Raman spectroscopy investigation of the H content of heated hard amorphous carbon layers

We revisit here how Raman spectroscopy can be used to estimate the H content in hard hydrogenated amorphous carbon layers. The H content was varied from 2 at.% to 30 at.%, using heat treatments of a a-C:H, from room temperature to 1300 K and was determined independently using ion beam analysis. We examine the correlation of various Raman parameters and the consistency of their thermal evolution with thermo-desorption results. We identify a weak band at 860 cm^− 1 attributed to H bonded to C(sp²). We show that the H_D/H_G parameter (height ratio between the D and G bands) is quasi-linear in the full range of H content and can thus be used to estimate the H content. Conversely, we show that the m/H_G parameter (ratio between the photoluminescence background, m, and the height of the G band), often used to estimate the H content, should be used with care, first because it is sensitive to various photoluminescence quenching processes and second because it is not sensitive to H bonded to C(sp²).     

Fabrication of tin-filled carbon nanofibres by microwave plasma vapour deposition and their in situ heating observation by environmental transmission electron microscopy

Sn-filled carbon nanofibres (CNFs) are fabricated by microwave plasma chemical deposition. Scanning electron microscopy observations revealed the existence of a Sn island under the CNFs. The structure of the CNFs is investigated, and the behaviour of Sn in the internal space of CNFs is revealed by performing in situ heating observations by environmental transmission electron microscopy (ETEM). ETEM observations reveal that they have low-crystallized carbon wall and Sn occupies not only the CNF’s internal space but also its carbon wall. The Sn inside the CNF is completely covered by the carbon wall. Further, the in situ heating observations reveal that Sn within the internal space and the carbon wall of the CNFs diffused to the outside during heating. Moreover, it is found that higher membered carbon rings and defects in the graphite layer act as diffusion routes between disordered carbon layers.     

Deposition of hard amorphous hydrogenated carbon films by radiofrequency parallel-plate hollow-cathode plasmas

Hard amorphous hydrogenated carbon (a-C:H) films were deposited by plasma decomposition of CH₄ gas in a RF parallel-plate hollow-cathode system. The deposition system was built by placing a metallic plate in parallel to and in electrical contact with an usual RF-PECVD planar cathode. Self-bias versus RF power curves were used to make an initial characterization of plasma discharges in nitrogen gas atmospheres, for pressures between 10 and 100 mTorr. The strongly increased power consumption to obtain the same self-bias in the hollow-cathode system evidenced an increase in plasma density. The a-C:H films were deposited onto Si single crystalline substrates, in the − 50 to − 500 V self-bias range, at 5, 10 and 50 mTorr deposition pressures. The film deposition rate was found to be about four times than that usually observed for single-cathode RF-PECVD-deposited films, under methane atmosphere, at similar pressure and self-bias conditions. Characterization of film structure was carried out by Raman spectroscopy on films deposited at 10 and 50 mTorr pressures. Gaussian deconvolution of the Raman spectra in its D and G bands shows a continuous increase in the I_D/I_G integrated band intensity ratio upon self-bias increase, obeying the expected increasing behavior of the sp² carbon atom fraction. The peak position of the G band was found to increase up to − 300 V self-bias, showing a nearly constant behavior for higher self-bias absolute values. On the other hand, the G band width showed a nearly constant behavior within the entire self-bias range. Nanohardness measurements have shown that films deposited with self-bias greater than 300 V are as hard as films obtained by the usual PECVD techniques, showing a maximum hardness of about 18 GPa. Films were also found to develop high internal compressive stress. The stress dependence on self-bias showed a strong maximum at about − 200 V self-bias, with a maximum stress value of about 5 GPa.     

