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Surface segregation in titanium carbide studied by low-energy Li+ ion scattering
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ELSEVIER
Surface
Science
303 (1994) 179-186
Surface segregation in titanium carbide studied by low-energy Li+ ion scattering R. Souda * , W. Hayami, T. Aizawa, S. Otani, Y. Ishizawa National Institute for Research in Inorganic Materials, 1-l Namiki, Tsukuba, Ibaraki 305, Japan
(Received 31 July 1993; accepted for publication
6 October
1993)
Abstract Low-energy Li’ ion scattering is employed to study surface segregation of dilute transition-metal impurities (0.6 wt% of W, Ta, 2, Nb and MO) in TiC by applying two scattering angles so as to perform the layer-by-layer analysis of the atomic concentration. A marked face dependence of the impurity distribution is observed in the first five
layers. At the close-packed (100) surface, the first layer is strongly enriched with the segregants substituted for the Ti atoms with the concentration of as large as 90 at%, though the deeper layers exhibit no marked enrichment of the segregants. At the TiC(111) surface, on the other hand, the segregants distribute not only at the topmost layer but also in the deeper
layers at least up to the fifth layer with almost the same concentration
1. Introduction In many surface systems, dissolved impurities segregate to interfaces such as free surfaces, grain boundaries, and phase boundaries, by elevating temperatures. This phenomenon plays an important role in many aspects of material properties, e.g. heterogeneous catalyst, corrosion, conductivity, and strength or embrittlement. Accordingly, a better understanding of impurity segregation is an essential prerequisite for eventual control of these important processes. So far, a body of investigations have been performed concerning metals and alloys [l], but little is known about segregation in compounds like transition-metal carbides (TMCs). TMCs have many unusual and
* Corresponding
author.
0039-6028/94/$07.00 0 1994 Elsevier Science SSDI 0039-6028(93)E0648-E
of about 6 at%.
interesting bulk properties arising mainly from their unique electronic structures and chemical bonding characters [2,3]. Recently, the surface properties of TMCs have received much attention because of the large variety of applications. For example, TIC is suggested as a highly stable field electron emitter [4,5] and also is a good catalyst for various reactions such as the production of hydrocarbons [6] and the thermal decomposition of nitrous oxides [7]. TiC crystallizes in the rocksalt structure and exhibits remarkable face-dependent chemical properties [8] because the polar (111) surface is terminated by the Ti atoms while the Ti and C atoms exist coplaner in the (100) surface [9]. In the previous work [lo], we have suggested that the oxygen atom preferentially bonds to surface carbon atoms on the (100) surface of TiC and ZrC but the metal-oxygen bond is more preferentially formed at the (100) surface of TaC and VC. This is probably because the
B.V. All rights reserved
density of states at the Fermi level, which is significant for determining chemical reactivity, has a more metal-d character for the latter. This assumption suggests that the chemical properties of, e.g. TiC is largely affected by segregation of group Va and Via transition-metal atoms. Of interest in this respect is the segregation of W since WC is known to behave similarly to Pt in its capacity to chemisorb hydrogen and catalyze the isomerization reaction [ll-131. We have investigated surface segregation in TiC including bulk impurities of W, Ta, MO, Nb, and Zr. This paper is focused on the structure analysis of the (100) and (111) surfaces of these crystals by using low-energy Li+ ion scattering. Low-energy ion scattering has been applied to the surface structure analysis especially under the impact collision mode (ICISS) [9]. However, ordinary ICISS using rare-gas ions is only capable of measuring the topmost layer because of the strong ion neutralization effect. Ion neutralization, in general, is very complicated depending on many factors such as ion energies, ion trajectories, and target species [14,15]. These obstacles have recently been resolved by using alkali-metal ions as a projectile 1161 or detecting both ions and neutrals [17-191, thus enabling us to obtain quantitative information about surface structures and compositions. It is demonstrated that the segregants are concentrated only on the topmost layer of the (100) surface while they are distributed in the deeper layers as well for the (111) surface.
2. Experimental The experiments were performed in an ultrahigh vacuum (UHV) system (base pressure of 8 x lo-” Torr), consisting of a main scattering chamber equipped with Lit and He+ ion sources, low-energy electron diffraction (LEED) optics, and a load lock system for sample transfer. A beam of Lit was generated by a thermionic-type ion source while the He+ ion was produced in a discharge-type ion source. The Li+ ions were incident upon a surface by changing glancing angles cy and reflected ions were detected in two different modes: As schematically shown in Fig.
UHV
chamber
\\ ti.2
gun
Fig. 1. Schematic of an experimental apparatus used for Lie-ICISS in two different modes. The rotatable hemispherical analyzer detects the scattered ions with @ ranging from 0 to 163” while the simple retarding field analyzer is utilized for detection of the ions scattered through 0 = 181”.
1, a rotatable hemispherical electrostatic energy analyzer was employed for detection of ions with scattering angle 0 ranging from 0” to 163”, while a simple retarding-field analyzer with a channeltron multiplier, placed as close as possible to the incident beam line, detected scattered ions with 0 of 181”. In the latter mode (referred to as 180”-ICISS), the energy spectrum could be obtained by numerical differentiation of the raw data. Thus, the information of scattered ions over a wide range of scattering angles could be obtained. In the present study, the measurements were made using a Li+ beam of 1 keV with two scattering angles of 160” and 180”. The single-crystal rods of TIC with bulk impurities (0.6 wt%,) of W, MO, Nb, and Ti were grown with a floating zone method [20]. The specimens were cut from the rods within an accuracy of 0.5” parallel to the (100) and (111) surfaces by spark erosion, and one face of the specimen was polished mechanically to a mirror finish. Two samples were mounted on a tandem sample holder so that the measurement could be performed simultaneously with a reference surface. The samples were cleaned by several flash heatings up to 1600°C in UHV. The surfaces thus prepared showed a sharp, well-defined 1 X 1 pattern in LEED and no visible oxygen contaminations in He+ ion scattering. The samples were then flash heated at 1700°C (totally for 150 s> to segregate
R. Souda et al. /Surface
the impurities to the surfaces. This treatment did not change the LEED pattern of the (100) surface but the (111) surface was faceted to some extent.
