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.     

Raman spectroscopy of carbon nanohoops

We present the Raman spectra for [n]Cycloparaphenylenes ([n]CPPs) of different sizes. A plethora of Raman modes are observed in these spectra, including modes that are analogous to those of the carbon nanotubes (CNTs) such as G-band, as well as Raman peaks that are unique for carbon nanohoops. In addition, we have calculated the theoretical Raman spectra of [n]CPPs for n = 4–20 using density functional theory (DFT), which are then compared to the experimental results for the assignment of different modes. The Raman peak positions are seen to be dependent on the size of the nanohoop from both the experimental and the calculated results.     

Long-term H-release of hard and intermediate between hard and soft amorphous carbon evidenced by in situ Raman microscopy under isothermal heating

We study the kinetics of the H release from plasma-deposited hydrogenated amorphous carbon films under isothermal heating at 450, 500 and 600 °C for long times up to several days using in situ Raman microscopy. Four Raman parameters are analyzed. They allow the identification of different processes such as the carbon network reorganization and the H release from sp³ or sp² carbon atoms and the corresponding timescales. Carbon reorganization with aromatization and loss of sp³ hybridization occurs first in 100 min at 500 °C. The final organization is similar at all investigated temperatures. Full H release from sp³ carbon occurs on a longer timescale of about 10 h while H release from sp² carbon atoms is only partial, even after several days. All these processes occur more rapidly with higher initial H content, in agreement with what is known about the stability of these types of films. A quantitative analysis of these kinetics studies gives valuable information about the microscopic processes at the origin of the H release through the determination of activation energies.     

Resonant Raman studies of tetrahedral amorphous carbon films

Resonant Raman scattering has been used to study the tetrahedral amorphous carbon films deposited by the filtered cathodic vacuum arc technique. The excitation wavelengths were 244, 488, 514 and 633 nm, corresponding to photon energies of 5.08, 2.54, 2.41 and 1.96 eV, respectively. In the visible Raman spectra only vibrational modes of sp²-bonded carbon (G and D peaks) are observed, while a wide peak (called the T peak) can be observed at approximately 1100 cm⁻¹ by UV-Raman spectra which is associated with the vibrational mode of sp³-bonded carbon. Both the position and the width of the G peak decrease almost linearly with increasing excitation wavelength, which is interpreted in terms of the selective ππ* resonant Raman scattering of sp²-bonded carbon clusters with various sizes. The G peak position in the UV-Raman spectra, the T peak position and the intensity ratios of I_D/I_G and I_T/I_G all exhibit maximum or minimum values at the carbon ion energy of 100 eV. The changes of these spectral parameters are discussed and correlated with the sp³ fraction of carbon atoms in the films.     

Raman investigation of single oxidized carbon nanotubes

The oxidation process of single-walled carbon nanotubes via nitric acid treatment was followed by IR-, UV-Vis-NIR, and single bundle Raman spectroscopy. The introduction of functional, oxygen-containing groups is revealed by an additional absorption band at 1725 cm⁻¹, characteristic of carbonyl stretch vibrations. No significant shift of the optical absorption bands could be detected after oxidation. The combination of atomic force microscopy and confocal scanning resonance-enhanced Raman microscopy was used to investigate thin bundles and, eventually, individual nanotubes in detail. These experiments enabled determination of the dependence of the Raman intensity of the G-line (around 1590 cm⁻¹) on the bundle height for both non-oxidized and oxidized tubes. The Raman cross-section of the oxidized tubes was found to be reduced by a factor of ˜4, compared to the pristine tubes. This observation is ascribed to all tubes within a bundle that are oxidized to the same degree.     

Low frequency excitations in amorphous polycarbonate studied by Raman spectroscopy

Low frequency excitations in amorphous polycarbonate-bisphenol A have been studied by Iaser Raman spectroscopy in the frequency region $\text{5}\text{cm}{}^{\mathrm{?1}}\text{< Δ}\text{v}\text{?}\text{< 200}\text{cm}{}^{\mathrm{?1}}$. Depolarized spectra were recorded at temperatures between ambient and 85 K, the reduced intensity spectra show no temperature dependence. Two bands are resolved in the reduced intensity spectrum: a strong band around 96 cm^?1 and a weak band around 45 cm^?1; these are attributed to the effects of longitudinal and transverse phonon waves originating in backbone motion. The low frequency Raman intensities provide information on the density of state g(ω) from which the specific heat, C_v, of polycarbonate has been calculated. This is found to vary with temperature in a manner similar to the calorimetrically measured C_v in the low temperature range ～1 K < T < 4 K.     

