Sintering Metal Nanoparticle Films

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Departmental Papers (MSE) Department of Materials Science & Engineering

January 2008

Sintering Metal Nanoparticle Films

Howard Wang State University of New York

Liwei Huang State University of New York

Zhiyong Xu State University of New York

Congkang Xu State University of New York

Russell J. Composto University of Pennsylvania, [email protected]

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Recommended Citation Wang, H., Huang, L., Xu, Z., Xu, C., Composto, R. J., & Yang, Z. (2008). Sintering Metal Nanoparticle Films. Retrieved from https://repository.upenn.edu/mse_papers/154

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This paper is posted at ScholarlyCommons. https://repository.upenn.edu/mse_papers/154 For more information, please contact [email protected]. Sintering Metal Nanoparticle Films

Abstract We have carried out several measurements in order to understand the process of metal nanoparticle (MNP) film sintering. Small angle neutron scattering has been used to reveal the average diameters of silver and gold nanoparticles (Ag-NPs and Au-NPs) used in this study to be 4.6 and 3.8 nm, respectively, with a size distribution of ca. 20%. Spun- cast Ag-NP and Au-NP films have been sintered at temperature ranges of 80-160ºC and 180-210º, respectively, for various times. The resulting film composition, morphology and electric resistance have been revealed. Upon sintering, the organic content in MNP films reduces to less than 10% while the overall film thickness educesr to about the half of the as-cast film thickness. The resistance of sintered Ag-NP films can arv y over more than 7 decades depending on the sintering temperature. The conductivity of Ag-NP films sintered at 150º is 2.4 times 10-8Ωm. The transport properties are affected by both the composition and morphology of sintered films.

Keywords metal nanoparticles, ion beam analysis, printable electronics, silver nanoparticles, sintering

Author(s) Howard Wang, Liwei Huang, Zhiyong Xu, Congkang Xu, Russell J. Composto, and Zhihao Yang

This conference paper is available at ScholarlyCommons: https://repository.upenn.edu/mse_papers/154 Sintering Metal Nanoparticle Films

Howard Wang'*, Liwei Huang', Zhiyong Xul, Congkang Xul, Russell J. Composto2, Zhihao Yang3 'Department of Mechanical Engineering, Center for Advanced Microelectronics Manufacturing, Binghamton University, SUNY, Binghamton, NY 13902 2Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104 3NanoMas Technologies, Inc, Vestal, NY 13850 Email:

Abstract applying a range of characterization methods including spectroscopic, thermal, microscopic and scattering techniques, We have carried out several measurements in order to we illustrate details of structures and behaviors of MNPs in understand the process of metal nanoparticle (MNP) film both solution suspensions and deposited films. We try to gain Small neutron has been used to sintering. angle scattering fundamental understandings of nanomaterials processing in reveal the average diameters of silver and gold nanoparticles device applications. The insights in the sintering mechanism (Ag-NPs and Au-NPs) used in this study to be 4.6 and 3.8 would offer guidance in materials design and processing nm, respectively, with a size distribution of ca. 20%. Spun- cast Ag-NP and Au-NP films have been sintered at 2. Materials temperature ranges of 80 - 160 °C and 180 - 210 °C, High-quality metal nanoparticles are obtained from respectively, for various times. The resulting film NanoMas Technologies, Inc, who has developed synthesis composition, morphology and electric resistance have been routines for large scale production of silver and gold MNP revealed. Upon sintering, the organic content in MNP films with ultra-small particle size (2 to 10 nm) and narrow reduces to less than 10% while the overall film thickness distribution. MNPs are stabilized with a thin layer of reduces to about the half of the as-cast film thickness. The surfactant molecules, which also render MNPs soluble in non- resistance of sintered Ag-NP films can vary over more than 7 polar organic solvents such as cyclohexane or toluene. UV- decades depending on the sintering temperature. The conductivity of Ag-NP films sintered at 1500C is 2.4 x 10-8 Qm. The transport properties are affected by both the composition and morphology of sintered films. Keywords Keywords, Metal nanoparticles, silver nanoparticles, sintering, printable electronics, ion beam analysis

