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Performance Enhancement of Metal Nanowire-based Transparent Electrodes by Electrically Driven Nanoscale Nucleation of Metal Oxides Yu-Jeng Shiau,a Kai-Ming Chianga and Hao-Wu Lin*a
Received 00th January 20xx, Accepted 00th January 20xx DOI: 10.1039/x0xx00000x www.rsc.org/
Solution-processed silver nanowire (AgNW) electrodes have been considered promising materials for next-generation flexible transparent conductive electrodes. Despite the fact that single AgNW has extremely high conductivities, the high junction resistance between nanowires limits the performance of the AgNW matrix. Therefore, post treatments are usually required to approach better NW-NW contact. Herein, we report a novel linking method that uses joule heating to accumulate sol-gel ZnO near nanowire junctions. The nanoscale ZnO nucleation successfully restrained the thermal instability of the AgNW under current injection and acted as an efficient tightening medium to realize good NW-NW contacts. A low process temperature ( 200 s), which caused a dramatic increase in Rsh. The most distinguished mechanism is that in the joule heating method. 50 The decrease in Rsh depended on Ag atom migration at the junctions, but in our new method, it relied on the formation of aggregated ZnO at the junctions rather than Ag atom movement. This advantage highlights the importance of ZnO formation, which can simultaneously protect the NW junctions and tighten the 55 crossed NWs together to approach low Rsh. The faster Rsh decrease in nucleated ZnO samples also indicated that lower energy was needed for ZnO formation than for diffusion of Ag atoms between Fig. 1 Experimental set up of the electrically driven nanoscale localized ZnO the junctions. nucleation. Red dots represent the AgNW junctions with high junction Another AgNW network with conventional hot-plate heated sol-gel resistance where the localized heat induces ZnO nucleation. 60 ZnO treatment was prepared as a reference. The experimental set 5 up is shown in Fig. S4†. Zinc acetate precursor was deposited on the pristine AgNW network, which was heat treated for 5 min at 150 °C
Rsh (ohm/sq)
1400
Fig. 2 Sheet resistance Rsh reduction at various current injections. (a) The time evolution of Rsh when constant currents of 200 mA, 400 mA, 600 mA, 800 mA, and 1 A were applied for 300 s. The inset shows a small current of 10 0.1 mA was applied for 300 s with no significant change to Rsh. (b) The final stable Rsh values are as a function of the applied current. The lowest Rsh of nearly 13 Ω sq-1 was found with an applied current ≥ 800 mA.
1200
Ag migration (I = 200 mA) ZnO nucleation (I = 200 mA)
1000 800 600 400 200 0
resulted in faster ZnO nucleation and a better ZnO tightening 15 quality, meaning Rsh dropped faster to a lower final R sh value. After tens of seconds, the Rsh was saturated and remained constant in all samples. There was no obvious sign of the electrical breakdown of AgNWs, even when under a continuous 1 A current injection for a prolonged time period. We infer that this was due to 20 the “self-stop” process of the formation of ZnO at the junction immediately lowering the junction resistance and lowering the heat generated at that junction as a result. The crystallized ZnO that covered the junction also prevented Ag migration. Clarification of the inability of the sol-gel ZnO to nucleate at room 25 temperature and contribute to the reduction of R sh by capillary force was performed by applying a current of 0.1 mA for 300 s to a ZnO precursor-treated AgNW network. The results of this clarification are shown in the inset in Fig. 2(a). No significant changes in the Rsh value were found, indicating that ZnO nucleation 30 at the junction resulted from electrical current flows > 200 mA. Fig. 2(b) illustrates the final stable Rsh values as a function of the applied -1 current. The lowest Rsh, approximately 13 Ω sq , was found in samples with applied currents ≥ 800 mA. To show the advantage of this method over traditional joule 35 heating, joule heated pristine AgNW networks without sol-gel ZnO were examined. The experimental set up is shown in Fig. S2† of the Supporting Information. Fig. S3† shows Rsh values as a function of time. By applying a current of 200–500 mA, the Rsh dropped from approximately 1300 Ω sq-1 to 101–67 Ω sq-1 within 150 s. Compared 40 to the localized ZnO nucleation samples, not only were the lowest Rsh values higher, but it also took a longer time to reach the lowest value. A more clear comparison can be found in the results of the samples with an applied current of 200 mA shown side-by-side in 45 Fig. 3. Notably, an unstable bumping Rsh value was found after the Rsh reached its lowest value (t = 60–200 s), and the electrical breakdown of AgNWs was found after an extended current
0
50
100
150
200
Time (s) 65
Fig. 3 Comparison of electrical characteristics between Ag migration and ZnO nucleation. Ag migration shows slower and lower sheet resistance Rsh reduction even with electrical breakdown after extended current injection, while ZnO nucleation at junction have faster and larger Rsh reduction.
