Complex Ink Research Articles

Simple and facile synthesis of a CuS film using a copper formate–thioacetamide complex ink

In this study, we developed a simple and facile synthesis method for producing CuS films at low temperatures. The method uses self-reducible complex inks comprising copper formate (Cuf) as the copper source and thioacetamide (TA) as both the sulfur source and complexing agent. The thermal properties of complex inks with different TA/Cuf ratios (0.5–2.0) were analyzed. The ink with a TA/Cuf ratio of 1 exhibited a significant decrease in the reduction temperature. The synthesis of a CuS film involved calcination of the ink at 140 °C; however, some residual Cuf was observed. Introducing hexanol to the ink, aimed at prolonging the liquid-phase reaction, yielded a pure CuS film that contained agglomerated particles. The thermal reduction pathway of Cuf to CuS was analyzed through thermogravimetric–mass spectrometric analysis, and the results revealed that the low-temperature synthesis was attributed to the formation of acetonitrile and formic acid during thermal decomposition of the ink.

Insight into the Types of Alkanolamines on the Properties of Copper(II) Formate-Based Conductive Ink.

Conductive inks are one of the most important functional materials for printed flexible electronic devices, and their properties determine the methods of subsequent patterning and metallization. In comparison with copper nanoparticle or nanowire inks, copper particle-free inks employing copper(II) formate (Cuf) as a precursor have attracted the interest of researchers due to their flexibility in preparation, excellent stability, and lower conversion temperature. Alkanolamines can provide Cuf with excellent solubility in alcohols while being less toxic and having a certain reducibility, making them preferable ligands in comparison with aliphatic amines and pyridine. However, there have been few studies on the effects of the alkanolamine types on the performance of Cuf inks. Also, the decomposition mechanism of copper-alkanolamine complex inks is not clear. In this work, different kinds of alkanolamines were chosen as ligands to formulate Cuf inks to address the mentioned issues. The influences of amine types on the stability, wettability, thermal decomposition behavior, and electrical performance of the formulated Cuf particle-free inks were investigated in detail. The results show that the utilization of alkanolamines could provide Cuf with excellent solubility in alcohols, resulting in an ink with good stability and favorable wetting properties. The thermal decomposition temperature and electrical performance of the formulated copper ink are largely dependent on the amine used. When amines with a longer carbon chain and more branches were utilized to prepare the ink, a decreased decomposition temperature was observed on the derived inks because of the steric hindrance effect. Copper films with good morphology and conductivity could be obtained at low temperatures by selecting the appropriate alkanolamine. Copper particle-free conductive ink from 2-amino-2-methyl-1-propanol demonstrated better morphology and electrical performance (16.09 μΩ·cm) and was successfully used for conductive circuits by direct-writing.

Rapid Sintering of Inkjet Printed Cu Complex Inks Using Laser in Air

Cu complex inks offer a facile and cost-effective alternative to commercial nanoparticle inks for printed electronics application. Cu complex inks are prepared by mixing Cu formate tetrahydrate (metal precursor) with amino-2-propanol (complexing agent) in a molar ratio of 1:2. A carrier solvent consisting of ethanol and ethylene glycol is then added to help adjust the rheological properties of the ink for printing purpose. The Cu complex ink having a viscosity of 12 mPa.s is then inkjet printed onto a polyimide substrate (thickness: 125 μm). After printing 10 layers, the ink is pre-dried under air at 100°C for 5 minutes followed by sintering using an IR laser (942 nm) to enable rapid thermal decomposition of the Cu formate. After sintering using a 100 W line beam laser scanned at 3 mm/sec under air; a contiguous and homogenous Cu metallic trace is formed with a minimum bulk resistivity of 4.8 μΩcm (2.84 times of bulk Cu). The laser sintered trace exhibited a highly compact and homogenous sintered structure after thermal decomposition compared to the conventional oven sintering process. Th result is remarkable and proves the importance of faster decomposition process. In contrast, sintering the same trace using slow ramp rate within the traditional oven sintering pr process, it is observed that a Cu metallic trace is formed with particles in the range of 2-5 μm that are sparsely distributed due to low metal content of Cu complex inks (< 10 metal wt.%). Due to sparse distribution of particles, a low bulk resistivity value of 102 μΩcm is obtained.