Substrate bias effect on structure of tetrahedral amorphous carbon films by Raman spectroscopy

Diamond-like carbon (DLC) films form a critical protective layer on magnetic hard disks and their reading heads. Now tetrahedral amorphous carbon films (ta–C) thickness of 2 nm are becoming the preferred means due to the highly sp³ content. In this paper, Raman spectra at visible and ultraviolet excitation of ta–C films have been studied as a function of substrate bias voltage. The spectra show that the sp³ content of 70 nm thick DLC films increases with higher substrate bias, while sp³ content of 2 nm ultra-thin films falls almost linearly with bias increment. And this is also consistent with the hardness measurement of 70 nm thick films. We proposed that substrate bias enhances mixing between the carbon films and either the Si films or Al₂O₃TiC substrate such that thin films contain less sp³ fraction. These mixing bonds are longer than C–C bonds, which inducing the hardness decreasing of ultra-thin DLC films with bias. But for 70 nm DLC, the effect of mixing layer can be negligible by compared to bias effect with higher carbon ion energy. So sp³ content will increase for thick films with substrate bias.     

Structures and morphologies of cast and plastically strained polyamide 6 films as evidenced by confocal Raman microspectroscopy and atomic force microscopy

Both confocal Raman microspectroscopy and atomic force microscopy (AFM) have been undertaken to study the crystalline and the morphological aspects of cast PA 6 films at a sub-microscopic scale. The percentages of the different crystalline structures present within PA 6 cast films, i.e. the monoclinic α, the pseudo-hexagonal β, and the monoclinic γ, have been measured by confocal Raman microspectroscopy. In cast films, the prevailing structure is the β one. AFM has been used to characterize the morphology of the PA 6 films. Simultaneously, the deformed state has been considered as well. Our main interest has been to follow the evolution of the percentage of each crystalline structure as a function of the plastic deformation mechanisms which are responsible of the yielding of PA 6 films: shear banding for temperatures T lower than 160 °C and formation of fibrils for      

Improved adhesion and tribological properties of fast-deposited hard graphite-like hydrogenated amorphous carbon films

Graphite-like hard hydrogenated amorphous carbon (a-C:H) was deposited using an Ar-C₂H₂ expanding thermal plasma chemical vapour deposition (ETP-CVD) process. The relatively high hardness of the fast deposited a-C:H material leads to high compressive stress resulting in poor adhesion between the carbon films and common substrates like silicon, glass and steel. A widespread solution to this problem is the use of an adhesion interlayer. Here we report on the changes in adhesion between the graphite-like a-C:H films and M2 steel substrates when different types of interlayers are used. Insignificant to very small improvements in adhesion were observed when using amorphous silicon oxide (a-SiO_x), amorphous organosilicon (a-SiC_xO_y:H_z) and amorphous hydrogenated silicon carbide (a-SiC_x:H_y) as adhesion layers. However, when sputtered Ti was used as an interlayer, the adhesion increased significantly. The dependence of the adhesive properties on the deposition temperature and interlayer thickness, as well as on the thickness of the a-C:H layer is presented and discussed. The low wear rates measured for the a-C:H/Ti/M2 stack suggest that these films are ideal for tribological applications.     

Dynamic behavior of carbon nanotube field emitters observed by in situ transmission electron microscopy

We examined dynamic behavior of field-emitting carbon nanotubes (CNTs) by in situ transmission electron microscopy (TEM). CNTs employed in the present study were multi-wall CNTs prepared by chemical vapor deposition, double-wall and single-wall CNTs produced by arc discharge. Orientation of CNTs, being random when no electric field was applied, were aligned parallel to the electric field and returned reversibly to their original direction when the field was turned off. In addition to this reversible behavior without serious structural damage in CNTs, sublimation and violent oscillation of CNTs were observed. When CNTs were bundled, branching of the bundle by electric static force was also observed.     

Raman microscopy of residual strains in carbon nanotube/epoxy composites

Carbon nanotube (CNT) strain variations with temperature, as measured with Raman microscopy, are reported for pristine and functionalized CNT/epoxy composites. The CNT strain is derived from the difference in frequencies of the CNT vibrational G⁺-mode in the composite and that of a relaxed CNT, and shown to serve as a measure of the local residual strains in the composites. The magnitudes of these strains vary with both CNT functionalization and CNT concentration. At room temperature and with the same local concentration of CNTs in the composite, the strains of oxidized and polyamidoamine-functionalized CNTs are found to be 2.5 times higher than that of the composite containing pristine CNTs. The higher residual strain of the composites loaded with functionalized CNTs reflects their better adhesion and integration in the polymer matrix. These findings are in accordance with the improved tensile properties measured for the functionalized CNT composites.     

