Droplet morphology analysis of drop-on-demand inkjet printing

As an accurate 2D/3D fabrication tool,inkjet printing technology has great potential in preparation of micro electronic devices.The morphology of droplets produced by the inkjet printer has a great impact on the accuracy of deposition.In this study,the drop-on-demand(DoD)inkjet simulation model was established,and the accuracy of the simulation model was verified by corresponding experiments.The simulation result shows that the velocity of the droplet front and tail,as well as the time to disconnect from the nozzle is mainly affected by density(ρ),viscosity(μ)and surface tension(σ)of droplets.When the liquid filament is about to disconnect from the nozzle,the filament length and filament front velocity are found to have a linear correlation withσ/ρμand ln(ρ/(μσ1/2)).

Additive Manufacture of Ceramics Components by Inkjet Printing

In order to build a ceramic component by inkjet printing, the object must be fabricated through the interaction and solidification of drops, typically in the range of 10–100 p L. In order to achieve this goal, stable ceramic inks must be developed. These inks should satisfy specific rheological conditions that can be illustrated within a parameter space defined by the Reynolds and Weber numbers. Printed drops initially deform on impact with a surface by dynamic dissipative processes, but then spread to an equilibrium shape defined by capillarity. We can identify the processes by which these drops interact to form linear features during printing, but there is a poorer level of understanding as to how 2D and 3D structures form. The stability of 2D sheets of ink appears to be possible over a more limited range of process conditions that is seen with the formation of lines. In most cases, the ink solidifies through evaporation and there is a need to control the drying process to eliminate the "coffee ring" defect. Despite these uncertainties, there have been a large number of reports on the successful use of inkjet printing for the manufacture of small ceramic components from a number of different ceramics. This technique offers good prospects as a future manufacturing technique. This review identifies potential areas for future research to improve our understanding of this manufacturing method.

Carbon-Based Flexible and All-Solid-State Micro-supercapacitors Fabricated by Inkjet Printing with Enhanced Performance

By means of inkjet printing technique, flexible and all-solid-state micro-supercapacitors(MSCs) were fabricated with carbon-based hybrid ink composed of graphene oxide(GO,98.0vol.%) ink and commercial pen ink(2.0vol.%). A small amount of commercial pen ink was added to effectively reduce the agglomeration of theGO sheets during solvent evaporation and the following reduction processes in which the presence of graphite carbon nanoparticles served as nano-spacer to separate GO sheets. The printed device fabricated using the hybrid ink,combined with the binder-free microelectrodes and interdigital microelectrode configuration, exhibits nearly 780%enhancement in areal capacitance compared with that of pure GO ink. It also shows excellent flexibility and cycling stability with nearly 100% retention of the areal capacitance after 10,000 cycles. The all-solid-state device can be optionally connected in series or in parallel to meet the voltage and capacity requirements for a given application.This work demonstrates a promising future of the carbonbased hybrid ink for directly large-scale inkjet printing MSCs for disposable energy storage devices.

An overview on the principle of inkjet printing technique and its application in micro-display for augmented/virtual realities

Augmented reality(AR)and virtual reality(VR)are two novel display technologies that are under updates.The essential feature of AR/VR is the full-color display that requires high pixel densities.To generate three-color pixels,the fluorescent color conversion layer inevitably includes green and red pixels.To fabricate such sort of display kits,inkjet printing is a promising way to position the color conversion layers.In this review article,the progress of AR/VR technologies is first reviewed,and in succession,the state of the art of inkjet printing,as well as two key issues-the optimization of ink and the reduction of coffee-ring effects,are introduced.Finally,some potential problems associated with the color converting layer are highlighted.

Drying-Mediated Self-Assembly of Graphene for Inkjet Printing of High-Rate Micro-supercapacitors

Scalable fabrication of high-rate micro-supercapacitors(MSCs)is highly desired for on-chip integration of energy storage components.By virtue of the special self-assembly behavior of 2D materials during drying thin films of their liquid dispersion,a new inkjet printing technique of passivated graphene micro-flakes is developed to directly print MSCs with 3D networked porous microstructure.The presence of macroscale through-thickness pores provides fast ion transport pathways and improves the rate capability of the devices even with solid-state electrolytes.During multiple-pass printing,the porous microstructure effectively absorbs the successively printed inks,allowing full printing of 3D structured MSCs comprising multiple vertically stacked cycles of current collectors,electrodes,and sold-state electrolytes.The all-solid-state heterogeneous 3D MSCs exhibit excellent vertical scalability and high areal energy density and power density,evidently outperforming the MSCs fabricated through general printing techniques.

