ARTICLE pubs.acs.org/JPCC
One-Step Fabrication of Copper Electrode by Laser-Induced Direct Local Reduction and Agglomeration of Copper Oxide Nanoparticle Bongchul Kang, Seungyong Han, Jongsu Kim, Seunghwan Ko, and Minyang Yang* Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Republic of Korea
bS Supporting Information ABSTRACT: Copper oxide (CuO) nanoparticle-based organic solutions are highly stable in air as well as cheaper compared to copper (Cu) nanoparticle solutions due to the absence of particle oxidation problems. Laser direct patterning via photothermochemical reaction of the CuO nanoparticles is suggested to efficiently fabricate Cu electrodes. CuO nanoparticles dispersed in ethylene glycol were instantaneously reduced to Cu nanoparticles by intense laser irradiation, and the Cu nanoparticles were simultaneously agglomerated and sintered to form conductors without additional processes. Finally, Cu electrodes with about 10 μm thickness and a specific electrical resistance of 31 μΩ 3 cm were successfully fabricated on a glass and flexible substrate. Furthermore, the maximum possible patterning rate was discussed in terms of the reduction rate of CuO. This conductor structuring method enables an environmentally friendly and cost-effective process as well as a low-temperature manufacturing sequence to realize large-area, flexible electronics on polymer substrates.
1. INTRODUCTION Recently, the cost of metal nanoparticle inks, such as gold (Au) and silver (Ag), used for photolithography-free electrode fabrication has increased due to continuous increases in the raw material prices.1 The copper nanoparticle (Cu NP) has been increasingly considered as a potential replacement ink material for the expensive noble metal NP ink.1 However, there is a critical problem because Cu is more easily oxidized in air due to the relatively low oxidation potential energy of Cu ions (0.34 V) compared to that of the noble metals (Au, 1.52 V; Ag, 0.799 V).2 Since, however, the Cu nanoparticle (NP) size decreases the affinity for Cu NP oxidation increases dramatically for increasing the surface area of NPs,2,3,8 it is difficult to synthesize Cu NPs with a minimal surface oxide layer. Therefore, the Cu NP oxidation problem causes the cost increase of the Cu NPs ink offsetting the advantage of the cheap Cu raw material. Moreover, an expensive gas chamber with an inert gas or a vacuum environment is additionally required to avoid oxidation during hightemperature sintering.912 Although the processes based on laser direct writing (LDW) of the Cu NPs ink have been studied to easily obtain a conductive Cu electrode,12,13 they also suffer from the high cost and oxidation of the Cu NP ink. The methods applying an electroless-plating on a prepatterned surface were demonstrated to realize selective copper metallization in insulators.1416 However, this hybrid method requires multistep processes as well as the use of environmentally unfriendly chemicals. In addition, Cu nanoparticles for conductive inks have been primarily synthesized in organic solvents (nonaqueous medium) under an inert atmosphere to prevent undesirable oxidation through chemical reduction mechanisms such as the polyol process.37,10 r 2011 American Chemical Society
The produced Cu NP are capped and dispersed in organic solvents that make the capping agents activated. Since, however, such Cu NP inks based on organic solvents with nonpolarity are not soluble in water, toxic organic solvents such as toluene were additionally required to selectively wash out the Cu NP inks. In this study, copper oxide (CuO) NPs dispersed in a reduction agent was directly converted to copper film by laserinduced direct local reduction, agglomeration, and sintering, as shown in Figure 1a. This approach is highly cost effective because air-stable CuO NPs enable easy and cheap ink production and is ecofriendly due to the use of a water-soluble reduction agent. Overall, it is a simple one-step electrode fabrication process which can finish the series of conventional printing processes such as inkjet and roll-to-roll printing including ink synthesis, patterning, and sintering in the air environment.2,811,17
2. EXPERIMENTAL METHODS There are two types of copper oxides: CuO and Cu2O. The band gap of CuO (∼1.2 eV) is lower than that of Cu2O (∼2.1 eV).18 Hence, Cu2O is reported to have a high transparency with a slightly yellowish color and usually absorbs wavelengths below 600 nm, while CuO strongly absorbs the whole visible spectrum range and is black in appearance. The irradiated photon energy should be larger than the band gap of these copper oxides for the laser to be absorbed by the material. If, however, Received: June 6, 2011 Revised: October 27, 2011 Published: October 27, 2011 23664
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Figure 1. (a) Schematics of the proposed process: Conversion of CuO NPs into Cu film by photochemical reduction and photothermal agglomeration. (Inset) Transmission electron microscope (TEM) image of CuO NP. (b) Schematics of the experimental setup.
