Nanoscale Periodic Structures Research Articles

Optical Characteristics of Stretchable Chiral Liquid Crystal Elastomer under Multiaxial Stretching

AbstractChiral liquid crystal elastomers (CLCEs) are soft photonic materials that exhibit both the photonic characteristics of nanoscale periodic helical structures and mechanical properties of rubber. Owing to its elasticity, the structural color of CLCEs can be tuned through mechanical deformations known as mechanochromism. Thus far, there is significant research attention to exploring the mechanochromism of CLCEs. However, most studies have only discussed the color shifting of CLCEs under uniaxial deformation. Therefore, the optical and chiral structural deformation behaviors of CLCEs under multiaxial stress are not well understood. This study investigates multiaxial (uniaxial, biaxial, and out‐of‐plane) stretching‐induced helical structure change and the resulting optical properties of CLCEs. The results confirm that uniaxial stretching leads to a loss of intrinsic circular polarization selectivity in CLCEs due to helix unwinding deformations, while biaxial and out‐of‐plane stretching maintain circular polarization.

Thermal conduction in titanium-chromium oxide natural superlattices with an ordered arrangement of nearly pristine interfaces

Thermal conduction was investigated in titanium-chromium oxide natural superlattices with an ordered arrangement of crystallographic shear planes, which were nearly pristine interfaces for thermal phonons. Thermal conductivity exhibited a clear maximum value at around 50 K, which indicates interface roughness as small as 20 pm based on the calculation of thermal conductivity by a modified Debye-Callaway model. With an ordered arrangement of nearly pristine interfaces at nanoscale, the wave contribution to phonon transport was estimated to be predominant at lower temperatures (less than 100 K) and as much as 30 % even at room temperature. We found that the interface roughness greatly influences the thermal conduction in natural superlattice systems, which may lead to variations in the reported value of thermal conductivity. We have demonstrated that the wave contribution of thermal conductivity become apparent in bulk crystal having nano scale periodic structure with pristine interfaces.

Control of Metal/Polymer Adhesion Strength by Metal Surface Structure for Sustainable Society

The relationship between the shear strength of the metal-resin and the periodic structure of the metal surface was investigated. Femtosecond laser processing was applied to the Cr-plated carbon tool steel surface to form irregularities with a pitch of about 20 μm and a depth of about 10 μm, and a nano-scale fine periodic structure (so-called LIPSS) was formed on the top of the peaks. The epoxy resin was then placed in a mold and pressed by a heat press onto a metal plate. Then, the molded epoxy resin was pressed from the side with an indenter to measure the shear force, and the shear strength define as the maximum shear force. As a result, a positive correlation was observed between the shear strength and the opening valley angle of the micro-periodic structure, and a negative correlation was observed between the shear strength and the aspect ratio, which is the depth of the micro-periodic structure divided by the pitch.

Mechanochromic Responses of Cholesteric Liquid Crystal Droplets with Nanoscale Periodic Helical Structures Showing Reversible and Tunable Structural Color

Cholesteric liquid crystal (CLC) droplets with nanoscale periodic helical structures are omnidirectional under planar anchoring, and light is selectively reflected from the droplet center at all directions. However, when CLC droplets are too small, random light scattering dominates. In this regard, we present a facile method to increase the selective Bragg reflection of CLC droplets and demonstrate the unique mechanochromic responses of CLC-droplet-dispersed polymer films. The films are originally opaque and white, when the films are bidirectionally stretched, spherical CLC droplets are squeezed into oblate shape and the point defect at the center of each CLC droplet transitions to a ring defect, greatly increasing the selective Bragg reflection region and resulting in the appearance of structural color. By tuning the helical pitches via the chiral dopant concentration from 1.7 to 3.0 wt %, the desired structural color is achieved. Smart sensors show color changes in response to hydration and dehydration are designed, paving the way for the developments of CLC-based functional materials and smart devices.

