Room Temperature Oxidation of Ruthenium - American Chemical

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Room Temperature Oxidation of Ruthenium Benjamin Herd, Jan C. Goritzka, and Herbert Over* Department of Physical Chemistry, Justus-Liebig-University, Heinrich-Buff-Ring 58, D-35392 Gießen, Germany ABSTRACT: With scanning tunneling microscopy (STM) we studied the initial oxidation of Ru(0001) at room temperature using atomic oxygen (O′) as oxidizing agent. Even at 300 K, Ru oxide clusters are formed and are located preferentially at single step edges and less likely at double steps and on the flat terrace. The observed Ru−O cluster formation indicates a nucleation and growth process. The cluster formation at room temperature implies that the supply of Ru atoms is not rate limiting in the initial oxidation of Ru(0001). A mobile surface Ru−O precursor species has to be postulated for explaining the observed nucleation and growth process. A mobile surface Ru−O precursor species is considered to be equally involved in the oxidation of Ru with molecular oxygen and the oxidation under electrochemical conditions.

1. INTRODUCTION The oxidation of platinum group metals (PGM) by molecular oxygen needs in general higher temperatures and higher oxygen partial pressures to proceed. For the case of ruthenium, temperatures above 500 K and pure oxygen pressures above 10−5 mbar are required to oxidize the surface region.1,2 Similar threshold values have been reported for the oxidation of the other PGM.3−7 During the formation of metal oxides, the metal−metal bonds have to be replaced by metal−oxygen bonds, a process which is thermally activated and therefore being rate limiting at low temperatures. Dissociative adsorption of oxygen can equally become rate determining in the gas phase oxidation of ruthenium since chemisorbed oxygen at saturation inhibits the further uptake of oxygen.8−10 Both bottlenecks may imply that oxidation at higher temperatures and higher partial pressures are needed to surpass these barriers. In a recent scanning tunneling microscopy (STM) study,11 high temperatures and high pressures have shown to be required in the oxidation of Ru(0001) with molecular oxygen in order to commence the formation of RuO2 clusters which precedes the growth of flat RuO2(110) regions. Quite in contrast, the oxidation of PGM under electrochemical (EC) conditions takes place already at room temperature and at anodic potentials where oxygen evolution sets in.12,13 Of course, one can simply envision the electrooxidation of PGM as solely being driven by the electrical potential so that higher temperatures are not required. However, this view is too simple as exemplified with the oxidation of ultrathin Ru layers in gas phase environment which has been reported to proceed even at room temperature14,15 under conditions of extreme ultraviolet lithography (EUVL). In EUVL, the next generation of lithography for the semiconductor industry,16 2 nm thick Ru capping layers have been applied to protect the multilayer mirrors in the exposure tool against oxidation. Unfortunately, under the conditions of a typical EUVL chamber with a partial pressure of water of 10−6 © 2013 American Chemical Society

mbar and EUV light striking the Ru surface, the ruthenium capping layer gradually develops into a surface oxide at room temperature, thereby deteriorating the optical reflectivity of the multilayer mirrors.17,18 It is worthwhile to note that such thin Ru capping layers oxidizes by exposure of molecular oxygen only above a threshold temperature of 473 K.19 Under EUVL conditions the oxidation of Ru capping layers proceeds via water rather than via molecular oxygen. The oxidation process of ruthenium under EUVL conditions is driven by secondary electrons released by EUV light striking the mirror surface. These electrons split weakly adsorbed water on the Ru capping layer surface until atomic oxygen is left on the surface. In this way atomic oxygen is accumulated on the Ru capping layer surface which leads to the oxidation of the 2 nm thick Ru film. A similar oxidation process takes place under electrochemical control so that oxidation under EUVL conditions may be considered a natural link between gas phase oxidation by molecular oxygen and electrooxidation. Common to both electron-stimulated oxidation processes in EUVL and electrochemistry (EC) is that the supply of the oxidizing agents does not impose a bottleneck. Therefore, in the present STM study of the gas phase oxidation of Ru(0001) we purposely switched off the bottleneck of oxygen supply by dosing atomic oxygen rather than molecular oxygen. The oxidation with atomic oxygen was carried out at 300 K in order to be as close as possible to reaction conditions met in electrooxidation and under EUVL conditions. It is demonstrated that atomic oxygen is able to form Ru−O clusters primarily at corroded single steps and also, but less frequently, on flat terraces, without invoking an electrochemical step in the oxidation process as encountered under EUVL and EC conditions. The formation of Ru−O clusters at room Received: April 29, 2013 Revised: June 24, 2013 Published: June 27, 2013 15148

