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Metal-oxide nanostructures play a fundamental role in a large number of technological applications, ranging from chemical sensors to data storage devices. As the size of the devices shrinks down to the nanoscale, it is mandatory to obtain sharp and good quality interfaces. Here, it is shown that a two-dimensional material, namely, graphene, can be exploited as an ideal buffer layer to tailor the properties of the interface between a metallic substrate and an ultrathin oxide. This is proven at the interface between an ultrathin film of the magnetoelectric antiferromagnetic oxide Cr2O3 and a Ni(111) single crystal substrate. The chemical composition of the samples has been studied by means of X-ray photoemission spectroscopy, showing that the insertion of graphene, which remains buried at the interface, is able to prevent the oxidation of the substrate. This protective action leads to an ordered and layer-by-layer growth, as revealed by scanning tunneling microscopy data. The structural analysis performed by low-energy electron diffraction indicates that the oxide layer grown on graphene experiences a significant compressive strain, which strongly influences the surface electronic structure observed by scanning tunneling spectroscopy.

Carbon forms (graphite, pyrolytic graphite, highly oriented pyrolytic graphite (HOPG), glassy carbon, carbon foam, graphene, buckypaper, etc) are a wide class of materials largely used in technology and energy storage. The huge request of carbon compounds with reliable and tunable physical and chemical properties is tackled by contriving new production protocols and/or compound functionalizations. To achieve these goals, new samples must be tested in a trial-and-error strategy with techniques that provide information in terms of both specimen quality and properties. In this work, we prove that electrochemical scanning probe techniques allow testing the performances of carbon compounds when are used as an electrode inside an electrochemical cell. Comparing the results with a reference sample (namely, HOPG) gives an insight on defects in the specimen structure, performances, and possible applications of the new samples. In this study, we concentrate on traditional carbon forms already employed in many fields versus new recently-developed specimens, in view of possible applications to the field of energy storage.

We give evidence of the formation of an ordered array of tetra-phenyl porphyrins (TPP) when these molecules are deposited on top of oxygen-passivated Fe(001), namely the Fe(001)-p(1 × 1)O surface. We also prove that they are magnetically coupled with the substrate. The ordered molecular packing, together with the magnetic coupling, are fundamental conditions for application in organic spintronic devices. The system is studied by means of spin-resolved photoemission spectroscopies and scanning tunneling microscopy.

The intercalation of graphite by electrochemical methods is an efficient strategy to produce massive graphene flakes. In fact, when graphite is biased inside an acidic solution, anions enter inside the stratified structure of the electrode and reduce the layer-to-layer interaction. Consequently, a gentle sonication is sufficient to disperse the graphene flakes inside the electrolyte. In view of an optimisation of the production protocol, a detailed analysis of the intercalation mechanism at the molecular length scale is mandatory. In the last 30 years, electrochemical (EC) scanning probe microscopies (e.g. EC-STM and in situ AFM) have been widely exploited in this research topic. In fact, these techniques have the possibility of combining the EC characterisation (e.g. cyclic-voltammetry, CV) with mechanical characterisation (e.g. adhesion and friction) and topography acquisition with high (molecular) lateral resolution. In this work, we investigate the tribological properties of the basal surface of graphite before and after the anion intercalation. By comparing the results acquired after the extraction of the graphite electrode from the EC cell with those collected inside the EC cell during the CV by an in situ AFM, we show how some features deriving from anisotropic friction can be exploited to unveil the very early stage of graphite exfoliation.

In situ atomic force microscopy (AFM) measures, with the scanner head immersed inside a liquid (namely an electrolyte), are—generally speaking—not easy to be acquired in real time, i.e., during electrochemical processes, the immersion of a massive and wide system (with respect to the electrochemical cell mean radius) can significantly perturb the electrolyte and the I/V profile of the electrochemical characterization (CV). Despite this fact, the information that can be obtained from an electrode morphological inspection is very precious, and the coupling between an AFM and an electrochemical cell is in any case successful. In this work, we discuss the surface morphological evolution of a graphite electrode when immersed inside an aqueous NaOH electrolyte. 1.0 M sodium hydroxide added inside electrochemical baths was shown to promote the intercalation of ions between the stratified graphite structure or graphene layers. Possible consequences of diluted NaOH electrolyte on the quality of the graphite basal plane have been never considered in detail yet. By means of an in situ AFM, we show the effects of CVs on the surface morphology of the graphite basal plane. Our result shows that a 1.0 M NaOH solution, generally used for promoting intercalation, changes the electrode surface quality as a consequence of a partial carbon corrosion.

