Sp2 Clusters Research Articles

Facile Synthesis of Carbon Film with High Carrier Concentration Using Coal Tar and the Application in Joule Heating

The excellent electrical, thermal, and mechanical properties of carbon films make them promising for application in functional devices. As the byproduct of the coking industry with low cost and large quantity, coal tar can be an alternative feedstock for preparing high-performance carbon films. This work adopts the negative-pressure chemical vapor deposition (NPCVD) approach to prepare thin films on a glass substrate with tunable electrical properties and thickness using coal tar as carbon source. The sheet resistance of as-prepared carbon films can be adjusted within 3 orders of magnitude due to its high carrier density (up to 2.1 × 1022/cm3). As potential Joule heater, carbon film is characterized by fast heating rate (up to 12.5 °C/s) and high temperature (up to 300 °C at 40 V in 24 s). The thickness and sp2 cluster size affect the carrier density and carrier mobility of carbon film. These results verify the feasibility of using coal tar to synthesis high-performance carbon films suitable for electronic applications.

Designing a sustainable fluorescent targeting probe for superselective nucleus imaging

Specific targeting of cell organelles is indispensable for bioimaging and diagnostics but remains a challenge due to the limitation of highly selective fluorescent targeting probes. Herein, we successfully designed a molecular fusion route to controllably synthesize bright orange fluorescent graphene quantum dots (GQDs). The relative nitrogen doping level of the GQDs reached 18.88 at%. The GQDs exhibited long-wavelength excitation fluorescence and photoluminescence (PL) stability. Femtosecond transient absorption spectroscopy elucidated that the sp2 cluster and the surface state's synergistic effect contributed to the PL. Furthermore, the most alluring discovery was that the GQDs had good biocompatibility and an unprecedented targeting capability for superselective nucleus imaging without staining cell organelles. Molecular dynamics simulations revealed that once the GQDs were transported across the plasma membrane, the higher the N-doping ratio of the GQDs was, and the easier the GQDs penetrated the membrane. The reason may stem from the compatibility of –NH2 with lipid molecules, which is higher than that of –OH. Enriched nitrogen doping in GQDs is beneficial for crossing the cell membrane to the target nucleus. Our findings could provide scientific theory and a technical basis to design nucleus-targeting probes in the future.

Electrical Conduction Properties of Hydrogenated Amorphous Carbon Films with Different Structures.

Hydrogenated amorphous carbon (a-C:H) films have optical and electrical properties that vary widely depending on deposition conditions; however, the electrical conduction mechanism, which is dependent on the film structure, has not yet been fully revealed. To understand the relationship between the film structure and electrical conduction mechanism, three types of a-C:H films were prepared and their film structures and electrical properties were evaluated. The sp2/(sp2 + sp3) ratios were measured by a near-edge X-ray absorption fine structure technique. From the conductivity–temperature relationship, variable-range hopping (VRH) conduction was shown to be the dominant conduction mechanism at low temperatures, and the electrical conduction mechanism changed at a transition temperature from VRH conduction to thermally activated band conduction. On the basis of structural analyses, a model of the microstructure of a-C:H that consists of sp2 and sp3-bonded carbon clusters, hydrogen atoms and dangling bonds was built. Furthermore, it is explained how several electrical conduction parameters are affected by the carrier transportation path among the clusters.

Photoluminescence quenching of thermally treated waste-derived carbon dots for selective metal ion sensing

In the present study, carbon-dots (CDs) were derived from the thermal oxidation of an agricultural waste, bitter tea residue, to obtain different sp2/sp3 ratios and electronic structures for metal sensing. The CDs obtained from calcination at 700 °C exhibited the highest photoluminescence (PL) quantum yield (QY) of 11.8% among all the samples treated at different temperatures. These CDs had a high degree of graphitization, which resulted in a strong π-π* electron transition, and hence in a high QY. The strong photoluminescence of the CDs could be used to sense the metal ions Ag+, Sr2+, Fe2+, Fe3+, Co2+, Ni2+, Cu2+, and Sn2+ by monitoring their PL intensity at an excitation wavelength of 320 nm. The metals inhibited the PL intensity in the order Ag+ > Fe2+, Fe3+, Ni2+ > Sr2+, Co2+, Cu2+, Sn2+, which demonstrated that the CDs exhibited high metal ion detection capability and selectivity. The detection of Fe3+ using CDs was performed in the range of 10–100 ppm with a LOD (limit of detection) value of 0.380 ppm. Theoretical calculations demonstrated that Ag+, Sr2+, and Sn2+ induced charge transfer excitation and that Fe2+ and Ni2+ induced d-d transitions via complexation with the sp2 clusters. The charge transfer excitation and d-d transitions hindered the π-π* transition of the sp2 clusters, leading to a quenching effect. On the other hand, Li+, Na+, and K+ ions did not alter the π-π* transition of the sp2 clusters, resulting in a negligible quenching effect. In summary, the oxidation level and electronic structure of CDs derived from bitter tea residue could be tailored, and the CDs were shown to be a facile, sustainable, and eco-friendly material for metal sensing.

