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Formation of oriented nanostructures in diamond using metallic nanoparticles
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2012 Nanotechnology 23 455302
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IOP PUBLISHING
NANOTECHNOLOGY
Nanotechnology 23 (2012) 455302 (7pp)
doi:10.1088/0957-4484/23/45/455302
Formation of oriented nanostructures in
diamond using metallic nanoparticles
H-A Mehedi, C Hebert, S Ruffinatto, D Eon, F Omnes and E Gheeraert
Institut Neel, CNRS and Universite Joseph Fourier, F-38042 Grenoble Cedex 9, France
E-mail: [email protected]
Received 9 July 2012, in final form 11 September 2012
Published 22 October 2012
Online at stacks.iop.org/Nano/23/455302
Abstract
A simple, fast and cost-effective etching technique to create oriented nanostructures such as
pyramidal and cylindrical shaped nanopores in diamond membranes by self-assembled
metallic nanoparticles is proposed. In this process, a diamond film is annealed with thin
metallic layers in a hydrogen atmosphere. Carbon from the diamond surface is dissolved into
nanoparticles generated from the metal film, then evacuated in the form of hydrocarbons and,
consequently, the nanoparticles enter the crystal volume. In order to understand and optimize
the etching process, the role of different parameters such as type of catalyst (Ni, Co, Pt, and
Au), hydrogen gas, temperature and time of annealing, and microstructure of diamond
(polycrystalline and nanocrystalline) were investigated. With this technique, nanopores with
lateral sizes in the range of 10–100 nm, and as deep as about 600 nm, in diamond membranes
were produced without any need for a lithography process, which opens the opportunities for
fabricating porous diamond membranes for chemical sensing applications.
(Some figures may appear in colour only in the online journal)
1. Introduction
metal film, (2) the carbon dissolution into nanoparticles
through the breaking of crystallinity at the diamond–metal
interface [15–17] and, finally, (3) the desorption of carbon
with the assistance of a hydrogen atmosphere. Hydrogen
gets catalytically dissociated into hydrogen atoms by metal
nanoparticles and gasifies the dissolved carbon in the
particles, and the amorphous carbon at the interface forms
methane [18] or some CHx complex.
In this study, the key parameters of the etching process
were investigated: type and thickness of the metallic layer; the
annealing temperature; the hydrogen pressure; the diamond
microstructure and the annealing time. We emphasized
the etching of [100]-oriented diamond planes, since it
is shown that metal-assisted diamond etching is slowest
for [111]-oriented planes and that they act as stopping
planes [13].
Diamond thin films are now easily produced on large areas
with nanocrystalline or polycrystalline microstructures, and
various applications are under development that take into
account the exceptional properties of diamond [1–4]. Among
them, chemical sensors are of particular interest, due to the
bio-compatibility of diamond, its chemical inertness and wide
potential window [5, 6]. In most cases, the device fabrication
process involves an etching step which is performed almost
exclusively by the reactive ion etching (RIE) technique with
the help of lithographic processes [7–9]. However, in the
case of sensors based on nanopores in a membrane [10–12],
this etching technique fails to create narrow (10–50 nm in
diameter) and long (typically longer than 300 nm) holes in
the diamond membrane due to the limitation of lithographic
resolution. Based on reported works [13, 14] on catalytic
etching of diamond, we have developed a fast and simple
etching method through the catalytic gasification of carbon
by metallic nanoparticles in a hydrogen atmosphere. In
this process, the etching of diamond proceeds with: (1)
the formation of nanoparticles from the initially deposited
0957-4484/12/455302+07$33.00
2. Methods and materials
The etching technique consists of two steps, (i) the deposition
of thin metallic layers on a diamond substrate and (ii) the
subsequent annealing in a hydrogen atmosphere. Metals (Ni,
1
c 2012 IOP Publishing Ltd Printed in the UK & the USA
Nanotechnology 23 (2012) 455302
H-A Mehedi et al
Figure 1. SEM images of the (100) surface of polycrystalline diamond after 10 min of annealing with a 3 nm film of (a) Ni, (b) Co, (c) Pt
and (d) Au, (800 ◦ C, 60 Torr H2 ); the corresponding enlarged images show etch pits with flat sidewall faces and sharply defined edges for
Ni, Co, Pt and finely dispersed nanoparticles for Au.
