Widespread approaches to fabricate surfaces with robust micro- and nanostructured topographies have been stimulated by opportunities to enhance interface performance by combining physical and chemical effects. In particular, arrays of asymmetric surface features, such as arrays of grooves, inclined pillars, and helical protrusions, have been shown to impart unique anisotropy in properties including wetting, adhesion, thermal and/or electrical conductivity, optical activity, and capability to direct cell growth. These properties are of wide interest for applications including energy conversion, microelectronics, chemical and biological sensing, and bioengineering. However, fabrication of asymmetric surface features often pushes the limits of traditional etching and deposition techniques, making it challenging to produce the desired surfaces in a scalable and cost-effective manner. We review and classify approaches to fabricate arrays of asymmetric 2D and 3D surface features, in polymers, metals, and ceramics. Analytical and empirical relationships among geometries, materials, and surface properties are discussed, especially in the context of the applications mentioned above. Further, opportunities for new fabrication methods that combine lithography with principles of self-assembly are identified, aiming to establish design principles for fabrication of arbitrary 3D surface textures over large areas.

Engineering of Micro‐ and Nanostructured Surfaces with Anisotropic Geometries and Properties

New synthetic innovations are rapidly being developed to address the demand for complex, next-generation nanomaterials with rigorously controlled architectures and interfaces. This Review highlights key strategies for the chemical transformation and stepwise synthesis of multicomponent inorganic nanostructures, with the existing nanoscale transformations categorized into classes of reactions that are related to those used in the synthesis of organic molecules. The application of concepts used in molecular synthesis—including site-selectivity, regio- and chemoselectivity, orthogonal reactivity, coupling reactions, protection/deprotection strategies, and procedures for separation and purification—to nanoscale systems is emphasized. Collectively, the resulting synthetic concept represents an emerging model for the synthesis of complex inorganic nanostructures on the basis of the guiding principles that underpin the multistep total synthesis of complex organic molecules and natural products.

Emerging strategies for the total synthesis of inorganic nanostructures.

Dr. K. Zhang, M. Ma, M. S. Jung, W. Kim, Prof. J. H. Park Department of Chemical and Biomolecular Engineering Yonsei University 50 Yonsei-ro , Seodaemun-gu , Seoul 120-749 Republic of Korea E-mail: lutts@yonsei.ac.kr Dr. L. Y. Wang, [+] Prof. H. Y. Lee National Creative Research Initiative Center for Smart Molecular Memory Department of Chemistry Energy Science and SAINT Sungkyunkwan University 2066 Seoburo, Jangan-Gu, Suwon , Gyeonggi-Do 440-746 Republic of Korea Prof. X. W. Sheng Department of Physics Anhui Normal University Anhui , Wuhu 241000 , P. R. China M. Ma SKKU Advanced Institute of Nanotechnology Sungkyunkwan University Suwon 440-746 , Republic of Korea W. Kim School of Chemical Engineering Sungkyunkwan University Suwon 440-746 , Republic of Korea

Tunable bandgap energy and promotion of H2O2 oxidation for overall water splitting from carbon nitride nanowire bundles

Considerable progress in the synthesis of anisotropic patchy nanoplates (nanoplatelets) promises a rich variety of highly ordered two-dimensional superlattices. Recent experiments of superlattices assembled from nanoplates confirm the accessibility of exotic phases and motivate the need for a better understanding of the underlying self-assembly mechanisms. Here, we present experimentally accessible, rational design rules for the self-assembly of the Archimedean tilings from polygonal nanoplates. The Archimedean tilings represent a model set of target patterns that (i) contain both simple and complex patterns, (ii) are comprised of simple regular shapes, and (iii) contain patterns with potentially interesting materials properties. Via Monte Carlo simulations, we propose a set of design rules with general applicability to one- and two-component systems of polygons. These design rules, specified by increasing levels of patchiness, correspond to a reduced set of anisotropy dimensions for robust self-assembly ...

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Self-Assembly of Archimedean Tilings with Enthalpically and Entropically Patchy Polygons

The morphology of thin films of molecular and polymeric semiconductors, which is structurally complex and heterogeneous across multiple length scales, is known to significantly affect the device performance. Yet, controlling the film structure is challenging, typically requiring chemical modification of the organic semiconductors, substrates, or the conditions under which the films are formed. Post-deposition processing offers an opportunity to decouple film formation from structural development, providing greater control over molecular ordering in organic semiconductor thin films. This review highlights recent advances in post-deposition processing, focusing specifically on methods that control three important aspects of the film structure – the in-plane and out-of-plane molecular orientations and molecular packing – and correlating these structural changes with device performance in organic thin-film transistors and solar cells.

