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Enhanced dispersibility and dispersion stability of dodecylamine-protected silver nanoparticles by dodecanethiol for ink-jet conductive inks
Applied Surface Science 292 (2014) 537–543
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Enhanced dispersibility and dispersion stability of dodecylamine-protected silver nanoparticles by dodecanethiol for ink-jet conductive inks Xueqin Zhou a,∗ , Wei Li a , Meilan Wu a , Shen Tang b , Dongzhi Liu a,∗ a b
School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China Shijiazhuang Ordnance Technical Institute, Shijiazhuang 050000, China
a r t i c l e
i n f o
Article history: Received 22 August 2013 Received in revised form 10 November 2013 Accepted 3 December 2013 Available online 11 December 2013 Keywords: Ag nanoparticles Conductive inks Inkjet printing Sintering Stability
a b s t r a c t This work studied dodecylamine-protected silver nanoparticles modified by a small amount of dodecanethiol as the co-protective agent. Contents of the dodecanethiol and the protective agent capping on the surface of silver nanoparticles were analyzed using the method of oxygen flask combustion and a thermogravimetric analysis instrument. Results of electrical property determination and transmission electron microscopy indicate that certain amount of capping dodecanethiol can slow down the spontaneous sintering process of silver nanoparticles. When capping DDT content of silver nanoparticles is 1.70 wt%, 10 wt% suspensions are stable under −18 ◦ C and can be stored stably at room temperature as long as 120 days. Furthermore, the silver nanoparticle concentration could be increased to 20 wt% with a stable storage time of 60 days at room temperature. Finally, stable polymer-free conductive inks with the silver nanoparticle concentration of 20 wt% were produced to fabricate patterns by ink-jet printing. The resistivity of the PI-supported patterns having been annealed at 130 ◦ C for 10 min is 7.2 cm. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Silver nanoparticle ink-jet conductive inks, which could deposit conductive lines on substrates in a cost-effective, time-saving and patterning-flexible way, have attracted considerable interests for their applications in production of electronic circuits [1] and electronic components [2]. Since the high surface energy of bare nanoparticles causes aggregation and flocculation, the surface of silver nanoparticles must be passivated, which was often accomplished by providing a steric barrier via a protective agent (PA) layer [3–6]. A concurred problem is that the printed patterns show poor conductivity due to an increase of the contact resistance between metallic particles by insulative PA [7]. Various methods were reported to remove PAs and sinter silver nanoparticles, therefore enhance film conductivity to meet the application requirement. Heating [3,8] is a simple and low cost process compared with other methods such as laser [9], microwave [10] and plasma [11]. But a heat-treatment temperature above 150 ◦ C would limit its application in fabricating of plastic flexible electronic devices due to the heat sensitivity of plastic substrates such as polyimide (PI) and poly(ethylene terephthalate) (PET) [12].
∗ Corresponding authors. Tel.: +86 22 2740 0911; fax: +86 22 2789 2283. E-mail addresses: [email protected] (X. Zhou), [email protected] (D. Liu). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.12.006
The dispersion stability and sintering conditions of silver nanoparticles are affected by capping PAs, which may be small molecular compounds with a long alkyl chain and polar head, such as alkanethiols [3] and alkylamines [4], or polymers bearing hydroxyl, amino or carbonyl groups, such as poly(vinyl pyrolidone) [5] and poly(acrylic acid) [6]. The polymers usually decompose at a temperature higher than 250 ◦ C. When the sintering process was carried out at 150 ◦ C, the polymer layer capping on the surface of silver nanoparticles only became thinner, resulting in a high resistance of the patterns [13]. On the other hand, the sintering temperature of silver nanoparticles using compound PAs significantly depends on their bonding energies with metal nanoparticles. Sintering of dodecanethiol-protected silver nanoparticles (Ag-DDT) requires a temperature no less than 200 ◦ C due to the strong bonding energy of Ag S, whereas the weak bonding energy of Ag N leads to a low heat-treatment temperature (<150 ◦ C) for dodecylamine-protected silver nanoparticles (Ag-DDA) [14]. Patterns obtained with Ag-DDA nanoparticles can even become conductive after air storage for less than 7 days [14]. However, such a weak bonding energy of Ag N would be adverse to the dispersion stability of silver nanoparticle inks. Different polymeric dispersants can delay the agglomeration or clustering, whereas the sintering temperature improves [15,16]. In this paper we prepared silver nanoparticles using a mixture of DDA and DDT as the PA (Ag-NS nanoparticles). Since the molecular structures of DDA and DDT are similar, we assume that a few
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amount of DDT will make little change of the sintering process of Ag-DDA nanoparticles, but strong Ag S bond may give a favorable effect on the ink stability. The dispersion stability and electrical properties of Ag-NS nanoparticle suspensions were investigated in detail. It is found that the precipitation of as-prepared silver nanoparticle suspensions at room temperature could be delayed by two months. Stable polymer-free silver nanoparticle inks with a silver nanoparticle concentration of 20% (by weight, expressed as wt% at the following) were obtained and the resistivity of inkjet-printed patterns was measured as 7.2 cm after 10 min heat-treatment at 130 ◦ C.
2. Experimental 2.1. Preparation and characterization of silver nanoparticles
2.2. Preparation and characterization of silver nanoparticle suspensions and films The synthesized silver nanoparticles were dispersed in cyclohexane with the different inputs by ultrasound treatment of 120 min and followed by filtration. Certain amount (Wt) of the suspensions was put in culture glass and the solvent was removed in vacuum at room temperature for 72 h. Then the residue (Wr) was weighted to calculate the silver nanoparticle concentrations (Wr/Wt).
Fig. 1. Plots of the DDT contents () and PA contents () in the nanoparticles against DDT amounts added during preparation.
