Single Walled Carbon Nanotube Magnetometer for Planetary Exploration
Jonathon A.
1a
Brame ,
Johnathan
1b
Goodsell ,
Bryan
2
Hicks ,
Dr.
1a
David Allred
and Dr. Stephanie A.
3
Getty
1 2007 ESMD Faculty/Student Research Team Participant, a Department of Physics & Astronomy, b Department of Mechanical Engineering, Brigham Young University, Provo, UT,
2
2007 Rocky Mountain NASA Space Grant Consortium Summer Internship, Department of Physics & Astronomy, Brigham Young University, Provo, UT,
3
Abstract
NASA Goddard Space Flight Center, Materials Engineering Branch, Code 541, Greenbelt, MD, Stephanie.A.Getty@nasa.gov
Applications
Fig. 4 – Surface of Mars
The small size and low mass of this device, coupled with high spatial
resolution makes it ideal for exploration of planetary magnetic fields as well as
localized magnetic mapping of fields such as those found on Mars. The design
concept can also be adapted to strain-sensing applications, including
acceleration, wind speed, temperature, and fluid flow measurements.
The goal of this project is to fabricate a low mass, strain-based magnetic sensor for use in
planetary science and exploration missions. The operating principle exploits the sensitivity
of Single Walled Carbon Nanotube (SWCNT) electrical properties to strain. The sensor
design resembles a classical compass with electronic readout. The sensor consists of a
magnetically responsive, high aspect-ratio Fe component suspended on a free-standing mat
of electrically contacted SWCNTs. During operation, torque on the Fe needle will transduce
ambient magnetic field strength into an electronic signal. We report on device fabrication
and preliminary measurements.
Background
SWCNTs are one-dimensional tubes of carbon atoms bonded hexagonally that have
outstanding electrical, thermal, and mechanical properties. Typical diameter is
approximately 1 nm, and the length of a nanotube can be thousands of times larger than the
diameter. The atomic structure of a SWCNT can be conceptualized by rolling up a single
sheet of graphite (graphene) and trimming off the overlap to form a seamless cylinder.
Courtesy Fuhrer Group
Prior Work
During the first year of the BYU-NASA collaboration program (2006), we were able to
do some preliminary work to enhance and validate the concept of the magnetometer as
well as increase our understanding of networks of SWCNTs. Some of the projects
included increasing SWCNT growth density, testing the strain-based resistance of
nanotube networks, and measuring the resistive response of nanotube networks to
changes in magnetic field.
Fig. 2a – SWCNTs grown
with iron nitrate catalyst
Fig. 2b – SWCNTs grown
with thin film iron catalyst
After growing carbon nanotubes on the SiO2 substrate, we use e-beam
lithography to define a pattern for gold evaporation. At right is a top and
side view of the device after the gold evaporation. Purple represents silicon
dioxide, Grey represents Silicon, Yellow represents gold, and Black
represents the nanotubes.
Fig. 5 – Earth’s
Magnetosphere
Fig. 6 – Mechanism
of operation
Concept
Single Walled Carbon Nanotubes (SWCNT) have been shown to have large
changes in resistance with applied strain (Tombler et al. Nature 2000). We
seek to extend that characteristic from a single tube to a network of tubes as
grown on the substrate. By suspending an iron needle on such a network of
nanotubes, a magnetic field will cause the needle to deflect, straining the
nanotubes and changing the net resistance of the network.
Fig. 1 – SWCNT atomic structure
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 use electron-beam lithography to open a pattern for the gold contact pads (deposited through thermal evaporation with a chromium under-layer for adhesion).
Once the contact pads are in place, another e-beam lithography step allows us to deposit the iron needle. We deposit a 300 nm Fe needle between two 30 nm
layers of chromium to protect the needle during the etching process. In order to free the needle from the substrate, we use wet chemical etching to create a trench
underneath the needle. In order to reduce the risk of damage due to surface tension as the device dries, we remove liquid in a Critical Point Dryer (CPD).
