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Infrared reflection absorption spectroscopic study for CO adsorption on molecular beam epitaxially grown Fe films on Cu(1 1 1)
Surface Science 472 (2001) 1±8
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Infrared re¯ection absorption spectroscopic study for CO adsorption on molecular beam epitaxially grown Fe ®lms on Cu(1 1 1) T. Tanabe, R. Buckmaster, T. Ishibashi, T. Wadayama *, A. Hatta Department of Materials Science, Graduate School of Engineering, Tohoku University, Aoba-yama 02, Sendai 980-8579, Japan Received 4 July 2000; accepted for publication 27 October 2000
Abstract Adsorption of CO at 90 K on molecular beam epitaxially grown Fe ®lms on Cu(1 1 1) has been investigated using IR re¯ection absorption spectroscopy (IRRAS). Re¯ection high energy electron diraction images reveal Fe ®lms up to 4 ML (monolayer) thickness have an fcc(1 1 1) structure and a mixture of fcc(1 1 1) and bcc(1 1 0) structures for 4±8 ML thick ®lms. A bcc(1 1 0) structure dominates the ®lms beyond 8 ML in thickness. The C±O stretching frequency at saturation coverages observed by IRRAS represents these structural dierences clearly. For 1±4 ML thick fcc Fe ®lms C±O stretch is located at 2000 cmÿ1 and the 6±8 ML thick ®lms yield two C±O stretch bands at 2000 and 2032 cmÿ1 , the latter of which is dominant for ®lms more than 10 ML thick. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Carbon monoxide; Iron; Copper; Molecular beam epitaxy; Re¯ection high-energy electron diraction (RHEED); Infrared absorption spectroscopy; Chemisorption; Surface structure, morphology, roughness, and topography
1. Introduction It has long been known that ultra-thin Fe ®lms grown on low index Cu single crystal surfaces are fcc owing to a quite low lattice mismatch between Cu and Fe. To date, a number of studies have been carried out on the lattice structures of Fe ®lms deposited on several single crystalline fcc metal substrates. For example, Kirschner and coworkers have reported their pioneering work for the
* Corresponding author. Tel.: +81-22-217-7319; fax: +81-22217-7318. E-mail address: [email protected] (T. Wadayama).
deposition process of fcc Fe on Cu(1 0 0) and Ni(1 0 0) using low energy electron diraction (LEED), scanning tunneling microscopy (STM), and surface magneto-optical Kerr eect [1,2]. Further, on the basis of LEED and STM measurements, Wuttig and coworkers [3,4] and Egelho and coworkers [5,6] made attempts to reveal the detailed structural change of the deposited Fe ®lm on Cu(1 0 0). However, their discussion has mainly been concerned with the correlation between the lattice structure and the magnetic properties. Also on Cu(1 1 1), Fe ®lm grows pseudomorphically with an fcc structure [7±10]. In the LEED study of Tian et al. [9], fcc Fe was found to grow with the lattice spacing of bulk Cu(1 1 1) up to a thickness of approximately 5 ML (monolayer), whereupon
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a structure of bcc begins to develop with the Kurdjumov±Sachs (K±S) orientation. However, the STM work of Kirschner and coworkers [10] has shown that the fcc±bcc transition takes place during thermal deposition of Fe ®lm in the range of 2±4 ML thickness: the saturation magnetization increases as the ®lm becomes thicker and takes on a more bcc-like character. KirschnerÕs group has reported the growth of Fe on stepped Cu(1 1 1) surfaces, the behavior of which is very similar to that on the ¯at Cu(1 1 1) surface [11,12]. In contrast, only a few papers have been reported from the view of surface chemistry on fcc Fe ®lms. Egawa and coworkers [13±15] have investigated the adsorption of CO, NO, and C2 H4 on fcc Fe ®lms on Rh(1 0 0) using ultra-violet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy, and thermal desorption spectroscopy. Furthermore, UPS study for adsorption and desorption behavior of CO on fcc Fe ®lms on Cu(1 0 0) has been reported by Radnik et al. [16]. In addition, we have previously investigated CO adsorption on molecular beam deposited fcc Fe thin ®lms on Cu(1 0 0) using infrared re¯ection absorption spectroscopy (IRRAS) [17, 18]. However, while great interest has been taken in the Fe/Cu(1 1 1) system from structural and magnetic viewpoints, no work has been made on the surface chemistry of Fe ®lm deposited on Cu(1 1 1). For this reason we have carried out an IRRAS investigation of CO adsorption on various Fe ®lms in thickness deposited on Cu(1 1 1) by molecular beam epitaxy. In this paper, adsorption behavior of CO on the deposited ®lms is discussed with their surface structures as revealed by re¯ection high energy electron diraction (RHEED). It is demonstrated that the C±O stretching frequency of adsorbed CO furnishes the measure of the surface structures. 2. Experimental Details of the experimental equipments and methods used in the present work are given elsewhere [17,18]. A Cu(1 1 1) crystal in less than 1° miscut range, which had been electropolished, was used as the substrate. The substrate surface was
