60
(a) (b)
(c)
Figure 1-9 (a)
Localized discharge spot can be seen at the gap with applying low voltage, (b)
plane-to-plane silicon electrodes with narrow gap. (c) micro-discharge in the gap of a comb actuator [39].
61
Figure 1-10 The breakdown discharge process of gas ionization sensor with CNTs as positive electrode [40].
Figure 1-11 The breakdown discharge process of gas ionization sensor with CNTs as negative electrode.
62
Figure 1-12 Schematic initially very small amount of free electrons, accelerated by a sufficiently strong electric field, give rise to electrical conduction through a gas by avalanche multiplication.
Figure 1-13 The Amplified mechanism of the electron flux in Townsend’s discharge [80].
63
Figure 1-14 The Paschen’s curve for air, two flat parallel copper electrodes, separated by 1 inch, for pressure between 3×10-2 torr and 760 torr. As the pressure is reduced below a torr (as shown in the diagram below) the curve of breakdown voltage versus pressure reaches a minimum, and then, as pressure is further reduced, rises steeply again.
64
Figure 1-15 The carbon nanotubes gas ionization sensor proposed by Modi et al [4].
Figure 1-16 The carbon nanotubes gas ionization sensor proposed by S J Kim et al [50].
65
Chapter 2
=============================================
Figure 2-1 The schematic flowchart for the fabrication of the Uniform CNTs film and Random oriented CNTs film: (a) RCA clean process to remove contamination particles on the silicon substrate. (b)Deposition of a 4-nm-thick Co catalyst layer for sample (A) and 4-nm-thick Co catalyst layer along with 1-nm-thick Ti capping layer for sample (B). (c)Loading the samples into thermal CVD to be pretreated in hydrogen environment and forming Co nanoparticles, and (d) synthesizing CNTs in the thermal CVD. It’s worth noting that the nonuniformity of Ti capping layer on the Co catalyst of sample (B) would limit the growth direction of CNTs and forming the random oriented CNTs film.
66
(a)
(b)
Figure 2-2 (a) Schematic picture and (b) photograph of thermal CVD. The process gases used here is hydrogen, nitrogen and ethylene.
67
Figure 2-3 The process parameters to synthesize CNTs. At the start, samples would be heated to the predetermined temperature of 600℃℃℃℃ in a nitrogen flow rate of 1000 standard cubic centimeter per minute (sccm) for an oxygen-free ambience. Prior to the CNTs growth, hydrogen gas with a flow rate of 300 sccm was fed into the reaction tube for 10 min to reduce the catalyst metal to the metallic phase, meanwhile transforming into nanoparticles. After the pretreatment step, the chamber would be heated again under nitrogen flow rate of 1000sccm to 700℃℃℃℃. Then, CNTs were grown at this temperature with hydrogen, nitrogen and ethylene, at a flow rate of 300 sccm, 500 sccm and 100 sccm for designated growth time. After that, samples were furnace-cooled to room temperature in nitrogen flow rate of 5000 sccm to fully exhaust the reaction and byproduct gases.
68
Figure 2-4 The schematic flowchart for the fabrication of CNTs-based film synthesized from the co-deposited catalyst. (a) At first, RCA clean process is used to remove contamination particles on the silicon substrate, and then (b) a
10-nm-thick aluminum supporting layer, a 1-nm-thick Titanium interlayer and a 4-nm-thick Co-Ti co-deposition layer were deposited sequentially. (c) Loading the samples into thermal CVD to be pretreated in hydrogen environment and forming nanoparticles, and (d) synthesizing CNTs in the thermal CVD.
69
Figure 2-5 The schematic flowchart for the fabrication of pillar array of vertical aligned CNTs bundles using co-deposited catalyst structure. (a) Firstly, the RCA clean process is used to remove contamination particles on the silicon substrate. (b) Secondly, the substrate was coated with photoresist and then the exposure was executed under the mask, and immediately (c) the development of the patterns was followed. (d)Co-Ti (40 Å)/Ti(10 Å) /Al (100 Å) as catalyst structure were deposited by magnetron sputtering and the lift-off process was used to define our circle patterns. (e) Loading the samples into chamber to be pretreated in hydrogen environment and forming the nanoparticles, and (f) synthesizing CNTs in the thermal CVD processes.
