High‐Performance Thin‐Film Transistors with DNA‐Assisted Solution Processing of Isolated Single‐Walled Carbon Nanotubes

2010 WILEY-VCH Verlag Gmb Two-dimensional networks of single-walled carbon nanotubes (SWNTs) have attracted much attention because of their excellent transport properties and expected scalable integration into devices such as thin-film transistors (TFTs) and chemical/ biological sensors. To realize these promising applications in such electronics, it is essential to develop sophisticated fabrication processes of the networks. Until now the networks of SWNTs have been fabricated by mainly using two different processes: dry and wet processes. The former includes direct synthesis and deposition of SWNTs on substrates using the chemical vapor deposition (CVD) method,, and the latter includes drop casting, spin coating, inkjet printing, and dip coating by using a solution of dispersed SWNTs. Even though the TFTs fabricated by the CVD process oftentimes show high device performance, the solution processes, in particular, receive increasing interests in recent years because of their low manufacturing cost and of high scalability. However, virtually all of the present solution processes have shown fatal drawbacks of aggregation of individual nanotubes during the network making. The aggregation basically causes i) a wide variation in bundle size and network morphology and ii) a bundling of metallic and semiconducting SWNTs. These obviously will degrade the device performance such as an on/ off current ratio, sensitivity and reproducibility. So far, most of the solution processes have incorporated the dispersion of bundled SWNTs. Since SWNT bundles may contain metallic SWNTs serving as a conductive wire, the network of bundled SWNTs tends to have conducting paths. Even though the enrichment of high-purity semiconducting SWNTs has been achieved, the TFT characteristics can still be degraded by a small number of metallic SWNTs which oftentimes creeps into bundled SWNT networks. To avoid the use of the bundles, several research groups have utilized aqueous solution of surfactant-wrapped individual SWNTs. However, the transistor characteristics of these individual SWNTs have never exceeded those of the bundled SWNTs for non-separated SWNTs. This is presumably because of the fact that such solution normally contains a large amount of free surfactants (usually hundreds times larger than SWNTs in weight) resulting in an inability to achieve effective inter-nanotube contacts. The development of a new and effective solution process of isolated SWNTs is, therefore, highly required to realize high performance TFT devices. One of the most promising candidates for realizing highperformance TFTdevices is to incorporate DNA-wrapped SWNTs as TFTchannels. It has been reported that individual SWNTs can easily be dispersed in water using DNA in the form of DNAwrapped SWNT hybrids (DNA-SWNTs). Importantly, they are completely stable without unbound and free DNA molecules unlike the surfactant-wrapped SWNTs case. Furthermore, the DNA-SWNTs can be sorted by chirality and length of nanotubes, which provides major advantages in the device applications. Here, we show that the DNA-SWNTs can provide an effective way to fabricate the uniform networks of highly isolated and structure-sorted nanotubes for TFTs of high performance. We have prepared a high purity aqueous solution of DNA-SWNTs using sonication, ultracentrifugation, and size-exclusion liquid chromatography (SEC). The SWNTs were wrapped by DNA molecules during the sonication in water. After removing unwrapped SWNTs through ultracentrifugation, the supernatant was collected. This supernatant contains the DNA-SWNTs and excessive amount of free DNA molecules (approximately 100 times the total weight of DNA-SWNTs), which disturb the inter-nanotube contacts. To remove the free DNA, the DNA-SWNTs were purified using SEC incorporating a newly developed HPLC column. Figure 1a shows typical SEC chromatograms of the supernatant, where the detecting UV wavelengths at 260 and 350 nm are used to trace DNA and SWNTs fraction, respectively. The DNA-SWNTs were completely separated from the free DNA because of their difference in size. The broad absorption band of DNA-SWNTs derives from a wide variety of their lengths. In SEC separations, the chromatographic retention time increases as the size of the molecule decreases. The DNA-SWNT hybrids, therefore, can be separated by their length. Figure 1b shows a histogram on the length distribution for a fraction of the retention time corresponding from 20 to 21min (hereafter refer to as fraction 20), which