Electrohydrodynamic Inkjet Printing Research Articles

Electrohydrodynamic Inkjet Printing of Three-Dimensional Perovskite Nanocrystal Arrays for Full-Color Micro-LED Displays.

Perovskite nanocrystal (PeNC) arrays are showing a promising future in the next generation of micro-light-emitting-diode (micro-LED) displays due to the narrow emission linewidth and adjustable peak wavelength. Electrohydrodynamic (EHD) inkjet printing, with merits of high resolution, uniformity, versatility, and cost-effectiveness, is among the competent candidates for constructing PeNC arrays. However, the fabrication of red light-emitting CsPbBrxI(3-x) nanocrystal arrays for micro-LED displays still faces challenges, such as low brightness and poor stability. This work proposes a design for a red PeNC colloidal ink that is specialized for the EHD inkjet printing of three-dimensional PeNC arrays with enhanced luminescence and stability as well as being adaptable to both rigid and flexible substrates. Made of a mixture of PeNCs, polymer polystyrene (PS), and a nonpolar xylene solvent, the PeNC colloidal ink enables precise control of array sizes and shapes, which facilitates on-demand micropillar construction. Additionally, the inclusion of PS significantly increases the brightness and environmental stability. By adopting this ink, the EHD printer successfully fabricated full-color 3D PeNC arrays with a spatial resolution over 2500 ppi. It shows the potential of the EHD inkjet printing strategy for high-resolution and robust PeNC color conversion layers for micro-LED displays.

Electrohydrodynamic Printing‐Based Heterointegration of Quantum Dots on Suspended Nanophotonic Cavities

AbstractNanophotonic structures are a foundation for the growing field of light‐based quantum networks and devices enabled by their ability to couple with and manipulate photons. Colloidal quantum dots (QDs) are uniquely suited to complement this range of devices due to their solution‐processability, broad tuneability, and near‐unity photoluminescence quantum yields in some cases. To bridge the gap between them, electrohydrodynamic inkjet (EHDIJ) printing serves as a highly precise and scalable nanomanufacturing method for deterministic positioning and deposition of attoliter‐scale QD droplets. This includes heterointegration in devices that are challenging to create by conventional subtractive semiconductor processing, such as QDs emitters coupled to substrate‐decoupled nanoscale resonant structures. In this work, the first successful application of EHDIJ printing for the integration of these colloidal QDs into suspended nanophotonic cavities is demonstrated, achieving selective single‐cavity deposition for cavity pairs as close as 100 nm apart. These results motivate the development of future suspended hetero‐integrated devices that utilize EHDIJ printing as a sustainable, additive, and scalable method for quantum photonics nanomanufacturing.

In situ preparation of SnO2/Pd@TiO2 bilayer sensor for highly sensitive and fast detection of H2S based on electrohydrodynamic jet printing

A micron-scale bilayer H2S sensor fabricated by electrohydrodynamic (EHD) jet printing in situ was evaluated. The prepared sensor consisted of a Pd@TiO2 cover layer (porous catalytic sensing layer), a SnO2 bottom layer (electron transport layer) and a Sn-Ti transition layer between them. The addition of TiO2 nanoparticles to the cover layer led to a porous structure after one-step sintering. The increase in specific surface area and the formation of heterojunction effectively improved the sensor sensitivity. The PdO nanoparticles loaded on the surface of TiO2 nanoparticles further improved the performance due to its catalysis. Since the kinetic energy of droplets of the same size generated by EHD inkjet printing is about 48 times that of inkjet printing, the film density is much higher than that of inkjet printing. The relatively dense bottom layer, and the thick and stable transition layer improved the long-term stability of the sensor. The detection limit of the sensor is 6 ppb, with linear range of 0.02–10 ppm H2S. The response and recovery times are 7 s and 45 s (2 ppm H2S), respectively. After 9 months of intermittent measurement, the response of the sensor to 2 ppm H2S decreased by only 10.6%.

Dual-Ligand Red Perovskite Ink for Electrohydrodynamic Printing Color Conversion Arrays over 2540 dpi in Near-Eye Micro-LED Display.

