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XPS study of mica surfaces
Journal of Electron Spectroscopy and Related Phenomena, 036%2048/93/$06.00 0 1993 - Elsevier Science Publishers
63 (1993) 289-306 B.V. All rights reserved
289
XPS study of mica surfaces’ Krishna Department
G. Bhattacharyya of Chemistry,
(First received
Gauhati
University,
Guwahati-781014,
India
13 July 1992; in final form 20 April 1993)
Abstract Muscovite mica surfaces were studied with X-ray photoelectron spectroscopy. The aircleaved mica surfaces showed a large Cls peak in the range 285.3-285.9eV indicating the presence of carbon species other than the adventitious. Normal emission and grazing emission spectra were utilised to calculate bulk and surface elemental ratios respectively for air-cleaved as well as vacuum-cleaved mica surfaces. Preferential attenuation of some peaks has been proposed for a few abnormal elemental ratios. Apparent shifts in binding energy due to sample charging were investigated for differently treated mica surfaces. Changes in the apparent shift are shown to be due to the existence of strong dipole fields at the surface resulting in a potential drop of as much as 3.6V across the dipole layer.
Introduction
The crystal structure of mica is very well established [l-3] and several excellent reviews [4-71 have covered the salient features of the structural characteristics of mica. The basic structure of mica, KAlz Sig AIOlo (OH),, consists of three layers: an octahedral layer (of mostly aluminium) is sandwiched between two identical tetrahedral layers, each of composition (Si, A%&&, with the vertices of both pointing inward. The neighbouring tetrahedra mutually share the basal oxygens which form a hexagonal mesh. A few unshared hydroxyl groups along with the apical oxygens make. the common plane of junction between the tetrahedral and the octahedral layers. The composite 2:l sheet is not electrically neutral as approximately one third of the tetravalent silicon atoms of the tetrahedral layers are substituted by trivalent aluminium atoms. The resultant charge is balanced by a layer of potassium ions in muscovite mica. The edge-on view of the layered structure of muscovite mica is shown in Fig. 1. One quarter of the tetrahedral sites is occupied by aluminium atoms, yet ‘The experimental part of this work was done at the Department College, London SW7 2AY, UK.
of Chemistry,
Imperial
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A
Si,Al 0,OH
2.120
A
2.258
ii
3.370
x
, I
SI,Al 0
A
, I
Al 0,OH
2.258
7 1
K+ v
Fig. 1. Edge-on view of the muscovite mica structure.
there is apparently no ordering of the silicon and the aluminium atoms [8]. One half of the tetrahedral sites have been proposed to be fully occupied by silicon atoms while the other half are occupied by Si,,, Alliz on average. Potassium ions sit on the cavities created by the opposing oxygen hexagons of the basal planes of the Si, Al tetrahedra. The average K-O bond length in muscovite is 3.09A, which is larger than the sum of the ionic radii (2.95A) of potassium and oxygen. This indicates that potassium ion is not in true 12-fold coordination and the structure is somewhat distorted. Detailed X-ray analysis [9] also shows that, (i) the silicon and the aluminium atoms of the tetrahedral layers are on two different planes, each parallel to the (001) plane and the Si-plane being 0.12 A closer to the octahedral plane, (ii) the apical oxygens and the hydroxyl groups also exist in two different planes, separated by 0.06 A, and one-third of the octahedral sites are vacant. The potassium ions are nearer to one oxygen basal plane (mean K-O distance, 2.812A) than to the other (mean K-O distance, 3.389 A) [lo-121. Easy cleavage of mica along the (001) plane is the potassium layer, excellent insulating properties, reasonable thermal stability and its chemically inert nature have made mica an important substrate for epitaxial
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vapour deposition studies [13,14].Most of the work on mica has centred around this line of interest. Poppa and Lee [15] observed no change in the oxygen signal on Auger analysis of freshly cleaved and annealed mica surfaces, but the latter surface showed appreciable depletion of potassium from the surface. Muller and Chang [16]found that a vacuum-cleaved mica surface charged to a potential greater than 1OOOVwhich could be avoided by cleaving mica in 10 Torr pressure of Hz, 02, etc., or by heating the surface at above 100°C after cleaving in vacuum. These authors also showed that all mica surfaces demonstrated secondary charging on being exposed to an electron beam. The air-cleaved mica surface has been known to have a carbon contamination [13,17]which may be due to carbon dioxide in the atmosphere 1181, while the vacuum-cleaved surface is almost contaminant-free. The problem of surface charging appears to have dissuaded workers from using XPS to study the mica surface. The few XPS studies [19,20] indicate that the photoelectron peaks are displaced by about 7.5 eV due to charging. The present work is aimed at using XPS to study the mica surface in some detail. Experimental Photoelectron spectra were obtained with a VG Scientific ESCALAB Mark II system [21]. The analysis chamber of the system, having the 150 o hemispherical analyser, was kept routinely at UHV in the region of 10-l’ Torr (1 Torr = 133.3 N me2), while the preparation chamber, used for sample cleaning, annealing and other preparation techniques, was at a slightly higher pressure in the range 1O-g-1O-8 Torr. Sample introduction was done through a fast entry air-lock in the preparation chamber. The X-ray source consists of a twin magnesium-aluminium anode which was operated with the filament at earth potential and the anode at a positive potential of up to 15kV. The X-ray gun is mounted on a linear motion drive and spectra were taken by placing the source as near as possible to the sample. Data acquisition and processing were carried out with an Apple 2e microcomputer equipped with VGS 1000 data system. 1 cm x 1 cm mica samples were cut from less than 0.25 mm thick muscovite (grade 5) sheet. On one side of the samples, corners were cut away to leave a raised octagonal area in the middle as shown in Fig. 2. This raised portion could be cleaved with the help of a silver wire loop attached to one edge through a very fine hole. The sample was kept pressed in position on a nickel stub with four tantalum clips spotwelded to the stub-holder. After locking the stub-holder in one of the high precision manipulators in either of the chambers, the top face of the sample could be cleaved with the help of a built-in manipulator. The sample could be cleaved once only in this way and if a freshly cleaved surface was required, a new sample was introduced.
292
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Gmple
63 (1993) 289-306
Tantalum clips
Portion to be cleaved
Metal stub
Fig. 2. Mica sample for XPS study.
Sample heating was done by placing the sample holder on a P8 probe in the preparation chamber. In this study, the achromatic Al KQ source was mainly used at an energy of lo-12 kV with an emission current of 5, 10 or 20 mA. The sample holder was kept earthed. The analyser was operated in the constant analyzer energy (CAE) mode with a 6mm slit and the channeltron was set to a voltage of 2.5-3.0 kV. Results Surface
and discussion charging
and the choice of a reference
A non-conducting sample charges up to an appreciable positive potential at its surface during XPS measurements. Such materials do not have sufficient delocalised, conduction-band electrons to neutralise the charged centres which build up from the cluster of positive holes created on photo-
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electron ejection. The resultant positive potential retards or drags the ejected photoelectrons, causing an apparent, additional positive shift, either subtracting from the uncharged kinetic energy or, correspondingly, adding on to the normal binding energies of the photoelectrons [ZZ]. The apparent binding energy EL in such cases, is related to the true binding energy Es by the relation
