With recent developments in single-walled carbon nanotube (SWNT) sorting technology, SWNT thin-film transistors (TFTs) fabricated by these sorted carbon nanotubes demonstrate high mobility and on/ off ratios. However, challenges still remain for their practical use, including:

Control of the threshold voltage for robust and low-power circuits
Reduction of the operational large hysteresis
Consistent device polarity (unipolar/ambipolar) in different environments.

Our first approach is to n-dope SWNTs using 1H-benzoimidazole derivatives processed via either vacuum evaporation or solution coating1. The threshold voltages of our polythiophene-sorted SWNTs TFTs can be continuously tuned over a wide range. Photoelectron spectroscopy (PES) measurements confirmed that the SWNT Fermi energy decreases with increased doping concentration. Utilizing this approach, we proceeded to fabricate SWNT complementary inverters by inkjet printing of the dopants with a noise margin of 28 V at VDD = 80 V (70% of 1/2 VDD) and a gain of 85. Additionally, equally robust SWNT CMOS inverters (noise margin 72% of 1/2 VDD), NAND and NOR logic gates with rail-to-rail output voltage swing and sub-nanowatts power consumption were fabricated onto a highly flexible substrate for the first time.

Our second approach is to utilize Poly(trifluoroethylene) (PTrFE) polymer as a top-gate dielectric to fabricate hysteresis-free devices at different sweep rates. These devices also exhibit excellent bias-stress stability under ambient conditions. Ambipolar SWNT transistors are observed when Poly(vinylidene-trifluoroethylene-chlorotrifluoroethylene), P(VDF-TrFE-CTFE), is utilized as a top-gate dielectric in air. Furthermore, we demonstrate the continuous tuning of threshold voltages of both unipolar and ambipolar SWNT TFTs by application of a second gate voltage.

Reference:

1. H. Wang, Z. Bao et al., Tuning the threshold voltage of carbon nanotube transistors by n-type molecular doping for robust and flexible complementary circuits, PNAS, 111(13), 4776-4781, 2014

2. H. Wang, Z. Bao et al., Highly Stable Carbon Nanotube Top-Gate Transistors with Tunable Threshold Voltage, Advanced Mateirals, doi: 10.1002/adma.201400540, 2014

Huiliang Wang

Stanford University

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