Wafer-scale, stretchable nanomeshes from an ultrathin-support-layer assisted transferKyung Jin Seo, Xun Han, Yi Qiang, Xuanyi Zhao, Yiding Zhong, Zhan Shi, and Hui Fang

Citation: Appl. Phys. Lett. 112, 263101 (2018); doi: 10.1063/1.5031040View online: https://doi.org/10.1063/1.5031040View Table of Contents: http://aip.scitation.org/toc/apl/112/26Published by the American Institute of Physics

Wafer-scale, stretchable nanomeshes from an ultrathin-support-layerassisted transfer

Kyung Jin Seo, Xun Han, Yi Qiang, Xuanyi Zhao, Yiding Zhong, Zhan Shi, and Hui Fanga)

Department of Electrical and Computer Engineering, Northeastern University, Boston, Massachusetts 02115,USA

(Received 27 March 2018; accepted 8 June 2018; published online 25 June 2018)

Metal nanomeshes possess unique electrical and mechanical properties for next-generation stretch-

able electronics. However, a critical unmet need lies in producing stretchable conductive nano-

meshes at large scale with high uniformity and intactness. Here, we present a wafer-scale

nondestructive transfer method by utilizing an ultrathin polyimide layer. This polyimide support

layer allows etchant vapor to transmit through to etch the sacrificial layer underneath, while being

continuous to support the nanomeshes during transfer before being removed completely after the

transfer. From this simple yet effective method, we developed 4-in.-wafer-scale gold nanomeshes

with low sheet resistance of 8.35 X/�, good transparency of 65% at 550 nm, and stretchability of

70%. Detailed vapor transmission studies reveal that etchant vapor indeed transmitted through the

support layer, with realistic sacrificial etching time needed for transfer. Together, these results pro-

vide a practical pathway towards fabricating large-scale nanomesh based stretchable electronics,

with applications ranging from on-skin electronics to implantable biomedical devices. We also

expect this ultrathin support layer approach to be generally applicable to the processing of many

other nanomaterials at large scale. Published by AIP Publishing. https://doi.org/10.1063/1.5031040

Stretchable electronics have emerged as strong contend-

ers for future consumer and bio-integrated electronics.1–8

Since the 1990s, paradigm shift from rigid electronics have

made significant strides, but still fall short of the require-

ments for many applications with more than bendability,

such as wrapping around irregular surfaces, applying them as

ultrasoft interfaces on to biological tissues, or even operating

around dynamically deforming subjects (e.g., a beating

heart).9 Stretchable electronics therefore have gained signifi-

cant interests for their ability to deform, bend, fold, and

stretch, which have led to promising demonstrations in vari-

ous areas, including displays,10 optoelectronics,11 on-skin

electronics12 and implantable biomedical devices.13,14 A crit-

ical component in stretchable electronics is their intercon-

nects, made of stretchable conductors. To achieve good

stretchability, researchers have developed different strategies,

such as conductive polymers,15 graphene networks,16 metal

nanoparticle/elastomer composites,17 liquid metal alloys,18

in-plane or out-of-plane metal micro-wavy structures,19 and

metallic nano-networks or nanomeshes.20–23 Of these strate-

gies, metal nanomesh structures are uniquely attractive due to

their nanoscale textures, enabling properties which can almost

be regarded as “intrinsic” for microelectronics. A nanomesh

is usually a form of interconnected network with its traces at

the nanoscale and mesh openings smaller than or comparable

with the resolution of conventional microfabrication.1,20,21

Compared to most intrinsically stretchable conductive poly-

mers, metal nanomeshes possess superior electrical conductiv-

ity. Compared to other stretchable approaches such as metal

nanoparticle/elastomer composites, liquid metal alloys and

metallic nano-networks, nanomeshes are more compatible

with standard micro/nano-fabrication. Compared to feature

sizes generally over 5 lm in microscale wavy or mesh struc-

tures, aggressive scaling of the mesh into 50 nm-scale enables

the resulting nanomesh to be advantageous for feature minia-

turization. Metal nanomeshes also possess large transparency

over a broad spectrum, enabling an important add-on property

to allow light to go through.

