Optical properties and electrical transport in intercalated amorphous carbon

Article abstract of DOI:10.1016/j.materresbull.2006.04.009

Iodine and sulfur incorporated amorphous carbon (a-C) films are prepared by vapor phase pyrolysis of carbon rich organic precursor, maleic anhydride. Transmission electron microscopy (TEM) analysis depicts the intercalation induced local structural ordering and evolution of nano-graphitic phases in the sample. An X-ray photoelectron spectroscopy (XPS) study on these samples confirms the presence of intercalates. The study of optical properties of the samples by UV-vis absorption spectrometry accounts for a decrease in optical bandgap and Urbach energy with impurity incorporation and increase in pyrolysis temperature. This refers to induced structural ordering, decrease in disorder and increase in sp² or π-sites in the sample. Electrical conductivity measurement on a-C samples reveals increase in conductivity with pyrolysis temperature induced by iodine and sulfur incorporation.

Full text of DOI:10.1016/j.materresbull.2006.04.009

Materials Research Bulletin 41 (2006) 2000–2006
www.elsevier.com/locate/matresbu
Optical properties and electrical transport in intercalated
amorphous carbon
*
Latha Kumari , S.V. Subramanyam
Department of Physics, Indian Institute of Science, Bangalore 560012, India
Received 23 July 2005; received in revised form 7 April 2006; accepted 18 April 2006
Available online 8 May 2006
Abstract
Iodine and sulfur incorporated amorphous carbon (a-C) films are prepared by vapor phase pyrolysis of carbon rich organic
precursor, maleic anhydride. Transmission electron microscopy (TEM) analysis depicts the intercalation induced local structural
ordering and evolution of nano-graphitic phases in the sample. An X-ray photoelectron spectroscopy (XPS) study on these samples
confirms the presence of intercalates. The study of optical properties of the samples by UV–vis absorption spectrometry accounts
for a decrease in optical bandgap and Urbach energy with impurity incorporation and increase in pyrolysis temperature. This refers
to induced structural ordering, decrease in disorder and increase in sp2 or p-sites in the sample. Electrical conductivity measurement
on a-C samples reveals increase in conductivity with pyrolysis temperature induced by iodine and sulfur incorporation.
# 2006 Elsevier Ltd. All rights reserved.
Keywords: A. Amorphous materials; B. Vapor deposition; C. Electron microscopy; D. Optical properties; D. Electrical properties
1. Introduction
Carbon is a remarkable element that exists in different forms ranging from insulator diamond to metallic graphite to
conducting/semiconducting nanotubes. The great versatility of carbon materials arises from the strong dependence of
their properties on the ratio of sp2 (graphite-like) to sp3 (diamond-like) bonds in the material. There are many forms of
sp2 bonded carbons with various degrees of graphitic ordering, ranging from microcrystalline graphite to glassy
carbon. In general, an amorphous carbon can have any mixture of sp3, sp2 and even sp sites, with the possible presence
of up to 60 at.% hydrogen [1–3]. Thus it involves both s-electrons and p-electrons.
The synthesis and physical properties of a-C films have been studied fairly extensively, but the mechanisms for the
formation of the structure of these films and their correlation with observed physical properties are still not fully
understood. Amorphous carbon is a material that greatly varies in its electrical properties depending on the deposition
method, dopant content and deposition temperature. Chemical inertness, low friction, and high wear resistance of
amorphous carbon films [4] have attracted attention for a wide range of applications, such as protective coatings for
computer hard disks, high-precision bearings, and microelectronic systems.
In amorphous carbons, the p-electronic states are less spaced in energy than the s-electronic states, which are
entirely responsible for the physical properties of a-C in the visible-near UV photon energy range. Hence, the
* Corresponding author. Tel.: +91 80 2293 2313; fax: +91 80 2360 2602.
E-mail address: latha@physics.iisc.ernet.in (L. Kumari).
0025-5408/$ – see front matter # 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2006.04.009

