Surface enhanced Raman spectroscopic characterization of molecular structures in diamond-like carbon films

Vibrational bands associated with the out of plane bending modes of CH groups attached to various aromatic ring structures have been identified in surface enhanced Raman spectra of diamond-like amorphous carbon films deposited by femtosecond laser ablation of graphite. The appearance of enhanced modes associated with specific ring structures is assigned to solo-, duo-, trio- and quartet-CH groups and to pendant rings. Analysis of these bands provides insight into the nature of the molecular ring components of fs-DLC films. The resulting composition consists of polycyclic aromatic carbon (PAH)-like ring molecules and polyyne chains stabilized in a sp³-bonded network.     

衬底温度对强流脉冲离子束烧蚀沉积类金刚石薄膜化学结构的影响

采用强流脉冲离子束(High-intensitypulsedionbeam,HIPIB)烧蚀技术在Si(100)基体上沉积类金刚石(Diamond-likecarbon,DLC)薄膜,衬底温度的变化范围为298～673K.利用Raman光谱和X射线光电子谱(XPS)对DLC薄膜的化学结合状态与衬底温度之间关系进行研究.薄膜XPS的C1s谱的解谱分析得出薄膜中含有sp³C(结合能为285.5eV)和sp²C(结合能为284.7eV)成分,根据解谱结果评价薄膜中sp³C含量.根据XPS分析可知,衬底温度低于473K时,sp³C的含量大约为40%左右;随着沉积薄膜时衬底温度的提高,sp³C的含量降低,由298K时的42.5%降到673K时的8.1%,从573K开始发生sp³C向sp²C转变.Raman光谱表明,随着衬底温度的提高,Raman谱中G峰的峰位靠近石墨峰位,G峰的半峰宽降低,D峰与G峰的强度比I_D/I_G增大,说明薄膜中的sp³C的含量随衬底温度增加而减少.     

Raman and SERS investigation of isolated sp carbon chains

Carbon chains with sp hybridization and a known distribution of lengths have been studied by Raman and Surface Enhanced Raman Scattering (SERS). With the support of density functional theory simulations the observed Raman features have been assigned to active modes of individual chains. SERS spectra show remarkable differences, attributed to the interaction between sp chains and Ag nanoparticles used as a SERS active medium. The results obtained for this model system permit in principle to explain a large number of Raman data from molecular and solid sp/sp² carbon systems.     

Nitrogen-doped diamond-like carbon as optically transparent electrode for infrared attenuated total reflection spectroelectrochemistry

This contribution describes the development of nitrogen-doped diamond-like carbon (N-DLC) thin films for multi-reflection mid-infrared (MIR) attenuated total reflectance (IR-ATR) spectroelectrochemistry. N-DLC coatings were deposited using pulsed laser deposition (PLD) involving the ablation of a high purity graphite target. The DLC matrix was further modified by ablating the target in the presence of nitrogen gas. This technique offers the advantage of depositing thin films at room temperature, thereby enabling coating of temperature-sensitive substrates including e.g., MIR waveguides. The resulting films were analyzed with X-ray photoelectron spectroscopy (XPS), and determined to be composed of carbon, nitrogen, and adventitious oxygen. Raman spectroscopic studies indicate that the addition of nitrogen induces further clustering and ordering of the sp(2)-hybridized carbon phase. The electrochemical activity of PLD fabricated N-DLC films was verified using the Ru(NH(3))(3+/2+) redox couple, and was determined to be comparable with that of other carbon-based electrodes. In situ spectroelectrochemical studies involving N-DLC coated zinc selenide (ZnSe) MIR waveguides provided evidence concerning the oxidation of N-DLC at anodic potentials in 1 M HClO(4) solutions. Finally, the electropolymerization of polyaniline (PAni) was performed at N-DLC-modified waveguide surfaces, which enabled spectroscopic monitoring of the electropolymerization, as well as in situ studying the structural conversion of PAni at different potentials.     

