Final Report Summary - EPREXINA (Electron paramagnetic resonance as a probe for extended interfaces in nanomaterials)
The goal of the project entitled EPREXINA was to determine the role of charge defects, surfaces and interfaces on nanomaterials properties and to propose a methodology for their control to design materials on demand. BaTiO3-based ferroelectrics as one of the most useful functional materials for applications ranging from advanced non-volatile computer memories, sensors, to micro-electromechanical machines, infrared detectors were objects of this proposal. Electron paramagnetic resonance (EPR), the most sensitive method for ions' valence state and local environment studies, was used for the charged defects characterisation. The materials used were 'pure' and 'doped' BaTiO3-based:
(i) powders,
(ii) ceramics,
(iii) single crystals,
(iv) composites (made of Mn-doped Ba1-xSrxTiO3 and MgO or of 'core-shell' architecture coated either by SiO2 or Al2O3),
(v) thin films and
(vi) multilayers.
Multilayers and thin films of different thickness were deposited on Si, SiO2, Al2O3 and SrTiO3 (single crystal) substrates. 3d5 doping cations were most often used because they have a long spin-lattice relaxation time and they can be studied by EPR in a wide range of temperature from liquid helium temperature to room temperature and above. For the defects identification and classification some 'pure' and doped barium titanate (BT) single crystals and 'bulk' ceramic samples with µm grains size were studied. Samples were obtained by sol-gel synthesis, magnetron sputtering, conventional sintering, spark plasma sintering. The combine analysis of all data obtained on nanosized samples (films, multilayers, ceramics and composites), as well as the comparison with single crystals spectra allowed proposing conclusions about the charge defects nature and their location in the studied materials: for nanosized BT powders defects located in the bulk of the grain and close to the surface were identified; in composite materials, the contribution from the defects located at the interface was separated. Along all the main results, some have been already published.
We demonstrated that both intrinsic and impurity cations can form paramagnetic defects at the composite interface that strongly influence the materials properties. High concentration of the charged defects at surface of nano powders and at interfaces in composites was revealed from EPR and a direct link to the dielectric properties was established. Based on a combined analysis BST ceramics with optimised dielectric properties of in broad frequency (kHzGHz) and temperature ranges were synthesized by SPS. We have shown that SPS allows interfaces control without interdiffusion between BST and MgO (up to 10 wt %). Addition of 4% MgO decreases the low-frequency dielectric losses while keeping high permittivity tunability. Combining the composite route with a chemical one, the substitution of Ti by Mn, allows further decrease of the high-frequency dielectric losses. Manganese ions (Mn2+ and Mn4+) are breaking the long-range correlation along the Ti-O chains due to the change in the ionic radius and mass, and even more efficiently by the creation of big Mn2+ - Vo charged defects as revealed from EPR measurements. Based on such controlled multiscale approaches an optimal material exhibiting suitable properties for practical applications was obtained: the composite made of Mn-doped BST + 4%MgO with high tunability (13 % under 1 kV / mm), low dielectric losses in a large frequency range (0.6 % at 10 kHz and 1.4 % at 1 GHz at RT) and high thermal stability of the permittivity. This result highlights the advantage of defects / interfaces control by EPR allowing thus the design of functional oxide-based multimaterials with adjustable dielectric characteristics through the material chemistry control.
We mapped out the relaxation activation energies with the amount of charged defects in films deposited by different methods revealing that the defects density is the key point in low temperature dielectric relaxation. BaTiO3-based ferroelectric thin films with different composition were grown using several modes: sputtering (with and without magnetron) and sol-gel. A clear relationship was established between the low temperature dielectric relaxation, the synthesis conditions and the defects concentration. Based on our EPR investigation, we ascribed the dielectric relaxation to the hopping of electrons among Ti3+-V(O) charged defects (the electrons hopping probability among Ti3+-V(O) centres is decreasing when the average distance between the centres increases). We then could correlate the increase of the dielectric activation energy to the decrease of the defects density. Optimised samples were then synthesised with low dielectric losses (0.027 at 0.3 kHz and 0.036 at 1 MHz) at room temperature. These results could be helpful in the process of ferroelectric films properties optimisation, for instance, to control their dielectric losses and leakage current.
