- Email: [email protected]
Structural investigation of the NiO(111) single crystal surface
ELSEVIER
Surface Science 392 ( 1997) Ll5%L20
Surface Science Letters
Structural investigation of the NiO( 111) single crystal surface A. Barbier *, G. Renaud CEA~Grenohle. De$zrtement de Recherche Fondamentale SW la Mat&e
Condens&, SP2M/IRS.
17. Rue dex Martyrs, 38054 Grenoble.
CPdes 9, France Received 23 June 1997: accepted
for publication
11 August
I997
_ Abstract
We report an in situ grazing incidence X-ray scattering study of the NiO( Ill ) single crystal surface. It evidences a relatively high stability of this surface. After an air anneal at 1300 K it is flat and almost not reconstructed. Annealing at 860 K in ultra-high vacuum leads to two domains: one covered by epitaxial and relaxed metallic Ni clusters associated with a highly ordered interfacial supercell on the coincidence site lattice, presumably a misfit dislocation network and one exhibiting a p(2 x 2) reconstruction. After oxidation the metallic Ni transforms again into NiO( 111) and only a p(2 x 2) reconstructed flat surface remains. In these conditions macroscopic ( 100) facetting was never observed. The genera1 behaviour of the NiO( 1I I ) surface is discussed and compared with other polar and non-polar oxide surfaces, with emphasis on the specific properties which could explain its stability. 0 1997 Elsevier Science B.V. Keywords: Nickel oxides; Oxidation;
Single crystal
epitaxy;
Single crystal
Ceramic surfaces are increasingly studied, both theoretically and experimentally, because they are involved in many fast innovating industrial sectors. Up to the very recent years they were mainly used as support for catalysis or thin film growth. Recent advances in their synthesis have extended the range of potential new applications. Indeed, ceramics as well as semiconductors or metals provide a large diversity of intrinsic properties: they might exhibit different gap widths, some of them are ferrimagnetic. ferromagnetic or anti-ferromagnetic. Unfortunately, because of their intrinsic insulating properties, only very little is known about the crystallographic structure of oxide surfaces. The NiO( 111) surface exhibits attractive physical properties and is thus particularly interesting. * Corresponding author. e-mail: [email protected]
Fax: (+ 33) 76885138;
0039-6028/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved PII SOO39-6028(97)00651-l
surface:
Surface
structure;
Wetting;
X-ray diffraction
NiO is an antiferromagnet and only a few years ago interfacial magnetic ordering effects on NiO based films were observed, including evidence of the interlayer exchange interaction [l] and antiferromagnetic ordering along the (111) planes in superlattices [2]. There are also hints of an enhanced reactivity of NiO( 111) films [3]. In spite of these interesting properties no experimental investigation of the NiO( 111) single crystal surface is available. Indeed, it is often thought that, like MgO( 11 l), the NiO( 111) surface is unstable because it is polar: the terminating plane of an ideally cut crystal contains only one ionic species OzP or Ni2+, so that a non-zero electrical charge and a dipolar momentum are left at the surface. It is thus subject to the classical Madelung problem, i.e., the divergence associated with the l/r term in the Coulomb potential of condensed systems. This has given rise, in the past, to the belief that the
A. Barbier. G. Renaud / Surface Science 392 (1997) LI5-L20
Coulomb interaction may be long-ranged and that polar surfaces must decompose in macroscopic (100) facets. Hence, most studies of ionic surfaces have been restricted to non-polar ones. On the other hand, recent theoretical works evidence that in many instances Coulombic effects seem to cancel almost completely at long range ( l/r5 behaviour). Wolf [4] demonstrated that the problem of the diverging surface potential may be simply overcome by summing over neutral shells of molecules; the polarization correction being avoided by choosing a basis molecule without dipole moment. He shows that within a simple cubic Bravais lattice with an octopole basis, (NiO),, a simple thermodynamic ground state at 0 K is found which corresponds to a p(2 x 2) reconstructed surface of finite energy, arranged in three atomic high micro-pyramids. Temperature effects and the nature of the terminating plane have been investigated by molecular dynamics simulations [ 51, which predict that the NiO( 111) surface could also be stabilized by oxidation, that the temperature should have only small effects on the overall behaviour and that Ni atoms should be on top of the pyramids. Possible surface relaxation yielding to even lower energy was not introduced. These very intriguing predictions rapidly led to trials in order to verify if there is or not a reconstruction on polar ( 111) surfaces, on NiO( 111) in particular. However, the NiO( 111) single-crystal surface is a good insulator at room temperature and its conductivity drops severely when the temperature decreases [ 61, preventing any investigation with electron based techniques due to charge build-up. It was thus very seductive to work on NiO( 111) ultra-thin films grown on single crystal substrates. A p(2 x 2) low energy electron diffraction (LEED) pattern was reported for 550 K grown films on Au( 111) [ 71 and for room temperature grown films on Ni ( 111) [ 81. However, these studies remain difficult because only a few layers can be grown on Au( 111) and because of the large lattice mismatch on the Ni( 111) substrates. On NiO/Ni ( 111) films, the pattern disappeared when water was introduced into the chamber due to hydroxyl adsorption and was restored upon annealing. However, this result stands in contradiction with a previous LEED work on the oxidation of
