Atomic Scale Insights into the Early Stages of Metal Oxidation: A

Article pubs.acs.org/JPCC

Atomic Scale Insights into the Early Stages of Metal Oxidation: A Scanning Tunneling Microscopy and Spectroscopy Study of Cobalt Oxidation A. Picone,*,† M. Riva,†,‡ A. Brambilla,† D. Giannotti,† O. Ivashko,†,§ G. Bussetti,† M. Finazzi,† F. Ciccacci,† and L. Duò† †

Dipartimento di Fisica, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italy Institute of Applied Physics, TU-Wien, Wiedner Hauptstraße 8-10, 1040 Vienna, Austria § Physik-Institut, Universität Zürich, Winterthurerstraße 190, CH-8057 Zürich, Switzerland ‡

ABSTRACT: Understanding the mechanisms driving the early stages of oxidation of metallic surfaces is of fundamental importance in fields such as nanocatalysis, electrochemistry, and nanoelectronics. In this work, the early stages of oxidation of ultrathin Co films deposited on the Fe(001)-p(1 × 1)O surface have been investigated by means of Auger electron spectroscopy, scanning tunneling microscopy, and scanning tunneling spectroscopy. The oxidation is initiated by homogeneous oxide nucleation over the atomically flat Co terraces, inducing the development of a smooth oxide wetting layer when the islands coalesce. Atomically resolved images reveal that the oxide wetting layer is highly defective, possibly due to the presence of oxygen vacancies. Constant current scanning tunneling microscopy images acquired in different tunneling conditions, as well as scanning tunneling spectroscopy, reveal the distinct electronic properties of the oxide nuclei with respect to the chemisorbed phase. The fundamental band gap develops since the early stages of oxide nucleation. Moreover, spectroscopic curves acquired in the near-field-emission regime reveal a significant lowering of the sample work function induced by the oxide development. Our results represent a remarkable case in which metal oxidation can be studied at the atomic-scale level.

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INTRODUCTION

closed oxide layer is formed, entirely covering the topmost layers of the metal surface. 3. Oxide thickening, during which the oxide scale growth takes place through migration of anions or cations across the oxide layer, leading to either oxygen or metal incorporation at the oxide/metal or oxygen/oxide interface, respectively. Although the chemisorbed oxygen phase and its influence on the structural,15−20 magnetic,19,21−29 and electronic21,30,31 surface properties have been deeply investigated for a large number of metals, much less is known about the oxide phase, especially concerning the early stages of oxide nucleation. In particular, the number of experimental investigations performed by means of scanning tunneling microscopy (STM) is scarce32−35 if compared to other surface-sensitive spatialaveraging techniques. This circumstance can be easily understood by considering that the oxidation process often induces the development of rough surfaces36−39 or even amorphous structures,40,41 rendering the atomic scale investigation by means of STM a challenging task. Moreover, the simultaneous

Metal oxidation plays a fundamental role in many fields of modern (nano)technology,1 spanning over different topics such as heterogeneous (nano)catalysis,2−4 surface functionalization or protection against corrosion,5−7 and microelectronic devices,8,9 just to name a few. From a more fundamental point of view, a detailed understanding of the transition between the metallic and the oxidic phases is far from being conclusively achieved, as testified by the large number of theoretical studies aiming to rationalize the subtle atomistic mechanisms at work during the early stages of surface oxidation.10−13 In a simplified picture, starting from a metallic surface, the oxidic phase develops through three steps: 1. Dissociation of molecular oxygen and formation of a chemisorbed layer. Oxygen molecules, impinging on the metallic surface from the gas phase, dissociate into atomic species. At the end of this process, oxygen atoms generally arrange in ordered overlayers, saturating the high-symmetry surface sites up to a critical coverage. 2. Oxide nucleation. The oxide nuclei either can develop by subsurface oxygen incorporation14 or alternatively are formed by metal atoms disrupted from step edges and oxygen from the gas phase. At the end of this process a © XXXX American Chemical Society

Received: January 13, 2016 Revised: February 15, 2016

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DOI: 10.1021/acs.jpcc.6b00384 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. Rocksalt CoO (a) and body-centered-tetragonal Co(001)-p(1 × 1)O (b) structures. The in plane lattice constant of Co(001)-p(1 × 1)O is abct Co = aFe = 287 pm. Red dotted lines indicate the Co(001)-p(1 × 1)O cell to be compared with the unit cell of CoO. The lattice misfit is given by f = (√2 abct Co − aCoO)/(√2 abct Co) = −4.9%.

