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An organometallic route to highly monodispersed silver nanoparticles and their application to ink-jet printing
Materials Chemistry and Physics 110 (2008) 316–321
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
An organometallic route to highly monodispersed silver nanoparticles and their application to ink-jet printing In-Keun Shim ∗ , Young Il Lee, Kwi Jong Lee, Jaewoo Joung Central R&D Institute, Samsung Electro-Mechanics 314, Maetan3-Dong, Youngtong-Gu, Suwon, Gyunggi-Do 443-743, Republic of Korea
a r t i c l e
i n f o
Article history: Received 10 October 2007 Received in revised form 5 February 2008 Accepted 13 February 2008 Keywords: Electronic materials Nanostructures Chemical synthesis
a b s t r a c t Highly monodispersed silver nanoparticles were successfully synthesized by thermolysis of silver alkanoate precursors and were characterized by X-ray diffraction, TGA/DTA and transmission electron microscopy. The results showed that these nanoparticles exhibit spherical shape with FCC crystal structure. The relationship between the carbon chain length and the monodispersity of the nanoparticles was investigated. Furthermore, the size of the particles was controlled by varying the concentration of the stabilizing surfactants. The silver nanoparticles were easily re-dispersed into n-tetradecane and printed onto various substrates using a Microfab head with a single nozzle. The ink-jet printed patterns were sintered at 250 ◦ C and their electrical resistivity was about 6 cm. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Over the past several years, many applications for nanomaterials that exploit their mesoscopic properties have been proposed. Nanoparticles, which exhibit properties different from the bulk materials, have been applied as catalytic, sensor, bio, photonic, and electronic materials [1–3]. With increasing demands for electronic devices using polymer-based printed circuit boards (PCBs), metal nanoparticles are especially in the limelight of industry because they can be used in novel applications to create conductive traces. There are many well-established manufacturing techniques for the fabrication of microelectronic devices such as photolithography, screen printing, nano-imprinting, micro-contact printing, and dip-pen lithography. More recently, ink-jet printing (IJP) is rapidly being developed for the display industry (OLED, spacer, color filter), the semiconductor industry (PCB, resister), and bio-medical technology (array, sensor) because it has the following advantages over conventional thin-film photolithography [4]: (1) (2) (3) (4) (5)
real time; high resolution; low cost; fast productivity; non-contact method.
Due to these merits, IJP is considered to be a promising tool for the formation of conductive patterns. However, many problems
∗ Corresponding author. Tel.: +82 437305622; fax: +82 437339408. E-mail address: [email protected] (I.-K. Shim). 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.02.020
remain to be solved in spite of the previously stated advantages, and many of these problems are derived from the nanoparticle inks. Nanoparticle inks are formed from nanoparticulate colloids that are stably dispersed in a liquid vehicle. Furthermore, given nanoparticles must fit certain criteria to be applied to IJP. In order to be used as a material in inks, the size of the nanoparticle must be about 50 nm or less with the necessity of high dispersion stability [5,6]. Moreover, the size and shape uniformity of the nanoparticles is very important because the conductivity strongly depends on the density of nanoparticles packed into the superstructure that results after drying the ink vehicle from a printed trace. A number of methods have been developed for preparation of silver nanoparticles such as the thermal decomposition method, polyol process, alcohol reduction, microemulsion method, radiolytic reduction, and sonochemical reduction [6–9]. In this study, we introduce a thermal decomposition method to achieve highly monodispersed silver nanoparticles with high precursor concentrations (up to 0.1 M) without the necessity of a size selection process (Fig. 1). The nanoparticle capping ligand is selected according to the intended sintering temperature of the ink.
2. Experimental 2.1. Materials The following chemicals were purchased from various companies and used without further purification: silver nitrate (99.9%, Kojima), sodium hydroxide (98%, Shinyo Pure Chemicals), dodecanoic acid (97%, Samchun Pure Chemicals), myristic acid (97%, Samchun Pure Chemicals), palmitic acid (97%, Samchun Pure Chemicals), n-hexanes (95%, Samchun Pure Chemicals), ethyl alcohol (95%, Samchun Pure Chemicals), dodecylamine (98%, Aldrich), 1-octadecene (90%, Aldrich), and n-tetradecane (99%, Kanto Chemicals).
