Sp2 Clusters Research Articles

PIP process greatly influencing the microstructure and electrical conductivity of polymer-derived SiCN ceramics

Precursor infiltration and pyrolysis (PIP) technique is widely used to densify porous ceramics. In this paper, PIP technique was used to fabricate dense polymer-derived SiCN ceramic bulks from porous SiCN preforms. Electrical conductivity and microstructure of dense SiCNs and the evolution with pyrolysis temperature were studied. Results were compared with that of porous SiCNs which were prepared by powder consolidation route without PIP process. Raman analysis showed that graphitization level of free-carbon in dense SiCNs is higher than that in porous SiCNs. As a consequence, the electrical conductivity of dense SiCNs was much higher than that of porous SiCNs. Free-carbon in porous SiCNs underwent a sp3-to-sp2 transition with increasing pyrolysis temperature throughout the temperature range of 1000 °C–1400 °C, whereas the sp3-to-sp2 transition was completed before 1200 °C in dense SiCNs. Main structural evolution of free-carbon in dense SiCNs was the growth of nanocrystalline graphite (nc-graphite) into ordered graphite when pyrolyzed at temperatures higher than 1200 °C. The growth activation energy of sp2 cluster size vs. pyrolysis temperature in the nc-graphite growth stage was found to be higher than that in the sp3-to-sp2 transition stage. The activation energy of conductivity increasing vs. pyrolysis temperature of the porous SiCNs was higher than that of the dense SiCNs, indicating that the electrical conductivity is more sensitive to the sp3-to-sp2 transition process than the sp2 cluster growth. This paper indicates that processing route can have great influence on the microstructure and electrical property of PDCs.

Failure of DLC Films Caused by Cyclic Sphere Indentation and Effects of Film Structures on Failure Mode

As a new strength evaluation method for hard coatings such as diamond-like carbon (DLC), the cyclic indentation test was proposed. Then, a test rig was fabricated and used for the evaluation of DLC. DLC and a Cr/C inclined interlayer for the improvement of the adhesion strength of DLC were prepared by the unbalanced magnetron (UBM) sputtering method on heat-treated and polished JIS SCM415 substrates. Si3N4 spheres having diameters of 10 mm were used for the cyclic indentation test. 10000 cycle indentations brought about DLC fracture and detachment in the indentation impression. Also ring cracks were observed around the impression. I(D)/I(G), which is the ratio of the D to G peak intensities, decreases towards the center of the impression. It was considered to indicate decreasing sp2 cluster size in DLC. On the other hand, compressive stress increased near the surface of DLC in the contact area, as indicated by the change in its G peak position. It seemed to be the cause of the fracture and detachment of DLC in the impression.

On the intriguing emission characteristics of size tunable carbon dots derived from functionalized multi-walled carbon nanotubes

Abstract Aqueous dispersions of carbon dots (C-dots) showing size tunable multi-colours in day light are obtained through surface functionalization of multi-walled carbon nanotubes (MWCNTs). The size of the C-dots varies with the functionalization temperature and smaller C-dots are found to be formed, at higher temperatures. The average size of the C-dots synthesized at temperatures of 100 °C, 80 °C and 60 °C lies in the range 6 nm–18 nm, based on transmission electron microscopy (TEM) images. The C-dots synthesized at these temperatures exhibit blue, green and golden yellow colours in day light without being excited by any external source. The structural characterizations using X-ray diffraction, Fourier transform infrared spectroscopy and Raman spectroscopy confirm the presence of a high density of functional groups on the surfaces of the C-dots, giving rise to high aqueous solubility. Compared to the Raman spectrum of the MWCNTs, the spectrum of the C-dots shows a significant increase in the intensity of the defect related D band with respect to that of the crystalline G-band, which indicates the presence of a high concentration of defects and the formation of smaller sp2 carbon clusters. The synthesized C-dots exhibit size dependent optical absorption and emission characteristics. The intriguing optical properties of these C-dots, which include the multi-colour emission at day light and the excitation wavelength and the pH dependent photoluminescence emission, can be mainly attributed to the formation of sp2 clusters of different sizes embedded in the sp3 carbon matrix.

