Solution processing simplifies the manufacturing of carbon-nanotube transistors

The electronic characteristics of carbon nanotubes (CNTs) make them promising components for high-performance devices, including field-effect transistors, field emitters, and sensors. Making such devices, though, depends on assembling the CNTs into hierarchical nanostructures by controlling the shape, location, and orientation of large arrays of nanotubes. In the past, manufacturers would have approached such a task through high-temperature chemical-vapor deposition. Recent research, however, indicates that solution-processable CNTs provide several advantages: low-cost fabrication, large-area coverage, and low-temperature processing on flexible substrates.

To provide the necessary current density for devices, parallel arrays of single-walled carbon nanotubes (SWNTs) must be created. Moreover, the alignment and surface density of CNTs must be increased to improve the performance of thin-film transistors (TFTs). We tried several solution-based techniques for aligning CNTs,¹ but the resulting surface density of the CNT array was low, limiting the possible applications. Here, we describe liquid-crystalline processing that uses tilted-drop casting of a nanotube solution on micropatterned geometries to produce dense arrays of CNTs with long-range order.²

Recent work shows that CNTs—like other anisotropic and one-dimensional molecules—might form a lyotropic liquid-crystalline phase, one where concentration affects the ordering.³ Above a critical concentration, the nanotubes showed a phase transition to the nematic liquid-crystal phase, where the molecules are arranged along a common axis. To fully exploit these phenomena in high-performance TFTs, a CNT layer requires the long-range order with low misalignment that our method creates. In addition, we showed that a densely packed, uniformly oriented, CNT array exhibits much better electrical performance than a random array of CNTs.

Figure 1(a) shows a schematic of the tilted-drop casting process. In this example, a solution of SWNTs was used with a silicon wafer modified with an amine-terminated, self-assembled monolayer: the latter was prepared according to a procedure described elsewhere.⁴

As the solvent evaporated, the liquid-solid-air contact line swept down the surface, creating a highly concentrated solution locally and leaving a dense layer of CNTs behind the receding line. As shown in Figure 1(b), an atomic force micrograph of the resulting SWNT film showed no long-range order. By confining the drying process in micropatterned geometries, we created an array of long-range nematic structures. To control this process, we used photolithography to form 5μm-periodic stripes of photoresist—depicted in Figure 1(c)—on top of an amine-terminated, self-assembled monolayer. This modification triggered a unidirectional, microfluidic flow pattern forming multiple stripes of densely packed and highly oriented SWNT films, as shown in Figure 1(d).

Further, we fabricated TFTs in which densely packed CNT layers served as semiconducting channels between the source and drain electrodes: see Figures 2(a) and (b). The hole mobility of oriented CNT-TFTs is in the range of 60–92cm²/Vs, five to six times larger than the random devices, as indicated in Figure 2(c). With a channel length of 20μm, some of our oriented CNT-TFTs even generated a mobility of 126cm²/Vs, comparable to the record mobility—125cm²/Vs—reported for similar devices created with chemical-vapor deposition.⁵ The transconductance is 0.02–0.2μS/μm (at V_D = 0.1V ) for oriented CNT-TFTs, which is seven times higher than that for the random transistors (0.003–0.03μS/μm)—as shown in Figure 2(d).

In conclusion, we developed a simple liquid-crystalline process for fabricating densely packed, uniaxially aligned SWNTs as a semiconducting layer in CNT-TFTs. Our devices show excellent electrical characteristics even without special optimization. Further, by using highly purified carbon nanotubes, our tilted-drop approach could provide simple fabrication of even-more efficient TFTs and other electronic microdevices with advanced electronic characteristics.