碳纤维微观结构表征： Raman光谱

Raman光谱作为材料微观结构表征的重要方法,其对碳材料的结构具有相当的敏感性,在0～3300cm^-1的波数范围内都有相当显著的谱峰响应。其中,理想石墨晶格面内CC键伸缩振动的G线和由无序结构引起的D线是认识碳材料纳米尺度结构特征的关键切入点。基于上述特征指标可以获得碳结构微观应力、晶态结构、石墨化度及结构不均匀性等一系列结构特征,是研究碳纤维微观结构及其形成、演变过程的重要技术手段。近年来,随着以“Mapping”技术为代表的系列新技术的成功应用,针对碳纤维微观结构的Raman光谱应用技术出现了一系列新的进展。本文以Raman光谱的碳纤维微观、介观层面的应用技术为切入点,综述了近年来Raman光谱在碳纤维微观应力/应变、晶态结构、石墨化度及结构不均匀性等方面的进展情况。     

Characterization of the anisotropy of pyrolytic carbon by Raman spectroscopy

The anisotropy of pyrolytic carbon is one of the most important properties for the development of TRISO (tristructural isotropic) coated fuel particles. Polarized Raman spectroscopy has been used to measure the anisotropy of pyrolytic carbon coatings ranging from high (highly oriented pyrolytic graphite) to low texture samples. Values obtained were correlated to the texture measured from the orientation angle (OA) of selected area electron diffraction patterns. The nodal behavior of the Raman intensity signals in graphite has been used to observe changes in the first order bands when the laser and scattered signal were polarized parallel or perpendicular to the graphene layers. Texture observed by Raman spectroscopy was largely affected by the orientation of the graphene layers inside growth features. Discrepancies in the values of texture measured between Raman spectroscopy and the OA was due to the difference of spatial resolution of each technique, 0.2 μm for OA and 2 μm for Raman spectroscopy. Both polarized Raman spectroscopy and nano-indentation were used to characterize the inner pyrolytic carbon in coated fuel particles before and after SiC deposition at 1500 °C.     

Raman spectroscopy on isolated single wall carbon nanotubes

A review is presented on the resonance Raman spectra from one isolated single wall carbon nanotube. The reasons why it is possible to observe the spectrum from only one nanotube are given and the important structural information that is provided by single nanotube spectroscopy is discussed. Emphasis is given to the new physics revealed by the various phonon features found in the single nanotube spectra and their connection to spectra observed for single wall nanotube bundles. The implications of this work on single wall carbon nanotube research generally are also indicated.     

Resonant Raman spectroscopy on enriched C carbon nanotubes

Isotopically enriched single-wall carbon nanotubes with different ¹³C concentrations were investigated by resonant Raman spectroscopy. Linear reductions of the Raman frequencies with an increase of ¹³C concentration are observed for the different nanotube Raman modes, and the effect of the reduced mass variation of the isotope mixture on the phonon frequencies is described through a simple harmonic oscillator model. In addition to the frequency dependence, the Raman linewidths as a function of the ¹³C concentration were also investigated and an expression describing this is presented. We observe an increase in the G band linewidth, as the ¹³C:¹²C ratio approaches unity. Measurements with different excitation energies were performed and the frequency dispersions of the D and G′ bands with laser energy were observed to be the same for ¹²C and ¹³C nanotubes, suggesting no changes in the electronic structure after isotope enrichment. Through analysis of the radial breathing modes in the Raman spectra obtained with different excitation energies, a relation between these modes frequency and the ¹³C nanotubes diameter was also established.     

Quantitative assessment of carbon nanotube dispersions by Raman spectroscopy

Aqueous dispersions of single wall carbon nanotubes (C-SWNTs), prepared using different dispersing agents, have been analysed by Raman spectroscopy. Normalising the spectra with respect to the area of the water O-H stretching transition eliminates the effects of photon scattering and absorption on the way through the dispersion, and the dispersions can be assessed quantitatively by comparison of the areas of the carbon nanotube G-band. The normalised G-band areas show linear concentration dependence according to Beer’s law. The influences of different dispersing agents and excitation wavelengths are discussed and the results are compared to the commonly used UV-Visible spectroscopic analysis. The method presented here is semi-quantitative and it is proposed to use the most effective dispersing agent found in this study, sodium dodecylbenzene sulfonate (SDBS), as a benchmark for future dispersion experiments.     

