Full text of DOI:10.1557/JMR.2000.0394

Processing of epitaxial LiMn2O4 thin film on MgO(110)
through metalorganic precursor
Yumi H. Ikuharaa) and Yuji Iwamotoa)
Fine Ceramics Research Association, Synergy Ceramics Laboratory, 2-4-1 Mutsuno, Atsuta-ku,
Nagoya, 456-8587, Japan
Koichi Kikuta and Shin-ichi Hirano
Graduate School of Engineering, Nagoyua University, Furo-cho, Chikusa-ku, Nagoya,
464-8603, Japan
(Received 21 April 2000; accepted 6 September 2000)
Epitaxial LiMn2O4 was successfully synthesized by coating a [Li–Mn–O] metalorganic
precursor solution onto MgO (110) substrates at temperatures as low as 350 °C.
Cross-sectional transmission electron microscopy observation revealed that the
orientation relationship between LiMn2O4 and MgO was (111)LiMn O //(111)MgO
,
4
(110)
//(110)MgO, and [112]
LiMn2O4
//[112]MgO, which resulted2in the (111)
LiMn2O4
LiMn2O4
planes growing perpendicular to the surface plane of MgO. The interface structure
consisted of (111) layers of Mn atoms in the LiMn2O4 crystal aligned with the Mg
atoms in the (111) planes of the MgO substrate when viewed along the [112] direction.
I. INTRODUCTION
thin film has not yet been prepared. Chemical process-
ing via a solution route using metalorganic precursors
is advantageous for preparing thin films with desired
stoichiometry, controlled purity, and compositional ho-
mogeneity.24 If an appropriate precursor solution and
substrate are chosen in terms of the lattice parameter
and crystal structure of the desired compound, micro-
structually controlled or highly oriented thin films can be
fabricated.
In this study, we report on the preparation of lithium
manganese oxide thin film using a metalorganic precur-
sor solution. In this process, we focus on the fabrication
of an epitaxial thin film with the (111) intercalated plane
perpendicular to the surface of the substrate. The micro-
structure of the epitaxial LiMn2O4 thin film was further
investigated by interface analysis methods.
In recent years, there has been considerable interest in
high energy density rechargeable batteries for use in
electronic devices. The increase in demand for light-
weight portable equipment means that microbatteries
must be developed as power sources. Intensive efforts
have been directed at fabricating the cell components of
these all-solid-state microbatteries, particularly by thin
film techniques.
11,12
LiMn2O4 1s3p–i1n5el,1–10 the layered compound LiCoO2
and LiNiO2
have been extensively investigated for
use as cathodic materials in lithium secondary batteries
because they all possess the characteristic electrochemi-
cal property of intercalation and deintercalation of
lithium ions into their crystal structure. In each case, it
has been demonstrated that the lithium ions are re-
versibly inserted between the layered planes. To facili-
tate the intercalation process in microbatteries, it is
beneficial to prepare a lithiated intercalation cathode film
with the intercalated plane perpendicular to the surface of
the substrate.
Several lithium intercalation compounds have been
fabricated in thin film form by techniques such as chemi-
cal vapor deposition, evaporation, and sputtering.16–23
These thin films all have an amorphous or random poly-
crystalline structure, since a microstructually controlled
II. EXPERIMENTAL PROCEDURE
A. LiMn2O4 thin film processing
Starting metalorganic precursors for the fabrication
of LiMn2O4 were LiOCH(CH3)2 and Mn(OC2H5)2
with 2-ethoxyethanol [C2H5OC2H4OH;(EGMEE)] as
the solvent. EGMEE was dried over molecular sieves
and distilled prior to use. The chemical modification re-
actions of the metalorganic precursors were carried out
under dry nitrogen atmosphere. The metalorganic pre-
cursors were dissolved in EGMEE and refluxed at
135 °C for 3 h. The alkoxy ligands in LiOCH(CH3)2 and
Mn(OC2H5)2 were modified to give LiOC2H4OC2H5 and
Mn(OC2H4OC2H5)4 by complete reaction.10,25 After the
a)Present address: Japan Fine Ceramics Center, Research and De-
velopment Laboratory, 2-4-1 Mutsuno, Atsuta-ku, Nagoya, 456-
8587, Japan.
