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Scanning tunneling microscopy of carbon- and sulfur-induced modifications of Ni(111) and Ni(110) surfaces
Vacuum/volume
Pergamon 0042-207X(95)001
15-8
Scanning tunneling microscopy induced modifications of Ni(lll) R Bicker and G H&z, Max-Planck-lnstitut 92, D-70174 Stuttgart, Germany
fur Metallforschung,
46/numbers S-IO/pages 1101 to 1104/1995 Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0042-207X/95$9.50+.00
of carbon- and sulfurand Ni(l10) surfaces lnstitut fur Werkstoffwissenschaft,
Seestral3e
On nickel sing/e crystals exhibiting (7 1 I), (I IO), and vicinal (I 10) surfaces, carbon or sulfur adsorbate layers have been produced under UHV conditions by annealing either purified samples in ethylene (6.7x lop4 Pa, 663-1000 K) or samples containing 5-7 ppm S in UHV at 823-l 123 K. The adsorbate-covered surfaces were analysed by Auger electron spectroscopy (AES) and imaged by scanning tunneling microscopy (STM). The adsorption of carbon on NitII 1) via ethylene dissociation was found to result in the epitaxial growth of a graphite monolayer. However, in the STM image, only three of six carbon atoms in the two-dimensional graphite lattice appear. In the case of the stepped vicinal to Ni(l10) surface Ni(771), carbon adsorption leads to the formation of uniformly distributed carbon islands with diameters between 30 and 100 A, but resolution of the structure on an atomic scale could not be achieved, Owing to sulfur segregation on Ni(1 II) at 823 K and approximately 0.2 ML (average value), a surface structure develops, which can be ascribed to the formation of a two-dimensional sulfide Ni,S involving surface reconstruction of the nickel substrate. On Ni(1 IO), sulfur segregation at 1043 K results in the growth of sulfur islands that exhibit at 0.4 ML a c(2x 2)-S overlayer structure.
1. Introduction It is well established that, on nickel surfaces, carbon monolayers (with two carbon atoms per one Ni atom) exhibiting the hexagonal structure of the basal plane of graphite can form under certain conditions of carbon activity and temperature and that this process, depending on the original surface orientation, may be accompanied by surface restructuring’-7. Furthermore, graphite monolayer formation and restructuring are found to be influenced by sulfur8-I”. The growth of the carbon monolayer on polycrystalline nickel owing to carburization in CHJHZ mixtures at 1150-1400 K has been studied by using the kinetic features of a subsequent decarburization in HZ as a probe for the coverage of the Ni surface with the graphite monolayer7~8~‘0. The surface areas that were covered with the graphite monolayer could be made visible via the precipitation of three-dimensional graphite. The distribution of these island-like areas indicated heterogeneous monolayer nucleation7,8. To obtain further information on the nucleation and growth of the graphite monolayer on Ni surfaces, on structural details of the carbon overlayer, and on surface restructuring, a scanning tunneling microscopy study was performed that also considered surface modifications induced by sulfur. In this paper, first results are presented referring to Ni single crystals exhibiting (11 I), (1 lo), and (771) surfaces that were covered with various amounts of carbon or sulfur. 2. Experimental Sample preparation and scanning tunneling microscopy were performed in an ultra-high vacuum (UHV) chamber
(STM) having
a base pressure below lo-* Pa. The UHV system is equipped with facilities for Auger electron spectroscopy (AES), quadrupole mass spectrometry, ion sputtering, and gas introduction. The design of the STM system used and the techniques involved are described elsewhere”~i2. The tips were electrochemically etched from polycrystalline tungsten wires. The Ni samples (3 mm x 9 mm; 0.5 mm thickness) were cut by spark erosion from a highpurity single crystal ingot. By using the Laue diffraction method, they were oriented to exhibit (11 l), (1 lo), and (711) surface planes. After mechanical and electrochemical polishing, the sample was introduced into the UHV system and for further preparation mounted on a manipulator. Heating was accomplished by electron bombardment ; the temperature was monitored by a chromel-alumel thermocouple, which was fixed to the sample holder. The sample was cleaned by successive cycles of Ar+ ion sputtering (3 keV, lo-’ Pa) and subsequent annealing in UHV up to 1123 K until the surface contaminations were reduced to values below the detection limit of AES. Carbon adsorbate layers on nickel were produced by annea!ing samples at temperatures between 663 and 1000 K in an ethylene (C,H,) atmosphere at 6.7 x 10m4 Pa for various times. Definite surface coverages with sulfur on nickel were established by an UHV annealing of samples containing some ppm S at temperatures between 823 and 1123 K. The surface composition of the prepared samples was determined by AES. After preparation, the Ni sample was transferred to the STM. The sample surfaces were imaged in both ‘constant current (topographic)’ and ‘constant height’ modes with tunneling currents of 0.2-0.9 nA and 1101
R Bdcker and G H&z:
Scanning tunneling
microscopy of modifications
of Ni(l11)
and Ni(l IO) surfaces
typical sample bias voltages ranging from +0.02 to f0.2 V relative to the tip. Calibration of images resolved on an atomic scale was achieved by using the known atomic distances in a Si(l1 l)-(7 x 7) reconstruction test image.
