Amorphous Carbon Samples Research Articles

Local structure of Amorphous carbon investigated by X-ray total scattering and RMC modeling

Amorphous carbon is a promising candidate as an energy storage material. In this paper, we performed an X-ray total scattering measurement, RMC modeling, and persistent homology analysis for amorphous carbon samples fabricated at two different heat treatment temperatures. According to the analysis of the nearest-neighbor carbon atoms and their angular histogram, the sample treated at higher temperature shows higher connectivity between carbon atoms than that treated at lower temperature. Furthermore, topological data analysis (persistent homology, PH) reveals quantitative results that relate ring structure and the connectivity between carbon atoms.

A machine learning approach for the automated classification of bulk sp2 and sp3 carbon materials

AbstractPrincipal component analysis (PCA) and linear discriminant analysis (LDA) were used to classify different types of carbon material based on their Raman spectra. The selected reference materials were highly oriented pyrolytic graphite (HOPG), diamond‐like carbon (DLC), glassy carbon (GC), hydrogenated graphite‐like carbon (GLCH), and hydrogenated polymer‐like carbon (PLCH). These materials vary in crystallinity, predominant carbon hybridization, and hydrogen content. The training dataset was Raman spectra collected from commercial samples (HOPG, DLC, GC) and samples synthesized in our laboratory (GLCH, PLCH). The Raman spectra were collected using 532 nm laser excitation. The classification model revealed that the first principal component (PC1) was the determinant source of information to separate the crystalline from the amorphous carbon samples. PC2 allowed the separation of amorphous material with different levels of hybridization (sp2 and sp3). Finally, both PC2 and PC3 contributed to separate materials with different levels of hydrogenation. The classification model was tested using a library of Raman spectra of carbon materials reported in the literature, and the results showed a high accuracy prediction (97%). The model presented here provides an avenue for automated classification of carbon materials using Raman spectroscopy and machine learning.

Nanoscale geometry determines mechanical biocompatibility of vertically aligned nanofibers

Vertically aligned carbon nanofibers (VACNFs) are promising material candidates for neural biosensors due to their ability to detect neurotransmitters in physiological concentrations. However, the expected high rigidity of CNFs could induce mechanical mismatch with the brain tissue, eliciting formation of a glial scar around the electrode and thus loss of functionality. We have evaluated mechanical biocompatibility of VACNFs by growing nickel-catalyzed carbon nanofibers of different lengths and inter-fiber distances. Long nanofibers with large inter-fiber distance prevented maturation of focal adhesions, thus constraining cells from obtaining a highly spread morphology that is observed when astrocytes are being contacted with stiff materials commonly used in neural implants. A silicon nanopillar array with 500 nm inter-pillar distance was used to reveal that this inhibition of focal adhesion maturation occurs due to the surface nanoscale geometry, more precisely the inter-fiber distance. Live cell atomic force microscopy was used to confirm astrocytes being significantly softer on the long Ni-CNFs compared to other surfaces, including a soft gelatin hydrogel. We also observed hippocampal neurons to mature and form synaptic contacts when being cultured on both long and short carbon nanofibers, without having to use any adhesive proteins or a glial monoculture, indicating high cytocompatibility of the material also with neuronal population. In contrast, neurons cultured on a planar tetrahedral amorphous carbon sample showed immature neurites and indications of early-stage apoptosis. Our results demonstrate that mechanical biocompatibility of biomaterials is greatly affected by their nanoscale surface geometry, which provides means for controlling how the materials and their mechanical properties are perceived by the cells. Statement of significanceOur research article shows, how nanoscale surface geometry determines mechanical biocompatibility of apparently stiff materials. Specifically, astrocytes were prevented from obtaining highly spread morphology when their adhesion site maturation was inhibited, showing similar morphology on nominally stiff vertically aligned carbon fiber (VACNF) substrates as when being cultured on ultrasoft surfaces. Furthermore, hippocampal neurons matured well and formed synapses on these carbon nanofibers, indicating high biocompatibility of the materials. Interestingly, the same VACNF materials that were used in this study have earlier also been proven to be capable for electrophysiological recordings and sensing neurotransmitters at physiological concentrations with ultra-high sensitivity and selectivity, thus providing a platform for future neural probes or smart culturing surfaces with superior sensing performance and biocompatibility.

