Dense Amorphous Carbon Research Articles

Preparation of high strength and rigidity carbon layer on SiO material surface by dynamic CVD method using gas carbon source C2H2

The main issue in the application of silicon-based negative electrode materials is the inevitable volume expansion, leading to negative electrode material fracture, which severely impacts the performance of batteries. This study employed Chemical Vapor Deposition (CVD) (C2H2@SiO), solid-phase coating method (Pitch@SiO), and liquid-phase coating method (RGFQ@SiO) to coat the surface of SiO materials with a dense amorphous carbon structure. Material property analysis revealed that C2H2@SiO has a relatively small specific surface area (1.8 m2 g−1) and surface carbon increment (2.4 %). It exhibited high reversible specific capacity (1563.4 mAh g−1), high initial Coulombic efficiency (81.45 %), and low volume expansion rate (9.3 %). Mechanical performance testing indicated that the surface carbon layer Young's modulus of C2H2@SiO was the highest (35 GPa), suggesting that using the CVD method to obtain a dense carbon layer can enhance the structural strength of the material. Based on the electrochemical-mechanical coupling theory simulation analysis of the stress during the charge-discharge process of electrode materials, the evolution of concentration and stress field during lithium insertion process was obtained, showing that the coating can further improve the electrochemical and mechanical performance of the negative electrode materials effectively. Additionally, the negative electrode sheets fabricated using sample C2H2@SiO were assembled into 18650 cylindrical batteries, exhibiting excellent cycling performance in 1000 cycles at 25 °C with a capacity retention rate of 85.7 %, along with good rate capability and high/low-temperature performance. The conclusions of this study provide certain guidance for the design and optimization of negative electrode materials for battery electrodes.

Improved Cycling Stability of LFP by W-Ti Co-Doping Strategy for Li-Ion Batteries

Olivine-type lithium iron phosphate (LFP) is used widely as a cathode material with excellent cycling stability, thermal stability, and low cost for lithium-ion batteries in power vehicles and energy storage. However, it has inherent drawbacks that limit its further development, such as low electronic conductivity and lithium ion diffusion rate. Here, secondary spherical particles LFP-W/Ti@C were synthesized using a simple sanding and spray drying method. The pyrolysis of glucose forms a uniform and dense amorphous carbon on the surface of LFP, and the constructed three-dimensional conductive network accelerates the transfer of electrons and ions while mitigating the volume expansion during the lithiation-delithiation process. In addition, the synergistic co-doping of W/Ti achieves nanosizing of primary particles with more active sites providing locations for lithium ions storage, while reducing polarization and internal resistance and enhancing the diffusion kinetics of lithium ion. The LFP-W/Ti@C obtained after optimization still has a discharge capacity of 107.7 mAh g−1 (with a cycle retention rate of up to 100%) after 500 cycles at 5 C. This simple W/Ti co-doping strategy provides excellent rate performance and shows great potential for fast charging in electric vehicles.

Tribological properties of graphene quantum dot hybrid polyethylene glycol lubricated molybdenum disulfide films

Herein, molybdenum disulfide (MoS2) films were obtained by magnetron sputtering, and graphene quantum dots (GQDs) prepared by the hydrothermal method were used as additives for polyethylene glycol (PEG) lubricants to investigate the tribological mechanisms under solid-liquid composite lubrication. The friction coefficient of GQDs hybridized PEG lubricated MoS2 solid-liquid composite films was reduced by 69.23%, and the wear rate was reduced by 79.31% as compared to solid lubrication (MoS2 film). This solid-liquid complex lubrication occurs mainly as abrasive wear. GQDs form a uniform and dense amorphous carbon film on the corresponding balls to protect the wear between the friction pair. This work demonstrates the great potential of zero-dimensional and two-dimensional materials for solid-liquid composite lubrication.

