Effect of dielectric confinement on optical properties of colloidal nanostructures

We review the effects caused by a large difference in the dielectric constants of a semiconductor and its surrounding in colloidal semiconductor nanostructures (NSs) with various shapes, e.g., nanocrystals, nanorods, and nanoplatelets. The difference increases the electron–hole interaction and consequently the exciton binding energy and its oscillator transition strength. On the other hand, this difference reduces the electric field of a photon penetrating the NS (the phenomenon is called the local field effect) and reduces the photon coupling to an exciton. We show that the polarization properties of the individual colloidal NSs as well as of their randomly oriented ensemble are determined both by the anisotropy of the local field effect and by the symmetry of the exciton states participating in optical transitions. The calculations explain the temperature and time dependences of the degree of linear polarization measured in an ensemble of CdSe nanocrystals.     

Effect of Gd doping on structural, electrical and magnetic properties of BiFeO3 electroceramic

Room temperature multiferroic electroceramics of Gd doped BiFeO₃ monophasic materials have been synthesized adopting a slow step sintering schedule. Incorporation of Gd nucleates the development of orthorhombic grain growth habit without the appearance of any significant impurity phases with respect to original rhombohedral (R3c) phase of un-doped BiFeO₃. It is observed that, the materials showed room temperature enhanced electric polarization as well as ferromagnetism when rare earth ions like Gd doping is critically optimized (x=0.15) in the composition formula of Bi_1+2xGd_2x/2Fe_1−2xO₃. We believe that magnetic moment of Gd⁺³ ions in Gd doped BiFeO₃ tends to align in the same direction with respect to ferromagnetic component associated with the iron sub lattice. The dielectric constant as well as loss factor shows strong dispersion at lower frequencies and the value of leakage current is greatly suppressed with the increase in concentration of x in the above composition. Addition of excess bismuth and Gd (x=0.1 and 0.15) caused structural transformation as well as compensated bismuth loss during high temperature sintering. Doping of Gd in BiFeO₃ also suppresses spiral spin modulation structure, which can change Fe-O-Fe bond angle or spin order resulting in enhanced ferromagnetic property.     

Effect of calcium substitution on structural,dielectric, ferroelectric,piezoelectric, and energy storage properties of BaTiO3

The effect of calcium substitution on the structural, dielectric, ferroelectric, piezoelectric, and energy storage properties of BaTiO₃ (BT) ceramics has been investigated. XRD confirmed the phase formation of Barium Calcium Titanate (BCT), and structural Rietveld refinement was used to estimate the lattice parameters. It is evident from the SEM data that the average grain size decreases as calcium is added. At Curie temperature (110 °C), BCT 0.10 ceramic has a good dielectric constant of 15834 and a very low dielectric loss of 0.009. According to the ferroelectric and piezoelectric investigations, BCT 0.10 exhibits maximum spontaneous polarization with the highest piezoelectric charge coefficient of 100 pC/N. BCT 0.10 has a maximum energy storage density of 96.8 mJ/cm³ and a good energy storage efficiency of 53.9%, which is around three times that of pure BaTiO₃. These results suggest that the BCT ceramic has good potential for energy storage applications.     

Effect of vanadium doping on structural, magnetic and optical properties of ZnO nanoparticles

V-doped ZnO nanoparticles were synthesized by heating metal acetates in organic solvent. All synthesized samples were annealed in air and reducing gas atmosphere at 600 °C for 8 h. The XRD patterns of both samples annealed in air and reducing atmosphere indicate that samples have polycrystalline wurtzite structure with increase in lattice constant with increase in V-doping. The particle sizes were calculated by using Scherrer's equation which lies in the range of 25-30 nm. The SEM images show that particles annealed in air and under reducing environment are spherical in nature. The EDX results reveal that samples contain V, Zn, and O contents only. The TPR results indicate that the system contains isolated VO_x, ZnO_x and bimetallic Zn: V (O_x) sites and indication of electronically excited bimetal sites. There is no signature of ferromagnetism in all samples annealed in air while room temperature ferromagnetism has been observed only under reducing atmosphere annealing. There is monotonically increase in saturation magnetization with V-doping concentration. UV-vis spectroscopy study shows that there is a linear increase in band gap energy with increase in V-doping, a direct evidence of change in magnetic properties due to V-doping and under reducing environment.     

