Fully Printed Carbon Nanotube Thin-Film Transistors for Pressure Sensing ApplicationsVipin Prajapati1,2, Joseph Andrews2, Martin Brooke2, Aaron Franklin2,3

1Department of Electrical Engineering, IIT Gandhinagar; 2Department of Electrical and Computer Engineering 3Department of Chemistry, Duke University, Durham NC

Introduction and Motivation

Fully printed and flexible electronics are excellent candidates for facilitating large area

displays, sensors, and actuators. Printed platforms with active surface areas on the order

of square centimetres can be manufactured at incredibly low costs, enabling economically

viable large area sensor networks. In particular, such large-area sensor platforms can be

used to improve transportation safety, or to enhance human-machine interface

applications. One specific material that lends itself particularly well to printing active

circuits is single-walled carbon nanotubes (CNTs). CNTs have been intensely studied for

use in printed thin-film transistors (TFTs) due to their high carrier mobility, excellent

chemical stability and mechanical flexibility, as well as compatibility with various

printing processes. CNTs are allotropes of carbon consisting of a single graphene sheet

rolled up into a cylindrical shape. Single nanotubes can exhibit ballistic transport with

current levels up to 25 µA with tensile strengths greater than 100 Gpa.3 These properties

make them ideal candidates for large area electronics, which must have favorable

electronic properties and be mechanically strong. A variety of applications, particularly

involving flexible electronics and Internet of Things (IoT) sensor networks would be

made possible with low-cost, printed CNT-TFTs that exhibit particularly high

performance. Since sensors for pressure sensing inside a tire should be flexible and cover

a large area while also being able to survive the harsh environment, CNT-TFTs are an

attractive option for their realization. CNT-TFTs are also well suited for pressure sensing

as they can have the entire active channel of CNTs exposed to the sensing environment,

which we hypothesize will enhance sensitivity to environmental pressure. Progress

towards achieving this pressure sensing application is achieved in this work.

Methods

Results

1. Cao, Changyong, et al. "Improving Contact Interfaces in Fully Printed Carbon

Nanotube Thin-Film Transistors." ACS nano 10.5 (2016): 5221-5229.

2. Franklin, Aaron D. "Nanomaterials in transistors: From high-performance to thin-

film applications." Science 349.6249 (2015): aab2750.

3. Chen, Kevin, et al. "Printed carbon nanotube electronics and sensor

systems." Advanced Materials 28.22 (2016): 4397-4414.

4. AEROSOL JET 300 Data Shee-

“http://www.optomec.com/wpcontent/uploads/2014/08/AJ_300_WEB_0216.pdf”

Conclusion

Through our initial investigations involving the various geometries of CNT-TFTs we

found that device performance of CNT-TFTs with width 200 µm and length 50 µm

yielded favorable characteristics for use in sensing applications. Also these transistors

have active surface area of 200 x 50 µm2, which is sufficient for a strong signal when

employed for sensing. This work is valuable in determining how varying both channel

length and channel width affect device performance. We have determined that for future

experiments, particularly with the goal of testing devices inside the pressure chamber, we

must use locally gated devices as opposed to the substrate gated devices printed in this

study. Although we do not have data proving the sensitivities to environmental pressure,

we hypothesize that the devices printed here will have a marked change in electrical

properties depending on the pressure of the environment that they are placed in.

In addition to the progress made with printing CNT-TFTs, we made an Embedded System

using an Arduino uno that can wirelessly transmit data to an Android cellphone. This can

be use to transmit sensor data from remote locations. For future work, back and top gated

CNT-TFTs will be printed using the same printing parameters as presented here. The

devices will then be studied as pressure sensors with their data being remotely transmitted

using the demonstrated Embedded System developed in this work.

References

Fig 1 : Aerosol Jet Printing Process4

Fig 3: Fully printed CNT-TFT- (a) Schematic diagram substrate gated CNT-TFT, (b)

SEM image of Ag-CNT interface, (c) Optical Image of CNT channel, (d) SEM image

of CNT channel, (e) AFM image of CNT channel

a1

c

b d

Laboratory of Electronics from Nanomaterials

e

Aerosol Jet Printing Process4

45 nm

0.0 1:Height 0.5µm

Fig 5: Pressure Chamber (a) outside view, (b) inside view with CNT-TFT

Fig 6: Wireless data transmission using Arduino Uno

ba

10-3

10-2

10-1

100

ID

(u

A)

-40 -30 -20 -10 0 10 20 30 40

VGS (V)

Lch = 50 um Lch = 100 um Lch = 200 um

a

Channel Width =100 µm

Fig 4: Electrical properties of CNT-TFTs with three different channel

lengths 50 µm, 100 µm and 200 µm - (a) and (b) Subthreshold transfer

(ID-VGS) characteristics with channel width 100 µm and 200 µm

respectively, (c) and (d) Transfer (ID-VGS) characteristics with channel

width 100µm and 200 µm respectively.

