Open Journal of Synthesis Theory and Applications, 2014, 3, 5-13
Published Online January 2014 (http://www.scirp.org/journal/ojsta)
OPEN ACCESS OJSTA
Doped Amorphous Carbon Films Prepared by Liquid
Canyan Che, Yang Li, Guifeng Zhang*, Dewei Deng
Key Laboratory for Materials Modification by Laser, Ion and Electron Beams, Ministry of Education,
School of Materials Science and Engineering, Dalian University of Technology, Dalian, China
Received September 22, 2013; revised October 30, 2013; accepted November 21, 2013
Copyright © 2014 Canyan Che et al. This is an open access article distributed under the Creative Commons Attribution License,
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It has theoretical significance and practical value to synthetize and modify amorphous carbon films by liquid
electro-deposition technique due to its low cost, simple equipment, and better operability in uniform deposition
of the films with large -area and complex shape work pieces. This article introduces the research situation of the
carbon films prepared by liquid phase electrochemical deposition according to the applied voltage, discusses the
influence of experimental parameters on the film properties, and describes possible reaction mechanisms. It
summarizes the research progress of amorphous carbon films doped with metal and nonmetals. Finally, existing
problems have been demonstrated and suggestions on research hotspots in the future are given.
Liquid Electrodeposition; Doping; Carbon Films
Amorphous carbon films have an extensive application
prosperity in the mechanical, electronic, chemical, mili-
tary and aerospace fields due to their low friction coeffi-
cient, good wear resistance, chemical inertness, and high
transmittance in a wide range from infrared to ultraviolet.
Recently, various physical vapor deposition (PVD) and
chemical vapor deposition (CVD) methods, including ion
beam deposition, pulsed laser deposition, and plasma
enhanced CVD, etc., are used to prepare t he films. How-
ever, the deficiencies of these techniques are more com-
plex equipment, higher cost, and more difficult in depo-
sition uniform films with large -area and irregular surface
. Maissel et al.  pointed out that the film materials
prepared in the gas phase can be also obtained from elec-
trochemical deposition in the liquid phase, and vice versa.
Hydrogenated carbon films were successfully synthe-
sized by Na mb  in 1992 from electrochemical deposi-
tion technique. Generally, using a high voltage, the films
were deposited on substrate as cathode and graphite or Pt
served as anode [4,5]. The report has attracted much at-
tention in view of the facts of low-cost, random choice of
substrates an d car bon sources, and easy doping.
2. Amorphous Carbon Films Prepared by
Cathodic Deposition Mode
2.1. Application of High-Voltage (1000 - 3000 V)
Some experiments have proved that hydrogenated amor-
phous carbon films with obvious characteristics of di-
amond structure can be prepared using liquid–phase de-
position under atmospheric pressure and low temperature.
Figure 1 shows SEM photographs of the films deposited
from organic solvents of methanol, ethanol, and iso-
propyl alcohol. These organic solvents possesses a lower
electrical conductivity and hence a higher voltage in the
range of 1000 - 3000 V should be applied between two
electrodes. All the same, the deposition rate is very low.
Two electrodes can be regarded as idea parallel plate
capacitor. Solution with a dielectric constant ε is pola-
rized under the action of external electric field. Induced
charges created on the electrodes and the density (also
*Corresponding a uthor.
C. Y. CHE ET AL.
OPEN ACCESS OJSTA
Figure 1. SEM photographs of the films depositd from var-
ious organic solvents of (a) methanol ; (b) ethanol ,
and (c) is opropyl alcohol  at 1000 V.
known as the polarization ability) is determined by the
is constant and E
electric field intensity. Obviously, under the same depo-
sition conditions P depends only on the dielectric con-
stants. It is therefore necessary to select organic solutions
with high dielectric constant . Carbon sources fre-
quently used contain methanol , alcohol [11,12],
methyl cyanide, DMF, DMSO [13,14], and so on. The
growth mechanism of carbon membrane can be briefly
described as follows: 1) the center of positive and nega-
tive charges in polarized molecules is non-coincidence
and electronic distribution tends to the polar functions.
