Manganese Oxide Thin Films on Au(111): Growth Competition

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Manganese Oxide Thin Films on Au(111): Growth Competition between MnO and Mn3O4 Christoph Johannes Moeller, Jade Baretto, Fernando Stavale, and Niklas Nilius J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04176 • Publication Date (Web): 30 Jul 2018 Downloaded from http://pubs.acs.org on August 1, 2018

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The Journal of Physical Chemistry

Manganese Oxide Thin Films on Au(111): Growth Competition between MnO and Mn3O4

C. Möller,1 J. Barreto,2 F. Stavale,2 N. Nilius 1,*

1

Carl von Ossietzky Universität, Institut für Physik, D-26111 Oldenburg, Germany 2

Centro Brasileiro de Pesquisas Físicas, 22290-180, Rio de Janeiro, RJ, Brazil

* Corresponding author: University of Oldenburg, [email protected], phone +49-441-798-3152

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Abstract The growth of manganese oxide on an Au(111) support has been examined over a wide range of preparation conditions and film thicknesses by means of scanning tunneling microscopy (STM), electron diffraction and photoelectron spectroscopy. Two oxide polymorphs were found to coexist on the gold surface. Whereas MnO-type structures prevail at oxygen-lean preparation conditions, annealing in oxygen gives Hausmannite Mn3O4 as dominant phase. Both polymorphs adopt square structures that only permit row-matched growth on the hexagonal gold, explaining the high tendency for oxide dewetting at elevated temperature. The manganese oxide films are thus polycrystalline and consist of a large number of sub-micrometer grains with different orientations and surface terminations. A variety of square and line patterns were identified on top of the oxide crystallites, all of them being compatible with either the primitive 5.8×5.8 Å2 cell of Mn3O4(001) or an ordered MnO(100) vacancy structure. STM conductance spectroscopy provides insight into the electronic properties of the Mn-O islands and yields the approximate size of the band gap as well as the energy position of localized Mn 3d levels in the gap region. Our work illuminates the structural and electronic properties of manganese oxide films grown on Au(111) and therefore complements earlier studies performed on Pt(111), Pd(100) and Ag(100) supports.


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1. Introduction An exceptionally high structural and chemical variability combined with unique magnetic properties puts the oxides of manganese among the most fascinating transition metal oxides.1 Key to this flexibility is the half-filled 3d electron shell of manganese. It promotes, on the one hand, a 2+ oxidation state of the Mn ions, as in rocksalt MnO, where only the 4s electrons contribute to chemical bonding and the 3d shell remains intact. Conversely, Mn oxidation states as high as 7+ might be stabilized, with all 4s and 3d electrons participating in bond formation. The respective materials, Mn2O7 and KMnO4, are known for their vivid chemical reactivity. Within these limits, several compounds with intermediate oxidation states exist, e.g. Mn3O4 (Hausmannite), Mn2O3 (Mn sesquioxide) and MnO2 (Mn dioxide). This chemical versatility explains the wide use of Mn-O in heterogeneous catalysis, especially in water oxidation reactions.2,3,4,5 Particularly interesting in this respect are MnO2 trilayers grown on Pt(111), a system that delivers reactive O-species for Mars-van-Krevelen-type oxidation reactions.6 The unique chemistry of manganese oxides is also encountered in biological systems,7,8 e.g. in the photosystem II, and, given their high ionic conductivity, in electrode materials for electrochemistry.9 The half-filled Mn 3d shell is finally responsible for fascinating magnetic properties, as found in antiferromagnetic Mn3O4 and multiferroic CaMn7O12.10,11 Most surface-science studies have been performed on oxygen-poor manganese oxides, in particular on MnO, Mn3O4, Mn2O3 and MnO2. The exploration started already in 1986 with a combined LEED and XPS investigation of single crystalline MnO(100).12 Later, the thin-film approach was exploited to overcome conductivity problems associated with the bulk material.13 The probably most comprehensive study was undertaken on Pd(100) supports.14,15 Here, a whole bunch of thin-film configurations were observed as a function of the oxidation conditions, including a Mn3O4 vacancy structure, rocksalt MnO and O-Mn-O trilayers. Epitaxial MnO(100) films were produced by reactive Mn deposition onto Ag(100),16,17 while formation of strongly faceted MnO(110) was revealed on Au(110).18 In other studies, the oxide growth involved spontaneous symmetry breaking, and hexagonal Mn-O structures were stabilized on square Pd(100) and Rh(100) supports.19,20 Also the reverse effect, i.e. growth of squarelattice MnO on hexagonal Pt(111), was reported.21 Apparently, details of oxidation procedure rather than the symmetry of the support govern the nature of the predominant Mn-O phase. 3 ACS Paragon Plus Environment


