Properties Of Nanostructured Carbon Films Research Articles

Nanostructured carbon films obtained by CH4 plasma deposition and annealing at high temperature: structural features and their effects on electrical and optoelectronic properties

This article is dedicated to the study of the structural, electrical, and optoelectronic properties of nanostructured carbon films obtained by methane plasma deposition, followed by annealing at high temperatures (650–800 °C). The conditions for obtaining the films affected the final physicochemical parameters. We studied the film morphology using atomic force microscopy, scanning electron microscopy, Raman spectroscopy, X-ray energy-dispersive analysis, and analysis of the current voltage (C-V) characteristics. The film thickness ranged from 20 to 150 nm, with a C/O ratio of 4:1. Structural studies have shown that the resulting nanostructured carbon films consist mainly of nanographite flakes, the lateral dimensions of which lie in the lateral size (La) range of 5 to 12 nm, and contain different fractional concentrations of sp3/sp2 crystalline phases of carbon. We have established that with an increase in the annealing temperature, the defectiveness of the carbon film structure increases; however, at the same time, the degree of graphitization increases, as indicated by the Raman spectroscopy data and the calculated values of layer resistances from the C-V characteristics. The values of photocurrents were calculated, from which it was found that the samples exhibited photosensitivity in the temperature range of room temperature to –173 °C, based on the temperature dependences of the C-V. The obtained results can be useful in creating day and night light sensors as well as temperature sensors suitable for use at low temperatures.

Tailoring the tribological properties of nanostructured carbon films under water lubrication

Carbon films have been considered suitable to be applied in water lubrication since they exhibit excellent friction-reducing and wear resistance, chemical inertness, etc. However, the basic understanding of tribological behaviors of carbon-based films under water lubrication still needs to be explored. In the present work, carbon films with different nanostructures were prepared by the electron cyclotron resonance (ECR) plasma nano-surface manufacturing system, and micro-textures with different sizes were prepared on the surface of carbon films by plasma etching. The influence of nanostructure and surface texture on the tribological properties of carbon films was investigated. The results show that different nanostructured carbon films can obtain lower friction coefficients and longer wear life under water lubrication than under dry condition. Due to low surface roughness, high hardness, and compact structure, the tribological properties of amorphous carbon (a-C) films under water lubrication are much better than those of graphene sheet-embedded carbon (GSEC) films. The prepared surface texture has a negative effect on the hard a-C film, but it can make the soft GSEC film generate soft wear debris at the initial stage. With the action of water, the soft wear debris is bonded on the surface of the contacting ball to form a silt-like transfer film, which increases the wear life by nearly three orders of magnitude. These results extend the basic understanding of the tribological behavior of carbon film under water lubrication, which is crucial in both fundamental and applied science.

Very thin N-doped nanostructured carbon films on quartz and sapphire substrate: Photoelectron emission properties

Very thin N-doped nanostructured carbon films were deposited on quartz and sapphire substrate by radio-frequency reactive magnetron sputtering using carbon target and gas mixture of Ar and N2 or N2+H2 reactive gasses. Rutherford backscattering spectroscopy and Elastic recoil detection analytical methods determined the concentration of elements in the films. Scanning electron microscopy was used to investigate the surface morphology of nanostructured very thin carbon films. Raman spectroscopy was used for the determination of chemical structural properties of the thin nanostructured carbon films. Pulsed laser induced electron emission method was used for the study of photoelectron emission properties of nanostructured carbon films. Measured bunch charge results of fabricated transmission photocathodes showed better photoelectron emission properties of very thin nanostructured carbon films prepared on sapphire substrates. Effects of substrate and technology of very thin nanostructured carbon films on the properties of photo-induced electron emitters as backside illuminated transmission photocathode are discussed.

Physical properties of chemical vapour deposited nanostructured carbon thin films

A simple thermal chemical vapour deposition technique is employed for the deposition of carbon films by pyrolysing the natural precursor “turpentine oil” on to the stainless steel (SS) and FTO coated quartz substrates at higher temperatures (700–1100 °C). In this work, we have studied the influence of substrate and deposition temperature on the evolution of structural and morphological properties of nanostructured carbon films. The films were characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM), contact angle measurements, Fourier transform infrared (FTIR) and Raman spectroscopy techniques. XRD study reveals that the films are polycrystalline exhibiting hexagonal and face-centered cubic structures on SS and FTO coated glass substrates respectively. SEM images show the porous and agglomerated surface of the films. Deposited carbon films show the hydrophobic nature. FTIR study displays C–H and O–H stretching vibration modes in the films. Raman analysis shows that, high ID/IG for FTO substrate confirms the dominance of sp 3 bonds with diamond phase and less for SS shows graphitization effect with dominant sp 2 bonds. It reveals the difference in local microstructure of carbon deposits leading to variation in contact angle and hardness, which is ascribed to difference in the packing density of carbon films, as observed also by Raman.

Investigation of electrical properties of nanostructured carbon films derived from block copolymers

Electrical properties of nanostructured carbon (ns-C) films fabricated by pyrolysis of PAN–b–PBA copolymers were investigated. Films having cylindrical morphology and pyrolyzed at 400, 500 and 600 °C were investigated. Both carbide forming (Zr, Ti) and non-carbide forming (Cu, Pt) metals spanning a wide range of electron work functions (4.1–5.5 eV) formed ohmic contacts to the ns-C films in the as-deposited state. The conductivity of the ns-C films increased roughly three orders of magnitude for every 100 °C increase in the pyrolysis temperature. Hall-effect measurements showed that the films pyrolyzed at 600 °C were n-type with a majority carrier concentration and mobility of 5.8 × 10 18 cm −3 and 0.97 cm 2/V s, respectively. Current–voltage measurements as a function of temperature ( I– V– T) were performed on films pyrolyzed at 600 °C, whereas films pyrolyzed at 400 and 500 °C were too resistive for reliable resistivity–temperature and Hall-effect measurements. The resistivity as a function of temperature was analyzed by using the reduced activation energy method and was determined to follow variable-range hopping (VRH) mechanisms at and below room temperature. The data indicates a crossover from Efros–Shklovskii VRH [J. Phys. C 8, (1975) L49] to Mott VRH [J. Non-Cryst. Solids 1, (1968) 1] at temperatures above 100 K.

