Electrical properties of amorphous carbon semiconductor prepared using a naphthalene precursor

Article abstract of DOI:10.1246/bcsj.20120201

An amorphous carbon (a-C) semiconductor was prepared by the pyrolysis of naphthalene in the presence of AlCl3 under a nitrogen gas flow. Electrical properties, such as the Seebeck coefficient (S) and electrical conductivity, of the a-C materials were studied as a function of the heat treatment temperature (7731273 K) and time (530 h). The electrical conductivity of a-C increased with the heating temperature. At a heating temperature of 873 K, a-C exhibits p-type semiconducting behavior (S = 0.41mVK^-1, α = 2.0 × 10 ^-5Scm^-1) and consists of a mixture of sp2-bonded carbon sheets (average sheet size: ca. 1.0 nm) and polycyclic aromatic hydrocarbons (PAH). After prolonged heat treatment (30 h) at 873 K, the electrical conductivity of a-C was increased by a factor of approximately 50 without loss of the semiconductivity (S = 0.20mVK^-1, α = 9.6 × 10 ^-4Scm^-1). The increase in the electrical conductivity of a-C is attributed to the decomposition of insulating PAH compounds by the prolonged heat treatment. Thus, precise control of the heating temperature and time has a significant effect on the p-type semiconductivity and electrical conductivity of a-C materials.

Full text of DOI:10.1246/bcsj.20120201

© 2013 The Chemical Society of Japan
Bull. Chem. Soc. Jpn. Vol. 86, No. 1, 45-50 (2013)
45
Selected Papers
Electrical Properties of Amorphous Carbon Semiconductor
Prepared Using a Naphthalene Precursor
Yasunori Inoue, Mayo Sugano, Shunsuke Kawakami, Masaaki Kitano,
³
Kiyotaka Nakajima, Hideki Kato, and Michikazu Hara*
Materials and Structures Laboratory, Tokyo Institute of Technology,
4259 Nagatsuta, Midori-ku, Yokohama, Kanagawa 226-8503
Received July 25, 2012; E-mail: mhara@msl.titech.ac.jp
An amorphous carbon (a-C) semiconductor was prepared by the pyrolysis of naphthalene in the presence of AlCl3
under a nitrogen gas flow. Electrical properties, such as the Seebeck coefficient (S) and electrical conductivity (·), of the
a-C materials were studied as a function of the heat treatment temperature (773-1273 K) and time (5-30 h). The electrical
conductivity of a-C increased with the heating temperature. At a heating temperature of 873 K, a-C exhibits p-type
semiconducting behavior (S = 0.41 mV K¹1, · = 2.0 © 10¹5 S cm¹1) and consists of a mixture of sp2-bonded carbon
sheets (average sheet size: ca. 1.0 nm) and polycyclic aromatic hydrocarbons (PAH). After prolonged heat treatment
(30 h) at 873 K, the electrical conductivity of a-C was increased by a factor of approximately 50 without loss of
the semiconductivity (S = 0.20 mV K¹1, · = 9.6 © 10¹4 S cm¹1). The increase in the electrical conductivity of a-C is
attributed to the decomposition of insulating PAH compounds by the prolonged heat treatment. Thus, precise control of
the heating temperature and time has a significant effect on the p-type semiconductivity and electrical conductivity of a-C
materials.
