Cu Salt Ink Formulation for Printed Electronics using Photonic

Article pubs.acs.org/Langmuir

Cu Salt Ink Formulation for Printed Electronics using Photonic Sintering Teppei Araki,*,† Tohru Sugahara,*,‡ Jinting Jiu,‡ Shijo Nagao,‡ Masaya Nogi,‡ Hirotaka Koga,‡ Hiroshi Uchida,§ Kenji Shinozaki,§ and Katsuaki Suganuma‡ †

Department of Adaptive Machine Systems, Graduate School of Engineering, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan ‡ Department of Advanced Interconnection Materials, Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan § Institute for Polymers and Chemicals Business Development Center, Showa Denko K. K., 5-1 Yawata Kaigan Dori, Ichihara, Chiba 290-0067, Japan S Supporting Information *

ABSTRACT: We formulate copper salt (copper formate/acetate/oleate) precursor inks for photonic sintering using high-intensity pulsed light (HIPL) based on the ink’s light absorption ability. The inks can be developed through controllable crystal field splitting states (i.e., the ligand weights and their coordination around the metal centers). The inks’ light absorption properties are extremely sensitive to the carbon chain lengths of the ligands, and the ink colors can drastically change. From the relationship between the ratios of C/Cu and the required sintering energies, it is possible to ascertain that the integral absorbance coefficients are strongly correlated with the photonic sintering behavior. These results suggest that the ink absorbance properties are the most important factors in photosintering. The wires formed by sintered copper formate complex ink via the HIPL method showed good electronic conduction, achieving a low resistivity of 5.6 × 10−5 Ω cm. However, the resistivity of the wires increased with increasing contains carbon chain length of the inks, suggesting that large amounts of residual carbon have negative effects on both the wire’s surface morphology and the electrical conductivity. We find in this study that high light absorptivity and low carbon inks would lead to a lower environmental load in future by reducing both energy usage and carbon oxide gas emissions. these techniques have been reported in the literature,16−18 although simultaneous high-speed sintering and large-area sintering have not yet been achieved for the mass-scale product quality of R2R printings. Photonic sintering processes by high-intensity pulsed light (HIPL) irradiation have recently been proposed for PE12−14 and have been demonstrated for room-temperature sintering of metal ink printed in a considerably large area within a fast time range from microseconds to a few seconds.15,18 The efficiency of photonic sintering is limited by the light absorption of the ink, which is an essential parameter needed to increase the energy transfer from light. Metal nanoparticle popularly used in inks for PE often exhibit a narrow absorption region of visible light and can be considered drawbacks for HIPL sintering.19 Tuning of light absorption of nanoparticle inks for photonic sintering, such as surface coating, addition of dispersive agents and reducing agent, or oxidization, have enabled photosintering.16,20,21 However, the cost of fabrication and

1. INTRODUCTION Reduction of industrial waste products while reducing energy consumption and greenhouse gas emissions such as CO2 has gained global attention. Printed electronics (PEs)1 is an emerging manufacturing technology intended to overcome these environmental issues. The recent progress in PE has resulted in the replacement of various applications such as integrated circuits (IC),2 solar cells,3 magnetic devices,4 radio frequency identification (RFID) tags,5 and printed circuit boards (PCB),6,7 realizing the wearable functions of flexible, stretchable, and transparent devices. For the wider adoption of PE in the future, however, numerous issues remain unsolved. To date, these issues include the selection of substrate materials, printing methods, ink materials, and sintering techniques. In particular, the development of ink materials in combination with corresponding sintering methods is the key for efficient roll-to-roll (R2R) printing processes that require a rapid sintering of printed ink. Various sintering techniques, instead of the normal heat treatment, are proposed to achieve high throughput R2R processes. These include converging laser beams,8 low-pressure argon-plasma exposure,9,10 microwave radiation,11,12 and photonic irradiation.13−15 Combinations of © 2013 American Chemical Society

