Symposium MA01 : Advanced Materials for Analog and Digital Functional Printing

Materials that either absorb or reflect in the near infrared (NIR) region have numerous applications. In particular, compounds that absorb in the NIR can be found in the market of smart windows, security inks, agricultural films, laser marking, laser welding and functional coatings, amongst others [1-2]. In this work we focus our attention in pigments for security inks and laser marking technology. There is a current interest in the industry to find non-toxic and non-expensive security inks for documents such as passports, visas, certificates, etc, to avoid counterfeiting and/or identity fraud. Moreover, these pigments can also be diluted in polymer compositions and used in the laser marking of plastics. It is an important industry considering the number of plastic objects that surround us that need marking [3].
Pigments used in this work are copper hydroxyphosphates with formula Cu₂PO₄OH. As a mineral it can be found in nature with the name libethenite, which belongs to adamite-type compounds and has space group Pnnm [4]. Several Cu₂PO₄OH samples were synthesized using different conditions and later characterized by XRD, FTIR, Raman, TGA, SEM and TEM. The optical properties were checked by UV-VIS-NIR spectroscopy. In order to assess the applicability of the materials as laser marking additives and as pigments in security inks, low density polyethylene testing plates and inks were tested.
The libethenite phase was detected in all samples after using different synthesis conditions. Interestingly, a sample was synthesized by using a low-cost synthetic procedure and was easily upscaled to 400 gr. After monitoring the temperature of the coated films, it was possible to observe a significant temperature decrease when compared to a resin-only coated film. The behaviour of the coated inks allows us to suggest that they could be used as security inks and be detected by a Raman detector.
In addition, several laser codes were successfully marked on testing plates. Because of the positive outcome of the laser-marking test we could suggest the use of libethenite for the marking of plastics.

References
[1] US Patent 20130264529 A1.
[2] US Patent 8778494 B2.
[3] R. Wissemborski, R. Klein, Welding and marking of plastics with lasers, LTJ 7 (2010) 19-22.
[4] I-S. Cho, D. W. Kim, S. Lee, C. H. Kwak, S-T. Bae, J. H. Noh, S. H. Yoon, H. S. Jung, D-W. Kim, K. S. Hong, Synthesis of Cu2PO4OH hierarchical superstructures with photocatalytic activity in visible light, Adv. Func. Mater. 18 (2008) 2154-2162.

This paper demonstrates a novel technique for printing conductive structures consisting of silver nanoparticles on plastic substrates. The main advantage of this new printing technique is that it employs a novel ink delivery method that can effectively reduce the size of the printed features and hence improve printing resolution. The resolution of current printing technologies is limited by the size of the nozzles used in the print heads and the spreading of ink after deposition. Here a combination of co-solvent and substrate interactions are used to reduce the feature size of solution printed circuits.

A variety of printing parameters and their effect on the printed features is studied. These factors include the relative solvent flow rates and surface energies of the nanoparticle inks, and substrate surface energies. A simple model is also developed to explain the observed trends. Understanding how these factors affect the printing process will allow us to better control the printing as needed required by different applications. Using this printing technique, silver nanoparticle lines down to widths of approximately 30 microns are achieved using a 300 micron print nozzle.

Transparent electrodes (TE) constitute a critical component within a wide range of devices including touch screens, solar cells, light emitting diodes (LEDs) or transparent heaters [1]. To date, the most commonly used transparent conductive material (TCM) is still indium tin oxide (ITO), however, the scarcity of indium and lack of flexibility of ITO have prompted the search for alternative materials. Among different types of emerging materials, the transparent electrodes based on silver nanowire (AgNW) networks exhibit excellent optical, electrical and mechanical properties fulfilling the requirements for many optoelectronic applications [2]. However, AgNW networks still suffer from thermal and electrical instabilities, requiring an effective and conformal protective layer. In addition, AgNW networks cannot act as an antireflective window or an effective collection layer in photovoltaic applications (because of free gaps between nanowires), which are usually ensured by a thin layer of metal oxide.
The aim of this contribution is to develop a composite electrode based on AgNW and metal oxide thin films such as zinc oxide (ZnO), aluminum oxide (Al₂O₃) or aluminum doped zinc oxide (AZO), which are printed by our home-made atmospheric pressure spatial atomic layer deposition system (AP-SALD) [3],[4]. We will show that a thin conformal ZnO coating deposited by APSALD technique can drastically enhance the stability of AgNW networks from 300°C up to 500°C. Similarly, while the electrical stability of uncoated AgNW network was 9V, ZnO coated network showed a 100% increase in electrical stability, up to 18V. The integration of the composite electrode to silicon heterojunction solar cell, as well as its implementation on flexible substrate, will be discussed.
In contrast to traditional fabrication processes, the APSALD technique relies on the possibility to print thin films in a vacuum-free, low-temperature (<200°C), low-cost and high throughput way (for instance compatible with roll-to-roll technology). Besides, the fabrication of AgNW networks involves also low-temperature processing steps and upscaling methods. Hence, the combination of both materials and their printable fabrication processes make this type of composite electrode very appropriate for future use, especially for flexible devices.
Ref:
[1] K. Ellmer, “Past achievements and future challenges in the development of optically transparent electrodes,” Nat. Photonics, vol. 6, Nov. 2012.
[2] D. Langley et al., “Flexible transparent conductive materials based on silver nanowire networks: a review,” Nanotechnology, vol. 24, 2013.
[3] D. Muñoz-Rojas and J. MacManus-Driscoll, “Spatial atmospheric atomic layer deposition: a new laboratory and industrial tool for low-cost photovoltaics,” Mater. Horiz., vol. 1, 2014.
[4] V.H. Nguyen et al., “Deposition of ZnO based thin films by atmospheric pressure spatial atomic layer deposition for application in solar cells,” J. Renew. Sustain. Energy, vol. 9, Mar. 2017.