Carbon has several different allotropic forms, such as graph-
ite, nanotubes, fullerenes, diamond, and amorphous carbon (a-
C), and these materials have attracted much attention due to
their unique structures and properties.1-4 Amorphous carbon (a-
C) is known as the main constituent of charcoal, soot (carbon
black), and activated carbon and has a number of structures
containing different configurations of threefold (sp2) and
fourfold (sp3) coordinate sites. Robertson et al. reported that
the band structure and optical properties of a-C films can be
continuously varied by changing the sp2 carbon content, where
sp2 sites are responsible for the optical and electrical properties,
whereas the sp3 regions govern the mechanical properties.5-7
The ratio of sp2 and sp3 sites in a-C materials is largely depend-
ent on the precursor materials, method of preparation, and heat-
ing conditions. For example, diamond-like amorphous carbon
containing a significant fraction of sp3 bonds has been prepared
by various deposition methods, such as focused ion beam
(FIB), sputtering, pulsed laser deposition, and plasma-enhanced
chemical vapor deposition (PECVD).8,9 Among these deposi-
tion methods, PECVD is utilized to control the sp3 bonding
fraction in a-C materials. However, the electrical properties of
a-C consisting of mainly sp2 bonds are yet to be clarified.
Carbonization of mesophase pitches would be a highly
controllable bottom-up approach for the electrical properties of
a-C consisting of mainly sp2 bonds because the structure of a-C
can be controlled by heating temperature and time. Mesophase
pitches, which are commonly referred to as petroleum or coal
tar pitches, are synthesized from pure aromatic hydrocarbons
such as naphthalene in the presence of an AlCl3 catalyst.10-14
These pitches are graphite-like amorphous carbon materials
with a high fraction of sp2 bonds and have thermotropic
behavior. While syntheses of graphitized carbon fibers using
mesophase pitches have been intensively studied, the semi-
conducting properties of these pitches have not been inves-
tigated in detail. A carbon atom with sp3 configuration forms a
strong · bond to an adjacent atom, whereas in the case of sp2
configuration, a carbon atom has three sp2 orbitals that not only
form · bonds, but also ³ bonds. These bonding characteristics
play an important role in the band structure of the carbon
materials. Therefore, it is of fundamental interest to investigate
the semiconducting behavior of a-C consisting of mainly sp2
carbon.
In this work, a-C materials are synthesized by heating
naphthalene in the presence of AlCl3 under a nitrogen gas
flow to prepare a-C materials with a significant fraction of sp2
bonds. The structural and electrical properties of the obtained
materials are studied as a function of the heat treatment tem-
perature and time.
Experimental
³ Present address: Institute of Multidisciplinary Research for
Advanced Materials, Tohoku University, 2-1-1 Katahira,
Aoba-ku, Sendai, Miyagi 980-8577
Synthesis. Naphthalene (NP) pitch was synthesized by
heating a mixture of naphthalene (Kanto Chemical Co.) and
Published on the web December 15, 2012; doi:10.1246/bcsj.20120201

46
Bull. Chem. Soc. Jpn. Vol. 86, No. 1 (2013)
Electrical Properties of Amorphous Carbon Semiconductor
AlCl3 (Kanto Chemical Co.) in a naphthalene:AlCl3 molar ratio
¹1
(A)
of 2:1 at 673 K for 15 h at a heating rate of 10 K min under
1.2
a nitrogen gas flow. The resulting pitch was washed with
1 M HCl and ethanol to remove residual AlCl3 and unreacted
naphthalene. After drying in an oven (353 K), the obtained
pitch (powder) was uniaxially pressed at 40 MPa to form a
pellet (10 mm diameter, 2 mm thick). The pellet was annealed
at 773-1273 K for 5 h or at 873 K for 15-30 h under a nitrogen
gas flow to produce a-C materials.
0.8
873
973
1273
0.4
Characterization. The Seebeck coefficient and electrical
conductivity were measured to clarify the electrical properties
of a-C. The electrical conductivity of a-C was measured using
the four probe method with a resistivity meter (MCP-T610,
Mitsubishi Chemical Analytech). The probe head used consists
of four metal tips arranged in a linear array with a 1.5 mm
gap between each tip. The Seebeck effect was measured at
298 K in air using an apparatus comprising two copper blocks
separated by a distance of 2 mm. The copper blocks were
cooled or heated using Peltier devices to control the difference
in temperature between the two copper blocks. X-ray powder
diffraction (XRD; Ultima-IV, Rigaku), Raman spectroscopy
(NRS-3100, Jasco), and Fourier transform infrared (FT-IR;
FT-IR 6100, Jasco) spectroscopy were used to characterize
the structure of a-C. The H/C atomic ratios of samples were
estimated by elemental analysis of C and H using a Vario
MICRO cube, Elementar. Raman spectroscopy was performed
under ambient conditions with a backscattering geometry using
a green laser diode (LD) excitation laser operating at 532.0 nm.
