Thermal surface chemistry of Fe(CO)5 on Pd(111) studied by FT-TPD and LITD-FTMS

Surface Science 436 (1999) L702–L706 www.elsevier.nl/locate/susc

Surface Science Letters

Thermal surface chemistry of Fe(CO) on Pd(111) studied by 5 FT-TPD and LITD-FTMS M. Noel Rocklein, Donald P. Land * 1 Shields Avenue, Department of Chemistry, University of California, Davis, CA 95616, USA Received 2 March 1999; accepted for publication 21 May 1999

Abstract Iron pentacarbonyl desorbs from Pd(111) in ultra-high vacuum at 153 and 170 K, corresponding to multilayer and monolayer desorption. Approximately 30% of the first saturation layer decomposes during the temperatureprogrammed desorption (TPD) experiment. Laser-induced thermal desorption shows submonolayer coverages to react under 150 K and then again near 200 K. TPD shows evolution of CO by a reaction-limited process near 260 K. The decomposition process is likely to involve a stepwise decarbonylation. © 1999 Published by Elsevier Science B.V. All rights reserved. Keywords: Chemisorption; Iron; Laser-induced thermal desorption (LITD); Low index single crystal surfaces; Palladium; Surface chemical reaction; Thermal desorption spectroscopy

1. Introduction There are several reasons for investigating the chemical interactions of organometallics with surfaces. The growth of thin metal films and filling of high aspect vias by metal organic chemical vapor deposition (MOCVD) are important to the microelectronics and magnetic storage industries. Interactions of organometallics with high-surfacearea materials are important for the generation of supported metal catalysts, and the resulting surface moiety is often responsible for observed catalytic chemistries. Both CVD and surface organometallic chemistry (SOMC ) are expected to benefit from * Corresponding author. Fax:+1-530-732-8995. E-mail address: [email protected] (D.P. Land)

fundamental research concerning reactions of organometallics on and with surfaces, e.g. by describing mechanistic details which produce pure versus contaminated film growth or active versus non-active catalysts. Laser-induced thermal desorption Fourier transform mass spectrometry (LITD-FTMS) aids in the identification of molecular adsorbates and can also be useful in obtaining rates of surface reactions by monitoring intermediate and product distributions through time-resolved, in situ measurements [1]. The application of LITD-FTMS to an organometallic/substrate system has only recently been attempted [2]. Iron pentacarbonyl is a highly reactive molecule that has proven to display interesting chemistry on a variety of surfaces. The existence of stable, partially decorbony-

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lated surface species has been inferred on Pt(111) from reflection absorption infrared spectroscopy (RAIRS) [3], on Ni(100) from X-ray photoelectron spectroscopy ( XPS) and 13CO adsorption temperature-programmed desorption (TPD) experiments [4], on Ag surfaces from RAIRS, XPS and TPD studies [5], and as clusters on Si(111) from high resolution electron energy loss spectroscopy (HREELS) and TPD data [6 ]. Decomposition intermediates are probably more common under photolytic conditions and are thought to occur on Si(111) [6,7], on C /Pt(111) [8], and on Ag surfaces [9–11], ads among others. Because of its thermal surface behavior, Fe(CO) was selected for preliminary studies 5 with the hope that LITD-FTMS might be able to differentiate between different surface moieties, if such species exist on Pd(111). Furthermore, ultrathin films of iron on palladium are extensively studied for their ferromagnetic properties [12–15]. These are typically produced by physical methods (evaporation or molecular beam epitaxy). The thermal surface chemistry of a simple, iron-containing MOCVD precursor on the Pd(111) surface may be relevant for future thin and ultrathin film applications. In this report, thermal desorption data are presented for Fe(CO) on Pd(111) using both 5 slow (~3 K s−1) and rapid (~11 K s−1) heating conditions. This is achieved by resistive heating during TPD experiments and by laser heating during LITD-FTMS experiments at variable surface temperatures, respectively. The rapid rate of surface heating during LITD often allows access to high-energy reaction pathways, such as desorption, before the reactant is significantly depleted through lower energy reaction paths. Thus, reactant or intermediate species, which would not normally desorb under slower heating methods, are sometimes found to desorb by LITD-FTMS.

