Polymer-assisted deposition of epitaxial oxide thin films

Grao en Química Xullo 2015 CURSO 2014-2015 Facultade deQuímica, University of Santiago de Compostela. Polymer-assisted deposition of epitaxial oxide thin films. Francisco Rivadulla Salgueiro Traballo Fin de Grao 2 3 Autorization Sheet Bachelator work (traballo de fin de grao) developed at CiQUS (Centro Singular de Investigación en Química Biolóxica e Materiais Moleculares, USC) and presented at Facultade de Química of the Universidade de Santiago de Compostela by FRANCISCO RIVADULLA SALGUEIRO, as a requisite to obtain the title of Grao en Química. This is presented after the authorization by Prof. FRANCISCO RIVADULLA FERNÁNDEZ and Ph.D. BEATRIZ RIVAS MURIAS, for its presentation and defense. Signed: Francisco Rivadulla Fernández Beatriz Rivas Murias Francisco Rivadulla Salgueiro 4 5 CONTENTS 1. SUMMARY .............................................................................................. 6 2. INTRODUCTION ................................................................................... 7 2.1. The importance of thin films. 2.2. Epitaxial growth and lattice mismatch. 2.3. Synthesis of thin films. 2.4. Polymer Assisted Deposition (PAD). 2.5. Thin-films of metallic oxides. 2.6. Perovskite structure. 2.7. Oxides synthesized, some characteristics and properties. 3. OBJECTIVES OF THIS WORK. ....................................................... 19 4. EXPERIMENTAL ................................................................................ 20 4.1. Spin-coating process. 4.2. Inductively Couple Plasma-Optical Emission Spectroscopy (ICP-OES). 4.3. Thermogravimetric Analysis (TGA). 4.4. Ultraviolet–visible Spectroscopy (UV-Vis). 4.5. X-ray Diffraction techniques (XRD). 4.6. Atomic Force Microscopy (AFM). 4.7. Electrical transport measurements. 4.8. Magnetic measurements. 5. RESULTS AND DISCUSSION. .......................................................... 29 5.1. Thermogravimetric characterization of polymers. 5.2. Preparation and deposition of the solutions. 5.3. Synthesis of BiFeO3 (BFO) thin films. 5.4. Synthesis of SrRuO3 (SRO) thin films. 5.5. Synthesis of La0.7Sr0.3MnO3 (LSMO) thin films. 5.6. Synthesis of a bilayer: BiFeO3 on La0.7Sr0.3MnO3 on SrTiO3 substrate (BFO/LSMO@STO). 6. CONCLUSIONS.................................................................................... 47 6 1. SUMMARY. The possibility of growing epitaxial thin-films has been intensely studied during the last decades. The development of the physical deposition techniques, such as Pulsed Laser Deposition (PLD), Sputtering or Molecular Beam Epitaxy (MBE) made possible the production films and multilayers of very high quality, although they have the serious drawback of the huge price of the equipment, particularly for small-laboratory basic research. For this reason, there is a great interest in developing more cost-effective chemical methods of deposition, which however must produce films of a quality similar to the physical techniques to meet the requirements of demanding applications. In this work we use a chemical method to grow metal-oxide thin films of a structural and morphological quality which is similar to that obtained by traditional physical highvacuum methods. The process basis is the preparation of solutions containing the metalcations precursors of the thin film; a soluble metal salt, a complexing agent and a polymer for retaining the cations are the basic components of the precursor solutions used in this study. By spin-coating the solutions over commercial monocrystalline substrates (like (001) oriented SrTiO3) and after a thermal treatment, thin-films of the materials of interest are obtained. A good control over the thickness and stoichiometry can be achieved through the control of the initial concentration of the solutions and the gaseous atmosphere during the annealing. In this study we obtained thin films epitaxially grown of BiFeO3, SrRuO3, La0.7Sr0.3MnO3 and a bilayer of BiFeO3/La0.7Sr0.3MnO3. These compositions were selected due to their scientific and technologic relevance. The structure and morphological characterization was carried out by X-ray and microscopy techniques. The electric and magnetic properties are also investigated to study the effect of the low dimensionality and epitaxial strain in these materials. 7 2. INTRODUCTION. 