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FIELD EMISSION STUDIES OF
NANOMATERIALS
TEO CHOON HOONG
(B.Sc. (Hons), NUS)
A THESIS SUBMITTED
FOR THE DEGREE OF MASTERS OF SCIENCE
DEPARTMENT OF PHYSICS
NATIONAL UNIVERSITY OF SINGAPORE
2007
FIELD EMISSION STUDIES OF
NANOMATERIALS
Submitted by:
TEO CHOON HOONG
(B.Sc. (Hons), NUS)
Supervisor:
A/PROF SOW CHORNG HAUR
A THESIS SUBMITTED
FOR THE DEGREE OF MASTERS OF SCIENCE
DEPARTMENT OF PHYSICS
NATIONAL UNIVERSITY OF SINGAPORE
2007
Acknowledgements
In these 2 years of research I have received help and support from many people
and without their help; I would not have such a fruitful time. I would like to thank my
supervisor, Associate Professor Sow Chorng Haur for patiently guiding me in the field
of nanotechnology for the past 3 years despite my “play hard” philosophy. He has
provided many constructive comments and ideas and is always there when help is
needed.
Another very important person I would like to thank is Dr Zhu Yanwu. Since
Honors year, he has been imparting his knowledge of field emission to me and
whenever I encounter difficult problems that I cannot solve, he always the one who
enlightens me.
I would also like to thank my lab buddies for making the days in colloid lab
enjoyable. Ah Mao for his weird theories, Sharon for her “I wonder how they would
think” mindset, Shuhua for her “That’s your problem” way of looking at things, Yilin
for his “Thou shall not kill” belief, Wei Kiong for his “Train till you see god” attitude,
Andrielle for her Donald Duck inspired voice and her full collection of vampire
drama series, Binni on his “Lab is my home” nature, Sheh Lit for his games and
gaming tips and Cheong on his “I’m very handsome” mentality.
Last of all, I would like to thank ASEAN foundation and the Japan-ASEAN
Solidarity Fund for supporting me financially.
List of Publications
1. Enhanced field emission of CuO nanowires films with large scale patterning by
focused laser beam by
Teo Choon Hoong, Zhu Yanwu, Sow Chorng Haur
Submitted
2. Field emission from hybrid CuO and CuCO3 nanosystems by
Teo Choon Hoong, Zhu Yanwu, Gao Xingyu, Andrew Wee Thye Shen and Sow
Chorng Haur
Solid State Communications. 145, 241 (2008)
3. Electron emission from a single CuO nanorod by
Teo Choon Hoong, Zhu Yanwu and Sow Chorng Haur
In preparation
4. Co3O4 Nanostructures with different morphologies and their Field Emission
properties by
Binni Varghese, Teo Choon Hoong, Zhu Yanwu, Mogolahalli V. Reddy, Bobba V.
R. Chowdari, Andrew Thye Shen Wee, Tan B. C. Vincent, Chwee Teck Lim and
Chorng-Haur Sow
Advanced Functional Materials. 17, 1932 (2007)
5. WO3-x nanorods synthesized on a hotplate: A simple and versatile technique by
F.C. Cheong, B. Varghese, Y.W. Zhu, C.H. Teo, E. P. S. Tan, L. Dai, V.B.C. Tan,
C.T. Lim, C.H. Sow
Submitted
CONTENTS
PAGE
Acknowledgements
i
List of publications
ii
Summary
vi
List of Figures
viii
List of Tables
x
Chapter 1 – Introduction
1
1.1 Motivations
1.2 Introduction to nanostructures
References
1
1
6
Chapter 2 - Theory of field emission; Fowler Nordheim theory
9
2.1 Tunneling current density
2.2 Fowler-Nordheim (FN) theory
2.3 FN equation and 1D Nanostructures Field Emitter
References
10
13
14
17
Chapter 3 – Experimental setup
18
3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
3.9
Growth of CuO nanorods
Field Emission measurement setup
Measurement of Field Emission from Single Nanorod
Scanning Electron Microscope (SEM)
High Resolution Transmission Electron Microscope (HRTEM)
X-Ray Diffraction (XRD)
X-Ray and UV Photoelectron Spectroscopy (XPS and UPS)
Sputtering Machine
Laser system
References
Chapter 4 – Enhanced field emission of CuO nanorod films from large scale
patterning by focused laser beam
18
20
22
25
27
29
29
30
31
33
34
4.1
4.2
4.3
4.4
Introduction
Experimental Details
Results and discussions
Conclusion
References
34
35
37
47
49
Chapter 5 – Field emission from hybrid CuO and CuCO3 nanosystems
51
5.1
5.2
5.3
5.4
51
52
54
61
62
Introduction
Experimental Details
Results and discussions
Conclusion
References
Chapter 6 – Electron emission from a single CuO nanorod
64
6.1
6.2
6.3
6.4
64
65
66
74
75
Introduction
Experimental Details
Results and discussions
Conclusion
References
Chapter 7 - Conclusions
77
Summary
The world of nanostructures has been intriguing to all. Nanostructures has
potential applications in numerous fields and can one day, change our world. It is an
active field of research that yields many unexpected and promising results for
researchers.
In this work, we aim to study the field emission properties of nanomaterials. We
have successfully used a focused laser beam to enhance the field emission properties
of CuO nanorods samples, synthesized a novel nanomaterial system and assembled
and test the distance dependence field emission properties of single CuO nanorod
samples using a probe station, a custom made stage and a field emission chamber.
Using a focused laser beam system, large scale laser patterning was carried out
on CuO nanorods samples and the laser patterning process created arrays of
micro-platforms. Nanorods by the edges of the platform do not face the screening
effect and thus, the exposed sides of these nanorods also contribute to field emission
leading to an improvement in the field emission performance. This demonstrates the
feasibility of using a focused laser beam for large scale micro-patterning and as a
mean to improve the field emission properties of nanostructures samples.
In addition, using a simple way of heating copper coated carbon paper in ambient,
novel hybrid CuO-CuCO3 nanosystems have also been synthesized. The percentage of
CuCO3 in the hybrid nanosystem can be adjusted by varying the thickness of the
copper coating. Field emission tests reveal that these nanosystems are among the
better performers compared to many samples and they are potential candidates for
future generation field emission devices.
Using a probe station, we were able to assemble single CuO nanorod samples onto an
etched tungsten tip. A custom made stage was used in conjunction with a field
emission chamber to test the field emission properties of the samples. Distance
dependence field emission tests show that the enhancement factor β and the turn-on
field ETO are dependent on the electrode separation distance d.
List of Figures
2.1 Potential-energy diagrams for electrons at a metal surface under an applied
field
9
2.2 Potential barrier of a general shape along the x-axis
10
2.3 The triangular barrier shape for the Fowler-Nordheim tunneling where q·φ is
the height of the potential barrier
13
2.4 Equipotential lines near a tip of a nanorod, darker grey scale represents a lower
potential area
15
3.1 (a) Hotplate with polished copper plates and tubes. (b) Freshly prepared copper
plates and tubes. (c) Copper plate and tubes after heating for 10 mins at 400°C.
(d) SEM image of the surface of the sample showing CuO nanorods
19
3.2 Growth mechanism of CuO nanorods with increasing time/temperature with
the black color region showing the molten state of Cu
20
3.3 Field emission measurement setup with emphasis on field emission chamber
22
3.4 Field emission measurement setup with emphasis on the vacuum system
22
3.5 Schematic of the probe station
24
3.6 Schematic of the setup for the measurement of field emission from single
nanorods
25
3.7 Schematic setup of a scanning electron microscope
26
3.8 Schematic setup of a transmission electron microscope
28
3.9 Schematic setup of the focused laser system
32
4.1 Side view SEM images of (a) and (b) S400-7, (c) and (d) S450-7, (e) and (f)
S450-5
40
4.2 (a) Diameter distribution for S400-7, (b) length distribution for S400-7, (c)
diameter distribution for S450-7, (d) length distribution for S450-7, (e)
diameter distribution for S450-5 and (f) length distribution for S450-5
41
4.3 Top view SEM images of (a) as-grown CuO nanorods, (b) - (e) laser patterned
CuO nanorods and (f) closed up view of the microballs in regions exposed to
the laser beam
44
4.4 (a) J-E plot for S400-7 before and after laser patterning, (b) corresponding FN
plot, (c) J-E plot for S450-7 before and after laser patterning, (d) corresponding
FN plot (e) J-E plot for S450-5 before and after laser patterning and (f)
corresponding FN plot.
45
5.1 SEM images of (a) pure carbon paper with inset at higher magnification, (b)
heated carbon paper, (c) heated Cu400_C with inset at higher magnification,
(d) heated Cu1800_C, (e) heated Cu4800_C, and (f) TEM image of a single
CuCO3 nanoparticle
55
5.2 (a). The XRD patterns for heated Cu400_C, heated Cu1800_C and heated
Cu4800_C (offset for clarity) (b) XPS O1s spectra for heated Cu400_C (top),
Cu1800_C (middle) and Cu4800_C (bottom) (offset for clarity) and (c) UPS
spectra of heated Cu400_C and Cu4800_C
57
5.3 (a) Current density vs applied field for heated Cu200_C to Cu4800_C, (b)
corresponding FN plots
60
6.1 (a) Optical image of anode and cathode, (b) SEM image of CuO nanorod (I) on
etched tungsten tip with inset at scale bar of 1 µm, (c) SEM image of CuO
nanorod (II) on etched tungsten tip with inset at scale bar of 1 µm and (d) SEM
image of CuO nanorod (III) on etched tungsten tip with inset at scale bar of 1
µm
67
6.2 (a) Current vs applied field for various electrode distances (CuO nanorod (I)),
(b) corresponding FN plot (offset for clarity), (c) enhancement factor, β vs
electrode distance, d with inset showing the relationship between 1/β and 1/d, 69
and (d) linear relationship between ETO and d
6.3 a) Current vs applied field for various electrode distances (CuO nanorod (II)),
(b) corresponding FN plot (offset for clarity), (c) enhancement factor, β vs
electrode distance, d with inset showing the relationship between 1/β and 1/d,
and (d) linear relationship between ETO and d
70
6.4 (a) Current vs applied field for various electrode distances (CuO nanorod (III)),
(b) corresponding FN plot (offset for clarity), (c) enhancement factor, β vs
electrode distance, d with inset showing the relationship between 1/β and 1/d,
and (d) linear relationship between ETO and d
71
List of Tables
4.1 Growth conditions and physical properties of the various CuO nanorods films
38
4.2 Field emission properties of the various CuO nanorods before and after laser
patterning
46
4.3 Enhancement factor and area factor of the various CuO nanorods before and
after laser patterning
46
5.1 Turn-on field and threshold field for various field emitters
60
6.1 Studies of field emission from CNT/CNTs from various groups
73
Chapter 1 – Introduction
1.1 Introduction to nanostructures
Nanostructures refer to structures with at least one dimension that is less than 100
nanometers (nm). Inside the nanostructures, electrons are confined in the nanoscale
dimension(s) but are free to move about in the other dimension(s). A simple way of
classifying nanostructures [1]:
Quantum well (2D): Electrons are confined in 1 dimension but are free to
move about in the other 2 dimensions
Quantum wire (1D): Electrons are confined in 2 dimensions but are free to
move about in 1 dimension. Quantum wires include nanotubes and nanorods.