the band gap of the oxide happens to be placed on the Cu absorption peaks, the Cu reduced from the copper oxide could be thermally damaged by consistently incident laser energy.19 Therefore, the absorption spectrum shift between Cu and copper oxide raw material is important to reduce the possibility for damage and the oxidation problem after Cu is formed by laser irradiation. On the basis of the above, it was concluded that CuO is a more suitable raw oxide material since the band gap of the CuO is in the relatively smaller absorption spectrum of the Cu than that of Cu2O. Near-infrared radiation (NIR) lasers with around 1000 nm wavelength are found to be appropriate for the current process considering the band gap of CuO. Preparation of CuO NP Solution. Cupric oxide (CuO) is commercially available in high-purity grade. CuO NP was prepared from an initial 97.5% grade NanoArc CuO powder supported by Alfa Aesar. NanoArc is a kind of physical vapor synthesis technique developed by Nanophase Technologies Corp. This process vaporizes precursors in plasma, followed by rapid quenching, condensation, and formation of metal oxide nanoparticles. The size of the nanoparticles is controlled by the condensation rate and the particle concentration in the quench zone.20 For high purity, the CuO powder was mixed with DI water and the powder was recovered by centrifugation. The collected powder was washed three times in distilled water and three times in ethanol and then vacuum dried at room temperature. The prepared CuO NP (60 wt %) was dispersed in a solution consisting of polyvinylpyrrolidone (PVP Mw 10 000, Aldrich, 13 wt %) and ethylene glycol (Aldrich, 27 wt %) with a ultrasonic wave. The size of the CuO NPs in the colloidal solution was measured to be approximately 200300 nm, as shown in the inset of Figure 1a. The viscosity of the CuO NP solution was adjusted to be appropriate for the coating process through adding distilled (DI) water (23 wt %) into the CuO NP solution at ambient condition. Finally, the CuO NP solution with a viscosity of 5000 cps was achieved. Coating Method. A soda lime glass (Paul Marienfeld GmbH & Co.) with 1 mm thickness and a polyimide (PI) film (SKCKOLON PI Co.) with 50 μm thickness were prepared as substrates in this study. To improve the wetting property of the prepared colloidal solution with polarity for each substrate, surface treatments such as a UVozone treatment for glass and a corona discharging for PI
film were conducted before coating the solution. The CuO NP solution was deposited on a soda lime glass and PI film by spin coating. According to required coating thickness, the spinning rate was adjusted in the range of 15004000 rpm for 30 s. After coating the solution, the specimens were dried in ambient condition for 30 min in order to evaporate the water included in the colloidal solution, since an excessive decrease of viscosity by a high water content could cause the undesirable mass flow by enhancement of marangoni flow during laser irradiation.21 Laser Processing Method. To investigate the effects of the laser operation mode, it was attempted that two types of lasers (continuous wave (CW) mode and pulsed mode) were used. Both employed lasers were a ytterbium-doped fiber laser with a 1070 nm wavelength. The laser beam spot with a 10/25 μm focal diameter (1/e2) was controlled by an xy galvanometer scanner and two kinds of f-theta lens with 63 and 256 mm focal lengths, as shown in Figure 1b. The laser power less than an average power of 0.4 W was carefully determined to avoid ablation or thermal damage in preliminary experiments. The effects of the other parameters, such as feeds, pulse width, and pulse repetition rate, were investigated to maximize the required performance of the Cu pattern including the chemical composition, physical connection, and morphology. The laser-induced direct reduction and agglomeration of CuO NP can happen selectively on the laser scanned spots, and the unexposed leftover region of the coated solution can be removed by spraying DI water to leave laser-processed Cu electrode patterns. Measurements for Physical and Chemical Characterization. To measure the shape and size of the Cu electrodes, an optical microscope and a scanning electron microscope (SEM) were used. In addition, an energy-dispersive spectrometry (EDS) measurement was conducted to quantitatively investigate the major chemical composition (C, O, Cu) ratio in the Cu electrodes. X-ray diffraction (XRD) data were collected to identify the conversion state of the colloids according to the laser irradiation.
