- Email: [email protected]
Quasi-single scattering of low-energy Ne+ ions from the Fe3O4 surfaces
ARTICLE IN PRESS
Vacuum 74 (2004) 253–257
Quasi-single scattering of low-energy Ne+ ions from the Fe3O4 surfaces W. Soszkaa, N.-T.H. Kim-Ngana,*, A. Koz"owskib b
a ! 30 084, Poland Institute of Physics, Pedagogical University, Podchorazych 2, Krakow Solid State Physics Department, Faculty of Physics and Nuclear Techniques, AGH University of Science and Technology, Krakow, Poland
Accepted 22 December 2003
Abstract The energy spectra of positive and negative ions emitted from single-crystalline Fe3O4 (0 0 1) and (1 1 1) surfaces under low-energy Ne+ ion bombardments were investigated in the temperature range from 85 to 300 K. The characteristics of the LEIS spectra of the two surfaces are very similar. The positive-charged-ion spectra have revealed the O+-recoil and the quasi-single scattering Ne+–Fe peak, while a large broad O1 recoil signal was observed in the negative-charged-ion ones. No change of the peak shape and of the energy position have revealed for the (0 0 1) surface, whereas the (1 1 1) one implies an appearance of a small peak around 150 K attributed to recoil-oxygen ions from the double-collisions. r 2004 Elsevier Ltd. All rights reserved. PACS: 68.35.p; 61.18.Bn Keywords: Magnetite; Surface composition; Low-energy ion scattering (LEIS)
1. Introduction Magnetite (Fe3O4), has been an attractive candidate for technological applications in many important fields of electronics and catalysis. Increasing attention has been focused on the investigations of the surface structure and properties of magnetite surfaces. Among different surface methods, low-energy ion scattering (LEIS) has *Corresponding author. Tel.: +48-12-6626314; fax: +48-126372243. E-mail address: [email protected] (N.-T.H. Kim-Ngan).
been used in a large extent to investigate the surface compositions and structures [1–3]. Recently, our LEIS investigations on several magnetite surfaces in a multiple scattering approximation [4] have revealed that a large change in the neutralization and re-ionization of ions was due to the metal–insulator phase transition of magnetite (the Verwey transition) and that the Auger neutralization of ions related to the electron localization at the Verwey transition gave a dominant contribution to the final scattered ion yield. We extend the use of the LEIS technique to a large-angle geometry with which we focus on
0042-207X/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2003.12.136
ARTICLE IN PRESS 254
W. Soszka et al. / Vacuum 74 (2004) 253–257
investigations of quasi-single scattering processes on the magnetite surfaces. The LEIS investigations on the magnetite (0 0 1) surface using different ion beams have been recently published [5,6]. In this paper, we focus on analyzing the LEIS data of the (0 0 1) and (1 1 1) magnetite surfaces under Ne+ ion bombardments.
2. Experimental details The experimental data were collected from a temperature-dependent LEIS-setup with largeangle geometry where the energy spectrum is largely dominated by the single scattering peaks; the incident angle C is in the range of 0–55 and the detection angle Y is in the range of 66–94 respect to the surface. Azimuthal scans can be taken at every 1 up to 730 with respect to the main direction. A Ne+ ion beam with primary energies of 4–8 keV was used. The operating pressure in the chamber is in the 109 mbar range. Target current density was in the range 106 A/ cm2. From the angle-dependent investigations of the LEIS spectra for the whole energy range and at different temperatures we found out that at the scattering-angle geometry with C ¼ 34 and Y ¼ 68 the strongest Ne+–Fe peak with a largest change of the peak-intensity with changing temperature was obtained. Besides, at a certain scattering-angle geometry, a strongest scattering signal was obtained for the (0 0 1) surface at the azimuthal angle of F ¼ 0 ; while it was at F ¼ þ12 for the (1 1 1) one. Those optimal angles were then used for the temperature-dependent investigations. Two samples were cut from the single crystals of magnetite along the (0 0 1) and (1 1 1) surfaces. (We address those surfaces as Fe3O4 (0 0 1)-sc and (1 1 1)-sc surface where the abbreviation means the single-crystalline sample.) The azimuthal angle F ¼ 0 was fixed at the main crystallographic surface direction: the [0 1 0] and the ½1 1% 0 direction, respectively, for the (0 0 1) and the (1 1 1) surface. The atomic arrangements in magnetite imply the square-shaped surface semi-channels on the (0 0 1) surface, e.g. with the channel height ( and the channel width l ¼ 4:20 A ( along h ¼ 1:05 A
the [0 1 0] crystallographic direction. On the (1 1 1) surface, the triangular-shaped channels were ( l ¼ 5:14 A ( along the ½1 1% 0 formed (h ¼ 0:61 A, direction). Not only the number of scattering center on the topmost surface layer, they also imply different interaction rate of incoming ions with surface atoms lying inside the channels. An annealing in situ was done at temperatures 500–600 K in UHV. Prior to LEIS analysis the target was cleaned by cycles of 6 keV Ar+ sputtering until a reproducible energy spectrum was obtained (after about 2 h). No contamination was detected.