Under the action of a high external electric field, the
charges are more away from the center, molecules will
be induced polarization; 2) positively and negatively
charged polar functional groups generate in the solution
by interrupting weak chemical bonds; 3) meanwhile, the
surface of the electrodes is activated, and the charged
particles move to the electrode and are captured on the
activity locations; 4) the positively charged groups are
captured and obtains the extra electron on the cathode,
and then dehydrogenation and deoxidization reactions
occur to form C-C bonds . It is also necessary for a
ideal reactant to have a low viscosity coefficient for the
ideal reactants. If the viscosity coefficient of electrolyte
is too high, negatively charged ions are difficult to move
away from the interfacial reaction zone during reaction,
which will lead to the accumulation of more and more
anions near the cathode. As a result, an electric layer is
formed in the contact zone between the substrate and the
organic reagent. The electric field in the electrical double
layer will offset the outfield, making polarization and
bond breaking in the region difficult, which is not con-
ducive to the grow th of carbon films. Basic properties o f
commonly used carbon sources are shown in Table 1.
2.2. Application of Mid-Voltage (200 - 800 V)
Inspired by atomic hydrogen effectively etching graphite
phase in vapor deposition, we attempted to add deionized
water into organic reagent. The results showed the depo-
sition voltage reduced or growth rate increased and the
Table 1. Basic properties of commonly used carbon sources.
(mPa·S) Dipole mo men t
Acetic acid 6.2020
The super scripts refer t o the measuri ng temperatu re and datas refer to refer-
C. Y. CHE ET AL.
OPEN ACCESS OJSTA
sp3 content in the films increased , and even nano-
diamond particles have been found in the deposited
fil ms .
We proposed a thermodynamic coupling model to ex-
plain the above phenomenon . The growth process of
carbon film can be divided into two steps: 1) organic
molecules are decomposed into the groups containing
carbon and there are captured by the electrode, 2) carbon
films grow through a series of reactions. Using methanol
as an example, carbon films are grown by generating
methyl and occurring dehydrogenation conversion. The
procedure is simply expressed as follows
is the change of Gibbs free energy. The in-
crement of Gibbs free energy is a criterion for judging
whether a reaction occurspontaneously at constant tem-
perature and constant pressure.
reaction (1) to the right with energy input and
means the methyl in thermodynamics can spontaneously
translate into carbon and hydrogen. Note that this reac-
tion usually needs to overcome an energy barrier, other-
wise the resultant may be metastable phase, such as
ethylene chain etc. H2O can be easily decomposed into H
ion and hydroxyl by inputting a lower energy, that is,
lower voltage because of its strong polarity. H ion ob-
tains electron to change into atomic hydrogen. Hydrogen
atoms near the cathode should have the following two
functions: interrupting the C-H bonds in the methyl to
form C-C bond, which is better for increasing growth
rate, and breaking chemical bonds in organic molecules
owing to large amounts of energy from the association of
H0.5H0300 K,101KPaGT P⋅→∆ <==
It is believed that three reactions (1), (2), and (3) could
be thermodynamic coupled when the above two
processes occur at the same time. The overall coupling
reaction is expressed as follows:
GGG xG∆=∆+∆+ ∆
represents the coupling coefficient, increased
with concentration of hydrogen atoms near the substrate.
The larger the coupling coefficient
, the smaller
the reaction (4) is more easily to right. Therefore, the
growth of carbon films can occur under a low voltage at
which water molecules can be polarized and decom-
2.3. Application of Low Voltage
Attempts had been made by others to prepare amorphous
carbon films using aqueous solution of acetic acid or
formic acid as electrolyte [19,20] under the pressure less
than 20 V. This result is easy to be explained based on the
above discussion because hydrogen ions can be sponta-
neously generated from the ionizatio n of aqueous solution
of acetic acid or formic acid. Therefore, the reaction (4)
can carry out to right only applying a low voltage. That
means a large amount of free ion hydrogen in liquid
phase guarantees to grow amorphous carbon films under
low external voltage. Effect of voltage on the carbon film
growth ca n be s ummarized in Table 2.
3. Amorphous Carbon Films Prepared by
Anodic Deposition Mode
So far only a few researchers attempt to prepare carbon
films by anode deposition mode. A brittle carbon film
has been successfully synthesized on the anode in 1997
by Novikov et al. , using anodic oxidation of aqueous
solution of ammonia acetylene under 2.0 - 3.0 V. Mean-
while, in 2001 they gained nano-diamond particles chos-
ing 50 mol% ammonium acetate in acetic acid solution as
electrolyte at the anodic bas emen t . In other reports
such as Shevchenko  and Li  et al. C2HLi/DMSO
and methanol were used as solutions.