Thermal desorption of hydrogen from carbon nanosheets

Carbon nanosheets are a unique nanostructure that, at their thinnest configuration, approach a single freestanding graphene sheet. Temperature desorption spectroscopy (TDS) has shown that the hydrogen adsorption and incorporation during growth of the nanosheets by radio frequency plasma-enhanced chemical vapor deposition are significant. A numerical peak fitting to the desorption spectra (300-1273 K) via the Polanyi-Wigner equation showed that desorption followed a second order process, presumably by the Langmuir-Hinshelwood mechanism. Six peaks provide the best fit to the TDS spectra. Surface desorption activation energies were determined to be 0.59, 0.63, and 0.65 eV for the external graphite surface layers and 0.85, 1.15, and 1.73 eV for desorption and diffusion from the bulk. In contrast to TDS data from previously studied a-C:H films [Schenk et al. J. Appl. Phys. 77, 2462 (1995)], a greater amount of hydrogen bound as sp(2) hybridized carbon was observed. A previous x-ray diffraction study of these films has shown a significant graphitic character with a crystallite dimension of L(a)=10.7 nm. This result is consistent with experimental results by Raman spectroscopy that show as-grown carbon nanosheets to be crystalline as commercial graphite with a crystallite size of L(a)=11 nm. Following TDS, Raman data indicate that the average crystallite increased in size to L(a)=15 nm.     

Oxidation and carbidation of laser-ablated amorphized Ti particles in carbon monoxide

IR laser ablation of hexagonal titanium in vacuum leads to amorphization of ablated Ti particles and when carried out in gaseous carbon monoxide it proceeds as reactive ablation involving particles amorphization, oxidation and carbidation. The films deposited in vacuum and in the presence of CO were examined by Fourier transform infrared, Raman and X-ray photoelectron spectroscopy, X-ray and electron diffraction and electron microscopy. The Ti films become oxidized upon contact with air and the Ti/C/O films are composed of Ti–O, Ti–C and C–O bonds-containing structures with Ti in Ti²⁺–Ti⁴⁺ state and incorporating crystalline rutile and elemental carbon. The ablation in vacuum represents a new approach to amorphous titanium and it is judged that hot ablated Ti particles are modified by reactions with CO decomposition products into amorphous Ti oxycarbides which undergo rapid post-pulse amorphization.     

Theory of femtosecond coherent anti-Stokes Raman scattering spectroscopy of gas-phase transitions

A theoretical analysis of coherent anti-Stokes Raman scattering (CARS) spectroscopy of gas-phase resonances using femtosecond lasers is performed. The time-dependent density matrix equations for the femtosecond CARS process are formulated and manipulated into a form suitable for solution by direct numerical integration (DNI). The temporal shapes of the pump, Stokes, and probe laser pulses are specified as an input to the DNI calculations. It is assumed that the laser pulse shapes are 70 fs Gaussians and that the pulses are Fourier-transform limited. A single excited electronic level is defined as an effective intermediate level in the Raman process, and transition strengths are adjusted to match the experimental Raman polarizability. The excitation of the Raman coherence is investigated for different Q-branch rotational transitions in the fundamental 2330 cm(-1) band of diatomic nitrogen, assuming that the pump and Stokes pulses are temporally overlapped. The excitation process is shown to be virtually identical for transitions ranging from Q2 to Q20. The excitation of the Raman coherences is also very efficient; for laser irradiances of 5x10(17) W/m2, corresponding approximately to a 100 microJ, 70 fs pulse focused to 50 microm, approximately 10% of the population of the ground Raman level is pumped to the excited Raman level during the impulsive pump-Stokes excitation, and the magnitude of the induced Raman coherence reaches 40% of its maximum possible value. The theoretical results are compared with the results of experiments where the femtosecond CARS signal is recorded as a function of probe delay with respect to the impulsive pump-Stokes excitation.     

大功率激光制备a-C和a-C∶H类金刚石膜的光学性能研究

采用大功率高重复频率准分子激光溅射热解石墨靶制备了类金刚石碳膜, 研究了实验条件对类金刚石膜光学性能的影响, 发现氢可以提高膜中sp3键的含量和膜的光学透过率. 在实验参数范围内, 膜的光学性能随着氢压的增加而提高. 根据类金刚石膜的反应沉积机理对上述结果进行了分析、解释.     