70
Figure 2-6 Mask design shows the array of 80µm in inter-pillar distance and 50µm in circle diameter defined in 1cm × 1 cm area.
Figure 2-7 The micrographs of samples were taken by scanning electron microscope (SEM, Hitachi S-4700I).
71
Figure 2-8 The high resolution transmission electron microscope (HRTEM;
JEOL JEM-2000EX) was used to examine the structure of CNTs.
72
Figure 2-9 High resolution confocal Raman microscope (HOROBA, Lab RAM HR) was also applied to analyze the crystallinity of the CNTs.
73
Figure 2-10 The gas ionization sensor measurement setups. The samples were put on a glass substrate with a spacer to control the distance between CNTs nanotips to the anode, and the anode was a glass coated with a indium-tin-oxide (ITO) film and green phosphor (P22). The samples were loaded into a vacuum chamber with anode applied to the high-voltage source measurement unit, Keithley 237, and the cathode applied to the high-current source measurement unit, Keithley 238, to measure the discharge current and the breakdown voltages of the samples.
74
Figure 2-11 Definition of the R/H ratio, where R is the inter-pillar distance (spacer) and H is the height of a CNT pillar.
75
Chapter 3
=============================================
Figure 3-1 The pictures shown here was the tilted images (about 45°) taken by scanning electron microscope (SEM) for the Uniform CNTs film grown by thermal CVD.
Figure 3-2 The pictures shown here was the cross-sectional image taken by SEM for the Uniform CNTs film grown by thermal CVD. The height of the vertical aligned CNTs is about 11.9µm.
76
Figure 3-3 The pictures shown here was the tilted images (about 45°) taken by SEM for the Random oriented CNTs film grown by thermal CVD.
Figure 3-4 The pictures shown here was the magnified (10000 times) tilted images (about 45°) taken by SEM for the Random oriented CNTs film grown by thermal CVD. It could be seen obviously that the CNTs tangled together and the growth direction is rarely vertical.
Figure 3-5 The pictures shown here was the cross SEM for the Random oriented CNTs film
the CNTs is quite nonuniform.
Figure 3-6 The Raman spectra analysis of the Uniform CNTs film and the ID/IG ratio which indicates the graphite
and 1.725.
77
The pictures shown here was the cross-sectional image taken by Random oriented CNTs film grown by thermal CVD. The height of quite nonuniform.
The Raman spectra analysis of the Uniform CNTs film and the ratio which indicates the graphite crystallinity of the samples
sectional image taken by grown by thermal CVD. The height of
The Raman spectra analysis of the Uniform CNTs film and the of the samples are 1.753
Figure 3-7 The Raman spectra analysis of the the ID/IG ratio which indicates the graphite and 1.755.
(a) Figure 3-8 (a)The micrographs of CNTs taken by
microscopy (TEM) obtained from the
multiwalled structure can be found with higher resolution.
78
The Raman spectra analysis of the Random oriented CNTs film and ratio which indicates the graphite crystallinity of the samples
(b)
The micrographs of CNTs taken by transmission electron (TEM) obtained from the Uniform CNTs film and (b) their multiwalled structure can be found with higher resolution.
CNTs film and of the samples are 1.789
ransmission electron and (b) their
79
(a) (b)
Figure 3-9 (a)The micrographs of CNTs taken by TEM obtained from the Random oriented CNTs film and (b) their multiwalled structure can also be found with higher resolution.
(a) (b)
Figure 3-10 The gas breakdown characteristics of the Radom oriented CNTs film under different gas pressures in the nitrogen environment. The pressures are (a) 0.0023 torr, (b) 0.0052 torr, (c) 0.079 torr, (d) 0.1 torr, (e) 0.2 torr, (f) 0.51 torr, (g) 0.8 torr, (h) 1.0 torr, (i) 2.1 torr and (j) 5.1 torr. They are integrated into (k).
80
(b) (d)
(e) (f)
(g) (h)
Figure 3-10 (cont.).
81
(i) (j)
(k)
Figure 3-10 (cont.).
82
Figure 3-11 Breakdown voltages vs. p×d characteristics of the Random oriented CNTs film under nitrogen environment (Paschen’s curve). It’s
apparently that the variations of the breakdown voltages are especially large at high voltage region and these variations are referred to the nonuniformity of the CNTs’ length. For those longer CNT, the difference of electrical field is larger, which leads to higher electron emission efficiency. Therefore, their breakdown voltages are lower.