100 95 90 85 Pristine AgNWs Best Joule heated AgNWs Electrical breakdown AgNWs Conventional sol-gel ZnO AgNWs Nanoscale ZnO nucleation AgNWs
80 75 70
400
600
800
1000
Wavelength (nm) 70
Fig. 4 Optical properties of AgNW films with different post treatments. The film with nanoscale localized ZnO nucleation showed almost the same transmittance as the pristine AgNW film, but with sheet resistance Rsh smaller by two orders of magnitude.
on a hot plate. As shown in Fig. 4, the lowest transmission was found in the hot plate heated sol-gel ZnO samples compared to nanoscale localized ZnO and joule heated samples. This result is quite reasonable since ZnO formed over the entire surface and more reflection or scattering of this surface was expected. A higher -1 Rsh (22 Ω sq ) compared to the best nanoscale localized ZnO 80 samples was found. We infer that the capillary force might have 75
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been averaged over the whole ZnO precursor surface during hot plate baking while the capillary force was localized to the NW junctions during joule heating, resulting in a better tightened quality. 5 The transmittance of the previously mentioned AgNWs post treatment is shown in Fig. 4, including pristine AgNWs (Film A), the best joule heated AgNWs before (Film B) and after (Film C) electrical breakdown, AgNWs with conventional sol-gel ZnO treatment (Film D), and AgNWs with nanoscale localized ZnO nucleation (Film E). 10 The disconnected, electrically broken down AgNWs show a transmittance of 94.3%, higher than that of pristine AgNWs. This was possibly due to some missing NW segments, which were observed later in the field emission scanning electron microscope (FE-SEM) image. However, the infinite Rsh of the broken down 15 AgNW network makes it useless as a TCE. The film with nanoscale localized ZnO nucleation showed almost the same transmittance as the pristine AgNW film (T = 92%), but with an Rsh two orders of magnitude lower. We then can infer that the ZnO that nucleates at the junction negligibly contributed to the transmittance loss over 20 the entire film. Tilt-angle (60°) FE-SEM images of the AgNW network under different post-treatments are shown in Fig. 5(a)-(e). In the case of the pristine AgNW film (Fig. 5(a)), the AgNW network randomly formed over the substrate area without a significant NW 25 aggregation. However, each AgNW sat gently on the others with a loose contact between them (see inset in Fig. 5(a)), which may have caused a high junction resistance. In the joule heating of pristine AgNW network case (Fig. 5(b)), it can be seen that, during the joule heating process, the heat 30 generated at junctions resulted in Ag atom migration from the top wire to the bottom wire under gravity. This phenomenon created increased contact between NWs and therefore significantly reduced both the junction resistance and Rsh. However, if a current was applied for an extended time period (t > 35 200 s, I = 200 mA), breakdown of the AgNW network was observed and is shown in Fig. 5(c). In contrast to the smooth nanowire surface of pristine AgNWs (Fig. 5(a)), Ag nanoparticles were found on the nanowire surface and glass substrate in the broken down samples. Some nanowires were even broken up into discontinuous 40 segments, destroying the continuous electrical pathway across the film. In the conventional sol-gel ZnO treatment case, fig. 5(d) shows that crystallized oxide phases of ZnO filled the entire surface with AgNWs embedded underneath. Fig. 5(e) shows the nanoscale localized nucleation of the ZnO sample. The zoomed-in SEM images 45 are shown in Fig. S5. The white arrow points out the ZnO formations. They were only found at the AgNW junctions, suggesting that by linking the AgNWs using our new method, a low Rsh can be realized without sacrificing transparency. Fig. 5(f) illustrates the AgNW junction properties before and after the 50 nanoscale localized ZnO nucleation. The relationship between optical transmittance and sheet 50 resistance can be expressed by Tinkham formula:
55
Fig. 5 Field emission scanning electron microscope (FE-SEM) images of different samples. (a) Tilt-angle (60°) SEM images of the pristine AgNW 60 network for both wide and narrow vision. (b) Joule heating treatment of pristine AgNW network. (c) Electrical breakdown of AgNW network. (d) Conventional sol-gel ZnO treatment under hot plate baking. (e) Electrically driven nanoscale ZnO nucleation. (f) Schematic diagram showing the change in the AgNW junction properties before and after the nanoscale localized 65 ZnO nucleation.