Received: May 30, 2013 Revised: August 5, 2013 Published: August 6, 2013 11192

dx.doi.org/10.1021/la402026r | Langmuir 2013, 29, 11192−11197

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conservation of the nanoparticle-based metal inks is relatively high. Therefore, further development of ink materials suitable for HIPL, possibly without metal nanoparticles, is needed for wider adoptions of PE for R2R mass productions. Several copper complex precursor inks, namely copper formate, acetate, and oleate are examined in this study and focuses on their tunable light absorption for use in HIPL sintering. The color of the complexes in the solution is dependent on the coordination states of the ligands (e.g., Jahn− Teller distortions).22 The cupric ions in these inks are coordinated with carboxyl and amine groups23,24 to form a complex, and their physical properties such as light absorption wavelength are determined by the crystal field splitting.25 By changing the crystal field splitting conditions, the photonic sintering process of these complex inks can be tuned for the HIPL sintering. Here, we show the successful conductive wires using the fabricated copper salt inks with high-speed photonic sintering. The effects of the ligands and carbon content of the inks for the HIPL sintering are discussed in detail.

2. EXPERIMENTAL SECTION

Figure 1. Absorption spectra of the three types of copper complex ink: copper formate Cu(HCOO)2, copper acetate Cu(CH3COO)2, and copper oleate Cu(C17H33COO)2. The inset photographs display the visual color of the inks.

Three types of copper complexes, copper formate tetrahydrate, copper acetate, and copper oleate, are purchased from Wako Pure Chemical Industries, Ltd. (Osaka, Japan) and are used as the sources of the copper metal inks. The starting materials are dissolved with 2,2′iminodiethanol (diethanolamine) with a mixing molar ratio of 0.1:0.2. Each solution is then diluted with 200 μL of ethanol. The light absorption properties of the copper complex inks are investigated through UV spectra recorded on a UV−vis−NIR spectrophotometer (V-670, JASCO, Japan). The prepared copper complex precursor inks are mask printed on a glass substrate, with width, length, and thickness of 3 mm, 20 mm, and 35 μm, respectively. The printed lines are then sintered by HIPL using a PulseForge3300 (Novacentrix Corp.) in ambient air without precure drying. The pulsed light is set at 230 V with 1400 μs exposure time, resulting in an irradiation energy of 2.69 J/cm2. The crystalline phases of the sintered wires were examined using Cu Kα radiation with an X-ray diffractometer (XRD, RINT RAPID II, Rigaku, Japan) with a 2θ range of 30°−120°. The surface microstructures of the wires were observed with a field-emission scanning electron microscopy (FE-SEM, JSM-6700F, JEOL, Japan), and the surface chemical bonding states and elemental compositions were analyzed by X-ray photoelectron spectroscopy (XPS, Kratos Axis NOVA, Shimadzu, Japan). The electrical resistivity of the wires was measured with a four-point probe method (Loresta-GP MCP-T600, Mitsubishi Chemical Corp., Japan). The line thickness was measured by cross-section observation with an optical microscope and was used to calculate resistivity.

oleate [Cu(C17H33COO)2] were weighed and dispersed in ethanol at the same molar ratio. The background noise of the measurement was reduced when using ethanol. Although the colors of formate and acetate inks are slightly different, their absorption peaks are almost the same. As indicated by the absorption of the inks, their colors differ slightly from light blue (formate ink) to dark blue (acetate ink) with increasing carbon content. The oleate ink is deep bluegreen (or moss green) and contains large amounts of carbon. It is well-known in complex liquid photonics that liquid colors vary sensitively when the ligand distribution changes at the central metal ions because of distortion field splitting of the electrical state (e.g., Jahn−Teller distortion).20 Carboxylic coppers have ligand molecules and become slightly photosensitive with ligand field splitting in the 650−850 nm range. In salt complex inks containing the amine-based solvent and the carboxyl group on the present study, the structure and the coordination state of ligand may not be explained. However, the relationship between UV absorbance spectra and colors of the inks was slightly observed. As seen in Figure 1, the absorption of the carboxylic acid copper (divalent copper compound) ink is located at about 700 nm and thus shows a light-blue color. The oleate copper ink shows gradual absorption at around 400−600 nm and thus displays a moss green color. This implies that the ligand coordination state around the copper ion has changed because the weight and electrical states of the carboxylic chain are different. In fact, copper oleate, which is more than three times heavier than copper formate, absorbs a wide range of wavelengths. Such optical characteristic of the oleate ink can be beneficial for photonic sintering. This is because both the pulsed light absorption band and the xenon lamp wavelength region are wide.20,21 Moreover, the copper formate and acetate inks can selectively absorb the visible light wavelengths. Thus, the light absorption properties of the copper salt inks are successfully controlled by changing the ligands and its distribution. This approach is expected to be promising for effective photonic sintering.