Quantum dot (0D): Electrons are confined in all 3 dimensions such as in a
nanocrystallite.
1.2 Motivations
In this work, the focus is on the 1D nanostructures. In recent years, 1D
nanostructures like nanotubes and nanorods have attracted much attention due to their
unique mechanical and electrical properties [2-5]. This enables them to find potential
application in fields such as biomedical [6], catalysts [7], sensor [8] and field
emission emitter [9].
Carbon Nanotubes (CNTs) are the most widely studied and have found numerous
potential applications [10-15]. Among the applications, extensive research has been
done on CNTs regarding their field emission properties [15-17] and this could lead to
a new generation of flat panel displays [18]. This is due to the nanotubes and
nanorods having sharp tip and high aspect ratio, capable of enhancing the local field
[19]. Other reasons for application in field emission also included the increased in
emission area due to the fact that the side of the nanotubes and nanorods are able to
field emit and the formation of unique forms of chemical compounds in the
nanostructures that serve as good emitter [20-25].
Apart from researching on CNTs in the area of field emission application, various
metal-oxide based nanorods have also been studied. Titanium-Oxide (TiO2) nanorods,
Zinc-Oxide (ZnO) nanorods and Indium-Oxide (In2O3) nanorods are among some of
the metal-oxide nanorods that have shown much potential in this area but it is not
cost-effective to grow them [26-30]. Recently, a simple method has been developed in
our lab to grow metal-oxide nanostructures by heating them in ambient air [31, 32].
This method has led to the easy and thus, economical growth of many promising 1D
nanomaterials such as Cupric-Oxide (CuO) nanorods, Cobalt-Oxide (CoO) nanorods,
Vanadium-Oxide (VO) nanorods and Tungsten-Oxide (WO) nanorods. This opens up
the possibility of developing economical and efficient flat panel displays.
Although field emitter films require a sufficiently high area density to be
functioning optimally, a highly dense film actually suffers from reduced field
emission performance caused by the screening effect due to the proximity of the
neighboring nanorods [33, 34]. Several methods have been employed to counter this
effect by controlling the density of the emitter film [30, 35-37]. CuO nanorods form the
backbone of this research as being a semiconductor, it has a lower surface potential
barrier than metals and narrow band gap which are favorable for field emission. While
CuO nanorods film is a potential source of field emitter, they too suffer from the
screening effect when they are highly dense. In the first part of the thesis, a technique
was introduced where large scale patterning of CuO nanorod films was carried out by
a focused laser beam. This laser patterning process creates micro-platforms, allowing
the nanorods along the edges of the platforms to field emit without facing the
screening effect. This also serves to increase the total emission area of the sample and
thereby, improving the field emission efficiency of the CuO nanorod films
While a single nanomaterial system may show potential as a field emitter, hybrid
nanosystems combining the properties of two or more different types of
nanostructures could further enhance the field emission properties and allow for the
tuning of field emission properties by varying the relative percentage of the individual
nanomaterials in the hybrids. In recent years, hybrid nanosystems such as ZnO
nanorods on carbon cloth and CNTs on carbon cloth have been developed and they
have shown excellent field emission properties [38-40]. Hybrid CuO and ZnO
nanostructures system have also been synthesized by directly heating brass in ambient
conditions and the field emission properties can be tuned by varying the percentages
of copper and zinc in brass [41].
In this work, a simple way of growing hybrid CuO and CuCO3 nanosystems is
introduced where the hybrid CuO-CuCO3 nanosystem is synthesized by directly
heating copper sputtered carbon paper in ambient. The relative concentration of
CuCO3 can also be varied by changing the thickness of copper on carbon paper. Field
emission properties of the nanosystems show the emission turn-on field and current
can be tuned by the coating thickness of copper. Their field emission properties
compared with other common field emitters will also be presented.
Nanorod films are highly suitable for field emission applications but a single
nanorod field emission test is required to understand the physics behind it as it
eliminates screening and edge effects that are found in films. Much effort has been
done to investigate the field enhancement factor dependence on electrode distance for
single CNT. Several models for the relationship between the enhancement factor β
and electrode separation distance d were proposed for individual carbon nanotube
field emission.
Among the models is the modified Miller model by Vallance and co-workers
where the model consists of a sphere floating between a ground plane and a charged
sphere. For this model, β decreases and approaches unity as d becomes very small.
This is because the geometry approaches that of two opposing infinite planes when
the separation is much smaller than the radius curvature at the cathode tip and the
geometric field enhancement is eliminated. As d increases and approaches infinity, β
will reach a constant value as it is then dependent only in its geometrical properties
[42]. Smith et al. proposed that β is independent of d when the anode to cathode
separation is greater than 3 times the height of the emitter away from the tip [43]. The
reason being as the anode plate moved away from the CNT tip, the parallel plate
approximation decreases and β becomes dependent on its geometrical properties
instead of the electrode separation. A linear relationship between β and d however,
was reported by Bai’s group [44].
In the final part of this work, a simple method to mount a single CuO nanorod
onto an etched tungsten tip will be introduced. The study of the field emission
properties of a single CuO nanorod is presented and the dependence between β and d
will be established. A relationship between the turn-on field, ETO (defined as the
electric field required to obtain 10 µA/cm2) and d is also presented. This study
provides an understanding to how β and ETO depends on d for the field emission of a
single metal-oxide nanorod.
This project is organized as follows. Chapter 2 introduces the theory of field
emission, the Fowler-Nordheim (FN) theory with brief derivation and the relationship
between the FN theory and 1D nanostructure field emitters. Chapter 3 details the
experimental procedures which includes the field emission setup, the various
characterization and patterning tools. Chapter 4 presents the large scale laser
patterning of CuO nanorod films and the effect on the field emission properties of the
samples. Chapter 5 shows a simple way of growing hybrid CuO-CuCO3 nanosystems
with potential as field emitters. Chapter 6 explores the field emission properties of a
single CuO nanorod and how the field emission properties depends on the electrode
separation distance d. Finally, Chapter 7 concludes the project.
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Chapter 2 – Theory of field emission; Fowler
Nordheim theory
Field emission is defined as the emissions of electrons from the surface of a
condensed phase e.g. metal into another phase e.g. vacuum [1]. To achieve a field
emission, a potential difference is applied across the sample giving rise to an external
field. This applied field distorts the potential of the sample enabling unexcited
electrons to tunnel through (See Figure 2.1).
Figure 2.1 Potential-energy diagrams for electrons at a metal surface under an applied
field [2]
The Fowler-Nordheim (FN) theory assumes that the resultant potential is
triangular and a relation between the current density (J), the applied electric field (E)
and the workfunction Φ of the material can be determined.
In this chapter, a brief derivation of the FN equation will be carried out and its
relation to the measurements collected in this project is also explored. The
introduction of the enhancement factor β into the FN equation will also be discussed.
2.1 Tunneling current density
In deriving the FN theory, we will first try to derive a tunneling current density
for an electron passing through a general potential (See Figure 2.2)
Figure 2.2 Potential barrier of a general shape along the x-axis
The vertical axis refers to the energy (E) in the band diagram. q·φ(x) is the shape
of the potential barrier, Ex is the electron energy along the x-direction and Ttun is the
tunneling distance.
The expression of the current density (J) induced by electrons tunneling in the
x-direction through a generic potential barrier is [3]:
+∞
J=q ∫ dv x vxN(vx)T(Ex)
0
(2.1)
Where q is the charge of an individual electron, T(Ex) is the tunneling probability,
that is the probability that one electron having energy Ex along the x-axis goes through
the potential barrier. N(vx)dvx is the density of electrons with velocity between vx and
vx+dvx along the x-axis. The integral is taken from 0 ∞ since the electrons are
trapped in the metal for x1.
This implies that the effective field F is greater than the average field. This is an
advantage of using nanowires as a field emitter; the same electric field applied will
result in a higher effective field thereby increasing the field emission current density.
Since β is mainly related to the geometry of the nanostructure, it will be
dependent on the morphology, length, l and diameter, d of the nanostructure [10]. For
1D nanostructure such as nanorod, β will be dependent on the aspect ratio (length
over diameter) of the nanorod. Several groups have tried to come up with a
relationship between β, l and d for CNT and single tip emitter [10-13] but they have
failed to agree on a common model. However, it is generally agreed that as the aspect
ratio of the nanorod increases, β increases as well.
Taking natural log on both sides for Equation 2.13, we get;
ln(
By plotting the graph of ln(
J
B
) = − + ln A
F
F2
(2.15)
1
F
J
) against and using the fact that β = we obtain the
E
E
E2
gradient of the slope as -K(φ)3/2/β where K is a constant while the y-intercept gives the
value of ln(αβ2/φ) where α is an area factor, which is equal to the ratio of an actual
emitting surface area to an overall surface area, describing the geometrical efficiency
of electron-field emission [14]. Knowing the workfunction, φ of the material, we can
obtain a value for the enhancement factor β and the area factor α.
References
[1] Robert Gomer. “Field emission and field ionization” pg 1, Harvard University
Press (1961)
[2] http://www.xintek.com
[3] http://sirad.pd.infn.it/people/candelori/PhD_Course_Radiation_Effects/
Conduzione_FN_SiO2.doc
[4] Richard L. Liboff. “Introductory Quantum Mechanics (3rd edition)” pg 664,
Addison-Wesley Publishing Company, Inc (1997)
[5] Richard L. Liboff. “Introductory Quantum Mechanics (3rd edition)” pg 269,
Addison-Wesley Publishing Company, Inc (1997)
[6] E. L. Murphy and R. H. Good, Phys. Rev. 102, 1464 (1956)
[7] R. H. Fowler and L. W. Nordheim, Proc. R. Soc. London, Ser. A 119, 173
(1928)
[8] L. W. Nordheim, Proc. R. Soc. London, Ser. A 121, 626 (1928)
[9] Y. W. Zhu, T. Yu, F. C. Cheong, X. J. Xu, C. T. Lim, V. B. C. Tan, J. T. L. Thong
and C. H. Sow, Nanotechnology. 16 88–92 (2005)
[10] Q. Zhao, H. Z. Zhang, Y. W. Zhu, S. Q. Feng, X. C. Sun, J. Xu, and D. P. Yu,
Appl Phys Lett 86, 203115 (2005)
[11] C. Liu, Y. Tong, H. M. Cheng, D. Golberg and Y. Bando, Appl Phys Lett 86,
223114 (2005)
[12] R. C. Smith, J. D. Carey, R. D. Forrest, and S. R. P. Silva, J. Vac. Sci. Technol.