At higher oxygen chemical potential, a transition between rocksalt MnO and spinel Mn3O4 occurs.22,23 While both structures crystallize in a cubic lattice, the O-content in Mn3O4 is 25% higher and Mn occupies both, octahedral and tetrahedral sites of the oxygen sub-lattice. Many Mn3O4 surfaces develop complex reconstructions, being induced by the polar nature of the bulk-cut terminations. The structure and thermodynamic stability of (001), (110) or (111)-derived surface terminations was therefore subject of several DFT calculations.24,25 Here, we discuss manganese-oxide films prepared by reactive Mn deposition onto Au(111). In contrast to earlier studies on Pd(100), Ag(100) and Rh(100), the hexagonal support may stabilize MnO(111), a Tasker-type III material with unusual properties.26,27 Another motivation to examine Mn-O surfaces is their large potential in binding and dissociating water molecules.28,29 By this means, a main drawback of our earlier model system, Cu2O/Au(111) that interacts only weakly with water is overcome.30,31 Our study finds the formation of square-lattice MnO(100) structures at oxygen-lean preparation conditions, despite a symmetry mismatch with hexagonal Au(111). With increasing O2 chemical potential, the rocksalt MnO transforms to spinel Mn3O4(001) that features improved registry with the support. The two oxide polymorphs were identified and characterized with X-ray photoelectron-spectroscopy (XPS), low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM).

2. Experimental methods The experiments have been performed in two ultrahigh-vacuum chambers (p ∼ 5×10-10 mbar). The German setup contained a custom-built Beetle-type STM, an XPS setup (ESCA-Lab 200) and standard facilities for sample preparation and analysis, such as LEED. The Brazilian chamber comprised a Phoibos 150 spectrometer (SPECS) and a LEED system. The XPS measurements were performed with Mg and Al Kα sources at 15 eV pass energy. Binding energies were calibrated with respect to the Au 4f 7/2 peak. The STM images were acquired at 100 K sample temperature in the constant current mode, using chemically etched gold tips. The oxide films were prepared by reactive Mn deposition from a Mo crucible onto a sputtered and annealed Au(111) single crystal at room temperature. The deposition rate was set to one monolayer (ML) per minute; the O2 pressure was varied between 10-6-10-8 mbar. To


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stimulate film crystallization, the samples were post-annealed at temperatures between 650 and 900 K either in oxygen or vacuum. The detailed growth conditions are specified later in the text.

Fig. 1: STM overview images (100 x 100 nm², 10 pA) of manganese oxide films on Au(111): (a) 0.25 ML (UB = 2.7 V), (b, c) 2.5 ML (UB = 3.7 V) and (d) 10 ML nominal coverage (UB = 8.7 V). Films in (a,b) are annealed in 5×10-7 mbar oxygen at 600 K, while those in (c,d) are vacuum-annealed to 800 K.

3. Experimental Results 3.1.

Morphology and composition of manganese oxide films

Room-temperature Mn deposition in 1×10-6 mbar O2 results in amorphous oxide films that homogenously cover the Au(111) surface, but do not produce distinct signatures in LEED and STM. Upon oxygen annealing at 600 K, oxide nano-islands with characteristic shapes nucleate along the gold step edges (Fig.1a). With increasing coverage, these clusters grow in size and agglomerate to large crystallites, exposing distinct square and line patterns on their atomically flat top facets (Fig. 1b). The mean island height amounts to 9 Å at this stage, however, higher clusters and uncovered gold regions are discernable as well. Apparently, the oxide shows little incentive to wet the Au(111) surface and its 5 ACS Paragon Plus Environment


initial polycrystalline texture prevails also at high coverage. Vacuum annealing amplifies the dewetting trend and produces large oxide crystallites with 50-100 nm diameter and 15-20 Å height, coexisting with bare Au(111) (Fig. 1c). In contrast to oxygen-annealed films, these islands often feature flat top facets without discernable atomic corrugation. Closed Mn-O films, being resistant against detwetting, can only be prepared by increasing the nominal Mn exposure above 10 ML (Fig. 1d). However, also these films are composed of sub-micrometer grains. Apparently, preparation of homogenous Mn-O films is challenging on the Au(111) support, suggesting either a low metal-oxide adhesion or an unfavorable atomic registry at the interface. Note that films thicker than 20 Å are nontransparent for low-energy electrons and thus inaccessible to low-bias STM imaging.