Kinetics of graphitization of thin diamond-like carbon (DLC) films catalyzed by transition metal

In this paper, we have studied the kinetics of graphitization at 773 K of thin diamond-like carbon (DLC) films coated with minute amount of Ni metallic particles. DLC films are deposited at room temperature by pulsed laser deposition (PLD) on a transparent quartz substrate, and Ni is deposited on the surface of DLC using molecular beam epitaxy technique at room temperature. The ultra-high vacuum thermal (range 573–873 K with 60 min annealing treatments) and kinetic (range 30–3760 min at 773 K) behaviors of the deposited films are investigated. Surface and interface characterizations indicate that the growth of graphitic sp2 clusters starts at temperatures lower than 573 K. The kinetics of graphitization is recorded at 773 K. Thus, the continuous growth of graphitic clusters leads to a long-range kinetics. These clusters are responsible for the increase in the electrical conductivity and carrier mobility, reaching values of 6.103 Siemens/cm and 20 V/cm2 × s, respectively. This continuous change is not only explained by the nucleation and growth of graphitic clusters, but also by some reorientation of them alongside both the surface and the quartz substrate. The obtained results demonstrate that thermally post-treated catalytic metal/DLC films are promising materials for conductive electrodes and sensing applications.Prime novelty and projected interestKinetics of graphitization at 773 K of thin diamond-like carbon (DLC) films coated with minute amount of Ni metallic particles;High surface electrical conductivities, which are quite promising and competitive in comparison to literature data dealing with transparent electrodes.Correlation between conductivity models and Raman measurements in Catalytic Metal/DLC material.

Temperature Dependent Electrical Transport Properties of High Carrier Mobility Reduced Graphene Oxide Thin Film Devices

We report temperature dependent electrical transport properties of high mobility reduced graphene oxide (RGO) thin films fabricated by pulse laser deposition. The temperature dependent (5K–350K) four terminal electrical transport property measurements confirm variable range hopping and thermally activated transport mechanism of the charge carriers at low (5K–210K) and high temperature (210K–350K) regions, respectively. The calculated localization length, the density of states near the Fermi level ( $E_{F}$ ), hopping energy, and Arrhenius energy gap provide useful information to explain the excellent electrical properties of the RGO films. Hall mobility measurement confirms ${p}$ -type characteristics of the thin films. The charge carrier Hall mobility can be engineered by tuning the growth parameters, and the measured maximum mobility was 1596 cm2v−1s−1. The optimization of the improved electrical property is well supported by structural properties such as the defect density, average size of sp2 clusters and degree of reduction, which were investigated by Raman spectroscopy and X-ray diffraction analysis.

Continuous wave laser induced nonlinear optical response of nitrogen doped graphene oxide

This study reports the investigations on linear and nonlinear optical (NLO) properties of graphene oxide (GO) and nitrogen doped graphene oxide (N-GO) obtained by a simple pyrolysis method. The third order nonlinear optical properties of GO and N-GO were studied using continuous wave (CW) laser operating at 532 nm wavelength by employing the Z-scan technique. The nature of the curves suggests that the GO and N-GO exhibits reverse saturable absorption (RSA) and negative nonlinear refractive (NLR) index. Further, nitrogen incorporation has significantly enhanced the nonlinear absorption coefficient (β) values. Variation in the localized sp2 cluster size within the sp3 domains are accounted for the optical energy gap which is determined by the Tauc plot. The fluorescence intensity and their relative quantum yields are largely influenced by nitrogen in sp2 sites studied using Light emitting diode (LED) as a source at an excitation wavelength 452 nm. This cost effective N-GO material with giant linear and nonlinear optical properties may become significant in bio-imaging, biosensor, optical switching devices and NLO based applications.

Semiempirical study on the absorption spectra of the coronene-like molecular models of graphene quantum dots.

Polycyclic aromatic hydrocarbons of the general formula C6n2H6n (coronene family) were used as molecular models of graphene quantum dots (GQDs). Absorption spectra of the model compounds were calculated by ZINDO/S method. The S0 → S1 transition energy (E1) was found to decrease with n as E1 = 4.75 × n-0.633 eV. This transition is forbidden in symmetric compounds but 'switches on' upon symmetry breaking. The energy of the first bright optical peak (Ebr) was found to decrease with n as Ebr = 6.31 × n-0.6 eV. The data obtained corroborate the earlier finding that the size-independent optical properties of GQDs are determined by relatively small isolated sp2 clusters separated by sp3 (oxygen-contained) 'defects' rather than the whole (corrupted) graphene sheets; such nanoparticles actually are not quantum dots. GQDs of pure (without defects) graphene sheets with fully π-conjugated sp2 systems should exhibit size-dependent optical properties due to the quantum confinement effect.