B 2, 23 (2005)
[13] X. Q. Wang, M. Wang, P. M. He, Y. B. Xu and Z. H. Li, J. Appl Phys 96, 11
(2004)
[14] I. S. Altman, P. V. Pikhitsa and M. Choi, Appl. Phys. Lett. 84, 1126 (2004)
Chapter 3 – Experimental setup
In this chapter, we will present the details of the experimental setup and sample
characterization technique used in this work. Section 3.1 provides the details of the
hotplate technique employed for the synthesis of the CuO nanorods. This is followed
by the details of the field emission measurement setup in Section 3.2. In Section 3.3,
we describe an approach developed in this work where a single nanorod was secured
onto the tip of a sharpened tungsten wire and subsequent measurement of the field
emission from the single isolated nanorod. Sections 3.4 to 3.9 give a brief overview of
the techniques used for the patterning and characterizations of the sample after the
synthesis.
3.1 Growth of CuO nanorods
In this work, CuO nanorods represent the main focus of our investigations. These
CuO nanorods were synthesized by a simple heating technique. A piece of copper
(99.999% purity, Sigma-Aldrich Pte Ltd) was used. To prepare the copper for the
growth of CuO nanorods, its surfaces were polished with sandpaper to remove any
dirt and the oxide layer. This mechanical polishing was sufficient to clean the copper
making it suitable for the growth.
The copper was then placed on a hotplate and heated at a temperature of 400°C to
500°C. Figure 3.1(a) shows a picture of the hotplate used and a few pieces of the
polished metallic copper can be seen on the hotplate. After heating for sometime, the
shining metallic pieces became dull and darkened as shown in Figure 3.1(b-c). After
heating for the required duration, the copper plates were left to cool to room
temperature. A black layer can be seen covering the copper plates and when viewed
under the SEM, a layer of vertically aligned CuO nanorods were found as shown in
Figure 3.1(d).
Figure 3.1 (a) Hotplate with polished copper plates and tubes. (b) Freshly prepared
copper plates and tubes. (c) Copper plate and tubes after heating for 10 mins at 400°C.
(d) SEM image of the surface of the sample showing CuO nanorods.
Figure 3.2 shows the proposed growth mechanism of the CuO nanorods. Even
though the copper plates were heated at a temperature much lower than the melting
point of the bulk metallic copper, the surface of these Cu plates could melt at a lower
temperature. Under suitable growth temperature, the surface of the copper melts and
the Cu atoms from the molten layer react with the oxygen in air to form CuO
molecules. The CuO molecules then condense to form CuO nanorods. Apart from
CuO molecules condensing to form CuO nanorods, there could also be surface
diffusion of CuO molecules contributing to the formation of CuO nanorods. As the
growth duration or temperature increases, more Cu atoms will be liberated from the
molten layer to react with oxygen to form CuO and the CuO then migrates upwards
and some condense on the CuO nanorods already formed, giving rise to longer and
thicker CuO nanorods.
Figure 3.2 Growth mechanism of CuO nanorods with increasing time/temperature
with the black color region showing the molten state of Cu.
3.2 Field Emission measurement setup
A dedicated field emission measurement system was setup and utilized in this
work. Figure 3.3 shows a schematic of the field emission measurement system. It
consists of a vacuum system with a main chamber that houses the sample. In order to
accurately measure the field emission current density of the nanorod samples, the tests
must be carried out in a high vacuum environment. To prepare the nanorod samples
for the experiment, a small piece of the sample was cut out and pasted onto a silicon
substrate with the aid of conducting copper tape. The nanorod sample acting as a
cathode was then mounted onto a sample mount and an Indium Tin Oxide (ITO) glass
plate covered with a layer of phosphor acting as an anode was placed on top of it. The
anode and cathode were separated by 100µm thick polymer films as spacer. The
anode and the cathode were connected to a Keithley 237 high voltage source
measurement unit. The Keithley 237 high voltage source measurement unit was
capable of supplying a voltage of 0-1100V and measuring the field emission current
of up to 5 decimal places at the same time. A PC system was utilized to interface with
the Keithley 237 for automated instrument control and data acquisition. The field
emission measurement setup achieved a high level of vacuum with the help of a
mechanical pump and a turbo pump (Figure 3.4). After 24 hours of pumping by the
turbo pump, a pressure of 8x10-7 torr could be reached. In the event that lower
pressure was desired or the pumping time was to be reduced, liquid nitrogen could be
introduced to a cold trap. This reduced the pressure of the system to around 1/3 of the
value (around 3x10-7 torr) that could be achieved by turbo pump alone.
Figure 3.3 Field emission measurement setup with emphasis on field emission
chamber. The sample is shown in black color in the diagram.
Figure 3.4 Field emission measurement setup with emphasis on the vacuum system.
3.3 Measurement of Field Emission from Single Nanorod
In order to carry out measurement of the field emission from a single CuO
nanorod, we adopted the following procedure to first secure a single nanorod onto a
sharp tungsten tip and then conduct field emission measurement from the assembled
single CuO nanorods. CuO nanorods were first grown by heating a polished copper
plate in ambient air at 400°C for 7 days. After the growth, the nanorods were placed
under a probe station (Cascade Microtech REL 3200, Figure 3.5) with precision
positioners (DCM 210 series) where an etched tungsten tip was held. A glass slide
with a double sided carbon tape on it was positioned beneath the tungsten tip and
using the controls of the probe station, the tip was then lowered until contact was
made between the tip and tape. The tip was then moved inwards into the tape,
piercing it and withdrawn. This effectively coats the tip with a layer of glue from the
carbon tape. The coated tungsten tip was then moved towards the nanorods until
contact between the nanorods and the tungsten tip was made. Once a nanorod was
found to be adhered onto the tungsten tip, the tip was then withdrawn and viewed
under scanning electron microscopy (SEM, JEOL JSM-6400F).
The setup for field emission measurement is illustrated in Figure 3.6. It consisted
of a fully UHV compatible micro piezo slides (MS 5) with MS controller unit (MSCU)
driver electronics which allows for movements in the x-y-z direction. The tungsten tip
with CuO nanorod acting as anode was placed on the slides and a commercially
available tungsten tip with diameter of 12.5µm acting as cathode was placed on a
custom made stage facing the nanorod. To align the 2 tips in the z direction, a long
working distance microscope (Seiwa SKZ-1 bonocular microscope) with a 45x zoom
was used to view the setup from the side and the height of the tungsten tip with CuO
nanorod was then adjusted until it was at the same level with the cathode tungsten tip.
The setup was placed in a field emission chamber and connected to a Keithley 237 high
voltage source measurement unit (SMU). All tests are carried under a pressure of
~8x10-7 torr and at room temperature. To view the movement of the anode tip in the x
and y directions for the field emission measurements, a color video camera with long
working distance microscope (JVC KY-F50E) connected to a television set was used
to view the setup from the top (See Figure 3.6). A position controller (Omicron
Nanotechnology CPR 5) was used to move the anode, changing the separation
between the electrodes d. During the alignment process, the errors for ∆x, ∆y and ∆z
were estimated to be ±5µm. At each electrode distance, the voltage was applied from
0 to 1100V in steps of 10V. To ensure repeatability of the results, the application of
voltage was repeated 20 times per electrode distance.
Microscope
TV screen
Precision
controllers
Movable stage
Figure 3.5 Schematic of the probe station
(a)
(b)
TV set
Viewing
window
Field
emission
chamber
Tungsten tip with
CuO nanowire
d
Purchased
tungsten tip
MS 5 X-Y-Z
Color video camera nanomanipulator
with long working
distance microscope
(JVC KY-F50)
Glass
slides
Glass slides with
polymer layer on top
Figure 3.6 Schematic of the setup for the measurement of field emission from single
nanorods.
3.4 Scanning Electron Microscope (SEM)
The SEM is capable of producing a magnified real time image of the surface of a
sample. Figure 3.7 shows the schematic setup of a typical SEM. A beam of electrons
is emitted from the electron gun and accelerated and focused onto a spot on the
surface of the sample by means of magnetic field from the condenser lenses. The
objective lens serves to limit the angular width of the electron beam thus, improves
the depth of field in an image. When the electron beams impinges on the sample
surface, several things can happen [1-2]:
The electrons could undergo elastically scattering in the sample with little or no
loss of energy and emerge from the sample as back-scattered electrons.
The electrons could be inelastically scattered in the sample, giving rise to
secondary electrons, auger electrons and X-rays
The electrons could be absorbed and give rise to visible light in a process known
as cathodoluminescence.
The electrons could also give rise to electric current in the sample.
Electron source
1st condenser lens
Condenser aperture
2nd condenser lens
Objective aperture
Scan coils
Objective lens
Sample
Figure 3.7 Schematic setup of a scanning electron microscope.
For the study of surface morphology, secondary electrons are used. The number
of secondary electrons depends on the energy of the primary electron beam, E0 and the
angle of tilt of the sample [3]. Secondary electrons are emitted from a sample depth of
1nm thus, for low E0 and an increasing angle of tilt φ relative to the sample, majority
of the electrons emitted from the samples are secondary electrons.
During the operation, the secondary electrons are collected and accelerated
towards the positively charged electrode of the detector and made to pass through a
scintillator. The electrons collide with the scintillator material and photons are
produced.
The photons then travel through a light pipe via total internal reflection to a
photomultiplier. On striking the photomultiplier, the photons are converted to highly
amplified electric signal which is then fed to a computer display [4].
During SEM operation, a sample with conducting surface must be used otherwise,
the electrons will accumulate on the surface of the sample and a charge-up will occur.
If the sample is non-conducting by nature, a very thin layer of conducting material e.g.
platinum is evaporated onto its surface.
For the SEM images obtained throughout this project, the JSM-6700F field
emission SEM is used. This FESEM uses a field emission cathode in the electron gun
which is capable of producing narrower probing beam resulting in improved spatial
resolution compared to the conventional SEM [5]. The JSM-6700F SEM is capable of
a magnification from x 25 to 650,000x.
3.5 High Resolution Transmission Electron Microscope (HRTEM)
The HRTEM is used to study samples at atomic resolution [6]. Figure 3.8 shows
the schematic setup of a typical HRTEM. The sample is irradiated with a beam of
electrons with energy ranging from 100-500keV. An image is formed with the
electrons transmitted through the sample by a sophisticated electron optic system.
The samples undergoing HRTEM studies need to be thin enough for the electron
beam to pass through and capable of withstanding high vacuum. Preparation
techniques such as ion beam milling and wedge polishing are frequently employed to
obtain a thin enough sample. For nanorods or nanotubes with small enough
dimensions, they can be suspended in a solvent such as alcohol and dispersed onto a
copper grid for HRTEM imaging.