1. Introduction Fig. 1. (a) SEM micrograph of ca. 5 nm silver nanoparticles, Printable electronics manufactured using roll-to-roll and (b) TEM micrograph of ca. 4 nm gold nanoparticles. processes will offer the unique opportunities for mass production of large format and inexpensive flexible 1..0 electronics [1]. A popular choice of the printable conductor o SANS Data precursor is metal nanoparticles (MNPs), which could be .5 Core-Shell Model Fitting printed to substrates from solutions and sintered to conductive 0..5- films at low temperatures because of the size effect [2]. Particularly, silver nanoparticles (Ag-NPs) are considered by .E many the most important precursor candidate for printed 0 0..0 X conductors for the good balance of cost and performance [3]. On the other hand, gold-nanoparticles (Au-NPs) are desirable C) -0..5 because Au has better chemical stability and high work Results: ± function. There have been extensive R&D efforts in Core radius: 23 1 A Core radius 5.5A methods for mass of -1. c: developing production high quality Ag- .0 Shell 6 ± 1 A and Au-NPs and exploring their applications in fabricating thickness: flexible devices [e.g., 3-6]. However, the understanding of -2.0 -1.5 -1.0 -0.5 the sintering process from precursor MNP to conductive Log (Q) [A] metallic film is rather limited. In this paper, we report Fig. 2. Small angle neutron scattering of silver nanoparticles in measurements in order to gain better understanding and better deuterated toluene reveals an ensemble-averaged Ag core control of the MNP application to conductive film formation. diameter of 4.6 nm with 20% dispersion, which is consistent with This study is possible because high-quality and large electron microscopy observation. The thickness of the organic quantities of uniform MNPs particles become available. By shell is found be ca. 0.6 nm in solutions.