Tailoring the oxygen content of graphene oxide by IR laser irradiation

Graphene oxide (GO) foils, synthesized with the Hummers’ method, were IR laser treated in air and under vacuum at some laser fluences (300 and 600 mJ/cm2 in air and 200 mJ/cm2 under vacuum). Depending on the laser fluence and the used environment, it was possible to control the oxidation or reduction degree of GO foils. The evolution of the structural and compositional characteristics of the as-prepared GO films was examined through different physical characterization techniques. In particular, scanning electron microscopy (SEM), surface profilometry, energy-dispersive X-ray spectroscopy (EDX), Rutherford backscattering spectrometry (RBS) and X-ray photoelectron spectroscopy (XPS) were used. A preliminary Raman analysis was also carried out on some GO foils. The results suggest the gradual restoration of sp2 clusters within the sp3 matrix under vacuum or in air at high laser fluences. In these conditions, the partial reestablishment of planarity and an increase in the D/G intensity ratio are also observed. In air, at low laser fluences, the IR laser irradiation causes GO films oxidation, increase in roughness and greater exfoliation availability. The useful deduced information about morphology and composition make the IR laser treatment a simple method for controlling the physical properties of GO film by tailoring the oxygen functional groups, and consequently, for the production of tailor-made highly conductive and defect-free graphene-like materials or GO composites with metals and organic molecules or GO doped with heteroatoms.

Graphene Oxide Quantum Dots Exfoliated From Carbon Fibers by Microwave Irradiation: Two Photoluminescence Centers and Self-Assembly Behavior.

Graphene oxide quantum dots (GOQDs) attract great attention for their unique properties and promising application potential. The difficulty in the formation of a confined structure, and the numerous and diverse oxygen-containing functional groups results in a low emission yield to GOQDs. Here, GOQDs with a size of about 5 nm, exfoliated from carbon fibers by microwave irradiation, are detected and analyzed. The exfoliated GOQDs are deeply oxidized and induce large numbers of epoxy groups and ether bonds, but only a small amount of carbonyl groups and hydroxyl groups. The subdomains of sp2 clusters, involving epoxy groups and ether bonds, are responsible for the two strong photoluminescence emissions of GOQDs under different excitation wavelengths. Moreover, GOQDs tend to self-assemble at the edges of their planes to form self-assembly films (SAFs) with the evaporation of water. SAFs can further assemble into different 3D patterns with unique microstructures such as sponge bulk, sponge ball, microsheet, sisal, and schistose coral, which are what applications such as supercapacitors, cells, catalysts, and electrochemical sensors need. This method for preparation of GOQDs is easy, quick, and environmentally friendly, and this work may open up new research interests about GOQDs.

Effect of nitrogen plasma afterglow on the surface charge effect resulted during XPS surface analysis of amorphous carbon nitride thin films

The aim of this paper is to investigate the relationship between the micro structure and the surface charge effect resulted during XPS surface analysis of amorphous carbon nitride thin films prepared by laser ablation method.The study results show that the charge effect coefficient (E) is not just a correction factor. We found that the changes in this coefficient value due to incorporation of nitrogen atoms into the carbon network are related to the spatial configurations of the sp2 bonded carbon atoms, order degree and sp2 clusters size. In addition, results show that the curve E vs. C(sp3)-N is a characteristic curve of the micro structure. This means that using this curve makes it easy to sorting the samples according to the micro structure (hexagonal rings or chains).

Graphene oxide as a water transporter promoting germination of plants in soil

Graphene oxide (GO) is a graphene derivative bearing various oxygen-containing functional groups attached to the basal plane and to the edges of the graphene lattice and hence has a unique structure in which numerous hydrophobic sp2 clusters are isolated within the hydrophilic sp3 C–O matrix. In this study, the hydrophilic nature and water-transporting properties of GO were exploited to promote germination and growth of plants. It was found that a low dose of GO significantly promoted the germination of spinach and chive in soil. The oxygen-containing functional groups of GO collected water, and the hydrophobic sp2 domains transported water to the seeds to accelerate the germination of plants. The strong interaction between GO and the surfaces of soil grains stabilized GO in the soil and prevented dissipation of GO. In addition, no GO was detected either on the surface or inside the cells of plants; this finding confirmed that GO was not phytotoxic. Therefore, GO may serve as a promising nontoxic additive to increase a plant yield.

