*3.3. Metal Nanowires (NWs)*

#### 3.3.1. Material Properties, Synthesis, and Ink Formation

Unlike NPs, NWs are differentiated by their large aspect ratios, with lengths often 1000 times greater than their widths [3,11,111]. As a result of these aspect ratios, NWs can from conductive networks with very minimal loading, exhibit minimal bending stiffness and exceptional yield strength approaching the theoretical value of E (Young's modulus)/10, high optical transmittance, and electrical conductivities that are dominated by

quantum effects [17,45,112]. When the NW widths become too small, conductivity is greatly diminished by edge effects from atoms at the material surface and scattering, setting a practical limit on widths for printed inks [112]. Compared to NPs, NW inks are significantly easier to synthesize because NWs in random orientations are much more resistant to agglomeration [11]. Unlike sintered NP sheets, these NW networks can stretch and deform when embedded in a polymer matrix [45]. NWs can also be made biocompatible because of the inability of small Ag particles to migrate into the skin [111]. In addition, NWs may be laser welded for the rapid formation of highly conductive sheets. Nanowires are traditionally synthesized through the polyol method for printed inks, but the template method is also widely employed [11,17,113]. In polyol synthesis, the solution temperature, PVP molar ratio to AgNO3, stirring rate, the introduction of platinum seeds or other nucleation agents, and the addition of chloride or bromide ions can all be used to control the material dimensions and AgNW quality [93,113,114]. Figure 7a shows SEM images of AgNWs synthesized in various PVP solutions along with quantitative measurements of average NW diameter and length, and this experiment demonstrates that PVP solutions must be carefully optimized for Polyol synthesis [93]. During this synthesis method, AgNWs are typically produced from Ag seeds reduced from AgNO3, and these seeds are capped by the presence of PVP [93,114]. Although the exact mechanism by which AgNWs are synthesized in the polyol process is not fully known, it is likely that the differential affinity of PVP to the <100> plane than <111> plane in silver leads to unidirectional growth [115]. Despite a rigorous theoretical model, empirical findings allow for precise control of material aspect ratios and purities [93].

AgNW inks typically contain much lower material loadings than AgNP inks, simplifying ink design. As a result, greater resolutions are often achievable. For instance, Liang et al. experimented with different material loadings in AgNW inks for high-resolution screen printing [11]. AgNWs with aspect ratios of 500 were mixed with (hydroxypropyl)methyl cellulose (HMC), Zonyl FC-300, and defoamer MO-2170 in a distilled water solution and sonicated [11]. HMC is a viscoelastic polymer with hydroxy groups that bind strongly to AgNWs to aid in dispersion and that serves as an emulsifier and thickening agent [11]. Zonyl FC-300 was used to decrease the surface tension of the ink and promote substrate wettability for high-resolution printing, and defoamer MO-2170 was necessary to prevent foaming during mechanical agitation. It was determined that a 6.6 wt.% AgNW ink had the greatest pseudo-plasticity and lowest viscoelasticity (i.e., the ink had the highest difference in viscosity during low and high shear and recovered viscosity the quickest after applied shear was removed), which allowed for screen printing of highly conductive (4.67 <sup>×</sup> <sup>10</sup><sup>4</sup> S/cm) 50 <sup>µ</sup>m width traces [11]. Likewise, Huang et al. investigated various material loadings for gravure printing AgNW inks, arriving at an optimal value of 5.0 wt.% that yielded 50 <sup>µ</sup>m width traces and 5.34 <sup>×</sup> <sup>10</sup><sup>4</sup> S/cm conductivity [50]. AgNWs were synthesized in the presence of PVP (50 mL 0.09 M in EG) and NaCl (150 µL 0.1 M in EG) and centrifuged with acetone and ethanol to remove the solvent and surfactant. The AgNWs were then dispersed in a Poly(ethylene oxide) solution. At 1.5 mm/s, the 5.0 wt.% ink demonstrated a capillary number of 1.09 and viscosity of 20.9 Pa.s, making it suitable for gravure printing [50]. Optical (left) and SEM (right) of the resultant prints are shown in Figure 7b [50]. It is also possible to pattern a nanowire precursor on a substrate and grow the nanowires in situ, and this process has been demonstrated using flexography for ZnO NW functionalization of electrochemical biosensors [116]. In this work, commercially available carbon and AgCl inks (Gwent, PontyPool, UK) were flexographically printed on flexible PI to form a conductive electrode, and the ZnO precursor ink (1.1 g of zinc acetate in 10 mL DI and 40 mL IPA) was printed with an anilox volume of 12 cm3/m<sup>2</sup> , anilox force of 125 N, printing force of 150 N and printing speed of 0.2 m/s. The ZnO wires were hydrothermally synthesized in situ in an aqueous solution of 10 mM hexamethylenetetramine to yield a flexible glucose sensor with a sensitivity of 1.2 <sup>±</sup> 0.2 <sup>µ</sup>A mM−<sup>1</sup> cm−<sup>2</sup> with a linear response to the addition of glucose over a concentration range of 0.1 mM to 3.6 mM [116]. Finally, inkjet printing AgNWs have been demonstrated, but the printing process must

be carefully controlled to prevent nozzle clogging. In a sheet-to-sheet process, Al-Milaji et al. created an AgNW ink for inkjet printing by synthesizing AgNWs with an average of diameter of 100 nm and length of 14.5 µm in a polyol process, then dispersing the resultant precipitate in ethanol [45]. The resultant ink was printed on an uncured liquid PDMS layer spin coated on PET, and the AgNW ink was absorbed into the PDMS to create a stretchable interconnect. The connectors demonstrated high reliability during strain and bending, but initial resistances were high (0.68 kΩ over 25 mm) [45]. In contrast, Finn et al. sonicated commercially purchased NWs to reduce particle length, dispersed in IPA, and optimized inkjet parameters to yield sheet resistances of 8 Ω/sq and conductivities of 105 S/m in traces with widths of 1–10 mm and thickness of 0.5–2 µm after curing at 110 ◦C [117]. In order to reduce clogging, a Dimatix printer with 16 nozzles of diameter 21.5 µm spaced 254 µm apart was used to create 10-pL droplets at 5 kHz with a spacing of 20 µm and 50% overlap [117].
