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Surface orientation effect in total electron yield on TiO2
ELSEVIER
Physica B 208&209 (1995) 81-83
Surface orientation effect in total electron yield on Ti02 B. P o u m e l l e c a'*, R. Cortrs b, J. B e r t h o n a aLab. TPCM, UA 446 CNRS, UPS Orsay, 91405 Orsay Cedex, France bLab. LPLE, UPR 15 CNRS, 10 rue Vauquelin, 75231 Paris Cedex, France
Abstract We show unusual dependence of the total electron yield on the illuminated surface orientation of TiO2. This requires care in recording angular dependencies and forces one to use always the same surface orientation. This in fact led us to publish wrong results in the past which we correct here.
1. Introduction
2. Experimental details
In recording the angular dependence of the Ti K edge in TiO2 ( D ~ , tetragonal structure), we have pointed out a dipole dependence in rotating the sample around the photon propagation axis [ l l . To record the quadrupole dependence and measure a sin(40) dependence where 0 is the direction of k or e in the (a, b) plane, Brouder [2] has found a variation of the intensity of the first prepeak on the same sample. Performing previously a similar experiment but keeping the e vector constant, we found [3] a larger variation extending on the whole spectrum. In this case, (0 01) and (110) illuminated faces were used on two samples. On the other hand, generally the quadrupole intensity leads at the most to only a few % variation for Q dependence at K edges of the first row transition metal [4]. In contrast, keeping the same face and the same e vector but rotating the sample around e, we have confirmed Brouder's result. The question is therefore: does the electronic yield depend on the crystal orientation of the illuminated surface all other things being constant?
To overcome doubts on effects that could arise from some non-controlled experimental parameters, we have made two measurements in keeping everything constant. Just the orientation of the illuminated surface was changed by moving the sample in the X-ray beam. We used a unique TiO2 single crystal cut following the planes (110) and (0 01) and carefully cleaned it. The crystal was allowed to move according to the X-ray beam slab in order to present either one or the other face. Two orientations of the above apparatus were used in order to check if there is an effect of the X-ray beam polarisation. The first orientation was such that e l l [ l i 0 , l and the other was 90 ° rotated. In this case e is 45 ° between [00 1] and [1 1 0,l. The total electron yield spectra were recorded on EXAFSII, D24 beam line at DCI-LURE-Orsay. Ring current: 280mA; beam energy: 1.85 GeV; monochromator: Si(3 1 1). Harmonic rejection by grazing mirror tuned at 4 mrad (cutoff 7500 eV). Io ion chamber filled with air and electron detector with atmospheric He. Count rate: 1 s per point. Electron collector biased at + 40 V. Elliptical polarisation minimised at 19 keV. Special attention was paid to linearity of the electron detector response since this quantity is of great importance here. Variation of spectra on slit width was studied
* Corresponding author.
0921-4526/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 9 2 1 - 4 5 2 6 ( 9 4 ) 0 0 8 2 4 - 8
82
B. Poumellec et al. ,/Physica B 208&209 (1995) 81 83
4.0-
Fig. la: e//[1-10]
~
÷
3.5-
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g e 45 ° between [001 ] and [-1-10] 45 ° between [001 ] and [110]
1.2-
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2.0-
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I 4970
I 4980 Energy (eV)
Fig. lb: e I I [1-10] k 45 ° between [001] and [ I I O]
~
I 4990
I 5000
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I
I
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/
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Fig. 2b:
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1.2-
I
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0 +
face (110) face (001)
g
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i
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t~
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/ 0,2-
0.0-
4950
/
0.6-
I 4960
49170
49180
I 4990
I 5000
Energy (eV)
Fig. 1. Spectra for polarisation parallel to [ l i 0 ] : (a)normalized current; (b) background subtracted and renormalized on the atomic absorption. and adjusted to render it negligible in respect to the observation described below. Vertical slit width placed before the ion chamber was adjusted in order to illuminate only the sample surface. Influence of the surface state of the crystal was also studied. As obtained from diamond cutting, optically polished or carbon coated was compared leading to the conclusion that as-obtainedfrom-cutting is the best solution maximising the electron current at around 4-20 pA. Finally, we have checked the repeatability all along the experiment.
