CARBON NANOTUBE ALIGNMENT, PH SENSOR APPLICATION AND NANOCOMPOSITE ...

129 Pages·2011·2.98 MB·English

CARBON NANOTUBE ALIGNMENT, PH SENSOR APPLICATION AND
NANOCOMPOSITE MICROMACHINING

BY
PENGFEI LI

A thesis submitted in partial fulfillment of
the requirements for the degree of
(cid:3)
MASTER OF SCIENCE IN MECHANICAL ENGINEERING
WASHINGTON STATE UNIVERSITY
School of Engineering and Computer Science
MAY 2011
To the Faculty of Washington State University:
The members of the Committee appointed to examine the thesis of PENGFEI LI
find it satisfactory and recommend that it be accepted.

___________________________________
Wei Xue, Ph.D., Chair

___________________________________
Stephen A. Solovitz, Ph.D.

___________________________________
Linda (Xiaolin) Chen, Ph.D.

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ACKNOWLEDGMENT
I would like to express my appreciation to my advisor, Dr. Wei Xue, for allowing me the
opportunity to work in his research group. This work would not have been completed
without his insightful guidance and patience for which I am deeply indebted.

I would like to thank Dr. Stephen A. Solovitz and Dr. Linda (Xiaolin) Chen for serving
on my thesis committee. I would also like to acknowledge Dr. Dave (Dae-Wook) Kim
and Dr. Jie Xu for their unselfish and unfailing support for this work.

I would like to thank Mr. Chad Swanson and Mr. Troy Dunmire for their precious insight
and valuable assistant. Without their help, I would not continue my research smoothly.

In addition, I would like to express my gratitude to all of my fellow students for their
support and friendship. In particular, the encouragement and insight I received from
David W. Binion, Wei Cui, Kan Kan Yeung, Berk Gonenc, Jason L. Juhala, and Caleb
M. Martin, is greatly appreciated. I feel honored to have shared this experience with each
of them.

Lastly, my warmest thanks belong to my friends, family and the one beyond everyone of
us, our omnipresent Lord, without you, I wouldn’t know where I am or where I should
go. Thank you so much Lord, for leading me through the misery and offering me the
strength. May your name be honored, and glorified.

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CARBON NANOTUBE ALIGNMENT, PH SENSOR APPLICATION AND
NANOCOMPOSITE MICROMACHINING
ABSTRACT
by Pengfei Li, M.S.
Washington State University
May 2011
Chair: Wei Xue
Dielectrophoresis has been used in the controlled deposition of carbon nanotubes with the
focus on the alignment of nanotube films and their applications in the last decade. In this
work we extend the research from the selective deposition of carbon nanotube films to
the alignment of nanotube bundles and individual nanotubes. Electrodes with ―teeth‖-like
patterns are fabricated to study the influence of the electrode width on the deposition and
alignment of carbon nanotubes. Due to the fact that only photolithography-based
techniques are used in the process, the fabrication cost is low and the devices are
inexpensive. The alignment of carbon nanotube films, bundles, and individual nanotubes
is achieved under the optimized experimental conditions. The microscopy inspection of
the samples demonstrates that the alignment of nanotube bundles and individual
nanotubes can be achieved using narrow electrodes and low-concentration nanotube
solutions.

With the assistance of dielectrophoresis, we fabricate and characterize pH sensors using
aligned single-walled carbon nanotubes (SWNTs). Electrodes with ―teeth‖-like patterns
are used to generate concentrated electric fields and strong dielectrophoretic forces for
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the SWNTs to deposit and align in desired locations. The devices are used as pH sensors
with the electrodes as the conducting pads and the dielectrophoretically captured SWNTs
as the sensing elements. When exposed to solutions with various pH values, the SWNTs
change their resistance accordingly. The SWNT-based sensors demonstrate a linear
relationship between the sensor resistance and the pH value in the range of 5-9. The
characterization of multiple sensors proves their pH sensitivity is highly repeatable.

We investigate the machinability of polymethylmethacrylate (PMMA)/multi-walled
carbon nanotube (MWNT) nanocomposites in focused ion beam (FIB) micromachining.
PMMA/MWNT nanocomposites are fabricated with a solution casting method.
Microscale pockets are created on the PMMA/MWNT nanocomposites to study the
material removal mechanism using FIB. It is observed that the material removal rate
increases with increasing input current and decreasing overlap%. Soft lithography is used
to translate the ion-milled pockets from nanocomposites into microscale posts on
polydimethylsiloxane (PDMS). A scanning electron microscope (SEM) is used to
investigate samples. Our results demonstrate an effective method for producing
microscale patterns on nanotube-based nanocomposites.

