Diamond & Related Materials 14 (2005) 16 – 22
www.elsevier.com/locate/diamond
Nanocrystalline diamond synthesized from C60
Natalia Dubrovinskaia*, Leonid Dubrovinsky, Falko Langenhorst,
Steven Jacobsen, Christian Liebske
Bayerisches Geoinstitut, Universit7t Bayreuth, Universittstrasse 30, 95440 Bayreuth, Germany
Received 27 January 2004; received in revised form 14 June 2004; accepted 15 June 2004
Available online 25 July 2004
Abstract
A bulk sample of nanocrystalline cubic diamond with crystallite sizes of 5–12 nm was synthesised from fullerene C60 at 20(1) GPa and
2000 8C using a multi-anvil apparatus. The new material is at least as hard as single crystal diamond. It was found that nanocrystalline
diamond at high temperature and ambient pressure kinetically is more stable with respect to graphitisation than usual diamonds.
D 2004 Elsevier B.V. All rights reserved.
Keywords: Nanodiamond
1. Introduction
The exceptional physical properties of diamond make it
not only the outstanding gem, but also an important material
in a wide variety of industries, where its extreme hardness,
toughness, high refractive index, transparency over a broad
spectral range and high thermal conductivity are exploited
[1]. Recently, the synthesis of nanocrystalline films of
diamond-like carbon (DLC) and polycrystalline cubic
diamond have become of increasing interest. These studies
particularly focused on designing diamond materials with
reduced grain dimensions [2–6].
Since the first artificial diamonds were manufactured
in the mid-1950s, various methods (ranging from direct
solid-state transformation of graphite under static or
shock pressure to chemical-vapour deposition) for diamond and DLC synthesis under variable pressure and
temperature conditions have been explored, resulting in
materials with properties approaching those of natural
diamonds [1,3,6–12]. Ultrafine diamonds with grain sizes
* Corresponding author. Tel.: +49 921553739; fax: +49 921553769.
E-mail address: [email protected]
(N. Dubrovinskaia).
0925-9635/$ - see front matter D 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.diamond.2004.06.017
of 5–10 nm were synthesised by explosives and may
display excellent properties as surface coating for metals
[9]. Polycrystalline cubic diamond synthesised by direct
conversion of graphite at static high pressures and
temperatures is ultrahard [3]. Compression of fullerene
C60 under non-hydrostatic pressures to 25–30 GPa at room
temperature has resulted in direct transformation to
diamond, probably with small crystallite sizes[13,14], but
the amount of synthesized material was insufficient for
detailed characterisation of the structure or mechanical
properties and the reproducibility of such experiments has
come under scrutiny [15,16]. Numerous nanocluster-based
phases with presumably 3D-polymerized fullerite structures have been synthesized from C60 and carbon singlewall nanotubes at pressures above 13 GPa and temperatures above 800 K [15–17]. These new materials show
impressive mechanical properties and some of them
appeared to be harder than diamond [15,16]. However,
subsequent studies did not confirm the exceptional hardness of fullerites [18–20].
In course of the systematic studies of direct transformation of low-density carbon materials into diamond,
we synthesized nanocrystalline diamond using C60 as a
starting material. The new material has a number of unusual
and potentially important properties.
N. Dubrovinskaia et al. / Diamond & Related Materials 14 (2005) 16–22
2. Experimental
We conducted a series of experiments in 1200- and 5000tonne multi-anvil presses, using powdered graphite, amorphous carbon, C60 and natural diamond as starting materials.
The samples were contained in a Pt capsule to avoid
undesirable reactions with carbon. The sample assembly
consisted of a MgO (+5 wt.% Cr2O3) octahedron (10 or 18
mm edge length) containing a LaCrO3 heater. The octahedron was compressed using 54 or 32 mm tungsten carbide
anvils with a truncation edge length of 11 or 4 mm
correspondingly and pyrophyllite gaskets. The temperature
was monitored with a W3Re/W25Re thermocouple located
axially with respect to the heater and with a junction close to
the Pt capsule. Experiments were conducted at pressures of
13–25 GPa and temperatures up to 2600 K (Table 1). In one
experiment, C60 was compressed to 20 GPa without heating.
The P–T uncertainties are estimated to be F1 GPa and F50
K, respectively. In each experiment, the sample was first
compressed to the desired pressure; the temperature was
then raised at ~100 K/min to the desired run temperature.