Fe(001) film was grown by means of molecular beam epitaxy (MBE). Fe(001)-p(1 × 1)O surfaces were realized by exposing a clean Fe(001) substrate held at 430 °C to 30 L (1 L = 1.3 × 10−6 mbar·s) of pure O2 (partial pressure: 2 × 10−7 mbar). The sample was then heated at 630 °C for 5 min to remove the excess oxygen from the surface. The resulting surface is characterized by a single layer of oxygen atoms, located in the 4-fold hollow sites of the Fe(001) lattice and completely saturating the surface.30 Co films about 5 ML thick were grown by means of MBE on the Fe(001)-p(1 × 1)O substrate, held at room temperature during Co deposition. After Co deposition, the sample was annealed at 200 °C for 5 min, in order to induce island coalescence and obtain atomically flat terraces. Oxygen exposure was performed by admitting molecular oxygen from a leak valve, with the oxygen partial pressure kept at 2 × 10−8 mbar for doses up to 100 L and raised to 4 × 10−8 mbar to reach the highest oxygen exposure considered in this study (500 L). The sample was kept at 200 °C during the oxidation process in order to facilitate the mobility of atomic species on the surface, inducing the formation of a smooth oxide overlayer. For the sample exposed to 500 L, a postannealing treatment at 300 °C for 5 min in UHV was also performed. Auger electron spectroscopy (AES) analysis was performed by means of an Omicron SPECTALEED with a retarding field analyzer (total acceptance angle 102°). A 3 keV, 20 μA electron beam was used, with a 3 V peak-to-peak modulation amplitude. The STM and STS (scanning tunneling spectroscopy) measurements were performed with an Omicron variable temperature STM in a UHV chamber connected to the preparation system. STM images were acquired in constant current mode with homemade electrochemically etched W tips. Tunneling spectra were acquired at liquid nitrogen temperature (77 K). Spectra in the region around the Fermi energy (EF) have been acquired at constant tip−sample separation (open feedback loop). Field emission resonance (FER) spectroscopy was performed at constant tunneling current (closed feedback loop).

presence of cationic and anionic species on the surface makes the interpretation of atomically resolved images a difficult task.42−45 However, the ultimate atomic-scale resolution provided by STM could shed light in many aspects of metal oxidation. For example, an open issue concerns the geometrical distribution of the oxide nucleation sites. In the literature, two types of oxide nucleation mechanisms have been recognized, i.e., the heterogeneous and the homogeneous ones. In the former case,46,47 the oxide develops from defects present at the surface, such as steps, kinks, or voids, while in the latter oxide patches start to grow on atomically flat terraces, without any preferential nucleation site.48,49 A second important issue is related to the electronic structure of the ultrathin oxide overlayer. What is the minimum thickness of the oxide film for which the electronic band gap starts to develop?50 How is it possible to experimentally distinguish between the chemisorbed and the oxidic phases?51 In this paper, we address these fundamental issues by discussing the early stages of oxidation of ultrathin Co films [Co coverages in the range of 5 equiv of monolayers (ML)a] grown on the Fe(001)-p(1 × 1)O surface. The choice of this system is motivated by the fact that high-quality Co ultrathin films can be epitaxially grown on the Fe(001)-p(1 × 1)O substrate, with oxygen floating on top of the growing film and acting as a surfactant.52−55 These films are characterized by a strained body-centered tetragonal (bct) crystal structure, with the same in-plane lattice constant of the Fe(001)-p(1 × 1)O surface. The resulting surface is a Co(001)-p(1 × 1)O phase which, in the present framework, can be associated with the chemisorbed oxygen phase on Co(001). Figure 1 compares the crystal structure of CoO with that of Co(001)-p(1 × 1)O. For the CoO(001)[110]||Co(001)[010] epitaxial orientation, a good lattice matching between metastable bct Co and rocksalt CoO is obtained, with a moderate misfit. Moreover, the CoO unit cell can be obtained by incorporation of oxygen atoms in subsurface octahedral sites of the Co(001)-p(1 × 1)O. For these reasons, the development of epitaxial and smooth Co oxide films is expected, making Co(001)-p(1 × 1)O an appealing substrate for the investigation of the oxidation mechanisms with atomic resolution.

