Scanning transmission electron microscopy in the dark field mode (STEM-DF) was used for chemical difference imaging (Z-contrast) of the nanowhiskers and the nanofibers (see Figure ). The heavier atoms (W) of the W18O49 nanowhiskers In the STEM-DF appear brighter than the silica-based nanofibers with lower atomic numbers (Si). Therefore, the concentration of tungsten is higher in the grown nanowhiskers as compared to that in the