2.1. The importance of the thin films. Nowadays, the development of synthesis method of high quality thin films is desirable due to its interesting applications in many areas of technology and in basic research. In this context, thin films are widely used in light emitting, in electronic devices of high temperature and high power, in microwave devices and micro-electromechanical, optical filters, photodiodes and sensors, cold cathodes, transparent transistors, photodetectors emitting diodes light and laser, magnetic storage memory; piezoelectric, ferroelectric devices and thermoelectric superconductors, etc. Apart from these widespread uses, thin films are important owing to the possibility of the appearance of new phenomena and improved properties induced by the strain substrate. This strategy is called strain engineering. For example, strained thin films of LaCoO3 shows a ferromagnetic-insulator behavior, with a TC~85-90 K1,2 although this material shows a diamagnetic response in bulk. Other interesting example is the verification of the theoretical prediction that strain can induced a polar phase in SrMnO3 compound,3 as it was demonstrated in SrTiO3 grown on Si(111).4 1 D. Fuchs, E. Arac, C. Pinta, S. Schuppler, R. Schneider, H. v. Löhneysen, Phys. Rev. B 75, 014434 (2008). 2 F. Rivadulla, Z. Bi, E. Bauer, B. Rivas-Murias, J.M. Vila-Fungueiriño, Q. X. Jia, Chem. Mater. 25, 55 (2013). 3 C. Becher, L. Maurel, U. Aschauer, M. Lilienblum, C. Magén, D. Meier, E. Langenberg, M. Trassin, J. Blasco, I. P. Krug, P. A. Algarabel, N. A. Spaldin, J. A. Pardo & M. Fiebig, Nature Nanotechnol., doi:10.1038/nnano.2015.108. 4 J.C. Woicik, E.L. Shirley, D.A. Fischer, S. Sambasivan, C.R. Ashman, C.S. Hellberg, P. Zschack, E. Karapetrova, P. Ryan, and H. Li, SRMS-5 Conference , Chicago July 30 – Aug 2 (2006). 8 2.2. Epitaxial growth and mismatch. That is epitaxial growth means that a material must maintain a consistent crystallographic orientation and commensurability to the substrate on which it is grown. When the composition of the film is different from the substrate, it is called heteroepitaxial growth. Homoepitaxial growth refers to equal composition materials in the film and substrate. It is possible to calculate the epitaxial stress caused on a material growth in a given substrate by the ƒ parameter (lattice mismatch). This can be calculated from the lattice parameters of the substrate and film: If the in-plane a,b cell parameters of the film undergo a compression (expansion)the out of plane c lattice parameter undergoes a corresponding expansion (compression), so the volume is maintained (elastic deformation, in the absence of stoichiometry variations).5 There are three types of growth by the value of parameter ƒ:  Growth in tensile stress or elongation (parameter values in the plane are lower than in the substrate). The film undergoes a compression parameter c out of the plane with a value to ƒ < 0.  Compressive stress growth (parameter values in the plane are higher than in the substrate). The film shows or undergoes an elongation in its parameter out of plane (c) with a value to ƒ > 0.  Relaxed growth. Too large a difference between the values of the film and substrate in-plane lattice parameters. This large difference causes the nearest layer to the substrate being affected by growth stress. As the film grows thicker, the layers are no longer affected by stress. The material fails to manifest characteristic properties due to tension and behaves like bulk material. 5 M. Opel, J. Phys. D: Appl. Phys 45, 033001 (2012). 9 Figure 1. Scheme of the three modes of thin-film epitaxial growth, depending on the lattice mistmatch with the substrate. a) ; b ) ; c) . There are numerous commercially available substrates for deposition of thin films in a wide range of lattice parameters. Figure 2. Commercial monocrystalline substrates with perovskite structure (pseudo-cubic or pseudotetragonal cells). 2.3. Synthesis of thin films. The synthesis methods of thin films can be classified into two main groups: physical and chemical techniques.  Physical techniques. The most used are molecular beam epitaxy (MBE), and Physical Vapour Deposition (PVD); where stand out Pulsed Laser Deposition (PLD), Electron Beam Physical Vapour Deposition (EBPVD); and sputtering deposition. All of them share the transfer of precursors to a substrate and their nucleation under high vacuum. 16 surrounded by eight octahedron with shared vertices, each of which contains a cation B in the center.15 Figure 6. Perovskite structure of mixed oxide and arrangement of the 8 octahedrons around the atom A.16 Generally, all mixed oxides with perovskite structure lose their cubic arrangement below a critical temperature (Tc), due to structural transitions that reduce their symmetry. These deviations from the cubic cell give rise to many types of compounds, we can quantify the distortion by “Goldschmidt tolerance factor”17, that predicts the symmetry of the structures based on the size of the ionic radii of the elements that it composes: where t s “Goldschmidt tolerance factor”, the rA, rB and rO are the ionic radii to cations A, B and oxygen atoms respectively. Thus, a value of t equal to unity is consistent with a cubic structural arrangement. Compression of the B-O bonds and elongation of the A-O bonds reduces t and gives place to structures such as tetragonal, orthorhombic or monoclinic. Conversely, compression of A-O and elongation of B-O stabilize the hexagonal symmetry. These distortions are accommodated by cooperative rotations of the octahedrons around a particular crystallographic axis, improving packaging and therefore decreasing the energy of the crystal.18 15 A. M. Glazer Acta Crystallogr. B 28, 3384 (1972); A. M. Glazer. Acta Crystallogr. A 31, 756 (1975). 