Electrohydrodynamic Printing of Microscale PEDOT:PSS-PEO

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Applications of Polymer, Composite, and Coating Materials

Electrohydrodynamic Printing of Microscale PEDOT:PSSPEO Features with Tunable Conductive/Thermal Properties Jinke Chang, Jiankang He, Qi Lei, and Dichen Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04051 • Publication Date (Web): 10 May 2018 Downloaded from http://pubs.acs.org on May 13, 2018

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ACS Applied Materials & Interfaces

Electrohydrodynamic Printing of Microscale PEDOT:PSS-PEO Features with Tunable Conductive/Thermal Properties Jinke Chang, Jiankang He*, Qi Lei, Dichen Li State key laboratory for manufacturing systems engineering, Xi’an Jiaotong University, Xi’an 710049, China * Corresponding author: [email protected]

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ABSTRACT:

Electrohydrodynamic (EHD) printing has been recently investigated as an effective technique to produce high-resolution conductive features. Most of the existing EHD printing studies for conductive features were based on metallic nanoparticle inks in a micro-dripping mode, which exhibited relatively low efficiency and commonly required high-temperature annealing process to achieve high conductivity. The EHD printing of high-resolution conductive features at relatively low temperature and in a continuous cone-jetting mode is still challenging since the conductive inks might connect the charged nozzle and the grounded conductive or semiconductive substrates to cause discharge and terminate the printing process. In this study, the EHD printing process of conductive polymers in a low temperature cone-jetting mode was explored to fabricate conductive microstructures. The smallest width of PEDOT:PSS lines were 27.25 ± 3.76 µm with a nozzle diameter of 100 µm. It was interesting to find that the electrohydrodynamically printed PEDOT:PSS-PEO features exhibited unique thermal properties when a DC voltage was applied. The conductive and thermal properties of the resultant features were highly dependent on the printing layer number. Microscale PEDOT:PSS features were further encapsulated into electrospun nanofibrous mesh to form a flexible sandwich structure. The EHD printing of PEDOT:PSS features with tunable conductive and thermal properties might be useful for the applications of flexible and wearable microdevices. Keywords: electrohydrodynamic printing, flexible electronics, PEDOT:PSS, microscale conductive features, tunable conductive/thermal properties


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1. Introduction With the increasing demands for complex functionalities and miniaturization of electronic microdevices such as microsensors,1 flexible electronics2 and energy-storage devices,3 the capability to fabricate micro/nano-scale conductive features in different substrate is highly required. Conventional microfabrication techniques such as lithography4 and transfer contact printing5 for such structures have various limitations such as complex procedures, difficulties in building complex three-dimensional (3D) structures and the requirement for costly equipment. Recently, high-resolution direct printing techniques such as direct ink writing6-8 and inkjet printing9-11 have been extensively investigated to fabricate high-resolution conductive features on different substrate. Microscale 3D conductive structures with continuous solids, high aspect ratio or spanning features have been printed through the control over ink composition, rheological behavior and printing parameters.12 Lewis et al.13 used direct ink printing to print 3D Li-ion microbatteries with the smallest feature size of 60 µm. However, the printing resolution is highly determined by the nozzle size of printing nozzle. It is commonly required to reduce nozzle size for the printing of high-resolution features, which might increase the difficulty in fabricating micro/nano-scale printing nozzles and the risk of nozzle clogging during the printing of viscous or nanoparticle-based conductive inks.14 Electrohydrodynamic (EHD) printing employs an electric field to eject tiny droplets or filaments onto a conductive or semi-conductive substrate with the size much smaller than the nozzle size.15,16 The size of the electrohydrodynamically printed features are commonly in the range of 50 nm-100 µm.17-19 For instance, Luo et al.20 electrohydrodynamically printed polyvinylidene fluoride (PVDF) fibers with an average diameter of 8 µm by using the nozzle diameter of 160 µm. To fabricate micro/nano-scale conductive structures, metal nanoparticles


