Printed electronics have been recognized as a one of emerging research field because of their attracting ability to fabricate flexible electronic devices [1-3]. Traditional electronic devices have been prepared by a broad range of substrates to induce electrically function [4,5]. Especially one of most importance is on the fabrication of electronic devices on bendable and flexible substrates compared with traditional technology [6]. Conventional approaches like electroless plating, vacuum deposition, and lithography etc. have required traditional processing with high-cost equipment under the harsh condition and then send out the toxic wastes by using unstable chemicals [7]. Compared with conventional approaches, the printed electronics have a strong point on faster, economic and environmental-friendly processing. Such strong points make an electronic system to be practical application for displays, photovoltaics, smart packaging, and sensors.

As a practical application of printed electronics, an inkjet printing requires conductive paste consisting of organic-inorganic composition, which metal, metal oxide and metal-organic complexes, have been carried for Roll-2-Roll fabrication [6,8]. Conductive pastes are generally composed of resin for mechanical and adhesive strength and conductive filler for an electrical path [7,9]. Typically, conductive nanoparticles are dispersed in the organometallic compound with conductive polymer. Physicochemical properties for the printable pattern should be considered for conductivity, optical transparency, stability to bending, and a good adhesion to substrate [6,10]. In particular, the combination of metal- and metal oxide particles and resin have become important ingredient for interconnect materials in electrically conductive pastes [6]. Because the electrical property of conductive pastes depends on the amount of solid metallic filler, a formulation by increasing metallic filler show higher conductivity, however, unfortunately, resulting in reducing the properties of paste like rheological property.

Considering above-mentioned requirements, the Ag flakes with a mixture of micro and nano-size have been widely used for good electrical contact between each other as an electrical-conductive filler [6,7]. Currently, the highly conductive Ag (σ = 6.3 x 10⁷ Ω^-1•m^-1) are a best candidate for the conductive ink in industrial applications [10]. Ag nanoparticles was filled up into the pore between Ag metal particles with micro size for a good electrical connect between each other [10]. A noble metal like Ag and Au have been studied as a conductive filler in paste formulation because they show higher conductivity and strong antioxidative stability. But, it is undesirable in practical market because of their intrinsically high cost. As such, recent literatures have reported the non-noble metal-based alloy as a conductive filler of practical pastes. Practical alternative materials should be considered in the aspect of inexpensive, highly conductive, and stable property in air.

In the current printed electronics, Cu is a good candidate of conductive materials for the shortcomings of noble metals including variation in cost and resources [11]. From the viewpoint of industrial production, Copper, Cu (σ = 5.96 x 10⁷ Ω^-1•m^-1), have recognized as a one of most promising candidates for conductive filler because their electrical properties including electrical resistivity and thermal conductivity like noble metal is properly inexpensive for the printed electronics industry [12]. Therefore, various methodologies of Cu nanomaterials have been reported to manipulate morphologies and properties by photo- and electro deposition, physiochemical treatment and wet chemical approaches etc. Given their shortcomings and advantages of methodologies, a solution-based approach has been widely chosen owing to the mild synthetic conditions, facile size control, and mass production with low-cost. However, when exposing into an air, copper nanoparticles are easily oxidized and resulting in non-conductivity. For attractive attempts to oxidation-stable copper nanoparticles, P. Zhang et al. have reported the copper nanoparticles with high stability by using bis(2-ethylhexyl)amine as the surfactant in the presence of carbon disulfide [13]. J. Li et al. investigated the Cu nanoparticles using a water/oleic acid mixed solvent [14]. The morphology of copper nanoparticles was controlled by the nuclei process and then, spherical copper nanoparticles with about 30 nm size was observed by using sodium hypophosphite and oleic acid as the reductant and the surfactant, respectively. Aqueous dodecyl sulfate solution was reported for reduction of copper ions by M.P. Pileni et. al [15]. The shape and size of final products was controlled by the critical micellar concentration.

Herein, the current work demonstrates air-stable Cu nanoparticle for oxidation using electron beam (e-beam) irradiation, not using chemical reducing agent, which is used for conductive metal filler. With the manipulation of an absorbed dose of electron beam [16], the air-stable copper nanoparticles with higher stability is prepared and formulated for conductive ink paste. The electrical conductivity of Cu conductive paste prepared under different sintering conditions is studied for elucidating the relationship between sheet resistance and volume fraction depending on heating temperature.