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Y.H. Ikuhara et al.: Processing of epitaxial LiMn2O4 thin film on MgO(110) through metalorganic precursor
solutions were cooled to room temperature, these two
solutions were condensed in a rotary evaporator and
added to fresh EGMEE solution to remove the subre-
acted products of (CH3)2 CHOH and C2H5OH.
A homogeneous metalorganic [Li–Mn–O] precursor
solution was prepared by vigorously mixing the
0.025 M LiOC2H4OC2H5 in EGMEE with 0.05 M
Mn(OC2H4OC2H5)4 in EGMEE for 1 h at room tempera-
ture, followed by repeated distillation using EGMEE.
The [Li–Mn–O] precursor solution was then condensed
to about 0.1 M and 0.3 M by placing it in a vacuum with
a rotary evaporator to remove solvent.
further at a rate of 5 °C/min to 350, 500, or 700 °C and
annealed for 1 h in oxygen atmosphere. X-ray diffraction
(XRD) measurements on heat-treated film were per-
formed with Cu K␣ radiation using a Rint2000 (Rigaku,
Tokyo, Japan) diffractometer equipped with monochro-
mator, operating at 30 kV and 50 mA. Pole-figure meas-
urements were performed with a pole-figure attachment
using the Schulz method to evaluate the film orientation.
C. Microstructure observation of LiMn2O4
thin films
The single-crystal MgO (110) substrates were cleaned
ultrasonically in ethanol and acetone, soaked in EGMEE,
then dried with nitrogen gas. LiMn2O4 precursor films
were fabricated from brown-colored [Li–Mn–O] metal-
organic precursor solution by spin coating (Model
K-359SD, Kyowariken Co. Ltd. Tokyo, Japan) onto the
cleaned single-crystal MgO (110) substrates (Tateho
Chemical. Co. Ltd. Tokyo, Japan) at a spinning speed of
2000 rpm for 20 s under flowing nitrogen gas.
The morphology of the precursor derived thin film
after being heat treated at 700 °C was characterized by
transmission electron microscopy (TEM). For cross-
B. Crystallization of LiMn2O4 thin films
After coating thin films onto the MgO (110) sub-
strates, samples were carefully transferred to a single-
zone tube furnace (Model KTF045, Koyo-Lindberg Co.
Ltd., Tokyo, Japan) and then heated to 170 °C for 30 min
to remove the organic ligands and subsequently heated
FIG. 2. X-ray diffraction patterns of the films fabricated by coating of
the 0.3 M precursor solution followed by heat treatment at (a) 350 °C,
(b) 500 °C, and (c) 700 °C.
FIG. 1. Geometry of the LiMn2O4–MgO interface. The interface
plane is parallel to the (110) plane of MgO and LiMn2O4. TEM ob-
servation was performed from cross-sectional directions: [112]L//
[112]M.
FIG. 3. X-ray diffraction patterns of the films fabricated by coating of
the 0.1 M precursor solution followed by heat treatment at (a) 350 °C,
(b) 500 °C, and (c) 700 °C, after one cycle, and (d) 700 °C with ad-
ditional coating and heating cycles.
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Y.H. Ikuhara et al.: Processing of epitaxial LiMn2O4 thin film on MgO(110) through metalorganic precursor
sectional observation by TEM, samples were prepared as
follows. Each sample was first oriented by x-ray Laue
back reflection from the substrate side and cut using a
diamond saw parallel to the (112) plane. Then two of
the cut pieces were glued together using Petropoxi 154
(Marto Inc., Tokyo, Japan) with the film surfaces fac-
ing each other. These ‘sandwiches’ were cut into bars
of length approximately 1.5 mm, polished to about
0.1 mm thick, and then attached with epoxy to Mo
rings for reinforcement. These were subsequently
dimpled to a thickness of about 20 ␮m and finally
thinned by ion beam sputtering at 4–5 kV, using a cold
stage (cooled with liquid N2) to minimize damage to
the specimens. Shields were used to protect the inter-
faces and to reduce the effect of the different milling
rates of LiMn2O4 and MgO. In all the micrographs
presented in this paper, the growth direction is upward.