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and nanowires such as copper, silver and gold were mixed with polymers for EHD printing due to the high chemical stability and high electrical conductivity.21-24 Rahman et al.25 presented the EHD printing of copper nanoparticles with minimum feature width of 12 µm. Park et al.26 developed Ag/Cu/Co nanoparticle inks and produced 3D conductive features with the smallest diameter of 700 nm. Lee et al27 electrohydrodynamically printed Ag nanowires for 1D and 2D patterns with an average diameter of 695 nm. However, most of the existing studies based on metal nanoparticles and nanowires were electrohydrodynamically printed in a micro-dripping mode and commonly required high-temperature annealing process for improved electrical conductivity. Moreover, when the conductive inks were electrohydrodynamically printed in a continuous cone-jetting mode, the conductive inks will connect the charged nozzle and the grounded conductive or semi-conductive substrate, which will cause discharge and immediately terminate the printing process. Therefore, electrohydrodynamic printing of conductive features in a continuous cone-jetting mode is still challenging. Poly(3,4-ethylenedioxythiophene):Polystyrene

Sulfonate

(PEDOT:PSS) is

a

kind

of

conductive polymer with high stability and flexibility as well as tunable conductivity at room temperature, which makes it a promising candidate for the fabrication of conductive filaments.28,29 However, the size of the electrohydrodynamically printed PEDOT:PSS lines ranged from 90 µm to 335 µm. It can not meet the increasing demands for high-resolution features. In this study, we explored to electrohydrodynamically print microscale PEDOT:PSSPEO conductive structures onto polymeric substrates in a low-temperature cone-jetting mode. The unique capability to flexibly tune the conductive and thermal properties of the electrohydrodynamically printed structures was further demonstrated.


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2. EXPERIMENTAL METHODS 2.1 Materials Polyethylene oxide (PEO) (Mw=4,000,000) and PEDOT:PSS conductive solution (1.0 wt. % in water) were bought from Sigma-Aldrich (United States). PEO was directly added into PEDOT:PSS conductive solution with a concentration of 1w/v% and stirred at room temperature for at least 5 hours to obtain a homogeneous mixed solution. The addition of PEO raised the viscosity and the surface tension of PEDOT:PSS inks (Figure S1), which were measured as 56.2 mN/m and 0.56 Pa·s by using a viscometer (NDJ-T, Fangrui, Shanghai, China) and a surface tension detector (JYW200B, KeCheng, Chengde, China). The sheet resistances of a PEDOT:PSS-PEO film (50 µm) and a PEDOT:PSS film (50 µm) were 202 Ω/sq and 61 Ω/sq, respectively, measured by a four-point probe resistance tester (Kaivo, FP001). This indicated that the addition of PEO increased the resistance of the printing inks. 2.2 EHD printing of microscale PEDOT:PSS-PEO features Figure 1 schematically shows the presented EHD printing strategy for microscale PEDOT:PSS-PEO structures. An insulating substrate was used instead of traditional conductive or semi-conductive substrate to prevent the discharging during EHD printing process (Figure S2). The nozzle-to-substrate distance decreased from millimeters to 100 µm to maintain enough electric force for the EHD printing of PEDOT:PSS-PEO inks in a continuous cone-jetting mode without electric discharge. A syringe (250 µL) filled with PEDOT:PSS-PEO inks was installed onto a micro-pump system (TJ-2A, Longer Pump, Baoding, China) to control the feeding rate. When a DC voltage of 3 kV was applied, the polymer inks was electrohydrodynamically printed from a nozzle with an inner diameter of 100 µm and deposited onto the insulating substrate. The temperature of the substrate was maintained at 55 oC to facilitate instant solidification of the


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printed inks. The resultant pattern was determined by the movement of XY stage (Xiamen Heidelstar Co., China). The printing head can move vertically to ensure a constant nozzle-tosubstrate distance for multiple layer printing.

Figure 1. EHD printing of high-resolution PEDOT:PSS-PEO features in a low-temperature cone-jetting mode. 2.3 Electrohydrodynamic printing of single-layer PEDOT:PSS-PEO lines Feeding rate and stage moving speed are two of important parameters to affect the widths of the electrohydrodynamically printed lines.20 Here we maintained the nozzle-to-substrate and voltage constant while the feeding speed and stage moving speed were changed in the range of 30-90 µL/h and 30-90 mm/s respectively. The resultant PEDOT:PSS-PEO lines were characterized by an inverted microscopy (ECLIPSE Ti, Nikon, Japan) and the size was measured in Image J software. For each condition, three samples were separately printed and nine points were totally measured. The presented EHD printing also enables to flexibly fabricate userspecific patterns such as “XJTU” pattern.


