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Observation of a new ridge structure along steps on the MgO(100) surface by non-contact atomic force microscopy
Surface Science 441 (1999) 529–541 www.elsevier.nl/locate/susc
Observation of a new ridge structure along steps on the MgO(100) surface by non-contact atomic force microscopy Ken-ichi Fukui, Yasuhiro Iwasawa * Department of Chemistry, Graduate School of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan Received 12 April 1999; accepted for publication 9 August 1999
Abstract The surface topography of an MgO(100) surface after annealing at high temperatures followed by Ar+-ion sputtering was examined by non-contact atomic force microscopy (NC-AFM ) and X- ray photoelectron spectroscopy ( XPS). The calcium impurity (210 ppm in the bulk) was segregated to the surface by annealing above 1200 K and agglomerated as CaO particles at monolayer or bilayer steps along the [011] direction after annealing at ~1600 K x for 1 h. The step was always accompanied by a band-like structure with a protrusive line running parallel to the step edge with a separation longer than 5 nm. The structure was stable on the surface after removal of the CaO particles x by Ar+-ion sputtering at ~1600 K, but eventually disappeared after repeated Ar+-ion sputtering and annealing cycles. This unusual ridge structure is probably due to long-range reconstruction induced by segregated calcium of less than 3% coverage. The calcium-induced structure on MgO(100) is also discussed in relation to the chemical reactivity for methane activation. © 1999 Published by Elsevier Science B.V. All rights reserved. Keywords: Atomic force microscopy; Calcium; Magnesium oxide; Surface segregation; Surface structure; X-ray photoelectron spectroscopy
1. Introduction Scanning probe microscopy (SPM ) has great potential to identify the key issues controlling physical and chemical surface processes and phenomena of interesting materials, discriminating sitespecific structural and dynamic events on an atomic scale. A practically important class of oxides (Al O , SiO , MgO, zeolites, etc.) is, how2 3 2 ever, out of range of the most established SPM technique of scanning tunneling microscopy (STM ) because they are non-conducting. Recent progress in non-contact atomic force microscopy (NC-AFM ) provides an opportunity to observe * Corresponding author. Fax: +81-3-5800-6892. E-mail address: [email protected] ( Y. Iwasawa)
atomic-scale singularities, such as adatoms and point vacancies, on semiconductor surfaces such as silicon [1] and InP [2]. We reported the first example of atom-resolved NC-AFM images of oxide surfaces with oxygen point defects on TiO (110)-(1×1) [3]. NC-AFM has great advan2 tages over contact-mode AFM for observing adsorbed molecules other than the singularities of the substrates by virtue of its much weaker interaction between the tip and the sample for imaging the structure. Individual formate ions ( HCOO−) [4] or acetate ions (CH COO−) [5] adsorbed on 3 TiO (110)-(1×1) were imaged as bright protru2 sions by NC-AFM. MgO is an insulating oxide with the rocksalt structure. It is widely used as a catalyst, adsorbent, etc. The (100) surface of MgO is the most stable
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face and can be easily exposed by cleavage. This surface is widely used as a substrate for epitaxial growth of metals and high-T superconducting c oxides [6 ]. Numerous studies on structural determination of the MgO(100) surface have been reviewed by Henrich and Cox [7]. Experimental studies of cleaved and annealed MgO(100) surfaces by dynamic low-energy electron diffraction (LEED), reflection high-energy electron diffraction (RHEED), impact collision ion-scattering spectroscopy (ICISS), surface extended energy-loss fine structure (SEELFS) and medium-energy ion scattering (MEIS) showed a small relaxation of no more than ±3% and rumpling less than ±5%, although some deviation from these values was reported in some cases. This system is also a subject of theoretical calculation by different methods. Relaxation of the first layer is expected to be less than ±3%, but surface rumpling has diverse values depending on the calculation. AFM studies of MgO(100) surfaces in contact mode focus on the morphology of the surface as a substrate for epitaxial growth of other oxides. Cleaved (100) faces revealed monolayer steps [8,9] or bilayer steps [9,10] along the [001] or [010] direction, and multilayer steps composed of highindex vicinal faces [11]. The changes in surface morphology following annealing at high temperatures in oxygen ambient were also examined [8,12–15]. Some groups reported rows along the [011] direction with a separation about 0.3 nm in a flat terrace of the MgO(100) surface [8,10,15]. Although this separation matches a spacing between oxygen anions of the surface, it may not reflect the atom position but the periodicity of the surface [16 ]. Segregation of calcium impurity to an MgO(100) surface by annealing has also been studied [17]. Calcium is a major impurity of MgO crystals and it segregates to the surface upon annealing above 1200 K. The surface concentration of calcium was monitored by Auger electron spectroscopy (AES ) and low-energy ionscattering spectroscopy (LEIS ) as a function of temperature between 1173 and 1723 K, and the heat of segregation was determined to be 50.3 kJ mol−1 [18], which showed good agreement with calculation [19]. The formation of particles
of a calcium compound upon annealing above 1300 K was suggested by AFM combined with macroscopic information from AES [13,14]. Preferential modification of steps by segregated calcium was suggested by reflection electron microscopy (REM ) combined with high-spatial-resolution electron energy-loss spectroscopy (EELS ) [20–22], but the structure was not determined. MgO is known to be an active catalyst, particularly with alkali metal additives such as lithium and sodium, for the oxidative coupling of methane to >C hydrocarbons [23–25]. Increased activity 2 and selectivity were also observed by the addition of another alkali earth metal such as barium [26,27] or calcium [28]. MgO/CaO mixed oxide showed high activity and selectivity to C hydro2 carbons at the MgO content of 85% [29–31]. Thus the structure of the calcium-segregated MgO(100) surface employed in this study may have a relation with such catalysis. In the present paper, we report NC-AFM topographies of MgO(100) surfaces after annealing at high temperatures. Combined with macroscopic information by X-ray photoelectron spectroscopy ( XPS), it was found that calcium agglomerates as particles at monolayer or bilayer steps along the [011] direction. The steps are always accompanied by a unusual ridge structure parallel to the steps. The structure is probably due to calcium-induced long-range reconstruction with less than 3% coverage of calcium on the surface.
2. Experimental The experiments were performed in an ultrahigh vacuum ( UHV ) atomic force microscope (JEOL JAFM4500XT ) equipped with an ion gun, LEED optics ( VG RVL109) and an XPS analyzer ( VG CLAM2). The base pressure was 1×10−8 Pa. Mechanochemically polished MgO(100) wafers of dimensions 6.5 mm×1 mm×0.25 mm ( Earth Chemical ) were purchased and kept in dry air. The crystals included 210 ppm of calcium as a major impurity. MgO samples were calcined in air at 1073 K for 2 h. A Si(111) wafer of dimensions 6.5 mm×1 mm×0.33 mm was attached to the rear of the MgO sample and used as a resistive heater
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up to 1600 K. The temperature of the MgO sample was measured with an infrared radiation thermometer (Minolta TR-630) through a view port. Ar+-ion sputtering was performed to remove contaminants on the surface at room temperature (3 keV, 3 min) unless otherwise noted. The binding energy in XPS excited by Mg Ka was calibrated by the O 1s level of oxygen anions of the MgO, which was supposed to be 530.0 eV. A positive shift of ca. 12 eV in binding energy was observed due to charge-up of the sample. For NC-AFM measurements, stiff silicon cantilevers with f =270–290 kHz and k=28–30 N m−1 0 (Nanosensors) were used as the force sensor. The oscillation amplitude of the cantilever was ~15 nm for all images presented in this paper. p–p NC-AFM measurements used the shift of the resonance frequency (Df ) of the cantilever as a feedback signal. As the cantilever approaches the sample, the attractive force begins to work between the tip and the sample, resulting in a negative shift of the resonance frequency. The feedback signal is applied to the Z piezo to keep the frequency shift constant, which gives a topographic image of the surface. A bias voltage (V ) was applied to the s silicon heater to obtain NC-AFM images. For contact-mode AFM, soft silicon cantilevers with k~0.2 N m−1 (Nanosensors) were used as the force sensor.
