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The atomic structure of single steps on the Si(111)- 3 3 ? -Sn surface and the dynamics of Sn adatoms in the vicinity of these steps were studied. The work was performed using scanning tunneling microscopy (STM) and ab initio calculations based on the density functional theory. The atomic structure model of the single steps consisting of Sn atomic chains along the steps was developed. This structure leads to the formation of potential double-wells near the steps acting as traps for Sn atoms and explains the fluctuating tunneling current recorded in these areas.

The atomic structure of well-ordered single steps on the $\mathrm{Si}(111)\ensuremath{-}\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}\ensuremath{-}\mathrm{Sn}$ surface and the dynamics of Sn adatoms in the vicinity of these steps was studied. The work was performed using low-temperature scanning tunneling microscopy (STM) and ab initio calculations based on the density-functional theory. The STM tip was used to record the tunneling current versus time on top of oscillating adatoms, keeping the feedback loop turned off. The dynamics of adatoms, detected as the telegraph noise present in the tunneling current, was registered near the steps at 80 K. The atomic structure model of the single steps consisting of Sn atomic chains along the steps was developed. This structure leads to the formation of potential double wells near the steps acting as traps for Sn atoms and explains the fluctuating current recorded in these areas.

We report a combined experimental and theoretical study on the formation of dumbbell silicene structures on Ag(110). High-resolution scanning tunneling microscopy (STM) reveals a rich tapestry of adatom-free and -decorated bidimensional silicene phases covering the whole Ag(110) surface. The most thermodynamically stable silicene models obtained from density-functional theory (DFT) perfectly reproduce all features observed by STM. These phases correspond to different Si buckled honeycomb reconstructions ((13 × 4), c(18 × 4), and c(8 × 4)) composed of two periodic motifs common to all structural models. DFT calculations show that these reconstructions are stabilized by the presence of ordered arrays of Si adatoms adsorbed on top of silicene in a dumbbell configuration. Grazing incidence X-ray diffraction (GIXD) measurements confirm the growth of a dumbbell silicene layer. The structure factor values are well reproduced by a (13 × 4) model with 4 Si adatoms per unit cell and a slight distortion of the hexagonal unit cell. Our STM-DFT-GIXD study demonstrates the formation of dumbbell silicene, a theoretically predicted two-dimensional Si allotrope. This opens up perspectives for tuning the peculiar properties of silicene.

Silicene, the 2D silicon allotrope analogue of graphene, was theoretically predicted in 1994 as a metastable buckled honeycomb silicon monolayer. Similarly to its carbon counterpart it was predicted to present an electronic structure hosting Dirac cones. In the last decade a great deal of work has been done to synthesize silicene and exploit its properties. In this paper we will review our research group activity in the field, dealing in particular with silicon-substrate interaction upon silicon deposition, and discuss the still debated silicene formation starting from the chemistry of silicon unsaturated compounds.

We report the discovery of a temperature-induced phase transition between the α and β structures of antimonene. When antimony is deposited at room temperature on bismuth selenide, it forms domains of α-antimonene having different orientations with respect to the substrate. During a mild annealing, the β phase grows and prevails over the α phase, eventually forming a single domain that perfectly matches the surface lattice structure of bismuth selenide. First-principles thermodynamics calculations of this van der Waals heterostructure explain the different temperature-dependent stability of the two phases and reveal a minimum energy transition path. Although the formation energies of freestanding α- and β-antimonene only slightly differ, the β phase is ultimately favored in the annealed heterostructure due to an increased interaction with the substrate mediated by the perfect lattice match.

We report the discovery of a temperature induced phase transition between the \alpha and \beta structures of antimonene. When antimony is deposited at room temperature on bismuth selenide, it forms domains of \alpha-antimonene having different orientations with respect to the substrate. During a mild annealing, the \beta phase grows and prevails over the \alpha phase, eventually forming a single domain that perfectly matches the surface lattice structure of bismuth selenide. First principles thermodynamics calculations of this van der Waals heterostructure explain the different temperature-dependent stability of the two phases and reveal a minimum energy transition path. Although the formation energies of free-standing \alpha- and \beta-antimonene only slightly differ, the \beta phase is ultimately favoured in the annealed heterostructure due to an increased interaction with the substrate mediated by the perfect lattice match.

In this paper we report on an x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) investigation of the Mn/GaAs(001) interface. The deposition of Mn in the submonolayer regime on the GaAs(001) surface at $390{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ induces a ($2\ifmmode\times\else\texttimes\fi{}1$) reconstruction which evolves to a ($2\ifmmode\times\else\texttimes\fi{}2$) by increasing the Mn amount. The combined information from XAS and XMCD indicates that the Mn atoms are located on surface sites possessing an almost square symmetry with a limited hybridization of the Mn electronic levels with the GaAs ones. The Mn atoms present a magnetic moment ranging between 4.9 and $4.2\phantom{\rule{0.28em}{0ex}}{\ensuremath{\mu}}_{B}$ depending on the surface reconstruction. The Mn/GaAs(001) interface shows a paramagnetic behavior with an average magnetic moment which decreases by increasing the Mn amount. This variation of the surface magnetization is interpreted as an antiferromagnetic coupling of the Mn magnetic moments as the surface density of Mn atoms is increased. A further increase of the Mn amount after the ($2\ifmmode\times\else\texttimes\fi{}2$) transition leads to the formation of Mn aggregates on the sample surface.

