Electronic and bonding properties of Fe- and Ni based hydrogenated amorphous carbon thin films by X-ray absorption, valence-band photoemission and Raman spectroscopy

Electronic and bonding properties of Me-based hydrogenated amorphous carbon (a-CH:Me, Me = Fe, Ni) thin films have been studied by X-ray absorption near-edge structure (XANES), valence-band photoemission (VB-PES) and Raman spectroscopy. Raman and XANES results show enhancement of the content of sp³-rich diamond-like carbon (DLC) by doping with Fe and Ni. The VB-PES spectrum of a-CH:Fe shows emergence of a prominent feature due to states of sp³-bonded clusters, indicating that a-CH:Fe induced enhancement of DLC structure. The nano-indentation measurement reveals that a-CH:Fe has a greatly enhanced hardness, while electrical resistance measurement shows that a-CH:Me reduces resistivity.     

Influence on the sp3/sp2 character of the carbon on the insertion of nitrogen in RFMS carbon nitride films

RFMS carbon nitride films have been elaborated at several substrate temperatures between 150 °C and 450 °C, where they evolve from a highly resistive to highly conductive comportment. Their local structure has been determined from X-ray photoemission, Raman and infrared spectroscopic results. The films composition has been measured by nuclear reaction analysis and elastic recoil detection.We will correlate the strong modifications of the electronic properties of the films to their well characterized structural changes. We will show how the substrate temperature acts on the incorporation of nitrogen in carboneous RFMS films and which is the resulting consequence on the sp³/sp² character of the carbon network.     

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.     

Removal of sp2-boron nitride transition layer in the growth of cubic boron nitride films

Practical application of cubic boron nitride (cBN) films is known to be hindered by its bad adhesion to substrates. One reason is that an sp²-bonded boron nitride (sp²-BN) transition layer is formed on the substrate surface at the early stage of deposition prior to the nucleation of cBN, which weakens the link between the cBN-rich layer and the substrate. In this study, we demonstrated the feasibility of removing this sp²-BN layer, by replacing it with a zirconium- (Zr-) rich composite layer. The method is to deposit a multilayer of Zr layer/sp²-BN layer/cBN-rich layer at 680 °C on a tungsten carbide substrate, followed by annealing the structure at 850 °C for 1 h. X-ray photoelectron spectroscopy, transmission electron microscopy and electron energy loss spectroscopy analyses showed that the Zr metal layer and sp²-BN layer reacted completely, to produce a composite interfacial layer consisting of metal Zr, Zr nitride, boride and oxide. The depth profiles of the elemental distributions and chemical states were investigated and discussed to reveal the diffusion of the elements associated with this deposition process.     

Hydrogen induced redox mechanism in amorphous carbon resistive random access memory

We investigated the bipolar resistive switching characteristics of the resistive random access memory (RRAM) device with amorphous carbon layer. Applying a forming voltage, the amorphous carbon layer was carbonized to form a conjugation double bond conductive filament. We proposed a hydrogen redox model to clarify the resistive switch mechanism of high/low resistance states (HRS/LRS) in carbon RRAM. The electrical conduction mechanism of LRS is attributed to conductive sp² carbon filament with conjugation double bonds by dehydrogenation, while the electrical conduction of HRS resulted from the formation of insulating sp³-type carbon filament through hydrogenation process.     

Analysis of the strong propensity for the delocalized diamagnetic π electronic structure of hydrogenated graphenes

The conformation and electronic structure of hydrogen-treated graphenes are investigated using the density-functional theory (DFT) method. We show that the overall energetics of the hydrogen chemisorption configuration can be analyzed with two energy components: the electronic pairing effect in the hyper-conjugated π electron network and the strain effect in the C–C bond at the boundary between sp³- and sp²-bonded regions. Some unpaired hydrogenation configurations can show magnetic ground states, but these were found to be unstable. The least strained paired configurations strongly favored the delocalized π electronic states. This suggests that appropriate annealing following a hydrogen plasma treatment of graphene can lead to a semiconducting state with a stable finite bandgap.     

