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.

4D Printing of Ultrastretchable Magnetoactive Soft Material Architectures for Soft Actuators.

Magnetoactive soft materials (MSMs) comprising magnetic particles and soft matrices have emerged as smart materials for realizing soft actuators. 4D printing, which involves fabricating 3D architectures that can transform shapes under external magnetic fields, is an effective way to fabricate MSMs-based soft actuators with complex shapes. The printed MSMs must be flexible, stretchable, and adaptable in their magnetization profiles to maximize the degrees of freedom for shape morphing. This study utilizes a facile 4D printing strategy for producing ultrastretchable (stretchability > 1000%) MSM 3D architectures for soft-actuator applications. The strategy involves two sequential steps: (i) direct ink writing (DIW) of the MSM 3D architectures with ink composed of NdFeB and styrene-isoprene block copolymers (SIS) at room temperature and (ii) programming and reconfiguration of the magnetization profiles of the printed architecture using an origami-inspired magnetization method (magnetization field, Hm = 2.7 T). Various differently shaped MSM 3D architectures, which can be transformed into desired shapes under an actuation magnetic field (Ba = 85 mT), are successfully fabricated. In addition, two different soft-actuator applications are demonstrated: a multifinger magnetic soft gripper and a Kirigami-shaped 3D electrical switch with conductive and magnetic functionalities. Our strategy shows potential for realizing multifunctional, shape-morphing, and reprogrammable magnetoactive devices for advanced soft-actuator applications.

Three-Dimensional Printing of Structural Color Using a Femtoliter Meniscus.

Structural colors are produced by the diffraction of light from microstructures. The collective arrangement of substructures is a simple and cost-effective approach for structural coloration represented by colloidal self-assembly. Nanofabrication methods enable precise and flexible coloration by processing individual nanostructures, but these methods are expensive or complex. Direct integration of desired structural coloration remains difficult because of the limited resolution, material-specificity, or complexity. Here, we demonstrate three-dimensional printing of structural colors by direct writing of nanowire gratings using a femtoliter meniscus of polymer ink. This method combines a simple process, desired coloration, and direct integration at a low cost. Precise and flexible coloration is demonstrated by printing the desired structural colors and shapes. In addition, alignment-resolved selective reflection is shown for displayed image control and color synthesis. The direct integration facilitates structural coloration on various substrates, including quartz, silicon, platinum, gold, and flexible polymer films. We expect that our contribution can expand the utility of diffraction gratings across various disciplines such as surface-integrated strain sensors, transparent reflective displays, fiber-integrated spectrometers, anticounterfeiting, biological assays, and environmental sensors.

Meniscus-Guided Micro-Printing of Prussian Blue for Smart Electrochromic Display.

Using energy-saving electrochromic (EC) displays in smart devices for augmented reality makes cost-effective, easily producible, and efficiently operable devices for specific applications possible. Prussian blue (PB) is a metal-organic coordinated compound with unique EC properties that limit EC display applications due to the difficulty in PB micro-patterning. This work presents a novel micro-printing strategy for PB patterns using localized crystallization of FeFe(CN)6 on a substrate confined by the acidic-ferric-ferricyanide ink meniscus, followed by thermal reduction at 120 °C, thereby forming PB. Uniform PB patterns can be obtained by manipulating printing parameters, such as the concentration of FeCl3 ·K3 Fe(CN)6 , printing speed, and pipette inner diameter. Using a 0.1 M KCl (pH 4) electrolyte, the printed PB pattern is consistently and reversibly converted to Prussian white (CV potential range: -0.2-0.5 V) with 200 CV cycles. The PB-based EC display with a navigation function integrated into a smart contact lens is able to display directions to a destination to a user by receiving GPS coordinates in real time. This facile method for forming PB micro-patterns could be used for advanced EC displays and various functional devices.

3D-printed NiFe-layered double hydroxide pyramid electrodes for enhanced electrocatalytic oxygen evolution reaction.

Electrochemical water splitting has been considered one of the most promising methods of hydrogen production, which does not cause environmental pollution or greenhouse gas emissions. Oxygen evolution reaction (OER) is a significant step for highly efficient water splitting because OER involves the four electron transfer, overcoming the associated energy barrier that demands a potential greater than that required by hydrogen evolution reaction. Therefore, an OER electrocatalyst with large surface area and high conductivity is needed to increase the OER activity. In this work, we demonstrated an effective strategy to produce a highly active three-dimensional (3D)-printed NiFe-layered double hydroxide (LDH) pyramid electrode for OER using a three-step method, which involves direct-ink-writing of a graphene pyramid array and electrodeposition of a copper conducive layer and NiFe-LDH electrocatalyst layer on printed pyramids. The 3D pyramid structures with NiFe-LDH electrocatalyst layers increased the surface area and the active sites of the electrode and improved the OER activity. The overpotential (η) and exchange current density (i0) of the NiFe-LDH pyramid electrode were further improved compared to that of the NiFe-LDH deposited Cu (NiFe-LDH/Cu) foil electrode with the same base area. The 3D-printed NiFe-LDH electrode also exhibited excellent durability without potential decay for 60 h. Our 3D printing strategy provides an effective approach for the fabrication of highly active, stable, and low-cost OER electrocatalyst electrodes.