3. Results The bulk-truncated structure of the MgO(100) surface is shown in Fig. 1. From the lattice parameter (a ) of 0.4211 nm, the spacing between the 0 layers is expected to be ca. 0.21 nm. In this figure, two steps with single- and double-atomic heights parallel to the [010] direction are shown schematically. Previous REM [21,22] and AFM [8–10] studies showed that the cleaved (100) surface has such steps parallel to the [010] and [001] directions. Fig. 2a shows an NC-AFM topograph of the MgO(100) surface after an as-received sample was calcined in air at 1073 K for 2 h and was admitted to the UHV chamber. Some large dips or scratches were found on the surface (such a region is not shown in Fig. 2a), but it was rather flat. A root-
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Fig. 1. Bulk-truncated structure of the MgO(100) surface with a single- and a double-atomic height steps, in top view and plane view.
mean-square (RMS) value, i.e. standard deviation of the height value, averaged for several images was 0.59 nm. Fig. 2b shows an NC-AFM topograph of the surface after Ar+-ion sputtering at room temperature (RT ) followed by UHV annealing at ~1400 K for 5 min. The surface is still rough (RMS 0.28 nm), which was also confirmed by diffuse (1×1) LEED spots with high background intensity. At a higher annealing temperature of ~1600 K, the (1×1) spots became sharp and the background intensity decreased. Fig. 2c shows an NC-AFM topograph after annealing the surface at ~1600 K for 5 min. Terraces had not yet developed; however, the surface became more flat (RMS 0.14 nm) than the surface of Fig. 2b. There were some particles on the surface, whose number increased with further annealing. Fig. 3 shows NC-AFM topographs after further annealing the surface of Fig. 2c at ~1600 K. In these images, particles are clearly found: lines run parallel to the [011] direction in Fig. 3a and the particles are located on them. These are more prominent in Fig. 3c. Although the area between any two such lines is still rough, these lines are considered to be steps by line-profile analysis. REM experiments showed that steps parallel to the [011] direction were preferred on an MgO(100) surface cleaved and annealed at high temperature [21]. When the surface was annealed as for long
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Fig. 2. Non-contact AFM topographs of MgO(100) surfaces after treatments at low temperature: (a) calcination in air at 1073 K for 2 h (Df~70 Hz, V =+0.5 V, 430 nm×430 nm); (b) s Ar+-ion sputtering and subsequent UHV annealing at ~1400 K for 5 min (Df ~100 Hz, V =+0.7 V, 800 nm× s 800 nm); (c) further UHV annealing at ~1600 K for 5 min (Df ~70 Hz, V =+0.7 V, 800 nm×800 nm). A cantilever with a s spring constant of 30 N m−1 and a resonant frequency of 284 kHz was used for these images.
as 1 h at ~1600 K, a much clearer step–terrace structure was observed ( Fig. 3b). A sharp (1×1) LEED pattern was observed for this surface ( Fig. 3e). The particles were also located at the steps, but several steps were bunched up at a cluster. As evident in Fig. 3d, they seem to locate at the cape. The origin of the particles became evident from XPS measurements. The only element which increased upon annealing the surface was calcium, which is the major impurity (210 ppm) of the MgO crystal. Contamination from the silicon heater or the sample holder was not observed by XPS even after a longer time of annealing at ~1600 K. The change in the Ca 2p , 2p XPS signal is shown 1/2 3/2 in Fig. 4. The calcium signal appeared after annealing the surface above 1200 K and increased at higher temperature (spectrum b). The signal decreased to the noise level upon sputtering the surface at room temperature (spectrum c), but increased again after annealing at ~1600 K (spectra d and e). The amount of calcium segregated to an MgO surface by annealing at 1673 K for 5 h was estimated to be 2.1×1014 ions cm−2 by Rutherford backscattering spectroscopy (RBS ) (19% replacement of magnesium atoms at the topmost layer if all the calcium signal came from the topmost layer) [32]. The equilibrium value for the calcium segregation between 1223 K and 1273 K was estimated by AES and LEIS, and corresponded to a 20% occupation of the surface cation sites [18]. The amount of calcium in Fig. 4e was estimated roughly from the XPS signal intensity ratio of (Ca 2p)/(Mg 2p) using relative crosssections and escape depths assuming a bulk-truncated structure. It corresponds to ca. 12% replacement of topmost-layer magnesium atoms by calcium. Owing to a large uncertainty of energy scale calibration, it may not be appropriate to quote the state of calcium. However, if the O 1s level of oxygen anions of the MgO is referred to 530.0 eV, the peak position for Ca 2p in Fig. 4 3/2 is estimated to be 347.0–347.3 eV. Binding energies lie in the range 346.1–347.3 eV for CaO. Thus, the calcium is probably oxidized to be CaO (x≤1) x on the surface. Fig. 5a shows a zoom-in NC-AFM image of the surface in Fig. 3b. Terraces were separated by
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Fig. 3. Non-contact AFM topographs of MgO(100) surfaces after further annealing of the surface of Fig. 2c at ~1600 K for (a) 20 min (Df~110 Hz, V =+0.7 V, 800 nm×800 nm) and (b) 1 h (Df~110 Hz, V =−0.7 V, 800 nm×800 nm). (c) A magnified image s s obtained from the same surface as in (a), but at a different place (Df~110 Hz, V =+0.7 V, 250 nm×250 nm). (d) A magnified image s of (b) (250 nm×250 nm). A cantilever with a spring constant of 30 N m−1 and a resonant frequency of 284 kHz was used for these images. (e) A well-contrasted (1×1) LEED pattern observed from the surface in (b).
single- or double-atomic height steps (0.2 nm or 0.4 nm) of the bulk structure as shown in Fig. 5b [distance between the (100) planes for the bulktruncated surface is 0.21 nm as shown in Fig. 1]. By looking closely at the steps, one can find that the steps are not so simple but have a band-like structure along them for all directions observed. The structure is clearer in the 3D view in Fig. 5c, where a protrusive line, indicated by an arrow in the figure, parallel to the step is observed accompa-
nied by slight depressive lines on both sides. Although one should be careful when determining the actual step edge from AFM images as discussed later, the protrusive line ca. 0.05 nm high from the terrace plane is away from the step edge at least by 5 nm. The distance of 5 nm corresponds to about 12 times the unit-cell dimension of the surface (0.42 nm). This kind of ridge structure has not been observed in previous AFM works with contact mode [8–15]. REM experiments showed
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Fig. 4. XPS of the Ca 2p , 2p region obtained on the 1/2 3/2 MgO(100) surfaces: (a) after calcination in air at 1073 K for 2 h; (b) after a cycle of Ar+-ion sputtering and UHV annealing at ~1600 K for 1 h; (c) after Ar+-ion sputtering of (b) at room temperature; (d) after UHV annealing of (c) at ~1600 K for 10 min; (e) after further UHV annealing of (d ) at ~1600 K for 2 h; (f ) after two cycles of Ar+-ion sputtering at ~1600 K for 1 h and UHV annealing at ~1600 K for 1 h.
the possibility of a calcium-modified structure at steps after annealing at high temperatures [20– 22]. Note that the terraces are rather flat compared with the protrusive lines along the steps, but have some structure with lines that are not necessarily correlated to the crystallographic direction. This is not an ordered structure but was reproducible by several scannings at the same area. Almost the same structure as that in Fig. 5 was obtained after annealing a sputtered MgO(100) sample in 1.0×10−4 Pa of O instead of UHV. 2 Fig. 6 shows contact-mode AFM images of an MgO(100) surface after Ar+-ion sputtering at RT and UHV annealing at ~1600 K for 50 min. These images were obtained on a different sample from that used for other NC-AFM images in the present paper. But it is confirmed that the ridge structure near steps similar to that in Figs. 3 and 5 was
Fig. 5. (a) Non-contact AFM topograph of the MgO(100) surface in Fig. 3b (Df~110 Hz, V =−0.7 V, 400 nm×400 nm). A s cantilever with a spring constant of 30 N m−1 and a resonant frequency of 284 kHz was used for this image. A line profile for the line in (a) and a three-dimensional (3D) view of the square region in (a) are shown in (b) and (c) (55.6 nm×55.6 nm), respectively. The step in (c) has a doubleatomic height. Arrow in (c) indicates the position of a protrusive line running parallel to the step.