(iii) the pattern in Figure l(a) is similar to that in most STM images of highly oriented pyrolytic graphite (HOPG)‘s’6 and similar to that found for graphite multilayers (3 nm thick) precipitated epitaxially on Ni(l1 1)i7.
3. Results and discussion
The array of carbon atoms in Figure l(a) does not show the six-fold symmetry of carbon atoms in the hexagonal basal plane of graphite but rather shows a three-fold symmetry. This means that as in the STM images of HOPG, only three of six carbon atoms in the two-dimensional graphite lattice appear (see Figure I(b)). In the case of HOPG, the structural STM results are discussed on the basis of the ABAB stacking sequence in the graphite lattice and the weak interlayer interactions that give rise to two non-equivalent carbon sites, A and B, within the twodimensional surface unit ce11’8-2”.This inequivalency of sites (carbon-site asymmetry) is thought to induce electronic effects resulting in a tunneling current that is higher for B-site atoms than for A-site atoms. Hence by STM, only the B atoms are ‘imaged”s22. Similar arguments may be put forward to understand the STM image (Figure 1(a)) of the graphite monolayer grown epitaxially on the Ni (111) surface. In this case, the weak interactions between the graphite monolayer and the Ni substrate cause an inequivalency of carbon sites in the monolayer with respect to the location of Ni atoms in the fee (111) substrate exhibiting an ABCABC stacking sequence. Two different carbon atom sites in the monolayer can be differentiated (Figure 1(b)) : carbon atoms that see a Ni atom directly below in the second substrate layer (hcp site) and carbon atoms that see a Ni atom directly below in the third substrate layer (fee site). Owing to this carbon-site asymmetry in the graphite monolayer, tunneling currents presumably occur which are very different for the two types of carbon atoms. Thus STM gives indication for only one type of carbon atoms in the graphite monolayer.
3.1 Graphite monolayer on Ni(ll1). After the annealing of a Ni single crystal with a (111) oriented surface in ethylene at 6.7 x 10m4 Pa and 1000 K, the STM image of a surface area of 30 A x 30 A (Figure l(a)) sh ows a triangular pattern in the array of bright spots, which can be attributed to the formation of a graphite monolayer. This interpretation is supported by the following findings : (i) AES
spectra indicate the presence of graphitic carbon on the surface, the degree of coverage being 0.6 ML (average value referring to a surface area with 100 pm diameter) ; (ii) the distance between the bright spots in the [ilO] direction in Figure l(a) (2.7 A) is comparable with the lattice parameter of graphite in the hexagonal basal plane (2.5 A); and
3.2 Carbon islands on the vicinal to Ni (110) surface Ni (771). The STM image of a Ni (77 1) surface in Figure 2 shows four (110) terraces, which are separated by monatomic steps (for characterization of an ideal Ni (771) surface, see the work of Koch et aF3). The numerous bright spots are assumed to be carbon islands
2.46A
2.L9A
Ni atoms beneath in second plane l in third plane o
Figure 1. (a) STM image (constant height mode) of a graphite monolayer on Ni (11 l), showin hexagonal symmetry ; crystal directions as indicated in Figure I(b) ; 30 x x 30 A, U, = 0.103 V, I, = 0.56 nA. @) Expected array of carbon atoms on Ni (111) in the grapbte monolayer; filled circles and open circles denote different carbon sites depending on whether, directly below the carbon atom, a Ni atom of the second (hcp site) or of the third lattice plane (fee site) is located. By STM, only either
the 0 carbon atoms or the @ carbon atoms are ‘visible’ as a triangular pattern. 1102
Figure 2. STM image (constant current mode) of the stepped vicinal to Ni (110) surface Ni (77 1) after annealing in ethylene at 663 K, showing carbon islands ; 1000 8, x 1000 A, U, = 0.2 V, Zt= 0.9 nA.
R Backer
and G H&z:
Scanning tunneling
microscopy
of modifications
of Ni(lll)
and Ni(l10)
surfaces
3.3
Formation of a c (2 x2)-S superstructure on Ni (110). After the annealing of a Ni (110) sample with a bulk sulfur content of 5 ppm at 1043 K, STM imaging shows sulfur islands with diameters of about 40 A. When the surface sulfur coverage is increased to about 0.4 ML by continued annealing at 1043 K, atomic resolution of the sulfur structure by STM can be achieved (Figure 3(a)). In Figure 3(b), the arrangement of the sulfur atoms on the Ni surface is modelled by assuming a c(2 x 2)-S superstructure on the non-reconstructed Ni (110) surface, which consists of sulfur atoms chemisorbed in two-fold hollow sites. These results confirm the findings of previous work28,“9. Moreover, in more recent STM investigations of the interaction of Ni (110) with sulfur3G32, at a sulfur coverage of 0.5 ML, a c(2 x 2)-S overlayer was observed at room temperature (RT). The c(2 x 2)-S phase belongs to a series of overlayers, which form at RT on Ni (110) with increasing sulfur coverage and which involve no surface restructurin$g-32.
