Shadowgraphic and emission imaging spectroscopic studies of the laser ablation of graphite in an Ar gas atmosphere

Laser ablation of graphite in an Ar atmosphere at 560 Torr was done using a nanosecond-pulse Nd:YAG laser (1064 nm) at a fluence of 12 J/cm². Dynamics in the ejection of carbon species and in their confinement near the graphite surface (<1 mm) due to their numerous collisions with Ar atoms were investigated by shadowgraphy, emission imaging, and emission spectroscopy at delay times of 0.01-100 7s following the laser irradiation. A shock wave was generated, and temporally and spatially dependent emissions from Ar⁺ and Ar were observed in addition to those from carbon species (C, C⁺, and C₂) and the Bremsstrahlung radiation from a hot plasma. We suggest that the dissipation of the kinetic and thermal energies of the carbon species, their backward motion, and their collisions with each other lead to the formation of clusters and particles through the interaction with Ar atoms.     

Growth of single-wall carbon nanotubes dependent on laser power density and ambient gas pressure during room-temperature CO2 laser vaporization

Single-wall carbon nanotubes (SWNTs) were synthesized by the irradiation of 20-ms CO₂ laser pulses onto a graphite–Co/Ni target at room temperature. We investigated the effect of laser power density (10–150 kW/cm²) and ambient Ar gas pressure (150–760 Torr) on the abundance of SWNTs with lengths of up to about 200 nm in soot-like carbonaceous deposits. For a constant power density (30 kW/cm²), depending on the Ar gas pressure, SWNTs with diameters of 1.2–1.4 nm were synthesized. Expansion behavior and temperature-fall rates of clusters and/or particles in laser plumes were also analyzed by high-speed video imaging and temporally and spatially resolved emission spectroscopy. The temperature-fall rates were estimated to be 171–427 K/ms. The SWNT growth on the time scale of a few milliseconds appeared to be related to some features of condensing clusters and/or particles, including resident densities, collision frequencies and temperatures. Received: 16 July 2001 / Accepted: 23 July 2001 / Published online: 30 August 2001     

Dynamics of single-wall carbon nanotube synthesis by laser vaporization

The key spatial and temporal scales for single-wall carbon nanotube (SWNT) synthesis by laser vaporization at high temperatures are investigated with laser-induced luminescence imaging and spectroscopy. Graphite/(Ni, Co) targets are ablated under typical synthesis conditions with a Nd:YAG laser at 1000 °C in a 2-in. quartz tube reactor in flowing 500-Torr Ar. The plume of ejected material is followed for several seconds after ablation using combined imaging and spectroscopy of Co atoms, C₂ and C₃ molecules, and clusters. The ablation plume expands in stages during the first 200 7s after ablation and displays a self-focusing behavior. Interaction of the plume with the background gas forms a vortex ring which segregates and confines the vaporized material within a ~1-cm³ volume for several seconds. Using time-resolved spectroscopy and spectroscopic imaging, the time for conversion of atomic and molecular species to clusters was measured for both carbon (200 7s) and cobalt (2 ms) at 1000 °C. This rapid conversion of carbon to nanoparticles, combined with transmission electron microscopy analysis of the collected deposits, indicate that nanotube growth occurs over several seconds in a plume of mixed nanoparticles. By adjusting the time spent by the plume within the high-temperature zone using these in situ diagnostics, single-walled nanotubes of controlled (~100 nm) length were grown and the first estimate of a growth rate on single laser shots (0.2 7m/s) was obtained.     

Single-walled carbon nanotube formation with double laser vaporization technique

Single-walled carbon nanotubes (SWNTs) were prepared with double laser vaporization of a graphite target and a metal/alloy target inside an electric furnace at 1200 ^°C ambient temperature with 500 torr Ar gas atmosphere. Each target was vaporized simultaneously with a different Nd:YAG laser. Several kinds of metal/alloy target (Ni, Co, Fe, and permalloy) were tested in order to see the difference in the resulting SWNT yield and the diameter distribution of them. The Raman spectra of SWNT-containing soot prepared by use of this technique with permalloy/carbon system indicated that permalloy gives almost the same yield as compared with Ni/Co carbon composite rod with single laser vaporization technique, though the diameter distribution of them is slightly different. Also, time-resolved images of the plume by carbon and permalloy nanoparticles after laser vaporization were collected using a high-speed video camera. These images suggest that the hot plumes due to carbon and permalloy nanoparticles do not mix together so extensively, at least in a few hundred microseconds after laser vaporization. The effect of time delay between two laser pulses on the yield and the diameter distribution of SWNTs was also presented and discussed.     

