Kim and Cho Nanoscale Research Letters 2012, 7:408 http://www.nanoscalereslett.com/content/7/1/408 Page 5 of 8 Figure 5 XRD patterns of porous Si. XRD patterns (a), (b), (c) and (d) were obtained from samples 1, 2, 3 and 4, respectively. In pattern (a), the raw curve is separated into three Lorentzian peaks (1) to (3). D ¼2π rﬃ2ﬃﬃ:ﬃ0ﬃﬃﬃ ; Δw 2 ð Þ information on the volume fraction of the nanocrystal- lites as well as the mean crystallite size. The Equation 2 was used to obtain the mean crystallite size [26,28]. where Δw is the shift of nc-Si peaks from that of bulk Si (521 cm−1). In Figure 6, the spectrum fits indicated that bulk Si, nc-Si and amorphous Si co-exist. In Figure 6a, the spectrum fit indicated the presence of nc-Si at 517.5 cm−1. The nanocrystallite size of sample 1 was esti- mated to be 4.7 nm. In the case of samples 2, 3 and 4, the fits for the nc-Si spectrum revealed peaks at 516.3, 510.2 and 505.9 nm, respectively. The nanocrystallite size was estimated to be 4.1, 2.7 and 2.3 nm, respectively. The presence of amorphous Si in the etched PS was also con- firmed from the TO mode at 480 cm−1 in the spectra [27]. The Raman spectra revealed the LO, LA and TA modes at 375, 310 and 150 cm−1, respectively. This observation of the Si nanocrystallite size and distribution by Raman spectroscopy matched the XRD results. The volume fraction of the Si nanocrys- tallites and amorphous phases in the porous Si was estimated from the Raman spectroscopy results. These values were obtained based on two signals: one (Ia) is the TO mode at 480 cm−1, and the other one (Ic) is nc-Si. The relative volume fraction of the crystallites in the PS was calculated using the equation, Xc ¼ Ic = ðIc þ πIaÞ , where η is the scattering factor and is regarded as approximately 1.0 for nanocrystal- lites [29]. The largest volume fraction of the nano- crystallites (approximately 5 nm) was observed for sample 1; the volume fraction of the nanocrystallites was approximately 71.3 %. For samples 2, 3 and 4, the relative volume fractions of the nanocrystallites were 41.7 %, 52.2 % and 50.5 %, respectively. In the Raman results, bulk Si peak is located at 521 cm−1. Peaks related to nanocrystallites (490 to 520 cm−1) and amorphous (480 cm−1) phases were separated from that of substrate bulk Si (521 cm−1) with high accuracy (95 % ± 2 %) by deconvolution as shown in Figure 6. Figure 7 shows a HRTEM image of sample 4. The image reveals an approximately 5-nm sized nanocrystal- lite embedded in the amorphous matrix. A nanocrystal was marked with black circle in Figure 6a. The fast Fou- rier transform (FFT) diffraction pattern (Figure 6b) clearly shows that an etched layer is rather amorphous and contains a small fraction of nanocrystallites. Optical features Figure 8 shows the PL spectra of the PS samples. In the PL spectra of samples 1, 2, 3 and 4, the maximum peak was observed near 722, 662, 643 and 615 nm, respect- ively. These wavelengths correspond to the band gap en- ergies of 1.71, 1.87, 1.93 and 2.02 eV. The size of the Si crystallites decreased with increasing wafer resistivity, as shown in the previous nanostructure section, and the optical band gap appeared to increase. This appears to be well matched to the relation of the optical band gap with the Si nanocrystallites, which was reported previ- ously in Blackwood's work [11]. Such correspondence also supports the fact that the observed PL is caused by the presence of nc-Si in the PS. The maximum intensity of the PL spectra of sample 1 is weaker than those of samples 3 and 4. In addition, the FWHM was asymmetric and wider than the other spec- tra. This trend was also confirmed by XRD. In the XRD patterns of sample 1 with a sponge-type structure, al- though the most prominent deconvoluted peak is related to the 5 nm-sized nanocrystallites, there are two other