3. Results and discussion 3.1. Segregation at the TiC(lO0) surface
Schematically shown in Fig. 2 is the atomic arrangement of the (111) and (100) surfaces of Tic. The (111) surface has an open structure compared to the close-packed (100) surface. Fig. 3 shows energy spectra of Li+ scattered from the (100) surface with no segregated impurities; the measurements were made in the [Olll azimuth at three glancing angles, 30”, 45”, and So”, with the fixed scattering angle of 160”. The spectra exhibit a surface peak of Ti, appearing around the elastic collision energy indicated by an arrow on the abscissa, as well as an extended background. The surface peaks is contributed to by the Li+ ions experienced quasi-single scattering by the surface atoms while the background originates essentially from multiple scattering including the deeper layers. The concept of quasi-single scattering and multiple scattering is somewhat crude, but the former is defined as a single large-angle scattering with a specific target atom accompanied by small-angle scattering by the adjacent atoms before and after the collision [91. It is notable that
(a)
lizi -
TiC(lll)
Science 303 (1994) 179-186
181
the surface peak width is apparently dependent upon CX.Shown in Fig. 4 are the Ti peak intensities with energies of 555 eV (corresponding to the peak position in Fig. 3c; indicated by solid circles with a solid line) and 530 eV (the peak position in Fig. 3b; indicated by open circles with a broken line) plotted as a function of (Y.The inset shows a schematic of the ion trajectories together with the shadow cones for Ti (open circles) and C (solid circles) atoms calculated using the Thomas-Fermi potential. The cutoff of the intensity at around (Y= 20” is due to the shadowing effect occurring on the Ti chain in the topmost layer, while the marked enhancement in intensity at around (Y= 45” arises from the focusing of the incident ion beam onto the second-layer Ti atoms by the topmost C atoms. In this configuration, the blocking effect on the outgoing trajectory narrows the width of this “focusing peak” and prevents the contribution from much deeper layers. The occurrence of the focusing effect lets us detect the second layer impurities, if any, with an enhanced sensitivity. The broader Ti peak in Fig. 3b indicates that the ions coming from the second layer lose more kinetic energy than those from the topmost layer. There are two possibilities for such inelastic scattering on the basis of nuclear and electronic stopping. According to the linear response theory 1211,the friction of the electron gas acts on a slow particle at a surface and the electronic stopping is proportional to the ion-path length. The elec-
(b)
TiC(100)
-(1211
Fig. 2. Atomic arrangement of (a) (111) and (b) (100) surfaces of Tic.
R. Souda
182
I
(a)
I
et al. /Surface
I
ti=30’
. (b)
~2-45’
(C)
Ll.80’
t
2oc
“600
Energy
(ev)
Fig. 3. Energy spectra of Li+ ions scattered from the TiC(100) surface with no segregated impurities. The measurements were made with use of a 1 keV Li+ beam in the [Oil] azimuth at the fixed scattering angle of 160”. The energy position for elastic binary collision is indicated by an arrow on the abscissa.
Science 303 (1494) 179-186
Typically shown in Fig. 5 are the energy spectra of Lit scattered from the Ta-segregated TiC(100) surface. A sharp Ta peak is recognized in addition to the structure coming from the matrix of TIC. The small peak at 745 eV is energetically assignable to Nb which is presumably included in the starting materials. The width of the Ta peak is as small as 15 eV in full width at half maximum (FWHM) and is almost constant at any glancing angles. The polar angle scans of the surface peak intensities are plotted in Fig. 6. Although the Ti peak exhibits marked cx dependence similar to the results of the Ta-free TiC(100) surface shown in Fig. 4, the Ta (855 eV> and Nb (745 eV> peak intensities show no structures assignable to the second-layer contribution. These segregants occupy the Ti sites at the topmost layers, which is confirmed by the occurrence of the steep shadowing effect in both polar and azimuthal angular scans of the intensity. The present result clearly indicates that the segregants are highly concentrated at the topmost layer by substitution of the Ti atoms and its distribution in the second layer is negligible small. The latter is in good accord with the narrow surface peak width of Ta and Nb compared to Ti. The atomic concentration is estimated from the spectrum in Fig. 5c where most of the ions
I
P,
tron-hole pair can also be excited in the collisional regime due to the electron promotion mechanism [15], where the energy loss is suggested to be enhanced in scattering from the second-layer Ti atoms coordinated by six C atoms rather than the first-layer Ti atoms [22]. On the other hand, the nuclear stopping is also likely in the framework of the quasi-single scattering from the deeper layer since the light elements such as the first-layer C atoms recoil so efficiently on the incoming and outgoing trajectories of Li+. Thus, the energy loss of the surface peak testifies the deeper-layer contribution in the present case.
