Principle and Application of Imaging Electron Energy Loss Spectroscopy

When electron beam is incident into specimen, a part of the electrons is inelastically scattered and loses a part of the energy. Elemental composition and atomic bonding state can be determined by analyzing the energy with the spectroscope attached under the electron microscope (Electron Energy Loss Spectroscopy). Because the ananlzing region can be selected from a part of the enlarged electron microscopic image, one can analyze very small region. Moreover, by selecting electrons with a specific loss energy by a slit so as to image them, element distribution in specimen can be visualized (Elemental Mapping).

An Example of Elemental Analysis by EELS

This shows EELS spectrum from the region of 1 um diameter of iron tetraphenylporphyrin monochloride crystal. Integral peak intensity is proportional to the number of each atom in the measured region. From the intensity and the cross-section of inelastic scattering, the elemental ratio is determined as C:N:Cl:Fe = 42.9:3.9:1.1:0.9. This value is corresponding well to the expected molecular composition (43:4:1:1). This result is obtained from the sample weight of 1X10-13 g.

Porphyrin Photo

An Example of States Analysis by EELS

Diamond, graphite and fullerene are the matters which consists of only carbon, so that, all of these specimens have absorption peaks around 284 eV in EELS corresponding to the existence of carbon atom. From the fine structure of the absorption peak, the difference in bonding state and local electronic state can be detected. The sharp peak at absorption edge corresponds to the exitation of carbon K-shell electron (1s electron) to empty anti-bonding pi-orbital. It is not observed for diamond, because of no pi-electron in it.

Carbon EELS

Elemental Distribution by Elemental Mapping

The distribution of element in specimen is clarified by selecting and imaging the electrons with a specific energy loss. Photo (a) is an electron microscopic elastic image of fine particles of PbCrO4 covered by SiO2 of about 15 nm thickness. (b) is elemental map by L-shell exitation electrons (100 eV) of silicon. Only the covering layer of SiO2 is clearly seen. (c) and (d) are elemental maps by K-shell exitation electrons of oxygen (532 eV) and L-shell exitation electrons of chromium (580 eV), respectively. The oxygen map shows the difference of oxygen concentration between the covering layer and the inner particle. In chromium map, only the particle of PbCrO4 can be seen.

Ceramic Photo

High Resolution Elemental Mapping

This is an example of high resolution elemental mapping of graphite intercalation compound of FeCl3. Photo (a) is elemental map by L-shell exiation electrons of chlorine (200 eV), monolayers of FeCl3 are seen. (b) is the intensity distribution along the direction crossing the iron chloride layer shown as 1,2 in (a). The layer thickness of FeCl3 is estimated to be 0.9 nm. As seen in model drawing of (c), two layers of FeCl3 (layer distance is 1.6 nm), in which three layers of graphite are inserted, are clearly distiguished. In each FeCl3 layer two layers of chlorine and one iron layer are stacked as shown in (d). The chlorine layers with a separation of 0.274 nm in the FeCl3 layer is not resolved and therefore the resolution in elemental mapping is estimated about 1 nm.

Elemental Mapping

Quantification of Elemental Mapping

The intensity of elemental map of thin film specimen is proportional to the number of existence atom. Photo (a) is the elastic high resolution electron microscopic image of carbon nanotube. (b) is the carbon map at the same region. As shown in the energy loss spectrum of (c), the intensity of carbon K-shell exitation superposes on the monotonous background component. This background component is removed by an extrapolation, which was decided from two different energy images taken at energyes before the absorption edge. The intensity profile of carbon map perpendicular to the tube axis is shown in figure (d). The intensity profile corresponds well to the calculated number distribution of carbon atom (solid line) based on the size and the shape of nanotube. The intensity dip at center part is corresponding to 20 carbon atoms.

Nanotube Photo

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