Electrohydrodynamically Printed d-f Transition Cerium(III) Complex.

The d-f transition rare earth complexes have recently emerged as a promising candidate for display applications due to the parity-allowed transition, high photoluminescence quantum yield (PLQY), short excited lifetime, and tunable emissions. Besides, inkjet printing has been regarded as an important technique for realizing full-color display. However, inkjet-printed d-f transition rare earth complexes have not been investigated. Herein, for the first time, we explored d-f transition cerium(III) complex 2-Me as the luminescent material by inkjet printing. With 1,2-dichlorobenzene as solvent and polystyrene as an additive, 2-Me film exhibits a similar emission peak and excited-state lifetime with 2-Me powder and a high PLQY of 45%, demonstrating the excellent stability of 2-Me ink. Finally, we suppressed the coffee ring effect and prepared the first inkjet-printed pattern ''HUST'' composed of d-f transition rare earth complex ink with uniform blue fluorescence. Our pioneering work provides a promising alternative for inkjet printing inks.

Decomposition mechanism and morphological evolution of in situ realized Cu nanoparticles in Cu complex inks

Controlling in situ nanoparticle formation and growth in Cu complex inks during decomposition and sintering process by modifying ink chemistry (complexing molar ratio, organic solvent) and influencing sintering parameters (temperature, ramp rate).

Oxygen Plasma-Induced Conversion of Silver Complex Ink into Conductive Coatings

The use of AgNO3-polyvinyl alcohol (PVA) ink and oxygen plasma to form conductive coatings on plastic substrates was studied. It was found that oxygen plasma can decompose silver complexes to form metallic silver without high-temperature heating. The AgNO3-PVA ratio and plasma parameters (time, power) were optimized to obtain uniform conductive coatings. The morphology and electrical characteristics of the coatings were evaluated. Composite coatings with high reflectivity and good adhesion were prepared with a resistivity of 1.66 × 10−6 Ω·m using MOD inks with a silver ion mass fraction of 5%, after 300 W plasma treatment of the PET substrate for 2 min (the chamber temperature was 37.3 °C). These results demonstrate the potential feasibility of silver MOD inks and oxygen plasma treatment for the production of silver connectors, electromagnetic shielding films, and antimicrobial coatings on low-cost plastic substrates.

Data‐Driven Ink Painting Brushstroke Rendering

AbstractAlthough digital painting has advanced much in recent years, there is still a significant divide between physically drawn paintings and purely digitally drawn paintings. These differences arise due to the physical interactions between the brush, ink, and paper, which are hard to emulate in the digital domain. Most ink painting approaches have focused on either using heuristics or physical simulation to attempt to bridge the gap between digital and analog, however, these approaches are still unable to capture the diversity of painting effects, such as ink fading or blotting, found in the real world. In this work, we propose a data‐driven approach to generate ink paintings based on a semi‐automatically collected high‐quality real‐world ink painting dataset. We use a multi‐camera robot‐based setup to automatically create a diversity of ink paintings, which allows for capturing the entire process in high resolution, including capturing detailed brush motions and drawing results. To ensure high‐quality capture of the painting process, we calibrate the setup and perform occlusion‐aware blending to capture all the strokes in high resolution in a robust and efficient way. Using our new dataset, we propose a recursive deep learning‐based model to reproduce the ink paintings stroke by stroke while capturing complex ink painting effects such as bleeding and mixing. Our results corroborate the fidelity of the proposed approach to real hand‐drawn ink paintings in comparison with existing approaches. We hope the availability of our dataset will encourage new research on digital realistic ink painting techniques.

