Superstructures of Self‐Organizing Thiophenes

The self‐assembling properties of π‐conjugated oligo‐, poly‐, and cyclothiophenes (see Figure) have been studied with different techniques. Scanning tunneling microscopy on perfectly ordered two‐dimensional monolayers at the solution–HOPG (highly oriented pyrolytic graphite) interface has successfully been applied. The submolecular resolution achieved in the STM images provides valuable lattice and molecular information. Supported by X‐ray structure analyses of the 3D bulk material and by theoretical calculations, molecular conformations, molecule–molecule, and molecule–substrate interactions of the oligo‐, poly‐, and cyclomers have been analyzed and discussed.     

Fabrication of Low‐Dimension Nanostructures Based on Organic Conjugated Molecules

The aim of this review is to explore the fundamental and technological incentive for the molecular design, synthesis, and aggregation behavior of conjugated organic molecules with photo‐electronic activity, and the fabrication of low‐dimensional organic conjugated nanomaterials by self‐assembly techniques. The properties of large oriented nanostructure arrays of organic charge transfer complexes based on conjugated molecules are also discussed. The dimension‐dependent emission properties have been observed, and conductivity, field emission properties, and sensing properties have been studied for the low dimensional nanostructures of nanoparticles, nanowires, nanorods, and nanostructure arrays.

     

A two‐and‐a‐half‐dimensional displacement‐based PML for elastodynamic wave propagation

This paper presents a perfectly matched layer (PML) technique for the numerical simulation of three‐dimensional linear elastodynamic problems, where the geometry is invariant in the longitudinal direction. Examples include transportation infrastructure, dams, lifelines, and alluvial valleys. For longitudinally invariant geometries, a computationally efficient two‐and‐a‐half‐dimensional (2.5D) approach can be applied, where the Fourier transform from the longitudinal coordinate to the wavenumber domain allows for the representation of the three‐dimensional radiated wave field on a two‐dimensional mesh. In this 2.5D framework, the equilibrium equations of a PML continuum are formulated in a weak form for an isotropic elastodynamic medium and discretized using a Galerkin approach. The 2.5D PML methodology is validated by computing the Green's displacements of a homogeneous halfspace, demonstrating that the 2.5D PML absorbs all propagating waves for different angles of incidence. Furthermore, the dynamic stiffness of a rigid strip foundation and the efficiency of a vibration isolating screen are computed. The examples demonstrate that the PML methodology is computationally efficient, especially when only the response of the structure or the near field response is of interest.Copyright © 2011 John Wiley & Sons, Ltd.     

Topology optimization using non‐conforming finite elements: three‐dimensional case

As in the case of two‐dimensional topology design optimization, numerical instability problems similar to the formation of two‐dimensional checkerboard patterns occur if the standard eight‐node conforming brick element is used. Motivated by the recent success of the two‐dimensional non‐conforming elements in completely eliminating checkerboard patterns, we aim at investigating the performance of three‐dimensional non‐conforming elements in controlling the patterns that are estimated overly stiff by the brick elements. To this end, we will investigate how accurately the non‐conforming elements estimate the stiffness of the patterns. The stiffness estimation is based on the homogenization method by assuming the periodicity of the patterns. To verify the superior performance of the elements, we consider three‐dimensional compliance minimization and compliant mechanism design problems and compare the results by the non‐conforming element and the standard 8‐node conforming brick element. Copyright © 2005 John Wiley & Sons, Ltd.     

Time‐domain BEM for three‐dimensional site response analysis of topographic structures

This paper presents a formulation of a time‐domain three‐dimensional boundary element method for site response analysis of topographic structures. The boundary element algorithm that uses the presented time‐convoluted traction kernels is applied to site response analyses of topographic structures. The seismic responses of canyon and ridge subjected to incident P and S waves are analyzed to demonstrate the accuracy of the kernels and the applicability of the presented boundary element algorithm for site response analysis of topographic structures. Seismic response analyses of three‐dimensional Gaussian‐shaped ridges show that the three‐dimensional axisymmetric ridge has a more amplification potential compared with three‐dimensional non‐axisymmetric elongated and two‐dimensional ridges, if the ridge is impinged by incident waves with wavelength of about the ridge's width. Copyright© 2009 John Wiley & Sons, Ltd.     

