Mechanical exfoliation and layer number identification of single crystal monoclinic CrCl3.

After the recent finding that CrI3, displays ferromagnetic order down to its monolayer, extensive studies have followed to pursue new two-dimensional (2D) magnetic materials. In this article, we report on the growth of single crystal CrCl3 in the layered monoclinic phase. The system after mechanical exfoliation exhibits stability in ambient air (the degradation occurs on a time scale at least four orders of magnitude longer than is observed for CrI3). By means of mechanical cleavage and atomic force microscopy (AFM) combined with optical identification, we demonstrate the systematic isolation of single and few layer flakes onto 270 nm and 285 nm SiO2/Si (100) substrates with lateral size larger than graphene flakes isolated with the same method. The layer number identification has been carried with statistically significant data, quantifying the optical contrast as a function of the number of layers for up to six layers. Layer dependent optical contrast data have been fitted within the Fresnel equation formalism determining the real and imaginary part of the wavelength dependent refractive index of the material. A layer dependent (532 nm) micro-Raman study has been carried out down to two layers with no detectable spectral shifts as a function of the layer number and with respect to the bulk.

Photothermal response of plasmonic nanofillers for membrane distillation.

Light-to-heat conversion in plasmonic nanoparticles (NPs) inside polymeric membranes is beneficial for improving the efficiency of membrane distillation for seawater desalination. However, the physical mechanisms ruling photothermal membrane distillation are unclear yet. Here, we model the plasmonic photothermal light-to-heat conversion from Ag, Au, and Cu nanofillers in polymeric membranes for membrane distillation. Photothermal effects in the cases of isolated metallic NPs and their assembly are investigated considering size effects and excitation sources. The increasing content of metallic NPs improves the efficiency of the light-to-heat conversion. For a polymeric membrane, filled with 25% Ag NPs, our model well reproduces the experimental temperature increase of 10 K. Specifically, we find that Ag NPs with a radius of around 30-40 nm are favorite candidates for membrane heating with excitation energy in the visible/near-UV range. The incorporation of a term associated with heat losses into the heat transfer equation well reproduces the cooling effect associated with vaporization at the membrane surface. Compared to Ag NPs, Au and Cu NPs show a broadened absorption cross section and their resonance has a nonlinear behavior with varying the excitation energy, better matching with sunlight radiation spectrum.

A few-layer graphene for advanced composite PVDF membranes dedicated to water desalination: a comparative study.

Membrane distillation is envisaged to be a promising best practice to recover freshwater from seawater with the prospect of building low energy-consuming devices powered by natural and renewable energy sources in remote and less accessible areas. Moreover, there is an additional benefit of integrating this green technology with other well-established operations dedicated to desalination. Today, the development of membrane distillation depends on the productivity-efficiency ratio on a large scale. Despite hydrophobic commercial membranes being widely used, no membrane with suitable morphological and chemical feature is readily available in the market. Thus, there is a real need to identify best practices for developing new efficient membranes for more productive and eco-sustainable membrane distillation devices. Here, we propose engineered few-layer graphene membranes, showing enhanced trans-membrane fluxes and total barrier action against NaCl ions. The obtained performances are linked with filling polymeric membranes with few-layer graphene of 490 nm in lateral size, produced by the wet-jet milling technology. The experimental evidence, together with comparative analyses, confirmed that the use of more largely sized few-layer graphene leads to superior productivity related efficiency trade-off for the membrane distillation process. Herein, it was demonstrated that the quality of exfoliation is a crucial factor for addressing the few-layer graphene supporting the separation capability of the host membranes designed for water desalination.

Adsorption-assisted transport of water vapour in super-hydrophobic membranes filled with multilayer graphene platelets.

The effects of confinement of multilayer graphene platelets in hydrophobic microporous polymeric membranes are here examined. Intermolecular interactions between water vapour molecules and nanocomposite membranes are envisaged to originate assisted transport of water vapour in membrane distillation processes when a suitable filler-polymer ratio is reached. Mass transport coefficients are estimated under different working conditions, suggesting a strong dependence of the transport on molecular interactions. Remarkably, no thermal polarization is observed, although the filler exhibits ultrahigh thermal conductivity. In contrast, enhanced resistance to wetting as well as outstanding mechanical and chemical stability meets the basic requirements of water purification via membrane distillation. As a result, a significant improvement of the productivity-efficiency trade-off is achieved with respect to the pristine polymeric membrane when low amounts of platelets are confined in spherulitic-like PVDF networks.

Indium selenide: an insight into electronic band structure and surface excitations.

We have investigated the electronic response of single crystals of indium selenide by means of angle-resolved photoemission spectroscopy, electron energy loss spectroscopy and density functional theory. The loss spectrum of indium selenide shows the direct free exciton at ~1.3 eV and several other peaks, which do not exhibit dispersion with the momentum. The joint analysis of the experimental band structure and the density of states indicates that spectral features in the loss function are strictly related to single-particle transitions. These excitations cannot be considered as fully coherent plasmons and they are damped even in the optical limit, i.e. for small momenta. The comparison of the calculated symmetry-projected density of states with electron energy loss spectra enables the assignment of the spectral features to transitions between specific electronic states. Furthermore, the effects of ambient gases on the band structure and on the loss function have been probed.