EB=E;3-C where C is the apparent shift in binding energy due to charging. The bremsstrahlung component of the achromatic X-rays produces a background of low energy electrons (~5 eV) by striking at the walls of the chamber and also at the X-ray gun window. These electrons neutralise a part of the surface charge and within a few seconds, an equilibrium steadystate static charge is established on the sample. The binding energies of constituents as recorded by the spectrometer are measured with respect to the vacuum level of the insulator rather than the Fermi level, and this introduces some amount of uncertainty in the values. The nature of the residual static charge and its effects on XPS measurements for insulating samples have been dealt with elaborately by Evans et al. [23], Lewis and Kelly [24], Swift [25], Swift et al. [26], Kohiki and Oki [27] and many others. The surface charging not only reduces the kinetic energy of the photoelectrons, but also broadens the peaks. Differential charging arising due to differences in insulating properties of the constituents enhances the peak broadening. The apparent shift in binding energy can be eliminated by neutralising the surface charge with a low energy electron beam from an electron flood gun [28-311. In the present work, the ESCALAB has a flood gun for use with the monochromator (footprint ~1 mm x 6mm) and is not suitable to adequately handle surface charging over the larger area irradiated by the twin anode. A suitable reference was thus necessary to assign binding energies to the constituents of mica. The Cls peak in the range 284.4284.8 eV, from an adventitious hydrocarbon layer due to pump oil contamination, has been widely used as a reference, but this suffers from a number of disadvantages arising from the uncertainty in the chemical state of the carbon species and the thickness of the carbon overlayer [25,32-351. Some workers also used a Cls binding energy of 285.0 eV or more as the reference [36-381, thus leaving a considerable amount of unreliability in the interpretation of binding energy data for the insulators. For mica, the Cls reference is not suitable as all air-cleaved surfaces contain some carbon contamination which is perhaps in a different chemical state from that of the adventitious carbon [18]. The vacuum-cleaved surface, however, is free from carbon contamination. Another charge referencing method consists in depositing a thin film of
294
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gold or other noble metals onto the insulator surface and then using Au4f or other suitable levels as reference_ The uncertainty about good electrical contact between the deposited film and the substrate and also the possibility of chemical interaction between the metal and the substrate, have, however, raised some questions about this method [39-411. In XPS measurements of zeolites, silica-aluminas and clay minerals, the Si2p peak has also been used as a convenient reference [42-451. Seyama and Soma [46] have also used the Si2s level as a reference for the montmorillonites. Kantschewa et al. [47] have shown that the potassium energy levels are insensitive to chemical shift and used the K2paj2 level as an internal reference for the XPS study of K,COs/y-Al& system. In the present work, K2p3,s binding energy was used as the reference level. The energy of this level was determined with respect to Pt4f7,2,5,2 levels by first depositing a platinum film on the mica surface and then estimating the apparent shift in these levels due to charging. The Si2p levels showed splitting after platinum deposition and were therefore not chosen for use as reference levels. A possible cause for this splitting might be the formation of platinum silicides which was, however, not investigated. Any possible impact of the shift of the platinum levels on calibration of the reference level was considered minimal and to have affected the calibration of other levels to the same extent. Platinum was evaporated onto both air-cleaved and vacuum-cleaved mica surfaces from a platinum source heated by electron bombardment (2 kV accelerating potential, 8 mA emission current, 12 min evaporation time). The evaporation source was previously calibrated to give a deposit of one monolayer of platinum on a nickel single crystal under identical conditions and the Pt4f,,s and 4fs/s levels were determined respectively to be 71.1 and 74.3 eV. On Table 1 Apparent Platinum levels
4f5/2
4f7/2
shift of PMf peaks on mica due to surface charging” Platinum on air-cleaved mica surface
Platinum on vacuum-cleaved mica surface
Normal emission
Normal emission
Grazing emission
Grazing emission
Apparent
Shift
Apparent
Shift
Apparent
Shift
Apparent
Shift
EBfeV
C/eV
EB/eV
CleV
EBleV
C/eV
EdeV
C/eV
80.30 77.20
6.00 6.10
81.25 73.00
6.95 6.90
80.50 77.40
6.20 6.30
81.50 78.25
7.20 7.15
“The shifts are calculated on the basis of Pt4f5j2 and 4f7,2 binding energies of 74.3 and 71.1 eV respectively. The values are found to have a variance of f0.3 eV from a number of measurements
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Table 2 Binding energies EB/eV of the principal constituents of muscovite mica obtained with an Al Ka source with K2p3jz energy = 293.75 * 0.20 eV as the reference and comparison with published data Data from
This worka Schultz et al. [48Jb Castle et al. [19]’ Wagner et al. [491d
Aluminium
Silicon
2P
2s
2P
2s
74.40
119.45
102.65 108.30
153.50 159.50
74.50
103.70e
74.05
102.16
Oxygen 1s
Carbon 1s
531.65
285.55
533.20
284.80
531.23
284.60
* The binding energies are mean values of a number of measurements on both air-cleaved and vacuum-cleaved mica surfaces. b These values were not corrected for charging. ’ These values were obtained assuming a constant shift of 7.5 eV due to charging. d Reference here was to Cls = 284.60 eV. e This appears to be a wrong value.
mica, the platinum peaks had an apparent shift of between 6 and 7 eV due to charging (Table l), the shift being greater for grazing emission XPS. The mean of the apparent shifts of the Pt4f levels was used to correct the K2p3j2 binding energy giving it a value of 293.75 f 0.20eV for both aircleaved and vacuum-cleaved surfaces. On the basis of K~P~,~ reference the binding energies of the principal constituents of mica are given in Table 2 along with a few published data due to other workers [19,48,49]. The agreement is reasonable. Tempere et al. [50] have reported A12s and Si2p energies in zeolites respectively in the ranges 119.3-119.9eV and 102.1-103.5 eV with Cls = 285.0eV reference. For a large number of zeolites and clay minerals, Barr [51] has shown the A12p and Si2p energies (referenced to Cls = 284.4 eV) to lie in the ranges of 73.2-74.6 eV and 100.9103.35 eV respectively. However, some uncertainty is inevitable in assigning binding energies to the constituents of an insulator and small shifts in core level peaks would be impossible to detect in these cases. Typical photoelectron
spectra
A typical photoelectron spectrum (grazing emission) of a muscovite sample cleaved in air immediately before introduction into the system is shown in Fig. 3. All the major constituents of mica could be seen to give XPS peaks. Some magnesium and sodium could also be detected from their X-ray induced Auger peaks (verified by taking the spectrum with the Mg Ka source when the peaks were displaced by 233 eV towards lower binding energy). This is in agreement with the fact that small amounts of
296 L”‘.“‘
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~“““‘.““““““““““““““~“““‘I”““““”
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63 (1993) 289-306
.I”“““‘I”“““‘I“’
-.
4 ”
0
t....,..~.I....l....l....r....l....~ 100 2ciJ 300 Bwdbg
. ..I...~r~..~‘..~.I..~~‘....I....’..~~’.~~.t....‘~-~. 402
500
6c0
700
800
900
loo0
energy(mcorrect.ed l/eV
Fig. 3. Photoelectron spectrum of an air-cleaved mica surface at grazing emission. The Al Kcu source was at 11 kV energy and 10 mA emission (maximum count rate 10 790, analyser energy 50 eV, step size 0.25 eV, time 40 s x 10 scans, signal to noise ratio 223:l). The binding energies are uncorrected.
sodium and magnesium replace potassium (interlayer positions) and aluminium (octahedral positions) respectively. The spectrum shows satellites of prominent peaks due to the Kas,a4 components of the X-radiation. The massive Cls peak in the spectra of air-cleaved mica samples was found to have a binding energy of 28525285.85 eV (K2p3j2 = 293.75 eV) with FWHM of 1.65-2.00 eV from a series of measurements. The Cls peak due to adventitious hydrocarbon contamination has been assigned an energy in the range 284.4-284.8 eV by various authors. It therefore appears that the Cls peak for the air-cleaved mica has at least some contribution from carbon species other than the adventitious hydrocarbon [18]. The mica surface, immediately after cleaving at 2 x 10-l’ Torr, yielded a photoelectron spectrum as shown in Fig. 4. The reduction in the Cls peak is a prominent feature of this spectrum. The small peak still appearing might have been picked up from the air-cleaved corners by the X-ray beam which has a diameter of almost 1 cm. All the other peaks become more prominent after the cleaving. Thus the carbon contamination was responsible for some degree of screening of the XPS signals of the air-cleaved surface.
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I”“““‘I”“““‘I,““‘,“l”‘l”‘,‘I”“““’1.“~”“’I.‘~“’.“I’.
63 (1993) 289-306
297 ‘I”‘,
t i
BindUg
ener~(uncorrected)/eV
Fig. 4. Photoelectron spectrum of a freshly vacuum-cleaved mica surface at grazing emission. The binding energies are uncorrected (Al Ka source at 11 kV, 10 mA emission, maximum count rate 19680, analyser energy 50eV, step size 0.25 eV, time 40 s x 10 scans, signal to noise ratio 634:l).
