Nanotechnology 24 (2013) 115306
M Stach et al
Figure 6. The simulated and measured ERs of different CR values
under different strains. The measured CRs (diamond) were first put
on the chart and proposed data points (triangle) were then put on the
chart as simulation parameters. To reflect real cases, the CR with a
smaller slope was set in the elastic region (y > -3.8%) to reflect
the metal's ductility. The CR was also set with a smaller slope after
y > -11.9% because of the delamination of the nanowires based
on physical evidence. The simulated ER values with y and CR as
parameters were put on the chart (circle) and compared with the
measured ERs (square). The simulation results fit the measured data
well, proving the tunable ERs and implying the importance of
considering crack and delamination defects.
showed reduced dimensions with an improved average ER
of 4.2 dB in the visible region. However, further straining
the WGP under a larger strain (y = -8.1%) in the plastic
region reduced the average ER to 2.5 dB in the visible region.
Figure 7. An SEM picture of the WGP at (a) x = 8% or
This change in ER could be explained by the combined
y = -3.8% and (b) x = 25% or y = -8.1%. Weak points in (a)
influences of pitch change, metal crack generation, nanowire
after straining and releasing appeared on the nanowires as buckling,
which further became cracks under larger strain. The cracks
delamination and randomly oriented nanowire fall-ons.
appearing in (b) implied that the weak points in (a) became cracks
Figure 6 shows the simulated and measured ER data
and the intact points in (a) showed cracks in (b) under severe
with different strains and metal crack ratios (CRs). The CR
straining, which enhanced the weak points. After delamination, the
was defined as the ratio of the length of the crack to the
nanowires fell on the structure without the originally specified
length of the structure in the y-direction (figure 1). In addition
orientation.
to the effect of pitches on ER values, this study considers
the influence of the CR. Along with the changes of the
the cracks (figure 7(b)). In such cases, the pitch continued
pitches of the WGP, the CR was gradually increased based
to reduce and the contact area and the adhesion force
on the results of the physical examination of the WGP. The
between the metal nanowires and the PEN substrate decreased
simulation changed the pitch and the CR of the WGP and
accordingly. Parts of the metal nanowires could not adhere
obtained a maximum ER, which agrees with the trend of the
to the PEN substrate, and randomly delaminated [22]. The
experimental data.
Figure 6 shows that the ER value increased before
partially delaminated metal nanowire fragments fell on the
reaching optimized strain, and decreased after it. The
substrate and overlapped with other intact metal nanowires
rise of the ER between
(figure 7(b)). The fragments contained part of the metal
y = 0% and y = -3.8% was
primarily because of the pitch modulation, as implied from
nanowires in the non-x-direction. This unexpectedly absorbed
the simulation results in figure 2. The setting of
and reflected a part of the TM mode transmittance without
y =
-3.8% shows weak points with a lower adhesion than the
reducing TE mode transmittance. Figures 2(b) and 6 show that
surroundings (figure 7(a)). Beyond this strain, weak points
the difference between TM mode transmittance and TE mode
gradually become cracks which reduce the absorption and
transmittance resulted in a reduced ER. This study defines the
reflection of the TE mode transmittance without influencing
optimized strain (OPTIMIZED) as the strain at which the largest
the TM mode transmittance. Consequently, the difference
ER occurred.
between TE and TM becomes smaller, which in turn
These results show that although the PEN entered its
decreases the ER. Furthermore, straining the WGP after y =
plastic region and could not completely return to its original
-3.8%, for example y = -8.1%, induced serious partial
state, the resulting WGP still provided a reversible ER. This
delamination of the metal nanowires instead of enlarging
characteristic provides users with two-way (both rise and fall
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