Transcript Composition Graded, Epitaxial Oxide Nanostructures

Composition Graded, Epitaxial Oxide Nanostructures: Fabrication and Properties
(NSF NIRT Grant # 0709293)
Efstathios I. Meletis1, Jiechao Jiang1, Chonglin Chen2, Amar S. Bhalla2, and Gemunu Gunaratne3
1
University of Texas at Arlington, Arlington, Texas;
2
3
University of Texas at San Antonio, San Antonio, Texas; University of Houston, Houston, Texas
(II) Double-layered Nanostructure of Ba(Zr,Ti)O3 Epilayer and
twin–coupled domain structures on MgO substrate
BACKGROUND
Perovskite oxides are of enormous fundamental interest and technological importance due to their
intriguing properties. These properties can be tailored for a wide range of applications in magnetic,
magneto-electronic, photonic, and spintronic technology. Many perovskite - type oxides have been
synthesized in the past in bulk form or as thin films. It is expected that nanostructures of these oxides
may offer enormous opportunities to explore intriguing physics and applications. However, synthesis
of one-dimensional perovskite -type oxides is a challenge and has seen very little success due to their
complex composition. Recently, we achieved fabrication of self-organized, ordered arrays of coherent,
orthogonal epitaxial (La, Sr)MnO3 nanopillars on (001) LaAlO3 by pulsed-laser deposition (PLD) [1],
which to the best of our knowledge, has been the first report on the fabrication by self-organization of
such epitaxial oxide nanopillars. The formation of the nanopillars depends strongly on the processing
temperature and oxide composition [2]. Such nanopillars exhibit novel magnetic properties different
from those of their bulk, thin film, or nanoparticle counterparts [3]. Furthermore, ferroelectric,
compositionally gradient thin films have been shown to tremendously enhance piezoelectric response
due to the build-in strain gradient. The coexistence of different properties that can be coupled in
nanocomposite thin films has recently stimulated much scientific and technological interest since the
coupling can provide new property tenability. However, major challenges exist in extending these
compositional variations from thin films to nanopillars since the fabrication of compositionally graded
and modulated composite nanopillars by self-organization has not yet been attempted.
.
We recently identified non-lead ferroelectric material, BaTiO3 and its modified
materials (Ba, Sr)TiO3 and Ba(Zr,Ti)O3 as another counterpart for the
composition graded nanostructures. These films exhibit high dielectric constant,
low dielectric loss tangent and large electric field tunability that have attracted
considerable attention for bypass capacitors, IR detectors, and tunable
microwave applications. We fabricated “structure” graded Ba(Zr,Ti)O3 films on
(001) MgO. Ba(Zr,Ti)O3 epilayer was first epitaxially grown on the substrate
followed by a layer of multi-oriented twin domain structures by sharing their {111}
planes with the epilayer. Such structure graded thin films show interesting
abnormal ferroelectric properties that do not exist in their bulk counterpart.
[001]
3
b a
3
Twin-4
b4
4
2
c
b2
c1
-
(111) 1
c4 (111)
a
a
b1
Twin-1
[010]
Epilayer
[100]
• Investigate the principles of formation of self-assembled, epitaxial nanopillars of ferromagnetic
(La,Sr)MnO3 and (La,Ca)MnO3, and ferroelectric (Ba,Sr)TiO3 and Ba(Ti,Zr)O3 perovskite-oxides;
• Fabricate compositionally graded and modulated composite (La,Sr)MnO3 and (La,Ca)MnO3, and
ferroelectric (Ba,Sr)TiO3 and Ba(Ti,Zr)O3 nanopillars;
• Characterize and study the mechanism of morphological evolution, structure and physical properties;
• Theoretically identify relationships between nanostructure characteristics and materials properties;
• Develop a tool for designing and exploring 1-D nanostructures of interest and other new materials.
Twin 2
Fig. 2 XTEM image of twin-coupled
structure on epitaxial Ba(Zr,Ti)O3 film on
(001) MgO. (a) Bright-field image, (b), (c)
and (d) dark-field images showing
presence of BZT epilayer and two twins.