Besides using high-resolution lithography tools, several

efforts have been devoted to develop soft lithography

approaches to achieve metal nanomesh patterns. Recent stud-

ies have demonstrated that metal nanomeshes transferred

from a process based on indium grain boundary lithography

can achieve up to 300% one-time strain, due to its nanoscale

spring-like structures.20,21 In this process, metal nanomeshes

were first formed between nanoscale indium grain bound-

aries on silicon (Si) wafers, then lifted off from substrates,

rendering a free-floating film of nanomeshes in diluted HF

solution, and finally picked up by a scooping method onto

polydimethylsiloxane (PDMS) substrate. We envision that

advanced lithography tools such as electron-beam and step-

per lithography can also achieve customized metal nano-

meshes on rigid substrates with similar or even better than

this final performance after transfer at a reasonable cost.

However, many targeted applications demand large-scale

devices with dimensions from centimeter to even meter, yet

there has not been any reliable manufacturing approaches to

transfer these envisioned large-scale nanomeshes without

altering or even damaging the nanomeshes. A key unmet

need that hinders the further development of stretchable

nanomesh based electronics is to achieve large-scale transfer

of them while maintaining their pattern and intactness, which

are critical to achieve reliable and uniform performance over

large-area stretchable electronics.

In this paper, we developed a 4-in.-wafer-scale nonde-

structive transfer method to achieve stretchable gold (Au)

nanomeshes using an ultrathin polyimide (PI) layer as

a)Author to whom correspondence should be addressed: h.fang@

northeastern.edu

0003-6951/2018/112(26)/263101/5/$30.00 Published by AIP Publishing.112, 263101-1

APPLIED PHYSICS LETTERS 112, 263101 (2018)

support. Critically, the support layer was thin enough to

allow etchant vapor to transmit through the nanomeshes and

etch the sacrificial layer underneath, while being continuous

to support the nanomesh during transfer. The nanomeshes on

the support layer also stayed with the source substrate even

after sacrificial-layer partial etching, enabling easy pick up

and transfer. After the transfer, the support layer can be

removed completely by selective etching [using O2 reactive

ion etching (RIE) for PI]. Due to the continuity of the origi-

nal support layer, this process prevents breaking of the nano-

meshes during processing and demonstrate excellent

uniformity. The resulting nanomeshes showed great stretch-

ability of 70% and excellent electrical conductivity with the

sheet resistance of 8.35 6 0.87 X/�. Systematic studies on

the etching of the PMMA sacrificial layer under the PI sup-

port layer revealed that acetone vapor indeed transmitted

through the PI, which did not jeopardize the etching effi-

ciency while enabling both temporal and spatial controllabil-

ity. In principle, this process should also be scalable as the

substrate size increases beyond 4-in. scale. While we

achieved the metal nanomeshes from a modified indium

grain boundary lithography, we expect this ultrathin-support-

layer approach is generally applicable to a vast majority of

fabrication processes of metal nanomeshes and possibly

other nano-systems. Together, these advances provide a

practical pathway towards achieving large-scale metal nano-

mesh-based stretchable electronics.

The fabrication of Au nanomeshes utilizes a modified

soft lithography method using indium (In) [Fig. 1(a)]. To

illustrate, we first deposited an engineered stack of PMMA/

PI/SiO2/PMMA/SiO2/In (from bottom to top) on a blank Si

wafer using combinations of spin-coating and electron-beam

deposition. Here, the PI serves as the ultrathin support layer

with versatile functions: (1) to allow etchant vapor to trans-

mit through for a controlled etching of the sacrificial layer;

(2) to maintain the original pattern and intactness of Au

nanomeshes over large-area. The PMMA at the bottom

serves as a sacrificial layer and the second PMMA layer on

top of the PI (and underneath In) serves a role similar to a

lift-off resist during In lift-off. We developed PMMA as the

sacrificial layer under In grains instead of the previous

reported K2SiO3 to avoid HF etching.20 The first SiO2 layer

is an etch stop which prevents the PI support layer from

being etched during PMMA dry etching. The second SiO2

layer is found to be important for the wetting of In grains to

form suitable grain shapes on PMMA. In film, when

FIG. 1. Fabrication of wafer-scale Au nanomeshes from an ultrathin polyimide support layer assisted transfer. (a) Fabrication of Au nanomesh on the ultrathin

