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Broadly speaking, two types of scanning probe microscopy technologies have been reported to be capable of resolving real space topography with atomic resolution: i) scanning tunneling microscopy and ii) atomic force microscopy. Although these techniques have real atomic resolution, they cannot directly distinguish the atomic species making up the specimen to be measured without using other relative speculation.
In recent years, we have developed and reported the use of scanning nonlinear dielectric microscopy (SNDM) for the measurement of the microscopic distribution of dielectric polarization. Thus, since the technique can sense the polarity of the specimen, if we can resolve a single electric dipole moment of an atom, we can expect to be able to directly distinguish atomic species. Most recently, we have succeeded in observing the Si(111)7×7 atomic structure using newly developed non-contact scanning nonlinear dielectric microscopy (NC-SNDM).[1] This is the first successful demonstration of the achievement of atomic resolution in a dielectric microscopic technique. Unfortunately, the quality of the image in Ref. [1] falls far short of clearly distinguishing the sign of a single-atom electric dipole moment. Nevertheless, advances in the resolution of SNDM have been expected to enable improved characterization of the single dipole moment at the atomic level.
In this paper, we clearly resolve the electric dipole moment distribution of Si atoms on Si(111)7x7 surface by NC-SNDM under ultrahigh vacuum conditions. The dc-bias voltage dependence of the atomic dipole moment on Si(111)7x7 surface was measured and the directions and the magnitudes of dipole moments of Si atoms on the surface were revealed.
Since the technique is applicable not only to semiconductors but also to both polar and non-polar dielectric materials, it is expected that NC-SNDM will contribute to the study of electric dipole moment distribution in insulating materials at the atomic level. This means that SNDM will open new possibilities for SPM to distinguish the atomic species making up condensed matter without using speculative methods.
[1]R. Hirose, K. Ohara. and Y. Cho, Nanotechnology 18, 084014 -1-5 (2007)
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