(a) (b)
Figure 3-12 The gas breakdown characteristics of the Uniform CNTs film under different gas pressures in the nitrogen environment. The pressures are (a) 0.0024torr, (b) 0.0049torr, (c) 0.078torr, (d) 0.1 torr, (e) 0.21torr, (f) 0.51 torr, (g) 0.82torr, (h) 1.0 torr, (i) 2.0torr and (j) 4.9torr. They are integrated into (k).
83
(b) (d)
(e) (f)
(g) (h)
Figure 3-12 (cont.).
84
(i) (j)
(k) Figure 3-12 (cont.).
85
Figure 3-13 Breakdown voltages vs. p×d characteristics of the Uniform CNTs film under nitrogen environment (Paschen’s curve). It’s apparently that the variations of the breakdown voltages are smaller than the Random oriented CNTs film. It was referred to the uniformity of the CNTs’ length of the Uniform CNTs film.
Figure 3-14 The stability test of gas breakdown characteristics under nitrogen environment with the Random oriented CNTs film and the Uniform CNTs The Vbr of the Random oriented
cycles, 68% in increase. And for
575V after 1000 cycles, 46% in increase
86
he stability test of gas breakdown characteristics under nitrogen environment with the Random oriented CNTs film and the Uniform CNTs
Random oriented CNTs film lifts up from 365V to 605V after 1000 cycles, 68% in increase. And for the Uniform CNTs film, it lifts up from 395V to 575V after 1000 cycles, 46% in increase.
he stability test of gas breakdown characteristics under nitrogen environment with the Random oriented CNTs film and the Uniform CNTs film.
up from 365V to 605V after 1000 niform CNTs film, it lifts up from 395V to
87
(a) (b)
(c) (d)
(e) (f)
Figure 3-15 The SEM images before and after stability tests: (a), (c) and (d) are the images of the Random oriented CNTs film before stability test, after 500 cycles stability tests and after 1000 cycles stability tests. And (b), (d) and (f) are the images of the Uniform CNTs film before stability test, after 500 cycles stability tests and after 1000 cycles stability tests.
88
Figure 3-16 The various surface energy of different metals, where one can find that the surface energy of cobalt is familiar with that of titanium.
Figure 3-17 The diagram of different surface energy metals reacting with Cobalt as they coalescence into nanoparticles.
89
(a)
Figure 3-18 The images of SEM displayed the roots of the CNTs for both (a) the conventional samples without co-deposited catalyst and (b) the proposed samples with co-deposited catalyst. The proposed samples were cleaved across the patterned area and a CNT immersed partially into the co-deposited metal layer on the cleaved edge was marked by a circle in (c).
Figure 3-18 (cont.).
90
(b)
(c)
91
(a) (b)
Figure 3-19 The catalyst after pretreatment in reducing gas environment: (a) without Al supporting layer and (b) with Al supporting layer, where
nanoparticles with small sizes could be achieved with Al supporting layer.
92
(a)
(b)
Figure 3-20 The Transmission Electron Microscopy (TEM) images of (a) using Co/Ti/Al catalyst structure and (b) using Co-Ti/Al co-deposited catalyst structure.
It’s obvious that the diameter of CNTs becomes smaller by using co-deposited catalyst structure.
93
(a) (b)
(c) (d)
(e) (f)
Figure 3-21 The gas breakdown characteristics of the CNTs-based film synthesized from the co-deposited catalyst structure under different gas pressures in the nitrogen environment. The pressures are (a) 0.0021 torr, (b) 0.0049 torr, (c) 0.082 torr, (d) 0.1 torr, (e) 0.18 torr, (f) 0.41 torr, (g) 0.8 torr, (h) 1.0 torr, (i) 2.1 torr and (j) 5.3 torr. The above is integrated into (k).
94
(g) (h)
(i) (j)
(k)
Figure 3-21 (cont.).
95
Figure 3-22 Breakdown voltages versus p×d characteristics of the CNTs-based film synthesized from the co-deposited catalyst structure under nitrogen
environment (Paschen’s curve). The variations of the breakdown voltages were improved obviously, which was related to the improvement of adhesion and contact resistance via the catalyst co-deposition.