3. RESULTS AND DISCUSSION Figure 2a shows the evolution of the elemental composition of the CuO NP solution before laser irradiation (left columns) and after CW laser irradiation (middle columns, 0.2 W power, 30 mm/s scan rate) and pulsed laser irradiation (right columns, 23665
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Figure 2. (a) Elemental composition comparison before laser irradiation (left columns) and after CW laser (middle colums) and pulsed laser (right columns) irradiation on the CuO NP solution. (b) XRD analysis before and after laser irradiation. (Inset) Transmission electron microscope (TEM) image of pulsed laser processed Cu film.
2 μJ pulse energy (0.2 W average power), 100 kHz repetition rate, and 30 mm/s scan rate). Figure 2a reveals that the laser process can successfully convert CuO into Cu. When the laser is irradiated, the CuO NPs photochemically can react with the ethylene glycol (C2H6O2) as follows 2H2 O
2HOðCH2 Þ2 OH w 2C2 H4 O CuO
w C4 H6 O2 þ 2Hþ þ 2e þ Cu2þ þ O2
w C4 H6 O2 þ H2 O þ Cu
ð1Þ
First, CuO NPs are heated in an instant by the optical absorption of CuO NPs when irradiating a laser on the coated solution. At the same time, the temperature of ethylene glycol surrounding the CuO NPs also increases near its own boiling point. If the temperature of the solution reaches around 160200 °C, the ethylene glycol starts to be dehydrated.7 Finally, acetaldehyde (C2H4O) is produced by dehydration of ethylene glycol (C2H6O2) due to the laser-induced local temperature rise and then the generated acetaldehyde reduces the CuO NPs to Cu NPs without decomposition of CuO NP (Supporting Information, Section 1).7 As soon as the CuO NPs were converted to Cu NPs, the Cu NPs can be agglomerated and sintered by the photothermal interaction between the continued incident of laser energy and the reduced Cu NPs. The residual secondary products are diacetyl (C4H6O2) and
water (H2O), which is chemically generated during the photochemical reduction. Diacetyl formation is explained by a duplicate oxidation of acetaldehyde previously produced by dehydration of ethylene glycol.22 However, these secondary products can be removed by evaporation during the laser processing and washing process to remove the unexposed regions. Even though the Cu NPs were chemically created by the laserinduced reduction of the CuO NPs, the Cu NPs could be reoxidized in the melting state when the NPs were agglomerated and sintered. The potential reoxidation problem may be alleviated by applying a short pulsed laser. To verify the above reasoning, the chemical compositions of the laser-irradiated regions were measured using an EDS analysis, as shown in Figure 2a. Figure 2a shows that the solution irradiated by the pulsed laser recorded a higher Cu content than that by the CW laser. Therefore, it could be concluded that intensive pulsed laser irradiation can minimize reoxidation of Cu NPs produced by laser-induced direct reduction. X-ray diffraction (XRD) data were collected to quantitatively measure the phase transformation of the conversed colloids, as shown in Figure 2b. The XRD pattern was acquired from the sample after the two-stage irradiation. The XRD pattern of the unirradiated products is identical to the single-phase CuO with a monoclinic structure. For the laser-processed specimen the diffraction peaks at 43.2° (43.3°), 50.4° (50.3°), and 74° (73.8°) correspond to formation of crystallized metallic copper, where the numbers in parentheses are 2θ of standard Cu. In addition, creation of crystallized Cu NPs and the grain boundary was observable from TEM observation shown in the inset of Figure 2b. The grain boundary was formed by thermal sintering of crystallized Cu NPs during laser irradiation. However, it could be observed that a small amount of Cu2O always exists as an intermediate crystalline phase