Probing the ferroelectric phase transition in sol–gel‐derived polycrystalline bismuth ferrite thin films

Polycrystalline BiFeO₃ (BFO) thin films were successfully grown on Pt/Ti/SiO₂/Si(100) and SrTiO₃ (STO) (100) substrates using the chemical solution deposition (CSD) technique. X‐ray diffraction (XRD) patterns indicate the polycrystalline nature of the films with rhombohedrally distorted perovskite crystal structure. Differential thermal analysis (DTA) was performed on the sol–gel‐derived powder to countercheck the crystal structure, ferroelectric (FE) to paraelectric (PE) phase transition, and melting point of bismuth ferrite. We observed a significant exothermic peak at 840 °C in DTA graphs, which corresponds to an FE–PE phase transition. Raman spectroscopy studies were carried out on BFO thin films prepared on both the substrates over a wide range of temperature. The room‐temperature unpolarized Raman spectra of BFO thin films indicate the presence of 13 Raman active modes, of which five strong modes were in the low‐wavenumber region and eight weak Raman active modes above 250 cm⁻¹. We observed slight shifts in the lower wavenumbers towards lower values with increase in temperature. The temperature‐dependent Raman spectra indicate a complete disappearance of all Raman active modes at 840 °C corresponding to the FE–PE phase transitions. There is no evidence of soft mode phonons. Copyright © 2008 John Wiley & Sons, Ltd.     

Raman spectroscopy of sol–gel derived titanium oxide thin films

Raman spectra of TiO₂ films prepared via the sol–gel process were studied by UV and visible Raman spectroscopy. The evolution of the phases of TiO₂ films during annealing was investigated, and the relative intensities of the Raman bands excited with 325 nm were found to be distinct from those of the bands excited with 514 nm. The transmittance and FTIR spectra of the films annealed at different temperatures were characterized. The crystallization process of the powders and thin films treated by different annealing methods were also studied with Raman spectroscopy. The results show that the change in the relative intensities is caused by the resonance Raman effect. The anatase to rutile transition of the powder occurs at 700 °C, while that of the thin film occurs at 800 °C. The analysis of Raman band shape (peak position and full width at half‐maximum) after conventional furnace annealing and rapid thermal annealing indicates the influence of the non‐stoichiometry and phonon confinement effect. Copyright © 2011 John Wiley & Sons, Ltd.     

Investigation of local structure of lead‐free relaxor Ba(Ti0.70Sn0.30)O3 by Raman spectroscopy

The local structure of Ba(Ti_0.70,Sn_0.30)O₃ (BSnT) was investigated using micro Raman spectroscopy from 80 to 500 K, which indicated three local phase sequences. The dielectric spectroscopy studies illustrated freezing of polar nano‐regions (PNR) below 95 K, merger of frequency dispersion at 220 K, and the origin of polar nano‐regions at 482 K. The temperature evolution of Raman spectra, basically integrated intensity, indicates non‐ergodic relaxor phase below 100 K, mature ergodic phase between 100 and 200 K, and the existence of ergodic polar clusters until 450 K, and above paraelectric phase. The observation of local phase sequences by Raman spectra are in agreement with dielectric spectroscopy anomalies. Copyright © 2008 John Wiley & Sons, Ltd.     

Raman micro‐spectroscopy for quantitative thickness measurement of nanometer thin polymer films

The sensitivity of far‐field Raman micro‐spectroscopy was investigated to determine quantitatively the actual thickness of organic thin films. It is shown that the thickness of organic films can be quantitatively determined down to 3 nm with an error margin of 20% and down to 1.5 nm with an error margin of 100%. Raman imaging of thin‐film surfaces with a far‐field optical microscope establishes the distribution of a polymer with a lateral resolution of ~400 nm and the homogeneity of the film. Raman images are presented for spin‐coated thin films of polysulfone (PSU) with average thicknesses between 3 and 50 nm. In films with an average thickness of 43 nm, the variation in thickness was around 5% for PSU. In films with an average thickness of 3 nm for PSU, the detected thickness variation was 100%. Raman imaging was performed in minutes for a surface area of 900 µm². The results illustrate the ability of far‐field Raman microscopy as a sensitive method to quantitatively determine the thickness of thin films down to the nanometer range. Copyright © 2015 John Wiley & Sons, Ltd.     