cleaned by repeated Ar sputtering followed by annealing under an ultra-high vacuum condition. RHEED, LEED, and Auger electron spectroscopy (AES) (OCI BDL600IR) were used to verify surface cleanliness and crystallographic order. In addition, the Cu surface underwent CO adsorption before depositing Fe, thereby con®rming that it yields such sharp C±O stretch band as reported by Pritchard et al. [19]. Fe of 99.999% in purity was deposited using a Knudsen cell onto the Cu surface at room temperature. Fe ®lm thicknesses in ML units were estimated from the number of regular oscillations in the RHEED intensity observed during the Fe ®lm/Cu(1 0 0) growth [17,18], since no clear oscillation has been observed in present Fe/Cu(1 1 1) system. The deposition rate of Fe was approximately 0.3 ML minÿ1 . The lattice structure of Fe ®lm surface during deposition was monitored by RHEED images. The deposited Fe surface was contaminated by carbon (less than 2% by AES). The deposited Fe ®lm was immediately cooled to approximately 90 K and was then exposed to CO at a pressure of approximately 5 10ÿ10 Torr. IRRAS spectra as a function of increasing CO exposure were recorded with 2 cmÿ1 resolution as the average of 300 scans using an FTIR spectrophotometer (Mattson RS-2) equipped with a liquid-N2 -cooled HgCdTe detector. After the C±O stretch band reached the saturation in intensity, the substrate was slowly warmed until complete desorption of CO from the surface. 3. Results and discussion 3.1. Fe ®lm growth on Cu(1 1 1) observed by re¯ection high energy electron diraction As is shown in Fig. 1, the initial RHEED image taken before Fe deposition exhibits a sharp streak pattern with atomic spacings consistent with those expected for the fcc Cu(1 1 1) surface; i.e., the spacing of adjacent atomic rows was estimated to in the h1 1 0i direction and 2.2 A in the be 1.3 A h1 1 2i direction. Essentially the same image was obtained for Fe ®lms less than 4 ML thick, indicative of the epitaxial growth of Fe on the Cu substrate. For higher Fe thicknesses, however,
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Cu(1 1 1) system with the K±S relation. The lattice spacings estimated from the new streaks (1.4 and in the h1 1 0i and h1 1 2i directions, respec2.0 A tively) agree well with those expected for bcc Fe(1 1 0) structure. Although no attempt has been made of discussion on the relation of lattice directions between the fcc and bcc phases, our RHEED observations for ®lms above 4 ML thick clearly show that the deposited ®lm surface consists of a mixture of fcc(1 1 1) and bcc(1 1 0) domains. Contrary to the case of Fe/Cu(1 0 0) system [3,4,18,25,26], the intensity of RHEED re¯exes during Fe deposition on Cu(1 1 1) showed no regular oscillation, indicating that no layer-by-layer growth of Fe took place on the Cu(1 1 1) substrate. This is consistent with the report by Kirschner and coworkers [11,27] that Fe growth on Cu(1 1 1) results in the formation of islands. Furthermore, as will be shown later, our IR spectra obtained for CO adsorbed on Fe/Cu(1 1 1) reveal that in the early stages of Fe deposition (1±6 ML), CO adsorbs not only on the deposited Fe but also on the Cu(1 1 1) surface.
Fig. 1. RHEED images of Cu(1 1 1) before and after Fe deposition, illustrating variations in the ®lm structure with increasing Fe layer thickness. The top shows the Cu(1 1 1) structure and atomic spacing in real space.
original streaks became discrete suggesting the deposited ®lm surfaces were atomically rough. Further, in the images of 6 and 21 ML thick ®lms, new streaks appeared, as indicated by arrows in the ®gure. The estimated spacing of adjacent in the h1 1 0i and atomic rows is 1.4 and 2.0 A h1 1 2i directions, respectively. The intensities of the new streaks increased with increasing Fe thickness. A number of investigations have been reported on the relation between the lattice directions of bcc ®lms and fcc substrates; there are speci®c relations, namely Nishiyama±Wassermann (N±W) [20,21] and K±S [22]. Several groups [9,12,23,24] have deduced from RHEED and LEED investigations that an epitaxial bcc(1 1 0) Fe ®lm grows on Fe/
3.2. CO adsorption on Fe/Cu(1 1 1) system observed by infrared re¯ection absorption spectroscopy Fig. 2 illustrates the IR spectra of CO adsorbed on (a) 2 ML, (b) 8 ML, and (c) 21 ML thick Fe ®lms at 90 K for various exposures in the region of C±O stretching vibration (mCO ). In the present study, no IRRAS-active CO stretch band was observed below 1900 cmÿ1 . As can be seen from Fig. 2(a), a band appears at 1930 cmÿ1 for 0.02 L exposure, which shifts to 2000 cmÿ1 after 0.96 L exposure and then saturates in intensity upon 1.40 L exposure. For 0.24 L exposure a sharp band emerges at 2075 cmÿ1 . With increasing CO exposure this band grows up without any shift in the frequency. Thus, at a saturated coverage two C±O stretch bands at 2075 and 2000 cmÿ1 are observed. The 2075 cmÿ1 band almost disappeared at 202 K, as can be seen in Fig. 3. It has been reported that CO adsorption on a clean Cu(1 1 1) surface yields a mCO band at 2073 cmÿ1 [11,28] and the desorption temperature of CO from the surface is about 200 K [19,29]. Further, it is known that for CO
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Fig. 2. IRRAS spectra of CO adsorbed on (a) 2 ML, (b) 8 ML, and (c) 21 ML Fe ®lms at 90 K as a function of increasing exposure. The ®lms were deposited on Cu(1 1 1) at room temperature.
