Nanostructured Carbon Films Research Articles

Nanoscale and Mesoscale Properties of Nanostructured Carbon Films

A multi‐scale investigation of nanostructured carbon films has been performed by means of inelastic light scattering (Raman and Brillouin scattering). Carbon films with different nano‐ and mesostructure have been deposited from supersonic cluster beams in a low energy deposition regime by exploiting aerodynamic focusing effects. Acoustic phonon propagation in the porous amorphous structure, where disorder acts as a damping factor, is investigated by Brillouin scattering. Depending on the nano‐ and meso‐structure, acoustic phonons can either propagate along the medium, which acts as an elastic continuum at the meso‐scale (i.e., hundreds of nm), or turn to overdamped oscillations localized by the structural disorder. Nevertheless, we show that it is always possible to measure the elastic constants of thin and porous films, when other techniques (e.g., nano‐indentation) become critical. At the nano‐scale, Raman scattering measurements show the typical structure of an amorphous carbon, where the structural disorder is affected by the primeval cluster mass distribution. The synthesis of cluster‐assembled carbon films and the in situ Raman characterization in a UHV system allowed to observe the presence of a relevant fraction of sp 1‐hybridized carbon chains (also known as carbynoid structures) embedded in the sp 2 amorphous network.

Plasma-enhanced Deposition of Nano-Structured Carbon Films

By pre-treating substrate with different methods and patterning the catalyst, selective and patterned growth of diamond and graphitic nano-structured carbon films have been realized through DC Plasma-Enhanced Hot Filament Chemical Vapor Deposition (PE-HFCVD). Through two-step processing in an HFCVD reactor, novel nano-structured composite diamond films containing a nanocrystalline diamond layer on the top of a nanocone diamond layer have been synthesized. Well-aligned carbon nanotubes, diamond and graphitic carbon nanocones with controllable alignment orientations have been synthesized by using PE-HFCVD. The orientation of the nanostructures can be controlled by adjusting the working pressure. In a Microwave Plasma Enhanced Chemical Vapor Deposition (MW-PECVD) reactor, high-quality diamond films have been synthesized at low temperatures (310°C-550°C) without adding oxygen or halogen gas in a newly developed processing technique. In this process, carbon source originates from graphite etching, instead of hydrocarbon. The lowest growth temperature for the growth of nanocrystalline diamond films with a reasonable growth rate without addition of oxygen or halogen is 260°C.

Surface characterisation of nano-structured carbon films deposited by Nd:YAG pulsed laser deposition

Nano-structured carbon films have been proved to be outstanding materials for different kinds of applications, mainly electronic, owing to their high electron field emission properties, but also energetic (hydrogen storage, fuel cells), catalytic (very high metal catalyst dispersion properties) or biological (very good bio-sensor support). In this work we studied the influence of experimental conditions (vs. substrate temperature, working pressure and inert sustaining gases) on the structure evolution of carbon films, from an almost amorphous material to nano-clustered graphene particles. The carbon films were grown by Pulsed Laser Ablation (Nd:YAG, 2nd harmonic: λ=532 nm, hν=2.33 eV, τ=7 ns, ν=10 Hz, Φ≈7 J/cm 2), from a pyrolytic graphite target on Si <100> substrates. PLD depositions were performed at increasing temperature, ranging from room temperature to 900 °C, both in vacuum and in He atmosphere, at ∼20 Pa. Near-edge X-ray absorption fine structure (NEXAFS) and Raman spectroscopy evidenced the aromatic ring formation and the clustering condensation process with increasing surface temperature. The surface roughness and morphology of films grown at different conditions have been characterised by atomic force microscopy (AFM) and scanning electron microscopy (SEM).

Structure, stability, and stress properties of amorphous and nanostructured carbon films

Structural and mechanical properties of amorphous and nanocomposite carbon are investigated using tight-binding molecular dynamics (TBMD) and Monte Carlo (MC) simulations. In the case of amorphous carbon (a-C), we show that the variation of sp 3 fraction as a function of density is linear over the whole range of possible densities and that the bulk moduli follow closely the power–law variation suggested by Thorpe. We also review earlier work pertained to the intrinsic stress state of tetrahedral (ta-C) amorphous carbon. In the case of nanocomposites, we show that the diamond inclusions are stable only in dense amorphous tetrahedral matrices. Their hardness is considerably higher than that of pure amorphous carbon films. Fully relaxed diamond nanocomposites possess zero average intrinsic stress.