L. Kumari, S.V. Subramanyam / Materials Research Bulletin 41 (2006) 2000–2006
2001
p-electronic states determine most of the optoelectronic properties of the a-C films. Effective doping can modify
optoelectronic properties, in particular optical band gap and photoconductivity, in semiconductor materials.
The bandgap in crystal is defined as the minimum energy gap between the occupied and empty states, which can
be either direct or indirect gap. In an amorphous semiconductor where there is no true gap, an arbitrary definition
must be used for the optical gap [5]. Experimentally, the two most common are the E04 gap defined as the energy at
which the optical absorption coefficient, a is 104 cmÀ1 and the Tauc gap [6]. The photon absorption in many
amorphous materials is found to obey the Tauc relation [6] discussed in detail by Davis and Mott, which is of the
form, ahn = B (hn À Eopt)n. Here a is the absorption coefficient, hn is the photon energy, B is a constant, Eopt (or Eg) is
the energy of the optical bandgap and the index, n is connected with the distribution of the density of states. The index
n = 1/2 corresponds to the direct allowed transition energy gap and n = 2 represents the indirect allowed transition
energy gap [7].
It is to be noted that for lower values of the absorption coefficient, a the Tauc’s relation gradually changes into an
exponential dependence as given by the Urbach law [8], a = a0 exp(hn/DE), where a0 is a constant and DE is the
Urbach energy which indicates the width of the band tail of the localized states in the bandgap. The Urbach energy is
one of standard measures of electronic disorder in amorphous semiconductors. Plot of ln a against hn accounts for the
determination of the values of Urbach energy. The Urbach energy can be calculated by determining the slope of the
linear region of the curves and taking their reciprocals.
In this communication, we present the synthesis of iodine and sulfur incorporated a-C films and their structural
characterization by TEM and XPS. Effects of intercalate incorporation on the optical properties and electrical
conductivity is also investigated. The data presented here is for saturation intercalated a-C films.
2. Experimental
The pure and intercalated (iodine and sulfur) a-C films were prepared by vapor phase pyrolysis technique using
temperature controlled home-built horizontal double zone furnace set-up. Carbon rich organic compound, maleic
anhydride was used as the precursor and iodine (crystals) and sulfur (powder) as intercalation agents. Both the
precursor and intercalate materials were taken in different weight ratios and were pyrolysed at various temperatures
(700–900 8C) in a quartz tube. The starting materials were vaporized in the low temperature zone and the film
deposition took place at high temperature zone. These films were deposited on unpolished quartz substrates and the
thickness of the films was of the order of 1–2 mm. Soon after the pyrolysis, the films in the quartz tube were annealed in
the presence of inert atmosphere (argon) to overcome any surface contamination. Detailed description of sample
preparation is given elsewhere [9]. The degree of iodination, I/P and degree of sulfur incorporation, S/P represents the
initial weight ratio of iodine and sulfur to precursor, respectively. The a-C samples discussed in the present work are
saturation intercalated with I/P = 1 and S/P = 0.96.
TEM and selected area electron diffraction (SAED) studies were performed on the powdered pure and intercalated
a-C samples using JEOL JEM-200CX operated at 200 kV. The samples used for the measurements were finely ground
˚
to a typical size of ꢀ500 A, later ultrasonically rinsed and were mounted on the copper grid.
XPS analysis of pure and intercalated a-C films was carried out employing Al Ka radiation (1486.6 eV) in
an ESCA-3 Mark II Spectrometer (VG Scientific Ltd., England) at pass energy of 50 eV. The XPS analysis was
performed at a base pressure of 10À9 Torr in the analyzer chamber. The binding energies were measured with a
precision of Æ0.2 eV. In order to avoid the specimen charging, the film was mounted on the sample probe
using silver paste. As these samples were highly conducting, no charging effect was observed during the
measurement.
Optical properties of the samples were studied by GBC Cintra UV–vis Spectrometer in the wavelength range, 200–
800 nm at room temperature. The fine powdered samples were mixed with paraffin oil, made into thin paste and
smeared lightly on the surface of a polished quartz substrate. Typically the thickness of the sample layer was of the
order of 2 mm. Another polished quartz substrate smeared with paraffin oil was used as a reference [10].
dc electrical conductivity measurements were performed on these films in a liquid helium cryostat (Janis
Supervaritemp Cryostat) from 300 K down to 1.3 K. The measurements were carried out by a standard four-probe
technique. The sample contacts were made using fine enamelled copper wires as contact leads and conducting silver
paste as contact material. The power dissipated into the sample was less than 1 mW at low temperatures in order to
avoid the heat-induced damage of the sample.