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Fig. 1. Synthetic procedure for silver nanoparticles.
2.2. Preparation of silver alkanoate Silver alkanoate complexes were prepared from silver nitrate with various sodium alkanoates. 0.5 mol of sodium hydroxide and an alkanoic acid were dissolved in 150 mL of deionized water. 200 mL of ethyl alcohol was added to the solution with 0.5 mol silver nitrate under vigorous stirring. When the solution became turbid, 350 mL of n-hexanes were added to the solution in order to extract the silver alkanoate complex from the aqueous phase. The resulting solution was then stirred for 2 h at room temperature. The organic phase was separated from the resulting biphasic solution using a separation funnel and washed with deionized water. Finally, we obtained pure silver alkanoate complexes after evaporation of n-hexanes in a vacuum oven. 2.3. Preparation of silver nanoparticles and conductive inks The following process is an optimized procedure for highly monodispersed silver nanoparticles, especially with regards to use of the silver dodecanoate precursor. 4 mmol of silver alkanoate was dissolved in 50 mL of 1-octadecene with 4 mmol of the corresponding alkanoic acid. The solution was heated to 210 ◦ C with a heating rate of 4 ◦ C min−1 and then the temperature was held for 2 h. After the silver alkanoate complex completely underwent thermal decomposition, the temperature was lowered to 70 ◦ C, and then 150 mL of ethanol was added to the solution for precipitation of the silver nanoparticles. Finally, the silver nanocrystals were centrifuged and washed several times with ethanol. For the preparation of conductive nanoparticle inks, the synthesized silver nanoparticles were re-dissolved into n-tetradecane with vigorous stirring. Finally, the silver nanoparticle inks were jetted onto various substrates using a Microfab head with a single nozzle, and the patterns were sintered in a convection oven at 250 ◦ C for 30 min.
3. Results and discussion To investigate the effect of the capping ligands with regards to resulting nanoparticle properties, the chain length was changed in the range of 12–16 carbons. Previous research in the literature proposed that the longer the carbon chain of the capping molecule, the more monodispersed the nanoparticles became [10]. Although the range of carbon chain lengths is not wide in our experiments, this trend could not be observed systematically. Fig. 2 shows TEM images of silver nanoparticles prepared from different silver alkanoate precursors. As shown in Fig. 2, the nanoparticles from each sample are of spherical morphology with an average size of about 6 nm. Because the silver dodecanate complex has the lowest thermal decomposition temperature, it is more advantageous than the other
silver alkanoate complexes with regards to the sintering process. Therefore, the synthetic strategy is focused on the silver dodecanate complex in this study. Fig. 3 shows monodispersed silver nanoparticles that were prepared by thermolysis of silver dodecanate while varying the mole ratio between silver dodecanate and additional dodecanoic acid. In particular, we can obtain highly monodispersed silver nanoparticles with an average size of about 6 nm that were prepared from a reaction mixture with a 1:1 silver precursor to corresponding acid mole ratio (sample 2). However, broad particle size distributions (PSDs) can readily be observed when the mole ratio of the reaction mixture of silver precursor to corresponding acid was 2:1 (sample 1). This difference in PSDs is principally derived from the concentration of capping molecules in comparison to that of the silver ions. A sufficient concentration of capping molecules results in efficient suppression in the growth of the silver nanoparticles during the reaction such that there is a greater level of control. As shown in Fig. 4a, all the nanocrystals of sample 2 are highly monodispersed in PSD. High-resolution TEM (HRTEM) images of these nanocrystals show distinct lattice fringe patterns, indicating the highly crystalline nature of the nanocrystals. The lattice spacing obtained from the micrograph above (Fig. 3c) is 0.24 nm, which matches well to the (1 1 1) plane for the face-centered-cubic (FCC) crystal structure of bulk silver. XRD patterns for the silver precursor and silver nanocrystals (sample 1) were measured in 2 from 30◦ to 100◦ . No crystalline signature for silver in the pristine silver dodecanate complex was observed in the XRD analysis, as shown in Fig. 5a, indicating that the complex is amorphous in nature. However, the diffraction pattern that was obtained after decomposition of silver dodecanate at 210 ◦ C was identified through comparison to the standard, pure, crystalline silver structure (JCPDS card no. 04-0783). The XRD pattern for sample 1 shows characteristic peaks at 2 = 38.1◦ , 44.3◦ , 64.4◦ , 77.5◦ , and 81.5◦ , respectively marked by the indices (1 1 1), (2 0 0), (2 2 0), (3 1 1), and (2 2 2). This confirms that the resultant particles are pure silver with an FCC crystal structure. The full width at half maximum (FWHM) of the (1 1 1) reflection from the X-ray data was used to calculate the average particle size by using the Debye–Scherrer formula [11]. The calculated average particle size was about 6.5 nm. Thus, the average particle size cal-
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Fig. 2. TEM images of the silver nanocrystals stabilized with dodecanoic acid (a), myristic acid (b) and palmitic acid (c).