Temperature‐dependent Raman spectroscopy of BaTiO3 nanorods synthesized by using a template‐assisted sol–gel procedure

In the present work, multiphase polycrystalline BTO nanorods were synthesized using template‐assisted sol–gel deposition and their structural evolution was studied using thermo Raman spectroscopy, X‐ray diffractometry and high‐resolution transmission electron microscopy (HRTEM). In the BTO nanorods, the tetragonal phase was the dominant one, while both Raman and HRTEM indicated a coexistence with the high‐temperature hexagonal polymorph. This phase was stable across the whole of the investigated temperature range (from −95 °C to 200 °C). The investigated nanorods underwent a diffuse phase transition from tetragonal to cubic with respect to the temperature, whereas the final phase‐transition temperature was shifted to higher values compared to that expected for BTO. The low‐temperature orthorhombic‐to‐rhombohedral phase transition was also shifted to higher temperatures. These differences could be explained by the strain induced by the presence of hexagonal nanolamellas intergrown within the tetragonal nanocrystals. This result indicates that the temperature of the ferroelectric phase transition in polycrystalline BTO nanorods can be manipulated by introducing a stable hexagonal phase. Copyright © 2012 John Wiley & Sons, Ltd.     

Self‐assembly and structure formation in liquid crystalline phthalocyanine thin films studied by Raman spectroscopy and AFM

This work presents an investigation of films prepared by doctor blade casting, the formation of self‐assembled microstructures of a liquid crystalline phthalocyanine with highly oriented molecules. Raman Spectroscopy in combination with atomic force microscopy is applied to study the structures within the films. By keeping the substrate at room temperature or at 353 K during coating, different geometric structures namely rods and islands form. Rod‐like structures are growing in coating direction, whereas directional growth of the islands is not observed. The distribution of the rod lengths varies widely, whereas the width appears more uniform. Annealing of the samples shows a different behavior of the two textures. Islands tend to melt, and rods smooth their structural form, which is extracted from Raman imaging in combination with atomic force microscopy. Additionally, Raman imaging gives insight into laterally different relative crystallinity. These observations are discussed in the context of the molecular orientation as probed by polarized Raman spectroscopy. These polarized Raman spectra indicate azimuthal alignment of the molecules within the rods (edge on alignment). This alignment occurs along and also perpendicular to the growth direction. In contrast to the alignment in the rods, the molecules inside the islands occurring at higher temperature do not show preferential molecular orientation. After annealing, no preferential molecular orientation is observed in rods because of the loss of anisotropy, too. Copyright © 2012 John Wiley & Sons, Ltd.     

Hard mode Raman spectroscopy and its application to ferroelastic and ferroelectric phase transitions

The application of Raman spectroscopy for the investigation of phase transitions focused traditionally on the observation of soft modes in displacive systems. The present furthergoing study on displacive and order-disorder systems is based on the observation of systematic changes of the scattering profiles of hard modes during the phase transition. It reveals the temperature evolution of the order parameters, the role of order-parameter fluctuations and phonon density of states effects in model systems like improper ferroelastic Pb₃(P_1-x As _x O₄)₂, lead diluted ferroelastic (Pb_1-x Ba _x )₃(PO₄)₂, pure ferroelastic As₂O₅ and the relaxor ferroelectric Pb(Sc_0.5Ta_0.5)O₃. Allied with supplementary experimental techniques hard-mode Raman spectroscopy (HMRS) is an ideal method for the investigation of order-parameter coupling effects and the characterization of structural phase transitions.     

