J.A. Leiro

Core-level XPS spectra of fullerene, highly oriented pyrolitic graphite, and glassy carbon

Abstract The C 1s spectra of fullerene C 60 , highly oriented pyrolitic graphite (HOPG) and amorphous carbon (a-C) have been measured using X-ray photoemission. The assumed background due to the inelastic scattering of electrons of these spectra has been subtracted by the Tougaard’s method. The relative intensities and the energy positions for the core-level satellites have been determined. For C 60 , a comparison of the low energy π type shake-up satellites gives good agreement between theory and experiment. Also, the energies of these features for fullerene and glassy carbon are very similar, whereas the corresponding energies for HOPG are somewhat larger, presumably, because of the higher density of the latter. Moreover, the atomic force microscopy (AFM) study indicates that the C 60 samples consist of a thick layer of large clusters on the Si(111) surface, which is in line with the molecular character of the XPS spectrum. Furthermore, the broad high energy satellite does not consist of a single plasmon but of many components due to collective excitations characteristic of molecules and solids. These features are discussed in the light of theoretical excitation energies.

XPS study of irradiation damage and different metal–sulfur bonds in dodecanethiol monolayers on gold and platinum surfaces

The X-ray induced irradiation damage is studied in the CH3(CH2)11SH monolayers on gold and platinum surfaces to get information on the metal–sulfur bonds. This is important because the sulfur–substrate bond is one of the crucial factors in thiol SAMs. The X-ray photoelectron spectra (XPS) of the S 2p and C 1s emission were analyzed. During the modification of the surface the thiol/platinum system behaves differently than the thiol/gold system. In the latter case the S 2p spectrum changes as a function of time during the measurement and two chemically different sulfur species, associated with Au-thiolates and irradiation induced sulfur species, can be identified in the spectra. On platinum, there are two chemically different sulfur species in pristine monolayers and their ratio remains unchanged as a function of measurement time. This trend is obviously surface induced and is related to the nature of the metal–sulfur bonds in the self-assembled thiol layers.

Self-Assembled Carbon Nanotubes on Gold: Polarization-Modulated Infrared Reflection−Absorption Spectroscopy, High-Resolution X-ray Photoemission Spectroscopy, and Near-Edge X-ray Absorption Fine Structure Spectroscopy Study

Recently we reported noncovalent functionalization of nanotubes in an aqueous medium with ionic liquid-based surfactants, 1-dodecyl-3-methylimidazolium bromide (1) and 1-(12-mercaptododecyl)-3-methylimidazolium bromide (2), resulting in positively charged single-wall carbon nanotube (SWNT)-1,2 composites. Thiolation of SWNTs with 2 provides their self-assembly on gold as well as templating gold nanoparticles on SWNT sidewalls via a covalent -S-Au bond. In this investigation, we studied the electronic structure, intermolecular interactions, and packing within noncovalently thiolated SWNTs and also nanotube alignment in the bulk of SWNT-2 dried droplets and self-assembled submonolayers (SAMs) on gold by high-resolution X-ray photoemission spectroscopy (HRXPS), C K-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and polarization-modulated infrared reflection-absorption spectroscopy (PM-IRRAS). HRXPS data confirmed the noncovalent nature of interactions within the nanocomposite of thiolated nanotubes. In PM-IRRAS spectra of SWNT SAMs on gold, the IR-active vibrational SWNT modes have been observed and identified. According to PM-IRRAS data, the hydrocarbon chains of 2 are oriented with less tilt angle to the bare gold normal in a SAM deposited from an SWNT-2 dispersion than those of 1 deposited from an SWNT-1 dispersion on the mercaptoethanesulfonic acid-primed gold. For both the dried SWNT-2 bulk and the SWNT-2 SAM on gold, the C K-edge NEXAFS spectra revealed the presence of CH-pi interactions between hydrocarbon chains of 2 and the pi electronic nanotube structure due to the highly resolved vibronic fine structure of carbon 1s --> R*/sigma*C-H series of states in the alkyl chain of 2. For the SWNT-2 bulk, the observed splitting and upshift of the SWNT pi* orbitals in the NEXAFS spectrum indicated the presence of pi-pi interactions. In the NEXAFS spectrum of the SWNT-2 SAM on gold, the upshifted values of the photon energy for R*/sigma*C-H transitions indicated close contact of 2 with nanotubes and with a gold surface. The angle-dependent NEXAFS for the SWNT-2 bulk showed that most of the molecules of 2 are aligned along the nanotubes, which are self-organized with orientation parallel to the substrate plane, whereas the NEXAFS for the SWNT-2 SAM revealed a more normal orientation of functionality 2 on gold compared with that in the SWNT-2 bulk.

