Direct growth of nanographene films by surface wave plasma chemical vapor deposition and their application in photovoltaic devices { Golap Kalita,* Madhu Sudan Kayastha, Hideo Uchida, Koichi Wakita and Masayoshi Umeno Received 3rd November 2011, Accepted 2nd February 2012 DOI: 10.1039/c2ra01024k Here, we report direct synthesis of nanographene films on silicon (n-Si) and glass (SiO 2 ) substrates by microwave assisted surface wave plasma (MW-SWP) chemical vapor deposition (CVD) and their application in photovoltaic devices. The technique is a metal catalyst free, rapid growth process and the film can be deposited on different substrates; thus simplifying the synthesis process for various device applications. The directly grown graphene film consists of triangular shaped nanographene domains with sizes of 80–100 nm in length. The nanographene domains interconnect to form a continuous film which shows metallic characteristics. A Schottky junction based photovoltaic device is fabricated with directly grown nanographene film on n-Si and a conversion efficiency of 2.1% is achieved. This finding shows that a transparent nanographene film can be deposited on different substrates and can be integrated for various devices. 1. Introduction Graphene is a perfect two-dimensional (2D) carbon sheet with hexagonal structures similar to benzene rings and is the basic building block of graphite, fullerene, and carbon nanotubes (CNTs). Since the discovery of graphene in 2004, 1 there has been an intense focus on graphene based materials, owing to their interesting electrical and optical properties for electronic device application. 2–5 There has been a lot of effort made to exploit the outstanding electrical, optical and mechanical properties of graphene for fabrication of transparent electrodes. 6–8 The synthesis of graphene has been achieved by various approaches, such as by thermal CVD on metal sheets and by the chemical reduction of graphene oxide which have been shown to be most suitable as transparent electrodes. 9–14 However thermal CVD graphene synthesis involves a metal catalytic layer and very high deposition temperature, typically around 1000 uC. 9,15 A catalyst free direct growth process at low-temperature can be far more advantageous for device applications. This can pave the way for direct integration of the graphene based materials with other suitable materials for device applications. Lot of studies has been carried out on fabrication of amorphous carbon (a-C) and Si heterojunction diodes and photovoltaic devices. 16–18 However, lack of ability to control the electrical and structural properties of a-C is a big disadvantage in the fabrication of heterojunction devices. Recent studies have reported the fabrication of CNT/Si heterojunction photovoltaic devices. 19,20 In a bulk CNT/ Si junction, there can be two types of heterostructures (Schottky and p–n heterojunction) as both metallic and semiconducting nanotubes coexist. Fabrication of a Schottky junction with a transferred CVD and mechanically exfoliated graphene on Si has also been demonstrated. 21,22 However, fabrication of large area solar cells with transferred graphene on brittle Si can have many disadvantages. There can be inhomogeneous contact formation, breaking of the graphene film during the transfer process and trapping of impurities at the interface. These problems can only be overcome with the direct growth of the graphene film for integration into a photovoltaic device. In this communication, we demonstrate the synthesis of nanographene films on n-Si and SiO 2 substrates by the MW-SWP CVD technique and the fabrication of a photovoltaic device. The directly grown graphene film consists of triangular shaped nanographene domains with sizes of 80–100 nm in length. The nanographene domains interconnect to form a continuous film which shows metallic behavior. 2. Experimental 2.1 Synthesis of the graphene films and characterization Nanographene films were deposited on n-Si (10 V cm) and SiO 2 substrates by using a MW-SWP-CVD technique (schematic figure in the ESI{). In the MW-SWP-CVD system a hollow quartz plate is used for plasma generation. It has been reported that by using a hollow plate plasma formation can be started-up smoothly without any density jump and the plasma density is two times higher than that of a flat plate. High density surface wave plasma can be generated in the meter scale and large area deposition can be achieved at a very fast rate. 23 For the deposition of the graphene film a gas mixture of C 2 H 2 and Ar with flow rates of 10 and 200 standard cubic centimeters per minute (sccm), respectively were used for all experiments. Deposition was carried out on the substrates without any pretreatment at a gas composition pressure of 45 Pa for a duration of 70–120 s. The power of the microwave used for Department of Electronics and Information Engineering, Chubu University, 1200, Matsumoto-cho, Kasugai-shi, 487-8501, Japan. E-mail: golapkalita@yahoo.co.in { Electronic supplementary information (ESI) available. See DOI: 10.1039/ c2ra01024k RSC Advances Dynamic Article Links Cite this: RSC Advances, 2012, 2, 3225–3230 www.rsc.org/advances COMMUNICATION This journal is ß The Royal Society of Chemistry 2012 RSC Adv., 2012, 2, 3225–3230 | 3225 Published on 08 March 2012. 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