Z.5A I/
-a I-In ti L
IS atoms
[ilo]
/ Ni atoms
Figure 3. (a) STM image (constant height mode) of a Ni (110) surface covered with sulfur (0.4 ML) ; crystal directions are rotated with respect to the directions given in Figure 3(b) ; 30 8, x 30 h;, U, = -0.1 V, 1, = 0.3 nA. (b) Proposed surface structure showing a c(2 x 2)-S superstructure.
with diameters of 2&30 A, which were formed during the preceding annealing treatment of the sample in ethylene (6.7 x 10m4 Pa, 663 K, 20 min). It may be seen that the (110) terraces are uniformly covered with the small carbon islands, indicating homogeneous nucleation of the islands. After an additional annealing of the sample at 933 K in UHV owing to coarsening, islands with diameters up to 100 A are observed. AES spectra give evidence that the structure in the carbon islands is graphite-like. However, structural resolution on an atomic scale by STM imaging could not be achieved. Considering the lattice misfit between a graphite monolayer and the (110) terraces of the Ni (771) surface, it is not likely that graphite monolayer formation occurs on surface areas consisting of more than a few rows of substrate atoms without surface restructuring. This is confirmed by previous results referring to graphitic carbon on Ni (771)24.25and Ni (11 0)26.27.In the present work, the carbon activity established on the Ni surface and in the near-surface region during the carburization annealing of the sample was obviously too low to induce surface restructuring as a prerequisite for carbon monolayer formation.
3.4 Sulfur adsorbate layers on Ni (111). Annealing treatments of a Ni (111) sample containing 7 ppm S (UHV, 823 K) resulted in sulfur coverages OS of 0.14.2 ML as determined by AES as average values related to a surface area with a diameter of 100 pm. Figure 4(a) shows that the adsorbed sulfur is arranged in a quadratic pattern, which does not reflect the three-fold rotational symmetry of the original fee Ni (111) substrate but does show the four-fold symmetry of fee (100) plane. This is considered as strong evidence for sulfur-induced surface restructuring. The structural model in Figure 4(b), describing the arrangement of sulfur atomsI’, follows the outline of a model proposed by Perdereau and 0udar29 on the basis of LEED data in which it is assumed that Ni adatoms sitting on Ni substrate atoms are located between the adsorbed sulfur atoms. The distance between next adjacent sulfur atoms is given as 4.81 A, which can be compared with the corresponding length of about 4.7 w as taken from the STM image in Figure 4(a). Perdereau and Ouda?’ interpreted the array of Ni and S atoms according to Figure 4(b) as an adsorbed two-dimensional Ni sulfide, N&S, involving surface restructuring. For a higher sulfur coverage of 0, z 0.5 ML, the same authorsz9 #claimed the formation of an adsorbed two-dimensional sulfide NiS also involving surface restructuring of the original Ni (111) substrate. In the present study, upon continued annealing of the Ni (111) sample in UHV at a higher temperature of 1123 K, the surface sulfur coverage 0s was increased up to about 0.4 ML (average value). Subsequent STM imaging of the surface revealed a rather complex arrangement of sulfur atoms”, which could not be described thoroughly by the above-mentioned structural modelZ9. It should be noted that 0udar33 later revised his hypothesis on the formation of two-dimensional Ni sulfides by taking into account diffraction data from more recent studies35,36 as well as his own work-function measurements33,34. In the meantime, further investigations37.38 have been carried out, which refer to the interaction of Ni (111) surfaces with sulf& at higher sulfur coverages of x 2)-S 0 s M 0.4 ML, considering especially the Ni (11 l)-~(54 structure. 4. Conclusions STM investigations (supported by AES measurements) of the adsorption or segregation of carbon or sulfur on various Ni single-crystal surfaces at 663-1123 K revealed different mechanisms of adsorbate-induced surface modifications depending on the original substrate surface orientation and the adsorbate. 1103
R Backer and G H&z:
Scanning tunneling
microscopy
of modifications
of Ni(l11)
and Ni(1 IO) surfaces
the fact that no large-area epitaxy between the graphite basal plane and the Ni (110) terraces exists. References
f2iil
I S atoms
/ Ni adatoms
Figure 4. (a) STM image (constant height mode) of a Ni (111) surface covered with sulfur; crystal directions as indicated in Figure 4(b) ; (0.2 ML, average value) ; 65 Ax 65 A, U, = - 0.1 V, Zt = 0.33 nA. (b) Proposed surface structure involving adsorbed two-dimensional N&S sulfide and reconstruction of the Ni substrate according to Perdererau and Oudar”.