Densification of sol–gel silica thin films induced by hard X‐rays generated by synchrotron radiation

In this article the effects induced by exposure of sol–gel thin films to hard X‐rays have been studied. Thin films of silica and hybrid organic–inorganic silica have been prepared via dip‐coating and the materials were exposed immediately after preparation to an intense source of light of several keV generated by a synchrotron source. The samples were exposed to increasing doses and the effects of the radiation have been evaluated by Fourier transform infrared spectroscopy, spectroscopic ellipsometry and atomic force microscopy. The X‐ray beam induces a significant densification on the silica films without producing any degradation such as cracks, flaws or delamination at the interface. The densification is accompanied by a decrease in thickness and an increase in refractive index both in the pure silica and in the hybrid films. The effect on the hybrid material is to induce densification through reaction of silanol groups but also removal of the organic groups, which are covalently bonded to silicon via Si—C bonds. At the highest exposure dose the removal of the organic groups is complete and the film becomes pure silica. Hard X‐rays can be used as an efficient and direct writing tool to pattern coating layers of different types of compositions.     

Leakage currents in ferroelectric thin films

The main mechanisms of leakage currents in thin lead zirconate titanate (PZT) ferroelectric films prepared by the sol–gel method are discussed. Four specific regions are determined in I–V dependencies. At very weak fields (10–20 kV/cm), the current falls with the voltage increase as a result of depolarization. In the low fields region (about 70–100 kV/cm), the leakage current decreases with the decrease of voltage ramp speed and its components are the ohmic and displacement currents. In the high fields region (≥130 kV/cm), the leakage current increases with the decrease of step voltage ramp in contrast to the previous case. Possible conductivity mechanisms are the Poole– Frenkel emission and hopping conduction. In the transition region between above-mentioned ones (from 80–90 to ～130 kV/cm), an abrupt unstable increase of current is observed caused by breakdown of reverse bias Schottky barrier. Depolarization currents are studied for sol–gel PZT films prepared at different preparation conditions.     

Non‐invasive depth profiling by space and time‐resolved Raman spectroscopy

Time‐resolved Raman spectroscopy, spatially offset Raman spectroscopy and time‐resolved spatially offset Raman spectroscopy (TR‐SORS) have proven their capability for the non‐invasive profiling of deep layers of a sample. Recent studies have indicated that TR‐SORS exhibits an enhanced selectivity toward the deep layers of a sample. However, the enhanced depth profiling efficiency of TR‐SORS, in comparison with time‐resolved Raman spectroscopy and spatially offset Raman spectroscopy, is yet to be assessed and explained in accordance to the synergistic effects of spatial and temporal resolutions. This study provides a critical investigation of the depth profiling efficiency of the three deep Raman techniques. The study compares the efficiency of the various deep Raman spectroscopy techniques for the stand‐off detection of explosive precursors hidden in highly fluorescing packaging. The study explains for the first time the synergistic effects of spatial and temporal resolutions in the deep Raman techniques and their impact on the acquired spectral data. Copyright © 2013 John Wiley & Sons, Ltd.     

Cesium doped and undoped ZnO nanocrystalline thin films: a comparative study of structural and micro‐Raman investigation of optical phonons

Undoped and cesium‐doped zinc oxide (ZnO) thin films have been deposited on sapphire substrate (0001) using the sol–gel method. Films were preheated at 300 °C for 10 min and annealed at 600 and 800 °C for 1 h. The grown thin films were confirmed to be of wurtzite structure using X‐ray diffraction. Surface morphology of the films was analyzed using scanning electron microscopy. The photoluminescence (PL) spectra of ZnO showed a strong ultraviolet (UV) emission band located at 3.263 eV and a very weak visible emission associated with deep‐level defects. Cesium incorporation induced a blue shift of the optical band gap and quenching of the near‐band‐edge PL for nanocrystalline thin film at room temperatures because of the band‐filling effect of free carriers. A shift of about 10–15 cm⁻¹ is observed for the first‐order longitudinal‐optical (LO) phonon Raman peak of the nanocrystals when compared to the LO phonon peak of bulk ZnO. The UV resonant Raman excitation at RT shows multiphonon LO modes up to fifth order. Copyright © 2010 John Wiley & Sons, Ltd.     

