Kim and Cho Nanoscale Research Letters 2012, 7:408 http://www.nanoscalereslett.com/content/7/1/408 this region. An approximately 100-μm-wide etched re- gion was neighbored by an approximately 100-μm-wide unetched region. In contrast, in Figure 1b, the etching reaction occurred on the Si surface homogeneously, and most of the etched surface consisted of microscale holes; the inset provides details of the column-type sur- face morphology. The wettability of etchants was examined by measur- ing the contact angle between the etchant and Si wafers. When the etchant with the highest solvent ratio (IPA, 66 %) was dropped on the Si surface, the etchant spread out rapidly. In contrast, in the case of electrolytes with an IPA ratio of 0 %, 5 % and 33 %, the etchant did not spread but rather stayed like a drop of water on a lotus leaf. When the IPA ratio of electrolytes was increased from 0 % to 33 %, contact angle was varied from 57° to 14° as shown in Figure 2a,b,c. In case of IPA ratio of 66 %, it was difficult to measure the contact angle for electrolytes because the etchant spread rapidly, as shown in Figure 2d. The variation in the contact angle between the etchant and Si wafer with the IPA contents suggests that a surfactant like IPA plays an important role in gen- erating a homogeneous morphology during electrochem- ical anodic etching. Figure 3 shows cross-section and plane view FE-SEM images of the PS. The morphologies of the PS samples 1 to 4 were examined to determine the role of the resistiv- ity of a Si wafer on the etching process. The etching was Page 3 of 8 carried out in electrolytes with the IPA ratio of 50 %. When the resistivity of Si wafer was increased from 0.005 to 15 Ω cm, the morphology varied from a ‘sponge’ (Figure 3d) to a ‘column’ (Figure 3f )-type structure. Brunauer Emmett Teller (BET) analysis was carried out to examine the variation in surface area with etched morphologies (Figure 4). When the morphology of PS changes from a sponge to column-type structure, the surface area decreased from 19.92 to 0.09 m2/g. The sponge-type and column-type structures showed the lar- gest and smallest surface area, respectively. The silicon wafers with resistivities of 0.005 to 15 Ω cm are expected to have doping concentrations of 3.62 × 1019 to 1.03 × 1015 atoms/cm3, respectively. The SCR width of the wafers under the etching conditions is critically determined by the distribution of dopants in the wafers. In addition, the charge carrier transfer type is also varied during the etching process with the doping concentra- tions [22]. As a result, such difference in the doping levels seems to bring up the morphological variation. Nanostructural features Figure 5 shows XRD patterns of the PS samples. The XRD peak at 33° of the sample with a resistivity of 0.005 Ω cm was assigned to the (200) plane of Si. The raw curve was deconvoluted into three Lorentzian peaks, and the results were used to estimate the nanocrystallite size. The fits of the XRD pattern are in good agreement Figure 2 Contact angle measurement results. The images in (a), (b), (c) and (d) were obtained when a drop of etchant with surfactant (IPA) ratios of 0 %, 5 %, 33 % and 66 % were placed on the surface of the Si wafers, respectively.