- Email: [email protected]
Safe limits for electron beam analysis of copper oxides
Ultramicroscopy 34 (1990) 27-32 North-Holland
27
Safe limits for electron beam analysis of copper oxides A m a n d a K. Petford-Long and Neil J. Long Department of Materials, University of Oxfora~ Parks Road, Oxfor~ OX1 3PH, England Received 5 February 1990; at Editorial Office 12 June 1990
As part of a wider study of transition metal oxides, CuO has been analysed using a STEM to determine the "safe" limits of electron beam dose rate and total electron dose to which the material can be exposed before electron-beam-induced reduction occurs. Electron energy loss spectroscopy has been used to determine the change in composition of the oxide.
1. Introduction
2. Experimental procedure
Oxides of the transition metals (TM) such as Cu [1] and Zn [2] are used as catalysts, and in recent years, electron microscopes have been extensively used to study such materials with the high spatial resolution necessary for analysing finely dispersed oxide particles. The electronbeam-induced reduction or sputtering of TM oxides, carbides and nitrides [3,4] is well known, but the rapidity with which this occurs is often not fully appreciated, resulting in erroneous analysis of the oxide composition. This is particularly important in catalysts containing both reduced and fully oxidised phases, where a correlation is needed between particle size and distribution and phase distribution. In this study we have carded out quantitative experiments, varying electron dose rate and total dose using a scanning transmission electron microscope (STEM). We hope to indicate the limits of dose rate and total dose for which no damage is observed during EELS microanalysis of CuO for typical dose rates which may be needed to obtain an adequate signal from small areas. Previous work [5] followed the reduction pathway of CuO to Cu, but the quantitative effect of dose rate, crystal thickness and size on the damage rate were not considered. In the present study, the reduction pathway has not been studied in detail, but more quantitative results have been obtained regarding damage rates.
The experiments were carried out in a VG HB501B STEM operated at 100 kV. The electron probe current was measured for a variety of operating conditions, and varied with size of the probe-forming aperture. The average current density at the specimen was determined by the image magnification (size of area scanned), increasing as a function of the square of the magnification. Electron energy loss spectroscopy (EELS) was used instead of energy-dispersive X-ray spectroscopy (EDS) for several reasons: Firstly it is difficult to obtain reliable values for the correction factors needed to convert O and Cu X-ray peak integrals into composition, since standards of all possible reduced copper oxides would be needed, which would themselves be beam sensitive. Secondly, there will be an appreciable contribution to the signal from surrounding indirectly excited material, whereas the EELS signal comes only from the area under the beam when very thin samples are used and the composition can be calculated without the need for standards. Thirdly, the lower beam current and short analysis time used in order to rninimise reduction during analysis meant that the X-ray spectra contained too few counts for a reliable estimate of the oxygen peak. Although the EELS background is high, several reliable computational methods are available to subtract the background, which are discussed later.
0304-3991/90/$03.50 © 1990 - Elsevier Science Publishers B.V. (North-Holland)
A.K. Petford-Long. N.J. Long / Safe limits for CuO analysis
28
Table 1 Conditions used during electron-beam-induced damage and analysis of copper oxides VOA (/xm)
OA (/xm)
Maguification
Current density
Total current
(A/cM)
(hA)
Damage Analysis
50 -
50
2 x 106 5 x 106
570 114
2.0 0.4
Damage Analysis
50 -
50
5 x 106 1 x 107
3570 714
2.0 0.4
Damage: 50/~m virtual objective aperture (VOA), magnification of 2 x 106 or 5 x 106. Analyse: 50/~m real objective aperture (OA), magnification of 5X106 or lX107.
