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j.matlet.2018.08.088

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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
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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: phyvikas@gmail.com, ksachdev.phy@mnit.ac.in
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 [1]. Flexibility, long-term stability, non-toxicity and cost-effective processing are other
important requirements depending on different applications [2]. The transparent conducting
electrode is a crucial component for optoelectronic devices such as liquid-crystal displays, touch
screens, OLEDs, photovoltaic [3]. 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 [4]. 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 [5]. 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 [6]. Recent reports have explored metal grids or meshes, but there is still scope for
advancement in this area [7] [8] [9].
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 [10]. 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 [6].
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 [11]. 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) [12] [13].
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 [12] 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 [8]; 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 [14]. 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 [11]. 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 [4].
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 [15]. 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.
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