离子注入ZnO薄膜的拉曼光谱研究

室温下,用80 keV N⁺和400 keV Xe⁺离子注入ZnO薄膜,注入剂量分别为5.0×10¹⁴—1.0×10¹⁷/cm²和2.0×10¹⁴—5.0×10¹⁵/cm².利用拉曼散射技术对注入前后的ZnO薄膜进行光谱测量和分析,研究了样品的拉曼光谱随离子注入剂量的变化规律.实验结果发现,未进行离子注入的样品在99,435 cm<     

Identification of Mn‐related Raman modes in Mn‐doped ZnO thin films

This article aims to investigate the Raman modes present in Mn‐doped ZnO thin films that are deposited using the magnetron co‐sputtering method. A broad band ranging from 500 to 590 cm⁻¹ is present in the Raman spectra of heavily Mn‐doped ZnO films. The multi‐peak‐fitting results show that this broad band may be composed of six peaks, and the peak at 528 cm⁻¹ could be a characteristic mode of Mn₂O₃. The results of this study suggest that the origin of the Raman peaks in Mn‐doped ZnO films may be due to three major types: structural disorder and morphological changes caused by the Mn dopant, Mn‐related oxides and intrinsic host‐lattice defects. Copyright © 2010 John Wiley & Sons, Ltd.     

Deep‐UV tip‐enhanced Raman scattering

We report for the first time the tip‐enhancement of resonance Raman scattering using deep ultraviolet (DUV) excitation wavelength. The tip‐enhancement was successfully demonstrated with an aluminum‐coated silicon tip that acts as a plasmonic material in DUV wavelengths. Both the crystal violet and adenine molecules, which were used as test samples, show electronic resonance at the 266‐nm excitation used in the experiments. With results demonstrated here, molecular analysis and imaging with nanoscale spatial resolution in DUV resonance Raman spectroscopy can be realized using the tip‐enhancement effect. Copyright © 2009 John Wiley & Sons, Ltd.     

FT‐Raman investigation of human dental enamel surfaces

FT‐Raman spectra of human enamel surfaces from sound, affected (with 1 cavity) and highly affected (with at least 3 cavities) tooth samples were analyzed by principal component analysis (PCA). Major differences between the unaffected and affected tooth samples seem to arise from the structural changes along the c‐axis of hydroxyapatite, the chief crystalline component of human dental enamel. Based on Fisher index calculations, the most discriminative value was obtained for the intensity of the only Raman active ν₂PO₄³⁻ (E₁) symmetric deformation mode at 428 cm⁻¹. Moreover, these changes can be observed through the whole tooth enamel surface, establishing a predisposition to caries correlated to chemical and structural composition of tooth enamel. No spectral changes regarding the CO₃²⁻ substitution were detected by both nondestructive FT‐Raman and FTIR (Fourier transform infrared) spectroscopy of the powdered teeth samples. Copyright © 2009 John Wiley & Sons, Ltd.     

Micro‐Raman investigation of transition‐metal‐doped ZnO nanoparticles

We measured the Raman spectra of ZnO nanoparticles (ZnO‐NPs), as well as transition‐metal‐doped (5% Mn(II), Fe(II) or Co(II)) ZnO nanoparticles, with an average size of 9 nm. A typical Raman peak at 436 cm⁻¹ is observed in the ZnO‐NPs, whereas Zn_1−xMn_xO, Zn_1−xFe_xO and Zn_1−xCo_xO presented characteristic peaks at 661, 665 and 675 cm⁻¹, respectively. These peaks can be related to the formation of Mn₃O₄, Fe₃O₄ and Co₃O₄ species in the doped ZnO‐NPs. Moreover, these samples were analyzed at various laser powers. Here, we observed new vibrational modes (512, 571 and 528 cm⁻¹), which are specific to Mn, Fe and Co dopants, respectively, and ZnO‐NPs did not reveal any additional modes. The new peaks were interpreted either as disorder activated phonon modes or as local vibrations of Mn‐, Fe‐ and Co‐related complexes in ZnO. Copyright © 2007 John Wiley & Sons, Ltd.     