Graphitic electrodes in ion batteries and graphite electrochemical exfoliation for graphene production are technological processes based on the anion intercalation mechanism in the stratified crystal structure upon immersion in acid solutions. To the detriment of the graphite mother-crystal, decomposition of the electrolyte solution or production of unexpected graphite intercalation compounds (GICs) is a side effect related to the anion intercalation mechanism. In this work, we studied the surface of highly oriented pyrolytic graphite (HOPG) samples treated with electrolyte solutions of 2 M HClO4 and 1 M H2SO4 in order to identify elemental and molecular species involved in the intercalation processes and, at the same time, find local surface defects through which solvated ions easily reach the HOPG subsurface. Results from cyclic voltammetry, X-ray photoelectron spectroscopy (XPS), and time-of-flight secondary ion mass spectrometry (ToF-SIMS) are described and discussed. Large blister formation and more general HOPG surface damage induced by treatment with acid solutions were revealed and mapped. In both HOPG samples, oxidized in 2 M HClO4 and 1 M H2SO4, the surface distribution of perchlorate and hydrogen sulfate ions gives direct evidence that the anion intercalation happens via surface defects among HOPG crystallites. These results pave the way for a complex intercalation process where more than one molecular species could be responsible for blister formation and carbon detriment.

The possibility of following electrochemical processes in situ and in real time using optical techniques is important in view of shining a light on the chemical processes at the surface. The interest grows if the optical apparatus is compact and can be employed in industrial quality-check protocols. Here, we show how graphite anion intercalation–an important chemical process to massively produce graphene flakes–can be monitored by a UV-vis spectrometer when the graphite works as an electrode immersed inside the electrochemical cell. Important information on the reversibility or quasi-reversibility of the reaction shows a clear visualization in optical color maps.

We present a multitechnique investigation of the structural and electronic properties of the prototypical system composed by ultra-thin films of magnetic molecules [Co-tetraphenyl-porphyrins (Co-TPP)] grown on a ferromagnetic substrate [oxygen passivated Fe(0 0 1), namely the Fe(0 0 1)-p(1 × 1)O surface]. Low Energy electron diffraction (LEED) and scanning tunneling microscopy (STM), coupled with first-principles calculations, reveal the formation of a commensurate superstructure at monolayer coverage, made by a square array of flat-lying TPP molecules. UV–photoemission and inverse photoemission spectroscopies (UPS and IPES) are used to investigate their electronic structure. Similar to our previous results on the Zn–TPP growth on Fe(0 0 1)–p(1 × 1)O, the passivation of the metallic surface is able to preserve the photoemission features characteristic of quasi-free molecules, opening the route towards an exploitation of single oxide layers as protective films in organic/inorganic junctions. X-ray photoemission (XPS) and near edge X-ray adsorption fine structure spectroscopies (NEXAFS), are used to reveal the details of the Co–TPP interaction with the substrate., We acknowledge the CINECA award under the ISCRA initiative, for the availability of high performance computing resources and support (Application No. HP10C2SVDP).

In the current interpretative model of graphite anion intercalation, the characteristic blisters, formed after the incipient surface oxidation, have the same structural behaviour of pristine graphite. However, this result is deduced from scanning tunneling microscopy data, which cannot give a chemical fingerprint of the electrode surface. Conversely, the contact potential difference between a reference tip and a sample depends on the chemical as well as on the structural conditions of the surface. As a consequence, in this paper, we combine a contact potential measurement with a scanning Kelvin force microscopy analysis, in order to extract information on electronic changes occurring to the electrode with a sub-micrometre investigation of the surface morphology after the incipient graphite oxidation process in diluted sulphuric acid. The results suggest that, as well as the structural properties, no significant electronic differences occur between blisters and the graphite basal plane after the intercalation process.