3. Results and discussions Similar characteristics of the LEIS spectra of scattered Ne+ ions from the two surfaces were observed. The positive-charged-ion spectra exhibited two well-defined peaks; the low-energy peak was in a good agreement with the energy of the O+-recoil atoms (E2(O)) while the high-energy peak was at the energy position for the Ne+–Fe scattering (E1(Fe) ) estimated from the elastic binary collision model [7]. Fig. 1 shows the LEIS spectra of the Fe3O4 (0 0 1) surface at different Ne+ ion primary energies. Unlike the cases of He+ and Ar+ ion bombardments where only small scattering peaks on a very high background were observed [5], the LEIS spectra of scattered
Fig. 1. Ne+ scattering spectra at different primary energies from the Fe3O4 (0 0 1) surface. C ¼ 34 ; Y ¼ 68 ; [0 1 0] azimuth (F ¼ 0 ), and target temperature T ¼ 300 K.
ARTICLE IN PRESS W. Soszka et al. / Vacuum 74 (2004) 253–257
Ne+ ions revealed an enormous Ne+–Fe scattering peak. The peak-intensity increased largely with increasing primary energy. The results indicated a very high survival probability of Ne+ ions scattered from Fe. A widening of the peak-width and an enhanced background at both the low- and the high-energy side of the binary Ne+–Fe peak with increasing primary energy, however, indicated a large contribution from quasi-scattering, multiple-scattering as well as from the re-ionization of the particles neutralized and scattered from the deeper layers at the final violent collision as they escape the magnetite surface. In Fig. 2 the LEIS spectra of the Fe3O4 (0 0 1) surface under 6 keV Ne+ ion bombardments at different detection angles were shown. The peak-intensity of the Ne+–Fe peak largely decreased with increasing the detection angle. A visible peak was still observed at Y ¼ 94 ; whereas the oxygen-recoil signal was disappeared at YX76 : It implied a step-like decrease in the scattered ion yield, Rþ ðYÞ; as shown in the inset of Fig. 2. The peak-intensity was found to increase enormously at low temperature. The LEIS spectra for 6 keV Ne+ ions at different temperatures were shown in Fig. 3. For the (0 0 1) surface, at 90 K the Ne+–Fe scattering peak intensity (I90 K ) was 2.5 times larger than that at room temperature (I300 K ) (Fig. 3a). A much less enhancement with an I90 K =I300 K ratio of 1.3 was found for the (1 1 1)
Fig. 2. Energy spectra of 6 keV Ne+ ions scattered from the Fe3O4 (0 0 1) surface at different detection angles. F ¼ 0 ; T ¼ 300 K. Inset: the normalized scattered ion yield, Rþ ðTÞ=Rþ 300 K ; as a function of the detection angles.
