Synergetic Effect of Blended Alkylamines for Copper Complex Ink To

Hence, copper MOD ink has great potential for application in printing electronics. ... heated in a vacuum oven (DZF-6020, Honghua Instrument Co., Ltd...
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Synergetic Effect of Blended Alkylamines for Copper Complex Ink To Form Conductive Copper Films Wen Xu and Tao Wang* State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China S Supporting Information *

ABSTRACT: Cu(II) complex ink consisting of copper formate (Cuf) and a primary alkylamine could yield highly conductive copper films at low heating temperatures without a reducing atmosphere. A synergetic effect of the blended alkylamines on the formation of conductive films was observed. It was found that blending two types of amines with different alkyl chain lengths as ligands could improve the conductivity of copper films, compared with using one of these amines alone. The decomposition mechanism of the Cuf−amine complex and the role of amines with different alkyl chain lengths were investigated. It was found that the decrease in the decomposition temperature and the formation of copper films were attributed to the activating effect and capping effect of the amine, and these two effects were dependent on the alkyl chain length. The relative intensity of the dual effects determined the decomposition rate of the complex and the nucleation and growth of particles. The use of blended amines with different alkyl chain lengths as ligands could balance the two effects and lead to appropriate nucleation and growth rates, so that densely packed copper films with low resistivity could be obtained at low heating temperature in a short time. The Cuf−butylamine−octylamine (Cuf−butyl−octyl) ink with 1:1 molar ratio of the amines showed the best performance. The understanding of the synergetic effect could provide guidance to the design of copper complex inks to control the morphology of the films.

1. INTRODUCTION Printed electronic technology is a burgeoning method in the electronics industry. Compared with the conventional photolithography/etching techniques, the printing technique is more convenient and environment-friendly, and shows advantages in flexible, wearable, large-area, and low-cost electronics fabrication.1,2 The core issue in the practical application of printed electronic technology lies in the development of conductive ink, which should be stable at room temperature, easily prepared, and meet the requirements of inkjet printing or screen printing. Currently, silver-based ink is commercialized and applied for the fabrication of simple electronics such as conductive circuits,3 radio frequency identification (RFID) tags,4 and sensors.5 However, the high cost and the low electromigration resistance of silver limit the large-scale use of silver-based ink. Bulk copper is a kind of cheap material with a low resistivity of 1.72 μΩ·cm, slightly higher than that of bulk silver (1.59 μΩ·cm), and has higher electromigration resistance.6 Owing to these advantages, copper-based ink has gained great attention in recent years. An increasing number of studies on nanocopper ink7−10 and copper MOD (metal organic decomposition) ink6,11−21 have been published. Copper MOD ink can be easily synthesized, avoiding the complicated preparation of nanoparticles, and it overcomes the drawbacks of nanocopper, such as oxidation and aggregation. © XXXX American Chemical Society

Hence, copper MOD ink has great potential for application in printing electronics. The main requirement for MOD ink is the formation of highly conductive films at low heating temperature. Copper formate (Cuf) is a common choice of metal source because it can form metallic copper with minimal organic residues at relatively low decomposition temperature (below 250 °C).18 However, Cuf cannot dissolve in organic solvents that are commonly used in printing process,19,20 and its decomposition temperature is too high for some heat-sensitive plastic substrates [polyethylene naphthalate(PEN) and polyethylene terephthalate (PET)].15,20 It is found that the transformation of Cuf into a complex with ligands can lower its decomposition temperature and improve the solubility in organic solvents. Currently, the ligands that are studied include ammonia,11,12 alkylamines,13−17 alkylol amines,6,18−20 and pyridines.21 Alkylamines, especially hexylamine and octylamine, are mostly used because they can yield dense copper films with low resistivity below 200 °C. Octylamine and its isomer are also used as cocomplexing agents to improve the conductivity of films. Shin et al. introduced octylamine to Cuf−AMP (2-amino-2-methylReceived: October 8, 2016 Revised: December 14, 2016 Published: December 20, 2016 A

