Nano Letters , 12 , 1184 (2012).

1. Gigahertz Flexible Transistors from DGU GrapheneMark C. Hersam, Northwestern University, DMR 1006391 Density gradient ultracentrifugation (DGU) allows single-layer graphene to be isolated in aqueous solution using non-covalently interacting surfactants. By avoiding covalent chemistry, DGU graphene preserves the superlative electronic properties of pristine graphene such as high charge carrier mobility. However, because it is in solution, DGU graphene can be directly assembled onto flexible substrates such as polyimide. In this study, the high frequency performance of DGU graphene transistors was evaluated in an international collaboration with Vincent Derycke (CEA Saclay) and Henri Happy (CNRS). The resulting transistors possessed cutoff frequencies of 2.2 GHz before de-embedding (8.7 GHz after de-embedding) during mechanical flexing, which is 1000x faster than competing flexible organic electronic materials. Flexible transistors based on DGU graphene operate at frequencies that are 1000x faster than competing flexible organic electronic materials. Nano Letters, 12, 1184 (2012).

2. Industrial Collaboration with IBM: 150 GHz SWCNT FETsMark C. Hersam, Northwestern University, DMR 1006391 While field-effect transistors (FETs) based on single-walled carbon nanotubes (SWCNTs) have excellent electrical characteristics at low frequencies, their high frequency performance is considerably less studied due to challenges in isolating and assembling sufficiently high purity semiconducting SWCNTs. In an industrial and international collaboration with IBM T. J. Watson Research Center, NanoIntegris, and Karlsruhe Institute of Technology, this work demonstrates a scalable, planar device platform that enables the electric-field driven assembly of aligned 99.6% pure semiconducting SWCNT arrays from solution. The resulting transistors show unprecedentedly high cutoff frequencies exceeding 150 GHz while maintaining current saturation and achieving power gain, thus enabling integration with conventional radio frequency circuitry. In collaboration with IBM, semiconducting SWCNTs are assembled into high frequency transistors using dielectrophoresis.