Patterning of Metal Films on Arbitrary Substrates by Using

May 16, 2016 - Patterning metal films on various substrates is essentially important and yet challenging for developing a wide variety of innovative d...
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Patterning of Metal Films on Arbitrary Substrates by Using Polydopamine as a UV-Sensitive Catalytic Layer for Electroless Deposition Lei Zhao, Daqun Chen, and Weihua Hu* Institute for Clean Energy & Advanced Materials, Faculty of Materials and Energy, Chongqing Key Laboratory for Advanced Materials and Technologies of Clean Energies, Southwest University, Chongqing 400715, China S Supporting Information *

ABSTRACT: Patterning metal films on various substrates is essentially important and yet challenging for developing a wide variety of innovative devices. We herein report a versatile approach to pattern metal (gold, silver, or copper) films on arbitrary substrates by using the bio-inspired polydopamine (PDA) thin film as a UV-sensitive adhesive layer for electroless deposition. The PDA film is able to be formed on virtually any solid surfaces under mild condition, and its rich catechol groups allow for electroless deposition of metal films with high adhesion stability. Upon UV irradiation, spatially selective oxidation of PDA film occurs and the local metal deposition is inhibited, thus facilitating successful patterning of metal films. Considering its versatility and simplicity, this strategy may demonstrate great applications in manufacturing various innovative devices.

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INTRODUCTION Patterned metal films are some of the indispensable components as interconnects, contacts, and electrodes in a wide variety of innovative devices such as micro total analysis systems, conformal displays, and wearable electronics.1−3 In the development of these devices, one major challenge is to pattern metal films on various substrates ranging from rigid inorganic substrates to soft elastomers with tremendously varying chemical, mechanical, and geometrical properties, which urges the development of “versatile” or substrate-independent metal patterning strategies. Nanolithography technology has been wellestablished for surface patterning in microelectronic industry, but it requires elaborate procedure and expensive infrastructure and is applicable to only a narrow range of specialized substrates with flat and rigid surfaces. Alternatively, patterning of metal film could be achieved via contact printing, molding, transfer printing, and embossing, but these techniques require careful tailoring of the surface/interface properties to ensure good adhesion; in some cases, postprinting processes such as chemical reduction, thermal annealing, and/or light irradiation are necessary to reduce the metal precursor, to stabilize the film, and/or to increase its electrical conductivity.3−7 Electroless deposition has attracted considerable interest for low-cost fabrication of metal nanostructures and films in recent years.3,8−10 Compared to electrochemical deposition and chemical/physical vapor deposition, it does not relies on expensive instruments and is simple to operate. Most importantly, it is compatible with different kinds of substrates, irrespective of their shapes or conductivities. To electrolessly deposit a patterned metal film, however, the substrate is also © XXXX American Chemical Society

needed to be prepatterned with catalyst (metal ions or metal particles in most cases) by using either tedious lithography or multistep printing for spatially defined seeding the metal films.4,11,12 Some self-assembled monolayer (SAM) films, depending on the nature of their end function groups, have demonstrated the ability to initiate or inhibit the metal growth during electrochemical/electroless deposition, thus offering an alternative means to grow metal pattern.13−16 However, this method is also far from versatile as it necessitates chemical specificity of the substrates by forming SAMs, which are available on only limited surfaces. We herein report a versatile approach to pattern metal films via electroless deposition. The word of versatile refers to two facts. (1) This strategy is applicable to arbitrary substrates, independent of their chemical nature, mechanical properties, wettability, or geometry. (2) Various metal films including gold, silver, and copper could be patterned. This strategy exploits the mussel-inspired polydopamine (PDA) film, which is a universal and multifunctional coating and could be easily formed on virtually any solid surfaces via oxidative polymerization of dopamine under mild conditions.17 Despite its broad applications, the formation mechanism and exact molecular structure of PDA film are still under debate. It is generally accepted that several indole intermediates are involved during the oxidative polymerization of PDA.18,19 To achieve metal patterning, as schematically shown in Figure 1, the substrate was first coated Received: March 22, 2016 Revised: May 5, 2016

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DOI: 10.1021/acs.langmuir.6b01118 Langmuir XXXX, XXX, XXX−XXX