The HRTEM used in this project is the JEOL JEM-3010F with an acceleration
voltage of 300kV. This HRTEM is capable of providing a resolution of 0.17nm.
Electron source
1st condenser lens
2nd condenser lens
Condenser aperture
Sample
Objective lens
Objective aperture
Selected area aperture
1st intermediate lens
2nd intermediate lens
Projector lens
Main screen (Phosphor)
Figure 3.8 Schematic setup of a transmission electron microscope
3.6 X-Ray Diffraction (XRD)
XRD is commonly used to measure to analyze the crystalline structure of a
sample. A monochromatic X-ray with wavelength λ is irradiated onto the sample and
for a single crystal, if the path difference between the beams reflected from parallel
atomic planes is an integer multiple of λ, the reflection intensity will be a maximum
due to constructive interference. This is known as the Bragg’s law:
2d sinθ = nλ
(3.1)
where d is the spacing of atomic planes, n is an integer and θ is Bragg angle, which is
the angle between the plane of the sample and the X-Ray source.
In our studies, the XRD used is Bruker Analytical X-Ray system, Cu Kα radiation
with λ=1.5406Å. In this system, the X-ray source and detector is fixed in position
while the sample is being rotated during the measurement. This effectively varies θ
and the range allowed for this system is from 20° to 140°.
3.7 X-Ray and UV photoelectron Spectroscopy (XPS and UPS)
XPS also known as Electron Spectroscopy for Chemical Analysis (ESCA) is
commonly used to analyze elemental composition. Carried out in Ultra-High Vacuum
(UHV), it does so by irradiating the sample with a beam of X-ray and if the X-ray
photon energy is high enough, it can cause the core level electrons to escape [7]. With
no energy loss, the kinetic energy of the electrons:
K = hf - EB
(3.2)
Where h is the planck’s constant, f is the frequency of the X-ray and EB is the binding
energy of the electrons.
The binding energy is a characteristic of the element which the electron originates
from and by analyzing the energy spectrum of electron, information on the elemental
composition of the sample can be obtained. The XPS system used in this project is the
ESCA MK II with Mg source.
By using ultraviolet photons, the electronic states of the valance band can be
studied and this technique is known as the Ultraviolet Photoelectron Spectroscopy
(UPS). UPS is useful in measuring density of states (DOS), Fermi level shift, work
function and chemical states.
3.8 Sputtering machine
Sputtering is a process where the atoms from a solid material are ejected due to
the bombardment of energetic ions on the material. In this work, the sputtering
machine used is the radio frequency (RF) Magnetron Denton Discovery 18. During
the sputtering process, argon (Ar) gas was introduced into the chamber and
maintained at 100 Standard Cubic Centimeter per Minute (SCCM) with a pressure of
10mTorr. An RF power of 100W was used to ionize the argon gas. The typical
sputtering rate of copper under these conditions is 20nm/min.
3.9 Laser system
In order to carry out micro-patterning onto nanostructures, a simple technique has
been developed in our lab for such purpose. Using a Helium Neon (He-Ne) laser of
power of 38mW with a beam width and beam divergence of 1.24mm and 0.65mrad
respectively together with an array of mirrors, a focused laser beam system is formed
(Figure 3.9). The first beam splitter (S1) directs the beam towards the objective lens
while an external light source (not shown in Fig 3.9) provided the illumination of the
samples. The beam were focus onto the sample using a Nikon 50X lens (numerical
aperture of 0.55) objective lens (L).
The nanostructure sample was then placed on a MICOS XY sample stage with 25
mm travel and a minimum step size of 50nm in the x and y directions. Control of the
stage was achieved by a MICOS motor controller and interfaced to the computer via
Microsoft Visual Basic software. During patterning process, light was collected via
the objective lens (L) and passed through S1 and S2 to be captured by a JVC CCD
camera which was connected to a TV and a computer for view and capturing
images/videos throughout the process [8]. The depth of nanostructure trimmed off
by the focused laser beam depends on the power of the laser and how well the laser
beam was being focused onto the sample surface.
CCD Camera
TV/Computer
M
He – Ne Laser
S1
L – Optical Lens
M – Mirror
S1 – Beam Splitter
M
B
L
L
Computer Control Stage
Figure 3.9 Schematic setup of the focused laser system
Nanostructure sample
(Mounted face-up)
References
[1] http://www.mse.iastate.edu/microscopy/college.html
[2] http://acept.la.asu.edu/PiN/rdg/elmicr/elmicr.shtml
[3] http://laser.phys.ualberta.ca/~egerton/SEM/sem.htm
[4] Lim San Hua, Honors thesis pg 25
[5] http://www.photometrics.net/fesem.html
[6] http://www.unl.edu/CMRAcfem/temoptic.htm
[7] http://en.wikipedia.org/wiki/X-ray_photoelectron_spectroscopy
[8] K. Y. Lim, C. H. Sow, J. Y. Lin, F. C. Cheong, Z. X. Shen, J. T. L. Thong, K. C.
Chin and A. T. S. Wee. Adv. Mater. 15, 300 (2003)
Chapter 4 – Enhanced field emission of CuO nanorod
films from large scale patterning by focused laser
beam
In this chapter, the field emission properties of CuO nanorods synthesized under
various growth conditions were investigated and presented. Large scale patterning of
the nanorods by focused laser beam resulting enhanced field emission properties will
also be discussed.
4.1 Introduction
In recent years, extensive research has been carried out on one dimensional
nanomaterials due to their unique mechanical and electrical properties [1-4]. The
sharp tip and high aspect ratio of the nanorods can effectively enhance the local
electric field and thus making them a suitable candidate for field emitters [5]. A
suitable material to be used as a field emitter is CuO nanorod. Zhu et al. [6]
demonstrated the feasibility of synthesizing CuO nanorods on a large scale with
promising field emission results. CuO nanorods are capable of achieving a high field
emission current density at relatively low field and exhibiting uniform field emission
distributions.
A common drawback for highly dense 1-D field emitter film is the screening
effect due to the proximity of the neighboring nanorods [7, 8]. Several methods have
been employed to control the density of Carbon Nanotubes (CNTs). These methods
include changing the composition of the catalyst [9], micro-contact printing [10] and
reduction of density by excimer laser treatment [11]. In the case for nanorods, the effect
of screening and their field emission properties have been investigated [12].
In this work, there are two main objectives. The first is to investigate the effect
of the physical properties such as density, length and diameter of as-grown CuO
nanorods on their field emission properties. The second objective is to find out how the
exposed edges of platforms created through large scale patterning by direct focused
laser nanofabrication will affects its field emission. We have found that the field
emission properties of as-grown CuO nanorods can be varied by changing their
physical properties and that large scale laser patterning is able to enhance their field
emission properties.
4.2 Experimental details
Vertically oriented CuO nanorods were synthesized by heating polished
copper plates on a hotplate in ambient air. The growth time was varied from 5 -7 days
and the growth temperature was varied from 400°C to 450°C. After the growth was
completed, the sample was left to cool to room temperature and the layer of CuO
nanorods was then extracted. The CuO nanorods were then characterized using
scanning electron microscopy (SEM, JEOL JSM-6400F).
Field emission tests were conducted for the as-grown CuO nanorods samples
using a two-parallel-plate configuration in a vacuum chamber with a pressure of about
1×10−6 Torr [6]. The peeled nanorod films were adhered onto a copper substrate
cathode by copper double-sided tape. Indium tin oxide (ITO) glass covered with a
layer of phosphor was employed as the anode. Two polymer films were used as
spacers and the distance between electrodes was kept at 200µm. A Keithley 237 high
voltage source measurement unit (SMU) was used to apply a voltage from 0 to 1100V
and to measure the emission current at the same time. All the measurements of the
field emission current density (J) and the applied field (E) were performed at room
temperature. To achieve stable measurements, the applied voltage was repeatedly
ramped up and down until there were no significant changes in the J-E plot in
between the ramps.
After the field emission tests, laser patterning was carried out using a focused
laser beam from a Helium-Neon laser [13, 14]. When CuO nanorods were illuminated
by the focused laser beam, high laser light absorption resulted in the melting of the
nanorods. The molten state of the CuO re-solidified into microballs and as a result, the
CuO nanorods became readily truncated by the laser beam. The stage with the
samples on it was programmed to move with respect to the laser and large scale
patterning on CuO nanorods was achieved. Further details regarding the laser
patterning were described in details in Chapter 3. In the 1st laser patterning process,
parallel micro-channels of width 10µm were patterned by laser onto the CuO
nanorods samples with the channel center to center distance separated at 30µm. The
choice of 10µm for channel width allowed for easy calculation of the remaining area
after the patterning. The number of channels cut onto the sample depended on its size
i.e. the larger the sample, the more channels cut. As a result, the laser patterning gave
st
rise to a parallel array of micro-platforms with a width of 20µm. After the 1
patterning process, field emission tests were conducted again under the same pressure.
The 2 samples with the highest nanorod density studied previously were subjected to
further laser patterning. More channels of width 10µm were cut orthogonally to the
previous platforms forming micro-squares with area of 400µm2. After the 2nd laser
patterning, the samples were subjected to the field emission tests. This allowed us to
investigate the effect of more exposed edges on the field emission properties. In this
work, we used the growth conditions of the samples to denote them i.e. samples
grown at 400°C for 7 days are denoted as S400-7. The 1st laser patterning process
removed 29%, 27% and 28% of the CuO nanorods in the samples S450-5, S400-7 and
S450-7 respectively. After the 2nd laser patterning, the percentages of the remaining
area of the samples covered with CuO nanorods were 54% and 53% for samples
S400-7 and S450-7 respectively.
4.3 Results and discussions
Three batches of CuO nanorods films with varying growth conditions were
synthesized and characterized. Table 4.1 lists the growth conditions and physical
properties of the as-grown nanorods. The side view SEM images of the CuO nanorods
are shown in Figure 4.1. Comparing Figure 4.1 (a), (b), (c) and (d), an increase in
growth temperature with the same duration results in a larger diameter and length of
the nanorods. Comparing Figure 4.1 (c), (d) and (e), (f), an increase in growth
duration while keeping temperature constant gives rise to longer nanorods with larger
diameter. From Table 4.1, it can also be observed that longer growth duration with the
same growth temperature results in a higher density while a higher growth
temperature with the same growth duration does not show significant change in the
density.
Sample
S400-7
S450-7
S450-5
Growth time (days)
7
7
5
Growth temperature (°C)
400
450
450
Density (x10 /cm )
5.0+0.4
4.8±0.4
4.5±0.4
Length (µm)
17±8
25±9
14±7
Diameter (nm)
100±30
190±40
90±30
Aspect ratio
170
132
156
8
2
Table 4.1 Growth conditions and physical properties of the various CuO nanorods
films.