High temperature activation and deactivation of single-walled carbon nanotube growth investigated by in situ Raman measurements

A decrease of nanotube yield is usually observed at high temperature. Here, we report on the origins of the activation and deactivation of the SWCNT growth in this high temperature range (between 700 °C and 850 °C) based on in situ Raman measurements. We observed that, at high temperature, carbon precursors such as ethanol and methane readily reduce metal oxide such as Co₃O₄. Once reduced, the size distribution of the catalyst particles quickly evolves at high temperature leading to a dramatic deactivation of the nanotube growth. An oxidizing pre-treatment has a stabilizing effect on the catalyst. In addition, we evidenced a threshold partial pressure of the carbon precursor to initiate the growth. This threshold partial pressure sets a second requirement for activating the nanotube growth in addition to the requirement of reducing the catalyst.     

Comparative study on isothermal kinetics of fullerol formation under conventional and microwave heating

The kinetic of fullerol formation in the presence of cetyl trimethyl ammonium bromide (CTAB) as a phase transfer catalyst both under conventional and microwave heating conditions was investigated. It was found that the activation energy (E_a) for the investigated processes under microwave heating was two to four times lower than for the same, conventionally heated process. Also, the pre-exponential factor (ln A) was 436–4200 times lower for the microwave heating process. The differences in the kinetic parameters (activation energy and pre-exponential factor) of the process under microwaves can be explained with the specific (non-thermal) activation of centres of the newly formed phase due to the electromagnetic energy absorption.     

Study of crystallization and isothermal thickening in polyethylene using SAXD,low frequency Raman spectroscopy and electron microscopy

This study contains a combined application of three different techniques for the study of polyethy-lenes crystallized from the melt under different circumstances, small-angle X-ray diffraction (SAXD), low frequency Raman spectroscopy to examine the longitudinal acoustic (LA) mode, and electron microscopy. In particular, the combination of SAXD and Raman methods enables the separation of the situation where there is only one lamellar structure which displays several orders in the SAXD pattern, from that where there is more than one type of lamellar thickness present. The superior power of the Raman method, which does not depend on the regularity in the lamellar stacking, becomes apparent. The multiplicity of the lamellar population could be associated with lamellae formed isothermally at the preselected crystallization temperature and with lamellae which originated from material which has remained uncrystallized at this temperature and formed subsequently with smaller lamellar thickness during cooling of the sample. The existence of the corresponding double lamellar population could be made directly visible using electron microscopy on freeze-cut and stained sections. The thinner lamellae in the double population could be extracted by solvents, removing the corresponding SAXD and Raman peaks, and leaving blank image areas in place of the thin lamellae in the electron-micrographs. These extracted thinner lamellae correspond to lower molecular weights as assessed by g.p.c. Thus molecular segregation during crystallization is involved. Furthermore the segregated texture units and their arrangement within the full morphology could now be identified. Pronounced changes in lamellar thickness with crystallization time were observed throughout and were associated in the early stages of crystallization with molecular fractionation and in the later stages with thickening of lamellae already present. An unexpected interrelation between nucleation density and the final lamellar thickness through the agency of isothermal lamellar thickening has been established. Examples are quoted which are contrary to the expected trend of lamellar thickness with crystallization temperature, but which are interpretable nevertheless in the light of the effect of isothermal lamellar thickening. The potential significance of all these findings and of this kind of approach for the characterization of crystalline bulk polymers is made throughout.     

