Urška
Trstenjak
*a,
Nina
Daneu
a,
Jamal
Belhadi
a,
Zoran
Samardžija
b,
Aleksander
Matavž
c,
Vid
Bobnar
cd,
Gertjan
Koster
ae and
Matjaž
Spreitzer
a
aAdvanced Materials Department, Jožef Stefan Institute, Jamova cesta 39, 1000 Ljubljana, Slovenia. E-mail: urska.trstenjak@ijs.si
bDepartment for Nanostructured Materials, Jožef Stefan Institute, Jamova cesta 39, 1000 Ljubljana, Slovenia
cCondensed Matter Physics Department, Jožef Stefan Institute, Jamova cesta 39, 1000 Ljubljana, Slovenia
dJožef Stefan International Postgraduate School, Jamova cesta 39, 1000 Ljubljana, Slovenia
eFaculty of Science and Technology and MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands
First published on 20th December 2022
0.67[Pb(Mg1/3Nb2/3)O3]–0.33[PbTiO3] (PMN–33PT) epitaxial thin films were prepared by pulsed-laser deposition (PLD) using ceramic targets, enriched with PbO (and MgO). The phase composition and crystal structure were analyzed by high-resolution X-ray diffraction. Accurate chemical analysis of the thin films was carried out using wavelength-dispersive X-ray spectroscopy, which revealed that the target-substrate material transfer is not fully stoichiometric. The largest deviations were found for Pb and Mg content. Our results show that it is possible to effectively tune the stoichiometry of the films via the use of custom-made ceramic targets, emphasizing their advantage over single-crystal targets in PLD growth of complex metal oxides. The functional response of the films, however, is the result of complex interactions between the crystal structure, microstructure and chemical composition of the films. Our results show that the sample, prepared from the target with 20 mol% PbO excess, with the largest deviations from the nominal stoichiometry, exhibits the highest longitudinal piezoelectric coefficient.
PMN–PT contains five diverse elements, the ratio of which needs to be kept precise in order to ensure superior electrical properties. First and foremost, the material synthesis should be done in such a way as to ensure that there is no Pb deficiency in the material. Owing to the high volatility of Pb, the high temperatures that are usually required to ensure crystalline products, can easily lead to loss of Pb during the synthesis or processing. Extensive Pb loss will result in the formation of Pb-deficient pyrochlore phases, which do not exhibit the desired functional properties and should therefore be avoided. In pulsed-laser deposition (PLD), the formation of these secondary phases is more often than not circumvented by adjusting the process parameters11–17 and/or making use of Pb-enriched targets.11,13,18 While there are reports on pyrochlore-free films grown using stoichiometric targets,17,19–21 the results are often not reproducible and the processes are limited to certain substrates. On some substrates13 or buffer layers,22 perfectly single-phase PMN–PT cannot be grown from stoichiometric targets. As it has been shown,10,22–24 the phase stability strongly depends on the morphology of the substrate surface. Additionally, the accuracy of the phase composition analysis is restricted by the detection limit of the technique, which is most commonly X-ray diffraction. Small pyrochlore inclusions might still be present, and some Pb sub-stoichiometry could be even accommodated by the perovskite structure. Small differences in the Pb content in the films could affect their functional properties and thereby explain the large variations in the reported values related to the dielectric, ferroelectric and piezoelectric performance of the films.
Furthermore, although fewer research has been done on the regulation the remaining constituents, it has been shown that the Mg/Nb ratio also plays an important role in the formation of the perovskite phase and on the functional properties of the material.11,21,25 As our previous study13 has shown, epitaxial PMN–PT thin films, prepared directly on SrTiO3 (STO) substrates from stoichiometric targets and targets with only PbO excess, exhibit Mg deficiency. It has also been shown that addition of excess MgO strongly influences the growth mechanism in bulk ceramic and single crystals.26,27
While stoichiometric transfer is commonly achieved in PLD, in complex materials such as PMN–PT, this may not always be the case. Therefore, it is important to analyze the composition of the resulting films. Chemical analysis of the thin films is seldom carried out due to technical difficulties related to sampling depth and signal overlap. Techniques such as X-ray photoelectron spectroscopy can only probe the sample surface, and etching processes rarely result in homogeneous removal of the material. Energy-dispersive X-ray spectroscopy (EDXS) of PMN–PT suffers from low accuracy due to low analytical sensitivity and peak overlaps. Additionally, the film thickness should be sufficient to avoid sampling the substrate in case it contains any of the elements present in PMN–PT (e.g.: SrTiO3) substrates.
