Polymer grafting surface as templates for the site-selective metallization

Applied Surface Science 274 (2013) 241–247

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Polymer grafting surface as templates for the site-selective metallization Fang Yang a,b , Peiyuan Li a,∗ , Xiangcheng Li c , Lini Huo a , Jinhao Chen b , Rui Chen a , Wei Na b , Wanning Tang b , Lifang Liang b , Wei Su b,∗∗ a

College of Pharmacy, Guangxi University of Chinese Medicine, Nanning 530001, China College of Chemistry and Life Science, Guangxi Teachers Education University, Nanning 530001, China c School of computer, electronics and information, Guangxi University, Nanning 530001, China b

a r t i c l e

i n f o

Article history: Received 27 September 2012 Received in revised form 18 January 2013 Accepted 25 February 2013 Available online 14 March 2013 Keywords: Metallic pattern Polymeric substrate Palladium Electroless plating

a b s t r a c t We report a simple, low-cost and universal method for the fabrication of copper circuit patterns on a wide range of flexible polymeric substrates. This method relies on procedures to modify the polymeric substrates with grafted polymer template to form surface-bound N-containing groups, which can bind palladium catalysts that subsequently initiate the site-selective deposition of copper granular layer patterns. The fabrications of patterned copper films were demonstrated on three kinds of flexible polymeric films including poly(imide) (PI), poly(ethylene naphthalate) (PEN) and poly(ethylene terephthalate) (PET) with minimum feature sizes of 200 ␮m. The films were characterized by ATR FT-IR, contact angle, XPS, XRD, TEM, SEM. Furthermore, the copper layered structure shows good adhesion with polymeric film. This method, which provides a promising strategy for the fabrication of copper circuit patterns on flexible polymeric substrates, has the potential in manufacturing conductive features adopted in various fields including modern electronics, opto-electronics and photovoltaic applications. © 2013 Elsevier B.V. All rights reserved.

1. Introduction The fabrication of metallic patterns on functional materials is extremely important for manufacturing processes of microelectronic devices [1–4]. Especially for optical device, biomedical and microelectronics applications, the surface metallization of flexible polymeric substrates has attracted increasing attention because of the polymers’ thermal plasticity that enables tuning of the nanocomposite microstructure, and the diversity of chemical bonds and structures available that provides additional possibilities for controlling physical and chemical properties [5–8]. The conventional patterning has been achieved by laser-induced deposition or chemical vapor deposition with a lithographic technique [9,10]. However, the adhesion of the metal pattern onto polymeric substrate is generally poor because the surface energy of polymer is low. Besides, these conventional patterning techniques do not modify polymer surfaces, and adhesion of the metal pattern onto polymeric substrate is due mainly to the physical anchoring effect, which is based on the roughness of the polymeric substrate [8,10,11]. Therefore, the development of a

∗ Corresponding author. Tel.: +86 771 2279416; fax: +86 771 2279416. ∗∗ Corresponding author. Tel.: +86 771 3908308; fax: +86 771 3908308. E-mail addresses: [email protected] (P. Li), [email protected] (W. Su). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.02.137

process for the fabrication metallic patterns with higher adhesion still remains a great challenge for chemists and material scientists. Recently, Akamatsu et al. [12] reported site-selective surface modification for the generation of copper micropatterns on a polyimide surface. Masuda and co-workers [13] fabricated a micropattern of copper thin film on a poly(ethylene terephthalate) substrate by electroless deposition using a self-assembled monolayer patterned with different functional groups (SH and OH terminal groups) as a template. Carmichael and co-workers [14] oxidized the various polymeric substrates to form surface-bound carboxylic acid groups, patterning of an aluminum porphyrin monolayers to bind a Pd/Sn colloidal catalyst that subsequently initiated the selective deposition of copper in an electroless plating solution. The above fabrication method can tune the polymer surface characteristics and enhance the adhesion between metal and polymeric substrate. However, diverse treatments needed for the fabrication of metal patterns onto different polymeric substrates result in more complicated and expensive manufacturing process. As a result, it is highly desirable to develop a simple, cost-effective and universal metallization method for different kinds of polymer materials. In this paper, we describe a facile method with potentially broad and perhaps universal scope to fabricate metal patterns onto polymeric substrates by electroless metal deposition, which is initiated by the palladium catalysts chemisorbed onto the modified