0
I
/
,
:4
I
20
I
I
60
40 K (deg
I
80
100
)
Fig. 4. The polar angle scans of 160”.ICISS azimuth of the TiC(lO0) surface. The Ti surface with energies of 555 eV (closed circles) and circles) are plotted against a with fixed @ of shows the ion trajectories and the calculated corresponding to each intensity variation.
along the 10111 peak intensity 530 eV (open 160”. The inset shadow cones
R. Souda et al. /Surface
(b)
Science 303 (1994) 179-186
183
thus determined as 15% Ti, 10% Nb, and 75% Ta. The strong enrichment of the segregants in the first layer relative to the second layer does not necessarily exclude the possibility of the oscillatory concentrations in the first three-four layers as is the case for, e.g. Pt-Ni alloy systems [23-251. We have employed 180”-ICISS for the purpose of detection of the deeper-layer concentration, and the results are shown in Figs. 7 and 8. The energy spectra in Fig. 7 are obtained by numerical differentiation of the raw data taken with the retarding analyzer. The energy resolution of this technique is below 25 eV as estimated from the FWHM of the Ta peak. The Nb peak is not clearly resolved because of the insufficient signal-to-noise ratio so that we focus on the Ti and Ta peaks. Displayed in Fig. 8 are the surface peak intensities of Ti and
01.45’
I
, 400
Nb 600
Ti 600 Energy
To
P
I
0
I
(eW
Fig. 5. Energy spectra of Li+ scattered gated TiC(100) surface. The measurements the same condition as in Fig. 3.
from the Ta-segrewere made under
are scattered from the topmost layer. The ion yield is, in reality, given by complicated factors such as the scattering cross section, the ion neutralization probability, and various apparatus parameters. Among them, neutralization is simplified considerably with use of the alkali-metal ions since the neutralization probability is almost constant over a wide energy range and is independent of the ion trajectories [141. From the comparison of the surface peak intensities with and without the segregants, the sensitivity factor of the ion detection, including both apparatus parameters and scattering cross sections, is estimated to be 2.1 and 1.7, respectively, for Ta and Nb relative to Ti. The first-layer composition is
Nb
0
20
40
60
80
100
(L (deg.)
Fig. 6. The polar angle scans for 160”-ICISS at the Ta-segregated TiC(100) surface. The surface peak intensities for (a) Ti (555 and 530 eV) and (b) Ta (855 eV) and Nb (745 eV) are plotted against (Y in the [Oil] azimuth.
184
R. Souda et al. /Surface
\ (b)
a=90'
n
n
I
Ti 600
PO0
200
Nb 800
To
I(lO(
Energy teVj Fig. 7. Energy spectra of Lit scattered from the Ta-segregated TiC(100) surface through the scattering angle of 180”. The spectra are obtained by numerical differentiation of the raw data taken with use of the retarding analyzer.
Ta integrated over the energy windows indicated by arrows in Fig. 7. The intensities are symmetrical with respect to (Y= 90”, reflecting the NaCltype bulk-crystal structure. The intensities remaining typically in the range of large ((Y> 16.5”) and small ((Y< 20”) glancing angles should be rather extrinsic since they can be ascribed to the contribution from the sample holder. As schematically shown in the inset of Fig. 8, the marked
Science 303 (1994) 179-186
peaks at (Y= 70” and 112” and the deep valley in between are due to focusing and shadowing effects, respectively, on the [loo] atomic rows. In this configuration, the ion beam can be concentrated on the third-layer atoms due to the double focusing effect by the first-layer Ti atoms and the second-layer C atoms so that the ion intensity is extremely enhanced and the focusing-peak width is considerably narrow. Even in this condition, the Ta peak intensity exhibits no marked focusing peaks, thus showing that the concentration of the Ta atom in the third layer is fairly small. Further investigation is performed using the samples including Zr, Nb, MO, and W impurities. The results are essentially the same as the present case. Therefore, the enrichment only of the first layer is concluded to be the general feature of the surface segregation at the TiC(100) surface. The concentration of the segregants at the topmost layer amounts to 90%, so that almost one monolayer precipitation can be achieved by this simple procedure. In this respect, the monolayer formation of WC or MoC with the rock-salt structure is of crucial importance since bulk WC or MoC crystallize in the hexagonal form. Moreover, as depicted in the introduction, these surfaces are intriguing in connection with the catalytic ability. Detailed studies of the chemical properties of the MO- or W-segregated TiC(100) surface are now in progress. 3.2. Segregation at the TiC(ll1)
0
20
40
60
80
100
120
l&O
I60
180
Fig. 8. The polar angle scans of 180”-ICISS along the [Oil] azimuth of the Ta-segregated TiC(100) surface. The inset shows schematics of ion trajectories corresponding to the marked increase in intensity.
surface
The heat treatment to segregate the impurities to the TiC(111) surface simultaneously induces the faceting of the surface to some extent. This is because the polar (111) surface is less stable than the neutral (100) surface. Fig. 9 displays the representative energy spectra of Li+ scattered from the Nb-segregated TiCXlll) surface, which is least faceted among the samples examined here. The measurements were made in the [lzl] azimuth at the glancing angles of 35”, 58”, and 80”, with the fixed scattering angle of 160”. In Fig. 9a, the ions are scattered mostly from the topmost-layer atoms since the incident angle of 35” corresponds to the channeling direction. This is also testified by the narrow surface peak width. On the other hand,
R. Souda et al. /Surface
Science 303 (1994)
185
179-186
even at smaller (Y.In reality, this surface is terminated in a large part by the flat plane as schematically shown in Fig. la. Consequently, the similarity in the surface peak intensities between Ti and Nb shows that Nb exist similarly in both first and third layers. The concentration of Nb in the present case is estimated to be about 6% from the surface peak intensities shown in Fig. 9. The distribution of Nb at the fifth layer can be investigated with use of 180”-ICISS. Not explicitly shown is the experimental result, but the polar angle scans of the Nb peak are found to be identical to those of the Ti peak. Thus, there exists no marked inclination in the impurity concentration at the TiCUll) surface. The same is essentially true for the other surfaces with the impurities of Zr, MO, and W, though the concentration is a little dependent on the species of the segregant or the degree of the faceting.
