The field emission properties of high aspect ratio diamond nanocone arrays fabricated by focused ion beam milling

Abstract

High aspect ratio diamond nanocone arrays are formed on freestanding diamond film by means of focused ion beam (FIB) milling technology and hot-filament chemical vapor deposition (HFCVD) method. The structure and phase purity of an individual diamond nanocone are characterized by scanning electron microscopy (SEM) and micro-Raman spectroscopy. The result indicates that the diamond cones with high aspect ratio and small tip apex radius can be obtained by optimizing the parameters of FIB milling and diamond growth. The diamond nanocone arrays were also used to study the electron field emission properties and electric field shielding effect, finding high emission current density, low threshold and weak shielding effect, all attributable to the high field enhancement factor and suitable cone density of the diamond nanocone emitter.

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The field emission properties of high aspect ratio diamond nanocone arrays fabricated by
focused ion beam milling
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2005 Sci. Technol. Adv. Mater. 6 799
(http://iopscience.iop.org/1468-6996/6/7/A16)
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The field emission properties of high aspect ratio diamond nanocone
arrays fabricated by focused ion beam milling
Z.L. Wang, Q. Wang, H.J. Li, J.J. Li, P. Xu, Q. Luo, A.Z. Jin, H.F. Yang, C.Z. Gu
*
Laboratory of Microfabrication, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China
Received 14 March 2005; revised 15 June 2005; accepted 15 June 2005
Available online 13 September 2005
Abstract
High aspect ratio diamond nanocone arrays are formed on freestanding diamond film by means of focused ion beam (FIB) milling
technology and hot-filament chemical vapor deposition (HFCVD) method. The structure and phase purity of an individual diamond
nanocone are characterized by scanning electron microscopy (SEM) and micro-Raman spectroscopy. The result indicates that the diamond
cones with high aspect ratio and small tip apex radius can be obtained by optimizing the parameters of FIB milling and diamond growth. The
diamond nanocone arrays were also used to study the electron field emission properties and electric field shielding effect, finding high
emission current density, low threshold and weak shielding effect, all attributable to the high field enhancement factor and suitable cone
density of the diamond nanocone emitter.
q2005 Elsevier Ltd. All rights reserved.
Keywords: Diamond nanocone arrays; FIB milling; Electron emission; Shielding effect
1. Introduction
The outstanding physical and chemical properties of
diamond, such as highest hardness, outstanding chemical
inertness and highest thermal conductivity, etc. make it an
excellent candidate for both mechanical and electronic
applications [1]. Furthermore, diamond as a material with
negative electron affinity is suitable for electron field
emission and display device applications. Electron field
emission from diamond has been intensely studied for the
last decade and observed to yield high emission current at a
low applied electric field [2–4]. However, most reported
diamond emitters are comprised of planar, irregular ion-
etched diamond film or non-uniformly diamond coated
silicon tips with low aspect ratio. This non-uniformity of
emitter microstructures leads to inconsistent or poor
emission behavior and long-term instability. It is known
that a good candidate for field emitter arrays should be a
structure of high aspect ratio and appropriate density to
gain enhanced field emission. Yet too much emitter density
is disadvantageous for improvement of emission because
of the shielding effect resulting from the proximity of
emitters, so the emitter with controllable density is required
[5]. Focused ion beam (FIB) milling technology is
convenient for controlling the density of patterns and
producing a hole geometry of high aspect ratio, and hence
diamond nanocone arrays with high aspect ratio and
controllable density can be achieved by filling chemical
vapor deposition (CVD) diamond grains in the FIB-milled
patterns, which can be expected to exhibit enhanced field
emission properties.
In this work, we report a novel method to obtain
enhanced field emission from high aspect ratio diamond
nanocone arrays with controlled density on a freestanding
diamond film. The diamond nanocone arrays are fabricated
via FIB milling and hot-filament chemical vapor deposition
(HFCVD) technologies on silicon substrate. The control-
lable aspect ratio and density of nanocone arrays can be
achieved by changing the pitches of milled hole patterns and
the parameters of ion beam. The electron field emission
from diamond nanocone arrays was studied and compared
with that from the back surface of diamond film, and the
electric field shielding effect induced by the proximity of
emitters is also discussed.
Science and Technology of Advanced Materials 6 (2005) 799–803
www.elsevier.com/locate/stam
1468-6996/$ - see front matter q2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.stam.2005.06.018
*
Corresponding author. Tel.: C86 10 82648197; fax: C86 10 82648198.
E-mail address: czgu@aphy.iphy.ac.cn (C.Z. Gu).
The STAM archive is now available from the IOP Publishing website http://www.iop.org/journals/STAM