In this study, we used different types of targets (stoichiometric, PbO-rich, PbO- and MgO rich) and analyzed the compositions of the resulting films with high precision and accuracy by quantitative electron-probe microanalysis using wavelength-dispersive X-ray spectroscopy (WDXS). As compared to EDXS the main benefits and advantages of WDXS are: (i) high energy resolution of 5–20 eV with peak overlaps being generally avoided, (ii) peak-to-background ratios (P/B) are high and consequently (iii) high analytical sensitivity can be achieved. This enables the execution of accurate measurements of elemental concentrations in the samples. However, the WDXS procedure is not that straightforward (compared to EDXS) and it has to be carefully optimized for each particular material by means of the use of best possible analytical conditions, verified standard materials, optimal X-ray spectroscopic measurements and most appropriate quantitative matrix-correction algorithms.
The results of the WDXS that was optimized for the analyses of the PMN–PT thin films enabled us to (i) understand the stoichiometry transfer from the target to the film, (ii) engineer the composition of the films to approach the nominal value and (iii) to study the influence of smaller off-stoichiometries on the structural and electrical properties of the PMN–PT thin films. This is the first systematic experimental study of the influence of full cation stoichiometry on the crystal- and microstructural, as well as electrical properties of PMN–PT epitaxial thin films. The findings of this study can be translated to and implemented in other material systems with a similarly high sensitivity to any perturbations in the structure.
The samples are labeled as XPYM, based on which target was used for their growth, where X and Y represent the quantities of PbO and MgO excess in mol%, respectively.
The surface structure of the samples was examined by atomic force microscopy (AFM, Digital Instruments DimensionTM 3100). The AFM images were processed using WSxM 4.0 software.21
The microstructure in cross-section was examined by scanning transmission electron microscopy (STEM) using a probe Cs-corrected scanning transmission electron microscope (Jeol ARM 200 CF, JEOL, Japan) operated at 200 kV. The lamellae were prepared by focused ion beam etching (FEI Helios NanoLab 650, Thermo Fisher Scientific, USA).
The chemical composition of the PMN–PT thin films was determined by means of quantitative electron probe microanalysis by wavelength-dispersive X-ray spectroscopy (WDXS) that was carried out using the Oxford Instruments INCA WAVE WDXS spectrometer attached to a scanning electron microscope (SEM, Jeol JSM-7600F). Special attention was paid to the optimization of low-voltage WDXS approach for the measurements of X-ray spectral lines Pb-Mα, Nb-Lα, Ti-Kα and Mg-Kα.
Using Monte-Carlo simulations we found that at electron-beam energy of 8 keV maximum depth of X-ray generation (X-ray range) was ≤0.30 μm for all analyzed elements. Consequently, it was verified that all X-rays were emitted only from the PMN–PT thin films (with thicknesses being >0.5 μm) without the influence of the SrTiO3 substrate. The overvoltage ratio at accelerating voltage of 8 kV was sufficient to provide efficient excitation of all analytical spectral lines.
The WDXS settings were precisely optimized by means of correct detector bias in the proportional counters, accurate peak and background positions and correct pulse-height analysis parameters. Focused point-beam measurements on the PMN–PT thin film samples were carried out at 8 kV accelerating voltage and electron dose of 4.5 μC per point, which ensured that sufficient number of X-ray counts were acquired to keep the standard counting error below ±2% relative for each element. Consequently, high analytical precision and accuracy were achieved. The standards used for quantification of Pb, Ti, Nb and Mg were pure stoichiometric oxides PbO, SrTiO3, Nb2O5 and MgO, whereas the oxygen content was calculated from the stoichiometry to nominal cation valences. The exact procedure for these measurements and analyses is described elsewhere.13,30,31
The longitudinal effective piezoelectric coefficient (d33,f) of the capacitors was measured using a double-beam laser interferometer (aixDBLI, aixACCT). The top electrode size of 600 μm was selected by taking into account the substrate thickness and its Poisson's ratio to avoid additional contributions to the measured strain arising from substrate deformation. d33,f was measured at different DC fields by superimposing an AC signal with an amplitude of 500 mV and a frequency of 1 kHz on a stepwise DC signal with a maximal amplitude of ±150 kV cm−1.
