Regulation of the Deposition Morphology of Inkjet-Printed Crystalline

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Regulation of the deposition morphology of inkjet-printed crystalline materials via polydopamine functional coatings for highly uniform and electrically conductive patterns Liang Liu, Siyuan Ma, Yunheng Pei, Xiao Xiong, Preeth Sivakumar, and Timothy J. Singler ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b03063 • Publication Date (Web): 11 May 2016 Downloaded from http://pubs.acs.org on June 5, 2016

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ACS Applied Materials & Interfaces

Regulation of the deposition morphology of inkjetprinted crystalline materials via polydopamine functional coatings for highly uniform and electrically conductive patterns Liang Liu, Siyuan Ma,‡ Yunheng Pei, Xiao Xiong, Preeth Sivakumar, Timothy J. Singler* Department of Mechanical Engineering, State University of New York (SUNY) at Binghamton, Binghamton, New York KEYWORDS: polydopamine, inkjet printing, plasma reduction, interfacial crystallization, coffee ring effect, reactive silver ink, deposition morphology control ABSTRACT

We report a method to achieve highly uniform inkjet-printed silver nitrate (AgNO3) and a reactive silver precursor patterns on rigid and flexible substrates functionalized with polydopamine (PDA) coatings. The printed AgNO3 patterns on PDA-coated substrates (glass & polyethylene terephthalate (PET)) exhibit a narrow thickness distribution ranging between 0.9 and 1µm in the line transverse direction and uniform deposition profiles in the line axial direction. The deposited reactive silver precursor patterns on PDA-functionalized substrates also

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show “dome-shaped” morphology without “edge-thickened” structure due to “coffee-stain” effect. We posit that the highly uniform functional ink deposits formed on PDA-coated substrates are attributable to the strong binding interaction between the abundant catecholamine moieties at the PDA surface and the metallic silver cations (Ag+ or Ag(NH3)2+) in the solutal inks. During printing of the ink rivulet and solvent evaporation, the substrate-liquid ink (S-L) interface is enriched with the silver-based cations and a solidification at the S/L interface is induced. The preferential solidification initiated at the S-L interface is further verified by the insitu visualization of the dynamic solidification process during solvent evaporation, and results suggest an enhanced crystal nucleation and growth localized at the S-L interface on PDA functionalized substrates. This interfacial interaction mediates solute transport in the liquid phase, resulting in the controlled enrichment of solute at the S-L interface and mitigated solute precipitation in both the contact line region and the liquid ink-vapor (L-V) interface due to evaporation. This mediated transport contributes to the final uniform solid deposition for both types of ink systems. This technique provides a complementary strategy for achieving highly uniform inkjet-printed crystalline structures, and can serve as an innovative foundation for highprecision additive delivery of functional materials.

1. Introduction Inkjet printing has shown significant potential for the low-cost and versatile fabrication of micro-scale functional material patterns which are critical to the areas of sensors,1 displays,2 batteries,3 and photovoltaics.4 One important application of this additive manufacturing technique is the patterning of electrically conductive structures.5 Metal organic and inorganic precursor solutions have been developed and used as printable inks for conductive pattern


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fabrication. The advantages of these solutal inks are: agglomeration-free and high printability,6 low temperature post-printing processing requirement (≤150 ℃ ),7 and high electrical conductivity.6,8 Nevertheless, it has to be admitted that these inks still face the challenges of ink concentration limit and deposition structural defects.9 To yield highly conductive patterns, post-printing processing methods are generally needed to initiate a decomposition reaction of the deposited solid, which helps to convert the precursor materials to metallic conductive patterns.6,7,9 Typical post-printing processing methods include thermal treatment,6–8 photonic radiation,10 and chemical reaction.11,12 Plasma treatment has recently been identified as another type of post-printing processing approach, the merits of which are low processing temperature,13 broad range of ink and substrate material pallet,14 as well as scale-up capability.15 It is widely recognized that a precise control over the pattern morphology of printed structure plays a critical role in fabricating electronics devices with optimal performance.16,17 For instance, Kim et al.18 demonstrated that the control over the deposition morphology of inkjet printed silver conductive lines facilitated the improvement of the electrical conductivity of the patterns after post-printing processing. Fukuda et al.19 recently reported the influence of the cross-sectional profile of inkjet printed silver electrode on the electrical performance of the fabricated thin film transistors. The result suggested a much better electrical breakdown strength and a lower gateleakage currents corresponding to the more uniformly deposited electrode materials. However, achieving a uniform deposition structure is a challenge for inkjet printing process. The reason is due to the so-called “coffee-stain” effect,20 where the replenishing convective flow to the contact line region would result in an excessive deposition of solid materials at the outer-edge of printed


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structure after solvent evaporation. Typical solutions to such a problem incorporate the liquid surrounding temperature control,21 environmental humidity control and evaporation rate manipulation,19 as well as applying co-solvent systems,18 et al. The reasons of applying these methods are trying to manipulate the evaporation process, weakening the convective flow strength, and introducing the Marangoni flow at the liquid ink-vapor (L-V) interface that counter-balances the convective transport process.18,19,21 Therefore, it is of great value and interest to develop a versatile approach to enhance the structural uniformity of inkjet printed patterns. In this work, we investigated the influence of a polydopamine (PDA) coating on the deposition morphology of inkjet printed patterns for two representative solutal ink systems: the AgNO3 aqueous solution precursor ink22 and the reactive silver ink.7 Inspired by the original investigation of Gross23 and Growther,24 we recently reported direct inkjet printing of silver nitrate (AgNO3) aqueous ink and using Argon plasma for post-printing processing.22 The results showed that plasma treatment triggered a reduction reaction within a superficial surface layer of the AgNO3 deposits, forming a “skin” layer of metallic silver on the surface of the AgNO3 layer. The deposited solid structure, nevertheless, exhibited a strong structural non-uniformity along the printed line axial direction. The maximum layer thickness was over 10µm, while less than 1µm layer thickness was exhibited in some other regions. The high irregularity of AgNO3 deposits was believed due to the challenge of a precise control over the crystallization process during solvent evaporation. Walker et al.7 recently developed a reactive silver ink formulation based on a modified Tollens’ reagent, where an ionic silver complex composed of silver acetate (CH3COOAg) and ammonium hydroxide (NH4OH) is reduced by formic acid (HCOOH) after inkjet printing and spontaneous precipitation of solute materials due to solvent evaporation. Highly conductive