The specimens were exposed to an alternating "damage" and "analyse" sequence, as shown in table 1. All damage and analysis was carried out using the reduced-area scan mode, with the total beam current kept constant during damage. The analyses were carried out at a higher magnification (but with a much lower beam current) than during "damage", on an area within that previously scanned, so that only material exposed to the high current density would be analysed. To reduce the acquisition time, because serial EELS collection was used, only 600 channels were scanned in windows around the O K edge and the Cu L2,3 edge. The spectra were collected with an effective collection half-angle of 7 mrad (corrected for incident beam divergence). Typically the specimens were damaged for 60 seconds, and the spectra were acquired during the following 60 seconds at 0.2 of the current density used during damage, and so on. Annular dark field (ADF) images were recorded so that changes in appearance of the sample could be correlated with changes in the EELS spectra. Basic spectral manipulation used modified programs based on those of Skiff [6]. Background fitting and edge stripping used Trebbia's programs [7] except when low-count statistics required the use of Egerton's graphical method and "h-factor" appraisal of goodness-of-fit [8]. The edge integration limits used were 100 eV (531-631 eV for the oxygen K edge and 926-1026 eV for the copper L edge). Changing the integration limits to exclude the near-edge fine structure in the edges did not
alter the results observed, within experimental error. The atomic fraction of each element was then calculated using the Cu and O partial cross sections calculated by Rez [9]. A quantitative measure of the C u : O ratio was obtained for each spectrum, with standard errors. The shape of the CuL2,3 edge gives a direct indication of the oxidation state of the Cu atoms, with the "white lines" seen for CuO decreasing in height as the oxide is reduced (the L2,3 edge for Cu metal shows no white-line intensity). However, the change in edge shape occurs very gradually, making it extremely difficult to extract an accurate measure of composition. The literature gives a wide range of experimental values for the ratio of the L: and L 3 white-line heights, e.g. refs. [5,10], ,10 2
1.50
I
I
I
I
/:=o
O
1.20-0.90--
Cu
0.60
0.30-0.00
I
450 x 102 1.50
I
I
600
750
900
I
I
I
I
1050
1200
I
f.=b~ns
1.20 ~ ~
0.90
0 rj
0.60 0.30
0.00 450
I 600
I 750
I 900
I 1050
1200
ENERGY LOSS [eV] Fig. 1. Electron energy loss spectra from CuO crystal before and after exposure to the electron beam. (6 rain damage at current density 570 A / c m 2 plus 7 min at a lower current density during acquisition of intermediate spectra.)
A.K. Petford-Lon$ N.J. Long / Safe limits for CuO analysis
for both CuO and Cu20, due probably to differences in the experimental conditions used in different laboratories. This is the primary reason for analysing both the Cu L2,3 and O K edges by measuring the peak integral under each edge. The specimens were of crushed bulk oxide deposited on a holey carbon film on a mesh grid. A comparison between specimens made by dry crushing and by crushing under high-purity acetone was made, but no difference in results was observed.
3. Results EELS spectra from CuO before and after 6 rain exposure to an electron beam of current density 570 A / c m 2 are shown in fig. 1. The shape of the CuL2,a edge has changed, indicating a change in valence state of the copper in the oxide, and the height of the O K edge has decreased, due to
29
oxygen loss. The oxygen atomic fraction in the crystal versus total electron dose for the same sequence is shown in fig. 2. Also shown in fig. 2 is the change in number of Cu counts with total electron dose. As damage rate may depend on the thickness of the analysed crystal, results were obtained for "thin" ( t < 1 0 urn) and "thick" ( t > 3 0 urn) crystals. The thickness of several crystals was measured using the low-loss region of the EELS spectrurn prior to starting the damage experiments, and the total number of counts in the Cu and O edges was noted. The thickness of damaged crystals could then be estimated by comparison of the number of Cu and O counts with those for a particular crystal thickness. A comparison of the change in oxygen content for "thin" and thick" crystals after exposure to the same total electron dose (at the same current density) can be seen by comparing figs. 2 (thin crystal) and 3 (thick crystal). Thick crystals are "'
0.6
0.5
0.4 tO
E 0.3 a o) x o
t-1 O O
""',,.. .o."°
i
..... -B...