Three-Dimensional Perovskite Nanopixels for Ultrahigh-Resolution Color Displays and Multilevel Anticounterfeiting.

Hybrid perovskites are emerging as a promising, high-performance luminescent material; however, the technological challenges associated with generating high-resolution, free-form perovskite structures remain unresolved, limiting innovation in optoelectronic devices. Here, we report nanoscale three-dimensional (3D) printing of colored perovskite pixels with programmed dimensions, placements, and emission characteristics. Notably, a meniscus comprising femtoliters of ink is used to guide a highly confined, out-of-plane crystallization process, which generates 3D red, green, and blue (RGB) perovskite nanopixels with ultrahigh integration density. We show that the 3D form of these nanopixels enhances their emission brightness without sacrificing their lateral resolution, thereby enabling the fabrication of high-resolution displays with improved brightness. Furthermore, 3D pixels can store and encode additional information into their vertical heights, providing multilevel security against counterfeiting. The proof-of-concept experiments demonstrate the potential of 3D printing to become a platform for the manufacture of smart, high-performance photonic devices without design restrictions.

3D-Printed Quantum Dot Nanopixels.

The pixel is the minimum unit used to represent or record information in photonic devices. The size of the pixel determines the density of the integrated information, such as the resolution of displays or cameras. Most methods used to produce display pixels are based on two-dimensional patterning of light-emitting materials. However, the brightness of the pixels is limited when they are miniaturized to nanoscale dimensions owing to their limited volume. Herein, we demonstrate the production of three-dimensional (3D) pixels with nanoscale dimensions based on the 3D printing of quantum dots embedded in polymer nanowires. In particular, a femtoliter meniscus was used to guide the solidification of liquid inks to form vertically freestanding nanopillar structures. Based on the 3D layout, we show high-density integration of color pixels, with a lateral dimension of 620 nm and a pitch of 3 µm for each of the red, green, and blue colors. The 3D structure enabled a 2-fold increase in brightness without significant effects on the spatial resolution of the pixels. In addition, we demonstrate individual control of the brightness based on a simple adjustment of the height of the 3D pixels. This method can be used to achieve super-high-resolution display devices and various photonic applications across a range of disciplines.

Air evolution during drop impact on liquid pool.

We elucidate the evolution of the entrained air in drop impact on a wide range of liquids, using ultrafast X-ray phase-contrast imaging. We elaborate the retraction mechanism of the entrapped air film in terms of liquid viscosity. We found the criterion for deciding if the entrapped air evolves into single or double bubbles, as determined by competition among inertia, capillarity, and viscosity. Low viscosity and low surface tension induce a small daughter droplet encapsulated by a larger air shell bubble, forming an antibubble. We demonstrate a phase diagram for air evolution regarding hydrodynamics.

3D-printed Cu2O photoelectrodes for photoelectrochemical water splitting.

Photoelectrochemical (PEC) water splitting is an alternative to fossil fuel combustion involving the generation of renewable hydrogen without environmental pollution or greenhouse gas emissions. Cuprous oxide (Cu2O) is a promising semiconducting material for the simple reduction of hydrogen from water, in which the conduction band edge is slightly negative compared to the water reduction potential. However, the solar-to-hydrogen conversion efficiency of Cu2O is lower than the theoretical value due to a short carrier-diffusion length under the effective light absorption depth. Thus, increasing light absorption in the electrode-electrolyte interfacial layer of a Cu2O photoelectrode can enhance PEC performance. In this study, a Cu2O 3D photoelectrode comprised of pyramid arrays was fabricated using a two-step method involving direct-ink-writing of graphene structures. This was followed by the electrodeposition of a Cu current-collecting layer and a p-n homojunction Cu2O photocatalyst layer onto the printed structures. The performance for PEC water splitting was enhanced by increasing the total light absorption area (A a) of the photoelectrode via controlling the electrode topography. The 3D photoelectrode (A a = 3.2 cm2) printed on the substrate area of 1.0 cm2 exhibited a photocurrent (I ph) of -3.01 mA at 0.02 V (vs. RHE), which is approximately three times higher than that of a planar photoelectrode with an A a = 1.0 cm2 (I ph = -0.91 mA). Our 3D printing strategy provides a flexible approach for the design and the fabrication of highly efficient PEC photoelectrodes.