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Langmuir 1-propanol) complex ink and got films with better physical contact among particles.19 Paquet et al. found that bis(3-butylpyridine) Cuf had poor film formation properties, whereas the combination of 3-butyl-pyridine and 2-ethyl-1-hexylamine strongly enhanced the conductivity of copper films.21 Yabuki17 reported the enhancement effect of octylamine in Cuf− alkylamine ink. Although these works17,19,21 reported that octylamine or its isomer could be used as the cocomplexing agent to lead the good control of particle morphology, they demonstrated phenomenological results and could not give explanations on how the combination of two different types of ligands could further promote the conductivity of films. To develop the copper MOD ink, the understanding of the thermal decomposition mechanism of copper complexes together with the formation of the copper film is important. Choi and Hong illustrated the roles of hexylamine during the thermal decomposition of the complex and mainly focused on the effects of the amine concentration on film formation.16 Yabuki found that the chain length of the amine coordinated to Cuf influenced the film structure and conductivity.17 Nevertheless, the effect of the molecular structure of alkylamine on the formation of the copper film has not been clarified systematically. In this work, the synergetic effect of blended alkylamines for copper complex ink to form highly conductive films was observed. It was found that blending two types of amines with different alkyl chain lengths as ligands could improve the conductivity of copper films, compared with using one of these amines alone. We investigated the thermal decomposition mechanism of Cuf coordinated by alkylamines with different chain lengths in detail, and elaborated the effects of the alkyl chain length on the nucleation and growth rates of copper particles, which determined the film formation. Based on the thermal decomposition investigations of Cuf−alkyamine and the effects of the alkyl chain length on the formation of the copper film, the mechanism of the synergetic effect was clarified. A better understanding of the synergetic effect could provide guidance to the design of copper complex inks.

diffraction (XRD, D8 Advance, Bruker) using Cu Kα radiation (λ = 1.5406 Å). A Fourier transform infrared (FTIR) spectrometer (Tensor 27, Bruker) was used to analyze the chemical structures of Cuf and copper complexes. The thermal decomposition behaviors of Cuf and the copper complexes were investigated using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC, Netzsch, STA 409PC) at the heating rate of 5 °C/min under a N2 atmosphere. To study the decomposition products during the heating process, TGA (Netzsch X70) coupled with mass spectrometry (MS, NETZSCH QMS 403 C) and FTIR spectrometry (VERTEX 70v) was applied. Transfer lines between the thermogravimetric instrument and the MS and FTIR spectrometer were heated to 200 °C. The chemical state of copper films was investigated using X-ray photoelectron spectroscopy (XPS, Esca Lab 250Xi, Thermo scientific) with Al Kα radiation, and the energy calibration was achieved by setting the C 1s line to 284.8 eV. The surface and cross-sectional morphology of the films were observed using field emission scanning electron microscopy (SEM, JEOL, JSM 7401). The volume resistivity of the copper film was calculated from the sheet resistance together with the thickness of the film. The sheet resistance of the film was measured using a 4-point probe (RTS-9, 4 Probes Tech). The thickness of the film was observed from the cross-sectional image from SEM. Because the densities of the films from complexes with different alkyl chain lengths were different, the film thickness ranged from 5 to 15 μm. An element analyzer (Vario EL III) was used to analyze the organic residues of the copper films.

3. RESULTS AND DISCUSSION 3.1. Resistivity and Morphology of the Copper Films from Different Complex Inks. Copper formate was chosen as the copper source for its self-reducible reaction. The complex could be easily formed by mixing Cuf and amine Cu(HCOO)2 + 2R‐NH 2 → Cu(R‐NH 2)2 (HCOO)2 (1)

The resistivity of the copper films heated at 200 °C from different complex inks is displayed in Figure 1. It was found that for complexes coordinated by one type of alkylamine alone, the resistivity of the yielded films decreased as the chain length increased, and Cuf−octyl yielded a film with relatively low resistivity of 8.32 μΩ·cm, which is 4.84 times the resistivity of bulk copper. However, the films from complexes coordinated by two types of amines with different alkyl chain lengths