16 http://www.bdigital.unal.edu.co/4261/2/2299928.20112.pdf 17 schm t, “ ch m sch t s s t E m t VIIIII” Sk ft Norske Videnskaps Akademi, Oslo (1926). 18 P K ys , “N v s p vsk t s b s bt s c c s xt m s p s ó y t mp t ”, U v s C mp t s (2014) 17 2.7. Oxides synthesized, some characteristics and properties. BiFeO3 In multiferroic materials, magnetism and ferroelectricity coexist, and if they are coupled they give rise to magnetoelectricity (or ferroelectromagnetism). Since most ferromagnets are conductive and ferroelectrics are insulating, ferromagnetic ferroelectrics are scarce, particularly at or above room temperature. BiFeO3 (BFO) is one of the most studied multiferroic materials. At room temperature, bulk BFO exhibits a rhombohedrally distorted perovskite structure, with pseudocubic lattice parameters ar = 3 965 Å αr = 89 4◦ O h s incommensurate G-type antiferromagnetic (AFM) structure (TN = 650 K) and ferroelectricity (TC = 1103 K) caused by 6s2 lone pair distortions of Bi3+ at the A-site. BFO is a material at room temperature with small energy band gap (2.5 eV) and a very m t f ct c p t ( 100 μC /cm2).19,20 SrRuO3 Ferromagnetic conductive oxide SrRuO3 (SRO), which has a Curie temperature of around 160 K, has been widely studied recently for electronic device applications because of its high electrical conductivity, crystal structure compatibility with many other technically important metal oxides, and high thermal and chemical stability. For example, epitaxial thin films of SRO have been used as bottom electrodes for ferroelectric capacitors, buffer layers for coated conductors, normal metal layers for superconductor Josephson junctions, and ferromagnetic metal layers in spin-polarized ferromagnetic tunnel junctions.21 Bulk SRO crystallizes in an orthorhombic structure with lattice parameters a = 5.5670 Å, b = 5.5304 Å, and c = 7.8446 Å; its lattice parameter is 3.930 Å in pseudocubic notation.22 19 G. A. Smolenskii, I. Chupis, Sov. Phys. Usp. 25, 475 (1982). 20 F. Kubel, H. Schmid, Acta Crystallogr. B 46, 698 (1990). 21 C. B. Eom, R. J. Cava, R. H. Fleming, J. M. Phillips, R. B. van Dover, J. H. Marshall, J. W. P. Hsu, J. J. Krajewski, W. F. Peck, Jr. Science 258:1766 (1992). 22 X. W. Wang, X. Wang, Y. Q. Zhang, Y. L. Zhu, Z. J. Wang et al., J. Appl. Phys. 107, 113925 (2010). 18 Reduced Curie temperatures have been observed for thin films of SrRuO3 deposited on substrates with mismatched lattice parameters and explained in terms of strain effects.23 La0.7Sr0.3MnO3 Doped perovskite manganites have renewed interest because they exhibit a variety of unique magnetic, electronic and transport behaviors. Half-metallic ferromagnetic materials appear as potential candidates for spintronic devices, and much work is under progress to synthesize magnetic oxides, such as La0.67Sr0.33MnO3 (LSMO). LSMO undergoes an anti-ferromagnetic and insulating behavior at high and low doping concentration (x) values. The system becomes ferromagnetic and metallic in a certain range of concentrations centered around x ≈ 0 33 w th h h st Curie temperature (Tc = 370K). So it is considered for the use in various devices such as magnetic field sensors and hard disk read heads. Moreover, due to the hole doping in LSMO, many interesting device applications have been proposed based on LSMO. LSMO is rhombohedral, in the pseudocubic description, the unit cell angle α and the lattice parameter of LSMO are 90.26° and 3.88 Å, respectively. The magnetic and transport properties of manganite thin films are very sensitive to its micro structure, growth conditions, and the lattice strain induced by the underlying substrate.24 23 J. J. Neumeier, A. L. Cornelius and J. S. Schilling, Physica B 198, 324 (1994). 24 M. C. Martin and G. Shirane, Phys. Rev B 53, 1428514290 (1996). 19 3. OBJECTIVES OF THIS WORK. The main objective of this project is to obtain thin films of nanometric thickness by a chemical deposition method, with the same quality as those obtained through physical deposition methods (Pulsed Laser Deposition or Molecular Beam Epitaxy). Chemical technique used in this project to obtain thin films is called the Polymer Assisted Deposition (PAD), a method based on the use of aqueous solutions of metal cations whose preparation involves a much lower cost than any equipment used in a physical technique. To obtain thin films of high quality must ensure certain important parameters: epitaxial growth, namely, growth of samples with the same orientation and structural relationship to the substrate on which is deposited; homogeneity and low surface roughness for films; coating large areas (cm2); and controlling the thickness and stoichiometry of the films. The physical properties (magnetic and electronic transport) will be used to test the validity of the films. 