A.2 Compression-assisted Plasmonic Welding
A.2.3 Electrical Properties
Table 8 and Figure 45 show the sheet resistance trends according with the processes. Sheet resistances were measured by van der Pauw four-point probe methods. The reduction of sheet resistance due to the press-assisted are noticeable especially in the high initial sheet resistance. Because we used AgNWs of very low concentration, very few wires form their junctions when it was coated only once or twice.
Accordingly, making junction which has lower resistance is dominant factor affecting the conductance in that cases. When it was coated once, the sheet resistances are measured 17,200 Ω sq , 22,000 Ω sq , and 1,580 Ω sq respectively for the groups of samples for non-welding, welding, and press- assisted welding. The remarkable reduction of sheet resistance reaching over ~91 % was measured comparing the non-welding and press-assisted welding groups of samples. On the other hand, the sheet resistance was rather increased ~28% for the welding-only samples compared to non-welding samples, and the increase of sheet resistant shows the vulnerability to break of wire during the welding. This implies that pressure makes it easier to make junction during the UV irradiation.
Dramatic increases of sheet resistance are observed depends on the number of coatings because the number of contacts of AgNWs. The contact resistance plays an important role to determine the overall sheet resistance in case of the lesser number of contacts. As the number of coating increased, the reduction of sheet resistance tends to decrease for press-assisted welding samples compared to non- welding samples. Nevertheless, as the number of coating increased, sheet resistances are still shown trends that are getting smaller. were shown. The press-assisted welding group of samples shows the lowest sheet resistance for the overall cases. When the processes were repeated 5 times, the sheet resistance was changed from 74.1 Ω sq into 46.9 Ω sq for the press-assisted welding while 71.3 Ω sq for the conventional welding process.
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Figure 46 Transmittance according to (a) the number of coating repetitions for each condition in wavelength of 570 nm, and (b) wavelength for each condition for the 5 times of coating.
Figure 46 (a) shows the transmittance of the coated AgNWs samples, which decreases linearly with the number of coatings. Welding points occupy very few portions of overall structure enough to be less than 1%. Therefore, the difference in permeability before and after welding is not noticeable. Also, there is no distinct difference between welding-only and press-assisted samples, which implies a low influence in the physical contact. When the AgNWs were coated on the substrate for 5 times, transparency is about 89% at the wavelength of 570nm. Because the transparency of glass substrate is measured in 91.5%, AgNWs merely affect to the transparency reaching 97.4% even for the 5-times coated samples. Here, only the samples of press-assisted welding were achieved less than 50 Ω sq which is commonly required for TCEs. Transmittance according to wavelength is shown in Figure 46 (b). Although drastic reduction of transmittance is observed less then wavelength of 400 nm, there is no unequal degree of transmittance over the visible light region which imply low distortion of light.
Because of the property of the random network of nanowires, there can be disproportion amount of AgNWs on the substrates. Therefore, overall area of sample was measured with spectrophotometer (Cary 5000 UV-Vis-NIR, Agilent).
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Figure 47 SEM image of coated AgNWs after (a), (b) plasmonic UV welding without compression, (c), (d) with compression.
There is a limit to the contact resistance which is reduced by welding. Generating heat locally at the welded portion using UV is a target of established welding techniques, but the joints are liable to be broken by the local heat before the welding points are generally generated in the sample. Figure 47 (a) and (b) are SEM images of the sample subjected to the conventional welding process. The catch is that the formation of numerous small round lumps of silver on the silver nanowire. UV promotes the chemical reaction such as oxidation on the surface of AgNWs. Junctions between AgNWs are formed and oxidized simultaneously. Consequently, it is restricted to make welding points on junctions before the breakage of wire. On the contrary, Figure 47 (c), (d) which are the SEM images of the AgNWs after compression-assisted UV welding shows distinctive clear image compared to image of conventional welding samples. Figure 47 (d) shows only junction points were melted slightly. Due to the compression, UV generated heats converged in junctions. Accordingly, it is advantageous that make welded junction before the wires are damaged.
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In summary, we report the advantage of exerting pressure during the UV welding in respect of sheet resistance. The number of welded junctions can be effectively increased by the compression. Pressure through sapphire window merely affect to transparency, which gives more than 97% transmittance with lower than 50 Ω sq of sheet resistance. Compression during UV welding reduces the gap between wires and helps to concentrate the heat induced by UV.
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Appendix B Capacitive Heart-Rate Sensing on Touch Screen Panel with Laterally Interspaced Electrodes
Integrating bionic sensors into mobile devices including fingerprint [102-104], skin moisture [105, 106], blood glucose level [107], blood pressure [108], and heartrate [109-113] are getting a lot of interest these days. Mobile bionic sensors have made significant advances in biometric authentication and health monitoring. In terms of accessibility, the Touch Screen Panel (TSP) is particularly advantageous in implementing biometric functions, but the actual embedding is quite difficult. [81, 114] Screen fingerprint sensors are one of the successful examples of such TSP-equipped bionic sensors. [104] In particular, many attempts have been made to implement heart rate sensors on mobile phones, smart watches and other wearable devices recently. [109, 110, 112, 113, 115] Portable heartrate sensors are extremely helpful to perceive and prevent cardiac disorders like arrhythmia. [116, 117] The commercialized heartrate sensors rely mostly on the optical measurements called photoplethysmography which exploits the difference in the amount of reflected light between systolic and diastolic phase of blood vessel. [109, 115, 118] Therefore, both light transmitter and receiver is required so that necessitate substantial amount of power and a dedicated space to be implemented. There exists an alternative way, electrocardiogram (ECG) which can can detect the electrical charges induced by cardiac cycles. [110, 119, 120] However, the system is somewhat complex and bulky, because ECG requires multiple electrical contacts in the human body.
In my work, it is demonstrated experimentally that heartbeat can be detected by analyzing the time- dependent capacitive signals when a certain portion of finger touches the TSP surface. When the finger touched the surface of TSP, a certain portion of the electric fields generated by the transmitter (TX) electrode is absorbed by the finger, and the mutual capacitance between transmitter and receiver (RX) electrode is reduces so that the finger touch can be sensed. [114, 121] Here, the periodic behavior of blood flow during cardiac cycles regularly modulates the dielectric constant of the finger. The fringing electric field on the top electrode is affected by the dielectric constant of the finger. In normal circumstances, the change of measured capacitance reflecting the modulation of finger dielectric constant is as small as comparable to the noises occurring during measurements. On the other hand, in case of TX and RX are laterally placed, the effect on the dielectric on the fringing electric field is much more magnified.[116] Additionally, because the penetration depth of the electric field is proportional to the spacing between TX and RX electrodes, it is adjustable to find the point where the heartbeat signal is at its maximum. We found that a proper separation of two interspaced electrodes can enhance the
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capacitance change enough to distinguish reliably the heartbeat signals from the noises. Moreover, it is proposed to the method for amplifying the penetration depth maintaining higher resolution using nonadjacent electrodes as TX and RX. This is very crucial to measure the heartbeat signal even in the environment of TSP with protective layer.
Figure 48 (a) Touch screen panel (TSP) fabrication process. (b) Schematic diagram of the operational principle of fabricated TSP and its equivalent circuit. (c) The photo of fabricated TSP and its
transmittance at the crossing area of top and bottom electrodes. The fabricated TSP has 6 × 6 crossed lines of top and bottom electrodes. The width of electrode (both top and bottom) is 1 mm and the gap between electrodes is 1.5 mm.
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