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Fig. 6. Contact AFM images (variable-force mode) of an MgO(100) surface after a cycle of Ar+-ion sputtering and UHV annealing at ~1600 K for 50 min: (a) 800 nm×800 nm, (b) 150 nm×150 nm. Repulsive force between the tip and the sample was set at ~5 nN to obtain these images. Sample bias was set at 0 V.
observed by NC-AFM. In Fig. 6a, calcium particles are apparent but steps are not so clear as in NC-AFM images in Fig. 3. The ridge was not observed even in the magnified image in Fig. 6b. In contact mode, many atoms of the tip and the sample contribute at the same time to imaging [16 ], resulting in difficulty imaging a singularity of the surface as compared with imaging in a noncontact mode. However, there remains a possibility that the ridge structure is an artifact which does
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not reflect the actual structure. This possibility is denied by the following results. NC-AFM topographs were obtained regulating the distance between the tip and the sample to keep the resonant frequency shift of the cantilever constant. The shift value relates strongly with the force gradient between the tip and the sample [1]. The force probably includes electrostatic force and many kinds of forces other than van der Waals’ force. The charge distribution near a step must be different from that inside a terrace, so then a charge effect may contribute to the appearance of the structure. In a p-type Si(111) surface, the charged area near a step was observed as a bright line ( larger attractive force) at positive sample bias voltages and as a dark line at negative bias voltages by NC-AFM [33]. Fig. 7 shows NC-AFM topographs of the surface in Fig. 3b at different sample bias voltages. The regulated bias voltage was applied to the silicon heater relative to the tip. The silicon heater is attached to the rear of the MgO sample, but the actual bias voltage of the sample is unknown because MgO is an insulator. The MgO sample rather act as a dielectric in an electric field. Positive or negative charge should be induced on the MgO surface depending on the bias voltage between the silicon heater and the tip. If the surface has a clear distribution of charge like Si(111), the native charge should affect the induced charge leading to a change in contrast of NC-AFM images at different bias voltages. For the two applied voltages in Fig. 7, similar structures with bands along steps were observed by NC-AFM. At higher sample bias voltages with positive or negative polarity, topographs only became ambiguous due to a large contribution of long-range electrostatic force and contrast inversion was not observed. These results suggest that the charging effect is not dominant in this case unlike for the Si(111) surface. Note that the structure inside the terraces is clearer in Fig. 7 with better tip condition than in Fig. 5a. There are some line features with small protrusions but they are not ordered. The CaO particles in Fig. 3b can be removed x from the surface by Ar+-ion sputtering at room temperature, but calcium segregates to the surface again upon annealing to smooth the surface. It is in equilibrium between the bulk and the surface.
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Fig. 7. Non-contact AFM topographs of the MgO(100) surface in Fig. 3b at difference sample bias voltage settings: (a) −0.7 V and (b) +0.7 V (Df~110 Hz, 137 nm×137 nm). A cantilever with a spring constant of 30 N m−1 and a resonant frequency of 284 kHz was used for this image.
The CaO particles can be effectively removed by x sputtering the surface at ~1600 K, at which temperature calcium segregation at the surface is favored for thermodynamic reasons. High-temperature annealing has an advantage to avoid roughening of the surface. Fig. 8a shows an NC-AFM topograph of the MgO(100) surface after two cycles of Ar+-ion sputtering at ~1600 K for 1 h and UHV annealing at ~1600 K for 1 h for the surface in Fig. 3b. The CaO particles have been x successfully removed and the capes have disappeared, but the ridge structure along the steps
Fig. 8. (a) Non-contact AFM topograph of an MgO(100) surface after two cycles of Ar+-ion sputtering at ~1600 K for 1 h and UHV annealing at ~1600 K for 1 h for the surface in Fig. 3b (Df~140 Hz, V =+0.7 V, 300 nm×300 nm). A cantiles ver with a spring constant of 28 N m−1 and a resonant frequency of 278 kHz was used for this image. A 3D view of the square region in (a) is shown in (b) (55.1 nm×55.1 nm). The step in (b) has a single-atomic height. The arrow in (b) indicates the position of a protrusive line running parallel to the step.
remains. Similar protrusive lines were observed as shown in the 3D image of Fig. 8b. The corresponding XPS signal of Fig. 4f shows that the amount of calcium on the surface is comparable to the noise level. The signal before removal of the CaO particles (Fig. 4e) has already been estimated x to be 12% coverage. Comparing these two signals, the amount of calcium drops to less than 3% coverage. However, we consider that a small
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Fig. 9. (a) Non-contact AFM topograph of a MgO(100) surface after further several cycles of Ar+-ion sputtering at room temperature and UHV annealing at ~1600 K for 1 h for the surface in Fig. 8a (Df~100 Hz, V =+0.7 V, 200 nm×200 nm). A cans tilever with a spring constant of 28 N m−1 and a resonant frequency of 278 kHz was used for this image. The line profile for the line in (a) is shown in (b).
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structure, keeping the minimum vertical component of the distance between the tip and the sample (d ) constant, because only the vertical component s of the force gradient between the tip and the sample is responsible for the frequency shift of the resonance frequency of the cantilever. The step structure becomes dull by using a dull tip as shown in Fig. 10a. The curvature c is determined by the 1 radius of the tip apex. Fig. 10b shows a case of so-called double tips which leads to a double overlapping image. This is not the case for the image in Fig. 5c. In Fig. 10c–e, a tip with a small protuberance at the apex was postulated. Such a protuberance is responsible for high-resolution imaging of surface structure in the flat region. To reproduce the protrusive line observed in Fig. 5c, a small projection on the surface should be considered as in Fig. 10d and e. Here, curvature c is 1 determined by the small protuberance of the tip; therefore, relatively small structures can be found on the terrace. Curvature c is determined by the 3 large tip apex in these cases, then the step structure becomes dull. Fig. 10e qualitatively reproduces the protrusive line in Fig. 5c. For a more precise discussion, however, we have to know the contribution of each atom on the tip and the sample to the frequency shift of the cantilever as a function of the distance between each atom.
4. Discussion amount of calcium impurity induces the ridge structure. Fig. 9 shows an NC-AFM topograph of an MgO(100) surface after repetition of Ar+-ion sputtering at RT and UHV annealing at ~1600 K for 1 h for several times for the surface in Fig. 8. Steps became straight and some pits formed on terraces. It should be noted that the ridge structure was not observed near the steps on this surface. We consider that further Ar+-ion sputtering reduced the amount of calcium left on the surface, resulting in disappearance of the ridge structure. Fig. 10 summarizes simple models for tip– sample regulation in NC-AFM operation near a step. They assume that the tip traces the substrate
4.1. Segregation of calcium impurity to an MgO surface It is well known that calcium impurity in MgO crystals segregates to the surface upon annealing above 1200 K [7]. Souda et al. [32] reported that calcium is highly concentrated in the outermost surface layer and no marked calcium enrichment was found in deeper layers up to the fourth layer by a combination of ICISS and RBS. As for the site of location of segregated calcium ions, magnesium cation sites are usually assumed. The (앀2×앀2) R45° reconstruction was predicted by calculation [34]. For this structure, magnesium cations at the topmost layer are replaced by
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calcium cations and half of the oxygen ions of the topmost layer are pushed up out of the surface. Experimentally, results of neutral beam incidence ion-scattering spectroscopy (NBISS ) suggest that the calcium ions lie 0.04±0.01 nm above the Mg– O plane [35]. Recent grazing-incidence X-ray scattering (GIXS) experiments show that the most likely structure for a calcium-segregated MgO(100) surface is the (앀2×앀2) R45° reconstruction [36 ], although the calcium concentration is different from that theoretically predicted [34]. However, other kinds of reconstruction have been proposed by RHEED [13,14] and REM [20–22] observations. The formation of particles near the steps has been reported on MgO(100) after annealing at high temperature in oxygen atmosphere. A darkfield image of electron transmission through a thin MgO(100) sample after annealing in oxygen at 1823 K for 22 h showed the presence of particles (ca. 70–300 nm in diameter from the image) at vicinal surfaces [20]. An analysis for these particles by high-spatial-resolution EELS showed a high concentration of calcium. Particles ca. 350–500 nm in diameter and ca. 30 nm in height were found at steps along the [011] and [01: 1] directions by contact-mode AFM after annealing the sample above 1273 K in air [13,14]. Although the size of the particles is different from those in Fig. 3b, we consider that the origin of the particles is the same; i.e., they are formed by segregated calcium. The difference may be due to the difference in terrace size, which was much smaller for Fig. 3 than those reported previously. Actually, we have observed that larger particles were accompanied by wider terraces, the size of which varied with position on a sample probably because of different sputtering conditions and distribution of temperature. The amount of calcium in the particles observed in Fig. 3b can be roughly estimated by the
Fig. 10. Models of traces of tips with different shapes at a simple step (a)–(c) or a modified step (d), (e). The vertical scale was elongated by a factor of three for these images. They assume that the tip traces the substrate structure, keeping the minimum vertical component of the distance between the tip and the sample (d ) s constant. The step height is assumed to be 0.4 nm. The radius of
the tip apex is set at 10 nm (typical value for the cantilevers used in the present study) in (a). Two apexes (double tip) with radii of 2.5 nm and 3.5 nm are assumed for (b). A small protuberance of radius of 0.2 nm is assumed at the tip apex of radius 10 nm. A small projection with 0.05 nm height is also assumed on the substrate at (c) the step edge and (d) 4 nm in from the step edge.