We report the first experimental study of graphene transferred on \b{eta}-Si3N4(0001)/Si(111). Our work provides a comprehensive quantitative understanding of the physics of ultrathin Si3N4 as a gate dielectric for graphene-based devices. The Si3N4 film was grown on Si(111) under ultra-high vacuum (UHV) conditions and investigated by scanning tunneling microscopy (STM). Subsequently, a graphene flake was deposited on top of it by a polymer-based transfer technique, and a Hall bar device was fabricated from the graphene flake. STM was employed again to study the graphene flake under UHV conditions after device fabrication and showed that surface quality is preserved. Electrical transport measurements, carried out at low temperature in magnetic field, revealed back gate modulation of carrier type and density in the graphene channel and showed the occurrence of weak localization. Under these experimental conditions, no leakage current between back gate and graphene channel was detected.

The possibility to intercalate noble gas atoms below epitaxial graphene monolayers coupled with the instability at high temperature of graphene on the surface of certain metals has been exploited to produce Ar-filled graphene nanosized blisters evenly distributed on the bare Ni(111) surface. We have followed in real time the self-assembling of the nanoblisters during the thermal annealing of the Gr/ Ni(111) interface loaded with Ar and characterized their morphology and structure at the atomic scale. The nanoblisters contain Ar aggregates compressed at high pressure arranged below the graphene monolayer skin that is decoupled from the Ni substrate and sealed only at the periphery through stable C-Ni bonds. Their in-plane truncated triangular shapes are driven by the crystallographic directions of the Ni surface. The nonuniform strain revealed along the blister profile is explained by the inhomogeneous expansion of the flexible graphene lattice that adjusts to envelop the Ar atom stacks.

We report a study of the interface between antimony and the prototypical topological insulator Bi2Se3. Scanning tunnelling microscopy measurements show the presence of ordered domains displaying a perfect lattice match with bismuth selenide. Density functional theory calculations of the most stable atomic configurations demonstrate that the ordered domains can be attributed to stacks of β-antimonene.

The surface electronic structure of Si(1 1 1)-[Formula: see text] has been studied by angle-resolved photo electron spectroscopy. Replicas of the S 1 surface state are found in correspondence with several [Formula: see text] unit cells in the reciprocal space. This observation resolves in a direct way the long-standing dichotomy between the structural and electronic properties of the system previously discussed on the basis of the [Formula: see text] or [Formula: see text] R30° surface models.

The chirality transfer control from a single molecule to the surface molecular superstructures and to the substrate is a challenging and important aspect of two-dimensional chiral nanostructures for tailoring the functionality of molecular--metal interfaces. However, how the chiral transfer takes place still remains an open question. In this paper, we combine data from scanning tunneling microscopy, low-energy electron diffraction and circular dichroism in the angular distribution of valence photoelectrons measurements to interpret the formation of extended chiral self-assembled domains obtained by adsorption of the chiral amino-alcohol alaninol on Cu(100). We find that the deposition of alaninol enantiomers results in the formation of isolated tetramers that are aligned along the directions of the substrate at low coverage, whereas a rotation of 14A degrees with respect to the Cu(100) unit vectors is observed when small clusters of tetramers are formed. This suggests that the reorientation originates from forces acting when tetramers pack together in the self-assembling process. Direct information on the chirality transfer from molecules to the substrate has been obtained by the dichroic behavior of a mixed molecule-copper valence state showing that the presence of molecular chiral domains induces asymmetric interaction with the substrate and locally transfers chiral character to the underlying metal atoms participating in the adsorption process.

The formation of 2D chiral monolayers obtained by self-assembly of chiral molecules on surfaces has been widely reported in the literature. Control of chirality transfer from a single molecule to surface superstructures is a challenging and important aspect for tailoring the properties of 2D nanostructures. However, despite the wealth of investigations performed in recent years, how chiral transfer takes place on a large scale still remains an open question. In this paper we report a coupling of scanning tunneling microscopy and low energy electron diffraction measurements with an original theoretical approach, combining molecular dynamics and essential dynamics with density functional theory, to investigate self-assembled chiral structures formed when alaninol adsorbs on Cu(100). The peculiarity of this system is related to the formation of tetrameric molecular structures which constitute the building blocks of the self-assembled chiral monolayer. Such characteristics make alaninol/Cu(100) a good candidate to reveal chiral expression changes. We find that the deposition of alaninol enantiomers results in the formation of isolated tetramers that are aligned along the directions of the substrate at low coverage or when geometrical confinement prevents long-range order. Conversely, a rotation of 14° with respect to the Cu(100) unit vectors is observed when small clusters of tetramers are formed. An insight to the process leading to a 2D globally chiral surface has been obtained by monitoring molecular assemblies as they grow from the early stages of adsorption, suggesting that the distinctive orientation of the self-assembled monolayer originates from a balance of cooperating forces which start acting only when tetramers pack together to form small clusters.