Co, Pt and Au) with thickness in the range of 1–10 nm were
deposited on diamond substrates by e-beam evaporation, the
thickness being measured by a quartz crystal oscillator. The
annealing step was carried out in a hot-filament-assisted CVD
diamond growth reactor at 750–900 ◦ C in 60 Torr flowing
hydrogen with 100 sccm (sccm denotes cubic centimeter
per minute at STP) for between a few minutes to several
hours. The base pressure of the furnace before annealing
was <10−6 Torr. The temperature was increased at a rate
of ∼200 ◦ C min−1 and measured with an infrared pyrometer
with a reading precision of ±10 ◦ C. After the annealing step,
the samples were cooled down for 30 min under a continuous
hydrogen flow.
Two types of diamond films were used for these
experiments, namely polycrystalline diamond with a grain
size of typically 50 µm and nanocrystalline diamond (grain
size 100 nm). Polycrystalline diamond was grown by Element
Six (electrochemical grade and boron doped CVD diamond).
Nanocrystalline diamond (undoped) with a thickness of about
1.5 µm was grown on silicon substrates in a microwave
plasma CVD reactor (5200 Seki Technotron MPCVD
reactor) using the bias-enhanced nucleation method [19]
with following parameters: 900 ◦ C temperature, 1000 W
microwave power, 0.3% methane in hydrogen, and pressure
of 30 Torr.
Annealed samples were characterized using a field
emission gun scanning electron microscope (FEG-SEM;
ZEISS Ultraplus) and Raman spectroscopy (HORIBA
Scientific, Raman spectroscopy system T64000, 514 nm
excitation).
During annealing, nanoparticles were generated from the
metallic layers, prior to etching, in order to minimize the
surface energy. This effect is well known and relates to the
melting point lowering effect of thin metal films and hydrogen
diffusion into the metal [20–23]. From the SEM observation,
we estimated the diameter of the resulting nanoparticles (Ni,
Co and Pt) to be approximately fifteen times larger than the
initially evaporated metal layer thickness.
Etching was observed for Ni, Co, and Pt, figures 1(a)–(c),
forming etch pits with flat sidewall faces and sharply
defined edges. In contrast, finely dispersed Au nanoparticles
were formed, figure 1(d), and neither etch pits nor nanochannels were apparently found on the diamond surface. Ni
nanoparticles showed higher etching activity than Co and Pt.
The observed difference in etching behavior between the
metals may be caused by the combined effect of the difference
in the size and melting point of metallic nanoparticles as well
as carbon solubility into the nanoparticles at the temperatures
in our experiment. The solubilities of carbon into Ni, Co
and Pt at 900 ◦ C estimated from M–C, metal–carbon, phase
diagrams are 1 at.%, 0.8 at.%, and 0.04 at.% respectively [24].
On the other hand, unlike the iron-group elements (Ni, Co),
the carbon solubility of Au is extremely low (0.018 at.%
at 800 ◦ C in bulk phase) [25]. It has recently been reported
that when the Au particle size becomes several tens of
nanometers, it has carbon solubility depending on the nature
of the substrate and source of carbon [26]. However, this
model of carbon solubility in nanosized Au particles would
not be similar to the uptake of carbon from diamond surface.
The observation also suggests that the size of the etched
structures depends on the size of the initially formed metal
nanoparticles. Moreover, the size can be adjusted to a certain
extent by the metal layer thickness, as shown in figure 2.
Nanoholes with a lateral size of 20 ± 5 nm, figure 2(a),
and 150 ± 10 nm, figure 2(b), were achieved by etching
nanocrystalline diamond with Ni particles that originated
from 1.5 nm to 10 nm Ni films respectively.
3. Results and discussion
3.1. Effect of type and thickness of metallic layer
In order to study the effect of metal type on the etching
process, Ni, Co, Pt, and Au with a thickness of 3 nm were
evaporated on polycrystalline diamond and annealed at 800 ◦ C
for 10 min. Figure 1 shows the SEM images of the (100)
surface of annealed polycrystalline diamond.
3.2. Effect of annealing temperature
To understand the influence of annealing temperature on the
etching process, polycrystalline diamond with a 3 nm film
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Nanotechnology 23 (2012) 455302
H-A Mehedi et al
Figure 2. SEM images of etched nanocrystalline diamond after 30 min of annealing with (a) 1.5 and (b) 10 nm Ni films, (800 ◦ C, 60 Torr
H2 ); the inset showing nanoholes with a lateral size of 20 ± 5 nm.
Figure 3. SEM images of a (100) surface of polycrystalline diamond after 30 min annealing at (a) 750 ◦ C, (b) 800 ◦ C, (c) 850 ◦ C and (d)
900 ◦ C, (3 nm Ni, 60 Torr H2 ).
of Ni was annealed at 750, 800, 850 and 900 ◦ C in 60 Torr
H2 atmosphere. Figure 3 shows the SEM images of a (100)
surface of polycrystalline diamond after 30 min of annealing.