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2.4 Electrohydrodynamic printing of multi-layer PEDOT:PSS-PEO features PEDOT:PSS-PEO walls with different layer number (5, 10, 15, 20, 40, 60 and 100) can also be electrohydrodynamically printed. After 10 layers were continuously printed, the nozzle was vertically moved up 8 µm to avoid significant change in nozzle-to-substrate distance. The morphology and dimension of the multilayer PEDOT:PSS-PEO features were characterized by a laser confocal microscope (OLS4000, Olympus). The presented process also enabled to print circular and curved PEDOT:PSS-PEO features with a layer number of 200 and 300. 2.5 Conductive property of electrohydrodynamically printed PEDOT:PSS-PEO features The capability to electrohydrodynamically print PEDOT:PSS-PEO features with multiple layers offers a simple and flexible way to tune the resistance of the printed structures. To measure the conductivity, ten parallel PEDOT:PSS-PEO straight lines (20 mm in length and 1 mm in spacing) were deposited onto an insulating glass substrate. Highly conductive silver coating was used to connect the printed lines at two end terminals. A digital universal meter (DT9205T, NJTY) was used to measure the resistance of the PEDOT:PSS-PEO patterns. The resistance data from thirty lines were totally quantified and was expressed as mean value ± standard deviation. When a 5V DC power was applied to LEDs through different layer PEDOT:PSS-PEO lines, the differences in luminescence intensity indicated the variation in line resistance. 2.6 Thermal property of the electrohydrodynamically printed PEDOT:PSS-PEO features To test the thermal properties of the PEDOT:PSS-PEO lines with different layer (5, 20, 40, 60 and 100), silver film was coated at both terminals of the printed lines and was connected to a DC power source. The voltage was changed from 2 V to 14 V and the corresponding thermal infrared images for different lines were captured by a thermal infrared camera (FLIR SYSTEMS,


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USA). The relationship between applied voltage and maximum heating temperature for PEDOT:PSS-PEO conductive lines with different layer was quantified. To demonstrate the effect of thermal property on the stability of feature resistance, the electrohydrodynamically printed PEDOT:PSS-PEO conductive lines with a layer number of 100 were periodically connected and disconnected with a DC power of 12 V. The feature resistance was continuously measured. PEDOT:PSS-PEO

conductive

lines

with

different

layer

number

were

further

electrohydrodynamically printed onto a flexible PET film and the corresponding thermal property was investigated. 2.7 Fabrication of PEDOT:PSS-PEO lines in flexible electrospun nanofibrous substrate Microscale PEDOT:PSS-PEO features can be further printed onto different substrates. Here flexible poly(lactic-co-glycolic acid) (Mw=200,000, LA:GA=50:50, PLGA) nanofibrous meshes were electrospun from 15 wt% PLGA solution in a house-made electrospinning equipment according to previously developed procedures. The applied voltage, flow rate, nozzle-tosubstrate distance and collecting rotating speed were fixed 15 kV, 1.4 mL/h, 10 cm and 3600 r/min, respectively.30 PEDOT:PSS-PEO conductive features with a layer number of 100 were electrohydrodynamically printed on the nanofibrous PLGA substrates. Another layer of PLGA nanofibrous films can be further electrospun on the top of the PEDOT:PSS-PEO conductive features. The microscopic morphology of the sandwich structures was characterized with SEM. The thermal properties of the sandwich structures at different environment temperature were characterized with a thermal infrared camera or thermoelectric couples. 3. Results and Discussion Figure 2a-b illustrates the morphology as well as the measured size of the PEDOT:PSS-PEO lines printed at different feeding rate and stage moving speed. When the moving speed was kept


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at 50 mm/s, the filament size gradually increased from 27.25 ± 3.76 µm to 47.95 ± 11.84 µm with the increase of feeding rate from 30 µL/h to 90 µL/h (Figure 2a). When the feeding rate was kept at 50 µL/h, the filament size obviously decreased from 71.91 ± 33.36 µm to 39.62 ± 7.65 µm with the increase of moving speed from 30 mm/s to 50 mm/s. However, the filament size slightly varied in the range 35.11 ± 5.24 µm to 39.62 ± 7.65 µm when the moving speed further increased to 90 mm/s (Figure 2b). To stably fabricate tiny conductive features and simultaneously consider stage vibration, the feeding rate and moving speed were finally selected as 50 µL/h and 50 mm/s for the subsequent experiment. Complex PEDOT:PSS-PEO features with “XJTU” pattern can be further printed (Figure 2c). The width of the conductive filament was about 30 µm in corresponding to the nozzle diameter of 100 µm.

