During structural observation of a sample with a scanning tunneling microscope (STM), photons are usually emitted from the tunneling gap between the STM tip and the sample. The energy spectrum, intensity angular distribution, and linear and circular polarizations of the emitted photons contain important information on the electronic and magnetic properties of the sample. Conversely, if the tunneling gap is irradiated with photons, not only a change in the electromagnetic condition of the tunneling gap but also a physical and/or chemical reaction in the sample occurs when necessary conditions are satisfied. Such a reaction during STM observation provides valuable information on the sample and is also useful for novel nanoscale structure fabrication.
In the present paper, we show several results of interest obtained by our group regarding the detection of photons emitted from the STM and the irradiation of photons to the tunneling gap of the STM:
1) The orbital symmetry of a single atom (Si) on a solid surface (Si(001)) can be determined by measuring the linear polarization of STM-induced photons coming from the atom.
2) The spin state of a magnetic nanostructure (Fe) can be analyzed by measuring the circular polarization of photons that are emitted when electrons from a nonmagnetic STM tip pass through the magnetic nanostructure, go into a selected substrate (GaAs) and then recombine with holes in the substrate.
3) The metallic and semiconducting states of individual carbon nanotubes (CNTs) can be distinguished in an STM image of the CNTs by observing the energy spectrum of STM-induced photons coming from individual CNTs.
4) During STM image observation of a sample, if the sample below the STM tip is irradiated with high-brilliance synchrotron X-rays and the photon energy of the X-rays (hv) is scanned, we can perform elemental analysis using the observed STM images, since a structure made of a certain element changes its image contrast when hv passes through the absorption edge of the element.
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