24
Silver nanoparticle concentration (%)
Ag-NS nanoparticles were prepared via a modified method of the literature [17]. Silver acetate (4.175 g, ≥99%, Tianjin Yingda Sparseness and Noble Reagent Chemical Factory) used as the precursor of silver nanoparticles was first dispersed in toluene (40 mL) and mixture of DDT and DDA (10.175 g, 55.0 mmol) as the PA was then added into the solution, followed by the addition of phenylhydrazine (1.35 g, 12.5 mmol) as the reducing agent. The reaction was completed within 1 h, yielding a dark-brown solution without precipitation. Mixture of acetone and methanol (1:1 by volume) was added into the solution to precipitate AgNS nanoparticles, which were filtrated, washed with mixture of acetone and methanol (1:1 by volume), and air dried. In our experiments, the designation of Ag-NS1, Ag-NS2, Ag-NS3, Ag-NS4 and Ag-NS5 refers to the silver nanoparticles prepared with DDT amount of 0.0253 g (0.125 mmol), 0.0506 g (0.25 mmol), 0.0759 g (0.375 mmol), 0.1012 g (0.5 mmol) and 0.1265 g (0.625 mmol), respectively. Ag-DDA nanoparticles were prepared in the same way except that only DDA (10.175 g, 55.0 mmol) was used as PA. Ag-DDT nanoparticles were prepared using a method modified from the literature [18]. Sodium borohydride (0.5 g, 13.2 mmol) dissolved in the mixture of 2-propanol (70 mL) and ethanol (15 mL) was added to a solution of silver nitrate (0.5 g, ≥99%, Tianjin Yingda Sparseness and Noble Reagent Chemical Factory) and dodecanethiol (0.7 mL) in 2-propanol (85 mL) with vigorous stirring at room temperature. After 15 min, the black Ag-DDT nanoparticles were separated by filtration, washed with water, toluene, ethanol and acetone in turn, and air dried. The content of PA capped on the surface of silver nanoparticles (denoted as PA content) was analyzed using a thermogravimetric analysis instrument (TGA, TGA-Q500) at a heating rate of 10 ◦ C/min under N2 atmosphere. The content of DDT capped on the surface of silver nanoparticles (denoted as DDT content) was measured by method of oxygen flask combustion and DX120 ion chromatography. Morphology of silver nanoparticles was observed by transmission electron microscopy (TEM, JEM-2100F, JEOL).
23 22 21
Ag-NS2
20 19
Ag-NS1
18 17 16
Ag-NS3
Ag-DDA
15 14
Ag-NS4
13 12 0.0
0.5
1.0
1.5
2.0
2.5
DDT Content (%) Fig. 2. Plots of the silver nanoparticle concentrations against DDT contents. The solvent is cyclohexane and the input of silver nanoparticles is 25% by weight.
The silver nanoparticle films were obtained by spin-coating the suspensions onto the glass slides, followed by a heat-treatment in air at various temperatures for 5–60 min. The electrical resistivity of the films was calculated from sheet resistance and film’s thickness, which were measured by RTS-9 four-point probe station and the Dektak6M measuring surface topography, respectively. 2.3. Ink-jet printing of silver nanoparticle conductive inks Ag-NS2 nanoparticles were dispersed in a 1:1 mixture of cyclohexane and dodecane by ultrasonic treatment for 120 min, followed by filtration through a 0.22 m syringe filter to form the conductive ink. The final Ag-NS2 concentration of inks was 10 wt% or 20 wt%. The viscosity and surface tension of conductive inks were measured by NDJ-1 and BZY-A at 25 ◦ C, respectively. The obtained conductive inks were printed on polyimide (PI) substrate by an ordinary commercial ink-jet printer Epson ME1+, which have drop-on-demand (DOD) piezoelectric head with 48 nozzles for black container. The resolution and droplet volume of Epson ME1+ are 2880 × 720 dpi and 6 pL, respectively. The ink-jet printed patterns were heattreated at 130 ◦ C and the sheet resistances were measured on RTS-9 four-point probe station.
X. Zhou et al. / Applied Surface Science 292 (2014) 537–543
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Fig. 3. Plots of the Ag-NS (a), Ag-DDA (b) and Ag-DDT (c) concentrations against PA contents at different inputs of silver nanoparticles. The solvent is cyclohexane.
Table 1 Stability of various silver nanoparticle suspensions at different temperatures.a Silver nanoparticle
Nanoparticle concentration
Room temperature 1d
7d
30 d
60 d
90 d
AgDDA
10 20
↓
↓
AgNS2
10 20
↓
AgNS3
10 20
↓
↓
5
↓
Ag-DDT a b
120 d
10 ◦ Cb
0 ◦ Cb
−18 ◦ Cb
↓ ↓
↓ ↓
ND
ND
ND
150 d
↓
“↓” represents precipitation, “” represents no precipitation, “ND” means not determination. The suspensions were kept at certain temperature for 24 h, then recovered to room temperature and centrifuged at 4000 rpm for 10 min.
Table 2 Resistivities ( cm) of various silver nanoparticle films kept at room temperature for various times.a Silver nanoparticle
0d
1d
2d
3d
4d
5d
6d
7d
Ag-DDA Ag-NS1 Ag-NS2 Ag-NS3 Ag-NS4 Ag-DDT
ND ND ND ND ND ND
ND ND ND ND ND ND
66.5 ND ND ND ND ND
14.64 130.2 ND ND 4550 ND
9.68 39.28 ND 4334.75 271.60 ND
7.15 34.02 1149.40 601.13 198.33 ND
6.38 27.82 309.14 74.71 60.08 ND
6.06 24.02 289.87 51.03 45.00 ND
a The films were prepared by suspensions with a silver nanoparticle concentration of 20 wt% except for Ag-DDT, of which the concentration is 5 wt%. “ND” means not determination due to the resistivity is higher than 10,000 cm.
3. Results and discussion 3.1. Contents of DDT and PA capping on the surface of silver nanoparticles Previous studies show that the bonding energy between thiol groups and silver nanoparticles is stronger than that of amino
groups [14]. Hence it is expected that silver nanoparticles would first chemsorb DDT molecules when both DDA and DDT are present, which was confirmed by the fact that the DDT content increases linearly with the improving DDT amount in preparation (Fig. 1). Furthermore, the DDT amount in preparation significantly influences the protective agent content of silver nanoparticles. The PA content is 9.0 wt% for Ag-DDA nanoparticles. This value was
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Fig. 4. TEM images of Ag-NS1 (a), Ag-NS2 (b), Ag-NS3 (c) and Ag-NS4 (d) nanoparticles after solvent evaporation in air at room temperature for 1 h.