Development of High Performance C-Rich a-SiC Semiconductors with Narrow-Optical Gap By Control of Its Phase Structure

1. Introduction For the expansion of application fields (ex. tailor-made photocatalysts and high-efficiency laser) of semiconductor, novel semiconductor materials with variable optical gaps in the range from 1.2 to 3.0 eV is necessary to be developed. The promising candidate of these semiconductor is carbon-rich amorphous silicon-carbon alloy (C-rich a-SiC).[1] The optical gaps (Eog) of C-rich a-SiC can be changed from 1.2 to 2.7 eV by changing the Si/C ratio. Hetero-junction solar cell comprising N-doped C-rich a-SiC with Eog = 2.7 eV and p-type Si have been reported to show the function of photon to electron conversion.[2] However, solar cells comprising N-doped C-rich a-SiC with Eog from 1.2 to 1.7 eV showed extremely lower conversion efficiencies. Therefore, all of these C-rich a-SiC with Eog from 1.2 to 2.7 eV may not be applied to photoelectronic devices. The objective of this study is to realize C-rich a-SiC with lower Eog (frim 1.2 to 1.7 eV) that have higher semiconductor property and are able to be applied to photoelectronic devices. a-SiC has a multiphase structure in which amorphous Si-C network (Si:C = x:1-x), sp2-hybridized carbon cluster (sp 2 clusters), and silicon-like clusters (Si-cluster) coexist. It has been reported that excess sp2 cluster worked as a recombination center of photoexcited carries. It results in the degradation of efficiencies of photon to electron conversion. In order to realize C-rich a-SiC with lower Eog and higher photon to electron conversion efficiency, the synthesis method that can enhance of ratio of C/Si in amorphous Si-C network and suppress the formation of sp 2 clusters in C-rich a-SiC is necessary to be developed. In this study, CVD synthesis using high-frequency and lower power r. f. plasma was tried to be adopted to induce two types of structural change at C-rich a-SiC. High-frequency and lower power r. f. plasma can avoid the formation of sp 2 clusters in C-rich a-SiC. It results in the semiconductor materials with lower Eog and higher photon to electron conversion efficiency.2. Experimental Nitrogen-doped C-rich a-SiC thin films were synthesized by radio frequency plasma enhanced chemical vapor deposition system (RF-PeCVD) (13.56 MHz, SAMCO Co., Ltd. Model BPD-1) using tetramethylsilane, 1,1,3,3-tetrametyldisilazane and n-hexanes as source materials. C-rich a-SiC thin films were deposited on glass plate and boron-doped silicon (100) substrate (resistivity of 2 W cm) after an in-situ sputter cleaning with argon ions. Substrate temperature was kept at 160 degree C during CVD deposition. The frequency of r.f. power supply was set at 60 MHz and r.f power were changed from 5 to 26 W. The deposition time was 80 minutes. Transmission spectra of N-doped C-rich a-SiC deposited on glass substrates were examined by UV-Vis spectrophotometer (JASCO Co., Ltd. V-670). The performance of heterojunction solar cells was investigated by current-voltage (I-V) measurement (KEYSIGHT, B2901A) with simulated AM 1.5 sunlight (ASAHI SPECTRA, HAL-320). 3. Results and Discussion Eog estimated from transmission spectra (Fig. 1) of resulting N-doped C-rich a-SiC were found to be in the range from 1.40 to 2.96 eV, as shown in Table 1. Eog was able to be controlled to lower value (1.4 eV) thorough the change of the S/C ratios in C-rich a-SiC (by changing the volume ratio of n-hexane in source materials). All hetero-junction solar cells comprising N-doped C-rich a-SiC with various Eog and p-type Si crystal were confirmed to show the function of photon-to electron conversion (Table 1). The improvement of function of photon to electron conversion at C-rich a-SiC with lower Eog (ca. 1.4 eV) might be caused by the suppression of the recombination of photo-exited electrons and holes. In Figure 1, the absorption peak of sp 2 clusters was observed at 1.1 eV and was clearly separated from the absorption peak of amorphous Si-C network (observed at 2.6 eV). The intensity decrease and peak shift to lower energy side of the absorption of sp 2 clusters in Fig. 1 indicates the decrease of the amount of sp 2 clusters and the enlargement of size of sp 2 clusters. These structural change might cause the suppression of recombination of photo-carriers. The relation between Eog and open circuit voltage of solar cell was consistent with estimated value from electronic structure of C-rich a-SiC. It was summarized that the development of C-rich a-SiC semiconductors with lower Eog (in the range from 1.4 to 1.7 eV) that are able to be applied to photo-electronic devices was successfully achieved. 4.