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in methanol and water, each for 5 min to remove possible contaminants. The green leaf and filter membrane were briefly rinsed with DI water. In a typical experiment, PDA film was deposited by immersing the substrate into a freshly prepared dopamine solution (2.0 mg mL−1 in 50 mM Tris buffer, pH 8.5) under ambient atmosphere for 7 h. After that, the substrate was removed from the solution, rinsed with copious deionized water, and dried with gentle N2 flow. Particularly, on PDMS surface the immersion step was repeated for four times to grow PDA film. UV Irradiation. A UV cleaner (BZZ250GF-TC, HuiWo Tech, China) equipped with a Hg−Xe lamp (110 W, main wavelength of 185 and 254 nm) was used for UV irradiation. The PDA-coated substrate was irradiated under UV for 15 min with a photomask covered on its surface. Substrates were washed with water and dried with N2 for subsequent electroless deposition. Our control experiments show that as-reported patterning method also works well on thinner PDA film grown by using the repeating immersion method.18 Namely, a substrate was dropped in a freshly prepared dopamine solution (2.0 mg mL−1 in 50 mM Tris buffer, pH 8.5) under ambient atmosphere for 20 min and removed from the solution, followed by dropping into another freshly prepared same solution for 20 min growth. The immersion step was repeated for three times. The UV irradiation time was 2 min for such thin PDA films. Electroless Deposition of Copper Film. A stock solution containing 50 mM ethylenediaminetetraacetic acid (EDTA), 50 mM CuCl2, and 0.1 M H3BO3, with pH of 7.0 adjusted by using NaOH solution was prepared, to which 0.1 M dimethylamine-borane (DMAB) of equal volume was added to initiate the electroless deposit of copper film, and PDA-coated substrates were immediately immersed into the solution for 2 h at 37 °C.17 Then substrates were removed from the solution and washed with DI water and dried with N2. Electroless Deposition of Silver Film. Electroless deposition of silver was carried out according to Formanek’s report.20 Aqueous ammonia solution (0.2 M) was added dropwise to an aqueous silver nitrate solution (5 mM) until the brown precipitate generated was completely dissolved. Glucose solution (1.67 mM) of equal volume was added into the silver nitrate solution, and the PDA-coated substrate was immediately immersed in the solution. After 10 min reaction, the substrate was removed from the solution, rinsed with DI water, and dried with N2. Electroless Deposition of Gold Film. Electroless deposition of gold was performed according to the reported method with minor

Figure 1. Schematic depicting the strategy of patterning metal film on arbitrary substrates by using PDA as a UV-sensitive catalytic layer.

with a PDA thin film by immersing it into a freshly prepared dopamine solution for hours, followed by exposing to UV irradiation with a photomask. After electroless deposition in a plating solution, patterned metal film was successfully grown on the substrate.

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EXPERIMENTAL SECTION

Chemicals and Materials. Tris(hydroxymethyl)aminomethane, dopamine hydrochloride, ethylenediaminetetraacetic acid (EDTA), copper(II) chloride (CuCl2), dimethylamine-borane (DMAB), and silver nitrate (AgNO3) were obtained from Sigma-Aldrich. Hydrochloric acid, boric acid (H3BO3), and sodium hydroxide (NaOH) were purchased from Aladdin (China). Polydimethylsiloxane (PDMS) was prepared by mixing 10 parts of backbone and 1 part of curing agent and cured at 70 °C for 2 h. All chemicals were of analytical grade and used without further purification. The deionized (DI) water (18.2 MΩ) was used in all experiments. Growing PDA Film. Before the PDA deposition, substrates including glass slide, polypropylene (PP) slice, poly(ethylene terephthalate) (PET) flake, and silicon wafer were ultrasonically cleaned

Figure 2. (a−d) SEM images and EDS Cu mapping (e) of TEM grid-patterned copper film on silicon. (f) XRD pattern of copper film on glass. B

