Carbon-dot doped, transfer-free, low-temperature, high mobility graphene using microwave plasma CVD.

Microwave plasma chemical vapor deposition is a well-known method for low-temperature, large-area direct graphene growth on any insulating substrate without any catalysts. However, the quality has not been significantly better than other graphene synthesis methods such as thermal chemical vapor deposition, thermal decomposition of SiC, etc. Moreover, the higher carrier mobility in directly grown graphene is much desired for industrial applications. Here, we report chemical doping of graphene (grown on silicon using microwave plasma chemical vapor deposition) with carbon dots to increase the mobility to a range of 363-398 cm2 V-1 s-1 (1 × 1 cm van der Pauw devices were fabricated) stable for more than 30 days under normal atmospheric conditions, which is sufficiently high for a catalyst-free, low-temperature, directly grown graphene. The sheet resistance of the graphene was 430 Ω â–¡-1 post-doping. The novelty of this work is in the use of carbon dots for the metal-free doping of graphene. To understand the doping mechanism, the carbon dots were mixed with various solvents and spin coated on graphene with simultaneous exposure to a laser. The significant information observed was that the electron or hole transfer to graphene depends upon the functional group attached to the carbon dot surface. Carbon dots were synthesized using the simple hydrothermal method and characterized with transmission electron microscopy revealing carbon dots in the range of 5-10 nm diameter. Doped graphene samples were further analyzed using Raman microscopy and Hall effect measurements for their electronic properties. This work can open an opportunity for growing graphene directly on silicon substrates with improved mobility using microwave plasma CVD for various electronic applications.

Laser-assisted graphene growth directly on silicon.

Controlled graphene growth on a substrate without the use of catalysts is of great importance for industrial applications. Here, we report thickness-controlled graphene growth directly on a silicon substrate placed in a low-density microwave plasma environment using a laser. Graphene is relatively easy to grow in high-density plasma; however, low-density plasma lacks the sufficient energy and environment required for graphene synthesis. This study reports that laser irradiation on silicon samples in a low-density plasma region nucleates graphene, and growth is controlled with laser exposure time and power. A graphene-silicon junction is thus formed and shows an enhanced (1.7 mA) short-circuit current as compared to one grown in high-density plasma (50µA) without the laser effects. Synthesized graphene is characterized by Raman spectroscopy, atomic force microscopy to investigate surface morphology and Hall effect measurements for electronic properties. The key aspect of this report is the use of a laser to grow graphene directly on the silicon substrate by ensuring that the bulk resistance of the silicon is unaffected by ion bombardment. Additionally, it is observed that graphene grain size varies in proportion to laser power. This report can help in the growth of large-area graphene directly on silicon or other substrates at reduced substrate temperatures with advanced electronic properties for industrial applications.

Direct Synthesis of Large-Area Graphene on Insulating Substrates at Low Temperature using Microwave Plasma CVD.

With a combination of outstanding properties and a wide spectrum of applications, graphene has emerged as a significant nanomaterial. However, to realize its full potential for practical applications, a number of obstacles have to be overcome, such as low-temperature, transfer-free growth on desired substrates. In most of the reports, direct graphene growth is confined to either a small area or high sheet resistance. Here, an attempt has been made to grow large-area graphene directly on insulating substrates, such as quartz and glass, using magnetron-generated microwave plasma chemical vapor deposition at a substrate temperature of 300 °C with a sheet resistance of 1.3k Ω/□ and transmittance of 80%. Graphene is characterized using Raman microscopy, atomic force microscopy, scanning electron microscopy, optical imaging, UV-vis spectroscopy, and X-ray photoelectron spectroscopy. Four-probe resistivity and Hall effect measurements were performed to investigate electronic properties. Key to this report is the use of 0.3 sccm CO2 during growth to put a control over vertical graphene growth, generally forming carbon walls, and 15-20 min of O3 treatment on as-synthesized graphene to improve sheet carrier mobility and transmittance. This report can be helpful in growing large-area graphene directly on insulating transparent substrates at low temperatures with advanced electronic properties for applications in transparent conducting electrodes and optoelectronics.