High Performance Field-Effect Transistors Via Aligned Polyfluorene-Sorted Single-Walled Carbon Nanotube Arrays

Semiconducting single-walled carbon nanotubes (s-SWCNTs) are promising materials for charge transport in field-effect transistor (FET) devices. In order to develop short channel s-SWCNT FETs with high on/off ratios relevant for logic applications, ultra-pure s-SWCNTs of 99.99999% semiconducting purity and higher are needed.  There have been many recent studies on the sorting of carbon nanotubes, but most reports have only reached semiconducting purity levels near 99%. In addition, aligned arrays of s-SWCNTs have the potential to improve the mobility and current output of FETs. However, unacceptable semiconducting purity levels (~99%), inter-nanotube charge screening, and inter-nanotube bundling have limited the optimization of such technologies in the short channel regime.^1,2 Polyfluorene polymers have been shown to selectively wrap s-SWCNTs on the basis of electronic type, chirality, and diameter. Here we assess the purity and electronic transport properties of aligned arrays of polyfluorene-sorted s-SWCNTs in FET devices.   

            We obtained high purity s-SWCNTs by dispersing nanotube powder in solutions of poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(6,6’-{2,2’-bipyridine)] (PFO-BPy) in toluene following procedures similar to Mistry et al.³ We fabricated well-aligned s-SWCNT arrays by dispersing droplets of of s-SWCNTs at an air water interface, leading to diffusion and self-assembly of nanotubes into well-aligned “stripes” spanning the entire substrate. Finally, we implemented the aligned films as the channel material in field-effect transistors at varying channel lengths ranging from 9 μm to 300 nm. We characterized the films using Raman spectroscopy, AFM and SEM, and found a high degree of alignment, monolayer thickness, and packing density of 50 tubes/micrometer. Previous works on aligned nanotube FETs have used nanotube densities of 200 tubes/micrometer; however, the proximity of nanotubes in these films cause inter-nanotube screening effects resulting lower conductance and mobility values than expected.¹ The polyfluorene polymer plays an important role in isolating s-SWCNTs by preventing direct contact, preventing inter-nanotube screening effects.⁴

The electronic properties of the aligned film FETs were measured demonstrating high values of on/off ratio, mobility, and conductance. In previous studies, device on/off ratios degraded substantially to less than 10³ at channel lengths shorter than 1 micrometer, indicating the presence of metallic nanotubes.⁵ Here we observe a different trend; as the channel length decreases from 9 μm to 300 nm the on/off ratio only decreases from 3x10⁷ to 2x10⁵. This data suggests that m-SWCNTs are far scarcer in our films than in other short channel s-SWCNT FET devices. We estimated purity by counting the number of nanotubes per channel in direct transport regime devices that have channel lengths of 500 nm and shorter. In fact, of the devices measured, encompassing approximately 1,000 nanotubes, zero exhibited metallic character (defined as having an on/off ratio of at least 10³), suggesting a s-SWCNT purity of greater than 99.9%.

We achieved consistently high performance devices indicated by high mobility in devices that also have high on/off ratios. The highest mobility was 45 cm²/Vs for a 2 μm channel length device with on/off ratio 2x10⁵. We observed high mobility values ranging from 16-45 cm²/Vs across a range of channel lengths. Previous works were limited by purity, leading to trade offs between high on/off ratio and mobility, especially in the direct transport regime.⁵ Here we demonstrate a route to achieve both a large on/off ratio and high mobility, which is particularly significant for short channel devices in which nanotubes directly span the channel.

References

¹ Q. Cao, et al., Nature Nanotechnology 8 (3), 180 (2013).

² M. Engel, et al., ACS Nano 2 (12), 2445 (2008).

³ K S. Mistry, et. al., ACS Nano 7 (3), 2231 (2013).

⁴ F. Léonard,  Nanotechnology 17 (9), 2381 (2006).

⁵ Nima Rouhi,  et al., ACSNANO 5 (11), 8471 (2011).