Nanocrystalline CuO films prepared by pyrolysis of
Cu-arachidate LB multilayers
M. Parhizkara,1 , Sukhvinder Singha , P.K. Nayaka , Nigvendra Kumarb , K.P. Muthec ,
S.K. Guptac , R.S. Srinivasad , S.S. Talwara , S.S. Majora,∗
a
d
Department of Physics, Indian Institute of Technology Bombay, Mumbai 400076, India
b Maharashtra College, Bombay University, Mumbai 400008, India
c TPPED, Bhabha Atomic Research Centre, Mumbai 400085, India
Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Mumbai 400076, India
Abstract
Nanocrystalline CuO films have been formed by the pyrolysis of copper arachidate LB multilayers. FTIR and UV–vis spectroscopic
investigations showed that the pyrolysis was initiated at ∼100 ◦ C and was complete at ∼300 ◦ C. XPS and electron diffraction studies confirm
the films to be single-phase copper (II) oxide. Analysis of the bright-field images showed that the film consists of uniformly distributed particles
of size 2.5 ± 1.5 nm. The optical absorption edge of the CuO film did not show a measurable blue shift attributable to the nanocrystalline nature
of the films. The absorption features are analysed in the background of the uncertainty prevailing in the nature and value of the absorption
edge of CuO.
Keywords: CuO; Langmuir–Blodgett; Copper arachidate; Nanocrystalline; Structure
1. Introduction
Cupric Oxide (CuO) is a semiconductor that has been
studied for photoconductive, photothermal and photoelectrochemical applications [1–4]. It has also been known to be
antiferromagnetic [5]. In recent years, it has attracted wide
interest due to its monoclinic unit cell and square-planar coordination [6], which is similar to that in high Tc cuprate
superconductors. Owing to the current interest in nanomaterials and their unique properties, the formation of nanocrystalline CuO has been achieved by rapid precipitation [7],
spin coating [8], solid-state reaction [9], sonochemical reaction [10], sol–gel techniques [11], solvothermal route [12],
electrochemical route [13] and controlled oxidation of copper nanoparticles within silica gels [14]. These studies reveal
interesting size-dependent effects on the structure, optical,
electrical and magnetic behaviour of nanocrystalline CuO.
Recently, very thin and homogenous metal oxide films
have been obtained by the oxidation of a precursor fatty acid
salt Langmuir–Blodgett (LB) multilayers [15–18]. In most
cases, the precursor LB multilayer is exposed to heat or UV
radiation in air, resulting in oxide formation and removal of
organic components. Using this method, very thin films of
CuO could be obtained by UV exposure followed by heat
treatment in air at 365 ◦ C of a precursor copper arachidate
LB multilayer [19]. It was also reported [19] that a direct heat
treatment of the copper arachidate (CuA) multilayer resulted
in a discontinuous film. Elastic recoil detection analysis was
used to calculate the Cu to O ratio.
In view of the increasing interest in transition metal oxide films, in particular copper oxide, a controlled heat treatment of CuA LB multilayers in oxygen ambient has been
carried out in the present work. The process of conversion
of the arachidate multilayer into CuO was studied by X-ray
diffraction, infrared spectroscopy and UV–vis spectroscopy.
278
The results show that the oxidation process and the removal
of organic components are completed at about 300 ◦ C. The
formation of a single-phase, nanocrystalline CuO film was
confirmed by transmission electron microscopy and X-ray
photoelectron spectroscopy studies.
2. Experimental
LB multilayers of copper arachidate were prepared by the
conventional LB deposition technique using a KSV 3000 instrument in a clean room. Arachidic (Aldrich, 99%) with
HPLC grade chloroform as solvent (1 mg/ml) was spread
on an aqueous subphase containing CuCl2 (10−4 M). Deionized and ultra-filtered water (Millipore) having a resistivity of 18.2 M cm was used to prepare the subphase. The
subphase temperature was kept constant at 20 ◦ C. The subphase pH of 10−4 M CuCl2 solution was found to be 6.1. The
monolayer was compressed with a constant barrier speed of
3 mm/min. The isotherm (not shown) exhibited a condensed
nature, in which the liquid condensed region was absent, indicating complete ionization of arachidic acid. The limiting
mean molecular area (LMMA) was found to be ∼20 Å2 comparable to the reported values for arachidate monolayers [20].