2. Experimental
For fabricating diamond nanocone arrays, first, a silicon
mold with high aspect ratio, inverted conical-shape hole
patterns was formed by FIB milling method. The
commercial FEI DB235 FIB system can produce a focused
Ga
C
-ion beam with a maximum energy of 30 keV and a
beam current of 1 pA-5 nA. The diameter, depth and pitch
of the hole pattern can be preset by a special software in this
system. Before FIB milling, a poly-methylmethacrylate
(PMMA) of 0.5 mm thickness was spin-coated on the silicon
wafer serving as a protective layer to prevent sputtered
silicon debris from re-deposition at the edge of etched holes
during FIB milling, and avoid any influence of the silicon
debris on the subsequent growth of diamond nanocones.
Second, the protective layer was dissolved in acetone and
the Si was thoroughly cleaned in ethanol, and the patterned
silicon substrate was then ultrasonically pretreated in an
ethanol solution containing nanodiamond powder in order
to enhance diamond nucleation and improve diamond film
growth on the patterned Si substrate. Third, diamond film
was deposited on the patterned silicon substrate by HFCVD,
the reaction gas is a mixture of CH
4
and H
2
, and the CH
4
concentration is 1.5%, the total pressure is 16 Torr. A
tantalum filament with diameter of 0.5 mm is heated to
2200 8C. The distance between the filaments and substrate is
10 mm. The substrate temperature of 850 8C is measured by
a thermocouple mounted on the substrate. The rate of
deposition for diamond film is 1 mm/h under the above
parameters. After deposition for 6 h, the FIB milled holes
were fully filled with diamond grains and formed a diamond
film of about 6 mm-thickness on the silicon surface. A
further step was to turn the diamond deposited wafer upside
down and glue it to another silicon wafer using conductive
epoxy. Finally, the diamond nanocones arrays, with precise
replications of the hole geometry, are formed on the back
surface of a freestanding diamond film after removing the
silicon substrate by wet chemical etching in a mixed acid
solution of HNO
3
and HF (1:3).
In the following we concentrate on the characterization
of the diamond nanocones and the detailed discussion of
their field emission properties. The detailed process
optimization and growth mechanism of diamond nanocones
in milled holes have been previously reported elsewhere [6].
The morphologies and microstructures of diamond nano-
cone arrays on the back surface of a freestanding diamond
film were characterized by scanning electron microscopy
(SEM), and micro-Raman spectroscopy was used to obtain
the phase purity information of an individual diamond cone.
The field emission characteristics were measured in a high
vacuum chamber with the basic pressure of better than 1!
10
K6
Pa. The distance between the anode, which is
sputtered indium tin oxide (ITO) glass, and the cathode of
diamond nanocone arrays was held at approximately
200 mm throughout the measurements.
3. Results and discusses
By optimizing the FIB milling conditions such as beam
current, initial diameter and depth of the hole, sputtering
dose and scan scheme, high aspect ratio inverted conical
shape hole patterns with controllable pitch can be achieved,
and hence high aspect ratio diamond nanocone arrays with
controllable pitch can be precisely replicated by filling
HFCVD diamond grains in the holes and forming an
diamond film that is freestanding after removing the silicon
substrate and bears the cones. Fig. 1(a) shows an SEM
micrograph of diamond nanocone arrays with a pitch of
20 mm. The total number of cones is about 6400 in the area
of 1.6 mm!1.6 mm. For each diamond cone, as shown in
the Fig. 1(b), the apex radius is about 100 nm, the base
radius is about 1 mm and the height is measured to be about
9mm. In order to obtain information about a diamond
nanocone’s phase purity, micro-Raman spectroscopy was
employed to characterize an individual cone. The detected
Raman spectrum is shown in Fig. 2, where the first order
diamond line at 1332 cm
K1
and a broad peak at about
1500 cm
K1
for sp
2
bonded carbon [7] were also detected.
Fig. 1. The SEM images of (a) diamond nanocone arrays on the surface of a
freestanding diamond film, which is used to measure field emission
properties. It has the array pitch of 20 mm and the total number of cones is
about 6400, and (b) one diamond nanocone in the array, which indicates
that the apex radius is estimated to be as small as 100 nm, the base radius is
about 1.0 mm and the height is measured to be about 9 mm.
Z.L. Wang et al. / Science and Technology of Advanced Materials 6 (2005) 799–803800