The length and diameter distributions for the 3 samples were obtained by
measuring the length and diameter of 400 individual CuO nanorods from the side
view SEM images of each sample. For the diameter distribution, 20 SEM images for
each sample were analyzed and for the length distribution, 5 SEM images from each
sample were analyzed. The SEM images were obtained from different locations for
better representation of the sample. The distributions are shown in Figure 4.2. From
Figure 4.2, it is observed that CuO nanorods grown at higher temperature have a
larger range in the length distribution as compared to CuO nanorods grown at a lower
temperature. The same applies to CuO nanorods grown at the same temperature but
for a longer duration.
For samples with the same growth duration but different temperature, a higher
temperature within the growth temperature range enables more Cu atoms to be
liberated from the surface, forming more CuO, which in turn favors longer CuO
nanorods formation. With the existence of longer nanorods and nanorods that just
started to form before cooling, the samples heated at a higher temperature give rise to
a larger spread in length. From this, it can be seen that control over the physical
properties of the samples can be achieved by changing the growth duration or
temperature.
Figure 4.1 Side view SEM images of (a) and (b) S400-7, (c) and (d) S450-7, (e) and (f)
S450-5
140
70
(a)
100
80
60
40
20
0
(b)
60
Number distribution
Number distribution
120
50
40
30
20
10
50
100
150
200
250
0
300
5
10
Diameter of CuO nanowires (nm)
Number distribution
100
70
(c)
80
60
40
20
0
50
100
150
200
250
35
40
40
30
20
0
300
0
5
80
70
80
60
40
10 15 20 25 30 35 40 45 50
Length of CuO nanowires (µm)
(e)
100
20
0
30
10
Number distribution
Number distribution
120
25
50
Diameter of CuO nanowires (nm)
140
20
(d)
60
Number distribution
120
15
Length of CuO nanowires ( µm)
(f)
60
50
40
30
20
10
50
100
150
200
250
Diameter of CuO nanowires (nm)
300
0
5
10
15
20
25
30
35
Length of CuO nanowires (µm)
Figure 4.2 (a) Diameter distribution for S400-7, (b) length distribution for S400-7, (c)
diameter distribution for S450-7, (d) length distribution for S450-7, (e) diameter
distribution for S450-5 and (f) length distribution for S450-5.
After the studies on the field emission properties of the as-grown CuO nanorods,
focused laser beam patterning was employed to create micro-patterns in the samples.
Figure 4.3 (a) shows top view SEM images of CuO nanorods before laser patterning
process. Figure 4.3 (b) and (c) show top view SEM images of CuO nanorods after the
1st laser patterning process and it can be observed that the platforms are periodically
spaced with equal dimensions. From Figure 4.3 (d) and (e), it can be observed that the
2nd laser patterning process created micro-squares that are also periodically spaced
and have equal dimensions. From Figure 4.3 (f), it can be observed that the CuO
nanorods melted to form microballs when exposed to laser radiation, consistent with
observations in a previous study [14].
The measured field emission current densities (J) versus applied field (E) for the
various CuO nanorod samples are shown in Figure 4.4. Table 4.2 summarizes the field
emission results of the various samples before and after laser patterning. For the
as-grown CuO nanorods films, S400-7 has the best field emission properties
compared to S450-7 and S450-5. The average aspect ratio for S450-5 (~156) is lower
than that of S400-7 (~170) and it has a lower density; thus, a higher aspect ratio and
density gives rise to better field emission performance. Comparing S400-7 with
S450-7, both samples have similar density but S450-7 has a lower aspect ratio (~132);
and from this, we can deduce that for samples with similar density, a higher aspect
ratio gives better field emission performance.
An experiment was carried out where the entire area of CuO nanorods were
truncated by focused laser beam and when the sample was subjected to field emission
tests, no field emission properties were detected. Thus, we can deduce that laser
truncated regions do not contribute to field emission and the enhanced current density
J presented in this work has taken into account the reduction in effective emitter area.
After the 1st laser patterning where micro-platforms were patterned onto the
samples, there is a significant improvement in the field emission current density. This
is a result of having more emission sites when the exposed sides of the nanorods
along the edges of the platforms contribute to the field emission. The ratio in the Table
4.2 is the ratio of the current density after patterning to the as-grown current density at
5.5V/µm. For the 2 samples with higher nanorod density, there is a significant
improvement in the turn-on field. The improvement can be attributed to higher area
density samples having more emission sites created after the laser patterning process
than lower area density samples and this reduces the field needed to achieve the turn
on current. Comparing how the 1st and 2nd laser patterning affects the field emission
of the samples, Figure 4.4 (c), (d), (e) and (f) show that the performance after 2nd laser
patterning process is still better than the as-grown field emission and there is no
significant change in the field emission current density for the samples after the 2nd
laser patterning process. This suggests that the increase in field emission performance
caused by more exposed edges of the micro-square platforms were offset by the
reduction in effective emitters such as long nanorods that were truncated in the
process.
Using the Fowler-Nordheim (FN) equation [6, 15, 16]
3
αA
Bφ 2
( βE avg ) 2 exp(−
)
J=
φ
βE avg
(1)
where J is the emission current density (A/cm2), Eavg is the average electric field
(V/µm), φ is the workfunction of the emitters (eV) with the workfunction of CuO
being 4.5eV [16] , α is the area factor and β is the enhancement factor. A and B are
constants and their values are 1.54x10-6 and 6.44x103 respectively. We can determine
α and β for the various samples before and after laser patterning. The results were
summarized in Table 4.3.
Figure 4.3 Top view SEM images of (a) as-grown CuO nanorods, (b) - (e) laser
patterned CuO nanorods and (f) closed up view of the microballs in regions exposed
to the laser beam
(a)
2
Current density of 450-5 (µA/cm )
60
50
As-grown
1st laser patterning
2
Ln( J/E )
40
30
20
10
0
0
1
100
3
4
5
6
(c)
90
80
60
1/E
(d)
-13.0
-13.2
2
40
-13.6
-13.8
30
-14.0
20
-14.2
10
-14.4
1
2
3
4
5
-14.6
6
0.18 0.19 0.20 0.21 0.22 0.23 0.24 0.25 0.26
Applied field, E (V/µm)
-12.6
(e)
90
1/E
-12.8
As-grown
1st laser patterning
2nd laser patterning
80
70
-13.0
2
50
40
-13.6
-13.8
-14.0
20
-14.2
10
-14.4
0
1
2
3
4
Applied field, E (V/µm)
As-grown
1st laser patterning
2nd laser patterning
-13.4
30
0
(f)
-13.2
60
Ln(J/E )
2
Current density of 450-7 (µA/cm )
100
As-grown
1st laser patterning
2nd laser patterning
-13.4
50
0
As-grown
1st laser patterning
0.18 0.19 0.20 0.21 0.22 0.23 0.24 0.25 0.26
-12.8
As-grown
1st laser patterning
2nd laser patterning
70
0
(b)
-12.6
Ln(J/E )
2
Current density of 400-7 (µA/cm )
2
Applied field, E (V/µm)
-13.0
-13.2
-13.4
-13.6
-13.8
-14.0
-14.2
-14.4
-14.6
-14.8
-15.0
-15.2
-15.4
-15.6
-15.8
5
6
-14.6
0.18 0.19 0.20 0.21 0.22 0.23 0.24 0.25 0.26
1/E
Figure 4.4 (a) J-E plot for S400-7 before and after laser patterning, (b) corresponding
FN plot, (c) J-E plot for S450-7 before and after laser patterning, (d) corresponding
FN plot (e) J-E plot for S450-5 before and after laser patterning and (f) corresponding
FN plot.
Sample
S400-7
S450-7
S450-5
Density (x10 /cm )
5.0
4.8
4.5
As-grown turn on field (V/µm)
Enhanced turn on field (V/µm)
(micro-platforms)
4.15
4.2
4.5
3.75
3.8
4.65
As-grown J (µA) at 5.5 V/µm
Enhanced J (µA) at 5.5 V/µm
(micro-platforms)
57
51
41
89
95
56
Ratio (micro-platforms)
Enhanced J (µA) at 5.5 V/µm
(micro-squares)
1.6
1.9
1.4
78
85
N.A
Ratio (micro-squares)
1.4
1.7
N.A
8
2
Table 4.2 Field emission properties of the various CuO nanorods before and after laser
patterning.
Sample
S400-7
S450-7
S450-5
As-grown enhancement factor (β0)
3500±100
3600±100
2500±100
Platform patterned enhancement factor (β1)
3400±100
3500±100
2000±100
Square patterned enhancement factor (β2)
3600±100
3600±100
N.A.
As-grown area factor (α0)
(9.0±0.2)x10-6
(7.4±0.1)x10-6
(5.5±0.1)x10-5
Platform patterned area factor (α1)
(2.1±0.1)x10-5
(1.8±0.1)x10-5
(3.9±0.1)x10-4
Square patterned area factor (α2)
(1.4±0.1)x10-5
(1.4±0.1)x10-5
N.A.
Table 4.3 Enhancement factor and area factor of the various CuO nanorods before and
after laser patterning.
Table 4.3 shows the enhancement factor β and area factor α before and after
laser patterning. From table 4.3, the samples show a slight fluctuation in the
enhancement factor and a significant increase in the area factor after the 1st laser
patterning. The 2nd laser patterning led to a decrease in area factor with the
enhancement factor fluctuating within the error. The 1st laser patterning enables the
exposed sides of the platforms to more effectively field emit without facing screening.
This led to an increase in the number of field emission sites and effectively increased
the area factor α1. α2 is larger than α0 and again, we can attribute this to the increased
in field emission sites after the laser patterning process. Although α2 is expected to be
larger than α1 due to the increase in exposed sides, it is in fact lower. A possible
reason for this could be that the effective emission sites such as long nanorods are not
evenly distributed across the samples and the 2nd laser patterning process removed
almost half the samples’ area and could have removed some areas that contain more
emission sites, thus lowering α2.
The enhancement factor of the sample is largely dependent on its geometry
and morphology. Since the laser-truncated area does not contribute to field emission
and the nanorods inside the strips and micro-squares are unaffected by the laser, the
enhancement factor before and after laser patterning should remain unchanged. The
enhancement factor for S450-5 before and after the laser patterning process shows a
difference and this could be caused by our experimental measurements.
4.4 Conclusion
In summary, we have synthesized 3 batches of CuO nanorods with various
physical properties and patterned them on a large scale using focused laser. Good
control over the physical properties of the nanorods can be achieved by changing the
growth duration or temperature. This study also demonstrates the feasibility of using
focused laser as a mean to conduct large scale micro patterning. The laser patterning
also improves the field emission properties of the nanorods and this is attributed to the
increased in emission sites for the exposed nanorods along the edges of the platforms.