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silver patterns (the same order as bulk silver conductivity) were achieved by heating under mild temperature (90°C, typically). However, detailed exploration of the deposition morphology associated with this promising ink is still absent. Surface functional coatings have been applied to regulate the nucleation, orientation, and deposition morphology of crystalline structures, due to the interfacial interactions between the coating layer and crystalline materials, such as lattice matching, van der Waals force, hydrogen bonding, electrostatic interaction et al.25 These interactions are identified as being able to promote the heterogeneous nucleation and following crystal growth of the solidification phase. Ryu et al.26 recently demonstrated the ability of surface-anchored polydopamine (PDA) moieties to facilitate the nucleation and crystal growth of hydroxyapatite, and they attributed this phenomenon to the binding interaction between the Ca+ mineral ions and the catecholamine functional groups of the PDA molecules grafted at the substrate surface, which helped with the enrichment of the Ca+ at the interface. Zhou et al.27 also explored the effect of PDA molecules in polybutylene succinate (PBS)/ramie fiber bio-composites system, where they introduced PDA coatings at the ramie fiber surface via immersion coating and found an improved interfacial strength of the composite material. They explained this result as the enhanced interfacial interaction between PBS and ramie fiber and the facilitated interfacial crystallization of PBS at the ramie fiber surface with the presence of PDA molecules. Inspired by these previous research investigations, herein, we demonstrate the use of PDA-functionalized substrates for regulating the deposition morphologies of inkjet printed AgNO3 and reactive silver patterns to achieve highly uniform structures with controllable thickness profiles. The deposited AgNO3 patterns on substrates with PDA coatings exhibited a regulated surface morphology with uniform material distribution along the printed line axis; the reactive silver patterns on PDA-coated substrates also


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showed a well-defined “dome-shaped” deposition profile with minimal structural nonuniformity. The crystallization process with solvent evaporation was investigated in-situ, and results suggested the morphological control mechanism of PDA functional coatings is probably related to the binding interaction between the PDA surface functional groups and metallic ions in liquid ink. This technique showed multi-substrates compatibility and was demonstrated for extended application in flexible electronics fabrication. 2. Experimental Details Materials used in experiments were soda lime glass microscope slides (VWR Plain, 75x25mm, VWR Scientific), polyethylene terephthalate (PET) films (Melinex ST506, 500 gauge, DuPont), Dopamine hydrochloride (DA) (99%, Alfa Aesar), Trizma base (Tris) (99.9%, Sigma-Aldrich), Silver Nitrate (AgNO3) (99%, Acros), Silver acetate (CH3COOAg) (99%, Sigma-Aldrich), formic acid (HCOOH) (≥88%, Sigma-Aldrich), Ammonium hydroxide (NH4OH) (30%, Fisher Scientific), cleaning solution (Micro-90, Cole-Parmer) and Isopropyl alcohol (IPA) (99%, BDH). All chemicals were used without further purification. Water was deionized and distilled before experiments. Glass substrates were cleaned by immersion ultrasonication (3510, Branson) with an aqueous solution consisting of 2% wt. Micro-90, followed by rinsing with copious amount of IPA and distilled deionized (DI) water. PET substrates were cleaned by thoroughly rinsing with IPA and DI water alternatively. The rinsed PET substrates were further treated by 300W Oxygen (O2) plasma (PE200RIE, PlasmaEtch) for 1min to enable inkjet printing of stable lines (see Supporting Information, Fig. S1). In order to form continuous polydopamine (PDA) coatings, the cleaned glass and PET substrates were immersed into DA aqueous solution (5mg/ml) with pH


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buffered at the value of 8.5 (1.2mg/ml Tris) to initiate the oxidative self-polymerization reaction.28 The immersion time for all the experiments was fixed at 24h, and substrates were subsequently rinsed with DI water and dried under a mild nitrogen stream. PDA-coated glass and PET substrates are indicated as “glass_PDA” and “PET_PDA”, respectively. Glass and PET substrates without PDA coatings were also used as substrates for comparison. To formulate printable AgNO3 ink, 25.4% wt. AgNO3 aqueous solution was prepared by dissolving 0.34g (0.002mol) solid AgNO3 into 1g of DI water, followed by 15min ultrasonication. The reactive silver ink was formulated by following a published protocol7 with minor modification: a close-top glass vial was used with continuous magnetic stirring under room temperature, 1g CH3COOAg was added into 2.5ml NH4OH aqueous solution (30%wt.) and homogenized for 1h to yield a transparent mixture. Then 0.2ml formic acid (88%wt.) was added into the reaction bath with a controlled infusion rate of 3µL/s by precision syringe pump (74900, Cole-Parmer). The mixture gradually changed color from light orange to brown and then black, indicating the formation of silver particles due to the spontaneous reduction reaction between silver ions and formic acid.7 The mixture was maintained undisturbed in the enclosed beaker for 12h, allowing for the continuation of reaction and sedimentation of suspended silver particles. The supernatant was then retracted and filtered through a syringe filter with 0.2µm pore size (02915-20, Cole-Parmer) and used as reactive silver ink. All inks described above were synthesized prior to experiments and used immediately. The printing process was performed via a customized single nozzle inkjet apparatus according to our previous reports.22 The system consists of a piezoelectric nozzle with orifice diameter of 80µm (MJ-AB-01-80-DLC, MicroFab) driven by a waveform generator (Jetdrive III, MicroFab). 5-line array patterns with 6mm single line length and 1.5mm inter-line spacing were inkjet


