Improving the Adhesion of Multi-Walled Carbon Nanotubes to Titanium by Irradiating the Interface with He+ Ions: Atomic Force Microscopy and X-ray Photoelectron Spectroscopy Study.

A complex study of the adhesion of multi-walled carbon nanotubes to a titanium surface, depending on the modes of irradiation with He+ ions of the "MWCNT/Ti" system, was conducted using atomic force microscopy and X-ray photoelectron spectroscopy. A quantitative assessment of the adhesion force at the interface, performed using atomic force microscopy, demonstrated its significant increase as a result of treatment of the "MWCNT/Ti" system with a beam of helium ions. The nature of the chemical bonding between multi-walled carbon nanotubes and the surface of the titanium substrate, which causes this increase in the adhesion of nanotubes to titanium as a result of ion irradiation, was investigated by X-ray photoelectron spectroscopy. It was established that this bonding is the result of the formation of chemical C-O-Ti bonds between titanium and carbon atoms with the participation of oxygen atoms of oxygen-containing functional groups, which are localized on defects in the nanotube walls formed during ion irradiation. It is significant that there are no signs of direct bonding between titanium and carbon atoms.

A Comparative XPS, UV PES, NEXAFS, and DFT Study of the Electronic Structure of the Salen Ligand in the H2(Salen) Molecule and the [Ni(Salen)] Complex.

A comparative study of the electronic structure of the salen ligand in the H2(Salen) molecule and the [Ni(Salen)] complex was performed using the experimental methods of XPS, UV PES, and NEXAFS spectroscopy along with DFT calculations. Significant chemical shifts of +1.0 eV (carbon), +1.9 eV (nitrogen), and -0.4 eV (oxygen) were observed in the 1s PE spectra of the salen ligand atoms when passing from a molecule to a complex, unambiguously indicating a substantial redistribution of the valence electron density between these atoms. It is proposed that the electron density transfer to the O atoms in [Ni(Salen)] occurred not only from the Ni atom, but also from the N and C atoms. This process seemed to be realized through the delocalized conjugated π-system of the phenol C 2p electronic states of the ligand molecule. The DFT calculations (total and partial DOS) for the valence band H2(Salen) and [Ni(Salen)] described well the spectral shape of the UV PE spectra of both compounds and confirmed their experimental identification. An analysis of the N and O 1s NEXAFS spectra clearly indicated that the atomic structure of the ethylenediamine and phenol fragments was retained upon passing from the free salen ligand to the nickel complex.

The Valence Band Structure of the [Ni(Salen)] Complex: An Ultraviolet, Soft X-ray and Resonant Photoemission Spectroscopy Study.

The valence band photoemission (VB PE) spectra of the [Ni(Salen)] molecular complex were measured by ultraviolet, soft X-ray and resonant photoemission (ResPE) using photons with energies ranging from 21.2 eV to 860 eV. It was found that the Ni 3d atomic orbitals' (AOs) contributions are most significant for molecular orbitals (MOs), which are responsible for the low-energy PE band at a binding energy of 3.8 eV in the VB PE spectra. In turn, the PE bands in the binding energies range of 8-16 eV are due to the photoionization of the MOs of the [Ni(Salen)] complex with dominant contributions from C 2p AOs. A detailed consideration was made for the ResPE spectra obtained using photons with absorption resonance energies in the Ni 2p3/2, N 1s, and O 1s Near-Edge X-ray Absorption Fine Structure (NEXAFS) spectra. A strong increase in the intensity of the PE band ab was found when using photons with an energy 854.4 eV in the Ni 2p3/2 NEXAFS spectrum. This finding is due to the high probability of the participator-Auger decay of the Ni 2p3/2-13d9 excitation and confirms the relationship between the PE band ab with the Ni 3d-derived MOs.

The Identification of Cu-O-C Bond in Cu/MWCNTs Hybrid Nanocomposite by XPS and NEXAFS Spectroscopy.