255
(a)
(b) Fig. 3. Energy spectra of 6 keV Ne+ ions at different temperatures: (a) for the Fe3O4 (0 0 1) surface (F ¼ 0 ) and (b) for the (1 1 1) one (F ¼ 12 ). Incidence C ¼ 34 ; detection angle Y ¼ 68 : The peak-intensity was normalized for the ion current of 1 nA.
surface (Fig. 3b). The results indicated a dominant contribution to the total scattered ion yield (Rþ ) from the ions scattered from the atoms inside the channels. Namely, the related interacting rate was much higher for the wide squared-shape channels on the (0 0 1) surface than that for the triangular ones on the (1 1 1) ones. At low temperatures, such a contribution was largely affected by a decrease of the neutralization degree related to the electron localization in magnetite, leading to a larger enhancement of Rþ for the squared-shape channels. At 90 K the Ne2+–Fe scattering peak became more visible for both surfaces. A very pronounced peak was observed in the energy spectra of the negative ions emitted from
ARTICLE IN PRESS 256
W. Soszka et al. / Vacuum 74 (2004) 253–257
the magnetite surfaces under Ne+ ion bombardments for the detection angles Y up to 80 . The energy position of the peak maximum was found at the expected energy of the recoil O1 ions from a binary collision with a quite small elastic energy loss. At 300 K the intensity of the O1-recoil peak was about 2.5 times bigger than that of the O+1recoil one. At higher detection angle (Y > 80 ) only a broad peak has revealed. An enormous enhancement of the O1-recoil peak and of the background at the high-energy side of the peak was found at 90 K as shown in Fig. 4. The results indicated that a large amount of oxygen ions were present on the outermost surface layer of magnetite and joined the binary collisions in a negativecharge state. A strong signal of both emitted O1 and O+1 ions indicated a strong charge-exchange effect on magnetite surfaces. We notice here that a well-known phenomenon in magnetite is the electron hopping between the Fe-octahedral sites of the spinel structure. In other words, there are a large amount of delocalized electrons in the hightemperature phase. However, no distinguished recoil O2 signal was detected in the LEIS spectra. No change of the peak shape and of the energy position for the O1-recoil peak recoil with changing temperature was observed for both two surfaces. However, the most striking feature is the appearance of a small peak around 150 K in the LEIS spectra of the (1 1 1) surface as shown in
Fig. 5. The larger peak represented the recoiloxygen ions from a single binary collision, while the additional peak was attributed to those resulting from the double-collisions. In analysing the oxygen emissions under Ne+ and Ar+ ion bombardments for the (0 0 1) surface we found out that the shadowing effect related to the screening action of oxygen ions implies a dominant contribution to R especially at low temperatures. The shadow effect on the magnetite surface was illustrated and discussed in detail in our recent publication [5]. The shadow radius RS at a distance d from an atom is expressed as: Rs ¼ 2:2½db2 =ð1 þ mÞð1=3Þ ; where b is the distance of closest approach in a head-on collision with an energy Eo ; m is the mass ratio between the surface atom and incoming ion [7]. We applied the calculations for the case of 6 keV Ne+ ions with an incident angle of c ¼ 34 using Ziegler– Biesack–Littmark approximation for the screening function [8]. The estimated values for the shadow radius for the Fe–O chain along the ½1 2% 1 ( direction with an atomic distance of l ¼ 3:43 A ( ( are: RS (ion-O)=0.764 A, RS (ion-Fe)=0.719 A. ( RS (ionIn comparison: RS (ion-O)=0.649 A, ( Fe)=0.610 A) for the [0 1 0] chain on the (0 0 1) ( We notice that unlike the surface with l ¼ 2:10 A. case of Ar+ ions, under Ne+ ion bombardments, the shadow radius is small and thus the ion-surface atom collision was not much effected by the
Fig. 4. Energy spectra of the recoil O1 ions from the Fe3O4 (0 0 1) surface under 6 keV Ne+ ion bombardments at 300 K (&) and at 90 K (:). C ¼ 34 ; Y ¼ 68 ; and F ¼ 0 :
Fig. 5. Temperature dependence of the LEIS spectra of the recoil O+1 ions from the Fe3O4 (1 1 1) surface under 6 keV Ne+ ion bombardments: C ¼ 34 ; Y ¼ 68 ; and F ¼ 12 : An additional oxygen peak from a double-collision was observed around 150 K.
ARTICLE IN PRESS W. Soszka et al. / Vacuum 74 (2004) 253–257
screening effect of the neighbour atom. Namely, for both two different types of channels the ions with different trajectories can always knock out the oxygen atoms leading to a weak surfacestructure dependence of the negative-charged ion emissions. However, the existence of the additional oxygen peak for the (1 1 1) surface indicated that a surface relaxation for the oxygen atoms was much enhanced in the case of the triangular-shaped channels providing a higher probability of oxygen recoil ions for the double-collisions.