The vacuum-cleaved surface also revealed the presence of some iron which is likely to be in either tetrahedral or octahedral positions. Relative
abundance
calculations
Problems associated with the use of XPS measurements to calculate relative abundances of constituent elements have been well documented [52-541. Peak area ratios, however, continue to be used for this purpose. Relative concentrations of any two elements, A and B, can be compared using the elementary relations
(2) where CA and cB are the COnCentratiOnS Of the tW0 deInentS, IA and 1s are the intensities of their prominent XPS peaks, and 23, and SB are the relative sensitivities for the peaks with respect to the spectrometer. Normalised peak area ratios for air-cleaved and vacuum-cleaved mica surfaces, calcu-
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lated on the above basis, are given in Table 3. For this purpose, each peak was scanned separately under the identical conditions of 10eV sweep width, 20 eV analyser energy, 0.05 eV step size, and 10 s x 25 scans. Background subtraction and peak area determination were done using the VGS 1000 programme. The relative sensitivity data for the ESCALAB were not determined and instead the following data of Wagner et al. [55] were used for calculations: Fls, 1.00 as the standard; Cls, 0.25; A12p, 0.185; Si2p, 0.27; K2p, 0.83. Table 3 has two sets of data for normal and grazing emission modes. The normal emission data, which are less surface sensitive, should represent bulk composition of mica whereas the grazing emission data should indicate the surface composition. According to Cox et al. [56] the mean free path A of electrons in a large number of insulating and semi-conducting oxides follows approximately the relation x = 0.35 x E3
(3)
where E is the kinetic energy of the photoelectron in electronvolts and X is in Angstriims. On this basis, X would be nearly 13A for electrons contributing to the SiXp and A12p peaks and 1.2Hifor those contributing to the K2p3i2 peak. For normal emission, the attenuation of photoelectrons originating at a distance D below the surface is equal to exp (-D/X) [57]. Thus at a depth of 10 A, which corresponds to the distance between cleavage planes in mica, the attenuation is 0.5. Although this attenuation is considerable, the normal emission XPS data should be fairly representative of the bulk structure of mica as far as A12p, Si2p and K2p,,, peaks are concerned. Thus, the K2ps@i2p ratio of 0.34-0.35 found in this work is close to the elemental ratio of 0.30 for the bulk mica. However, a large discrepancy exists between the experimental A12p/Si2p ratio (x0.55) and that calculated on Table 3 Normalised
peak area ratios in muscovite
Ratioa
Normal
A12pf Si2p K&/2
ISi%
ClsjSiZp A12p(AC)/A12p(VC) Si2p(AC)/SiBp(VC) Cls(AC)/Cls(VC) Air-cleaved
emission
mica surface;
data
Grazing
emission
Air-cleaved
Vat-cleaved
Air-cleaved
Vat-cleaved
surface
surface
surface
surface
0.54 0.34 4.54
0.57 0.35 0.15
0.30 0.24 3.29
0.69 0.30 0.25
0.33
K~P,/,(AC)/K~P,,~(VC)
‘AC,
mica from XPS intensity
0.26
0.34 0.34 10.64 VC, vacuum-cleaved
0.60 0.48 7.93 mica surface.
K.G. BhattacharyyaiJ. Electron Spectrosc. Relat. Phenom. 63 (1993) 289-306
299
the basis of mica composition (0.87). The reason for this is not clear, but the discrepancy may be due to one or all of the following possibilities. (a) The relative sensititivity factor used for A12p may not be appropriate as a large part of aluminium in mica is in octahedral coordination, a type of bonding not found in the reference compounds used in determining the relative sensitivity data by Wagner et al. [55]. (b) The attenuation of photoelectrons originating from the octahedral layer at normal emission may be greater than supposed because of steric blocking by the tetrahedral layer of atoms immediately above. (c) Wertheim [58] has recently pointed out that the relative sensitivity factors do not provide a reliable basis for quantitative chemical analysis. The grazing emission data for the vacuum-cleaved surface showed a much larger A12p/Si2p ratio of 0.69. In this mode, the photoelectrons leave the surface at an angle of approximately 10 o to the surface plane and the attenuation will be equal to exp (-D/X sin loo). Even at a depth of 3 A below the surface, the attenuation is 0.26 and the photoelectrons will originate effectively from the top two layers of atoms in mica. Only the tetrahedrally coordinated layer with the associated potassium inter-layer ions are thus likely to be detected in grazing emission. For this layer, the elemental ratios should be Al/Si = 0.29 and K/Si = 0.30. The K2p3#i2p ratio from this work agrees well with this value, but the A12p/Si2p value (0.69) for the vacuum-cleaved surface is more than twice as large. This anomalous result is also likely to be due to steric blocking arising from the particular structure of mica. The aluminium atoms of the tetrahedral layer, being 0.12A closer to the surface [9] than the silicon atoms, apparently block the photoelectrons emerging at grazing angle from the silicon atoms and thus reduce the Si2p signal with consequent apparent enrichment of the A12p signal. Significantly, the A12p/Si2p ratio for the aircleaved surface at grazing emission is almost equal to the theoretical value for the tetrahedral layer. The presence of the carbon layer seems to have caused the tetrahedral Si and Al atoms to relax so that they are in the same plane equidistant from the surface. The reduction in the intensities of normal emission XPS peaks for the aircleaved surface by a factor of three in comparison to those for the vacuumcleaved surface cannot possibly be attributed to the carbon overlayer alone. A layer of carbon, about 10 A thick, would be required to give this tremendous attenuation. Such massive deposition of carbon was unlikely although the XPS spectrum indicated a considerable amount of carbon on the air-cleaved surface. It is also likely that the carbon atoms sit directly above the Al, Si, and K atoms in such a way as to screen photoelectrons emitted from these atoms at normal incidence. The influence of the carbon overlayer on the XPS peaks at grazing emission is different. As seen from Table 3, the influence now appears to be much more
300 Table 4 Apparent micaa Mica surfweb
K.G. BhattacharyyalJ.
En
FWHM
1.65
79.20
2.85
1.65
(74.40) 80.50
1.65
A
290.35
B
(285.55) 292.05
E
(286.00) 292.55 (285.60) 291.70 (285.80) 292.70 (285.90)
Si2p
A12p
Cls FWHM
D
63 (1993) 289-306
shift of core level binding energies (in eV) of the principal XPS peaks of muscovite
En
C
Electron Spectrosc. Relat. Phenom.
2.20 1.90 2.05
(74.50) 81.40 (74.45) 80.30 (74.40) 81.45 (74.45)
1.80 1.85 1.90
EB 107.45 (102.65) 108.60 (102.55) 109.50 (102.55) 108.50 (102.60) 109.65 (102.65)
a All data refer to grazing emission; corrected values of Es are given in parentheses reference). b See text.
FWHM 2.05 2.00 2.05 2.05 2.05
(K2p3,z
pronounced in the case of the AlZp. For the air-cleaved surface, the AlZp intensity dropped by a factor of four, while the Si2p and K2p3i2 intensities were much less affected. The larger value of Cls/SiZp peak area ratio for the air-cleaved surface in normal emission compared to that in grazing emission shows the carbon contamination to be bulky and non-homogeneous. Surface charging
and apparent binding energy shift
It was observed that the apparent shifts in binding energy due to charging were much more for the vacuum-cleaved surface than for the air-cleaved surface. Another interesting observation was that heating of the samples further shifted the peaks to higher binding energy while exposure to water vapour or ammonia decreased the shifts. The apparent shift oscillated between 4.5 and 7.0 eV depending upon the history of the mica surface. The apparent and corrected binding energies of the principal peaks for the mica surfaces (at grazing emission) are given in Table 4. The following surfaces were studied in sequence: A, air-cleaved surface; B, freshly vacuum-cleaved surface; C, surface after heating to 600K for 30min; D, surface exposed to 18OOL of water vapour (1 L = lop6 Torr s); E, surface reheated to 600 K for 30mins. Each peak was scanned individually under the same set of conditions (10 eV sweep width, 20 eV CAE, 10 s x 25 scans, 0.05 eV step width, same beam energy and emission current). Repeating the sequence produced identical results. The peaks corresponding to mica surfaces A, B, and C are shown in Fig. 5,
K.G.
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=P~/z -J&i
Electron
Spectrosc.
Rdat.
298.55 (293.75)
1.85
299.80 (293.75) 300.70 (293.75) 299.65 (293.75) 300.75 (293.75)
1.80 1.80 1.80 1.85
&3
FWHM
301.40
1.90
(296.6) 302.65 (296.6) 303.40 (296.45) 302.45 (296.55) 303.50 (296.50)
63 (1993) 289-306
301
01s
=P~/z
FWHM
Htenom.