Friction wear characteristics of a femtosecond laser-patterned amorphous carbon thin film

In this study, an amorphous carbon thin film on a cemented carbide substrate was prepared with surface micropatterns using femtosecond laser machining. The micropatterns that were considered were made of lines, dotted lines, and zigzag dotted lines. We performed a tribology test using a ball-on-disk friction wear test and investigated the coefficient of friction (COF) and wear amount as a function of pattern size and shape. The micropattern made using femtosecond laser patterning exhibited better friction coefficient properties than the bare amorphous carbon samples. Moreover, the pattern size and shape influenced the friction coefficient value of the patterned samples. The dotted line pattern shape showed superior results in friction coefficient characteristics compared to those of other micropatterns.

Tridimensional N, P-Codoped Carbon Sponges as Highly Selective Catalysts for Aerobic Oxidative Coupling of Benzylamine.

Two tridimensional N-doped porous carbon sponges (3DC-X) have been prepared by using cetyltrimethylammonium chloride (CTAC) and cetyltrimethylammonium bromide (CTAB) as soft templates and alginate to replicate the liquid crystals formed by CTA+ in water. Alginate is a filmogenic polysaccharide of natural origin having the ability to form nanometric defectless films around objects. Subsequent pyrolysis at 900 °C under an Ar flow of the resulting CTA+–polysaccharide assemblies result in 3DC-1 and 3DC-2, with the N percentages of 0.48 and 0.36 wt % for the materials resulting from CTAC and CTAB, respectively. Another four 3DC materials were obtained via pyrolysis of the adduct of phytic acid and chitosan, rendering an amorphous, N and P-codoped carbon sample (3DC-3 to 3DC-6). The six 3DC samples exhibit a large area (>650 m2 × g–1) and porosity, as determined by Ar adsorption. The catalytic activity of these materials in promoting the aerobic oxidation of benzylamine increases with the specific surface area and doping, being the largest for 3DC-4, which is able to achieve 73% benzylamine conversion to N-benzylidene benzylamine in solventless conditions at 70 °C in 5 h. Quenching studies and hot filtration tests indicate that 3DC-4 acts as a heterogeneous catalyst rather than an initiator, triggering the formation of hydroperoxyl and hydroxyl radicals as the main reactive oxygen species involved in the aerobic oxidation.

Pomegranate-like S@N-doped graphitized carbon spheres as high-performance cathode for lithium-sulfur battery

To obtain high-performance sulfur cathode for lithium-sulfur (Li-S) battery, porous graphitized carbon spheres with N doping have been fabricated via an in situ Fe catalysis as sulfur host. The in situ generated Fe nanoparticles play double roles as graphitization catalyst and pore template. The electrical conductivity of the graphitized sample (5.6 S cm−1) is about one order of magnitude higher than that of the amorphous carbon sample (0.63 S cm−1). Correspondingly, the pomegranate-like S@N-doped graphitized carbon cathode displays very good electrochemical performance. A capacity of 600 mAh g−1 is obtained after 150 charge/discharge cycles at a current density of 1 C with a sulfur content of 73 wt%. The pomegranate-like S@N-doped graphitized carbon is promising cathode candidate for high-performance Li-S battery.

Evaluation of Tribological Performance of Hydrogenated Dlc by Surface Texturing in the Presence of Palm Based Tmp Ester at Different Temperatures

Surface textured and DLC coated (hydrogenated amorphous carbon) samples were assessed for their coating performance at 40 oC, 80 oC and 125 oC. As a result, textured a-C:H DLC demonstrated higher but stable coefficient of friction (COF) at high temperatures as compared to un-textured DLC samples. However, textured DLC samples showed higher wear resistance compared to un-textured DLC coating. The enhancement can be elucidated by the lower graphitization of textured DLC samples.