3.1. Amorphous Carbon Films Prepared by
Anodic Deposition Mode
It is generally believed that the only chemical bonds with
lowest energy, for example, methyl and hydroxyl in
CH3OH, can be interrupted under high voltage. However,
we think the bond polarity is equally important in addi-
tion to bond energy. The bond with a stronger polarity or
bigger electronagativity difference is more easily inter-
rupted. Bond energy and electronegativity difference of
typical chemical bonds are shown in Table 3.
The bond energies of three types of bonds in methanol,
H-CH2OH, CH3O-H, and CH3-OH, are 411 kJ·mol−1,
Table 2. Effect of voltage on the carbon film growth.
Electrolyte Voltage values Voltage function
Pure organic reagents
Polarization and decomposition
of organic reagents
with deionized water
200 V - 800 V
Polarization and decomposition
of deionized water
Electrolyte with a
large amount of H
ions <20V Trapping hydrogen ions
C. Y. CHE ET AL.
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Table 3. Bond energy and electronegativity difference of
typical chemical bonds.
Chemical bond C-C C-O C-H O-H
Bond energy (kJ /mol) 346 358 411 459
Electronegativity difference 0 1 0.4 1.4
459 kJ·mol−1, and 358 kJ·mol−1, respectively. It is clear
bond possesses a lowest energy and moderate
electronegativity, and both the bond energy and electro-
negativity of the
bond are largest. Therefore, in-
terruption of both bonds of
in a strong electric field:
. The radicals
to anode to form amorphous carbon film by a series of
dehydrogenati on and deoxidization reactions.
3.2. Effect of Deionized Water
In contrast, we have also investigated the influence of
deionized water on growth of carbon films on anode.
Silicon wafer is elected for the substrate to prevent the
anode oxidation and the deposition voltage is kept at 800
V. Infrared spectra of the films deposited from various
concentration methanol in water are shown in Figure 2.
It can be seen that the films contain large number of O-H
bonds and its content increases with the increase of dei o-
nized water, so also for the O-Si-O, which demonstrates
that the substrate as anode is easy to be oxidized. OH−
ions generated by electrolyzing water captured on the
anode are not conductive to carbon film growth on the
anode, which is different with cathodic deposition mode.
4. Doped Carbon Films Prepared by Liquid
4.1. Thin Films Doped with Metal
Microhardness, tribological property, adhesive strength,
chemical stability, and electrical conductivity of the films
can be generally enhanced by doping metals . Com-
pared with the gas phase method, doping metals into the
carbon films becomes much easier for liquid phase depo-
sition technique. Amorphous carbon films doped with
metal have been successfully obtained using metal salt
solution as electrolyte or nanoparticles as dop ant.
4.1.1. Metal Salt Electrolyte
Wan et al.  prepared diamond-like carbon film at
1200 V and 55˚C - 60˚C for 10 h using methanol as car-
bon source and ferric acetylacetonate as dopant. The re-
sults showed that 10% doped Fe with preferred orienta-
tion distributed in the DLC matrix and the sp3 content
and the electrical conductivity were enhanced. It is well
Figure 2. IR spectrogram of the deposited carbon films
from volume ratio of H2O to CH3OH of (a) 0; (b) 1; and (c)
understood that methyl CH3+ and Fe ion Fe3+ move to
cathode to form the Fe-doped DLC film under high vol-
A brown Cu-doped DLC film can also be deposited at
1600 V and 60˚C using from methyl cyanide as carbon
as dopant , where
the solubility of copper salt solution, current density, and
deposition time are 1.5 mM, 2.0 - 5.5 mA·cm −2, and 20
min, respectively. It was said that the sp3 content in the
film was increased to a certain extent. In order to achieve
a co-deposition of Cu and carbon, dimethylsulfoxide
(DMSO) with a large dipole moment and dielectric con-
stant was selected as carbon source .
C. Y. CHE ET AL.
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Carbon films doped metals can be also achieved by
anode deposition mode. A light yellow
film with a
Ru/ C /N
atomic ratio of 0.28/0.33/1 has
be synthesized using a mix solution of
methyl cyanide at a voltage of 1200 V and a temperature
of 55˚C for a deposition time of 10 h. The nanoruthenium
particles with (101) preferred orientation were not dis-
tributed evenly throughout the doped film. Nevertheless,
the resistivity of the Ru-doped film decreased greatly
from 108 Ω·m to 100 Ω·m. Yu et al.  used cathode
deposition mode instead to prepare Ru-doped amorphous
carbon films, containing the evenly distributed nano-
Ruthenium particles with a diameter range of 2 - 4 nm.
Ma et al.  claimed that doping Ni can improve adhe-
sive strength and wear resistance. Experiments indicate
that it is also easy to realize co-doping of metals (for
example, Cu and Ag ) in electrochemical deposition.
