Investigation of phases in the carbon films deposited by electrolysis of ethanol liquid phase using Raman scattering

Article abstract of DOI:10.1016/S0009-2614(00)00041-5

A carbon film was deposited on Si substrate from an ethanol liquid phase by electrolysis at low temperature (60°C) in ambient atmosphere, and various carbon phases in the film were investigated by micro-Raman spectroscopy. Raman spectroscopy analysis confirmed the crystalline diamond structure by the presence of a sharp peak at 1331 cm^-1. Meanwhile, short-chain polyethylene, hydrogenated amorphous diamond-like and glassy carbon phases were found in the films.

Full text of DOI:10.1016/S0009-2614(00)00041-5

2
5 February 2000
Chemical Physics Letters 318 Ž2000. 471–475
www.elsevier.nlrlocatercplett
Investigation of phases in the carbon films deposited by
electrolysis of ethanol liquid phase using Raman scattering
Z. Sun a,), Y. Sun , X. Wang
b
c
a
School of Electrical and Electronic Engineering, Nanyang Technological UniÕersity, Singapore 639798, Singapore
b
Department of Chemistry, New York UniÕersity, New York, NY 10003, USA
c
Department of Physics, Texas Tech UniÕersity, Lubbock, TX 79409, USA
Received 24 September 1999; in final form 5 January 2000
Abstract
A carbon film was deposited on Si substrate from an ethanol liquid phase by electrolysis at low temperature Ž608C. in
ambient atmosphere, and various carbon phases in the film were investigated by micro-Raman spectroscopy. Raman
y
1
spectroscopy analysis confirmed the crystalline diamond structure by the presence of a sharp peak at 1331 cm
.
Meanwhile, short-chain polyethylene, hydrogenated amorphous diamond-like and glassy carbon phases were found in the
films. q 2000 Elsevier Science B.V. All rights reserved.
1
. Introduction
carbon films from an organic solution Žethanol. was
made by Namba w2x, then the carbon films were
deposited from water–ethylene glycol solution by
Suzuki et al. w3x, and diamond-like carbon films were
deposited from methanol solution by Wang et al. w4x.
Recently, the growth of diamond-like carbon films
by an electrochemical method using a solution of
acetylene in liquid ammonia as the electrolyte was
reported by Novikov and Dymont w5x. All the experi-
ments implied that some fine diamond crystals might
be grown by electroplating techniques at low temper-
Diamond is the thermodynamically stable phase at
high pressure and high temperature ŽHPHT., thus
HPHT Ž)50 kbar and 15008C. technique has been
developed to synthesize diamond particles in various
size Žfrom nm to mm. since the 1950s w1x. Currently,
non-equilibrium process at low pressure by chemical
vapor deposition ŽCVD. has been widely used for
synthesis of polycrystalline diamond films at the
temperature of 700–10008C. Because most of the
thin film materials can be deposited in liquid phases
at low temperatures based on the electroplating tech-
niques, growth of carbon films which possessed
diamond-like structure on Si substrates was at-
tempted using various organic solutions by electroly-
sis w2–5x. The first attempt to grow diamond-phase
atures Ž-708C.. However, the lack of the sharp
y1
single peak at 1332 cm
in the Raman spectrum
which is a reliable measurement for identifying crys-
talline diamond in the above experiments leads us to
make a further investigation. Here we report that the
carbon films can be deposited on Si using a simple
and economic process by electrolysis of ethanol liq-
)
Ž
.
uid phase at low temperatures 608C in ambient
Corresponding author. Fax: 65-791-2687; e-mail:
ezsun@ntu.edu.sg
atmosphere, various carbon phases including dia-
0
009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.
PII: S0009-2614Ž00.00041-5

