITO) electrodes, due to the different surface tensions of the two components on ITO and glass. This self-assembly of the liquid-crystal droplets may be utilized to manufacture $e.g.$ switchable photonic crystals, sensors, or microlens arrays.

Dielectric properties of liquid crystal formed by laterally fluorine-substituted banana-shaped molecules

ABSTRACT Dielectric spectroscopy in the frequency range from 10−2 to 107 Hz was applied to investigate the dynamics of the relaxation processes in a banana-shaped liquid crystal compound with fluorinated chain segments. The dielectric spectra indicate the presence of two relaxation processes and a conductivity contribution. The high-frequency process shows Arrhenius-like temperature dependence in the B4 and SmCP phases and can be assigned to the hindered rotation of the bent-shaped molecules around their long molecular axis. The low-frequency process can be related to the dynamics of the periodic superstructures in B4 phase and fluctuations of the modulated ferroelectric domains in the SmCP phase. In the B7 (or SmCG) phase, both processes show the nonlinear dependence of the relaxation frequencies especially close to the isotopic phase transition. The dielectric measurements confirm the existence of the SmCP – B7 (or SmCG) second order or weakly first-order phase transition.

DNA-Functionalized, Bivalent Proteins.

Bivalent DNA conjugates of β-galactosidase (βGal), having pairs of oligonucleotides positioned closely on opposing faces of the protein, have been synthesized and characterized. These structures, due to their directional bonding characteristics, allow for the programmable access of one-dimensional protein materials. When conjugates functionalized with complementary oligonucleotides are combined under conditions that support DNA hybridization, periodic wire-type superstructures consisting of aligned proteins form. These structures have been characterized by gel electrophoresis, cryo-transmission electron microscopy, and negative-stain transmission electron microscopy. Significantly, melting experiments of complementary building blocks display narrowed and elevated melting transitions compared to the free duplex DNA, further supporting the formation of the designed binding mode, and unambiguously characterizing their association as DNA-mediated. These novel structures illustrate, for the first time, that directional DNA bonding can be realized with only a pair of DNA modifications, which will allow one to engineer directional interactions and realize new classes of superstructures not possible simply through shape control or isotropically functionalized materials.

Role of Absorbing Nanocrystal Cores in Soft Photonic Crystals: A Spectroscopy and SANS Study.

Periodic superstructures of plasmonic nanoparticles have attracted significant interest because they can support coupled plasmonic modes, making them interesting for plasmonic lasing, metamaterials, and as light-management structures in thin-film optoelectronic devices. We have recently shown that noble metal hydrogel core-shell colloids allow for the fabrication of highly ordered 2-dimensional plasmonic lattices that show surface lattice resonances as the result of plasmonic/diffractive coupling (Volk, K.; Fitzgerald, J. P. S.; Ruckdeschel, P.; Retsch, M.; König, T. A. F.; Karg, M. Reversible Tuning of Visible Wavelength Surface Lattice Resonances in Self-Assembled Hybrid Monolayers. Adv. Optical Mater. 2017, 5, 1600971, DOI: 10.1002/adom.201600971). In the present work, we study the photonic properties and structure of 3-dimensional crystalline superstructures of gold hydrogel core-shell colloids and their pitted counterparts without gold cores. We use far-field extinction spectroscopy to investigate the optical response of these superstructures. Narrow Bragg peaks are measured, independently of the presence or absence of the gold cores. All crystals show a significant reduction in low-wavelength scattering. This leads to a significant enhancement of the plasmonic properties of the samples prepared from gold-nanoparticle-containing core-shell colloids. Plasmonic/diffractive coupling is not evident, which we mostly attribute to the relatively small size of the gold cores limiting the effective coupling strength. Small-angle neutron scattering is applied to study the crystal structure. Bragg peaks of several orders clearly assignable to an fcc arrangement of the particles are observed for all crystalline samples in a broad range of volume fractions. Our results indicate that the nanocrystal cores do not influence the overall crystallization behavior or the crystal structure. These are important prerequisites for future studies on photonic materials built from core-shell particles, in particular, the development of new photonic materials from plasmonic nanocrystals.

Semiconducting large bandgap oxides as potential thermoelectric materials for high-temperature power generation?

Semiconducting large bandgap oxides are considered as interesting candidates for high-temperature thermoelectric power generation (700–1,200 °C) due to their stability, lack of toxicity and low cost, but so far they have not reached sufficient performance for extended application. In this review, we summarize recent progress on thermoelectric oxides, analyze concepts for tuning semiconductor thermoelectric properties with view of their applicability to oxides and determine key drivers and limitations for electrical and thermal transport properties in oxides based on our own experimental work and literature results. For our experimental assessment, we have selected representative multicomponent oxides that range from materials with highly symmetric crystal structure (SrTiO3 perovskite) over oxides with large densities of planar crystallographic defects (TinO2n−1 Magneli phases with a single type of shear plane, NbOx block structures with intersecting shear planes and WO3−x with more defective block and channel structures) to layered superstructures (Ca3Co4O9 and double perovskites) and also include a wide range of their composites with a variety of second phases. Crystallographic or microstructural features of these oxides are in 0.3–2 nm size range, so that oxide phonons can efficiently interact with them. We explore in our experiments the effects of doping, grain size, crystallographic defects, superstructures, second phases, texturing and (to a limited extend) processing on electric conductivity, Seebeck coefficient, thermal conductivity and figure of merit. Jonker and lattice-versus-electrical conductivity plots are used to compare specific materials and material families and extract levers for future improvement of oxide thermoelectrics. We show in our work that oxygen vacancy doping (reduction) is a more powerful driver for improving the power factor for SrTiO3, TiO2 and NbOx than heterovalent doping. Based on our Seebeck-conductivity plots, we derived a set of highest achievable power factors. We met these best values in our own experiments for our titanium oxide- and niobium oxide-based materials. For strontium titanate-based materials, the estimated highest power factor was not reached; further material improvement is possible and can be reached for materials with higher carrier densities. Our results show that periodic crystallographic defects and superstructures are most efficient in reducing the lattice thermal conductivity in oxides, followed by hetero- and homovalent doping. Due to the small phonon mean free path in oxides, grain boundary scattering in nanoceramics or materials with nanodispersions is much less efficient. We investigated the impact of texturing in Ca3Co4O9 ceramics on thermoelectric performance; we did not find any improvement in the overall in-plane performance of a textured ceramic compared to the corresponding random ceramic.

