Current-voltage Characteristics of Single CdSe Colloidal Nanodots Measured by Conductive-tip Atomic Force Microscopy
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Current-voltage Characteristics of Single CdSe Colloidal Nanodots Measured by Conductive-tip Atomic Force Microscopy
Ichiro Tanaka1, Eri Kawasaki1, O. Ohtsuki1, K. Uno1, M. Hara2, H. Asami3, T. Murase3, 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 investigated current-voltage characteristics of individual CdSe colloidal nanodots by conductive-tip atomic force microscopy (AFM). The colloidal nanodots were spun-coat and scattered on a self-assembled monolayer of thiophene molecules formed on Au (111) surfaces for single dot measurements. A thin SiO2 layer was deposited on the sample surface in order to prevent the dots being moved by the tip during measurement. We imaged the topography of isolated single dots by AFM operated in contact mode, and measured current-voltage characteristics with the conductive tip positioned on single dots; large conductivity changes which suggest resonant tunneling through a quantized energy level in the dot was observed even at room temperature.
INTRODUCTION Since the colloidal chemical techniques were developed to synthesize highly monodisperse II-VI and III-V semiconductor nanodots (NDs), they have attracted much attention because of their highly efficient photoluminescence (PL) with size-dependent spectra which are attractive for applications in novel optical devices [1, 2]. Moreover, it has been reported that the PL intensity of CdSe colloidal NDs increases as a function of the photo-excitation energy and duration [3]. Since this memory effect strongly depends on the preparation of the NDs, the main contribution is attributed to electron trap states at the interface between the CdSe nanocrystals and organic ligands. In order to study the interfacial properties, electronic investigation has advantages over optical since interface traps are usually non-radiative. Recently, tunneling spectroscopy using a low-temperature scanning tunneling microscope (STM) was performed to investigate electronic properties of semiconductor NDs at 4.2 K, and resonant tunneling through the energy levels in NDs
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accompanied with charging effect was reported [4-7]. Here, we adopted conductive-tip atomic force microscopy (AFM) for the electronic study of single CdSe colloidal NDs, and measured their current-voltage (I-V) characteristics at room temperature [8, 9].
EXPERIMENT The CdSe colloidal nanodots were synthesized by injecting precursor that contains Se dissolved in tributylphosphine and dimethyl cadmium into tri-n-octylphosphine oxide (TOPO) at temperatures ranging from 300 to 350 . TOPO acts as hot soap where nucleation and growth occur, and caps the CdSe ND surfaces with excellent passivity. The obtained NDs were dispersed in toluene, and spun-coat on substrates at 1000 rpm for 60 sec. T
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