978-1-4244-2054-4/08/$25.00 2008 IEEE Energy (MeV) Energyr (MeV) visible absorption spectroscopy shows narrow absorption 0.5 l.o 1.5 2.0 lo 1.0 1.2 60 peaks centered at ca. 420 and 510 nm for Ag- and Au-NPs in 190 °C - (b) 50- (a) dilute cyclohexane solutions, respectively. X-ray diffraction detector indicates that both MNPs have the same crystalline structures N40 H 180 OC_ A ca as the bulk metals. The size and uniformity of particles are 190 °C 180 °C examined using electron microscopy and small angle neutron ' 20 D scattering (SANS). Figure 1 shows a SEM micrograph of ca. As-cast 10 Si S 5 nm Ag-NPs (a), and a TEM micrograph of ca. 4 nm Au-NPs Au S (b). SANS measurement of Ag-NP in toluene is shown in 50 100 150 200 250 300 350 400 100 150 200 250 Channel Channel Fig. 2, in which the SANS data (open circles) are fitted with a Fig. 3. (a) RBS spectra of Au-NP films showing increasing Au core-shell model to give the average Ag-NP characteristics. density upon sintering. (b) A blow-up view of the Si and S front The ensemble-averaged Ag-NP core diameter is found to be showing diminishing S signals with sintering. 4.6 +0.2 nm with a standard deviation dispersion of 1.1 rnm, or + 20%, which is consistent with electron microscopy 3.2. FRES observation. The thickness of the organic shell is found be The loss of hydrocarbon coating molecules upon sintering ca. 0.6 nm in solutions and 0.3 nm in the dried state. We is also evident from the FRES measurements. Figure 4a emphasize the importance of characterizing bulk properties shows hydrogen peaks of FRES spectra of as-cast and for achieving better performances in device applications. sintered Ag- and Au-NP films. Figure 4a shows FRES spectra of Ag-NP films after various times at 80 °C. The H- 3. Experiment signal decreases with sintering time, indicating that sintering at a temperature as low as 80 °C for a time as short as a few Films of Ag-NPs and Au-NPs were spun-cast from minutes is sufficient to drive majority of hydrocarbon away cyclohexane solutions with 10% and 500 MNP solid mass, from the film. Figure 4b shows Au-NP films sintered at in respectively. MNP films were sintered air at various various temperatures for 3 min. Apparently, the variation of temperatures for various times. As-cast and sintered films residual hydrocarbon content is sensitive only to a certain were characterized using ion beam analysis, electron microscopy, and direct-current I-V measurements. Two types Energy (MeV) Energy (MeV) 0.4 0.5 0.6 0.7 0.9 0.4 0.5 0.6 0.7 0.8 0.9 of ion beam measurements were carried out using a 10 T" 10 collimated 2.0 MeV He+ beam. The hydrogen content in the (a) b As-ast film was measured using forward recoil spectrometry (FRES), while the compositional profiles of heavier elements were measured using Rutherford backscattering spectrometry 4 4 1800 (RBS). In RBS, the incident beam was normal to the film, back scattered He ions were detected at 50 from the surface 10 min<10 C normal. In FRES, the incident beam was 750 from the surface 0 00 100 5 150 200 50 100 2 150 normal, recoiled hydrogen atoms were detected at the Channel Channel reflection direction. Forward scattered He ions were Fig. 4. The hydrogen peak in FRES spectra of (a) Ag-NP films specular sintered at 80 °C for various times, and (b) Au-NP films sintered stopped using an 8 ptm thick Mylar foil. at various temperatures for 3 min. The H-content decreases with longer sintering time or higher sintering temperature. 3.1. RBS range of sintering temperatures. Typical RBS spectra are shown in Fig. 3 for as-cast and The fraction of residual hydrogen (equivalently the annealed Au-NP films. The Au signals from the Au-NP films organic content) in sintered films is obtained from the and Si signals from the substrates are clearly visible in Fig. integrated total H counts normalized by the as-cast ones. The 3a. Upon sintering, the width of Au peaks becomes narrower time variations of residual H fraction in Ag-NP films sintered suggesting thinner Au-NP films (or more accurately smaller at 80 °C and 100 °C are shown in a semi-logarithm scale in area atomic density) and the peak height increases indicating Fig. 5a. The symbols are experimental data and the dashed a higher Au density. The effect appears more prominent at lines are guide to the eye. The organic content decreases with the higher sintering temperature. Figure 3b is a blow-up view the logarithmic sintering time, typically to below 10% after a near the Si edges, where S signals are resolved. Since S is few minutes at higher sintering temperatures. Figure 5b part of the thiol group binding the surfactant molecules to the shows the residual H fraction in Au-NP films after sintered at Au-NP surfaces, the diminishing S signal upon sintering various temperatures for 3 min. Apparently there is a indicates that the hydrocarbon coating molecules have been transition regime between 170 °C and 190 °C. The organic driven away from the film. That causes the reduced area content loss at 170 °C is ca. 10%, while it becomes more than atomic density. Consequently, the Si front moves to the 9000 at temperatures above 190 °C. This behavior is due to a higher energies because of less energy loss for He+ ions well-defined activation energy for the desorption of surfactant scattered off the Si at the substrate surface in annealed films. from Au-NP surfaces. be critical in determining the transport properties. The 1.or 1.2 - morphology is in turn sensitive to particle characteristics and 0.8- * 80 °C 1 0 *1000°C . the thermal history. .0 L) 08 L'5 0.6 The effect of sintering temperature is illustrated in Fig. 7b for Ag-NP films after 3 min sintering at various temperatures. -FU 0.4- - --- X 0 The reduction of organic materials is of one decade at low and ._ --o 0.2- high temperatures, whereas the resistance varies over more

