Structural Color Patterns by Electrohydrodynamic Jet Printed Photonic

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Structural Colour Patterns by Electrohydrodynamic Jet Printed Photonic Crystals Haibo Ding, Cun Zhu, Lei Tian, Cihui Liu, Guangbin Fu, Luoran Shang, and Zhongze Gu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11409 • Publication Date (Web): 25 Jan 2017 Downloaded from http://pubs.acs.org on January 25, 2017

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ACS Applied Materials & Interfaces

Structural Colour Patterns by Electrohydrodynamic Jet Printed Photonic Crystals Haibo Ding, Cun Zhu, Lei Tian, Cihui Liu, Guangbin Fu, Luoran Shang, Zhongze Gu* State Key Laboratory of Bioelectronics, School of Biological Science and Medical Engineering, Southeast University, Nanjing 210096, China; Keywords: photonic crystals; colloidal crystals; structural colour; electrohydrodynamic jet printing; self-assembly.

Abstract

In this work, we demonstrate the fabrication of photonic crystal patterns with controllable morphologies and structural colours utilizing electrohydrodynamic jet (E-jet) printing with colloidal crystal inks. The final shape of photonic crystal units is controlled by the applied voltage signal and wettability of the substrate. Optical properties of the structural colour patterns are tuned by the self-assembly of the silica nanoparticle building blocks. Using this direct printing technique, it is feasible to print customized functional patterns composed of photonic crystal dots or photonic crystal lines according to relevant printing mode and predesigned tracks. This is the first report for E-jet printing with colloidal crystal inks. Our results exhibit promising applications in display, biosensors and other functional devices.

ACS Paragon Plus Environment

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INTRODUCTION

Inspired by natural opals, photonic crystals (PhCs) possess a unique mechanism that contributes to brilliant and non-fading structural colours with tunable ability,1-3 which are excellent candidates for a broad range of apllications, such as optical devices,4 biosensors,5 display materials,6 and information carriers.7 Thanks to the periodic structures in sub-micrometer scale, the self-assembled colloidal crystals create a forbidden gap to reflect light with a specific wavelength in the visible region. Thus, they control the light propagation and generate an obvious structural colour.8-11 Tremendous effort has been devoted to the fabrication of structural colour materials. However, it remains a challenge for researchers to obtain structural colour units with controllable shapes and improved optical properties for practical applications. Researchers have proposed effective solutions to realize the self-assembly of colloidal crystals in small controllable quantities within the micrometer range, such as the template-assisted selfassembly,12,13

wettability

effect,14,15

photolithography,16,17

inkjet

printing,18,19

and

microfluidics.20,21 Among these techniques, inkjet printing has an advantage in precisepositioned micro-droplet arrangement, which shows versatile operability for direct writing of complex patterns.22 In particular, the microstructure of each printed unit could be controlled by the wettability of the substrate, which plays an important role in the optical properties of the assembled colloidal crystals. Nevertheless, the inkjet printing technique is still confronted with the following issues: limited resolution and clogging. Electrohydrodynamic jet (E-jet) a new printing technique based on inkjet printing has been proven to be a useful means to micro/nanomanufacturing.23-27 Compared to classical inkjet printing, the key advantage of E-jet printing lies in its ability to generate uniform droplets or lines with sizes much smaller than the inner


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diameters of the nozzles. In addition, the electric field between the nozzle and the substrate has a positive effect on shape control of assembled nanoparticles in the droplets.28-30 Using the electrohydrodynamic phenomenon, the generation of uniform latex droplets and PhC beads has been successfully realized with electrospray.31,32 However, it is still difficult to precisely control the deposition of colloidal inks for PhC patterns with electrohydrodynamic technique due to several limitations for the combination of E-jet printing and colloidal crystal inks. First, electrical breakdown is unavoidable for large-scale fabrication using E-jet printing system due to close distance between the nozzle and the substrate.33 Secondly, droplet deflection and retreat during the flight period to substrates also affect consistent ejection of latex droplets.34 Herein, we demonstrate the construction of PhC patterns with structural colours and controllable microstructures via E-jet printing by using colloidal crystals inks for the first time. We systematically investigated the E-jet printing mechanism with colloidal inks, as well as the influence of the materials, substrates and other printing parameters such as the applied voltage signal on the printing stability, precision and final shape of the microstructures. The generation of discontinuous or continuous latex jets during printing was regulable by the applied electrical field between the nozzle and the substrate. On the Basis of the wettability of the target substrate, the printed colloidal nanoparticles was able to self-assemble into PhC dots, rings and lines. According to the predesigned tracks, we have directly printed customized patterns with brilliant structural colours. Interestingly, the resultant colourful patterns showed angle dependence or independence property at different viewing angles, which relied on the morphologies of assembled nanoparticles. We believe this work provides a promising strategy to fabricate microstructures in demand for display and optical applications.