Fig.2: LEED pattern of (a) 5 ML manganese oxide on Au(111) annealed in oxygen and (b) same film after vacuum annealing to 800 K. The LEED in (a) comprises two staggered 12-spot rings that are best visible at 40 eV electron energy and reduce to a faint inner ring at 90 eV. These inner reflexes disappear upon vacuum annealing (b). Both diffraction patterns are compatible with three square lattices, rotated by 60°. The lattice parameters are determined to 5.8 Å and 3.1 Å for oxygen and vacuum-annealed films, respectively, using the position of Au(111) reflexes as internal reference (see insets).

LEED measurements provide first insight into the structure of Au(111)-supported manganese oxides. For oxygen-annealed films of 5 ML nominal thickness, two staggered 12-spots rings are observed at 40 eV electron energy (Fig. 2a). At higher energy, these rings move to the center and the second-order reflexes become visible. After vacuum annealing, the inner rings completely disappear from the diffraction pattern and only two staggered outer rings, comprising 12 spots each, remain detectable (Fig. 2b). The reflexes hereby locate at nearly identical positions as the second-order spots of vacuum6 ACS Paragon Plus Environment

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annealed films, indicating a reduction of the unit-cell size by a factor of two. Both spot patterns are compatible with square oxide lattices, occurring in three domains rotated by 60°.32 An alternative interpretation, two hexagonal oxide domains tilted by 30°, does not satisfy the experimental data, as the second-order spots are not at their expected positions. By optimizing the electron energy, the Au(111) diffraction spots become detectable as well, providing an internal reference to determine the oxide lattice parameters (Fig. 2, insets). The relative distance between oxide and overlayer spots hereby experience marginal changes for the different preparation procedures. Correctly accounting for the symmetry factor between hexagonal and square lattices yields real-space dimensions of 5.8 Å and 3.1 Å for oxygen- and vacuum-annealed films, respectively. The rotational alignment of the gold and oxide spots further suggests a fixed correlation between both lattices, in which one oxide unit-cell vector always aligns with an Au〈110〉 direction. In general, the granular nature of the oxide films, as seen in the STM images, gives rise to rather diffuse LEED patterns with low spot intensities Nonetheless, two different lattice structures can be revealed, providing evidence for the presence of two oxide polymorphs on the gold surface.

Fig. 3: XP spectra of 5 ML manganese oxide taken in the Mn 2p and O 1s region. The upper spectrum was acquired on a vacuum-annealed film and shows the Mn 2p doublet and the characteristic Mn2+ satellite. In the lower spectrum taken on an oxygen-annealed film, the 2p states are shifted to higher binding energy and the satellite disappears, indicative for Mn3O4 formation. The two components of the spin-split Mn 2p state were fitted with two Gaussians to improve agreement with experimental data.22 The O 1s peak position hardly depends on details of the oxidation procedure and only the overall intensity is higher in oxidized films.



