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Photonic crystals are extensively explored to replace inorganic pigments and organic dyes as coloring elements in printing, painting, sensing, and anti-counterfeiting due to their brilliant structural colors, chemical stability, and environmental friendliness. However, most existing photonic-crystal-based pigments can only display monochromatic colors once made, and generating multicolors has to start with designing different building blocks. Here, a novel photonic pigment featuring highly tunable structural colors in the entire visible spectrum, made by the magnetic assembly of monodisperse nanorods into body-centered-tetragonal photonic crystals, is reported. Their prominent magnetic and crystal anisotropy makes it efficient to generate multicolors using one photonic pigment by magnetically controlling the crystal orientation. Further, the combination of angle-dependent diffraction and magnetic orientation control enables the design of rotation-asymmetric photonic films that display distinct patterns and encrypted information in response to rotation. The efficient multicolor generation through precise orientational control makes this novel photonic pigment promising in developing high-performance structural-colored materials and optical devices.

Photothermal actuators have attracted increasing attention due to their ability to convert light energy into mechanical deformation and locomotion. This work reports a freestanding, multidirectional photothermal robot that can walk along a predesigned pathway by modulating laser polarization and on-off switching. Magnetic-plasmonic hybrid Fe3 O4 /Ag nanorods are synthesized using an unconventional templating approach. The coupled magnetic and plasmonic anisotropy allows control of the rod orientation, plasmonic excitation, and photothermal conversion by simply applying a magnetic field. Once the rods are fixed with desirable orientations in a bimorph actuator by magnetic-field-assisted lithography, the bending of the actuator can be controlled by switching the laser polarization. A bipedal robot is created by coupling the rod orientation with the alternating actuation of its two legs. Irradiating the robot by a laser with alternating or fixed polarization synergistically results in basic movement (backward and forward) and turning (including left-, right-, and U-turn), respectively. A complex walk along predesigned pathways can be potentially programmed by combining the movement and turning modes of the robots. This strategy provides an alternative driving mechanism for preparing functional soft robots, thus breaking through the limitations in the existing systems in terms of light sources and actuation manners.

Responsive optical nanomaterials that can sense and translate various external stimuli into optical signals, in the forms of observable changes in appearance and variations in spectral line shapes, are among the most active research topics in nanooptics. They are intensively exploited within the regimes of the four classic optical phenomena-diffraction in photonic crystals, absorption of plasmonic nanostructures, as well as color-switching systems, refraction of assembled birefringent nanostructures, and emission of photoluminescent nanomaterials and molecules. Herein, a comprehensive review of these research activities regarding the fundamental principles and practical strategies is provided. Starting with an overview of their substantial developments during the latest three decades, each subtopic discussion is led with fundamental theories that delineate the correlation between nanostructures and optical properties and the delicate research strategies are elaborated with specific attention focused on working principles and optical performances. The unique advantages and inherent limitations of each responsive optical nanoscale platform are summarized, accompanied by empirical criteria that should be met and perspectives on research opportunities where the developments of next-generation responsive optical nanomaterials might be directed.

This current research progress on the fabrication of hollow nanostructures by using self-templating methods is reviewed. After a brief introduction to the unique properties and applications of hollow nanostructures and the three general fabrication routes, the discussions are focused on the five main self-templating strategies, including galvanic replacement, the Kirkendall effect, Ostwald ripening, dissolution-regrowth, and the surface-protected hollowing process. Some newly developed synthetic routes are selected and discussed in detail. In conclusion, a summary and the perspectives on the directions that might lead the future development of this exciting field are presented.