The luminescent carbon-bearing microinclusion enigma in the Kimi Unit, Rhodope, Greece: Raman microscopic point analyses and mapping with different lasers

The Kimi Unit of the Rhodope Metamorphic Province (RMP), NE Greece, experienced ultrahigh-pressure metamorphism (UHPM), as documented by the unequivocal presence of diamond microinclusions in metapelitic garnet porphyroblasts. Certain peculiar lozenge-shaped 2-8 microm sized inclusions in diamond-bearing garnets reveal a broad composite and asymmetric triplet band (phase XXX) at approximately 1331 cm(-1) in their Raman spectra acquired with a 632.8 nm He-Ne laser, initially attributed to an sp(3)-hybridized C-polymorph. These have been meticulously re-investigated by means of combined 2-wavelength (514.5 nm/632.8 nm laser) Raman microscopy. Raman mapping has been extensively employed in order to examine the spatial distribution of phase XXX and of other phases in these polyphase inclusions and to explore for additional Raman bands. The triplet band at approximately 1331 cm(-1) measured with the 632.8 nm laser shifts to much higher wavenumbers ( approximately 4966 cm(-1)) when excited with a 514.5 nm Ar(+) laser, proving that the XXX triplet is not a real Raman band but a luminescence one at approximately 691.1 nm. Numerous hypotheses on the nature of the mysterious phase XXX (e.g. Cr(3+)-bearing mineral, carbonate, C polymorph, gas, organic phase) are explored and discussed but all are shown to be unsatisfactory. It is suggested that XXX occurs as nanocrystals that luminesce strongly giving the appearance (in Raman maps) of being larger.     

Fast-and ultra-fast laser pulse induced reactions between carbon dioxide and methane

The direct excitation of CO2 using fast (nanosecond) and ultrafast (femtosecond) pulsed lasers was investigated.A gas reaction cell was used to excite CO2 in a 50:50 mixture of CO2 and CH4 using nano-and femtosecond laser systems,to induce a reaction between these two compounds.FT-IR spectra showed that CO was formed using the nanosecond and femtosecond laser systems.It was also found that hydrocarbons,containing C-C bonds were formed.For both the nanosecond and femtosecond laser,it was found that more C-C higher hydrocarbons were formed after 5 h compared to 3 h of irradiation.Irradiation at pressures of 250,350 and 500 kPa with the femtosecond laser system showed the expected increase in the amount of CO formed with an increase in pressure.Results from this study showed that carbon dioxide and methane can be activated successfully using nanosecond laser pulses at 2000 nm and femtosecond laser pulses at 795 or 2000 nm and that these activated species react to form CO and C-C containing products.     

Fast- and ultra-fast laser pulse induced reactions between carbon dioxide and methane

The direct excitation of CO_2 using fast (nanosecond) and ultrafast (femtosecond) pulsed lasers was investigated. A gas reaction cell was used to excite CO_2 in a 50:50 mixture of CO_2 and CH_4 using nano- and femtosecond laser systems, to induce a reaction between these two compounds. FT-IR spectra showed that CO was formed using the nanosecond and femtosecond laser systems. It was also found that hydrocarbons, containing C-C bonds were formed. For both the nanosecond and femtosecond laser, it was found that more C-C higher hydrocarbons were formed after 5 h compared to 3 h of irradiation. Irradiation at pressures of 250, 350 and 500 kPa with the femtosecond laser system showed the expected increase in the amount of CO formed with an increase in pressure. Results from this study showed that carbon dioxide and methane can be activated successfully using nanosecond laser pulses at 2000 run and femtosecond laser pulses at 795 or 2000 nm and that these activated species react to form CO and C-C containing products.     