Ni(lll)whereonlyc(2x2)and(7~7)reconstructions were reported [9]. To overcome the charge effect problem we have investigated the NiO( 111) single-crystal surface by grazing incidence X-ray scattering (GIXS). Our results, performed on a single crystal surface, confirm experimentally the latest theoretical predictions [4,5]. The GIXS experiments were carried out on the French CRG-IF beamline, BM32 [lo], of the ESRF (European Synchrotron Radiation Facility, Grenoble, France). The beam energy was set to 18 keV to avoid fluorescence background from the Ni K-edge. The beamline optics was doubly focused. The beam size at the sample was 0.3 mm(H) x 1 mm(V). The vertical sample was in a ultra-high vacuum (UHV) chamber (base pressure 3 x lo-” mbar) equipped with large Be windows and coupled to an horizontal z-axis diffractometer. The incidence angle was set to the critical angle for total external reflection (0.2”). The NiO( 111) sample, purity 99.9%, was provided, aligned to better than 0. lo and polished by Crystal GmbH (Berlin, Germany). The surface basis vectors describe the triangular lattice that is appropriate for ( 111) surfaces, they are related to the bulk ones by: a, =[ 110]/2, b, = [0 ill/2 and c, = [ 1111. The h and k indexes are chosen to describe the in-plane momentum transfer, Q,, and / the perpendicular one, Q,. All three indexes are expressed in the reciprocal lattice units (r.1.u.) of NiO( 111). The as-polished sample yielded a flickering and fuzzy RHEED pattern exhibiting thin rings typical of a polishing induced polycrystalline surface texture. Since GIXS needs single crystals of high bulk and surface quality, these ones had to be first re-crystallized. To check the stability of the bulk lattice against decomposition and follow the improvement of crystal quality, in-plane rocking scans of the (110) Bragg peak were performed on another sample during in situ annealing at increasing temperatures until melting (2200 K). Fig. la, shows that the near surface crystalline quality improves drastically during this treatment: the initial mosaic spread of ci 2” falls to 0.1” without change in the structure since the integrated intensity remains constant. Other crystals were air annealed
A. Barbier, G. Renaud I Surface Science 392 (1997j L15-L20
e [r.l.u.]
6
Fig. I, ( I I( ) CTR of the NiO( 1 I I ) surface: A, after air annealing at 1273 K; 0. after outgassing in UHV at 860 K: and ~~ , after oxidation at 860 K. Inset (a): monitoring of the rocking curve of the (I IO) Bragg reflection during in situ annealing until melting. Inset (b): rocking scan on the air annealed surface across the I I/ CTR at / = 1.5, the width is limited by the mosaic spread of the crystal. Insets ordinates are linear.
up to 1800 K. The temperature of 1300 K seems to be the higher limit of stability for the ( 111) surface since annealing at higher temperatures leads to macroscopic steps running over the whole surface. The crystal used in the present study was thus first annealed in air at 1273 K for 3 h and introduced in the chamber just after cooling. The RHEED pattern consisting of sharp points and many Kikuchi lines was less flickering but a significant background remained, probably due to charge build-up. Remarkably, during laser alignment, no modification of the sharp laser spot was observed upon reflection, indicating that, even after this high temperature annealing, the surface remains of high optical quality. The mosaic spread of the sample measured on in- and out-of-plane Bragg reflections was 0.3-0.4”. The detector slits were adapted to integrate the intensity over this angular width. Without outgassing, strong crystal truncation rods [ 111 (CTRs) (Fig. 1) were measured perpendicularly to the surface up to the highest accessible QZ values (&=6). At the out-of-phase conditions (in between Bragg reflections) the rod is still intense and sharp (Fig. 1b), indicating a surface of very
slight roughness, with large atomically flat terraces. The widths of all rocking scans are limited by the mosaic spread of the crystal. No oscillations were observed by X-ray specular reflectivity measurements, only an inflection at - 1.5 which could correspond to a very thin (< 10 A) adsorbed layer on top of the surface. This behaviour indicates that although adsorbed gases may be present, the oxide surface below is flat. Since NiO is insulating at room temperature AES (Auger electron spectroscopy) measurements were not possible at this stage. In-plane measurements (Fig. 2a and 2d) along the high symmetry directions show that the surface structure is almost (1 x 1) although a weak contribution of the p( 2 x 2) reconstruction is already present in Fig. 2d. At this stage no contribution from ordered adsorbates, metallic Ni or from (100) facets could be detected. The surface was next annealed at 860 K for 30 min resulting in desorption of the typical air contaminants. This leads to a slight decrease of the intensities of the CTRs (Fig. 1), indicating a very limited surface roughening. AES measurements
A. Barbier, G. Renaud 1 Surface Science 392 (1997) LIS-L20
(c)
0
Dislocation Rod
t [r.l.u.]