Nano-scale periodic structures and gap distribution in Pb-doped Bi2223 cuprate superconductors observed by STM/STS

The first systematic study of the nano-scale periodic local density of states (LDOS) and the inhomogeneous gap distributions for the Pb doped Bi2Sr2Ca2Cu3O10 + y (PbBi2223) microcrystals was carried out using the low-temperature scanning tunneling microscopy/spectroscopy. The spatially inhomogeneous gap distributions with patchy structures of several nm size reveal the average gap magnitude Δave ≃ 46 meV with the standard deviation σ = 8 meV. Those features are similar to properties of the widely investigated two-layer Bi-based cuprates. The energy dispersive types of the LDOS modulations are identified as being due to the quasiparticle interference. The relevant scattering vectors seem to testify that the PbBi2223 superconductors are ones, which was shown earlier to be most probable for Bi2212. Three types of energy non-dispersive modulations manifest themselves. Among them, the modulation in the diagonal direction with the period of ∼2.1a0 [ (2π/a0)] seems to be observed for the first time.

A semi-analytical approach for the characterization of ordered 3D nanostructures using grazing-incidence X-ray fluorescence

Following the recent demonstration of grazing-incidence X-ray fluorescence (GIXRF)-based characterization of the 3D atomic distribution of different elements and dimensional parameters of periodic nanoscale structures, this work presents a new computational scheme for the simulation of the angular-dependent fluorescence intensities from such periodic 2D and 3D nanoscale structures. The computational scheme is based on the dynamical diffraction theory in many-beam approximation, which allows a semi-analytical solution to the Sherman equation to be derived in a linear-algebraic form. The computational scheme has been used to analyze recently published GIXRF data measured on 2D Si3N4 lamellar gratings, as well as on periodically structured 3D Cr nanopillars. Both the dimensional and structural parameters of these nanostructures have been reconstructed by fitting numerical simulations to the experimental GIXRF data. Obtained results show good agreement with nominal parameters used in the manufacturing of the structures, as well as with reconstructed parameters based on the previously published finite-element-method simulations, in the case of the Si3N4 grating.

Triazine-Based Two-Dimensional Organic Polymer for Selective NO2 Sensing with Excellent Performance.

Gas sensors with high sensitivity, fast response/recovery, good selectivity, and room-temperature operation are highly desirable for practical use. However, most of the existing gas sensing materials, either conventional metal oxide semiconductors or advanced inorganic two-dimensional (2D) polymers, can hardly satisfy the above requirements. Herein, we demonstrate an organic 2D polymer derived from a covalent triazine framework (CTF), which possesses nanoscale thickness, intrinsic and periodic pore structures, and abundant functional groups with excellent gas sensing performance. The as-prepared triazine-based 2D polymer (T-2DP) exhibits selective recognition to NO2 with an ultrahigh sensitivity of 452.6 ppm-1, which outperforms most other 2D nanomaterials and its CTF matrix. The sensing effect is superfast (35-47 s) and fully reversible operated at room temperature. The superior comprehensive gas sensing performance of T-2DP and the underlying mechanism was experimentally studied and further discussed by comparison with that of CTF and widely investigated inorganic 2D polymers including graphene and MXene. As a proof of concept, a flexible NO2 chemiresistor based on T-2DP was fabricated to demonstrate its potential for integration into wearable electronics. The scientific findings in this work may propose a new route for the design of high-performance gas sensing materials on the basis of organic 2D polymers in next-generation wearable electronic devices.

Fabrication of Regular Hierarchical Structures with Superhydrophobic and High Adhesion Performances on a 304 Stainless Steel Surface via Picosecond Laser

Hierarchical structures significantly influence the development of metal surface wettability. In this study, three kinds of hierarchical structures formed by the superimposition of different nanoscale (quasi-) periodic structures on micro-column arrays were fabricated on 304 stainless steel surfaces via picosecond laser irradiation. Scanning Electron Microscope (SEM) and Confocal Laser Scanning Microscope (CLSM) were used to characterize the created hierarchical structures. An optical contact angle meter was used to analyse the wetting performances. The results show that the surfaces of these fabricated samples have superhydrophobic properties and strong adhesion performances, which can be attributed to the formation of hierarchical structure that causes a reduction in the liquid-solid contact area and the change in the direction of surface tension. By controlling the dimensionof the nanotextures on the micro-column arrays, the hydrophobic property of 304 stainless steel surfaces can be greatly improved.