T (λ ) = (1 +
188.5 σ OP (λ ) −2 ) RS σ DC
(1)
where σOp (λ) is the optical conductivity and σDC is the DC conductivity of the film. A significant index of σDC/σOp has been quantified by a figure of merit (FOM), where higher σDC/σOp values indicate higher transmittance for a given Rsh or lower Rsh for a given transmittance. Researches usually use the FOM value to evaluate the performance of different kinds of TCEs, such as ITO (σDC/σOp = 30 75 160), PEDOT:PSS (σDC/σOp = 50),51 graphene (σDC/σOp = 70),18 and 52 single-walled carbon nanotubes (SWNTs) (σDC/σOp = 24). Fig. 6 presents the transmittance (λ = 550nm) plots as a function of Rsh of the localized ZnO nucleation samples (red circles). Each sample was prepared using various initial AgNW concentrations −1 80 (1.25–20 mg·cc from right to left). A series of dotted grey lines represents the curves according to the Tinkham equation, which corresponds to σDC/σOp values of 100, 200, 300, 400, and 500. The commercial ITO shows a value of σDC/σOp = 160, which is inferior to the nanoscale ZnO nucleation samples (σDC/σOp = 340). Comparison 85 of the transmittance as a function of the R sh from this study and the 53-56 results collected from other state-of-the-art AgNW-based TCEs is shown in Fig. 6. The results in this paper are highly comparable with the best performance TCEs nowadays. Table 1 summarizes the Rsh, transmittance at 550 nm and FOM using different post 90 treatments presented in this work. The process temperature of AgNW-based TCEs is another important issue to be addressed. For traditional thermal annealing of pristine AgNW or sol-gel metal oxide treatment, in order to trigger the linking process, high temperature (> 100 °C) is needed to 95 effectively reduce the Rsh. Moreover, thermal annealing of pristine AgNW causes a rapid increase of the Rsh value at 200 °C (Table S1 in ESI†). Hence, both techniques cannot be implemented on low glass transition temperature (Tg) substrates such as polyethylene terephthalate (PET, Tg = 70 °C) and polyethylene naphthalate (PEN, 100 Tg = 120 °C). To evaluate the process temperature of the electrically driven ZnO nucleation, the in-situ substrate temperature during the treatment was measured using a thermal coupler. 70
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ARTICLE to its loosely adhesion to the glass substrate while the ZnO nucleation AgNW TCE was only slightly peeled off. Transparent film heaters are widely used in various applications, such as window defogging, outdoor displays, heating source of 57-60 sensors, and other heating systems. To show the potential 40 application of the ZnO nucleation AgNW TCE, transparent film heaters were demonstrated. Pristine AgNW and ZnO nucleation treated AgNW TCE were used to fabricate transparent film heaters. Fig. 9(a) and (b) show the experimentally measured substrate temperature as a function of time under 3-15 V applied voltage on 45 pristine AgNW and ZnO nucleation treated AgNW TCE respectively. As shown in Fig. 9(b), the substrate temperature reached 31, 52, 109, and 157 °C when the applied voltage was set as 3 V, 9 V, 12 V, and 15V respectively. The ZnO nucleation treated AgNW film heater has good performance at relatively low input voltage. The film 50 heater generated high temperature of up to 157 °C is suitable for removing frost on outdoor windows quickly. Furthermore, the transmittance of 92% is higher than most of the AgNW-based transparent heaters nowadays. Fig. 9(a) shows that the substrate temperature reached 27, 30, 55 36, and 50 °C when the applied voltage was set as 3 V, 9 V, 12 V, and 15V respectively. The small temperature changes of film heater is due to the high resistance of the pristine AgNW TCE (Rsh ~ 1100 Ω -1 sq ). At an input voltage of 20 V, a temperature drop was observed at the temperature of 87 °C resulted from the electrical breakdown. 60 The comparison between pristine and treated AgNW clearly shows that the ZnO nucleation at junction not only effectively reduces Rsh but also protects the AgNW from electrical breakdown resulting to a stable temperature profile of the transparent heater. To demonstrate the universal applicable of the linking method, 65 AgNWs with average diameter of 35 nm and average length of 15 μm were also utilized in this treatment and the Rsh characteristics are shown in Fig. S7†. Compared with a 90 nm-diameter AgNW network of the similar initial Rsh, resembling reduction order of Rsh was found using an 800 mA applied current. The same final Rsh 70 independent of the diameter of AgNW is because that the Rsh are mainly contributed by the junction resistances rather than the resistance of AgNW itself. Using the same magnitude of electrical current generates similar amount of heat and the tighten abilities of ZnO precursor at the junctions, resulting in a common final Rsh. 75 To examine the large area uniformity of the AgNW TCE, Rsh as a function of welding area using this treatment is shown in Fig. S8†. Rsh of nine divided areas were measured. Welding areas of 4, 9, 16 2 -1 cm show average Rsh of 14.2, 19.6, 20.5 Ω sq and small standard -1 deviations of 0.80, 1.01, 2.11 Ω sq respectively. The uniform Rsh 80 found in this test indicates that the linking method can readily be used in large area fabrication.