Na2FePO4F/C composite cathode with excellent ions migration property for high-performance sodium-ion batteries

For sodium-ion batteries, polyanionic cathode materials have been mainly studied due to their advantages of high safety, low-cost and environmental friendliness. However, some shortcomings of low intrinsic conductivity and slow reaction kinetics limit its further application. In our study, a uniform and dense amorphous carbon layer is coated onto the surface of fluorophosphate (Na2FePO4F/C) via high-energy ball milling and heat-treatment process. According to the results of SEM and TEM, it is seen that the surface of the active particles would be composited with a carbon layer. When the heat treatment temperature is 650 °C, the ratio of ID/IG of Raman is the largest, indicating that the highest disorder degree of the carbon layer. The obtained Na2FePO4F/C-650 has the largest initial discharge specific capacity (120 mAh·g−1 at 0.1 C), and the specific capacity retention rate is 76.54% at 10 C for 1000 cycles. Furthermore, the sodium-ion diffusion coefficient of Na2FePO4F/C-650 measured using AC impedance is up to 4.91 × 10−12 cm2·s−1. It could be resulted from the fact that amorphous carbon layer can absorb more sodium ions and is beneficial to ion diffusion kinetics according to the DFT calculation. This study has proposed a new idea to modify cathode material and facilitated the commercial application of Na2FePO4F.

Evolution of photoresist layer structure and surface morphology under fluorocarbon‐based plasma exposure

AbstractIon bombardment of photoresist materials during plasma etching results in the formation of a surface dense amorphous carbon (DAC) layer that contributes to both etch resistance and the development of surface roughness. Real‐time ellipsometric measurements/analysis reveals that a C4F8‐containing plasma interacts with an Ar‐plasma‐formed DAC layer to produce a modified DAC/fluorocarbon (FC) layer by FC deposition/diffusion of fluorine into the surface. The depletion of the DAC layer via modification and ion bombardment causes the etch rate of the bulk layer to increase. As the modified surface layer is formed, a noticeable decrease in surface roughness decrease is observed. These findings provide an understanding of the mechanisms of atomic layer etching processes in photoresist materials.

Validation of etching model of polypropylene layers exposed to argon plasmas

AbstractThin layers of polypropylene (PP) have been treated by argon low‐temperature plasmas in an inductively coupled plasma setup. The etched thickness of PP was monitored in situ by means of single‐wavelength ellipsometry. The ellipsometric model of the polymer surface exposed to plasma consists of a UV‐modified layer, a dense amorphous carbon layer because of ion bombardment, and an effective medium approximation layer, which accounts for moderate surface roughness. The etching behavior has been compared to a model based on argon ion beam irradiation experiments. In this approach, surface processes are described in terms of etching yields and crosslinking probabilities as a function of incident fluxes and energies of Ar ions and UV photons. The ion beam model fits well with the plasma etching results.

In situ atomic-scale observation of irradiation induced carbon nanocrystalline formation from dense carbon clusters

We present a direct observation of the transformation of dense amorphous carbon clusters into diamond nanocrystalline under electron beam irradiation by in situ transmission electron microscopy, where the surrounding carbon matrix did not significantly change. Our findings provide clear and convincing evidence for the diamond nanocrystalline evolving from energetic amorphous carbon sites. Furthermore, graphitization of amorphous carbons usually demands a high temperature combined with high pressure. Hence, graphitization of amorphous carbons at relatively low temperatures is highly desired. Here we offer a useful method for catalyst-free graphitization of amorphous carbons by employing moderate electron beam irradiation, without external heating being applied.