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exceeding 108 W cm−2 and allows for over 40 independent samplings. The heating rate and temperature rise are maximized by increasing the power density to within 80% of that required to ablate the surface. The palladium single crystal (Aremco Products, Inc., Ossining, NY ) is oriented within 1° of the 111 face and has been polished, ultimately with 0.05 mm deagglomerated alumina. Initial cleaning in vacuum involves high temperature sputtering at ~1180 K with ~500 eV argon ions to remove sulfur, as confirmed by Auger spectroscopy. Sputtering before every experiment removed iron. Annealing above 1180 K or higher removes embedded argon (which is evident if present during LITD experiments) and aids in surface reconstruction, according to low energy electron diffraction (LEED). Carbon contamination can be removed by repeated heat cycles in oxygen (ca. 10−6 Torr), and flashing to between 800 and 1000 K. The presence of carbon was monitored by saturating the surface with O at room temperature and 2 observing the ratio of CO to O at 760 and 800 K 2 during TPD. Several dark, freeze–pump–thaw cycles were performed on Fe(CO) stored in a foil-wrapped, 5 glass bulb using a dry-ice/acetone bath. After an initial flush, Fe(CO) vapors were stored in a 5 stainless steel, diffusion-pumped foreline (base pressure<5×10−5 Torr), and these vapors showed no significant degradation when held overnight, as monitored by FTMS. The FTMS cubic ion analyser cell and ultrahigh vacuum system (base pressure<2×10−10 Torr) have been described elsewhere [16 ]. Exposures for TPD and LITD experiments are not corrected for ion gauge sensitivity. All TDS data have been normalized to unit experimental parameters (i.e. 1 mA ionization current, 1 ms ionization period).

3. Results and discussion 2. Experimental During the LITD experiment, a Nd:YAG laser emits a 5 ns, 10 mJ pulse of 1064 nm light that is focused to a 1 mm diameter spot on the 12 mm diameter Pd surface. This produces a power density

Below 100 K, the clean Pd(111) surface was exposed to various amounts of Fe(CO) by back5 filling the chamber. The thermal desorption traces of m/z 56 are shown in Fig. 1 since Fe+ is the base peak formed in our mass spectrometer during

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Fig. 1. Fe+ TPD signal at 3 K s−1 for various exposures of Fe (CO) onto Pd(111) below 100 K. Traces have been offset by 5 0.02 V s increments.

70 eV electron ionization of Fe(CO) . Exposures 5 at and above 10 Langmuir (1 L=10−6 Torr s) produce two molecular desorption peaks of iron pentacarbonyl, at 153 and 170 K, corresponding to multilayer and monolayer coverages. Since FTMS is a true multiplex technique, the entire mass spectrum is always monitored. No other ironcontaining species were observed during TPD from the clean surface. This is in contrast to the Pt(111) surface which is reported to produce an additional feature – a small, broad, recombinative molecular desorption peak centerd between 255 and 265 K [3,17]. The thermal desorption data indicate that some decomposition is occurring. Each of the traces represented in Fig. 1 have been integrated and the difference in area between the 10 and 15 L TPD traces, divided by the difference in exposure, can be used to calculate a sensitivity factor (signal observed per unit exposure). Subtraction of the desorption quantity contained within the multilayer peak from the respective exposure shows saturation to occur between 9.5 and 12 L. Furthermore, an average of 2.9±0.5 L (95% confidence, N=4) does not desorb, indicating that

about 30% of the first layer decomposes under slow heating conditions. Accordingly, complete decomposition should occur for exposures below ~3 L, as confirmed by the absence of any ironcontaining desorption products in TPD after 1.5 and 0.75 L exposures. Fig. 2 shows the FTMS signal for m/z 28 or CO+, as a function of surface temperature. Carbon monoxide desorbs from clean Pd(111) in UHV under 500 K and we observe a broad CO desorption peak in our spectra around 470 K. Evidently, there is not a significant quantity of surface carbon since its presence can delay the desorption of CO until 540 K [18]. The increase in the amount of CO desorbing at 470 K (seen up to 3 L) is due largely to an increase in Fe(CO) decomposition. 5 However, some ambiguity exists concerning the relative amounts of CO displaced from the surface during exposure or produced from reactions on the stainless steel walls of the UHV chamber [19]. Also, no recombinant CO desorption, which would indicate CMO bond breaking, was observed after a 0.6 L exposure. For exposures at and above 3 L, a second, broad CO desorption peak near 260 K is always

Fig. 2. CO+ TPD signal at 3 K s−1 for various exposures of Fe(CO) onto Pd(111) below 100 K. Traces have been offset 5 by 0.02 V s increments.

