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Applications of the near-edge and low-loss fine structure in the analysis of diamond
Ultramicroscopy 28 (1989) 43-46 North-Holland, Amsterdam
43
APPLICATIONS OF THE NEAR-EDGE AND LOW-LOSS FINE STRUCTURE IN THE ANALYSIS OF DIAMOND S.D. BERGER, J. BRULEY and L.M. BROWN Cavendish Laboratory, Madingley Roa~ Cambridge CB3 0HE, UK and
D.R. McKENZIE School of Physics, University of Sydney, Sydney, NSW, Australia Received at Editorial Office 3 November 1988; presented at Workshop March 1988
We have used electron energy loss spectroscopy (EELS) to analyse the electronic, chemical and structural properties of diamond and diamond-like material.
I. Voidites in diamond Voidites are nanonmeter-size octahedral faceted inclusions found on the cube planes of diamond; I
~2
they are. associated with dislocations loops. In electron microscopy studies the voidites appear to be the dissociation product of the more common platelet defects [1]. EELS analysis of "voidites" in diamond reveals them to be composed of nitrogen, see fig. 1. The near-edge structure of the nitrogen K edge is compared directly to standard spectra of gaseous molecular nitrogen and ammonia published in the literature [2,3] (see fig. 2). Quantitative analysis reveals that the nitrogen in the voidites is very dense (half the atomic density of the diamond) and is independent of voidite size. We can calculate the pressure required to maintain this density and it seems probable that the diamonds are formed somewhat deeper in the earth's mantle than previously estimated. This work is discussed in ref. [4].
2. EELS analysis of vacmmt-arc-deposited diamond-like films Energy Loss / eV Fig. 1. EELS recorded from a single voidite and a defect-free region of diamond. The nitrogen and carbon edges appear at 400 and 290 eV, respectively.
Electron energy loss spectroscopy measurements have been made on amorphous diamondlike carbon films produced by condensing the
0304-3991/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
S.D. Berger et aL / Application of near-edge and low-loss fine structure in analysis of diamond
44
- - ' T - NH 3 /'/"
\
/
',
r
,
N2
<
i
i
i!
/
,/ _~
r
400
420
440
\
!
!"
/
460
/
Energy Loss / eV
Fig. 2. The voidite spectrum after the removal of the background reveals the near-edge structure of the nitrogen absorption edge. The standard spectra from gaseous molecular nitrogen [2] and ammonia [3] are illustrated. Both are convoluted with a top-hat function of width 2 eV to mimic bur spectral resolution.
17
2'o
i
i
~
~0
AE (eV}
Fig. 4. Valence loss EELS from the various allotropes of carbon: (a) natural diamond; (b) amorphous diamond-like carbon, a-D; (c) graphitized carbon black; (d) amorphous carbon. The a-D spectrum is characterized by its high bulk plasmon energy (30.2 eV) and its low qt to ~r* intensity within the 0 to 7 eV loss region.
300
320
AE (eY)
31.0
350
Fig. 3. EELS in the carbon K-edge region from (a) graphitized
carbon (dashed line) and a-carbon (solid line) and Co) diamond (dashed line) and a-D (solid line). The fraction of sp2 bonded carbon present within the a-D film can be estimated from the intensity of the ls to ~r* peak at 285.5 eV, see ref. [5].
p l a s m a stream f r o m a filtered v a c u u m arc. T h e results are c o m p a r e d with spectra f r o m d i a m o n d , graphitized c a r b o n a n d a m o r p h o u s c a r b o n , see figs. 3 a n d 4. A l t h o u g h the energy loss spectra reveal the presence of a small q u a n t i t y of sp 2 b o n d e d m a t e r i a l i n the d i a m o n d - l i k e film~ the high p l a s m o n f r e q u e n c y a n d shape of the K edge show that the m a t e r i a l is essentially a n a m o r p h o u s form of d i a m o n d . T h e fraction of sp 2 b o n d e d c a r b o n was q u a n t i f i e d a n d f o u n d to b e of the order of 15%. It was n o t possible to d e t e r m i n e if the sp 2
S.D. Berger et al. / Application of near-edge and low-loss fine structure in analysis of diamond
45
carbon was on the surface or throughout the bulk. This work is discussed in detail in ref. [5].
