Rapid Fabrication of Two‐ and Three‐Dimensional Colloidal Crystal Films via Confined Convective Assembly

We have developed a self‐assembly method for fabricating well‐ordered two‐dimensional (2D) and three‐dimensional (3D) colloidal crystal films. With a minute amount of a polystyrene colloidal suspension and without any special equipment, the proposed method can be used to rapidly deposit high‐quality colloidal crystal films over a large surface area. By controlling the lift‐up rate of the substrate, we modulate the meniscus thinning rate, which determines whether the colloidal particles are assembled into two or three dimensions. The proposed method can be used to fabricate not only monolayered colloidal crystals with colloidal particles of various sizes, but also multilayered colloidal crystals. In addition, the method enables us to fabricate binary colloidal crystals by consecutively depositing large and small particles.     

Molecular Mimetic Self‐Assembly of Colloidal Particles

This article presents an overview of the current progress in molecular mimetic self‐assembly of colloidal particles. Firstly, the recent study of colloidal particles at interfaces is highlighted, underlining the mesoscopic mimicry of the surface activity of amphiphilic molecules using colloidal particles. Secondly, various strategies developed thus far to impart colloidal particles with anisotropy in terms of chemical composition, surface chemistry and particle morphology, which are regarded as mesoscopic atoms and molecules, are reviewed. Thirdly, an overview of the current theoretical and experimental results of using the rules of molecular synthesis and self‐assembly to direct self‐assembly of colloidal particles is presented. Finally, the experimental challenges associated with molecular mimetic self‐assembly of colloidal particles are outlined, giving a rather conservative conclusion of the status quo of this new research field with a very optimistic outlook.     

Synthetic Self‐Limiting Structures Engineered with Defective Colloidal Clusters

One of the key challenges in the study of self‐assembly with synthetic particles is how to build finite‐sized constructs that resemble self‐limiting structures such as well‐known proteins and biomolecules found in nature. Inspired by this concept, a novel method for realizing self‐limiting self‐assembly of colloidal clusters by establishing design rules to obtain desired final structures using a bottom‐up assembly approach is presented. The constructs identified in this work will be “locked” in a well‐defined configuration as the structure morphologies will not allow them to grow any further. The approach presented here provides two distinct advantages. First, the self‐limiting characteristics of the resulting constructs are preserved no matter how many building blocks are present within the system. Second, the setup of interparticle interactions reflected by interaction matrices are much simpler compared to finite‐sized structures of simple spherical particles which may require engineering pairwise interactions between as many particle types as the number of particles present in the system. The self‐assembly process as well as phase transformation and kinetics of several intriguing finite‐sized configurations are studied. Possible extensions of this concept to produce sophisticated multi‐phased structures where different types of finite‐sized and large assemblies may be present at once are also discussed.     

Transferable Crack‐Free Colloidal Crystals on an Elastomeric Matrix with Surface Relief

Crack‐free three‐dimensional (3D) colloidal silica crystals are fabricated on an elastomeric polydimethylsiloxane (PDMS) stamp via the lift‐up method. A surface relief structure is fabricated on the PDMS substrate to enable the formation of colloidal crystal assemblies that cannot be achieved on a plane PDMS substrate owing to the hydrophobic nature of its surface. Four samples of uniform silica particles having different sizes are prepared for colloidal crystal assembly on PDMS substrates with various relief patterns. This strategy not only provides a means for the assembly of crack‐free colloidal crystals on a soft hydrophobic surface via the lift‐up method but enables the transfer of the crack‐free colloidal crystals onto a curved surface.     

Dimer‐Based Three‐Dimensional Photonic Crystals

The self‐assembly of polystyrene dimer‐ and spherocylinder‐shaped colloids is achieved via controlled drying on glass and silicon substrates. 3D monoclinic colloidal crystal structures are determined from scanning electron microscopy images of sections prepared using focused ion‐beam (FIB) milling. Full photonic bandgaps between the eighth and ninth bands are found for a systematic range of colloidal dimer shapes explored with respect to the degree of constituent lobe fusion and radius ratio. The pseudogap between bands 2 and 3 for spherocylinder‐based monoclinic crystals is also probed using normal incidence reflection spectroscopy.     

