Morphology and defect structure of the CeO2(1 1 1) films grown on Ru(0 0 0 1) as studied by scanning tunneling microscopy

The morphology of ceria films grown on a Ru(0 0 0 1) substrate was studied by scanning tunneling microscopy in combination with low-energy electron diffraction and Auger electron spectroscopy. The preparation conditions were determined for the growth of nm-thick, well-ordered CeO₂(1 1 1) films covering the entire surface. The recipe has been adopted from the one suggested by Mullins et al. [D.R. Mullins, P.V. Radulovic, S.H. Overbury, Surf. Sci. 429 (1999) 186] and modified in that significantly higher oxidation temperatures are required to form atomically flat terraces, up to 500 Å in width, with a low density of the point defects assigned to oxygen vacancies. The terraces often consist of several rotational domains. A circular shape of terraces suggest a large variety of undercoordinated sites at the step edges which preferentially nucleate gold particles deposited onto these films. The results show that reactivity studies over ceria and metal/ceria surfaces should be complemented with STM studies, which provide direct information on the film morphology and surface defects, which are usually considered as active sites for catalysis over ceria.     

Si nucleation on Si(1 1 1)-7 × 7: From cluster pairs to 2D islands

Nucleation of 2D islands in Si/Si(1 1 1)-7 × 7 molecular beam epitaxy is studied using scanning tunneling microscopy (STM). A detailed analysis of the population of small amorphous clusters coexisting on the surface with epitaxial 2D islands has been performed. It is shown that small clusters tend to form pairs. The pairs serve as precursors for 2D islands as confirmed by direct STM observations of the smallest 2D islands covering two adjacent half-unit cells of the 7 × 7 reconstruction. It is proved with scaling arguments that the critical nucleus for 2D island formation consists not only of the pair itself, but also includes additional adatoms not belonging to the stable clusters.     

Growth of Ce on Rh(1 1 1)

We have studied the growth of cerium films on Rh(1 1 1) using STM (scanning tunneling microscopy), LEED (low energy electron diffraction), XPS (X-ray photoelectron spectroscopy) and AES (Auger electron spectroscopy). Measurements of the Ce films after room temperature deposition showed that Ce is initially forming nanoclusters in the low coverage regime. These clusters consist of 12 Ce atoms and have the shape of pinwheels. At a coverage of 0.25 ML (monolayer, ML) an adatom layer with a (2 × 2) superstructure is observed. Above 0.4 ML, Rh is diffusing through pinholes into the film, forming an unstructured mixed layer. Annealing at 250 °C leads to the formation of ordered Ce-Rh compounds based on the bulk compound CeRh₃. At a coverage of 0.1 ML, small ordered (2 × 2) surface alloy domains are observed. The exchanged Rh atoms form additional alloy islands situated on the pure Rh(1 1 1) surface, showing the same (2 × 2) superstructure as the surface alloy. At a coverage of 0.25 ML, the surface is completely covered by the surface alloy and alloy islands. The (2 × 2) structure is equivalent to a (1 1 1)-plane of CeRh₃, contracted by 6%. Annealing a 1 ML thick Ce layer leads to a flat surface consisting of different rotational domains of CeRh₃(1 0 0). The Rh needed for alloy formation comes from 50 Å deep pits in the substrate. Finally we show that LEIS (low energy ion scattering) is not suitable for the characterization of Ce and CeRh films due to strong effects of neutralization.     

STM study of acetylene reaction with Si(1 1 1): observation of a carbon-induced Si(1 1 1)√3×√3R30° reconstruction

