The formed oxide covers Torin 2 all the internal surface of the porous nanowires and leads to expansion of the volume of the Si nanostructures composing the SiNW skeleton (Figure 3b). With the additional HF dip, the SiO2 layer from the internal porous Si surface is dissolved, selleck screening library leading to full dissolution of the upper length of the nanowires, which is highly porous (Figure 3c). This proves that the whole volume of the SiNWs is fully porous and that there is no single-crystal Si core
in the nanowires. This was an open question in the literature . The fact that after the first HF/piranha treatment the length of the SiNWs is only slightly reduced, while after the additional HF dip the NWs www.selleckchem.com/products/tpx-0005.html almost disappear, except of a short nanowire base, indicates that the SiNW porosity is not homogeneous throughout their length, but it is higher at their top and it gradually decreases from the top to the bottom. In addition, the fact that the above chemical treatment did not dissolve the porous Si layer underneath the SiNWs means that the porosity of this layer is lower than that of the SiNWs’ tops. Consequently, in the as-grown sample, this layer is not expected to have
a significant contribution to the PL spectrum. Photoluminescence spectra PL spectra were obtained from the as-formed samples and from samples after different chemical treatments. PL was excited by a HeCd laser line at 325 nm. The results are summarized in Figure 4 for a sample etched for 60 min. The PL peak is broad, with a maximum at approximately 1.9 eV and a full width at half maximum (FWHM) of approximately 380 meV in the case of the as-formed sample. By immersing the as-etched sample into an HF solution, the PL peak was red-shifted from 1.73 to 1.80 eV while the PL FWHM increased from 412 to 447 meV. In addition, the PL intensity increased by a factor of 2. The HF dip was then followed by a piranha treatment that oxidizes the internal Si surface, forming an oxide shell around the nanostructures composing
the porous nanowire skeleton. This treatment old caused a shift of the PL wavelength to approximately the initial peak energy and the initial FWHM. In addition, the PL intensity was doubled. Finally, after an additional HF treatment, the PL intensity was increased by 50 times, without any significant wavelength shift. These results will be discussed below. Figure 4 PL spectra from the as-grown sample etched for 60 min and samples after different chemical treatments. The spectrum from the as-grown sample is denoted by (1), the sample after an HF dip by (2), after HF/piranha by (3), and after HF/piranha/HF by (4). The vertical dashed line is a guide to the eye. From time-resolved PL measurements, the PL decay time at room temperature was found to be in the 19- to 23-μs range.