This work investigates scalable synthesis and patterning of structured nanoparticles in polymer matrix nanocomposites using a chemical infusion technique as well as laser-assisted processing. A chemical infusion process using various metal and metal oxide precursors has been used to create functional nanoparticles in a solid polymer matrix. Under specific processing conditions, these nanocomposites consist of monodisperse nanoparticles distributed throughout the bulk of the polymer matrix. While patterned nanocomposites can be produced by applying a mask on the polymer matrix to physically hinder the precursor diffusion and the subsequent particle formation, a laser direct-write method using fiber-coupled laser sources with motorized stages is used to create various macroscopic patterns as well as complicated nanostructures. In particular, localized decomposition of supplementary precursor species in the immediate vicinity of the nanoparticles is achieved using laser irradiation, which results in the formation of secondary structures near the seed nanoparticles. Both continuous laser and femtosecond laser sources are used to study the temporal and spatial effects on producing patterned materials.

A major challenge of designing and fabricating electrodes and sensors for wearable and flexible electronic devices is the ability to make conductive composites that has a good dispersion of metal nanoparticles in a matrix that is often insulative. Poly(dimethylsiloxane) (PDMS) is one of the most commonly used elastomeric matrices used since it conforms well to a given shape leading to good contact between the conductive electrode sensor and sensing surface. Consequently, we can integrate both adhesion and conducting functions by creating a conductive PDMS composite that are loaded with conductive fillers such as Ag nanoparticles. The conductivity of these composites which dictates its sensing performance depends on intrinsic properties of filler and matrix, filler loading and filler-matrix interaction. A continuous electrical pathway is needed for these conductive filler-polymer networks so as to achieve a sufficiently low resistivity and reliable sensing. Therefore, the filler amount would need to exceed the percolation threshold concentration. For our Ag-particle based composite, the percolation threshold is estimated to be ~7-8 vol% based on existing literature. Generally, the percolation threshold limit depends on the filler morphology, strength of interaction of individual particles within the agglomerate and dispersion characteristics of the filler agglomerates. Although higher aspect ratio for fillers leads to lower percolation thresholds, a potential benefit of spherical nanoparticles is that the higher mobility of spherical nanoparticles would allow conductive pathways that were lost upon deformation to be more easily recovered. Polyacrylic acid (PAA) is typically used as a particle stabilizer for inks used for direct writing. However, PAA-modified Ag nanoparticles do not mix with PDMS due to incompatible interfacial chemistries, which also leads incomplete PDMS curing as the carboxylic functional groups interfered detrimentally with the catalyst used for PDMS curing. Thus, the modification of the Ag-PAA particles surface chemistry was needed to improve its dispersion and enable the curing of Ag-PDMS mixture.

In this presentation, a single-step process for modifying the existing PAA-functionalized Ag nanoparticles through an inter-polymer complexation between PAA and PVP was presented and the interfacial characteristics of the PAA-PVP-modified nanoparticles system were investigated. The formation of intermolecular hydrogen bond between PAA and PVP, and the increased thermal stability and intermolecular interactions as PAA complexed with PVP were also verified. This surface modification via complex formation allowed the mixture to cure for Ag loading as high as 25 vol% to form Ag-PDMS composites, whilst the unmodified Ag-PAA PDMS system cannot be cured once it exceed 7 vol% of Ag. With the higher Ag loading, we were able to lower electrical resistivity from ~110 Ω.cm (at 7 vol%) to ~6 Ω.cm (pass percolation threshold).

A particular sub-branch of 3D printing is extrusion-based printing (EBP) of metallic and ceramic colloidal materials, where highly concentrated suspensions of particles are deposited to construct near-net shaped parts with complex geometries. Large-area magnets, transformers, magnets with complex shapes, constant-flux inductors, and electrochemical devices can be realized through EBP. However, polymer-based composites of Neodymium Iron Boron (NdFeB) family, iron, and NiCuZn ferrite are the only magnetic systems that are formulated as inks for EBP. The presence of high amounts of additives (5-8 wt. % to 69 wt. %) intrinsically lowers the loading of magnetic particles and limits the performance of fabricated objects due to the reduction in the magnetic material filling factor. To harness the full potential of EBP, there is a need for maximum purity inks. We systematically pursued a particle-specific approach to design an additive that can cater the surface charge and its distribution in iron oxide particles (IOPs) to stabilize and control the viscosity of suspensions of IOPs. We synthesized poly(ethylene glycol) (PEG)-grafted copolymers of N-[3(dimethylamino)propyl]methacrylamide (DMAPMA) and acrylic acid (AA) and investigated the effect of i) comonomer ratios, and ii) the density of PEG side chains on the stability of suspensions. The optimized ink contained 81 wt. % of IOPs in the presence of 1.15 wt. % of a single additive (by weight of IOPs) in a fully aqueous medium. To demonstrate the printability of various geometries, we printed three different shapes of magnetic cores (rectangular, thick-walled toroidal, and thin-walled toroidal cores) and a porous lattice structure. We characterized the electrical and magnetic properties of magnetic cores through impedance spectroscopy (IS) and vibrating sample magnetometry (VSM), respectively. To the best of our knowledge, this fully aqueous ink is the first of its kind for EBP in terms of comprising a magnetic material of choice at highest amounts through the minimum use of a single additive.