6
Fig. 3. Out-of-plane measurements on the UHV annealed surface:(a)(1.17,1.17,~),(b)(1.5,0,~)and(c)(1.17,2.5,~).The curves were shifted for the sake of clarity. Ordinate scale is logarithmic.
i
h [r:l.u.]
;
’
Fig. 2. In-plane measurements along the (h, h, 0.06) (a, b and c) and the (h, 0,0.06) (d, e and f) directions of the NiO( 111) surface after air annealing (a and d), UHV annealing (b and e) and after in situ oxidation (c and f). Note that the strong CTRs of NiO are present in the (h, 0,0.06) direction showing again that the topmost surface planes of NiO( 111) are very well defined. The curves were shifted for the sake of clarity but the relative intensities are comparable. Ordinate scale is logarithmic.
showed only a very small amount of residual C and S. The in-plane measurements (Fig. 2b and 2e) reveal two new and strong features: extra peaks at each half integer value corresponding to the p(2 x 2) reconstruction; and Bragg peaks at the exact positions expected for epitaxial relaxed fee Ni( 111) (h = 1.17 and 2.24 in Fig. 2b). Out-ofplane scans (Fig. 3a) confirm the three-dimensional character of the metallic Ni. The width in e of the Ni Bragg peaks is constant, - 0.18, corresponding to an average island height of 19 Ni layers. This contrasts with the almost constant intensity of the p(2 x 2) rods (Fig. 3b) up to large QZs, which indicates that the p(2 x 2) reconstruction is essentially two-dimensional. Because the intensities of the CTRs do not vary significantly the NiO( 111) surface remains flat and the Ni clusters are likely to form on top of the surface, rather than below.
Only an extremely weak signal (50 counts per second) was found along the rod directions of hypothetical (100) facets. The exploration of the rest of the reciprocal space evidenced a succession of regularly spaced rods corresponding to a very well defined network of extra rods (Fig. 4). Its periodicity is exactly 0.2 in the NiO( 111) reciprocal lattice which corresponds to a 5 x 5 superstructure on the coincidence network between NiO and Ni. Indeed the lattice mismatch between five periods of
Fig. 4. Scans through the (5 x 5) DN around one of the rods (1.17,2.57,0.47). This representation was chosen for ease of data processing although the real angle between [ho] and [Ok] is 60”. Ordinate scale is linear. The intensity of the largest peak is 8500 counts per second. The same result was obtained for e=o.7.
A. Barbier, G. Renaud / Surjke
NiO and six periods of Ni is only 1.3%. The corresponding rods have only significant intensity for G>O.2, and extend to large es with a smooth maximum around L’= 0.5 (Fig. 3~). This is typical of a superstructure located at the interface between Ni ( 111) islands and the NiO( 111) surface, with a limited extension in the Ni clusters. The small lateral extension (the widths in h and k are only 0.05 corresponding to an average domain size of 60 A) shows that the periodicity is very well defined. This superstructure is likely to be an interfacial network of misfit dislocations relaxing the lattice parameter misfit [ 121. The lack of asymmetry or interference on the NiO CTRs (Fig. l), indicates that no significant amount of Ni is pseudomorphic [12]. From all these observations it is obvious to state that the surface is reduced, when heated in UHV, because of lack of oxygen. Metallic threedimensional epitaxial Ni clusters form on top of the surface, the presence of the dislocation network (DN) implies that the interface between these clusters and the NiO( 111) surface is not p( 2 x 2) reconstructed. In regions not covered by Ni, the surface undergoes the p(2 x 2) reconstruction, which is very likely to be the predicted octopolar reconstruction. At this temperature, no macroscopic facetting is observed. In order to verify our assumption on the lack of the surface was heated again in oxygen, 2.5 x lop6 mbar partial pressure of oxygen at 860 K during 45 min. This oxidation leads to vanishing of the metallic Ni clusters and of the associated DN. Only the p(2 x 2) reconstruction remains with its two-dimensional character (Fig. 2c and 2f ). Again, no macroscopic facetting is observed. The signal on the NiO rods increases (Fig. 1) showing that the surface is flatter than in any of the previous situations, this seems only possible if the Ni clusters are not replaced statically by NiO clusters. We may reasonably think that the surface is completely reconstructed and flat at this stage. From our experiment we have obtained a rather unexpected result: we must conclude that, below 1300 K, this surface is stable and of high optical quality for wavelengths between hard X-rays and visible light, provided no oxygen vacancies are produced. Roughening occurs when the surface
Science 392 (1997) LlS-L20