The multilayer deposition was carried out at a surface pressure of 20 mN/m. The compressed monolayer was transferred
by vertical dipping method at a speed of 3 mm/min. Typically
about 100–200 monolayers were transferred on quartz and
CaF2 substrates, respectively.
The as-deposited CuA multilayers were annealed in a
quartz tubular furnace in the temperature range 100–600 ◦ C
in a stream of oxygen at atmospheric pressure. A temperature controller (±5 ◦ C) was used to set the temperature at the
required value.
IR spectra were obtained with a Perkin Elmer Spectrum1
FTIR spectrometer and UV–vis spectra with a Shimadzu UV160A spectrophotometer. X-ray reflection (XR) studies were
performed with Philips PW1820 powder diffractometer using Cu K␣ radiation in the 2θ range of 4–16◦ . Transmission
electron microscopy (TEM) studies were carried out using
a PHILIPS Model CM200 Supertwin Transmission Electron
microscope operated at 200 kV. For TEM studies, CuO films
were lifted off from quartz substrates in a dilute HF solution
and transferred onto copper grids. X-ray photoelectron spectroscopic measurements were carried out with an XPS setup incorporated in the RIBER MBE EVA 32 R&D system.
The XPS system comprises of a twin anode (Mg/Al) source
(Model CX 700) and an electron energy analyzer (Model
MAC 2). The excitation used was Mg K␣ (1253.6 eV) and
the binding energies were referenced using Au 4f7/2 peak.
3. Results and discussion
The XR patterns in the 2θ range of 4–16◦ for the asdeposited and heat-treated CuA multilayers on quartz sub-
Fig. 1. X-ray reflection patterns of a copper arachidate multilayer on a quartz
substrate. In (a) as-deposited state and after heat treatments at (b) 100 ◦ C,
(c) 190 ◦ C, (d) 200 ◦ C.
strate are shown in Fig. 1. The XR pattern (a) for the asdeposited multilayer exhibits well-defined Bragg peaks corresponding to (0 0 l) reflections. The average bilayer period
calculated from the peak positions was found to be 55 Å. Considering the total length of arachidate molecule to be ∼27.5 Å,
the bilayer period of 55 Å indicates molecular packing with
alkyl chains of copper arachidate nearly perpendicular to
the layer [21]. Heat treatment of the CuA multilayer up to
∼100 ◦ C (Fig. 1b) showed an increase in the intensity of the
Bragg peaks, without any change in their positions. Further
heat treatments in the range 100–190 ◦ C showed that the intensities of the Bragg peaks decreased. Fig. 1c shows the XR
pattern of the CuA multilayer heat-treated at 190 ◦ C. However, when the CuA multilayer was heat-treated at 200 ◦ C,
the Bragg peaks disappeared (Fig. 1d) suggesting complete
destruction of the layered structure and this is attributed to
melting of the arachidate salt. Similar observations have earlier been reported on LB multilayers of several fatty acid salts
[22].
Fig. 2a and b shows the FTIR spectra in the regions,
2800–3000 and 1300–1700 cm−1 , respectively, for the CuA
multilayer on CaF2 substrate in the as-deposited state and after oxidation treatment at different temperatures. The spectra
of the as-deposited multilayer show intense bands at 2916 and
279
Fig. 3. UV–vis transmittance (T) and reflectance (R) spectra of (a) as deposited copper arachidate multilayer and after heat treatment at (b) 100 ◦ C,
(c) 200 ◦ C, (d) 300 ◦ C, (e) 400 ◦ C, 500 ◦ C.
Fig. 2. FTIR transmittance spectra of an as-deposited copper arachidate
multilayer () and after heat treatment at 100 ◦ C (䊉), 200 ◦ C () and 300 ◦ C
(—).