4
72
Z. Sun et al.rChemical Physics Letters 318 (2000) 471–475
mond, polyethylene, hydrogenated diamond-like and
glassy carbon in the films are confirmed by micro-
Raman spectroscopy analysis.
2
. Experimental
The carbon films were synthesized using electro-
plating technique using ethanol as an electrolyte w2x.
In our deposition system, the Pt wire of 0.6 mm
diameter was used as the anode electrode. The smooth
Si substrate Žresistivity, 30V cm; size, 10=30=0.3
3
mm . was mounted on the negative electrode. The
distance between the two electrodes was about 5
mm. Prior to the deposition, the Si substrate was
dipped into the mixture of dilute HF–HNO3 solution
for 10 min in order to get rid of the native oxide
layer. Then the substrate was ultrasonically cleaned
in deionized water for 30 min. The potential applied
Fig. 2. A typical Raman spectrum of the carbon film deposited by
electrolysis of ethanol.
on Si was about 0.2–0.5 mm hy1. The thickness of
the carbon films was about 2–4 mm. After deposi-
tion, the samples were investigated by scanning elec-
tron microscopy ŽSEM. and micro-Raman spec-
troscopy ŽAr ion laser, 514.5 nm for excitation, the
laser power on the sample is less than 2 mW..
to the substrates was changed from 0 to 1 kV and the
y2
current density from 0 to 10 mA cm
under a
constant temperature. The analytically pure ethanol
Ž99.5%. was used as an electrolyte, the purity of the
solution affected the resistivity of the electrolyte.
The resistivity of the solution can be slightly tuned
by adding a small amount of methanol. The current
density increases linearly with the applied potential
for the electrolyte w2x. The typical deposition condi-
3. Results and discussion
The carbon films deposited by electrolysis mainly
consist of fine diamond-like phase carbon w2,4x. All
tion was under a temperature of 608C with the
applied potential of 800 V, and the current density of
the films appeared white in color. The film morphol-
ogy and roughness is related to the electrolysis depo-
sition conditions, such as the composition of the
solvent, the applied current and voltage, and the
temperature of the solvent. From SEM observation,
the films deposited from ethanol by electrolysis were
mainly composed of grains of several tens of
nanometers in size, a few crystals in several microm-
eters were frequently observed, as shown in Fig. 1.
A typical Raman spectrum of the carbon film from
electrolysis of ethanol at 800 V is shown in Fig. 2.
y2
8
mA cm . The deposition rate of the carbon films
y1
Two broad peaks around 1350 and 1600 cm ap-
pear in the Raman spectrum, respectively. These two
y1
peaks are related to D band at 1355 cm
and G
y1
band at 1580 cm
of the graphite, indicating that
2
some sp amorphous carbon phases exited in the
film w6–8x. The carbon films deposited under various
potential Ž400–1000 V. have been investigated, the
Fig. 1. A SEM image of the carbon film deposited by electrolysis
of ethanol.
Raman spectra of the films are quite similar to that

Z. Sun et al.rChemical Physics Letters 318 (2000) 471–475
473
shown in Fig. 2, indicating that the structures of the
carbon films deposited under different potential are
similar.
with a natural single-crystalline diamond which ex-
y1
hibits only one sharp peak at 1332.5 cm and the
y1
line width of the peak is 4.3 cm
under our mea-
To investigate the structure of the film in details,
micro-Raman spectroscopy Žthe size of focused laser
spot was less than 2 mm. was used in different local
surement condition, the downshift peak position and
broader of the peak width in our film may be due to
the fine grains of diamond in the film w9x. Also, the
y1
area of the films. In most cases, the spectra of the
films measured by micro-Raman spectroscopy were
similar to the spectrum shown in Fig. 2. However, in
some areas, the micro-Raman spectrum is quite dif-
ferent. Fig. 3a shows one of the micro-Raman spec-
tra detected in a local area of a film Ždeposited at
broad peaks around 1346 and 1600 cm which are
2
related to sp carbon phase can be seen in the Raman
spectrum. It is noted that the Raman spectra showing
diamond features were detected only on a few sam-
ples in a large number of the carbon films prepared
under potential of 400–1000 V by micro-Raman
00 V., a sharp peak at 1330.7 cmy1, which is
8
measurement, the line width of diamond Raman peak
y1
assigned to crystalline diamond. The line width of
in the range of 7.6–18.4 cm
has been observed.
y1
the peak is 7.6 cm , which indicates that the quality
The amount of good quality crystals in the carbon
films is still small. However, the diamond crystalline
of the diamond crystals is quite good. Comparing
Fig. 3. Micro-Raman spectra of local area in the carbon films deposited by electrolysis of ethanol: Ža. diamond crystals, Žb. diamond-like
carbon, Žc. polyethylene crystals, Žd. glassy carbon.

