Suppressed screening effects in curvilinear tetrahedral diamond field emitter arrays fabricated on anodic aluminum oxide

Article abstract of DOI:10.1149/2.011201jes

Curvilineary tetrahedral diamond arrays of nano-tip with different pitches are firstly fabricated and the suppressed screening effects are also investigated. The curvilinear tetrahedral diamond arrays are fabricated by hot-filament chemical vapor deposition (HFCVD) on anodic aluminum oxide (AAO) template. The AAO template is fabricated by hard anodization. The diamond nano-particles are adhered onto the barrier layer of AAO template by Polyethylenimine pretreatment to increase the nucleation density of diamond. The diamond film is then deposited on the barrier layer of AAO by HFCVD. Curvilinear tetrahedral diamond arrays of nano-tip are developed after the removal of AAO template by wet chemical etching. The field emission effects of different diamond nano-tip arrays are measured by field emission meter. The morphology and quality of curvilinear tetrahedral diamond nano-tip array are examined by scanning electron microscopy and Raman spectroscopy, respectively. Innovative curvilinear tetrahedral structures are obtained successfully with the tip radius of 15 nm-30 nm, and the height of 100 nm-200 nm at different pitches from 50 nm to 300 nm. As a result, the lowest turn-on electric field of 5.4 V/μm and the highest field emission current density of 8.4 μA/cm ² can be obtained at the smallest pitch-height ratio of 0.25-1 without screening effect.

Full text of DOI:10.1149/2.011201jes

Journal of The Electrochemical Society, 159 (1) K1-K4 (2012)
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013-4651/2012/159(1)/K1/4/$28.00 © The Electrochemical Society
Suppressed Screening Effects in Curvilinear Tetrahedral Diamond
Field Emitter Arrays Fabricated on Anodic Aluminum Oxide
z
Hung-Yin Tsai and Chih-Cheng Yeh
Department of Power Mechanical Engineering, National Tsing Hua University, Hsinchu 300, Taiwan R.O.C.
Curvilineary tetrahedral diamond arrays of nano-tip with different pitches are firstly fabricated and the suppressed screening effects
are also investigated. The curvilinear tetrahedral diamond arrays are fabricated by hot-filament chemical vapor deposition (HFCVD)
on anodic aluminum oxide (AAO) template. The AAO template is fabricated by hard anodization. The diamond nano-particles are
adhered onto the barrier layer of AAO template by Polyethylenimine pretreatment to increase the nucleation density of diamond.
The diamond film is then deposited on the barrier layer of AAO by HFCVD. Curvilinear tetrahedral diamond arrays of nano-tip
are developed after the removal of AAO template by wet chemical etching. The field emission effects of different diamond nano-tip
arrays are measured by field emission meter. The morphology and quality of curvilinear tetrahedral diamond nano-tip array are
examined by scanning electron microscopy and Raman spectroscopy, respectively. Innovative curvilinear tetrahedral structures are
obtained successfully with the tip radius of 15 nm-30 nm, and the height of 100 nm-200 nm at different pitches from 50 nm to
2
3
00 nm. As a result, the lowest turn-on electric field of 5.4 V/μm and the highest field emission current density of 8.4 μA/cm can
be obtained at the smallest pitch-height ratio of 0.25-1 without screening effect.
2011 The Electrochemical Society. [DOI: 10.1149/2.011201jes] All rights reserved.
©
Manuscript submitted July 11, 2011; revised manuscript received September 20, 2011. Published December 6, 2011.
Diamond is a suitable kind of material for field emission thanks
to its material properties such as mechanical hardness, chemical in-
ertness, and negative electron affinity (NEA). He et al. indicated that
the electric field at apex of emitter can be dramatically enhanced by
decreasing the size of the emitter apex.
acid and ethanol solution (1:4 volumetric ratio) for 5 minutes to re-
duce the surface roughness and scratches on the aluminum sheet.