Fig. 3. Variation of C±O stretch bands for CO adsorbed on Cu(1 1 1) deposited with 2 ML Fe, as a function of increasing substrate temperature. Initially CO was saturated at 90 K.
adsorption on Cu single crystalline surfaces the peak shift of mCO with increasing exposure is relatively small because of the counterbalance of the opposite shifts due to the eect of back donation of electrons from Cu to the CO 2p antibonding orbital and that of dipole±dipole coupling between adsorbed CO molecules [30]. The little shift of the 2075 cmÿ1 band shown in Fig. 2(a) can be explained similarly. Thus, considering that the peak frequency as well as the desorption temperature is in good agreement with the literature [11,19,28,29], the 2075 cmÿ1 band can safely be attributed to CO adsorbed on the Cu(1 1 1) substrate. As is seen from Fig. 2(b), no band was observed at 2075 cmÿ1 on the 8 ML thick ®lm, an indication of complete Fe coverage on the Cu(1 1 1) surface. It should be noted that the peak position of mCO for CO adsorbed on the Cu(1 1 1) surface (Fig. 2(a)) is essentially the same as that for CO on the clean Cu(1 1 1) surface, revealing that the uncovered Cu(1 1 1) surface is chemically little in¯uenced by the deposited Fe islands. Kirschner and coworkers [11,27] reported morphological changes in the deposited Fe islands on Cu(1 1 1) using STM. Their results show that the Fe ®lm grows through
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Table 1 Frequency of C±O stretch bands for CO adsorbed on various Fe surfaces Surface
mCO frequency (cmÿ1 )
bcc Fe(1 0 0) [31] bcc Fe(1 1 0) [32] bcc Fe(1 1 1) [33] bcc poly Fe [17] fcc Fe(1 0 0)/Cu(1 0 0) [17] fcc Fe(1 1 1)/Cu(1 1 1)a bcc Fe(1 1 0)/Cu(1 1 1)a
1900±2055 (on 1890±1985 (on 1940±2015 (on 2055 (on top) 1920±2048 (on 1930±2000 1940±2032
a
top), 1180±1245 (lying down) top), 1180±1245 (lying down) top), 1735±1860 (shallow hollow), 1325±1575 (deep hollow) top and bridge)
Present work.
aggregation of the deposited Fe islands along steps on the substrate. Taking this fact into account, the uncovered Cu surface noted above is considered to be dominated by the terrace of Cu(1 1 1) upon which aggregation of Fe islands does not easily occur, as is rationalized by the fact that the peak position of adsorbed CO remains unchanged even after 6 ML thick deposition. The mCO frequencies thus far reported for CO adsorbed on low Miller index Fe crystal surfaces are summarized in Table 1, where our data for CO adsorption on MBE deposited Fe ®lms on Cu(1 0 0) and Cu(1 1 1) are also presented. On referring to Table 1 the mCO frequencies observed in the present work are found to be ascribed to CO on the on-top sites of the ®lms. It is signi®cant to note that the 8 ML thick Fe ®lm exhibits two mCO bands (Fig. 2(b)), the high- and low-frequency bands being attributable to adsorption on the fcc(1 1 1) and bcc(1 1 0) surfaces, respectively. It is also evident from Fig. 2(c) that the 21 ML thick ®lm gives only a single dominant band at the CO saturation coverage (1.15 L) that must be ascribed to CO adsorbed on the bcc(1 1 0) surface. Thus, it may be concluded that the surface structural changes during the deposition judged by RHEED experiments are justi®ed by the IRRAS observations of the adsorbed CO. Fig. 3 shows a series of IRRAS spectra for CO adsorbed on the 2 ML thick Fe ®lm during elevating substrate temperatures. The band for CO adsorbed on the uncovered Cu(1 1 1) surface almost diminishes at 202 K. In contrast, the band due to adsorption at the deposited fcc Fe surface shifts to lower frequency with increasing substrate temperature and almost disappears at 399 K, re-
vealing the occurrence of CO decomposition or desorption between 350 and 399 K. The UPS study of Radnik et al. [16] showed that the desorption/decomposition temperature of CO is 370 K on the epitaxial fcc Fe ®lm on Cu(1 0 0) while it is 300 K on bulk bcc Fe(1 1 0). In accord with this study, our previous IRRAS investigation for CO adsorption on Fe/Cu(1 0 0) [17] has shown that adsorbed CO is thermally more stable on the fcc Fe(1 0 0) surface than on the polycrystalline bcc Fe ®lm surface. If merely CO decomposition is responsible for the disappearance of the IR bands, it follows that both fcc Fe(1 1 1) and Fe(1 0 0) ®lm surfaces are less active than bcc Fe surfaces investigated in the past. Further, it should be noted here that absorption intensities of the band due to adsorbed CO on the Fe increased during elevating the substrate temperature from 202 to 249 K. This temperature range is just above the desorption temperature of CO adsorbed on Cu(1 1 1). Thus, the migration of the adsorbed CO on the uncovered Cu(1 1 1) surface to the Fe ®lm surface seems to take place at that temperature range. IRRAS spectra for CO adsorbed at saturation on Fe ®lms with dierent thicknesses on Cu(1 1 1) are depicted in Fig. 4, where three absorption features are observed at 2075, 2032, and 2000 cmÿ1 . As mentioned earlier, the 2075 cmÿ1 band is due to mCO for adsorption on the Cu(1 1 1) substrate surface while the latter two bands are on the deposited Fe ®lm surface. As is seen from Fig. 4, though the 2075 cmÿ1 band does not shift with deposition from 1 to 8 ML thickness, its intensity falls o approximately proportional to the square inverse of Fe ®lm thickness. On the basis of intensity analysis for the CO band, the proportion of
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®lm of bcc Fe grown on Cu(1 1 1) gave rise to a mCO band that is the same in the shape and frequency as the mCO band observed on the bcc Fe(1 1 0) ®lm surface obtained for Fe/Cu(1 0 0) system [18]. 3.3. Correlation between the deposited Fe structure and the mCO band frequency
Fig. 4. IRRAS spectra for CO adsorbed at saturation on Cu(1 1 1) at 90 K deposited with various Fe ®lms in thickness.
