High Aspect Ratio Silver Nanogrids by Bottom-Up Electrochemical Growth as Transparent Electrode

Abstract

A scalable selective-area electrochemical method is reported for the fabrication of interconnected metal nanostructures. In this work, the fabrication of silver nanowire grids for the application of transparent electrodes is explored. The presented method is based on a through-the-mask electrodeposition method, where the mask is made by using substrate conformal imprint lithography. We find that the nucleation density of the silver nanoparticles is the key parameter for successful homogeneous void-free filling of the template. We independently controlled the density of the silver nuclei and their growth by using a double potential pulse. The silver nanowire grids show high transmission (95.9%) and low sheet resistance (as low as 3.7 Ω/sq), resulting in a superior figure of merit (FoM). Due to the bottom-up nature of this technique, arbitrarily high aspect ratio nanowires can be achieved, therefore decreasing the sheet resistance without affecting transmittance and carrier collection. The presented method can be generalized to the large-area nanofabrication of any well-defined nanostructure design of any metal transparent electrode for multiple applications.

Figure 1

(a) Schematic representation of the SCIL imprinting procedure. (b) Schematic representation of the RIE of the mask to etch the residual layer of the sol–gel and transfer the pattern to the PMMA layer. (c) Schematic representation of the electrochemical filling of the trenches. (d) Schematic representation of the influence of the nucleation density. The top and bottom rows show the effect of a too low and a sufficiently large nucleation density, respectively. (e) SEM image of a typical template-assisted electrodeposited Ag NW grid having a pitch of 2 μm. The inset shows the crossing of two NWs in more detail.

Figure 2

(a) Width of the wires as seen from the top (SEM) vs the grid height obtained from the transferred charge for a pitch of 2 (black circles) and 4 μm (red squares). The inset shows the reconstructed height profile for the 2 μm pitch, including the definition of the inner angle α. The error bars correspond to the standard deviation of the Gaussian fit to the width distribution obtained from the SEM images. (b) Cross-sectional SEM image of a Ag NW grid having a pitch of 2 μm, a height of 219 nm, a top width of 98 nm, and a base width of 78 nm. The inner angle α between the ITO substrate and the Ag NW is 92.6°. (c) Height of the Ag NW grids vs the growth time for a pitch of 2 (black circles) and 4 μm (red squares). The height obtained from AFM and the transferred charge are represented by closed and open markers, respectively. The error on the height obtained from the transferred charge method is propagated from the error on the width, and the error on the height obtained from AFM is the standard deviation of the Gaussian fit to the height distribution.

Figure 3

(a) Normalized resistivity vs the height of the Ag NW grids as obtained from the transferred charge for a pitch of 2 (black circles: thick ITO, green triangles: thin ITO) and 4 μm (red squares). The dashed blue line represents the best fit to the electron scattering model using the parameters λ = 58 nm, w = 94 nm, R = 0.20, p = 0, and d = 32 nm. (b) Normalized transmission spectra for a pitch of 2 μm for three different heights (41, 131, and 228 nm). The inset shows the ratio of the AM1.5G weighted transmittance spectra T_AM1.5G by its geometrical shading T_geo.

Figure 4

Performance characterization of the Ag NW grids, including similar systems found in the literature (electroless Ag nanogrids, evaporated Ag nanogrids, random Ag NWs, EHD printed Ag nanogrids, aligned Ag NWs, electrospun Ag NWs). The data from this work are represented by red squares, black circles, and green triangles for the 2, 4, and 2 μm on thin ITO samples, respectively. Five different values of FoM are represented by the dashed gray lines.