Hydrogen adsorption on single-walled carbon nanotubes studied by core-level photoelectron spectroscopy and Raman spectroscopy

We have investigated the adsorption of atomic hydrogen on vertically aligned carbon nanotube (CNT) films using in situ synchrotron-radiation-based core-level (CL) photoelectron spectroscopy and Raman spectroscopy. From C 1s CL spectra, we identified a CL peak component due to C-H bonds of carbon atoms in single-walled carbon nanotubes (SWCNTs). We also found the suppression of π-plasmon excitation, indicating that the hydrogen adsorption deforms the bonding structure. Raman spectra of the SWCNT film indicated that the radial-breathing-mode intensities of SWCNTs decreased due to the adsorption-induced bonding-structure deformation. Moreover, the decrease for small-diameter SWCNTs was more severe than that for large-diameter SWCNTs. Our results strongly suggest that the hydrogen adsorption, which induces the structure deformation from sp² to sp³-like bonding, depends on the diameter of SWCNTs.     

Electrical and spectroscopic investigations on the reduction mechanism of graphene oxide

Ultra-large graphene oxide sheets of monolayer thickness were obtained by removing ultrasonication during the conventional oxidation process of graphite. In situ electrical conductivity measurement during the reduction by hydrazine monohydrate vapor and thermal annealing revealed the existence of an onset temperature at which electrical conduction started to occur. Corresponding X-ray photoelectron spectra indicated that an approximately 65% restoration of the sp² network led to the occurrence of electrical conduction. Reduction by ring-opening of epoxide group by hydrazine treatment seemed to restore the sp² network, while thermal annealing left a considerable number of defects in the graphene sheet through the formation of volatile gases.     

Dose and wavelength dependent study of graphene oxide photoreduction with VUV Synchrotron radiation

Photoreduction has so far received less attention than other approaches used to reduce graphene oxide (GO) like thermal and chemical reduction, although its potential is huge. The mechanism of deoxygenation is still questioned. In photolithographic applications, this means that one cannot predict whether the minimum feature size is close to the diffraction limit or larger. In this work we have studied GO photoreduction with vacuum ultraviolet Synchrotron radiation. GO has been exposed to extreme-UV and soft-X-ray radiation and the ratio between C–O and sp² C–C bonds has been measured by photoemission spectroscopy as a function of dose and number of photons. The deoxygenation rate has been demonstrated to be proportional to the imaginary part f₂ of the oxygen scattering factor. Experiments at the wavelengths where the f₂ of oxygen is close to its maximum (namely close to 2 nm and between 20 nm and 60 nm) have demonstrated that GO can be rapidly reduced (even few minutes exposure time) with doses well below those needed for photothermal heating (the corresponding temperature rise is ∼1 μK) and without damaging the basal plane (the in-plane sp² C–C bonds are not broken). Efficient nanometer scale GO photopatterning is therefore demonstrated to be possible.     

Surface amorphisation of TiZrN coatings by laser carburisation

ABSTRACT

The enhancement of wear resistance by laser carburisation on TiZrN coatings was investigated in terms of bond state. Graphite paste was used to cover the TiZrN coatings and then pulsed laser ablation was used to solidify the carbon. Tribometer test showed that the friction coefficient was reduced from 0.64 to 0.17 after laser carburisation. The doped carbon was analysed as a mixture with sp²C and sp³C bonds using an X-ray photoelectron spectroscopy depth profile. The sp²/sp³ ratio was 2.29 at the surface and increased up to 2.91 in the depth direction. To verify the effect of the hybrid bonds on the atomic order, annealing was carried out to the carburised specimens. As the ratio increased to 3.28, graphitisation resulting in transition from sp³ to sp² was confirmed, and variation in the lattice structure was demonstrated by a reduction in the lattice constant (4.45?Å to 4.40?Å) using Rietveld refinement.     

Gas sensing mechanism of carbon nanotubes: From single tubes to high-density networks

Gas sensing in carbon nanotubes is still poorly understood. Possible mechanisms are charge transfer between adsorbed gas molecules and the nanotubes or gas-induced changes at the interface between the nanotubes and their metal contacts. For carbon nanotube networks, it is also important to understand how defects and junctions formed by crossing nanotubes affect adsorption. Previous work demonstrates that for devices made with a single carbon nanotube, the response was mainly due to modifications at the nanotube/metal contact interfaces rather than molecular adsorption on the nanotube. Here it is shown that in networks of carbon nanotubes the gas sensitivity is due to both the nanotube/metal contacts and the carbon nanotube network. The network effects are dominated by gas-induced changes in nanotube/nanotube junctions, rather than gas adsorption on regions of the nanotubes away from the junctions.     