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L. Kumari, S.V. Subramanyam / Materials Research Bulletin 41 (2006) 2000–2006
3. Results and discussion
3.1. TEM analysis
TEM analysis of pure and intercalated samples helps in understanding the effect of iodine and sulfur incorporation
and increase in pyrolysis temperature on the structural properties. Fig. 1 represents the TEM image and SAED pattern
of (a) a-C, (b) a-C:I and (b) a-C:S samples prepared at 900 8C. From the micrographs (Fig. 1b and c), it is evident that
the iodine and sulfur incorporation accounts for the clustering (size ꢀ300–1200 nm) of carbon atoms and evolution of
nano-graphitic phase in the a-C sample, which has relevance with SEM analysis [9,11]. However, TEM image of a-C
sample shows a clear amorphous structure (Fig. 1a). Inset of Fig. 1b and c represents SAED patterns of iodine (a-C:I)
and sulfur (a-C:S) incorporated a-C sample showing a discrete ring pattern (refers to polycrystalline phase), which
infers the induced structural ordering, whereas SAED of a-C sample depicts a diffused ring pattern (inset of Fig. 1a),
confirming its amorphous nature.
To evaluate the d-spacing values, the distance, r of a diffraction spot/ring from the undiffracted spot, which is
inversely proportional to the d-spacing of the diffraction planes is measured. Next with the known value of camera
˚
constant (17 mm A) and measured r-values, the d-spacing values are determined. The electron diffraction ring patterns
(inset of Fig. 1b and c) denoted as 1–3 are indexed to (0 0 2), (1 0 0) and (1 1 0) peaks of graphite respectively, with
comparable d-spacing values. The d-spacing values of some of these diffraction rings are also in agreement with the d-
spacing values (standard index files) corresponding to carbon–iodine composites and sulfur embedded in the carbon
network, respectively [10]. However, in most cases, electron diffraction provides complementary information in
correspondence with X-ray diffraction [12], which is also witnessed for the present system [11]. A small variation in
the d-spacing values from standard data are observed which can be attributed to ring diffuse due to two phase system
(both amorphous and crystalline) as indicated previously in XRD analysis [9,11] and hence, it hinders the accurate
measurement of r values.
3.2. XPS analysis
XP spectra of a-C, a-C:I and a-C:S films deposited at 900 8C on unpolished quartz substrate are recorded employing
Al Ka radiation. The survey spectra (Fig. 2) show the presence of iodine and sulfur together with oxygen and also the
expected carbon in the intercalated films. Oxygen atoms may be incorporated into the sample during the pyrolysis
process or due to surface oxidation. For all the spectra, the most intense peak at around 283 eV belongs to C 1s
photoemission and the line shape at about 533 eV can be assigned to O 1s signal. A shoulder adjacent to C 1s main peak
on the lower binding energy side corresponds to C 1s satellite peak. In the survey spectrum for a-C:I film, I(3d5/2, 3/2
)
peaks at around 620 and 631 eV, respectively can be attributed to iodine atoms in the sample. Thus XPS analysis infers
that iodine atoms get incorporated via other atoms (probably carbon atoms) and not as pure iodine molecule [13]. In the
survey spectrum of a-C:S sample, the two intense peaks with a binding energy of about 162.4 and 224.7 eVare attributed
to S 2p and S 2s line shapes, respectively. A small shift in binding energy of these peaks with reference to standard data is
observed for samples prepared with intercalate incorporation at various pyrolysis temperatures, which accounts for the
different chemical environment. The lineshapes of the O 1s and C 1s give information about the chemical bonding
environment [14]. A peak shift in C 1s of a-C:I and a-C:S film in comparison with a-C on to the higher binding energy
region refers to charge transfer (or interaction) between host (carbon) and guest species (intercalates). The discrepancy
in the higher binding energy region of C 1s peak is due to C 1s emission associated with the C O bond. The drawback of
˚
XPS technique is that it can probe only the top few layers (ꢀ20–50 A) and hence is restricted to only surface analysis.
3.3. Optical studies
The optical absorption spectra of pure and intercalated a-C samples were recorded using UV–vis absorption
spectrometry in the wavelength range, 200–800 nm at room temperature. To determine the optical bandgap energy,
(ahn)1/2 as a function of photon energy, hn (Tauc’s plot) was plotted (not shown). The intercept of these curves on the
photon energy axis gives the optical bandgap energy (or indirect transition energy gap). The variation of optical
bandgap energy as a function of pyrolysis temperature for pure, iodinated (I/P = 1) and sulfonated (S/P = 0.96) a-C
samples, respectively is as shown in Fig. 3. Inset in Fig. 3 gives a list of optical bandgap energy for a-C:I and a-C:S