Fabrication of Hierarchical Pillar Arrays from Thermoplastic and Photosensitive SU‐8

By exploiting the thermoplastic and photosensitive nature of SU‐8 photoresists, different types of hierarchical pillar arrays with variable aspect ratios are fabricated through capillary force lithography (CFL), followed by photopatterning. The thermoplastic nature of SU‐8 enables the imprinting of micropillar arrays with variable aspect ratios by CFL using a single poly(dimethylsiloxane) mold, simply by tuning the initial film thickness of SU‐8 on a substrate. The pillar array is subsequently photopatterned through a photomask, followed by post‐exposure baking above the glass transition temperature (T_g) of SU‐8. The pillars in the exposed region become highly crosslinked and, therefore, neither soluble nor able to reflow above T_g, whereas the pillars in the unexposed regions can reflow and flatten out. Two developing strategies are investigated after UV exposure of the SU‐8 pillar arrays including i) solvent development and drying and ii) thermal reflow to create bilevel hierarchical structures with short pillars and single‐level, dual‐scaled, high‐aspect‐ratio (up to 7.7) pillars in a microdot array, respectively.     

c‐Type method of unified CAMG and FEA. Part 1: Beam and arch mega‐elements—3D linear and 2D non‐linear

Computer‐aided mesh generation (CAMG) dictated solely by the minimal key set of requirements of geometry, material, loading and support condition can produce ‘mega‐sized’, arbitrary‐shaped distorted elements. However, this may result in substantial cost saving and reduced bookkeeping for the subsequent finite element analysis (FEA) and reduced engineering manpower requirement for final quality assurance. A method, denoted as c‐type, has been proposed by constructively defining a finite element space whereby the above hurdles may be overcome with a minimal number of hyper‐sized elements. Bezier (and de Boor) control vectors are used as the generalized displacements and the Bernstein polynomials (and B‐splines) as the elemental basis functions. A concomitant idea of coerced parametry and inter‐element continuity on demand unifies modelling and finite element method. The c‐type method may introduce additional control, namely, an inter‐element continuity condition to the existing h‐type and p‐type methods. Adaptation of the c‐type method to existing commercial and general‐purpose computer programs based on a conventional displacement‐based finite element method is straightforward. The c‐type method with associated subdivision technique can be easily made into a hierarchic adaptive computer method with a suitable a posteriori error analysis. In this context, a summary of a geometrically exact non‐linear formulation for the two‐dimensional curved beams/arches is presented. Several beam problems ranging from truly three‐dimensional tortuous linear curved beams to geometrically extremely non‐linear two‐dimensional arches are solved to establish numerical efficiency of the method. Incremental Lagrangian curvilinear formulation may be extended to overcome rotational singularity in 3D geometric non‐linearity and to treat general material non‐linearity. Copyright © 2003 John Wiley & Sons, Ltd.     

Light‐Driven ZnO Brush‐Shaped Self‐Propelled Micromachines for Nitroaromatic Explosives Decomposition

Self‐propelled micromachines have recently attracted lots of attention for environmental remediation. Developing a large‐scale but template‐free fabrication of self‐propelled rod/tubular micro/nanomotors is very crucial but still challenging. Here, a new strategy based on vertically aligned ZnO arrays is employed for the large‐scale and template‐free fabrication of self‐propelled ZnO‐based micromotors with H₂O₂‐free light‐driven propulsion ability. Brush‐shaped ZnO‐based micromotors with different diameters and lengths are fully studied, which present a fast response to multicycles UV light on/off switches with different interval times (2/5 s) in pure water and slow directional motion in aqueous hydrogen peroxide solution in the absence of UV light. Light‐induced electrophoretic and self‐diffusiophoretic effects are responsible for these two different self‐motion behaviors under different conditions, respectively. In addition, the pH of the media and the presence of H₂O₂ show important effects on the motion behavior and microstructure of the ZnO‐based micromotors. Finally, these novel ZnO‐based brush‐shaped micromotors are demonstrated in a proof‐of‐concept study on nitroaromatic explosive degradation, i.e., picric acid. This work opens a completely new avenue for the template‐free fabrication of brush‐shaped light‐responsive micromotors on a large scale based on vertically aligned ZnO arrays.     