Spectroscopic studies of nanocrystalline diamond materials

The structural and electronic properties of nanocrystalline diamond films synthesized by a modified hot-filament chemical vapour deposition process were investigated by both bulk- and surface-sensitive techniques. Diffraction and microscopy data show the films to consist of diamond grains with an average crystallite size of about 10–15 nm and a root-mean-square roughness of similar size. Carbon core-level excitations in transmission electron energy-loss spectroscopy reveal an sp² content below 5%. The low energy loss spectra are quite similar to that of diamond crystal. The high sp³ content in the films was also confirmed by C 1s photoelectron plasmon energy loss features in X-ray photoemission experiments and by X-ray excited Auger-electron spectroscopy. We find that the hydrogen covered diamond surface gets contaminated after storage for several months under ambient conditions. Heating up to 500°C in vacuo is required to desorb the adsorbate layer.     

XPS spectra of thin CNx films prepared by chemical vapor deposition

Carbon nitride (CN_x) thin films with an N/C ratio of 0.605:0.522 have been synthesized using different sources as a ksilol, CCl₄, N₂ and NH₃ by PECVD (plasma enhanced chemical vapor deposition) and hot filament-CVD reactors. X-ray photoelectron spectroscopy (XPS) analyses, which give C_1s peaks with a maximum at 285.7 eV and 287 eV, typical for C–N bonds and sp² hybridization and CN bonds and sp³ hybridization, respectively. The observed and N_1s peaks with a maximum at about 399 eV suggest the existence of different C–N bonds and polycrystallite structure in the amorphous carbide matrix. The concentration of the different CN bonds varies, depending on the deposition technique.     

Understanding the thermal evolution of defects in carbon-implanted ZnO single crystal

Defects and impurities play a major role in controlling the electrical and optical properties of semiconductor materials. Herein, hydrothermally grown ZnO single crystals have been implanted with carbon (C) dopants at room temperature and then annealed in argon atmosphere at various temperatures between 400 and 800 °C. The thermal evolution of C-related defects and their effects on the structural, optical and electrical properties of ZnO single crystals were systematically characterized and discussed. The results show ion implantation induces serious lattice disorder, and post-implantation annealing could promote the lattice renormalization, accompanied by an increase in crystal quality and average visible transmittance. Furthermore, it is found that the diffusion of octahedral carbon interstitial (C_i) along parallel to c-axis facilitates the growth of carbon sp² clusters due to its low migration barrier during annealing, which energetically contribute to the decrease of the resistivity. Meanwhile, abundant C_i will be able to enter into V_Zn to form C_Zn or combine with lattice O to form (C_O)_O donor defects upon annealing, dominating the increase of electron carrier concentration and enhancing the anomalous Raman mode at 510-525 cm⁻¹. These findings strengthen the fundamental understanding of the donor behavior of C impurities in ZnO.     

Dry-pressed anodized titania nanotube/CH3NH3PbI3 single crystal heterojunctions: The beneficial role of N doping

Highly ordered, anodically grown TiO₂ nanotubes on titanium supports were annealed in ammonia atmosphere in order to incorporate nitrogen doping (≤2?at.%) in the titanium oxide lattice. FESEM micrographs revealed nanotubes with an average outer diameter of 101.5?±?1.5?nm and an average wall thickness of about 13?nm. Anatase crystals were formed inside the tubes after annealing in ammonia atmosphere for 30?min. With further annealing, rutile peaks appeared due to the thermal oxidation of the foil and rise as the duration of heat treatment was increased. The concentration and chemical nature of nitrogen in the nanotube arrays can be correlated to the optical response of dry-pressed heterojunctions of doped TiO₂/CH₃NH₃PbI₃ single crystals. The N-TiO₂/perovskite heterojunction with the highest amount of interstitial nitrogen exhibited an improved photocurrent, indicating the importance of the semiconductor doping-based heterojunction optimization strategies to deliver competitive levels of halide perovskite-based optoelectronic devices to be envisioned for urban infrastructures.     

Room temperature dc electrical conductivity studies of electron-beam irradiated carbon nanotubes

The influence of electron-beam (E-beam) irradiation on the electrical (electronic) properties of single- (SW) and multi-walled (MW) carbon nanotube grown by microwave chemical vapor deposition is investigated. These films were subjected to a constant energy of 50 keV (50 A/cm²) from a scanning electron microscope gun for 2.5, 5.5, 8.0, and 15 h continuously — such conditions resemble increased temperature and/or pressure regime, enabling a degree of structural fluidity. To assess the structural modifications and electrical properties, the films were analyzed before and after irradiation. The experiments show that with increased exposure to ≥ 8–9 h, occasionally found individual bundles of single-wall nanotubes tend to collapse or pinch, graphitize/amorphize, and oxidize within the area of the electron-beam focus. Dramatic improvement in the I–V properties for single-walled (from semiconducting to quasi-metallic) and relatively small but systematic behavior for multi-walled with increasing exposure is discussed in terms of the critical role of controlled introduction of defects. The contact resistance decreases by orders of magnitude when exposed to electron beam and for all of the measurements the values ranged between 80 Ω and 10 kΩ at room temperature. These results also indicated that multi-walled nanotubes tend to reach a state of saturation degradation assessed by four-probe conductivity measurements. It is suggestive that there may be local gradual re-organization, i.e. sp^2 + δ, sp³ C ⇔ sp² C. More importantly, they provided a contrasting comparison between metallic/semiconducting (single/double-wall) and invariably metallic (multi-wall) carbon nanotubes.     