Role of the dense amorphous carbon layer in photoresist etching

The development of new photoresists for semiconductor manufacturing applications requires an understanding of the material properties that control the material's plasma etching behavior. Ion bombardment at ion energies of the order 100 s of eV is typical of plasma-based pattern-transfer processes and results in the formation of a dense amorphous carbon (DAC) layer on the surface of a photoresist, such as the PR193-type of photoresist that currently dominates the semiconductor industry. Prior studies have examined the physical properties of the DAC layer, but the correlation between these properties and the photoresist etching behavior had not been established. In this work, the authors studied the real-time evolution of a steady-state DAC layer as it is selectively depleted using an admixture of oxygen into an argon plasma. Observations of the depletion behavior for various DAC layer thicknesses motivate a new model of DAC layer depletion. This model also correlates the impact of the DAC layer thickness with the etch rate of the bulk photoresist. The authors find that up to a 40% depletion of the DAC layer thickness does not have a significant impact on the bulk photoresist etch rate. However, further depletion results in an exponential increase in the etch rate, which can be up to ten times greater at full depletion than for the fully formed DAC layer. Thus, with these trends the authors show that the photoresist etch rate is controlled by the thickness of the DAC layer. Furthermore, thickness loss of the DAC layer in an O2-containing plasma coincides with a chemical modification of the layer into an oxygen-rich surface overlayer with properties that are intermediate between those of the DAC layer and the bulk photoresist. Support for this interpretation was provided via x-ray photoelectron spectroscopy characterization. Atomic force microscopy was used to gauge the impact on surface roughness as the DAC layer is formed and depleted. The trends established in this work will provide a benchmark in our development of new photoresists, which will be suitable for pattern transfer processes that will ultimately be a part of enabling smaller semiconductor device feature sizes and pitches.

Formation of nanometer-thick delaminated amorphous carbon layer by two-step plasma processing of methacrylate-based polymer

The authors show that extended He plasma pretreatment (PPT) of methacrylate-based 193 nm photoresist (PR) material in conjunction with a subsequent biased Ar plasma treatment can lead to blister formation at the polymer surface due to delamination of an ultrathin, ion-induced, dense, amorphous carbon (DAC) layer formed by low energy ion bombardment. For our experimental conditions, the delaminated layer is 1–2 nm thick and primarily composed of sp2-hybrized amorphous carbon. A He or Ar plasma process alone will not lead to this phenomenon, and so far the authors have only observed it for a methacrylate polymer. A possible mechanism of the formation of the ultrathin layer that is consistent with all observations is as follows: During He plasma pretreatment, volatile species are produced by ultraviolet/vacuum ultraviolet radiation-induced photolysis of the polymer pendant groups, e.g., adamantyl and chain-scissioning of the polymer backbone to a depth of greater than 100 nm. While volatile products formed close to the polymer surface can diffuse out during He PPT, those formed deep within the polymer bulk cannot and their concentration will become significant for extended He PPT. During the biased Ar plasma treatment step, a DAC surface layer is generated by Ar+ ion bombardment within the first seconds of plasma exposure. The thickness is dependent on ion energy and in the range of one to several nanometers. This layer appears to be impermeable to gaseous products formed in the PR material. Thus, volatile species diffusing to the surface can accumulate underneath the DAC layer, causing a loss of adhesion and subsequent delamination of this layer from the PR bulk film. The authors also report surface and electrical characterizations of the ultrathin DAC layer using optical microscopy, transmission electron microscopy, Raman and x-ray photoemission spectroscopy, and two-point probe techniques.

Principles for designing sputtering-based strategies for high-rate synthesis of dense and hard hydrogenated amorphous carbon thin films

In the present study we contribute to the understanding that is required for designing sputtering-based routes for high rate synthesis of hard and dense amorphous carbon (a-C) films. We compile and ...

Disorder and optical gaps in strained dense amorphous carbon and diamond nanocomposites

We employ empirical tight-binding simulations on strained tetrahedral amorphous carbon and diamond nanocomposite networks. For each applied strain, the optoelectronic properties are monitored through the absorption coefficient and the dielectric function. These lead to the optical gaps and are able to quantify the amount of disorder in the structures. We compare our results to those of unstrained nanostructured diamond and amorphous carbon (a-C) phases and link the degree of disorder in these materials to their structural details as a function of the external load. The atomic rearrangements and distortions imposed by the external strain in these structures are directly observable in their optoelectronic properties. We thoroughly discuss the interplay between increased external strain, structural and topological disorder, atomic rearrangements and their effect on the calculated optoelectronic properties.

