Search

Tellurium switch operates memories through crystalline-liquid-crystalline phase changes

Fe-N-C catalysts were synthesized from a nitrogen and iron wet impregnation of carbon black pearls followed by pyrolysis steps. Three different nitrogen sources (dopamine, imidazole and benzimidazole), and two different pyrolysis atmospheres (Ar and NH3) were used. The obtained materials were characterized in terms of structure, morphology, surface chemistry, and electrochemical properties. Electrodes with a high porosity and accessible active sites were obtained tailoring the synthesis parameters, as indicated by Raman and X-ray photoelectron spectroscopies, and cyclic voltammetry with rotating ring disk electrode. Pyrolysis under ammonia atmosphere led to high electrochemical active surface area (ECSA) and the use of imidazole as nitrogen-rich organic precursor improved oxygen reduction reaction (ORR) activity in alkaline pH. This can be ascribed to the modification of surface chemistry of the electrocatalysts triggered by the N-rich organic precursor and pyrolysis atmosphere. The catalyst obtained by using imidazole and pyrolyzed in NH3 had a variety of iron-, oxygen- and nitrogen-functional groups, nitrogen being mainly distributed in imine-, pyridinic- and pyrrolic-N. In addition, durability tests showed a stable ECSA and ORR activity after cycling of the prepared electrocatalysts outperforming durability of Pt-based materials in alkaline environment and indicating applicability in anion exchange membrane fuel cells.

In the path of direct energy conversion and wastewater disposal through microbial fuel cells (MFCs), the oxygen reduction reaction (ORR) plays a pivotal role. However, kinetic limitations hinders the spread of such technology requiring the use of a catalyst to develop efficient and cost effective devices. Herein we report a facile method for the preparation of iron-based catalyst supported on graphene oxide (GO) obtained by electrochemical exfoliation of graphite in aqueous solution of ammonium sulfate. Two different strategies to include nitrogen functionalities on/in GO matrix have been used, such as one-step nitrogen-doping in solution and post treatments based on annealing with ammonia gas. Iron (II) phthalocyanine (FePc) was used as iron source and deposited on GO by pyrolysis-free impregnation. Tuning the adjustable parameters governing the materials preparation allowed producing GO nanosheets with unique morphology and surface properties for enhancing the interaction with FePc. By combining the use of microscopy, electrochemical and spectroscopic techniques, a correlation between structure and surface chemistry of the prepared materials with catalytic activity towards ORR was established. The applicability of iron-based materials as ORR cathodes was evaluated by assembling single chamber air-cathodes MFCs, which power and voltage generation over time were acquired. The obtained results demonstrated that FePc/GO-based electrocatalysts can be used for electricity generation and waste treatment at the cathode side of MFCs.

In this study, a generalized guideline is identified to predict the interaction between two-dimensional (2D) layered materials and substrate surfaces. Additionally, the van der Waals (vdW) heterostructures commensurability, the phase formation and the strain relaxation are identified during interface growth. To achieve such a general overview, the case of Ge-Sb-Te (GST) alloys on InAs(111) is studied. In this system, low-lattice mismatch conditions are fulfilled to avoid relaxation due to formation of misfit dislocations and allow to correctly identify vdW epitaxy. At the same time, the substrate can be efficiently prepared into self- and un-passivated surfaces to clarify the role of the surface interaction. Furthermore, the GST epilayer exhibits two different highly ordered 2D structures and a three-dimensional disordered structure, allowing to directly infer the nature of the epitaxy. This study opens the way for the design and mastering of vdW epitaxial growth of 2D heterostructures as well as hybrid 2D and non-layered materials.

The deposition of calcium fluoride (CaF2) on Si(111) at temperatures above 570 °C has been studied with scanning tunneling microscopy. At such temperatures, triangular calcium fluoride islands are formed both on terraces and along the phase domain boundaries of the (7 × 7) reconstruction of the Si(111) substrate. In addition to the formation of islands, we observe that CaF2 molecules react with the substrate inducing large areas of its surface to reconstruct into (√3 × √3) and c(2 × 4) phases. Upon annealing at 600 °C, the abovementioned areas of (√3 × √3) and c(2 × 4) turn into the stable (3 × 1) phase upon desorption of fluorine. Calcium fluoride islands are stable at this temperature. Depositions of calcium fluoride performed with Si substrate kept at higher temperature, namely at 680 °C, lead directly to the formation of (3 × 1) phase due to the complete desorption of fluorine, without passing through the formation of the metastable (√3 × √3) and c(2 × 4) phases. If CaF2/Si(111) is brought at even higher temperatures, Ca also starts desorbing and the (7 × 7)-Si(111) reconstruction can eventually be restored.