2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials. Copper formate tatrahydrate (98%) was purchased from Alfa Aesar. Propylamine (99.5%), butylamine (99.5%), amylamine (99%), hexylamine (99%), and octylamine (99%) were purchased from Aladdin. Ethanol (99.7%) was purchased from Tong Guang Fine Chemicals Company. To be brief, the amine mentioned above is abbreviated as propyl, butyl, amyl, hexyl, and octyl in the following section. Copper formate tetrahydrate was dried at 0.001 MPa, 90 °C for 5 h to get anhydrated copper formate. Other chemicals were used as received without further purification. 2.2. Synthesis of Copper Complex Ink and Conductive Film. The Cuf−amine complex ink was prepared as follows. First, the amine was dissolved in ethanol. Then anhydrated copper formate was added into the amine solution and stirred at 30 °C for 30 min, forming the deep blue copper complex ink. The molar ratio of Cuf, amine, and ethanol was fixed as 1:2:6.5. As for the blended amine complex ink, two kinds of amine were added equimolarly. To obtain the conductive copper film, 400 μL of ink was dropped onto the glass substrate (2.5 × 2.5 cm) and spread out on the whole surface with a pipette. Then the ink-coated substrate was dried to remove the ethanol and heated in a vacuum oven (DZF-6020, Honghua Instrument Co., Ltd.) at 0.001 MPa. 2.3. Characterization. The copper complex was obtained by drying the ink at 0.001 MPa, 45 °C for 8 h. The crystalline structures of the coated films and the complexes were identified using X-ray

Figure 1. Resistivity of copper films heated at 200 °C for 40 min from different Cuf−amine complex inks. B

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Figure 2. SEM images of copper films from different Cuf−amine complex inks heated at 200 °C for 40 min. (a) Cuf−propyl, (b) Cuf−butyl, (c) Cuf−amyl, (d) Cuf−hexyl, (e) Cuf−octyl, and (f) Cuf−butyl−octyl.

3.2. Decomposition of Cuf−Amine Complexes with Different Alkyl Chain Lengths. To figure out the mechanism of the synergetic effect of the blended alkylamines on the formation of copper films, the understanding of the decomposition of Cuf−amine complexes with different alkyl chain lengths is essential. The chemical structures of the Cuf−amine complexes with different alkyl chain lengths were investigated using FTIR spectrometry. For the sake of convenience, only the FTIR spectra of Cuf−butyl, Cuf−hexyl, and Cuf−octyl are demonstrated in Figure 3a. The spectra of Cuf−propyl and Cuf−amyl are shown in Figure S2. We found that all complexes had similar structures (Figure 3a), which showed the features of both Cuf and amine. No peaks related to ethanol were detected, proving that the solvent had been removed in the drying process. The peaks around 3250 and 3150 cm−1 belonged to the stretching vibration of N−H. It should be noted that these two peaks in complexes were red-shifted by 100 cm−1 compared with the pure amine (Figure 3b), indicating the bonding of amino to Cu(II).16,22 The C−H stretching vibration of complexes appearing between 2950 and 2840 cm−1 was stronger than that of Cuf. The double sharp peaks around 1570 and 1390 cm−1 represent the asymmetric and symmetric stretching vibration of carboxylate, respectively. The deformation vibration of N−H appeared at 1460 cm−1. A series of small peaks between 1200 and 1020 cm−1 were attributed to the stretching vibration of C−N. The similar structures implied that the decomposition of Cuf−amines had a similar mechanism. The decompositions of Cuf−amine complexes and Cuf were investigated using TGA (Figure 4). It was found that the existence of amine could promote the complete decomposition of Cuf, decreasing the final decomposition temperature by 45− 75 °C. With the increase of the alkyl chain length of amine, the final decomposition temperature of the copper amine complex tended to elevate. The decomposition residues of Cuf−amines and Cuf were metallic copper, identified using XRD patterns. From Figure 2, it could be found that the structure of the film was related to the alkyl chain length of the amine. In fact, the size of particles was associated with the decomposition rate of the complex. It could be seen from DTGA in Figure 4b that the decomposition of the copper−amine complex went through two or three stages (two stages for Cuf−propyl and Cuf−butyl, and three stages for Cuf−amyl, Cuf−hexyl, and Cuf−octyl), along with the nucleation and growth of copper. The transition