The aluminum sheet was placed in a holder with circular opening of
2 cm in diameter that can limit the reaction area. Prior to hard anodiza-
tion, the protection layer was formed on the sample using moderate
1
11
The tetrahedral diamond nano-tips of curvilinear profile were
formed by hot-filament chemical vapor deposition (HFCVD) on the
barrier layer of an anodic aluminum oxide (AAO) template. AAO is
a self-assembling template with regular nano-pore array on the top
and barrier layer at the bottom. The pore array has been commonly
anodization to stabilize the reaction. The conditions of moderate
anodization are 40 V for 10 minutes in 0.3 M oxalic acid and 25 V
for 20 minute in sulfuric acid. Then the voltage was slowly increased
to a higher constant voltage for hard anodization. Hard anodization
results in high temperature on the sample surface and may cause the
sample burnt. Therefore, a powerful low temperature bath of special
design with vigorous stirring and addition of ethanol into electrolyte
2
used as a template to fabricate structures such as nano-rod array and
3
nano-dot array, while the barrier layer was rarely applied to fabricate
12
structures. In this study, the use of the barrier layer as a template to
fabricate the innovative tetrahedral diamond nano-tips of curvilinear
profile was unprecedentedly proposed for the first time.
as coolant was used. The temperature of electrolyte can be lowered
◦
to −5 C to maintain the stability at high-field anodization. After hard
anodization was finished, the aluminum substrate was removed by a
mixture of copper dichloride and hydrochloric acid (1:4 volumetric
ratio); thereupon, the freestanding barrier layer of the AAO template
was obtained.
An AAO template fabricated by hard anodization has many ad-
vantages over conventional moderate anodization. Hard anodization
4
at high voltage (over self assembling voltage ) results in high growth
5
rate (30–50 μm/h) and better regularity than moderate anodization.
The diamond film was then deposited on the barrier layer of the
AAO template by HFCVD method. Since growing diamond onto
alumina is heteroepitaxial, it is hard to nucleate on the AAO template
directly. To enhance the nucleation density of diamond, diamond nano-
powders (4–12 nm in diameter) were adhered to the barrier layer of the
AAO template by Polyethylenimine (PEI) pretreatment as diamond
nucleation sites. Method of PEI treatment is described as follows: the
AAO template was dipped into PEI solution for an hour; then the
AAO template was dipped into the mixture of diamond nano-powders
(4–12 nm in diameter) and water for an hour. The diamond nano-
powder/water mixture is 0.001 g/ml. Before the AAO template was
dipped into the mixture, diamond nano-powders had been uniformly
dispersed in water by ultrasonic vibration.
The screening effect of field emission is induced by high density
emitter array, which retards the emission of electrons from emitters
due to the interaction of electric field between emitters. The screening
effect can be diminished by increasing the pitch of emitters; however,
increasing the pitch also decreases the number of emitters per unit
area, which reduces the emission current density. To obtain better
field emission result, it is important to optimize the distance between
emitters. Carbon nanotubes are the most popular material as field
emitters. There have been many researches on increasing field emis-
sion current of carbon nanotubes by optimizing the intertube distance;
however, the screening effect cannot be avoided.6–8 Wang et al. fab-
ricated conical diamond tips arrays with the apex radius of 100 nm
and discussed the effect of emitter density on the field-screening. Tsai
9
The HFCVD process was carried out using a mixture of 2%
methane and 98% hydrogen at gas pressure of 80 torr for 5 hours.
10
et al. fabricated diamond nano-tips of a tip radius of about 20–30 nm
on porous anodic alumina by HFCVD. In the current study, we devel-
oped innovative tetrahedral diamond nano-tips of curvilinear profile
which suppress the screening effect.
◦
The tungsten filaments were heated to 2000 C, and the distance be-
tween substrate and the tungsten filaments was 10 mm. The substrate
◦
temperature was 600∼700 C. The thickness of the diamond layer af-
ter the hot-filament chemical vapor deposition was about 8 μm. After
deposition, the AAO template was removed by 6 M NaOH solution,
and the diamond nano-tip array was then developed.