PI: 1. we deposit an engineered stack of PMMA/PI/SiO2/PMMA/SiO2/In with PI as an ultrathin support layer. The first SiO2 layer acts as an etch stop and the

second one acts crucially for adhesion and wetting of In grains, 2. diluted nitric acid etches small In grains and widens the gap between large grains, 3. RIE

etches the mask stack up to the PI layer, and 4. E-beam evaporator deposits Au where Au in between the grains creates nanomesh traces. (b) Sheet resistance

mapping of 4-in.-wafer-scale Au nanomeshes with 30 nm-thickness and 140 nm-width. (c) Transfer with an ultrathin PI support on stretchable substrates fol-

lowing steps in (a): 5. lift-off remaining SiO2 dry etching finalizes nanomesh on PI, 6. acetone soaking etches PMMA to release Au nanomeshes on PI from

the substrate. The PI support helps to maintain nanomesh intactness over large-area and prevent breaking during transfer. 7. PDMS directly picks up the Au

nanomesh on PI, 8. flip, and 9. RIE removes the PI to complete the process.

263101-2 Seo et al. Appl. Phys. Lett. 112, 263101 (2018)

deposited at right thickness, creates nano-sized grains with

irregular structures which can serve as a lift-off mask leading

to stretchable nanomeshes from In grain boundaries. After In

deposition, a diluted nitric acid solution (20%) selectively

cleaned the small In nanoparticles at the grain boundaries

and also widened the gap between grains controllably. A

reactive ion etching (RIE) process then etched PMMA layer

with In as the mask, leaving also lateral undercuts facilitating

the lift-off. After e-beam evaporation deposited Au, lift-off

in acetone with a gentle sonication, followed by SiO2 etching

with RIE finalized the process of achieving Au nanomeshes

on PI. The resulting nanomeshes showed excellent electrical

conductivity with a sheet resistance of 8.35 6 0.87 X/� and

with high uniformity at 4-in. wafer-scale [Fig. 1(b)]. These

steps are standard in micro/nanofabrication techniques and

simple to follow. Other tools such as an e-beam writer and

stepper can produce the nanomesh patterns in fewer steps.

Then, soaking of the samples in acetone released the Au

nanomeshes on PI from the Si substrate [Fig. 1(c)]. Time

control during this acetone soaking process is needed to not

completely release the layers from the Si substrate to float

around in the acetone. The process utilized direct transfer by

using a PDMS substrate (elastomer base and curing agent

with 30:1 ratio) to pick up the Au nanomeshes on PI.

Deposition of a thin Ti/SiO2 sticking layer to the sample,

and UV/ozone (UVO3) treatments to both the sample and the

PDMS receiving substrate resulted in chemical bonding

interface between the two, leading to better transfer and per-

manent adhesion. Then, O2-based RIE completely etched the

PI layer to achieve Au nanomeshes on PDMS. Additionally,

we also tried to fabricate nanomeshes on PDMS substrates

directly. However, PDMS swelled during the acetone sonica-

tion as expected, causing destructive damage to the nano-

meshes. By using the PI support, the resulting wafer-scale Au

nanomeshes can achieve good totality and uniformity. A

transfer of a Mona Lisa figure formed by the patterning of the

Au nanomeshes on the 4 in. wafer demonstrated the capabil-

ity of the support layer (Fig. 2). We used Mona Lisa figure as

a visually appealing example to demonstrate 4-in. wafer-scale

transfer of arbitrary shaped nanomesh features. Detailed SEM

characterization of wafer-scale Au nanomeshes transferred

on PDMS reveals excellent uniformity without observing

obvious defects.