Duration of heating varied from several minutes to 1.5 h.
The samples were either quenched by switching off the
power to the furnace, or gradually cooled with rate ~10 K/
min, and then slowly decompressed. Upon completion of
each experiment, the capsule was carefully removed and the
treated material was mechanically cleaned of platinum.
Samples from the 5000-tonne press came out as compact
solid cylinders about 1.8 mm in diameter and 3 mm in
height. The chemical composition and texture of the
samples was analysed using a LEO-1530 scanning electron
microscope and a PHILIPS CM20 FEG analytical transmission electron microscope (ATEM), operating at 200 kV.
Optical spectra were collected using a Bruker IFS 120 HR
high-resolution FTIR spectrometer coupled with a Bruker
IR microscope and Raman spectrometers (XY Dilor,
operating with 514 nm laser and LabRam, operating with
632 nm incident laser). X-ray diffraction data were obtained
17
in-house using a STOE STADI P diffractometer (CoKa
radiation, 40 kV, 20 mA) and synchrotron radiation facilities
at the Advanced Photon Source (APS) at Argonne National
Laboratory (GSECARS, sector 13, Chicago, USA; monochromatic radiation wavelength 0.3301 2), and at the ID30
beam line at ESRF (monochromatic radiation wavelength
0.3738 2) in angle-dispersive mode.
3. Results and discussion
The C60 sample compressed at room temperature to 20
GPa came out as a black non-transparent brittle cylinder. Xray diffraction and Raman spectroscopy show, in good
agreement with previous observations [15], that fullerene
preserves its structure (lattice parameter of the cubic cell
reduced from 14.041(3) 2 for pristine material to 13.856(3)
2 for the product phase) after high-pressure treatment. All
carbon phases processed at simultaneous high pressures and
temperatures transformed to transparent (in the case of
amorphous carbon, graphite and C60) or semi-transparent
(when diamond powder was used) materials. Samples
obtained from powdered diamond and amorphous carbon
are slightly grey in colour, while materials obtained from
graphite and C60 are yellowish. SEM and ATEM studies
show that at the conditions of our experiments, platinum did
not react with carbon, the samples were not contaminated
and contained only pure carbon. X-ray powder diffraction
patterns of all recovered samples are dominated by reflections from diamond (Fig. 1), however, materials synthesized
at temperatures below 2600 K from amorphous carbon,
graphite and diamond exhibit several additional small
reflections at ~2.19, 1.92 and 1.50 2 (Fig. 1). These
reflections correspond to lonsdaleite (2H diamond polytype)
and could result from either a small amount of this phase or
disorder of the diamond structure along [111]. Indeed,
electron diffraction images of the natural Ia diamond
recovered after treatment at 20(1) GPa and 2300(50) K
Table 1
Experimental conditions and results
No.
Sample
P, GPa
T, K
Heating, min
Cooling
Starting material
Product
1
2
3
4
5
6
7
8
9
10
11
H1927
S3199
Z274
Z298
Z296
Z277
S3174
Tro1
Tro2
Li1
Li2
20
20
20
20
13
20
20
25
24
24
20
2300
2300
2300
2300
2400
300
2300
2600
2500
2450
2500
60
60
60
60
60
–
60
20
20
15
95
quenched
quenched
quenched
slow
slow
–
quenched
quenched
quenched
quenched
quenched
amorphous carbon
powdered diamond
C60
C60
C60
C60
C60
graphite
graphite
graphite
graphite
PolyDa+Lb
SingleDc+L
NanoDd+6He
NanoD+6H
SingleD
C60
NanoD
PolyD
PolyD
PolyD+L
PolyD+L
a
b
c
d
e
Polycrystalline diamond (here: crystallite sizeN50 nm).
Lonsdaleite.
Diamond single crystals of a few micron size.
Nanocrystalline diamond (here: crystallite size of 5–12 nm).
New 6H diamond polytype.
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N. Dubrovinskaia et al. / Diamond & Related Materials 14 (2005) 16–22
Fig. 1. Examples of diffraction patterns of carbon materials synthesized (a) from graphite at 24 GPa and 2400 K; (b) from natural diamond at 20 GPa and 2300
K; and (c) from C60 at 20 GPa and 2300 K (bDQ for diamond reflections, bLQ for lonsdaleite (or 2H diamond polytype), b6HQ for 6H diamond polytype). Insert
shows enlarged plot of pattern c with vertical bars corresponding to the position and intensities of the diffraction lines of 6H polytype of diamond.
