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Two-dimensional (2D) magnets such as chromium trihalides CrX3 (X = I, Br, Cl) represent a frontier for spintronics applications and, in particular, CrCl3 has attracted research interest due its relative stability under ambient conditions without rapid degradation, as opposed to CrI3. Herein, mechanically exfoliated CrCl3 flakes are characterized at the atomic scale and the electronic structures of pristine, oxidized, and defective monolayer CrCl3 phases are investigated employing density functional theory (DFT) calculations, scanning tunneling spectroscopy (STS), core level X-ray photoemission spectroscopy (XPS), and valence band XPS and ultraviolet photoemission spectroscopy (UPS). As revealed by atomically resolved transmission electron microscopy (TEM) and energy dispersive X-ray (EDX) analysis, the CrCl3 flakes show spontaneous surface oxidation upon air exposure with an extrinsic long-range ordered oxidized O–CrCl3 structure and amorphous chromium oxide formation on the edges of the flakes. XPS proves that CrCl3 is thermally stable up to 200 °C having intrinsically Cl vacancy-defects whose concentration is tunable via thermal annealing up to 400 °C. DFT calculations, supported by experimental valence band analysis, indicate that pure monolayer (ML) CrCl3 is an insulator with a band gap of 2.6 eV, while the electronic structures of oxidized and Cl defective phases of ML CrCl3, extrinsically emerging in exfoliated CrCl3 flakes, show in-gap spin-polarized states and relevant modifications of the electronic band structures.

In this work we shed light on the early stage of the chemical vapor deposition of graphene on Ge(001) surfaces. By a combined use of m-Raman and x-ray photoelectron spectroscopies, and scanning tunneling microscopy and spectroscopy, we were able to individuate a carbon precursor phase to graphene nucleation which coexists with small graphene domains. This precursor phase is made of C aggregates with different size, shape and local ordering which are not fully sp(2) hybridized. In some atomic size regions these aggregates show a linear arrangement of atoms as well as the first signature of the hexagonal structure of graphene. The carbon precursor phase evolves in graphene domains through an ordering process, associated to a re-arrangement of the Ge surface morphology. This surface structuring represents the embryo stage of the hills-and-valleys faceting featured by the Ge(001) surface for longer deposition times, when the graphene domains coalesce to form a single layer graphene film. (C) 2018 Elsevier Ltd. All rights reserved.

We systematically investigate the chemical vapor deposition growth of graphene on Ge(110) as a function of the deposition temperature close to the Ge melting point. By merging spectroscopic and morphological information, we find that the quality of graphene films depends critically on the growth temperature improving significantly by increasing this temperature in the 910–930 °C range. We correlate the abrupt improvement of the graphene quality to the formation of a quasi-liquid Ge surface occurring in the same temperature range, which determines increased atom diffusivity and sublimation rate. Being observed for diverse Ge orientations, this process is of general relevance for graphene synthesis on Ge.

By combining scanning probe microscopy with Raman and x-ray photoelectron spectroscopies, we investigate the evolution of CVD-grown graphene/Ge(001) as a function of the deposition temperature in close proximity to the Ge melting point, highlighting an abrupt change of the graphene's quality, morphology, electronic properties and growth mode at 930 degrees. We attribute this discontinuity to the incomplete surface melting of the Ge substrate and show how incomplete melting explains a variety of diverse and long-debated peculiar features of the graphene/Ge(001), including the characteristic nanostructuring of the Ge substrate induced by graphene overgrowth. We find that the quasi-liquid Ge layer formed close to 930 degrees is fundamental to obtain high-quality graphene, while a temperature decrease of 10 degrees already results in a wrinkled and defective graphene film., in press

Using high-resolution scanning tunneling microscopy, we investigate the periodicity of mixed arrays of single and triple steps and the atomic structure of triple steps on three vicinal Si(111) samples with off-angles 1.5 ∘ ≤ θ ≤ 9.45 ∘ towards the [ 1 ¯ 1 ¯ 2 ] direction. The comparison between the (556) and (557) triple-step structure demonstrates that only the number of 7x7 terraces is affected by miscut. At variance with the singular Si(111) case, Ge epitaxy is strongly correlated to step morphology, resulting, at the critical coverage and small misorientation, in the nucleation of holed nanoislands from step-edge protrusions. During step-flow growth, stress-driven bunching of atomic steps (ripples) is observed on the (557) surface, indicating that this mechanism of stress relief of Si(001) vicinal surfaces works also on Si(111) surfaces.