1.90 1.85 1.85 1.85
EB
FWHM
536.45
2.30
(531.65) 537.55 (531.50) 538.55 (531.60) 537.45 (531.55) 538.65 (531.65)
2.35 2.30 2.35 2.35
which clearly shows the apparent shifts for each peak. The surfaces D and E produced peaks in almost the same positions as B and C respectively. The K2p3,~~,~ doublet was well resolved with an energy separation of 3 eV. The Si2p peak showed a shoulder on the low binding energy side with an energy separation of more than 3 eV from the principal peak. This shoulder was found to correspond to an energy of 98 eV after correction for apparent shift and could be assigned to Al KL2,3L2,3 Auger peak induced by the bremsstrahlung component of the X-radiation (the characteristic lines of an Al Ka: source are too near to the kinetic energy of an Al KL2,3L2,3 electron to excite this Auger transition). Many authors [59,60] have discussed the appearance of such bremsstrahlung induced Auger peaks in XPS measurements using an achromatic source. When the spectra were taken with the Mg source this shoulder on the Si2p peak disappeared proving conclusively that it was the Al KL2,3L2,3 Auger peak. The shifts in the apparent binding energies appear to be associated with the presence or absence of adsorbed layers on the mica surface. Thus the large increase in apparent binding energy of all the photoemission peaks after cleaving in vacuum may be due to the absence of the carbon contamination layer. Comparing the data for the air-cleaved and the vacuumcleaved surfaces, the carbon layer is responsible for a drop of approximately 1.2 eV in the binding energies of A12p, SiBp, K2p312,1j2and 01s core electrons. Again, when the vacuum-cleaved surface was heated, there was a rise of nearly 1.0 eV in the apparent binding energy. This may be due to removal of water from the surface. Readsorption of water reversed the process and
302
K.G. BhattacharyyulJ. Electron Spectrosc. Relat. Phenom. 63 (1993) 289-306
the apparent binding energy was restored to more or less the previous value. Such behaviour is unknown for conducting samples [61]. The presence of an adsorbed layer on the surface could cause some change in the steady state charge present on the surface during the photoionisation process. The rate of neutralisation of the positive charge by the stray electrons generated by the bremsstrahlung radiation might have been affected. A more likely explanation, however, would be due to change in the dipole fields of the surface. Quite large dipole fields can be expected on the mica surface because of the presence of potassium ions with the corresponding negative charge residing on the tetrahedral aluminium atoms [16]. Molecules adsorbing on the surface will be in a strong field and may become polarised, thus reducing the potential drop across the dipole layer. Alternatively adsorption might cause some movement of the potassium ions with a resultant change in the dipole moment. The uncertainty about the position of potassium ions relative to the aluminium atoms makes it difficult to calculate the potential drop arising from the dipole layer. Considering the dipole layer as a parallel plate condenser, an appropriate estimation of the potential drop can, however, be made from the relationship T/‘=oct where CTis the charge density, d the charge separation, &ois the permittivity of vacuum and E, is the relative permittivity of the medium. The number of potassium ions perm’ on the mica surface is about 2 x 1018. With d = 1 A and E, = 1, the potential drop across the dipole layer is 3.6V. Thus a change of l.O-1.2V due to an adsorbed layer is quite feasible. With the above model, both carbon contamination and water have to be adsorbed in such a way so as to make the outer end of the dipole layer more positive. If the adsorbate molecules were merely polarised in the surface field, this would require the initial fields to be negative outwards, i.e. for the potassium ions to lie deeper in the surface than the corresponding negative charge. However, a small relaxation of the potassium ions outward, induced by adsorption, would produce the desired result, irrespective of the initial direction of the field. Table 4 also shows that the FWHM for all the peaks is much larger compared to that in the case of a conducting sample. The Si2p, 01s and K2p3,2,1,2 peaks have a constant FWHM value for the mica surfaces of different history, but this is not true for Cls and A12p peaks. The A12p peak for the air-cleaved surface was very small and broad. The Cls peak was very small after cleaving and it showed a tendency to broaden after the sample was heated. The 01s and the SiZp peaks were exceptionally broad indicating they might be composite peaks.
K.G.
Bhattacharyya/J.
Electron Spectrosc.
Relat. Phenom.
Al ?Q
c
Msx.Count Rate 464
Max.count
63 (1993) 289-306
IS
,, :
Fate 2009
I1
’\ \ ',
I I
\
I
\ \
I I
3
I
/'A -..-.._.._ _.-...s.“.
285
85
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i
‘.., A__.,,
‘k,
-S.&L I
I
I
1
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EL.%&/.C” C ?,
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,’ ___--
303
295
290
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106
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542
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Fax.Comt
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Rate 1045
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3INDING ENEFGY (UNCORRECTED) / eV ---+ Fig. 5. Apparent shift of grazing emission XPS peaks of muscovite mica due to surface charging: A, air-cleaved surface; B, vacuum-cleaved surface; C, surface after heating to 600 K for 30min in a vacuum. (Non-linear background subtracted.)
It was not possible in this work to resolve Si2~3,~,~,~ and A12p3,2,1,2 doublets because of closeness of their binding energies (~1 eV difference). Although aluminium occurs in both tetrahedral and octahedral environments, no splitting was observed in A12p and A12s peaks. Similarly no
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splitting was observed for the silicon photemission peaks, although silicon occupies two types of site, one with an aluminium neighbour and the other without. Anderson and Swartz [62] have made similar observations for a number of aluminosilicates. A qualitative study of the extent of charging on other insulator surfaces such as molecular sieve 13x, potassium carbonate and silicon carbide (a small amount of the powdered sample was pressed onto a rhenium foil, held by tantalum tags to the stub holder) showed a binding energy shift of between 0.5 and 2eV depending upon the thickness of the layer. It does appear that the sheet structure of mica is responsible for the substantial surface charging in mica. Also the amount of charging did not particularly depend on the thickness of the mica sample; the cleaving actually increased the charging, Conclusions Use of XPS to study the mica surface yields some interesting results. It is shown that XPS data at normal and grazing emission can be used to calculate the relative abundance of various constituents respectively for the bulk and the surface of mica. Discrepancies can be explained on the basis of selective attenuation of some signals. The apparent shifts of the photoelectron peaks due to surface charging do not depend upon the thickness of the sample, but on whether or not there is an adsorbed layer. An adsorbed layer reduces the charging and hence the shift is also reduced. This may be due to changes in the dipole layer brought about by the adsorbate layer. Acknowledgement The author is grateful to Dr. D.O. Hayward, Department of Chemistry, Imperial College for allowing the use of all the facilities in his laboratory and also for assisting with the interpretation of the results.
References 1 2 3 4 5 6
C. Mauguin, C.R. Acad. Sci. (Paris), 185 (1927) 288; 186 (1928) 879, 1131. L. Pauling, Proc. Natl. Acad. Sci., USA, 16 (1930) 123. W.W. Jackson and J. West, Z. Kristallogr., 76 (1930) 211; 85 (1933) 160. L. Bragg, G.F. Claringbull and W.H. Taylor, Crystal Structure of Minerals, Cornell University Press, New York, 1965, Chapter 13. SW. Bailey, in G.W. Brindley and G. Brown (Eds.), Crystal Structure of Clay Minerals and their X-Ray Identification, Mineralogical Society, London, 1980, Chapter 1. S.W. Bailey, Clays Clay Miner., 32 (1984) 81.
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27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45
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W.A. Deer, R.A. Howie and J. Zussman, An Introduction to Rock-forming Minerals, ELBS/Longman, Essex, 1985, pp. 193-205. H.S. Yoder and H.P. Eugster, Geochim. Cosmochim. Acta, 8 (1955) 225. L. Gatineau and J. Mering, Clay Miner. Bull., 3 (1958) 238. E.W. Radoslovich, Nature (London), 183 (1959) 253. E.W. Radoslovich, Acta Crystallogr., 13 (1960) 919; Am. Mineral., 47 (1962) 617. E.W. Radoslovich and K. Norrish, Am. Mineral., 47 (1962) 599. H. Poppa and A.G. Elliot, Surf. Sci., 24 (1971) 149. B. Lewis and J.C. Anderson, Nucleation and Growth of Thin Films, Academic Press, New York, 1978, p. 29. H. Poppa and E.H. Lee, Thin Solid Films, 32 (1976) 223. K. Muller and C.C. Chang, Surf. Sci., 8 (1968) 455; 14 (1969) 39. M-J. Dowsett, R.M. King and E.H.C. Parker, J. Vat. Sci. Technol., 14 (1977) 711; Surf. Sci., 71 (1978) 541. K.G. Bhattacharyya, Langmuir, 5 (1989) 1155. J.E. Castle, L.B. Hazel1 and R.H. West, J. Electron Spectrosc, Relat. Phenom., 16 (1979) 97. C.D. Wagner, D.E. Passoja, H.T. Hillery, T.G. Kinisky, H.A. Six, W.T Jansen and J.A. Taylor, J. Vat. Sci. Technol., 21 (1982) 933. K.G. Bhattacharyya, Ph.D. Thesis, University of London, 1984, pp. 177-187. T.L. Barr, J. Vat. Sci. Technol. A, 7 (1989) 1677. S. Evans, R.G. Pritchard and J.M. Thomas, J. Phys. C, 10 (1977) 2483. R.T. Lewis and M.A, Kelly, J. Electron Spectrosc, Relat. Phenom., 20 (1980) 105. P. Swift, Surf. Interface Anal., 4 (1982) 47. P. Swift, D. Shuttleworth and M.P. Seah, in D. Briggs and M.P. Seah (Eds.), Practical Surface Analysis by Auger and X-Ray Photoelectron Spectroscopy, John Wiley, Chichester, 1983, p. 437. S. Kohiki and K. Oki, J. Electron Spectrosc. Relat. Phenom., 33 (1984) 375. D.A. Huchital and R.T. McKeon, Appl. Phys. Lett., 20 (1972) 158. G. Muller, Appl. Phys., 10 (1976) 317. C.P. Hunt, C.T.H. Stoddart and M.P. Seah, Surf. Interface Anal., 3 (1981) 157. S. Luck, R. Foerch, K.D. Sales and D.S. Urch, J. Chem. Sot., Faraday Trans. 1,85 (1989) 4129. ES. Brandt, D.F. Untereker, C.N. Reilley and R.W. Murray, J. Electron Spectosc. Relat. Phenom., 14 (1978) 113. R.J. Bird and P. Swift, J. Electron Spectrosc. Relat. Phenom., 21 (1980) 227. C.D. Wagner, Applied Surface Analysis, ASTM STP 699, American Society for Testing Materials, Philadelphia, 1980. R.P. Vasquez, J. Electron Spectrosc. Relat. Phenom., 50 (1990) 167. J. Nakatani, S. Ozawa and Y. Ogino, J. Phys. Chem., 93 (1989) 3255. K.C. Dash, B. Folkesson, R. Larsson and M. Mohapatra, J. Electron Spectrosc. Relat. Phenom., 49 (1989) 343. P.J.C. Chappal, D.R. Williams and G.A. George, J. Colloid Interface Sci., 134 (1990) 385. M.F. Ebel and H. Ebel, J. Electron Spectrosc. Relat. Phenom., 3 (1974) 169. C.R. Grinnard and H.M. Riggs, Anal. Chem., 46 (1974) 1306. Y. Uwamine, T. Ishizuka and H. Yamatera, J. Electron Spectrosc. Relat. Phenom., 23 (1981) 55. J.J. Ogilvie and A. Wolberg, Appl. Spectrosc., 26 (1972) 401. D.T. Clark and H.R. Thomas, J. Polym. Sci., 14 (1976) 167. C. Defosse and P. Canesson, J. Chem. Sot., Faraday Trans. 1, 72 (1976) 2565. R. Borade, A, Sayari, A. Adnot and S. Kaliaguine, J. Phys. Chem., 94 (1990) 5989.
