The Microstructions Of Allotropic Carbon Materials Studied By Positiron Annihilation Techniques

The microstructure of allotropic carbon materials, such as graphite,nanophase C, diamond films, and C₆₀ films, have been investigated bypositron annihilation techniques. The following experimental results havebeen abtained.1. Microdefects and the distribution of electron momentum ingraphite and nanophase C(1) The remarkable anisotropic distribution of electronic momentumwas found in single crystalline graphite. Furthermore, the S parameter islinear to cos²Î¸. However, this phenomenon was not found in nanophaseC.(2) With the increase of the temperature from 25K to 295K, the openvolume of defects increase, and the average free electron density decreasein graphite and nanophase C. The electronic density in nanophase C alsohas a linear relationship with temperatures.(3) The defects near to the surface layer of nanophase C may absorbthe hydrogen impurity and gives rise to the decrease of S parameter. The content of hydrogen impurity decreases with the increase of the distantfrom the surface.2. Microdefects in the undoped, B-doped and S-doped diamondfilms before and after annealing(1) The defects in undoped diamond film would recover afterannealing at temperature below 600℃. The open volume of defect willincrease after annealing at temperatures above 900℃due to the thermalvacancies migrating to merge together.(2) The density of defect in low B-doped diamond film will decreaseafter annealing at temperature below 400℃, however it will increaseafter annealing at temperature above 600℃.(3) The defects in high B-doped diamond film are easy to recover.The S parameters of this sample decrease after annealing at defferenttemperature.(4) The S parameters of S-doped diamond film keep unchangedafter annealing at temperature from 200 up to 1000℃, that is, thestructure of this film is quite stable ever at high temperature.(5) The density of defect in the S-doped diamond film is higher thanthat in undoped diamond films. And the density of defect in the B-dopeddiamond film is lower than that of in undoped diamond film. Thus, theaddition of small amount of B atoms will decrease the density of defect inthe film. 3. Microdefects in C₆₀ films deposited at defferent energy before andafter annealing(1) As the deposition energy below 200ev, the C₆₀ film is mainlypristine C₆₀, the C₆₀ molecules maintain their structure and preservemolecular identity.(2) As the deposition energy close to 250ev, the intermolecularcohesion getting stronger. The open volume of defect in C₆₀ filmincreases due to the polymerized fullerenes occuring in the film.(3) The film structure depends on the energy of the incident ions. Asdeposition energy higher than 300ev, the polymerized fullerenes becomelarger, on the other hand, the structure of C₆₀ molecules will be destroyedinto pieces, and the amorphous will form on the film. At meantime, thematerial is more compact at high deposition energy. The open volumedefect in the film depends on the competition between the above twofactors.(4) After the annealing at 600℃for lh, the amorphous carbon willbecome in graphitic clusters, and the polymerized C₆₀ cages aretransformed back to pristine or small polymerized C₆₀.