(a)
I
LOO
l’
Nb600
600
Energy
Ti
peak
1
CeV)
Fig. 9. Energy spectra of Lif scattered from the Nb-segregated TiC(111) surface obtained at the fiied scattering angle of 160”. The measurements were made in the [l%l azimuth.
the spectrum for LY= 58” is fairly broad for both matrix and segregant. It is notable that the shape of the Nb peak changes quite similarly to that of the Ti peak. The polar angle scans of the peak intensities are plotted in Fig. 10 for (a) Ti and (b) Nb peaks at two energy positions, A and B, indicated by arrows in Figs. 9a and 9b. In apparent contrast to the results for the (100) surface, the intensities of the Nb peak exhibit the angular variation quite similar to those of the Ti peak. The marked structure in the range of 50” < (Y< 90” comes from the shadowing and focusing effects of the Li+ ions on the third-layer atoms, while the plateau for cr < 50” arises mostly from the first layer. The steep intensity drop at (Y= 12” is due to the shadowing effect on the first-layer Ti atoms. If the surface were faceted considerably with the (1001 face, the shadowing should not occur so rapidly and the intensity would remain
40
60
60
100
60
60
100
a (deg. )
0
20
LO OL (deg.)
Fig. 10. The polar angle scans of 160”-ICISS along the [l?l] azimuth of the Nb-segregated TiC(111) surface. The surface peak intensities at the energy positions A and B, indicated by arrows in Figs. 9a and 9b, are plotted against a.
186
R. Souda et al. /Surface
Science 303 (I 994) 179-186
4. Conclusion
6. References
Low-energy Li + ion scattering is well suited for studying depth dependent surface segregation effects. By applying two different scattering angles to achieve focusing of the ion beam onto the desired surface layers, the layer-by-layer analysis of the atomic concentration is performed with a considerable sensitivity. The demonstration is made for the segregation of dilute transitionmetal impurities (0.6 wt%) in Tic. On the TiC(100) surface, the segregants (Ta, W, Zr, Nb, and MO) exist at the topmost layer by substitution of the Ti atoms and their concentrations amount to 90%. On the other hand, the segregants distribute with almost equal amounts at least in the first five layers of the TiC(ll1) surface though the heat treatment simultaneously induces faceting of the surface to some extent. According to the bond breaking theory, the component with the lowest bond strength will segregate. This effect is more pronounced on the loosely packed surfaces [24]. Since the cations in the (100) [(ill)] surfaces have 5 (3) nearest neighbor C atoms, the impurity should be concentrated on the topmost layer of the (111) surface more efficiently. In reality, the present results are in apparent contradiction to this simple picture. This is presumably because the nature of the bonding of the transition-metal carbide is rather specific, exhibiting significant covalent and ionic character in addition to the metallic bond. Also possible is that the faceting as well as the resulting lattice deformation may buffer the strain energy at the TiC(111) surface, and hence the impurities can be distributed in the deeper layers.
Segrega[II W.C. Johnson and J.M. Blakely, in: Interfacial tion (American Society for Metals, Metals Park, OH, 1979). Metal Carbides and Nitrides, Vol. L-21LE. Toth, Transition 7 (Academic Press, New York, 19711. 131 K. Schwarz, CRC Crit. Rev. in Solid State Mater. Sci. 13 (1987) 211. [41 C. Oshima, M. Aono, R. Nishitani and S. Kawai, Appl. Phys. Lett. 35 (19791 493. El Y. lshizawa, S. Aoki, C. Oshima and S. Otani, J. Phys. D 22 (1989) 1763. J. [61 I. Kojima, E. Miyazaki, Y. Inoue and I. Yasumori, Catal. 59 (1979) 472. G.K. Ivakhnyuk, N.V. Sirotinkin, A.A. [71 V.V. Samonin, Titov, V.P. Gusev, D.N. Gavrilov and N.F. Fedorov, Zh. Prikl. Khim. (Leningrad) 55 (1982) 453. [81 C. Oshima, M. Aono, S. Zaima, Y. Shibata and S. Kawai, J. Less-Common Met. 82 (1981) 69. [91 M. Aono and R. Souda, Jpn. J. Appl. Phys. 24 (1985) 1249. S. Otani, Y. Ishizawa and C. 1101 R. Souda, T. Aizawa, Oshima, Surf. Sci. 256 (1991) 19. Acta 15 (19701 1273. [Ill H. Bohm, Electrochim. [I21 R.B. Revy and M. Boudart, Science 181 (1973) 547. J. Catal. 39 (1975) 298; 48 [131 P.N. Ross and P. Stonehart, (1977) 42. 1141 A.L. Boers, Nucl. Instrum. Methods B 2 (19841 353. Ml R. Souda, T. Aizawa, C. Oshima and Y. Ishizawa, Nucl. Instrum. Methods B 45 (1990) 364. [161 H. Niehus and G. Comsa, Surf. Sci. 140 (1984) 18. [171 H. Niehus, Surf. Sci. 166 (19861 L107. [18] M. Katayama, E. Nomura, N. Kanekama, H. Soejima and M. Aono, Nucl. Instrum. Methods B 33 (19881 857. [19] K. Sumitomo, T. Kobayashi, F. Shoji and K. Oura, Phys. Rev. Lett. 66 (1991) 1193. [20] S. Otani, T. Tanaka and Y. Ishizawa, J. Cryst. Growth 92 (1988) 359. [21] P.M. Echenique, R.M. Nieminen, J.C. Ashley and R.H. Ritchie, Phys. Rev. A 33 (1986) 897. [22] R. Souda, T. Aizawa, S. Otani and Y. Ishizawa, Surf. Sci. 232 (19901 219. [23] Y. Gauthier, W. Hoffmann and M. Wuttig, Surf. Sci. 223 (19901 239. [24] S. Deckers, F.H.P.M. Habraken, W.F. van der Weg, A.W. Denier van der Con, B. Pluis, J.F. van der Veen and R. Baudoing, Phys. Rev. B 42 (19901 419. [25] J. Eymery and J.C. Joud, Surf. Sci. 231 (19901 419.