Influence of post‐annealing on a diamondlike carbon film analyzed by Raman spectroscopy

The influence of a post‐annealing treatment on the chemical structure of a diamondlike carbon (DLC) film was clarified by Raman spectroscopy. The DLC films were synthesized by ionized deposition. The structures were elucidated via Raman analysis in conjunction with the sp2 cluster model. The as‐prepared DLC film consisted of a dielectric matrix including sp3 carbon, where sp2 clusters were floating. When the post‐annealing treatment commenced, especially between 450 and 600°C, carbon─hydrogen bonds were cleaved, and the hydrogen atoms were desorbed from the film, creating defects or dangling bonds. The defects were reactive in growing sp2 clusters that were strained with numerous defects because of the restricted degrees of freedom in the solid. As the post‐annealing temperature further increased, the clusters became dominant and the strain was gradually dissolved.

The assembly of a composite based on nano-sheet graphene oxide and montmorillonite

Graphene oxide (GO) nano-sheets were synthesized using a modified Hummers’ method from graphite powder. The Raman spectrum of GO displayed a D-band at 1359 cm−1 and a G-band at 1594 cm−1. The ID/IG value of GO was calculated to be 0.97, suggesting the formation of new sp2 clusters upon reduction. A method was designed to investigate the assembly of the GO/montmorillonite (MMT) composite. After the addition of GO, the typical peaks of montmorillonite in FT-IR spectra shifted, indicating the assembly between GO and MMT. The D-band and G-band reduced sharply in the GO/MMT composite. More importantly, the D-band (1344 cm−1) and G-band (1574 cm−1) shifted significantly and the ID/IG value of the GO/MMT composite was calculated to be 1.13, showing a change in the GO structure. In the addition of 0.04 wt% GO to MMT, the value of interlayer space (d) was up to 13.0 Å measured by XRD due to the insertion of GO into MMT. The evident increases in contents of carbon atoms (26.59%) and nitrogen atoms (3.44%) indicate that GO was successfully combined with MMT. The nano-pores and clay sheets were not observed in the SEM image of GO/clay, but obvious wrinkles, while flexible sheets were observed in the typical scanning electron microscopy images of GO. This further proves that GO was combining with clay. The TEM image shows that the GO nano-sheets were tiled on the surface of MMT sheets. This observation suggests that a stable assembly structure was formed between GO sheets and MMT sheets. The change in particle size of MMT with the addition of GO shows that interaction occurred between GO sheets and MMT sheets, which was further confirmed by the results of zeta potential. Adsorption and insertion were the main mechanisms to assemble GO and MMT.

Investigations on optical properties of ZnO decorated graphene oxide (ZnO@GO) and reduced graphene oxide (ZnO@r-GO)

The present investigation is based on the production of reduced graphene oxide (r-GO) from the graphene oxide using Hummer's (GO) and improved Hummer's methods (IGO) at elaborated conditions, named as GO and IGO, respectively hereafter. In contrast to previously known techniques, the presented process does not generate toxic gas. Meanwhile, the reduction temperature can be easily controlled. This approach provides a more significant amount of hydrophilic oxidized graphene as compared to GO and IGO with the use of additional KMnO4. Thus synthesized IGO was used to produce r-GO by thermal treatment. The morphological characteristics show that the obtained samples have a wrinkled paper-like morphology with severely folded lines. However, r-GO has double layers and multilayer at the edges. All the products (GO, IGO, and r-GO) have been decorated with ZnO nanoparticles (NPs). The XRD patterns of ZnO@GO composites have confirmed the characteristic peaks of wurtzite ZnO indicating the formation of ZnO nanoparticles onto the surface of graphene. The microscopic studies confirm the random growth/decoration of ZnO NPs on the surface of GO/IGO/r-GO sheets. However, in IGO and r-GO, loading/growth of ZnO NPs are less as compared to ZnO@GO. Overall structural studies indicate the oxidation of graphite and reduction of graphene oxide into r-GO sheets and ZnO decoration. Upon UV excitations, a bright blue emission has been exhibited by the GO that originates from geminate recombination of localized e-h pairs in sp2 clusters those primarily act as the luminescent centers. The noteworthy enhancement in the emission intensities after the incorporation of ZnO nanoparticles on the surface of GO is observed. The improved synthesis method and low-temperature reduction technique of GO may be essential for the large-scale production of r-GO as well as the construction of devices composed of ZnO@GO/IGO/r-GO.

