Nanowires

Harrick Plasma → Applications → Materials → Metals → Nanowires

Conductive nanowires have been extensively studied recently for their potential use as transparent conducting electrodes in flexible electronics, wearable biosensors (wearable electronics), organic light emitting diodes OLEDs, and solar cells [1-3].

Although indium tin oxide (ITO) is a widely used transparent conducting electrode in current device manufacturing, it is not an ideal choice for flexible electronics because of its relative brittleness, more expensive production cost (conventionally by vacuum deposition), and increased difficulty in sourcing the indium metal.

Due to their nanometer scale, nanowires are intrinsically optically transparent and mechanically flexible to meet most of the criteria for electrodes used in flexible electronics, which include high optical transmission, high electrical conductivity, and flexibility and structural stability to withstand continuous and repeated deformation cycles.

Recently, researchers have been investigating various materials and methods to fabricate nanowires. In particular, silver and gold nanowires (AgNW, AuNW) are favorable alternatives to ITO because they can be deposited by simpler and lower-cost methods, such as spin, dip, or spray coating from solution; drop casting, or self-assembly. Research has also focused on fabricating nanowires that can be precisely patterned, cover large surface area with relative smoothness, and can be scaled for mass production.

One critical piece in fabricating nanowires from solution is to have clean, hydrophilic surfaces so that the solution can readily wet and distribute uniformly across the surface with strong adhesion, which is critical for optimal device performance. This can be achieved by plasma treatment to remove residual organic contamination and introduce oxygen containing polar functional groups (hydroxyl, carbonyl, carboxyl) to increase surface hydrophilicity.

In this note, we explore the use of plasma treatment to facilitate nanowire fabrication and improve electrical properties for optimal device performance.

Fabricating nanowires

To prepare substrates surfaces for fabricating nanowires, plasma treatment is commonly performed prior to the deposition step. Plasma cleaning renders surfaces hydrophilic and promotes uniform distribution of the nanowire solution onto the surface. In some cases, an intermediate layer may be deposited on the plasma-treated substrate to further facilitate adhesion of nanowire to the substrate.

Plasma-treated, hydrophilic surfaces have also been exploited to create patterned nanowire networks (Figure 1). In Chen et al.’s work, AgNWs were deposited on flexible polydimethylsiloxane PDMS substrates. As PDMS is inherently hydrophobic, air plasma was first applied to PDMS through a patterned mask. Plasma rendered hydrophilic the exposed PDMS to create a patterned hydrophilic/hydrophobic surface which defined the nanowire deposition areas. The solution-cast nanowires adhered to the hydrophilic areas but dewetted from the untreated, hydrophobic areas.

In another example, Gao et al. used O₂ plasma to render PDMS hydrophilic prior to stretching to form well-defined, periodic wrinkles (the periodicity and height of the wrinkle were controlled by the magnitude of the applied strain). The wrinkled PDMS served as a periodic template and was then spray coated with AgNW dispersion to form a patterned nanowire layer.

Following patterning, the nanowires can be further embedded into a flexible optical coating (e.g. Ecoflex, UV curable polyurethan optical adhesive, PDMS) to form a nanowire/polymer composite. The resulting nanowire composite is optically transparent, flexible, and stretchable yet is able to maintain a relatively smooth, continuous wire network with repeated mechanical strain cycles, all of which are critical to be used in flexible/wearable electronics. For Chen et al., their patterned AgNW composite demonstrated optical and electrical properties roughly comparable to that of ITO, with average sheet resistance R_S of ~15 Ω/sq and ~85% optical transmittance at 550 nm. For Gao et al., the patterned AgNWs exhibited R_S of 18 Ω/sq and 78% optical transmittance.

Figure 1. An example of patterning nanowires on PDMS substrate, an inherently hydrophobic material. Plasma renders the exposed surfaces hydrophilic. Nanowire solution coats the hydrophilic areas to create a patterned nanowire layer.

Improving nanowire electrical properties

Plasma treatment has been employed to improve nanowire electrical properties and yield highly conductive electrodes. Li et al. applied Ar plasma to AgNW spin-coated from solution, to remove residual polyvinylpyrrolidone PVP surfactant (from the deposition process). In addition, plasma treatment fused adjacent AgNWs to improve electrical contact between nanowires. Gong et al. also used Ar plasma in a similar manner on self-assembled AuNWs to remove excess oleylamine OA ligand (from the self-assembly process) and promote intimate contact at the nanowire junctions, thereby reducing electrical resistance.

From Li et al’s study, Figure 2 shows the resulting AgNW sheet resistance with varying plasma treatment times and indicates that R_S decreased from at least 25 Ω/sq (as deposited) to 7-8 Ω/sq with up to 15 min plasma. However, longer plasma exposure resulted in greater R_S. SEM images showed breaks and discontinuity along the length of the nanowires, suggesting melting and damage to AgNW with excessive plasma exposure.

By adjusting treatment time, Li et al. found that 15 min plasma produced optimal nanowire performance, with R_S of ~7 Ω/sq while maintaining 78% optical transmittance at 550 nm, and electron work function of 5.84 eV. As a point of reference, conventional heating of AgNWs (150°C for 15 min) produced R_S of 20-25 Ω/sq, ~72% optical transmittance at 550nm, and work function of 5.5 eV. Other researchers reported that longer thermal annealing times (up to 200 min) may be required to further reduce AgNW sheet resistance [4]. Li et al.’s work demonstrated that plasma treatment can be a crucial step to enhance electrical properties of AgNW deposited from solution for optimal device performance.

Figure 2. Sheet resistance R_S of solution cast AgNW with Ar plasma treatment. Ar plasma removed residual organic PVP (from coating process) and enhanced contact between adjacent nanowires. Data from Li J, Tao Y, Chen S, Li H, Chen P, Wei M-z, Wang H, Li K, Mazzeo M and Duan Y. “A flexible plasma-treated silver-nanowire electrode for organic light-emitting devices”. Sci. Rep. (2017) 7(1): 16468. 10.1038/s41598-017-16721-7