3. Results The electron current normalised to the incident intensity is shown in Figs. l(a) and 2(a). In the first case, the polarisation is e l l [ l i 0 ] and k 45 ° between [ 0 0 1 ] and
[iio]. In the second case e is 45 ° between [0 0 1] and [ i i 0]. In Fig. l(a), the current collected from the face (001) is
0,04950
I
I
4960
4970
I
4980 Energy(eV)
4990
5000
50 0
Fig. 2. Spectra for polarisation 45 ° between [00 1] and [110]: (a) normalized current; (b) background subtracted and renormalized on the atomic absorption.
about twice that collected from the face (1 1 0). In Fig. 2(a), the observation is reversed. On the other hand, in Fig. l(a), the modulation (especially after the edge) is about the same for both faces whereas a net difference is observed in Fig. 2(a). We emphasise that for both orientations of the apparatus, the incident angle of the beam to the surface and the direction of electron collector are the same, i.e. the geometry is exactly the same. Thus only the polarisation is different. The question is now about the shape of the spectra. Are they different or does the above discrepancy disappear after subtraction of the background and renormalisation? In Fig. l(b), we see that the first prepeak and the shoulder on the edge do not have the same intensity nor the modulation after the edge. It appears that face (00 1) gives less contrasted modulations than the other. In Fig. 2(b), for another polarisation state, similar observations can be made. The first prepeak is the most affected
B. Poumellec et al. / Physica B 208&209 (1995) 81 83
the remaining part of the spectrum changes also but to a less extent than for e l l [ l l 0 ] . For polarisation ell[001.] (not shown here), we have not detected any dependence of the shape of the spectrum on the face orientation. Thus, we deduce that dependence on the face takes place especially on the part of the spectrum of ell[1 1 0] polarisation state.
83
necessary to keep the same illuminated surface orientation. This had not been made on our first investigation [3] and yielded wrong results in regard to the quadrupole part. Aware of this problem, we confirm that only the first prepeak of the Ti K edge of TiO2 exhibits a quadrupole dependence as was previously mentioned by Brouder [2], but we emphasise that it has a very low symmetry. This will be shown in another paper.
4. Conclusion References
We point out a dependence of the electron yield on the orientation of the surface for all polarisation. This is an unusual result but to be sure of that we have paid much attention to record consecutive spectra, while keeping everything constant. We show that this phenomenon gives rise to a dependence of the shape of the spectra for the part of the polarisation ell [1 i 0 ] , i.e. no dependence is detected for e II [001). This phenomenon is not easily explainable although it is worth noticing that channelling can arise in the direction 1-00 1] for low energy electrons ( ~ 100 eV). Therefore, when measuring the angular dependence of TiO2 but perhaps of other oxides, it is
[1] B. Poumellec,R. Cortes, G. Tourillon and J. Berthon, Phys. Stat. Sol. B 164 (1991) 319. I-2] C. Brouder, J.P. Kappler and E. Beaurepaire, in: 2nd European Conference on Progress in X-ray Synchrotron Radiation Research, 1990, Rome (Societa Italiena di Fisica, Bologna, 1990). [-3] B. Poumellec, R. Cortes, G. Tourillon and J. Berthon, in: 2nd European Conference on Progress in X-ray Synchrotron Radiation Research, 1990, Rome (Societa Italiena di Fisica, Bologna). [4] G. Hildebrandt, J.D. Stephenson and H. Wagenfeld, Z. Naturf. 30 (1975) 697.