Experimental
Aluminum sheet (99.5% purity, 0.5 mm thickness) was cleaned
by ultrasonic cleaner in acetone as well as de-ionized water for
The morphologyof diamond nano-tip arrays and the barrier layer of
the AAO templates were examined by field emission scanning electron
microscopy (FESEM). The quality of diamond was examined using
Raman spectroscopy with a laser wavelength of 514 nm. The field
emission properties were measured in a vacuum chamber that was
1
0 minutes sequentially and then dried by nitrogen gas. After cleaning,
◦
the aluminum was electropolished at 10 C and 15 V in a perchloric
z
−6
E-mail: hytsai@pme.nthu.edu.tw
maintained at 5 ×10 torr. Samples were placed at cathode, and the
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Journal of The Electrochemical Society, 159 (1) K1-K4 (2012)
Figure 3. FESEM image shows a curvilinear tetrahedral diamond tip, and the
apex is about 15–30 nm in radius. (The scale bar is 100 nm.)
The average size of cells is about 50 nm and 300 nm at 45 V and
5
5 V, respectively.
The surface of diamond nano-powders has a negative charge in
water or in organic solvents, and the PEI is a polymer with posi-
tive charges. Dipping an AAO template into PEI solution made the
template surface positively charged. Diamond nano-powders were
adhered on the surface of the barrier layer after immersing the AAO
13
Figure 1. FESEM images show the morphology of the barrier layer after
anodization in (a) 0.3 M oxalic acid at 80 V; (b) 0.3 M oxalic acid at 100 V;
template into diamond solution.
The diamond nano-powders were adhered to the barrier layer of
the AAO template by PEI pretreatment that increased the nucleation
density of diamond. Figure 2a is the FESEM image of the barrier layer
of the AAO template on which diamond nucleates without pretreat-
ment, and Figure 2b is the result with PEI pretreatment. The nucleation
experiment was carried out at the condition described in experimental
section for 1 hour so that the state of nucleation could be observed.
As examined, diamond is hard to nucleate on the AAO template di-
rectly without pretreatment. The nucleation density of diamond was
increased after the PEI pretreatment, and a continuous film can be ob-
tained by further CVD deposition. PEI pretreatment does not damage
the sample morphology but improves the nucleation density.
The fabricated diamond nano-tip using the barrier layer of the
AAO template is shown in Fig. 3. The shape of tip is a curvilinear
tetrahedron with the height of about 100–200 nm and the apex of
about 15–30 nm in radius. Figure 4 shows the diamond nano-tip with
different pitches from 50-300 nm. There is some debris on the sample
surface which results from incomplete removal of AAO template.
Figure 5 shows the Raman spectrum of diamond nano-tip arrays. The
(c) 0.3 M oxalic acid at 120 V; (d) 0.3 M oxalic acid at 140 V; (e) 0.3 M sulfuric
acid at 45 V;(f) 0.3 M sulfuric acid at 55 V. (The scale bars are 1 μm.)
distance between two electrodes was 70 μm. The area of the anode
was 1.5 cm × 0.5 cm and the field emission effect was an average
result within this area. The applied voltage was varied in the range of
0–1100 V.
Results and Discussion
In this study, we varied the pitch size of the diamond nano-tip
array by changing the barrier layer morphology of AAO templates.
The applied voltages in 0.3 M oxalic acid were 80 V, 100 V, 120 V,
and 140 V. The applied voltages in 0.3 M sulfuric acid were 45 V and
5
5 V. Because the low current density during reaction in the 0.3 M
oxalic acid at 80 V made the growth rate lower than that at other
conditions, the time of anodization was extended to 5 hours. The rest
samples were treated for 1 hour. The thickness of AAO templates
ranges from 30 to 50 μm.