Investigation on photoluminescence emission of (reduced) graphene oxide paper

In order to contrastively investigate optical properties of graphene oxide (GO) and reduced graphene oxide (rGO) paper, GO is prepared by improved Hummer method and controlled reduced using hydration hydrazine to obtain good dispersive rGO in organic solvent. Finally, GO and rGO paper are obtained by vacuum filtration method. Samples morphology and optical properties are analyzed by scanning electron microscopy (SEM) images, Raman spectra, absorbance spectra and photoluminescence (PL) spectra. Results indicate that there are large numbers of localized states in both GO and rGO paper, and optical gaps of two samples are 0.62 eV. In PL spectra of GO paper, we observe three emission peaks at 565, 578 and 608 nm, respectively whose intensity decreases evidently after reduced, which is due to the decrease of oxide functionalized groups and expansion of sp2 clusters. PL emission will gradually decrease during GO are reduced.

Lifetime modulation of graphene oxide film by laser direct writing for the fabrication of micropatterns

The strong, broad and tunable fluorescence emission of graphene oxide (GO) has shown the exciting optical applications in many areas, such as fluorescence imaging in living cell, high sensitive detection of heavy metal ions, and the fabrication of optoelectronic devices. However, the intrinsic heterogeneous fluorescence intensity resulting from the variability in the power density of excitation laser and the non-uniform thickness of GO film, hinders its further applications in the micropatterning, information storage and display technology, which requires homogeneous fluorescence emission. In contrast to the fluorescence intensity, the fluorescence lifetime of GO is determined by the intrinsic nature of chromophores, rather than the film thickness or excitation power density. Here we report that the fluorescence lifetime is homogeneous for GO film, which eliminates the anisotropic optical properties of GO film. By reducing the GO film through the irradiation from a 405 nm continuous-wave laser at a certain power density on a home-built scanning confocal microscope, we find that the lifetime can be precisely modulated by controlling the duration of laser irradiation. It is determined that the lifetime gradually decreases with the increase of duration. As reported in the previous researches, the GO fluorescence originates from the graphene-like confined sp2 clusters and sp3 domains consisting of oxygen-containing functional groups, where the lifetime of sp3 domain is about 1.4 ns, and that of sp2 domain is 0.14 ns. During the photoreduction, the long-lived sp3 domains will decrease or convert into short-lived sp2 domains, resulting in the decrease of lifetime. Hence, by controlling the reduction degree or the ratio of the two domains, the lifetime of GO film can be determined. More importantly, the lifetime distributions of the reduction areas are very narrow, leading to a relatively homogenous background. The precise manipulation of lifetime can be used to fabricate micropatterns with high contrast. Combining with laser direct writing with features of maskless, facile processing ability and high spatial resolution, many versatile micropatterns, such as quick response code, barcode, graphic, alphabet, and numbers can be readily created based on the modulation of fluorescence lifetime. By using three optimized durations of laser irradiation, three distributions with narrow widths are obtained. Based on this processing, the micropatterns with three colors are determined, which indicates that the multimode optical recording can be created on the GO film based on the modulation of fluorescence lifetime. Furthermore, the multilayer micropatterns are also created. The robust and versatile micropatterns with film-thickness and excitation-power-independent features show their promising applications in electronics, photonics, display technology and information storage.

Chemical structure and electrical characteristics of diamondlike carbon films

The relationship between the chemical structure and electrical characteristics of diamondlike carbon (DLC) films has been clarified. The DLC films were formed in atmospheres with different ratios of methane to argon under the photoemission-assisted Townsend discharge. The dependence of the dielectric constant of the films on methane concentration in the synthesis was arch-like with a maximum. The breakdown strength was constant irrespective of the synthesis atmosphere and was approximately the same for both electrical polarizations. Raman spectra of the films were deconvoluted into five active bands. Raman analysis in conjunction with the sp2 cluster model elucidated the DLC structure. The sp2 cluster model comprises sp2 clusters floating in a dielectric matrix sea. The sp2 clusters were rather aliphatic for films formed in low methane concentration. The clusters grew to become aromatic with increasing methane concentration. Defects or dangling bonds increased similarly, but they were terminated with hydrogen for films formed in high methane concentration. The hydrogen-terminated bonds occupied large amounts of space in the DLC films, causing internal strain. The dielectric constant of the whole DLC film was determined by the size of sp2 clusters, dielectric constant of the matrix sea material, and space volume induced by the hydrogen–carbon bonds. The breakdown strength was determined by the balance between the size of sp2 clusters and density of dangling bonds in the matrix sea. However, because the dependences of these factors on methane concentration were opposing, the breakdown strength was approximately constant irrespective of methane concentration.