the uncovered to the entire Cu(1 1 1) surface area can be estimated to be 51% (1 ML), 22% (2 ML), 5% (4 ML), and 3% (6 ML) for the respective layers. The results are consistent with the formation of islands which was revealed by the RHEED observation that no regular oscillation occurred in the RHEED intensity during Fe deposition, as mentioned in Section 3.1. An interesting feature, which can be seen in Fig. 4, is the peak position of mCO . For the ®lms from 1 to 4 ML in thickness the mCO band is positioned at 2000 cmÿ1 . However, for the 6 and 8 ML thick ®lms the 2000 cmÿ1 band is strongly aected by the additional band at 2032 cmÿ1 which arises from the fcc±bcc transition, as veri®ed by the RHEED images shown in Fig. 1. At the saturated coverage of adsorbed CO on the deposited Fe above 8 ML thick, a mCO band at 2032 cmÿ1 can be seen, a manifestation of the steady growth of bcc Fe(1 1 0) structure following the completion of the phase transition. We may point out here that the 21 ML
Possibly the most important result derived from the present work is a close correlation between the Fe ®lm structures as revealed by RHEED and the mCO frequencies observed with IRRAS. As Fig. 4 shows, the frequency for saturated CO adsorption changes as the surface structure varies from fcc(1 1 1) (1±4 ML) to a mixture of fcc(1 1 1) and bcc(1 1 0) (4±8 ML) followed by changing to bcc(1 1 0) (above 8 ML). Including the data for low exposures, the mCO band of CO adsorbed at the fcc (1 1 1) surface is in the range 1930±2000 cmÿ1 whereas for adsorption on the bcc(1 1 0) surface it ranges from 1940 cmÿ1 or less to 2032 cmÿ1 (Fig. 1). It has been found by Erley [32] using high resolution electron energy loss spectroscopy (HREELS) that CO adsorption on bulk bcc Fe(1 1 0) yields a mCO band in the region 1890±1985 cmÿ1 . Thus, it is clear that the mCO frequency of adsorbed CO diers on the bulk crystal surface and on the thin ®lm surface. This dierence is of unquestionable interest because it may imply that surface electronic structure or surface density of states in the thin ®lm is dierent from that in the bulk crystal. Although the Fe coverage at which the fcc±bcc phase transition occurs is not exactly known, the layer thickness at which a bcc structure begins to develop as well as the thickness at which the bcc structure is formed overall is compatible with literatures [9,10]. Another noticeable feature is that the mCO band of CO at the bcc ®lm surface (Fig. 2(c)) is very broad in comparison to the mCO band observed on the Cu(1 1 1) substrate surface. Band broadening is also observed for the mCO band for CO on the 2 ML thick fcc ®lm (Fig. 2(a)). These probably indicate that the surfaces are not atomically smooth. It has already been mentioned that in that case no regular oscillation was observed in the RHEED intensity and the ®lm consists of islands as revealed by the appearance of another mCO band due
T. Tanabe et al. / Surface Science 472 (2001) 1±8
to the adsorbed CO on the uncovered Cu(1 1 1) substrate surface. Further, STM studies show that Fe ®lms deposited on Cu(1 1 1) have a threedimensional island structure [34,35]. Thus, it is most likely that a large contribution to the observed bandwidth comes from the inhomogeneous distribution of adsorption sites or variations in the chemical environment of the deposited Fe over the surface.
4. Conclusions In this paper adsorption behavior of CO at 90 K on Fe ®lms deposited by MBE onto Cu(1 1 1) was investigated. The results clearly demonstrate a simple correlation of the C±O stretching frequency with the surface lattice structure of the Fe ®lm, as revealed by RHEED. Fe ®lms of 1±4 ML gave rise to RHEED images correspond to an fcc(1 1 1) whereas saturated CO adsorption on their surfaces exhibited a C±O stretch band at 2075 cmÿ1 due to the adsorption at the Cu(1 1 1) substrate surface in addition to another band at 2000 cmÿ1 due to the bonding to the fcc Fe(1 1 1) ®lms. The ®lms beyond 8 ML thick showed the RHEED image representing a bcc(1 1 0) with a C±O stretch band at 2032 cmÿ1 for the saturated adsorption. At the intermediate Fe thicknesses in which the RHEED image consists of fcc(1 1 1) and bcc(1 1 0) lattice structures, two IR bands were observed at 2000 and 2032 cmÿ1 . These results clearly demonstrate that IRRAS observation of CO adsorbed on Fe ultra-thin ®lm on Cu substrate is a suitable guide for investigating change in surface structure of MBE system.
Acknowledgements This work was supported by a grant-in-aids (T.W. No. 11450280) from the Ministry of Education, Science, Sports and Culture of Japan. One of the authors (R.B.) wishes to express appreciation to the Japanese Government and Tohoku University for making this research experience possible.