Gold Contacts
The surface of Mars and the
Earth’s magnetospherePrime subjects for magnetometry measurements.
SWNT
Magnetometer
Fabrication
• Persistent
magnetic
moment of
Fe “needle”
Iron Needle
• Compass
mechanism
After electrically testing the gold contacts to make sure that the nanotube
network contacts the gold pads, we deposit a thin iron needle between the
gold contacts (represented by brown in the diagram). We encase the iron
needle between two 30 nm chromium evaporations to help passivate the iron
in preparation for the wet etching.
• Torque
bends
SWCNTs
ST5 Fluxgate
• Monitor
electrical
conductance
vs. field
Max Op
450°C
100°C
Temp
Sensor
1cm x 1cm x
4cm x 4cm x 6cm
Dimensions
100nm
Sensor
-5
75 g
10 g
Mass
Sensor Op
-3
-2
50 mW
10 - 10 mW
Power
Table 1. Comparison of SWCNT Magnetometer to miniaturized
fluxgate magnetometer flown on ST5 mission (UCLA)
Trench Etching
The etching process involves first immersing the device in a Buffered Oxide
Etch (BOE) to etch the SiO2. We use photoresist to shield the rest of the
substrate while using an optical microscope to expose the area surrounding
the needle. After the BOE, we etch the silicon with KOH at 65º C to make a
trench deep enough to allow the iron needle to rotate out of the plane of the
substrate.
Results
Fig. 7 – Intrinsic magnetic field
response of SWCNT network
At right is an image taken with a scanning
electron microscope of a successful device. The
needle is suspended over the trench by a mat of
nanotubes (note the rolled mat of tubes on the
left end of the device).
Increased Growth Density: The figure above on the left shows characteristic growth of
SWCNTs using an iron nitrate solution catalyst. On the right is a characteristic growth using
and indirect evaporation of thin-film iron, maximized at a growth temperature of 950°C.
SWCNTs
Fig. 9 – NanoCompass fabrication
procedure
Fig. 10 – Prototype NanoCompass
Conclusions and Future Work
Au Electrode
Au Electrode
Fe Needle
•
Completed device for testing
•
More robust etching process
•
Measurement of resistance of suspended nanotube networks
over a trench (with no needle)
•
Fig. 3 – Strain response of
flexible SWCNT network
device on parylene
substrate
Future Work
Reproduce dense SWCNT growth at BYU
Magnetic Field (T)
Make electronic measurements of device with AFM
Magnetic Testing: Above is a graph of the resistance of a nanotube network with a changing
applied magnetic field. It is important that a magnetic field does not change the inherent
resistance of the nanotubes so that we can isolate the resistance change due to the torque from
the iron needle. No changes were seen in magnetic fields up to 0.35 Tesla.
Strain-Based Resistance: At left is a graph of the
resistance of a nanotube network as it was stretched from
slack through equilibrium to a maximum of 20 µm. The
measurements were made on a mat of nanotubes that had
been lifted off the SiO2 substrate into a flexible Parylene
substrate through a process developed last summer.
Fig. 8 – Preliminary magnetic
field response of NanoCompass
prototype
0
0.04
0.08
0.12 0.16 0.20
Magnetic Field (T)
0.24
0.28
At left is a plot of current measured by the magnetometer
device versus changing magnetic field up to 0.3T. Device
response is shown for increasing and decreasing magnetic
field over one cycle. No conclusive device response was
seen. Optimization of the device design is underway to
increase sensor response to magnetic field, including
segmented electrodes to isolate SWCNTs that experience
largest strain and a needle with reduced width for higher
magnetic coercivity.
Further strain testing measurements
Acknowledgements
ESMD Program, Rocky Mountain NASA Space Grant Consortium
BYU- Dr. Robert Davis, Dr. Aaron Hawkins
GSFC- Rachel Bis, Stacy Snyder, C. Taylor, D. Dove,
C. Hoffman, L. Wang