Laser-induced carbon deposition from supercritical benzene

Article abstract of DOI:10.1016/S0009-2614(00)01284-7

A catalyst-free deposition of carbon onto a Si substrate immersed into benzene under supercritical pressure and temperature is induced by radiation of a Cu vapor laser. Raman analysis shows the signal typical of that for glassy carbon with a broadened D-peak. HRTEM indicates that the carbonaceous species have the shape of hollow rolls of 3-5 nm in diameter with multilayered walls with interlayer distance of 0.9 nm. The remaining liquid shows a strong luminescence in the visible, which is tentatively attributed to formation of polyaromatic compounds.

Full text of DOI:10.1016/S0009-2614(00)01284-7

22 December 2000
Chemical Physics Letters 332 (2000) 231±235
www.elsevier.nl/locate/cplett
Laser-induced carbon deposition from supercritical benzene
A.V. Simakin, E.D. Obraztsova, G.A. Shafeev *
General Physics Institute of the Russian Academy of Sciences, 38 Vavilov street, 117942 Moscow, Russia
Received 4 September 2000; in ®nal form 26 October 2000
Abstract
A catalyst-free deposition of carbon onto a Si substrate immersed into benzene under supercritical pressure and
temperature is induced by radiation of a Cu vapor laser. Raman analysis shows the signal typical of that for glassy
carbon with a broadened D-peak. HRTEM indicates that the carbonaceous species have the shape of hollow rolls of 3±
5 nm in diameter with multilayered walls with interlayer distance of 0.9 nm. The remaining liquid shows a strong
luminescence in the visible, which is tentatively attributed to formation of polyaromatic compounds. Ó 2000 Elsevier
Science B.V. All rights reserved.
Carbon deposition has attracted signi®cant at-
tention during the last decade due to large variety
of its allotropic modi®cations, such as fullerenes
[1], carbon nanotubes [2], onion-like structures,
etc. Various techniques have been applied to de-
compose carbon-containing compounds, either
with or without catalyst, such as arc discharge
between graphite electrodes, laser ablation, chem-
ical vapor deposition, to mention a few. Our pre-
vious studies on laser decomposition of
hydrocarbons were focused on laser pyrolysis of
aromatic compounds, such as benzene, toluene, or
cumene, at the solid±liquid interface [3]. These
compounds have the lowest energy of decompo-
sition compared to other hydrocarbons. At ambi-
ent pressure, the carbon deposited on a Si
substrate from liquid benzene is a glassy carbon
with well-de®ned D and G peaks in the Raman
spectrum [3]. Unlike a pure liquid benzene [4], the
decomposition is believed to be a thermal process.
The decomposition of the liquid hydrocarbon
starts in laser-irradiated area of a Si wafer im-
mersed into the liquid at the estimated peak tem-
perature of the wafer of 800°C. As the irradiation
proceeds, the liquid itself commences to absorb the
laser radiation due to continuous generation of
glassy carbon nanoparticles in the bulk. This
means that the system demonstrates a positive
feedback between the laser intensity and absorp-
tion. The calculations show that a carbon nano-
particle inside the laser beam is heated up to ca
1000°C during the laser pulse, which is quite suf-
®cient to provide the decomposition of benzene
around it. If the liquid benzene is exposed to laser
radiation through a transparent solid (say, a re-
actor window), the products of its pyrolysis can be
e€ectively quenched on the solid. This results in
deposition of a diamond-like ®lm on the trans-
parent window with sp3 fraction amounting to
80% [5,6]. The deposition of the diamond-like ®lm
proceeds from a gas bubble that expands in the
vicinity of the interface solid±liquid. The bubble
* Corresponding author. Fax: +7-095-135-0376.
E-mail address: shafeev@kapella.gpi.ru (G.A. Shafeev).
0009-2614/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 0 0 9 - 2 6 1 4 ( 0 0 ) 0 1 2 8 4 - 7

