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The Ni(100)(2×2)p4g–N reconstruction determined by surface X-ray diffraction
Surface Science 433–435 (1999) 317–321 www.elsevier.nl/locate/susc
The Ni(100)(2×2)p4g–N reconstruction determined by surface X-ray diffraction E. Dudzik a,1, A.G. Norris a, R. McGrath a, *, G. Charlton b, G. Thornton b, B. Murphy c, T.S. Turner c, D. Norman c a Surface Science Research Centre, The University of Liverpool, Liverpool L69 3BX, UK b Department of Chemistry, Manchester University, Manchester M13 9PL, UK c CLRC Daresbury Laboratory, Warrington WA4 4AD, UK
Abstract The ‘clock’ reconstruction of the Ni(100) (2×2)p4g–N system has been measured using surface X-ray diffraction. ˚ was found. This value is smaller than An in-plane displacement of the surface layer Ni atoms of d =0.30±0.01 A xy that measured for this system using photoelectron diffraction (PED) and surface extended X-ray absorption fine structure (SEXAFS). Possible reasons for this are discussed. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Nickel; Nitrogen; Single crystal surfaces; Surface relaxation and reconstruction; X-ray scattering
1. Introduction So-called ’clock’ reconstructions are a type of reconstruction found in a variety of transitionmetal adsorbate systems: simple adsorbate overlayers such as Ni(100)–N [1–4], Ni(100)–C [4–8] and Rh(100)–O [9,10]; in systems where the adsorbate goes subsurface, e.g. Cu(100)–D [11]; and systems where surface alloying takes place, e.g. Cu(100)–Pd [12–17] and Pd(100)–Al [18,19]. The unifying characteristic is that at a certain adsorbate coverage surface, metal atoms move tangentially in-plane with respect to the adsorbate atoms in alternate clockwise and anticlockwise rotations. The result of this displacement is a longer nearest* Corresponding author. Fax: +44-151-708-0662. E-mail address: [email protected] (R. McGrath) 1 Present address: CLRC Daresbury Laboratory, Warrington WA4 4AD, UK.
neighbour distance and hence a reduction of adsorbate-induced stress. Fig. 1 shows the directions of movement of the in-plane metal atoms in the case of a simple adsorbate such as N. The structures of several of these systems, including Ni(100) (2×2)p4g–N, have been measured using a variety of structural techniques. The in-plane displacement d (see Fig. 1) for the xy Ni(100)–N system was measured using PED ˚ ) [4] and SEXAFS (0.77±0.10 A ˚) (0.55±0.20 A [1,2]. Here, we present a new structural determination of this reconstruction using the surface X-ray diffraction (SXRD) technique. This technique is particularly sensitive to in-place displacement because of the large in-plane momentum transfer due to the grazing incidence geometry. We compare the value of d obtained with previous measxy urements of this and similar systems and discuss possible reasons for differences in the values obtained.
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to determine the roughness of the surface. Occasionally, further sputter/anneal cycles were found to be necessary to remove polishing damage not visible by LEED. The Ni(100) (2×2)p4g–N reconstruction was prepared by sputtering the clean Ni(100) surface in 5×10−5 Torr of N , at a 2 current of 2 mA and a voltage of 500 V for 10 min. At this stage, the p4g reconstruction is visible as a ‘fuzzy’ LEED pattern. An anneal to 600 K for about 5 min then produces a sharper LEED pattern. The p4g reconstruction remained stable over several days at a base pressure of 2×10−10 Torr; the quality of the reconstructed surface was monitored using the peak shape and structure factor of the (0.5 1.5 0.3) fractional order diffraction peak. The scanning tunnelling microscopy (STM ) measurements were performed in UHV using Omicron instruments at Liverpool and Manchester universities. Fig. 1. Top view and side view (in a 110 azimuth) of the (2×2)p4g structure formed by C or N on Ni(100), showing the structural fit parameters used in this work. The adsorbate is represented by the filled circles and the Ni by open circles. The unbroken lines in the side view indicate the z-positions of the unreconstructed bulk lattice planes. The square in the top view shows the bulk unit cell.