The lack of stability of red perovskite nanocrystals (PeNCs) remains the main problem that restricts their patterning application. In this work, the dual-ligand passivation strategy was introduced to stabilize PeNCs and inhibit their halogen ion migration during high-voltage electrohydrodynamic (EHD) inkjet printing. The as-printed red arrays exhibit the highest emisson intensity and least blue shift compared with samples with other passivation strategies under a high electric field during EHD inkjet printing. Combining with blue and green PeNC inks, single-color and tricolor color conversion layer arrays were successfully printed, with minimum pixel size of 5 μm and the highest spatial resolution of 2540 dpi. The color coordinate of CsPbBrI2 NCs arrays are located close to the red point, with a color gumat of 97.28% of Rec. 2020 standard. All of these show great potential in the application of color conversion layers in a near-eye micro-LED display.

Electrohydrodynamically printed microlens arrays with controllable curvature based on surface functionalization

Precisely controlling the optical characteristics of the microlens array (MLA) is a key issue for the MLA's fabrication and its applications. A cost-effective method for fabricating MLA with different curvatures and diameters by surface functionalization and electrohydrodynamic inkjet (E-jet) printing is proposed. The surface with self-assembled monolayers has low surface energy, which greatly improves the contact angle of the microlens. The UV-ozone treatment and neutral density filters with different optical densities are used to control the wettability of the substrates. Based on this method, the curvatures of the microlens can be controlled easily and the numerical aperture (NA) of the microlens can be adjusted from 0.18 to 0.49. Compared with traditional fabrication techniques, the method we demonstrated is flexible, low-cost, rapid, and capable of fabricating MLA with large area, high packing density, desirable NA, and diameters.

Thin-film temperature sensors with grid-type circuits for temperature distribution measurements

Measuring the temperature of satellites is crucial for thermal control. Efficient heat control is possible when the location of the heat source can be quickly identified and the heat transfer area can be measured. Grid-type thin-film temperature sensors have been proposed for this purpose. Grid-type thin-film temperature sensors use electrohydrodynamic (EHD) inkjet printing technology, a next-generation printing technology. Sensors with high sensing resolution can be manufactured using these technologies. However, the EHD inkjet printing technology is sensitive to printing conditions, and determining the optimal conditions is crucial. Therefore, we produce inks for application in EHD inkjet printing technology and conduct experiments to validate their characteristics. In addition, we determine the optimal conditions for printing and manufacturing grid-type thin-film temperature sensors. The heating element is placed at an arbitrary location and the location of heat generation is detected using the manufactured temperature sensors. We have obtained visualization data showing the heat distribution with time. As a result, the production of a grid-type thin film temperature sensor using the printing process was confirmed. This temperature sensor was able to track the heat source and check the distribution process through which heat is transferred. Therefore, it can be expected to be used in fields that require temperature monitoring.

20.1: Invited Paper: Research on Oxide Thin Film Transistors for Wearable Sensors

With the rapid development of the Internet of Things, sensors as an important foundation in the era of smart interconnection, have a significant potential in the emerging fields of smart home, wearable devices, and smart mobile terminals. Flexible sensor has been attracting a great attention due to its portability, miniaturization and long endurance. The high consumption and signal crosstalk are urgent issues for the high‐integration sensing array. These issues could be effectively addressed by active‐matrix thin film transistors (TFT) backplane. In this work, the feasibility of oxide TFT for flexible sensor was discussed, in which high‐quality oxi de semiconductor films, oxide dielectric films, and printing Ag electrodes and wires were investigated, respectively. The device performance of flexible Si‐doped SnO2 TFT was optimized by modulating oxygen partial pressure, which could achieve a relative good electrical properties at room temperature. To implement low‐consumption application, high quality of high‐k ZrO2 dielectric film was investigated by solution‐processed method. The dielectric properties of ZrO2 film with the mircowave‐as sisted or deep ultraviolet irradiation‐assisted annealing process was similar to that of the thermal ann ealing process. The morphology and conductivity of Ag electrodes and wires printed by electrohydrodynamic (EHD) inkjet printing technology were studied. High conductivity of Ag electrodes and wires was achieved at a low curing temperature. These results imply that oxide TFT has a significant potential in the application of flexible sensor array.

