Template Assisted Nucleation of Cobalt and Gold Nano-clusters on an Ultrathin Iron Oxide Film
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Being the basic building blocks for nano-magnetic and nano-catalytic devices, regular arrays of nano-clusters play a crucial role in modern nanotechnology. One of the possible fabrication methods of periodic nanostructures consists in exploiting nano-patterned substrates as templates for the self-assembly of the deposited atoms. Here, we have investigated the templating properties of a Moiré superlattice formed at the interface between a FeO(111)-like ultrathin film and a Ni/Fe(001) substrate. Co and Au, representative of elements with high and low oxygen affinity, respectively, have been deposited on the iron oxide film. Scanning tunneling microscopy reveals that Co nucleates preferentially along the corrugated regions of the Moiré superstructure, forming stripes with high aspect ratio. On the other hand, Au atoms nucleate randomly distributed three-dimensional islands on the FeO(111) surface.
KeywordsOxide Self-assembly Scanning tunneling microscopy Diffusion Moiré
The investigation of the chemical and physical properties of nano-sized clusters is a crucial issue of modern nanotechnology, since small particles are the core components of a number of nano-catalytic and nano-electronics devices [1, 2, 3, 4, 5]. Apart from being important for the optimization of the device performances, such field is interesting also from a fundamental point of view. When the size of the clusters shrinks below a certain value, the extreme confinement of the electrons can induce different properties from those of bulk materials. For instance, Au clusters with diameters smaller than 6 nm are active in the oxidation of CO, while bulk gold is chemically inert . Moreover, due to finite size effects, small clusters composed by ferromagnetic metals can become superparamagnetic , or elements that are non-magnetic in the bulk can display magnetism .
Among the systems composed by nano-clusters, those in which the particles form monodisperse ensembles (i.e. with a narrow size distribution) and arrange in periodic arrays deserve particular consideration, since:
The homogeneous size distribution and the well-defined geometrical relationship with the substrate make the experimental investigation by means of spatial averaging techniques feasible.
The interaction between regularly-spaced clusters can give rise to collective properties, which are not merely the sum of those of the single particles .
The most used bottom-up strategy for obtaining regular arrays of clusters relies on the self-assembly of atoms deposited on nanostructured metallic , graphitic  or oxidic  substrates. Narrowing the discussion to oxide substrates, Al2O3 , FeO(111)  and TiOx  ultrathin films, just to cite a few, have been used to drive nanoparticles nucleation.
Here, we use a nanostructured FeO(111)-like [in the following simply addressed to as FeO(111)] film as a template for the growth of submonolayer films of Co and Au, representative of metals with high and low oxygen affinity, respectively. The FeO(111) film, obtained after the oxidation of a Ni/Fe(001) bilayer, is characterized by two mutually orthogonal arrays of surface undulations, arising from the superposition between the square and hexagonal lattices of the metallic substrate and of the oxide overlayer, respectively [16, 17]. We observe that Co nucleation is driven by such a mesoscopic Moiré pattern, while the templating effect is not efficient in the case of Au, which nucleates in randomly distributed clusters.
2 Experimental details
The experiments were performed in an ultrahigh vacuum (UHV) system (base pressure 1 × 10− 10 mbar) composed by two chambers. The FeO(111) film has been obtained after the following preparation steps :
A 500 nm thick Fe(001) film was grown on a MgO(001) substrate.
A 3 ML thick metastable body-centered-cubic Ni film was grown on the Fe(001) surface .
The Ni/Fe(001) sample was treated with two cycles of oxidation at room temperature (RT), followed by annealing at 200 °C for 5 min in UHV. During the first cycle the oxygen dose was 50 L (1 L = 10− 6 Torr s), while during the second it was 100 L. The oxygen exposure at RT induces the segregation of Fe atoms from the bulk to the surface, where their oxidation occurs.
The sample was then annealed in UHV at 300 °C for 5 min in order to improve the surface order. After this step, the Ni film is completely buried underneath the iron oxide layer. Based on Auger electron spectroscopy results (not shown), we estimate a thickness of about 3–4 ML for the FeO(111) film.