Nanoparticle and 2-D materials are useful for printed electronics as they can be incorporated in all kind of inks which are compatible to various printing processes. Metallic nanoparticles based inks are already well established for conducting layers, and now the concept is being tried with oxide nanoparticels and 2-D materials, etc. Avoiding agglomeration of nanoparticles and low temperature annealing are two key challenges in this effort. This presentation discusses results on two such inks based on ZnO nanoparticles and graphene. Our interest is to develop these inks for fabricating all solution processed polymer light emitting diodes (PLEDs).

Solution processed ZnO layer is being developed as an alternative to LiF which acts as an electron injection layer and is thermally evaporated in our standard PLED stack (ITO/PEDOT (40-60nm)/MEH- PPV(60-80nm)/LiF/Al. Many applications use zinc oxide film prepared by sol-gel technique using 2-methoxyethanol and ethanolamine. This technique requires annealing at temperatures above 250^o C which can damage organic layers in PLEDs. In order to reduce the annealing temperature, we have adopted the approach to first synthesize nanoparticles of ZnO which can be annealed at low temperatures. We varied temperature during synthesis from 25, 40 and 60^o C, TEM image analysis shows nanoparticle size changes from 12, 6 and 4nm, respectively. Dynamic light scattering measurements show size in 100s of nm indicating aggregation and poor film formation. To suppress aggregation repeated purification (up to three times) process led to narrow particle size distribution and reduced particle size. This colloidal solution was used to make thin films on glass using spin coating. Film morphology and structure is studied using AFM and XRD and thickness is measured using profilometer.

Graphene is an excellent candidate for transparent conducting electrodes because of properties such as excellent optical transparency, high electrical conductivity together with flexibility. For this purpose, optimization in ink formulation will be reported to form films of graphene and PEDOT:PSS.

Paper electronics using solution processing is rapidly evolving and the possibility of extending the available information beyond the printed graphic is very interesting [1]. However, paper imposes special requirements for manufacturing electronic components by being rough and absorptive, the need for recyclability and/or compostability. For paper electronics to be truly recyclable and/or disposable the components need be environmentally friendly and bio-compatible, and preferably operating at low voltages. Here we propose an environmentally safe approach to such active circuits and electronic functionalities, by making ion-modulated transistors (IMTs) on paper [2].

We have further developed the paper electronics platform towards a fully printable paper-based test array for material-cell interaction studies [4]. The paper-based cell growth platform has been shown to provide an efficient and excellent high-throughput and low-cost platform for studying the effect of materials and surfaces on cell functions.

References
[1] D. Tobjörk and R. Österbacka, Advanced Materials, 23, 1935-1961 (2011).
[2] F. Pettersson, T. Remonen, D. Adekanye, et al., ChemPhysChem 16, 1286-1294 (2015)
[3] E. Rosqvist, E. Niemelä, et al., Manuscript in preparation. (2018)

Printed organic field-effect transitors (OFETs) have been considered for many novel applications towards large area and flexible electronics, since they can enable pervasive integration of electronic functionalities in all sorts of appliances, their portability and wearability. Applications are countless: from personal devices (e.g. wearable health monitoring devices) to large-area sensors (e.g. electronic skin, bio-medical devices), and smart tagging of products with radio-frequency identification tags. However, printed OFETs fabricated with scalable tools fail to achieve the minimum speed required for example to drive high-resolution displays or to read the signal from a real-time imager, where a transition frequency (f_T), i.e. the highest device operative frequency, above 10 MHz is required. In this contribution we present effective strategies to increase f_T in polymer devices by combining only printing and digital, laser-based direct-writing techniques.
By controlling the self-assembling properties of conjugated copolymers, in combination with simple, roll-to-roll compatible coatings, it is possible to achieve well-ordered and efficient charge-transport nanostructures over large-areas. In particular, by exploiting the one-dimensional self-assembly of model conjugated polymers, such as naphthalene diimide based co-polymers, highly controlled printed anisotropic thin films with excellent transport properties are demonstrated. By controlling the ink flow directionality with a bar-coating deposition technique, shear-aligned thin films with a highly oriented functional surface are realized at a coating speed of few meters per minute, without the need for additional post-processing steps. This approach produces a marked FET mobility anisotropy and greater performance uniformity with respect to the spin-coating deposition, and excellent electron mobility along the printing direction. The simple adoption of this fast coating approach allows to strongly enhance the highest operational frequency of FET devices, achieving a transition frequency in the MHz range.
By combining printing and laser-based direct-writing techniques, additional strategies to boost the transition frequency of polymer based devices can be pursued. First, by combining inkjet printing and femtosecond laser ablation to obtain small channel lengths, all-polymer FETs operating in the MHz regime can be fabricated on plastic without the use of any mask. In particular, an engineered layout of the contacts allows to achieve a transition frequency of 4.9 MHz. Alternatively, narrow, micron-scale metallic electrodes can be sintered on plastic through femtosecond laser sintering. The combination of such electrodes with fast-coated polymers allow to achieve the higher transition frequency for a mask-less fabricated polymer transistor to date, reaching 20 MHz.