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Fig. 1 Comparison of XRD patterns for PMN–PT thin films, prepared from different targets with a magnified region around the (002) PMN–PT peaks. |
The calculated chemical formulae (empirical) along with the formula for the reference stoichiometric PMN–33PT composition are given in Table 1 and the results are also schematically presented in Fig. 2. The measurements reveal that targets with different amount of PbO and MgO excess always result in the formation of A-site excess PMN–PT thin films, even in the sample prepared from the target without PbO and MgO excess (0P0M), whereas the amount of B-site cations is almost always lower than the stoichiometric composition (see Table 1). The oxygen concentrations were calculated based on the overall amount of the cations and the values are close to the nominal value in all samples. This supports the hypothesis that the compensation mechanism is based on the tendency of the system to maintain overall electroneutrality. The Mg/Nb ratio is lower than the theoretical ratio in all samples, except for 20P20M. Eremenko et al.32 showed how the local Mg/Nb ratio in PMN affects the magnitude of Pb displacement. A larger concentration of surrounding Mg leads to a stronger Pb displacement. They also showed how the polar nanoregions are associated with the Pb displacements. The correlation between the Mg/Nb ratio and the electrical properties of the samples in this study will be discussed in Section 3.5.
Sample | Chemical formula | A-site cations [at%] | A-site charge | B-site cations [at%] | B-site charge | Mg/Nb |
---|---|---|---|---|---|---|
0P0M | Pb1.01Mg0.195Nb0.46Ti0.326O2.99 | 20.2 | 2.02 | 19.6 | 3.99 | 0.42 |
20P0M | Pb1.05Mg0.185Nb0.45Ti0.317O3.00 | 20.9 | 2.09 | 19.0 | 3.89 | 0.41 |
10P10M | Pb1.02Mg0.207Nb0.45Ti0.329O3.01 | 20.4 | 2.04 | 19.7 | 3.98 | 0.46 |
20P10M | Pb1.02Mg0.213Nb0.45Ti0.323O3.01 | 20.4 | 2.04 | 19.7 | 3.96 | 0.48 |
20P20M | Pb1.01Mg0.228Nb0.44Ti0.334O3.02 | 20.2 | 2.02 | 20.0 | 3.99 | 0.52 |
Stoichiometric reference | Pb1.00 Mg0.222Nb0.45Ti0.333O3.00 | 20.0 | 2.00 | 20.0 | 4.00 | 0.50 |
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Fig. 2 Concentration of individual elements, as determined by WDXS analysis. The dashed line marks the PMN–33PT stoichiometric values. |
The films prepared from different targets have different chemical compositions and the results, shown in Fig. 2 reveal that the overall chemical composition of the target is not directly transferred to the thin films. The possible reasons for non-stoichiometric transfer are manifold. Firstly, loss of Pb during target sintering and sub-nominal Mg content due to its hygroscopic nature can cause non-stoichiometry in the targets. To avoid this issue, we sintered the targets in a PbO-rich environment and ensured dry conditions for the MgO powder. Excluding these extrinsic factors, non-stoichiometric transfer can occur during the PLD process. This can happen in either of the three phases: (i) ablation, (ii) transfer and (iii) nucleation. During the ablation, inhomogeneous fluence, incongruent ablation or target degradation can present an issue. In the transfer stage, preferential scattering of lighter elements can occur. In the last stage – nucleation – the sticking coefficient of different species can vary, leading to non-stoichiometry, especially owing to the volatile nature of Pb-based components. In view of achieving pure-perovskite growth, we have addressed these aspects in our previous work, where we selected an optimal fluence value for the ablation stage,13 along with the process pressure for the transfer stage,13 and the temperature13 and surface roughness22 for the nucleation stage. Nevertheless, as this study shows, non-negligible variations in the stoichiometry exist even for films that are phase-pure according to XRD.
The WDXS results show that while the amount of Nb is fairly constant and close to the nominal (stoichiometric) value in all samples, the amounts of Ti and Mg vary with the target composition, the highest differences are observed for Mg. In sample 0P0M, Mg and Ti are both substoichiometric. In the case of deposition using only Pb-rich target (sample 20P0M), a higher fraction of Pb from the target remains in the film, while the concentrations of Mg and Ti drop even more significantly; the amount of Mg is only ∼83% and the amount of Ti ∼95% of the value expected for the stoichiometric PMN–33PT composition. Based on the observations in our previous study22 and other literature reports on the need for Mg-rich targets,11,21,25 we prepared targets with MgO excess in addition to the PbO excess. Surplus of MgO in the targets is directly reflected in increased Mg content in the films. While Mg (and Ti) concentrations in the samples 10P10M and 20P10M were still slightly substoichiometric, the higher Mg excess in the target used for the preparation of the 20P20M sample resulted in overcompensation of Mg, whereas the concentrations of all other cations were very close to the expected (stoichiometric) values.