2. Experimental The SXRD measurements were performed in the new INGRID (INstrument for GRazing Incidence Diffraction) endstation on beamline 9.4 of the Daresbury Synchrotron Radiation Source (SRS ). This chamber allows the use of the 5-circle surface X-ray diffractometer with an ultra-high vacuum ( UHV ) sample environment. The out-ofplane detector angle can be set to two fixed positions at c=0 and c=15. An X-ray wavelength of ˚ was selected with the Si(111) double crystal 0.9 A monochromator. Ni samples were prepared by repeated sputter and anneal cycles until Auger electron spectroscopy (AES) measurements showed no visible contamination and ‘sharp’ (1×1) low-energy electron diffraction (LEED) patterns were obtained. SXRD scans of fractional order or anti-Bragg peaks were then used in between sputtering and annealing cycles to monitor in-plane order until the domain size was maximized
3. Data acquisition X-ray diffraction data were taken both in the in-plane geometry (with a low perpendicular momentum transfer) and along diffraction rods. At each point in reciprocal space, a scan was made through the Bragg condition by rotating the sample around its surface normal. The structure factor at each point was then determined by taking the square root of the integrated intensity of the peak after applying angular corrections for the polarization, the Lorentz factor and the illuminated area [20]. Where possible, data were taken from symmetry equivalent rods and averaged. Although the error in the structure factors produced by the integration process was usually quite low – of the order of 2% (depending on the signal/noise level of each scan) – the reproducibility between symmetry equivalent rods was found to be of the order of 10%, so that this higher systematic error was used for the structure factor data sets to allow for imperfections of the alignment matrix and the crystal. Data calculated from structural models were fitted to these structure factor data sets using a least-squares fit. This was done with the program ‘’ [21]. All Miller indices hkl given here refer to the
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the p4g LEED pattern are the (0 0.5) in the LEED notation and the (0.5 0.5) in the bulk notation.
4. Results
Fig. 2. In-plane data for the Ni(100) (2×2)p4g–N surface taken at l=0.3. The radius of each circle is proportional to the structure factor; empty semicircles represent data, and filled semicircles represent structure factors calculated using the best fit model.
bulk unit cell (a =[100] , a =[010] , a =[001] ), 1 c 2 c 3 c which is rotated by 45° with respect to the surface unit cell commonly used in LEED, and which is larger by a factor 앀2. Thus, the missing spots in
Data were taken along the (11), (22) and (20) diffraction rods and in the in-plane geometry. The ˚) comparatively small lattice constant of Ni (3.52 A meant that no higher perpendicular momentum transfer than l=2.5 could be reached. Two fractional order overlayer rods were measured as well, the (0.5 1.5) and the (0.5 2.5). The in-plane data and the results of the least-squares fit for the Ni(100) (2×2)p4g–N system are shown in Fig. 2. It was generally found during fitting that the values for the Debye–Waller temperature factors had only a minimal influence on x2, so that the errors on these were of the order of 100%. For this reason, the temperature factors for both Ni and N were ˚ 2 [22]. kept fixed at the value for bulk Ni, 0.37 A The structural parameters used in the fit are shown in Fig. 1. Following previous results by Gauthier and co-workers for the Ni(100) (2×2)p4g–C system [7], a second-layer buckling was introduced as well as the top-layer in-plane and out-of-plane displacement and the N out-ofplane displacement, a total of five structural fit
Table 1 Structural fit parameters for Ni(100)(2×2)p4g–N (this work) surface and results from previous structural measurements of this and similar systemsa System
h a
Ni(100)–N
0.5
Ni(100)–C
0.5
Rh(100)–O Cu(100)–D Pd(100)–Al Cu(100)–Pd
0.5 0.5 ≥0.5 1.0
Method
d a
d xy
d z
SXRD (this work) SEXAFS [1,2] PED [4] PED [4] LEED [5] SEXAFS [6 ] LEED [7] LEIS [10] LEIS [11] LEIS [18,19] LEED [13] LEIS [16,17]
0.20±0.10 – 0.25±0.05 0.25±0.05 0.30±0.12 0.2±0.2 0.31±0.05 0.6±0.1 – – – –
0.30±0.01 0.77±0.10 0.55±0.20 0.55±0.20 0.35±0.05 – 0.45±0.07 0.2±0.10 0.23 0.5±0.1 0.28 0.25±0.10
0.17±0.01 – 0.15±0.10 0.15±0.10 0.20±0.05 – 0.19±0.04 – – – – –