Transimittance (%)
95
σDC/σOp = 400 σDC/σOp = 500 σDC/σOp = 300
90 σDC/σOp = 200 σDC/σOp = 100 ZnO nucleation, this work ITO, ref. 30 AZO/AgNW/AZO, ref. 53 ITO NP/AgNW, ref. 56 ZnO/AgNW/ZnO, ref. 55 AgNW, ref. 54
85 80 75
10
100
1000
Rsh (ohm/sq) Fig. 6 Transmittance (λ = 550 nm) as a function of the sheet resistance Rsh of localized ZnO nucleation samples (red circles) with various initial AgNW concentrations. A series of dotted grey lines represents the curves according 5 to Tinkham equation, which corresponds to σDC/σOp values of 100, 200, 300, 400, and 500.
1340 I = 200 mA I = 1000 mA
50 40
1320 1300 80 60
30
40 20 10
20 0
50
100
150
Rsh (ohm/sq)
60
Temperature (oC)
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100
0 200
Time (s) Fig. 7 Substrate temperature and sheet resistance Rsh as a function of time 10 measured with current injections of 200 and 1000 mA. Blue arrows indicate the time where the applied current was terminated. The electrically driven nanoscale ZnO nucleation shows a low process temperature of 45 °C and 49 °C respectively.
Fig. 7 shows that for current injections under 200 mA and 1000 mA, with sufficient injection time to reach their lowest Rsh, the maximum substrate temperatures were only 45 °C and 49 °C respectively. Although the temperature at the junctions may locally reach higher than 45–49 °C, but a global low substrate temperature 20 is desirable for heat sensitive substrates compared to the traditional sol-gel method (T > 150 °C). To demonstrate the advantage of the low process temperature in our method, we fabricated both conventional sol-gel ZnO treated AgNWs and electrically driven nanoscale ZnO nucleation treated AgNWs on PEN 25 substrates. Fig. S6† clearly shows that the former technique involving the high process temperature of 150 °C was needed caused the flexible substrate to deform while the sample utilized in the latter technique, which remained flat and smooth in appearance. 30 The nanoscale ZnO nucleation treated AgNW TCE was also 38 subjected to adhesion test using 3M Scotch tape. Fig. 8(a) and (b) show the image of joule heated AgNW and ZnO nucleation AgNW TCEs after peeling the film repeatedly with the tape. The image clearly shows that joule heated AgNW TCE was easily peeled off due 15
Fig. 8 Image of (a) Joule heated AgNW TCE and (b) nanoscale ZnO nucleation treated AgNW TCE after the adhesion test. The red rectangles indicate the
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Fig. 9 Time dependent temperature profiles of (a) pristine AgNW and (b) ZnO nucleation AgNW transparent film heaters under various applied voltage.
Table 1 Sheet resistance Rsh, transmittance, and figure of merit (FOM) under different post treatments in this study. Film
Post treatment
Sheet resistance (Ω sq-1)
Average total transmittance (%),[@550 nm]
Figure of merit
A
Pristine AgNWs
1300
92.3, [92.0]
3.4
B
Best joule heated AgNWs
67
92.8, [93.0]
76.1
C
Electrical breakdown AgNWs
─
94.4, [94.3]
0
D
Conventional sol-gel ZnO AgNWs
22
88.1, [87.9]
128.6
E
Nanoscale ZnO nucleation AgNWs
13
91.9, [92.0]
340
Conclusions We demonstrated a novel AgNW matrix linking method by electrically forming nanoscale localized ZnO nucleation at wire junctions. This technique is superior to the conventional methods: pristine AgNW matrix joule heating may result in a narrow process window, an unstable lowest Rsh and even worth, and an electrical breakdown of AgNWs with extended treatment time; adding metal oxide precursor on to the high temperature annealed AgNW matrix can both reduce the transmittance of the film and make it unsuitable for heat sensitive substrates. Yet, by using the technique proposed in this report, the fastest Rsh reduction and lowest and most stable final Rsh value (13 Ω sq-1) can be realized without scarifying transmittance (T = 92% at 550 nm) of the film. The nanoscale localized ZnO nucleation at the AgNW junction was further confirmed by the FE-SEM image. The low global process temperature of 49 °C is suitable for many temperature sensitive substrates. With the low sheet resistance, high transmittance, good stability, low process energy consumption, and low process temperature, AgNW TCEs fabricated with the proposed technique are believed to be a promising platform for next-generation optoelectronic devices.
Acknowledgements The authors would like to acknowledge the financial support of the Ministry of Science and Technology of Taiwan (102-2221-E007-125-MY3, 101-2112-M-007-017-MY3, 103-2633-M-007001), and the Low Carbon Energy Research Center, National Tsing Hua University.
Notes and references
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area where the peeling tests were applied. (c) Rsh as a function of the number of peeling tests on different treated TCEs.