The stoichiometry of the Mn-O films has been determined with XPS. Fig.3 compares two data sets acquired on a 5 ML-film that was first exposed to 1×10-6 mbar O2 at 600 K and later vacuum-annealed at 800 K. Both spectra are dominated by the Mn 2p doublet, whereby each component is split again into two sub-peaks of different spin configuration. Surprisingly, the respective components, fitted after Shirley-background correction, systematically shift to 0.6 eV lower binding in vacuum- as compared to oxygen-annealed films. Moreover, a pronounced shoulder appears at 647 eV in spectra of the reduced film, not observed before vacuum annealing. Both findings indicate the initial presence of Mn3O4 (Hausmannite) on the surface, which gets reduced to MnO (Manganosite) upon vacuum annealing.22,23,33 Distinction between the two oxide phases remains ambiguous at this point, as both polymorphs may coexist on the Au(111). However, concurrent MnO and Mn3O4 growth was reported earlier on Pd(100), Ag(100) and Pt(111) supports with Mn3O4-type structures prevailing at oxygenrich conditions.14,16,21 In those studies, the oxide stoichiometry was pinpointed either by synchrotronbased XPS, vibrational spectroscopy or high-resolution LEED. The O 1s peak is shown for completeness in Fig. 3b. Its binding energy of 530 eV agrees well with reported O 1s positions in oxygen-poor manganese oxides and no signs for surface hydroxylation are found.23 Moreover, the integrated O 1s intensity is found to be 15% lower in reduced as compared to oxidized film, in agreement with a reduced oxygen content in MnO with respect to Mn3O4. Summarizing this first chapter, reactive Mn deposition on Au(111) results in Mn3O4 and MnO-type structures at oxidizing and reducing conditions, respectively. Both oxides adopt square-lattice configurations with either 5.8 or 3.1 Å periodicity, occurring in three rotational domains due to the symmetry mismatch with the hexagonal Au(111). Further insight into the surface configuration of both oxide polymorphs comes from the atomically resolved STM images presented in the next paragraph.

3.2. Atomic structure of manganese oxide islands and thin films We start our discussion with samples prepared at low Mn exposure (Fig. 4). Panels (a-e) show a selection of characteristic oxide islands, being oxidized in 10-6 mbar O2 and vacuum-annealed to 700 K afterwards. The overall island shapes are compatible with a square atomic lattice, as concluded from predominant 90° and 135° angles. At least one island edge always follows an Au〈110〉 lattice vector, 8 ACS Paragon Plus Environment

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demonstrating the template effect of the gold surface. The island top facets typically exhibit square patterns with 5.8 Å periodicity (Fig. 4a,b) or line patterns with 5.8 or 8.2 Å spacing (Fig. 4c,d). All these patterns can be traced back to a fundamental 5.8 Å square lattice, whereby the 8.2 Å periodicity corresponds to the cell diagonal (√2×5.8 Å). Only in rare cases, distinct atomic features are absent on the oxide surface (Fig. 4e). Typical island heights vary between 2-4 Å and are therefore distinctively lower than the vertical unit-cell size of orthorhombic Mn3O4 (9.3 Å). Unambiguous distinction between MnO and Mn3O4 configurations is thus unreliable in the limit of ultra-thin islands and we concentrate our discussion on thicker, more bulk-like aggregates first.

Fig. 4 (a-e): Selection of manganese oxide islands on Au(111) prepared by Mn deposition in 1×10-6 mbar O2 and vacuum annealing to 700 K (0.25 ML, 10 x 10 nm2, UB = 0.5 V). (f) Pyramidal Mn-O island produced by vacuum annealing to 900 K, i.e. at strongly reducing conditions (UB = 9.0 V).

Figure 5a depicts the corner of a 15 Å high oxide crystallite, prepared via oxygen annealing. Its pronounced 5.8 Å square pattern corresponds to the staggered 12-spot rings seen in the LEED pattern of Fig. 2a. A similar pattern is resolved in Fig. 5b, only that the respective island has a different orientation and a domain boundary cuts through its center. In both cases, one oxide unit-cell vector follows an Au〈110〉 direction, indicating again a defined interface registry with Au(111). On an even larger crystallite of 20 Å height, a different type of square lattice is detected (Fig. 5c). Its unit cell is rotated 9 ACS Paragon Plus Environment


by 45°, expanded to 8.2 × 8.2 Å2 (times factor √2) and corresponds to a c(2×2) configuration with respect to the former lattice. Both terminations seem to originate from the same square structure and only a deviating surface reconstruction and/or contrast scheme gives rise to the different STM appearance. Also the various line patterns depicted in Fig. 5(d-f) can be associated to a reconstructed 5.8 Å square lattice. For example, the 17 Å line spacing in panel (e) corresponds to three times the fundamental cell size, while a transition from an 8.2 Å square to a 12 Å line pattern is revealed on the island in panel (d).

Fig. 5: Selection of atomic patterns found on large manganese oxide crystallites prepared by annealing in 5×10-7 mbar O2 at 600 K, (a-c) 10 x 10 nm2 and (d-f) 15 x 15 nm2 (2.5 ML, UB = 2.7 V). The square and line patterns are tentatively assigned to different surface terminations of Mn3O4(001).