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deposited on substrates with drop-to-drop spacing of 50µm and printing frequency of 50Hz. All inkjet experiments were performed under room temperature and 25 ± 2% relative humidity as controlled by an environmental chamber surrounding the inkjet nozzle and substrate. A radio frequency (RF, 13.56 MHz) plasma system (PE200RIE, PlasmaEtch) was used to process the printed AgNO3 patterns to form electrically conductive paths. Plasma processes were conducted by varying the treatment time from 0.5 to 2min, with fixed RF power (900W), pressure (100mtorr) and Argon gas flow rate (20sccm). A regular convection oven was used to thermally process the printed reactive silver patterns to form conductive paths. Heat treatments were performed by varying the time from 0 to 30min, with temperature controlled at 120°C. 3. Characterization Ink surface tension and viscosity were measured by a bubble pressure tensiometer (BP100, Kruss) and a rotary viscometer (TA1000, TA Instrument) with 60mm aluminum cone-plate probe, respectively. Ink wetting contact angles (CAs) on substrates were measured by a goniometer using sessile droplet method.29 To confirm the expected material chemistry of the PDA coating layer on the surfaces of substrates, substrates were characterized by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR, Nicolet 8700, Thermo). Top view images of the solidified line patterns were acquired by optical microscope (OM, AxioImager M1m, Zeiss). Surface profiles were obtained by contact profilometry (Dektak 8, Veeco) to evaluate deposit morphology. Materials compositions were confirmed by X-ray diffraction (XRD, XDS2000, Scintag). Detailed morphologies were characterized by field


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emission scanning electron microscopy (SEM, Supra55VP, Zeiss) from both top and crosssectional views. A modified four-point probe (4-P probe) method11,22 was used to measure the electrical resistance of each 5-line array of functionalized patterns after plasma and thermal treatments. Two silver electrodes were sputtering deposited (CRC-600, Torr International) on both ends of each array with 4mm distance between electrodes’ inner edges. Averaged single line resistance was calculated by sequentially scratching discontinuities in the 5 lines in each array and measuring the resistance change before and after each scratch. The resistivity was calculated with a method based on our recent publication of the Argon plasma-processed AgNO3 material system.22 The mechanical durability of the conductive lines was assessed by measuring the line electrical resistance subject to cyclic bending deformation using a customly-designed cyclic bending tester.11 A sequential pair of concave and convex deformations was defined as one cycle. The frequency of 100 cycles per minute and bending radius of curvature of 2.5 mm were used as bending parameters. Resistance values of the lines following each 1000 bending cycles were recorded up to 10000 cycles. 4. Results and Discussion A schematic representation of the polydopamine (PDA) coating process is shown in Fig. 1(a). According to Lee et al.,28 trace amounts of the oxygen dissolved in water trigger the selfpolymerization reaction of dopamine hydrochloride (DA) in the aqueous phase under a suitable pH condition (pH = 8.5, typically). The surfaces of coated substrates exhibited a gradually deepening dark brown color with increasing immersion time (see Supporting Information, Fig.


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S2), and the PDA layer thickness can be controlled by simply varying the substrate immersion time. In this study, to ensure the complete surface coverage of PDA, 24 hours immersion time was adopted for all experiments. The measured density, viscosity, surface tension, and wetting contact angle (CA) of the AgNO3 and reactive silver inks on glass and PET substrates with and without PDA coatings are listed in the Supporting Information (SI). Histogram of the inks’ CAs (Figs. 1(b)) provides a direct comparison of the wetting behavior of the two inks on glass and PET substrates with and without PDA coatings. For AgNO3 ink (Figs. 1(b), left), the measured CAs on uncoated glass and PET substrates were 55.7° and 42.3°, respectively. These values increased to 61.5° and 57.5° after PDA coating, suggesting the PDA coating rendered the substrates slightly less wettable by AgNO3 ink. For reactive silver ink (Figs. 1(b), right), however, the PDA coating rendered glass and PET substrates more wettable by reactive silver ink. The CAs of the reactive silver ink droplets were 42.3° and 45.3° on uncoated glass and PET, respectively. These values then decreased to 29.8° and 28.1° on glass_PDA and PET_PDA, respectively. Additionally, the measured CAs on glass_PDA and PET_PDA were very similar with each other by using the same type of ink, suggesting the PDA coating process is substrate independent.28 Furthermore, the material chemistry of the PDA coatings on glass and PET substrates were verified by ATR-FTIR spectroscopy (Fig. 1(c)) which showed characteristic absorption peaks of the PDA molecules on the surfaces of substrates after the coating process. The glass_PDA and PET_PDA exhibited a broadened absorbance peak in the wavenumber range from 3600 cm-1 to 3100cm-1, which is attributable to the N-H and O-H stretching vibrations.30 The two peaks at 1625 cm-1 and 1500cm-1 are the N-H bending and aromatic ring C=C resonance vibrations.31 These characteristic absorbance peaks are consistent with the reported FTIR spectra of the PDA molecular structures.32


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Figure 1. (a) Schematic of the PDA coating process on glass and PET substrates; (b) Histogram of the wetting CAs of the AgNO3 and reactive silver ink droplets on glass and PET substrates with and without PDA coatings (scale bars of the ink sessile droplet images represent 1000µm); (c) ATR-FTIR characterization of the glass and PET substrates with and without PDA coatings. Figures 2(a) and (b) show a schematic representation of the 5-line array inkjet deposition process on substrates with and without PDA coatings, and a temporal image sequence exhibiting a printed AgNO3 liquid rivulet on glass_PDA substrate, respectively. The liquid rivulet consisted of an initial bulge located at the printing starting position and a straight liquid rivulet following the bulge. According to Thompson et al.,33 the first several landed drops will always merge together to form the initial bulge, as a consequence of the contact angle hysteresis and strong surface tension driven wetting within a time-scale between two sequentially printed droplets. The initial bulge is further pronounced due to its relatively low pressure that drives a net internal fluid flow back to the starting position of the line. The optical microscopic (OM) images of the


