How to realize ultimate spatial and temporal resolutions by laser-combined scanning tunneling microscopy?
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How to realize ultimate spatial and temporal resolutions by laser-combined scanning tunneling microscopy? Hidemi Shigekawa*, Osamu Takeuchi, Masahiro Aoyama, Yasuhiko Terada, Hiroyuki Kondo and Haruhiro Oigawa Institute of Applied Physics, CREST, 21st COE, University of Tsukuba, Tsukuba 305-8573 Japan * http://dora.ims.tsukuba.ac.jp/ Abstract By combining scanning tunneling microscopy (STM) and the optical pump-probe technique using a femtosecond pulse laser, we have developed a new microscopy, shaken pulse-pair-excited STM (SPPX-STM), that enables us to observe the dynamics of electronic structures with the ultimate spatial and temporal resolutions. 1. Introduction At present, smaller and faster are the keywords in nanoscale science and technology. Actually, important and interesting phenomena occurring in various systems, such as functional materials, electronic devices, signal transfer in biosystems, and chemical reactions, are observed at several tens of nanometer to the single-molecule level in space and in the several tens of picosecond to subpicosecond range in time. However, it is extremely difficult to realize spatial and temporal resolutions simultaneously on this scale. Therefore, to determine the possibilities of realizing such resolutions, it is necessary to develop a new method. That is what we have been challenging to do, namely, the development of a new microscopy, the laser-combined scanning tunneling microscopy (STM) [1-6]. STM has an excellent spatial resolution on the subangstrom scale, but, its temporal resolution is limited to ~100kHz. In contrast, the ultrashort pulse laser technique enables the observation of electronic dynamics in the femtosecond range, but its spatial resolution is generally limited by wavelength. Therefore, it is desired to combine these two techniques to realize the ultimate spatial and temporal resolutions simultaneously. 2. Electronic structures on nanoscale. In the world of nanofactories, as illustrated in Fig. 1, elemental blocks with various characteristics are integrated and organized on a designed stage to produce desired or new functions in a system on the macroscopic scale. However, to realize such a goal, the characterization and control of the structures of each
Figure 1. Schematic model of nanoscale system.
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element are essential. Let us see two examples. Figures 2(a) and 2(b) show an STM image of a Si nanoparticle of ~3nm diameter on a graphite surface and the cross sections of the same region obtained at different bias voltages, respectively. Inhomogeneous structures are observed in the cross sections. Since STM provides information on the electronic structures at the observed bias voltage, the results indicate that there are complex electronic structures even at the single-molecule level. How can complex local dynamics work to produce a systematic function on the macroscopic scale?
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Figure 2. STM image of Si nanoparticle (a) and cross section of the same region obtained at different bias voltages (b).
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