4
74
Z. Sun et al.rChemical Physics Letters 318 (2000) 471–475
structure of the carbon films deposited from electrol-
ysis of ethanol were also detected by Namba using
X-ray photoemission spectroscopy and transmission
electron diffraction w2x. More recently, we found
that, using this kind of carbon film as an intermedi-
ate layer, diamond nucleation could be enhanced
greatly on Si substrate by CVD process, and some
epitaxial growth was also observed w10x, suggesting
the n value of w–CH CH –x chain. So the electroly-
2
2
n
sis polyethylene consists of amorphous carbon with
short w–CH CH –x chains. Such short-chain poly-
2
2
n
ethylene was frequently detected in the prepared
carbon films by micro-Raman spectroscopy. This
results show that the short chain of polyethylene
structure together with diamond-like structure has
been formed in the deposition process, suggesting
that the formation of diamond structure may relate to
the polyethylene structure during electrolysis of
ethanol. However, in some local area of the film,
some glassy carbon phases were identified by
micro-Raman measurement, the spectrum is shown
that high diamond-like phases may exist in the elec-
trolysis carbon films. The Raman scattering is more
2
3
sensitive to sp carbon phase than to sp carbon
2
phase, and the intensity of the sp carbon phase is
much larger than sp carbon phase in the spectra
3
y1
Žexcited by 514.5 nm. w6x. Thus, the Raman spectra
of the electrolysis carbon films mainly show
graphite-like features in most cases, even some dia-
mond crystals existed in the carbon films.
in Fig. 3d. The broad peaks at 1360 and 1600 cm
appeared in the spectrum, the line width is 40 and 50
y1
cm , respectively, which indicates that some fine
grain of graphite crystals Žnanocrystals. or glassy
carbon existed in the film w6,7x. Thermodynamically,
graphite is the stable form of carbon phase at ambi-
ent pressures and temperatures, so formation of
graphtic phases is favored in the deposition process.
Using ethanol as an electrolyte to grown diamond
crystals on Si substrate by electrolysis, we proposed
that three possible steps might occur based our ex-
periment results.
We investigated a large number of the samples by
micro-Raman spectroscopy, some spectra which
show diamond-like carbon Žamorphous., poly-
ethylene w–CH CH –x and glassy carbon features
2
2
n
were also observed, as shown in Fig. 3b–d, respec-
tively. In Fig. 3b, a broad peak around 1500 cmy1
which was assigned to amorphous diamond-like car-
bon w6–8x can be seen, the weak peaks around 1295,
y1
1
450 cm
are from the –CH – various stretch
2
vibration mode w11x. This result indicates that some
alkyl structures –CH – existed in the film, and the
3.1. Formation of polyethylene [–CH CH –] in
2
2
2
n
film is hydrogenated diamond-like carbon. In Fig.
ethanol solution
3
2
c, several sharp peaks at 1062, 1129, 1296, 1440,
850, 2880, 2930 cm can be observed, which are
y1
Electrolysis of ethanol to form alkyl chain w–
CH –x structure which was found in the resulted
corresponding to polyethylene crystal. The broad
features around 1340, 1600 and 2722 cmy1 related
2
n
carbon film ŽFig. 2b,c. may suggest that this struc-
ture originated from the precursor solution. In the
electrolysis process, ethanol may lose either hydro-
to amorphous carbon phase also appeared in the
spectrum. The bands at 1062 and 1129 cm
weak intensity are from C–C stretchings. The bands
at 1440 and 1296 cm
y1
with
q
q
gen ŽH . or H O and form ethyl ŽCH CH ., as
y1
2
3
2
with medium-strong inten-
shown in Eqs. Ž1. and Ž2.. If some ethyl Žion. gets an
electron Že. near the cathode ŽSi., a polymerization
sity are –CH – in-plane twisting mode. The asym-
2
metric and symmetric CH2 stretchings appear
strongly between 2930 and 2850 cmy1 in the spec-
reaction may possibly occur and form a short-chain
polyethylene structure on the negatively biased elec-
trode, as show in Eq. Ž3..
trum. Comparing with the intensity of a Raman
spectrum of polyethylene polymer, w–CH CH –x
Žn is )1000., the intensity of the peaks in the
spectrum of electrolysis polyethylene is much
weaker, indicating that the conjugation length of
w–CH CH –x chains in electrolysis polyethylene is
2
2
n
y
q
CH CH OH
H CH CO qH
Ž
Ž
1
2
.
.
3
2
3
2
q
q
CH CH OHqH
CH CH qH O
3
2
3
2
2
2
2
n
much smaller than that in the polyethylene polymer,
because the relative Raman intensity increases with
2
e
q
2
n CH CH q
2w–CH CH –x qnH
Ž
3
.
3
2
2
2
n
2

Z. Sun et al.rChemical Physics Letters 318 (2000) 471–475
475
3
.2. Formation of silicon carbide interface in the
of the carbon atoms, stabilization of the diamond-like
carbon structure on the growth surface, and abstrac-
tion of hydrogen from surface hydrocarbons Žpoly-
ethylene..
initial deposition process
When the Si substrate was treated in HF–HNO3,
the Si surface can be possibly terminated by hydro-
gen or dangling bond. In the deposition process, the
SiC interlayer may possible form, as shown in Eqs.
Ž4.–Ž6.. Such alkyl chain structure is favored to form
a monolayer of SiC on the hydrogen terminated Si
surface w12,13x, and enhance diamond nucleation
w14x.
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