Complete optical transmission through nano-scale periodic metallic structure with three-stepped slits

Metallic structures with periodic array of subwavelength slits are well-known to lead to extraordinary optical transmission (EOT). Excellent power transmission originates from surface waves excited by incident transverse magnetic wave. Here we show that the metallic structure with array of three-stepped slits can transmit almost the entire power of incident wave into the substrate at desired optical frequency for low-temperature-grown gallium arsenide (LT-GaAs) substrate. Transmitted power of the proposed structure is studied both by an analytical model as well as numerical method by finite element (FEM). It is found that existence of three corners in the proposed structure can favorably confine generated carriers in the vicinity of the electrodes as this structure is used in photoconductive antenna (PCA); this feature contributes to increase terahertz (THz) current and subsequently THz radiation power of PCAs.

Differences in the optical properties of valve and girdle band in a centric diatom.

Diatoms are phototrophic single-celled microalgae encased in a cell wall (frustule) made of amorphous silicate. The frustule comprises two valves connected by a variable number of girdle bands, all exhibiting periodic micro/nanoporous structures. We studied the optical properties in water of girdle bands from the centric diatom Coscinodiscus granii, a frustule part that so far has received little attention by the scientific community. We show that valves and girdle bands exhibit different optical properties, as valves attenuate shorter wavelengths and girdle bands attenuate longer wavelengths of the visible light spectrum. Girdle bands show iridescent coloration in dependence of the light direction. Although the biological meaning of periodic nanoscale structures of frustules is still a matter of debate, the differences of valve and girdle band optical properties indicate that living diatoms are complex optical systems, where valves, girdles and pigments modulate light inside the cell.

Electrochemical Applications of Microphase‐Separated Block Copolymer Thin Films

AbstractIn this review, we discuss electrochemical applications of electrode‐supported thin films derived from microphase‐separated block copolymers (BCPs). In contrast to conventional homopolymers used for electrode modification, BCPs afford periodic nanoscale structures (microdomains) of uniform dimensions (5–100 nm) and predictable morphologies, which can be controlled by adjusting the lengths of individual homopolymer segments. BCPs usable for electrochemical applications consist of two or more segments that form solute‐permeable, redox‐active or etchable microdomains and a chemically inert matrix. The resulting microdomains afford nanoscale molecular/charge transport pathways toward the underlying electrode, whereas the matrix serves as a scaffold to improve film stability in solution. These polymeric thin films provide a unique means for fabricating nanowire arrays via template‐based electrochemical synthesis, designing electrochemical sensors with unique selectivity, and constructing electrochemically‐tunable functional materials.

New Metamaterials with Combined Subnano - and Mesoscale Topology for High-efficiency Catalytic Combustion Chambers of Innovative Gas Turbine Engines

The object of the study is a catalytic combustion chamber that provides a highly efficient combustion process through the use of effects: heat recovery from combustion, microvortex heat transfer, catalytic reaction and acoustic resonance. High efficiency is provided by a complex of related technologies: technologies for combustion products heat transfer (recuperation) to initial mixture, catalytic processes technology, technology for calculating effective combustion processes based on microvortex matrices, technology for designing metamaterials structures and technology for obtaining the required topology product by laser fusion of metal powder compositions. The mesoscale level structure provides combustion process with the use of a microvortex effect with a high intensity of heat and mass transfer. High surface area (extremely high area-to-volume ratio) created due to nanoscale periodic structure and ensures catalytic reactions efficiency. Produced metamaterial is the first multiscale product of new concept which due to combination of different scale level periodic topologies provides qualitatively new set of product properties. This research is aimed at solving simultaneously two global problems of the present: ensure environmental safety of transport systems and power industry, as well as the economy and rational use of energy resources, providing humanity with energy now and in the foreseeable future.

Formation of slantwise orientated nanoscale ripple structures on a single-crystal 4H-SiC surface by time-delayed double femtosecond laser pulses

An effective control of the nanoscale periodic ripple structures on a single-crystal 4H-SiC surface is carried out by temporally delayed double collinear femtosecond laser pulse trains with different linear polarizations. The observed evolution of the ripple slant angle shows a dramatic decrease within an initial time delay range of ~10 ps between the two laser pulses, but keeps almost steady for larger time elapse. Through the detailed analyses, we propose a theoretical model to reveal the underlying physics: regarding the double femtosecond laser pulses, the first laser pulse transfers the semiconductor into nonequilibrium states associated with a transient refractive index grating on the surface, which subsequently tends to change the direction of the surface plasmon excitation of the second laser pulse. These behaviors can eventually result in the slantwise oriented laser intensity fringes and corresponding periodic ablation of grooves. Such investigations provide additional insights into the ripple formation processes, which is very important for control of nanomanufacturing materials by femtosecond lasers.