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RESULTS AND DISCUSSION Surface Chemistry and Oxide Nucleation. Figure 2 shows the effects of the thermal treatments performed on 5 ML Co(001)-p(1 × 1)O before oxygen exposure. Due to the interrupted Co atomic flux, the as-deposited sample is characterized by a fractional Co coverage, with the surface

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EXPERIMENTAL SECTION Samples were prepared in an ultrahigh vacuum (UHV) system (base pressure 1 × 10−10 mbar) by starting from UHV-cleaned MgO(001) single crystal substrates, over which a 400 nm thick B


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Figure 2. Effect of the annealing treatments at 200 °C on the 5 ML Co(001)-p(1 × 1)O films. (a, b) Mesoscopic morphology of the asgrown sample and postannealed sample, respectively. Tunneling parameters I = 1 nA and V = 1 V, image size 200 × 200 nm2. The inset of panel b displays the atomically resolved surface, characterized by a 4-fold symmetry (image size 1.6 × 1.6 nm2, tunneling parameters I = 5 nA and V = 0.1 V). (c, d) Topographic line scans corresponding to the horizontal white lines in panels a and b, respectively. In both cases the measured topographic height is approximately 140 pm, corresponding to a single ML (i.e., two exposed surface layers).

exposing mainly two atomic layers, separated by a high density of monatomic steps (Figure 2a and Figure 2c). After the annealing procedure, the number of monatomic steps decreases, leaving a surface characterized by large atomically flat terraces (Figure 2b and Figure 2d). Atomically resolved images acquired on the terraces reveal that oxygen atoms arrange in an ordered square overlayer, in registry with the substrate (see inset of Figure 2b). AES data (not shown), acquired before and after the annealing procedure, are identical in terms of peak positions and relative intensities, indicating that annealing does not lead to a significant interdiffusion of Co atoms into the Fe bulk. Figure 3 shows the evolution of AES peaks upon Co deposition and oxygen exposure, in both the high (Figure 3a) and low (Figure 3b) kinetic energy regions. Spectra acquired on the Fe(001)-p(1 × 1)O and 5 ML Co(001)-p(1 × 1)O (spectra i and ii, respectively) are reported, along with those acquired on the oxidized samples (from spectra iii to spectra v). The spectrum of oxidized Fe (vi), acquired on the Fe(001) sample exposed to 50 L of oxygen, is also reported for reference. In the high kinetic energy region (Figure 3a), the oxygen peak (KLL transition) intensity detected in spectrum i corresponds to a single oxygen layer adsorbed on the Fe(001)p(1 × 1)O surface. After Co deposition, the oxygen KLL peak is unchanged (see spectrum ii), indicating that the oxygen floats on top of the growing film, in agreement with previous investigations.54 Upon exposure to 20 L (spectrum iii) and 500 L (spectrum iv) of oxygen, a noticeable increase of the oxygen peak intensity is observed. UHV annealing at 300 °C on the 500 L dosed sample does not alter the oxygen peak intensity (spectrum v). The peaks acquired in the low kinetic energy region, corresponding to Co and Fe MNN transitions, are more surface sensitive. Moreover, they are particularly sensitive to the local atomic chemical environment; therefore their shape

Figure 3. Evolution of AES spectra upon oxygen exposure on 5 ML Co(001)-p(1 × 1)O in the high (a) and low (b) kinetic energy region. Spectra i and ii refer to the pristine Fe(001)-p(1 × 1)O substrate and to the 5 ML Co(001)-p(1 × 1)O sample, respectively. Spectra iii and iv were acquired after exposure to 20 and 500 L of molecular oxygen, respectively. During these oxidation cycles the sample was kept at a temperature of 200 °C. Spectrum v has been acquired on the 500 L dosed sample (spectrum iv) after annealing it at 300 °C in UHV conditions. Spectrum vi is a reference spectrum of oxidized Fe, obtained by exposing the Fe(001) surface to 50 L of molecular oxygen. Continuous and dashed lines in panel b correspond to spectra acquired with the Auger electron beam at normal and grazing (77°) emission with respect to the sample surface, respectively.