L. Kumari, S.V. Subramanyam / Materials Research Bulletin 41 (2006) 2000–2006
2003
Fig. 1. TEM images of (a) a-C, (b) a-C:I and (c) a-C:S samples. Inset is the SAED patterns.
samples prepared at 900 8C with different I/P and S/P ratios, respectively. It is evident from the analysis that both an
increase in pyrolysis temperature with constant I/P and S/P values and an increase in intercalate content at a particular
preparation temperature accounts for a considerable reduction in the optical band gap. The decrease in bandgap energy
can be interpreted as due to addition of intercalates, which influences the dangling bond states or affects the states
within the bandgap and also accounts for the local structural ordering. Moreover, intercalate incorporation may affect
the carbon hybridization or introduce more defect states in the gap, which thereby accounts for the bandgap reduction.

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L. Kumari, S.V. Subramanyam / Materials Research Bulletin 41 (2006) 2000–2006
Fig. 2. XPS survey spectra of pure and intercalated a-C films.
This also infers an increase in sp2 or p-sites, which strongly influences the optical bandgap [10]. Optical studies also
depict a decrease in Urbach energy, which has dependence on disorder, inferring an increase in sp2 or p-sites. Urbach
energy values for a-C, a-C:I and a-C:S samples prepared at 900 8C are 0.145, 0.11 and 0.125 eV, respectively. A small
increase in the bandgap and Urbach energy is noticed for saturation sulfur incorporated samples, which indicates the
excess sulfur incorporation induced impurity scattering or discontinuities. The optical spectra of all samples show a
long tail. The appearance of this exponential tail is due to lack of long-range order in the system. The tail is generally
attributed to disorder in the material that leads to a tail in the valence and conduction bands.
3.4. Electrical resistivity
The conductivity of intercalated a-C films depends on the pyrolysis temperature and also intercalates concentration
[9,11]. The resistivity ratio, rr = r(1.3 K)/r(300 K) defines the measure of disorder in the a-C system [15]. As the
Fig. 3. Optical bandgap vs. pyrolysis temperature plot for pure and intercalated a-C samples. Inset is a list of optical bandgap energy as a function of
intercalates content.