culated using the Debye–Scherrer formula corroborates well with the particle sizes seen in the TEM image (Fig. 3b). Thermogravimetric analyses for the samples were conducted with a 10 ◦ C min−1 ramp. Upon heating while purging with air, samples underwent rapid weight loss at around 210 ◦ C due to the decomposition of the capping molecules in the presence of the silver nanocrystals and silver ions. The TGA data corresponds to strong exothermic DTA peaks at around 210 ◦ C, and the rest of the exothermic peaks, in the temperature range of 270–310 ◦ C, are from the pyrolysis of dodecanoate and the remaining carbon residues. These thermal analyses indicate the minimum thermal decomposition temperature of the reaction mixtures. As shown in Fig. 6a, the residue weight percent of silver dodecanate is 38.9%, and it is similar in value with the theoretical mole weight percent of silver in the silver dodecanoate complex. The TGA results also yield evidence that silver dodecanoate prepared by the two-phase method is in a pure form without any significant amount of unreacted dodecanoic acid remaining. Fig. 6c shows that the residue percent of silver remaining is 85.4%, and this value corroborates well with the calculated amount of capping molecule on the surface of the nanoparticles. Fig. 7 shows the FT-IR spectrum of silver dodecanate at room temperature. In the high frequency region above 2800 cm−1 , the spectrum exhibits at least three strongly absorbing peaks at 2848 cm−1 , 2917 cm−1 , and 2956 cm−1 for the symmetric and asymmetric stretching modes of the methylene-groups and asymmetric stretching of the methyl-group, respectively. The peak at 2871 cm−1 is probably due to the symmetric stretching of the methyl-group. There are two shoulder bands at ca. 2895 cm−1 and 2964 cm−1 . The former band is from the Fermi resonance absorption due to the d+ mode and the latter one is due to the r− mode [12,13]. In the low frequency region below 1600 cm−1 , some “fingerprint” stretching modes for the carboxylate-group and three different bending modes (scissoring, rocking, and wagging modes) for the methylene-groups in silver dodecanate are observed. The absence of any peak corresponding to a carbonyl group near 1700 cm−1 is notable in providing evidence that the as prepared silver dodecanoate was synthesized in a relatively pure form without contamination by excess free acid. The FT-IR spectrum of silver nanoparticles shown in Fig. 8 gives some important information about the interaction between the surface ligands and the metal nanoparticles. This absorption spectrum pattern is similar to that of self-assembled monolayers (SAMs) of stearic acid on a bulk silver substrate [14,15]. As stated in the given references, the carboxylate-group gives rise to a symmetric stretch at about 1439 cm−1 , and the presence of this band supports that the silver metal nanoparticles are passivated by a uniform layer of dodecanoate. In Fig. 7, the as (COO− ) band is more intense than the s (COO− ) band. However, the as (COO− ) band becomes weaker than the s (COO− ) band in the FT-IR spectrum of the nanoparticles shown in Fig. 8. From this data, the weakness of the as (COO− ) band in the spectrum of nanoparticles can be interpreted as the carboxylategroup being symmetrically chemisorbed on the surfaces of silver nanoparticles via the two oxygen atoms [16]. All of these observations suggest that the polarizing nature of the silver cation in silver dodecanoate more readily holds the negative charge of the carboxylate anion to one oxygen atom at room temperature whereas the more neutrally charged, or less negatively charged, state of the silver atoms on the surfaces of the nanoparticles is less polarizing such that bidentate binding occurs with the negative charge of the carboxylate anion delocalized between the two oxygen atoms. The inset of Fig. 9 shows a stable silver nanoparticle ink sample. For preparation of an ink-jettable ink, the synthesized silver
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Fig. 3. HRTEM images of monodispersed silver nanocrystals stabilized with dodecanoic acid. (a) and (b) Sample 1; (c) and (d) sample 2.