232
A.V. Simakin et al. / Chemical Physics Letters 332 (2000) 231±235
contains the mixture of benzene vapors and gas-
eous products of reaction [5], and the pressure in
the bubble drops with its expansion from several
kbars to ambient one.
The luminescence of the remaining liquid was
registered using a pulsed excitation at 532 nm.
Below the critical temperature, the medium in
the cell is very inhomogeneous due to evaporation
of the liquid and its condensation on the glass
window as small drops. Upon further increase of
temperature, the liquid passes through the stage of
critical opalescence, and then the medium becomes
quite transparent. No detectable decomposition of
benzene is observed without laser irradiation at
temperature 300±310°C during 2 h. We conclude
that the possible catalytical action of steel walls of
the cell on decomposition of the hydrocarbon can
be neglected. The decomposition starts immedi-
ately upon laser exposure of the Si wafer. The
observation of the process on a screen shows that
decomposition of the hydrocarbon commences in
the bulk of the cell after several minutes of the
irradiation. This is due, undoubtedly, to the ac-
cumulation of carbon particles in the gas.
One might expect new features of laser decom-
position of aromatic hydrocarbons upon transi-
tion to supercritical conditions. In a sense, this
process should be close to laser-induced chemical
vapor deposition (LCVD), though the latter is
carried out typically at low pressure of the pre-
cursor, up to 1 Torr. On the other hand, the crit-
ical pressure of benzene is about pcr ˆ 40 atm at
Tcr ˆ 288°C, so that the density of the benzene
molecules may greatly exceed that during the de-
composition at ambient pressure. The elevated
pressure may change the kinetics of aggregation of
carbon clusters in laser-exposed area of the sub-
strate compared to normal pressure conditions.
High density of benzene and, eventually, of car-
bonaceous species in supercritical conditions is
also characteristic for carbon deposition via abla-
tion of a graphitic target with a high repetition rate
ps laser [7], though the source of carbon in our
case are the benzene molecules. In case of laser
ablation, the resulting carbon consists of cluster-
assembled carbon nanofoam. This Letter describes
our preliminary results on morphological features
of carbon deposited by laser pyrolysis of super-
critical benzene.
The carbon deposit on the Si wafer has an is-
land-like structure (see Fig. 1). The lateral dimen-
sions of each island are much smaller than the laser
spot on the wafer (5±8 against 50 lm). The
adherence of the deposit to Si surface is fairly good.
A signi®cant amount of carbon is formed also
in the bulk of the supercritical benzene. After
A stainless steel cell with a glass window was
®lled with several cm3 of liquid benzene of ana-
lytical purity. A single crystal Si wafer was placed
on the bottom of the cell, the latter was heated
from the bottom by the external heater. The tem-
perature of the cell was controlled by a calibrated
thermocouple. The beam of a copper vapor laser
(wavelength of 510.6 nm, pulse duration of 20 ns,
repetition rate of 10 kHz) was introduced into the
cell through the glass window with a 10 cm focal
distance lens. Typical laser ¯uence on a Si surface
was 0:5 J=cm2. To irradiate the extended area of
the Si wafer, the cell was mounted on a computer-
driven X±Y stage allowing its displacement under
the laser beam with controlled scanning velocity in
the range 0.3±3 mm/s. The samples of Si with de-
posited carbon were characterized by scanning
electron microscopy (SEM), Raman spectroscopy,
high-resolution electron microscopy (HRTEM).
Fig. 1. SEM view of carbon deposit on Si. Scale bar denotes
20 lm.

A.V. Simakin et al. / Chemical Physics Letters 332 (2000) 231±235
233
cooling down of the cell, the remaining liquid
contains the carbon suspension. The suspension
decants in several hours and can be characterized
by Raman spectroscopy and HRTEM.
shows another shape, in particular, of carbon ®l-
aments of hundreds nm long. The Raman signal
also varies signi®cantly over di€erent portions of
carbon. Some portions of carbon are Raman-
silent, while the others are characteristic of glassy
carbon. The latter spectrum shows that carbon
deposited in supercritical conditions is more dis-
ordered than that observed by us previously dur-
ing deposition from liquid benzene at atmospheric
pressure in similar experimental conditions [3]. In
Fig. 2 shows the HRTEM view of carbon
formed in the cell. One can see carbon species that
form a kind of rolls. They are empty inside, and
their average diameter is 3±5 nm (Fig. 2a). The
walls of the rolls are composed of several carbon
layers. The measured interlayer distance is 0.9 nm
in a wide range of laser ¯uences (Fig. 2b). To the
best of our knowledge, this type of carbon struc-
tures has not been observed so far. At the same
time, due to signi®cant inhomogeneity of temper-
ature inside the laser beam, the carbon residue
1
particular, the D peak centered at 1310 cm is
signi®cantly broadened, probably, due to the
presence of other carbons. No Raman signal is
observed in a low-shift domain, which should
correspond to carbon planes with 0.9 nm spacing.
Fig. 2. HRTEM view of carbon rolls formed in the bulk of supercritical benzene upon laser irradiation. (a) Several rolls, space bar
denotes 3 nm; (b) enlarged view of a roll, interlayer distance is 0.9 nm.

