Formation of hydrogen-capped polyynes by laser ablation of C60 particles suspended in solution

Laser ablation of C₆₀ particles suspended in hexane or methanol solution has been studied using a Nd:YAG laser (266, 355, 532, and 1064 nm). Insoluble product analysis by Raman spectroscopy showed that C₆₀ particles transformed to graphite-like ones. On the other hand, soluble product analysis by HPLC coupled with UV absorption spectroscopy and GC/MS demonstrated that linear hydrogen-capped polyynes (C_nH₂: n=8, 10, 12) were formed. The dominant C_nH₂ polyyne was C₈H₂ in all cases. The relative abundance of polyynes decreased with increasing wavelength of the Nd:YAG laser, except for 266-nm irradiation in hexane, where the relative abundance of polyynes at 355 nm was greater than that at 266 nm. It was therefore concluded that photochemical processes are more important than thermal ones for the formation of polyynes. The relative abundance of polyynes in hexane was greater than that in methanol. The dependence of relative abundance of polyynes on the particles concentration, laser irradiation time, and laser power was measured in order to obtain information on formation mechanism of polyynes. These results and reported laser photochemical processes of C₆₀ molecules in the gas phase suggested that C₂ radicals produced from C₆₀ are polymerized and hydrogenated to form C₈H₂ and much smaller amounts of C₁₀H₂ and C₁₂H₂.     

Laser ablation of diamond particles suspended in ethanol: Effective formation of long polyynes

Polyynes were synthesized by laser ablation of diamond particles with various sizes suspended in ethanol. Chain length distributions and total yields of polyynes produced were compared with those produced from graphitized diamond particles and graphite particles. The relative amounts of long polyynes such as C₁₄H₂ and C₁₆H₂ produced from diamond particles were found to be larger than those from graphitized diamond particles and graphite particles. From the change of the chain length distribution with the laser irradiation time, it is concluded that the long polyynes are produced directly from diamond particles at the initial stage of ablation. Furthermore, the total yield of polyynes was found to increase with the size of diamond particles and decrease as their graphitization proceeds. Possible mechanisms of these results are discussed.     

Quantitative evaluation of sp/sp hybridization ratio in cluster-assembled carbon films by in situ near edge X-ray absorption fine structure spectroscopy

We present a near edge X-ray absorption fine structure spectroscopy characterization of nanostructured carbon films containing carbynoid species. By a careful data analysis and normalization of the spectra at the carbon K-edge we have quantitatively evaluated the extent of valence sp hybridization of the films. A sp/sp² ratio between 10% and 25% has been obtained. This result allowed the evaluation of the ratio between the sp and sp² Raman cross section at different excitation laser wavelengths.     

Laser desorption-ionization time of flight mass spectrometry of various carbon materials

The possibilities of commercially available MALDI-TOF mass spectrometric instrumentation equipped with 337 nm nitrogen laser in carbon materials analysis have been examined. Laser desorption-ionization of synthetic diamond, graphite, glassy carbon, carbon nano-tubes, and diamond-like thin layer results in the formation of two positively charged ( and odd-numbered 44-346) and one negatively charged carbon cluster set (). Fullerene C₆₀ was analyzed for comparison. From the clusters mass spectra, several similarities, concerning “magic number” ions with high signal intensities, can be observed. It is concluded that differing carbon materials are subjected by UV-laser pulses to similar physico-chemical changes. Heavy carbon clusters of low stability and characteristic loss of neutral C₃ particle in case of the smaller clusters during post-source decay (PSD) implies that the species observed do not possess stable fullerene structure, which might have been expected for C_n species with n ? 20.     