dx.doi.org/10.1021/jp404239y | J. Phys. Chem. C 2013, 117, 15148−15154

The Journal of Physical Chemistry C

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temperature implies the introduction of a mobile surface Ru−O precursor species which is considered to be equally involved in the oxidation of Ru with molecular oxygen and the oxidation under EUVL and EC conditions.

2. EXPERIMENTAL DETAILS The experiments were conducted in a home-built threechamber ultrahigh-vacuum (UHV) system; details have been described recently.11 Briefly, this main chamber is furnished with a mass spectrometer and a dual X-ray source together with a semispherical analyzer (PSP Vacuum Technology) to perform X-ray photoelectron spectroscopy (XPS) experiments. The sample can be transferred from the analysis chamber to the STM chamber (VT-STM, Omicron). We used homemade tungsten tips. All STM images presented in this paper are taken in the constant current mode at 298 K. The Ru(0001) sample was cleaned by Ar-ion sputtering for 10 min (p(Ar) = 10−5 mbar, 1.5 kV, 15 mA) and roasting the sample at 950 K in 10−7 mbar of oxygen in order to remove carbon contamination originating from carbon segregation from bulk Ru. Since the highest sample temperature achieved with the present sample holder is limited to 1000 K, still about 0.25 ML of adsorbed O remains on the Ru(0001) surface after sample cleaning. A temperature of 1000 K is not sufficient to anneal the bubbles induced by Ar-ion sputtering.20 Oxidation of the sample was performed by exposing atomic oxygen from a thermal cracker (Oxford Applied Research: TC50 - Universal Thermal Cracker). Thermally cracked atomic oxygen has previously been used to study the oxidation of Pt(111).21 The quoted exposures of atomic oxygen O′ are determined by the actual exposure of molecular oxygen times the cracking efficiency determined by mass spectrometry. For an O2 pressure of 2 × 10−8 mbar the cracking efficiency is about 30%, so that the partial pressure of atomic oxygen O′ is estimated to be 0.6 × 10−8 mbar. The given exposure of atomic oxygen O′ is based on this partial pressure estimate times the exposure time. One langmuir corresponds to an exposure of 1.33 × 10−6 mbar·s. Alternatively, one could have used an oxygen plasma source for producing atomic oxygen, such as reported in the literature22,23 or ozone.24 The application of NO2 is not meaningful since the dissociation of NO2 and the release of NO requires oxidation temperatures above 500 K.25,26

Figure 1. STM images of the Ru(0001) surface which was exposed to various doses of O′ at room temperature. Large STM images show a surface region of 200 nm × 200 nm, while the insets in (b, c) indicate magnified STM images of 50 nm × 50 nm and the inset in (a) a magnified STM image of the (1 × 1)O (4 nm × 4 nm). Various line scans are indicated. (a) 20 langmuirs of O′ (U = 1.3 V, I = 1 nA), (b) 40 langmuirs of O′ (U = 1.0 V, I = 1 nA), and (c) 80 langmuirs of O′ (U = 0.9 V, I = 1 nA).