20 4. EXPERIMENTAL. 4.1. Spin-coating process. Spin-coating has been used for decades for fabrication of thin films. The process itself consists of depositing a small amount of solution onto the center of a substrate, rotating at high speed.25 In our case we used a model spin-coater WS-650-23 NPP Laurell. The technique is widely used and it is based mainly on centrifugal force achieved due to the high rotational speed of the substrate, fixed to the base of the spin coater by vacuum. This causes the spreading of the solution throughout the substrate surface, reaching a homogeneous solution layer covering the entire substrate. The quality of the spin-coated films depends on several factors:  Solution: volume, viscosity, concentration and surface tension.  Substrate: roughness, hydrophobic effects and superficial dimension.  Other conditions: rotational speed, acceleration, dwell time, temperature and environmental humidity. Th v m f s t s f p s t s st y s b tw 20 40 μL with a metal concentration of 120 - 300 mM. The environmental conditions were 17 - 25 °C of temperature and 50 - 70 % of humidity. The substrates were 5 x 5 mm of size and a rotational speed of 2000 - 4500 rpm. 4.2. Inductively Couple Plasma-Optical Emission Spectroscopy (ICPOES). This technique is used to determine the metal concentration from solutions used in thin film deposition. In inductively coupled plasma optical emission spectrometry (ICP25 Massey et al. US 4267212 A “ ss S y D p s t ”, IBM Technical Disclosure Bulletin 14, 10 (1972). 21 OES), the samples are transported into the plasma by a current flow of liquid through a quartz tube becoming an aerosol upon arrival at the nebulizer, where the liquid is separated into scattered drops and enters the plasma. Once in the plasma, the sample undergoes several consecutive processes: desolvation, vaporization, atomization and excitation or ionization. Atoms or ions introduced in the plasma emit a characteristic wavelength, which becomes useful information about concentration.26 There are several intense lines which can identify and quantify the most atoms of the periodic table elements (over 70 elements); however, it depends on the selection matrix or concentration in which our analyte are (detection limits). Samples must be properly filtered solutions, and the volume required will depend on the number of items to be measured, generally require 10 to 15 mL of filtered solution.27 The model of own ICP-OES spectrometer is PerkinElmer AS93-plus (general service of USC, CACTUS Lugo). 4.3. Thermogravimetric Analysis (TGA). Thermogravimetric analysis (TGA) in inert atmosphere is used as a method for empirically assessing the thermal stability of PEI. Equipment used for our analysis, TA Instruments Q5000(general equipment of CIQUS), allows to analyze our polymers in the same conditions which they are subjected to during the heat treatment in the furnace (muffle or tube) in air or oxygen atmosphere; with temperature ranges and heating ramps also similar to those used in that process. When working with polymers, is advisable to work with the same amounts in the times of performing the analysis to avoid drifts in the data (similarly by using a heating ramp). 26 Douglas A. Skoog, F. James Holler, Stanley R. Crouch. 2008 “Principios de Análisis Instrumental”. 6ª Ed. México: Cengage Learning. ISBN-10: 970-686-829-1. 27 http://www.usc.es/gl/investigacion/riaidt/analise/icpo.html 22 4.4. Ultraviolet–visible Spectroscopy. Molecular absorption spectroscopy is based on measuring the transmittance (T) or the absorbance (A) of solutions found in transparent cells with length b. In the moderatelow concentration range, the concentration of the analyte is linearly related to the value of the absorbance, according to the Beer´s law: Where A is the absorbance, T transmittance, and is the radiant incident and transmitted intensities, respectively, is the analyte´s molar absorptivity, b pathlength cell and c is the concentration of the analyte to be measured. At higher concentrations the degree of interactions between solvent-solute and solutesolute is too high and may affect the absorption of the analyte; the closeness of ions in solution and other species also distorting the absorptive capacity of the analyte.26 This technique provided us with information about the concentrations of ruthenium in the different solutions, and also allowed us to study the complexation equilibria RuEDTA through the variation in the complex transition band. In this research, we have used a Cary series, 300 UV-Vis Spectrophotometer (Agilent Technologies). 