Diagnostics of laser-ablated carbon plumes by photoionization using a tunable laser

Tunable laser pulses at wavelengths from 250 to 880 nm were fired at plumes produced by laser ablation of a carbon target in He atmosphere. We observed an electrical current due to photoionization, which roughly reflected the behavior of carbon clusters in the plume. The photoionization current had dynamic temporal variations, comprising a rapid increase at the beginning (T_D<0.2 ms, where T_D is the time after a YAG laser pulse for laser ablation irradiates the target), a gradual increase for 0.2D<3 ms, and a slow decrease for T_D>7 ms. The increasing phase of the photoionization current was synchronized with the decreasing phase of C₂ radical density. For He gas pressures lower than 0.8 Torr no photoionization current was detected. The growth rate of the photoionization current was higher for a higher He gas pressure.     

Optical characterisation of the photolytic decomposition of ferrocene into nanoparticles

Optical emission from the photolytic dissociation of ferrocene Fe(C₅H₅)₂, often abbreviated as FeCp₂, in argon atmosphere was studied. The dissociation was performed by using an ArF excimer laser, operating at a wavelength of 193 nm. Two pressure regions were examined. At low (0.1 mbar) pressure, several emission lines of Fe could be identified, however no C, C₂, or CH emission lines/bands were found. At a higher (20 mbar) pressure of the FeCp₂/Ar gas mixture, a broadband emission identified as blackbody radiation was observed. This blackbody radiation originates from nanoparticles with a mean size of 30 nm, which consist of both metallic iron and amorphous carbon. The initial colour temperature of the particles was 2600 K.     

Synthesis of single wall carbon nanotubes by using arc discharge technique in nitrogen atmosphere

Single wall carbon nanotubes (SWNTs) were prepared with arc discharge technique using Ni/Co carbon composite rod in He, Ar, and N₂ atmosphere, respectively. The yield and the diameter distribution of them were compared with each other. The results show that N₂ atmosphere at low pressure gives the highest yield for the formation of SWNTs, almost comparable to that obtained with laser furnace technique. It also declares that He atmosphere seems to make SWNTs having smaller diameter distribution than those obtained in N₂ and Ar atmosphere. These findings were summarized and used for the discussion related to the formation mechanism of SWNTs.     

A novel route to aligned nanotubes and nanofibres using laser-patterned catalytic substrates

We describe the generation of aligned carbon nanotube bundles and films by pyrolysis of solid organic precursors (for example 2-amino-4,6-dichloro-s-triazine, s-triaminotriazine) at 950-1050 °C over laser-patterned thin metal (Fe, Co, Ni) films, deposited on silica substrates. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) studies reveal that surface roughness of the laser-etched catalytic substrates plays a key role in achieving control of nanotube growth. We believe that, during the etching process, the energised (ablated) metal clusters condense and recrystallise evenly, possibly as the metal oxide, within the edges or surface of the eroded regions. During pyrolysis these catalytic particles, embedded in the silica substrates, are responsible for carbon agglomeration and subsequent tube axial growth, suggesting that nanotube alignment strongly depends upon the etching conditions (for example laser power, pulse duration, and focal distance). The pyrolysed products (usually nanotubes or nanofibres) were characterised by SEM, high-resolution transmission electron microscopy (HRTEM), electron energy loss spectroscopy (EELS) and energy dispersive X-ray spectroscopy (EDX). Samples containing only small amounts of amorphous carbon and other carbonaceous particles are notably absent. We observe that the degree of graphitisation is dependent upon the catalyst and the organic precursor. Interestingly, a nitrogen content З% was detected within the nanofibres, which exhibit corrugated graphite-like morphologies. This pyrolytic method may be used to advantage in generating aligned heteroatomic nanostructures such as B_xC_yN_z systems.     

Plasma characterization and room temperature growth of carbon nanotubes and nano-onions by excimer laser ablation

The deposition of carbon nanotubes and carbon nano-onions at room temperature using excimer laser radiation to ablate mixed graphite-metal targets is described. Our deposition conditions are in contrast to other investigations on the pulsed laser deposition of carbon nanotubes that have employed high temperatures and high pressures. We find that the formation of these carbon nanostructures is dependent on the ambient gas employed during ablation. In the presence of O₂ gas, carbon nanotubes and nano-onions are produced, while inert atmospheres such as Ar yield amorphous carbon. High-resolution, in situ, time-resolved emission spectroscopy has been used to track the evolution of species (C₂, C₃, Ni/Co) in the ablation plume. Spectral fits on low and high-resolution spectra reveal that the vibrational-rotational temperatures for C₂ produced in O₂ remain at ∼5000 K for nearly 20 μs but drop rapidly in Ar. Details of the formation of carbon nanotubes and nano-onions, and in situ time-resolved optical emission spectroscopy are described.     