The first stages of acetylene reaction with the Si(1 1 1)7 × 7 reconstructed surface kept at 600 °C are studied by recording scanning tunneling microscopy (STM) images during substrate exposure at a C₂H₂ pressure of 2 × 10⁻⁴ Pa (2 × 10⁻² mbar). We observed the progressive substitution of the 7 × 7 reconstruction with a carbon induced Si(1 1 1)√3×√3R30° reconstruction characterized by an atomic distance of 0.75 ± 0.02 nm, very close to that of the silicon 7 × 7 adatoms. This means that a carbon enrichment of the silicon outermost layers occurs giving rise to the formation of a Si-C phase different from the √3×√3R30° reconstruction typical of Si terminated hexagonal SiC(0 0 0 1) surface with an atomic distance of 0.53 nm. To explain STM images, we propose a reconstruction model which involves carbon atoms in T₄ and/or S₅ sites, as occurring for B doped Si(1 1 1) surface. Step edges and areas around the silicon surface defects are the first regions involved in the reaction process, which spreads from the upper part of the step edges throughout the terraces. Step edges therefore, progressively flakes and this mechanism leads, for the highest exposures, to the formation of large inlets which makes completely irregular the straight edge typical of the Si(1 1 1)7 × 7 terraces. These observations indicate that there occurs an atomic diffusion like that driving the meandering effect. Finally, the formation of a few crystallites is shown also at the lowest acetylene exposures. This is the first STM experiment showing the possibility to have carbon incorporation in a Si(1 1 1) matrix for higher amounts than expected, at least up to 1/6 of silicon atomic layer.     

Intermixing at the interface of the system Nb/Fe(1 1 0) investigated by STM

The growth of niobium on the Fe(1 1 0) surface at a deposition temperature between room temperature (RT) and 680 K was studied using in situ STM and LEED. At RT we observe no indication of intermixing. Although a final roughness of only 1.7 Å is reached, the crystalline quality is low. At elevated growth temperatures the development of a surface alloy was observed, whose formation is ascribed to an exchange mechanism through which Nb adatoms are incorporated into the Fe surface. These Nb atoms arrange themselves in chains along the [0 0 1] direction. The expelled Fe atoms form islands on the Nb/Fe-alloy substrate. At higher coverage additionally a Nb wetting layer and intermixed 3D islands evolve.     

STM and LEED observations of a c(2 × 2) Ge overlayer on Pt(1 0 0)

We have investigated surface structures formed by deposition of 0.2 and 0.5-ML Ge on Pt(1 0 0) by using scanning tunneling microscopy (STM) and low electron energy diffraction (LEED). In addition, their temperature dependence and reactivity to CO have been studied. We observed the formation of disordered domains for Ge adatom coverages below 0.25-ML and complete c(2 × 2) structures at 0.25 to 0.5-ML Ge after annealing at 600-1200 K. Deposition of 0.2-ML Ge on a clean, hexagonally reconstructed (5 × 20)-Pt(1 0 0) substrate at 400 K lifts the reconstruction and ejects excess Pt atoms from the first layer into the adlayer. After annealing this surface to 600 K, the deposited Ge formed Ge adatoms on flat terraces and on round Pt adislands with incomplete c(2 × 2) structures, in addition to the presence of clean (1 × 1)-Pt(1 0 0) domains that were several nanometers across. Some domains of the unreconstructed (5 × 20)-Pt(1 0 0) surface still remained. After the deposition of 0.5-ML Ge and annealing at 600 K, disordered Ge domains disappeared and a c(2 × 2) Ge overlayer was produced all over the surface. Square terraces with square domains of the clean (1 × 1)-Pt(1 0 0) surface extended for nanometers. Annealing this surface to 900 K produced disordered Ge domains, and this was associated with an increase in Ge vacancies. When surfaces with 0.2-ML Ge were heated to 900 or 1200 K, or when a surface with 0.5-ML Ge was heated to 1200 K, larger domains of (5 × 20)-Pt(1 0 0) were formed with the agglomeration of disordered Ge adatoms. Pt clusters were observed in the Ge domains, and we consider these to be composed of those excess Pt atoms formed by lifting the reconstruction of the (5 × 20)-Pt(1 0 0) surface upon Ge agglomeration during cooling. A paper published elsewhere [T. Matsumoto, C. Ho, M. Batzill, B.E. Koel, Physical Review B, submitted for publication.] describes Na⁺-ion scattering spectroscopy (Na⁺-ISS) and X-ray photoelectron diffraction (XPD) experiments that distinguish between Ge present in an overlayer from incorporation into the top Pt layer to form a surface alloy for the surface structures reported here. Furthermore, these investigations revealed that disordered Ge adatoms observed herein might be associated with incomplete c(2 × 2) structures. Therefore, our observations of the formation of complete and incomplete domains of c(2 × 2) Ge adatoms indicate that interactions between Ge adatoms are repulsive at nearest neighbor distances and attractive at second-nearest neighbor distances. Regarding the reactivity of these surfaces, CO does not chemisorb on a Pt(1 0 0) surface with a c(2 × 2)-Ge overlayer and no measurable CO uptake was observed under UHV conditions at 220 K.     

