Optical Absorption Studies of LiCoO2 Thin Films Grown by Pulsed ...

International Journal of Pure and Applied Physics
ISSN 0973-1776 Volume 6, Number 3 (2010), pp. 365–370
© Research India Publications
http://www.ripublication.com/ijpap.htm
Optical Absorption Studies of LiCoO 2 Thin Films
Grown by Pulsed Laser Deposition
M.C. Rao
Department of Physics, Andhra Loyola College, Vijayawada – 520008, India.
E-mail: raomc72@gmail.com
Abstract
Thin films of LiCoO 2 were prepared by pulsed laser deposition technique.
Two important deposition parameters such as substrate temperature and
oxygen partial pressure during the thin film deposition were controlled. The
optical absorbance of LiCoO 2 thin films have been studied in the vicinity of
the fundamental absorption edge. We observed a gradual increase of the
optical absorption from 400 to 1000 nm with the appearance of a prominent
step at 750 nm. The variation of the optical absorption near the fundamental
edge allows to determine the energy gap between d-bands of LiCoO 2 . These
plots gave the band gap energy in the range 2.38 - 2.30 eV for films grown
at temperatures in the range 300 - 700 0 C, respectively, with an accuracy ±
0.02 eV. These results suggest that the open and porous structured LiCoO 2
PLD films find potential applications as binder free electrode in the fabrication
of all solid state microbatteries.
Keywords: LiCoO 2 thin films, PLD, Optical properties.
Introduction
Lithiated transition metal oxides such as LiMO 2 (Where M = Co, Ni, Mn etc.) have
received considerable attention in recent years as high voltage positive electrode
materials for use in secondary lithium batteries. Among these, the high cycling
stability and high cell potential against lithium makes LiCoO 2 an attractive cathode
material in the fabrication of all solid state rechargeable microbatteries [1, 2]. Its
theoretical specific capacity and energy densities are 274 mAh/g and 1070 Wh/kg
respectively. Experimentally it has been observed that the material delivers only
capacity around 140 mAh/g due to structural considerations.
The layered LiCoO 2 consists of a close packed network of oxygen ions with Li
and Co ions on alternative (111) planes of the cubic rock - salt sublattice. The edges

366 M.C. Rao
of CoO 6 octahedral were shared to form CoO 2 sheets and lithium ions can move in
two-dimensional directions between CoO 2 sheets. Thus the layered LiCoO 2 has an
anisotropic structure and there by electrochemical lithium insertion / extraction
behaviour must depend strongly on the orientation of the microcrystallites. The
growth of LiCoO 2 thin films with preferred orientation is known to be crucial. Several
thin film deposition techniques such as RF sputtering [1, 3], pulsed laser deposition
[3-7], electrostatic spray deposition [8] and chemical vapour deposition [9] were
employed for the growth of LiCoO 2 thin films. A brief literature survey reveals that it
is difficult to grow stoichiometric and stable c - axis oriented LiCoO 2 thin films by
several physical vapour deposition methods due to many growth kinetic processes
which occur in vacuum or at low oxygen partial pressures.
Recently, pulsed laser deposition (PLD) has been widely recognized as a very
promising, versatile and efficient method for the deposition of metal oxide thin films
[10]. When PLD is carried out in the atmosphere of a chemically reactive gas
(a process known as reactive pulsed laser deposition (RPLD)), the flux of the laser
ablated material interacts with the gas molecules all along the transit from the target to
the collector surface. The resulting deposited layer was found to have a chemical
composition substantially the same as the base or starting material. Preliminary
investigations on pulsed laser deposited LiCoO 2 thin films were carried out by
Julien et al. [4]. Iriyama et al. [5] prepared thin films of LiCoO 2 by PLD and studied
the electrochemical performance. They observed that the reactivity in single-phase
region at potentials more positive than 4.0 V was lower than that of randomly oriented
films. Poly-crystalline layered R 3m
phase thin films of LiCoO 2 were grown by PLD
by Julien et al. [11]. These LiCoO 2 cathode active films were found to deliver a
specific capacity of 195 mC/μm cm 2 in the voltage range 2.0 - 4.2 V. However the
investigations on the physical properties of LiCoO 2 films that are essentially depend
upon the deposition parameters give a scope for effective utilization of these thin
films in the fabrication of microbatteries. Hence in the present study the influence of
deposition parameters on the optical properties of pulsed laser deposited LiCoO 2 thin
films were reported.
Experimental
LiCoO 2 films were grown by pulsed laser deposition technique on quartz substrates
maintained at temperatures in the range 300 - 700 0 C. LiCoO 2 target was prepared by
sintering a mixture of high purity LiCoO 2 and Li 2 O powders (Cerac products) with
excess of Li i.e. Li/Co > 1.0 by adding Li 2 O [11]. The mixture was crushed and
pressed at 5 tonns.cm -2 to make tablets of 2 mm thick and 13 mm diameter. To get
quite robust targets, the tablets were sintered in air at 800 0 C. The typical substrates
i.e. quartz glass were cleaned using HF solution. The target was rotated at 10 rotations
per minute with an electric motor to avoid depletion of material at any given spot. The
laser used in these experiments is the 248 nm line of a KrF excimer laser (Luminics
PM 882) with 10 ns pulse with a repetition rate of 10 Hz. The rectangular spot size of
the laser pulse was 1x3 mm and the energy 300 mJ. The target substrate distance was
4 cm. The deposition temperature was maintained with thermocouple and temperature