4.5. X-ray Diffraction techniques (XRD). The X-ray diffraction is one of the physical phenomena that occur when interacting Xray beam of a particular wavelength, with a crystalline substance. The X-ray diffraction is based on coherent scattering of X-ray beam by the sample (the wavelength of the radiation is maintained) and constructive interference of the waves are in phase and which are dispersed in certain directions of space. The phenomenon of diffraction can 23 be described by Bragg's Law, which predicts the direction in which constructive interference occurs between beams of X-rays scattered coherently by crystal: where is the distance between planes (it is the order of the X-ray´s wavelength). The equipment used was a PANalytical Empyream with beam incident wavelength of 1.540598 Å (general services of USC). Its basic components are: an emission source of X-rays copper anode, a mobile platform and a detector sample. Platform allows the correction of the inclination by optimizing the position of the sample, also looking reflections of interest. We can do various actions with that equipment:  ω. Angle of incidence of X-rays with the sample surface.  2θ. Angle between the X-ray and detector.  ψ. Tilting of the sample.  φ. Sample rotation in the plane.  x,y. Sample displacement in the plane.  z. Vertical displacement of the sample. Figure 7. Principal equipment geometry angles to measurements. Our study was conducted with different configurations of the X-ray source, the sample or the detector to obtain different information about our films:  θ - θ measurements. Such measurements give information about the orientation and cell parameters out of the plane of the film. In our case the substrates are in an orientation (001) and if samples grow oriented with respect 24 to the substrate STO (001), only the reflections in planes (00l) should be observable. The cell parameter of the samples can be obtained according to: where a is the corresponding cell parameter and h , k and l are the Miller´s indices of the plane. To make adjustments to the calculation of these parameters we were used the program HighScore Plus.  X-ray Reflectometry (XRR). It is a technique that uses X-ray radiation to measure the reflected intensity at low angles. It is ωθ scans, maintaining a constant ratio of ω θ . As the signal obtained corresponds to the interference of the reflected radiation between layers of different density, determines the film thickness. For angles of incidence ( θ ) below the critical angle, penetration is very low and total external reflection occurs. Above this critical angle increases penetration and, due to interference between the x-ray beams constructively and destructively dispersed, oscillations that provide information about the film thickness are detected (the period of oscillations is inversely proportional to film thickness). The decay in signal intensity indicates the roughness, so that the greater the roughness of our film, the higher will be the intensity decay with the angle. The thickness can be obtained by making an adjustment with Fourier´s Transform with X'Pert Reflectivity program, which uses the equation: where is the maximum position, is the critical position, m is the order of the peak, e is the film´s thickness and λ is the wavelength of X-rays. Thickness values can be obtained plotting - and taking the slope of the linear fit ( p ):  Reciprocal Space Mapping (RSM). It allows obtaining images in two dimensions around a reflection. When performed on a ( ω θ ) symmetrical reflection, we only get information about cell parameter out of the plane of the film; but if we do it around an asymmetric reflection, also provides information 25 about the cell parameters in the plane, giving valuable information to get a stress analysis between the substrate and the film. For the RSM it is necessary to make a series of sweeps in θ for a range of values to ω with ψ and φ constants, that is, the position of the detector is varied to different angles of incidence of the X-ray. Coordinates in reciprocal space are (parallel to surface) and (perpendicular to the surface), and are described by the equations defining the parameters in and out of the plane as: where h, k and l are Miller´s indices of the corresponding planes in the pseudocubic system and λ it is the wavelength of incident X-ray radiation. 4.6. Atomic Force Microscopy (AFM). AFM is a kind of technique within the Scanning Probe Microscopy (SPM). AFM microscopy probes the surface of a sample with a tip very sharp, a few microns in length and less than 100 Å in diameter. The tip is located at the end of the cantilever of 100 to 200 microns long and the force (the most common being the van der Waals forces) between the tip and the sample surface causes the cantilever to bend or flex. These deflections are transmitted to a photodiode (detector) via a laser (on the top of the cantilever) and a topographic map of the surface is obtained. This type of measurement can be applied to any material, independently of their electrical conductivity. 