Electrohydrodynamically Printed High‐resolution Arrays Based on Stabilized CsPbBr3 Quantum Dot Inks

AbstractHigh‐resolution perovskite quantum dots (PeQDs) arrays printed by electrohydrodynamic (EHD) inkjet printing show great application prospects in fields of micro‐display. Commonly, EHD inkjet requires a polar system for printing but it easily quenches the PeQDs. Meanwhile, the mismatch of solvent parameters causes uneven surface morphology and printing instability. In this work, a morphology uniformization strategy of a nonpolar binary solvent system of 1,3,5‐triethylbenzene and n‐tetradecane is proposed to prepare the didodecyldimethylammonium bromide (DDAB)/PbBr2 passivated PeQDs inks. When the solvent volume ratio is 5:5, the photoluminescence quantum yield (PLQY) of the ink reaches 94.06%. Benefiting from the match of boiling point, surface tension, and dielectric constant of ink, after the EHD inkjet printing, the line width of continuous smooth line arrays is as narrow as 2.0 µm and the coffee effect of dot arrays is well inhibited. Meanwhile, derived from the strong electrostatic interaction between the π‐electron structure of 1,3,5‐triethylbenzene and DDA+, the ink exhibits high stability. Moreover, the high‐resolution logo and miniature quick responsecode by EHD inkjet printing reach 22 718 dpi, far higher than that of the reported micro‐LED. This work paves the way for high‐resolution pixel arrays of micro‐display.

E-nose based on a high-integrated and low-power metal oxide gas sensor array

To improve the poor selectivity of a semiconductor gas-sensor system, an array of sensors can be utilized. However, this increases the system’s size and power consumption. To overcome these limitations, we propose a high-integrated and highly selective electronic nose (E-nose) comprising two independent gas-sensing elements on a low-power microhotplate (MHP). Pd-SnO2 nanoflowers and Pd-WO3 microparticles were prepared and printed on a bridge-structured MHP 20 µm apart and over an area of 110 µm × 45 µm. This was achieved using electrohydrodynamic inkjet printing aided by in-situ infrared laser curing to form an array of sensors. A power of 17 mW was required to increase the temperature of the MHP to 300 °C, which is the optimal operating temperature of the two gas sensors. Thus, a high response and low cross-sensitivity were achieved for hydrogen, ammonia, hydrogen–ammonia, ethanol, acetone, ethanol–acetone, toluene, and formaldehyde. A wavelet transform was used to reduce the noise and dimensionality of the signals from the gas-sensor array. Qualitative identification of the eight gases with an accuracy of 99.86% was achieved using the k-nearest neighbor (kNN) model. A p neighbors back propagation neural network (pN-BPNN) model was established to remove interfering samples to quantitatively estimate the gas concentration. The quantitative identification accuracy of pN-BPNN was higher than that of the standard back propagation neural network (BPNN) model with the average absolute percentage error of hydrogen detection in the range of 15–500 ppm, decreasing from 5.44% to 2.08%.

Evaluation of Temperature Sensors for Detection of Heat Sources Using Additive Printing Method.

Electrohydrodynamic (EHD) inkjet printing is an efficient technique for printing multiple sensors in a multifaceted area. It can be applied to various fields according to the shape of the printing result and the algorithm employed. In this study, temperature sensors capable of detecting heat sources were fabricated. Inks suitable for EHD inkjet printing were produced, and optimal parameters for printing were determined. Printing was performed using the corresponding parameters, and various printing results were obtained. Furthermore, an experiment was conducted to confirm the temperature measurement characteristics of the results and the tolerance of the sensor. Grid-type sensors were fabricated based on the results, and the sensor characteristics were confirmed in an orthogonal form. Heat was applied to arbitrary positions. Resistance to changes due to heat was measured, and the location at which the heat was generated was detected by varying the change in resistance. Through this study, efficient heat control can be achieved, as the location of the heat source can be identified quickly.