3. RESULTS AND DISCUSSION In the following we will scrutinize the Ru−O cluster formation at room temperature with STM. Figure 1a displays STM images of a Ru(0001) surface which was exposed to 20 langmuirs of O′ at room temperature. Single-atomic step edges are decorated by small Ru−O clusters and slightly corroded. On the flat terrace a (1 × 1)O phase (inset Figure 1a) is fully developed,10,27 on which a few clusters are present. Double steps are substantially less decorated by Ru−O clusters than single atomic steps (cf. Figure 1a). The line scan in Figure 1a shows a typical cluster located at the rim of the top terrace at a single step. The height above the top terrace is about 5 Å, while the lateral size of the cluster is about 4 nm. The found height does exclude a simple subsurface phase at the step edge, where oxygen penetrates into the top Ru layer and lift the Ru atoms by 1.5 Å.28 A height of 5 Å suggests that either a O−Ru−O trilayer28 or a single RuO2 layer constitutes the Ru−O cluster. There are few clusters also located on the terrace

with an apparent height of 5 Å, but corrosion on the terraces has not been observed. From the formation of Ru−O clusters even at room temperature one can clearly conclude that the supply of Ru atoms in the initial oxidation of Ru(0001) does not represent the bottleneck. Therefore, the activation of molecular oxygen is considered to be the rate-determining step in the initial oxidation of Ru(0001) with molecular oxygen. Ru atoms experience a high cohesion energy in bulk Ru and on the Ru(0001). Even Ru atoms at the step edges and at kink sites are strongly bound so that the mobility of Ru atoms along the step edge is even at 800 K exceedingly low. In order to produce Rucontaining clusters, Ru has to be transformed first into a mobile Ru-containing species. This process is realized by the formation of Ru−O precursors, where the strong O−Ru bonding compensates for the lost Ru−Ru back-bonding to the substrate, thereby creating a mobile precursor species. 15149



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Figure 2. (a) Thermal desorption spectra from Ru(0001) which is exposed to increasing amounts of atomic oxygen O′ at room temperature. (b) Corresponding XP spectra of O 1s as a function of O′ exposure. The difference XP spectrum is derived from the 20 and 40 langmuirs of O′ spectra.

Figure 3. Annealing experiments: STM images of the Ru(0001) surface which was exposed to 80 langmuirs of O′ at room temperature and subsequently annealed to 600 K (U = 1 V, I = 1 nA) (a), 700 K (U = 1 V, I = 1 nA) (b), and 800 K (U = 0.8 V, I = 1 nA) (c) for 15 s. Large STM images show a surface region of 100 nm × 100 nm, while the inset presents magnified STM images of 15 nm × 15 nm. The line scan indicates a double and single step in the STM image (c).

near region of Ru(0001) is quantified when exposing the surface to various amounts of atomic oxygen at room temperature. As reference structure with well-defined coverage we choose the Ru(0001)-(1 × 1)O phase with a nominal Ocoverage of 1 ML (monolayer).8,9 According to this calibration, already an exposure of 10 langmuirs of O′ results in a Ru(0001) surface which is covered with 1 ML of oxygen, while an exposure of 19 langmuirs of O′ leads to a slightly higher coverage of 1.2 ML. Higher exposures of 42 and 78 langmuirs of O′ result in oxygen coverages of about 1.5 and 1.9 ML, respectively. Most of the oxygen desorbs at a temperature of about 1000 K, which has also been observed by Böttcher et al.30 Above 19 langmuirs of O′ the TD spectrum reveals a low-temperature desorption peak at 800 K which has not been observed in TDS for high temperature exposures of molecular oxygen.9 We suggest that this desorption feature is related to the release of oxygen during decomposition of Ru−O clusters seen in Figure 1. For comparison reasons the oxygen content at the Ru(0001) surface was also determined by XPS (cf. Figure 2b). From the O 1s spectra it is clear that only little more oxygen than a (1 × 1)O is accumulated at the surface when exposing 40 langmuirs of O′. This results fits also with the STM experiments which indicate a coverage of about 2−5% of Ru−O clusters covering the Ru(0001)-(1 × 1)O surface. Therefore, the clusters observed in STM can be considered to consist of Ru and oxygen, justifying the use of the phrase “Ru−O cluster” throughout the article.

