X-ray diffraction and Raman study of nanogranular BaTiO3-CoFe2O4thin films deposited by laser ablation on Si/Pt substrates

Nanocomposite thin films composed by (BaTiO3)1-x-(CoFe2O4)x with different cobalt ferrite concentrations (x) have been deposited by pulsed laser ablation on platinum covered Si(001) substrates. The films structure was studied by X-ray diffraction and Raman spectroscopy. It was found that the CoFe2O4 phase unit cell was compressed along the growth direction of the films, and it relaxed with increasing x. The opposite behavior was observed in the BaTiO3 phase where the lattice parameters obtained from the X-ray measurements presented a progressive distortion of its unit cell with increasing x. The presence of the strain in the films induced a blueshift of the Raman peaks of CoFe2O4 that decreased with increasing CoFe2O4 concentration. Cation disorder in the cobalt ferrite was observed for lower x, where the nanograins are more isolated and subjected to more strain, which was progressively decreased for higher CoFe2O4 content in the films.This work has been financially supported by the Portuguese Foundation for Science and Technology (FCT), through the project POCI/CTM/60181/2004 and the Bilateral Research Action nºB48-06 between CRUP (Portugal) and British Council (UK)

Nanogranular BaTiO3–CoFe2O4 thin films deposited by pulsed laser ablation

Detailed structural and magnetic measurements were performed on nanostructured composite thin films of cobalt ferrite (CoFe2O4 - magnetostrictive) dispersed in a barium titanate (BaTiO3 - piezoelectric) matrix, with different CoFe2O4 concentrations (ranging from x=20% to x=70%). The films were deposited by laser ablation on platinum covered Si(100). Their structure was studied by X-ray diffraction and Raman spectroscopy. The magnetic properties were measured with a SQUID magnetometer. The nanocomposite films were polycrystalline and composed by a mixture of tetragonal-BaTiO3 and CoFe2O4 with the cubic spinel structure. The lattice parameter of the CoFe2O4 phase varied from 8.26Å (x=20%) to 8.35Å (x=70%), and, comparing with bulk CoFe2O4, it was under compressive stress that relaxed as its concentration progressively increased. In the tetragonal-BaTiO3 phase, the lattice parameter a was contracted relative to the bulk phase and decreases with x. The lattice parameter c increased from 4.088 Å (x=20%) to 4.376 Å (x=70%), so that the BaTiO3 c axes was increasingly expanded as the quantity of the barium titanate phase was reduced. This behavior was the opposite of that observed in CoFe2O4. The magnetic measurements showed that the coercive fields decreased from 6.6 kOe (x=20%) to 2.3 kOe (x=70%) which was attributed to the progressive relaxation of the stress in the films as well as to the increase of particle agglomeration in bigger polycrystalline clusters with increasing cobalt ferrite concentration. For higher temperatures T=300 K the reduction of magnetocrystalline anisotropy induced a strong reduction of the coercive field.This work has been financially supported by the Portuguese Foundation for Science and Technology (FCT), through the project POCI/CTM/60181/2004

Structural and magnetic properties of nanogranular BaTiO3-CoFe2O4 thin films deposited by laser ablation on Si/Pt substrates

Thin film nanogranular composites of cobalt ferrite (CoFe2O 4) dispersed in a barium titanate (BaTiO3) matrix were deposited by laser ablation with different cobalt ferrite concentrations (x). The films were polycrystalline and composed by a mixture of tetragonal- BáTiO3 and CoFe2O4 with the cubic spinnel structure. A slight (111) barium titanate phase orientation and (311) CoFe2O4 phase orientation was observed. As the concentration of the cobalt ferrite increased, the grain size of the BaTiO 3 phase decreased, from 91nm to 30nm, up to 50% CoFe 2O4 concentration, beyond which the BaTiO3 grain size take values in the range 30-35nm. On the other hand the cobalt ferrite grain size did not show a clear trend with increasing cobalt ferrite concentration, fluctuating in the range 25nm to 30nm. The lattice parameter of the CoFe2O4 phase increased with increasing x. However, it was always smaller than the bulk value indicating that, in the films, the cobalt ferrite was under compressive stress that was progressively relaxed with increasing CoFe2O4 concentration. The magnetic measurements showed a decrease of coercive field with increasing x, which was attributed to the relaxation of the stress in the films and to the increase of particle agglomeration in bigger polycrystalline clusters with increasing cobalt ferrite concentration.This work has been financially supported by the Portuguese Foundation for Science and Technology (FCT), through the project POCI/CTM/60181/2004

ZnO thin films implanted with Al, Sb and P : optical, structural and electrical characterization

In this work we report a study on the structure, optical and electrical properties of P, Sb and Al implanted ZnO thin films that had been produced by r.f. magnetron sputtering. The influence of the different replacing atoms on the structure and properties of the films has been explored. Looking for the best annealing conditions, two different annealing temperatures (300ºC and 500ºC) have been employed. Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction, transmittance and d.c conductivity measurements have been used to characterize the samples. X-ray diffraction and Raman scattering patterns confirm that after annealing, doped films keep a polycrystalline nature with (002) preferred orientation. These films remain very transparent and the electrical conductivity increases significantly after the 500ºC annealing, reaching values of 10.9 (cm)-1 in the P-doped, 10.33 (cm)-1 in the Al-doped and 0.56 (cm)-1 in the Sb-doped sample

Impact of disorder on optical phonons confined in CdS nano-crystallites embedded in a SiO2 matrix

