AFM manipulation of nanoparticles based on dielectrophoresis

In this study, dielectric nanoparticles, Al₂O₃ (10~30nm), are trapped and immobilized on an indium-tin oxide (ITO) glass substrate by an atomic force microscope (AFM) based on dielectrophoretic (DEP) force. In the simulation results, the DEP force decreases rapidly away from the apex of the AFM tip, and the effective depth is only about 20μm. Therefore, an open reservoir with a 20μm depth is fabricated from thick photoresist (SU-8), and the hydrophobicity of the ITO surface is modified by an hexamethyldisilazane (HMDS) coating. By applying AC voltage to the ITO substrate and grounding the AFM tip, a non-uniform electric field can be constructed at a depth of 15-20 μm in the Al₂O₃ nanoparticle suspension of 5%, 8% and 10% concentration, respectively. As the experimental results show, nanoparticles can be trapped at the location of the AFM tip and immobilized on the ITO surface by a positive DEP force with an AC signal in the range of 10 kHz to 10MHz. In addition, the cluster of Al₂O₃ nanoparticles can be kept stationary without diffusion even though the AFM tip is stopped by the application of the AC signal and lifted up to the suspension surface. On the contrary, a negative DEP occurs when the AC signal applied ranges between 1 to 10 kHz, and the nanoparticles are repelled from the AFM tip and suspended in the suspension. In general, we demonstrate the technology of patterning nanoparticles by using an AFM tip with DEP force. By integrating the excellent positioning ability of the AFM and the controllable DEP force, cluster arrays of nanoparticles can be accomplished in the near future.