Edge Near Edge X-ray Absorption Fine Structure Spectra Research Articles

Cationic vacancies and anomalous spectral-weight transfer in Ti1−xTaxO2thin films studied via polarization-dependent near-edge x-ray absorption fine structure spectroscopy

We report the electronic structures of Ta-doped anatase TiO 2 thin films grown by pulsed laser deposition (PLD) with varying magnetization using a combination of first-principles calculations and near-edge x-ray absorption fine structure (NEXAFS) spectroscopy. The roles of Ta doping and Ti vacancies are clarified, and the observed room-temperature ferromagnetism is attributed to the localized magnetic moments at Ti vacancy sites ferromagnetically ordered by electron charge carriers. O K -edge spectra exhibit significant polarization dependence which is discussed and supported by first-principles calculations in relation to both the crystal symmetry and the formation of defects. In particular, anomalous spectral-weight transfer across the entire O K edge for the ferromagnetic thin film is associated exclusively with the occurrence of Ti vacancies and strong correlation effects, which result in the enhancement of the direct interaction between oxygen sites and of the anisotropy of the e g - p σ hybridizations in the out-of-plane component. Our results show that O K -edge NEXAFS spectra can provide reliable experimental probes capable of revealing cationic defects that are intimately related to the ferromagnetism in transition metal oxides.

Ferromagnetism in Chemically-synthesized Co-doped ZnO

We report room temperature ferromagnetism (RTFM) in Co-doped ZnO (Zn1−xCoxO; x = 0.01 − 0.05) powder synthesized using a co-precipitation method. Our magnetometry data reveal a weak FM behavior at RT in the range 0.01 < x < 0.05. FM by magnetic FM moment steadily decreases with increasing dopant. The O K-edge near-edge X-ray absorption fine structure (NEXAFS) spectra indicate that O vacancies increases with increasing Co concentration and that the sample with x ≥ 0.03 has more broadening at 535 and 540 eV, which may be due to the oxygen vacancies. Co in the ZnO host reveals a +2 oxidation state via the K edge NEXAFS spectra.

Direct observation of oxygen induced room temperature ferromagnetism in MoO2 thin films by x-ray magnetic circular dichroism characterizations

We report the element specific polarized near edge x-ray absorption fine structure (NEXAFS) and x-ray magnetic circular dichroism (XMCD) experiments on well characterized undoped MoO2 thin films that show ferromagnetism at room temperature. The polarization dependent of O K edge NEXAFS spectra indicate a strong hybridization of O 2p-4d Mo orbitals followed by a strong anisotropy in the electronic properties. An unquenched orbital magnetic moment within the O 2p shell is clearly evident from the XMCD O K edge, which is ferromagnetically coupled to the neighboring Mo moments as confirmed by Mo M3,2 edge XMCD experiment.

Substitutional nitrogen incorporation through rf glow discharge treatment and subsequent oxygen uptake on vertically aligned carbon nanotubes

By employing polarization- and temperature- dependent near- edge x- ray- absorption fine structure (NEXAFS) studies, we show that vertically aligned carbon nanotubes when subjected to low- pressure radio- frequency (rf) glow discharge can easily be doped with nitrogen atoms at predominantly graphitelike substitutional sites and at the same time preserve their vertical alignment in contrast to commonly used wet chemical functionalization methods. The O K edge NEXAFS spectra and angle- dependent x- ray phoelectron spectroscopy measurements established the presence of a significant amount of oxygen containing functional groups localized at the outer part of the walls. The amount of oxygen adsorbed on the carbon nanotubes is dependent on the introduction of nitrogen sites and exposed edge plane defects generated by plasma N treatment. Thermal gravimetric analysis measurements revealed a lower decomposition temperature for the plasma treated carbon nanotubes, which has its origin in the presence of a large fraction of active sites induced by rf glow discharge plasma treatment that accelerates the oxidation rate of N- doped carbon nanotubes. Our results indicate that rf low- pressure glowdischargemediated doping of nanotubes is a promising route to control the electronic structure of nanotubes and their reactivity due to introduction of active site defects.

Orientation of Thin Films Synthesized from Silicon Phthalocyanine Dichloride on a Highly Oriented Pyrolytic Graphite Investigated Using Near Edge X-ray Absorption Fine Structure

Molecular orientation of thin films of silicon phthalocyanine (SiPc) compounds on highly oriented pyrolytic graphite (HOPG) was investigated by near edge X-ray absorption fine structure (NEXAFS) and X-ray photoelectron spectroscopy (XPS). The films were prepared by a casting method using solution of SiPc dichloride. XPS results showed that the chlorine atoms in SiPc dichloride were substituted by oxygen atoms when the film was heated in the air. The orientation of the molecules with respect to the substrate plane was investigated by the polarization dependences of the Si K edge NEXAFS spectra. For the sample heated in the air, two clear peaks appeared in the NEXAFS spectra at around 1847.2 and 1852.4 eV, which were assigned to Si 1s→σSi–N* and Si 1s→σSi–O*, respectively. The intensities of the resonance peaks showed strong polarization dependence. A quantitative analysis of the polarization dependence revealed that the Si–N bond was lying down while the Si–O bond was out of the plane.

NEXAFS study and electrical properties of nitrogen-incorporated tetrahedral amorphous carbon films

Tetrahedral amorphous carbon nitride (ta-C:N) films obtained by filtered cathodic vacuum arc deposition have been investigated by Near edge X-ray absorption fine structure (NEXAFS) spectroscopy. The C K (carbon K) edge NEXAFS spectra clearly revealed the increase of C N bonds with the increase of nitrogen concentration. The C K edge NEXAFS analysis further showed that the content of the surface defect (mainly C–H bonds) decreased with the increase of nitrogen in the films. As the nitrogen concentration increased a pronounced change in the π* features were observed at the N K (nitrogen K) edge. The origin of the N K edge π* peaks are mainly due to the formation of C N bonds and nitrogen substitution in the graphite network. Polarization dependent NEXAFS measurements showed an angular distribution of CN bonds. The electrical resistivity of the ta-C:N films decreased at higher nitrogen concentration and this may be from the development of graphite-like structures in these films. The electrical measurements were confirmed further with electrochemical measurements.

NEXAFS measurements of the molecular ordering in the boundary layers of liquid crystalline free standing films

By near edge X-ray absorption fine-structure (NEXAFS) spectroscopy a finite molecular tilt angle in the surface layer of a free standing film in the liquid crystalline smectic A phase of C7 was directly detected. Analysis of the angular dependent intensities of the oxygen K edge NEXAFS spectra leads to an average tilt angle of the molecules in the surface layer of about 34°, which is characteristic for the bulk smectic C* phase of C7.