presented lower resistivity than that from complexes coordinated with one type of these amines alone, which indicated that the mix of alkylamines with a long chain and a short chain as ligands could improve the conductivity of the copper films obtained. Notably, the film from Cuf−butyl−octyl showed the lowest resistivity of 4.28 μΩ·cm, which is only 2.49 times the resistivity of bulk copper. When we changed the heating temperature to 160 and 180 °C, the films derived from Cuf− butyl−octyl still showed the best conductivity compared with other complexes with one type of amine alone (shown in Figure S1). Obviously, there was a synergetic effect of blended amines with different alkyl chain lengths for copper complex inks on the formation of copper films. The SEM images of copper films heated at 200 °C for 40 min from copper complex inks coordinated with different alkylamines are shown in Figure 2. Films derived from Cuf−propyl and Cuf−butyl consisted of small particles (50−150 nm) piling in large particles (500−1000 nm) that had poor physical contact between particles, resulting in a loose structure with high resistivity. As the alkyl chain length increased, the particle size decreased and tended to be uniform, forming copper films with dense structures and low resistivities. The copper film obtained from Cuf−octyl contained particles in a narrow size distribution (150−250 nm), resulting in better conductivity than that from other complexes with one type of ligand. However gaps and voids still existed in the film. The film from Cuf−butyl−octyl contained closely contacted particles. The smaller particles (80−120 nm) attaching to the bigger particles (200−250 nm) could fill the gaps and intensify the contact between particles, forming denser structures than that from Cuf−octyl. The decrease in the amount of octylamine also reduced the organic residues of the films at the same heating temperature, as shown in Table 1. A higher film density and less organic residues contributed to the improvement of the conductivity of the copper film. Table 1. Elemental Analysis of the Copper Films Derived from Various Cuf−Amine Complex Inks Heated at 200 °C for 40 min sample

carbon (wt %)

hydrogen (wt %)

nitrogen (wt %)

total (wt %)

Cuf−butyl Cuf−octyl Cuf−butyl−octyl

0.2530 0.4647 0.4075

0.2275 0.2817 0.2200

0.0915 0.1157 0.0795

0.5720 0.8621 0.7070 C

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Figure 4. (a) TGA and (b) differential TGA (DTGA) of Cuf and different Cuf−amine complexes in N2. The different stages of the decomposition of copper−amine complexes are marked by dash lines. Figure 3. (a) FTIR spectra of Cuf−amine complexes. (b) FTIR spectra comparison of Cuf, Cuf−octyl, and octylamine.

shown in Figure 5a1−a2. Neither MS nor FTIR detected any CO2, H2O, H2, or amine in this period. In Figure 5a1−a2, the first decomposition stage that occurred from 100 to 120 °C was the exothermic process. No amine but CO2 (MS: m/z = 44, 28, 12. FTIR: sharp peaks near 2350 cm−1) and H2O (MS: m/z = 18. FTIR: small band near 3750 cm−1) were observed using MS and FTIR, as shown in Figure 5b1−b2,c1−c2. All of these indicate that the carboxyl dissociated from Cuf first, leading to the reduction of Cu(II). The TG−DSC plot of Cuf in Figure 5a3 suggests that the carboxyl dissociation together with copper nucleation releases heat, which conforms to the thermal behaviors of Cuf−amines between 100 and 120 °C. As the temperature went up, the reaction entered into the second stage (endothermic process). Peaks related to amine (peaks at 3250 and 3150 cm−1) were observed using FTIR in Figure 5c1−c2, representing the dissociation of amine from Cuf complexes. The release of the decomposition products of Cuf− butyl tailed off around 155 °C, which was almost 30 °C lower than that of Cuf−octyl. The fragment (m/z = 30, CH2NH2+) of amine only occurred around 175 °C, seen in Figure 4b2, corresponding to the third decomposition stage of Cuf−octyl, which should be the release of octylamine (boiling point: 175− 177 °C) with heat absorption. However, this fragment (m/z = 30) was absent during the decomposition of Cuf−butyl, as shown in Figure 5b1. It was inferred that the volatile products during the second stage, namely the simultaneous dissociation of carboxyl and amine, might be in other complicated forms different from the single CO2 or amine. Only when the