increased to 9.6 wt% and 10.6 wt% for Ag-NS1 and Ag-NS2 at DDT content of 0.76 wt% and 1.58 wt%, respectively. When the DDT content was improved to 2.33 wt%, the PA content was found as 10.2 wt% (Ag-NS3). Further increasing DDT amount caused a decreased PA content (Ag-NS4). The initial increase of PA contents can be reasonably explained by the interaction between alkyl chains of DDA and DDT. Further increase of DDT amount make it possible to form an almost all-trans zigzag configuration among alkyl chains of capping DDT molecules as found in Ag-DDT nanoparticles [14], which is against their interaction with DDA alkyl chains. But results of IR and XRD determination did not give any clear information of this supposition due to too low DDT content in Ag-NS nanoparticles. 3.2. Dispersibility and dispersion stability of silver nanoparticles A sufficient amount of metal nanoparticles in the ink is vital to have continuous metal nanoparticles aligned in the printed pattern to allow the conductivity to meet application requirements [19]. A minimal nanometal concentration is reported to be far more than 10 wt% for 10 nm size particles [20]. To explore the effect of
capping DDT on the dispersibility of silver nanoparticles, the silver nanoparticles were dispersed in cyclohexane via ultrasound treatment and filtration and then the silver nanoparticle concentrations of suspensions were plotted against DDT contents (Fig. 2). Under the same conditions, the more silver nanoparticles inputted the higher silver nanoparticle concentration of suspensions. Besides, existence of capping DDT also affects the Ag-NS concentration of suspensions. When the inputs of silver nanoparticles are the same, there is a maximal silver nanoparticle concentration, which was found for Ag-NS2 nanoparticles. Fig. 3 gives the plots of the silver nanoparticle concentration against the PA content. When same amounts of silver nanoparticles were inputted into cyclohexane under the same dispersion conditions, the final Ag-NS4 and Ag-NS3 concentrations were found lower than Ag-NS1 concentration though the PA content is higher for Ag-NS4 or Ag-NS3 than Ag-NS1 (Fig. 3a). This can be also explained by the interaction among alkyl chains, which causes an increase of Ag-DDA concentrations (Fig. 3b) and a decrease of AgDDT concentrations (Fig. 3c) with the rising PA contents. Furthermore, Ag-NS2 nanoparticles exhibit excellent dispersion stability. As shown in Table 1, 10 wt% and 20 wt% Ag-NS2
X. Zhou et al. / Applied Surface Science 292 (2014) 537–543
Fig. 5. TEM images of Ag-NS1 (a), Ag-NS2 (b), Ag-NS3 (c) and Ag-NS4 (d) nanoparticles after air storage at room temperature for 24 h.
Fig. 6. TEM images of Ag-NS2 nanoparticles after air storage at room temperature for 48 h (a), 72 h (b) and 96 h (c).
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22
(a)
20
Ag-NS1 Ag-NS2 Ag-NS3 Ag-NS4
Resistivity ( ⏐.cm)
18 16 14 12 10 8 6 4
0
10
20
30
40
50
60
Time (min)
Fig. 8. The Ag-NS2 conductive ink and the ink-jet printed pattern on PI substrate.
30
(b)
Resistivity ( ⏐.cm)
25
3.3. Electrical properties of silver nanoparticle films
Ag-NS1 Ag-NS2 Ag-NS3 Ag-NS4
20
15
10
5 90
100
110
120
130
Temperature
11
(c)
10
Resistivity ( ⏐ .cm)
9 8 7 6
y =3.36+0.25*exp(x/0.76) 2 R =0.9968
5 4 3 0.0
0.5
1.0
1.5
2.0
2.5
DDT Content (%) Fig. 7. Plots of the Ag-NS film resistivities against heat-treatment times at 130 ◦ C (a), the heat-treatment temperatures for 60 min (b) and the DDT contents when heated at 130 ◦ C for 60 min (c). The silver nanoparticle concentrations are 20 wt%.
suspensions in cyclohexane could be stored stably without precipitation at room temperature for 120 and 60 days, respectively. The occurrence of sediments was delayed by 60 and 30 days, respectively, in comparison with 10 wt% and 20 wt% Ag-DDA suspensions, which show sediments on the bottom within 60 and 30-day storage, respectively. Besides, when the temperature was reduced to 0 ◦ C or −18 ◦ C, silver nanoparticle suspensions may freeze for the melting point of cyclohexane is 6.5 ◦ C. After thawing at room temperature and centrifuging at 4000 rpm for 10 min, both Ag-NS2 and Ag-NS3 suspensions did not show any sediments, which was found in Ag-DDA suspensions with the same treatment.
All Ag-NS films, which are insulative upon preparation, can become conductive after air storage at room temperature within 7 days (Table 2). Simultaneously, the color of Ag-NS films changes from bluish violet to silver white. These phenomena are similar to Ag-DDA films [14], indicating that Ag-NS nanoparticles could also sinter spontaneously. However, Ag-NS films, having been kept in the air at room temperature for 7 days, exhibit higher resistivity than Ag-DDA film (Table 2). This is assigned to the presence of capping DDT due to the strong Ag S bonding energy could prohibit the interparticle interaction and coalesce among Ag-DDT nanoparticles. The spontaneous sintering process was observed by TEM. Fig. 4 shows the TEM images of Ag-NS nanoparticles obtained by dropping the Ag-NS-in-cyclohexane suspensions on the carbon-coated copper TEM grids and drying in air at room temperature for 60 min. All Ag-NS nanoparticles are spherical and the size is about 5 nm. Obviously all these nanoparticles initially form locally ordered structure. Nanoparticles coalescence occurs one day later for AgNS1, Ag-NS3, and Ag-NS4 (Fig. 5), and only four days later for Ag-NS2 (Fig. 6). These results indicated that a small amount of capping DDT could decrease the interparticle interaction and slow down the spontaneous coalescence process of Ag-NS2 nanoparticles. However, the DDT content should be not more than 2.33 wt%, otherwise the obtained nanoparticles such as Ag-NS3 and Ag-NS4, the decreased PA content would prompt the coalescence process. Heat-treatment can quickly reduce the resistivity of silver nanoparticle films and the higher the temperature the faster the onset resistivity decreases (Fig. 7a). A temperature above 110 ◦ C has less significant influence on the resistivity of Ag-NS1, Ag-NS2 and Ag-NS3 films when the heating time was 60 min (Fig. 7b). But the resistivity of Ag-NS4 films continues to decrease even at 130 ◦ C. Besides it is found that the resistivity of Ag-NS films after 60 min heat-treatment at 130 ◦ C improves exponentially with DDT contents (Fig. 7c). It could be explained by the presence of capping DDT which only dissociates at a temperature above 150 ◦ C. But even if the DDT content is 2.54 wt%, the resistivity of Ag-NS4 films heated at 130 ◦ C for 60 min can reach 10.3 cm, which is about 6.5 times the bulk Ag resistivity (Ag bulk = 1.6 cm). 3.4. Inkjet printing Above results indicate that Ag-NS2 nanoparticles might be a potential candidate to produce ink-jet conductive inks. Hence corresponding conductive inks were prepared. 1-Dodecane was used in combining with cyclohexane to reduce the evaporation rate of solvent and thus prohibit the nozzles of printer from clogging. No