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NC-AFM image. The particles have average dimensions of 2.3 nm height and 55 nm diameter, which make a volume of 3.7×103 nm3 for the averaged particle. But the particle diameter observed by SPM is the convolution of the particle shape and the tip shape, similar to the results in Fig. 10. The particle diameter observed is generally larger than the real diameter. Assuming the same story as in Fig. 10 with tip radius of 10 nm, the real diameter of the averaged particle is estimated to be 50 nm. Then the volume should be 3.0×103 nm3 for the averaged particle. If we assume that the particle consists of CaO, then the calcium content is 1.1×105 Ca particle−1. Considering the density of the particles, the concentration of calcium on the surface is 3.2×1014 Ca cm−2. This is in the same order as that estimated by XPS (1.4×1014 Ca cm−2). This estimation is compatible with the XPS results after removal of the CaO particles ( Fig. 4f ) which x showed that the calcium signal was no higher than the noise level. Then we can conclude that most of calcium, which is segregated from the bulk to the surface, is favored to agglomerate as particles and does not make the (앀2×앀2) R45° structure under the conditions we used. Double-line features which aligned along the [011] direction or the [01: 1] direction with an average length of 80±30 nm were observed by REM in the terrace region on a MgO(100) sample with 270 ppm calcium after annealing at 1823 K for 24 h, but were not observed on a sample with 40 ppm calcium [21]. A (앀2×앀2) R45° structure was suggested by GIXS for a calcium-segregated MgO(100) surface after annealing in air at 1800 K [36 ]. Such structures were not observed at any stage of the treatments in the present study, possibly due to the different concentration of calcium in the bulk or the different annealing temperature.
4.2. Origin of the ridge structure near the steps Modification of steps on MgO(100) along the [011] direction by segregated calcium has been suggested by REM experiments [20–22]. Steps parallel to the [001] and [010] directions on a cleaved MgO(100) surface changed their directions
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to the [011] direction after annealing the sample above 1823 K for 24 h in an oxygen atmosphere, accompanied by broadening of the steps along into grayish bands [20–22]. The width of the bands was wider for the sample with the higher concentration of calcium in the bulk [20] or for the sample annealed at higher temperature for a longer time [21]. High-spatial-resolution surface analysis by EELS showed that calcium was accumulated at the decorated steps [21,22]. A change in step direction by annealing MgO(100) was also observed by contact-mode AFM [13]. These results support the hypothesis that the ridge structure is induced by segregated calcium. The calcium-induced ridge structure always appeared along the steps and CaO particles were x also located on the steps. Fig. 3c shows that the CaO particles were formed on straight steps along x the [011] direction. The ridge structure at the steps begins to appear in Fig. 3c. The change from Fig. 3a to Fig. 3b by further annealing suggest that CaO particles pinned the motion of the steps. x It seems that all the steps retreat inside the upper terraces except for the position of the particles where some steps bunch up. At 1600 K, components of MgO have some mobility to smooth the surface structure but evaporation may occur at the same time [37]. Evaporation from the step edges is probably more favorable energetically than evaporation from inside the terraces. The CaO x particles may inhibit the evaporation, resulting in pinning of the step motion. It is not clear from the experimental results whether calcium segregates preferentially from the bulk through the step region or diffuses to the step region after random segregation at the whole surface. In either case, the problem is how calcium can induce such longrange reconstruction of more than 5 nm wide with the direction parallel to the steps, even to the curved one. We have estimated that the calcium coverage for the band like structure in Fig. 5 or 8 is less than 3%. If we assume that such calcium is concentrated in the band region of 5 nm width, the coverage of calcium in the region is 29%. At present, it is not apparent whether other faint line features observed further inside the terrace relate to segregated calcium or not. They might be due to slight roughening of the surface by high-temper-
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ature annealing. The height of the protrusive line from the terrace plane is ca. 0.05 nm, which is close to the value of 0.04±0.01 nm for protrusion of calcium ions above the Mg–O plane suggested by NBISS [35]. Again it is not clear that the replacement of magnesium ions by calcium ions occurs only at the sites in the 5 nm region from the step edge. The mechanism for the formation of such peculiar structure has not been determined in the present paper. It may be of a general interest whether such long-range reconstruction with a determined separation from a step edge occurs for other systems. High activity and selectivity to C hydrocarbons 2 by oxidative coupling of methane have been reported on MgO/CaO catalysts [29–31]. The performance showed a maximum at the mixture of 85% MgO/CaO. Thermal desorption [31] and infrared spectroscopy [30] of adsorbed CO sug2 gested that surface Lewis basicity has a maximum at the 85% MgO/CaO catalyst, showing the same trend with reactivity. Strong base sites are sometimes considered as the site of methane activation by heterolytic rupture of the CMH bond, although homolytic mechanism with participation of different oxygen species is usually assumed [24]. Infrared spectroscopy of the adsorbed CO sug2 gests that most of the surface is covered with MgO for the 85% MgO/CaO catalyst with little modification by highly dispersed or dissolved calcium ions [30]. An increase in lattice distortion by the unusual ridge reconstruction in Fig. 3b may be the reason for the high reactivity of the calcium-doped MgO catalyst for oxidative coupling of methane.
5. Conclusions We have measured the topography of MgO(100) surfaces after annealing at high temperatures followed by Ar+-ion sputtering by NC-AFM. After annealing the sputtered MgO(100) surface to ~1600 K for 1 h, flat terraces appeared with monolayer or bilayer steps along the [011] direction. XPS measurements indicated that calcium impurity was segregated from the bulk to the surface and agglomerated as CaO particles at the steps. The steps were always x
accompanied by the band-like structure with a protrusive line ca. 0.05 nm high running parallel to the step edge with a separation of more than 5 nm, as evident from 3D images of the NC-AFM topographs. The ridge structure remained on the surface after removal of the CaO particles by x Ar+-ion sputtering at ~1600 K, but eventually disappeared after repeated Ar+-ion sputtering and annealing cycles. This unusual ridge structure is probably due to long-range reconstruction induced by segregated calcium of less than 3% coverage.
Acknowledgements This work was supported by CREST (Core Research for Evolutional Science and Technology) of Japan Science and Technology Corporation (JST ).
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K.-i. Fukui, Y. Iwasawa / Surface Science 441 (1999) 529–541 [16 ] F.F. Abraham, I.P. Batra, Surf. Sci. 209 (1989) L125. [17] J. Nowotny, L.-C. Dufour, Surface and Near Surface Chemistry of Oxide Materials, Elsevier, Amsterdam, 1988. [18] R.C. McCune, P. Wynblatt, J. Am. Ceram. Soc. 66 (1983) 111. [19] E.A. Colbourn, W.C. Mackrodt, P.W. Tasker, J. Mater. Sci. 18 (1983) 1917. [20] P.A. Crozier, M. Gajdardziska-Josifovska, J.M. Cowley, Microsc. Res. Technol. 20 (1992) 426. [21] M. Gajdardziska-Josifovska, P.A. Crozier, M.R. McCartney, J.M. Cowley, Surf. Sci. 284 (1993) 186. [22] P.A. Crozier, M. Gajdardziska-Josifovska, Ultramicroscopy 48 (1993) 63. [23] J.S. Lee, S.T. Oyama, Catal. Rev. Sci. Eng. 30 (1988) 249. [24] E.N. Voskresenskaya, V.G. Roguleva, A.G. Anshits, Catal. Rev. Sci. Eng. 37 (1995) 101. [25] J.H. Lunsford, Angew. Chem. Int. Ed. Engl. 34 (1995) 970. [26 ] D. Dissanayake, J.H. Lunsford, M.P. Rosynek, J. Catal. 143 (1993) 286.