undergoes a reduction, that is, oxygen desorption. but this phenomenon is reversible. Our structural investigations confirm both thcoretical predictions [4,5]. The NiO( 111)-( 1 x 1) surface is stable when adsorbed species are present (air), or when metallization occurs. It remains stable up to 1300 K provided it is kept in an oxygen rich environment. When the surface is clean it undergoes a p( 2 x 2) reconstruction, presumably the predicted octopolar one which seems to be the stable thermodynamic ground state. Besides the stability of the surface some other major conclusions arise. The formation of Ni clusters shows that the surface begins its reduction in UHV at rather low temperature, and that Ni has a high surface mobility. Moreover, Ni does not wet the reconstructed surface of its own oxide. This implies that the flat p(2 x 2) reconstructed surface is the lowest energy state. This energetic behaviour is confirmed also by the smoothing of the surface during oxidation. In some way, the Ni atoms must spread out over the surface when oxidation proceeds, it is thus a dynamic process and although Ni does not wet the reconstructed NiO( 111) surface, NiO molecules wet it. It is thus likely that the homoepitaxial growth of NiO on NiO( 111) may be of a nearly layer by layer type. On the other hand, the DN is quite surprising in itself because, although such networks are expected, and were observed, at metal/oxide interfaces with small lattice mismatches [ 121, for the large 16% misfit between Ni and NiO, alternative relaxation processes would be expected. Moreover since the (5 x 5) rods are very well defined, the DN is very well ordered and the epitaxial relationship between Ni and NiO( 111) is clearly cube on cube. Such a behaviour can only be explained by the strong interaction between Ni and the NiO( 11 I )(1 x 1) surface and by the way the interface was formed: by reduction rather than by deposition. However, the reconstructed surface has a lower energy than the Ni/NiO( 111) interface with its DN, which explains that Ni does not spread over the entire surface. How can we understand our results in view of other dense oxide surfaces? The a-A&0,( 0001) surface shows a similar behaviour. When it is unreconstructed only one Al plane is present on top
A. Barbier, G. Renaud / Surface Science 392 (1997) L15-L20
[ 131 whereas after UHV high temperature annealing, oxygen reduction occurs and several Al( 111) planes are observed [ 141. The main differences with the NiO( 111) system seems to be the better wetting of Al on its oxide, the smaller misfit of 4% and the fact that A&O, is non-polar. During UHV reduction Al layers spread over the surface of its oxide and form well ordered two-dimensional reconstructions which may be compared to the DN of Ni on NiO( 111). On the other hand on MgO( 111), which is polar as NiO( 111 ), strong facetting was observed after heating in UHV [ 151. Since the NiO( 111) surface is more stable, either the polar character of the surface may no longer be considered as only responsible for the facetting process, or a competitive process prevents facetting in the present case. Indeed, MgO is a relatively simple oxide while NiO is a strongly correlated one which could exhibit very unusual behaviours [ 161. Another difference between Mg and Ni is their sublimation temperature: it is probable that Ni does not evaporate at 1300 K while Mg does. During reduction, oxygen leaves the oxide surface in both cases. If the metal remains on the surface, the process may be reversible, which is the case for NiO( 111 ), while if not, the crystal starts to decompose and, according to Wulf’s theorem, the low index facets will be favoured, which is the case of MgO( 111). This is virtually equivalent to chemical etching of metals where facetting occurs in a similar way: removal of the metal atoms from the high index faces. We may thus suggest that the stability of metal oxide surfaces at a given temperature depends on the ability of the metal to evaporate. Since NiO( 111) is stable, many other orientations should also be stable. This point may open new perspectives of investigation of ionic systems. We can now understand why the NiO( 111) surface remains stable upon annealing in air: at elevated temperature, the surface is either metallic, or stabilized by the p(2 x 2) reconstruction because of oxygen; when cooled down, the possible metallic Ni oxidizes into NiO. Finally light gas adsorption stabilizes the ( 1 x 1) structure at room temperature. Since our surface was exposed to air during several hours after annealing and showed reminiscence of the p(2 x 2) reconstruction, the contamination pro-
cess must be relatively slow. This again, as well as the absence of evolution of the p( 2 x 2) reconstruction during the time of the experiment, is another indication of its very high stability. Since all CTRs are nearly symmetric, no significant surface relaxation is expected. A more compredescription needs complimentary hensive quantitative measurements of CTRs. Finally, the ease of preparation of this substrate as well as its intrinsic properties let us expect that single crystal NiO( 111) surfaces will become a popular substrate for growth of heterostructures with new properties. Moreover our observations validate the thin film approach and may be of use for preparing high quality NiO( 111) thin films.