X. Zhou et al. / Applied Surface Science 292 (2014) 537–543
sediment has been observed yet for two months. The viscosity and the surface tension were 1.2 cP and 23.8 mN/m, respectively, for 10 wt% inks, and 1.3 cP and 24.6 mN/m, respectively, for 20 wt% inks. The ink-jet printed patterns on PI films (Fig. 8) were heated at 130 ◦ C for 10 min. The thickness and resistivities of obtained PIsupported patterns were measured as about 250 nm, 30.2 cm and 7.2 cm for 10 wt% and 20 wt% inks, respectively. Resistivities of the patterns could be further reduced to 13.7 cm and 5.5 cm, respectively, as extending the heating-time at 130 ◦ C to 60 min. 4. Conclusion Dodecylamine (DDA) was used combined with a few amount of dodecanethiol (DDT) as the PA to prepare silver nanoparticles. Dispersion stability and electrical properties of Ag-NS nanoparticles were found to be closely associated with DDT content, which was found to increase linearly with the rising amount of DDT added in preparation. The capping DDT could also significantly influence the PA content and slow down the spontaneous coalescence of silver nanoparticles in solid state at room temperature, leading to an obviously enhanced stability of Ag-NS2 suspensions when comparing to Ag-DDA suspensions. Electrical properties of Ag-NS films could be improved quickly by heat-treatment at temperature above 90 ◦ C. It is found that the resistivity of Ag-NS films heated at 130 ◦ C for 60 min improves exponentially with DDT content. Finally 20 wt% stable polymer-free conductive inks were obtained with AgNS2 nanoparticles and the resistivity of inkjet-printed patterns on PI was measured as 7.2 cm and 5.5 cm after heat-treatment at 130 ◦ C for 10 min and 60 min, respectively, showing great potentials in the application of flexible printed electronic circuits and devices. Acknowledgement The authors would like to thank for the financial support of the Doctoral Programs Foundation of China (No. 20110032110018). References [1] S. Jeong, H.C. Song, W.W. Lee, Y.G. Choi, B.H. Ryu, Preparation of aqueous Ag ink with long-term dispersion stability and its inkjet printing for fabricating conductive tracks on a polyimide film, Journal of Applied Physics 108 (2010) 102805.
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[2] L. Xie, M. Mantysalo, A.L. Cabezas, Y. Feng, F.F. Jonsson, L.R. Zheng, Electrical performance and reliability evaluation of inkjet-printed Ag interconnections on paper substrates, Materials Letters 188 (2012) 68–72. [3] S.K. Volkman, S. Yin, T. Bakhishev, K. Puntambekar, V. Subramanian, M.F. Toney, Mechanistic studies on sintering of silver nanoparticles, Chemistry of Materials 23 (2011) 4634–4640. [4] L. Polavarapu, K.K. Manga, K. Yu, P.K. Ang, H.D. Cao, J. Balapanuru, K.P. Loh, Q.H. Xu, Alkylamine capped metal nanoparticle “inks” for printable SERS substrates, electronics and broadband photodetectors, Nanoscale 3 (2011) 2268–2274. [5] S. Li, P. Liu, Q.S. Wang, Study on the effect of surface modifier on self-aggregation behavior of Ag nanoparticle, Applied Surface Science 263 (2012) 613–618. [6] S. Magdassi, M. Grouchko, O. Berezin, A. Kamyshny, Triggering the sintering of silver nanoparticles at room temperature, ACS Nano 4 (2010) 1943–1948. [7] A. Denneulin, A. Blayo, C. Neuman, J. Bras, Infra-red assisted sintering of inkjet printed silver tracks on paper substrates, Journal of Nanoparticle Research 13 (2011) 3815–3823. [8] B.Y. Ahn, E.B. Duoss, M.J. Motala, X. Guo, S. Park, Y. Xiong, J. Yoon, R.G. Nuzzo, J.A. Rogers, J.A. Lewis, Omnidirectional printing of flexible, stretchable, and spanning silver microelectrodes, Science 323 (2009) 1590–1593. [9] H. Alemohammad, O. Aminfar, E. Toyserkani, Morphology and microstructure analysis of nano-silver thin films deposited by laser-assisted maskless microdeposition, Journal of Micromechanics and Microengineering 18 (2008) 115015. [10] J. Perelaer, R. Jani, M. Grouchko, A. Kamyshny, S. Magdassi, U.S. Schubert, Plasma and microwave flash sintering of tailored silver nanoparticle ink, yielding 60% bulk conductivity on cost-effective polymer foils, Advanced Materials 24 (2012) 3993–3998. [11] S. Wunscher, S. Stumpf, A. Teichler, O. Pabst, J. Perelaer, E. Beckert, U.S. Schubert, Localized atmospheric plasma sintering of inkjet printed silver nanoparticles, Journal of Materials Chemistry 22 (2012) 24569–24576. [12] T.H.J. van Osch, J. Perelaer, A.W.M. de Laat, U.S. Schubert, Inkjet printing of narrow conductive tracks on untreated polymeric substrates, Advanced Materials 20 (2008) 343. [13] L.X. Mo, D.Z. Liu, X.Q. Zhou, L.H. Li, Preparation and conductive mechanism of the ink-jet printed nanosilver films for flexible display, in: Proceedings of the Second International Congress on Image and Single Processing, 1–9, 2009, pp. 3159–3163. [14] L.X. Mo, D.Z. Liu, W. Li, L.H. Li, L.C. Wang, X.Q. Zhou, Effects of dodecylamine and dodecanethiol on the conductive properties of nano-Ag films, Applied Surface Science 257 (2011) 5746–5753. [15] A. Kosmala, R. Wright, Q. Zhang, P. Kirby, Synthesis of silver nano particles and fabrication of aqueous Ag inks for inkjet printing, Materials Chemistry and Physics 129 (2011) 1075–1080. [16] A. Kosmala, Q. Zhang, R. Wright, P. Kirby, Development of high concentrated aqueous silver nanofluid and inkjet printing on ceramic substrates, Materials Chemistry and Physics 132 (2012) 788–795. [17] Y. Li, Y. Wu, B.S. Ong, Facile synthesis of silver nanoparticles useful for fabrication of high-conductivity elements for printed electronics, Journal of the American Chemical Society 127 (2005) 3266–3267. [18] S.K. Volkman, Y. Pei, D. Redinger, S. Yin, V. Subramanian, Ink-jetted silver/copper conductors for printed RFID applications, Materials Research Society Symposium Proceedings 814 (2004) 151–156. [19] J. Chung, S. Ko, N.R. Bieri, C.P. Grigoropoulos, D. Poulikakos, Conductor microstructures by laser curing of printed gold nanoparticle ink, Applied Physics Letters 84 (2004) 801–803. [20] N.S. Kim, K.N. Han, Future direction of direct writing, Journal of Applied Physics 108 (2010) 102801.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Enhanced dispersibility and dispersion stability of dodecylamine-protected silver nanoparticles by dodecanethiol for ink-jet conductive inks Xueqin Zhou a,∗ , Wei Li a , Meilan Wu a , Shen Tang b , Dongzhi Liu a,∗ a b
