Flexible and transparent electrodes imprinted from metal nanostructures: morphology and opto-electronic performance

Abstract

We directed the self-assembly of nanoscale colloids via direct nanoimprint lithography to create flexible transparent electrodes (FTEs) with metal line widths below 3 μm in a roll-to-roll-compatible process. Gold nanowires and nanospheres with oleylamine shells were imprinted with soft silicone stamps, arranged into grids of parallel lines, and converted into metal lines in a plasma process. We studied the hierarchical structure and opto-electronic performance of the resulting grids as a function of particle geometry and concentration. The performance in terms of optical transmittance was dominated by the line width. Analysis of cross-sections indicated that plasma sintering only partially removed the insulating ligands and formed lines with thin conductive shells and a non-conductive core. We provide evidence that the self-assembly of high-aspect nanowires can compensate for defects of the stamp and substrate irregularities during imprinting, while spheres cannot. The wire-based electrodes thus outperformed the sphere-based electrodes at ratios of optical transmittance to sheet resistance of up to ≈ 0.9% Ω_sq ^-1, while spheres only reached ≈ 0.55% Ω_sq ^-1.

Fig. 1. (A): Sheet resistances R_sh and optical transmittances for wire- and sphere-based electrodes directly after plasma sintering. Data for electrodes with R_sh that exceeded the scale are not included (Section 2 of the ESI has the complete data set). (B): Dependence of with concentration c_Au. All graphs show average values from three measurements, the standard deviation, and a fit (dashed lines). Light colours represent the lowest and dark colours the highest Au concentrations in the printed colloids.

Fig. 2. Typical morphological features of lines printed from AuNW at c_Au = 6 mg mL⁻¹ before and (purple flash) after plasma. (A): TEM image of dried and self-assembled wires. (B): SEM image (top-view) of a printed line. (C): SEM image (top-view) of a printed, plasma-sintered line; the white arrow indicates one of the typical crack-like defects which start forming at 6 mg mL⁻¹ in the bleeded parts. (D): Height trace of a printed, plasma-sintered line. (E): TEM image of a cross-section of a printed, plasma-sintered line. Inset: A shell formed by the plasma. The red dashed circles indicate gaps within the shell.

Fig. 3. Typical morphological features of lines printed from AuNP at c_Au = 30 mg mL⁻¹ before and (purple flash) after plasma. (A): TEM image of dried and self-assembled spheres. (B): SEM image (top-view) of a printed line. (C): SEM image (top-view) of a printed, plasma-sintered line. (D): Height trace of a printed, plasma-sintered line. (E): TEM image of a cross-section of a printed, plasma-sintered line. Inset: A shell formed by the plasma.

Fig. 4. Topography maps of (A) wire- and (B) sphere-based printed lines after plasma sintering. White arrows indicate defects similar to those in Fig. 2C. (C) Average conductor width w, (D) maximum conductor height h_max and (E) cross-sectional area A_shell as a function of gold concentration c_Au. Standard deviations are indicated as error bars, the dashed lines are fits by linear regression to better illustrate the (piece-wise) linear behaviour.

Fig. 5. Effect of particle concentration c_Au on (A) the conductance G of a representative square electrode sub-section, (B) the cross-sectional area A_shell and (C), the shell conductivity σ_shell. Standard deviations are indicated as error bars, the dashed lines are fits by linear regression to better illustrate the piece-wise linear relations.

Fig. 6. Development of the optical transmittance for (A) wire- and (B) sphere-based electrodes (experiment and model) with concentration c_Au, the belonging standard deviation and a data fit (dashed lines).