12. "-,..
.-
--.
8
",.l~f. o"
0.2
0.1
dose (C/sq nm)
Fig. 2. Graphs showing change in oxygen atomic fraction (top curve) and change in total number of counts in copper peak (bottom curve, arbitrary units) for CuO crystal with total electron dose at a current density of 570 A / c m 2 (thin crystal). Same damage sequence as fig. 1.
30
A.K. Petford-Long, N.J. Long / Safe limits for CuO analysis 0.6
5000 0.5 4000
0.4 C O
aooo ~,
E 0.3
0 0
tO
i
D.
8
O
2OOO
0.2
|000
0.1
• 0.5E10
'
" 1.0E10 dose
' (C/sq
' 1.5E10'
' 2.0E10
'
nm)
Fig. 3. Graph showing change in oxygen atomic fraction and total number of Cu counts (arbitrary units) for CuO crystal with electron dose (current density 570 A/cm2, thick crystal).
more resistant to oxygen loss, as would be expected, especially if the reduction is associated with a surface effect. Data for exposure to an electron beam of current density 3570 A / c m 2 is shown in fig. 4. This shows the change in oxygen content and total number of copper counts for a case in which an electron-beam-damage-induced hole formed in the specimen (current density 3570 A/cm2). In contrast to the earlier figures, the data in fig. 4 initially shows a relatively constant composition ( O : C u ratio) and a rapid decrease in the total number of Cu counts, followed by a loss of oxygen and further Cu loss. An A D F image corresponding to the damage sequence shown in fig. 4, after exposure to the electron beam, is shown in fig. 5. Bright contrast arising from copper metal which has diffused away from the irradiated area during "damage" can be seen around the edges of a darker electron-beam-damage-induced hole in the specimen.
4. D i s c u s s i o n
The data presented in this study were recorded using a STEM in which the probe is scanned over an area and the EELS data is acquired in serial mode. The probe has a finite size and current distribution, so that for a fixed probe the material under the beam will see a variation in current density. The " s p o t " mode (stationary electron probe) was not used for the following reasons: (a) A detailed analysis of the probe shape and current distribution would be needed since composition changes sensitive to the absolute beam current would be convoluted in the EELS spectrum. (b) A probe size of 2 nm or less and total beam current 1 nA would give a current density of approximately 30 000 A / c m 2, resulting in an unacceptably high damage rate or such short acquisition times that the signal will be too weak for adequate statistics for composition measurement. Previous work has shown that analysis of both the Cu and
A.K. Petford-Long, N.J. Long / Safe limits for CuO analysis
31
0.6
4500 4OOO
0.5
3500 0.4 tO
2so0 O
0.3 ~ ~4 0
0.2
"''",}
........... L................ m.............. -m..............
m.,. .~,, ,..,~.l-~ 1500
1000 0.1
500
1E10'
2E10'
3E10'
4E10' 5E10' 6E10' dose (C/sq nm)
7E10'
8E10'
9E10'
Fig. 4. Graph showing change in oxygen atomic fraction and total number of copper counts (arbitrary units) with total electron dose at a current density of 3570 A / c m 2.
Fig. 5. Annular dark-field image of CuO crystal after electronbeam-induced damage at a current density of 3570 A / f r o 2.