3. RESULTS AND DISCUSSION Light absorption, refraction, and diffraction of inks are determined ink colors. The carboxylic acid copper inks have transparent characteristics. The inks have each peak areas of absorption wavelength, and the areas will be staining the inks. The color of the complex ink depends on the state of the crystal field splitting between the central metal and the ligands because the solution absorption wavelength changes with the stable coordination geometry in the complex. Accordingly, the absorption wavelength of the ink is an important parameter for photonic sintering. The absorption characteristics of the copper carboxylate inks in this study are shown in Figure 1. The colors are associated with the absorbance; the insets show images of the inks and their colors. The absorption of each of the inks, which were prepared from copper formate [Cu(HCOO)2], copper acetate [Cu(CH3COO)2], and copper 11193


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the open symbols indicate the estimated absorption energy from the absorption coefficients which are integrated into the absorption spectra from the range of 300−1200 nm in Figure 1. Thus, the open symbols, which suggest the absorption ability, show net values calculated from the ratio of absorption coefficient to formate ink; here, the ratio values of formate, acetate, and oleate ink are represented as 1.0, 0.8, and 1.3, respectively. For instance, the oleate ink (green △ in Figure 2) requires over 50 J/cm2 of energy for sintering. In fact, the oleate ink only needs low photosintering energy (▲) because it has a high absorption ability to the light. On the contrary, the acetate ink demanded much photosintering energy due to low absorption ability. These results show that the ink absorbance is an important factor because the solid symbols form a linear line, indicating a correlation between the amount of the C/Cu ratio and the required net sintering energy. Although the oleate inks show good absorption because of the optimum coordination state of their ligands, the long carbon chains which surround the copper ions require increased energy for photosintering. Consequently, in inks used for photonic sintering, the band gap and electrical state around the metal ions are important factors, which are strongly related to the coordination state (i.e., the weight, length, and density of the ligands). Figure 3 shows XRD patterns of the three inks after photonic sintering. The Bragg reflections of the samples were indexed

The three Cu complex inks were mask printed to form lines on a glass substrate in order to investigate the crystal structure, microstructure, and physical properties after photonic sintering without precure drying. The pulse energy is 2.69 J/cm2 per one pulse and the total energy is calculated from the cumulative number of pulses. The total sintering energies required for formate, acetate, and oleate inks on glass substrates are 16.1, 26.9, and 40.4 J/cm2, respectively, which increase with increasing carbon chain content in the inks. The chemical composition of a material is determined by the physical and chemical states (i.e., the equilibrium state) when the substance is synthesized by chemical reaction in solution. Chemical reactions encompass changes that involve the formation and breakage of chemical bonds between atoms because of changes in the electron distribution and lead to the transformation of the initial substances into another substance. During a photochemical reaction, coordination bonds of the complex are cleaved by high-intensity light and materials are then decomposed by condensation of the molecules. Three steps are required to form the Cu complex precursor ink from the bulk copper compounds. Initially, the ligand field (LF) around the copper ions absorbs light, and the electrons are excited under exposure to HIPL. Then, copper complex ions are divided into simple copper ions and molecules by a photochemical reaction. Next, the copper ions move off along the surrounded ligands and aggregate to form the nucleus. Finally, the assembled nuclei behave like bulk copper and obtain electrons from ligands or other reduction agents. It is therefore necessary for nucleation that the ions obtain enough energy from moving around the molecules or ligands. Thus, increased sintering energy is needed with increasing carbon chain length. Figure 2 shows the photo sintering energy as a function of the C/Cu ratio for each ink. The solid symbols indicate the used total irradiation energy in the present experiment, while