[110]0
[001]4
[111]2
[001]3
[110]0
10
2
Polarization (c/cm )
[111]0
[110]2
[110]3
[111]3
Twin 3
[001]
[111]0
[001]1
[111]4
[111]1
[110]1
[001]1
Twin 1
[111]0
[111]1
[111]3
[001]3
[110]0
[110]4
[111]2
[111]4
6
[111]0
[001]2
4
[110]0
2
[001]4
Twin 4
0
-2
-4
-6
-4
-2
0
2
4
Applied field (V)
Fig. 4 Hysteresis loop measurement of BZT
film exhibiting interesting abnormal
properties due to the formation of the twins.
We have systematically investigated the effects of temperature, pressure, laser energy and
frequency and post-annealing on the microstructure formation of epitaxial (La,Sr)MnO3 thin
films. We are able to fabricate (La,Sr)MnO3 continuous epilayer (Fig. 1a) and discrete epitaxial
nanopillars (Figs. 1b and c) by manipulating the experimental conditions and parameters and
confirmed the repeatability for achieving a variety of designed nanostructures. A roadmap for
fabricating various distinct epitaxial nanostructures has been established.
Fig. 3 Schematic illustration of the epilayer
structure and the four possible oriented twin
domains (up) and their crystallographic
orientation relationships between the
epilayer and the twin domains (down).
Black blocks (squares, triangles and
ellipses) represent the zone axes of the
epilayer, while those filled with coarse slope,
horizontal, fine slope and vertical lines
represent the zone axes of Twin-1, Twin2, Twin-3 and Twin-4, respectively.
Twins
Fig. 5 Plan-view TEM (a) bright-field and
dark-field images (b), (c), (d) and (e)
showing presence of twin domain Twin-1
(T1), Twin-2 (T2), Twin-3 (T3), and Twin4 (T4), respectively.
Fig. 7 Cross-section TEM (a) bright-field image and (b)
EDP of the PSTO/MgO interface; (c) bright-field image
and (d) EDP of the LCMO/MgO interface.
Fig. 8 (a) EDP and (b) HRTEM of plan-view
PSTO/MgO interface; (c) EDP and (c) HRTEM
of plan-view LCMO/MgO interface.
(IV) Theory and Modeling of Self-assembling of Nanostructured Films
Knowledge obtained from these series of investigations is used for theoretical modeling for
further precisely controlling the formation of the nanopillar structures. Structures formed during
the growth of an epilayer on a substrate are determined by minimizing the energy of the
configuration, which consists of (1) elastic energy of the epilayer, due to the requirement that it
be commensurate with the substrate, (2) the surface energy of the epilayer, and (3) the wetting
potential [6]. We assume that the substrate lattice is unchanged, and hence that there is no
associated energy. The spatio-temporal dynamics of the epilayer is typically described using the
evolution of its height h(x,y) via
Epilayer
Fig. 6 HRTEM image of a planview TEM sample showing
coexistence of epilayer and twins.
where the diffusion is along the surface h(x,y) [1]. Unfortunately, the expressions for the terms
on the right (the free energy density, curvature, wetting energy etc.) in terms of h(x,y) and its
derivatives are very complicated. The analysis can be simplified by using the “small slope”
expansion, which will be valid close to the Stransky-Krastonow instability, where the
homogeneous solution destabilizes to a patterned array [7]. Under these conditions, the
previous equation reduces to
Educational Outreach
UTA established a working relationship with the Society of Hispanic Professional Engineers (SHPE) sponsoring
six (6) Hispanic students to participate in Pre-college Symposia and developing a Latino Summer Camp on UTA
campus during the Summer of 2008.
References:
[1] J.C. Jiang, E.I. Meletis and K.I. Gnanasekar, “Self-organized, ordered array of coherent orthogonal column nanostructures in epitaxial La0.8Sr0.2MnO3 thin films”, Appl. Phys.
Lett. vol 80, 4831-4833, 2002.
[2] J.C. Jiang, K.I. Gnanasekar and E.I. Meletis, “Composition and Growth Temperature Effect on the Microstructure of Epitaxial La1-xSrxMnO3 Thin Films on (100) LaAlO3”, J.
Mater. Res., vol. 18, 2556-2561, 2003.
[3] J.C. Jiang, L.L. Henry, K.I. Gnanasekar, C.L. Chen and E.I. Meletis, “Self-Assembly of Highly Epitaxial (La,Sr)MnO3 Nanorods on (001) LaAlO3”, Nano Letters, vol. 14, 741745, 2004.
[4] J.C. Jiang, Z. Yuan, C.L. Chen and E.I. Meletis, “Interface Modulated Structure of Highly Epitaxial (Pb,Sr)TiO3 Thin Films on (001) MgO” Appl. Phys. Lett., vol. 90, Art. No.