The laser was focused on a 1 ¯m spot with a 100© objective
lens, and a laser power of 100 mW was employed. Raman
spectra were measured with reduced powers down to 0.1-0.2
0.0
0.0
1.0
2.0
3.0
4.0
Temperature difference, ΔT/K
3.0
1.0
(B)
-1.0
-3.0
-5.0
-7.0
σ
673
873
1073
1273
Heating temperature/K
¹1
Figure 1. (A) Seebeck voltage as a function of temperature
difference (¦T) for the a-C materials heated at various tem-
peratures. (B) Electrical conductivity (log ·) as a function
of the heating temperature.
mW at a resolution of 1 cm to avoid damaging the sample.
XRD patterns of each sample were measured using a dif-
fractometer equipped with a copper target X-ray tube set to
¹1
40 kVand 20 mA. Samples were scanned at a rate of 0.1° min
with an open divergence slit, an open scattering slit and a
1.0 mm receiving slit. The parallel beam method was used for
all samples.
Table 1. Electrical Conductivity (·) and Seebeck Coeffi-
cient (S) of the a-C Materials Heated at Various Temper-
atures and Times
FT-IR spectra were recorded at room temperature using
a spectrometer equipped with an MCT detector at a 4 cm
resolution. 64 scans were collected for each spectrum. The
samples were pressed with KBr powder to form pellets (50 mg,
20 mm diameter) with an a-C:KBr weight ratio of 1:25.
¹1
·
/S cm
S
Heating temperature, time
¹1
¹1
/mV K
¹6
¹5
a)
773 K, 5 h
873 K, 5 h
973 K, 5 h
1273 K, 5 h
2.0 © 10
2.0 © 10
0.3
1.5 © 102
®
0.41
0.02
0.01
Results and Discussion
¹4
Seebeck Coefficient and Electrical Conductivity.
Figure 1A shows the Seebeck voltage as a function of the
temperature difference (¦T) for a-C heated at various tem-
peratures. The Seebeck voltage of a-C heated at 773 K could
not be measured due to its high resistivity. In the case of a-C
heated at 873 K, the Seebeck voltage gradually increased with
increasing ¦T, which indicates p-type semiconductivity. There-
fore, the room-temperature electrical conductivity of the a-C
material prepared by heating polymerized naphthalene at 873 K
involves holes as charge carriers. Small Seebeck voltages of
less than 0.1 mV were obtained for heating temperatures higher
than 973 K. Figure 1B shows the electrical conductivity of a-C
heated at various temperatures. The electrical conductivity of
a-C was significantly increased with the heating temperature,
873 K, 15 h
873 K, 30 h
1.8 © 10
9.6 © 10
0.39
0.20
¹4
a) Immeasurable level.
and that for the sample heated at 1273 K was approximately
eight orders of magnitude higher than that for the sample
heated at 773 K. The Seebeck coefficients calculated from the
slopes in Figure 1A and the electrical conductivity of a-C
are summarized in Table 1. The optimum heating temperature
to form semiconducting a-C materials was determined to be
873 K. These results suggest that the heating temperature plays
an important role in the conversion of insulating a-C to a p-type
semiconductor material. To evaluate the conduction mechanism

Y. Inoue et al.
Bull. Chem. Soc. Jpn. Vol. 86, No. 1 (2013)
47
(A)
5 h
15 h
30 h
1273 K
973 K
1.2
0.8
0.4
0.0
873 K
773 K
NP
1440 cm−1
1280 cm−1
0.0
1.0
2.0
3.0
2000 1800 1600 1400 1200 1000
Raman shift/cm−1
Temperature difference, ΔT/K
Figure 2. Seebeck voltage as a function of temperature
difference (¦T) for a-C heated at 873 K for 5-30 h.