96
Figure 3-23 The stability test of gas breakdown characteristics under nitrogen environment with the Random oriented CNTs film, the Uniform CNTs film and the CNTs film with co-sputter catalyst. One can find that the Vbr of the CNTs film synthesized from Co-Ti co-deposited catalyst structure lifts up from 375V to 435V after 1000 cycles, 16% in increase, which is further improved than the first two CNTs film.
97
(a)
(b)
Figure 3-24 The SEM images before and after stability tests: (a), (b) and (c) are the images of the CNTs film synthesized from Co-Ti co-deposited catalyst
structure before stability test, after 500 cycles stability tests and after 1000 cycles stability tests, respectively. The pull-off and evaporation of CNTs were not as serious as the conventional two samples, which are associated to the
improvement of adhesion and contact resistance via the catalyst co-deposition.
98
(c)
Figure 3-24 (cont.).
99
Figure 3-25 Simulation of the equipotential lines of the electrical field for tubes of different distances between each other [81].
100
Figure 3-26 (a) Simulation of the equipotential lines of the electrical field for tubes of 1 µm height and 2 nm radius, for distances of 4, 1, and 0.5 µm between tubes; along with (b) the corresponding changes of the field enhancement factor ββ
ββ and emitter density, and (c) current density as a function of the distance [53].
101
Figure 3-27 Definition of the R/H ratio, where R is the inter-pillar distance (spacer) and H is the height of a CNT pillar.
Figure 3-28 The cross-sectional view of CNTs film synthesized from Co-Ti co-deposited catalyst structure for 50.8 µm in height.
Figure 3-29 The pictures shown here w the cross-sectional image (b)
CNTs synthesized from the Co
thermal CVD. Here the R/H ratio is 0.61.
102
(a)
(b)
pictures shown here were the tilted image (a) (about 45°
sectional image (b) taken by the SEM for the48.8 µm high pillar CNTs synthesized from the Co-Ti co-deposited catalyst structure grown by
ere the R/H ratio is 0.61.
about 45°)and high pillar-like deposited catalyst structure grown by
103
(a)
(b)
Figure 3-30 Breakdown voltages vs. p×d characteristics of (a) the film sample (50.8 µm) and (b) the pattern sample (48.8 µm, R/H =0.61) under nitrogen environment (Paschen’s curve). One could find in (c) that there is no much difference of breakdown voltages between the film sample and the pattern sample at this R/H ratio.
Figure 3-30 (cont.).
104
(c)
105
Figure 3-31 The cross-sectional view of CNTs film synthesized from Co-Ti co-deposited catalyst structure for 11.3 µm in height.
(a)
Figure 3-32 The pictures shown here were the tilted image (a) (about 45°)and the cross-sectional image (b) taken by the SEM for the 12.5 µm high pillar-like CNTs synthesized from the Co-Ti co-deposited catalyst structure grown by thermal CVD. Here the R/H ratio is 2.40.
Figure 3-32 (cont.).
Figure 3-33 Breakdown voltages vs. p×d characteristics of (11.3 µm) and (b) the pattern sample (12.5
environment (Paschen’s curve).
voltages of the patterned sample begin to lower than R/H ratio.
106
(b)
(a)
Breakdown voltages vs. p×d characteristics of (a) the
) and (b) the pattern sample (12.5 µm, R/H =2.40) under nitrogen s curve). One could found in (c) that the breakdown patterned sample begin to lower than that of film sample
the film sample , R/H =2.40) under nitrogen
he breakdown film sample at this
Figure 3-33 (cont.).
107
(b)
(c)
108
Figure 3-34 The cross-sectional view of CNTs film synthesized from Co-Ti co-deposited catalyst structure for 13.1 µm in height.
(a)
Figure 3-35 The pictures shown here were the tilted image (a) (about 45°)and the cross-sectional image (b) taken by the SEMfor the 10.3µmhigh pillar-like CNTs synthesized from the Co-Ti co-deposited catalyst structure grown by thermal CVD. Here the R/H ratio is 2.91.
Figure 3-35 (cont.).