International audience; In Accumulated Low Schottky Barrier metal oxide semiconductor field effect transistors (MOSFET) on SOl structures, very thin silicide layers are used for ohmic contacts. Silicide contacts form due to metallurgical reaction of metal with semiconductor. In order to get a broad vision of the most interesting metal/silicide solution several material stacks were evaluated including platinum and iridium. In this work cross-sectional transmission electron microscopy was used to study the process of silicide formation in metal/Si and metal/SiGe/Si structures. Novel structure architectures have been developed. They are mainly based on the use of a very thin film SOl substrate with typical active silicon layer thicknesses ranging between 2 and 20 nm for 10-40 nm gate length MOSFETs. In Accumulation LowSchottky Barrier -SOI MOSFET structures very thin silicides are used for ohmic contacts. Silicide contacts form due to metallurgical reaction of metal with semiconductor.

An extended formation of faceted pit-like defects on Ge(001) and Ge(111) wafers was obtained by thermal cycles to T> 750 {\deg}C. This temperature range is relevant in many surface-preparation recipes of the Ge surface. The density of the defects depends on the temperature reached, the number of annealing cycles performed and correlates to the surface-energy stability of the specific crystal orientation. We propose that the pits were formed by preferential desorption from the strained regions around dislocation pile-ups. Indeed, the morphology of the pits was the same as that observed for preferential chemical etching of dislocations while the spatial distribution of the pits was clearly non-Poissonian in line with mutual interactions between the core dislocations., Comment: European Union (EU), Horizon 2020, Far-Infrared Lasers Assembled using Silicon Heterostructures,FLASH,Grant number: 766719

We present a joint experimental and theoretical study of the electronic properties of the rebonded-step reconstructed Ge/Si(1 0 5) surface which is the main strained face found on Ge/Si(0 0 1) quantum dots and is considered a prototypical model system for surface strain relaxation in heteroepitaxial growth. Using a vicinal surface as a model system for obtaining a stable single-domain film structure with large terraces and rebonded-step surface termination, we realized an extended and ordered Ge/Si planar hetero-junction suitable for direct study with angle-resolved photoemission spectroscopy. At the coverage of four Ge monolayers photoemission spectroscopy reveals the presence of 2D surface and film bands displaying energy-momentum dispersion compatible with the 5 × 4 periodicity of the system. The good agreement between experiment and first-principles electronic structure calculations confirms the validity of the rebonded-step structural model. The direct observation of surface features within 1 eV below the valence band maximum corroborates previously reported analysis of the electronic and optical behavior of the Ge/Si hetero-interface.

Graphene, the wonder material of the 21st century, has not yet achieved the expected outcomes in terms of applications in nanoelectronics. This is not surprising, as large-scale graphene growth is still mostly limited to CVD on metallic foils, followed by graphene transfer to the semiconductor substrate required for electronic devices, which is cumbersome and difficult to automatize. Moreover, graphene is gapless, and this is still seriously limiting its applications. This book is an attempt to dispel this pessimistic outlook, summarizing the latest achievements in the direct growth of graphene on semiconductors.SiC is the ideal semiconductor for graphene growth, which is typically achieved by thermal graphitization. Through high-temperature annealing in a controlled environment, it is possible to decompose the topmost SiC layers, obtaining quasi-ideal graphene by Si sublimation. Graphene on SiC with record electronic mobilities has been demonstrated, opening the way for applications in nanoelectronics, by exploiting selective growth on patterned structures and gap opening by quantum confinement. The book opens with a chapter on the significance and challenges of graphene growth on semiconductors, by Mishra et al., drawing a picture of the perspectives of this technology.The three following chapters, by Aristov et al., Gupta et al., and Tomellini et al., respectively, are dedicated to an up-to-date analysis of the synthesis of graphene on SiC in ultrahigh vacuum. The fifth chapter, by Forti, Starke, and Coletti, is a review of the effect of atomic intercalation at the SiC–graphene interface, with an in-depth discussion of the doping effects and of the electronic properties of lateral superlattices. The sixth chapter, written by the de Heer and Berger group, reporting graphene growth on SiC by sublimation, summarizes the whole history of graphene growth on SiC by confined controlled sublimation, up to the latest developments in the growth of templated graphene nanostructures. We hope that this book can be of inspiration to the many scientists striving to improve the growth of graphene on semiconductors and to the young researchers from industry and academia approaching this fascinating world. The developments sketched here show a promising outlook for the future, with many exciting outcomes on the horizon, from the artificial opening of a gap to the creation of 2D field-effect transistors with nanodimensions.

