Conductive-tip atomic force microscopy of CdSe colloidal nanodots

Electronic properties of individual CdSe colloidal nanodots have been investigated by conductive-tip atomic force microscopy (AFM). Submonolayer-thick films of the colloidal nanodots were fabricated on a self-assembled monolayer of alkanethiol molecules formed on Au(1 1 1) surfaces for single dot measurements. First, we simultaneously imaged the topography and conductivity of isolated single dots by AFM operating in contact mode with a conductive tip under appropriate bias voltages. In the current image, it is found that the dot regions have higher electric resistances due to tunneling resistance through the CdSe dots. We found a 10-nm scale electric inhomogeneity around the dots, which may correspond to the previously reported etch-pits of Au(1 1 1) surfaces formed during the deposition of the alkanethiol molecules. Then, current–voltage characteristics were measured with the conductive tip positioned on the single dots; large changes in the conductivity which suggest resonant tunneling through the quantized energy level in the dot were observed even at room temperature.