Co and Au films were evaporated by means of molecular beam epitaxy, with the substrate kept at RT during the growth. The deposition rate was calibrated by means of a quartz microbalance. The coverages, expressed in equivalent monolayers (ML), are referred to the atomic density of the surface where the films are deposited [i.e. about 1.2 × 1015 atoms/cm2 for the Fe(001) surface and about 1.7 × 1015 atoms/cm2 for the FeO(111) surface]. Low-energy electron diffraction (LEED) measurements were performed by means of an Omicron SPECTALEED. Scanning tunneling microscopy (STM) experiments have been performed with an Omicron variable temperature instrument. STM images have been acquired at RT in the constant-current mode with home-made electrochemically-etched W tips.
3 Results and Discussion
Figure 1 shows the main crystallographic and morphological features of the FeO(111) surface. The LEED pattern of Fig. 1a reveals the presence of two hexagonal lattices, with one side of the hexagonal unit cell aligned along either the  or the  direction of the Fe(001) substrate. The analysis of the LEED pattern reported in reference  indicates that the surface unit mesh corresponds to a slightly distorted hexagon with basis vectors (3.06 ± 0.07) Å and (3.22 ± 0.11) Å long. It is worth to remark that rock-salt monoxide surfaces oriented along the (111) direction are polar, i.e. alternating layers of oppositely charged ions induce a divergent electric dipole moment perpendicular to the surface. For this reason, bulk-terminated polar surfaces are not stable. Generally, (111) oriented surfaces of transition metal mono-oxides undergo a octopolar p(2 × 2) reconstruction in order to compensate the electrostatic instability . In our case the comparison of the structural parameters of the ultrathin Fe oxide film with those of a nominal bulk-terminated FeO(111) surface (aFeO = 3.04 Å) allows us to exclude that the surface is reconstructed. However, we would like to stress that ultrathin oxide films have been suggested to be able to sustain uncompensated polarity , as recently experimentally demonstrated by Gurgul et al., who observed stable unreconstructed FeO(111) films as thick as 16 ML .
As expected from the fourfold symmetry of the Fe(001) substrate, the two equivalent domains of the FeO(111) film are rotated by 90° with respect to each other. For each domain, the diffraction spots are split along one of the main crystallographic directions of the Fe(001) surface, while in the corresponding orthogonal direction the spots are elongated. Such an observation suggests that two orthogonal nanometric periodic superstructures cover the surface. The large scale STM image of Fig. 1b reveals that the surface is atomically flat, with only few three-dimensional clusters, which are remnant of the surface roughening induced by the oxidation of the Ni film during the FeO(111) preparation (see experimental details). As suggested by the LEED pattern, the surface topography is characterized by a periodic superstructure, formed by bright wavy lines running along the  and  crystallographic directions of the Fe(001) substrate. Figure 1c shows a magnification of the FeO(111) surface, in which two periodic patterns modulate the surface topography. The first one, visible also on the large scale image, is characterized by a corrugation of about 60 pm and a periodicity of about 8 nm, as inferred from line profile of Fig. 1d, corresponding to the black continuous line drawn in Fig. 1c. The second type of surface undulation, orthogonal with respect to the first one, is characterized by a shorter periodicity and a lower corrugation of about 2.36 nm and 20 pm, respectively, as inferred from the topographic profile measured along the broken red lines in Fig. 1c and d. In a previous publication, the formation of this Moiré superstructure has been ascribed to the shift of the atomic registry at the interface between the hexagonal and square lattices of the FeO(111) film and of the Fe(001) substrate, respectively .
Figure 2 displays the surface topography after the deposition of about 0.6 ML of Co on the FeO(111) surface. Co atoms form stripes running along the  and  directions of the Fe(001) substrate, while the regions between the stripes are almost uncovered. Figure 2b displays the Fast Fourier Transform of the STM image, from which it is possible to infer a periodicity of about 8.3 nm for the Co nanostructures [see the line profile of Fig. 2c]. The spacing between the Co stripes is close to the long periodicity measured on the pristine FeO(111) surface, suggesting a preferential Co nucleation on the highly corrugated regions of the iron oxide surface.
Figure 3(a) displays a closer view of the Co covered surface. In this image, the low corrugation periodicity is visible and does not show any sizable Co nucleation, while the high corrugation lines are completely covered by Co. The apparent height of the Co stripes is about 160 pm, as displayed in the line scan of Fig. 3b. It is well known that the tunneling process is strongly influenced by the surface electronic properties [22, 23], therefore it is not possible to obtain an accurate estimation of the topographic height from STM measurements. However, considering that the interlayer spacing of metallic Co and Co oxide ranges from 200 pm (for hexagonal-close-packed cobalt along the c axis) to 246 pm (for CoO along the  direction), it is reasonable to assume that the apparent height of the Co stripes corresponds to one atomic layer.