Atmospheric-pressure Air Plasma Treatment of Polyester Fabrics for Inkjet Printing with Pigment Inks

Without any preprocessing,polyester fabric has lower ability to hold on water and inks due to the smooth morphology of polyester fibers. Therefore, patterns directly printed with pigment inks have poor color yields and bleed easily. Pretreatments of polyester fabric were carried out with atmospheric air plasma under different experimental conditions. After plasma treatment the samples were printed with magenta pigment ink. Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) analyses indicated that the enhanced color performance was mainly contributed by not only the etching effect but also oxygen-containing polar groups induced onto fiber surfaces through plasma treatment. Thereby the surface modification of polyester fabrics using atmospheric-pressure air plasma offers a potential way to fabric pretreatment for pigment inkjet printing with the advantages of environmental friendly and energy saving over traditional pretreatment methods.

Single droplet formation by controlling the viscoelasticity of polymer solutions during inkjet printing

Inkjet printing has emerged as a potential solution processing method for large-area patterned films.During inkjet printing,a single droplet without satellite droplet is required for high-quality film.Herein,we propose a strategy for obtaining a single droplet by adjusting the reduced concentration(c/c^(*),where c^(*)is the critical overlap concentration)in the range of 1.0-1.5.Droplet formation can be categorized into three distinct regimes:(1)c/c^(*)<1.0,satellite droplet;(2)c/c^(*)=1.0-1.5,single droplet;(3)c/c^(*)>2.0,no droplet.Furthermore,an inertial-capillary balance led to the 2/3-power scaling of the minimum radius with time for the solutions of c/c^(*)<1.0.However,for the solutions of c/c^(*)=1.0-1.5,the ligament radius decreased exponentially with time.Moreover,the Weissenberg number was higher than the critical value of 0.5,indicating that the polymer chains underwent coil-stretch transition.The viscoelastic-capillary balance dominated instead of the inertial-capillary balance.The resulting viscoelastic resistance reduced the length of the ligament and increased the velocity difference between the satellite and main droplets.Consequently,a single droplet was formed.In addition,the law can be successfully generalized to various molecular weights,molecular structures and solvents.

Enhanced photoluminescence of a microporous quantum dot color conversion layer by inkjet printing

Owing to their high color purity,tunable bandgap,and high efficiency,quantum dots(QDs)have gained significant attention as color conversion materials for high-end display applications.Moreover,inkjet-printed QD pixels show great potential for realizing full-color mini/micro-light emitting diode(micro-LED)-based displays.As a color conversion layer,the photoluminescence intensity of QDs is limited by the insufficient absorptance of the excitation light due to the lack of scattering.Conventional scatterers,such as titanium dioxide microparticles,have been applied after additional surface engineering for sufficient dispersity to prevent nozzle clogging in inkjet printing process.In our work,as an alternative approach,we use inkjet printing for depositing a phase separating polymer ink based on polystyrene(PS)and polyethylene glycol(PEG).QD/polymer composite pixels with scattering micropores are realized.The morphology of the micropores can be tailored by the weight ratio between PS and PEG which enables the manipulation of scattering capability.With the presence of the microporous structure,the photoluminescence intensity of the QD film is enhanced by 110%in drop-cast films and by 35.3%in inkjet-printed QD pixel arrays compared to the reference samples.

Graphene-Based Conducting Inks for Direct Inkjet Printing of Flexible Conductive Patterns and Their Applications in Electric Circuits and Chemical Sensors

A series of inkjet printing processes have been studied using graphene-based inks. Under optimized conditions, using water-soluble single-layered graphene oxide （GO） and few-layered graphene oxide （FGO）, various high image quality patterns could be printed on diverse flexible substrates, including paper, poly（ethylene terephthalate） （PET） and polyimide （PI）, with a simple and low-cost inkjet printing technique. The graphene-based patterns printed on plastic substrates demonstrated a high electrical conductivity after thermal reduction, and more importantly, they retained the same conductivity over severe bending cycles. Accordingly, flexible electric circuits and a hydrogen peroxide chemical sensor were fabricated and showed excellent performances, demonstrating the applications of this simple and practical inkjet printing technique using graphene inks. The results show that graphene materials--which can be easily produced on a large scale and possess outstanding electronic properties--have great potential for the convenient fabrication of flexible and low-cost graphene- based electronic devices, by using a simple inkjet printing technique.