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Observation of a new ridge structure along steps on the MgO(100) surface by non-contact atomic force microscopy Ken-ichi Fukui, Yasuhiro Iwasawa * Department of Chemistry, Graduate School of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan Received 12 April 1999; accepted for publication 9 August 1999
Abstract The surface topography of an MgO(100) surface after annealing at high temperatures followed by Ar+-ion sputtering was examined by non-contact atomic force microscopy (NC-AFM ) and X- ray photoelectron spectroscopy ( XPS). The calcium impurity (210 ppm in the bulk) was segregated to the surface by annealing above 1200 K and agglomerated as CaO particles at monolayer or bilayer steps along the [011] direction after annealing at ~1600 K x for 1 h. The step was always accompanied by a band-like structure with a protrusive line running parallel to the step edge with a separation longer than 5 nm. The structure was stable on the surface after removal of the CaO particles x by Ar+-ion sputtering at ~1600 K, but eventually disappeared after repeated Ar+-ion sputtering and annealing cycles. This unusual ridge structure is probably due to long-range reconstruction induced by segregated calcium of less than 3% coverage. The calcium-induced structure on MgO(100) is also discussed in relation to the chemical reactivity for methane activation. © 1999 Published by Elsevier Science B.V. All rights reserved. Keywords: Atomic force microscopy; Calcium; Magnesium oxide; Surface segregation; Surface structure; X-ray photoelectron spectroscopy
1. Introduction Scanning probe microscopy (SPM ) has great potential to identify the key issues controlling physical and chemical surface processes and phenomena of interesting materials, discriminating sitespecific structural and dynamic events on an atomic scale. A practically important class of oxides (Al O , SiO , MgO, zeolites, etc.) is, how2 3 2 ever, out of range of the most established SPM technique of scanning tunneling microscopy (STM ) because they are non-conducting. Recent progress in non-contact atomic force microscopy (NC-AFM ) provides an opportunity to observe * Corresponding author. Fax: +81-3-5800-6892. E-mail address: [email protected] ( Y. Iwasawa)
atomic-scale singularities, such as adatoms and point vacancies, on semiconductor surfaces such as silicon [1] and InP [2]. We reported the first example of atom-resolved NC-AFM images of oxide surfaces with oxygen point defects on TiO (110)-(1×1) [3]. NC-AFM has great advan2 tages over contact-mode AFM for observing adsorbed molecules other than the singularities of the substrates by virtue of its much weaker interaction between the tip and the sample for imaging the structure. Individual formate ions ( HCOO−) [4] or acetate ions (CH COO−) [5] adsorbed on 3 TiO (110)-(1×1) were imaged as bright protru2 sions by NC-AFM. MgO is an insulating oxide with the rocksalt structure. It is widely used as a catalyst, adsorbent, etc. The (100) surface of MgO is the most stable
0039-6028/99/$ – see front matter © 1999 Published by Elsevier Science B.V. All rights reserved. PII: S0 0 39 - 6 0 28 ( 99 ) 0 08 9 7 -3
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face and can be easily exposed by cleavage. This surface is widely used as a substrate for epitaxial growth of metals and high-T superconducting c oxides [6 ]. Numerous studies on structural determination of the MgO(100) surface have been reviewed by Henrich and Cox [7]. Experimental studies of cleaved and annealed MgO(100) surfaces by dynamic low-energy electron diffraction (LEED), reflection high-energy electron diffraction (RHEED), impact collision ion-scattering spectroscopy (ICISS), surface extended energy-loss fine structure (SEELFS) and medium-energy ion scattering (MEIS) showed a small relaxation of no more than ±3% and rumpling less than ±5%, although some deviation from these values was reported in some cases. This system is also a subject of theoretical calculation by different methods. Relaxation of the first layer is expected to be less than ±3%, but surface rumpling has diverse values depending on the calculation. AFM studies of MgO(100) surfaces in contact mode focus on the morphology of the surface as a substrate for epitaxial growth of other oxides. Cleaved (100) faces revealed monolayer steps [8,9] or bilayer steps [9,10] along the [001] or [010] direction, and multilayer steps composed of highindex vicinal faces [11]. The changes in surface morphology following annealing at high temperatures in oxygen ambient were also examined [8,12–15]. Some groups reported rows along the [011] direction with a separation about 0.3 nm in a flat terrace of the MgO(100) surface [8,10,15]. Although this separation matches a spacing between oxygen anions of the surface, it may not reflect the atom position but the periodicity of the surface [16 ]. Segregation of calcium impurity to an MgO(100) surface by annealing has also been studied [17]. Calcium is a major impurity of MgO crystals and it segregates to the surface upon annealing above 1200 K. The surface concentration of calcium was monitored by Auger electron spectroscopy (AES ) and low-energy ionscattering spectroscopy (LEIS ) as a function of temperature between 1173 and 1723 K, and the heat of segregation was determined to be 50.3 kJ mol−1 [18], which showed good agreement with calculation [19]. The formation of particles
of a calcium compound upon annealing above 1300 K was suggested by AFM combined with macroscopic information from AES [13,14]. Preferential modification of steps by segregated calcium was suggested by reflection electron microscopy (REM ) combined with high-spatial-resolution electron energy-loss spectroscopy (EELS ) [20–22], but the structure was not determined. MgO is known to be an active catalyst, particularly with alkali metal additives such as lithium and sodium, for the oxidative coupling of methane to >C hydrocarbons [23–25]. Increased activity 2 and selectivity were also observed by the addition of another alkali earth metal such as barium [26,27] or calcium [28]. MgO/CaO mixed oxide showed high activity and selectivity to C hydro2 carbons at the MgO content of 85% [29–31]. Thus the structure of the calcium-segregated MgO(100) surface employed in this study may have a relation with such catalysis. In the present paper, we report NC-AFM topographies of MgO(100) surfaces after annealing at high temperatures. Combined with macroscopic information by X-ray photoelectron spectroscopy ( XPS), it was found that calcium agglomerates as particles at monolayer or bilayer steps along the [011] direction. The steps are always accompanied by a unusual ridge structure parallel to the steps. The structure is probably due to calcium-induced long-range reconstruction with less than 3% coverage of calcium on the surface.
2. Experimental The experiments were performed in an ultrahigh vacuum ( UHV ) atomic force microscope (JEOL JAFM4500XT ) equipped with an ion gun, LEED optics ( VG RVL109) and an XPS analyzer ( VG CLAM2). The base pressure was 1×10−8 Pa. Mechanochemically polished MgO(100) wafers of dimensions 6.5 mm×1 mm×0.25 mm ( Earth Chemical ) were purchased and kept in dry air. The crystals included 210 ppm of calcium as a major impurity. MgO samples were calcined in air at 1073 K for 2 h. A Si(111) wafer of dimensions 6.5 mm×1 mm×0.33 mm was attached to the rear of the MgO sample and used as a resistive heater
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up to 1600 K. The temperature of the MgO sample was measured with an infrared radiation thermometer (Minolta TR-630) through a view port. Ar+-ion sputtering was performed to remove contaminants on the surface at room temperature (3 keV, 3 min) unless otherwise noted. The binding energy in XPS excited by Mg Ka was calibrated by the O 1s level of oxygen anions of the MgO, which was supposed to be 530.0 eV. A positive shift of ca. 12 eV in binding energy was observed due to charge-up of the sample. For NC-AFM measurements, stiff silicon cantilevers with f =270–290 kHz and k=28–30 N m−1 0 (Nanosensors) were used as the force sensor. The oscillation amplitude of the cantilever was ~15 nm for all images presented in this paper. p–p NC-AFM measurements used the shift of the resonance frequency (Df ) of the cantilever as a feedback signal. As the cantilever approaches the sample, the attractive force begins to work between the tip and the sample, resulting in a negative shift of the resonance frequency. The feedback signal is applied to the Z piezo to keep the frequency shift constant, which gives a topographic image of the surface. A bias voltage (V ) was applied to the s silicon heater to obtain NC-AFM images. For contact-mode AFM, soft silicon cantilevers with k~0.2 N m−1 (Nanosensors) were used as the force sensor.
