Applied Surface Science 258 (2012) 7384–7388 Contents lists available at SciVerse ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc Synthesis of colourless silver precursor ink for printing conductive patterns on silicon nitride substrates Qijin Huang, Wenfeng Shen, Weijie Song ∗ Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo, Zhejiang 315201, China a r t i c l e i n f o Article history: Received 21 February 2012 Received in revised form 5 April 2012 Accepted 5 April 2012 Available online 11 April 2012 Keywords: Ink-jet printing Silver precursor ink Conductive silver patterns a b s t r a c t Silver precursor ink was synthesised by a simple and environmentally friendly method based on chemical reduction. The stability, particle size, viscosity and surface tension of the ink were adjusted by adding polyvinylpyrrolidone (PVP) and ethylene glycol (EG). The silver patterns were fabricated on the silicon nitride substrate and were characterised by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), and electrical measurements. The thickness of the sample printed three times was approximately 0.66 m, and it increased to 2.43 m after 12 printings. The ink-jet-printed silver patterns exhibited good conductivity when the samples were sintered at temperatures above 200 ◦ C. The resistivity value was observed to decrease to 3.1 cm after sintering at 500 ◦ C for 60 min, twice the value of bulk silver (1.6 cm). The low resistivity of silver patterns suggests applications for ink-jet printing of electronics devices. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Ink-jet printing is now widely used in many applications beyond its conventional use in desktop publishing . Its ability to deliver a precise amount of material in a rapid, reproducible fashion to pre-determined locations under computer control is a desirable feature for such applications as producing conductive patterns for electronic devices [2–4]. A major challenge in applying ink-jet processes for the preparation of conductive patterns is formulating suitable ink. In the case of inks for metal conductive patterns, the content of the ink must be adjusted to provide the required resolution while still providing good adhesion and the desired electronic properties for conducting patterns [5–7]. There are two main types of ink that are used to obtain conducting patterns , which are nanoparticle (NP) ink and metal-organic decomposition (MOD) ink. Most studies have focused on the ink-jet printing of conductive patterns with silver nanoparticle ink because of the desirable conductivity and anti-oxidation properties of the silver patterns [9,6,10]. However, the silver nanoparticle ink is composed of a suspension of silver nanoparticles, which can deposit inside the nozzle chamber and ultimately clog the nozzle, limiting the application of this technology on a large scale [11,12]. Furthermore, silver nanoparticles are generally synthesised in an organic medium that is primarily composed of hazardous organic solvents. A variety of toxic wastes are generated throughout the complicated ∗ Corresponding author. Tel.: +86 574 87913375. E-mail address: [email protected] (W. Song). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.04.037 synthesis process, including the particle separation, washing and dispersion steps. Therefore, a non-toxic and stable silver ink with superior stability against aggregation is of great importance in this technology. In this work, we report the synthesis of a stable silver precursor ink by a simple and environmentally friendly method. The ink is colourless and transparent, similar to the “true solution”, so that it can be printed directly onto the silicon nitride substrate with a common colour ink-jet printer. The as-printed patterns deposited with a range of printing cycles were thermally treated at various sintering temperatures and for sintering times. Their surface microstructure and resistivity evolution were investigated. The result shows that the lowest resistivity of sintered silver patterns is twice the value of bulk silver. This approach involves a simple and universal method for easily controlling the conductive pattern’s shape, thickness and resistivity. The successful creation of this type of silver precursor ink provides an easy technique for fabricating electronic devices using ink-jet printing. 