5. Acknowledgement This work was supported in part by the Special Coordination Funds Promoting for Science and Technology.
&
,.:,...
‘.:.:...:.:...:. .... ,. .. .. .. . ........ .,.: ,.... ,,..~,::. :..:,,
;i:;;;,:;:::::y$:. :\. ;.’
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‘.’
.,. >::. :::.:
,.
‘surface science
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ELSEVIER
Surface
Science
303 (1994) 179-186
Surface segregation in titanium carbide studied by low-energy Li+ ion scattering R. Souda * , W. Hayami, T. Aizawa, S. Otani, Y. Ishizawa National Institute for Research in Inorganic Materials, 1-l Namiki, Tsukuba, Ibaraki 305, Japan
(Received 31 July 1993; accepted for publication
6 October
1993)
Abstract Low-energy Li’ ion scattering is employed to study surface segregation of dilute transition-metal impurities (0.6 wt% of W, Ta, 2, Nb and MO) in TiC by applying two scattering angles so as to perform the layer-by-layer analysis of the atomic concentration. A marked face dependence of the impurity distribution is observed in the first five
layers. At the close-packed (100) surface, the first layer is strongly enriched with the segregants substituted for the Ti atoms with the concentration of as large as 90 at%, though the deeper layers exhibit no marked enrichment of the segregants. At the TiC(111) surface, on the other hand, the segregants distribute not only at the topmost layer but also in the deeper
layers at least up to the fifth layer with almost the same concentration
1. Introduction In many surface systems, dissolved impurities segregate to interfaces such as free surfaces, grain boundaries, and phase boundaries, by elevating temperatures. This phenomenon plays an important role in many aspects of material properties, e.g. heterogeneous catalyst, corrosion, conductivity, and strength or embrittlement. Accordingly, a better understanding of impurity segregation is an essential prerequisite for eventual control of these important processes. So far, a body of investigations have been performed concerning metals and alloys [l], but little is known about segregation in compounds like transition-metal carbides (TMCs). TMCs have many unusual and
* Corresponding
author.
0039-6028/94/$07.00 0 1994 Elsevier Science SSDI 0039-6028(93)E0648-E
of about 6 at%.
interesting bulk properties arising mainly from their unique electronic structures and chemical bonding characters [2,3]. Recently, the surface properties of TMCs have received much attention because of the large variety of applications. For example, TIC is suggested as a highly stable field electron emitter [4,5] and also is a good catalyst for various reactions such as the production of hydrocarbons [6] and the thermal decomposition of nitrous oxides [7]. TiC crystallizes in the rocksalt structure and exhibits remarkable face-dependent chemical properties [8] because the polar (111) surface is terminated by the Ti atoms while the Ti and C atoms exist coplaner in the (100) surface [9]. In the previous work [lo], we have suggested that the oxygen atom preferentially bonds to surface carbon atoms on the (100) surface of TiC and ZrC but the metal-oxygen bond is more preferentially formed at the (100) surface of TaC and VC. This is probably because the
B.V. All rights reserved
density of states at the Fermi level, which is significant for determining chemical reactivity, has a more metal-d character for the latter. This assumption suggests that the chemical properties of, e.g. TiC is largely affected by segregation of group Va and Via transition-metal atoms. Of interest in this respect is the segregation of W since WC is known to behave similarly to Pt in its capacity to chemisorb hydrogen and catalyze the isomerization reaction [ll-131. We have investigated surface segregation in TiC including bulk impurities of W, Ta, MO, Nb, and Zr. This paper is focused on the structure analysis of the (100) and (111) surfaces of these crystals by using low-energy Li+ ion scattering. Low-energy ion scattering has been applied to the surface structure analysis especially under the impact collision mode (ICISS) [9]. However, ordinary ICISS using rare-gas ions is only capable of measuring the topmost layer because of the strong ion neutralization effect. Ion neutralization, in general, is very complicated depending on many factors such as ion energies, ion trajectories, and target species [14,15]. These obstacles have recently been resolved by using alkali-metal ions as a projectile 1161 or detecting both ions and neutrals [17-191, thus enabling us to obtain quantitative information about surface structures and compositions. It is demonstrated that the segregants are concentrated only on the topmost layer of the (100) surface while they are distributed in the deeper layers as well for the (111) surface.
2. Experimental The experiments were performed in an ultrahigh vacuum (UHV) system (base pressure of 8 x lo-” Torr), consisting of a main scattering chamber equipped with Lit and He+ ion sources, low-energy electron diffraction (LEED) optics, and a load lock system for sample transfer. A beam of Lit was generated by a thermionic-type ion source while the He+ ion was produced in a discharge-type ion source. The Li+ ions were incident upon a surface by changing glancing angles cy and reflected ions were detected in two different modes: As schematically shown in Fig.
UHV
chamber
\\ ti.2
gun
Fig. 1. Schematic of an experimental apparatus used for Lie-ICISS in two different modes. The rotatable hemispherical analyzer detects the scattered ions with @ ranging from 0 to 163” while the simple retarding field analyzer is utilized for detection of the ions scattered through 0 = 181”.
1, a rotatable hemispherical electrostatic energy analyzer was employed for detection of ions with scattering angle 0 ranging from 0” to 163”, while a simple retarding-field analyzer with a channeltron multiplier, placed as close as possible to the incident beam line, detected scattered ions with 0 of 181”. In the latter mode (referred to as 180”-ICISS), the energy spectrum could be obtained by numerical differentiation of the raw data. Thus, the information of scattered ions over a wide range of scattering angles could be obtained. In the present study, the measurements were made using a Li+ beam of 1 keV with two scattering angles of 160” and 180”. The single-crystal rods of TIC with bulk impurities (0.6 wt%,) of W, MO, Nb, and Ti were grown with a floating zone method [20]. The specimens were cut from the rods within an accuracy of 0.5” parallel to the (100) and (111) surfaces by spark erosion, and one face of the specimen was polished mechanically to a mirror finish. Two samples were mounted on a tandem sample holder so that the measurement could be performed simultaneously with a reference surface. The samples were cleaned by several flash heatings up to 1600°C in UHV. The surfaces thus prepared showed a sharp, well-defined 1 X 1 pattern in LEED and no visible oxygen contaminations in He+ ion scattering. The samples were then flash heated at 1700°C (totally for 150 s> to segregate
R. Souda et al. /Surface
the impurities to the surfaces. This treatment did not change the LEED pattern of the (100) surface but the (111) surface was faceted to some extent.
