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J.L. Andújar

Bio: J.L. Andújar is an academic researcher from University of Barcelona. The author has contributed to research in topics: Thin film & Plasma-enhanced chemical vapor deposition. The author has an hindex of 23, co-authored 86 publications receiving 1628 citations.


Papers
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TL;DR: In this paper, the influence of negative bias voltage applied to substrates and the nitrogen background pressure (up to 10−3 Torr) on film properties was studied by scanning electron microscopy (SEM) and electron energy loss spectroscopy (EELS).
Abstract: Tetrahedrally bonded amorphous carbon (ta-C) and nitrogen doped (ta-C:N) films were obtained at room temperature in a filtered cathodic vacuum arc (FCVA) system incorporating an off-plane double bend (S-bend) magnetic filter. The influence of the negative bias voltage applied to substrates (from −20 to −350 V) and the nitrogen background pressure (up to 10−3 Torr) on film properties was studied by scanning electron microscopy (SEM), electron energy loss spectroscopy (EELS), Raman spectroscopy, X-ray photoemission spectroscopy (XPS), secondary ion mass spectroscopy (SIMS) and X-ray reflectivity (XRR). The ta-C films showed sp3 fractions between 84% and 88%, and mass densities around 3.2 g/cm3 in the wide range of bias voltage studied. In contrast, the compressive stress showed a maximum value of 11 GPa for bias voltages around −90 V, whereas for lower and higher bias voltages the stress decreased to 6 GPa. As for the ta-C:N films grown at bias voltages below −200 V and with N contents up to 7%, it has been found that the N atoms were preferentially sp3 bonded to the carbon network with a reduction in stress below 8 GPa. Further increase in bias voltage or N content increased the sp2 fraction, leading to a reduction in film density to 2.7 g/cm3.

154 citations

Journal ArticleDOI
TL;DR: The relationship between metal-induced (W, Mo, Nb and Ti) structures and the surface properties of Me-DLC thin films is discussed in this article, which shows the possibilities of controlling the amorphous carbon films structure and surface properties by introducing metal in the DLC matrix.
Abstract: The relationship between metal-induced (W, Mo, Nb and Ti) structures and the surface properties of Me–DLC thin films is discussed. Nanocomposite films were deposited on c–Si wafers by pulsed-DC reactive magnetron sputtering controlling the gas ratio CH4/Ar. The sputtering process of metals such as Ti, Nb and Mo (unlike the tungsten) in the presence of methane shows a low reactivity at low methane concentration. The deposition rate and the spatial distribution of sputtered material depend of Z-ratio of each metal. The surface contamination of metal targets by carbon, owing to methane dilution, limits the incorporation of metals into DLC films according to an exponential decay. Results of electron probe microanalysis and X-ray photoelectron spectroscopy indicate a C rich Me/C composition ratio for low relative methane flows. According to the depth profile by secondary ion mass spectrometry, the films are systematically homogeneous in depth, whereas at high carbon contents they exhibit a metal-rich interfacial layer on the substrate. Moreover, high resolution transmission electron microscopy has evidenced important structural modifications with respect to DLC standard films, with marked differences for each Me/C combination, providing nanodendritic, nanocrystallized or multilayered structures. These particular nanostructures favour the stress decrease and induce significant changes in the tribological characteristics of the films. This study shows the possibilities of controlling the amorphous carbon films structure and surface properties by introducing metal in the DLC matrix. © 2007 Elsevier B.V. All rights reserved.