Effect of cobalt doping on structural,optical and redox properties cerium oxide nanoparticles

Cobalt-doped ceria nanoparticles were synthesized using the polyol method under co-precipitation hydrolysis. The structural, morphological, optical and redox properties were observed to investigate the influence of different concentration of cobalt ion doping on the prepared CeO₂ nanomaterials in terms of X-ray diffraction, field-emission transmission electron microscopy, thermogravimetric analysis, Fourier-transform infrared spectroscopy, UV/vis absorption spectroscopy and temperature program reduction techniques. The optical band gap energy was calculated from the optical absorption spectra for doped ceria nanoparticles, which have been found to be 2.68, 2.77, and 2.82 eV for the 2, 4, and 7 mol% Co ion-doped CeO₂ nanoparticles, respectively. As observed, the band gap energies increases as the doping Co ion concentrations increased, which could be due to significant increased oxygen vacancies with Co doping. The synergistic interaction between Co and CeO₂ was the main factor responsible for high catalytic activity of cobalt-doped CeO₂ model catalysts.     

Specific features of structural, dielectric, and optical properties of CdTiO3

The optical and dielectric properties and the structure of crystalline and ceramic perovskite CdTiO₃ modifications have been investigated. The multiplication of the unit cell along three perovskite axes is found. The new possible space groups of symmetry D _2h ¹⁸ and D _2h ²¹ are determined. It is elucidated how small radiation doses affect the structure and properties of crystalline and ceramic perovskite modifications. Analysis of the intensities of diffraction reflections demonstrate that the defect formation leads to an increase in the antiparallel displacements of Cd atoms in the unit cell.     

Unravelling the effect of Ni doping on the structural,optical and dielectric properties of nanocrystalline SnO2

Nanocrystalline pristine and Ni doped (x= 0.05, 0.10 and 0.15) SnO₂ samples are synthesized via sol-gel process. An extensive microstructure, optical and dielectric studies are performed through several analytical techniques. Analysis of the x-ray diffraction (XRD) data through Rietveld refinement confirms tetragonal crystal structure with P4₂/mnm space group for all the samples without any impurity or secondary phase. Scanning electron microscopy (SEM) images exhibit smooth surface morphology with agglomeration of the particles and energy dispersive x-ray spectra (EDS) confirm elemental composition of the samples. The average crystallite/particles size of the samples is found to increase for 5% Ni doped SnO₂ but reduces on further increase in the Ni content. Fourier transform infrared (FTIR) spectra show vibrational modes of the functional groups present in the samples. UV-visible absorption spectra indicate a red shift on Ni doping in SnO_2, ascribed to the trapping of excitons by the oxygen vacancies and thereby creating extra energy states within the bandgap. The band gap is found to diminish from 3.81 eV for pristine SnO₂ to 3.57 eV for the sample with 15% Ni concentration. Frequency dependent dielectric measurements at room temperature reveal higher values of the dielectric constant at lower frequencies that can be described on the basis of Maxwell-Wagner theory. The results demonstrate that the oxygen vacancies play a crucial role and concentration of Ni has strong influence on the microstructure, optical and dielectric behaviour of SnO₂.     

Effect of Mg doping on the structural and dielectric properties of strontium titanate ceramics

The influence of Mg incorporation into A- and B-sites of the perovskite lattice of SrTiO₃ ceramics on the structural, microstructural and low-frequency dielectric properties is studied in this work. Compounds in the systems Sr_1-xMg_xTiO₃ and SrTi_1-yMg_yO_3- were synthesised by a conventional solid-state method. The solid solubility of Mg is restricted to x<1% for A-site occupancy (Sr_1-xMg_xTiO₃) and to y<15% for B-site occupancy (SrTi_1-yMg_yO_3-). The lattice parameter is found to increase with Mg content for the SrTi_1-yMg_yO_3- system, while it is almost invariant in the Sr_1-xMg_xTiO₃ one. The dependence on the lattice-site occupancy is also verified for the grain growth of ceramics. For SrTi_1-yMg_yO_3- the average grain size markedly decreases with increasing Mg content. For Sr_1-xMg_xTiO₃ the inverse dependence is observed. Contrary to expectations, Mg does not induce ferroelectricity or relaxor-like behaviour in strontium titanate, located in either A- or B-sites of the SrTiO₃ lattice. Moreover, fitting the dielectric behaviour to Barretts law demonstrates that B-site doping drives the system away from the ferroelectric instability. In Mg-doped strontium titanate ceramics the dielectric permittivity and dielectric losses decrease. The results are discussed based on the correlation between cation-site occupancy, charge and chemical stoichiometry in both systems. PACS 61.10.Nz; 68.37.Hk; 68.37.Lp; 77.22.Ch; 77.22.Gm; 77.84.Dy     

