Setup and Procedures: Silver chloride (AgCl) was added to a heated solution of polyvinylpyrrolidone (PVP) and ethylene glycol. The PVP acts as a surfactant for the silver from the AgCl, such that the growth of the nanowires is controlled. Through this, the length of the silver nanowires is allowed to increase while the diameter stays the same. A reaction occurred after AgNO3 was added to the solution to form these nanowires. Samples of the silver nanowires after different amounts of reaction time (6, 12, 18, and 24 minutes) were created through drop-casting. Image analysis was then run using a Scanning Electron Microscope (SEM).

Analysis and Discussion: Figure 1 attached gives you a picture of what the silver nanowires looked like under the SEM. Table 1 gives average physical (calculated using Fiji) and electric characteristics of the nanowires in each sample.

Figure 1. AgNW samples at reaction times of (A) 6, (B) 12, (C) 18, (D) 24 minutes. Scale bar provided.

The first observation I make is that objects other than solely silver nanowires are formed—particularly in Figure 1A and 1B. These are likely clumps of silver which simply did not form nanowires as not enough reaction time had occurred. In addition, I should note that the places on the sample where SEM analysis was performed were not at random. Our group specifically chose locations where nanowires (even if just a few in the case of the 6 minute reaction) could be found and thus easily analyzed later. However, just qualitatively, there were more nanowires in each sample as the reaction time increased. Particularly for the 18 minute and 24 minute samples, there were a plethora of nanowires and not many silver clumps.

Table 1. Average length, diameter, and aspect ratios of four AgNW samples, depending on the reaction time.

Nanowires tend to have aspect ratios of 1000:1 (length to diameter), far greater than ours. This could be because we did not allow enough time for the reaction to proceed—as the data show, the aspect ratio tended to increase with reaction time. The length of the nanowires also greatly increases with time, particularly between 12 and 18 minutes. This is because the PVP in the reaction does not stick to the ends of the nanowires, so nucleation occurs there to allow them to grow infinitely in length. High aspect ratio allows for incredible flexibility, and nanowires are virtually indestructible. Bulk silver has a resistance of about 1.6 ✕ 10-8 Ωm, so the resistance of silver nanowires is far larger. The increase in resistance as reaction time proceeds could be explained by the longer time the nanowires have in contact with PVP, which has a low conductivity as a polymer. Forourpurposes,wewouldwanttominimizetheresistivity in our nanowires. Thus, if we are to proceed with this research, we might want to figure out a way to minimize the amount of time the nanowires spend with PVP, while still allowing them to grow (because increased aspect ratio allows for better flexibility and strength).

Conclusion & Potential Use: For use in solar cells, we should leverage the fact that the aspect ratio/flexibility grows as reaction time increases, but try to figure out a way to change the reaction mechanism such that the resistivity does not increase. Generally, silver nanowires have fantastic conductivity, and as they are more conductive than bulk silver, could be used in highly-efficient and extremely flexible solar panels.