Growth of carbon nanotubes on Fe or Ni-coated Si substrate by feeding with carbon from a graphite ablation plume

By feeding with carbon clusters from the ArF excimer laser (=193 nm) ablation of graphite target, carbon nanotubes (CNTs) were grown on Fe and Ni films deposited on SiO₂/Si substrates, which were set inside a quartz tube with Ar gas pressure of 500 Torr operating at 1100 °C. Optical emission spectroscopic observation of the ablation plume of graphite and Ni/Y catalyst was performed in the Ar gas for a pressure range of 0–600 Torr at room temperature and 1000 °C. The emission band intensity of C₃ (ù_u) at the distance of 2 mm from the target increased with increasing Ar gas pressure. PACS 79.20.Ds; 81.07.De; 39.30.+w     

Formation of single-wall carbon nanotubes in Ar and nitrogen gas atmosphere by using laser furnace technique

The formation of single-wall carbon nanotubes (SWNTs) by using laser vaporization technique in different ambient gas atmosphere was investigated. SWNTs were prepared with Rh/Pd (1.2/1.2 atom%)-carbon composite rod in Ar and nitrogen gas atmosphere, respectively. Raman spectra of raw carbon materials including SWNTs and photoluminescence mapping of dispersed SWNTs in a surfactant solution demonstrate that the diameter distribution of SWNTs prepared in Ar atmosphere is narrower than those obtained by using CVD technique (e.g. HiPco nanotube), even when the ambient temperature is as high as 1150 ^○C. It was also found that nitrogen atmosphere gives wider diameter distribution of SWNTs than that obtained with Ar atmosphere. Furthermore, the relative yield of fullerenes (obtained as byproducts) is investigated by using HPLC (high-performance liquid chromatography) technique. It was found that the relative yield of higher fullerenes becomes lower, when nitrogen is used as an ambient gas atmosphere. Based on these experimental findings, a plausible formation mechanism of SWNTs is discussed.     

Raman spectroscopy of SWNTs produced by a XeCl excimer laser ablation at high temperatures

Synthesis of single-wall carbon nanotubes (SWNTs) was carried out by an ablation method using a XeCl excimer laser. It was irradiated onto a graphite target containing Co and Ni at the temperatures of 1073, 1173, 1273, 1373, 1473, 1523 and 1623 K under the atmosphere (0.1 MPa) of Ar gas with the flow rate of 12 ml/min. The measurement by a scanning/transmission electron microscope and Raman spectroscopy found the formation of SWNTs with the diameter of about 1.3 nm and the length of about 2 μm in ablated carbonaceous soot. The ratio of peak intensity of 1590 cm⁻¹ (G band) to that of 1335 cm⁻¹ (D band) in the high frequency Raman spectra increased with increasing the ambient temperature. The radial breathing mode (RBM) in the low frequency Raman spectra shows that the mean diameter of SWNTs increased with increasing the ambient temperature.     

New aspects on pulsed laser deposition of aligned carbon nanotubes

We have grown carbon nanotubes (CNT) by pulsed laser deposition (PLD) at 1000 °C in Ar atmosphere. A Nd/YAG laser was used for irradiation of a graphite target containing Ni and Co rods. High-resolution scanning electron microscopy (HRSEM) and transmission electron microscopy (TEM) images showed that “closed” carbon nanotubes were grown between clusters of metallic particles, so that the individual nanotubes were arranged in parallel to each other forming a shape of “Rope-Bridge”. The nanotubes structure was analyzed by high-resolution transmission electron microscopy (HRTEM) and their type was found to be of MWNT, containing about five SWNT. Total diameter was 5-20 nm and their length was about 1 μm. High homogeneous distribution carbon nanotubes were grown and different structures were observed such as well-aligned carbon nanotubes, bamboo-like and Y-junction carbon nanotubes.     

Temperature distribution above a heated surface in a CVD reactor with hydrocarbon-containing gaseous mixtures

Pyrolysis of C₂H₂ and CH₄ gases near a heated surface was investigated using coherent anti-Stokes Raman spectroscopy (CARS). Gas temperature and hydrocarbon density spatial profiles above the surface of deposition were measured. Although the measured hydrocarbon density distribution above the surface showed a gradual decrease upon approaching the surface, the observed temperature distribution revealed the presence of a temperature maximum at distances of 300-600 7m from the surface. The appearance of this maximum is explained by the release of condensation energy in the region where intensive homogeneous nucleation of carbon clusters takes place.     

