Kim and Cho Nanoscale Research Letters 2012, 7:408 http://www.nanoscalereslett.com/content/7/1/408 Page 6 of 8 Figure 6 Raman spectra of porous Si. Raman spectra (a), (b), (c) and (d) were obtained from samples 1, 2, 3 and 4, respectively. The presence of amorphous Si in the etched PS can be confirmed from the transverse optical (TO) mode at 480 cm−1 in the spectra; longitudinal optical (LO), longitudinal acoustical (LA) and transverse acoustical (TA) modes are located at 375, 310 and 150 cm−1, respectively. groups of crystallites that can extend the PL to a longer wavelength, as shown in Figure 8. The FWHM of sample 4 is approximately 165 nm, whereas that of sample 1 is about 262 nm. Such a sig- nificant difference in size distribution of nanocrystallites Figure 7 HRTEM images were obtained from the porous Si. Micrograph (a) was obtained from sample 4. The nanocrystallites are marked with black circles. (b) FFT pattern of the image shown in (a). between sample 1 and sample 4 might come from the fact that the doping concentration of sample 1 is ap- proximately 104 times higher than that of sample 4. As a result, sample 1 has a larger concentration of doping- related atomic defects, which could be the starting point of anodic chemical etching. On the other hand, in sam- ple 4, the distance between the defects related to doping is much larger than that of sample 1. The nanocrystal- lites produced in the region between the doping-related defect sites appear to be of approximately 2 nm; ap- proximately 2-nm-sized crystallites must be dominant in the size distribution of sample 4. On the other hand, the distance between such doping-related defects in sample 1 is much smaller than that of sample 4. The dominance of a particular size in the size distribution of nanocrys- tallites in sample 1 is hardly expected compared to that of sample 4. From the variation in the size distribution of the nano- crystallites with the doping concentration, it is highly likely that the size of the nanocrystallites is related to