Organorhodium Dimer Dopes Single Wall Carbon Nanotubes

Pentamethyl rhodacene and a section of a single wall carbon nanotube.

In newly reported research in Nano Letters, single-walled carbon nanotube (SWCNT) were n-doped with a pentamethylrhodacene dimer. SWCNT transistors are among the most developed nanoelectronic devices for high-performance computing applications. While p-type SWCNT transistors are easily achieved through adventitious adsorption of atmospheric oxygen, n-type SWCNT transistors require extrinsic doping schemes. Existing n-type doping strategies for SWCNT transistors suffer from one or more issues including environmental instability, limited carrier concentration modulation, undesirable threshold voltage control, and/or poor morphology. In particular, commonly employed benzyl viologen n-type doping layers possess large thicknesses, which preclude top-gate transistor designs that underlie high-density integrated circuit layouts. To overcome these limitations, we report here the controlled n-type doping of SWCNT thin-film transistors with a solution-processed pentamethylrhodocene dimer. The charge transport properties of organorhodium-treated SWCNT thin films show consistent n-type behavior when characterized in both Hall effect and thin-film transistor geometries. Due to the molecular-scale thickness of the organorhodium adlayer, large-area arrays of top-gated, n-type SWCNT transistors are fabricated with high yield. This work will thus facilitate ongoing efforts to realize high-density SWCNT integrated circuits.

These existing SWCNT n-type doping approaches have processing and stability limitations. For instance, substitutional doping decreases carrier mobility, low work-function metals oxidize in ambient conditions, and metal oxides/nitrides have shown limited ability to tune the electronic properties of SWCNT-based devices. However, recent progress in benzyl viologen dopant and hybrid organic/inorganic encapsulation layers has enabled the fabrication of complementary metal-oxide-semiconductor (CMOS) static random access memory circuits based on SWCNT TFTs.(35) Although this result demonstrates the suitability of SWCNTs for sophisticated electronic circuits, the tuning of the n-type SWCNT TFT electronic properties is not easily achieved. Furthermore, the thick layer required for benzyl viologen doping does not allow for top-gate device structures, ultimately leading to nonoptimal circuit layout and resulting limitations in integrated circuit density and complexity. As a step toward overcoming these issues, we introduce here the use of a molecular n-type dopant, pentamethylrhodocene dimer ((RhCp*Cp)2), for producing n-type SWCNT TFTs that can be controllably doped by solution-processing methods. From both Hall effect and TFT measurements, the resulting SWCNT thin-film electronic characteristics show consistent and tunable n-type behavior. Importantly, the resulting n-type SWCNT TFTs possess complete p-type carrier suppression analogous to the n-type carrier suppression induced by adsorbed atmospheric dopants in p-type SWCNT TFTs. Since the organorhodium adlayer is molecularly thin, this approach also enables the demonstration of high-yield n-type SWCNT TFTs fabricated in a top-gate device structure, thus creating future opportunities for improved SWCNT integrated circuit layout and function.

Controlled n-Type Doping of Carbon Nanotube Transistors by an Organorhodium Dimer

Michael L. Geier†, Karttikay Moudgil‡, Stephen Barlow‡, Seth R. Marder‡, and Mark C. Hersam*†§∥

† Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, United States. ‡ School of Chemistry and Biochemistry, Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia 30332, United States. § Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States. ∥ Department of Electrical Engineering and Computer Science, Northwestern University, Evanston, Illinois 60208, United States

Nano Lett., 201616 (7), pp 4329–4334

DOI: 10.1021/acs.nanolett.6b01393

Publication Date (Web): June 2, 2016