Polyurethanes (PU) are versatile materials that are used in a variety of applications. They can be found in the fields of insulation, cushioning and light-weight materials. Recently, PU materials gained entry into the field of inkjet printable materials. It has successfully been shown that PU materials can be fabricated using reactive inkjet printing. Further on, porous PU materials are of specific interest in the fields of adsorber materials or as scaffolds in tissue engineering. Due to the versatility of PU materials, it is possible to formulate numerous inks, yielding diverse material properties. The combination of printing technology and porous structures has previously been shown using a DLP printer in combination with a UV-curable O/W-emulsion, yielding conductive porous objects. Thus, a combination of polyurethane foam chemistry and ink formulation is the goal. We investigated inks that can be used to obtain polyurethane foams via reactive inkjet printing. Generally, two inks are necessary to produce polyurethane foam via reactive inkjet printing. The first ink containing an isocyanate reactive component often called polyol and the second ink containing the isocyanate component. As inkjet printing is limiting the selection of materials, mostly due to the materials viscosity, low viscous isocyanate monomers were chosen. Furthermore, the polyol ink consists of a variety of components such as main polymer, a crosslinker, a blowing agent and catalysts.
We tested selected materials for their general usage as PU reactants with caution to inkjet printing. Therefore, low viscous materials such as poly(ethylene glycol) as the linear bulk polymer and 1,6-hexamethylene diisocyanate (HDI) as the reactive isocyanate component were chosen. The materials were characterized by means of rheology and tensiometry to assure the usability in an inkjet printhead. We will show the influence of the concentration of dibutyltin dilaurate (DBTL) and iron(III) chloride as the main gelling catalyst by means of creamtime measurements. Selected materials were printed using a Fujifilm Dimatix DMP-3000. The printed structures were characterized by FT-IR spectroscopy as well as light- and scanning electron microscopy.

Proposed applications of drop-on-demand inkjet printing extend beyond the printing of text and graphics to include rapid manufacturing of thin electrical devices. The most common material for printing electronics is silver nanoparticles. However, silver is expensive, and metal nanoparticle films require post-processing to remove polymeric stabilizers and recover high conductance. The post-processing step restricts substrate choice and slows throughput. These limitations are shared by other traditional electronic materials such as copper, gold, and aluminum. Oxidation of nanoparticles is another challenge that severely reduces or prevents electrical conductance, especially for copper and aluminum, and necessitates specialized processing and handling.

Carbon nanomaterials are a promising alternative to metallic nanoparticles because of their high electrical conductivity and chemical stability. Current carbon nanomaterial inks suffer from low concentrations because it is difficult to disperse carbon nanomaterials in common, environmentally-safe solvents. Low particle concentration prevents sufficient percolation in printed materials, so electrical conductivity is low. Electrostatic stabilization improves nanoparticle stability – and thus increases concentration – in liquid media. The surface charges of nanoparticles in a liquid form an electrical double layer with dissolved ions. When two particles approach each other, their double layers overlap resulting in a repulsive electrostatic force. The zeta potential characterizes the magnitude of the electric potential between the particle double layer and the bulk liquid and is measurable with dynamic light scattering techniques.

Here we investigate the electrostatic stability of a novel carbon nanomaterial, hollow carbon nanospheres¹ (HCNS), in water and ethylene glycol. The HCNS consist of concentric graphene spheres and are made from charred cellulose, the byproduct of a biofuel production process. We measure the zeta potential and particle size distribution as a function of pH to identify solvent parameters that maximize the electrostatic stability of the carbon nanomaterial. We observe an increased sensitivity of zeta potential to changes in pH for HCNS in water over those in ethylene glycol. The maximum observed zeta potential of HCNS in ethylene glycol is 50% less than those in water at the same pH. The results suggest an aqueous based HCNS ink is more stable than one based in ethylene glycol. By performing pH-titrations with HCNS, we identify the presence and concentration of ionizing surface groups as a function of pH and deduce their relative contributions to the electrical double layer. We also compare the maximum concentration of HCNS in water and ethylene glycol to commercial metal nanoparticle inks.

¹M. J. Wagner, J. Cox, T. McKinnon, and K. Gneshin, “Hollow carbon nanosphere based secondary cell electrodes.” U.S. Patent 8262942, issued September 11, 2012.

New materials are of high interest in additive manufacturing and in particular with 3D Printing. In this, talk we report on new properties and materials from Hierarchically porous thermoplastic polyurethane (TPU) that is 3D printed by direct viscous solution printing. The viscous materials are composed of powder mixture suspended in TPU polymer solution. In a 4D printing process, the printed objects undergo phase separation and chemical etching to generate three-level porous structure which transfers un-compressible plastics into super-elastic and lightweight elastic foam. Mechanical properties and porous structures are tunable through ink composition and computer design. This technique, for the first time, enables mold-free fabrication of TPU foam with complex 3D architectures. It is potentially a universal route to be applied to many other thermoplastic polymers for 4D printing of super compressible materials.