One of the problems that graphene is facing in electronic applications is its quality, which is still far from the level required to obtain an industrial scale production of reliable nanoscale devices, and it is ultimately related to the growth method. Understanding the kinetics of the graphene growth is an outstanding problem in physics, whose solution is the key to achieve full control on the graphene properties . This chapter deals with the microscopic phenomena occurring during epitaxial graphene growth on SiC by high temperature annealing. We discuss recent experimental and theoretical findings on the Si diffusion in the SiC matrix and on the kinetics of graphene growth as a function of temperature and of time. The theoretical framework developed in this chapter will help to understand the physical basis of the graphene formation, which is essential for a perfect control of the graphene quality, helping to define the optimal conditions for the growth of a continuous monolayer or bilayer graphene on SiC.

International audience; We fabricated flat, two-dimensional germanium sheets showing a honeycomb lattice that matches that of germanene by depositing submonolayers of Ge on graphite at room temperature and subsequent annealing to 350 degrees C. Scanning tunneling microscopy shows that the germanene islands have a small buckling with no atomic reconstruction and does not give any hints for alloy formation and hybridization with the substrate. Our density functional theory calculations of the structural properties agree well with our experimental findings and indicate that the germanene sheet interacts only weakly with the substrate underneath. Our band structure calculations confirm that the Dirac cone of free-standing germanene is preserved for layers supported on graphite. The germanene islands show a small but characteristic charge transfer with the graphite substrate which is predicted by our ab initio simulations in excellent agreement with scanning tunneling spectroscopy measurements.

© 2015 Elsevier Ltd. All rights reserved. We followed by X-ray Photoelectron Spectroscopy (XPS) the time evolution of graphene layers obtained by annealing 3C SiC(111)/Si(111) crystals at different temperatures. The intensity of the carbon signal provides a quantification of the graphene thickness as a function of the annealing time, which follows a power law with exponent 0.5. We show that a kinetic model, based on a bottom-up growth mechanism, provides a full explanation to the evolution of the graphene thickness as a function of time, allowing to calculate the effective activation energy of the process and the energy barriers, in excellent agreement with previous theoretical results. Our study provides a complete and exhaustive picture of Si diffusion into the SiC matrix, establishing the conditions for a perfect control of the graphene growth by Si sublimation.

A few layers of functionalized graphene (FLG) have been obtained by the oxidative unzipping reaction of single-walled carbon nanotubes. A detailed Scanning Tunneling Microscopy and Transmission Electron Microscopy study shows that several different morphologies are observed, depending on the substrate upon which they are deposited, thus providing useful hints for a real world application of these carbon nanostructures. In addition, a combined Scanning Tunneling Spectroscopy investigation has shown that the electronic properties of these FLG are affected by their functionalization degree, opening interesting perspectives for their use in sensor and biosensor devices. © Taylor & Francis Group, LLC.

Spatial organization of Ge islands, grown by physical vapor deposition, on prepatterned Si(001) substrates has been investigated. The substrates were patterned prior to Ge deposition by nanoindentation. Characterization of Ge dots is performed by atomic force microscopy and scanning electron microscopy. The nanoindents act as trapping sites, allowing ripening of Ge islands at those locations during subsequent deposition and diffusion of Ge on the surface. The results show that island ordering is intrinsically linked to the nucleation and growth at indented sites and it strongly depends on pattern parameters. © 2011 Elsevier B.V.

Controlled synthesis of carbon nanotubes (CNTs) is highly desirable for nanoelectronic applications. To date, metallic catalyst particles have been deemed unavoidable for the nucleation and growth of any kind of CNTs. Ordered arrays of nanotubes have been obtained by controlled deposition of the metallic catalyst particles. However, the presence of metal species mixed with the CNTs represents a shortcoming for most electronic applications, as metal particles are incompatible with silicon semiconductor technology. In the present paper we report on a metal-catalyst-free synthesis of CNTs, obtained through Ge nanoparticles on a Si(001) surface patterned by nanoindentation. By using acetylene as the carbon feed gas in a low-pressure Chemical Vapor Deposition (CVD) system, multi-walled carbon nanotubes (MWNT) have been observed to arise from the smallest Ge islands. The CNTs and the Ge three-dimensional structures have been analysed by SEM, EDX and AFM in order to assess their elemental features and properties. EDX and SEM results allow confirmation of the absence of any metallic contamination on the surface, indicating that the origin of the CNT growth is due to the Ge nanocrystals.

The photocurrent generation of nanometric Ge droplets has been investigated by using electrochemical measurements. Dots have been grown by annealing Ge layers of different nominal thicknesses (ranging from 0.5 to 5 nm) deposited on clean SiO2 surface at room temperature. The photocurrent signals for the largest dots show features, which can be ascribed to the Ge nanocrystals direct electronic transitions. Only in the case of 0.5 nm Ge film a broad and intense feature at 2.4 eV has been observed while the peaks at higher energy result to be dramatically reduced. Since this last sample is characterized by (5±1) nm nanodots size (the smallest among the measured samples) the appearance of this broad band in the photocurrent active spectrum can be ascribed to quantum confinement effect.