force associated with the expansion of the film, we show that the blown film method can be used to biaxially deform the embedded spherical polymer colloids into disk-shaped ellipsoids in a uniform manner. Figure 1 schematically outlines the procedure. In the first step, monodispersed polystyrene (PS) microspheres are dispersed in a diluted aqueous solution of polyvinyl alcohol (PVA). [34] A thin composite PS/PVA film is obtained by slowly evaporating the solvent. The film is then mounted on top of the wide opening of a ceramic filtering funnel, and sealed with a steel clamp. To improve the sealing, a polyethylene film, such as those used to package food, may be sandwiched between the funnel and the composite film. The system is then immersed into a hot silicon oil bath at � 1608C. The composite film softens and becomes deformable at this temperature, which is much higher than the Tgs of both PVA (� 858C) and PS (� 958C). Air is then injected into the system through the small opening at the end of the funnel using a syringe, allowing the film to blow into a dome-like shape. The funnel is removed from the oil bath and allowed to cool in air with the pressure maintained so that the stretched PVA films do not show apparent shrinkage upon cooling. The softened PS colloids experience a uniform 2D stretching force because of their small sizes compared to the PVA film, and eventually evolve into disk-shapes during the bubble expansion. It has been suggested that the transformation of the embedded particles is facilitated by the strong adhesion arising from the hydrogen bonding between the hydroxyl groups on the PVA and the surface groups on the latex particle surface. [25,29] The deformed shapes are locked in place when the particles are cooled below their Tg. Finally, dissolving the PVA film in isopropanol/water at 608C releases the stretched PS particles which can be collected by centrifugation. The transformation from spherical colloids to disks can be controlled by the extent of bubble expansion, which can be simply represented by the ratio of the height of the bubble (H) to the original film size (D), which is the diameter of the funnel opening. Practically, the height of the bubble can be controlled by the amount of air injected into the funnel through the syringe. As shown in the scanning electron microscopy (SEM) images in Figure 2, PS spheres with radius R0 of � 290nm can be uniformly transformed into disk-shaped spheroids with the major axis (Rd )o f� 350nm, � 450nm, and eventually � 575nm at different H/D ratios. Along with the expansion in the major axis, the dimension in the minor axis decreases during the stretching, as clearly demonstrated in the insets of Figure 2. Because very few particles are standing in a perfectly vertical orientation, it is difficult to survey many of them to accurately obtain the average value of the minor axis. We therefore only determined the average value of the major axis by measuring

Polymers are widely used in the chemical synthesis of colloidal nanocrystals, and their roles are generally documented as steric stabilizers or capping agents. In particular, poly(vinylpyrrolidone) (PVP) has received special attention because of its high chemical stability, nontoxicity, and excellent solubility in many polar solvents. Although the repeating unit of PVP has been extensively investigated for its coordination capability, the end groups of PVP remain largely unexplored in terms of functionality and reactivity.

This paper discusses the potential use of non-spherical colloids (see Figure) as the building blocks in self-assembly to fabricate three-dimensional photonic crystals with bandgaps located in the optical regime. The methods of producing non-spherical colloids as monodisperse samples, and with well-defined shapes and tightly controlled dimensions are presented, and the methods employed to organize these non-spherical colloids into 3D crystalline lattices are discussed.

This article presents an overview of current research activities that center on monodispersed colloidal spheres whose diameter falls anywhere in the range of 10 nm to 1 μm. It is organized into three parts: The first part briefly discusses several useful methods that have been developed for producing monodispersed colloidal spheres with tightly controlled sizes and well-defined properties (both surface and bulk). The second part surveys some techniques that have been demonstrated for organizing these colloidal spheres into two- and three-dimensionally ordered lattices. The third part highlights a number of unique applications of these crystalline assemblies, such as their uses as photonic bandgap (PBG) crystals; as removable templates to fabricate macroporous materials with highly ordered and three-dimensionally interconnected porous structures; as physical masks in lithographic patterning; and as diffractive elements to fabricate new types of optical sensors. Finally, we conclude with some personal perspectives on the directions towards which future research in this area might be directed.

Templating against two-dimensional (2D) regular arrays of square pyramidal pits etched in Si(100) wafers has been exploited to fabricate colloidal crystals with their (100) planes oriented parallel to the substrates (see Figure for an SEM image). The capability and feasibility of this method have been demonstrated by crystallizing 1.0, 0.48, and 0.25 μm polystyrene beads into 3D opaline lattices having such an orientation over areas as large as several square centimeters. Like their (111)-oriented cousins, these long-range ordered lattices of spherical colloids are useful in many areas such as photonics and porous materials. In particular, the ability to generate large colloidal crystals with adjustable spatial orientations will allow one to systematically investigate their photonic band structures in an effort to elucidate the structure–property relationship.