In recent years, interest in molecular doping of organic semiconductors has grown significantly. Doped organic semiconductors have applications in many device applications, including thermoelectrics, photovoltaics, and LEDs. However, doping also often has significant effects on film morphology or solubility, suggesting that doping processes could also play a role in device fabrication. As an example, doping induced solubility control (DISC) patterning can generate topographic features sizes below 300 nm, better than generally achievable by photolithography. The same process can also be used to generate bilayers of mutually soluble polymers. We recently reviewed prospects for these types of processes in Jacobs and Moule, Adv. Mater. 2017, 1703063.

In order to feasibly use molecular dopants to control film solubility or for patterning, we must develop sequential methods for adding or removing dopants to/from films. Several groups have studied sequential doping processes, however relatively little attention has been given to the reverse process—removing dopants from doped films. These dedoping processes, which are similar to compensation doping in inorganic semiconductors, are also important in device applications. Compensation doping leaves behind immobile ions, while dedoping involves subsequent removal of the compensated charges. In organic semiconductor device applications, dedoping processes are likely preferable to compensation due to the low doping efficiencies typically observed.

Here, we discuss dedoping mechanisms in organic semiconductors, focusing on poly(3-hexylthiophene) : 2,3,5,6-tetrafluoro-7,7’,8,8’-tetracyanoquinodimethane (P3HT:F4TCNQ) as a model system. We identify a series of reactions between F4TCNQ and amines that allow for quantitative dedoping, leaving film fluorescence yield unchanged, or in some cases higher than as-cast films (see Jacobs et al. Chem. Mater. 2017, 29, 832). Along with absorption, conductivity, and fluorescence microscopy measurements, our results suggest that primary amines react with intrinsic doping defects in P3HT, providing a simple method to improve film quality. We also present an analogous photochemical reaction between F4TCNQ and tetrahydrofuran (see Fuzell and Jacobs, et al., J. Phys. Chem. Lett. 2016, 7, 4297). This reaction allows for direct modulation of film doping at sub-micron length scales using focused laser light (Jacobs et al., Adv. Mater. 2017, 29, 1603221), and thus could be used to directly fabricate nanoscale devices (e.g. transistors). Similar reactions are almost certainly widespread in all classes of molecular dopants.

Conductive films have attracted much attention in the printed electronics industry. To date, expensive conductive silver inks have been utilized widely in these conductive films, which ultimately increase the cost. Hence the alternative low-cost copper inks will be of great interest in the future. This paper will present how to synthesize antioxidative conductive copper inks applicable to flexible substrates. The antioxidative conductive copper inks were prepared by dispersing the antioxidative copper particles in diethylene glycol with the bisphenol-F type BEF170 epoxy resin as a binder and the Methyl-5-norbornene-2,3-dicarboxylic anhydride (NMA) as a curing agent, then were coated on PI and PET substrates to form the copper films, followed by sintering by intense pulsed light in air . We found that the formation of three-dimensional structure between BFE170 binder and curing agent NMA don’t affect the conductivities of copper films, and meanwhile can enhance the adhesion strength on PI and PET substrates. The lowest resistivities of 535 μΩcm on PI substrates and 865μΩcm on PET substrates were determined by using the four-point probe method and the highest adhesion of no peeling after the peel-off test with 3M Scotch 600 tape were achieved with the copper ink composed of BEF170 epoxy resin binder mixed with curing agent NMA in an equivalent ratio of 1:1.

Thin film transistors (TFTs) are important electronic switching devices made by depositing thin films of an active semiconductor as well as the dielectric layer and metal electrode over a supporting substrate, which primarily used for large-area flat panel displays. TFT can be made using a wide variety of semiconductor materials from conventional amorphous silicon to recently organic and inorganic materials.
Metal oxide semiconductors are promising candidates for making transparent thin-film transistor circuitry, which has been extensively studied due to their favorable field-effect mobility, excellent environmental and thermal stability, high optical transparency, and high uniformity in large scale fabrication applications. Furthermore, the availability of solution-processed metal-oxide semiconductors open an avenue to fabricate metal oxide TFTs by direct printing processes. However, the majority of solution-processed metal oxide semiconductors are n-channel materials.
The availability of P-type semiconductor channel materials will enable Complementary Metal-Oxide-Semiconductor (CMOS) circuits which offer the benefits of lower power consumption, less heat generation, higher circuit density, higher noise-margin, and simpler architecture over circuits built on unipolar TFTs. Low temperature fabrication of printed p-type CuI TFTs was reported for the first time by our group [1]. One layer of CuI thin film was fabricated by inkjet printing the Copper iodide precursor ink directly onto the device substrate followed by immediate crystallization of CuI nanoparticles as the solvent evaporated. The substrate temperature during inkjet printing was varied in order to obtain continuous CuI films with large grain size for improved device performance. In this presentation, we will report the fabrication and characterization of p-type thin film transistors on flexible substrates by the combination of inkjet printing and photonic sintering.
[1] Choi, Chang-Ho, Jenna Y. Gorecki, Zhen Fang, Marshall Allen, Shujie Li, Liang-Yu Lin, Chun-Cheng Cheng, and Chih-Hung Chang. "Low-temperature, inkjet printed p-type copper (I) iodide thin film transistors." Journal of Materials Chemistry C 4, no. 43 (2016): 10309-10314.
This work was supported by the US National Science Foundation [CMMI #1537196, CBET# 1449383] and the Walmart Manufacturing Innovation Foundation.