Three‐dimensional Displacement and Shape Measurement with a Diffraction‐assisted Grid Method

The grid method is a full‐field optical technique for computing surface displacements and strains of a material by analyzing the phase of grid lines patterned on the specimen. To date, most experiments using the grid method have measured only two‐dimensional in‐plane deformations. Here, the grid method is extended to three dimensions by using a crossed grid pattern and a diffraction grating which enables acquiring images from multiple viewing angles on a single camera. In‐plane displacements and strains are computed using the conventional grid method, and the corresponding three‐dimensional (3D) displacements—including out‐of‐plane displacements or shapes—are computed by analyzing the images collected at different viewing angles. The technique is demonstrated by measuring 3D rigid body motion, the 3D displacements of a membrane in a pressure‐bulge experiment, and the out‐of‐plane curvature of a cylindrical specimen.     

Flexible High‐Conductivity Carbon‐Nanotube Interconnects Made by Rolling and Printing

Applications of carbon nanotubes (CNTs) in flexible and complementary metal‐oxide‐semiconductor (CMOS)‐based electronic and energy devices are impeded due to typically low CNT areal densities, growth temperatures that are incompatible with device substrates, and challenges in large‐area alignment and interconnection. A scalable method for continuous fabrication and transfer printing of dense horizontally aligned CNT (HA‐CNT) ribbon interconnects is presented. The process combines vertically aligned CNT (VA‐CNT) growth by thermal chemical vapor deposition, a novel mechanical rolling process to transform the VA‐CNTs to HA‐CNTs, and adhesion‐controlled transfer printing without needing a carrier film. The rolling force determines the HA‐CNT packing fraction and the HA‐CNTs are processed by conventional lithography. An electrical resistivity of 2 mΩ · cm is measured for ribbons having 800‐nm thickness, while the resistivity of copper is 100 times lower, a value that exceeds most CNT assemblies made to date, and significant improvements can be made in CNT structural quality. This rolling and printing process could be scaled to full wafer areas and more complex architectures such as continuous CNT sheets and multidirectional patterns could be achieved by straightforward design of the CNT growth process and/or multiple rolling and printing sequences.     

One‐dimensional analysis of thin‐walled closed beams having general cross‐sections

This paper presents a new one‐dimensional theory for static and dynamic analysis of thin‐walled closed beams with general cross‐sections. Existing one‐dimensional approaches are useful only for beams with special cross‐sections. Coupled deformations of torsion, warping and distortion are considered in the present work and a new approach to determine sectional warping and distortion shapes is proposed. One‐dimensional C⁰ beam elements based on the present theory are employed for numerical analysis. The effectiveness of the present theory is demonstrated in the analysis of thin‐walled beams having pillar sections of automobiles and excavators. Copyright © 2000 John Wiley & Sons, Ltd.     

Mechanical properties of rock specimens containing pre‐existing flaws with 3D printed materials

Three‐dimensional printing (3DP) technology has undergone a rapid development in the last few years and become a useful tool in many research fields. This study applied 3DP technology to prepare solid specimens simulating rock‐type materials combined with computed tomography scanning and 3D image processing. 3DP specimens with pre‐existing flaws in different inclination angles were fabricated and then conducted a series of mechanical experiments to study the influence of number and inclination angle of pre‐existing flaw on strength and failure patterns under uniaxial compression. The experimental results indicated that 3DP specimens had similar mechanical properties with rock‐type materials. The 3DP specimens with 2 pre‐existing flaws had lower compressive strength with an average of 4.26 MPa, whereas compressive strength of specimens with one flaw was no less than 5.08 MPa. Different inclination angles led to various failure patterns and compressive strengths, which took on a V‐shaped curve with the increase of inclination angles. This study demonstrated that 3DP technology provided a new perspective for conducting laboratory experimental research of rock mechanics.     