nanoparticles were dissolved into n-tetradecane. Their solid contents and viscosity are about 20 wt% and 5 cps. The dispersed silver nanoparticles can be suspended for several weeks in the stationary state and more than 3 min under centrifugation at 3000 rpm without sedimentation.
Patterning on various substrates was achieved using an automated IJP process. As shown in Fig. 9, the nanosilver ink was jetted onto a polyimide film, photo-paper, and a silicon wafer using a Microfab head with an optimized waveform. Finally, the specific electrical resistance was about 6 cm, which was achieved by
Fig. 4. HRTEM images and electron diffraction pattern of monodispersed silver nanocrystals. HRTEM image and electron diffraction patterns reveal the highly crystalline nature of the silver nanocrystals.
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Fig. 5. X-ray diffraction patterns of silver dodecanoate (a) and silver nanoparticles (b).
Fig. 6. TGA and DTA of silver dodecanoate (a) and (b), and silver nanoparticles (c) and (d).
Fig. 7. FT-IR spectrum of silver dodecanate at room temperature.
Fig. 8. FT-IR spectrum of silver nanoparticle.
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monodispersed silver nanocrystals to be obtained in gram scale quantities with a single reaction and without the necessity of a size-selective post-processing step. Second, high yields of greater than 75% are possible when using this synthetic method. The nanoparticles were successively re-dispersed into organic solvents and also worked well when formulated into ink-jettable inks. Silver nanoparticles formulated into inks are useful as inkjettable materials because they have relatively low sintering temperatures due to depressed melting points and reasonably high conductivity measurements. Furthermore, the IJP of nanomaterials can be applied to the low-cost fabrication of electronics. Acknowledgements
Fig. 9. Conductive silver patterns on various substrates (silver nanoparticle ink in inset).
pyrolysis of the surface ligands on the nanoparticles and the ink solvent at 250 ◦ C for 30 min. 4. Conclusions Highly monodispersed 6 nm silver nanocrystals were synthesized in the high-boiling solvent 1-octadecene with silver dodecanate and dodecanoic acid. The resulting silver nanocrystals show high crystallinity such that even a lattice fringe pattern can be observed and measured within the HRTEM images. We can control the PSDs by varying the mole ratio of reaction mixture species, and the ideal mole ratio was 1:1 silver dodecanate to dodecanoic acid. Conclusively, the purity of the silver precursor and mole ratio of reaction mixture species are key factors in synthesizing monodispersed silver nanocrystals. In addition the synthetic procedures developed in this study offer several very important advantageous features over other conventional methods for the synthesis of monodispersed nanocrystals. First, this process allows
The authors would like to acknowledge the Analytical Research Group at Samsung Electro-Mechanics and give thanks to Nathan E. Stott for corrections made to the original manuscript. References [1] D.I. Gittins, D. Bethell, D.J. Schiffrin, R.J. Nichols, Nature 408 (2000) 67. [2] T.S. Ahmadi, Z.L. Wang, T.C. Green, A. Henglein, M.A. El-Sayed, Science 272 (1996) 1924. [3] H. Imahori, S. Fukuzumi, Adv. Mater. 13 (2001) 1197. [4] P. Calvert, Chem. Mater. 13 (2001) 3299. [5] Y. Li, Y. Wu, B.S. Ong, J. Am. Chem. Soc. 127 (2005) 3266. [6] K.J. Lee, Y.I. Lee, I.K. Shim, J. Joung, Y.S. Oh, J. Colloid Interface Sci. 304 (2006) 92. [7] Y. Socol, O. Abramson, A. Gedanken, Y. Meshorer, L. Berenstein, A. Zaban, Langmuir 18 (2002) 4736. [8] A. Henglein, Chem. Mater. 10 (1998) 444. [9] H. Sato, T. Hirat, I. Komasawa, J. Chem. Eng. Jpn. 29 (1996) 501. [10] B.L.V. Prasad, S.I. Stoeva, C.M. Sorensen, K.J. Klabunde, Langmuir 18 (2002) 7515. [11] A. Guinier, X-ray Diffraction in Crystals, Imperfect Crystals and Amorphous Bodies, W.H. Freeman, San Francisco, 1963. [12] R.G. Snyder, S.L. Hsu, S. Krimm, Spectrochim. Acta A 34 (1978) 395. [13] I.R. Hill, I.W. Levin, J. Chem. Phys. 70 (1979) 842. [14] S.J. Ahn, D.H. Son, K. Kim, J. Mol. Struct. 324 (1994) 223. [15] S.J. Lee, K. Kim, Vib. Spectrosc. 18 (1998) 187. [16] S.J. Lee, S.W. Han, H.J. Choi, K. Kim, J. Phys. Chem. B 106 (2002) 28.