a All measurements are in angstroms. h is the adsorbate coverage in ML, d is the height of the adsorbate above the top bulk a a metal layer position, d is the in-plane top metal displacement and d is the vertical metal top layer displacement as shown in Fig. 1. xy z
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parameters. There were three further fit parameters, i.e. the scale factor and the occupancies for the top layer Ni and the N. The temperature factors were kept fixed as described above. The data set contained 149 reflections, of which 73 were not symmetry equivalent. The fit achieved a x2 of 0.9. The structural parameters found by the fit are shown in Table 1 (top row). The in-plane displace˚ . The height of the adsorment is d =0.30±0.01 A xy ˚ ; the bate above the surface is d =0.20±0.10 A N rather large percentage error reflects the fact that the N is a comparatively weak scatterer so that its contribution to the overall signal is quite weak. The upward relaxation of the top layer is ˚ . The results for the buckling of d =0.17±0.01 A z the second layer show that the Ni underneath the hollows in which the N is adsorbed follows this upward relaxation of the top layer Ni ˚ ), while the Ni underneath (d =0.11±0.07 A b1 those fourfold hollows into which the top layer Ni atoms move is displaced downwards (d =−0.07 b2 ˚ ). This is similar to the buckling ±0.07 A found in the Ni(100)–C system using LEED [7]. The occupancy found for the top layer Ni was 0.94±0.05 and for the N 0.85±0.16, again with a large percentage error for the N. The slightly reduced occupancy of the top layer Ni presumably reflects the fact that some surface defects are produced by the N sputtering during the preparation of the p4g surface.
D), and for Cu(100)–Pd (here, the adsorbate intermixes with the substrate and surface alloying takes place). The values of d for the Ni(100)–N case as xy measured by PED and SEXAFS are large compared to our determination of this system and other determinations of similar systems. Our confidence in the SXRD results stems from the fact
5. Discussion and summary In Table 1, we compare the results of our measurements with those of previous studies of Ni(100)–N and other systems where the p4g reconstruction occurs. The main discrepancy with previous Ni(100)–N measurements is in the in-plane displacement d , which is smaller than that xy obtained for the Ni(100)–N system using PED ˚ ) and SEXAFS (0.77 A ˚ ). All other mea(0.55 A sured parameters fall within the error bars of the previous results. The result is close to the values of d obtained for the Rh(100)–O system, for the xy Cu(100)–D system (which is somewhat dissimilar in that the reconstruction is driven by subsurface
Fig. 3. Two images of the Ni(100)p4g(2×2)–N surface result˚ 2 image ing from different sample preparations. (a) 400×400 A obtained after sputtering and annealing to 600 K. (b) ˚ 2 image obtained after sputtering and annealing to 400×400 A 520 K.
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that this technique is especially sensitive to in-place displacement because of the large in-plane momentum transfer q due to the grazing incidence parallel geometry. PED is certainly more sensitive to vertical displacements, due to the dominance of the backscattering of electrons by atoms directly behind the emitting atom, whereas SEXAFS measures in-plane displacements only by deduction from measured bond-lengths. A possible explanation for the differences could be the nature of the surface being measured. We have found using scanning tunnelling microscopy (STM ) [23] that the quality of the surface varies considerably with preparation, in particular the annealing stage after N sputtering which repairs 2 the sputtering damage. Large terraces of width ˚ can be formed at 600 K, but if a lower ≥400 A annealing temperature is used (of order 520 K ), surfaces are formed with terrace widths of typically ˚ and large numbers of pits and defects (see 50 A Fig. 3). Measurement on a very defected surface may result in different average structural values for the surface because of the influence of steps and defects. By measuring peak widths from diffraction peaks arising from the reconstruction, using SXRD, we are able to tune the annealing temperature so that surfaces of a large terrace width are formed. In summary, we have measured the structural parameters of the Ni(100) (2×2)p4g–N reconstruction using SXRD. The results overall agree well with other determinations, although the value of d is smaller than that deduced from PED or xy SEXAFS. The reasons for these differences have been discussed.
