Using an atomic force microscope combined with an inverted optical microscope, we are studying directly the charge and dipole moment of single CdSe nanocrystals and their corresponding effect on the nanocrystal's photoluminescence. Previously, the electrostatic polarization of semiconductor nanocrystals has been either indirectly measured or inferred, but not measured directly. We now have the ability to measure electrostatic properties directly, and in real time. In particular, we use a technique called electrostatic force microscopy (EFM) to measure the dielectric constant and electrostatic polarization of single CdSe nanocrystals in dry air at room temperature. We are working on combining EFM with single nanocrystal optical spectroscopy to directly see if the nanocrystal's charge state affects its optical properties. In route to this challenging experiment, we have first used our AFM/Optical microscope apparatus to correlate the luminescence and the topographic images of 200 nm polymer dye spheres. The figure shows on the bottom the optical fluorescent image, and on top the corresponding topographical image. The inset is a high resolution AFM image of one of the dye spheres.