Spectral Dependence of the Optical Rectification Effect in Nanographite Films

The spectral dependence of the optical rectification effect in nanostructured carbon (nanographite) films obtained by plasmachemical deposition was studied in a wavelength range from 266 to 1900 nm. In this range, the amplitude of the electric signal observed when films were irradiated by nanosecond light pulses of constant power increases in inverse proportion to the laser wavelength. The experimental results confirm the assumptions made previously concerning the mechanism of the observed effect. It is suggested that nanographite films are promising materials for detectors of ultrashort laser pulses in the IR, visible, and UV spectral intervals and for generators of electromagnetic radiation operating in the terahertz frequency range.

Room temperature photoluminescence from nanostructured amorphous carbon

Visible room-temperature photoluminescence (PL) was observed from hydrogen-free nanostructured amorphous carbon films deposited by pulsed laser ablation in different background pressures of argon (PAr). By varying PAr from 5to340mTorr, the film morphology changed from smooth to rough and at the highest pressures, low-density filamentary growth was observed. Over the same pressure regime an increase in the ordering of sp2 bonded C content was observed using visible Raman spectroscopy. The origin of the PL is discussed in terms of improved carrier localization within an increased sp2 rich phase.

Elastic moduli of nanostructured carbon films

We have computed the elastic constants of nanostructured carbon films as obtained from classical molecular dynamics simulations of a cluster beam deposition process. The calculations show that the elastic constants of the deposited films are related to the average size of the clusters by a power law. This allows us to extrapolate the present theoretical data to the scale of the experimental results obtained by Brillouin scattering.

Optical rectification effect in nanostructured carbon films

Electrically conducting nanostructured carbon films obtained by chemical vapor deposition and composed of nanodimensional graphite crystals exhibit the effect of optical rectification on exposure to nanosecond pulsed laser radiation. Experiments show that the amplitude and polarity of the pulsed voltage strongly depend on the angle of incidence and polarization of the laser radiation and on the spatial orientation of a carbon film with electrodes relative to the laser beam. Under the optimum conditions corresponding to maximum amplitude of the response signal, the factor of conversion of the laser pulse power into electric voltage was about 500 and 650 mV/MW at a laser wavelength of 1064 and 532 nm, respectively.

Thermophoretic control of building units in the plasma-assisted deposition of nanostructured carbon films

The possibility of the thermophoretic control of the plasma-grown building units in the plasma-assisted deposition of various carbon-based nanostructures on Ni-based catalyzed Si substrates is reported. It is experimentally demonstrated that varying the near-substrate temperature gradient, one can selectively deposit or levitate the carbon-based nanoparticles grown in the low-temperature reactive plasmas of Ar+H2+CH4 gas mixtures. When the nanoparticles are levitated in the plasma presheath, the arrays of vertically aligned carbon nanotips are assembled, whereas the enhanced deposition of the building units from the gas phase favors the formation of polymorphous nanostructured carbon films. The experimental observations are supported by the one-dimensional model of the nanoparticle dynamics in the near-electrode area. It is shown that the thermophoretic force is indeed a crucial factor that controls the deposition of the plasma-grown fine particles. The experimental and computation results suggest that it is likely that the aligned carbon nanotip structures are predominantly grown by the molecular or radical units, whereas the plasma-grown nanoparticles are presumably the most important component of polymorphous carbon films.

Effects of high-temperature hydrogenation treatment on sliding friction and wear behavior of carbide-derived carbon films

In this study, we investigated the effects of a high-temperature hydrogenation treatment on the sliding friction and wear behavior of nanostructured carbide-derived carbon (CDC) films in dry nitrogen and humid air environments. These films are produced on the surfaces of silicon carbide substrates by reacting the carbide phase with chlorine or chlorine–hydrogen gas mixtures at 1000 to 1100 °C in a sealed tube furnace. The typical friction coefficients of CDC films in open air are in the range of 0.2 to 0.25, but in dry nitrogen, the friction coefficients are 0.15. In an effort to achieve lower friction on CDC films, we developed and used a special hydrogenation process that was proven to be very effective in lowering friction of CDC films produced on SiC substrates. Specifically, the films that were post-hydrogen-treated exhibited friction coefficients as low as 0.03 in dry nitrogen, while the friction coefficients in humid air were ~0.2. The wear of Si 3N 4 counterface balls was hard to measure after the tests, while shallow wear tracks had formed on CDC films on SiC disks. Detailed mechanical and structural characterizations of the CDC films and sliding contact surfaces were done using a series of analytical techniques and these findings were correlated with the friction and wear behaviors of as-produced and hydrogen-treated CDC films.