Optical Absorption Studies of LiCoO 2 Thin Films Grown 367
controller. During the deposition pure oxygen was introduced into the deposition
chamber and desired pressure was maintained with a flow controller. The optical
properties of the films were recorded using Hitachi U 3400 UV-VIS-NIR double
beam spectrophotometer with accuracy limits ± 0.2 nm.
Results and Discussion
Pulsed laser deposited LiCoO 2 films are pin-hole free as revealed from optical
microscopy and well adherent to the substrate surface. The thickness of LiCoO 2 films
is 250 nm. The influence of oxygen partial pressure (pO 2) and deposition temperature
(Ts) on the optical properties of the films is systematically studied. The chemical
compositional studies made on LiCoO 2 films revealed that a minimum of 100 mTorr
oxygen partial pressure is required to grow nearly stoichiometric films.
The study of optical absorption, particularly the absorption edge, has proved to be
very useful for elucidation of the electronic structure of the materials. It is possible to
determine whether the optically induced transition is direct or indirect and allowed or
forbidden by analysis of the absorption edge.
The optical absorption coefficient of the films is evaluated using the relation
α = 1/t ln (T/(1-R) 2 ) (1)
Where T is the transmittance, R is the reflectance and t the thickness of the film.
For an incident photon energy greater than the band gap and above the
exponential tail the optical absorption follows a power law [12] of the form
α(hv) = B(hv - E g ) n (2)
Where B is a constant, E g is the band gap of the material and n is the exponent.
The exponent n determines the type of electronic transition causing the absorption and
take the values 1/2, 3/2, 2 and 3 for direct allowed, direct forbidden, indirect allowed
and indirect forbidden transitions respectively [13].
The optical absorbance of LiCoO 2 thin films have been studied in the vicinity of
the fundamental absorption edge. The electronic structure of the Co-3d bands of
layered rock salt LiCoO 2 consists of three Co-t 2g (valence bands) and two Co-e g
(conduction bands). Theoretical investigations have shown that the Co-e g bands are
empty and their peak position lies at around 1.7 eV above the top to the Co-t 2g bands
[14]. The density of states due to the Co-e g bands consist of a prominent peak at ~2.2
eV above the Fermi level E F in the Co-t 2g band [15].
Fig. 1 shows the optical absorption spectra related in the wavelength range 300-
1200 nm for the films deposited at substrate temperatures 300 and 700 0 C on quartz
substrate. We observed a gradual increase of the optical absorption from 400 to 1000
nm with the appearance of a prominent step at 750 nm. This feature is more
pronounced for the well-crystallized film grown at 700 0 C. Plots of the optical
absorbance, (αhν) 2 versus photon energy for LiCoO 2 films are shown in Fig. 2.
The variation of the optical absorption near the fundamental edge allows to
determine the energy gap between d-bands of LiCoO 2 . These plots gave the band gap
energy in the range 2.38 - 2.30 eV for films grown at temperatures in the range
300 - 700 0 C, respectively, with an accuracy ± 0.02 eV. These results are in good
agreement with theoretical estimations [14, 15] and experimental data reported by

368 M.C. Rao
Kushida and Kuriyama[13]. The d-d transition from E F in the Co-t 2g bands to the Coe
g bands in LiCoO 2 was observed at 2.1 eV. Intercalation of Li in LiCoO 2 increases
the Co-O bond length leading to a σ overlap between the O-2p and Co-3d orbitals,
thereby pushing the antibonding bands (e g bands) down [15]. Thus the textured films
allow for investigation of the d-d electronic transition in LiCoO 2 . Our experimental
data support the above theoretical prediction.
Figure 1: Optical absorption spectra of LiCoO 2 deposited at different substrate
temperatures in pO 2 = 100 mTorr.
Figure 2: Plots of (αhν) 2 vs. hν of LiCoO 2 thin film deposited at different substrate
temperatures in pO 2 = 100 mTorr.

Optical Absorption Studies of LiCoO 2 Thin Films Grown 369
Conclusion
Thin films of LiCoO 2 were prepared by pulsed laser deposition. PLD films were
found to be uniform with regard to the surface topography, thickness and well
adherent to the substrate surface. The optical absorbance of LiCoO 2 thin films have
been studied in the vicinity of the fundamental absorption edge. The variation of the
optical absorption near the fundamental edge allows to determine the energy gap
between d-bands of LiCoO 2 . The band gap energy was measured in the range
2.38 - 2.30 eV for films grown at temperatures in the range 300 - 700 0 C, respectively,
with an accuracy ± 0.02 eV. These results suggest that the open and porous structured
LiCoO 2 PLD films find potential applications as binder free electrode in the
fabrication of all solid state microbatteries.
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