O edges is necessary - impossible with a fixed probe without using a parallel data collection system [11]. Increasing the collector aperture size increases the number of electrons detected, but not necessarily the peak-to-background ratio [12]; the easiest solution is thus to scan the probe so that the beam is averaged over a larger area than the probe size (for magnifications less than 1 × 107× ). The averaged current density and damage rate are drastically reduced, allowing longer count times, thereby improving the statistics. Due to the way in which an analogue time-base (as used here) operates, there will be areas at the start of the frame and along the top of the frame which receive a significantly higher dose than the rest of the scanned area analysed, which has undergone uniform irradiation (this will give a systematic error in the calculated dose). In this study we have considered the average current density in
32
A.K. Petford-Long, N.J. Long / Safe limits for CuO analysis
the scanned area as the total integrated beam current dividing by the area of the scan. Care has to be taken that an area is not scanned during "damage" at a very low magnification and then analysed at a very high magnification, as it might be possible to analyse an entirely unexposed region of material between the low magnification scan lines. A possible electron-beam-induced mechanism which explains the observed results is surface reduction of the CuO to a reduced oxide or Cu by an electron-stimulated-desorption reaction such as the Knotek-Feibelman mechanism (known to result in the reduction of maximal valence oxides [13]), followed by migration of Cu over the particle surface to the edges of the scanned area. Migration of Cu away from the scanned area will give a linear decrease in the total number of Cu counts and will also reveal fresh surface layers of oxide which can be reduced, leading to further migration, and so on until a hole is formed. The relatively constant oxygen content, followed by a sharp drop, will also be explained by this model - initially the ratio of unreduced oxide to surface Cu will be very high, so that the majority of the EELS signal will be from bulk CuO. As reduction proceeds, the crystal will get thinner, but migration of Cu away from the analysed area will mean that the signal still comes predominantly from CuO, with only a small effect from the surface metal layer. The apparent oxygen content will therefore only drop slightly below 50 at%. When the majority of the oxide under the beam has been reduced, the atomic percentage of oxygen will fall rapidly. The apparent oxygen content will only drop to zero provided that complete reduction of a substantial region under the electron beam occurs before migration of copper results in the formation of a hole through the crystal.
5. Conclusions It is possible to obtain chemical information from CuO at a high spatial resolution (1-2 nm)
using STEM and EELS, provided that the counting time used to collect the spectra is long enough to ensure adequate statistics for composition measurement. This is important since the choice of "safe" beam current, current density and operating conditions must be made with statistics in mind. For specimens with a thickness of 10 nm or more, keeping the incident electron beam current density below 570 A / c m 2 would allow analysis time of up to approximately 4 min - ample time to collect EELS spectra with sufficient counts to obtain a realistic measure of composition.
Acknowledgement We are grateful to Prof. Sir Peter Hirsch for provision of laboratory space.
References , [1] Y. Yokamoto, K. Fukino, T. Imanaka and S. Teranishi, J. Phys. Chem. 87 (1983) 3740. [2] T. van Herwijnen and W.A.J. de Jong, J. Catal. 63 (1980) 83. [3] P.A. Crozier, J.N. Chapman, A.J. Craven and J.M. Titchmarsh, in: Analytical Electron Microscopy - 1984, Eds. D.B. Williams and D.C. Joy (San Francisco Press, San Francisco, 1984) p. 79. [4] L.E. Thomas, in: Analytical Electron Microscopy - 1984, Eds. D.B. Williams and D.C. Joy (San Francisco Press, San Francisco, 1984) p. 247. [5] N.J. Long and A.K. Petford-Long, Ultramieroscopy 20 (1986) 151. [6] M. Skiff, PhD Thesis, Arizona State University, 1986. [7] P. Trebbia, Ultramicroscopy 24 (1988) 399. [8] R.F. Egerton, Electron Energy Loss Spectroscopy in the Electron Microscope (Plenum, New York 1986). [9] R.D. Leapman, P. Rez and D.F. Mailers, J. Chem. Phys. 72 (1980) 1232. [10] T.G. Sparrow, B.G. Williams, C.N.R. Rao and J.M. Thomas, Chem. Phys. Lett. 108, (1984) 547. [11] See, for example, R.F. Egerton, J. Electron Mierosc. Tech. 1 (1984) 37. [12] D.C. Joy in: Principles of Analytical Electron Microscopy, Eds. D. Joy, D. Romig and J. Goldstein (Plenum, New York, 1986) p. 256. [13] M.L. Knotek and P.J. Feibelman, Phys. Rev. Lett. 40 (1978) 964.
