M. Gabás

Preparation and characterization of transparent ZnO thin films obtained by spray pyrolysis

b ´´ Abstract Structural, optical, chemical and electrical properties of thin films of ZnO obtained by spray pyrolysis over Pt or silica substrates are determined at temperature ranges between 223 and 373 K. The thin films are pure ZnO with a preferred crystalline orientation of ( 002 ). Grain-boundary barriers are created by the band bending and the mobility of charge carriers is limited by the thermal field emission of electrons at the grain-boundary barriers. The density of ionized acceptor atoms and the width of the space charge, both obtained from the capacitance-voltage (C-V) method, are consistent with the theory of the depletion layer in the Schottky barrier device. The density of states of defect, obtained from admittance spectroscopy, only presents a maximum at approximately 0.44 eV, whose position in the band gap does not change under bias. 2002 Elsevier Science B.V. All rights reserved.

Unraveling the conduction mechanism of Al-doped ZnO films by valence band soft x-ray photoemission spectroscopy

We report on the correlation between the electrical behavior and valence band spectra of undoped and Al-doped ZnO films, obtained by using x-ray photoelectron spectroscopy. Although Al-doping can induce a conductivity increase of two orders of magnitude, we show that the gap persists and there is no semiconductor–metal transition upon doping. For the 3% Al-doped ZnO film, we measure a reduction in the band gap of ∼150meV with respect to the undoped and the 1% doped films. Our results suggest that the band conduction mechanism proposed for undoped ZnO at room temperature still dominates the conduction process in doped films.

Electrochemical Growth of Diverse Iron Oxide (Fe3O4, α-FeOOH , and γ-FeOOH) Thin Films by Electrodeposition Potential Tuning

Magnetite, goethite, and lepidocrocite thin films have been electrochemically grown on titanium substrates by the anodic oxidation of ferrous ions in a 0.01 M FeSO 4 (NH 4 ) 2 SO 4 ·6H 2 O + 0.04 M CH 3 COOK, pH 6.0, aqueous solution. It is demonstrated that the deposition potential can be used as a tool to tune the obtainment of the different pure phases of the iron oxide-oxyhydroxides thin films. Results of an exhaustive structural characterization, a morphological study, and X-ray photoelectron spectroscopy characterization are presented.

Differences in n-type doping efficiency between Al- and Ga-ZnO films

A careful and wide comparison between Al and Ga as substitutional dopants in the ZnO wurtzite structure is presented. Both cations behave as n-type dopants and their inclusion improves the optical and electrical properties of the ZnO matrix, making it more transparent in the visible range and rising up its electrical conductivity. However, the same dopant/Zn ratio leads to a very different doping efficiency when comparing Al and Ga, being the Ga cation a more effective dopant of the ZnO film. The measured differences between Al- and Ga-doped films are explained with the hypothesis that different quantities of these dopant cations are able to enter substitutionally in the ZnO matrix. Ga cations seem to behave as perfect substitutional dopants, while Al cation might occupy either substitutional or interstitial sites. Moreover, the subsequent charge balance after doping appear to be related with the formation of different intrinsic defects that depends on the dopant cation. The knowledge of the doped-ZnO films microstructure is a crucial step to optimize the deposition of transparent conducting electrodes for solar cells, displays, and other photoelectronic devices.

XAS study of the reversible reactivity mechanism of micro- and nanostructured electrodeposited Cu2O thin films towards lithium

Highly uniform electrodeposited Cu2O electrodes for Li-ion batteries, with variable particle size, morphology and thickness, have been studied in the charged and discharged steps as a function of the number of cycling processes, by means of ex situ X-ray absorption spectroscopy (XAS) at the Cu K edge. Both, fingerprint analysis of the X-ray Absorption Near Edge Structure spectra (XANES) and the radial atomic distribution obtained by the analysis of the Extended X-ray Absorption Fine Structure (EXAFS) spectra, have allowed us to quantify the bimodal distribution (Cu and Cu2O) of the electrodes. These results correlate qualitatively well with their different electrochemical response, establishing a deep insight into the reversible reactivity mechanism of these films towards lithium in Li batteries. Our results demonstrate that the reversibility of the reduction process upon the number of cycles depends on the thickness, as it was suggested in previous works but also on the morphology of the electrodes. Higher reversibility corresponds to smaller thickness electrodes. For similar thickness, the reversibility is higher for porous and nanometric particle electrodes than for compact and micrometric particle electrodes. The thinnest film exhibited an increase of capacity after several recycling steps, which agrees with the increase of the Cu2O content.

