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PHYS.PLASMAS

The evolution of the aggregation of condensed matter from single atoms to three-dimensional structures represents an important topic in nanoscience, since it contains essential information to achieve tailor- made growth of nanostructured materials. Such an issue is particularly important in the case of the formation of nano-oxides, which have strong potentialities in heterogeneous catalysis reactions. Herein, we show that a low-nuclearity graphene supported Ag cluster (formed by 11 atoms) shows a local AgO4 configuration, spatial charge distribution and two oxidation states that are similar to those of bulk AgO. This result indicates that bulk oxide conformations can start to develop already for extremely small atomic aggregates (

The interaction between a Ni(111) substrate covered by a complete Gr monolayer and H atoms occurs through two parallel routes leading to the hydrogen chemisorption on graphene and, at much lower rate but still with some ease, to the intercalation of H atoms below it. This latter reaction determines a direct interaction of the H atoms with the metal surface and eventually the H diffusion into the Ni bulk under the Gr cover. In this study we have combined high-resolution X-ray photoelectron spectroscopy, thermal programmed desorption, and density functional theory calculations to establish how the chemisorption and intercalation yields and their interplay depend on temperature and to find out how graphene affects the amount and the evolution of the hydrogen diffused in the Ni bulk. We found that between 150 and 320 K, hydrogen chemisorption on Gr is independent of temperature and that Gr lifting, which signals the H intercalation below it, does not occur below 180−200 K, being limited by an energy barrier of the order of 150 meV. For the heavily hydrogenated samples, when H atoms diffuse also into the Ni bulk, the Gr cover plays a key role for H storage because it strongly enhances the amount of H loaded in the interface with respect to the bare Ni(111) substrate. This behavior, possibly exhibited also by other graphene/metal interfaces provided that intercalation of H below graphene can readily occur, might foster the design of innovative materials to be applied for H storage.

We report on a novel approach to determine the relationship between the corrugation and the thermal stability of epitaxial graphene grown on a strongly interacting substrate. According to our density functional theory calculations, the C single layer grown on Re(0001) is strongly corrugated, with a buckling of 1.6 Angstrom, yielding a simulated C 1s core level spectrum which is in excellent agreement with the experimental one. We found that corrugation is closely knit with the thermal stability of the C network: C-C bond breaking is favored in the strongly buckled regions of the moire´ cell, though it requires the presence of diffusing graphene layer vacancies.

By using ab initio molecular dynamics calculations, we show that even where the graphene lattice constant contracts, as previously reported for freestanding graphene below room temperature, the average carbon-carbon distance increases with temperature, in both free and supported graphene. This results in a larger corrugation at higher temperature, which can affect the interaction between graphene and the supporting substrate. For a weakly interacting system as graphene/Ir(111), we confirm the results using an experimental approach which gives direct access to interatomic distances.

We have addressed the problem of determining a reliable experimental descriptor of surface chemical reactivity by measuring Pd and Ru 3d(5/2) core-level shifts of the Pd(n)/Ru(0001) pseudomorphic overlayer system (n = 0-3) by high resolution X-ray photoelectron spectroscopy. We find a linear relationship between the calculated theoretical changes of the d-band center position projected on each Ru and Pd atomic layer (which is, according to the Hammer and Norskov d-band model, a good theoretical descriptor of chemical reactivity) and the corresponding core-level shifts, both for the Ru atomic planes and for the Pd overlayers. Core-level shifts, therefore, should be considered as reliable experimental descriptors of chemical reactivity in the same sense (and with similar limitations) of the theoretical descriptor d-band center. Final-state effect contributions to the shifts do not obscure this trend.

Pilonidal disease (PD) is a relatively common, benign but challenging condition of the natal cleft. This consensus statement was drawn up by a panel of surgeons, identified by the Italian Society of Colorectal Surgery (SICCR) as having a “special interest” in PD, with the aim of recommending the best therapeutic options according to currently available scientific evidence. A three-step modified-Delphi process was adopted, implying: (1) choice of the panelists; (2) development of a discussion outline and of target issues; and (3) a detailed systematic review of the current literature. The agreement/disagreement level was scored on a five-point Likert scale as follows: “A + : strongly agree; A–: agree; N: unsure/no opinion; D–: disagree; D + : strongly disagree. Each panelist contributed to the production of this manuscript, and the final recommendations were reviewed by the Clinical Practice Guidelines Committee.