modifications can be considered as a fingerprint of metal oxidation. Spectrum i is characteristic of Fe(001)-p(1 × 1)O, displaying a small shoulder at lower kinetic energies with respect to the main metallic Fe peak, readily associated with surface Fe−O bonds.56 After Co deposition, the Co MNN transition at 52 eV emerges, as evident from spectrum ii. Oxygen exposure induces the development of a second peak, located at lower energies with respect to the metallic one (spectra iii and iv). This peak, characterized by a maximum at about 43 eV, is associated with the development of Co oxide.57,58 In the spectra acquired at grazing emission, the oxide-related peak is more pronounced, suggesting that oxide regions cover the metallic layer. More importantly, up to a dose of 500 L, the spectra do not exhibit any feature that can be ascribed to Fe oxides (compare spectra iii and iv with the Fe oxide reference spectrum vi).59,60 This result is of particular importance for the interpretation of the STM data, since it indicates that the features observed on the surface layer are exclusively related to Co oxide. Notice that the phenomenology observed in the present case is different from that found in the C


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The Journal of Physical Chemistry C case of postoxidized Ni/Fe(001) films, in which case the oxygen exposure induces segregation of Fe from the substrate to the surface layer, where its oxidation takes place.60,61 However, spectrum v reveals that some Fe oxide is present on the surface after the UHV annealing treatment at 300 °C, indicating that a high temperature annealing would trigger the redox reaction at the oxide/metal interface. Figure 4 displays constant current STM images acquired after oxygen dosing. At the lowest oxygen exposure considered

evolution strongly resembles that observed in the case of oxide layers grown by metal deposition in oxygen atmosphere, as recently observed in the case of epitaxial RuO2(110) deposited on a TiO 2 (110) substrate. 62 STM images at higher magnification reveal the presence of atomic-scale features inside the oxide islands (Figure 4c and Figure 4e), measured as 90 pm deep depressions with respect to the oxide patches (Figure 4d). A quantitative analysis of STM images reveals that the dark spots cover about 20% of the atomic sites of the wetting layer. In order to obtain further information on the nature of these atomic-like defects, the autocorrelation function of their two-dimensional geometrical distribution has been evaluated, as displayed in Figure 4f. Sharp peaks corresponding to the atomic positions of a square lattice with 0.29 ± 0.015 nm periodicity are visible, indicating that the dark spots are in registry with the substrate. Moreover, neither nearest nor nextnearest neighbors are present in the autocorrelation function, pointing toward a repulsive interaction between the dark atomic-scale features. These considerations suggest that the atomic-scale depressions imaged by STM are defects in the Co oxide lattice, due to either cationic or anionic vacancies. Figure 5 displays the evolution of the surface morphology at higher oxygen doses. After exposure to 30 L of oxygen, the surface is completely covered by the wetting layer, while islands belonging to the second layer start to nucleate (Figure 5a). The plot in Figure 5c displays the layer completion for the first two oxide layers as a function of oxygen exposure. The growth rate of the second-layer islands is much slower compared to that of the first layer, as expected from the presence of the passivating oxide wetting layer. Figure 5b displays a detail of a second-layer island. The atomic-scale dark defects are well visible on the wetting layer, while they appear as faint dark spots on the second-layer islands. Further insights in their character are provided by the analysis of the oxide electronic properties. Electronic Properties. In order investigate the electronic properties of the oxide islands, constant current STM measurements in variable tunneling conditions were performed. The apparent height of the oxide islands with respect to the substrate has been found to be strongly dependent on the tunneling parameters. Figure 6a shows an oxide island imaged by switching the tunneling parameters between the forward and backward scan line. The oxide island is imaged as a 90 pm high protrusion with respect to the substrate for tunneling parameters V = 1.5 V and I = 10 pA, while it appears as a 103 pm deep depression at V = 0.6 V and I = 400 pA. We performed a series of constant-current images similar to those reported in Figure 6a by changing the tunneling current from 10 pA to 500 pA in steps of 50 pA and the applied voltage from −2 to 2 V in steps of 200 mV. The results are summarized in the diagram of Figure 6b, displaying the oxide apparent height with respect to the substrate as a function of the tunneling parameters. For both polarities of the applied sample voltage, a well-defined region exists (marked by the red dotted line in Figure 6b) around EF, in which oxide islands are imaged as depressions with respect to the substrate. We also performed measurements by keeping the tunneling current constant at a low value (10 pA) and varying the applied voltage from −3 to 3 V (Figure 6c). The oxide apparent height as a function of the applied voltage steadily increases, from zero up to about 190 pm (50 pm) for positive (negative) sample bias. In order to rationalize these experimental observations we recall that, in the applied experimental conditions, during the scan the feedback loop keeps the tunneling current constant by