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EXPERIMENTAL SECTION

Preparation of Colloidal Crystal Inks. Three solvents were chosen as the dispersant for colloidal crystal inks, including deionized water (DI water), ethylene glycol (EG) and capryl alcohol (CA). EG, CA, and ethyl alcohol were purchased from Sinopharm Chemical Reagent Co., Ltd, China. Silica nanoparticles modified with charged groups were purchased from Nanjing Dongjian Biological Technology Co., Ltd, China. For each ink, as-prepared monodisperse silica nanoparticles were first purified via centrifugation, followed by dispersion in corresponding solvents with sonication for 30 min. The concentrations of silica nanoparticles in each ink were 0.01 g/ml, 0.02 g/ml, 0.05 g/ml, 0.1 g/ml and 0.2 g/ml, respectively. Construction of E-jet Printing System. The system composed of four main parts: printing stage, printing nozzle, voltage generator, and the control unit. The printing stage consisted of a translation stage and an electrically conducting support. An ITO glass (10 cm in length, 10 mm in width) was rested on a computer-controlled two-axis translation stage to serve as the electrode for E-jet printing (Figure S1a). The nozzle was fixed at a translation stage perpendicular to the conducting support that could adjust the distance between the nozzle and the substrate. The voltage generator was a voltage amplifier (TREK, 609E), which could magnify the low analog voltage signal from the control unit to a high output voltage. The control unit generated all the control signals of the stage and the voltage based on the LabVIEW program. A CCD camera was employed to measure the distance between the nozzle and the substrate and to observe the ejection of nozzle during the printing process in real-time. For pulsating jet mode, the applied voltage was an alternating pulse signal and the E-jet printer was equipped with single substrate electrode. The nozzle electrode and syringe pump were unnecessary (Figure S1b). Printing nozzle was a tapered capillary with a hydrophobic surface


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coating (Figure S1c). The distance between nozzle and substrate was kept at 150 µm. Ink was introduced into the capillary based on the capillary effect before printing. For continuous jet mode, the applied voltage was a constant voltage signal. The nozzle electrode and the syringe pump were necessary. A metal needle with plain end was used as the nozzle (Figure 6a), which was connected to a syringe pump to generate steady ejecting streams. Besides, nozzle electrode was connected to ground electrode of voltage amplifier for safety considerations. The nozzle-to-substrate distance was 1500 µm. Preparation of Printing Substrates. Opaque silicon wafers were prepared for hydrophobic substrates. Transparent Indium tin oxide (ITO) glasses were prepared for hydrophilic substrates. ITO glasses (ITO-B001) were purchased from Zhuhai Kaivo Optoelectronic Technology Co., Ltd, China. Silicon wafers were purchased from Suzhou Wenhao Chip Technology Co., Ltd, China. Both ITO glasses and silicon wafers were cut into 3 cm in length and 3 cm in width, then cleaned by ultrasonic washing. For hydrophobic substrates, 100 µL of amorphous fluoropolymer solution (DuPont) were dropped onto the silicon wafers and spin coated at a speed of 3000 rpm for 120 s, followed by heating at 300 °C for 6 h. Preparation of Capillary Nozzles. The glass capillaries (1B100F-3) were purchased from World Precision Instruments, Inc., USA. They were tapered to obtain an inner diameter of 0.05– 0.5 mm using a capillary puller (Sutter Instrument, P-97) and a microforge (Narishige, MF-830). Then the capillary nozzles were dipped into trimethoxy (octadecyl) silane solution (5 vol% in acetone) for 12 hours to obtain a hydrophobic surface coating in order to prevent inks from wetting the sidewall of nozzles. Trimethoxy (octadecyl) silane was purchased from SigmaAldrich Co.


