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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)
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