The aim of this work is to understand the surface interactions of Reduced Graphene Oxide (RGO) with a water soluble corrole using graphene as a versatile substrate for sensor devices. Scanning Tunneling Microscopy (STM) and UV-Vis Spectroscopy are employed for understanding the morphology and microstructure of the deposited corrole molecules on RGO at atomic level. Other surface characterization technique like X-Ray Photoelectron Spectroscopy (XPS) is also used for better understanding of the chemical distribution, the molecular environment and the hybridization of the corrole molecules on RGO.

Starting with the basic definition, a short description of a few relevant physical quantities playing a role in the growth process of heteroepitaxial strained systems, is provided. As such, the paper is not meant to be a comprehensive survey but to present a connection between the Stranski-Krastanov mechanism of nanostructure formation and the basic principles of nucleation and growth. The elastic field is described in the context of continuum elasticity theory, using either analytical models or numerical simulations. The results are compared with selected experimental results obtained on GeSi nanostructures. In particular, by tuning the value of quantities such as vicinality, substrate orientation and symmetry of the diffusion field, we elucidate how anisotropic elastic interactions determine shape, size, lateral distribution and composition of quantum dots.

Here, we present a reliable process to deposit transparent conductive films on silicon oxide, quartz, and sapphire using a solid carbon source. This layer consists of partially ordered graphene flakes with a lateral dimension of about 5 nm. The process does not require any catalytic metal and exploits a high current arc evaporation (Φ-HCA) to homogeneously deposit a layer of carbon on heated substrates. A gas atmosphere consisting of Argon or Argon/Hydrogen blend acting as a buffer influences the morphology of the growing film. scanning tunneling microscopy, transmission electron microscopy, and Raman spectra were used for a thorough characterization of the samples in order to optimize the growth parameters. The best carbon layers have a surface resistance of 5.7 × 103 Ω◻ whereas the optical transparency of the coatings is 88% with an excellent homogeneity over areas of several cm2. Such results are compatible with most semiconductor fabrication processes and make this method very promising for various industrial applications.

We first investigate the spontaneous formation of nanometric and highly dense (∼3×10 12 cm −2 ) Ge droplets on SiO 2 /Si(0 0 1) by scanning tunnelling microscopy and spectroscopy. Ge dots are grown using a two-step process: first, Ge deposition on the clean SiO 2 surface at room temperature (RT) and second, thermal annealing at 500 °C. Ge dots appear free of germanium oxides. Then we show the ordering of Ge nanocrystals (NCs) on SiO 2 /Si(0 0 1) substrates patterned by focused ion beam (FIB). Lateral ordering of the ultra-dense Ge nanodots was achieved by the combination of the following technological steps: (a) use of a FIB to create ordered two-dimensional (2D) arrays of regular holes on a field oxide on the silicon substrate, (b) chemical cleaning and restoring of the Si surface in the holes, (c) further oxidation to transfer the pattern from the field oxide to the silicon substrate, (d) removal of the field oxide and thermal re-oxidation of the sample in order to create a tunnelling oxide of homogeneous thickness on the patterned silicon surface and (e) self-assembly of the 2D arrays of Ge dots on the patterned tunnelling oxide. We show that Ge NCs are ordered in the FIB holes whatever is their dimension/density.

During the past few decades, the field of nanoscience and nanotechnology has undergone a revolution that parallels the extraordinary advances in surface science instruments and techniques. The nano-era is posited as having begun in the early 1980s with the invention of scanning probe microscopy which allowed one to monitor, measure, and manipulate matter at the nanoscale level. Dramatic new insights have come from the application of this and other new experimental tools. The ability to precisely control atoms and build molecules at extremely small length scales is leading to unprecedented breakthroughs in electronics [1, 2], photonics [3, 4], medicine [5], and energy production [6]. In particular, the formation of periodically ordered structures offers the possibility of direct fabrication of semiconductor quantum nanostructures in which a narrow gap material is embedded into a matrix of a wide-gap material providing a confined potential for carriers. Periodic structures of such inclusions create a superlattice comprising quantum wells, quantum wires, and quantum dots (QDs). QDs, the ultimate limit of low-dimensional structures, have become a fascinating subject both for the basic research and for device application. The most powerful and common method to obtain high-quality QDs consists in exploiting the Stranski–Krastanov (SK) growth mode during strained heteroepitaxial growth, in which QDs spontaneously form on top of a thin wetting layer (WL). A prototype system for studying the fascinating and complex phenomena related to the strain-driven heteroepitaxy and for investigating the properties of QDs’ spontaneous formation is Ge/Si. As a model system, Ge on vicinal Si(001) makes it possible to investigate strain relaxation, growth instabilities, and three-dimensional (3D) island formation and self-organization. Moreover, by changing the substrate miscut, it is possible to tune both the energetic and the kinetic factors governing the growth of single nanostructures [7]. There is a huge literature on the SK growth of Ge on Si surfaces and many excellent monographs and review papers surveying the different aspects of the subject have been written [7–11]. Most of them are aimed at researchers with some