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solidified AgNO3 crystals on glass, glass_PDA, PET and PET_PDA substrates under low (5x objective) and high (50x objective) magnifications are shown in Figs. 2(c), (f), (i), and (l) respectively. Two directions for the printed rivulet are defined for better data presentation, the axial and transverse directions, referring to a direction along and orthogonal to the printed line axis, respectively (Fig. 2(a)). The transverse surface profiles of AgNO3 deposits for a 400µm long axial section of the line can be seen in Figs. 2(d), (g), (j), and (m). The long-range thickness distribution of the AgNO3 deposits was characterized at multiple sites using contact profilometry measurements over 500µm axial spatial intervals, and the averaged AgNO3 layer thicknesses for each of the different measurement sites were calculated and plotted versus axial position (Figs. 2(e), (h), (k), and (n)). The AgNO3 deposits on uncoated glass and PET substrates (Figs. 2(c) and (i)) exhibited a significantly random material distribution. Some regions had continuous coverage by AgNO3 solid while the substrate surface was exposed at other locations of the same line. The surface profile measurements of the deposited AgNO3 also revealed significant shortrange (Figs. 2(d) and (j)) and long-range (Figs. 2(e) and (k)) thickness variations. The maximum peak height for a typical transverse line scan was approximately 4µm on glass and 2.3µm on PET, whereas minimum values of zero thickness were observed for deposits on both of the uncoated substrates. However, the AgNO3 lines on glass_PDA and PET_PDA exhibited very good deposition uniformity (Figs. 2(f) and (l)). The transverse thickness profiles were generally distributed within the range of 0.9 to 1µm (Figs. 2(g) and (m)) with very little variation along the line axis (Figs. 2(h) and (n)). In addition, it was noted that a clue of “edge-thickened” deposition of the AgNO3 lines on glass_PDA and PET_PDA (Figs. 2 (g) and (m)), for which the AgNO3 layer thicknesses on the edges of the printed lines were slightly greater than the centerline region (in the transverse direction), which was resulted from the enhanced evaporation at the contact lines


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due to “coffee-stain” effect.20 Therefore, in comparison with the non-uniform and random AgNO3 crystalline patterns on uncoated substrates, the glass and PET with PDA coatings were capable of regulating the deposited AgNO3 structures with diminished thickness variation in the line transverse direction and long-range structural uniformity in the line axial direction. The reason for the formation of a morphologically more uniform AgNO3 crystalline structure on PDA-coated substrate surface is probably due to the strong binding interaction between the PDA surface functional groups, namely the catecholamine groups, and the metallic ions (Ag+) in the liquid ink. According to Ryu et al.,26 the abundant catecholamine moieties in the PDA layer are capable of forming coordination bounding with different types of metallic ions, including Ag+. A direct result of such a binding interaction is the interfacial enrichment with the metallic ions, which would then facilitate the precipitation (anions are attracted to the interface due to electrostatic interaction) and crystal growth at the interface. In our experiment, this process is better represented by the schematic of Fig. 3. The printed AgNO3 ink rivulet typically experiences a 3-stage evolution before solidification: rivulet formation, evaporation, and nucleation. These three stages are depicted by Fig. 3(a)-(c) on substrate without PDA coatings and by Fig. 3(d)-(f) on substrate with PDA coatings, respectively. For the ink rivulet on uncoated substrate surface, the Ag+ and NO3- ions would be homogeneously dispersed in the beginning when the rivulet is inkjet dispensed (Fi. 3(a)). The effect of evaporation will be dominant as time is increased, which would lead to an enriched ionic species at the liquid ink-vapor (L-V) interface, as can be seen in Fig. 3(b) (inserted figure showing rivulet cross-section “B”). It is noted that the three-phase contact line might be even more enriched than the other interfacial areas, due to the higher evaporation flux at this region.20 In the third stage, a few spots along the L-V interface will form initial nucleuses and facilitate subsequent crystal growth due to


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prolonged evaporation and solute supersaturation (Fig. 3(c)). Presumably, the nucleation and crystallization process at the L-V interface is highly random, and the direction of crystal growth can either protrude out of the L-V interface or extend into the liquid region, which would probably be the reason for the formation of discontinuous AgNO3 crystalline structure with significant thickness variation (Fig. 2(c)-(e) and (i)-(k)). However, for the ink rivulet on substrate with PDA coatings, the strong binding interaction between the catecholamine moieties and Ag+ species starts to dominate, as exhibited in Fig. 3(e) (inserted figures showing rivulet crosssection “D” (lower left) and interfacial binding process (upper right)). The mutual attraction between the catecholamine functional groups and the Ag+ ions would exert an even higher solute enrichment at the substrate-liquid ink (S-L) interface than the evaporatively driven solute accumulation at L-V interface (Fig. 3(f), upper). Therefore, it would be highly possible that the initial nucleuses are formed at and subsequent crystals are grown from the S-L interface on substrate with PDA coatings (Fig. 3(f), lower). It is thus well-expected that such an enhanced interfacial crystallization induced by PDA functional layer is more favorable for the controlled formation of AgNO3 crystalline patterns with improved structural uniformity (Fig. 2(f)-(h) and (l)-(n)). Indeed, the enhanced interfacial crystallization due to the presence of PDA coatings was experimentally verified by previous literature reports,26,27 and also observed by our in-situ visualization of the rivulet solidification process (Fig. 9(a)). For the reactive silver ink, OM images of the deposition patterns and surface profiles on substrates with and without PDA coatings were compared (Fig. 4). There were no significant differences in the deposition structure between glass and PET, and between glass_PDA and PET_PDA. The reactive silver lines on glass and PET (Figs. 4(a) and (e)) were characterized by two dark parallel lines along the contact line region and a shiny crystalline centerline region.


