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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]

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]

We propose the optimization of the magnetic remanence and the thermal stability of Mn phthalocyanine coupled with a ferromagnetic substrate, by exploiting interlayer exchange coupling within an advanced organic spin interface architecture, constituted by a FeCo film covered by a graphene membrane, hosting the MnPc molecular layer. The challenge to obtain magnetic remanence for molecular systems stable up to room temperature has been accomplished thanks to a super-exchange path, mediated by the π orbital of the organic ligands of the molecule and of the graphene sheet, favoring an antiferromagnetic (AFM) alignment for the MnPc molecules with the FeCo film. This spin interface with a strong AFM coupling mediated by a graphene spacer is optimized against thermal fluctuations, presenting a well defined remanence even at room temperature, as demonstrated by the persistent dichroic signal in temperature-dependent circularly polarized x-ray absorption spectra. [ABSTRACT FROM AUTHOR]

We report on the electronic properties of an artificial system obtained by the intercalation of equiatomic FeCo layers under graphene grown on Ir(111). Upon intercalation, the FeCo film grows epitaxially on Ir(111), resulting in a lattice-mismatched system. By performing density functional theory calculations, we show that the intercalated FeCo layer leads to a pronounced corrugation of the graphene film. At the same time, the FeCo intercalated layers induce a clear transition from a nearly undisturbed to a strongly hybridized graphene π-band, as measured by angle-resolved photoemission spectroscopy. A comparison of experimental results with the computed band structure and the projected density of states unveils a spin-selective hybridization between the π band of graphene and FeCo-3d states. Our results demonstrate that the reduced dimensionality, as well as the hybridization within the FeCo layers, induces a narrowing and a clear splitting of Fe 3d-up and Fe 3d-down-spin bands of the confined FeCo layers with respect to bulk Fe and Co. [ABSTRACT FROM AUTHOR]

Iron-phthalocyanine and cobalt-phthalocyanine chains, assembled along the Au(110)-(1×2) reconstructed channels, present a strong interaction with the Au metallic states, via the central metal ion. X-ray photoemission spectroscopy from the metal-2p core-levels and valence band high-resolution ultraviolet photoelectron spectroscopy bring to light signatures of the interaction of the metal-phthalocyanine single-layer with gold. The charge transfer from Au to the molecule causes the emerging of a metal-2p core level component at lower binding energy with respect to that measured in the molecular thin films, while the core-levels associated to the organic macrocycle (C and N 1s) are less influenced by the adsorption, and the macrocycles stabilize the interaction, inducing a strong interface dipole. Temperature Programmed Desorption experiments and photoemission as a function of temperature allow to estimate the adsorption energy for the thin-films, mainly due to the molecule-molecule van der Waals interaction, while the FePc and CoPc single-layers remain adsorbed on the Au surface up to at least 820 K. [ABSTRACT FROM AUTHOR]

Iron oxide nanostructures, a promising alternative to carbon-based anode in lithium-ion batteries, can be produced using a hard template route. This procedure guarantees the formation of Fe2O3 nanowires with comparable diameter and size (average diameter 8 nm) with a dominant cubic c-phase at the surface. Lithium exposure of the iron oxide nanowires in ultra-high-vacuum (UHV) conditions induces reduction of the Fe ion, leading to a Fe3O4 and then to a Fe2+ phase, as determined by means of core-level photoemission spectroscopy. Mild annealing of Fe2O3 in UHV determines an oxygen content reduction for the nanowires at lower temperature with respect to the bulk phase. The morphology and the evolution of the electronic properties upon reduction have been compared to those of micro-sized bulk-like grains, to unravel the role of the reduced size and surface-volume ratio. [ABSTRACT FROM AUTHOR]

We report on the influence of defects in the electronic properties of a pentacene single-layer grown on a benzenethiolate self-assembled-monolayer (Bt-SAM) adsorbed on Cu(100) at 300 K, using high-resolution ultraviolet photoelectron spectroscopy. The weak interaction between pentacene and the underlying Bt-SAM is deduced by the lineshape and evolution of the highest-occupied molecular orbital (HOMO). Closer inspection of the pentacene HOMO band reveals a skewed feature, where the contribution in the low binding energy side is found originating from states associated to structural defects that can be reduced by controlled mild-temperature annealing (355 K). Presence of such defects influences also the pentacene HOMO onset energy and hole-injection barrier that can be reduced to typical value of this organic heterostructure after the annealing. [ABSTRACT FROM AUTHOR]

The growth of pentacene on suitable metallic templates is studied by means of low-energy electron diffraction and ultraviolet photoelectron spectroscopy. Highly ordered pentacene single layers can be prepared by deposition on filled d-band metal templates kept at 370 K. The presence of the steps for the Cu(119) vicinal surface and of the Au troughs for the Au(110)-(1×2) surface allows the formation of commensurate long-range ordered structures with (3×7) and (3×6) periodicities, respectively. A detailed analysis of the molecular induced electronic states evolution is performed for different growth morphologies. The adsorption energy of the ordered molecular single layers on the Au(110) surface is lower (1.90 eV) than on the Cu vicinal surface (2.36 eV), where the steps enhance the molecule adsorption energy. [ABSTRACT FROM AUTHOR]

The C K-edge photoabsorption and 1s core-level photoemission of pentacene (C22H14) free molecules are experimentally measured, and calculated by self-consistent-field and static-exchange approximation ab initio methods. Six nonequivalent C atoms present in the molecule contribute to the C 1s photoemission spectrum. The complex near-edge structures of the carbon K-edge absorption spectrum present two main groups of discrete transitions between 283 and 288 eV photon energy, due to absorption to π* virtual orbitals, and broader structures at higher energy, involving σ* virtual orbitals. The sharp absorption structures to the π* empty orbitals lay well below the thresholds for the C 1s ionizations, caused by strong excitonic and localization effects. We can definitely explain the C K-edge absorption spectrum as due to both final (virtual) and initial (core) orbital effects, mainly involving excitations to the two lowest-unoccupied molecular orbitals of π* symmetry, from the six chemically shifted C 1s core orbitals. [ABSTRACT FROM AUTHOR]

The authors present a combined photoemission and scanning-tunneling spectroscopy study of the filled electronic states, the molecular energy gap, and the energy level diagram of highly ordered arrays of pentacene deposited on the Cu(119) vicinal surface. The states localized at the interface are clearly singled out, comparing the results at different pentacene thicknesses and with gas-phase photoemission data. The molecular gap of 2.35 eV, the hole injection barrier of 1.05 eV, and the electron injection barrier of 1.30 eV determine the energy level diagram of the states localized at the pentacene molecules. [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., (© 2023 Author(s). Published under an exclusive license by AIP Publishing.)