Stability Bounds for Micron Scale Ag Conductor Lines Produced by Electrohydrodynamic Inkjet Printing.

Continuous conducting lines of width 5–20 μm have been printed with a Ag nanoparticle ink using drop-on-demand (DOD) electrohydrodynamic (EHD) inkjet printing on Si and PDMS substrates, with advancing contact angles of 11° and 35°, respectively, and a zero receding contact angle. It is only possible to achieve stable parallel sided lines within a limited range of drop spacings, and this limiting range for stable line printing decreases as the contact angle of the ink on the substrate increases. The upper bound drop spacing for stable line formation is determined by a minimum drop overlap required to prevent contact line retraction, and the lower bound is governed by competing flows for drop spreading onto an unwetted substrate and a return flow driven by a Laplace pressure difference between the newly deposited drops and the fluid some distance from the growing tip. The upper and lower bounds are shown to be consistent with those predicted using existing models for the stability of inkjet printed lines produced using piezoelectric droplet generators. A comparison with literature data for EHD printed lines finds that these limiting bounds apply with printed line widths as small as 200 nm using subfemtoliter drop volumes. When a fine grid pattern is printed, local differences in Laplace pressure lead to the line width retracting to the minimum stable width and excess ink being transported to the nodes of the grid. After printing and sintering, the printed tracks have a conductivity of about 15%–20% of bulk Ag on the Si substrate, which correlates with a porosity of about 60%.

Electrohydrodynamic (EHD) inkjet printing flexible pressure sensors with a multilayer structure and periodically patterned Ag nanoparticles

Electrohydrodynamic (EHD) inkjet printing flexible pressure sensors with a multilayer structure and periodically patterned Ag nanoparticles

Direct Patterning of Perovskite Nanocrystals on Nanophotonic Cavities with Electrohydrodynamic Inkjet Printing.

Overcoming the challenges of patterning luminescent materials will unlock additive and more sustainable paths for the manufacturing of next-generation on-chip photonic devices. Electrohydrodynamic (EHD) inkjet printing is a promising method for deterministically placing emitters on these photonic devices. However, the use of this technique to pattern luminescent lead halide perovskite nanocrystals (NCs), notable for their defect tolerance and impressive optical and spin coherence properties, for integration with optoelectronic devices remains unexplored. In this work, we additively deposit nanoscale CsPbBr3 NC features on photonic structures via EHD inkjet printing. We perform transmission electron microscopy of EHD inkjet printed NCs to demonstrate that the NCs' structural integrity is maintained throughout the printing process. Finally, NCs are deposited with sub-micrometer control on an array of parallel silicon nitride nanophotonic cavities and demonstrate cavity-emitter coupling via photoluminescence spectroscopy. These results demonstrate EHD inkjet printing as a scalable, precise method to pattern luminescent nanomaterials for photonic applications.

Electrohydrodynamic ink-jet printing characteristics of inks for temperature measurements

Temperature sensors have high usability owing to their small size and high resolution. These can be achieved with electrohydrodynamic (EHD) ink-jet printing technology, but it is important to find applicable inks. Therefore, inks capable of measuring temperature were produced using polymer-based and surfactant-based solutions in this study. The temperature measurement performance of the inks was evaluated, in a high-temperature environment. Inks capable of sensing temperature were applied to an EHD ink-jet printer. Printable inks were derived, and experiments were conducted to determine the stability of the meniscus shapes and printed lines under varying voltage and height conditions. In addition, experiments were conducted to assess the variation in the line thickness with the printing speed by determining the optimum voltage and height parameters. Through these experiments, inks capable of measuring temperature were manufactured, and the optimal printing conditions for EHD ink-jet printing were obtained.

