Anomalous deformation behavior of Ag nanowires on Au electrode in low-temperature environments

We discovered that Ag nanowires (AgNWs) on an Au electrode exhibited an anomalous deformation behavior despite a low-temperature environment of 65 °C. Most AgNWs on the Au electrode were deformed after heating. In contrast, AgNWs on the Cr and Ag electrodes exhibited a few changes and maintained their initial shape. The deformation behavior of AgNWs on metal electrodes has not yet been reported and is currently difficult to explain using known processes such as diffusion and alloying. Nonetheless, they evidently depend on the electrode material. The findings of this study are crucial for the design of AgNW-based electronic devices.

We discovered that Ag nanowires (AgNWs) on an Au electrode exhibited an anomalous deformation behavior despite a low-temperature environment of 65 °C.Most AgNWs on the Au electrode were deformed after heating.In contrast, AgNWs on the Cr and Ag electrodes exhibited a few changes and maintained their initial shape.The deformation behavior of AgNWs on metal electrodes has not yet been reported and is currently difficult to explain using known processes such as diffusion and alloying.Nonetheless, they evidently depend on the electrode material.The findings of this study are crucial for the design of AgNW-based electronic devices.© 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd A g nanowires (AgNWs) are the most well-known and commercially available metal NWs.AgNW mesh films exhibit remarkable physical properties such as high optical transparency, electrical conductivity and mechanical flexibility.Hence, they are considered promising candidate materials for functional electrodes such as flexible transparent electrodes. 1,2)On the other hand, AgNWs have some issues such as lack of long-term stability, which hinders their use as commercial electrode material.Generally, AgNWs are chemically synthesized via the polyol method. 3)Polyvinylpyrrolidone (PVP) functions as a shape-stabilizing agent as well as a passivation layer for the AgNWs.The degradation of AgNWs typically progresses via atomic diffusion and/or chemical reactions at the areas where the PVP layer was removed or at defects in the PVP layer.UV light, 4,5) temperature 6,7) and atmospheric agents (moisture, oxygen, sulfur, chlorine) contribute to the degradation of AgNWs. 8,9)][12] Most previous studies have investigated the degradation mechanism of AgNWs with respect to a specific degradation factor.AgNW degradation behavior due to complex degradation factors during real-world usage has not been fully elucidated.
In our previous study, we reported a functional device using the percolation conduction of printed AgNWs. 13)While investigating the long-term stability of the AgNW device through an environmental test, we discovered that AgNWs exhibit an anomalous deformation behavior; AgNWs in contact with certain metals undergo rapid deformation despite a low-temperature environment.In this paper, we report our findings on this anomalous behavior, which has been hitherto undocumented in the literature.AgNWs in contact with quartz and Cr retained their shape after 52 h under 65 °C.In contrast, a substantial number of AgNWs deformed when they were in contact with Au.The problem of AgNW deformation in the presence of specific electrode materials within the general operating temperature range of an electronic device is a crucial aspect for device design since the use of AgNWs in electronic devices as conducting materials necessitates a connection between the AgNWs and the electrodes.
We used AgNWs dispersed in isopropyl alcohol (Sigma-Aldrich, diameter: 60 nm, length: 10 μm).We used a quartz substrate and fabricated Au, Ag and Cr (>99.99%)electrodes via the vacuum evaporation method.The surface of the quartz substrate was cleaned via O 2 plasma dry cleaning for 300 s under a power of 100 W and O 2 flow rate of 7 cm 3 min −1 .Patterned Au electrodes were fabricated by first depositing 8 nm of Cr as an adhesion layer on the quartz substrate using a metal mask, and then 24 nm of Au was deposited on the Cr layer.The Ag and Cr electrodes were fabricated by depositing 50 nm of Ag and Cr onto the quartz substrate, respectively.AgNW dispersion was then spin coated onto each electrode.The patterned Au electrode substrate was treated again to plasma cleaning under the same conditions before spin-coating the AgNWs to ensure a uniform surface wettability on the electrode pattern and substrate.Finally, all AgNW-coated electrodes were thermally treated in a thermostatic chamber at 65 °C.