2.1. Preparation of Cu nanoparticles with oxidation-stability

Copper sulfate pentahydrate (CuSO₄ㆍ5H₂O, 99.9%, Aldrich) was used as metal precursor, ethylene glycol (EG, JUNSEI) and poly(N-vinylpyrrolidone, PVP, Mw = 220,000, JUNSEI) were used as a solvent and as a dispersing agent, respectively. All chemical reagents were used with a pure grade. Copper nanoparticles were prepared using electron beam irradiation in EG. Typically, PVP of 1.7 g was added in EG of 100 mL at 40 ℃. 0.2 M of copper sulfate was mixed to the prepared PVP solution in the presence of N₂ gas with stirred for 30 min. The mixture was irradiated with the different absorbed doses of electron beam at the room temperature (RT) and a synthetic process was conducted as a batch process. Under the e-beam irradiation, the precursor solution was stirred in order to diffuse the solvated electrons in the reaction mixture. The reacted solution was filtered with 10,000 rpm and treated by formic acid and dried at 40 ℃ in vacuum oven. In current work, the electron beam accelerator was carried out in the Korea Atomic Energy Research Institute (KAERI). Under the electron beam energy with 1 MeV and current of 1 mA, various absorbed doses were performed by accelerator.

2.2. Preparation of conductive paste using Cu metal filler

The as-prepared Cu nanoparticles through e-beam irradiation were used as metal fillers for producing conductive pastes. Epoxy, acrylic, polyester, and urethane resins were purchased from SK-Cytec and used as binders. Ethyl carbitol acetate, butyl carbitol acetate, triethylene glycol monoethyl ether, and terpineol were purchased from JUNSEI and used as solvents. We swelled 15 wt% of the binder in 15 wt% solvent for 30 min at a rotation of 1,100 rpm and at a revolution of 1350 rpm using a paste mixer (DAEHWA Tech. co., PDM-1KV). The solvent and epoxy binder were mixed with the same wt%. The mixture was subjected to an antifoaming process at a rotation speed of 100 rpm for 30 min and a revolution speed of 12,000 rpm using a paste mixer. Then, we added 70 wt% Cu nanoparticles and processed the paste using a three-roll mill.

2.3. Characterization

The size, morphology and the crystallographic information of final samples were characterized by transmission electron microscopy (TEM), JEOL 2200FS, which equipped with selected area electron diffraction (SAED). UV-vis analysis was carried out with a X-ma 6300PC Spectrophotometer of Human Corporation, Korea). The crystalline structure of final products was characterized by X-ray powder diffraction (XRD). The XRD patterns were recorded with a PHILIPS (Netherlands) X'Pert-MPD diffractometer using Cu Kα irradiation at 40 and 150 kV. The surface oxidation state in the final samples was analized by X-ray photoelectron spectroscopy (XPS, Multilab 2000) from Thermo Electron Corporation, England. The resistivity of the paste was measured on a 24203A Source Meter with 4-point probes.

In the initial step, a radiolysis on aqueous solution generates many species. Among these active species, the solvated electron (e_solv) and hydrogen atom (H∙) act as a strong reductive one [17]. Under irradiation of electron beam, Cu cation are generated in the CuSO₄ solution. As the irradiation dose increase for the reduction of Cu ion, an abundant amount of neutral copper (Cu₀) atom can be produced through the following reaction:

During irradiation, the Cu atoms were nucleated and aggregated continuously into cluster. And then, each copper cluster were grown from metallic copper clusters.

Based on the radiolysis mechanism of the ethylene glycol, the process of ionization and excitation of ethylene glycol generate the solvated electron, the radical and protonated ethylene glycol molecules [17]. 3 kinds of reduction species from EG allow metal cation to form metallic species.

Such reduction reaction is conveniently conducted under ambient condition. In the preparation process of nanoparticles, PVP was acted as the surfactant to manipulate the size and morphologies of the final product.