Figure 4 focuses on the surface topography acquired after the deposition of an increasing amount of Au on the FeO(111) surface. At a coverage lower than 0.05 ML, small Au clusters are present on the surface, preferentially nucleating at the step edges and defect sites, as shown in Fig. 4a. Increasing the coverage up to 1.4 ML leads to an increase of the nuclei density, reaching the value of 0.016 nm− 2 [see Fig. 4b]. Unlike Co, Au grows in a Volmer–Weber mode, as testified by the large apparent height of about 1.4 nm of the Au clusters [see Fig. 4c]. Further Au deposition up to a coverage of 2.6 ML mainly induces an increase of the islands density, up to 0.035 nm− 2 in Fig. 4d, but no long range order can be recognized in their geometrical distribution, indicating that the Moiré pattern of FeO(111) does not influence their nucleation.
In order to rationalize the different templating effect of the FeO(111) surface for the nucleation of Co and Au, it is worth recalling the main physical mechanisms which have been proposed as driving forces for the self-organization of metals on solid surfaces:
Repulsive interaction between diffusing atoms and misfit dislocations, as observed in the case of Fe islands grown on the dislocations pattern formed by a bilayer of Cu deposited on Pt(111) .
Preferential nucleation at point or line defects, as shown in the case of Pd, Rh, Co, Ir deposited on Al2O3 .
Displacement of atomic species from the substrate, as suggested for the preferential nucleation of Fe, Co and Ni at the elbows of the Au(111) herringbone reconstruction .
Heterogeneity of the surface potential and repulsive interaction between charged species, which have been proposed as driving forces for the self-organization of Au atoms on the Moiré superstructure formed by 1 ML of FeO(111) on Pt(111) .
Variation of the lattice mismatch along the Moiré unit cell, as suggested for the growth of periodically arranged Fe and Cr islands on the coincidence lattice formed at the MgO/Mo(001) interface .
In our case, Co nucleates following the Moiré pattern of the FeO(111) surface, therefore we can exclude the mechanisms involving surface defects. Also the displacement of atomic species from the substrate is unlikely, because this mechanism would promote the formation of point nucleation centers and not the stabilization of elongated structures. Finally, the mechanisms involving the charge transfer from the substrate to the adatoms are expected to be more efficient at low temperatures. Since the deposition was performed at RT, in a first approximation we can neglect the charge transfer process.
The different behavior observed for Co and Au suggests that the templating effect is related to the chemical interaction between the iron oxide substrate and the adlayers. Indeed, it is likely that Co atoms deposited on the FeO(111) film react with the topmost layer of oxygen by forming a Co oxide film. Previous investigations on Co films deposited on the oxygen passivated Fe(001)-p(1 × 1)O surface revealed that oxygen floats on top of the growing film, forming a CoO(100) single layer  We suggest that a similar mechanism is effective after Co deposition on the FeO(111) surface, resulting in the stabilization of one-layer-thick stripes of CoO(111). Such an interpretation is reinforced by considering that the mismatch between metallic cobalt (lattice constant 250 pm) and the FeO(111) surface would be about 19%, while the lattice constants of the isostructural FeO and CoO bulk compounds are very similar. Following this interpretation, the formation of CoO(111) seems to be energetically favored in the highly corrugated regions of the Moiré pattern. Such a preferential nucleation could be due to a better lattice matching and/or to an energetically more favored substitution of Fe-O bonds with Co-O bonds in the regions where the pronounced surface buckling increases the Fe-O distance. Due to the low oxygen affinity of Au, which does not form any stable oxidic compound, the discussed mechanism is not effective, therefore the nucleation of Au is not influenced by the FeO(111) Moiré.
The FeO(111) ultrathin film formed by the oxidation of Ni/Fe(001) bilayers has been used as a template for the growth of Co and Au films. In the former case, Co nucleates preferentially along the highly corrugated regions of the Moiré pattern characterizing the FeO(111) surface, forming regularly spaced stripes with a high aspect ratio. On the other hand, Au forms three-dimensional islands that are evenly distributed on the surface, indicating that the templating effect of the FeO(111) film is not effective in the latter case. Following our experimental results and the analysis of the literature, we suggest that the driving force inducing the self-organization of Co is the preferential nucleation of CoO on the highly corrugated regions of the FeO(111) surface.
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