From Nanofibers to Nanorods: Nanostructure of Peptide‐Drug Conjugates Regulated by Polypeptide‐PEG Derivative and Enhanced Antitumor Effect

Peptide‐drug conjugates (PDCs) are a type of self‐assembled prodrug with good potential for drug delivery due to their excellent biocompatibility, high drug loading, and permanent controllable release. However, most PDCs tend to self‐assemble into filamentous nanostructures in water and under physiological conditions, making them unsuitable as intravenous formulations due to the entanglement of long fibers and the risk of thrombus. Injected PDCs also face challenges in overcoming the complex physiological environment to reach the target site. To expand their clinical use, it is necessary to control the properties of PDC, including the self‐assembled structure and physiological performance, to avoid the above problems. Based on assembly mechanism studies of PDCs, a new method for regulating PDC morphology is developed by controlling intermolecular interactions in the assembly process. This method can alter the final morphology of PDCs from nanofibers to nanorods, and the introduced macromolecules endow the PDC with new characteristics that facilitate stable and high‐efficiency access to the target site.     

Pseudobilayer Vesicle Formation via Layer‐by‐Layer Assembly of Hydrophobically Modified Polymers on Sacrificial Substrates

A bilayer of a hydrophobically modified polyelectrolyte, octadecyl poly(acrylamide) (PAAm), sandwiched between the layers of a hydrophilic polyelectrolyte, poly(ethyleneimine) (PEI), is prepared by the sequential electrostatic–hydrophobic–electrostatic‐interaction‐driven self‐assembly on planar and colloid substrates. This process results in a PEI/[PAAm]₂/PEI‐multilayer‐coated substrate. The removal of a PAA/PEI/[PAAm]₂/PEI‐multilayer‐coated decomposable colloidal template produces hollow capsules. Irregular hydrophobic domains of the [PAAm]₂ bilayer in the PEI/[PAAm]₂/PEI‐multilayer capsule are infiltrated with a lipid to obtain a uniform, distinct hydrophobic layer, imparting the capsule with a pseudobilayer vesicle structure.     

Fabrication of Multicomponent Microsystems by Directed Three‐Dimensional Self‐Assembly

We have developed a directed self‐assembly process for the fabrication of three‐dimensional (3D) microsystems that contain non‐identical parts and a statistical model that relates the process yield to the process parameters. The self‐assembly process uses geometric‐shape recognition to identify different components, and surface tension between liquid solder and metal‐coated areas to form mechanical and electrical connections. The concept is used to realize self‐packaging microsystems that contain non‐identical subunits. To enable the realization of microsystems that contain more than two non‐identical subunits, sequential self‐assembly is introduced, a process that is similar to the formation of heterodimers, heterotrimers, and higher aggregates found in nature, chemistry, and chemical biology. The self‐assembly of three‐component assemblies is demonstrated by sequentially adding device segments to the assembly solution including two hundred micrometer‐sized light‐emitting diodes (LEDs) and complementary metal oxide semiconductor (CMOS) integrated circuits. Six hundred AlGaInP/GaAs LED segments self‐assembled onto device carriers in two minutes, without defects, and encapsulation units self‐assembled onto the LED‐carrier assemblies to form a 3D circuit path to operate the final device. The self‐assembly process is a well‐defined statistical process. The process follows a first‐order, non‐linear differential equation. The presented model relates the progression of the self‐assembly and yield with the process parameters—component population and capture probability—that are defined by the agitation and the component design.     

Oriented Assembly of Gold Nanorods on the Single‐Particle Level

Non‐spherical colloidal nanoparticles have great potential for applications owing to their enhanced directional properties. However, the lack of methods to precisely assemble them on surfaces has hindered exploitation of their properties for planar devices. Here, the oriented assembly of short gold nanorods with lengths below 100 nm from colloidal suspensions is demonstrated. A locally induced phase transition confines the colloidal nanorods at a receding three‐phase contact line that is controllably moved over a nanostructured surface in a capillary assembly process. Dedicated topographical trapping sites allow for aligned assembly of the nanorods on the single‐particle level. The feasibility of this method is demonstrated by assembling nanorods into long‐range‐ordered, non‐close packed arrays that could serve as anti‐counterfeit labels by virtue of their distinct optical appearance in the far‐field. Furthermore, oriented nanorod dimers that are deterministically assembled have the potential to function as nano‐plasmonic antenna devices.     