Conventional and high-resolution electron microscopic
observations were performed using a JEOL 2010 elec-
tron microscope (JEOL Co, Ltd., Tokyo, Japan) operat-
ing at 200 kV with a point to point resolution of about
0.194 nm. The LiMn2O4 and MgO interfaces were char-
acterized from the [112]LiMn O //[112]MgO direction
(Fig. 1).
2
4
III. RESULTS AND DISCUSSION
A. Synthesis of LiMn2O4 thin films
Firstly, thin film formation was investigated by chang-
ing the concentration of the [Li–Mn–O] metalorganic
precursor solutions. Figures 2(a), 2(b), and 2(c) show the
XRD patterns of the thin films after heat treatment at
350, 500, and 700 °C, respectively, in oxygen atmos-
phere using the 0.3 M precursor solution. Single phase
lithium manganese spinel thin film could be prepared by
heat treatment at a temperature as low as 350 °C, where
peaks corresponding to (111), (220), (311), (400), (511),
and (440) reflections were detected. These results coin-
cide with those obtained by powder processing.10 XRD
analysis revealed that polycrystalline LiMn2O4 thin films
were prepared at all temperatures up to 700 °C. The
FIG. 4. Pole figures of LiMn2O4 films prepared on MgO(110) substrates by coating of the 0.1 M precursor solution followed by heat treatment
at (a) 350 °C, (b) 500 °C, and (c) 700 °C, and the corresponding overwritten x-ray diffraction patterns of films heat treated at (d) 350 °C, (e)
500 °C, and (f) 700 °C.
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Y.H. Ikuhara et al.: Processing of epitaxial LiMn2O4 thin film on MgO(110) through metalorganic precursor
strong peak at 2␪ ס 18.76° corresponds to the (111)
plane with an interplanar distance of d ס 0.476 nm. It
therefore appears that using a higher concentration solu-
tion results in a large number of nuclei forming in the
upper part of the film where they are not constrained to
grow epitaxially during heat treatment.
When a lower concentration precursor solution such as
0.1 M was used for the spin coating, only the (110) peak
could be detected in samples prepared at 350, 500, and
700 °C [Figs. 3(a), 3(b), and 3(c)] by XRD. This indi-
cates that the (110) plane of LiMn2O4 is highly oriented
on the MgO(110) substrate, since in polycrystalline
LiMn2O4 powder (JCPDS Card No. 35-782), the ratio of
the intensity of the (110) peak to that of the maximum
FIG. 5. Selected-area diffraction pattern of the LiMn2O4–MgO sys-
tem projected along [112]L//[112]M cross-sectional directions.
LiMn2O4 thin film was fabricated by coating the 0.1 M precursor
solution followed by heat treatment at 700 °C.
FIG. 7. Bright-field transmission electron micrograph of the LiMn2O4
film on the MgO(110) substrate heat treated at 700 °C, obtained with
the electron beam aligned along the [112]L//[112]M direction. The
growth occurred in an upward direction.
FIG. 6. Schematic representation of the orientation relationship be-
tween LiMn2O4 and MgO substrate. The (110) face of LiMn2O4 is
parallel to the (110) face of MgO.
FIG. 8. Cross-sectional high-resolution electron micrograph of the
LiMn2O4–MgO interface heat treated at 700 °C, with the incident
electron beam parallel to [112]L//[112]M.
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Y.H. Ikuhara et al.: Processing of epitaxial LiMn2O4 thin film on MgO(110) through metalorganic precursor
FIG. 9. (a) Projected LiMn2O4/MgO interfacial structure models of (a) and (b) along the [112]L//[112]M direction; (c) and (d) are computer-
simulated images based on the structural models of (a) and (b), respectively.
peak (111) is 1:100. Furthermore, with additional 0.1 M
solution coating and heating at 700 °C, the intensity of
the (110) peak increased as shown in Fig. 3(d), because
the volume of oriented LiMn2O4 increased.
about crystal structure. The crystallographic oriented
structure of the LiMn2O4 thin film prepared from 0.1 M
precursor solution on MgO(110) was characterized using
the (400) reflection.