2. Experimental details 2.1. Materials All reagents – AgNO3 , dextrose, ammonium hydroxide, polyvinylpyrrolidone (PVP, Mw = 3000), ethanol, ethylene glycol (EG), and 2-methoxyethanol (ME) – were analytical-grade reagents, and they were purchased from Sinopharm Chemical Reagent Co., Ltd. Deionised water was used in all of the experiments. Q. Huang et al. / Applied Surface Science 258 (2012) 7384–7388 7385 Fig. 1. (a) Optical image and (b) particle size distribution of silver precursor ink after one month of storage. 2.2. The preparation of silver precursor ink and silver conductive patterns 3. Results and discussion 3.1. The physical properties of silver precursor ink AgNO3 solution was prepared by dissolving silver nitrate (0.05 M) in a mixed solvent (50 ml) of deionised water and ethanol with a volume ratio of 1:3. NH4 OH was added to the AgNO3 solution drop by drop with stirring until a colourless and transparent solution was formed. The pH value of the transparent solution was approximately 9. Subsequently, 2-methoxyethanol and ethylene glycol were incorporated into the transparent solution to adjust its viscosity and surface tension. PVP (0.02 mM) was added as a ﬁlmforming agent. Then, 10 ml of dextrose solution (10 g of dextrose dissolved in 100 ml of deionised water) was incorporated as a weak reducing agent. After stirring for 10 min, the solution was ﬁltered. A colourless and transparent silver precursor ink was obtained, and the ﬁnal pH of the silver precursor was approximately 8.5. 2.3. Inkjet printing and heat treatment of silver patterns The ink-jet printing technique used in this paper was similar to the technique used in our previous work [13,14]. The modiﬁed printer setup consisted of a drop-on-demand DOD ink-jet nozzle manufactured from Seiko Epson Corp. The modiﬁed printer has a piezoelectric head with 90 openings of size about 28 m, and each droplet volume is of the order of 3 pl. The silver precursor ink was loaded into the cartridges and then printed on a silicon nitride substrate using our modiﬁed ink-jet printer. If the ink was printed onto a substrate more than once, the sample was allowed to dry in air for 4 min between printings. When printing was complete, the samples were kept in air at 50 ◦ C. After 4 h, the samples were sintered at different temperatures in the range from 150 ◦ C to 500 ◦ C. 2.4. Characterisations The crystal structure and chemical composition of the silver patterns were investigated by X-ray diffraction (XRD) using a Bruker AXS D8 Advance diffractometer with Cu K␣, = 0.1542 nm. The surface morphology of the samples was observed by a Hitachi S4800 ﬁeld emission scanning electron microscope (FESEM) using an operating voltage of 8 kV. The viscosity of the silver precursor ink was measured with a Brookﬁeld Viscometer DV-II+ pro with a UL/Y adapter at 25 ◦ C. The resistivity was detected by the 4-Point Sheet Resistance Test System, Lucas-Signatone Pro4-4000. To make ink-jet-printable ink, the properties of this ink, including its stability, particle size, viscosity and surface tension must be formulated to ﬁt the physical and rheological requirements of ﬂuid ﬂow during the printing process. The stability of the silver precursor ink is a key factor in applications involving printing conductive patterns using a common colour printer. To prevent the rapid precipitation of Ag, a low-molecular-weight organic compound, ME, and a high-molecular-weight organic compound, PVP, were added to the reaction media as excellent stabilising agents that can free Ag+ ions gradually by forming coordination complex Ag+ ions. It was observed that the silver precursor ink demonstrated good performance and stability over an extended period with cold-storage at temperatures of 10 ◦ C and below. Fig. 1 shows the optical image and particle size distribution of silver precursor ink after one month of cold-storage. As shown in Fig. 1(a), it was observed that after a month of storage, the ink remained colourless and transparent, without any precipitates. From Fig. 1(b), it can be seen that the average particle size increased slowly from 250.2 nm to 581.1 nm after a month of storage. To form a well-shaped drop, the viscosity of the ink needs to be adjusted to a suitable range. The surface tension plays an important role in the interaction between the printer nozzle and the ink and in the spreading