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Characterization. The photographs of the E-jet printer and the patterns were obtained with a digital camera (Canon, EOS 5D Mark II). The microstructures of the PhC dots were characterized by scanning electron microscope (SEM, HITACHI, S-300N). The optical images of nozzle, PhC dots and lines were taken by an optical microscope (OLYMPUS, BX51) equipped with a CCD camera (Media Cybernetics Evolution MP 5.0). The diameter of final structures and the spacing distance were measured using ImageJ. The reflection spectra of the PhC patterns were measured using a tungsten halogen source (Ocean Optics, LS-1) and an optical spectrometer (Ocean Optics, USB 65000). The static contact angles were measured on a contact-angle system (JC2000D, Shanghai Zhongcheng Digtal Technology Apparatus Co. Ltd) with latex droplets (2 µL). Each contact angle was measured by averaging five independent measurements.

RESULTS AND DISCUSSION

Material Selection. For colloidal PhCs, we need stabilized suspensions with silica or polystyrene colloids dispersed in either water or organic solvents. The selection of dispersant that simultaneously meets the demands of electrohydrodynamic technique and self-assembly process is crucial to E-jet printing with colloidal crystal inks. DI water was eliminated due to frequent spray during E-jet printing. In this case, the positions and final microstructures of the generated dots could not be precisely controlled, as shown in Figure S2a-b. Alcoholic solutions are good solvents for dispersing colloids since they offer applicable boiling points and viscosities based on molecular structures. EG and CA were then chosen in this study for their performance. Other than EG and CA, other alcoholic solutions could be used in E-jet printing, such as butanediol (Figure S2c). Both EG inks and CA inks could satisfy the demands for the pulsating jet mode.


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EG was a better choice for its lower boiling point, which was beneficial to the self-assembly of colloidal crystals. On the other hand, it was difficult to print EG inks into continuous streams, making CA the only choice for continuous jet mode for its high viscosity. To avoid swelling or dissolution of the colloids, we chose silica nanoparticles rather than polymeric spheres for their chemical stability in alcoholic solutions. The original wettability of ITO glasses and silica wafers was tested with a droplet of pure EG (Figure S3), which showed a similar contact angle. The silica wafers were flatter than the ITO glasses, which was better for a hydrophobic coating. It was easy to distinguish the hydrophilic substrates from hydrophobic substrates according to the transparency. Mechanism of E-jet Printing. In a typical E-jet printing system, the voltage applied between nozzle and conducting support creates the electrohydrodynamic phenomenon that drives flow of inks out of the nozzle. As shown in Figure 1a, our strategy offered pulsating jet mode and continuous jet mode, which were responsible for the generation of latex droplets and continuous streams respectively. When using the pulsating jet mode, an E-jet printer for drop-on-demand

printing was equipped with single electrode. In so doing, it improved the operation stability for latex droplets and simplified the fabrication of the nozzles. When using the continuous jet mode, we realized the direct writing of continuous lines via near-field electrospinning. With the evaporation of the dispersants, dots, rings and lines consisting of colloidal particles were assembled depending on the wettability of the target substrates (Figure 1b). Thus, functional patterns with structural colours and specific microstructures were fabricated (Figure 1c).