Electron field-emission mechanism in nanostructured carbon films: A quest

An open question to the community about the general consensus on the field-emission mechanism in carbon-based materials led to this study. By applying the Fowler–Nordheim (FN) model for carbon-based films, despite the fact that the microstructure and the resulting physical properties of the films can be tuned by scanning various process parameters, providing, in turn, from almost insulating (less defective) to semiconducting (highly defective) films and even a mixture of the two, the material can be categorized as electrically heterogeneous nanostructured carbon. The electrical heterogeneity arises from the different carbon hybridizations (sp2- versus sp3-bonded carbon). In an attempt to tackle these issues, we have performed a comprehensive analysis of I–V data obtained from filament-assisted chemical-vapor-deposition-grown sulfur-incorporated nanocomposite carbon thin films with different microstructures. Studies of the augmentation of the field-emission properties in this material indicated various roles of sulfur in modifying the film properties [Gupta et al., Appl. Phys. Lett. 80, 3446 (2002)]. The I–V data were fitted to various mathematical forms: I=AV2 exp(−B/V) [FN model], I=C exp(aV1/2/kT) [Schottky model], and I=Vn (n&amp;gt;1, for high fields) [space-charge-limited current (SCLC) model]. The goodness of fit along with the theoretical justification(s) on the electron field-emission results were taken into consideration to provide favorable indications for accepting or discarding any particular model. These findings suggest that there is an apparent crossover from SCLC to FN behavior as a function of film microstructure occurring due to the impurity incorporation as the microstructure transits smoothly from microcrystalline to nanocrystalline carbon. Other evidence in support of the aforementioned suggestion is based on the concept of percolation occurring in this nanocomposite carbon (a mix of conducting–insulating/semiconducting) material, whereby the electrons are allowed to tunnel from one conductive cluster to another separated by an insulating matrix, which is demonstrated through electrical conductivity measurements.

Combined atomistic and continuum methods to map electric properties of nanostructured carbon films

In this work we investigate the structure and the surface electric reaction field of nanostructured amorphous carbon films by means of a combined atomistic and continuum approach. The aim of the present investigation is to infer a connection between the film deposition protocol and the local electrostatic field arising when the sample is subjected to an external uniform electric field. The suggested combined methodology in principle permits to characterizing both film surface electrostatics and possible interaction between film surface and other molecular aggregates of interest.

Deposition of Nanostructured Diamond-like Carbon Films on Al Substrate by Facile Electrochemical Route

Abstract Diamond-like carbon (DLC) films were deposited on Al substrate by electrolysis of dimethylsulfoxide (DMSO) at relatively low voltage (150 V). The morphology of the sample was investigated by SEM, indicating that the film was composed of compact grains about 100 nm. The analysis of the films by XPS, FTIR, and Raman spectroscopy revealed that the deposits are typical hydrogenated DLC films.

Chemical and thermal stability of carbyne-like structures in cluster-assembled carbon films

Nanostructured carbon films consisting of sp chains (polyynes and polycumulenes) embedded in an sp 2 matrix are grown using supersonic carbon cluster beam deposition in ultrahigh vacuum at room temperature. All the specimens have been analyzed by in situ Raman spectroscopy. The use of different excitation wavelengths (532 and 632.8 nm) confirms the presence of distinct carbynoid species. Chemical stability of the sp species has been studied by exposing the as-deposited films to 500 mbar of H 2 , He, N 2 , and dry air. Gas exposure produces an exponential decay of the carbynoid fraction slightly affecting the sp 2 component. Helium, hydrogen, and nitrogen do not chemically interact with the sp chains whereas oxygen reacts with the carbynoids species causing their fast and almost complete destruction. The films have been also thermally annealed at 20°, 100°, 150°, and 200°C. The amount of carbynoid species is rapidly and strongly reduced at temperature larger than room temperature. The relevance for material science and interstellar chemistry of the production of a bulk form of carbon where sp and sp 2 hybridizations coexist is addressed.