Increasing the O′ exposure to 40 langmuirs (cf. Figure 1b) results in a dense decoration of single atomic steps with Ru−O clusters and an increased population of Ru−O clusters on the terraces. There is no indication that the corrosion at the step edges has continued. A line scan through a cluster on the terrace indicate an apparent height of 5.3 Å and a lateral size of 3 nm. Increasing the O′ exposure to 80 langmuirs (cf. Figure 1c) does not result in a higher density of clusters at single steps. However, the density of clusters at double steps has increased although their density is still significantly lower than found at single atomic steps. Most evidently from STM the number of clusters on the terraces has dramatically increased (cf. Figure 1). Comparing these room temperature results with those of oxidation experiments at higher temperatures, the presence of clusters on the terraces is a peculiar behavior at room temperature. Obviously, the Ru−O precursor is not only mobile along the step edges, but it also moves across the terraces. We expect that diffusion of such Ru−O precursor species is much slower at room temperature than at elevated temperatures. This allows the formation of more and smaller Ru−O clusters along the step edges and on the terraces. Only when Ru−O precursor molecules meet, Ru−O nuclei are formed, which further grow to Ru−O clusters if the Ru−O nucleus is critical.29 This nucleation process is less likely on the terrace than at the step edges with its confinement to one dimension. Figure 2a summarizes the thermal desorption spectroscopy (TDS) experiments where the capacity of oxygen in the surface 15150



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reduce the Ru−O clusters consists of chemical reduction by CO exposure. In Figure 5 we summarize these reduction

The XPS results contrast corresponding TDS experiments which estimate a cumulative oxygen amount of 1.5 ML for 40 langmuirs of O′. This is not necessarily a contradiction since both characterization techniques probe different depth ranges of the surface. While TDS probes the oxygen content in a surface region several 100 nm deep, XPS is a method which is sensitive to oxygen in the 1−2 nm thick selvedge region of Ru(0001). To study the thermal stability of the Ru−O clusters, we exposed the Ru(0001) surface to 80 langmuirs of O′ at room temperature. Subsequently, we annealed the sample to various temperatures (600, 700, and 800 K). Annealing the sample to 600 K (cf. Figure 3a) indicates that the clusters both on the terrace and at the step edges are slightly affected by this heat treatment. The lateral size distribution of the clusters increases a little while the height distribution narrows as indicated in the statistical distributions depicted in Figure 4. Heating the sample

Figure 5. Reduction experiments with CO at 600 K: the Ru(0001) was exposed to 40 langmuirs of O′ at room temperature (a) and then exposed to 100 langmuirs of CO at 600 K (b). Subsequently, the surface was just annealed to 600 K for 15 min to directly compare chemical reduction effect and annealing effects (c). Lastly, the surface was exposed to an additional 200 langmuirs of CO at 600 K (d). STM images show a surface region of 150 nm × 150 nm; U = 1 V, I = 1 nA.

experiments, addressing the stability of Ru−O clusters against chemical reduction by CO. In these experiments we start from a Ru−O cluster precovered surface, which was produced by exposing the Ru(0001) surface to 40 langmuirs of O′ at 300 K (cf. Figure 5a). The specific reduction temperature of 600 K was chosen to be high enough that chemical reduction is facile but the chosen temperature is low enough to exclude further temperature induced aggregation and agglomeration processes at the surface. Chemical reduction by exposing 100 langmuirs of CO at 600 K leads to a decrease in cluster density at the step edges (at both single and biatomic steps) (cf. Figure 5b). At the same time the averaged cluster size reduces form 4.5 to 3.5 nm, and the height distribution of the clusters narrows and shows a small maximum at around 4.3 Å which fits to a two-layer Ru cluster without incorporated oxygen. Annealing now this surface to 600 K for an additional 15 min does not change significantly the distributions in Figure 6 or the appearance of the clusters in STM images (cf. Figure 5c), indicating that annealing alone to 600 K does not further reduce and modify the clusters. Additional exposure to 200 langmuirs of CO at 600 K (cf. Figure 5d) leads to dissolution of most of the clusters at the step edges and subsequent integration of the released Ru atoms into the step edge. No clusters are found with STM to be located at the bottom edge of the steps. This may indicate that clusters of the bottom terrace in touch with the step are prone to dissolve and are subsequently incorporated into the step. Simple annealing to 600 K will essentially not dissolve Ru−O clusters. Therefore, an alternative view could be that CO