after laser irradiation by the diffraction peak at 36.6°. It was estimated that this is due to reoxidation of reduced Cu NPs during sintering among the Cu NPs. At this time, reoxidation to CuO has not occurred since a high thermal energy above 400 °C is required for conversion from Cu to CuO.23 As a quantitative comparison, while the Cu2O phase occupied about 6.3 wt % over the whole composition, the Cu content was recorded 93.7%. In conclusion, it was shown that the laser-processed pattern was dominantly composed with the crystallized Cu NPs. The physical connection between the Cu NPs is also important to obtain highly conductive electrodes. The sintering between the NPs strongly depends on processing duration that is determined by the laser pulse width. The sintering state in the Cu electrodes from the various pulsed width (4, 50, 100, 200 ns) was characterized using SEM observation. Here, the pulse energy, repetition rate (Rp), and scan rate (ν) were fixed to be 2 μJ, 100 kHz, and 30 mm/s, respectively. Figure 3a illustrates that better fusion between the Cu NPs were observed at a longer pulse width than 50 ns. In addition, the pattern width was not changed at a laser pulse width longer than 50 ns (Figure 3a(ii), 3a(iii), and 3a(iv)), while the 4 ns pulse width (Figure 3a(i)) generated a narrower pattern. It was inferred that the narrow line width for the 4 ns pulse width case reflects the weak thermal energy diffusion due to the poor connection between the Cu NPs compared with longer pulse width cases. Since, however, the longer pulse width can make Cu NPs oxidized more easily, as shown in Figure 3b, it could be concluded that a pulse width of 50 ns is appropriate for reducing the CuO NPs and for simultaneously sintering the reduced Cu NPs. 23666
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Figure 3. (a) SEM images at 1000 magnification and 10 000 magnification of the copper patterns at various laser pulse widths: (i) 4, (ii) 50, (iii) 100, and (iv) 200 ns. (b) Variation of the elemental composition with the corresponding pulse width.
Figure 4. SEM images of the copper patterns at a pulse energy of 2 μJ, a scan rate of 30 mm/s, and a repetition rate of (a) 50, (b) 100, (c) 200, and (d) 300 kHz.
Since ν is closely correlated with the Rp in the pulsed laser operation, the feeds per pulse (Fp = ν/Rp) were used as a parameter to characterize the processing rate in this study. Experimental results showed that the pattern line width was broadened and the void on the surface was increased with a decreasing Fp due to an increase of the incident energy per unit volume. For that reason the void formation on the surface of the electrode was considerably reduced with decreasing Fp, as shown in Figure 4. However, it was not possible for the voids to be completely removed due to evaporation of the residual materials such as diacetyl and water chemically formed during the reduction procedure of eq 1. The maximum Fp for the dense surface was found to be around 100 nm (ν 30 mm/s, Rp 300 kHz). When ν exceeded 30 mm/s under the fixed Fp (100 nm) and pulse energy (2 μJ), the center of the electrode became gradually hollow with increasing ν, as shown in SEM images (Figure 5a, ν 100 mm/s, Rp 1000 kHz, Fp 100 nm). This phenomenon has not been observed in the conventional laser direct writings (LDWs) using metallic ink.8,21,24 To investigate the cause of the central hollow structure formation, the chemical composition both in the edge of the hollow region (mark A in Figure 5a) and in the edge of the structure (mark B in Figure 5a) was measured by an EDS. Figure 5b revealed that more oxygen was detected at the
Figure 5. (a) SEM image of the copper pattern at a scan rate of 100 mm/s and a pulse repetition rate of 1000 kHz. (b) Elemental composition of the pattern at the center and edge detecting position.