Acknowledgements This work was supported by EPSRC grant GR/K/23225. The authors would like to thank S. Bennett, C. Muryn, J.M.C. Thornton and M.J. Scantlebury for help in the SXRD data
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collection, and Th. Bertrams, A. Munz and P.W. Murray for assistance with STM imaging.
References [1] L. Wenzel, D. Arvanitis, W. Daum, H.H. Rotermund, J. Sto¨hr, K. Baberschke, H. Ibach, Phys. Rev. B 36 (1987) 7689. [2] D. Arvanitis, K. Baberschke, L. Wenzel, Phys. Rev. B 37 (1988) 7143. [3] F.M. Leibsle, Surf. Sci. 297 (1993) 98. [4] A.L.D. Kilcoyne, D.P. Woodruff, A.W. Robinson, Th. Lindner, J.S. Somers, A.M. Bradshaw, Surf. Sci. 253 (1991) 107. [5] J.H. Onuferko, D.P. Woodruff, B.W. Holland, Surf. Sci. 87 (1979) 3573. [6 ] M. Bader, C. Ocal, B. Hillert, J. Haase, A.M. Bradshaw, Phys. Rev. B 35 (1987) 5900. [7] Y. Gauthier, R. Baudoing-Savois, K. Heinz, H. Landskron, Surf. Sci. 251/252 (1991) 493. [8] C. Klink, L. Olesen, F. Besenbacher, I. Stensgaard, E. Lægsgaard, N.D. Lang, Phys. Rev. Lett. 71 (1993) 4350. [9] J.R. Mercer, P. Finetti, F.M. Leibsle, R. McGrath, V.R. Dhanak, A. Baraldi, K.C. Prince, R. Rosei, Surf. Sci. 352 (1996) 173. [10] Y.G. Shen, A. Qayyum, D.J. O’Conner, B.V. King, Phys. Rev. B 58 (1998) 10025. [11] M. Foss, F. Besenbacher, C. Klink, I. Stensgaard, Surf. Sci. 297 (1993) 98. [12] T.D. Pope, K. Griffiths, P.R. Norton, Surf. Sci. 306 (1994) 294. [13] T.D. Pope, M. Vos, H.T. Tang, K. Griffiths, I.V. Mitchell, P.R. Norton, W. Liu, Y.S. Li, K.A.R. Mitchell, Z.-J. Tian, J.E. Black, Surf. Sci. 337 (1995) 79. [14] P.W. Murray, I. Stensgaard, E. Lægsgaard, F. Besenbacher, Phys. Rev. B 52 (1995) R14404. [15] P.W. Murray, I. Stensgaard, E. Lægsgaard, F. Besenbacher, Surf. Sci. B 365 (1996) 591. [16 ] J. Yao, Y.G. Shen, D.J. O’Conner, B.V. King, Surf. Sci. 359 (1996) 65. [17] Y.G. Shen, J. Yao, D.J. O’Conner, B.V. King, R.J. MacDonald, J. Phys.: Condens. Matter 8 (1996) 4903. [18] Y.G. Shen, D.J. O’Conner, R.J. MacDonald, J. Phys.: Condens. Matter 9 (1997) 8345. [19] Y.G. Shen, D.J. O’Conner, R.J. MacDonald, J. Phys.: Condens. Matter 9 (1997) 9459. [20] R. Feidenhans’l, Surf. Sci. Rep. 10 (1989) 105. [21] E. Vlieg, J.F. van der Veen, S.J. Gurman, C. Norris, J.E. MacDonald, Surf. Sci. 210 (1989) 301. [22] N.M. Butt, J. Bashir, B.T.M. Willis, G. Heger, Acta Crystallogr. A 44 (1988) 396. [23] A.G. Norris, M.J. Scantlebury, A. Munz, T.H. Bertrams, P. Finetti, E. Dudzik, P.W. Murray, R. McGrath, Surf. Sci. 424 (1999) 74.