Additive manufacturing of thin electrolyte layers via inkjet printing of highly-stable ceramic inks

Inkjet printing is a promising alternative for the fabrication of thin film components for solid oxide fuel cells(SOFCs) due to its contactless, mask free, and controllable printing process. In order to obtain satisfying electrolyte thin layer structures in anode-supported SOFCs, the preparation of suitable electrolyte ceramic inks is a key. At present, such a kind of 8 mol% Y_(2)O_(3)-stabilized ZrO_(2)(8 YSZ) electrolyte ceramic ink with long-term stability and high solid loading(> 15 wt%) seems rare for precise inkjet printing, and a number of characterization and performance aspects of the inks, such as homogeneity, viscosity, and printability, should be studied. In this study, 8 YSZ ceramic inks of varied compositions were developed for inkjet printing of SOFC ceramic electrolyte layers. The dispersing effect of two types of dispersants, i.e., polyacrylic acid ammonium(PAANH4) and polyacrylic acid(PAA), were compared. The results show that ultrasonic dispersion treatment can help effectively disperse the ceramic particles in the inks. PAANH4 has a better dispersion effect for the inks developed in this study. The inks show excellent printable performance in the actual printing process. The stability of the ink can be maintained for a storage period of over 30 days with the help of initial ultrasonic dispersion. Finally, micron-size thin 8 YSZ electrolyte films were successfully fabricated through inkjet printing and sintering, based on the as-developed high solid loading 8 YSZ inks(20 wt%). The films show fully dense and intact structural morphology and smooth interfacial bonding, offering an improved structural quality of electrolyte for enhanced SOFC performance.

Inkjet printing for electroluminescent devices： emissive materials, film formation, and display prototypes

Inkjet priming （IJP） is a versatile technique for realizing high-accuracy patterns in a cost-effective manner. It is considered to be one of the most promising candidates to replace the expensive thermal evaporation technique, which is hindered by the difficulty of fabricating low-cost, large electroluminescent devices, such as organic light- emitting diodes （OLEDs） and quantum dot light-emitting diodes （QLEDs）. In this invited review, we first introduce the recent progress of some printable emissive materials, including polymers, small molecules, and inorganic colloidal quantum dot emitters in OLEDs and QLEDs. Subsequently, we focus on the key factors that influence film formation. By exploring stable ink formulation, selecting print parameters, and implementing droplet deposition control, a uniform film can be obtained, which in turn improves the device performance. Finally, a series of impressive inkjet-printed OLEDs and QLEDs prototype display panels are summarized, suggesting a promising future for IJP in the fabrication of large and high-resolution flat panel displays.

Reduced graphene oxide with a highly restored π-conjugated structure for inkjet printing and its use in all-carbon transistors

An inkjet-printed graphene film is of great importance for next-generation flexible, low cost and high performance electronic devices. However, due to the limitation of the inkjet printing process, the electrical conductivity of inkjet-printed graphene films is limited to N10 S＇cm-1, and achieving a high conductivity of the printed graphene films remains a big challenge. Here, we develop a ＂weak oxidation- vigorous exfoliation＂ strategy to tailor graphene oxide （GO） for meeting all the requirements of highly conductive inkjet-printed graphene films, including a more intact carbon plane and suitable size. The -conjugated structure of the resulting graphene has been restored to a high degree, and its printed films show an ultrahigh conductivity of -420 S-cm-I, which is tens of times higher than previously reported results, suggesting that, aside from developing a highly efficient reduction method, tuning the GO structure could be an alternative way to produce high quality graphene sheets. Using inkjet-printed graphene patterns as source/drain/gate electrodes, and semiconducting single-walled carbon nanotubes （SWCNTs） as channels, we fabricated an all-carbon field effect transistor which shows excellent performance （an on/off ratio of -104 and a mobility of -8 cm2＂V-l＇s-1） compared to previously reported CNT-based transistors, promising the use of nanocarbon materials, graphene and SWCNTs in printed electronics, especially where high performance and flexibility are needed.