Summary Introduction: The aim of this study is to investigate the nanocrystallization of steels caused by the transformation from the austenitic to the martensitic phase induced by a severe plastic deformation (SPD) treatment. In this framework, we applied an air blast shot peening treatment, which is a simple protocol widely used for industrial purposes. Methods: AISI 286 and AISI 316 specimens were peened for different times and polished using diamond pastes in order to remove corrugations higher than 1 μm. The characterization of the steel surfaces was performed by atomic force microscopy (AFM) operating in contact mode. Additional EDXD measurements were performed to confirm the phase transition. Results and Discussion: An AFM-based characterization at nanometric level of the steel surfaces is provided. When the peening exceeds a threshold time that, as expected, depends on the steel composition, a uniform nanostructuration is detected. It is well known that such rearrangement is associated to the growth of a martensitic phase. To date, AFM has been employed in this field only for few applications and to solve specific problems. On the other hand, our results demonstrate that this is a useful technique for the characterization of hardened surfaces, especially when non-destructive sample preparation treatments are required. Moreover, we show that AFM can be a useful tool also for in situ industrial diagnostics of metallic parts.

The Si/Ag(110) interface is attracting increasing interest since the recent claim that the one-dimensional (1D) nanostructures formed at this surface would be silicene nanoribbons. Lately, the synthesis of multilayer silicene nanoribbons was reported as well. In this paper, we report on the scanning tunneling microscopy, low-energy electron diffraction, and density functional theory study of the 1D nanostructures forming on Ag(110) upon Si deposition at temperatures above 460 K. Two different kinds of nanostructures are here described: nanodikes and nanotrenches, respectively protruding from and engraved on the Ag(110) substrate. The first kind of structure was recently reported to be constituted by multistacks of silicene nanoribbons. We show here that nanodikes and nanotrenches are the result of silver faceting stabilized by silicon atoms. Indeed, such nanostructures share the very same facet crystallographic features, namely inclination and reconstruction: the top and base parts are Ag(110) planes, while their sides are Ag(221) and Ag(22$\overline{1}$) facets stabilized by Si atoms. Finally, density functional theory calculations suggest that Si atoms may substitute Ag atoms in such facets, casting doubts on the usual assumption that Ag is a suitable substrate for silicene growth because it is inert against silicon.

The low-temperature phase of Sn/Ge(1 1 1) 3 × 3 is studied by means of first-principle calculations at the DFT level and in the GW approximation. Experimental STM images taken at 80 K are presented and compared with theoretical predictions. This analysis allows to give support to a geometry in which one Sn adatom is higher than the other two in the surface unit cell and excludes the complementary geometry in which two Sn adatoms are higher than the third. Concerning the very low temperature phase (T < 30 K), we show that from a purely energetic point of view a sqrt3xsqrt3 geometry cannot be excluded, being almost degenerate with the 3 × 3 phase.

Using Surface Differential Reflectivity (SDR) we have studied the optical properties of a PbSe(100) surface obtained by cleaving a PbSe sample in ultra-high-vacuum environment. SDR showed the existence of a gap of approximately 1.6 eV. The derived surface dielectric function showed, in the energy range of the present study (1.2–3.6 eV), three optical transitions observed at energies of 1.59, 2.01 and 2.39 eV. By putting together angle resolved photoemission data and theoretical calculations, these transitions have been, tentatively, assigned to different points in the surface Brillouin zone. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report the discovery of a nearly-free electronlike resonant state for a $\ensuremath{\beta}{\text{-Si}}_{3}{\text{N}}_{4}(0001)\ensuremath{-}8\ifmmode\times\else\texttimes\fi{}8$ layer grown on Si(111), as observed by angle-resolved photoemission and scanning tunneling spectroscopy. Comparison with measurements performed on a $\mathrm{Si}(111)\ensuremath{-}7\ifmmode\times\else\texttimes\fi{}7$ surface helped us in the identification of the band. It is found that this parabolic state is degenerate with surface projected bulk bands of the Si(111) substrate.