Scanning microwave microscopy (SMM) is based on the interaction between a sample and an electromagnetic evanescent field, in the microwave frequency range. SMM is usually coupled with a scanning probe microscopy (SPM) technique such as in our case, a scanning tunneling microscope (STM). In this way, the STM tip is used to control the distance between the probe and the sample while acting as an antenna for the microwave field. Thanks to the peculiarity of our home-made setup, the SMM is a suitable method to study blisters formed on HOPG surface as consequence of an electrochemical treatment. Our system has a “broad-band” approach that opens the way to perform local microwave spectroscopy over a broad frequency range. Moreover, microwaves have the ability to penetrate into the sample allowing the sub-surface characterization of materials. The application of the SMM to characterize blisters formed on the HOPG surface provides information on the sub-layer structures.

Graphite intercalation via chemical strategies is a common procedure to delaminate stratified crystals and obtain a suspension of graphene flakes. The intercalation mechanism at the molecular level is still under investigation in view of enhancing graphene production and reducing damage to the original pristine crystal. The latter, in particular, can undergo surface detriment due to both blister evolution and carbon dissolution. The role of the electrolyte temperature in this process has never been investigated. Here, by using an in-situ atomic force microscopy (AFM) apparatus, we explore surface morphology changes after the application of fast cyclic-voltammetries at 343 K, in view of de-coupling the crystal swelling phenomenon from the other electrochemical processes. We find that blisters do not evolve as a consequence of the increasing temperature, while the quality of the graphite surface becomes significantly worse, due to the formation of some adsorbates on possible defect sites of the electrode surface. Our results suggest that the chemical baths used in graphite delamination must be carefully monitored in temperature for avoiding undesired electrode detriment.

Phosphoric acid is an inorganic acid used for producing graphene sheets by delaminating graphite in (electro-)chemical baths. The observed phenomenology during the electrochemical treatment in phosphoric acid solution is partially different from other acidic solutions, such as sulfuric and perchloric acid solutions, where the graphite surface mainly forms blisters. In fact, the graphite surface is covered by a thin layer of modified (oxidized) material that can be observed when an electrochemical potential is swept in the anodic current regime. We characterize this particular surface evolution by means of a combined electrochemical, atomic force microscopy and Raman spectroscopy investigation.

In view of large-scale applications, electrochemical exfoliation of graphite for the production of graphene sheets must follow chemical processes that ensure high quality of the products—wide-size graphene foils, single- or few-layer thickness, and low level of defectivity—in order to guarantee high electrical transport and good mechanical properties. Understanding the exfoliation process of graphite at the atomic scale, that is, the intercalation of graphene layers in the electrolyte solution, is fundamental to really be able to control and optimize such processes. This can be obtained, for instance, by investigation of the exfoliated graphite—the surface of the original crystal left behind in the chemical solution—and by real-time monitoring of graphite surface morphological and structural modifications during the exfoliation process. Here, we monitor graphite surface changes as a function of the electrochemical potential by both electrochemical (EC) atomic force microscopy and EC scanning tunneling mic...

The realization of nanometer-scale structures through bottom-up strategies can be accomplished by exploiting a buried network of dislocations. We show that, by following appropriate growth steps in ultra-high vacuum molecular beam epitaxy, it is possible to grow nano-structured films of CoO coupled to Fe(001) substrates, with tunable sizes (both the lateral size and the maximum height scale linearly with coverage). The growth mode is discussed in terms of the evolution of surface morphology and chemical interactions as a function of the CoO thickness. Scanning tunneling microscopy measurements reveal that square mounds of CoO with lateral dimensions of less than 25 nm and heights below 10 atomic layers are obtained by growing few-nanometers-thick CoO films on a pre-oxidized Fe(001) surface covered by an ultra-thin Co buffer layer. In the early stages of growth, a network of misfit dislocations develops, which works as a template for the CoO nano-structuring. From a chemical point of view, at variance with typical CoO/Fe interfaces, neither Fe segregation at the surface nor Fe oxidation at the buried interface are observed, as seen by Auger electron spectroscopy and X-ray Photoemission Spectroscopy, respectively.