4.1.2. Metal Nanoparticles as Dopant
In fact it is difficult to con trol the co-deposition of metal
and carbon because of an obvious difference in ioniza-
tion properties between metal salts and organic solvents.
Consequently, some researchers use metal nanoparticles
as dopant. During the deposition process electromagnetic
stirring should be essential to ensure uniformity. Gold
nanoparticles (50 nm) were uniformly embedded in
amo r phous carbon film on silicon substrate using me-
thanol solution at the voltage of 1200 V for the deposi-
tion time of 5 h . Under the same deposition condi-
tions, the resistivity decreased from 108 Ω·cm for the
film without Au-doping to 104 Ω. cm with doped Au,
which means that transition of amorphous carbon films
from insulator to semiconductor should be also possible
by doping metal nanoparticles.
Adding Palladium nanoparticles and using methanol
and dehydrolinalool (C10H16O) as carbon source, the
amo rphous carbon film, in which Pd particles with a size
of 1 - 5 nm evenly distributed can be successfully achieved
4.2. Non-Metal Dopants
Doping nonmetal into DLC films is directed to enhanc-
ing the content of sp2 and improving optical and elec-
trical properties. A low emission current greatly limits
the application of amorphous films as cold cathode.
Doping N into amorphous carbon films can elevate Fer-
mi level and lower work function, and then improve
emission effect [35,36]. Carbamide and C60 derivant (C60
[(NH2)2CNCN]5) can be used as doping agent . Typ-
ically, different N doping contents have been obtained
from a mixed solution with various molar ratios of me-
thanol to carbamide in the range of 1:2000 - 1:500 .
The deposition voltage fixed at 600 V, the reaction time
was 4 h, and the reactor temperature was controlled at
60˚C. Analysis showed that nitrogen was doped into the
film in the form of C=N and C-N bonds and the sp2 con-
tent increased. Meanwhile, the field emission current
density came to 59.5 μA /mm2 while electric field was
about 24 V/mm, which fulfill requirement of field emis-
sion display on electronic emitter.
Doping phosphorus even with a small amount for ex-
ample 1% can strongly decrease the resistivity by 6 - 7
orders of magnitude , improve the critical density of
field emission and band gap was also increased. Moreo-
ver, it has been found that P-doped amorphous carbon
films decreased platelet adsorption rate and activity, in-
hibited blood coagulation, and then may improve biolog-
ical compatibility .
Wan et al.  used a mixed solution of methanol and
three phenyl phosphate with the molar ratio of 1000:1 to
prepare P-doped carbon films. Phosphorus combined
with carbon in the form C=P bond and the graphitization
degree of the films enhanced confirmed by XPS and
Raman spectroscopy. The experiments of the field emis-
sion characteristics showed that the threshold electric
field and current density for the undoped- and doped-P
films were 12 V/mm, 45 .7 μA/mm2, and 9.5 V/mm, 12.6
μA/mm2, respectively . Similarly, the emission pr op-
erties can be also improved by doping sulfur using car-
bon disul f i de  or thiophene [4 4] as dopant.
For dopin g fl uorine, 2,2,2-Trifluoroethanol
, TEF)is a good candidate dopant , be-
ing identical in structure and property with methanol .
After fluorine doping, the amorphous carbon film is
present as nano-scale bamboo shoots and the structure is
closely packed and highly preferred orientation. The
contact angle of the F-doped film with water is 145˚.
This is attributed to its special structure and low surface
energy of the
, which is confirmed that fluorine ex-
ists in main forms of
(x = 1,2) in the
film by infrared analysis.
4.3. Other Dopants
In order to decrease internal stress simultaneously on the
premise of ensuring hardness, the allotropes of carbon
including C60 and carbon nanotube were doped into DLC
films. Hu et al.  had doped C60 nano particles into the
amorphous carbon films. The concentration of C60 nano
particles in the electrolyte was 0.28 mg·mL−1. D eposition
voltage, temperature, and reactive time were 1600 V,
50˚C, and 10 h, respectively. The result showed that the
film hardness although little changed, the internal stress
was effectively reduced.