3.3. Interpretation of Mn-O thin-film structures on Au(111) The MnO and Mn3O4 bulk structures are good starting points to analyze the various surface configurations observed in STM. MnO has a lattice parameter of 4.4 Å; its primitive cell is smaller by a factor of √2 (3.1 Å). Mn3O4, on the other hand, crystallizes in the orthorhombic structure with a 5.8 × 5.8 Å2 base plane and a vertical c vector of 9.4 Å. LEED indeed reveals both in-plane parameters, the 3.1 Å of MnO(100) for thick, vacuum-annealed films and the 5.8 Å of Mn3O4(001) for samples exposed to oxygen (Fig. 2). STM, on the other hand, exclusively finds periodicities that are compatible with a 10 ACS Paragon Plus Environment

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fundamental cell size of 5.8 Å. The fact that no intrinsic MnO(100) lattice spacing is found in STM suggests that the rocksalt surface either reconstructs or the dense-packed unit cells are simply not resolved in our STM data. We have indications for both scenarios. Given their different lattice symmetries, the square MnO(100) can adjust to hexagonal Au(111) only along one Au〈110〉-type direction, while no registry is reached along the orthogonal Au〈112〉 (Fig. 6a). The corresponding growth scheme, known as row matching, is indeed found in our data, where one oxide unit-cell vector always aligns with an Au〈110〉 direction. Moreover, many Mn-O islands feature elongated shapes (Fig. 4e) or expose different line structures, following the direction of commensurate growth (Figs 4c,d). Row matching is a common growth phenomenon for oxide films on symmetry mismatched supports, and was reported for Al10O13/NiAl(110) and FeO(111)/Ag(001), for example.34,35 To connect the lattice periodicity of 5.8 Å observed in STM to the MnO(100) bulk structure, a times-two reconstruction combined with a 7% lattice compression would be required. Compressive strain is indeed known to promote lattice restructuring at the interface to the mismatched support. A typical response is the insertion of a vacancy pattern into the interfacial oxide plane, which leads for example to a c(4×2) reconstruction in NiO(100) monolayers grown on Pd(100).36,37 Similar vacancy structures were found for CoO and MnO layers on Ir(100) and Pd(100) surfaces, respectively.38,39 The 5.8 Å periodicity observed here would be compatible with a (2×2) vacancy array in the strained MnO(100), whereby the Mn vacancies align with an Au〈110〉 direction and the STM contrast would arise from those MnO4 units that are sandwiched between two vacancy sites (Fig. 6a). In principle, the MnO(100) vacancy model could be extended to describe the stripe patterns found on many oxide crystallites (e.g. in Figs. 4 and 5). For this purpose, additional Mn defects need to be inserted into the interfacial oxide layers. By arranging them in stripes running along Au〈110〉, the system would also be able to avoid unfavorable Mn binding positions in the Au(111) surface, such as top sites (Fig. 6a). The periodicity of the emerging oxide stripes would then be governed by the registry between oxide ad-layer and gold substrate in the orthogonal Au〈112〉 direction and range between 5 and 20 Å in agreement with the measurements. We note that a similar mechanism was proposed for Mn-O line patterns observed on Pd(100),14 as well as MnO2, NiO2 or CoO2 oxide stripes on Ir(100), suggesting a certain generality of the mechanism.40 11 ACS Paragon Plus Environment


Both, the MnO-vacancy and the deduced stripe model fail to rationalize the observed oxide corrugation in the limit of thick layers. A strain-induced (2×2) vacancy pattern is expected to be localized at the metal-oxide interface and its visibility should fade away with increasing layer thickness.37 Our STM images show, however, pronounced square and line patterns even on oxide crystallites thicker than 20 Å. In this thickness range, the bulk structure of MnO should be established and, given the high thermodynamic stability of rocksalt (100), no surface reconstruction is expected anymore.16 Consequently, large oxide islands with well-resolved surface corrugation cannot be associated to MnO(001), a conclusion that becomes invalid for those crystallites without resolved surface pattern.

Fig. 6: (a) Tentative models for MnO(100)-derived vacancy structures and MnO2-type stripe configurations with different spacing. (b) Bulk unit cell and four different surface terminations of Mn3O4(001). The latter are derived by cutting the unit cell at the indicated layer positions.