Optical and electrical properties of N-DLC films deposited by atmospheric pressure DBD plasma: Effect of deposition time

In this study, at atmospheric pressure, a dielectric barrier discharge (DBD) plasma was used to deposit the diamond-like carbon film doped by nitrogen atoms (N-DLC) on a glass substrate. The nitrogen gas was sent to the n-hexane container by bubbling technique and finally, n-hexane/ nitrogen vapor was guided to the plasma gap. The composition and the structure of the deposited films were studied by FTIR and Raman spectra and EDX analysis. The effect of deposition time on optical, electrical, and mechanical properties of coated films was investigated. Changes in N-DLC film characteristics were assigned to the decreased hydrogen participation rate during the growth process and increasing both sp2 cluster size and sp2 content. The surface morphology of films was analyzed by FESEM and AFM assay.

Optical properties of reduced graphene oxide sheets

In this paper, we are investigating the Raman and photoluminescence properties of reduced graphene oxide sheets (rGO). Moreover, graphene oxide (GO) sheets are synthesized using Hummer’s method and further reduced into graphene sheets using D-galactose. Both GO and rGO are characterized by UV-vis spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Thermogravimetric (TGA) analysis. Raman analysis of rGO shows the restoration of graphitic domains in GO after re- duction. The photoluminescence of rGO showed emission in the UV region which is blue shifted along with luminescent quenching as compared to GO. This blue shift and quenching in photoluminescence arise due to the newly formed crystalline sp2 clusters in rGO which created percolation pathways between the sp2 clusters already present.

A thermal method for obtention of 2 to 3 reduced graphene oxide layers from graphene oxide

In this work, a thermal reduction method was developed to obtain reduced graphene oxide (rGO) with 2 or 3 layers from graphene oxide (GO). The GO X-ray diffraction (XRD) patterns presented diffraction peak at 2θ = 10°, which is related to (002) reflection. After heat treatment under nitrogen (N2(g)) atmosphere, this peak was shifted to 2θ = 25°, presenting an interlayer distance of 3.8 A, associated to GO reduction. BET analysis of modified GO samples identified an average pore diameter of 45.38 A and surface area of 23.06 m2/g. In the case of rGO1, rGO2 and rGO3 samples, they presented surface areas from 32.47 to 612.74 m2/g and an average pore diameter of 108.21–149.54 m2/g. Thermogravimetric analysis (TGA) indicated a higher mass loss between 150 and 230 °C. Raman spectra showed ID/IG ratios of rGO samples were higher than GO (1.36-GO; 1.45-rGO1, 1.87-rGO3) due to reducing GO and increasing sp2 clusters. XPS analysis revealed that the main carbon species in the samples were sp2-type bonds (14.99 at% for the GO and 47.85 at% for rGO3). The FTIR spectra of rGO1, rGO2 and rGO3 samples presented peaks at 3454.22 cm−1 (hydroxyl) and 1077.43 cm−1 (C–O).

