On the limit of single-walled carbon nanotube random network conductivity

Aalto University, P.O. Box 11000, FI-00076 Aalto www.aalto.fi Author Kimmo Mustonen Name of the doctoral dissertation On the limit of single-walled carbon nanotube random network conductivity Publisher School of Science Unit Department of Applied Physics Series Aalto University publication series DOCTORAL DISSERTATIONS 194/2015 Field of research Nanomaterials Manuscript submitted 5 October 2015 Date of the defence 7 December 2015 Permission to publish granted (date) 4 November 2015 Language English Monograph Article dissertation (summary + original articles) Abstract Single-walled carbon nanotubes (SWCNTs) are one of the most interesting emerging materials for practical applications. As transparent conductive films (TCFs) and thin film transistors (TFTs) they provide prospects for both improved flexibility and conductivity over established metal oxide and silicon-based materials. Technologically crucial performance optimizations, however, require a coherent picture how the SWCNT network properties, specifically sheet conductance, absorbance and spatial uniformity, emerge from individual nanotubes.Single-walled carbon nanotubes (SWCNTs) are one of the most interesting emerging materials for practical applications. As transparent conductive films (TCFs) and thin film transistors (TFTs) they provide prospects for both improved flexibility and conductivity over established metal oxide and silicon-based materials. Technologically crucial performance optimizations, however, require a coherent picture how the SWCNT network properties, specifically sheet conductance, absorbance and spatial uniformity, emerge from individual nanotubes. Here, a new kind of floating catalyst approach based on a spark discharge generator (SDG) is presented for the synthesis of predominantly individual SWCNTs in the gas phase. In this process, Brownian diffusion is identified as the major cause behind nanotube gas-phase aggregation (bundling). This can be avoided by limiting the SWCNT number concentration down to ~10 cm, yielding a high fraction of 60-80% of individual tubes on substrates. For mostly individual 3-4 μm long SWCNTs, the observed aggregation rate matches a mobility diameter of 20 nm. The synthesized tubes exhibit a pre-eminence of near-armchair chiralities, up to 70% having chiral angles ≥20°, with an unconventionally high fraction of semiconducting tube species, 80%, at a growth temperature of 750 °C. Furthermore, by optical and electrical characterization of networks fabricated from individual tubes and small diameter bundles, unambiguous experimental evidence of the detrimental nature of SWCNT bundling on TCF performance is found. The performance loss is explained to be due to gratuitous absorbance in large diameter bundles, without a compensating conductivity gain. An absorbance-conductance model is presented, assuming that the Beer-Lambert law applies independent of the TCFs’ internal geometry, whereas at room temperature a significant charge carrier transport is allowed only through metallic-metallic tube junctions. The maximum network conductivity is expected where the nanotube lengthwise resistances between the junctions become as large as the junction resistances, providing the ultimate performance limit for metallicity-mixed SWCNT networks of 80 Ω/☐ at 90% transparency. For all-metallic and doped networks, the limit is expected at 25 Ω/☐. In correspondence, nitric acid treated TCFs fabricated using individual 4 μm long SWCNTs are demonstrated with a sheet resistance of 63 Ω/☐ at 90% transparency. Finally, random-network TFTs fabricated from the individual tubes approach the uniformity of ideal computer-simulated systems. The TFTs exhibit On/Off current ratios between 10 and 10 and simultaneous charge carrier mobilities up to 100 cm Vs combined with a fabrication yield of >99%. The normalized On-current shows standard deviation of ~25%, showing unprecedently high uniformity for random network TFTs. !