Figure 4. (a, b) Constant-current STM images of the 5 ML Co(001)p(1 × 1)O film exposed to 2 L (a) and 7 L (b) of O2. Dark islands, referred to in the text as oxide islands, nucleate homogeneously over the atomically flat Co terrace (a), then coalesce into a percolated layer at increasing oxygen exposure (b). Images size is 200 × 200 nm2. Tunneling parameters are I = 1 nA, V = 1.1 V and I = 1 nA, V = 1 V for panels a and b, respectively. (c) Atomically resolved image of an oxide island, in which atomic-scale dark spots are visible over the oxide island. Tunneling parameters are I = 1 nA and V = 1 V. (d) Topographic height corresponding to the white line scan in panel c. (e) STM image of the completed oxide wetting layer. (f) Twodimensional autocorrelation function of the geometrical distribution of atomic-scale dark defects. The absence of first- and second-nearest neighbors is evident.

(2 L), Co terraces are covered by randomly distributed dark islands (Figure 4a), referred to as oxide islands in the following. Interestingly, the homogeneous nucleation of oxide islands has been recently observed in the oxidation of the Zr(0001) surface.38 In that case, the STM images were acquired at a stage of oxidation in which the substrate was fully covered by oxide; thus it was not possible to image the metal substrate between the oxide clusters. Upon increasing the oxygen exposure up to 7 L, the surface fraction covered by the oxide islands increases, leading to film percolation and the formation of a closed oxide wetting layer (Figure 4b and Figure 4e). This morphological D


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Figure 5. (a) Mesoscopic morphology resulting from the exposure of the Co(001)-p(1 × 1)O film to 30 L of molecular oxygen, in which islands nucleating on top of the oxide wetting layer are visible. Image size is 200 × 200 nm2. Tunneling parameters are I = 50 pA and V = 1 V. (b) Blow-up of a second-layer island, acquired at V = 1 V, I = 50 pA. The atomic-scale dark spots are well visible on the wetting layer, while they appear as faint features on second-layer islands. (c) Fraction of layer completion versus oxygen exposure for the first (black squares) and second (red circles) oxide layer.

Figure 6. (a) Constant current images acquired by switching the tunneling conditions between the forward and the backward scan. The oxide island protrudes 90 pm with respect to the substrate in the forward scan (tunneling parameters V = 1.5 V, I = 10 pA), while in the backward scan (tunneling parameters V = 0.6 V, I = 400 pA) the oxide island is imaged as a 103 pm depression with respect to the substrate. (b) Apparent height of oxide islands with respect to the substrate as a function of the tunneling parameters. The red dotted line marks the region where the oxide islands are imaged as depressions. (c) Measured apparent height of oxide islands at I = 10 pA and variable bias voltage.

As already discussed for heteroepitaxial ultrathin oxide films stabilized on metallic substrates, a strong variation of the relative height between the metallic substrate and an oxide island with the applied bias voltage can be considered as a fingerprint of the development of an electronic band gap in the oxide.64−70 In this frame, when the applied tip/sample voltage brings the tip EF above (below) the conduction (valence) band edge, the oxide electronic states are available for the tunneling process. On the other hand, when the applied voltage falls inside the oxide gap, a negligible density of states of the latter contributes to the tunneling process. In this case, the current is provided by electrons tunneling directly from the underlying metal substrate into the tip. According to this picture, the data of Figure 6b suggest that the Co oxide islands are partially embedded in the metallic substrate. As a matter of fact, at high bias the images reflect the surface topography, yielding a positive corrugation of the oxide islands. On the other hand, for

adjusting the tip/surface distance. Generally, the measured height differences between different regions of the surface originate from either sample topography or the variation of the local density of states. As a matter of fact, the tunneling current can be approximated as63 I=