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Fig. 1. Experimental scheme: (A) the formation of active radicals on the surface of the pre-irradiated flexible polymeric substrate; (B) the formation of P-4VP polymertemplated surface of the substrate; (C) the formation of patterned Pd catalyst onto the P-4VP polymer-template through a screen printed mask; (D) selectively deposition of copper onto the Pd catalyst-modified areas.

surface of the polymeric substrate (Fig. 1). To strongly immobilize the catalyst onto the substrate, surface modification with nitrogen (N)-containing molecules has attracted substantial investigative interest [15,16]. Kimura et al. demonstrated a route to deposite nickel layer patterns using photocross-linked partially quaterized poly(vinyl pyridine) thin films on insulate substrates [16]. However, adhesion of the modified polymer template onto polymeric substrate is mainly relied on the physical anchoring effect, which may affect the adhesion between the metal layer and polymer substrate. The addition of chemical bonding may help to enhance the metal/polymer adhesion. Here, we introduced the pyridine centers onto the polymeric surface by the graft copolymerization of pyridine-containing monomers via gamma irradiation. The main advantage of the application of irradiation applied to polymeric substrate is the formation of strong bridges between macromolecules [17], which contributes to the highly adhesive metallic layers formed onto the modified surface of various flexible polymeric substrates such as poly(imide) (PI), poly(ethylene naphthalate) (PEN) and poly(ethylene terephthalate) (PET). 2. Experimental 2.1. Materials Deionized water of 18 M resistivity was used for all experiments. Dimethylamine borane (DMAB) was purchased from Shenyu Chemical Ltd. (China), 4-vinylpyridine (4-VP) was purchased from J&K Scientific Ltd. (China), PdCl2 was provided by Beijing Jiuzhoumol Ltd. (China), the electroless Cu bath was purchased from Nanjing Delei Technology Ltd. (China). All of these reagents were used as received unless otherwise noted. Poly(ethylene terephthalate) (PET), polyimide (PI) and poly(ethylene naphthalate) (PEN) films were purchased from Toray–DuPont. The polymeric films were rinsed with ethanol under ultrasonification and dried with nitrogen for 1 min prior to use. 2.2. Preparation of copper film on flexible substrates The process of fabricating the copper pattern onto the polymeric films is schematically presented in Fig. 1. The polymeric films cleaned with ethanol were sealed in a polyethylene bag and subjected to ␥-ray irradiation at a dose rate of 30 kGy. After irradiation, the pre-irradiated films were sealed and placed in a refrigerator at 4 ◦ C for future use. In a typical preparation process, a mixed solution containing 10 mL of 4-vinylpyridine monomer, 88.6 mL of H2 O, and