Characterization of doped single-wall carbon nanotubes by Raman spectroscopy

Single-wall carbon nanotubes were grown by thermal chemical vapor deposition using either boron- or nitrogen-containing feedstocks or both. Carrier doping was evidenced by hardenings of the G band in Raman spectra, and the estimated carrier concentration reached ∼0.4%. In the G′ and D band spectra, a doping-induced component was observed at the high- or low-energy side of the original one. However, the appearance of the new component did not always coincide with the carrier doping. The doped SWCNTs often show radial breathing mode peaks in the off-resonance region, indicating a defect-induced modification of absorption spectrum.     

Raman spectroscopy measurements of DC-magnetron sputtered carbon nitride (a-C:N) thin films for magnetic hard disk coatings

As a protective coating for hard disks in magnetic storage applications, amorphous carbon nitride (a-C:N) thin films have proved superior to DLC (diamond-like carbon) a-C:H films in terms of durability, wear-resistance and adhesion properties. In this study, we present Raman spectroscopy investigations of a-C:N films which were produced by DC-magnetron sputtering systems. The layers were deposited with a variable nitrogen content, thickness and substrate temperature. Raman measurements were carried out with two different excitation lasers at wavelengths of 488 and 532 nm. The spectra show that besides the typical carbon D- and G-bands, two other characteristic bands are present at approximately 690 and 1090 cm⁻¹. The meaning and identification of these bands is not clear in the literature. In order to obtain more information, the films were also characterized by various analytical techniques, e.g. time-of-flight secondary ion mass spectrometry (ToF-SIMS), Auger electron spectroscopy (AES), ellipsometry, and n+k optical measurements. The Raman G-band position shows a systematic shift with the varying nitrogen content of the films. A comparison of layer thickness and the total area of D-, G- and 1090 cm⁻¹ bands also shows a significant correlation. The results offer Raman spectroscopy as a possible monitoring tool for carbon nitride coatings in the production of magnetic hard disk drives.     

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.     

Evaluation of thermal conductivity of the constituent layers in TRISO particles using Raman spectroscopy

The thermal conductivity of individual layer in the tristructural-isotropic fuel particle was evaluated using Raman spectroscopy. In this method, laser acted simultaneously as an excitation source and a heating source. A three-dimensional point-heating model was developed to estimate the local temperature rise in the probing volume of the laser. The thermal conductivity can be evaluated based on the dependences of the Raman peak position on the temperature and laser power. The calculated thermal conductivities were 8.9 ± 0.2 W/m °C, 13.9 ± 1.5 W/m °C and 11.9 ± 0.9 W/m °C for the buffer, the inner and the outer PyC layers, respectively. Contrastly, the thermal conductivity of the SiC layer was 4.1 W/m °C, which is much lower than the reference value, e.g. 168 W/m °C reported by López-Honorato et al. (J. Nucl. Mater. 378(1) 35–39, 2008). The uncertainty of employing Raman spectroscopy to determine thermal conducitvity was discussed.     

Structural investigation of nanocrystalline diamond/amorphous carbon composite films

Nanocrystalline diamond/amorphous carbon (NCD/a-C) composite films have been prepared by microwave plasma chemical vapor deposition (MWCVD) from methane/nitrogen mixtures. The complex nature of the coatings required the application of a variety of complementary analytical techniques in order to elucidate their structure. The crystallinity of the samples was studied by selected-area electron diffraction (SAED). The diffraction patterns revealed the presence of diamond crystallites within the films. From the images taken by transmission electron microscopy (TEM) the crystallite size was determined to be on the order of 3–5 nm. The results were confirmed by X-ray diffraction (XRD) measurements exhibiting broad (111) and (220) peaks of diamond from which the average size of the crystallites was calculated. The grain boundary width is 1–1.5 nm as observed by TEM images which corresponds to a matrix volume fraction of about 40–50%. This correlates very well with the crystalline phase content of about 50% in the films estimated from their density (2.75 g/cm³ as determined by X-ray reflectivity). The bonding structure of the composite films was studied by electron energy loss spectroscopy (EELS) in the region of carbon core level. The spectra were dominated by a peak at 292 eV indicating the diamond nature of the investigated films. In addition, the spectra of NCD/a-C films possessed a shoulder at 284 eV due to the presence of a small sp² bonded fraction. This phase was identified also by X-ray photoelectron spectroscopy (XPS). The sp²/sp³ ratio was on the order of 10% as determined by deconvolution of the C1s XPS peak.     

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.