Electronic structure of Al- and Ga-doped ZnO films studied by hard X-ray photoelectron spectroscopy

Al- and Ga-doped sputtered ZnO films (AZO, GZO) are semiconducting and metallic, respectively, despite the same electronic valence structure of the dopants. Using hard X-ray photoelectron spectroscopy we observe that both dopants induce a band in the electronic structure near the Fermi level, accompanied by a narrowing of the Zn 3d/O 2p gap in the valence band and, in the case of GZO, a substantial shift in the Zn 3d. Ga occupies substitutional sites, whereas Al dopants are in both substitutional and interstitial sites. The latter could induce O and Zn defects, which act as acceptors explaining the semiconducting character of AZO and the lack of variation in the optical gap. By contrast, mainly substitutional doping is consistent with the metallic-like behavior of GZO.

Micro- and nanoparticle generation during nanosecond laser ablation: correlation between mass and optical emissions.

The particulate emission during nanosecond ablation of gold targets was investigated at various fluences (10-100 Jcm(-2)) and vacuum levels (0.05-750 Torr). Atomic emission spectra were acquired during the ablation process and post-mortem characterization of particle spatial distribution was performed using scanning electron microscopy. The discussion of the results in the context of existing theoretical models permitted the identification of four distinct mass removal mechanisms. While the presence, shape and intensity of atomic emission lines is a telltale of the nanoparticle formation process, the fluctuations of the emission signal over a number of laser shots was linked to the production of microscopic debris.

Modulation of the exfoliated graphene work function through cycloaddition of nitrile imines

After the feasibility of the 1,3-dipolar cycloaddition reaction between nitrile imines and exfoliated graphene by density functional theory calculations was proved, very few-layer graphene was effectively functionalized using this procedure. Hydrazones with different electronic properties were used as precursors for the 1,3-dipoles, and microwave irradiation as an energy source enabled the reaction to be performed in a few minutes. The anchoring of organic addends on the graphene surface was confirmed by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis. Ultraviolet photoelectron spectroscopy (UPS) was used to measure the work function and band gap of these new hybrids. Our results demonstrate that it is possible to modulate these important electronic valence band parameters by tailoring the electron richness of the organic addends and/or the degree of functionalization.

Particle formation and plasma radiative losses during laser ablation suitability of the Sedov-Taylor scaling.

Deviations of the Sedov-Taylor scaling at three different laser ablation regimes (500 mJ in a 0.8 mm spot, 50 mJ in a 0.8 mm spot and 500 mJ in a 2.5 mm spot) were investigated using Schlieren photography in combination with optical scattering and optical emission spectrometry, among others. For each case, the time evolution of the shock front was related to the formation, expansion and properties of the plasma. Both, the time scale of the different radiative processes and that observed for vapor condensation into nanoparticles and sub-micron particles are compatible with the divergences found between the model and experimental data.

Effects of Li+ inclusion on the magnetic properties of mixed Cr/Fe oxide pillared α-zirconium phosphate materials

Abstract The magnetic properties of mixed Cr/Fe 80:20 oxide pillared α-zirconium phosphate with different Li+ exchanged contents were studied by a.c. susceptibility measurements as a function of both frequency and temperature. Also studied are magnetisation measurements as a function of both temperature and applied field. We found relaxational behaviour, which can be ascribed to the presence of very fine aggregates of mixed Fe/Cr oxide. In our study, the effect that Li+ inclusion exerts in the material is clearly observed, probably modifying the particle size and morphology. Magnetic interactions play an important role in these compounds and their presence is deduced from experimental data.