Strain-Based Resistance of Single-Walled Carbon Nanotubes

AbstractThe 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.

Jonathon A. Brame1, Johnathan Goodsell1, Dr. Stephanie A. Getty2

1 2006 ESMD Faculty/Student Research Team Participant, Department of Physics, Provo, UT, jon.brame@gmail.com

2 NASA Goddard Space Flight Center, Materials Engineering Branch, Code 541, Greenbelt, MD, Stephanie.A.Getty@nasa.gov

FabricationAfter 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 Fe Thin Film SWCNTs

Parylene Gold ContactsEtching (KOH)

Fig 4

Fig 3

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).

Top view

BackgroundThe 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.

Fig. 1 Conductivity versus strain in a SWCNT depressed by an AFM tip (Tombler et al. Nature, 2000). Fig 2 Drastic change in conductivity due to stretching was

measured for a semi-conducting SWCNT (Cao et al. Physical Review Letters, 2003).

Stretching ResultsThe 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.

Resistance Steps

0

800

1600

2400

3200

4000

0 4 8 12 16 20 24 28

Stretch (µm)

Resis

tance (kΩ

)

-Stretch-Release

slack in device

Fig 10 A step pattern was used for testing resistance while stretching, stretching twice by 4 µm, then releasing back 4 µm

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).

Level 2~450 kΩ

Level 1~300 kΩ

Growth ResultsUsing 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).

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.)

1.0 e65.0 e4SWCNTs

2.4e345Parylene

Young’s Modulus (MPa)

Tensile Strength (MPa)

1.0 e65.0 e4SWCNTs

2.4e345Parylene

Young’s Modulus (MPa)

Tensile Strength (MPa)

Table 1

ApplicationsA 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).

Fig 6 Earth’s magnetosphere

Fig7 Possible planetary magnetic exploration

Conclusions and Future Work• 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

Fabrication of micro-magnetometer

Fig 5

AcknowledgementsESMD Program, Rocky Mountain Space Grant ConsortiumBYU- Dr. David Allred, Johnathan GoodsellFisk University- Melissa HarrisonC. Taylor, D. Dove, C. Hoffman, L. Wang

Figure 9 The resistance of the nanotube device remained constant through a changing magnetic field