of the pico-litre droplet over the substrate surface. Inks possessing a surface tension on the order of 25–50 mN/m and a Newtonian viscosity of 1–20 mPa s were shown to be most suitable for ink-jet printing . In this work, ME and EG were incorporated into the reaction media to obtain suitable viscosity and surface tension values for the ink-jet printing process. The physical properties of the silver precursor ink and the ﬂuids used to prepare the ink are summarised in Table 1. The silver precursor ink’s surface tension and viscosity in our study were 34.5 mN/m and 3.2 mPa s, respectively. Table 1 Physical properties of ﬂuids used to prepare the silver precursor ink. DI water Ethanol Ethylene glycol 2-methoxyethanol The ink Surface tension (mN m−1 ) Viscosity (mPa s) Boiling point (◦ C) 72.8 22.3 48.5 27.2 34.5 1 1.2 19 1.6 3.2 100 78.4 198 125 – 7386 Q. Huang et al. / Applied Surface Science 258 (2012) 7384–7388 Fig. 2. Optical images of silver ﬁlms sintered at (a) 50 ◦ C and (b) 350 ◦ C. The number of printing cycles was eight. 3.2. Morphology and microstructure of the silver patterns The silver precursor ink was used to prepare electrically conductive thin patterns. Initially, ﬁlms with a size of 15 mm × 15 mm were printed eight times on a silicon nitride substrate. Fig. 2 shows the optical images of the silver ﬁlms sintered at (a) 50 ◦ C and (b) 350 ◦ C. With heat treatment, the ﬁlm changed colour from dark brown into silvery white. The silver ﬁlms was well adhered to the substrate without cracks after sintering, and the nonconductive ﬁlm became conductive. Fig. 3 shows the XRD patterns of the silver thin ﬁlms that were formed by printing precursor ink eight times on the silicon nitride substrate and subsequently sintering the sample at 350 ◦ C for 5 min in an ambient atmosphere. Peaks can be observed clearly at 38.2◦ , 44.4◦ , and 64.5◦ , and they were attributed to the diffraction from the (1 1 1), (2 0 0) and (2 1 1) crystalline planes of the face-centred structure of silver, respectively, according to the Silver Joint Committee on Powder Diffraction Standards Database (File NO.87-0509). Fig. 3 also showed that there were no silver oxide peaks in the XRD pattern. It could be conﬁrmed that the oxidation of silver does not occur during the sintering process. Silicon peaks were also observed in the XRD image due to the beam intensity, which was able to break through the silver ﬁlm. The thickness of the silver thin ﬁlm pattern could be controlled by changing the number of printing cycles . In this work, conductive silver thin ﬁlms were prepared by printing silver precursor ink patterns numerous times, and the ﬁlms were sintered at the temperature of 350 ◦ C for 5 min. The relationship between the thickness and the number of printing cycles was studied. As seen in Fig. 4, the thickness of the silver ﬁlm increased linearly with the number of printing cycles. The thickness of the sample after three printing cycles was approximately 0.66 m, and it increased to 2.43 m when the ﬁlm was printed 12 times. 3.3. Electronic properties of the silver patterns With the evaporation of the solvents during sintering, the silver clustered into small agglomerates with many voids, as shown in Fig. 5(a). However, when the number of printings increased to six and nine, as depicted in Fig. 5(b) and (c), respectively, the silver nanoparticles grew; the voids diminished, and the solvents were removed, which meant that the silver ﬁlms became denser, thus, resulting in better conductivity. The ﬁlm’s thickness increased to 2.43 m after twelve printing cycles. The microstructure observations from Fig. 5(d) clearly demonstrated that most of the voids disappeared, and consequently, the electrical resistivity of the ﬁlms decreased. The silver pattern exhibits a relatively low resistivity (∼3.3 cm) after drying at 350 ◦ C for 5 min in air. With the increase of drying time to 10 min and 30 min at 350 ◦ C, the resistivity of the silver ﬁlms after twelve printing cycles decreased slowly. They were 3.3 and 3.2 cm, respectively. To investigate the effect of the sintering time on the silver patterns further, the sintering temperature was ﬁxed at 500 ◦ C and the number of printing cycles was set at twenty. Fig. 6 shows an SEM image of the ink-jet-printed ﬁlms as a function of the sintering time. After the pattern was sintered for 10 min, the