The Ni(100)(2×2)p4g–N reconstruction determined by surface X-ray diffraction E. Dudzik a,1, A.G. Norris a, R. McGrath a, *, G. Charlton b, G. Thornton b, B. Murphy c, T.S. Turner c, D. Norman c a Surface Science Research Centre, The University of Liverpool, Liverpool L69 3BX, UK b Department of Chemistry, Manchester University, Manchester M13 9PL, UK c CLRC Daresbury Laboratory, Warrington WA4 4AD, UK
Abstract The ‘clock’ reconstruction of the Ni(100) (2×2)p4g–N system has been measured using surface X-ray diffraction. ˚ was found. This value is smaller than An in-plane displacement of the surface layer Ni atoms of d =0.30±0.01 A xy that measured for this system using photoelectron diffraction (PED) and surface extended X-ray absorption fine structure (SEXAFS). Possible reasons for this are discussed. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Nickel; Nitrogen; Single crystal surfaces; Surface relaxation and reconstruction; X-ray scattering
1. Introduction So-called ’clock’ reconstructions are a type of reconstruction found in a variety of transitionmetal adsorbate systems: simple adsorbate overlayers such as Ni(100)–N [1–4], Ni(100)–C [4–8] and Rh(100)–O [9,10]; in systems where the adsorbate goes subsurface, e.g. Cu(100)–D [11]; and systems where surface alloying takes place, e.g. Cu(100)–Pd [12–17] and Pd(100)–Al [18,19]. The unifying characteristic is that at a certain adsorbate coverage surface, metal atoms move tangentially in-plane with respect to the adsorbate atoms in alternate clockwise and anticlockwise rotations. The result of this displacement is a longer nearest* Corresponding author. Fax: +44-151-708-0662. E-mail address: [email protected] (R. McGrath) 1 Present address: CLRC Daresbury Laboratory, Warrington WA4 4AD, UK.
neighbour distance and hence a reduction of adsorbate-induced stress. Fig. 1 shows the directions of movement of the in-plane metal atoms in the case of a simple adsorbate such as N. The structures of several of these systems, including Ni(100) (2×2)p4g–N, have been measured using a variety of structural techniques. The in-plane displacement d (see Fig. 1) for the xy Ni(100)–N system was measured using PED ˚ ) [4] and SEXAFS (0.77±0.10 A ˚) (0.55±0.20 A [1,2]. Here, we present a new structural determination of this reconstruction using the surface X-ray diffraction (SXRD) technique. This technique is particularly sensitive to in-place displacement because of the large in-plane momentum transfer due to the grazing incidence geometry. We compare the value of d obtained with previous measxy urements of this and similar systems and discuss possible reasons for differences in the values obtained.
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to determine the roughness of the surface. Occasionally, further sputter/anneal cycles were found to be necessary to remove polishing damage not visible by LEED. The Ni(100) (2×2)p4g–N reconstruction was prepared by sputtering the clean Ni(100) surface in 5×10−5 Torr of N , at a 2 current of 2 mA and a voltage of 500 V for 10 min. At this stage, the p4g reconstruction is visible as a ‘fuzzy’ LEED pattern. An anneal to 600 K for about 5 min then produces a sharper LEED pattern. The p4g reconstruction remained stable over several days at a base pressure of 2×10−10 Torr; the quality of the reconstructed surface was monitored using the peak shape and structure factor of the (0.5 1.5 0.3) fractional order diffraction peak. The scanning tunnelling microscopy (STM ) measurements were performed in UHV using Omicron instruments at Liverpool and Manchester universities. Fig. 1. Top view and side view (in a 110 azimuth) of the (2×2)p4g structure formed by C or N on Ni(100), showing the structural fit parameters used in this work. The adsorbate is represented by the filled circles and the Ni by open circles. The unbroken lines in the side view indicate the z-positions of the unreconstructed bulk lattice planes. The square in the top view shows the bulk unit cell.