27
Safe limits for electron beam analysis of copper oxides A m a n d a K. Petford-Long and Neil J. Long Department of Materials, University of Oxfora~ Parks Road, Oxfor~ OX1 3PH, England Received 5 February 1990; at Editorial Office 12 June 1990
As part of a wider study of transition metal oxides, CuO has been analysed using a STEM to determine the "safe" limits of electron beam dose rate and total electron dose to which the material can be exposed before electron-beam-induced reduction occurs. Electron energy loss spectroscopy has been used to determine the change in composition of the oxide.
1. Introduction
2. Experimental procedure
Oxides of the transition metals (TM) such as Cu [1] and Zn [2] are used as catalysts, and in recent years, electron microscopes have been extensively used to study such materials with the high spatial resolution necessary for analysing finely dispersed oxide particles. The electronbeam-induced reduction or sputtering of TM oxides, carbides and nitrides [3,4] is well known, but the rapidity with which this occurs is often not fully appreciated, resulting in erroneous analysis of the oxide composition. This is particularly important in catalysts containing both reduced and fully oxidised phases, where a correlation is needed between particle size and distribution and phase distribution. In this study we have carded out quantitative experiments, varying electron dose rate and total dose using a scanning transmission electron microscope (STEM). We hope to indicate the limits of dose rate and total dose for which no damage is observed during EELS microanalysis of CuO for typical dose rates which may be needed to obtain an adequate signal from small areas. Previous work [5] followed the reduction pathway of CuO to Cu, but the quantitative effect of dose rate, crystal thickness and size on the damage rate were not considered. In the present study, the reduction pathway has not been studied in detail, but more quantitative results have been obtained regarding damage rates.
The experiments were carried out in a VG HB501B STEM operated at 100 kV. The electron probe current was measured for a variety of operating conditions, and varied with size of the probe-forming aperture. The average current density at the specimen was determined by the image magnification (size of area scanned), increasing as a function of the square of the magnification. Electron energy loss spectroscopy (EELS) was used instead of energy-dispersive X-ray spectroscopy (EDS) for several reasons: Firstly it is difficult to obtain reliable values for the correction factors needed to convert O and Cu X-ray peak integrals into composition, since standards of all possible reduced copper oxides would be needed, which would themselves be beam sensitive. Secondly, there will be an appreciable contribution to the signal from surrounding indirectly excited material, whereas the EELS signal comes only from the area under the beam when very thin samples are used and the composition can be calculated without the need for standards. Thirdly, the lower beam current and short analysis time used in order to rninimise reduction during analysis meant that the X-ray spectra contained too few counts for a reliable estimate of the oxygen peak. Although the EELS background is high, several reliable computational methods are available to subtract the background, which are discussed later.
0304-3991/90/$03.50 © 1990 - Elsevier Science Publishers B.V. (North-Holland)
A.K. Petford-Long. N.J. Long / Safe limits for CuO analysis
28
Table 1 Conditions used during electron-beam-induced damage and analysis of copper oxides VOA (/xm)
OA (/xm)
Maguification
Current density
Total current
(A/cM)
(hA)
Damage Analysis
50 -
50
2 x 106 5 x 106
570 114
2.0 0.4
Damage Analysis
50 -
50
5 x 106 1 x 107
3570 714
2.0 0.4
Damage: 50/~m virtual objective aperture (VOA), magnification of 2 x 106 or 5 x 106. Analyse: 50/~m real objective aperture (OA), magnification of 5X106 or lX107.