Similarly, carbon nanotube is also easily doped in
DLC films using liquid phase deposition. A typical exp e-
rimental condition is that N,N-d imethylformamide (DMF,
99.5%) is used as electrolyte, in which the content of
carbon nanotube is 0.53 mg· mL −1, and deposition voltage,
C. Y. CHE ET AL.
OPEN ACCESS OJSTA
temperature, and deposition time are 1400 V, 50 ˚C, and
5 h, respectively . As is known to all, carbon nano-
tube possesses excellent mechanical properties including
high elastic modulus and toughness because of its special
structure . The DLC film after doping carbon nano-
tube has changed in property. The internal stress de-
creased from 1.2 GPa to 0.83 GPa, hardness increased
from 10.28 GPa to 12.47 GPa, and Young modulus de-
creased from 253.64 GPa to 206.66 GPa. The spectrum
analysis showed that the ratio of sp2 in the film in-
Zhang et al.  prepared lamellar nano oxidized
graphene from liquid phase, and then doped them into
DLC film, where methanol used as reactant and the con-
tent of the oxidized graphene was 1 mg/ml. Deposition
voltage and time were 1200 V and 10 h, respectively.
The properties of the DLC films are obviously improved
by doping graphite. The hardness and Young modulus
are greatly increased from 5.1 GPa and 137.4 GPa to
10.1 GPa and 171.1 GPa, respectively, and the friction
coefficient is also decreased significantly. The electrical
resistivity decreases from 108 Ω·cm to 102 Ω·cm. The
explanation could be the inhibition of plastic deformation
and the relaxation of internal stress by the lamellar gra-
An attempt has been made to dope ZrO2 into the car-
bon films in our recent research work under low voltage.
In the experiments a mixed solution of acetic acid, alco-
hol, and deionized water with a volume ratio of 1:4:5 is
selected as electrolyte. The presence of acetic acid can
provide free hydrogen ions in abundance and alcohol is
helpful to get a better suspension with nano ZrO2 par-
ticles. Raman spectra of the films deposited at a pulsed
voltage of 100 - 200 V and a deposition time of 30min
show that there are ZrO2 characteristic strong peaks lo-
cated between 200 - 700 cm−1 and a broad peak centered
at 1580 cm−1, attributing to amorphous carbon as showed
in Figure 3. The film doped ZrO2 nano particles is very
smooth and compact, as shown in Figure 4.
5. Existing Problems
Liquid electro-deposition technique is a promising me-
thod to synthesis amorphous carbon films on the com-
plicated workpiece with large area under low temperature.
However, there are still some problems to be solved.
1) Mechanical properties and adhesive strength
The recent researches focus on chemical components,
morphology, and micro-structure. There is few report on
the properties of the films deposited from electro-depo-
sition because they are poor due to lack of high-energy
ion bombardment during the film growth. There is an
obviously difference in mechanical properties with the
films by vapor phase deposition , although some im-
provements have been made by doping metal and in-
Figure 3. Raman spectra of the films doped with ZrO2 ((a)
0.6 mg/L; (b) 0.2 mg/L) .
Figure 4. SEM photograph of the film doped with ZrO2
troducing additive agent. Table 4 shows the micro-hard-
ness of the carbon films deposited using various depos i-
2) The ratio of
The properties of DLC films strongly depend on the
. The films deposited by liquid
phase method contain too much graphite phase and their
C. Y. CHE ET AL.
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Table 4. Micro-hardness of the carbon films.
Films type Preparation
a-C:H r, f 2000 - 4500 
a-C:H r, f 3000 - 5000 
a-C:H Ion beam 3000 - 5000 
a-C:H Magnet-Sputtering 2500 - 4300 
a-C:H DC -3000 
Dense Carbon Ion beam ≤6000 
nano-particles Electro-deposition ~673 
quality is difficult to control. It is necessary to try to find
out suitable carbon sources, additive agents, and deposi-
tion proce s s.
3) The exploration of deposition mechanism
Currently, study on the growth mechanism of carbon
films from liquid phase deposition is still in the initial
stage. Although some experimental results have been
slightly analyzed, it is necessary to establish a perfect
theory to guide experimental researches on film growth
With the further research on growth of amorphous car-
bon films by electro-deposition, the theory and technol-
ogy in this field will be gradually improved. The tech-
nique will possess a huge development potential and be
widely used to prepare new materials because of unique
properties and low cost. Research works in future should
be more intensive in the following aspects: 1) selection
of carbon sources, additives and dopants; 2) mechanism
of formation and stability of amorphous or diamond-like
carbon with diamond phase structure; and 3) application
of the films.
The project is financially supported by the Cultivation
Fund of the Key Scientific and Technical Innovation
Project, Ministry of Education (N01707015) and by Ma-
jor Project of Chinese National Programs for Fundamen-
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