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Alkali metal doping of multi-walled carbon nanotubes is of great interest, both fundamentally to explore the effect of dopants on quasi-one-dimensional electrical systems and for energy applications such as alkali metal storage. We present an investigation with complementary photoemission and Raman spectroscopies, fully carried out in an ultra-high vacuum, to unveil the electronic and vibrational response of a forest of highly aligned multi-walled carbon nanotubes by in situ potassium doping. The charge donation by the alkali adatoms induces a plasmon mode, and the density of states undergoes an energy shift consistent with electron donation and band filling of the multi-walled carbon nanotube band structure. The π-states in the valence band and the Raman peaks unveil an evolution that can be ascribed to charge donation and partially to a tensile strain exerted by the K adatoms on the carbon lattice. All these effects are thermally reversible, fostering these materials as a potential system for electronic charge harvesting. [ABSTRACT FROM AUTHOR]

The growing demand for improved electrochemical performance in energy storage systems has stimulated research into advanced two-dimensional (2D) materials for electrodes. In this work, we obtain a layered MXene compound by exfoliating a titanium aluminum carbide precursor using tetramethylammonium hydroxide (TMAOH) ions in a full room temperature process followed by manual shaking. The hexagonal crystal structure and composition of the layered materials are characterized using different techniques. X-Ray diffraction shows the formation of 2D nano-sheets before and after the TMAOH treatment via its characteristic (002) diffraction peak, bringing to light an increase in the interlayer spacing after treatment. Scanning electron microscopy images confirm the layered morphology, whose composition is determined by energy dispersive x-ray analysis for the bulk material and by x-ray photoelectron spectroscopy for the surface of the obtained compounds. This study demonstrates a promising route to enhance delamination of this MXene 2D material in a low-cost room-temperature approach. [ABSTRACT FROM AUTHOR]

A detailed inverse photoemission study unveils the unoccupied electronic structure induced by the adsorption of CuPc and CoPc phthalocyanines on Au(110) reconstructed channels. The different behavior in the two systems is related to the different intermixing of orbitals with the underlying gold states. Broadening of the density of states at the Fermi level is detected after CoPc adsorption, absent in the case CuPc. A detailed comparison with the element-selective X-ray absorption spectroscopy enlightens and complements the IPES results and confirms a surface-driven intermixing of the CoPc orbitals involved in the interaction, with the out-of-plane Co 3dz2 orbital strongly hybridized with the gold electronic states. Moreover, the contribution of the 3d empty states to the IPES data is reported for FePc, CoPc, and CuPc thin films. [ABSTRACT FROM AUTHOR]

Highly aligned multi-wall carbon nanotubes were investigated with scanning electron microscopy (SEM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) before and after bombardment performed using noble gas ions of different masses (argon, neon and helium), in an ultra-high-vacuum (UHV) environment. Ion irradiation leads to change in morphology, deformation of the carbon (C) honeycomb lattice and different structural defects in multi-wall carbon nanotubes. One of the major effects is the production of bond distortions, as determined by micro-Raman and micro-X-ray photoelectron spectroscopy. We observe an increase in sp 3 distorted bonds at higher binding energy with respect to the expected sp 2 associated signal of the carbon 1s core level, and increase in dangling bonds. Furthermore, the surface damage as determined by the X-ray photoelectron spectroscopy carbon 1s core level is equivalent upon bombarding with ions of different masses, while the impact and density of defects in the lattice of the MWCNTs as determined by micro-Raman are dependent on the bombarding ion mass; heavier for helium ions, lighter for argon ions. These results on the controlled increase in sp 3 distorted bonds, as created on the multi-wall carbon nanotubes, open new functionalization prospects to improve and increase atomic hydrogen uptake on ion-bombarded multi-wall carbon nanotubes. [ABSTRACT FROM AUTHOR]

The dispersion of the electronic states of epitaxial graphene (Gr) depends significantly on the strength of the bonding with the underlying substrate. We report on empty electron states in cobalt-intercalated Gr grown on Ir(111), studied by angle-resolved inverse photoemission spectroscopy and x-ray absorption spectroscopy, complemented with density functional theory calculations. The weakly bonded Gr on Ir preserves the peculiar spectroscopic features of the Gr band structure, and the empty spectral densities are almost unperturbed. Upon intercalation of a Co layer, the electronic response of the interface changes, with an intermixing of the Gr π* bands and Co d states, which breaks the symmetry of π/σ states, and a downshift of the upper part of the Gr Dirac cone. Similarly, the image potential of Ir(111) is unaltered by the Gr layer, while a downward shift is induced upon Co intercalation, as unveiled by the image state energy dispersion mapped in a large region of the surface Brillouin zone. [ABSTRACT FROM AUTHOR]