The present work displays a route to design strain gradients at the interface between substrate and van der Waals bonded materials. The latter are expected to grow decoupled from the substrates and fully relaxed and thus, by definition, incompatible with conventional strain engineering. By the usage of passivated vicinal surfaces we are able to insert strain at step edges of layered chalcogenides, as demonstrated by the tilt of the epilayer in the growth direction with respect of the substrate orientation. The interplay between classical and van der Waals epitaxy can be modulated with an accurate choice of the substrate miscut. High quality crystalline GexSb2Te3+x with almost Ge1Sb2Te4 composition and improved degree of ordering of the vacancy layers is thus obtained by epitaxial growth of layers on 3–4° stepped Si substrates. These results highlight that it is possible to build and control strain in van der Waals systems, therefore opening up new prospects for the functionalization of epilayers by directly employing vicinal substrates.

The disordering process in crystalline GeSb2Te4 films has been studied by means of ion irradiation with 150 keV Ar+ ions. The effect of the interfaces and the role of the crystal microstructure has been investigated. The disordering path observed in a randomly oriented polycrystalline material with trigonal structure involves the transition to the disordered rocksalt structure (at fluence 7x1013 cm-2) and then to the amorphous phase (at 1.5x1014 cm-2). In GeSb2Te4 epitaxially grown on Si(111) the formation of the disordered rocksalt phase occurs at much higher fluence (3x1014 cm-2) and it is preceded by the conversion of the stable phase into the ordered rocksalt structure (at 5x1013 cm-2), with the formation of ordered vacancy layers, associated to a local variation of the stoichiometry. Even by increasing the fluence up to 3.5x1014 cm-2, the films remains mainly crystalline. The observed behaviour has been attributed to the effect of the interfaces and suggests the possibility to promote switching between the crystalline phases by interface engineering.

Tissue engineering is a highly interdisciplinary field of medicine aiming at regenerating damaged tissues by combining cells with porous scaffolds materials. Scaffolds are templates for tissue regeneration and should ensure suitable cell adhesion and mechanical stability throughout the application period. Chitosan (CS) is a biocompatible polymer highly investigated for scaffold preparation but suffers from poor mechanical strength. In this study, graphene oxide (GO) was conjugated to chitosan at two weight ratios 0.3% and 1%, and the resulting conjugates were used to prepare composite scaffolds with improved mechanical strength. To study the effect of GO oxidation degree on scaffold mechanical and biological properties, GO samples at two different oxygen contents were employed. The obtained GO/CS scaffolds were highly porous and showed good swelling in water, though to a lesser extent than pure CS scaffold. In contrast, GO increased scaffold thermal stability and mechanical strength with respect to pure CS, especially when the GO at low oxygen content was used. The scaffold in vitro cytocompatibility using human primary dermal fibroblasts was also affected by the type of used GO. Specifically, the GO with less content of oxygen provided the scaffold with the best biocompatibility.

In this study, 1D structures based on α-MnO2 nanowires and β-MnO2 nanorods were fabricated by a single-step hydrothermal synthesis and evaluated as cathode components for microbial fuel cells (MFCs). The role of crystallinity and morphology on electrocatalytic activity was investigated by combining X-ray diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy with an electrochemical analysis of oxygen reduction reaction at the surface of MnO2-based structures. Among the synthesized composites, β-MnO2 nanorods showed higher ORR activity thanks to the synergy between the rod-like morphology and the nitrogen presence in/on the carbon support which provides a more accessible surface for oxygen adsorption and consequent reduction. MFCs assembled with β-MnO2 nanorod cathode allowed achieving a maximum power density of 524 ± 3 mW m−2 and superior cycling stability as compared to α-MnO2 nanowires and control Pt/C.