The possibility of depositing precisely mass-selected Ag clusters (Ag-1, Ag-3, and Ag-7) on Ru(0001) was instrumental in determining the importance of the in-plane coordination number (CN) and allowed us to establish a linear dependence of the Ag 3d(5/2) core-level shift on CN. The fast cluster surface diffusion at room temperature, caused by the low interaction between silver and ruthenium, leads to the formation of islands with a low degree of ordering, as evidenced by the high density of low-coordinated atomic configurations, in particular CN = 4 and 5. On the contrary, islands formed upon Ag-7 deposition show a higher density of atoms with CN = 6, thus indicating the formation of islands with a close-packed atomic arrangement. This combined experimental and theoretical approach, when applied to clusters of different elements, offers the perspective to reveal nonequivalent local configurations in two-dimensional (2D) materials grown using different building blocks, with potential implications in understanding electronic and reactivity properties at the atomic level.

Older and newer forms of vulnerability are manifesting themselves today, denouncing a suffering which is both material and existential, individual and societal. It is particularly evident within urban contexts and amplified by the crumbling of collective belonging and the increasing scarcity of places of participation where responsibility and the commitment to building one’s own personal biography are increasingly delegated to each individual, both materially and symbolically. Two separate pedagogical studies will explore some processes of signification of the experiences connected to new poverty, emphasizing the risks of simplification and normativity identified in some of the current interpretations in the common sense, such as the problematic proximity which these are in danger of sharing with the interpretative premises that shape the same local educational services dedicated to new poverty itself., Encyclopaideia, V. 25 N. 59 (2021)

X-ray photoelectron spectroscopy is a powerful analytical tool to fingerprint the atomic oxidation state. However, the well-established trend that is found for bulk, ultra-thin films and surface oxides, which is usually characterized by an incremental binding energy shift upon increasing oxidation state, may not apply at the sub-nanoscale where reduced dimensionality and quantum size effects play a relevant role. Here we investigate through a combined experimental and theoretical approach the oxidation of size-selected Ag7 and Ag11 clusters supported on graphene, revealing an anomalous Ag 3d5/2 core level shift trend upon increasing O coverage. We show that the negative core level shift trend typical of Ag reverts back in the case of the highest Ag(III) oxidation state, an effect that is due to the peculiar electronic structure of Ag nano-oxides. Our results highlight the great care needed to extend at the zero-dimensional scale the knowledge acquired for the spectral interpretation of 3D and 2D materials.

In this work, the melting line of calcium has been characterized both experimentally, using synchrotron X-ray diffraction in laser-heated diamond-anvil cells, and theoretically, using first-principles calculations. In the investigated pressure and temperature range (pressure between 10 and 40 GPa and temperature between 300 and 3000 K) it was possible to observe the face-centred phase of calcium and to confirm (and characterize for the first time at these conditions) the presence of the body-centred cubic and the simple cubic phase of calcium. The melting points obtained with the two techniques are in excellent agreement. Furthermore, the present results agree with the only existing melting line of calcium obtained in laser-heated diamond anvil cells, using the speckle method as melting detection technique. They also confirm a flat slope of the melting line in the pressure range between 10 and 30 GPa. The flat melting curve is associated with the presence of the solid high-temperature body-centered cubic phase of calcium and to a small volume change between this phase and the liquid at melting. Reasons for the stabilization of the body-centered face at high-temperature conditions will be discussed.

In this work, we characterize the adsorption of pentacene molecules on Ir(111) and their behaviour as a function of temperature. While room temperature adsorption preserves the molecular structure of the five benzene rings and the bonds between carbon and hydrogen atoms, we find that complete C–H molecular break up takes place between 450 K and 550 K, eventually resulting in the formation of small graphene islands at temperatures larger than 800 K. Most importantly a reversible temperature-induced dehydrogenation process is found when the system is annealed/cooled in a hydrogen atmosphere with a pressure higher than 5 × 10−7 mbar. This novel process could have interesting implications for the synthesis of larger acenes and for the manipulation of graphene nanoribbon properties., In this work, we characterise the adsorption of pentacene molecules on Ir(111) and their dissociation behaviour as a function of temperature.

The transport properties of iron under Earth's inner core conditions are essential input for the geophysical modelling but are poorly constrained experimentally. Here we show that the thermal and electrical conductivity of iron at those conditions remains high even if the electron-electron-scattering (EES) is properly taken into account. This result is obtained by ab initio simulations taking into account consistently both thermal disorder and electronic correlations. Thermal disorder suppresses the non-Fermi-liquid behavior of the body-centered cubic iron phase, hence, reducing the EES; the total calculated thermal conductivity of this phase is 220 Wm$^{-1}$K$^{-1}$ with the EES reduction not exceeding 20%. The EES and electron-lattice scattering are intertwined resulting in breaking of the Matthiessen's rule with increasing EES. In the hexagonal close-packed iron the EES is also not increased by thermal disorder and remains weak. Our main finding thus holds for the both likely iron phases in the inner core., 8 pages, 4 figures, published main text. For supplementary material see https://www.nature.com/articles/s41467-020-18003-9