3. Results The bulk-truncated structure of the MgO(100) surface is shown in Fig. 1. From the lattice parameter (a ) of 0.4211 nm, the spacing between the 0 layers is expected to be ca. 0.21 nm. In this figure, two steps with single- and double-atomic heights parallel to the [010] direction are shown schematically. Previous REM [21,22] and AFM [8–10] studies showed that the cleaved (100) surface has such steps parallel to the [010] and [001] directions. Fig. 2a shows an NC-AFM topograph of the MgO(100) surface after an as-received sample was calcined in air at 1073 K for 2 h and was admitted to the UHV chamber. Some large dips or scratches were found on the surface (such a region is not shown in Fig. 2a), but it was rather flat. A root-
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Fig. 1. Bulk-truncated structure of the MgO(100) surface with a single- and a double-atomic height steps, in top view and plane view.
mean-square (RMS) value, i.e. standard deviation of the height value, averaged for several images was 0.59 nm. Fig. 2b shows an NC-AFM topograph of the surface after Ar+-ion sputtering at room temperature (RT ) followed by UHV annealing at ~1400 K for 5 min. The surface is still rough (RMS 0.28 nm), which was also confirmed by diffuse (1×1) LEED spots with high background intensity. At a higher annealing temperature of ~1600 K, the (1×1) spots became sharp and the background intensity decreased. Fig. 2c shows an NC-AFM topograph after annealing the surface at ~1600 K for 5 min. Terraces had not yet developed; however, the surface became more flat (RMS 0.14 nm) than the surface of Fig. 2b. There were some particles on the surface, whose number increased with further annealing. Fig. 3 shows NC-AFM topographs after further annealing the surface of Fig. 2c at ~1600 K. In these images, particles are clearly found: lines run parallel to the [011] direction in Fig. 3a and the particles are located on them. These are more prominent in Fig. 3c. Although the area between any two such lines is still rough, these lines are considered to be steps by line-profile analysis. REM experiments showed that steps parallel to the [011] direction were preferred on an MgO(100) surface cleaved and annealed at high temperature [21]. When the surface was annealed as for long
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Fig. 2. Non-contact AFM topographs of MgO(100) surfaces after treatments at low temperature: (a) calcination in air at 1073 K for 2 h (Df~70 Hz, V =+0.5 V, 430 nm×430 nm); (b) s Ar+-ion sputtering and subsequent UHV annealing at ~1400 K for 5 min (Df ~100 Hz, V =+0.7 V, 800 nm× s 800 nm); (c) further UHV annealing at ~1600 K for 5 min (Df ~70 Hz, V =+0.7 V, 800 nm×800 nm). A cantilever with a s spring constant of 30 N m−1 and a resonant frequency of 284 kHz was used for these images.
as 1 h at ~1600 K, a much clearer step–terrace structure was observed ( Fig. 3b). A sharp (1×1) LEED pattern was observed for this surface ( Fig. 3e). The particles were also located at the steps, but several steps were bunched up at a cluster. As evident in Fig. 3d, they seem to locate at the cape. The origin of the particles became evident from XPS measurements. The only element which increased upon annealing the surface was calcium, which is the major impurity (210 ppm) of the MgO crystal. Contamination from the silicon heater or the sample holder was not observed by XPS even after a longer time of annealing at ~1600 K. The change in the Ca 2p , 2p XPS signal is shown 1/2 3/2 in Fig. 4. The calcium signal appeared after annealing the surface above 1200 K and increased at higher temperature (spectrum b). The signal decreased to the noise level upon sputtering the surface at room temperature (spectrum c), but increased again after annealing at ~1600 K (spectra d and e). The amount of calcium segregated to an MgO surface by annealing at 1673 K for 5 h was estimated to be 2.1×1014 ions cm−2 by Rutherford backscattering spectroscopy (RBS ) (19% replacement of magnesium atoms at the topmost layer if all the calcium signal came from the topmost layer) [32]. The equilibrium value for the calcium segregation between 1223 K and 1273 K was estimated by AES and LEIS, and corresponded to a 20% occupation of the surface cation sites [18]. The amount of calcium in Fig. 4e was estimated roughly from the XPS signal intensity ratio of (Ca 2p)/(Mg 2p) using relative crosssections and escape depths assuming a bulk-truncated structure. It corresponds to ca. 12% replacement of topmost-layer magnesium atoms by calcium. Owing to a large uncertainty of energy scale calibration, it may not be appropriate to quote the state of calcium. However, if the O 1s level of oxygen anions of the MgO is referred to 530.0 eV, the peak position for Ca 2p in Fig. 4 3/2 is estimated to be 347.0–347.3 eV. Binding energies lie in the range 346.1–347.3 eV for CaO. Thus, the calcium is probably oxidized to be CaO (x≤1) x on the surface. Fig. 5a shows a zoom-in NC-AFM image of the surface in Fig. 3b. Terraces were separated by
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Fig. 3. Non-contact AFM topographs of MgO(100) surfaces after further annealing of the surface of Fig. 2c at ~1600 K for (a) 20 min (Df~110 Hz, V =+0.7 V, 800 nm×800 nm) and (b) 1 h (Df~110 Hz, V =−0.7 V, 800 nm×800 nm). (c) A magnified image s s obtained from the same surface as in (a), but at a different place (Df~110 Hz, V =+0.7 V, 250 nm×250 nm). (d) A magnified image s of (b) (250 nm×250 nm). A cantilever with a spring constant of 30 N m−1 and a resonant frequency of 284 kHz was used for these images. (e) A well-contrasted (1×1) LEED pattern observed from the surface in (b).
single- or double-atomic height steps (0.2 nm or 0.4 nm) of the bulk structure as shown in Fig. 5b [distance between the (100) planes for the bulktruncated surface is 0.21 nm as shown in Fig. 1]. By looking closely at the steps, one can find that the steps are not so simple but have a band-like structure along them for all directions observed. The structure is clearer in the 3D view in Fig. 5c, where a protrusive line, indicated by an arrow in the figure, parallel to the step is observed accompa-
nied by slight depressive lines on both sides. Although one should be careful when determining the actual step edge from AFM images as discussed later, the protrusive line ca. 0.05 nm high from the terrace plane is away from the step edge at least by 5 nm. The distance of 5 nm corresponds to about 12 times the unit-cell dimension of the surface (0.42 nm). This kind of ridge structure has not been observed in previous AFM works with contact mode [8–15]. REM experiments showed
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Fig. 4. XPS of the Ca 2p , 2p region obtained on the 1/2 3/2 MgO(100) surfaces: (a) after calcination in air at 1073 K for 2 h; (b) after a cycle of Ar+-ion sputtering and UHV annealing at ~1600 K for 1 h; (c) after Ar+-ion sputtering of (b) at room temperature; (d) after UHV annealing of (c) at ~1600 K for 10 min; (e) after further UHV annealing of (d ) at ~1600 K for 2 h; (f ) after two cycles of Ar+-ion sputtering at ~1600 K for 1 h and UHV annealing at ~1600 K for 1 h.
the possibility of a calcium-modified structure at steps after annealing at high temperatures [20– 22]. Note that the terraces are rather flat compared with the protrusive lines along the steps, but have some structure with lines that are not necessarily correlated to the crystallographic direction. This is not an ordered structure but was reproducible by several scannings at the same area. Almost the same structure as that in Fig. 5 was obtained after annealing a sputtered MgO(100) sample in 1.0×10−4 Pa of O instead of UHV. 2 Fig. 6 shows contact-mode AFM images of an MgO(100) surface after Ar+-ion sputtering at RT and UHV annealing at ~1600 K for 50 min. These images were obtained on a different sample from that used for other NC-AFM images in the present paper. But it is confirmed that the ridge structure near steps similar to that in Figs. 3 and 5 was
Fig. 5. (a) Non-contact AFM topograph of the MgO(100) surface in Fig. 3b (Df~110 Hz, V =−0.7 V, 400 nm×400 nm). A s cantilever with a spring constant of 30 N m−1 and a resonant frequency of 284 kHz was used for this image. A line profile for the line in (a) and a three-dimensional (3D) view of the square region in (a) are shown in (b) and (c) (55.6 nm×55.6 nm), respectively. The step in (c) has a doubleatomic height. Arrow in (c) indicates the position of a protrusive line running parallel to the step.
