SEEiNG -
PN-III-P1-1.1-TE-2016-2433
Strain - engineered complex oxide thin films for energy generation - Applications
Stages
STAGE 1 (YEAR -2018)
Evaluation of physico-chemical properties of BFO thin films with concentration gradient of the dopant element
Objectives
Obtaining epitaxial thin films of BYFO on different monocrystalline substrates (SrTiO3, GdScO3, etc.) with concentration gradient of the dopant.
Activities
1.1. Epitaxial thin films deposition of BYFO on different monocrystalline substrates (SrTiO3, GdScO3, etc.)
1.2 Investigation of chemical and physical properties of BYFO thin films obtained by PLD (AFM, SEM, PFM, Kelvin Probe, XRD, SE, SIMS)
1.3 Management, results analysis, dissemination, editing
Results delivered per stage
The report will include details on the chemical and structural investigation of BYFO thin films deposited on various monocrystalline substrates by PLD
Creating the project web page, networking.
Results:
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Thin layers of BFO with various concentrations of the doping element (Y) - 3%, 5% and 10% - were obtained and their physico-chemical properties were determined depending on the concentration of the doping element.
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Also, in order to impose compression and extension deformation conditions on the crystalline structure of the perovskitic material, two types of monocrystalline supports were used that have lower network constant values (SrTiO3, cubic, c-3.905 A ) or bigger (GdScO3, ortorombic, c- 5, 48 A) than that of the material to be deposited in the form of a thin layer (BYFO, pseudocubic, c- 3.94 A). In addition, to characterize from an electrical point of view the thin layers obtained by PLD, a monocrystalline support of SrTiO3 type doped with Nb was also used.
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Fig.1 : AFM imagines on BYFO/STON at 3%, 5% and 10 %Y
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The mosaicity of BYFO / STON thin films was obtained by ω-scan measurements of symmetrical reflections. Figure 2b. shows the higher value of the width at half height for BYFO / STON thin films compared to that of BFO / STON undoped film.
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The optical properties of the BYFO / STON thin films were determined from spectro-ellipsometry analyzes. Samples with different thicknesses and doping deposited at a variable number of laser pulses were analyzed
Fig. 2:Dispersion curves of refractive indices and extinction coefficients calculated from SE for BYFO3% / STON and BYFO10% STON films
Fig.3: a. X-ray diffraction patterns of the BYFO thin films with different Y dopant; b. Ω-scan diffractograms (002) corresponding to BYFO 3% / STON, BYFO 5% / STON, BYFO 10% / STON samples compared to BFO / STON undoped sample
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EDX analyzes performed around an area with extensive defects showed that in this area the Y content is higher than in other defect-free areas in the thin film section.
STAGE 2 (YEAR 2019)
Determining the effect of the monocrystalline substrare type on the growth dynamics of BYFO and KBiFe2O5 thin films and their structural, electrical and optical properties
Objectives
Obtaining BYFO and KBiFe2O5 thin films on different monocrystalline substrates and evaluating their structural, electrical and optical properties.
Activities
2.1 Parametric experimental study for optimizing the conditions for obtaining BYFO and KBiFe2O5 thin films on various monocrystalline substrates
2.2 Investigation of the chemical and physical properties of BYFO and KBiFe2O5 thin films obtained by PLD (AFM, SEM, PFM, Kelvin Probe, XRD, SE, SIMS, HR-TEM)
2.3. Management, results analysis, dissemination, editing
2.4. Management, results analysis, editing.
Results delivered per stage
Activity report
Updating the web page
Presentation at an international conference
Writing a scientific paper and sending it for publication in ISI-rated journals.
Results:
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thin layers of bismuth ferrite with Y concentration gradient were obtained and their physico-chemical properties were determined according to the experimental conditions for obtaining the thin layers. Also, thin layers of bismuth and potassium ferrites - KBiFe2O5 (KBFO) were obtained on various types of monocrystalline substrates.
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Y:BiFeO3
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- the purpose of the activity was to determine the effect of the yttrium concentration gradient on the functional properties of the films and especially on the optical properties
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- the AFM imagines indicates the presence on the surface of the films of elongated crystals, rectangular in shape, preferentially oriented along two mutually perpendicular directions. The crystallite dimensions and the roughness value of the RMS surface, determined from the topography images, increase with increasing number of pulses. Measurements of the local piezoelectric response under the action of an external electric field (PFM - piezoforce microscopy) indicate the presence of well-defined piezoelectric domains
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- in the X-ray spectrum can be seen only reflections (00l) of both the BFO film and the BYFO film, without the appearance of other parasitic reflections. This indicates an epitaxial increase in BFO / STON film and BYFO3% / BFO / film for both very thin films.