32 Thermal treatment is the last step. The steps and final treatment temperature depend on the film to synthesize and the decomposition temperature of the used polymer. The thermal treatment sometimes requires a different atmosphere than the air, for this purpose a tube furnace is used with a mass flow controller in order to know exactly the used flow. 5.3. Synthesis of BiFeO3 (BFO) thin films. The concentration of the precursor solutions was 200,034mM and 119,878 mM for Fe and Bi cations respectively. We have mixed 0.72 ml of Bi-EDTA-PEI solution and 1.2 ml of Fe-EDTA-PEI (stoichimoetry proportion) obtaining a final solution prepared for deposition with a total cationic concentration of ~150 mM. The mixture was spin-coated with the following deposition conditions:  20 μ f f s t  Rotation speed: 3000 rpm during 20 seconds.  Substrate SrTiO3 (001), with drying temperature of 120 °C. Finally, after the deposition the films were annealing at 650ºC during 3 hours (heating and cooling ramps of 2 °C /min) under two different atmospheres: in air and in nitrogen in order to study the effect of these different atmospheres in the synthesis of BFO. We have chosen a reducing nitrogen atmosphere taking into account the work of H. Liu et al. where the authors obtain pure BFO in bulk under H2 and N2 atmospheres.29 Moreover, previous studies in the group showed no important differences in the decomposition temperature of PEI under N2 with respect to air. After the heat treatment, the next step is their structural characterization of the films using X-ray diffraction. Figure 11 shows the XRD diffractogram for one of our samples. Only the (00l) reflections appear for the STO substrate as well as the film, meaning that the film grows in the same orientation than the STO substrate. No secondary reflections of impurities 29 H. Liu, Y. Pu, X. Shi, QU. Yuan, Ceram. Int. 39, S217–S220 (2013). 33 or missorientations are observed. From this diffractogram we can obtain the value of the out of the plane lattice parameter for our samples. 20 40 60 80 100 120 STO(002) STO(003) STO(004) STO(001) BFO(004) BFO(003) BFO(002) BFO(001) Intensity (a.u.) 2 (º) Figure 11. XRD diffractogram of BFO film treated in air at 650 °C. To better display the graphical information provided by the XRD diffractogram, we do a more detailed measurement of the peak corresponding to the plane (002) because it is the most intense, and therefore tends to give us the same information but more clearer. It is true that sometimes peaks at higher angles such as the plane (004) give a greater separation between the signals of the substrate and the film, but in our particular case, we have taken the first option as a compromise of intensity and position with respect to the peak of the substrate. In figure 12, X-ray diffractograms around the (002) reflection and XRR measurements are displayed for both samples (in air and nitrogen atmosphere). We have also made reflectivity measurements of the films to obtain the values of the thickness, the interface roughness, and density of our materials (figure 12 b and c). 34 42 44 46 48 50 Intensity (a.u.) 2 (º) STO(002) BFO(002) a) 0 1 2 3 2 (º) b) Fit Intensity (a.u.) Thickness = 10.9 nm Density = 8.35 g/cm3 Roughness = 1.48 nm 0 1 2 3 c) Thickness = 10.7 nm Density = 8.35 g/cm3 Roughness = 1.5 nm Fit Intensity (a.u.) 2 (º) Figure 12. a) X-Ray diffractograms around the peak (002) for BFO @ STO in air (top) and N2 (bottom) at 650 ºC; b) XRR diffractogram for BFO grown in air atmosphere at 650 ° C over STO (001); c) XRR diffractogram for BFO grown in N2 at 650 ºC on STO (001). Through the fitting of the XRR measurments, we obtain the following parameters:  The thickness of the films: in both atmospheres is around 11 nm.  The roughness is about 1.5 nm.  Material density is 8.35 g/cm3 for both samples From the XRD diffractogram we can obtain the out of the plane lattice parameter value using the formula described in the X-Ray Diffraction section (Experimental Part), the values are 4.029(1) Å and 4.051(1) Å for the sample grown in air and nitrogen atmosphere respectively. We see that the lattice parameter out of the plane for the film is greater than the BFO in bulk (3.964 Å)19,20, so the material undergoes an in-plane compressive stress. To confirm this, we have performed the reciprocal space maps (RSM) measurement (figure 13). We observe a good matching between the films and the substrate (in plane parameter = 3.905 Å) and therefore conducting an in plane compressive stress. 