Directed Self‐Assembly of Gradient Concentric Carbon Nanotube Rings

Hundreds of gradient concentric rings of linear conjugated polymer, (poly[2‐methoxy‐5‐(2‐ethylhexyloxy)‐1,4‐ phenylenevinylene], i.e., MEH‐PPV) with remarkable regularity over large areas were produced by controlled “stick‐slip” motions of the contact line in a confined geometry consisting of a sphere on a flat substrate (i.e., sphere‐on‐flat geometry). Subsequently, MEH‐PPV rings were exploited as a template to direct the formation of gradient concentric rings of multiwalled carbon nanotubes (MWNTs) with controlled density. This method is simple, cost effective, and robust, combining two consecutive self‐assembly processes, namely, evaporation‐induced self‐assembly of polymers in a sphere‐on‐flat geometry, followed by subsequent directed self‐assembly of MWNTs on the polymer‐templated surfaces.     

Hierarchical Self‐Organization of Nanomaterials into Two‐Dimensional Arrays Using Functional Polymer Scaffold

Fabrication of two and three‐dimensional nanostructures requires the development of new methodologies for the assembly of molecular/macromolecular objects on substrates in predetermined arrangements. Templated self‐assembly approach is a powerful strategy for the creation of materials from assembly of molecular components or nanoparticles. The present study describes the development of a facile, template directed self‐assembly of (metal/organic) nanomaterials into periodic micro‐ and nanostructures. The positioning and the organization of nanomaterials into spatially well‐defined arrays were achieved using an amphiphilic conjugated polymer‐aided, self‐organization process. Arrays of honeycomb patterns formed from conjugated C₁₂PPPOH film with homogenous distribution of metal/organic nanomaterials. Our approach offers a straightforward and inexpensive method of preparation for hybrid thin films without environmentally controlled chambers or sophisticated instruments as compared to multistep micro‐fabrication techniques.     

Multicolor Printing Using Electric‐Field‐Responsive and Photocurable Photonic Crystals

Efficient and large scale printing of photonic crystal patterns with multicolor, multigrayscale, and fine resolution is highly desired due to its application in smart prints, sensors, and photonic devices. Here, an electric‐field‐assisted multicolor printing is reported based on electrically responsive and photocurable colloidal photonic crystal, which is prepared by supersaturation‐induced self‐assembly of SiO₂ particles in the mixture of propylene carbonate (PC) and trimethylolpropane ethoxylate triacrylate (ETPTA). This colloidal crystal suspension, named as E‐ink, has tunable structural color, controllable grayscale, and instantly fixable characteristics at the same time because the SiO₂/ETPTA‐PC photonic crystal has metastable and reversible assembly as well as polymerizable features. Lithographical printing with photomask and maskless pixel printing techniques are developed respectively to efficiently prepare multicolor and high‐resolution photonic patterns using a single‐component E‐ink.     

Cover Picture: Fabrication of Multicomponent Microsystems by Directed Three‐Dimensional Self‐Assembly (Adv. Funct. Mater. 5/2005)