of the first stage and the second stage occurred at 120 °C. The first stage occurred from 100−120 °C for all complexes, but the decomposition rate slowed down in the first stage with the increase in the alkyl chain length of the amine. For complexes with a short alkyl chain, the decomposition rate reached the maximum at 120 °C and then slowed down. For complexes with a long alkyl chain, the decomposition rate reached the extreme point at 120 °C in the first stage, but the maximum rate occurred in the second stage. Thus, the mass loss in the first stage of the complexes with a short chain was greater than that of the complexes with a long chain. Cuf−butyl and Cuf−octyl were taken as examples for complexes with a short alkyl chain and a long alkyl chain, respectively, to study the reaction mechanism. The thermal effects of Cuf−amine complexes and Cuf investigated using thermogravimetry (TG)−DSC are displayed in Figure 5a1−a3. TG−MS−FTIR was applied to analyze the gaseous products, as shown in Figure 5b1−b3,c1−c3. We found that the decomposition of Cuf−amine complexes was accompanied by a series of endothermic and exothermic processes, different from that of Cuf that only went through the exothermic process, as shown in Figure 5a 3 . 23 The main gaseous products of the decomposition of the complex including CO2, H2O, H2, and amine did not emerge concurrently, and CO2 was generated first from 100 °C. Endothermic peaks appeared before 100 °C, whereas no obvious change in mass loss was observed, as D

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Figure 5. TG−DSC−MS−FTIR analysis of the decomposition of Cuf−butyl (a1, b1, c1), Cuf−octyl (a2, b2, c2), and Cuf (a3, b3, c3).

Figure 6. (a) Optical images of the Cuf−butyl complex at RT, 110, and 120 °C. (b) XRD patterns of the Cuf−butyl complex at RT, 110, and 120 °C. (c) Partial magnification of the XRD pattern of Cuf−butyl at 110 °C.

decomposition of carboxyl was finished, could the characteristic fragment (m/z = 30, CH2NH2+) of amine be detected, which implied that the reduction of copper was completed in the

second stage. For Cuf−octyl, the growth of the copper particles was accompanied by the release of the undecomposed octylamine in the third decomposition stage. E

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Figure 7. SEM images of copper films from different Cuf−amine complex inks heated for 10 min at 120, 140, 160, and 180 °C. (a1−a4) for Cuf− butyl, (b1−b4) for Cuf−octyl, and (c1−c4) for Cuf−butyl−octyl. The corresponding particle size distributions are displayed in (d−f).

During the thermal decomposition, the color of the copper complex changed from blue to green, and finally to the color of

copper (Figure 6a). In Figure 6b, the XRD patterns of the Cuf−butyl complex at room temperature (RT), 110, and 120 F

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Langmuir °C are illustrated to track the reaction along with the color change. Compared to Cuf, it was found that the XRD pattern of the Cuf−butyl complex at RT only showed strong peaks below 15°, which indicated the coordination of amine to Cuf. When heated at 110 °C for 10 min, peaks related to Cu2O and Cu were detected (Figure 6c), whereas the structure of amine bonded to Cuf (peaks below 15°) still existed, as shown in Figure 6b. At 120 °C, only metallic copper was detected in the film, indicating the collapse of the Cuf−amine structure. The corresponding XRD patterns of Cuf−octyl and Cuf−butyl− octyl are demonstrated in Figure S4, and they indicate the same behaviors. It could be concluded that the existence of amine activated the dissociation of carboxyl beginning at 100 °C and led to the reduction of Cu2+ to Cu0. The bond between the amine and Cuf weakened during this period. The newly yielded copper nuclei were covered by the free amine and incompletely decomposed Cuf. When the heating temperature was elevated to 120 °C, the amine started to dissociate from Cuf, and the thermal behavior of the reaction changed from exothermic to endothermic. It was reported that amines could not only assist in reducing Cuf but also cap the copper particles and restrain their growth.16 The reason why the decomposition of Cuf could be promoted by amines was that amines could attack the carboxyl and transfer the precursor into active atomic monomers.24,25 The endothermic process under 100 °C should be the sign for the activation of the precursor. The decomposition of carboxyl breaking out from 100−120 °C marked the nucleation of copper. Compared with Figure 5a1−a2, it was found that the complex with a short alkyl chain (Cuf−butyl) had a higher enthalpy than the complex with a long alkyl chain (Cuf−octyl) during this exothermic process. In Figure 4b, the decomposition rate of the complex with a short alkyl chain from 100 to 120 °C was faster than the complex with a long alkyl chain, which implied that the nucleation of copper was more adequate. During the following endothermic process, the amines that remained in the films acted as the capping agent, hindering the subsequent growth of copper nuclei. The amines with a short alkyl chain as ligands showed stronger activating effect, bringing about more nuclei at low temperature, and were easier to release, leaving less organic residues in copper films. On the other hand, the capping effect of short chain amines was not strong enough to block the aggregation of nanoparticles, resulting in a porous structure with wide particle size distribution. As the alkyl chain length of amine increased, the activating effect declined, whereas the capping effect was enhanced. Amines with a long alkyl chain as ligands could restrain the growth of copper particles and yield films with narrow particle size distributions, although their relatively high boiling points made them harder to release, resulting in a higher decomposition temperature. 3.3. Synergetic Effect of Blended Amines. The decomposition of Cuf−butyl−octyl and the formation of copper films were observed to figure out why blended-amine complexes showed better performance. The surface morphology of the films and the particle size distributions from Cuf− butyl−octyl at 120, 140, 160, and 180 °C for 10 min is shown in Figure 7, together with that of Cuf−butyl and Cuf−octyl. The corresponding resistivity is displayed in Figure S6, and it can be seen that the film from Cuf−butyl−octyl showed the lowest resistivity. In Figure 8, it is shown that the decomposition of Cuf−butyl−octyl also went through three stages like Cuf−octyl, but the TG−DSC plot of Cuf−butyl−