Physica B 208&209 (1995) 81-83
Surface orientation effect in total electron yield on Ti02 B. P o u m e l l e c a'*, R. Cortrs b, J. B e r t h o n a aLab. TPCM, UA 446 CNRS, UPS Orsay, 91405 Orsay Cedex, France bLab. LPLE, UPR 15 CNRS, 10 rue Vauquelin, 75231 Paris Cedex, France
Abstract We show unusual dependence of the total electron yield on the illuminated surface orientation of TiO2. This requires care in recording angular dependencies and forces one to use always the same surface orientation. This in fact led us to publish wrong results in the past which we correct here.
1. Introduction
2. Experimental details
In recording the angular dependence of the Ti K edge in TiO2 ( D ~ , tetragonal structure), we have pointed out a dipole dependence in rotating the sample around the photon propagation axis [ l l . To record the quadrupole dependence and measure a sin(40) dependence where 0 is the direction of k or e in the (a, b) plane, Brouder [2] has found a variation of the intensity of the first prepeak on the same sample. Performing previously a similar experiment but keeping the e vector constant, we found [3] a larger variation extending on the whole spectrum. In this case, (0 01) and (110) illuminated faces were used on two samples. On the other hand, generally the quadrupole intensity leads at the most to only a few % variation for Q dependence at K edges of the first row transition metal [4]. In contrast, keeping the same face and the same e vector but rotating the sample around e, we have confirmed Brouder's result. The question is therefore: does the electronic yield depend on the crystal orientation of the illuminated surface all other things being constant?
To overcome doubts on effects that could arise from some non-controlled experimental parameters, we have made two measurements in keeping everything constant. Just the orientation of the illuminated surface was changed by moving the sample in the X-ray beam. We used a unique TiO2 single crystal cut following the planes (110) and (0 01) and carefully cleaned it. The crystal was allowed to move according to the X-ray beam slab in order to present either one or the other face. Two orientations of the above apparatus were used in order to check if there is an effect of the X-ray beam polarisation. The first orientation was such that e l l [ l i 0 , l and the other was 90 ° rotated. In this case e is 45 ° between [00 1] and [1 1 0,l. The total electron yield spectra were recorded on EXAFSII, D24 beam line at DCI-LURE-Orsay. Ring current: 280mA; beam energy: 1.85 GeV; monochromator: Si(3 1 1). Harmonic rejection by grazing mirror tuned at 4 mrad (cutoff 7500 eV). Io ion chamber filled with air and electron detector with atmospheric He. Count rate: 1 s per point. Electron collector biased at + 40 V. Elliptical polarisation minimised at 19 keV. Special attention was paid to linearity of the electron detector response since this quantity is of great importance here. Variation of spectra on slit width was studied
* Corresponding author.
0921-4526/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 9 2 1 - 4 5 2 6 ( 9 4 ) 0 0 8 2 4 - 8
82
B. Poumellec et al. ,/Physica B 208&209 (1995) 81 83
4.0-
Fig. la: e//[1-10]
~
÷
3.5-
3.0-
g e 45 ° between [001 ] and [-1-10] 45 ° between [001 ] and [110]
1.2-
+
=_ .~
1,4-
J
:~
"2 z.5-
1.o-
~
0.8-
2.0-
~ 0.6-
1.5-
i0.4-
1.0I 4960
4950
1.4-
I 4970
I 4980 Energy (eV)
Fig. lb: e I I [1-10] k 45 ° between [001] and [ I I O]
~
I 4990
I 5000
0.2-
4950 ~ ~;b, ~
I
I
4960
4970
I
4980 Energy (eV)
1.Z-
1.o-
1,0-
2
_~
;# %V'
0.8o
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i
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/
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I
I
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5010
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Fig. 2b:
1 A - e 45 ° between [001] and [-1-10] k 45 ° between [001] and [110]
1.2-
I
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0 +
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g
.Q '~
0.6-
i
0.4-
t~
0.4zo
0.2-
/ 0,2-
0.0-
4950
/
0.6-
I 4960
49170
49180
I 4990
I 5000
Energy (eV)
Fig. 1. Spectra for polarisation parallel to [ l i 0 ] : (a)normalized current; (b) background subtracted and renormalized on the atomic absorption. and adjusted to render it negligible in respect to the observation described below. Vertical slit width placed before the ion chamber was adjusted in order to illuminate only the sample surface. Influence of the surface state of the crystal was also studied. As obtained from diamond cutting, optically polished or carbon coated was compared leading to the conclusion that as-obtainedfrom-cutting is the best solution maximising the electron current at around 4-20 pA. Finally, we have checked the repeatability all along the experiment.
