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Depth profiled 57Fe conversion electron mössbauer spectra a new method for distinguishing overlayer and substrate signals
Surface Science 79 (1979) L333-L336 0 North-Holland Publish~g company
SURFACE SCIENCE LETTERS DEPTH PROFILED
“Fe
CONVERSION
ELECTRON MOSSBAUER SPECTRA
ANEWMETHODFORDISTINGUISHINGOVERLAYERANDSUBSTRATE SIGNALS M.J. TRICKER * Department of Chemistry, Herder-WattUniversity,Ricmrton. Edinburgh EH14 4AS, UK
and L. ASH and W. JONES Edward Davies Chemical Laboratories, UniversityCoUegeof Wales,Aberystwyth, SY.23 INE, UK Received 6 April 1978; manuscript received in final form 8 August 1978
57Fe conversion electron Mossbauer spectroscopy (CEMS) has been shown to be a useful technique for the characterisation of new phases of the order of tens to a few hundreds of nanometers thick, formed on the surfaces of iron containing substrates and in studies of thick specimens where conventional transmission Mossbauer spectra are unobtainable [I] **. In the simplest CEMS experiment, a He/Ch, flow proportional counter is used to obtain a depth integrated Mossbauer spectrum of the outer 300 nm of the sample by detection of back-scattered electrons [2,3]. As an example, we show in fig. la, the s’Fe CEM spectrum of a lightly oxidised unenriched iron foil (30 min at 350°C in air) in which the spectral components arising from the iron substrate and Fes04 and Fe,Os oxide overlayers can be disting uished. It is not however possible to extract information relating to the nature of the iron environment as a function of depth from experiments of this type although it has been shown possible to depth profile the outermost 100 nm of 57Fe enriched specimens fca. 90% enrichment) by the use of P-ray spectrometers [4-61. Here we report a simple procedure, involving the use of He/Ch4 flow detectors, which allows the signal from thick substrates to be clearly dist~guished from that arising from the overlayers, for samples containing natural abundances of 57Fe (ca. 2%). The method relies on the recent observation that there is a component in the total back-scattered electron spectrum (recorded using a He/CH4 flow counter) * Author to whom correspondence should be addressed. ** Applications of the technique in the areas of corrosion and oxidation of metals and alloys, surface phase analysis of steels, ion implantation, surface chemistry and geochemistry are reviewed in ref. [ 1 ] . L333
L334
M.J. Tricker et al. / Depth profiled 5 ‘Fe CEMS
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VELOCITY Fig. 1.
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M.J. Tricker et al. /Depth
Table
0
CEMS
L335
1
Thickness of gold on sample (nm)
13 23 35 46
profiled “Fe
Ratio of iron/oxide signal a
0.81
1.00 1.15 1.33 1.40
a Using the peaks A and B indicated in fig. 1.
which arises from the detection of photoelectrons, produced in surface regions by y- and X-rays, back-scattered from deep within the sample, subsequent to resonant absorption processes, and that this component can be enhanced by evaporation of an inert overlayer a few tens of nanometers thick onto the surface of the sample [7]. As an example of the method we show in figs. lb, c the progressive changes in the CEM spectrum of the lightly oxidised iron foil caused by the evaporation of successive layers of gold onto the specimen. It can be seen (see table 1) that the signal of the iron substrate is enhanced relative to the signal from the oxide overlayers as the gold overlayer increases in thickness. Clearly, therefore, evaporation of ca. 50 nm of gold onto an unknown sample allows the substrate signal to be distinguished from that of the overlayer( It should be noted that it would not normally be possible to distinguish the substrate from overlayer signal by a combination of transmission Miissbauer and CEMS techniques for thick substrates or by the use of a P-ray spectrometer for samples containing natural abundances of iron or for overlayers thicker than ca. 100 nm. The new method described above overcomes these difficulties. However is should be further noted that similar depth profile information for overlayers on thick substrates may be obtained by the combined use of CEMS methods and techniques where the outermost few microns of the sample is probed by the direct detection of back-scattered y- or X-photons by the use of a torroidal Ar/CH4 proportional counter [8]. The new method described here is complimentary to this approach but has the advantage that only one (He/CH4) detector is required. It is clear, therefore, that the use of the new method will be justified in many situations and in particular when it is required to distinguish the MGssbauer spectrum arising from a thin overlayer(s) from that of a thick substrate where the composition of these regions is not known. Such situations may be met for example in the examination of weathered minerals or in studies of grossly corroded metal or alloys where zoning of the corrosion product overlayers occurs.
L336 References [ l] M.J. Tricker, in: Surface and Defect Properties of Solids, Vol. 6 (Chem. Sot. London. 1977) p. 106. [ 21K.R. Swanson and Y.J. Spijkerman, J. Appl. Phys 41(1970) 3155. [3] J.M. Thomas, M.J. Tricker and A.P. ~r~terbottom, J. Chem. Sot., Faraday II, 71 (197.5) 1708. [4] W. Jones, J.M. Thomas, R.K. Thorpe and M.J. Tricker, AppI. Surface Sci. 1 (1978) 388. [5] U. Biiverstam, C. Bohm, T. Ekdahl, D. Liljequist and B. RingstrGm, in: MGssbauer Effect Methodology, Vol. 9, Ed. J.J. Gruverman (Plenum, New York, 1974) p. 259. [6] J.P. Schunk, JM. Friedt and Y. Llabador, Rev. Phys. Appl. 10 (1975) 121. [7] M.J. Tricker, TX. Cranshaw and L. Ash, Nud. Ins&. Methods 143 (1977) 307. IS] A. Sette-Camera and W. Keune, Corrosion Sci. 15 (1975) 441.