In recent years strain engineering is proposed in chalcogenide superlattices (SLs) to shape in particular the switching functionality for phase change memory applications. This is possible in Sb2Te3/GeTe heterostructures leveraging on the peculiar behavior of Sb2Te3, in between covalently bonded and weakly bonded materials. In the present study, the structural and thermoelectric (TE) properties of epitaxial Sb2+xTe3 films are shown, as they represent an intriguing option to expand the horizon of strain engineering in such SLs. Samples with composition between Sb2Te3 and Sb4Te3 are prepared by molecular beam epitaxy. A combination of X-ray diffraction and Raman spectroscopy, together with dedicated simulations, allows unveiling the structural characteristics of the alloys. A consistent evaluation of the structural disorder characterizing the material is drawn as well as the presence of both Sb-2 and Sb-4 slabs is detected. A strong link exists among structural and TE properties, the latter having implications also in phase change SLs. A further improvement of the TE performances may be achieved by accurately engineering the intrinsic disorder. The possibility to tune the strain in designed Sb2+xTe3/GeTe SLs by controlling at the nanoscale the 2D character of the Sb2+xTe3 alloys is envisioned.

We present an in-depth investigation of structural and electronic properties of GaAsBi epilayers. High (001) crystalline order is achieved using careful molecular beam epitaxy and surface preparation procedures. High surface order allows us to use x-ray, ultraviolet, and angle-resolved photoemission spectroscopy at variable photon energies and to disentangle electronic effects of an atomically thin Bi-rich surface layer with $(2\ifmmode\times\else\texttimes\fi{}3)$ symmetry from those of Bi atoms incorporated in the GaAs bulk matrix. The influence of bulk-integrated Bi concentrations on the GaAs band structure becomes visible in angle-resolved photoemission after removing Bi-rich surface layers by a brief and mild ion bombardment and subsequent annealing treatment. Experimental observations are supported by density functional theory simulations of the valence band structure of bulk and surface-reconstructed GaAs with and without Bi. Bi-induced energy shifts in the dispersion of GaAs heavy and light hole bulk bands are evident both in experiment and theory, which are relevant for modulations in the optical band gap and thus optoelectronic applications.

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.

Several multilayer InAs/GaAs(001) samples were grown in a Molecular Beam Epitaxy chamber under critical growth conditions in order to induce the self-assembling of chains of InAs Quantum Dots over mounded GaAs surfaces. As the number of deposited layers was increased, an increasing in-line ordering was observed. Finite Element Method simulations confirmed this trend which is driven mainly by the propagation of the elastic strain held through the layers. On the other hand, the morphological features of the surface contribute to improving the alignment of the InAs Quantum Dots in every chain. (C) 2015 Elsevier B.V. All rights reserved.

Surface transfer doping of hydrogen-terminated diamond induced by high work function V2O5 oxide was investigated on samples with (100) and (111) surface crystal-orientations. An enhancement of sheet hole density and a decrease in sheet resistance were obtained in the case of (111) diamond as compared to (100). In particular, a sheet resistance as low as 1.8 kΩ/◻ and a sheet hole concentration of 1.1 × 1014 cm−2 were obtained by Hall effect measurements for V2O5/H-(111) oriented diamonds, the latter being about twice as high as the one obtained for V2O5/H-(100) oriented diamonds. This was confirmed by capacitance-voltage measurements on metal/V2O5/H-diamond diodes fabricated on the investigated samples, also resulting in the determination of the depth profiles of hole accumulation layers at the diamond surface. X-ray photoelectron spectroscopy measurements of the C1s core level shift were used to determine the differences in surface band bending, leading to a different hole accumulation layer formation efficiency at the V2O5/H-diamond interface. An upward band bending of 0.7 eV and 0.3 eV in response to the surface transfer doping induced by a 10 A thick V2O5 layer was measured for (111) and (100) diamond surfaces, respectively. This is a further confirmation that V2O5 is more effective in surface transfer doping for H-(111) oriented diamond. The obtained results are very promising in view of the development of high-power metal oxide field effect transistors based on the H-diamond surface.Surface transfer doping of hydrogen-terminated diamond induced by high work function V2O5 oxide was investigated on samples with (100) and (111) surface crystal-orientations. An enhancement of sheet hole density and a decrease in sheet resistance were obtained in the case of (111) diamond as compared to (100). In particular, a sheet resistance as low as 1.8 kΩ/◻ and a sheet hole concentration of 1.1 × 1014 cm−2 were obtained by Hall effect measurements for V2O5/H-(111) oriented diamonds, the latter being about twice as high as the one obtained for V2O5/H-(100) oriented diamonds. This was confirmed by capacitance-voltage measurements on metal/V2O5/H-diamond diodes fabricated on the investigated samples, also resulting in the determination of the depth profiles of hole accumulation layers at the diamond surface. X-ray photoelectron spectroscopy measurements of the C1s core level shift were used to determine the differences in surface band bending, leading to a different hole accumulation layer formation eff...