Figure 3-36 Breakdown voltages vs. p×d characteristics of (13.1 µm) and (b) the pattern sample (10.3
environment (Paschen’s curve).
voltages of patterned sample are greatly lower than film sample
109
(b)
(a)
Breakdown voltages vs. p×d characteristics of (a) the
) and (b) the pattern sample (10.3µm, R/H =2.91) under nitrogen s curve). One could found in (c) that the breakdown voltages of patterned sample are greatly lower than film sample at this R/H ratio.
the film sample , R/H =2.91) under nitrogen
he breakdown at this R/H ratio.
Figure 3-36 (cont.).
110
(b)
(c)
111
Figure 3-37 The cross-sectional view of CNTs film synthesized from Co-Ti co-deposited catalyst structure for 5.95 µm in height.
(a)
Figure 3-38 The pictures shown here were the tilted image (a) (about 45°)and the cross-sectional image (b) taken by the SEMfor the 5.95µmhigh pillar-like CNTs synthesized from the Co-Ti co-deposited catalyst structure grown by thermal CVD. Here the R/H ratio is 5.04.
Figure 3-38 (cont.).
Figure 3-39 Breakdown voltages vs. p×d characteristics of (5.95 µm) and (b) the pattern sample 5.95
environment (Paschen’s curve).
voltages of patterned sample is lower than fi 2.91 at this R/H ratio.
112
(b)
(a)
Breakdown voltages vs. p×d characteristics of (a) the ) and (b) the pattern sample 5.95 µm, R/H =5.04) under nitrogen
s curve). One could found in (c) that the breakdown voltages of patterned sample is lower than film sample but not so much as R/H =
the film sample , R/H =5.04) under nitrogen
he breakdown lm sample but not so much as R/H =
Figure 3-39 (cont.).
113
(b)
(c)
114
Figure 3-40 The comparison of the breakdown voltages characteristics of the film samples with different height. The lowest breakdown voltages were obtained at the height of CNTs around 11.3 µm to 13.1 µm. The breakdown voltage would increase when the height of the CNTs was too long or too short.
115
Figure 3-41 The comparison of the breakdown voltages characteristics of the pattern samples with different R/H ratio. It’s obviously that the lowest
breakdown voltages were approached as the R/H ratio was around 2.91. If the R/H ratio increases to 5.04 and 11.4, the breakdown voltages will raise up gradually.
116
(a)
(b)
Figure 3-42 (a)The local electrical field distribution and (b) the emission corner of the film sample and the patterned sample for a constant
anode-to-cathode distance and the same gas pressure.
117
(a)
(b)
Figure 3-43 Corresponding changes of the field enhancement β as a function of the R/H ratio when considering (a) the screening effect, (b) the aspect ratio effect and (c) both of the two effects.
118
(c) Figure 3-43 (cont.).
(a)
Figure 3-44 The breakdown characteristics of different gases for (a)the film sample with 11.3 µm in CNTs’ height and (b)the patterned sample with R/H = 2.91. Different gas molecules have different mean free path, ionization energy and recombination rate, which causes their different breakdown characteristics in Paschen’s curve.
119
(b) Figure 3-44 (cont.).
(a)
Figure 3-45 Discharge current versus breakdown voltage curves for Ar, N2, Air, O2 and CO2 of the film sample with 11.3 µm in CNTs’ height at p×d product value around (a)8×10-4 torr cm and (b) 8×10-3torr cm, showing distinct
breakdown voltages; carbon dioxide displays the highest and argon the lowest.
120
(b) Figure 3-45 (cont.).
(a)
Figure 3-46 Discharge current versus breakdown voltage curves for Ar, N2, Air, O2 and CO2 of the patterned sample with R/H = 2.91 at p×d product value
around (a)8×10-4 torr cm and (b) 8×10-3torr cm, showing distinct breakdown voltages; carbon dioxide displays the highest and argon the lowest. It’s noticeably that the breakdown voltage windows of (b) were wider and at relatively low voltages.
Figure 3-46 (cont.).
Figure 3-47 Breakdown voltages of Ar and CO function of concentration
patterned sample with R/H = 2.91.Breakdown voltage increases with increasing CO2 concentration in the mixture, and decreases with increasing
concentration.