35 3.7 3.8 3.9 4.0 4.1 3.8 3.9 4.0 4.1 BFO a) c (Å) a (Å) STO 3.7 3.8 3.9 4.0 4.1 3.8 3.9 4.0 4.1 STO BFO b) c (Å) a (Å) Figure 13. Reciprocal space maps of BFO samples grown at 650 ° C in a) air atmosphere and b) N2 atmosphere. We can also see that the values obtained for the out of plane lattice parameters in figure 13 present similar values to those obtained by XRD, 4.032 Å and 4,053 Å respectively for air and nitrogen atmosphere. On the other hand, with these two parameters we can calculate the film volumes, and obtain information about whether the growth of our film involves an elastic deformation or present some kind of defect in the structure. The volume values are displayed in table X. We observe that there is a slight difference between the bulk and film volumes, that can mean small cation vacancies. Atmosphere Cell volume in bulk (Å3) Cell volume in film (Å3) Air 62.29 61.45 N2 62.29 61.77 Table 3. Values for cell volume for the different BFO films synthesized at 650 °C. After analysis of the data obtained from the diffractograms of XRD, XRR and RSM, a study of the surface of the BFO films was performed by AFM (atomic force microscopy). This study is useful to determine the surface roughness of our materials, and obtain the values of Rq (roughness mean square). 36 Figure 14. AFM images in no contact mode for BFO films in a) air atmosphere and b) N2 atmosphere. Rq values for both samples are 1.4 nm to BFO synthesized in air atmosphere and 2.6 nm to BFO synthesized in N2 atmosphere. Moreover, the surface of the samples is not uniform due to segregation in our film are appreciated (white spots, figure 14). So we have carried out the synthesis of a new film at a lower temperature in order to try to avoid this segregation, considering the possibility of segregation of Bi and Fe oxides. Taking also into account that the roughness is smaller (AFM) and the better defined oscillations in XRR of the samples synthesized in air, we decided to synthesized a new BFO film annealing at 600 °C in air. The value obtained for parameters in the plane (a and b) is 3,905 Å and out of the plane parameter (c) is 4.03(1) Å, so we have a cell volume of 61.45 Å3. This film is slight thicker than those synthesized at 650ºC, around 15 nm, and its roughness is around 1.68 nm (figure 15). 0 1 2 3 Fit Thickness = 14.75 nm Density = 8.30 g/cm3 Roughness = 1.68 nm 2 (º) Intensity (a.u.) Figure 15. XRR diffractogram for the BFO sample grown at 600 ° C in air atmosphere. b) a) 37 Figure 16. AFM image in no contact mode to BFO sample grown at 600 °C in air atmosphere. The AFM image (figure 16) shows greater homogeneity in this sample than the samples synthesized at 650ºC, and less segregation. So the choice of lower the temperature it has had a positive impact on the growth of film. 5.4. Synthesis of SrRuO3 (SRO) thin films. Prior to the SRO film growth we have carried out a study of stability of ruthenium (Ru) as our intention was to make a deposition using the same method of preparation of the solutions for the BFO. We started with the preparation of solutions of ruthenium and EDTA concentrations in the range of 10-5 M, and via UV-Vis spectroscopy we performed a study of absorption bands corresponding to different Ru complexes. We studied the optimal pH for obtaining a complex with appropriate negative charges to establish the necessary interactions with the positive amino groups of the PEI and so achieve maximum retention of Ru cations. As seen in figure 17, as the pH increases, the Ru-EDTA complex varies, obtaining a molecule with greater negative charge as the pH increases, as the complex lose H+. UV-Vis spectroscopy, figure 17 b, sh ws th t p 7.4 the absorption band undergoes a change. The appearance of two bands in a different position than those for 38 lower pH indicates that the complex is undergoing a change in its structure that causes the change electronic transitions between states. 200 300 400 500 0.00 0.07 0.14 0.21 0.28 0.35 2.0x10-5 5.0x10-5 8.0x10-5 1.1x10-4 0.0 0.1 0.2 0.3 0.4 Absorbance (a.u) (nm) a) Absorbance (a.u.) [EDTA] (M) 200 250 300 350 b) Normalized absorbance (a.u)  (nm) pH = 2.56 pH = 4.82 pH = 5.8 pH = 7.37 pH = 8.7 Figure 17. a) Representation of the absorption bands of EDTA in aqueous solution at different pH; b) Representation of the absorption bands Ru-EDTA complex in aqueous solution at different pH. Also a study of aqueous solutions of EDTA was performed in the concentration range of 10-5-10-4 M to confirm that the spectrum of EDTA did not suffer evolution with pH variation (figure 17 a)). The changes in the Ru-EDTA complex can be rationalized according to:30 Figure 18. Complexes Ruthenium-EDTA equilibrium in water under pH changes. The process carried out for the preparation of Ru-EDTA-PEI solution was the same as for the respective Bi and Fe, however, when attempting to stabilize the dissolution of RuEDTA-PEI, underwent hydrolysis of Ru and thus, the appearance of a precipitate mixture of oxide and hydroxide of Ru. The cause was the high pH at which forced us to work complexation with EDTA to obtain a negatively charged complex. 