ﬁlm consisted of slightly sintered particles with a grain diameter of ∼0.4 m as seen in Fig. 6(a), and its resistivity reached a value of 4.3 cm. As the (111) 2.8 · 2.6 · 2.4 · * * (311) (200) *Si · 2.2 Thickness(μm) Intensity(arb.units) Ag 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 30 40 50 60 2 Theta(degree) Fig. 3. The X-ray diffraction analysis of the silver pattern obtained with 8 printing cycles after heating at 350 ◦ C for 5 min. 2 4 6 8 10 12 Printing times(N) Fig. 4. The relationship between the thickness of the ﬁlms and the number of printing cycles after sintering at 350 ◦ C for 5 min. Q. Huang et al. / Applied Surface Science 258 (2012) 7384–7388 7387 Fig. 5. SEM images showing the microstructure evolution of the silver patterns as a function of the number of printing cycles: (a) 3, (b) 6, (c) 9, and (d) 12. The samples were sintered at 350 ◦ C for 5 min. The scale bar is 1 m. Fig. 6. An SEM image of ink-jet-printed Ag ﬁlms sintered at 500 ◦ C for different lengths of time: (a) 10 min (b) 30 min and (c) 60 min. The number of printing cycles was 20. The scale bar is 200 nm. (d) EDS result of ink-jet-printed Ag ﬁlms sintered at 500 ◦ C for 60 min. The number of printing cycles was 20. 7388 Q. Huang et al. / Applied Surface Science 258 (2012) 7384–7388 and the number of printing cycles. When the silver ﬁlms were printed three and twelve times, their thicknesses were 0.66 m and 2.43 m, respectively. After sintering at 500 ◦ C for 60 min, the resistivity of silver conductive ﬁlms deposited using 20 printing cycles was 3.1 cm, which was twice as high as that of bulk silver. Moreover, this silver precursor ink promise enormous potential for directly printing conductive features by a common colour inkjet printer, making ink-jet printing of low-cost electronics devices a possibility. 25 Resistivity (μΩ·cm) 20 15 10 Acknowledgement 5 0 100 This work has been supported by Ningbo Natural Science Foundation (grant no. 2011A610009) and Zhejiang Provincial Natural Science Foundation of China (grant no. Y12E020042). 200 300 Temperature (ºC ) 400 500 Fig. 7. Resistivity variations of ink-jet-printed ﬁlms as a function of sintering temperature. The number of printing cycles was 12. The sintering time was 1 h. sintering time increased to 30 min, larger particles formed and the porosity became less as seen in Fig. 6(b), resulting in good contact and a lower resistivity (∼3.1 cm). Even when the ﬁlm was sintered at 500 ◦ C for 60 min, as exhibited in Fig. 6(c), small voids were still observed, which caused the ﬁlm’s resistivity (∼3.1 cm) to be identical to that obtained for 30 min of sintering. Further sintering does not bring about a signiﬁcant decrease in the resistivity, which is consistent with the percolation theory [16–18]. The key aspects of this theory model are that microstructures consisting of large grains are more conductive than microstructures consisting of small grains and that less porosity are more favourable to conductivity . Fig. 6(d) also shows the EDS results of silver patterns. It can be seen that the weight ratio of C/Ag in the pattern sintered at 500 ◦ C for 60 min is approximately 0.044. This may be interpreted as indicating that when the organic residues are almost completely removed, the resistivity of the ﬁlms are stable, even with increasing sintering time. The sintering temperature is an important factor that inﬂuences the resistivity of the silver conductive pattern. Fig. 7 shows the resistivity change of the silver pattern as a function of the sintering temperature from 150 ◦ C to 500 ◦ C for 1 h. When the ﬁlm was sintered at 150 ◦ C, the electrical resistivity had a higher value of 24.7 cm. By increasing the sintering temperature, the solvents and the majority of capping molecules in the ink were gradually removed, and the electrical resistivity decreased until it reached a value of 3.3 cm at 350 ◦ C, which was two times higher than that of bulk Ag (1.6 cm). The change of the resistivity was not clear when the sintering temperature was increasing continuous. The electrical resistivity of the silver ﬁlm reached minimum values of 3.1 cm at 500 ◦ C. 4. Conclusions A stable and colourless silver precursor ink has been synthesised for directly printing silver conductive patterns with a modiﬁed colour ink jet printer. 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