The specimens were exposed to an alternating "damage" and "analyse" sequence, as shown in table 1. All damage and analysis was carried out using the reduced-area scan mode, with the total beam current kept constant during damage. The analyses were carried out at a higher magnification (but with a much lower beam current) than during "damage", on an area within that previously scanned, so that only material exposed to the high current density would be analysed. To reduce the acquisition time, because serial EELS collection was used, only 600 channels were scanned in windows around the O K edge and the Cu L2,3 edge. The spectra were collected with an effective collection half-angle of 7 mrad (corrected for incident beam divergence). Typically the specimens were damaged for 60 seconds, and the spectra were acquired during the following 60 seconds at 0.2 of the current density used during damage, and so on. Annular dark field (ADF) images were recorded so that changes in appearance of the sample could be correlated with changes in the EELS spectra. Basic spectral manipulation used modified programs based on those of Skiff [6]. Background fitting and edge stripping used Trebbia's programs [7] except when low-count statistics required the use of Egerton's graphical method and "h-factor" appraisal of goodness-of-fit [8]. The edge integration limits used were 100 eV (531-631 eV for the oxygen K edge and 926-1026 eV for the copper L edge). Changing the integration limits to exclude the near-edge fine structure in the edges did not
alter the results observed, within experimental error. The atomic fraction of each element was then calculated using the Cu and O partial cross sections calculated by Rez [9]. A quantitative measure of the C u : O ratio was obtained for each spectrum, with standard errors. The shape of the CuL2,3 edge gives a direct indication of the oxidation state of the Cu atoms, with the "white lines" seen for CuO decreasing in height as the oxide is reduced (the L2,3 edge for Cu metal shows no white-line intensity). However, the change in edge shape occurs very gradually, making it extremely difficult to extract an accurate measure of composition. The literature gives a wide range of experimental values for the ratio of the L: and L 3 white-line heights, e.g. refs. [5,10], ,10 2
1.50
I
I
I
I
/:=o
O
1.20-0.90--
Cu
0.60
0.30-0.00
I
450 x 102 1.50
I
I
600
750
900
I
I
I
I
1050
1200
I
f.=b~ns
1.20 ~ ~
0.90
0 rj
0.60 0.30
0.00 450
I 600
I 750
I 900
I 1050
1200
ENERGY LOSS [eV] Fig. 1. Electron energy loss spectra from CuO crystal before and after exposure to the electron beam. (6 rain damage at current density 570 A / c m 2 plus 7 min at a lower current density during acquisition of intermediate spectra.)
A.K. Petford-Lon$ N.J. Long / Safe limits for CuO analysis
for both CuO and Cu20, due probably to differences in the experimental conditions used in different laboratories. This is the primary reason for analysing both the Cu L2,3 and O K edges by measuring the peak integral under each edge. The specimens were of crushed bulk oxide deposited on a holey carbon film on a mesh grid. A comparison between specimens made by dry crushing and by crushing under high-purity acetone was made, but no difference in results was observed.
3. Results EELS spectra from CuO before and after 6 rain exposure to an electron beam of current density 570 A / c m 2 are shown in fig. 1. The shape of the CuL2,a edge has changed, indicating a change in valence state of the copper in the oxide, and the height of the O K edge has decreased, due to
29
oxygen loss. The oxygen atomic fraction in the crystal versus total electron dose for the same sequence is shown in fig. 2. Also shown in fig. 2 is the change in number of Cu counts with total electron dose. As damage rate may depend on the thickness of the analysed crystal, results were obtained for "thin" ( t < 1 0 urn) and "thick" ( t > 3 0 urn) crystals. The thickness of several crystals was measured using the low-loss region of the EELS spectrurn prior to starting the damage experiments, and the total number of counts in the Cu and O edges was noted. The thickness of damaged crystals could then be estimated by comparison of the number of Cu and O counts with those for a particular crystal thickness. A comparison of the change in oxygen content for "thin" and thick" crystals after exposure to the same total electron dose (at the same current density) can be seen by comparing figs. 2 (thin crystal) and 3 (thick crystal). Thick crystals are "'
0.6
0.5
0.4 tO
E 0.3 a o) x o
t-1 O O
""',,.. .o."°
i
..... -B...
12. "-,..