We investigated the degradation of the AgNW mesh film coated on the patterned Au electrode under 65 °C to study its degradation behavior at the maximum operating temperature region of a typical electronic device.We monitored the electrical resistance of the AgNW mesh film between the patterned Au electrodes during thermal treatment.Although the resistance of the AgNW mesh film reached a minimum after 3 h, it began to increase slightly after 52 h.We anticipated that this increase in the resistance marked the initiation of the AgNW degradation and we attempted to observe the degradation patterns via field-emission scanning electron microscopy (FE-SEM; JEOL JSM-7200F) and atomic force microscopy (AFM; Bruker MultiMode8).
Figure 1 shows the FE-SEM images of the AgNW mesh film coated on the patterned Au electrode substrate after 52 h of heating.We discovered that the AgNWs in contact with the Au electrode exhibited a peculiar and notable deformation behavior.Figure 1(a) shows the SEM image of the AgNW mesh film at the interface between the Au electrode and the quartz substrate.While the AgNWs on the quartz substrate retain their shape, those on the Au electrode have completely lost their shape and only their residue appears on the electrode.Figure 1(b) shows the opposite side of the Au electrode shown in Fig. 1(a).The Cr adhesion layer sticks out between the Au electrode and quartz substrate due to the difference in the position of the evaporation source during vacuum evaporation.AgNWs on the Cr layer and quartz substrate retain their shape.Some of the AgNWs stacked on top of other AgNWs maintained their shape, even on the Au electrode.Based on the above results, the worsening of the electrical connection between the AgNW mesh film and the Au electrode is anticipated to be reason behind the increased resistance of the AgNW mesh film on the Au electrode.The heat treatment at 65 °C represents a sufficiently low-temperature condition for Ag materials.These results suggest that the reported anomalous deformation of AgNWs occurs only when they are in contact with specific metal electrodes.
Figure 2 shows the AFM images of AgNWs on the Au electrode before and after heating.From Fig. 2(a), it can be seen that the AgNWs on the Au electrode before heating retain their shape and the height of single AgNWs is around 50 nm.However, as shown in Fig. 2(b), the height of AgNWs in direct contact with the Au surface decreased to several nanometers after heating.On the other hand, the AgNWs stacked on other AgNWs retain their initial shape.Based on the SEM and AFM results, it is confirmed that the volume of AgNWs in contact with the Au electrode surface is significantly decreased after 65 °C heating.
To compare the deformation behavior of the AgNWs on each electrode, each AgNW-coated electrode substrate was thermally treated for 5 and 24 h, respectively.Figures 3(a 3(c) and 3(d), the AgNWs on the Cr electrode retain their shape.Remarkably, a few particles appear on the surfaces of the AgNWs after 24 h of treatment [Fig.3(d)], which may be attributed to the AgNW degradation due to the atmosphere. 9)As shown in Figs.3(e) and 3(f), the AgNWs on the Ag electrode do not show a deformation behavior similar to that shown by the AgNWs on the Au electrode.However, some low-brightness areas are observed on the Ag electrode at the intersection of the AgNWs.In addition, some AgNWs in the low-brightness areas are unnaturally curved.Although particles do not appear on the surface of the AgNWs on the Ag electrode after 24 h [Fig.3(f)], the lowbrightness areas of the Ag electrode tend to expand, and the diameter of the AgNWs in the areas decreases.
As evident from Fig. 3, the deformation of AgNWs on the Au electrode had progressed rapidly within 5 h under the low temperature of 65 °C.It can be assumed that the degradation of the AgNWs on the Cr electrode was mainly influenced by the atmosphere rather than the Cr electrode.Some kind of  reaction occurred between the AgNWs and the Ag electrode on account of them sharing the same metal.Hence, the deformation of the AgNWs on the Ag electrode may be attributed to the Ag electrode instead of the atmosphere.
The application of AgNWs as transparent electrode materials is well-known, which implies that AgNWs are mostly arranged on transparent substrates.Hence, a comprehensive understanding of the degradation mechanisms of AgNWs on opaque and conductive metal substrates has not yet been achieved.In particular, the anomalous deformation behavior of AgNWs on Au electrodes has not been previously reported.[16][17][18] Although the deformation mechanism of AgNWs on Au electrodes is currently unclear, we discuss it here by systematically compiling the previously reported characteristics of Ag and Au materials.