DOI: 10.1021/acs.langmuir.6b01118 Langmuir XXXX, XXX, XXX−XXX

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Figure 3. Optical photographs of patterned large-area copper films on glass (a), colored PP sheet (b), green leaf (c), PET flake (d), and PDMS film (e). Note that each substrate was only patterned on top side and therefore a complete copper film was grown on the bottom side. modification.21 Briefly, the PDA-coated substrate was immersed in a freshly prepared solution containing 0.01 wt % HAuCl4 and 0.4 mM hydroxylamine hydrochloride for 30 min reaction. Then the substrate was rinsed with DI water and dried by N2. Characterizations. UV−vis spectroscopy was performed with a UV-2550 (Shimadzu, Japan). SEM images were obtained from a JSM6510LV (JEOL, Japan) or a JSM-7800F (JEOL, Japan). Raman spectra were collected on a Renishaw Raman microscopy (Invia Reflex). XPS spectra were measured on an Escalab 250xi system from Thermo Scientific. XRD patterns were collected on Shimadzu XRD-7000 diffractometer. FTIR spectra were recorded on Nicolet FTIR 6700 spectrophotometer (Thermo Nicolet) under ATR mode. Sheet resistance of metal film was measured by a Keithley 2400 source meter with a four-point probe meter (ST2558B-F03, SuZhou Jingge Electronic Co., Ltd., China).

unveils the spatially defined distribution of copper element on the substrate (Figure 2e). The X-ray diffraction (XRD) pattern (Figure 2f) confirms the nature of metallic copper film (JCPDS No. 04-0836), and its thickness is determined to be around 100 nm according to the cross-section view of SEM (Figure S1). The sheet resistance of resultant copper film reaches 7.7 ± 0.8 Ω/□ according to four-point probe measurement. It is also worth noting that the thickness of copper film could be finely tuned in certain range by simply changing the parameters for electroless plating such as plating duration and concentration of bath solution in case that metal film of different thickness is needed. One may notice the slight nonspecific deposition of copper on irradiated area in Figure 2d, which may be efficiently inhibited by optimizing the parameters of electroless deposition to slow down the kinetics of copper nucleation and/or growth. This strategy is applicable to a broad variety of substrates, ranging from inorganic and rigid glass and silicon, polymeric flexible PET flake, PP sheet, to soft elastomer PDMS film and even green leaf with curved surface (see Figure 3 and more in Figure S2). Large-area (wafer-sized) patterned films could be also obtained by using this strategy. The versatility of this strategy originates from the universal PDA film, which is an adherent polymeric film mimicking the adhesive foot proteins secreted by mussels and could be formed on virtually all types of solid surfaces.17,22 Notably, all the deposited copper films display good adhesion on various substrates. For example, the patterns deposited on PET flake keep adherent and conformable even after several hundred times of bending and releasing. Thus, this approach offers great promise for use in stretchable and flexible electronics after necessary structural optimization.23 The good

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RESULTS AND DISCUSSION By using a transmission electron microscopy (TEM) grid (125 μm pitch) as a photomask, we first demonstrate the patterning of copper film on silicon surface. As shown as the scanning electron microscopy (SEM) images in Figure 2, after 2 h electroless deposition in a copper plating solution, the resultant copper film exactly duplicates the pattern of the TEM grid. The copper deposits only in the shadowed areas (with pristine PDA film) to form continuous and uniform copper film (bright frameworks in Figure 2a,b; also see its surface details in Figure 2c), while in the exposed areas no copper film was formed (dark circles in Figure 2a,b) except for a few metal clusters scattering on the substrate. Well-defined boundary of copper film between the exposed and shadowed areas was clearly observed (Figure 2d). Elemental mapping by energy dispersive X-ray spectrometry (EDS) further C

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Figure 4. SEM images (a−d) and EDS Au mapping (e) of TEM grid-patterned gold film; SEM images (f, g) and EDS Ag mapping (h) of TEM gridpatterned silver film on glass; (i) an optical photograph of large-area silver pattern on glass.

adhesion possibly stems from the high interfacial adhesive strength of mussel-inspired PDA film adhering to substrates as well as its strong binding toward deposited metal film.22,24−27 We also notice that the thickness of PDA film (the thickness is typically ca. 30 nm for 7 h immersion17) does not significantly influence the patterning efficiency. Patterning could be achieved regardless of the thickness of PDA film, while longer irradiation time is needed for thicker film. With this strategy, other metal films could also be patterned by simply changing the plating solution. Figure 4 shows the SEM images and EDS mapping images of patterned gold and silver films using the same TEM grid as a photomask (also see XRD patterns of these metal films in Figure S3). It is observed that, similar to copper film, both the gold and silver film exactly inherit the pattern of the photomask. Metals are selectively deposited only in the shadowed areas (namely, on pristine PDA surface). Interestingly, the gold film is formed by separated gold nanoparticles with dimension in several hundred nanometers, which exhibits broad absorbance in 500−800 nm wavelength range (Figure S4). It is also possible to obtain continuous gold film pattern by e.g. galvanic replacement on sacrificial copper pattern or directly optimizing the conditions of gold deposition. For silver pattern, continuous film comprising large-sized silver particles is formed on the substrate and its sheet resistance is measured to be ca. 0.8 Ω/□ (Figure S5, for 30 min plating). The difference in microscopic structures of copper, silver, and gold films is very likely arising from the different surface energies of three metals, as reviewed recently by Chen and co-workers.28,29