∫E

E F + eV

ρs (E) ρt (E − eV ) T (z ,E ,V ) dE

F

(1)

with the barrier transmission coefficient ⎡ 2z ⎛ eV ⎞⎟ ⎤ ⎥ T (z ,E ,V ) = exp⎢ − 2m⎜ϕ − E + ⎝ 2 ⎠⎦ ⎣ ℏ

(2)

where ρt and ρs are the tip and sample density of states, respectively, ϕ is the average work function of tip and sample, z is the tip/sample separation, m is the electron mass, e is the electron charge, and ℏ is the reduced Planck constant. E


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emitter (i.e., the tip) is dominant, while in the latter case it is absent, since the density of states around the emitter EF (i.e., the oxide) is negligible, yielding a lower oxide apparent height at negative sample bias. According to the model of Figure 7, the asymmetry is expected to be stronger for oxide islands in which EF is close to the conduction band edge (n-doped oxide), while for p-doped oxides the effect should be weaker. Further insights into the electronic structure of the oxide islands come from spectroscopic measurements. In particular, Figure 8a displays the FER spectra acquired over a Co oxide

bias falling inside the oxide band gap, the tip follows the contour of the oxide/metal interface, placed below the Co(001)-p(1 × 1)O surface, resulting in depressed oxide islands. It should be noted that at very low tunneling currents, the oxide islands are always imaged as protrusions, although their apparent height is strongly reduced around EF. This fact can be explained considering that the oxide layer considerably lowers the work function of the Co(001)-p(1 × 1)O substrate (see below). As a consequence, the tunneling process is more efficient above the oxide islands, compensating the larger width of the tunneling barrier above the oxide islands. This effect is expected to be more pronounced at low tunneling currents, i.e., at larger absolute tip−surface separation, since the voltage drop across the insulating layer is negligible. On the other hand, at higher current, i.e., when the tip−surface separation is smaller, the lower vacuum level above the oxide islands is counterbalanced by the band bending induced by the electric field present between the sample and the tip. In order to keep the current constant, the tip has to move closer to the surface when it is placed above an oxide island. Another aspect to be discussed is the asymmetry in the oxide apparent height measured at positive and negative voltages. Such an effect can be explained by the asymmetry in the tunneling process for electrons flowing either from or toward the sample, respectively. As can be inferred from the expression of T(z,E,V), electrons possessing an energy closer to the emitter EF are those injected more efficiently in the tunnel junction. At positive sample bias, electrons tunnel from the occupied electronic states of the tip into the empty states of the oxide conduction band (Figure 7a), while at negative sample bias electron tunneling occurs from the occupied oxide valence band into the unoccupied states of the tip (Figure 7b). In the former case, the contribution of electrons around EF of the

Figure 8. (a) FER spectra acquired over Co oxide islands (red line) and over the Co(001)-p(1 × 1)O terrace (black line). The order n of each peak is indicated. (b) Energy position of the FER peaks as a function of n. (c) STS (scanning tunneling spectroscopy) spectra acquired at constant sample−tip distance. The set point before the feedback loop switch off was set to V = 2.8 V and I = 10 pA. The horizontal dashed line marks the tunneling current detection limit of the experimental setup. The minimum value of the detectable tunneling current has been determined as the root-mean-square of the current signal acquired with the tip out of tunneling contact.

island and over a Co(001)-p(1 × 1)O terrace. The FER spectra have been acquired by keeping the feedback loop closed (i.e., at constant tunneling current) and increasing the bias applied between tip and sample. When the tip−sample bias exceeds the sample work function, the electron transport across the tip− sample junction is dominated by the presence of discrete electron states confined in the vacuum region between the tip and the sample. These states are detected as strong peaks in dI/dV spectra, often referred to as Gundlach oscillations.71 Assuming a one-dimensional tunnel junction between tip and sample, the position of each FER peak En is given by72 Figure 7. Schematic band diagram of the tip/oxide/metal tunnel junction at positive (a) and negative (b) applied sample voltages. The length of the arrows schematically indicates the current intensity from the respective electronic states. Dashed arrows indicate the absence of tunneling current from the oxide layer due to the presence of the electronic gap. The voltage drop inside the oxide layer is neglected.