Figure 3. (a) X-ray diffraction patterns from the photonic sintered ink wires: copper formate Cu(HCOO)2, copper acetate Cu(CH3COO)2, and copper oleate Cu(C17H33COO)2. In the inset, relative copper and carbon contents obtained by X-ray photoelectron spectroscopy (XPS) are presented. (b) Color photographs of each photosintered wire. Scale bar shows 1 mm. The photosintering pulse energy is 2.69 J/cm2 per one pulse, and the total energy is calculated from the cumulative number of pulses. The total sintering energies of formate, acetate, and oleate are 16.1 (6 pulses irradiation), 26.9 (10 pulses), and 40.4 (15 pulses) J/cm2, respectively.

with a unit cell in the F-centered cubic symmetry, Fm3̅m (no. 225) space group. The structure based on formate ink showed good agreement between the ideal and observed reflection patterns; the results are shown in Figure 3a. A small number of impurity phases were observed for all the samples. However, the phase corresponding to copper oxide was not detected. The XRD peaks of the photosintered bulk copper decreased with

Figure 2. Accumulated photonic energy required for sintering the printed ink in the unit area is presented as a function of C/Cu content ratio of each ink. Solid symbols indicate the used total energy from the irradiation lamp, and open ones indicate the net absorption energy corrected by the absorption coefficient of difference from formate ink. The absorption coefficients are derived from integrating the absorption spectra in Figure 1 from 300 to 1200 nm. 11194


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the Supporting Information). Mixed CuO and Cu2O exist on the surfaces of wires that used formate and acetate inks because a Cu2+ satellite peak is strongly detected at 944 eV by XPS. In the oleate ink wires, Cu2O is dominant because the satellite peak is significantly smaller than that of the others. However, the results of this analysis do not agree with the colors of the wires. Typically, Cu2O has a bright orange color, while the CuO displays a black color. Hence, the oleate ink wire included something that changed to the black color. The XPS results indicated not only a low copper content in the oleate ink wiring but also high carbon content, shown in the inset graph of Figure 3 and Table 1, which strongly indicates that the wiring has high carbon residue content. Figure 4a shows the resistivity in the step graph patterns that decreases with increasing photonic sintering energy. The resistivities obtained for the formate, acetate, and oleate inks are 5.6 × 10−5 Ω cm, 2.1 × 10−2 Ω cm, and a value outside measurement range (OMR), respectively. The increasing electrical resistivity of the wires are given that the increasing carbon chain length (formate, acetate, and oleate) contained in inks are required for the increasing total photosintering energies (i.e., 16.1, 26.9, and 40.4 J/cm2, respectively), as seen in Figure 4a. These electrical properties may include the influence of residual carbon. However, for in-depth discussion about morphology and microstructure of the wires with carbon behavior on the glass substrate, FE-SEM images of each wire surface were observed. Figure 4, panels b−d, show FE-SEM images of copper wires prepared using the photosintered formate ink, acetate ink, and

increasing carbon content in each of the inks. These results indicate that the relative amount of copper to carbon contained in the inks is strongly involved but is not the only factor. Optical microscopy images of the three Cu wires are shown in Figure 3b. In the images, the formate line shows a bright copper color, while the metallic luster of the acetic and oleate wires are lost, appearing much darker. This result suggests that the carbide content from the inks remains as residual carbon in the sintered wire. Approximately 50−60 at. % of carbon atoms are detected on the wire surfaces by XPS, while the copper content is low, as shown in the inset graph of Figure 3a and Table 1. Table 1. Measured Atomic Ratios of the Surface of PhotoSintered Cu Wires by XPS atoms

C

Cu

N

O

Na

Si

analyzed radiation

C 1s

Cu 2p3/2

N 1s

O 1s

Na 1s

Si 2p

49

6.7

4.4

40

NDa

ND

46

6.1

4.7

39

0.9

3.6

58

0.8

1.5

28

0.5

12

Cu(CHCOO)2 Cu(CH3COO)2 Cu(C17H33COO)2 a

ND stands for “cannot collect data”.

