Peter Hammer

Electronic structure of hydrogenated carbon nitride films

Hydrogen-induced changes on the electronic and structural properties of amorphous carbon nitride (a-CNx:H) prepared by ion beam assisted deposition are investigated by x-ray photoelectron, ultraviolet photoelectron, infrared, and Raman spectroscopies. Two series of specimen are studied: films with a constant nitrogen content (C/N=26%) grown at 150 °C using different hydrogen partial pressures between 0% and 70% and samples deposited at different substrate temperatures (150–500 °C) with fixed H2 partial pressure of 60%. The pronounced changes of the N 1s and C 1s core level spectra on increasing hydrogen incorporation (up to 17 at. %) are interpreted as due to the formation of terminating NH and CH bonds accompanied by modifications of the local C–N bonding structure. Corresponding changes are observed in the He II valence band spectra showing a recession of the leading edge of more than 0.9 eV while the optical band gap widens from 0 to more than 1 eV. Consistent with these results, the information obtain...

SYNTHESIS OF CARBON NITRIDE FILMS AT LOW TEMPERATURES

Carbon nitride films (CNx) have been deposited by sputtering a graphite target with nitrogen ions. Films were grown both with and without the presence of an assisting focused N2 ion beam. The sputter beam voltage was varied between 150 and 1500 V and the applied assisting beam voltage from 80 to 500 V. The substrate was held at fixed temperatures between 80 and 673 K. The coatings were characterized with respect to their electrical, optical, and structural properties. The nitrogen content was measured by x-ray photoelectron spectroscopy (XPS) and a maximum nitrogen concentration of 44 at. % was obtained for a nonassisted sample deposited at 140 K. The chemical structure was investigated by XPS and Fourier transform infrared spectroscopy. Reduction of the substrate temperature in conjunction with low sputter beam voltages (<200 V) caused the optical band gap to increase up to 2.2 eV, the sheet conductivity to decrease to less than 10−9 (Ω cm)−1 and the density to be reduced to 1.6 g/cm3. The increasing tra...

Incorporation of nitrogen in carbon nanotubes

Abstract Nitrogen-doped carbon nanotubes were obtained by the arc-discharge technique in a helium–nitrogen atmosphere and using iron–nickel–cobalt catalysts, The samples were analyzed using spectroscopic techniques (Raman, EELS, X-ray photoelectron spectroscopy) and transmission electron microscopy (TEM). Pure helium atmosphere conditions led to bundles of single-wall nanotubes with diameters of ∼1.5 nm. The presence of nitrogen during tube formation produced irregular and thickly textured tubes. TEM micrographs showed that N suppresses the formation of bundles of single-wall nanotubes, giving rise to nested nanofibers. Quantum-chemical calculations were carried out to study the influence of substitutional N on the tube conformation. The calculations show that the combination of hexagons and pentagons at low N concentration produces kinks that account for the irregular shaped nanotubes.

Photocatalytic degradation of methylene blue by TiO2–Cu thin films: Theoretical and experimental study

In this work the effect of doping concentration and depth profile of Cu atoms on the photocatalytic and surface properties of TiO(2) films were studied. TiO(2) films of about 200 nm thickness were deposited on glass substrates on which a thin Cu layer (5 nm) was deposited. The films were annealed during 1s to 100°C and 400°C, followed by chemical etching of the Cu film. The grazing incidence X-ray fluorescence measurements showed a thermal induced migration of Cu atoms to depths between 7 and 31 nm. The X-ray photoelectron spectroscopy analysis detected the presence of TiO(2), Cu(2)O and Cu(0) phases and an increasing Cu content with the annealing temperature. The change of the surface properties was monitored by the increasing red-shift and absorption of the ultraviolet-visible spectra. Contact angle measurements revealed the formation of a highly hydrophilic surface for the film having a medium Cu concentration. For this sample photocatalytic assays, performed by methylene blue discoloration, show the highest activity. The proposed mechanism of the catalytic effect, taking place on Ti/Cu sites, is supported by results obtained by theoretical calculations.

Ion beam deposited carbon nitride films: characterization and identification of chemical sputtering

Abstract Carbon nitride films were synthesized by ion beam assisted sputtering. A graphite target was sputtered by argon or nitrogen ions and the growing film was simultaneously bombarded by a focused nitrogen ion beam of energies between 100–800 eV at 100 and 400 °C. It has been found that film growth occurs only if the ion-to-atom arrival ratio is smaller than a critical value of about 1.8 and it appears to be almost independent of the assisting beam energy. This effect, limiting the film growth, is a consequence of a chemical reaction between carbon and nitrogen forming volatile CN compounds. Experimental evidence was obtained by monitoring the gas evolved during the deposition process with a quadrupole gas analyzer. The maximum value of nitrogen content measured by Auger electron spectroscopy was about 35 at.%. All films were investigated by Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. The latter technique shows a preferential bonding of N to sp 3 -hybridized C. Hardness measurements with values up to 20 GPa were measured using a depth sensing nanoindenter.