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Surface profile measurements (Figs. 4(b) and (f)) confirmed that the reactive silver line patterns on glass and PET exhibited “edge-thickened” deposition profile due to “coffee-stain” effect.20 However, uniformly deposited solid structures with some surface texture were formed for lines on PDA-coated substrates, and no “edge-thickened” structure was observed. Profilometry measurements also exhibited “dome-shaped” morphologies on both glass_PDA and PET_PDA (Figs. 4(d) and (h)). Thus, the PDA coatings facilitated the morphological uniformity improvement of the deposited reactive silver patterns by forming a “dome-shaped” transverse thickness profile on glass_PDA and PET_PDA substrates, in contrast with the non-uniform “edge-thickened” deposits on substrates without PDA coatings. The formation of highly uniform reactive silver ink deposits on PDA-functionalized substrates would also be due to the strong binding interaction between the surface-anchored catecholamine functional groups and the ionic species in the solutal ink, as exhibited in the schematic of Fig. 5. According to Walker et al.,7 the composition in the reactive silver ink includes diamminesilver cations (Ag(NH3)2+), ammonium cations (NH4+), acetate anions (CH3CO2-), and formate anions (HCO2-). Along with inkjet printing and solvent evaporation, the Ag+ metallic ions are liberated from the original complex state (Ag(NH3)2+), allowing for a redox reaction with HCO2- (Fig. 5(a), eq. (1)). After ink solidification, the additional AgCH3CO2 could be further reduced by HCO2NH4 in a solid state reaction under elevated temperature (Fig. 5(a), eq. (2)). For the ink rivulet on uncoated substrate, the reduction would probably be initiated at the contact line region (Fig. 5(c)-(d)), due to the locally accelerated evaporation20 and thus the induced redox reaction. Presumably, the observed thicker deposition at the line edges on uncoated substrate would probably consists of a metallic silver material, and the centerline region would be mainly deposited with AgCH3CO2 and HCO2NH4. For the ink rivulet on PDA-coated substrate,


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however, the strong binding interaction between the catecholamine surface functional groups and the metallic cations would influence the solute concentration profile. After printing and rivulet formation, there would be a Ag(NH3)2+ species enrichment at the substrate-liquid ink (S-L) interface, which dominants over the evaporatively driven solute enrichment at the contact line region (Fig. 5(e)-(f)). The localized Ag(NH3)2+ concentration at the S-L interface would be further increased with prolonged evaporation time, finally reach the solubility limit of Ag(NH3)2CH3CO2 and lead to the formation of free Ag+ ions. Therefore the redox reaction between Ag+ and HCO2- would be triggered at the S-L interface, as shown in Fig. 5(g). The existence of PDA coatings would result in a material redistribution in the printed ink rivulet, with confined solute enrichment at the S-L interface. Therefore, the evaporatively driven solute accumulation at the contact line and the subsequent “edge-thickened” deposition would be mitigated. The above proposed material deposition process in the as-printed reactive silver deposits was further verified by SEM analysis (Fig. 7(d)), and the PDA coating mediated S-L interfacial solidification process was also identified by in-situ visualization of the ink rivulet evolution with evaporation (Fig. 9(b)).


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Figure 2. (a) Schematic representation of the 5-line array deposition process of AgNO3 ink on substrates with and without PDA coatings; (b) Time-resolved image sequence showing the actual


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inkjet printing process of AgNO3 line pattern on PDA-coated glass substrate (scale bars shown represent 500µm); (c, f, i, j) Optical micrographs of the AgNO3 deposits on glass, glass_PDA, PET, and PET_PDA (scale bars shown represent 200µm and 50µm for low and high magnification images, respectively); (d, g, j, m) The surface profiles of AgNO3 deposits in a 400µm length region on substrates with and without PDA coatings; (e, h, k, n) The long-range thickness distribution of the AgNO3 deposits with 500µm spatial intervals in the line axial direction on substrates with and without PDA coatings.

Figure 3. Schematic of the AgNO3 aqueous ink rivulet evolution process: (a) Inkjet printing AgNO3 ink rivulet on substrate surface without PDA coatings; (b) AgNO3 ink rivulet


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evaporation with time on uncoated surface; (c) Cross-sectional view of the AgNO3 rivulet on uncoated surface when the initial crystals are formed; (d) Inkjet printing AgNO3 ink rivulet on substrate surface with PDA coatings; (e) AgNO3 ink rivulet evaporation with time on PDAcoated surface; (f) Cross-sectional view of the AgNO3 rivulet on PDA-coated surface when the initial crystals are formed; Further experiments were conducted to investigate the influence of the PDA coatings on the final morphologies and electrical properties of the printed conductive patterns after post-printing processing. 900W Argon (Ar) plasma was used to process the AgNO3 precursor patterns, and the XRD data (Fig. 6(c)) confirmed that the AgNO3 lines were reduced to metallic silver with increasing plasma processing time (see Supporting Information, Fig. S3), consistent with our previous report.22

Figure 4. (a, c, e, g) Low (20x objective) and high (50x objective) magnification optical micrographs of the printed reactive silver patterns on glass and PET substrates with and without PDA coatings (scale bars shown represent 100µm and 50µm for low and high magnification images, respectively); (b, d, f, h) Surface profiles of the reactive silver deposits on glass and PET substrates with and without PDA coatings.