Structural Transformations of Amorphous Carbon (Glassy Carbon) at High Shock Pressures

Amorphous carbon (glassy carbon) samples were shock compressed up to 80 GPa and temperatures up to 1700 K for several microseconds. Glassy carbon samples before and after an explosive action are analyzed by X-ray diffraction, electron microscopy, and electron-probe microanalysis. It is shown that as a result of microsecond shock pressure exposure, glassy carbon is compacted to ρCG ≈ 2.3(5) g/cm3 and is partly transformed into a graphite-like nanomaterial with a cellular structure. At the level of crystallites, the density of glassy carbon increases via a decrease in the interplanar spacings and an increase in the crystallite thickness and width. Spheres from 20 nm to 80 μm in diameter are found to be formed during shock-wave compression of glassy carbon in a copper container and high-temperature shock heating posteffects. Spheres 20 μm in diameter consist of a copper-rich core and a carbon shell.

Structure of amorphous carbon quenched from liquid in the pressure range 1–40 GPa: Molecular dynamic modeling

It is well known, that quenching from the liquid state is the basis of many methods for creating new materials with unique properties. Liquid and amorphous carbon are a mixture of atoms with different states of hybridization (sp1, sp2, sp3) owing to polymorphism. It is claimed that there is a tendency of growth of sp1 atoms in liquid and amorphous carbon at decreasing pressure. Great interest has been shown recently to carbyne and pseudocarbynes, consisting of sp1-hybridizing atoms. These materials have unique optical and mechanical properties. In the present work a pressure dependence of the structure of amorphous carbon, quenched from liquid is studied by molecular dynamics simulation in the pressure range 1–40 GPa. The interaction between carbon atoms was determined by two bond-order potentials: Airebo and ReaxFF. These two potentials take into account the type of a chemical bond as well as breaking and formation of new chemical bonds during the modeling process. We study a bulk quenching from liquid carbon in the NPT ensemble at a constant pressure and determine the distribution of chemical bonds sp1–sp2–sp3 in amorphous carbon during the quenching. Quenched liquid structure modeling at a pressure of 1 GPa and the structure of an amorphous carbon sample obtained experimentally at a pressure of 25 MPa by Raman spectroscopy showed that the sp1 fraction of carbon was significant.

Theoretical investigation of magnetic properties in interfaces of magnetic nanoparticles and amorphous carbons

Based on the experimental finding of the exchange bias in amorphous carbon samples with embedded Co nanoparticles and on the graphited character of the amorphous carbon interface confirmed by molecular dynamics simulations we have proposed the interface of graphited carbon to be antiferromagnetic. A theoretical model, which comprises the Kondo interactions in the interfaces of Co nanoparticles and the induced antiferromagnetic interactions in the graphited carbons, is employed to evaluate the ferromagnetism of the interfaces of Co nanoparticles. We have shown that the ferromagnetism of interfaces of Co nanoparticles will be enhanced by the increase of antiferromagnetic interaction as well as the increase of electron density in the graphited carbons. In particular, we found that the antiferromagnetic interactions in graphited carbons will change the spin-wave excitation in interfaces of Co nanoparticles from the quasiacoustic mode to the quasioptical one.

Structural modification of Ga+ and N+ ion implanted ta-C films

Thin-film samples (d ˜ 40 nm) of tetrahedral amorphous carbon (ta-C) deposited by filtered cathodic vacuum arc (FCVA) were implanted with Ga+ at ion energy E = 20 keV and ion fluences D = 3 × 1014÷3 × 1015 cm2 and N+ with the same energy and a dose D = 3 × 1014 cm-2. The Ga+ ion beam induced a structural modification of the implanted material. This resulted in a considerable change of its structural properties, manifested as the formation of a new phase under non-equilibrium conditions, which could be accompanied by considerable changes in the ta-C films optical properties. The N+ implantation also resulted in a modification of the surface structure. These effects were explored using transmission (TEM) and scanning (SEM) electron microscopy.