Metal-tetraphenyl-porphyrin (M-TPP) molecules typically self-assemble forming square-like superlattices, as dictated by their shape. The dependence of the adsorption properties on the central atom is systematically studied for Co-, Ni-, and Zn-TPP adsorbed on oxygen passivated Fe(001), namely the Fe(001)- p ( 1 × 1 ) O surface. It is found by low energy electron diffraction (LEED) and scanning tunneling microscopy (STM) that despite the weak molecule–substrate interaction, preserving many features of quasi-free molecules, the self-assembled structure switches from the ( 5 × 5 ) R 37 ° superlattice of Co-TPP and Ni-TPP to the plain ( 5 × 5 ) of Zn-TPP. Ab initio calculations based on density functional theory (DFT) are used to investigate the adsorption properties of the different molecules and the possible overlayers formed. Adsorption energies, structures, and electronic properties are reported, discussing the bonding mechanisms and the magnetic character. Only moderate energy differences are found, suggesting that subtle effects may steer the selection of the structure among overlayers with similar properties although differing substantially as for the LEED and STM experimental results.

Ion intercalation inside graphite crystal is a well-established method for graphene production. Pure acid electrolytes swell the stratified crystal structure and a gentle sonication disperses graphene flakes inside the solution. In view of a better environment-friendly graphite delamination procedure, diluted electrolytes can represent a viable option. Unfortunately, the presence of solvated ions between graphite planes induces oxidation of carbon atoms and the following development of gases in the buried layers of the crystal. Being not permeable, graphite entraps the gases that deform the crystal by producing characteristic bubbles called blisters. The latter represent a detriment of the original specimen and influence the quality of the produced graphene flakes. Here, we will address key properties of blister formation and growth by proving, in particular, that blisters evolve when the applied electrochemical potential value is reduced (cathodic regime), i.e. after the intercalation, conversely to what has been believed for long time. This finding requires a refinement of the current anion intercalation interpretative model.

Systems comprising single and multilayer graphene deposited on metals and immersed in acid environments have been investigated, with the aim of elucidating the mechanisms involved, for instance, in hydrogen production or metal protection from corrosion. In this work, a relevant system, namely chemical vapor deposited (CVD) multilayer graphene/Ni (MLGr/Ni), is studied when immersed in a diluted sulfuric electrolyte. The MLGr/Ni electrochemical and morphological properties are studied in situ and interpreted in light of the highly oriented pyrolytic graphite (HOPG) electrode behavior, when immersed in the same electrolyte. Following this interpretative framework, the dominant role of the Ni substrate in hydrogen production is clarified.

Anion intercalation in graphite was studied since the early 1980s, with the aim of changing the graphite transport properties of a carbon electrode. The Li-ion batteries renewed these researches as well as the interest of low-cost graphene production via (electro-) chemical strategies. In this last case, anion species intercalate inside the stratified graphite crystal structure, expand the layer–layer distance, and facilitate the graphite delamination. Electrochemical-scanning probe microscopy (EC-SPM) offers the possibility of following directly the evolution during the different involved EC processes.

Graphite is the primary sorce of electrodes used in both industrial production and basic research. Under electrical stress graphitic electrodes can undergo degradations, delmainations and aging processes due to the intercalation of chemical species between the graphene layers. All these events cause significant loss of performance and activity. Surface protection could represent the solution to avoid degradative intercalations, for example by mean of overlayed thin films, if suitable materials and conditions are met. In this contribution, we will cover the current state-of-the-art about the surface protection of a higly oriented pyrolytic graphite (HOPG) electrode with small molecules. The use of porphirins will be discussed, considering aspects related to deposition protocols, surface interactions and protections and the morphological and chemical stabilities of the molecules. Finally it will be presented a short overview on the synthesis of porphyrins evidencing the multifacet potentiality of this class of compounds as protecting agents.

When used as substrates, thin metal-oxide (MO) layers can perturb the physical and chemical properties of molecules in contact with the surface. To study the molecule-MO layer interaction, we focused our investigation on a prototypical interface, namely zinc tetraphenylporphyrin (ZnTPP) film on Fe(001)-p(1 × 1)O. In a previous study, we found that no significant change of the electronic structure takes place at the monolayer (ML) coverage either in the core level photoemission spectra or in the highest occupied molecular orbitals (HOMOs). However, molecules showed the occurrence of a commensurate (5 × 5) diffraction pattern that indicates a certain degree of interaction with the substrate. In order to better understand the effective molecule/metal decoupling operated by the FeO layer, we performed a combined investigation based on a scanning tunneling microscopy (STM) study of the self-assembled ZnTPP molecular layer and on a near edge X-ray absorption fine structure spectroscopy (NEXAFS). Molecules are found to lie almost parallel to the substrate, even if the central macrocycle displays a characteristic small saddle-like distortion (