2849 cm−1 due to CH2 asymmetric and symmetric vibrations
of the alkyl chain of the CuA. The strong absorption band at
1587 cm−1 is assigned to the asymmetric stretching vibrations of the carboxylate group. The presence of this band and
the complete absence of C O stretching band of unionized
carboxylic acid at ∼1700 cm−1 confirm that the as-deposited
multilayer consists of CuA and not a mixture of arachidic
acid and salt. The bands at ∼1470, ∼1413 and ∼1315 cm−1
are assigned to CH2 scissoring, COO− symmetric stretch and
CH2 wagging modes of arachidate salt, respectively [23]. The
asymmetric COO− band is normally observed for divalent
fatty acid salts, such as cadmium arachidates at ∼1540 cm−1
[23]. However, it is known to shift to higher frequencies depending on the size, mass and electronegativity of the metal
[24].
The spectrum of the CuA multilayer subjected to oxidation treatment at 100 ◦ C shows that in general, the bands become sharper and more intense compared to the as-deposited
multilayer, suggesting improvement in structural order due
to the annealing effect of heat treatment [25]. On oxidation
treatments at higher temperatures, the intensities of the bands
decrease and eventually disappear for multilayers treated at
300 ◦ C or above. This indicates the removal of all organic
moieties by pyrolysis in the films subjected to oxidation treatment at 300 ◦ C or above.
Fig. 3 shows the transmission and reflection spectra of
the as-deposited and heat-treated CuA multilayers on quartz
substrates, in the wavelength range 300–1100 nm. The asdeposited CuA multilayer is transparent in the visible region
and exhibits an absorption edge at ∼300 nm. A similar behaviour is seen with a small reduction in transmission after heat treatment at 100 ◦ C. However, the heat treatment at
200 ◦ C results in a noticeable drop in transmission in the region 300–400 nm, with an increase in overall reflection, indicating the beginning of oxide formation. The films subjected
to heat treatment at 300 ◦ C show significant loss of transmission in the wavelength region 300–700 nm. Subsequent heat
treatments at 400 and 500 ◦ C did not show any significant
change in the UV–vis spectra. These observations suggest
that the oxidation process starts at ∼200 ◦ C and is completed
at ∼300 ◦ C. This conclusion is consistent with the FTIR data
discussed above.
The structure and composition of the CuA multilayer heattreated at temperatures of 300 ◦ C and above was investigated
by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The XR patterns of the heattreated multilayers were featureless, which is attributed to the
small thickness of the oxide film formed by the heat treatment.
The bright-field image and the diffraction pattern of the
CuA film obtained by heat treatment at 400 ◦ C are shown in
Fig. 4a and b, respectively. The diffused rings in the diffraction pattern were indexed to reflections from the planes of
CuO indicating the presence of CuO phase in the oxidized
film. The presence of a small quantity of Cu2 O could not be
ruled out on the basis of TEM data, since all the reflections
from cubic Cu2 O lie close to a large number of reflections
from monoclinic CuO. Further, the diffused nature of the
diffraction rings makes it quite difficult to unambiguously
rule out the presence of reflections from Cu2 O. The brightfield image of the film shows a uniform array-like distribution
of nanoparticles and the image analysis showed the particle
size to be 2.5 ± 1.5 nm. The nanocrystalline nature of the film
280
Cu 2p3/2 peak in CuO is reported at 933.2 eV as compared to
932.4 eV in Cu2 O [26]. The separation between the Cu 2p3/2
peak and its first satellite is about 8 eV and the 2p peaks are
broad, with 2p3/2 having a peak width of ∼3.5 eV. In comparison, the 2p core levels of Cu2 O are reported to be sharp (Cu
2p3/2 peak width ∼1.9 eV) [26]. These features are characteristics of materials such as copper dihalides or CuO having
a d9 configuration in ground state [26]. Thus the existence
of strong satellite peaks for Cu 2p confirms the presence of
CuO phase in the film. Furthermore, the symmetric nature of
the 2p core level peaks and the absence of any shoulder on
the lower energy side confirms the absence of Cu2 O in the
film.
