Effect of the crystallographic c-axis orientation on the tribological properties of the few-layer PtSe2

Two-dimensional (2D) transition metal dichalcogenides are potential candidates for ultrathin solid-state lubricants in low-dimensional systems owing to their flatness, high in-plane mechanical strength, and low shear interlayer strength. Yet, the effects of surface topography and surface chemistry on the tribological properties of 2D layers are still unclear. In this work, we performed a comparative investigation of nanoscale tribological properties of ultra-thin highly-ordered PtSe2 layers deposited on the sapphire substrates with the in-plane and out-of-plane crystallographic orientation of the PtSe2 c-axis flakes, and epitaxial PtSe2 layers. PtSe2 c-axis orientation was found to has an impact on the nanotribological, morphological and electrical properties of PtSe2, in particular the change in the alignment of the PtSe2 flakes from vertical (VA) to horizontal (HA) led to the lowering of the coefficient of friction from 0.21 to 0.16. This observation was accompanied by an increase in the root-mean-square surface roughness from 1.0 to 1.7 nm for the HA and VA films, respectively. The epitaxial films showed lower friction caused by lowering adhesion when compared to other investigated films, whereas the friction coefficient was similar to films with HA flakes. The observed trends in nanoscale friction is attributed to a different distribution of PtSe2 structure

A ferromagnetic Eu–Pt surface compound grown below hexagonal boron nitride

One of the fundamental applications for monolayer-thick 2D materials is their use as protective layers of metal surfaces and in situ intercalated reactive materials in ambient conditions. Here we investigate the structural, electronic, and magnetic properties, as well as the chemical stability in air of a very reactive metal, Europium, after intercalation between a hexagonal boron nitride (hBN) layer and a Pt substrate. We demonstrate that Eu intercalation leads to a hBN-covered ferromagnetic EuPt2 surface alloy with divalent Eu2+ atoms at the interface. We expose the system to ambient conditions and find a partial conservation of the di-valent signal and hence the Eu–Pt interface. The use of a curved Pt substrate allows us to explore the changes in the Eu valence state and the ambient pressure protection at different substrate planes. The interfacial EuPt2 surface alloy formation remains the same, but the resistance of the protecting hBN layer to ambient conditions is reduced, likely due to a rougher surface and a more discontinuous hBN coating

Physical Delithiation of Epitaxial LiCoO2 Battery Cathodes as a Platform for Surface Electronic Structure Investigation

We report a novel delithiation process for epitaxial thin films of LiCoO2(001) cathodes using only physical methods, based on ion sputtering and annealing cycles. Preferential Li sputtering followed by annealing produces a surface layer with a Li molar fraction in the range 0.5 < x < 1, characterized by good crystalline quality. This delithiation procedure allows the unambiguous identification of the effects of Li extraction without chemical byproducts and experimental complications caused by electrolyte interaction with the LiCoO2 surface. An analysis by X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) provides a detailed description of the delithiation process and the role of O and Co atoms in charge compensation. We observe the simultaneous formation of Co4+ ions and of holes localized near O atoms upon Li removal, while the surface shows a (2 × 1) reconstruction. The delithiation method described here can be applied to other crystalline battery elements and provide information on their properties that is otherwise difficult to obtainThis work was supported by the Spanish MICINN (grant nos. PID2021-123295NB-I00 and PID2020-117024GB-C43), MAT2017-83722-R, “María de Maeztu” Programme for Units of Excellence in R&D (CEX2018-000805-M), within the framework of UE M-ERA.NET 2018 program under StressLIC Project (grant no. PCI2019-103594) and by the Comunidad Autónoma de Madrid (contract no. PEJD-2019- PRE/IND-15769 and S2108-NMT4321). The authors acknowledge Elettra Sincrotrone Trieste for providing access to its synchrotron radiation facilities. They thank Ignacio Carabias from the Diffraction Unit CAI UCM for his experimental support and helpful comments. The research leading to this result has been supported by the project CALIPSOplus under Grant Agreement 730872 from the EU Framework Programme for Research and Innovation HORIZON 2020. M.J., P.M., I.P., and F.B. acknowledge funding from EUROFEL (RoadMap Esfri). The work at the University of Maryland was supported by ONR MURI (Award No. N00014-17-1-2661). The work at Sandia National Laboratories was supported by the Laboratory-Directed Research and Development (LDRD) Program and the DOE Basic Energy Sciences Award number DE-SC0021070. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the US Department of Energy’s National Nuclear Security Administration under contract DE-NA 000352