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Infrared re¯ection absorption spectroscopic study for CO adsorption on molecular beam epitaxially grown Fe ®lms on Cu(1 1 1) T. Tanabe, R. Buckmaster, T. Ishibashi, T. Wadayama *, A. Hatta Department of Materials Science, Graduate School of Engineering, Tohoku University, Aoba-yama 02, Sendai 980-8579, Japan Received 4 July 2000; accepted for publication 27 October 2000
Abstract Adsorption of CO at 90 K on molecular beam epitaxially grown Fe ®lms on Cu(1 1 1) has been investigated using IR re¯ection absorption spectroscopy (IRRAS). Re¯ection high energy electron diraction images reveal Fe ®lms up to 4 ML (monolayer) thickness have an fcc(1 1 1) structure and a mixture of fcc(1 1 1) and bcc(1 1 0) structures for 4±8 ML thick ®lms. A bcc(1 1 0) structure dominates the ®lms beyond 8 ML in thickness. The C±O stretching frequency at saturation coverages observed by IRRAS represents these structural dierences clearly. For 1±4 ML thick fcc Fe ®lms C±O stretch is located at 2000 cmÿ1 and the 6±8 ML thick ®lms yield two C±O stretch bands at 2000 and 2032 cmÿ1 , the latter of which is dominant for ®lms more than 10 ML thick. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Carbon monoxide; Iron; Copper; Molecular beam epitaxy; Re¯ection high-energy electron diraction (RHEED); Infrared absorption spectroscopy; Chemisorption; Surface structure, morphology, roughness, and topography
1. Introduction It has long been known that ultra-thin Fe ®lms grown on low index Cu single crystal surfaces are fcc owing to a quite low lattice mismatch between Cu and Fe. To date, a number of studies have been carried out on the lattice structures of Fe ®lms deposited on several single crystalline fcc metal substrates. For example, Kirschner and coworkers have reported their pioneering work for the
* Corresponding author. Tel.: +81-22-217-7319; fax: +81-22217-7318. E-mail address: [email protected] (T. Wadayama).
deposition process of fcc Fe on Cu(1 0 0) and Ni(1 0 0) using low energy electron diraction (LEED), scanning tunneling microscopy (STM), and surface magneto-optical Kerr eect [1,2]. Further, on the basis of LEED and STM measurements, Wuttig and coworkers [3,4] and Egelho and coworkers [5,6] made attempts to reveal the detailed structural change of the deposited Fe ®lm on Cu(1 0 0). However, their discussion has mainly been concerned with the correlation between the lattice structure and the magnetic properties. Also on Cu(1 1 1), Fe ®lm grows pseudomorphically with an fcc structure [7±10]. In the LEED study of Tian et al. [9], fcc Fe was found to grow with the lattice spacing of bulk Cu(1 1 1) up to a thickness of approximately 5 ML (monolayer), whereupon
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a structure of bcc begins to develop with the Kurdjumov±Sachs (K±S) orientation. However, the STM work of Kirschner and coworkers [10] has shown that the fcc±bcc transition takes place during thermal deposition of Fe ®lm in the range of 2±4 ML thickness: the saturation magnetization increases as the ®lm becomes thicker and takes on a more bcc-like character. KirschnerÕs group has reported the growth of Fe on stepped Cu(1 1 1) surfaces, the behavior of which is very similar to that on the ¯at Cu(1 1 1) surface [11,12]. In contrast, only a few papers have been reported from the view of surface chemistry on fcc Fe ®lms. Egawa and coworkers [13±15] have investigated the adsorption of CO, NO, and C2 H4 on fcc Fe ®lms on Rh(1 0 0) using ultra-violet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy, and thermal desorption spectroscopy. Furthermore, UPS study for adsorption and desorption behavior of CO on fcc Fe ®lms on Cu(1 0 0) has been reported by Radnik et al. [16]. In addition, we have previously investigated CO adsorption on molecular beam deposited fcc Fe thin ®lms on Cu(1 0 0) using infrared re¯ection absorption spectroscopy (IRRAS) [17, 18]. However, while great interest has been taken in the Fe/Cu(1 1 1) system from structural and magnetic viewpoints, no work has been made on the surface chemistry of Fe ®lm deposited on Cu(1 1 1). For this reason we have carried out an IRRAS investigation of CO adsorption on various Fe ®lms in thickness deposited on Cu(1 1 1) by molecular beam epitaxy. In this paper, adsorption behavior of CO on the deposited ®lms is discussed with their surface structures as revealed by re¯ection high energy electron diraction (RHEED). It is demonstrated that the C±O stretching frequency of adsorbed CO furnishes the measure of the surface structures. 2. Experimental Details of the experimental equipments and methods used in the present work are given elsewhere [17,18]. A Cu(1 1 1) crystal in less than 1° miscut range, which had been electropolished, was used as the substrate. The substrate surface was