The assembling of metal phthalocyanines on the rippled moiré superlattice of graphene/Ir(111) intercalated with one Co layer is driven by the site-dependent polarization field induced by the incommensurate graphene-Co interface. We have performed an X-ray absorption and photoemission study to unveil the role of the metallic centers and of the organic ligands in the molecule-Co interaction process mediated by graphene. Notably, we consider different electronic molecular orbitals, i.e. phthalocyanines with Cu and Mn metallic ions. The spectroscopic response suggests almost unaltered CuPc molecular states upon adsorption, and the rippled graphene carpet decouples completely the electronic interaction between the molecules and the Co layer, while a slight hybridization is present for MnPcs. MnPc molecules, trapped in the valleys of the moiré graphene superlattice, slightly intermix, through the orbitals protruding out of the molecular plane, with the underlying Co, while the organic ligands are almost unaltered. Graphene acts as an interlayer and mediates the interaction between metal phthalocyanines and the metallic substrate, preventing a strong chemical intermixing and enabling the assembly of almost unaltered molecules, preserving their electronic/magnetic state. [ABSTRACT FROM AUTHOR]

We report on the assembly of a highly ordered array of copper tetrameric clusters, coordinated into a metal-organic network. The ordered cluster array has been achieved by the deposition of tetrahydroxyquinone molecules on the Cu(111) surface at room temperature, and subsequent thermally activated dehydrogenation with the formation of tetraoxyquinone tetra-anions with a 4 × 4 periodicity. The supramolecular organic network acts as a spacer for the highly ordered two-dimensional network of copper tetramers at the very surface. [ABSTRACT FROM AUTHOR]

Amorphous SiO2 thin films are one of the two components of the highly reflective mirror coatings of gravitational-wave detectors. For this study, layers of amorphous SiO2 on crystalline Si substrates were produced by ion-beam sputtering (IBS), using accelerated neutralized argon ions as sputtering particles, as is the case for the actual mirror coatings of gravitational-wave detectors. The aim of this study is to investigate the possible presence of various defects in the materials in order to improve the coating quality. We provide evidence that, due to the synthesis method, about 0.2 wt.% of Ar is present in the coatings, and it can be released by means of thermal treatments, starting around 400 °C. The time and temperature to obtain the total release of Ar increases with the coating thickness; for a thickness of 100 nm, all argon is released below 600 °C, while an isotherm of one hour at 900 °C is necessary for a coating 5 μm thick. Besides the Ar atoms left from the synthesis, other defects, such as Si clusters and silicon dangling bonds, are present in the coatings. The concentration of both of them is strongly reduced by thermal treatments either in vacuum or in air. The overall thickness of the coating is slightly increased after thermal treatments, as witnessed by the change of the period of interference fringes. [ABSTRACT FROM AUTHOR]

Manganese-phthalocyanines form assembled chains with a variety of ordered super-structures, flat lying along the Au(110) reconstructed channels. The chains first give rise to a ×5 symmetry reconstruction, while further deposition of MnPc leads to a ×7 periodicity at the completion of the first single layer. A net polarization with the formation of an interface dipole is mainly due to the molecular π-states located on the macrocycles pyrrole rings, while the central metal ion induces a reduction in the polarization, whose amount is related to the Mn-Au interaction. The adsorption-induced interface polarization is compared to other 3d-metal phthalocyanines, to unravel the role of the central metal atom configuration in the interaction process of the d-states. The MnPc adsorption on Au(110) induces the re-hybridization of the electronic states localized on the central metal atom, promoting a charge redistribution of the molecular orbitals of the MnPc molecules. The molecule-substrate interaction is controlled by a symmetry-determined mixing between the electronic states, involving also the molecular empty orbitals with d character hybridized with the nitrogen atoms of the pyrrole ring, as deduced by photoemission and X-ray absorption spectroscopy exploiting light polarization. The symmetry-determined mixing between the electronic states of the Mn metal center and of the Au substrate induces a density of states close to the Fermi level for the ×5 phase. [ABSTRACT FROM AUTHOR]

Layered molybdenum disulphide (MoS2) crystals in combination with graphene create the opportunity for the development of heterostructures with tailored surface and structural properties for energy storage applications. Herein, 2D heterostructures are developed by growing MoS2 on epitaxial and self‐standing nanoporous graphene (NPG) using chemical vapor deposition (CVD). The effect of substrate as well as different CVD growth parameters such as temperature, amount of sulfur and MoO3 precursors, and argon flow on the growth of MoS2 is systematically investigated. Interestingly, various structures of MoS2 such as monolayer triangular islands, spirals, standing sheets, and irregular stacked multilayered MoS2 are successfully developed. The growth mechanism is proposed using different advanced characterization techniques. The formation of a continuous wetting layer with grain boundaries over the surface prior to formation of any other structures is detected. As a proof of principle, MoS2/NPG is employed for the first time as anode material in potassium ion battery. The electrode delivers a specific capacity of 389 mAh g−1 with over 98% stability after 200 cycles. The porous structures clearly facilitate the ion transport which is beneficial for the ion battery. These encouraging results open new opportunities to develop hierarchical heterostructures of 2D‐materials for next‐generation energy storage technologies. [ABSTRACT FROM AUTHOR]