−
1
−1
peaks at 1332 cm and 1580 cm represent first order diamond
band and graphite band, respectively. The shape of Raman spectrum
is similar to all six kinds of structures. The diamond peaks are 1334-
Figure 1a–1d show FESEM pictures of the morphology of the
barrier layer formed by 0.3 M oxalic acid at different voltages (80 V,
−
1
−1
1339 cm , a little above the value of 1332 cm , which indicates that
the diamond structures are under compressive stress. The full width
at half maximum (FWHM) of the diamond peak represents the quality
14
1
00 V, 120 V, and 140 V). The average size of cells is about 100 nm
at 80 V to 300 nm at 140 V. Figure 1e and 1f are the barrier layer
formed by 0.3 M sulfuric acid at different voltages (45 V and 55 V).
−
1
of diamond. Rather than about 2 cm in nature diamond, the FWHM
Figure 2. FESEM images of diamond nucleation on
AAO templates (a) without pretreatment; (b) with
PEI pretreatment. (The scale bars are 10 μm.)
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Journal of The Electrochemical Society, 159 (1) K1-K4 (2012)
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Figure 4. FESEM images of diamond nano-tip arrays made by different AAO
templates: (a) 0.3 M oxalic acid at 80 V; the pitch size is 160–260 nm;
b) 0.3 M oxalic acid at 100 V; the pitch size is 160–260 nm; (c) 0.3 M
oxalic acid at 120 V; the pitch size is 180–280 nm; (d) 0.3 M oxalic acid at
40 V; the pitch size is 200–300 nm; (e) 0.3 M sulfuric acid at 45 V, the pitch
size is 50–100 nm;(f) 0.3 M sulfuric acid at 55 V, the pitch size is 80–130 nm.
The scale bars are 1 μm.)
(
1
(
−
1
in our diamond structures are 70–80 cm which suggests possible
15
graphite and disordered structure at grain boundary of diamond film.
In the current study, field emission characteristics of diamond
nano-tip arrays with different pitches are to be investigated. To com-
pare the field emission characteristics of diamond nano-tip arrays,
diamond tip arrays with three different pitch-height ratios (0.25–1,
0
.5–2, 1–3) are fabricated. The conditions of our samples are shown
in Table I. Figure 6 shows the relationship between the emission cur-
rent density and the applied electric field for three kinds of pitch-height
Figure 6. The relationship between emission current density and applied
electric field for three kinds of pitch-height ratios.(a)0.25–1; (b) 0.5–2;
(c)1–3.The inset shows Fowler-Nordheim plot.
Figure 5. Raman spectrum of diamond nano-tip arrays formed by different
AAO templates.
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Journal of The Electrochemical Society, 159 (1) K1-K4 (2012)
Table I. The conditions of samples.
Pitch size
(nm)
Tip height
(nm)
Turn on electric field
pitch- height ratio
Templates
(V/μm)
0.25∼1
0.5∼2
1∼3
0.3 M sulfuric at acid 45 V
0.3 M oxalic acid at 80 V
0.3 M oxalic acid at 140 V
50∼100
100∼200
200∼300
100∼200
100∼200
100∼200
5.4
10.1
11.1
ratios. The 1st, 2nd, and 3rd in this figure mean the measurement has
been proceeded for three times at different points on the same sam-
ple. The turn-on electric field is defined as the electric fields required
to produce the current density of 0.1 μA/cm . The turn-on electric
field of pitch-height ratios of 0.25–1, 0.5–2 and 1–3 are 5.4 V/μm,
linear profile with the smallest pitch-height ratio of 0.25–1 and no
screening effect is shown in this study. The turn-on electric field is
5.4 V/μm with the field emission current density of 8.4 μA/cm at
the applied electric field of 9.5 V/μm. The screening effect is reduced
since the tetrahedral diamond nano-tips with curvilinear profile make
the equipotential lines much smooth from the top to the bottom of the
tips with the profile of the curvilinear tetrahedron. The curved mor-
phology of diamond nano-tip arrays has further potential to highly
reduce the screening effect and provide large emission current; it can
be used as a cold cathode for practical applications in the near future.