When plasmonics meets membrane technology.

In this review, we present the applications of thermoplasmonics in membrane processes. We discuss the influence of the heat capacity of the solvent, the amount of plasmonic nanoparticles in the membrane, the intensity of the light source and the transmembrane flow rate on the increase of permeability. Remarkably, thermoplasmonic effects do not involve any noticeable loss of membrane rejection. Herein, we consider application feasibilities, including application fields, requirements of feed, alternatives of light sources, promising thermoplasmonic nanoparticles and scaling up issues.

The influence of chemical reactivity of surface defects on ambient-stable InSe-based nanodevices.

We demonstrate that, in contrast to most two-dimensional materials, ultrathin flakes of InSe are stable under ambient conditions. Despite their ambient stability, InSe-based nanodevices show an environmental p-type doping, suppressed by capping InSe with hexagonal boron nitride. By means of transport experiments, density functional theory and vibrational spectroscopy, we attribute the p-type doping assumed by uncapped InSe under an ambient atmosphere to the decomposition of water at Se vacancies. We have estimated the site-dependent adsorption energy of O2, N2, H2O, CO and CO2 on InSe. A stable adsorption is found only for the case of H2O, with a charge transfer of only 0.01 electrons per water molecule.

Interplay of Surface and Dirac Plasmons in Topological Insulators: The Case of Bi_{2}Se_{3}.

We have investigated plasmonic excitations at the surface of Bi_{2}Se_{3}(0001) via high-resolution electron energy loss spectroscopy. For low parallel momentum transfer q_{∥}, the loss spectrum shows a distinctive feature peaked at 104 meV. This mode varies weakly with q_{∥}. The behavior of its intensity as a function of primary energy and scattering angle indicates that it is a surface plasmon. At larger momenta (q_{∥}~0.04 Å^{-1}), an additional peak, attributed to the Dirac plasmon, becomes clearly defined in the loss spectrum. Momentum-resolved loss spectra provide evidence of the mutual interaction between the surface plasmon and the Dirac plasmon of Bi_{2}Se_{3}. The proposed theoretical model accounting for the coexistence of three-dimensional doping electrons and two-dimensional Dirac fermions accurately represents the experimental observations. The results reveal novel routes for engineering plasmonic devices based on topological insulators.

Evidence of composite plasmon­phonon modes in the electronic response of epitaxial graphene.

The electronic response of quasi-freestanding graphene on Pt(111) has been measured by high-resolution electron energy loss spectroscopy. Loss spectra reveal the existence of three distinct excitations: a dispersing feature due to the ordinary sheet plasmon and two dispersionless modes at 0.2 and 0.5­0.6 eV. The latter two features are assigned to the coupled plasmon­phonon excitation and to an interface plasmon, respectively. The complex interactions of plasmons with other particles have significant fundamental and practical implications on the electronic response of graphene and their knowledge is essential for tailoring upcoming graphene-based plasmonic devices.

Interplay between single-particle and plasmonic excitations in the electronic response of thin Ag films.

High-resolution electron energy loss spectroscopy is used to study the electronic properties of thin Ag layers on Ni(111). In addition to the ordinary surface plasmon at 3.8 eV, we observe a broad feature at 7-8 eV, whose nature is investigated as a function of scattering geometry and primary electron beam energy. Loss measurements unambiguously indicate that this mode has spectral components from both free-electron Ag plasmonic excitations (free-electron surface plasmons and multipole plasmons) and single-particle transitions.

Alkali-induced hydrogenation of epitaxial graphene by water splitting at 100 K.

The coadsorption of potassium with water at 100 K on graphene/Pt(111) has been studied by high-resolution electron energy loss spectroscopy. The adsorption of alkali metals induces water splitting and the formation of C-H and C-OH groups. Such finding is of great interest for tailoring graphene-Pt electro-catalysts. Furthermore, the alkali-promoted dissociation of water molecules offers the possibility to attain a partial hydrogenation of the graphene sheet even at low temperature.

Helium, neon and argon diffraction from Ru(0001).

We present an experimental and theoretical study of He, Ne and Ar diffraction from the Ru(0001) surface. Close-coupling calculations were performed to estimate the corrugation function and the potential well depth in the atom-surface interaction in all three cases. DFT (density functional theory) calculations, including van der Waals dispersion forces, were used to validate the close-coupling results and to further analyze the experimental results. Our DFT calculations indicate that, in the incident energy range 20-150 meV, anticorrugating effects are present in the case of He and Ar diffraction, whereas normal corrugation is observed with Ne beams.

The adsorption and co-adsorption of oxygen and carbon monoxide on Pt3Ni(111): a vibrational study.