81 citations

Journal ArticleDOI
TL;DR: In this paper, the preparation of metal containing hydrogenated amorphous carbon (a-C:H) thin films by means of reactive magnetron sputtering with pulsed d.c.f. bias using different gas mixtures of methane and argon was discussed.
Abstract: We discuss the preparation of metal containing hydrogenated amorphous carbon (a-C:H) thin films by means of reactive magnetron sputtering with pulsed d.c. power of a metal target (W, Mo, Nb, Ti) and r.f. bias using different gas mixtures of methane and argon. The obtained samples comprised a thickness between 100 and 600 nm and a low internal stress when little quantities of metal were incorporated. The chemical composition was analysed by X-ray Photoelectron Spectroscopy (XPS). Depth profile composition was qualitatively displayed by secondary ion mass spectrometry (SIMS). The optical characterization of the films was carried out by transmittance measurements in the visible range and showed a relationship between the optical band gap and the composition. Structural information was obtained by X-ray diffraction (XRD), whose results showed a shift of the Bragg peaks from carbide crystallites as the metal amount increased. Peak width calculations situated particle sizes in the nanometric range. Surface topography from atomic force microscopy (AFM) measurements is also discussed. Surface energy was measured by the contact angle technique. The internal stress of the films was obtained by profilometry and a relationship with their structure was found. The results are compared with those corresponding to metal-free (pure) a-C:H films.

73 citations

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TL;DR: In this paper, the effects of the different metal induced structures on the electrical, optical and mechanical properties of diamond-like carbon (DLC) thin films were studied, where samples were prepared at room temperature by pulsed-DC reactive magnetron sputtering, as a suitable way to vary their composition.
Abstract: The addition of transition metal (Mo, Nb, Ti, W) atoms within the matrix of diamond-like carbon (DLC) films leads to the improving of their mechanical properties. In this work, we study the effects of the different metal induced structures on the electrical, optical and mechanical properties of DLC thin films. The samples were prepared at room temperature by pulsed-DC reactive magnetron sputtering, as a suitable way to vary their composition. X-ray photoelectron spectroscopy (XPS) evidenced a smooth variation of the composition with relative methane flow. Metal-DLC (Me-DLC) films are generally uniform in composition, as confirmed by secondary ion mass spectrometry (SIMS). Cross-section micrographs of the films, observed by transmission electron microscopy (TEM), showed relevant structural differences, which led to strong variations of physical properties. Among these variations, we can point to a better control of roughness, measured by atomic force microscopy (AFM), and stress. Indentation essays on W-DLC films have been performed in order to obtain the dependence of hardness with W concentration. Optical properties determined by UV–VIS spectroscopic ellipsometry evidenced the influence of methane dilution. The electrical measurements show a thermally activated conductivity, whose range of values and activation energy depend on the metal abundance. Me-DLC films suppose an advance in hard and wear-resistant coatings development.

68 citations

Journal ArticleDOI
TL;DR: In this paper, a detailed in situ spectroellipsometric analysis of the nucleation and growth of hydrogenated amorphous silicon (a:Si:H) is presented, where real time ellipsometric trajectories are recorded, using fixed preparation conditions, at various photon energies ranging from 2.2 to 3.6 eV.
Abstract: A detailed in situ spectroellipsometric analysis of the nucleation and growth of hydrogenated amorphous silicon (a:Si:H) is presented. Photoelectronic quality a‐Si:H films are deposited by plasma‐enhanced chemical vapor deposition on smooth metal (NiCr alloy) and crystalline silicon (c‐Si) substrates. The deposition of a‐Si:H is analyzed from the first monolayer up to a final thickness of 1.2 μm. In order to perform an improved analysis, real time ellipsometric trajectories are recorded, using fixed preparation conditions, at various photon energies ranging from 2.2 to 3.6 eV. The advantage of using such a spectroscopic experimental procedure is underlined. New insights into the nucleation and growth mechanisms of a‐Si:H are obtained. The nucleation mechanism on metal and c‐Si substrates is very accurately described assuming a columnar microstructural development during the early stage of the growth. Then, as a consequence of the incomplete coalescence of the initial nuclei, a surface roughness at the 10–...