2
2
1
0.1 V/μm and 11.1 V/μm, respectively. The best emission current
2
density of 8.4 /μA/cm at the applied electric field of 9.5 V/μm can
be obtained from the tip array with the smallest pitch-height ratio. The
6–8
result is contrary to the conventional prediction for CNT or conical
9
diamond with needle-like emitters that the tip array with smaller pitch
may lead to stronger screening effect and the field emission current
should be reduced. We suggest that the screening effect can be re-
duced since the tetrahedral diamond nano-tips with curvilinear profile
make the equipotential lines much smooth from the top to the bottom
of the tips with the profile of the curvilinear tetrahedron. The results
agree with the simulated equipotential lines with different shapes of
References
1. J. He, P. H. Cutler, and N. M. Miskovsky, Appl. Phys. Lett., 59, 1644 (1991).
2
3
4
5
6
. Z. F. Zhou, Y. C. Zhou, Y. Pan, and X. G. Wang, Mater. Lett., 62, 3419 (2008).
. H. Masuda and M. Satoh, Jpn. J. Appl. Phys., 35, L126 (1996).
. S. Ono, M. Saito, M. Ishiguro, and H. Asoh, J. Electrochem. Soc., 151, B473 (2004).
. W. Lee, R. Ji, U. Gosele, and K. Nielsch, Nature Mater., 5, 741 (2006).
. L. Nilsson, O. Groening, C. Emmenegger, O. Kuettel, E. Schaller, and L. Schlapbach,
Appl. Phys. Lett., 76, 2071 (2000).
16
emitters by Wu et al.
Conclusions
7. J. S. Suh, K. S. Jeong, J. S. Lee, and I. T. Han, Appl. Phys. Lett., 80, 2392 (2002).
8
9
. R. C. Smith and S. R. P. Silva, Appl. Phys. Lett., 94, 133104 (2009).
. Z. L. Wang, Q. Luo, J. J. Li, Q. Wang, P. Xu, Z. Cui, and C. Z. Gu, Diamond Relat.
Mater., 15, 631 (2006).
0. H. Y. Tsai, H. C. Liu, J. H. Chen, and C. C. Yeh, Nanotechnology, 22, 235301 (2011).
1. K. Schwirn, W. Lee, R. Hillebrand, M. Steinhart, K. Nielsch, and U. G o¨ sele K, ACS
Nano, 2, 302 (2008).
2. Y. Li, M. Zheng, L. Ma, and W. Shen, Nanotechnology, 17, 5101 (2006).
13. J. J. Adair and R. K. Singh, U.S. Pat. 5485804 (1996).
4. G. W. Bak, K. Fabisiak, L. Klimek, M. Kozanecki, and E. Staryga, Opt. Mater., 30,
The diamond nano-tips with innovative tetrahedral structures of
curvilinear profile are firstly obtained from the barrier layer of the
AAO template. The AAO templates are fabricated using hard an-
odization that provides faster growth rate and more regularized struc-
ture than conventional method. PEI is used to adhere diamond nano-
powders as nucleation sites on the barrier layer of the AAO template
and to help deposit diamond film on alumina directly. We have fab-
ricated different tip arrays of pitch sizes from 50 nm to 300 nm with
tip radius of 15 nm-30 nm, and height of 100 nm-200 nm. The best
result is obtained from the tetrahedral diamond nano-tips of curvi-
1
1
1
1
770 (2008).
1
1
5. Q. Hu, R. K. Joshi, and A. Kumar, Appl. Surf. Sci., 256, 6233 (2010).
6. H. C. Wu, H. Y. Tsai, H. T. Chiu, and C. Y. Lee, ACS Appl. Mater. Interfaces, 2, 3285
(2010).
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