Thermal stability of microhardness and internal stress of hard a-C films with predominantly sp2 bonds

Annealing in vacuum at temperatures up to 820 °C was used to study the thermal stability of mechanical properties of magnetron-sputtered thick (approx. 1.5 μm) a-C films. A predominance of sp² bonds was characteristic for all these films. The microhardness, internal stress, electron diffraction, Raman and optical spectra of three sets of films with different initial microhardness (H≈50, 20 and 10 GPa, respectively) were compared. Annealing of the hardest film up to 500 °C led to an increase in microhardness accompanied by a decrease in internal stress. Internal stress did not relax completely for hard films, even after annealing up to 820 °C, and the microhardness remained rather high (∼40 GPa). Both the high internal stress and the specific film nanostructure are responsible for the high microhardness of these sp²-bonded films.     

XPS studies of amorphous SiCN thin films prepared by nitrogen ion-assisted pulsed-laser deposition of SiC target

Amorphous SiCN films were prepared on Si (100) substrates by nitrogen ion-assisted pulsed-laser ablation of an SiC target. The dependence of the formed chemical bonds in the films on nitrogen ion energy and the substrate temperature was investigated by an X-ray photoelectron spectroscopy (XPS). The fractions of sp² CC, sp³ CC, and sp² CN bonds decreased, and that of NSi bonds increased when the nitrogen ion energy was increased without heating during the film preparation. The fraction of sp³ CN bonds was not changed by the nitrogen ion irradiation below 200 eV. Si atoms displaced carbon atoms in the films and the sp³ bonding network was made between carbon and silicon through nitrogen. This tendency was remarkable in the films prepared under substrate heating, and the fraction of sp³ CN bonds also decreased when the nitrogen ion energy was increased. Under the impact of high-energy ions or substrate heating, the films consisted of sp² CC bonds and SiN bonds, and the formation of sp³ CN bonds was difficult.     

Low-temperature synthesis of multiwalled carbon nanotubes by graphite antenna CVD in a hydrogen-free atmosphere

Multiwalled carbon nanotubes (MWCNTs) were synthesized at 390 °C in a hydrogen-free atmosphere by graphite antenna chemical vapor deposition, which provides carbon radicals by the etching of the antenna itself in a He- or Ar-based noble gas plasma. An increase in the number of defects in the graphite layers of the CNTs was observed with increasing hydrogen partial pressure. The results of X-ray photoelectron spectroscopy analysis suggested that these defects were caused by the transformation from an sp² to an sp³ structure in the graphite layers of CNTs due to hydrogen radicals.     

Effect of Mn2+ doping on the lattice and the microwave dielectric properties of MgTa2O6 ceramics

A series of Mn²⁺-doped Mg_1-xMn_xTa₂O₆ (x = 0.02, 0.04, 0.06, 0.08, 0.10, 0.12) ceramics were synthesized by solid-state reaction method. The influence of introducing Mn–O bonds as a partial replacement for Mg–O bonds on the lattice and microwave dielectric properties was systematically investigated. XRD and Rietveld refinement confirm that Mn²⁺ occupies the 2a Wyckoff position and forms a pure trirutile phase. Moreover, based on the chemical bond theory, the dielectric constant is mainly affected by the ionicity of the Ta–O bond. The lattice and dielectric properties remain relatively stable with Mn²⁺ doping below 0.1, but excessive Mn²⁺ doping leads to pronounced distortion of the lattice, which is not beneficial for lattice stability and microwave dielectric properties. Introducing an appropriate amount of Mn–O bonds with high bond dissociation energy facilitates MgO6 octahedron stability, which improves the thermal stability of the lattice. Accordingly, the microwave dielectric properties for 0.06 Mn²⁺-doped MgTa₂O₆ ceramics were determined: ε_r = 28, Q × f = 105,000 GHz (at 7.5 GHz), τ_f = 19.5 ppm/^°C.     