Speckle Suppression by Decoherence in Fluctuation Electron Microscopy.

We compare experimental fluctuation electron microscopy (FEM) speckle data with electron diffraction simulations for thin amorphous carbon and silicon samples. We find that the experimental speckle intensity variance is generally more than an order of magnitude lower than kinematical scattering theory predicts for spatially coherent illumination. We hypothesize that decoherence, which randomizes the phase relationship between scattered waves, is responsible for the anomaly. Specifically, displacement decoherence can contribute strongly to speckle suppression, particularly at higher beam energies. Displacement decoherence arises when the local structure is rearranged significantly by interactions with the beam during the exposure. Such motions cause diffraction speckle to twinkle, some of it at observable time scales. We also find that the continuous random network model of amorphous silicon can explain the experimental variance data if displacement decoherence and multiple scattering is included in the modeling. This may resolve the longstanding discrepancy between X-ray and electron diffraction studies of radial distribution functions, and conclusions reached from previous FEM studies. Decoherence likely affects all quantitative electron imaging and diffraction studies. It likely contributes to the so-called Stobbs factor, where high-resolution atomic-column image intensities are anomalously lower than predicted by a similar factor to that observed here.

Ion implantation induced modification of ta-C films

Thin film samples (d~40nm) of tetrahedral amorphous carbon (ta-C), deposited by filtered cathodic vacuum arc (FCVA), have been implanted with N+ at ion energy E = 20 keV and ion dose D = 3.1014 cm−2. The induced structural modification of the implanted material results in a considerable change of its optical properties, best manifested by a significant shift of the optical absorption edge to lower photon energies as obtained from optical transmission measurements. This shift is accompanied by a considerable increase of the absorption coefficient (photo-darkening effect) in the measured photon energy range (0.5÷3.0 eV). These effects could be attributed both to additional defect introduction and increased graphitization, as confirmed by IR and Raman measurements. The optical contrast thus obtained (between implanted and unimplanted film material) could be made use of for information archiving, in the area of high-density optical data storage while using focused ion nano-beams.

Raman microscopy as a defect microprobe for hydrogen bonding characterization in materials used in fusion applications

AbstractWe present the Raman microscopy ability to detect and characterize the way hydrogen is bonded with elements that will be used for ITER's plasma facing components.For this purpose we first use hydrogenated amorphous carbon samples, formed subsequently to plasma‐wall interactions (hydrogen implantation, erosion, deposition...) occurring inside tokamaks, to demonstrate how this technique can be used to retrieve useful information. We pay attention in identifying which spectroscopic parameters are sensitive to the local structure (sp3/sp2) and which gives information on the hydrogen content using isothermal and linear temperature ramp studies on reference samples produced by plasma enhanced chemical vapor deposition. We then focus on the possibility to use this fast, non‐destructive and non‐contact technique to characterize the influence of hydrogen isotope implantation in few nanometers of graphite and beryllium as C is still used in the JT‐60 tokamak and Be is used in JET and will be used as plasma‐facing component in the future reactor ITER. We also pay attention on implantation in tungsten oxide which may be formed accidently in the machine. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Jump in the electrical conductivity of shock-compressed glassy carbon

The effect of high dynamic pressures on the electrical conductivity of the amorphous conducting carbon phase (glassy carbon) has been studied. The electrical conductivity of glassy carbon samples has been measured under the condition of shock compression and subsequent release wave. The history of the shock loading of glassy carbon has been calculated with the developed semiempirical equations of state. It has been shown the electrical conductivity of glassy carbon samples in the compression phase at a pressure of 45(5) GPa decreases abruptly by two orders of magnitude. In the relief phase, partially reversible change in the electrical conductivity of an amorphous carbon sample occurs. The recorded effect has been treated as a result of a partially reversible physicochemical transformation of shock-compressed amorphous carbon.