Figure 8. (a) TG/DSC analysis of Cuf−butyl−octyl. (b) DTGA of Cuf−butyl, Cuf−octyl, and Cuf−butyl−octyl. The decomposition stages are marked by dash lines.

octyl was not the simple superposition of that of Cuf−butyl and Cuf−octyl. From Figures 5a1−a2 and 8a, it can be found that the enthalpy of Cuf−butyl−octyl in the first decomposition stage from 100−120 °C was the highest in comparison with Cuf−butyl and Cuf−octyl. The weight loss in the first stage for Cuf−butyl, Cuf−octyl, and Cuf−butyl−octyl was 18.40, 9.12, and 16.53%, respectively, which was attributed to the dissociation of carboxyl. The weight ratio of carboxyl in Cuf− butyl, Cuf−octyl, and Cuf−butyl−octyl was 30.02, 21.85, and 25.29%, so the calculated decomposition ratio of carboxyl for these three complexes in this stage was 61.29, 41.74, and 65.36%, respectively, which signified that the copper nucleation from Cuf−butyl−octyl was accelerated compared with Cuf− butyl and Cuf−octyl. Besides, the blended amines as ligands also showed a good ability for morphology control. As shown in Figure 7, the film that formed at 120 °C from Cuf−butyl consisted of lots of big copper particles in a wide size ranging from 50 to 600 nm, and the particle size distribution was extended to 50−1200 nm for the film formed at 180 °C. By contrast, the film from Cuf−octyl consisted of more uniform particles distributing mainly in 50−150 nm during the heat treatment. It was indicated that both the nucleation and growth of particles generated from complexes with a short alkyl chain were faster than that from complexes with a long alkyl chain. The strong activating effect of butylamine induced the fast decomposition of Cuf−butyl in the first stage and led to high nucleation rate, but the drastic growth of the nuclei decreased G

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Langmuir the concentration of the Cu monomers sharply, yielding the loose-structure film that consisted of particles with wide size distribution due to Ostwald ripening. On the contrary, the slow decomposition of Cuf−octyl in the first stage brought about a low nucleation rate, and the growth of the particle was confined because of the strong capping effect. Cuf−butyl−octyl took the advantages of both butylamine and octylamine. Because of the synergism of blended amines, the copper nucleation from Cuf− butyl−octyl was adjusted to an adequate rate in the first stage, as well as the delayed growth of the particle because of the existence of octylamine. Thus, the structure of the film from Cuf−butyl−octyl had a mean particle size smaller than that from Cuf−butyl but larger than that from Cuf−octyl, as shown in Figure 7d−f. The particle size distribution in the film from Cuf−butyl−octyl was not as uniform as that from Cuf−octyl, but a more densely packed structure with the smaller particles filling the space between bigger particles could be obtained because of the balance of the high nucleation rate and the weakened Ostwald ripening effect. Besides, the replacement of partial octylamine by butylamine also reduced the organic residues of the films at the same heating temperature. A higher film density and less organic residues contributed to the improvement of the conductivity of the films. To find out the appropriate blending ratio, we changed the blending ratio and set the molar ratio of Cuf and amines to 1:2. The resistivity of the films heated at 200 °C for 40 min from Cuf−butyl−octyl with different molar fractions of octylamine is shown in Figure 9. It was found that the blend of 50 mol %