Silicon reactivity at the Ag(111) surface M. Sattaa, S. Colonnab, R. Flamminib, A. Cricentib, F. Ronci*b a CNR-Istituto per lo Studio dei Materiali Nanostrutturati, Dipartimento di Chimica, Università di Roma "La Sapienza", P.le Aldo Moro 5, I-00185, Roma, Italy b CNR-Istituto di Struttura della Materia, Via Fosso del Cavaliere 100, I-00133 Roma, Italy *e-mail: fabio.ronci@ism.cnr.it Keywords: Silicon, silver, silicene, STM, plane-wave DFT Introduction In the last years a large number of papers reported the synthesis of silicene, a 2D silicon allotrope analogue of graphene, on Ag(110) and Ag(111) substrates [1], attracting a large interest in the scientific community. The modelization of silicene on silver substrates is generally based on the assumption of a complete immiscibility between silicon and silver. Recent results, however, revealed that in both cases the silver substrate is significantly altered upon silicon deposition, inducing either substrate reconstructions [2], faceting [3] or domains growth inside the first Ag(111) layer rather than on top of it [4-5]. Results and Discussion In this talk our recently published results on the Si/Ag(111) system [6] will be described. Through a combined DFT and STM study, the basic exchange mechanism between Si and the topmost layer Ag atoms is explained. We provide a description of the nucleation process, demonstrating that the impinging Si atoms can actually penetrate the first silver layer by exchanging position with Ag atoms. Indeed, despite the quite high energy barrier, the Si/Ag exchange process is made possible by exploiting the excess energy released upon the Si atoms adsorption process. Figure 1: Schematic picture describing the silicon atoms adsorption and thermalization processes at the Ag(111) surface Conclusions We demonstrate that Si atoms are embedded and confined in the first Ag(111) layer where they act as seeds for the growth of recessed islands. Our results demonstrate that surface reactivity cannot be neglected when describing the Si/Ag(111) interface formation. References: [1]H. Oughaddou, H. Enriquez, M. R. Tchalala, H. Yildirim, A. J. Mayne, A. Bendounan, G. Dujardin, M. Ait Ali, A. Kara; Silicene, a promising new 2D material; Progr. Surf. Sci.; 90(2015) 46 and references therein. [2]R. Bernard, T. Leoni, A. Wilson, T. Lelaidier, H. Sahaf, E. Moyen, L. Assaud, L. Santinacci, F. Leroy, F. Cheynis, A. Ranguis, H. Jamgotchian, C. Becker, Y. Borensztein, M. Hanbücken, G. Prévot, and L. Masson; Growth of Si ultrathin films on silver surfaces: Evidence of an Ag(110) reconstruction induced by Si; Phys. Rev. B; 88 (2013) 121411. [3]F. Ronci, G. Serrano, P. Gori, A. Cricenti, S. Colonna; Silicon-induced faceting at the Ag(110) surface; Phys. Rev. B; 89 (2014) 115437. [4]J. Sone, T. Yamagami, Y. Aoki, K. Nakatsuji, H. Hirayama; Epitaxial growth of silicene on ultra-thin Ag(111) films; New J. Phys. 16 (2014) 095004. [5]G. Prévot, R. Bernard, H. Cruguel, Y. Borensztein; Monitoring Si growth on Ag(111) with scanning tunneling microscopy reveals that silicene structure involves silver atoms; Appl. Phys. Lett.; 105 (2014) 213106. [6]M Satta, S. Colonna, R. Flammini, A. Cricenti, F. Ronci; Silicon Reactivity at the Ag(111) Surface; Phys. Rev. Lett.; 115 (2015) 026102.

Using a combination of scanning tunneling microscopy (STM) imaging and total-energy calculations within density functional theory, we evaluate a range of new and existing reconstruction models for Si nanoribbons that form on Ag(110). We propose two models that are consistent with experimental estimates of the Si coverage and the previously reported formation of Ag missing rows. The models yield STM images in excellent agreement with experiment, and are thermodynamically stable. This work provides clear evidence for a strongly bound Si-Ag reconstruction on Ag(110), and does not find any support for models based on silicene.

The modelization of the Si/Ag(110) and Si/Ag(111) systems is generally based on the assumption of a complete immiscibility between silicon and silver. Recent results, on the contrary, demonstrated that in both cases the silver substrate is altered upon silicon deposition through Si-Ag exchange processes, either inducing substrate reconstructions [1] and faceting [2], or suggesting that Si-Ag reactivity must be taken into account. In this talk, I will at first survey our recent results on the Si/Ag(110) system [5], showing novel structural models for nanoribbons obtained considering missing-row reconstructions of the Ag(110) substrate. On the basis of STM measurements, Si coverage evaluation and density functional theory simulations, our models result to be thermodynamically stable, consistent with the experimental Si and Ag coverages, and yield simulated STM images in agreement with the measured data; in contrast, silicene-based models yield unsatisfactory results. Next, I'll discuss about our recently published paper [6] on the Si/Ag(111) that describes, through a combined DFT and STM study, the basic exchange mechanism between Si and the topmost layer Ag atoms that results in the growth of the well-known different kind of reconstructed domains inside, rather than on top, the first Ag(111) layer. [1] R. Bernard, T. Leoni, A. Wilson, T. Lelaidier, H. Sahaf, E. Moyen, L. Assaud, L. Santinacci, F. Leroy, F. Cheynis, A. Ranguis, H. Jamgotchian, C. Becker, Y. Borensztein, M. Hanbücken, G. Prévot, and L. Masson, Phys. Rev. B 88, 121411 (2013). [2] F. Ronci, G. Serrano, P. Gori, A. Cricenti, and S. Colonna, Phys. Rev. B 89, 115437 (2014). [3] J. Sone, T. Yamagami, Y. Aoki, K. Nakatsuji, and H. Hirayama, New J. Phys. 16, 095004 (2014). [4] G. Prévot, R. Bernard, H. Cruguel, and Y. Borensztein, Appl. Phys. Lett. 105, 213106 (2014). [5] C. Hogan, S. Colonna, R. Flammini, A. Cricenti, and F. Ronci, submitted. [6] M Satta, S. Colonna, R. Flammini, A. Cricenti, and F. Ronci, Phys. Rev. Lett. 115, 026102 (2015).