Structure and properties of plasma sprayed BaTiO3 coatings: Spray parameters versus structure and photocatalytic activity

International audiencePlasma spraying enables the creation of layers with thickness in a millimeter range adhering on various substrates. This paper provides a study of phase composition, optical properties and photocatalytic activity of BaTiO3 coatings prepared by atmospheric plasma spraying. The spraying was carried out by a direct current gas-stabilized plasma gun. BaTiO3 was fed into the plasma jet as a feedstock powder prepared by a reactive sintering of micrometer-sized powders of BaCO3 and TiO2. Microstructure and phase composition are reported and discussed in connection with optical properties and photocatalytic activity. The spraying was carried out by a direct current gas-stabilized plasma gun which normally utilizes spray distance (SD) in frames from 100 to 150 mm. Besides conventional SD 100 mm also extremely high SD 190 mm was used. The color of the sprayed coating is different for each SD and also differs from sintered BaTiO3. X-ray diffraction and also SAD mode of HR-TEM show certain content of amorphous fraction in the coating. The hydrogen content in the coating was found to be higher in the coatings than in the sintered bulk. The diffuse reflectance was measured by UV-VIS spectrophotometry and corresponding band-gap energy was estimated. X-ray photoelectron spectroscopy confirms specific stoichiometric and structural disorder observed also at bang-gap evaluation and by Raman spectroscopy. HR-TEM images for crystalline and amorphous zones are given. Photocatalytic decomposition of acetone was tested and BaTiO3 coatings compared with a sintered bulk

Thiolate-Capped Silver Nanoparticles: Discerning Direct Grafting from Sulfidation at the Metal–Ligand Interface by Interrogating the Sulfur Atom

International audienceGrafting thiol-bearing molecules at the surface of silver nano-particles (AgNPs) is a successful strategy to tune their optical and antibacterial properties. The capping layer generated from self-assembly of the ligands at the nanoparticle surface determines the range of possible applications of the resulting material. In particular, direct grafting of the thiol heads to surface Ag(I) can occur, with various hybridizations of the S atom, sp versus sp 3. Alternatively, a passivating Ag 2 S layer can form. We make use of S K-edge X-ray absorption near edge structure (XANES) and synchrotron-based X-ray photoelectron spectroscopy (XPS) to probe the metal−ligand interface in different thiol-capped AgNPs. The use of cryogenic conditions for XAS analyses reveals a peculiar spectral signature for thiolates chemisorbed on the AgNPs surface, unambiguously distinguished from that of Ag 2 S. Ab initio simulations of XANES spectra and XPS analyses are used to predict the grafting mode, suggesting that different ligand architectures promote slightly different proportions of sp/sp 3 sites, and a dramatic variability in the stability of the nanomaterial that can evolve toward either self-assembly or dissolution of the AgNPs

Vylepšené uspořádání kvazi-dvourozměrného MoS2 přechodem z amorfního na krystalický stav počínající z amorfního MoS2+ bohatého na síru