The development of ink based on water-soluble complexes (WSC) is one of actual and worthwhile directions for development of ink-jet printing technology. Printing using the WSC-based ink implies local deposition of the complex compound in the form of a necessary pattern on the substrate and subsequent decomposition of the deposited complex ink to the final functional material (metal, oxide, etc.) by subsequent treatment (UV, IR, etc.). WSC-based inks, which are true solutions, would significantly solve the problem of aging inherent for inks based on colloidal solutions. Since the use of the WSC-based inks eliminates any possibility of aggregation of particles, which has a favorable effect on the uniformity and trouble-free operation of the print heads of printers. Moreover, the creation of WSC-based inks with a low decomposition temperature will allow using a wide range of organic flexible materials as substrates.
The report presents the results of our research, which was directed on the development Ag-based ink for creating conductive paths of electronic circuits. The [Ag (NH₃)₂]2CO₃-based ink has been developed and the decomposition processes of the Ag-based complex to the metal under low-temperature treatment (T < 120°C) have been investigated.
The [Ag(NH₃)₂]2CO₃ complex was produced by the interaction of Ag₂CO₃ (1 g) with a 25% aqueous solution of ammonia (3 ml). Further, ammonium formate (AF) was added to the prepared solution in various molar ratios of Ag₂CO₃/NH₄HCO₂ (from 1:1 to 1:1.6). The prepared solutions had a viscosity of 2.13 mPa^.s and a Ag content of 27 wt%.
Then the solutions were layered to a glass substrate and heated to 100°C, resulting in a Ag recovery reaction according to the equation:
[Ag(NH₃)₂]2CO₃+NH₄HCO₂ → 2Ag+5NH₃+2CO₂+H₂O.
The XRD analysis revealed that as a result of thermal decomposition, a mixture of silver and silver oxide was formed on the substrate, where the proportion of silver oxide in the layers decreased with increasing content of AF in the solution. At a ratio Ag₂CO₃/NH₄HCO₂ = 1:1.6, silver oxide is not detected, which determines this molar ratio as the most optimal for the WSC-based silver ink.
Further to optimize the rheological properties of the ink, various polyols (ethylene glycol, diethylene glycol, glycerin) were used as an additive to the [Ag (NH₃)₂]2CO₃ complex solution (of 10 vol.%). It has been found that the additives of the polyols increased the viscosity and wetting coefficient. According to the study of the shape of the formed microdroplets of ink on the surface of the PET substrate the best wetting takes place in the case of the addition of ethylene glycol.
Finally, it can be concluded that the ink with the addition of ethylene glycol are most appropriate to technological requirements for ink-jet printing. The developed ink provided the highest wetting ratio necessary for both free passage of ink through the channels of the print head and for uniformly forming layers on heat-sensitive substrates.

"Printing" typically boils down to synthesis and spatial allocation of droplets and particles after a given pattern. Synthesis and spatial arrangement of nano-and microparticles of different size is important in designing nanostructured materials with functional properties. Wet synthesis methods lack flexibility to create various sizes of particles (particle libraries) using fixed conditions without the repetition of the steps of the method with a new set of parameters. Here, we report a new method where we arrange particulates from a liquid by using the competition and interplay of diffusion, chemical reaction and subsequent precipitation. The patterns develop by self-organization and needs no further manipulation than initial allocation of chemical sinks and sources. The synthesis method is based on nucleation and particle growth in the wake of a moving chemical front in a gel matrix. The process yields well-separated regions (bands) filled with mono-disperse nano-and microparticles, with the size of particles varying from band to band in a predictable way. The smallest particle size which we can report is below 300 nm. The smallest band separation is 25 micrometer. This method represents a new approach and a promising tool for competitive, fast, up-scalable, low-cost printerless printing of various sizes of colloidal particles with magnetic, optical, electrical and catalytic properties.

Applications such as the Internet of Things (IoT), flexible radars and 5G telecommunications will require new forms of electronics in the microwave and millimeter frequency domains. These applications may require form factors other than planar, rigid printed circuit boards, namely electronics that are flexible, conformable, wearable or embedded in 3D objects. Printed electronics is an additive manufacturing approach that uses electronic materials to fabricate circuits directly from CAD files. This emerging technology requires development of new electronic materials in the form of printable inks or filaments. This talk will describe research on application of conductive and dielectric inks to printed electronics, including the challenges of using these new electronic materials. These challenges include material characterizion at frequencies up to 30 GHz, printability, and thermal processing. In addition, a novel ferroelectric ink, developed to print varactors and phase shifters for steerable antenna arrays and frequency-agile systems, will be discussed. This ferroelectric ink can be tailored to have a relative permittivity (e_r) as high as 55 with a very low loss tangent (~10^-3) and a capacitance tunability up to 10% at microwave frequencies. The use of this novel material to print varactors and phase shifters (required for flexible, conformable radar systems) will be described.