Investigation of TiC-C coatings by X-ray photoelectron spectroscopy

Titanium carbide-based coatings deposited by arc-technology in C₂H₂/Ar atmosphere were studied by X-ray photoelectron spectroscopy. It was found that, in addition to the cubic phase of TiC _x O _y oxycarbide, the films contain carbon in the amorphous, presumably graphite-like state. In carbon C1s spectra, bands at 282.0, 284.4, and 286.0 eV correspond to the TiC _x state, amorphous carbon, and C-C bonds, respectively. The maximum at 283.0 eV was interpreted as the C state in titanium carbide nanoforms, i.e., Ti₁₄C₁₃ clusters or Ti₈C₁₄ carbohedra. The phase ratio was varied by coating deposition conditions, i.e., TiC/a-C deposition by Ti cathode sputtering in C₂H₂/Ar, and composite Ti/C target sputtering in Ar and C₂H₂/Ar. When using the Ti cathode and C₂H₂/Ar gas mixture, the ratio of carbide and amorphous a-C phases was estimated as 1: 1; the surface layer ～15 nm thick was enriched with amorphous carbon. It was assumed that TiC/a-C composite coatings with an additional a-C film on the surface would have an increased stability in reaction media and good biocompatibility.     

Large Relative Raman Shift for Molecules Adsorbed on Metallic Nano-particles

The enhancement of two order-of-magnitudes is observed in surface-enhanced Raman spectroscopy （SERS） of gases （CO, C2H2, C2H4, etc） adsorbed on nitric acid-roughened metal foil. In addition, some Raman lines of gases adsorbed on these active substrates show larger frequency shifts and linewidth broadening, compared with the Raman spectroscopy of free gases. Using the two-oscillator electromagnetic model, we explain this phenomenon. It is related to the large non-regular particles on the active substrate we prepared. It is found that the parameters of the surface-plasmon dispersion, the distance of molecules from the surface and the radius of particles play crucial roles on the relative large Raman shifts.     

Stability and Growth Modes of Ni--C Clusters: A Study based on All-Electron Density Function Theory

Growth modes of the free-standing NiCN （N ≤ 8） and Ni2CN （N ≤ 8） dusters are investigated by the allelectron density functional theory. The results reveal that there are two competing modes for the growth of these clusters： the linear chain and the ring structure without transannular bonds. The lowest-energy geometries of NiCN （N ≤ 8） are the linear chains with the Ni atom at one end, except for NiC2 and NiCT. The Ni2CN （N ≤ 8） clusters all prefer to the linear chains with the two Ni atoms at the two ends. Miilliken population analysis indicates that the total spin of the lowest-energy cluster show significant odd-even alternation. The NiMCN （M = 1,2） clusters with the even N are one and those with the odd-N are zero.     

Single-walled carbon nanotubes produced by cw CO2-laser ablation: study of parameters important for their formation

The production of single-walled carbon nanotubes (SWNTs) using the cw CO₂-laser ablation technique is reported. Different metals and metal concentrations in the carbon targets as well as different buffer gases and gas pressures have been used in order to study their influence on the formation of SWNTs. It is shown that the conditions near the evaporation zone, i.e. especially the local temperature environment induced by the laser radiation as well as the used metals play a key role in the formation process of SWNTs. Employing a very simple experimental setup the cw CO₂-laser ablation technique easily provides the environment favorable for the growth of high quality SWNT material under a wide range of experimental conditions.     

Deposition of C60 polymer films under various plasma conditions

We studied the deposition of C₆₀ polymer films under different Ar plasma conditions. The films were deposited at a pressure range between 1.3 and 40 Pa, and the input power was varied from 10 to 70 W. The films were investigated by Raman spectroscopy to confirm the C-C valence states of the polymeric phases. C₆₀ polymers were formed under various experimental conditions. However, the depositions resulted in non-uniform films consisting of unpolymerized C₆₀, dimers, linear chains and polymeric planes. Amorphous carbon was found in the films deposited at 13 and 20 Pa (50 W input power) and 13 Pa (70 W).     

Formation of single-wall carbon nanohorn aggregates hybridized with carbon nanocapsules by laser vaporization

Single-wall carbon nanohorn (SWNH) aggregates hybridized with carbon nanocapsules (CNCs) were fabricated at a high yield (∼70%). The carbon was laser-vaporized for 2 s into an Ar gas atmosphere with one of the following: Fe, Al, Si, Co, Ni, Cu, Ag, La₂O₃, Y₂O₃, and G₂O₃. By optimizing the Ar gas pressure and metal content, we were able to produce hybridized SWNH structures for Fe, Co, Ni, Cu, and Ag. Possible mechanisms for governing hybrid production, which occurs with smaller CNCs (<100 nm) with only certain metals and carbide, are discussed on the basis of thermal and catalytic graphitization. PACS 61.46.Df; 68.37.Lp; 81.16.Mk