Low-temperature sintering of silver-ammonia complex organic composite ink shows high conductivity for humidity sensors

The development of flexible electronic devices is faced with the challenge of difficult fabrication of complex functional structures on flexible substrates. In this paper, a high viscosity ink of silver-ammonia complex was developed based on the reductive ink principle. Using the electrohydrodynamic direct writing, composite fiber patterned printing was achieved on a flexible substrate. By low-temperature sintering of precursor fibers for reduction, a dense silver structure was formed on the surface, resulting in the formation of Ag/PEO conductive fibers. This approach avoided the high temperature damage to the flexible substrate and the adhesion of materials, while demonstrating good conductivity. The Ag/PEO conductive fibers exhibited different sensitivities to H2O molecules in low, medium, and high humidity environments, which were used to design a flexible Ag structure humidity sensor (ASHS) as a functional material. The ASHS showed a linear response relationship and a fast response speed in an environmental humidity range of 11 % to 75 %RH, enabling real-time monitoring of human respiration. This study demonstrates a promising application prospect of the silver-ammine complex ink in the field of flexible electronic devices.

Nanoscale contact state design for nodal energy transfer at crossed Ag nanowires

The thermal conduction properties of the cross-junction formed by one pair of Ag nanowires were characterized using experiments and simulations. By laser-sintering crossed nanowires on a substrate on which the Ag ion complex ink was dispersed, we succeeded in visualizing the heat flow trajectory of the junction according to the application of energy. To theoretically understand the energy transfer behavior at the junctions, Ag nanowires with penta-twinned cross-sections were modeled by all-atom molecular dynamics simulations. The equilibrium and non-equilibrium simulation results showed the structural stability at the interface junction, convergence of heat flux in an unsteady state, and effective thermal conductivity of the entire system in a steady state. In particular, we focused on the dependence of the diameter on the nanoscale region, which is the most important design variable for Ag nanowires. Accordingly, a structure–property relationship governed by the nanoscale geometry of the junction was identified.

Unraveling the Mechanism of Plasma‐Induced Curing of Particle‐Free Complex Inks for Manufacturing of High‐Performance Flexible Copper Films and Patterns

AbstractAlthough cost‐effective copper inks show great potential in printed flexible electronics, the poor stability and high sintering temperatures have hindered their practical application seriously. Herein, the mechanism of plasma‐induced curing of particle‐free complex copper inks is investigated systematically to realize the low‐temperature fabrication of high‐performance copper films and patterns for advanced flexible electronics. The relationships among the ink formulations, plasma characteristics, microstructures, and the properties of the printed copper patterns are identified; the mechanisms of plasma‐induced curing of copper complex inks are summarized. The fabricated copper films exhibit high conductivity (4.1 µΩ cm), excellent flexibility (bending at a radius of 2 mm), and environmental stability because of the dense microstructures and robust interface adhesion, comparable to that of electroless‐plated films. Moreover, intricate copper antenna patterns on flexible substrates can be obtained rapidly using direct writing and gravure printing methods. These results provide a feasible low‐temperature route to fabricate high‐performance copper films and patterns, promoting the practical application of low‐cost copper inks in printed flexible electronics.

E-Textile by Printing an All-through Penetrating Copper Complex Ink.

Wearable electronics is an emerging field in academics and industry, in which electronic devices, such as smartwatches and sensors, are printed or embedded within textiles. The electrical circuits in electronics textile (e-textile) should withstand many cycles of bending and stretching. Direct printing of conductive inks enables the patterning of electrical circuits; however, while using conventional nanoparticle-based inks, printing onto the fabric results in a thin layer of a conductor, which is not sufficiently robust and impairs the reliability required for practical applications. Here, we present a new process for fabricating robust stretchable e-textile using a thermodynamically stable, solution-based copper complex ink, which is capable of full penetrating the fabric. After printing on knitted stretchable fabrics, they were heated, and the complex underwent an intermolecular self-reduction reaction. The continuously formed metallic copper was used as a seed layer for electroless plating (EP) to form highly conductive circuits. It was found that the stretching direction has a significant role in resistivity. This new approach enables fabricating e-textiles with high stretchability and durability, as demonstrated for wearable gloves, toward printing functional e-textile.