A number of studies have proposed that silicene--a single layer of a buckled honeycomb structure of Si atoms--is responsible for the one-dimensional nanoribbon (NR) structures observed to form when silicon is evaporated on Ag(110) [1]. Recent works using Raman and surface differential reflectance spectroscopy have cast doubts upon this interpretation, however [2,3]. Here we propose new reconstruction models for Ag(110)/Si nanoribbon structures based on new Si coverage measurements and evaluate them using density functional theory simulations[4]. Our models are shown to be thermodynamically stable, are consistent with the experimental Si and Ag coverages [5], and yield simulated STM images in excellent agreement with the measured data; in contrast, silicene-based models yield unsatisfactory results. This work provides clear evidence for strongly bound Si-Ag reconstructions on Ag(110), and finds little support for silicene-like structures [1]. [1] A. Kara et al, Journal of Superconductivity and Novel Magnetism 22, 259 (2009); P. De Padova, et al, Applied Physics Letters 96, 261905 (2010); B. Aufray et al, Applied Physics Letters 96, 183102 (2010); C. Lian and J. Ni, Physica B: Condensed Matter 407, 4695 (2012). [2] E. Speiser et al, Applied Physics Letters 104, 161612 (2014). [3] Y. Borensztein, et al, Physical Review B 89, 245410 (2014). [4] C. Hogan et al, in preparation. [5] R. Bernard et al, Physical Review B 88, 121411 (2013)

In-situ X-ray diffraction studies have been performed on a Li4/3Ti5/3O4 electrode upon cycling in a Li cell, by using a very high energy (87.5 keV) synchrotron beam. The real time structural changes of its crystalline lattice were observed over two complete cycles of the cell. The high-resolution measurements allowed us to precisely monitor the extremely small breathing movement of the structure and to plot the curve of the lattice parameter as a function of the lithiation degree. The investigation revealed an unexpected behavior in the structural evolution upon cycling, which was attributed to the reversible passage from a monophasic to a biphasic domain upon insertion. Furthermore, the structural evolution turned out to be slightly different in the first and in the second cycle. This suggests that irreversible rearrangements, like the ones observed for every other insertion compound, occur also in this case, although on an extremely smaller scale.

Spontaneous reactions occurring at the surface of LiNi0.8Co0.2O2 and LiMn2O4-based electrodes during the storage in organic non-aqueous electrolytes have been investigated by diffuse reflectance FTIR technique. It is found that both materials spontaneously form different inorganic and organic compounds on their surface when in contact with electrolyte solutions. The nature of these self-acting reactions is moreover found to be similar to that of the processes occurring during electrochemical cycling of the electrodes. Reaction mechanisms and the final products depend on both electrode surface chemistry and the nature of electrolyte used. It appears that the spontaneous reactions are initiated by lithium deintercalation from the electrode active material. The influence of different factors, e.g. degree of lithiation of the active material, roughness of the electrode surface and temperature on the reaction rate is discussed.

Spontaneous reactions occurring at the surface of LiNiyCo(1−y)O2-based electrodes during contact with non-aqueous organic electrolytes have been investigated by FTIR and Raman spectroscopy. It is found that several types of compounds and/or functional groups are formed on the electrode surface and that these compounds appear to be dependent on the type of electrolyte used. Thus, for a LiClO4-propylene carbonate (PC) electrolyte, the main reaction is formation of Li-carbonate, whereas in the case of LiPF6-ethylene carbonate/dimethyl carbonate (EC/DMC) electrolyte formation of P-, O- and F-containing compounds dominate. Spectroscopic data also show a variation of the LiNi(1−y)CoyO2 crystalline structure during storage in an electrolyte which probably is due to a spontaneous deintercalation of the Li ions. An analysis of the newly formed species is presented and possible reaction mechanisms are discussed.

The in situ observation of the structural changes induced in electrodic active materials upon cycling is commonly performed by angular dispersive X-ray diffraction (ADXD). Here an alternative approach based on the energy dispersive X-ray diffraction (EDXD) is described, together with its application to the study of the well-known LiNi0.8Co0.2O2 cathode material. In the first part, we briefly describe the technique and its application to the in situ study of electrode materials. In the second part, we discuss how the experimental data have to be processed in order to separate the signal produced by the active material only from that of the other parts of the cell. In the third part, a dense sampling of the changing structure of LiNi0.8Co0.2O2 during the first three charge−discharge cycles is shown in terms of the lattice parameters time evolution. The results confirm what was already reported in the literature on the behavior of similar materials, providing, at the same time, detailed information on the ev...

The investigation of structural changes of intercalation materials during cycling is of primary importance to understand the factors that limit their cyclability. In particular in-situ measurements, allowing the simultaneous acquisition of structural and electrochemical data, consent to directly correlate the cycling performance of the material under test to its structure evolution and vice versa.The energy dispersive X-ray diffraction (EDXD) technique has been applied to the study of the structural evolution of LiNi0.8Co0.2O2 during several charge−discharge cycles performed with increasing anodic limits. This way of monitoring the effect of Li ions intercalation−deintercalation process on the host materials lattice parameters proved to be a valid alternative to the traditional angle dispersive X-ray diffraction (ADXD) method, solving some of its problems, first of all the question of the X-ray absorption.The results described in this paper confirm what reported in previous literature works based on in-si...

Layered compounds, such as LiNiO2 and LiCoO2 , have been extensively studied as active cathodic materials in lithium-ion batteries. Mixed oxides having general formula LiNiyCo1−yO2 represent a good compromise between the limited cyclability of LiNiO2 and the high cost of LiCoO2. However, recent studies have demonstrated that LiNiyCo1−yO2 compounds are thermally unstable in their charged state, undergoing exothermic reactions that might cause thermal runaway and safety concern. The stability of the compounds may be greatly controlled by doping with a suitable metal, M = Al, Mg. In this work we further investigate the role of the doping metal on the thermal, electrochemical and structural characteristics of the LiNiyCo1−y−zMzO2 electrode materials. These materials were prepared using a soft chemistry route, to achieve the proper control of the chemical homogeneity and of the microstructural properties of the final samples. The thermal behavior of the doped LiNiyCo1−y−zMzO2, where M = Al, was studied using differential scanning calorimetry. The structural properties upon cycling were investigated by a recently, in-house developed, in situ energy dispersive X-ray diffraction (EDXD) technique. The reversibility and rate capabilities of the cathodes in lithium cells were characterized using electrochemical equipment.