3. Results The electron current normalised to the incident intensity is shown in Figs. l(a) and 2(a). In the first case, the polarisation is e l l [ l i 0 ] and k 45 ° between [ 0 0 1 ] and
[iio]. In the second case e is 45 ° between [0 0 1] and [ i i 0]. In Fig. l(a), the current collected from the face (001) is
0,04950
I
I
4960
4970
I
4980 Energy(eV)
4990
5000
50 0
Fig. 2. Spectra for polarisation 45 ° between [00 1] and [110]: (a) normalized current; (b) background subtracted and renormalized on the atomic absorption.
about twice that collected from the face (1 1 0). In Fig. 2(a), the observation is reversed. On the other hand, in Fig. l(a), the modulation (especially after the edge) is about the same for both faces whereas a net difference is observed in Fig. 2(a). We emphasise that for both orientations of the apparatus, the incident angle of the beam to the surface and the direction of electron collector are the same, i.e. the geometry is exactly the same. Thus only the polarisation is different. The question is now about the shape of the spectra. Are they different or does the above discrepancy disappear after subtraction of the background and renormalisation? In Fig. l(b), we see that the first prepeak and the shoulder on the edge do not have the same intensity nor the modulation after the edge. It appears that face (00 1) gives less contrasted modulations than the other. In Fig. 2(b), for another polarisation state, similar observations can be made. The first prepeak is the most affected
B. Poumellec et al. / Physica B 208&209 (1995) 81 83
the remaining part of the spectrum changes also but to a less extent than for e l l [ l l 0 ] . For polarisation ell[001.] (not shown here), we have not detected any dependence of the shape of the spectrum on the face orientation. Thus, we deduce that dependence on the face takes place especially on the part of the spectrum of ell[1 1 0] polarisation state.
83
necessary to keep the same illuminated surface orientation. This had not been made on our first investigation [3] and yielded wrong results in regard to the quadrupole part. Aware of this problem, we confirm that only the first prepeak of the Ti K edge of TiO2 exhibits a quadrupole dependence as was previously mentioned by Brouder [2], but we emphasise that it has a very low symmetry. This will be shown in another paper.
4. Conclusion References
We point out a dependence of the electron yield on the orientation of the surface for all polarisation. This is an unusual result but to be sure of that we have paid much attention to record consecutive spectra, while keeping everything constant. We show that this phenomenon gives rise to a dependence of the shape of the spectra for the part of the polarisation ell [1 i 0 ] , i.e. no dependence is detected for e II [001). This phenomenon is not easily explainable although it is worth noticing that channelling can arise in the direction 1-00 1] for low energy electrons ( ~ 100 eV). Therefore, when measuring the angular dependence of TiO2 but perhaps of other oxides, it is
[1] B. Poumellec,R. Cortes, G. Tourillon and J. Berthon, Phys. Stat. Sol. B 164 (1991) 319. I-2] C. Brouder, J.P. Kappler and E. Beaurepaire, in: 2nd European Conference on Progress in X-ray Synchrotron Radiation Research, 1990, Rome (Societa Italiena di Fisica, Bologna, 1990). [-3] B. Poumellec, R. Cortes, G. Tourillon and J. Berthon, in: 2nd European Conference on Progress in X-ray Synchrotron Radiation Research, 1990, Rome (Societa Italiena di Fisica, Bologna). [4] G. Hildebrandt, J.D. Stephenson and H. Wagenfeld, Z. Naturf. 30 (1975) 697.