3D Nanoprinting of Perovskites.

As competing with the established silicon technology, organic-inorganic metal halide perovskites are continually gaining ground in optoelectronics due to their excellent material properties and low-cost production. The ability to have control over their shape, as well as composition and crystallinity, is indispensable for practical materialization. Many sophisticated nanofabrication methods have been devised to shape perovskites; however, they are still limited to in-plane, low-aspect-ratio, and simple forms. This is in stark contrast with the demands of modern optoelectronics with freeform circuitry and high integration density. Here, a nanoprecision 3D printing is developed for organic-inorganic metal halide perovskites. The method is based on guiding evaporation-induced perovskite crystallization in mid-air using a femtoliter ink meniscus formed on a nanopipette, resulting in freestanding 3D perovskite nanostructures with a preferred crystal orientation. Stretching the ink meniscus with a pulling process enables on-demand control of the nanostructure's diameter and hollowness, leading to an unprecedented tubular-solid transition. With varying the pulling direction, a layer-by-layer stacking of perovskite nanostructures is successfully demonstrated with programmed shapes and positions, a primary step for additive manufacturing. It is expected that the method has the potential to create freeform perovskite nanostructures for customized optoelectronics.

3D printing of highly conductive silver architectures enabled to sinter at low temperatures.

Silver (Ag) nanoparticle-based inks are frequently used in printed electronics to form conductive patterns, but often require high-temperature sintering to achieve the optimum electrical conductivity, hindering their use in substrates with poor heat resistance. Herein, a three-dimensional (3D) printing strategy to produce highly conductive Ag 3D architectures that can be sintered at low temperatures is reported. This strategy is based on the additive deposition of Ag nanoparticles and microflakes via extrusion-based 3D printing with the Ag ink that involves poly(acrylic acid) (PAA)-stabilized Ag nanoparticles, Ag microflakes, and NaCl - a destabilizing agent. The designed Ag inks are stable and suitable for ink-extrusion 3D printing. In chemical sintering, Cl- can detach PAA from the Ag nanoparticle surface, enabling nanoparticle coalescence and sintering. An elevated annealing temperature induces increased NaCl density in the printed patterns and accelerates the surface and grain boundary diffusion of Ag atoms, contributing to enhance chemical sintering. On annealing at ∼110 °C for 30 min, the printed structures exhibited an electrical conductivity of ∼9.72 × 104 S cm-1, which is ∼15.6% of that of bulk Ag. Complicated Ag architectures with diverse shapes were successfully fabricated on polymeric substrates. Several structural electronic applications were demonstrated by hybrid 3D printing combining our extrusion-based 3D printing and conventional fused deposition modeling (FDM).

Electroless Deposition-Assisted 3D Printing of Micro Circuitries for Structural Electronics.

Three-dimensional (3D) printing is a next-generation free-form manufacturing technology for structural electronics. The realization of structural electronic devices necessitates the direct integration of electronic circuits into 3D objects. However, creating highly conductive, high-resolution patterns in 3D remains a major challenge. Here, we report on a metallic 3D printing method that incorporates electroless deposition (ELD) into the direct ink writing method. Our approach consists of two steps: (1) direct ink writing of catalyst microstructures with a functional catalyst ink containing Ag ions and (2) ELD of Cu onto the printed catalyst structures. High-quality, stable Cu 3D printing is achieved through the design of the Ag catalyst ink; hydroxypropyl cellulose is added as both a rheological modifier (printing) and dissolution inhibitor (ELD). As a result, various two-dimensional (2D) and 3D Cu micro circuitries with high conductivity (∼65% of bulk) can be directly integrated onto 3D plastic substrates without the need for high-temperature annealing. A hybrid strategy that combines ELD-assisted 3D printing and conventional fused deposition modeling enables full fabrication of structural electronic devices. This 3D printing strategy can be a low-cost and facile method for obtaining highly conductive metallic 2D and 3D microstructures in structural electronics.

Flexible Strain Sensors Fabricated by Meniscus-Guided Printing of Carbon Nanotube-Polymer Composites.