Inkjet printing technology for increasing the I/O density of 3D TSV interposers

Interposers with through-silicon vias(TSVs)play a key role in the three-dimensional integration and packaging of integrated circuits and microelectromechanical systems.In the current practice of fabricating interposers,solder balls are placed next to the vias;however,this approach requires a large foot print for the input/output(I/O)connections.Therefore,in this study,we investigate the possibility of placing the solder balls directly on top of the vias,thereby enabling a smaller pitch between the solder balls and an increased density of the I/O connections.To reach this goal,inkjet printing(that is,piezo and super inkjet)was used to successfully fill and planarize hollow metal TSVs with a dielectric polymer.The under bump metallization(UBM)pads were also successfully printed with inkjet technology on top of the polymer-filled vias,using either Ag or Au inks.The reliability of the TSV interposers was investigated by a temperature cycling stress test(−40℃ to+125℃).The stress test showed no impact on DC resistance of the TSVs;however,shrinkage and delamination of the polymer was observed,along with some micro-cracks in the UBM pads.For proof of concept,SnAgCu-based solder balls were jetted on the UBM pads.

Inkjet printing of 2D polyaniline for fabricating flexible and patterned electrochromic devices

Conjugated organic polymer(COP)-based electrochromic devices(ECDs)exhibit promising applications in digital and color displays.However,fabricating COP-based ECDs integrated with excellent electrochromic performance,customized patterns,and flexibility remains challenging.In this study,we report inkjet-printed,flexible,and patterned ECDs based on two-dimensional(2 D)polyaniline(PANI)sheets,which are evenly dispersed in formic acid(FA)enabling high-precision,stable inkjet printing.The pristine lamellar structure of PANI sheets,which combine nanoscale thickness and an appropriate doping ratio,and the additive-free ink composition endow the printed PANI electrodes and ECDs with high performance.The fabricated PANI electrode exhibits a high optical contrast(76%at a wavelength of 750 nm),a good coloration efficiency(CE)of 259.1 cm^(2) C^(-1),and a short coloration/bleaching time(1.8/2.4 s),simultaneously integrated with pseudocapacitance and mechanical flexibility.Moreover,the 2 D lamellar PANI ink developed in this study can be printed into various designed patterns,particularly for electrochemically controlled,addressable electrochromic displays.This work highlights 2 D lamellar PANI as a promising electrochromic material for flexible and patterned ECDs.

Effects of the actuation waveform on the drop size reduction in drop-on-demand inkjet printing

In this study the effects of the actuation waveforms on the droplet generation in a drop-on-demand inkjet printing are studied systematically by numerical simulations.Two different types of waveforms,namely the unipolar and bipolar actuations,are investigated for three fluids with different physical properties.We focus on two key parameters,which are the dwell time and the velocity amplitude.For the unipolar driving,the ejection velocity and the ejected liquid volume are both increased as the velocity amplitude becomes larger.The dwell time only has minor effects on both the ejection velocity and the ejected liquid volume.The ejection velocity decreases significantly for large liquid viscosity,while the influences of viscosity on the ejected liquid volume are much weaker.Four different droplet morphologies and the corresponding parameter ranges are identified.The droplet radius can be successfully reduced to about 40%e of the nozzle exit radius.For the bipolar waveforms,same droplet morphologies are observed but with shifted boundaries in the phase space.The minimal radius of stable droplet produced by the bipolar waveforms is even smaller compared to the unipolar ones.

Zwitterion-assisted transition metal dichalcogenide nanosheets for scalable and biocompatible inkjet printing

Inkjet printing of two-dimensional(2D)transition metal dichalcogenide(TMD)nanosheets fabricated by liquid-phase exfoliation(LPE)allows simple,mass-producible,and low-cost photo-electronic devices.Many LPE processes involve toxic and environmentally hazardous solvents;however,dispersants have restricted the extent of applications of 2D-TMD inks.Herein,various 2D-TMD nanosheets,including MoS2,MoSe2,WS2,and WSe2,in addition to few-layered graphene,are inkjet-printed using a LPE process based on zwitterionic dispersants in water.Zwitterions with cationic and anionic species are water-soluble,while alkyl chain moieties associated with two ionic species adhere universally on the surface of TMD nanosheets,resulting in high throughput liquid exfoliation of the nanosheets.The zwitterion-assisted TMD nanosheets in water are successtully employed as an ink without the need for additives to adjust the viscosity and surface tension of the ink for use in an office inkjet printer;this gives rise to A4 scale,large-area inkjet-printed images on diverse substrates,such as metals,oxides,and polymer substrates patchable onto human skin.Combination with conductive graphene nanosheet inks allowed the development of mechanically flexible,biocompatible-printed arrays of photodetectors with pixelated MoSe2 channels on a paper exhibiting a photocurrent ON/OFF ratio of approximately 1038 and photocurrent switching of 500 ms.