The results of the research of a composite based on multi-walled carbon nanotubes (MWCNTs) decorated with CuO/Cu2O/Cu nanoparticles deposited by the cupric formate pyrolysis are discussed. The study used a complementary set of methods, including scanning and transmission electron microscopy, X-ray diffractometry, Raman, and ultrasoft X-ray spectroscopy. The investigation results show the good adhesion between the copper nanoparticles coating and the MWCNT surface through the oxygen atom bridge formation between the carbon atoms of the MWCNT outer graphene layer and the oxygen atoms of CuO and Cu2O oxides. The formation of the Cu-O-C bond between the coating layer and the outer nanotube surface is clearly confirmed by the results of the O 1s near edge X-ray absorption fine structure (NEXAFS) and X-ray photoelectron spectroscopy (XPS) of the Cu/MWCNTs nanocomposite. The XPS measurements were performed using a laboratory spectrometer with sample charge compensation, and the NEXAFS studies were carried out using the synchrotron radiation of the Russian-German dipole beamline at BESSY-II (Berlin, Germany) and the NanoPES station at the Kurchatov Center for Synchrotron Radiation and Nanotechnology (Moscow, Russia).

Comparative Study of the Structural Features and Electrochemical Properties of Nitrogen-Containing Multi-Walled Carbon Nanotubes after Ion-Beam Irradiation and Hydrochloric Acid Treatment.

Using a set of microscopic, spectroscopic, and electrochemical methods, a detailed study of the interrelation between the structural and electrochemical properties of the as-prepared nitrogen-containing multi-walled carbon nanotubes (N-MWCNTs) and their modified derivatives is carried out. It was found that after treatment of nanotubes with hydrochloric acid, their structure is improved by removing amorphous carbon from the outer layers of N-MWCNTs. On the contrary, ion bombardment leads to the formation of vacancy-type structural defects both on the surface and in the bulk of N-MWCNTs. It is shown that the treated nanotubes have an increased specific capacitance (up to 27 F·g-1) compared to the as-prepared nanotubes (13 F·g-1). This is due to an increase in the redox capacitance. It is associated with the reversible Faraday reactions with the participation of electrochemically active pyridinic and pyrrolic nitrogen inclusions and oxygen-containing functional groups (OCFG). Based on the comparison between cyclic voltammograms of N-MWCNTs treated in HCl and with an ion beam, the peaks on these curves were separated and assigned to specific nitrogen inclusions and OCFGs. It is shown that the rate of redox reactions with the participation of OCFGs is significantly higher than that of reactions with nitrogen inclusions in the pyridinic and pyrrolic forms. Moreover, it was established that treatment of N-MWCNTs in HCl is accompanied by a significant increase in the activity of nitrogen centers, which, in turn, leads to an increase in the rate of redox reactions involving OCFGs. Due to the significant contribution of redox capacitance, the obtained results can be used to develop supercapacitors with increased total specific capacitance.

Honeycomb Boron on Al(111): From the Concept of Borophene to the Two-Dimensional Boride.

A great variety of two-dimensional (2D) boron allotropes (borophenes) were extensively studied in the past decade in the quest for graphene-like materials with potential for advanced technological applications. Among them, the 2D honeycomb boron is of specific interest as a structural analogue of graphene. Recently it has been synthesized on the Al(111) substrate; however it remains unknown to what extent does honeycomb boron behave like graphene. Here we elucidate the structural and electronic properties of this unusual 2D material with a combination of core-level X-ray spectroscopies, scanning tunneling microscopy, and DFT calculations. We demonstrate that in contrast to graphene on lattice-mismatched metal surfaces, honeycomb boron cannot wiggle like a blanket on Al(111), but rather induces reconstruction of the top metal layer, forming a stoichiometric AlB2 sheet on top of Al. Our conclusions from theoretical modeling are fully supported by X-ray absorption spectra showing strong similarity in the electronic structure of honeycomb boron on Al(111) and thick AlB2 films. On the other hand, a clear separation of the electronic states of the honeycomb boron into π- and σ-subsystems indicates an essentially 2D nature of the electronic system in both one-layer AlB2 and bulk AlB2.

Electronic structure of the [Ni(Salen)] complex studied by core-level spectroscopies.

The nature and structure of occupied and empty valence electronic states (molecular orbitals, MOs) of the [Ni(Salen)] molecular complex (NiO2N2C16H14) have been studied by X-ray photoemission and absorption spectroscopy combined with density functional theory (DFT) calculations. As a result, the composition of the high-lying occupied and low-lying unoccupied electronic states has been identified. In particular, the highest occupied molecular orbital (HOMO) of the complex is found to be predominantly located on the phenyl rings of the salen ligand, while the states associated with the occupied Ni 3d-derived molecular orbitals (MOs) are at higher binding energies. The lowest unoccupied molecular orbital (LUMO) is also located on the salen ligand and is formed by the 2pπ orbitals of carbon atoms in phenyl groups of the salen macrocycle. The unoccupied MOs above the LUMO reflect σ- and π-bonding between Ni and its nearest neighbours. All valence states have highly mixed character. The specific nature of the unoccupied Ni 3d-derived σ-MO is a consequence of donor-acceptor chemical bonding in [Ni(Salen)].

