A noncatalytic chemical vapor deposition mechanism is proposed, where high precursor concentration, long deposition time, high temperature, and flat substrate are needed to grow large-area nanocrystalline graphene using hydrocarbon pyrolysis. The graphene is scalable, uniform, and with controlled thickness. It can be deposited on virtually any nonmetallic substrate that withstands ∼1000 o C. For typical examples, graphene grown directly on quartz and sapphire shows transmittance and conductivity similar to exfoliated or metalcatalyzed graphene, as evidenced by transmission spectroscopy and transport measurements. Raman spectroscopy confirms the sp 2 -C structure. The model and results demonstrate a promising transfer-free technique for transparent electrode production.Graphene, a monolayer of sp 2 carbon atoms, has received much attention. The use of graphene in transparent electronics is one of its most promising applications. 1 Apart from its high transparency/conductivity, the flexibility and ease of integration on a variety of substrates are obvious advantages. 30-inch graphene has been demonstrated by virtue of the recent advances in chemical vapor deposition (CVD), 2 which is compatible with the existing semiconductor technology. 3 Nevertheless, the necessity of etching the metal catalyst and transfer of graphene to foreign substrates hampers wide industrialization. Thus, efforts are put into the metal-free growth of graphene on materials including SiO 2 , 4-6 Al 2 O 3 , 7 MgO, 8 etc. To date, however, the graphitization process on dielectrics is poorly understood while the experiments are largely experience based. Recently, we have shown that large-area uniform graphene can be grown on virtually any hightemperature substrates, on Si 3 N 4 or HfO 2 , for instance. 9 In this letter, we provide a broader and deeper insight, discussing in detail the growth mechanism of graphene directly on insulators. As an example, we show our results on graphene on quartz and sapphire, hinting at the future prospects of the transfer-free graphene for transparent electrodes.Bulk graphite is usually made at >3000 o C and catalysis is needed to lower the temperature. 10 However, nanoscale graphene flakes form without any catalysts at merely ∼1000 o C. For example, carbon black which is produced massively by natural gas pyrolysis 11-14 is nothing else but chaotically connected graphene flakes. 11 Although widely used in industry, this process has so far been overlooked for making large-area graphene. Here, we show that macroscopically amorphous carbon black can be turned into textured graphene thin films when a) Electronic mail: jiesu@chalmers.se. the carbon precursor concentration and growth temperature are high, also requiring a relatively long deposition time and flat substrate. Fig. 1(a) illustrates the CVD reactor used in this work. The middle region is heated to 1000 o C while the left and right sides are <600 o C. There is no material deposition in the right zone because of the reduced CH 4 decomposition efficiency at lo...