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Chapter 5 – Field emission from hybrid CuO and
CuCO3 nanosystems
In this chapter, we present a simple way of synthesizing hybrid CuO and CuCO3
nanosystems. Methods of characterizing the samples will also be presented and the
field emission properties of the samples will be discussed.
5.1 Introduction
Hybrid nanosystems combining the properties of two or more different types of
nanostructures could further enhance the field emission properties and allow for the
tuning of field emission properties by varying the relative percentage of the individual
nanomaterials in the hybrids. Recently, hybrid CuO and ZnO nanorods nanostructures
have been synthesized by directly heating brass in ambient conditions and the field
emission properties can be tuned by varying the percentages of copper and zinc in
brass [1]. Carbon cloth possesses field emission property due to the large quantity of
potential emitting sites on its protruding carbon fibers and is a candidate for large area
field emission flat cathode [2]. Taking advantage of the rough geometry of the carbon
cloth, nanomaterials grown on carbon cloth are expected to exhibit enhanced field
emission properties due to the extra geometrical enhancement from the multistage
effect [3]. Ren and coworkers have grown ZnO nanorods and CNTs on top of carbon
cloth and achieved excellent field emission properties with ultra low turn-on and
threshold field [4-6]. Carbon paper, unlike carbon cloth, has no field emission
properties by itself due to its planar geometry and lack of protruding fibers. Thus, it
allows us to characterize the field emission current performance of the samples on
carbon paper substrate more accurately.
On the other hand, to the best of our knowledge, there has been no field emission
study on hybrid nanosystem with CuO and CuCO3 reported. To further enrich the pool
of field emission candidates and to achieve better controllability of field emission,
hybrid CuO-CuCO3 nanosystem was synthesized by directly heating copper sputtered
carbon paper in ambient in this work. The relative concentration of CuCO3 can be
varied by changing the thickness of copper on carbon paper. Field emission properties
of the hybrids show the emission turn-on field and current can be tuned by the coating
thickness of copper.
5.2 Experimental details
As-purchased Toray carbon papers with thickness of 0.28mm were cut into 1cm x
1cm pieces and placed in a radio-frequency plasma-assisted sputtering machine
(Denton Vacuum Discovery 18) and the chamber was evacuated to a base pressure of
10−6 Torr. After that, Ar plasma with a power of about 100 W was induced to bombard a
pure Cu (Angstrom Sciences, 99.999%) target. The deposition rate was ~20nm/min and
last for 10 minutes (~200nm), 20 minutes (~400nm), 90 minutes (~1800nm), 120
minutes (~2400nm) and 240 minutes (~4800nm) under room temperature (25°C). We
shall use the notation Cu200_C, Cu400_C, Cu1800_C, Cu2400_C and Cu4800_C to
denote these samples. After the sputtering process, the copper sputtered carbon papers
were then heated at 400°C for 3 days using a hotplate in ambient condition. For
comparison, a few pieces of pure carbon papers were heated together with the copper
coated samples. After the growth process, scanning electron microscopy (SEM, JEOL
JSM-6400F), X-Ray diffraction system (XRD, Bruker Analytical X-Ray system, Cu
Kα radiation, λ=1.5406Å), transmission electron microscope (TEM, JEOL
JEM-3010F, 300kV), X-ray photoelectron spectroscopy (XPS, ESCA MK II; Mg
source) and ultraviolet photoelectron spectroscopy (UPS, Surface, Nanostructure and
Interface Science (SINS) beamline at Singapore Synchrotron Light Source (SSLS)) [7]
were used to characterize the samples. Field emission tests for the various samples
were carried out using a two-parallel-plate configuration in a vacuum chamber at a
pressure of 1x10-6 Torr [8]. The samples were adhered onto a Cu substrate cathode by
double-sided copper tape. Indium tin oxide (ITO) glass covered with a layer of
phosphor was employed as the anode. A polymer film was used as a spacer and the
distance between electrodes was kept at 200µm. A Keithley 237 high voltage source
measurement unit (SMU) was used to apply a voltage from 0 to 1100 V and to
measure the emission current at the same time. All the measurements were performed
at room temperature.
5.3 Results and discussions
Figure 5.1 (a) and (b) show the SEM images of pure carbon paper before and
after heating respectively. It can be observed that the carbon paper fibers undergo no
significant change after heating at 400°C for 3 days. Figure 5.1(c) shows particle-like
structures with average size of 180nm growing on the C fibers with the exception of a
few short nanorods on the heated Cu400_C sample. From Figure 5.1 (d), nanorods
can be seen starting to grow on the heated Cu1800_C sample. From Figure 5.1 (e),
high density of nanorods with 2.7µm and 160nm in length and diameter respectively
can be seen covering the heated Cu4800_C sample. In general, it can be concluded
that the density of the nanorods increases as the thickness of copper increases. TEM
studies of the heated Cu400_C sample shows a mixture of CuO and CuCO3
nanoparticles from the measurement of interface distance in the high-resolution TEM
images. In Figure 5.1 (f), a lattice spacing of 2.65 Å was obtained from the particle,
consistent with the (-1 1 2) spacing of single crystalline CuCO3
Figure 5.1 SEM images of (a) pure carbon paper with inset at higher magnification,
(b) heated carbon paper, (c) heated Cu400_C with inset at higher magnification, (d)
heated Cu1800_C, (e) heated Cu4800_C, and (f) TEM image of a single CuCO3
nanoparticle
From the XRD patterns of the various samples in Figure 5.2(a), the majority of
the peaks for all samples correspond to CuO and a distinct CuCO3 peak can be seen at
54.7° for heated Cu400_C. The height of the CuCO3 peak decreases for heated
Cu1800_C and is completely suppressed when the thickness of Cu is 4800nm. We can
infer from the XRD results that the nanorods seen in the SEM images correspond to
CuO nanorods and as the thickness of copper coating increases, CuO nanorods
dominate completely. Figure 5.2 (b) shows the XPS O1s spectra of the three samples.
The main peak at 530 eV is attributed to CuO [9] and the peak at 531.5 eV can be
attributed to CuCO3 [10]. However, there is a possible contribution to the peak at
531.5 eV due to oxygen adsorbed onto the surface of CuO nanorods at 531.6 eV [11]
As such, no attempt was made to deconvolute the spectra. Figure 5.2 (c) shows the
low kinetic energy part of the UPS spectra for the Cu400_C and Cu4800_C samples
measured at normal emission angle. In this measurement, a negative bias of 5V was
applied to the sample to eliminate the influence of the analyzer workfunction. From
the low energy cutoff in this graph, the workfunction can be determined to be 4.3eV
and 4.5eV for the Cu400_C and Cu4800_C sample respectively. This suggests that
hybrid CuO and CuCO3 nanosystem has a lower workfunction than CuO and can be
attributed to the presence of CuCO3 nanoparticles. The high current density for heated
Cu400_C is consistent with measured low workfunction.
CuO
CuCO3
(a)
Intensity (a.u)
Heated
Cu4800_C
Intensity (a.u)
(b)
528
Heated
Cu1800_C
30
40
50
60
70
80
532
534
Heated Cu400_C
Heated Cu4800_C
(c)
Intensity (a.u)
Heated
Cu400_C
530
Binding energy (eV)
3.8 4.0 4.2 4.4 4.6 4.8 5.0
2θ (degree)
2
3
4
5
6
7
8
9
Energy (eV)
Figure 5.2(a). The XRD patterns for heated Cu400_C, heated Cu1800_C and heated
Cu4800_C (offset for clarity) (b) XPS O1s spectra for heated Cu400_C (top),
Cu1800_C (middle) and Cu4800_C (bottom) (offset for clarity) and (c) UPS spectra
of heated Cu400_C and Cu4800_C.
Figure 5.3 (a) shows the field emission current density (J) versus applied field (E)
for the different samples. To achieve stable measurements, the applied voltage was
repeatedly ramped up and down until there were no significant changes in the J-E plot
in between the ramps. Field emission tests for pure carbon paper, heated carbon paper
and as sputtered copper coated carbon paper showed no measurable field emission.
The field required to achieve a field emission current density of 10 µA/cm2 are: 2.05
V/µm for heated Cu400_C, 2.18 V/µm for heated Cu200_C, 2.45 V/µm for heated
Cu1800_C, 3.75 V/µm for heated Cu2400_C and 5.18 V/µm for heated Cu4800_C. It
can also be observed that the maximum current density in our setup is the highest for
the heated Cu400_C sample reaching 1.02 mA/cm2 at a low field of 3.18 V/µm. The
maximum current density achievable also decreases as the thickness of copper
deviates from the 400nm range. The field emission fluorescence image, however, is
not uniform and this could be attributed to the uneven nature of the carbon paper
and/or the uneven thickness of materials on the carbon fibers. From FN plots in
Figure 5.3 (b), it can also be observed that besides the heated Cu4800_C sample, the
rest have two different slopes. The change in the slope indicated turning on of
different emitting species and could be due to defects or emitters with different
chemical nature or morphology. Using the FN equation [9, 12, 13]
3
αA
Bφ 2
( βE avg ) 2 exp(−
)
J=
φ
βE avg
(5.1)
Where J is the emission current density (A/cm2), Eavg is the average electric field
(V/µm), φ is the workfunction of the emitters (eV), α is the area factor and β is the
enhancement factor. A and B are constants and their values are 1.54x10-6 and 6.44x103
respectively.
Considering only the best and worst field emitter and from the workfunctions
obtained, we determine the enhancement factor: β1 = 2960 in the high E-field region
and β2 = 2029 in the low E-field region for the heated Cu400_C sample and β = 1601
for the heated Cu4800_C sample. The area factor α is calculated to be ~1.3x10-2 cm2
and ~5.2x10-4 cm2 for the high E-field region of heated Cu400_C and Cu4800_C
respectively. These results suggest that the best field emission performance of heated
Cu400_C sample could be attributed to both high enhancement factor and large
effective emission area. Furthermore, the enhancement factor and field emitting area
decrease as the concentration of CuCO3 decreases. This is contrary to higher aspect
ratio having higher enhancement factor [15] and is due to the lower work function of
CuCO3 nanoparticles and could also be due to the higher number of defects in these
nanoparticles. The higher number of defects present in CuCO3 could also account for
the increase in the field emitting area for samples with higher CuCO3 concentration.
From these results, we can also conclude that as the copper coating decreases below
the 400nm range, the concentration of CuCO3 decreases and thus, leading to a
decrease in field emission performance.
Table 5.1 shows the turn-on field (defined as the field required to generate 10
µA/cm2) and threshold field (defined as the field required to generate 1 mA/cm2) for
hybrid CuO and CuCO3 nanosystem together with selected samples. Samples with *
in the threshold field column indicate samples that cannot reach 1mA/cm2 and in
place is the maximum current density achievable. From the table, it can be seen that
the field emission properties of hybrid CuO and CuCO3 nanosystem is among the
better performers compared to many samples.