The results of XRD analyses and WDXS measurements indicate that all films prepared from targets with different amounts of PbO and MgO excess are perovskites but have different average chemical compositions. The overall excess of A-site cations in perovskite-type thin films, e.g. BaTiO3, can be compensated by the formation of extended defects such as Ruddlesden-Popper (RP) phases, threading dislocations and twins,33,34 whereas the deficiency on B-sites in compounds where this structural sites are occupied by two or more heterovalent cations, may lead to the areas of local structural and chemical disorder/heterogeneity, e.g. the formation of (hierarchical) polar nanoregions (heterogeneous polar states) that enhance the properties of relaxor ferroelectrics.35 Larger deviations from stoichiometric composition would lead to larger inhomogeneities (sample 20P0M). The signature of Pb-rich PMN–PT thin films are also the broad peaks appearing at ∼24.5° and at ∼42.5° in the θ–2θ XRD measurements.
Wu and Davies36 showed that deviations in the stoichiometry as small as 1 mol% can induce large changes in the extent and stability of the B-site cation ordering of BaZn1/3Nb2/3O3. Furthermore, they showed that the off-stoichiometry also induces changes in the sintering and microstructure, as well as the dielectric properties. We used RSM and STEM to study the effects of different target compositions on structural and microstructural properties of the samples.
The pseudo-cubic lattice parameters were calculated from the RSM data using the equation , where a and c are the in-plane and out-of-plane lattice parameters, respectively (Fig. 4a). The a and c values, together with the calculated apc values and the FWHMs are given in the ESI† (Section S1). The bulk pseudo-cubic value, which was calculated as
from the literature,37 is marked with the grey dashed line in Fig. 4a). These results show that the unit cell dimensions of all samples are larger in comparison to the bulk PMN–33PT. As the c/a ratios calculated from the RSMs vary, this may indicate a variation in the domain structure/orientation. The structure of bulk PMN–33PT at room temperature is monoclinic.37 Different orientations of the monoclinic structure would produce different in- and out-of-plane lattice parameters in the films. However, due to substrate constraints, the phase diagram in thin films is different38 and the monoclinic phase has been shown to be absent in thin films.39 In principle, the different c/a ratios could also indicate that samples 20P10M and 20P20M are compressively strained, while samples 0P0M, 20P0M and 10P10M are under tensile strain. However, based on the difference in the TECs, the STO substrate is expected to compressively strain the film.40,41 This, together with the observation of expanded unit cell volume, implies that in the latter three films the distortion is brought about by (local) non-stoichiometry. Note that the c/a ratios are close to 1, and that the peaks are very broad, yielding a large distribution of values, especially for a.
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Fig. 4 (a) Pseudocubic lattice parameters of the PMN–PT layers. (b) FWHM of the PMN–PT layers along the ω and θ–2θ direction. |
While the intensity and shape of the STO and LNO spots are comparable in all samples, the RSMs of PMN–PT layers reveal spots with different shapes implying differences in the structural properties of the samples. By rule, the spots from PMN–PT layers are broader, sometimes also asymmetric; the broadest spot was observed in sample 20P0M, while the one from sample 20P20M exhibited the highest asymmetry. There is a correlation between the deviation from stoichiometry on B-sites as determined by WDXS and the size of the RSM spots of the PMN–PT layers (sample 20P0M), as larger stoichiometry deviation is related to larger inhomogeneities.
To compare the crystalline quality and homogeneity of the samples, the full-widths-at-half-maximums (FWHMs) of the peaks were determined from the θ–2θ and rocking-curve (ω) measurements (Fig. 4b). The former is a measure of the variation of d-spacing across the thickness of the films, while the latter is a measure of crystal quality or mosaicity, i.e. the tilting of individual crystallites with respect to the[00l] direction. Both FWHM values are highest in sample 20P0M and lowest in samples 0P0M and 10P10M. While the FWHM value from the rocking curve is commonly used as a quantitative measure of crystallinity and the density of defects, it does not necessarily correlate with the functional properties of PMN–PT, since a wider distribution can stem from the presence of different ferroelectric domains, which contribute to the functional response.
