HIGH-ADHESION METALLIC NETWORKS
In article 1900056, Jinwei Gao and co-workers present a high adhesion metallic network, coated with root-mimicking dendritic nanowires, inspired by the ground-gripping
functionality of bamboo roots. This network shows not only good optoelectronic performance (≈ 85% transparency and ≈1.5 Ω sq
21sheet resistance), but also excellent
adhesion to flexible substrates and enhanced haze, which is of benefit to the effective window electrodes of OLEDs and solar cells.
https://www.onlinelibrary.wiley.com/doi/10.1002/admt.201970043
DOI: 10.1002/admt.201900056
mers,[6,7] carbon materials (graphene[8,9]
and carbon nanotubes[10,11]), metal nano
wires,[12–14] and metallic networks.[15–18]
Among those, metallic networks with their good optoelectronic properties and excellent mechanical flexibility are the most promising.
Previously, some of the coauthors of this work have developed metallic net
works based on the cracking technology.[15,18,19] This technology employs thin films of a sacrificial material (deposited on a substrate), which cracks while drying. After metal deposition and liftoff of the sacrificial material, a continuous network of metallic nano/micro ribbons forms directly on the substrate.
This technology produces excellent TCEs on solid substrates, outperforming ITO. While this technology is compatible with flexible substrates, adhesion remains a problem due to addi
tional stresses experienced during substrate flexing. There are two failure mechanisms due to flexing: metal film fracture and delamination from the substrate. Fracture has been thor
oughly studied in the context of tensile strain and bending
etc.[15,18] Recent strategies to improve the nanowire network–
substrate adhesion include mechanical pressing,[20] plasmonic laser nanowelding,[21] nanosoldering,[22] additional solvent treat
ment,[23] polymer encapsulation, and TiO2 gel coverage.[24,25]
These harsh processes cause typically some damage to the net
work, and therefore, strategies for simultaneous improvement of adhesion while maintaining device performance remain a challenge. Recently, metallic films with nanopile interlocking have been fabricated, and have some potential to resolve this problem.[26] Here, we developed a highadhesion (HA) flexible TCE, based on a metallic crack network combined with the bamboo root idea. This HA network shows excellent optoelec
tronic properties, combined with superb adhesion to flexible substrates.
Figure 1a shows the rhizome structures on the bamboo roots, which through a characteristic root interlocking, provides exceptional mechanical support and stability for the bamboo plant. Figure 1b shows schematic of our highadhesion net
work, with the dendritic nanowires resembling the rhizome structures (see the zoomin panel in Figure 1a). Figure 1c illus
trates steps of our fabrication process, which begins with the spin coating of a polyimide (PI) layer on a silicon wafer (Step 1).
After solidification of PI, a sacrificial crack mask (nail polish) is spin coated on the PI surface, and subsequently selfcracked during drying (Step 2). The cracking pattern is then copied onto PI film by using plasma etching (Step 3). The electroplating
conducting oxides, show superior optoelectronic performance and mechani cal
flexibility, their adhesion to flexible substrates remains a problem. In this work, a high adhesion metallic network inspired by the ground-gripping func- tionality of bamboo roots is developed. This network shows not only excellent optoelectronic performance but also superb adhesion to flexible substrates could be beneficial for flexible electronics.
Dr. G. Dong, Prof. M. Pan
School of Mechanical and Automotive Engineering South China University of Technology
Guangzhou 510640, China E-mail: mqpan@scut.edu.cn
Dr. G. Dong, S. Liu, Prof. K. Kempa, Prof. J. Gao
Institute for Advanced Materials and Guangdong Provincial Key Laboratory of Quantum Engineering and Quantum Materials Academy of Advanced Optoelectronics
South China Normal University Guangzhou 510006, China E-mail: gaojinwei@m.scnu.edu.cn Prof. G. Zhou
Guangdong Provincial Key Laboratory of Optical Information Materials and Technology and Institute of Electronic Paper Displays
South China Academy of Advanced Optoelectronics South China Normal University
Guangzhou 510006, China Prof. J.-M. Liu
Laboratory of Solid State Microstructures Nanjing University
Nanjing 210093, China Prof. K. Kempa Department of Physics Boston College Chestnut Hill MA 02467, USA
The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/admt.201900056.
Highperformance transparent conductive electrodes (TCE) are essential components of various optoelectronic devices, such as flat panel displays,[1] touch screens,[2] solar cells,[3] photovoltaic cells,[4] and organic lightemitting diodes.[5] Transparent con
ducting oxides (TCO) in general, and indium tin oxide (ITO) in particular, are the most common materials for TCE. However, due to high mechanical brittleness, their application in flex
ible electronics/optoelectronics is very limited. This, combined with TCO increasing expense (e.g., due to scarcity of indium) makes the search for their replacement urgent. Many potential
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(the electroplating processes is shown in the Experimental Sec
tion) and washingout of the sacrificial layer, follows (Step 4). In Step 5, the array is coated first with UV glue, and then with a flexible polyethylene terephthalate (PET) film. Finally, in Step 6 the film stack is pilledoff the Si wafer. This is the final product:
a high adhesion metallic network, anchored to the flexible sub
strate with the metallic nanodendrites. This is a low tempera
ture, solution based process, which involves no high vacuum processing.