(9 Upon the segregation
of sulfur, a simple c(2 x 2)-S overlayer forms on Ni (110) at 1043 K and 0, w 0.4 ML, whereas, on Ni (111) at 823 K and OS z 0.2 ML (average value), a reconstructed surface phase is observed, which can be described as an adsorbed two-dimensional sulfide N&S. (ii) Adsorption of carbon on Ni (111) at 1000 K leads to the epitaxial growth of a graphite monolayer that exhibits the structure of the hexagonal basal plane of graphite. However, as is found for highly oriented pyrolytic graphite (HOPG), only three of six carbon atoms of the two-dimensional graphite lattice are ‘imaged’ by STM. In contrast, on Ni (771) at 663-1000 K, only carbon islands form, but no graphite monolayer formation is detected. This is a consequence of
1104
‘J C Shelton, H R Pan1 and J M Blakely, SurfSci, 43,493 (1974). ‘M Eizenberg and J M Blakely, SurfSci, 82,228 (1979). 3M Eizenberg and J M Blakely, J Chem Phys, 71, 3467 (1979). 4R Kikowatz and K Flad and G Horz. J Vuc Scz Technol, A5, 1009 (1987). ‘G Hljrz, In Proceedings of Symposium on High Temperature Materials : Chenzistrv-IV (Edited bv Z A Munir and D Cubicciotty), Vol. 88-5, p. 111. Electrochemical Society, Pennington NJ, USA (1988). 6H-H Angermann and G Horz, Vacuum, 41,139 (1990). ‘H-H Angermann and G H&z, Surf Sci, 251/252, 809 (1991). ‘G H&z and H-H Angermann, Fresenius J Anal Chem, 341,408 (1991). 9H-H Angermann and G Hiirz, 2 Metallkd, 82,950 (1992). “H-H Angermann and G Hiirz, Appl SurfSci, 70/71, 163 (1993). “R Backer, PhD Thesis, University of Stuttgart (1994). “R Backer, R Kirchheim and G Hiirz, to be pubhshed. 13H-S Gtintherodt and R Wiesendanger (Editors), Scanning Tunneling Microscopy I (Springer Series in Surface Sciences 20). Springer-Verlag, Berlin, Germany (1992) p 132. ‘%-I Park and C F Quate, Appl Phys Left, 48, 112 (1986). “M Kuwabara, D R Clarke and D A Smith, Appl Phys Lett, 56, 2396 (1990). r6G Binnig H Fuchs, Ch Gerber, H Rohrer, E St011 and E Tosatti, Europhys iett, 1, 31 (1986). 17SYamazaki M Tanaka, S Tanaka, M Fuginami and R Uemori, J Vat Sci Technol, i9, 883 (1991). “1 P Batra, N Garcia, H Rohrer, H Salemmk, E Stoll and S Ciraci, Surf Sci, 181, 126 (1987). 191P Batra and S Ciraci. J Vat Sci Technol, A6,313 (1988) “D Tomanek S G Louie, H J Mamm, D W Abraham, R E Thomson, E Ganz and J Clarke, Phys Rev, B35,7790 (1987). “D Tomanek and S G Louie, Phys Rev, B37,8327 (1988). *‘J Tersoff, Phys Rev Lett, 57,440 (1986). ‘3R Koch, M Borbonus, 0 Haase and K H Rieder, Appl Phys, A55,417 (1992). “R Koch, 0 Haase, M Borbonus and K H Rieder, Phys Rev, B45, 1525 (1992). “0 Haase, R Koch, M Borbonus and K H Rieder, Ultramicroscopy, 4244,451 (1992). 26L Papagno and L S Caputi, Phys Rev, B29, 1483 (1984). 27L Papagno M Conti, L S Caputi, J Anderson and G J Lapeyre, Surf Sci, 219, L5i5 (1989). “D R Huntley, SudSci, 240, 13 (1990). 29M Perdererau and J Oudar, SuvfSci, 20,80 (1970). ‘OF Besenbacher, I Stensgaard, L Ruan, J K Nsrskov and K W Jacobsen, Surf Sci, 272, 334 (1992): 3’F Besenbacher. L Ruan and I Stensgaard, Phys Rev Lett, 69, 3523 (1992). 32L Ruan, I Stensgaard, E Laegsgaard and F Besenbacher, Surf Sci, 296, 275 (1993). 33J Oudar In Proceedmgs of Fourth International Conference on Solid Surfaces and Third European Conference on Surface Science, Cannes, Frances, 22-26 Sept. (1980), Vol. 1, p. 645. Supplement to Revue Le Vide, les Couches Minces, No. 201. 34J Oudar, Catal Rev-Sci Engng, 22, 171 (1980). 3*T Edmonds, J J McCarrol and R C Pitkethly, J Vat Sci Technol, 8,68 (1971). 36W Erley and H Wagner, J Catal, 53,287 (1978). 37Y Kitajim, T Yokoyama, T Ohta, M Funabashi, N Kosugi and H Kuroda, SurfSci, 214, L261 (1989). “Y-S Ku and S H Overbury, SurfSci, 276,262 (1992).
