I. Selective Dispersion of Semiconducting Carbon Nanotubes with Conjugated Polymers
Single-walled carbon nanotubes (SWNTs) have excellent electrical properties for a wide range of applications, such as transistors, solar cells, photodetectors and sensors. However, for all these applications, it is necessary to separate semiconducting (SC) SWNTs from the metallic ones. Sorting SWNTs using conjugated polymers has been demonstrated to be a simple and low-cost process to effectively sort large quantity of SC-SWNTs. My work aimed to improve both the sorting-yield and SC-SWNT purity for the sorting process as well as to understand the mechanism of selective SC-SWNT dispersion. I found that dispersion yield increases with polymer side-chain length, as a result of increased surface contact predicted from molecular dynamics simulation in explicit toluene (left figure). [ACS Nano, 2014] By exploring sorting with different solvents, we found that only non-polar solvents allow dispersion of semiconducting SWNTs, I used these data and several previous evidences to propose a new mechanism to explain the selective dispersion of semiconducting SWNTs. [Small, 2014]
In addition, the dispersion and sorting of higher mobility large-diameter SWNTs has been difficult due to strong van der Waals interactions between the lower curvature walls of the tubes. I managed to sort large-diameter SC-SWNTs by using planar backbone polymers. [ACS Nano, 2013] By modifying polymer structure and polymer/SWNT ratios, we achieved very high purity of semiconducting large-diameter SWNTs. We fabricated transistors with simultaneous mobilities above 40 and on/off ratios above 10^4. [Small, 2015] I also wrote a review article summarizing the effects of various polymer and solvents on sorting of carbon nanotubes as well as their electronics applications. [Nano today, 2015]
II. Improving Carbon Nanotube Transistor Characteristics for Flexible Circuits
Flexible electronics have attracted increasing attention recently due to the plethora of possible and realized applications in radio-frequency identification cards, flexible displays and digital processors. With our high-purity sorting of SC-SWNTs by conjugated polymers, high mobility and on/off ratios thin-film transistors (TFTs) were demonstrated. However, challenges still remain for their practical use in circuit application: including: i) control of the threshold voltage for robust and low-power circuits; ii) reduction of the operational large hysteresis; iii) consistent device polarity (unipolar/ambipolar) in different environments.
Our first approach was to n-dope SWNTs using 1H-benzoimidazole derivatives processed via either vacuum evaporation or solution coating. The threshold voltages of our polythiophene-sorted SWNTs TFTs could be continuously tuned over a wide range. 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 (top figure). [PNAS, 2014]
Our second approach was to utilize fluorinated Poly(trifluoroethylene) (PTrFE) polymer as a top-gate dielectric to fabricate hysteresis-free devices at different sweep rates. These devices also exhibited excellent bias-stress stability under ambient conditions, important for the long-term operation of SWNT TFTs. Furthermore, we demonstrated the continuous tuning of threshold voltage by application of a second gate voltage. [Advanced Materials, 2014]
Our first approach was to n-dope SWNTs using 1H-benzoimidazole derivatives processed via either vacuum evaporation or solution coating. The threshold voltages of our polythiophene-sorted SWNTs TFTs could be continuously tuned over a wide range. 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 (top figure). [PNAS, 2014]
Our second approach was to utilize fluorinated Poly(trifluoroethylene) (PTrFE) polymer as a top-gate dielectric to fabricate hysteresis-free devices at different sweep rates. These devices also exhibited excellent bias-stress stability under ambient conditions, important for the long-term operation of SWNT TFTs. Furthermore, we demonstrated the continuous tuning of threshold voltage by application of a second gate voltage. [Advanced Materials, 2014]
III.Fabrication of Shape-Controlled, Self-Wrapped Carbon Nanotube Electronics
Conventional electronics are two-dimensional (2D) and fabricated on flat surfaces. However, many real-world objects are three-dimensional (3D). It is therefore highly desirable to fabricate 3D electronics with controlled shapes or achieve conformal coating onto the 3D electronics. In this work, I developed a bilayer system of shape memory polymers and flexible carbon nanotube electronics on polyimide for achieving shape-controlled 3D electronics. I demonstrated the temperature-assisted fabrication of 3D complex structures and developed an analytical model for predicting the shape evolution of the my devices. Through local heating by electrical metal heaters, I achieved multi-step folding using the bilayer system. Additionally, the electronics can be conformally shrunk on various irregular objects used in daily life. Various shrunken SWNT devices such as transistors, hydrogen sensors, temperature sensors or memory devices were demonstrated. [Advanced Science, 2015]