Pulsed laser deposition of glass-like cluster assembled carbon films

Carbon films have been synthesized at room temperature in helium atmosphere, at high pressure, on (1 0 0) Si substrates by pulsed KrF excimer laser ablation of highly oriented pyrolitic graphite. By changing laser power density (from 8.5 to 19 MW mm⁻²) and gas pressure (from 0.6 Pa to 2 kPa), nanometer sized cluster assembled films were obtained. Film morphology, as studied by scanning electron microscopy, changes with increasing helium pressure, from dense columns, to node-like morphology, then to an open dendritic structure. Carbon coordination was studied by visible Raman spectroscopy in all films. They are structurally disordered, sp² coordinated and belong to the family of glass-like carbons. The deduced film coherence length agrees with the average size of carbon aggregates that build up the films, as measured by transmission electron microscopy in representative samples. The average number of carbon atoms per cluster, that depends on helium (high) pressure, was obtained by a simple model.     

Transformation of fullerene peapods to double-walled carbon nanotubes induced by UV radiation

Double-walled carbon nanotubes were prepared by XeCl-laser irradiation of fullerene (C₆₀ or C₇₀) peapods. Raman spectroscopy evidences less defect structure of outer tubes, as compared to those in double-walled carbon nanotubes grown by thermal treatment of peapods. The diameter distribution also differs from that of the thermally prepared nanotubes. At the given laser fluence, the conversion of C₇₀@SWCNT into double-walled carbon nanotubes was more efficient than the corresponding conversion of C₆₀@SWCNT.     

Synthesis of polyynes in a submerged electric arc in organic solvents

The products formed by the electric arc between graphite electrodes submerged in organic solvents like acetonitrile, n-hexane and methanol consist of a series of polyynes having the general chemical structure: H(CC)_mH (with m an integer 1,2,3,…m). The polyynes were separated through liquid chromatography (HPLC) and identified through their characteristic electronic absorption spectra recorded by a diode-array detector. When acetonitrile was arced at −40 °C, the entire polyynes series to m=9 (chain with 18 carbon atoms) were produced. Instead, at room temperature the entire polyynes series to m=8 (chain with 16 carbon atoms) were obtained. Similar results were obtained by arcing graphite electrodes in n-hexane and in methanol. The purest polyynes mixture is obtained in methanol while in n-hexane and in acetonitrile by-products are formed together with the polyynes series. In particular, in acetonitrile, a series of monocyanopolyynes was detected together with the normal polyynes series.The polyynes are formed by the vaporization of the elemental carbon from the graphite electrodes and dissolved in the solvent where the electrodes are submerged. The demonstration that the polyynes series H(CC)_mH is hydrogen-capped is based on several experimental data from electronic and FT-IR spectroscopy as well as their reactivity with a specific reagent for terminal acetylenes. Some chemical properties of the polyynes solutions (hydrogenation, oxidation) are also treated and discussed.     

Effects of substrate temperatures on improved surface-enhanced Raman scattering

In this work, electrochemical methods were used to prepare surface-enhanced Raman scattering (SERS)-active gold substrates to investigate the effect of substrate temperatures on improved SERS performances. The results indicate that the SERS enhancement capabilities are gradually raised from 25 °C to a maximum at 40 °C and monotonically decreased from 40 to 100 °C. The SERS of rhodamine 6G (R6G) adsorbed on the SERS-active substrate at 40 °C exhibits a higher intensity by 4-fold of magnitude, as compared with that of R6G adsorbed on the SERS-active substrate at 25 °C. Also, SERS of polypyrrole (PPy) deposited on the roughened Au substrate treated at 35 °C exhibits the highest intensity by 3-fold of magnitude, as compared with that of PPy deposited on the same roughened Au substrate at room temperature.     

Structural changes in soot particles induced by diode laser irradiation

The effects of laser radiation on soot in a vacuum were studied by Raman spectroscopy and transmission electron microscopy. It was found that defective carbon onions could be formed simply by irradiation of focused common diode lasers with intensity much lower (10³-10⁶ W/cm²) than those used in the previous work. A modified Melton model was introduced to analyze the formation of onions. The results show a threshold value of laser intensity (5.49 × 10³ W/cm²), above which sublimation and rearrangement play a dominant role in the formation of onions. Such onions can also be formed with laser intensities below the threshold since neighboring graphene layers may merge and grow by capturing interstitial carbon atoms. The deposited soot is damage resistant to the diode laser radiation in air due to the rapid formation of carbon onions.     