Study on the carbon fragment anions produced by femtosecond laser ablation of solid C60

Production of the anions (negative ions) has been observed by femtosecond laser ablation (fsLA) of solid C(60) with a time-of-flight (TOF) mass spectrometer. In contrast to C(60)(+), production of C(60)(-) due to an electron capture is found very limited because of the small electron affinity of the C(60) molecule. Narrow TOF peaks of small carbon fragment anions C(n)(-) (n ≤ 23) suggest instantaneous production of the fragment anions through dissociative ionization of C(60). Production of the mono-hydrogenated carbon fragment anions C(n)H(-) has been observed and also the abrupt change in the yield of C(n)H(-) has been observed at n = 10, which is attributed to the structural change of the carbon fragments from a linear chain to a monocyclic ring. The results are found similar to those obtained for the carbon fragments produced by nanosecond laser ablation (nsLA) of solid C(60), which demonstrates that the thermalization in an ablation plasma washes away any difference in the nature of carbon fragments produced by fsLA and nsLA.     

Substrate-induced Raman frequency variation for single-walled carbon nanotubes

We report on the monotonic Raman frequency shift and intensity variation when a laser spot moves along the same single-walled carbon nanotube (SWNT) for both the radial-breathing mode (RBM) and the G-band. Our substrates are Si wafers coated with thermal oxide, and trenches with widths of 1-80 mum are etched in the SiO2 by photolithography and reactive ion etching. SWNTs are grown by chemical vapor deposition and lie on top of the SiO2 and across the trenches. When the laser spot moves from the middle of the trench to the SiO2 region along the nanotube, we observe a clear upshift in the RBM and G-band frequencies and a decrease of intensity. The effect is more significant with large ( approximately 2 nm) diameter nanotubes and appears to be chirality dependent. These studies provide important information about environmental effects on single-walled carbon nanotube resonant Raman spectroscopy.     

A novel approach to carbon hollow spheres and vessels from CCl4 at low temperatures

Carbon hollow spheres (400-600 nm) and vessels (400 nm x 3000 nm) have been synthesized from sp3-CCl4 at 190 and 230 degrees C, respectively. The HRTEM images and Raman spectra reveal the sp2 nature of the as-obtained products, indicating that the transformation from carbon sp3 to sp2 occurs in the reactions. The possible mechanism has also been proposed.     

Bulk synthesis of linear carbon chains confined inside single-wall carbon nanotubes by vacuum discharge

Carbyne, an infinite carbon chain, has attracted much interest and induced significant controversy for many decades. Recently, the presence of linear carbon chains (LCCs), which were confined stably inside double-wall carbon nanotubes (DWCNTs) and multiwall carbon nanotubes (MWCNTs), has been reported. In this study, we present a novel method to produce LCCs in a film of carbon nanotubes (CNTs). Our transmission electron microscopy and Raman spectroscopy revealed the formation of a bulk amount of LCCs after electric discharge of CNT films, which were used as field emission cathodes. The LCCs were confined inside single-wall CNTs as well as DWCNTs. Furthermore, two or three LCCs in parallel with each other are encapsulated when the inner diameter of CNT is larger than approximately 1.1 nm.     

The chemical composition and bonding structure of amorphous hydrogenated carbon nitride film on aluminum surface deposited by electrodeposition

Amorphous hydrogenated carbon nitride films with excellent adhesion to the substrates were deposited on the surfaces of aluminum plates by electrodeposition with acetonitrile as carbon source. The as‐obtained films were detailed characterized by Raman, XPS and FTIR. The results show that the as‐obtained films are mainly made up of sp2 C, sp3 C, C? N, C?N bonds and a spot of sp3? CH₂, sp3? CH₃, sp3? CH and sp2? CH bonds. The atomic ratio of N/C is evaluated to be about 2.98 at.% by XPS. Raman analysis indicated that I_D/I_G ratios decreased from 0.827 to 0.675 when the deposition time increased from 10 to 20 h. At the end, on the basis of the detailed analysis, results of Raman, FTIR and XPS, the possible deposition mechanism was discussed. Copyright © 2010 John Wiley & Sons, Ltd.     

