Nano-ionic materials made of strontium titanate (SrTiO
Increasing worldwide concerns about environmental degradation, global
warming, and the need for new renewable energy sources and safety issues
regarding, e.g., nuclear power lead to a search for new materials for sensor,
energy conversion, and storage applications. Currently used materials such as
yttrium stabilized zirconia (YSZ) yield an outstanding performance such as
fast ionic transport
This work focuses on investigation of nano-ionic structures comprising alternating layers of metal oxides with oxygen deficiency and excess. Consequently, an increase in concentrations of mobile defects in the form of oxygen vacancies and interstitials and thus enhanced ionic conductivity is expected. The Debye screening length at the interface of nano-ionic layers should be on the order of several tens of nanometers resulting in effects on the oxygen transport visible by oxygen exchange experiments and impedance spectroscopy.
Total conductivity of different STF
compositions at 850
The transport properties in these materials are investigated using oxygen
exchange experiments and impedance spectroscopy as a function of temperature
and of oxygen partial pressure. The material systems selected for this work
are strontium titanate (SrTiO
STO single crystals show a perovskite structure with a closely packed anion
sublattice. Because of size constraints, strontium titanate cannot
accommodate interstitial defects
The stoichiometric composition of STF solid solutions is defined as
SrTi
Schematic illustration of an interface between STF layer and STO. The oxygen depth profiles along the interface are expected to be much broader close to the interface (a) than in bulk STO or STF (b).
STO is a perovskite structure with well-studied electrochemical properties
(e.g.,
At low oxygen partial pressures, oxygen atoms tend to leave the crystal
lattice, generating oxygen vacancies and free electrons compensating the
charge
Total conductivity in STO as a
function of oxygen partial pressure and temperature as reported by several
authors
At intermediate oxygen partial pressures the ionic conductivity dominates,
with oxygen vacancies being the prevalent mobile charge carriers.
At high oxygen partial pressures the oxygen atoms fill the structural
vacancies. As a result of this reaction holes are generated to compensate the
charge,
The total conductivity of STO,
As mentioned above, thermally generated anion Frenkel defects in STF cause an
enhanced concentration of charge carriers in the vicinity of the interface,
i.e., oxygen interstitials
Diffusion model (not to scale) used to simulate the 2-D diffusion process through the space charge zone of STO and crystal bulk. The dotted line covering most of the outer boundaries represents an area with zero flow; the arrows inside the area represent places where depth profiles are acquired (see below).
Since the thickness of the space charge zone is several orders of magnitude
lower than the sum of thicknesses of all other components of the sample, its
measurable impact on the conductivity is expected to be relatively low. The
increase in
In order to estimate the influence of the space charge layer on the overall
oxygen transport and, in turn, on the oxygen partial pressure-dependent
conductivity, a 2-D model of the interface, including the STO substrate, is
created. The model shown in Fig. The oxygen flow from outside into the sample occurs in a small section at the left border of the model area only.
In practice, the oxygen access is enabled by an initial crater crated by ion
etching as depicted in Fig. The oxygen diffusion coefficients in the STO space charge zone and in the STO substrate equal The thickness of the space charge zone is chosen to be 30 nm. The thickness of the STO substrate is about 1
The oxygen diffusion in the model described above is simulated using Comsol
Multiphysics software
Results of the simulation are subsequently compared with experimental
results. Here, cross sections and line scans (denoted by vertical arrows in
Fig.
A scheme of a sample with two STF layers and an STO layer deposited on the substrate as well as the initial SIMS crater for oxygen access.
A scheme of a layer sequence deposited on a STO substrate is shown in
Fig.
All layers are synthesized on a (100) face of single crystalline
10
The diffusion path for oxygen is opened by initial SIMS analysis. Thereby a
rectangular crater of approximately 500
The diffusion runs in
After pre-annealing, the samples are quickly removed from the hot zone of the
furnace and the atmosphere is replaced by a
The diffusion runs are performed at temperatures from 500 up to
700
The SIMS measurements are performed with a Hiden Analytical SIMS Workstation.
Here, a primary Ar
After diffusion runs the samples are analyzed for the second time using the
scanning mode of the SIMS system. Here, a larger area of
2.1
To determine the electrical impedance, STF20
The samples are annealed at temperatures between 500 and 700
Scheme (not to scale) of the electrode arrangement for conductivity measurements. Two wires are attached to the sample using Pt paste at the ends of the rod. Due to the position of the electrodes, the components of the structure are connected in parallel.
The complex impedance is measured with a Solartron SI1260 gain phase analyzer
in a frequency range from 1 Hz to 1 MHz. Subsequently, the impedance
spectra are fitted with an electronic circuit consisting of a resistor
The reference sample used for comparison purposes only is a STO substrate
without additional layers. Further samples consist of a STO substrate with a
100 nm STF20 and a LAO film on top of it (see
Fig
High-resolution TEM image of a STF50
In order to confirm the quality of prepared samples, high-resolution TEM
images of the interface are acquired. Here, the specimens are cut into
5
A HR-TEM image of the interface between substrate and deposited STF50 (see
Fig.