Modulating the defects of graphene blocks by ball-milling for ultrahigh gravimetric and volumetric performance and fast sodium storage

Dense carbon materials with fast sodium storage performance are strongly desired for developing high-energy and high-power devices, but remain challenging because of the sluggish Na+ transport kinetics. Herein, we report that the defect density and sp2 cluster size of dense graphene blocks (DGB) can be elaborately modulated by ball-milling to achieve both high gravimetric and volumetric capacities and outstanding rate performance for Na+ storage. The loose graphene flakes are cut into small platelets with enriched defects and simultaneously densified by mechanical forces, leading to abundant active sites for Na+ storage, controlled sp2 size as conductive networks, and large interlayer spacing for fast Na+ transport. The DGB performs a novel capacitive Na+ storage with high capacities of 507 mAh g−1 and 397 mAh cm−3 at 50 ​mA ​g−1, and an ultrahigh rate of 181 mAh g−1 at 10 ​A ​g−1. It also shows a remarkable cycle stability due to the strongly-coupled layer structure. The comprehensive performance is superior to most of the reported carbons. The Na-ion capacitor delivers an ultrahigh energy density of 45 ​Wh kg−1 even at 14,205 ​W ​kg−1. Our work broadens the avenue for preparing advanced carbon materials for compact Na+ storage.

Corrosion resistance of amorphous carbon film in 3.5 wt% NaCl solution for marine application

In this work, we fabricated the tetrahedral amorphous carbon (ta-C) films with thicknesses of 20 nm, 40 nm and hydrogenated diamond-like carbon (DLC:H) film with thickness of 1.1 μm on 316 stainless steel substrate (316SS). The electrochemical corrosion performances of two typical amorphous carbon (a-C) films in 3.5 wt% NaCl solution were focused. Results revealed that both ultrathin ta-C films exhibited superior corrosion resistance than that of DLC:H film, because the high content of sp3 bonded carbon and low porosity in ta-C films significantly suppressed the diffusion of corrosion media. However, a thickness threshold beyond 20 nm was proposed to obtain the superior corrosion resistance for ta-C films due to the emerged pitting of 316SS. For DLC:H film, the deterioration of corrosion resistance could be attributed to the hydrogen-containing interface, the large sp2 clusters with high electron transport, and the higher porosity density in carbon matrix.

Transient Depolarization Spectroscopic Study on Electronic Structure and Fluorescence Origin of Graphene Oxide.

It is well-established that the electronic states of graphene oxide (GO) consist of sp2 clusters with different sizes and the surrounding sp3 matrix according to recent reports. However, addressing the excitation energy migration/redistribution among those electronic states in GO-based complex systems from spectroscopic experiments is still a challenge. Here, we combine the time-resolved absorption and fluorescence depolarization experiments to reveal the excitation energy migration processes in electronic states in GO. We demonstrate that, in sp3 domains of GO, there are charge-transfer states between sp3-hybridized carbon atoms and the oxygen-containing functional groups, and the energy redistribution and charge migration in sp3 matrix occur on the time scale from subpicoseconds to tens of picoseconds. In contrast, the electronic states of sp2 clusters in GO are rather localized and dominantly contribute to the excitation-wavelength-dependent red fluorescence of GO.

Modeling Electrical Switching Behavior of Carbon Resistive Memory

The amorphous carbon-based resistive memory has recently attained vast attention due to its non-volatility, fast switching speed, long data retention, and multilevel recording. However, the memory switching mechanism of amorphous carbon media remains mysterious and thus severely restricted its application prospect. To resolve this issue, a comprehensive three-dimensional model by simultaneously solving the current continuity equation, heat transfer equation, and mass concentration equation, is developed to model the physical conversions between sp2 and sp3 clusters. According to simulations, electric field was considered as the sole critical factor that determines the formation of conductive sp2 filament during the ‘SET’ process, whereas the ‘RESET’ process is mainly attributed to the induced high temperature that accelerates the growth of sp3 cluster and causes the rupture of the sp2 filament. It was additionally found that the sp2 filament was preferably formed inside the region having larger sp2 concentrations, and its rupture usually initiates from the filament center. The threshold voltages of carbon resistive memory for different thickness and different sp2 fractions were also calculated and exhibited good agreement with experimental measurements.