The present work displays a route to design strain gradients at the interface between substrate and van der Waals bonded materials. The latter are expected to grow decoupled from the substrates and fully relaxed and thus, by definition, incompatible with conventional strain engineering. By the usage of passivated vicinal surfaces we are able to insert strain at step edges of layered chalcogenides, as demonstrated by the tilt of the epilayer in the growth direction with respect of the substrate orientation. The interplay between classical and van der Waals epitaxy can be modulated with an accurate choice of the substrate miscut. High quality crystalline Ge

In this paper we examine experimentally and theoretically how the As flux direction and intensity influences the recently observed selective growth of InAs quantum dots on the rippled surface of GaAs, where the dots line up only against well determined slopes of the mounded GaAs surface facing the impinging Arsenic flux. We have previously shown that the observed phenomenon is exclusively related to the As constituent, challenging the widespread belief that As plays only a minor role in the dot formation. The selective growth is obtained by Molecular Beam Epitaxy at high growth temperature and under a high As/In flux ratio. To interpret the experimental results, we use a newly developed kinetic model incorporating new features: (i) anions are explicitly considered; (ii) cations and anions follow a different kinetics; (iii) the dot surface is distinguished by the dot bulk and atoms are transferred back and forth between the two phases depending on the surface interaction with the environment. We find that even a very small difference in the As flux intensity impinging over two substrate regions produces a cation current flow from one region to the other, so that the dots form only on the region exposed to the largest flux.

Working under critical conditions for dot nucleation in a Molecular Beam Epitaxy chamber, we were able to drive the formation of InAs dot chains to precise locations in multilayered samples grown on a rippled GaAs(001) surface. We discussed the role of the elastic field and the surface curvature in determining the dot arrangement at each stacked layer, proving a new mechanism of self-organization of the dots. In particular, we succeeded in controlling the interplay between elastic and curvature effects and we showed how a selection process is achievable in the chain formation. The role of the stress field was also studied by means of Finite Element Method simulations, and we gained a valuable understanding of the interlayer dot correlations for dot arrays with variable cap thicknesses. We proved the existence of an anisotropy in the cap formation of isolated dots, which appeared to be directly related to our peculiar growth geometry and experimental set-up. Copyright © 2017 American Scientific Publishers All rights reserved.

Scientific Reports 6: Article number: 23843; published online: 01 April 2016; updated: 16 August 2016 In this Article, Stefania Privitera and Emanuele Rimini are incorrectly listed as being affiliated with ‘Paul-Drude-Institut für Festkörperelektronik, Hausvogteiplatz 5-7, 10117 Berlin, Germany’. The correct affiliation is listed below