Strong optical nonlinearity of CdS/nitrogen-doped reduced graphene oxide nanocomposites using Z-scan technique

CdS/nitrogen-doped reduced graphene oxide nanocomposites with different reduced graphene oxide contents were prepared using a hydrothermal method and characterized by XRD, FTIR, and SEM analyses. The successful formation of nanocomposites has been confirmed by X-ray powder diffraction and indicated that the average crystalline sizes decreased as the reduced graphene oxide contents increased. The SEM results approved the anchoring of CdS nanoparticles to nitrogen-doped graphene oxide surface. FTIR spectra confirmed the successful reduction of graphene oxide. CdS/nitrogen-doped reduced graphene oxide nanocomposites exhibited strong nonlinear absorption and refraction response under a nanosecond Nd:YAG laser at 532 nm which is due to the reversed saturable absorption and self-defocusing phenomena, respectively. Using the open-aperture Z-scan technique, the two-photon absorption coefficients, β, of nanocomposites are calculated of the order of 10−6 cm/W that is 10 times larger than β of graphene oxide. The nonlinear refractive indices, n2, for these nanocomposites are measured in the order of 10−12 cm2/W with a negative sign. The results showed that the nonlinear optical response of CdS/nitrogen-doped graphene oxide nanocomposites is third-order and susceptibility, χ(3), was calculated in the order of 10−8 esu and increased by adding reduced graphene oxide to nanocomposites, which can be related to the variation in contents of sp2 domains and sp2 clusters in the CdS/nitrogen-doped reduced graphene oxide due to the various reduced graphene oxide levels. The results suggest that CdS/nitrogen-doped graphene oxide may be promising candidates for nonlinear optical applications.

Cluster-Type Structure of Amorphous Smooth Hydrocarbon CD x Films (x ~ 0.5) from T-10 Tokamak

Structure of smooth hydrocarbon CD x films with a high deuterium ratio x ~ 0.5 redeposited from T-10 tokamak D-plasma discharges (NRC Kurchatov Institute, Moscow) has been studied. For the first time, small and wide angle X-ray scattering technique using synchrotron radiation and neutron diffraction have been employed. A fractal structure of CD x films is found to consist of mass-fractals with rough border, surface fractals (with rough surface), plane scatterers and linear chains forming a branched and highly cross-linked 3D carbon network. The found fractals, including sp2 clusters, are of typical size ~1.60 nm. They include a C13 fragment consisting of three interconnected aromatic rings forming a minimal fractal sp2 aggregate 9 × C13. These graphene-like sp2 clusters are interconnected and form a 3D lattice which can be alternatively interpreted as a highly defective graphene layer with a large concentration of vacancies. The unsaturated chemical bonds are filled with D, H atoms, linear sp2 C=C, C=O, and sp3 structural elements like C-C, C-H(D), C-D2,3, C-O, O-H, COOH, C x D(H) y found earlier from the infrared spectra of CD x films, which are binding linear elements of a carbon network. The amorphous structure of CD x films has been confirmed by the results of earlier fractal structure modeling, as well as by researches with X-ray photoelectron spectroscopy which allow finding a definite similarity with the electron structure of their model analogues — polymeric a-C:H and a-C:D films with a disordered carbon network consisting of atoms in sp3 + sp2 states.

Correlation study of microstructure and photoluminescence properties of a-C:H thin films deposited by plasma-enhanced chemical vapor deposition technique

Hydrogenated amorphous carbon (a-C:H) thin films are deposited by plasma-enhanced chemical vapor deposition (PE-CVD) technique with different frequency (RF) powers at a low substrate temperature (fixed at 250°C). In this work, the composition, the types of chemical banding, carbon hybridization structure and optical properties of the a-C:H thin films have been studied by FTIR, XPS, UV–Vis spectrum and PL spectrum testing techniques. The results show that the hydrogen content and carbon hybridization structure play an important role in the evolution of photoluminescence (PL) properties with increasing RF power. It is found that with the decrease of sp3/sp2 ratio, the peak energy for main PL emissions peaks (mode 2) showed a red shift. PL emission intensity of the a-C:H thin films is influenced by the hydrogen content and carbon sp2 cluster. The enlargement of sp2 rich clusters and the increase of the hydrogen content will lead to an enhancement of PL emission intensity, which is a consequence that the domain of sp2 rich clusters creates more radiative recombination centers with the increase of RF power. Furthermore, different full width at half maximum (FWHM) of PL emissions peaks can be attributed to band-to-band recombination in various size sp2 rich clusters.