234
A.V. Simakin et al. / Chemical Physics Letters 332 (2000) 231±235
pression is derived for the case when the particle
The peculiar feature of the deposition from
radius R is much lower than the heat propagation
length in the surrounding medium during the laser
pulse of duration tp: R2 ꢀ altp, where al stands for
the heat di€usivity of benzene. The term
benzene under supercritical conditions is the
modi®cation of the remaining liquid. Indeed, after
the end of irradiation and cooling down, the re-
maining liquid shows a strong luminescence in the
visible. The spectrum of luminescence is shown in
Fig. 3. This spectrum is characteristic of dyes
containing several aromatic dyes. Their formation
might be attributed to high pressure and temper-
ature in the laser beam. More information on the
structure of the luminescent species can be ob-
tained using the liquid phase chromatography,
which is ongoing.
The speci®c carbon structures observed in the
present work might be attributed to highly non-
equilibrium conditions of laser-induced decompo-
sition of supercritical benzene. The essential
feature of the process is the high repetition rate of
laser pulses, which favors the accumulation of
carbon species in the medium. The decomposition
of benzene starts on the Si surface, then the ab-
sorption of the gas increases with the time of ir-
radiation. Small carbon particles become the
centers of benzene decomposition. Indeed, the
temperature T of the particle can be estimated
using the following relation [5]:
h
i
p
ka ˆ exp
0:2 n2 ‡ k2
1
ꢀ
ꢁ
ꢂꢃ
8pkR
 1 exp
k
takes into account the decrease of the e€ective
cross-section of the particle of radius R ꢀ k com-
pared to its geometric cross-section. Here n and k
stand for the refractive index and extinction co-
ecient of glassy carbon, respectively. At laser
wavelength k ˆ 510:6 nm, for a bulk glassy carbon
n ˆ 2 and k ˆ 0:7 [8]. For radius R of several
nanometers the particle is heated to 1000 K at a
laser ¯uence of 1 J=cm2. This temperature rise is
gained on the background temperature in the cell of
300°C. Thus, the overall temperature of the carbon
particle is high enough to cause the decomposition
of benzene molecules around the particle. The
temperature of the gas inside the laser beam is es-
sentially inhomogeneous, its maxima coincide with
the absorbing carbon particles. The speci®c mor-
phology of carbon presented in Fig. 2 might be due
to vortex-like motion of carbon nanoparticles in-
side the laser beam. Indeed, the formation of gas-
eous products of benzene decomposition, mainly
CH4 and C2H2 [6] may induce the instability of gas
¯ow around the carbon particle. This point, how-
ever, requires further studies. The high pressure
around the absorbing carbon particle favors the
formation of multi-ring compounds, as con®rmed
by strong luminescence of the remaining liquid.
Thus, the characteristic feature of laser-assisted
carbon deposition from supercritical benzene is the
speci®c form of carbon presented as empty rolls of
3±5 nm in diameter. The walls of these rolls are
made of several carbon planes with an interlayer
distance of 0.9 nm. The decomposition of benzene
in our experimental conditions seems to proceed
via thermal heterogeneous reaction on a Si sub-
strate in the ®rst stages of irradiation and then on
carbon nanoparticles in the bulk of gaseous ben-
zene. The carbon deposited from supercritical
benzene might possess interesting ®eld emission
Rjka
4kltp
T ˆ
;
where R stands for the particle radius, j is the laser
¯uence, kl the heat conductivity of the surrounding
liquid, and tp is the laser pulse duration. This ex-
Fig. 3. Luminescence spectrum of the liquid remaining in the
cell. The excitation wavelength is 532 nm. The sharp peak is due
to scattered excitation light.

A.V. Simakin et al. / Chemical Physics Letters 332 (2000) 231±235
235
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Acknowledgements
We acknowledge the help of M. Lavergne
(HRTEM), S.V. Lavrischev (SEM), and
A.G. Vitukhnovsky (luminescence measurements).
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