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observed. According to Kuhn et al. [20] no significant CO desorption occurs from Pd(111) below 300 K for saturating exposures made at 90 K and at 10−6 Torr. Further, no CO desorption states were observed near 260 K on the Fe(100) [20] and Fe(111) [21] surfaces. (Data obtained for CO on Fe(110) were not relevant since exposures were conducted at 280 K [22].) It is likely, then, that palladium-bound iron species are undergoing decomposition near 260 K. The CO desorption may occur directly from the iron center or promote CO displacement from the palladium surface because of a positive volume change during the reaction. Above ~320 K, the evolution of CO appears to resemble that from clean Pd(111) at higher exposures. This is consistent with the Pt(111) system since no iron carbonyl surface intermediates are observed by RAIRS above 240 K [3]. However, the shape of the peak depends on both the CO coverage [23] and the surface heterogeneity and we are therefore unable to determine whether the presence of iron or iron carbonyl species is affecting the CO desorption profile between 320 and 420 K. The LITD-FTMS temperature survey shown in Fig. 3 sheds light on the decomposition process. Following a submonolayer, 5 L exposure, the surface was slowly heated to different temperatures and then held constant while several LITD spectra were obtained. The mean intensities (±2× standard error of mean) are shown for the dominant ions observed at each temperature. Note that the large errors near 150 K are due to signals that are changing as a function of time. Not only does the loss of Fe+ occur 20 K below normal thermal desorption for submonolayer coverages, the increase in absolute CO+ coverage implies that surface reaction is occurring; if Fe(CO) desorp5 tion were the sole cause, then CO+ magnitude would be expected to decrease owing to the lost contribution from molecular cracking. ( There is, however, an alternative explanation – an increase in CO+ magnitude might arise if, following any slow desorption from the monolayer at 150 K, laser-induced thermal decomposition occurred as surface crowding was decreased. The ambiguity should be resolved in future work using RAIRS.) The LITD experiment also detects a small

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Fig. 3. Dominant ions formed during LITD-FTMS temperature survey of Pd(111) after 5 L Fe(CO) exposures at less than 5 100 K.

amount of surface-bound iron species above the aforementioned TDS desorption temperatures – up to 196 K for this experiment and up to 205 K for a 12 L exposure. The nature of this remnant surface species is not entirely understood, owing in part to its minimal detection by electron ionization. We have previously described a chemical ionization technique using CH+ for this system 5 [2] but it did not yield any iron-containing LITDFTMS signal above 160 K. Above 205 K, only CO+ is observed with LITD-FTMS, which eventually disappears above 500 K, in accordance with TPD data. A final observation is that lower exposures (submonolayer) typically show a larger CO+/Fe+ ratio than do higher exposures (>monolayer) in LITD at 100 K. In fact, no Fe+ signal is detected during LITD at or above 100 K for a 2.3 L exposure. This supports the notion that a significant fraction of submonolayer exposures results in chemisorption at 100 K; either the laser induces thermal decomposition for low coverages or partial or complete decomposition occurs upon adsorption for small exposures. Again, this question will be addressed in future RAIRS work. 4. Conclusion This preliminary data allow us to hypothesize a surface reactivity scheme. A significant fraction

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(30%) of the first saturation layer is irreversibly adsorbed, or has decomposed, at 100 K. During a temperature ramp of 3 K s−1, exposures in excess of 3 L and less than 10–12 L result in a single, molecular desorption peak at 170 K. An increase in desorbed CO at 150 K under LITD indicates an increased amount of decomposition at that temperature. A small amount of iron-containing desorption products is observed until ~200 K where another decomposition step must occur. Further decarbonylation occurs upon resistive heating to produce a broad CO desorption band near 260 K. The remaining CO desorbs by 500 K.

Acknowledgement The authors appreciate the funding from the National Science Foundation for support of this work, under Grant No. CHE-9612732.

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