Figure 10. Resistivity of the films heated at 160 and 180 °C for different times for Cuf−butyl−octyl with 50 mol % octylamine.

at 160 °C for 20 min was 22.94 and 21.40 μΩ·cm, respectively. These results demonstrate that Cuf−butyl−octyl could yield highly conductive copper films on both flexible and rigid substrates at relatively low temperature in short time, which was suitable for printed electronic process. In this study, we mainly focused on the effects of the alkyl chain length of amine on the decomposition of the copper complex and copper film formation, and disclosed the synergetic effect of blended amines. For practical applications, the durability of the films should be promoted to sustain abrasion and flex. Our further study will focus on the adhesion and cohesion of copper films.

4. CONCLUSIONS The synergetic effect of blended amines as ligands to copper complex ink to form highly conductive copper films was observed. Cuf coordinated by blended amines with a long alkyl chain and a short alkyl chain could improve the conductivity of copper films, compared with using one of the two amines alone. It was found that the decomposition of all Cuf−amine complexes with different alkyl chain lengths went through a series of endothermic and exothermic processes. The first endothermic process was a sign of the activation of the copper complex by the amine. The following exothermic process occurring from 100 to 120 °C for all complexes marked the copper nucleation. The growth of copper particles and the dissociation of the ligand occurred in the next endothermic process. Complexes with a short alkyl chain demonstrated stronger activating effect to accelerate the copper reduction, leading to a high nucleation rate. However, the capping effect was not strong enough to hinder the particle growth. The yielded film was loose and porous due to Ostwald ripening. As the alkyl chain length of amine increased, the activating effect declined, whereas the capping effect was enhanced. The slow decomposition of complexes with a long alkyl chain induced the slow nucleation of copper particles, but the growth was confined to form uniform copper films with lower resistivity compared to complexes with a short alkyl chain. Blended amines as ligands to Cuf displayed the synergism of long chain and short chain amines, showing both strong activating effect in copper nucleation and good capping effect in particle growth. More densely packed film of smaller particles filling the space between bigger particles could be obtained due to the balance of the fast nucleation and the weakened Ostwald ripening effect. Cuf−butyl−octyl (50 mol % octylamine) complex ink

Figure 9. Resistivity of films heated at 200 °C for 40 min from Cuf− butyl−octyl with different molar fractions of octylamine.

octylamine and 50 mol % butylamine showed the best balance of the activating effect and the capping effect, yielding the lowest resistivity of 4.28 μΩ·cm. The resistivity of copper films derived from Cuf−butyl−octyl with 50 mol % octylamine heated at 160 and 180 °C for different times is displayed in Figure 10. As the heating time increased, the resistivity of the film gradually decreased because of the release of the organic residues. It should be noted that the resistivity mainly decreased from 10 to 20 min, and did not change much from then on. Thus, heating for 20 min was long enough to form a highly conductive film. The ink also showed good performance when applied to flexible substrates including PET, PEN, and polyimide (PI). The resistivity of the film on PI treated at 180 °C for 20 min was 9.69 μΩ·cm. Because PET and PEN were heat sensitive, the heating temperature was set to 160 °C. The resistivity of the films on PET and PEN treated H

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Langmuir

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showed the best performance, yielding densely packed copper films with low resistivity at low heating temperature in short time on both flexible and rigid substrates, which show potential for printed electronic process. The understanding of the synergetic effect of blended amines could provide guidance to the design of other types of copper complex inks.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b03668. Comparison of the resistivity of copper films at different heating temperatures for 10 min from Cuf−butyl, Cuf− octyl, and Cuf−butyl−octyl, FTIR spectra of Cuf−propyl and Cuf−amyl, TG−MS−FTIR of Cuf−butyl−octyl, XRD patterns of Cuf−octyl and Cuf−butyl−octyl complexes at RT, 110, and 120 °C, cross-sectional images and resistivity comparisons of copper films heated for 10 min at 120, 140, 160, and 180 °C, and XPS analysis of copper films heated at 120 °C for 10 min (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +86 10 62784877. Fax: +86 10 62784877. ORCID

Tao Wang: 0000-0002-5714-4852 Notes

The authors declare no competing financial interest.

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REFERENCES

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DOI: 10.1021/acs.langmuir.6b03668 Langmuir XXXX, XXX, XXX−XXX