4. Summary Our LEIS investigations of magnetite surfaces using Ne+ ion beams have revealed a high survival probability of Ne+ ions scattered from Fe and a large contribution from quasi- and multiplescattering. An enormous enhancement of the peak-intensity of the Ne+–Fe peak of the (0 0 1) surface indicated a dominant contribution from the neutralization process related to a change in the electron localization degree of magnetite. Moreover, the interacting rate of the incoming ions with the atoms inside the channels was higher for the squared-shape channels than for the
257
triangular ones. A very strong signal from emitted oxygen ions indicated that a large amount of negative-charged oxygen ions was present on the outermost surface layer of magnetite. Our LEIS investigations provided an evidence for the presence of mixed Fe–oxygen layers on these magnetite surfaces.
References [1] Pedemonte L, Bracco G, Robin A, Heiland W. Nucl Instrum Methods B 2002;193:563–7 (and references therein). [2] Brongersma HH, Gildenpfennig A, Denier van der Gon AW, van de Grampel RD, Jansen WPA, Knoester A, Laven J, Viitanen MM. Nucl Instrum Methods B 2002;190:11–8. [3] Boerma DO. Nucl Instrum Methods B 2001;183:73–87. [4] Kim-Ngan N-TH, Soszka W. Nucl Instrum Methods B 2002;193:391–5 (and references therein). [5] Kim-Ngan N-TH, Soszka W. Surf Sci 2003;536:24–32. [6] Kim-Ngan N-TH, Soszka W, Jaglo G, Goc-Jaglo D. J Magn Magn Matter 2004;268:49–56. [7] Parilis ES, Kishinevsky LM, Turaev NYu, Baklitzky BE, Umarov FF, Verleger VKh, Nizhnaya SL, Bitensky IS. Atomic collisions on surfaces. North-Holland: Amsterdam, London, New York, Tokyo; 1993. [8] Ziegler JF, Biersack JP, Littmark U. In: Ziegler JF, editor. Stopping powers and ranges of ions in matter. New York: Pergamon Press; 1985.
Vacuum 74 (2004) 253–257
Quasi-single scattering of low-energy Ne+ ions from the Fe3O4 surfaces W. Soszkaa, N.-T.H. Kim-Ngana,*, A. Koz"owskib b
a ! 30 084, Poland Institute of Physics, Pedagogical University, Podchorazych 2, Krakow Solid State Physics Department, Faculty of Physics and Nuclear Techniques, AGH University of Science and Technology, Krakow, Poland
Accepted 22 December 2003
Abstract The energy spectra of positive and negative ions emitted from single-crystalline Fe3O4 (0 0 1) and (1 1 1) surfaces under low-energy Ne+ ion bombardments were investigated in the temperature range from 85 to 300 K. The characteristics of the LEIS spectra of the two surfaces are very similar. The positive-charged-ion spectra have revealed the O+-recoil and the quasi-single scattering Ne+–Fe peak, while a large broad O1 recoil signal was observed in the negative-charged-ion ones. No change of the peak shape and of the energy position have revealed for the (0 0 1) surface, whereas the (1 1 1) one implies an appearance of a small peak around 150 K attributed to recoil-oxygen ions from the double-collisions. r 2004 Elsevier Ltd. All rights reserved. PACS: 68.35.p; 61.18.Bn Keywords: Magnetite; Surface composition; Low-energy ion scattering (LEIS)
1. Introduction Magnetite (Fe3O4), has been an attractive candidate for technological applications in many important fields of electronics and catalysis. Increasing attention has been focused on the investigations of the surface structure and properties of magnetite surfaces. Among different surface methods, low-energy ion scattering (LEIS) has *Corresponding author. Tel.: +48-12-6626314; fax: +48-126372243. E-mail address: [email protected] (N.-T.H. Kim-Ngan).
