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  1. Abstract

    Drop‐casting manganese oxide (MnO2) hollow nanospheres synthesized via a simple surface‐initiated redox route produces thin films exhibiting angle‐independent structural colors. The colors can rapidly change in response to high‐humidity dynamic water vapor (relative humidity > 90%) with excellent reversibility. When the film is triggered by dynamic water vapor with a relative humidity of ≈100%, the color changes with an optimal wavelength redshift of ≈60 nm at ≈600 ms while there is no shift under static water vapor. The unique selective response originates from the nanoscale porosity formed in the shells by randomly stacked MnO2nanosheets, which enhances the capillary condensation of dynamic water vapor and promotes the change of their effective refractive index for rapid color switching. The repeated color‐switching tests over 100 times confirm the durability and reversibility of the MnO2film. The potential of these films for applications in anti‐counterfeiting and information encryption is further demonstrated by reversible encoding and decoding initiated exclusively by exposure to human breath.

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  2. Abstract

    Photonic crystals (PCs) constructed from colloidal building blocks have attracted increasing attention because their brilliant structural colors may find broad applications in paints, sensors, displays, and security devices. However, producing high‐quality structural colors on flexible substrates such as textiles in an efficient and scalable manner remains a challenge. Here a robust and ultrafast approach to produce industrial‐scale colloidal PCs by the shear‐induced assembly of liquid colloidal crystals of polystyrene beads pre‐formed spontaneously over a critical volume fraction is demonstrated. The pre‐crystallization of colloidal crystals allows their efficient assembly into large‐scale PCs on flexible fabric substrates under shear force. Further, by programming the wettability of the fabric substrate with hydrophilic–hydrophobic regions, this shear‐based assembly strategy can conveniently generate pre‐designed patterns of complex structural colors. This assembly strategy brings structural coloration to flexible fabrics at a scale suitable for commercial applications; therefore, it holds the potential to revolutionize the coloration technology in the textile industry.

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  3. Abstract

    The large‐scale fabrication of two‐dimensional periodic nanostructures in high quality holds great promises for building novel nanoscale devices. They have been conventionally produced using interface‐mediated self‐assembly strategies such as the Langmuir‐Blodgett and oil‐water interfacial assembly methods, which however are limited by the stringent requirements for the hydrophilicity/hydrophobicity of the particle surface. In this article, we review the recent progress on the air‐liquid interfacial self‐assembly to demonstrate that such strategies are simple, inexpensive, effective, scalable, and versatile in the fabrication of two‐dimensional periodic nanostructured arrays. They can be successfully employed to assemble nanoparticles of various compositions and surface properties into periodic nanostructured monolayer films, which find many important applications, for example, in the construction of colorimetric sensors for gas, chemical, and biomedical detection.

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  4. Organizing the colloidal particles into 3D superstructures is a promising strategy for fabricating functional metamaterials with novel optical, electric, and catalytic properties. The rich surface properties of the colloidal particles provide many ways to manipulate their assembly behavior. Emulsion droplets are ideal microspaces for confining colloidal self-assembly, offering many advantages such as versatility, scalability, and controllability over size, shape, and composition. In this review, we first introduce recently developed strategies for the emulsion-confined assembly of colloidal particles into 3D superstructures by manipulating the interfacial properties of the emulsion droplets and colloidal particles, then demonstrate the novel collective properties of the assembled superstructures and highlight some of their unique optical and catalytic properties and applications in bioimaging, diagnosis, drug delivery, and therapy. 
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  5. Expensive instruments and complicated data processing are often required to discriminate solvents with similar structures and properties. Colorimetric sensors with high selectivity, low cost, and good portability are highly desirable to simplify such detection tasks. Herein, we report the fabrication of a photonic crystal sensor based on the self-assembled resorcinol formaldehyde (RF) hollow spheres to realize colorimetric sensing of polar solvents, including homologs and isomers based on the saturated diffusion time. The diffusion of solvent molecules through the photonic crystal film exhibits a unique three-step diffusion profile accompanied by a dynamic color change, as determined by the physicochemical properties of the solvent molecules and their interactions with the polymer shells, making it possible to accurately identify the solvent type based on the dynamic reflection spectra or visual perception. With its superior selectivity and sensitivity, this single-component colorimetric sensor represents a straightforward tool for convenient solvent detection and identification. 
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  6. null (Ed.)
  7. Morphological and magnetic anisotropy can be combined in colloidal assembly to create unconventional secondary structures. We show here that magnetite nanorods interact along a critical angle, depending on their aspect ratios and assemble into body-centered tetragonal colloidal crystals. Under a magnetic field, size-dependent attractive and repulsive domains develop on the ends and center of the nanorods, respectively. Our joint experiment-computational multiscale study demonstrates the presence of a critical angle in the attractive domain, which defines the equilibrium bonding states of interacting rods and leads to the formation of non–close-packed yet hard-contact tetragonal crystals. Small-angle x-ray scattering measurement attributes the perfect tetragonal phase to the slow assembly kinetics. The crystals exhibit brilliant structural colors, which can be actively tuned by changing the magnetic field direction. These highly ordered frameworks and well-defined three-dimensional nanochannels may offer new opportunities for manipulating nanoscale chemical transformation, mass transportation, and wave propagation. 
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  8. null (Ed.)
    Responsive chromic materials are highly desirable in the fields of displays, anti-counterfeiting, and camouflage, but their advanced applications are usually limited by the unrealized delicate and independent tunability of their three intrinsic attributes of color. This work achieves the separate, continuous, and reversible modulation of structural color brightness and hue with an aqueous suspension of dual-responsive Fe 3 O 4 @polyvinylpyrrolidone (PVP)@poly( N -isopropyl acrylamide) (PNIPAM) flexible photonic nanochains. The underlying modulation mechanism of color brightness was experimentally and numerically deciphered by analyzing the morphological responses to stimuli. When an increasing magnetic field was applied, the random worm-like flexible photonic nanochains gradually orientated along the field direction, due to the dominant magnetic dipole interaction over the thermal motion, lengthening the orientation segment length up to the whole of the nanochains. Consequently, the suspension displays increased color brightness (characterized by diffraction intensity). Meanwhile, the color hue (characterized by diffraction frequency) could be controlled by temperature, due to the volume changes of the interparticle PNIPAM. The achieved diverse color modulation advances the next-generation responsive chromic materials and enriches the basic understanding of the color tuning mechanisms. With versatile and facile color tunability and shape patterning, the developed responsive chromic liquid promises to have attractive potential in full-color displays and in adaptive camouflages. 
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