Lithium polymer electrolytes formed by dissolving a lithium salt LiX in poly(ethylene oxide) PEO may find useful application as separators in lithium rechargeable polymer batteries. The main problems which are still to be solved for a complete successful operation of these materials are the reactivity of their interface with the lithium metal electrode and the decay of their conductivity at temperatures below 70°C. In this paper we demonstrate that a successful approach for overcoming these problems is the dispersion of selected, low-particle size ceramic powders in the polymer mass with the aim of developing new types of nanocomposite PEO-LiX polymer electrolytes characterized by enhanced interfacial stability as well as by improved ambient temperature transport properties.

Lithium polymer electrolytes formed by dissolving a lithium salt LiX in poly(ethylene oxide) PEO, may find useful application as separators in lithium rechargeable polymer batteries. The main problems, which are still to be solved for a complete successful operation of these materials, are the reactivity of their interface with the lithium metal electrode and the decay of their conductivity at temperatures below 70°C. In this paper we demonstrate that a successful approach for overcoming these problems, is the dispersion of selected ceramic powders in the polymer mass, with the aim of developing new types of composite PEO–LiX polymer electrolytes characterized by enhanced interfacial stability, as well as by improved ambient temperature transport properties.

In the development of rechargeable lithium polymer batteries it is of paramount importance to control the passivation phenomena occurring at the lithium electrode interface. It is well estabilished that the type and the growth of the lithium passivation layer is unpredictably influenced by the presence of liquid components and/or impurities in the electrolyte. Therefore, one approach to improve the stability of the lithium interface is the use of liquid-free, highly pure electrolytes. The electrochemical properties of a composite polymer electrolyte obtained by hot pressing a mixture of polyethylene oxide (PEO), a lithium salt (lithium tetrafluoroborate, LiBF4) and a powdered ceramic additive (γ-LiAlO2), will be presented and discussed. The electrochemical characterization included the determination of the ionic conductivity, the anodic break-down voltage and, most importantly, the stability of the lithium metal electrode interface and the lithium stripping-plating process efficiency. The main feature of this dry, true solid-state electrolyte is a very good compatibility with the lithium metal electrode, demonstrated by a very high lithium cycling efficiency, which approaches a value of 99%.

Several types of lithium ion conducting polymer electrolytes have been synthesized by hot-pressing homogeneous mixtures of the components, namely, poly(ethylene oxide) (PEO) as the polymer matrix, lithium trifluoromethane sulfonate (LiCF{sub 3}SO{sub 3}), and lithium tetrafluoroborate (LiBF{sub 4}), respectively, as the lithium salt, and lithium gamma-aluminate {gamma}-LiAlO{sub 2}, as a ceramic filler. This preparation procedure avoids any step including liquids so that plasticizer-free, composite polymer electrolytes can be obtained. These electrolyte have enhanced electrochemical properties, such as an ionic conductivity of the order of 10{sup {minus}4} S/cm at 80--90 C and an anodic breakdown voltage higher than 4 V vs. Li. In addition, and most importantly, the combination of the dry feature of the synthesis procedure with the dispersion of the ceramic powder, concurs to provide these composite electrolytes with an exceptionally high stability with the lithium metal electrode. In fact, this electrode cycles in these dry polymer electrolytes with a very high efficiency, i.e., approaching 99%. This in turn suggests the suitability of the electrolytes for the fabrication of improved rechargeable lithium polymer batteries.

A Li–Mn–O defective spinel phase material has been synthesized through repeated grinding and heating (T

We present scanning tunneling microscopy (STM) measurements at the TaSe2 surface showing a first order process leading to a surface Mott transition. This metal to insulator transition is triggered by the commensurate charge density wave (CDW) which set up at 475 K. The electronic structure rearrangement due to the new periodicity gives rise to a half filled sub-band prone to the localization phenomena. This transition results fully developed below 250 K in agreement with previous photoemission measurements.

Chiral self-assembled molecular networks (SAMNs) are important for technological and fundamental reasons. In spite of the large number of works in this field, the mechanism of chirality transfer from single molecules to large-scale two-dimensional (2D) networks is not fully understood yet. This work reports on the self-assembly of simple amino alcohols with different chirality measures on Cu(100). Ethanolamine (2-amino-1-ethanol), alaninol (2-amino-1-propanol) and butanolamine (2-amino-1-butanol) adsorbed on Cu(100) have been investigated with scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), photoelectron spectroscopy (PES) and density functional theory (DFT) calculations. This study addresses the role of the single-molecule handedness in the global chirality expression, showing how the chirality measure of the precursors plays an important role in the formation of globally chiral superstructures. © 2014 Elsevier B.V. All rights reserved.

