Current Images of CdSe Colloidal Nanodots Observed by Conductive-tip Atomic Force Microscopy
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Current Images of CdSe Colloidal Nanodots Observed by Conductive-tip Atomic Force Microscopy
Ichiro Tanaka1, Eri Kawasaki1, O. Ohtsuki1, M. Hara2, H. Asami3, and I. Kamiya3 1 Department of Materials Science & Chemistry, Wakayama University, 930 Sakaedani, Wakayama 640-8510, Japan 2 Frontier Research System, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan 3 Science and Technology Research Center, Mitsubishi Chemical Corp., 1000 Kamoshida-cho, Aoba-ku, Yokohama 227-8502, Japan
ABSTRACT We have fabricated submonolayer-thick films of CdSe colloidal nanodots in order to investigate electronic properties of individual nanodots by conductive-tip atomic force microscopy (AFM). Topographic and current images of isolated single CdSe colloidal dots on single crystalline Au (111) surface which was covered with dodecanethiol self-assembled monolayer were obtained 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 also found 10 nm-scale electric inhomogeneity around the dots, which may corresponds to the previously reported etch-pits of Au (111) surfaces formed during the deposition of the dodecanethiol molecules.
INTRODUCTION Semiconductor colloidal nanodots have been one of major research interests since they exhibit quantum effect or nano-size effect which is potentially applicable for novel optical and opto-electronic devices [1, 2]. Recently, highly monodisperse II-VI and III-V semiconductor nanodots have been synthesized by colloidal chemical techniques [3, 4]. Because they are free from the lattice mismatched substrate, the choice of materials for colloidal dots is wider than that for self-assembled quantum dots formed by Stranski-Krastanov mode growth by molecular beam epitaxy or metal organic chemical vapor deposition. The optical properties of the colloidal dots have been widely investigated, and it has been recognized that the surface termination may be playing an important role for the properties. We have reported an optical memory effect of CdSe colloidal nanodots H9.19.1
where the photoluminescence (PL) intensity of the nanodots spun-coat on glass substrates depends on the photoexcitation energy and duration [5]. This effect is strongly influenced by the preparation process of the sample, and the main contribution is therefore attributed to electron trap states at the interface between the CdSe nano crystals and organic molecules covering them. In order to study the interface properties of colloidal nanodots, electronic investigation have advantages over optical since interface traps are known to be non-radiative. We thus adapted conductive-tip atomic force microscopy (AFM) [6, 7] for nano scale electronic study of CdSe colloidal nanodots. In this paper, we report on fabrication of sub-monolayer thick CdSe colloidal nanodot film for electronic investigation of individual nanodots, and observation of current images of single CdSe col
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