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
An organometallic route to highly monodispersed silver nanoparticles and their application to ink-jet printing In-Keun Shim ∗ , Young Il Lee, Kwi Jong Lee, Jaewoo Joung Central R&D Institute, Samsung Electro-Mechanics 314, Maetan3-Dong, Youngtong-Gu, Suwon, Gyunggi-Do 443-743, Republic of Korea
a r t i c l e
i n f o
Article history: Received 10 October 2007 Received in revised form 5 February 2008 Accepted 13 February 2008 Keywords: Electronic materials Nanostructures Chemical synthesis
a b s t r a c t Highly monodispersed silver nanoparticles were successfully synthesized by thermolysis of silver alkanoate precursors and were characterized by X-ray diffraction, TGA/DTA and transmission electron microscopy. The results showed that these nanoparticles exhibit spherical shape with FCC crystal structure. The relationship between the carbon chain length and the monodispersity of the nanoparticles was investigated. Furthermore, the size of the particles was controlled by varying the concentration of the stabilizing surfactants. The silver nanoparticles were easily re-dispersed into n-tetradecane and printed onto various substrates using a Microfab head with a single nozzle. The ink-jet printed patterns were sintered at 250 ◦ C and their electrical resistivity was about 6 cm. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Over the past several years, many applications for nanomaterials that exploit their mesoscopic properties have been proposed. Nanoparticles, which exhibit properties different from the bulk materials, have been applied as catalytic, sensor, bio, photonic, and electronic materials [1–3]. With increasing demands for electronic devices using polymer-based printed circuit boards (PCBs), metal nanoparticles are especially in the limelight of industry because they can be used in novel applications to create conductive traces. There are many well-established manufacturing techniques for the fabrication of microelectronic devices such as photolithography, screen printing, nano-imprinting, micro-contact printing, and dip-pen lithography. More recently, ink-jet printing (IJP) is rapidly being developed for the display industry (OLED, spacer, color filter), the semiconductor industry (PCB, resister), and bio-medical technology (array, sensor) because it has the following advantages over conventional thin-film photolithography [4]: (1) (2) (3) (4) (5)
real time; high resolution; low cost; fast productivity; non-contact method.
Due to these merits, IJP is considered to be a promising tool for the formation of conductive patterns. However, many problems
∗ Corresponding author. Tel.: +82 437305622; fax: +82 437339408. E-mail address: [email protected] (I.-K. Shim). 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.02.020
remain to be solved in spite of the previously stated advantages, and many of these problems are derived from the nanoparticle inks. Nanoparticle inks are formed from nanoparticulate colloids that are stably dispersed in a liquid vehicle. Furthermore, given nanoparticles must fit certain criteria to be applied to IJP. In order to be used as a material in inks, the size of the nanoparticle must be about 50 nm or less with the necessity of high dispersion stability [5,6]. Moreover, the size and shape uniformity of the nanoparticles is very important because the conductivity strongly depends on the density of nanoparticles packed into the superstructure that results after drying the ink vehicle from a printed trace. A number of methods have been developed for preparation of silver nanoparticles such as the thermal decomposition method, polyol process, alcohol reduction, microemulsion method, radiolytic reduction, and sonochemical reduction [6–9]. In this study, we introduce a thermal decomposition method to achieve highly monodispersed silver nanoparticles with high precursor concentrations (up to 0.1 M) without the necessity of a size selection process (Fig. 1). The nanoparticle capping ligand is selected according to the intended sintering temperature of the ink.