These results show that it is possible to successfully
coat multiple layers of the oriented thin film by careful
selection of the substrate and precursor concentration.
Figure 4 shows pole figures of the LiMn2O4 on MgO
(110) substrates heat treated at (a) 350 °C, (b) 500 °C,
and (c) 700 °C when the (400) plane of LiMn2O4 was set
normal to the x-ray direction, where the LiMn2O4 the
MgO (110) substrate is inclined at 43° to the horizontal.
Figures 4(d), 4(e), and 4(f) show the corresponding over-
written XRD patterns in the range of ␣-scan from 0° to
90° with the range of ␤-scan from 0° to 360°. The term
B. Crystal orientation of LiMn2O4 thin film
To confirm the epitaxy of the thin film, pole-figure
measurements were performed, as this is a powerful
method for obtaining the three-dimensional information
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Y.H. Ikuhara et al.: Processing of epitaxial LiMn2O4 thin film on MgO(110) through metalorganic precursor
subscripts L and M, respectively, to simplify identi-
fication of crystal planes and directions in each
material.
Figure 5 shows the selected-area diffraction patterns of
the cross-sectional LiMn2O4 thin film on MgO substrate
heat treated at 700 °C with the incident electron beam
parallel to [112]L//[112]M directions. According to this
diffraction pattern, the orientation relationship between
LiMn2O4 and MgO is
(111)L//(111)M
(110)L//(110)M
[112]L//[112]M
,
,
.
Since the lattice parameters of LiMn2O4 and MgO are
0.8247 and 0.4213 nm, respectively, the area of the
(110) face of the MgO unit cell is very nearly one-
quarter that of the LiMn2O4 unit cell. Thus, (110)L can
fit very well onto the surface of (110)M. Here, the mis-
fit parameter f ס 2 [d(222)L − d(111)M]/[d(111)M] be-
tween the parallel (222)L and (111)M planes, which are
perpendicular to the interface, is 2%. The question as to
whether Mn atoms align on the Mg atoms or not will be
addressed later.
The orientation relationship is indicated in Fig. 6,
which shows schematically the interface between (110)L
and (110)M. Cross-sectional directions such as [112]L//
[112]M, which lie in the (110)M plane, are exactly par-
allel to the interface and are suitable directions for high-
resolution electron microscopy (HREM) observations.
Figure 7 shows a bright-field image of the LiMn2O4
film heat treated at 700 °C on the MgO(110) substrate
obtained with the electron beam aligned along the
[112]L//[112]M zone axis. It can be seen that the
LiMn2O4–MgO interface has a wavy structure and is not
atomically smooth in some regions, while the thickness
of the heat-treated film is about 100 nm.
Figure 8 is a cross-sectional HREM image of the in-
terface region. According to the selected area diffraction
pattern in Fig. 5 and the HREM image in Fig. 8, (111)L
is grown epitaxially perpendicular to the (110)M surface,
which corresponds to the results of the pole figure. The
thermal decomposition of the precursor and nucleation of
the LiMn2O4 occurred at low temperature (approxi-
mately 200 °C),25 and the epitaxial growth was acceler-
ated on the surface of the MgO(110) substrate at
temperatures up to 700 °C. In the next section, we dis-
cuss the atomic structure model at the LiMn2O4/MgO
interface based on the orientation relationship results and
the HREM image.
FIG. 10. Experimental HREM images of the LiMn2O4–MgO interface
heat treated at 700 °C, projected along the [112]L//[112]M direction.
The inset is the computer-simulated image shown in Fig. 9(c).
␤ is the rotation axis perpendicular to the film plane, and
␣ is the rotation axis perpendicular to ␤ and ␪. There are
six reflections; (400), (400), (040), (040), (004), (004),
for {400} and their angles to the (220) plane are 45°,
135°, 45°, 135°, 90°, and 90°, respectively. Since only
(400) and (040) have angles smaller than 90°, these re-
flections with two fold symmetry between LiMn2O4 and
MgO (110) appear as two spots along ␤ at ␣ ס 45° as
shown in Figs. 4(a), 4(b), and 4(c), indicating that ori-
ented LiMn2O4 thin film on MgO(110) was prepared by
heat treatment at 350, 500, and 700 °C. The combination
of XRD (Fig. 3) and pole figure analysis (Fig. 4) results
show clearly that epitaxial growth takes place even at
350 °C. Nevertheless, the degree of orientation is not as
high for the film formed at 350 °C, compared with those
formed at higher temperatures.