Directed three‐dimensional self‐assembly to assemble and package integrated semiconductor devices is demonstrated by Jacobs and Zheng on p. 732. The self‐assembly process uses geometrical shape recognition to identify different components and surface‐tension between liquid solder and metal‐coated areas to form mechanical and electrical connections.The components (top left) self‐assemble in a turbulent flow (center) and form functional multi‐component microsystems (bottom right) by sequentially adding parts to the assembly solution. The technique provides, for the first time, a route to enable the realization of three‐dimensional heterogeneous microsystems that contain non‐identical parts, and connecting them electrically. We have developed a directed self‐assembly process for the fabrication of three‐dimensional (3D) microsystems that contain non‐identical parts and a statistical model that relates the process yield to the process parameters. The self‐assembly process uses geometric‐shape recognition to identify different components, and surface tension between liquid solder and metal‐coated areas to form mechanical and electrical connections. The concept is used to realize self‐packaging microsystems that contain non‐identical subunits. To enable the realization of microsystems that contain more than two non‐identical subunits, sequential self‐assembly is introduced, a process that is similar to the formation of heterodimers, heterotrimers, and higher aggregates found in nature, chemistry, and chemical biology. The self‐assembly of three‐component assemblies is demonstrated by sequentially adding device segments to the assembly solution including two hundred micrometer‐sized light‐emitting diodes (LEDs) and complementary metal oxide semiconductor (CMOS) integrated circuits. Six hundred AlGaInP/GaAs LED segments self‐assembled onto device carriers in two minutes, without defects, and encapsulation units self‐assembled onto the LED‐carrier assemblies to form a 3D circuit path to operate the final device. The self‐assembly process is a well‐defined statistical process. The process follows a first‐order, non‐linear differential equation. The presented model relates the progression of the self‐assembly and yield with the process parameters—component population and capture probability—that are defined by the agitation and the component design.     

Layer‐by‐Layer Growth of Multicomponent Colloidal Crystals Over Large Areas

Self‐assembly of different sized colloidal particles into multicomponent crystals results in novel material properties compared to the properties of the individual components alone. The formation of binary and, for the first time, ternary colloidal crystals through a simple and inexpensive confined‐area evaporation‐induced layer‐by‐layer (LBL) assembly method is reported. The proposed method produces high quality multicomponent colloidal crystal films over a broad range of particle size‐ratios and large surface areas (cm²) from silica/polystyrene colloidal suspensions of low concentration. By adjusting the size‐ratio and concentration of the colloidal particles, complex crystals of tunable stoichiometries are fabricated and their structural characteristics are further confirmed with reported crystal analogues. In addition, complex structures form as a result of the interplay of the template layer effect, the surface forces exerted by the meniscus of the drying liquid, the space filling principle, and entropic forces. Thus, this LBL approach is a versatile way to grow colloidal crystals with binary, ternary, or more complex structures.     

Artificial Defect Engineering in Three‐Dimensional Colloidal Photonic Crystals

Artificial defect engineering in 3D colloidal photonic crystals is of paramount importance in terms of device applications. Over the past few years, we have carried out a great deal of research on introducing artificial defects, including point, line, and planar defects, in 3D colloidal photonic crystals by using “bottom‐up” self‐assembly in combination with “top‐down” micromachining techniques. In this Feature Article, we summarize our research results regarding the engineering of artificial defects in self‐assembled 3D photonic crystals, along with other important research breakthroughs in the literature. The significant advancements in the engineering of defects as reviewed here together with the encouraging reports on the fabrication of perfect colloidal crystals without unwanted defects will collectively lead to technological applications of self‐assembled 3D photonic crystals in the near future.     

Self‐Assembled SERS Substrates with Tunable Surface Plasmon Resonances

The fabrication of surface‐enhanced Raman spectroscopy (SERS) substrates that are optimized for use with specific laser wavelength–analyte combinations is addressed. In order to achieve large signal enhancement, temporal stability, and reproducibility over large substrate areas at low cost, only self‐assembly and templating processes are employed. The resulting substrates consist of arrays of gold nanospheres with controlled diameter and spacing, properties that dictate the optical response of the structure. Tunability of the extended surface plasmon resonance is observed in the range of 520–1000 nm. It is demonstrated that the enhancement factor is maximized when the surface plasmon resonance is red‐shifted with respect to the SERS instrument laser line. Despite relying on self‐organization, site‐to‐site enhancement factor variations smaller than 10% are obtained.     

