Strain-Based Resistance of Single-Walled Carbon Nanotubes
Jonathon A.
1 2006
2
1
Brame ,
Johnathan
1
Goodsell ,
Dr. Stephanie A.
2
Getty
ESMD Faculty/Student Research Team Participant, Department of Physics, Provo, UT, jon.brame@gmail.com
NASA Goddard Space Flight Center, Materials Engineering Branch, Code 541, Greenbelt, MD, Stephanie.A.Getty@nasa.gov
Abstract
Applications
A device of this scale capable of measuring strain would be very useful in
the nano-technology industry. Specifically this nano-sensor is being
developed to create a micro-scale vector magnetometer for use in
magnetospheric science and planetary magnetic study and mapping (see
Figures 6 & 7).
Background
The remarkable effect of strain on the conductivity of
SWCNTs has been demonstrated through local
deformation (see Figure 1), and through stretching of the
tube structure (see Figure 2). We seek to extend those
results to an array of SWCNTs in a strain sensor.
Stretching Results
-Stretch
-Release
4000
The initial results of the stretch-testing show
evidence of reversible, strain-based change in
resistance in SWCNT devices. Figure 10
shows both the characteristic resistance
changes with stretching/releasing, as well as
several distinct resistance “levels” possibly
activated by individual nanotube contacts
changing in the stretching process.
3200
Level 2~450 kΩ
2400
1600
slack in device
800
0
0
Fig 6 Earth’s magnetosphere
Fig. 1
Conductivity
versus strain in a
SWCNT
depressed by an
AFM tip
(Tombler et al.
Nature, 2000).
Resistance Steps
Resistance (kΩ)
The goal of this project is to fabricate devices to test the strain-based change in resistance of
Single-Walled Carbon Nanotubes (SWCNT) for use in micro-scale, high resolution
magnetometry. To do this, we must first fabricate a device with electrically contacted
SWCNTs, then release the device onto a flexible substrate for strain testing. We report
progress in growth techniques, testing techniques, and comparisons between Chemical
Vapor Deposition (CVD) grown tubes and commercially available SWCNTs.
Fig7 Possible
planetary
magnetic
exploration
Growth Results
Using thin film iron catalyst has shown marked improvement over the
initial iron nitrate catalyzed CVD growths (see Figure 8), yielding initial
SWCNT resistance decrease of several orders of magnitude.
Additionally, magnetic tests were performed on tubes grown using this
new method to ensure that there is no inherent magnetic effect from the
iron catalyst (Figure 9).
Fig 2 Drastic change in conductivity due to stretching was measured
for a semi-conducting SWCNT (Cao et al. Physical Review Letters,
2003).
Fabrication
4
Level 1~300 kΩ
Tensile
Table 1 Strength
(MPa)
Parylene 45
Young’s
Modulus
(MPa)
2.4e3
SWCNTs 5.0 e4
1.0 e6
8
12
16
20
Fe Thin Film
Fig 10 A step pattern was used for testing resistance while
stretching, stretching twice by 4 µm, then releasing back 4 µm
Table 1 Due to the vastly different stretching characteristics of
parylene and SWCNTs, uniform stress is not distributed evenly
throughout the sample during a stretching. Most of the strain
is absorbed in the parylene, while the nanotubes may slip
within the substrate rather than stretch as desired. As the
parylene stretches, however, it should cause enough
displacement of the tubes to change the resistance, since
nanotubes resistance are subject to change through bending as
well as stretching (see Tombler et al.)
Parylene
Etching (KOH)
Conclusions and Future Work
SWCNTs
Gold Contacts
Fig 3
Fig 8 The SEM image on the left shows SWCNTs grown with Iron Nitrate
catalyst, while the image on the right shows SWCNTs grown using the thin film
iron catalyst technique (Note that the image on the right is at twice the
magnification as the image on the left).
•
Increased Growth Density of SWCNT
•
Transfer of CVD grown SWCNTs onto flexible substrate
•
Fabrication of device to measure stretch-based resistance changes
•
Development of methodology for stretching SWCNTs
•
Preliminary results from stretch-testing
•
Preliminary comparisons of CVD and commercial SWCNTs
•
Future Work
Joint-effort testing between NASA and BYU
Continuation of stretch-testing
Further testing of commercially available tubes
Top view
Fig 4
Once the SWCNTs were released
onto the flexible substrate, the
device was mounted onto a frame
made of ceramic chips with gold
contact pads around the outside.
The contact pads on the frame
were then wire-bonded to the gold
contacts on the nanotubes (see
Figure 4). Once the sample was
attached and contacted to the
frame, the whole device was
mounted onto a probe station for
resistance testing (see Figure 5).
Fabrication of micro-magnetometer
Acknowledgements
Figure 9 The resistance of the nanotube device
remained constant through a changing magnetic field
Fig 5
28
Stretch (µm)
After depositing a thin film layer of Iron, SWCNTs were grown on the SiO2 substrate through a CVD process using methane and ethylene as the feed gases. Next
we used a shadow-masked gold evaporation to establish electrical contact with the nanotubes and coated the whole device with Parylene. Once the SiO2 is etched
away from behind, we are left with an electrically contacted SWCNT device on a flexible substrate. (see Figure 3)
SiO2
24
ESMD Program, Rocky Mountain Space Grant Consortium
BYU- Dr. David Allred, Johnathan Goodsell
Fisk University- Melissa Harrison
C. Taylor, D. Dove, C. Hoffman, L. Wang