Investigating the PMMA etching further sheds light on

the exact mechanism by which the acetone dissolving of

PMMA occur under the PI support layer. We prepared nano-

mesh samples of different PI thicknesses (300 nm to 2.4 lm)

with 500-nm-thick PMMA underneath and put them into ace-

tone at room temperature. We purposely utilized ultrathin PI

layer such that the acetone vapor can transmit through and

etch the PMMA. Many evidences support that the acetone

etching of PMMA under the ultrathin PI was dominated by

vertical etching instead of lateral [Fig. 3(a)]. First, if vertical

etching is dominating, then the PMMA etching will be

primarily from top to down. Indeed we found from the cross-

sectional SEM image of a ready-to-transfer sample after

acetone etching that there is a �100-nm gap between the PI

and remaining PMMA [Figs. 3(b) and 3(c)]. As mentioned

previously, the PI layer (and the above nanomeshes) was still

attaching with the substrate even when it was ready to trans-

fer, although over soaking in acetone for longer time did lift

off the layers completely. The reason for this attaching might

be attributed to that the as-dissolved PMMA can still serve as

a weak link to hold the PI. This attaching facilitates the trans-

fer and provides excellent spatial control over the process.

Another evidence comes from the PI relieving kinetics.

Figure 3(d) plots the time needed to transfer the PI with Au

nanomeshes as a function of different PI thicknesses. We

notice two trends in this plot. Initially, as the thickness of PI

decreases from microscale to �700 nm, the time to transfer

also decreases. This trend indicates that, instead of lateral

undercut, it is the acetone vapor transmission through the PI

layer that plays a dominating role in partially removing the

underlying PMMA layer; if lateral undercut is dominating,

the time would be similar for all thicknesses. Theoretically,

the relationship of acetone vapor transmission rate (VTR)

through the support layer can be described as

VTR ¼ P � Dp=t; (1)

where P is permeability of the substance through a specific

support layer, Dp is the partial pressure difference between

above and below the support layer, and t is the thickness of

the support layer. Since this rate is normally defined as

g/m2 day, if we assume certain amount of acetone is needed

to etch the PMMA, the etching time and the support layer

thickness theoretically will have a linear relationship, which

is in a good agreement with the experimental trend. As the

PI layer thickness enters the sub-500 nm regime, the time to

transfer appears to saturate at �5 min. This saturation sug-

gests that PMMA dissolution itself is dominating the pro-

cess. In other words, the PMMA dissolution is not fast

FIG. 2. Transfer and uniformity of

wafer-scale Au nanomeshes. The

wafer-scale nanomesh (patterned as a

Mona Lisa figure) demonstrated

stretchability with SEM images at dif-

ferent locations to show high

uniformity.

263101-3 Seo et al. Appl. Phys. Lett. 112, 263101 (2018)

enough to act as an infinite sink for the acetone vapor, which

also in turn slows down the acetone transmission.

To further quantify the acetone vapor transmission

through the ultrathin PI, we estimated the vapor transmission

from the above equation. Assuming we need to etch 100 nm

of PMMA for the transfer, the area-normalized weight of

PMMA would be 1.18� 10�5 g/cm2. From literature, we

estimated the acetone permeability through PI as

1.948� 10�16 g cm/(cm2 s Pa) at room temperature,24 a satu-

rated acetone vapor pressure (Dp) of 53.32 kPa, and a solu-

bility of PMMA in acetone of 15% weight.25 Dividing the

acetone weight needed to dissolve the aforementioned

PMMA by the acetone VTR, therefore, yields the time

needed to transfer. With a PI thickness of 2 lm, we achieved

a time of 1515 s from the calculation, which is in close agree-

ment with the experimental data. In addition to the previous

evidences, this match of the time from theory and experi-

ment further proves that the acetone vapor transmission

through the ultrathin PI leads to PMMA etching. This trans-

mission not only provides homogeneous PMMA etching rate

across the entire sample surface, but also accelerates the

relieving and transfer process. The transfer time can also be

well controlled with different PI thicknesses, providing cus-

tomizability to both the process and the final structure.

