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

Kimmo Mustonen

Research output: ThesisDoctoral ThesisCollection of 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. 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 ~105 cm-3, 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 104 and 106 and simultaneous charge carrier mobilities up to 100 cm2 V-1s-1 combined with a fabrication yield of >99%. The normalized On-current shows standard deviation of ~25%, showing unprecedently high uniformity for random network TFTs.
Translated title of the contributionSatunnaissuunnattujen hiilinanoputkiverkkojen sähkönjohtavuuden raja
Original languageEnglish
QualificationDoctor's degree
Awarding Institution
  • Aalto University
Supervisors/Advisors
  • Kauppinen, Esko, Supervising Professor
  • Nasibulin, Albert, Thesis Advisor
Publisher
Print ISBNs978-952-60-6531-1
Electronic ISBNs978-952-60-6532-8
Publication statusPublished - 2015
MoE publication typeG5 Doctoral dissertation (article)

Keywords

  • SWCNT
  • TCF
  • TFT
  • bundling mechanism

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