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Fig.1a.: X-ray diffractograms for BYFO 3% / BFO / STON heterostructures;b. X-ray diffractograms for BYFO 3% / BFO / STON heterostructures around maximum diffraction (002)
Fig 2: Dispersion curves of refractive indices and extinction coefficients (top) and Tauc plotting (bottom) for BFO / BYFO samples deposited at varying number of pulses.
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K:BiFeO3
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- the purpose was to observe the effect of the substrate type on the properties of the films =>In this case the number of pulses was kept constant at 15,000, the thicknesses of the samples being similar. The substrates used were Pt / Si, single crystals of SrTiO3, Nb: SrTiO3, GdScO3 and RuSrO3 / SrTiO3. In the case of platinum silicon substrate (Pt / Si) the number of pulses was only 10,000.
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a. Pt/Si b.SrTiO3 c. DyScO3 d. Nb:SrTiO3 e.SrRuO3/SrTiO3
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- Piezoelectric response microscopy (PFM) was performed only on samples deposited on STON and SRO / STO conductive supports, by applying a voltage of 10V (down 10 V, up -10 V) for STON and 5 V for SRO / STO (figure 8). There was a controlled change in the amplitude and orientation of the piezoelectric response in the regions where the local electric field was applied.
Fig.2: PFM response for KBFO sample deposited on Nb: SrTiO3, poling (down 10 V, up -10 V) (up) and KBFO deposited on SRO / STO (down), poling (down 5 V, up -5 V)
Fig 3: X-ray diffractograms corresponding to thin layers of KBFO deposited on different substrates
- In the case of thin layers of KBFO / Pt (111) /Si there is no epitaxial growth but the perovskitic rhombohedral phase of KBFO appears. =>Impurities appear which are induced by the appearance of platinum-based compounds in the support (platinum silicides).
- For substrates with smaller misfit such as DSO or SRO, network constants close to them and a lower crystalline deformation are obtained.
- In the case of thin layers of KBFO / SRO / STO parasitic reflections appear, unlike the samples deposited on STON / STO for which the presence of parasitic phases is not observed.The thin layers of KBFO / STO or KBFO / STON have higher network constants and are not yet relaxed.
- From the dispersion curves for thin layers of KBFO obtained by PLD we can observe the influence of the type of substrate. In the case of KBFO deposited on Pt / Si, the lowest values for the refractive index are obtained (n ~ 2.2-2.4), and in the dispersion curve of the extinction coefficients a residual absorption is observed on the whole measured spectrum.
- For KBFO / DyScO3, the values of the refractive index increase slightly compared to the sample deposited on the platinum, and the first characteristic oscillation of the bismuth ferrite also appears.
- In the case of films deposited on Nb: SrTiO3, the first absorption peak is shifted to wavelengths in the visible range and using Tauc plotting a value of Egdirect = 2.1 eV is obtained. In the case of KBFO deposited on undoped STO, the first absorption peak moves even further to longer wavelengths, its center being very close to 680 nm, at the border between visible and near infrared, the calculated value of Egdirect being 1.58 eV
STAGE 3 (YEAR 2020)
Evaluation of photoelectrochemical properties of water molecule dissociation of BYFO and KBiFe2O5 thin films with deformation gradients and different crystalline orientations
Activities
3.1. Evaluation of photocatalytic and photoelectrochemical properties of BYFO and KBiFe2O5 thin films with gradient deformations and different crystalline orientations.
3.2. Management, results analysis, results editing
Results delivered per stage
Updating the web page
Presentation at an international conference
Scientific publication of an article in an ISI rated journal.
Final activity report
Results:
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analysis of photoelectrochemical properties: heterostructures based on BFO / BYFO, as well as thin layers of KBFe2O5 were evaluated from a photoelectrochemical point of view for the dissociation of the water molecule in a system of 3 electrodes: thin layer done by PLD (working electrode), counter-electrode or auxiliary electrode (Pt wire), reference electrode (Ag / AgCl 3.5M, KCl).
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the photoelectrochemical properties of KBFO films with different doping degrees were studied: KBFO (25% - K: Bi = 1: 2) obtained and analyzed structurally and optically in stage II of the project and films with different KBFO stoichiometry (50% - K: Bi = 1: 1) obtained in this stage.
Fig.1. Potentiodynamic measurements obtained for thin layers of
KBFO (0.25) / STON having different thicknesses
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KBFO (50%) thin layers were tested for 900 s in the absence / presence of an external electric field
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the potentiostatic determinations of the current densities as a function of the pH of the electrolytic solution for the thin layers of KBiFe2O5 / STON are presented. In the acid medium (pH = 2.6) a maximum value of ~ 0.013 mA / cm2 is obtained. This value doubles in the basic environment (0.03 mA / cm2).
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important parameters in the efficiency of photoelectrochemical properties are the choice of the substrate and the thickness of the thin layers
Fig.2: Potentiodynamic measurements in ON / OFF irradiation conditions and continuous irradiation for BFO / BYFO 5% heterostructures