3.3. Effect of hydrogen pressure
Figure 4 shows SEM micrographs of a (100) surface of
polycrystalline diamond annealed with a 3 nm Ni film for
10 min at 800 ◦ C in vacuum, 60 Torr Ar, 60 Torr H2 , and 1 atm
H2 .
Neither etch pits nor nano-channels were apparently
found on the diamond surface when there was no hydrogen
in the furnace, i.e. under vacuum, figure 4(a), and Ar
atmosphere, figure 4(b). Moreover, in both cases Ni
nanoparticles were covered by overlayers, as apparent
in the SEM images observed by an in-lens energy and
angle selective backscatter detector (EsB), right panels of
figures 4(a) and (b). The overlayers were confirmed as
carbon by energy dispersive x-ray analysis (EDX), and most
likely could be amorphous carbon, since nickel carbides
become unstable at temperatures as low as 350 ◦ C [27]. This
observation indicates that when there is no hydrogen, and
thereby no desorption, carbon absorbed by the Ni can lead
to the nucleation and growth of inactive carbon layers that
The observation suggests that etching occurs typically
in the range of 800–850 ◦ C. The absence of etching at
750 ◦ C, figure 3(a), could be due to the very low solubility
of carbon into Ni at this temperature. On the other hand,
the density of etched nanostructures decreased dramatically
at 850 ◦ C, figure 3(c). It seems that, at this temperature,
some of the particles are not active, in particular the smallest
particles. The effect is even stronger at 900 ◦ C, where no
etching was observed at all, instead particles started to form
clusters, figure 3(d). We attribute this to the random nucleation
and growth of carbon overlayers on the metal particles,
poisoning them at higher temperatures, where the rate of
carbon diffusion to the metal–gas interface was the highest. A
high surface concentration of carbon enhances the probability
of nucleation and the subsequent growth of an inactive carbon
layer.
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Nanotechnology 23 (2012) 455302
H-A Mehedi et al
Figure 4. SEM images of a (100) surface of polycrystalline diamond after 10 min annealing in (a) vacuum, (b) 60 Torr Ar, (c) 60 Torr H2
and (d) 1 atm H2 , (3 nm Ni, 800 ◦ C); the right panels of images (a) and (b) are SEM images observed by an in-lens energy and angle
selective backscatter detector (EsB), showing the presence of carbon overlayers.
eventually decrease the catalytic activity of the metal. So,
hydrogen is instrumental in the etching process. Moreover,
the higher the pressure of hydrogen gas in the furnace, the
faster the diamond is etched, figure 4(d). We attributed this
to the increased concentration of atomic hydrogen, generated
catalytically, that, indeed, enhances the desorption rate.
nanoparticles [30], and molten Ni [13], as catalysts. It has
been shown that Ni-assisted diamond etching is slowest
for the [111]-oriented diamond planes and that they act as
stopping planes [13]. Since the uptake of carbon atoms from
diamond by the metal nanoparticles occurs from the weakly
bonded atoms, the carbon atoms of the closest packing plane,
the {111} planes, are the most stable against the dissolution
of carbon in nanoparticles, as compared to the other crystal
planes [30]. As a result, etch pits walled with four {111} planes
are formed on the {100} planes, whereas {111} show etching
pits with equilateral triangles and hexagons [13].
3.4. Effect of diamond microstructure
Polycrystalline diamond and nanocrystalline diamond were
annealed with a 3 nm Ni film at 800 ◦ C under 60 Torr
hydrogen atmosphere. After 30 min of annealing, etching was
observed on both microstructures of diamond. No particular
effect related to the grain boundaries was observed. Etch
pits with an inverted four-sided pyramid were formed on
[100]-oriented planes of polycrystalline diamond, figure 5(a),
whereas a mixture of pyramidal, hexagonal, and trigonal
shaped etch pits were observed on nanocrystalline diamond,
figure 5(b). Note that the nanocrystalline diamond sample
exhibits a mixture of [111]-oriented and [100]-oriented
planes, and the etching behavior is different depending on the
crystallographic orientation.
The orientation dependence of diamond etching has been
studied before using molten Ce [28], Fe particles [29], Co
3.5. Change in microstructure during etching
Two diamond samples, [100]-oriented single crystalline
(CVD diamond plate, Element Six) and nanocrystalline
diamond (1.5 µm thickness, 100 nm grain size), were
annealed with 3 nm Ni for 10 min at 800 ◦ C. In order to
detect an eventual change in microstructure, such as the
appearance of graphitic phases on the surface, both samples
were then characterized by Raman spectroscopy using 514 nm
excitation, and the spectra were compared with those recorded
before annealing.