At this point, Mn3O4-type structures (Fig. 6b) come into play, the growth of which at oxygen-rich conditions is strongly suggested by LEED and XPS (Figs. 2,3). For those films, the Mn3O4 lattice parameters can be read off from the STM data at various points. The respective crystallites often feature a unique height of 9 Å, matching the c-parameter of orthorhombic Mn3O4 (Fig. 7a). Moreover, their top facets are covered by square lattices of 5.8 and 8.2 Å periodicity, or parallel stripes with 12 and 17 Å spacing, incompatible with a compact MnO(100) surface.16,17 These reconstructions disappear after vacuum annealing at 800 K, leaving behind the featureless oxide crystallites associated to rocksalt MnO (Fig. 7b). All these observations support the idea of Mn3O4 formation in preparations performed at oxygen excess.


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Fig. 7: Large-scale topographies of a 2.5 ML thick manganese oxide film annealed in (a) oxygen at 600 K and (b) vacuum at 800 K (100 x 100 nm2, UB = 3.7 V). The film in (a) is dominated by corrugated oxide islands of 9 Å height, indicative for Mn3O4, while tall and featureless MnO-type crystallites prevail in (b).

The different structural motifs found on the surface of oxygen-rich crystallites can be traced back to the 5.8×5.8 Å2 basal plane of the Mn3O4, and reflect either the primitive cell or a c(2×2), (2×1) or (3×1) reconstruction. The large structural variability relates to different positions at which the orthorhombic Mn3O4 cell can be cut into (001)-oriented slices (Fig. 6b).22 Atomic planes comprising only Mn2+ ions hereby alternate with Mn2O4 stripes of 5.8 Å periodicity and orthogonal orientation. Other terminations are formed by adding/removing ionic species from the different planes, for instance to compensate for polarity or respond to more oxidizing / reducing environments. Which surface configuration gets finally realized depends on the thermodynamic stability of a respective plane at the temperature and O2 pressure during preparation. DFT calculations suggest that Mn2O4 stripe-terminations are thermodynamically preferred over a wide range of oxygen chemical potentials,41 and corresponding line patterns are indeed observed on many of our Mn3O4 islands (Figs. 3 and 4). A more detailed analysis of the observed surface patterns requires ab-initio thermodynamic calculations and is beyond the scope of this paper.42 Our data nonetheless demonstrate that the amount of MnO versus Mn3O4 structures on Au(111) can be determined by adjusting appropriate preparation conditions. In a last paragraph, we focus on a deviating island type, being produced by vacuum-annealing the MnO films to 900 K (Fig. 4f). These islands are characterized by pyramidal instead of square shapes and exhibit heights larger than 20 Å. Their geometry suggests that the underlying lattice is of hexagonal 13 ACS Paragon Plus Environment


and not of square shape, as before. A plausible configuration would be a MnO pyramid with a (111) base plane and (100) side facets. While the former ensures good registry with hexagonal Au(111), the side planes suppress polarity and increase the thermodynamic stability of the crystallite. We refrain from further discussions of MnO(111)-derived structures and refer to the literature for additional information.19,20,43

3.4. Electronic properties of manganese oxide islands Differential conductance (dI/dV) spectroscopy has been exploited to gain insight into the local density of states (LDOS) of the various oxide crystallites observed in this study. The oxides of manganese differ in the oxidation state of their Mn ions, hence the d-band occupation. While MnO has a halffilled Mn 3d band (3d5), two Mn3+ (3d4) and one Mn2+ ion (3d5) coexist in spinel Mn3O4 and MnO2 exclusively contains Mn4+ (3d3) ions. These electron configurations in conjugation with different oxygen-binding geometries lead to variations in the oxide band gap that decreases from MnO (3.5 V) to MnO2 (2.25 V) and Mn3O4 (1.9 V).44,45,46

Fig. 8: (a) Differential conductance curves measured on Mn3O4 (upper curves) and MnO-type islands (lower curve) and of 9 and 15 Å thickness, respectively. While the latter features a plain band gap of 3.2 V, the former exhibit characteristic gap states being associated to localized Mn 3d states (see arrows). (b) Topography and dI/dV maps of small Mn-O islands taken at different bias voltages. Apart from the one at 1.6 V, all maps were acquired at bias positions inside the band gap and feature either distinct gap states or wavy patterns due to the Shockley surface state of Au(111) (0.25 ML, 50 x 50 nm2).