Channel Width =200 µm

Ink Preparation

Sample Preparation: We used a 300nm silicon wafer as our substrates. First, we cut the

wafer in squares of dimension 1.5x1.5mm. Next, we cleaned these chips with Acetone for 5

min in ultrasonic tub, rinsed with DI water, then ultrasonicated the substrates again for 5

min in IPA (Iso Propyl Alcohol) and finally dried the samples with N2 gas. Lastly, the chip

was placed in an oxygen plasma for 4 min at an RF power of 100 watts.

Printing Silver Ink: The silver ink was prepared with the following volumes: 1 ml Ag

nanoparticle ink provided by UT-Dots, 600 µl Xylene and 400 µl terpineol. This ink is use

for printing source and drain. The source and drain of CNT-TFTs were printed with

following printing parameters: Sheath flow rate of 25 sccm, carrier flow rate of 20 sccm,

atomizer current of 450 mA and a print speed 2 mm/s. Than The samples kept in a oven for

1 hour at 200 °C.

Printing CNT Ink: The chip was first immersed into Poly-L-Lysine (PLL) solution (0.1%

w/v in water; Sigma-Aldrich) for 5 min and then rinsed with DI water in order to enhance

CNT adhesion to the SiO2. The semiconducting channel of the CNT-TFTs was printed with

the following printing parameters: Sheath flow rate of 40 sccm, carrier flow rate of 23

sccm, atomizer current of 450 mA and a print speed of 2 mm/s. They amount of print

passes was varied from 1 to 3 for different devices. The samples kept in a oven for 10 mins

at 150 °C. Than samples were rinsed with toluene for 30s, then rinsed with DI Water and

IPA, and finally dried using N2 gas.

Printing Dielectric Ink: A dielectric ink, Xdi from Xerox, was used as a printed dielectric

for top and bottom gate devices. It was printed using the following parameters: Sheath flow

rate of 20 sccm, carrier flow rate of 25 sccm, an atomizer current of 450 mA and a speed of

10 or 8mm/s.

e

Fig 2: Printing process of different types of CNT-TFTs (a) Substrate Gated CNT-TFT

(b)Bottom Gated CNT-TFT (c) Top Gated CNT-TFT

First Ag Gate

Printed

Si Si

Si

300nm SiO2

SWCNT Channel

Si

Si

Dielectric

Si

Substrate gated CNT-TFT

Top gated CNT-TFT

Ag Source and drain

Si

PLL

Ag nanoparticle

300nm SiO2

Bottom gated CNT-TFT

99% semiconducting SWCNT Dielectric Layer

PLL(Poly-L-Lysine)

a

b c

A liquid sample is atomized, creating a

dense aerosol composed of droplets with

diameters between approximately 1 and 5

microns.

The aerosol is transported to the deposition

head using an inert carrier gas.

The aerosol is focused within the

deposition head by an annular sheath gas.

The resulting high velocity jet is deposited

onto planar or 3D substrates, creating

features ranging from 10 microns to

millimetres in size.

A motion-control system allows for the

creation of complex patterns on the

substrate.

First Ag source and drain, then the CNT

channel are printed on a silicon wafer with

an SiO2 surface for making substrate gated

devices.

10-3

10-2

10-1

100

ID

(u

A)

-40 -30 -20 -10 0 10 20 30 40

VGS (V)

Lch = 50 um Lch = 100 um Lch = 200 um

0.5

0.4

0.3

0.2

0.1

0.0

ID

(u

A)

-40 -30 -20 -10 0 10 20 30 40

VGS (V)

Lch = 50 um Lch = 100 um Lch = 200 um

0.5

0.4

0.3

0.2

0.1

0.0

ID

(u

A)

-40 -30 -20 -10 0 10 20 30 40

VGS (V)

Lch = 50 um Lch = 100 um Lch = 200 um

c d

b

Fig 7: Picture of OPTOMEC Aerosol Jet Printer

Si