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H. Seyama and M. Soma, J. Chem. Sot., Faraday Trans. 1, 80 (1984) 237. M. Kantschewa, E.V. Albano, G. Ertl and H. Knozinger, Appl. Catal., 8 (1983) 71. H.D. Schultz, C.J. Vesely and D.W. Langer, Appl. Spectrosc., 28 (1974) 374. C.D. Wagner, D.E. Passoja, H.T. Hillery, T.G. Kinisky, H.A. Six, W.T. Jansen and J.A. Taylor, J. Vat. Sci. Technol., 21 (1982) 933. J.E. Tempere, D. Delafosse and J.P. Contour, Chem. Phys. Lett., 33 (1975) 95. T.L. Barr, Appl. Surf. Sci., 15 (1983) 1. J.M. Adams, S. Evans, P.I. Reid, J.M. Thomas and M.J. Walters, Anal. Chem., 49 (1977) 2001. M.H. Koppelman, in J.W. Stucki and W.L. Banwart (Eds.), Advanced Chemical Methods for Soil and Clay Minerals Research, D. Reidel, New York, 1980, p. 169. C. Defosse and P.G. Rouxhet, in J.W. Stucki and W.L. Banwart (Eds.), Advanced Chemical Methods for Soil and Clay Minerals Research, D. Reidel, New York, 1980, p. 169. C.D. Wagner, L.E. Davis, M.V. Zeller, J.A. Taylor, R.M. Raymond and L.H. Gale, Surf. Interface Anal., 3 (1981) 221. P.A. Cox, R.G. Egdell, C. Harding, W.R. Patterson and P.J. Tavener, Surf. Sci., 123 (1982) 179. H. Ebel, C. Pohn, R. Svagera, M.E. Wernle, M.F. Ebel and A. Jablonski, J. Electron Spectrosc. Relat. Phenom., 50 (1990) 109. G.K. Wertheim, J. Electron Spectrosc. Relat. Phenom., 50 (1990) 31. J.E. Castle and R.H. West, J. Electron Spectrosc. Relat. Phenom., 18 (1980) 355. C.D. Wagner and A.T. Taylor, J. Electron Spectrosc. Relat. Phenom., 20 (1980) 83. C.R. Brundle and A.F. Carley, Chem. Phys. Lett., 33 (1975) 41. P.R. Anderson and W.E. Swartz, Jr., Inorg. Chem., 13 (1974) 2294.
63 (1993) 289-306 B.V. All rights reserved
289
XPS study of mica surfaces’ Krishna Department
G. Bhattacharyya of Chemistry,
(First received
Gauhati
University,
Guwahati-781014,
India
13 July 1992; in final form 20 April 1993)
Abstract Muscovite mica surfaces were studied with X-ray photoelectron spectroscopy. The aircleaved mica surfaces showed a large Cls peak in the range 285.3-285.9eV indicating the presence of carbon species other than the adventitious. Normal emission and grazing emission spectra were utilised to calculate bulk and surface elemental ratios respectively for air-cleaved as well as vacuum-cleaved mica surfaces. Preferential attenuation of some peaks has been proposed for a few abnormal elemental ratios. Apparent shifts in binding energy due to sample charging were investigated for differently treated mica surfaces. Changes in the apparent shift are shown to be due to the existence of strong dipole fields at the surface resulting in a potential drop of as much as 3.6V across the dipole layer.
Introduction
The crystal structure of mica is very well established [l-3] and several excellent reviews [4-71 have covered the salient features of the structural characteristics of mica. The basic structure of mica, KAlz Sig AIOlo (OH),, consists of three layers: an octahedral layer (of mostly aluminium) is sandwiched between two identical tetrahedral layers, each of composition (Si, A%&&, with the vertices of both pointing inward. The neighbouring tetrahedra mutually share the basal oxygens which form a hexagonal mesh. A few unshared hydroxyl groups along with the apical oxygens make. the common plane of junction between the tetrahedral and the octahedral layers. The composite 2:l sheet is not electrically neutral as approximately one third of the tetravalent silicon atoms of the tetrahedral layers are substituted by trivalent aluminium atoms. The resultant charge is balanced by a layer of potassium ions in muscovite mica. The edge-on view of the layered structure of muscovite mica is shown in Fig. 1. One quarter of the tetrahedral sites is occupied by aluminium atoms, yet ‘The experimental part of this work was done at the Department College, London SW7 2AY, UK.
of Chemistry,
Imperial
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A
Si,Al 0,OH
2.120
A
2.258
ii
3.370
x
, I
SI,Al 0
A
, I
Al 0,OH
2.258
7 1
K+ v
Fig. 1. Edge-on view of the muscovite mica structure.
there is apparently no ordering of the silicon and the aluminium atoms [8]. One half of the tetrahedral sites have been proposed to be fully occupied by silicon atoms while the other half are occupied by Si,,, Alliz on average. Potassium ions sit on the cavities created by the opposing oxygen hexagons of the basal planes of the Si, Al tetrahedra. The average K-O bond length in muscovite is 3.09A, which is larger than the sum of the ionic radii (2.95A) of potassium and oxygen. This indicates that potassium ion is not in true 12-fold coordination and the structure is somewhat distorted. Detailed X-ray analysis [9] also shows that, (i) the silicon and the aluminium atoms of the tetrahedral layers are on two different planes, each parallel to the (001) plane and the Si-plane being 0.12 A closer to the octahedral plane, (ii) the apical oxygens and the hydroxyl groups also exist in two different planes, separated by 0.06 A, and one-third of the octahedral sites are vacant. The potassium ions are nearer to one oxygen basal plane (mean K-O distance, 2.812A) than to the other (mean K-O distance, 3.389 A) [lo-121. Easy cleavage of mica along the (001) plane is the potassium layer, excellent insulating properties, reasonable thermal stability and its chemically inert nature have made mica an important substrate for epitaxial
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vapour deposition studies [13,14].Most of the work on mica has centred around this line of interest. Poppa and Lee [15] observed no change in the oxygen signal on Auger analysis of freshly cleaved and annealed mica surfaces, but the latter surface showed appreciable depletion of potassium from the surface. Muller and Chang [16]found that a vacuum-cleaved mica surface charged to a potential greater than 1OOOVwhich could be avoided by cleaving mica in 10 Torr pressure of Hz, 02, etc., or by heating the surface at above 100°C after cleaving in vacuum. These authors also showed that all mica surfaces demonstrated secondary charging on being exposed to an electron beam. The air-cleaved mica surface has been known to have a carbon contamination [13,17]which may be due to carbon dioxide in the atmosphere 1181, while the vacuum-cleaved surface is almost contaminant-free. The problem of surface charging appears to have dissuaded workers from using XPS to study the mica surface. The few XPS studies [19,20] indicate that the photoelectron peaks are displaced by about 7.5 eV due to charging. The present work is aimed at using XPS to study the mica surface in some detail. Experimental Photoelectron spectra were obtained with a VG Scientific ESCALAB Mark II system [21]. The analysis chamber of the system, having the 150 o hemispherical analyser, was kept routinely at UHV in the region of 10-l’ Torr (1 Torr = 133.3 N me2), while the preparation chamber, used for sample cleaning, annealing and other preparation techniques, was at a slightly higher pressure in the range 1O-g-1O-8 Torr. Sample introduction was done through a fast entry air-lock in the preparation chamber. The X-ray source consists of a twin magnesium-aluminium anode which was operated with the filament at earth potential and the anode at a positive potential of up to 15kV. The X-ray gun is mounted on a linear motion drive and spectra were taken by placing the source as near as possible to the sample. Data acquisition and processing were carried out with an Apple 2e microcomputer equipped with VGS 1000 data system. 1 cm x 1 cm mica samples were cut from less than 0.25 mm thick muscovite (grade 5) sheet. On one side of the samples, corners were cut away to leave a raised octagonal area in the middle as shown in Fig. 2. This raised portion could be cleaved with the help of a silver wire loop attached to one edge through a very fine hole. The sample was kept pressed in position on a nickel stub with four tantalum clips spotwelded to the stub-holder. After locking the stub-holder in one of the high precision manipulators in either of the chambers, the top face of the sample could be cleaved with the help of a built-in manipulator. The sample could be cleaved once only in this way and if a freshly cleaved surface was required, a new sample was introduced.