been used in a large extent to investigate the surface compositions and structures [1–3]. Recently, our LEIS investigations on several magnetite surfaces in a multiple scattering approximation [4] have revealed that a large change in the neutralization and re-ionization of ions was due to the metal–insulator phase transition of magnetite (the Verwey transition) and that the Auger neutralization of ions related to the electron localization at the Verwey transition gave a dominant contribution to the final scattered ion yield. We extend the use of the LEIS technique to a large-angle geometry with which we focus on
0042-207X/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2003.12.136
ARTICLE IN PRESS 254
W. Soszka et al. / Vacuum 74 (2004) 253–257
investigations of quasi-single scattering processes on the magnetite surfaces. The LEIS investigations on the magnetite (0 0 1) surface using different ion beams have been recently published [5,6]. In this paper, we focus on analyzing the LEIS data of the (0 0 1) and (1 1 1) magnetite surfaces under Ne+ ion bombardments.
2. Experimental details The experimental data were collected from a temperature-dependent LEIS-setup with largeangle geometry where the energy spectrum is largely dominated by the single scattering peaks; the incident angle C is in the range of 0–55 and the detection angle Y is in the range of 66–94 respect to the surface. Azimuthal scans can be taken at every 1 up to 730 with respect to the main direction. A Ne+ ion beam with primary energies of 4–8 keV was used. The operating pressure in the chamber is in the 109 mbar range. Target current density was in the range 106 A/ cm2. From the angle-dependent investigations of the LEIS spectra for the whole energy range and at different temperatures we found out that at the scattering-angle geometry with C ¼ 34 and Y ¼ 68 the strongest Ne+–Fe peak with a largest change of the peak-intensity with changing temperature was obtained. Besides, at a certain scattering-angle geometry, a strongest scattering signal was obtained for the (0 0 1) surface at the azimuthal angle of F ¼ 0 ; while it was at F ¼ þ12 for the (1 1 1) one. Those optimal angles were then used for the temperature-dependent investigations. Two samples were cut from the single crystals of magnetite along the (0 0 1) and (1 1 1) surfaces. (We address those surfaces as Fe3O4 (0 0 1)-sc and (1 1 1)-sc surface where the abbreviation means the single-crystalline sample.) The azimuthal angle F ¼ 0 was fixed at the main crystallographic surface direction: the [0 1 0] and the ½1 1% 0 direction, respectively, for the (0 0 1) and the (1 1 1) surface. The atomic arrangements in magnetite imply the square-shaped surface semi-channels on the (0 0 1) surface, e.g. with the channel height ( and the channel width l ¼ 4:20 A ( along h ¼ 1:05 A
the [0 1 0] crystallographic direction. On the (1 1 1) surface, the triangular-shaped channels were ( l ¼ 5:14 A ( along the ½1 1% 0 formed (h ¼ 0:61 A, direction). Not only the number of scattering center on the topmost surface layer, they also imply different interaction rate of incoming ions with surface atoms lying inside the channels. An annealing in situ was done at temperatures 500–600 K in UHV. Prior to LEIS analysis the target was cleaned by cycles of 6 keV Ar+ sputtering until a reproducible energy spectrum was obtained (after about 2 h). No contamination was detected.
3. Results and discussions Similar characteristics of the LEIS spectra of scattered Ne+ ions from the two surfaces were observed. The positive-charged-ion spectra exhibited two well-defined peaks; the low-energy peak was in a good agreement with the energy of the O+-recoil atoms (E2(O)) while the high-energy peak was at the energy position for the Ne+–Fe scattering (E1(Fe) ) estimated from the elastic binary collision model [7]. Fig. 1 shows the LEIS spectra of the Fe3O4 (0 0 1) surface at different Ne+ ion primary energies. Unlike the cases of He+ and Ar+ ion bombardments where only small scattering peaks on a very high background were observed [5], the LEIS spectra of scattered
Fig. 1. Ne+ scattering spectra at different primary energies from the Fe3O4 (0 0 1) surface. C ¼ 34 ; Y ¼ 68 ; [0 1 0] azimuth (F ¼ 0 ), and target temperature T ¼ 300 K.