3. Results and discussion 3.1. Segregation at the TiC(lO0) surface
Schematically shown in Fig. 2 is the atomic arrangement of the (111) and (100) surfaces of Tic. The (111) surface has an open structure compared to the close-packed (100) surface. Fig. 3 shows energy spectra of Li+ scattered from the (100) surface with no segregated impurities; the measurements were made in the [Olll azimuth at three glancing angles, 30”, 45”, and So”, with the fixed scattering angle of 160”. The spectra exhibit a surface peak of Ti, appearing around the elastic collision energy indicated by an arrow on the abscissa, as well as an extended background. The surface peaks is contributed to by the Li+ ions experienced quasi-single scattering by the surface atoms while the background originates essentially from multiple scattering including the deeper layers. The concept of quasi-single scattering and multiple scattering is somewhat crude, but the former is defined as a single large-angle scattering with a specific target atom accompanied by small-angle scattering by the adjacent atoms before and after the collision [91. It is notable that
(a)
lizi -
TiC(lll)
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181
the surface peak width is apparently dependent upon CX.Shown in Fig. 4 are the Ti peak intensities with energies of 555 eV (corresponding to the peak position in Fig. 3c; indicated by solid circles with a solid line) and 530 eV (the peak position in Fig. 3b; indicated by open circles with a broken line) plotted as a function of (Y.The inset shows a schematic of the ion trajectories together with the shadow cones for Ti (open circles) and C (solid circles) atoms calculated using the Thomas-Fermi potential. The cutoff of the intensity at around (Y= 20” is due to the shadowing effect occurring on the Ti chain in the topmost layer, while the marked enhancement in intensity at around (Y= 45” arises from the focusing of the incident ion beam onto the second-layer Ti atoms by the topmost C atoms. In this configuration, the blocking effect on the outgoing trajectory narrows the width of this “focusing peak” and prevents the contribution from much deeper layers. The occurrence of the focusing effect lets us detect the second layer impurities, if any, with an enhanced sensitivity. The broader Ti peak in Fig. 3b indicates that the ions coming from the second layer lose more kinetic energy than those from the topmost layer. There are two possibilities for such inelastic scattering on the basis of nuclear and electronic stopping. According to the linear response theory 1211,the friction of the electron gas acts on a slow particle at a surface and the electronic stopping is proportional to the ion-path length. The elec-
(b)
TiC(100)
-(1211
Fig. 2. Atomic arrangement of (a) (111) and (b) (100) surfaces of Tic.
R. Souda
182
I
(a)
I
et al. /Surface
I
ti=30’
. (b)
~2-45’
(C)
Ll.80’
t
2oc
“600
Energy
(ev)
Fig. 3. Energy spectra of Li+ ions scattered from the TiC(100) surface with no segregated impurities. The measurements were made with use of a 1 keV Li+ beam in the [Oil] azimuth at the fixed scattering angle of 160”. The energy position for elastic binary collision is indicated by an arrow on the abscissa.
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Typically shown in Fig. 5 are the energy spectra of Lit scattered from the Ta-segregated TiC(100) surface. A sharp Ta peak is recognized in addition to the structure coming from the matrix of TIC. The small peak at 745 eV is energetically assignable to Nb which is presumably included in the starting materials. The width of the Ta peak is as small as 15 eV in full width at half maximum (FWHM) and is almost constant at any glancing angles. The polar angle scans of the surface peak intensities are plotted in Fig. 6. Although the Ti peak exhibits marked cx dependence similar to the results of the Ta-free TiC(100) surface shown in Fig. 4, the Ta (855 eV> and Nb (745 eV> peak intensities show no structures assignable to the second-layer contribution. These segregants occupy the Ti sites at the topmost layers, which is confirmed by the occurrence of the steep shadowing effect in both polar and azimuthal angular scans of the intensity. The present result clearly indicates that the segregants are highly concentrated at the topmost layer by substitution of the Ti atoms and its distribution in the second layer is negligible small. The latter is in good accord with the narrow surface peak width of Ta and Nb compared to Ti. The atomic concentration is estimated from the spectrum in Fig. 5c where most of the ions
I
P,
tron-hole pair can also be excited in the collisional regime due to the electron promotion mechanism [15], where the energy loss is suggested to be enhanced in scattering from the second-layer Ti atoms coordinated by six C atoms rather than the first-layer Ti atoms [22]. On the other hand, the nuclear stopping is also likely in the framework of the quasi-single scattering from the deeper layer since the light elements such as the first-layer C atoms recoil so efficiently on the incoming and outgoing trajectories of Li+. Thus, the energy loss of the surface peak testifies the deeper-layer contribution in the present case.
