Three-Dimensional Printing of Structural Color Using a Femtoliter Meniscus

3D-architected gratings for polarization-sensitive, nature-inspired structural color

Structural coloration, a color-generation mechanism often found in nature, arises from light-matter interactions such as diffraction, interference, and scattering, with micro- and nanostructured elements. Herein, we systematically study anisotropic, 3D-architected grating structures with polarization-tunable optical properties, inspired by the vivid blue of Morpho butterfly wings. Using two-photon lithography, we fabricate multilayered gratings, varying parameters such as height (through scanning speed and laser power), periodicity, and number of layers. In transmission, significant color transitions from blue to brown were identified when varying structural parameters and incident light polarization conditions (azimuthal angle and ellipticity). Based on thin film diffraction efficiency theory in the Raman-Nath regime, optical characterization results are analytically explained, evaluating the impact of each parameter variation. Overall, these findings contribute to technological implementations of polarization-sensitive, 3D-architected gratings for structural color applications.

3D-Printing of Freestanding Pure MXene Microarchitectures

Since their discovery, titanium-based MXenes (Ti3C2Tx) have attracted significant attention. Several studies have presented versatile, cost-effective, and scalable approaches for fabricating Ti3C2Tx-based functional components. However, most previous studies only allowed the realization of 2D patterns or required diverse additives to produce 3D architectures. Herein, a 3D-Printing approach for producing 3D microarchitectures composed entirely of Ti3C2Tx. Ti3C2Tx additive-free aqueous ink consists of 0.1 wt.% Ti3C2Tx nanosheets is proposed. The diameter (ds) of the printed Ti3C2Tx 3D microarchitectures can be determined by controlling the meniscus channel size, which is influenced by the diameter (dp) of the micropipette opening and pipette-pulling rate (v). Through optimized control of the pipette, a minimum ds of 1.3 µm is obtained, and complex shapes such as zigzag, helix, bridge, and pyramid shapes can be implemented. To demonstrate the feasibility of realizing functional Ti3C2Tx 3D components, three electrical components are demonstrated: 3D micro-interconnects and 3D transducers for photodetectors and humidity sensors. It is believed that this facile approach can be used for nano 3D-Printing as well as micro printing of Ti3C2Tx architectures.

Operando Colorations from Real-Time Growth of 3D-Printed Nanoarchitectures

While artificial 3D nanostructures can generate precise and flexible coloration, their real-time color changes during 3D nanoprinting remain unexplored owing to the inherent challenges of in situ transient measurements and observations. In this study, a 3D-printing system which supports the operando observation/measurement of the color dynamics of subwavelength metallic nanoarchitectures fabricated in real time is developed and evaluated. During 3D printing, the dimensions and geometries of the 3D nanostructures grow over time, producing a large library of optical spectra associated with real-time color changes. Only a timer is needed to define the expected colors from a single 3D print run. Fin-like nanostructures are used to toggle colors based on the polarization effect and produce color gradients. Based on structural coloration, nanoarchitectures are designed and printed to animate desired color patterns. Moreover, the resulting color dynamics can also serve as an operando identifier for real-time structural information during 3D nanoprinting. A single print run enables the efficient creation of a comprehensive library of desired colorations owing to the flexibility in time-dependent controllability and 3D geometries at the subwavelength scale. 3D nanoprinted plasmonic structures exhibiting time-varying colorations (4D printing of colors) uniquely redefines the coloring stategy, offering considerable potential for numerous applications.

Physical Unclonable Functions with Hyperspectral Imaging System for Ultrafast Storage and Authentication Enabled by Random Structural Color Domains

Physical unclonable function (PUF) is attractive in modern encryption technologies. Addressing the disadvantage of slow data storage/authentication in optical PUF is paramount for practical applications but remains an on-going challenge. Here, a highly efficient PUF strategy based on random structural color domains (SCDs) of cellulose nanocrystal (CNC) is proposed for the first time, combing with hyperspectral imaging system (HIS) for ultrafast storage and authentication. By controlling the growth and fusion behavior of the tactoids of CNC, the SCDs display an irregular and random distribution of colors, shapes, sizes, and reflectance spectra, which grant unique and inherent fingerprint-like characteristics that are non-duplicated. Based on images and spectra, these fingerprint features are used to develop two sets of PUF key generation methods, which can be respectively authenticated at the user-end and the manufacturer-front-end that achieving a high coding capacity of at least 22304. Notably, the use of HIS greatly shortens the time of key reading and generation (≈5 s for recording, 0.5-0.7 s for authentication). This new optical PUF labels can not only solve slow data storage and complicated authentication in optical PUF, but also impulse the development of CNC in industrial applications by reducing color uniformity requirement.