Largely suppressed
In order to verify the effect of the diffusion barrier, samples with and
without the opening (SIMS crater) in the Au
Fe profile at the interfaces of a
50 nm thick STF50 sandwiched between LAO and STO before and after 10 h of
annealing at 600
The second aspect investigated by SIMS is the thermal stability of the thin
STF layer as well as the multilayer structures. As shown in
Fig.
The increase in annealing temperature to 700 infinite depth in the direction of the STO substrate; a finite Fe source in the form of the STF layer; different diffusion coefficients in STF and STO; and a ca. 10 nm wide interface area where
The starting point of the simulation is the as-prepared Fe profile in STF and
STO.
The STF20
Mean oxygen concentration
Although no detailed information regarding the temperature dependence of Fe
diffusion is gathered, the activation energy
The broadening of the interface limits the long-term application of the
structures to about 600–650
An attempt to estimate the mean oxygen concentration
Initial SIMS profile of an
as-prepared sample with following structure:
Au
The SIMS measurements prior to annealing of the samples confirm the existence
of all layers in the case of single- as well as multi-layer structures. As
shown in an exemplary depth profile (see
Fig.
Normalized
Two-dimensional
If the plateau is attributed to the space charge zone, its width should be
comparable with the Debye length
Normalized
The diffusion coefficients of oxygen for both temperatures are determined
using an analytical solution of Fick's second law, as well as numerically as
described in Sect.
The 3-D analysis of the samples annealed at 600
Cross section of a 3-D
Intentionally, the center of the crater sputtered prior to the oxygen
exchange experiments just reaches the STO substrate. The latter is used as
indicator of a sufficiently deep crater to enable oxygen exchange at the
upper STF
One can observe that
Since such multilayer stacks might be applied as fast ionic conductors, a
comparison of oxygen diffusivity between the STF
Two cross sections of oxygen distribution showing the influence of the oxygen diffusion coefficient at the interface on the shape of the concentration depth profile.
In order to estimate the range of measurable oxygen diffusion coefficients
along the interface, the numerical model of STO described in
Sect.
Simultaneous diffusion of oxygen along the interface and in the bulk requires
the consideration of two different experimental aspects. First, for a given
temperature, the annealing time has to be large enough to enable oxygen
diffusion in the bulk. An underestimated time results in a natural abundance
of the tracer measured through the entire sample. On the other hand, the
overestimated annealing time results in a sample saturated with the
Impact of the oxygen diffusion
coefficient along the interface on the oxygen depth profile acquired
100
Since the 3-D SIMS measurement revealed a uniform distribution of
It must be noted that this approach allows one only to determine the lowest
possible limit for
Exemplary Nyquist plot of STF20 on STO
impedance measured at 600
The numerical approach not only enabled simulation on the entire oxygen
concentration profile, including the STO
The measured impedance spectra as shown in Fig.
Exemplary plot showing the change in the
resistance of STF20 on the STO sample (see Sect.
The determination of the conductivity is performed in two different gas
buffer compositions, CO
After the set temperature stabilizes, the
Minimum of electronic conductivity, ionic conductivity and oxygen partial pressure at the intrinsic
point obtained from the fit of Eq. (
The total conductivity plotted against oxygen partial pressure shows a shape
typical of the predominant
Using Eq. (
The results show that there is only a slight difference in the conductivity
measured in two different gas mixtures (see Table ionic conductivity in the intermediate range with
The activation energies for
The band-gap enthalpy
Total electrical conductivity of
the STF20 layer on the STO substrate as a function of oxygen partial pressure
and temperature. The measurements are performed in CO
The measurement in hydrogen and carbon monoxide containing atmospheres is
shown in Fig.
At oxygen partial pressures below 10
At an oxygen partial pressure of 1 mbar and a temperature of 690
Total electrical conductivity of the
STF20
Another explanation of conductivity enhancement is the contribution of the
space charge zone. In order to match the resistance of the measured sample
with the resistance of STO and resistance of STF20, one would require an
additional element with
Assuming that the ionic contribution in STF20 films on STO is attributed to
Frenkel defects, the oxygen diffusion coefficient can be calculated from the
conductivity data. The 3-D SIMS measurements reveal a 30 nm thick layer of
enhanced oxygen diffusivity in STO in the vicinity of the interface. Using
this number and the ionic contribution to
Epitaxial structures containing interleaved STO and STF layers grown
epitaxially on STO substrates are investigated by oxygen tracer diffusion
experiments and impedance spectroscopy at different temperatures and oxygen
partial pressures. Three-dimensional element profiles show increased
Conductivity measurements in H
The authors declare that they have no conflict of interest.
This joint research project was financially supported by the state of Lower Saxony and the Volkswagen Foundation, Hannover, Germany. Edited by: J. Zosel Reviewed by: two anonymous referees