MEMS piezo-resistive force sensor based on DC sputtering deposited amorphous carbon films

The rapid growing demand of micro-electromechanical system (MEMS) sensors brings an urgent need for high performance and low cost sensitive materials. In this work, amorphous carbon (a-C) film was in-situ deposited on silicon substrate as strain sensitive component using economical direct current (DC) magnetron sputtering process and the a-C sensor was systematically designed, fabricated and tested. By adjusting the negative bias voltage in the range of 0–400 V, the gauge factor (GF) of the a-C film was adjusted within the range of 3.3–6.9. What’s more, the film’s sp2 cluster size played an important role in their piezo-resistive performance and conductivity, which illustrated the thick-film resistors (TFRs) theory. Additionally, CAFM results also supported the applying of TFRs theory in this work. Benefiting from the outstanding performance of a-C film, the MEMS force sensor, consisted a Wheatstone full-bridge with four a-C piezo-resistors, had a sensitivity of 9.8 μV/V/mN and non-linearity about 2.0% FS in the testing range of 0–210 mN, while it also showed a good repeatability. These investigations provided deeper insight into the piezo-resistive behavior of a-C film and contributed to the development of high performance and more economical sensitive materials for MEMS sensors.

On the adhesion and wear resistance of DLC films deposited on nitrile butadiene rubber: A Ti-C interlayer

To promote the adhesion strength between diamond-like carbon (DLC) films and nitrile butadiene rubber (NBR) substrates, titanium-doped carbon (Ti-C) films prepared by dual-target magnetron sputtering under varied substrate bias voltages were used as an interlayer on the rough NBR. The surface topography and structure of Ti-C films were investigated by atomic force microscopy (AFM) and Raman spectra. Raman analysis indicates that the increase of substrate bias voltage leads to an increase of the number or the size of sp2 clusters in the Ti-C interlayer. The adhesion strength and tribological properties of DLC films coated on NBR substrate were scrutinized by a scratch tester and a ball-on-disk tribometer, respectively. It was found that DLC film with a Ti-C interlayer at a certain bias voltage exhibited superior wear resistance with a low coefficient of friction (CoF) during the sliding of 6000 laps. No clear damages in the coatings were observed from the wear tracks. Besides, the scratch test also revealed a reliable adhesion when the interlayer was prepared at −150 V, as confirmed by a scratch crack width of ~50 μm as compared to that of the pure DLC film increasing to ~ 120 μm. Therefore, a Ti-C interlayer could significantly enhance the adhesion and wear resistance of DLC thin films deposited on NBR.

Influence of deposition temperature on the structure, optical and electrical properties of a-C films by DCMS

Amorphous carbon films have been considered as strongly promising semiconductor materials, due to their superior mechanical properties, specific bond structures, as well as facile deposition over large area with device integrated processes. To figure out the effect of sp2/sp3 ratio and size of sp2 clusters on opto-electrical behavior, we fabricated a series of H-free amorphous carbon (a-C) films by magnetron sputtering with a controllable deposition temperature from 30 to 400 °C. Result showed that all the films displayed a strong light absorption in the ultraviolet and visible regions, where the transmittance was less than 5% in the wavelength range of 200–750 nm, demonstrating the typical semiconductor behavior in temperature from 5 to 350 K, exhibiting a lower resistivity at higher temperature. In addition, their transmittance, optical bandgap, resistivity and activation energy showed similar tendency with increasing deposition temperature, which was discussed in terms of the evolution of sp2/sp3 ratio and sp2 cluster in the a-C films. These studies provide a new route to tune optical and electrical properties of a-C films for their opto-electrical applications.

Understanding the thermal evolution of defects in carbon-implanted ZnO single crystal

Defects and impurities play a major role in controlling the electrical and optical properties of semiconductor materials. Herein, hydrothermally grown ZnO single crystals have been implanted with carbon (C) dopants at room temperature and then annealed in argon atmosphere at various temperatures between 400 and 800 °C. The thermal evolution of C-related defects and their effects on the structural, optical and electrical properties of ZnO single crystals were systematically characterized and discussed. The results show ion implantation induces serious lattice disorder, and post-implantation annealing could promote the lattice renormalization, accompanied by an increase in crystal quality and average visible transmittance. Furthermore, it is found that the diffusion of octahedral carbon interstitial (Ci) along parallel to c-axis facilitates the growth of carbon sp2 clusters due to its low migration barrier during annealing, which energetically contribute to the decrease of the resistivity. Meanwhile, abundant Ci will be able to enter into VZn to form CZn or combine with lattice O to form (CO)O donor defects upon annealing, dominating the increase of electron carrier concentration and enhancing the anomalous Raman mode at 510-525 cm−1. These findings strengthen the fundamental understanding of the donor behavior of C impurities in ZnO.