Fully Additive Electrohydrodynamic Inkjet‐Printed TiO2 Mid‐Infrared Meta‐Optics

AbstractAdditive manufacturing at the micron and sub‐micron scale is a rapidly expanding field with electrohydrodynamic inkjet (EHDIJ) printing proving to be a critical fabrication technique that will enable continued advancement. Increasing the range of materials that can be used with EHDIJ printing to create micron and sub‐micron scale features is critical for increasing the variety of devices that can be fabricated with this method. Ceramic, semiconducting, and hybrid organic–inorganic materials are essential for meta‐optics and micro‐electromechanical systems devices, yet these materials are vastly underexplored for applications in EHDIJ printing. A novel printing solution is presented containing a titania alkoxide precursor that is compatible with EHDIJ printing and capable of producing final printed features of 1 µm and below; the highest resolution features ever reported for this family of materials and this method. This solution is used to fabricate the first EHDIJ printed and functioning mid‐infrared meta‐optics lens, capable of focusing 5 µm light.

Temperature-Sensing Inks Using Electrohydrodynamic Inkjet Printing Technology.

Temperature measurement is very important for thermal control, which is required for the advancement of mechanical and electronic devices. However, current temperature sensors are limited by their inability to measure curved surfaces. To overcome this problem, several methods for printing flexible substrates were proposed. Among them, electrohydrodynamic (EHD) inkjet printing technology was adopted because it has the highest resolution. Since EHD inkjet printing technology is limited by the type of ink used, an ink with temperature-sensing properties was manufactured for use in this printer. To confirm the applicability of the prepared ink, its resistance characteristics were investigated, and the arrangement and characteristics of the particles were observed. Then, the ink was printed using the EHD inkjet approach. In addition, studies of the meniscus shapes and line widths of the printed results under various conditions confirmed the applicability of the ink to the EHD inkjet printing technology and the change in its resistance with temperature.

A review on multi nozzle electrohydrodynamic inkjet printing system for MEMS applications

New microproducts require to utilize the variety of materials. Their complex three-dimensional microstructures have big aspect ratios. This ability to fabricate geometrically complicated 3D microstructures provides some additional profits to the additive manufacturing systems over traditional methods. Among the enormous variety of micro-products, depending on the mixtures of usefulness of the product and fundamentals of operation, the foremost types are micro-opto-electro-mechanical systems, micro electromechanical systems, micro-optical electronics systems and microelectronic products. Electrohydrodynamic inkjet printing is an innovative high-resolution technology of inkjet printing which has the benefits of being a non-contact, maskless, additive and direct-write methods. The resolution of printing of EHD surpasses by approx. two orders of magnitude compared to the general inkjet printing methods. It has been used mostly in cases of nano or micro manufacturing of very small objects for modelling of a big range of constituents on different substrates having the alternatives of either Drop-On-Demand mode or continuous mode. It is considered to be a capable substitute to thermal and piezoelectric based inkjet printing methods since it has a unique quality of producing small jet or droplets in comparison to the nozzle orifice. Several advantages in fine patterning are presented by EHD inkjet printing processes, but the little manufacturing speed of EHD inkjet printing is an unadorned disadvantage that has been hindering its probable extensive uses in the industry of electronics. To overcome this restriction, the direct printing of colloidal solutions with the help of multiple nozzle EHD inkjet printing method is used. This review offers a short account of multiple-nozzle electrohydrodynamic inkjet printing of colloidal solutions for its application in MEMS.

CFD-based numerical modeling to predict the dimensions of printed droplets in electrohydrodynamic inkjet printing

