Photoelectron spectromicroscopy experiments at the UVSOR facility1

ELSPEC 3411

Journal of Electron Spectroscopy and Related Phenomena 92 (1999) 165–169

Photoelectron spectromicroscopy experiments at the UVSOR facility1 T. Kinoshita a,*, K.G. Nath b, Y. Haruyama a, M. Watanabe a2, S. Yagi a3, S.-i. Kimura a, A. Fanelsa c a UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan Department of Structural Molecular Science, Graduate University for Advanced Studies, Okazaki 444-8585, Japan c Institut fu¨r Angewandte Physik, Heinrich-Heine-Universita¨t Du¨sseldorf, Du¨sseldorf D-40225, Germany

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Abstract Photoelectron spectromicroscopy experiments have been started at the UVSOR facility of the Institute for Molecular Science. The commercial system (FISONS instruments, ESCALAB 220i-XL) has been connected to two beamlines which cover the photon energy range of 10 eV–5 keV. It is expected that spatial resolution of 2 mm for the imaging mode and 20 mm for the spectroscopic mode can be achieved. In conjunction with monochromatized (and polarized) synchrotron radiation light from the UVSOR storage ring, it is planned to undertake some experiments using this apparatus, not only for surface science but also for spectroscopy of small samples. As a demonstration of the apparatus, the magnetic domain image of Fe(110) surface with magnetic dichroism effect is shown. The photoemission spectra from small organic materials (DI-DCNQI) 2-M (M = Ag, Cu) are also presented. q 1998 Elsevier Science B.V. All rights reserved Keywords: Microscopy; Photoelectron spectroscopy; Imaging; Magnetic domain; Organic materials

1. Introduction Recent advances of photoelectron microscopy are very productive. Especially in the third generation synchrotron radiation (SR) light sources, the development of the photoelectron microscope with high spatial resolution becomes one of the most important * Corresponding author: e-mail: [email protected]; fax:+81564-54-7079. 1 Presented at the Todai Symposium 1997 and the 6th ISSP International Symposium on Frontiers in Synchrotron Radiation Spectroscopy, Tokyo, Japan, 27–30 October 1997. 2 Present address: Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK) Oho 1-1, Tsukuba 305, Japan. 3 Present address: Department of Material Science, Faculty of Science, Hiroshima University, Kagamiyama 1-3, HigashiHiroshima 724, Japan.

plans in the community of the researchers of SR. Combining spectroscopy technique with microscope technique may give us a lot of advantages such as element-specific imaging of sample surfaces, possibility of studying very small samples, getting precise information from inhomogeneous surfaces and so on. For these kinds of studies, not only spatial resolution but also energy resolution (of photons and/ or electrons) is the important factor. That is why we are using the term spectromicroscopy according to the review by Tonner et al. [1] and references therein. So far, two types of photoelectron spectromicroscopies have been developed to achieve micro-analysis. One combines a micro-beam of photons obtained by a zone plate system or a mirror system with a conventional photoelectron analyzer. To get an element-specific image of the sample, the photoelectron

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T. Kinoshita et al./Journal of Electron Spectroscopy and Related Phenomena 92 (1999) 165–169

signal is recorded from every position of the sample surface which is moved in very small steps. This is called a scanning photoelectron microscope. The other uses a special electron energy analyzer and an electrostatic lens elements and/or a magnetic lens elements to obtain a magnified image of the photoelectrons, in which light with normal beam size (several mm) can be used. There are also two possibilities of getting an element-specific image. One takes photoelectron signals as a function of photon energy without electron energy analysis, which has been established as photoelectron emission microscope (PEEM). Another analyzes the energy of photoelectrons, but spatial resolution is poorer than that of PEEM. Now the spatial resolution of these microscopes is almost coming to the region of several or several tenths of a nanometre [1]. Although it is rather difficult to construct photoelectron spectromicroscope equipment with high performance at the UVSOR facility, a second generation SR light source, there still exist many interesting subjects for photoelectron spectromicroscopy. In this paper, we introduce our photoelectron spectromicroscope equipment (with an electron energy analyzer) and show two examples of our results. One is magnetic domain imaging with linearly polarized or unpolarized light, and another is photoemission spectroscopy of small samples (DIDCNQI) 2-M (M = Ag, Cu).