Nitrogen implantation of suspended graphene flakes: Annealing effects and selectivity of sp2 nitrogen species

Nitrogen inclusion in both chemical vapour deposition and exfoliated few-layer graphene flakes was performed by nitrogen ion implantation in ultra-high vacuum. Inclusion of up to ∼20 at.% nitrogen can be reached through this clean technique with absence of oxygen species in the final product, while maintaining a largely sp²-carbon network. The nitrogen inclusion was observed by scanning X-ray photoelectron microscopy (SPEM) with energy resolution of 0.2 eV and spatial resolution of 10 nm. SPEM can be used to follow the evolution of nitrogen species: pyridinic, graphitic, and pyrrolic, at different doping stages and annealing temperatures. Variations in the ratio between sp² nitrogen species was observed for increasing treatment time; annealing results in quenching of the sp³ component, suggesting the graphitic nitrogen as the most thermal stable species. The occurrence of graphitic species together with the absence of pyrrolic is indicative of N-incorporation into a hexagonal graphene-based lattice. Ion irradiation followed by annealing performed in a controlled way is a promising strategy to fine control the nature of the nitrogen species grafted to the graphene while focusing on selected applications.     

Thermal performance of vertically-aligned multi-walled carbon nanotube array grown on platinum film

Vertically-aligned carbon nanotube array is expected to inherit high thermal conductivity and mechanical compliance of individual carbon nanotube and serve as thermal interface material. In this paper, vertically-aligned multi-walled carbon nanotube arrays have been directly grown on Pt film and the thermal performance has been studied by using laser flash technique. The determined thermal diffusivity decreases from 0.187 to 0.135 cm² s⁻¹ and the thermal conductivity increases from 1.8 to 3.1 W m⁻¹ K⁻¹ as temperature increases from 243.2 to 453.2 K. The fracture surface of the array peeled off the Pt film was characterized by scanning electron microscopy. It has been illustrated that the tearing surface is not smooth but fluffy with torn carbon nanotubes, indicating strong interfacial bonding and consequent small interface resistance between carbon nanotube array and Pt film. According to Raman spectra and transmission electron microscopy image, the possible mechanisms responsible for the thermal transport degradation are low packing density, twist, and the presence of impurities, amorphous carbon, defects and flaws. The influence of intertube van der Waals interactions has been studied by comparing the phonon dispersion relations and is expected to be not significant.     

Tuning electronic properties of carbon nanotubes by nitrogen grafting: Chemistry and chemical stability

Plasma-based methods were used to graft nitrogen atoms to the hexagonal lattice of vertically aligned carbon nanotubes (v-CNTs). The nitrogen grafting (as pyridinic, pyrrolic and graphitic) was mediated by the creation of defects induced by energetic species present in the nitrogen plasma. We investigated the effect of adding nitrogen atoms via plasma treatment on the electronic properties of both v-CNT tips and sidewalls using ultraviolet and X-ray photoemission spectroscopy and spectromicroscopy. Site selective nitrogen grafting near the tips, up to a depth of 4 μm, was evaluated, beyond which the properties of the v-CNTs remain unperturbed. The N 1s XPS spectra recorded on the v-CNT tips showed three components related to nitrogen grafted as pyridinic, pyrrolic or graphitic. During thermal heating, we observed variations in the intensity ratio of these components due to the different thermal stability of the nitrogen grafting configurations; the most stable were the sp² pyridinic and graphitic nitrogen. The area ratio variation of these components was accompanied by a change in the density of states at the Fermi energy level, thus suggesting that the nitrogen functionalization strategy employed can be used to activate the v-CNT tips allowing the tuning of electronic properties by controlling the grafting of different nitrogen species.     

Hydrogen incorporation,bonding and stability in nanocrystalline diamond films

The hydrogen concentration in hot filament and microwave plasma CVD nanocrystalline diamond films is analysed by secondary ion mass spectrometry and compared to the film grain size. The surface and bulk film carbon bonds are analysed respectively by X-ray photoelectron spectroscopy (XPS) and ultra-violet Raman spectroscopy. XPS results show the presence of the hydrogenated p-type surface conductive layer. The respective intensities of the 1332 cm^− 1 diamond peak, of the G and D bands related to sp² phases, and of the 3000 cm^− 1 CH_x stretching mode band, are compared on Raman spectra. The samples are submitted to thermal annealing under ultra-high vacuum in order to get hydrogen out-diffusion. XPS analysis shows the surface desorption of hydrogen. Thermal annealing modifies the sp² phase structure as hydrogen out diffuses.     

Diamond-like carbon (DLC) films as electrochemical electrodes

Amorphous carbon can be any mixture of carbon bonds of sp³, sp², and even sp¹, with the possible presence of hydrogen. The group of mixture, of which there is a high fraction of diamond-like (sp³) bonds, is named diamond-like carbon (DLC). Unlike the crystalline carbon materials: diamond, graphite, carbon nanotube, fullerene and graphene, DLC can be deposited at room temperature without catalyst or surface pretreatment. Furthermore, its properties can be tuned by changing the sp³ content, the organization of sp² sites and hydrogen content, and also by doping. This paper firstly reviewed the electrochemical properties of DLC films and their applications.