In this study, we present a significant advance in the growth of epitaxial GeTe-Sb2Te3 alloys on Si(111)-(√3 × √3)R30°-Sb reconstructed surface by means of Molecular Beam Epitaxy. By employing X-ray diffraction and Raman spectroscopy, we show how phase, composition, and vacancy ordering can be tailored by tuning the growth parameters such as Ge and Te elemental fluxes as well as the substrate temperature. The effect of each parameter on the GeSbTe composition and phase is also discussed. A very surprising finding is that GeSbTe alloys are more ordered when grown at lower substrate temperatures. In addition, it was possible to fabricate ordered GeSbTe even for substrate temperatures as low as 120 °C. In situ high-energy electron diffraction is employed to monitor the crystallinity and surface roughness of GeSbTe films at different stages of growth. Thus, we identify the deposition parameter ranges whereby high structural quality GeSbTe with flat surfaces can be obtained.In this study, we present a significant advance in the growth of epitaxial GeTe-Sb2Te3 alloys on Si(111)-(√3 × √3)R30°-Sb reconstructed surface by means of Molecular Beam Epitaxy. By employing X-ray diffraction and Raman spectroscopy, we show how phase, composition, and vacancy ordering can be tailored by tuning the growth parameters such as Ge and Te elemental fluxes as well as the substrate temperature. The effect of each parameter on the GeSbTe composition and phase is also discussed. A very surprising finding is that GeSbTe alloys are more ordered when grown at lower substrate temperatures. In addition, it was possible to fabricate ordered GeSbTe even for substrate temperatures as low as 120 °C. In situ high-energy electron diffraction is employed to monitor the crystallinity and surface roughness of GeSbTe films at different stages of growth. Thus, we identify the deposition parameter ranges whereby high structural quality GeSbTe with flat surfaces can be obtained.

A combination of far-infrared and Raman spectroscopy is employed to investigate vibrational modes and the carrier behavior in amorphous and crystalline ordered GeTe-Sb2Te3 alloys (GST) epitaxially grown on Si(111). The infrared active GST mode is not observed in the Raman spectra and vice versa, indication of the fact that inversion symmetry is preserved in the metastable cubic phase in accordance with the F"Equation missing"m3 space group. For the trigonal phase, instead, a partial symmetry break due to Ge/Sb mixed anion layers is observed. By studying the crystallization process upon annealing with both the techniques, we identify temperature regions corresponding to the occurrence of different phases as well as the transition from one phase to the next. Activation energies of 0.43 eV and 0.08 eV for the electron conduction are obtained for both cubic and trigonal phases, respectively. In addition a metal-insulator transition is clearly identified to occur at the onset of the transition between the disordered and the ordered cubic phase.

Scientific reports 6, 23843 (2016). doi:10.1038/srep23843, Phase Change Materials (PCMs) are unique compounds employed in non-volatile random access memory thanks to the rapid and reversible transformation between the amorphous and crystalline state that display large differences in electrical and optical properties. In addition to the amorphous-to-crystalline transition, experimental results on polycrystalline GeSbTe alloys (GST) films evidenced a Metal-Insulator Transition (MIT) attributed to disorder in the crystalline phase. Here we report on a fundamental advance in the fabrication of GST with out-of-plane stacking of ordered vacancy layers by means of three distinct methods: Molecular Beam Epitaxy, thermal annealing and application of femtosecond laser pulses. We assess the degree of vacancy ordering and explicitly correlate it with the MIT. We further tune the ordering in a controlled fashion attaining a large range of resistivity. Employing ordered GST might allow the realization of cells with larger programming windows., Published by Nature Publishing Group, London

The application of the Voronoi tessellation for studying the nucleation process at the basis of thin film formation dates back to the end of nineties. In particular, Poissonian and spatial correlated nuclei distributions were investigated. The growth of stressed interfaces, however, has evidenced that elastic strain can modify the spatial distribution of nuclei giving rise to new challenges at the level of stochastic geometry. Here we report a study of a 2D nucleation of dots randomly arranged in lines and/or belts on the surface. The results are compared to the experimental case of multilayer structures of InAs quantum dots grown on GaAs(001). (C) 2015 Elsevier B.V. All rights reserved.

The selective growth of InAs quantum dot chains self-ordered at mesoscopic distances on a rippled GaAs(001) substrate is obtained by tuning the direction of the As flux at high temperature and high As/In flux ratio. Microphotoluminescence experiments demonstrate that such isolated quantum dot chains show single quantum dot emission properties.