2. Experimental The SXRD measurements were performed in the new INGRID (INstrument for GRazing Incidence Diffraction) endstation on beamline 9.4 of the Daresbury Synchrotron Radiation Source (SRS ). This chamber allows the use of the 5-circle surface X-ray diffractometer with an ultra-high vacuum ( UHV ) sample environment. The out-ofplane detector angle can be set to two fixed positions at c=0 and c=15. An X-ray wavelength of ˚ was selected with the Si(111) double crystal 0.9 A monochromator. Ni samples were prepared by repeated sputter and anneal cycles until Auger electron spectroscopy (AES) measurements showed no visible contamination and ‘sharp’ (1×1) low-energy electron diffraction (LEED) patterns were obtained. SXRD scans of fractional order or anti-Bragg peaks were then used in between sputtering and annealing cycles to monitor in-plane order until the domain size was maximized
3. Data acquisition X-ray diffraction data were taken both in the in-plane geometry (with a low perpendicular momentum transfer) and along diffraction rods. At each point in reciprocal space, a scan was made through the Bragg condition by rotating the sample around its surface normal. The structure factor at each point was then determined by taking the square root of the integrated intensity of the peak after applying angular corrections for the polarization, the Lorentz factor and the illuminated area [20]. Where possible, data were taken from symmetry equivalent rods and averaged. Although the error in the structure factors produced by the integration process was usually quite low – of the order of 2% (depending on the signal/noise level of each scan) – the reproducibility between symmetry equivalent rods was found to be of the order of 10%, so that this higher systematic error was used for the structure factor data sets to allow for imperfections of the alignment matrix and the crystal. Data calculated from structural models were fitted to these structure factor data sets using a least-squares fit. This was done with the program ‘’ [21]. All Miller indices hkl given here refer to the
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the p4g LEED pattern are the (0 0.5) in the LEED notation and the (0.5 0.5) in the bulk notation.
4. Results
Fig. 2. In-plane data for the Ni(100) (2×2)p4g–N surface taken at l=0.3. The radius of each circle is proportional to the structure factor; empty semicircles represent data, and filled semicircles represent structure factors calculated using the best fit model.
bulk unit cell (a =[100] , a =[010] , a =[001] ), 1 c 2 c 3 c which is rotated by 45° with respect to the surface unit cell commonly used in LEED, and which is larger by a factor 앀2. Thus, the missing spots in
Data were taken along the (11), (22) and (20) diffraction rods and in the in-plane geometry. The ˚) comparatively small lattice constant of Ni (3.52 A meant that no higher perpendicular momentum transfer than l=2.5 could be reached. Two fractional order overlayer rods were measured as well, the (0.5 1.5) and the (0.5 2.5). The in-plane data and the results of the least-squares fit for the Ni(100) (2×2)p4g–N system are shown in Fig. 2. It was generally found during fitting that the values for the Debye–Waller temperature factors had only a minimal influence on x2, so that the errors on these were of the order of 100%. For this reason, the temperature factors for both Ni and N were ˚ 2 [22]. kept fixed at the value for bulk Ni, 0.37 A The structural parameters used in the fit are shown in Fig. 1. Following previous results by Gauthier and co-workers for the Ni(100) (2×2)p4g–C system [7], a second-layer buckling was introduced as well as the top-layer in-plane and out-of-plane displacement and the N out-ofplane displacement, a total of five structural fit
Table 1 Structural fit parameters for Ni(100)(2×2)p4g–N (this work) surface and results from previous structural measurements of this and similar systemsa System
h a
Ni(100)–N
0.5
Ni(100)–C
0.5
Rh(100)–O Cu(100)–D Pd(100)–Al Cu(100)–Pd
0.5 0.5 ≥0.5 1.0
Method
d a
d xy
d z
SXRD (this work) SEXAFS [1,2] PED [4] PED [4] LEED [5] SEXAFS [6 ] LEED [7] LEIS [10] LEIS [11] LEIS [18,19] LEED [13] LEIS [16,17]
0.20±0.10 – 0.25±0.05 0.25±0.05 0.30±0.12 0.2±0.2 0.31±0.05 0.6±0.1 – – – –
0.30±0.01 0.77±0.10 0.55±0.20 0.55±0.20 0.35±0.05 – 0.45±0.07 0.2±0.10 0.23 0.5±0.1 0.28 0.25±0.10
0.17±0.01 – 0.15±0.10 0.15±0.10 0.20±0.05 – 0.19±0.04 – – – – –
a All measurements are in angstroms. h is the adsorbate coverage in ML, d is the height of the adsorbate above the top bulk a a metal layer position, d is the in-plane top metal displacement and d is the vertical metal top layer displacement as shown in Fig. 1. xy z