1.4 g (NH4 )2 ·Fe(SO4 )2 ·6H2 O was added to a 500 mL three-necked flask. The reactor was purged with nitrogen for 10 min to eliminate oxygen. Then the pre-irradiated film was added to the solution. The grafting reaction was carried out at 80 ◦ C for 3 h with constant stirring under argon gas. After the grafting reaction was complete, the grafted modified films were cleaned with ultrasonic vibration with methanol for 10 min to remove the residual monomer and homopolymer. The obtained product was dried in vacuum at 60 ◦ C. Next, the inert ink was solidified on the polymeric surface to form a fine-precision invert circuit pattern as a mask via a simple screen-printing process [18]. The modified sample substrate was subsequently activated at 50 ◦ C for 30 min by immersion in a hydrochloric acid solution containing 0.5 g/L of PdCl2 , followed by gentle rinsing with Milli-Q water. Next, this substrate was immediately immersed in a 0.1 M DMAB aqueous solution at 50 ◦ C for 30 min to activate the doped Pd catalyst. Finally, the activated substrate was immersed in an electroless Cu bath at 50 ◦ C for 30 min. Finally, the substrate was carefully rinsed with Milli-Q water and dried with Nitrogen gas. 2.3. Characterization Infrared spectra were obtained in the 4000–750 cm−1 range using a Nicolet 670 Fourier-transform infrared (FT-IR) instrument equipped with an attenuated total reflectance (ATR) attachment. Contact angles were determined using an optical contact-angle meter (JC2000C1, Shanghai Powereach Digital Technology Equipment Co., Ltd., China) in the drop/sessile down mode. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Kratos Axis Ultra DLD multi-technique X-ray photoelectron spectroscope with a Mg K␣ source operated at 14.0 kV and 25 mA. All of the binding energies were referenced to the C 1s peak at 284.6 eV of the surface adventitious carbon. The cross-sectional microstructure of the film was observed using transmission electron microscopy (TEM, H500, HITACHI). For cross-sectional TEM observations, the samples were sectioned into ca. 100 nm thick slices with the conventional microtome technique using a diamond knife (Leica, Ultracut R). The adhesion between copper films and the polymeric substrate was tested using the ASTM D3359B02 tape test. XRD patterns (10◦ ≤ 2 ≤ 95◦ ) were recorded on a Rigaku D/Max 2500 PC diffractometer equipped with a Cu K␣ radiation ( = 0.154056 nm) source. The surface morphology of the films was observed by scanning electron microscopy (SEM, S-3400N, HITACHI) and atomic force microscopy (AFM, Veeco Co.). For SEM imaging, Au (1–2 nm) was sputtered onto the grids to prevent charging effects and to improve image clarity. 3. Results and discussion 3.1. Mechanism of the interaction between polymer template and substrate The interaction between the modified polymer template and the polymeric film surface is important for it can affect the adhesion of copper layer onto the polymer substrate. In this study, the preirradiation method, which has attracted increasing attention for it can be used to introduce a variety of functional groups and does not require chemical initiators or catalysts, was employed [19]. In the pre-irradiation method, polymeric matrix is first irradiated to generate free radical stable at ambient temperature in order to initiate covalent bonds with monomers after irradiation [20]. Then free radicals initiate grafting polymerization as shown in reactions (reaction (1–4)). P → P•

(1)

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Fig. 2. (A) ATR-FTIR spectra in the 4000–750 cm−1 region of (a) bare and (b) P-4VP polymer-modified PI film, and photographs of a water droplet on the surface of (B) bare and (C) P-4VP polymer-modified PI film.

P • + nM → PMn•

(2)

P • + mM → PMm•

(3)

PMn• + PMm• → PMn + mP

(4)

In the equations above, P, M, and n/m represent the polymeric film (PI, PET or PEN), the monomer of 4-vinylpyridine (4-VP), and the number of monomer, respectively. During graft reaction process, the free radicals formed on PI macrostructure initiate the radical reaction with 4-VP monomer. The free radicals act as reactive centers, and chemical bonds form between polymeric film and the resultant poly(4-vinylpyridine) polymer (P-4VP). The formation of chemical bond builds a bridge between the polymeric substrate and grafted polymer [21], therefore, these two heterogeneous surfaces can tightly bind, which is benefit for the enhancement of the metal/polymer adhesion. 3.2. Surface properties of the modification film We describe here the fabrication of patterned copper films onto a PI substrate as an example of this patterning method. ATR FT-IR measurements were performed to investigate the structural change in the PI film before and after P-4VP treatment. Fig. 2A presents FTIR spectra of PI film before and after surface modification. Fig. 2Aa shows a typical IR spectrum of bare PI film. The absorption bands at 1776 and 1716 cm−1 are assigned to the symmetric and asymmetric stretching vibrations of the carbonyl groups coupled through the five-membered imide ring. [22,23] The band at 1500 cm−1 is due to the phenyl groups of ODA part of the PI polymer. [24] The peak at 1376 cm−1 is attributed to the ring stretching vibration of imide C N C, whereas the band at 1247 cm−1 is assigned to the Ar O Ar stretching vibration of aromatic ether [25]. After modification, some new peaks appear in the IR spectrum (Fig. 2Ab). The new peak at 2920 cm−1 is due to the aliphatic C H stretching vibration of poly(4-vinyl pyridine) [26], and the broad band at 3700–3000 cm−1 is attributed to the N H stretching vibration of pyridyl group [27]. The IR results indicate that after polymerization treatment the pyridine groups have been grafted on the surface of PI film.