Reflectance anisotropy spectroscopy (RAS) has been applied to study the oxidation of decapped InAs(0 0 1) surfaces at roomtem perature. (2 � 4) and (4 � 2) surfaces were prepared in ultra high vacuumby annealing, and then exposed to increasing amounts of molecular oxygen. During exposure to gas, RAS spectral structures at 2.4 eV in (2 � 4) and at 1.7 eV in (4 � 2) are progressively reduced. After about 1 � 10 4 L, a broad residual anisotropy spectrumis still m easured for (2 � 4), while relic anisotropy is nearly null for (4 � 2). Residual features are visible at bulk critical points (E1, E1 þ D1 and E 0 ). The different behavior versus gas contamination evidences the origin of RAS spectral structures, allowing interpretation in terms of surface-state or bulk transitions (the latter modified by the surface). By using Kramers–Kronig relations, the anisotropy of the imaginary part of the surface dielectric function (De 00 ) between [ 1 ] and [1 1 0] directions of the substrate has been obtained fromRAS data. We conclude that the main RAS structures (at 2.4 eV in (2 � 4), at 1.7 eV in (4 � 2)) are related to the surface reconstruction, although only the latter is a transition involving true surface states. We observe bulk contributions in coincidence with critical points (E1, E1 þ D1 and E 0 ). 2002 Elsevier Science B.V. All rights reserved.

The structure of Cu thin layers deposited on the MgO(0 0 1) surface obtained by cleavage in ultra high vacuum has been investigated by in situ X-ray absorption measurements as a function of the metal coverage. By exploiting the polarized nature of the synchrotron radiation it has been possible to measure the Cu–Cu bond length parallel to the substrate surface for coverages in the monolayer range. Extended X-ray absorption fine structure measurements have evidenced a Cu–Cu distance close to that of the bulk metal, resulting from a weak film–substrate interaction irrespective of the copper thickness. Copper oxidation on the MgO surface has not been evidenced. Our results give a direct confirmation that Cu grows in form of clusters and that the film–substrate interaction is weak as previously reported.

Microbial fuel cells (MFCs) can be considered as an efficient and flexible platform for integrated waste treatment and energy recovery [1]. The extensive research work devoted to this technology over the past decade demonstrates the promising outlook of MFCs, but practical application is still limited by the high costs associated to the materials used for device assembly, such as cathode materials which accounts for over 50% of the overall MFC capital cost. The sluggish kinetics of oxygen reduction reaction (ORR) has led to the use of expensive catalysts, such as platinum, which is not suitable to be applied to sustainable technologies. Hence, a great variety of materials have been developed for MFC cathodes, including nitrogen-doped activated carbons and non platinum group metal catalysts. Such materials allow achieving ORR rate comparable to Pt, the morphology and structure of the catalysts playing an important role on the efficiency and durability of ORR active sites [2,3]. The development of new carbon nanostructures with highly tunable morphology and structure has led to the use of graphene for several applications, including as component for MFC cathodes [4,5]. However, challenges, such as complexity in synthesis and costs, still limit the applicability of graphene as cathode component of MFCs. Therefore, a facile and efficient approach to develop graphene based catalysts can be considered a promising direction to achieve sustainable wastewater treatment and bioenergy production by BESs. In this work, we report a facile method for large-scale preparation of ORR catalysts based on graphene oxide (GO) obtained by electrochemical oxidation of graphite in aqueous solutions of inorganic salts. We developed different strategies to include nitrogen functionalities in GO structure, including post treatments based on annealing with ammonia gas and one-step nitrogen-doping of GO. By combining the use of atomic force microscopy with electrochemical and spectroscopic techniques, we correlated the different morphology and surface chemistry of GO with catalytic activity towards ORR. Differences in catalytic activity obtained by supporting iron on GO surface were also elucidated, investigating the nature of ORR active sites. The applicability of GO-based materials as ORR cathodes of MFCs was evaluated by assembling single chamber air-cathodes MFCs operating with sodium acetate in phosphate buffer solution. Coulombic efficiency, polarization and power density curves, and voltage generation cycles over time were acquired. The body of results demonstrated the potential ability of GO electrocatalysts to substitute platinum for ORR in MFCs. Acknowledgements. The present work was carried out with the support of the “European Union's Horizon 2020 research and innovation programme” (under H2020-FTIPilot-2015-1, Grant Agreement n. 720367-GREENERNET), the University of Rome Tor Vergata (under the Research Call “Consolidate the Foundations”, project name: BEST WATER), and CNPq - Conselho Nacional de Desenvolvimento Científico e Tecnológico, Brazil. References. [1] A. Rinaldi, B. Mecheri, V. Garavaglia, S. Licoccia, P. Di Nardo, E.Traversa. Energ. Environ. Sci., 1 (2008) 417-429. [2] C. Santoro, A. Serov, L. Stariha, M. Kodali, J. Gordon, S. Babanova, O. Bretschger, K. Artyushkova, P. Atanassov, Energ. Environ. Sci. 9 (2016) 2346-2353. [3] A. Iannaci, B. Mecheri, A. D'Epifanio, M. J. Lázaro Elorri, S. Licoccia. Int J Hydrogen Energ. 41 (2016) 19637-19644. [4] H. Yuan, Z. He. Nanoscale 7 (2015) 7022-7029. [5] K. Parvez, S. Yang, Y. Hernandez, A. Winter, A. Turchanin, X. Feng, K. Müllen. ACS Nano. 6 (2012) 9541-9550.