Figure 4. Statistical distribution of the lateral size (left) and the height (right) of clusters prepared by exposing the Ru(0001) surface to 80 langmuirs of O′ (atomic oxygen) and then annealing to various specific temperatures (600, 700, and 800 K) each for 15 s.

to 700 K changes the cluster distribution on the surface profoundly. The density of clusters both at the step edge and on the terrace diminishes (cf. Figure 3b). This observation is reconciled with the O2 desorption feature at 800 K. The size and height distributions narrow in comparison with the 600 K annealing. A magnified STM image of the clusters at the step edges discloses an internal structure which is reminiscent of an agglomeration of small clusters (cf. inset in Figure 3b). With further annealing to 800 K the step edges are freed from most of the clusters, while the number of clusters on the terrace does not decrease further (cf. Figure 3c). However, the lateral size distribution of the remaining clusters becomes squeezed, while the height distribution exhibits maxima at 2 and 4 Å. From this behavior one may imply that the Ru−O cluster has lost oxygen, thus transforming some of the Ru−O clusters to a pure Ru clusters. Upon annealing to 800 K, the step edges appear less corroded than after exposure of 80 langmuirs of O′ at 300 K, although the step edges still expose many kinks. Simply heating the sample to various temperatures is a quite harsh method for reducing Ru−O clusters. A milder method to 15151



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rate determining, in particular at low temperatures. The most surprising result of this study is however that atomic O′ is able to corrode Ru(0001) even at room temperature forming Ru−O clusters both at the step edges (preferentially at single atomic steps) and on the terraces. Corrosion has shown to be confined to the step edges; no corrosion has been observed on the terraces. This finding is plausible since undercoordinated Ru atoms are less strongly bound to the surface than fully coordinated Ru atoms in the terrace. The mobile Ru atoms required for the formation of Ru−O clusters are most likely originating from the step edges which is reconciled with the observed corrosion and the location of Ru−O clusters primarily at the step edges. But even the corrosion of single step edges by atomic O′ at room temperature requires the introduction of a mobile Ru−O precursor species since undercoordinated Ru atoms from step edges are not mobile at temperatures lower than 800 K. The observation of clusters at step edges and on the terraces implies that the Ru−O precursor species are mobile along the step edges and on the terraces, respectively. Diffusion of the Ru−O precursor should be much faster on the terrace than at the step edges, since the bonding energies of the Ru−O precursor are expected to be much lower on the terrace than at the step edge due to lower coordination of Ru atoms in the terraces. How do the Ru−O precursors form? From our STM study only beyond a critical coverage of chemisorbed oxygen at the step edges (realized by little some 10 langmuirs of O′ at room temperature and which presumably constitutes a (1 × 1)O1,8), the Ru atoms at the step edges are sufficiently destabilized by chemisorbed O so that additional O′ uptake is able to generate the mobile Ru−O precursors for the cluster formation. The chemical nature of the postulated mobile Ru−O precursor species is unknown and calls for ab initio calculations. This postulated precursor species may be reminiscent of RuO3 and RuO4both are considered as mobile precursor species in the growths of RuO2 single crystals by chemical vapor transport in a flowing gas reaction system.31 However, it is also conceivable that even a RuO2 precursor species may be mobile on the surface, in particular along the step edges. Although less probable, Ru−O cluster are also formed on the terraces at room temperature. One can expect that Ru−O clusters are formed at the step edges when Ru−O precursor species meet at the step edge, thus forming a critical nucleus, and the critical nucleus grows in size. On the terrace the mobility of the Ru−O precursor is higher but also the freedom to move in various directions is higher, so that it is not very likely that two Ru−O precursor meet to form a critical nucleus on the terrace. If a critical nucleus of Ru−O is formed, we expect that this cluster is not mobile, neither on the terrace nor at the step edge since the reaction temperature of 300 K is too low. Increasing the temperature to 450 K does not allow for cluster formation on the terraces,32 but clusters are found at single atomic steps. This finding may be rationalized by a critical nucleus which is larger at 450 K than at room temperature. Since several Ru−O precursor species have to meet before the critical nucleus decays, cluster growth on the terraces at 450 K is quite improbable, whereas this process is still likely at step edges due to the geometric constraint to one dimension. The resulting Ru−O clusters contain both Ru and oxygen as evidenced by the CO reduction experiments at 600 K and the O 1s XPS measurements. From TDS and XPS we propose that the Ru−O clusters are not oxides but rather an aggregation of