Three-Dimensional Visualization and Characterization of Polymeric Self-Assemblies by Transmission Electron Microtomography.

Self-assembling structures and their dynamical processes in polymeric systems have been investigated using three-dimensional transmission electron microscopy (3D-TEM). Block copolymers (BCPs) self-assemble into nanoscale periodic structures called microphase-separated structures, a deep understanding of which is important for creating nanomaterials with superior physical properties, such as high-performance membranes with well-defined pore size and high-density data storage media. Because microphase-separated structures have become increasingly complicated with advances in precision polymerization, characterizing these complex morphologies is becoming increasingly difficult. Thus, microscopes capable of obtaining 3D images are required. In this article, we demonstrate that 3D-TEM is an essential tool for studying BCP nanostructures, especially those self-assembled during dynamical processes and under confined conditions. The first example is a dynamical process called order-order transitions (OOTs). Upon changing temperature or pressure or applying an external field, such as a shear flow or electric field, BCP nanostructures transform from one type of structure to another. The OOTs are examined by freezing the specimens in the middle of the OOT and then observing the boundary structures between the preexisting and newly formed nanostructures in three-dimensions. In an OOT between the bicontinuous double gyroid and hexagonally packed cylindrical structures, two different types of epitaxial phase transition paths are found. Interestingly, the paths depend on the direction of the OOT. The second example is BCP self-assemblies under confinement that have been examined by 3D-TEM. A variety of intriguing and very complicated 3D morphologies can be formed even from the BCPs that self-assemble into simple nanostructures, such as lamellar and cylindrical structures in the bulk (in free space). Although 3D-TEM is becoming more frequently used for detailed morphological investigations, it is generally used to study static nanostructures. Although OOTs are dynamical processes, the actual experiment is done in the static state, through a detailed morphological study of a snapshot taken during the OOT. Developing time-dependent nanoscale 3D imaging has become a hot topic. Here, the two main problems preventing the development of in situ electron tomography for polymer materials are addressed. First, the staining protocol often used to enhance contrast for electrons is replaced by a new contrast enhancement based on chemical differences between polymers. In this case, no staining is necessary. Second, a new 3D reconstruction algorithm allows us to obtain a high-contrast, quantitative 3D image from fewer projections than is required for the conventional algorithm to achieve similar contrast, reducing the number of projections and thus the electron beam dose. Combining these two new developments is expected to open new doors to 3D in situ real-time structural observation of polymer materials.

Geometric Prepatterning-Based Tuning of the Period Doubling Onset Strain During Thin-Film Wrinkling

Wrinkling of thin films is an easy-to-implement and low-cost technique to fabricate stretch-tunable periodic micro and nanoscale structures. However, the tunability of such structures is often limited by the emergence of an undesirable period-doubled mode at high strains. Predictively tuning the onset strain for period doubling via existing techniques requires one to have extensive knowledge about the nonlinear pattern formation behavior. Herein, a geometric prepatterning-based technique is introduced that can be implemented even with limited system knowledge to predictively delay period doubling. The technique comprises prepatterning the film/base bilayer with a sinusoidal pattern that has the same period as the natural period of the system. This technique has been verified via physical and computational experiments on the polydimethylsiloxane (PDMS)/glass bilayer system. It is observed that the onset strain can be increased from the typical value of 20% for flat films to greater than 30% with a modest prepattern aspect ratio (2·amplitude/period) of 0.15. In addition, finite element simulations reveal that (i) the onset strain increases with increasing prepattern amplitude and (ii) the delaying effect can be captured entirely by the prepattern geometry. Therefore, one can implement this technique even with limited system knowledge, such as material properties or film thickness, by simply replicating pre-existing wrinkled patterns to generate prepatterned bilayers. Thus, geometric prepatterning is a practical scheme to increase the operating range of stretch-tunable wrinkle-based devices by at least 50%.