cleaned by repeated Ar sputtering followed by annealing under an ultra-high vacuum condition. RHEED, LEED, and Auger electron spectroscopy (AES) (OCI BDL600IR) were used to verify surface cleanliness and crystallographic order. In addition, the Cu surface underwent CO adsorption before depositing Fe, thereby con®rming that it yields such sharp C±O stretch band as reported by Pritchard et al. [19]. Fe of 99.999% in purity was deposited using a Knudsen cell onto the Cu surface at room temperature. Fe ®lm thicknesses in ML units were estimated from the number of regular oscillations in the RHEED intensity observed during the Fe ®lm/Cu(1 0 0) growth [17,18], since no clear oscillation has been observed in present Fe/Cu(1 1 1) system. The deposition rate of Fe was approximately 0.3 ML minÿ1 . The lattice structure of Fe ®lm surface during deposition was monitored by RHEED images. The deposited Fe surface was contaminated by carbon (less than 2% by AES). The deposited Fe ®lm was immediately cooled to approximately 90 K and was then exposed to CO at a pressure of approximately 5 10ÿ10 Torr. IRRAS spectra as a function of increasing CO exposure were recorded with 2 cmÿ1 resolution as the average of 300 scans using an FTIR spectrophotometer (Mattson RS-2) equipped with a liquid-N2 -cooled HgCdTe detector. After the C±O stretch band reached the saturation in intensity, the substrate was slowly warmed until complete desorption of CO from the surface. 3. Results and discussion 3.1. Fe ®lm growth on Cu(1 1 1) observed by re¯ection high energy electron diraction As is shown in Fig. 1, the initial RHEED image taken before Fe deposition exhibits a sharp streak pattern with atomic spacings consistent with those expected for the fcc Cu(1 1 1) surface; i.e., the spacing of adjacent atomic rows was estimated to in the h1 1 0i direction and 2.2 A in the be 1.3 A h1 1 2i direction. Essentially the same image was obtained for Fe ®lms less than 4 ML thick, indicative of the epitaxial growth of Fe on the Cu substrate. For higher Fe thicknesses, however,
T. Tanabe et al. / Surface Science 472 (2001) 1±8
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Cu(1 1 1) system with the K±S relation. The lattice spacings estimated from the new streaks (1.4 and in the h1 1 0i and h1 1 2i directions, respec2.0 A tively) agree well with those expected for bcc Fe(1 1 0) structure. Although no attempt has been made of discussion on the relation of lattice directions between the fcc and bcc phases, our RHEED observations for ®lms above 4 ML thick clearly show that the deposited ®lm surface consists of a mixture of fcc(1 1 1) and bcc(1 1 0) domains. Contrary to the case of Fe/Cu(1 0 0) system [3,4,18,25,26], the intensity of RHEED re¯exes during Fe deposition on Cu(1 1 1) showed no regular oscillation, indicating that no layer-by-layer growth of Fe took place on the Cu(1 1 1) substrate. This is consistent with the report by Kirschner and coworkers [11,27] that Fe growth on Cu(1 1 1) results in the formation of islands. Furthermore, as will be shown later, our IR spectra obtained for CO adsorbed on Fe/Cu(1 1 1) reveal that in the early stages of Fe deposition (1±6 ML), CO adsorbs not only on the deposited Fe but also on the Cu(1 1 1) surface.
Fig. 1. RHEED images of Cu(1 1 1) before and after Fe deposition, illustrating variations in the ®lm structure with increasing Fe layer thickness. The top shows the Cu(1 1 1) structure and atomic spacing in real space.
original streaks became discrete suggesting the deposited ®lm surfaces were atomically rough. Further, in the images of 6 and 21 ML thick ®lms, new streaks appeared, as indicated by arrows in the ®gure. The estimated spacing of adjacent in the h1 1 0i and atomic rows is 1.4 and 2.0 A h1 1 2i directions, respectively. The intensities of the new streaks increased with increasing Fe thickness. A number of investigations have been reported on the relation between the lattice directions of bcc ®lms and fcc substrates; there are speci®c relations, namely Nishiyama±Wassermann (N±W) [20,21] and K±S [22]. Several groups [9,12,23,24] have deduced from RHEED and LEED investigations that an epitaxial bcc(1 1 0) Fe ®lm grows on Fe/
3.2. CO adsorption on Fe/Cu(1 1 1) system observed by infrared re¯ection absorption spectroscopy Fig. 2 illustrates the IR spectra of CO adsorbed on (a) 2 ML, (b) 8 ML, and (c) 21 ML thick Fe ®lms at 90 K for various exposures in the region of C±O stretching vibration (mCO ). In the present study, no IRRAS-active CO stretch band was observed below 1900 cmÿ1 . As can be seen from Fig. 2(a), a band appears at 1930 cmÿ1 for 0.02 L exposure, which shifts to 2000 cmÿ1 after 0.96 L exposure and then saturates in intensity upon 1.40 L exposure. For 0.24 L exposure a sharp band emerges at 2075 cmÿ1 . With increasing CO exposure this band grows up without any shift in the frequency. Thus, at a saturated coverage two C±O stretch bands at 2075 and 2000 cmÿ1 are observed. The 2075 cmÿ1 band almost disappeared at 202 K, as can be seen in Fig. 3. It has been reported that CO adsorption on a clean Cu(1 1 1) surface yields a mCO band at 2073 cmÿ1 [11,28] and the desorption temperature of CO from the surface is about 200 K [19,29]. Further, it is known that for CO
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T. Tanabe et al. / Surface Science 472 (2001) 1±8
Fig. 2. IRRAS spectra of CO adsorbed on (a) 2 ML, (b) 8 ML, and (c) 21 ML Fe ®lms at 90 K as a function of increasing exposure. The ®lms were deposited on Cu(1 1 1) at room temperature.