Structure of the dense amorphous carbon phase synthesized in a mixture with diamond as a result of shock compression of carbon black

Soft X-ray emission and absorption spectroscopy methods were used in the present work to study experimentally the electronic structure of the dense amorphous carbon phase synthesized in a mixture with diamond as a result of shock compression of carbon black. The X-ray emission C Kα bands, representing the energy distribution of the C 2p-like states, have been recorded for the amorphous carbon phase and, for comparison, for diamond, lonsdaleite, graphite, and carbon black. For the amorphous carbon phase, diamond and lonsdaleite the spectra of quantum yields of the X-ray photoeffect in the area of the C K absorption edge have been recorded as well. Total and partial densities of states of the carbon H-6 and bct-4 phases, diamond and lonsdaleite have been calculated using the first-principles self-consistent full potential linearized augmented plane wave (FP-LAPW) method. Comparison on a common energy scale of the X-ray emission C Kα bands and C K quantum yields of the dense amorphous carbon phase as well as the theoretical curves of partial C 2p-like states of the carbon H-6 and bct-4 phases reveals that, the electronic structure of the amorphous carbon phase under consideration is well described by that of the carbon bct-4 phase. The experimental X-ray emission and absorption spectroscopy data indicate that the amorphous carbon phase studied is metallic, being in agreement with the results of the theoretical FP-LAPW calculations.

Diamond nucleation by energetic pure carbon bombardment

Deposition of 1000 eV pure carbon ions onto Si(001) held at 800 degrees C led to direct nucleation of diamond crystallites, as proven by high-resolution transmission electron microscopy and electron energy loss spectroscopy. Molecular dynamic simulations show that diamond nucleation in the absence of hydrogen can occur by precipitation of diamond clusters in a dense amorphous carbon matrix generated by subplantation. Once the diamond clusters are formed, they can grow by thermal annealing consuming carbon atoms from the amorphous matrix. The results are applicable to other materials as well.

Understanding of the Phase Transformation from Fullerite to Amorphous Carbon at the Microscopic Level

We study the shock-induced phase transformation from fullerite to a dense amorphous carbon phase by tight-binding molecular dynamics. For increasing hydrostatic pressures P, the C60 cages are found to polymerize at P<10 GPa, to break at P approximately 40 GPa, and to slowly collapse further at P>40 GPa. By contrast, in the presence of additional shear stresses, the cages are destroyed at much lower pressures (P<30 GPa). We explain this fact in terms of a continuum model, the snap-through instability of a spherical shell. Surprisingly, the relaxed high-density structures display no intermediate-range order.

Corrosion properties of ion beam modified fullerene thin films on iron substrates

The corrosion behavior of fullerene coated iron samples was investigated by using electrochemical techniques. Fullerene was deposited on iron substrates via thermal evaporation under high vacuum conditions. Subsequent sample preparation included heat treatment as well as Ar + ion irradiation under various conditions, which were carried out to modify the fullerene coatings. Changes in the structure of the films due to the treatment were analyzed by Raman spectroscopy. To reveal the samples’ corrosion behavior, cyclovoltammetric measurements were performed in a weakly acidic aqueous medium particularly with regard to possible corrosion protective properties. As the results show, Ar-ion treatment of the fullerene films leads to considerably increased corrosion resistance, which is due to formation of a dense amorphous carbon network. In contrast, heat treatment of the fullerene films increases the corrosion rates, presumably due to formation of iron carbide, which leads to galvanic corrosion.

In situ discovery of shock-induced graphite-diamond phase transition in gneisses from the Ries Crater, Germany

Reflected-light microscopy and fine-scale lasts microRaman spectroscopy of shocked garnet-cordierite-sillimanite gneisses in suevites of the Ries meteorite impact crater, Germany, led to the discovery of impact diamonds in their pristine graphite-diamond assemblages. Graphite-diamond textural relations permit a clear determination of the solid-state nature of the formation of diamond from graphite, which is estimated to have occurred at a peak-shock pressure between 30 and 40 GPa. Shock-induced transformations were promoted only in unkinked and undeformed graphite booklets at the graphite-garnet, graphite-sillimanite, or graphite-rutile interfaces, where the difference in shock impedance is very high. Reverberations of shock waves with short wavelengths similar to the grain sizes at the phase boundaries are probably important constraints for dynamic graphite-diamond phase transformation. Raman spectroscopic investigations of hard transparent carbon platelets intercalated between fine-grained diamond and deformed graphite revealed the platelets to be Raman inactive. The platelets are either dense amorphous carbon or an unknown dense crystalline carbon phase that is Raman inactive.