The moderate sp
2
contents of the diamond nanocone may
originate mainly from the high density of grain boundaries
and impurity at the interface between the diamond film and
the silicon substrate during the early stage of film growth.
In order to evaluate the field emission properties from
diamond nanocone arrays, the field electron emissions from
diamond nanocone arrays on a freestanding diamond film
and the back surface of diamond film were measured and
compared. The as-deposited diamond nanocone arrays
shown in Fig. 1(a) were used for the study of field emission.
At the same time, a sample of diamond film deposited on
silicon substrate without patterned holes was also prepared
for studying the field emission from the back surface of
diamond film. As a result, an enhanced field emission from
diamond nanocone arrays was observed. The current versus
electric field (I–E) curves and Flower–Nordheim (F–N)
plots of field emissions from the diamond nanocone arrays
and the back surface of diamond film are shown in Fig. 3(a)
and (b), respectively. For the diamond nanocone arrays, the
emission current increased rapidly at an applied voltage of
about 2 V/mm and reached 88 mAat15V/mm. For the back
surface of diamond film, the emission characteristic shifted
to the high voltage region, the emission current began to
increase gradually at about 9 V/mm and only reached 10 mA
at 15 V/mm. So the resulting emission current from as-
formed cones is about 78 mA (88–10 mA) at 15 V/mm, and
hence the average emission current from an individual
diamond cone is estimated to be about 13 nA. As shown in
the characterization from SEM, the apex radius of the cone
is about 60 nm. Therefore, the average emission current
density per cone was about 5.8!10
4
mA/cm
2
if the
majority of the electrons is considered to be emitted from
the hemispherical apex of the cone [8]. It is obvious higher
than the emission current density of w5!10
2
mA/cm
2
at
16.5 V/mm from carbon nanotube (CNT) emitters, which
suggests that the diamond nanocone can be a electron source
with high brightness [9–11]. The results indicated that
diamond nanocone arrays on a freestanding diamond film
have a greatly enhanced field emission property compared
with that of the back surface of diamond film. This can be
attributed to the enhancement of electron emission due to
the increase of sp
2
bonded carbon on the surface of diamond
nanocone [12], and besides, the cone structure of diamond
nanocone arrays, such as the high aspect ratio that can be
calculated from the ratio of height (9 mm) and apex radius
(100 nm) of a cone and the appropriate density of diamond
nanocone arrays, which improves the field enhancement
factor of diamond nanocone emitters. In the following we
will discuss the enhanced electron field emission due to the
high aspect ratio of as-deposited cone arrays, and the
shielding effect is also discussed therein.
It is known that the field enhancement factor bis a key
parameter, which reflects the enhanced electron emission
due to the localized electronic states by the geometrical
configuration of the emitters. In theoretical case, bcan be
expressed as h/r, where his the height of emitter and ris the
tip radius of it. Yet in fact, the value of bcan be modified by
other effects such as shielding effect due to the proximity of
emitters. So we have to introduce an effective field
enhancement factor b
eff
to describe the actual field
enhancement, which can be obtained from the measured
1000 1100 1200 1300 1400 1500 1600 1700
Raman Intensity (a.u.)
Wavenumber(cm-1)
1500cm–1
1332cm–1
Fig. 2. Micro-Raman spectrum from the surface of a single diamond cone.
0
8 10121416642
20
40
60
80
100
(a)
Current(µA)
Electric field (V/µm)
diamond cones arrays
backsurface of diamond film
0.06 0.12 0.18 0.24 0.30 0.36 0.42
–4.8
–4.0
–3.2
–2.4
–1.6
–0.8
(b) diamond cones array
backsurface of diamond
Ln(Current/Electric Field2)
1/Electric field (µm/V)
Fig. 3. (a) The curves of current versus electric field (I–E) curves and (b) the
Flower–Nordheim (F–N) plots of field emission from the diamond cones
array and the back surface of diamond film.
Z.L. Wang et al. / Science and Technology of Advanced Materials 6 (2005) 799–803 801