Electrohydrodynamic (EHD) inkjet printing is a type of potential non-contact micro/nanoscale manufacturing technology. The printed droplet dimension plays an important role in EHD inkjet printing due to its significant influence on printing quality and the resolution of patterns. The complexity of the mechanism and the limited process optimization techniques present a challenge in obtaining the desired printing resolution, eventually becoming an expensive and time-consuming endeavor. In this study, a CFD model is proposed to investigate the mechanism of the cone-jet printing process in EHD inkjet printing. The complete cone-jet printing process is successfully simulated. A further analysis predicts the jetting diameter and printed droplet diameter with different operating parameters and substrates. The simulation has a satisfactory agreement with experiments in predicting the printing behavior and printing quality (jetting diameter, printed droplet diameter). Such advancement in modeling can be utilized to guide the quick determination of operation parameters for the desired printing resolution when given a new ink. • Electrohydrodynamic inkjet printing is a non-contact micro/nano manufacturing technique that utilizes electric force to generate droplets. • A computational fluid dynamics (CFD) model is proposed to investigate the cone-jet printing mechanism in EHD printing. • A complete cone-jet printing cycle with four phases is simulated: cone formation, jet generation, jet break and droplet expansion. • A further analysis predicts the jetting diameter and printed droplet diameter using different operating parameters and substrates. • The modeling advancement can guide the quick determination of operation parameters for desired printing resolution given a new ink. Electrohydrodynamic (EHD) inkjet printing is a type of potential non-contact micro/nanoscale manufacturing technology. The printed droplet dimension plays an important role in EHD inkjet printing due to its significant influence on printing quality and the resolution of patterns. The complexity of the mechanism and the limited process optimization techniques present a challenge in obtaining the desired printing resolution, eventually becoming an expensive and time consuming endeavor. Recent developments in computational fluid dynamics (CFD) bring an effective alternative to alleviate the aforementioned challenges. In this study, a CFD model is proposed to investigate the mechanism of the cone-jet printing process in EHD inkjet printing. The complete cone-jet printing process is successfully simulated with four phases: Taylor cone formation, cone-jet generation, jet break and droplet expansion. A further analysis predicts the jetting diameter and printed droplet diameter with different operating parameters and substrates. The simulation has a satisfactory agreement with experiments in predicting the printing behavior and printing quality (jetting diameter, printed droplet diameter). Such advancement in modeling can be utilized to guide the quick determination of operation parameters for the desired printing resolution when given a new ink.

Fabrication of circuits by multi-nozzle electrohydrodynamic inkjet printing for soft wearable electronics

Wearable electronic devices are evolving from current rigid configurations to flexible and ultimately stretchable structures. These emerging systems require soft circuits for connecting the various working units of the overall system. This paper presents fabrication of soft circuits by electrohydrodynamic (EHD) inkjet-printing technique. Multi-nozzle EHD printing head is employed for rapid fabrication of electric circuits on a wide set of materials, including glass substrate (rigid), flexible polyethylene terephthalate (PET) films, and stretchable thermoplastic polyurethane (TPU) films. To avoid the effects of substrate materials on the jettability, the proposed multi-nozzle head is equipped with integrated individual counter electrodes (electrodes are placed above the printing substrate). High-resolution circuits (50 ± 5 µm) with high electrical conductivity (0.6 Ω □−1) on soft substrate materials validate our well-controlled multi-nozzle EHD printing approach. The produced circuits showed excellent flexibility (bending radius ≈ 5 mm radius), high stretchability (strain ≈ 100%), and long-term mechanical stability (500 cycles at 30% strain). The concept is further demonstrated with a soft strain sensor based on a multi-nozzle EHD-printed circuit, employed for monitoring the human motion (finger bending), indicating the potential applications of these circuits in soft wearable electronic devices.Graphic

Fabrication of microvascular constructs using high resolution electrohydrodynamic inkjet printing

Fabrication of the intricate anatomy of vasculature within engineered tissue remains one of the key challenges facing the field of tissue engineering. We report the use of electrohydrodynamic (EHD) inkjet printing to create hydrogel-based microvascular tissues with hierarchical and branching channels, whose minimum feature size of 30 μm approaches the physical scale of native capillary blood vessels. The principle relies on the use of complementary thermoreversible gelling properties of Pluronic F127 (PF-127) and gelatin methacryloyl, which served as sacrificial templates and permanent matrices respectively. Human dermal fibroblasts and human umbilical vein endothelial cells were successfully co-cultured within the engineered microvascular tissue constructs for up to 21 days, and attained high cell viability. Tissue specific morphology was maintained on perfusion. The ability to create cellularised, vascularised proto-tissues with high spatial resolution using EHD inkjet printing, provides a new strategy for developing advanced vascular models with the potential to impact upon an extensive range of biomedical applications.