It is well-known that copper forms surface oxidation layers a few nanometers thick, which may be closely related to the copper wire color. The binding energy for all the wires has a copper oxide peak at 933 eV from XPS (Figures S1and S2 of

Figure 4. (a) Electrical resistivity of the three wires made of different inks. OMR stands for an outside measurement range. (b−d) FE-SEM surface images recorded for each sintered wires, presented with a magnified top view in the inset. Scale bars show 10 and 1 μm, respectively. The total sintering energies of each wire on glass substrates are 16.1 (formate), 26.9 (acetate), and 40.4 (oleate) J/cm2. 11195


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complex inks will lead to environmentally friendly conditions, such as resource savings, energy savings, and cost-effectiveness for next-generation PEs. Therefore, it is suggested that this study can provide a design guideline for ink to be used for photonic sintering, and the concept will lead to the reduction of carbon and carbon dioxide gas.

oleate ink on glass substrates. The formate ink wire surface morphology is more densely sintered than that of the other wires. The sintered surface morphology of the acetate ink in Figure 4c exhibits higher roughness than that of formate in Figure 4b. The oleate ink wire shows a nonuniform surface morphology and observed places where the copper compounds are peeling, as shown in Figure 4d. The magnified images of the wire surfaces show a similar microstructure to that of the acetate ink wire (see the inset of Figure 4, panels b and c). It appears to have a craterlike topography on the magnified surface; however, a networked microstructures has been completely formed, as shown in the insets of Figure 4, panels b and c. This is implied that electrically conductive paths exist on the surfaces, which agree with the conductivity mentioned above. On the other hand, the magnified surface of oleate ink wire showed some so thin and defective parts, as shown Figure 4d and its inset, which means the electrical conductive networking may disconnect. The craters and the defects of these wires may be created because of carbon oxide gas evaporation at the photonic sintering. In addition, the copper compound of wire spreads on the substrate with photosintering because these inks contain a large amount of the carbon, and as a result, the wire microstructure formed due to the generation of carbon oxide gas. Therefore, the oleate inks contain a lot of carbon constituents, which were evaporated at the moment when exposed to the HIPL. The oleate Cu wire (i.e., conduction path) was cut due to the volatile instantaneous of many impurities. From this result, it is clear that the wire width increased with increasing carbon content in each ink (see inset of Figure 3), and this result should account for carbon residues or copper oxides. In any case, the electrical resistivities are different and increase by an order of magnitude with the increasing carbon contents of the inks. This suggests that low carbon content is desirable when forming wire lines for photonic sintering. Moreover, the Cu formate ink provided a highly conductive wire (10−5 Ω cm) with high-speed (milliseconds to a few seconds) photonic sintering in ambient conditions.

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ASSOCIATED CONTENT

S Supporting Information *

XPS spectra and cross-sectional image. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*T.S.: e-mail, [email protected]. T.A.: e-mail, [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Young Scientists (B) Grant 25810140.

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REFERENCES

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4. CONCLUSION We have demonstrated the use of copper complex precursor inks fabricated by light sintering for next-generation PEs. Three types of copper complex precursor inks were investigated for their light absorbance for photonic sintering properties of R2R PEs. The light absorption characteristics of the copper complex inks can be controlled by the coordination states of ligands around the center of the metal elements. This was determined using ligand field theory and its parameters relating to complex compounds. The copper oleate ink had the most favorable light absorption properties because of the optimized coordination of the carbon chain on the carboxylate. However, the carbon content in the inks seriously affected the sintering conditions and wire properties after photonic sintering. Large amounts of carbon damaged the wire surfaces when carbon was evaporated in the photosintering process. In addition, residual carbon in wire lines disrupted the electrically conductive path. Wires based on formate ink show metal copper color and a good Cu diffraction pattern after photosintering, and the resistivity reached 5.6 × 10−5 Ω cm. These results show that the carbon content of the inks should be reduced. The current study proposes that the control of the coordination states of ligands and the carbon content of 11196


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