center than at the edge. The oxygen distribution result implies that there were more CuO NPs in the central hollow region than in the edges. This unique central structure formation may be explained by the time required for reduction of CuO NPs. The simultaneous increase and proportional relation between ν and Rp induces only a decrease in the pulse time duration (Tp) between each pulse without changing the Fp and the energy input per unit volume. In the temporal aspect of laser pulses if the Tp between laser pulses were not enough long compared with the time required for reduction of CuO NP, the existence probability 23667
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Figure 6. (a) Cross-sectional SEM image and (b) photograph of various copper electrodes on a glass substrate.
of unreduced CuO NP in the laser-irradiated region could increase for a pulse time duration (Tp). In other words, when the following laser pulse is irradiated to the film after irradiating a prior pulse, the laser-irradiated region could include relatively many CuO NPs that are not converted to Cu NP yet due to lack of time for reduction. In this case, because the unreduced CuO NPs more strongly absorb the irradiated NIR laser than the converted Cu NPs by the band gap difference, the unreduced CuO NPs that is heated and excited by the prior pulse could violently interact with the following laser pulse. Therefore, the CuO NPs in the laser-irradiated region are removed bit by bit per every pulse by laser ablation. This situation begins from the center in the laser-irradiated region as a result of the relatively high energy density due to the Gaussian distribution of the laser beam. In addition, since the feeds per pulse (Fp) is much smaller than the laser beam diameter, as mentioned in the above lasing conditions, this situation is repetitively occurring in the same spatial region. Finally, the central hollow structure is created by continued accumulation of the quantity of the ablated CuO NPs in the same region. In the line patterning a series of the repetition of this procedure caused the line-shaped hollow structure in the electrode, as shown in Figure 5a. Therefore, the time interval (Tp) between pulses should be longer than the time required for reduction of CuO NPs to prevent the occurrence of such unordinary structure. In other words, the maximum ν was limited by the reduction rate of the CuO NPs. From the above relationship, the minimum time required for complete reduction estimated from Tp experiments was found to be approximately 3.3 μs. On the basis of the above experiments, it was determined that the optimized patterning condition was a pulse width of 50 ns, ν of 30 mm/s, and Rp of 300 kHz. Figure 6 shows the Cu electrode with a thickness of approximately 10 μm created by the optimized conditions on a glass substrate. Although the laser energy cannot directly penetrate to the bottom of such a thick film, the reduction could occur by diffusion of the heat energy coming from the upper layer because formation of acetaldehyde only depends on the temperature of ethylene glycol (Supporting Information, Section 2).7 This result shows that the achievable electrode is thicker than
Figure 7. (a) Photograph and microscopic image of copper electrodes on a polyimide film. (b) SEM image of the Cu pattern of approximately 11 μm using a 10 μm spot diameter on a polyimide substrate at a pulse energy of 0.3 μJ. (c) Temperature distribution image of the heater using an infrared camera at an electrical input of 30 V and 0.15 A. (Inset) Schematic illustration of the microheater.
