Mar 13, 2009 - Nanoscale chemical imaging using scanning tunneling microscopy is ... excitation of the probed element by a synchrotron radiation light.
PRL 102, 105503 (2009)
PHYSICAL REVIEW LETTERS
week ending 13 MARCH 2009
Nanoscale Chemical Imaging by Scanning Tunneling Microscopy Assisted by Synchrotron Radiation Taichi Okuda,1,* Toyoaki Eguchi,1 Kotone Akiyama,1 Ayumi Harasawa,1 Toyohiko Kinoshita,2 Yukio Hasegawa,1 Masanori Kawamori,3 Yuichi Haruyama,3 and Shinji Matsui3 1
The Institute for Solid State Physics (ISSP), The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa 277-8581, Japan 2 Japan Synchrotron Radiation Research Institute (JASRI), 1-1 Koto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan 3 Laboratory of Advanced Science and Technology for Industry, University of Hyogo, 3-1-2 Koto, Kamigori-cho, Ako-gun, Hyogo 678-1205, Japan (Received 2 December 2008; published 13 March 2009) Nanoscale chemical imaging using scanning tunneling microscopy is demonstrated with a core-level excitation of the probed element by a synchrotron radiation light. Pronounced element-specific contrasts were observed in the spatial resolution of 10 nm on checkerboard-patterned Ni and Fe samples in differential photoinduced current images taken with the scanning tunneling microscopy tip under the synchrotron radiation irradiation whose photon energies are above and below the Ni (Fe) L absorption edge. The local detection of the photoinduced secondary electrons through the surface barrier lowered by the proximate tip and/or via the tunneling process probably plays an important role in achieving the highspatial resolution. DOI: 10.1103/PhysRevLett.102.105503
PACS numbers: 61.05.a, 68.37.Ef
The determination of the atomic structure and chemical composition of materials is one of the most fundamental and important issues in solid state physics. To this end, various kinds of microscopic and element-sensitive analysis techniques have been developed and scientists have obtained ways to observe materials’ structures in an atomic resolution and analyze elemental compositions with very high precision. However, the method to analyze the elemental species of individual atoms with an atomic spatial resolution has not been established so far. If such a method is realized, one can directly determine the atomic structures of reconstructed and reacted materials, localized nanostructures, adsorbed molecules, and so on, and the impact on material science would be tremendous and enormous contributions to the development of nextgeneration devices are expected. For the surface chemical imaging, techniques utilizing core-electron excitation are often used since the energy level of core electrons can be used as ‘‘fingerprints’’ of elements. Micro-Auger spectroscopy, nanobeam photoemission spectroscopy, and photoemission electron microscopy (PEEM) are examples of the chemical imaging techniques using the core-electron excitation. However, the spatial resolutions of the techniques are in principle limited by the size of the electron or photon beams or aberration of the electron lenses and cannot reach an atomic resolution. Recently, chemical imaging in an atomic resolution has been reported by scanning transmission electron microscope combined with electron energy loss spectroscopy [1]. The techniques, however, can be utilized only for the materials with columnar structure. On the contrary, scanning probe microscopy (SPM), for instance, scanning tunneling microscopy (STM), atomic force microscopy (AFM), and so on, can achieve the 0031-9007=09=102(10)=105503(4)
atomic resolution easily and is nowadays a quite popular tool to reveal atomically resolved surface topography. Because STM detects valence or conduction electrons via the tunneling process, however, it is quite difficult to obtain the chemical information. One exceptional work is inelastic electron tunneling spectroscopy proposed by Stipe et al. [2]. The mass difference of H and D atoms in a halfdeuterated acetylene molecule was successfully detected with the method. However, the method is available only for the light elements and difficult to apply for various elements. In order to overcome this shortcoming of SPM and append the function of the elemental sensitivity, we have developed synchrotron radiation assisted STM (SRSTM) [3–5]. In the SRSTM measurements, photoinduced secondary electrons that are produced by core-electron excitation followed by various decay processes such as the Auger decay are detected with a STM probe tip [3–5]. Since the intensity of the secondary electrons emitted from the surface is proportional to the absorption probability of the core-level electrons, one can obtain the ‘‘fingerprint’’ of the element by measuring the secondary electron intensity as a function of the excitation photon energies ( x-ray absorption spectrum). In previous papers, we have observed the photocurrent image of Ni microdots formed on a Au substrate and demonstrated quite high spatial resolution (