Fig. 3. Variation of C±O stretch bands for CO adsorbed on Cu(1 1 1) deposited with 2 ML Fe, as a function of increasing substrate temperature. Initially CO was saturated at 90 K.
adsorption on Cu single crystalline surfaces the peak shift of mCO with increasing exposure is relatively small because of the counterbalance of the opposite shifts due to the eect of back donation of electrons from Cu to the CO 2p antibonding orbital and that of dipole±dipole coupling between adsorbed CO molecules [30]. The little shift of the 2075 cmÿ1 band shown in Fig. 2(a) can be explained similarly. Thus, considering that the peak frequency as well as the desorption temperature is in good agreement with the literature [11,19,28,29], the 2075 cmÿ1 band can safely be attributed to CO adsorbed on the Cu(1 1 1) substrate. As is seen from Fig. 2(b), no band was observed at 2075 cmÿ1 on the 8 ML thick ®lm, an indication of complete Fe coverage on the Cu(1 1 1) surface. It should be noted that the peak position of mCO for CO adsorbed on the Cu(1 1 1) surface (Fig. 2(a)) is essentially the same as that for CO on the clean Cu(1 1 1) surface, revealing that the uncovered Cu(1 1 1) surface is chemically little in¯uenced by the deposited Fe islands. Kirschner and coworkers [11,27] reported morphological changes in the deposited Fe islands on Cu(1 1 1) using STM. Their results show that the Fe ®lm grows through
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Table 1 Frequency of C±O stretch bands for CO adsorbed on various Fe surfaces Surface
mCO frequency (cmÿ1 )
bcc Fe(1 0 0) [31] bcc Fe(1 1 0) [32] bcc Fe(1 1 1) [33] bcc poly Fe [17] fcc Fe(1 0 0)/Cu(1 0 0) [17] fcc Fe(1 1 1)/Cu(1 1 1)a bcc Fe(1 1 0)/Cu(1 1 1)a
1900±2055 (on 1890±1985 (on 1940±2015 (on 2055 (on top) 1920±2048 (on 1930±2000 1940±2032
a
top), 1180±1245 (lying down) top), 1180±1245 (lying down) top), 1735±1860 (shallow hollow), 1325±1575 (deep hollow) top and bridge)
Present work.
aggregation of the deposited Fe islands along steps on the substrate. Taking this fact into account, the uncovered Cu surface noted above is considered to be dominated by the terrace of Cu(1 1 1) upon which aggregation of Fe islands does not easily occur, as is rationalized by the fact that the peak position of adsorbed CO remains unchanged even after 6 ML thick deposition. The mCO frequencies thus far reported for CO adsorbed on low Miller index Fe crystal surfaces are summarized in Table 1, where our data for CO adsorption on MBE deposited Fe ®lms on Cu(1 0 0) and Cu(1 1 1) are also presented. On referring to Table 1 the mCO frequencies observed in the present work are found to be ascribed to CO on the on-top sites of the ®lms. It is signi®cant to note that the 8 ML thick Fe ®lm exhibits two mCO bands (Fig. 2(b)), the high- and low-frequency bands being attributable to adsorption on the fcc(1 1 1) and bcc(1 1 0) surfaces, respectively. It is also evident from Fig. 2(c) that the 21 ML thick ®lm gives only a single dominant band at the CO saturation coverage (1.15 L) that must be ascribed to CO adsorbed on the bcc(1 1 0) surface. Thus, it may be concluded that the surface structural changes during the deposition judged by RHEED experiments are justi®ed by the IRRAS observations of the adsorbed CO. Fig. 3 shows a series of IRRAS spectra for CO adsorbed on the 2 ML thick Fe ®lm during elevating substrate temperatures. The band for CO adsorbed on the uncovered Cu(1 1 1) surface almost diminishes at 202 K. In contrast, the band due to adsorption at the deposited fcc Fe surface shifts to lower frequency with increasing substrate temperature and almost disappears at 399 K, re-
vealing the occurrence of CO decomposition or desorption between 350 and 399 K. The UPS study of Radnik et al. [16] showed that the desorption/decomposition temperature of CO is 370 K on the epitaxial fcc Fe ®lm on Cu(1 0 0) while it is 300 K on bulk bcc Fe(1 1 0). In accord with this study, our previous IRRAS investigation for CO adsorption on Fe/Cu(1 0 0) [17] has shown that adsorbed CO is thermally more stable on the fcc Fe(1 0 0) surface than on the polycrystalline bcc Fe ®lm surface. If merely CO decomposition is responsible for the disappearance of the IR bands, it follows that both fcc Fe(1 1 1) and Fe(1 0 0) ®lm surfaces are less active than bcc Fe surfaces investigated in the past. Further, it should be noted here that absorption intensities of the band due to adsorbed CO on the Fe increased during elevating the substrate temperature from 202 to 249 K. This temperature range is just above the desorption temperature of CO adsorbed on Cu(1 1 1). Thus, the migration of the adsorbed CO on the uncovered Cu(1 1 1) surface to the Fe ®lm surface seems to take place at that temperature range. IRRAS spectra for CO adsorbed at saturation on Fe ®lms with dierent thicknesses on Cu(1 1 1) are depicted in Fig. 4, where three absorption features are observed at 2075, 2032, and 2000 cmÿ1 . As mentioned earlier, the 2075 cmÿ1 band is due to mCO for adsorption on the Cu(1 1 1) substrate surface while the latter two bands are on the deposited Fe ®lm surface. As is seen from Fig. 4, though the 2075 cmÿ1 band does not shift with deposition from 1 to 8 ML thickness, its intensity falls o approximately proportional to the square inverse of Fe ®lm thickness. On the basis of intensity analysis for the CO band, the proportion of
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®lm of bcc Fe grown on Cu(1 1 1) gave rise to a mCO band that is the same in the shape and frequency as the mCO band observed on the bcc Fe(1 1 0) ®lm surface obtained for Fe/Cu(1 0 0) system [18]. 3.3. Correlation between the deposited Fe structure and the mCO band frequency
Fig. 4. IRRAS spectra for CO adsorbed at saturation on Cu(1 1 1) at 90 K deposited with various Fe ®lms in thickness.
