Formation Of Carbon Nanoclusters Research Articles

Crystal-Chemical Features of Diamonds Implanted with Helium Ions

Implantation of high-energy helium ions into crystals of natural colorless diamonds colors them bright yellow, brown, or black, depending on the implantation dose. The nature of the induced fantasy color and crystal-chemical features of the implanted diamonds have been investigated by IR spectroscopy and Raman spectroscopy. It is established that the radiation effect of helium ions on the diamond crystal structure results in the transformation of nitrogen-vacancy centers in its bulk with the formation of new A-type color centers, as well as the formation of carbon nanoclusters with the sp2 hybridization of bonds (which is typical of graphite) in black diamonds.

Effects of Thermal Annealing on the Structural and Optical Properties of Carbon-Implanted SiO2

Amorphous carbon (a-C) nanoclusters were synthesized by the implantation of carbon ions (C-) into thermally grown silicon dioxide film (-500 nm thick) on a Si (100) wafer and processed by high temperature thermal annealing. The carbon ions were implanted with an energy of 70 keV at a fluence of 5 x 10(17) atoms/cm2. The implanted samples were annealed at 1100 degrees C for different time periods in a gas mixture of 96% Ar+4% H2. Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and High Resolution Transmission Electron Microscopy (HRTEM) were used to study the structural properties of both the as-implanted and annealed samples. HRTEM reveals the formation of nanostructures in the annealed samples. The Raman spectroscopy also confirms the formation of carbon nano-clusters in the samples annealed for 10 min, 30 min, 60 min and 90 min. No Raman features originating from the carbon-clusters are observed for the sample annealed further to 120 min, indicating a complete loss of implanted carbon from the SiO2 layer. The loss of the implanted carbon in the 120 min annealed sample from the SiO2 layer was also observed in the XPS depth profile measurements. Room temperature photoluminescence (PL) spectroscopy revealed visible emissions from the samples pointing to carbon ion induced defects as the origin of a broad 2.0-2.4 eV band, and the intrinsic defects in SiO2 as the possible origin of the -2.9 eV bands. In low temperature photoluminescence spectra, two sharp and intense photoluminescence lines at -3.31 eV and -3.34 eV appear for the samples annealed for 90 min and 120 min, whereas no such bands are observed in the samples annealed for 10 min, 30 min, and 60 min. The Si nano-clusters forming at the Si-SiO2 interface could be the origin of these intense peaks.

Optical and dielectric properties of 55 MeV carbon beam-irradiated polycarbonate films

Polycarbonate films were irradiated with 55 MeV carbon (C 5+) beam at different fluences from 1×1011 to 1×1013 ions/cm2. The structural, optical and dielectric properties are studied. The X-ray diffraction pattern shows the decrease in crystallinity and increase in average intermolecular spacing with an increase in fluence due to the formation of carbon nanoclusters in the polymer matrix. UV–VIS absorption spectra show that the band gap energy decreases upon irradiation. It depends on the ion fluence and growth of carbon nanoclusters. The Fourier transform infrared spectra evidenced cross-linking and chain scissoring both at high fluences. The dielectric constant decreases while dielectric loss and alternating current conductivity increase with fluence.

FORMATION OF NANOCLUSTERS IN EXPANDING LASER PLUME

Formation of carbon nanoclusters in a laser-pulse created plume expanding in vacuum and in a noble gas environment at various pressures was studied. Experiments were performed with carbon nanoclusters formed by laser ablation of graphite targets with 12-picosecond 532 nm laser pulses at MHz-range repetition rate in a broad range of ambient He , Ar , Kr , and Xe gas pressures from 2 × 10-2 Torr to 1500 Torr. The experimental results confirmed our theoretical prediction that the average size of the nanoparticles depends weakly on the type of the ambient gas, and is determined exclusively by the single-laser pulse parameters. The most important finding relates to the fact that in vacuum the cluster size is mainly determined by hydrodynamic expansion of the plume, while in the ambient gas it is controlled by atomic diffusion in the gas.

Raman investigation of low temperature AlGaAs/GaAs(100) heterostructures

Abstract Method of Raman backscattering allowed one to study the substructure of epitaxial low temperature MOCVD AlGaAs/GaAs(1 0 0) heterostructures. It is shown that experimental data received during work correlate with results of the structural researches accomplished in the previous work. The assumption that at high concentration of a carbon acceptor atoms concentrate on defects of a crystal lattice of AlGaAs solid solution with formation of carbon nanoclusters is confirmed.