3.Studies of the fundamental absorption edge by EELS Experiments reveal that in large band gap materials such as diamond the electron energy loss spectrum shows a characteristic parabolic rise just above the energy band gap. Furthermore, the contribution of indirect transitions to the spectrum leads to an orientation dependent rise whereas that of direct transitions leads to one which is independent of the specimen orientation, see fig. 5. This behaviour is to be contrasted with the spectrum derived from optical absorption experiments which reveals features related only to direct transitions. It is possible to produce energy-filtered images within the band gap, as shown in fig. 6. The image shows the difference between insulating mineral clusters, d, and graphite-like particles, b, which appear bright in both images. Also clearly visible are surface states. The graphite particles almost certainly result from specimen preparation, which
Fig. 6. Energy-filtered band gap (3 eV) and annular dark field images of diamond, illustrating variations in the local density of states with energies less than the band gap, see ref. [6].
ii
is by burning. This work is described in detail in refs. [6,7].
Acknowledgements I'o
;s t,~) Energy Loss
Fig. 5. Orientation d~oc~denc~ of the low energy loss spectra of diamond. The shape of the spectra can be related to a simple joint density of states, derived by taking into account the finite momentum transfer for inelastic scattering, see ref.
[71.
The authors are grateful to Adam Hilger, Institute of Physics Publishing Ltd., for permission to reproduce figs. 5 and 6 from reference [6]; and to The Editor of Philosophical Magazine for permission to reproduce figs. 3 and 4 from ref. [5] and figs. 1 and 2 from ref. [4].
46
S.D. Berger et al. / Application of near-edge and low-loss fine structure m analysis of diamond
References [1] J.C. Barry, L.A. Bursill, J.L. Hutchinson, A.R. Lang, G.M. Rackham and N. Sumida, Phil. Trans. Roy. Soc. London A321 (1987) 361. [2] A.P. Hitchcock and L.E. Brion, J. Electron Spectrosc. Related Phenomena 18 (1980) 1. [3] R.N.S. Sodhi and C.E. Brion, J. Electron Speetrosc. Related Phenomena 36 (1985) 18. [4] J. Bruley and L.M. Brown, Phil. Mag., in press.
[5] S.D. Berger, D.R. McKenzie and P.J. Martin, Phil. Mag. Letters 57 (1988) 285. [6] J. Bruley, L.M. Brown and S.D. Berger, in: Electron Microscopy and Analysis 1985, Inst. Phys. Conf. Ser. 78, Ed. G.J. Tatlock (Hilger/Inst. Physics, London-Bristol, 1986)ch. 14, p. 561. [7] J. Bruley and L.M. Brown, in: Analytical Electron Microscopy, Ed. G.W. Lorimer (Inst. of Metals, London, 1988).
43
APPLICATIONS OF THE NEAR-EDGE AND LOW-LOSS FINE STRUCTURE IN THE ANALYSIS OF DIAMOND S.D. BERGER, J. BRULEY and L.M. BROWN Cavendish Laboratory, Madingley Roa~ Cambridge CB3 0HE, UK and
D.R. McKENZIE School of Physics, University of Sydney, Sydney, NSW, Australia Received at Editorial Office 3 November 1988; presented at Workshop March 1988
We have used electron energy loss spectroscopy (EELS) to analyse the electronic, chemical and structural properties of diamond and diamond-like material.
I. Voidites in diamond Voidites are nanonmeter-size octahedral faceted inclusions found on the cube planes of diamond; I
~2
they are. associated with dislocations loops. In electron microscopy studies the voidites appear to be the dissociation product of the more common platelet defects [1]. EELS analysis of "voidites" in diamond reveals them to be composed of nitrogen, see fig. 1. The near-edge structure of the nitrogen K edge is compared directly to standard spectra of gaseous molecular nitrogen and ammonia published in the literature [2,3] (see fig. 2). Quantitative analysis reveals that the nitrogen in the voidites is very dense (half the atomic density of the diamond) and is independent of voidite size. We can calculate the pressure required to maintain this density and it seems probable that the diamonds are formed somewhat deeper in the earth's mantle than previously estimated. This work is discussed in ref. [4].
2. EELS analysis of vacmmt-arc-deposited diamond-like films Energy Loss / eV Fig. 1. EELS recorded from a single voidite and a defect-free region of diamond. The nitrogen and carbon edges appear at 400 and 290 eV, respectively.
Electron energy loss spectroscopy measurements have been made on amorphous diamondlike carbon films produced by condensing the
0304-3991/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
S.D. Berger et aL / Application of near-edge and low-loss fine structure in analysis of diamond
44
- - ' T - NH 3 /'/"
\
/
',
r
,
N2
<
i
i
i!
/
,/ _~
r
400
420
440
\
!
!"
/
460
/
Energy Loss / eV
Fig. 2. The voidite spectrum after the removal of the background reveals the near-edge structure of the nitrogen absorption edge. The standard spectra from gaseous molecular nitrogen [2] and ammonia [3] are illustrated. Both are convoluted with a top-hat function of width 2 eV to mimic bur spectral resolution.