TEM images in Fig. 1 show typical morphologies of copper nanoparticles prepared by different absorbed doses of electron beam irradiation. As-prepared copper nanoparticles appear spherical shape with size range from 5 nm to 47 nm. With an irradiating 100 kGy of the absorbed dose, the size of copper nanoparticles is less than 7 nm, while the size of copper nanoparticles was 47 nm as the absorbed dose increases to 300 kGy. Microscopic observation indicates that the sizes in as-prepared copper nanoparticles increase as the absorbed dose increases. The UV-vis spectra[Fig. 1(f)] of copper nanoparticle depending on different absorbed doses show that the plasmon absorption band of copper nanoparticles was observed at around 570 nm as the irradiation increases. It indicates that metal nanoparticles were formed and stabilized by PVP after the metal atoms coalesce in the reducing process of the Cu²⁺ cations. Because their size-dependent optical properties, a red shift of plasmon absorption in the prepared copper nanoparticles shows an increase in the size of Cu nanoparticles.

Figure 1. Typical TEM images of Cu nanoparticles by e-beam irradiation with different absorbed doses (a) 100 kGy. (b) 200 kGy. (c) 300 kGy. (d) SAED pattern and (e) XRD of Cu nanoparticles by 300 kGy of irradiation. UV-vis spectra of Cu nanoparticles with different absorbed doses. (red) 100 kGy. (green) 200 kGy. (blue) 300 kGy. Scale bar is 40 nm.

The as-prepared Cu nanoparticles via e-beam irradiation with 300 kGy of the absorbed dose were characterized using SAED and XRD. The four fringes in the SAED pattern of the prepared sample correspond to the (111), (200), (220), and (311) planes in the face- centered cubic (FCC) structure of metallic Cu, thus agreeing with the XRD results. The XRD patterns of the prepared samples do not contain the peaks for copper oxide. The diffractions angles of 44.28, 50.40, 74.12, and 89.90° in the XRD patterns correspond to the diffraction angles of the FCC phase (the standard powder diffraction card, ASTM 03-1005). The XRD results show that Cu metallic nano-particles could be crystallized using e-beam irradiation without employing any chemical reducing agent and special experimental conditions such as high vacuum or high temperature.

In the current work, PVP was used as both the dispersing and capping agent. The surface oxidation state of copper nanoparticle prepared by using PVP show the copper peak (Cu2s, Cu2p_1/2, Cu2p_3/2), the carbon peak (C1s), oxygen peak (O1s) and nitrogen peak (N1s) in XPS spectra of Figure 2(a). The photoelectron peaks of copper in Figure 2(b) shows that doublet peaks centered at 933.2 and 953.0 eV, which match to Cu2p_3/2 and Cu2p_1/2 in Cu-O. The peaks centered at binding energies of 285.8 eV (Fig. 2(c)), 531.8 eV (Fig. 2(d)), and 400.2 eV (Fig. 2(e)) show C1s, O1s and N1s, respectively, indicating C-C bonding, O 1s from the carboxyl C=O bonding and C-N bonding. The strong interaction in the PVP enclosed the surface of the copper nanoparticle indicates that chemical adsorption between the metal core and an oxygen atom in the PVP as shown in Fig. 2(f). Figure 3 shows the XRD patterns of copper nanoparticles after exposing to an ambient condition for 75 days. XRD pattern of samples depending on exposing period did not show the peak related oxides even exposed to 75 days. It shows that the prepared Cu nanoparticles are air-stable. This result reveals that the presence of Mw 220,000 of PVP induce Cu nanoparticles to have good oxidation stability from oxidation in air during the nucleation and growth step even the thin layer of oxide would be form on the surface of Cu nanoparticles.

Figure 2. (a) XPS spectra of prepared Cu nanoparticles by 300 kGy of irradiation. (b) Cu2p, (c) C1s, (d) O1s, (e) N1s, and (f) illustration of prepared copper nanoparticles with air-stable.

Figure 3. X-ray diffraction patterns of copper nanoparticles depending on aging time.