2. Experimental 2.1. Materials The following chemicals were purchased from various companies and used without further purification: silver nitrate (99.9%, Kojima), sodium hydroxide (98%, Shinyo Pure Chemicals), dodecanoic acid (97%, Samchun Pure Chemicals), myristic acid (97%, Samchun Pure Chemicals), palmitic acid (97%, Samchun Pure Chemicals), n-hexanes (95%, Samchun Pure Chemicals), ethyl alcohol (95%, Samchun Pure Chemicals), dodecylamine (98%, Aldrich), 1-octadecene (90%, Aldrich), and n-tetradecane (99%, Kanto Chemicals).
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Fig. 1. Synthetic procedure for silver nanoparticles.
2.2. Preparation of silver alkanoate Silver alkanoate complexes were prepared from silver nitrate with various sodium alkanoates. 0.5 mol of sodium hydroxide and an alkanoic acid were dissolved in 150 mL of deionized water. 200 mL of ethyl alcohol was added to the solution with 0.5 mol silver nitrate under vigorous stirring. When the solution became turbid, 350 mL of n-hexanes were added to the solution in order to extract the silver alkanoate complex from the aqueous phase. The resulting solution was then stirred for 2 h at room temperature. The organic phase was separated from the resulting biphasic solution using a separation funnel and washed with deionized water. Finally, we obtained pure silver alkanoate complexes after evaporation of n-hexanes in a vacuum oven. 2.3. Preparation of silver nanoparticles and conductive inks The following process is an optimized procedure for highly monodispersed silver nanoparticles, especially with regards to use of the silver dodecanoate precursor. 4 mmol of silver alkanoate was dissolved in 50 mL of 1-octadecene with 4 mmol of the corresponding alkanoic acid. The solution was heated to 210 ◦ C with a heating rate of 4 ◦ C min−1 and then the temperature was held for 2 h. After the silver alkanoate complex completely underwent thermal decomposition, the temperature was lowered to 70 ◦ C, and then 150 mL of ethanol was added to the solution for precipitation of the silver nanoparticles. Finally, the silver nanocrystals were centrifuged and washed several times with ethanol. For the preparation of conductive nanoparticle inks, the synthesized silver nanoparticles were re-dissolved into n-tetradecane with vigorous stirring. Finally, the silver nanoparticle inks were jetted onto various substrates using a Microfab head with a single nozzle, and the patterns were sintered in a convection oven at 250 ◦ C for 30 min.
3. Results and discussion To investigate the effect of the capping ligands with regards to resulting nanoparticle properties, the chain length was changed in the range of 12–16 carbons. Previous research in the literature proposed that the longer the carbon chain of the capping molecule, the more monodispersed the nanoparticles became [10]. Although the range of carbon chain lengths is not wide in our experiments, this trend could not be observed systematically. Fig. 2 shows TEM images of silver nanoparticles prepared from different silver alkanoate precursors. As shown in Fig. 2, the nanoparticles from each sample are of spherical morphology with an average size of about 6 nm. Because the silver dodecanate complex has the lowest thermal decomposition temperature, it is more advantageous than the other
silver alkanoate complexes with regards to the sintering process. Therefore, the synthetic strategy is focused on the silver dodecanate complex in this study. Fig. 3 shows monodispersed silver nanoparticles that were prepared by thermolysis of silver dodecanate while varying the mole ratio between silver dodecanate and additional dodecanoic acid. In particular, we can obtain highly monodispersed silver nanoparticles with an average size of about 6 nm that were prepared from a reaction mixture with a 1:1 silver precursor to corresponding acid mole ratio (sample 2). However, broad particle size distributions (PSDs) can readily be observed when the mole ratio of the reaction mixture of silver precursor to corresponding acid was 2:1 (sample 1). This difference in PSDs is principally derived from the concentration of capping molecules in comparison to that of the silver ions. A sufficient concentration of capping molecules results in efficient suppression in the growth of the silver nanoparticles during the reaction such that there is a greater level of control. As shown