NOVELTY - Graphite oxide is exposed to source of ultraviolet light, visible light and/or infrared radiant energy to form graphene.    USE - Manufacture of graphene used for producing heat for destroying unwanted cells or tissue, photovoltaic cell and light emitting diode (all claimed).    ADVANTAGE - The graphene is efficiently manufactured by simple method.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) method of producing heat by photothermal energy conversion, which involves exposing photothermally active material to ultraviolet light, visible light, or infrared radiant energy. The photothermally active material is chosen from graphite oxide, partially reduced graphite oxide, graphene, metal nanocomposite of graphite oxide, metal nanocomposite of partially reduced graphite oxide, metal nanocomposite of graphene, semiconductor nanocomposite of graphite oxide, semiconductor nanocomposite of partially reduced graphite oxide, and semiconductor nanocomposite of graphene;    (2) apparatus for producing heat by photothermal energy conversion, which has container (A) to accommodating photothermally active material(s), container (B) to accommodating heatable medium for containing heatable medium and conduit(s) for transporting heated medium to location where heat is to be released from heated medium;    (3) apparatus for desalinating seawater, which has container (A), container to accommodating seawater for containing seawater, condenser for condensing water vapor and a receptacle for receiving condensed water vapor;    (4) method for destroying unwanted cells or tissue in to-be-processed component, which involves contacting photothermally active material at or near unwanted cells or tissue, and exposing the photothermally active material to source of ultraviolet light, visible light, or infrared radiant energy;    (5) photovoltaic cell, which has a transparent conducting layer, a photoabsorbing layer comprising semiconductor nanocomposite(s) of graphene and back electrode; and    (6) light-emitting-diode, which has a substrate and semiconductor nanocomposite of graphene contacted with substrate. The semiconductor nanocomposite of graphene is doped with impurities to create a p-n junction on substrate.