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parameters. There were three further fit parameters, i.e. the scale factor and the occupancies for the top layer Ni and the N. The temperature factors were kept fixed as described above. The data set contained 149 reflections, of which 73 were not symmetry equivalent. The fit achieved a x2 of 0.9. The structural parameters found by the fit are shown in Table 1 (top row). The in-plane displace˚ . The height of the adsorment is d =0.30±0.01 A xy ˚ ; the bate above the surface is d =0.20±0.10 A N rather large percentage error reflects the fact that the N is a comparatively weak scatterer so that its contribution to the overall signal is quite weak. The upward relaxation of the top layer is ˚ . The results for the buckling of d =0.17±0.01 A z the second layer show that the Ni underneath the hollows in which the N is adsorbed follows this upward relaxation of the top layer Ni ˚ ), while the Ni underneath (d =0.11±0.07 A b1 those fourfold hollows into which the top layer Ni atoms move is displaced downwards (d =−0.07 b2 ˚ ). This is similar to the buckling ±0.07 A found in the Ni(100)–C system using LEED [7]. The occupancy found for the top layer Ni was 0.94±0.05 and for the N 0.85±0.16, again with a large percentage error for the N. The slightly reduced occupancy of the top layer Ni presumably reflects the fact that some surface defects are produced by the N sputtering during the preparation of the p4g surface.
D), and for Cu(100)–Pd (here, the adsorbate intermixes with the substrate and surface alloying takes place). The values of d for the Ni(100)–N case as xy measured by PED and SEXAFS are large compared to our determination of this system and other determinations of similar systems. Our confidence in the SXRD results stems from the fact
5. Discussion and summary In Table 1, we compare the results of our measurements with those of previous studies of Ni(100)–N and other systems where the p4g reconstruction occurs. The main discrepancy with previous Ni(100)–N measurements is in the in-plane displacement d , which is smaller than that xy obtained for the Ni(100)–N system using PED ˚ ) and SEXAFS (0.77 A ˚ ). All other mea(0.55 A sured parameters fall within the error bars of the previous results. The result is close to the values of d obtained for the Rh(100)–O system, for the xy Cu(100)–D system (which is somewhat dissimilar in that the reconstruction is driven by subsurface
Fig. 3. Two images of the Ni(100)p4g(2×2)–N surface result˚ 2 image ing from different sample preparations. (a) 400×400 A obtained after sputtering and annealing to 600 K. (b) ˚ 2 image obtained after sputtering and annealing to 400×400 A 520 K.
E. Dudzik et al. / Surface Science 433–435 (1999) 317–321
that this technique is especially sensitive to in-place displacement because of the large in-plane momentum transfer q due to the grazing incidence parallel geometry. PED is certainly more sensitive to vertical displacements, due to the dominance of the backscattering of electrons by atoms directly behind the emitting atom, whereas SEXAFS measures in-plane displacements only by deduction from measured bond-lengths. A possible explanation for the differences could be the nature of the surface being measured. We have found using scanning tunnelling microscopy (STM ) [23] that the quality of the surface varies considerably with preparation, in particular the annealing stage after N sputtering which repairs 2 the sputtering damage. Large terraces of width ˚ can be formed at 600 K, but if a lower ≥400 A annealing temperature is used (of order 520 K ), surfaces are formed with terrace widths of typically ˚ and large numbers of pits and defects (see 50 A Fig. 3). Measurement on a very defected surface may result in different average structural values for the surface because of the influence of steps and defects. By measuring peak widths from diffraction peaks arising from the reconstruction, using SXRD, we are able to tune the annealing temperature so that surfaces of a large terrace width are formed. In summary, we have measured the structural parameters of the Ni(100) (2×2)p4g–N reconstruction using SXRD. The results overall agree well with other determinations, although the value of d is smaller than that deduced from PED or xy SEXAFS. The reasons for these differences have been discussed.
Acknowledgements This work was supported by EPSRC grant GR/K/23225. The authors would like to thank S. Bennett, C. Muryn, J.M.C. Thornton and M.J. Scantlebury for help in the SXRD data
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collection, and Th. Bertrams, A. Munz and P.W. Murray for assistance with STM imaging.
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