We use selfcracking mask as a temple to generate metallic networks.[15] The cracking mechanism has been detailed.[14,17,35]
Different inexpensive materials, e.g., egg white, are discussed in Figure S1 in the Supporting Information. We chose the nail polish, since it provides the best morphology; the crack pattern generated by the nail polish is finer and more uniform (see Figure 2a), than the corresponding eggwhite film (Figure S2 in Supporting Information). 2b,c shows microscopic images of the morphologies of network, b) before and c) after transfer
ring to the flexible substrate. A photograph of the final network on a flexible PET substrate presented in Figure 2d shows that the network has a significant haze. While this prevents dis
play applications of this TCE, it is advantageous for the flexible light source and photovoltaic applications. In principle, we can
control the thickness and depth of the dendritic nanowires by controlling the reaction time, and the reaction solution. In this manuscript, the typical length scale of the dendritic nanowire is about 5–10 µm in length and 100–500 nm in thickness, and roughly 45° angle between wires.
Optoelectronic performance (transmittance vs sheet resist
ance) of our highadhesion networks is shown in Figure 3a and Table S1 in Supporting Information. For comparison, we also list those data.[13,25–34] Our networks outperform other net
works, with a very high figure of merit[9,33] ranging from 700 to 1400. Figure 3b shows transmittance and the haze[15] versus radiation wavelengths. Due to the lightscattering dendritic structure (Figure 2b and Figure S3, Supporting Information), the HA networks show significant haze of ≈20%, as compared with ITO (≈2%), and CNN without dendrite structures (denoted as “network”) (≈8%). The details for optoelectronic measure
ment are shown in the Experimental Section.
Figure 4 confirms excellent mechanical bending stability of our networks. The detailed procedures for bending test are shown in the Experimental Section. Figures S4 and S5 in the Supporting Information also show the setup for bending test. Figure 4a shows sheet resistance of our network (crack nail polish) and ITO, subjected to multiple bending events.
Figure 1. Schematic diagram of high-adhesion transparent networks and the fabrication processing. a) Photograph of a bamboo root, with the enlarged image on the right showing details of the hierarchical branches. b) Schematic of a high-adhesion metallic network which mimics the bamboo root.
The enlarged image schematically shows the cross-section of a single line of the network. c) Schematic of the fabrication processes using a solution method, including six steps.
While ITO shows strong resistance fluctuations during bending, and a permanent large resistance increase after bending, our HA network is 3–4 orders of magnitude more stable in both respects, as shown in Figure 4b.
Figure 5a shows the results of the adhesion peeroff test (using 3M tape) on our HA crack network, and simple crack networks made with different metal deposition techniques:
thermal evaporation, sputtering, and the electroplating (the samples fabricated with thermal evaporation and sputtering are shown in the Experimental Section). The peeloff experi
mental details are shown in the Experimental Section and Figures S6 and S7 in Supporting Information. It is clear, that
the HA network is by far the best, with no sign of damage even after 100 pilloff events. This conclusion holds also for the ultrasonication test (shown in Figure 5e, the experimental details and setup are shown in the Experimental Section and Figures S8 and S9 in Supporting Information), with the HA network surviving (with no sign of damage) 1200 s of ultrasoni
cation. The optical images of the networks, taken at the end of the tests, and shown in Figure 5b–d,f,h, are consistent with the analysis above.
In conclusion, by mimicking the roots of bamboo, we have fabricated highadhesion crack metallic networks on flexible substrate. In addition to excellent optoelectronic performance, Figure 2. Network Morphology and photograph. a) Optical microscope image of the cracking pattern with nail polish as a crack template (pro- cessing Step 2). b) SEM image of the network after electroplating (processing Step 4). The inset shows a magnification of the network fragment.
c) SEM image of the final network, with dendritic nanowires embedded in the flexible PET substrate (processing Step 6). d) Photograph of the final network on a flexible PET substrate.
Figure 3. Optoelectronic performance of our high-adhesion networks. a) Optical transmittance versus the corresponding sheet resistance (at 550 nm wavelength), for various samples (a circuit indicates the data set in this paper). b) Optical transmittance (left panel) and haze (right panel) as a func- tion of wavelength for our network with or without roots, along with ITO film of 150 nm thickness.