30 H. C, Bajaj and R. van Eldik, Inorg. Chem. 27, 4052-4055 (1988). 39 So, it was necessary to prepare a new solution to which the pH is exhaustively controlled by determining the hydrolysis-point around p 5.5. The solution was filtered with MILLIPORE-equipment described above, with filter 10 kDa as solutions of other metals, and after sending an aliquot of the solution to ICP, the ruthenium concentration was 120.88 mM, it was not a very high value, but enough to try deposited this solution. The pH of the solution after the filtration process was 5.21 and it wasn´t appreciated the appearance of any precipitate. The final molarity of cations was 150.7 mM. Then Sr-Ru solution was concentrated to a value of 200.00 mM by solvent evaporation. The conditions of deposition to conduct were:  20 μ f s t  Rotation speed: 4500 rpm during 20 seconds.  Substrate SrTiO3 (001), with drying temperature of 140 °C. The heat treatment was 700 °C in O2 atmosphere and these conditions were maintained throughout successive depositions.31 However, the results obtained by the XRR and XRD diffractograms showed no signs of being we obtain the desired product. Furthermore, electrotransport tests showed us an insulating film, so that the product obtained was not SrRuO3 (conductive). Therefore, since they have not been successful, we decided to carry out the synthesis using PAA as retention polymer, one polymer subject studied by TGA. We must be careful with the time of preparation of the solutions and the time between preparation and deposition of solutions for Ru-PAA and Sr-Ru-PAA, as they undergo gelification after a few minutes. 0.5 ml of Ru-PAA solution were taken and 0.3634 ml of Sr-PAA obtaining a concentration of cations, 269.51 mM in the deposition solution. Although if heat treatment conditions are maintained (growing temperature and atmosphere), the deposition conditions change:  30 μ f s t  Rotation speed: 2000 rpm during 20 seconds.32 31 H. M. Luo, M. Jain, S. A. Baily, T. M. McCleskey, A. K. Burrell, E. Bauer, R. F. DePaula, P. C. Dowden, L. Civale, and Q. X. Jia, J. Phys. Chem. B 111, 7497-7500 (2007). 40  Substrates SrTiO3 (001) and LaAlO3 (001), with drying temperature of 140 °C. In this case the heating ramp in O2 atmosphere has 3 steps:  2 °C/min from 25 ° C to 150 ° C in this step is maintained 10 min.  3 °C/min from 150 ° C to 460 ° C keeping it at that temperature for 30 min.  3 °C/min from 460 ° C to 700 ° C, the final growth temperature, and this temperature is maintained for 1 hour. The cooling ramp is 5 °C per minute to room temperature. In figure 19 we can see the results of X-ray diffraction patterns of the peak detailed film (002) grown on two different substrates. 42 44 46 48 50 52 a) LAO(002) SRO(002) Intensity (a.u.) 2 (º) 42 44 46 48 50 52 b) STO(002) SRO(002) Intensity (a.u.) 2 (º) Figure 19. a) Detailed XRD diffractogram to SRO of peak (002) grown on LaAlO3 (LAO)(001)at 700 ° C and under an O2 atmosphere; b) Detailed XRD diffractogram to SRO of peak (002) grown on STO (001)at 700 ° C and under an O2 atmosphere. We tried a deposition on LaAlO3, LAO (apc= 3.79 Å)32 to probe the effect of a larger compression over the easiness of synthesis of the film. We obtained lattice parameter values outside the plane of 3.918(2) Å for the film grown over LAO and 3.945(1) Å for the film grown over STO, knowing that the lattice parameter value of SRO in bulk is 3.93 Å.22 These values indicate that the film grows with a compression tension out of the plane to STO but relaxed on LAO. In the RSM for the sample over STO, it can be seen that the sample adjusts its lattice parameter to the substrate (figure 20). 32 S. Y. Park and Y. P. Lee, J. Korean Phys. Soc. 45, 47-45 (2004). 41 3.7 3.8 3.9 4.0 4.1 3.8 3.9 4.0 4.1 SRO STO c (Å) a (Å) Figure 20. RSM diffractogram of SRO film synthesized on STO (001) by the method of PAD using polyacrylic acid as a polymer. Resistance measurements of the samples were also carried out to check the electrical transitions of the film, and therefore its quality. The SRO has a metallic behavior with a transition around 140 K as can be seen in figure 21. The SRO has a Curie temperature about 160 K in bulk.21 Data obtained from the electrical transport study indicate a slight reduction in the position of the electrical transition temperature due to the strain on the film to adjust its lattice parameters on the plane to the STO. However, the electrical measurement on LAO sample shows that this film is insulator, meaning that the product obtained was not stoichiometric SrRuO3. 100 150 200 250 300 100 120 140 Resistance () Temperature (K) Figure 21. Graphical representation of the electrical behavior of a film of SRO synthesized by the method of PAD as a function of temperature.