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Our dI/dV curves, acquired with a lock-in amplifier, reproduce this trend to a certain degree (Fig. 8a). The largest gap (3.2 eV) is measured on top of the tall, featureless oxide crystallites (see Fig. 7b), being assigned to MnO according to their preparation at reducing conditions. The gap size decreases to ∼2.4 eV for corrugated Mn3O4-type islands formed in oxygen-rich environments (see Fig. 5). In both cases, the valence-band onsets measured between -1.5 and -1.0 V are in good agreement with nonlocal data acquired with photoelectron spectroscopy.22,23 Additional dI/dV intensity inside the gap region is observed for Mn3O4-type crystallites with different surface configurations. On islands covered with 5.8 Å square lattices, an in-gap peak at -1.0 V is detected, indicative for an occupied state. On islands exposing a stripe termination, a similar gap-state appears at -0.5 V, i.e. closer to the Fermi level. The localized states are tentatively assigned to the Mn 3d levels that occur at different energies according to their electron occupation.47,48 In response to crystal-field and spin effects, the Mn 3d states split into t2g and eg manifolds, each one with spin-up and spin-down components.49 In antiferromagnetic MnO, shell closing is achieved for one spin-component (t2g↑↑↑, eg↑↑), stabilizing the respective levels at energies well below EF. As a consequence, the valence-band edge is dominated by the O 2p states and no extra peak appears in the gap region (Fig. 8a, lowest curve). In contrast, Mn3O4 contains Mn species with lower d-occupation, causing the occupied eg component to shift into the gap (Fig. 8a, topmost curve).50 The gap state with highest energy is revealed for Mn3O4 islands exposing a pronounced line pattern, possibly related to MnO2-derived nano-stripes.22,40 It should be noted that also defects and low-coordinated edge ions might be responsible for the observed in-gap states and a definite interpretation requires theory support. Differential conductance maps of ultrasmall oxide islands prepared at oxygen-rich conditions are finally reported in Fig. 8b. The map taken at 1.6 V depicts the LDOS distribution just at the onset of the conduction band. While the bigger islands have already developed high dI/dV intensity, smaller islands remain dark as their gap might be larger. At 0.6 V, the interior of the oxide islands appears dark, while bright features become visible along the island boundaries. The latter might be related to lowcoordinated edge ions, exhibiting localized gap states. At 0.3 V, i.e. in the middle of the gap, absolute conductance values are very low and the contrast needs to be enhanced to identify the oxide islands. This makes the Au(111) surface state discernable as well, producing the pronounced wave pattern seen 15 ACS Paragon Plus Environment


in the dI/dV map. Conductance maps at negative voltages did not produce an exploitable contrast and localized Mn 3d levels could therefore not be identified in real-space measurements.

4. Conclusions Manganese oxide islands and thin films of different morphology and stoichiometry have been prepared on an Au(111) support and explored by means of LEED, XPS and STM. MnO(100) is obtained as dominant oxide phase if the final annealing step is performed in vacuum. The respective surfaces exhibit either quadratic (2×2) vacancy patterns or compact rocksalt lattices without discernable corrugation in the limit of low and high thickness, respectively. Annealing in oxygen, on the other hand, gives rise to Mn3O4 formation, as identified by a characteristic LEED and XPS signature. STM reveals large oxide crystallites covered by a variety of atomic patterns. Although no theoretically-confirmed surface terminations are available, all structures seem compatible with different (001) cuts through the Mn3O4 unit cell and their reconstructions. STM conductance spectroscopy finally provides insight into the size of the oxide band gap and the energy position of the Mn 3d levels localized in the gap region. Given its narrow gap size, especially Mn3O4 might be relevant for photo-induced chemical processes, e.g. the splitting of water. Experiments on the chemical response of Mn3O4(001) require however the fabrication of homogenous, single-phase films and therefore ask for an optimization of the above preparation schemes.

Acknowledgments This work is dedicated to Hajo Freund, whose scientific excellence and personal integrity helped us finding our ways through the challenging world of science. We are grateful for many years of assistance and fruitful collaborations. Direct financial support for this work comes from the DFG grants Ni 650-3 and Ni 650-4. ‘Understanding photocatalytic processes at the nanoscale’. J.B and F.S are grateful for the support from CNPq, FAPERJ, CAPES, Alexander von Humboldt Foundation and the MaxPlanck Partnergroup Program.

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Manganese Oxide Thin Films on Au(111): Growth Competition

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