the uncovered to the entire Cu(1 1 1) surface area can be estimated to be 51% (1 ML), 22% (2 ML), 5% (4 ML), and 3% (6 ML) for the respective layers. The results are consistent with the formation of islands which was revealed by the RHEED observation that no regular oscillation occurred in the RHEED intensity during Fe deposition, as mentioned in Section 3.1. An interesting feature, which can be seen in Fig. 4, is the peak position of mCO . For the ®lms from 1 to 4 ML in thickness the mCO band is positioned at 2000 cmÿ1 . However, for the 6 and 8 ML thick ®lms the 2000 cmÿ1 band is strongly aected by the additional band at 2032 cmÿ1 which arises from the fcc±bcc transition, as veri®ed by the RHEED images shown in Fig. 1. At the saturated coverage of adsorbed CO on the deposited Fe above 8 ML thick, a mCO band at 2032 cmÿ1 can be seen, a manifestation of the steady growth of bcc Fe(1 1 0) structure following the completion of the phase transition. We may point out here that the 21 ML
Possibly the most important result derived from the present work is a close correlation between the Fe ®lm structures as revealed by RHEED and the mCO frequencies observed with IRRAS. As Fig. 4 shows, the frequency for saturated CO adsorption changes as the surface structure varies from fcc(1 1 1) (1±4 ML) to a mixture of fcc(1 1 1) and bcc(1 1 0) (4±8 ML) followed by changing to bcc(1 1 0) (above 8 ML). Including the data for low exposures, the mCO band of CO adsorbed at the fcc (1 1 1) surface is in the range 1930±2000 cmÿ1 whereas for adsorption on the bcc(1 1 0) surface it ranges from 1940 cmÿ1 or less to 2032 cmÿ1 (Fig. 1). It has been found by Erley [32] using high resolution electron energy loss spectroscopy (HREELS) that CO adsorption on bulk bcc Fe(1 1 0) yields a mCO band in the region 1890±1985 cmÿ1 . Thus, it is clear that the mCO frequency of adsorbed CO diers on the bulk crystal surface and on the thin ®lm surface. This dierence is of unquestionable interest because it may imply that surface electronic structure or surface density of states in the thin ®lm is dierent from that in the bulk crystal. Although the Fe coverage at which the fcc±bcc phase transition occurs is not exactly known, the layer thickness at which a bcc structure begins to develop as well as the thickness at which the bcc structure is formed overall is compatible with literatures [9,10]. Another noticeable feature is that the mCO band of CO at the bcc ®lm surface (Fig. 2(c)) is very broad in comparison to the mCO band observed on the Cu(1 1 1) substrate surface. Band broadening is also observed for the mCO band for CO on the 2 ML thick fcc ®lm (Fig. 2(a)). These probably indicate that the surfaces are not atomically smooth. It has already been mentioned that in that case no regular oscillation was observed in the RHEED intensity and the ®lm consists of islands as revealed by the appearance of another mCO band due
T. Tanabe et al. / Surface Science 472 (2001) 1±8
to the adsorbed CO on the uncovered Cu(1 1 1) substrate surface. Further, STM studies show that Fe ®lms deposited on Cu(1 1 1) have a threedimensional island structure [34,35]. Thus, it is most likely that a large contribution to the observed bandwidth comes from the inhomogeneous distribution of adsorption sites or variations in the chemical environment of the deposited Fe over the surface.
4. Conclusions In this paper adsorption behavior of CO at 90 K on Fe ®lms deposited by MBE onto Cu(1 1 1) was investigated. The results clearly demonstrate a simple correlation of the C±O stretching frequency with the surface lattice structure of the Fe ®lm, as revealed by RHEED. Fe ®lms of 1±4 ML gave rise to RHEED images correspond to an fcc(1 1 1) whereas saturated CO adsorption on their surfaces exhibited a C±O stretch band at 2075 cmÿ1 due to the adsorption at the Cu(1 1 1) substrate surface in addition to another band at 2000 cmÿ1 due to the bonding to the fcc Fe(1 1 1) ®lms. The ®lms beyond 8 ML thick showed the RHEED image representing a bcc(1 1 0) with a C±O stretch band at 2032 cmÿ1 for the saturated adsorption. At the intermediate Fe thicknesses in which the RHEED image consists of fcc(1 1 1) and bcc(1 1 0) lattice structures, two IR bands were observed at 2000 and 2032 cmÿ1 . These results clearly demonstrate that IRRAS observation of CO adsorbed on Fe ultra-thin ®lm on Cu substrate is a suitable guide for investigating change in surface structure of MBE system.
Acknowledgements This work was supported by a grant-in-aids (T.W. No. 11450280) from the Ministry of Education, Science, Sports and Culture of Japan. One of the authors (R.B.) wishes to express appreciation to the Japanese Government and Tohoku University for making this research experience possible.
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