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Figure 5. Schematic of the reactive silver ink rivulet evolution process: (a) Chemical equations representing the reduction reaction in reactive silver ink system;7 (b) Inkjet printing reactive silver ink rivulet on substrate surface without PDA coatings; (c) Reactive silver ink rivulet evaporation with time on uncoated surface; (d) Cross-sectional view of the reactive silver rivulet on uncoated surface when the initial reduction reaction is triggered by evaporation; (d) Inkjet printing reactive silver ink rivulet on substrate surface with PDA coatings; (e) Reactive silver ink rivulet evaporation with time on PDA-coated surface; (f) Cross-sectional view of the reactive


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silver rivulet on PDA-coated surface when the initial reduction reaction is triggered at the substrate-liquid ink (S-L) interface; The OM images of the plasma-processed AgNO3 lines with treatment time ranging from 0 to 2min for the glass and PET substrates with and without PDA coatings are shown in Fig. 6(a). For uncoated glass and PET, the plasma-processed AgNO3 showed highly irregular and discontinuous morphologies, which was due to both the non-uniformity of the as-printed AgNO3 patterns and the physical etching by plasma ion bombardment during the post-printing processing. For glass_PDA and PET_PDA, however, the patterns after plasma processing retained structural continuity with uniform surface morphology. The top view SEM images (Fig. 6(b)) show detailed surface structures of plasma-processed patterns versus treatment time. Interestingly, the surface morphology of the patterns on glass_PDA differed from that on PET_PDA under the same plasma processing time. For glass_PDA, the patterns showed a roughened surface morphology with increasing plasma processing time, and the 2min plasma-processed sample even exhibited a highly porous and “granular” surface morphology with considerable structural boundaries, which shows clear evidence of strong plasma ion bombardment and physical etching effect.34 For PET_PDA, no obvious porosity or structural boundary was shown on the surfaces of the patterns up to 1min of plasma processing time. The only exception was the sample at 2min plasma processing time, where the surface became less continuous and a relatively large area of porosity was exposed, which were probably resulted from the large scale reaction-driven material volumetric shrinkage due to density mismatch ( ߩ஺௚ேைయ = 4.35g/cm-3 and ߩ஺௚ = 10.49g/cm-3).24 The surface profiles of the AgNO3 deposits on PDA-coated substrates before and after plasma are shown in Fig. 6(d). The uniform surface morphology was maintained after plasma processing with only slightly roughened surface texture, and a height decrease from the


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original 1µm to 0.6~0.7µm after 2min plasma processing was observed. The cross-sectional SEM images (Fig. 6(e)) showed that after 0.5min plasma processing, a distinct top layer of metallic silver (‫ܮ‬஺௚ ) was formed on the deposited AgNO3 lines, and the total layer (‫ܮ‬௧௢௧௔௟ ) was converted to silver after 2min plasma processing. The silver top layer thickness (߬஺௚ ) exhibited an approximately linear growth behavior with increasing plasma processing time for both glass_PDA and PET_PDA (Fig. 6(f)), while the total layer thickness (߬௧௢௧௔௟ ) decreased due to the reaction-driven material volumetric shrinkage and the plasma etching effect.22 The measured ߬஺௚ was 146 ± 30nm and 226 ± 50nm at 0.5min plasma time, and further increased to 463 ± 80nm and 551 ± 70nm at 2min plasma time, for structures on glass_PDA and PET_PDA respectively. To study the thickness evolution of the deposits on PDA-coated substrates with varied plasma processing time, two parameters, the silver top layer thickness growth rate (߬ሶ஺௚ ) and the total layer thickness reduction rate (߬ሶ ௧௢௧௔௟ ), were investigated. A detailed analysis (see Supporting Information, Fig. S4) revealed a 12% smaller ߬ሶ஺௚ and an 18% larger ߬ሶ ௧௢௧௔௟ for patterns on glass_PDA than those on PET_PDA with the same plasma processing time. The larger value of ߬ሶ ௧௢௧௔௟ on glass_PDA, however, is not be solely due to the reaction-driven material shrinkage; otherwise ߬ሶ஺௚ on glass_PDA would exceed that on PET_PDA by assuming the same silver top layer density on both PDA-coated substrates, which is obviously contradictory to the observed results. It is quite possible that the patterns on glass_PDA experienced excessive plasma ion bombardment and physical etching, which resulted in an additional consumption of the silver top layer material, and therefore, the observed smaller ߬ሶ஺௚ and larger ߬ሶ ௧௢௧௔௟ on glass_PDA. The excessive plasma etching effect for samples on glass_PDA was also supported by top view SEM images (Fig. 6(b)).


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Figure 6. (a) Optical micrographs of the plasma-processed AgNO3 lines on substrates with and without PDA coatings (scale bars shown represent 50µm); (b) Top view SEM images of the patterns with increasing plasma treatment time on PDA-coated substrates (scale bars shown represent 200nm); (c) XRD characterization of the as-deposited and Ar plasma-processed AgNO3 patterns on glass substrate; (d) Surface profiles of the deposits on PDA-coated substrates before and after 2min plasma; (e) Cross-sectional view SEM images of the patterns with increasing plasma treatment time on PDA-coated substrates (scale bars shown represent 200nm); (f) Total layer thickness (߬௧௢௧௔௟ ) and silver top layer thickness (߬஺௚ ) as a function of plasma processing time for patterns on PDA-coated substrates.


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Fig. 7(a) shows the OM images of thermally treated reactive silver patterns on substrates with and without PDA coatings. For patterns on any single type of substrate (glass, PET, glass_PDA, or PET_PDA), although no considerable morphological change along with thermal treatment could be detected via OM observation, XRD characterization (Fig. 7(b)) revealed a significant transition of material composition from the original AgCH3COO-rich phase of the as-printed patterns to the final of metallic silver phase after heat treatment, suggesting a thermally triggered reduction reaction, which is in consistence with a previous report.7 Fig. 7(c) shows the surface profiles of the reactive silver patterns before and after thermal processing on substrates with and without PDA coatings. For all of the samples, thermal treatment led to a decreased height of the deposited materials, which was due to the chemical reaction driven material formation and condensation, as well as the removal of volatile reactants and reaction products. The “domeshaped” deposition morphology was unchanged after thermal treatment for patterns on PDAcoated substrates, while the “edge-thickened” structure was still obvious for patterns on substrates without PDA coatings, suggesting the PDA coating layer facilitated a well-controlled deposition structure even after full functionalization via thermal treatment. Detailed morphologies were characterized by SEM (Fig. 7(d)). For the as-printed structures (Fig. 7(d), left), the two parallel dark lines along the contact line regions on uncoated substrates were observed in optical micrographs (Fig. 7(a), for glass and PET) and were actually structures mainly consist of silver nanoparticles due to evaporation induced reduction reaction (Fig. 7(d), upper left).7 From high magnification top and cross-sectional view SEM images of the as-printed structure, it was clear that the line pattern consists of a particulate region at the line edge and a relatively flat interior region between the two parallel edges. The formation of the condensed silver particulate region on the line edge was probably due to the enhanced liquid evaporation at