292
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Tantalum clips
Portion to be cleaved
Metal stub
Fig. 2. Mica sample for XPS study.
Sample heating was done by placing the sample holder on a P8 probe in the preparation chamber. In this study, the achromatic Al KQ source was mainly used at an energy of lo-12 kV with an emission current of 5, 10 or 20 mA. The sample holder was kept earthed. The analyser was operated in the constant analyzer energy (CAE) mode with a 6mm slit and the channeltron was set to a voltage of 2.5-3.0 kV. Results Surface
and discussion charging
and the choice of a reference
A non-conducting sample charges up to an appreciable positive potential at its surface during XPS measurements. Such materials do not have sufficient delocalised, conduction-band electrons to neutralise the charged centres which build up from the cluster of positive holes created on photo-
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electron ejection. The resultant positive potential retards or drags the ejected photoelectrons, causing an apparent, additional positive shift, either subtracting from the uncharged kinetic energy or, correspondingly, adding on to the normal binding energies of the photoelectrons [ZZ]. The apparent binding energy EL in such cases, is related to the true binding energy Es by the relation
EB=E;3-C where C is the apparent shift in binding energy due to charging. The bremsstrahlung component of the achromatic X-rays produces a background of low energy electrons (~5 eV) by striking at the walls of the chamber and also at the X-ray gun window. These electrons neutralise a part of the surface charge and within a few seconds, an equilibrium steadystate static charge is established on the sample. The binding energies of constituents as recorded by the spectrometer are measured with respect to the vacuum level of the insulator rather than the Fermi level, and this introduces some amount of uncertainty in the values. The nature of the residual static charge and its effects on XPS measurements for insulating samples have been dealt with elaborately by Evans et al. [23], Lewis and Kelly [24], Swift [25], Swift et al. [26], Kohiki and Oki [27] and many others. The surface charging not only reduces the kinetic energy of the photoelectrons, but also broadens the peaks. Differential charging arising due to differences in insulating properties of the constituents enhances the peak broadening. The apparent shift in binding energy can be eliminated by neutralising the surface charge with a low energy electron beam from an electron flood gun [28-311. In the present work, the ESCALAB has a flood gun for use with the monochromator (footprint ~1 mm x 6mm) and is not suitable to adequately handle surface charging over the larger area irradiated by the twin anode. A suitable reference was thus necessary to assign binding energies to the constituents of mica. The Cls peak in the range 284.4284.8 eV, from an adventitious hydrocarbon layer due to pump oil contamination, has been widely used as a reference, but this suffers from a number of disadvantages arising from the uncertainty in the chemical state of the carbon species and the thickness of the carbon overlayer [25,32-351. Some workers also used a Cls binding energy of 285.0 eV or more as the reference [36-381, thus leaving a considerable amount of unreliability in the interpretation of binding energy data for the insulators. For mica, the Cls reference is not suitable as all air-cleaved surfaces contain some carbon contamination which is perhaps in a different chemical state from that of the adventitious carbon [18]. The vacuum-cleaved surface, however, is free from carbon contamination. Another charge referencing method consists in depositing a thin film of
294
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gold or other noble metals onto the insulator surface and then using Au4f or other suitable levels as reference_ The uncertainty about good electrical contact between the deposited film and the substrate and also the possibility of chemical interaction between the metal and the substrate, have, however, raised some questions about this method [39-411. In XPS measurements of zeolites, silica-aluminas and clay minerals, the Si2p peak has also been used as a convenient reference [42-451. Seyama and Soma [46] have also used the Si2s level as a reference for the montmorillonites. Kantschewa et al. [47] have shown that the potassium energy levels are insensitive to chemical shift and used the K2paj2 level as an internal reference for the XPS study of K,COs/y-Al& system. In the present work, K2p3,s binding energy was used as the reference level. The energy of this level was determined with respect to Pt4f7,2,5,2 levels by first depositing a platinum film on the mica surface and then estimating the apparent shift in these levels due to charging. The Si2p levels showed splitting after platinum deposition and were therefore not chosen for use as reference levels. A possible cause for this splitting might be the formation of platinum silicides which was, however, not investigated. Any possible impact of the shift of the platinum levels on calibration of the reference level was considered minimal and to have affected the calibration of other levels to the same extent. Platinum was evaporated onto both air-cleaved and vacuum-cleaved mica surfaces from a platinum source heated by electron bombardment (2 kV accelerating potential, 8 mA emission current, 12 min evaporation time). The evaporation source was previously calibrated to give a deposit of one monolayer of platinum on a nickel single crystal under identical conditions and the Pt4f,,s and 4fs/s levels were determined respectively to be 71.1 and 74.3 eV. On Table 1 Apparent Platinum levels
4f5/2
4f7/2
shift of PMf peaks on mica due to surface charging” Platinum on air-cleaved mica surface
Platinum on vacuum-cleaved mica surface
Normal emission
Normal emission
Grazing emission
Grazing emission
Apparent
Shift
Apparent
Shift
Apparent
Shift
Apparent
Shift
EBfeV
C/eV
EB/eV
CleV
EBleV
C/eV
EdeV
C/eV
80.30 77.20
6.00 6.10
81.25 73.00
6.95 6.90
80.50 77.40
6.20 6.30
81.50 78.25
7.20 7.15
“The shifts are calculated on the basis of Pt4f5j2 and 4f7,2 binding energies of 74.3 and 71.1 eV respectively. The values are found to have a variance of f0.3 eV from a number of measurements
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Table 2 Binding energies EB/eV of the principal constituents of muscovite mica obtained with an Al Ka source with K2p3jz energy = 293.75 * 0.20 eV as the reference and comparison with published data Data from
This worka Schultz et al. [48Jb Castle et al. [19]’ Wagner et al. [491d
Aluminium
Silicon
2P
2s
2P
2s
74.40
119.45
102.65 108.30
153.50 159.50
74.50
103.70e
74.05
102.16
Oxygen 1s
Carbon 1s
531.65
285.55
533.20
284.80
531.23
284.60
* The binding energies are mean values of a number of measurements on both air-cleaved and vacuum-cleaved mica surfaces. b These values were not corrected for charging. ’ These values were obtained assuming a constant shift of 7.5 eV due to charging. d Reference here was to Cls = 284.60 eV. e This appears to be a wrong value.
mica, the platinum peaks had an apparent shift of between 6 and 7 eV due to charging (Table l), the shift being greater for grazing emission XPS. The mean of the apparent shifts of the Pt4f levels was used to correct the K2p3j2 binding energy giving it a value of 293.75 f 0.20eV for both aircleaved and vacuum-cleaved surfaces. On the basis of K~P~,~ reference the binding energies of the principal constituents of mica are given in Table 2 along with a few published data due to other workers [19,48,49]. The agreement is reasonable. Tempere et al. [50] have reported A12s and Si2p energies in zeolites respectively in the ranges 119.3-119.9eV and 102.1-103.5 eV with Cls = 285.0eV reference. For a large number of zeolites and clay minerals, Barr [51] has shown the A12p and Si2p energies (referenced to Cls = 284.4 eV) to lie in the ranges of 73.2-74.6 eV and 100.9103.35 eV respectively. However, some uncertainty is inevitable in assigning binding energies to the constituents of an insulator and small shifts in core level peaks would be impossible to detect in these cases. Typical photoelectron
spectra
A typical photoelectron spectrum (grazing emission) of a muscovite sample cleaved in air immediately before introduction into the system is shown in Fig. 3. All the major constituents of mica could be seen to give XPS peaks. Some magnesium and sodium could also be detected from their X-ray induced Auger peaks (verified by taking the spectrum with the Mg Ka source when the peaks were displaced by 233 eV towards lower binding energy). This is in agreement with the fact that small amounts of
296 L”‘.“‘
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Electron
Spectrosc.
~“““‘.““““““““““““““~“““‘I”““““”
Relat. Phenom.
63 (1993) 289-306
.I”“““‘I”“““‘I“’
-.