Recent advances in inkjet printing synthesis of functional metal oxides

lnkjet printing （IJP） synthesis has emerged as a useful technique for the fabrication of functional metal oxides in the fields of nanotechnology and materials science. In this paper, we will review the fundamental state-of-the-art principles of the special ink formulations used for IJP synthesis of functional metal oxides and the applications of these oxides.

Inkjet printing of epitaxially connected nanocrystal superlattices

Access to a blossoming library of colloidal nanomaterials provides building blocks for complex assembled materials.The journey to bring these prospects to fruition stands to benefit from the application of advanced processing methods.Epitaxially connected nanocrystal(or quantum dot)superlattices present a captivating model system for mesocrystals with intriguing emergent properties.The conventional processing approach to creating these materials involves assembling and attaching the constituent nanocrystals at the interface between two immiscible fluids.Processing small liquid volumes of the colloidal nanocrystal solution involves several complexities arising from the concurrent spreading,evaporation,assembly,and attachment.The ability of inkjet printers to deliver small(typically picoliter)liquid volumes with precise positioning is attractive to advance fundamental insights into the processing science,and thereby potentially enable new routes to incorporate the epitaxially connected superlattices into technology platforms.In this study,we identified the processing window of opportunity,including nanocrystal ink formulation and printing approach to enable delivery of colloidal nanocrystals from an inkjet nozzle onto the surface of a sessile droplet of the immiscible subphase.We demonstrate how inkjet printing can be scaled-down to enable the fabrication of epitaxially connected superlattices on patterned sub-millimeter droplets.We anticipate that insights from this work will spur on future advances to enable more mechanistic insights into the assembly processes and new avenues to create high-fidelity superlattices.

Efficient emission of quasi-two-dimensional perovskite films cast by inkjet printing for pixel-defined matrix light-emitting diodes

Quasi-two-dimensional(quasi-2D)perovskites are promising materials for potential application in light-emitting diodes(LEDs)due to their high exciton binding energy and efficient emission.However,their luminescent performance is limited by the low-n phases that act as quenching luminescence centers.Here,a novel strategy for eliminating low-n phases is proposed based on the doping of strontium bromide(SrBr_(2))in perovskites,in which SrBr_(2)is able to manipulate the growth of quasi-2D perovskites during their formation.It was reasonably inferred that SrBr_(2)readily dissociated strontium ions(Sr^(2+))in dimethyl sulfoxide solvent,and Sr^(2+)was preferentially adsorbed around[PbBr_(6)]^(4−)through strong electrostatic interaction between them,leading to a controllable growth of quasi-2D perovskites by appropriately increasing the formation energy of perovskites.It has been experimentally proved that the growth can almost completely eliminate low-n phases of quasi-2D perovskite films,which exhibited remarkably enhanced photoluminescence.A high electroluminescent efficiency matrix green quasi-2D perovskite-LED(PeLED)with a pixel density of 120 pixels per inch fabricated by inkjet printing technique was achieved,exhibiting a peak external quantum efficiency of 13.9%,which is the most efficient matrix green quasi-2D PeLED so far to our knowledge.

Efficient red perovskite quantum dot light-emitting diode fabricated by inkjet printing

Perovskite quantum dots(PeQDs)are considered potential display materials due to their high color purity,high photoluminescence quantum yield(PLQY),low cost and easy film casting.In this work,a novel electroluminescence(EL)device consisting of the interface layer of long alkyl-based oleylammonium bromide(OAmBr),which passivates the surface defects of PeQDs and adjusts the carrier transport properties,was designed.The PLQY of the OAmBr/PeQD bilayer was significantly improved.A high-performance EL device with the structure of indium tin oxide/poly(3,4-ethylenedioxythiophene)polystyrene sulfonate/poly(bis(4-phenyl)(2,4,6-trimethylphenyl)amine)/OAmBr/PeQDs/2,2′,2′′-(1,3,5-benzinetriyl)-tris(1-phenyl-1H benzimidazole)/LiF/Al was constructed using a spin-coating method.A peak external quantum efficiency(EQE)of 16.5%at the emission wavelength of 646 nm was obtained.Furthermore,an efficient matrix EL device was fabricated using an inkjet printing method.A high-quality PeQD matrix film was obtained by introducing small amounts of polybutene into the PeQDs to improve the printing process.The EQE reached 9.6%for the matrix device with 120 pixels per inch and the same device structure as that of the spin-coating one.