"The Art and Science of Multi-Tip Nanoprobes"

Shuji Hasegawa, University of Tokyo, Japan

A multi-tip scanning tunneling microscope (STM) can be used as a very versatile tool for electronic transport characterization at nanometer scale [1]. The STM tips are employed as current sources, voltage pick-up probes, and field-gate electrodes. This tool can be used for conductivity measurements by four-point probe method, scanning tunneling potentiometry, and multi-tip tunneling spectroscopy (transconductance and Green's function measurements), which are interesting and important not only for fundamental physics research, but also for as testing electrical characteristics of nanometer-scale electronic devices and bio-materials. Several groups are developing different types of multi-tip STMs. Some companies begin to deliver products of the multi-probe scanning probe microscopes with conductive atomic-force-microscope cantilevers. But the apparatus and operating system are not yet fully developed, and still have much room for evolution in many aspects.

In my talk, our custom-made four-tip STM's operating in ultrahigh vacuum are introduced; four tips of STM are driven independently in an organic way by a single PC with aid of scanning electron microscope (SEM) for precisely positioning the tips with arbitrary arrangements on specified areas on the sample surface. The spacing between the tips can be changed from 20 nm to 1 mm. I will introduce two important points for the machine; one is the operating system which enables automatic recognition and arrangement of tips by using SEM, and the other is carbon nanotube tips [2] which makes possible it to reduce the tip spacing down to 20 nm [3] .

Some application examples will be introduced by using the machine, mainly about the electrical conductivity measurements of the topmost atomic layers of crystal surfaces. The apparatus is also useful for measuring conductivity of individual nanometer-scale objects, such as silicide nanowires and carbon nanotubes [3].

[1] R. Hobara, N. Nagamura, S. Hasegawa, I. Matsuda, Y. Yamamoto, K. Ishikawa, and T. Nagamura, Review of Scientific Instruments 78, 053705 ( 2007).

[2] H. Konishi, Y. Murata, W. Wongwiriyapan, M. Kishida, K. Tomita, K. Motoyoshi, S. Honda, and M. Katayama, S. Yoshimoto, K. Kubo, R. Hobara, I. Matsuda, and S. Hasegawa, M. Yoshimura, J.-G. Lee and H. Mori, Review of Scientific Instruments 78, 013703 (2007).

[3] S. Yoshimoto, Y. Murata, K. Kubo, K. Tomita, K. Motoyoshi, T. Kimura, H. Okino, R. Hobara, I.  Matsuda, S. Honda, M. Katayama, and S. Hasegawa, Nano Letters 7, 956 (2007).