NOVELTY - The method comprises replacing a portion of air in a reactor having a substrate with a buffer gas by reducing the reactor pressure to a pressure of 1-50 mTorr, flowing the buffer gas through the reactor and then heating the reactor, and passing the buffer gas through a methanol liquid precursor and an oxygen-containing hydrocarbon liquid precursor respectively to bring the methanol liquid precursor and an oxygen-containing hydrocarbon liquid precursor into contact with the substrate for a time of 20 minutes sufficient to form a graphene film.    USE - The method is useful for forming a single-layer graphene film, which is useful in electronic devices including radio frequency devices, lithium ion batteries, transparent electrodes/displays, solar cells, organic photovoltaic devices, field emission tips, chemical sensors, magnetic sensors and optoelectronic devices.    ADVANTAGE - The method is capable of energy-efficiently forming the high quality and purity graphene film with improved electrical and physical properties, excellent electron mobility and high grain uniformity and without any defects.    DETAILED DESCRIPTION - The method comprises replacing a portion of air in a reactor having a substrate with a buffer gas by reducing the reactor pressure to a pressure of 1-50 mTorr, flowing the buffer gas through the reactor and then heating the reactor, and passing the buffer gas through a methanol liquid precursor and an oxygen-containing hydrocarbon liquid precursor respectively to bring the methanol liquid precursor and an oxygen-containing hydrocarbon liquid precursor into contact with the substrate for a time of 20 minutes sufficient to form a graphene film. A ratio of the buffer gas partial pressure to the methanol liquid precursor partial pressure is 50:1-2:1. The graphene film has a single grain continuity of 1 mu m2 and a grain size of 10-30 mu m2. The heating step comprises heating the reactor to a temperature of 600-1200 degrees C that is sufficient for decomposition of the methanol liquid precursor and is less than the substrate melting point. The method further comprises etching the substrate, annealing the substrate by heating and maintaining the reactor at a temperature of 900-1200 degrees C and by heating or cooling the reactor to a temperature that is sufficient for decomposition of the methanol liquid precursor and less than the substrate melting point before passing the buffer gas through the methanol liquid precursor, and cooling the graphene film disposed on the substrate to about room temperature. The buffer gas has a flow rate of 100-500 standard cubic centimeters per minute (sccm). The reactor is pumped down to a pressure of 10-40 mTorr. A ratio of the intensities of G':G peaks as measured from a Raman Spectrum of the graphene film is 5. The substrate has a thickness of 50-50 mu m. An INDEPENDENT CLAIM is included for a graphene film.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic view of a reactor for preparing a graphene film.