Single-dot tunneling spectroscopy of CdSe colloidal nano-dot by atomic force microscopy

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Recently, highly monodisperse colloidal semiconductor nano-dots have been attracting much attention since they exhibit unique optical properties. We have been interested in the electronic properties of colloidal nano-dots since they seem to be important to explain optical ones. In particular, the interface energy states are likely correlated with non-linear optical properties such as optical memory effect of the dots. It is therefore important to investigate the current-voltage (I-V) characteristics of the colloidal nano-dots in order to reveal the energy levels of the dots by tunneling spectroscopy.For the study of the electron transport through single CdSe colloidal nanodots, we have employed conductive-tip atomic force microscopy. (AFM) We have shown that current-voltage characteristics of single CdSe colloidal nanodot were measured even at room temperature by conductive-tip AFM. However, we found that the measured dot heights by contact-mode AFM were much smaller than the average d … More iameter of the dots estimated from optical absorption measurement. We speculated that the dots slightly changed their positions when the metal-coated Si tips came in contact for contact-mode AFM measurements. This position fluctuation seemed to be caused by the large capillary force between the tip and sample, and prevents us from accurate measurement of the dot height and the current through the dot.We therefore adopted conductive carbon nano-tube(CNT) tips for conductive-tip AFM of CdSe colloidal nano-dots, and found that the capillary force between the CNT tip and sample is reduced to be about one thirtieth of that for the metal-coated Si tips, and that the dot height measurement is much improved in accuracy. As a result, I-V characteristics of single colloidal nano-dots have been obtained at room temperature without extra dot anchoring process which was required for metal-coated Si tips. In the I-V curves, clear conductance changes were observed which suggested resonant electron tunneling through the nano-dot.Also, the formation of two-dimensional islands consist of colloidal nano-dots was investigated in order to stabilize the dot position during conductive-tip AFM measurements. We fabricated single-dot height films of the colloidal nano-dots by horizontal lift-off process on Au (111) films evaporated on mica substrates, and confirmed the existence of 100-nm-scale two-dimensional islands of the dots by contact-mode AFM. I-V measurements of these islands will be performed. Less