Nonplanar titanyl phthalocyanine (TiOPc) molecules, characterized by a central, highly dipolar Ti-O group, can offer further degrees of freedom in tailoring the properties of hybrid organic-inorganic structures. Here, we combine scanning tunneling microscopy and low-energy electron diffraction measurements with ab initio density functional theory calculations to investigate the interaction of TiOPc molecules with the Ag(100) surface. Isolated molecules are adsorbed with the macrocycle parallel to the surface in two different configurations: with the O atom pointing outward (UP configuration) or with the O atom pointing toward the surface (DOWN configuration). A different interaction of UP and DOWN molecules with the surface can account for their different orientation on the surface as well as for the observation of marked chiral effects only for UP molecules. Self-assembled domains of randomly mixed UP and DOWN molecules form at the monolayer coverage, driven by a subtle interplay between the molecule-surface interaction, the intermolecular dipolar attraction, and the side interaction between adjacent molecules. Chiral patterns of isolated molecules, observed by STM, are transferred to these domains. Remarkably, theoretical calculations disclose an interfacial nature of such chiral properties. Finally, a metallic behavior observed for the monolayer disappears for TiOPc molecules in a second layer, which are electronically decoupled from the substrate.

Lithiated transition metal oxides having a layered structure and general formula LiMO2, have been extensively studied as positive electrode active materials for lithium or lithium-ion batteries. In particular, lithium nickel dioxide (LiNiO2) and lithium cobalt dioxide (LiCoO2) present a layered structure with high diffusion coefficients for the lithium ion. This latter property is very important in order to realize practical devices having high discharge rates. LiNiO2, compared with LiCoO2, has the advantage to be a cheaper material with a higher specific capacity for lithium cycling, but its stability upon cycling can be greatly influenced by the displacement of Ni ions from the Ni layers to the Li planes as the content in lithium is reduced over a certain value. Recently, solid solutions such as LiNixCo1−xO2 have been proposed to offer a compromise between stability, cost and capacity. In this work we have studied LiNi0.5Co0.5O2 prepared by the Complex Sol-Gel Process (CSGP). The advantage of this procedure toward the solid-state process is the high homogeneity in composition and in particle dimension of the synthesized compounds. The samples have been characterized electrochemically using chronopotentiometric, voltammetric and impedance measurements in liquid electrolyte. The results indicates that CSGP-synthesized LiNi0.5Co0.5O2 shows good cyclability (after 1000 cycles about 2/3 of the initial capacity can still be cycled) only if the anodic potential is limited to about 4.2 V. The quite low values of the specific capacity (∼70 mAh/g at C/1 charge-discharge rate) can be justified by the non-complete calcination reaction, as suggested by X-ray measurements. Kinetic properties have been evaluated by Electrochemical Impedance Spectroscopy measurements, which have shown quite high values for the lithium chemical diffusion coefficient (10−7÷10−8 cm2s−1) and its unexpected decrease as deintercalation proceeds from x=0.5 in LiNi0.5Co0.5O2.

Interest in the Si/Ag(110) system, which forms highly ordered linear nanostructures coined 'silicon nanoribbons', was recently boosted by the claim that such nanoribbons may be formed by silicon atoms arranged in a 2D honeycomb structure as in graphene, i.e. silicene. Despite such a revived interest, many discrepancies still exist in the recently reported results. This paper reports on a systematic investigation by scanning tunneling microscopy and low-energy electron diffraction of the Si/Ag(110) system as a function of the amount of deposited silicon and the deposition temperature. This reveals a complex interplay between these two factors, resulting in a rich array of possible self-assembled nanostructures and surface reconstructions. Several novel findings and clarification of the contradictory results reported in the literature are discussed in this work. In particular, the deposition temperature is demonstrated to be a key parameter to control the width of the Si nanoribbons produced. Recently, massive linear nanostructures were reported to be 'multilayer silicene', forming once the deposited silicon amount exceeds full coverage. However, we show that such nanostructures are also observed at low silicon coverage, demonstrating that their formation is exclusively determined by a deposition temperature higher than 460 K. On the other hand, for Si amounts higher than one monolayer the surface presents a novel c(8 × 4) reconstruction, which is responsible for the ×4 periodicity detected by LEED measurements, previously attributed to the 1.6 nm-wide nanoribbons overlayer or to 'multilayer silicene'. Finally, the large collection of acquired data also allowed us to single out image artifacts that may explain the contradictory results appearing in previous papers.

The growth of Mn nanostructures on a 1D grating of silicon nano-ribbons is investigated at atomic scale by means of scanning tunneling microscopy, low energy electron diffraction and core level photoelectron spectroscopy. The grating of silicon nano-ribbons represents an atomic scale template that can be used in a surface-driven route to control the combination of Si with Mn in the development of novel materials for spintronics devices. The Mn atoms show a preferential adsorption site on silicon atoms, forming one-dimensional nanostructures. They are parallel oriented with respect to the surface Si array, which probably predetermines the diffusion pathways of the Mn atoms during the process of nanostructure formation. © 2013 IOP Publishing Ltd.

The recently reported synthesis of silicene in the form of nanoribbons on Ag(110) or 2D epitaxial sheets on Ag(111) aroused considerable interest in the scientific community. Both overlayers were reported to display signatures of Dirac fermions with linearly dispersing electronic bands. In this work, we study the electronic structure of these adsorbate systems within density functional theory. We show that the conical features apparent in angle-resolved photoelectron spectroscopy measurements are not due to silicon but to the silver substrate, as an effect of band folding induced by the Si overlayer periodicity.