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Figure 1 Schematic illustration of the E-jet printing with colloidal crystal inks. (a) Two printing modes with pulse or constant signals. (b) Microstructures based on self-assembly of colloidal particles on different substrates. (c) Optical images of the printed PhC dots, rings, and lines corresponding to the applied printing mode and the target substrate. In continuous jet mode, substrate electrode was connected to the conductive nozzle. However, we cancelled the nozzle electrode in our E-jet printing system under pulsating jet mode in order to ensure operation stability for colloidal inks. An alternating signal with positive working potential and negative bias potential was connected with conducting support below target substrate. Due to the alternating pulse signals, positive or negative charges were induced on the meniscus based on the potential connecting to the conduct which generated a Taylor cone at the nozzle head.33,34 No jetting was obtained under the start-up voltage for jetting. During the positive period of the signal, negative charges were aggregated on the meniscus of the nozzle head (Figure 2a). At sufficient high voltage, the electrostatic stress overcame the capillary tension at the apex of the liquid cone and a negatively charged droplet was ejected onto the substrate. After the ejection, the net charge in the ink changed to positive, which suppressed


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continuous generation of droplets with induced negative charges. Therefore, it was unnecessary to use a conductive nozzle. The capillary nozzle offered a smaller diameter. The aggregation, ejection, and neutralization phase during one E-jet printing circle were captured by the CCD camera. It should be emphasized that e-spray would form multiple uncontrollable jets if the voltage exceeds a threshold.23 The bias potential did not have any effect on jetting performance for its low amplitude but played an important role in space charge neutralization. The mechanism of our E-jet printing under pulsating jet mode was verified by using colloidal crystals with different surface charges from the ink. For the signal shown in Figure 2a, the EG inks containing silica nanoparticles with either positive or negative charges were ejected. Only one latex droplet was observed in a cycle. Besides, we concluded that ejection was achieved with sufficient high working voltage with an appropriate duration because the surface charge of the nanoparticles had little effect on the net charge of ink. Moreover, all silica nanoparticles with different charges were assembled into ordered structures (Figure 2b). Despite the size of resultant PhC dots, no obvious difference was observed in term of morphology. It should be attributed to a prolonged processing volatilization time compared to that of E-jet printing. For a PhC pattern consisting of 1000 dots, it took about 17 minutes to print at a frequency of 1 Hz.

The

volatilization process together with the assembly of colloidal crystals required more than one hour at room temperature. The external electrical field was removed after printing. Thus, only one droplet was deposited at the same position. The final assembled shape of each droplet resulted from the three-phase contact line behaviors on the substrate.


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Figure 2 (a) Mechanism of E-jet printing under pulsating jet mode and images captured by the CCD camera during one cycle. (b) SEM images of PhC dots fabricated in pulsating jet mode using silica nanoparticles modified with positive (NH2+) and negative (HSO3-) charges. All scale bars refer to 10 µm. Effect of the wettability of substrate. Wettability plays an important role in controlling the final shape of microstructures.35-37 We systematically investigated the influence of wettability of substrate on the morphology of resultant patterns by using different dispersants for the inks. All inks containing silica nanoparticles with a diameter of 265 nm were printed to the ITO glasses and the wafers with a Teflon layer using pulsating jet mode. The surface tension of EG (48.4 mN/m) was higher than that of CA (29.0 mN/m), leading to a larger contact angle for silica inks with the same concentration. The dispersing silica nanoparticles brought unobvious effect on the final homogeneous ink which is shown in Figure 3a-b. The ink droplets deposited on the Teflon layer had a higher contact angle than those deposited on the ITO layer for the same ink. It is not difficult to find that hydrophilic surface contributes to a ring structure, which can be explained by the coffee-ring effect, while hydrophobic surface contributes to a dome structure with a high


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height-to-diameter ratio. Thus, the substrate with higher contact angle is beneficial to the generation of dot patterns with smaller diameter. Despite their morphologies, both rings and dots exhibited vivid structural colours, indicating a successful self-assembly of silica nanoparticles in the inks during the E-jet printing (Figure 3g-j). In the following experiments, we chose silica nanoparticles in EG with a concentration of 0.1 g/mL as the inks for pulsating jet mode and silica nanoparticles in CA with a concentration of 0.2 g/mL for continuous jet mode.