4 ”
0
t....,..~.I....l....l....r....l....~ 100 2ciJ 300 Bwdbg
. ..I...~r~..~‘..~.I..~~‘....I....’..~~’.~~.t....‘~-~. 402
500
6c0
700
800
900
loo0
energy(mcorrect.ed l/eV
Fig. 3. Photoelectron spectrum of an air-cleaved mica surface at grazing emission. The Al Kcu source was at 11 kV energy and 10 mA emission (maximum count rate 10 790, analyser energy 50 eV, step size 0.25 eV, time 40 s x 10 scans, signal to noise ratio 223:l). The binding energies are uncorrected.
sodium and magnesium replace potassium (interlayer positions) and aluminium (octahedral positions) respectively. The spectrum shows satellites of prominent peaks due to the Kas,a4 components of the X-radiation. The massive Cls peak in the spectra of air-cleaved mica samples was found to have a binding energy of 28525285.85 eV (K2p3j2 = 293.75 eV) with FWHM of 1.65-2.00 eV from a series of measurements. The Cls peak due to adventitious hydrocarbon contamination has been assigned an energy in the range 284.4-284.8 eV by various authors. It therefore appears that the Cls peak for the air-cleaved mica has at least some contribution from carbon species other than the adventitious hydrocarbon [18]. The mica surface, immediately after cleaving at 2 x 10-l’ Torr, yielded a photoelectron spectrum as shown in Fig. 4. The reduction in the Cls peak is a prominent feature of this spectrum. The small peak still appearing might have been picked up from the air-cleaved corners by the X-ray beam which has a diameter of almost 1 cm. All the other peaks become more prominent after the cleaving. Thus the carbon contamination was responsible for some degree of screening of the XPS signals of the air-cleaved surface.
K.G.
BhattacharyyalJ.
C,““““1”““”
Electron
Spectrosc.
Relat. Phenom.
I”“““‘I”“““‘I,““‘,“l”‘l”‘,‘I”“““’1.“~”“’I.‘~“’.“I’.
63 (1993) 289-306
297 ‘I”‘,
t i
BindUg
ener~(uncorrected)/eV
Fig. 4. Photoelectron spectrum of a freshly vacuum-cleaved mica surface at grazing emission. The binding energies are uncorrected (Al Ka source at 11 kV, 10 mA emission, maximum count rate 19680, analyser energy 50eV, step size 0.25 eV, time 40 s x 10 scans, signal to noise ratio 634:l).
The vacuum-cleaved surface also revealed the presence of some iron which is likely to be in either tetrahedral or octahedral positions. Relative
abundance
calculations
Problems associated with the use of XPS measurements to calculate relative abundances of constituent elements have been well documented [52-541. Peak area ratios, however, continue to be used for this purpose. Relative concentrations of any two elements, A and B, can be compared using the elementary relations
(2) where CA and cB are the COnCentratiOnS Of the tW0 deInentS, IA and 1s are the intensities of their prominent XPS peaks, and 23, and SB are the relative sensitivities for the peaks with respect to the spectrometer. Normalised peak area ratios for air-cleaved and vacuum-cleaved mica surfaces, calcu-
K.G.
298
BhattacharyyafJ.
Electron
Spectrosc.
ReEat. Phenom.
63 (1993) 289-306
lated on the above basis, are given in Table 3. For this purpose, each peak was scanned separately under the identical conditions of 10eV sweep width, 20 eV analyser energy, 0.05 eV step size, and 10 s x 25 scans. Background subtraction and peak area determination were done using the VGS 1000 programme. The relative sensitivity data for the ESCALAB were not determined and instead the following data of Wagner et al. [55] were used for calculations: Fls, 1.00 as the standard; Cls, 0.25; A12p, 0.185; Si2p, 0.27; K2p, 0.83. Table 3 has two sets of data for normal and grazing emission modes. The normal emission data, which are less surface sensitive, should represent bulk composition of mica whereas the grazing emission data should indicate the surface composition. According to Cox et al. [56] the mean free path A of electrons in a large number of insulating and semi-conducting oxides follows approximately the relation x = 0.35 x E3
(3)
where E is the kinetic energy of the photoelectron in electronvolts and X is in Angstriims. On this basis, X would be nearly 13A for electrons contributing to the SiXp and A12p peaks and 1.2Hifor those contributing to the K2p3i2 peak. For normal emission, the attenuation of photoelectrons originating at a distance D below the surface is equal to exp (-D/X) [57]. Thus at a depth of 10 A, which corresponds to the distance between cleavage planes in mica, the attenuation is 0.5. Although this attenuation is considerable, the normal emission XPS data should be fairly representative of the bulk structure of mica as far as A12p, Si2p and K2p,,, peaks are concerned. Thus, the K2ps@i2p ratio of 0.34-0.35 found in this work is close to the elemental ratio of 0.30 for the bulk mica. However, a large discrepancy exists between the experimental A12p/Si2p ratio (x0.55) and that calculated on Table 3 Normalised
peak area ratios in muscovite
Ratioa
Normal
A12pf Si2p K&/2
ISi%
ClsjSiZp A12p(AC)/A12p(VC) Si2p(AC)/SiBp(VC) Cls(AC)/Cls(VC) Air-cleaved
emission
mica surface;
data
Grazing
emission
Air-cleaved
Vat-cleaved
Air-cleaved
Vat-cleaved
surface
surface
surface
surface
0.54 0.34 4.54
0.57 0.35 0.15
0.30 0.24 3.29
0.69 0.30 0.25
0.33
K~P,/,(AC)/K~P,,~(VC)
‘AC,
mica from XPS intensity
0.26
0.34 0.34 10.64 VC, vacuum-cleaved
0.60 0.48 7.93 mica surface.
K.G. BhattacharyyaiJ. Electron Spectrosc. Relat. Phenom. 63 (1993) 289-306
299
the basis of mica composition (0.87). The reason for this is not clear, but the discrepancy may be due to one or all of the following possibilities. (a) The relative sensititivity factor used for A12p may not be appropriate as a large part of aluminium in mica is in octahedral coordination, a type of bonding not found in the reference compounds used in determining the relative sensitivity data by Wagner et al. [55]. (b) The attenuation of photoelectrons originating from the octahedral layer at normal emission may be greater than supposed because of steric blocking by the tetrahedral layer of atoms immediately above. (c) Wertheim [58] has recently pointed out that the relative sensitivity factors do not provide a reliable basis for quantitative chemical analysis. The grazing emission data for the vacuum-cleaved surface showed a much larger A12p/Si2p ratio of 0.69. In this mode, the photoelectrons leave the surface at an angle of approximately 10 o to the surface plane and the attenuation will be equal to exp (-D/X sin loo). Even at a depth of 3 A below the surface, the attenuation is 0.26 and the photoelectrons will originate effectively from the top two layers of atoms in mica. Only the tetrahedrally coordinated layer with the associated potassium inter-layer ions are thus likely to be detected in grazing emission. For this layer, the elemental ratios should be Al/Si = 0.29 and K/Si = 0.30. The K2p3#i2p ratio from this work agrees well with this value, but the A12p/Si2p value (0.69) for the vacuum-cleaved surface is more than twice as large. This anomalous result is also likely to be due to steric blocking arising from the particular structure of mica. The aluminium atoms of the tetrahedral layer, being 0.12A closer to the surface [9] than the silicon atoms, apparently block the photoelectrons emerging at grazing angle from the silicon atoms and thus reduce the Si2p signal with consequent apparent enrichment of the A12p signal. Significantly, the A12p/Si2p ratio for the aircleaved surface at grazing emission is almost equal to the theoretical value for the tetrahedral layer. The presence of the carbon layer seems to have caused the tetrahedral Si and Al atoms to relax so that they are in the same plane equidistant from the surface. The reduction in the intensities of normal emission XPS peaks for the aircleaved surface by a factor of three in comparison to those for the vacuumcleaved surface cannot possibly be attributed to the carbon overlayer alone. A layer of carbon, about 10 A thick, would be required to give this tremendous attenuation. Such massive deposition of carbon was unlikely although the XPS spectrum indicated a considerable amount of carbon on the air-cleaved surface. It is also likely that the carbon atoms sit directly above the Al, Si, and K atoms in such a way as to screen photoelectrons emitted from these atoms at normal incidence. The influence of the carbon overlayer on the XPS peaks at grazing emission is different. As seen from Table 3, the influence now appears to be much more
300 Table 4 Apparent micaa Mica surfweb
K.G. BhattacharyyalJ.
En
FWHM
1.65
79.20
2.85
1.65
(74.40) 80.50
1.65
A
290.35
B
(285.55) 292.05
E
(286.00) 292.55 (285.60) 291.70 (285.80) 292.70 (285.90)
Si2p
A12p
Cls FWHM
D
63 (1993) 289-306
shift of core level binding energies (in eV) of the principal XPS peaks of muscovite
En
C
Electron Spectrosc. Relat. Phenom.
2.20 1.90 2.05
(74.50) 81.40 (74.45) 80.30 (74.40) 81.45 (74.45)
1.80 1.85 1.90
EB 107.45 (102.65) 108.60 (102.55) 109.50 (102.55) 108.50 (102.60) 109.65 (102.65)
a All data refer to grazing emission; corrected values of Es are given in parentheses reference). b See text.
