Mapping the Valence States of Transition-Metal Elements Using Energy-Filtered Transmission Electron Microscopy

Abstract

The properties of transition-metal oxides are related to the presence of elements with mixed valences, such as Mn and Co. Spatial mapping of the valence-state distribution of transition-metal elements is a challenge to existing microscopy techniques. In this letter, using the valence-state information provided by the white lines observed in electron energy-loss spectroscopy in a transmission electron microscope (TEM), an experimental approach is demonstrated to map the valence-state distributions of Mn and Co using the energyfiltered TEM. An optimum spatial resolution of ∼ 2 nm has been achieved for two-phase Co oxides with sharp boundaries. This provides a new technique for quantifying the valence states of cations in magnetic oxides.Many physical and chemical properties of inorganic materials are determined by the elements with mixed Valences in the structural unit, 1 by which we mean that an element has two or more different valences while forming a compound. Transitionand rare-earth-metal elements with mixed valences are unique for initiation of electronic, structural, and/or chemical evolutions. We have demonstrated previously the valence states of Mn and Co in their oxides by applying electron energy-loss spectroscopy (EELS) for quantitative determination. [2][3][4][5] In EELS, the L ionization edges of transition-metal, rare-earth, and actinide compounds usually display sharp peaks at the near-edge region. These threshold peaks are known as white lines. The unoccupied 3d states form a narrow energy band, the transition of a 2p-state electron to the 3d levels, leading to the formation of white lines observed experimentally. EELS experiments have shown that a change in the valence states of cations introduces significant variation in the intensity ratio of the white lines, giving the possibility of identifying the occupation number of 3d or 4d electrons (or cation valence states) using the measured white line intensities. 6,7 The information obtained using EELS is an integration over the spatial region illuminated by the incident electron beam. In this paper, we introduce a new experimental approach which can give the distribution maps of cation valences in real space, allowing a direct identification of cations with different valence states. This is useful for studying nanocomposite magnetic oxide materials with mixed valences. Figure 1a shows an EELS spectrum of MnO 2 acquired at 200 kV using a Hitachi HF-2000 transmission electron microscope equipped with a Gatan 666 parallel-detection electron energyloss spectrometer. The oxygen K edge and the Mn L edge are clearly seen. The two sharp peaks are the L 3 and L 2 white lines, the intensity ratio of which is very sensitive to the valance state of Mn. The sharp white lines are the dominant EELS fine structures of transition-and rare-earth-metal elements. EELS analysis of the valence state is carried out in reference to the spectra acquired from standard specimens with known cation valence states. If a series of EELS spectra are acquired from several standa...

Analytical SuperSTEM for extraterrestrial materials research

“…Because of the short acquisition times, the requirement of high specimen stability that is a prerequisite for X-ray maps, acquired over minutes or hours, is less important with EFTEM imaging. Second, just as EELS can probe the local electronic structure of specimens, energy-selected images showing variations in local electronic structure can be formed (Batson et al 1994;Kimoto et al 1999;Wang et al 1999). Third, quantitative chemical images with spatial resolution of 1 nm and sensitivity for some elements at the ~1% level are possible (Elbert et al 2001).…”

Determination of chromium valence over the range Cr(0)–Cr(VI) by electron energy loss spectroscopy

“…[59][60][61][62]). Further, it has been demonstrated that of Cr(VI) make it a particular environmental concern, electron energy-fitered TEM imaging can produce valence Geochemical and microbiological processes of Cr specific maps of metals [63][64][65]. Determination of oxidation oxidation/reduction can drive both precipitation-dissolustate by EELS (as well as XPS and XAS) is accomplished by tion and ion absorption-desorption reactions in the analyzing valence-induced differences in the fine structure of environment.…”

Transmission electron microscopy analysis of some transition metal compounds for energy storage and conversion

“…In the EELS core-loss spectrum, the integral intensity ratio of the L 3 (2p 3/2 → 3d) and L 2 (2P 1/2 → 3d) excitation peaks is sensitive to the change in the valence states of transition metal cations [132][133][134][135]. Therefore, the oxidation states of transition metal cations can be identified by measuring the integrated intensities of L 2,3 edges.…”