Conversion of Fullerenes to Diamond

The present invention relates generally to methods for the conversion of fullerenes to diamond or diamond films. More particularly, the invention is concerned with the manufacture of nonhydrogenic fullerenes as precursors for the synthesis of diamond or diamond-like films. Because of their thermodynamic instability with respect to diamond, the conversion of fullerenes to diamond has significant advantages over presently used methods for producing synthetic diamonds.

The prior art methods of manufacturing diamond can be divided into two main approaches. In the first or high-pressure method, graphite powder is heated to about 1500.degree. C. under 60 kbar of pressure in the presence of an iron catalyst. Under this extreme, but equilibrium, condition of pressure and temperature, graphite can be converted to diamond. About 75 tons of diamond are produced industrially each year in this way. The second or low pressure method of producing diamond artificially involves producing a mixture usually of a few percent of methane in hydrogen gas. A hot filament or a microwave discharge is used to dissociate the methane molecule to form the methyl radical, CH.sub.3, and the hydrogen molecule is dissociated to form hydrogen atoms. Diamond or diamond-like films can be grown this way epitaxially on diamond nuclei. Such films, however, always contain small contaminating amounts (0.1-1%) of hydrogen which are undesirable for many applications.

The usefulness and novelty of fullerene precursors for diamond synthesis stem from several of their properties: Fullerene precursors are thermodynamically unstable with respect to diamond and, therefore, stable only in a kinetic sense. In addition, since the fullerenes are molecular entities, they are volatile with C.sub.60 having a vapor pressure of 10.sup.-4 Torr at 500.degree. C. Fullerenes are also allotropes of carbon; that is, they contain no hydrogen; and therefore, diamonds produced from fullerene precursors are hydrogen-defect free. Another useful characteristic of fullerene is the chemical bond in C.sub.60 is intermediate between graphite (sp.sup.2) and diamond (sp.sup.3). Furthermore, fragmentation involving carbon-carbon bond breakage occurs via the elimination of C.sub.2 groups. Recent scanning tunneling microscope studies have shown C.sub.2 groups to be intimately involved in the growth of epitaxial diamond films, particularly of the "dimer rows" of the 2.times.1 reconstructed <100> surface. It has also been determined both the positive C.sub.60.sup.+ and negative C.sub.60.sup.- ions are stable entities that can be accelerated to kilovolt energies under an applied electrostatic potential. The so-called LUMO (lowest unoccupied molecular orbital) of the fullerenes also is an antibonding three-fold degenerate orbital that can accept up to six electrons from electron donors such as the alkali metals. The resultant repulsion between the delocalized electrons weakens the carbon-carbon bonds of the C.sub.60 cage and provides a mechanism for the fullerene to undergo diamond transformation.

It is therefore an object of the invention to provide a novel method of making fullerene precision materials for the synthesis of diamonds and diamond films.

It is another object of the invention to provide a new method of converting condensed phase fullerenes to diamond by the application of heat and pressure but at lower temperatures and pressures than used in the conversion of graphite to diamond.

It is a further object of the invention to provide an improved method of applying heat and pressure to fullerenes in the presence of alkali or alkaline earth metals, such as potassium or calcium or other elements that act as charge transfer or complexing agents, to facilitate the conversion of condensed phase fullerenes to the diamond phase under milder conditions than those required for the transformation of graphite to diamond.

It is an additional object of the invention to provide a new method of growing diamond films on diamond nuclei by decomposing fullerene vapor onto heated substrates.

It is another object of the invention to provide a novel method of growing diamond films on diamond nuclei by decomposing fullerene vapor onto heated substrates in the presence of alkali or alkaline earth metal vapors.

It is a further object of the invention to provide an improved method of implanting kilovolt energy ionized fullerene molecules on surfaces so that fragmentation and carbon carbon bond breakage occurs with resultant diamond growth on diamond or fullerene nuclei.

It is an additional object of the invention to provide a new method of implanting multikilovolt fullerene ions to form refractory metal surfaces and subsurface carbides.

Other objects, features and advantages of the present invention will be readily apparent from the following description of the preferred embodiments thereof, taken in conjunction with the accompanying drawings described below wherein like elements have like numerals throughout the several views.