Figure 3 (a, b) Relationship between concentrations of silica particles in colloidal inks and the contact angles of (a) ITO glasses and (b) silicon wafers with Teflon layers. (c, d) Optical image of the latex droplets with concentrations of (c) 0.1 g/ml in EG, and (d) 0.2 g/ml in CA on the ITO glasses. (e, f) Optical image of the latex droplet with the concentrations of (e) 0.1 g/ml in EG, and (f) 0.2 g/ml in CA on silicon wafers with Teflon layers. (g - j) Optical images of the printed PhC microstructures by using the corresponding substrates and inks in (c-f). Control of the E-jet Printing Precision in Pulsating Jet Mode. Since the diameter of the printing dots is associated with the volume of the jetting droplet, we investigated the effect of the


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nozzles and the applied voltage. The aim of these experiments was to achieve stable jetting droplets with a high precision in the pulsating jet mode. Inks were held in the capillary because of the comprehensive balance of the gravity of the inks, the electric field force, and the capillary force. Considering the volume gap between each jetting droplet and the remaining inks in the capillary, it was easy to keep the balance during E-jet printing. The ink was supplemented to the meniscus at the nozzle head by the electric field force and the gravity force during the printing process. Here, the relevant parameters are the inner diameter of the nozzles and the volume of ink in the capillary. First, we fixed the height of ink at 5 cm while using six kinds of nozzles with inner diameter of 61 µm, 127 µm, 179 µm, 189 µm, 231 µm, and 324 µm, respectively. The velocity of the translation stage was 100 µm/s. All the other relevant parameters are summarized in Table 1. The tuning range of the diameter of PhC dots could be adjusted from 8.4 to 27.5 µm (Figure 4a). In general, decreasing the inner diameter of nozzles creates jetting droplets with a smaller radius. Then we used a nozzle with an inner diameter of 127 µm and filled the nozzles with different volume of inks which was tuned by the height of ink in the capillary. From the data shown in Figure 4b, the volume of inks had no significant effect on the final diameter of PhC dots, which also proved the feasibility of E-jet printing without a pump in pulsating jet mode. The printing precision was determined by the parameters of the voltage signal which contained working potential, duty cycle, and frequency of the voltage. According to the mechanism of E-jet printing in pulsating jet mode, higher working potential would result in a bigger jetting droplet. Hence increasing the diameter of the PhC dot. The diameter of assembled dots increased from 10.2 µm to 24.2 µm when the working potential increased from 1 kV to 2 kV (Figure 4c). Also,


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the structures of the dots may depend on the pulse width (w) when the working potential of the voltage is fixed, w = D/f,

(1)

where D is the duty cycle of working potential, and f is the frequency of the voltage. Only one droplet could be generated in one cycle. Thus, longer duration of the working voltage almost had no effect on the volume of jetting droplets which was verified by increasing the duty cycle of working potential (Figure 4d). On the contrary, an obvious decrease in the final diameter of PhC dots was found when using a voltage with higher frequency. as shown in Figure 4e. Compared to tuning the duty cycle, the adjustment of frequency brought about a change in the total period for a jetting. Less time was left for the relaxation of the nozzle meniscus under higher frequency, leading to a decrease in volume of droplets. However, limited by the weak restorability of colloidal inks, a frequency exceeded the critical value would result in unstable e-spray. For our EG inks, the highest frequency could be set up to 10 Hz, which remained an obstacle to rapid fabrications. Up to now, the resolution for E-jet printing has reached below 100 nm using a nozzle with a diameter of 1 µm. 25 There are other ways to reduce the size of final structures, such as increasing the contact angle of the substrate, 38 reducing the concentration of silica nanoparticles in the colloidal inks or using a sharper nozzle. However, those approaches were unapplicable to our colloidal crystals inks. When printing the colloidal crystal ink with a concentration of 0.02 g/ml, the PhC dots with a diameter of 6.2 µm and quasi-ordered structures would be achieved (Figure S4). The self-assembly of silica nanoparticles would become even poorer with further reduction in the size of PhC dots, which is destructive for the structural colour display. Sharper nozzles may result in clogging problem because the diameter of the containing silica nanoparticle is 200


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- 300 nm. Based on the above results, an array of PhC dots (Figure 4f) was obtained with an average diameter of 11.8 µm and a standard deviation of 0.789 µm (with 30 dot diameter measurements) which was an appropriate value for high-resolution pixels using colloidal crystals for display. The positioning accuracy relied on three components: the movement of the stage, jet straightness, and the wettability of the substrate. From statistical analysis (Figure S5b), it is seen that the positioning accuracy was mainly affected by the positioning error along the movement direction. We consider it was attributed to the aggregation of charges on the meniscus, which might induce the shift of the drying droplets on the hydrophobic Teflon substrate.

