When the normal load was increased to 2 mN, a slight groove with a depth of about 0.5 nm was formed on the GaAs surface. However, when the normal load exceeded 10 mN, the scratching damage became severe and the depth of the groove increased to 23 nm at 30 mN. After etching in H2SO4 aqueous solution for 30 min, there was no visible etching difference on the wearless scratched surface, as shown in Figure 4b. However, the protuberance piled up gradually from the groove area when the normal load increased
from 2 to 30 mN. Therefore, the critical load for the friction-induced fabrication on the GaAs surface is 2 mN, under which the Hertzian contact pressure P c is estimated as 4.85 GPa [17, 18]. Such contact pressure was very close to the critical Hertzian contact pressure for the initial yield of GaAs surface . The height of those protuberances was plotted in Figure 5. It can be seen that EPZ015666 cell line the height of these protuberances increased with the normal load during scratching. When the load was 30 mN, the height of nanostructures could get to 75 nm. Since the protuberance formed only in the wear area, the fabrication mechanism could be related to the deformation of the substrate induced by the mechanical interaction. The detailed generation mechanism of the protrusive nanostructures on the GaAs surface will be discussed in the next section. Figure 4 Effect of normal load on the fabrication of GaAs
surface by scratching and post-etching. (a) AFM images of selleck kinase inhibitor before the scratches created on the GaAs surface under various normal loads. (b) AFM images of the nanolines on the GaAs surface after etching in H2SO4 aqueous solution for 30 min. The cross-sectional profiles were plotted
below for the comparison. Figure 5 Effect of normal load on the height of the nanostructure on the GaAs surface. Mechanism of the friction-induced selective etching on GaAs surface Effect of surface oxide on the friction-induced selective etching Extensive work has shown that various nanostructures can be produced on monocrystalline silicon and quartz surfaces by the friction-induced selective etching method [20, 21]. Guo et al.  suggested that both the tribochemical reaction and the transmutation of crystal structure on the scanned area can result in friction-induced selective etching. To investigate whether the tribochemical reaction played the role in the selective etching of the GaAs surface, X-ray photoelectron spectroscope was used to detect the possible change of chemical composition on the original surface, scratched surface, and this website post-etching surface, respectively. The variation in the bonding states of Ga was presented in Figure 6. On the original surface, it was observed that there were two Ga3d peaks, i.e., Ga-O (Ga2O3) bond at 20.05 eV and Ga-As bond at 18.74 eV , which meant that a native oxide layer existed on the sample surface. On the scratched area, the signal of Ga-O was a little stronger than that on the original surface.