L. Kumari, S.V. Subramanyam / Materials Research Bulletin 41 (2006) 2000–2006
2005
disorder in the system increases, rr increases. The disorder in the system can be tuned as a function of increase in
impurity concentration at a particular pyrolysis temperature and also with an increase in pyrolysis temperature at a
constant intercalates content, which depicts a decreasing trend. Variation of room temperature conductivity as a
function of pyrolysis temperature for pure and intercalated a-C samples is shown in Fig. 4. The room temperature
conductivity also depicts an increasing trend with an increment in intercalate content at constant pyrolysis temperature
[9] and increase in pyrolysis temperature with constant intercalate concentration. The increase in room temperature
conductivity as a function of increase in pyrolysis temperature can be attributed to the evolution of nanographitic phase
and more ordered structural rearrangements in the a-C system. However, an increase in sulfur concentration
(optimum) accounts for an increment in the conductivity and further increase in sulfur accounts for a decrease in the
conductivity [11]. This concludes that the addition of excess sulfur acts as impurity scattering center within the carbon
network and shows a tendency to aggregate at grain boundaries. Also most of the incorporated sulfur atoms may
remain electrically inactive (unionized). Hence, large amount of sulfur in the sample accounts for a low mobility and
low carrier concentration, depicting a decrease in conductivity.
Temperature dependence of resistance in a-C and a-C:S samples prepared at 900 8C shows a semiconductor like
behavior, whereas the a-C:I sample depicts a very interesting resistivity conduct at low temperatures. It shows a clear
positive temperature coefficient of resistance (PTCR) at low temperatures with a crossover (negative TCR to PTCR)
temperature of T* ꢀ 26 K (inset of Fig. 4), which is a typical behavior of disordered metal. Earlier work [16] on doped
semiconductors suggest that at low temperatures, the electrical conductivity depends on the density of states close to
Fermi energy, D(EF) and the radius of localization of the charge carriers bound to impurity states (the ‘Bohr radius’,
aÃB). At very high doping levels, the large overlap of the wavefunctions leads to impurity-band conduction and may
result in a finite conductivity at T = 0 K and the system becomes metallic. Moreover, iodine incorporation and
pyrolysis temperature are expected to change the a-C ‘‘backbone’’ in such a way that the structural modification [9] in
the system accounts for a transition. Iodine being an acceptor impurity can also induce a transition due to impurity
band conduction and accounts for a shift in the Fermi energy [10]. This kind of behavior was previously observed in
PAN-based fiber [17] and pyrocarbon [18]. From the electrical transport studies, it is evident that the intercalated a-C
samples are less disordered and more conducting than the pure a-C. This infers that the intercalate incorporation into
the a-C system induces a local ordering or structural rearrangement, which also depicts an increase in sp2 or p-sites,
accounting for a more conducting sites. Earlier work on sulfur incorporation into nanocomposite carbon thin film
accounts for the enhancement of the spatial connectivity of the sp2-bonded carbon network, which thereby exhibits the
drastic increase in the conductivity [19]. Also intercalates incorporation is expected to bring about charge transfer [20–
22], resulting in the enhancement of the density of states, thereby increasing the conductivity of the sample [9,10]. The
overall conductivity of the sample may be understood as due to the hopping or tunneling of charge carriers between the
remote conductive chains [23]. Iodinated samples are found to be more conducting and ordered than the pure and
sulfur incorporated a-C samples.
Fig. 4. Conductivity variation with pyrolysis temperature for a-C, a-C:I and a-C:S films. Inset is a-C:I film prepared at 900 8C showing PTCR at low
temperatures.

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4. Conclusions
Iodine and sulfur incorporated a-C samples are prepared by vapor phase pyrolysis technique. Structural studies on
these samples depict an intercalate incorporation and pyrolysis temperature induced structural ordering and increase in
sp2 or p-sites. XPS analysis confirms the presence of intercalates in the intercalated a-C samples. Optical bangap and
urbach energy values shows decreasing trend with an increase in the pyrolysis temperature at constant intercalate
content and increase in intercalate concentration at a particular pyrolysis temperature, depicting an increase in sp2 or
p-sites. A monotonic increase in room temperature conductivity is noticed as a function of intercalates content and
pyrolysis temperature, which also depicts a decrease in disorder or more ordered structural rearrangement in the
system.
Acknowledgements
We thank DST, CSIR and STC for the financial support. We are grateful to Dr. Subbanna (late) for TEM
micrographs and useful discussions. Our sincere thanks to Prof. M.S. Hegde for providing XPS facility.
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