(B)
of the a-C material, dependence of electrical conductivity on
temperature was measured (Figure S3). It was clearly seen that
the conductivity increases with temperature, which is a typical
feature of thermal activated conduction in semiconductor
materials. This behavior could also be interpreted as thermally
activated hopping mechanism in electron transport through the
a-C material. The temperature dependence of the conductivity
(·) was modeled by an Arrhenius-type equation:
30 h
15 h
5 h
· ¼ ·0 expðꢀEa=kBTÞ
ð1Þ
where ·0 is a constant, Ea is the activation energy, and kB is
Boltzmann constant. Based on the slopes in Figure S3, the
activation energy of a-C heated at 873 K is determined to be
160 meV, which is consistent with the reported values.15,16
The a-C heated at temperatures above 973 K exhibited much
higher electrical conductivity than that of 873 K; however, the
semiconducting behavior disappeared. The activation energy
for the electrical conductivity may decrease with increasing the
heating temperature because high conductivity samples show
quasimetallic behavior.15,16
Figure 2 shows the Seebeck voltage as a function of
temperature difference (¦T) for a-C heated at 873 K for 5-
30 h. All slopes shown in Figure 2 are positive, which indicates
that a-C has p-type semiconductivity after prolonged heat
treatment. Table 1 shows that the Seebeck coefficient of a-C
gradually decreases and the electrical conductivity increases by
more than an order of magnitude as the heating time increases.
Thus, the electrical conductivity of a-C can be successfully
improved without loss of its semiconductivity.
1800
1600
Raman shift/cm−1
1400
1200
1000
Figure 3. (A) Raman spectra of the a-C materials heated at
various temperatures for 5 h, and (B) at 873 K for 5-30 h.
of G bands into two well-resolved peaks is observed in the
Raman spectra of mesophase pitch or polycyclic aromatic
hydrocarbons (PAH) such as pyrene and coronene. The lengths
of their covalent C-C bonds are similar, but not identical, with
that for graphite. The distorted structure of the PAHs leads to
a significant variation in their C-C bond stress, which results
in splitting of the G band. The D bands of NP and a-C heated
¹1
at 773-873 K show several components at 1150-1500 cm
¹1
(Figure S1). The shoulder peaks around 1280 and 1440 cm
are attributed to the symmetric vibrational C-C sp2 bond and
C-H bending vibration in the finite domain size of PAH,
respectively.18-20 The other peaks at 1250 and 1376 cm are
¹1
most likely associated with the A1g type vibration of the
specific benzene skeletons of molecules present in the pitch.20
These results imply that a-C heated at lower than 873 K
contains small sp2 carbon sheets, C-H bonds, aryl-aryl bonds,
and a naphthenic structure.21 The shoulder peaks around 1280
Raman Spectroscopy. To clarify the correlation between
the electrical properties and the structure of a-C, detailed
characterization of the material was conducted. Figure 3 shows
Raman spectra of the a-C materials prepared by heating
¹1
polymerized NP under various conditions. The spectra of all
and 1440 cm decreased with an increase in the heating
¹1
samples have two broad peaks at ca. 1600 and ca. 1350 cm
,
temperature and then diminished at heating temperatures
greater than 973 K, which indicates that the PAH structures
are decomposed or carbonized into carbon sheets. Furthermore,
heat treatment higher than 973 K results in the broadening of
the G bands. This may be due to the existence of the D¤ band
(1620 cm¹1), which appears as a shoulder of the graphite E2g
which are attributed to the G (stretching mode of graphite) and
D (breathing mode) bands. The peak observed around 1600
¹1
cm is deconvoluted into two components centered at 1604
¹1
and 1576 cm (Figure S1), the latter of which is close to the
allowed E2g mode for a graphite single crystal.7,17 The splitting

48
Bull. Chem. Soc. Jpn. Vol. 86, No. 1 (2013)
Electrical Properties of Amorphous Carbon Semiconductor
c
b
a
(A)
(B)
0.4
0.2
973 K
873 K
773 K
973 K
873 K
773 K
NP
NP
1200
1000
800
600
4000
3000
2000
1000
Wavenumber/cm−1
Wavenumber/cm−1
(C)
c
a
c
c
c
b
b
c
c
c
b
b
b
c
¹1
Figure 4. FT-IR spectra of a-C heated at various temperatures for 5 h: (A) 400-4000 and (B) 500-1200 cm (C) Correlation
¹1
between infrared frequencies in the range of 700-900 cm (a, b, and c) and the position of hydrogen atoms bonded to PAHs.