Diamond can be formed using compounds of the alkali metals with C.sub.60 (e.g., K.sub.3 C.sub.60, K.sub.4 C.sub.60, K.sub.6 C.sub.60). These carbon-alkali metals are mixed with conventional diamond powder and placed in a molybdenum or tantalum container that is sealed by electron beam welding in an inert atmosphere. The capsule is placed in a tetrahedral anvil apparatus and subjected to pressures of 5-20 kbar and temperatures of 500.degree.-1000.degree. C. These conditions are maintained for 1-10 hours after which the pressure and temperature are lowered to ambient conditions. The container is then removed, and the mixture of diamond, uncreacted C.sub.60 and excess potassium placed in a vessel. Heating the vessel to 500.degree. C. in a vacuum sublimes out the C.sub.60 and potassium, leaving diamond behind. A commercially more economical procedure is to substitute a mixture of C.sub.60 and C.sub.70 for pure C.sub.60, and the same results can be achieved.

In another form of the invention, compounds of the alkali metals with C.sub.60 (e.g., K.sub.3 C.sub.60, K.sub.4 C.sub.60, K.sub.6 C.sub.60) are combined in a suitable refractory vessel, or container, such as Ta or Mo, with a diamond seed crystal. The seed crystal is disposed on a heater button or platform with power supplied by a separate power supply. The vessel is then placed in a furnace and heated in the furnace to a temperature of 500.degree.-700.degree. C. The diamond seed crystal is heated by the separate power supply to 1000.degree.-1200.degree. C. In the vapor phase, charge transfer from the alkali metal atoms to C.sub.60 produces C.sub.6.sup.- which converts to diamond epitaxially on the seed crystal. Growth is continued until the diamond crystal has reached the desired size.

In a further form of the invention positive or negative ions of C.sub.60, C.sub.70 or higher fullerenes or mixtures of fullerenes are produced in a conventional ion source. Well-known methods for producing the ions can be used: laser ablation, electron attachment, electron bombardment or photoionization. A means is provided for electrostatically accelerating the ions to kilovolt energies. A variety of commercially available ion sources, including ion implanters, can be used for this purpose. Because there is only one positive charge per C.sub.60 or C.sub.70 molecule, space charge limitations are overcome; and fullerene ion currents can be obtained which are about two orders of magnitude larger than equivalent carbon ion currents using graphite as the source of carbon. Bombardment of surfaces covered with diamond nuclei or fullerene coatings with fullerene ions having energies from about 250 eV to about 2.5 KeV results in the formation of diamond films. Bombardment of surfaces, and particularly refractory metal surfaces, such as tungsten, with fullerene ions having energies from about 2.5 keV to at least about 250 MeV results in implantation of carbon and the formation of surface or subsurface carbides. The following nonlimiting examples illustrate other features of the invention.

Fullerenes with formulas C.sub.60 to C.sub.266 were prepared in a plasma arc discharge between graphite electrodes. Almost one-third of the material extractable with toluene and higher boiling point organic solvents is composed of C.sub.84 to C.sub.200. The fullerene compositions were characterized by conventional mass spectrometric measurements.

Potassium-C.sub.60 compounds were synthesized using plasma arc discharge produced soot which was processed by solvent extraction and exchange column liquid chromatography to produce pure C.sub.60. The preparation was carried out with a conventional Schlenk apparatus. Small potassium (K) chips were added to a solution of C.sub.60 in toluene and reacted for two hours. The black precipitate that formed was filtered off, placed on a stainless steel sample mount, and inserted into a laser desorption time-of-flight mass spectrometer. No C.sub.60.sup.+ was seen which is in contrast to the positive ion spectrum from the original C.sub.60 sample. Thus, K.sub.x C.sub.60 has formed an ion-like bonding with the C.sub.60 forming the anion and the K, the cation.

The above mentioned conventional ion producing and ion accelerating mass spectrometer has also been used to characterize the photoionization and photofragmentation of C.sub.60 and C.sub.70. Photoionization performed without fragmentation to produce C.sub.60 + from neutral C.sub.60 molecules was accomplished by using laser beam photons of 10.5 eV, approximately 3 eV above the ionization potential of C.sub.60. The photoions were accelerated to several keV and detected by a channel-plate detector. By increasing laser powers to higher intensities, various photofragments were produced. These photofragments included C.sub.58, C.sub.56. . . C.sub.32. . . C.sub.10, C.sub.8, . . . C. Thus, the higher the laser power, the lower the nuclearity of the carbon cluster that is produced.