Ultrasmall Ge islands with low diameter-to-height aspect ratio on Si(1 0 0)-(2 × 1) surfaces

Scanning tunneling microscopy (STM) and high resolution cross-sectional transmission electron microscopy (XTEM) studies have been used to investigate the formation of Ge nanocrystals grown on Si(1 0 0)-(2 × 1) surfaces by molecular beam epitaxy (MBE). We observe relatively high density of Ge islands where small ‘pyramids’, small ‘domes’ and facetted ‘domes’ of various sizes co-exist in the film. As revealed from XTEM images, a large fraction of islands, especially dome-shaped Ge islands have been found to have an aspect ratio of ∼1 (diameter):1 (height). Observation of truncated-sphere-shaped Ge islands with a narrow neck contact with the wetting layer is reported.     

A route to continuous ultra-thin cerium oxide films on Cu(1 1 1)

The growth and morphology of ultra-thin CeO₂(1 1 1) films on a Cu(1 1 1) substrate were investigated by means of low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). The films were grown by physical vapor deposition of cerium in an oxygen atmosphere at different sample temperatures. The preparation procedure is based on a modification of a previous method suggested by Matolin and co-workers [1], involving growth at elevated temperature (520 K). Here, LEED shows good long range ordering with a “(1.5 × 1.5)” superstructure, but STM reveals a three-dimensional growth mode (Vollmer-Weber) with formation of a closed film only at larger thickness. Using a kinetically limited growth process by reactive deposition at low sample temperatures (100 K) and subsequent annealing, we show that closed layers of ceria with atomically flat terraces can be prepared even in the regime of ultra-thin films (1.5 ML). Closed and atomically flat ceria films of larger thickness (3 ML) are obtained by applying a multistep preparation procedure, in which successive ceria layers are homoepitaxially grown on this initial film. The resulting overlayers show strong similarities with the morphology of CeO₂(1 1 1) single crystal surfaces, suggesting the possibility to model bulk ceria by thin film systems.     

The effect of embedded Pb on Cu diffusion on Pb/Cu(1 1 1) surface alloys

We have used scanning tunneling microscopy and low-energy electron microscopy to measure the thermal decay of two-dimensional Cu, Pb-overlayer, and Pb-Cu alloy islands on Pb-Cu(1 1 1) surface alloys. Decay rates covering 6-7 orders of magnitude are accessible by applying the two techniques to the same system. We find that Cu adatom diffusion across the surface alloy is rate-limiting for the decay of both Pb and Pb-Cu islands on the surface alloy and that this rate decreases monotonically with increasing Pb concentration in the alloy. The decrease is attributed to repulsive interactions between Cu adatoms and embedded Pb atoms in the surface alloy. The measured temperature dependences of island decay rates are consistent with first-principles calculations of the Cu binding and diffusion energies related to this “site-blocking” effect.     

Scanning tunneling microscopy study on initial stage of atomic hydrogen adsorption on the Si(1 1 1) 7 × 7 surface

Initial hydrogen adsorption on the Si(1 1 1) 7 × 7 surface was studied by scanning tunneling microscopy (STM) in an ultrahigh vacuum. Room temperature adsorbed hydrogen on the adatom in the 7 × 7 reconstruction led to depression of adatoms in the STM images. The hydrogen uptake curve at the adatom site as a function of hydrogen exposure time was well represented by Langmuir adsorption. No preferential adsorption was seen among four inequivalent adatoms in the 7 × 7 reconstruction. Adsorption of the adjacent center and corner adatoms respectively showed ∼10% higher adsorption. Even though the number of reacted adatoms in the half unit of the 7 × 7 reconstruction was statistically random, the number of reacted adatoms in the nearest neighbor half unit was enhanced as the number of reacted sites increased in the half unit.     