Figure 6. Statistical distribution of the lateral size (left) and the height (right) of clusters prepared by exposing the Ru(0001) surface to 40 langmuirs of O′ (atomic oxygen) at room temperature and exposing the surface to various doses of CO at 600 K: 100 langmuirs of CO at 600 K for 15 min, additional annealing at 600 K for 15 min, and additional exposure of 200 langmuirs of CO at 600 K for 15 min.

molecules are not only reducing the Ru−O clusters but are also corroding the small metal clusters via the formation of Ru carbonyls which are mobile along the step edge and facile to be incorporated into the step edge concomitant with CO release. The lateral size of the remaining clusters on the terraces does not change very much, but the height distribution transforms clearly into a bimodal distribution with a pronounced peak at 2.2 Å and a smaller peak at 4.3 Å (cf. Figure 6. These two height values of clusters are consistent with a metallic cluster where the interlayer distance is a multiple of 2.1 Å as in bulk Ru(0001). In order to determine whether the observed clusters are oxide or metal clusters, one can inspect carefully the O 1s XP spectrum (cf. Figure 2b). The O capacity increases slightly with O′ exposure above 1 ML; however, there is no signature of oxidic O 1s feature in the difference spectrum of Figure 2b. In principle, Ru-3d spectra may provide some information about the oxidation state of Ru in such Ru−O clusters, as observed in the initial oxidation of Ru(0001).31 Ru-3d XP spectra (not shown) do not contain enough information on the surface due to the low surface sensitivity (high photon energy). From the O1s-spectra, we presume therefore that the Ru−O precursor species agglomerate to form Ru−O clusters without transforming them into a stoichiometric Ru oxide. If the molecular Ru−O precursor species would consist of RuO2+x, then the transformation to RuO2 requires the loss of oxygen as well as the connection and rearrangement of new Ru−O bonds, a process which may need activation and therefore higher temperatures.

4. DISCUSSION OF THE RESULTS AND CONCLUSIONS 4.1. Oxidation of Ru(0001) at Room Temperature Using Atomic Oxygen. Using atomic oxygen as the oxidizing agent, the supply of isolated Ru atoms is assumed to become 15152



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Ru−O precursors. These Ru−O clusters are stable up to 600 K in UHV and at 800 K O2 desorption from the clusters sets in. 4.2. Comparison with Electrooxidation. Even at room temperature corrosion and oxide cluster formation are observed with STM when the Ru(0001) surface is exposed to atomic O′ at room temperature. This finding brings gas-phase oxidation closer to electrooxidation, a process which takes place at room temperature but under potential control. Our gas phase oxidation results may add to our understanding on the electrooxidation of Ru(0001). The electrical potential in the electrooxidation process is primarily used to decompose water and supply the electrode surface with an oxidizing agent, i.e., atomic O′.13,33 The consecutive oxidation process by uncharged surface oxygen is essentially not driven by the external potential and is therefore likely to be similar to the oxidation via atomic oxygen. With surface science methods the electrooxidation of a Ru(0001) model electrode was studied utilizing a closed sample-transfer system between the electrochemical cell and the UHV chamber.34 When the Ru(0001) electrode is immersed in a 0.1 M HClO4 solution and the electrode potential was kept at 1.12 V (vs Ag/AgCl) for 2 min, RuO2 grows epitaxially with its (100) plane parallel to the surface of Ru(0001) as deduced from ex-situ RHEED.34 The RuO2(100) film is very rough, growing as three-dimensional clusters with an average size of 2 nm, whereas the nonoxidized Ru(0001) surface is still flat. The electrooxidation of Ru(0001) in 0.05 M H2SO4 was studied also by in-situ STM.35 Clearly, the oxidation starts at the step edges forming small clusters (size about 2−3 nm) at a potential of 1.17 V versus RHE. STM images indicate smooth regions (chemisorbed O on Ru(0001)), oxide islands along the step edge (size about 2−3 nm), and larger oxide island on the terraces (5 nm). The authors concluded that the Ru atoms necessary for oxide growth stem from the dissolution of step edges.35 Also in the electrooxidation process of Ru(0001) a mobile Ru containing precursor species is required for the formation of Ru−O clusters. While under UHV conditions there are not too many Ru-containing precursor compounds conceivable, under electrochemical conditions many more complexes can be thought of such as Ru−O, Ru−OH, Ru−H2O, and Ru−O, OH complexes. Therefore, also the process of electrooxidation needs further input concerning the mobile precursor species from DFT calculations. For the process of electrooxidation of Ru(0001) these coordination complexes may not even stay on the electrode surface, but rather these can be wandering within the electrolyte and then redeposited at a different place on the electrode surface.