Structural modifications and electrical properties in ion-irradiated polyimide

The ion beam-induced modification of the electrical conductivity and the related structural features have been studied for thin films of poly-iso-quinazolindione (PIQ). The films, deposited onto specially designed test patterns, were irradiated by using 600 keV Ar + ions in the fluence range between 1 × 10 14 to 1 × 10 15 ions/cm 2. The beam-induced chemical and structural modifications have been investigated by using X-ray Photoelectron Spectroscopy (XPS) and Raman Spectroscopy, while the modification of the electrical properties was followed by performing a complete set of I/V measurements. In particular, we obtained the evidence of the occurrence of a true semiconducting state for samples irradiated at fluence around 6 × 10 14 ions/cm 2, which exhibited the characteristic I/V features of a Schottky diode. Samples irradiated at higher fluence showed a good conductivity, with a saturation value of 4 × 10 2 Ω cm. The XPS data demonstrate that the modification of the electrical properties is due to the progressive formation with increasing ion fluence of a dense amorphous carbon network, while Raman data suggest the existence of different structural features, respectively for the semiconducting and the conducting phases.

Tetrahedral amorphous carbon films prepared by magnetron sputtering and dc ion plating

Highly tetrahedral, dense amorphous carbon (ta-C) films have been deposited using rf sputtering of graphite by an unbalanced magnetron with intense dc Ar-ion plating at low temperatures (&amp;lt;70 °C). The ratio of the argon ion flux to neutral carbon flux Φi/Φn is about 5. The film density and compressive stress are found to pass through a maximum of 2.7 g/cm3 and 16 GPa, respectively, at an ion plating energy of about 100 eV. Experiments with higher ion flux ratios of Φi/Φn=10 show that it is possible to deposit carbon films with densities up to 3.1 g/cm3 and sp3 contents up to 87%. Deposition of ta-C in this experiment when the energetic species is Ar appears to require a minimum stress of 14 GPa to create significant sp3 bonding, which contrasts with the continuous increase in sp3 content with stress when the energetic species is C ions themselves. These results are used to discuss possible deposition mechanisms.

Electronic structure of dense amorphous carbon.

The tight-binding method is used to calculate the electronic densities of states of various models of a-C. Both models generated using Tersoff's potential and quantum mechanically derived forces show \ensuremath{\pi} and ${\mathrm{\ensuremath{\pi}}}^{\mathrm{*}}$ states in the \ensuremath{\sigma} gap in agreement with experimental data. The former models, however, have an additional large peak at the Fermi level, which is shown to derive mainly from misoriented ${\mathit{p}}_{\mathrm{\ensuremath{\pi}}}$ orbitals and dangling bonds. The models do not support the presence of large aromatic ring structures. We find that embedding the local \ensuremath{\pi} electron systems of small clusters in the rigid ${\mathit{sp}}^{3}$ network leads to reduced level splittings and is thus consistent with the observed small optical gaps \ensuremath{\le}2.5 eV.

Carbon films of amorphous and oriented graphitic structure from fullerene ion beam deposition

Energetic fullerene deposition has been performed with an intense, highly ionized beam in the kiloelectron-volt energy range to form carbon thin films. The deposited material is amorphic and essentially free of hydrogen that is bonded to carbon. The films show graphitic nanocrystallites embedded in amorphous carbon which are preferentially oriented with the c-axis parallel to the substrate surface. With increasing substrate temperature, the size of the crystallites increases together with a decrease in the percentage of sp[sup 3]-bonded carbon. There is evidence that the primary deposition product is mainly sp[sup 3]-bonded carbon which then relaxes to form the oriented, graphitic nanocrystallites. The method of fullerene ion beam deposition has been used to form a dense amorphous carbon film on a single-crystalline fullerite film, where the crystalline quality of the fullerite film is only insignificantly affected by the deposition process. 38 refs., 8 figs.