(Fig. 2) show asterism of (111) reflections indicating
disordering of the diamond structure. The results of our high
pressure and temperature experiments on amorphous and
crystalline graphite are in agreement with the results of
Irifune et al. [3]. However, X-ray and electron diffraction
patterns from the samples synthesized from C60 contain (in
addition to the diamond-structure reflections at 2.0497,
1.2552 and 1.0705 2) diffraction lines at 2.141, 1.9195,
1.6292, 1.3670, 1.1578 and 1.0519 2 (Figs. 1 and 3). All
these lines can be indexed in the framework of a hexagonal
unit cell with lattice parameters a=2.510(1) 2 and
c=12.301(3) 2. Such diffraction data are thus consistent
with theoretically predicted [16] 6H diamond-like polytype
(Fig. 4).
While diamond powder treated at high pressures and
temperatures and the material synthesized from C60 at 13
GPa and 2400 K are relatively coarse-grained with crystallites larger than 1 Am (producing single spots in electron
diffraction images, Fig. 2), samples obtained from amor-
phous carbon, graphite and C60 are so finely crystalline that
they give continuous Debye-Scherrer lines (Fig. 3c). TEM
images, as well as estimation from the broadening of X-ray
diffraction lines by the Williamson-Hall method [21,22],
show that bulk samples of diamond obtained at P=20 GPa
and T=2300 K from C60 consist of crystallites of 5–12 nm
(Fig. 3a). Additional synthesis experiments indicate that
nanocrystalline diamond forms independent of the cooling
rate (Table 1). Due to the very small grain size of the
nanocrystalline material, high-resolution TEM (HRTEM)
images were difficult to achieve, but where possible,
HRTEM images show that the nanocrystalline diamond
synthesised from C60 has an ideal structure, free of stacking
faults or other defects (Fig. 3b). The 6H polytype could not
however be imaged, probably due to its small abundance in
the sample. The EELS spectra are also perfectly compatible
with the diamond structure (Fig. 5) and suggest that sp3
hybridization prevails even across the numerous grain
boundaries.
N. Dubrovinskaia et al. / Diamond & Related Materials 14 (2005) 16–22
Fig. 2. Electron diffraction image of the natural Ia diamond recovered after
treatment at 20(1) GPa and 2300(50) K. Asterism of (111) reflections gives
evidence for disorder of the diamond structure.
19
Fig. 6 shows Raman spectra of carbon materials
synthesized at various conditions. While the first-order
diamond Raman line at ~1330 cm 1 dominates the spectra
of polycrystalline diamond and lonsdaleite produced from
graphite and amorphous carbon, this line is completely
absent in the spectra of the material synthesized from C60.
The latter spectra are dominated by broad bands at about
500, 1100 and 1420 cm 1. The features at 500 and 1100
cm 1 were reported for bamorphous diamondQ [17,18].
They may originate from the breakdown of the selection
rules due to the small size of crystallites and are
characteristic of the vibrational density of state of diamond
[17,18]. The broad band at 1420 cm 1 closely resemble
those reported for bsuperhard fulleritesQ [15,19,20]. How-
Fig. 3. A transmission-electron micrograph (a, b) and electron diffraction images (c–e) from the nanocrystalline diamond produced by direct conversion of C60
at 20 GPa and 2300 K. HRTEM image (insert at the lower right corner) shows that individual crystallites, which comprised the bulk of the nanocrystalline
sample synthesised from C60, have the ideal diamond structure, free of stacking faults or other defects. The letter bDQ denotes electron diffraction lines from
diamond and 6H from diamond polytype.
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N. Dubrovinskaia et al. / Diamond & Related Materials 14 (2005) 16–22
Fig. 4. Structures of lonsdaleite, diamond and the new 6H polytype of diamond.
ever, we do not have full and satisfactory explanation of the
observed Raman spectra.
The nanocrystalline diamond-like material synthesized
from C60 exhibits an absorption similar to that of diamonds
in the visible/near-infrared range (Fig. 7). Note that two- and
three-phonon absorption of the new nanocrystalline material
is weaker than that in bulk diamond (Fig. 7).