Phase transitions in ferroelectric-paraelectric superlattices

Within the phenomenological Landau–Ginzburg–Devonshire theory, we discuss the paraelectric-ferrolectric transition in superstructures consisting of ferroelectric and paraelectric layers of equal thickness. The polar axis of the ferroelectric is perpendicular to the layer plane as expected in fully strained BaTiO3/SrTiO3 superstructures on SrTiO3 substrates with pseudomorphic electrodes. We concentrate on the electrostatic effects and do not take into account the boundary conditions other than the electrostatic ones. We find that when the ferroelectric phase transition in the superstructures is into a multidomain state, both its temperature and its character, i. e., the profile of the polarization appearing at the phase transition is strongly influenced by the nature of the near-electrode region. This is also the case for the layer thickness separating the single-and multidomain regimes of the transition. Such a finding makes us question the idea that these superstructures can be thought of as infinite systems, i.e., periodic superstructures similar to a crystal. The irrelevance of this idea in certain conditions is demonstrated by comparing the phase transitions in two different superstructures consisting of ferroelectric and paraelectric layers of the same thickness. In one of them, the ferroelectric layer is in immediate contact with an ideal metallic electrode, whereas at the other boundary, it is the paraelectric layer that is in contact with the electrode. In another superstructure, one paraelectric layer is split in two equal parts which are placed as the first and last layer between the electrodes and the ferroelectric layers which are closest to the electrodes. We show (with some formal reservations) that the phase transition temperature in the first superstructure can be over 100 °C more than in the second one if the material parameters of BaTiO3/SrTiO3 are used for the estimations. Moreover, the profile of the polarization arising at the phase transition is inhomogeneous along the superstructure and has the maximum amplitude in the ferroelectric layer contacting the electrode. We argue that this situation is general and results in smearing of the phase transition anomalies for the layer thicknesses corresponding to multidomain transitions. The work is mainly analyical but numerical methods have been used to support some statements that have been put forward as hypotheses.

Self-organized, gratinglike nanostructures in polymer films with embedded metal nanoparticles induced by femtosecond laser irradiation

The self-organized formation of periodic superstructures in thin plasma polymer films containing noble metal nanoparticles upon femtosecond laser irradiation has been studied in detail. By applying several hundred laser pulses on average per spot, the nanostructure of the metal layer is persistently changed into an approximate line grating with periodical changes in particle size and shape as observed by scanning and transmission electron microscopy. The formation of the structures is only possible within rather narrow ranges of laser intensity and metal content. The orientation of the lines is given by the laser polarization, while their spatial periodicity depends on laser wavelength and incidence angle. These observations give evidence that interference of incident light with light scattered into the film plane is the main mechanism controlling the nanostructure formation. We also discuss the optical spectra of the irradiated regions, in particular the observed dichroism and its relation to the prepared periodic structures.

1D periodic bimetallic superstructures by co-assembly of ternary block copolymer/nanoparticle blends

The formation of hierarchical 1D periodic bimetallic superstructures by co-assembly of ternary block copolymer/particle blend systems is reported. The results point to the relevance of entropy-driven morphology selection processes in enthalpic neutral block copolymer/particle blends that should render the presented approach applicable to a wide range of polymer/particle compositions.

Spontaneous Formation of Complex Periodic Superstructures under High Interferential Illumination of Small‐Molecule‐Based Photochromic Materials

AbstractA series of push‐pull azo compounds containing bulky substituents are synthesized, yielding fully amorphous materials with glass‐transition temperatures above 200 °C. Thin films are subjected to holographic illumination and show superior bulk photomigration in terms of speed and efficiency compared to materials exhibiting similar electronic and photochromic properties in the solid state. The reported results give evidence that a microscopic consideration of the free volume rather than macroscopic parameters like the glass‐transition temperature should to be adopted to interpret the matrix stiffness and its deformation ability. Irradiation performed at higher laser intensity produces periodic superstructures whose height is five to six times as high as the initial film thickness. The surface tension and instability effects are put forward to interpret the growth of such superstructures.

Moiré Patterns in Superimposed Nanoporous Thin Films Derived from Block-Copolymer Assemblies

Nanoscale rotation moire patterns are observed in double-layer nanoporous thin films produced with use of block-copolymer self-assembly. Periodic hexagonal moire superstructures appear when the films possessing long-range order are superimposed at small misorientation angles. Overlapping films misoriented by angles close to 30° generate aperiodic quasi-crystal-like superstructures with 5-fold symmetries. The stacking of the disordered nanoporous films produces labyrinth-like patterns. Block-copolymer moire superstructures are promising as nanolithography masks with controllable morphology and periodicity.