Metal-organic compounds, such as copper and silver carboxylates, form a new class of conductive inks with unique properties that offer new opportunities in printable electronics. These molecular inks are compounds that upon heating decompose into their metallic state yielding conductive traces. These materials distinguish themselves from conventional conductive inks used in printable electronic in that they do not contain metal nanoparticles or flakes, and are printed and processed as molecular precursors. Thus their molecular nature affords benefits over conventional inks such as improved printability, robust mechanical properties, high electrical conductivity and the ability to use these inks in unusual ways. Our first research goal is to understand how the metal-organic compounds decompose to form metal films. With an understanding of the mechanism of decomposition, the molecular structure of the compounds can be optimized to improve the properties of the ink. Second the compounds must be formulated to be compatible with various printing techniques, such as inkjet, screen and aerospray printing. Third, the method of processing the inks, whether thermally or photonically must be optimized in order to yield metal traces with properties that meet the demands of printable electronics. This presentation will highlight the progress we have made in understanding and optimizing copper formate and silver carboxylate inks and showcase their use in unique applications.

Chemically modified graphene (CMG) based materials such as graphene oxide (GO) and reduced graphene oxide (rGO) have attracted considerable attention as precursors for the fabrication of filtration membranes, RF circuits, supercapacitor and battery electrodes as well as anti-corrosive and thermal protection coatings. In this regard, we have developed a unique bulk coagulation procedure for the synthesis of transition metal incorporated GO and rGO colloidal dispersions (aqueous and non-aqueous) with controllable rheological properties. In particular, using these dispersions/inks in conjunction with traditional coating techniques as well as with ink-jet and 3-D paste-printing techniques, we demonstrate the ability to fabricate a wide variety of devices and structures. Particular attention is paid to optimizing ‘graphene-ink’ properties, such as viscosity, composition and conductivity for the targeted printing and characterization of CMG derived optically active thin films, plasmonic metamaterials and RF circuits. The versatility of using CMG based inks and pastes in the context of additive fabrication of multifunctional ‘material real-estates’ will also be highlighted.

In this talk we will discuss the state of the art and potential future directions in paper-based electronics with special emphasis to the work developed at CENIMAT|i3N, covering electronic devices, smart displays, printed electronics, sensors and diagnostic tests.

We have been observing a rapid and growing interest concerning the utilization of biological materials for a wide range of applications. One of the most representative example is cellulose, not only in the form of raw material mainly for pulp and paper production, but also in the development of advanced materials/products with tailor-made properties, especially the ones based on nanostructures.

5 years ago paper electronics was pure science fiction, but today we have already several paper-based electronics like integrated circuits, supercapacitors, batteries, fuel cells, solar cells, transistors, microwave electronics, digital logic/computation, displays, force-sensing MEMS, user interfaces, transparent substrates, substrates with high strength, wearable devices, and new rapid diagnostic test sensors. These devices with their associated physics and processing will play an important and relevant to our society ongoing efforts to in environmental sustainability, safety, communication, health, and performance.

In modern daily life, the large area display have been the major information delivery media for internet and broadcasting. Furthermore, as represented by the recent development of high performance thin-film solar cell, ubiquitous sensor array for personal health care, and internet of things, the emerging large area electronics applications requires development of high performance semiconducting materials for emerging applications. Metal chalcogenide have been known as promising materials for electronics and solar cell applications. Although there have been limited success of solution processing of metal chalcogenides, The general strategy for obtaining high-quality, large-area metal chalcogenide semiconductor films from soluble precursors is still under development. In this talk, I will present the recent development of high performance metal chalcogenide semiconductors with chalco-gel based soluble molecular precursor. The optimized TFT device could exhibit a maximum field-effect mobility greater than 300 cm²V^-1s^-1 with an on/off current ratio of > 10⁷ and a good operational stability (threshold voltage shift < 0.5 V at positive gate-bias stress of 10 ks). In addition, metal-chalcogenide-based seven-stage ring oscillators operating at an operating speed of ~2.6 MHz (propagation delay of < 27 ns/stage) was demonstrated.

Membranes are ubiquitously used but today’s microporous membranes have random pore morphology and tortuous paths which may not suit certain applications. Recent demands in purification processes, cell biology research, and stenciling applications are driving the development of new classes of membranes with very uniform pore architecture (size, distribution, shape, order, density) and straight pore channels.
A plethora micro-nanofabrication methods which enable excellent morphological control have been reported. Amongst the traditional top-down approaches, mold-based imprinting and replica molding techniques are widely adopted for replicating highly dense and ordered pattern from a prefabricated mold onto target material at elevated temperature and pressure or under ultraviolet radiation. However, imprinting process typically leaves behind thin residual layer which requires complex and laborious etching procedures for its removal.
The last two decades have witnessed significant progress in soft lithography which is well known for its remarkable simplicity and good patterning fidelity. Micromolding in Capillaries (MIMIC) is a soft lithography variant capable of generating numerous residue-layer free polymeric structures with ordered dimensions from ~1 μm to >100 μm. The significance of MIMIC process is the in-situ through-thickness pore formation without the need for complex procedures and expensive post etching processes. Instead, conformal contact between the patterned mold and support substrate, and the ensuing capillary flow into the formed channel network are important governing factors. As a result, MIMIC has been increasingly explored as a membrane fabrication technique but like many other microfabrication methods, it is inherently batch-based with manual steps.
To facilitate the transition from batch to continuous production of residue-layer free membranes with precise pore architecture, a novel roll-to-roll (R2R) platform was designed and developed based on MIMIC. The core idea of the R2R platform is to adopt a roller comprises a series of flexible and reusable patterned molds, and to allow each fabrication step to be automated with precise positioning and reproducible force control. The versatile process can also fabricate multilevel, multiscale patterned structures that are difficult to achieve with existing fabrication methods. Complex structures such as ordered porous membranes with integrated nanoporous top layer and double-side surface patterned membranes will be shown as examples.
As a production platform, the first-of-its-kind R2R system offers time, reproducibility and economic incentives over slower, non-automated MIMIC process carried out in batch or, over laborious, expensive post etching processes. The capillary driven, room temperature and solvent-free process is also comparatively a more sustainable printing process for potential scale up production of a number of membrane products.