References [ 1 ] S. Soeya, S. Nakamura, T. Imagawa, S. Narishige, J. Appl. Phys. 77 (1995) 5838 and references therein [2] D.M. Lind, S.-P. Tay, S.D. Berry, J.A. Borchers, R.W. Erwin, J. Appl. Phys. 73 (1993) 6886. [3] D. Cappus, C. Xu, D. Ehrhch, B. Dillmann, C.A. Ventrice et al., Chem. Phys. 177 (1993) 533. [4] D. Wolf, Phys. Rev. Lett. 68 (1992) 3315. [ 51 P.M. Oliver, G.W. Watson, S.C. Parker, Phys. Rev. B 52 (1995) 5323 and references therein. [6] J.E. Keem, J.M. Honig, L.L. Van Zandt, Phil. Mag. B 37 ( 1978) 537. [7] C.A. Ventrice, Jr, Th. Bertrams, H. Hannemann, A. Brodde, H. Neddermeyer, Phys. Rev. B 49 (1994) 5773. [8] F. Rohr, K. Wirth, J. Libuda, D. Cappus, M. Baumer, H.J. Freund, Surf. Sci. 315 (1994) L977. [9] W.-D. Wang, N.J. Wu, P.A. Thiel, J. Chem. Phys. 92 ( 1990) 2025. [lo] ESRF Beamline Handbook (http://www.esrf.fr). [ll] I.K. Robinson, D.J. Tweet, Rep. Prog. Phys. 55 (1992) 599 and references therein. [12] P. Guenard, G. Renaud, B. Villette, Physica B 221 (1996) 205; P. Guenard, G. Renaud, B. Villette, M.-H. Yang, C.P. Flynn, Scripta Metall. Mater. 31 (1994) 1221. [ 131 P. Gutnard, G. Renaud, A. Barbier, M. Gautier-Soyer, Mater. Res. Sot. Symp. Proc. 437 (1996) 15. [14] G. Renaud, B. Villette, I. Vilfan, A. Bourret, Phys. Rev. Lett. 73 (1994) 1825. [IS] V.E. Henrich, Surf. Sci. 57 (1976) 385. [16] J.J.M. Pothuizen, 0. Cohen, G.A. Sawatzky, Mater. Res. Sot. Symp. Proc.‘401 (1996) 501; V.I. Anisimov, F. Aryasetiawan, A.I. Lichtenstein, J. Phys. Condens. Matter 9 ( 1997) 767.
Surface Science 392 ( 1997) Ll5%L20
Surface Science Letters
Structural investigation of the NiO( 111) single crystal surface A. Barbier *, G. Renaud CEA~Grenohle. De$zrtement de Recherche Fondamentale SW la Mat&e
Condens&, SP2M/IRS.
17. Rue dex Martyrs, 38054 Grenoble.
CPdes 9, France Received 23 June 1997: accepted
for publication
11 August
I997
_ Abstract
We report an in situ grazing incidence X-ray scattering study of the NiO( Ill ) single crystal surface. It evidences a relatively high stability of this surface. After an air anneal at 1300 K it is flat and almost not reconstructed. Annealing at 860 K in ultra-high vacuum leads to two domains: one covered by epitaxial and relaxed metallic Ni clusters associated with a highly ordered interfacial supercell on the coincidence site lattice, presumably a misfit dislocation network and one exhibiting a p(2 x 2) reconstruction. After oxidation the metallic Ni transforms again into NiO( 111) and only a p(2 x 2) reconstructed flat surface remains. In these conditions macroscopic ( 100) facetting was never observed. The genera1 behaviour of the NiO( 1I I ) surface is discussed and compared with other polar and non-polar oxide surfaces, with emphasis on the specific properties which could explain its stability. 0 1997 Elsevier Science B.V. Keywords: Nickel oxides; Oxidation;
Single crystal
epitaxy;
Single crystal
Ceramic surfaces are increasingly studied, both theoretically and experimentally, because they are involved in many fast innovating industrial sectors. Up to the very recent years they were mainly used as support for catalysis or thin film growth. Recent advances in their synthesis have extended the range of potential new applications. Indeed, ceramics as well as semiconductors or metals provide a large diversity of intrinsic properties: they might exhibit different gap widths, some of them are ferrimagnetic. ferromagnetic or anti-ferromagnetic. Unfortunately, because of their intrinsic insulating properties, only very little is known about the crystallographic structure of oxide surfaces. The NiO( 111) surface exhibits attractive physical properties and is thus particularly interesting. * Corresponding author. e-mail: [email protected]
Fax: (+ 33) 76885138;
0039-6028/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved PII SOO39-6028(97)00651-l
surface:
Surface
structure;
Wetting;
X-ray diffraction
NiO is an antiferromagnet and only a few years ago interfacial magnetic ordering effects on NiO based films were observed, including evidence of the interlayer exchange interaction [l] and antiferromagnetic ordering along the (111) planes in superlattices [2]. There are also hints of an enhanced reactivity of NiO( 111) films [3]. In spite of these interesting properties no experimental investigation of the NiO( 111) single crystal surface is available. Indeed, it is often thought that, like MgO( 11 l), the NiO( 111) surface is unstable because it is polar: the terminating plane of an ideally cut crystal contains only one ionic species OzP or Ni2+, so that a non-zero electrical charge and a dipolar momentum are left at the surface. It is thus subject to the classical Madelung problem, i.e., the divergence associated with the l/r term in the Coulomb potential of condensed systems. This has given rise, in the past, to the belief that the
A. Barbier. G. Renaud / Surface Science 392 (1997) LI5-L20
Coulomb interaction may be long-ranged and that polar surfaces must decompose in macroscopic (100) facets. Hence, most studies of ionic surfaces have been restricted to non-polar ones. On the other hand, recent theoretical works evidence that in many instances Coulombic effects seem to cancel almost completely at long range ( l/r5 behaviour). Wolf [4] demonstrated that the problem of the diverging surface potential may be simply overcome by summing over neutral shells of molecules; the polarization correction being avoided by choosing a basis molecule without dipole moment. He shows that within a simple cubic Bravais lattice with an octopole basis, (NiO),, a simple thermodynamic ground state at 0 K is found which corresponds to a p(2 x 2) reconstructed surface of finite energy, arranged in three atomic high micro-pyramids. Temperature effects and the nature of the terminating plane have been investigated by molecular dynamics simulations [ 51, which predict that the NiO( 111) surface could also be stabilized by oxidation, that the temperature should have only small effects on the overall behaviour and that Ni atoms should be on top of the pyramids. Possible surface relaxation yielding to even lower energy was not introduced. These very intriguing