Simple Patterning of Large-Area Graphene By Metal Mask and Sacrificial Polymer Layer

Graphene has attracted a lot of attention due to its extraordinary properties such as low resistivity, high mechanical/chemical stability, and high mobility. One particular area that has aroused recent interest is its use as an electrode material for organic electronics such as organic field-effect transistors (OFETs), organic flexible solar cell, and organic light-emitting diode (OLED). This is because, graphene has a high work function which matches to the highest occupied molecular orbital (HOMO) level of organic semiconductor, and has a strong π-π interaction with organic semiconductors which can reduce the injection barrier at the graphene/organic interface for enhanced the charge injection efficiency. However, for practical applications in organic electronics, large-area graphene film needs to be produced either via chemical vapor deposition (CVD) technique or via thin film of reduced graphene oxide (RGO) sheets and they need to be patterned in a specific dimension. The commonly used patterning technique is selectively etching of unwanted regions of graphene using metal etch mask and then the removal of the metal via harmful acid treatment.

In this work, we report a new approach for a simple patterning of graphene film by metal mask using a sacrificial methyl methacrylate (PMMA) polymer layer. Our method includes both spin-coated polymer layer as sacrificial layer and metal deposition by shadow mask as etching mask. After the graphene is placed on Si substrate, PMMA layer is spin-coated on top of the graphene film, and then the metal is evaporated through shadow mask. After the samples are exposed under plasma etching, the patterned graphene is obtained. The metal is then easily removed along with the with the PMMA layer by a acetone lift off. The size and shape of patterned graphene is the exactly same as the metal mask. We show that this new method can apply to different types of graphene film such as chemical vapor deposition (CVD) graphene and reduced graphene oxide (RGO) demonstrating as a reliable processing technique. In contrast to conventional metal etch mask patterning, our method does not include any harmful acid treatment of removing metal mask, making it an environmental-friendly method. We also fabricated OFETs by thermally depositing pentacene using patterned graphene electrode. Our devices show excellent device performance with a maximum mobility of 0.33cm²/Vs and current on-off ratio of ~10⁵. We believe that this approach can open the new pathway for manufacturing other carbon-based thin film devices.