These results indicate that nucleation and crystalliza-
tion commence at a low temperature on the MgO(110)
substrate by this method using metalorganic precursor
solution and that the crystallization proceeds at higher
temperatures while maintaining the homogeneity of the
crystal structure.
C. Orientation relationship between LiMn2O4 thin
film and MgO
Both LiMn2O4 and MgO have cubic crystal struc-
tures; LiMn2O4 is a spinel (space group, Fd3m) while
MgO has a sodium chloride structure (space group,
Fm3m). In this section, the orientation relationship be-
tween LiMn2O4 film and MgO substrate being heat
treated at 700 °C was investigated by transmission
electron microscopy (TEM). In the following dis-
cussion, LiMn2O4 and MgO are denoted by the
D. Atomic structure of the LiMn2O4–MgO
interface
Since the misfit between LiMn2O4 and MgO is about
2%, the model was constructed to expand the LiMn2O4
so as to exactly fit the MgO atomic alignment of the
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Y.H. Ikuhara et al.: Processing of epitaxial LiMn2O4 thin film on MgO(110) through metalorganic precursor
substrate to form a coherent interface. Two possibilities
were considered for the position of Mn atoms on the
(110) surface of MgO. Figures 9(a) and 9(b) show two
projected interfacial structure models along the [112]L//
[112]M beam directions using supercells where Mn at-
oms on the (110) surface of MgO are located directly
above the Mg atoms (model I), or on the O atoms (model
II), in the projected image. In constructing the supercells
in Fig. 9, the separation between the two crystals was
assumed to be such that the shortest interatomic distance
corresponds to the bond length in an MgO crystal. On
this basis, the shortest interatomic distance in model I (or
II) of Fig. 9 is 0.210 nm. Computer simulations were
performed using the supercells. The supercell lattice pa-
rameters were 1.823 × 2.828 × 2.000 nm3 for model I and
model II.
sectional TEM reveals that the orientation relationship
between MgO and LiMn2O4 is (111)L//(111)M, (110)L//
(110)M [112]L//[112]M, which results in the (111)L
planes growing perpendicular to the surface plane of
MgO. Since the area of one face of the cubic MgO unit
cell is very nearly quarter that of the LiMn2O4 unit
cell face, this orientation relationship results in a good
fit between the Mn atomic layer of the film and the
Mg atomic layer of the substrate at the interface.
Hence, when the epitaxial LiMn2O4 film is prepared
on MgO(110), the (111) layers of Mn atoms align
themselves above the (111) layers of Mg atoms in the
substrate.
ACKNOWLEDGMENTS
This work was entrusted by NEDO as part of the Syn-
ergy Ceramics Project under the Industrial Science and
Technology Frontier (ISTF) Program promoted by AIST,
MITI, Japan. The authors are members of the Joint Re-
search Consortium of Synergy Ceramics.
Figures 9(c) and 9(d) are computer-simulated images
of the interface models shown in Figs. 9(a) and 9(b) re-
spectively, for a crystal thickness of 9 nm and a defocus
of −7 nm. These calculations were actually performed
over a range of thickness from 5 to 15 nm, and a range of
defocus from −5 to −30 nm. The best fits to the experi-
mental images in the direction [112]L//[112]M were ob-
tained for model I. This indicates that there is a good
match between model I and the experimental image. A
comparison of the simulated HREM images of model I
with the experimental images in the [112] directions is
shown in Fig. 10. As can be seen, there are reasonable
matches between the simulated and experimental images,
indicating that model I is closer to the actual structure of
the LiMn2O4–MgO interface. Hence, in the present sys-
tem, the Mn atoms in the (111) layers are most likely
aligned with the Mg atoms in the (111) planes of the
MgO substrate when viewed in the [112] direction. This
can be understood in terms of the close match between
the spinel and sodium chloride crystal structures, which
encourages the LiMn2O4 crystallites to grow essentially
as an extension of the cubic MgO crystal.
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