Effect of laser intensity on yield and physical characteristics of single wall carbon nanotubes produced by the Nd:YAG laser vaporization method

We report on the effect of the Nd:YAG laser intensity on diameter distribution, yield and physical characteristics of single-wall carbon nanotubes (SWNTs) while comparing three different laser configurations (namely: (i) single 532 nm pulse; (ii) single 1064 nm pulse; and (iii) 532 nm followed by the 1064 nm double pulse). The carbon SWNTs were synthesized at a furnace temperature of 1150 °C and characterized by means of laser micro-Raman spectroscopy and high resolution transmission electron microscopy (HRTEM). Regardless of the laser configuration used, it is found that both the yield and the structural characteristics of the SWNTs are highly sensitive to the laser intensity. Indeed, by combining Raman analyses together with HRTEM observations we were able to point out the existence of an optimal laser intensity which leads not only to the highest yield of SWNTs and the largest bundles but also to the lowest level of amorphous and, or disordered sp² carbon in the deposits. While the optimal laser intensity was found to increase from 1.7 to 2.9×10⁹ W/cm² when the laser wavelength is changed from 1064 to 532 nm, the double pulse configuration offered a larger process latitude since high yield of SWNTs was obtained over the (0.8–3.5)×10⁹ W/cm² laser intensity range centered around the optimal value of 2.3×10⁹ W/cm². Moreover, it is shown that the increase of the laser intensity (from 0.5 to 5.6×10⁹ W/cm²) favors the growth of large nanotubes (1.4 nm-diam.) to the detriment of smaller ones (1.1 nm-diam.). A tendency to form larger nanotubes was also observed when increasing the furnace temperature from 1000 to 1150 °C. Finally, the laser intensity effect is interpreted in terms of near-surface or deep laser energy absorption in the graphite target.     

Effect of ambient gas ionisation on the morphology of a pulsed laser deposited carbon film

The paper reports a study of the mechanism of ambient gas ionisation and its effects on a growing carbon film. Such an ionisation occurs during the expansion through an inert gas of an ablation plume generated by laser irradiation of a target. Charge transfer reactions between ablated ions and inert gas atoms lead to the formation of a charged layer in contact with plume front. The energy lost by fast ablated ions associated with plume slowing down is calculated. In the case of carbon ablated in helium and argon atmospheres, where ionisation plays a very different role, we discuss the microstructure changes observed in the deposited films in terms of dramatic differences of plume dynamics.     

Surface-enhanced Raman scattering of suspended monolayer graphene

Abstract

The interactions between phonons and electrons induced by the dopants or the substrate of graphene in spectroscopic investigation reveal a rich source of interesting physics. Raman spectra and surface-enhanced Raman spectra of supported and suspended monolayer graphenes were measured and analyzed systemically with different approaches. The weak Raman signals are greatly enhanced by the ability of surface-enhanced Raman spectroscopy which has attracted considerable interests. The technique is regarded as wonderful and useful tool, but the dopants that are produced by depositing metallic nanoparticles may affect the electron scattering processes of graphene. Therefore, the doping and substrate influences on graphene are also important issues to be investigated. In this work, the peak positions of G peak and 2D peak, the I_2D/I_G ratios, and enhancements of G and 2D bands with suspended and supported graphene flakes were measured and analyzed. The peak shifts of G and 2D bands between the Raman and SERS signals demonstrate the doping effect induced by silver nanoparticles by n-doping. The I_2D/I_G ratio can provide a more sensitive method to carry out the doping effect on the graphene surface than the peak shifts of G and 2D bands. The enhancements of 2D band of suspended and supported graphenes reached 138, and those of G band reached at least 169. Their good enhancements are helpful to measure the optical properties of graphene. The different substrates that covered the graphene surface with doping effect are more sensitive to the enhancements of G band with respect to 2D band. It provides us a new method to distinguish the substrate and doping effect on graphene.