High-resolution electron energy loss spectroscopy has been used to investigate the adsorption and co-adsorption of oxygen and CO on the Pt(3)Ni(111) surface. For the sake of comparison, similar measurements have also been performed on the Pt(111) surface. We find that CO adsorbs at the same manner on both surfaces. By contrast, significant differences between the two surfaces exist concerning the adsorption of O and the co-adsorption of O with CO.

Plasmonic modes confined in nanoscale thin silver films deposited onto metallic substrates.

Collective electronic excitations in nanoscale thin Ag layers adsorbed on Cu(111) and Ni(111) at room temperature have been investigated by high-resolution electron energy loss spectroscopy. Surface plasmon was found to be confined within grains on Ag thin films on Cu(111) nanostructured in islands. Annealing removed surface plasmon confinement and induced a negative linear term of the dispersion relation. On the other hand, on flat thin films on Ni(111) the dispersion of Ag surface plasmon is fully quadratic. Landau damping processes of the plasmonic excitation were found to be dependent on the growth mode. Ag multipole surface plasmon at 7.7 eV was observed only under stringent kinematic conditions enhancing surface sensitivity.

CO-promoted formation of the alkali-oxygen bond on Ni(111).

High-resolution electron energy loss spectroscopy was used to study the coadsorption of alkali metals (Na, K) and oxygen on clean and CO-modified Ni(111) surfaces. We unambiguously show that on an alkali-precovered surface, the alkali-O bond was not formed upon O(2) exposure. On the contrary, the alkali-O bond was readily observed by exposing to O(2) the Ni(111) surface precovered with an alkali+CO phase. This enhanced oxidation rate of alkali metals in the presence of CO molecules was ascribed to the short-range CO-induced modification of the electronic charge of alkali-metal adatoms.

Collective excitations in nanoscale thin alkali films: Na/Cu(111).

The relationship between electron quantum confinement and the energy dispersion of the surface plasmon in nanoscale thin Na layers adsorbed on Cu(111) at room temperature have been studied by high resolution electron energy loss spectroscopy. Screening effects due to electron quantum confinement occurring in this system cause the lowering of the surface plasmon frequency and, moreover, make the energy range of its dispersion curve larger than in thick alkali films. Landau damping of the plasma excitation was unexpectedly very efficient at small momenta. The dispersion curve of the Na surface plasmon was found to depend on the primary electron beam energy. Multipole surface plasmon at 4.70 eV was observed only for higher impinging energies.

Comparative vibrational study on alkali coadsorption with CO and O on Ni(111) and Cu(111).

High-resolution electron energy loss spectroscopy was used to investigate alkali (Na, K) coadsorption with CO and O on Cu(111) and Ni(111). Measurements provided new insights in these systems. A CO-induced weakening of the alkali-substrate bond was revealed on both substrates. The effect is more pronounced for the Na+CO/Ni(111) system. Submonolayers of alkalis were found to promote the preferential population of the subsurface site for O/Cu(111) but not for O/Ni(111).

Alkali-promoted CO dissociation on Cu(111) and Ni(111) at room temperature.

The coadsorption of alkalis (K, Na) and CO on Cu(111) was investigated by high-resolution electron energy loss spectroscopy. Measurements performed at room temperature showed that CO adsorption is partially dissociative on a potassium-precovered Cu(111) surface and fully dissociative for Na/Cu(111). Carbon monoxide molecules occupy adsorption sites directly adjacent to those of alkali adatoms, as suggested by the absence of a threshold alkali precoverage for CO dissociation. On the contrary, for alkali+CO/Ni(111) a threshold alkali precoverage for CO dissociation was found to exist.

Evidences of alkali-induced softening of the oxygen-substrate bond.

The interaction of oxygen with alkalis (Na, K) on Ni(111) was studied by high-resolution electron energy loss spectroscopy. Loss measurements revealed for the first time a softening of the O-Ni bond and, simultaneously, a strengthening of the alkali-Ni bond in the alkali+O coadsorbed phase, in perfect agreement with recent theoretical calculations. The weakening of the O-Ni bond was ascribed to the alkali-induced filling of the O 2p(z) antibonding orbitals. Different physical mechanisms were discussed for explaining the strengthening of the alkali-substrate bond whenever alkalis are coadsorbed with O adatoms.

High resolution electron energy loss measurements of Na/Cu(111) and H2O/Na/Cu(111): dependence of water reactivity as a function of Na coverage.

Collective electronic excitations occurring in Na layers grown on Cu(111) and in H2O/Na/Cu(111) have been investigated at room temperature by high resolution electron energy loss spectroscopy. Loss spectra taken for a coverage between 0.55 and 0.70 ML of Na are characterized by a feature at 3.0 eV assigned to a Mie resonance. Further increasing the Na coverage leads to the appearance of the Na surface plasmon at 3.9 eV. Water molecules dissociate on Na layers as shown by the appearance of the OH-Na vibration. Upon water adsorption, relevant effects on both electronic excitations and vibrational modes were observed as a function of Na coverage.