International audience; Driven by the perspective of large-scale, high-quality graphene production via chemical routes, the investigation of electrochemical anion intercalation between the basal graphite planes has seen a renewed interest among the scientific community. At relatively high electrochemical potentials, when oxidation occurs, graphite electrodes undergo significant anion intercalation processes. The latter swell the uppermost graphite layers (i.e., graphene sheets), reduce the interplane interaction and favor the graphite delamination in liquid. Different intercalation stages are observed in a perchloric acid electrolyte, which are usually interpreted in terms of different perchlorate penetration depths. Nonetheless, the understanding of the morphological changes occurring at the electrode surface during the different intercalation stages is still not completely clear. We combine different microscopy techniques including optical, scanning electron and electrochemical atomic force microscopies to analyze the morphological evolution of the graphite surface at different length scales as a function of the applied electrochemical potential. Whereas both carbon dissolution and blisters affect the surface on the micrometer scale as soon as intercalation starts, we find that the graphite surface is cracked on the sub-millimeter scale only when intercalation at a higher potential is reached, inducing a significant aging of the electrode surface.

The development of graphene products promotes a renewed interest toward the use of graphite in addition to the historical one for its proven viability as battery electrode. However, when exposed to harsh conditions, the graphite surface ages in ways that still need to be fully characterized. In applications to batteries, to optimize the electrode performances in acid solutions, different surface functionalizations have been studied. Among them, aromatic molecules have been recently proposed. In this communication, we report on the protective effect exerted by a physical-vapor-deposited porphyrin layer. Metal-free tetra-phenyl-porphyrins were deposited on a highly oriented pyrolytic graphite crystal to study the modifications that occur during anion intercalation in graphite. The graphite electrode was plunged in an electrolyte solution of 1 M sulfuric acid and subjected to cyclic voltammetry. The results indicate that blister formation, the characteristic swelling of graphite surface induced by anion intercalation, is significantly perturbed by the porphyrin overlayer; the process is inhibited in those areas where the protective porphyrin film is present. We ascribe the inhibition of the anion intercalation to the protective porphyrin wetting layer.

Interfaces between organic semiconductors and ferromagnetic metals offer intriguing opportunities in the rapidly developing field of organic spintronics. Understanding and controlling the spin-polarized electronic states at the interface is the key toward a reliable exploitation of this kind of systems. Here we propose an approach consisting in the insertion of a two-dimensional magnetic oxide layer at the interface with the aim of both increasing the reproducibility of the interface preparation and offering a way for a further fine control over the electronic and magnetic properties. We have inserted a two-dimensional Cr4O5 layer at the C-60/Fe(001) interface and have characterized the corresponding morphological, electronic, and magnetic properties. Scanning tunneling microscopy and electron diffraction show that the film grows well ordered both in the monolayer and multilayer regimes. Electron spectroscopies confirm that hybridization of the electronic states occurs at the interface. Finally, magnetic dichroism in X-ray absorption shows an unprecedented spin-polarization of the hybridized fullerene states. The latter result is discussed also in light of an ab initio theoretical analysis.

The effects of oxygen exposure on ultrathin films of Ni and Cr deposited on Fe(001) are studied by means of low energy electron diffraction, Auger electron spectroscopy and scanning tunneling microscopy. In the case of the Ni/Fe(001) oxidation we find that, since the first stages of oxygen exposure, Fe atoms segregate toward the sample surface, where their oxidation takes place. Post-annealing treatments in ultra-high-vacuum conditions promote the Fe oxidation and improve the order of the overlayer, leading to the stabilization of an atomically flat FeO(111)-like film covering the sample. On the other hand, the oxidation of the Cr/Fe(001) follows a different path. At low oxygen exposure only the Cr film is oxidized, while upon increasing the oxygen dose Fe oxidation takes place. The morphology resulting from the oxidation of such a system is rough and successive thermal treatments do not result in surface ordering.