Chirality expression modifications occurring for a simple chiral amino-alcohol, D-Alaninol, adsorbed on Cu(100), when passing from low to high molecular coverage, are studied by means of scanning tunneling microscopy (STM) and theoretical modeling. At low coverage, D-Alaninol molecules organize in tetramers that are aligned to Cu(100) unit vectors whereas, increasing the molecular amount on the surface, the interplay of supramolecularity and chirality induces a rotation of the tetramers of 14 degrees with respect to the substrate lattice vectors. This behaviour is analyzed by means of a theoretical approach that combines classical molecular dynamics (MD) and density functional theory (DFT). The force field adopted in MD calculations is parametrized by fitting it to results obtained at DFT level on a single adsorbed alaninol molecule on the Cu(100) surface. The configurations of isolated and surrounded tetramers most sampled by MD are extracted by Principal Component Analysis. Further DFT relaxation with semiempirical dispersion correction (DFT-D) and considering also the possible dehydrogenation of both functional groups of alaninol, give rise to a few structures that are discussed in terms of their energetics and by comparing the resulting simulated STM images with the experimental ones.

A combined scanning tunneling microscopy and low-energy electron diffraction investigation of the Mn/GaAs(001) interface formation is reported. The interface, grown on a (2× 4) reconstructed substrate produced by molecular beam epitaxy, was studied as a function of Mn evaporation with thickness ranging from 1/8 ML to 1 ML. The interaction of Mn atoms with the semiconductor surface is strong and leads to surface reconstructions involving a rearrangement of the two outmost atomic layers of the substrate. For Mn thickness lower than 1/2 ML, the surface is characterized by a (2 ×1) periodicity. Conversely, when the Mn deposition is increased to 1/2 ML the surface reconstruction is strongly dependent on the preparation procedure. If Mn deposition is performed on the substrate at 390 C, a fully ordered surface characterized by a clear (2 ×2) reconstruction is obtained, whereas, annealing the sample after Mn deposition, gives a disordered surface with a (2 ×1) symmetry. An intermediate phase between (2 ×1) and (2 ×2) is found for Mn depositions in between 1/4 and 1/2 ML. No further structural evolution was observed for both preparation methods above 1/2 ML coverage. © 2011 American Institute of Physics.

A review of the activity of scanning probe microscopy at our Institute is presented, going from instrumentation to software development of scanning tunnelling microscopy, atomic force microscopy and scanning near-field optical microscopy (SNOM). Some of the most important experiments in material science and biology performed by our group through the years with these SPM techniques will be presented. Finally, infrared applications by coupling a SNOM with a free electron laser will also be presented.

A very high-energy synchrotron radiation source ~87 keV! was utilized for in situ sampling of the structural changes occurring in the electrodic materials of a Li-ion cell during charge–discharge cycling. The real-time evolution of their crystal lattice was obtained as a function of the degree of Li intercalation. As a result, new information on two electrodic materials, Li–Ti ‘‘zero strain’’ and Li–Ni–Co oxide, both of extreme interest for generation of rechargeable batteries, was gained. The actual change of the Li–Ti oxide lattice parameter upon cycling was observed in greater detail than before, and provided evidence of unexpected behavior in some intervals of the cycle. In the Li– Ni–Co sample, a new phase formed during the early stages of cycling that remained stable in the subsequent cycles was revealed.

An in-situ energy dispersive X-ray diffraction (EDXD) analysis has been run on the Li[Li1/3Ti5/3]O4 compound upon Li intercalation–deintercalation process. The results confirm that this process is accompanied by a very small variation of the host lattice parameter, i.e., confined between 1‰ over the entire cycle. This value, which agrees with previous literature information, concurs to demonstrate that Li[Li1/3Ti5/3]O4 may indeed be considered as a zero-strain intercalation compound, this being a characteristic of key technological importance since lattice strains upon cycling are among the main causes of capacity decays in lithium battery electrodes. In addition, this work confirms that EDXD is a quite convenient technique for electrochemical measurements since, allowing in-situ lattice parameter determinations, may lead to a complete evaluation of the intermediate stages of the intercalation process and, possibly, to detect differences among the various cycles.

After almost three decades since the invention of the scanning tunnelling microscope (STM) its application to the study of dynamic processes at surfaces is attracting a great deal of interest due to its unique capacity to observe such processes at the atomic level. The alpha-phase of group IV adatoms on Ge(111) and Si(111) is the ideal playground for the analysis of critical phenomena and represents a prototype of a two-dimensional electron system exhibiting thermally activated peculiar Sn adatom dynamics. This paper will relate the study of adatom dynamics at the alpha-Sn/Ge(111) and alpha-Sn/Si(111) surfaces, discussing in detail the methods we used for such kinds of time-resolved measurements. The microscope tip was used to record the tunnelling current on top of an oscillating Sn adatom, keeping the feedback loop turned off. The dynamics of the adatoms is detected as telegraph noise present in the tunnelling versus time curves. With this method it is possible to increase the acquisition rate to the actual limit of the instrument electronics, excluding piezo movement and feedback circuitry response time. We put emphasis on the statistical data analysis which allows the localization of the sample areas that are involved in dynamical processes.