Figure 2. Characterization of PEDOT:PSS-PEO lines printed at different process parameters. (a) Influence of feeding rate on filament morphology and size at the moving speed of 50 mm/s, (b) influence of moving speed on filament morphology and size at the feeding rate of 50 µL/h,


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(c) EHD printing of PEDOT:PSS-PEO conductive features with a “XJTU” pattern. Scale bars in Figure 1a-b indicate 50 µm. Figure 3a-d shows the 3D profiles of multi-layer PEDOT:PSS-PEO walls. The wall widths slightly fluctuated in the range of 49.50 ± 10.09 µm to 62.50 ± 13.12 µm while the wall height significantly increased from 0.77 ± 0.16 µm to 57.25±8.11 µm as the printed layer raised from 5 to 100. However, the wall height did not linearly increased (Figure 3e). In the first 20 layers, the average layer thickness was about 180 nm, close to that of one-layer filament (157 ± 25 nm). The average layer thickness further increased to 0.57 µm during the printing of the next 80 layers. The difference in average layer thickness might be due to the transform of collecting substrate from glass or polymer substrates to newly printed features. The wall cross-sectional areas increased from 23.32 ± 3.99 µm2 to 1227.32 ± 302.02 µm2 (Figure 3f), which showed a similar variation trend with feature height. The resistance of PEDOT:PSS-PEO lines can be flexibly adjusted by the printing layers as shown in Figure 3f. During the printing of 100-layer features, the resistance sharply reduced from 16.02 ± 1.70 kΩ/cm to 2.79 ± 0.37 kΩ/cm in the first 20 layers and further decreased from 1.46 ± 0.22 kΩ/cm to 0.77 ± 0.05kΩ/cm in the next 80 layers. The PEDOT:PSS-PEO lines can be flexibly bended and the corresponding resistance slightly changed (Figure S3). When 5 V voltage was applied to LEDs through electrohydrodynamically printed PEDOT:PSS-PEO lines with different layers, the light exhibited a clear gradient in their luminescence intensity as shown in Figure 3g-h.


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Figure 3. Electrohydrodynamic printing of multi-layer PEDOT:PSS-PEO features with tunable conductive properties.

(a-d) Microscopic images of electrohydrodynamically printed

PEDOT:PSS-PEO features with different layer number. (e-f) Relationship of printing layers with feature dimension and resistance. (g-h) Verification on the resistance tunability for multi-layer PEDOT:PSS-PEO features.


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Besides straight lines, the presented electrohydrodynamically printing method can print multilayer curved structures such as circular features (3mm in radius) with a layer number of 200 (Figure

4a-b).

More

complex

curved

features

can

also

be

programmed

and

electrohydrodynamically printed with a layer number of 300 (Figure 4c-d). Figure 4e-f shows the SEM images of the multi-layer features with smooth lateral surface. The height of the printed features are about 250 µm and the bottom width was about 190 µm. Figure 4g-h shows the microscopic interface between different layers in the electrohydrodynamically printed PEDOT:PSS-PEO filaments. Corrugation-like microstructures were clearly observed among neighbor layers, which might be due to the uneven surface of previously printed lines.

Figure 4. EHD printing of multi-layer curved PEDOT:PSS-PEO features. (a-b) Optical photos of circular features with a layer number of 200. (c-d) Optical photos of 300-layer curved features. (e-f) SEM image of the microstructures of the printed multi-layer PEDOT:PSS-PEO features. (gh) SEM image of the side view of the multi-layer PEDOT:PSS-PEO features. It was interesting to find that the electrohydrodynamically printed microscale PEDOT:PSSPEO features exhibited excellent thermal properties even when low voltage was applied. Figure 5a illustrates typical infrared thermal images of the PEDOT:PSS-PEO filaments with different


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printing layers. The highest temperature at the printed line area uniformly distributed and gradually increased as the applied voltage as well as the layer number increased. For instance, the maximum temperatures of the 60-layer conductive filaments were 39.4 oC and 118 oC in corresponding to the applied voltage of 4 V and 12 V. At the same applied voltage of 12 V, the maximum temperatures of the 2-layer and 100-layer conductive filaments were 31.1 oC and 190 o

C, respectively. The exothermic properties are better than the metal-based or graphene-based

heaters.21, 31,32 However, PEDOT:PSS-PEO films did not exhibit similar thermal property as the electrohydrodynamically printed lines (Figure S4). The difference in thermal property might be resulted from the different microstructures between thin films and the EHD-printed features. Figure