FWHM 2.05 2.00 2.05 2.05 2.05
(K2p3,z
pronounced in the case of the AlZp. For the air-cleaved surface, the AlZp intensity dropped by a factor of four, while the Si2p and K2p3i2 intensities were much less affected. The larger value of Cls/SiZp peak area ratio for the air-cleaved surface in normal emission compared to that in grazing emission shows the carbon contamination to be bulky and non-homogeneous. Surface charging
and apparent binding energy shift
It was observed that the apparent shifts in binding energy due to charging were much more for the vacuum-cleaved surface than for the air-cleaved surface. Another interesting observation was that heating of the samples further shifted the peaks to higher binding energy while exposure to water vapour or ammonia decreased the shifts. The apparent shift oscillated between 4.5 and 7.0 eV depending upon the history of the mica surface. The apparent and corrected binding energies of the principal peaks for the mica surfaces (at grazing emission) are given in Table 4. The following surfaces were studied in sequence: A, air-cleaved surface; B, freshly vacuum-cleaved surface; C, surface after heating to 600K for 30min; D, surface exposed to 18OOL of water vapour (1 L = lop6 Torr s); E, surface reheated to 600 K for 30mins. Each peak was scanned individually under the same set of conditions (10 eV sweep width, 20 eV CAE, 10 s x 25 scans, 0.05 eV step width, same beam energy and emission current). Repeating the sequence produced identical results. The peaks corresponding to mica surfaces A, B, and C are shown in Fig. 5,
K.G.
BhattacharyyafJ.
=P~/z -J&i
Electron
Spectrosc.
Rdat.
298.55 (293.75)
1.85
299.80 (293.75) 300.70 (293.75) 299.65 (293.75) 300.75 (293.75)
1.80 1.80 1.80 1.85
&3
FWHM
301.40
1.90
(296.6) 302.65 (296.6) 303.40 (296.45) 302.45 (296.55) 303.50 (296.50)
63 (1993) 289-306
301
01s
=P~/z
FWHM
Htenom.
1.90 1.85 1.85 1.85
EB
FWHM
536.45
2.30
(531.65) 537.55 (531.50) 538.55 (531.60) 537.45 (531.55) 538.65 (531.65)
2.35 2.30 2.35 2.35
which clearly shows the apparent shifts for each peak. The surfaces D and E produced peaks in almost the same positions as B and C respectively. The K2p3,~~,~ doublet was well resolved with an energy separation of 3 eV. The Si2p peak showed a shoulder on the low binding energy side with an energy separation of more than 3 eV from the principal peak. This shoulder was found to correspond to an energy of 98 eV after correction for apparent shift and could be assigned to Al KL2,3L2,3 Auger peak induced by the bremsstrahlung component of the X-radiation (the characteristic lines of an Al Ka: source are too near to the kinetic energy of an Al KL2,3L2,3 electron to excite this Auger transition). Many authors [59,60] have discussed the appearance of such bremsstrahlung induced Auger peaks in XPS measurements using an achromatic source. When the spectra were taken with the Mg source this shoulder on the Si2p peak disappeared proving conclusively that it was the Al KL2,3L2,3 Auger peak. The shifts in the apparent binding energies appear to be associated with the presence or absence of adsorbed layers on the mica surface. Thus the large increase in apparent binding energy of all the photoemission peaks after cleaving in vacuum may be due to the absence of the carbon contamination layer. Comparing the data for the air-cleaved and the vacuumcleaved surfaces, the carbon layer is responsible for a drop of approximately 1.2 eV in the binding energies of A12p, SiBp, K2p312,1j2and 01s core electrons. Again, when the vacuum-cleaved surface was heated, there was a rise of nearly 1.0 eV in the apparent binding energy. This may be due to removal of water from the surface. Readsorption of water reversed the process and
302
K.G. BhattacharyyulJ. Electron Spectrosc. Relat. Phenom. 63 (1993) 289-306
the apparent binding energy was restored to more or less the previous value. Such behaviour is unknown for conducting samples [61]. The presence of an adsorbed layer on the surface could cause some change in the steady state charge present on the surface during the photoionisation process. The rate of neutralisation of the positive charge by the stray electrons generated by the bremsstrahlung radiation might have been affected. A more likely explanation, however, would be due to change in the dipole fields of the surface. Quite large dipole fields can be expected on the mica surface because of the presence of potassium ions with the corresponding negative charge residing on the tetrahedral aluminium atoms [16]. Molecules adsorbing on the surface will be in a strong field and may become polarised, thus reducing the potential drop across the dipole layer. Alternatively adsorption might cause some movement of the potassium ions with a resultant change in the dipole moment. The uncertainty about the position of potassium ions relative to the aluminium atoms makes it difficult to calculate the potential drop arising from the dipole layer. Considering the dipole layer as a parallel plate condenser, an appropriate estimation of the potential drop can, however, be made from the relationship T/‘=oct where CTis the charge density, d the charge separation, &ois the permittivity of vacuum and E, is the relative permittivity of the medium. The number of potassium ions perm’ on the mica surface is about 2 x 1018. With d = 1 A and E, = 1, the potential drop across the dipole layer is 3.6V. Thus a change of l.O-1.2V due to an adsorbed layer is quite feasible. With the above model, both carbon contamination and water have to be adsorbed in such a way so as to make the outer end of the dipole layer more positive. If the adsorbate molecules were merely polarised in the surface field, this would require the initial fields to be negative outwards, i.e. for the potassium ions to lie deeper in the surface than the corresponding negative charge. However, a small relaxation of the potassium ions outward, induced by adsorption, would produce the desired result, irrespective of the initial direction of the field. Table 4 also shows that the FWHM for all the peaks is much larger compared to that in the case of a conducting sample. The Si2p, 01s and K2p3,2,1,2 peaks have a constant FWHM value for the mica surfaces of different history, but this is not true for Cls and A12p peaks. The A12p peak for the air-cleaved surface was very small and broad. The Cls peak was very small after cleaving and it showed a tendency to broaden after the sample was heated. The 01s and the SiZp peaks were exceptionally broad indicating they might be composite peaks.
K.G.
Bhattacharyya/J.
Electron Spectrosc.
Relat. Phenom.
Al ?Q
c
Msx.Count Rate 464
Max.count
63 (1993) 289-306
IS
,, :
Fate 2009
I1
’\ \ ',
I I
\
I
\ \
I I
3
I
/'A -..-.._.._ _.-...s.“.
285
85
i’,.2
i
‘.., A__.,,
‘k,
-S.&L I
I
I
1
&I
EL.%&/.C” C ?,
.f \ .N ..,
,’ ___--
303
295
290
si 2p
I
I
106
I
I
537
111
542
I”, ;“..
Fax.Comt
!
I
Rate 1045
_
._.--$-
.’
\
3INDING ENEFGY (UNCORRECTED) / eV ---+ Fig. 5. Apparent shift of grazing emission XPS peaks of muscovite mica due to surface charging: A, air-cleaved surface; B, vacuum-cleaved surface; C, surface after heating to 600 K for 30min in a vacuum. (Non-linear background subtracted.)
It was not possible in this work to resolve Si2~3,~,~,~ and A12p3,2,1,2 doublets because of closeness of their binding energies (~1 eV difference). Although aluminium occurs in both tetrahedral and octahedral environments, no splitting was observed in A12p and A12s peaks. Similarly no
304
K.G.
BhattacharyyalJ.
Ebctron
Spectrosc.
Relat. Phenom.
63 (1993) 289-306
splitting was observed for the silicon photemission peaks, although silicon occupies two types of site, one with an aluminium neighbour and the other without. Anderson and Swartz [62] have made similar observations for a number of aluminosilicates. A qualitative study of the extent of charging on other insulator surfaces such as molecular sieve 13x, potassium carbonate and silicon carbide (a small amount of the powdered sample was pressed onto a rhenium foil, held by tantalum tags to the stub holder) showed a binding energy shift of between 0.5 and 2eV depending upon the thickness of the layer. It does appear that the sheet structure of mica is responsible for the substantial surface charging in mica. Also the amount of charging did not particularly depend on the thickness of the mica sample; the cleaving actually increased the charging, Conclusions Use of XPS to study the mica surface yields some interesting results. It is shown that XPS data at normal and grazing emission can be used to calculate the relative abundance of various constituents respectively for the bulk and the surface of mica. Discrepancies can be explained on the basis of selective attenuation of some signals. The apparent shifts of the photoelectron peaks due to surface charging do not depend upon the thickness of the sample, but on whether or not there is an adsorbed layer. An adsorbed layer reduces the charging and hence the shift is also reduced. This may be due to changes in the dipole layer brought about by the adsorbate layer. Acknowledgement The author is grateful to Dr. D.O. Hayward, Department of Chemistry, Imperial College for allowing the use of all the facilities in his laboratory and also for assisting with the interpretation of the results.
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