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Performance Enhancement of Metal Nanowire-based Transparent Electrodes by Electrically Driven Nanoscale Nucleation of Metal Oxides Yu-Jeng Shiau,a Kai-Ming Chianga and Hao-Wu Lin*a
Received 00th January 20xx, Accepted 00th January 20xx DOI: 10.1039/x0xx00000x www.rsc.org/
Solution-processed silver nanowire (AgNW) electrodes have been considered promising materials for next-generation flexible transparent conductive electrodes. Despite the fact that single AgNW has extremely high conductivities, the high junction resistance between nanowires limits the performance of the AgNW matrix. Therefore, post treatments are usually required to approach better NW-NW contact. Herein, we report a novel linking method that uses joule heating to accumulate sol-gel ZnO near nanowire junctions. The nanoscale ZnO nucleation successfully restrained the thermal instability of the AgNW under current injection and acted as an efficient tightening medium to realize good NW-NW contacts. A low process temperature ( 200 s), which caused a dramatic increase in Rsh. The most distinguished mechanism is that in the joule heating method. 50 The decrease in Rsh depended on Ag atom migration at the junctions, but in our new method, it relied on the formation of aggregated ZnO at the junctions rather than Ag atom movement. This advantage highlights the importance of ZnO formation, which can simultaneously protect the NW junctions and tighten the 55 crossed NWs together to approach low Rsh. The faster Rsh decrease in nucleated ZnO samples also indicated that lower energy was needed for ZnO formation than for diffusion of Ag atoms between Fig. 1 Experimental set up of the electrically driven nanoscale localized ZnO the junctions. nucleation. Red dots represent the AgNW junctions with high junction Another AgNW network with conventional hot-plate heated sol-gel resistance where the localized heat induces ZnO nucleation. 60 ZnO treatment was prepared as a reference. The experimental set 5 up is shown in Fig. S4†. Zinc acetate precursor was deposited on the pristine AgNW network, which was heat treated for 5 min at 150 °C
Rsh (ohm/sq)
1400
Fig. 2 Sheet resistance Rsh reduction at various current injections. (a) The time evolution of Rsh when constant currents of 200 mA, 400 mA, 600 mA, 800 mA, and 1 A were applied for 300 s. The inset shows a small current of 10 0.1 mA was applied for 300 s with no significant change to Rsh. (b) The final stable Rsh values are as a function of the applied current. The lowest Rsh of nearly 13 Ω sq-1 was found with an applied current ≥ 800 mA.
1200
Ag migration (I = 200 mA) ZnO nucleation (I = 200 mA)
1000 800 600 400 200 0
resulted in faster ZnO nucleation and a better ZnO tightening 15 quality, meaning Rsh dropped faster to a lower final R sh value. After tens of seconds, the Rsh was saturated and remained constant in all samples. There was no obvious sign of the electrical breakdown of AgNWs, even when under a continuous 1 A current injection for a prolonged time period. We infer that this was due to 20 the “self-stop” process of the formation of ZnO at the junction immediately lowering the junction resistance and lowering the heat generated at that junction as a result. The crystallized ZnO that covered the junction also prevented Ag migration. Clarification of the inability of the sol-gel ZnO to nucleate at room 25 temperature and contribute to the reduction of R sh by capillary force was performed by applying a current of 0.1 mA for 300 s to a ZnO precursor-treated AgNW network. The results of this clarification are shown in the inset in Fig. 2(a). No significant changes in the Rsh value were found, indicating that ZnO nucleation 30 at the junction resulted from electrical current flows > 200 mA. Fig. 2(b) illustrates the final stable Rsh values as a function of the applied -1 current. The lowest Rsh, approximately 13 Ω sq , was found in samples with applied currents ≥ 800 mA. To show the advantage of this method over traditional joule 35 heating, joule heated pristine AgNW networks without sol-gel ZnO were examined. The experimental set up is shown in Fig. S2† of the Supporting Information. Fig. S3† shows Rsh values as a function of time. By applying a current of 200–500 mA, the Rsh dropped from approximately 1300 Ω sq-1 to 101–67 Ω sq-1 within 150 s. Compared 40 to the localized ZnO nucleation samples, not only were the lowest Rsh values higher, but it also took a longer time to reach the lowest value. A more clear comparison can be found in the results of the samples with an applied current of 200 mA shown side-by-side in 45 Fig. 3. Notably, an unstable bumping Rsh value was found after the Rsh reached its lowest value (t = 60–200 s), and the electrical breakdown of AgNWs was found after an extended current
0
50
100
150
200
Time (s) 65
Fig. 3 Comparison of electrical characteristics between Ag migration and ZnO nucleation. Ag migration shows slower and lower sheet resistance Rsh reduction even with electrical breakdown after extended current injection, while ZnO nucleation at junction have faster and larger Rsh reduction.
