Nassiopoulou et al. Nanoscale Research Letters 2011, 6:597 http://www.nanoscalereslett.com/content/6/1/597 Experimental details The substrates used in this work were p-type (100) Si wafers with resistivity ranging from 1 to 2 Ω cm. SiNWs were formed with the single-step MACE techni- que that consists of immersing the sample in an HF/ AgNO3 aqueous solution for a process time that deter- mines the SiNW length. With this technique, the mechanism of SiNW formation involves two different processes that occur simultaneously: (a) deposition of Ag nanoparticles on the Si surface and (b) catalytic etching of Si at the sites where the Ag nanoparticles have been deposited. The composition of the AgNO3/ HF/H2O solution used was 0.67 g:35 ml:182 ml. Experi- ments were carried out in a temperature ranging from 20°C to 55°C. Confined areas on the Si wafer were defined by photolithography using the AZ5214 photore- sist. This photoresist was used as the masking material for SiNW formation on confined areas, and it was found to constitute an excellent masking material since it with- stands the MACE solution for a long time and it is easily removed with acetone after the end of the process. Square-shaped windows in the photoresist having a sur- face area ranging from 2 × 2 μm2 to 400 × 400 μm2 were used in our experiments. SiNWs were characterized by FE-SEM using a JSM- 7401F microscope (JEOL, Tokyo, Japan). The Ag den- drite-shaped structures that usually grow on the SiNW surface during the single-step MACE process were removed in an HNO3/water solution with a volume ratio of 1:1. Results and discussion SiNW formation and morphology on large surface areas In order to understand the experimentally observed dif- ferences of the etch rate between large areas and con- fined areas, it is necessary to recall here the mechanism involved in the one-step MACE process. In this process, the SiNWs are produced by simply immersing the wafer into an HF/AgNO3 aqueous solution of a given concen- tration for an appropriate time. The reaction that takes place is a galvanic displacement reaction. A galvanic cell is established when the Si wafer is immersed into the solution because the reduction potential of the Ag+/Ag couple is more positive than the flat band potential of Si. Two simultaneous processes occur in the galvanic displacement reaction at the Si surface: (a) reduction of Ag+ ions by hole injection into Si, which produces metallic Ag deposits (cathodic reaction, electron-con- suming type) and (b) oxidation of Si by the injected holes (anodic reaction, electron-releasing or hole-con- suming type). In this process, the bonding electrons of surface Si atoms are transferred to Ag+ ions in the aqu- eous HF solution as described in detail by Peng et al. [26]. The oxidized Si is dissolved by the HF, leading to Page 2 of 8 Si etching and resulting in pore or NW formation. Ag+ reduction and Si oxidation result in the deposition of silver atoms on the cathodic sites of the Si surface, forming nanoscale Ag nuclei at the beginning of the process, with higher electronegativity than Si and thus, strongly attracting electrons from Si to become nega- tively charged, providing a catalytic surface for further Ag+ reduction. A quasi-Schottky Ag/Si interface is formed, with a relatively low-energy barrier for holes. Charge transfer from the Ag nuclei to Ag+ ions in the solution occurs by hole injection through the quasi- Schottky Ag/Si interface. Typical scanning electron microscopy [SEM] images of the SiNWs formed by a single-step MACE process are shown in Figure 1: in a1 (plane view) and a2 (cross sec- tion), we see the case of short NWs (5-μm long), which are relatively straight and separated from each other, while in b1 (plane view) and b2 (cross section), we see the case of longer NWs (20-μm long). It is depicted that after a certain length (a few micrometers), the longer NWs tend to merge together at their tops and form bundles of NWs, as depicted in both the top view (b1) and the cross section (b2) of Figure 1. It is worth noting that the etching process by MACE occurs vertically to the Si surface of (100) wafers due to the fact that the reaction rate is much higher in [100] crystallographic orientations than in other orientations. Consequently, when a Si wafer is immersed into the etching solution, SiNWs are formed not only on the front Si wafer surface, but also on the equivalent [100] crystallographic orientations of the sidewalls and the back side of the wafer. In each case, the SiNWs grow perpendicularly to the Si surface. In order to avoid NW formation on the back side of the wafer, it is necessary to cover the backside Si surface with a masking material that prevents Si etching by MACE. The photoresist AZ 5214 was found to be an appropriate choice in this respect. The above effect is illustrated in the SEM images of Figure 2: in (a), we see a cross-sectional SEM image of the front Si surface; in (b), from the sidewalls; and in (c), from the backside surface of the wafer. From this last image, it is depicted that the backside wafer roughness is reproduced at the interface between SiNWs and Si substrate, the NW length being constant and perpendicular to the surface. On the surface of the already formed SiNWs, we do not expect any significant etching due to depletion of the NWs from carriers that occurs during etching. Formation of SiNWs on confined areas on the Si substrate We investigated the growth of SiNWs by MACE on confined areas on the Si substrate of a surface area that varied from 400 × 400 μm2 down to 2 × 2 μm2, and we compared the results with those from non-patterned