possible (spectroscopic mode). Recently, FISONS Instruments modified the system and developed a new system called the ESCALAB 220i-XL. We connected this modified equipment to the UVSOR beamlines. The concept of the equipment is almost similar to the ESCASCOPE, but the performance is very much advanced. Especially, by using the additional magnetic lens (XL lens), the count rate of photoelectron signals and the spatial resolutions became better. In addition to the commercial system, we attached some components. The schematic view of the system is shown in Fig. 1. It consists of a hemispherical electron analyzer with a radius of 150 mm, an electrostatic lens system, a magnetic lens system, an X-ray tube (MgK a, AlK a) for photoexcitation, a sputtering gun, and so on. In order to prepare well-ordered surfaces, the sample preparation chamber with low energy electron diffraction (LEED) optics, the evaporators, the quartz thickness monitor and the sample transfer system are attached to the main chamber. It is expected from the commercial argument that spatial resolution of 2 mm for the imaging mode and 20 mm for the spectroscopic mode can be achieved. The experiments are performed at the two beamlines BL5B [3] and BL7A[4] at the UVSOR. By using these two beamlines, we can perform photoelectron

2. Photoelectron spectromicroscopy equipment at the UVSOR facility Since it is rather difficult to obtain a micro-beam of photons with enough intensity at the UVSOR for bending magnet beamlines, we have chosen a way of magnifying the photoelectron image by an electrostatic and magnetic lens system. As reviewed by Holldack and Grunze [2], a commercial system of photoelectron spectromicroscopy is now available. They used the ESCASCOPE (FISONS Instruments) for X-ray photoelectron spectromicroscopy and performed some successful experiments at the BESSY. In their system, lateral imaging for defined kinetic energy electrons by multi-channel plate with a florescence screen is possible (imaging mode). Photoemission spectroscopy for the defined area is also

Fig. 1. Schematic view of the photoelectron spectromicroscopy equipment at the UVSOR (FISONS ESCALAB 220i-XL). To impinge SR light from oblique angle, the whole system is rotated by tilting the platform of the system. For preparation and characterization of samples, such as a LEED optics, evaporators, an ion gun, a quartz thickness monitor etc. are attached.

T. Kinoshita et al./Journal of Electron Spectroscopy and Related Phenomena 92 (1999) 165–169

spectromicroscopy experiments with a very wide range photon energy region (from 10 eV to 5 keV). To impinge the SR light at an oblique incidence angle, we set-up the whole system on a rotary stand and connect the port to the end port of the beamline via a bellow tube. The maximum rotation angle is a = 7.58, which is limited by the inner bore radius of the port for SR incidence in the analyzer chamber.

3. Application to magnetic domain imaging with linearly polarized or unpolarized light In this section, we show the magnetic domain image with magnetic dichroism. The usual size of magnetic domains of ferromagnetic materials is known to be several hundred mm, which is suitable for the observation by the photoelectron spectromicroscope. There were several experimental examples in which the magnetic domains of the ferromagnetic films and surfaces were observed by photoelectron microscope. Most of the experiments were to observe the magnetic circular dichroism (MCD) effect in the spectra around absorption edges or in the photoemission spectra or in the Auger signals [1,2,5]. Recently, the possibility of observing the magnetic domain using linearly polarized light has also been suggested. Actually, the new type of magnetic linear dichroism (MLD) was observed [6] in total electron yield (absorption) spectroscopy. The magnetic domain image of the Fe(001) surface based on this effect is shown in [7], where the commercial PEEM system was used for imaging of secondary electrons. It has been shown that the combination of the MLD with the MCD gives us the overall information about the local magnetic moment of each magnetic domain. Not only the MLD effect of absorption edges but also the MLD effect at photoemission peaks can be used for the magnetic domain imaging. Namely, the MLD [8] or magnetic linear dichroism in angular distribution (MLDAD) [9] effect of photoemission spectra may be useful for the magnetic domain imaging. We have succeeded in observing the magnetic domains of the Fe(110) surface using the MLD effect around the Fe 2p 3/2 photoemission peak at BL7A [10]. The observed image is similar to that shown below (in Fig. 2a), but the contrast is clearer.