Nanostructured substrates are an interesting path towards the production of quantum dots devoted to microelectronic applications. In this work we report results on the effects of different nanopatterning methods to obtain lateral ordering of Ge islands grown on Si. By using Scanning Tunneling Microscopy we have studied in real-time the wetting layer growth and islands formation on nanopatterned Si substrates at 500 °C. We compare results obtained on Si substrates nanopatterned by using two different techniques: STM lithography and natural patterning induced by surface instabilities such as step bunching. Different issues on both cases have been addressed: substrate preparation, Ge dots placement and growth mode. We have observed that, on Si(001), the Ge islands nucleate near the holes and on Si(111) step bunching can guide the growth of aligned rows of islands.

Quartz crystal microbalances (QMB) have been coated by thin films of different metal complexes of protoporphyrin IX dimethyl ester (M-PPIX), deposited by electropolymerization technique. The structure of these films has been characterized by scanning tunnelling microscopy (STM). The sensory behaviour of the obtained QMB have been studied in the presence of four volatile organic compounds (VOC).

We compare, by high-resolution electron-energy-loss spectroscopy (HREELS), the electronic structure of the GaAs(110)1x1 surface and that of the GaAs(001)2x4 As-rich surface in the energy-loss region 0.5-5 eV. The HREEL spectra are interpreted in terms of realistic calculations. The spectral features above the gap are assigned to electronic transitions involving surface and/or bulk states. Losses at energies within the gap are associated to defect states at the surface. [S0163-1829(98)51236-2].

We show that SiGe islands are transformed into nanoholes and rings solely by annealing treatments and without Si capping. Rings are produced by a rapid flash heating at temperatures higher than the melting point of Ge, whereas nanoholes are produced by several minutes of annealing. The rings are markedly rich in Si with respect to the pristine islands, suggesting that the evolution path from islands to rings is driven by the selective dissolution of Ge occurring at high temperatures. © 2013 Elsevier B.V. All rights reserved.

Common features of Ge growth on vicinal Si(001) and Si(111) surfaces are discussed. We show that surface vicinality has a twofold effect on three-dimensional islanding in this system. As long as the atomic structure of the singular surface is preserved, epitaxial islands undergo a morphological change which is dictated by the misorientation from the singular surface. When also the surface reconstruction is so affected by miscut that surface and interfacial energies are drastically modified, the islanding pathway diverges from that occurring on flat surfaces. © 2013 Elsevier B.V.

Ge/Si alloys were prepared in UHV by solid phase epitaxy on Si(111) substrates. The alloy formation, as a function of the evaporation rate and the Ge layer thickness has been followed in situ by RHEED and scanning tunneling microscopy. The 5 × 5 surface reconstruction appeared after annealing at 450°C Ge layers (up to 10 A thick), obtained from a low rate Knudsen cell evaporator. In this case a nearly flat and uniform layer of reconstructed alloy was observed. When using an e-gun high rate evaporator we needed to anneal the Ge layer up to 780°C to obtain a 5 × 5 reconstruction. The grown layer was not flat, with many steps and Ge clusters; at high coverages (10 A and more) large Ge islands appeared. Moreover, we then succeeded in visualizing at atomic resolution the top of some of these Ge islands which displayed a 2 × 1 reconstruction, probably induced from the high compressive strain due to the lattice mismatch with the substrate. We suggest that this unusual behavior could be connected to the high evaporation rate, which helped the direct formation of Ge microcrystals on the Si substrate during the deposition process.

X-ray photoemission electron microscopy (XPEEM) is used to investigate the chemical composition of Ge/Si individual islands obtained by depositing Ge on Si(111) substrates in the temperature range 460–560 °C. We are able to correlate specific island shapes with a definite chemical contrast in XPEEM images, at each given temperature. In particular, strained triangular islands exhibit a Si surface content of 5%–20%, whereas it grows up to 30%–40% for “atoll-like” structures. The island’s stage of evolution is shown to be correlated with its surface composition. Finally, by plotting intensity contour maps, we find that island centers are rich in Si.

By using step-bunched Si(111) surfaces as templates, we demonstrate the self-assembly of an ordered distribution of Ge islands without lithographic patterning. Initially, islands nucleate and evolve at step edges, up to complete ripening, forming long ribbons. Subsequently, island nucleation takes place at the center of flat terraces. Ge islands appear to be regularly spaced in scanning tunneling microscope images. The exploitation of this effect provides a possible route to grow ordered arrays of semiconducting nanostructures.