The synthesis of stoichiometric two-dimensional (2D) transition-metal dichalcogenides (TMDC) over large areas remains challenging. Using a combination of X-ray diffraction and X-ray absorption spectroscopy, we demonstrate the advantages of using a thin amorphous layer of S-rich MoS2 (MoS4 in this paper) for the growth of well-ordered crystalline MoS2 films via annealing at 900 degrees C. In contrast to the crystallization of stoichiometric amorphous MoS2, the crystallization of the as-depo sited amorphous MoS4 phase shows the strong preferred ordering of layered MoS2 on a Si/SiOx nontemplating substrate with the dominant (002) crystallographic plane and accompanying Kiessig fringes, which indicate the improved crystallinity of the MoS2 layers. A similar effect can only be achieved by the templated crystallization of an amorphous MoS2 thin film deposited on a c-plane sapphire substrate. We suggest that the crystal growth improvement originates from the lower coordination number (CN) of the Mo atoms in the MoS4 amorphous phase (CN = 4) in comparison with that of amorphous MoS2 (CN = 6) and the gradual release of free sulfur atoms from the thin film during crystallization.Syntéza stechiometrických dvourozměrných (2D) dichalkogenidů přechodných kovů (TMDC) na velké plochy je stále výzvou. Pomocí kombinace rentgenové difrakce a rentgenové absorpční spektroskopie demonstrujeme výhody použití tenké amorfní vrstvy MoS2 bohatého na S (MoS4 v tomto článku) pro růst dobře uspořádaných krystalických filmů MoS2 prostřednictvím žíhání při 900 °C. Na rozdíl od krystalizace stechiometrického amorfního MoS2, krystalizace amorfní MoS4 fáze vykazuje silně preferované uspořádání vrstveného MoS2 na Si/SiOx netemplátovém substrátu s dominantní (002) krystalografickou rovinou a doprovodnými Kiessigovými oscilacemi, které indikují vylepšené uspořádání vrstev MoS2. Podobného efektu lze dosáhnout pouze templátovou krystalizací amorfního tenkého filmu MoS2 naneseného na safírovém substrátu s orientací v rovině c. Domníváme se, že zlepšení růstu krystalů je způsobeno nižším koordinačním číslem (CN) atomů Mo v amorfní fázi MoS4 (CN = 4) ve srovnání s amorfním MoS2 (CN = 6) a postupným uvolňováním volných atomů síry z tenkého filmu během krystalizace

Insights about the interaction of methotrexate loaded hydrophilic gold nanoparticles: spectroscopic, morphological and structural characterizations

Gold nanoparticles (AuNPs) are promising carriers in the field of nanomedicine and represent a very intriguing approach in drug delivery applications, due to their small size and enhanced properties. This work aims to highlight the interaction between functionalized AuNPs and the immune-system suppressant drug Methotrexate (MTX) at molecular level. Small and monodisperse (&lt;2RH&gt;=5 ± 1 nm) gold nanoparticles were prepared by a simple chemical route using hydrophilic thiol 3-mercapto-1-propanesulfonate (3MPS) as a functionalizing/capping agent and act as a platform for post-synthesis conjugation of MTX via non-covalent interaction. The AuNPs-3MPS@MTX bioconjugate and the AuNPs alone were characterized to investigate their optical, chemical, and morphological properties. Moreover, NMR, AFM, SAXS, HR-TEM and SR-XPS data confirmed the spherical shape of AuNPs and allowed to determine the mechanisms behind such drug-nanoparticle physicochemical interactions. These analyses define the overall structure of drug-loaded AuNPs-3MPS and drug location on the colloidal nanoparticles surface. Based on the experimental data, it is notable to assert that MTX was successfully loaded on the negatively charged nanoparticles surface via electrostatic interactions. The physicochemical behavior leads to the formation of large clusters with close packed arrangement of AuNPs-3MPS@MTX. This self-assembling property is of importance for delivery purpose affecting the drug-loaded nanoparticle size, functionality, and morphology. Knowledge of how these systems behave will aid in increasing drug efficacy and in understanding the pharmacodynamics and pharmacokinetic properties, opening to new physicochemical insight for therapy and drug delivery system

Low temperature, solution-processed perovskite solar cells and modules with an aperture area efficiency of 11%

Planar perovskite solar cells and modules were realized by using low temperature solution-process fabrication procedures. The photovoltaic performance was improved by optimizing a SnO2 electron transport layer and its interface with the perovskite layer. We achieved a power conversion efficiency (PCE) of 17.3% on small area cell (0.09 cm2) with negligible hysteresis and a steady-state PCE equal to 17.4%. Furthermore, shelf life tests showed a relative decrease of only 5% in PCE from its initial value after 1000 h of storage in dark conditions in air (RH 20%). Up-scaling of the technology was implemented entirely in air with fabrication of modules with a high aperture ratio of 91%. The modules delivered a maximum PCE of 13.1% obtained on an active area of 13.8 cm2 and of 11.9% on an aperture area of 15.2 cm2 representing state of art performance for fully low temperature solution processed planar perovskite solar modules