In conclusion, this project allowed enforcing the complementary skills of the research groups in Bordeaux and Kiev to achieve the first detailed investigations of surface / interface defects influence on nanosized materials properties to tailor, at the nanoscale, novel material systems with radically new or enhanced properties and performance based upon an improved understanding of materials nanostructure.
(i) powders,
(ii) ceramics,
(iii) single crystals,
(iv) composites (made of Mn-doped Ba1-xSrxTiO3 and MgO or of 'core-shell' architecture coated either by SiO2 or Al2O3),
(v) thin films and
(vi) multilayers.
Multilayers and thin films of different thickness were deposited on Si, SiO2, Al2O3 and SrTiO3 (single crystal) substrates. 3d5 doping cations were most often used because they have a long spin-lattice relaxation time and they can be studied by EPR in a wide range of temperature from liquid helium temperature to room temperature and above. For the defects identification and classification some 'pure' and doped barium titanate (BT) single crystals and 'bulk' ceramic samples with µm grains size were studied. Samples were obtained by sol-gel synthesis, magnetron sputtering, conventional sintering, spark plasma sintering. The combine analysis of all data obtained on nanosized samples (films, multilayers, ceramics and composites), as well as the comparison with single crystals spectra allowed proposing conclusions about the charge defects nature and their location in the studied materials: for nanosized BT powders defects located in the bulk of the grain and close to the surface were identified; in composite materials, the contribution from the defects located at the interface was separated. Along all the main results, some have been already published.
We demonstrated that both intrinsic and impurity cations can form paramagnetic defects at the composite interface that strongly influence the materials properties. High concentration of the charged defects at surface of nano powders and at interfaces in composites was revealed from EPR and a direct link to the dielectric properties was established. Based on a combined analysis BST ceramics with optimised dielectric properties of in broad frequency (kHzGHz) and temperature ranges were synthesized by SPS. We have shown that SPS allows interfaces control without interdiffusion between BST and MgO (up to 10 wt %). Addition of 4% MgO decreases the low-frequency dielectric losses while keeping high permittivity tunability. Combining the composite route with a chemical one, the substitution of Ti by Mn, allows further decrease of the high-frequency dielectric losses. Manganese ions (Mn2+ and Mn4+) are breaking the long-range correlation along the Ti-O chains due to the change in the ionic radius and mass, and even more efficiently by the creation of big Mn2+ - Vo charged defects as revealed from EPR measurements. Based on such controlled multiscale approaches an optimal material exhibiting suitable properties for practical applications was obtained: the composite made of Mn-doped BST + 4%MgO with high tunability (13 % under 1 kV / mm), low dielectric losses in a large frequency range (0.6 % at 10 kHz and 1.4 % at 1 GHz at RT) and high thermal stability of the permittivity. This result highlights the advantage of defects / interfaces control by EPR allowing thus the design of functional oxide-based multimaterials with adjustable dielectric characteristics through the material chemistry control.
We mapped out the relaxation activation energies with the amount of charged defects in films deposited by different methods revealing that the defects density is the key point in low temperature dielectric relaxation. BaTiO3-based ferroelectric thin films with different composition were grown using several modes: sputtering (with and without magnetron) and sol-gel. A clear relationship was established between the low temperature dielectric relaxation, the synthesis conditions and the defects concentration. Based on our EPR investigation, we ascribed the dielectric relaxation to the hopping of electrons among Ti3+-V(O) charged defects (the electrons hopping probability among Ti3+-V(O) centres is decreasing when the average distance between the centres increases). We then could correlate the increase of the dielectric activation energy to the decrease of the defects density. Optimised samples were then synthesised with low dielectric losses (0.027 at 0.3 kHz and 0.036 at 1 MHz) at room temperature. These results could be helpful in the process of ferroelectric films properties optimisation, for instance, to control their dielectric losses and leakage current.
In conclusion, this project allowed enforcing the complementary skills of the research groups in Bordeaux and Kiev to achieve the first detailed investigations of surface / interface defects influence on nanosized materials properties to tailor, at the nanoscale, novel material systems with radically new or enhanced properties and performance based upon an improved understanding of materials nanostructure.