Expansion-limited aggregation of nanoclusters in a single-pulse laser-produced plume

Formation of carbon nanoclusters in a single-laser-pulse created ablation plume was studied both in vacuum and in a noble gas environment at various pressures. The developed theory provides cluster radius dependence on combination of laser parameters, properties of ablated material, and type and pressure of an ambient gas in agreement with experiments. The experiments were performed on carbon nanoclusters formed by laser ablation of graphite targets with 12 picosecond 532 nm laser pulses at MHz-range repetition rate in a broad range of ambient He, Ar, Kr, and Xe gas pressures from $2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}$ to 1500 Torr. The experimental results confirmed our theoretical prediction that the average size of the nanoparticles depends weakly on the type of the ambient gas used, and is determined exclusively by the single laser pulse parameters even at the repetition rate as high as 28 MHz with the time gap 36 ns between the pulses. The most important finding relates to the fact that in vacuum the cluster size is mainly determined by hydrodynamic expansion of the plume while in the ambient gas it is controlled by atomic diffusion in the gas. We demonstrate that the ultrashort pulses can be used for production of clusters with the size less than the critical value, which separates the particles with properties drastically different from those of a material in a bulk. The presented results of experiments on formation of carbon nanoclusters are in close agreement with the theoretical scaling. The developed theory is applicable for cluster formation from any monatomic material, such as silicon for example.

Modeling of Processes of the Formation and Transformations of Nanoclusters in a Rapidly‐Cooling Mixture of a Vapor and an Inert Gas

As the initial state, we consider a high‐temperature vapor consisting of atoms (or a mixture of atoms and small clusters). It is assumed that during a rapid expansion of the vapor and its mixing with a buffer gas (e.g., helium), it cools down to the temperature when the dissociation of clusters run slowly, while the processes of nucleation (association) of particles dominate. Such approach was employed, e.g., for modeling of processes of formation of carbon nanoclusters (CNC) in a carbon arc.

Hydrogen-enhanced clusterization of carbon in crystalline silicon

The influence of hydrogen impurities on the formation of carbon nanoclusters in carbon-enriched (>1017 cm−3) float-zone (FZ) silicon has been studied by means of EPR along with uniaxial-stress experiments. Hydrogen was incorporated by proton implantation at room temperature. As object of research, we used EPR as a signal of the earlier not identified EPR defect with S=1/2 and a C2 symmetry of g-tensor (denoted as PK4), which was observed in fast neutrons irradiated silicon. We have found that hydrogen implantation in FZ silicon with elevated content of carbon upon subsequent annealing above ∼250 °C generate most prominent EPR signal of the defect. We have revealed reliably three sets of hyperfine (hf) satellites: (1) one equivalent Si atom with strong localization (∼42%) of the resonant wave function of tetragonal symmetry; (2) four equivalent Si atoms with ∼16% of the resonant wave function; (3) weak hfi, which can be caused by an isotope of 13C from four equivalent carbon atoms. Uniaxial stress experiments reveal very large value (∼2.8 eV) of the activation energy for atomic reorientation that, along with data of piezospectroscopic tensor, are in good agreement with the cluster nature of the defect. We suggest that the presence of hydrogen leads to a strongly coordinated formation of nanocluster, which include carbon and self-interstitial atoms. It is suggested that the electronic structure of this defect corresponds to the double donor in a positive charge state.

Distinctive features of time-resolved photoluminescence of porous silicon coated with a layer of diamondlike carbon

Time-resolved photoluminescence from porous silicon coated with a diamondlike carbon film is investigated. The intensity of the photoluminescence from the carbon film is obserd to increase after deposition, and there is an accompanying change in the intensity and a shortwavelength shift of the photoluminescence band of porous silicon that depends on the porosity of its original layers. These changes are explained by the formation of carbon nanoclusters on the surface of the silicon filaments.

Catalytic role of nickel, palladium, and platinum in the formation of carbon nanoclusters

Carbon clusters were prepared by arc discharge from graphite anodes containing nickel, palladium, and platinum at conditions favoring nanotube production. All three metals led to very little encapsulation of themselves or their compounds into the carbon nanoclusters under the conditions studied. However, the characters of the samples, collected from the cathodic deposits as well as from the soot, were different for the three metal-filled rods. Nickel-filled anode stimulated the growth of single-walled nanotubes and also catalyzed the formation of strings of spherical carbon clusters. The growth mechanism of the strings of spherical clusters is proposed. Platinum-filled anode produced the highest density of nanotubes in the slag. Palladium-filled anode generates the same yield of fullerenes as pure graphite but shifts the ratio of fullerenes by enhancing the yield of C 70 and higher-order fullerenes.