Transfer and characterization of large-area CVD graphene for transparent electrode applications

The growth of chemical vapor deposited graphene on copper is approaching industrial maturity. A subsequent transfer of the graphene layer from its catalytic growth substrate is required for integration into optoelectronic devices and similar applications. It is well established that defects such as cracks, line defects, and wrinkles all contribute to lowering the quality and usability of graphene.

This means that the development of transfer methods that does not introduce damage to the graphene layer and is non-destructive towards the catalytic growth substrate are of high importance. This thesis addresses key issues for industrial integration of large area graphene for optoelectronic devices.

This is done through optimization of existing characterization methods and development of new transfer techniques. A method for accurately measuring the decoupling of graphene from copper catalysts is introduced. The method is based on Raman spectroscopy, a standard characterization tool in the graphene community.

By measuring when the graphene is fully decoupled from its growth substrate we are able to transfer graphene by mechanical peeling from 12 inch diameter copper thin films. Additionally, results from an electrochemical transfer method and from a transfer method based on interfacial Cu oxidation in alkaline solution are presented.

Both methods leave the copper catalyst intact for regrowths of graphene. The structural integrity of the transferred graphene is retained by these transfer methods and the electrical properties of graphene after transfer are superior compared to the standard etching transfer method. Spatial mapping of the electrical properties of transferred graphene is performed using terahertz time-domain spectroscopy (THz-TDS).

The non-contact nature of THz-TDS and the fact that it is an accurate and reliable probe of the graphene sheet conductivity makes it an interesting candidate for characterization of graphene production in industrial settings. Here we show that the electrical properties of graphene are measurable by THz-TDS on different substrates such as silicon wafers, glass, and polymer films, which only increases the suitability of THz-TDS for characterization of graphene in industrial settings.