Low-magnification STEM images of the samples (Fig. 5a) reveal that the thickness of the PMN–33PT layers varies from ∼630 nm in sample 20P20M to ∼1 μm in sample 20P0M. All PMN–33PT layers are dense and contain two types of defects; threading dislocations that extend from the interface with the underlying LNO layer upwards and stand-alone V-shaped defects that occur inside the layers. The density of the V-shaped defects is the highest in sample 0P0M, whereas they are not common in the remaining samples where threading dislocations are the prevailing type of extended defects.
Typical for threading dislocations (Fig. 5b) is the 1/2 shift of the out-of-plane (001) lattice planes (all indices given as pseudocubic). Their formation is related to the presence of RP defects in the underlying LNO layer which contains a high density of RP defects that extend mainly perpendicular to the substrate (in the growth direction, with out-of-plane shift) as marked on Fig. 5b. The RP defects in LNO can represent nucleation sites for threading dislocations in the epitaxial PMN–PT layer as observed in Fig. 5b. RP defects in LNO are crystallographic planar defects with well-defined local structure (rock salt-type) composed of double La-O layer and can occur along the three equivalent {100} planes of the pseudocubic structure. In PMN–PT, the dislocations deviate to other lattice planes, which is probably related to the instability of the rock salt structure in lead-oxides. Eventually, with the layer thickness, some of the dislocations are neutralized by alignment of domains with identical shift and therefore it is typically observed that the density of threading dislocations in PMN–PT layer gradually decreases with the layer thickness. Threading dislocations in PMN–PT with the local RP-stacking represent local Pb enrichments. Therefore, the difference in the density of these dislocations across the film thickness implies the presence of a compositional gradient.
Sample 0P0M predominantly contains V-shaped defects, whereas threading dislocations are visible only at the contact with LNO. High-resolution STEM analysis of the V-shaped defects (Fig. 5c) reveals that they are characterized by 1/2 shift of the in-plane lattice planes. Although they are not bound to any well-defined crystallographic plane, it occasionally seems that they follow the {012} planes of the pseudocubic perovskite structure and from symmetric V-shapes with apparently lowest energy. Also in this type of defects the RP-type stacking indicates local Pb-rich enrichment. This is in agreement with the results of WDXS analyses, which showed Pb excess in all samples, even in this one, where the target was stoichiometric without any Pb or Mg excess.
The presence of threading dislocations at the contact with LNO indicates that this type of extended defects forms during the layer growth. The formation of V-shaped defects is not that common in the samples prepared form targets that contained excess of PbO or a combination of PbO and MgO. Using the stoichiometric PMN–33PT target results in more pronounced island-type growth. The junction between the individual islands is manifested as the V-shaped defects. We assume that this change in the growth mode is related to the wetting mechanism; however, further research is needed to confirm this hypothesis.
Differences in the stoichiometry can directly affect the permittivity and polarization through the formation of localized screening charges. While the maximum polarization values of the remaining samples are comparable, several distinguishing features can be observed. Firstly, while all samples exhibit relaxor-like P–E loops, sample 10P10M exhibits the smallest coercive field and the loop of sample 20P20M is the widest. Secondly, the loops are shifted laterally, i.e. exhibit imprint. Imprint (self-bias) could be brought about by the asymmetric electrode configuration (LNO and Au)10; however, the imprint should then be the same for all of the samples. Here, a positive self-bias is present in sample 20P0M, while the rest of the samples exhibit a negative self-bias, exhibiting a clear trend with the change in the Pb content. Another possible explanation for the built-in bias is due to a strain gradient in the layer.19 The strain gradient could arise from a stoichiometry gradient throughout the thickness of the film, as a result of inhomogeneous ablation, i.e. depletion of certain chemical elements during ablation. Depending on the initial target composition, this effect can be more or less pronounced. The compositional gradient can also stem from the growth mode, as observed in our microstructural analysis. The density of the threading dislocations, which represent local Pb enrichments, is diminishing with the film thickness as the structural domains are aligning during growth. Additionally, different defects can be accumulated at these boundaries between the crystallites. Recently, intrinsic complex polar defect formation through high-energy ion bombardment resulting in PMN–PT films with electrical imprint has been reported.42,43 It has been shown the defect dipoles or defect complexes are formed to preserve charge neutrality, which is in line with our observations on the compensation mechanism. The imprint and enhanced electrical resistivity are highly useful for energy storage applications. Our current study shows that it is possible to achieve similar properties in PMN–PT thin films without the use of ion-bombardment, simply by tuning the composition of the PLD targets.