School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China Shijiazhuang Ordnance Technical Institute, Shijiazhuang 050000, China
a r t i c l e
i n f o
Article history: Received 22 August 2013 Received in revised form 10 November 2013 Accepted 3 December 2013 Available online 11 December 2013 Keywords: Ag nanoparticles Conductive inks Inkjet printing Sintering Stability
a b s t r a c t This work studied dodecylamine-protected silver nanoparticles modified by a small amount of dodecanethiol as the co-protective agent. Contents of the dodecanethiol and the protective agent capping on the surface of silver nanoparticles were analyzed using the method of oxygen flask combustion and a thermogravimetric analysis instrument. Results of electrical property determination and transmission electron microscopy indicate that certain amount of capping dodecanethiol can slow down the spontaneous sintering process of silver nanoparticles. When capping DDT content of silver nanoparticles is 1.70 wt%, 10 wt% suspensions are stable under −18 ◦ C and can be stored stably at room temperature as long as 120 days. Furthermore, the silver nanoparticle concentration could be increased to 20 wt% with a stable storage time of 60 days at room temperature. Finally, stable polymer-free conductive inks with the silver nanoparticle concentration of 20 wt% were produced to fabricate patterns by ink-jet printing. The resistivity of the PI-supported patterns having been annealed at 130 ◦ C for 10 min is 7.2 cm. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Silver nanoparticle ink-jet conductive inks, which could deposit conductive lines on substrates in a cost-effective, time-saving and patterning-flexible way, have attracted considerable interests for their applications in production of electronic circuits [1] and electronic components [2]. Since the high surface energy of bare nanoparticles causes aggregation and flocculation, the surface of silver nanoparticles must be passivated, which was often accomplished by providing a steric barrier via a protective agent (PA) layer [3–6]. A concurred problem is that the printed patterns show poor conductivity due to an increase of the contact resistance between metallic particles by insulative PA [7]. Various methods were reported to remove PAs and sinter silver nanoparticles, therefore enhance film conductivity to meet the application requirement. Heating [3,8] is a simple and low cost process compared with other methods such as laser [9], microwave [10] and plasma [11]. But a heat-treatment temperature above 150 ◦ C would limit its application in fabricating of plastic flexible electronic devices due to the heat sensitivity of plastic substrates such as polyimide (PI) and poly(ethylene terephthalate) (PET) [12].
∗ Corresponding authors. Tel.: +86 22 2740 0911; fax: +86 22 2789 2283. E-mail addresses: [email protected] (X. Zhou), [email protected] (D. Liu). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.12.006
The dispersion stability and sintering conditions of silver nanoparticles are affected by capping PAs, which may be small molecular compounds with a long alkyl chain and polar head, such as alkanethiols [3] and alkylamines [4], or polymers bearing hydroxyl, amino or carbonyl groups, such as poly(vinyl pyrolidone) [5] and poly(acrylic acid) [6]. The polymers usually decompose at a temperature higher than 250 ◦ C. When the sintering process was carried out at 150 ◦ C, the polymer layer capping on the surface of silver nanoparticles only became thinner, resulting in a high resistance of the patterns [13]. On the other hand, the sintering temperature of silver nanoparticles using compound PAs significantly depends on their bonding energies with metal nanoparticles. Sintering of dodecanethiol-protected silver nanoparticles (Ag-DDT) requires a temperature no less than 200 ◦ C due to the strong bonding energy of Ag S, whereas the weak bonding energy of Ag N leads to a low heat-treatment temperature (<150 ◦ C) for dodecylamine-protected silver nanoparticles (Ag-DDA) [14]. Patterns obtained with Ag-DDA nanoparticles can even become conductive after air storage for less than 7 days [14]. However, such a weak bonding energy of Ag N would be adverse to the dispersion stability of silver nanoparticle inks. Different polymeric dispersants can delay the agglomeration or clustering, whereas the sintering temperature improves [15,16]. In this paper we prepared silver nanoparticles using a mixture of DDA and DDT as the PA (Ag-NS nanoparticles). Since the molecular structures of DDA and DDT are similar, we assume that a few
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amount of DDT will make little change of the sintering process of Ag-DDA nanoparticles, but strong Ag S bond may give a favorable effect on the ink stability. The dispersion stability and electrical properties of Ag-NS nanoparticle suspensions were investigated in detail. It is found that the precipitation of as-prepared silver nanoparticle suspensions at room temperature could be delayed by two months. Stable polymer-free silver nanoparticle inks with a silver nanoparticle concentration of 20% (by weight, expressed as wt% at the following) were obtained and the resistivity of inkjet-printed patterns was measured as 7.2 cm after 10 min heat-treatment at 130 ◦ C.
2. Experimental 2.1. Preparation and characterization of silver nanoparticles
2.2. Preparation and characterization of silver nanoparticle suspensions and films The synthesized silver nanoparticles were dispersed in cyclohexane with the different inputs by ultrasound treatment of 120 min and followed by filtration. Certain amount (Wt) of the suspensions was put in culture glass and the solvent was removed in vacuum at room temperature for 72 h. Then the residue (Wr) was weighted to calculate the silver nanoparticle concentrations (Wr/Wt).
Fig. 1. Plots of the DDT contents () and PA contents () in the nanoparticles against DDT amounts added during preparation.