The water contact angle measurement was conducted to further investigate the surface change of the PI film before and after P-4VP modification. The photographs of a water droplet on the surface of bare and surface-modified PI film are shown in Fig. 2. For the bare PI film, the contact angle is 29◦ (Fig. 2B). After polymerization treatment, the value of contact angle increases to 46◦ (Fig. 2C). Based on the water contact angle results combined with the IR spectra, the P-4VP polymeric template has been successfully immobilized on the PI film surface.

3.3. Surface properties of Pd-modified PI film The P-4VP polymer-templated PI film was subsequently immersed into a PdCl2 solution to absorb Pd ion, followed by immersed into a DMAB solution to activate the ion-doped PI film. DMAB was employed as a reducing agent because it contains no metallic cations that may cause serious problems in terms of the electrical conductivity and electromigration resistance of the resulting copper interconnection. Fig. 3 presents the SEM images of the Pd2+ ion-doped PI film before and after activation. For the ion-doped PI film, the surface is very smooth without little particles. After DMAB treatment, the Pd ions adsorbed on the surface of PI film are activated by electrons generated through self-oxidative decomposition of DMAB that proceeds at the interface between the film and solution (Fig. 3A). As can be seen form the SEM image of Pd-activated PI film, there are plenty of nanoparticles with diameters of approximately 50 nm assembled on the substrate surface (Fig. 3B). The water contact angle measurement was conducted to further investigate the surface change. The contact angle of Pd-doped PI film is 45◦ (Fig. 3C), which is similar with the P-4VP polymer-modified PI film (46◦ , Fig. 2C), while the value for Pd-activated PI film is 51◦ (Fig. 3D). The formation process of palladium NPs on the P-4VP polymertemplated flexible polymeric substrate is presented in Fig. 4. First, the Pd ions were chemisorbed and immobilized onto the N-containing P-4VP polymeric template on the surface of flexible polymeric substrate to form macromolecular metal complexes. Subsequently, the Pd ions were reduced by the self-oxidative decomposition of the reducing agent (DMAB). Finally, the Pd NPs

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Fig. 3. SEM images of the (A) Pd-doped and (B) Pd-activated PI film. All scale bars = 200 nm. And Photographs of a water droplet on the surface of Pd-doped PI film (C) before and (D) after DMAB activation.

are formed and assembled on the P-4VP polymer-templated substrate. XPS analysis was conducted to monitor the presence of palladium species within the PI film (Fig. 5). While no peaks in the palladium region were observed at the bare PI without any treatment, the P-4VP polymer-templated PI film exhibited two peaks at 343 and 338 eV, respectively, corresponding to the Pd 3d3/2 and 3d5/2 binding energies for Pd2+ (Fig. 5B) [28]. After reduction, the two peaks appeared at 341 and 336 eV, respectively, corresponding to the Pd 3d3/2 and 3d5/2 binding energies for Pd0 (Fig. 5D) [29]. The observed peak positions suggest that the incorporated Pd2+ species were reduced by DMAB to active Pd0 species within the N-containing P-4VP polymer templates onto the PI substrate. 3.4. Selective surface metallization The SEM image of the copper patterns on PI substrate is presented in Fig. 6A. After electroless plating, a smooth and neat metal film, which is composed of numerous copper nanoparticles with

Fig. 4. Formation of palladium particles on the flexible polymeric substrate.