Figure 4 (a - e) Correlation between the diameter of resultant dots and the parameters of E-jet printing in pulsating jet mode: (a) the diameter of nozzles, (b) the height of inks in the capillary, (c) the amplitude of working potential, (d) the duty cycle and (e) the frequency. (f) Optical images of the array of PhC dots with a diameter of 10 µm. Table 1 Parameters in investigation of the precision for E-jet printing in pulsating jet mode. Figure

The inner

The height of

The working

The bias

The duty

The

diameter of

the ink in the

potential of

potential of

cycle of

frequency of


14

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nozzles [µm]

capillary [cm]

voltage [kV]

voltage [kV]

voltage [%]

voltage [Hz]

4a

60 - 325

5

1.3

-0.1

15

1

4b

127

1-6

1.3

-0.1

15

1

4c

127

3

1-2

-0.1

15

1

4d

127

3

1.3

-0.1

5 – 50

1

4e

127

3

1.3

-0.1

15

1-8

4f

61

3

1.3

-0.1

15

1

Fabrication of Structural Colour Patterns. A letter pattern of “SEU” was printed by using pulsating jet mode. To achieve a higher resolution, we chose printing PhC dots (Figure 5) rather than PhC rings (Figure S6c). The PhC rings may play important roles in other areas, such as biosensors, and tissue engineering scaffolds. The SEM images showed detailed surface structure of these dots where monodisperse silica nanoparticles formed a predominantly hexagonal symmetry (Figure 5b). Because of the periodic arrangement of the silica nanoparticles, the resultant PhC dots possessed photonic band gap, leading to vivid iridescent colours and characteristic reflection peaks of the E-jet printing patterns. The diameter of each dot was 30 µm and the hight was 6.9 µm (Figure 5c). Because the height/diameter ratio of these dots was only 0.23, the letter pattern showed an angle-dependent reflection colour (Figure 5a), which was similar to the PhC films.6,38 With the decrease of viewing angles, the reflection peaks changed from red to blue, which showed obvious blue-shift in reflection spectra (Figure 5d and 5e). Various colours could be achieved by using inks containing silica particles with different diameters (Figure S6a and S6b) and complex patterns could be generated by designing the motion curve of the stage. Due to the nature of homemade system, an obstacle for multiple colours was that only one kind of ink was filled in the nozzle, and there would be replacement error if we changed different inks for one pattern. An E-jet printing system with multiple nozzles


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can solve the problem which would be added to our system in the future to enable printing multiple materials on the same substrate.

Figure 5 (a) Photographs of letter patterns “SEU” (Southeast University) observed at various viewing angles. The white dots are deposited dust, which could be avoided in clean room. (b) Top-view and (c) side-view SEM images of the PhC dots in the letter pattern. (d) Reflection spectra of the letter pattern detected at specified viewing angles. (e) The relationship between the reflectance peaks and the viewing angles. Fabrication of Continuous PhC Lines. Continuous PhC lines are significant to the fabrication of biochips, microfluidics, and other functional devices. The coalescence of two neighboring drops was utilized for fabricating lines in drop-on-demand printing mode.39,40 Though PhC lines could be achieved by the pulsating jet mode (Figure S7), it was difficult to achieve the coalescence of droplets and getting rid of the coffee-ring effect at the same time. The