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Fig. 6. Contact AFM images (variable-force mode) of an MgO(100) surface after a cycle of Ar+-ion sputtering and UHV annealing at ~1600 K for 50 min: (a) 800 nm×800 nm, (b) 150 nm×150 nm. Repulsive force between the tip and the sample was set at ~5 nN to obtain these images. Sample bias was set at 0 V.
observed by NC-AFM. In Fig. 6a, calcium particles are apparent but steps are not so clear as in NC-AFM images in Fig. 3. The ridge was not observed even in the magnified image in Fig. 6b. In contact mode, many atoms of the tip and the sample contribute at the same time to imaging [16 ], resulting in difficulty imaging a singularity of the surface as compared with imaging in a noncontact mode. However, there remains a possibility that the ridge structure is an artifact which does
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not reflect the actual structure. This possibility is denied by the following results. NC-AFM topographs were obtained regulating the distance between the tip and the sample to keep the resonant frequency shift of the cantilever constant. The shift value relates strongly with the force gradient between the tip and the sample [1]. The force probably includes electrostatic force and many kinds of forces other than van der Waals’ force. The charge distribution near a step must be different from that inside a terrace, so then a charge effect may contribute to the appearance of the structure. In a p-type Si(111) surface, the charged area near a step was observed as a bright line ( larger attractive force) at positive sample bias voltages and as a dark line at negative bias voltages by NC-AFM [33]. Fig. 7 shows NC-AFM topographs of the surface in Fig. 3b at different sample bias voltages. The regulated bias voltage was applied to the silicon heater relative to the tip. The silicon heater is attached to the rear of the MgO sample, but the actual bias voltage of the sample is unknown because MgO is an insulator. The MgO sample rather act as a dielectric in an electric field. Positive or negative charge should be induced on the MgO surface depending on the bias voltage between the silicon heater and the tip. If the surface has a clear distribution of charge like Si(111), the native charge should affect the induced charge leading to a change in contrast of NC-AFM images at different bias voltages. For the two applied voltages in Fig. 7, similar structures with bands along steps were observed by NC-AFM. At higher sample bias voltages with positive or negative polarity, topographs only became ambiguous due to a large contribution of long-range electrostatic force and contrast inversion was not observed. These results suggest that the charging effect is not dominant in this case unlike for the Si(111) surface. Note that the structure inside the terraces is clearer in Fig. 7 with better tip condition than in Fig. 5a. There are some line features with small protrusions but they are not ordered. The CaO particles in Fig. 3b can be removed x from the surface by Ar+-ion sputtering at room temperature, but calcium segregates to the surface again upon annealing to smooth the surface. It is in equilibrium between the bulk and the surface.
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Fig. 7. Non-contact AFM topographs of the MgO(100) surface in Fig. 3b at difference sample bias voltage settings: (a) −0.7 V and (b) +0.7 V (Df~110 Hz, 137 nm×137 nm). A cantilever with a spring constant of 30 N m−1 and a resonant frequency of 284 kHz was used for this image.
The CaO particles can be effectively removed by x sputtering the surface at ~1600 K, at which temperature calcium segregation at the surface is favored for thermodynamic reasons. High-temperature annealing has an advantage to avoid roughening of the surface. Fig. 8a shows an NC-AFM topograph of the MgO(100) surface after two cycles of Ar+-ion sputtering at ~1600 K for 1 h and UHV annealing at ~1600 K for 1 h for the surface in Fig. 3b. The CaO particles have been x successfully removed and the capes have disappeared, but the ridge structure along the steps
Fig. 8. (a) Non-contact AFM topograph of an MgO(100) surface after two cycles of Ar+-ion sputtering at ~1600 K for 1 h and UHV annealing at ~1600 K for 1 h for the surface in Fig. 3b (Df~140 Hz, V =+0.7 V, 300 nm×300 nm). A cantiles ver with a spring constant of 28 N m−1 and a resonant frequency of 278 kHz was used for this image. A 3D view of the square region in (a) is shown in (b) (55.1 nm×55.1 nm). The step in (b) has a single-atomic height. The arrow in (b) indicates the position of a protrusive line running parallel to the step.
remains. Similar protrusive lines were observed as shown in the 3D image of Fig. 8b. The corresponding XPS signal of Fig. 4f shows that the amount of calcium on the surface is comparable to the noise level. The signal before removal of the CaO particles (Fig. 4e) has already been estimated x to be 12% coverage. Comparing these two signals, the amount of calcium drops to less than 3% coverage. However, we consider that a small
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Fig. 9. (a) Non-contact AFM topograph of a MgO(100) surface after further several cycles of Ar+-ion sputtering at room temperature and UHV annealing at ~1600 K for 1 h for the surface in Fig. 8a (Df~100 Hz, V =+0.7 V, 200 nm×200 nm). A cans tilever with a spring constant of 28 N m−1 and a resonant frequency of 278 kHz was used for this image. The line profile for the line in (a) is shown in (b).
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structure, keeping the minimum vertical component of the distance between the tip and the sample (d ) constant, because only the vertical component s of the force gradient between the tip and the sample is responsible for the frequency shift of the resonance frequency of the cantilever. The step structure becomes dull by using a dull tip as shown in Fig. 10a. The curvature c is determined by the 1 radius of the tip apex. Fig. 10b shows a case of so-called double tips which leads to a double overlapping image. This is not the case for the image in Fig. 5c. In Fig. 10c–e, a tip with a small protuberance at the apex was postulated. Such a protuberance is responsible for high-resolution imaging of surface structure in the flat region. To reproduce the protrusive line observed in Fig. 5c, a small projection on the surface should be considered as in Fig. 10d and e. Here, curvature c is 1 determined by the small protuberance of the tip; therefore, relatively small structures can be found on the terrace. Curvature c is determined by the 3 large tip apex in these cases, then the step structure becomes dull. Fig. 10e qualitatively reproduces the protrusive line in Fig. 5c. For a more precise discussion, however, we have to know the contribution of each atom on the tip and the sample to the frequency shift of the cantilever as a function of the distance between each atom.