0
I
/
,
:4
I
20
I
I
60
40 K (deg
I
80
100
)
Fig. 4. The polar angle scans of 160”.ICISS azimuth of the TiC(lO0) surface. The Ti surface with energies of 555 eV (closed circles) and circles) are plotted against a with fixed @ of shows the ion trajectories and the calculated corresponding to each intensity variation.
along the 10111 peak intensity 530 eV (open 160”. The inset shadow cones
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(b)
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thus determined as 15% Ti, 10% Nb, and 75% Ta. The strong enrichment of the segregants in the first layer relative to the second layer does not necessarily exclude the possibility of the oscillatory concentrations in the first three-four layers as is the case for, e.g. Pt-Ni alloy systems [23-251. We have employed 180”-ICISS for the purpose of detection of the deeper-layer concentration, and the results are shown in Figs. 7 and 8. The energy spectra in Fig. 7 are obtained by numerical differentiation of the raw data taken with the retarding analyzer. The energy resolution of this technique is below 25 eV as estimated from the FWHM of the Ta peak. The Nb peak is not clearly resolved because of the insufficient signal-to-noise ratio so that we focus on the Ti and Ta peaks. Displayed in Fig. 8 are the surface peak intensities of Ti and
01.45’
I
, 400
Nb 600
Ti 600 Energy
To
P
I
0
I
(eW
Fig. 5. Energy spectra of Li+ scattered gated TiC(100) surface. The measurements the same condition as in Fig. 3.
from the Ta-segrewere made under
are scattered from the topmost layer. The ion yield is, in reality, given by complicated factors such as the scattering cross section, the ion neutralization probability, and various apparatus parameters. Among them, neutralization is simplified considerably with use of the alkali-metal ions since the neutralization probability is almost constant over a wide energy range and is independent of the ion trajectories [141. From the comparison of the surface peak intensities with and without the segregants, the sensitivity factor of the ion detection, including both apparatus parameters and scattering cross sections, is estimated to be 2.1 and 1.7, respectively, for Ta and Nb relative to Ti. The first-layer composition is
Nb
0
20
40
60
80
100
(L (deg.)
Fig. 6. The polar angle scans for 160”-ICISS at the Ta-segregated TiC(100) surface. The surface peak intensities for (a) Ti (555 and 530 eV) and (b) Ta (855 eV) and Nb (745 eV) are plotted against (Y in the [Oil] azimuth.
184
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\ (b)
a=90'
n
n
I
Ti 600
PO0
200
Nb 800
To
I(lO(
Energy teVj Fig. 7. Energy spectra of Lit scattered from the Ta-segregated TiC(100) surface through the scattering angle of 180”. The spectra are obtained by numerical differentiation of the raw data taken with use of the retarding analyzer.
Ta integrated over the energy windows indicated by arrows in Fig. 7. The intensities are symmetrical with respect to (Y= 90”, reflecting the NaCltype bulk-crystal structure. The intensities remaining typically in the range of large ((Y> 16.5”) and small ((Y< 20”) glancing angles should be rather extrinsic since they can be ascribed to the contribution from the sample holder. As schematically shown in the inset of Fig. 8, the marked
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peaks at (Y= 70” and 112” and the deep valley in between are due to focusing and shadowing effects, respectively, on the [loo] atomic rows. In this configuration, the ion beam can be concentrated on the third-layer atoms due to the double focusing effect by the first-layer Ti atoms and the second-layer C atoms so that the ion intensity is extremely enhanced and the focusing-peak width is considerably narrow. Even in this condition, the Ta peak intensity exhibits no marked focusing peaks, thus showing that the concentration of the Ta atom in the third layer is fairly small. Further investigation is performed using the samples including Zr, Nb, MO, and W impurities. The results are essentially the same as the present case. Therefore, the enrichment only of the first layer is concluded to be the general feature of the surface segregation at the TiC(100) surface. The concentration of the segregants at the topmost layer amounts to 90%, so that almost one monolayer precipitation can be achieved by this simple procedure. In this respect, the monolayer formation of WC or MoC with the rock-salt structure is of crucial importance since bulk WC or MoC crystallize in the hexagonal form. Moreover, as depicted in the introduction, these surfaces are intriguing in connection with the catalytic ability. Detailed studies of the chemical properties of the MO- or W-segregated TiC(100) surface are now in progress. 3.2. Segregation at the TiC(ll1)
0
20
40
60
80
100
120
l&O
I60
180
Fig. 8. The polar angle scans of 180”-ICISS along the [Oil] azimuth of the Ta-segregated TiC(100) surface. The inset shows schematics of ion trajectories corresponding to the marked increase in intensity.
surface
The heat treatment to segregate the impurities to the TiC(111) surface simultaneously induces the faceting of the surface to some extent. This is because the polar (111) surface is less stable than the neutral (100) surface. Fig. 9 displays the representative energy spectra of Li+ scattered from the Nb-segregated TiCXlll) surface, which is least faceted among the samples examined here. The measurements were made in the [lzl] azimuth at the glancing angles of 35”, 58”, and 80”, with the fixed scattering angle of 160”. In Fig. 9a, the ions are scattered mostly from the topmost-layer atoms since the incident angle of 35” corresponds to the channeling direction. This is also testified by the narrow surface peak width. On the other hand,
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even at smaller (Y.In reality, this surface is terminated in a large part by the flat plane as schematically shown in Fig. la. Consequently, the similarity in the surface peak intensities between Ti and Nb shows that Nb exist similarly in both first and third layers. The concentration of Nb in the present case is estimated to be about 6% from the surface peak intensities shown in Fig. 9. The distribution of Nb at the fifth layer can be investigated with use of 180”-ICISS. Not explicitly shown is the experimental result, but the polar angle scans of the Nb peak are found to be identical to those of the Ti peak. Thus, there exists no marked inclination in the impurity concentration at the TiCUll) surface. The same is essentially true for the other surfaces with the impurities of Zr, MO, and W, though the concentration is a little dependent on the species of the segregant or the degree of the faceting.
