Accepted Manuscript Nature-Inspired Bilayer Metal Mesh for Transparent Conducting Electrode Application Neha Sepat, Vikas Sharma, Devendra Singh, Garima Makhija, Kanupriya Sachdev PII: DOI: Reference: S0167-577X(18)31292-8 https://doi.org/10.1016/j.matlet.2018.08.088 MLBLUE 24799 To appear in: Materials Letters Received Date: Revised Date: Accepted Date: 12 July 2018 13 August 2018 16 August 2018 Please cite this article as: N. Sepat, V. Sharma, D. Singh, G. Makhija, K. Sachdev, Nature-Inspired Bilayer Metal Mesh for Transparent Conducting Electrode Application, Materials Letters (2018), doi: https://doi.org/10.1016/ j.matlet.2018.08.088 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Nature-Inspired Bilayer Metal Mesh for Transparent Conducting Electrode Application Neha Sepat1, Vikas Sharma1, 3,*, Devendra Singh1, Garima Makhija1, Kanupriya Sachdev1, 2,* Department of Physics, Malaviya National Institute of Technology, Jaipur, 302017, India Materials Research Centre, Malaviya National Institute of Technology, Jaipur, 302017, India 3Department of Physics, Indian Institute of Technology Delhi, New Delhi, 110016, India 1 2 * Email: email@example.com, firstname.lastname@example.org Nature-inspired structures as transparent conducting electrodes are exciting alternatives to conventional TCEs because they provide higher transmittance and conductivity at low temperatures on flexible substrates. The current work is focussed to develop a metal mesh structure with the help of plant leaf vein as a template. Bilayer metal mesh of thickness < 100 nanometer was deposited and transferred to flexible plastic substrate at room temperature. Scanning electron microscopy images were used to obtain the ratio of open area space and covered space of the electrodes. The bilayer metal mesh structure shows high optical transmittance (>85%) and electrical resistivity of the order of 10-4 ? cm. The metal mesh TCE based on leaf vein template opens up new ways of obtaining large charge transfer resolving the junction resistance problem encountered in case of metal nanowires. KEYWORDS: Transparent Conducting Electrodes (TCEs); Bilayer Metal Mesh; Electrical properties; XPS. Introduction The primary requirement for TCEs is that it should allow transport of both electrons and photons . Flexibility, long-term stability, non-toxicity and cost-effective processing are other important requirements depending on different applications . The transparent conducting electrode is a crucial component for optoelectronic devices such as liquid-crystal displays, touch screens, OLEDs, photovoltaic . Over the last decade a persistent increase in optoelectronic devices, has led to exploring new materials and techniques for enhancement of required properties of a TCE. Indium tin oxide (ITO) is the most widely used TCE, but it has several drawbacks viz, depleting availability, high fabrication cost and mechanical brittleness, which restricts its application. To overcome these drawbacks, many ITO alternatives have been developed and tested, such as oxides other than ITO, metal nanowire networks, conducting polymers, carbon nanotubes and graphene, thin metal film and metal grids . Carbon-based TCEs provide excellent mechanical flexibility, but their electrical performance is limited. Metal nanowires, which own excellent conductivity and flexibility and can be manufactured using lowcost techniques, suffer from high junction resistance and low electrical uniformity throughout the electrode network . Metal mesh TCEs are seen to exhibit extremely low sheet resistance with high optical transmittance and good mechanical flexibility. Also they provide a uniformly distributed metal network over the entire electrode surface and do not suffer from junction resistance . Recent reports have explored metal grids or meshes, but there is still scope for advancement in this area   . In this paper, we demonstrate a facile and inexpensive new strategy to develop high-performance flexible bilayer metal mesh inspired by natural network structure. These networks achieve excellent current delivery with minimal