100 95 90 85 Pristine AgNWs Best Joule heated AgNWs Electrical breakdown AgNWs Conventional sol-gel ZnO AgNWs Nanoscale ZnO nucleation AgNWs
80 75 70
400
600
800
1000
Wavelength (nm) 70
Fig. 4 Optical properties of AgNW films with different post treatments. The film with nanoscale localized ZnO nucleation showed almost the same transmittance as the pristine AgNW film, but with sheet resistance Rsh smaller by two orders of magnitude.
on a hot plate. As shown in Fig. 4, the lowest transmission was found in the hot plate heated sol-gel ZnO samples compared to nanoscale localized ZnO and joule heated samples. This result is quite reasonable since ZnO formed over the entire surface and more reflection or scattering of this surface was expected. A higher -1 Rsh (22 Ω sq ) compared to the best nanoscale localized ZnO 80 samples was found. We infer that the capillary force might have 75
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been averaged over the whole ZnO precursor surface during hot plate baking while the capillary force was localized to the NW junctions during joule heating, resulting in a better tightened quality. 5 The transmittance of the previously mentioned AgNWs post treatment is shown in Fig. 4, including pristine AgNWs (Film A), the best joule heated AgNWs before (Film B) and after (Film C) electrical breakdown, AgNWs with conventional sol-gel ZnO treatment (Film D), and AgNWs with nanoscale localized ZnO nucleation (Film E). 10 The disconnected, electrically broken down AgNWs show a transmittance of 94.3%, higher than that of pristine AgNWs. This was possibly due to some missing NW segments, which were observed later in the field emission scanning electron microscope (FE-SEM) image. However, the infinite Rsh of the broken down 15 AgNW network makes it useless as a TCE. The film with nanoscale localized ZnO nucleation showed almost the same transmittance as the pristine AgNW film (T = 92%), but with an Rsh two orders of magnitude lower. We then can infer that the ZnO that nucleates at the junction negligibly contributed to the transmittance loss over 20 the entire film. Tilt-angle (60°) FE-SEM images of the AgNW network under different post-treatments are shown in Fig. 5(a)-(e). In the case of the pristine AgNW film (Fig. 5(a)), the AgNW network randomly formed over the substrate area without a significant NW 25 aggregation. However, each AgNW sat gently on the others with a loose contact between them (see inset in Fig. 5(a)), which may have caused a high junction resistance. In the joule heating of pristine AgNW network case (Fig. 5(b)), it can be seen that, during the joule heating process, the heat 30 generated at junctions resulted in Ag atom migration from the top wire to the bottom wire under gravity. This phenomenon created increased contact between NWs and therefore significantly reduced both the junction resistance and Rsh. However, if a current was applied for an extended time period (t > 35 200 s, I = 200 mA), breakdown of the AgNW network was observed and is shown in Fig. 5(c). In contrast to the smooth nanowire surface of pristine AgNWs (Fig. 5(a)), Ag nanoparticles were found on the nanowire surface and glass substrate in the broken down samples. Some nanowires were even broken up into discontinuous 40 segments, destroying the continuous electrical pathway across the film. In the conventional sol-gel ZnO treatment case, fig. 5(d) shows that crystallized oxide phases of ZnO filled the entire surface with AgNWs embedded underneath. Fig. 5(e) shows the nanoscale localized nucleation of the ZnO sample. The zoomed-in SEM images 45 are shown in Fig. S5. The white arrow points out the ZnO formations. They were only found at the AgNW junctions, suggesting that by linking the AgNWs using our new method, a low Rsh can be realized without sacrificing transparency. Fig. 5(f) illustrates the AgNW junction properties before and after the 50 nanoscale localized ZnO nucleation. The relationship between optical transmittance and sheet 50 resistance can be expressed by Tinkham formula:
55
Fig. 5 Field emission scanning electron microscope (FE-SEM) images of different samples. (a) Tilt-angle (60°) SEM images of the pristine AgNW 60 network for both wide and narrow vision. (b) Joule heating treatment of pristine AgNW network. (c) Electrical breakdown of AgNW network. (d) Conventional sol-gel ZnO treatment under hot plate baking. (e) Electrically driven nanoscale ZnO nucleation. (f) Schematic diagram showing the change in the AgNW junction properties before and after the nanoscale localized 65 ZnO nucleation.
