A high-quality nitride crystal can be produced efficiently by charging a nitride crystal starting material that contains tertiary particles having a maximum diameter of from 1 to 120 mm and formed through aggregation of secondary particles having a maximum diameter of from 100 to 1000 μm, in the starting material charging region of a reactor, followed by crystal growth in the presence of a solvent in a supercritical state and/or a subcritical state in the reactor, wherein the nitride crystal starting material is charged in the starting material charging region in a bulk density of from 0.7 to 4.5 g/cm.sup.3 for the intended crystal growth.

A group 13 nitride crystal having a hexagonal crystal structure and containing at least a nitrogen atom and at least a metal atom selected from a group consisting of B, Al, Ga, In, and Tl. The group 13 nitride crystal includes a first region disposed on an inner side in a cross section intersecting c-axis, a third region disposed on an outermost side in the cross section and having a crystal property different from that of the first region, and a second region disposed at least partially between the first region and the third region in the cross section, the second region being a transition region of a crystal growth and having a crystal property different from that of the first region and that of the third region.

A method of arranging nanocrystals is provided, which includes a first process of putting barium titanate nanocrystals and/or strontium titanate nanocrystals, and a nonpolar solvent into a container, a second process of collecting a supernatant liquid including the barium titanate nanocrystals and/or the strontium titanate nanocrystals from the container, and a third process of immersing a substrate having an uneven structure into the supernatant liquid, and pulling up the substrate so as to coat the surface of the uneven structure with the supernatant liquid by using a capillary phenomenon, and to arrange the nanocrystals on the uneven structure.

Single-crystal diamond is composed of carbon in which a concentration of a carbon isotope .sup.12C is not lower than 99.9 mass % and a plurality of inevitable impurities other than carbon. The inevitable impurities include nitrogen, boron, hydrogen, and nickel, and a total content of nitrogen, boron, and hydrogen of the plurality of inevitable impurities is not higher than 0.01 mass %. In order to manufacture single-crystal diamond, initially, a hydrocarbon gas in which a concentration of the carbon isotope .sup.12C is not lower than 99.9 mass % is subjected to denitrification.

Single-crystal diamond is composed of carbon in which a concentration of a carbon isotope .sup.12C is not lower than 99.9 mass % and a plurality of inevitable impurities other than carbon. The inevitable impurities include nitrogen, boron, hydrogen, and nickel, and a total content of nitrogen, boron, and hydrogen of the plurality of inevitable impurities is not higher than 0.01 mass %. In order to manufacture single-crystal diamond, initially, a hydrocarbon gas in which a concentration of the carbon isotope .sup.12C is not lower than 99.9 mass % is subjected to denitrification.

Techniques for processing materials in supercritical fluids including processing in a capsule disposed within a high-pressure apparatus enclosure are disclosed. The disclosed techniques are useful for growing crystals of GaN, AlN, InN, and their alloys, including InGaN, AlGaN, and AlInGaN for the manufacture of bulk or patterned substrates, which in turn can be used to make optoelectronic devices, lasers, light emitting diodes, solar cells, photoelectrochemical water splitting and hydrogen generation devices, photodetectors, integrated circuits, and transistors.

A method for producing a GaN crystal is provided. In the method, front surfaces of a plurality of tiling GaN seeds closely arranged side by side on a flat surface of a plate are planarized. An aggregated seed is formed by arranging the tiling GaN seeds closely side by side on a susceptor of an HVPE apparatus in the same arrangement as when fixed on the plate, with the front planarized surfaces facing upward. A bulk GaN crystal is grown epitaxially on the aggregated seed by an HVPE method.

Provided is a method for synthesizing TiO.sub.2 nanocrystal, comprising: adjusting the pH value of a colloidal suspension of tetratitanic acid nanosheet as a precursor to 5-13; and subjecting the precursor to a hydrothermal reaction to obtain the TiO.sub.2 nanocrystal. The TiO.sub.2 nanocrystal synthesized by the method is anatase-type, and the exposed crystal facet thereof is {010} crystal facet. The method has advantages of low cost, no pollution, simple synthesizing process, strong controllability, short production period and good reproducibility, and is suitable for industrial production.

Provided is a method for synthesizing TiO.sub.2 nanocrystal, comprising: adjusting the pH value of a colloidal suspension of tetratitanic acid nanosheet as a precursor to 5-13; and subjecting the precursor to a hydrothermal reaction to obtain the TiO.sub.2 nanocrystal. The TiO.sub.2 nanocrystal synthesized by the method is anatase-type, and the exposed crystal facet thereof is {010} crystal facet. The method has advantages of low cost, no pollution, simple synthesizing process, strong controllability, short production period and good reproducibility, and is suitable for industrial production.

According to an embodiment of the present invention, a method for preparing a graphite-titanium oxide composite comprises (S1) a surface-modifying graphite with benzyl alcohol or a cellulose-based material using a sol-gel method, (S2) distributing the surface-modified graphite in a solvent, adding a titanium precursor to the solvent, and mixing the titanium precursor with the surface-modified graphite to obtain a graphite-titanium mixture, and (S3) thermally treating the graphite-titanium mixture to grow a titanium oxide on a surface of the graphite.