4. Discussion amount of calcium impurity induces the ridge structure. Fig. 9 shows an NC-AFM topograph of an MgO(100) surface after repetition of Ar+-ion sputtering at RT and UHV annealing at ~1600 K for 1 h for several times for the surface in Fig. 8. Steps became straight and some pits formed on terraces. It should be noted that the ridge structure was not observed near the steps on this surface. We consider that further Ar+-ion sputtering reduced the amount of calcium left on the surface, resulting in disappearance of the ridge structure. Fig. 10 summarizes simple models for tip– sample regulation in NC-AFM operation near a step. They assume that the tip traces the substrate
4.1. Segregation of calcium impurity to an MgO surface It is well known that calcium impurity in MgO crystals segregates to the surface upon annealing above 1200 K [7]. Souda et al. [32] reported that calcium is highly concentrated in the outermost surface layer and no marked calcium enrichment was found in deeper layers up to the fourth layer by a combination of ICISS and RBS. As for the site of location of segregated calcium ions, magnesium cation sites are usually assumed. The (앀2×앀2) R45° reconstruction was predicted by calculation [34]. For this structure, magnesium cations at the topmost layer are replaced by
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calcium cations and half of the oxygen ions of the topmost layer are pushed up out of the surface. Experimentally, results of neutral beam incidence ion-scattering spectroscopy (NBISS ) suggest that the calcium ions lie 0.04±0.01 nm above the Mg– O plane [35]. Recent grazing-incidence X-ray scattering (GIXS) experiments show that the most likely structure for a calcium-segregated MgO(100) surface is the (앀2×앀2) R45° reconstruction [36 ], although the calcium concentration is different from that theoretically predicted [34]. However, other kinds of reconstruction have been proposed by RHEED [13,14] and REM [20–22] observations. The formation of particles near the steps has been reported on MgO(100) after annealing at high temperature in oxygen atmosphere. A darkfield image of electron transmission through a thin MgO(100) sample after annealing in oxygen at 1823 K for 22 h showed the presence of particles (ca. 70–300 nm in diameter from the image) at vicinal surfaces [20]. An analysis for these particles by high-spatial-resolution EELS showed a high concentration of calcium. Particles ca. 350–500 nm in diameter and ca. 30 nm in height were found at steps along the [011] and [01: 1] directions by contact-mode AFM after annealing the sample above 1273 K in air [13,14]. Although the size of the particles is different from those in Fig. 3b, we consider that the origin of the particles is the same; i.e., they are formed by segregated calcium. The difference may be due to the difference in terrace size, which was much smaller for Fig. 3 than those reported previously. Actually, we have observed that larger particles were accompanied by wider terraces, the size of which varied with position on a sample probably because of different sputtering conditions and distribution of temperature. The amount of calcium in the particles observed in Fig. 3b can be roughly estimated by the
Fig. 10. Models of traces of tips with different shapes at a simple step (a)–(c) or a modified step (d), (e). The vertical scale was elongated by a factor of three for these images. They assume that the tip traces the substrate structure, keeping the minimum vertical component of the distance between the tip and the sample (d ) s constant. The step height is assumed to be 0.4 nm. The radius of
the tip apex is set at 10 nm (typical value for the cantilevers used in the present study) in (a). Two apexes (double tip) with radii of 2.5 nm and 3.5 nm are assumed for (b). A small protuberance of radius of 0.2 nm is assumed at the tip apex of radius 10 nm. A small projection with 0.05 nm height is also assumed on the substrate at (c) the step edge and (d) 4 nm in from the step edge.
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NC-AFM image. The particles have average dimensions of 2.3 nm height and 55 nm diameter, which make a volume of 3.7×103 nm3 for the averaged particle. But the particle diameter observed by SPM is the convolution of the particle shape and the tip shape, similar to the results in Fig. 10. The particle diameter observed is generally larger than the real diameter. Assuming the same story as in Fig. 10 with tip radius of 10 nm, the real diameter of the averaged particle is estimated to be 50 nm. Then the volume should be 3.0×103 nm3 for the averaged particle. If we assume that the particle consists of CaO, then the calcium content is 1.1×105 Ca particle−1. Considering the density of the particles, the concentration of calcium on the surface is 3.2×1014 Ca cm−2. This is in the same order as that estimated by XPS (1.4×1014 Ca cm−2). This estimation is compatible with the XPS results after removal of the CaO particles ( Fig. 4f ) which x showed that the calcium signal was no higher than the noise level. Then we can conclude that most of calcium, which is segregated from the bulk to the surface, is favored to agglomerate as particles and does not make the (앀2×앀2) R45° structure under the conditions we used. Double-line features which aligned along the [011] direction or the [01: 1] direction with an average length of 80±30 nm were observed by REM in the terrace region on a MgO(100) sample with 270 ppm calcium after annealing at 1823 K for 24 h, but were not observed on a sample with 40 ppm calcium [21]. A (앀2×앀2) R45° structure was suggested by GIXS for a calcium-segregated MgO(100) surface after annealing in air at 1800 K [36 ]. Such structures were not observed at any stage of the treatments in the present study, possibly due to the different concentration of calcium in the bulk or the different annealing temperature.
4.2. Origin of the ridge structure near the steps Modification of steps on MgO(100) along the [011] direction by segregated calcium has been suggested by REM experiments [20–22]. Steps parallel to the [001] and [010] directions on a cleaved MgO(100) surface changed their directions
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to the [011] direction after annealing the sample above 1823 K for 24 h in an oxygen atmosphere, accompanied by broadening of the steps along into grayish bands [20–22]. The width of the bands was wider for the sample with the higher concentration of calcium in the bulk [20] or for the sample annealed at higher temperature for a longer time [21]. High-spatial-resolution surface analysis by EELS showed that calcium was accumulated at the decorated steps [21,22]. A change in step direction by annealing MgO(100) was also observed by contact-mode AFM [13]. These results support the hypothesis that the ridge structure is induced by segregated calcium. The calcium-induced ridge structure always appeared along the steps and CaO particles were x also located on the steps. Fig. 3c shows that the CaO particles were formed on straight steps along x the [011] direction. The ridge structure at the steps begins to appear in Fig. 3c. The change from Fig. 3a to Fig. 3b by further annealing suggest that CaO particles pinned the motion of the steps. x It seems that all the steps retreat inside the upper terraces except for the position of the particles where some steps bunch up. At 1600 K, components of MgO have some mobility to smooth the surface structure but evaporation may occur at the same time [37]. Evaporation from the step edges is probably more favorable energetically than evaporation from inside the terraces. The CaO x particles may inhibit the evaporation, resulting in pinning of the step motion. It is not clear from the experimental results whether calcium segregates preferentially from the bulk through the step region or diffuses to the step region after random segregation at the whole surface. In either case, the problem is how calcium can induce such longrange reconstruction of more than 5 nm wide with the direction parallel to the steps, even to the curved one. We have estimated that the calcium coverage for the band like structure in Fig. 5 or 8 is less than 3%. If we assume that such calcium is concentrated in the band region of 5 nm width, the coverage of calcium in the region is 29%. At present, it is not apparent whether other faint line features observed further inside the terrace relate to segregated calcium or not. They might be due to slight roughening of the surface by high-temper-
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ature annealing. The height of the protrusive line from the terrace plane is ca. 0.05 nm, which is close to the value of 0.04±0.01 nm for protrusion of calcium ions above the Mg–O plane suggested by NBISS [35]. Again it is not clear that the replacement of magnesium ions by calcium ions occurs only at the sites in the 5 nm region from the step edge. The mechanism for the formation of such peculiar structure has not been determined in the present paper. It may be of a general interest whether such long-range reconstruction with a determined separation from a step edge occurs for other systems. High activity and selectivity to C hydrocarbons 2 by oxidative coupling of methane have been reported on MgO/CaO catalysts [29–31]. The performance showed a maximum at the mixture of 85% MgO/CaO. Thermal desorption [31] and infrared spectroscopy [30] of adsorbed CO sug2 gested that surface Lewis basicity has a maximum at the 85% MgO/CaO catalyst, showing the same trend with reactivity. Strong base sites are sometimes considered as the site of methane activation by heterolytic rupture of the CMH bond, although homolytic mechanism with participation of different oxygen species is usually assumed [24]. Infrared spectroscopy of the adsorbed CO sug2 gests that most of the surface is covered with MgO for the 85% MgO/CaO catalyst with little modification by highly dispersed or dissolved calcium ions [30]. An increase in lattice distortion by the unusual ridge reconstruction in Fig. 3b may be the reason for the high reactivity of the calcium-doped MgO catalyst for oxidative coupling of methane.
5. Conclusions We have measured the topography of MgO(100) surfaces after annealing at high temperatures followed by Ar+-ion sputtering by NC-AFM. After annealing the sputtered MgO(100) surface to ~1600 K for 1 h, flat terraces appeared with monolayer or bilayer steps along the [011] direction. XPS measurements indicated that calcium impurity was segregated from the bulk to the surface and agglomerated as CaO particles at the steps. The steps were always x
accompanied by the band-like structure with a protrusive line ca. 0.05 nm high running parallel to the step edge with a separation of more than 5 nm, as evident from 3D images of the NC-AFM topographs. The ridge structure remained on the surface after removal of the CaO particles by x Ar+-ion sputtering at ~1600 K, but eventually disappeared after repeated Ar+-ion sputtering and annealing cycles. This unusual ridge structure is probably due to long-range reconstruction induced by segregated calcium of less than 3% coverage.
Acknowledgements This work was supported by CREST (Core Research for Evolutional Science and Technology) of Japan Science and Technology Corporation (JST ).
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