Xitong Zhang

Mechanism of the Defect Formation in Supported Graphene by Energetic Heavy Ion Irradiation: the Substrate Effect

Although ion beam technology has frequently been used for introducing defects in graphene, the associated key mechanism of the defect formation under ion irradiation is still largely unclear. We report a systematic study of the ion irradiation experiments on SiO2-supported graphene, and quantitatively compare the experimental results with molecular dynamic simulations. We find that the substrate is, in fact, of great importance in the defect formation process, as the defects in graphene are mostly generated through an indirect process by the sputtered atoms from the substrate.

Growth and structure of C60 thin films on NaCl, glass and mica substrates

Abstract We studied the growth and structure of C60 thin film condensed on NaCl, glass and mica substrates by transmission electron microscopy. Highly ordered, (111) textured and epitaxial thin films were obtained on (001) NaCl and mica respectively. Various deposition parameters including different substrate temperatures, deposition rates and film thicknesses were experimented with. The orientational order and nature of defects present in the films were assessed by electron diffraction, bright and dark field images. We ascribe the abnormal reflections in the electron diffraction patterns which had been thought by some workers to belong to the diffraction of an h.c.p. structure phase to the existence of stacking disorder in the f.c.c. structure.

Study of microstructure of epitaxial fullerenes films

Abstract A comprehensive study was made on the structure of epitaxial thin films of C 60 and C 70 by means of transmission electron microscopy. Both the films show similar face-centered cubic structure and are epitaxial on (001) mica with close-packed plane parallel to the substrate surface. Two main kinds of defects-stacking faults and twins-were observed and are discussed. The effect of the remaining C 70 impurity on the crystal orientation of C 60 films was studied by comparing different samples made from high-purity fullerene and C 60 /C 70 mixtures. The results show that there is a higher density of planar defects in the films containing larger amounts of impurities: moreover, some faint anomalous reflections located at so-called 2a 0 fcc reciprocal lattice points were also detected, probably as a result of C 70 contamination. Finally, it is found that stacking disorders can be easily increased by keeping the high-quality pure C 60 film in air at room temperature for a few weeks, implying the instability of the crystal orientation of the epitaxial fullerene films.

Surface enhanced electronic transport. A new method to probe the possible interactions between C60 and non-alkali metals

Abstract By measuring the resistance in situ during the deposition of C 60 on ultrathin metal layers in a UHV system, we find the resistance decreases sharply in most cases. This enhanced electronic transport in metal—C 60 multilayers (bilayers), may be related to charge transfer from the metal to C 60 and surface bonding effects, and hence can be used as a new possible approach for probing the interfacial interactions between C 60 and metals.

The bias-temperature effect in a rectifying Nb/C60/p-Si structure: evidence for mobile negative charges in the solid C60 film

Solid C60 film was grown on a p-type Si substrate and a rectifying Nb/C60/p-Si structure was prepared. Capacitance-voltage (C-V) measurements showed that for temperatures above 260 K the C-V curve of the Nb/Co60/p-Si structure shifted along the voltage axis depending on biasing conditions. We analysed this effect to reveal the existence of mobile negative charges in the C60 layer and determine the density of the mobile charges.

A study of interface structure between diamond film and silicon substrate

Abstract Diamond thin films have been successfully grown on Si(100) substrates at low temperatures from 315°C to 685°C by a microwave plasma chemical vapour deposition system. It is found that when oxygen is added to the CH 4 H 2 gas mixtures at low temperature (below 600°C) the first step is the formation of SiO 2 on the silicon substrate, and a diamond film is formed on SiO 2 .

Comparison of electronic energy loss in graphene and BN sheet by means of time-dependent density functional theory

Time-dependent density functional theory combined with Ehrenfest dynamics are employed to calculate electronic energy loss of energetic ions in two-dimensional graphene and white graphene (BN) targets. Special attention is paid to the effects of different electronic structures on their stopping power. Our results show that the energy transferred to the graphene target is much larger than to BN for both H(+) and He(2+) projectiles. Since the energy is mainly deposited into the electronic degree of freedom, it means that the electronic structure of the target plays an important role in determining the collision process. Our analysis indicates that more excited electrons are observed in graphene compared to BN. At low energies, a velocity proportional relation is found in the electronic energy loss of H(+) and He(2+) in both graphene and BN. In particular, a threshold velocity is observed for He(2+). Finally, we have compared the energy transfer from neutral and charged projectiles when they collide with graphene and BN and the results show that charged projectiles damage the targets more severely.

Synthesis of K3C60 single crystal thin films with high critical currents

Abstract High quality K3C60 single crystal thin films have been synthesized on (001) mica substrates. The normal state electrical resistivity shows classic metallic temperature dependence and can be fit to a functional form of ϱ(T)=a+bT2. The film has a sharp transition to superconductivity near 22K, with a 10% to 90% width less than 700 mK. The experimental data near Tc have poor agreements with the Aslmozov-Larkin type fluctuation. Furthermore, critical current density up to 50 000 A/cm2 has been obtained at 4.7 K by continuous DC measurements, the preliminary analysis indicate that Jc is proportional to 1-(T/Tc)3.

Rubidium-doped epitaxial C60 thin films: synthesis and electronic transport

Epitaxial thin films of C60 have been synthesized and doped with rubidium. Above 80 K, the sample resistivity shows classic metallic temperature dependence just like that of K3C60 single crystals. However, the temperature coefficient changes sign at about 80 K, and the resistivity increases by 5% before the superconducting state near 30 K is reached. As for different samples, the zero-resistance temperature varies sharply with the sample lowest resistivity and can be fitted to a functional form of Tc0=a+b rho -5. The critical current density Jc is 103-104 A cm2 at approximately= 5 K, 0 T and it is found that Jc is proportional to (1-T/Tc)alpha , where alpha is in the range 1.3-2.0. Furthermore, the voltage-current characteristic of the films is very similar to that of high-Tc superconductors and remains a problem for further research on this new kind of material.

Theoretical investigation on radiation tolerance of

Ternary phases with layered hexagonal structures, as candidate materials used for next-generation nuclear reactors, have shown great potential in tolerating radiation damage due to their unique combination of ceramic and metallic properties. However, materials behave differently in amorphization when exposed to energetic neutron and ion irradiations in experiment. We first analyze the irradiation tolerances of different (MAX) phases in terms of electronic structure, including the density of states (DOS) and charge density map. Then a new method based on the Bader analysis with the first-principle calculation is used to estimate the stabilities of MAX phases under irradiation. Our calculations show that the substitution of Cr/V/Ta/Nb by Ti and Si/Ge/Ga by Al can increase the ionicities of the bonds, thus strengthening the radiation tolerance. It is also shown that there is no obvious difference in radiation tolerance between and due to the similar charge transfer values of C and N atoms. In addition, the improved radiation tolerance from Ti3AlC2 to Ti2AlC (Ti3AlC2 and Ti2AlC have the same chemical elements), can be understood in terms of the increased Al/TiC layer ratio. Criteria based on the quantified charge transfer can be further used to explore other phases with respect to their radiation tolerance, playing a critical role in choosing appropriate MAX phases before they are subjected to irradiation in experimental test for future nuclear reactors.