Infrared analysis of deuterated carbon-nitrogen films obtained by dual-ion-beam-assisted-deposition

The isotopic effect on the infrared spectra is used to determine the existence of nitrogen–hydrogen bonds in amorphous carbon–nitrogen alloys (a-CNx) prepared by dual-ion-beam-assisted deposition. The deuteration experiments and the evolution of the infrared spectra upon atmospheric exposure show that hydroxyls are incorporated from atmospheric moisture.

Study of the Chemical Composition and Microstructure of Ion Beam‐deposited CNx Films Including an XPS C 1s Peak Simulation

The chemical composition and microstructure of dual ion beam-deposited CNx films with nitrogen contents in the range 20–33 at.% have been examined by Fourier transform infrared spectroscopy (FTIR) and x-ray photoelectron spectroscopy (XPS). The FTIR spectra together with other published data have been used to construct a model microstructure of the CNx films. The XPS N 1s peak is composed of two components corresponding to N–sp2 C and N–sp3 C bonds. Using the quantified N 1s data and making certain assumptions, the model microstructure has been used as the basis of a C 1s peak simulation in which nine C components have been combined. At an N content of 20 at.% the simulation agrees well with the experimentally recorded XPS C 1s peak. The FTIR spectra, XPS N 1s peak shape and C 1s peak simulations support a chemical structure of N substitution in an amorphous sp2/sp3 hybridized carbon structure. Above an N content of ∽20%, a new (nitrile-like) chemical form also emerges in the material, its abundance increasing with N content. It is proposed that this new structure occurs due to the presence, in the surface region of the condensing film, of CN dimers, which are formed when a C atom is surrounded by many N atoms. This molecule becomes trapped in the material and bonds to atoms at neighbouring sites.

Titanium boron nitride coatings of very high hardness

Abstract Ti-B-N coatings of variable composition have been sputter deposited from a heterogeneously composed Ti-BN target, consisting of a boron nitride base plate on which small Ti platelets were regularly arranged. By varying the number of platelets the concentration ratio c Ti / c B of the target and therefore also of the coating could easily be varied. Inhomogeneities in the chemical composition were in the per cent range. Very hard coatings of up to 55 GPa were obtained by sputtering with a substrate bias of - 150V at a substrate temperature of 400°C. The hardness maximum was found at a chemical composition where both the TiB 2 and the TiN phases coexist in equal concentrations. All coatings show very broad diffraction peaks indicating average grain sizes in the nanometre range where the Hall-Petch relation is no more valid. With coatings composed of very small grains an inverse Hall-Petch effect was observed. The tribological performance of the Ti-B-N coatings is promising; however, in general it is worse than that observed with TiN coatings. The main reason seems to be the relatively low cohesive strength of the material.

Hard graphitic-like amorphous carbon films with high stress and local microscopic density

In this work, we report unusual properties of amorphous carbon films prepared by ion beam-assisted deposition using different noble gases (neon, argon, and krypton). Independent of the noble gas ions used, the intrinsic compressive stress and plasmon energy increase sharply with the assisting ion beam energies up to 100 eV. Above this energy, the material properties depend on the mass of the ion. The highest values of stress (∼12 GPa) and plasmon energy associated with the C 1s core electron (29.5 eV) are of the same order of magnitude as those reported for highly tetrahedral amorphous carbon films. Structural results, however, indicate that the material is composed of a hard, highly stressed, and locally dense graphite-like network, i.e., a predominantly sp2-bonded material. It is suggested that the ion bombardment compacts the film structure by reducing the interplanar cluster distances, generating high compressive stress and high local density. The differences in the properties of the films introduced ...

Electrical conductivity of amorphous hydrogenated carbon

Abstract Amorphous hydrogenated carbon films (a-C: H) were deposited in a capacitively coupled r.f. discharge reactor. The precursor gas and the amount of doping and inert gas added during deposition were changed in a systematic manner. The d.c. conductivity was measured in the temperature range 80 to 500 K. The conductivity behaviour of the investigated samples, as well as to our knowledge that of all samples investigated by other authors, can be described in a uniform way by a characteristic temperature Θ (≊ 1500 K). At this temperature the extrapolated conductivity of all samples should be the same. This behaviour will be discussed in the context of different conductivity models as, for example the variable-range hopping, transport in band-tail states and hopping between graphite-like clusters, a statistical shift of the Fermi level, and the multiphonon tunnelling model. It is shown that only the latter model can, under certain conditions, satisfactorily explain the common behaviour of the samples inve...