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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
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