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the contact line, which is the driving force for metallic silver ion reduction in the reactive silver ink functionalization mechanism. In contrast, the as-printed structure on PDA-coated substrate was quite different (Fig. 7(d), lower left). Only a narrow particle region (approximately 200nm in width) can be observed at the line edge, and the remaining region of the line was covered by a relatively smooth “dome-shaped” region as shown in the high magnification top and crosssectional view SEM images. The effect of thermal treatment on the line morphology was also investigated in detail (Fig. 7(d), right). For uncoated substrates (Fig. 7(d), upper right), the heat treatment caused coarsening of the silver nanoparticles in the line edge region, suggesting the continuation of chemical reaction as well as thermal sintering leading to enlarged nanocrystals. Meanwhile, the interior region also exhibited a noticeable morphological transition from the original flat and smooth surface to the final particulate layer structure with observable structural boundaries for substrates with and without PDA coatings (Fig. 7(d), right). Cross-section SEM images revealed a densely packed particulate structure in the bulk of the deposits following thermal treatment (Fig. 7(d), right), indicating the thermally induced reduction reaction in the solid state. The electrical resistivities of the reduced AgNO3 and reactive silver patterns after post-printing processing were calculated based on the four-point probe (4-P probe) characterization technique (Figs. 8(a)-(b)). For plasma-reduced AgNO3 patterns (Fig. 8(a)), the resistivities on glass_PDA showed a decreasing trend with increasing plasma treatment time, with the values of 1.8×10-6 Ω•m and 6.3×10-7Ω•m after 0.5 and 2min plasma treatment, respectively. The resistivities of PET_PDA decreased with increasing plasma processing time to a minimum of 6.8×10-8Ω•m (at 1min) and then increased to 1.2×10-7Ω•m (at 2min). The PET_PDA showed a lower resistivity than the glass_PDA for the same plasma treatment time, which was probably due to the excessive


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plasma ion bombardment and physical etching on the glass_PDA (see Supporting Information, Figs. S4). The lowest electrical resistivity achieved on PET_PDA was only 4 times greater than bulk silver (at 1min), which was 10-times smaller than the lowest value achieved on glass_PDA, corresponding to 40 times greater than bulk silver (at 2min). The electrical resistivities for samples on uncoated glass and PET substrates were unmeasurable for all treatment times, because of the discontinuous nature of the as-printed and plasma-processed patterns.


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Figure 7. (a) Optical micrographs of the reactive silver deposits corresponding to different thermal treatment times ranging between 0 and 30min (scale bars represent 50µm); (b) XRD characterization of the as-printed and thermally treated patterns on substrates with and without PDA coatings; (c) Surface profiles of the as-printed and thermally treated line patterns on substrates with and without PDA coatings; (d) Top and cross-sectional view SEM images of the reactive silver patterns before and after thermal treatment on substrates with and without PDA coatings. For thermally treated reactive silver patterns (Fig. 8(b)), the as-printed structures possessed electrical resistivity of about 2.07×10-6Ω•m for both glass and PET groups, corresponding to two orders of magnitude greater than bulk silver. The electrical resistivity decreased drastically with thermal treatment, roughly a 10-times decrease of the resistivity after only 5 minutes of thermal processing under 120°C. Further treatment resulted in a much slower decrease of the resistivity, and the lowest value achievable was 2.7×10-7Ω•m after 30min thermal treatment, which is about 17 times greater than bulk silver. The electrical resistivities achieved on PDA-coated substrates were comparable to those on uncoated substrates, with the lowest values of 2.05×10-7Ω•m and 2.08×10-7Ω•m after 30min thermal treatment, on glass_PDA and PET_PDA respectively, corresponding to about 13 times greater than bulk silver. Conductive arrays of plasma-treated (2min, 900W) AgNO3 precursor patterns and thermally treated (30min, 120°C) reactive silver patterns on PET_PDA were subjected to cyclic bending (Figs. 8(c)), and it was observed that the resistances increased to 1.7 and 1.8 times the original values by bending up to 10000 cycles for reduced AgNO3 and reactive silver patterns respectively, which were comparable with published reports11,35 and reasonably robust for flexible microelectronics applications. Digital images of the 4-P probe electrical property


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characterization apparatus, the plasma-processed AgNO3 and thermally treated reactive silver patterns on PET_PDA substrate, as well as the cyclic bending testing apparatus for mechanical reliability characterization of the conductive patterns are exhibited in Figs. 8(d)-(e), (f)-(g), and (h)-(j), respectively.

Figure 8. (a) The calculated electrical resistivities of plasma-reduced AgNO3 patterns with varied plasma treatment time on PDA-coated substrates (electrical resistance unmeasurable for glass and PET due to non-uniform deposition and broken lines following plasma processing); (b) The electrical resistivities of thermally treated reactive silver patterns for different thermal treatment times on substrates with and without PDA coatings; (c) Cyclic bending test results of plasma-reduced AgNO3 and thermally treated reactive silver patterns on PET_PDA; (d)-(e) Photographs of the 4-P probe characterization apparatus; (f)-(g) Photographs of the plasmareduced AgNO3 and thermally treated reactive silver patterns on PET_PDA; (h)-(j) Photographs of the cyclic bending test apparatus.


