Ink Process Research Articles

(Invited) Processing and Applications of Monodisperse Two-Dimensional Nanomaterial Inks

Two-dimensional nanomaterials have emerged as promising candidates for next-generation electronics and optoelectronics [1], but advances in scalable nanomanufacturing are required to exploit this potential in real-world technology. This talk will explore methods for improving the uniformity of solution-processed two-dimensional nanomaterials with an eye toward realizing dispersions and inks that can be deposited into large-area thin-films [2]. In particular, density gradient ultracentrifugation allows the solution-based isolation of graphene [3], boron nitride [4], montmorillonite [5], and transition metal dichalcogenides (e.g., MoS2, WS2, ReS2, MoSe2, WSe2) [6,7] with homogeneous thickness down to the atomically thin limit. Similarly, two-dimensional black phosphorus is isolated in organic solvents [8] or deoxygenated aqueous surfactant solutions [9] with the resulting phosphorene nanosheets showing field-effect transistor mobilities and on/off ratios that are comparable to micromechanically exfoliated flakes. By adding cellulosic polymer stabilizers to these dispersions, the rheological properties can be tuned by orders of magnitude, thereby enabling two-dimensional nanomaterial inks that are compatible with a range of additive manufacturing methods including inkjet [10], gravure [11], screen [12], and 3D printing [13]. The resulting printed two-dimensional nanomaterial structures show promise in several applications including photodiodes [14], anti-ambipolar transistors [15], gate-tunable memristors [16], and heterojunction photovoltaics [17,18]. [1] D. Jariwala, et al., ACS Nano, 8, 1102 (2014). [2] E. B. Secor, et al., Journal of Physical Chemistry Letters, 6, 620 (2015). [3] A. A. Green, et al., Nano Letters, 9, 4031 (2009). [4] J. Zhu, et al., Nano Letters, 15, 7029 (2015). [5] J. Zhu, et al., Advanced Materials, 28, 63 (2016). [6] J. Kang, et al., Nature Communications, 5, 5478 (2014). [7] J. Kang, et al., Nano Letters, 16, 7216 (2016). [8] J. Kang, et al., ACS Nano, 9, 3596 (2015). [9] J. Kang, et al., Proc. Nat. Acad. Sci. USA, 113, 11688 (2016). [10] E. B. Secor, et al., Journal of Physical Chemistry Letters, 4, 1347 (2013). [11] E. B. Secor, et al., Advanced Materials, 26, 4533 (2014). [12] W. J. Hyun, et al., Advanced Materials, 27, 109 (2015). [13] A. E. Jakus, et al., ACS Nano, 9, 4636 (2015). [14] D. Jariwala, et al., Proc. Nat. Acad. Sci. USA, 110, 18076 (2013). [15] D. Jariwala, et al., Nano Letters, 15, 416 (2015). [16] V. K. Sangwan, et al., Nature Nanotechnology, 10, 403 (2015). [17] D. Jariwala, et al., Nano Letters, 16, 497 (2016). [18] T. A. Shastry, et al., ACS Nano, 10, 10573 (2016).

Optimizing aerosol jet printing process of silver ink for printed electronics

Aerosol jet is an additive manufacturing technique that writes electronic inks directly onto the surface. The process has great potential for printed electronics as it is non-contact method that works well with variety of materials- metals, insulators, semiconductors, epoxy and encapsulation materials for conformal writing. This paper explores the effect of some parameters like ultrasonic power and atomizer flow on overall print quality, thus establishing a process window.

Effects of color mixing components on offset ink and printing process

ABSTRACTIn printing, the most important element is to obtain the correct substrate and ink combination. Transparent white and opaque white are used for the lightening of printing inks, but they are also an important component for the mixing of inks. The primary objective of this study was to assess the result of using transparent and opaque white in mineral oil-based offset printing inks on the rheological and printability properties of these inks as well as the ideal mixing ratios in terms of print quality. Transparent and opaque white were added into the standard offset magenta ink variable ratios. Differences in ink viscosity, tack, and flow (rheological) values affect such parameters as adhesion, transferability, and printing stability in these inks. Prints were made on matt-coated papers of the same grammage with these inks, and contact angle, solid ink density, color strength, and print gloss values of these ink films were measured. Color differences () of inks in comparison to the standard ink were calculated and the effects of transparent and opaque white on the rheological and printability properties of the ink were presented. Finally, the ideal mixture ratios of transparent and opaque white into the ink were determined in terms of print quality and recommendations were presented.