5b

shows

the

measured

data

of

the

maximum

temperature

for

the

electrohydrodynamically printed features with different layer at specific applied voltage, which provides a unique capability to flexibly tune thermal properties of the electrohydrodynamically printed features. To evaluate the stability of the electrohydrodynamically printed features during cyclic heatingcooling process, the changes in resistance of the PEDOT:PSS-PEO features with 100 layers were calculated from time-dependent current changes during the thermal heating process under an applied voltage of 12 V. As shown in Figure 5c, the resistance of the printed features was relatively stable during the 10 cycles of heating-cooling process. For each cycle, about 5% rises in feature resistance was found when the temperature gradually increased from room temperature to maximum temperature (about 190 oC). Although PEO have a lower melting temperature of 65 o

C, PEDOT:PSS-PEO features could stably maintain their morphology during one hour heating

process (200 oC) (Figure S5). Figure 5d shows the electrohydrodynamically printing of parallel conductive features with a gradient layer number of 20, 40, 60, 80 and 100 on a flexible PET


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(polyethylene terephthalate) film. When the printed features were connected with DC power sources of 4 V and 6 V, gradient distribution of maximum temperature was achieved as shown in Figure 5e.

Figure 5. Characterization of thermal properties of the PEDOT:PSS-PEO conductive features. (a) Infrared thermal images of multi-layer PEDOT:PSS-PEO features under specific applied voltage. (b) Systematic quantification of maximum temperature of multi-layer filaments under different applied voltage of 2-14 V. (c) Resistance stability of multi-layer PEDOT:PSS-PEO features during cycling heating. (d) EHD printing of microscale conductive lines with gradient layer number on a flexible PET film. (e) Infrared thermal images to show gradient distribution of maximum temperature of the printed features.


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To demonstrate the potential applications of the presented EHD printing for functional flexible electronics, microscale PEDOT:PSS-PEO features were encapsulated into electrospun nanofibrous films (Figure 6a). Microscopic images of the resultant micro/nanofibrous structures indicated that nanoscale fibers were parallel along with the surface of the collecting roller while the electrohydrodynamically printed conductive lines were perpendicular to electrospun nanofibers (Figure 6b-d). The microscale features were tightly boned with the bottom nanofibrous membranes. Another layer of nanofibers can be further electrospun to embed the printed conductive features inside as shown in Figure 6e-f. When the sandwich structure was connected to a DC power of 2 V, the electrohydrodynamically printed features inside the nanofibrous membrane clearly appeared in the infrared images (Figure 6g) due to thermal properties. To accurately measure the temperature of the nanofibrous structures, a thin thermocouple electrode (diameter 0.1 mm) was wrapped inside the flexible sandwich structures. A thermometer was connected with the electrode to record the changes in temperature as shown in Figure 6h. The temperature can be rapidly increased from room temperature (22 oC) to 40 oC in one minute as the voltage of 6V was applied (Figure 6i). Even when the sandwich structure was placed at the environment temperature of 7 oC, the printed PEDOT:PSS-PEO features exhibited similar heating efficiency. The unique capability to tune the conductive and thermal properties might find wide applications for the EHD printing of flexible and wearable microdevices.


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Figure 6. EHD printing of microscale PEDOT:PSS-PEO features in flexible nanofibrous structures. (a) Schematic for the fabrication of micro/nanoscale sandwich structures. (b-d) SEM images of PEDOT:PSS-PEO features on the top of the electrospun flexible PLGA nanofibers. (ef) Sandwich structure with conductive PEDOT:PSS-PEO features in the middle of nanofibers. (g) Infrared thermal image of the sandwich structure, (h) setup to measure the temperature of the sandwich structure. (i) Heating process when the sandwich structure was connected with 6V voltage and placed at different environment temperature (22 oC and 7 oC).


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4. Conclusion In summary, we presented a novel EHD printing strategy and successfully addressed the issue of discharging in a cone-jetting mode by employing an insulating substrate and a short nozzle-tosubstrate distance of 100 µm. The strategy enabled our EHD printing process to directly fabricate microscale PEDOT:PSS-PEO features with tunable conductive and thermal properties onto flexible polymeric substrates in a low temperature (

Electrohydrodynamic Printing of Microscale PEDOT:PSS-PEO

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