Chemical interactions between metal and silicates at the core-mantle boundary (CMB) are now thought to lead to transfer of oxygen into Earth's liquid core. Establishing the nature and extent of this transfer is important for constraining the conditions under which the core formed, the origin of a stably stratified region below the CMB and the possible precipitation of oxides within the core. Previous models of FeO transfer have considered a solid mantle; however, several lines of evidence suggest that the lowermost mantle could have remained above its solidus long after core formation was complete, which would allow much faster mass transfer. We investigate this scenario by developing a time-dependent model of FeO exchange between a diffusive stratified layer at the top of the core and a long-lived molten magma ocean. Core FeO concentration, c ¯ F e O c , is evolved subject to a time-dependent mass flux at the CMB, radius r cmb , which depends on the FeO concentration at the bottom ( c ¯ F e O m ( r cmb ) ) and top ( c ¯ F e O m ( r bulk ) ) of the chemical boundary layer above the CMB. Coupled core-magma ocean evolution arises because c ¯ F e O m ( r cmb ) and c ¯ F e O c ( r cmb ) are linked through the partition coefficient P = c ¯ F e O c ( r cmb ) / c ¯ F e O m ( r cmb ) . c ¯ F e O m ( r bulk ) is held constant in No Crystallization (NC) models and evolves in Middle-Out Crystallization (MOC) models according to the basal magma ocean model of Labrosse et al. (2007) , generalised to account for FeO loss to the core. In the first 1 Gyr, FeO transfer in all models with ≥10% FeO in the magma ocean and P ≥ 5 produces pure FeO compositions at the CMB, stably stratified layers of 60 − 80 km and accounts for 15 − 50 % of the total present-day core oxygen content. In NC models the magma ocean does not completely freeze in 4 Gyr, in which time the stable layer reaches 120 − 150 km and FeO transfer can account for all of the present-day O in the core. However, in MOC models FeO loss to the core causes the magma ocean to completely freeze in the first 1-3 Gyrs following core formation. Our results suggest that the present-day core composition may not provide a strong constraint on models of core formation and that FeO could have precipitated at the top of the core.

The thermal conductivity (κ) of Earth's core is a critical parameter that controls predictions of core cooling rate, inner core age and the power available to the geodynamo. However, the values of core thermal conductivity inferred from recent studies span a wide range due to the challenges of extrapolating to the pressure-temperature-composition (P-T-C) conditions of the core liquid. In particular, extrapolations of κ from direct experimental determinations are lower than ab initio calculations conducted at core conditions. We have performed density functional theory (DFT) calculations to determine the thermal conductivity and resistivity (ρ) of solid FeSi alloys with two compositions, 4 mol % and 15 mol % Si, at a range of temperatures (850-4350 K) and pressures (60-144 GPa) for ease of comparison with recent directly measured κ values. In agreement with recent experiments, our calculations show that for the larger Si composition the resistivity of the mixture increases substantially, compared to pure Fe, reaching its saturated value already at the lowest temperature investigated. As a result, the thermal conductivity of the mixture is also correspondingly reduced. We also analysed the effect of possible errors in the DFT calculations due to the neglect of electron-electron scattering (EES) processes. Our results show that experimental and EES-corrected DFT calculations of κ are actually consistent within uncertainties when compared directly at overlapping P-T-C conditions. We present new core thermal history models using our EES-corrected estimates of κ=75−81 W m−1 K−1 at core-mantle boundary (CMB) conditions, which support previous determinations of late inner core formation around 400-700 Myrs ago and an early molten lower mantle.