diameters of 150 nm, is formed on the PI substrate. Fig. 6B presents an optical photograph of the resulting copper circuit pattern. The width of the linear copper pattern is 200 ␮m, which is in close agreement with the screen-grid specifications. The copper film on the PI film surface is compact and neat, and the edges of the pattern lines are almost straight and uniform. The results show that the copper circuit patterns with a width of 200 ␮m have been successfully fabricated onto the PI surface. The cross-sectional image of the copper layer on the substrate was examined by TEM. The TEM image of the copper patterns on PI substrate (Fig. 7A) verified the SEM result. A compact copper film with a thickness of approximately 350 nm is immobilized on the surface of PI substrate. The contact angle of copper film on PI substrate is 230.0◦ (Fig. 7B), which is larger than that of Pd-activated PI film before copper metallization. This higher hydrophobicity is relied on the increasing surface roughness. The copper nanoparticles formed on the PI film during the electroless plating process increase the apparent hydrophobic by entrapping air, as the lotos effect. [30,31] Based on the contact angle data, we can deduce that the difference in contact angle between the PI films before and after electroless plating treatment is attributed to the difference in intrinsic wettability between the two ionizable species [32]. The adhesion of copper wire/substrate is essentially important in the metal/polymer system. The adhesion of the copper films on PI, PET and PEN substrates was evaluated by a standard peel adhesion test (ASTM D3359B-02) [14]. Briefly, the copper film was scorced into a 1 mm × 1 mm square lattice pattern. Then the tape was sticked to the cut surface and subsequently peeled off. After the peel adhesion test, there was no removal of copper film. The edges of the cuts are completely smooth, and none of the squares of the lattice is detached. The result demonstrates that adhesion of the copper patterns/flexible polymeric substrates achieve the 5B ASTM adhesion classification, the highest level in this standard peel adhesion test. This strong adhesion between the copper film and the polymeric substrate is probably due to the P-4VP polymeric template chemically bound to the surface of

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Fig. 5. The wide-scan and (A) Pd 3d core-level (B) XPS spectra of Pd-doped PI film, and the wide-scan (C) and Pd 3d core-level (D) XPS spectra of Pd-activated PI film.

Fig. 6. (A) SEM image of copper patterns on PI substrate. The scale bar = 200 nm. (B) Optical photograph of copper circuit patterns with the lines of 200 ␮m width.

Fig. 7. (A) Cross-sectional TEM image of copper patterns. The scale bar = 200 nm. (B) Photographs of a water droplet on the surface of copper pattern.

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the non-modified PI, PET and PEN substrates that lack appropriate groups for the adsorption and immobilization of catalysts. In this study, the fabrication of P-4VP polymer-templated surfaces of the flexible polymeric film by graft polymerization was carried out with the assistant of radiation technology. Free radicals (P• ) in flexible polymer chain were produced after the flexible polymeric film was irradiated by 60 Co-␥ ray [20]. The grafted N-containing polymeric templates can provide sites for the subsequent chemisorptions of palladium catalysts. Therefore, the copper pattern can be tightly deposited on the P-4VP modified polymeric substrates. This simple and versatile method for direct site-selective plating of durable copper layers on a flexible polymeric film possesses great potential for applications in the electronics industry. 4. Conclusions

Fig. 8. XRD patterns of the bare (A) and copper-coated (B) PI substrate.

the substrate. The Pd species incorporated within the template can initiate the deposition of copper film onto the PI substrate, consequently, a ternary metal–template–substrate layered structure connected through chemical bonding is formed that results in the strong adhesion between the substrate and the copper film. Fig. 8 shows the XRD patterns of the bare (a) and copper-coated (b) PI film. The two broad diffraction peaks at 2 = 13.7◦ , 22.3◦ and 26.2◦ are characteristic peaks of bare PI films. Four sharp diffraction peaks emerge at 2 = 43.5◦ , 50.5◦ , 74.3◦ , and 89.9◦ , and the cor˚ responding lattice-plane distances are 2.08, 1.81, 1.28 and 1.09 A, respectively. These results correspond to Bragg reflections from the (1 1 1), (2 0 0), (2 2 0) and (3 1 1) planes, respectively, of the facecentered-cubic structure of crystalline copper [33,34]. These peaks are in close agreement with the previously reported data (JCPDS 040836). These results indicate that the copper pattern on the PI substrate is homogeneous and smooth. For comparison, a control experiment was conducted to investigate the copper electroless deposition on a non-poly(4vinylpyridine)-modified polymer surface. In short, the preirradiated substrate (PI/PET/PEN) was activated by immersion in a hydrochloric acid solution containing PdCl2 . Next, the substrate was immediately immersed in a DMAB aqueous solution, subsequently immersed in an electroless Cu bath, and rinsed with Milli-Q water and dried. However, no copper metal was formed on the substrate without P-4VP modification. The metallization of flexible polymeric substrate enables the fabrication of devices, i.e. conformal displays, wearable electronics and bioelectronic devices such as sensors and artificial nerves. Electroless deposition of metal is of special interest in the fabrication of metal patterns on insulating substrate such as polymeric films, nevertheless, it is difficult for the electroless deposition to undergo on non-modified PI, PET and PEN substrates. Electroless deposition, which is based on the reduction of metallic ions from solution onto the substrate to be metalized without application of any electric current, generally requires the employ of a catalyst that lowers the activation energy of metal formation by serving as a temporary electron bridge between the reducing agent and metallic ions. After deposition of the catalyst grains, the metal deposition process becomes autocatalytic [35]. The catalyst deposition is the most crucial stage of the electroless deposition [36]. The catalyst must be strongly bound to the substrate. However, the introduction of catalyst is difficult onto

