Length-dependent performances of sodium deoxycholate-dispersed single-walled carbon nanotube thin-film transistors
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Zhenfeng Wang and Jun Wei Singapore Institute of Manufacturing Technology, Singapore 638075 (Received 28 June 2012; accepted 4 September 2012)
The material characteristics of single-walled carbon nanotubes (SWCNTs) influence the performance of SWCNT thin-film transistors (TFTs). In this study, a density gradient ultracentrifugation method was used to sort surfactant (sodium deoxycholate)-dispersed SWCNTs by length. SWCNTs of 150 6 33 nm and 500 6 91 nm long were fabricated into TFTs. The results show that the performance of SWCNT-TFTs is tube length dependent. TFTs fabricated using 500-nm long tubes have maximum on/off ratio around 105 with the mobility at ;0.15 cm2/(V s), which is much higher than that of TFTs using 150-nm long tubes. Shorter tubes need higher tube density to form semiconducting paths, leading to lower on/off ratio and high contact resistance. Surfactant-wrapped SWCNTs will bundle into ropes of different size when tube density is high. It is critical to control tube length as well as surfactant residue content to build high performance SWCNT-TFTs.
I. INTRODUCTION
Single-walled carbon nanotubes (SWCNTs) can be visualized as a seamless cylinder rolled up by 1-atom-thick planar graphene sheet. They can be either metallic (m) or semiconducting (s) depending on the rolling direction of the graphene sheet. m-SWCNTs can carry an electrical current density, which is three orders of magnitude higher than a typical metal, such as copper or aluminum at ;4 109 A/cm2,1 and the carrier mobility of individual s-SWCNTs is ten times better than that of silicon at ;10,000 cm2/(V s2).2 Such exceptional electronic properties inspire intensive research on many of their potential applications, including conductive films, interconnects, transistors, logic gates, and sensors. Most of these applications need uniform m- or s-SWCNTs; e.g., SWCNT-based transistors need high purity s-SWCNTs. However, none of the current synthesis methods can produce SWCNTs with uniform metallicity or structure. To overcome this barrier, significant research efforts have been dedicated to separate and enrich SWCNTs according to their diameter, metallicity, and chirality.3–5 SWCNT networks have shown great potentials in realizing the transistor applications of SWCNTs.6,7 Compared with transistors using individual tubes, SWCNT networks average the electronic properties of many SWCNTs, and the chance of forming conducting paths is significantly reduced. Besides, it is also much easier to achieve scalable production a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.321 1004
J. Mater. Res., Vol. 28, No. 7, Apr 14, 2013
http://journals.cambridge.org
Downloaded: 13 Mar 2015
of SWCNT thin films at fairly low cost comparing with positioning and aligning many individual tubes.8 SWCNT structures, such as metallicity, diameter, and length, can strongly affect the performance of SWCNT thin-film transistors (SWCNT-TFTs). They not only directly decide the electronic properties but also influence the formation of
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