Pergamon 0042-207X(95)001
15-8
Scanning tunneling microscopy induced modifications of Ni(lll) R Bicker and G H&z, Max-Planck-lnstitut 92, D-70174 Stuttgart, Germany
fur Metallforschung,
46/numbers S-IO/pages 1101 to 1104/1995 Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0042-207X/95$9.50+.00
of carbon- and sulfurand Ni(l10) surfaces lnstitut fur Werkstoffwissenschaft,
Seestral3e
On nickel sing/e crystals exhibiting (7 1 I), (I IO), and vicinal (I 10) surfaces, carbon or sulfur adsorbate layers have been produced under UHV conditions by annealing either purified samples in ethylene (6.7x lop4 Pa, 663-1000 K) or samples containing 5-7 ppm S in UHV at 823-l 123 K. The adsorbate-covered surfaces were analysed by Auger electron spectroscopy (AES) and imaged by scanning tunneling microscopy (STM). The adsorption of carbon on NitII 1) via ethylene dissociation was found to result in the epitaxial growth of a graphite monolayer. However, in the STM image, only three of six carbon atoms in the two-dimensional graphite lattice appear. In the case of the stepped vicinal to Ni(l10) surface Ni(771), carbon adsorption leads to the formation of uniformly distributed carbon islands with diameters between 30 and 100 A, but resolution of the structure on an atomic scale could not be achieved, Owing to sulfur segregation on Ni(1 II) at 823 K and approximately 0.2 ML (average value), a surface structure develops, which can be ascribed to the formation of a two-dimensional sulfide Ni,S involving surface reconstruction of the nickel substrate. On Ni(1 IO), sulfur segregation at 1043 K results in the growth of sulfur islands that exhibit at 0.4 ML a c(2x 2)-S overlayer structure.
1. Introduction It is well established that, on nickel surfaces, carbon monolayers (with two carbon atoms per one Ni atom) exhibiting the hexagonal structure of the basal plane of graphite can form under certain conditions of carbon activity and temperature and that this process, depending on the original surface orientation, may be accompanied by surface restructuring’-7. Furthermore, graphite monolayer formation and restructuring are found to be influenced by sulfur8-I”. The growth of the carbon monolayer on polycrystalline nickel owing to carburization in CHJHZ mixtures at 1150-1400 K has been studied by using the kinetic features of a subsequent decarburization in HZ as a probe for the coverage of the Ni surface with the graphite monolayer7~8~‘0. The surface areas that were covered with the graphite monolayer could be made visible via the precipitation of three-dimensional graphite. The distribution of these island-like areas indicated heterogeneous monolayer nucleation7,8. To obtain further information on the nucleation and growth of the graphite monolayer on Ni surfaces, on structural details of the carbon overlayer, and on surface restructuring, a scanning tunneling microscopy study was performed that also considered surface modifications induced by sulfur. In this paper, first results are presented referring to Ni single crystals exhibiting (11 I), (1 lo), and (771) surfaces that were covered with various amounts of carbon or sulfur. 2. Experimental Sample preparation and scanning tunneling microscopy were performed in an ultra-high vacuum (UHV) chamber
(STM) having
a base pressure below lo-* Pa. The UHV system is equipped with facilities for Auger electron spectroscopy (AES), quadrupole mass spectrometry, ion sputtering, and gas introduction. The design of the STM system used and the techniques involved are described elsewhere”~i2. The tips were electrochemically etched from polycrystalline tungsten wires. The Ni samples (3 mm x 9 mm; 0.5 mm thickness) were cut by spark erosion from a highpurity single crystal ingot. By using the Laue diffraction method, they were oriented to exhibit (11 l), (1 lo), and (711) surface planes. After mechanical and electrochemical polishing, the sample was introduced into the UHV system and for further preparation mounted on a manipulator. Heating was accomplished by electron bombardment ; the temperature was monitored by a chromel-alumel thermocouple, which was fixed to the sample holder. The sample was cleaned by successive cycles of Ar+ ion sputtering (3 keV, lo-’ Pa) and subsequent annealing in UHV up to 1123 K until the surface contaminations were reduced to values below the detection limit of AES. Carbon adsorbate layers on nickel were produced by annea!ing samples at temperatures between 663 and 1000 K in an ethylene (C,H,) atmosphere at 6.7 x 10m4 Pa for various times. Definite surface coverages with sulfur on nickel were established by an UHV annealing of samples containing some ppm S at temperatures between 823 and 1123 K. The surface composition of the prepared samples was determined by AES. After preparation, the Ni sample was transferred to the STM. The sample surfaces were imaged in both ‘constant current (topographic)’ and ‘constant height’ modes with tunneling currents of 0.2-0.9 nA and 1101
R Bdcker and G H&z:
Scanning tunneling
microscopy of modifications
of Ni(l11)
and Ni(l IO) surfaces
typical sample bias voltages ranging from +0.02 to f0.2 V relative to the tip. Calibration of images resolved on an atomic scale was achieved by using the known atomic distances in a Si(l1 l)-(7 x 7) reconstruction test image.