gas mixture serves in the crystal growth from vapor phase as the mobile precursor species. The oxidation of Ru(0001) by molecular and atomic oxygen proceeds also via a mobile surface Ru−O precursor species. For molecular oxygen the temperature required for the formation of Ru−O precursor species is 500 K, which is significantly lower than for the growth of RuO2 single crystal by deposition form the vapor phase. However, when using atomic O′, the temperature for the formation of surface Ru−O precursor can be further reduced to 300 K. The oxidation of Ru(0001) at room temperature by exposing atomic oxygen O′ are summarized as follows: (i) Single atomic steps are slightly corroded, and no corrosion is observed on the terrace. During corrosion mobile surface Ru−O precursor species are formed. (ii) Ru−O clusters form via nucleation and growth of Ru−O precursor species preferentially at single atomic step edges and less frequently on the terrace. With increasing O′ exposure the steps are fully decorated with clusters and the terraces become more populated by clusters. Multiple steps are able to form clusters but with a lower concentration than at single atomic steps. (iii) The Ru−O clusters can be reduced by CO exposure at 600 K. The reduction of Ru−O clusters leads to clusters of the same lateral size but with a bimodal height distribution with maxima at 2.2 and 4.4 Å, indicating that the reduced clusters consist of pure Ru. (iv) Ru−O cluster are thermally stable up to 600 K in UHV, and these clusters can be chemically reduced by CO exposure at 600 K. (v) Very likely the mobile precursor species consists of Ru and O, whose exact chemical composition and structure await future ab initio calculations. TDS and XPS suggest that most of Ru−O clusters consist of aggregation of Ru−O precursor species rather than a well-defined Ru-oxide. We conclude from the present study for the oxidation with molecular oxygen: (i) Ru supply is not the bottleneck in the initial oxidation of Ru(0001) with molecular oxygen, and it does not need higher temperatures to form a Ru-containing species as mobile precursor species. (ii) Mobile Ru−O precursor species are also pivotal for the oxidation of Ru(0001) by molecular oxygen. (iii) The threshold temperature of 500 K in the oxidation process of Ru(0001) by molecular oxygen is needed to activate molecular oxygen over the saturated chemisorbed oxygen phase rather than to activate Ru atoms for the formation of Ru−O precursor species. The mobile Ru-containing precursor species is equally important for the oxidation under EUVL and EC conditions.

5. CONCLUSIONS The most intriguing results of the present oxidation study is the formation of Ru−O precursor species by atomic O′ even at room temperature. Just for comparison reasons, we may compare this result with the growth of RuO2 single crystals by deposition from the vapor phase in which molecular O2 is used as transporting agent.31 Oxygen flow (1 bar) is passed over polycrystalline ruthenium of RuO2 at 1600 K, which results in a (equilibrium) mixture of RuO3/RuO4 in the gas. At the outlet zone of the reactor, which is kept at a lower temperature, say 1000 K, the RuO3/RuO4 gas mixture decomposes and RuO2 crystallizes via a nucleation and growth mode. The RuO3/RuO4

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected], Fax ++49641-9934559, Tel ++49-641-9934550. The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Daniel Langsdorf for fruitful discussions and valuable help in some of the STM experiments.

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REFERENCES

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Room Temperature Oxidation of Ruthenium - American Chemical

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