As mentioned previously, the AgNWs that contacted with the Au electrode lost their shape, and the residue of the AgNWs was left on the Au electrode surface.Hence, we first assessed the possibility of AgNW diffusion into the Au electrodes.Ag and Au can easily inter-diffuse at high temperatures and form an all-proportional solid solution.Most real-life measurement data regarding volume diffusion are based on phenomena, such as the Kirkendall effect and impurity diffusion, which occur under high-temperature conditions.Therefore, the diffusion coefficient for the diffusion of Ag in Au (D Ag in Au ) at 65 °C was estimated by extrapolating a reference Arrhenius plot (Volume D1, 2, and 3 in Fig. 4). 19,20)his revealed that D Ag in Au at 65 °C was less than 1 × 10 −30 m 2 s −1 .The diffusion length (L) for a material is expressed as L = 2(Dt) 1/2 , where t and D denote the duration of diffusion and diffusion coefficient of the material, respectively.At D Ag in Au = 1 × 10 −30 m 2 s −1 , the values of L for a t of one hour, day, and year are 1.20 × 10 −13 , 5.88 × 10 −13 and 1.12 × 10 −11 m, respectively.Therefore, it is difficult for Ag to diffuse into Au under low temperatures such as 65 °C.However, if the sample is maintained under a high pressure, Ag can diffuse into Au more easily even at low temperatures.At RT (assumed to be 25 °C) and under 1 GPa, D Ag in Au (Volume D4) is 2.18 × 10 −14 m 2 s −1 (L = 1.78 × 10 −5 m at t = 1 h), 21) and at 150 °C and under 2 MPa, D Ag in Au (Volume D5) is 3.15-49.7 × 10 −19 m 2 s −1 (L = 6.73-26.8× 10 −8 m at t = 1 h). 22)n Ag nanowhisker coated with Au was reported to become a hollow structure of Au after heating at 170 °C for 30 min because the Ag diffused to the inside surface of the coated Au, to the interface between the Au and substrate and to the Au grain boundaries. 23)In general, the diffusion rate increases in the order volume diffusion < grain boundary 045001-3 © 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd diffusion < surface diffusion.The above study reported that the effective value of D Ag in Au for grain boundary diffusion (Grain boundary D1) is 1.40 × 10 −14 m 2 s −1 at 170 °C (L = 1.42 × 10 −5 m at t = 1 h).However, in the above study, the Au grain boundaries were distinctly visible from the SEM images.In contrast, our Au electrode on the Cr layer did not exhibit any distinct grain boundaries [Fig.1(a)].Some grain boundaries were observed in our Ag electrode [Figs.3(c) and 3(f)].However, the deformation rate of AgNWs on the Au electrodes was significantly higher than that on the Ag electrode.
The diffusion coefficient for surface diffusion depends not only on the composition of the surface but also on the crystal lattice at the surface.We could not find any papers that reported the surface diffusion coefficient of Ag on Au at 65 °C.However, the self-surface diffusion coefficient of Ag on Ag (111) under the same conditions is estimated to be in the range of 2-3 × 10 −9 m 2 s −1 . 24,25)The surface diffusion coefficient of Au on Ag (110) at 65 °C is estimated to be in the range of 1.36-2.37× 10 −11 m 2 s −1 . 26)The surface diffusion coefficient of Ag on Au (110) is considered to be an order of magnitude higher than that of Au on Ag (110). 26,27)herefore, accurately estimating the surface diffusion coefficient of Ag on Au is difficult at present.It is presumably in the range of 10 −11 -10 −9 m 2 s −1 (L = 3.79-37.9× 10 −4 m at t = 1 h) at 65 °C (Surface D1).Surface diffusion is a phenomenon by which an isolated atom or an atomic cluster is adsorbed onto the surface and is transported to a stable position through hopping.The phenomenon is considered during vapor phase growth and sintering.The release of Ag atoms from AgNWs through sublimation is not plausible under 65 °C, as evidenced from the observation that AgNWs on both the Cr layer and quartz substrate retained their initial shape after heating.
The actual value of D Ag in Au under the experimental conditions is unknown.However, at 65 °C, D Ag in Au = 5.00 × 10 −20 m 2 s −1 if L = 60 nm (diameter of AgNW) and t = 5 h.All estimated D Ag in Au values are plotted in Fig. 4. If the anomalous deformation of AgNWs on the Au electrode is due to diffusion, D Ag in Au at 65 °C should be comparable to the grain boundary diffusion coefficient.