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with ≈85% transparency and ≈1.5 Ω sq−1 of sheet resistance, the arrays show exceptional adhesion, as confirmed by the 3M tape and ultrasonication tests. This strongly enhanced adhe
sion results from the dendritic nanowire structure, forming on the network lines/ribbons during electroplating. This dendritic structure enhances haze (≈20%), which prevents display applications of this TCE, but is advantageous for the flexible solar cells.
Experimental Section
Preparation of Silicon Substrates and the PI film: P-type, polished crystalline silicon wafers (Sheng Shun Electronic Co., Shenzhen, China) were washed with deionized water, acetone (Aladdin Reagent Co.), and ethanol, followed by drying in nitrogen. This was followed by plasma
cleaning for 100 s (Figure S10a, Supporting Information). The PI solution (Aladdin Reagent Co.) was spin coated on the wafer surface (500 rpm for 10 s, and 3000 rpm for 20 s). After curing for 30–50 min at 80–85 °C, the PI solid film formed.
Preparation of Cracking Templates: Egg white and distilled water were mixed in the ratio of 2:1, and centrifuged at 3500–5000 rpm for 5–10 min.
Nail polish was used without further purification. Egg white, water solution, and the nail polish were spin coated on the surface of the PI film (500 rpm for 10 s, then spinning at 3000 rpm for 20 s). The surface of PI film was changed from hydrophobic to hydrophilic by plasma treatment (30–60 s), improving the coating quality (the contact angles are shown in Figure S10b,c in the Supporting Information). Cracking networks formed automatically at room temperature in 6–10 min (humidity: 20–30%, line width: 4–20 µm, spacing between: 40 and 100 µm, thickness: 10 and 40 µm). The mechanism of the crack formation was due to the stress release.[35] During the etching process (400–600 s), the PI layer was selectively removed. Finally, the sacrificial layer was removed, either with the deionized water in the case of the egg Figure 4. Bending flexibility performance. a) Sheet resistance variations under multiple bending for ITO film (red line), and our HA network (blue line).
b) Sheet resistance variations under multiple bending of our HA network.
Figure 5. Adhesion performance of crack networks on the PET substrate, made with different metal deposition techniques: thermal evaporation, sput- tering, and the electroplating in this paper. a) Sheet resistance versus number of peel-off events, in the 3M tape test. SEM images of networks after 3M tape, and ultrasonication tests, respectively: b,f) for thermal evaporation, c,g) sputtering, d,h) electroplating. e) Sheet resistance versus ultrasoniction time. The scale bars in (b–d) and (f–h) are 100 µm.
samples were tested using optical spectrometer (Maya 2000 Pro, Ocean Optics, China) with a test wavelength range of 400–800 nm. The sample size was 30 mm × 30 mm. The sheet resistance of the sample was measured by the Van der Pauw method.[38]
Mechanical Property Test (Bending, Peel-Off, and Sonication): Bending Test—The test was done by a home-made machine (Figures S4 and S5, Supporting Information) which included a stepper electrode, a DC power supply, a control system, and a motion device. First, the test sample was fixed on the mobile station, and both ends of the sample were connected to the keithley 2400 through wires. The keithley 2400 real- time recorded the resistance, and the control system controlled the left and right movement of the mobile station. Since the sample was fixed on the mobile station, the sample was bending with the movement of the motion device. The bending radius of the HA network was 6 mm, and the speed of the bending was 5 mm s−1.
Peel-Off Test: The test sample was fixed and both ends of the sample were connected to the keithley 2400 through wires. The keithley 2400 real-time recorded the resistance. The data was recorded with keithley 2400, while peering action was recorded by manual operation. The 3M tape (Model 244, 3M Company USA) was cut into a bent shape with a width of 5 mm. The detachment angle was about 45 °C, and peeling off speed was about once within 3 s (Figures S6 and S7, Supporting Information).
Sonication Test: Both ends of the sample were connected to the keithley 2400 (Keithley 2400, USA) through wires, and the sample was placed in deionized water of the ultrasonic equipment (Kunshan ultrasonic instrument Co., Ltd, KQ-100DB), and then the equipment was turned on. The resistance was continuously recorded with the sonication time.
The parameters of sonication test were sonication frequency: 40 kHz, power output: 100 W, which is the maximum power output (at 40 kHz and 100 W) (Figures S8 and S9, Supporting Information).
Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements
G.P.D. and S.L. contributed equally to this work. The authors thank the financial support from NSFC-Guangdong Joint funding, China (No. U1801256), National Key R&D Program of China (No. 2016YFA0201002), Guangdong Provincial Foundation (2016KQNCX035), NSFC grant (No. 51803064, 51571094, 51431006, 51561135014, U1501244), Program for Chang Jiang Scholars and Innovative Research Teams in Universities (No. IRT_17R40), and Guangdong Innovative Research Team Program (No. 2013C102). The authors also thank the support from the Guangdong Provincial Engineering Technology Research Center for Transparent Conductive Materials.
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