质子束轰击下MWCNTs无定形结构的转变

在室温下用70keV质子束多壁碳纳米管(MWCNTs),研究了多壁碳纳米管从石墨结构向无定形结构的转变过程。轰击后,MWCNTs外围的石墨结构变成均匀的无定形结构包覆着内部的石墨结构。增大照射量,这种无定形化的过程继续向MWCNTs内层推进,直至形成一个中空的无定形纳米线结构。质子束轰击引入的MWCNTs的结构转变,是从外层向内层逐步推进的石墨结构向无定形结构的转变过程。本文对这种结构演化的机理进行了讨论。     

Carbon nanostructures produced through ion irradiation

Several nanostructures we produced by ion irradiation have been reviewed in this paper. By using ions to irradiate two ultrahigh molecular weight polyethylene targets respectively, it was found that small fullerenes C20 and C26 were grown, adding two members to the fullerene family. Meanwhile, crystalline diamonds also have been produced by Ar+ ions irradiation of graphite. In the experiment of double ions Ni+ and Ar+ irradiation, nanoscale ar-gon bubbles formed. On the other side, when multi-wall carbon nanotubes were irradiated by C+, many MWCNTs evolved to amorphous carbon nanowires and amorphous carbon nanotubes. And there are possible welding in the crossed nanotubes.     

利用粒子辐照技术实现碳纳米管向金刚石的转变

采用低能氢等离子体和中能C^ 离子束辐照技术相结合的方法,实现了碳纳米管向金刚石纳米晶粒的转变,完成了一个从有序(碳纳米管)到无序(无定形碳纳米线)再到有序(金刚石纳米晶)的转变过程。利用透射电子显微镜(TEM)、选区电子衍射(SAD)和拉曼光谱(Raman)等研究了晶粒的微观结构,并对纳米金刚石晶粒的生成机理进行了初步探讨。     

Structural characterization of amorphous Fe–Si and its recrystallized layers

We have synthesized amorphous Fe–Si thin layers and investigated their microstructure using transmission electron microscopy (TEM). Si single crystals with (1 1 1) orientation were irradiated with 120 keV Fe⁺ ions to a fluence of 4.0 × 10¹⁷ cm⁻² at cryogenic temperature (120 K), followed by thermal annealing at 1073 K for 2 h. A continuous amorphous layer with a bilayered structure was formed on the topmost layer of the Si substrate in the as-implanted specimen: the upper layer was an amorphous Fe–Si, while the lower one was an amorphous Si. After annealing, the amorphous bilayer crystallized into a continuous β-FeSi₂ thin layer.     

Shortening of multi-walled carbon nanotubes by γ-irradiation in the presence of hydrogen peroxide

Multi-walled carbon nanotubes (MWCNTs) were effectively cut with a facile and mild cutting method by using γ-irradiation in the presence of hydrogen peroxide. The TEM, Raman and XRD results showed that the structural integrity of the cut MWCNTs was preserved with a little surface functionalization.     

Electron-beam radial distribution analysis of irradiation-induced amorphous SiC

Advanced electron microscopy techniques have been employed to examine atomistic structures of ion-beam-induced amorphous silicon carbide (SiC). Single crystals of 4H-SiC were irradiated at a cryogenic temperature (120 K) with 300 keV Xe ions to a fluence of 10¹⁵ cm⁻². A continuous amorphous layer formed on the topmost layer of the SiC substrate was characterized by energy-filtering transmission electron microscopy in combination with imaging plate techniques. Atomic pair-distribution functions obtained by a quantitative analysis of energy-filtered electron diffraction patterns revealed that amorphous SiC networks contain heteronuclear Si–C bonds, as well as homonuclear Si–Si and C–C bonds, within the first coordination shell. The effects of inelastically-scattered electrons on atomic pair-distribution functions were discussed.     

Radiation effects of pyrochlore-rich synroc by heavy-ion irradiation

Heavy-ion irradiation is commonly used to study radiation damage of high level radioactive waste (HLW) forms, but S ion was never used before. In this investigation, 100 MeV ^32S ions produced by tandem accelerator was used to study radiation effects on pyrochlore-rich synroc which contained simulated actinides. The amorphization and amorphous doses were determined by X-ray diffractometer (XRD) and transmission electron microscopy/select area electron diffraction (TEM/SAED). The vacancy defects induced by heavy-ion irradiation were characterized by using positron annihilation technique (PAT). The experimental results show that the amorphous dose is 0.5 dpa, the defects produced by heavy-ion irradiation are mainly voids, and irradiation could continue to intensify the vacancy defects even after the amorphous dose was reached.     