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TABLE OF CONTENTS
ACKNOWLEDGMENT.............................................................................................. iii
ABSTRACT ................................................................................................................. iv
LIST OF TABLES ........................................................................................................ x
LIST OF FIGURES ..................................................................................................... xi
CHAPTER 1 INTRODUCTION ............................................................................... 1
1.1 Motivation ..................................................................................................... 1
1.2 Carbon nanotubes.......................................................................................... 2
1.2.1 Overview ............................................................................................... 2
1.2.2 Carbon nanotube pH sensors ................................................................. 7
1.3 Carbon nanotube alignment .......................................................................... 9
1.3.1 Alignment methods................................................................................ 9
1.3.2 Dielectrophoretic method .................................................................... 10
1.4 Objectives ................................................................................................... 12
CHAPTER 2 PATTERN DESIGN AND ELECTRODE FABRICATION ............ 16
2.1 Introduction ................................................................................................. 16
2.2 Dielectrophoresis ........................................................................................ 17
2.3 Photomask design ....................................................................................... 20
2.4 Photolithography ......................................................................................... 23
2.5 Wet etching ................................................................................................. 26
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2.6 Conclusion .................................................................................................. 27
CHAPTER 3 SELECTIVE DEPOSITION AND ALIGNMENT OF SINGLE-
WALLED CARBON NANOTUBES ............................................................................... 29
3.1 Introduction ................................................................................................. 29
3.2 Material preparation .................................................................................... 31
3.3 Electrodes fabrication ................................................................................. 32
3.4 Results and discussion ................................................................................ 36
3.4.1 Deposition and alignment of the SWNTs ............................................ 36
3.4.2 Electrical characteristics of the aligned SWNTs ................................. 40
3.5 Conclusion .................................................................................................. 44
CHAPTER 4 SELECTIVE DEPOSITION AND ALIGNMENT OF MULTI-
WALLED CARBON NANOTUBES ............................................................................... 45
4.1 Introduction ................................................................................................. 45
4.2 Experiments ................................................................................................ 45
4.3 Results and discussion ................................................................................ 46
4.3.1 Deposition and alignment of the MWNTs .......................................... 46
4.3.2 Electrical characteristics of the aligned MWNTs ................................ 48
4.3.3 CNT deposition without an electric field ............................................ 52
4.4 Conclusion .................................................................................................. 53
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CHAPTER 5 DIELECTROPHORESIS ALIGNED SINGLE-WALLED CARBON
NANOTUBES AS PH SENSORS.................................................................................... 54
5.1 Introduction ................................................................................................. 54
5.2 Materials and experiments .......................................................................... 56
5.3 Results and discussion ................................................................................ 58
5.3.1 Dielectrophoresis ................................................................................. 58
5.3.2 pH sensitivity ....................................................................................... 60
5.3.3 Sensing repeatability............................................................................ 64
5.3.4 Response time ...................................................................................... 66
5.3.5 Long-term stability .............................................................................. 68
5.4 Conclusion .................................................................................................. 69
CHAPTER 6 MULTI-WALLED CARBON NANOTUBE NANOCOMPOSITES
MICROMACHINING ...................................................................................................... 71
6.1 Introduction ................................................................................................. 71
6.2 Experimental methods ................................................................................ 73
6.2.1 Workpiece material .............................................................................. 73
6.2.2 FIB experimental design and procedures ............................................ 74
6.2.3 Soft lithography materials and procedures .......................................... 78
6.3 Results and discussion ................................................................................ 80
6.3.1 Machined features ................................................................................ 80
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6.3.2 The machined feature geometries ........................................................ 84
6.3.3 Material removal rate (MRR) .............................................................. 87
6.4 Conclusion .................................................................................................. 89
CHAPTER 7 CONCLUSION AND FUTURE WORK .......................................... 91
7.1 Conclusion .................................................................................................. 91
7.2 Future work ................................................................................................. 93
7.2.1 A lab-on-a-chip device using a carbon nanotube sensor array ............ 93
7.2.2 Large-scale deposition of graphene with dielectrophoresis ................ 94
7.2.3 Dielectrophoretic deposition and ion sensitivity of graphene ............. 95
BIBLIOGRAPHY ....................................................................................................... 97

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LIST OF TABLES
Table 1-1: Mechanical properties of CNTs and other common materials. ......................... 4
Table 4-1: The calculated resistances of the deposited SWNTs and MWNTs from the
electrical characterization ..................................................................................... 51
Table 5-1: Sensor response time under different pH values. ............................................ 68
Table 6-1: Experimental design summary. ....................................................................... 78
Table 6-2: Average machining time, machined depth, and measured aspect ratio. .......... 83

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