sensor

ekusoyt

sensor

Carbon Nanotube Resonator Sensors for

Wireless Sensing Systems

Anh-Vu Pham

DMRC

University of California at Davis

Department of Electrical and Computer Engineering

Davis, CA 95616


Outline

Overview of Carbon Nanotubes

Carbon Nanotubes Resonator Sensors

Experimental Results

Future Work

Conclusions


Carbon Nanotubes

• Graphene sheet rolled into a cylinder

• Carbon nanotube cylinder ~1-2μm to centimeter long

• Tube diameters ~ 1 nm (Single Walled

Nanotubes) & 25nm (Multi Walled Nanotubes)

• Concentric cylinders: MWNT with an interlayer spacing of 3.4Å

• Metallic or semi-conducting carbon nanotubes

Graphene Sheet

Nanotube

MWNT

Single Wall Carbon Nanotube


Overview of Wireless Sensor Systems

• Passive sensor devices and zero power consumption

• Wireless transceivers interrogate and detect RF signals

• Signal transducing in the sensor device

Transmitting

Antenna

Modulator

Passive Sensor

Device

Power Amplifiers

Receiving

Antenna

Oscillator

Low Noise

Amplifier

Filter Mixer Filter


RF Electromagnetic Resonator Sensor

• Circular disk resonator on duroid board

• Random assembly of carbon nanotubes

• Single walled carbon nanotube (SWNT) and multiwalled

carbon nanotube (MWNT) coating

Duroid Board

Resonator disk

coated with

Carbon

Nanotubes

Carbon

Nanotubes

Cu disk

Microstrip

Feedline

Duroid Substrate

Top view of circular resonator

Front view of resonator


Experimental Setup

• 1-port RF measurements using network analyzers

• Gas inlet connection and pressure gauge

• Exhaust outlet

Testing

Chamber

Testing chamber and sensor circuit

G

A

S

C

Y

L

I

N

D

E

R

Pressure

Gauge

Network

Analyzer

Testing setup

To

Vacuum

pump


Initial Analysis of Nanotube Sensor

• Circular disk resonator without nanotubes

• No change in electrical response without nanotubes

• Absence of adsorbent surface

In Vacuum

In Ammonia

Return Loss (dB)

0

-5

-10

-15

-20

-25

-30

Resonant freq. of

3.88 GHz

-35

3.5 3.7 3.9 4.1 4.3

Frequency (GHz)


Response of As-Prepared SWNT Sensor to NH 3

• ~1.4 nm diameter and ~1-2μm length

• Room temperature operation

• Resonant frequency shift when exposed to NH 3

-10

In Vacuum (before) In Ammonia In Vacuum (after)

Return Loss (dB)

-15

-20

-25

-30

-35

Shift of 6.25 MHz

-40

3.88 3.9 3.92 3.94 3.96 3.98 4

Frequency (GHz)


Response of As-Prepared MWNT Sensor to NH 3

• ~25 nm diameter and ~1-2μm length

• Room temperature operation

• Smaller Resonant frequency shift

In Vacuum (before) In Ammonia In Vacuum (after)

-10

Return Loss (dB)

-15

-20

-25

Shift of 3.75 MHz

-30

3.83 3.86 3.89 3.92 3.95

Frequency (GHz)


Response Time and Recovery Time

• Response of carbon nanotube resonator sensor to NH 3

• Fast response time: ~1 to 2 minutes

• Fast recovery time: ~1 to 2 minutes

• Response and recovery time for CO 2 : ~ 45 s

4.627

4.626

4.625

Freq. (GHz)

4.624

4.623

4.622

4.621

4.62

4.619

NH 3

0 100 200 300 400 500 600

Time (sec)


Comparison of Sensitivity

• Surface area is key to gas sensors

• Higher sensitivity for SWNT sensor

• Similar response at lower gas concentration

Frequency Shift in MHz

6

5

4

3

2

1

f o ~ 4 GHz

+ : SW NT coated sensor

response to am m onia

Δ : MW NT coated sensor

response to am m onia

0

0

400

800

Ammonia (ppm)

1200

Sensitivity of RF Carbon Nanotube Sensor to NH 3


Degassed Experimental Setup

To network

analyzer

Coaxial

cable

Resonant

circuit

Testing

chamber

Gas inlet

To vacuum

pump

Heating tape

Pressure

regulator

Second testing chamber

Schematic of the second testing setup

• Measurements of degassed nanotube sensor

• Second testing chamber used for degassing nanotubes


Response of the degassed SWNT to O 2

• SWNTs degassed at ~10 -5 Torr, 125 º C

• Response to air dominated by O 2

• Frequency shift equal to the degassed shift

In Air Degassed In Oxygen

-20

Return Loss (dB)

-30

-40

-50

Δω 0

= 2.3 MHz

-60

3.87 3.88 3.89 3.90 3.91

Freq (GHz)

Δω = 2.3 MHz

Δω 0

= Frequency shift upon degassing SWNTs

Δω = Frequency shift upon He exposure


Frequency Shifts for different Gases

• Shift more pronounced for polar gases

• Shift for polar gases more than the degassed shift

• Maximum shift observed for NH 3

Response of sensor in different gases

In Ammonia

In CO

Polar gases

Non-polar

gases

In Oxygen

Gas

In Nitrogen

In Ar

In He

Degassed

In Air

3.883 3.884 3.885 3.886 3.887 3.888 3.889 3.89

Freq shift (GHz)

Response of the SWNT sensor to various polar

and non-polar gases


Summary of Performance

• Change in ε r results in resonant frequency shift

• Ultra high sensitivity ~4000 Hz/ppm

• Room temperature operation

• Fast response time ~45 s to few minutes

• Fast recovery time ~45 s to few minutes v.s. 10

hours

• Physisorption and chemisorption

• Suitable for wireless detection platforms


Sensitivity of As-prepared SWNT to Organic Solvents

• Significant resonant frequency Shift

with SWNT coating

• Monitor biological agents using

spectral responses

Frequency Shift with Methanol Applied, Nanotubes Present

-10.00

Initial (in Air) Shifted (in Methanol) Recovered (in Air)

-12.00

-14.00

-16.00

Return Loss (dB)

-18.00

-20.00

-22.00

-24.00

-26.00

f

= 7.164 MHz

-28.00

-30.00

3.850 3.860 3.870 3.880 3.890 3.900 3.910 3.920 3.930

Frequency (GHz)


Sensitivity of As-prepared SWNT to Organic Solvents

Comparison of Frequency Shifts Induced By Various Solutions

Response to water (no nanotubes)

Response to methanol (no nanotubes)

Response to isopropyl alcohol (no nanotubes)

Response to water (nanotubes present)

Response to methanol (nanotubes present)

Response to isopropyl alcohol (nanotubes present)

8.00

Wih Nanotubes

7.00

Frequency Shift Measured(MHz)

6.00

5.00

4.00

3.00

2.00

Without Nanotubes

With Nanotubes

Without Nanobubes

With Nantubes

1.00

Without Nanotubes

0.00

Water Methanol Isopropyl Alcohol


Next Generation Carbon Nanotube Sensors

• Vertically aligned carbon nanotubes

• Increase surface area and sensitivity

• Multi-wall Carbon Nanotubes

• Functionalization of carbon nantobues

Nano Lab

Next generation sensor using vertically aligned carbon nanotubes


Conclusions

• New nano-based sensor Platform

• High sensitivity and fast response and recovery time

• Room temperature operation

• Potential for new toxic and biological agents

• Zero power consumption

• Highly suitable for wireless sensor networks


Acknowledgement

National Science Foundation

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