(iii) the pattern in Figure l(a) is similar to that in most STM images of highly oriented pyrolytic graphite (HOPG)‘s’6 and similar to that found for graphite multilayers (3 nm thick) precipitated epitaxially on Ni(l1 1)i7.
3. Results and discussion
The array of carbon atoms in Figure l(a) does not show the six-fold symmetry of carbon atoms in the hexagonal basal plane of graphite but rather shows a three-fold symmetry. This means that as in the STM images of HOPG, only three of six carbon atoms in the two-dimensional graphite lattice appear (see Figure I(b)). In the case of HOPG, the structural STM results are discussed on the basis of the ABAB stacking sequence in the graphite lattice and the weak interlayer interactions that give rise to two non-equivalent carbon sites, A and B, within the twodimensional surface unit ce11’8-2”.This inequivalency of sites (carbon-site asymmetry) is thought to induce electronic effects resulting in a tunneling current that is higher for B-site atoms than for A-site atoms. Hence by STM, only the B atoms are ‘imaged”s22. Similar arguments may be put forward to understand the STM image (Figure 1(a)) of the graphite monolayer grown epitaxially on the Ni (111) surface. In this case, the weak interactions between the graphite monolayer and the Ni substrate cause an inequivalency of carbon sites in the monolayer with respect to the location of Ni atoms in the fee (111) substrate exhibiting an ABCABC stacking sequence. Two different carbon atom sites in the monolayer can be differentiated (Figure 1(b)) : carbon atoms that see a Ni atom directly below in the second substrate layer (hcp site) and carbon atoms that see a Ni atom directly below in the third substrate layer (fee site). Owing to this carbon-site asymmetry in the graphite monolayer, tunneling currents presumably occur which are very different for the two types of carbon atoms. Thus STM gives indication for only one type of carbon atoms in the graphite monolayer.
3.1 Graphite monolayer on Ni(ll1). After the annealing of a Ni single crystal with a (111) oriented surface in ethylene at 6.7 x 10m4 Pa and 1000 K, the STM image of a surface area of 30 A x 30 A (Figure l(a)) sh ows a triangular pattern in the array of bright spots, which can be attributed to the formation of a graphite monolayer. This interpretation is supported by the following findings : (i) AES
spectra indicate the presence of graphitic carbon on the surface, the degree of coverage being 0.6 ML (average value referring to a surface area with 100 pm diameter) ; (ii) the distance between the bright spots in the [ilO] direction in Figure l(a) (2.7 A) is comparable with the lattice parameter of graphite in the hexagonal basal plane (2.5 A); and
3.2 Carbon islands on the vicinal to Ni (110) surface Ni (771). The STM image of a Ni (77 1) surface in Figure 2 shows four (110) terraces, which are separated by monatomic steps (for characterization of an ideal Ni (771) surface, see the work of Koch et aF3). The numerous bright spots are assumed to be carbon islands
2.46A
2.L9A
Ni atoms beneath in second plane l in third plane o
Figure 1. (a) STM image (constant height mode) of a graphite monolayer on Ni (11 l), showin hexagonal symmetry ; crystal directions as indicated in Figure I(b) ; 30 x x 30 A, U, = 0.103 V, I, = 0.56 nA. @) Expected array of carbon atoms on Ni (111) in the grapbte monolayer; filled circles and open circles denote different carbon sites depending on whether, directly below the carbon atom, a Ni atom of the second (hcp site) or of the third lattice plane (fee site) is located. By STM, only either
the 0 carbon atoms or the @ carbon atoms are ‘visible’ as a triangular pattern. 1102
Figure 2. STM image (constant current mode) of the stepped vicinal to Ni (110) surface Ni (77 1) after annealing in ethylene at 663 K, showing carbon islands ; 1000 8, x 1000 A, U, = 0.2 V, Zt= 0.9 nA.
R Backer
and G H&z:
Scanning tunneling
microscopy
of modifications
of Ni(lll)
and Ni(l10)
surfaces
3.3
Formation of a c (2 x2)-S superstructure on Ni (110). After the annealing of a Ni (110) sample with a bulk sulfur content of 5 ppm at 1043 K, STM imaging shows sulfur islands with diameters of about 40 A. When the surface sulfur coverage is increased to about 0.4 ML by continued annealing at 1043 K, atomic resolution of the sulfur structure by STM can be achieved (Figure 3(a)). In Figure 3(b), the arrangement of the sulfur atoms on the Ni surface is modelled by assuming a c(2 x 2)-S superstructure on the non-reconstructed Ni (110) surface, which consists of sulfur atoms chemisorbed in two-fold hollow sites. These results confirm the findings of previous work28,“9. Moreover, in more recent STM investigations of the interaction of Ni (110) with sulfur3G32, at a sulfur coverage of 0.5 ML, a c(2 x 2)-S overlayer was observed at room temperature (RT). The c(2 x 2)-S phase belongs to a series of overlayers, which form at RT on Ni (110) with increasing sulfur coverage and which involve no surface restructurin$g-32.