Field emission properties of nano-structured carbon films with carbon needles deposited on Si using high-power-density microwave-plasma CVD method

Nano-structured carbon films (NCFs) were fabricated on Si wafer chips with hydrogen–methane gas mixture by creating high-power-density microwave (MW) plasma in a quartz tube with a low-output MW power source. Carbon sheets with many nano-protuberances were complicatedly twisted each other in the center region of the NCF while carbon needles were additionally formed in the fringe region of the NCF. These NCFs yielded field emission (FE) characteristics with the FE current density of 68 mA/cm 2 at a macroscopic electric field, E, of 10 V/μm. Since the FE currents tended to be saturated even in a medium E region, no simple Fowler–Nordheim (F–N) model was applicable. A model considering the F–N mechanism, statistic effects of FE tip structures and a space-charge-limited-current effect has been applied successfully to explain all the FE data observed for E < 10 V/μm. It turned out that E-dependent parameters such as the effective total emission area and the effective field enhancement factor should be employed to obtain sufficient quantitative agreements between the model and the experiments.

Carbon nano-structures containing nitrogen and hydrogen prepared by ion beam assisted deposition

In this paper, we report the synthesis and some properties of nano-structured carbon films grown on nanometric nickel particles at 700°C by ion beam assisted deposition (IBAD). In situ photoelectron emission spectroscopy (XPS) reveals the presence of nitrogen in the carbon layer coating the nickel particles. Atomic force microscopy (AFM) displays regular dome-like structures with a mean width, height, and density of ∼170nm, ∼22nm, and ∼1.4×109domes/cm2, respectively. Raman spectroscopy shows the characteristic frequencies associated with graphite and disordered structures. Disorder increases on increasing nitrogen content. High-resolution transmission electron microscopy (HRTEM) confirms the presence of multiwall well-organized graphite layers covering the nickel particles. The field emission properties of the structures are reported.

Statistical modeling of field-enhancement-factor distribution of nanostructured carbon films

We measured the field-emission properties of nanostructured carbon films (NSCFs), carbon nanotubes, and carbon nanoparticles, using a microtip and a large area phosphor-screen anode. The normal distribution of turn-on electric fields which was measured using a microtip anode from 100 square cells of 30×30μm2 for each NSCF indicated an asymmetric distribution of field-enhancement factor β with a large-β tail. The direct deduction of the field-enhancement factors via fitting of current-versus-voltage curves to the simplified Fowler-Nordheim (FN) equation with a series-resistance effect confirmed the asymmetric β distribution. Moreover, field-emission features observed using a large area phosphor screen, such as a slight curvature in the FN plots at the low voltage regime and applied-field-dependent exponential increase of emission-site densities, turned out to be consistent with the asymmetric β distribution.

Statistical analysis of field electron emission from nanostructured carbon films

The field-emission properties of nanostructured carbon films (NSCFs), such as carbon nanotubes and carbon nanoparticles, were measured using a microtip and a large-area phosphor-screen anode. Except at small bias-voltage range, the current-versus-voltage curves measured with a large-area anode were fitted well to a simplified Fowler-Nordheim (FN) equation with a series-resistance effect. From the mapping of the turn-on field using a microtip anode, it was deduced that the field-enhancement factor β of NSCFs had an asymmetric distribution with a large-β tail. The asymmetric β distribution led to explanations of the slight curvature in FN plots at the low-voltage regime and of the exponential increase of emission-site densities dependent on the applied field.

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.

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.

CVD growth and field emission properties of nanostructured carbon films

An investigation of the growth mechanisms, electronical and structural properties, and field emissions of carbon films obtained by chemical vapour deposition showed that field emissions from films composed of spatially oriented carbon nanotubes and plate-like graphite nanocrystals exhibit non-metallic behaviour. The experimental evidence of work function local reduction for carbon film materials is reported here. A model of the emission site is proposed and the mechanism of field emission from nanostructured carbon materials is described. In agreement with the model proposed here, the electron emission in different carbon materials results from sp3-like defects in an sp2 network of their graphite-like component.

Field emission site densities of nanostructured carbon films deposited by a cathodic arc

The field emission properties of nanostructured carbon films deposited by cathodic vacuum arc have been investigated by measuring both the emission currents and the emission site density. The films have an onset field of 3 V/μm. The emission site density is viewed on a phosphor anode and it increases rapidly with applied field. It is assumed that the emission occurs from surface regions with a range of field enhancement factors but with a constant work function. The field enhancement factor is found to have an exponential distribution.

Field Emission Site Densities of Nanostructured Carbon

ABSTRACTThe field emission properties of nanostructured carbon films deposited by cathodic vacuum arc in a He atmosphere have been studied by measuring the emission currents and the emission site density. The films have an onset field of ∼3 V/μm. The emission site density is viewed on a phosphor anode and it increases rapidly with applied field. It is assumed that the emission occurs from surface regions with a range of field enhancement factors but with a constant work function. The field enhancement factor is found to have an exponential distribution.