The introduction of a two-dimensional oxide layer at the interface between an organic semiconductor and a ferromagnetic metal (spinterface) can help in tailoring the formation of spin-polarized hybridized interface states. Here, we consider the case of a Cr 4 O 5 monolayer at the C 60 / Fe ( 001 ) interface, which is already known to feature the occurrence of spin-polarized states in the fullerene molecules. In this work, we employ scanning tunneling microscopy/spectroscopy and photoemission spectroscopy to show that the C 60 / Cr 4 O 5 / Fe ( 001 ) spinterface is characterized by the formation of a well-ordered fullerene monolayer and of strongly hybridized interface states. These experimental results are discussed in terms of state-of-the-art ab initio calculations of the structural, electronic, and magnetic properties at the interface.

C60 molecules coupled to metals form hybrid systems exploited in a broad range of emerging fields, such as nanoelectronics, spintronics, and photovoltaic solar cells. The electronic coupling at the C60/metal interface plays a crucial role in determining the charge and spin transport in C60-based devices; therefore, a detailed understanding of the interface electronic structure is a prerequisite to engineering the device functionalities. Here, we compare the electronic and structural properties of C60 monolayers interfaced with Fe(001) and oxygen-passivated Fe(001)-p(1 × 1)O substrates. By combining scanning tunneling microscopy and spectroscopy, Auger electron spectroscopy, photoemission and inverse photoemission spectroscopies, we are able to elucidate the striking effect of oxygen on the interaction between Fe(001) and C60. Upon C60 deposition on the oxygen-passivated surface, the oxygen layer remains buried at the C60/Fe(001)-p(1 × 1)O interface, efficiently decoupling the fullerene film from the metal...

Using cyclic voltammetry (CV), reflectance anisotropy spectroscopy (RAS), and in situ electrochemical scanning tunneling microscopy (EC-STM), we have studied the structure and structural transition...

The sublimation of free base porphyrin (namely H2TPP) in ultra-high vacuum on graphite results in a three dimensional clusters growth. On the other hand, a two dimensional layer of free base porphyrin can be obtained by a post growth chemical treatment of the sample, based on hydrochloric vapor exposure. In this work, the morphological modifications induced by the sublimation of Fe atoms on the porphyrins two dimensional film are investigated by means of scanning tunneling microscopy. We provide the evidence that the porphyrins film undergoes to a recrystallization process, induced by the interaction with Fe atoms. A new treatment with hydrochloric vapor removes the three dimensional porphyrins clusters again.

In this paper, we report on the characterization of the magnetic properties of layered Fe/CoO/Fe(001) magnetic structures by means of Magneto-Optical Kerr Effect. Hysteresis loops were acquired on samples with variable CoO thickness, from 1 nm to 4 nm, and at different temperatures, from 30 K to room temperature. This characterization offers the opportunity of exploiting the differences in the layer-dependent sensitivity of Kerr rotation and Kerr ellipticity in order to disentangle the contribution of the different Fe layers in the hysteresis loops. Moreover, it allows us to give a detailed overview of the magnetic behavior of the trilayers.

A remarkable enhancement of atomic diffusion is highlighted by scanning tunneling microscopy performed on ultrathin metastable body-centered tetragonal Co films grown on Fe(001). The films follow a nearly perfect layer-by-layer growth mode with a saturation island density strongly dependent on the layer on which the nucleation occurs, indicating a lowering of the diffusion barrier. Density functional theory calculations reveal that this phenomenon is driven by the increasing capability of the film to accommodate large deformations as the thickness approaches the limit at which a structural transition occurs. These results disclose the possibility of tuning surface diffusion dynamics and controlling cluster nucleation and self-organization.

The oxygen-saturated Fe(001)-$p(1\ifmmode\times\else\texttimes\fi{}1)$O surface has been used as a template to stabilize two-dimensional Cr oxides on Fe(001). Cr deposition at 400 ${}^{\ensuremath{\circ}}$C leads to two different well-ordered phases, depending on the amount of Cr deposited. In the submonolayer regime a novel $c(4\ifmmode\times\else\texttimes\fi{}2)$ overlayer self-assembles on the Fe(001)-$p(1\ifmmode\times\else\texttimes\fi{}1)$O surface, saturating for a coverage of about $0.75$ monolayers. This phase becomes unstable for higher coverages, when a $(\sqrt{5}\ifmmode\times\else\texttimes\fi{}\sqrt{5})R{27}^{\ensuremath{\circ}}$ superstructure emerges. The structural and electronic details of the two one-layer-thick oxides are studied by combining high-resolution scanning tunneling microscopy, low-energy electron diffraction, Auger electron spectroscopy, and density functional theory.