58 citations


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TL;DR: In this paper, the structures of various types of amorphous carbon films and common characterization techniques are described, which can be classified as polymer-like, diamond-like or graphite-like based on the main binding framework.
Abstract: Amorphous and nanocrystalline carbon films possess special chemical and physical properties such as high chemical inertness, diamond-like properties, and favorable tribological proprieties. The materials usually consist of graphite and diamond microstructures and thus possess properties that lie between the two. Amorphous and nanocrystalline carbon films can exist in different kinds of matrices and are usually doped with a large amount of hydrogen. Thus, carbon films can be classified as polymer-like, diamond-like, or graphite-like based on the main binding framework. In order to characterize the structure, either direct bonding characterization methods or the indirect bonding characterization methods are employed. Examples of techniques utilized to identify the chemical bonds and microstructure of amorphous and nanocrystalline carbon films include optical characterization methods such as Raman spectroscopy, Ultra-violet (UV) Raman spectroscopy, and infrared spectroscopy, electron spectroscopic and microscopic methods such as scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy, transmission electron microscopy, and electron energy loss spectroscopy, surface morphology characterization techniques such as scanning probe microscopy (SPM) as well as other characterization methods such as X-ray reflectivity and nuclear magnetic resonance. In this review, the structures of various types of amorphous carbon films and common characterization techniques are described.

1,004 citations

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TL;DR: Experimental and theoretical results indicate that the bonded case cannot, while the separated one can, turn the inert CNTs into ORR electrocatalysts, demonstrating the crucial role of the doping microstructure on ORR performance.
Abstract: Two kinds of boron and nitrogen co-doped carbon nanotubes (CNTs) dominated by bonded or separated B and N are intentionally prepared, which present distinct oxygen reduction reaction (ORR) performances. The experimental and theoretical results indicate that the bonded case cannot, while the separated one can, turn the inert CNTs into ORR electrocatalysts. This progress demonstrates the crucial role of the doping microstructure on ORR performance, which is of significance in exploring the advanced C-based metal-free electrocatalysts.

816 citations

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TL;DR: The BOLS correlation mechanism has been initiated and intensively verified as discussed by the authors, which has enabled the tunability of a variety of properties of a nanosolid to be universally reconciled to the effect of bond order deficiency of atoms at sites surrounding defects or near the surface edges of the nano-material.
Abstract: This report deals with the mechanism behind the unusual behavior of nanostructures in mechanical strength, thermal stability, acoustics (lattice dynamics), photonics, electronics, magnetism, dielectrics, and chemical reactivity and its indication for designing and fabricating nanostructured materials with desired functions. A bond-order-length-strength (BOLS) correlation mechanism has been initiated and intensively verified, which has enabled the tunability of a variety of properties of a nanosolid to be universally reconciled to the effect of bond order deficiency of atoms at sites surrounding defects or near the surface edges of the nanosolid. The BOLS correlation indicates that atomic coordination imperfection causes the remaining bonds of the under-coordinated atom to contract spontaneously associated with bond strength gain and the intraatomic trapping potential well depression. Consequently, localized densification of charge, energy and mass occurs to the surface skin, which modify the atomic coherency (the product of bond number and the single bond energy), electroaffinity (separation between the vacuum level and the conduction band edge), work function, and the Hamiltonian of the nanosolid. Therefore, any detectable quantity can be functionalized depending on the atomic coherency, electroaffinity, work function, Hamiltonian or their combinations. For instances, the perturbed Hamiltonian determines the entire band structure such as the band-gap expansion, core-level shift, Stokes shift (electron-phonon interaction), and dielectric suppression (electron polarization); The modified atomic coherency dictates the thermodynamic process of the solid such as self-assembly growth, atomic vibration, phase transition, diffusitivity, sinterbility, chemical reactivity, and thermal stability. The joint effect of atomic coherency and energy density dictates the mechanical strength (surface stress, surface energy, Young's modulus), and compressibility (extensibility, or ductility) of a nanosolid. Most strikingly, a combination of the new freedom of size and the original BOLS correlation has allowed us to gain quantitative information about the single energy levels of an isolated atom and the vibration frequency of an isolated dimer, and the bonding identities in the metallic monatomic chains and in the carbon nanotubes. A survey and analysis of the theoretical and experimental observations available to date demonstrated that the under-coordinated atoms in the surface skin of 2-3 atomic layers dictate the performance of nanostructures yet atoms of the interior remain as they are in the bulk counterpart. Further extension of the BOLS correlation and the associated approaches to atomic defects, impurities, liquid surfaces, junction interfaces, and amorphous states and to the temperature domain would be more challenging, fascinating, and rewarding.