A molecular dynamics study of the clustering of implanted potassium in multiwalled carbon nanotubes

We have studied the low energy irradiation of carbon nanotubes (CNT) with K ions using classical molecular dynamics simulations with analytical potentials. The studied CNTs had diameters of about 0.5–1.2 nm and single or multiple walls. The average penetration depth and probabilities to introduce an impurity atom into CNT were studied with simulations on irradiating the CNT with single K ion. The number of potassium clusters, their average sizes and the damage produced into the CNT due to the irradiation were studied using multiple K ion irradiations. We found that the K ions are mobile in CNTs right after the implantation event and that they cluster together. For CNTs with 1–3 coaxial tubes, the highest ratio of K atoms in clusters per total number of K ions was obtained by using an irradiation energy of about 100 eV. Also the least damage per K ion was found to be produced into the CNT with this energy when those energies high enough for the ion to penetrate the outermost wall of the CNT were considered.     

AFM and STM investigation of carbon nanotubes produced by high energy ion irradiation of graphite

Carbon nanotubes (CNT) were produced by high energy, heavy ion irradiation (215 MeV Ne, 246 MeV Kr, 156 MeV Xe) of graphite. On samples irradiated with Kr and Xe ions large craters were found by atomic force microscopy, these are attributed to sputtering. Frequently one or several CNTs emerge from the craters. Some of the observed CNTs showed a regular vibration pattern. No other carbon based materials, like amorphous carbon or fullerenes were evidenced. Focused ion beam cuts were used to compare CNTs with surface folds on graphite.     

Plasma Syntheses of Carbon Nanotube-Supported Pt-Pd Nanoparticles

It is reported that the highly dispersed Pt nanoparticles on carbon nanotubes can be synthesized under mild conditions by in situ plasma treatment.The carbon nanotube was pretreated by O₂ plasma to transform into oxide carbon nanotubes（O-CNTs）,and then it was mixed with the precursors（the mixture of H₂ PtCl₆and PdCl₆）.After that,the O-CNTs and the precursors were simultaneously treated by H₂ plasma.The precursors were transformed into Pt-Pd nanoparticles（NPs）and the O-CNTs transformed into CNT.The synthesized CNT-based Pt-Pd nanoparticles were characterized by scanning electron microscopy,transmission electron microscopy,X-ray diffraction and X-ray photoelectron spectroscopy.All the analysis showed that the Pt-Pd nanoparticles were deposited on CNT as a form of face-centered cubical structure.     

Electron transferring from titanium ion irradiated carbon nanotube arrays into vacuum under low applied fields

Field emission characteristics of carbon nanotube arrays synthesized by thermal chemical vapor deposition on iron ion pre-bombarded silicon substrate are enhanced by titanium ion irradiation. A pronounced degradation of turn-on electric field of 0.305 V/μm and threshold field, of which the lowest value is only 1.054 V/μm, about 0.482 V/μm at the dose of 5 × 10¹⁶ ions/cm² is as an expression of this enhancement. Scanning electron microscopy, Raman spectroscopy, x-ray photoelectron spectroscopy, Photoelectron spectrometer and transmission electron microscopy are measured for comparison before and after the Ti ion irradiation of the carbon nanotube arrays, and the results reveal that the formation of carbon nanorod/nanotube heterostructure during ion irradiation plays a dominative role in the promotion of the field emission properties. However, high-dose irradiating transaction on carbon nanotube arrays will exert repulsive effects on the field emission characteristics for the introduction of severe structural damage. Additionally, the longtime eminent stability behaviors under high applied fields have provided a possibility for the potential application of field emission flat panel display or electron emitters based on carbon nanotube arrays.     

Enhanced field emission from nano-graphite coated carbon nanotubes

An effective method by low energy carbonhydrogen ion treatment to enhance field emission of the carbon nanotubes (CNTs) is demonstrated. Comparing with control, field emission (FE) currents of the CNTs by carbonhydrogen ion irradiation increased, and the turn-on field and the threshold field decreased significantly. The structure characteristic revealed by transmission electron microscopy demonstrates that CNTs are coated by nano-graphite particles after being treated with low energy carbonhydrogen ion and that there are large quantities of defects and grain boundaries in the coated layer. It is considered that the coating layer can decrease the effective surface work function of CNTs and correspondingly increase field emission. In addition, the defects, the grain boundaries and the C-H dipoles forming in the process of the low energy ions irradiation can effectively enhance the field emission.     

Etude au microscope electronique des defauts de structure dans l'oxyde de beryllium fritte irradie

Irradiated beryllium oxide was examined by electron microscopy, by means of a method which permits study of samples in which structural damage prevents fractographic examination.

Irradiation reduces intergranular cohesion and, as has already been established for magnesia, the beryllium oxide crystallites are honeycombed by groups of point defects which greatly perturb the lattice. Examination of samples annealed at 900–1500° C after irradiation has confirmed that these defects group themselves in linear arrays of small loops which gradually become elongated in the basal plane of the crystal.

Microhardness measurements suggest that dislocations, if indeed they continue to exist, become much less mobile after irradiation.     