ARTICLE IN PRESS W. Soszka et al. / Vacuum 74 (2004) 253–257
Ne+ ions revealed an enormous Ne+–Fe scattering peak. The peak-intensity increased largely with increasing primary energy. The results indicated a very high survival probability of Ne+ ions scattered from Fe. A widening of the peak-width and an enhanced background at both the low- and the high-energy side of the binary Ne+–Fe peak with increasing primary energy, however, indicated a large contribution from quasi-scattering, multiple-scattering as well as from the re-ionization of the particles neutralized and scattered from the deeper layers at the final violent collision as they escape the magnetite surface. In Fig. 2 the LEIS spectra of the Fe3O4 (0 0 1) surface under 6 keV Ne+ ion bombardments at different detection angles were shown. The peak-intensity of the Ne+–Fe peak largely decreased with increasing the detection angle. A visible peak was still observed at Y ¼ 94 ; whereas the oxygen-recoil signal was disappeared at YX76 : It implied a step-like decrease in the scattered ion yield, Rþ ðYÞ; as shown in the inset of Fig. 2. The peak-intensity was found to increase enormously at low temperature. The LEIS spectra for 6 keV Ne+ ions at different temperatures were shown in Fig. 3. For the (0 0 1) surface, at 90 K the Ne+–Fe scattering peak intensity (I90 K ) was 2.5 times larger than that at room temperature (I300 K ) (Fig. 3a). A much less enhancement with an I90 K =I300 K ratio of 1.3 was found for the (1 1 1)
Fig. 2. Energy spectra of 6 keV Ne+ ions scattered from the Fe3O4 (0 0 1) surface at different detection angles. F ¼ 0 ; T ¼ 300 K. Inset: the normalized scattered ion yield, Rþ ðTÞ=Rþ 300 K ; as a function of the detection angles.
255
(a)
(b) Fig. 3. Energy spectra of 6 keV Ne+ ions at different temperatures: (a) for the Fe3O4 (0 0 1) surface (F ¼ 0 ) and (b) for the (1 1 1) one (F ¼ 12 ). Incidence C ¼ 34 ; detection angle Y ¼ 68 : The peak-intensity was normalized for the ion current of 1 nA.
surface (Fig. 3b). The results indicated a dominant contribution to the total scattered ion yield (Rþ ) from the ions scattered from the atoms inside the channels. Namely, the related interacting rate was much higher for the wide squared-shape channels on the (0 0 1) surface than that for the triangular ones on the (1 1 1) ones. At low temperatures, such a contribution was largely affected by a decrease of the neutralization degree related to the electron localization in magnetite, leading to a larger enhancement of Rþ for the squared-shape channels. At 90 K the Ne2+–Fe scattering peak became more visible for both surfaces. A very pronounced peak was observed in the energy spectra of the negative ions emitted from
ARTICLE IN PRESS 256
W. Soszka et al. / Vacuum 74 (2004) 253–257
the magnetite surfaces under Ne+ ion bombardments for the detection angles Y up to 80 . The energy position of the peak maximum was found at the expected energy of the recoil O1 ions from a binary collision with a quite small elastic energy loss. At 300 K the intensity of the O1-recoil peak was about 2.5 times bigger than that of the O+1recoil one. At higher detection angle (Y > 80 ) only a broad peak has revealed. An enormous enhancement of the O1-recoil peak and of the background at the high-energy side of the peak was found at 90 K as shown in Fig. 4. The results indicated that a large amount of oxygen ions were present on the outermost surface layer of magnetite and joined the binary collisions in a negativecharge state. A strong signal of both emitted O1 and O+1 ions indicated a strong charge-exchange effect on magnetite surfaces. We notice here that a well-known phenomenon in magnetite is the electron hopping between the Fe-octahedral sites of the spinel structure. In other words, there are a large amount of delocalized electrons in the hightemperature phase. However, no distinguished recoil O2 signal was detected in the LEIS spectra. No change of the peak shape and of the energy position for the O1-recoil peak recoil with changing temperature was observed for both two surfaces. However, the most striking feature is the appearance of a small peak around 150 K in the LEIS spectra of the (1 1 1) surface as shown in