optical shading. These mesh structures find applications in photovoltaics, transparent heaters, photoelectrochemical devices and other flexible and transparent energy and optoelectronics devices. Experimental Details Fabrication: The bilayer metal meshes were fabricated using leaf vein network as template. Description of fabrication of vein structure template is given in figure S1 (supplementary data). A brief description is given here. Leaves of Ficus Religiosa, Hamelia and Cassia fistula were immersed in 0.5 mg ml-1 Na2CO3 solution at 800 C for varying periods depending on the thickness and roughness of the leaves. The green part of the leaves was removed by a soft brush followed by washing and drying. Obtained vein skeleton were then pasted on glass substrates for metal deposition. Schematic description of vein structure metal mesh fabrication process is shown in figure 1 (a-upper part). The sputtering of the metal layer was done by the standard sputtering target with 99.9% purity using DC sputtering. To address issues of oxidation and environmental stability, a bilayer combination was chosen for transparent electrode fabrication. The vein template underneath was removed by heating with vein side downward. To protect the sample from dangling glass as weight was used. The produced metal mesh structure was washed and transferred to PET substrate and heat treated to improve adhesion. There were two combinations Ag-Au and Ag-Cu which were investigated for this bio-inspired metal mesh structure. The optical, electrical and structural properties were measured using standard characterization facilities. Results and Discussion Scanning Electron Microscopy (SEM): The SEM results (Figure 1 (b-g, lower part) show that the metal mesh follows the morphology of leaves vein structure. Closed area ratio (CAR) and Open area ratio (OAR) of the leaves network was calculated through water shading tool of ImageJ. The insets of figure 1 provides area fraction of the open area ratio and the covered area ratio of the respective mesh structure . Hamelia leaf has maximum OAR and interconnectivity of veins. For both bilayers uniform and smooth coatings are seen. It is observed that in the Hamelia leaf the vein structure is more interconnected as compared to others. For Ficus Religiosa and Cassia Fistula the template structures are open ended. These vein structure of two leaves (FR and CF) is denser than hamelia leaf, but with less interconnection. The vein size also varies from nm thickness to micrometre thickness result in a mesh structure of metal with same dimensions . Figure1. (a) Description of the mechanism of vein structure used as a template for fabrication of metal grid and SEM image of (b) Ag-Au metal mesh using Hamelia leaf, (c) Ag-Au metal mesh using Cassia Fistula and (d) Ag-Au metal mesh using Ficus Religiosa(e) Ag-Cu metal mesh using Hamelia, (f) Ag-Cu metal mesh using Cassia Fistula (g) Ag-Cu metal mesh using Ficus Religiosa leaves vein structure.In set images are showing the area fraction of the open area ratio (OAR) to the covered area ratio (CAR) of respective metal meshes. Optical and Electrical Properties :Optical transmittance is an essential property for transparent and conductive electrodes . The comparison of transmittances for metal meshes based on different leaves networks are shown in figure 2. Optical properties of metal grids having Hemelia leave vein structure are superior as compared to that of Cassia Fistula and Ficus Religiosa vein structured metal mesh. The difference in transmittance of metal meshes is due to the difference in open area ratio of the respective leaves vein network. Furthermore, the transmittance of metal mesh is nearly constant in the near UV to the visible range of light. Obtained average transmittance using Hamelia leaves, for Ag-Cu metal mesh is 91.51 % and 80.24% for Ag-Au metal mesh in the wavelength range 350 to 800 nanometer. The obtained value of transparency for a metal mesh using Hamelia leaves vein structure is good enough to be compared to that of commercially available transparent electrodes (ITO)  . Figure 