In conclusion, we have demonstrated a novel, simple and universal method for the site-selective deposition of copper granular layer patterns on a variety of flexible polymeric substrates such as poly(imide) (PI), poly(ethylene naphthalate) (PEN) and poly(ethylene terephthalate) (PET). The method relies on the use of grafted polymer template incorporating palladium catalysts, which can be acted as adhesive interlayers for fabricating precise copper patterns on flexible polymers. Moreover, the copper–template–substrate layered structure produced good adhension between the resulting copper layer and the flexible polymeric substrate. This facile method with a great potential for manufacturing mico/nano devices such as microelecrtonics and biosensors, is suitable for cost-effective fabrication of flexible electronic devices. Acknowledgments This research is supported by the National Natural Science Foundation of China (Grant No. 20961001, 21261005, 51263002), Guangxi Natural Science Foundation (Grant No. 2012GXNSFAA053194; 2010GXNSFB013014), Key Project of the Chinese Ministry of Education (Grant No. 2010168), Scientific Research Fund of Guangxi Provincial Education Department (Grant No. 201106LX273) and Guangxi University of Chinese Medicine (Grant No. P2012024). References [1] O. Seitz, M. Dai, F.S. Aguirre-Tostado, R.M. Wallace, Y.J. Chabal, Copper-metal deposition on self assembled monolayer for making top contacts in molecular electronic devices, Journal of the American Chemical Society 131 (2009) 18159–18167. [2] A. Garcia, T. Berthelot, P. Viel, A. Mesnage, P. Jegou, F. Nekelson, S. Roussel, S. Palacin, ABS polymer electroless plating through a one-step poly(acrylic acid) covalent grafting, ACS Applied Materials & Interfaces 2 (2010) 1177–1183. [3] D. Schaubroeck, J.D. Baets, T. Desmet, S.V. Vlierberghe, E. Schacht, A. Calster, Introduction of amino groups on the surface of thin photo definable epoxy resin layers via chemical modification, Applied Surface Science 255 (2009) 8780–8787. [4] S. Yang, D. Wu, S. Qi, G. Cui, R. Jin, Z. Wu, Fabrication of highly reflective and conductive double-surface-silvered layers embedded on polymeric films through all-wet process at room temperature, Journal of Physical Chemistry B 113 (2009) 9694–9701. [5] B.Y. Ahn, E.B. Duoss, M.J. Motala, X. Guo, S.I. Park, Y. Xiong, J. Yoon, R.G. Nuzzo, J.A. Rogers, J.A. Lewis, Omnidirectional printing of flexible, stretchable, and spanning silver microelectrodes, Science 20 (2009) 1590–1593. [6] J.H. Ahn, H.S. Kim, K.J. Lee, S. Jeon, S.J. Kang, Y. Sun, R.G. Nuzzo, J.A. Rogers, Heterogeneous three-dimensional electronics by use of printed semiconductor nanomaterials, Science 314 (2006) 1754–1757. [7] N.S. Baek, J.H. Lee, Y.H. Kim, B.J. Lee, G.H. Kim, I.H. Kim, M.A. Chung, S.D. Jung, Photopatterning of cell-adhesive-modified poly(ethyleneimine) for guided neuronal growth, Langmuir 27 (2011) 2717–2722. [8] S. Ikeda, H. Yanagimoto, K. Akamatsu, H. Nawafune, Copper/polyimide heterojunctions controlling interfacial structures through an additive-based, all-wet chemical process using ion-doped precursors, Advanced Functional Materials 17 (2007) 889–897.

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