051904 (2007).
[5] J. C. Jiang, J. He, E.I. Meletis, J. Liu, Z. Yuan, and C. L. Chen, “Two-dimensional Modulated Interfacial Structures of Highly Epitaxial Ferromagnetic (La,Ca)MnO3 and
Ferroelectric (Pb,Sr)TiO3 Thin Films on (001) MgO” Journal of Nano Research, vol. 3, 59-66 (2008).
[6] B. J. Spencer, P. W. Voohees, and S. H. Davis, “Morphological Instability in Epitaxially Strained Dislocation-Free Solid Films: Linear Stability Theory,”
J. Appl. Phys. 73, 4955 (1993).
[7] A. A. Golovin, M. S. Levine, T. V. Savina, and S. H. Davis, “Faceting Instability in the Presence of Wetting Interactions: A Mechanism for the
Formation of Quantum Dots,” Phys. Rev. B 70, 235342 (2004).
[8] F. Shi, P. Sharma, D J. Kouri, F. Hussain, and G. H. Gunaratne, “Nanostructures with Long-Range Order in Monolayer Self-Assembly,” Phys. Rev. E 78, 025203 (2008).
The nature of the interfacial structure is very important in understanding the growth
mechanism of epitaxial films and nanopillars. Cross-section TEM has been widely used to
study the interfacial structure of heteroepitaxial films and has been turned out to be a very
effective technique for such studies. The lattice misfit induced strain energy can be partially
or fully released at the interface between the epitaxial film and substrate by edge
dislocation formation which can be periodically distributed along the interface. However, the
interfacial structure information obtained using cross-section TEM is limited in onedimensional space. More local information is needed in order to completely understand the
influence of the substrate surface characteristics and film/substrate interface on the
microstructure of epitaxial films. As a part of this project, we recently developed a method
using plan-view TEM to study the interface structure in 2D space, which is able to provide
critical and valuable information that is lacking from the cross-section TEM analysis [4]. We
have fabricated and studied epitaxial (La,Ca)MnO3 and (Pb,Sr)TiO3 films on MgO substrate.
The lattice mismatch near the interface regions obtained using the new method was found
to be -8.0% for (La,Ca)MnO3/MgO and -7.14% for PbTiO3/MgO. Both values are larger than
those obtained using cross-section TEM (-6.4% for (La,Ca)MnO3/MgO and -6.2 % for
PbTiO3/MgO). The (Pb,Sr)TiO3 film is well commensurate with the substrate over large
areas, whereas (La,Ca)MnO3 film is only locally commensurate with the substrate [5].
MgO
(I) Epitaxial (La,Sr)MnO3 Layer and Nanopillar Structures
Figure 1. XTEM image of
epitaxial (La,Sr)MnO3
continuous film (a) and
nanopillars (b) on (001)
LaAlO3 substrate. (c)
Plan-view TEM of
epitaxial (La,Sr)MnO3
nanopillars.
(III) Two-dimensional Interfacial Structure of the Epitaxial Oxide Films on MgO
[001]2
8
The specifications and long term
vision have been discussed with
the project team members
during the kickoff meeting (Oct.
1 s t , 2 0 0 7 , U TA ) . T h e s e
specifications were updated
during the 2nd (May 26, 2008,
UTSA) and 3rd (Oct. 16, 2008, UH)
project meeting according to the
project feedback mechanism.
Twin-2
(111)
-(111)
c3
OBJECTIVES
Project flow chart for interaction between team members
and overall contribution to Design of new materials.
a2
Twin-3
UH
Fig. 10 2008 Latino Summer Camp on UTA campus: demonstration of temperature
effects on the mechanical behavior of engineering materials.
In deriving this equation, we have scaled h(x,y) by
the height L at which the homogeneous layer
destabilizes. The control parameters L, g, p, and q
can be evaluated n terms of the mechanical
parameters of the substrate and the epilayer.
Under the model dynamics, a uniform (but noisy)
deposition of atoms on a substrate gives selfassembled quantum-dot arrays. In order to form
large-scale perfect arrays, we use a technique
that we proposed previously; namely masking of
the deposition [3]. Properties of the mask can be
determined from the spatio-temporal dynamics of
the formation of a disordered pattern in the
absence of the mask [8].
Fig. 9 Ordered quantum- dot array.