Design and Feasibility Evaluation of WGPs Chip as a Physical Force Sensor

Wire grid polarizers (WGPs) are composed of 1-D nanoscale periodic structures and are widely used in liquid crystal display devices to enhance brightness and improve the utilization rate of the backlight source. This letter proposes a new design of a WGPs chip, which can be used in a physical force sensor. In this way, the WGPs can fabricate on many types of microstructure, such as cantilever beam, thin film, and bridge. Then, infrared light was served as the signal source, and illuminated WGPs chip with the optimum angle. When a physical force is applied to the microstructure, the incident angle of infrared light will be varied, resulting in different transmission signal output. As a result, this system can be used for designing the physical force-measuring module operated by optical contactless sensing approach. Furthermore, the WGPs structure introduced in this letter was defined by laser interference lithography and deposited Al by E-beam evaporation.

Controlling domain orientation of liquid crystalline block copolymer in thin films through tuning mesogenic chemical structures

ABSTRACTControlling the macroscopic orientation of nanoscale periodic structures of amphiphilic liquid crystalline block copolymers (LC BCPs) is important to a variety of technical applications (e.g., lithium conducting polymer electrolytes). To study LC BCP domain orientation, a series of LC BCPs containing a poly(ethylene oxide) (PEO) block as a conventional hydrophilic coil block and LC blocks containing azobenzene mesogens is designed and synthesized. LC ordering in thin films of the BCP leads to the formation of highly ordered, microphase‐separated nanostructures, with hexagonally arranged PEO cylinders. Substitution on the tail of the azobenzene mesogen is shown to control the orientation of the PEO cylinders. When the substitution on the mesogenic tails is an alkyl chain, the PEO cylinders have a perpendicular orientation to the substrate surface, provided the thin film is above a critical thickness value. In contrast, when the substitution on the mesogenic tails has an ether group the PEO cylinders assemble parallel to the substrate surface regardless of the film thickness value. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 532–541

Fabrication of high aspect ratio nanoscale periodic structures by the soft X-ray interference lithography

Nanoscale periodic structures have been utilized in the scintillator field to obtain enhanced light extraction efficiency. Sufficient structure depth is necessary to achieve better extraction efficiency. Recently, a soft X-ray interference lithography (SXIL) has been developed in the Shanghai Synchrotron Radiation Facility (SSRF). SXIL can be used to fabricate a high aspect ratio pattern due to the uniform distribution of the beam dose at the photoresist depth. A grating mask with a new photon stop layer was attempted, mainly consisting of Perm alloy, and it was optimized for the SXIL to increase the entire service life. Preliminary results suggest that PMMA structure with an aspect ratio of up to 3 has been successfully manufactured using SXIL techniques. Therefore, this technique has been studied to fabricate the artificial nanostructure on the scintillator in the high efficiency radiation detector area.

Design and structural study of a triple-shape memory PCL/PVC blend

Abstract Poly(e-caprolactone) (PCL) and poly (vinyl chloride) (PVC) are miscible with each other, and their glass transition temperatures differ a lot. With this feature, we tried to design a triple-shape memory material by simply blending them together. After carefully examining the glass transition broadness and the tensile behavior of various PCL/PVC blends, PCL-30% (the weight fraction of PCL was 30%) was picked out as the best candidate. It fulfills two criterions that it has a wide glass transition range and it has no strain or stress induced crystallization. Subsequent triple-shape memory procedure proves that it could function as an effective shape memory blend. The structure was studied by SAXS/WAXS and nano-scale periodic structure was found, which was composed of crystalline and amorphous PCL regions of 4.7 nm and 8.4 nm respectively. Such periodic domains would orient along the stretch direction, but it kept stable and could restore to its original state after the stress was released. Dichromic FTIR discerned the orientation of PCL chains at low temperature and PVC chains at high temperature. It can be concluded that both the nano-scale PCL crystalline structures and physical entanglements served as the fixed phase, while the amorphous PVC and PCL regions played as the reversible phase at high and low switch temperatures respectively.

Role of Acoustoelectric Interaction in the Formation of Nanoscale Periodic Structures of Adsorbed Atoms

The role of acoustoelectric effects in the formation of nanoscale structures of adatoms, resulting from the self-consistent interaction of adatoms with a surface acoustic wave and the electronic subsystem, is studied for the case of charged and uncharged adatoms. It is shown that an increase in the doping level of a semiconductor with donor impurities at a fixed average adatom concentration results in an increase in the critical temperature below which self-organization processes occur.