By exploiting the misfit strain of Ge on Si epitaxy, we examine the significant changes induced by surface stress in the polar structural phase diagram of Si(001) surfaces. Under compressive strain, the atomic and mesoscale structures of the vicinal Si(001) surfaces are converted into a new singular (105) face which does not exist on the strain-free equilibrium shape of Si and Ge. The observed structural modifications of substrates have far-reaching implications for the Stranski-Krastanov evolutionary path of three-dimensional islanding. © 2012 American Physical Society.

The strain-induced self-assembly of suitable semiconductor pairs is an attractive natural route to nanofabrication. To bring to fruition their full potential for actual applications, individual nanostructures need to be combined into ordered patterns in which the location of each single unit is coupled with others and the surrounding environment. Within the Ge/Si model system, we analyze a number of examples of bottom-up strategies in which the shape, positioning, and actual growth mode of epitaxial nanostructures are tailored by manipulating the intrinsic physical processes of heteroepitaxy. The possibility of controlling elastic interactions and, hence, the configuration of self-assembled quantum dots by modulating surface orientation with the miscut angle is discussed. We focus on the use of atomic steps and step bunching as natural templates for nanodot clustering. Then, we consider several different patterning techniques which allow to harness the natural self-organization dynamics of the system, such as: scanning tunneling nanolithography, focused ion beam and nanoindentation patterning. By analyzing the evolution of the dot assembly by scanning probe microscopy, we follow the pathway which leads to lateral ordering, discussing the thermodynamic and kinetic effects involved in selective nucleation on patterned substrates.

A morphological model of the Volmer-Weber type is developed for the reactive interface formed between Fe and a ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8}$ single-crystal surface which takes into account microscopic aspects of the growth such as clustering, metal oxidation via the superconductor substrate, and diffusion through the cluster of elemental species released in the metal-surface redox reaction. The experimental findings of a local chemical analysis by core emission spectroscopy and an atomic-scale structural characterization by scanning tunneling microscopy are in good quantitative agreement with the proposed model indicating that the basic microscopic assumptions are correct.

Semiconductor epitaxial nanostructures have been recently proposed as the key building blocks of many innovative applications in materials science and technology. To bring their tremendous potential to fruition, a fine control of nanostructure size and placement is necessary. We present a detailed investigation of the self-ordering process in the prototype case of Ge/Si heteroepitaxy. Starting from a bottom-up strategy (step-bunching instabilities), our analysis moves to lithographic techniques (scanning tunneling lithography, nanomechanical stamping, focused ion beam patterning) with the aim of developing a hybrid approach in which the exogenous intervention is specifically designed to suit and harness the natural self-organization dynamics of the system. Copyright © 2011 American Scientific Publishers All rights reserved.

We present a novel approach to engineer the growth of strained epitaxial films based on tailoring the elastic-interaction potential between nanostructures with substrate vicinality. By modeling the island-island interaction energy surface within continuum elasticity theory, we find that its fourfold symmetry is broken at high miscuts, producing directions of reduced elastic-interaction energy. As a consequence, it is possible to direct the Ge island growth on highly misoriented Si(001) substrates towards desired pathways. © 2011 American Physical Society.

The pair interaction between Ge islands on vicinal Si(001) substrates is investigated by scanning tunneling microscopy measurements as a function of the miscut angle. By the analysis of the nearest-neighbor island distributions, we assess the dependence of the local strain field on the substrate misorientation. We support our results by modeling elastic relaxation for different shapes and arrangements of islands with finite element calculations. © 2010 The American Physical Society.

In the present paper, we report on a metal-catalyst-free synthesis of carbon nanotubes (CNTs) on a pre-patterned Si(001) surface. Arrays of triangular-shaped holes were created by nanoindentation in specific sites of the sample. After germanium deposition and chemical vapor deposition (CVD) of acetylene, a few CNTs nucleated and grew from germanium nanoparticles. This result illustrate how it is possible to control the growth of CNTs without the use of a metal catalyst, by leading the assembly of Ge nanoparticles with a patterning technique.

A complete description of Ge growth on vicinal Si(001) surfaces in the angular miscut range 0\ifmmode^\circ\else\textdegree\fi{}--8\ifmmode^\circ\else\textdegree\fi{} is presented. The key role of substrate vicinality is clarified from the very early stages of Ge deposition up to the nucleation of 3D islands. By a systematic scanning tunneling microscopy investigation we are able to explain the competition between step-flow growth and 2D nucleation and the progressive elongation of the 3D islands along the miscut direction [110]. Using finite element calculations, we find a strict correlation between the morphological evolution and the energetic factors which govern the ${105}$ faceting at atomic scale.