The oxidation properties and electrical conductivity of conductive Cu paste prepared through the e-beam irradiation were studied under different sintering conditions in Fig. 4.. A copper paste was printed on to a glass using the screen printing, and sintered under air, nitrogen gas, and formic acid. Figure 4 shows the changes in the color of the copper paste under the three different sintering atmospheres at 200 °C for 30 min. The copper paste by heat-treated under air and N₂ gas was changed to a blue color from original a reddish brown color. It is expected that Cu paste would be oxidized during heat-treatment process based on color change of original paste. However, the copper paste by heat-treated under a formic acid atmosphere shows original reddish-brown color with Cu. SEM images of each samples was compared for morphologies depending on sintering conditions. SEM image of heat-treated paste on glass under air condition shows typical morphology of aggregated nanoparticles. Even heat-treated samples under N₂ condition was aggregated with smaller particles. Interestingly, porous thin film was prepared in Cu paste film on glass after heat-treatment under formic acid. In addition, the Cu nanoparticles were well connected to each other after the heat treatment in a formic acid atmosphere. In this work, formic acid has a role of reducing atmosphere following reaction.

Figure 4. Photograph and top-viewed SEM image of sintered copper paste under different atmosphere.

As such, unexpected minor oxides generated during formulation process of conductive paste could be reduced. Figure 5 shows the XRD patterns of the heat-treated copper paste under the different atmosphere. The metallic copper particles were transformed into the copper oxide (Cu₂O) when the heat treatment was conducted in air or under N₂ gas. On the other hand, the Cu nanoparticles were not oxidized under the annealing process in formic acid. Therefore, formic acid is a useful material to prevent the oxidation and conduct the reducing under the sintering process.

Figure 5. The comparison of typical XRD pattern of (a) as-prepared and (b) sintered copper paste under air, (c) N₂ and (d) Formic acid atmosphere.

Figure 6(a) shows the sheet resistance of a sintered copper paste under the different atmosphere compositions, sintering temperatures, and sintering times. It has well-reported that the sheet resistance is increased as temperature and duration for heat-treatment increases because oxidation would be occurred on the metal surface. Such relationship between sheet resistance and sintering condition appear in our result. Sheet resistance of typical 3 kinds of paste increased as heating temperature and time increased. The Cu paste with heat-treated under air and N₂ show over 1.39 MΩ∙cm² of sheet resistance. However, when copper paste was sintered for 15 min at 200 ℃ under the formic acid, the sheet resistance was recorded to 5.95 Ω∙cm². Even sheet resistance was consistently increased as heating temperature was over at 250 ℃, it is worth noted that Cu paste with such sheet resistance is sufficiently to replace conventional conductive silver paste. The volume fractions of metal and metal oxides in the prepared conductive paste by treated under the formic acid were determined to evaluate the quantity of oxidation as shown in Figure 6(b). As relationship between sheet resistance and heat condition, the volume fraction and crystalline domain size of Cu₂O, oxides phase increased with increasing temperature and duration. At a higher sintering temperature, the Cu nanoparticles aggregated with each other and a crystal growth appeared in the matrix, while the amount of oxidation of the Cu nanoparticles simultaneously increased in the Cu₂O crystalline state. Together with the tendency of the sheet resistance on heat-treatment, it indicates that sheet resistance is increased as the Cu₂O volume fraction and crystalline domain size increase.

Figure 6. (a) The sheet resistance of sintered copper paste under different atmosphere, sintering temperatures, and duration. Atomic structure (blue ball; Cu atom, red ball; O atom) of Cu metal (black square) and Cu₂O (red square). (b) Volume fraction and crystalline domain size of Cu paste according to the calcination temperature.

In this work, air-stable Cu nanoparticles were prepared through e-beam irradiation without a chemical reductant. This method is suitable for mass production of nanoparticles owing to its simplicity and fast reaction time. The sizes of the Cu nanoparticles could be changed by adjusting the absorbed dose of the e-beam irradiation. The oxidation state of the Cu surface was confirmed through XPS analysis. The XPS results revealed that PVP enabled the synthesis of air-stable Cu nanoparticles capable of maintaining oxidative stability even after exposure to air for 75 days. The as-prepared Cu nanoparticles were used as fillers for preparing conductive pastes. Among the Cu con-ductive pastes coated on glass substrates under various heat treatment conditions and atmospheres, the paste prepared through heat treatment under formic acid formed a porous, thin film on the glass substrate with well-connected particles. Such structural features show that the prepared Cu conductive paste can replace conventional conductive pastes prepared from noble metals based on the relationship between the sheet resistance, Cu₂O volume fractions, and heat treatment conditions.