Dual-probe scanning tunneling microscope and a carbon nanotube ring transistor
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Dual-probe scanning tunneling microscope and a carbon nanotube ring transistor Taishi Shigematsu, Hiroyuki Watanabe, Chikara Manabe, Kei Shimotani, and Masaaki Shimizu Ecology Research Laboratory, Corporate Research Laboratory, Fuji Xerox Co., Ltd. 1600, Takematsu, Minamiashigara-shi, Kanagawa-ken, 250-0111, JAPAN ABSTRACT For measuring molecular device, we developed a dual-probe scanning tunneling microscope (D-STM) composed of two STM systems in which a carbon nanotube (NT) was used for STM tip. Using D-STM, we fabricated a NT ring device. The NT ring device showed a switching behavior with applying gate bias. Furthermore, in STM imaging for various gate biases, we could observe directly hole injection into the NT ring. INTRODUCTION For realizing molecular electronics [1], it is important to elucidate electric property of nanometer scale device. In this study, we have developed a dual-probe scanning tunneling microscope (D-STM) system which was composed of two individual STM systems. Our D-STM system can measure the electric properties of a sample with a spatial resolution of ~10 nm. Using the D-STM system, we have investigated the electric properties of a NT ring [2]. EXPERIMENTS D-STM system We have developed a D-STM system that was composed of two individual STM systems. Figure 1(a) shows a schematic of the D-STM system. Our D-STM has two structural features which are "carbon nanotube probe" and "active damper system". In the D-STM measurements for a nanometer-scale sample, we must use STM probes whose radiuses of curvatures are smaller than the sample size, since "interference between two probes" becomes a serious problem. Therefore, we used carbon nanotubes (NTs) for STM probes [3,4] as shown in Fig.1 (b). The diameter of NTs used in this study is less than 10 nm, so that we can measure nanometer-scale samples (~20 nm in size) without the interference.
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(a)
(b) PZT1
PZT2
XYZ
XYZ
NT W tip
PZT A spring
1.5 µm
1st NT probe
sample
2nd NT probe
Fig. 1 (a) Schematic of D-STM system. (b) Scanning electron microscopy (SEM) image of NT probe. Multi-walled NTs was attached to the apex of W tip using micromanipulator system [5]. A mechanical vibration of the D-STM system prevents observation of a sample. In order to reduce the mechanical vibration, we developed the active damper system. The active damper system consists of a small piezoelectric actuator (PZT-A in Fig. 1(a)) and three springs. First, we measured the frequency of a characteristic vibration of the D-STM system by monitoring the tunneling current from the first NT probe to a sample. Next, we vibrated PZT-A at the same frequency of the characteristic vibration (~ 100 Hz). Then, we changed the phase of driving oscillator by π/2 to offset the characteristic vibration. This simple damper system could reduce the amplitude of mechanical vibration from several nm to less than 0.1 nm. RESULTS AND DISCUSSION NT ring transistor Using the D-STM system, we fabricated a carbon nanotube (NT) ring device and measured its electric properties.
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