17
2'o
i
i
~
~0
AE (eV}
Fig. 4. Valence loss EELS from the various allotropes of carbon: (a) natural diamond; (b) amorphous diamond-like carbon, a-D; (c) graphitized carbon black; (d) amorphous carbon. The a-D spectrum is characterized by its high bulk plasmon energy (30.2 eV) and its low qt to ~r* intensity within the 0 to 7 eV loss region.
300
320
AE (eY)
31.0
350
Fig. 3. EELS in the carbon K-edge region from (a) graphitized
carbon (dashed line) and a-carbon (solid line) and Co) diamond (dashed line) and a-D (solid line). The fraction of sp2 bonded carbon present within the a-D film can be estimated from the intensity of the ls to ~r* peak at 285.5 eV, see ref. [5].
p l a s m a stream f r o m a filtered v a c u u m arc. T h e results are c o m p a r e d with spectra f r o m d i a m o n d , graphitized c a r b o n a n d a m o r p h o u s c a r b o n , see figs. 3 a n d 4. A l t h o u g h the energy loss spectra reveal the presence of a small q u a n t i t y of sp 2 b o n d e d m a t e r i a l i n the d i a m o n d - l i k e film~ the high p l a s m o n f r e q u e n c y a n d shape of the K edge show that the m a t e r i a l is essentially a n a m o r p h o u s form of d i a m o n d . T h e fraction of sp 2 b o n d e d c a r b o n was q u a n t i f i e d a n d f o u n d to b e of the order of 15%. It was n o t possible to d e t e r m i n e if the sp 2
S.D. Berger et al. / Application of near-edge and low-loss fine structure in analysis of diamond
45
carbon was on the surface or throughout the bulk. This work is discussed in detail in ref. [5].
3.Studies of the fundamental absorption edge by EELS Experiments reveal that in large band gap materials such as diamond the electron energy loss spectrum shows a characteristic parabolic rise just above the energy band gap. Furthermore, the contribution of indirect transitions to the spectrum leads to an orientation dependent rise whereas that of direct transitions leads to one which is independent of the specimen orientation, see fig. 5. This behaviour is to be contrasted with the spectrum derived from optical absorption experiments which reveals features related only to direct transitions. It is possible to produce energy-filtered images within the band gap, as shown in fig. 6. The image shows the difference between insulating mineral clusters, d, and graphite-like particles, b, which appear bright in both images. Also clearly visible are surface states. The graphite particles almost certainly result from specimen preparation, which
Fig. 6. Energy-filtered band gap (3 eV) and annular dark field images of diamond, illustrating variations in the local density of states with energies less than the band gap, see ref. [6].
ii
is by burning. This work is described in detail in refs. [6,7].
Acknowledgements I'o
;s t,~) Energy Loss
Fig. 5. Orientation d~oc~denc~ of the low energy loss spectra of diamond. The shape of the spectra can be related to a simple joint density of states, derived by taking into account the finite momentum transfer for inelastic scattering, see ref.
[71.
The authors are grateful to Adam Hilger, Institute of Physics Publishing Ltd., for permission to reproduce figs. 5 and 6 from reference [6]; and to The Editor of Philosophical Magazine for permission to reproduce figs. 3 and 4 from ref. [5] and figs. 1 and 2 from ref. [4].
46
S.D. Berger et al. / Application of near-edge and low-loss fine structure m analysis of diamond
References [1] J.C. Barry, L.A. Bursill, J.L. Hutchinson, A.R. Lang, G.M. Rackham and N. Sumida, Phil. Trans. Roy. Soc. London A321 (1987) 361. [2] A.P. Hitchcock and L.E. Brion, J. Electron Spectrosc. Related Phenomena 18 (1980) 1. [3] R.N.S. Sodhi and C.E. Brion, J. Electron Speetrosc. Related Phenomena 36 (1985) 18. [4] J. Bruley and L.M. Brown, Phil. Mag., in press.
[5] S.D. Berger, D.R. McKenzie and P.J. Martin, Phil. Mag. Letters 57 (1988) 285. [6] J. Bruley, L.M. Brown and S.D. Berger, in: Electron Microscopy and Analysis 1985, Inst. Phys. Conf. Ser. 78, Ed. G.J. Tatlock (Hilger/Inst. Physics, London-Bristol, 1986)ch. 14, p. 561. [7] J. Bruley and L.M. Brown, in: Analytical Electron Microscopy, Ed. G.W. Lorimer (Inst. of Metals, London, 1988).