.-
--.
8
",.l~f. o"
0.2
0.1
dose (C/sq nm)
Fig. 2. Graphs showing change in oxygen atomic fraction (top curve) and change in total number of counts in copper peak (bottom curve, arbitrary units) for CuO crystal with total electron dose at a current density of 570 A / c m 2 (thin crystal). Same damage sequence as fig. 1.
30
A.K. Petford-Long, N.J. Long / Safe limits for CuO analysis 0.6
5000 0.5 4000
0.4 C O
aooo ~,
E 0.3
0 0
tO
i
D.
8
O
2OOO
0.2
|000
0.1
• 0.5E10
'
" 1.0E10 dose
' (C/sq
' 1.5E10'
' 2.0E10
'
nm)
Fig. 3. Graph showing change in oxygen atomic fraction and total number of Cu counts (arbitrary units) for CuO crystal with electron dose (current density 570 A/cm2, thick crystal).
more resistant to oxygen loss, as would be expected, especially if the reduction is associated with a surface effect. Data for exposure to an electron beam of current density 3570 A / c m 2 is shown in fig. 4. This shows the change in oxygen content and total number of copper counts for a case in which an electron-beam-damage-induced hole formed in the specimen (current density 3570 A/cm2). In contrast to the earlier figures, the data in fig. 4 initially shows a relatively constant composition ( O : C u ratio) and a rapid decrease in the total number of Cu counts, followed by a loss of oxygen and further Cu loss. An A D F image corresponding to the damage sequence shown in fig. 4, after exposure to the electron beam, is shown in fig. 5. Bright contrast arising from copper metal which has diffused away from the irradiated area during "damage" can be seen around the edges of a darker electron-beam-damage-induced hole in the specimen.
4. D i s c u s s i o n
The data presented in this study were recorded using a STEM in which the probe is scanned over an area and the EELS data is acquired in serial mode. The probe has a finite size and current distribution, so that for a fixed probe the material under the beam will see a variation in current density. The " s p o t " mode (stationary electron probe) was not used for the following reasons: (a) A detailed analysis of the probe shape and current distribution would be needed since composition changes sensitive to the absolute beam current would be convoluted in the EELS spectrum. (b) A probe size of 2 nm or less and total beam current 1 nA would give a current density of approximately 30 000 A / c m 2, resulting in an unacceptably high damage rate or such short acquisition times that the signal will be too weak for adequate statistics for composition measurement. Previous work has shown that analysis of both the Cu and
A.K. Petford-Long, N.J. Long / Safe limits for CuO analysis
31
0.6
4500 4OOO
0.5
3500 0.4 tO
2so0 O
0.3 ~ ~4 0
0.2
"''",}
........... L................ m.............. -m..............
m.,. .~,, ,..,~.l-~ 1500
1000 0.1
500
1E10'
2E10'
3E10'
4E10' 5E10' 6E10' dose (C/sq nm)
7E10'
8E10'
9E10'
Fig. 4. Graph showing change in oxygen atomic fraction and total number of copper counts (arbitrary units) with total electron dose at a current density of 3570 A / c m 2.
Fig. 5. Annular dark-field image of CuO crystal after electronbeam-induced damage at a current density of 3570 A / f r o 2.