Figure 5.3 (a) Current density vs applied field for heated Cu200_C to Cu4800_C, (b)
corresponding FN plots
Sample
Hybrid CuO and CuCO3
(400nm)
CuO nanorods[8]
ZnO agavelike
nanoneedles [14]
Carbon Nanotubes [15]
GaN nanobelts [16]
Hybrid CuO and ZnO
(40% Zn) [1]
GaAs nanorods [17]
Cu2S nanorods [18]
CNT on carbon cloth [4]
ZnO nanorods on carbon
cloth [3]
Turn-on
field
(V/µm)
2.05
3.5-4.5
2.4
0.75
1.3
3
2.0
6
< 0.2
0.2
Threshold field
(V/µm)
3.18
*450µA/cm2 at
7V/µm*
4.3
1.6
2.3
*470µA/cm2 at
5.5V/µm*
6.5
12
< 0.4
0.7
Table 5.1 Turn-on field and threshold field for various field emitters.
5.4 Conclusion
In conclusion, we have successfully synthesized hybrid CuO-CuCO3
nanosystems by simple heating of copper-coated carbon paper in ambient and the
relative concentrations can be varied by the thickness of copper on carbon paper. Field
emission tests reveal these hybrid nanosystems as promising field emitter and a
potential candidate for future generation field emission devices.
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Chapter 6 – Electron field emission from a single CuO
nanorod
In this chapter, a simple technique of assembling a single CuO nanorod onto an
etched tungsten tip using a probe station is presented. A custom-made stage is used to
measure the field emission properties of the assembled single nanorod and from the
data collected, several interesting results are obtained. These findings are then
compared with the existing results from the field emission of single CNT.
6.1 Introduction
One-dimensional (1D) nanostructures such as nanotubes and nanorods have
unique mechanical and electrical properties [1-4] thus, enabling them to find
applications in numerous fields [5-8]. One of the main applications for nanostructures
is in the area of field emission. One-dimensional (1D) nanostructures are particularly
prominent in this area due to their ability to enhance local electric field due to their
sharp tips and high aspect ratio [9] as discussed in Chapter 1.
In terms of applications, nanorods films are highly suitable but to investigate the
physics behind the field emission process, the study of a single nanorod is desirable as
it eliminates the contributions from effects such as screening and edge effects found in
films. In recent years, field emission properties of single 1D nanostructures have been
reported and many studies have been carried out to investigate the field enhancement
factor dependence on the electrode distance for single carbon nanotubes (CNTs).
However, there appear to be inconsistencies between the studies and the methods of
testing the field emission properties of a single 1D nanostructure remains challenging,
requiring expensive equipments in some cases. Metal-oxide nanostructures have been
shown to be promising field emitters [10-14] and with the combination of easy growth
technique [15, 16], they are a source for future field emitters. To date, there are very
few reports on the field emission studies of single metal-oxide nanostructures and in
this chapter, we report a simple way to assemble single CuO nanorod on a tungsten tip
together with its field emission properties and the dependence of its field
enhancement factor on the distance between the electrodes.
6.2 Experimental details
CuO nanorods were grown by heating a polished copper plate in ambient at
400°C for 7 days. After the growth, the assembly of a single CuO nanorod onto an
etched tungsten tip was carried out using a probe station. More details with regards to
the process can be found in Chapter 3. In this chapter, we present the field emission
properties of 3 CuO nanorods and they are labeled as CuO nanorod (I), CuO nanorod (II)
and CuO nanorod (III) respectively.
A custom made stage was used in conjunction with the field emission chamber for
the field emission tests of the single CuO nanorod. Further details regarding the setup
and process can be found in Chapter 3.
6.3 Results and discussions
Figure 6.1 (a) shows the optical image of the anode and cathode taken from the
camera. Figure 6.1 (b), (c) and (d) shows the SEM image of the CuO nanorod (I), (II)
and (III) attached to a tungsten tip respectively. It can be observed that while there
were some stray nanorods attached to the main nanorod for CuO nanorod (I) and (II),
the tip of the main nanorod was still more than 10µm away from the shorter nanorods
and thus, the longer CuO nanorod with length of ~30µm and diameter of ~230nm for
I and length ~25µm and diameter ~230nm for II would still be the main contributing
factor to the field emission measurements. One interesting observation made during
the field emission tests was that the CuO nanorod tends to align itself in the direction
of the applied field. This alignment usually took place around the turn-on field region.
This further suggests that the main contributing factor would be the longer nanorod.
Figure 6.1 (a) Optical image of anode and cathode, (b) SEM image of CuO nanorod (I)
on etched tungsten tip with inset at scale bar of 1 µm, (c) SEM image of CuO nanorod
(II) on etched tungsten tip with inset at scale bar of 1 µm and (d) SEM image of CuO
nanorod (III) on etched tungsten tip with inset at scale bar of 1 µm.
Figure 6.2 (a), 6.3 (a) and 6.4 (a) shows the graph of measured field emission
current (I) vs applied field (E) for various electrode distances for CuO nanorod (I), (II)
and (III) with the corresponding FN plots shown in Figure 6.2 (b), 6.3 (b) and 6.4 (b).
In determining our electric field strength, we use the voltage divided by the electrode
distance (V/d) [17]. It can be observed that as the electric field increases, the
maximum field emission current achieved increases too.
The Fowler-Nordheim (FN) theory [15, 18, 19] is useful for calculating the
enhancement factor β of single field emitter. From the FN equation, β was calculated
and plotted against the electrode distance d as shown in Figure 6.2 (c), 6.3 (c) and 6.4
(c). From the β vs d graphs, it can be observed that β decreases and approaches a
constant value as d decreases. The inset shows a linear relationship between 1/β and
1/d. It can also be noted from Figure 6.2 (d), 6.3 (d) and 6.4 (d) that the turn on field,
ETO (field needed to achieve a field emission current of 1nA) increases as the
electrode distance d decreases. These results could be explained by the fact that as the
nanorod approaches the cathode, the electric field between the tip of the nanorod and
the cathode approximates to that of a parallel plate, eliminating geometric field
enhancement thus, β decreases and the field needed to produce 1nA (ETO) increases.
As the distance between the nanorod and cathode increases, the geometry of the
nanorod comes into play and allows for geometric field enhancement and therefore, β
increases and ETO decreases.
-10
(a)
-20
313µm
297µm
281µm
266µm
250µm
234µm
(b)
-30
-40
Ln( J/E2)
Field emission current, I (nA)
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
0.0 0.5 1.0 1.5
-50
313µm
297µm
281µm
266µm
250µm
234µm
-60
-70
-80
-90
2.0 2.5 3.0 3.5 4.0 4.5 5.0
0.22
0.24
Applied field, E ( V/µm)
4.4
0.0020
0.0015
6000
5000
4.6
(c)
1/β
Enhancement factor, β
7000
4000
3000
0.0010
0.0005
0.0000
0.0032
0.0036
0.0040
0.0044
1/d ( µm-1)
2000
1000
0
220
0.28
0.30
0.32
1/E ( µm/V)
Turn on field, ETO ( V/µm)
8000
0.26
(d)
4.2
4.0
3.8
3.6
3.4
3.2
3.0
2.8
2.6
240
260
280
300
320
Electrode distance, d ( µm)
220
240
260
280
300
320
Electrode distance, d ( µm)
Figure 6.2 (a) Current vs applied field for various electrode distances (CuO nanorod
(I)), (b) corresponding FN plot (offset for clarity), (c) enhancement factor, β vs
electrode distance, d with inset showing the relationship between 1/β and 1/d, and (d)
linear relationship between ETO and d.
(a)
16
Field emission current, I (nA)
-20
375µm
359µm
344µm
328µm
313µm
297µm
281µm
266µm
250µm
14
12
10
8
6
-30
375µm
359µm
344µm
328µm
313µm
297µm
281µm
266µm
250µm
-50
-60
4
2
0
(b)
-40
Ln( J/E2)
18
-70
0
1
2
3
4
5
0.25
Applied field, E ( V/µm)
6000
(c)
4000
0.0006
0.0004
0.0002
3000
0.0025
0.0030
0.0035
0.0040
1/d ( µm-1)
2000
1000
Turn on field, ETO ( V/µm)
1/ β
Enhancement factor, β
0.0008
240 260 280 300 320 340 360 380 400
Electrode distance, d ( µm)
0.35
0.40
0.45
1/E ( µm/V)
0.0012
0.0010
5000
0.30
4.2
4.0
3.8
3.6
3.4
3.2
3.0
2.8
2.6
2.4
2.2
2.0
(d)
240 260 280 300 320 340 360 380 400
Electrode distance, d ( µm)
Figure 6.3 (a) Current vs applied field for various electrode distances (CuO nanorod
(II)), (b) corresponding FN plot (offset for clarity), (c) enhancement factor, β vs
electrode distance, d with inset showing the relationship between 1/β and 1/d, and (d)
linear relationship between ETO and d.
(a)
250µm
200µm
175µm
150µm
125µm
100µm
-20.8
-21.2
-21.6
250µm
200µm
175µm
150µm
125µm
100µm
-22.0
-22.4
-22.8
-23.2
0
4000
1
2500
2000
3
4
5
6
7
8
0.08
9 10 11 12
Applied field, E ( V/µm)
9
(c)
0.0016
8
0.0014
0.0012
3500
0.0010
1/β
3000
2
0.0008
0.0006
0.0004
0.0002
0.004
0.006
0.008
0.010
1/d ( µm-1)
1500
1000
500
80 100 120 140 160 180 200 220 240 260
Electrode distance, d ( µm)
Turn on field, ETO ( V/µm)
4500
Enhancement factor, β
(b)
-20.4
Ln( I/E2)
Field emission current, I (nA)
65
60
55
50
45
40
35
30
25
20
15
10
5
0
0.12
0.16
0.20
0.24
1/E ( µm/V)
(d)
7
6
5
4
3
2
80
120
160
200
240
Electrode distance, d ( µm)
Figure 6.4 (a) Current vs applied field for various electrode distances (CuO nanorod
(III)), (b) corresponding FN plot (offset for clarity), (c) enhancement factor, β vs
electrode distance, d with inset showing the relationship between 1/β and 1/d, and (d)
linear relationship between ETO and d.
Several models for the relationship between β and d were proposed for individual
carbon nanotube field emission. From the work of Vallance and co-workers, β and d
followed the modified Miller model where the model consists of a sphere floating
between a ground plane and a charged sphere. For this model, β decreases and
approaches unity as d becomes very small. This is because the geometry approaches
that of two opposing infinite planes when the separation is much smaller than the
radius curvature at the cathode tip and the geometric field enhancement is eliminated.
As d increases and approaches infinity, β will reach a constant value as it is then
dependent only in its geometrical properties [20].