0.0 0.0 10 1 100 0 50 100 150 200 250 than 7 decades. Furthermore, both residual H and resistance time (min) Temperature (oC) measurements indicate the somewhat optimal processing Fig. 5. The residual hydrogen analysis for (a) Ag-NP films temperatures, ca. 200 °C for Au-NPs and ca. 130 °C for Ag- sintered at 80 and 100 °C for various times, and (b) Au-NP NPs, respectively. films sintered at various temperatures for 3 min. The H-content decreases with longer sintering time or higher sintering 10° lo0U temperature. 10, (a) 107ko (b) 10

10lo 10I 10 U) 104 3.3. Morphology 0 10 'a c5 103 Typical morphologies of sintered films are illustrated in 1010o - oo 1021 o Fig. 6. Sintered Ag-NP films usually are smooth with 10' 100° D 10' isolated holes, whereas sintered Au-NP films show long lo'° 10-' l 10-' | 7 0 5 10 15 20 80 100 120 140 160 cracks. This could be understood since the volume reduction Annealing time (min) Temperature (oC) is larger in Au-NP films than in Ag-NP films. Thermal Fig. 7. The electrical resistance of Ag-NP films (a) sintered at gravimetric analysis indicates that both Ag- and Au-NPs lose various temperatures for various times, and (b) sintered at about 10% mass upon fully sintering. Since Au has higher various temperatures for 3 min. density, the volume fraction of organics is higher in Au-NP films than in Ag-NP films. Upon sintering, larger volume 4. Conclusions shrinkage in Au-NP films causes higher tendency toward We have carried out a series of measurements in order to cracking than hole formation. For device application, crack understand processes in sintering uniform metal morphology is not desirable since it would greatly deteriorate nanoparticles. The composition, morphology and electric the electrical transport properties. A simple mending resistance of sintered Ag- and Au-NP films have been approach is printing multiple layers with sequential printing revealed. Upon sintering, the organic content in MNP films and sintering of each layer. Usually a second layer could reduces to less than 10% while the overall film thickness improve the transport by 4 times to more than one order of reduces to half. The conductivity of Ag-NP films sintered at magnitude. A challenging task is to control the volume 150°C was found to be 2.4 x 10-8 Qm. The transport shrinkage to occur normal to the film surface in stead of in properties are due mostly to the morphology of sintered films, lateral directions. which could vary due to particle characteristics and ...... ,11...,..- '-...... E-.. .I..= } W '.. W. processing conditions. Acknowledgments This research was funded by the Center for Advanced Microelectronics Manufacturing at the State University of New York at Binghamton I~~~~~ ~~~~~~~~~ I iReferences 1. Crawford, G. P. Ed. Flexible Flat Panel Display, Wiley (2005) 2. Buffat, Ph., Borel, J-P., "Size Effect on the Melting Temperature of Gold Particles", Phys. Rev. A, 13, pp 2287-2298, 1976. Fig. 6. SEM micrographs of (a) Ag-NP film after 5 min 3. Gasman, L., "Silver Powders and Inks for Printable at 100 °C, and (b) Au-NP film after 6 min at 190 'C. Electronics 2007-1014", NanoMarkets report NA-3022, 2007. 3.4. Resistance 4. Park JW, Baek SG., "Thermal Behavior of Direct-Printed Electric resistance measurements on sintered Ag-NP films Lines of Silver Nanoparticles", Scripta Materialla, 55, pp are shown in Fig. 7. The time dependent resistance data as 1139-1142, 2006. shown in Fig. 7a could be compared to the corresponding residual H content data in Fig. 5a. The comparison indicates 5. Wakuda D, et. al. "Novel Method for Room Temperature that resistance of sintered film is sensitive to both the residual Sintering of Ag Nanoparticle Paste in Air",. Chem Phys. organic content and the sintering temperature. However, Lett, 441, pp 305-308, 2007. films with similar organic content at 80 °C and 100 °C can 6. Kim, D. et. al. "Ink-Jet Printing of Silver Conductive differ in electric resistance for several orders of magnitude. Tracks on Flexible Substrates", Mol. Cryst. Liq. Cryst., This observation suggests that morphology of sintered films Vol. 459, pp. 45-55, 2006