predictions rapidly led to trials in order to verify if there is or not a reconstruction on polar ( 111) surfaces, on NiO( 111) in particular. However, the NiO( 111) single-crystal surface is a good insulator at room temperature and its conductivity drops severely when the temperature decreases [ 61, preventing any investigation with electron based techniques due to charge build-up. It was thus very seductive to work on NiO( 111) ultra-thin films grown on single crystal substrates. A p(2 x 2) low energy electron diffraction (LEED) pattern was reported for 550 K grown films on Au( 111) [ 71 and for room temperature grown films on Ni ( 111) [ 81. However, these studies remain difficult because only a few layers can be grown on Au( 111) and because of the large lattice mismatch on the Ni( 111) substrates. On NiO/Ni ( 111) films, the pattern disappeared when water was introduced into the chamber due to hydroxyl adsorption and was restored upon annealing. However, this result stands in contradiction with a previous LEED work on the oxidation of
Ni(lll)whereonlyc(2x2)and(7~7)reconstructions were reported [9]. To overcome the charge effect problem we have investigated the NiO( 111) single-crystal surface by grazing incidence X-ray scattering (GIXS). Our results, performed on a single crystal surface, confirm experimentally the latest theoretical predictions [4,5]. The GIXS experiments were carried out on the French CRG-IF beamline, BM32 [lo], of the ESRF (European Synchrotron Radiation Facility, Grenoble, France). The beam energy was set to 18 keV to avoid fluorescence background from the Ni K-edge. The beamline optics was doubly focused. The beam size at the sample was 0.3 mm(H) x 1 mm(V). The vertical sample was in a ultra-high vacuum (UHV) chamber (base pressure 3 x lo-” mbar) equipped with large Be windows and coupled to an horizontal z-axis diffractometer. The incidence angle was set to the critical angle for total external reflection (0.2”). The NiO( 111) sample, purity 99.9%, was provided, aligned to better than 0. lo and polished by Crystal GmbH (Berlin, Germany). The surface basis vectors describe the triangular lattice that is appropriate for ( 111) surfaces, they are related to the bulk ones by: a, =[ 110]/2, b, = [0 ill/2 and c, = [ 1111. The h and k indexes are chosen to describe the in-plane momentum transfer, Q,, and / the perpendicular one, Q,. All three indexes are expressed in the reciprocal lattice units (r.1.u.) of NiO( 111). The as-polished sample yielded a flickering and fuzzy RHEED pattern exhibiting thin rings typical of a polishing induced polycrystalline surface texture. Since GIXS needs single crystals of high bulk and surface quality, these ones had to be first re-crystallized. To check the stability of the bulk lattice against decomposition and follow the improvement of crystal quality, in-plane rocking scans of the (110) Bragg peak were performed on another sample during in situ annealing at increasing temperatures until melting (2200 K). Fig. la, shows that the near surface crystalline quality improves drastically during this treatment: the initial mosaic spread of ci 2” falls to 0.1” without change in the structure since the integrated intensity remains constant. Other crystals were air annealed
A. Barbier, G. Renaud I Surface Science 392 (1997j L15-L20
e [r.l.u.]
6
Fig. I, ( I I( ) CTR of the NiO( 1 I I ) surface: A, after air annealing at 1273 K; 0. after outgassing in UHV at 860 K: and ~~ , after oxidation at 860 K. Inset (a): monitoring of the rocking curve of the (I IO) Bragg reflection during in situ annealing until melting. Inset (b): rocking scan on the air annealed surface across the I I/ CTR at / = 1.5, the width is limited by the mosaic spread of the crystal. Insets ordinates are linear.
up to 1800 K. The temperature of 1300 K seems to be the higher limit of stability for the ( 111) surface since annealing at higher temperatures leads to macroscopic steps running over the whole surface. The crystal used in the present study was thus first annealed in air at 1273 K for 3 h and introduced in the chamber just after cooling. The RHEED pattern consisting of sharp points and many Kikuchi lines was less flickering but a significant background remained, probably due to charge build-up. Remarkably, during laser alignment, no modification of the sharp laser spot was observed upon reflection, indicating that, even after this high temperature annealing, the surface remains of high optical quality. The mosaic spread of the sample measured on in- and out-of-plane Bragg reflections was 0.3-0.4”. The detector slits were adapted to integrate the intensity over this angular width. Without outgassing, strong crystal truncation rods [ 111 (CTRs) (Fig. 1) were measured perpendicularly to the surface up to the highest accessible QZ values (&=6). At the out-of-phase conditions (in between Bragg reflections) the rod is still intense and sharp (Fig. 1b), indicating a surface of very
slight roughness, with large atomically flat terraces. The widths of all rocking scans are limited by the mosaic spread of the crystal. No oscillations were observed by X-ray specular reflectivity measurements, only an inflection at - 1.5 which could correspond to a very thin (< 10 A) adsorbed layer on top of the surface. This behaviour indicates that although adsorbed gases may be present, the oxide surface below is flat. Since NiO is insulating at room temperature AES (Auger electron spectroscopy) measurements were not possible at this stage. In-plane measurements (Fig. 2a and 2d) along the high symmetry directions show that the surface structure is almost (1 x 1) although a weak contribution of the p( 2 x 2) reconstruction is already present in Fig. 2d. At this stage no contribution from ordered adsorbates, metallic Ni or from (100) facets could be detected. The surface was next annealed at 860 K for 30 min resulting in desorption of the typical air contaminants. This leads to a slight decrease of the intensities of the CTRs (Fig. 1), indicating a very limited surface roughening. AES measurements
A. Barbier, G. Renaud 1 Surface Science 392 (1997) LIS-L20
(c)
0
Dislocation Rod
t [r.l.u.]