First study of humidity sensors based on nanostructured carbon films produced by supersonic cluster beam deposition

We have investigated the feasibility of humidity sensors based on nanostructured carbon (ns-C) films produced by supersonic cluster beam deposition (SCBD). The ns-C films have been obtained using pure He as the buffer gas in the pulsed microplasma cluster source (PMCS). Films of different thickness (1–10 μm) were deposited on Au contacts: resistive- and capacitive-type devices were manufactured. Resistive sensor show a flat response for relative humidity (%RH) up to 60%. Preliminary measurements performed on capacitive-type devices at room temperature in the relative humidity range 10–70% showed a linear dependence of the capacitance on RH%. Fast rise times and a sensitivity of the order of 0.1–0.5 pF/%RH favourably compare with standard commercial sensors and other advanced capacitive devices made with polymers.

Fullerene-Based Carbon Nanostructures for Methanol Oxidation

Films of C60 clusters were electrophoretically deposited on optically transparent electrode surfaces. These C60 films constitute a new class of carbon electrodes with properties that differ from graphite and diamond electrodes. The electrophoretically deposited C60 cluster film is highly porous and is quite stable to oxidative potentials. Hence C60 film provides an electrochemical window to carry out oxidation processes. Upon electrodeposition of platinum particles, these nanostructured carbon films show remarkable activity toward methanol oxidation. The dependence of methanol oxidation on the amount of platinum and C60 in a half-cell reaction demonstrates the role of fullerene nanoclusters as new form of carbon support.

Gas exposure and thermal stability of linear carbon chains in nanostructured carbon films investigated by in situ Raman spectroscopy

In situ Raman spectroscopy from nanostructured carbon films, produced by supersonic cluster beam deposition, has revealed a high fraction of sp 1 linear carbon chains (also known as carbynoid structures) embedded in a sp 2 amorphous matrix. The presence of sp 1 linear carbon chains is revealed by a large structured band with a main peak at nearly 2100 cm −1 and a shoulder at 1980 cm −1, assigned to polyyne (alternating single–triple bonds) and polycumulene (double bonds) moietes respectively. The stability of these species has been analysed following the temporal evolution of the characteristic sp 1 Raman band intensity under exposure to several gases (He, N 2, H 2 and dry air). The carbynoid species appear to be very reactive to oxygen-rich environment, whereas the effects of other gases are less destructive. Actually, exposure to oxygen reveals different decay mechanisms with respect to other gases. The thermal stability in ultra high vacuum has been also studied up to 200 °C. We observe different decay kinetics as a function of annealing temperature, showing thresholds in the chain stability.

Spatially resolved valence band study of nanostructured carbon films containing transition metal nanocrystals

We studied the electronic structure of nanostructured C films produced by ultrasonic cluster beam deposition with the intention to find out the bonding configuration of the C network and to determine the modification induced by transition metal cluster inclusion in the film. Exploiting the energy and spatial resolution of the Spectromicroscopy beamline at Elettra Synchrotron, we were able to follow the valence band features depicted in 95.0 eV-excited photoemission spectra with a 0.5 μm lateral resolution. The spectroscopic results allowed to conclude that nanostructured C films have a mixed sp 2/sp 3 bond configuration and an amorphous-like structure and that those features are uniformly distributed in the film. The inclusion of metal cluster (Ni, Ti and Mo) in the film strongly modifies the valence band spectra. The surface stoichiometry of the embedded metal clusters and the keeping of their metallic nature are determined by a detailed analysis of the spatial dependence of the ultraviolet photoemission spectroscopy (UPS) spectra and by a comparison with previous studies on metal-carbide and -oxide surfaces.

Diamond-like nanostructured carbon film deposition using thermionic vacuum arc

Diamond-like nanostructured carbon films were deposited by a new type of vacuum arc technique, named thermionic vacuum arc (TVA) in a high-vacuum (10 −6 mbar) chamber. This is our first report in an international journal on TVA use for nanostructured carbon film deposition. TVA technology has two main advantages: (1) the continuous bombardment of just depositing thin film by energetic carbon ions generated in carbon vapors plasma and (2) the ions energy can be fully controlled and changed at will even during deposition. The structures of the films have been characterized by high-resolution transmission electron microscopy, selected area electrons diffraction and fast Fourier transmission. The measured interatomic distances reveal a diamond-like nanostructure carbon film with an average grain size approximately 3–5 Å and a d-spacing of 0.286 nm, which corresponding to a rhomboedral lattice of diamond/carbon (ASTM 0015—pattern no 79-1473). The results demonstrate that the TVA is suitable for the preparation of diamond-like nanostructured coatings with high quality.

Diamond-like nanostructured carbon film deposition using thermionic vacuum arc

Diamond-like nanostructured carbon film deposition using thermionic vacuum arc