in Fig. 4a, all the nanocrystals of sample 2 are highly monodispersed in PSD. High-resolution TEM (HRTEM) images of these nanocrystals show distinct lattice fringe patterns, indicating the highly crystalline nature of the nanocrystals. The lattice spacing obtained from the micrograph above (Fig. 3c) is 0.24 nm, which matches well to the (1 1 1) plane for the face-centered-cubic (FCC) crystal structure of bulk silver. XRD patterns for the silver precursor and silver nanocrystals (sample 1) were measured in 2 from 30◦ to 100◦ . No crystalline signature for silver in the pristine silver dodecanate complex was observed in the XRD analysis, as shown in Fig. 5a, indicating that the complex is amorphous in nature. However, the diffraction pattern that was obtained after decomposition of silver dodecanate at 210 ◦ C was identified through comparison to the standard, pure, crystalline silver structure (JCPDS card no. 04-0783). The XRD pattern for sample 1 shows characteristic peaks at 2 = 38.1◦ , 44.3◦ , 64.4◦ , 77.5◦ , and 81.5◦ , respectively marked by the indices (1 1 1), (2 0 0), (2 2 0), (3 1 1), and (2 2 2). This confirms that the resultant particles are pure silver with an FCC crystal structure. The full width at half maximum (FWHM) of the (1 1 1) reflection from the X-ray data was used to calculate the average particle size by using the Debye–Scherrer formula [11]. The calculated average particle size was about 6.5 nm. Thus, the average particle size cal-
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Fig. 2. TEM images of the silver nanocrystals stabilized with dodecanoic acid (a), myristic acid (b) and palmitic acid (c).
culated using the Debye–Scherrer formula corroborates well with the particle sizes seen in the TEM image (Fig. 3b). Thermogravimetric analyses for the samples were conducted with a 10 ◦ C min−1 ramp. Upon heating while purging with air, samples underwent rapid weight loss at around 210 ◦ C due to the decomposition of the capping molecules in the presence of the silver nanocrystals and silver ions. The TGA data corresponds to strong exothermic DTA peaks at around 210 ◦ C, and the rest of the exothermic peaks, in the temperature range of 270–310 ◦ C, are from the pyrolysis of dodecanoate and the remaining carbon residues. These thermal analyses indicate the minimum thermal decomposition temperature of the reaction mixtures. As shown in Fig. 6a, the residue weight percent of silver dodecanate is 38.9%, and it is similar in value with the theoretical mole weight percent of silver in the silver dodecanoate complex. The TGA results also yield evidence that silver dodecanoate prepared by the two-phase method is in a pure form without any significant amount of unreacted dodecanoic acid remaining. Fig. 6c shows that the residue percent of silver remaining is 85.4%, and this value corroborates well with the calculated amount of capping molecule on the surface of the nanoparticles. Fig. 7 shows the FT-IR spectrum of silver dodecanate at room temperature. In the high frequency region above 2800 cm−1 , the spectrum exhibits at least three strongly absorbing peaks at 2848 cm−1 , 2917 cm−1 , and 2956 cm−1 for the symmetric and asymmetric stretching modes of the methylene-groups and asymmetric stretching of the methyl-group, respectively. The peak at 2871 cm−1 is probably due to the symmetric stretching of the methyl-group. There are two shoulder bands at ca. 2895 cm−1 and 2964 cm−1 . The former band is from the Fermi resonance absorption due to the d+ mode and the latter one is due to the r− mode [12,13]. In the low frequency region below 1600 cm−1 , some “fingerprint” stretching modes for the carboxylate-group and three different bending modes (scissoring, rocking, and wagging modes) for the methylene-groups in silver dodecanate are observed. The absence of any peak corresponding to a carbonyl group near 1700 cm−1 is notable in providing evidence that the as prepared silver dodecanoate was synthesized in a relatively pure form without contamination by excess free acid. The FT-IR spectrum of silver nanoparticles shown in Fig. 8 gives some important information about the interaction between the surface ligands and the metal nanoparticles. This absorption spectrum pattern is similar to that of self-assembled monolayers (SAMs) of stearic acid on a bulk silver substrate [14,15]. As stated in the given references, the carboxylate-group