T (λ ) = (1 +
188.5 σ OP (λ ) −2 ) RS σ DC
(1)
where σOp (λ) is the optical conductivity and σDC is the DC conductivity of the film. A significant index of σDC/σOp has been quantified by a figure of merit (FOM), where higher σDC/σOp values indicate higher transmittance for a given Rsh or lower Rsh for a given transmittance. Researches usually use the FOM value to evaluate the performance of different kinds of TCEs, such as ITO (σDC/σOp = 30 75 160), PEDOT:PSS (σDC/σOp = 50),51 graphene (σDC/σOp = 70),18 and 52 single-walled carbon nanotubes (SWNTs) (σDC/σOp = 24). Fig. 6 presents the transmittance (λ = 550nm) plots as a function of Rsh of the localized ZnO nucleation samples (red circles). Each sample was prepared using various initial AgNW concentrations −1 80 (1.25–20 mg·cc from right to left). A series of dotted grey lines represents the curves according to the Tinkham equation, which corresponds to σDC/σOp values of 100, 200, 300, 400, and 500. The commercial ITO shows a value of σDC/σOp = 160, which is inferior to the nanoscale ZnO nucleation samples (σDC/σOp = 340). Comparison 85 of the transmittance as a function of the R sh from this study and the 53-56 results collected from other state-of-the-art AgNW-based TCEs is shown in Fig. 6. The results in this paper are highly comparable with the best performance TCEs nowadays. Table 1 summarizes the Rsh, transmittance at 550 nm and FOM using different post 90 treatments presented in this work. The process temperature of AgNW-based TCEs is another important issue to be addressed. For traditional thermal annealing of pristine AgNW or sol-gel metal oxide treatment, in order to trigger the linking process, high temperature (> 100 °C) is needed to 95 effectively reduce the Rsh. Moreover, thermal annealing of pristine AgNW causes a rapid increase of the Rsh value at 200 °C (Table S1 in ESI†). Hence, both techniques cannot be implemented on low glass transition temperature (Tg) substrates such as polyethylene terephthalate (PET, Tg = 70 °C) and polyethylene naphthalate (PEN, 100 Tg = 120 °C). To evaluate the process temperature of the electrically driven ZnO nucleation, the in-situ substrate temperature during the treatment was measured using a thermal coupler. 70
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ARTICLE to its loosely adhesion to the glass substrate while the ZnO nucleation AgNW TCE was only slightly peeled off. Transparent film heaters are widely used in various applications, such as window defogging, outdoor displays, heating source of 57-60 sensors, and other heating systems. To show the potential 40 application of the ZnO nucleation AgNW TCE, transparent film heaters were demonstrated. Pristine AgNW and ZnO nucleation treated AgNW TCE were used to fabricate transparent film heaters. Fig. 9(a) and (b) show the experimentally measured substrate temperature as a function of time under 3-15 V applied voltage on 45 pristine AgNW and ZnO nucleation treated AgNW TCE respectively. As shown in Fig. 9(b), the substrate temperature reached 31, 52, 109, and 157 °C when the applied voltage was set as 3 V, 9 V, 12 V, and 15V respectively. The ZnO nucleation treated AgNW film heater has good performance at relatively low input voltage. The film 50 heater generated high temperature of up to 157 °C is suitable for removing frost on outdoor windows quickly. Furthermore, the transmittance of 92% is higher than most of the AgNW-based transparent heaters nowadays. Fig. 9(a) shows that the substrate temperature reached 27, 30, 55 36, and 50 °C when the applied voltage was set as 3 V, 9 V, 12 V, and 15V respectively. The small temperature changes of film heater is due to the high resistance of the pristine AgNW TCE (Rsh ~ 1100 Ω -1 sq ). At an input voltage of 20 V, a temperature drop was observed at the temperature of 87 °C resulted from the electrical breakdown. 60 The comparison between pristine and treated AgNW clearly shows that the ZnO nucleation at junction not only effectively reduces Rsh but also protects the AgNW from electrical breakdown resulting to a stable temperature profile of the transparent heater. To demonstrate the universal applicable of the linking method, 65 AgNWs with average diameter of 35 nm and average length of 15 μm were also utilized in this treatment and the Rsh characteristics are shown in Fig. S7†. Compared with a 90 nm-diameter AgNW network of the similar initial Rsh, resembling reduction order of Rsh was found using an 800 mA applied current. The same final Rsh 70 independent of the diameter of AgNW is because that the Rsh are mainly contributed by the junction resistances rather than the resistance of AgNW itself. Using the same magnitude of electrical current generates similar amount of heat and the tighten abilities of ZnO precursor at the junctions, resulting in a common final Rsh. 75 To examine the large area uniformity of the AgNW TCE, Rsh as a function of welding area using this treatment is shown in Fig. S8†. Rsh of nine divided areas were measured. Welding areas of 4, 9, 16 2 -1 cm show average Rsh of 14.2, 19.6, 20.5 Ω sq and small standard -1 deviations of 0.80, 1.01, 2.11 Ω sq respectively. The uniform Rsh 80 found in this test indicates that the linking method can readily be used in large area fabrication.