In situ characterization of material extrusion printing by near-infrared spectroscopy

Material extrusion printing of reactive resins and inks present a unique challenge due to the time-dependent nature of the rheological and chemical properties they possess. As a result, careful print optimization or process control is important to obtain consistent, high quality prints via additive manufacturing. We present the design and use of a near-infrared (NIR) flow through cell for in situ chemical monitoring of reactive resins during printing. Differences between in situ and off-line benchtop measurements are presented and highlight the need for in-line monitoring capability. Additionally, in-line extrusion force monitoring and off-line post inspection using machine vision is demonstrated. By combining NIR and extrusion force monitoring, it is possible to follow cure reaction kinetics and viscosity changes during printing. When combined with machine vision, the ability to automatically identify and quantify print artifacts can be incorporated on the printing line to enable real-time, artificial intelligence-assisted quality control of both process and product. Together, these techniques form the building blocks of an optimized closed-loop process control strategy when complex reactive inks must be used to produce printed hardware.

Complex ink flow mechanisms in micro-direct-ink-writing and their implications on flow rate control

Despite its simplicity, low cost, and ability to process a wide range of materials, direct-ink-writing (DIW) is an additive manufacturing process with low resolution and accuracy, in the multiple hundred microns to millimeter range. One of the main sources for this issue is the difficulty with accurately controlling ink flow rate at smaller size scales. Towards addressing this limitation, this paper elucidates complex ink flow mechanisms that renders flow rate control difficult and explores printing implementations to increase flow rate accuracy in direct-ink-writing at the micro scale. To this end, a DIW system utilizing hybrid pressure and velocity-controlled extrusion is used to obtain pressure-flow rate relationships for a water-based sodium carboxymethyl cellulose (NaCMC) solution ink, as a function of printing nozzle diameter (510–100 µm). These studies showed that the transient response of piston velocity-controlled extrusion significantly slows down with decreasing nozzle diameter. For pressure-controlled extrusion, the wall slip increases with decreasing nozzle diameter and the constant slip velocity assumption no longer holds as nozzle size decreases below a certain diameter. To contextualize the influence of such behavior on flow rate accuracy, temperature-controlled parallel plate rheometry was performed on the inks and rheological ink models were accordingly determined. It was shown that the associated flow rate predictions under predicted flow rates due to lack of wall-slip consideration, particularly for smaller nozzle sizes. Lastly, an iterative pressure-controlled DIW implementation was explored to address the accuracy issues for micro-DIW. Our results indicated significant improvement in the transient response and flow rate accuracy for nozzle diameters as small as 100 µm using this approach compared to both of the conventional pressure and velocity control approaches.

Printed Low-Hysteresis Stretchable Strain Sensor Based on a Self-Segregating Conductive Composite

The hysteresis of stretchable composites raises significant challenges in the accuracy, reliability, and stability of stretchable strain sensors. Herein, we report a self-segregating conductive composite that overcomes these issues in printed stretchable strain sensors. This printable composite possesses self-segregation between polydimethylsiloxane and carbon black, which was induced by a deep eutectic solvent. Compared with that of the conventional composites with a random conductive network, our spontaneously formed conductive architecture exhibits superior electromechanical performance: (i) low hysteresis and high sensitivity, (ii) high conductivity and low elastic modulus, and (iii) excellent reliability and stability. Moreover, the composite can be applied directly to a simple stencil printing process without any complex ink synthesis and post-treatment of the fabricated device. This work provides a materials design strategy for achieving low mechanical and electrical hysteresis in a conductive composite. The fabricated sensors exhibit comprehensive performance capabilities appropriate for whole body human motion detection as a proof of concept.