6
Fig. 3. Out-of-plane measurements on the UHV annealed surface:(a)(1.17,1.17,~),(b)(1.5,0,~)and(c)(1.17,2.5,~).The curves were shifted for the sake of clarity. Ordinate scale is logarithmic.
i
h [r:l.u.]
;
’
Fig. 2. In-plane measurements along the (h, h, 0.06) (a, b and c) and the (h, 0,0.06) (d, e and f) directions of the NiO( 111) surface after air annealing (a and d), UHV annealing (b and e) and after in situ oxidation (c and f). Note that the strong CTRs of NiO are present in the (h, 0,0.06) direction showing again that the topmost surface planes of NiO( 111) are very well defined. The curves were shifted for the sake of clarity but the relative intensities are comparable. Ordinate scale is logarithmic.
showed only a very small amount of residual C and S. The in-plane measurements (Fig. 2b and 2e) reveal two new and strong features: extra peaks at each half integer value corresponding to the p(2 x 2) reconstruction; and Bragg peaks at the exact positions expected for epitaxial relaxed fee Ni( 111) (h = 1.17 and 2.24 in Fig. 2b). Out-ofplane scans (Fig. 3a) confirm the three-dimensional character of the metallic Ni. The width in e of the Ni Bragg peaks is constant, - 0.18, corresponding to an average island height of 19 Ni layers. This contrasts with the almost constant intensity of the p(2 x 2) rods (Fig. 3b) up to large QZs, which indicates that the p(2 x 2) reconstruction is essentially two-dimensional. Because the intensities of the CTRs do not vary significantly the NiO( 111) surface remains flat and the Ni clusters are likely to form on top of the surface, rather than below.
Only an extremely weak signal (50 counts per second) was found along the rod directions of hypothetical (100) facets. The exploration of the rest of the reciprocal space evidenced a succession of regularly spaced rods corresponding to a very well defined network of extra rods (Fig. 4). Its periodicity is exactly 0.2 in the NiO( 111) reciprocal lattice which corresponds to a 5 x 5 superstructure on the coincidence network between NiO and Ni. Indeed the lattice mismatch between five periods of
Fig. 4. Scans through the (5 x 5) DN around one of the rods (1.17,2.57,0.47). This representation was chosen for ease of data processing although the real angle between [ho] and [Ok] is 60”. Ordinate scale is linear. The intensity of the largest peak is 8500 counts per second. The same result was obtained for e=o.7.
A. Barbier, G. Renaud / Surjke
NiO and six periods of Ni is only 1.3%. The corresponding rods have only significant intensity for G>O.2, and extend to large es with a smooth maximum around L’= 0.5 (Fig. 3~). This is typical of a superstructure located at the interface between Ni ( 111) islands and the NiO( 111) surface, with a limited extension in the Ni clusters. The small lateral extension (the widths in h and k are only 0.05 corresponding to an average domain size of 60 A) shows that the periodicity is very well defined. This superstructure is likely to be an interfacial network of misfit dislocations relaxing the lattice parameter misfit [ 121. The lack of asymmetry or interference on the NiO CTRs (Fig. l), indicates that no significant amount of Ni is pseudomorphic [12]. From all these observations it is obvious to state that the surface is reduced, when heated in UHV, because of lack of oxygen. Metallic threedimensional epitaxial Ni clusters form on top of the surface, the presence of the dislocation network (DN) implies that the interface between these clusters and the NiO( 111) surface is not p( 2 x 2) reconstructed. In regions not covered by Ni, the surface undergoes the p(2 x 2) reconstruction, which is very likely to be the predicted octopolar reconstruction. At this temperature, no macroscopic facetting is observed. In order to verify our assumption on the lack of the surface was heated again in oxygen, 2.5 x lop6 mbar partial pressure of oxygen at 860 K during 45 min. This oxidation leads to vanishing of the metallic Ni clusters and of the associated DN. Only the p(2 x 2) reconstruction remains with its two-dimensional character (Fig. 2c and 2f ). Again, no macroscopic facetting is observed. The signal on the NiO rods increases (Fig. 1) showing that the surface is flatter than in any of the previous situations, this seems only possible if the Ni clusters are not replaced statically by NiO clusters. We may reasonably think that the surface is completely reconstructed and flat at this stage. From our experiment we have obtained a rather unexpected result: we must conclude that, below 1300 K, this surface is stable and of high optical quality for wavelengths between hard X-rays and visible light, provided no oxygen vacancies are produced. Roughening occurs when the surface
Science 392 (1997) LlS-L20
