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Extensive piezoresponse force microscopy (PFM) and magnetic force microscopy (MFM) measurements in conjunction with piezoresponse spectroscopy have been carried out on pellets of Bi0.9A0.1FeO2.95 (A = Ba, Ca) and Bi0.9Ba0.05Ca0.05FeO2.95 co-doped ceramic samples in order to characterize their ferroelectric and magnetic nature and correlate the findings with our recent far-infrared spectroscopic studies on these samples. We are able to clearly discern the switching behavior of the 71° and 109° ferroelectric domains as distinct from that of the 180° domains in both pristine and Ba-doped bismuth ferrite samples. While substitution of Ba at the Bi site in bismuth ferrite does not affect the ferroelectric and magnetic properties to a great extent, Ca-doped samples show a decrease in their d 33 values with a concomitant increase in their magnetic behavior. These results are in agreement with the findings from our far-infrared studies.
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The spin wave resonances of BiFeO3 ceramics have been followed at low temperature through far-infrared reflectance measurements. Following the scheme of Fishman et al (2015 Phys. Rev. B 92 094422) we have been able to assign all the spin wave modes observed. A complete lifting of the degeneracies of all these modes is seen at 250 K concomitant with the increase in single-ion anisotropy. For the first time, all the spin wave modes have been observed in the infrared spectra of BiFeO3. Correlated changes in the strength and frequencies of spin wave excitations with the reported magnetic transitions at low temperature are observed. A simultaneous increase in anharmonicity of the magnetic cycloid and single-ion anisotropy with decreasing temperature results in a partial suppression of the spin wave excitations. An increase in the magnetoelectric coupling is also observed below 150 K.
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Diisopropylammonium bromide (DIPAB) can be crystallized either in an orthorhombic (P212121) or in a monoclinic (P21) structure at room temperature depending on synthesis conditions. The non-polar orthorhombic structure exhibits a subtle, irreversible transformation into the ferroelectric monoclinic-II (m-II) phase above ~421K. At a slightly higher temperature of 426K this m-II (P21) phase reversibly transforms into a disordered, paraelectric monoclinic-I (P21/m) structure. We synthesized DIPAB in the orthorhombic structure, heated it to obtain the m-II phase and carried out a systematic study of their Raman and IR spectra. We obtained the phonon irreducible representations from factor group analysis of the orthorhombic and m-II structures based on the reported structural information. DIPAB is an organic molecular crystal, and the vibrational spectra in the intramolecular region (200-3500cm-1) of the two different phases are identical to each other, indicating weak inter-molecular interactions in both crystalline structures. In the low wavenumber region (10-150cm-1) the Raman spectra of the two phases are different due to their sensitivity to molecular environment. We also carried out first principles calculations using Gaussian 09 and CASTEP codes to analyze the vibrational frequencies. Mode assignments were facilitated by isolated molecule calculations that are also in good agreement with intramolecular vibrations, whereas CASTEP (solid state) results could explain the external modes.
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Graphene oxide (GO) based nanocomposites have gained considerable attention in the field of material science due to their excellent physicochemical and biological properties. Incorporation of nanomaterials into GO sheets prevents the formation of π-π stacking bond thereby giving rise to composites that show the improved properties compared to their individual counterparts. In this work, reduced graphene oxide (rGO) - hydroxyapatite (HAP) nanocomposites were synthesized by ultrasonic method. Increasing the c/a ratio of HAP in the diffraction pattern of rGO/HAP nanocomposites indicates the c-axis oriented grown HAP nanorods interacting with rGO layers. Shift in wavenumber (15cm-1) and increase of full width at half maximum (45cm-1) of G band in Raman spectra of the rGO/HAP nanocomposites are observed and attributed to the tensile strain induced due to the intercalated HAP nanorods between the rGO layers. Atomic force microscopy (AFM) and phase imaging studies revealed the intercalation of HAP nanorod with diameter 30nm and length 110-120nm in rGO sheets was clearly perceived along with improved elasticity compared to pristine HAP. 13C-NMR results proved the synergistic interaction between both components in rGO/HAP nanocomposite. The novel properties observed and the microscopic mechanism responsible for this are a result of the structural modification in rGO layers brought about by the intercalation of HAP nanorods.