Stabilization of the thermal decomposition process of self-reducible copper ion ink for direct printed conductive patterns

An alkylamine is added to stabilize the thermal decomposition process and to improve the surface morphology of printed patterns. The adhesion and mechanical stability of the copper thin films are also investigated.

Conductive grid patterns prepared by microcontact printing silver nanoparticles ink

Metal grid transparent conductive electrodes have been developed by traditional printing techniques, but lower resolution of grid line has limited its performance. In this study, conductive grid patterns with ~10 µm width were created on the polyethylene terephthalate (PET) substrate by microcontact printing (µCP) silver nanoparticles ink. We have found that the composition of silver nanoparticles ink and the O2 plasma treated time of polydimethylsiloxane (PDMS) stamp were critical for the µCP process. Specially, the hole size of stamp has great influence on the patterns shape when using grid-structured stamp. This is because the break of liquid bridge formed between two adjacent lines of stamp depends on the hole size of stamp during the dry process of ink. Larger hole was beneficial for the formation of grid patterns with smooth edge. Conductive grid patterns with typical sheet resistance of 610 Ω/□ and 74% transparency were acquired over the entire 1 cm square area using 40 µm hole stamp. This method will lead to a facile strategy to prepare conductive electrode.

Optimizing inkjet printing process to fabricate thick ceramic coatings

This article describes the use of Taguchi optimization and ANOVA techniques on inkjet printing process to determine optimal parameters for fabrication of thick ceramic coatings over glass substrates. Stable nanoparticle suspensions are synthesized through high energy milling of precursor powders with adequate quantities of binder and suspending solvent. Most often, inkjet printing process is being used for developing fine and thin layers (<10µm). However, an attempt is made to fabricate thick ceramic films by varying only IJP process parameters and without multiple layer deposition, thereby reducing efforts in ink synthesis and processing time of coated substrates. Three parameters of IJP were varied for developing a model that was used for precisely predicting the printed layer thickness under varying process parameters. ANOVA technique showed that open time interval in combination with nano particle concentration in the ink could potentially lead to thick coatings. The higher volume % of solvent in the diluted suspension ink under the influence of substrate heating contributed significantly to coffee stain effect with irregular surface coatings. However, increasing the concentration of nanoparticles in the diluted ink resulted in substantial improvement in thickness of the layer with simultaneous control of coating defects.

Ink-jet printed highly conductive pristine graphene patterns achieved with water-based ink and aqueous doping processing

We report an efficient inkjet printing of water-based pristine GNPs graphene ink and a facile aqueous halogen doping process that provides significant and thermally stable conductivity enhancement of printed patterns. Highly concentrated aqueous graphene ink populated by few-layer pristine graphene flakes is obtained by means of scalable shear exfoliation process with the aid of bromine intercalation. The as-printed GNP films which has been merely treated by drying at 100 °C exhibits DC conductivity (σDC) of ∼1400 S/m likely due to bromine doping effect. This value is significantly increased to ∼3 × 104 S/m when an additional treatment by means of dipping in aqueous iodine solution is applied prior to the drying. As contrast, σDC is increased to ∼2.4 × 104 S/m when a mere annealing at elevated temperature in air is employed. When the aqueous iodine doping process and annealing at elevated temperature is combined, an unprecedented value of σDC ∼105 S/m is achieved. The availability of water-based GNPs inks and low-temperature doping scheme for efficient and reliable conductivity enhancement has offered a pathway for the application of GNPs in different printed electronics devices.

All-printed strain sensors: Building blocks of the aircraft structural health monitoring system

Characterization of all-printed strain gages to assess their suitability for structural state monitoring of large structures is presented. Strain sensor response, transverse strain sensitivity and long-term reliability are key performance parameters of printed strain sensors on flexible substrates. These key performance parameters are evaluated for inkjet and screen printed strain sensors on polyethylene-terephthalate (PET) flexible substrates. More specifically, printing characteristics of commercially available inks, gage factor of serpentine strain sensors with transverse strain and temperature sensitivity, and sensor reliability under unidirectional tensile and fatigue loading is assessed. Maximum strain to which both inkjet and screen printable formulations can be reliably used for long-term repeatable measurements is recommended based on tensile and fatigue testing. Variation in gage factor is attributed to micro- and macro-scale fracture of printed traces under mechanical loading. Substrate, ink and printing process parameters are identified to further improve strain sensing characteristics of low-cost, large area strain mapping systems. Reliable, low-cost, and large-area strain mapping systems are sought for continuous or on-demand real-time diagnosis and prognosis of complex structural components.