To investigate the underlying mechanism of this PDA-based patterning strategy, X-ray photoelectron spectroscopy (XPS) of PDA film on silicon was collected before and after the UV irradiation. XPS peaks originating from C, O, and N atoms were measurable on the survey spectrum of pristine PDA. After UV illumination, the intensities of C 1s and N 1s are significantly decreased, as shown in Figure S6, possibly suggesting the oxidation of PDA film. On the high-resolution spectra shown in Figure 5, the pristine PDA film demonstrates a broad C 1s peak, which could be fitted with four peaks assigned to C−C/C−H, C−O/C−N, CO/CN, and π → π* species, respectively.18 After UV irradiation, the peak intensity decreases significantly and the peaks from C−O/C−N and CO/CN almost disappear, implying the diminishment of the carbon−oxygen covalent bonds in the PDA film upon UV irradiation, possibly via the formation of gaseous carbon dioxide and water. At the same time, the relative intensity of N 1s signal becomes negligible upon the UV irradiation, further indicating the oxidation of the PDA film. UV irradiation has been widely used to clean various surfaces due to its intrinsic ability of oxidizing organic contaminants to water and carbon dioxide in the presence of molecular oxygen.30,31 Therefore, it is not surprising that the PDA film was oxidized upon UV illumination. Actually the PDA pattern produced by UV irradiation could be directly observed with SEM (Figure S7). Additional Fourier transform infrared (FTIR) spectra and Raman spectra also confirm the oxidation of PDA and the destruction of its active catechol group upon UV irradiation (Figures S8 and S9).32−35 Together with XPS data, D

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Figure 5. High-resolution XPS C 1s (a, b) and N 1s (c, d) peaks of PDA film deposited on silicon before (a, c) and after UV irradiation (b, d).

stability; when exposing to UV irradiation, however, it is oxidized and loses its catalytic ability to complex metal ions and to initiate the film growth, thus enabling patterning of metal films. Considering its versatility and simplicity, this approach may demonstrate great applications in manufacturing various innovative devices.

these results unambiguously confirm that UV irradiation induces oxidation of PDA film. It could be concluded that this patterning approach relies on the UV-induced spatially selective oxidation of PDA film. In its pristine state, PDA acts as a catalytic layer for electroless deposition of metal films. The two hydroxyl groups of the catechol moiety present on PDA film show strong chelation affinity toward various metal ions, and the formed strong bidentate complexes would facilitate the initial nucleation and sequential growth of the metal films on the substrate in electroless deposition process, as demonstrated previously.17,35,36 When exposing to the UV irradiation, on the other side, these catechol groups were oxidized, and the PDA film lost its catalytic activity for electroless deposition; the nucleation and local growth of the metal were inhibited (as shown in Figure 1), thus enabling successful patterning of metal films. It has also been reported that the PDA possesses redox activity to directly reduce metal (gold and silver) ions.17,37 Our control experiments unveil that without the exogenous reducing reagents no silver or copper film could be formed on PDA film. For gold, only sparse nanoparticles formed on the surface. It suggests that the main role of PDA in present work is catalytic layer for metal chelating and nucleation.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b01118.

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UV−vis, XPS, Raman, FTIR spectra, optical photographs, SEM images, and XRD patterns (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (W.H.Hu). Notes

The authors declare no competing financial interest.

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CONCLUSION

ACKNOWLEDGMENTS

We gratefully acknowledge the financial support from National Natural Science Foundation of China (No. 21273173), Fundamental Research Funds for the Central Universities (XDJK2015B014), and Chongqing Key Laboratory for Advanced Materials and Technologies of Clean Energies.

In summary, we developed a facile approach for patterning of metal films on arbitrary substrates by using the PDA film as a UVsensitive and universal catalytic layer for electroless deposition. Bio-inspired PDA film, in its pristine state, acts as a catalytic layer for electroless deposition of metal films with high adhesion E

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DOI: 10.1021/acs.langmuir.6b01118 Langmuir XXXX, XXX, XXX−XXX