Room-temperature high-precision printing of flexible wireless electronics based on MXene inks

Wireless technologies-supported printed flexible electronics are crucial for the Internet of Things (IoTs), human-machine interaction, wearable and biomedical applications. However, the challenges to existing printing approaches remain, such as low printing precision, difficulty in conformal printing, complex ink formulations and processes. Here we present a room-temperature direct printing strategy for flexible wireless electronics, where distinct high-performance functional modules (e.g., antennas, micro-supercapacitors, and sensors) can be fabricated with high resolution and further integrated on various flat/curved substrates. The additive-free titanium carbide (Ti3C2Tx) MXene aqueous inks are regulated with large single-layer ratio (>90%) and narrow flake size distribution, offering metallic conductivity (~6, 900 S cm−1) in the ultrafine-printed tracks (3 μm line gap and 0.43% spatial uniformity) without annealing. In particular, we build an all-MXene-printed integrated system capable of wireless communication, energy harvesting, and smart sensing. This work opens a door for high-precision additive manufacturing of printed wireless electronics at room temperature.

Dry Printing and Additive Nanomanufacturing of Flexible Hybrid Electronics and Sensors

AbstractThe growing demand for flexible and wearable hybrid electronics has triggered the need for advanced manufacturing techniques with versatile printing capabilities. Complex ink formulations, use of surfactants/contaminants, limited source materials, and the need for high‐temperature heat treatments for sintering are major issues facing the current inkjet and aerosol printing methods. Here, the nanomanufacturing of flexible hybrid electronics (FHE) by dry printing silver and indium tin oxide on flexible substrates using a novel laser‐based additive nanomanufacturing process is reported. The electrical resistance of the printed lines is tailored during the print process by tuning the geometry and structure of the printed samples. Different FHE designs are fabricated and tested to check the performance of the devices. Mechanical reliability tests including cycling, bending, and stretching confirm the expected performance of the printed samples under different strain levels. This transformative liquid‐free process allows the on‐demand formation and in situ laser crystallization of nanoparticles for printing pure materials for future flexible and wearable electronics and sensors.

Self-Organizing, Environmentally Stable, and Low-Cost Copper–Nickel Complex Inks for Printed Flexible Electronics

Cost-effective copper conductive inks are considered as the most promising alternative to expensive silver conductive inks for use in printed electronics. However, the low stability and high sintering temperature of copper inks hinder their practical application. Herein, we develop rapidly customizable and stable copper-nickel complex inks that can be transformed in situ into uniform copper@nickel core-shell nanostructures by a self-organized process during low-temperature annealing and immediately sintered under photon irradiation to form copper-nickel alloy patterns on flexible substrates. The complex inks are synthesized within 15 min via a simple mixing process and are particle-free, air-stable, and compatible with large-area screen printing. The manufactured patterns exhibit a high conductivity of 19-67 μΩ·cm, with the value depending on the nickel content, and can maintain high oxidation resistance at 180 °C even when the nickel content is as low as 6 wt %. In addition, the printed copper-nickel alloy patterns exhibit high flexibility as a consequence of the local softening and mechanical anchoring effect between the metal pattern and the flexible substrate, showing strong potential in the additive manufacturing of highly reliable flexible electronics, such as flexible radio-frequency identification (RFID) tags and various wearable sensors.

Electrochemical reduction of carbon dioxide to C2 products using a copper-sulfide, nanoparticle electrode synthesized by thermal decomposition of sulfur and copper-amine complex ink

A copper sulfide electrode was synthesized by thermal decomposition of sulfur and copper-amine complex ink.

Complex Ink Flow Mechanisms in Micro-Direct-Ink-Writing and Their Implications on Flow Rate Control

Complex Ink Flow Mechanisms in Micro-Direct-Ink-Writing and Their Implications on Flow Rate Control

Patterning microporous paper with highly conductive silver nanoparticles via PVP-modified silver–organic complex ink for development of electric valves

The stabilization of silver cations generated from silver-reactive ink by PVP on HEC-modified Whatman No. 1 paper and enhanced electrowetting properties for developing a paper-based microfluidic device equipped with an electrical valve.