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In-situ OM observations of the solidification processes of the printed AgNO3 and reactive silver liquid rivulets were performed to elucidate the interaction between the PDA layer and the crystalline materials deposited, as well as to understand the underlying mechanism for the formation of crystallization patterns with well-defined morphologies. AgNO3 precursor ink on substrates with and without PDA coatings were first investigated (Fig. 9(a)). The AgNO3 rivulet showed a unidirectional crystallization process with evaporation, where the AgNO3 solid nucleated at the tip of the contact line followed by a rapid dendritic crystal propagation opposing the printing direction along the line axis.22 High magnification image sequences (Fig. 9(a), left), taken by a high resolution oil objective focused at the substrate-liquid ink (S-L) interface (400nm focal depth), revealed the detailed dynamic crystallization process. It was observed that for uncoated glass, dendritic AgNO3 crystals grew at the solidification front. However, those dendrites were blurred and defocused, suggesting these structures were not located at the S-L interface. It is possible that these crystals propagated along the liquid-vapor (L-V) interface of the rivulet, due to the evaporation induced AgNO3 concentration increase and the nucleation and crystal growth beyond its solubility limit. On the other hand, all images showed a well-focused crystal growth front with clear textured structure on glass_PDA, with the AgNO3 structures propagating along the S-L interface.


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Figure 9. (a) In-situ OM images and schematics of the solidifying AgNO3 ink rivulet on pristine glass and glass_PDA substrates (scale bars shown represent 50µm for low (5x objective) and high (63x oil objective with 400nm focal depth) magnification images, respectively); (b) In-situ OM images and schematics of the solidifying reactive silver ink rivulet on pristine glass and glass_PDA substrates (scale bars shown represent 500µm and 50µm for low and high magnification images, respectively). A similar time-resolved OM observation was performed for the reactive silver ink (Fig. 9(b)). The results suggested that, while to the contact line initiated nucleation and crystal propagation on the pristine glass substrate (Fig. 9(b), left), the solidification on glass_PDA exhibited a nucleation originated at the centerline region, with dendritic crystal structures forming and propagating from these nucleation sites and merging with each other to form fully solidified and continuous structures (Fig. 9(b), right). Therefore, the PDA functional coating led to an enhanced nucleation and crystal growth at the S-L interface, which was dominant over the normal evaporation driven solidification processes, for instance, at the L-V interface or in the vicinity of


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the contact line where the evaporation rate is the most significant. The observed enhancement of the interfacial crystallization facilitated by the PDA functional coating for both AgNO3 precursor and reactive silver inks can possibly be explained by the strong affinity between PDA layer surface functional groups and silver ions (Ag+) in solutal inks.28,36,37 The PDA molecule generally exhibits a universal binding capacity and attraction interaction with various metal ions including Ag+.27,36,37 Locally concentrated Ag+ near the PDA surface (S-L interface with PDA functional coating) is a possible reason for the initial nucleus formation and further crystal growth starting at the S-L interface.27 5. Conclusions The inkjet printing of two types of functional inks: the 25.4% wt. silver nitrate (AgNO3) aqueous solution precursor ink and the reactive silver ink were performed on glass and polyethylene terephthalate (PET) substrates with and without polydopamine (PDA) coatings, in order to assess the influence of the PDA layer on the deposition morphologies of the solidified structures. Contact profilometry results showed that a uniformly deposited AgNO3 solid structure with narrow thickness distribution (0.9-1µm) was achieved on glass_PDA (PDA-coated glass) and PET_PDA (PDA-coated PET) substrates, in contrast with the highly random and discontinuous deposition on substrates without PDA coatings. At the same time, the surface profiles of the deposited reactive silver patterns on PDA-coated substrates also exhibited “dome-shaped” morphologies, which were dissimilar to the non-uniform “edge-thickened” surface profiles on uncoated substrates. Fully converted metallic silver patterns were produced by 900W Argon plasma treatment of the printed AgNO3 precursor patterns up to 2min. The lowest resistivity achieved on PET_PDA substrate was 4 times greater than bulk silver versus 40 times greater than bulk silver on glass_PDA. As a comparison, non-conductive path was formed on uncoated glass


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and PET substrates, which was due to the non-uniform and discontinuous nature of the AgNO3 precursor film by inkjet deposition. The electrical performance of the thermally treated reactive silver patterns were also investigated with varied heat treatment time up to 30min with a temperature at 120°C. Results showed that the lowest electrical resistivity achieved on PDAcoated substrates was 13 times greater than bulk silver versus 17 times greater than bulk silver on uncoated substrates. Cyclic bending test was performed for the samples on flexible PET_PDA substrates, showing less than 2 times increase of the final pattern resistance after 10000 bending cycles. In-situ optical microscope observation of the solidification process revealed that on PDAcoated substrates, the AgNO3 crystals preferentially propagated along the solid-liquid ink (S-L) interface, and the nucleation and crystal growth of the reactive silver ink were initiated at the centerline region (S-L interface) of the rivulet and dominated over the evaporation driven crystallization process. These phenomena were due to the strong binding interaction between the PDA surface functional groups and the Ag+ metallic ions in the solutal inks, and would probably be the cause of the well-controlled deposition morphologies on PDA-coated substrates. ASSOCIATED CONTENT Supporting Information Tables of the physical properties and wetting contact angles of the two ink systems, figure showing the wetting property of O2 plasma-treated PET substrate, figures exhibiting the optical property of PDA-coated glass and PET substrates, XRD data of Ar plasma-processed AgNO3 patterns, and detailed discussion about the morphological evolution of Ar plasma-processed AgNO3 patterns. This materials is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION


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Corresponding Author *Email: [email protected]. Tel: +01-607-777-4330. Fax: +01-607-777-4620. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. ACKNOWLEDGMENT The authors would like to acknowledge the financial support by the Center for Advanced Microelectronics Manufacturing (CAMM), and the Advanced Diagnostics Laboratory (ADL) at SUNY Binghamton. REFERENCES (1)

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Regulation of the Deposition Morphology of Inkjet-Printed Crystalline

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