Three‐dimensional measurement and cohesive element modelling of deformation and damage in a 2.5‐dimensional woven ceramic matrix composite

A cohesive element numerical model, which reproduces the three‐dimensional microstructure of a 2.5‐dimensional silicon‐nitrogen‐oxide fibre/fabric‐reinforced boron nitride ceramic matrix composite (SiNO/BN) is applied to simulate the failure of specimens that are observed in situ during diametral compression testing. Measurements of deformation by image correlation of two‐dimensional optical surface observations and three‐dimensional X‐ray computed tomographs are used to fit the simulation's elastic properties for the matrix and fibre tows. The observed patterns of damage nucleation and propagation are correctly simulated using a local tensile strain criterion.     

Finite element methods for the non‐linear mechanics of crystalline sheets and nanotubes

The formulation and finite element implementation of a finite deformation continuum theory for the mechanics of crystalline sheets is described. This theory generalizes standard crystal elasticity to curved monolayer lattices by means of the exponential Cauchy–Born rule. The constitutive model for a two‐dimensional continuum deforming in three dimensions (a surface) is written explicitly in terms of the underlying atomistic model. The resulting hyper‐elastic potential depends on the stretch and the curvature of the surface, as well as on internal elastic variables describing the rearrangements of the crystal within the unit cell. Coarse grained calculations of carbon nanotubes (CNTs) are performed by discretizing this continuum mechanics theory by finite elements. A smooth discrete representation of the surface is required, and subdivision finite elements, proposed for thin‐shell analysis, are used. A detailed set of numerical experiments, in which the continuum/finite element solutions are compared to the corresponding full atomistic calculations of CNTs, involving very large deformations and geometric instabilities, demonstrates the accuracy of the proposed approach. Simulations for large multi‐million systems illustrate the computational savings which can be achieved. Copyright © 2003 John Wiley & Sons, Ltd.     

Microcontact Printing with Laser Direct Writing Carbonization for Facile Fabrication of Carbon‐Based Ultrathin Disk Arrays and Ordered Holey Films

A nanometer‐thick carbon film with a highly ordered pattern structure is very useful in a variety of applications. However, its large‐scale, high‐throughput, and low‐cost fabrication is still a great challenge. Herein, microcontact printing (µCP) and direct laser writing carbonization (DLWc) are combined to develop a novel method that enables ease of fabrication of nanometer‐thick and regularly patterned carbon disk arrays (CDAs) and holey carbon films (HCFs) from a pyromellitic dianhydride‐oxydianiline‐based polyamic acid (PAA) solution. The effect of PAA concentration and pillar lattice structure of the polydimethyl siloxane stamp are systematically studied for their influence on the geometrical parameter, surface morphology, and chemical structure of the finally achieved CDAs and HCFs. Within the PAA concentration being investigated, the averaged thickness of CDAs and HCFs can be tailored in a range from a few tens to a few hundred of nanometers. The µCP+DLWc‐enabled electrically conductive CDAs and HCFs possess the characteristics of ease‐of‐fabrication, nanometer‐thickness, highly regular and controlled patterns and structures, and the ability to form on both hard and soft substrates, which imparts usefulness in electronics, photonics, energy storage, catalysis, tissue engineering, as well as physical, chemical, and bio‐sensing applications.     

From Flatland to Spaceland: Higher Dimensional Patterning with Two‐Dimensional Materials

The creation of three‐dimensional (3D) structures from two‐dimensional (2D) nanomaterial building blocks enables novel chemical, mechanical or physical functionalities that cannot be realized with planar thin films or in bulk materials. Here, we review the use of emerging 2D materials to create complex out‐of‐plane surface topographies and 3D material architectures. We focus on recent approaches that yield periodic textures or patterns, and present four techniques as case studies: (i) wrinkling and crumpling of planar sheets, (ii) encapsulation by crumpled nanosheet shells, (iii) origami folding and kirigami cutting to create programmed curvature, and (iv) 3D printing of 2D material suspensions. Work to date in this field has primarily used graphene and graphene oxide as the 2D building blocks, and we consider how these unconventional approaches may be extended to alternative 2D materials and their heterostructures. Taken together, these emerging patterning and texturing techniques represent an intriguing alternative to conventional materials synthesis and processing methods, and are expected to contribute to the development of new composites, stretchable electronics, energy storage devices, chemical barriers, and biomaterials.     