The nanomeshes demonstrated high stretchability,

uniformity, and yield. We performed strain tests of the

achieved Au nanomeshes with 1 in.� 1 in. size, 30-nm-thick

Au, and �140-nm-wide mesh width on �1-mm-thick 30:1

PDMS substrates. Figure 4(a) shows its one-time stretching

performance. As strain is applied, the sample resistance

remained nearly unchanged at its stretched state for up to 40%

strain, then increased with strain. The resistance remained

similar as the original value after release from strain of up to

60%. However, over 70% strain, the released nanomeshes

have a dramatic resistance increase, possibly due to the non-

reversible breaking of certain nanomesh traces. The nano-

meshes also demonstrated reliability over continuous cyclic

strain of 30% for 500 cycles [Fig. 4(b)]. The resistance of the

stretched state gradually increased; however, the final R/R0 at

the released state remained close to 1 with a final value of 1.4

after 500 cycles, showing only a slight change. Here, R is the

resistance at certain strain and R0 is the initial resistance. This

phenomenon is consistent with previous Au nanomesh studies,

where a process of necking-recontact-cold welding was postu-

lated to cause the recovering of the nanomeshes.20 This robust

stretchability level is already promising for many applications

ranging from epidermal electronics (�30%),26,27 brain activ-

ity mapping (5%),28 to cardiac mapping (20–30%).29 The

cyclic strain study protocols here are also on par with many

previous studies regarding stretchable conductors.11,12,17

Compared to the previous Au nanomeshes from grain bound-

ary lithography with up to 150% cyclic stretchability,21 we

attribute the performance discrepancy to two reasons. The first

is that here before the transfer the PDMS was not pre-strained

for the easiness of processing. Second, in our process, possibly

due to e-beam deposition, the Au nanomesh trace width varies

FIG. 3. Studies of acetone vapor transmission through the ultrathin PI support layer. (a) Schematic image of acetone vapor transmission, showing that PMMA

etching under the ultrathin PI is dominated by vertical etching instead of lateral. Tilted view SEM of: (b) before acetone vapor etching, (c) after acetone vapor

etching, showing �100 nm gap between PI and PMMA. (d) Time needed to transfer as a function of PI thickness.

263101-4 Seo et al. Appl. Phys. Lett. 112, 263101 (2018)

substantially along the trace compared to the previous reports

using sputtering deposition. This non-uniformity makes the

narrowing of the trace width difficult and hard to achieve

ultra-narrow traces for highly stretchable patterns. However,

we note that the grain boundary lithography method, in gen-

eral, yields non-homogenous nanomesh patterns, and we

expect in the future to achieve engineered nanomesh pattern

from industry lithography tools with even higher stretching

performance and homogeneity. We expect that this ultrathin

support layer approach is readily translational to these meth-

ods. The transmittance spectrum shows the nanomeshes have

a moderate transparency of 65% at 550 nm [Fig. 4(c)].

In this paper, we have demonstrated a nondestructive

transfer method to achieve stretchable metal nanomeshes at

the 4 in.-wafer scale, further scalable as the substrate size

increases. By utilizing an ultrathin PI layer to support the Au

nanomeshes, we demonstrated that acetone vapor can effi-

ciently transmit through them, enabling etching of the under-

neath PMMA sacrificial layer and the following transfer, and

bonding to PDMS substrates. The resulting wafer-scale nano-

meshes demonstrated great mechanical stretchability, high

electrical conductivity, and moderate optical transparency.

This ultrathin-support-layer approach is in principle applica-

ble to transfer many nanomaterial networks. We also expect

this approach to enable fabrications of many large-scale

stretchable electronic devices, with applications ranging from

on-skin electronics to implantable biomedical devices.

This work was supported by Northeastern University.

We would like to acknowledge the George J. Kostas

Nanoscale Technology and Manufacturing Research Center

at Northeastern University for technical support. We would

like to thank S. Kar for using their UV-Vis Spectrometer for

transmittance tests.

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FIG. 4. Stretchability and transmittance of Au nanomeshes. (a) Strain test.

BT means before transfer. The sample resistance remained nearly unchanged

at its stretched state for up to 40% strain, then increased with strain. The

resistance remained similar as the original value after release from strain of

up to 60%. However, over 70% strain, the released nanomeshes have a dra-

matic resistance increase, possibly due to the non-reversible breaking of cer-

tain nanomesh traces. (b) Cyclic strain test. The nanomeshes also

demonstrated reliability over continuous cyclic strain of 30% for 500 cycles.

A process of necking-recontact-cold welding was postulated to cause the

recovering of the nanomeshes. (c) Transmittance spectra of Au nanomeshes.

263101-5 Seo et al. Appl. Phys. Lett. 112, 263101 (2018)