that using conventional inkjet printing methods requiring use of Cu colloidal solutions with low viscosity.911 To investigate the process applicability to the flexible substrates, this process was also demonstrated on a polyimide substrate, as shown in Figure 7a. Although the pattern width could be controlled by changing the pulse energy in the fixed laser spot, achieving a smaller pattern width than when the laser spot diameter was limited. However, the resolution can be further improved by decreasing the laser spot diameter with a tighter focusing lens. Thus, we achieved a pattern resolution down to approximately 11 μm using a laser spot diameter of 10 μm at a pulse energy of 0.3 μJ (ν 30 mm/s, Rp 1000 kHz), as shown in Figure 7b. Although a low resistivity below 10 μΩ 3 cm25,26 can be expected from the fusion state among Cu NPs at high convergence efficiency above 90%, the specific electrical resistance of the Cu patterns was measured to be 31 μΩ 3 cm. The resistance discrepancy is due to the occurrence of voids, as mentioned above. However, this resistivity value is low enough for highquality electronic application, and also, it is as good as the values from previous LDWs using silver inks27,28 and Cu ink13 as well as 23668
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The Journal of Physical Chemistry C inkjet printing.10 The reliability of the electrodes is an important evaluation index for the current technique to be applied in the real electronics industry. To investigate the adhesion property of the laser-processed pattern a peeling test which is the so-called “scotch tape” method was conducted using a conventional adhesive tape (3M). The laser-processed Cu pattern showed better adhesion characteristics than the unirradiated CuO film. This comes from the adhesion enhancement caused by slight melting at the interface between the Cu pattern and the polymer substrate. If finer CuO nanoparticles can be applied to this method than that used in this study, the low densification problem of the Cu electrode causing a depression of cohesion and electrical conductivity can be resolved by the thermodynamic size effect.29 According to the laser ablation synthesis in solution (LASiS) method, Cu NPs with a diameter of 330 nm can be uniformly produced by irradiating laser pulses to the bulk Cu plate in solution such as water and acetone.30 If the Cu NPs produced by LASiS were stored in DI water more than 2 weeks, the Cu NPs can be perfectly oxidized to CuO NPs.31 Finally, the ultrafine CuO NP can be ecofriendly produced without any toxic solvents and complicated chemical synthesis procedures. To demonstrate the applicability of this method for real devices, it was attempted that a flexible microheater device was produced, as shown in the inset of Figure 7c. According to a temperature distribution of the produced heater measured by an infrared camera (FLIR Co.), it could be verified that the laser-processed microheater shows a uniform temperature of approximately 354 K without any electrical disconnections even on bending, as shown in Figure 7c. It shows that this method is more favorable at producing a more compact device and reducing production costs compared to photolithogrphy. Finally, this result represents that this method can be directly applied to the manufacture of flexible electronics alternatives to photolithography.
4. CONCLUSIONS We propose a simple one-step fabrication of copper electrodes by laser-induced reduction of the copper oxide. To avoid reoxidation during the agglomeration and sintering of the Cu NPs a pulsed laser was applied in this study. The pulse width, scanning rate, and pulse repetition rate were optimized to maximize the density of the electrode and the electrical conductivity. Finally, copper electrodes with above 10 μm thickness and a specific electrical resistance of 31 μΩ 3 cm were successfully created on a glass and flexible substrate in air. The diverse substrate material choice including flexible polymer and cost-effective copper patterning with low resistivity makes our approach a strong potential approach for flexible electronics fabrication on a heat-sensitive polymer substrate. Therefore, this method is expected to contribute to continuing advance of fabrication of flexible electronic devices, such as flexible PCB,32 flexible display,33,34 flexible solar cell,35 and flexible energy storage devices.36 ’ ASSOCIATED CONTENT
bS
Supporting Information. Detailed information on the laser-induced decomposition of CuO NP and thick pattern formation. This material is available free of charge via the Internet at http://pubs.acs.org.
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’ AUTHOR INFORMATION Corresponding Author
*Phone: 82-42-350-3224. Fax: 82-42-350-5224. E-mail: myyang@ kaist.ac.kr.
