Thermochimica Acta 542 (2012) 52–56 Contents lists available at SciVerse ScienceDirect Thermochimica Acta journa l h o me page: www.elsevier.com/locate/tca Application of the specific thermal properties of Ag nanoparticles to high-resolution metal patterning Yong Son a , Junyeob Yeo a , Cheol Woo Ha a , Jinhwan Lee a , Sukjoon Hong a , Koo Hyun Nam b , Dong-Yol Yang a,∗ , Seung Hwan Ko a,∗ a School of Mechanical Engineering & Aerospace System, Korea Advanced Institute of Science and Technology, 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea b Department of Physics, Ewha Womans University, Daehyun-dong 11-1, Seodaemun-gu, Seoul 120-750, Republic of Korea a r t i c l e i n f o Article history: Available online 15 March 2012 Keywords: Silver nanoparticles Low-melting temperature Thermal property In situ TEM TGA–DSC Direct laser sintering process a b s t r a c t Metal nanoparticles exhibit specific electronic, chemical and optical properties due to the thermodynamic size effect, which cannot be observed in bulk materials. Ag NPs show size dependent melting temperature depression phenomena. In this study, the thermal sintering behavior of the self-assembled monolayer protected Ag NPs has been observed using in situ transmission electron microscopy. The thermal charac- teristics of the Ag NPs have also been examined with a thermogravimetric analysis, a differential scanning calorimetry and a thermal conductivity measurement. These assessments have shown that the melting of the Ag NPs starts at 150 ◦ C, which is much lower than the melting temperature of bulk silver (960 ◦ C). The measured thermal conductivity of the Ag NPs (0.37 W/(m K)) is also lower than that of bulk silver (429 W/(m K)). These specific thermal properties of the Ag NPs can be applied to a low-temperature and a high-resolution direct-metal patterning process. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Recently, metal nanoparticles (NPs) have attracted a consider- able amount of research interest due to their remarkable electronic, chemical and optical properties [1,2]. One of the most interesting properties is their low melting temperature due to thermodynamic size effect. The low melting temperature nanomaterial ink enabled the development of several solution processible direct patterning processes such as screen printing [3], inkjet printing [4–7], nanoim- printing [8,9], and the selective laser sintering of metal NPs for the applications in the low temperature direct-metal patterning. This new solution processible direct metal patterning processes have been studied as an alternative to the conventional metal vacuum deposition which is usually expensive, time consuming, environ- mentally not friendly. In previous research, metal patterns in uniform shapes were fabricated by inkjet printing or continuous wave (CW) laser irradi- ation processes. The smallest critical feature size is one of the most important components to realize the high performance electronics. However, the minimum feature size through an inkjet is limited to the micro-scale size due to the jetting instability and nozzle size, and it is very difficult to demonstrate sub-wavelength-scale pat- terns when using CW laser irradiation due to the large heat diffusion ∗ Corresponding author. E-mail addresses: dyyang@kaist.ac.kr (D.-Y. Yang), maxko@kaist.ac.kr (S.H. Ko). that occurs during that process [10–12]. While solution processi- ble deposition and patterning process themselves have been well studied, more fundamental understanding on how each individual metal nanoparticle behaves during the developed process lacks. In this study, uniform Ag NPs were synthesized and their ther- mal properties along with the in situ direct TEM (transmission electron microscope) observation were examined. These direct experimental observations will provide valuable fundamental basic data to better understand the nanoparticle based solution based deposition and ultimately provide the feasibility of the fabrication of sub-wavelength-scale metal patterns using a direct femtosecond laser sintering process. 2. Experimental Ag NPs were synthesized from a two-phase method which was modified from a process reported by Kogel and Fitzmaurice [13]. Aqueous silver ions (AgNO 3 ) were added to toluene containing a long-chain alkylammonium surfactant to form the two-phase system. 0.2 M tetroactylammonium bromide ((C 8 H 17 ) 4 NBr) was mixed with 20.4 mL of toluene, followed by the addition of 30 mM AgNO 3 solution in 30 mL deionized (DI) water. After vigorous stir- ring for 1 h, silver ions (Ag + ) were transferred to an organic phase, while the aqueous phase was removed. While the mixture was stirred, 0.16 g of dodecanethiol (C 12 H 25 SH) was added as a capping agent. After 15 min, 0.43 M aqueous sodium borohydride (NaBH 4 ) mixed in 24 mL DI water was added rapidly (∼10 s) to the organic 0040-6031/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.tca.2012.03.004