Formation and Characterization of a Well-Packed Particulate Structure Consisted of Cu Nanoparticles By Capillary Force for Low Resistivity at Low Temperature

Cu nanoparticles (NPs) have been studied extensively to develop nano ink for printable electronics on flexible substrates because of its low cost, high conductivity and low sintering temperature. Although there are many synthesis methods with low resistivity which normally require high temperature, high pressure, toxic regent and special equipment, it was difficult to achieve the practical use of Cu NPs because the synthesis methods are expensive. We have developed a facile and green synthesis method of Cu NPs by using water, citric acid, ascorbic acid as solvent, complex/capping agent and reducing agent, respectively. The particle size of the citric acid modified Cu NPs can be controlled from 30 nm to 10 µm by varying pH and concentrations of Cu ion and ascorbic acid. For low resistivity, a high packing density of the Cu NPs is required to increase the interparticulate junction area before heat treatment because the well-packed particulate structure leads to a low temperature densification reaction of metal NPs. We have succeeded in increasing the interparticulate junction area by combining with the Cu NPs and the micro-sized particles (MPs). The bimodal size distribution leads to the high packing density because small particles are placed in voids of packing large particles. However, the resistivity of the Cu particles (32 µΩ・cm) after heat treatment at 200oC is far from that of bulk Cu (1.7 µΩ・cm) because the Cu MPs prevent low temperature sintering of the mixture. Therefore, a well-packed particulate structure consisted only of Cu NPs is require for low resistivity at low temperature. In this study, capillary forces generated from solvents evaporation were utilized to obtain the high packing density of the Cu NPs for low resistivity at low temperature because capillary forces have been applied to obtain closely packed various NPs. For characterizations of Cu films prepared from the Cu NPs, XRD, FT-IR, SEM and XPS measurement were carried out. Besides, the electrical resistivity of Cu films after heat treatment was measured. Since the citric acid modified Cu NPs can well disperse in alcohols and boiling points of alcohols are relatively low as compared other organic solvents, alcohols are suitable to form stable Cu ink and use capillary forces. Cu films were prepared from drop casting of the Cu alcohol ink and doctor blade process using Cu NPs paste consisted of the Cu NPs and α-terpineol as a traditional method. The weight of these Cu films is controlled to be approximately the same. After dry treatment of the Cu films, the thickness of the Cu film (8 µm) prepared from the Cu alcohol ink is much thinner than that (30 µm) of prepared from the Cu paste. The high packing density of the Cu NPs arises from capillary forces during alcohol evaporation. After heat treatment at 200oC under Ar+H2 (98%+2%), the resistivities of the Cu films obtained from the Cu ink and the Cu paste are 7 µΩ・cm and 80 µΩ・cm, respectively. It was found that the well-packed particulate structure consisted only of Cu NPs by capillary forces facilitates a densification by heat treatment, resulting in the low resistivity at low temperature. In our presentation, detailed results about the Cu films prepared from the Cu ink will be introduced. This work was supported by JSPS KAKENHI Grant-in-Aid for Young Scientists (B) 15K16155.

Thermal processing of aqueous AZO inks towards functional TCO thin films

Aluminium doped zinc oxide is considered an interesting, earth abundant alternative to the transparent conductor indium tin oxide. In this respect, we synthesized Al-ZnO (AZO) thin films by a chemical solution deposition approach. Using water-based inks, the AZO thin films were printed in one single step on glass substrates in non-vacuum conditions. The drying process and the humidity of the atmosphere proved to be crucial during the thermal processing of the wet deposited thin films. To improve the understanding of the phenomena involved, the influence of these processing parameters on the final properties of the AZO thin films are investigated by FTIR, XRD, SEM, 1H and 27Al NMR and resistivity measurements. Optimal conditions were obtained by drying the wet thin film at 120 °C, followed by heating in a tube furnace until 500 °C under a 200 ppm O2/N2 atmosphere with a humidity controlled by bubbling the gas flow through 2 glass bottles containing water at 20 °C. AZO thin films with an resistivity of 2.54 × 10−2 Ω cm were obtained.