24
Silver nanoparticle concentration (%)
Ag-NS nanoparticles were prepared via a modified method of the literature [17]. Silver acetate (4.175 g, ≥99%, Tianjin Yingda Sparseness and Noble Reagent Chemical Factory) used as the precursor of silver nanoparticles was first dispersed in toluene (40 mL) and mixture of DDT and DDA (10.175 g, 55.0 mmol) as the PA was then added into the solution, followed by the addition of phenylhydrazine (1.35 g, 12.5 mmol) as the reducing agent. The reaction was completed within 1 h, yielding a dark-brown solution without precipitation. Mixture of acetone and methanol (1:1 by volume) was added into the solution to precipitate AgNS nanoparticles, which were filtrated, washed with mixture of acetone and methanol (1:1 by volume), and air dried. In our experiments, the designation of Ag-NS1, Ag-NS2, Ag-NS3, Ag-NS4 and Ag-NS5 refers to the silver nanoparticles prepared with DDT amount of 0.0253 g (0.125 mmol), 0.0506 g (0.25 mmol), 0.0759 g (0.375 mmol), 0.1012 g (0.5 mmol) and 0.1265 g (0.625 mmol), respectively. Ag-DDA nanoparticles were prepared in the same way except that only DDA (10.175 g, 55.0 mmol) was used as PA. Ag-DDT nanoparticles were prepared using a method modified from the literature [18]. Sodium borohydride (0.5 g, 13.2 mmol) dissolved in the mixture of 2-propanol (70 mL) and ethanol (15 mL) was added to a solution of silver nitrate (0.5 g, ≥99%, Tianjin Yingda Sparseness and Noble Reagent Chemical Factory) and dodecanethiol (0.7 mL) in 2-propanol (85 mL) with vigorous stirring at room temperature. After 15 min, the black Ag-DDT nanoparticles were separated by filtration, washed with water, toluene, ethanol and acetone in turn, and air dried. The content of PA capped on the surface of silver nanoparticles (denoted as PA content) was analyzed using a thermogravimetric analysis instrument (TGA, TGA-Q500) at a heating rate of 10 ◦ C/min under N2 atmosphere. The content of DDT capped on the surface of silver nanoparticles (denoted as DDT content) was measured by method of oxygen flask combustion and DX120 ion chromatography. Morphology of silver nanoparticles was observed by transmission electron microscopy (TEM, JEM-2100F, JEOL).
23 22 21
Ag-NS2
20 19
Ag-NS1
18 17 16
Ag-NS3
Ag-DDA
15 14
Ag-NS4
13 12 0.0
0.5
1.0
1.5
2.0
2.5
DDT Content (%) Fig. 2. Plots of the silver nanoparticle concentrations against DDT contents. The solvent is cyclohexane and the input of silver nanoparticles is 25% by weight.
The silver nanoparticle films were obtained by spin-coating the suspensions onto the glass slides, followed by a heat-treatment in air at various temperatures for 5–60 min. The electrical resistivity of the films was calculated from sheet resistance and film’s thickness, which were measured by RTS-9 four-point probe station and the Dektak6M measuring surface topography, respectively. 2.3. Ink-jet printing of silver nanoparticle conductive inks Ag-NS2 nanoparticles were dispersed in a 1:1 mixture of cyclohexane and dodecane by ultrasonic treatment for 120 min, followed by filtration through a 0.22 m syringe filter to form the conductive ink. The final Ag-NS2 concentration of inks was 10 wt% or 20 wt%. The viscosity and surface tension of conductive inks were measured by NDJ-1 and BZY-A at 25 ◦ C, respectively. The obtained conductive inks were printed on polyimide (PI) substrate by an ordinary commercial ink-jet printer Epson ME1+, which have drop-on-demand (DOD) piezoelectric head with 48 nozzles for black container. The resolution and droplet volume of Epson ME1+ are 2880 × 720 dpi and 6 pL, respectively. The ink-jet printed patterns were heattreated at 130 ◦ C and the sheet resistances were measured on RTS-9 four-point probe station.
X. Zhou et al. / Applied Surface Science 292 (2014) 537–543
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Fig. 3. Plots of the Ag-NS (a), Ag-DDA (b) and Ag-DDT (c) concentrations against PA contents at different inputs of silver nanoparticles. The solvent is cyclohexane.
Table 1 Stability of various silver nanoparticle suspensions at different temperatures.a Silver nanoparticle
Nanoparticle concentration
Room temperature 1d
7d
30 d
60 d
90 d
AgDDA
10 20
↓
↓
AgNS2
10 20
↓
AgNS3
10 20
↓
↓
5
↓
Ag-DDT a b
120 d
10 ◦ Cb
0 ◦ Cb
−18 ◦ Cb
↓ ↓
↓ ↓
ND
ND
ND
150 d
↓
“↓” represents precipitation, “” represents no precipitation, “ND” means not determination. The suspensions were kept at certain temperature for 24 h, then recovered to room temperature and centrifuged at 4000 rpm for 10 min.
Table 2 Resistivities ( cm) of various silver nanoparticle films kept at room temperature for various times.a Silver nanoparticle
0d
1d
2d
3d
4d
5d
6d
7d
Ag-DDA Ag-NS1 Ag-NS2 Ag-NS3 Ag-NS4 Ag-DDT
ND ND ND ND ND ND
ND ND ND ND ND ND
66.5 ND ND ND ND ND
14.64 130.2 ND ND 4550 ND
9.68 39.28 ND 4334.75 271.60 ND
7.15 34.02 1149.40 601.13 198.33 ND
6.38 27.82 309.14 74.71 60.08 ND
6.06 24.02 289.87 51.03 45.00 ND
a The films were prepared by suspensions with a silver nanoparticle concentration of 20 wt% except for Ag-DDT, of which the concentration is 5 wt%. “ND” means not determination due to the resistivity is higher than 10,000 cm.
3. Results and discussion 3.1. Contents of DDT and PA capping on the surface of silver nanoparticles Previous studies show that the bonding energy between thiol groups and silver nanoparticles is stronger than that of amino
groups [14]. Hence it is expected that silver nanoparticles would first chemsorb DDT molecules when both DDA and DDT are present, which was confirmed by the fact that the DDT content increases linearly with the improving DDT amount in preparation (Fig. 1). Furthermore, the DDT amount in preparation significantly influences the protective agent content of silver nanoparticles. The PA content is 9.0 wt% for Ag-DDA nanoparticles. This value was
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X. Zhou et al. / Applied Surface Science 292 (2014) 537–543
Fig. 4. TEM images of Ag-NS1 (a), Ag-NS2 (b), Ag-NS3 (c) and Ag-NS4 (d) nanoparticles after solvent evaporation in air at room temperature for 1 h.
