Atomically resolved imaging with a capability of elemental identification is one of the ultimate goals in the development of microscopy. Using scanning tunneling microscopy (STM), which provides us atomically resolved surface images, many attempts have been performed for elementally contrasted images. However, since STM basically probes electronic states near the Fermi energy, it is difficult to obtain definite "fingerprints" of elements. In this study, we report on a new method to obtain element specific images by STM combined with synchrotron radiation (SR) light. We found that, by exciting core electrons with the soft X-ray irradiation and detecting emitted electrons with the STM probe tip, X-ray absorption spectra bearing elemental information can be obtained [1, 2]. A sample was irradiated with a light coming from an undulator of SR (Photon Factory, KEK, Japan) at an angle of 4°. As a sample, an array of square Ni dots was fabricated on a Au-coated Si(001) wafer by electron-beam lithography. A tungsten tip covered with a glass layer except an area less than 5 µm from the tip apex was used for the STM imaging and the photo-induced current detection so that an undesired background component in the photo-induced current can be eliminated [3]. We found that the photo-induced current is enhanced when the photon energy is just above the Ni absorption edges. From the photo-induced current measured during the tip scanning over the surface, images showing a spatial distribution of Ni were obtained. An estimated spatial resolution of the chemical imaging is less than 20 nm, better than that achieved by photoemission electron microscopy [4]. The origin of the local sensitivity of elements observed in the photo-induced current is, we presume, due to a local reduction in a potential barrier on the sample by the proximity of the probe tip. The spatial resolution can be improved with high intensity soft X-ray beam line. Our results open the possibility of atomic-scale element specific imaging using SR-STM.
[1] T. Matsushima et al., Rev. Sci. Instrum. 75, 2149 (2004).[2] T. Okuda et al., J. Electron Spectros. Relat. Phenom. 144-147C, 1157 (2005).[3] K. Akiyama et al., Rev. Sci. Instrum. 76, 083711 (2005).[4] T. Eguchi et al., Appl. Phys. Lett. 89, 243119 (2006). |