We present a detailed scanning tunneling microscopy study which describes the morphological transition from ripple-to-dome islands during the growth of Ge on the vicinal Si(1 1 10) surface. Our experimental results show that the shape evolution of Ge islands on this surface is markedly different from that on the flat Si(001) substrate and is accomplished by agglomeration and coalescence of several ripples. By using published data of surface energy and finite-element analysis, we provide a meaningful explanation of our experimental observations. © 2010 The American Physical Society.

We report on measurements of step-step interaction on a flat Si (111) - (7×7) surface and on vicinal Si(001) surfaces with miscut angles ranging between 0.2 and 8°. Starting from scanning tunneling microscopy images of these surfaces and describing steps profile and interactions by the continuum step model, we measured the self-correlation function of single steps and the distribution of terrace widths. Empirical parameters, such as step stiffness and step-step interaction strength, were evaluated from the images. The present experiment allows to assess the dependence of the step-step repulsion on miscut angle, showing how parameters drawn from tunneling images can be used to interpolate between continuum mesoscopic models and atomistic calculations of vicinal surfaces. © 2009 The American Physical Society.

We have grown Ge quantum dots by Physical Vapor Deposition (PVD) on step bunched Si(111) surfaces, vicinal Si(001) surfaces and oxidized Si substrates. Control of one- and two-dimensional ordering of the islands was obtained combining top-down patterning techniques (Focused Ion Beam milling), naturally occurring instabilities and anisotropies typical of Si surfaces. Real-time study of growth kinetics and self-organization of the islands has been accomplished using Scanning Tunnelling Microscopy imaging in UHV. A software routine was used to analyze the in-plane ordering of the islands on selected images. We focused on the study of the first nucleation stages of the dots on high-miscut vicinal Si(001) surfaces, in an attempt to correlate the observed behavior with the properties of these surfaces. The relevance of this research to quantum-dots based technology is also discussed. © 2008 Elsevier Ltd. All rights reserved.

EELFS (Extended Energy Loss Fine Structure) measurements in the reflection mode have been performed at the O K edge of YBa2Cu3O7-δ thin films and sintered pellets in order to study the radial distribution function and its modification after loss of oxygen induced by thermal heating. The Fourier analysis of the extended energy loss signal shows a O-Cu distance in the ceramic oxide slightly smaller than that in CuO.

We examine the time evolution of the O 1s core-level and valence-band spectra of high-quality Pb-free and -doped Bi2Sr2CaCu2O8(001) single crystals cleaved at 20 K in ultrahigh vacuum. A component line at 533.5 eV of the O 1s spectrum is unambiguously identified as extrinsic and separated by the doublet structure related to the normal-state electronic structure of this compound. Evidence is given of the surface physisorption of carbon monoxide at low temperature (

The characterization of the surface electronic structure as a function of the oxygen content by means of photoemission is presented for sintered powders, films, and single crystals of YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}}. Core-level line shapes of O, Ba, and Cu are strongly influenced by the oxygen stoichiometry, which is varied by heating cycles {ital in} {ital vacuo} and in O{sub 2} atmosphere. The evolution of the core levels following oxygen in- and out-diffusion was followed by x-ray-diffraction measurements. The results indicate that the observed spectral changes are related to the oxygen deficiency in the bulk more than to extrinsic contamination of the surface and point to the importance of charge redistribution and oxygen ordering in the basal plane.

We explore the use of the pair distribution function to study the self-organization process of Ge quantum dots on both nanopatterned and nonpatterned oxidized Si(001) surfaces. Dots formation and ordering upon annealing of a Ge thin film are analyzed. The method we use is not limited to this case study. We show how it can be applied to determine short and long range self-ordering of nanostructures. We support our results by applying a software routine to simulate patterns of dots to finally spot the relevant physical aspects of Ge islands self-assembly.

Surface patterning is expected to influence the nucleation site of deposited nanostructures. In the present study, clean Si and SiO2 surfaces were patterned by a nanolithographic process using a Focused Ion Beam (FIB). Ge was evaporated in ultra high vacuum at 873 K on these substrates, resulting in the formation of island arrays. Based on scanning tunneling microscopy and atomic force microscopy images, a statistical analysis was performed in order to highlight the effect of patterning on the size distribution of islands compared to a non-patterned surface. We find that the self-organization mechanism on patterned substrates results in a very good arrangement and positioning of Ge nanostructures, depending on growth conditions and holes distance, both on Si and SiO2 surfaces.