Z.5A I/
-a I-In ti L
IS atoms
[ilo]
/ Ni atoms
Figure 3. (a) STM image (constant height mode) of a Ni (110) surface covered with sulfur (0.4 ML) ; crystal directions are rotated with respect to the directions given in Figure 3(b) ; 30 8, x 30 h;, U, = -0.1 V, 1, = 0.3 nA. (b) Proposed surface structure showing a c(2 x 2)-S superstructure.
with diameters of 2&30 A, which were formed during the preceding annealing treatment of the sample in ethylene (6.7 x 10m4 Pa, 663 K, 20 min). It may be seen that the (110) terraces are uniformly covered with the small carbon islands, indicating homogeneous nucleation of the islands. After an additional annealing of the sample at 933 K in UHV owing to coarsening, islands with diameters up to 100 A are observed. AES spectra give evidence that the structure in the carbon islands is graphite-like. However, structural resolution on an atomic scale by STM imaging could not be achieved. Considering the lattice misfit between a graphite monolayer and the (110) terraces of the Ni (771) surface, it is not likely that graphite monolayer formation occurs on surface areas consisting of more than a few rows of substrate atoms without surface restructuring. This is confirmed by previous results referring to graphitic carbon on Ni (771)24.25and Ni (11 0)26.27.In the present work, the carbon activity established on the Ni surface and in the near-surface region during the carburization annealing of the sample was obviously too low to induce surface restructuring as a prerequisite for carbon monolayer formation.
3.4 Sulfur adsorbate layers on Ni (111). Annealing treatments of a Ni (111) sample containing 7 ppm S (UHV, 823 K) resulted in sulfur coverages OS of 0.14.2 ML as determined by AES as average values related to a surface area with a diameter of 100 pm. Figure 4(a) shows that the adsorbed sulfur is arranged in a quadratic pattern, which does not reflect the three-fold rotational symmetry of the original fee Ni (111) substrate but does show the four-fold symmetry of fee (100) plane. This is considered as strong evidence for sulfur-induced surface restructuring. The structural model in Figure 4(b), describing the arrangement of sulfur atomsI’, follows the outline of a model proposed by Perdereau and 0udar29 on the basis of LEED data in which it is assumed that Ni adatoms sitting on Ni substrate atoms are located between the adsorbed sulfur atoms. The distance between next adjacent sulfur atoms is given as 4.81 A, which can be compared with the corresponding length of about 4.7 w as taken from the STM image in Figure 4(a). Perdereau and Ouda?’ interpreted the array of Ni and S atoms according to Figure 4(b) as an adsorbed two-dimensional Ni sulfide, N&S, involving surface restructuring. For a higher sulfur coverage of 0, z 0.5 ML, the same authorsz9 #claimed the formation of an adsorbed two-dimensional sulfide NiS also involving surface restructuring of the original Ni (111) substrate. In the present study, upon continued annealing of the Ni (111) sample in UHV at a higher temperature of 1123 K, the surface sulfur coverage 0s was increased up to about 0.4 ML (average value). Subsequent STM imaging of the surface revealed a rather complex arrangement of sulfur atoms”, which could not be described thoroughly by the above-mentioned structural modelZ9. It should be noted that 0udar33 later revised his hypothesis on the formation of two-dimensional Ni sulfides by taking into account diffraction data from more recent studies35,36 as well as his own work-function measurements33,34. In the meantime, further investigations37.38 have been carried out, which refer to the interaction of Ni (111) surfaces with sulf& at higher sulfur coverages of x 2)-S 0 s M 0.4 ML, considering especially the Ni (11 l)-~(54 structure. 4. Conclusions STM investigations (supported by AES measurements) of the adsorption or segregation of carbon or sulfur on various Ni single-crystal surfaces at 663-1123 K revealed different mechanisms of adsorbate-induced surface modifications depending on the original substrate surface orientation and the adsorbate. 1103
R Backer and G H&z:
Scanning tunneling
microscopy
of modifications
of Ni(l11)
and Ni(1 IO) surfaces
the fact that no large-area epitaxy between the graphite basal plane and the Ni (110) terraces exists. References
f2iil
I S atoms
/ Ni adatoms
Figure 4. (a) STM image (constant height mode) of a Ni (111) surface covered with sulfur; crystal directions as indicated in Figure 4(b) ; (0.2 ML, average value) ; 65 Ax 65 A, U, = - 0.1 V, Zt = 0.33 nA. (b) Proposed surface structure involving adsorbed two-dimensional N&S sulfide and reconstruction of the Ni substrate according to Perdererau and Oudar”.
