Tunneling Current-Distance Characteristic of Scanning Vibrating Probe / 1-Alkanethiol Self-Assembled Monolayer (SAM) / A
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Tunneling Current-Distance Characteristic of Scanning Vibrating Probe / 1-Alkanethiol Self-Assembled Monolayer (SAM) / Au (111) Structure Yuhsuke Yasutake1, Yasuo Azuma1, Kouhei Nagano1, and Yutaka Majima1, 2 1 Department of Physical Electronics, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan 2
Organization and Function, PRESTO, Japan Science and Technology Agency,
2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan ABSTRACT We report measurements of the tunneling current – distance (I-d) dependence above alkanethiol self-assembled monolayers (SAMs) on Au (111) substrates with high probe voltage. From the semilogarithmic I-d plots of hexanethiol and octanethiol SAMs, a kink in the tunneling current slopes is clearly observed, which shows the point where the STM tip contacts the end of the SAMs. The conductance decay constants of the vacuum layer and the alkanethiol SAMs are estimated from the tunneling current slopes. We also discuss the contact conductance of alkanethiol SAMs. INTRODUCTION Nanomechanical single-electron devices such as electron shuttles have attracted much attention toward precision measurement, metrology, and the development of new devices utilizing the softness of organic molecules [1-4]. When conductive nanoparticles are introduced as Coulomb islands in nanomechanical double-barrier tunneling junction (DBTJ), the number of electrons is quantized by Coulomb blockade (CB) phenomena and can be modulated by nanomechanical motion. In the DBTJ that consist of scanning tunneling microscopy (STM) tip/vacuum/nanoparticle/insulator/metal substrate, there are two tunneling resistances, R1 and R2; one is between STM tip and a nanoparticle (R1), and the other is between a nanoparticle and metal substrate (R2). The polarity of electric charge on nanoparticles depends on the tunneling resistance ratio of R1/R2. To realize an electron shuttle in which a single electron is transported one by one in accordance with the nanomechanical vibration in the DBTJ, the control of R1/R2 is very important. It should be noted that a SAM can be
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used as tunneling barrier that directly corresponds to R2. When a nanoparticle is placed on the alkanethiol SAM, the tunneling resistance R2 can be controlled by choosing alkyl chain length of alkanethiol SAM [5-9]. It should be noted that if the nanomechanical single electron devices want to be used even at room temperature, operation voltage should be larger than 1V to overcome the thermal fluctuation. At that time, the tunneling regime should change from direct tunneling to Fowler-Nordheim tunneling [9]. As a result, the clarification of the I-d characteristic under high voltage is important. As a nano-scale modulation method, nanomechanical electrode vibration can be applied using a STM tip. For the creation of a nanoelectromechanical single-electron device, it is important to detect a single electron motion under nanomechanical vibration of DBTJ. Recently, we have developed a nanomechanical simultaneous measuring system to meas
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