Bone marrow is a highly complex tissue, in which the regenerative systems of blood and bone are closely interlaced. Hematopoietic stem cells (HSCs) are at the root of blood formation and give rise to all different types of blood cells including erythrocytes and immune cells. Nowadays, HSCs are the only stem cells routinely applied in the clinics to treat patients that suffer from hematological disorder such as leukemia. Therefore, being able to manipulate or multiply HSCs in vitro is highly desirable from a clinical point of view.
In vivo HSCs are tightly controlled by their local microenvironment – their so-called stem cell niche in the bone marrow. In this niche HSCs receive all the signals that they need to maintain their stem cell properties (i.e. the abilities to differentiate and to self-renew). These signals include not only biological signals such as growth-factors, cell-cell contacts or cell-matrix interactions but also physical stimuli such as matrix stiffness, nanopatterning or the three-dimensional architecture of the environment. During the last years, we could show that all these parameters are able influence HSC behavior. Using conventional approaches to create 3D cell culture scaffolds, we found that an appropriate 3D architecture of the environment is particularly crucial in the development of in vitro systems that allow targeted HSC proliferation and differentiation. Such conventional approaches, however, do not allow a full biomimicry of the bone marrow. Additive manufacturing might be the key to achieve this goal.
Which are the requirements for a perfect bone marrow analog? How could 3D printing – including printing of stiff inorganic material, soft matrices and cells - help of fulfill these requirements? These questions will be addressed in order to evaluate, how bioprinting could help us to do another step towards a synthetic bone marrow analog that allows the in vitro multiplication of HSCs.

Digital 3D manufacturing techniques are being successfully adapted for tissue engineering applications. Each technique requires printable matrices, so called bioinks that match with the requirements of the process and meet the needs of cells. We provide bioinks based on biomolecules from the native extracellular matrix and present a well-controlled procedure to produce gelatin derivatives with defined degrees of modification.[1]
Double chemical functionalization of gelatin by methacryloylation and acetylation enabled control over the viscous behavior of gelatin solutions and the photochemical crosslinking. [2] For the first time we can quantify the total amounts of coupled methacrylate functions (methacrylic acid coupled to hydroxyl groups of the biomolecule), methacrylamide functions (methacrylic acid coupled to amino groups) and acetic functions based on 2D NMR.[1] E.g. application of 10-fold molar excess of methacrylic anhydride in relation to free amino groups resulted in GM10, gelatin derivatives with high degrees of metharcyloylation (DM) of 0.958 ± 0.068 mmol methacryl-functions per gram gelatin. Application of 5-fold or 2-fold excess lead to GM5 or GM2 with 0.338 ± 0.022 mmol/g or 0.618 ± 0.032 mmol/g, respectively.
It is of note that controlled addition of acetic anhydride yielded GM2A8 or GM5A5 with the same DM as GM2 or GM5, respectively, while the total degrees of modification (DMA) were equal to the DMA of GM10.
Viscosity analysis of gelatin solutions showed that the viscosity decreased with increasing DMA from 6.17 ± 2.20 mPas (GM2) to 2.42 ± 0.205 mPas (GM2A8) for 10 % solutions and even stronger from 110.2 ± 31.5 mPas (GM2) to 9.0 ± 0.4 mPas (GM2A8) for 20 % solutions.[2] The upper viscosity limit reported for inkjet printing is approx. 10 mPas. We reached the ink-jet printable range by double functionalization, which was not possible with simple functionalization at comparable DM. Through specification of the degrees of methacryloylation and acetylation we can provide bioinks with various viscosities, gelling behavior and crosslinking potential for different printing methods. Inkjet printing of low viscous GM10 inks loaded with chondrocytes indicated that the presented bioink is generally applicable for inkjet processing of viable cells.[2] Stable hydrogel crosslinking was achieved with layerwise extrusion printing and irradiation of bioinks composed of GM5A5 and GM2A8, photoinitiator and gradients of methacrylated chondroitin sulfate and hyaluronic acid. Promotion of osteogenic differentiation of mesenchymal stem cells was achieved in hydroxyapatite containing formulations of GM2, GM5 and methacrylated hyaluronic acid.[3]

1 Claassen et al., Biomacromolecules, 2017 under revision.
2 Hoch et al., J Mater Chem B, 2013. 1(41) 5675-5685.
3 Wenz et al., Biofabrication, 2017 accepted.