An innovative multilayer growth of InAs quantum dots on GaAs(100) is demonstrated to lead to self-aggregation of correlated quantum dot chains over mesoscopic distances. The fundamental idea is that at critical growth conditions is possible to drive the dot nucleation only at precise locations corresponding to the local minima of the Indium chemical potential. Differently from the known dot multilayers, where nucleation of new dots on top of the buried ones is driven by the surface strain originating from the dots below, here the spatial correlations and nucleation of additional dots are mostly dictated by a self-engineering of the surface occurring during the growth, close to the critical conditions for dot formation under the fixed oblique direction of the incoming As flux, that drives the In surface diffusion.

The sum rule for the imaginary part of the dielectric function e″ can help in discriminating surface and bulk-related optical transitions, by means of the number of electrons taking part in the transitions. This is particularly simple in the region below the main critical points of the bulk band structure (below 3.5 eV for Si, below 2.2 eV for Ge), since there the surface differential reflectivity is roughly proportional to e″ of the surface. We discuss the cases of Ge(1 1 1)-2×1 and Si(1 1 1)-2×1 investigated by surface differential reflectivity in the energy range 0.3–3.5 eV, with light linearly polarized parallel and perpendicular to the chains of the Pandey model. A key point is that the sum rule holds for each polarization separately (termed α and β), implying that: ∫ωe α ″ d ω=∫ωe β ″ d ω . In these results, the effect of the sum rule can be easily recognized (crossing of the curves and seemingly equal areas on both sides of the crossing point). Similar considerations should also hold for reflectance anisotropy spectroscopy (RAS). It will be shown, however, that a compensation of positive and negative signals, in compliance with the sum rule, is actually expected only if RA spectra are multiplied by a suitable function. Moreover, the compensation is not exact, when the bulk is strongly absorbing. The results of a numerical simulation valid for various materials and surface oscillators are reported. They help to understand why in some cases (for instance in GaAs(0 0 1)-2×4 and −4×2), the sum rule is apparently not obeyed.

EXAFS (Extended X-ray Absorption Fine Structure) and XANES (X-Ray Absorption Near Edge Structure) spectroscopies have been used to probe the local bonding of the artificially layered superconducting (BaCuO2)2/(CaCuO2)n (n = 2, 3, 4) superlattices. Fluorescence-yield measurements have been performed above the Cu and Ba K-edges. This study shows that the charge reservoir (CR) block of the Ba/Ca superlattice contains oxygen vacancies randomly distributed in the CuO2 planes and that the two apical oxygen atoms are displaced out of the Ba plane giving rise to buckled BaO planes. These oxygen atoms have a distorted pyramidal and octahedral coordination around Cu at Ba/Ca interface and in the (BaCuO2) block, respectively.

Polarised extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) spectroscopies have been used to probe the local bonding of the infinite layer compounds CaCuO2 and SrCuO2 and of artificially layered superconducting (BaCuO2)2/(CaCuO2)n and nonsuperconducting (BaCuO2)2/(SrCuO2)2 superlattices. Fluorescence-yield measurements have been performed at T=77 K above the Cu and Ba K-edges. The XANES spectrum of CaCuO2 is described in terms of mixing of electronic configurations in the final state. EXAFS results show that the charge reservoir (CR) block of the Ba/Ca superlattice contains 0.6 oxygen vacancies per unit cell randomly distributed in the CuO2 plane sandwiched between the BaO planes and that the two apical oxygens are not in the Ba plane but are shifted toward Cu by 0.21 A. These oxygens have a pyramidal and distorted octahedral coordination with Cu at the Ba/Ca interface and in the (BaCuO2)2 block, respectively. The results clarify the origin of the oxygen doping mechanism in these artificially layered compounds.