Microstructure effect on field emission from tetrahedral amorphous carbon films annealed in nitrogen and acetylene ambient

Tetrahedral amorphous carbon (ta-C) films have been deposited by filtered cathodic vacuum arc technique. The samples were then annealed at various temperatures in nitrogen and acetylene ambient. The surface morphologies and microstructure of the films were characterized using atomic force microscopy, scanning electron microscopy, visible and ultraviolet Raman spectroscopy. A thin layer of amorphous carbon was deposited on the surface of the ta-C films after annealed at 700 and 800 °C while submicro crystalline pyrolytic graphite was formed on the surface of the ta-C film annealed at 900 °C. The surface layer was found to enhance the sp² clustering of the underlying ta-C layer. Field emission results reveal that the sp² cluster size plays an important role in electron field emission properties. The threshold field decreases as the sp² cluster size increases. For the film annealed at 800 °C, the lowest threshold field and the largest cluster size concurred.     

Characteristics and surface energy of silicon-doped diamond-like carbon films fabricated by plasma immersion ion implantation and deposition

Diamond-like carbon (DLC) films doped with different silicon contents up to 11.48 at.% were fabricated by plasma immersion ion implantation and deposition (PIII-D) using a silicon cathodic arc plasma source. The surface chemical compositions and bonding configurations were determined by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The results reveal that the sp³ configuration including Si–C bonds increases with higher silicon content, and oxygen incorporates more readily into the silicon and carbon interlinks on the surface of the more heavily silicon-doped DLC films. Contact angle measurements and calculations show that the Si-DLC films with higher silicon contents tend to be more hydrophilic and possess higher surface energy. The surface states obtained by silicon alloying and oxygen incorporation indicate increased silicon oxycarbide bonding states and sp³ bonding states on the surface, and it can be accounted for by the increased surface energy particularly the polar contribution.     

Effects of short-range order on the electronic structure and optical properties of amorphous carbon

We have calculated the joint density of states and absorption coefficient of amorphous carbon, a-C:H, treating it as a ternary alloy of sp² carbon, sp³ carbon and hydrogen. These optical properties have been calculated in terms of the above compositions and a short-range order parameter determining the degree of clustering of the sp² (or sp³) carbon atoms. Our calculations show that this parameter has a significant effect on the Tauc gap, which seems to be considerably smaller than the mobility gap. By comparing, however, our theoretical values of the Tauc gap with experiments we deduce that the degree of clustering found in most real samples is very small.     

Bias enhanced diamond nucleation onto 3C–SiC(100) surfaces studied by high resolution X-ray photoelectron and high resolution electron energy loss spectroscopies

The nature of hydrogen and carbon bonding configuration formed onto 3C–SiC(100) surfaces by the diamond bias enhanced nucleation process consisting of stabilization and biasing stages were investigated by high resolution electron energy loss spectroscopy and high resolution X-ray photoelectron spectroscopy. During the stabilization stage a sp³-CH_x bonded carbonaceous mono-layer is formed onto a hydrogenated 3C–SiC(100)–C–H terminated surfaces. After the biasing stage a hydrogenated nano-diamond film is formed. It was determined that hydrogen is strongly bonded to these nano-diamond surfaces and boundaries in sp³-C–H and sp²-C–H mono-hydride configuration. In addition, CH_x (x > 1) weakly bonded surface or sub-surface species were detected. Regions which are not fully covered by the nano-diamond film expose the SiC surface covered with a very thin carbonaceous layer.     

