Nassiopoulou et al. Nanoscale Research Letters 2011, 6:597 http://www.nanoscalereslett.com/content/6/1/597 to 50°C. The SiNW formation rate was found to be lin- ear with temperature in both cases, with an activation energy derived from an Arrhenius plot and is equal to 0.37 eV in the case of non-patterned areas (in good agreement with Cheng et al. [30]) compared to 0.38 eV in the case of lithographically patterned areas. The two Arrhenius plots are almost parallel to each other, show- ing that the thermal activation process is the same in both cases (see Figure 6). In order to understand the mechanism responsible for the increase of the formation rate of Si NWs on litho- graphically patterned areas compared to the non-pat- terned ones, we performed a series of experiments and examined different factors that could be at the origin of the different reaction kinetics in the two cases, as follows: Effect of surface area In order to elucidate the effect of surface area localiza- tion on the formation rate of SiNWs, we performed a series of experiments on a single wafer with different lithographically defined surface areas. The wafer was immersed into the solution for 30 min at a temperature of 30°C for a reaction time of 1 h. A comparative study of cross-sectional SEM images of the SiNWs from the different surface areas of the sample showed that there was no significant difference in the SiNW length in Page 6 of 8 small and large confined surface areas in the examined range from 50 × 50 μm2 to 400 × 400 μm2. Effect of silver dendrite density During the MACE process, Ag is initially deposited on the Si surface, and a galvanic cell is established when Si etching is initiated; Ag nanoparticles proceed into the Si substrate through Si etching. These particles are found at the interface of the etched area with the Si substrate. On the other hand, silver continues to be deposited on the Si surface, forming dendrite nanostructures, which are very dense in the case of large non-confined areas. In the case of confined areas, the dendrite nanostruc- tures formed on the Si surface are localized on Si areas not covered by the photoresist. They grow locally and are less dense, thus allowing for an easier interaction of the Si surface with the metal ions from the chemical solution. This is illustrated in Figure 7, where we see tilted cross-sectional SEM images from a non-patterned (a) and a 2 × 2-μm2 lithographically defined surface area (b). In (a), the dendrites are very dense and completely cover the etched area, while in (b), they protrude from the small etched areas, leaving more free spaces between them, thus facilitating the interaction of Si with the che- mical solution. We performed experiments by changing the inter-distance between small etched areas, and we found again that there was no significant difference Figure 6 Arrhenius plots of the formation rate of SiNWs on non-patterned and on lithographically defined areas. Arrhenius plots of the etch rate of Si by MACE as a function of temperature in the case of non-patterned areas (dark squares) and in the case of lithographically defined confined areas (red circles). From these plots, the process activation energy is calculated at approximately 0.37 eV in the first case and at approximately 0.38 eV in the second case.