Visualization of Microcapillary Tips Using Waveguided Light

The microcapillary, a glass tube with a nano/micrometer scale aperture, is used for manipulating small objects across diverse disciplines. A primary concern in using the microcapillary involves tip breakage upon contact. Here, we report a method for visualizing the microcapillary tip, enabling precise and instant determination of its contact with other objects. Illumination directed to the back aperture of the microcapillary induces waveguiding through the glass wall, enabling the visualization of the tip through scattering. We demonstrate that the tip scattering is sensitive to contact with an adjacent object owing to the near-field interaction of the waveguided light, providing a clear distinction between the contact and noncontact states. The key advantage of our method encompasses its minimal influence, irrespective of conductivity, and applicability to nanoscale systems. The versatility of our method is shown by the application to a wide range of tip diameters, various substrate and in-filling materials.

Emission Directionality of 3D-Printed Photonic Nanowires

Photonic devices can be advanced by increasing the density of the integrated optical components. As the integration density increases, the potential for signal interference between adjacent components, optical crosstalk, becomes a concern. To address the crosstalk issue, it is crucial to identify the emission directionality of the integrated optical components. In this study, we investigate the emission directionality of 3D printed light-emitting nano/microwires. We experimentally and numerically showed that when the diameter is reduced below the single-mode cutoff, the emission becomes noticeably directional. In addition, our demonstrations on pairs of closely positioned wires show that optical crosstalk can be effectively avoided by reducing the diameter to the nanoscale to exploit the strong directionality of its emission. We expect that our study can be applied to various fundamental research and applications in the fields of photonics, optical communication, sensing, and imaging, where the directionality of the emissions is crucial.

Hydrogen Bond-Mediated Self-Shielded Moisture-Responsive Structural Color for Time-Temperature Indicating

Effective monitoring of the time-temperature history of biological reagents, chemical drugs, and perishable foods during cold chain storage is crucial for ensuring their quality and efficacy. Time-temperature indicators (TTIs) are developed to assess the cumulative impact of time and temperature on product quality. However, current indicators face challenges related to complex wrapping procedures, narrow tracking ranges, susceptibility to photobleaching, and pre-use instability, hampering widespread use. Herein, the first moisture-responsive 1D photonic crystal (1DPC) TTIs featuring robust structural colors, customizable time-temperature ranges, and reliable renewability are demonstrated. The indicators exhibit distinct color-changing responsiveness toward water vapor, which remains observable after prolonged storage at low temperatures. Significantly, the moisture responsiveness gradually diminishes at elevated temperatures over time due to ambient water-induced hydrogen bond formation, effectively shielding the indicator from external stimuli. This property enables the naked-eye inspection of product efficacy during cold chain storage. Additionally, the endowed flexibility of the TTI facilitates its easy attachment to targets, functioning as a convenient indicator label. Remarkably, the indicator can be stably stored for an extended period at room temperature before use, thereby showcasing substantial market potential.

Three-Dimensionally Printed, Vertical Full-Color Display Pixels for Multiplexed Anticounterfeiting

Information encryption strategies have become increasingly essential. Most of the fluorescent security patterns have been made with a lateral configuration of red, green, and blue subpixels, limiting the pixel density and security level. Here we report vertically stacked, luminescent heterojunction micropixels that construct high-resolution, multiplexed anticounterfeiting labels. This is enabled by meniscus-guided three-dimensional (3D) microprinting of red, green, and blue (RGB) dye-doped materials. High-precision vertical stacking of subpixel segments achieves full-color pixels without sacrificing lateral resolution, achieving a small pixel size of ∼μm and a high density of over 13,000 pixels per inch. Furthermore, a full-scale color synthesis for individual pixels is developed by modulating the lengths of the RGB subpixels. Taking advantage of these unique 3D structural designs, trichannel multiplexed anticounterfeiting Quick Response codes are successfully demonstrated. We expect that this work will advance data encryption technology while also providing a versatile manufacturing platform for diverse 3D display devices.