(a)
I
LOO
l’
Nb600
600
Energy
Ti
peak
1
CeV)
Fig. 9. Energy spectra of Lif scattered from the Nb-segregated TiC(111) surface obtained at the fiied scattering angle of 160”. The measurements were made in the [l%l azimuth.
the spectrum for LY= 58” is fairly broad for both matrix and segregant. It is notable that the shape of the Nb peak changes quite similarly to that of the Ti peak. The polar angle scans of the peak intensities are plotted in Fig. 10 for (a) Ti and (b) Nb peaks at two energy positions, A and B, indicated by arrows in Figs. 9a and 9b. In apparent contrast to the results for the (100) surface, the intensities of the Nb peak exhibit the angular variation quite similar to those of the Ti peak. The marked structure in the range of 50” < (Y< 90” comes from the shadowing and focusing effects of the Li+ ions on the third-layer atoms, while the plateau for cr < 50” arises mostly from the first layer. The steep intensity drop at (Y= 12” is due to the shadowing effect on the first-layer Ti atoms. If the surface were faceted considerably with the (1001 face, the shadowing should not occur so rapidly and the intensity would remain
40
60
60
100
60
60
100
a (deg. )
0
20
LO OL (deg.)
Fig. 10. The polar angle scans of 160”-ICISS along the [l?l] azimuth of the Nb-segregated TiC(111) surface. The surface peak intensities at the energy positions A and B, indicated by arrows in Figs. 9a and 9b, are plotted against a.
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4. Conclusion
6. References
Low-energy Li + ion scattering is well suited for studying depth dependent surface segregation effects. By applying two different scattering angles to achieve focusing of the ion beam onto the desired surface layers, the layer-by-layer analysis of the atomic concentration is performed with a considerable sensitivity. The demonstration is made for the segregation of dilute transitionmetal impurities (0.6 wt%) in Tic. On the TiC(100) surface, the segregants (Ta, W, Zr, Nb, and MO) exist at the topmost layer by substitution of the Ti atoms and their concentrations amount to 90%. On the other hand, the segregants distribute with almost equal amounts at least in the first five layers of the TiC(ll1) surface though the heat treatment simultaneously induces faceting of the surface to some extent. According to the bond breaking theory, the component with the lowest bond strength will segregate. This effect is more pronounced on the loosely packed surfaces [24]. Since the cations in the (100) [(ill)] surfaces have 5 (3) nearest neighbor C atoms, the impurity should be concentrated on the topmost layer of the (111) surface more efficiently. In reality, the present results are in apparent contradiction to this simple picture. This is presumably because the nature of the bonding of the transition-metal carbide is rather specific, exhibiting significant covalent and ionic character in addition to the metallic bond. Also possible is that the faceting as well as the resulting lattice deformation may buffer the strain energy at the TiC(111) surface, and hence the impurities can be distributed in the deeper layers.
Segrega[II W.C. Johnson and J.M. Blakely, in: Interfacial tion (American Society for Metals, Metals Park, OH, 1979). Metal Carbides and Nitrides, Vol. L-21LE. Toth, Transition 7 (Academic Press, New York, 19711. 131 K. Schwarz, CRC Crit. Rev. in Solid State Mater. Sci. 13 (1987) 211. [41 C. Oshima, M. Aono, R. Nishitani and S. Kawai, Appl. Phys. Lett. 35 (19791 493. El Y. lshizawa, S. Aoki, C. Oshima and S. Otani, J. Phys. D 22 (1989) 1763. J. [61 I. Kojima, E. Miyazaki, Y. Inoue and I. Yasumori, Catal. 59 (1979) 472. G.K. Ivakhnyuk, N.V. Sirotinkin, A.A. [71 V.V. Samonin, Titov, V.P. Gusev, D.N. Gavrilov and N.F. Fedorov, Zh. Prikl. Khim. (Leningrad) 55 (1982) 453. [81 C. Oshima, M. Aono, S. Zaima, Y. Shibata and S. Kawai, J. Less-Common Met. 82 (1981) 69. [91 M. Aono and R. Souda, Jpn. J. Appl. Phys. 24 (1985) 1249. S. Otani, Y. Ishizawa and C. 1101 R. Souda, T. Aizawa, Oshima, Surf. Sci. 256 (1991) 19. Acta 15 (19701 1273. [Ill H. Bohm, Electrochim. [I21 R.B. Revy and M. Boudart, Science 181 (1973) 547. J. Catal. 39 (1975) 298; 48 [131 P.N. Ross and P. Stonehart, (1977) 42. 1141 A.L. Boers, Nucl. Instrum. Methods B 2 (19841 353. Ml R. Souda, T. Aizawa, C. Oshima and Y. Ishizawa, Nucl. Instrum. Methods B 45 (1990) 364. [161 H. Niehus and G. Comsa, Surf. Sci. 140 (1984) 18. [171 H. Niehus, Surf. Sci. 166 (19861 L107. [18] M. Katayama, E. Nomura, N. Kanekama, H. Soejima and M. Aono, Nucl. Instrum. Methods B 33 (19881 857. [19] K. Sumitomo, T. Kobayashi, F. Shoji and K. Oura, Phys. Rev. Lett. 66 (1991) 1193. [20] S. Otani, T. Tanaka and Y. Ishizawa, J. Cryst. Growth 92 (1988) 359. [21] P.M. Echenique, R.M. Nieminen, J.C. Ashley and R.H. Ritchie, Phys. Rev. A 33 (1986) 897. [22] R. Souda, T. Aizawa, S. Otani and Y. Ishizawa, Surf. Sci. 232 (19901 219. [23] Y. Gauthier, W. Hoffmann and M. Wuttig, Surf. Sci. 223 (19901 239. [24] S. Deckers, F.H.P.M. Habraken, W.F. van der Weg, A.W. Denier van der Con, B. Pluis, J.F. van der Veen and R. Baudoing, Phys. Rev. B 42 (19901 419. [25] J. Eymery and J.C. Joud, Surf. Sci. 231 (19901 419.
5. Acknowledgement This work was supported in part by the Special Coordination Funds Promoting for Science and Technology.