121
(b)
(a)
Breakdown voltages of Ar and CO2 gases in mixture with air as a concentration for (a) film sample with 11.3 µm in CNTs’ height patterned sample with R/H = 2.91.Breakdown voltage increases with increasing
concentration in the mixture, and decreases with increasing Ar
gases in mixture with air as a m in CNTs’ height and (b) patterned sample with R/H = 2.91.Breakdown voltage increases with increasing
Ar
Figure 3-47 (cont.).
122
(b)
123
Chapter 4
=============================================
Figure 4-1 Extension of the linear region in the right side of the Paschen’s curve
124
Reference
[1] Sander J. Tans, Alwin R. M. Verschueren & Cees Dekker, “Room-temperature transistor based on a single carbon nanotube,” NATURE | VOL 393 | 7 MAY 1998
[2] R. Martel, T. Schmidt, H. R. Shea, T. Hertel, and Ph. Avouris, “Single- and multi-wall carbon nanotube field-effect transistors,” APPLIED PHYSICS LETTERS VOLUME 73, NUMBER 17
[3] Auble, D.L.; Meyers, T.P. , "An open path, fast response infrared absorption gas analyzer for H2O and CO2,” Boundary-Layer Meteorology 59(3):pp.243–256.
[4] Ashish Modi, Nikhil Koratkar, Eric Lass, Bingqing Wei& Pulickel M. Ajayan, ” Miniaturized gas ionization sensors using carbon nanotubes,” Nature Vol 424 10 JULY 2003
[5] Iijima S.,” Helical microtubules of graphitic carbon ,” Nature Vol. 354 (6348), pp.56–58, 1991.
[6] Baughman, Ray H., Zakhidov, Anvar A., de Heer, and Walt A. ,” Carbon Nanotubes—The Route Toward Application ,” Science, Vol.297 pp.787-790, 2002.
[7] Douglas R. Kauffman and Alexander Star, “Carbon Nanotube Gas and Vapor Sensors” Angew. Chem. Int. Ed. 2008, 47, pp.6550 – 6570
[8] M. S. Dresselhaus, G. Dresselhaus, K. Sugihara, L. I. Spain, and H. A. Goldberg,
“Graphite fibers and filaments,” Springer-Verlag, New York, pp. 185-186, 1998 [9] P. M. Ajayan, “Nanotubes from carbon,” Chem. Rev., Vol. 99, pp. 1787-1790,
1999.
[10] S. Iijima and T. Ichihashi, “Single-shell carbon nanotubes of 1-nm diameter,”
Nature, Vol. 363, pp. 603-605, 1993
[11] D. S. Bethune, C. H. Kiang, M. S. de Vries, G. Gorman, R. Savoy, J. Vazquez,
125
and R. Beyers, “Cobalt-catalyzed growth of carbon nanotubes with single-atomic-layer walls,” Nature, Vol. 363, pp. 605-607, 1993 [12] “http://en.wikipedia.org/wiki/Image:CNTnames.png” 2006 [13] “http://online.itp.ucsb.edu/online/qhall_c98/dekker/” 2006
[14] Y. K. Kwon, T. H. Lee, S. G. Kim, P. Jund, D. Tomanek, and R. E. Smalley.
“Morphology and Stability of Growing Multiwall Carbon Nanotubes" Phys Rev.
Lett., Vol. 79, pp. 2065-2068, 1997
[15] Y. Saito, “Carbon Nanotubes: Preparation and Physical Properties,” Asia Display/IDW’01, pp. 11-14, 2001
[16] Min-Feng Yu et al., “Strength and Breaking Mechanism of Multiwalled Carbon Nanotubes Under Tensile Load,” Science, Vol. 287, pp. 637-640, 2000.
[17] Philip G. Collins and Phaedon Avouris, “Nanotubes for Electronics- Scientific American December”, Vol. 69, 8 pages., 2000
[18] Gamaly EG and Ebbesen TW,” Mechanism of carbon nanotube formation in the arc discharge ,” Phys Rev B, 1995,52(3), pp.2083–2089.
[19] Guo T, Nikolaev P, Thess A, Colbert DT, and Smalley RE.,” Catalytic growth of single-walled nanotubes by laser vaporization ,” Chem Phys Lett, Vol. 243,
[19] Guo T, Nikolaev P, Thess A, Colbert DT, and Smalley RE.,” Catalytic growth of single-walled nanotubes by laser vaporization ,” Chem Phys Lett, Vol. 243,