Magnesium doping in hierarchical ZnO nanostructures and studies on optical properties

Magnesium doping in hierarchical zinc oxide nanostructures has been carried out using an aqueous method. The XRD results confirmed the hexagonal wurtzite structure for the magnesium doped zinc oxide nanoparticles. On doping with Mg²⁺, there is a change in morphology of the hierarchical nanostructures to nanorods. The optical absorption and photoluminescence properties of the nanostructures depend on the magnesium doping level. A blue shift of the band gap absorption and the near band edge emission is observed on Mg doping.     

Effect of rare-earth donor Ce3+ doping at A-site of PLSZNT on dielectric and piezoelectric properties

Dielectric and piezoelectric properties of [Pb_0.976La_0.014−xCe_xSr_0.01][Zr_0.57Ti_0.43]_{(0.9975−((0.014−x)/4)−(x/4))}Nb_0.002O₃ (PLCSZNT) ceramic compositions for 0 ⩽ x ⩽ 1 mol% were investigated. The XRD analysis showed the presence of single rhombohedral phase. Grain size and density increased until 0.6 mol% Ce and further Ce concentration inhibited the grain growth. The stability of rhombohedral phase has been supported by tolerance factor and average electronegativity difference. The room temperature dielectric response (ε_RT) increased up to 0.6 mol% combined with a significantly reduced dielectric loss (Tan δ) and low Curie temperature (T_c). The higher piezoelectric properties associated with low Ce concentration are attributed to rhombohedral phase. The optimum dielectric and piezoelectric properties were found in 0.6 mol% Ce composition which could be suitable for possible piezoelectric applications.     

Role of Fe doping on structural and vibrational properties of ZnO nanostructures

In this report, Raman and Fourier Transform Infrared (FTIR) measurements were carried out to study the phonon modes of pure and Fe doped ZnO nanoparticles. The nanoparticles were prepared by sol–gel technique at room temperature. The X-ray diffraction measurements reveal that the nanoparticles are in hexagonal wurtzite structure and doping makes the shrinkage of the lattice parameters, whereas there is no alteration in the unit cell. Raman measurements show both E₂^lowE_{2}^{\mathrm{low}} and E₂^HighE_{2}^{\mathrm{High}} optical phonon mode is shifted towards lower wave number with Fe incorporation and explained on the basis of force constant variation, stress measurements, respectively. In addition, Fe related local vibrational modes (LVM) were observed for higher concentration of Fe doping. FTIR spectra reveal a band at 444 cm⁻¹ which is specific to E ₁ (TO) mode; a red-shift of this mode in Fe doped samples and some surface phonon modes were observed. Furthermore, the observation of additional IR modes, which is considered to have an origin related to Fe dopant in the ZnO nanostructures, is also reported. These additional mode features can be regarded as an indicator for the incorporation of Fe ions into the lattice position of the ZnO nanostructures.     

Effect of uniaxial stress on the dielectric properties of BaTiO3+0.1wt.%Eu2O3 ceramics

ABSTRACT

BaTiO₃+0.1wt.%Eu₂O₃ ceramics were prepared by a solid-state reaction method. The dielectric behavior of these ceramics as a function of uniaxial pressure has been systematically studied. The external stress showed obvious effects on these properties. An increase of the Curie point (T_c) and decrease of the Curie–Weiss temperature (T₀) was observed with increasing pressure, resulting in an increase in the first-order nature of the phase transformation (T_c–T₀ increases). Broadening and flattening of the permittivity versus temperature curves near their maximum was found. The pressure behavior of thermal hysteresis and the ??/?T vs. T plot suggests that the phase transition changes to second-order type with increasing pressure. Furthermore, the Curie–Weiss constant obtained from a modified Curie–Weiss law strongly decreases with increasing pressure, suggesting that the mechanism of phase transition is going to order–disorder type.     