slope of F–N plots as shown in Fig. 3(b). Using original F–N
equation [13]:
JZ
AE2
ft2ðyÞexp KBf3=2VðyÞ
E

A
cm2;(1)
the slope of F–N plots S(E ln(J/E
2
)) can be expressed as:
SZ
KBF3=2
eff
beff
;(2)
where Bis a constant with the value 6.831!10
3
eV
K3/
2
Vmm
K1
, and the effective work function F
eff
of diamond
is 0.08 eV, as introduced in Ref. [14]. The calculated b
eff
is
about 45. For the back surface of the freestanding diamond
film, b
eff
is about 2, which shows that the back surface of
diamond film also has a certain field enhancement capability
due to the surface is not very flat.
It is well known that shielding effect is disadvantageous
for field emission, especially for densely distributed emitter
arrays. In the following we analyze the shielding effect on
the field emission of as-formed diamond cone arrays. The
local field without shielding effect can be expressed as:
Elocal Z
bV
d;(3)
where dis the distance between the cathode and anode, bis
the aspect ratio (h/r) of the as-formed emitter, and Vis the
applied voltage. While for the case that the field
enhancement is fully shielded, the local field can be
expressed as:
Elocal Z
V
d:(4)
when considering the actual case, which has to be a
compromise between those limited cases, the following
phenomenological formula can be employed, as used in Ref.
[15]:
Elocal Z
sbV
dCð1KsÞV
d;(5)
where sis a parameter describing the degree of the
screening effect, which ranges from 0 for very densely
arranged emitters to 1 for very sparsely arranged emitters.
And it is a function of the height and inter-distance of the
emitter in the arrays. From Eq. (5) we can have:
beff ZsbC1Kszsb:(6)
using b
eff
(45) calculated from F–N plots and b(90)
estimated from SEM observation, the sfor cone arrays
can be calculated to be about 0.5, which shows the shielding
effect is rather weak. For comparison, severe shielding
effect (sis about 0.06) of the field emission properties for
densely aligned carbon nanotubes has been studied in Ref.
[15]. Therefore, it can be concluded the diamond cone
density used in our experiment is quite advantageous for
decreasing shielding effect.
4. Conclusions
High aspect ratio diamond nanocone arrays with
controllable density and aspect ratio were successfully
fabricated by FIB milling and CVD diamond. With
optimized FIB milling conditions, diamond nanocones
with high aspect ratio and small tip apex radius can be
obtained. The Raman spectrum shows the dominant
diamond line at 1332 cm
K1
and a weak peak at about
1500 cm
K1
for moderate sp
2
bonded carbon. The electron
emission property from the diamond nanocone arrays on a
freestanding diamond film was studied and compared with
that from the back surface of diamond films. The results
show that the diamond nanocone arrays have obviously
enhanced emission compared with that from the back
surface of the diamond films at the same applied electric
fields. This can be attributed to the enhancement of
electron emission due to the high aspect ratio and the
increase of sp
2
bonded carbon on the surface of diamond
nanocone. Weak shielding effect resulted from the
appropriate patterning density also improves the field
enhancement factor of diamond nanocone emitters. The
fact that the applied voltage of 15 V/mm can produce the
emission current density of about 5.8!10
4
mA/cm
2
for such sparse diamond nanocone arrays suggests that
it can be a candidate electron source with super-high
brightness.
Acknowledgements
This work was supported by the State Key Development
Program for Basic Research of China (Grant No.
2002CB613500) and National Center for Nanoscience and
Technology, China.
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