Directing the film structure of organic semiconductors via post-deposition processing for transistor and solar cell applications

Nanowires (NWs) have a large aspect ratio and large available surface area, which have made them a potential platform for applications in biosensors as well as electronic/optic and power storage de...

Assembly of Gold Nanowires on Gold Nanostripe Arrays: Simulation and Experiment

Anisotropic particles assemble to spontaneously form columnar arrays. Hybrid nanotube/nanowire particles (silica nanotubes partially filled with metallic cores) deposit with their denser metallic ends towards the surface, orienting them vertically. Up to 84% are observed to be standing over a 0.64 cm(2) area within 15 min. Standing percentage is found to be dependent on particle surface concentration.

Vertical arrays of anisotropic particles by gravity-driven self-assembly

Columnar arrays of anisotropic nano- and microparticles, in which the long axes of the particles are oriented perpendicular to the substrate, are of interest for photovoltaics and other applications. Array assembly typically requires applied electric or magnetic fields and/or controlled drying, which are challenging over large areas. Here, we describe a scalable approach to self-assemble multicomponent nanowires into columnar arrays. Self-assembly of partially etched nanowires (PENs) occurred spontaneously during sedimentation from suspension, without drying or applied fields. PENs, which have segments that are either gold or “empty” (solvent-filled) surrounded by a silica shell, were produced from striped metal nanowires by first coating with silica and then removing sacrificial segments by acid etching. Electrostatic repulsion between the particles was necessary for array assembly; however, details of PEN surface chemistry were relatively unimportant. The aspect ratio and relative center of mass (COM) o...

Self-Assembly of Segmented Anisotropic Particles: Tuning Compositional Anisotropy To Form Vertical or Horizontal Arrays

Understanding how micro- and nanoparticles interact is important for achieving bottom-up assembly of desired structures. Here, we examine the self-assembly of two-component, compositionally asymmetric nanocylinders that sediment from solution onto a solid surface. These particles spontaneously formed smectic arrays. Within the rows of an array, nanocylinders tended to assemble such that neighboring particles had the same orientation of their segments. As a probe of interparticle interactions, we classified nanocylinder alignments by measuring the segment orientations of many sets of neighboring particles. Monte Carlo simulations incorporating an exact expression for the van der Waals (vdW) energy indicate that differences in the vdW interactions, even when small, are the key factor in producing observed segment alignment. These results point to asymmetrical vdW interactions as a potentially powerful means of controlling orientation in multicomponent cylinder arrays, and suggest that designing for these interactions could yield new ways to control self-assembly.

Asymmetric van der Waals forces drive orientation of compositionally anisotropic nanocylinders within smectic arrays: experiment and simulation.

Nanowires (NWs) have a large aspect ratio and large available surface area, which have made them a potential platform for applications in biosensors as well as electronic/optic and power storage devices. However, the realization of many of these applications requires the wires to be positioned and oriented through an assembly process. Hence, it is essential to develop assembly strategies and techniques to achieve a variety of desired structures. In this work, we explored NW assembly using a patterned NW–substrate interaction. Experimentally, silica-coated Au nanowires (diameter of 340 nm, lengths of 2.4 and 4.4 μm, 30 nm SiO₂ shell) are allowed to self-assemble onto microfabricated Au features that create a series of “stripes” on a glass substrate (feature height of 50–200 nm; widths of 2.4, 4.5, and 4.8 μm). We observe a rich variety of patterns, with NWs concentrated atop the Au features and oriented perpendicular and diagonal to the stripe axes. We develop a model of this system by considering the relevant van der Waals and electrostatic interactions among NWs and between the NWs and stripes. Monte Carlo simulations of the assembly were performed based on this model, and good agreement with the experiment was achieved. An interesting finding from this work is that extra repulsion at the NW ends plays an important role in determining whether NWs order with their long axes parallel or perpendicular to the Au stripe axis. The simplicity of our approach makes this platform a promising way to achieve more elaborate nanoparticle assemblies in the future.

David J. Kirby

Papers

Assembly of Gold Nanowires on Gold Nanostripe Arrays: Simulation and Experiment

Vertical arrays of anisotropic particles by gravity-driven self-assembly

Self-Assembly of Segmented Anisotropic Particles: Tuning Compositional Anisotropy To Form Vertical or Horizontal Arrays

Asymmetric van der Waals forces drive orientation of compositionally anisotropic nanocylinders within smectic arrays: experiment and simulation.

Assembly of Gold Nanowires on Gold Nanostripe Arrays: Simulation and Experiment