mode at 1580 cm¹1. It is previously seen in microcrystalline
graphite with disordered structure.22,23 Figure 3B shows that
the peaks due to PAH structures disappear after heat treatment
for more than 15 h. Therefore, prolonged heat treatment at
873 K also leads to the decomposition of PAH structures in the
a-C material.
Correlation of Electrical Properties with Carbon Struc-
ture. Figure 5 shows XRD patterns of the a-C materials
heated under various conditions. All XRD patterns have two
broad diffraction peaks around 25-26 and 45°, which are
assigned to the carbon (002) and (10) planes, respectively.
These diffraction patterns are typical of a-C with a turbostratic
structure.28 Figure 5A shows that the intensity of the (002) and
(101) diffraction peaks grow at the boundary between 973 and
1273 K, which suggests that the expansion and stacking of
carbon sheets are accompanied by decomposition of the PAH
structures in NP during the heat treatment.29 In contrast, there
was no noticeable difference in the XRD patterns of the a-C
materials heated at 873 K for various times (Figure 5B), which
suggests that the decomposition of PAH proceeds in preference
to the expansion and stacking of carbon sheets at 873 K.
Table 2 summarizes the composition of the a-C materials
determined by elemental analysis. The H/C ratio was signifi-
cantly decreased with an increase in the heating temperature,
and is due to the expansion of carbon sheets and the pyrolysis
of PAH, as evidenced by the Raman and FT-IR spectra, as well
as the XRD patterns (Figures 3A, 4A, and 5A). However, the
H/C ratio of a-C heated at 873 K was only slightly decreased
after prolonged heat treatment. This is not attributed to the
growth of carbon sheets, but to the decomposition of PAH. If
the carbon sheet is made of sp2 (trivalent) carbon atoms that
form a hexagonal network isotropically, then the mean size of
the carbon sheets can be estimated from the H/C values and
the diameter of a 6-membered carbon ring. The carbon sheet
IR Spectroscopy. Figure 4 shows FT-IR spectra of the a-C
materials heated under various conditions. Although aliphatic
C-H (2850-3000 cm¹1), aromatic C-H (3000-3100 cm¹1),
aromatic C-C stretching (ca. 1600 cm¹1), and methylene C-H
in-plane bending (ca. 1440 cm¹1) are observed in the spectrum
of NP, these peaks disappear after heat treatment at more than
773 K (Figure 4A). Furthermore, the peaks due to the out-
of-plane vibration of aromatic C-H bonds are also observed
¹1
around 700-900 cm (Figure 4B), which can be used to esti-
mate the concentration of aromatic C-H bonds, and also to
follow the changes in aromatic substitution. The band around
¹1
880 cm is identified as the penta-substituted aromatic ring
containing isolated C-H bonds. Two vibrations (835 and 810
cm¹1) are attributed to the aromatic C-H of the two or three
adjacent hydrogens, and the band at 750 cm¹1 is due to the four
adjacent C-H groups of the naphthalene ring (Figure 4C).24-27
The intensity of these peaks decreased with an increase in the
heating temperature (Figure 4B), which indicates that the
PAH structures were decomposed by heat treatment. Figure S2
shows the same tendency as the results presented in Figure 4.