Vibrational characterization of dinaphthylpolyynes: a model system for the study of end-capped sp carbon chains

We perform a systematic investigation of the resonance and vibrational properties of naphthyl-terminated sp carbon chains (dinaphthylpolyynes) by combined multi-wavelength resonant Raman (MWRR) spectroscopy, ultraviolet-visible spectroscopy, and Fourier-transform infrared (FT-IR) spectroscopy, plus ab initio density functional theory (DFT) calculations. We show that the MWWR and FT-IR spectroscopies are particularly suited to identify chains of different lengths and different terminations, respectively. By DFT calculations, we further extend those findings to sp carbon chains end-capped by other organic structures. The present analysis shows that combined MWRR and FT-IR provide a powerful tool to draw a complete picture of chemically stabilized sp carbon chains.     

Electrodeposition of nanostructured diamond-like films by oxidation of lithium acetylide

Diamond-like carbon (DLC) films have been deposited by anodic oxidation of 4 M solution of lithium acetylide in dimethylsulfoxide on the surface of stainless steel or nickel electrode at room temperature and moderate anodic current densities (0.2–2.0 mA/cm²) in the range of electrode potentials 0.3–2.5 V (vs. sat. Ag|AgCl reference electrode). Electrodeposited DLC coatings represented complete and optically transparent films of a thickness 50–100 nm having dark island inclusions with a diameter 0.8–5.0 μm. The concentration and average size of these particles increased with the prolongation of deposition time. Micro-Raman spectra obtained by the focusing of laser beam onto these dark inclusions are characterized by a broad peak centered at 1500 cm⁻¹ and weak peak at 1200 cm⁻¹. With a defocused laser beam, there appear two well-distinguished peaks on the integrated Raman spectra – at 1530 and 1130 cm⁻¹. Analysis of Raman spectra with the use of a Breit–Wigner–Fano lineshape and spectrum deconvolution indicates that the electrodeposited films consist of diamond-like nanostructured carbon with a high content (70–80%) of sp³ phase.     

Growth and characterization of ultrathin carbon films on electrodeposited Cu and Ni

Ultrathin carbon films were grown on different types of metallic substrates. Free‐standing foils of Cu and Ni were prepared by electroforming, and a pure Ni film was obtained by galvanic displacement on a Si wafer. Commercial foil of Ni 99.95% was used as a reference substrate. Carbon films were grown on these substrates by chemical vapour deposition in a CH₄‐H₂ atmosphere. Obtained films were characterized by Raman spectroscopy, X‐ray photoelectron spectroscopy (XPS), Auger electron spectroscopy, and ultraviolet photoemission spectroscopy. The XPS at grazing collection angle was used to determine the thickness of carbon films. Depending on the deposition parameters, the films of graphene or graphite were obtained on the different substrates. The uniformity of graphene and its distribution over the sample area were investigated from Raman data, optical images, and XPS chemical maps. The presence of graphene or graphite in the films was determined from the Raman spectra and Auger peak of C KVV. For this purpose, the D parameter, which is a fingerprint of carbon allotropes, was determined from C KVV spectra acquired by using X‐rays and electron beam. A formation of an intermediate layer of metal hydroxide was revealed in the samples with graphene overlayer.     

Raman characterization of amorphous carbon films

Amorphous carbon films, deposited with the LASER-ARC technique, have been characterized using Raman scattering experiments at an excitation wavelength of 633 nm provided by a He-Ne laser. To distinguish between the homogeneous amorphous film and incorporated particles area resolved measurements have been carried out due to the laser spot diameter of 1 m. Typical diamondlike (DLC) films, grown near room temperature, show a broad Raman band between 1000 cm^–1 and 1800 cm^–1 fitted very well by two gaussian distributions. Films deposited at higher substrate temperatures reveal more graphitic features in the spectra. The spectra of particles consists of a graphite-like portion originated from the graphitic structure of the particle and a diamond-like portion caused by the covering DLC film. The degree of disorder and diamond-likeness in the film structure is quantified by the peak position, the full width at half maximum (FWHM) and intensity relation of the fitted D- and G-peaks.