However, due to the limitations of our technology and facility, we did not obtain any conclusive evidence that substantiates diffusion as the cause of deformation at 65 °C.
Next, we investigated the low-temperature reactions of Ag and Au due to size effects, such as melting-point depression.Ag core/Ag shell nanoparticles (8 nm diameter and 1 nm thick, respectively) must be heated above 200 °C to alloy them. 28)An alloying between the AgNWs and Au electrode is not feasible at 65 °C, given that the temperature is significantly lower than 200 °C and the diameter of the AgNWs used in this work was 60 nm.Ag and Au nanomaterials in direct contact without a capping material are reported to merge during transmission electron microscopy (TEM) at RT. 29) However, the above reaction can be considered an atomic diffusion phenomenon assisted by the high-energy electron beam irradiation employed in TEM (∼200 kV). 30)Although we used an accelerating voltage less than or equal to 15 kV for conducting the FE-SEM observations, only the AgNWs on the Au electrode significantly deformed.Evidently, the deformation of AgNWs was not induced by the low-energy electron beam employed in FE-SEM.
As discussed above, the anomalous deformation behavior of AgNWs on the Au electrode at 65 °C does not follow the same patterns observed in previous studies and the deformation mechanism remains unclear.Even Ag nanomaterials smaller than AgNWs must be heated to approximately 200 °C to form an alloy with Au.At a temperature of 65 °C, Ag cannot easily diffuse into Au via volume diffusion without the assistance of high-energy sources such as pressure or electron beams.The deformation of AgNWs on the Au electrode may be due to grain boundary diffusion.However, it is difficult to explain the deformation behavior by a simple grain boundary diffusion mechanism because the behavior between Au and Ag electrodes is extremely different.Unfortunately, the research equipment in our lab does not have sufficient performance capability to observe the nano-or atom-scale elemental distribution.It needs highresolution elemental distribution analysis equipment and technique to reveal whether AgNWs diffuse into Au at 65 °C.
In summary, we discovered that AgNWs on an Au electrode exhibit an anomalous deformation behavior under low-temperature conditions.Most AgNWs on the Au electrode underwent significant deformation, whereas those on the Cr electrode underwent only minor degradation.AgNWs on the Ag electrode exhibited a slow deformation.However, the deformation behavior was different to that observed on the Au electrode.The deformation behavior exhibited by AgNWs on metal electrodes has not been previously reported since AgNWs are expected to be transparent electrode materials.As per previous reports on Ag nanomaterials, volume diffusion, surface diffusion, alloying and sintering of AgNWs with Au are unlikely to occur at a temperature of 65 °C.The possibility of grain boundary diffusion cannot be ruled out.However, the deformation rate of AgNWs on the Au electrode with no grain boundaries was significantly higher than that on the Ag electrode with several grain boundaries.A comprehensive understanding of the deformation mechanisms of AgNWs on metal electrodes remains elusive.Nonetheless, the deformation evidently depends on the electrode material.It is crucial to understand the Fig. 4. Estimated diffusion coefficients for the diffusion of Ag into Au under various diffusion modes and conditions.(Volume D1, 19) D2, 20) D3, 20) D4, 21) D5, 22) Grain boundary D1, 23) Surface D1 26,27) ).

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© 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd deformation mechanism of AgNWs on metal electrodes in low-temperature environments to prevent issues while AgNW functional electrodes in electronic devices.
) and 3(b) show the SEM images of AgNWs on the Au electrode.The images were obtained in charged sample mode because the samples were prepared using a diluted AgNW dispersion, and the AgNWs did not form a conductive mesh network on the insulating quartz substrate.As evident from Figs. 3(a) and 3(b), the thermally treated AgNWs on the Au electrode exhibit significant deformation within 5 h.As shown in Figs.

Fig. 1 .
Fig. 1.FE-SEM images of the AgNW mesh film coated on the patterned Au electrode after 52 h of heating.Film covering (a) quartz-Au and (b) quartz-Cr-Au areas.

Fig. 2 .
Fig. 2. AFM images of the AgNWs on the Au electrode (a) before and (b) after 65 °C heating.