These results are in good agreement with the Raman spectra
obtained.

Y. Inoue et al.
Bull. Chem. Soc. Jpn. Vol. 86, No. 1 (2013)
49
(A)
(B)
1273 K
30 h
973 K
873 K
15 h
5 h
773 K
NP
20
40
60
80
20
40
60
80
2θ
/degree
2 /degree
θ
Figure 5. (A) XRD patterns of the a-C materials heated at various temperatures for 5 h, and (B) at 873 K for 5-30 h.
H/C = 0.5
H/C = 0.3
ca. 0.74 nm
ca. 1.23 nm
H/C = 0.2
ca. 2.21 nm
Figure 6. Relationship between the carbon framework size and H/C ratio.
Table 2. Elemental Analyses for the a-C Materials Heated at
Various Temperatures and Times
type semiconducting behavior decreases. These results indi-
cates that a-C materials consisting of small carbon sheets (ca.
1.0 nm) exhibit p-type semiconductivity, while further increase
in the carbon sheet size leads to quasimetallic conduction
(Figure 1 and Table 1). The sheet size of a-C heated at 873 K
remained unchanged after prolonged heat treatment up to
30 h. Therefore, enhancement of the electrical conductivity of
a-C semiconductors after prolonged heat treatment (Table 1)
at 873 K is not due to expansion of the carbon sheet size, but to
decomposition of the PAH structures in a-C.
Figure 7 shows a schematic diagram of the carbonization
process for the a-C materials. Polymerized naphthalene is
composed of PAHs bonded to one another, which are converted
into a-C during heat treatment above 773 K. The a-C changes
from insulator to conductor through to p-type semiconductor
with an increase in the heating temperature, accompanied by
an increase in the average carbon sheet size. These results
indicate that the carbon sheet size has a significant effect on the
semiconducting behavior of a-C (i.e., a-C containing 1-nm-
sized carbon sheets exhibits p-type semiconductivity). a-C
heated at 873 K for 5 h consisted of a mixture of PAH sheets
and small carbon sheets, of which the electrical conductivity
was very low due to the presence of PAHs with high resistivity.
Heating temperature, time
H/C
O/C
NP
0.54
0.43
0.32
0.19
0.08
0.03
0.01
0.01
0.01
0.00
773 K, 5 h
873 K, 5 h
973 K, 5 h
1273 K, 5 h
873 K, 15 h
873 K, 30 h
0.29
0.26
0.01
0.01
frameworks expected for a-C materials with specific H/C ratios
are illustrated in Figure 6. The average size of the carbon
sheets of the a-C materials heated at 773-873 K was deter-
mined to be ca. 1.0 nm, and that for the a-C materials heated at
greater than 973 K was more than 2.2 nm. The a-C materials
with small carbon sheets contain a lot of molecular defect
sites that generate holes as a majority carrier in valence band,
resulting in p-type semiconducting behavior.30 An increase in
the carbon sheet size leads to a decrease in the defect sites
and the special connectivity of each carbon sheets. This would
enhance electron transfer between carbon sheets although p-

50
Bull. Chem. Soc. Jpn. Vol. 86, No. 1 (2013)
Electrical Properties of Amorphous Carbon Semiconductor
NP
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Supporting Information
Band-fitting of the Raman spectrum of a-C heated at 873 K,
FT-IR spectra of the a-C materials heated at 873 K for 5-30 h,
and temperature dependence of conductivity for the a-C
material heated at 873 K for 5 h. This material is available
free of charge on the web at http://www.csj.jp/journals/bcsj/.
30 H. A. Yu, T. Kaneko, S. Otani, Y. Sasaki, S. Yoshimura,
Carbon 1998, 36, 137.