’ ACKNOWLEDGMENT This work was supported by the second stage of the Brain Korea 21 (BK 21) and KAIST Institute (KI) for Optical Science and Technology (OST). ’ REFERENCES (1) Das, R. Replacing printed silver with copper. Printed electronics world 2008. http://www.printedelectronicsworld.com/articles/replacingprinted-silver-with-copper-00001002.asp. (2) Cho, M. S.; Choi, W. H.; Kim, S. G.; Kim, I. H.; Lee, Y. J. Nanosci. Nanotechnol. 2010, 10, 6888–6891. (3) Sun, J.; Jing, Y.; Jia, Y.; Tillard, M.; Belin, C. Mater. Lett. 2005, 59, 3933–3936. (4) Bonet, F.; Guery, C.; Guyomard, D.; Urbina, R. H.; Elhsissen, K. T.; Tarascon, J. M. Int. J. Inorg. Mater. 1999, 1, 47–51. (5) Park, B.; Jeong, S.; Kim, D.; Moon, J.; Lim, S.; Kim, J. J. Colloid Interface Sci. 2007, 311, 417–424. (6) Kyler, J. C.; Reveles, J. U.; Michael, D. S.; Shiv, N. K.; Everett, E. C. J. Phys. Chem. C 2011, 115, 2656–2664. (7) Fievet, F.; Lagier, J. P.; Blin, B. Solid State Ionics 1989, 32/33, 198–205. (8) Dong, T. Y.; Wu, H. H.; Huang, C.; Song, J. M.; Chen, I. G.; Kao, T. H. Appl. Surf. Sci. 2009, 255, 3891–3896. (9) Jeong, S.; Woo, K.; Kim, D.; Lim, S.; Kim, J. S.; Shin, H.; Xia, Y.; Moon, J. Adv. Funct. Mater. 2008, 18, 679–686. (10) Lee, Y.; Choi, J.; Lee, K.; Stott, N. E.; Kim, D. Nanotechnology 2008, 19, 415604. (11) Woo, K.; Kim, D.; Kim, J. S.; Lim, S.; Moon, J. Langmuir 2009, 25, 429–433. (12) Kim, T; Hwang, J; Moon, S. Jpn. J. Appl. Phys. 2010, 49, 05EA09. (13) Watanabe, A.; Miyashita, T. J. Photopolym. Sci. Technol. 2007, 20, 115–116. (14) Xu, J.; Liao, Y.; Zeng, H.; Zhou, Z.; Sun, H.; Song, J.; Wang, X.; Cheng, Y.; Xu, Z.; Sugioka, K.; Midorikawa, K. Opt. Express 2007, 15, 12743–12748. (15) Kind, H.; Geissler, M.; Schmid, H.; Michel, B.; Kern, K.; Delamarche, E. Langmuir 2000, 16, 6367–6373. (16) Akamatsu, K.; Ikeda, S.; Nawafune, H.; Yanagimoto, H. J. Am. Chem. Soc. 2004, 126, 10822–10823. (17) Jeong, S.; Kim, D.; Moon, J. J. Phys. Chem. C 2008, 112, 5245–5249. (18) Rafea, M A; Roushdy, N. J. Phys. D 2009, 42, 015413. (19) Johnson, P. B.; Christy, R. W. Phys. Rev. 1972, 6, 4370–4379. (20) Sarkas, H.; Murray, P. G.; Fay, A.; Brotzman, R. W. NSTINanotech 2004, 3, 496–498. (21) Kang, B.; Ko, S.; Kim, J.; Yang, M. Opt. Express 2011, 19, 2573–2579. (22) Blin, B.; Fievet, F.; Beaupere, D.; Figlarz, M. Nouv. J. Chim. 1989, 13, 67. (23) Kevin, M.; Ong, W. L.; Lee, G. H.; Ho, G. W. Nanotechnology 2011, 22, 235701. (24) Ko, S.; Pan, H.; Grigoropoulos, C. P.; Luscombe, C. K.; Frechet, J. M. J.; Poulikakos, D. Appl. Phys. Lett. 2007, 90, 141103. (25) Watanabe, A.; Kobayashi, Y.; Konno, M.; Yamada, S.; Miwa, T. Jpn. J. Appl. Phys. 2005, 44, 740–742. (26) Kang, B.; Kno, J.; Yang, M. J. Micromech. Microeng. 2011, 21, 075017. (27) Li, X.; Zeng, X.; Li, H.; Qi, X. Thin Solid Films 2005, 483, 270–275. 23669
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