Non-resonant Raman spectroscopy studies of a set of CdS films annealed at different temperatures were performed and showed a direct correlation between the width of the Raman peak produced by CdS-like optical phonons and the crystalline quality of the semiconductor phase probed by x-ray diffraction (XRD) and transmission electron microscopy (TEM). In order to decribe the Raman lineshape a model proposed by Trallero-Giner et al (1998 Phys. Rev. B 57 4664) was used, which considers optical phonons confined in small semiconductor spheres with a size distribution. The model is shown to give a good reproduction of the spectra of samples where the semiconductor phase is most crystalline. However, it required too large values of phonon damping to fit the spectra of several other samples, which, according to XRD and TEM data, do contain CdS nano-crystallites. This large broadening of the Raman peak was considered as inhomogeneous, i.e. associated with disorder. Numerical lattice dynamics calculations were performed for 2D binary clusters of arbitrary shape and three kinds of disorder were considered, (i) random variation of the Cd–S bond frequency from one nano-crystallite to another, (ii) cluster shape irregularities and (iii) fluctuations of the nearest-neighbour interaction constant within one cluster. It is shown that ‘ensemble disorder’ (i) can be responsible for a shoulder above the bulk CdS phonon frequency observed for some of our samples. The effect of shape disorder (ii) is similar to that of the size dispersion producing some inhomogeneous broadening of the peak. In addition, it gives rise to an extra low-frequency mode originating from the top of the acoustic band. The force constant’s disorder (iii) is shown to result in a stronger asymmetric broadening of the Raman peak.Fundação para a Ciência e a Tecnologia (FCT

Ferroelectric characterization of aligned barium titanate nanofibres

We report the synthesis, structural and ferroelectric characterization of continuous well-aligned nanofibres of barium titanate produced by the electrospinning technique. The fibres with average diameter of 150–400 nm consist of connected nanoparticles of BaTiO 3 stacked together to form the shape of a long filament. The tetragonal phase in the obtained nanofibres was revealed by the x-ray diffraction and Raman spectroscopy and has been also confirmed by the second harmonic generation (SHG) and piezoresponse force microscopy (PFM). The temperature dependence of the SHG in the vicinity of the paraelectric–ferroelectric phase transition suggests that barium titanate nanofibres are indeed ferroelectric with an apparent glass-like state caused by metastable polar nanoregions. The existence of domain structure and local switching studied by PFM present clear evidence of the polar phase at room temperature.Fundação para a Ciência e a Tecnologia (FCT

Resonant raman scattering in ZnO : Mn and ZnO: Mn : Al thin films grown by RF sputtering

Raman spectroscopy results obtained under visible (non-resonant) and UV (resonant) excitation for nanocrystalline ZnO, ZnO:Mn and ZnO:Mn:Al thin films grown by radio frequency magnetron sputtering are presented and compared. The origin of the multiple longitudinal optical (LO) phonon Raman peaks, strongly enhanced under resonance conditions, and the effects of the dopants on them are discussed in the framework of the ‘cascade’ model. It is suggested that the observed suppression of the higher-order LO phonon lines for ZnO:Mn:Al is caused by the dissociation of excitons in the heavily n-type doped material. On the basis of the cascade model interpretation of the higher-order Raman peaks in the resonant spectra, the LO phonon frequencies for wavevectors away from the gamma point are evaluated and compared topreviously published phonon dispersion curves.Financial support from the FCT through project PTDC/FIS/72843/2006 is acknowledged

Electrical and raman scattering studies of ZnO:P and ZnO:Sb thin films

A study on the structure, electrical and optical properties of ZnO thin films produced by r.f. magnetron sputtering and implanted either with phosphorous (P) or antimony (Sb) is reported in this work. Raman spectroscopy, X-ray diffraction, optical transmittance and Hall effect measurements have been employed to characterize the samples. X-ray diffraction and Raman scattering patterns confirm that, after a 500ºC annealing, the doped films keep a polycrystalline nature with (002) preferred orientation. These films are very transparent and Hall effect results show that all have p-type conduction, despite doping ion and dose. The electric resistivity reaches values of 0.012 (cm) and 0.042 (cm) for the P and Sb-doped samples, respectivel

Raman study of doped-ZnO thin films grown by rf sputtering

A Raman spectroscopy study of doped versus undoped ZnO layers is presented. The layers were grown by RF magnetron sputtering and the doping with Al, Sb and Mn was achieved by ion implantation with subsequent annealing. First-order Raman response measured under λ=488 nm excitation is discussed. It is shown that doping with any of the impurities used in this work produces a strong enhancement of the longitudinal optical (LO) phonon band, which is attributed to the intra-band Fröhlich mechanism. In addition, doping with Mn results in an extra mode located at 530 cm-1, tentatively attributed to a local vibrational mode of Mn substituting Zn in the lattice sites.FCT through project PTDC/FIS/72843/200

Raman study of insulating and conductive ZnO: (Al, Mn) thin films

Raman spectroscopy results obtained for undoped and Al- and/or Mn-doped ZnO thin films produced by RF-sputtering are reported. The effect of the doping method (either co-sputtering or ion implantation), the dopant type and its concentration on the Raman-active vibrational modes in these films were studied in detail. The results are discussed with focus on the peak shifts and broadening, and on the doping-induced relaxation of the symmetry selection rules. A particular attention is paid to the 520-530 cm-1 Raman band observed in all Mn containing samples and a simple theoretical model and arguments are presented in support of its relation to the local (gap) phonon mode produced by Mn atoms substituting Zn in the cationic sublattice of the ZnO crystal.Karlsruhe Nano Micro Facility (KNMF), a Helmholtz Research Infrastructure at KI