Fig. 5. The larger peak represented the recoiloxygen ions from a single binary collision, while the additional peak was attributed to those resulting from the double-collisions. In analysing the oxygen emissions under Ne+ and Ar+ ion bombardments for the (0 0 1) surface we found out that the shadowing effect related to the screening action of oxygen ions implies a dominant contribution to R especially at low temperatures. The shadow effect on the magnetite surface was illustrated and discussed in detail in our recent publication [5]. The shadow radius RS at a distance d from an atom is expressed as: Rs ¼ 2:2½db2 =ð1 þ mÞð1=3Þ ; where b is the distance of closest approach in a head-on collision with an energy Eo ; m is the mass ratio between the surface atom and incoming ion [7]. We applied the calculations for the case of 6 keV Ne+ ions with an incident angle of c ¼ 34 using Ziegler– Biesack–Littmark approximation for the screening function [8]. The estimated values for the shadow radius for the Fe–O chain along the ½1 2% 1 ( direction with an atomic distance of l ¼ 3:43 A ( ( are: RS (ion-O)=0.764 A, RS (ion-Fe)=0.719 A. ( RS (ionIn comparison: RS (ion-O)=0.649 A, ( Fe)=0.610 A) for the [0 1 0] chain on the (0 0 1) ( We notice that unlike the surface with l ¼ 2:10 A. case of Ar+ ions, under Ne+ ion bombardments, the shadow radius is small and thus the ion-surface atom collision was not much effected by the
Fig. 4. Energy spectra of the recoil O1 ions from the Fe3O4 (0 0 1) surface under 6 keV Ne+ ion bombardments at 300 K (&) and at 90 K (:). C ¼ 34 ; Y ¼ 68 ; and F ¼ 0 :
Fig. 5. Temperature dependence of the LEIS spectra of the recoil O+1 ions from the Fe3O4 (1 1 1) surface under 6 keV Ne+ ion bombardments: C ¼ 34 ; Y ¼ 68 ; and F ¼ 12 : An additional oxygen peak from a double-collision was observed around 150 K.
ARTICLE IN PRESS W. Soszka et al. / Vacuum 74 (2004) 253–257
screening effect of the neighbour atom. Namely, for both two different types of channels the ions with different trajectories can always knock out the oxygen atoms leading to a weak surfacestructure dependence of the negative-charged ion emissions. However, the existence of the additional oxygen peak for the (1 1 1) surface indicated that a surface relaxation for the oxygen atoms was much enhanced in the case of the triangular-shaped channels providing a higher probability of oxygen recoil ions for the double-collisions.
4. Summary Our LEIS investigations of magnetite surfaces using Ne+ ion beams have revealed a high survival probability of Ne+ ions scattered from Fe and a large contribution from quasi- and multiplescattering. An enormous enhancement of the peak-intensity of the Ne+–Fe peak of the (0 0 1) surface indicated a dominant contribution from the neutralization process related to a change in the electron localization degree of magnetite. Moreover, the interacting rate of the incoming ions with the atoms inside the channels was higher for the squared-shape channels than for the
257
triangular ones. A very strong signal from emitted oxygen ions indicated that a large amount of negative-charged oxygen ions was present on the outermost surface layer of magnetite. Our LEIS investigations provided an evidence for the presence of mixed Fe–oxygen layers on these magnetite surfaces.
References [1] Pedemonte L, Bracco G, Robin A, Heiland W. Nucl Instrum Methods B 2002;193:563–7 (and references therein). [2] Brongersma HH, Gildenpfennig A, Denier van der Gon AW, van de Grampel RD, Jansen WPA, Knoester A, Laven J, Viitanen MM. Nucl Instrum Methods B 2002;190:11–8. [3] Boerma DO. Nucl Instrum Methods B 2001;183:73–87. [4] Kim-Ngan N-TH, Soszka W. Nucl Instrum Methods B 2002;193:391–5 (and references therein). [5] Kim-Ngan N-TH, Soszka W. Surf Sci 2003;536:24–32. [6] Kim-Ngan N-TH, Soszka W, Jaglo G, Goc-Jaglo D. J Magn Magn Matter 2004;268:49–56. [7] Parilis ES, Kishinevsky LM, Turaev NYu, Baklitzky BE, Umarov FF, Verleger VKh, Nizhnaya SL, Bitensky IS. Atomic collisions on surfaces. North-Holland: Amsterdam, London, New York, Tokyo; 1993. [8] Ziegler JF, Biersack JP, Littmark U. In: Ziegler JF, editor. Stopping powers and ranges of ions in matter. New York: Pergamon Press; 1985.