Beryllium doping graphene,graphene-nanoribbons,C60-fullerene,and carbon nanotubes

Beryllium substitutional doping within graphene, graphene nanoribbons, and carbon nanotubes are investigated using first-principles density functional theory calculations. Nanoribbons with armchair and zigzag edges, semiconducting (10,0) and metallic (6,6) carbon nanotubes, and C₆₀ fullerene structures are analyzed. Binding energy, doping energy, band structure, electronic density of states (DOS), and magnetic ordering are calculated. Our results demonstrate that conversely to perfect graphene, Be-doped graphene reveals a semiconducting behavior with an indirect band gap of 0.298 eV. Formation energy analysis reveals that Be into graphene and ribbons is more energetically favorable, but the energies involved are larger than those obtained for B- and N-doped nanocarbons. For nanoribbons, two different ways of incorporating the Be atom are explored (dopant placed in the center or edge), demonstrating that armchair nanoribbons preserve the semiconducting behavior with a reduced band-gap whereas that zigzag nanoribbons exhibit a half-metallic behavior with magnetic order along the edges. Results on Be-doping zigzag (10,0) semiconducting and armchair (6,6) metallic nanotubes and C₆₀ fullerene reveal the appearance of additional electronic states around the Fermi level. We envisage that the present investigation could motivate the realization of future experiments to introduce Be into sp² graphite-like lattice using high temperature chemical vapor deposition method.     

Studies of interfacial layers between 4H-SiC (0 0 0 1) and graphene

The region between epitaxial graphene and the SiC substrate has been investigated. 4H-SiC (0 0 0 1) samples were annealed in a high temperature molecular beam epitaxy system at temperatures between 1100 and 1700 °C. The interfacial layers between the pristine SiC and the graphene layers were studied by X-ray photoelectron spectroscopy. Graphene was found to grow on the SiC surface at temperatures above 1200 °C. Below this temperature, however, sp³ bonded carbon layers were formed with a constant atomic Si concentration. C1s and Si2p core level spectra of the graphene samples suggest that the interface layer we observe has a high carbon concentration and its thickness increases during the graphitization process. A significant concentration of Si atoms is trapped in the interface layer and their concentration also increases during graphitization.     

Vibrational studies of microcrystalline diamond and diamond-like carbon by high resolution electron energy loss spectroscopy

High resolution electron energy loss spectroscopy (HREELS) has been applied to investigate the vibrational states of microcrystalline diamond and diamond-like carbon (DLC) films prepared in a low pressure inductively coupled plasma. The CO additive to a CH₄/H₂ plasma exhibits different phonon density of states. Without CO additive, the HREELS spectrum shows a faint peak at ∼1500 cm⁻¹ due to CC stretching vibration of sp² bonds, indicating that the sample is mainly composed of DLC. On the other hand, the HREELS profiles show a peak at ∼1100 cm⁻¹ assigned to CC stretching vibration of sp³ sites with CO additive. The intensity of the peak becomes strong and a shoulder centered at ∼700 cm⁻¹ corresponding to the bending vibration of sp³ bonded carbons appears with increasing CO additive. It consequently implies that the CO additive brings about the decrease of the fraction of sp² bonded carbons in the resultant films, and it is qualitatively in agreement with the previous characterizations by Raman spectroscopy, transmission electron microscopy, and reflection high energy electron diffraction.     

Band engineering magneto-resistance effect in Co:a-C films

Co-doped amorphous carbon (Co:a-C) films, which comprise a homogeneously mixed carbon matrix phase with approximately 5 at.% cobalt, were deposited on quartz glass by radio frequency magnetron sputtering. A bias-dependent positive magnetoresistance (PMR) with a peak at a particular voltage was observed. The electronic structures were examined by ultraviolet photoelectron spectroscopy and X-ray absorption near edge spectroscopy. The spectroscopic results reveal that Co doping promotes the graphitization in the a-C matrix and the 3d orbital of Co is hybridized with sp² states (DOSs) in the Co:a-C. The PMR effect is related to the modulation of the DOS of the Co:a-C films at the Fermi level by Zeeman splitting.     