Scanning tunneling microscopy and spectroscopy investigations of copper phthalocyanine adsorbed on Al2O3/Ni3Al(1 1 1)

Low temperature scanning tunneling microscopy (LT-STM) and scanning tunneling spectroscopy (STS) have been used to investigate adsorbed copper phthalocyanine (C₃₂H₁₆N₈Cu) molecules on an ordered ultrathin Al₂O₃ film on the Ni₃Al(1 1 1) surface as a function of coverage and annealing temperature. For sub-monolayer coverage and a deposition temperature of 140 K two different planar molecular adsorption configurations rotated by 30° with respect to each other were observed with submolecular resolution in the STM images. The template effect of the underlying oxide film on the CuPc orientation, however, is only weak and negligible at higher coverages. For θ_CuPc ≈ 1 ML, before completion of the first layer, the growth of a second layer was already observed. The measured spacing of 3.5 Å between first and second layer corresponds to the distance between the layers in the α-modification of crystalline CuPc. The molecules deposited at 140 K are thermally stable upon prolonged annealing to temperatures up to 250 K. By the use of STS the lowest unoccupied molecular orbital (LUMO) of the adsorbed copper phthalocyanine molecules has been identified at an energy of 1.2 eV above E_F. The lateral distribution of the electronic states of the CuPc has been analyzed and mapped by STS.     

Dynamics of copper atoms on Si(1 1 1)-7 × 7 surfaces

This study investigated the dynamics of copper atoms adsorbed on Si(1 1 1)-7 × 7 surfaces between 300 K and 623 K using a variable-temperature scanning tunneling microscope (STM). The diffusion behavior of copper clusters containing up to ∼6 atoms into a particular half unit cell of the 7 × 7 reconstructed Si(1 1 1) surface was considered. The movements and the formation of copper clusters were tracked in detail. The activation energies and pre-exponential factors for various diffusion paths were estimated. Finally, the Cu-etching-Si process and the quasi-5 × 5 incommensurated phase of Cu/Si islands were discussed.     

Scanning tunneling microscopy study on a BC3 covered NbB2(0 0 0 1) surface

We have investigated a BC₃ covered NbB₂(0 0 0 1) surface using scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and low energy electron diffraction (LEED). The STM images reveal characteristic features of a Moiré pattern reflecting an incommensurate relation of the BC₃ sheet with the substrate: bright protrusions with the periodicity of the substrate lattice are modulated in intensity with the periodicity of the BC₃ lattice. As a result, the surface exhibits nm-scale patchy regions with either the √3 × √3 or the 1 × 1 structure of the substrate. The two-dimensional Fourier transformation pattern of the STM image is consistent with the LEED pattern proving the epitaxial and incommensurate relationship between BC₃ surface sheet and substrate. No feature of a predicted superconducting gap was found in STS spectra measured at 5 K.     

Epitaxial growth of ultra-thin NiO films on Cu(1 1 1)

The very first stages of the growth of NiO on Cu(1 1 1) is examined on a microscopic scale. The paper focuses on the morphological and structural characterization of nanostructures formed in the 0-1 Å thickness range. Ultra-thin NiO films, obtained through evaporation of a Ni rod under an oxygen atmosphere were grown at 550 K. In the early stages of the growth the oxide film morphology shows 10-30 nm large, monolayer high, islands with a partial incorporation of metallic Ni in the first Cu(1 1 1) surface plane. The first layer is formed by an epitaxial atomic layer exhibiting a STM contrast similar to the one observed on adsorbed oxygen on Cu(1 1 0). A NiO cluster nucleation and coalescence mechanism is proposed in order to explain the formation of the second NiO layer. A α-Ni₂O₃ hexagonal phase, or a structural distortion of the NiO(1 1 1)()R30° structure could both explain the complex LEED patterns.     

Reactivity to sulphur of clean and pre-oxidised Cu(1 1 1) surfaces

The reactivity of clean and pre-oxidised Cu(1 1 1) surfaces exposed to sulphur (H₂S) has been studied at room temperature by Auger electron spectroscopy, low energy electron diffraction and scanning tunneling microscopy. On the clean surface, the sulphur-saturated surface structure is dominated by the or so-called “zigzag” superstructure. It is shown that a single orientation domain is favoured by the slight misorientation (∼2°) of the surface with respect to the (1 1 1) plane. Scanning tunneling microscopy measurements also revealed two minority structures. Pre-oxidation was performed by exposure to 1.5 × 10⁴ L of O₂ at 300 °C. Under exposure to H₂S (1 × 10⁻⁷ mbar) at room temperature, the oxygen is totally substituted by sulphur. Once initiated, sulphur adsorption seems to propagate to cover the whole surface on the O-covered surface faster than on the clean Cu(1 1 1). At saturation by adsorbed sulphur, the surface is completely covered by the superstructure of highest coverage. This enhanced uptake of sulphur is assigned to the surface reconstruction of the copper surface induced by the pre-oxidation, causing a stronger reactivity of the Cu atoms released by the decomposition of the oxide.     