Synthesis of mesoporous hollow carbon microcages by combining hard and soft template method for high performance supercapacitors

Carbon materials have been widely used in electrochemistry filed such as supercapacitors because of the good conductivity, abundant sources and low prices. Further improving the electrochemical performance of carbon materials still attracts the interest of researchers. In this work, nitrogen-doped mesoporous hollow carbon microcages (N-MHCC) are successfully prepared by combining hard and soft template method. This hierarchically porous structure (meso- & micro-pore) of N-MHCC provides a large number of active centers, sufficient space and reaction interface, promoting the rapid diffusion of electron and electrolyte ions transport. By comparing the electrochemical performance of nitrogen-doped hollow carbon microcages (N-HCC) and N-MHCC, it can be calculated that N-MHCC shows high specific capacitance of 210.66 F g^?1 at 0.5 A g^?1 while N-HCC only shows 132.6 F g^?1. The cycle retention rate of N-MHCC is as high as 96.92 % at 5 A g^?1 after 4000 cycles. Furthermore, the simple preparation method and attractive performance make N-MHCC a promising candidate for high performance supercapacitors.     

软硬链段添加碳纳米管/炭黑对聚氨酯纳米纤维性能的影响

以聚丙二醇(PPG 2000)和二苯基甲烷二异氰酸酯(MDI-50)为原料,利用预聚法合成聚氨酯(PU)。在预聚过程中加入碳纳米管(CNT),合成碳纳米管/聚氨酯(CNT/PU);在扩链过程中加入炭黑(CB),合成炭黑/聚氨酯(CB/PU)。结合静电纺丝制备CNT/PU、CB/PU纳米纤维。采用SEM、TEM、FTIR、DSC、XRD对两种纳米纤维的结构进行了表征,对纳米纤维膜的电性能、力学性能及传感性能进行了测试。结果表明,由于CNT/CB在聚氨酯软硬链段的选择性嵌入,聚氨酯纳米纤维的微相分离程度分别增大/减小;相对纯PU纳米纤维膜,CNT/PU纳米纤维膜的断裂伸长率降低,CB/PU纳米纤维膜的断裂强度提高。导电填料(CNT、CB)的含量以聚氨酯的质量(PPG 2000、MDI-50和无水乙二胺的总质量)为基准,当CNT的含量为6.0%时,CNT/PU纤维膜的电阻率为1.22Ω·m,传感系数GF为45.64(拉伸应变在80%～120%);当CB含量为12.5%时,CB/PU纤维膜的电阻率为0.14Ω·m,传感系数GF为167.43(拉伸应变在80%～120%)。     

Linear carbon chains encapsulated in multiwall carbon nanotubes: Resonance Raman spectroscopy and transmission electron microscopy studies

In this paper we report the characterization of linear carbon chains encapsulated in multiwalled carbon nanotubes by using Raman spectroscopy and transmission electron microscopy. The chains are characterized by strong vibrational peaks around 1850 cm⁻¹ and both the frequency and intensity of these peaks were found to be dependent on laser excitation energy. Furthermore, resonance Raman spectroscopy was used for constructing the resonance window of the linear carbon chains. The Raman spectroscopy data showed that long chains have lower highest occupied molecular orbital–lowest unoccupied molecular orbital energy gaps and weaker carbon–carbon bonds. Besides the spectroscopy evidence for the linear carbon chain, we used scanning transmission electron microscopy/electron energy loss spectroscopy analysis of the nanotube cross section to unambiguously show the existence of a 1D structure present within the innermost carbon nanotube with an unprecedented clarity compared to previous reports on this kind of system.     

Influence of heating and laser irradiation on the Raman D band in single-wall carbon nanotubes

Fine structure of the Raman D band in single-wall carbon nanotubes (SWNTs) was investigated by heating and laser irradiation. It is shown that the D band is composed of three components at ～ 1313, 1340, and 1355 cm^? 1, denoted by D₁, D₂, and D₃, respectively. The D₁ and D₂ intensities significantly increase with laser irradiation in air and vacuum, respectively. The D₃ intensity drastically increases with heating in air. From these results, it is suggested that the fine structure of the D band is attributed to different kinds of defects introduced in SWNTs.     

Nano-patterning of through-film conductivity in anisotropic amorphous carbon induced using conductive atomic force microscopy

Direct nanometer patterning of through-film electrical conductivity on carbon films is crucial in the development of carbon materials for nanotechnology. Typically, nanometer topographical surface modification can be created using scanning probe microscopy techniques producing surface artifacts which do not extend through the film. Here, we demonstrate the direct nano-patterning of through-film conductivity on highly oriented anisotropic carbon films by applying single voltage pulses (6 V) to a conductive atomic force tip in contact with the film. The thus induced conductivity is at least four orders of magnitude higher than that of the nominal area.     