increased to 9.6 wt% and 10.6 wt% for Ag-NS1 and Ag-NS2 at DDT content of 0.76 wt% and 1.58 wt%, respectively. When the DDT content was improved to 2.33 wt%, the PA content was found as 10.2 wt% (Ag-NS3). Further increasing DDT amount caused a decreased PA content (Ag-NS4). The initial increase of PA contents can be reasonably explained by the interaction between alkyl chains of DDA and DDT. Further increase of DDT amount make it possible to form an almost all-trans zigzag configuration among alkyl chains of capping DDT molecules as found in Ag-DDT nanoparticles [14], which is against their interaction with DDA alkyl chains. But results of IR and XRD determination did not give any clear information of this supposition due to too low DDT content in Ag-NS nanoparticles. 3.2. Dispersibility and dispersion stability of silver nanoparticles A sufficient amount of metal nanoparticles in the ink is vital to have continuous metal nanoparticles aligned in the printed pattern to allow the conductivity to meet application requirements [19]. A minimal nanometal concentration is reported to be far more than 10 wt% for 10 nm size particles [20]. To explore the effect of
capping DDT on the dispersibility of silver nanoparticles, the silver nanoparticles were dispersed in cyclohexane via ultrasound treatment and filtration and then the silver nanoparticle concentrations of suspensions were plotted against DDT contents (Fig. 2). Under the same conditions, the more silver nanoparticles inputted the higher silver nanoparticle concentration of suspensions. Besides, existence of capping DDT also affects the Ag-NS concentration of suspensions. When the inputs of silver nanoparticles are the same, there is a maximal silver nanoparticle concentration, which was found for Ag-NS2 nanoparticles. Fig. 3 gives the plots of the silver nanoparticle concentration against the PA content. When same amounts of silver nanoparticles were inputted into cyclohexane under the same dispersion conditions, the final Ag-NS4 and Ag-NS3 concentrations were found lower than Ag-NS1 concentration though the PA content is higher for Ag-NS4 or Ag-NS3 than Ag-NS1 (Fig. 3a). This can be also explained by the interaction among alkyl chains, which causes an increase of Ag-DDA concentrations (Fig. 3b) and a decrease of AgDDT concentrations (Fig. 3c) with the rising PA contents. Furthermore, Ag-NS2 nanoparticles exhibit excellent dispersion stability. As shown in Table 1, 10 wt% and 20 wt% Ag-NS2
X. Zhou et al. / Applied Surface Science 292 (2014) 537–543
Fig. 5. TEM images of Ag-NS1 (a), Ag-NS2 (b), Ag-NS3 (c) and Ag-NS4 (d) nanoparticles after air storage at room temperature for 24 h.
Fig. 6. TEM images of Ag-NS2 nanoparticles after air storage at room temperature for 48 h (a), 72 h (b) and 96 h (c).
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X. Zhou et al. / Applied Surface Science 292 (2014) 537–543
22
(a)
20
Ag-NS1 Ag-NS2 Ag-NS3 Ag-NS4
Resistivity ( ⏐.cm)
18 16 14 12 10 8 6 4
0
10
20
30
40
50
60
Time (min)
Fig. 8. The Ag-NS2 conductive ink and the ink-jet printed pattern on PI substrate.
30
(b)
Resistivity ( ⏐.cm)
25
3.3. Electrical properties of silver nanoparticle films
Ag-NS1 Ag-NS2 Ag-NS3 Ag-NS4
20
15
10
5 90
100
110
120
130
Temperature
11
(c)
10
Resistivity ( ⏐ .cm)
9 8 7 6
y =3.36+0.25*exp(x/0.76) 2 R =0.9968
5 4 3 0.0
0.5
1.0
1.5
2.0
2.5
DDT Content (%) Fig. 7. Plots of the Ag-NS film resistivities against heat-treatment times at 130 ◦ C (a), the heat-treatment temperatures for 60 min (b) and the DDT contents when heated at 130 ◦ C for 60 min (c). The silver nanoparticle concentrations are 20 wt%.
suspensions in cyclohexane could be stored stably without precipitation at room temperature for 120 and 60 days, respectively. The occurrence of sediments was delayed by 60 and 30 days, respectively, in comparison with 10 wt% and 20 wt% Ag-DDA suspensions, which show sediments on the bottom within 60 and 30-day storage, respectively. Besides, when the temperature was reduced to 0 ◦ C or −18 ◦ C, silver nanoparticle suspensions may freeze for the melting point of cyclohexane is 6.5 ◦ C. After thawing at room temperature and centrifuging at 4000 rpm for 10 min, both Ag-NS2 and Ag-NS3 suspensions did not show any sediments, which was found in Ag-DDA suspensions with the same treatment.
All Ag-NS films, which are insulative upon preparation, can become conductive after air storage at room temperature within 7 days (Table 2). Simultaneously, the color of Ag-NS films changes from bluish violet to silver white. These phenomena are similar to Ag-DDA films [14], indicating that Ag-NS nanoparticles could also sinter spontaneously. However, Ag-NS films, having been kept in the air at room temperature for 7 days, exhibit higher resistivity than Ag-DDA film (Table 2). This is assigned to the presence of capping DDT due to the strong Ag S bonding energy could prohibit the interparticle interaction and coalesce among Ag-DDT nanoparticles. The spontaneous sintering process was observed by TEM. Fig. 4 shows the TEM images of Ag-NS nanoparticles obtained by dropping the Ag-NS-in-cyclohexane suspensions on the carbon-coated copper TEM grids and drying in air at room temperature for 60 min. All Ag-NS nanoparticles are spherical and the size is about 5 nm. Obviously all these nanoparticles initially form locally ordered structure. Nanoparticles coalescence occurs one day later for AgNS1, Ag-NS3, and Ag-NS4 (Fig. 5), and only four days later for Ag-NS2 (Fig. 6). These results indicated that a small amount of capping DDT could decrease the interparticle interaction and slow down the spontaneous coalescence process of Ag-NS2 nanoparticles. However, the DDT content should be not more than 2.33 wt%, otherwise the obtained nanoparticles such as Ag-NS3 and Ag-NS4, the decreased PA content would prompt the coalescence process. Heat-treatment can quickly reduce the resistivity of silver nanoparticle films and the higher the temperature the faster the onset resistivity decreases (Fig. 7a). A temperature above 110 ◦ C has less significant influence on the resistivity of Ag-NS1, Ag-NS2 and Ag-NS3 films when the heating time was 60 min (Fig. 7b). But the resistivity of Ag-NS4 films continues to decrease even at 130 ◦ C. Besides it is found that the resistivity of Ag-NS films after 60 min heat-treatment at 130 ◦ C improves exponentially with DDT contents (Fig. 7c). It could be explained by the presence of capping DDT which only dissociates at a temperature above 150 ◦ C. But even if the DDT content is 2.54 wt%, the resistivity of Ag-NS4 films heated at 130 ◦ C for 60 min can reach 10.3 cm, which is about 6.5 times the bulk Ag resistivity (Ag bulk = 1.6 cm). 3.4. Inkjet printing Above results indicate that Ag-NS2 nanoparticles might be a potential candidate to produce ink-jet conductive inks. Hence corresponding conductive inks were prepared. 1-Dodecane was used in combining with cyclohexane to reduce the evaporation rate of solvent and thus prohibit the nozzles of printer from clogging. No
X. Zhou et al. / Applied Surface Science 292 (2014) 537–543
sediment has been observed yet for two months. The viscosity and the surface tension were 1.2 cP and 23.8 mN/m, respectively, for 10 wt% inks, and 1.3 cP and 24.6 mN/m, respectively, for 20 wt% inks. The ink-jet printed patterns on PI films (Fig. 8) were heated at 130 ◦ C for 10 min. The thickness and resistivities of obtained PIsupported patterns were measured as about 250 nm, 30.2 cm and 7.2 cm for 10 wt% and 20 wt% inks, respectively. Resistivities of the patterns could be further reduced to 13.7 cm and 5.5 cm, respectively, as extending the heating-time at 130 ◦ C to 60 min. 4. Conclusion Dodecylamine (DDA) was used combined with a few amount of dodecanethiol (DDT) as the PA to prepare silver nanoparticles. Dispersion stability and electrical properties of Ag-NS nanoparticles were found to be closely associated with DDT content, which was found to increase linearly with the rising amount of DDT added in preparation. The capping DDT could also significantly influence the PA content and slow down the spontaneous coalescence of silver nanoparticles in solid state at room temperature, leading to an obviously enhanced stability of Ag-NS2 suspensions when comparing to Ag-DDA suspensions. Electrical properties of Ag-NS films could be improved quickly by heat-treatment at temperature above 90 ◦ C. It is found that the resistivity of Ag-NS films heated at 130 ◦ C for 60 min improves exponentially with DDT content. Finally 20 wt% stable polymer-free conductive inks were obtained with AgNS2 nanoparticles and the resistivity of inkjet-printed patterns on PI was measured as 7.2 cm and 5.5 cm after heat-treatment at 130 ◦ C for 10 min and 60 min, respectively, showing great potentials in the application of flexible printed electronic circuits and devices. Acknowledgement The authors would like to thank for the financial support of the Doctoral Programs Foundation of China (No. 20110032110018). References [1] S. Jeong, H.C. Song, W.W. Lee, Y.G. Choi, B.H. Ryu, Preparation of aqueous Ag ink with long-term dispersion stability and its inkjet printing for fabricating conductive tracks on a polyimide film, Journal of Applied Physics 108 (2010) 102805.