SURFACE SCIENCE LETTERS DEPTH PROFILED
“Fe
CONVERSION
ELECTRON MOSSBAUER SPECTRA
ANEWMETHODFORDISTINGUISHINGOVERLAYERANDSUBSTRATE SIGNALS M.J. TRICKER * Department of Chemistry, Herder-WattUniversity,Ricmrton. Edinburgh EH14 4AS, UK
and L. ASH and W. JONES Edward Davies Chemical Laboratories, UniversityCoUegeof Wales,Aberystwyth, SY.23 INE, UK Received 6 April 1978; manuscript received in final form 8 August 1978
57Fe conversion electron Mossbauer spectroscopy (CEMS) has been shown to be a useful technique for the characterisation of new phases of the order of tens to a few hundreds of nanometers thick, formed on the surfaces of iron containing substrates and in studies of thick specimens where conventional transmission Mossbauer spectra are unobtainable [I] **. In the simplest CEMS experiment, a He/Ch, flow proportional counter is used to obtain a depth integrated Mossbauer spectrum of the outer 300 nm of the sample by detection of back-scattered electrons [2,3]. As an example, we show in fig. la, the s’Fe CEM spectrum of a lightly oxidised unenriched iron foil (30 min at 350°C in air) in which the spectral components arising from the iron substrate and Fes04 and Fe,Os oxide overlayers can be disting uished. It is not however possible to extract information relating to the nature of the iron environment as a function of depth from experiments of this type although it has been shown possible to depth profile the outermost 100 nm of 57Fe enriched specimens fca. 90% enrichment) by the use of P-ray spectrometers [4-61. Here we report a simple procedure, involving the use of He/Ch4 flow detectors, which allows the signal from thick substrates to be clearly dist~guished from that arising from the overlayers, for samples containing natural abundances of 57Fe (ca. 2%). The method relies on the recent observation that there is a component in the total back-scattered electron spectrum (recorded using a He/CH4 flow counter) * Author to whom correspondence should be addressed. ** Applications of the technique in the areas of corrosion and oxidation of metals and alloys, surface phase analysis of steels, ion implantation, surface chemistry and geochemistry are reviewed in ref. [ 1 ] . L333
L334
M.J. Tricker et al. / Depth profiled 5 ‘Fe CEMS
(Cl
*
. I *
.
.
.
*
l
.
*
‘*
(b)
, .
.* . .
.
‘
.
..
A
: L
.
1
.
(a)
VELOCITY Fig. 1.
nun s4
M.J. Tricker et al. /Depth
Table
0
CEMS
L335
1
Thickness of gold on sample (nm)
13 23 35 46
profiled “Fe
Ratio of iron/oxide signal a
0.81
1.00 1.15 1.33 1.40
a Using the peaks A and B indicated in fig. 1.
which arises from the detection of photoelectrons, produced in surface regions by y- and X-rays, back-scattered from deep within the sample, subsequent to resonant absorption processes, and that this component can be enhanced by evaporation of an inert overlayer a few tens of nanometers thick onto the surface of the sample [7]. As an example of the method we show in figs. lb, c the progressive changes in the CEM spectrum of the lightly oxidised iron foil caused by the evaporation of successive layers of gold onto the specimen. It can be seen (see table 1) that the signal of the iron substrate is enhanced relative to the signal from the oxide overlayers as the gold overlayer increases in thickness. Clearly, therefore, evaporation of ca. 50 nm of gold onto an unknown sample allows the substrate signal to be distinguished from that of the overlayer( It should be noted that it would not normally be possible to distinguish the substrate from overlayer signal by a combination of transmission Miissbauer and CEMS techniques for thick substrates or by the use of a P-ray spectrometer for samples containing natural abundances of iron or for overlayers thicker than ca. 100 nm. The new method described above overcomes these difficulties. However is should be further noted that similar depth profile information for overlayers on thick substrates may be obtained by the combined use of CEMS methods and techniques where the outermost few microns of the sample is probed by the direct detection of back-scattered y- or X-photons by the use of a torroidal Ar/CH4 proportional counter [8]. The new method described here is complimentary to this approach but has the advantage that only one (He/CH4) detector is required. It is clear, therefore, that the use of the new method will be justified in many situations and in particular when it is required to distinguish the MGssbauer spectrum arising from a thin overlayer(s) from that of a thick substrate where the composition of these regions is not known. Such situations may be met for example in the examination of weathered minerals or in studies of grossly corroded metal or alloys where zoning of the corrosion product overlayers occurs.
L336 References [ l] M.J. Tricker, in: Surface and Defect Properties of Solids, Vol. 6 (Chem. Sot. London. 1977) p. 106. [ 21K.R. Swanson and Y.J. Spijkerman, J. Appl. Phys 41(1970) 3155. [3] J.M. Thomas, M.J. Tricker and A.P. ~r~terbottom, J. Chem. Sot., Faraday II, 71 (197.5) 1708. [4] W. Jones, J.M. Thomas, R.K. Thorpe and M.J. Tricker, AppI. Surface Sci. 1 (1978) 388. [5] U. Biiverstam, C. Bohm, T. Ekdahl, D. Liljequist and B. RingstrGm, in: MGssbauer Effect Methodology, Vol. 9, Ed. J.J. Gruverman (Plenum, New York, 1974) p. 259. [6] J.P. Schunk, JM. Friedt and Y. Llabador, Rev. Phys. Appl. 10 (1975) 121. [7] M.J. Tricker, TX. Cranshaw and L. Ash, Nud. Ins&. Methods 143 (1977) 307. IS] A. Sette-Camera and W. Keune, Corrosion Sci. 15 (1975) 441.