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Nanotechnology 23 (2012) 455302
H-A Mehedi et al
Figure 5. SEM images of (a) (100) surface of polycrystalline diamond and (b) nanocrystalline diamond after 30 min annealing with 3 nm
Ni film, (800 ◦ C, 60 Torr H2 ), showing the formation of nanopores with no obvious dependence on the diamond microstructure.
Figure 6. Raman spectra of (a) single crystal diamond ([100]-oriented, CVD diamond, Element Six), (b) nanocrystalline diamond (1.5 µm
thickness, 100 nm grain size), showing no change in the diamond structure after the catalytic etching by Ni nanoparticles, (3 nm Ni, 800 ◦ C,
10 min, 60 Torr H2 ).
Figure 6(a) shows Raman spectra for a [100]-oriented
diamond crystal recorded before (red curve) and after (blue
curve) annealing. Both spectra show a similar characteristic
diamond line at 1332.4 cm−1 , and no graphitic component is
observed before or after treatment. On the other hand, D and
G peaks, which lie at 1337 and 1590 cm−1 respectively, were
observed for the nanocrystalline diamond sample, figure 6(b).
The band at 1337 cm−1 is assigned to the sp3 -bonded carbon
in diamond (D band) and the broad peak around 1590 cm−1
is attributed to the sp2 -bonded carbon (G band). The peak
intensity ratio of the D band to the G band (ID /IG ) is a
measure of the amount of graphitic impurities. The measured
ID /IG values of the treated and untreated samples were
1.02 and 1.01 respectively. The observation suggests that the
diamond structure was maintained for both samples (single
crystalline and nanocrystalline diamond) after the catalytic
etching by Ni nanoparticles.
According to figure 7(a), etching proceeded rapidly
within first 60 min, then slowed down, and eventually was
saturated after 90 min of annealing. After 6 h of annealing, the
mean depth was estimated to be about 520 nm. The observed
reduction and saturation of the etching rate for long processing
times could be explained by the increase in the distance from
the top surface down to the metal nanoparticle. As the Ni
nanoparticle moves deeper into the diamond, away from the
source of molecular hydrogen, the carbon desorption from the
nanoparticle becomes less effective, eventually leading to the
saturation of the catalyst with carbon.
4. Conclusion
The careful study of the process parameters shows that
nanopores can be formed in diamond, whatever its
microstructure. The process requires a catalyst with high
carbon solubility, such as nickel, in a hydrogen atmosphere at
a temperature of 800–850 ◦ C. The etching process is stopped
for catalyst particles located deep inside the crystal volume,
probably because of a limited gas exchange between the
surface and the particle, leading to carbon saturation. After
optimization of the process parameters, nanopores as long
as 600 nm were formed in nanocrystalline diamond, which
will allow the fabrication of porous diamond membranes for
chemical sensor applications.
3.6. Effect of annealing time
Nanocrystalline diamond with a thickness of about 1.5 µm
was annealed with a 3 nm Ni film for 6 h at 800 ◦ C. After
every 10 min, the cross section of the sample was observed
in SEM and the depth of etch pits was roughly estimated
from the observation, figure 7(a). Figure 7(b) shows the SEM
micrograph, of the cross section of the etched nanocrystalline
diamond after 90 min of annealing, observed by an EsB
detector. The white spots correspond to Ni nanoparticles.
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Nanotechnology 23 (2012) 455302
H-A Mehedi et al
Figure 7. (a) Variation of etch depth with annealing time measured for nanocrystalline diamond after 6 h of annealing with 3 nm Ni film,
(800 ◦ C, 60 Torr H2 ). Bottom and top line of the box represent the 25th and 75th percentile of each data set. The band and square inside
refer to the median and mean respectively; minimum and maximum values are indicated by the cross. Whiskers refer to mean ± standard
deviation. After every 10 min the cross section of the etched sample was observed by SEM and pore depth was estimated roughly from the
SEM image. Note that the Ni particles that were present on the diamond surface after annealing were excluded from the depth measurement.
(b) Corresponding SEM image recorded by using an EsB detector after 90 min, revealing Ni nanoparticles embedded into the crystal
volume.
Acknowledgments
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atmospheric pressure experiments. The work was supported
by the ANR 2010-BLAN-812-03 (VLOC) program.
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