Mixed ink of copper nanoparticles and copper formate complex with low sintering temperatures

A kind of copper mixed ink with low sintering temperatures was developed. The mixed ink was composed of copper nanoparticles and copper formate 3-dimethylamino-1,2-propanediol (DMAPD) complex. TG-MS showed a two-step decomposition process of the mixed ink below 200 °C. The copper formate DMAPD complex decomposed to copper nanoparticles at about 140 °C. The freshly formed small copper nanoparticles dispersed around the large copper nanoparticles of the ink, and then sintered at relatively low temperatures to form connections between larger nanoparticles. The copper films were obtained by thermally sintering of the mixed ink films at different temperatures. The mixed ink with 38 wt% of copper formate complex gave a film with the lowest resistivity of 18 μΩ cm after sintering at 200 °C, which is less than one thirtieth of that of the film without copper formate complex at the same sintering temperature. The morphology analysis showed the dense copper films from the mixed ink, in contrast to the loose film from the organic decomposition ink made of copper formate complex.

Novel Use of Preoperative Epidermal Coloring of Very Small Dermatological Specimens-Protocol for Reduction of Lost Specimens.

Small tissue biopsies are often difficult to visualize and can be easily lost or mishandled. The authors hypothesized that full epidermal surface coloration of small skin lesions with a sterile skin marker (gentian violet ink) before performing shave biopsy would make small gross specimens easier to identify without impacting microscopic appearance. Live evaluation of 4 inked and 4 noninked gross (2-3 mm) specimens in covered and uncovered formalin-containing jars by 50 consecutive health care personnel demonstrated that inked specimens were significantly (P < 0.001) easier to visualize than noninked specimens. Additionally, a blinded dermatopathologist evaluated 25 inked and 25 noninked specimens microscopically. Utilization of this inking process did not interfere with histopathologic assessment or impede diagnosis. This pilot study describes an easily implementable quality improvement measure that may decrease the rate of loss and mishandling of specimens.

Colloidal Wurtzite Cu2SnS3 (CTS) Nanocrystals and Their Applications in Solar Cells

In the development of low-cost, efficient, and environmentally friendly thin-film solar cells (TFSCs), the search continues for a suitable inorganic colloidal nanocrystal (NC) ink that can be easily used in scalable coating/printing processes. In this work, we first report on the colloidal synthesis of pure wurtzite (WZ) Cu2SnS3 (CTS) NCs using a polyol-mediated hot injection route, which is a nontoxic synthesis method. The synthesized material exhibits a random distribution of CTS nanoflakes with an average lateral dimension of ∼94 ± 15 nm. We also demonstrate that CTS NC ink can be used to fabricate low-cost and environmentally friendly TFSCs through an ethanol-based ink process. The annealing of as-deposited CTS films was performed under different S vapor pressures in a graphite box (volume; 12.3 cm3), at 580 °C for 10 min using a rapid thermal annealing (RTA) process. A comparative study on the performances of the solar cells with CTS absorber layers annealed under different S vapor pressures was cond...

Dissolution of Sn, SnO, and SnS in a Thiol-Amine Solvent Mixture: Insights into the Identity of the Molecular Solutes for Solution-Processed SnS.

Binary solvent mixtures of alkanethiols and 1,2-ethylenediamine have the ability to readily dissolve metals, metal chalcogenides, and metal oxides under ambient conditions to enable the facile solution processing of semiconductor inks; however, there is little information regarding the chemical identity of the resulting solutes. Herein, we examine the molecular solute formed after dissolution of Sn, SnO, and SnS in a binary solvent mixture comprised of 1,2-ethanedithiol (EDT) and 1,2-ethylenediamine (en). Using a combination of solution (119)Sn NMR and Raman spectroscopies, bis(1,2-ethanedithiolate)tin(II) was identified as the likely molecular solute present after the dissolution of Sn, SnO, and SnS in EDT-en, despite the different bulk material compositions and oxidation states (Sn(0) and Sn(2+)). All three semiconductor inks can be converted to phase-pure, orthorhombic SnS after a mild annealing step (∼350 °C). This highlights the ability of the EDT-en solvent mixture to dissolve and convert a variety of low-cost precursors to SnS semiconductor material.

EFFECT OF HLB NUMBERS OF SURFACTANT ON THE RHEOLOGICAL PROPERTIES OF HMX-BASED ENERGETIC INK

Energetic ink, combined with energetic materials and additives through adaptive three-dimensional printing technology, is a promising method of replacing traditional charge styles, and its rheological property is an important factor in the preparation process. Sorbitan monooleate (Span-80) and polyoxyethylene sorbitan monooleate (Tween-80) are two surfactants that have been commonly used to improve the rheological properties of suspensions. The influence of surfactants in the viscosity of energetic ink is a significant issue that is worth exploring. The effects of the surfactant percentage and hydrophilic−lipophilic balance (HLB) numbers on ink viscosity were obtained based on the the addition of different contents of Span-80 and Tween-80 to an ultrafine cyclotetramethylenete tranitramine (HMX)/alcohol nylon/ethanol system. The results are presented using the rules of viscosity, the non-Newtonian indexes that reflected the rheological stability, and the yield values that reflect the degree of difficulty from the static state to flow. The best HLB number that effectively increased the rheological property in this study was chosen according to the results, which may provide some reference for additives in the preparation process of energetic ink.