AES and core level photoemission in the study of a-C and a-C:H

The state of the art in the use of two probes of the occupied electron states, namely C KVV Auger emission and C 1s photoemission, in the study of a-C and a-C:H is reviewed, with particular attention to the issue of deriving the sp² fraction. The local character of the two probes justifies decomposition of the relative spectra into an sp² and an sp³ component. While however decomposition of the C 1s spectrum relies upon a theoretical basis and allows accounting for disorder effects in the amorphous state, no theory is available to support C KVV spectrum decomposition which has therefore to rely upon a purely empirical basis. In addition, the introduction of disorder related effects is not straightforward for this spectrum. A real validation of the sp² fraction measurement is lacking for both techniques, though there are indications that both allow qualitative or even semi-quantitative (C 1s spectrum) understanding of the electronic structure of amorphous carbon systems. Beside the sp² fraction evaluation, other pieces of information, concerning the spatial organization of the sp² sites, are possibly extracted from these spectra.     

Graphitic carbon growth on crystalline and amorphous oxide substrates using molecular beam epitaxy

We report graphitic carbon growth on crystalline and amorphous oxide substrates by using carbon molecular beam epitaxy. The films are characterized by Raman spectroscopy and X-ray photoelectron spectroscopy. The formations of nanocrystalline graphite are observed on silicon dioxide and glass, while mainly sp² amorphous carbons are formed on strontium titanate and yttria-stabilized zirconia. Interestingly, flat carbon layers with high degree of graphitization are formed even on amorphous oxides. Our results provide a progress toward direct graphene growth on oxide materials.PACS: 81.05.uf; 81.15.Hi; 78.30.Ly.     

Structural and mechanical characterization of platelet graphite nanofibers

Platelet graphite nanofibers have been characterized by scanning electron microscopy, transmission electron microscopy, electron diffraction, X-ray photoemission spectroscopy, and atomic force microscopy. The results show that the graphene sheets are stacked parallel to each other and are perpendicular to the fiber axis; the interlayer spacing is 0.34 nm. A small fraction of carbon atoms are bonded to oxygen. Solid-state nuclear magnetic resonance shows that hydrogenated carbons are under the detection limit (<5%) and that the nanofibers are dominated by sp²-bonded carbons. Mechanical measurements were made on individual nanofibers by nanoindentation.     

An analysis of microstructural variations in carbon black modified by oxidation or ultrasound

The microstructure and electronic structure of modified carbon black (CB) were investigated by Raman spectroscopy, transmission electron microscopy, electron energy loss spectroscopy and ultraviolet spectroscopy. The modified CB samples include oxidised CB and ultrasound-treated CB under different modification conditions. Typical parameters, such as graphene layer size, the ratio of sp²/sp³-hybridised carbon atoms, energy gap (E_g), and π–π^∗ band position, provide information on the microstructure and electronic structure, and these parameters also allow discrimination between different modified CB samples to achieve a desired structure. Oxidation conditions could be carefully chosen to prevent excessive corrosion and form an ordered structure. However, ultrasound has a reverse effect; the graphite layers of the CB samples were exfoliated, and a disordered microstructure was visible. The results indicate that increasing sp²-island size in CB samples decreases the optical gap and increases ultraviolet absorption.     

Atomic oxygen functionalization of double walled C nanotubes

The interaction of atomic oxygen with double walled C nanotubes at room temperature was studied by high resolution photoemission spectroscopy with synchrotron radiation. The nature of the chemical species formed on the nanotube sidewalls was followed from the initial adsorption up to advanced oxidation stages, whereas the thermal evolution of the O-related chemical species was monitored by fast photoemission. At the beginning of oxidation O atoms preferentially chemisorb forming C-O-C bonds, in ether and epoxy structures, which originate different components in the O1s spectra and exhibit different thermal stability. The onset of sp² lattice distortion is attested by the appearance of C-C bonds intermediate between sp² and sp³ configurations. The formation of double and triple C-O bonds is favored at later oxidation stages, and is accompanied by increasing lattice amorphization and decreasing emission in the Fermi level region. After annealing at 950 °C the O1s signal disappears and the presence of lattice defects emerges from the C1s line shape. This result, together with the chemical inertness of the deoxygenated nanotubes towards CO and O₂ adsorption suggests that the dangling bonds are promptly healed by thermal annealing and only stable topological defects are retained in the nanotube lattice.