Mixed layer formation of copper overlayers on Ni(1 1 0) surface

Copper overlayer formation on the Ni(1 1 0) surface was studied by scanning tunneling microscopy (STM) in an ultrahigh vacuum. Atom-resolved STM images showed that initially deposited Cu is replaced with surface Ni atoms forming atom-size depressions on the Ni(1 1 0) terraces and a Ni-rich quasi-one-dimensional island along the direction. Further Cu deposition yields a mosaic structure on the islands, indicating Cu/Ni mixed layer formation. From the quantitative measurement of the Cu/Ni ratio on the substrate and the islands, impinging Cu will be replaced with surface Ni whereas expelled Ni and directly impinging Cu to the island form the mixed island. The number of Cu atoms in the islands, however, more than the directly impinging Cu, indicate significant Cu/Ni replacement at the periphery of the island.     

Signatures of epitaxial graphene grown on Si-terminated 6H-SiC (0 0 0 1)

Epitaxial graphene layers thermally grown on Si-terminated 6H-SiC (0 0 0 1) have been probed using Auger electron spectroscopy, Raman microspectroscopy, and scanning tunneling microscopy (STM). The average multilayer graphene thickness is determined by attenuation of the Si (L₂₃VV) and C (KVV) Auger electron signals. Systematic changes in the Raman spectra are observed as the film thickness increases from one to three layers. The most striking observation is a large increase in the intensity of the Raman 2D-band (overtone of the D-band and also known as the G′-band) for samples with a mean thickness of more than ∼1.5 graphene layers. Correlating this information with STM images, we show that the first graphene layer imaged by STM produces very little 2D intensity, but the second imaged layer shows a single-Lorentzian 2D peak near 2750 cm⁻¹, similar to spectra acquired from single-layer micromechanically cleaved graphene (CG). The 4-10 cm⁻¹ higher frequency shift of the G peak relative to CG can be associated with charge exchange with the underlying SiC substrate and the formation of finite size domains of graphene. The much greater (41-50 cm⁻¹) blue shift observed for the 2D-band may be correlated with these domains and compressive strain.     

Morphology and electronic structure of bcc Co(1 1 0) and fcc/hcp Co(1 1 1) on Fe(1 1 0) investigated by STM and STS

We report on the growth of ultrathin epitaxial Co films on Fe(1 1 0) examined by scanning tunneling microscopy and spectroscopy (STM and STS). At room temperature Co forms pseudomorphic, ideally ordered body-centered cubic (bcc) layers for the first two monolayers as confirmed by atomically resolved STM images. This is in contrast to the related case of Co/Cr(1 1 0) where a superstructure occurs in the second layer. The third monolayer forms a close-packed structure and causes a transformation of the buried second monolayer into a close-packed structure. The Fe(1 1 0) substrate strongly influences the electronic structure of the first Co monolayer as concluded from the dI/dU spectra. This influence is less important for the second monolayer. The measured local density-of-states function for the bcc Co double layer is in agreement with theoretical predictions for bcc Co.     

Interpretation of initial stage of 3C-SiC growth on Si(1 0 0) using dimethylsilane

The initial stage of cubic silicon carbide (3C-SiC) growth on a Si(0 0 1) surface using dimethylsilane (DMS) as a source gas was observed using scanning tunneling microscopy (STM) and reflection high-energy electron diffraction (RHEED). It was found that the dimer vacancies initially existing on the Si(0 0 1)-(2 × 1) surface were repaired by the Si atoms in DMS molecules, during the formation of the c(4 × 4) surface. From the STM measurement, nucleation of SiC was found to start when the Si surface was covered with the c(4 × 4) structure but before the appearance of SiC spots in the RHEED pattern. The growth mechanism of SiC islands was also discussed based on the results of RHEED, STM and temperature-programmed desorption (TPD).