Surface XPS core-level shifts of FeS2

The XPS S 2p core-level spectra of pyrite FeS2 have been measured to study the surface core-level shifts (SCLSs). In addition to the previously found features on the low binding energy (BE) side of the line, evidence for a new structure on the high BE side is given. This feature seems to be surface sensitive. Also it is found that the final state effect may give a significant contribution to the SCLS.

Surface atomic forces and multilayer relaxation of W(001), W(110) and Fe/W(110)

Abstract Calculations of the atomic forces, magnetic moments and relaxations of the surfaces of W and of a Fe monolayer on W have been performed in the framework of the full potential LAPW method and in terms of the Car–Parrinello approach. The surface is found to be contracted by about 4% for W(100) and about 0.8% for W(110) as compared with the bulk spacing in agreement with recent LEED experiments. Estimates of surface energy of W(001) and W(110) as well as surface core-level shifts are in accordance with experiments, too. A qualitative Pettifor-like charge-transfer model has been used to discuss the relaxations of the surface of tungsten and iron monolayer. Quantitative estimates show that deposition of the Fe monolayer to the tungsten surface decreases the surface tension discernibly relieving the tensile surface stress of the clean W(110) surface.

Surface segregation and core-level shift of a PdRh alloy studied by XPS

Abstract The surface core-level shift and surface segregation of a Pd93Rh7 alloy have been considered in the light of X-ray photoemission spectroscopy. A core-level shift of the Pd3d line to a lower binding energy as far as the surface atoms are concerned has been observed, which can be expected on the basis of the Z + 1 approximation. Also, surface segregation of Pd atoms has been found by using different take-off angles between the momentum of the escaping photoelectron and the surface of the PdRh alloy. This effect has been discussed in terms of the surface density of electronic states and the d-band occupancy of an alloy.

Preparation of Thin Melanin-Type Films by Surface-Controlled Oxidation.

The preparation of thin melanin films suitable for applications is challenging. In this work, we present a new alternative approach to thin melanin-type films using oxidative multilayers prepared by the sequential layer-by-layer deposition of cerium(IV) and inorganic polyphosphate. The interfacial reaction between cerium(IV) in the multilayer and 5,6-dihydroxyindole (DHI) in the adjacent aqueous solution leads to the formation of a thin uniform film. The oxidation of DHI by cerium(IV) proceeds via known melanin intermediates. We have characterized the formed DHI-melanin films using scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), UV-vis spectroscopy, and spectroelectrochemistry. When a five-bilayer oxidative multilayer is used, the film is uniform with a thickness of ca. 10 nm. Its chemical composition, as determined using XPS, is typical for melanin. It is also redox active, and its oxidation occurs in two steps, which can be assigned to semiquinone and quinone formation within the indole structural motif. Oxidative multilayers can also oxidize dopamine, but the reaction stops at the dopamine quinone stage because of the limited amount of the multilayer-based oxidizing agent. However, dopamine oxidation by Ce(IV) was studied also in solution by UV-vis spectroscopy and mass spectrometry in order to verify the reaction mechanism and the final product. In solution, the oxidation of dopamine by cerium shows that the indole ring formation takes place already at low pH and that the mass spectrum of the final product is practically identical with that of commercial melanin. Therefore, layer-by-layer formed oxidative multilayers can be used to deposit functional melanin-type thin films on arbitrary substrates by a surface-controlled reaction.

Atomic structure and chemical bonding of boro- and azafullerene dumb-bells

Abstract The FP-LMTO calculations of dimers (C 59 N) 2 , (C 59 B) 2 , (C 59 NB), hydroazafullerene C 59 NH and hydroborofullerene C 59 BH have been made to specify the ground state geometry, parameters of the electronic structure and bonding energies. The boron atom introduced in the cage of the fullerene causes more disturbances on the atomic structure than the corresponding nitrogen atom would do. The density of states of the azafullerene dumb-bell is slightly shifted to higher binding energies in comparison with that of the borofullerene one and it shows an extra feature at the bottom of the spectrum.

Cr Impurity Effect on Antiphase Boundary in FeAl Alloy

In the FeAl alloy, the electronic structures of (110) and (121) antiphase boundaries (APBs) and the effect of Cr on them are calculated using the recursion method. The density of states, the interaction energy between the atoms, and the energy of APBs are presented. On the basis of these results, the effects of Cr and APB on the physical properties of the FeAl alloy are discussed.

Rippled relaxation of ordered NiAl(110) surface

Abstract The electronic structure of ordered NiAl(110) surface has been calculated using the continued fraction method. The interaction energy between atoms is presented. On the basis of these results, the rippled relaxation, which previously has been observed in LEED experiments for the (110) surface of the NiAl alloy, is discussed.