775 citations

Journal ArticleDOI
TL;DR: In this paper, a general framework for the interpretation of infrared and Raman spectra of amorphous carbon nitrides is presented, which can be used to explain the large dynamic charge of the more delocalized bonding which occurs in more than two bonded networks.
Abstract: A general framework for the interpretation of infrared and Raman spectra of amorphous carbon nitrides is presented. In the first part of this paper we examine the infrared spectra. The peaks around 1350 and 1550 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ found in the infrared spectrum of amorphous carbon nitride or hydrogenated and hydrogen-free amorphous carbon are shown to originate from the large dynamic charge of the more delocalized \ensuremath{\pi} bonding which occurs in more ${\mathrm{sp}}^{2}$ bonded networks. The IR absorption decreases strongly when the \ensuremath{\pi} bonding becomes localized, as in tetrahedral amorphous carbon. Isotopic substitution is used to assign the modes to $\mathrm{C}=\mathrm{C}$ skeleton modes, even those modes around 1600 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ which become strongly enhanced by the presence of hydrogen. The infrared spectrum of carbon nitride may resemble the Raman spectrum at some excitation energy, but the infrared activity does not primarily result from nitrogen breaking the symmetry. In the second part we examine the Raman spectra. A general model is presented for the interpretation of the Raman spectra of amorphous carbon nitrides measured at any excitation energy. The Raman spectra can be explained in terms of an amorphous carbon based model, without need of extra peaks due to CN, NN, or NH modes. We classify amorphous carbon nitride films in four classes, according to the corresponding N-free film: $a\ensuremath{-}\mathrm{C}:\mathrm{N},$ $a\ensuremath{-}\mathrm{C}:\mathrm{H}:\mathrm{N},$ $ta\ensuremath{-}\mathrm{C}:\mathrm{H}:\mathrm{N},$ and $ta\ensuremath{-}\mathrm{C}:\mathrm{N}.$ We analyze a wide variety of samples for the four classes and present the Raman spectra as a function of N content, ${\mathrm{sp}}^{3}$ content, and band gap. In all cases, a multiwavelength Raman study allows a direct correlation of the Raman parameters with the N content, which is not generally possible for single wavelength excitation. The G peak dispersion emerges as a most informative parameter for Raman analysis. UV Raman enhances the ${\mathrm{sp}}^{1}$ CN peak, which is usually too faint to be seen in visible excitation. As for N-free samples, UV Raman also enhances the C-C ${\mathrm{sp}}^{3}$ bonds vibrations, allowing the ${\mathrm{sp}}^{3}$ content to be quantified.

674 citations

Journal ArticleDOI
TL;DR: An overview of dynamic self-organization phenomena in complex ionized gas systems, associated physical phenomena, and industrial applications is presented in this paper, where the most recent experimental, theoretical, and modeling efforts to understand the growth mechanisms and dynamics of nano- and micron-sized particles, as well as the unique properties of the plasma-particle systems (colloidal, or complex plasmas) and the associated physical effects are reviewed and the major technological applications of micro- and nanoparticles are discussed.
Abstract: An overview of dynamic self-organization phenomena in complex ionized gas systems, associated physical phenomena, and industrial applications is presented. The most recent experimental, theoretical, and modeling efforts to understand the growth mechanisms and dynamics of nano- and micron-sized particles, as well as the unique properties of the plasma-particle systems (colloidal, or complex plasmas) and the associated physical phenomena are reviewed and the major technological applications of micro- and nanoparticles are discussed. Until recently, such particles were considered mostly as a potential hazard for the microelectronic manufacturing and significant efforts were applied to remove them from the processing volume or suppress the gas-phase coagulation. Nowadays, fine clusters and particulates find numerous challenging applications in fundamental science as well as in nanotechnology and other leading high-tech industries.

322 citations