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continuous jet mode provided a simple strategy for the fabrication of continuous patterns. Clean ITO glasses were selected as target substrates for the fabrication of PhC lines because of the hydrophobic effect on CA which could cause a surface tension to crack the streams into droplets during the volatilization. As a result, continuous PhC lines with adjustable width were directly written using our E-jet printing system. One remarkable difference between pulsating jet mode and continuous jet mode was the tunable effect of the voltage amplitude. Here, higher DC voltage minimized lateral distribution of the electric field lines and resulted in a sharped Taylor cone. Consequently, a thinner stream could eject from the apex of the cone when increasing the voltage (Figure 6a). The effects of the voltage of power supply, flow rate of syringe pump, and the velocity of translation stage are shown in Figure S8 respectively. The width of obtained PhC lines could be down to 100 µm (Figure 6b and 6c). These results show that the flow rate and velocity had a direct effect on the width. Increasing the flow rate meant more inks were accumulated at the nozzle head, which brought about a higher pressure to overcome the capillary tension. On the other hand, the jet streams in the air were stretched by a drawing force derived from the motion stage and tuned by velocity. Within the mechanical strength of the CA streams, a higher velocity led to a thinner width of the generated PhC lines. Using the same system, a more obvious effect from these factors could be seen in near-field electrospinning of polyethylene oxide (PEO) solution and we realized a higher resolution of 1 µm (Figure S9). Utilizing the continuous jet mode, a pattern consisting of parallel lines was obtained. As shown in Figure 6d and 6e, the arrays of parallel lines show excellent angle independence property, displaying constant colours and reflection peaks at different viewing angles. This phenomenon could be attributed to the amorphous structure of silica nanoparticles generated during printing process.41,42 Owing to the high viscosity and high boiling point of the dispersant


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CA in the ink, it became an obstacle for the assembly of nanoparticles with highly ordered closepacked structures.

Subsequently resulting in a quasi-amorphous array of silica nanoparticles

with short-range ordering and long-range disordering (Figure 6f and 6g). Compared with the angle-independent PhCs using spherical symmetry of the elements,43-45 the single-step E-jet printing method can directly fabricate arbitrary patterns with angle-independent structural colours.

Figure 6 (a) Optical images captured by the CCD camera during continuous jet mode. A metal needle with an internal diameter of 500 µm was used as the nozzle. (b, c) Photographs of the graphic patterns fabricated by continuous jet mode. (d, e) Photographs and reflection spectra of the array of parallel lines observed at various viewing angles. (f, g) SEM images of the PhC lines and the nanoparticles in the lines.


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Conclusions

In summary, a practical approach has been demonstrated for the construction of structural colour patterns by using colloidal crystal inks and E-jet printing. Colloidal latex and streams were generated by specialized voltage signals. Customized patterns on the other hand could be directly written with the movement of translation stage. The final shapes of PhC units could be controlled by the wettability of the substrates. Optical properties of the structural colour patterns could be tuned by the self-assembly of the silica particles. As our strategy could satisfy the fabrication of functional patterns with controllable morphology and optical property, we hope our findings will contribute to optical applications as well as intelligent devices.

ASSOCIATED CONTENT Supporting Information Available Schematic illustration of the E-jet printer; Photograph of the E-jet printer; Optical image of the capillary nozzle; Optical images of the PhC dots fabricated by using DI water and butanediol as the solvent; Images of EG droplets on a silica wafer and an ITO glass; SEM image of a PhC dot when printing the colloidal crystal ink with a dilute concentration; Statistical analysis of the diameter of the droplets and positioning accuracy; Photograph of letter pattern “SEU” which consisted of PhC rings; PhC lines fabricated by pulsating jet mode; Correlation between the widths of the lines and the parameters of E-jet printing in continuous jet mode; Optical images of the printed polyethylene oxide (PEO) lines in continuous jet mode. This material is available free of charge via the Internet at http://pubs.acs.org.


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Corresponding Author [email protected] Notes The authors declare no competing financial interests. Acknowledgement This research was supported by the National Natural Science Foundation of China (Grants 21327902, 21635001, and 21501026), the Natural Science Foundation of Jiangsu Province of China (Grant BK20140626), the Research Innovation Program for College Graduates of Jiangsu Province (KYLX_0189), and the Jiangsu Planned Projects for Postdoctoral Research Funds (Grant 1601012B). We thank Prof. Yongan Huang and Dr. Yanqiao Pan from Huazhong University of Science and Technology for helping with the construction of E-jet printer.

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Graphical Abstract or Table of Contents


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Structural Color Patterns by Electrohydrodynamic Jet Printed Photonic

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