Effect of Fe-ion implantation doping on structural and optical properties of CdS thin films

We report on effects of Fe implantation doping-induced changes in structural, optical, morphological, and vibrational properties of cadmium sulfide thin films. Films were implanted with 90 keV Fe⁺ ions at room temperature for a wide range of fluences from 0.1×10¹⁶ to 3.6×10¹⁶ ions cm⁻² (corresponding to 0.38–12.03 at.% of Fe). Glancing angle X-ray diffraction analysis revealed that the implanted Fe atoms tend to supersaturate by occupying the substitutional cationic sites rather than forming metallic clusters or secondary phase precipitates. In addition, Fe doping does not lead to any structural phase transformation although it induces structural disorder and lattice contraction. Optical absorption studies show a reduction in the optical band gap from 2.39 to 2.17 eV with increasing Fe concentration. This is attributed to disorder-induced band tailing in semiconductors and ion-beam-induced grain growth. The strain associated with a lattice contraction is deduced from micro-Raman scattering measurements and is found that size and shape fluctuations of grains, at higher fluences, give rise to inhomogeneity in strain.     

Effect of Zn and Mg doping on structural,dielectric and magnetic properties of tetragonal CuFe2O4

The Cu_1?xA_xFe₂O₄ (A = Zn, Mg; x = 0.0, 0.5) ferrites were successfully synthesized by chemical co-precipitation method. X-ray diffraction and Raman spectroscopy reveals that all the ferrite samples are in single-phase with tetragonal structure for CFO and cubic spinel structure for CZFO and CMFO samples. SEM micrograph shows the variation of grain size with Zn and Mg doping in parent CFO sample. Frequency dependent dielectric response confirms the dielectric polarization and electrical conduction mechanism in the present series with a maximum value of dielectric constant and loss tangent for CZFO sample. The anomaly ～493 K in temperature dependent dielectric constant and dielectric loss is assigned to tetragonal to cubic phase transition in CFO sample. The magnetic measurement explored that the saturation value (M_s) is maximum for CZFO as compared to CFO and CMFO ferrites samples.     

Effect of aluminium doping and annealing on structural and optical properties of cerium oxide nanocrystals

Nanocrystalline fluorite-like structures of Ce_1−xAl_xO_2−δ compounds were prepared by the chemical precipitation method using cerium chloride and aluminium chloride as precursors. The prepared samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and diffuse reflectance spectroscopy (DRS). The effects of aluminium doping concentration and annealing on particle size, lattice parameter and band gap energies were investigated. The particle size of Al-doped CeO₂ samples were found to decrease with Al concentration and it increases from 6 to 20 nm as annealing temperature increases to 900 °C.     

Effect of iron doping and annealing on structural and optical properties of cerium oxide nanocrystals

Nanocrystalline fluorite-like structures of Ce_1−xFe_xO_2−δ compounds were prepared by chemical precipitation method using cerium chloride and iron chloride as precursors. The prepared samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and diffuse reflectance spectroscopy (DRS). The effects of iron doping concentration and annealing on particle size, lattice parameter and band gap energies were investigated. The particle size of Fe-doped CeO₂ samples were found to decrease with iron concentration and it increases from 9 to 26 nm as annealing temperature increases to 900 °C.     

Effect of Ca substitution on structural,magnetic and dielectric properties of BiFeO3

Bi_1-xCa_xFeO₃ (x = 0.40, 0.50) compounds are synthesized by conventional solid-state reaction method. The effect of Ca²⁺ substitution is investigated on structural, dielectric and magnetic properties. Rietveld refinement confirms that crystal structures of both the samples are tetragonal with P4mm symmetry. The highest values of remnant magnetization (M_r ) and coercive field (H_c ) are 0.002 emu/g and 0.23 kOe, respectively, for x = 0.50. The electric modulus formalism is used to distinguish and separate out the relaxation process which is dominated by the transport phenomenon. The relaxation process has activation energy ～0.32 eV which is found to be related to both the hopping mechanisms, i.e. electronic and oxide ions.