O edges is necessary - impossible with a fixed probe without using a parallel data collection system [11]. Increasing the collector aperture size increases the number of electrons detected, but not necessarily the peak-to-background ratio [12]; the easiest solution is thus to scan the probe so that the beam is averaged over a larger area than the probe size (for magnifications less than 1 × 107× ). The averaged current density and damage rate are drastically reduced, allowing longer count times, thereby improving the statistics. Due to the way in which an analogue time-base (as used here) operates, there will be areas at the start of the frame and along the top of the frame which receive a significantly higher dose than the rest of the scanned area analysed, which has undergone uniform irradiation (this will give a systematic error in the calculated dose). In this study we have considered the average current density in
32
A.K. Petford-Long, N.J. Long / Safe limits for CuO analysis
the scanned area as the total integrated beam current dividing by the area of the scan. Care has to be taken that an area is not scanned during "damage" at a very low magnification and then analysed at a very high magnification, as it might be possible to analyse an entirely unexposed region of material between the low magnification scan lines. A possible electron-beam-induced mechanism which explains the observed results is surface reduction of the CuO to a reduced oxide or Cu by an electron-stimulated-desorption reaction such as the Knotek-Feibelman mechanism (known to result in the reduction of maximal valence oxides [13]), followed by migration of Cu over the particle surface to the edges of the scanned area. Migration of Cu away from the scanned area will give a linear decrease in the total number of Cu counts and will also reveal fresh surface layers of oxide which can be reduced, leading to further migration, and so on until a hole is formed. The relatively constant oxygen content, followed by a sharp drop, will also be explained by this model - initially the ratio of unreduced oxide to surface Cu will be very high, so that the majority of the EELS signal will be from bulk CuO. As reduction proceeds, the crystal will get thinner, but migration of Cu away from the analysed area will mean that the signal still comes predominantly from CuO, with only a small effect from the surface metal layer. The apparent oxygen content will therefore only drop slightly below 50 at%. When the majority of the oxide under the beam has been reduced, the atomic percentage of oxygen will fall rapidly. The apparent oxygen content will only drop to zero provided that complete reduction of a substantial region under the electron beam occurs before migration of copper results in the formation of a hole through the crystal.
5. Conclusions It is possible to obtain chemical information from CuO at a high spatial resolution (1-2 nm)
using STEM and EELS, provided that the counting time used to collect the spectra is long enough to ensure adequate statistics for composition measurement. This is important since the choice of "safe" beam current, current density and operating conditions must be made with statistics in mind. For specimens with a thickness of 10 nm or more, keeping the incident electron beam current density below 570 A / c m 2 would allow analysis time of up to approximately 4 min - ample time to collect EELS spectra with sufficient counts to obtain a realistic measure of composition.
Acknowledgement We are grateful to Prof. Sir Peter Hirsch for provision of laboratory space.
References , [1] Y. Yokamoto, K. Fukino, T. Imanaka and S. Teranishi, J. Phys. Chem. 87 (1983) 3740. [2] T. van Herwijnen and W.A.J. de Jong, J. Catal. 63 (1980) 83. [3] P.A. Crozier, J.N. Chapman, A.J. Craven and J.M. Titchmarsh, in: Analytical Electron Microscopy - 1984, Eds. D.B. Williams and D.C. Joy (San Francisco Press, San Francisco, 1984) p. 79. [4] L.E. Thomas, in: Analytical Electron Microscopy - 1984, Eds. D.B. Williams and D.C. Joy (San Francisco Press, San Francisco, 1984) p. 247. [5] N.J. Long and A.K. Petford-Long, Ultramieroscopy 20 (1986) 151. [6] M. Skiff, PhD Thesis, Arizona State University, 1986. [7] P. Trebbia, Ultramicroscopy 24 (1988) 399. [8] R.F. Egerton, Electron Energy Loss Spectroscopy in the Electron Microscope (Plenum, New York 1986). [9] R.D. Leapman, P. Rez and D.F. Mailers, J. Chem. Phys. 72 (1980) 1232. [10] T.G. Sparrow, B.G. Williams, C.N.R. Rao and J.M. Thomas, Chem. Phys. Lett. 108, (1984) 547. [11] See, for example, R.F. Egerton, J. Electron Mierosc. Tech. 1 (1984) 37. [12] D.C. Joy in: Principles of Analytical Electron Microscopy, Eds. D. Joy, D. Romig and J. Goldstein (Plenum, New York, 1986) p. 256. [13] M.L. Knotek and P.J. Feibelman, Phys. Rev. Lett. 40 (1978) 964.