The morphology and the electronic structure of ultrathin Ni films grown on the oxygen-passivated Fe(001)-$p(1\ifmmode\times\else\texttimes\fi{}1)$O surface are investigated by means of scanning tunneling microscopy and spectroscopy. We observe that oxygen efficiently floats on top of the Ni film and has an ``anti-surfactant'' effect on the Ni/Fe(001) interface, promoting the formation of two-layer-high islands since the first stages of growth, whereas the Ni growth on the oxygen-free Fe(001) surface proceeds in a layer-by-layer mode. Our experimental results show that the effectiveness of the oxygen surfactant action in the growth of metal thin films strongly depends on the choice of the substrate and overlayer combination.

Domains of different surface reconstruction-negatively or positively buckled isomers-have been previously observed on highly n-doped Si(111)-2 × 1 surfaces by angle-resolved ultraviolet photoemission spectroscopy and scanning tunneling microscopy/spectroscopy. At low temperature, separate domains of the two isomer types are apparent in the data. It was argued in the previous work that the negative isomers have a lower energy of their empty surface states than the positive isomers, providing a driving force for the formation of the negative isomers. In this work we show that the relative abundance of these two isomers shows considerable variation from sample to sample, and it is argued that the size of the isomer domains is likely to be related to this variation. A model is introduced in which the electrostatic effect of charge transfer between the domains is computed, yielding total energy differences between the two types of isomer. It is found that the transfer of electrons from domains of positive isomers to negative ones leads to an energetic stabilization of the negative isomers. The model predicts a dependence of the isomer populations on doping that is in agreement with most experimental results. Furthermore, it accounts, at least qualitatively, for the marked lineshape variation from sample to sample observed in photoemission spectra.

We report evidence that the body-centered cubic (bcc)-face-centered cubic (fcc) transition that occurs during Ni film growth on a Fe(001) substrate is preceded by a pre-martensitic phase, as demonstrated by low-energy electron diffraction. The corresponding film superstructure is characterized by a displacement of Ni atoms along the main h100i crystallographic axes of iron, without any rotation of the unit cell with respect to the (001) plane, in contrast with the martensitic transition that shows four fcc Ni domains with the Nih211i crystallographic directions aligned with the Feh110i axes. In addition, the martensitic transition is detected not at 6ML, as previously believed, but above 20ML if the Ni sample is rigorously kept at room temperature. The surface morphology of the bcc-fcc transition is characterized by the development of Ni mounds oriented along the h110i directions, as shown by scanning tunneling microscopy.

A long-standing puzzle regarding the $\mathrm{Si}(111)\ensuremath{-}2\ifmmode\times\else\texttimes\fi{}1$ surface has been solved. The surface energy gap previously determined by photoemission on heavily $n$-doped crystals was not compatible with a strongly bound exciton known from other considerations to exist. New low-temperature angle-resolved photoemission and scanning tunneling microscopy data, together with theory, unambiguously reveal that isomers with opposite bucklings and different energy gaps coexist on such surfaces. The subtle energetics between the isomers, dependent on doping, leads to a reconciliation of all previous results.

The ordering properties of tetraphenylporphyrin molecules sublimated on a highly oriented pyrolitic graphite surface have been investigated. The morphological information obtained by scanning tunneling microscopy has been linked with the optical spectra by reflectance anisotropy spectroscopy. Measurements were performed in situ as a function of the nominal coverage.

Cr ultrathin films have been prepared on the oxygen-passivated Fe(001)-p(1 × 1)O surface, resulting in monolayer range chromium oxide layers. We discuss the presence of first nearest-neighbor Cr pairs, as seen by scanning tunneling microscopy, in the early stages of interface growth and compare the results with previous experimental observations on the Cr/Fe oxygen-free interface. We then derive the magnetic properties of the Cr oxide ultrathin films by analyzing new x-ray magnetic circular dichroism measurements conducted on samples grown both at room temperature and at 400 °C. The ultrathin Cr oxide film forms in both cases a ferromagnetic layer which is antiferromagnetically coupled to Fe. © 2013 AIP Publishing LLC.