Thin films and artificial materials deposition, applied to the study of high temperature superconductors, can be a powerful method to achieve better comprehension of the conduction mechanisms in these materials and, in perspective, to obtain new superconducting compounds. We simulate the layered structure of the superconducting oxides by alternating infinite layer compounds ( CaCuO 2, SrCuO 2, BaCuO 2) both pure and doped either with chemical substitutions or stoichiometry variations. By the first way, we obtained superlattices with a semiconducting electrical behaviour, probably because of the disorder induced by such high chemical substitutions. The second way led to superconducting films with transition temperatures around 80 K and low anisotropy. Furthermore, their artificial nature offers an unique possibility of analysing the influence of the structural properties on the superconducting behaviour, such as the correlation between the number of CuO 2 planes contained in the infinite layer block ( CaCuO 2) and the transport properties.

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].

Progress in tailoring the size, shape and positioning of Quantum Dots on the substrate is crucial for their potential applications in new optoelectronic devices for nano-photonics as well as in quantum information and computation. Using Molecular Beam Epitaxy in pulsed deposition mode we demonstrate that the nucleation of InAs Quantum Dots can be selectively guided on the GaAs(001) surface by a suitable choice of the kinetic parameters for the growth of both the GaAs buffer layer and the InAs Quantum Dots. By developing a two-species rate-equation kinetic model we show that the positioning of the Quantum Dots on only one side of mounds of the GaAs buffer can be traced back to the very small As flux gradient between the two mound slopes $\left( {\Delta F_A /F_A \approx 1 - 5\% } \right)$ caused by the proper tilting of the incoming As flux. Such gradient originates, at the relatively high growth-temperature, a net cation flow from one slope of the mound to the other that is responsible for the selective growth.

Educazione, innovazione, conoscenza, creatività, ricerca scientifica e didattica. Ecco alcuni dei pilastri culturali che sostengono il progetto degli Stage a Tor Vergata, destinato a studentesse e studenti del IV e V anno della scuola secondaria di secondo grado. Trenta studenti, meritevoli ma soprattutto motivati, vengono inseriti per cinque giorni e in due distinte fasi formative, in tre gruppi di ricerca, del Dipartimento di Fisica dell'Università di Roma Tor Vergata, attivi in settori d'avanguardia della scienza dei materiali e dei materiali innovativi per l'astrofisica sperimentale. Per cinque giorni "vivono" nei laboratori di ricerca, esplorano e apprendono le più recenti novità e tecnologie nella ricerca di materiali innovativi per lo sviluppo dell'ICT (Information and Communication Technology), della conversione fotovoltaica e dell'astrofisica dello spazio. Lavorano in piccoli gruppi, guidati da docenti universitari, identificano gli obiettivi a cui giungere, si distribuiscono incarichi e responsabilità, si confrontano, organizzano, gestiscono e presentano le fasi della ricerca tenendo in considerazione la teoria di riferimento.

Here we show a new effect due to the Arsenic flux in the Molecular Beam Epitaxy growth of InAs Quantum Dots on GaAs(001) at temperatures higher than 500 °C and an high As/In flux ratio. We show that, by changing and tuning the direction of the As flux on a rippled substrate, a selective growth can be obtained where the dots form only on some appropriately orientated slopes of a sequence of mounds elongated along the[-110] surface direction. Since the relative As flux intensity difference over the two opposite mound slopes is very small (2-5%) the observed large effect cannot be explained simply as a pure shadowing effect and reveals instead that As, whose contribution to the modeling of growth has often been ignored or underestimated, probably for a lack of knowledge, plays a fundamental role at these growth conditions. To explain our experiment we have developed a kinetic model which explicitly takes into account the coupling between cations (In) and anions (As), and find that the very small surface gradient in the anion flux, due to the oblique evaporation on the mounded surface, is responsible for a massive drain of cations toward the surfaces anion-rich areas, thus generating the selective growth of Quantum Dots. We expect a comparable behavior for the anions of other III-V and II-VI compound semiconductors.