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Since the unpolarized light can be considered as the incoherent superposition of s- and p-polarized lights, it is also possible to observe the magnetic dichroism effect in angle-resolved photoemission by using a conventional X-ray tube or a discharge lamp [11,12]. The effect is called as MDAD. In [12], the MDAD signal caused by Fe 3p photoemission spectra from the different sample positions was observed. We have taken the magnetic domain image of the Fe(110) surface by recording Fe 2p photoelectrons at E B = 706 eV and at E B = 704 eV binding energies as shown in Fig. 2(a). As discussed below (Fig. 2b), the maximum asymmetry difference of the MDAD was observed at this condition. The acceptance angle of photoelectrons was less than 68. The emission angle of the photoelectrons was about 58 from the surface normal along the (100) direction, in which the maximum of the MDAD effect has been reported [11,12]. By considering the direction of easy axis of magnetization (100), the expected directions of the magnetic moment for the domains ‘‘A’’ (the bright area) and ‘‘B’’ (the dark area) are indicated. The imaging process accords to the way discussed in [2]. Fig. 2(b) shows the Fe 2p photoemission spectra (the detection area being 50 mm) from two different positions of the sample surface. The MDAD effect is observed reflecting the direction of the magnetic moment of each magnetic domain (‘‘A’’ or ‘‘B’’). Because the MDAD effect is not so large as shown in Fig. 2, the contrast of the image is not clear. This is one of the reasons why the zone boundary of two magnetic domains is not clearly observed. The MLD effect is also not as large as the MCD. Therefore, we plan to measure the MCD using a new beamline for circularly polarized undulator light at the UVSOR facility (BL5A) [13]. The experiment would be valuable for further studies of magnetic thin films.

4. Photoemission spectroscopy of very small samples As an example of the photoelectron spectroscopy experiments of very small samples, we show here the valence band photoemission spectra of (DI-DCNQI) 2M (M = Ag, Cu). Although some photoemission results for similar organic materials, (DMeDCNQI)-M salts, have been reported [14], no

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Fig. 2. (a) Magnetic domain image of Fe(110) surface by using MDAD effect. Al K a X-ray line was used for photoelectron excitation. The unpolarized light was impinged from the upper left. The Fe 2p electrons at E B = 706 eV and at E B = 704 eV were corrected. After subtraction and normalization of the two sets of data, we got the magnetic domain image. The direction of the magnetic moment for each magnetic domain (‘‘A’’ (bright) or ‘‘B’’ (dark)) is indicated (see text). (b) Small area (50 mm) photoemission spectra from two different regions (‘‘A’’ and ‘‘B’’) of the Fe (110) surface excited by AlK a X-ray line. The intensity is normalized so as to obtain the same background count rates of the two spectra for both the higher binding energy side and the lower binding energy side. The MDAD signal can be observed reflecting the magnetic domain information.

photoemission studies have been performed for (DIDCNQI) 2-M. It is rather difficult to synthesize big samples for these kinds of materials. The size of samples used here was smaller than 1000 × 100 mm 2. It is expected from the NMR, conductivity and magnetic susceptibility measurements [15] that the (DI-DCNQI) 2-Ag should show the localized nature of Ag 4d electrons, whereas that of Cu should show the itinerant nature of Cu 3d electrons. Fig. 3 shows the comparative photoemission spectra of (DI-DCNQI) 2-Ag and -Cu taken at hn = 70 eV. The detection area of the photoemission spectra was 50 mm, which is smaller than the sample size. The

needle-shaped samples are mounted like bridges on the 1 mmf diameter holes of the sample holder. The photoelectrons were counted only when the detection area was fixed just on the sample, whereas no electrons were detected when the detection area was not on the sample. It is noticed from the figure that the Ag 4d bands are located around a 5 eV binding energy and localized within a 2.5 eV width. On the other hand, the Cu 3d bands seem to be situated around 3.5 eV and show a broader band width. The result is a direct evidence of the localized nature of Ag 4d electrons and the itinerant nature of Cu 3d electrons. We have also measured the photon energy

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Science, Sports and Culture. We thank Professor Kronmller for lending us a Fe(110) single crystal. We also would like to express gratitude to Professor Kisker and Dr Hillebrecht for their encouragement and discussion. Professor Kanoda and Dr Hiraki are acknowledged for providing the DCNQI samples. The staff members of the UVSOR facility are acknowledged for their experimental support.

References

Fig. 3. Valence band photoemission spectra for (DI-DCNQI) 2-Ag and (DI-DCNQI) 2-Cu samples taken at hn = 70 eV. The detection area was 50 mm.

dependence and the polarization dependence of the spectra, which will be described elsewhere [16]. It is known that these kinds of organic materials are easily damaged by radiation. If we use a micro-beam with high intensity, the spectral features may be immediately changed. In this sense, there still exists an advantage in performing the photoemission measurements for such kinds of small organic materials at the UVSOR facility using our spectromicroscopy system.

5. Summary We have started photoelectron spectromicroscopy experiments at the UVSOR facility. The image of the magnetic domain of the Fe(110) surface with magnetic dichroism effect at the Fe 2p 3/2 core level photoemission was presented. The valence band photoemission spectra for the small samples (DIDCNQI) 2-M (M = Ag, Cu) were also shown.

Acknowledgements This work is partially supported by a Grant-in-Aid for Scientific Research from Ministry of Education,

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