⎛ 3π ℏe ⎞2/3 2/3 2/3 ⎟ F n En = ϕ + ⎜ ⎝ 2 2m ⎠

(3)

where ϕ is the sample work function (when applying a positive bias to the sample) and F is the electric field between the tip F


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The Journal of Physical Chemistry C and the surface. Figure 8b displays the experimental En values as a function of n2/3, along with the best fitting line. According to eq 3, the local work function can be determined by considering the intersection between the line fitting the experimental data and the energy axis. The first peak is excluded from the fitting procedure in order to reduce image charge effects. The extrapolated ϕ values are 3.4 ± 0.1 eV and 4.3 ± 0.1 eV for the Co oxide island and the Co(001)-p(1 × 1)O surface, respectively. This result reveals a strong decrease of ϕ induced by oxide nucleation, in line with previous results about oxide layers supported on different metal substrates.73−75 Figure 8c shows the I(V) spectra acquired at constant tip/ sample distance. In this case, the feedback loop was switched off and the tunneling current I was acquired as a function of the applied voltage V, with the tip stabilized over either the Co(001)-p(1 × 1)O substrate (black line) or the oxide islands (red line). The tunneling set point before switching off the feedback was settled at high bias voltage (V = 2.8 V) and low tunneling current (I = 10 pA), in order to obtain a large tip/ sample separation during acquisition of the I(V) curves, which minimizes the contribution from the underlying metallic substrate in the spectrum of the oxide island. The spectrum acquired over oxide islands displays a region in which the tunneling current is constant at a negligible value. On the other hand, the current in the spectrum acquired on the Co(001)p(1 × 1)O substrate drops to zero exclusively in a small region around zero applied voltage.76 On the basis of these measurements, the valence and conduction band edges of the oxide are placed at −1.5 ± 0.2 eV and 0.8 ± 0.05 eV, respectively. The estimated band gap width of about 2.3 eV compares well with that reported for bulk CoO,77 indicating that already at this early stage the oxidic electronic structure is well developed. Moreover, the position of EF does not correspond to the center of the band gap, being closer to the conduction band minimum than to the valence band maximum. The asymmetric position of EF with respect to valence and conduction band edges, along with the strong difference between the apparent height for positive and negative voltages (see Figure 6) and the significant decrease of ϕ over the oxide islands, provides a hint about the nature of the atomic scale features imaged inside the oxide islands. These lattice defects are likely due to oxygen vacancies, which are well-known electron donors. In this picture, the oxide islands are n-doped, with EF closer to the conduction band. Correspondingly, the apparent height of oxide islands is higher when the electronic states of the conduction band are involved in the tunneling process. Additionally, due to the presence of defects, a charge flow is expected to occur from the oxide to the metal substrate.78,79 The electrons transferred from the oxide islands toward the metallic substrate would create an outward dipole directed normal to the surface. The presence of such a dipole can explain the lower value of the work function of the oxide islands compared to that of the Co(001)-p(1 × 1)O terraces.

vacancies. By combining tunneling-dependent topographic images and scanning tunneling spectroscopy, it was possible to describe the different electronic structure of the oxidic and the chemisorbed phases. Since the early stages of oxide nucleation, the CoO band gap is fully developed and a lowering of about 0.9 eV of the work function occurs. As a final remark, we mention that the smooth morphology and the well-defined surface chemistry of the CoO/Co/ Fe(001)-p(1 × 1)O structure can represent a model system for the study of the spin structure of complex antiferromagneticoxide/ferromagnetic-metal layered structures80 with atomic scale resolution.81

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the Italian Ministry of University and Research through the FIRB Project RBAP115AYN.

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ADDITIONAL NOTE One equivalent monolayer (1 ML) equals the amount of Co atoms required to completely saturate the adsorption sites on the Fe substrate, i.e., about 12.2 atoms/nm2. The nominal thickness of 1 ML of Co corresponds to about 140 pm. a

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CONCLUSIONS In summary, the early stages of oxidation of ultrathin Co films deposited on the Fe(001)-p(1 × 1)O surface have been investigated. Atomically resolved STM images show that the oxidation process follows a homogeneous nucleation, with islands developing over atomically flat terraces and coalescing into a smooth wetting layer. Such a wetting layer is found to be highly defective, most probably due to the presence of oxygen G


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Atomic Scale Insights into the Early Stages of Metal Oxidation: A

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