Role of Ni substitution on structural, magnetic and electronic properties of epitaxial CoCr2O4 spinel thin films.

Cubic spinel CoCr2O4 has recently attained attention due to its multiferroic properties. However, the Co site substitution effect on the structural and magnetic properties has rarely been studied in thin film form. In this work, the structural and magnetic properties of Co1-x Ni x Cr2O4 (x= 0, 0.5) epitaxial thin films deposited on MgAl2O4 (100) and MgO (100) substrates to manipulate the nature of strain in the films using pulsed laser deposition (PLD) technique are presented. The epitaxial nature of the films was manifested through x-ray diffraction (XRD), reciprocal space mapping (RSM) and Rutherford backscattering spectrometry (RBS) measurements. Raman measurements revealed a disappearance of characteristic A 1 g and F 2 g modes of the CoCr2O4 with increase in the Ni content. Atomic force microscopy (AFM) and field emission scanning electron microscopy (FE-SEM) studies show a modification of the surface morphology upon Ni substitution. Magnetic measurements disclose that the ferrimagnetic Curie temperature (T C) of the CoCr2O4 in thin film grown on MgAl2O4 (100) and MgO (100) substrates were found to be 100.6 ± 0.5 K and 93.8 ± 0.2 K, respectively. With Ni substitution the T C values were found to be enhanced to 104.5 ± 0.4 K for MgAl2O4 (100) and 108.5 ± 0.6 K for MgO (100) substrates. X-ray photoelectron spectroscopy (XPS) suggests Cr3+ oxidation states in the films, while Co ions are present in a mixed Co2+/Co3+ oxidation state. The substitution of Ni at Co site significantly modifies the line shape of the core level as well as the valence band. Ni ions are also found to be in a mixed 2+/3+ oxidation state. O 1s core level display asymmetry related to possible defects like oxygen vacancies in the films.

Quantitative resonant soft x-ray reflectivity of ultrathin anisotropic organic layers: Simulation and experiment of PTCDA on Au.

Resonant soft X-ray reflectivity at the carbon K edge, with linearly polarized light, was used to derive quantitative information of film morphology, molecular arrangement, and electronic orbital anisotropies of an ultrathin 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) film on Au(111). The experimental spectra were simulated by computing the propagation of the electromagnetic field in a trilayer system (vacuum/PTCDA/Au), where the organic film was treated as an anisotropic medium. Optical constants were derived from the calculated (through density functional theory) absorption cross sections of the single molecule along the three principal molecular axes. These were used to construct the dielectric tensor of the film, assuming the molecules to be lying flat with respect to the substrate and with a herringbone arrangement parallel to the substrate plane. Resonant soft X-ray reflectivity proved to be extremely sensitive to film thickness, down to the single molecular layer. The best agreement between simulation and experiment was found for a film of 1.6 nm, with flat laying configuration of the molecules. The high sensitivity to experimental geometries in terms of beam incidence and light polarization was also clarified through simulations. The optical anisotropies of the organic film were experimentally determined and through the comparison with calculations, it was possible to relate them to the orbital symmetry of the empty electronic states.

Adsorption geometry variation of 1,4-benzenedimethanethiol self-assembled monolayers on Au(111) grown from the vapor phase.

1,4-benzenedimethanethiol was chemisorbed from the vapor phase onto Au(111). The chemisorption geometry, molecular orientation, and bonding properties were studied at different degrees of surface coverage by photoelectron spectroscopy, metastable deexcitation spectroscopy, and near-edge x-ray absorption fine structure spectroscopy at the carbon K edge. Two main chemisorption regimes were identified: at low coverage the molecules adopt a flat configuration, then, as the molecular density of the first layer increases, the reduction of the available chemisorption sites induces the newly bonded molecules to assume a vertical alignment, with only one of the sulphur head groups interacting with the substrate. Experimental results were interpreted on the basis of theoretical calculations that we performed on the free molecule concerning the molecular orbitals' density of states and simulated x-ray absorption.

NEXAFS study of multi-walled carbon nanotubes functionalization with sulfonated poly(ether ether ketone) chains.

Functionalization of multi-walled carbon nanotubes (MWNTs) surface by sulfonated poly (ether ether ketone) SPEEK chains using a direct attachment reaction was investigated. A two step method was performed. MWNTs were oxidized by a nitric acid treatment to generate carboxyl groups on their surface. The grafting reaction of sulfonated groups of SPEEK with carboxyl groups present on the surface of oxidized MWNTs readily proceeds by using hexane diamine as an interlinking molecule. Transmission electron microscopy (TEM) shows that tubes are wrapped by polymer chains. Near edge X-ray absorption fine structure spectroscopy (NEXAFS) at the C K-edge, O K-edge, and N K-edge and X-ray photoelectron spectroscopy (XPS) were used to give evidence of covalent functionalization of MWNTs by SPEEK macromolecules.

Spectroscopic study of nitrogen doping of multi-walled carbon nanotubes.

Combined spatially resolved electron-energy loss spectroscopy (EELS) and high resolution near-edge X-ray absorption fine structure (NEXAFS) spectroscopy have been used to investigate the nitrogen doping of multi-walled carbon nanotubes (N-MWNT). EELS indicates that most of the tubes are nitrogen-doped. NEXAFS spectroscopy reveals pyridine-like and nitrile N structures. High resolution NEXAFS experiments show that the main nitrogen concentration originates from a high amount of molecular N2 encapsulated into only a small quantity of tubes.

Electronic structure of C60 on Au(887).

We present an analysis of the electronic structure of C60 adsorbed on a vicinal Au(111) surface at different fullerene coverages using photoemission, x-ray absorption, and scanning tunneling microscopy/spectroscopy (STS). STS provides a straightforward determination of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels with respect to the Fermi energy. At C60 coverages of 0.5 and 1 ML a 2.7 eV wide HOMO-LUMO gap is found. The near-edge x-ray absorption fine structure (NEXAFS) spectrum for the 0.5 ML C60 nanomesh structure displays a significant intensity at the low energy side of the LUMO exciton peak, which is explained as due to absorption into HOMO-LUMO gap states localized at individual C60 cluster edges. From 0.5 to 1 ML we observe a rigid shift of the HOMO-LUMO peaks in the STS spectra and an almost complete quenching of the gap states feature in NEXAFS.

High-pressure ground state of SmB6: electronic conduction and long range magnetic order.

High-pressure 149Sm nuclear forward scattering of synchrotron radiation and specific heat measurements have been performed on the intermediate valent Kondo insulator SmB6. The results show that at a critical pressure p(c) approximately = 6 GPa, where the charge gap closes, a first order transition occurs to a magnetically ordered state, which shows typical features of trivalent samarium compounds. The similarity with SmS stresses the role of local correlations and gives important insight into the debate on the local or itinerant character of the f electrons in heavy fermion systems.

Pressure-induced magnetic order in golden SmS.

High pressure 149Sm nuclear forward scattering experiments have been performed on the nonmagnetic semiconductor SmS. We present the first clear evidence that the closure of the insulating gap at p(Delta) approximately 2 GPa coincides with the appearance of magnetic order. The pressure-induced magnetic phase transition has some first order character and suggests that the Sm ions are nearly trivalent at p(Delta). A Gamma(8) quartet crystal field ground state with a value of approximately 0.5 micro(B) for the samarium magnetic moment is inferred from our results. Considerable magnetic short range order is observed above the ordering temperature inferred from macroscopic measurements.