We report a detailed investigation of interdiffusion processes that occur during the growth of germanium nanostructures on the (111)-oriented surface of silicon. In particular, X-ray Absorption Fine Structure (XAFS) measurements performed ex situ show that a Ge(1-x)Si(x) alloy forms during deposition, with average composition x varying between 0.25 and 0.50, depending on substrate temperature and total coverage. By fitting the Si nearest-neighbor numbers around Ge as a function of the deposited thickness with a simple model, the effective vertical composition profile in the growth direction has been estimated. The latter has been described with a static effective diffusion length of (10.0 +/- 1.5) nm at 530 degrees C and (5 +/- 1) nm at 450 degrees C, which is interpreted as the dominance of surface transport processes in the intermixing dynamics. The analysis of the data on Ge-Ge bond length indicates a decrease of the Ge-Ge atomic distances with increasing Ge fraction, confirming previous theoretical predictions for strained epilayers. The XAFS results are compared to morphological information obtained by scanning tunneling microscopy investigations carried out in situ, yielding a satisfactory description for the epitaxy of this system.

Quantum dots (QDs) grown on semiconductors surfaces are actually the main researchers' interest for applications in the forecoming nanotechnology era like nanotransistors and nanomemories. Novel approaches to form ordered patterns of homogeneous nanostructures consist of natural patterning induced by surface instabilities, as step bunching of Si(111) or misoriented Si(001) surfaces, and of nanolithographic techniques, as focused ion beam (FIB) or patterning by scanning tunneling microscopy (STM). Based on the analysis of STM images and movies we report on growth and arrangement of Ge islands on nanopatterned silicon. Several issues are discussed: substrate nanostructuring using different techniques, wetting layer growth, transition up to 3D islands formation and arrangement of QDs to form nanomemories.

Quantum dots (QDs) grown on semiconductors surfaces are actually the main researchers' interest for applications in the forecoming nanotechnology era. New frontiers in nanodevices technology rely on the precise positioning of the nucleation site and on controlling the shape and size of the dots. In this paper we will review some recent studies regarding the control of the nucleation process on semiconductor surfaces. After a brief review of the theory of free nucleation on surfaces and of the role of steps and defects, a few novel approaches to form ordered patterns on surfaces are explored: natural patterning induced by surface instabilities (as step bunching or step meandering), in situ substrate patterning by Scanning Tunneling Microscopy (STM), high resolution patterning by Focused Ion Beam (FIB). Growth of epitaxial layers of semiconductors (Ge/Si(100) or InAs/GaAs(100)) on these patterned surfaces has been studied by STM or Atomic Force Microscopy (AFM) unveiling the way in which the first atoms start to aggregate and identifying their exact nucleation site. Control of the dot size to match the patterning typical wavelength has been achieved by using surfactants on misoriented substrates. STM images acquired in real time allowed to identify the mechanism of Ge cluster formation on patterned Si(100), and to follow the island transition from pre-pyramid to pyramid. Nucleation of Ge islands on SiO2 layers has been obtained thanks to FIB tight patterning, achieving island densities of 3.5x10^10/cm^2.

Quantum dots (QDs) grown on semiconductors surfaces are actually the main researchers' interest for applications in the forecoming nanotechnology era. New frontiers in nanodevice technology rely on the precise positioning of the nucleation site and on controlling the shape and size of the dots. Novel approaches to form ordered patterns of homogeneous nanostructures are explored: natural patterning induced by surface instabilities (as step bunching of Si(111) or misoriented Si(001) surfaces), standard patterning with high resolution lithographic techniques, implantation of Ga+ ions by Focused Ion Beam (FIB), or in situ substrate patterning by Scanning Tunneling Microscopy (STM). Based on the analysis of STM images we report on growth and arrangement of Ge islands on Si(001) substrates nanopatterned using several different approaches. The first is a natural method based on the regular step bunching that occurs on Si(111) surfaces with different annealing treatments. The second is based on the self organization of a Si(001) misoriented surface covered by a thin layer of a GeSi alloy. The third exploit an array of holes produced by STM lithography. The forth is a tight pattern created by FIB. We analyze the resulting distribution of islands resulting from all these approaches.

The stoichiometry of Ge/Si islands grown on Sis111d substrates at temperatures ranging from 460 to 560 degrees Celsius was investigated by x-ray photoemission electron microscopy sXPEEMd. By developing a specific analytical framework, quantitative information on the surface Ge/Si stoichiometry was extracted from laterally resolved XPEEM Si 2p and Ge 3d spectra, exploiting the chemical sensitivity of the technique. Our data show the existence of a correlation between the base area of the self-assembled islands and their average surface Si content: the larger the lateral dimensions of the 3D structures, the higher their relative Si concentration. The deposition temperature determines the characteristics of this relation, pointing to the thermal activation of kinetic diffusion processes.