Usually pure electrostatic charges cannot be utilized alone to form macro-sized three-dimensional electromagnetic traps for trapping charged particles. These traps need additional dynamic electromagnetic fields. We report here the observation of a CdSe nanocrystal floating over a carbon film in a transmission electron microscope. The nanocrystal was most likely trapped in a naturally formed, nanometre-sized 3D Coulomb potential well. The results indicated the possibility of developing a novel nanodevice for the trapping and manipulation of charged nanoparticles.