Micellar‐Mediated Block Copolymer Ordering Dynamics Revealed by In Situ Grazing Incidence Small‐Angle X‐Ray Scattering during Spin Coating

Spin coating is one of the most versatile methods to generate nanostructured block copolymer (BCP) thin films which are highly desired for many applications such as nanolithography or organic electronics. The self‐assembly pathways through phase separation, both in solvent and in bulk, strongly influence the final BCP structure obtained after spin coating. As a demonstration, the formation of highly ordered in‐plane lamellae is elucidated herein by using in situ grazing incidence small‐angle X‐ray scattering. A key step in this complex fast organization process is the formation of intermediate micellar phases triggered by solvent affinity toward one of the block. Indeed, directional coalescence of a short‐lived intermediate hexagonal structure of cylindrical micelles enables the development of a final highly ordered lamellar structure, predominantly oriented parallel to the substrate surface. These results suggest that the existence of such transient micellar phases is a crucial process in order to produce highly ordered structures with a specific orientation directly after the BCP thin film deposition; and should be the focus of further optimization for the directed self‐assembly and, more generally, in the bottom‐up nanostructure fabrication.     

Controlled Assembly of Poly(3‐hexylthiophene): Managing the Disorder to Order Transition on the Nano‐ through Meso‐Scales

Self‐assembly of conjugated organic semiconductors into ordered, larger scale entities is a critical process to achieve efficient charge transport at the nano‐ through macro‐scales, and various methodologies aimed at enhancing molecular ordering have been introduced. However, mechanistic understanding is limited. Here, a mechanistic elucidation of poly(3‐hexylthiophene) (P3HT) molecular self‐assembly is proposed based on experimental demonstration of controlled, solution‐based P3HT self‐assembly into rod‐like polycrystalline nanostructures. The synergistic combination of nonsolvent addition and ultrasonication facilitates rod‐like P3HT nanostructure formation in solution. Importantly, through sequential application of both treatments, nanostructure length can be easily modulated, and the assembly process is shown to follow a simple 2‐step crystallization model, which depends upon nucleation followed by growth. Through arrays of experimental results, the validity of 2‐step crystallization is confirmed and is proposed as a comprehensive platform to understand self‐assembly processes of conjugated polymers into larger, ordered mesoscale entities.     

A Universal Approach to Fabricate Various Nanoring Arrays Based on a Colloidal‐Crystal‐Assisted‐Lithography Strategy

In this paper a convenient and universal strategy for preparing nanoring arrays of different compositions based on a colloidal‐crystal‐template strategy is reported. Large‐area arrays of polystyrene, magnetite, Au, Si, magnetite nanoparticle/polystyrene and Au/polystyrene double‐layer composite nanorings are prepared. Many kinds of nanoring structures, including Fe₃O₄ nanoparticle/polystyrene and Au/polystyrene double‐layer nanorings, can be released from the substrates, resulting in free‐standing composite nanorings, which might be used as self‐assembly building blocks and ultrasensitive bio‐ and chemical sensors.     

Direct Writing of Three‐Dimensional Polymer Scaffolds Using Colloidal Gels

Polymer scaffolds intended to provide a substrate for cell attachment and proliferation benefit if the geometric architecture, mechanical properties, and surface chemistry are controllable within the range applicable for the target tissue. Such scaffolds may be made bioinductive through the inclusion of surface proteins and release of growth factors. Furthermore, the polymer support may be formed of biodegradable polymers for use as tissue‐engineering scaffolds. In this study, a new scaffold‐fabrication technique based on the direct writing of polymer colloidal‐gel‐based inks is described. The colloidal approach allows for the modular design of inks where the structure and composition of the colloidal particles, surface adsorbed molecules, and dissolved species may be easily controlled. Polyacrylate latex particles are formulated into colloidal gels by using a thermoreversible gel‐forming poly(ethylene oxide)–poly(propylene oxide) block‐copolymer adsorbed layer. The resulting colloidal gels are laced with the model protein bovine serum albumin (BSA) either dissolved in the solvent phase of the ink or dispersed in chitosan nanoparticles as a second colloid. Ink development and rheological characterization are presented along with demonstration of assembly of mesoporous scaffolds. After assembly and drying of the scaffold structure, the drug‐release kinetics are measured upon re‐exposure to an aqueous environment. Protein activity appears to be unaffected by the processing route of these scaffolds. Finally, the assembly of heterogeneous scaffolds is demonstrated to illustrate the possibilities for staged or heterogeneous drug release. This approach to scaffold fabrication offers a new route for scaffold assembly from water‐insoluble polymers while allowing the inclusion of sensitive biomolecules without risk of denaturation.