3. Optical transmittance spectra for the (a) Ag-Cu metal mesh and (b) Ag-Au metal mesh using three different leaves. Resistivity and a Sheet resistance of the (c) Ag-Cu metal mesh and (d) Ag-Au metal mesh using Hamelia leaf vein structure. The electrical properties of metal mesh were investigated using four-point probe measurement  and the results are presented in figure 2. The sheet resistance of Hamelia sample was obtained as 5.7 ??? for Ag-Cu metal mesh and 14.7 ??? for Ag-Au metal mesh. The conductivity of metal mesh is influenced by mesh width and thickness as well as material resistivity ; the resistivity of the metal mesh using Cassia Fistula and Ficus Religiosa leaves vein structures is higher than that of Hamelia leave vein based metal mesh. Ag-Au Ag-Cu Metal Mesh Metal-Mesh Sheet resistance(?/?) 14.7 5.70 Optical transmittance at 550nm 80.1% 91.67% Average transmittance for 350-800nm 80.24% 91.51% The figure of merit (??) (�-3) 7.4 73 Table 1. Sheet resistance, transmittance and Figure of merit of Ag-Au metal mesh and Ag-Cu metal mesh using Hamelia vein structure. The data listed in Table 1 summarizes the optoelectronic parameters of the samples. The optical transmittance for Ag-Au is 80.24% and for Ag-Cu is 91.51% for metal mesh using Hamelia leaf vein network. To compare the performance of the different metal meshes Figure of merit (FOM) was obtained using the following equation given by Haacke as FOM = Tav10/Rs, where Tav is average transmittance and Rs is the sheet resistance . Higher vakue of FOM corresponds to more efficient metal mesh. FOM calculated for Ag-Au is 7.4 �-3 ?-1 and for Ag-Cu is 7.3 � 10-2 ?-1. X-ray Photoelectron Spectroscopy: XPS was done to determine the elemental composition and the chemical state of the elements of the metal meshes. Figure 3 gives the high-resolution XPS spectrum of Ag-Au and Ag-Cu metal meshes, where the raw data spectrum (black) is fitted using Casa XPS software. Figure 3(a) represents the Ag core level spectrum of the Ag-Au metal mesh on Hamelia leaf structure. The most intense peaks positioned at 374.14 eV and 368.12 eV are attributed to Ag 3d3/2 and Ag 3d5/2 respectively . Figure 3(b) represents the Au 4f highresolution spectrum of the Ag-Au metal mesh, where the 4f photoemission is split between two peaks positioned at 87.68 eV and 84 eV corresponding to Au 4f5/2 and Au 4f7/2 respectively . Similarly, figure 3(c) represents Ag 3d high-resolution spectrum of the Ag-Cu metal mesh. The peaks positioned at 374.22 eV and 368.22 eV correspond to Ag 3d3/2 and Ag 3d5/2 respectively. Figure 3(d) represents Cu 3p high-resolution spectrum of the Ag-Cu metal mesh, the peaks positioned at 952.4 eV and 932.6 eV correspond to Cu 2p1/2 and Cu 2p3/2 respectively . The XPS results affirm the existence of metals in their natural state. (a) (b) (c) (d) Figure 3. XPS spectra of (a) Ag 3d of Ag-Au, (b) Au 4f of Ag-Au, (c) Ag 3d of Ag-Cu, (d) Cu 3p of Ag-Cu Metal mesh having Hamelia vein structure. Conclusions Through this study, we have demonstrated development of nature-inspired transparent conducting networks. The metal meshes were successfully fabricated using leaves vein structures and investigated for their optical and electrical properties for TCE application. The obtained average transparency of these metal meshes is about 80 %-90% with resistivity values in the range of 10-4 ?-cm. Metal meshes using Hamelia leaves vein structure have outstanding electrical and optical properties in comparison to those of Cassia Fistula and Ficus Religiosa leaves. The metal grid electrode using Hamelia vein network with exceptional properties show promise for current & future applications. Acknowledgements Authors are thankful to Materials Research Centre, MNITJ for providing characterization facilities for this work. References  K. Ellmer, Nat. Photonics 6, 808 (2012).  A. Kumar and C. Zhou, ACS Nano 4, 11 (2010).  G. U. Kulkarni, S. Kiruthika, R. Gupta, and K. Rao, Curr. Opin. Chem. Eng. 8, 60 (2015).  V. Sharma, R. Vyas, P. Bazylewski, G. S. Chang, K. Asokan, and K. 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