Additive Manufacturing of 3D‐Architected Multifunctional Metal Oxides

Additive manufacturing (AM) of complex three‐dimensional (3D) metal oxides at the micro‐ and nanoscales has attracted considerable attention in recent years. State‐of‐the‐art techniques that use slurry‐based or organic–inorganic photoresins are often hampered by challenges in resin preparation and synthesis, and/or by the limited resolution of patterned features. A facile process for fabricating 3D‐architected metal oxides via the use of an aqueous metal‐ion‐containing photoresin is presented. The efficacy of this process, which is termed photopolymer complex synthesis, is demonstrated by creating nanoarchitected zinc oxide (ZnO) architectures with feature sizes of 250 nm, by first patterning a zinc‐ion‐containing aqueous photoresin using two‐photon lithography and subsequently calcining them at 500 ºC. Transmission electron microscopy (TEM) analysis reveals their microstructure to be nanocrystalline ZnO with grain sizes of 5.1 ± 1.6 nm. In situ compression experiments conducted in a scanning electron microscope show an emergent electromechanical response: a 200 nm mechanical compression of an architected ZnO structure results in a voltage drop of 0.52 mV. This photopolymer complex synthesis provides a pathway to easily create arbitrarily shaped 3D metal oxides that could enable previously impossible devices and smart materials.     

Rapid and Low‐Cost Prototyping of 3D Nanostructures with Multi‐Layer Hydrogen Silsesquioxane Scaffolds

A layer‐by‐layer (LBL) method can generate or approximate any three‐dimensional (3D) structure, and has been the approach for the manufacturing of complementary metal‐oxide‐semiconductor (CMOS) devices. However, its high cost precludes the fabrication of anything other than CMOS‐compatible devices, and general 3D nanostructures have been difficult to prototype in academia and small businesses, due to the lack of expensive facility and state‐of‐the‐art tools. It is proposed and demonstrated that a novel process that can rapidly fabricate high‐resolution three‐dimensional (3D) nanostructures at low cost, without requiring specialized equipment. An individual layer is realized through electron‐beam lithography patterning of hydrogen silsesquioxane (HSQ) resist, followed by planarization via spinning SU‐8 resist and etch‐back. A 4‐layer silicon inverse woodpile photonic crystal with a period of 650 nm and a 7‐layer HSQ scaffold with a period of 300 nm are demonstrated. This process provides a versatile and accessible solution to the fabrication of highly complex 3D nanostructures.     

Modelling three‐dimensional piece‐wise homogeneous domains using the α‐shape‐based natural element method

In this paper, the application of the natural element method (NEM) to the numerical analysis of two‐ and three‐dimensional piece‐wise homogeneous domains is presented. The NEM differs from other meshless methods in its capability to accurately reproduce essential boundary conditions along convex boundaries. The α‐shape‐based extension of this method (α‐NEM) generalizes this behaviour to non‐convex domains, enables us to construct models entirely in terms of the initial cloud of points and allows us to simulate material discontinuities in a straightforward manner. In the following sections, simple and effective algorithms are presented for the construction of α‐shapes in domains composed of various materials. Examples are presented in two‐ and three‐dimensional cases in the context of linear elastostatics showing good performance even with the simple numerical quadrature used. Copyright © 2002 John Wiley & Sons, Ltd.     

Numerical accuracy of a Padé‐type non‐reflecting boundary condition for the finite element solution of acoustic scattering problems at high‐frequency

The present text deals with the numerical solution of two‐dimensional high‐frequency acoustic scattering problems using a new high‐order and asymptotic Padé‐type artificial boundary condition. The Padé‐type condition is easy‐to‐implement in a Galerkin least‐squares (iterative) finite element solver for arbitrarily convex‐shaped boundaries. The method accuracy is investigated for different model problems and for the scattering problem by a submarine‐shaped scatterer. As a result, relatively small computational domains, optimized according to the shape of the scatterer, can be considered while yielding accurate computations for high‐frequencies. Copyright © 2005 John Wiley & Sons, Ltd.