Piezoresistive behavior of amorphous carbon films for high performance MEMS force sensors

In this study, microelectromechanical systems (MEMS) force sensors based on H-free amorphous carbon (a-C) films with controlled piezoresistive behavior were fabricated by a facile magnetron sputtering technique. By adjusting the substrate bias voltage from 0 V (floating state) to –350 V, the gauge factor (GF) of the a-C film was modulated in the range of 1.4–12.1. Interestingly, the GF showed a strong dependence on the sp2 content and the sp2 cluster size of the film, which was consistent with the theory of thick film resistors. In addition, the sensitivity of a-C based MEMS force sensors reached 80.7 μV/V/N in the force range of 0–1.16 N, with a nonlinearity of approximately 1.3% full scale and good repeatability in over 5000 test cycles.

Influence of particulate on surface energy and mechanical property of diamond-like carbon films synthesized by pulsed laser deposition

We report a detailed structural study of particulate laden diamond-like carbon (DLC) films synthesized by pulsed laser deposition (PLD) and its influence on surface energy and mechanical properties. A detailed analysis of various parameters such as: Dispersion (G), I(D)/I(G), T-peak position and FWHM(G) obtained from UV Raman spectroscopy for flat and particulate region of all DLC specimens indicate that the vibrational properties vary in both the regions depending on growth mechanism and modification of particulates in the plasma, respectively. The influence of these heterogeneous vibrational properties on surface energy and hardness is extensively studied by contact angle measurement and nano-indentation, respectively. DLC surface energy is observed to be influenced by sp2 cluster size, sp3 content, bond length and angle disorder of sp2 clusters found in the particulate regions, whereas indentation hardness (H) and modulus (E) of DLC films at 0.3 mN applied indentation load follows the evolution of sp3 content in the flat region.

Correlation study of structural, optical, and hydrophobicity properties of diamond-like carbon films prepared by an anode layer source

Diamond- like carbon (DLC) thin films were deposited using anode layer source system on glass substrate at substrate temperature ranging from room temperature (RT) to 375 °C. An anode layer source is a specific ion gun, which can be fed with carbon precursors like propane to deposit hard and highly defect-free hydrogenated DLC thin films. The morphological, structural, optical, and hydrophobicity properties of DLC thin films were characterized by atomic force microscopy (AFM), field emission scanning electron microscopy (FE-SEM), Raman spectroscopy, UV- Vis spectrophotometer, and water contact angle (CA) measurement. The surface roughness was found to increase with substrate temperature. Disorder (D) and graphite (G) peak positions have been observed to respectively move toward 1400 cm−1 and 1546 cm−1, with substrate temperature, indicating a graphitization of DLC thin films, which may probably be ascribed to the sp2 cluster sites growth. The full width at half maximum (FWHM) of G peak increased upon increasing the substrate temperature. From the FWHM values of G peak, the clusters became more ordered with increasing the substrate temperature of DLC thin films. The G peak position changed by increasing the substrate temperature that in turn led to increment in the ID/IG ratio from 0.71 to 0.85, and also the graphite cluster size increased from 1.07 to 1.24 nm. Optical transmittance in UV and near infrared regions of the prepared DLC thin films found to decrease with the increase of substrate temperature. The refractive index (n) and extinction coefficient (k) of the films have been found to increase with substrate temperature. The CA of DLC thin films slightly increased with increasing substrate temperature from 80° at RT to 94° at 300 °C.

Chemical structure analysis of diamond-like carbon by Raman spectroscopy

The chemical structure of diamond-like carbon (DLC) films was analyzed by Raman spectroscopy. The samples were DLC films synthesized by photoemission-assisted Townsend discharge (PATD). Group theory of the fundamental molecular structures suggested that the Raman spectrum consists of five bands with specific vibration modes, and the lineshape was represented by a modified Voigt-type formula. Analysis of the areas and positions of the bands resulted in the chemical structure of the DLC films with the sp2 cluster model. The model comprises conjugated and conductive clusters of sp2 carbons (sp2 clusters) floating in a non-conjugated and dielectric matrix of sp2 carbon, sp3 carbon, and hydrogen. The sp2 clusters were rather aliphatic for DLC films formed in low concentration of methane. The clusters grew to become aromatic with increasing methane concentration. The number of defects or dangling bonds increased similarly but were terminated with hydrogen for the films formed in a high methane concentration. The essential structure of DLC is the result of the development of random conjugation represented by the sp2 cluster model. We consider that DLC is a carbonaceous material in which conjugation increases slowly with time during the deposition process and which exhibits dielectric characteristics.