Hardness measurements of superhard materials like
diamond or DLC are problematic, because the measurements are based on the assumption that the tested material
plastically deforms [19,20,23]. Obviously, the hardness of
the indenter should exceed the hardness of the tested
material and this requirement limited our ability to measure
Fig. 6. Raman spectra of carbon materials obtained from powder of natural Ia
diamond treated at 20 GPa and 2200 K (line a), and synthesized from graphite
(line b) and C60 (line c) at the same pressure and temperature conditions.
Fig. 5. The EELS spectra from ptestine C60 (bottom); from polycrystalline
diamond obtained from graphite at 20 GPa and 2300 K; from the
nanocrystalline diamond produced by direct conversion of C60 at the same
conditions. Spectrum of the nanodiamond sample (top) treated at 1800 K in
forming gas atmosphere does not show any sign of sp2-bonded carbon.
Fig. 7. IR spectra of carbon materials obtained from powder of natural Ia
diamond treated at 20 GPa and 2200 K (line a), and synthesized from graphite
(line b) and C60 (line c) at the same pressure and temperature conditions.
N. Dubrovinskaia et al. / Diamond & Related Materials 14 (2005) 16–22
21
While our experimental results generally support the
observations reported recently by Irifune et al. [3], there is
clear difference between the materials synthesized from
graphite and C60. While Irifune et al. [3] reported that
according X-ray diffraction and Raman spectroscopic
measurements, their polycrystalline diamond (with crystallite sizes 10–200 nm) was pure cubic diamond, Raman
spectra from our nanodiamond are completely different (Fig.
6) from those of cubic diamond. Irifune et al. [6] did not
observe in their samples 6H diamond polytype.
4. Conclusions
Fig. 8. Examples of Raman spectra of natural and nanodiamond treated at
ambient pressure and various temperatures for 2 h in an inert atmosphere
(Ar+2% H2). While natural diamond rapidly graphitises at 1300 8C (broad
bands at 1320 and 1600 cm 1), nanodiamond does not show any sign of
graphitisation even after heating at 1500 8C, and only treatment at 1600 8C
results in some minor changes in the Raman spectra.
the hardness of synthesized nanodiamond. It is known, for
example, that the hardness of the (111) face of the type IIa
diamond (bhardestQ diamond face [19]) so far has not been
measured [23], because it is not possible to make an
indentation on this face. In our case, a tip of the diamond of
a Vickers-type indenter does not make any scratches or
indentations on the surfaces of nanodiamonds at loads up to
500 g. Even using SEM we can not see the mark of indentor.
So, we conclude that nanodiamond material is at least as
hard as usual bulk diamond.
Badziag et al. [24] were the first to suggest that
nanodiamond clusters less than about 5 nm in size may
actually be more stable than graphite clusters of the same
size. We treated natural Ia diamond and synthetic nanodiamond made from C60 at temperatures between 1000 and
1900 K at 100 K steps in an inert (Ar+2% H2) atmosphere.
The samples were heated for 2 h and gradually cooled
after holding at each temperature. Graphitisation was
monitored by EELS and Raman spectroscopy because
both methods are very sensitive to the presence of sp2bonded carbon in the material. The first sign of partial
graphitisation was noticed for natural diamond at 1100 K
and at 1500 K graphitisation became rapid and the sample
became not-transparent (Fig. 8). At the same time, nanodiamond does not show any sign of graphitisation (EELS
spectra do not show any sign of sp2-bonded carbon), even
after heating at 1800 K (Fig. 5), and only treatment at
1900 K results in some changes in Raman spectra (Fig. 8).
These experiments suggest that nanodimanod is kinetically
more stable against graphitisation than bulk single crystal
diamond.
A bulk sample of nanocrystalline cubic diamond with
crystallite sizes of 5–12 nm was synthesised from fullerene
C60. Its properties were studied using X-ray diffraction,
Raman, IR spectroscopy and HRTEM. It was found that the
material has a unique Raman spectrum different from that of
usual diamond and the thermal stability of nanodiamond
produced from C60 is at least 300 K higher than that of
normal diamond. Such properties of the new material could
relate to the small dimensions and sharp size distribution of
crystallites (5–12 nm).
Acknowledgments
We thank F. Seifert for encouraging discussions. Help of
D. Frost, F. Gaillard, R. Trfnnes, V. Prakapenka and W.
Crichton with experiments is highly acknowledged.
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