## 2 Methodology  
The method used for the fabrication of silver nanosphere arrays decorated on a polycarbonate substrate with self-organized, hexagonal close-packed nanocone is illustrated schematically in Figure 1. First, a nanocone-shaped groove array structure of aluminum substrate was fabricated by using anodic aluminum self-assembly technique [18; 19; 20]. To form a cone-shaped cavity, the anodization and pore widening processes were alternately repeated several times, which creates a top widened and bottom narrow holes, the diameter of these holes are reduced gradually from the top to the bottom of these holes. The fabrication process of the nanocone cavity array of the aluminum substrate including electrolytic polishing, anodic treatment, removal of anodic aluminum oxide. Briefly, a 99.999% purity aluminum substrate was polished in a solution mixed with perchloric acid and anhydrous alcohol in a volume ratio of 1:3.5 and applied with a voltage of 25 V for 2 min. The polished aluminum substrate was anodic oxidation treated in 0.3 M oxalic acid solution with 50 volts for two hours. The anodic aluminum oxide layer was removed by placing the substrate in a 5 wt% phosphoric acid solution at temperature 35 \({}^{\circ}\)C for 1 h. The anodization and anodic aluminum oxide removal processes were alternately repeated 5 times to obtain the desired nanocone cavity array structure.  
Then a Nickel replica of the nanocone-shaped groove array was obtained by electroforming the nanostructured AAO layer. The nanocone-shaped groove array structured Nickel mold was used as a  
Figure 1: Schematic illustration of the fabrication processes. The nanostructured mold was pressed on a Polycarbonate (pc) plastic substrate using a hot-press molding machine. The nanostructured-nickel mold was heated and pressed on the pc surface for several minutes. After cool down to room temperature, the pc substrate with replicated nanostructure was released from the master mold.  
master mold. Figure 1. illustrated the hot pressing process. The nanocone-shape groove structured Nickel mold was imprinted on a Polycarbonate(pc) plastic substrate using a hot-press molding machine for heating and pressing the nanostructured nickel mold on pc surface for several minutes. After cool down to room temperature, the pc substrate was released from the master mold. Using this method, we were able to produce many nanocone arrays (moth eye like structure as illustrated in Figure 1) structured pc substrates with low-cost and same quality.  
The silver nanoparticles were formed on top of nanocones using sputtering. A dc magnetron sputtering system (Cressington 108 Sputter Coater) was used. Due to the self-aggregating nature of nanoscale metallic materials, the sputtered silver nanoparticles tend to self-aggregate at the tip of the nanocones, and finally form silver nanospheres at the tip of each nanocone (as illustrated in Figure 1). The diameter and interspacing of the nanospheres can be varied by adjusting the sputtering parameters. In order to obtain a uniform and a closely packed array of nanospheres, the appropriate sputtering parameter is needed. In this study, we varied the sputtering duration while the sputtering current and gas pressure were kept at constant (20 mA and 0.02 mbar respectively). The nanostructured polycarbonate substrates were sputtered for different durations to obtain nanospheres with various diameters, and at the same time, the interparticle gaps of the nanospheres were changed accordingly.