(9 Upon the segregation
of sulfur, a simple c(2 x 2)-S overlayer forms on Ni (110) at 1043 K and 0, w 0.4 ML, whereas, on Ni (111) at 823 K and OS z 0.2 ML (average value), a reconstructed surface phase is observed, which can be described as an adsorbed two-dimensional sulfide N&S. (ii) Adsorption of carbon on Ni (111) at 1000 K leads to the epitaxial growth of a graphite monolayer that exhibits the structure of the hexagonal basal plane of graphite. However, as is found for highly oriented pyrolytic graphite (HOPG), only three of six carbon atoms of the two-dimensional graphite lattice are ‘imaged’ by STM. In contrast, on Ni (771) at 663-1000 K, only carbon islands form, but no graphite monolayer formation is detected. This is a consequence of
1104
‘J C Shelton, H R Pan1 and J M Blakely, SurfSci, 43,493 (1974). ‘M Eizenberg and J M Blakely, SurfSci, 82,228 (1979). 3M Eizenberg and J M Blakely, J Chem Phys, 71, 3467 (1979). 4R Kikowatz and K Flad and G Horz. J Vuc Scz Technol, A5, 1009 (1987). ‘G Hljrz, In Proceedings of Symposium on High Temperature Materials : Chenzistrv-IV (Edited bv Z A Munir and D Cubicciotty), Vol. 88-5, p. 111. Electrochemical Society, Pennington NJ, USA (1988). 6H-H Angermann and G Horz, Vacuum, 41,139 (1990). ‘H-H Angermann and G H&z, Surf Sci, 251/252, 809 (1991). ‘G H&z and H-H Angermann, Fresenius J Anal Chem, 341,408 (1991). 9H-H Angermann and G Hiirz, 2 Metallkd, 82,950 (1992). “H-H Angermann and G Hiirz, Appl SurfSci, 70/71, 163 (1993). “R Backer, PhD Thesis, University of Stuttgart (1994). “R Backer, R Kirchheim and G Hiirz, to be pubhshed. 13H-S Gtintherodt and R Wiesendanger (Editors), Scanning Tunneling Microscopy I (Springer Series in Surface Sciences 20). Springer-Verlag, Berlin, Germany (1992) p 132. ‘%-I Park and C F Quate, Appl Phys Left, 48, 112 (1986). “M Kuwabara, D R Clarke and D A Smith, Appl Phys Lett, 56, 2396 (1990). r6G Binnig H Fuchs, Ch Gerber, H Rohrer, E St011 and E Tosatti, Europhys iett, 1, 31 (1986). 17SYamazaki M Tanaka, S Tanaka, M Fuginami and R Uemori, J Vat Sci Technol, i9, 883 (1991). “1 P Batra, N Garcia, H Rohrer, H Salemmk, E Stoll and S Ciraci, Surf Sci, 181, 126 (1987). 191P Batra and S Ciraci. J Vat Sci Technol, A6,313 (1988) “D Tomanek S G Louie, H J Mamm, D W Abraham, R E Thomson, E Ganz and J Clarke, Phys Rev, B35,7790 (1987). “D Tomanek and S G Louie, Phys Rev, B37,8327 (1988). *‘J Tersoff, Phys Rev Lett, 57,440 (1986). ‘3R Koch, M Borbonus, 0 Haase and K H Rieder, Appl Phys, A55,417 (1992). “R Koch, 0 Haase, M Borbonus and K H Rieder, Phys Rev, B45, 1525 (1992). “0 Haase, R Koch, M Borbonus and K H Rieder, Ultramicroscopy, 4244,451 (1992). 26L Papagno and L S Caputi, Phys Rev, B29, 1483 (1984). 27L Papagno M Conti, L S Caputi, J Anderson and G J Lapeyre, Surf Sci, 219, L5i5 (1989). “D R Huntley, SudSci, 240, 13 (1990). 29M Perdererau and J Oudar, SuvfSci, 20,80 (1970). ‘OF Besenbacher, I Stensgaard, L Ruan, J K Nsrskov and K W Jacobsen, Surf Sci, 272, 334 (1992): 3’F Besenbacher. L Ruan and I Stensgaard, Phys Rev Lett, 69, 3523 (1992). 32L Ruan, I Stensgaard, E Laegsgaard and F Besenbacher, Surf Sci, 296, 275 (1993). 33J Oudar In Proceedmgs of Fourth International Conference on Solid Surfaces and Third European Conference on Surface Science, Cannes, Frances, 22-26 Sept. (1980), Vol. 1, p. 645. Supplement to Revue Le Vide, les Couches Minces, No. 201. 34J Oudar, Catal Rev-Sci Engng, 22, 171 (1980). 3*T Edmonds, J J McCarrol and R C Pitkethly, J Vat Sci Technol, 8,68 (1971). 36W Erley and H Wagner, J Catal, 53,287 (1978). 37Y Kitajim, T Yokoyama, T Ohta, M Funabashi, N Kosugi and H Kuroda, SurfSci, 214, L261 (1989). “Y-S Ku and S H Overbury, SurfSci, 276,262 (1992).