Printed strain sensors have promising potential as a human-machine interface (HMI) for health-monitoring systems, human-friendly wearable interactive systems, and smart robotics. Herein, flexible strain sensors based on carbon nanotube (CNT)-polymer composites were fabricated by meniscus-guided printing using a CNT ink formulated from multiwall nanotubes (MWNTs) and polyvinylpyrrolidone (PVP); the ink was suitable for micropatterning on nonflat (or curved) substrates and even three-dimensional structures. The printed strain sensors exhibit a reproducible response to applied tensile and compressive strains, having gauge factors of 13.07 under tensile strain and 12.87 under compressive strain; they also exhibit high stability during ∼1500 bending cycles. Applied strains induce a contact rearrangement of the MWNTs and a change in the tunneling distance between them, resulting in a change in the resistance (Δ R/ R0) of the sensor. Printed MWNT-PVP sensors were used in gloves for finger movement detection; these can be applied to human motion detection and remote control of robotic equipment. Our results demonstrate that meniscus-guided printing using CNT inks can produce highly flexible, sensitive, and inexpensive HMI devices.

Precise Placement of Microbubble Templates at Single Entity Resolution.

Microbubbles have been used as a soft template to produce hollow structures for diverse applications in chemistry, materials science, and biomedicine. It is a challenge, however, to control their size and position at single-entity level. We report on an on-demand method to produce and place a single microbubble with programmed size and position. The method exploits scanning an electrolyte-filled micropipette to place a hydrogen (H2) bubble, generated by water electrolysis, on the desired position. The bubble growth is self-limited after the bubble size fits to the pipet aperture, yielding well-controlled bubble size. The bubble growth dynamics within the pipet is successfully investigated by a methodology that combines phase-contrast X-ray imaging and electric-current measurement. We show that the microbubbles, accurately controlled in size and position, can be used for the fabrication of various polypyrrole microcontainer arrays. We expect the scanning-pipet strategy could be generalized for manipulating various soft materials at will.

Nanowires: Quantitative Probing of Cu(2+) Ions Naturally Present in Single Living Cells (Adv. Mater. 21/2016).

Quantitative probing of the Cu(2+) ions naturally present in single living cells is accomplished by a probe made from a quantum-dot-embedded-nanowire waveguide. After inserting the active nanowire-based waveguide probe into single living cells, J. H. Je and co-workers directly observe photoluminescence (PL) quenching of the embedded quantum dots by the Cu(2+) ions diffused into the probe as described on page 4071. This results in quantitative measurement of intracellular Cu(2+) ions.

Quantitative Probing of Cu(2+) Ions Naturally Present in Single Living Cells.

Quantitative probing of Cu(2+) ions naturally present in single living cells is realized by developing a quantum-dot-embedded nanowire-waveguide probe. The intracellular Cu(2+) ion concentration is quantified by direct monitoring of photoluminescence quenching during the insertion of the nanowire in a living neuron. The measured intracellular Cu(2+) ion concentration is 3.34 ± 1.04 × 10(-6) m (mean ± s.e.m.) in single hippocampal neurons.

A light-driven supramolecular nanowire actuator.

A single photomechanical supramolecular nanowire actuator with an azobenzene-containing 1,3,5-tricarboxamide derivative is developed by employing a direct writing method. Single nanowires display photoinduced reversible bending and the bending behavior follows first-order kinetics associated with azobenzene photoisomerization. A wireless photomechanical nanowire tweezers that remotely manipulates a single micro-particle is also demonstrated.

Four-dimensional visualization of rising microbubbles.

Four-dimensional imaging, which indicates imaging in three spatial dimensions as a function of time, provides useful evidence to investigate the interactions of rising bubbles. However, this has been largely unexplored for microbubbles, mostly due to problems associated with strong light scattering and shallow depth of field in optical imaging. Here, tracking x-ray microtomography is used to visualize rising microbubbles in four dimensions. Bubbles are tracked by moving the cell to account for their rise velocity. The sizes, shapes, time-dependent positions, and velocities of individual rising microbubbles are clearly identified, despite substantial overlaps between bubbles in the field of view. Our tracking x-ray microtomography affords opportunities for understanding bubble-bubble (or particle) interactions at microscales - important in various fields such as microfluidics, biomechanics, and floatation.

Light propagation in conjugated polymer nanowires decoupled from a substrate.

Light-emitting conjugated polymer nanowires are vertically grown and remotely manipulated into a freestanding straight or curved structure in three-dimension. This approach enabled us to eliminate substrate coupling, a critical issue in nanowire photonics in the past decade. We for the first time accomplished characterization of propagation and bending losses of nanowires completely decoupled from a substrate.

Single inorganic-organic hybrid nanowires with ambipolar photoresponse.

We report for the first time single nanowires (NWs) with ambipolar (positive/negative) photoresponse that changes sign depending on the illumination wavelength. The single NWs were grown by the meniscus-guided method using inorganic (ZnO nanoparticles)-organic (PEDOT:PSS) hybrid materials. The ambipolar photoresponse of the single NWs enabled us to develop an unprecedented spectrum-discriminating NW photodetector array.