Atomic-scale structure of irradiated GaN compared to amorphised GaP and GaAs

We have compared the atomic-scale structure of ion irradiated GaN to that of amorphised GaP and GaAs. While continuous and homogenous amorphised layers were easily achieved in GaP and GaAs, ion irradiation of GaN yielded both structural and chemical inhomogeneities. Transmission electron microscopy revealed GaN crystallites and N₂ bubbles were interspersed within an amorphous GaN matrix. The crystallite orientation was random relative to the unirradiated epitaxial structure, suggesting their formation was irradiation-induced, while the crystallite fraction was approximately constant for all ion fluences beyond the amorphisation threshold, consistent with a balance between amorphisation and recrystallisation processes. Extended X-ray absorption fine structure measurements at the Ga K-edge showed short-range order was retained in the amorphous phase for all three binary compounds. For ion irradiated GaN, the stoichiometric imbalance due to N₂ bubble formation was not accommodated by Ga–Ga bonding in the amorphous phase or precipitation of metallic Ga but instead by a greater reduction in Ga coordination number.     

Energy loss of protons in carbon nanotubes: Experiments and calculations

We have studied the energy loss of protons in multi-walled carbon nanotube (MWCNT) samples, both experimentally and theoretically. The experiments were done in transmission geometry, using 6 and 10 keV proton beams, with the MWCNT targets dispersed on top of a ～20 nm-thick holey carbon coated TEM grid (amorphous carbon film, a-C). The energy loss of protons interacting with the MWCNTs and the amorphous carbon film is obtained after analyzing the signals coming from both types of carbon allotropes. The electronic energy loss of protons is calculated using the dielectric formalism, with the target energy loss function built from optical data. Comparison of experimental and theoretical data indicates that model calculations appropriate for three-dimensional (bulk) targets substantially overestimate the energy loss to MWCNTs. In contrast, a recent parameterization of the dielectric function of MWCNTs predicts significantly lower stopping power values compared to the bulk models, which is more in line with the present experimental data when considering the additional stopping mechanisms that are effective in the keV range.     

Visible photoluminescence of SiO2 implanted with carbon and silicon

The structures formed after sequential implantation of silicon plus carbon in amorphous SiO₂ and annealing presented strong photoluminescence bands in the deep red (1.4–1.6 eV) and green (2.0–2.2 eV) regions of the visible spectrum. The energy and intensity of the bands depended strongly on the temperature and duration of annealing. Different behaviours with post-processing were encountered for the red and green bands, including deexcitation kinetics and structural origin. The FTIR, Raman and HRTEM measurements showed that silicon crystallites were reponsible for the red emitting band while carbon aggregates were probably the origin of the green one.     

Loop formation by ion irradiation in yttria stabilized zirconia

Yttria stabilized zirconia (YSZ) is a candidate material focused as optical and insulating materials in nuclear reactors. Therefore, it is useful to investigate defect formation during irradiation, in order to assess YSZ resistance to radiation damage. In the present study, in situ transmission electron microscopy (TEM) observations were performed on YSZ during 30 keV Ne⁺ ion irradiation in the temperature range of 723–1123 K (using 100 K intervals). Results revealed that damage evolution morphology depends on irradiation temperature. For irradiations below 1023 K, defect clusters and bubbles were formed simultaneously. On the other hand, at 1123 K, only bubbles were formed in the initial stage of irradiation. Loops formed later following the bubble formation. It was also observed that, in the early stage of irradiation above 923 K, larger bubbles were formed along the loop planes compared with other areas.

TEM observations indicated that dislocation loops formed on three kinds of crystallographic planes: namely, {1 0 0}, {1 1 1} and {1 1 2} planes.     

Nucleation and growth of defect clusters in CeO2 irradiated with electrons

Nucleation and growth process of defect clusters in cerium dioxide (CeO₂) with fluorite-type crystal structure has been investigated in situ under electron irradiation by using high voltage transmission electron microscopy. Planar defect clusters were formed with electron irradiation ranging from 200 to 1000 keV at temperatures below 450 K. The defect clusters were determined to be faulted-interstitial type dislocation loops lying on {1 1 1} planes. The growth rate of dislocation loops was found to increase with decreasing electron energy. An analysis of the fluence dependence of the growth process of dislocation loops suggests an increase in the vacancy mobility with decreasing electron energy. The rate of the electronic excitation is discussed in terms of the radiation-induced diffusion of oxygen-ion vacancies.     

Depth profiling of heavy-ion-mixed TiN films

A survey is given on the modifications of 30–300 nm thin TiN films on various substrates under high-fluence irradiation with 80–700 keV Ar, Kr and Xe ions. In particular, the effects of sputtering, interface mixing and blister formation were investigated. The results were obtained by combining several depth-profiling techniques, such as resonant nuclear reaction analysis (RNRA), Rutherford backscattering spectroscopy (RBS) and proton-induced X-ray emission (PIXE). Scanning electron microscopy and adhesion tests were also applied to the films. Optimal interface mixing parameters are proposed on the basis of the measured sputtering and mixing rates.