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.

Transfer free graphene growth on SiO2 substrate at 250 °C.

Low-temperature growth, as well as the transfer free growth on substrates, is the major concern of graphene research for its practical applications. Here we propose a simple method to achieve the transfer free graphene growth on SiO2 covered Si (SiO2/Si) substrate at 250 °C based on a solid-liquid-solid reaction. The key to this approach is the catalyst metal, which is not popular for graphene growth by chemical vapor deposition. A catalyst metal film of 500 nm thick was deposited onto an amorphous C (50 nm thick) coated SiO2/Si substrate. The sample was then annealed at 250 °C under vacuum condition. Raman spectra measured after the removal of the catalyst by chemical etching showed intense G and 2D peaks together with a small D and intense SiO2 related peaks, confirming the transfer free growth of multilayer graphene on SiO2/Si. The domain size of the graphene confirmed by optical microscope and atomic force microscope was about 5 µm in an average. Thus, this approach will open up a new route for transfer free graphene growth at low temperatures.

Influence of oxygen on nitrogen-doped carbon nanofiber growth directly on nichrome foil.

The synthesis of various nitrogen-doped (N-doped) carbon nanostructures has been significantly explored as an alternative material for energy storage and metal-free catalytic applications. Here, we reveal a direct growth technique of N-doped carbon nanofibers (CNFs) on flexible nichrome (NiCr) foil using melamine as a solid precursor. Highly reactive Cr plays a critical role in the nanofiber growth process on the metal alloy foil in an atmospheric pressure chemical vapor deposition (APCVD) process. Oxidation of Cr occurs in the presence of oxygen impurities, where Ni nanoparticles are formed on the surface and assist the growth of nanofibers. Energy-dispersive x-ray spectroscopy (EDXS) and x-ray photoelectron spectroscopy (XPS) clearly show the transformation process of the NiCr foil surface with annealing in the presence of oxygen impurities. The structural change of NiCr foil assists one-dimensional (1D) CNF growth, rather than the lateral two-dimensional (2D) growth. The incorporation of distinctive graphitic and pyridinic nitrogen in the graphene lattice are observed in the synthesized nanofiber, owing to better nitrogen solubility. Our finding shows an effective approach for the synthesis of highly N-doped carbon nanostructures directly on Cr-based metal alloys for various applications.

Opening of triangular hole in triangular-shaped chemical vapor deposited hexagonal boron nitride crystal.

In-plane heterostructure of monolayer hexagonal boron nitride (h-BN) and graphene is of great interest for its tunable bandgap and other unique properties. Here, we reveal a H2-induced etching process to introduce triangular hole in triangular-shaped chemical vapor deposited individual h-BN crystal. In this study, we synthesized regular triangular-shaped h-BN crystals with the sizes around 2-10 µm on Cu foil by chemical vapor deposition (CVD). The etching behavior of individual h-BN crystal was investigated by annealing at different temperature in an H2:Ar atmosphere. Annealing at 900 °C, etching of h-BN was observed from crystal edges with no visible etching at the center of individual crystals. While, annealing at a temperature ≥ 950 °C, highly anisotropic etching was observed, where the etched areas were equilateral triangle-shaped with same orientation as that of original h-BN crystal. The etching process and well-defined triangular hole formation can be significant platform to fabricate planar heterostructure with graphene or other two-dimensional (2D) materials.