Charge injection experiments by electrostatic force microscopy are performed on single semiconductor nanoparticles. Different methods of detecting the stored charge are used. Although the amount of charge stored in particles of realistic shape can be determined quantitatively, we present here a qualitative comparison between Q (V ) hysteresis curves observed on silicon and GaN quantum dots, in dry nitrogen and in ultra high vacuum. For silicon dots in dry atmosphere, we find a hysteresis behavior entirely different from the one observed on GaN dots in ultra high vacuum. The contribution of interface states to hysteresis is discussed