Asunto(s)
Nanocompuestos , Fenómenos Químicos , Durapatita , Grafito , ÓxidosRESUMEN
Thin films of nanocrystalline TiO2 were synthesized by spray pyrolysis technique in the temperature range 300 degrees C to 550 degrees C in steps of 50 degrees C. The films were coated on glass and quartz substrates by ultrasonic nebulization of titanium-oxy-acetyl acetonate followed by pyrolysis. The structure and morphology of the thin films were characterized by X-ray Diffraction (XRD), Raman Spectroscopy (RS) and Scanning Electron Microscopy (SEM), while the optical band gaps were measured by Spectroscopic Ellipsometry (SE) and UV-Visible spectroscopy. XRD investigations revealed distinct crystal structures of the films synthesized above and below 300 degrees C. While films grown at substrate temperature 300 degrees C were amorphous, those grown at 350 dgrees C and above showed tetragonal anatase crystal structure. The morphological investigations from SEM showed that the films deposited at 350 degrees C were porous and exhibited flower like morphology. The microstructures of the films grown on quartz at 450 degrees C were found to be uniform and dense. The nominal grain sizes evaluated from High Resolution SEM (HRSEM) studies were approximately 20 nm and compared well with the grain sizes calculated from XRD. The band gap values calculated from ellipsometry studies were approximately 3.7 eV and 3.95 eV for the films grown at 450 degrees C and 350 degrees C, respectively. This is in good agreement with those obtained from UV-Visible spectroscopy.
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Cerium oxide (CeO2) thin films were prepared on (100) Si and glass substrates using pulsed laser ablation at different oxygen partial pressures (2.5 x 10(-5)-3.5 x 10(-1) mbar) and at a substrate temperature of 873 K. XRD studies on the films showed that the films are polycrystalline having fluorite structure. The oxygen partial pressure has a dominant effect on the thickness, crystallite size and preferred orientation of the films. At an oxygen partial pressure of 3.5 x 10(-2) mbar, the intensity of (200) reflection is maximum and film possesses the maximum crystallite size of about 50 nm. Though all the films are highly transparent in the visible region, films prepared at an oxygen partial pressure of 3.5 x 10(-1) mbar, exhibits maximum transmittance (>90% @ 632 nm). The optical band gap is found to decrease from 3.66 to 3.42 eV with the increasing oxygen partial pressure.
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Low temperature infrared absorption measurements have been carried out on nanocrystalline La0.67Ca0.33MnO3 powder across the magnetic and the insulator-to-metal phase transitions. Interesting changes are observed in the temperature dependence of the mid-IR polaronic background and the stretching mode of the MnO, octahedron. Upon cooling below 250 K, the overall absorbance increases, but, unlike in the case of microcrystalline LCMO, the stretching mode is not completely screened even at 5 K. The temperature dependence of the stretching mode parameters points to a much reduced phonon-polaron coupling as compared to that in the bulk material.
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Nanocrystalline chromiuim nitride has been synthesised by direct gas phase nitridation of nanocrystalline chromia at 1100 degrees C in ammonia-atmosphere. XRD of this material showed formation of single phase CrN with particle size around 20 nm. AFM studies showed particle distribution along with some soft agglomerated nanostructures. Nanocrystalline Cr2O3 and partially-as well as fully--converted nanocrystalline CrN were also investigated using various spectroscopic techniques like XPS, FT-IR, and Raman for gaining insight into the conversion pathways. Spectroscopic investigations of these materials clearly indicate that complete conversion of CrN occurs by nitriding at 1100 degrees C for 4 hrs. The salient spectroscopic features of these nanocrystalline materials with respect to their microcrystalline counterparts are discussed.