Modeling thermal stress in thin films produced by photonic curing

A thermal stress model is developed to simulate stresses produced in the processing of thin film metal inks due to arbitrary time dependent temperature variations through the film stack. The current model employs and extends the method developed by Hsueh (2002), which models stresses in thin films where the temperature of each layer in the film stack was assumed to be uniform at a temperature different from the initial temperature. The model results in two integrals involving temperature distribution through the film stack. The thermal stress model developed is validated using two cases where analytical temperature solutions are available. The first is a constant surface heat flux on a single layer. The second is a constant surface temperature on a three-layer film stack. Both cases begin from an initial uniform temperature. The integrals appearing in the expressions for stress distributions are computed using the analytical solutions of the temperature distributions and are used to compute the stress distributions through the film stack. The analytical solutions are compared to numerical results produced by solving the transient heat conduction equation using a previously developed finite volume method. In the numerical solutions, the integrals involving temperature are approximated using the midpoint rule. The computed numerical results compare very well with the analytical solutions for both cases studied.

Investigation of DLP Projector as an Energy Light Source to Cure Silver Conductive Ink

Conductive ink has been widely used in many application of electronic parts. It normally comes in liquid form and will need to be cured usually by conventional oven or laser irradiation after it is drawn to expose its metallic contents. But, these curing processes suffer from numerous problems consist of laser is a costly process and a warping defect when cured using conventional oven. Thus, alternative process is prominent and DLP projector has been seen capable of polymerised photopolymer materials. By making use of its potential associated with a proper control of the light intensity, exposure time and distance, it could provides sufficent heat to cure conductive ink. This study investigates the curing process of silver conductive ink on polymer substrate using DLP projector light with a variation of curing time and distance. Electrical resistance, hardness and adhesion level of cured sample were measured to determine its functionality. The conductive ink is deposited on the substrate using a doctor blade method and Microsoft Powerpoint slide is used to shape the light image produced by the projector. It was found that the conductivity and curing level of the sample cured were higher at the distance which getting the sharpest light image. The performances of the samples were also greater as the curing time increased. DLP projector is able to cure the conductive track efficiently with shorter curing time apart from the cheap preparation and setup cost.

Flexible large area organic light emitting diode fabricated by electrohydrodynamics atomization technique

Large area organic light emitting diode (OLED) has been fabricated on flexible polyethylene terephthalate (PET) substrate using electrohydrodynamics spray system with active area of 3 × 3 cm2. This solution processing of organic inks and quantum dots is performed at room temperature and atmospheric pressure in a single step processing compatible with roll-to-roll processing system. The thin film characterizations are performed by SEM, TEM, spectroscopes and semiconductor device analyzer. CdSe/ZnS quantum dots are used as emissive layer while PEDOT:PSS and MEH-PPV are used as hole and electron transport layers. OLED produced red light at the wavelength of 634 nm. A life time of 1 h at 0.3 lx was achieved by the OLED device.

Solid Oxide Steam Electrolysis Anodes with Precisely Controlled Microstructure for Performance Analysis

Understanding the effect of oxygen electrode structure on electrochemical performance of a solid oxide electrolysis cell is of great importance, especially considering oxygen electrode delamination issues that are presently limiting their commercialization. To study the structure-performance relationship a closely managed production method has been developed to create controlled and repeatable electrode structures. The electrode deposition method uses a solvent based ink sprayed using an air brush. The ink formulation, processing, and deposition have been strictly controlled. The sintering method chosen to calcine the structure was carefully constrained. Morphological differences caused by the adjustment to the ink formulation, spraying, and sintering parameters were observed.

Dispersion behaviour of graphene oxide and reduced graphene oxide

The dispersion behaviour of graphene oxide (GO) and chemically reduced GO (rGO) has been investigated in a wide range of organic solvents. The effect of the reduction process on the GO solubility in eighteen different solvents was examined and analysed, taking into consideration the solvent polarity, the surface tension and the Hansen and Hildebrand solubility parameters. rGO concentrations up to ∼9μg/mL in chlorinated solvents were achieved, demonstrating an efficient solubilization strategy, extending the scope for scalable liquid-phase processing of conductive rGO inks for the development of printed flexible electronics.