The absorption spectrum of the CuO film obtained by heat
treatment at 400 ◦ C of a CuA multilayer consisting of 200
monolayers was calculated from the transmission and reflection data shown in Fig. 3. The absorption coefficient (α) was
calculated using the simple relation for a strongly absorbing film, neglecting reflections from the film–substrate and
substrate–air interfaces:
T = (1 − R)e−αt
Fig. 4. (a) Bright-field image and (b) the selected area diffraction pattern
of a copper oxide film obtained by heat treatment of a copper arachidate
multilayer at 400 ◦ C.
is responsible for the diffused nature of the rings seen in the
electron diffraction pattern of the oxide film.
In order to confirm the absence of Cu2 O phase, XPS studies were carried out on the above film. Fig. 5 shows the
Cu 2p core level XPS spectrum. The peaks corresponding
to the core level 2p3/2 and 2p1/2 transitions of copper at 933.5
and 953.2 eV, respectively, along with the satellites on higher
binding energy side are seen. These values compare well with
the reported values for Cu 2p levels in CuO. For example, the
Fig. 5. Cu 2p core level X-ray photoelectron spectrum of a copper oxide
film obtained by heat treatment of a copper arachidate multilayer at 400 ◦ C.
The thickness (t) of the heat-treated oxide film was taken
to be 40 nm based on earlier data on oxidation of copper
arachidate LB multilayers [19]. The plots of α versus hν in the
energy range of 1−5 eV for films heat-treated at temperatures
above 300 ◦ C were found to be similar. Hence, only a typical
plot has been shown in Fig. 6a for the film heat-treated at
400 ◦ C. Fig. 6b shows an expanded version of this plot in the
energy range 1.55–1.65 eV. The values of α ≤ 1.6 eV were
found to be smaller than 1000 cm−1 , the experimental error
in the measurement of α. A significantly sharp increase in
α is clearly seen in Fig. 6b for energy values greater than
∼
=1.6 eV.
Limited studies have been reported on the electronic structure of CuO and its optical properties. Most of the results on
polycrystalline and thin-film samples report an energy gap in
the range 1.2–1.85 eV, with some early reports mentioning
the absorption edge to be too shallow to define an energy
gap [3,4,27–29]. Ghijsen et al. [26] have studied the electronic structure of CuO by electron spectroscopic techniques
to demonstrate that CuO is a charge transfer gap insulator
with a bandgap of 1.4 eV. A limited number of band structure
calculations have also been carried out for CuO, of which,
Ching et al. [30] have reported that CuO has a direct gap of
1.6 eV, which agrees well with the present results. The absorption spectrum of CuO films obtained in the present work also
shows a good agreement with the spectroscopic ellipsometry
results of Ito et al [31]. There is a general agreement, both in
the nature of the α versus hν curve as well as the magnitude
of α in the energy range studied. It may be mentioned that
owing to the ambiguities associated with the electronic structure and optical properties of CuO, we have not attempted to
describe the absorption edge through the usual dependence
of α ∼ (ν − ν0 )x .
281
4. Conclusions
Single-phase copper (II) oxide thin films have been formed
by the pyrolysis of copper arachidate LB multilayers. FTIR
and UV–vis spectroscopic investigations showed that the oxidation and the removal of organic moieties were complete
at ∼300 ◦ C. TEM images showed the film to consist of a
uniformly distributed particles of size 2.5 ± 1.5 nm. The absorption edge of the CuO films was found to be 1.6 eV and did
not show any noticeable blue shift due to the nanocrystalline
size of the films. This absence of blue shift is attributed the
relatively large effective mass of the holes and electrons in
CuO and the uncertainty in the value of the absorption edge
of CuO prevailing in literature.
Acknowledgments
The financial support for this work from the Department
of Science and Technology, Government of India, is gratefully acknowledged. One of the authors Sukhvinder Singh is
thankful to CSIR, New Delhi, for junior research fellowship.
References
Fig. 6. (a) Absorption coefficient (α) vs. photon energy plot for a copper oxide film obtained by heat treatment of a copper arachidate multilayer at 400 ◦ C and (b) an expanded version in the photon energy range of
1.55–1.65 eV.
It is also interesting to note that the absorption edge of
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significant blue shift in the absorption edge of the CuO films
in spite of their nanocrystalline nature.
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