▎ 摘　　要

NOVELTY - Preparation of a graphene sheet (140) comprises: forming a film comprising a graphitizing catalyst (100); heat-treating a gaseous carbon source (120) in the presence of the graphitizing catalyst to form graphene (130); and cooling the graphene to form the graphene sheet. USE - The graphene sheet is useful in a graphene substrate (claimed), a membrane, a hydrogen storage medium, an optical fiber and an electrical device. ADVANTAGE - The economical method prepares large-sized graphene sheet, which has a desired thickness. The prepared graphene sheet can be applied to various fields in various applications without limitation on the shape of a substrate even if the graphene sheet has a complicated or topographically modulated structure. The heat-treatment and cooling method is performed as a single cycle but a dense graphene sheet having many layers may be formed by repeating the method several times. DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for: (1) the graphene sheet prepared by the method or derived from a polycyclic aromatic molecule in which many carbon atoms are covalently bound to each other, where the graphene sheet comprises 1-300 layers and each of the width and length of the graphene sheet is 1 mm or greater; and (2) a graphene substrate comprising: a substrate; and the graphene sheet formed on the substrate, where the graphene sheet is derived from the polycyclic aromatic molecule. DESCRIPTION OF DRAWING(S) - The diagram shows a schematic illustration of the method of preparing the graphene sheet. Graphitizing catalyst (100) Gaseous carbon source (120) Graphene (130) Graphene sheet. (140)