Synthesis of Tic Thin Films by CVD from Toluene and Titanium Tetrachloride with Nickel as Catalyst

Titanium Carbide TiCx was deposited onto quartz substrates by the chemical vapour deposition (CVD) method. TiCx thin films were obtained from titanium tetrachloride (TiCl4) and toluene (C6H5CH3) as carbon source. Deposits were carried out in the range of substrate temperatures from 300 to 1100 C, with a source gas molar ratio (C6H5CH3/TiCl4 + C6H5CH3 ) of 0.8. There was two deposit processes; the first was to carry out without nickel as catalyst and the second with nickel as catalyst. Results showed that the effect of nickel as catalyst is very relevant to obtain TiCx. TiC0.87 thin films, 0.5 m thickness, were obtained for deposition temperatures above 900C using nickel as catalyst. In this case a cubic structure, with a lattice parameter of 4.327 A and a hardness of 2900 Vickers, was obtained at 11000C. whilst for substrate temperatures between 300 to 900°C with nickel as catalyst amorphous carbon samples were obtained. For the samples prepared without the presence of the nickel as catalyst only C amorphous thin films were obtained for all deposit temperatures.

Study on Raman Spectroscopy and Purification of B-C-N Compound

Electrophoretic technology is used to purify BC3.3N in an amorphous carbon and hexagonal boron nitride sample. The results are acquired using Raman spectroscopy and field emission scanning electron microcopy with energy dispersive spectroscopy, which show that pure BC3.3N is obtained from graphite and hexagonal boron nitride. The structural characteristics of this compound are studied by Fourier transform infrared spectroscopy. A simple method for purifying B-C-N compounds is presented.

A study of raman and rutherford backscattering spectra of amorphous carbon films modified with platinum

Raman spectra have been measured in the spectral range from 1000 to 1800 cm−1 on samples of amorphous carbon modified with platinum in amounts comparable with that of carbon. Also, Rutherford backscattering spectra have been studied. It is shown that, as the platinum concentration is raised to ∼0.5 × 1022 at cm−3, the average size of graphene clusters increases. As the platinum concentration increases further, the graphene clusters become smaller in size.

Compositional characterization of carbon electrode material: A study using simultaneous TG–DTA–FTIR

Present work describes the application of thermal methods, especially the evolved gas analysis (EGA) for the compositional characterization of carbon electrode material with respect to its organic, amorphous and graphitic carbon content. Trace levels of organic carbon present in the amorphous carbon samples were determined qualitatively by using FTIR absorption spectroscopy. Amorphous and graphitic carbon content in synthetic mixture samples were determined quantitatively using simultaneous TG–DTA–FTIR measurements. FTIR system was calibrated using the measured absorption signal of the evolved carbon dioxide due to the decomposition of cadmium carbonate. Inter-comparison studies using TG–FTIR measurements show that simultaneous FTIR spectroscopy is an effective complementary quantitative measurement technique for thermogravimetric analysis involving the complex decomposition reaction processes.

Field emission studies of amorphous carbon deposited on copper nanowires grown by cathodic arc plasma deposition

The field emission properties of amorphous carbon/copper nanowire (CuNW)/Si composites have been investigated. The CuNWs (50–80 nm in diameters) were deposited anodically within the pores of a porous anodic aluminum oxide (AAO) nano-template by electrolysis of a CuSO 4-H 2SO 4 solution at room temperature using DC current. Two kinds of structure, amorphous carbon/Si and amorphous carbon/CuNWs/Si, were used. Both of them can emit electrons in vacuum towards an anode at a very low turn-on field. Field emission from the amorphous carbon/CuNWs/Si shows a fourfold decrease in turn-on field compared with the amorphous carbon/Si. Results indicated that the electrons were emitted under the effect of an enhanced field because of the geometry of the amorphous carbon sample.