Pulsed laser deposition of perovskite relaxor ferroelectric thin films

Structural, dielectric and ferroelectric properties of thin films of La-doped lead zirconate titanate (PLZT) and sodium bismuth titanate-barium titanate (NBT-BT) perovskite relaxor ferroelectric have been investigated. PLZT films were deposited on Pt/Si substrates in oxygen atmosphere by pulsed laser deposition (PLD) and radio frequency (RF) discharge-assisted PLD, using sintered targets with different La content and Zr/Ti ratio, near or at the boundary relaxor ferroelectric. The films are polycrystalline with perovskite cubic or slightly rhombohedral structure. A slim ferroelectric hysteresis loop, typical for relaxors, has been measured for all film sets. Dielectric characterization shows a large value of capacitance tunability and low dielectric loss. However, common problems related to lead diffusion into the metallic electrode layer do not allow one to obtain high capacitance values, due to the formation of an interface layer with low dielectric constant. Lead-free NBT-BT thin films have been deposited on single crystal (1 0 0)-MgO substrates starting from targets with composition at the morphotropic phase boundary between rhombohedral and tetragonal phase. Films deposited by PLD are polycrystalline perovskite with a slight (1 0 0) orientation. Capacitance measurements were performed using interdigital metallic electrodes deposited on the film's top surface and showed high relative dielectric constant, on the order of 1300.     

High P–T Raman study of transitions in relaxor multiferroic Pb(Fe0.5Nb0.5)O3

The vibrational and structural properties of Pb(Fe_0.5Nb_0.5)O₃ have been investigated using Raman spectroscopy up to 40 GPa at 300 K and from 300 to 415 K at selected pressures. The measurements reveal three phase transitions, at 5.5, 8.7, and 24 GPa at room temperature. The temperature dependences of the spectra indicate transitions at 1.5 GPa, at 335 and 365 K. The results are consistent with the appearance of an intermediate tetragonal P4mm phase between the ferroelectric R3m and paraelectric Pm‐3m phases. A P–T phase diagram is proposed that allows further insight into the magnetoelectric coupling present in this material. Copyright © 2015 John Wiley & Sons, Ltd.     

Synthesis and high-frequency magnetic properties of sol–gel derived Ni–Zn ferrite–forsterite composites

Ni_0.5Zn_0.5Fe₂O₄–forsterite composites were synthesized by a sol–gel method. X-ray diffraction and SEM were used to characterize the crystallization behavior of the composite samples which were heat treated at temperatures varying from 800 to 1100°C. The results showed that Ni_0.5Zn_0.5Fe₂O₄ and forsterite (Mg₂SiO₄) can co-crystallize and the crystallites grow even more larger with increasing heat-treatment temperature. High-frequency (10 MHz–1 GHz) magnetic and dielectric properties of the composite samples were presented. Permeability increased with heat-treatment temperature. Quality factor was found to be two orders higher than that of equivalent pure, bulk ferrite.     

Measurement and calculation of the Q‐branch spectrum of nitrogen using inverse Raman spectroscopy and cw Raman‐induced polarization spectroscopy

The techniques of inverse Raman spectroscopy, Raman‐induced polarization spectroscopy (RIPS), and optical heterodyne RIPS (OHD‐RIPS) are compared by probing the Q‐branch of the nitrogen molecule. The signal is measured employing either a photomultiplier tube (low background level–RIPS) or a photodetector (high background level–IRS and OHD‐RIPS). The measurements are performed using atmospheric mixtures of N₂ Ar with concentrations varying from 0 to 79% N₂. This strategy permits estimation of detection limits using the different techniques. Pump and probe energy levels are varied independently to study signal dependence on laser irradiance. A theoretical treatment is presented on the basis of the Raman susceptibility equations, which permits the calculation of spectra for all three techniques. Calculated Q‐branch spectra are compared with the measured spectra for the interactions of a linearly polarized probe beam with a linearly or circularly polarized pump beam. The polarizer angle in the detection path for OHD‐RIPS has a dramatic effect on the shape of the spectrum. The calculated and experimental OHD‐RIPS spectra are in good agreement over the entire range of investigated polarizer angles. Detection limits using these techniques are analyzed to suggest their applicability for measuring other species of importance in combustion and plasma systems. Copyright © 2007 John Wiley & Sons, Ltd.