Transimittance (%)
95
σDC/σOp = 400 σDC/σOp = 500 σDC/σOp = 300
90 σDC/σOp = 200 σDC/σOp = 100 ZnO nucleation, this work ITO, ref. 30 AZO/AgNW/AZO, ref. 53 ITO NP/AgNW, ref. 56 ZnO/AgNW/ZnO, ref. 55 AgNW, ref. 54
85 80 75
10
100
1000
Rsh (ohm/sq) Fig. 6 Transmittance (λ = 550 nm) as a function of the sheet resistance Rsh of localized ZnO nucleation samples (red circles) with various initial AgNW concentrations. A series of dotted grey lines represents the curves according 5 to Tinkham equation, which corresponds to σDC/σOp values of 100, 200, 300, 400, and 500.
1340 I = 200 mA I = 1000 mA
50 40
1320 1300 80 60
30
40 20 10
20 0
50
100
150
Rsh (ohm/sq)
60
Temperature (oC)
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100
0 200
Time (s) Fig. 7 Substrate temperature and sheet resistance Rsh as a function of time 10 measured with current injections of 200 and 1000 mA. Blue arrows indicate the time where the applied current was terminated. The electrically driven nanoscale ZnO nucleation shows a low process temperature of 45 °C and 49 °C respectively.
Fig. 7 shows that for current injections under 200 mA and 1000 mA, with sufficient injection time to reach their lowest Rsh, the maximum substrate temperatures were only 45 °C and 49 °C respectively. Although the temperature at the junctions may locally reach higher than 45–49 °C, but a global low substrate temperature 20 is desirable for heat sensitive substrates compared to the traditional sol-gel method (T > 150 °C). To demonstrate the advantage of the low process temperature in our method, we fabricated both conventional sol-gel ZnO treated AgNWs and electrically driven nanoscale ZnO nucleation treated AgNWs on PEN 25 substrates. Fig. S6† clearly shows that the former technique involving the high process temperature of 150 °C was needed caused the flexible substrate to deform while the sample utilized in the latter technique, which remained flat and smooth in appearance. 30 The nanoscale ZnO nucleation treated AgNW TCE was also 38 subjected to adhesion test using 3M Scotch tape. Fig. 8(a) and (b) show the image of joule heated AgNW and ZnO nucleation AgNW TCEs after peeling the film repeatedly with the tape. The image clearly shows that joule heated AgNW TCE was easily peeled off due 15
Fig. 8 Image of (a) Joule heated AgNW TCE and (b) nanoscale ZnO nucleation treated AgNW TCE after the adhesion test. The red rectangles indicate the
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Fig. 9 Time dependent temperature profiles of (a) pristine AgNW and (b) ZnO nucleation AgNW transparent film heaters under various applied voltage.
Table 1 Sheet resistance Rsh, transmittance, and figure of merit (FOM) under different post treatments in this study. Film
Post treatment
Sheet resistance (Ω sq-1)
Average total transmittance (%),[@550 nm]
Figure of merit
A
Pristine AgNWs
1300
92.3, [92.0]
3.4
B
Best joule heated AgNWs
67
92.8, [93.0]
76.1
C
Electrical breakdown AgNWs
─
94.4, [94.3]
0
D
Conventional sol-gel ZnO AgNWs
22
88.1, [87.9]
128.6
E
Nanoscale ZnO nucleation AgNWs
13
91.9, [92.0]
340
Conclusions We demonstrated a novel AgNW matrix linking method by electrically forming nanoscale localized ZnO nucleation at wire junctions. This technique is superior to the conventional methods: pristine AgNW matrix joule heating may result in a narrow process window, an unstable lowest Rsh and even worth, and an electrical breakdown of AgNWs with extended treatment time; adding metal oxide precursor on to the high temperature annealed AgNW matrix can both reduce the transmittance of the film and make it unsuitable for heat sensitive substrates. Yet, by using the technique proposed in this report, the fastest Rsh reduction and lowest and most stable final Rsh value (13 Ω sq-1) can be realized without scarifying transmittance (T = 92% at 550 nm) of the film. The nanoscale localized ZnO nucleation at the AgNW junction was further confirmed by the FE-SEM image. The low global process temperature of 49 °C is suitable for many temperature sensitive substrates. With the low sheet resistance, high transmittance, good stability, low process energy consumption, and low process temperature, AgNW TCEs fabricated with the proposed technique are believed to be a promising platform for next-generation optoelectronic devices.
Acknowledgements The authors would like to acknowledge the financial support of the Ministry of Science and Technology of Taiwan (102-2221-E007-125-MY3, 101-2112-M-007-017-MY3, 103-2633-M-007001), and the Low Carbon Energy Research Center, National Tsing Hua University.
Notes and references
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magnitude.
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