Gli ambivalenti esiti delle trasformazioni economiche, sociali, culturali della contemporaneità hanno dato vita a un’area in cui si incrociano i lembi di differenti forme di fragilità, economica, sociale, relazionale, e di sofferenza urbana (Saraceno, 2010): una zona grigia di disagio diffuso (Iori e Rampazi, 2008; Tramma, 2015) che non supera quelle soglie di malessere conclamato che la collocherebbero nell’area della grave emarginazione, oggetto di attenzione dei servizi e delle politiche educative. I processi che hanno contribuito al diffondersi di vulnerabilità sociale (Ranci, 2002) hanno modificato anche alcuni tratti della condizione di povertà contemporanea: di questi elementi di novità mira a rendere conto il concetto di nuove povertà, una fascia dai confini incerti, di povertà grigie (Dovis e Saraceno, 2011), esito dell’ampliamento del rischio di impoverimento a fasce di popolazione prima considerate protette: un rischio connesso a eventi sempre meno eccezionali e sempre più legati ai “normali” corsi di vita. La ricerca pedagogica intorno alle implicazioni educative dei processi di impoverimento si è focalizzata, attraverso metodi biografici (Merril & West, 2012), sulle storie di vita e di formazione di quindici uomini e donne coinvolti da recenti processi di impoverimento. La cornice teorica della pedagogia sociale (Tramma, 2010) ha indirizzato l’esplorazione di queste traiettorie biografiche intorno alle dimensioni educative, formali e informali, che contribuiscono a definire lo scivolamento in situazioni di fragilità e/o a prevenirlo e attutirlo; l’analisi pedagogica si è concentrata sulle dimensioni di vulnerabilità intorno a cui si articolano le criticità dei percorsi di vita e sulle modalità, i significati, le rappresentazioni tramite cui i soggetti attraversano e rielaborano la propria storia e la propria condizione, facendo emergere il ruolo di un clima educativo diffuso che, contribuendo a letture sempre più individualizzate e iper-responsabilizzanti della propria vita e della condizione di impoverimento, ostacola la comprensione critica dei propri percorsi di vita e delle dinamiche del presente, fattore indispensabile perché si aprano per i soggetti possibilità di cambiamento e di azione verso il miglioramento delle condizioni di vita, individuali e collettive. La tensione trasformativa (Baldacci, 2001) della ricerca pedagogica qui presentata concerne la possibilità di contribuire alla riflessione pedagogica sulle nuove povertà, in vista di individuare orientamenti teorici e metodologici per un’intenzionalità educativa capace di promuovere percorsi di uscita dalla povertà e di intervenire in termini preventivi nelle situazioni a rischio di scivolamento. Economic, social and cultural transformations of these times have produced ambivalent results while giving rise to a newfound and widespread form of distress where economic, social and relational fragility and urban suffering intertwine (Saraceno, 2010): this grey area of distress (Iori and Rampazi, 2008; Tramma, 2015) does not exceed standard thresholds of overt malaise that would place it within the traditional boundaries regarding severe marginality and exclusion – areas often touched upon by educational services and policies. The elements that contributed to the spread of social vulnerability (Ranci, 2002) – including the precariousness of living and working conditions, the weakening of the systems in place offering social protection and the erosion of the social fabric – have played a role in altering some traits of contemporary poverty: the concept of new poverty aims to account for new elements of a population dealing with uncertain borders, grey poverty (Dovis and Saraceno, 2011), meaning that those who were not previously considered at risk of poverty are now implicated: a risk related to events increasingly frequent and increasingly linked to those paths of life seen as “normal”. The pedagogical research on the educational implications of impoverishment processes focuses, through biographical methods (Merril & West, 2012), on the life stories and educational biographies of fifteen men and women who have recently become impoverished. The theoretical framework concerning social pedagogy (Tramma, 2010) has directed the exploration of these biographical trajectories in and around both formal and informal educational dimensions: these contribute to falling into fragile situations, but also preventing and/or reducing their eventual impact. The pedagogical analysis focuses on the vulnerability areas around which life’s critical aspects are concentrated, and on the representations and meanings through which impoverished people live and rework their own story and condition, highlighting the key role of a contemporary educational climate. Such an educational climate promotes increasingly individualised and notably hyper-responsible representations of one’s own life and impoverishment, while at the same time hindering the critical understanding of one’s life path, losing focus on the dynamics of the present – something indispensable to promote with the subjects changes for the improvement of individual and collective living conditions. The transformative tension (Baldacci, 2001) as part of this pedagogical research aims to contribute to the pedagogical reflection on new poverty in order to identify theoretical and methodological orientations for educational actions able to promote fresh paths for subjects, both in terms of preventive interventions with people at risk of poverty, and in terms of paths out of poverty.

Transfer of oxygen between Earth's core and lowermost mantle is important for determining the chemistry and nature of stratification on both sides of the core‐mantle boundary (CMB). Previous studies have found that oxygen enters the metal when Fe‐O liquid equilibrates with representative lower mantle materials. However, experiments have not yet been conducted at CMB pressure‐temperature conditions. Here we use density functional theory to obtain the first estimates of oxygen partitioning between liquid Fe‐O‐Si metals and ferropericlase at CMB conditions. Our method successfully reproduces experimentally derived partitioning data at 134 GPa and 3200 K, while our calculations show a strong increase of oxygen partitioning into metal with temperature and a weaker increase with pressure, consistent with previous work. At CMB conditions of 135 GPa and 4000–4700 K oxygen partitioning into metal is higher than previous estimates and increases strongly with metal oxygen concentration. Analysis of the lower mantle chemical boundary layer shows that oxygen transport through the solid is severely limited even with the enhanced partitioning and is unlikely to explain the thickness of a stably stratified layer below the CMB inferred from seismology. However, if the lower mantle was molten in early times, as suggested by core evolution models with high thermal conductivity, then the mass flux and stable layer thickness are significantly increased.