undergoes a reduction, that is, oxygen desorption. but this phenomenon is reversible. Our structural investigations confirm both thcoretical predictions [4,5]. The NiO( 111)-( 1 x 1) surface is stable when adsorbed species are present (air), or when metallization occurs. It remains stable up to 1300 K provided it is kept in an oxygen rich environment. When the surface is clean it undergoes a p( 2 x 2) reconstruction, presumably the predicted octopolar one which seems to be the stable thermodynamic ground state. Besides the stability of the surface some other major conclusions arise. The formation of Ni clusters shows that the surface begins its reduction in UHV at rather low temperature, and that Ni has a high surface mobility. Moreover, Ni does not wet the reconstructed surface of its own oxide. This implies that the flat p(2 x 2) reconstructed surface is the lowest energy state. This energetic behaviour is confirmed also by the smoothing of the surface during oxidation. In some way, the Ni atoms must spread out over the surface when oxidation proceeds, it is thus a dynamic process and although Ni does not wet the reconstructed NiO( 111) surface, NiO molecules wet it. It is thus likely that the homoepitaxial growth of NiO on NiO( 111) may be of a nearly layer by layer type. On the other hand, the DN is quite surprising in itself because, although such networks are expected, and were observed, at metal/oxide interfaces with small lattice mismatches [ 121, for the large 16% misfit between Ni and NiO, alternative relaxation processes would be expected. Moreover since the (5 x 5) rods are very well defined, the DN is very well ordered and the epitaxial relationship between Ni and NiO( 111) is clearly cube on cube. Such a behaviour can only be explained by the strong interaction between Ni and the NiO( 11 I )(1 x 1) surface and by the way the interface was formed: by reduction rather than by deposition. However, the reconstructed surface has a lower energy than the Ni/NiO( 111) interface with its DN, which explains that Ni does not spread over the entire surface. How can we understand our results in view of other dense oxide surfaces? The a-A&0,( 0001) surface shows a similar behaviour. When it is unreconstructed only one Al plane is present on top
A. Barbier, G. Renaud / Surface Science 392 (1997) L15-L20
[ 131 whereas after UHV high temperature annealing, oxygen reduction occurs and several Al( 111) planes are observed [ 141. The main differences with the NiO( 111) system seems to be the better wetting of Al on its oxide, the smaller misfit of 4% and the fact that A&O, is non-polar. During UHV reduction Al layers spread over the surface of its oxide and form well ordered two-dimensional reconstructions which may be compared to the DN of Ni on NiO( 111). On the other hand on MgO( 111), which is polar as NiO( 111 ), strong facetting was observed after heating in UHV [ 151. Since the NiO( 111) surface is more stable, either the polar character of the surface may no longer be considered as only responsible for the facetting process, or a competitive process prevents facetting in the present case. Indeed, MgO is a relatively simple oxide while NiO is a strongly correlated one which could exhibit very unusual behaviours [ 161. Another difference between Mg and Ni is their sublimation temperature: it is probable that Ni does not evaporate at 1300 K while Mg does. During reduction, oxygen leaves the oxide surface in both cases. If the metal remains on the surface, the process may be reversible, which is the case for NiO( 111 ), while if not, the crystal starts to decompose and, according to Wulf’s theorem, the low index facets will be favoured, which is the case of MgO( 111). This is virtually equivalent to chemical etching of metals where facetting occurs in a similar way: removal of the metal atoms from the high index faces. We may thus suggest that the stability of metal oxide surfaces at a given temperature depends on the ability of the metal to evaporate. Since NiO( 111) is stable, many other orientations should also be stable. This point may open new perspectives of investigation of ionic systems. We can now understand why the NiO( 111) surface remains stable upon annealing in air: at elevated temperature, the surface is either metallic, or stabilized by the p(2 x 2) reconstruction because of oxygen; when cooled down, the possible metallic Ni oxidizes into NiO. Finally light gas adsorption stabilizes the ( 1 x 1) structure at room temperature. Since our surface was exposed to air during several hours after annealing and showed reminiscence of the p(2 x 2) reconstruction, the contamination pro-
cess must be relatively slow. This again, as well as the absence of evolution of the p( 2 x 2) reconstruction during the time of the experiment, is another indication of its very high stability. Since all CTRs are nearly symmetric, no significant surface relaxation is expected. A more compredescription needs complimentary hensive quantitative measurements of CTRs. Finally, the ease of preparation of this substrate as well as its intrinsic properties let us expect that single crystal NiO( 111) surfaces will become a popular substrate for growth of heterostructures with new properties. Moreover our observations validate the thin film approach and may be of use for preparing high quality NiO( 111) thin films.
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