In situ quantification of minor compounds in pressurized carbon dioxide using Raman spectroscopy

In this work, we present an optical approach that allows quantification of compounds dissolved down to a sub 10 ppm (molar) level in supercritical carbon dioxide (scCO₂) by in situ Raman spectroscopy. The quantification strategy is based on the consideration of Raman signal intensity ratios and of weak Raman signals of CO₂. The first measure makes the quantification strategy independent of the refractive index, laser power fluctuations and changes in alignment, and is thus robust against variations of the operation conditions. The second measure circumvents the classical challenges related to the dynamic range limit of detectors, which is inherent to the quantification of minor compounds. Example solubilities of the wax alkyl ketene dimer (AKD) in CO₂ were quantified at 333 K and a range of pressures from 10 MPa to 18 MPa. The solubilities are between 0.1 mg g⁻¹ and 1.5 mg g⁻¹ corresponding to ∼9 ppm and ∼132 ppm (molar), respectively.     

Transmission electron microscopy study of the microstructure of carbon/carbon composites reinforced with in situ grown carbon nanofibers

Introduction of carbon nanofibers (CNFs) into carbon/carbon (C/C) composites is an effective method to improve the mechanical properties of C/C composites. In situ grown CNFs reinforced C/C composites as well as conventional C/C composites without CNFs were fabricated by chemical vapor infiltration. Transmission electron microscopy investigations indicate that the entangled CNFs (30–120 nm) formed interlocking networks on the surface of carbon fibers (CFs). Moreover, a thin high-textured (HT) pyrocarbon (PyC) layer (～20 nm) was deposited on the surface of CFs during the growth of CNFs. We find the microstructure of C/C composites depends strongly on the local distribution density (LDD) of CNFs. In regions of low CNF LDD, a triple-layer structure was formed. The inner layer (attached to CF) is HT PyC (～20 nm), the middle layer (150–200 nm) is composed of HT PyC coated CNFs (HT/CNFs) and medium-textured PyC, and the outmost layer (several microns) is composed of HT/CNFs and micropores. In regions of high CNF LDD, a double-layer structure was formed. The inner layer is HT PyC (～20 nm), and the outer layer is composed of HT/CNFs, isotropic PyC and nanopores. However, only medium-textured PyC and micropores were found in the matrix of the conventional C/C composites.     

Tribological properties of hydrogenated amorphous carbon under dry and lubricated conditions

Diamond-like carbon (DLC) coatings are considered as potential surface coatings for many engineering applications including gears and engine parts. It is important to know whether conventional extreme pressure (EP) additive-containing oil can work with DLCs and provide tribological performance as effective as they provide on steel surfaces. This study examines the friction and wear properties of hydrogenated amorphous carbon (a-C:H) under dry, base oil (BO)- and fully formulated gear oil (FF)-lubricated conditions. Pronounced graphitisation occurs due to rubbing under dry condition and provides low friction but promotes wear. However, BO and FF greatly suppress graphitisation and wear. Tribofilms formed from FF in a-C:H/steel contact appear to exhibit superior antiwear properties than those formed in steel/steel contact.     

Annealing-induced structural changes of carbon onions: High-resolution transmission electron microscopy and Raman studies

The first- and second-order Raman spectra of carbon nano-onions (CNOs), produced via annealing of detonation nanodiamonds with a mean grain size of ∼5 nm in the argon ambience at the maximal temperature of annealing process (T_MAX) varying from 1500 to 2150 °C, are analyzed together with the high-resolution transmission electron microscopy (HRTEM) images. The combined analysis provides a deep insight into the annealing-induced atomic-scale structural modifications of the CNO nanoparticles. The Raman and HRTEM data unambiguously demonstrate the reduction in the number of defects in the CNO structure, as well as indicate the conversion from the diamond sp³-bonded carbon phase to the sp²-bonded carbon phase with increasing T_MAX and its almost full completion for T_MAX = 1600 °C.