Finally, pinching of the loops can be observed (most pronounced in samples 10P10M and 20P20M), which points to the presence of defect dipoles that have a polarization direction opposite to the one of the PMN–PT matrix.
Our P–E measurements confirm the findings of Eremenko et al.32 regarding the influence of the local Mg/Nb ratio on Pb displacement, according to which it is expected that a higher Mg/Nb ratio would produce the largest displacement, leading to higher polarization values. As shown in Section 3.2., sample 20P20M had the highest Mg/Nb ratio, while the ratio was the lowest in sample 20P0M.
On the other hand, the highest piezoelectric response was measured in sample 20P0M (Fig. 7). Note that sample 20P0M exhibited the highest deviation from the nominal stoichiometry and also had the highest FWHM values in XRD measurements, whereas the stoichiometry of sample 20P20M, for example, was close to the nominal composition and the bulk apc values and the FWHMs were relatively low. This supports the hypothesis that the overall composition is a sum of the matrix composition and the Pb-rich defects. This should be taken into consideration when tuning the stoichiometry in order to maximize the functional response. Furthermore, to achieve high piezoelectric coefficients, order and disorder need to be combined. Barriers in the form of chemical modulation need to be distributed throughout the material in a way that disrupts the long-range polarization.44 The local chemical environment together with substrate-induced strain plays a crucial role in enhancing the competition between the ordered and disordered regions and affecting the overall susceptibility to applied stimuli.45
Note also that both sample 20P0M and sample 20P20M contained pyrochlore (in very low quantities). Therefore, the presence of a small concentration of pyrochlore inclusions does not play a crucial role in the functional response of the material.
The reported di-, ferro- and piezoelectric properties are comparable to other literature reports on PLD-derived epitaxial PMN–PT thin films around the MPB.10,11,46–49
Based on WDXS results, the target-substrate material transfer is not fully stoichiometric in this type of complex material. The largest deviations were found for Pb and Mg content, the concentration of which was highly correlated. The highest off-stoichiometry was measured in sample 20P0M, which was prepared from the target with PbO excess only. Of the 20 mol% excess added, approximately 5 mol% excess remained in the film, while Mg was depleted by 16 mol%. The WDXS results emphasize the importance of chemical analysis of the films, as it is clear that pure-perovskite films can still exhibit a certain degree of variation in their chemical composition. Nevertheless, the average composition alone cannot be used to predict the functional response, as it represents the sum composition of the matrix and extended defects. Specifically, the sample prepared from the target without PbO addition did not exhibit Pb deficiency, but its dielectric breakdown strength was very low. Based on our observations, it seems that this is a result of a relatively high concentration of Pb-rich RP-type defects, while the perovskite matrix is depleted of Pb.
Different target compositions give rise to different concentrations or even different types of complex defects (polar, non-polar, etc.). The formation of complex defects influences the electrical characteristics of the material and the shape of the hysteresis loops. All of the samples exhibited relaxor-like behavior; however, the widths of the P–-E loops differed, as well as the directions and magnitudes of the imprint. The latter show a correlation with the Pb concentration. By tuning the imprint via the use of compositionally modified targets, we can design materials with enhanced energy harvesting and energy storage performance without the need to incorporate additional dopants, buffer layers, or perform ex situ treatments.
We found that the sample with one of the highest deviations from the bulk lattice parameters and with the highest deviation from the nominal stoichiometry exhibited the highest d33,f values. Furthermore, this sample (20P0M) also had the highest surface roughness and FWHM values in XRD measurements, which imply lower crystalline quality. On the other hand, higher rocking curve values could indicate the presence of different ferroelectric domains, which contribute to polarization rotation and enhance the piezoelectric response.
Our findings emphasize the importance of a complete structural and chemical analysis of such complex thin films. For enhancing the functional response, it is important to properly correlate the results of the structural and chemical analyses. In order to deepen the understanding of such relaxor-ferroelectric thin films, further studies are required to elucidate the defect structures and local chemical environments.
Footnote |
† Electronic supplementary information (ESI) available: See supplementary data for: values extracted from XRD data, AFM images of the samples’ surface and STEM cross-sections of the LNO layer. See DOI: https://doi.org/10.1039/d2tc04070k |
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