PACS

78.67.Wj (optical properties of graphene); 74.25.nd (Raman and optical spectroscopy); 63.22.Rc (phonons in graphene)     

Characterizing various types of defects in nuclear graphite using Raman scattering: Heat treatment,ion irradiation and polishing

Raman spectroscopy has proved to be an appropriate technique to probe defects in carbon-based materials owing to its high sensitivity, most often focused on the commonly used I_D/I_G parameter. However, this ratio may be activated by various types of defects and in a completely independent manner. Therefore, discriminating between defects is challenging. The central idea of the present work is to provide a better understanding of the Raman response to the various types of defects that may appear in nuclear graphite (carbon–carbon composite) during its manufacturing process, its operation in the nuclear reactor, or even during its preparation process such as polishing which is usually used prior to Raman characterization. This work also demonstrates the discrimination of the defect types using the combination of the I_D/I_G and FWHM(G), two structural disorder indicators evolving differently according to the type and the concentration of the introduced defects into the carbon network. The ion-beam irradiation was used here as an effective way for creating defects that could be similar to those created by neutrons in the nuclear reactor.     

Rapid identification of bacteria utilizing amplified dielectrophoretic force-assisted nanoparticle-induced surface-enhanced Raman spectroscopy

Dielectrophoresis (DEP) has been widely used to manipulate, separate, and concentrate microscale particles. Unfortunately, DEP force is difficult to be used in regard to the manipulation of nanoscale molecules/particles. For manipulation of 50- to 100-nm particles, the electrical field strength must be higher than 3 × 10⁶ V/m, and with a low applied voltage of 10 V_p-p, the electrode gap needs to be reduced to submicrons. Our research consists of a novel and simple approach, using a several tens micrometers scale electrode (low cost and easy to fabricate) to generate a dielectrophoretic microparticle assembly to form nanogaps with a locally amplified alternating current (AC) electric field gradient, which is used to rapidly trap nanocolloids. The results show that the amplified DEP force could effectively trap 20-nm colloids in the nanogaps between the 5-μm particle aggregates. The concentration factor at the local detection region was shown to be approximately 5 orders of magnitude higher than the bulk solution. This approach was also successfully used in bead-based surface-enhanced Raman spectroscopy (SERS) for the rapid identification of bacteria from diluted blood.     

Improved surface-enhanced Raman scattering based on Ag-Au bimetals prepared by galvanic replacement reactions

In this work, the improved surface-enhanced Raman scattering (SERS) of Rhodamine 6G (R6G) adsorbed on Ag-Au bimetals synthesized via galvanic replacement of Ag with Au was first investigated. First, silver substrates were roughened by triangular-wave oxidation-reduction cycles (ORC) in aqueous solutions containing 0.1 M KCl. At the same time, Cl⁻ and Au-containing nanocomplexes in solutions were prepared by treating gold substrates with the similar electrochemically roughening procedures. Then the roughened Ag substrates were incubated in the Cl⁻ and Au-containing solutions for different couples of minutes to undergo the galvanic replacement reactions. Encouragingly, the SERS of R6G adsorbed on this roughened Ag substrate modified by the replacement of Ag with Au for 3 min exhibits a higher intensity by one order of magnitude and a better resolution, as compared with the SERS of R6G adsorbed on an unmodified roughened Ag substrate. The increased SERS effect can be ascribed to the compositions of complexes formed on the substrates. In this optimum condition, the atomic ratio of Ag to Au is ca. 6.6.     

A comparison between Raman spectroscopy and surface characterizations of multiwall carbon nanotubes

The distribution of graphene units with an axial symmetry gives rise to different types of carbon filaments: nanotubes, nanofilaments and classical fibers. In this work the surfaces of different multiwalled nanotubes are characterized by two complementary techniques: chemical ones based on Total Surface Area and Active Surface Area measurements, associated with a physical approach the Raman scattering spectroscopy. From analysis of Raman data we deduce the values of the in-plane coherence lengths, identified as L₁ the planar projection of graphene sheets, and we propose an analysis for the observed line-width behavior related to the graphitization step. From the surface chemical properties we establish a general relationship between the density of functional surface groups and the in plane coherence length L₁ for all types of MWNT. This analysis allows us to show the influence of both, the structural organization and the different treatments on the interfacial characteristics of these nanocarbons.