Smith et al. suggested that β is independent of d when the anode to cathode
separation is greater than 3 times the height of the emitter away from the tip [21]. The
reason being as the anode plate moved away from the CNT tip, the parallel plate
approximation decreases and β becomes dependent on its geometrical properties
instead of the electrode separation. Bai’s group however, found a linear relationship
between β and d [22]. In all cases, the experiments were backed by simulation results.
Bonard’s group showed that β followed the Edgcombe and Valdré model saturating at
high d [23].
Possible reasons for the discrepancies between the various results could be due to
the electrode distance d or the vacuum pressure. Since the various groups conducted
the experiments under different electrode distance range, the results obtained are
limited by the range and might not be valid outside those range. Under different
vacuum pressure, the number of gas molecules between the anode and cathode differs
and the probability that an emitted electron colliding with the gas molecules differs as
well. Under such circumstances, the field emission current obtained will be different
under different vacuum levels. For Bai’s case, the d ranges from 1.2 to 46.8µm and
the experiment was carried out at around 10-7 torr. In the case for Vallance, it is from
1.4 to 13.5µm at a pressure of about 3x10-8 torr. Smith performed the experiment for d
ranging from 1 to 60µm at a pressure level of
Smith et al.
1 - 60
-5
< 10
Single CNT
Tip-Tip
Not
Bonard et al.
0.06 - 5
stated
Non linear, saturating at high
Single CNT
Tip-Tip
CNTs on Fe
Zhong et al.
700 - 5200
-8
1x10
3h (height of CNT)
tip
d
1/β linearly proportional to
Tip-Flat
1/d
Table 6.1 Studies of field emission from CNT/CNTs from various groups
From our experiments, the minimum electrode distance was 100µm with the
maximum being 375µm thus, the relationships obtained here is most accurate within
this range. Owing to our equipments’ limitations, we were unable to increase the
electrode distance further as we were unable to detect any field emission current
beyond the stated maximum distance. Attempts to perform the field emission test
under lower electrode distances have failed as the nanorod dropped off, possibly
under the influence of the high field. This was due to the weak bonding between the
glue and the nanorod thus, we were unable to detect any current below the minimum
stated electrode distance.
From the graphs of β vs d, it is clear that as d decreases, β decreases to a constant
value. At higher d however, β is also expected to reach a constant value as it is not
physically possible for β to approach an infinitely large value as d increases. With β
approaching constant values as d decreases to 0 or increases to ∞, ETO is expected to
approach constant values at extreme ranges too.
6.4 Conclusion
In summary, single CuO nanorod on tungsten tip have been assembled using a
simple technique and this technique could be applied to assemble other types of single
nanostructures. From their field emission tests, it is observed that β reaches a constant
value as d decreases. A linear relationship between 1/β and 1/d is obtained. A linear
relationship is also obtained for ETO and d. We propose that β should reach a constant
value as d increases and that the linear relationship for ETO is limited to a certain range
i.e. ETO should eventually reach a plateau as d increases or decreases. This study
provides an understanding on the field emission properties of single metal-oxide
nanorod.
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Chapter 7 – Conclusions
In the first part of the project, vertically oriented CuO nanowires were
synthesized by heating a metallic copper on a hotplate in ambient air. It is found that
control over the density, length and diameter can be achieved by changing the growth
duration or temperature. From the field emission studies of the as-grown CuO
nanowires with various physical properties, samples with higher nanowire density and
aspect ratio gives the best field emission properties. After the studies on the field
emission properties of the as-grown CuO nanowires, focused laser beam patterning
was employed to create large scale micro-patterns in the samples. After the 1st laser
patterning where micro-platforms are patterned onto the samples, there is a significant
improvement in the field emission current density. This is a result of having more
emission sites when the exposed sides of the nanowires along the edges of the
platforms contribute to the field emission. The 2nd laser patterning process created
micro-squares on the 2 samples with highest nanowire density. There is no significant
change in the field emission current density for the samples after the 2nd laser
patterning process. This suggests that the increase in field emission performance
caused by more exposed edges of the micro-square platforms were offset by the
reduction in effective emitters such as long nanowires that were truncated in the
process. This study shows that good control over the physical properties of the
nanowires can be achieved by changing the growth duration or temperature and large
scale laser patterning is a feasible method to improve the field emission properties of
nanowires films
In the second part of the project, we have synthesized a hybrid nanosystem
consisting of CuO and CuCO3 nanostructures. Hybrid CuO nanostructures with
CuCO3 nanoparticles were grown by simple heating of copper-coated carbon paper in
ambient air. Various techniques were used to characterize the samples and the relative
concentration of CuCO3 nanoparticles formed can be tuned by varying the thickness
of copper on carbon paper. 5 different thickness of copper were coated onto the
carbon paper namely: 200nm, 400nm, 1800nm, 2400nm and 4800nm. In general, the
concentration of CuCO3 increases as the thickness of the copper decreases until it
reaches a maximum at around 400nm and decreases below that. Field emission
studies show that the optimum field emission properties, superior to many common
field emitters were from the samples with 400nm thick of copper coating and the
worst performers were from the 4800nm coated samples. This suggests that samples
with the highest concentration of CuCO3 are the best field emitters while samples
with the least CuCO3 concentration are the worst. UPS measurements samples with
400nm and 4800nm revealed that samples with 400nm of copper coating has a lower
workfunction than that of 4800nm and from Fowler-Nordheim (FN) analysis, the
superior field emission properties of the 400nm coated samples could also be
attributed to the high enhancement factor and large effective emission area. This is
due to the lower work function of CuCO3 nanoparticles and could also be due to the
higher number of defects in these nanoparticles. The higher number of defects present
in CuCO3 could also account for the increase in the field emitting area for samples
with higher CuCO3 concentration. From this study, we have successfully synthesized
hybrid CuO-CuCO3 nanosystems by simple heating of copper-coated carbon paper in
ambient air and the relative concentrations can be varied by the thickness of copper on
carbon paper. Field emission tests reveal these hybrid nanosystems as promising field
emitters and potential candidates for future generation field emission devices.
In the final part of the project, the field emission properties of single CuO
nanorod were investigated. A simple method was used to attach a single CuO nanorod
onto an etched tungsten tip. From the field emission studies of the 3 batches of single
CuO nanorod, it was observed that as the electric field increases, the maximum field
emission current achieved increases. From the FN equation, β is calculated and
plotted against the electrode distance d and it can be observed that β decreases and
approaches a constant value as d decreases. A linear relationship of 1/β vs 1/d was
obtained. Plotting the turn-on field ETO vs d, a linearly decreasing trend was obtained.
These observations can be explained by the fact that as the nanorod approaches the
cathode, the electric field between the tip of the nanorod and the cathode
approximates to that of a parallel plate, eliminating geometric field enhancement thus,
β decreases and the field needed to produce 1nA (ETO) increases. As the distance
between the nanorod and cathode increases, the geometry of the nanorod comes into
play and allows for geometric field enhancement and therefore, β increases and ETO
decreases.
For future experiments, it is worthwhile to investigate if the direct heating of
various metal coated carbon papers would give rise to interesting hybrid nanosystems
with good field emission properties. In addition, the study of the field emission
properties of more single metal-oxide nanorods will provide us with a better
understanding of how β and ETO varies with d for various materials and if a general
trend can be observed.
[...]... d 71 List of Tables 4.1 Growth conditions and physical properties of the various CuO nanorods films 38 4.2 Field emission properties of the various CuO nanorods before and after laser patterning 46 4.3 Enhancement factor and area factor of the various CuO nanorods before and after laser patterning 46 5.1 Turn-on field and threshold field for various field emitters 60 6.1 Studies of field emission from... relative concentration of CuCO3 can also be varied by changing the thickness of copper on carbon paper Field emission properties of the nanosystems show the emission turn-on field and current can be tuned by the coating thickness of copper Their field emission properties compared with other common field emitters will also be presented Nanorod films are highly suitable for field emission applications... 133107 (2006) Chapter 2 – Theory of field emission; Fowler Nordheim theory Field emission is defined as the emissions of electrons from the surface of a condensed phase e.g metal into another phase e.g vacuum [1] To achieve a field emission, a potential difference is applied across the sample giving rise to an external field This applied field distorts the potential of the sample enabling unexcited... ratio, capable of enhancing the local field [19] Other reasons for application in field emission also included the increased in emission area due to the fact that the side of the nanotubes and nanorods are able to field emit and the formation of unique forms of chemical compounds in the nanostructures that serve as good emitter [20-25] Apart from researching on CNTs in the area of field emission application,... color region showing the molten state of Cu 3.2 Field Emission measurement setup A dedicated field emission measurement system was setup and utilized in this work Figure 3.3 shows a schematic of the field emission measurement system It consists of a vacuum system with a main chamber that houses the sample In order to accurately measure the field emission current density of the nanorod samples, the tests... implies that the effective field F is greater than the average field This is an advantage of using nanowires as a field emitter; the same electric field applied will result in a higher effective field thereby increasing the field emission current density Since β is mainly related to the geometry of the nanostructure, it will be dependent on the morphology, length, l and diameter, d of the nanostructure [10]... nanorod films While a single nanomaterial system may show potential as a field emitter, hybrid nanosystems combining the properties of two or more different types of nanostructures could further enhance the field emission properties and allow for the tuning of field emission properties by varying the relative percentage of the individual nanomaterials in the hybrids In recent years, hybrid nanosystems such... was capable of supplying a voltage of 0-1100V and measuring the field emission current of up to 5 decimal places at the same time A PC system was utilized to interface with the Keithley 237 for automated instrument control and data acquisition The field emission measurement setup achieved a high level of vacuum with the help of a mechanical pump and a turbo pump (Figure 3.4) After 24 hours of pumping... in black color in the diagram Figure 3.4 Field emission measurement setup with emphasis on the vacuum system 3.3 Measurement of Field Emission from Single Nanorod In order to carry out measurement of the field emission from a single CuO nanorod, we adopted the following procedure to first secure a single nanorod onto a sharp tungsten tip and then conduct field emission measurement from the assembled... nanostructure field emitters Chapter 3 details the experimental procedures which includes the field emission setup, the various characterization and patterning tools Chapter 4 presents the large scale laser patterning of CuO nanorod films and the effect on the field emission properties of the samples Chapter 5 shows a simple way of growing hybrid CuO-CuCO3 nanosystems with potential as field emitters .. .FIELD EMISSION STUDIES OF NANOMATERIALS Submitted by: TEO CHOON HOONG (B.Sc (Hons), NUS) Supervisor: A/PROF SOW CHORNG HAUR A THESIS SUBMITTED FOR THE DEGREE OF MASTERS OF SCIENCE... factor and area factor of the various CuO nanorods before and after laser patterning 46 5.1 Turn-on field and threshold field for various field emitters 60 6.1 Studies of field emission from CNT/CNTs... of two or more different types of nanostructures could further enhance the field emission properties and allow for the tuning of field emission properties by varying the relative percentage of