Single-Phase Borophene on Ir(111): Formation, Structure, and Decoupling from the Support.

Artificial two-dimensional (2D) materials, which host electronic or spatial structure and properties not typical for their bulk allotropes, can be grown epitaxially on atomically flat surfaces; the design and investigation of these materials are thus at the forefront of current research. Here we report the formation of borophene, a planar boron allotrope, on the surface of Ir(111) by exposing it to the flux of elemental boron and consequent annealing. By means of scanning tunneling microscopy and density functional theory calculations, we reveal the complex structure of this borophene, different from all planar boron allotropes reported earlier. This structure forms as a single phase on iridium substrate in a wide range of experimental conditions and may be then decoupled from the substrate via intercalation. These findings allow for production of large, defect-free borophene sheets and advance theoretical understanding of polymorphism in borophene.

One Precursor but Two Types of Graphene Nanoribbons: On-Surface Transformations of 10,10'-Dichloro-9,9'-bianthryl on Ag(111).

On-surface synthesis has emerged in the last decade as a method to create graphene nanoribbons (GNRs) with atomic precision. The underlying premise of this bottom-up strategy is that precursor molecules undergo a well-defined sequence of inter- and intramolecular reactions, leading to the formation of a single product. As such, the structure of the GNR is encoded in the precursors. However, recent examples have shown that not only the molecule, but also the coinage metal surface on which the reaction takes place, plays a decisive role in dictating the nanoribbon structure. In this work, we use scanning probe microscopy and X-ray photoelectron spectroscopy to investigate the behavior of 10,10'-dichloro-9,9'-bianthryl (DCBA) on Ag(111). Our study shows that Ag(111) can induce the formation of both seven-atom wide armchair GNRs (7-acGNRs) and 3,1-chiral GNRs (3,1-cGNRs), demonstrating that a single molecule on a single surface can react to different nanoribbon products. We additionally show that coadsorbed dibromoperylene can promote surface-assisted dehydrogenative coupling in DCBA, leading to the exclusive formation of 3,1-cGNRs.

From Graphene Nanoribbons on Cu(111) to Nanographene on Cu(110): Critical Role of Substrate Structure in the Bottom-Up Fabrication Strategy.

Bottom-up strategies can be effectively implemented for the fabrication of atomically precise graphene nanoribbons. Recently, using 10,10'-dibromo-9,9'-bianthracene (DBBA) as a molecular precursor to grow armchair nanoribbons on Au(111) and Cu(111), we have shown that substrate activity considerably affects the dynamics of ribbon formation, nonetheless without significant modifications in the growth mechanism. In this paper we compare the on-surface reaction pathways for DBBA molecules on Cu(111) and Cu(110). Evolution of both systems has been studied via a combination of core-level X-ray spectroscopies, scanning tunneling microscopy, and theoretical calculations. Experimental and theoretical results reveal a significant increase in reactivity for the open and anisotropic Cu(110) surface in comparison with the close-packed Cu(111). This increased reactivity results in a predominance of the molecular-substrate interaction over the intermolecular one, which has a critical impact on the transformations of DBBA on Cu(110). Unlike DBBA on Cu(111), the Ullmann coupling cannot be realized for DBBA/Cu(110) and the growth of nanoribbons via this mechanism is blocked. Instead, annealing of DBBA on Cu(110) at 250 °C results in the formation of a new structure: quasi-zero-dimensional flat nanographenes. Each nanographene unit has dehydrogenated zigzag edges bonded to the underlying Cu rows and oriented with the hydrogen-terminated armchair edge parallel to the [1-10] direction. Strong bonding of nanographene to the substrate manifests itself in a high adsorption energy of -12.7 eV and significant charge transfer of 3.46e from the copper surface. Nanographene units coordinated with bromine adatoms are able to arrange in highly regular arrays potentially suitable for nanotemplating.