gives rise to a symmetric stretch at about 1439 cm−1 , and the presence of this band supports that the silver metal nanoparticles are passivated by a uniform layer of dodecanoate. In Fig. 7, the as (COO− ) band is more intense than the s (COO− ) band. However, the as (COO− ) band becomes weaker than the s (COO− ) band in the FT-IR spectrum of the nanoparticles shown in Fig. 8. From this data, the weakness of the as (COO− ) band in the spectrum of nanoparticles can be interpreted as the carboxylategroup being symmetrically chemisorbed on the surfaces of silver nanoparticles via the two oxygen atoms [16]. All of these observations suggest that the polarizing nature of the silver cation in silver dodecanoate more readily holds the negative charge of the carboxylate anion to one oxygen atom at room temperature whereas the more neutrally charged, or less negatively charged, state of the silver atoms on the surfaces of the nanoparticles is less polarizing such that bidentate binding occurs with the negative charge of the carboxylate anion delocalized between the two oxygen atoms. The inset of Fig. 9 shows a stable silver nanoparticle ink sample. For preparation of an ink-jettable ink, the synthesized silver
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Fig. 3. HRTEM images of monodispersed silver nanocrystals stabilized with dodecanoic acid. (a) and (b) Sample 1; (c) and (d) sample 2.
nanoparticles were dissolved into n-tetradecane. Their solid contents and viscosity are about 20 wt% and 5 cps. The dispersed silver nanoparticles can be suspended for several weeks in the stationary state and more than 3 min under centrifugation at 3000 rpm without sedimentation.
Patterning on various substrates was achieved using an automated IJP process. As shown in Fig. 9, the nanosilver ink was jetted onto a polyimide film, photo-paper, and a silicon wafer using a Microfab head with an optimized waveform. Finally, the specific electrical resistance was about 6 cm, which was achieved by
Fig. 4. HRTEM images and electron diffraction pattern of monodispersed silver nanocrystals. HRTEM image and electron diffraction patterns reveal the highly crystalline nature of the silver nanocrystals.
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Fig. 5. X-ray diffraction patterns of silver dodecanoate (a) and silver nanoparticles (b).
Fig. 6. TGA and DTA of silver dodecanoate (a) and (b), and silver nanoparticles (c) and (d).
Fig. 7. FT-IR spectrum of silver dodecanate at room temperature.
Fig. 8. FT-IR spectrum of silver nanoparticle.
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monodispersed silver nanocrystals to be obtained in gram scale quantities with a single reaction and without the necessity of a size-selective post-processing step. Second, high yields of greater than 75% are possible when using this synthetic method. The nanoparticles were successively re-dispersed into organic solvents and also worked well when formulated into ink-jettable inks. Silver nanoparticles formulated into inks are useful as inkjettable materials because they have relatively low sintering temperatures due to depressed melting points and reasonably high conductivity measurements. Furthermore, the IJP of nanomaterials can be applied to the low-cost fabrication of electronics. Acknowledgements
Fig. 9. Conductive silver patterns on various substrates (silver nanoparticle ink in inset).
pyrolysis of the surface ligands on the nanoparticles and the ink solvent at 250 ◦ C for 30 min. 4. Conclusions Highly monodispersed 6 nm silver nanocrystals were synthesized in the high-boiling solvent 1-octadecene with silver dodecanate and dodecanoic acid. The resulting silver nanocrystals show high crystallinity such that even a lattice fringe pattern can be observed and measured within the HRTEM images. We can control the PSDs by varying the mole ratio of reaction mixture species, and the ideal mole ratio was 1:1 silver dodecanate to dodecanoic acid. Conclusively, the purity of the silver precursor and mole ratio of reaction mixture species are key factors in synthesizing monodispersed silver nanocrystals. In addition the synthetic procedures developed in this study offer several very important advantageous features over other conventional methods for the synthesis of monodispersed nanocrystals. First, this process allows
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