543
[2] L. Xie, M. Mantysalo, A.L. Cabezas, Y. Feng, F.F. Jonsson, L.R. Zheng, Electrical performance and reliability evaluation of inkjet-printed Ag interconnections on paper substrates, Materials Letters 188 (2012) 68–72. [3] S.K. Volkman, S. Yin, T. Bakhishev, K. Puntambekar, V. Subramanian, M.F. Toney, Mechanistic studies on sintering of silver nanoparticles, Chemistry of Materials 23 (2011) 4634–4640. [4] L. Polavarapu, K.K. Manga, K. Yu, P.K. Ang, H.D. Cao, J. Balapanuru, K.P. Loh, Q.H. Xu, Alkylamine capped metal nanoparticle “inks” for printable SERS substrates, electronics and broadband photodetectors, Nanoscale 3 (2011) 2268–2274. [5] S. Li, P. Liu, Q.S. Wang, Study on the effect of surface modifier on self-aggregation behavior of Ag nanoparticle, Applied Surface Science 263 (2012) 613–618. [6] S. Magdassi, M. Grouchko, O. Berezin, A. Kamyshny, Triggering the sintering of silver nanoparticles at room temperature, ACS Nano 4 (2010) 1943–1948. [7] A. Denneulin, A. Blayo, C. Neuman, J. Bras, Infra-red assisted sintering of inkjet printed silver tracks on paper substrates, Journal of Nanoparticle Research 13 (2011) 3815–3823. [8] B.Y. Ahn, E.B. Duoss, M.J. Motala, X. Guo, S. Park, Y. Xiong, J. Yoon, R.G. Nuzzo, J.A. Rogers, J.A. Lewis, Omnidirectional printing of flexible, stretchable, and spanning silver microelectrodes, Science 323 (2009) 1590–1593. [9] H. Alemohammad, O. Aminfar, E. Toyserkani, Morphology and microstructure analysis of nano-silver thin films deposited by laser-assisted maskless microdeposition, Journal of Micromechanics and Microengineering 18 (2008) 115015. [10] J. Perelaer, R. Jani, M. Grouchko, A. Kamyshny, S. Magdassi, U.S. Schubert, Plasma and microwave flash sintering of tailored silver nanoparticle ink, yielding 60% bulk conductivity on cost-effective polymer foils, Advanced Materials 24 (2012) 3993–3998. [11] S. Wunscher, S. Stumpf, A. Teichler, O. Pabst, J. Perelaer, E. Beckert, U.S. Schubert, Localized atmospheric plasma sintering of inkjet printed silver nanoparticles, Journal of Materials Chemistry 22 (2012) 24569–24576. [12] T.H.J. van Osch, J. Perelaer, A.W.M. de Laat, U.S. Schubert, Inkjet printing of narrow conductive tracks on untreated polymeric substrates, Advanced Materials 20 (2008) 343. [13] L.X. Mo, D.Z. Liu, X.Q. Zhou, L.H. Li, Preparation and conductive mechanism of the ink-jet printed nanosilver films for flexible display, in: Proceedings of the Second International Congress on Image and Single Processing, 1–9, 2009, pp. 3159–3163. [14] L.X. Mo, D.Z. Liu, W. Li, L.H. Li, L.C. Wang, X.Q. Zhou, Effects of dodecylamine and dodecanethiol on the conductive properties of nano-Ag films, Applied Surface Science 257 (2011) 5746–5753. [15] A. Kosmala, R. Wright, Q. Zhang, P. Kirby, Synthesis of silver nano particles and fabrication of aqueous Ag inks for inkjet printing, Materials Chemistry and Physics 129 (2011) 1075–1080. [16] A. Kosmala, Q. Zhang, R. Wright, P. Kirby, Development of high concentrated aqueous silver nanofluid and inkjet printing on ceramic substrates, Materials Chemistry and Physics 132 (2012) 788–795. [17] Y. Li, Y. Wu, B.S. Ong, Facile synthesis of silver nanoparticles useful for fabrication of high-conductivity elements for printed electronics, Journal of the American Chemical Society 127 (2005) 3266–3267. [18] S.K. Volkman, Y. Pei, D. Redinger, S. Yin, V. Subramanian, Ink-jetted silver/copper conductors for printed RFID applications, Materials Research Society Symposium Proceedings 814 (2004) 151–156. [19] J. Chung, S. Ko, N.R. Bieri, C.P. Grigoropoulos, D. Poulikakos, Conductor microstructures by laser curing of printed gold nanoparticle ink, Applied Physics Letters 84 (2004) 801–803. [20] N.S. Kim, K.N. Han, Future direction of direct writing, Journal of Applied Physics 108 (2010) 102801.